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Kinetics of direct reduction of unagglomerated iron-ore with coal char Roman-Moguel, Guillermo Julio 1984

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KINETICS OF DIRECT REDUCTION OF UNAGGLOMERATED WITH COAL CHAR  IRON-ORE  by GUILLERMO JULIO ROMAN-MOGUEL B. E n g . ( M e t a l l u r g y ) I n s t i t u t o P o l i t e c n i c o N a c i o n a l , M e x i c o , 1977 M.Sc. ( M e t a l l u r g y ) I m p e r i a l C o l l e g e , London, 1978  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department o f M e t a l l u r g i c a l E n g i n e e r i n g  We a c c e p t t h i s t h e s i s as c o n f o r m i n g to t h e required standard  c  THE UNIVERSITY OF BRITISH COLUMBIA December, 1984 © G u i l l e r m o J u l i o Roman-Moguel  L  In p r e s e n t i n g  this thesis i n p a r t i a l fulfilment of the  r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y of B r i t i s h Columbia, I agree that it  freely  t h e L i b r a r y s h a l l make  a v a i l a b l e f o r r e f e r e n c e and study.  I further  agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s  thesis  f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e h e a d o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . understood that for  financial  copying o r p u b l i c a t i o n of t h i s  D e p a r t m e n t o f >\e-TAtU)Q<^tc<SL.  £Nc=>'  The U n i v e r s i t y o f B r i t i s h C o l u m b i a 1956 Main M a l l V a n c o u v e r , Canada  V6T 1Y3  DE-6 (3/81)  .iMsiUfray  thesis  g a i n s h a l l n o t be a l l o w e d w i t h o u t my  permission.  Date  It i s  S-Hi \c\%S  M&EB. i M  (L,  written  i i  ABSTRACT  The k i n e t i c s of d i r e c t r e d u c t i o n of a commercial  unagglomerated  iron ore, with low-rank coal chars, have been investigated i n the tempera t u r e range of 800-950°C (1073-1223 K) using a l a b o r a t o r y - s i z e rotary reactor.  The variables studied were temperature, coal t y p e , p a r t i c l e  size  of c o a l and ore, f i x e d carbon-to-iron r a t i o , r o t a t i o n a l speed-and percent filling.  In addition the e f f e c t s of a c a t a l y s t on the Boudouard r e a c t i o n  and of i n e r t gas f l u s h i n g on the reduction rate were determined.  Mixing  studies at room temperature and at reduction temperature y i e l d e d the best mixing conditions p r i o r t o the k i n e t i c s determinations. Agglomeration between the reduced p a r t i c l e s was also studied. The m i x i n g experiments at room temperature y i e l d e d the f o l l o w i n g . The degree of mixing depends almost e n t i r e l y on the c o a l - t o - o r e s i z e r a tio.  Best mixing i s achieved with values of t h i s r a t i o of 1 and smaller,  f o r ore p a r t i c l e s larger than 254 ym; f o r s m a l l e r s i z e s than t h i s , good m i x i n g can be o b t a i n e d at h i g h e r coal-to-ore s i z e r a t i o s .  At reduction  temperature, improvement i n the reduction rate was not obtained e i t h e r by f u r t h e r i n c r e a s i n g the f i x e d carbon-to-iron r a t i o from 0.32 t o 0.64 or by varying the r o t a t i o n a l speed from 7 t o 20 r.p.m.. In t h e k i n e t i c s e x p e r i m e n t s , the o v e r a l l reduction process was found t o be c o n t r o l l e d up t o 0.5 to 0.8 f r a c t i o n a l r e d u c t i o n by t h e Boudouard r e a c t i o n , depending on the p a r t i c l e s i z e and temperature; from then  i ii  on, the k i n e t i c s were c o n t r o l l e d e s s e n t i a l l y by t h e r e d u c t i o n  reaction.  The a c t i v a t i o n energies obtained were 224 kJ/mole f o r the Boudouard react i o n , using sub-bituminous coal char, and 264 kJ/mole u s i n g l i g n i t e c h a r ; these values correspond t o that of a catalyzed r e a c t i o n .  coal  The cata-  l y t i c e f f e c t of the coal ash on t h e Boudouard r e a c t i o n was found t o be much l a r g e r than the respective e f f e c t of m e t a l l i c i r o n .  The presence of  a diluent gas extended the f r a c t i o n a l reduction over which Boudouard react i o n control i s exerted. The a c t i v a t i o n energy obtained f o r the reduction of w u s t i t e by CO i s 116.4 kJ/mole.  The analysis of the Poo/Pet^  produced by the reaction proved t o be a powerful t o o l  r a  tio  i n e l u c i d a t i n g the  rate c o n t r o l l i n g step. Smaller ore p a r t i c l e s were found t o agglomerate considerably more, in the non-catalyzed experiments; the addition of a c a t a l y s t f o r the Boudouard r e a c t i o n a l s o produced  larger agglomerates.  In neither case d i d  agglomeration retard the reduction rate t o a considerable extent.  No ac-  c r e t i o n growth was observed on the reactor w a l l . Estimative c a l c u l a t i o n s showed that s i m i l a r throughputs can be obtained by processing the unagglomerated concentrate, as compared t o operat i o n s which u t i l i z e indurated p e l l e t s under the same c o n d i t i o n s .  An ad-  vantage of a process using concentrates i s the lower temperature at which i t can be operated.  iv  TABLE OF CONTENTS  ABSTRACT  i i  LIST OF TABLES  viii  LIST OF FIGURES  x  LIST OF SYMBOLS  xvi  ACKNOWLEDGEMENTS  xix  Chapter 1  INTRODUCTION  1  2  REVIEW OF PREVIOUS WORK  4  2.1 Introduction  4  2.2 Direct-reduction Rotary K i l n s 2.2.1 Operation of reduction k i l n s 2.2.2 Accretion formation  10 11 15  2.3 K i n e t i c s studies of iron-oxide reduction with s o l i d reductants 2.4 Laboratory  scale studies i n rotary reactors  21 31  3  SCOPE OF THE PRESENT WORK AND OBJECTIVES  41  4  EXPERIMENTAL  44  4.1 Introduction  44  4.2 Materials and t h e i r preparation 4.2.1 Iron-ore concentrate 4.2.2 Low rank coals 4.2.3 Catalyst 4.3 Reduction apparatus 4.3.1 Heating system 4.3.2 Sealing system 4.3.3 Building materials and dimensions 4.3.4 Rotating system  45 45 48 52 52 54 54 57 61  V  4.3.5 Feeding, sampling and emptying systems 4.3.6 Temperature monitoring and control 4.3.7 Gas flow measurement and analysis  5  61 63 64  4.4 Apparatus f o r Room-Temperature Mixing Experiments  65  4.5 Coal Charring Equipment  67  4.6 Experimental Design and Variables 4.6.1 Variables i n room-temperature mixing experiments . 4.6.2 Variables i n reduction experiments 4.6.3 Experimental design  69 69 71 72  4.7 Experimental Procedures 4.7.1 Room-temperature mixing experiments 4.7.2 Coal charring experiments 4.7.3 Reduction experiments  74 74 76 77  RESULTS OF ROOM-TEMPERATURE MIXING EXPERIMENTS  83  5.1 E f f e c t of p a r t i c l e - s i z e r a t i o  87  5.2 Effect of r o t a t i o n a l speed  88  5.3 E f f e c t of fixed-carbon t o iron r a t i o  90  5.4 Effect of percent loading  93  5.5 Preliminary discussion of r e s u l t s from roomtemperature mixing experiments 6  RESULTS OF REDUCTION EXPERIMENTS 6.1 Results of coal d e v o l a t i l i z a t i o n experiments 6.1.1 Temperature measurement i n the coal bed 6.1.2 Effect of soak time and p a r t i c l e s i z e 6.1.3 Discussion of coal d e v o l a t i 1 i z a t i o n r e s u l t s 6.2 Calculation of f r a c t i o n a l reduction  93 103 103 105 105 109 I l l  6.3 Results of experiments t o determine variables ranges ... 6.3.1 Effect of fixed-carbon-to-iron r a t i o 6.3.2 E f f e c t of r o t a t i o n a l speed 6.3.3 Effect of percent loading 6.3.4 Preliminary discussion of r e s u l t s from experiments to determine variables ranges  115 116 116 116  6.4 Results of main experimental block 6.4.1 Effect of temperature: the base case  122 122  120  vi  6.4.2 E f f e c t o f Cp-j /Fe s t o i c h i o m e t r i c r a t i o 6.4.3 Reduction o f f i n e r p a r t i c l e s under well-mixed conditions 6.4.4 Reduction under bed segregation c o n d i t i o n s 6.4.5 R e p r o d u c i b i l i t y t e s t s 6.4.6 Preliminary d i s c u s s i o n o f r e s u l t s from the main experimental block  126  6.5 Results of comparative experiments 6.5.1 Reduction using a c a t a l y s t f o r Boudouard r e a c t i o n . 6.5.2 Reduction with l i g n i t e 6.5.3 Reduction with graphite 6.5.4 E f f e c t of i n e r t - g a s f l u s h i n g 6.5.5 Reduction o f hematite p e l l e t s 6.5.6 P r e l i m i n a r y d i s c u s s i o n o f r e s u l t s from the comparative experiments  148 148 151 151 151 155  STUDY OF PARTICLES AGGLOMERATION DURING REDUCTION  160  7.1 P a r t i c l e s i z e d i s t r i b u t i o n a f t e r reduction  160  7.2 Scanning e l e c t r o n microscope observations  166  x  7  126 130 130 134  157  7.3 Preliminary d i s c u s s i o n on p a r t i c l e s agglomerat i o n during reduction 8  171  OVERALL DISCUSSION OF RESULTS AND PROPOSED MECHANISMS  175  8.1 T e s t i n g o f a v a i l a b l e k i n e t i c s r e l a t i o n s h i p s f o r the reduction o f iron-oxides with carbon 8.1.1 Boudouard r e a c t i o n as rate c o n t r o l l i n g mechanism . 8.1.2 Reduction r e a c t i o n as rate c o n t r o l l i n g mechanism . 8.2 Overall view o f the reduction process  175 177 191 193  8.3 Estimative c a l c u l a t i o n s f o r the operation o f an i n d u s t r i a l - s i z e rotary k i l n 9  SUMMARY AND CONCLUSIONS  LIST OF REFERENCES  199 203 207  APPENDICES A  HEAT BALANCE CALCULATIONS TO DIMENSION ROTARY REACTOR ..  219  B  CALIBRATION CURVES FOR FLOWMETERS ....'  224  C  CONDITIONS FOR ROOM-TEMPERATURE MIXING EXPERIMENTS  229  vii  D E F  LISTING OF PROGRAM TO PROCESS REDUCTION EXPERIMENTS DATA AND SAMPLE OUTPUT  230  SUMMARY OF OVERALL MASS-BALANCES FOR REDUCTION EXPERIMENTS  243  CALCULATIONS FOR THE OXIDATION OF SiC HEATING ELEMENT ..  244  vii i  LIST OF TABLES  Chapter 2 I II III  Page  Direct reduction of iron-bearing material with fluid reductants^'H'^  g  Direct reduction of iron-bearing materials with solid reductants9>ll>13  7  Main operational features of rotary-kiln direct reduction p l a n t s '  13  Analysis of accretions formed in a pilot-plant size rotary k i l n  18  4 1  IV  4 6  7 9 - 7 9  V Phases encountered in the accretions formed during direct reduction in a rotary k i l n ^ " 7  VI VII  77  18  Studies on reduction kinetics of iron oxides with carbonaceous materials  26  Characteristics of laboratory-size rotary kiln expermiments for reduction of iron oxides  33  Chapter 4 VIII  IX  Chemical analysis of spiral iron-ore concentrate, full size range and two size fractions: -420 +300 and -106 +74 ym  47  Random loose and packed bulk densities of the ironore concentrate and the Forestburg coal, for different particle sizes  50  X Proximate and ultimate analyses of Forestburg coal and Saskatchewan lignite. Mean particle sizes: 718, 180 and 90 vm XI XII XIII  51  Thermophysical properties of refractory materials used in the reactor  59  Summary of variables and levels tested in the mixing and reduction experiments using Forestburg coal . .  70  Certified grade gas standard composition  79  ix  XIV  Gas chromatograph operating conditions  79  Chapter 5 XV  Angle of repose f o r d i f f e r e n t coal p a r t i c l e sizes and ore/coal mixtures with 254 ym ore p a r t i c l e s i z e ...  101  Chapter 6 XVI  XVII  XVIII XIX XX  XXI  Temperatures at four locations i n the coal bed, according to Figure 6.1, f o r p a r t i c l e sizes of -841 +600, -420 +300, -210 +149 and -106 +74 um  107  Proximate and ultimate analyses of Forestburg coal and Saskatchewan l i g n i t e a f t e r 10 hours charring t r e a t ment. Mean p a r t i c l e s i z e s : 718, 180 and 90 \im  110  Ash composition of Forestburg lignite  110  coal and Saskatchewan  Retention times of the product gases at d i f f e r e n t sections of the reactor  137  Stoichiometric r a t i o s f o r reduction of Fe203 with carbon under two types of reaction c o n t r o l : Boudouard and reduction  142  Conditions and r e s u l t s f o r reduction of hematite p e l l e t s  156  Chapter 8 XXII  Reaction rate constants f o r Stages I to I I I during reduction experiments  185  X  LIST OF FIGURES  Chapter 2 2.1  2.2  2.3  2.4 2.5  2.6  Page  Flowsheet f o r general d i r e c t reduction process (Modified from reference 15)  5  Main features of the SL/RN rotary k i l n process (Modified from reference 9)  9  Formation of accretions with 10% f i n e s i n three dimensional p l o t t i n g ' ^  19  Q u a l i t a t i v e progress of the formation of accretions as a function of reduction time and furnace l e n g t h ^ .  19  Sub-processes i n the reduction of iron-oxide with carbon  23  R e a c t i v i t i e s of d i f f e r e n t carbonaceous materials as a function of temperature-^!  37  Chapter 4 4.1  P a r t i c l e s i z e d i s t r i b u t i o n of s p i r a l iron-ore concentrate  46  4.2  SEM  49  4.3  Overall view of experimental  4.4  Top view of rotary reactor showing sealing areas  55  4.5  (A) Side view of open rotary reactor  58  photograph of s p i r a l iron-ore concentrate  (lOOx)  set-up f o r reduction t e s t s ..  (B) Central cross-sections of rotary reactor 4.6  Sampling probe configuration  4.7  Equipment f o r room-temperature mixing experiments (A) Gen-  53  58 62  eral view; (B) Detail view of a c r y l i c blade  66  4.8  Schematic view of coal d e v o l a t i 1 i z a t i o n equipment  68  4.9  Graph showing bed volume and depth as a function of percent loading f o r a reactor 14 cm i n diameter Reaction chamber temperature as a function of time  75 81  4.10  xi  Chapter 5 5.1  D e f i n i t i o n of boundaries of degree of mixing. Conditions: dp = 358 m; C F j / F e = 0.45; 15 r.p.m.; 20% loading; c f / a ~ f o r (A), 4.464; (B), 2.000; (C), 1.285; (D), 1,000; (E), 0.714; and ( F ) , 0.503. (A) and (B), segregated bed; (C) and (D), t r a n s i t i o n a l condition and (E) and ( F ) , well-mixed bed  84  E f f f e c t of d'c/cTpe s i z e r a t i o on the degree of mixing f o r 90 m iron-ore p a r t i c l e s . Conditions: Cp-j /Fe = 0.45; 15 r.p.m. and 20% loading  89  x  e  c  5.2  Fe  x  5.3  Effect of r o t a t i o n a l speed on the degree_ of_ mixing f o r 180 m iron-ore p a r t i c l e s . Conditions: d /dp = 0.5; CFix/Fe = 0.45 and 20% loading c  5.4  e  Effect of fixed-carbon to iron r a t i o on the degree of mixing f o r 358 m iron-ore p a r t i c l e s . Conditions: d /dp = 4; 20% loading and 15 r.p.m c  5.5  c  94  Predominance area diagram f o r the degree of mixing as a function of cT /cIp s i z e r a t i o and iron-ore mean p a r t i c l e s i z e , a p . Conditions: CF-j /Fe = 0.45; 5 t o 15 r.p.m. and 20% loading  95  Predominance area diagram f o r the type of bed motion as a function of cTc/dpe s i z e r a t i o and iron-ore mean • p a r t i c l e s i z e , cTp . Conditions: Cp-j /Fe = 0.45; 5 and 15 r.p.m., and 20% loading  96  Void s i z e between p a r t i c l e s as a function of coal part i c l e s i z e i n a loose bed  99  c  e  e  e  5.8  92  e  x  5.7  e  Effect of percent loading on the degree of mi_xing f o r 358 ym iron-ore p a r t i c l e s . Conditions: d /dp = 4.0; C F i / F e = 0.45 and 15 r.p.m  5.6  91  x  x  Chapter 6 6.1  6.2  6.3  Temperature at four l o c a t i o n s i n the coal bed during d e v o l a t i l i z a t i o n treatment  106  Percent hydrogen remaining i n char, a f t e r d e v o l a t i l i z a t i o n treatment at 900°C, f o r four coal p a r t i c l e s i z e s : 716, 358, 180 and 90 ym  108  Gas composition as a function of reaction time f o r a reduction experiment; conditions as shown  113  xii  6.4  P l o t of f r a c t i o n a l reduction versus time showing the e f f e c t of Cp-jx/Fe r a t i o ; conditions as shown  117  P l o t of f r a c t i o n a l reduction versus time showing the e f f e c t of r o t a t i o n a l speed at two C p i / F e r a t i o s ; conditions as shown  118  Plot of f r a c t i o n a l reduction versus time showing the e f f e c t of percent f i l l at two Cf-\ /Fe r a t i o s ; conditions as shown  119  6.7  Variables i n main experimental block  123  6.8  P l o t of f r a c t i o n a l reduction versus time showing the e f f e c t of temperature: the base case. Conditions as shown  124  Change i n PQQ/PQQ r a t i o with f r a c t i o n a l reduction during experiments of base case; conditions as shown  125  P l o t of f r a c t i o n a l redution versus time showing the e f f e c t of s t o i c h i o m e t r i c Cpix/Fe r a t i o ; conditions as shown  127  Change i n PQQ/PQQ r a t i o with f r a c t i o n a l reduction during experiments with s t o i c h i o m e t r i c Cpi /Fe r a t i o ; cond i t i o n s as shown  128  6.5  x  6.6  x  6.9  2  6.10 6.11  x  6.12  Plot of f r a c t i o n a l reduction versus time showing the e f f e c t of f i n e r p a r t i c l e s ; conditions as shown 129  6.13  Change i n Pco/ COo r a t i o with f r a c t i o n a l reduction during experiments with f i n e r p a r t i c l e s ; conditions as shown p  131  6.14  Plot of f r a c t i o n a l reduction versus time showing the e f f e c t of bed segregation; conditions as shown 132  6.15  Change i n P c o / C O o r a t i o with f r a c t i o n a l reduction during experiments with segregated bed; conditions as shown ..  133  P l o t of f r a c t i o n a l reduction versus time f o r experimental r e p r o d u c i b i l i t y t e s t s ; conditions as shown  135  Stoichiometric Cp-j /Fe r a t i o as a function of temperature f o r the reduction of Fe203 with carbon  143  F r a c t i o n a l reduction rate as a function of f r a c t i o n a l reduction f o r experiments of the base case  146  6.16  6.17  6.18  p  x  xi i i  6.19  Plot of f r a c t i o n a l reduction versus time showing the e f f e c t of a c a t a l y s t of the Boudouard r e a c t i o n ; conditions as shown 149  6.20  Change i n PQQ/PQ,Q r a t i o with f r a c t i o n a l reduction during catalyzed and ^ - f l u s h i n g experiments; conditions as shown 0  150  6.21  Plot of f r a c t i o n a l reduction'versus time f o r reduction with Saskatchewan l i g n i t e ; conditions as shown 152  6.22  Change i n Pco/PcO? r a t i o with f r a c t i o n a l reduction f o r experiments wixh l i g n i t e ; conditions as shown  153  Plot of f r a c t i o n a l reduction versus time showing the eff e c t s of N2~flushing on reduction with Forestburg coal and reduction with graphite; conditions as shown  154  6.23  Chapter 7 7.1  7.2  7.3  P a r t i c l e size d i s t r i b u t i o n a f t e r reduction of 90 pm p a r t i c l e s  161  P a r t i c l e size d i s t r i b u t i o n a f t e r reduction, case; (b) catalyzed experiments  (a) base  P a r t i c l e size d i s t r i b u t i o n a f t e r reduction, gated bed; (b) Stoichiometric Cpi /Fe  (a) segre-  P a r t i c l e s i z e d i s t r i b u t i o n a f t e r reduction, case; (b) L i g n i t e reductant  (a) base  162  163  x  7.4  7.5  7.6  7.7  7.8  164  Agglomerates average s i z e as a function of reduction temperature. Conditions of experiments as shown  167  Agglomeration r e l a t i v e t o the o r i g i n a l ore size as a function of temperature. Conditions of experiments as shown  168  Agglomerate formed during reduction of 90 vm i r o n ore p a r t i c l e s (lOOx)  169  Iron whiskers produced during reduction j o i n i n g two reduced p a r t i c l e s (800x)  169  7.9  S i l i c a t e p a r t i c l e between two reduced grains (800x)  170  7.10  Agglomerate formed during reduction under catalyzed conditions (lOOx)  170  XIV  7.11  Non-metallic p a r t i c l e s between reduced iron grains during reduction under catalyzed conditions (200x) . . .  173  Chapter 8 8.1  8.2  Plot of In (1-fc) vs t for the base case experiments (Boudouard control)  Plot of In (1-fc) vs t for the stoichiometric C p j / F e experiments  (Boudouard control)  x  8.3  8.4  8.5  8.7  180  Plot of In ( 1 - f r ) vs t for the finer p a r t i c l e s experiments (Boudouard control)  181  Plot of ln ( 1 - f r ) vs t for the segregated bed experiments iBoudouard control)  182  Plot of ln (1-fc) vs t for the catalyzed and N2~flushed experiments  8.6  179  (Boudouard control)  Plot of ln ( 1 - f r ) vs t for the l i g n i t e experiments (Boudouard control)  183  reductant 184  Arrhenius plots for base case, stoichiometric C p i / F e , l i g n i t e reductant and catalyzed experiments. Stage II (Boudouard control)  187  Arrhenius plots for base case, segregated bed and finer p a r t i c l e s experiments. Stage II (Boudouard control) .  190  Arrhenius plots for base case, stoichiometric Cp-j /Fe and segregated bed experiments. Stage I (Boudouard control)  192  Plot of 1 - ( l - f R ) l / 3 i f Stage III (Reduction control)  194  x  8.8  8.9  8.10  8.11  8.12  8.13  x  v  s  o r  D  a  s  e  c  a  s  e  experiments.  Plot of 1 - ( l - f R ) l / 3 vs t for segregated bed experiments. Stage III (Reduction control)  195  Plot of 1 - ( l - f R ) l / 3 t f catalyzed Stage III (Reduction control)  196  v  s  o r  experiments.  Arrhenius plots for base case, segregated bed and catalyzed experiments. Stage III (Reduction control)  197  XV  Appendix A A.l  Heat losses and surface temperature for different inner diameters and insulation thicknesses  223  Appendix B B.l  B.2  B.3  B.4  Calibration curve for gas-standard flowrate in flowmeter Gilmont #1  225  Calibration curve for gas-standard flowrate in flowmeter Gilmont #2  226  Calibration curve for gas-standard flowrate in flowmeter Gilmont #3  227  Calibration curve for gas-standard flowrate in flowmeter Gilmont #4  228  XVI  LIST OF SYMBOLS A  Bed surface area [m^]  Cpi  F i x e d carbon i n coal and char  x  Cp-j /Fe x  Fixed c a r b o n - t o - i r o n r a t i o by weight  (C)  t  Amount o f carbon reacted up to time t , [mole]  d/\gg  Average s i z e of agglomerate, [ym], i n Equations (7.1) and (7.2)  d  Coal mean p a r t i c l e s i z e , [ym]  c  d /dp c  dp  e  e  Coal to ore s i z e r a t i o Iron ore mean p a r t i c l e s i z e [ym]  di  Average s i z e of the openings between two s c r e e n s [ y m ] , E q u a t i o n (7.2)  Eff  E f f i c i e n c y o f s e a l i n g i n the r e a c t o r  (Eff)-,  E f f i c i e n c y o f s e a l i n g i n the r e a c t o r a t beginning o f experiment  (Eff)f  E f f i c i e n c y o f s e a l i n g i n the r e a c t o r a t end o f experiment  fR  Fractional reduction  ll  S p e c i f i c r a t e constant i n Equations (2.8) and (2.9)  k  O v e r a l l r a t e constant i n Equations (2.3) through (2.5), [cm^/s]  kg  Rate constant f o r Boudouard r e a c t i o n  kp  Rate constant f o r r e d u c t i o n r e a c t i o n  Ki  E q u i l i b r i u m constant i n Equation (6.11)  K2  E q u i l i b r i u m constant i n Equation (6.12)  K3  E q u i l i b r i u m constant i n Equation (6.13)  1  Average v o i d s i z e , [ym]  m  c  Molar f l o w r a t e o f C, [mol/min], Equation (6.10)  xvi i  M o l a r f l o w r a t e o f CO, [ m o l / m i n ] , E q u a t i o n s  mco  (6.5),  (6.9)  and  (6.10)  M o l a r f l o w r a t e o f CO2, [ m o l / m i n ] , E q u a t i o n s  rfiQQ  (6.6),  (6.9) and  (6.10) ITIQ  Molar f l o w r a t e o f 0£, [mol/min], Equation  (Ox)t  Moles o f oxygen removed up t o time t [mole], Equation (6.1)  (0x)j t o  (6.9)  T o t a l moles o f oxygen i n c o n c e n t r a t e and c o a l a s h , E q u a t i o n (6.1)  PQQ  P a r t i a l pressure of CO  PQQ  P a r t i a l pressure o f CO2  PH  P a r t i a l pressure o f H2  QQQ  CO f l o w r a t e [£/min], Equations (6.3) and (6.5)  QC0 QR  2  0 T  c 0  2  f l o w r a t e [£/min], Equations  (6.4)  and (6.6)  Gas f l o w r a t e measured by rotameter, [ V m i n ] , Equation  (6.2)  QTot  C o r r e c t e d gas f l o w r a t e [ V m i n ] , Equation  R  Kiln radius  RQ  O v e r a l l r a t e constant [ m i n ] i n Equations (2.6) and (2.7)  RQQ  Rate o f CO formation [mol/min] i n Equation  (2.8)  RQQ^  Rate o f CO2 formation [mol/min] i n Equation  (2.9)  r  I n i t i a l r a d i u s of oxide p a r t i c l e  0  (6.12)  -1  T  Temperature  t  Reaction time  V  Bed volume [m ]  X-j  Amount of m a t e r i a l r e t a i n e d between two s i e v e s  Z  Volume o f product formed per u n i t volume o f s o l i d consumed, i n Equation (2.5)  3  xvi l i  LIST OF SYMBOLS  Greek  p  6  F r a c t i o n o f CO u t i l i z e d i n reduction, Equations (2.8) and (2.9)  <>f  Mole f r a c t i o n o f CO i n e q u i l i b r i u m with FeO/Fe or CO/CO?, Equat i o n (6.16)  corr  C o r r e c t i o n f a c t o r f o r d i f f e r e n c e i n d e n s i t i e s between standard and product gas  a  P r o p o r t i o n a l i t y constant, Equation (5.1)  to  Rotational  T  Retention time o f gases i n reactor [ s ]  speed  $r,c  Angle o f repose o f c o a l , [°]  $r,mix  Angle o f repose o f ore/coal mixture, [°]  xix  ACKNOWLEDGEMENTS  I wish t o e x p r e s s my most s i n c e r e g r a t i t u d e to Dr. J.K. Brimacombe f o r h i s guidance and unswerving  support throughout  the diverse  d i f f i c u l t i e s t h i s research e n t a i l e d . To Dr. G.G. Richards I extend my warmest a p p r e c i a t i o n f o r useful d i s c u s s i o n s and f o r h i s help and support d u r i n g t h e l a s t , c r u c i a l s t a g e s o f t h i s work.  To Mr. P. Wenman, my  g r a t i t u d e f o r a l l h i s i n t e r e s t and help throughout the d u r a t i o n o f t h i s task cannot be o v e r s t a t e d . The f i n a n c i a l s u p p o r t o f CONACYT and IPN, Mexico, and NSERC, Canada, through t h e s c h o l a r s h i p s they p r o v i d e d me w i t h a t d i f f e r e n t stages o f t h i s work, i s g r a t e f u l l y acknowledged.  In t h i s regard, Mr. S.  Martinez a l s o deserves my very s p e c i a l thanks. The d i s c u s s i o n s and a s s i s t a n c e o f f e l l o w graduate students:  Dr.  A. Bustos ( f o r teaching me the " t r i a l and e r r o r " method), Mr. R. V a r v a l hol , Mr. H. C a s t i l l e j o s and Mr. C. Schvezov are a l s o s i n c e r e l y acknowledged.  Dr. G. Cabanas deserves my personal thanks f o r s t e e r i n g me i n t o  graduate s t u d i e s . F i n a l l y , t o my f a t h e r Vincente f o r e v e r y t h i n g he has given me, and to my wife Mayra f o r her patience and support, I g r a t e f u l l y dedicate t h i s work.  XX  How many roads must a man walk down Before you c a l l him a man? R. Zimnermann  1  CHAPTER 1  INTRODUCTION  D i r e c t Reduction processes, t h a t i s , f o r t h e r e d u c t i o n o f i r o n o x i d e s i n t h e s o l i d s t a t e , p l a y an i n c r e a s i n g l y important part i n t o day's ironmaking i n d u s t r y . The obvious advantages o f c i r c u m v e n t i n g t h e s m e l t i n g o f t h e o x i d e s , w i t h t h e consequent lowering o f the heat r e quirements f o r the o v e r a l l process and ease i n handling o f the p r o d u c t s are c l e a r .  Furthermore,  t h e c o m m e r c i a l l y proved c a p a b i l i t y o f these  processes t o operate on a s m a l l e r s c a l e than t h e b l a s t f u r n a c e , even down t o 40(10)3 t o n n e s p e r y e a r , and t o use indigenous reductant r e sources such as low-rank c o a l s or natural gas, makes them a t t r a c t i v e a t l e a s t i n r e g i o n s which do n o t have a s t a b l e scrap supply.  However i n  d i r e c t r e d u c t i o n processes, which operate a t 900 to 1150°C the r e d u c t i o n k i n e t i c s a r e s l o w e r and t h e t h r o u g h p u t per u n i t volume o f r e a c t o r i s consequently lower than i n the b l a s t furnace.  Therefore the need t o a c -  c e l e r a t e t h e r e d u c t i o n r a t e and minimize i n h e r e n t o p e r a t i o n a l problems is strong. 1  Rotary k i l n D i r e c t Reduction processes, o f which the SL/RN type i s the most widely used, exemplify the f o r e g o i n g c o n s i d e r a t i o n s .  With  p l a n t s r a n g i n g from a one u n i t , 30(10)^ ton/year, to nine u n i t s , 2(10)^ ton/year, i n c o u n t r i e s with such d i f f e r e n t r e d u c t a n t raw m a t e r i a l s as  2  India and A u s t r a l i a , the commercial strated.  c a p a b i l i t y of t h i s process i s demon-  So f a r however, t h i s process has operated using mainly e i t h e r  iron-ore p e l l e t s or lumps, with the notable exception of f i n e ores which contain r e l a t i v e l y high c o n c e n t r a t i o n s of r e f r a c t o r y - m e t a l o x i d e s , such as  Ti02  or  The presence of these oxides hinders the formation of  V2O5.  a c c r e t i o n s on the k i l n ' s inner wall i n s p i t e of the small p a r t i c l e s i z e . These a c c r e t i o n s , as w i l l be d e s c r i b e d i n Chapter 2, represent the main operational problem i n the r o t a r y k i l n processes.  The use o f p e l l e t s ,  on the o t h e r hand, diminishes the extent of t h i s problem but r e s u l t s i n a comparatively slower r e d u c t i o n rate and adds, with the p e l l e t i z a t i o n s t a g e , a f u r t h e r 30 p e r c e n t as a minimum to the cost2>3 f the D i r e c t 0  Reduced Iron (DRI)  product.  T h i s work was thus aimed at the study of the reduction k i n e t i c s of an unagglomerated  i r o n - o r e concentrate, using low rank c o a l s as r e -  d u c t a n t i n a l a b o r a t o r y - s i z e r o t a r y r e a c t o r . At the same time the poss i b l e formation of agglomerates  a g a i n s t the r e a c t o r w a l l was  as a f i r s t step i n the development of a process of t h i s k i n d .  examined, This work  a l s o complements other k i l n research e f f o r t s i n t h i s department, namely: the motion c h a r a c t e r i s t i c s of the s o l i d s bed and the p a r t i c l e s segregat i o n , by H e n e i n ,  4  GorogS and B a r r ,  the heat t r a n s f e r c h a r a c t e r i s t i c s of the process by 6  the o v e r a l l mathematical  modelling of the SL/RN pro-  7  c e s s by Venkateswaran, and the reduction k i n e t i c s of t i t a n i f e r o u s ores by  Sucre-Garcia.8 T h i s t h e s i s i s o r g a n i z e d a l o n g the f o l l o w i n g l i n e s .  The most  r e l e v a n t p a r t of the extensive l i t e r a t u r e on the subject i s r e v i e w e d i n Chapter  2.  This i n c l u d e s general papers on D i r e c t Reduction and r o t a r y  3  k i l n o p e r a t i o n s , fundamental s t u d i e s on iron-oxide reduction with s o l i d reductants and l a b o r a t o r y - s c a l e s t u d i e s i n r o t a r y r e a c t o r s i n c l u d i n g r e d u c t i o n k i n e t i c s and the a c c r e t i o n formation.  In Chapter 3, the objec-  t i v e s of t h i s work are presented i n d e t a i l , and the e x p e r i m e n t a l n i q u e s a r e l a i d out i n Chapter 4.  tech-  The r e s u l t s of room-temperature mix-  ing experiments, of the reduction experiments and the study o f p a r t i c l e agglomeration,  with p r e l i m i n a r y d i s c u s s i o n s i n each case, are  presented  i n Chapters 5, 6 and 7, r e s p e c t i v e l y . F i n a l l y , the o v e r a l l d i s c u s s i o n of the r e s u l t s and proposed mechanisms, are presented i n Chapter 8.  4  CHAPTER 2  REVIEW OF PREVIOUS WORK  2.1 I n t r o d u c t i o n The d i r e c t r e d u c t i o n o f i r o n oxide, a t the fundamental  and i n -  d u s t r i a l l e v e l s , i s a broad f i e l d o f research and has been t h e s u b j e c t o f s e v e r a l s t u d i e s i n t h e l a s t t h i r t y y e a r s , owing to i t s i n c r e a s i n g economic s i g n i f i c a n c e . 9 - 1 4 A general flowsheet f o r the d i r e c t - r e d u c t i o n p r o c e s s e s i n commercial  o p e r a t i o n i s shown i n Figure 2.1.15 The pro-  cesses can be c l a s s i f i e d i n t o two groups: those which u t i l i z e f l u i d r e a c t a n t s , i n s h a f t furnaces or f l u i d i s e d beds, and processes that employ s o l i d coal as reductant, u s u a l l y i n r o t a r y k i l n r e a c t o r s . A summary o f d i r e c t - r e d u c t i o n p r o c e s s e s , a c c o r d i n g to t h i s c l a s s i f i c a t i o n , i s presented i n Tables I and I I . P r o c e s s e s t h a t u t i l i z e f l u i d r e d u c t a n t s and s h a f t f u r n a c e s account f o r over 85 percent o f the d i r e c t - r e d u c e d i r o n (DRI) p r o d u c e d t o d a y . 16  They can be d e s c r i b e d b r i e f l y as f o l l o w s :  Iron ore lumps or  p e l l e t s are charged t o the t o p o f t h e v e r t i c a l s h a f t and as they des c e n d , c o n t a c t a r i s i n g s t r e a m o f r e d u c i n g g a s . The i r o n oxide i s reduced i n the upper s e c t i o n o f the r e a c t o r , while c a r b o n i z a t i o n and  5  Iron ore  Iron ore pellets  Pel letizer  I I  [_Fine_ concentrate  (I) Sized coal I 1 K2) CO a H mixture I  I I i  Off-gas,dust, heat recovery >•  1  2  i i Fuel Direct  pretreatment  reduction  vessel  reactor 1  »  Fossil fuel (1) Coal (2) Natural (3) Fuel oil  2.1  gas Oirect  Flowsheet f o r general d i r e c t r e d u c t i o n ( M o d i f i e d from r e f e r e n c e 15)  reduced  process  iron  9  1  TABLE I: DIRECT REDUCTION OF IRON-BEARING MATERIALS WITH FLUID REDUCTANTS .* .!3  PROCESS NAME  REACTOR Ld.xL  FEED [mm]  REDUCTANT  Cn]  T (REDUCTION) [°C]  ARMCO  Shaft 5 x 27.4  Pellets, Ore (5-20)  Nat.Gas  FIOR  Fluid Bed  Concentrate (<5)  Nat.Gas  880  HIB  Fluid Bed 6.7 x 52  Concentrate (<2)  Nat.Gas  870  HyL  Fixed Bed  Pellets, Ore (9-16)  Nat.Gas  870-1030  MIDREX  Shaft 4.9 x 28  Pellets, Ore (5-20)  Nat.Gas  NSC  Shaft 2.5 x ?  Lump, Pellets  Sha f t 7 x?  Lump, Pellets  PUROFER  Oil  Hydrocarbons  * COST including capital cost in U.S. Oils in 1982.  760-800  ENERGY CONMSMP. Fuel Electric [GCal] [KWh]  PROD. RATE Ton DRI m3 Day  COST* $ Ton DRI  MET. [%]  PECULIARITIES AND COMMENTS  136.66  92  Steam reformed natural gas.  2.83  37  1.77  4.00  45  1200/Vol  N.A.  93  3 Reactors In CC.series 10 Atm. Aux. Equipment  40  0.30  N.A.  70  2 Reactors 1n series. 2 Atm. Product Briquetted  2.40  45  0.85  132.56  92  Batch. 4 reactors In Cycle. Top inlet of Gas.  850-900  2.67  125  1.97  136.03  92  Countercurrent gas-solid.  1000  3.00  180  500/Vol  N.A.  950-1000  3.50  270  960/Vol  N.A.  Counterpressure: 5 Atm. Texaco Gasification process. 92  Rectangular section. Product briquetted.  9  TABLE II.  PROCESS NAME  REACTOR Ld.xL [•]  11  DIRECT REDUCTION OF IRON-BEARING MATERIALS UITHI SOLID REDUCTANTS . »  FEED [mm]  REDUCTANT (REDUCTION) [°C]  KAWASAKI  Rotary Kiln 5 x 50  Waste dust/ Coke Breeze sludge (10-15)  KINGLORMETOR  Fixed Bed 0.45x0.6x12  Lump ore or Pellets (6-25)  KOHO  Rotary K1ln 3.3 x 24  KRUPPCODIR  ENERGY Fuel [GCal]  CONMSMP. Electric [KWh]  PROD. RATE fTon DRI(  L«J|)jy J  COST J Ton DRI  13  MET. [i]  PECULIARITIES AND COMMENTS  1100-1200  3.85  115  0.59  N.A.  95  Pelletl zed Dust. Gratekiln plant.  Coal/Char  1050  3.80  80  1.16  N.A.  92  Preheatlng-ReductlonCool Ing  Pellets with Carbon (5-16)  Char from BF Dust  1150  2.85  110  0.67  N.A.  74  GMndlng-PelletlzIngReduction Pellets from Dust. Bentonlte.  Rotary Kiln 4.1 x 73.5  Lump Ore (5-25)  Anthracite <10 mm  3.90  55  0.42  124.78  93  Any reductant can be used.  SDR  Rotary K1ln 4.5 x 71  Pelletlzed waste Iron dust (>6)  3.69  700  0.41  N.A.  93  Product used as BF Feed.  SL/RN  Rotary K1ln 3.6 x 50 6 x 125  Lump (3-20) Pellets (8-10) Fines (0.04-3)  3.90  85  0.42  93  Simpler. Any solid reductant. Underbed air Injection.  SPM  Rotary Kiln 3.9 x 80  Waste Sludge Fine  1200  2.10  220  0.54  N.A.  77  Reduction and Agglomeration simultaneously. M1x-F1lter-dry-feed. Scrapper bar.  Rotary Kiln 2.5 x 45  Lump or Sinter  1050-1100  3.00  35  0.45  116.23  93  Alternate Injection of gas (bottom) and air (top).  ACCAR  Coke fines (ground) Coal (<20)  Anthracite  Coal (20%) Gas (80%)  * COST including capital cost in U.S. Dlls. in 1982.  950-1150  1150  900-1100  107.71  8  c o o l i n g o f the p r o d u c t o c c u r i n the l o w e r p a r t . 17,19  A v a r i a t i o n of  t h i s process i s the r e t o r t system, which operates on a batch b a s i s w i t h the gas i n j e c t e d from above.18 Rotary k i l n processes,!'20 on the other hand, make use of a hori z o n t a l r o t a t i n g c y l i n d e r , as d e p i c t e d i n Figure 2.2.  This r e f r a c t o r y -  l i n e d r e a c t o r , w i t h a diameter ranging from 4 to 6 meters and a length from 60 to 125 meters, i s f e d w i t h the i r o n - c o n t a i n i n g m a t e r i a l , the c a r b o n a c e o u s r e d u c t a n t and, i f necessary, a d e s u l p h u r i z i n g agent.  The  charge moves c o u n t e r c u r r e n t l y to the gas f l o w i n g through the f r e e b o a r d , e s s e n t i a l l y i n plug flow.  A i r i s introduced i n t o the freeboard to burn  the carbon monoxide e v o l v i n g from the bed, to provide heat f o r the p r o c e s s , p a r t i c u l a r l y the endothermic Boudouard r e a c t i o n .  Extra fuel i s  i n j e c t e d from the discharge end d u r i n g s t a r t - u p o f the k i l n .  In t h e  f i r s t s e c t i o n of the k i l n , the preheating zone, the charge i s heated up to o p e r a t i o n a l temperature, most of the v o l a t i l e s from the coal a r e r e l e a s e d and some r e d u c t i o n takes p l a c e .  In the remaining length of the  k i l n , the r e d u c t i o n zone, f u r t h e r r e d u c t i o n i s c a r r i e d out and m e t a l l i z a t i o n i s achieved. The l e n g t h of the zones i s determined, among other f a c t o r s , by the r e a c t i v i t y of the charge.  A v a r i a t i o n to t h i s mode o f  o p e r a t i o n i s f o u n d i n the ACCAR process21-22 where l i q u i d or gaseous f u e l s are i n j e c t e d beneath the bed near the p r o d u c t end o f the k i l n . The t e m p e r a t u r e d i s t r i b u t i o n o f gas and s o l i d s and the r e d u c t i o n sequence i n s i d e a r e d u c t i o n k i l n a l s o are presented i n F i g u r e 2.2. Because the aim of t h i s work i s to study the reduction k i n e t i c s of f i n e i r o n - o r e under c o n d i t i o n s s i m i l a r to t h o s e found i n r o t a r y k i l n s , these c o n d i t i o n s are f i r s t reviewed i n t h i s Chapter.  This i s  Main f e a t u r e s of the SL/RN r o t a r y k i l n (Modified from r e f e r e n c e 9)  process  10  f o l l o w e d by an assessment  of a common problem i n r e d u c t i o n k i l n s :  the  formation of a c c r e t i o n s on the i n s i d e w a l l . Then s t u d i e s o f the k i n e t i c s o f i r o n - o x i d e r e d u c t i o n with s o l i d r e d u c t a n t s , u t i l i z i n g thermobalances, are summarized to provide an i n s i g h t i n t o the p o s s i b l e mechanisms i n the complex r e a c t i o n system.  F i n a l l y i n the l a s t s e c t i o n of the  chapter, d i f f e r e n t s t u d i e s i n v o l v i n g l a b o r a t o r y - s c a l e r o t a r y r e a c t o r s are c r i t i c a l l y examined.  2.2 D i r e c t - r e d u c t i o n Rotary K i l n s I n d u s t r i a l research r e l a t e d to r o t a r y - k i l n d i r e c t reduction can be c l a s s i f i e d , according to the i r o n - c o n t a i n i n g raw m a t e r i a l  utilized  and the purpose of the product, i n t o three general areas:  i)  Reduction o f p e l l e t a n d / o r lump f e e d w i t h the p r o d u c t  being  charged to an e l e c t r i c furnace f o r steel making.23-47 ii)  Treatment of i r o n and steelmaking waste m a t e r i a l s such as fume, dusts and sludge, where the product obtained i s to be fed to the blast  iii)  4  furnace. 