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Factors affecting the mechanical properties of blast furnace coke Grant, Michael G. K. 1986

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FACTORS AFFECTING THE MECHANICAL PROPERTIES OF BLAST FURNACE COKE  by MR. MICHAEL G.K. GRANT, B.A.SC. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE  in THE FACULTY OF GRADUATE STUDIES Metals and Materials Engineering  We accept t h i s thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA December 1986 ©  Mr. Michael G.K. Grant, B.A.Sc, 1986  In  presenting  degree  at  this  the  thesis in  University of  partial  fulfilment  of  of  department  this or  publication of  thesis for by  his  or  that the  her  representatives.  It  this thesis for financial gain shall not  Department of The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3  for  an advanced  Library shall make  it  agree that permission for extensive  scholarly purposes may be  permission.  DE-6(3/81)  requirements  British Columbia, I agree  freely available for reference and study. I further copying  the  is  granted  by the  understood  that  head  of  copying  my or  be allowed without my written  Abstract The influence of coking conditions, with respect to position in a commercial coke-oven, on the mechanical behaviour of blast furnace coke has been studied. This involved the determination of density, porosity, the characterization of microstructure and assessing the influence of a l l three on the compressive strength of coke. The p l a s t i c flow properties were also investigated at temperatures greater than 1000°C. Three coke batches, originating in a 5m commercial coke-oven at three d i f f e r e n t positions with respect to height (0.8m, 3.3m  and 5m below the coal l i n e ) , along with  three coke batches produced in a 460mm test-oven, were supplied by Energy, Mines and Resources (CANMET) in Ottawa. A- warf coke batch was also provided as a control sample. Several hundred c o r e - d r i l l e d specimens (*1.3cm diameter and 1.3cm  length) were produced from the seven coke batches. The  bulk density of each c y l i n d r i c a l coke specimen  was  determined. Also, a detailed microstructural analysis, using a L e i t z Image Analyzer, of the f l a t faces of the coke cylinders was performed to quantitatively characterize the pore and c e l l wall size, and the pore geometry. The compressive strength of each coke cylinder was determined both at ambient temperature and at 1400°C. In addition, the p l a s t i c flow behaviour of the commercially produced coke batches was studied.  Results indicate that the coke product bulk density was affected by the coke-oven pressure ( s t a t i c load). Studies of the  test-oven coke batches revealed that coke bulk density  increased with higher oven presure. Furthermore, the pore size was found to be larger for cokes produced at lower oven pressures. The c e l l wall size did not appear to be affected by coke-oven pressure. The bulk density of the commercially produced samples increased with depth below the coal l i n e . This was attributed to a higher temperature and s t a t i c load that existed at the bottom of the battery. The pore size was larger in cokes extracted from higher regions. No c o r r e l a t i o n of c e l l wall size with depth below the coal l i n e was found. However, an oven size effect on the pore and wall size was noticed. Both the pore and wall size was smaller in the  test-oven coke batches. The compressive strength of coke was higher in batches  subjected to higher coke-oven pressures. Similarly,' the compressive strength of commercial coke batches was higher for coke batches extracted from regions near the sole of the coke-oven, than that for coke batches extracted from higher regions. It was concluded that high oven pressures resulted in cokes exhibiting a lower porosity and small pores which had the combined e f f e c t of producing stronger coke. Coke strength was generally shown to be higher at 1400°C than at room temperature. The test-oven cokes were always stronger than cokes produced in the 5m commercial coke-oven.  iii  Constant load tests revealed  that coke exhibited  p l a s t i c flow behaviour at temperatures above 1000°C. The time dependent strain data was interactive-double-Kelvin  described using  an  element v i s c o - e l a s t i c model.  