8-60  Treatment  o f f i n e m a t e r i a l s , e s p e c i a l l y ironsand c o n c e n t r a t e s ,  to produce a charge f o r the e l e c t r i c steelmaking  In each case the achievement temperature,  furnace.61-71  of a smooth operation r e q u i r e s c o n t r o l o f  b u l k d e n s i t i e s / s i z e of ore and coal f o r good bed mixing,  coal r e a c t i v i t y , ore r e d u c i b i l i t y , r e s i d e n c e time o f m a t e r i a l s i n the k i l n , amount of f i n e s present, s o f t e n i n g p o i n t of coal ash and a i r profile.  In a d d i t i o n pneumatic coal i n j e c t i o n at the discharge end o f the  11  k i l n , a i r i n j e c t i o n under the bed of the feed end of the k i l n , e x i t gas v e l o c i t y , m i n e r a l o g i c a l c h a r a c t e r i s t i c s of feed m a t e r i a l s and w a s t e - g a s heat u t i l i z a t i o n are important f a c t o r s .  2.2.1 Operation of r e d u c t i o n k i l n s A summary41»46 f 0  ^  pi ants p r e s e n t l y i n operation i s presented  e  i n Table III and gives an idea of the range of composition of the i r o n c o n t a i n i n g m a t e r i a l s t r e a t e d , as w e l l as of the d i f f e r e n t reductants used.  I t i s important to emphasize t h a t to date no p l a n t has p r o c e s s e d  f i n e i r o n - o r e concentrates s u c c e s s f u l l y . In the cases where f i n e mater i a l s have been processed, v i z . , at New Zealand Steel65 and Western T i tanium L t d . ,  6 6  the concentrate has a Ti02 content l a r g e r than 55 and of  8 percent, r e s p e c t i v e l y . This r e f r a c t o r y oxide a i d s i n the p r e v e n t i o n of a c c r e t i o n formation, even though the p a r t i c l e s i z e of -250 + 90 ym i s smal 1. Of the important aspects of k i l n operation mentioned above, heat t r a n s f e r i s one of the most c r i t i c a l f o r several reasons. the g e n e r a t i o n o f gaseous  F i r s t l y , both  r e d u c t a n t , and oxide reduction have to take  place i n s i d e the same v e s s e l , c f . , F i g u r e 2.1.  The former  process,  c o n s i s t i n g e s s e n t i a l l y o f the Boudouard or S o l u t i o n - l o s s r e a c t i o n , i s s t r o n g l y endothermic whereas the oxide r e d u c t i o n r e a c t i o n i s s l i g h t l y exothermic as f o l l o w s :  C + C0  2  A H° = 86232 J/mole CO  = 2 CO  2O3 + 3 CO = 2 Fe + 3 C0  2  (2.1)  A H° = -9355 J/mole CO....(2.2)  12  Secondly, the heat flow patterns i n the r o t a r y k i l n are exceedingly complex. 5  F i n a l l y , the c y c l i c a l nature of the process renders temperature  measurement d i f f i c u l t so t h a t c o n t r o l o f t e m p e r a t u r e i s f r a u g h t w i t h problems. O p e r a t i o n o f a r e d u c t i o n k i l n from a thermal standpoint i s a f f e c t e d by the f o l l o w i n g parameters:  i)  The c o a l r e a c t i v i t y :  t h i s parameter has a strong i n f l u e n c e on  the r a t e of g a s i f i c a t i o n and depends on the g e o l o g i c a l o r i g i n of the c o a l . tion.  More r e a c t i v e c o a l s permit lower temperature opera-  Moreover, the Boudouard r e a c t i o n has a s t r o n g i n f l u e n c e  on the o v e r a l l r e d u c t i o n k i n e t i c s , ii)  The ore r e d u c i b i l i t y :  t h i s parameter i s dependent on the mine-  r a l o g i c a l c h a r a c t e r i s t i c s o f the raw m a t e r i a l s and on t h e i r treatment p r i o r t o r e d u c t i o n , e.g., p e l l e t i z a t i o n .  Again a  higher r e d u c i b i l i t y allows lower temperature o p e r a t i o n , iii)  The s o f t e n i n g p o i n t of the coal ash and i t s important e f f e c t on the a c c r e t i o n development:  a lower s o f t e n i n g p o i n t f a c i l i t a t e s  the formation of compounds with wustite with low m e l t i n g p o i n t . T h i s t o p i c w i l l be d i s c u s s e d i n more d e t a i l i n a subsequent section.  Only l i m i t e d c o n t r o l can be exerted over the above mentioned a s p e c t s , as compared with the a i r p r o f i l e , which i s a very i m p o r t a n t p r o c e s s v a r i able.  Meadowcroft  and Brimacombe  73  have a s c e r t a i n e d t h a t to achieve a  41  TABLE III. OPERATIONAL FEATURES OF ROTARY-KILNS DIRECT REDUCTION PLANTS . COMPANY  46  START UP DATE  KILN SIZE [M]  KILN UNITS  ORE  1969  2.4 x 30  1  Ilmenlte concentrate  Subbltumlnous  1968/80  4.0 x 60  Lump ore  Bituminous  New Zealand Steel I  1969  4.0 x 75  1  Beach Sand concentrate  Lignite  Acos F1no* .Iratlnl  1973  3.6 x 50  1  Pellets/lump ore  Bituminous  Stelco**  1975  6.0 x 125  1  Pellets  Subbltumlnous  360000  Nippon Kokan**  1974  6.0 x 70  1  Waste oxide pellets  Bituminous  400000  Slderperu  1980  2.9 x 62  3  Pellets  Coke/Anthracite  120000  Unldo/SIIL  1980  3.0 x 40  2  Lump ore  Bituminous  Hlghveld II*  1983  4.0 x 60  3  Lump ore  Bituminous  600000***  Iscor*  1984  4.6 x 80  4  Lump Ore  Bituminous  720000  New Zealand Steel II*  1984  4.6 x 65  4  Beach Sand Concentrate  Lignite  900000  Dunswart-Krupp  1973  4.6 x 73  1  Lump ore  Subbltumlnous  150000  Western Titanium Hlghveld I  * Under construction  10  In operation as required  *** Prereduction  COAL  CAPACITY (Ton/year DRI) 15000 2000000*** 175000  60000  35000  14  smooth o p e r a t i o n t h i s v a r i a b l e must be c a r e f u l l y c o n t r o l l e d . A l s o , the bed composition, i . e . , f i x e d c a r b o n - t o - i r o n r a t i o ( C f - j / F e ) , a f f e c t s the x  bed thermal c h a r a c t e r i s t i c s .  For example, i n c r e a s i n g the Cp-j /Fe r a t i o x  from 0.17 t o 0.23 has been reported t o d e c r e a s e 80°C t h e bed temperature.  6 6  under t h e s e circumstances a s i g n i f i c a n t change i n k i l n opera-  t i o n c o n d i t i o n s c o u l d be undetected, i f o n l y gas t e m p e r a t u r e monitored.  i s being  C o n s e q u e n t l y , thermocouples attached to the k i l n wall must  have a quick response, i n order t o record a c c u r a t e l y the temperature o f the s o l i d s and f r e e b o a r d . M i x i n g o f t h e c h a r g e i n a r o t a r y k i l n must be o p t i m i z e d t o achieve a good o p e r a t i o n .  In t h i s regard, d e n s i t i e s and p a r t i c l e s i z e s  of both ore and coal are o f the utmost importance, and an optimum combination which y i e l d s the best mixing c o n d i t i o n s must be sought.  A neces-  s a r y c o n d i t i o n f o r optimum m i x i n g i s t h a t the ore p a r t i c l e s should be l a r g e r than the coal.46  i t must be s t r e s s e d however that t h i s holds f o r  r e l a t i v e l y l a r g e p a r t i c l e s , i . e . , i r o n - o r e p e l l e t s o f about 12 mm, and may not be completely true f o r f i n e r s i z e s o f both coal and o r e . From the standpoint o f r e a c t i o n k i n e t i c s alone the ore and c o a l p a r t i c l e s i z e should be as small as p o s s i b l e ( a t t h e i r optimum s i z e r a t i o , d /dp , f o r mixing). c  e  U t i l i z a t i o n o f f i n e i r o n concentrates has  been r e s t r a i n e d though, because of inherent o p e r a t i o n a l problems, ly:  name-  enhancement o f a c c r e t i o n formation and high l e v e l s o f dust c a r r y  over i n the o f f - g a s e s , a t present working  temperatures.  I t has been r e c o g n i z e d that the r o t a r y k i l n i s not very energy efficient;  4 4  thus e f f o r t s have been d i r e c t e d a t u t i l i z i n g as much o f the  heat a v a i l a b l e as p o s s i b l e . A i r i n j e c t i o n under t h e b e d , a t t h e f e e d  15  end of the k i l n , has been implemented i n order to burn out most of the v o l a t i l e s e v o l v i n g from the coal a t t h i s stage. i s h e a t e d more r a p i d l y , which s h o r t e n s  In a d d i t i o n , the charge  the preheating s e c t i o n of the  k i l n , and brings about an i n c r e a s e i n the t h r o u g h p u t of the  reactor.  M o r e o v e r , waste-gas heat u t i l i z a t i o n has been optimized, to preheat  the  feed p r i o r to charging.71 In a n o t h e r area of p r o c e s s  improvement, coal i n j e c t i o n at the  discharge end of the k i l n has been employed to s u s t a i n the reduction r e a c t i o n , and to maintain h i g h l y reducing c o n d i t i o n s , which would minimize product r e o x i d a t i o n at t h i s l a s t stage.  2.2.2  Accretion  formation  A c c r e t i o n s are s o l i d agglomerates which adhere to the r e f r a c t o r y wall i n s i d e the k i l n .  The a c c r e t i o n i s formed i n i t i a l l y by s i n t e r i n g of  f i n e c h a r g e p a r t i c l e s a g a i n s t the r e f r a c t o r y s u r f a c e , and l a t e r by s i n t e r i n g of the p a r t i c l e s t h e m s e l v e s .  An i n c r e a s i n g r a t e o f a c c r e t i o n  growth can d i m i n i s h the k i l n c a p a c i t y to the point where the operation has to be shut down and the a c c r e t i o n s removed.  T e m p e r a t u r e measure-  ment, the importance of which was discussed i n the previous s e c t i o n , i s obviously impaired by a c c r e t i o n growth, which leads to s e r i o u s control problems.  process  The main mechanism i n the s i n t e r i n g process i s the  p a r t i a l f u s i o n and d i s s o l u t i o n of the f i n e r p a r t i c l e s . 7 4  This  process  i s c o n t r o l l e d p r i m a r i l y by the l o c a l temperature and the chemical  compo-  s i t i o n of the f i n e s , where the l a t t e r determines t h e i r c a p a b i l i t y to form binary, ternary and even quaternary  compounds.  These phenomena can  16  o c c u r r e a d i l y i n the coal based processes, owing to the f o l l o w i n g f a c 7 5  tors: "  i)  8 5  Poor c r y s t a l l i z a t i o n of the ore which causes i t to d i s i n t e g r a t e more e a s i l y to produce f i n e s .  F o r example,  h e m a t i t e i s more  l i k e l y to i n c r e a s e a c c r e t i o n growth than magnetite, s i n c e the volume i n c r e a s e a s s o c i a t e d with the Fe203 to Fe304 transformat i o n and the r e s u l t i n g i n t e r n a l s t r e s s e s , enhances the f i n e s generation. ii)  C o m p o s i t i o n of the f i n e s present, o r g i n a t i n g from the d i f f e r e n t components i n the bed, which determines the e q u i l i b r i u m s o f t e n i n g and m e l t i n g t e m p e r a t u r e s .  These components are coal ash,  i r o n - o r e gangue, i r o n oxides and lime or dolomite, iii)  L o c a l f u s i o n of these f i n e s caused by the thermal c o n d i t i o n s of the process. Among the important thermal c o n d i t i o n s a r e c y c l i c h e a t i n g and c o o l i n g due to the k i l n r o t a t i o n , the temperature d i f f e r e n c e between the wall and c h a r g e and p o s s i b l e hot s p o t s caused by the burner flame.  iv)  The h i g h s u r f a c e t e n s i o n of the Ca0-Si02 and CaO-FeO-Si02 s y s tems which causes the p a r t i c l e s t o c o a l e s c e more r e a d i l y and remain i n the l i q u i d phase,  v)  F a c t o r s o f a n c i l l a r y i m p o r t a n c e i n c l u d e the v i s c o s i t y of the softened phase, i n t e r f a c i a l t e n s i o n between wall and p a r t i c l e s , c o m p r e s s i v e f o r c e s due to charge h e i g h t , mechanical adhesion of the s o l i d p a r t i c l e s and dust s e d i m e n t a t i o n from the k i l n exhaust system.  gas-  17  An i n s i g h t i n t o these f a c t o r s has been p r o v i d e d by L e i s t e r e t al .75-77 - j  n a  c h a r a c t e r i z a t i o n of the m a t e r i a l s used f o r d i r e c t reduc-  t i o n i n r o t a r y k i l n s , where the importance o f the i n t e r a c t i o n between c o a l a s h , c a l c i n e d dolomite and hematite ores with respect to the s o f tening p o i n t of the mix was assessed. T h i s study a l s o i n c l u d e d an e v a l u a t i o n of the species present i n the a c c r e t i o n s from an i n d u s t r i a l s i z e rotary k i l n .  The ranges of composition observed were s i m i l a r t o t h o s e 8 -  obtained from a study i n a p i l o t - p l a n t s c a l e o p e r a t i o n ? ^ and are pres e n t e d i n T a b l e IV.  Gudenau e t a l . ,  8 2  and S c h l e b u s c h ,  81  c a r r i e d out a  study i n a d i r e c t l y - f i r e d r o t a r y drum, l a r g e enough (800 mm i.d.) to a l low f o r the s e p a r a t i o n of the reducing atmosphere w i t h i n the charge from the o x i d i s i n g gases i n the f r e e b o a r d . They assessed the e f f e c t o f t i m e of r e d u c t i o n , c h a r g e t e m p e r a t u r e , and temperature d i f f e r e n c e between charge and wall on a c c r e t i o n growth, f o r d i f f e r e n t combinations o f p e l l e t s , l i g n i t e and a n t h r a c i t e . A s i m i l a r study was performed, but on f i n e ores with the same reductants i n an i n d i r e c t l y heated r o t a r y r e a c tor, by Wenzel e t a l .  8 3  8  and Grosse-Daldrup. ^  The most important f i n d i n g s from both s t u d i e s can be summarized as f o l l o w s :  i)  A c c r e t i o n formation i n c r e a s e s s t r o n g l y with temperature and a l s o with the temperature g r a d i e n t between w a l l and c h a r g e , c f . , Figure 2.3.  ii)  Two t y p e s o f s i n t e r i n g " b r i d g e s " are p o s s i b l e a c c o r d i n g to the 8  extent of r e d u c t i o n . ^  8 3  For a lower degree of r e d u c t i o n , the  18  TABLE IV. ANALYSIS OF ACCRETIONS FORMED IN A PILOT-PLANT SIZE ROTARY KILN78,79 Position  Feedstock  SiO?  Al  0O3  CaO [%]  FeO [%]  Fe  Layer c l o s e s t to charge (top)  ore pellets  10-15 8-12  10-15 10-15  1 1-2  Middle l a y e r  ore pellets  35-45 38-50  50-60 50-60  3 3-5  1 1  nil 0.5  ore pellets  30-40 35-40  55-65 55-65  2-3 2-4  1 1  nil nil  Layer c l o s e s t t o kiln lining (bottom)  40-50 38-50  TABLE V. PHASES ENCOUNTERED IN THE ACCRETIONS FORMED DURING DIRECT REDUCTION IN A ROTARY KILN 75-77 Phase  Wustite Iron Silica F a y a l i te Gehlenite Anorthite Hercynite Eutectic 1 Eutectic 2 Eutectic 3 Eutectic 4 Eutectic 5 Eutectic 6  Formula  FeO Fe Si0  Melting Point [°C]  Comments  1369 1565 2  1723  2FeO-Si02  2CaO-Al203*Si02 CaO-Al203«2Si02 FeO-Al2O3  FeO + 27% CaO 2FeO'Si02 + FeO 2FeO-Si02 + S i 0 2 CaO + Si02 + FeO FeO + AI2O3 + S i 0 2 CaO + AI2O3 + S i 0 2  1205 1593 1553 1547 1070 1175 1180 1105 1070 1165  in Fe presence Si02  as t r i d i m i t e Olivine line Fayalite/Cordierite/Hercynite Anorthite/Calcium S i l i c a t e  19  Accretion  growth, d (mm)  120 40  -7  Temperature difference  Bed temperature (°C)  wall/bed(°C)  2.3  Formation o f a c c r e t i o n s with 10% f i n e s i n three dimensional p l o t t i n g 7 4  Preheat FeO zone zone  Metallization zone Oxidation zone  o c o (J  <  Kiln length Reduction time  2.4  Q u a l i t a t i v e progress o f the formation o f a c c r e t i o n s as a f u n c t i o n o f r e d u c t i o n time and furnace leng  20  l e a d i n g phase i n the bridges j o i n i n g the agglomerates i s a s i l i cate with a g l a s s y s t r u c t u r e , w h i l e a t h i g h e r r e d u c t i o n s t h i s l e a d i n g phase becomes a network o f m e t a l l i c i r o n g r a i n s formed a t the s t a r t o f reduction.80-84  A qualitative representation of  t h i s sequence i n the k i l n i s p r e s e n t e d i n F i g u r e 2.4, where a maximum i s seen a t the preheating (low r e d u c t i o n ) end ( s i l i c a t e b r i d g e ) , and a second maximum appears a t higher r e d u c t i o n  (iron  bridge) where some o f the v i t r e o u s s t r u c t u r e i s s t i l l present, iii)  A range o f s o f t e n i n g temperatures between 1020 and 1060°C was observed75-77,85 depending on the oxide phases present; a summary o f the phases most l i k e l y t o appear i s shown i n T a b l e V. G e h l e n i t e and a n o r t h i t e were found i n very small amounts i n the agglomerates, while h e r c y n i t e was found i n a few c a s e s b u t i n r e l a t i v e l y l a r g e amounts.  I t i s i n t e r e s t i n g to observe t h a t  m u l l i t e , which i s the product o f the decomposition o f k a o l i n i t e present i n ash, i n v a r i a b l y occurred coupled with anorthite. This suggests t h a t the l a t t e r i s a product o f t h e r e a c t i o n between t h e k a o l i n i t e and t h e CaO from t h e dolomite.  At about  1120°C a n o n - c r y s t a l l i n e phase was observed t o form, composed o f FeO, S i 0 2 , Al2O3 and CaO, while a t higher Si02 c o n t e n t s , a t e n dency t o form c r y s t a l s was r e p o r t e d , iv)  With r e s p e c t t o s i z e , t h e f i n e r f r a c t i o n s o f t h e r e l a t i v e l y l i g h t components, i . e . , coal ash and ore gangue, p r e f e r e n t i a l l y d e p o s i t e d on t h e w a l l ; t h e h a r d e r and l a r g e r components apparently fall  off.  8 2  21  v)  A c c r e t i o n s were p r o d u c e d by c o m b i n a t i o n s of f a c t o r s , such as high ash contents i n the coal p l u s h i g h t e m p e r a t u r e s ,  lignite  ash plus f i n e ore at medium temperature; f i n e ore plus high temperature; and p e l l e t s and a n t h r a c i t e lumps c o a t e d w i t h f i n e s plus high temperatures of wall and c h a r g e . vi)  82  F i n a l l y , the interweaving of the i r o n whiskers, produced d u r i n g the r e d u c t i o n , was proved to be the main component of the i r o n bridges during r e d u c t i o n of iron-sands concentrates with l i g n i t e and a n t h r a c i t e .  8 3  From the above s e c t i o n s , the degree of complexity of the o v e r a l l r o t a r y k i l n process i s c l e a r l y e v i d e n t .  I t i n v o l v e s at d i f f e r e n t stages  o f the p r o c e s s , d i f f e r e n t mechanisms of p a r t i c l e s mixing, heat t r a n s f e r and r e a c t i o n k i n e t i c s ; the l a t t e r i s the s u b j e c t of the next s e c t i o n .  2.3 K i n e t i c s s t u d i e s of i r o n - o x i d e r e d u c t i o n with s o l i d reductants In order to improve the process k i n e t i c s of the r o t a r y k i l n , i n terms o f the a s p e c t s d i s c u s s e d i n Section 2.2, the mechanism of i r o n oxide r e d u c t i o n with carbon must be understood.  This mechanism c o n s i s t s  of chemical r e a c t i o n and mass-transfer processes and has been s t u d i e d by 8  numerous i n v e s t i g a t o r s . 6 - 1 0 4 j  n  e  r e a c t i o n r a t e s , f o r s p e c i f i c iron-and  carbon-containing m a t e r i a l s , have been obtained.  In a d d i t i o n , d i f f e r e n t  mathematical models, of isothermal non-moving systems, have been proposed w i t h d i f f e r e n t degrees of a p p l i c a b i l i t y .  1 0 5 - 1 1 0  T h e i r aim has been  to assess the importance of each of the i n d i v i d u a l subprocesses i n v o l v e d i n the o v e r a l l r e d u c t i o n r e a c t i o n , i . e . , E q u a t i o n s (2.1)  and  (2.2)  22  coupled  together,  i n order to obtain a f u n c t i o n a l r e l a t i o n s h i p between  them and the o v e r a l l reduction r a t e . The o v e r a l l r e d u c t i o n r e a c t i o n has been recognized to proceed v i a the gaseous intermediates, CO and C O 2 .  The i n d i v i d u a l subprocesses,  s c h e m a t i c a l l y shown in Figure 2.5, are the f o l l o w i n g (numbers  correspond  to those i n f i g u r e ) :  (1) D i f f u s i o n o f CO t h r o u g h the porous reduced i r o n - l a y e r , i n the p a r t i a l l y reduced iron-oxide p a r t i c l e . (2) Chemical r e a c t i o n at i r o n / i r o n - o x i d e i n t e r f a c e . ( 3 ) D i f f u s i o n outward of the gaseous reduction product, C O 2 , through the product l a y e r . ( 4 ) Transport of CO2 through the gas surrounding the p a r t i c l e s . (5) Boudouard r e a c t i o n . (6) Transport of CO through the gas surrounding  the p a r t i c l e s .  (7) D i r e c t s o l i d - s o l i d r e d u c t i o n .  The most important  f a c t o r s which may have an i n f l u e n c e on these  subprocesses, are:  (A) Temperature. (B)  Stoichiometry.  (C) Geometry and s t r u c t u r e of the p a r t i c l e s : e.g., p o r o s i t y , t o r tuosity, crystallinity. (D) S t r u c t u r e of the bed of p a r t i c l e s ; e.g., fraction.  p a r t i c l e s i z e , void  Sub-processes i n the r e d u c t i o n o f i r o n - o x i d e with carbon  24  (E) Mixing of the r e a c t a n t s ; e.g., f i x e d bed, moving bed. (F) Presence of i m p u r i t i e s ; c a t a l y z e r s , x i n h i b i t e r s . (G) Gas  The  properties.  i n f l u e n c e o f the above f a c t o r s  on the o v e r a l l  reaction  r a t e , has been the subject of many research e f f o r t s with varying degrees of success.  Earlier investigators, Jander,  86  Ginstling,  8 6 a n  d  have obtained general r e l a t i o n s h i p s f o r d i f f u s i o n c o n t r o l l e d i n p a r a l l e l , and they are,  1 _(l-f )l/3 R  =  Carter,  8 7  reactions  respectively:  _k_  t  (  2.3)  o  2  1 - 2/3 f - (1 - f ) / R  3  =  JL r  t  (2.4)  o  2  Z - [1 + ( Z - l ) f ] 2 / 3 . ( Z - l ) ( l - f ) / R  3  R  <2 5)  HI^>  -  These r e l a t i o n s h i p s were obtained with the f o l l o w i n g assumptions:  i)  The r e a c t i n g p a r t i c l e s are s p h e r i c a l i n shape and remain so  du-  r i n g the r e a c t i o n , ii) iii)  D i f f u s i o n obeys F i c k ' s One  law.  o f the gaseous products d i f f u s e s i n t o the p a r t i c l e s of the  s o l i d s with which i t r e a c t s .  25  iv)  The volume of the p a r t i c l e s remains constant, except i n the l a s t case, Equation (2.5), where the volume change i s considered.  With r e s p e c t to the present iron-oxide/carbon r e a c t i o n system, the mechanisms e l u c i d a t i n g how the o v e r a l l r e a c t i o n d e v e l o p s verse c o n d i t i o n s have been p r o g r e s s i v e l y r e v e a l e d .  under d i -  A summary of the ex-  perimental c o n d i t i o n s of the most r e l e v a n t s t u d i e s i s presented i n Table VI.  In an experimental  vacuum, Y u n  8  study of Fe£03 reduction with pure carbon under  e s t a b l i s h e d t h a t the most d i f f i c u l t stage of r e d u c t i o n ,  namely the step with the highest a c t i v a t i o n energy, 'FeO' to Fe.  This was l a t e r confirmed by R o s s  8 9  i s the reduction o f  when studying the mech-  anisms of oxygen m i g r a t i o n , through the i r o n l a y e r , during the r e d u c t i o n o f c o a r s e l y c r y s t a l l i s e d ores.  It was a l s o found that s o l i d - s t a t e d i f -  f u s i o n of i r o n and oxygen through the s o l i d - p h a s e l a t t i c e becomes ext r e m e l y important i n the f i n a l reduction stage, i . e . , above 80 percent. On the other hand, i n a study of Fe203 reduction with g r a p h i t e , i n a n i trogen atmosphere, Otsuka and K u n i i  9 0  showed that the r a t e - l i m i t i n g step  in the o v e r a l l reduction r e a c t i o n i s the Boudouard r e a c t i o n . basis of t h i s f i n d i n g , R a o  91  On  the  developed a model assuming that the g a s i f i -  c a t i o n of carbon obeys f i r s t order k i n e t i c s , and formulated the f o l l o w ing equation: dWr  -  = R W  (2.6)  dt In t h i s E q u a t i o n W  c  constant.  i s the amount of carbon and R  c  i s an o v e r a l l rate  Thus f o r the o v e r a l l r e d u c t i o n , based on the stoichiometry of  TABLE VI. STUDIES ON REDUCTION KINETICS OF IRON OXIDES WITHI CARBONACEOUS MATERIALS IRON OXIDE SIZE IN [vm]  AUTHOR YEAR REF.#  REDUCTANT (% C) SIZE IN Urn]  Fe203 (R.G.) <147; 0.5 q  Graphite (R.G.) <147  BICKNESE 1966, 92  FeO  Coconut charcoal 95  OTSUKA 1969,  Fe203 (99%) 43 to 147; 0.8 g  YUN 1961,  83  90  (Synthetic) <1 12 g  Electrode Graphite (99.8%); 43 to 208  EL-GUINDY 1970, 103  FeO-T10 (pure) Finely ground 0.8 g  RAO 1971,  Fe20 (R.G.) 80% <1; 0.8 g  Amorphous Carbon (R.G.);-44,135,270  FRUEHAN 1977, 96  Fe203, FeO (R.G.) <74; 0.8 g  Coconut charcoal, char & coke; 74, 500, 1600  ABRAHAM 1979, 93  Fe 0 43 4  Electrode Graphite 68  WRIGHT 1981, 94  Fe 03 (BF Pellets) 1200, 12 g  Coal char (82.3%) -800 + 100  SUNDAR MURT1 1982, 104  Fe0>Cr203 (pure) <45 0.8 g  Graphite (99.9%) N.A.  SEATON 1983, 99  Fe203 « Fe 04 (BF) 1400. N.A.  Bituminous char (83.3); <47  RAO 1984,  Fe3<>4 (Concentrate) Coke (85.6) <250; 8g <50  2  3  91  116  2  3  (R.G.) 1800 11 9  2  3  Graphite N.A.  REDUCTANT PRE-TREATMEMT Degassed  C/Fe, C/0  EXPERIMENTAL SYSTEM  [kcal/mole]  Balance Vacuum  7001100  Balance C02 wt.  9801165  13.9  O.U-0.62 0.83  Gas Analysis 200 cm/m1n N2  10501150  (0.2 f) 15-24 (0.6 F) 55-75  None  0.08-1.16 0.28-4.05  Balance  None  0.16-0.96 0.37-2.23  Balance 600 cnt /min Ar  8501087  72  Balance 1000 cm3/n1n Ar or He  9001200  70-80  N.A.  None  1.42 3.30  T RANGE [°C]  N.A. N.A. I . 8 8 N 2 + CO  0.16-0.23 1 hr at 600°C 0.56-0.93  None  20 h, up to 850^ None  N.A.  2 h. at 600°C  0.65 1.50  0.36 1.25  0.79-1.71 2.44-5.97 0.31 & 0.35 0.72 A 0.65  0.25 0.65  3  64 * 6  1075-1140 Non-1 sot.  3  Fe203 apart from C. CSZ cell  Char around pel let  8801042 Non I sot.  (0.2 f) 72 (0.6 F) 55  900-  70-80  57  1150 1300  Balance  Horizon. Tube 800 cm/min N2  Gas Analysis 200 cm /min N  N.A.  8001200  3  2  9001000  F e 0 : 30-57 Fe 04 : 38 2  3  3  73 42-75 (Cat)  ro  27  Equations  (2.1) and (2.2), and c o n s i d e r i n g that a l l the CO was consumed  in the r e d u c t i o n , the rate Equation was found to be  log (1.743 - f ) = l o g 1.743 R  Equation  (2.7) f i t t e d the experimental  -  R 2.303  t  (2.7)  r e s u l t s well except f o r the e a r l y  p a r t o f the r e a c t i o n , i n agreement with e a r l i e r f i n d i n g s .  8 6 - 9 0  By way  of e x p l a n a t i o n , i t was suggested that the c a r b o n g a s i f i c a t i o n d i d not follow Equation  (2.6)  d u r i n g the e a r l y stages, but was c o n t r o l l e d by  some other mechanism. The c a t a l y s i s of the Boudouard r e a c t i o n by i r o n and L i 2 O was a l s o demonstrated, as well as the i n h i b i t i n g e f f e c t of FeS additions. c a t i o n , was  The chemical  r e a c t i o n c o n t r o l , exerted by the carbon g a s i f i -  a l s o i n f e r r e d by Bicknese and C l a r k  9 2  from the  observation  that there was no measurable d i f f e r e n c e i n the r a t e of r e d u c t i o n d i f f e r e n t coal p a r t i c l e sizes.  for  They also introduced the concept t h a t ,  in a s t a t i c system, the r e l a t i v e rates of the reduction r e a c t i o n and the g a s i f i c a t i o n o f c a r b o n depend upon the r e a c t i v i t i e s of the oxide  and  carbonaceous m a t e r i a l . More r e c e n t experimental been undertaken.  s t u d i e s , a p p l i c a b l e to t h i s work, have  In one i n s t a n c e , the r e d u c t i o n and the g a s i f i c a t i o n  r e a c t i o n s are i s o l a t e d from one another, but contained i n the same vessel .  93  I t was  f o u n d t h a t the o v e r a l l rate of reduction i s an order of  magnitude lower under these c o n d i t i o n s , as compared to the c a s e where both s o l i d s are i n t i m a t e l y m i x e d .  Thus the e f f e c t s of mixing and/or  i r o n c a t a l y s i s have been i l l u s t r a t e d .  In another work, by Wright e t  28  al.,  9 4  t h e r e a c t a n t s were kept separate u n t i l the d e s i r e d r e a c t i o n tem-  perature was reached, then the f i n e reductant was poured over the i r o n oxide p e l l e t .  In t h i s c a s e , the r e s u l t s agreed well with an e a r l i e r  study i n a l a b o r a t o r y k i l n by the same a u t h o r s by F r u e h a n ,  9 6  Turkdogan e t a l . ,  9 7  9 5  and with those of works  and Gransden e t a l .  9 8  I t was d e t e r -  mined, from the a c t i v a t i o n energies obtained f o r these s i m i l a r r e a c t i o n systems, t h a t the o v e r a l l r e d u c t i o n r e a c t i o n i s c o n t r o l l e d by the chemical r e a c t i o n of the carbon g a s i f i c a t i o n up to about 1000°C, whereupon mass t r a n s f e r mechanisms s t a r t to assume c o n t r o l . The above f i n d i n g has been f u r t h e r c o r r o b o r a t e d by Seaton e t al.  9 9  They have a l s o mentioned that heat t r a n s f e r e f f e c t s , manifested  as t e m p e r a t u r e g r a d i e n t s through the Fe2t)3 p e l l e t s , play a p a r t i n the reduction r e a c t i o n . Among other aspects s t u d i e d , the p r e t r e a t m e n t o f the carbonaceous m a t e r i a l has been found to have a n o t i c e a b l e e f f e c t on t h e r e d u c t i o n r e a c t i o n , 100-101  a n (  j  an  i n c r e a s e i n the r e d u c t i o n r a t e ,  through the c a t a l y s i s of the Boudouard r e a c t i o n by a l k a l i has a l s o been reported by Rao and H a n . l °  carbonates,  2  Studies on the r e d u c t i o n k i n e t i c s o f FeO, o f m a t e r i a l s whose main components were T i 0 2 and mechanisms however.103-104  0 2 0 3 ,  have y i e l d e d d i f f e r e n t r e a c t i o n  S o l i d - s t a t e r e d u c t i o n c o n t r o l i s r e p o r t e d by  E l - G u i n d y and D a v e n p o r t , 103 t o have predominated i n the r e d u c t i o n o f F e 0 - T i 0 2 up to 1020°C, whereupon gaseous r e d u c t i o n by CO takes over the control.  An important f i n d i n g i n t h i s study i s the o b s e r v e d i n f l u e n c e  of the i n e r t - g a s f l u s h i n g on the r e d u c t i o n r a t e . Back d i f f u s i o n of the i n e r t gas i n t o the s o l i d s bed was r e c o g n i z e d , even a t low f l o w r a t e s such as 20 cm3/rnin.  In the r e d u c t i o n of Chromite, on the other hand,  29  Sundar M u r t i and Seshadri'104 r e p o r t the r e d u c t i o n to be c o n t r o l l e d by d i f f u s i o n of oxygen towards the surface of the p a r t i c l e s , i n the temper a t u r e range from 1150 to 1300°C. The complex nature of the two p a r a l l e l r e a c t i o n s , i r o n r e d u c t i o n and carbon g a s i f i c a t i o n , has l e d to the development of more t h e o r e t i c a l models w i t h d i f f e r i n g d e g r e e s o f c o m p l e x i t y .  These models have had  v a r y i n g l e v e l s of success i n r e p r e s e n t i n g the experimental r e s u l t s ,  un-  der g i v e n c o n s t r a i n t s . Thus, a general mathematical model of r e a c t i o n between s o l i d s , through gaseous i n t e r m e d i a t e s , was proposed by Sohn and Szekely.105  T h e i r model i s r e s t r i c t e d to the case of chemical c o n t r o l  and under the f o l l o w i n g assumptions:  i)  A porous p e l l e t made up of u n i f o r m l y d i s t r i b u t e d p a r t i c l e s of any of the f o l l o w i n g geometries:  flat p l a t e , long c y l i n d e r or  sphere. ii)  Steady-state c o n d i t i o n s with r e s p e c t to gas c o n c e n t r a t i o n .  The r a t e of r e a c t i o n was expressed as a f u n c t i o n o f FeO c o n c e n t r a t i o n , c a r b o n c o n c e n t r a t i o n , p a r t i c l e s s i z e r a t i o , e q u i l i b r i u m constants f o r the FeO r e d u c t i o n and Boudouard r e a c t i o n and the shape of the p a r t i c l e s . On the other hand, by a l s o c o n s i d e r i n g mass-transfer mechanisms, Raol06_ 109 d e v e l o p e d a model which f i t t e d h i s experimental r e s u l t s from a prev i o u s s t u d y r e a s o n a b l y well.91 lowing c o n d i t i o n s :  The model was e s t a b l i s h e d f o r the f o l -  30  i)  The o v e r a l l r e a c t i o n was assumed t o proceed i n three stages: i n i t i a t i o n , propagation and t e r m i n a t i o n ,  ii)  The gaseous  d i f f u s i o n was assumed to be u n i d i r e c t i o n a l i n t o a  stagnant i n e r t gas; t h i s gas i s considered n o t t o d i f f u s e back i n t o the s o l i d s bed. iii)  There were no s t r u c t u r a l changes i n the sample,  iv)  The gas-phase c o m p o s i t i o n s were t h o s e f o r e q u i l i b r i u m c o n d i tions.  The r a t e s o f CO and CO2 p r o d u c t i o n were e s t a b l i s h e d on the b a s i s of a two-step r e a c t i o n sequence, formation and d i f f u s i o n , based on E q u a t i o n (2.6).  The equations f o r the r a t e s a r e : 2 (1 - 6)  P  h  R C O  =  T1  p  m  1  +  m  C Q 2  a  P  -(2.8)  C0  2  (23 - 1) I P a  R  C0  C 0 2  =  (2.9)  ?  I  where  +  m  a  P  C0  2  6 denotes the f r a c t i o n of CO u t i l i z e d i n r e d u c t i o n , 1\ i s a spe-  c i f i c r a t e constant and m i s a f u n c t i o n o f the i n t r i n s i c r a t e constants a  and of the e q u i l i b r i u m constant.  By c o n s i d e r i n g t h e mechanism o f v i s -  cous f l o w , s l i p f l o w and o r d i n a r y and surface d i f f u s i o n , the d i f f e r e n t i a l equations e s t a b l i s h e d were numerically s o l v e d . Again, t h e o v e r a l l r e d u c t i o n p r o c e s s was found to be mass-transfer c o n t r o l l e d , determined  31  by the a v a i l a b i l i t y of CO, above 1000°C.  These r e s u l t s were c o r r o b o r a t -  ed by a s i m i l a r model, where the e v a l u a t i o n o f the d i f f u s i v e f l o w  was  s i m p l i f i e d to one i n v o l v i n g molecular d i f f u s i o n and forced gas flow to 11  obeying P o i s e u i l i e ' s e q u a t i o n . ^ Summarizing, as shown i n Table VI, the majority of the reduction k i n e t i c s s t u d i e s share the f o l l o w i n g s i m i l a r i t i e s :  small sample w e i g h t  ( l e s s than 12 g) of pure m a t e r i a l s and c o n s i d e r a b l e amounts of i n e r t gas f l u s h i n g during the r e a c t i o n ; and s t a t i c s o l i d s c o n d i t i o n s i n a l l cases. The o v e r a l l r e d u c t i o n r e a c t i o n has been proved to proceed through gaseous i n t e r m e d i a t e products, CO and  C02»  c o n t r o l l i n g i t at lower t e m p e r a t u r e s .  At h i g h e r t e m p e r a t u r e s ,  the  with the Boudouard r e a c t i o n mass-  t r a n s f e r mechanisms such as gas d i f f u s i o n play an i n c r e a s i n g l y important role.  The a c t i v a t i o n e n e r g i e s o b t a i n e d ,  f o r the Boudouard r e a c t i o n ,  v a r y i n the range of 15 to 80 kcal/mole at temperatures between 800  and  1165°C, with most of the values f a l l i n g i n the narrower range o f 45 t o 75 kcal/mole.  This range agrees with t h a t reported f o r the o x i d a t i o n of  c a r b o n a c e o u s m a t e r i a l s with  CO2,  1 1 1  the higher values corresponding  purer, l e s s r e a c t i v e , m a t e r i a l s such as coke and graphite,97,112-113 the lower values to those of c h a r s  1 1 4  and of c a t a l y z e d r e a c t i o n s .  1 1 5  to a n f  "  j  1 1 8  2.4 Laboratory s c a l e s t u d i e s i n r o t a r y r e a c t o r s Several s t u d i e s have been performed i n l a b o r a t o r y - s i z e r o t a r y reactors.8,80-81,95,110-129  j h e i r p r i n c i p a l aim was to determine the  best c o n d i t i o n s f o r the r e d u c t i o n of i r o n - b e a r i n g m a t e r i a l s , under r o t a t i n g c o n d i t i o n s , with r e l a t i v e l y l a r g e r s i z e samples.  An o v e r a l l view  of the c h a r a c t e r i s t i c s and operational d e t a i l s of each r o t a r y r e a c t o r i s  32  summarized i n Table VII; a b r i e f account of t h e i r f i n d i n g s i s given  be-  low. The e a r l i e s t study by Williams et a l .  1 1 9  t a i l e d d e s c r i p t i o n since the range of o p e r a t i n g t e s t e d was  broad.  i s worthy of a more def a c t o r s and  variables  The study o f d i r e c t reduction of i r o n ore, c a r r i e d  out p r o g r e s s i v e l y i n muffles, i n d i r e c t l y - h e a t e d r o t a t i n g t u b e s o f d i f f e r e n t s i z e s and d i r e c t l y f i r e d drums and p i l o t - p l a n t s i z e k i l n s , y i e l d ed the f o l l o w i n g important  i)  findings:  Under s u i t a b l e c o n d i t i o n s , sponge i r o n could be produced on a commercial s c a l e a t about 900°C, with a reduction degree than 95  ii)  percent.  The Boudouard r e a c t i o n appeared to be the rate c o n t r o l l i n g step in the  iii)  higher  process.  Since the process r e q u i r e s a large heat input, and as the charge i s a poor heat conductor, heat t r a n s f e r i n t o the bed appeared to be a l i m i t i n g f a c t o r i n a successful l a r g e - s c a l e o p e r a t i o n .  iv)  Although the gas stream above the r o l l i n g charge in the k i l n  was  o x i d i z i n g w i t h r e s p e c t to i r o n , the hot carbon p a r t i c l e s maintained a s t r o n g l y reducing atmosphere within the charge, v)  Rapid heating and thorough s t i r r i n g were obtained; no d i f f e r e n c e was observed between 1 and 12 r.p.m; a l t h o u g h  t h i s does not  agree with more recent s t u d i e s . vi)  Low-grade, non-coking bituminous and l i g n i t i c c o a l s proved to be good r e d u c i n g agents; coking c o a l s could not be used since they caked and formed a c c r e t i o n s .  TABLE VII. CHARACTERISTICS OF LABORATORY-SIZE ROTARY KILN EXPERIMENTS FOR IRON ORE REDUCTION  AUTHOR Year, Ref.#  DxL [mn]  L/D  3/ [mm] p  WILLIAMS 1927, 119  483 x 838  1.74  2.35  THEMELIS 1964, 120  915 x 152  1.67  N.A.  MOELLER 1967, 122  145 x 1450 10.0 14.5  MEADOWCROFT 1973, 123  152 x 610  4.0  12  ^c^Fe  c  FIx/  p e  0.45 N.A.  0.375  Wide range  0.40 - 0.56  Max. F i l l . [t]  COMMENTS  1.5-12  900 - 1050 Heat then charge. Internal firing with fuel o i l .  12  0.4 -1.5  800 - 1100 Charge ore under Inert atmosphere, heat up, charge coal. External firing. 1000- 1050  Preheat to 800 then charge. Char. External heating. Flow of N£. Gas analysis.  1010- 1120  Induction heating.  1.5-5  800 - 1000  External electric heating. Continuous gas analysis. Continuous feed. Gas reduction.  1 - 10  900 - 1150 Preheat to 600°, charge, then heat to T. Induction heating.  16  25 x 245  9.69  1.25  REUTER 1975, 121  150 x 680  4.53  9  PETERSON 1976, 124  178 x 381  2.14  15  0.95  MORRISON 1978, 95  485 x 280  0.58  12  0.39  0.3 - 1.0  11  SUCRE 1979, 8  64 x 310  4.8  0.15  3.12  0.39 - 0.91  13  HAUSLER 1951, 129  150 x 680  4.53  0.4  0.5  16  0.58  T [°C]  18  NIXON 1973, 128  0.36 - 1.25 0.4 - 0.8  [r.p.m.]  16  13  1020 N.A.  1000 -1150  10 - 30- 950 - 1070 8  920 - 1000  Green pellets. External electric heating. 5 /min N2External electric heating. Charge ore then heat the charge char. Heat then charge. Induction heating. Induction heating. 833 cm /min N23  oo oo  34  vii)  At ore s i z e s smaller than 5 mm,  the reduction rate hardly v a r i e d  with r e s p e c t to the p a r t i c l e s i z e , viii)  Best m i x i n g r e s u l t s were o b t a i n e d when the reductant and ore were of the same s i z e ; f i n e r coal f l o a t e d on top of the  ix)  bed.  When f i n e i r o n - o r e , l e s s than 147 ym i n s i z e , was processed, i t s i f t e d through the bed and s i n t e r e d on the hot k i l n w a l l s .  A l t h o u g h the c o n d i t i o n s of the experiments did not allow a very p r e c i s e e v a l u a t i o n o f the r e s u l t s of the p r o c e s s ,  e.g.,  points (v),  ( v i i ) and ( v i i i ) , good general trends were e s t a b l i s h e d . Other studies have been performed on several aspects of the process on the l a b o r a t o r y s c a l e .  