Acknowledgements The author wishes to express his gratitude to his research supervisor, Dr. A.CD. encouragement  Chaklader for his advice and  during t h i s project. Thanks are also extended  to the f a c u l t y , staff and fellow graduate students. The assistance of J . Arblaster and A. Darnbrough are e s p e c i a l l y appreciated. F i n a n c i a l assistance from the Energy, Mines and Resources (CANMET) in Ottawa, i s g r a t e f u l l y  v  acknowleged.  Table of Contents Abstract  ii  Acknowledgements  v  Table of Contents  vi  L i s t of Tables  x  L i s t of Figures  xii  L i s t of Abbreviations and Symbols 1.  xvi  Introduction  1  1.1 Role of Coke in the Blast Furnace  3  1.1.1 Coke as a Fuel  4  1.1.2 Coke as a Reducing Agent  4  1.1.3 Coke as a Physical Support  for the Burden ..6  1.2 Coke Characteristics necessary for the Blast Furnace  10  1.3 Production of Blast Furnace Coke  13  1.3.1 Coal C h a r a c t e r i s t i c s 1.3.1.1 Coal Chemistry 1.3.1.2 Coal Rank  13 13 .....13  1.3.1.3 Coal Minerals  15  1.3.1.4 Reflectance  17 18  1.3.2 Coal Pyrolysis 1.3.2.1 Pyrolysis Reaction  18  1.3.2.2 F l u i d i t y during Pyrolysis  19  1.3.3 Conventional Cokemaking Process 1.4 Testing Methods for Coke  20 24  1.4.1 Porosity and True Specific Gravity  24  1.4.2 Tumbler Tests  27  1.4.3 Reactivity Tests  29  vi  1.4.4 Coke Strength after G a s i f i c a t i o n  30  1.4.5 Shatter Tests  31  1.5 Previous Strength Tests and Analysis  32  1.5.1 High Temperature Tumbler Tests  32  1.5.2 Fundamental Strength Tests  33  1.5.2.1 Ambient Temperature Tests  33  1.5.2.2 High Temperature Tests  36  1.5.3 Pore Structure and Coke Strength  37  1.5.4 Coke Mechanical Properties i n Relation to Position in Coke Oven  41  1.6 Objectives of this Research Project 2.  Experimental  ...44  2.1 Sample I d e n t i f i c a t i o n 2.1.1  41  Positioning of Coke Lumps  48 48  2.1.2 Selection of Test Lumps  51  2.1.3  51  Sectioning of Test Lumps  2.1.4 Core D r i l l i n g of Coke Specimens  53  2.2 Preparation of Test Specimens  53  2.3 Bulk Density Measurement  53  2.4 Test Sample Selection  55  2.5 Microstructural Analysis  55  2.5.1  Photomicrograph Preparation  57  2.5.2 Image Analysis  59  2.5.3  61  True Density  2.6 Mechanical 2.6.1  Tests  62  Compression Test Apparatus  63  2.6.2 Procedure of Testing  63  2.6.3  65  Experimental  Parameters vii  2.7 Graphitization Tests  65  Results and Analysis  67  3.1 General Comments  67  3.2 Density  67  3.2.1 Bulk Density  67  3.2.2 True Density  72  3.3 Microstructure  72  3.3.1 S e n s i t i v i t y  74  3.3.2 Edge E f f e c t s  77  3.3.3 C e l l Wall Size  79  3.3.4 Pore Size and Geometry  81  3.3.5 Representation of Pore Size and Geometry ..83 3.4 Compressive Strength of Coke Batches  ....85  3.4.1 Coke Strength as a Function of Variables ..85 3.4.2 Bulk Density  85  3.4.3 Porosity  87  3.4.4 Microstructure  91  3.4.4.1 C e l l Wall Size  91  3.4.4.2 Pore Size  94  3.4.4.3 Number of Pores  94  3.4.4.4 Pore Perimeter  100  3.4.5 S t a t i s t i c a l Aspects of Coke Fracture  100  3.4.6 Theoretical Aspects of Coke Fracture  111  3.4.7 S t a b i l i t y and Hardness  115  3.5 High Temperature Creep  119  3.5.1 Graphitization  119  3.5.2 Isothermal Constant Load Tests  120  viii  3.5.3  C r e e p B e h a v i o u r o f Coke  3.5.4  Visco-elastic  3.5.5 A n a l y s i s  126  Models  ..127  of Creep Data  127  Discussion 4.1  137  Bulk Density  ........138  4.1.1  Reliability  of Bulk D e n s i t y  4.1.2  Oven P r e s s u r e  4.1.3  True D e n s i t y  Results  139  1  141  4.2 M i c r o s t r u c t u r e 4.3 E f f e c t  4.4  138  ..143  of Coking c o n d i t i o n s  on M i c r o s t r u c t u r e  Wall Thickness  ..145  4.3.1  Cell  4.3.2  P o r e S i z e a n d Geometry  4.3.3  Summary o f M i c r o s t r u c t u r a l D e p e n d e n c e on Coking C o n d i t i o n s ...152  Coke S t r e n g t h  145 .........146  1 52  4.4.1  Effect  of Temperature  153  4.4.2  Effect  of M i c r o s t r u c t u r e  154  4.5 C r e e p o f Coke  157  Conclusion  160  References  163  ix  L i s t of Tables Nominal Cokemaking Capacity of the Western World i n 1981 Typical Properties of Blast Furnace Coke ASTM, JIS and ISO Standard Drum Tests Coking Conditions For Test Oven Cokes Characteristics of the As-received Test Oven Cokes As-received Algoma Coke Properties Values of Fixed Compression Test Properties Estimated Variation