S i m i l a r i t y c r i t e r i a were a p p l i e d to s c a l e  down a pre-reduction k i l n , by Themelis e t al.,120 h e r e i t was W  concluded  that the rate of generation and mass t r a n s f e r of CO, are the most import a n t s t e p s i n the r e d u c t i o n p r o c e s s  and depend s t r o n g l y on the s i z e ,  d e n s i t y , and r e a c t i v i t y of the coal used. v i o u s l y important  in determining  The density and s i z e are  the mixing patterns of the p a r t i c l e s  w i t h i n the bed and t h e r e f o r e were the s u b j e c t o f two 4  d i e s . »121  ob-  further stu-  in h i s i n v e s t i g a t i o n of t r a n s p o r t i n and mixing behaviour of  r o t a r y k i l n beds f o r the production of sponge i r o n , Reuterl21  evaluated  the e f f e c t s of r o t a t i o n a l speed, c y l i n d e r s i z e , m i x i n g t i m e , t e m p e r a t u r e , p a r t i c l e shape and s u r f a c e c o n d i t i o n s , percent loading and the s i z e and density r a t i o s of the raw m a t e r i a l s on the r e d u c t i o n o f the i r o n o x i d e . Optimum r e s u l t s were reported f o r the reduction of p e l l e t s when the d / d c  F e  r a t i o was 0.36,  and the C - /Fe r a t i o was 0.4 at 1050°C. Fl  x  A l s o , the mixing patterns f o r a broad range of p a r t i c l e s and  cylinder  35  4  s i z e s , at room temperature, were studied in depth by Henein who conclusively determined the boundaries between slumping and r o l l i n g motions in the bed of s o l i d s . of  He also investigated the kidney-shaped segregations  smaller p a r t i c l e s formed in the centre of the bed; he showed as well  that  2  the Froude Number by i t s e l f  (w R/g) is not s u f f i c i e n t  for scaling  up purposes. The  influence  of coal  r e a c t i v i t y on the SL/RN process also has  been studied on a laboratory scale. cibility  of iron oxide p e l l e t s ,  The r e a c t i v i t y , coupled with  was studied by Moeller et a l . .  coal size of -1 + 0.5 mm, previously devol a t i 1 i z e d ,  1 2 2  reduFor a  they examined  the  following:  i) ii) iii)  Influence of ash on r e d u c i b i l i t y . Reactivity of coal as a function of ash content, Softening behaviour of the ash.  Dolomite was used as a desulphurizer and the reaction was conducted i n an e l e c t r i c a l l y heated cylinder and monitored by gas analysis of CO. A high ash content was found to raise the temperature at which the ing  of the charge commences but, at the same time, to decrease produc-  tivity. ly  The s u i t a b i l i t y of a broad range of Canadian c o a l s , p a r t i c u l a r -  lignite  and s u b - b i t u m i n o u s ,  Meadowcroft et of  soften-  al..*  2 3  for the SL/RN process was assessed by  A standard test was performed by reacting 2 Kg  -15 + 9 mm f i r e d p e l l e t s and 675 g of fixed carbon, as charred c o a l ,  in an externally heated rotary reactor.  It was found that the lower the  rank of the c o a l , the higher the  of r e d u c t i o n o b t a i n e d ;  degree  also,  36  t a k i n g i n t o account the gas composition, the temperature and f i x e d c a r bon content of the char, the r e l a t i v e r e a c t i v i t y of l i g n i t e i s 180 times g r e a t e r than t h a t o f b i t u m i n o u s coal which, i n t u r n , i s 12 times more r e a c t i v e than a n t h r a c i t e .  The r e a c t i v i t i e s o f a b r o a d range o f c o a l s  have been r e p o r t e d , i n g r a p h i c a l form, by Reuter e t a l . .  2 8  These are  shown i n F i g u r e 2.6 and c o r r o b o r a t e the f i n d i n g s o f Meadowcroft  et  al..123 Further aspects c o n s i d e r e d i n l a b o r a t o r y - s i z e k i l n s , when reducing i r o n - o x i d e p e l l e t s , have been:  i)  S t a t i s t i c a l optimization.95  ii)  Comparison of green vs. f i r e d pel l e t s . 1 2 4  iii)  Comparison of f i r e d p e l l e t s vs. ore lumps.125  iv)  Tendencies towards s i n t e r i n g of f i n e m a t e r i a l s .  v)  8 0  Heat t r a n s f e r character!'sties. 126  vi)  A c c r e t i o n b u i l d up on the k i l n w a l l .  8 1  A set of isothermal e x p e r i m e n t s was p e r f o r m e d by M o r r i s o n e t a l . , 9 5 where the e f f e c t s of three independent v a r i a b l e s , a t f i v e l e v e l s each, were s t a t i s t i c a l l y e v a l u a t e d . The v a r i a b l e s  were:  time, from 60  to 240 minutes; temperature, from 1000 to 1150°C and C p i / F e r a t i o , from x  0.3 to 1.0.  Optimum c o n d i t i o n s , f o r a r e d u c t i o n higher than 95 percent,  were o b t a i n e d between 1040 and 1100°C and at C F i / F e r a t i o between 0.7 x  and 0.9.  I t was suggested t h a t long r e s i d e n c e times, high C p i / F e r a x  t i o s and r e l a t i v e l y lower temperatures w i l l produce a well r e d u c e d p e l let.  On the other hand, the e f f e c t of heating r a t e and type of p e l l e t  37  1  f  20 Brown coal,Rhein,Germany ^> Huntly,New Zealand  I 0 Forestburg,Canada  \  Gallup,U.S. "\_ C h a r c o a l , B r a z i l X>^ Sama, Turkey  O^eao . B r a z i l  K  Q S i n g a r en i , Ind i a  „ „ , OCharquedas,Brazil Cotgrave,England ^ ^ ' Blair Athol,Australia O n  Tavistock,South A f r i c a  0  >  Anthrac ite,Germany  0.5 Coke Breeze  0.3 900  1  I  950  1000  Temperature  2.6  of  1  1050 reduction (°C)  R e a c t i v i t i e s o f d i f f e r e n t carbonaceous materials as a function o f temperature* ! . 3  ^»  1100  38  1  was t e s t e d by P e t e r s o n and P r a s k y , ^ on a sample o f 700 g o f p e l l e t s (>67 p e r c e n t Fe) and 800 g o f l i g n i t e ( C p i = 33 percent) a t 1020°C and x  5 r.p.m., under a stream of 5 £/min o f n i t r o g e n . The h i g h e s t m e t a l l i z a t i o n o b t a i n e d was 96 percent and the main d i f f i c u l t y encountered, when green p e l l e t s were reduced, was dust formation. Another s e t o f comparat i v e experiments, but t h i s time between p e l l e t s and ore lumps, was performed by C h a t t e r j e e and C h a k r a v a r t y .  125  By changing one v a r i a b l e a t a  time, they t e s t e d the f o l l o w i n g : temperatures from 900 to 1150°C, times o f r e d u c t i o n from 120 t o 150 minutes and ore/coal r a t i o from 1 t o 1.2; the best c o n d i t i o n s found were f o r a p e l l e t / c o a l r a t i o o f 1.2, f o r 150 minutes a t 1050°C. The r e d u c t i o n o f f i n e r i r o n - o x i d e m a t e r i a l s has not been given much a t t e n t i o n t o date.  In t h e i r study o f the r e d u c t i o n o f s i n t e r , -3 +  0.8 mm, performed i n a c o n t i n u o u s l y f e d r o t a t i n g r e a c t o r and using gaseous reducing agents, M u l l e t e t a l .  1 2 7  and N i x o n  1 2 8  claimed t h a t a y i e l d  of 90 percent r e d u c t i o n c o u l d e a s i l y be obtained without s t i c k i n g o f the c h a r g e o r soot f o r m a t i o n . This study was aimed a t developing a process s i m i l a r t o the ACCAR p r o c e s s .  2 1 - 2 2  The r e s u l t s were analyzed by l i n e a r  curve f i t t i n g the percent r e d u c t i o n t o the temperature, time, o r e s i z e , presence o f unreacted hydrocarbons i n the reducing gas, r a t i o o f H/C i n the gas and t o the reducing c a p a c i t y o f the gas; out o f these v a r i a b l e s , o n l y t h e temperature showed a l o g a r i t h m i c r e l a t i o n s h i p with the percent reduction.  I t was suggested t h a t s t i c k i n g o f the c h a r g e i s r e l a t e d t o  the formation o f "hot spots," i . e . , l o c a l i z e d heating a t c e r t a i n p o i n t s , owing t o the heat t r a n s f e r c h a r a c t e r i s t i c s o f the process.  39  In a study o f the r e d u c t i o n o f t i t a n i f e r o u s f i n e ores, SucreGarcia  8  concluded  that a mixed c o n t r o l governed the r e d u c t i o n - and c a r -  bon g a s i f i c a t i o n r e a c t i o n s a t temperatures below 1000°C.  The  used were d i f f e r e n t -149 ym t i t a n i f e r o u s ores and -600 +149 l i g n i t e , and the v a r i a b l e s studied were r o t a t i o n a l s p e e d , r a t i o , t e m p e r a t u r e and p r e - o x i d a t i o n of the ores.  reactants ym  charred  char-to-ore  The k i n e t i c s of the  r e a c t i o n s were followed by a n a l y s i s of the e x i t gases f o r CO.  Accre-  t i o n s were observed to form, and hardly any change i n the reduction rate was observed with a change i n the r o t a t i o n a l speed from 18 to 30 r.p.m., i n d i c a t i n g t h a t good mixing'was achieved.  F i n a l l y , the p o s s i b i l i t y of  using a f i n e i r o n - o r e i n d i r e c t r e d u c t i o n was  r e c e n t l y examined by  r i a u s l e r , 1 2 9 - j p i l o t - p l a n t r o t a r y k i l n s , a scanning e l e c t r o n microscope n  and r o t a r y c y l i n d e r s .  The operational l i m i t a t i o n s were l o o k e d a t  and  the most important f i n d i n g s were:  i)  The s i z e f r a c t i o n smaller than 300 ym should be kept at the lowest level possible,  ii) iii)  S u f f i c i e n t reduction was achieved with l i g n i t e coal at 900°C. Between 700 and 800°C a s t r o n g d i s i n t e g r a t i o n of the  mineral  takes place, enhancing a c c r e t i o n s , iv)  The agglomerating behaviour of the sponge i r o n i s a d v a n t a g e o u s since i t y i e l d s up to 90 percent m e t a l l i z a t i o n ,  v)  The agglomeration phenomenon decreases with the increment i n the amount o f h i g h r e a c t i v i t y c o a l ; i n other words, an absence of reductant at the end of the r e d u c t i o n p r o c e s s w i l l p r o d u c e a strong s i n t e r i n g tendency in the charge due to r e o x i d a t i o n .  40  vi)  A d d i t i o n o f i r o n - s a n d s t o t h e charge of f i n e i r o n c o n c e n t r a t e i n h i b i t s the agglomeration t e n d e n c i e s .  In summary, as can be seen i n T a b l e V I I , most of the s t u d i e s have focussed on the r e d u c t i o n of p e l l e t s i n e x t e r n a l l y heated r e a c t o r s , a t t e m p e r a t u r e s from 900 to 1150°C.  As might be expected, the h i g h e s t  r e d u c t i o n has been o b t a i n e d a t higher temperatures and higher Cp-j /Fe x  ratios.  At these temperatures however, the formation of u n d e s i r a b l e ac-  c r e t i o n s i s more l i k e l y .  The amounts of excess carbon used, with r e -  spect to the ' s t o i c h i o m e t r i c ' v a l u e , are high which p r o d u c e s a f a v o u r able e f f e c t on the r e d u c t i o n r a t e but may reduce the r e a c t o r throughput. Furthermore, the rate of the r e d u c t i o n process v a r i e s c o n s i d e r a b l y , e s p e c i a l l y a t the d e s i r e d l o w e r t e m p e r a t u r e s , between p e l l e t s and f i n e i r o n - o r e c o n c e n t r a t e s . F i n a l l y , the formation of a c c r e t i o n s on the r e a c t o r w a l l i s d i f f e r e n t i n the m e t a l l i c chamber used i n most of the experiments as compared to i t s r e f r a c t o r y material c o u n t e r p a r t .  41  CHAPTER 3  SCOPE OF THE PRESENT WORK AND OBJECTIVES  It becomes apparent from the foregoing d i s c u s s i o n t h a t the  ma-  j o r i t y of the previous research has been performed, e i t h e r on the reduct i o n k i n e t i c s of very small amounts of rather pure m a t e r i a l s Under s t a t i c c o n d i t i o n s , i . e . , thermobalances, or on r e l a t i v e l y l a r g e r o t a r y r e a c t o r s u s u a l l y i n v o l v i n g p e l l e t s , where the o p e r a t i o n a l p a r a m e t e r s f o r r e d u c t i o n have been t e s t e d .  C l e a r l y , n e i t h e r of the above two systems  providesthe reduction k i n e t i c s information necessary  t o a i d i n the  de-  v e l o p m e n t o f a p r o c e s s f o r the r e d u c t i o n o f unagglomerated i r o n ore f i n e s , with a low rank coal i n a r o t a r y k i l n , which was the aim o f t h i s work.  The d i f f e r e n c e s i n the reduction k i n e t i c s of the above mentioned  two systems, when compared to the present work, are based e s s e n t i a l l y on the f o l l o w i n g c o n s i d e r a t i o n s : 1)  The p a r t i c l e s s i z e , and consequently  the r e a c t i n g surface  area, d i f f e r s g r e a t l y between an average 12 mm d i a m e t e r p e l l e t and p a r t i c l e s to be s t u d i e d here, with s i z e s smaller than 0.5 mm.  the  Logical-  l y , mass t r a n s f e r mechanisms w i l l play a l a r g e r r o l e i n the c a s e o f the reduction of p e l l e t s with the consequently 2)  lower o v e r a l l r a t e .  Good mixing c o n d i t i o n s are a c h i e v e d  in a r o t a t i n g system,  f a c i l i t a t i n g intimate contact of the r e a c t i n g s o l i d s with t h e i r gaseous  42  r e a c t a n t s ; t h i s may be contrasted to s t a t i c - b e d systems where concentrat i o n gradients within the s o l i d s m i x t u r e are more l i k e l y to d e v e l o p . F u r t h e r m o r e , the use of i n e r t gases to f l u s h the bed i n the l a t t e r case i s l i k e l y to have an e f f e c t on the l a t e r stages of r e d u c t i o n . 3)  The r e a c t i v i t y of coal s t r o n g l y depends on i t s rank and, to  a l e s s e r extent, on the d e v o l a t i l i z a t i o n t r e a t m e n t a p p l i e d to i t .  In  g e n e r a l , i t can be s a i d t h a t the lower the coal rank the higher i t s r e a c t i v i t y , which enhances the p o s s i b i l i t y of i t s g a s i f i c a t i o n a t l o w e r temperatures.  However, d i f f e r e n t r e a c t i v i t i e s f o r c o a l s of the same  rank and subject to i d e n t i c a l d e v o l a t i l i z a t i o n treatments have been  ob-  s e r v e d ; t h i s d i f f e r e n c e can be a t t r i b u t e d to the g e o l o g i c a l o r i g i n of the c o a l .  Thus, i t becomes necessary to u t i l i z e , i n the k i n e t i c s study;  the c o a l t y p e t h a t would most l i k e l y be used f o r an i n d u s t r i a l s c a l e process, since the i n f l u e n c e of the coal r e a c t i v i t y on the Boudouard r e a c t i o n , and consequently of utmost importance.  on the reduction process, has been proved to be A d e v o l a t i l i z a t i o n treatment f o r the c o a l , p r i o r  to the r e d u c t i o n experiments, i s a l s o necesary.  The presence of v o l a -  t i l e s would make the r e d u c t i o n k i n e t i c s more d i f f i c u l t to f o l l o w  and  analyze. Bearing  i n mind t h e s e c o n s i d e r a t i o n s , the o b j e c t i v e s of t h i s  work were:  i)  To study the r e d u c t i o n k i n e t i c s of an unagglomerated iron-ore concentrate, u t i l i z i n g d e v o l a t i l i z e d low-rank c o a l s as r e d u c t a n t , under well mixed c o n d i t i o n s approximating those of an i n dustrial scale rotary k i l n .  43  ii)  To e v a l u a t e t h e e f f e c t s o f s e v e r a l v a r i a b l e s on the reduction k i n e t i c s , namely temperature, f i x e d c a r b o n - t o - i r o n  r a t i o , par-  t i c l e s i z e , r o t a t i o n a l speed, type o f c o a l , percent l o a d i n g and the presence o f a c a t a l y s t f o r t h e c a r b o n  g a s i f i c a t i o n reac-  t i on. iii)  To c h a r a c t e r i z e t h e agglomeration  o f the f i n e p a r t i c l e s w i t h i n  the s o l i d s bed and a g a i n s t the furnace wall during r e d u c t i o n , iv)  To d e t e r m i n e t h e most a p p r o p r i a t e o v e r a l l reduction k i n e t i c s model, i n v o l v i n g a l s o the carbon g a s i f i c a t i o n r e a c t i o n .  Before the reduction experiments were c a r r i e d out, i t was essent i a l t o define the optimum s o l i d s mixing c o n d i t i o n s , a t room temperature and i n the same r e a c t o r , as an a i d to understanding  the bed behaviour i n  the reduction t e s t s . The ranges o f some o f t h e o p e r a t i o n a l  variables  were s e l e c t e d a c c o r d i n g t o s i m i l a r i n d u s t r i a l c o n d i t i o n s and, although every e f f o r t was made t o achieve mixing c o n d i t i o n s i n the bed that were the same as i n a l a r g e k i l n , no attempt was made to obtain complete s i m i l a r i t y between the l a b o r a t o r y r e a c t o r and i t s i n d u s t r i a l c o u n t e r p a r t ; the g e o m e t r i c been proved  and heat t r a n s f e r c h a r a c t e r i s t i c s o f a r o t a r y k i l n have d i f f i c u l t to scale u p .  4 , 6 7  '  1 2 0  Nonetheless,  the reduction  k i n e t i c s r e s u l t s obtained w i l l be t h e same r e g a r d l e s s o f t h e r e a c t o r s c a l e , provided the same well mixed c o n d i t i o n s p r e v a i l i n the bed and an e q u i v a l e n t heat source i s f u r n i s h e d t o maintain bed temperature.  Simi-  l a r l y , t h e d e v o l a t i l i z a t i o n treatment a p p l i e d t o the coal was performed under the same temperature c o n d i t i o n s as t h o s e p r e s e n t i n t h e r o t a r y kiln.  44  CHAPTER 4  EXPERIMENTAL  4.1 I n t r o d u c t i o n A d e t a i l e d a c c o u n t o f the experimental techniques i s presented in t h i s chapter.  I t i n c l u d e s the c h a r a c t e r i s t i c s o f t h e raw m a t e r i a l s  used and t h e i r p r e p a r a t i o n , a d e s c r i p t i o n o f the apparatus used i n the mixing and r e d u c t i o n e x p e r i m e n t s , t h e p r o c e d u r e s f o r t h e e x p e r i m e n t s t h e m s e l v e s and t h e e x p e r i m e n t a l d e s i g n f o r t h e v a r i a b l e s and l e v e l s tested. The m a t e r i a l s s t u d i e d , a s p i r a l i r o n - o r e concentrate from Carol Lake Mine, Quebec, and a sub-bituminous coal from Forestburg C o l l i e r i e s , A l b e r t a , were s e l e c t e d f o r t h i s study on the b a s i s o f t h e i r being r e a d i l y a v a i l a b l e and i n p l e n t i f u l r e s e r v e s ; m o r e o v e r , t h e r e l a t i v e l y h i g h r e a c t i v i t y o f the l a t t e r renders i t more s u i t a b l e , as was pointed out i n the previous chapter, f o r an i n d u s t r i a l o p e r a t i o n a t lower temperatures, with i t s obvious advantages. A l a b o r a t o r y - s i z e r o t a r y k i l n was designed f o r the purposes o f the mixing and the r e d u c t i o n experiments. was i t s heating system:  The main f e a t u r e o f t h e k i l n  an a x i a l l y p o s i t i o n e d s i l i c o n c a r b i d e r a d i a t i n g  element which provided the energy r e q u i r e d t o c a r r y o u t t h e r e a c t i o n s but, a t the same time, d i d not i n t e r f e r e with the atmosphere produced by  45  the r e a c t i o n s . T h i s l a t t e r f a c t , c o u p l e d w i t h the hermetic s e a l i n g achieved i n the e n t i r e system, allowed a c a r e f u l g a s - f l o w measurement and a n a l y s i s to be performed throughout each experiment i n order to f o l low the k i n e t i c s of the o v e r a l l r e d u c t i o n r e a c t i o n . The s i z e of the r e a c t i o n chamber was l a r g e and able to c o n t a i n a maximum of approximately 1.5 kg of s o l i d s .  This amount was b i g enough to p e r m i t the  assessment  o f the e f f e c t s of the mixing v a r i a b l e s throughout the r e d u c t i o n and the mixing experiments; c l e a r l y , t h i s c o u l d not be a c h i e v e d u t i l i z i n g the t r a d i t i o n a l thermogravimetric approach. The room-temperature mixing experiments were h e l p f u l not only to determine the optimum mixing c o n d i t i o n s , as pointed out i n the p r e v i o u s chapter, but a l s o to a i d i n minimizing the number of experimental l e v e l s during the subsequent r e d u c t i o n t e s t s .  Nevertheless, d e t e r m i n a t i o n s t o  o b t a i n the b e s t m i x i n g c o n d i t i o n s , i n the r e d u c t i o n experiments, were performed as w e l l . Furthermore, an experimental design was u t i l i z e d i n both c a s e s to a i d i n r e d u c i n g the l a r g e number o f experiments which otherwise would have been necessary.  4.2 M a t e r i a l s and t h e i r p r e p a r a t i o n  4.2.1  Iron-ore concentrate  The i r o n o r e used i n the e x p e r i m e n t s was a commercial concentrate from Carol Lake Mine, Quebec.  spiral  The s i z e d i s t r i b u t i o n  and  c h e m i c a l composition of the concentrate are shown i n Figure 4.1 and Table VIII r e s p e c t i v e l y .  The main i r o n - b e a r i n g m i n e r a l o g i c a l component  was s p e c u l a r hematite which had a high degree of c r y s t a l i n i t y , as shown  Particle  size  (/im)  P a r t i c l e size d i s t r i b u t i o n of spiral iron-ore concentrate  47  TABLE VIII.  ~---~_J^terial Species Fe (Total)  CHEMICAL ANALYSIS OF SPIRAL IRON-ORE CONCENTRATE, FULL SIZE RANGE AND TWO SIZE FRACTIONS, -420 +300 and -106 +74 ym  F u l l s i z e range [<716 ym] 65.80  -420 +300 ym d = 358 ym  -106 +74 ym 3" = 90 ym  66.7  66.5  11.9  8.3  FeO  -  P  0.009  0.006  0.013  Mn  0.12  0.10  0.08  4.70  3.60  2.90  0.20  0.10  0.10  CaO  -  0.40  0.30  MgO  -  0.40  0.30  0.002  0.002  0.005  0.005  0.005  0.005  Si0  2  Al 0 2  3  S Na ° 2  K ° Fe 0 2  2  H0 2  3  (calc)  3.50  82.20  85.90  0.06  0.20  48  i n Figure 4.2.  The second most important i r o n species was magnetite and  the gangue m a t e r i a l s were b a s i c a l l y s i l i c e o u s , mostly q u a r t z . The o r e was o v e n - d r i e d o v e r n i g h t and then s i e v e d , i n t o narrow s i z e f r a c t i o n s , i n a Gil son p i l o t - p l a n t f a c i l i t y . c h o s e n , among the e i g h t o b t a i n e d , were f o u r : 300 ym, - 210 + 149 ym and - 106 + 74 ym.  The s i z e f r a c t i o n s  - 841 + 600 ym, - 420 +  The amounts o f m a t e r i a l r e -  q u i r e d i n each s i z e f r a c t i o n were r a t h e r l a r g e owing to the number of experiments planned.  Each f r a c t i o n was sampled  f o r chemical a n a l y s i s  and i t s b u l k d e n s i t y was measured f o l l o w i n g the technique reported by Henein.  4  The chemical a n a l y s i s and bulk d e n s i t i e s , i n the random loose  and packed c o n d i t i o n s , f o r the d i f f e r e n t f r a c t i o n s r e l e v a n t to the experiments, are presented i n Table VIII and Table IX, r e s p e c t i v e l y .  4.2.2 Low rank c o a l s The c o a l used i n most o f the experiments was a 'C type c o a l , from Forestburg C o l l i e r i e s , A l b e r t a .  sub-bituminous  The p r o x i m a t e  u l t i m a t e analyses of the coal are presented i n Table X.  and  The coal i n the  a s - r e c e i v e d c o n d i t i o n was d r i e d o v e r n i g h t i n a warmed-floor  room, then  c r u s h e d i n a hammer m i l l and screened i n t o narrow s i z e f r a c t i o n s , using the f a c i l i t y mentioned above.  Again, each s i z e f r a c t i o n was sampled f o r  c h e m i c a l a n a l y s i s and i t s b u l k d e n s i t y was d e t e r m i n e d with the same technique as f o r the ore d e s c r i b e d above; bulk d e n s i t i e s f o r the s i z e f r a c t i o n s u t i l i z e d i n the experiments are presented i n Table IX. A l i g n i t i c c o a l , from Estevan, Saskatchewan, was used i n a few comparative experiments; sented i n Table X.  i t s proximate and u l t i m a t e analyses are pre-  4.2  SEM photograph of s p i r a l i r o n - o r e concentrate (lOOx)  TABLE IX. RANDOM LOOSE AND PACKED BULK DENSITIES OF THE IRON-ORE CONCENTRATE AND THE FORESTBURG COAL, FOR DIFFERENT PARTICLE SIZES  Material  Mean P a r t i c l e S i z e  Density 3  [ y. m]  Iron ore  Coal  [kg/m ]  Loose  Dense  358  2455  2747  254  2439  2717  180  2384  2632  90  2504  2703  716  632  682  180  652  725  90  653  728  51  TABLE X.  MeanWticle s i z e [ym]  PROXIMATE AND ULTIMATE ANALYSES OF FORESTBURG COAL AND SASKATCHEWAN LIGNITE MEAN PARTICLE SIZES 718, 180 AND 90 ym  718 %  Forestburg 180 90 Proximate A n a l y s i s %  Lignite  %  180 %  H0 2  22.54  15.22  15.37  28.5  Ash  26.10  37.00  33.70  11.0  Volatiles  34.10  29.70  32.00  27.4  Fixed C  39.80  33.30  34.30  32.7  Ultimate A n a l y s i s (d.b)  Carbon  51.91  44.53  45.52  60.80  Hydrogen  3.65  3.25  3.09  4.00  Nitrogen  1.10  0.96  0.98  0.10  Sulphur  0.62  0.57  0.60  0.70  Ash  26.10  37.00  33.70  15.90  Oxygen  16.62  13.69  16.11  18.50  52  4.2.3 C a t a l y s t Lithium, sodium and potassium carbonates of chemical p u r i t y were u t i l i z e d to prepare a c a t a l y s t mixture a c c o r d i n g t o the t e c h n i q u e r e p o r t e d by R a o .  1 3 0  An e q u i m o l a r m i x t u r e o f the t h r e e carbonates was  ground, melted and held a t 700°C f o r three hours under an i n e r t - g a s a t mosphere.  The melt was c o o l e d , s o l i d i f i e d and ground again to a p a r t i -  c l e s i z e l e s s than 150 vm, then stored under dry c o n d i t i o n s .  The t o t a l  amount o f c a t a l y s t needed f o r the c a t a l y z e d experiments was produced i n a s i n g l e batch, to ensure t h a t the m i x t u r e had the same p r o p e r t i e s i n every experiment.  4.3 Reduction Apparatus P r i o r to d e s c r i b i n g the apparatus i n d e t a i l , a brief outline w i l l be given to provide an o v e r a l l p i c t u r e .  The r e a c t o r , shown i n F i g -  ure 4.3, c o n s i s t e d e s s e n t i a l l y o f a s h o r t c y l i n d r i c a l steel s h e l l , i n side o f which r e f r a c t o r y m a t e r i a l was c a s t t o give shape t o the i n t e r n a l w o r k i n g chamber.  The r e a c t o r c o u l d be s p l i t i n t o two s e c t i o n s , along  i t s a x i s , to empty i t s contents a f t e r each experiment, and then b o l t e d back t o g e t h e r b e f o r e t h e n e x t t e s t .  The steel s h e l l had openings at  both ends on i t s a x i s , through which a h e a t i n g element was  positioned.  Two s t e e l p i p e s , f i r m l y supported by the support frame were attached to each of the openings of the r e a c t o r , to provide a route f o r the f l u s h i n g o f the i n e r t gas before the experiment was s t a r t e d , and f o r the e x i t i n g of the gaseous r e a c t i o n p r o d u c t s i n t o the f l o w r a t e measurement and analysis train.  The frame a l s o supported the t r u n n i o n s upon which the  Sampling point-*  To exhaust hood flowmeters  Steel shell  Castable support clamp Teflon insert  4.3  •• •%•''•:•:/ AI Q shield 2  3  v.  AUO.  -HlLSiljcone rubber gasket 1_  SiC heating element  Overall view of experimental set-up f o r r e d u c t i o n t e s t s  54  r e a c t o r r o t a t e d , powered by a chain and sprocket and motor system, l o cated beneath the r e a c t o r .  The r e a c t o r had t h r e e r a d i a l o p e n i n g s f o r  purposes of s o l i d s feeding and sampling and i n t r o d u c i n g the thermocouple probe and the a c c r e t i o n s probe, r e s p e c t i v e l y .  4.3.1  Heating system The c o r e o f the h e a t i n g system was a s i l i c o n carbide r a d i a t i n g  element p o s i t i o n e d along the a x i s of the r e a c t i o n chamber, as shown i n F i g u r e 4.3.  A r e c r y s t a l l i z e d alumina tube, supported from one of the  chamber ends, surrounded the element to p r o t e c t i t from f a l l i n g cles.  The h e a t i n g element was secured i n p o s i t i o n by two  copper b u s - b a r s ,  parti-  water-cooled  one a t each end, which i n t u r n were h e l d by s t e e l  b r a c k e t s , w i t h T e f l o n i n s u l a t i o n s l e e v e s , secured to the frame of the support assembly.  Obviously a l l the d i f f e r e n t r o t a t i n g and s t a t i c parts  o f the s y s t e m had to be c o n c e n t r i c a r o u n d the heating element, since clearances between some parts were of the order of 5 mm.  Even a m i n o r  m i s a l i g n m e n t c o u l d be o f d i s a s t r o u s consequences f o r the f r a g i l e e l e ment, as was witnessed i n one i n s t a n c e .  The power s o u r c e was a m u l t i -  tap A.C. transformer, with p r o p o r t i o n a l c o n t r o l , capable of d e l i v e r i n g 3 kW.  4.3.2  S e a l i n g system S e a l i n g was  one o f the most important design f e a t u r e s , as well  as the most d i f f i c u l t to achieve, c o n s i d e r i n g the c h a r a c t e r i s t i c s of the r o t a t i n g high temperature system, Figure 4.4.  The system had to be a i r  t i g h t because the reduction k i n e t i c s were to be assessed through  the  4.4  Top view o f r o t a r y reactor showing s e a l i n g areas .  56  measurement of the flow rate and composition of gaseous products.  Thus  i f e i t h e r a i r leaked i n or gas leaked out, s e r i o u s e r r o r s would be i n troduced i n the k i n e t i c s e v a l u a t i o n . D i f f e r e n t m a t e r i a l s and c o n f i g u r a t i o n s were t e s t e d i n terms of both minimum leakage and minimum  bending  s t r e s s on the heating element at the e x i t of the end p i p e s , regions A-A' i n Figure 4.4.  The material chosen was a s i l i c o n e sponge, c a p a b l e o f  w i t h s t a n d i n g r e l a t i v e l y high temperatures  200°C) without l o s i n g i t s  e l a s t i c p r o p e r t i e s . Thus s i l i c o n e sponge s l i c e s were l o c a t e d i n regions A-A  1  t o p r o v i d e s e a l s . Another key l o c a t i o n to be sealed was the j o i n t  between the s t a t i c end-pipes flanges and the r o t a t i n g r e a c t o r , B-B' Figure 4.4.  in  Here 15 x 15 mm s i l i c o n e sponge r i n g s were f i x e d by g l u e i n g  one of t h e i r faces unto a groove i n the steel s h e l l of the k i l n , w h i l e the opposite face was l u b r i c a t e d with a high-temperature  M0S2 grease and  s l i d compressed a g a i n s t the face of each end-pipe f l a n g e . T h i s compress i o n was a c h i e v e d by s i m u l t a n e o u s l y h a n d - f a s t e n i n g  both  end-pipes  a g a i n s t the r i n g s through the use o f two t h r e a d e d r o d s w i t h n u t s , as shown i n F i g u r e 4.4.  The t h i r d i m p o r t a n t l o c a t i o n where the s e a l i n g  sponge was used, seen as C-C  i n Figure 4.4, was i n the j o i n t between  the two s e c t i o n s of the steel s h e l l comprising the r e a c t o r body. Here, a s t r i p of the material was glued to the edge o f one o f the s h e l l s s e c t i o n s b e f o r e the r e a c t o r was c l o s e d ; the s t r i p had to be replaced a f t e r every experiment, as opposed to the other s e a l s which l a s t e d f o r several r u n s , b e c a u s e the combined e f f e c t o f compression stroyed i t s e l a s t i c i t y .  and temperature  de-  The r e s t of the system gas l i n e s were s e a l e d  w i t h s t a n d a r d vacuum g r e a s e .  The e f f i c i e n c y of s e a l i n g achieved, as  57  w i l l be e x p l a i n e d i n d e t a i l i n s e c t i o n 4.7 of t h i s chapter, was q u a n t i f i e d i n every experiment and was higher than 95 percent i n a l l cases.  4.3.3  B u i l d i n g m a t e r i a l s and dimensions The body o f the r o t a r y r e a c t o r i s shown i n F i g u r e 4.5 A and B  and c o n s i s t e d o f a c y l i n d r i c a l s t e e l s h e l l 9.3 mm t h i c k , 512 mm long and 406 mm i n t e r n a l diameter.  I t could be s p l i t apart i n t o two s e c t i o n s , as  shown i n F i g u r e 4.5 A; the s e c t i o n s were j o i n e d a t t h e i r edges by b o l t ing the flanges together. in diameter  The c i r c u l a r openings a t each end were 54 mm  and t h e grooves around them f o r s e a l i n g purposes were 6 mm  deep, 12.4 mm wide and 120 mm i n circumference. The i n t e r n a l r e f r a c t o r y l i n i n g , shown with f i n a l dimensions i n Figure 4.5 B, c o n s i s t e d of two c o n c e n t r i c l a y e r s of c a s t a b l e m a t e r i a l s . The i n n e r w o r k i n g l a y e r was 20 mm t h i c k made from C a s t o l a s t G, a high alumina material with thermophysical  p r o p e r t i e s p r e s e n t e d i n T a b l e XI  which can withstand higher temperatures  but possesses a r e l a t i v e l y high  thermal c o n d u c t i v i t y . The e x t e r n a l l a y e r was 113 mm t h i c k c a s t from P l i c a s t LWI24, w i t h p r o p e r t i e s presented i n Table XI, which provided the necessary thermal i n s u l a t i o n . The thickness o f the r e f r a c t o r y l a y e r s was determined by performing a heat balance on the system given the following constraints:  i) ii)  A maximum inner face temperature  o f 1100°C (1373 K ) .  The need t o c o n t a i n about 1.5 kg o f charge m a t e r i a l s i n t h e r e a c t i o n chamber a t 20 percent f i l l i n g .  Cold junction ''dewar  Steel tire  at  \  T/c  Slip ring  End ^pipe  0-Brocket  AI Q tube 2  3  Rollers  L Figure 4.5.  (A) Side view of open rotary reactor.  400 mm  •112  mm *| Charge  _fc±.  /  PliCQSl  /  L.W.I. 2 4  TT  N  406  " Feeding port -280 mm •!  mm  thermocouple  ' y probe ~ M Figure 4.5.  54  4^.  140 140  180 I8C  mm  Al 0 castable 2  3  II  (B) Central cross-sections of rotary reactor  59  TABLE XI. THERMOPHYSICAL PROPERTIES OF REFRACTORY MATERIALS USED IN THE REACTOR  P l i c a s t LW1 24R  Composition  Castolast G  46% A 1 0 , 38% S i 0 , 1.5% F e 0 , 1.5% T i 0 , 9% CaO, 1% MgO, 2% A l k a l i e s 2  3  2  2  Density  Thermal c o n d u c t i v i t y  Maximum temperature  1282 Kg/m  3  0.40 W/m K ( a t 815°C)  1050°C  2  3  94% A 1 0 6% Phosphates 2  3  2650 Kg/m  3  8.8 W/m K (1000°C)  1200°C  60  iii)  A reasonably  low e x t e r n a l s h e l l t e m p e r a t u r e o f approximately  200°C (573 K). This was t o enable use o f s i l i c o n e base mater i a l s to seal the system.  The heat balance c a l c u l a t i o n s are presented i n Appendix A. The ends o f t h e r e a c t i o n chamber were c a s t i n a 45° c o n i c a l s h a p e , as shown i n Figure 4.5 B, to minimize dead-zones which could a f f e c t the mixing o f the p a r t i c l e s as the r e a c t o r r o t a t e d . The r e f r a c t o r i e s , once they were c a s t i n s i d e the s h e l l , were f i r e d using the heating element f o l l o w i n g the manufacturer's s p e c i f i c a tions.  However, when p r e l i m i n a r y t e s t s were conducted with the r e a c t o r  s e a l e d , c o n s i d e r a b l e amounts o f water were found t o d e p o s i t a t t h e c o n densers  and even a t the e x i t end-pipe.  This was concluded t o be due to  the f a c t that the external l a y e r s o f the r e f r a c t o r y were n e v e r a b l e t o r e a c h t h e t e m p e r a t u r e necessary to r e l e a s e t h e i r water of c r y s t a l l i z a t i o n , i . e . , about 400°C, thus the water could only be slowly r e l e a s e d by d i f f u s i o n i n t o t h e r e a c t i o n chamber. Consequently, the whole r e a c t o r was baked i n a g a s - f i r e d furnace, a t 500°C f o r 72 h o u r s , t o e n s u r e a l l the w a t e r had been e l i m i n a t e d .  Even then, i n subsequent t r i a l s , t r a c e  amounts o f water were s t i l l found which, most l i k e l y , r e s u l t e d from a d s o r p t i o n from t h e s u r r o u n d i n g s open.  d u r i n g i n t e r v a l s when the r e a c t o r was  This f i n a l p r o b l e m was f u l l y overcome by k e e p i n g  the r e a c t o r  c l o s e d and a t 500°C d u r i n g i d l e p e r i o d s ; needless to say that maintenance o f a dry r e a c t o r was e s s e n t i a l f o r the study o f reduction coal.  using  61  4.3.4  Rotating system A 1/4 h.p. v a r i a b l e speed motor with a 16 tooth d r i v i n g sprock-  e t , shown i n Figure 4.3, was used to impart the r o t a t i o n to the r e a c t o r . A s t a n d a r d c h a i n , 25.4 mm p i t c h , l i n k e d the d r i v i n g sprocket to i t s 80 tooth counterpart that was b o l t e d to the r e a c t o r .  Rotation was  achieved  v i a s t e e l t i r e s , seen i n F i g u r e 4.5 A, which rested on r o t a t i n g s t e e l trunnions supported by s t e e l b a l l bearings whose p i l l o w blocks were s e cured to the frame of the s t r u c t u r a l assembly.  Special care was paid to  the f a c t that several attachments were to be protruding from the  shell,  so enough c l e a r a n c e had to be allowed f o r them to r o t a t e f r e e l y .  The  maximum r o t a t i o n a l speed t h a t could be i m p a r t e d t o the r e a c t o r was  25  r.p.m..  4.3.5  Feeding, sampling and emptying systems The r e a c t o r was provided with one feeding p o r t , shown i n  4.5 B, which a l s o c o u l d be used f o r s o l i d s sampling.  Figure  The port was  de-  signed i n such a way as to feed the charging m a t e r i a l s i n a t a n g e n t i a l d i r e c t i o n , thus a v o i d i n g any p o s s i b l e damage to the f r a g i l e heating e l e ment by thermal shock and chemical a t t a c k .  A sampling probe,  described  below, c o u l d a l s o be i n t r o d u c e d through t h i s port i n t o the s o l i d s bed without the p o s s i b i l i t y of touching the element. The s a m p l i n g  probe, presented  i n Figure 4.6, c o n s i s t e d of two  c o n c e n t r i c s t a i n l e s s s t e e l p i p e s , one of 9.3 mm and the other of 25.4 diameter.  mm  The o u t e r p i p e , s l i g h t l y t a p e r e d f o r ease of i n t r o d u c t i o n  through the feeding p o r t , was c l o s e d at i t s top by a threaded cap which  62  4.6  Sampling probe configuration  63  allowed the movement of the inner pipe f o r purposes o f opening and c l o s ing the probe to take a s o l i d s sample. inner pipe  Argon gas was blown through  and e x i t e d c l o s e to the s o l i d s sample to f a c i l i t a t e quench-  ing and c e s s a t i o n of the r e a c t i o n . The i n e r t gas e x i t e d the o u t e r at p o i n t B  the  5  shown i n F i g u r e 4.6.  The s i z e of the sample obtained  pipe was  between 1 and 3 grams. One more p r o b e , i t s l o c a t i o n shown i n F i g u r e 4.5 B, was c a s t i n t o the r e f r a c t o r y to c h a r a c t e r i z e the p o s s i b l e formation of a c c r e t i o n s a g a i n s t the w a l l .  The probe was made with the same r e f r a c t o r y t h i c k n e s s  as the r e s t of the k i l n , to transmit the same heat by c o n d u c t i o n ; i t s exposed area at the r e a c t i o n chamber was 4.5  2  cm .  In order to empty the r e a c t o r contents and to v i s u a l l y i n s p e c t the r e a c t i o n chamber a f t e r every experiment,  the short end of the reac-  t o r c o u l d be separated from the l a r g e r s e c t i o n , as shown i n F i g u r e  4.5  A, by f a s t e n i n g the former with two b o l t e d brackets to i t s corresponding end-pipe f l a n g e .  The end-pipe r e s t e d on a heavy p l a t f o r m t h a t had  the  c a p a b i l i t y to r o l l , l o n g i t u d i n a l l y with respect to the heating element, on four b a l l bearings over the s t r u c t u r a l assembly, t o a l l o w a s e p a r a t i o n of approximatley 25 cm between the two s e c t i o n s .  4.3.6  Temperature monitoring and c o n t r o l A quick response thermocouple probe, l o c a t e d a t p o i n t A i n F i g -  ure 4.5 B, was ment. 131  implemented a c c o r d i n g to previous work i n t h i s depart-  T h i s probe was used to monitor the temperature  of the system  during the t e s t s and contained three i n d i v i d u a l t h e r m o c o u p l e s :  two  at  the inner surface of the r e a c t i o n chamber and the other protruding 15 mm  64  i n t o t h e chamber.  The d u p l i c a t e i n n e r - s u r f a c e thermocouples ensured  c o r r e c t temperature measurement a t t h i s p o i n t . The t h e r m o c o u p l e s were c h r o m e l - a l umel type 'K' wires o f 0.76 mm diameter and t h e i r beaded ends were p r o t e c t e d from e r o s i o n and chemical attack by a p p l y i n g a t h i n l a y e r , l e s s than 1 mm t h i c k , o f alumina ceramic p a i n t . The t h i c k n e s s of the c o a t i n g , given the r e l a t i v e l y high c o n d u c t i v i t y o f the alumina, i n t r o d u c e d a n e g l i g i b l e t h e r m a l r e s i s t a n c e to the heat flow.  The c o l d  j u n c t i o n f o r the thermocouples, shown i n Figure 4.5 A, was an i c e f i l l e d thermos f l a s k r o t a t i n g with the r e a c t o r . The v o l t a g e s i g n a l s from any of the three thermocouples were t r a n s m i t t e d t o a s l i p r i n g , w i t h t h e h e l p o f a m u l t i p o s i t i o n s w i t c h , which i n t u r n t r a n s m i t t e d them to a c h a r t r e c o r d e r . The e l e c t r i c signal was a l s o f e d t o t h e c o n t r o l l e r o f the power source.  4.3.7 Gas flow measurement and a n a l y s i s The gas flow measurement and sampling set-up i s shown i n F i g u r e 4.3.  Above the g a s - e x i t end-pipe a s o l i d s t r a p and c o o l e r were l o c a t e d .  This was a water c o o l e d copper p i p e , 31 mm i n t e r n a l diameter, t h a t c o n t a i n e d an i n t e r w o v e n mesh of copper i n s i d e to t r a p s o l i d p a r t i c l e s ent  t r a i n e d by the gas stream.  A subsequent g l a s s condenser trapped most o f  the remaining dust and c o o l e d the gas down to room temperature. The gas flow was d i r e c t e d , v i a tygon t u b i n g , i n t o a s e t o f b a l l rotameters, Gilmont s e r i e s 1, 2, 3 and 4.  The rotameters were p r e v i o u s -  l y c a l i b r a t e d , f o r a standard gas mixture of composition s i m i l a r to t h a t e x p e c t e d t o be obtained from the experiments, u t i l i z i n g a l i q u i d - s e a l e d gas meter f o r the higher flows and the s o a p - b u b b l e method f o r t h e low  65  flow r a t e s . pendix B.  C a l i b r a t i o n curves f o r the rotameters are presented i n Ap-  The gas f l o w was d i v e r t e d a c c o r d i n g t o i t s m a g n i t u d e ,  by  changing the tygon tubing coming from the second condenser, i n t o the app r o p r i a t e flowmeter with the help of quick t e f l o n connectors which every r o t a m e t e r had a t i t s i n l e t end.  The flows were always measured i n be-  tween 15 and 80 percent o f the maximum  respective flowmeter c a p a c i t y  w h i c h , f o r the s e t o f flowmeters, encompassed a flow rate range between 3  20 and 36000 c m / m i n .  The p r e s s u r e head a t the flowmeter's i n l e t was  measured with a u-tube manometer and ranged between 0.5 and 4 cm o f wat e r ; the gas temperature was a l s o measured a t t h i s point with a thermometer.  The gas sampling p o i n t , shown i n F i g u r e 4.3, was l o c a t e d a f t e r  the s e t o f rotameters. The r e t e n t i o n time from the e x i t o f the r e a c t o r up to t h i s point v a r i e d , a c c o r d i n g to flow r a t e , between a few s e c o n d s and 8 minutes since the condensers and gas l i n e s had a volume o f approx3  i m a t e l y 3000 cm .  3  The gas samples, of 60 cm , were taken with s y r i n g e s  through a rubber s e c t i o n of the gas l i n e , b e f o r e the gaseous p r o d u c t s were exhausted i n t o a fume hood.  The sampling procedure i s e x p l a i n e d i n  d e t a i l i n s e c t i o n 4.7.3 of t h i s c h a p t e r .  4.4 Apparatus f o r Room-Temperature Mixing Experiments The same r o t a r y r e a c t o r , with some m o d i f i c a t i o n s f o r v i s u a l  ob-  s e r v a t i o n , was u t i l i z e d f o r the mixing experiments as was mentioned a t the beginning of t h i s c h a p t e r . The arrangement, shown i n F i g u r e 4.7, c o n s i s t e d o f removing the s h o r t s e c t i o n o f the r e a c t o r and r e p l a c i n g i t with a t r a n s p a r e n t , c o n i c a l l y shaped s e c t i o n , that corresponded to that  66  4.7  (b) Equipment f o r room-temperature mixing experiments (A) General view; (B) D e t a i l view o f a c r y l i c blade .  67  end of the r e a c t i o n chamber.  It was p o s s i b l e i n t h i s manner to observe  the way the s o l i d s bed behaved under d i f f e r e n t c o n d i t i o n s . An a u x i l i a r y d e v i c e , shown i n Figure 4.7 B, was used to observe the degree of mixing at the c e n t r a l c r o s s s e c t i o n o f the bed.  The  de-  v i c e c o n s i s t e d of a transparent p l a s t i c blade i n the shape of a segment s i m i l a r to the c r o s s s e c t i o n a l area of the bed.  A handle  perpendic-  u l a r to the blade was used to p o s i t i o n and hold i t i n place while a photograph was taken.  A 35 mm camera, with a macro l e n s , was u t i l i z e d t o  record the p a r t i c l e s d i s t r i b u t i o n behind the blade.  4.5 Coal Charring Equipment A s c h e m a t i c view o f the set-up to perform the coal d e v o l a t i l i z a t i o n i s presented i n F i g u r e 4.8.  The set-up c o n s i s t e d i n a p r e - o x i d -  i s e d s t a i n l e s s - s t e e l t r a y , able to c o n t a i n 1 kg of c o a l . The t r a y was placed on the bone-ash f l o o r of a t o p - f i r e d gas furnace, above the e x i t of the p i p e which c a r r i e d i n e r t gas.  The coal c o n t a i n i n g t r a y was co-  vered with a l a r g e r , heavy Inconel t r a y t h a t , placed upside down a g a i n s t the furnace f l o o r , acted as a l i d to maintain an i n e r t atmosphere around the c o a l . A small opening was l e f t i n the bone ash a t one o f the c o r n e r s , as shown i n F i g u r e 4.8, to allow the v o l a t i l e products to escape from the system and to be burnt o f f by the furnace flame.  Four  chromel-  alumel thermocouples, marked i n Figure 4.8 as A,B, and C,D, were placed along the l o n g i t u d i n a l mid-section of the c o a l bed: two a t the c e n t e r and the o t h e r two a t one of the ends of the bed.  In each of the l o c a -  t i o n s , one thermocouple was p o s i t i o n e d a t the bottom and the o t h e r a t the mid-height of the bed.  It was p o s s i b l e , i n t h i s way, to record the  68  Top  view  Thermocouples  Gos exit  Burner  Inconel Coal Bone ash floor \  bed  K ~ IT  Inert gas Side  4.8  —1 ^S.S.'trdy  view  Schematic view of coal d e v o l a t i l i z a t i o n equipment  cover  69  t e m p e r a t u r e g r a d i e n t s c r u d e l y w i t h i n the bed during the coal c h a r r i n g experiments.  4.6 Experimental  Design and V a r i a b l e s  The v a r i a b l e s and t h e i r l e v e l s , summarized i n Table XII and des c r i b e d below, were chosen i n order t o obtain an o v e r a l l p i c t u r e o f t h e r e d u c t i o n p r o c e s s , under f e a s i b l e c o n d i t i o n s f o r an i n d u s t r i a l - s i z e operation.  In p a r t i c u l a r , e f f e c t s on the mixing p a t t e r n s , on the reduc-  t i o n k i n e t i c s and on the agglomeration and coal p a r t i c l e s were o f concern. forthcoming  sub-sections.  c h a r a c t e r i s t i c s o f the i r o n - o r e  The v a r i a b l e s are presented  i n the  The sequence o f t e s t i n g the d i f f e r e n t l e v e l s  and the r a t i o n a l e , are presented i n the ensuing Chapters 5 and 6.  4.6.1 V a r i a b l e s i n room-temperature mixing experiments The room-temperature m i x i n g e x p e r i m e n t s were p e r f o r m e d u s i n g only mixtures  o f i r o n o r e and F o r e s t b u r g c o a l ; the mixtures with the  other carbonaceous m a t e r i a l s were assumed t o f o l l o w the same m i x i n g beh a v i o u r , s i n c e t h e i r d e n s i t i e s and s u r f a c e c h a r a c t e r i s t i c s were very similar.  i)  The v a r i a b l e s and t h e i r r e s p e c t i v e l e v e l s t e s t e d were:  Coal to ore s i z e r a t i o , dc/d*F . e  For each o f the four ore p a r t i -  c l e s i z e s , * 358, 254, 280 and 90 ym,  four coal p a r t i c l e s i z e s  * P a r t i c l e s i z e h e r e a f t e r d e f i n e d as the a r i t h m e t i c mean value between the two c l o s e s t U.S. s e r i e s screen openings.  70  TABLE X I I . SUMMARY OF VARIABLES AND LEVELS TESTED. USING FORESTBURG COAL  Experiments Mixing  Variable  Symbol  Levels  Iron-ore mean p a r t i c l e s i z e [ m]  d/p  e  Coal to ore s i z e r a t i o  d /dp  e  F i x carbon to i r o n r a t i o  Cpj /Fe  358, 254, 180, 90  y  c  4, 2, 1, 0.5 0.45, 0.58,  x  0.71, 0.84 Rotational speed [r.p.m.]  w  5, 15  Percent volume f i l l e d [%]  % L  12, 20  Reduction T  Temperature o f r e a c t i o n [°C] Rotational speed [r.p.m.] F i x carbon to i r o n r a t i o  800, 850. 900, 950 •  a)  Cpix/Fe  7, 11, 14, 17, 20 0.16. 0.24, 0.32 0.48, 0.64  Percent volume f i l l e d [%]  % L  7, 14  Iron-ore mean p a r t i c l e s i z e [ym]  "d/  358, 90.  Coal to ore s i z e r a t i o  d /d c  F  Fe  0.5, 1, 2  71  were used i n o r d e r t o obtain d / d p r a t i o s o f approximately 4, c  e  2, 1 and 0.5. Adding together t h e w e i g h t o f t h e s e s i z e f r a c t i o n s accounts f o r approximately three quarters o f the weight o f the a s - r e c e i v e d o r e . ii)  F i x e d c a r b o n t o i r o n r a t i o , CFix/Fe.  Four l e v e l s were t e s t e d :  0.45, 0.58, 0.71 and 0.84 which correspond to 180, 260, 340 and 400 p e r c e n t e x c e s s o f c a r b o n , r e s p e c t i v e l y , according to the stoichiometry o f Equations (2.1) and (2.2). iii)  R o t a t i o n a l s p e e d , u> • Two l e v e l s were t e s t e d , 5 and 15 r.p.m. which cover the range o f i n d u s t r i a l c o n d i t i o n s , when t h e Froude Number ( F r = 2 R/g) - j w  S u s e c  j  a s  a  similarity criterion.  The me-  dian p o i n t , 10 r.p.m., was t e s t e d i n some experiments. iv)  P e r c e n t l o a d i n g , % L.  Two l e v e l s were t e s t e d , 12 and 20 per-  cent, which again covers the range o f volume f i l l i n g  i n indus-  t r i a l operation.  4.6.2 V a r i a b l e s i n r e d u c t i o n experiments The v a r i a b l e s t e s t e d i n the r e d u c t i o n experiments were: i)  Coal t y p e .  The r e d u c t i o n k i n e t i c s were c h a r a c t e r i z e d f u l l y f o r  the F o r e s t b u r g , s u b - b i t u m i n o u s  coal.  Comparative  t e s t s were  p e r f o r m e d w i t h t h e Saskatchewan L i g n i t e and, i n one case, with electrode-grade graphite, ii)  Reduction Temperature,  T.  Four i s o t h e r m a l temperatures were  t e s t e d , 800, 850, 900 and 950°C.  Although they are comparative-  l y low w i t h r e s p e c t to present i n d u s t r i a l o p e r a t i o n s , the r e a c t i v i t y of the Forestburg coal r e p o r t e d i n the 1iterature!23  72  i n d i c a t e d t h a t carbon g a s i f i c a t i o n , and the corresponding reduct i o n , were f e a s i b l e a t these t e m p e r a t u r e s a t r e a s o n a b l e r a t e s and with l e s s p a r t i c l e agglomeration, iii)  Fixed carbon to i r o n r a t i o , C p i / F e . x  tested:  Again f i v e l e v e l s were  0.16, 0.24, 0.32, 0.48 and 0.64 which c o r r e s p o n d t o 0,  50, 100, 200 and 300 percent excess carbon with r e s p e c t to the minimum s t o i c h i o m e t r i c r e q u i r e m e n t s o f E q u a t i o n s (2.1) and (2.2). iv)  Coal t o o r e s i z e r a t i o , d / d p c  e a  Two l e v e l s , 0.5 and 2, were  t e s t e d f o r the 358 ym ore p a r t i c l e s and one l e v e l , 1, was t e s t e d f o r the 90 ym ore p a r t i c l e s , v)  Rotational speed, co . F i v e speeds were t e s t e d , 7, 11, 14, 17 and 20 r.p.m. u n t i l no f u r t h e r a p p r e c i a b l e e f f e c t was o b s e r v e d on the r e d u c t i o n r a t e .  vi)  P e r c e n t l o a d i n g , % L.  Two l e v e l s were t e s t e d , 7 and 14 per-  cent.  4.6.3 Experimental design As can r e a d i l y be o b s e r v e d from t h e number o f v a r i a b l e s and l e v e l s i n v o l v e d f o r each type o f the experiments c o n s i d e r e d above, the number o f p o s s i b l e e x p e r i m e n t a l c o m b i n a t i o n s i s r a t h e r high:  larger  than 250 f o r the mixing t e s t s and about 600 f o r t h e r e d u c t i o n e x p e r i ments.  C l e a r l y , some c o n s i d e r a t i o n s had to be a p p l i e d i n order to r e -  duce those f i g u r e s and y e t obtain the o v e r a l l p i c t u r e o f the experimental system here s t u d i e d . Thus:  73  i)  D u r i n g the room-temperature  m i x i n g e x p e r i m e n t s , the extreme  values of the v a r i a b l e s were f i r s t t e s t e d and, i f a s i g n i f i c a n t e f f e c t on the m i x i n g was not observed, the intermediate values were d i s c a r d e d . The f u l l range of the v a r i a b l e s was c h e c k e d a t l e a s t once f o r one i r o n - o r e s i z e however, ii)  In the r e d u c t i o n experiments, the e f f e c t of the r o t a t i o n a l  speed  was a s s e s s e d o n l y a t the Cp-j /Fe r a t i o s of 0.16 and 0.64 and x  900°C of temperature.  T h i s temperature e n s u r e d an a p p r e c i a b l e  r a t e and the two v a l u e s o f the Cp-j /Fe r a t i o enclose the f u l l x  range to be t e s t e d , iii)  A l s o i n the r e d u c t i o n experiments, the e f f e c t of Cp-j /Fe r a t i o x  was e v a l u a t e d , a g a i n , a t the high temperature (900°C) to e n s u r e an a p p r e c i a b l e r a t e a t a l l the Cp-j /Fe l e v e l s . x  however, the r o t a t i o n a l r.p.m.) because  In t h i s case  speed was k e p t a t the l o w e r l e v e l  the e f f e c t o f Cp-i /Fe was x  (7  g r e a t e r a t low  speeds.  The above c o n s i d e r a t i o n s reduced the number of experiments to a manageable s i z e , and r e p l a c e d the experimental design which had o r i g i n a l l y been planned, f o r the r e d u c t i o n experiments.  This was a f r a c t i o n a l  f a c t o r i a l design of four v a r i a b l e s a t f o u r l e v e l s e a c h , t h a t was  also  discarded f o r the f o l l o w i n g reasons:  i)  The i n t e r a c t i o n s between some p a i r s of v a r i a b l e s c o u l d not be neglected, as i s assumed i n t h a t kind of d e s i g n .  74  ii)  In o r d e r to f u l f i l l the symmetry of the design, some v a r i a b l e s would have had to have been t e s t e d a t a number o f l e v e l s t h a t was not necessary,  iii)  In order to perform the s t a t i s t i c a l a n a l y s i s of the experimental design, and thus make f u l l p r o f i t o f i t s p r o p e r t i e s , a t l e a s t one r e p l i c a t i o n of each experiment would have been r e q u i r e d and so, again, the number of experiments would have been doubled.  4.7 Experimental Procedures A d e s c r i p t i o n o f each o f the t h r e e t y p e s of experiments, f o r mixing-, c h a r r i n g - and r e d u c t i o n s t u d i e s , i s presented i n t h i s s e c t i o n .  4.7.1 Room-temperature mixing experiments This group of experiments was c a r r i e d out i n the equipment s c r i b e d i n s e c t i o n 4.4 above.  de-  The aim of the testwork was, as has been  stated i n s e c t i o n 4.1 of t h i s chapter, to determine optimum m i x i n g c o n d i t i o n s o f the c o a l - o r e mixtures previous to the r e d u c t i o n experiments. This was achieved through v i s u a l o b s e r v a t i o n of the t y p e o f bed  motion  and p h o t o g r a p h i c r e c o r d i n g of the d i s t r i b u t i o n of s o l i d p a r t i c l e s .  The  sequence of operation was as f o l l o w s . The i r o n - o r e and c o a l p a r t i c l e s mixture was prepared, with the predetermined d / d p c  ponent i n t o a 1000 cm  e  3  s i z e r a t i o , by adding small amounts of each comgraduated c y l i n d e r , and hand mixing them c a r e f u l -  ly each time, u n t i l the volume necessary f o r the d e s i r e d percent l o a d i n g was f i l l e d .  A g r a p h , shown i n Figure 4.9, was prepared i n advance f o r  easy r e f e r e n c e to e s t a b l i s h the r e l a t i o n s h i p s amongst percent l o a d i n g ,  75  4.9  Graph showing bed volume and depth as a f u n c t i o n of percent l o a d i n g f o r a r e a c t o r 14 cm i n diameter .  76  bed volume and bed depth f o r the r e a c t o r . The weights of each component were r e c o r d e d f o r subsequent t e s t s with the same d / d p and Cpix/Fe r a c  tios.  e  The mixture was then f e d i n t o the r e a c t o r and the r o t a t i o n s t a r t -  ed a t the d e s i r e d r.p.m. (measured with a hand tacometer), kept f o r 30 minutes and stopped.  The bed behaviour was t a k e n to be t h a t o b s e r v e d  d u r i n g the l a s t f i v e m i n u t e s o f r o t a t i o n .  There i s evidence i n the  1 i t e r a t u r e l Z l t h a t complete mixing i s obtained i n l e s s than 25 r e v o l u t i o n s ; t h i s c o n d i t i o n was achieved even a t the low r o t a t i o n a l i . e . , 5 r.p.m.  speeds,  The type of bed motion observed was recorded a c c o r d i n g  to the c l a s s i f i c a t i o n of s l i p p i n g , slumping and r o l l i n g r e p o r t e d by nein.  4  He-  The t r a n s p a r e n t end of the chamber was removed and the p l a s t i c  blade was c a r e f u l l y placed a t the c e n t r a l c r o s s s e c t i o n area o f the s o l i d s bed, as has been shown i n F i g u r e 4.7 A.  The p a r t i c l e s which r e -  mained i n f r o n t were brushed away with great c a r e , i n order not t o d i s t u r b t h e bed b e h i n d the b l a d e , and a photograph was taken.  (A g r e a t  advantage f o r t h i s purpose was the f a c t that there was a l a r g e c o n t r a s t i n c o l o r between the two t y p e s o f p a r t i c l e s . )  The r e a c t o r was then  emptied and the m a t e r i a l s d i s c a r d e d .  4.7.2 Coal c h a r r i n g experiments One k i l o g r a m o f c o a l , t h a t had been s t o r e d a f t e r screening i n t i g h t l y c l o s e d p l a s t i c bags, was put i n t o the s t a i n l e s s steel t r a y which then was placed i n the set-up d e s c r i b e d i n s e c t i o n 4.5 and shown i n F i g ure 4.8.  3  The argon flow was s t a r t e d a t 1500 cm /min and the gas f i r e d  furnace was l i t .  The valves f o r gas and a i r were c o n t r o l l e d i n order to  produce a heating rate of approximately 9°C/min, which has been reported  77  t o be the h e a t i n g r a t e f o r the m a t e r i a l s i n an i n d u s t r i a l s i z e r o t a r y kiln,  1  u n t i l the center region of the coal bed temperature reached 900°C  (1173 K); t h i s was achieved i n approximately 100 m i n u t e s , and the c o a l was then kept at t h i s temperature f o r the d e s i r e d p e r i o d of time, a f t e r which, the furnace was turned o f f and the char was allowed to cool overn i g h t with the argon f l o w r a t e maintained at 1000 cm^/min. Once the char reached room temperature, i t s weight was recorded, a 10 gram sample was taken and s t o r e d again i n a s e a l e d p l a s t i c bag.  4.7.3 Reduction experiments The c o r e o f t h i s study was the r e d u c t i o n experiments.  Several  p r e l i m i n a r y t r i a l s had to be c a r r i e d out, with an ever i n c r e a s i n g degree of success, u n t i l the optimum procedure was a t t a i n e d ; the d e s c r i p t i o n of t h i s procedure f o l l o w s . The amount of the i r o n - o r e and coal mixture, f o r a given Cpix/Fe and d / d p c  e  r a t i o s , was prepared f o l l o w i n g the same procedure as the one  used f o r the room-temperature mixing experiments, j u s t d e s c r i b e d i n sect i o n 4.7.1.  The only n o t i c e a b l e d i f f e r e n c e was the weight r a t i o of ma-  t e r i a l s , s i n c e the f i x e d carbon content i n the char had o b v i o u s l y changed during the d e v o l a t i l i z a t i o n process. The r e a c t o r , which was kept at 500°C during the i d l e periods to prevent r e - a d s o r p t i o n of m o i s t u r e , was s e t the n i g h t p r e v i o u s to the e x p e r i m e n t , a t 75°C above the d e s i r e d t e s t temperature.  Thus by the  time the experiment was s t a r t e d the l a r g e r e f r a c t o r y mass of the r e a c t o r had already a t t a i n e d near s t e a d y - s t a t e thermal e q u i l i b r i u m with i t s surroundings.  The reason f o r superheating the r e f r a c t o r y was the f a c t t h a t  78  when the charge m a t e r i a l s were i n t r o d u c e d , a sharp drop i n the set temperature i n the r e a c t i o n chamber was o b s e r v e d .  The r e a c t o r was  then  sealed, paying s p e c i a l a t t e n t i o n to the c r i t i c a l s e a l i n g areas, designed by A-A' and B-B  1  i n f i g u r e 4.4.  Argon flow was s t a r t e d to f l u s h the a i r  from the system f o r about 45 minutes; meanwhile the a i r and standard gas samples of composition shown i n T a b l e X I I I , were a n a l y z e d i n the gas chromatograph,  a P e r k i n - E l m e r , Sigma 38 dual column gas  whose operating c o n d i t i o n s are presented i n T a b l e XIV.  chromatograph The c h r o m a t o -  graph had a peak a r e a i n t e g r a t o r and a c h a r t r e c o r d e r attached to i t . The columns were a 0.3 x 180 cm f i l l e d with Porapak N' m a t e r i a l f o r CO2 1  a n a l y s i s and a 0.3 x 300 cm column f i l l e d with a molecular s i e v e , MS 5A, for the a n a l y s i s of hydrogen, oxygen, n i t r o g e n , and carbon monoxide.  At  l e a s t three samples of each gas were run and the r e s p e c t i v e peak a r e a counts f o r each gas component were averaged.  The r e l a t i v e e r r o r between  the counts was, i n almost a l l cases, smaller than one percent. The a i r - t i g h t n e s s e f f i c i e n c y of the r e a c t o r , h e r e a f t e r r e f e r r e d to as e f f i c i e n c y , was then a s s e s s e d by comparing  the amount o f argon  g o i n g i n t o t h a t g o i n g o u t o f the system, using two i d e n t i c a l flowmet e r s , at an i n p u t f l o w r a t e o f 7000 cm3/min; the e f f i c i e n c y was c a l c u l a t ed, by s u b t r a c t i n g the output from the input and d i v i d i n g by the i n p u t , and r e c o r d e d as the s t a r t i n g v a l u e .  The c o l d j u n c t i o n f l a s k was  filled  with crushed i c e and the two thermocouples at the chamber face were compared a g a i n s t each other, as a check to ensuring both were working. t h i s p o i n t , the temperature c o n t r o l was turned down to the s e t  At  tempera-  t u r e and, i m m e d i a t e l y , t h e f e e d i n g p o r t opened and the s o l i d s mixture q u i c k l y but s t e a d i l y poured i n t o the r e a c t o r with the help of a f u n n e l .  TABLE X I I I .  CERTIFIED-GRADE STANDARD GAS COMPOSITION  Gas  %  Ar  1.07  H  4.04  2  C0  2  CO  20.10 74.79  TABLE XIV. GAS CHROMATOGRAPH OPERATING CONDITIONS  C a r r i e r gas Oven temperature I n j e c t o r and d e t e c t o r temperatures Pressure o f i n j e c t i o n  Argon 105°C 130°C 22 p . s .  80  The p o r t was t i g h t l y c l o s e d , the r o t a t i o n s t a r t e d to the d e s i r e d r o t a t i o n a l speed and t h e argon f l o w s t o p p e d ; t h e measurements were t h u s initiated.  The chamber and s o l i d s temperature, shown i n F i g u r e 4.10,  dropped s h a r p l y but, because of the superheating provided, r e c o v e r e d t o the  s e t t e m p e r a t u r e w i t h i n 10 minutes and a t the higher temperatures,  before 6 minutes. The gas f l o w r a t e measurement a t every time i n t e r v a l , was f o l lowed by a gas sample, w i t h i n a maximum of 10 seconds.  The f i r s t meas-  urement and sample were taken, i n v a r i a b l y , 2 minutes a f t e r the s t a r t and then, i n most c a s e s , every 2 minutes u n t i l 20 minutes, e x c e p t when t h e r e a c t i o n was expected to proceed f a s t e r , whereupon the measurements were taken every minute.  From then on, the r e a d i n g s were s p a c e d a t l a r g e r  i n t e r v a l s , up t o a maximum o f 30 minutes which was maintained f o r the r e s t of the experiment.  The gas flow was p r o g r e s s i v e l y d i v e r t e d t o t h e  smaller c a p a c i t y flowmeters as the r e a c t i o n rate dropped, but s i n c e most 3  of t h e gas f l o w r a t e s produced were between 2000 and 12000 cm /min and at the higher rates there was some small entrainment o f v e r y f i n e c o a l d u s t , two f l o w m e t e r s w i t h t h i s c a p a c i t y were used, a l t e r n a t e l y being cleaned i n between measurements. The gas samples s t a r t e d to be analyzed i n t h e gas chromatograph before 30 minutes had elapsed from the time of taking them, which i s well w i t h i n the c o n s i d e r e d safe storage p e r i o d o f one h o u r .  1 3 2  The measurements continued u n t i l the r e a c t i o n p r a c t i c a l l y s t o p 3  ped, i . e . , a t flow rates of about 80 cm /min.  The argon flow was s t a r t -  ed, the power turned o f f and t h e e f f i c i e n c y o f t h e system d e t e r m i n e d again; the o v e r a l l e f f i c i e n c y was c o n s i d e r e d to be the a r i t h m e t i c mean  950  T Charge  introduction  5 Time  4.10  10 (min)  Reaction chamber temperature as a f u n c t i o n o f time oo  82  o f the i n i t i a l and f i n a l v a l u e s .  The r o t a t i o n was stopped then, and the  r e a c t o r l e f t t o cool o v e r n i g h t with an argon flow o f approximately 1000 cm^/mi n. The r e a c t o r was s p l i t a p a r t , a f t e r the argon flow had been stopped, and the contents were c a r e f u l l y emptied onto a t r a y . They were immediately t r a n s f e r r e d i n t o a f l a s k t h a t was t i g h t l y c l o s e d with a rubber stopper, t o avoid any r e o x i d a t i o n o f t h e f r e s h p r o d u c t s .  The s e a l ,  shown by C - C i n f i g u r e 4.4, was r e p l a c e d , the r e a c t o r c l o s e d back and the power s t a r t e d again. The r e a c t i o n s o l i d products were sieved i n t o narrow s i z e f r a c t i o n s , from 2000 ym down t o 74 pm, i n standard T y l e r screens.  The d i f -  ference between the i r o n - o r e and coal p a r t i c l e s s i z e was a b i g advantage f o r t h i s purpose, except a t the f i n e s t s i z e s ( s m a l l e r than 140 pm) when ;  a sample had t o be taken and separated m a g n e t i c a l l y to o b t a i n the products d i s t r i b u t i o n ; t h i s was a l s o t h e c a s e when t h e e x p e r i m e n t s d /dp c  e  o f 1 were p e r f o r m e d .  with  In a l l cases, a sample o f about 20 grams  was taken from each s i z e f r a c t i o n , t o be examined under t h e s c a n n i n g e l e c t r o n m i c r o s c o p e ; t h e s e samples were k e p t i n s i d e t i g h t l y sealed vials.  83  CHAPTER 5  RESULTS OF ROOM-TEMPERATURE MIXING EXPERIMENTS  The r e s u l t s o f the room-temperature mixing experiments a r e p r e sented i n t h i s chapter.  The experiments were performed i n the apparatus  d e s c r i b e d i n S e c t i o n 4.4 (Figure 4.7) f o l l o w i n g the technique p r e s e n t e d i n S e c t i o n 4.7.1. A summary o f the experimental c o n d i t i o n s and r e s u l t s i s t a b u l a t e d i n Appendix C, and only a synopsis o f the r e s u l t s , t o i l l u s t r a t e t h e o b s e r v e d p a t t e r n s , t h e t y p e o f bed motion and degree of mixing, i s presented i n the forthcoming s e c t i o n s . The t y p e o f bed motion and the corresponding mixing o f the part i c l e s were determined by v i s u a l o b s e r v a t i o n . Bed motion was c a t e g o r i z e d a c c o r d i n g to the d e f i n i t i o n s o f s l i p p i n g , slumping, r o l l i n g and c a t a r a c t i n g , as given by Henein e t al..133  p a r t i c l e mixing was c h a r a c t e r -  ized i n terms o f three d e f i n e d 'degrees' o f m i x i n g , as i l l u s t r a t e d i n Figure 5.1, namely:  i)  S e g r e g a t e d c o n d i t i o n : i n which there e x i s t e d a sharply defined core, or "kidney,"  4  o f i r o n - o r e p a r t i c l e s w i t h i n the bed, (A)  and (B) i n Figure 5.1.  (A)  (B) D e f i n i t i o n of boundaries o f degree o f m i x i n g . C o n d i t i o n s d =358ym; C_. /Fe=0.45; 15 r.p.m.; 20% l o a d i n g ; d /d„ re r IX c Fe f o r ( A ) , 4.464; (B) , 2.000 ; (C) , 1. 285; (D) , 1,000; (E) 0.714; a n d ( F ) , 0.503. (A) a n d ( B ) , s e g r e g a t e d b e d ; (C) a n d ( D ) , t r a n s i t i o n a l c o n d i t i o n a n d (E) and ( F ) , w e l l mixed bed.  F i g u r e 5.1  (Continued)  F i g u r e 5.1  (Continued)  87  ii)  Transitional condition:  i n which some regions i n the c r o s s -  s e c t i o n of the bed showed a h i g h e r c o n c e n t r a t i o n o f i r o n - o r e p a r t i c l e s , without well defined boundaries, ure iii)  (C) and (D) i n F i g -  5.1.  Well mixed c o n d i t i o n :  i n which a l l the i r o n - o r e p a r t i c l e s were  evenly d i s t r i b u t e d over the c r o s s - s e c t i o n o f the bed, (F) i n Figure  (E)  and  5.1.  The d e g r e e o f m i x i n g was estimated a t the c o n c l u s i o n of the experiment and a l s o checked a g a i n s t photographs which were taken. It i s important  to s t a t e a t t h i s p o i n t t h a t a x i a l segregation  was not measured w i t h i n the s o l i d s bed.  A x i a l s e g r e g a t i o n was  not  ex-  p e c t e d f o r t h i s r e a c t o r s i z e ; 134 m o r e o v e r , the c o n i c a l shape of the chamber ends, and the c a r e f u l l e v e l i n g of the r e a c t o r p r i o r to each p e r i m e n t , helped to minimize a x i a l e f f e c t s . This f a c t was  ex-  corroborated  by the observation of the same s i z e of segregation kidney, a t the t r a n s parent end of the r e a c t o r and at the c e n t r a l c r o s s - s e c t i o n of the s o l i d s bed, during the experiments.  5.1 E f f e c t of p a r t i c l e - s i z e r a t i o The s i n g l e most important v a r i a b l e to a f f e c t bed motion patterns and degree of mixing was the d i f f e r e n c e i n s i z e between the i r o n - o r e and c o a l p a r t i c l e s . C o n s e q u e n t l y , o f the v a r i a b l e s s t u d i e d , the l a r g e s t number o f v a l u e s were t e s t e d f o r the d / d p s i z e r a t i o throughout the c  experimental  programme.  e  Nine v a l u e s o f d ^ / d p were s t u d i e d , i n the e  range between 4 and 0.5, f o r the four s i z e s of i r o n - o r e p a r t i c l e s .  For  88  the t e s t s i n v o l v i n g t h e l a r g e s t i r o n - o r e p a r t i c l e s i z e , 358 ym, the other v a r i a b l e s were maintained constant a t the f o l l o w i n g values: t i o n a l speed a t 5 and 15 r.p.m., C F i x / percent l o a d i n g a t 12 and 20 percent.  F e  rota-  r a t i o a t 0.45 and 0.84, and  For the remaining three i r o n - o r e  p a r t i c l e s i z e s , the other v a r i a b l e s were t e s t e d only a t 5 and 15 r.p.m., Cpix/Fe r a t i o o f 0.45 and 20% percent l o a d i n g .  I t was observed during these t e s t s t h a t r o l l i n g motion  predomi-  n a t e d a t t h e l a r g e r v a l u e s o f dc/cfFe and o f i r o n - o r e p a r t i c l e s i z e . Slumping, on the other hand, a p p e a r e d a t l o w e r l e v e l s o f t h e s e ables.  An uneven type o f motion, a l t e r n a t i n g between unsteady  vari-  slumping  and c a t a r a c t i n g , was observed, a t 15 r.p.m. f o r the s m a l l e s t d / d p r a c  e  t i o , i n each t e s t ; an assessment o f t h i s behaviour i s p r e s e n t e d i n t h e discussion section. The d / d p c  e  s i z e r a t i o exerted a strong i n f l u e n c e on the degree  of p a r t i c l e mixing as w e l l , as can be seen i n Figure 5.1.  The best mix-  ing was obtained f o r the smaller values o f both 3 /dp and i r o n - o r e parc  ticle size.  e  As shown i n F i g u r e 5.2 r e l a t i v e l y good mixing, was obtained  even a t a d / c i p o f 4 f o r the small coal p a r t i c l e s i z e . c  e  Segregation i n  t h e bed was p r e s e n t with l a r g e r values o f d / d p and i r o n - o r e p a r t i c l e c  e  s i z e ; see F i g u r e 5.1, (A) to (D).  5.2 E f f e c t o f r o t a t i o n a l speed At 5 and 15 r.p.m. l e v e l s , there was a n o t i c e a b l e e f f e c t o f the r o t a t i o n a l speed on the type o f bed motion.  The four i r o n - o r e p a r t i c l e  s i z e s (358, 254, 180 and 90 ym) were t e s t e d , a t a l l the other v a r i a b l e  89  E f f e c t of d /d s i z e r a t i o on the degree of mixing f o r 90 ym i r o n - 8 r e p a r t i c l e s . Conditions: / 0.4 5; 15 r.p.m. and 20% l o a d i n g . c  F  F  i  x  e  =  90  l e v e l s , . f o r each r o t a t i o n a l speed.  At 5 r.p.m. the bed motion a l t e r n a t -  ed, w i t h i n each experiment, between slumping and r o l l i n g a t t h e l a r g e r v a l u e s o f o r e s i z e and cT /^Fe r a t i o . c  At the s m a l l e r v a l u e s , however,  the bed motion was e s s e n t i a l l y slumping; some uneven bed motion was obs e r v e d i n t e s t s i n v o l v i n g t h e s m a l l e s t coal p a r t i c l e s (90 ym). At 15 r.p.m., on the other hand, t h e bed motion o b s e r v e d was r o l l i n g under most c o n d i t i o n s .  Some slumping motion was o b t a i n e d , however, when the  s m a l l e s t coal p a r t i c l e s were t e s t e d .  Some experiments were c a r r i e d o u t  a t 10 r.p.m., t o a i d i n d e f i n i n g more s h a r p l y t h e s l u m p i n g - r o l l i n g boundary, but they revealed the same motion as a t 15 r.p.m.. The d e g r e e o f mixing o f p a r t i c l e s was not a f f e c t e d by r o t a t i o n al speed i n the range o f 5 to 15 r.p.m. even though d i f f e r e n t t y p e s o f bed motion were o b s e r v e d .  T h i s i s c l e a r l y i l l u s t r a t e d i n F i g u r e 5.3,  which shows close-up photographs o f the bed, without the p l a s t i c b l a d e , a f t e r being subjected to the minimum and maximum r o t a t i o n a l speeds.  5.3 E f f e c t o f f i x e d - c a r b o n to i r o n r a t i o The C p i x / F e has no a p p r e c i a b l e e f f e c t on the type o f bed motion over the range 0.45 to 0.84. The l e v e l s a t which t h e o t h e r v a r i a b l e s were m a i n t a i n e d d u r i n g t h e s e t e s t s , were ^ / d p e °f 4, 2, 1, and 0.5; c  percent l o a d i n g o f 12 and 20; r o t a t i o n a l speed of 5 and 15 r.p.m.. The d e g r e e o f m i x i n g o f p a r t i c l e s was a f f e c t e d by the C p i / F e a  x  r a t i o , as shown i n F i g u r e 5.4, but only with respect to the depth of the c o a l s u r f a c e l a y e r , i n t h e experiments where s e g r e g a t i o n was present; e  the c o a l s u r f a c e l a y e r was s l i g h t l y deeper when C p i / F was 0.84. x  e f f e c t was not observed when well-mixed c o n d i t i o n s were present.  This  5 r.p.m.  15 r.p.m.  5.3  E f f e c t o f r o t a t i o n a l s p e e d on t h e d e g r e e o f m i x i n g f o r 180 um i r o n - o r e p a r t i c l e s . C o n d i t i o n s : d / d = 0.5; C„. /Fe = 0.4 5 a n d 2 0 % l o a d i n g . Fix c  F e  92  C: . /Fe Fix' 5.4  -  0.84  E f f e c t o f f i x e d - c a r b o n t o i r o n r a t i o on t h e d e g r e e m i x i n g f o r 358 ym i r o n - o r e p a r t i c l e s . C o n d i t i o n s : d /d =4; 20% l o a d i n g and 15 r.p.m.  of  93  5.4 E f f e c t o f percent l o a d i n g At l e v e l s o f 12 and 20 percent, no a p p r e c i a b l e e f f e c t o f percent l o a d i n g on the type of bed motion was observed v i s u a l l y . T h i s was t h e c a s e f o r t h e extreme v a l u e s o f each o f t h e o t h e r v a r i a b l e s , namely ~d /dp o f 4 and 0.5, C p i / F e o f 0.45 and 0.84 and r o t a t i o n a l speeds o f 5 c  e  x  and 15 r.p.m.. S i m i l a r l y , percent l o a d i n g had no e f f e c t on the degree o f mixing of p a r t i c l e s , as i l l u s t r a t e d i n Figure 5.5. A s e g r e g a t e d  bed i s found  a t 12 and 20 p e r c e n t f i l l , w i t h o t h e r c o n d i t i o n s held constant.  The  same r e s u l t s were obtained when the p a r t i c l e s were well mixed.  5.5 P r e l i m i n a r y d i s c u s s i o n o f r e s u l t s from room-temperature mixing experiments The r e s u l t s o f the room-temperature mixing experiments a r e d i s cussed i n t h i s s e c t i o n , only w i t h i n the context o f relevance to the subsequent reduction experiments, t h a t were c a r r i e d out using the same r e a c t o r and m a t e r i a l s .  Further d i s c u s s i o n as to the a p p l i c a b i l i t y o f the  mixing r e s u l t s obtained, t o d i f f e r e n t s i z e s o f systems and m a t e r i a l s , i s presented i n Chapter 8. It i s apparent  from t h e r e s u l t s obtained that the s i n g l e most  important v a r i a b l e a f f e c t i n g the mixing and motion o f the bed i s the r a t i o o f p a r t i c l e s i z e s ; t h i s e f f e c t i s i l l u s t r a t e d by the c o n s t r u c t i o n o f predominance-area diagrams shown i n Figures 5.6 and 5.7, (The points designated by the l e t t e r s F, G, R and S i n Figure 5.6 w i l l be discussed l a t e r i n the t e x t ) .  I t i s seen that the areas c o r r e s p o n d i n g  to segrega-  t i o n and r o l l i n g motion and to slumping motion and a well-mixed nearly c o i n c i d e n t .  bed are  20%  5.5  loading  E f f e c t o f percent l o a d i n g on the degree o f mixing f o r 358 ym i r o n - o r e p a r t i c l e s . C o n d i t i o n s : d /d =4.0; C /Fe=0.4 5 and 15 r.p.m. c Fe Fix  1  1  >  1  \ \  Transition \  \  o  v \  •  •  \ .  Segregated \  \ Well  1  mixed  \ \ *  \  *  s  _  OF  O  "  C  .  € ©  O  o  oR  J 100  200  o  L 300  d (fj.m) F e  6  Predominance area diagram f o r the degree o f mixing as a f u n c t i o n o f d / d p size r a t i o and i r o n - o r e mean p a r t i c l e s i z e , d p . C o n d i t i o n s : Cp-| /Fe = 0.45; 5 to 15 r.p.m. and 20% loading . c  e  e  x  400  5 rpm Slumping^ Transition  9  \\ \ \ \  \  \  v  Rolling  « \ \ transition ©  Slumping o  \  s  ^  \  s  ©  N  v  ^  o 100  ©  •  •  •  ©  — .  — -  o 200 d (/xm)  o  300  Fe  Predominance areajdiagram f o r the type o f bed motion as a f u n c t i o n o f d / d p s i z e r a t i o and i r o n - o r e mean p o a r t i c l e s i z e , a p . C o n d i t i o n s : Cf^/Fe = 0.45; 5 and 15 r.p.m., and 20% loading . c  e  e  ©  -© 400  97  T h i s b e h a v i o u r can be explained i n terms of a p e r c o l a t i o n anism operating during r o t a t i o n a l  movement of the bed.  mech-  A c c o r d i n g to  t h i s mechanism, heavier and/or f i n e r p a r t i c l e s s i f t through the l i g h t e r and/or l a r g e r p a r t i c l e s , s e g r e g a t i n g i n t o a c o r e i n the s o l i d s these differences  and  the p a r t i c l e shape and surface  bed;  characteristics  have been reported to be among the main f a c t o r s which f a v o u r s e g r e g a 4  tion. '  1 3 5  By p r o g r e s s i v e l y  d e c r e a s i n g the s i z e of the l i g h t e r p a r t i c l e s ,  the mixing i s a f f e c t e d because,  at a c r i t i c a l s i z e ratio, the v o i d s  be-  tween the l i g h t e r p a r t i c l e s w i l l be too small to allow the heavier part i c l e s to pass through. 5.6,  f o r iron-ore  T h i s was  the c a s e , as can be seen i n F i g u r e  p a r t i c l e s l a r g e r than 254 ym, where a c r i t i c a l  of about 1.0 was obtained.  13  1 2 1  c  e  According to geometric p r i n c i p l e s alone, the  c r i t i c a l s i z e r a t i o should be between 2.41 homogeneous s p h e r e s ,  d /dp  and 6.46  for orderly-arranged  or greater than 1.2 f o r i r r e g u l a r , homogeneous  solids. **  The c r i t i c a l r a t i o i s t h e r e f o r e expected to be lower, i f the  difference  i n d e n s i t i e s also i s c o n s i d e r e d ; t h i s was c l e a r l y the case i n  the p r e s e n t s t u d y .  A l s o , a c r i t i c a l s i z e r a t i o of about 0.5 has been  reported f o r mixtures of iron-ore p e l l e t s and  coal.  1 2 1  In the present study, the c r i t i c a l s i z e r a t i o f o r the ore mixture was estimated as f o l l o w s .  coal/iron-  F i r s t , a sample of coal  particles  with 1595 ym mean p a r t i c l e s i z e was mounted i n r e s i n ; vacuum was to ensure that the r e s i n f i l l e d the i n t e r p a r t i c l e v o i d s . 20 mm, men;  applied  A g r i d of 20 x  with 2 mm d i v i s i o n s , was drawn on the f l a t surface of the  speci-  the d i s t a n c e s between the edges of the p a r t i c l e s were measured,  with a s t e r e o s c o p i c m i c r o s c o p e , i n the two  perpendicular  directions  98  a l o n g the l i n e s o f the g r i d .  The averaging of over a hundred measure-  ments y i e l d e d a v o i d s i z e of 1.025  mm.  Thus, i f i t i s assumed  that  s m a l l e r p a r t i c l e s m a i n t a i n the same shape, the average void s i z e , £, should be r e l a t e d to the mean coal p a r t i c l e s i z e , d , as c  a  = a  cT  follows  (5.1)  .  c  where a i s a p r o p o r t i o n a l i t y constant t h a t depends m a i n l y on  particle  shape; i n the p r e s e n t c a s e , a has a v a l u e of 0.643. By applying above r e l a t i o n s h i p to d i f f e r e n t p a r t i c l e s i z e s , the v o i d s i z e  the  corre-  sponding to each p a r t i c l e s i z e i s obtained; t h i s i s shown i n Figure  5.8.  Also shown i n Figure 5.8 are the t h e o r e t i c a l values c a l c u l a t e d  two  d i f f e r e n t p a r t i c l e shapes: dimensions 0.5 d  c  x d  c  based on the f o l l o w i n g  i) ii)  for  a sphere with diameter '"d and a slab with c  x 2 cf . c  The t h e o r e t i c a l void s i z e s were obtained  assumptions:  The number of voids i s the same as the number of p a r t i c l e s , The voids are s p h e r i c a l , with the l a r g e s t d i a m e t e r to f i t between p a r t i c l e s .  iii)  The a v e r a g e s i z e of the voids i s obtained by d i v i d i n g the measured void f r a c t i o n by the number of p a r t i c l e s .  For example, f o r c o a l p a r t i c l e s w i t h d  c  = 0.716  mm the void s i z e i s  0.283 mm f o r s p h e r i c a l p a r t i c l e s , and 0.566 mm f o r s l a b - l i k e p a r t i c l e s . I t i s seen i n F i g u r e 5.8  t h a t the measured void s i z e s f i t between the  two t h e o r e t i c a l l y c a l c u l a t e d v a l u e s .  1500  i  r  /I  lOOOr-  y/  Slab shaped particles /  600  /  /  1  / Spherical  /  400 ^J§_  o I —  S  particles /  2 00  in  / I  A /  •a o  >  100 /  i  /  / /  60  / / /  30 50  100  200 400 _ 1000 Coal particle size , dc (^im)  2000  Void s i z e between p a r t i c l e s as a f u n c t i o n o f coal part i c l e s i z e i n a loose bed .  -i  100  T h i s d i a g r a m can t h u s be used to p r e d i c t the s i z e of the ore p a r t i c l e s that w i l l r e s u l t i n s e g r e g a t i o n , size.  f o r a given coal p a r t i c l e  From knowledge of the coal s i z e , the corresponding  be obtained.  v o i d s i z e can  If the ore p a r t i c l e s are smaller than t h i s , they w i l l tend  to s e g r e g a t e ; i f they are l a r g e r , they w i l l stay well mixed. p l e , the void s i z e s f o r 0.356 and 0.127 0.082 mm  For exam-  mm coal p a r t i c l e s are 0.235 and  r e s p e c t i v e l y ; thus a 0.254 mm ore p a r t i c l e w i l l segregate i n a  bed c o n t a i n i n g 0.356 mm coal p a r t i c l e s and be well mixed i n the presence of 0.127  mm coal p a r t i c l e s . These two c o n d i t i o n s can be a s c e r t a i n e d  re-examining Figure 5.6, points G and R r e s p e c t i v e l y . dence i s s e l f  The  by  correspon-  explanatory.  At ore s i z e s smaller than t h i s however, the well-mixed boundary i n Figure 5.6 i s d i s p l a c e d toward higher d / d p r a t i o s . Therefore c  e  other  f a c t o r s must have exerted an i n c r e a s i n g l y stronger i n f l u e n c e on bed mixing.  Among the l i k e l y f a c t o r s may be the e l e c t r o s t a t i c charging and/or  an increased f r i c t i o n c o e f f i c i e n t of the coal p a r t i c l e s , that were c o r respondingly  smaller.  The former f a c t o r would cause the coal p a r t i c l e s  to clump together hindering the f r e e m o t i o n of the ore p a r t i c l e s and l e a v i n g them randomly disseminated through the bed.  The l a t t e r f a c t o r  would produce a s i m i l a r e f f e c t by the r e s p e c t i v e i n c r e a s e of the  fric-  t i o n c o e f f i c i e n t between ore and coal p a r t i c l e s . This mechanism would a l s o e x p l a i n the correspondence between slumping and well-mixed  behavi-  our that was obtained always f o r smaller coal p a r t i c l e s i z e s . Results of angle of repose measurements, using the cone method,  4  are presented i n Table XV f o r d i f f e r e n t p a r t i c l e s i z e s of coal by i t s e l f and mixed w i t h 254 ym ore p a r t i c l e s , at a C  F i x  / F e of 0.32.  A slight  101  t e n d e n d y o f i n c r e a s i n g angle of repose, f o r the coal p a r t i c l e s by thems e l v e s , i s obtained as coal p a r t i c l e s i z e decreases.  An increased  angle  of r e p o s e was obtained f o r the ore/coal mixtures, at l a r g e r coal s i z e s , as would be e x p e c t e d .  T h i s e f f e c t however was  not p r e s e n t  f o r the  smaller coal p a r t i c l e s s i z e s . From the f o r e g o i n g , i t i s apparent t h a t the type of bed motion i s p r i m a r i l y determined by the coal p a r t i c l e s i z e , f o r the range t e s t e d ,  TABLE XV ANGLE OF REPOSE FOR DIFFERENT COAL PARTICLE SIZES AND ORE/COAL MIXTURES WITH 254 m ORE PARTICLE SIZE d [vm]  d /dp  1016 510 358 254 127  4.0 2.0 1.4 1.0 0.5  c  dp  c  e  $  r j C  [°]  e  = 254 vm,  37.0 37.0 37.5 38.0 38.0  $r,mix [°]  $r,mix-$r,c [°]  39.5 38.5 38.5 38.0 38.0  2.5 1.5 1.0 0.0 0.  $ ,Fe = 35° r  whereas the p a r t i c l e s s i z e r a t i o , d / d p , determines the degree of mixc  e  ing a t coal s i z e s l a r g e r than 254 ym; a t s m a l l e r s i z e s , e l e c t r o s t a t i c and f r i c t i o n a l forces become predominant. The above d i s c u s s i o n i s based on experiments at room temperature •and when no r e a c t i o n i s t a k i n g p l a c e .  However, the main d i f f e r e n c e with  respect to a r e a c t i n g system, to be encountered i n the reduction e x p e r i ments, i s the d e c r e a s i n g  c o a l p a r t i c l e s i z e as the r e a c t i o n s proceed.  102  This w i l l only but enhance the degree of mixing of the bed since the ore p a r t i c l e s are expected to m a i n t a i n a p p r o x i m a t e l y t h e i r i n i t i a l  size.  F i n a l l y , the gases g e n e r a t e d by the r e a c t i o n may produce an e f f e c t on the bed mixing; t h i s w i l l be discussed i n Chapter 8.  103  CHAPTER 6  RESULTS OF REDUCTION EXPERIMENTS  The r e s u l t s of the reduction experiments are p r e s e n t e d chapter.  in this  The r e s u l t s of the coal d e v o l a t i l i z a t i o n experiments f o l l o w i n g  the technique o u t l i n e d i n Section 4.7.2  are p r e s e n t e d  first.  This i s  followed by the p r e s e n t a t i o n of the r e s u l t s of the reduction experiments arranged i n t o three s e c t i o n s :  the p r e l i m i n a r y runs to d e t e r m i n e the  r a n g e s o f v a r i a b l e s , the main e x p e r i m e n t a l experiments.  block and the comparative  A l l three sets were performed i n the equipment d e s c r i b e d  i n S e c t i o n 4.3,  f o l l o w i n g the procedure e s t a b l i s h e d i n Section 4.7.3.  6.1 Results of coal d e v o l a t i l i z a t i o n experiments A d e v o l a t i l i z a t i o n treatment of the coal was  deemed  p r i o r to the reduction experiments, f o r the f o l l o w i n g reasons.  necessary, Firstly,  the presence of the v o l a t i l e c o n s t i t u e n t s i n the c o a l , i n c l u d i n g m o i s ture,  would complicate unduly the assessment of the reduction k i n e t i c s  owing t o the presence of p a r a l l e l r e a c t i o n schemes. j e c t i v e o f t h i s work was  M o r e o v e r , the  t o study the r e d u c t i o n k i n e t i c s with  ob-  carbon  alone, as i t would occur i n the reduction s e c t i o n o f an i n d u s t r i a l r o tary k i l n .  As was  s t a t e d e a r l i e r , Section 2.2.1, most of the v o l a t i l e  c o n s t i t u e n t s i n the coal are r e l e a s e d i n the p r e h e a t i n g s e c t i o n o f the  104  kiln.  I t i s important to note however, t h a t the p o s s i b i l i t y of a small  degree of reduction by hydrogen always e x i s t s i n the r e d u c t i o n s e c t i o n , s i n c e a t r a c e amount of hydrogen i s r e t a i n e d by the char.  Similarly, in  the present study, a small f r a c t i o n of r e d u c t i o n by hydrogen i s poss i b l e ; t h i s i s discussed i n d e t a i l i n Section 6.4.6. The second reason f o r d e v o l a t i l i z i n g the coal i s r e l a t e d to experimental c o n s i d e r a t i o n s . duction experiments  The gas flowrate measurements during the r e -  would be impaired by the d e p o s i t i o n o f  low-melting  p o i n t compounds, i n the gas l i n e s as the gases discharged from the react o r cooled down.  These compounds would be r e l e a s e d i n c o n s i d e r a b l e  amounts i f raw c o a l were u s e d .  Experiments  t h e r e f o r e were conducted  with the aim of e l i m i n a t i n g the major f r a c t i o n o f the v o l a t i l e s under temperature  c o n d i t i o n s s i m i l a r to those i n an i n d u s t r i a l r o t a r y k i l n .  A  heating r a t e of 9°C/min was adopted f o r these t e s t s based on a v a i l a b l e information.^ tried.  Coal soak p e r i o d s , at 900°C, from one to ten hours were  The p a r t i c l e s i z e s s t u d i e d were those to be u t i l i z e d i n the r e -  duction experiments,  namely:  - 841 + 600, - 420 + 300, - 210 + 149, and  - 106 + 74 ym. Two methods were adopted to evaluate v o l a t i l e s r e l e a s e : cal analyses and w e i g h t l o s s o f the sample.  P r o x i m a t e and  chemi-  ultimate  a n a l y s e s p r o v i d e d a p r e c i s e v a l u e of the v o l a t i l e matter and hydrogen remaining i n the char, r e s p e c t i v e l y . These were p e r f o r m e d Inc..  by S t e l c o  The weight-loss method, on the other hand, provided a reasonable  estimate of the evolved v o l a t i l e s .  T h i s method p e r m i t t e d the c o n s i s -  t e n c y o f the c h a r r i n g treatment f o r a l l coal used i n the reduction experiments to be  monitored.  105  6.1.1. Temperature measurement i n the coal bed Considering the s i z e o f the coal samples and t h e n a t u r e o f t h e f u r n a c e , d e s c r i b e d e a r l i e r , temperature g r a d i e n t s were expected to develop i n s i d e the coal bed. The measurement o f the t e m p e r a t u r e a t d i f f e r e n t l o c a t i o n s i n t h e bed, was t h e r e f o r e aimed a t e v a l u a t i n g these gradients s i n c e they might i n f l u e n c e c h a r homogeneity.  A temperature  versus time p l o t , f o r an experiment with a 10 hour soaking-time i s shown i n Figure 6.1. Thermocouple t h o s e i n F i g u r e 4.8.  l o c a t i o n s are a l s o shown and c o r r e s p o n d t o  The temperature a t each p o i n t during the soaking  p e r i o d i s presented i n Table XVI f o r the four p a r t i c l e s i z e s t r e a t e d . The h e a t i n g r a t e ranged from 8.8°C/min a t the bottom side l o c a t i o n , p o i n t B i n Figure 6.1, up t o 10°C/min p o i n t D.  at the central  position,  As i s shown i n Table XVI, a maximum temperature d i f f e r e n c e o f  40°C was observed between these p o i n t s f o r the l a r g e s t c o a l p a r t i c l e s . S i m i l a r c o n d i t i o n s were o b t a i n e d with the s m a l l e r p a r t i c l e s , although the temperature g r a d i e n t s decreased s l i g h t l y with p a r t i c l e s i z e .  6.1.2 E f f e c t o f soak time and p a r t i c l e s i z e V o l a t i l e and hydrogen r e l e a s e i n c r e a s e d w i t h soak t i m e , as was t o be e x p e c t e d . release.  F i g u r e 6.2 shows t h e asymptotic p a t t e r n o f hydrogen  The major f r a c t i o n o f t h e hydrogen was e v o l v e d d u r i n g t h e  h e a t i n g - u p p e r i o d and the t o t a l f r a c t i o n was p r a c t i c a l l y independent of p a r t i c l e s i z e , w i t h i n the range t e s t e d . A minimum amount o f hydrogen r e t a i n e d , about 0.3 percent, was obtained with 10 hours o f treatment.  106  6.1  Temperature a t four l o c a t i o n s i n the coal bed during d e v o l a t i l i z a t i o n treatment .  107  TABLE XVI. TEMPERATURES MEASURED AT FOUR LOCATIONS IN THE COAL BED, FOR PARTICLE SIZES OF - 841 + 600, - 420 + 300, - 210 + 149 and - 106 + 74 ym  Particle s i z e ( m) y  - 841 + 600  Location  - 420 + 300  - 210 + 149  - 106 + 74  Temperature (°C)  A  920  923  920  919  B  883  889  888  890  C  907  910  911  909  D  923  920  921  921  108  15  Particle size o 0.716 mm • 0.358 " A 0.180" Q 0.040 "  3.0  25  CVJ 5*  1.5  1.0  A  0.5  1 120  240  360  Charring  6.2  480  600  720  time (min)  Percent hydrogen remaining i n char, a f t e r d e v o l a t i l i z a t i o n treatment a t 900°C, f o r four coal p a r t i c l e s i z e s : 716, 358, 180 and 90 ym .  109  The p r o x i m a t e and u l t i m a t e a n a l y s e s , f o r the d i f f e r e n t char p a r t i c l e s i z e s , are presented i n Table XVII, ash composition i s presented i n T a ble XVIII.  6.1.3 D i s c u s s i o n o f coal d e v o l a t i l i z a t i o n r e s u l t s The 10 hour treatment was adopted as the s t a n d a r d f o r t h e c o a l samples  t o be used i n a l l the r e d u c t i o n experiments.  From the trend i n  Figure 6.2, i n order t o reduce the hydrogen content a f u r t h e r 0.1 p e r c e n t , a t l e a s t f i v e a d d i t i o n a l hours o f treatment would have been r e quired.  P r a c t i c a l c o n s i d e r a t i o n s however prevented t h i s .  Depending  on  the C F i / F e r a t i o t o be used i n the r e d u c t i o n experiments, a maximum r e x  duction by hydrogen o f 4.7 p e r c e n t might be e x p e c t e d , based on s t o i c h i o m e t r i c c o n s i d e r a t i o n s alone, f o r a 0.3 percent H2 content. S i m i l a r l y 0.45 p e r c e n t H2, o b t a i n e d w i t h a 5 hour t r e a t m e n t , might  produce  c l o s e to 7 percent r e d u c t i o n whereas 0.2 percent H » to be obtained with 2  a 15 hour t r e a t m e n t , would y i e l d about 3 p e r c e n t r e d u c t i o n .  These  amounts o f r e d u c t i o n are o b v i o u s l y smaller when the e q u i l i b r i u m f o r the H  2  - H2° - CO - CO2 system i s c o n s i d e r e d .  A d i s c u s s i o n on t h i s i s pre-  sented i n Section 6.4.6. The range o f heating r a t e s , 8.8 to 10°C/min, a t d i f f e r e n t l o c a t i o n s i n the coal bed have been demonstrated i n the l i t e r a t u r e ! 3 7 - 1 3 8 t o a f f e c t n e i t h e r the rate o f v o l a t i l e s r e l e a s e n o r t h e s t r u c t u r e o f t h e c h a r t o any g r e a t d e g r e e .  S i m i l a r l y , the temperature g r a d i e n t s found  i n s i d e the bed are not considered s i g n i f i c a n t a t t h e c h a r r i n g ture o f 900°C.  tempera-  Gradients might have played a more important r o l e had  110  TABLE XVII.  PROXIMATE AND ULTIMATE ANALYSES OF FORESTBURG COAL AND SASKATCHEWAN LIGNITE AFTER 10 HOUR CHARRING TREATMENT. MEAN PARTICLE SIZES: 718, 180 and 90 m y  Coal Mean P a r t i c l e s i z e [ym]  718  Forestburg 180  Lignite 180  90  Proximate A n a l y s i s  H0 Ash Volatiles Fixed C  0.62 49.50 2.50 48.00  2  0.83 56.10 3.10 40.84  0.58 52.10 2.50 45.40  0.08 28.40 4.64 66.96  U l t i mate Analysi s (d.b.) Carbon Hydrogen Nitrogen Sulphur Ash Oxygen  42. 76 0. 31 0. 29 0. 71 56. 10  48.07 0.51 0.48 0.66 49.50 0.78  TABLE XVIII.  Fe 03 2  P2O5  46. 72 0. 33 0. 34 0. 71 52. 10  69.57 0.43 0.40 0.85 28.40 0.35  -  -  ASH COMPOSITION OF FORESTBURG COAL AND SASKATCHEWAN LIGNITE IN % WEIGHT Si02  Al2O3  CaO  MgO  NagO  K 0 2  Ti02  Forestburg 6.00 Lignite  0.83 55.80  2 1 . 2 0 12.30 1.80 1.23 0.68  3.60 0.36 51.80 23.60  1 1 . 1 0 2.80 4.60 0.99  0.19 0.64  Ill  the t e m p e r a t u r e been lower, o f the order o f 600°C, or f o r s h o r t e r soaking periods.137-138 The t e m p e r a t u r e o f 900°C chosen f o r the d e v o l a t i l i z a t i o n t r e a t ments was lower than the maximum value o f 950°C t o be t e s t e d i n some o f the r e d u c t i o n experiments. This d i f f e r e n c e , however, should not r e s u l t i n a major change i n the r e a c t i v i t y of the charred product f o r t h e f o l lowing reason. I t has been demonstrated that n e i t h e r g r a p h i t i z a t i o n nor a f u r t h e r change i n pore surface area i s obtained by t r e a t i n g s u b - b i t u minous c o a l s beyond 900°C, a t l e a s t up t o 1 3 0 0 ° C . l  11  T h e r e f o r e , since  pore surface area e x e r t s a v e r y s t r o n g i n f l u e n c e on r e a c t i v i t y , t h i s property i s not expected t o change d r a m a t i c a l l y by i n c r e a s i n g the t r e a t ment temperature by 50°C. The l a r g e s t o p e n i n g o f t h e p o r e s i n t h e s e c o a l s , hence h i g h e s t r e a c t i v i t y ,  1 3 7  i s o b t a i n e d by t r e a t i n g them a t  700°C. Therefore, d e v o l a t i l i z a t i o n a t t h i s temperature would be advocated f o r reduction purposes, i f i t were not f o r the f a c t that the major p o r t i o n o f the hydrogen i s not r e l e a s e d unless heated above t h i s temperature.  1 1 1  M o r e o v e r , i f t h e c o a l had been d e v o l a t i l i z e d a t 700°C, the  hydrogen content remaining i n the char would have been r e l a t i v e l y l a r g e and a l s o t h e reduction achieved v i a hydrogen. By c o n s i d e r i n g the above f a c t o r s a temperature o f 900°C was t h e r e f o r e adopted f o r the d e v o l a t i l i z a t i o n o f the coal samples throughout a l l the reduction program.  6.2 C a l c u l a t i o n o f f r a c t i o n a l reduction The r e s u l t s o f t h e r e d u c t i o n experiments are presented as the s t a n d a r d p l o t s o f f r a c t i o n a l r e d u c t i o n , fR, versus time. The slopes o f  112  the r e d u c t i o n c u r v e s represent the f r a c t i o n a l rates of r e d u c t i o n .  The  f r a c t i o n a l reduction was c a l c u l a t e d using the f o l l o w i n g r e l a t i o n s h i p : (0x)  t  (6.1) (Ox)  Tot  where ( O x ) ^ i s the moles o f oxygen removed up to time t , and ( 0 x ) j t i s o  the t o t a l moles of oxygen i n the o r i g i n a l concentrate.  ( 0 x ) y t was obo  t a i n e d from the chemical composition of the o r e , Table VIII, and a l s o by c o n s i d e r i n g t h e Fe203 content i n the coal ash, Table XVIII. c a l c u l a t e d by the f o l l o w i n g  i)  (Ox)t was  procedure:  The CO and CO2 mole f r a c t i o n s , Xco and XQO » were obtained from the r e s u l t s of gas a n a l y s i s .  ( I t was assumed the gas sample had  the same composition as that coming o f f the bed, as w i l l be d i s cussed i n Section 6.4.6.) An example of the gas c o m p o s i t i o n a f u n c t i o n o f reduction ii)  as  time i s shown i n Figure 6.3.  The measured gas f l o w r a t e , QR t> was c o r r e c t e d f o r the s e a l i n g 0  e f f i c i e n c y o f the system, E f f , as defined i n Section 4.7.3, and f o r the density d i f f e r e n c e with respect to the standard g a s , as p r o v i d e d by t h e manufacturer of the flowmeters, using  Equation  (6.2): corr Q Tot where E.f f  2  (6.2)  10  80-  £ o o o  u  T=850°C C f j / F e = a32  60  x  \ O  40  20  d  $  0  c  co/co ,  (NJ  O  /d =0.5 rpm = I 4 %fill = l4 Fe  u 4  2  O  — A  H  toCr-Q- Q - 4 - o  40  2  1  h 80  120 Time  6.3  8  CO  160  200  i 240  280  (min)  Gas composition as a f u n c t i o n of r e a c t i o n time f o r a reduction experiment; c o n d i t i o n s as shown .  M  114  and s  172  (1 - V ) p.  conr  p  1 / 2  (1 - s t d )  whereupon the flowrate o f each gaseous s p e c i e s c o u l d be c a l c u lated,  Q0  <W ' C0  =  X  C  Q  C0  = 2  Q  ( 6  X  T o t ' C0  ( 6 2  '  '  3 )  4 )  together with t h e i r corresponding molar r a t e s ,  iii)  m  C0 = QCO P T / f  R T  m  C0  =  Q  2  C0  P 2  6  5  ( - )  R T  T/ f  (6-6)  The molar rates of oxygen and carbon were c a l c u l a t e d by c o n s i dering t h a t ,  C + 1/2 0 = CO  (6.7)  C + 0 = C0  (6.8)  2  2  2  Thus %  = 1/2 \ o  + "to  (6.9)  2 m  C  = \ o  +  \ o  2  (6.10)  115  iv)  Then, by numerical  integration  1 3 9  of the oxygen and carbon molar  r a t e s t h e i r r e s p e c t i v e amounts removed up to time t , (Ox)-t and (C)t»  c o u l d be o b t a i n e d .  By s u b s t i t u t i n g ( 0 x )  (6.1) the f r a c t i o n a l reduction was  t  i n Equation  obtained.  A computer program was implemented to perform the c a l c u l a t i o n s described above; the program l i s t i n g and a sample o u t p u t are i n A p p e n d i x D. species:  presented  The program a l s o gave the o v e r a l l mass balance f o r each  i r o n , oxygen, hydrogen, carbon and i n e r t m a t e r i a l s .  The  over-  a l l material balances c l o s e d w i t h i n 6 percent i n a l l cases, the majority being w i t h i n 4 percent.  A summary of the mass balances i s p r e s e n t e d  in  Appendix E.  6.3 Results of experiments to determine v a r i a b l e s ranges The purpose o f t h i s s e t o f e x p e r i m e n t s was t o d e t e r m i n e the r a n g e s o f o p e r a t i o n a l v a r i a b l e s w i t h i n which an e f f e c t on the reduction rate could be obtained.  These v a r i a b l e s were:  r a t i o , r o t a t i o n a l speed and percent l o a d i n g .  fixed  carbon-to-iron  The approach followed  to change p r o g r e s s i v e l y the l e v e l of the t e s t e d v a r i a b l e , i n  was  separate  experiments, u n t i l no f u r t h e r e f f e c t was observed on the reduction r a t e . In each case, the other v a r i a b l e s were m a i n t a i n e d  at constant l e v e l s  c h o s e n under the c o n s i d e r a t i o n s s t a t e d i n Section 4.6.3. The p a r t i c l e s i z e r a t i o was maintained at 0.5 i n a l l c a s e s , c o r r e s p o n d i n g mixed c o n d i t i o n s .  This i s shown by point R i n Figure 5.6.  ture of 900°C was used i n a l l t e s t s , f o r the reasons 4.6.2.  t o wel 1A tempera-  stated in Section  116  6.3.1 E f f e c t o f f i x e d - c a r b o n - t o - i r o n r a t i o The e f f e c t o f C p i / F e x  r a t i o on the reduction r a t e , f o r values  between 0.16 and 0.64, i s i l l u s t r a t e d i n Figure 6.4. The c o n d i t i o n s f o r t h e s e t e s t s were: r o t a t i o n a l speed o f 7 r.p.m. and 14 percent l o a d i n g . I t . can be seen t h a t the reduction rates beyond C p i / F e = 0.32 are very x  s i m i l a r , e s p e c i a l l y up t o 0.5 f r a c t i o n a l r e d u c t i o n . the r a t e i s c o n s i d e r a b l y slower.  A t 0.16 however,  A small divergence o f the 0.48 curve  i s observed a t a l a t e r stage o f r e d u c t i o n .  6.3.2 E f f e c t o f r o t a t i o n a l speed The e f f e c t o f t h e r o t a t i o n a l speed on the reduction r a t e , f o r values from 7 t o 20 r.p.m., i s shown i n F i g u r e 6.5. C p j / F e were t e s t e d : x  cases.  Two v a l u e s o f  0.64 and 0.16; 14 percent f i l l i n g was used i n a l l  I t can be seen t h a t p r a c t i c a l l y no d i f f e r e n c e between t h e r a t e s  was p r o d u c e d by changes i n the r o t a t i o n a l speed.  At the 0.16 l e v e l , a  s l i g h t l y f a s t e r r e d u c t i o n r a t e was p r e s e n t a t t h e h i g h e r speed, 14 r.p.m.  The e x p e r i m e n t a t 17 r.p.m. was i n t e r r u p t e d due to o p e r a t i o n a l  problems i n the d r i v i n g motor.  6.3.3 E f f e c t o f percent l o a d i n g The e f f e c t o f 7 and 14 percent l o a d i n g i s shown i n Figure 6.6, for Cp-j /Fe r a t i o s o f 0.64 and 0.16. The r o t a t i o n a l speed was maintainx  ed constant a t 7 r.p.m..  I t can be seen t h a t a t t h e l o w e r v a l u e o f  C p i / F e , t h e i n i t i a l r a t e was f a s t e r a t 7 percent loading than a t 14 x  percent.  The opposite was true a t l a r g e r f r a c t i o n s reduced.  At the  117  ,b 0  l  1  1—i  i  i  l  80  I  I  160 Time  6.4  1  r  1  L_  240 (min)  P l o t of f r a c t i o n a l r e d u c t i o n versus time showing the e f f e c t o f Cpix/Fe r a t i o ; c o n d i t i o n s as shown .  Time  (min)  P l o t o f f r a c t i o n a l reduction versus time showing the e f f e c t o f r o t a t i o n a l speed a t two Cp-jx/Fe r a t i o s ; c o n d i t i o n s as shown .  ! ^  }C /Fe=0.64 fix  14%  C „/Fe =0.16 fi  Temp-900°C rpm -7 d /d =05 c  0  1  80  Fe  1  240  160 Time (min)  P l o t of f r a c t i o n a l r e d u c t i o n versus time showing the e f f e c t o f percent f i l l a t two C p i x / ratios; c o n d i t i o n s as shown . F e  120  0.64 v a l u e o f t h e Cp - /Fe r a t i o , on the other hand, p r a c t i c a l l y no e f n  X  f e c t o f the percent l o a d i n g was  observed.  6.3.4 P r e l i m i n a r y d i s c u s s i o n of r e s u l t s from e x p e r i ments to determine v a r i a b l e s ranges" The f i x e d c a r b o n - t o - i r o n r a t i o showed a strong e f f e c t on the r e duction rate as was to be expected.  The value o f 0.32 c o u l d be taken as  the optimum, since no improvement was achieved by i n c r e a s i n g the Cp-j /Fe x  value t o 0.64.  The s l i g h t l y slower rate shown a t t h e 0.48 v a l u e might  be a t t r i b u t e d to e i t h e r experimental e r r o r or d i f f e r e n c e s i n the s o l i d s composition.  This might  a l s o account f o r the r e l a t i v e l y l a r g e d i f f e r -  ence o b t a i n e d when the o v e r a l l mass balance was performed, 5.4 percent, as shown i n Appendix E. The optimum value of 0.32, i . e . 100 percent exc e s s w i t h respect to the minimum s t o i c h i o m e t r i c requirements, i s c o n s i derably lower than the f i g u r e s reported f o r f i x e d - b e d r e d u c t i o n  (Table  V I ) , where e f f e c t s o f t h e Cp-j /Fe r a t i o were s t i l l found a t values as x  high as 0.96.  This can undoubtedly  be a t t r i b u t e d to two f a c t o r s :  bet-  t e r m i x i n g i n t h e r o t a r y r e a c t o r , and t h e char r e a c t i v i t y was higher than those of the carbonaceous m a t e r i a l s used i n some o f t h e s t u d i e s 9 0  mentioned e a r l i e r . "  9 3  When the value of 0.32 f o r Cp-j /Fe i s compared x  to those studied i n s i m i l a r works using r o t a r y r e a c t o r s (Table V I I ) , i t i s found t h a t i t i s again lower than those studied p r e v i o u s l y .  This can  be due to a more intimate contact between the r e a c t i n g gases and s o l i d s , a c h i e v e d i n the reduction o f the f i n e m a t e r i a l s i n t h i s work. value was adopted f o r use i n the main experimental dard C F i / F e r a t i o . x  The 0.32  b l o c k , as the s t a n -  121  The e f f e c t c a u s e d by the percent l o a d i n g on the r e d u c t i o n r a t e can be analyzed as f o l l o w s . F i r s t l y , f o r shallower beds the p a r t i c l e s have been r e p o r t e d to spend longer o v e r a l l times a t the surface of the bed.4.121  S e c o n d l y , the a r e a to volume r a t i o (A/V) has been found to  e x e r t a s t r o n g e f f e c t on the heat t r a n s f e r i n t o the bed;? the shallower the bed, the l a r g e r the A/V r a t i o becomes with i t s c o n s e q u e n t ment of heat t r a n s f e r i n t o the bed. was 48 m  -1  enhance-  At 14 percent f i l l i n g the A/V r a t i o _1  whereas at 7 percent, i t was 85 m .  Therefore, at the lower  Cp-jx/Fe l e v e l the shallower bed reacted f a s t e r i n i t i a l l y due to the f a c t t h a t , although the temperature at the c e n t e r o f the bed was 900°C, an i n c r e a s e d f r a c t i o n o f t h e r e a c t i o n was o c c u r r i n g at the bed's s u r f a c e because of the combined e f f e c t s of longer exposure times at the s u r f a c e and o f the s l i g h t l y h i g h e r temperature p r e v a i l i n g t h e r e . The slowing down of the rate as the r e a c t i o n advanced i s but a consequence  o f the  i n i t i a l l y f a s t e r consumption  e  o f carbon..  At the higher C F i / F x  ratio  however, the l a r g e excess of c a r b o n must have overcome t h e s e e f f e c t s s i n c e the i r o n - o r e p a r t i c l e s were surrounded by more coal char. Fourteen percent l o a d i n g was adopted f o r the e x p e r i m e n t s o f the main b l o c k f o r two reasons.  F i r s t l y , t h i s value i s c l o s e s t to the range used i n an i n -  dustrial operation.  S e c o n d l y , as was mentioned above, a 0.32 C p i / F e x  r a t i o was to be used i n t h e s e e x p e r i m e n t s and t h e r e f o r e the p e r c e n t f i l l i n g was not expected to play a p a r t . The l a c k o f an e f f e c t on the r e d u c t i o n r a t e of r o t a t i o n a l speed i s i n agreement with what was found i n the room-temperature periments.  m i x i n g ex-  In t h o s e e x p e r i m e n t s , good mixing c o n d i t i o n s had been ob-  t a i n e d a t speeds as low as 5 r.p.m. p r o v i d e d t h a t the r i g h t p a r t i c l e  122  s i z e r a t i o was u t i l i z e d .  N o n e t h e l e s s , t o ensure t h a t good g a s - s o l i d  c o n t a c t i n g was achieved, a r o t a t i o n a l speed o f 14 r.p.m. was adopted f o r a l l the subsequent r e d u c t i o n t e s t s .  6.4 Results o f main experimental block T h i s s e t o f e x p e r i m e n t s was designed t o assess the e f f e c t s o f temperature and p a r t i c l e s i z e r a t i o , on the r e d u c t i o n r a t e , i n o r d e r t o d e t e r m i n e the r a t e c o n t r o l l i n g step f o r the o v e r a l l r e d u c t i o n r e a c t i o n . Based on the d i s c u s s i o n given i n S e c t i o n 6.3.4, t h e s e e x p e r i m e n t s were c a r r i e d out a t a r o t a t i o n a l speed o f 14 r.p.m., 14 percent l o a d i n g and a C p i / F e r a t i o o f 0.32. x  The s t o i c h i o m e t r i c Cp-j /Fe r a t i o , 0.16, was x  t e s t e d f o r only one s e t o f temperatures. s t u d i e d i s shown i n F i g u r e 6.7.  A summary o f t h e c o n d i t i o n s  The r e s u l t s are presented as f r a c t i o n a l  r e d u c t i o n - v e r s u s - t i m e p l o t s and as P c o / C 0 p  2  ratio-versus-fractional-  reduction plots.  6.4.1 E f f e c t o f temperature:  the base case  The c o n d i t i o n s f o r t h i s sub-set o f experiments were: i r o n - o r e p a r t i c l e mean s i z e o f 358 ym and dc/cTpg o f 0.5, which are i n the w e l l mixed r e g i o n i n Figure 5.6; these r e s u l t s were taken as t h e base c a s e . A d i s c u s s i o n on t h i s c h o i c e i s given i n S e c t i o n 6.4.6. The e f f e c t o f temperature, from 800 t o 950°C, on the rate o f r e d u c t i o n and on the gas composition i s shown i n F i g u r e s 6.8 and 6.9, r e spectively.  I t i s seen t h a t the r e d u c t i o n r a t e i n c r e a s e d with  t u r e , as was t o be e x p e c t e d .  tempera-  The f r a c t i o n a l r e d u c t i o n obtained was  greater than 0.9 f o r temperatures above 850°C.  At 800°C, the r e d u c t i o n  Temperature (°C) / d Fe 0.5  0.5  1.0  2.0  800  3 58/1 m  850  900  950  —  V a r i a b l e s i n main experimental block  /Fe  0.32  0.32  90/xm  358/im  f j x  0.16  —  358/xm  C  -*  0.32  124  Time  .8  (min)  P l o t of f r a c t i o n a l r e d u c t i o n versus time showing the e f f e c t of temperature: the base case. C o n d i t i o n s as shown .  125  100  950 °C Boudouard equil 900°C  50  850°C  0.2  .9  0.4 Fractional  r a  0.6 0.8 reduction  .0  Change i n Pcn/Pco? t i o with f r a c t i o n a l r e d u c t i o n during experiments of^base case; c o n d i t i o n s as shown .  126  was c o n s i d e r a b l y slower and t h e r e f o r e the f r a c t i o n a l reduction d i d not reach 0.5.  Temperature produced a marked e f f e c t on gas c o m p o s i t i o n a s  With i n c r e a s i n g t e m p e r a t u r e t h e Pco/' C02 3  well.  r a t 1 0 S  w e r e  s r n  'fted  0  f u r t h e r away from the values i n e q u i l i b r i u m w i t h t h e i r o n o x i d e s . d i s c u s s i o n on t h e shapes o f t h e Pco/ CG2 p  c u r v e s  1 S  A  given i n Section  6.4.6. 6.4.2 E f f e c t o f Cp-j /Fe s t o i c h i o m e t r i c r a t i o x  Conditions i n these e x p e r i m e n t s were i d e n t i c a l t o t h o s e  just  d e s c r i b e d i n Section 6.4.1, except f o r the C p j / F e r a t i o t h a t was 0.16. x  Results o f f r a c t i o n a l r e d u c t i o n obtained from 850 t o 950°C a r e shown, compared t o those o f the base case, i n Figure 6.10. a Cpi /Fe x  I t i s seen t h a t a t  o f 0.16 slower r e d u c t i o n r a t e s were obtained a t a l l tempera-  tures and f u l l reduction was not accomplished. The e f f e c t on gas c o m p o s i t i o n observed that the  PCQ/PQQ^  r a t 1 0 S  i s shown i n Figure 6.11.  It i s  remained c l o s e r to the reduction e q u i -  l i b r i a values, over a smaller extent o f r e d u c t i o n , as compared t o t h e base case, Figure 6.9.  6.4.3 Reduction o f f i n e r p a r t i c l e s under well-mixed  conditions  The only d i f f e r e n c e s i n t h e c o n d i t i o n s o f t h e s e  experiments,  when compared t o t h e base case, are the i r o n - o r e mean p a r t i c l e s i z e of 90 ym and the d / d p r a t i o o f 1.0. c  well-mixed fractional 6.12.  e  These c o n d i t i o n s are a l s o w i t h i n the  r e g i o n , as shown by p o i n t F i n F i g u r e 5.6.  Results o f the  r e d u c t i o n o b t a i n e d from 850 t o 950°C, are shown i n Figure  An i n t e r e s t i n g e f f e c t i s observed:  a t 850°C the reduction rate  127  6.10  P l o t of f r a c t i o n a l redution versus time showing the e f f e c t of s t o i c h i o m e t r i c Cp-,- /Fe r a t i o ; c o n d i t i o n s as shown x  128  100  T 950 °C T  50  Boudouard equil _  900°C 850°C  20  950Q,—°-o o  CJ  O O  o. P 0  ,900°C  CL \  o o  0_  ' —o  C /Fe=0.l6 fix  =0  5  d_c/d e d = 358 /xm F  0.5  Fe  i  ' Fe 0 / FeO equil 3  4  0.2 f 0.  0  0.2  0.4  Fractional  .11  r  0.6  0B  1.0  reduction  Change i n Pcn/Pco? a t i o with f r a c t i o n a l r e d u c t i o n during experiments with s t o i c h i o m e t r i c Cpix/Fe r a t i o ; cond i t i o n s as shown .  129  Time  6.12  (min)  P l o t of f r a c t i o n a l r e d u c t i o n versus time showing the e f f e c t of f i n e r p a r t i c l e s ; c o n d i t i o n s as shown .  130  i s s l o w e r than t h a t of the standard case whereas a t 950°C the opposite was o b t a i n e d . At 900°C a behaviour s i m i l a r to the l a t t e r p r e v a i l e d . The gas compositions obtained are shown i n F i g u r e 6.13.  At a l l  t h r e e t e m p e r a t u r e s i t i s seen t h a t the Pco/ C0 r a t i o s remained c l o s e r p  2  to the FeO/Fe e q u i l i b r i u m values than i n the base c a s e , t h r o u g h o u t a l l the r e d u c t i o n .  6.4.4 Reduction under bed s e g r e t a t i o n c o n d i t i o n s F o r t h e s e e x p e r i m e n t s the d / d p was changed to 2.0 as opposed c  to 0.5 f o r the base case.  e  For the i r o n - o r e p a r t i c l e s i z e used, 358  ym,  bed s e g r e g a t i o n c o n d i t i o n s were expected, as shown by p o i n t S i n F i g u r e 5.6.  Results of f r a c t i o n a l r e d u c t i o n o b t a i n e d from 850 t o 950°C are  shown i n F i g u r e 6.14.  I t can be seen t h a t the r e d u c t i o n r a t e s were  c l e a r l y slower than those of the base case; t h i s e f f e c t was more marked the lower the temperatures. The e f f e c t o f bed s e g r e g a t i o n , i . e . , l a r g e r coal p a r t i c l e s , on gas c o m p o s i t i o n i s shown i n F i g u r e 6.15.  It i s seen t h a t the Pco/PCGv,  r a t i o s remained c l o s e r to the FeO/Fe e q u i l i b r i u m v a l u e s , than t h o s e o f the base case, during the i n i t i a l stages of r e d u c t i o n . At l a t e r stages, they showed a stronger d e p a r t u r e , from t h e s e e q u i l i b r i u m v a l u e s , t h a n those of the base case.  6.4.5 R e p r o d u c i b i l i t y t e s t s T e s t s o f experimental r e p r o d u c i b i l i t y were performed throughout the main experimental block.  Tests were repeated f o r the base c a s e a t  800°C and 900°C and f o r the s t o i c h i o m e t r i c C p ^ / F e r a t i o a t 900°C.  The  131  100 50-  Boudouard equil 900 °C  20  850°C  I0  Ov o  950°C  (VI  900°C  O  o  9 50°C  Q. \  850°C 'FeO'/Fe equil  o 0°  /  C /Fe =0.32  "  fjx  d /d ,= 1.0 d =90^m c  0.5  F  Fe  **i ' FeJD./FeO' equil • 950°C ~ !  0.2 0  4  -/i  1 0  0.2  0.4  Fractional  6.13  0.6  0.8  ID  reduction  Change i n Pco/ CQo ' ' t h f r a c t i o n a l r e d u c t i o n during experiments with f i n e r p a r t i c l e s ; c o n d i t i o n s as shown p  rat1  0  wl  132  Time  6.14  (min)  P l o t of f r a c t i o n a l r e d u c t i o n versus time showing the e f f e c t of bed segregation; c o n d i t i o n s as shown .  