of Mean Bulk Density and Oven Wall Pressure with Depth Below Coal Line Variation of True Density as a Function of Coking Conditions Variation of the Arithmetic Mean Values of the Pore-parameters Measured using the L e i t z TAS PLUS Image Analyzer for CPR-264 as a Function of the Number of Fields Analyzed Comparison of Porosity Values as Measured Using the Image Analyzer with Those Values Obtained From Pynometry The E f f e c t of Edges on Microstructural Parameters Values of CPR-264 Comparison of C e l l Wall Size Values For Different Cokes Mean and Standard Deviation of the Coke Porous Parameters for  XV  XVI  XVII XVIII  a l l Seven Coke Batches  82  Cummulative 80% Finer Values of the Seven Pore Parameters for the Seven Coke Batches  84  Mean Compressive Strength Values for the Seven Coke Batches at Ambient Temperature and at 1400°C  86  Parameters for the Equation: a=-ne+b  92  Parameters for the Equation: 0=o exp(-be)  92  Weibull Modulus Values of Seven Coke Batches at Ambient Temperature and at 1400°C  110  Calculated C r i t i c a l Stress Intensity Factors Using a Multiple Flaw Model  114  Conditions for Creep Tests  123  Probability that the Assumption of D i f f e r i n g Bulk Densities i s Erroneous for Comparisons Between Coke Batches  140  C e l l Wall Size Values as a Function of Coking Conditions  147  o  XIX  XX  XXI XXII  XXIII  XXIV  Pore Size and Geometry Values as a Function of Coking Conditions  xi  148  L i s t of Figures Figure  Page  1  Schematic Representation of the Ironmaking Blast Furnace  5  2  Effect of Coke Size and Uniformity on Bulk Resistance to Air Flow  8  Schematic Representation of the Layers of Coke, Ore and Flux as They Occur in the Blast Furnace  9  Schematic Representation of the C o a l i f i c a t i o n Process  14  Origins of Different Coal Macerals  16  ASTM Free Swelling Index  21  Schematic Representation of the Charge in a Conventional Coke-oven at Some Intermediate Stage During the Heat  22  Coke Lumps as They Would Be Positioned in a Coke-Oven Battery  23  Arrangement of Typical Commercial Coke-Oven Batteries  25  Cross-section of a Typical Coke-oven Battery (showing the r a i l car)  26  The Schematic Representation of the Diametral Compression Test  34  The Origin and Corresponding ASTM Properties of the Algoma Coke Batches  46  The Positions of Coke Lumps as They Were Assumed to be in the Coke-oven  49  An Example of Coke Lumps Selected for Further Study  52  3  4 5 6 7a  7b  7c 7d  8 9  10  11  xii  12  Schematic Representation of the Procedure for Sectioning Coke Lumps  54  Schematic Representation of the L e i t z TAS PLUS Image Analyzer  56  14  A Typical Coke Microstructure  58  15  A Flow Chart of the Software Used to Quantify Coke Microstructure  60  A Schematic Representation of the Compressive Testing Apparatus  64  The D i s t r i b u t i o n of Bulk Density Values for a l l Seven Coke Batches  69  A Summary of Mean Bulk Density Values For A l l Seven Coke Batches  70  Bulk Density versus Estimated Oven Pressure  71  Mean Compressive Strength vs Mean Bulk Density  88  Compressive Strength vs Bulk Density for CPR-264 at Room Temperature  89  Mean Compressive Strength vs Coke Porosity  90  Mean Compressive Strength vs C e l l Wall Size  93  Mean Compressive Strength vs Pore Area  95  Mean Compressive Strength vs Pore Equivalent Diameter  96  Mean Compressive Strength vs Pore Mean Chord Length  97  Mean Compressive Strength vs Pore Maximum Feret's Diameter  98  Mean Compressive Strength vs Number of Pores Per F i e l d  99  13  16 17 18a 18b 19a 19b  20 21a 21b 21c 21d 21e 21f  xiii  Mean Compressive Strength vs Pore Perimeter Weibull D i s t r i b u t i o n Plot of CPR-264 Mean Compressive Strength Values Weibull D i s t r i b u t i o n Plot of CPR-265 Mean Compressive Strength Values Weibull D i s t r i b u t i o n Plot of CPR-266 Mean Compressive Strength Values Weibull D i s t r i b u t i o n Plot of CPR-267 Mean Compressive Strength Values Weibull D i s t r i b u t i o n Plot of CPR-268 Mean Compressive Strength Values Weibull D i s t r i b u t i o n Plot of CPR-269 Mean Compressive Strength Values Weibull D i s t r i b u t i o n Plot of CPR-270 Mean Compressive Strength Values Mean Compressive Strength vs {Fmax ' exp[-2(Fmax/Fmin) « ep]} -0  5  0  5  Calculated C r i t i c a l Stress Intensity Factors Using Ambient Temperature Compressive Strength Data Calculated