133  100 1  1  ' 950°C '  50  —  Boudouard equil 900°C  20  —  850°C  y/  —  9 50°C, 900°C^ • 850°C o  10  CM  O  o Q.  0.5  dp= 358 fj. m e  Fe 0 /Fe0 equil 3  4  0.2 0.1  0.2  0.4 Fractional  6.15  0.6  0.8  1.0  reduction  Change i n Pco/ COo r a t i o with f r a c t i o n a l reduction during experiments with segregated bed; c o n d i t i o n s as shown . p  134  f r a c t i o n a l reduction r e s u l t s are shown i n Figure 6.16.  It i s seen that  the agreement between the repeated e x p e r i m e n t s i s w i t h i n 1.5 e x c e p t i n the 900°C base case experiment.  The unusual  percent,  pattern followed  by experiment number R12 r e q u i r e d that i t be r e p e a t e d .  Experiment  R58  was the r e s u l t and was used f o r a l l subsequent c a l c u l a t i o n s . 6.4.6  P r e l i m i n a r y d i s c u s s i o n of r e s u l t s from the main experimental block" B e f o r e the r e s u l t s presented above can be d i s c u s s e d , c o n s i d e r a -  t i o n must be given to several c h a r a c t e r i s t i c s of the r e a c t i n g s y s t e m , namely:  i)  Sample gas a n a l y s i s as r e l a t e d to the time-lag, i . e . , the time the gases leave the s o l i d s - b e d to the moment of  ii) iii) iv)  sampling,  P o s s i b l e reduction by hydrogen remaining i n the char, Stoichiometry of the o v e r a l l reduction r e a c t i o n , P o s s i b l e e f f e c t o f the SiC heating element o x i d a t i o n on the gas r e a c t i o n s (discussed i n Appendix F ) .  The t i m e - l a g i s a f u n c t i o n o f the gas flowrate f o r a given v o l ume of r e a c t o r and gas t r a i n .  The r e t e n t i o n t i m e o f the gaseous p r o -  d u c t s i n the gas f l o w s y s t e m c o u l d t h e r e f o r e vary, w i t h i n one e x p e r i ment, from a few seconds up to several minutes with the d e c l i n e of react i o n r a t e s , as was m e n t i o n e d i n S e c t i o n 4.3.7. The longer r e t e n t i o n times are of concern because there e x i s t e d the p o s s i b i l i t y of i n t e r m i x i n g o f the gaseous products from d i f f e r e n t stages of the r e a c t i o n . gas l i n e was comprised e s s e n t i a l l y of three s e c t i o n s :  The  the end-pipe, the  135  1.0 0.9 0.8 0.7 o*  fjx  AA  1/  0.6  900°C, C /Fe = 0.32  900°C,C /Fe = 0.16 • • 800°C,C /Fe = 0.32 fjx  fjx  I V  0.5  ^ R 52 :B^-R45  Lll  rpm %fill d /d c  =14 =14 F c  = 0.5  d = 358 ^.m Fe  0  80  160  240  Time (min)  6.16  P l o t of f r a c t i o n a l r e d u c t i o n versus time f o r experimental r e p r o d u c i b i l i t y ; c o n d i t i o n s as shown .  136  c o o l e r and the p l a s t i c t u b i n g , as were d e s c r i b e d i n S e c t i o n 4.3.7.  Re-  s u l t s of c a l c u l a t i o n s of the r e t e n t i o n times i n each s e c t i o n o f the gas s y s t e m , f o r the l o w e r gas f l o w r a t e s a t 0.80, 0.90 and 0.95 f r a c t i o n a l r e d u c t i o n are presented i n Table XIX. each s e c t i o n were:  The assumed gas t e m p e r a t u r e s i n  950°C i n the f r e e b o a r d , 500°C i n the end p i p e , 100°C  i n the c o o l e r and room temperature i n the p l a s t i c t u b i n g .  I t can be  o b s e r v e d t h a t the r e t e n t i o n time i n the e n t i r e system was about 7 minu t e s a t fR = 0.95, 2 minutes a t f ^ = 0.90 and l e s s than 1 minute a t f ^ = 0.80.  Furthermore, i f i t i s c o n s i d e r e d t h a t the gases s p e n t o n l y  about 60 p e r c e n t o f t h i s t i m e i n the f r e e b o a r d , where mixing might be more l i k e l y to occur because i n the gas l i n e s plug flow would be expected t o d o m i n a t e , the mixing e f f e c t i s even s m a l l e r . Since samples were taken every 30 minutes a t these r e a c t i o n stages, the gas m i x i n g e f f e c t can be s a f e l y neglected below a f r a c t i o n a l r e d u c t i o n o f 0.95. The extent of r e d u c t i o n c a r r i e d out by the trace amounts o f hydrogen remaining i n the char i s c o n s i d e r e d next.  F i r s t , the worst case  in t h i s regard, i . e . , the l a r g e s t amount o f hydrogen c o n t a i n e d i n the i r o n - o r e / c h a r admixture, i s the 0.32 Cf-\ /Fe x  imum w e i g h t r a t i o o f c h a r t o o r e was u s e d .  experiments where the maxIn t h i s c a s e , the c h a r  weight was 313 g with 0.31 percent hydrogen, which y i e l d s 0.485 moles of hydrogen p o t e n t i a l l y i n v o l v e d i n the r e d u c t i o n . Since the o n l y p r o d u c t o f r e d u c t i o n by hydrogen c o u l d be w a t e r vapor, the maximum number of oxygen moles removed from Fe2°3 i s 0.243, which corresponds to 4.7 percent o f the t o t a l oxygen present. I t i s r e c o g n i z e d t h a t r e d u c t i o n by hydrogen i s more e a s i l y achieved than with carbon monoxide.  The extent  TABLE XIX. RETENTION TIME OF THE PRODUCT GASES AT DIFFERENT SECTIONS IN THE REACTOR  Section  D [cm]  V [cm ]  [cnw/min]  3  [s]  Freeboard  11.5  3435  0.80 0.90 0.95  8200 2870 820  25 72 251  End-pipe  7.6  817  0.80 0.90 0.95  5190 1820 519  9 27 94  Cooler  2.5  147  0.80 0.90 0.95  2510 880 250  4 10 35  120  0.80 0.90 0.95  2000 800 200  4 9 36  Tubing  0.9  138  o f t h i s can be e s t i m a t e d c o n s i d e r i n g , by way o f i l l u s t r a t i o n , the f o l lowing e q u i l i b r i a ! 4 0  :  Q?7  FeO + CO = Fe + C 0  2  FeO + H = Fe + H 0 2  P ,  - C0 / C0 2  ; K;j  2  P  ,  ; 27  P  P  = H 0/ H 2  P  C0  ?  + H = CO + hLO  ; K:?  2  27  2  = 0.443  (6.11)  = 0.633  (6.12)  P  C0 * H 0 2  = r  C0  K 2  H  = 1.428  (6.13)  2  Methane may a l s o be formed b u t to such a small extent as to be s a f e l y neglected.  In the present case, the molar r a t i o o f c a r b o n t o h y d r o g e n  i s given by C  40.6% / 12  =  0.31%  H  „.  //2  R 9 p  ^.o^o-  S i n c e t h e r e w i l l be one mole o f (CO + CO2) f o r each mole o f carbon, and one mole o f ( H + H 0) f o r each mole o f hydrogen, then 2  2  P K  + P  C0  *C0  ?  — = 21.828 P  H  + 2  P  H 0 2  A l s o , s i n c e the gas mixture i s only comprised o f the four gases, (PCO  +  PC0 ) + ( P H + PH 0) = 1 atm 2  2  2  Therefore, by s o l v i n g simultaneously these two r e l a t i o n s h i p s , t h e f o l lowing i s obtained: (PCO and  (P  H 2  +  p  C0 ) = ° 2  9 5 6 at  "i  + P ) = 0.044 atm H  0  139  F i n a l l y , by r e c a l l i n g the e q u i l i b r i u m constants, K i and K , the p a r t i a l 2  pressure (and volume f r a c t i o n ) can be obtained, namely: = 0.6625  PCO p  p  co  2  = 0.2937 = 0.0268  H 2  PH 0 = 0.0170 2  This value f o r the hydrogen volume f r a c t i o n l i e s w e l l w i t h i n t h e range of values obtained i n the gas analyses; see Figure 6.3. However, an i n crease i n the hydrogen content i s observed as t h e r e a c t i o n p r o c e e d e d . T h i s can be e x p l a i n e d  by t h e f a c t t h a t t h e CO contents were a l s o i n -  c r e a s i n g , through the Boudouard r e a c t i o n , and thus the e q u i l i b r i u m given by E q u a t i o n  (6.13) tends t o be d i s p l a c e d to the reactants s i d e .  More-  over, upon the c o o l i n g o f the gases along the gas l i n e s , t h e d i s p l a c e ment o f t h e e q u i l i b r i u m to the reactants s i d e , i n Equation a l s o favoured.  i)  (6.13), was  From the foregoing, the f o l l o w i n g can be concluded:  The r a t e o f r e d u c t i o n i s a f f e c t e d , t o a s i m i l a r extent f o r a given temperature f o r a l l c o n d i t i o n s t e s t e d , by t h e h y d r o g e n contents o f the char.  The maximum extent o f reduction by hydro-  gen i s 1.8 percent, ii)  The  Pco/Pco  r a t i o s a r e s i m i l a r l y a f f e c t e d to a small  c o n s i d e r a t i o n to t h i s was given when the Pco/ CO p  w e r e  extent;  Plotted.  140  The s t o i c h i o m e t r y o f the o v e r a l l reduction r e a c t i o n , o f obvious importance t o the amount o f carbon r e q u i r e d to reduce a given oxide, can be analyzed as f o l l o w s .  The two r e a c t i o n s ,  C + C 0 = 2 CO  (2.1)  2  F e 0 + 3C0 = 2 Fe + 3 C 0 2  3  (2.2)  2  have been a s c e r t a i n e d t o occur i n p a r a l l e l .  B u t , t o what e x t e n t  will  each gaseous s p e c i e s , CO and C 0 , be u t i l i z e d to c a r r y out i t s respec2  tive reaction?  Two l i m i t i n g cases can be e s t a b l i s h e d by combining  Equa-  t i o n s (2.1) and (2.2), namely: 1/3 F e 0 + C 2  3  = 2/3 Fe +  CO  1/3 F e 0 + 1/2 C = 2/3 Fe + 1/2 C 0 2  Equation  3  2  (C/Fe)  w t  = 0.32  (6.14)  (C/Fe)  w t  = 0.16  (6.15)  (6.14) c o n s i d e r s t h a t C 0 r e a c t s instantaneously with carbon 2  whereas Equation (6.15) i s obtained by assuming that the reduction by CO occurs much more r a p i d l y than carbon g a s i f i c a t i o n .  C l e a r l y , the o v e r a l l  reduction r e a c t i o n given by Equation (6.15) w i l l be the more c a r b o n e f f i c i e n t o f t h e two, and a l s o w i l l  s e t t h e minimum carbon r e q u i r e d to  c a r r y out the complete r e d u c t i o n o f the oxide.  I t can thus be i n f e r r e d  t h a t the " s t o i c h i o m e t r i c " p r o p o r t i o n o f carbon and F e 0 3 , w i l l depend on 2  the r e l a t i v e rates o f both  r e d u c t i o n and g a s i f i c a t i o n r e a c t i o n s as has  been t h e o r e t i c a l l y d i s c u s s e d b e f o r e . other hand, that i n experimental  1 0 5  I t i s a l s o evident, on the  (and i n d u s t r i a l ) r e a c t i o n systems t h e  gaseous phase w i l l be formed o f both CO and C 0 . 2  ing o v e r a l l reduction r e a c t i o n can be p o s t u l a t e d ,  Therefore, the f o l l o w -  141  1/3  F e  ') 2 ° 3  +  +  (• *)  C  =  2 / 3  R  (  + 4, Fe + *C0 + | CO,2 (6.16)  where R i s the e q u i l i b r i u m CO/COg r a t i o f o r e i t h e r the reduction or the Boudouard r e a c t i o n s and <j> i s the CO mole f r a c t i o n corresponding equilibrium.  t o that  The s t o i c h i o m e t r i c r a t i o c a l c u l a t i o n s can thus be perform-  ed under the f o l l o w i n g assumptions:  i)  When t h e Boudouard r e a c t i o n i s rate c o n t r o l l i n g , R and <)> w i l l be those o f the reduction e q u i l i b r i u m ; conversely, when t h e r e duction r e a c t i o n c o n t r o l p r e v a i l s , R and <t> w i l l be those o f the Boudouard e q u i l i b r i u m ,  ii)  R and <j> i n t h e r e d u c t i o n e q u i l i b r i u m are taken to be those o f w u s t i t e r e d u c t i o n s i n c e t h i s i s the slowest stage i n the Fe203 reduction sequence.  R e s u l t s o f these c a l c u l a t i o n s are presented i n Table XX and Figure 6.17 f o r each type of c o n t r o l l i n g r e a c t i o n , a t the t e m p e r a t u r e s r e l e v a n t t o t h i s work.  From these r e s u l t s , a Cp-j /Fe o f 0.16 was adopted f o r t h i s x  work as the " s t o i c h i o m e t r i c " value.  An a l t e r n a t e treatment o f the s t o i -  c h i o m e t r y o f t h e o v e r a l l reduction r e a c t i o n could be developed, on the basis o f the concept o f the "gas u t i l i z a t i o n f a c t o r , " Section 2.3, Equations  B , mentioned i n  (2.8) and (2.9).  The c h o i c e o f the experiments o f the base case was based on the following.  With respect t o Cp-j /Fe, the 0.32 value was s e l e c t e d because x  142  TABLE XX.  STOICHIOMETRIC RATIOS FOR REDUCTION OF F e 0 WITH CARBON, UNDER TWO TYPES OF REACTION CONTROL: BOUDOUARD AND REDUCTION 2  3  BOUDOUARD CONTROL C0/C0  2  Molar Ratio C/Fe  C/O  Weight Ratio Fix/ c  p e  Limit  0  0.750  0.500  0.161  800  1.841  1.109  0.739  0.238  850  1.976  1.123  0.749  0.241  900  2.155  1.139  0.759  0.245  950  2.311  1.152  0.768  0.248  1000  2.448  1.163  0.775  0.250  REDUCTION CONTROL C0/C0  2  Molar Ratio C/Fe  C/0  Weight Ratio Fix/ c  p e  800  3.598  1.232  0.821  0.265  850  17.868  1.425  0.950  0.306  900  37.462  1.462  0.975  0.314  950  70.429  1.479  0.986  0.318  1000  120.000  1.488  0.992  0.320  Limit  »  1.500  1.000  0.322  800  850  900  950  Temperature (°C)  17  S t o i c h i o m e t r i c C p j / F e r a t i o as a f u n c t i o n of temperature for the reduction of Fe203 with carbon . x  1000  144  no f u r t h e r enhancement i n the r e d u c t i o n r a t e was obtained by i n c r e a s i n g the amount o f coal char.  With r e s p e c t t o p a r t i c l e s i z e , the 358 ^m o r e  p a r t i c l e s were t h e l a r g e s t , i n a c c o u n t a b l e weight p r o p o r t i o n , i n the concentrate s i z e range (see F i g u r e 4.1); s m a l l e r p a r t i c l e s s h o u l d be r e d u c e d f a s t e r , as was p r o v e d with the experiments using 90>wm p a r t i cles.  The c h a r p a r t i c l e s ( d = 180 ym) mixed with the 358 y m ore were c  the l a r g e s t p o s s i b l e , with which a well mixed bed was s t i l l obtained, as shown by p o i n t R i n Figure 5.6. The d / d p r a t i o o f 0.5 provided then c  the most ' d i f f i c u l t ' s i z e o f ore t o r e d u c e .  e  The 90 y m o r e p a r t i c l e s ,  c o n v e r s e l y , p r o v i d e d t h e s m a l l e s t s i z e o f ore t o be reduced; they a c count f o r more than 40 percent o f the t o t a l c o n c e n t r a t e w e i g h t .  This  ore s i z e was t e s t e d mainly due t o i t s expected enhancing e f f e c t on the agglomeration growth. The e f f e c t o f temperature on the r e d u c t i o n r a t e presented common f e a t u r e s throughout the main experimental block, as i l l u s t r a t e d by F i g ure 6.8, namely:  i)  A p r o g r e s s i v e l y i n c r e a s i n g o v e r a l l r e d u c t i o n r a t e was produced by i n c r e m e n t s i n t h e t e m p e r a t u r e  o f r e a c t i o n , from 800 t o  950°C. ii)  F r a c t i o n a l r e d u c t i o n higher than 0.85 was obtained, i n a l l cases b u t t h e s t o i c h i o m e t r i c Cf^ /Fe x  t  f o r temperatures a t and above  850°C. iii)  A t 900 and 950°C, the r e d u c t i o n r a t e s e x h i b i t e d a constant period up t o about 0.6 f r a c t i o n a l r e d u c t i o n . they slowed down.  Beyond t h i s p o i n t ,  145  iv)  At 850°C the r e d u c t i o n r a t e s showed a decrease at about 0.2.  =  The rates then s l i g h t l y i n c r e a s e d up t o a b o u t 0.6  from  where they again decayed, v)  At 800°C the reduction rates were slow.  The reduction obtained  i n most cases d i d not proceed much beyond f ^ = 0.3 which c o r r e sponds to the s t a r t of wustite r e d u c t i o n .  The drop i n r e d u c t i o n r a t e , a t about 0.2 f r a c t i o n a l reduction was not confined to the t e s t s a t 850°C.  T h i s phenomenon can be  seen  b e t t e r by p l o t t i n g f r a c t i o n a l reduction rate as a f u n c t i o n of f r a c t i o n reduced.  This i s i l l u s t r a t e d i n F i g u r e 6.18  f o r the base case  only.  S i m i l a r p a t t e r n s were a l s o o b t a i n e d f o r a l l other c o n d i t i o n s .  It i s  seen t h a t a drop i n the rates occurred between 0.1 and 0.2 reduction.  fractional  T h i s was f o l l o w e d by a r e c o v e r y p e r i o d up to about 0.3.  From then on, a f a i r l y constant r a t e p r e v a i l e d , up to d i f f e r e n t e x t e n t s o f r e d u c t i o n , whereupon the r a t e s d e c l i n e d u n t i l complete reduction was achieved. fore  90  98  »  S i m i l a r patterns i n the reduction rate have been observed  be-  and they can be explained on the b a s i s of the stage-wise r e -  duction of Fe203 as f o l l o w s . A f r a c t i o n a l r e d u c t i o n o f 0.1 c l o s e l y corresponds to the comp l e t e r e d u c t i o n of Fe203 to Fe304.  This value should be 0.111  f o r pure  Fe203 b u t , s i n c e the ore contained a f r a c t i o n of Fe304 (Table V I I I ) , a value of 0.101  i s obtained.  S i m i l a r l y , the f r a c t i o n a l r e d u c t i o n c o r r e -  sponding  t o the n o n - s t o i c h i o m e t r i c  Feo.9530,  i s found between 0.221  range o f w u s t i t e , F e n . 8 8 1 °  and 0.273 i n s t e a d of 0.243 and 0.300.  t0  Fractional reduction  18  F r a c t i o n a l reduction rate as a f u n c t i o n o f f r a n c t i o n a l reduction f o r experiments o f the base case .  147  A c c o r d i n g l y , t h e drop i n rate between 0.1 and about 0.2 f r a c t i o n a l r e duction corresponds  t o r e d u c t i o n o f Fe304 t o F e g g i 0 . n<  ^  nis  ro  °- P  sharper, and p r o p o r t i o n a l l y s m a l l e r , the higher t h e t e m p e r a t u r e .  1 S  Sub-  s e q u e n t l y , i n f u r t h e r r e d u c i n g t o Feo.9530, a recovery i n the r a t e i s observed, again, o f a l a r g e r magnitude with higher t e m p e r a t u r e s .  This  i s f o l l o w e d by a p e r i o d o f reasonably constant r a t e , t o l a r g e r extents of reduction the lower the temperature;  f i n a l l y , a decaying r a t e i s o b -  tained. When t h e gas c o m p o s i t i o n s 6.11,  o b t a i n e d a r e examined, Figures 6.9,  6.13 and 6.15, three stages i n t h e PCO/POQ^ r a t i o are encountered  in most experiments.  An i n i t i a l stage, up t o the appearance o f w u s t i t e ,  where t h e r a t i o s i n c r e a s e with f r a c t i o n a l r e d u c t i o n . by a r e a s o n a b l y constant Pco/ CO p  This i s followed  P ^ r i o d , i n d i c a t i v e o f Boudouard reac-  t i o n c o n t r o l by i t s p r o x i m i t y t o t h e o x i d e s e q u i l i b r i a . was  s h o r t e r a t 950° than a t 900°C and 850°C.  This period  The length o f t h i s p e r i o d  a l s o v a r i e d with the c o n d i t i o n s o f the experiment.  I t was l o n g e s t f o r  the f i n e r p a r t i c l e s , followed i n decreasing order by the base case, the s t o i c h i o m e t r i c Cp-j /Fe and the segregated bed experiments. x  t h i r d stage o f i n c r e a s i n g Pco/ CO p  r a  Finally, a  t i o i s observed.  From the foregoing o b s e r v a t i o n s , i t i s evident that a correspondence e x i s t s between the stages i n the reduction r a t e s and Pco/ C0 p  2  t i o s , as would be expected. Chapter  ra  "  The a n a l y s i s o f t h i s c o r r e s p o n d e n c e , i n  8, w i l l a i d i n d e t e r m i n i n g  t h e r a t e c o n t r o l l i n g mechanism a t  each stage o f the reduction r e a c t i o n .  D e t a i l e d c o n s i d e r a t i o n w i l l be  given t o the p a r t i c l e s s i z e as w e l l .  148  6.5 Results of comparative  experiments  The aim of these experiments was to determine the e f f e c t s of add i t i o n a l parameters, on the r e d u c t i o n r a t e s , i n order to f u r t h e r a s s e s s the s t a g e s o b s e r v e d i n the e x p e r i m e n t s o f the main b l o c k . meters t e s t e d s e p a r a t e l y were:  The para-  a c a t a l y s t f o r the Boudouard r e a c t i o n ,  use o f l i g n i t e as r e d u c t a n t , use o f g r a p h i t e as reductant, i n e r t - g a s f l u s h i n g and r e d u c t i o n of hematite p e l l e t s .  Operational c o n d i t i o n s were  the same as i n the main experimental b l o c k , namely:  14 r.p.m., 14 per-  cent l o a d i n g and 0.32 Cp - /Fe. n  X  6.5.1 Reduction using a c a t a l y s t f o r the Boudouard r e a c t i o n The e f f e c t on the r e d u c t i o n r a t e s o f a c a t a l y s t f o r the Boud o u a r d r e a c t i o n i s shown i n F i g u r e 6.19.  The c a t a l y s t was an equimolar  mixture of l i t h i u m , sodium and p o t a s s i u m c a r b o n a t e s , p r e p a r e d as exp l a i n e d i n S e c t i o n 4.23 and added i n an amount of 5 wt percent with r e spect to the charge.  I t i s seen i n F i g u r e 6.19  that reduction rates  were c o n s i d e r a b l y f a s t e r than i n the base case.  The high degree of r e -  duction achieved a t 800°C i s e s p e c i a l l y remarkable. The gas c o m p o s i t i o n f o r t h e s e e x p e r i m e n t s i s shown i n F i g u r e 6.20. case.  The Pco/ CO p  r a t  i o s are markedly d i f f e r e n t than those of the base  The second stage, c h a r a c t e r i z e d by constant Pco/PfJOg  n o n - e x i s t e n t a t e i t h e r temperature with the c a t a l y s t .  r a  t i o s , was p  The Pr,o/ C0  ra 2  ~  t i o s were higher than i n the base case, throughout the e n t i r e r e d u c t i o n , a t both temperatures and the r a t i o s i n c r e a s e d f a s t e r , than those of the base case, i n the l a s t stage.  149  Time  6.19  (min)  P l o t o f f r a c t i o n a l reduction versus time showing the e f f e c t of a c a t a l y s t of the Boudouard r e a c t i o n ; c o n d i t i o n s as shown .  100 50  900 °C  20 Boudouard  equil  900°C (catalyzed)' 0  /  0  800°C (N flush) 2  800°C_ _ /• • ^©-®800 C _ e^r^ (catalyzed) e  O 0°  ©rT  . 3DD1Q. _ _  o 0_°  0.5  0.20.  0  0.2  0.4  0.6  0.8  .0  Fractional reduction  20  Change i n Pcrj/PcOo t"»° with f r a c t i o n a l reduction durin c a t a l y z e d and ^ - f 9 experiments; c o n d i t i o n s as shown . ra  1 u s r n n  151  6.5.2 Reduction with l i g n i t e F r a c t i o n a l r e d u c t i o n p l o t s , using l i g n i t e , are shown i n F i g u r e 6.21. The r e d u c t i o n r a t e s were c o n s i s t e n t l y s l o w e r than t h o s e o f t h e base case; t h i s can be seen by comparing F i g u r e s 6.8 and 6.21. This e f f e c t was more marked the lower the temperature. The gas c o m p o s i t i o n , shown i n F i g u r e 6.22, d i f f e r e d from the base c a s e p r i m a r i l y i n t h a t the Pco/ CO p  c o n s t a i r t  periods were s l i g h t l y  s h o r t e r and c l o s e r t o the FeO/Fe e q u i l i b r i a f o r the l i g n i t e c o a l .  6.5.3 Reduction with g r a p h i t e R e d u c t i o n w i t h a low r e a c t i v i t y carbonaceous m a t e r i a l , such as g r a p h i t e , i s expected t o be minimal i n the range o f temperatures t e s t e d . N o n e t h e l e s s , an e x p e r i m e n t was conducted with t h i s m a t e r i a l to v e r i f y t h i s h y p o t h e s i s . Temperature was v a r i e d from 875 t o 950°C d u r i n g t h e t e s t w i t h the r e s u l t s shown i n Figure 6.23.  I t i s seen t h a t the reduc-  t i o n r a t e was extremely slow, even a t 950°C.  The gas a n a l y z e d was a l -  most pure C02*  6.5.4 E f f e c t o f i n e r t - g a s f l u s h i n g 3  In t h i s e x p e r i m e n t a 4000 cm /min f l o w o f n i t r o g e n was blown 1  over the bed s u r f a c e , shown by N-N  i n F i g u r e 4.3.  r e d u c t i o n r a t e i s shown i n F i g u r e 6.23.  The e f f e c t on the  I t i s seen t h a t the r e a c t i o n  slowed down a f t e r a f r a c t i o n a l r e d u c t i o n o f 0.8, and stopped completely, j u s t above a f r a c t i o n a l r e d u c t i o n o f 0.9. The corresponding e f f e c t on gas composition i s shown i n F i g u r e 6.20.  A s l i g h t decrease i n Pco/ COo r a t i o i s seen during the r e d u c t i o n p  152  A 850°C X  Lignite C /Fe=0.32 dc/d =0.5 d = 358/xm rpm =14 % f i l l = 14 fix  Fe  F e  240  160 Time  6.21  (min)  P l o t o f f r a c t i o n a l reduction versus time f o r r e d u c t i o n with Saskatchewan l i g n i t e ; c o n d i t i o n s as shown .  100  T 950°C Boudouard equil  50  900°C  20  850°C  10 N  O  950°C  o—o—o  5  0°  \  o  950°C  0.°  FeO'/Fe equil  8  50°C  Lignite C /Fe - 0.32 fjx  d /d , = 0.5 c  0.2 0.1  950°C Fe 0 /'FeO'equil 3  F  dp = 358/i.m e  4  0.2  0.4  0.6  0B  1.0  Fractional reduction  .22  Change i n Pco/ CQo r a t i o with f r a c t i o n a l r e d u c t i o n f o r experiments with l i g n i t e ; c o n d i t i o n s as shown . p  154  6.23  P l o t of f r a c t i o n a l reduction versus time showing the e f f e c t s of N - f l u s h i n g on r e d u c t i o n with Forestburg coal and reduction with g r a p h i t e ; c o n d i t i o n s as shown . 2  155  o f w u s t i t e , up t o a f r a c t i o n a l r e d u c t i o n of about 0.8, whereupon the ratio increased dramatically.  6.5.5  Reduction of hematite p e l l e t s Commercial hematite p e l l e t s (67 percent i r o n ) were reduced, t o -  gether with the concentrate, i n several of the experiments.  Two  pellets  o f d i f f e r e n t s i z e s , 14 and 19 mm i n diameter, were reduced i n each of these t e s t s .  P e l l e t s were r e d u c e d a t e v e r y t e m p e r a t u r e ,  950°C, at l e a s t once.  from 800 t o  The extent of r e d u c t i o n of the p e l l e t s was d e t e r -  mined from t h e i r t o t a l weight l o s s , assumed to be oxygen removed.  A  summary o f the c o n d i t i o n s t e s t e d and the r e s u l t s obtained are presented i n Table XXI compared to the r e d u c t i o n of the r e s p e c t i v e c o n c e n t r a t e s . A t 8 0 0 ° C , d u r i n g the segregated-bed c o n d i t i o n s , the f r a c t i o n a l reduction of the smaller p e l l e t was p r a c t i c a l l y the same as t h a t o f the c o n c e n t r a t e ; the l a r g e r p e l l e t though, was 10 percent l e s s reduced.  On  the other hand, during the c a t a l y z e d experiment at the same temperature, both the l a r g e and small p e l l e t s were 40 percent l e s s reduced than the concentrate but were a t l e a s t 15 percent more reduced than t h e i r c o u n t e r p a r t s i n the s e g r e g a t e d - b e d  c o n d i t i o n s . At 850°C, during both the  base case and the f i n e r p a r t i c l e s r e d u c t i o n , p e l l e t s were r e d u c e d  at  l e a s t 50 percent l e s s than the concentrates with a very small d i f f e r e n c e in r e d u c t i o n between p e l l e t s .  At 900°C, during the r e d u c t i o n of f i n e r  p a r t i c l e s , the d i f f e r e n c e i n the extent of reduction was about 40 percent; again a minor d i f f e r e n c e was seen between p e l l e t s a t t r i b u t a b l e  to  t h e i r s i z e d i f f e r e n c e . F i n a l l y , at 950°C, the extent of p e l l e t reduct i o n , during the r e d u c t i o n of the f i n e r p a r t i c l e s was p r a c t i c a l l y the  156  TABLE XXI. CONDITIONS AND RESULTS FOR REDUCTION OF HEMATITE PELLETS  T  I n i t i a l weight  rc]  d [ym]  [g]  Final Pellets  800  100  10.8293 6.6536  0.4358 0.4328  0.830 0.830  Catalyzed  90  9.9766 5.8585  0.1919 0.2912  0.297 0.297  Fine Mixture  90  9.7635 6.4264  0.3955 0.4203  0.932 0.932  Fine Mixture  180-  7.5026  0.4150  0.913  Base Case  900  90  8.5451 6.8137  0.4617 0.4358  0.861 0.861  Fine Mixture  950  716  12.4747 5.5047  0.7838 0.9294  0.973 0.973  Segregated  90  9.7947 6.7192  0.9592 0.9687  0.997 0.997  Fine Mixture  850  c  f  R. Ore  Bed c o n d i t i i  157  same f o r both the concentrate and p e l l e t s , without any e f f e c t of p e l l e t s i z e . However, during the segregated-bed experiment at the same temperat u r e , the smaller p e l l e t was 15 percent more reduced than the l a r g e r but 5 percent l e s s than the r e s p e c t i v e  6.5.6  concentrate.  P r e l i m i n a r y d i s c u s s i o n of r e s u l t s from the comparative experiments The f a c t that the presence of a c a t a l y s t f o r the Boudouard reac-  t i o n i n c r e a s e d the r e d u c t i o n r a t e , through i t s e f f e c t on gas composit i o n , i s not s u r p r i s i n g . That r e a c t i o n has been i d e n t i f i e d as the r a t e determining step by several of the i n v e s t i g a t i o n s reviewed i n Chapter 2. However, when comparing t h i s work with t h o s e p r e v i o u s l y r e p o r t e d , mixing  the  c o n d i t i o n s , p a r t i c l e s s i z e and carbonaceous material r e a c t i v i t y  are found to be s i g n i f i c a n t l y d i f f e r e n t . In a d d i t i o n an i n e r t - g a s d i l u e n t was  u t i l i z e d i n most o f the e a r l i e r s t u d i e s .  It might be a n t i c i -  pated that d i f f e r e n t c o n t r o l l i n g mechanisms may be i n v o l v e d i n the  pre-  s e n t i n v e s t i g a t i o n , p a r t i c u l a r l y i n the l a t e r reduction stages.  This  may a l s o be i n f e r r e d from a c a r e f u l study of the Pco/PCGv,  r a t  i ° obtained  i n the experiment when a nitrogen f l u s h was used, see F i g u r e 6.20.  In  t h i s t e s t , the r a t i o followed very c l o s e l y the shape of the e q u i l i b r i u m l i n e s f o r the oxide r e d u c t i o n .  This i n d i c a t e s that the Boudouard r e a c -  t i o n p l a y e d a more i m p o r t a n t  r o l e under d i l u e n t - g a s c o n d i t i o n s .  can be analyzed as f o l l o w s .  There i s e v i d e n c e  t h a t back d i f f u s i o n o f  i n e r t gas i n t o the s o l i d s bed i s p o s s i b l e i n the thermogravimetric periments reviewed  i n S e c t i o n 2.3.103  This  j h i s was  ex-  estimated to be the  158  case, by c a l c u l a t i o n s based on o r d i n a r y molecular d i f f u s i o n i n t o a s t a g nant gas f o r two o f the t h e r m o g r a v i m e t r i c r e d u c t i o n s t u d i e s r e v i e w ed. 96,102  i these experiments, 8 and 0.85 g samples were reduced under n  i n e r t gas flows of 200 and 2000 cm^/min, r e s p e c t i v e l y . Inert gas flows of 15 and 1400 £/min would have had t o have been used i n the p r e s e n t s t u d y i n o r d e r t o o b t a i n s i m i l a r r a t i o s of i n e r t gas to sample weight. A l o w e r f l o w o f 4000 cm^/min was used i n the present study, based on evidence t h a t i n e r t gas d i f f u s i o n i n t o the bed i s more e a s i l y a c h i e v e d i n a r o t a r y k i l n . 8 M o r e o v e r t h i s lower flow would correspond b e t t e r with the range of nitrogen flow, v i a i n j e c t e d a i r , i n an i n d u s t r i a l r o tary k i l n . 7  T h e r e f o r e , the c o n d i t i o n s f o r n i t r o g e n d i f f u s i o n i n t o the  bed were p o s s i b l e i n the experiment.  The d i l u t i o n of the r e a c t i n g gases  w i t h i n the bed f a v o u r e d t h e Boudouard r e a c t i o n to be r a t e c o n t r o l l i n g over a l a r g e r extent of the r e d u c t i o n . The l o w e r r e d u c t i o n r a t e obtained when l i g n i t e was used as r e ductant can only be e x p l a i n e d on the b a s i s o f the lower ash c o n t e n t o f t h i s m a t e r i a l as compared w i t h the sub-bituminous coal char.  The  c a t a l y t i c e f f e c t of ash on the Boudouard r e a c t i o n has been a s s e r t e d bef o r e . ! 14,117 T h e r e f o r e , s i n c e the Boudouard r e a c t i o n c o n t r o l s the overa l l r e d u c t i o n process to a c e r t a i n extent, a slower g a s i f i c a t i o n r e a c t i o n w i l l r e t a r d the r e d u c t i o n . This stronger c o n t r o l by the Boudouard r e a c t i o n , during the f i r s t h a l f of the r e d u c t i o n process, can be c o r r o b o r a t e d by the i n c r e a s e d c l o s e n e s s o f the Pco/Pc^ r a t i o s to the 'FeO'/Fe e q u i l i b r i a i n the l i g n i t e experiments over the base c a s e , F i g u r e s 6.22  and 6.9 r e s p e c t i v e l y . The minimal r e d u c t i o n obtained with  159  g r a p h i t e , even a t 950°C, again shows the importance o f the Boudouard r e a c t i o n when such a l o w - r e a c t i v i t y material i s used. on t h e e f f e c t o f t h e c a r b o n a c e o u s - m a t e r i a l  Further d i s c u s s i o n  r e a c t i v i t y on the o v e r a l l  r e a c t i o n k i n e t i c s i s t o be given i n Chapter 8. The e x p e r i m e n t s  w i t h p e l l e t s were c a r r i e d out t o compare the  t o t a l extent o f r e d u c t i o n o f l a r g e r p a r t i c l e s with t h a t o f the f i n e o r e . The p e l l e t s were expected to remain i n s i d e the bed, as was corroborated by v i s u a l i n s p e c t i o n a f t e r the experiments had ended.  It i s therefore  assumed t h a t the p e l l e t s encountered the same reducing c o n d i t i o n s as the ore concentrate.  In g e n e r a l , the higher t h e t e m p e r a t u r e ,  the smaller  the d i f f e r e n c e i n r e d u c t i o n between p e l l e t s and c o n c e n t r a t e . t o be expected s i n c e the Pco/Pcc^  r a t l 0 S  i n  t n e  This was  bed increased with tem-  p e r a t u r e , and t h e r e f o r e the reducing p o t e n t i a l was higher.  160  CHAPTER 7  STUDY OF PARTICLES AGGLOMERATION DURING REDUCTION  One of the o b j e c t i v e s of t h i s work was to c h a r a c t e r i z e the glomeration  ag-  o f the f i n e p a r t i c l e s w i t h i n the s o l i d s bed and on the f u r -  nace wall during r e d u c t i o n .  Since no a c c r e t i o n s were formed on the r e -  a c t o r w a l l , most l i k e l y due to the r e l a t i v e l y low temperatures of operat i o n , only the agglomeration this chapter.  of p a r t i c l e s w i t h i n the bed i s examined i n  The r e s u l t s are p r e s e n t e d as a comparison between the  p a r t i c l e s i z e d i s t r i b u t i o n of the r e d u c e d charge.  p r o d u c t and the o r i g i n a l  These are complemented with observations using a scanning e l e c -  tron microscope.  7.1 P a r t i c l e s i z e d i s t r i b u t i o n a f t e r reduction The p a r t i c l e s i z e of the i r o n - o r e c o n c e n t r a t e was  - 410 + 305 ym ( d p  e  was  - 107 + 74 ym (dp  = 90 ym).  e  = 358 m) y  before  reduction  i n a l l cases but one, where the s i z e The r e s u l t s of the p a r t i c l e s i z e d i s -  t r i b u t i o n a f t e r reduction under d i f f e r e n t c o n d i t i o n s are shown i n F i g ures 7.1 t h r o u g h  7.4.  In each f i g u r e , the narrow s i z e range of the  s t a r t i n g material i s shown by v e r t i c a l d o t t e d l i n e s .  The  percentage  u n d e r s i z e a t which the p a r t i c l e s - s i z e d i s t r i b u t i o n crosses the uppers i z e l i m i t , provides a q u a l i t a t i v e measure of the agglomeration  growth.  100  15 II A - i * K - ^ H  /  / ^  800°C  80  /  A /  /  /  /  850,900°C /  /  60  o  /950°C  40  20 /  /i - a - P —  60  100  200  Q  '  I  |  400 Agglomerate  I  L  1000 2000 size (ft-m )  P a r t i c l e s i z e d i s t r i b u t i o n a f t e r reduction of 90 y m p a r t i c l e s .  4000  8000  200  2  400  1000  2000 400 1000 Agglomerate size (/xm)  Particle size d i s t r i b u t i o n after reduction, case; (b) c a t a l y z e d experiments .  (a) base  3000  P a r t i c l e size d i s t r i b u t i o n after reduction. (b) S t o i c h i o m e t r i c C_. /Fe  (a) Segregated  bed  Agglomerate  .4  size ifim)  Particle size distribution after reduction. case; (B) L i g n i t e reductant .  (A) base  165  On t h i s b a s i s , the l a r g e s t agglomeration of p a r t i c l e s was obt a i n e d when the dp  e  = 90 ym ore was reduced, as shown i n Figure 7.1.  It  i s seen t h a t more agglomeration was produced the higher the temperature. A t 900 and 850°C however, the extent of agglomeration was very s i m i l a r . A few of the agglomerates were as l a r g e as 10 mm. c l e s agglomerated to a l e s s e r e x t e n t .  In Figure 7.2 the p o s i t i v e e f f e c t  of the c a t a l y s t on agglomeration i s i l l u s t r a t e d . pronounced  a t h i g h e r temperature.  At 800°C, the p a r t i -  T h i s e f f e c t was more  By comparing Figure 7.3 with Figure  7.2(A) the e f f e c t s of C p i / F e and bed segregation can be seen. x  Slightly  more agglomeration was produced when the s t o i c h i o m e t r i c Cp-j /Fe was x  ployed.  em-  Bed s e g r e g a t i o n , on the other hand, i n c r e a s e d the agglomerating  t e n d e n c y o n l y a t 850°C; at 900°C the extent of agglomeration was pract i c a l l y the same whereas at 950°C l e s s agglomeration was obtained i n the segregated bed.  F i n a l l y , the e f f e c t of using a l i g n i t e reductant i s i l -  l u s t r a t e d i n Figure 7.4.  I t i s seen t h a t agglomerate s i z e s are l a r g e r  w i t h l i g n i t e than t h o s e w i t h sub-bituminous coal at temperatures from 850 to 905°C. A q u a n t i t a t i v e measure o f the agglomeration can be given by the following r e l a t i o n s h i p ^ :  Agglomeration = ^ 2 9 dFe  (7.1)  where the average diameter of the agglomerates, d^gg, i s given by  166  X-j i s the amount of material r e t a i n e d between two sieve s i z e s and d - i s n  the average s i z e of the openings i n those s i e v e s . A summary of the c a l c u l a t i o n s based on the above, f o r a l l c o n d i t i o n s t e s t e d , i s presented i n F i g u r e s 7.5 and 7.6 f o r cT gg and ^Agg/^Fe r e s p e c t i v e l y . In these f i g A  ures the e f f e c t s described above can be c l e a r l y seen. o f s m a l l e r ore p a r t i c l e s on a g g l o m e r a t i o n , terms i s e v i d e n t .  i n a b s o l u t e and r e l a t i v e  I t i s a l s o seen t h a t t e m p e r a t u r e  marked e f f e c t when segregation i n the bed was  7.2 Scanning e l e c t r o n microscope  The strong e f f e c t  d i d not e x e r t a  present.  observations  The reduced products were examined under a scanning e l e c t r o n microscope. 90  An example of the agglomerates formed during the reduction of  m p a r t i c l e s i s shown i n Figure 7.7.  I t was d e t e r m i n e d  by the  ob-  s e r v a t i o n of several samples from non-catalyzed c o n d i t i o n s t h a t the agglomerates were held together p r i m a r i l y by the i r o n whiskers which grow as a  product of the r e d u c t i o n ; t h i s i s shown i n Figure 7.8.  This pho-  tograph shows two reduced g r a i n s , one on top and the o t h e r a t the tom, j o i n e d together.  bot-  R e l a t i v e l y few non-metallic p a r t i c l e s were found  between the reduced g r a i n s .  P a r t i c l e s which were found i n t h i s r e g i o n ,  as i l l u s t r a t e d i n F i g u r e 7.9, a p p e a r e d to be e n t a n g l e d by the i r o n whiskers r a t h e r than being bonded to the r e d u c e d  grains.  A different  p a t t e r n was observed when the agglomerates formed during reduction with the c a t a l y s t were examined.  As shown i n Figure 7.10, c o n s i d e r a b l y more  non-metallic p a r t i c l e s were found between the reduced g r a i n s .  1200  Catalyzed  1000  800  E =L  600 Base case  cn ° 4 0 0 |_  Stoichiometric'! (C /Fe) f j x  d  F e  =358/i. m  2 0 0 d 0  800  1  850  1  900 Temperature (°C)  .5  Agglomerates average s i z e as a f u n c t i o n of reduction temperature. Conditions of experiments as shown .  F e  = 90/x m  L 950  800  850  900  Temperature (°C)  Agglomeration r e l a t i v e to the o r i g i n a l ore s i z e as a f u n c t i o n of temperature. C o n d i t i o n s of experiments as shown .  950  7.7  7.8  Agglomerate formed during r e d u c t i o n o f 90 ym i r o n ore p a r t i c l e s (lOOx)  Iron whiskers produced during reduction j o i n i n g two reduced p a r t i c l e s (800x)  170  7.9  7.10  S i l i c a t e p a r t i c l e between two reduced g r a i n s (800x)  Agglomerate formed during r e d u c t i o n under c a t a l y z e d c o n d i t i o n s (lOOx)  171  7.3 P r e l i m i n a r y d i s c u s s i o n on p a r t i c l e s agglomeration during reduction Before the agglomeration of p a r t i c l e s w i t h i n the bed i s d i s c u s s ed, c o n s i d e r a t i o n must be given to the a b s e n c e o f a c c r e t i o n growth on the i n n e r wall of the r e a c t o r .  As was mentioned above, one of the main  reasons f o r the lack of these a c c r e t i o n s i s the r e l a t i v e l y low t e m p e r a t u r e s a t which the experiments  were c a r r i e d out.  The highest tempera-  ture t e s t e d , 950°C, i s s t i l l c o n s i d e r a b l y below the melting p o i n t of any o f the p o s s i b l e phases to be formed between the high-alumina wall and the components of the charge.  These phases, E u t e c t i c s 5 and 6 g i v e n i n  T a b l e V, have melting temperatures  of 1070 and 1165°C r e s p e c t i v e l y . In  a d d i t i o n , the smoothness of the r e f r a c t o r y surface may have hindered the growth of a c c r e t i o n s . The l a r g e agglomeration  growth, r e l a t i v e to the i n i t i a l p a r t i c l e  s i z e , when the 90 ym p a r t i c l e s were reduced can be explained as f o l l o w s . In C h a p t e r  2, the mechanisms o f agglomeration  during the reduction of  i r o n ores were e s t a b l i s h e d to be s i l i c a t e and i r o n 'bridges' j o i n i n g the particles together.  At the temperatures  t e s t e d however, the s i l i c a t e  bridges w i l l play but a minimal r o l e i n the n o n - c a t a l y z e d as was confirmed by the microscopic o b s e r v a t i o n s . of i r o n bridges w i l l p r i m a r i l y determine  experiments,  Therefore the number  the a g g l o m e r a t e g r o w t h .  The  number o f bridges w i l l i n turn be d i r e c t l y p r o p o r t i o n a l to the s p e c i f i c surface area (area per u n i t mass) of the reduced i n d i v i d u a l p a r t i c l e s . The r a t i o of s p e c i f i c surface area of the 90 ym with respect to the 358 ym p a r t i c l e s i s obtained as f o l l o w s . The volume of a 90 ym p a r t i c l e i s  172  64 t i m e s s m a l l e r than that of a 358 ym p a r t i c l e s .  Therefore, by d i v i d -  ing the s p e c i f i c surface area o f 64 small p a r t i c l e s by that of one l a r g e p a r t i c l e , a r a t i o of 4 i s obtained.  T h e o r e t i c a l l y then, the 90 ym par-  t i c l e s should agglomerate 4 t i m e s more, w i t h r e s p e c t t o the o r i g i n a l p a r t i c l e s i z e , than the 358 ym p a r t i c l e s .  This i s the case at 850 and  900°C, see Figure 7.6.  e  D i v i d i n g the d g g ? d p of the 90 ym p a r t i c l e s by A  that of the 358 ym p a r t i c l e s (base c a s e ) , r a t i o s o f 3.19  and 3.96  are  o b t a i n e d f o r 900 and 850°C r e s p e c t i v e l y . At 950°C however, t h i s r a t i o i s 6.28.  This i s l a r g e r agglomeration  t o a s t r o n g e r e f f e c t of temperature  than p r e d i c t e d and i s l i k e l y  due  on the bonds between smaller p a r t i -  cles. The l a r g e degree of agglomeration  obtained when the c a t a l y s t was  used can only be explained i n terms of an e f f e c t of the c a t a l y s t on s u r f a c e o f the n o n - m e t a l l i c p a r t i c l e s . At the temperatures c a t a l y s t was  the  t e s t e d the +  i n the l i q u i d s t a t e . Moreover, the a l k a l i i o n s , L i , Na  +  +  and K may have r e a d i l y d i s s o l v e d the surface of the non-metallic p a r t i c l e s , forming phases which tended to bond the i r o n p a r t i c l e s t o g e t h e r . This i s confirmed  by e x a m i n i n g Figure 7.11.  It i s seen t h a t the non-  m e t a l l i c p a r t i c l e s between i r o n g r a i n s p o s s e s s a more rounded s h a p e ; even some d r o p l e t s are observed, which would i n d i c a t e the molten s t a t e of the c a t a l y s t . The smaller degree of agglomeration when 358  obtained f o r other v a r i a b l e s  m ore p a r t i c l e s were reduced, can be explained on the b a s i s of  the c h a r a c t e r i s t i c c o n d i t i o n s of each t e s t . The s l i g h t i n c r e a s e i n agg l o m e r a t i o n f o r Cpix/Fe  =  0*16 over the base case r e f l e c t s the enhanced  p r o b a b i l i t y that reduced grains would contact each other given t h a t  7.11  Non-metallic p a r t i c l e s between reduced i r o n g r a i n s during reduction under c a t a l y z e d c o n d i t i o n s (200x)  174  t h e r e were fewer c h a r p a r t i c l e s i n the bed.  Very l i k e l y agglomeration  would have proceeded f u r t h e r had the reduction been c o m p l e t e d .  During  r e d u c t i o n under s e g r e g a t e d - b e d c o n d i t i o n s a t 850°C, the i r o n g r a i n s would have been c l o s e r together i n the core and t h e r e f o r e c o n t a c t e d each o t h e r more e a s i l y w i t h the consequent enhanced growth of agglomerates. At 900 and 950°C however, t h i s was not observed. The a g g l o m e r a t e s o b t a i n e d under a l l c o n d i t i o n s discussed above were e a s i l y broken up by a p p l y i n g a g e n t l e p r e s s u r e . M o r e o v e r , t h e i r s i z e s s h o u l d not c a r r y dramatic consequences f o r the reduction p r o c e s s . This w i l l be f u r t h e r discussed i n Chapter 8.  175  CHAPTER 8  OVERALL DISCUSSION OF RESULTS AND PROPOSED MECHANISMS  An o v e r a l l d i s c u s s i o n o f the r e d u c t i o n k i n e t i c s i n t h e r o t a r y r e a c t o r i s p r o v i d e d i n t h i s c h a p t e r , based on the p r e l i m i n a r y d i s c u s sions given i n Sections 5.5, 6.3.4, 6.4.6, and 6.5.6. This i s o b t a i n e d by a p p l y i n g a v a i l a b l e k i n e t i c s r e l a t i o n s h i p s , f o r d i f f e r e n t rate-cont r o l l i n g mechanisms, to the d i s t i n c t s t a g e s o b s e r v e d r a t e s and gas compositions.  i n the reduction  A c t i v a t i o n energies f o r the two r e a c t i o n s ,  carbon g a s i f i c a t i o n and oxide r e d u c t i o n , are a l s o obtained and compared t o t h e v a l u e s reported i n the l i t e r a t u r e .  F i n a l l y , estimative c a l c u l a -  t i o n s are made as t o how an i n d u s t r i a l - s c a l e r o t a r y k i l n would p e r f o r m , when p r o c e s s i n g t h e m a t e r i a l s u t i l i z e d i n t h i s study.  P a r t i c l e mixing  and agglomeration c h a r a c t e r i s t i c s are i n c o r p o r a t e d i n the estimations as well.  8.1 T e s t i n g o f a v a i l a b l e k i n e t i c s r e l a t i o n s h i p s f o r the r e d u c t i o n o f i r o n - o x i d e s with carbon The complexity o f the two r e a c t i o n s , i r o n - o x i d e r e d u c t i o n and carbon  g a s i f i c a t i o n , o c c u r r i n g i n p a r a l l e l , does n o t allow a s i n g l e  mechanism t o properly d e s c r i b e t h e r e d u c t i o n path o v e r t h e r a n g e s o f t e m p e r a t u r e , gas composition, p a r t i c l e s i z e and chemical composition o f  176  p r a c t i c a l and e x p e r i m e n t a l  importance.  140  Among the k i n e t i c s subpro-  cesses e x i s t i n g i n the p a r a l l e l r e a c t i o n s scheme, shown i n F i g u r e 2.5, p r e v i o u s workers have s i n g l e d out the Boudouard r e a c t i o n and the d i f f u sion o f the gases through the reduced l a y e r o f the o x i d e p a r t i c l e s t o be r a t e c o n t r o l l i n g over a major f r a c t i o n o f the reduction process.  The  r e l a t i v e i n f l u e n c e exerted by each o f these subprocesses on t h e o v e r a l l reduction r a t e , depends p r i m a r i l y on the temperature the r e a c t i v i t y and p a r t i c l e s i z e o f each m a t e r i a l .  o f operation and on In a d d i t i o n , t h e  i n t e r m i x i n g o f t h e r e a c t i n g s o l i d s and gases and the presence o f a d i l u t i n g i n e r t gas could be important. A c c o r d i n g t o t h e c h a r a c t e r i s t i c s o f t h e present  experimental  system, gaseous d i f f u s i o n mechanisms should play b u t a minimal r o l e on the o v e r a l l reduction rate mostly owing to the p a r t i c l e s i z e s u t i l i z e d . A l s o , the p o s s i b l e i n f l u e n c e o f hydrogen on the o v e r a l l reduction can be d i s c a r d e d based on the f o l l o w i n g . According to Equation (6.12) the r e duction o f FeO could be c a r r i e d out by hydrogen thus producing water vapour.  The l a t t e r would i n t u r n r e g e n e r a t e hydrogen by the water-gas  r e a c t i o n , Equation (6.13), a l l t h i s happening w i t h i n the ore p a r t i c l e s . However, i f t h i s was the p r e v a i l i n g sequence o f r e d u c t i o n , an i n c r e a s e in the amount o f char ( r e l a t i v e to ore) and t h e r e f o r e i n t h e amount o f hydrogen present, would have increased the reduction r a t e .  This was not  observed when doubling the amount o f char during t h e e x p e r i m e n t s , c . f . Figure 6.4. Therefore, the o v e r a l l reduction rate w i l l l i k e l y be determined by the i n d i v i d u a l r e a c t i o n rates o f carbon g a s i f i c a t i o n and o x i d e r e d u c t i o n , a t d i f f e r e n t stages o f the process.  The s h i f t i n c o n t r o l l i n g  177  mechanism can be i n f e r r e d from the observed changes i n the gas composit i o n , e x e m p l i f i e d by Figure 6.9, as well as from the patterns  presented  i n the f r a c t i o n a l reduction r a t e , i l l u s t r a t e d by Figure 6.18, at d i f f e r ent extents of r e d u c t i o n .  A c c o r d i n g l y , the o v e r a l l r e d u c t i o n sequence  w i l l be t r e a t e d as three d i s t i n c t stages:  Stage I, from the beginning  of the r e a c t i o n to the end of the n o n - s t o i c h i o m e t r i c range o f w u s t i t e ; S t a g e I I , from the beginning of reduction of wustite to the end of the constant r a t e p e r i o d ; and Stage I I I , from the end o f the c o n s t a n t  rate  p e r i o d onwards. In terms of f r a c t i o n a l r e d u c t i o n , Stage I ends at f ^ = 0.273 depending on the composition of the ore and Stage II e x t e n d s from fR = 0.273 to varying extents of r e d u c t i o n , which changes with the cond i t i o n s of the system. The two chemical r e a c t i o n s , Boudouard and r e d u c t i o n , can be cons i d e r e d to occur n e i t h e r i n s e r i e s nor i n p a r a l l e l s i n c e both  chemical  p o t e n t i a l s are b e i n g generated by the two r e a c t i o n s o c c u r r i n g s i m u l t a neously.  Therefore, the Boudouard r e a c t i o n having the h i g h e s t a c t i v a -  t i o n e n e r g y o f the two, the o v e r a l l process can be assumed to be cont r o l l e d by t h i s r e a c t i o n . As the r e d u c t i o n p r o c e s s a d v a n c e s however, t h e r e w i l l be l e s s wustite a v a i l a b l e and the reduction r e a c t i o n becomes i n c r e a s i n g l y more important, t h e r e f o r e rate c o n t r o l l i n g s i n c e t h e r e i s always an excess of carbon  8.1.1  present.  Boudouard r e a c t i o n as r a t e c o n t r o l l i n g mechanism When the Boudouard r e a c t i o n i s assumed to be the  rate-determin-  i n g s t e p i n the o v e r a l l r e d u c t i o n process, as i s assumed i n t h i s case f o r Stages I and I I , the r e s u l t s o f f r a c t i o n a l reduction can be analyzed  178  •in terms o f the f r a c t i o n a l consumption o f carbon.  Therefore, by c o n s i -  d e r i n g t h e Boudouard r e a c t i o n to obey f i r s t order kinetics,91,96-97 r a t e o f carbon consumption  dW - — dt  t n e  i s given by  = kg W  (8.1)  c  where W i s the mass o f carbon remaining and kg i s the r a t e constant f o r c  - 1  the Boudouard r e a c t i o n i n m i n .  I n t e g r a t i n g Equation (8.1) and ex-  p r e s s i n g i t i n terms o f f r a c t i o n a l conversion o f carbon, f , y i e l d s c  In (1 - f ) = - K t c  B  (8.2)  Thus, by p l o t t i n g l n (1 - fr;) vs t t h e value o f the rate constant i s obtained.  This i s presented i n F i g u r e s 8.1 through 8.6 f o r t h e d i f f e r -  ent c o n d i t i o n s s t u d i e d ; a summary o f the r a t e constants obtained i s g i v en i n T a b l e XXII.  In those f i g u r e s , the f r a c t i o n a l r e d u c t i o n , f ^ , c o r -  responding to the departure from l i n e a r i t y i s i n d i c a t e d . Care must be e x e r c i s e d i n a s s e s s i n g t h i s p o i n t o f departure, given the l o g a r i t h m i c nature o f the p l o t s .  When comparing  t h e s e f i g u r e s w i t h t h o s e o f gas  c o m p o s i t i o n , F i g u r e s 6.9, 6.11, 6.13, 6.15, 6.20 and 6.22, the c l o s e c o r r e s p o n d e n c e between the values o f fR a t the departure from l i n e a r i t y and t h o s e a t which, t h e Pco/ C0 p  r a t 1 2  ° s t a r t e d to i n c r e a s e i s e v i d e n t .  Taking the base case as an example, the p o i n t s o f departure from l i n e a r i t y i n Figure 8.1 occur a t f o f 0.59, 0.61 and 0.74, a t 950, 900 and R  179  P l o t o f l n ( l - f ) vs t f o r the base case experiments (Boudouard c o n t r o l ) . c  Time  (min)  P l o t o f In (1-fr,) vs t f o r the s t o i c h i o m e t r i c C p i / F e experiments (Boudouard c o n t r o l ) . X  181  8.3  P l o t of In ( 1 - f r ) vs t f o r the f i n e r p a r t i c l e s experiments (Boudouard control) .  182  8.4  P l o t of l n ( 1 - f r ) vs t f o r the segregated bed experiments (Boudouard c o n t r o l ) .  183  I  _  900°C, • cat Catalysed 8  -1.5  2.0  8.5  50  N flushed 2  100 150 Time (min)  200  P l o t o f In (1-frj) vs t f o r the c a t a l y z e d and ^ - f l u s h e d experiments (Boudouard c o n t r o l )  250  184  8.6  P l o t of l n ( 1 - f r ) vs t f o r the l i g n i t e reductant experiments (Boudouard c o n t r o l ) .  185  TABLE XXII.  REACTION RATE CONSTANTS FOR STAGES I THROUGH I I I DURING REDUCTION EXPERIMENTS  Conditions  Base case  S t o i c h i o m e t r i c Cp-j /Fe x  Finer particles  Segregated bed  Lignite  Catalyzed  Temp [°C]  Stage I k (10)2  950 900 850 950 900 850 950 900 850 950 900 850 950 900 850 900 800  2.28 1.51 0.62 2.71 1.62 0.48 3.71 1.64 0.50 1.55 1.05 0.32 1.92 1.11 0.49 2.60 1.11  B  Stage II k (10)2 B  4.85 2.05 0.73 5.06 2.26 0.79 8.72 2.10 0.59 3.02 1.53 0.41 3.87 1.30 0.41 4.75 0.39  Stage I I I k (10)3 R  8.75 5.40 3.26  -  5.70 3.72 2.23  5.13 2.00  186  850°C r e s p e c t i v e l y . The c o r r e s p o n d i n g values of f ^ f o r the change i n gas composition, shown i n Figure 6.9, are 0.45, 0.60 and 0.70.  The r e -  l a t i v e l y l a r g e d i f f e r e n c e observed at 950°C i s due to the closeness of the p o i n t s i n Figure 8.1 whereby the p o i n t of d e p a r t u r e from  linearity  cannot be seen very c l e a r l y . The r a t e constants of Stage II w i l l be analyzed f i r s t , i n terms of a c t i v a t i o n energy, f o r the f o l l o w i n g reason.  The r e d u c t i o n o f wus-  t i t e a c c o u n t s f o r over two t h i r d s of the t o t a l , and represents the most d i f f i c u l t step i n the Fe203/Fe304/'FeO' r e d u c t i o n sequence. The a c t i v a t i o n energy, AE, f o r the Boudouard r e a c t i o n i s o b t a i n 4  ed by c o n s t r u c t i n g Arrhenius p l o t s of l n Kg vs 10 /T.  These are shown  i n F i g u r e 8.7 f o r c o n d i t i o n s o f the base c a s e , the s t o i c h i o m e t r i c Cp-j /Fe, r e d u c t i o n w i t h l i g n i t e and the c a t a l y z e d experiments  during  x  Stage I I . The a c t i v a t i o n energy f o r the Boudouard r e a c t i o n with F o r e s t burg c o a l i s e s t a b l i s h e d to be about 224 KJ/mole (53.5 kcal/mole)  from  the base case and s t o i c h i o m e t r i c Cp-j /Fe experiments.  well  x  This agrees  with the reported v a l u e s , i n C h a p t e r 2, f o r s i m i l a r c o a l c h a r s , and catalyzed cokes.  The s l i g h t l y higher a c t i v a t i o n energies obtained f o r  l i g n i t e and the c a t a l y z e d r e a c t i o n , 264 KJ/mole (63.1 kcal/mole) and  257  KJ/mole (61.4 kcal/mole) r e s p e c t i v e l y , can be r e a d i l y explained i n terms of the c a t a l y s i s of the Boudouard r e a c t i o n . The c a t a l y t i c e f f e c t e x e r t e d by a l k a l i oxides and i r o n on the Boudouard r e a c t i o n has been recognized.  Since both components are p r e -  sent i n most of the r e d u c t i o n experiments, a l k a l i oxides i n the coal ash and i r o n as the product of the r e a c t i o n , the o v e r a l l c a t a l y t i c has to be the a d d i t i o n of the e f f e c t s of each.  effect  Fresh i r o n i s present i n  8.7  Arrhenius p l o t s f o r base case, s t o i c h i o m e t r i c Cp-jx/Fe, l i g n i t e reductant and c a t a l y z e d e x p e r i ments. Stage II (Boudouard c o n t r o l ) .  188  i n c r e a s i n g l y l a r g e r q u a n t i t i e s as the r e d u c t i o n proceeds.  However, the  p o i n t s o f c o n t a c t with the coal p a r t i c l e s a r e l i m i t e d t o t h e e x t e r n a l s u r f a c e o f the p a r t i c l e s ; furthermore, i t has been e s t a b l i s h e d t h a t the i r o n c a t a l y s t i s d e a c t i v a t e d more e a s i l y a t lower t e m p e r a t u r e s , o f t h e order of 900°C.  141  The a l k a l i oxides, on the other hand, although pre-  sent as a very small f r a c t i o n o f the coal ash, are much more f i n e l y d i v i d e d , and evenly d i s t r i b u t e d through the coal p a r t i c l e s .  Therefore, i n  s p i t e o f the r a t h e r d i f f e r e n t amounts o f each o f the c a t a l y s t compon e n t s , t h e c a t a l y t i c e f f e c t due to the a l k a l i oxides i n the coal ash i s l i k e l y the most important. ing.  T h i s hypothesis i s supported by the f o l l o w -  F i r s t l y , a s t r o n g c a t a l y t i c e f f e c t o f i r o n on the o x i d a t i o n o f  g r a p h i t e has been observed,142  D  U  t the i r o n had t o be impregnated i n t o  the carbon by using a s a l t s o l u t i o n (20 percent FeCl3 i n H 2 O ) . When the i r o n was i n simple c o n t a c t w i t h t h e g r a p h i t e t h e r a t e i n c r e a s e d o n l y slightly.  In the same study, the c a t a l y t i c e f f e c t o f i r o n on more reac-  t i v e forms o f carbon was l e s s s i g n i f i c a n t ; t h i s n e g l i g i b l e e f f e c t a l s o was c o r r o b o r a t e d by Fruehan,96 when reducing i r o n oxides with coke and coal c h a r s .  Secondly, the a c t i v a t i o n energy f o r l i g n i t e g a s i f i c a t i o n i s  about 10 p e r c e n t h i g h e r than t h a t f o r t h e F o r e s t b u r g  sub-bituminous  c o a l , c . f . , F i g u r e 8.7. On examination o f the ash contents and composit i o n s o f each c o a l c h a r , shown i n Tables XVII and XVIII, and by c o n s i dering t h a t 313 g o f Forestburg coal char and 272 g o f l i g n i t e char were used t o o b t a i n a C p i x / F e o f 0.32 i n each case, i t can be a s c e r t a i n e d t h a t there was about 60 percent more K 0 when using Forestburg coal char 2  and about 60 percent more Na 0 when using l i g n i t e . 2  However, the K 0 i s 2  189  a c o n s i d e r a b l y stronger c a t a l y s t than N a 2 0  102  and t h e r e f o r e enhances the  Boudouard r e a c t i o n i n the Forestburg c o a l . The A r r h e n i u s p l o t s f o r Stage II (Boudouard c o n t r o l ) , f o r the segregated bed and f i n e r p a r t i c l e s are compared t o the base case i n F i g ure 8.8.  The s p r e a d o f t h e p o i n t s f o r t h e s e g r e g a t e d  b e d , and the  s l i g h t l y l a r g e r a c t i v a t i o n energy obtained o f 232.8 KJ/mole (55.6 k c a l / mole) can both be e x p l a i n e d by the f o l l o w i n g . F i r s t l y , i t can be seen i n Figures 6.15 and 8.4 t h a t c o n t r o l by t h e Boudouard r e a c t i o n s o l e l y was exerted only during a small f r a c t i o n o f the t o t a l r e d u c t i o n . f o r e , the e v a l u a t i o n o f the rate constants was more u n c e r t a i n . l y , the de-mixing  ThereSecond-  s t a t e o f the s o l i d s bed hindered the small c a t a l y t i c  e f f e c t o f i r o n . The r e l a t i v e l y high a c t i v a t i o n energy obtained f o r t h e experiments  w i t h f i n e r p a r t i c l e s o f 277.8 KJ/mole (66.2 kcal/mole) can  only be explained i n terms o f the coal p a r t i c l e s i z e .  A t 90 ym, more  ash p a r t i c l e s may have r e a d i l y been l i b e r a t e d from the char and t h e r e fore t h e i r c a t a l y t i c e f f e c t on the Boudouard r e a c t i o n diminished. F i n a l l y , by o b t a i n i n g the a c t i v a t i o n energy f o r Stage I o f r e duction, when m e t a l l i c i r o n was not y e t present, the separate e f f e c t o f the ash can be estimated.  catalytic  The Arrhenius p l o t i s shown i n F i g -  ure 8.9 and the a c t i v a t i o n energy obtained i s 231.1 KJ/mole (55.2 k c a l / mole).  T h i s v a l u e i s only s l i g h t l y higher (about 3 percent) than that  f o r Stage II when m e t a l l i c i r o n i s already present.  This  corroborates  the s t r o n g e r c a t a l y t i c e f f e c t exerted by the coal ash when compared to that o f m e t a l l i c i r o n and that the Boudouard r e a c t i o n a l s o c o n t r o l s t h e Stage I o f r e d u c t i o n .  The l a t t e r would be expected since the a c t i v a t i o n  A r r h e n i u s p l o t s f o r base case, segregated bed and f i n e r p a r t i c l e s experiments. Stage II (Boudouard c o n t r o l )  191  energies  f o r the Fe203-Fe3U4 and Fe3U4 to 'FeO' reduction by CO are 69  KJ/mole (16.5 k c a l / mole) and 73 KJ/mole (17.1  kcal/mole) respective-  l .143 y  The l e v e l l i n g o f f of the temperature dependence of KB i n Stage I 9 4  a t the h i g h e r t e m p e r a t u r e , t h a t has been observed bef o r e , » due, a t l e a s t i n p a r t , to gas d i f f u s i o n e f f e c t s i n t o both the  9 6  >  99  is  iron-oxide  and c o a l char p a r t i c l e s which a t low e x t e n t s of r e a c t i o n are not so porous as l a t e r i n the r e a c t i o n .  This s l i g h t d i f f u s i o n e f f e c t can  a s c e r t a i n e d f u r t h e r by o b s e r v i n g i n Figure 8.8 the stronger e x i s t i n g during the experiments under s e g r e g a t e d  be  curvature  bed c o n d i t i o n s which  i n v o l v e d l a r g e r coal p a r t i c l e s .  8.1.2  Reduction r e a c t i o n as rate c o n t r o l l i n g mechanism It has been shown i n the previous s e c t i o n that the Boudouard r e -  a c t i o n does not c o n t r o l the o v e r a l l r e d u c t i o n process over the f u l l range of r e d u c t i o n ; the extent of t h i s c o n t r o l depends on the mental c o n d i t i o n s .  experi-  Therefore, i n the l a t e r stages of the process,  the  r e s i s t a n c e s of the two r e a c t i o n s act i n s e r i e s and the o v e r a l l r a t e o f reduction declines.  A k i n e t i c equation f o r reduction control i s given  i n terms of f r a c t i o n a l reduction by145 1 - (1 - f ) R  1 / 3  = k  R  t  where KR i s the rate constant i n m i n  (8.3) -1  f o r the reduction of 'FeO' by  since i n Stage III wustite w i l l be the o n l y i r o n - c o n t a i n i n g  species.  Thus, by p l o t t i n g l - ( l - f R ) l / 3 vs t the values of the rate contants be obtained.  CO,  can  This i s shown i n Figures 8.10 through 8.12 and a summary  •  8.9  Arrhenius p l o t s f o r base case, s t o i c h i o m e t r i c Cp-j /Fe and segregated bed experiments. Stage I (Boudouard control) x  193  o f the v a l u e s of KR i s given i n Table XXII.  It can be seen t h a t a r e a -  s o n a b l y good l i n e a r f i t i s obtained i n going from the fR value f o r the end o f the Boudouard c o n t r o l by i t s e l f up to above fR = 0.80 i n a l l cases.  The segregated-bed  c o n d i t i o n , Figure 8.11, and the c a t a l y z e d ex-  periments, Figure 8.12, have been chosen as examples because they showed a stronger departure from l i n e a r i t y , when the Boudouard r e a c t i o n  was  c o n s i d e r e d to be the rate l i m i t i n g step, and t h e r e f o r e t h e i r rate const a n t s could be evaluated with more ease. The a c t i v a t i o n energy f o r the reduction r e a c t i o n i s a l s o o b t a i n 4  ed through the c o n s t r u c t i o n of Arrhenius p l o t s of In KR VS 10 /T. are shown i n Figure 8.13. 'FeO'  These  The energy of a c t i v a t i o n f o r the reduction of  by CO i s found to be 116.4 KJ/mole (27.8 kcal/mole) f o r the base  case and s l i g h t l y lower f o r the segregated-bed  condition.  f a l l s w i t h i n the range reported by Themelis and G a u v i n  146  This value of 63 to 126  KJ/mole (15 to 30 kcal/mole), and match very c l o s e l y the value by Y u *  4 5  o f 113 K J / m o l e (27 kcal/mole).  reported  The a c t i v a t i o n energy f o r the  c a t a l y z e d r e a c t i o n i s about 15 percent lower and a l s o f i t s w e l l w i t h i n the reported values.  8.2 Overall view of the r e d u c t i o n process It has been shown i n the preceding s e c t i o n s t h a t the Boudouard r e a c t i o n c o n t r o l s the o v e r a l l r a t e o f r e d u c t i o n , a t temperatures  of  900°C and lower and under well-mixed c o n d i t i o n s , up to extents o f f r a c t i o n a l reduction which vary from 0.50 to 0.80.  At 950°C, however, the  194  8.10  1 / 3  P l o t of 1 - ( l - f ) vs t f o r base case experiments. Stage III (Reduction c o n t r o l )  8.11  1  3  P l o t of 1 - ( 1 - f ) / vs t f o r segregated bed e x p e r i ments. Stage III (Reduction c o n t r o l )  196  8.12  1  3  P l o t of 1 - ( 1 - f ) / vs t f o r c a t a l y z e d experiments. Stage III (Reduction c o n t r o l )  197  T(°C) -3  950  900  850  1  1  1 AE o o  800 1  kJ/mole kcal/mole 116.4 27.8 108.9 26.0 98.1 23.4 -  —  jvBase  -6 Segregated  \. Catalyzed  -7 ao  1  1  8.5  9.0  9.5  I0 /T(K ) 4  8.13  _I  Arrhenius p l o t s f o r base case, segregated bed and c a t a l y z e d experiments. Stage III (Reduction c o n t r o l )  198  e x t e n t o f c o n t r o l by t h e Boudouard r e a c t i o n does not go beyond fR = 0.50. This i s but an obvious p o s i t i v e e f f e c t o f temperature on t h e k i n e t i c s o f the c a r b o n - g a s i f i c a t i o n r e a c t i o n . The reduction r e a c t i o n , on the other hand, p l a y s a more import a n t r o l e i n the second part o f the o v e r a l l reduction r e a c t i o n up to fR values o f about 0.95, a t temperatures o f 900°C and lower.  At 950°C t h e  c o n t r o l by the reduction r e a c t i o n s t a r t s e a r l i e r (fR = 0.50) l i k e l y due again to the strong e f f e c t o f temperature on the k i n e t i c s o f the Boudouard r e a c t i o n .  A t t h i s temperature however, the i n c r e a s i n g control ex-  e r t e d by t h e r e d u c t i o n  r e a c t i o n i s somewhat more uncertain above fR =  0.85 ( i n d i c a t e d i n Figures 8.10 and 8.13). on t h e f o l l o w i n g c o n s i d e r a t i o n s .  This can be e x p l a i n e d  based  At that f r a c t i o n a l reduction the par-  t i a l l y reduced p a r t i c l e s already must have agglomerated.  Agglomeration  o f f i n e r e d u c e d p a r t i c l e s has been demonstrated to s e t i n from the beginning of the wustite reduction.129  The i n c r e a s e i n p a r t i c l e s i z e  would produce an increase not only of the d i f f u s i o n e f f e c t s , but also o f the mass t r a n s f e r e f f e c t s through an increased boundary l a y e r thickness around the agglomerated p a r t i c l e s , because o f t h e i r l a r g e r s i z e .  The  boundary l a y e r e f f e c t would be f u r t h e r enhanced by the lower r e a c t i o n r a t e , hence lower gas v e l o c i t y , present a t t h i s l a s t stage o f r e d u c t i o n . T h i s i n t e r p r e t a t i o n i s supported by the r e l a t i v e l y sharper drop observed i n the reduction rate of f i n e r p a r t i c l e s , Figure 6.12, which e x h i b i t ed the g r e a t e s t agglomeration, Figure 7.1. The c a t a l y z e d experiments aided i n c l a r i f y i n g the r e a c t i o n mechanisms o f t h e ' u n c a t a l y z e d  1  r e d u c t i o n process.  The c a t a l y s t presence  a c c e l e r a t e d the reduction through i t s e f f e c t on the Boudouard  reaction;  199  reasonable  r e d u c t i o n was thus o b t a i n e d  even a t 800°C.  However, the  u t i l i z a t i o n o f t h i s c a t a l y s t i n an i n d u s t r i a l process i s hindered by i n h e r e n t o p e r a t i o n a l problems, as w i l l be discussed i n the f o l l o w i n g section.  8.3 Estimative c a l c u l a t i o n s f o r the operation of an i n d u s t r i a l - s i z e r o t a r y k i l n The reduction o f iron-oxide with carbon i n an i n d u s t r i a l r o t a r y k i l n i n v o l v e s phenomena o f p a r t i c l e mixing, gas flow, heat t r a n s f e r and r e a c t i o n k i n e t i c s . These phenomena are s t r o n g l y i n t e r r e l a t e d and can change a l o n g t h e r e a c t i o n path; t h i s renders an i n t e g r a t e d a n a l y s i s o f the process d i f f i c u l t .  For i n s t a n c e , t h e d e g r e e o f m i x i n g i n t h e bed  not o n l y w i l l a f f e c t t h e r e a c t i o n k i n e t i c s but a l s o the heat t r a n s f e r mechanisms w i t h i n the bed; heat t r a n s f e r w i l l a f f e c t t h e a g g l o m e r a t i o n growth and t h e r e b y  the reduction k i n e t i c s and the p a r t i c l e s mixing be-  haviour; and so on. M i x i n g o f i r o n o r e and c o a l p a r t i c l e s together, a t the s i z e s u t i l i z e d i n t h i s work, had n o t been s t u d i e d b e f o r e .  As d i s c u s s e d i n  Section 5.5, not only the density and s i z e r a t i o s are important but a l s o other f a c t o r s exert  an i n c r e a s i n g e f f e c t  as the coal  size  decreases.  These are the f r i c t i o n c o e f f i c i e n t and p o s s i b l y e l e c t r o s t a t i c forces between the p a r t i c l e s . Since these are a l l i n t e n s i v e p r o p e r t i e s , t h e i r e f f e c t i n a l a r g e r s i z e o p e r a t i o n i s l i k e l y to p e r s i s t . However, one f u r t h e r f a c t o r has t o be considered with respect t o p a r t i c l e m i x i n g i n an i n d u s t r i a l o p e r a t i o n .  T h i s i s the p a r t i a l f l u i d i z a t i o n o f the bed  200  caused by the p r o p o r t i o n a l l y l a r g e r flowrates of gases evolved.  The bed  surface area to volume r a t i o , A/V f o r i n s t a n c e , i n a 2.1 m diameter 1  i s about 4 n r * whereas that f o r the present study i s 48 n r .  kiln  Therefore,  tendency to f l u i d i z e the top l a y e r s of the bed would be l a r g e r i n an i n d u s t r i a l k i l n ; f l u i d i z a t i o n of t h i s kind has been observed b e f o r e .  6 6  Al-  so, there would be a propensity f o r the c o a l p a r t i c l e s t o move t o the upper l a y e r s i n the bed, e s p e c i a l l y as the r e a c t i o n s proceed and the coal p a r t i c l e s decrease i n s i z e .  The f l u i d i z a t i o n t e n d e n c y c o u l d be  d i m i n i s h e d by u s i n g s l i g h t l y l a r g e r coal p a r t i c l e s , since the minimum f l u i d i z a t i o n v e l o c i t y i s i n f l u e n c e d by (d~ )3. c  Thus, during the f i r s t  stages of r e d u c t i o n where l a r g e volumes o f gases are g e n e r a t e d ,  the  l a r g e r coal p a r t i c l e s would not be entrained as e a s i l y . As the r e a c t i o n proceeds, the coal becomes f i n e r but the gas f l o w r a t e s then are and the reduced p a r t i c l e s w i l l have s t a r t e d to agglomerate.  lower  These com-  bined f a c t o r s may provide a well mixed bed along the k i l n . The s i z e of operation against which the r e s u l t s of the base case from t h i s work are compared below, i s t h a t of a p i l o t - p l a n t operation of Stelco Inc.  7  The k i l n , 35 m long by 2.1 m i n t e r n a l diameter, i s capable  of reducing to 95 percent completion 113 t o n s o f h e m a t i t e  p e l l e t s per  day a t 14 p e r c e n t f i l l i n g (or 200 ton/day at 25 percent f i l l i n g ) , at a 1000°C and a Cp-j /Fe o f 0.43. x  Furthermore,  the movement of m a t e r i a l s  w i l l be assumed to be i n plug flow. In order to assess the throughput of a 2.1 x 35 m k i l n using the present m a t e r i a l , i t s o p e r a t i o n a l parameters must be d e t e r m i n e d vance.  i n ad-  F i r s t , w i t h r e g a r d to r o t a t i o n a l speed, a c r i t e r i o n has been  developed to scale-up k i l n s based on the modified Froude number, ^  2  R/g,  201  and the r a t i o of diameters of the model and prototype r a i s e d to the oneh a l f power, as follows133. '  2 U  1/2  R  /  R  D  1/2 (8.4)  M  By s u b s t i t u t i n g numerical values i n Equation 8.4, a r o t a t i o n a l speed of 0.9 r.p.m. would be necessary i n the l a r g e k i l n i n o r d e r t o o b t a i n the same bed behaviour as t h a t i n the l a b o r a t o r y k i l n at 7 r.p.m.  Secondly,  a v a i l a b l e r e l a t i o n s h i p s e x i s t to determine the average r e t e n t i o n t i m e , T^,  i n reduction kilns,146 such as 0.148 T  where l  r  k -  K,  mV (8.5)  SDN  i s the angle of repose of the s o l i d s mixture, L i s the k i l n  >m  length i n m, S i s the k i l n slope i n percent, D i s the k i l n diameter i n m and N i s the number of r e v o l u t i o n s per minute.  Thus, by s u b s t i t u t i n g the  k i l n dimensions and the angle of repose of the m a t e r i a l s from T a b l e in Equation  XV  ( 8 . 5 ) , a r e t e n t i o n time of 360 min i s obtained f o r a k i l n  slope of 4.5 percent.  This i s the time r e q u i r e d t o reduce  the c o n c e n -  t r a t e 95 p e r c e n t a t 850°C, as shown i n Figure 6.8, by a l s o c o n s i d e r i n g t h a t the reduction zone i n the k i l n comprises about one h a l f o f the t o tal  length.  D i v i d i n g the volume of the bed, of 17 m  3  f o r 14 percent 3  f i l l i n g , by the r e t e n t i o n t i m e , a volumetric flowrate of 68 m /day i s obtained.  The measured density of the i n i t i a l ore/char mixture i s  1.28  3  t o n / m and the d e n s i t y o f the p r o d u c t s a f t e r 95 percent reduction i s 3  3  2.43 ton/m ; t h e r e f o r e , an average value of 1.86 ton/m can be c o n s i d e r ed.  Then by m u l t i p l y i n g the volumetric flowrate by the average d e n s i t y ,  202  a k i l n t h r o u g h p u t o f 126 ton/day i s o b t a i n e d ; t h i s i s e q u i v a l e n t to 83 t o n s o f o r e p r o c e s s e d per day a t a Cp-j /Fe o f 0.32. By r e c a l l i n g t h a t x  the same s i z e o f k i l n i s capable o f p r o c e s s i n g 113 t o n o f p e l l e t s p e r day a t 1000°C, t h e t h r o u g h p u t o f t h e k i l n p r o c e s s i n g concentrate a t 850°C i s about 25 percent lower.  However, the same c a l c u l a t i o n s f o r r e -  d u c t i o n o f the concentrate a t 900°C show t h a t a throughput o f 125 t o n / day can be e a s i l y achieved, which i s about 10 p e r c e n t h i g h e r than t h a t of the k i l n p r o c e s s i n g p e l l e t s . On b a s i s o f the above e s t i m a t e s , s i m i l a r throughputs can be obt a i n e d by t r e a t i n g f i n e ores and p e l l e t s .  The c l e a r advantage i n t h e  f o r m e r c a s e i s the c o n s i d e r a b l y lower temperature o f o p e r a t i o n with the consequent decrease i n o p e r a t i o n a l problems, e s p e c i a l l y lower a c c r e t i o n formation and waste-gas f l o w r a t e s .  203  CHAPTER 9  SUMMARY AND CONCLUSIONS  The k i n e t i c s o f d i r e c t r e d u c t i o n o f a commercial  unagglomerated  i r o n o r e , w i t h low-rank coal c h a r s , have been i n v e s t i g a t e d i n the temperature range o f 800-950°C (1073-1223 K). The r e a c t o r u t i l i z e d was a l a b o r a t o r y - s i z e b a t c h r o t a r y k i l n , capable o f p r o c e s s i n g about 1 k i l o gram o f ore/char mixture.  The o v e r a l l r e d u c t i o n k i n e t i c s were f o l l o w e d  by measurement o f gas a n a l y s i s and gas f l o w r a t e s . The extreme mean part i c l e s i z e s , 358 and 90 vm, from t h e f u l l s i z e range o f t h e i r o n o r e concentrate were reduced with three d i f f e r e n t coal char p a r t i c l e s i z e s . The mixing o f the char/ore mixture was d e t e r m i n e d a t room temperature i n the same r e a c t o r under d i f f e r e n t c o n d i t i o n s p r i o r to the r e duction experiments.  The c o n d i t i o n s s t u d i e d were:  r a t i o from 0.5 t o 4.0; r o t a t i o n a l  char to ore s i z e  speed o f 5 and 15 r.p.m.; percent  f i l l i n g o f 12 and 20 percent and f i x e d c a r b o n - t o - i r o n r a t i o from 0.45 t o 0.84. The mixing s t u d i e s y i e l d e d the f o l l o w i n g :  i)  u  With an ore p a r t i c l e s i z e of 254 m and l a r g e r , t h e degree o f bed mixing depends almost completely on the c o a l - t o - o r e s i z e r a t i o ; best mixing i s achieved a t a r a t i o o f 1 and s m a l l e r .  204  ii)  At ore p a r t i c l e s i z e s s m a l l e r than 254 ym, the degree of bed mixing i s b e l i e v e d to be a f f e c t e d more s t r o n g l y by other f a c t o r s such as e l e c t r o s t a t i c and f r i c t i o n f o r c e s between the coal particles.  Good mixing i s obtained even at a c o a l - t o - o r e s i z e r a -  t i o of 4 f o r 90 ym ore p a r t i c l e s , iii)  Rotational speed, percent loading and f i x e d - c a r b o n - t o - i r o n r a t i o have n e g l i g i b l e e f f e c t s on the degree o f m i x i n g w i t h i n  the  ranges t e s t e d and f o r t h i s r e a c t o r s i z e , iv)  The bed m o t i o n c h a r a c t e r i s t i c s were a l s o d e l i n e a t e d .  Rolling  motion predominated a t higher c o a l - t o - o r e s i z e r a t i o f o r l a r g e r p a r t i c l e s s i z e whereas slumping was obtained with a smaller coal p a r t i c l e s i z e ; bed m o t i o n depended a l m o s t c o m p l e t e l y  on c o a l  particle size.  The m i x i n g c o n d i t i o n s were f u r t h e r t e s t e d i n the reduction experiments. At a f i x e d temperature and r o t a t i o n a l speed, no i n c r e a s e i n the t i o n r a t e was o b s e r v e d  reduc-  by v a r y i n g the f i x e d carbon-to-iron r a t i o from  0.32 to 0.64; a slower rate was obtained a t a value of 0.16.  At a f i x -  ed t e m p e r a t u r e and f i x e d carbon-to-iron r a t i o s of 0.16 and 0.64, no improvement i n the reduction rate was obtained by i n c r e a s i n g the r o t a t i o n al speed from 7 to 20 r.p.m..  A bed with 7 percent f i l l i n g y i e l d e d a  f a s t e r reduction rate i n i t i a l l y ,  over a bed w i t h 14 p e r c e n t  filling.  This was due to combined e f f e c t s of mixing and heat t r a n s f e r . Having d e l i n e a t e d the best mixing c o n d i t i o n s , r e d u c t i o n e x p e r i ments were c a r r i e d out to d e t e r m i n e t h e r a t e c o n t r o l l i n g steps, and t h e i r energies of a c t i v a t i o n , comprising the o v e r a l l r e d u c t i o n  process.  205  The P c o / C 0 2 p  r a t 1 0  P  r o v e  d  t o  D e  a  powerful tool e l u c i d a t i n g the r a t e  c o n t r o l l i n g s t e p . The o v e r a l l r e d u c t i o n p r o c e s s was found t o be c o n t r o l l e d up to 0.5 to 0.8 f r a c t i o n a l r e d u c t i o n by the Boudouard r e a c t i o n , depending on the p a r t i c l e s i z e and temperature; from then on, the o v e r a l l r e a c t i o n was c o n t r o l l e d e s s e n t i a l l y by the r e d u c t i o n r e a c t i o n . The a c t i v a t i o n energies obtained were 224 kJ/mole m o l e ) f o r the Boudouard  (53.5 k c a l / -  r e a c t i o n , using sub-bituminous coal char, and  264 kJ/mole (63.1 k c a l / m o l e ) u s i n g l i g n i t e c o a l c h a r .  These v a l u e s  agree w e l l w i t h p r e v i o u s works and c o r r e s p o n d to t h a t of a c a t a l y z e d r e a c t i o n ; t h i s was c o r r o b o r a t e d by a r t i f i c i a l l y c a t a l y z i n g the Boudouard r e a c t i o n with a mixture of a l k a l i ( L i , Na and K) carbonates.  The energy  of a c t i v a t i o n obtained i n the r e d u c t i o n of w u s t i t e by CO was 116.4 k j / mole (27.8 k c a l / m o l e ) ; t h i s a l s o agrees with p r e v i o u s l y reported v a l u e s . The c a t a l y t i c e f f e c t of the coal ash, e s p e c i a l l y due to the K 0 2  content, on the Boudouard r e a c t i o n , was found to be much l a r g e r than the r e s p e c t i v e e f f e c t of m e t a l l i c i r o n produced by the r e d u c t i o n . In a d d i t i o n , the presence of a d i l u e n t i n e r t gas blown over the bed, was  found  to e x t e n d the f r a c t i o n a l r e d u c t i o n over which Boudouard c o n t r o l i s exerted. The 90 urn ore p a r t i c l e s were found to agglomerate much more than those of 358 ym s i z e .  The main c a u s e o f a g g l o m e r a t i o n , i n the non-  c a t a l y z e d e x p e r i m e n t s , was d e t e r m i n e d by SEM o b s e r v a t i o n s to be the whisker i r o n growth on the s u r f a c e of the reduced p a r t i c l e s ; t h i s agrees with p r e v i o u s l y reported studies.  The a d d i t i o n of a c a t a l y s t a l s o i n -  creased the agglomeration by p a r t i a l l y m e l t i n g the ash and gangue materials.  In n e i t h e r case d i d agglomeration r e t a r d the r e d u c t i o n r a t e to a  206  considerable extent.  No a c c r e t i o n growth was observed on the r e a c t o r  w a l l , l i k e l y as a consequence o f the low temperature  of o p e r a t i o n .  F i n a l l y , e s t i m a t i v e c a l c u l a t i o n s showed t h a t s i m i l a r  throughputs  can be obtained by processing the unagglomerated c o n c e n t r a t e , as comp a r e d t o o p e r a t i o n s which u t i l i z e indurated hematite p e l l e t s under the same c o n d i t i o n s .  However, the advantage o f using c o n c e n t r a t e s  l i e s on  the f a c t t h a t t h e p r o c e s s can be o p e r a t e d a t a temperature  at least  100°C lower.  207  LIST OF REFERENCES  1  Venkateswaran, V., & Brimacombe, J.K., "Mathematical Model of the SL/RN D i r e c t R e d u c t i o n P r o c e s s , " Met. T r a n s . 8B(3):387-398, (1977).  2  Lownie, H.W., J r . , " P r o s p e c t s f o r t h e F u t u r e o f D i r e c t Reduced I r o n " i n D i r e c t Reduced I r o n — T e c h n o l o g y and Economics of Production and Use, ISS-AIME, pp. 202-211, (1980).  3  Schnabel, W., Husain, R., & Schlebusch, D.W., " F l e x i b i l i t y of SL/RN Coal Based D i r e c t R e d u c t i o n i n R e s p e c t o f Raw M a t e r i a l s and F u e l s , " MPT-Metall. 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(1984), pp. 28.  46  Schlebusch, D., "The Coal Based SL/RN D i r e c t R e d u c t i o n P r o c e s s , " L u r g i Symposium on D i r e c t Reduction, Moscow, March (1984), pp. 38.  47  S c h n a b e l , W., Schlebusch, D., & Elsenheimer, G., "SL/RN Coal-based D i r e c t R e d u c t i o n — T h e S t a t e of the A r t , " P r o c . 42nd Ironmak. Conf., A t l a n t a , U.S.A., A p r i l (1983), pp. 163-170.  48  Kanda, Y., Toyozawa, H., Yamada, Y., & Nakamura, M., " D i r e c t Reduct i o n Process f o r Iron I n d u s t r i e s Waste F i n e s , " ISS-AIME Ironmak. 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An Introduction to Coal Technology. demic Press, (1979).  139  P o l l a r d , J.H. A Handbook of Numerical and S t a t i s t i c a l Techniques. Cambridge: Cambridge U n i v e r s i t y Press, (1977).  140  Ross, H.U. "Physical Chemistry," In D i r e c t Reduced Iron-Techno!ogy and Economics o f P r o d u c t i o n and Use, ISS-AIME, ppTI 9-26,  New York: Aca-  (1980).  141  Walker, P.L., S h e l e f , M., & Anderson, R.A. Chemi s t r y and Physi cs o f C a r b o n , V o l . 4, pp. 287-383. New Y o r k : M a r c e l Dekker, (1968).  142  Turkdogan, E.T., & V i n t e r s , J . V .  Carbon 1 0 ( 1 ) : 9 7 - l l l , (1972).  143  Yu, K.O., & G i l l i s , P.P., "Mathematical S i m u l a t i o n of D i r e c t Reduct i o n , " Met. Trans. 12B(3):111-120, (1981).  218  144  L e v e n s p i e l , 0. Chemical Reaction E n g i n e e r i n g . New York: John Wiley & Sons, (1972), pp. 350-351.  145  Doraiswamy, L.K., & Sharma, M.M. Heterogeneous Reactions: A n a l y s i s , Examples and R e a c t o r DesTgn--Vol ume I: Gas-Sol i d and S o l i d - S o l i d R e a c t i o n s . New York: John Wiley & Sons, (1984), 459 pp.  146  T h e m e l i s , N.J., & Gauvin, "A G e n e r a l i z e d Rate Equation f o r the Reduction of Iron Oxides," Trans. AIME, 227(4):290-300, (1963).  147  Hinze, J.W., & T r i p p , W.C. i n "Metal-Slag-Gas R e a c t i o n s and P r o c e s s e s , " A. F o r o u l i s & W.W. Smeltzer ( E d s . ) . The E l e c t r o c h e m i cal S o c i e t y (1975), pp. 391-408.  219  APPENDIX A.  HEAT BALANCE OF THE LABORATORY KILN  The heat that the Glo-Bar element has to provide in excess, accounting for the need of the reaction, the heat losses and the heat stored, for different kiln characteristics, is calculated as follows:  q  +  in  q  q  e l  q  =  %ut  II.  gen  -  i n  Vn  q  q  r e d  =  q  ~  +  red  q  sto  ^ (ID  e l  0  +  ^gas  (  ^loss  = AHj/At  I  V  )  (V)  = aFA ( T ^ - T ^ ) :  where 1 F BR A  and  F  g R  =  (1 T,  + +  J 1  1  +  A  i i - i (- - 1) A E 2  2  F  2  2  =  ; in this case F = 1. g  A  1 + 2  2 F  A" ~ M  B  l  Several different dimensions, for the element and the cylinder, were tried. Cylinder:  The data presented here are for the optimum found. 14 cm X. 32.8 cm, E  2  2  = 0.5; A = 1450 cm ; 2  220' Element:  2.54 cm X 35.6 cm;  A /A 1  At  Ej = 0.8; A  1  = 284 cm  = 0.196; Fg = 1; F * 0.692  2  T = 1250°C = 1523 K 2  $el  =  5  ,  6  7  2  4  4  (0.692) 284 cm [(1523K) - (1373) ]  4  cm k  q  e l  = 2035W 2  Load = 7.17 W/cm  At  J  = 1300°C = 1573K  l  q , = 2860 W el M  2  Load = 10.1 W/cm  Both temperatures and loads are within safe operational range, according to tables provided by the manufacturer.  III.  qg  e n  i s considered to be zero, since no a i r w i l l be introduced to  help the combustion of CO.  IV.  q .=q out gas  H  M  +Q loss H  n  o,gas i s taken to be 20% of the heat consumed by the r e a c t i o n .  q-j  o s s  i s c a l c u l a t e d considering conduction through the r e f r a c t o r y  l i n i n g and convection from the external s h e l l . data used are ( n e g l e c t i n g end e f f e c t s ) :  The formula and  221  c q  ]  o  s  s  1m (R /R ) 3  ,  2  a  In (R /R ) 4  2TTLK  where  2TTLK  2  4  —  2  T = 1100°C =  In (Rg/R )  3  +  - — — 2TTLK  +  3  + 2TTL h  4  1373K  Too = 20°C = 2 9 3 K  cm  R  2  = 7  R  3  = 8cm ( 1 cm of alumina)  R  4  = 1 4 to 3 0 cm (Castable plicast)  R  5  = R + 0 . 9 5 cm ( 0 . 9 5 cm of steel shell)  L  4  = 33  cm  k  = 8.8  (10)"  2  2  k  =  (IO)"  3  3  k  4  = 0 . 4 3 W./cm°C (at 200°C)  P„ =  r  3.4  W/cm°C (at 1000°C) W/cm°C (at 700°C)  0.706  3 Gr =  Si  T  ^ 5  "  T O  °)  L  W  v 2  N  E  R  E  L  and  =  7 T R  5 0  Nu = 0 . 5 3 ( G r ' P r ) -  25  The calculations were performed iteratively, for a given diameter, 2 R , and with an initial assumed temperature of the external shell, 4  Tr-. A q, was .Qbta.ined, ;;ahdaa"new Tr-! was back'calculated from q, b ^loss b ^loss repeating until a reasonable convergence. in Figure A l . V.  q  r e ( J  at 900°C Fe 0 2  A H  900°C  =  AH  3  + 3C = 2Fe + 3C0  298 r!173 +  J  298  A C p d t  + Z  The results are presented  222  AH  2 9 8  = 3(-26.42kcal) - (-197kcal) = 117740 cal/mole  ACp = 2Cp(Fe) + 3Cp(C0) - Cp(Fe 0 ) - 3Cp(C) 2  3  3  2Cp(Fe) = 17.746 + 2.948(10)" T  -113.84T~  0.330(10)V  3  3Cp(C0) = 20.370 + 2.940(10)" T -  2  3  5  - Cp(Fe 0 ) = 23.490 - 18.600(10)" T - 3.550(10) T~ 2  2  3  3  5  - 3Cp (C) = -0.078 - 27.920(10)" T + 1.062(10) T  3  AC  1/2  5  = 14.548-40.632(10)" T + 4.282(10) T"  2  -2  6  + 12.465(10)" T  - 113.84T"  1/2  6  + 12.465(10)" T  r  ^ACpdT = 14.548 (Tg-Tj) - 40.632(10)~  3  2  2  (T - T ) + 4.282(10)  -113,84 [ 2 ( T  1 / 2 2  {1-1)  5  T  2 7 2  6  3  T  l 2  3  - T ^ ) + 12.465(10)" ( T - T )  JACpdt = -9619.3 cal/mole L q  t  r e d  = 2(1220) + 2(160) = 2760 cal/mole = 117740-9619.3 + 2760 = 110881 cal/mole  f o r 95% reduction, and transforming to grams of Fe reduced: • _ 110881(0 95) _ q q o C a l red 2(55,85) gFe red  q  y y j  _ .  q 4 f i  J g Fe red  So i t can be said that the element w i l l provide enough heat to f u l f i l l the requirements of the system.  2  2  6  8  10  12  14  16  Refractory thickness  A.l  18  (cm)  Heat l o s s e s and surface temperature f o r d i f f e r e n t inner diameters and i n s u l a t i o n thicknesses  20  22  APPENDIX B.  CALIBRATION CURVES FOR FLOWMETERS  20  B.l  40  60 Scale reading  80  C a l i b r a t i o n curve f o r gas-standard f l o w r a t e i n flowmeter Gilmont #1  20  B.2  40  60 Scale reading  C a l i b r a t i o n curve f o r gas-standard f l o w r a t e i n flowmeter Gilmont #2  100  Scale  B.3  reading  C a l i b r a t i o n curve f o r gas-standard f l o w r a t e i n flowmeter Gilmont #3 ro ro  Scale  B.4  reading  C a l i b r a t i o n curve f o r gas-standard f l o w r a t e i n flowmeter Gilmont #4  229  APPENDIX C. CONDITIONS FOR ROOM-TEMPERATURE Run #  Ore d / d p C p i / F e size [ym]  C 01 C 02 C 03  358  c  c c c c c c c c c c c c c c c c  04 05 06 07 08 10 16 17 18 20 22 24 25 26 31 32  c c c c c  34 42 44 50 58  254  c c c c  66 74 82 90  180  c c c c  98 104 106 113  c 79  e  4.464  X  0.45 0.58 0.58 0.71 0.84  2.000 1.285  0.45 0.84 0.45  1.000 0.714 1.285 0.503  0.84 0.45 0.84  3.984 2.008 1.414 1.000 0.504  0.45  3.984 1.996 1.000  0.45  Load- Ore wt Coal wt Bed M i x Bed Mix ing Motion deg Motion deg [%] [ g ] [ g ]  3.984 2.825 1.000 0.325  + + + + + +  R R R R R R  + s +  R R  s s s s s s s  R R R R R R  12 20 12 20 12 20 12 20 20 20 12 20 20 20 20 12 20 12 20  386 644 318 530 270 450 235 391 651 396 387 645 654 654 391 398 663 242 404  289 483 307 512 319 532 328 547 484 554 290 484 491 491 548 298 497 339 565  S S S S S S S S  20  650 656 656 665 668  487 492 492 498 500  R R R S  646 638 658 658  484 478 493 493  R R R S  20  0.500 90  MIXING EXPERIMENTS  0.45 0.84 0.45  20 20 20 12  646 387 666 398  484 541 499 298  0.45  20  638  478  * Some behaviour a t 10 r.p.m. S = Slumping a = Segregated R = Rolling T = Transitional C = Cataracting M = Well mixed  + + + + + +  R s + C s + C + s +  + +  S S R  S + + +  S S S  0  a a a a a a a a a  T T T R M R M M M M S+ M S a a a  T M  R R R R R R R R R* R R* R* + C +S R* R R C +R + C  R R R S + R S + C  a a a a a 0  a a a a  T T T M M M M M M a a a  T M  R R +S R +S S + C  T M M  s s  T S+R + C T S+R + C M S + R M S  T T M M  s  M  M  S +  R  S  •  a  T M M  R  a  APPENDIX D LISTING OF PROGRAM TO PROCESS REDUCTION EXPERIMENTS DATA AND SAMPLE OUTPUT  231  Listing 1 2 3 4 5 6 7 8 Q  10  A4 1  o f EXPERCO C C C C C C  c C c  c  a t 16:01:23 o n DEC 23, 1984 f o r CCid=GM07  Page  1  EXPERIMENTS OF DIRECT REDUCTION OF UNAGGLOMERATED IRON ORE WITH COAL  THIS PROGRAM C A L C U L A T E S , FOR THE REDUCTION EXPERIMENTS THE OUTLET GAS COMPOSITION, THE FLOWRATES OF EACH GAS, THE BOUDOUARD AND REDUCTION RATES AND THE FRACTIONAL REDUCTION OF THE IRON OXIDE AND CARBON REACTED. '*** NOMENCLATURE AND UNITS * * *  1  12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 3 1 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 5 1 52 53 54 55 56 57 58  c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c c  FRACTION OF ASH IN CHAR FRACTION OF FIXED CARBON IN CHAR F E 2 0 3 IN CHAR ASH. AS FRACTION OF ASH CARBON IN CHAR AT START OF REACTION. IN MOLE CARBON MOLECULAR WEIGHT, IN G/MOLE CARBON MONOXIDE MOLECULAR WEIGHT, IN G/MOLE CARBON DIOXIDE MOLECULAR WEIGHT, IN G/MOLE CHAR WEIGHT FED IN, IN G F I X E D CARBON TO IRON RATIO CARBON MONOXIDE TO CARBON DIOXIDE RATIO CARBON MONOXIDE TO HYDROGEN RATIO CARBON MONOXIDE FLOWRATE. IN L/MIN CARBON DIOXIDE FLOWRATE, IN L/MIN CARBON MONOXIDE RATE OF PRODUCTION, IN MOLE/MIN CARBON OIOXIDE RATE OF PRODUCTION. IN MOLE/MIN CARBON RATE OF TRANSFER, IN MOLE/MIN CARBON REACTED FROM START OF REACTION, IN MOLE OXYGEN TRANSFERRED, EXTRAPOLATING, IN MOLE TOTAL OXYGEN TRANSFERRED, EXTRAPOLATING, IN MOLE CARBON TRANSFERRED, EXTRAPOLATING, IN MOLE TOTAL CARBON TRANSFERRED, EXTRAPOLATING, IN MOLE F E 2 0 3 MOLECULAR WEIGHT, IN G/MOLE FEO MOLECULAR WEIGHT, IN G/MOLE F E 2 0 3 FRACTION IN ORE FEO FRACTION IN ORE FRACTION OF GAS MEASURED, AT EXPERIMENT START FRACTION OF GAS MEASURED, AT EXPERIMENT END AVERAGE FRACTION OF GAS MEASURED FRACTIONAL REDUCTION OF IRON OXIDES FRACTIONAL REACTION OF CARBON FRACTIONAL RATE OF REDUCTION, 1/MIN FRACTIONAL RATE OF BOUDOUARD, 1/MIN TOTAL IRON INPUT, IN MOLE FRACTION OF GANGUE IN ORE TOTAL GAS FLOWRATE, IN L/MIN FRACTION OF HYDROGEN IN CHAR HYDROGEN FLOWRATE. IN L/MIN RATE OF HYDROGEN RELEASED, IN MOLE/MIN HYDROGEN RELEASED FROM START OF REACTION, IN MOLE INERT MATERIALS IN ORE AND CHAR RUN NUMBER TIME OF REACTION, IN MIN TEMPERATURE OF REACTION. IN DEGREES CENTIGRADE ROOM TEMPERATURE, IN K HYDROGEN TRANSFERRED PER TIME INTERVAL, IN MOLE OXYGEN REACTED PER TIME INTERVAL. IN MOLE CAR80N REACTED PER TIME INTERVAL, IN MOLE  ASH CFIX CFE203 CIN CMWT COMWT C02MWT CHARWT CTOFE C0C02 C0H2 COFLW C02FLW CORATE C02RAT CRATE CAC EX02 EX02AC EXC EXCAC FE203M FEOMWT FFE203 FFEO FRABEG FRAEND FRAGAS FRARED FRACAR FRAT02 FRATEC FEIN GANGUE GA5FLW H2CHAR H2FLW H2RATE H2AC INERT I RUN ITIME ITEMP I TGAS M0LH2 M0L02 MOLC  •232  Listing 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116  o f EXPERCO C C C C C C C C C C C C C C C C * C C C C C C C C C C C  a t 16:01:23 o n DEC  23,  1984 f o r CCid=GM07  Page  WEIGHT OF IRON ORE, IN G OXYGEN MOLECULAR WEIGHT, IN G/MOLE TOTAL OXYGEN IN OXIDES AT START OF REACTION,IN MOLE RATE OF OXYGEN TRANSFER, IN MOLE/MIN OXYGEN TRANSFERRED FROM START OF REACTION, IN MOLE ATMOSPHERIC PRESSURE FRACTION OF HYDROGEN IN OUTLET GAS FRACTION OF CARBON MONOXIDE IN OUTLET GAS FRACTION OF CARBON DIOXIDE IN OUTLET GAS FRACTION OF OXYGEN IN OUTLET GAS FRACTION OF NITROGEN IN OUTLET GAS GAS CONSTANT, IN ATM*L/MOLE*K HYDROGEN DENSITY, IN G/CM3 CO D E N S I T Y , IN G/CM3 C02 DENSITY, IN G/CM3 ARGON DENSITY, IN G/CM3 ROTAMETER READING GAS D E N S I T Y , IN G/CM3 CORRECTION FACTOR FOR DENSITY SUM OF GAS FRACTIONS, ACCORDING TO ANALISYS S I Z E RATIO OF ORE TO CHAR GAS FLOWRATE OF STANDARD GAS. IN L/MIN TOTAL REDUCTION OF ORE, AFTER EXTRAPOLATION TOTAL CARBON REACTION, AFTER EXTRAPOLATION ******  DECLARATION  OF  VARIABLES  2  OREWT 02MWT 02IN 02RATE 02AC P PCTH2 PCTCO PCTC02 PCT02 PCTN2 R R0H2 ROCO R0C02 ROAR ROTAM ROGAS ROCORR SIGMA SIZERA STDFLW TOTRED TOTCAR  **********  IMPLICIT REAL*8(A-H,0-Z) REAL*8 M0L02,MOLC,M0LH2,INERT INTEGER I , I RUN,ITIME.I TEMP,ITGAS.N,KTH2,KTCO,KTC02,KT02,KTN2 INTEGER KTSH2,KTSCO,KTSC02,KTS02,KTSN2 DIMENSION T I M E ( 3 0 ) , V O L ( 3 0 ) , I T I M E ( 3 0 ) , P C T H 2 ( 3 0 ) , P C T C O ( 3 0 ) DIMENSION P C T C 0 2 ( 3 O ) , P C T 0 2 ( 3 0 ) , P C T N 2 ( 3 0 ) , S I G M A ( 3 0 ) , G A S F L W ( 3 0 ) DIMENSION H 2 F L W ( 3 0 ) , C O F L W ( 3 0 ) , C 0 2 F L W ( 3 0 ) , H 2 R A T E ( 3 0 ) , C O R A T E ( 3 0 ) DIMENSION C 0 2 R A T ( 3 0 ) , 0 2 R A T E ( 3 0 ) , C R A T E ( 3 0 ) , C 0 C 0 2 ( 3 0 ) , C 0 H 2 ( 3 0 ) DIMENSION M 0 L 0 2 ( 3 0 ) , M O L C ( 3 0 ) , M 0 L H 2 ( 3 0 ) , H 2 A C ( 3 0 ) , F R A T 0 2 ( 3 0 ) DIMENSION 0 2 A C ( 3 0 ) , C A C ( 3 0 ) , F R A R E D ( 3 0 ) , F R A C A R ( 3 0 ) , K T H 2 ( 3 0 ) DIMENSION K T C O ( 3 0 ) , K T C 0 2 ( 3 0 ) , K T 0 2 ( 3 0 ) , K T N 2 ( 3 0 ) , F R A T E C ( 3 0 ) DIMENSION P C C H 2 ( 3 0 ) , P C C C O ( 3 0 ) , P C C C 0 2 ( 3 0 ) , S I G M A C ( 3 0 ) , P C T A R ( 3 0 ) DIMENSION C 0 C C 0 2 ( 3 0 ) , C 0 C H 2 ( 3 0 ) , P C F H 2 ( 3 0 ) , P C F C O ( 3 0 ) , P C C A R ( 3 0 ) DIMENSION P C F C 0 2 ( 3 0 ) , S I G M A F ( 3 0 ) , C 0 F C 0 2 ( 3 0 ) , C 0 F H 2 ( 3 0 ) DIMENSION ROGAS(30),ROTAM(30),ROCORR(30),STDFLW(30),BOUCON( 30)  .  C C C C  ******  10 20' C  30 40  READING  AND  WRITING  OF DATA  *******  OPERATIONAL R E A D ( 5 , 1 0 ) I R U N , I TEMP,ITGAS,P,R,CTOFE,SIZERA,OREWT READ(5,20)CHARWT,CFIX,ASH,GANGUE,FRABEG,FRAEND,F ET FORMAT(1X,I2,2X,I3,2X,I3,2X,F4.2,2X,F5.3,2X,F4.2,2X, 1 F5.3.2X,FS. 1 ) FORMAT( 1X.F5. 1 , 2 X , F 5 . 3 , 2 X , F 5 . 3 , 2 X , F 6 . 4 , 2 X , F 4 . 2 , 2 X , F 4 . 2 , 1 2X.F5.3) STOCHIOMETRIC: READ(5,3O)02MWT,FE203M,FE0MWT,CMWT,C0MWT,C02MWT READ(5,40)H2MWT,FFE203,FFEO,CFE203,H2CHAR,FEOUT.CINOUT READ(5.45)R0H2,R0C0,R0C02,R0AR,R0STD FORMAT(1X.F5.2,2X,F6.2,2X,F5.2,2X,F5.2,2X,F5.2,5X,F5.2) FORMAT(1X,F4.2,2X.F6.4.2X.F6.4,2X.F6.4.2X,F6.4,  233  Listing 1 17 1 18 1 19 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174  o f EXPERCO  at  16:01:23 o n DEC  23,  1984  f o r CCid=GM07  Page  3  1  45 C  50 60 C  65 75 80 C  100 105  1 10  130 135  140 150 160  165 C C C  2X,F6.2,2X,F6.2) FORMAT(1X,E10.4.2X,E10.4,2X,E10.4.2X,E10.4,2X,E10.4) CHROMATOGRAPH STANDARDS READ(5,50)KTSH2,KTSC0,KTSC02,KTS02,KTSN2 READ(5,60)STDH2,STDCO,STDC02,STD02,STDN2 FORMAT( 1X.15,2X, 15,2X,15,2X,15,2X,15) FORMAT(1X,F4.2,2X,F5.2,2X,F5.2,2X,F5.2,2X,F5.2) EXPERIMENTAL DATA READ(5,75)N DO 65 1=1,N READ(5,80-)ITIME( I ) , ROTAM( I ) , STDFLW( I ) , 1 KTH2(I),KTCO(I),KTC02(I),KT02(I),KTN2(I) CONTINUE FORMAT(1X,12) FORMAT(1X,I3.2X.F4.1,1X.F6.3, 1 3X,I5.2X. I5.2X, I5.2X,I5.2X,15) WRITING OF DATA W R I T E ( 6 , 105)IRUN,ITEMP,ITGAS,CTOFE,SIZERA,OREWT WRITE(6,110)CHARWT,CFIX,FET.ASH,GANGUE,FRABEG,FRAEND W R I T E ( 6 , 1 3 0 ) F F E 2 0 3 , FFEO,CFE203,H2CHAR W R I T E ( 6 , 135)R0H2,R0C0,R0C02,R0AR,R0STD WRITE(6,14O)KTSH2,KTSC0,KTSC02,KTS02.KTSN2 WRITE(6,150)STDH2,STDCO,STDC02,STD02,STDN2 WRITE(6,160) DO 100 I=1,N WRITE(6,165)I,ITIME(I),ROTAM(I),STDFLW(I),KTH2(I), 1 KTCO(I),KTC02(I),KT02(I),KTN2(I) CONTINUE FORMAT( 1H1,///,5X, 'EXPERIMENT R' . I 2 . / / . 5 X , 'ITEMP. . .. ' , 13, 1 1X, ' C ,/,5X, 'ITGAS. . . . ' , I3.2X, 'K' ,/,5X, 'CTOFE• •. ' .t F6 . 1 . 1X, 2 F 4 . 2 . / . 5 X , 'SIZERA. . . ',F5.3,/.5X, 'OREWT. 3 'G' ) FORMAT(5X,'CHARWT...',F5.1,1X,'G',/,5X,'CFIX '. F5 .3./.5X. 1 ' FET ' ,F5.3,/,5X, 'ASH ' , F5.3./.5X, FRAEND 2 'GANGUE...',F5.3./,5X,'FRABEG.. . ' , F4 . 2 , / , '5X , 3 F4.2) FORMAT(5X, ' F F E 2 0 3 . . . ' ,F6.4,/,5X, 'FFEO ' , F 6 . 4 ,5X / ., 1 ' C F E 2 0 3 . . . ' ,F6.4,/,5X, 'H2CHAR. . . ' ,F6.4) 3X , FORMAT(//,5X , 'GAS D E N S I T I E S : ' ,/,5X, 'R0H2. . . ' .E10.4, ,10.4. 1 'ROCO . . . ' ,E10.4,3X, 'R0C02 . . ' ,E10.4,3X, 'ROAR..' .E 2 3X, 'ROSTD. . ' ,E10.4) FORMAT(//,5X. 'GAS STANDARDS: './,5X, 'KTSH2. . ',16,3X, 'KTSCO..', 1 16.3X, 'KTSC02 . ' , 16,3X, 'KTS02. . ' ,16,3X, 'KTSN2 .16) FORMAT(5X, 'STDH2. . . ' , F5.2,3X, 'STDCO. . . ' ,F5.2,3X , ' ,F5.2) 1 'STDCO . . . ' , F5.2,3X, 'STD02. . . ' ,F5.2,3X, 'STDN2 FORMAT(//,5X.'DATA:',//,2X.' I',3X,'ITIME',2X,'ROTAM', 2X, KT02',4X 1 ' STDFLW ' , 4X , ' KTH2',3X,' KTCO ' , 3X , ' K.TC02 ' , 3X 2 '. KTN2 ' ,/,8X, 'MIN' ,6X, '%' ,3X, 'L/MIN' ) FORMAT(2X,I 2,4X, I 3,4X,F4. 1, 1X,F6.3,4X, 16, 1 2X, 16, 2X , I6..3X . I6.3X, 16) ******  CALCULATION  OF  GAS  COMPOSITION AND  WRITE(6,220) DO 2 0 0 I=1,N PCTH2(I)=STDH2*KTH2(I)/KTSH2 P C T C O ( I ) =STDCO*KTCO(I)/KTSCO PCTC02(I ) = STDC02*KTC02(I)/KTSC02  DENSITY  ************  234  Listing 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 21 1 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232  o f EXPERCO a t  16:01:23 o n DEC  23,  1984  f o r CC1d=GM07 P a g e  4  PCT02(I)=STD02*KT02(I)/KTS02 PCTN2(I)=STDN2*KTN2(I)/KTSN2 SIGMA(I)=PCTH2(I)+PCTC0(I)+PCTC02(I)+PCT02(I)+PCTN2(I) PCTAR(I)=100.DO-SIGMA(I) C0C02(I)=PCTCO(I )/PCTC02(I) C0H2(I)=PCTCO(I)/PCTH2(I) WRITE(6,240)ITIME(I ) ,PCTH2(I),PCTCO(I),PCTC02(I),PCT02(I), 1 PCTN2(I),SIGMA(I),PCTAR(I),C0C02(I),C0H2(I) CORREC=(SIGMA(I ) - P C T 0 2 ( I ) - P C T N 2 ( I ) + P C T A R ( I ) ) / 1 0 0 . D O P C C H 2 ( I )=PCTH2( I )/CORREC P C C C O ( I ) = P C T C O ( I )/CORREC P C C C 0 2 ( I ) = P C T C 0 2 ( I )/CORREC P C C A R ( I ) = P C T A R ( I )/CORREC SIGMAC(I)=PCCH2(I)+PCCCO(I)+PCCC02(I) C0CC02(I)=PCCCO(I )/PCCC02(I) C0CH2(I)=PCCCO(I)/PCCH2(I) CORREF=SIGMAC(I)/100.D0 PCFH2(I)=PCCH2(I)/CORREF P C F C O ( I ) = P C C C O ( I )/CORREF P C F C 0 2 ( I )=PCCC02( I )/CORREF SIGMAF(I)=PCFH2(I)+PCFCO(I)+PCFC02(I) C0FC02(I) = (PCFCO(I ) + PCFH2(I))/(PCFC02(I)-PCFH2(I ) ) C0FH2(I)=PCFCO(I )/PCFH2(I) ROGAS(I) = ((PCCH2(I)*R0H2)+(PCCCO(I)*ROCO)+(PCCC02(I)'R0C02)+ 1 (PCCAR(I)*ROAR))/100.DO CONTINUE 200 WRITE(6,245) DO 215 I = 1 , N WRITE(6,250)ITIME(I),PCCH2(I),PCCCO(I),PCCC02(I), 1 SIGMAC(I ) , P C C A R ( I ) , C 0 C C 0 2 ( I ) , C 0 C H 2 ( I ) , R O G A S ( I ) 2 15 CONTINUE WRITE(6,255) DO 219 1=1,N W R I T E ( 6 , 2 6 0 ) I T I M E ( I ) , P C F H 2 ( I ) , P C F C O ( I ) ,PCFC02( I ) , 1 SIGMAF(I ) , C 0 F C 0 2 ( I ) , C 0 F H 2 ( I ) 219 CONTINUE 220 FORMAT(1H1,//,5X,'GAS COMPOSITION',//,5X.'ORIGINAL', 1 / / , 2 X , ' I T I M E ' , 3 X , 'PCTH2' ,3X, 'PCTCO' ,3X, 'PCTC02 ' .3X , 2 'PCT02' ,3X, 'PCTN2',3X, ' SIGMA' ,3X, 'PCTAR' ,3X, 'C0/C02' 3 3X,'C0/H2',/,3X,'MIN') 240 F O R M A T ( 3 X , I 3 , 4 X . F 5 . 2 , 3 X , F 5 . 2 , 4 X , F 5 . 2 , 3 X , F 5 . 2 . 1 3X,F5.2,3X,F6.2,3X,F5.2,4X,F5.2,3X,F5.2) 245 FORMAT(/,5X,'WITHOUT AIR'.74X,'ROGAS(G/CM3)'./) 250 F O R M A T ( 3 X , 1 3 , 4 X , F 5 . 2 , 3 X , F 5 . 2 . 1 4X,F5 . 2, 19X,F6.2,3X,F5.2,4X,F5.2,3X,F5.2,9X,E10.4) 255 FORMAT(1H1,/.5X.'WITHOUT ARGON' ,/) 260 F O R M A T ( 3 X , I 3 , 4 X , F 5 . 2 , 3 X , 1 F5.2.4X.F5.2, 19X.F6.2.12X,F5.2.3X,F5.2) C C C  *************  C A L C U L A T I O N OF  GAS  FLOWRATES  FRAGAS=(FRABEG+ FRAEND)/2.DO WRITE(6,310) DO 300 I=1,N ROCORR(I) = (DSORT( 1 . D O / R O G A S ( I ) ) ) / ( D S Q R T ( GASFLWfI)=STDFLW(I)*ROCORR(I)/FRAGAS H2FLW(I)=GASFLW(I )*PCCH2(I)/100.DO COFLW(I)=GASFLW(I)*PCCCO(I)/100.DO  *************  1.DO/ROSTD))  235  Listing 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290  o f EXPERCO  at  16:01:23 o n DEC  23,  1984 f o r CCid=GM07  Page  5  C02FLW(I)=GASFLW(I ) *PCCC02(I)/100.D0 W R I T E ( 6 , 3 2 0 ) I T I M E ( I ) ,GASFLW(I),H2FLW(I),COFLW( I ) ,C02FLW( I ) 300 CONTINUE 310 FORMAT( 1H 1 ,//,5X , ' GAS FLOWRATES' ,//, 1 2X,'ITIME',5X,'GASFLW,5X,'H2FLW,5X,'COFLW,5X, 2 ' C 0 2 F L W ,/3X, 'MIN' ,7X, 'L/MIN' ,5X, 'L/MIN' ,5X, 'L/MIN' , 3 6X,'L/MIN',/) 320 F0RMAT(3X,I 3 , 6 X , F 6 . 3 , 5 X , F 5 . 3 , 5 X , F 6 . 3 , 5 X , F 5 . 3 ) C C * * * * * * * * * * * * * * * * * * * CALCULATION OF MOLAR RATES * * * * * * * * * * * * * * * * * * * C C OXYGEN AND CARBON AMOUNTS IN FEED 0 2 I N = ( ( O R E W T * ( ( F F E 2 0 3 * 4 8 . 0 0 / 1 5 9 . 7 0 ) + (FFEO*16.DO/7 1.85) 1 ))+(CHARWT*ASH*CFE203*48.00/159.70))/32.00 C I N = CHARWT*CFIX/12.DO FEIN=((0REWT*FET)+(CHARWT*ASH*CFE203))/55.85 WRITE(6,410) DO 400 1=1,N H2RATE(I)=H2FLW(I)/(R*ITGAS) CORATE(I)=COFLW(I)/(R*ITGAS) C02RAT(I)=C02FLW(I)/(R*ITGAS) 02RATE(I)=CORATE(I)/2.DO+C02RAT(I) CRATE(I)=C0RATE(I)+C02RAT(I) FRAT02(I)=02RATE(I)/(02IN*60.DO) FRATEC(I)=CRATE(I)/(CIN*60.DO) WRITE(6,420)ITIME(I),H2RATE(I),CORATE(I),C02RAT(I),02RATE(I), 1 C R A T E ( I ) , F R A T 0 2 ( I ) ,FRATEC(I ) 4 0 0 CONTINUE 4 1 0 FORMAT(//,5X,'MOLAR RATES',//, 1 2X, ' I T I M E ' ,5X, 'H2RATE',5X, 'CORATE' ,5X, 'C02RAT' ,5X, 2 '02RATE' ,5X, 'CRATE' ,8X, 'FRAT02', 10X, ' F R A T E C . 3 /,3X,'MIN',5X,'MOL/MIN',4X, 4 'MOL/MIN',4X,'MOL/MIN',4X,'MOL/MIN',4X,'MOL/MIN', 5 7X, ' 1 / S E C , 11X, ' 1 / S E C ,/) 420 FORMAT(3X,I3,6X,F5.3,6X,F5.3,6X,F5.3,6X,F5.3,6X, 1 F5.3.6X.E10.4.6X.E10.4) C C * * * * * C A L C U L A T I O N OF FRACTIONAL REDUCTION AND CARBON REACTED * * * * * * * * * C ( I N T E G R A T I O N IS PERFORMED USING TRAPEZOIDAL RULE) C C F I R S T INTERVAL APPROXIMATION WRITE(6,510) M0L02(1)=02RATE(1)/2.DO*ITIME(1) 02AC( 1 )=M0L02( 1 ) FRARED(1)=02AC(1)/02IN MOLC(1)=CRATE(1)/2.00*ITIME(1) CAC(1)=MOLC( 1 ) FRACAR(1)=CAC( 1 )/CIN M0LH2(1)=H2RATE(1)/2.DO*ITIME(1) H2AC(1)=M0LH2(1) 8=1.DO-FRACAR(1) BOUCON(1)=DLOG(B) W R I T E ( 6 , 5 2 0 ) I T I M E ( 1 ) ,M0L02( 1 ),02AC( 1 ) , F R A R E D ( 1 ) , M O L C ( 1 ) . 1 CAC( 1),BOUCON( 1 ) , F R A C A R ( 1 ) , M 0 L H 2 ( 1 ) , H 2 A C ( 1) C INTEGRATION DO 500 d=2,N M0L02(J) = ((02RATE(J- 1 )+02RATE(J))/2.DO)*(ITIME(J) 1 -ITIME(d-l))  .236  Listing 291 292 293 294 295 296  297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348  of  EXPERCO a t  1  1 6 : 0 1 : 2 3 o n DEC 2 3 ,  1984 f o r  CCid=GM07  Page  6  02AC(d)=02AC(d~1)+M0L02(J) FRARED(d)=02AC(d)/02IN MOLC(d)=((CRATE(J-1)+CRATE(J))/2.DO)*(ITIME(J)-ITIME(d-1)) CAC(d)=CAC(d-1)+MOLC(J) FRACAR(d)=CAC(d)/CIN M0LH2(d)=((H2RATE(d-1)+H2RATE(d))/2.D0)*(ITIME(d)  -ITIME(J-O)  H2AC(d)=H2AC(d-1)+M0LH2(d) B=1.DO-FRACAR(d) BOUCON(d)=DLOG(B) W R I T E ( 6 . 5 2 0 ) 1 T I M E ( d ) , M 0 L 0 2 ( d ) , 0 2 A C ( J ) , F R A R E D ( d ) , MOLC ( d ) , 1 C A C ( d ) , FRACAR(d') .BOUCON(d) , M 0 L H 2 ( d ) , H 2 A C ( d ) 5 0 0 CONTINUE C LAST INTERVAL EXTRAPOLATION (ONE AND A HALF TIMES LAST INTERVAL) ITIMEX=1 . 5 * ( I T I M E ( N ) - I T I M E ( N - 1 ) ) ITIMFI=ITIME(N)+ITIMEX EX02=(02RATE(N)/2,DO)*ITIMEX EX02AC=02AC(N)+EX02 T0TRED=EX02AC/02IN EXC=(CRATE(N)/2.D0)*ITIMEX EXCAC=CAC(N)+EXC TOTCAR=EXCAC/CIN EXH2=H2RATE(N)*ITIMEX EXH2AC=H2AC(N)+EXH2 BC=1.DO-TOTCAR BOUCOT=DLOG(BC) WRITE(6,520)ITIMFI,EX02,EX02AC,TOTRED,EXC,EXCAC, 1 T0TCAR,B0UC0T,EXH2,EXH2AC 5 1 0 FORMAT( 1 H 1 , / / / , 5 X , ' F R A C T I O N A L R E D U C T I O N ' , / / / , 1 2X,'ITIME',5X,'M0L02',5X,'02AC',5X,'FRARED',5X,'MOLC', 2 7X, ' C A C ' , 5 X , ' F R A C A R ' , 5 X , ' L N ( 1 - F ) ' , 5 X , ' M 0 L H 2 ' , 5 X , ' H 2 A C ' , / , 3 3X,'MIN',/) 520 F O R M A T ( 3 X , I 3 , 6 X , F 5 . 3 , 5 X , F 5 . 3 , 5 X , F 5 . 3 , 5 X , F 5 . 3 , 4 X , F G . 3 , 1 5X,F5.3,5X,E10.3,5X,F5.3,5X,F5.3) C C * * * * * * MASS BALANCE * * * * * * * * C TOTFED=OREWT+CHARWT FEIN=FEIN*55.85 02IN=02IN*32.DO H2IN=CHARWT*H2CHAR CIN=CIN*12.D0 INERT=(OREWT*GANGUE)+(CHARWT*ASH*(1.D0-CFE203)) CININ=CIN+INERT T0TIN=FEIN+02IN+H2IN+CIN+INERT 020UT=EX02AC*32.DO H20UT=EXH2AC*2.DO CIN0UT=CIN0UT+(EXCAC*12.DO) T0T0UT=FE0UT+020UT+H20UT+CIN0UT XTOT=(TOTFED-TOTOUT)/T0TFED XFE = ( F E I N - F E O U T ) / F E I N X02=(02IN-020UT)/02IN XH2=(H2IN-H20UT)/H2IN XCIN=(CININ-CINOUT)/CININ XTOTAL=(TOTIN-TOTOUT)/TOTIN WRITE(6,62O)T0TFED 620 FORMAT( / / / / / , 7 X , ' * * * * MASS BALANCE * * * * ' / / , 2 4 X , ' F E ' , 1 7X.'02',7X,'H2',7X,'C,6X,'INERT',4X,  237  Listing 349 350 351 352 353 354 355 356 357 358 359 360 361 362  o f EXPERCO a t 2  16:01:23 o n DEC  23,  1984  f o r CCid=GM07 Page  7  'C+INERT' ,3X, 'TOTAL' ,//,5X, 'FEED' .67X.F7.2) WRITE(6,630)FEIN,02IN.H2IN,CIN,INERT,CININ.TOTIN, 1 FE0UT,020UT,H20UT,CIN0UT,T0T0UT, 2 XT0T,XFE,X02,XH2,XCIN.XT0TAL 630 FORMAT( 5 X , ' I N * ' , 1 3 X , F 6 . 2 , 3 X , F 6 . 2 , 3 X , F 6 . 2 , 3 X , 1 F6.2,3X,F6.2,3X,F6.2,3X,F7.2,/,5X, 2 'OUT **' , 1 1 X , F 6 . 2 , 3 X . F 6 . 2 , 3 X , F 6 . 2 . 2 1 X , 3 F6.2,3X,F7.2,///,5X, '(FEED-OUT)/FEED' , 4 5 6 X , F 6 . 3 , / , 5 X , ' ( I N - O U T ) / I N ' ,6X , 5 F6.3,3X,F6.3,3X,F6.3,21X,F6.3, 6 3 X , F 6 . 3 , / / , 5 X , ' * FROM SOLIDS ANALYSES',/,5X, 7 '** FROM GAS ANALYSIS AND SOLIDS SEPARATION') RETURN END  EXPERIMENT  R58  ITEMP....900 C ITGAS....294 K CTOFE....0.32 S I Z E R A . . .0.500 OREWT.... 6 0 0 . 0 G CHARWT...313.0 G CFIX 0.408 FET 0.667 ASH 0.561 GANGUE...0.058 FRABEG...0.99 FRAEND...0.96 FFE203...0.8220 FFEO 0. 1 190 CFE203...0.0460 H2CHAR...0.0031  GAS D E N S I T I E S : R0H2. . .0.8300E-04  GAS STANDARDS: KTSH2.. 4540 STDH2... 4.04  ROCO. . .0. 1 165E-02  KTSCO.. 7435 STDCO . . .74.79  R0C02. .0. 1831E-02  ROAR. . .0. 1665E-02  KTSC02. 1924 KTS02.. 2103 STDCO . . .20.10 STD02...21.00  KTSN2.. 6992 STDN2. . .78.00  DATA: I 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16  ITIME MIN 2 4 6 8 10 12 14 16 21 25 30 35 41 66 96 126  ROTAM °  17 42 62 50 37 34 37 37 38 38 35 26 18 56 40 20  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  STDFLW L/MIN 1 600 4 650 7 550 5 800 4 050 3 550 4 050 4 050 4 200 4 200 3 800 2 650 1 750 1 100 0 710 0 285  KTH2  KTCO  KTC02  385 911 14 50 1669 1569 1591 1700 1800 1855 1958 2108 2403 3075 4620 5615 7600  0 3475 5606 6376 6552 6315 6600 6700 6852 6961 6720 6719 7190 7272 7033 7300  3407 4124 2468 2131 2054 1946 1900 1900 1873 1917 2032 1919 1584 1555 1433 1200  KT02 0 0 0 0 0 60 0 0 0 0 0 0 0 0 7 1 94  KTN2 252 1 0 787 0 0 200 0 0 0 O 0 0 0 0 235 270  ROSTD. .0. 1260E-02  GAS  COMPOSITION  ORIGINAL ITIME MIN 2 4 6 8 10 12 14 16 21 25 30 35 41 66 96 126  PCTH2 0 . 34 0,.81 1 .. 29 1 .. 49 1 .40 1 . 42 1 .51 1 .60 1 .65 1 . 74' 1 . 88 2 . 14 2 . 74 4.1 1 5 .00 6 . 76  -PCTCO 0 34 56 64 65 63 66 67 68 70 67 67 72 73 70 73  .0 .96 . 39 . 14 .91 . 52 . 39 . 40 .93 .02 .60 . 59 . 33 . 15 . 75 . 43  PCTC02 35 .59 43 .08 25 . 78 22 . .26 21 .46 20 . 33 19 .85 19..85 19 .57 20 .03 21 , 23 20 .05 16 .55 16 . 25 14 .97 . 12 . 54  PCT02 0 .0 0 .0 0 .0 0 .0 0 .0 0 .60 0..0 0..0 0 .0 0 .0 0..0 0..0 0. 0 0..0 0..71 0..94  PCTN2  SIGMA  PCTAR  28 , 12 0..0 8 .78 . 0..0 0..0 2 . 23 0..0 0..0 0 .0 0 .0 0. 0 0..0 0. 0 0..0 2 .62 . 3 .01  64 .06 78 .85 92 . 24 87 .89 88 .76 88 . 10 87 . 75 88.. 85 90 . 14 91 . 79 90 , 70 89 .77 91 .61 93 .51 94 .04 96..68  35 .94 21 . 15 7 .76 . 12 . 1 1 1 1.24 1 1.90 12 . 25 1 1. .15 9 . 86 8 .21 . 9 . 30 10.. 23 8 . 39 6 .49 . 5 .96 . 3 . 32  C0/C02 0 0 2 2 3 3 3 3 3 3 3 3 4 4 4 5  .0 .81 . 19 .88 .07 . 12 . 34 . 40 .52 .50 . 18 . 37 . 37 .50 . . 73 .86 .  C0/H2 0 .0 43 . 12 43 .70 43 . 18 47 .20 44 .87 43 . 89 42 .08 41 .76 40 . 19 36 .04 3 1 .61 26 . 43 17 . 79 14 . 16 10 .86  WITHOUT AIR 2 4 6 8 10 12 14 •16 21 25 30 35 41 66 96 126  0.. 48 0 .81 1. 4 1 1 .. 49 1 .40 1 .46 1 .51 . 1 .. 60 1 .65 . 1 . 74 1 .. 88 2 . 14 2 . 74 4 .1 1 5 . 17 7 .04  R0GAS(G/CM3 0 34 61 64 65 65 66 67 68 70 67 67 72 73 73 76  .0 .96 . 82 . 14 .91 . 37 . 39 . 40 .93 .02 .60 . 59 .33 . 15 . 18 . .45  49 . 52 43 .08 28 . 26 22 . 26 21 .46 20 .92 19 .85 . 19 .85 19 .57 20 .03 21 .23 20..05 16 .55 16 . 25 15 .49 . 13 .05 .  50..00 78 .85 91 .50 87 . .89 88 . 76 87 . 75 87 . 75 88 . .85 90. 14 91 . , 79 90..70 89 . .77 91 . .61 93 , .51 93 . .84 96 . .55  50..00 21 . 15 8 .50 . 12 .11 1 1 24 . 12 . 25 12 .25 1 1 15 . 9 .86 8 ,21 9. 30 10. 23 8 .39 6 .49 6. 16 3 .45  0..0 0 .81 2 . 19 2 .88 3 ,07 . 3 . 12 3 , 34 3 .40 3 .52 3 .50 ' 3 .18 3 . 37 4 .37 . 4 .50 4 .73 . 5 .86  0 43 43 43 47 44 43 42 41 40 36 31 26 17 14 10  .0 . 12 .70 . 18 . 20 .87 .89 .08 . 76 . 19 .04 .61 . .43 . 79 . 16 . 86  0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.  1740E-02 1549E-02 1380E-02 1358E-02 1349E-02 1350E-02 1342E-02 1336E-02 1327E-02 1321E-02 1333E-02 1327E-02 1288E-02 1261E-02 1243E-02 1193E-02  CO VO  WITHOUT 2 4 6 8 10 12 14 16 21 25 30 35 4 1 66 96 126  ARGON  0 .95 1 .03 1 . 55 1 .69 • 1 ..57 1 .66 1 . 72 1 .80 . 1 .83 1 .90 2 .07 2 . 38 2 .99 4 .40 5 .51 7 . 29  0..0 44 .33 67 . 56 72 .98 74 . 25 74 . 50 75 .66 75 .86 76 .46 76 . 28 74 . 53 75 .29 78 .95 78 .23 77 .99 79 . 19  99 .05 54 .64 30,. 89 25 , . 33 24 . , 17 23 . 84 22 .62 22 . 34 21 .71 , 21 .82 , 23 , . 40 22 . 33 18 .06 17 . 37 16 .50 13 .52 .  100..00 100 0 0 100,,00 100, 0 0 100..00 100..00 100 .00 100..00 100 .00 100 .00 100 .00 100 0 0 100 .00 100 .00 100 .00 100..00  0..01 0 . 85 2 .. 36 3 16 3 . 35 3 . 43 3 .70 . 3 . 78 3 .94 , 3 .92 , 3 , 59 3 ,89 5 . 43 6 . 37 7 .59 13 . , 89  0 .0 43 . 12 43 . 70 43 . 18 47 . 20 44 . 87 43 . 89 42 .08 4 1 . 76 40.. 19 36 . .04 31 61 26 . 43 17 .79 14 . 16 10 .86  HH c o c n c n - . u i o u '  — cn.uroococn.uro  O O O O O O O O O O O O O O O O b b b b o b b b b b b b b b b b O O O O O O O O O O O O O O O O  »-» -1  Z 3 m  2 o >  2  73  o tIo \  XI  -^rorororocoucocouroro.fc.b.-i.o  2 > -I z m  O O O O O O O O O O O O O O O O  o o r o  b b b b b - - - ^ ^ b - -- -- o b \2 — rocoui-JOroro-.-.co-'Uicocn Oro*coJiCoroOco--uiOtoOro  O O O O O O O O O O O O O O O O o o o o b b b o o o o b b b o b O O O - ' M U U U O U C O U O l O v l M touioororocouifkcoco-^cnco-j-ico  O O O O O O O O O O O O O O O O o b o o b o b o o b b b - - > - ' 0  O - ^ r o c o u i o o c o c o c o o o ^ i c o f o c o o r o c n c n u i c D i D - j a i i . o c o c o . - ' C o r o c o c o  73  >  2  o n r- o v. ro  ro to CO CO  to ro ro Ul CO CO co Ul CO CO .b _ t CO CO _>. CO oo <n O Ul O Ul £Ul  O m mm m m m m m m m m m m m m m O OOO O O O b O O b O O O b O  4i£.J^COCOCOCOCOCOCOCOCOCOU3COJ^  ^ r o  o  co c n & r o  z  m  OO-'-'NU* co  O O  *. ^  CJ . t »u i ~ j  - J  - J c n - J t o _i. O O c n O - J CO CO CO U M NI t I D O CD c o r o - * CO CO O CO CO Ul Ul CO 4 i - * c o - * -sj CO CD CD CO O CD  O O O O O O O O O O O O O O O O o o o o o b o o o o o o o - ^ o g rocox^j^ui-j-jcncncrioiuicoocoo  -'Oocnco-j-'-cococn-'-'Cnuiuiui-o  O O O - ' - ' M M M M M U r O U ^ - ' O rouiooto^iuicocD-Jcncjcncnuiuio  r\  CO  73  (/)  -  z c  r- o \ o 2 -n  n r- o \ ro 2 -n i-i r—  ro 73 > H  o o r- 73 \ > 2 —i  \  -n 73  (/)> OO m H  ro  — 73 C/) >  m -i O m O  O-.-'iouicococDco-o-jcoioococn CO-'00(DCOO^UO(OCOO>1IO(I1(D co^coi»-.uico-'-'COff)-'Cn-'Coro  a K.  s I-n z  O O O O O O O O O O O O - r o - O  r~ \ 2 >-•  o >  C/l  >  (/l  o o  O O O O O O O O O O O O O O O O  i i i i i i O O O O O O O O O O O O O O O O  z  cn  n t~  >  H m cn  H-1 -t HH HH  z c  4^ CO CO CO CO CO CO CO CO CO CO CO CO  _.4>.ff)-^-»rorororoio->.roco4^ro4^ - J t o o i o u i - . t i i i o i o co-ro ro co — M O t ( O O U I D ^ C O O l - J - O — ro oo co - J C O C O U I C D ~ 4 — - c O N a u i r o c o c o c o — co rnrTirnrnnirnrnrTirnrnrnrnrnrfirnrn  ff> CO  ro o _* Ul  cocorocococnt.co-'-JO^Ni-iO O-o-tn^oroaicocorooaiuicoi.  O O O O O O O O O O O O O O O O . ro u i - J to CO CO co -J O O CO ro CO OO -J - j A J — t .u cn -— CO O ff) ro  .u c o c o r CO — Ul O  c o c n  •-t > Z -I  2 73  Z m  O O O O O O O O O O O O O O O O O O O O O -•• — — to ro — O - . r o A < n c o ^ u i u i 4 i . A r o * . o - J c o r o -»-jfouicno~<-ucn.t>0">uiu)0">coco  ro  Z  «  > m—i  FRACTIONAL  ITIME MIN 2 4 6 8 10 12 14 16 2 1 25 30 35 41 66 96 126 17 1  ****  REDUCTION  MOL02  02AC  0. 029 0. 137 0. 290 0. 311 0. 220 0. 169 0. 167 0. 178 0. 459 0. 380 0. 456 0. 364 0. 294 0.,794 0. 608 0. 333 0.. 144  0 .029 0 . 165 0.. 455 0 . 766 0 .986 1 .. 154 1 .321 1 . 499 1 .959 2 . 339 2 . 795 3 . 159 3 .453 4 . 248 4 .856 5 . 189 5 . 333  MASS BALANCE  FRARED  0..006 0. 032 0. 087 0. 147 0. 189 0. 222 0. 254 O. 288 0 . 376 0.. 449 0,"537 0..607 0..663 0 . 816 0..933 0..997 1 .025 .  MOLC  0..029 0. 168 0..416 0..482 0..351 0..271 0.. 270 0 290 0 . 750 0. 623 0., 742 0.. 590 0.. 485 1 .341 1..032 0., 572 0 . 251  CAC  0 ,029 0 . 197 0 .612 1 .094 1 .. 445 1 .716 . 1 .986 2 . 276 3 .027 3 .649 4 . 391 4 . 981 5 .466 6 .807 7 .839 . 8 .411 8 .661  FRACAR  LN(1-F)  003 0. 018 0. 058 0. 103 0 136 0. 161 0. 187 0.,214 0,, 284 0., 343 0, 4 13 0. 468 0.,514 0..640 0.,737 0. 790 0.,814  0. 270E-02  -o. 186E-01 -o. 593E-01  -0. 108E+00 -0. 146E+00 -0. 176E+00 -0. 207E+00 -o. 24 1E+00 -0. 335E+00 -0. 420E+00 -0. 532E+00 -0. 631E+00 -0. 721E+00 -0. 102E+01 -0. 133E+01 -0. 156E+01 -0. 168E+01  MOLH2  0 ,000 0 .002 0,.006 0,.008 0 .006 0,.004 0 .005 o .005 0 ,014 0 .012 • 0 ,015 0 .013 0,.013 0,.049 0 .052 0 .037 0 .039  H2AC  0, 0 0 0 0.,002 0. 008 0. 016 0. 022 0. 026 0. 031 0. 0 3 6 0 050 0. 062 0. 077 0..090 0., 103 0, 152 0., 205 0, 24 1 0, 281  **** FE  02  FEED IN * OUT **  408.28 436.00  166.57 170.65  0.97 0.56  (FEED-OUT )/FEED (IN-OUT)/IN  -0.068  -0.025  0.421  * FROM SOLIDS ANALYSES ** FROM GAS ANALYSIS AND  H2  SOLIDS SEPARATION  C  127.70  INERT  C+INERT  TOTAL  202.56  330.26 265.44  913.00 906.07 872.65  0.196  0.044 0.037 ^ 4=>  APPENDIX E  SUMMARY OF OVERALL MASS-BALANCES FOR REDUCTION EXPERIMENTS* Conditions  Temperature [°C]  (In-0ut)/I  950 900 850 800  0.060 0.037 0.031 0.041  S t o i c h i o m e t r i c C - /Fe  950 900 850 800  0.039 0.051 0.027 0.022  Finer p a r t i c l e s  950 900 850 800  -0.037 0.033 0.025 0.022  Segregated bed  950 900 850 800  0.048 0.056 0.035 0.014  Lignite  950 900 850  0.038 0.022 -0.021  Catalyzed  900 800  -0.020 -0.058  N --flushed = 0.48 CC -i x / = 0.64 c . • /Fe = 0.16; 7 r.p.m. 0.64; = /Fe 7 r.p.m. C i 0.64; = / F e 11 r.p.m. Ci 0.64; = / F e 20 r.p.m. CFI 1% f i l l C F I /Fe = 0.16; C / F e = 0.64; 7% f i l l  900 900 900 900 900 900 900 900 900  0.022 0.054 0.031 0.027 0.015 0.041 0.040 0.032 0.037  Base case  Fl  2  F  p e  F  F  X  F  X  F  X X  X  F i x  x  *A11 experiments a t 14 percent f i l l , 14 r.p.m. and 0.32 C - /Fe l e s s otherwise i n d i c a t e d . Fl  x  244  APPENDIX F .  CALCULATIONS FOR THE OXIDATION OF S i C HEATING ELEMENT.  The  reactions  c o n s i d e r e d to take  and  their respective  constants,  (1)  free  place  energy  inside  equations  the r o t a r y r e a c t o r  and e q u i l i b r i u m  are:  C+ C 0 = 2 C 0  AG£=39810-40.87T  2  2  K^tPco) (Pco ) 2  (2)  SiC+3C0 =Si0 +4C0 2  2  AG°=-24930-41.49T  4  K =(Pco) 2  2  (Pco )  3  2  (3)  SiC+2C0 =Si0+3C0  3  AG°=99520=82.79T  K =(Pco) (PsiO) 0  (Pco )^ 2  The  calculated Pco/Pco  ed below  for a total  temperatures  T (K)  relevant  (1) Pco/Pco  2  2  ratios  f o r these e q u a t i o n s  p r e s s u r e o f one a t m o s p h e r e , to t h i s  (2) Pco/Pco  and f o r t h e  (3) Pco/Pco  2  PsiO  1. 73 (10) -4  48.69  3. 94 (10) -4 -4 7. 21(10) -4 1. 33(10) -4 2. 14 (10) -4 3. 2 3 (10) -4 4 . 70(10)  4 . 7 6 U 0 )  1200  49.05  3.51(10)  4  17 3.34  4  2  11.38  11.46  1300  174.63  2.63(10)  4  1400  523.00  2.06(10)  4  519.00 1346.00  1500  1356.00  1.71(10)  4  1600  3124.00  1.43(10)  4  3101.00  1.22(10)  4  6477.00  6525.00  tabulat-  case.  1100  1700  are  245 Thermodynamically SiC  h e a t i n g element,  generated.  then, the p o s s i b i l i t y e x i s t s o f the o x i d a t i o n o f the  i n t o e i t h e r S i O o r SiC>2, by the small f r a c t i o n o f  CO2  K i n e t i c a l l y , on the other hand, the r e a c t i o n i s very slow and i t s  r a t e c a n be a s s e s s e d w i t h a v a i l a b l e d a t a . 1 4 7  p  l a m i n a r f l o w o f gas a t  o r  5  1477 °C, and under an oxygen p r e s s u r e o f I O " atm [these a r e the extreme oxi d i s i n g c o n d i t i o n s t h a t c o u l d be f a c e d by the system], t h e r a t e c o n s t a n t f o r 4  o x i d a t i o n o f the S i C element i s 1 0 ~ g/cm^min..  In t h i s case the element sur-  2  f a c e a r e a i s 284 cm , and by c o n s i d e r i n g s t o i c h i o m e t r y , the r a t e o f CO produced w i l l be:  1 0  "  4  Hh-  X  2 8 4  ™  X  1  4x28 g/mo1  x  22  '  4  iioT  =  ^(lO)"  3  ^  (STP)  w h i c h , f o r an e x p e c t e d CO e v o l u t i o n from r e a c t i o n s (1) and (2) o f about 5 V m i n , r e p r e s e n t s about 1 percent o f the t o t a l gas to be analyzed.  I t i s a l s o w o r t h w h i l e mentioning t h a t a f t e r some o x i d a t i o n has taken p l a c e , the element s u r f a c e w i l l be p a s s i v a t e d slowing some more the r a t e .  

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