C r i t i c a l Stress Intensity Factors Using Compressive Strength Data Obtained at 1400°C S t a b i l i t y and Hardness vs Mean Compressive Strength Normalized Creep Curves Produced at 1500°C and 6.0Mpa Creep of CPR-270 at 1500°C Under Varying Conditions of Applied Stress xiv  Creep of CPR-270 at 9.29Mpa Under Varying Conditions of Temperature Single-Kelvin Unit Interactive-double-Kelvin Unit Three-non-interactive-Kelvin Units Variation in V i s c o - e l a s t i c Model Mechanical Parameters as a Function of Temperature For CPR-270 Variation in V i s c o - e l a s t i c Model Mechanical Parameters as a Function of Applied Pressure for CPR-270 Temperature H i s t o r i e s in the Algoma No.9 Coke-oven Battery at the Top, Middle and Sole Positions Pore Size as a Function of Estimated Coking Temperature Pore Size as a Function of Estimated Oven Pressure C e l l Wall Size and Number of Pores Per F i e l d vs Pore Size  xv  L i s t of Abbreviations and Symbols  A  Fraction of the t o t a l creep a t t r i b u t e d to the f i r s t exponent of the experimental creep equation  a  Crack half-length  ASTM  American Standard for Testing Materials  A.T.  Ambient Temperature  B  BCRA  Fraction of the t o t a l creep attributed to the second exponent of the experimental creep equation B r i t i s h Carbonization Research Association  CSR  Coke Strength after Reaction in 100%  CSR*  Coke Strength after Reaction in 10%  ECE  European Commission for Europe  Fmax  Maximum Feret's diameter  Fmin  Minimum Feret's diameter  H.V.  High v o l a t i l e bituminous coal  ISO  International Standards Organization  JIS  Japanese Industrial  K KJC L.V. L.V.D.T. m M^  Standards  Total creep in v i s c o - e l a s t i c model (at t=») Calculated c r i t i c a l stress intensity factors Low v o l a t i l e bituminous coal Linear Voltage D i f e r e n t i a l Transformer Weibull modulus Spring constant in the i - t h spring of the v i s o - e l a s t i c model xvi  C0 C0  2  2  M.V. P P(a)  Medium V o l a t i l e Bituminous Oven-pressure Probability of f a i l u r e  s  Laplace operator  t  Time  U (t)  Coal  c  Step function of input stress  Vf  Volume fraction of porosity  a,P  Exponention constants of the f i r s t and second terms of creep equation  e  Strain  «p  Percent porosity determined using pynometric and bulk density values  «I  Percent porosity determined using the image analyzer  rji  V i s c o s i t y of the i - t h dashpot in the v i s c o - e l a s t i c model  Pb  Bulk Density  Mpb  Mean bulk density  o-pb  Standard deviation of bulk density values  a o  0  Applied Stress Nominal applied stress  xvi i  1. INTRODUCTION The strength properties of coke from ambient temperature to * 1800°C i s very important for e f f i c i e n t blast furnace operations. This research program was designed to determine the strength properties of coke both at ambient and high temperatures and then to correlate the strength properties with other coke parameters. The coke samples used in t h i s study were supplied by CANMET, Ottawa, but were prepared both in a conventional coke-oven at the Algoma Steel Corporation at Sault Ste. Marie, Ontario and in a test-oven at the Energy Research Laboratories in Ottawa. However, before going into d e t a i l s of the research project, i t w i l l be worthwhile to outline some background on: 1.  the importance and functions of coke,  .2.  how  i t i s produced,  3.  s i g n i f i c a n t properties normally determined for evaluating coke c h a r a c t e r i s t i c s , and  4.  performance.  It i s important to know some of these aspects as the relevance of t h i s research project w i l l be apparent with t h i s background. Coke i s a carbonaceous product made from a special type of coal known as metallurgical c o a l . Table I shows the nominal coke-making capacity of the Western World in 1981 (1). At least ninety-two percent of coke produced in the Western World in 1981 was from carbonizing coal and seventy-eight percent of that was used for the steel  1  2  Steel Industry  COUNTRY  mmt(l)  Coal Industry mmt(l)  %  Algeria Argentina Australia  1. 20 1. 28 5.,20  100.0 too,.0 95.0  Austria Belgium Brazil  2. 35 6. SO <t. 18  100. ,0 97, .2 96 ,5  -  Canada Chile Colombia  6.. U 0..32 0. 36  98 .2 100, .0 100, .0  0.11  Egypt France Germany F.R.  1.,00 5. 35 7.,88  100, .0 i»6 .6 2, .5  0..50 15..09 0.,«0  100, .0 100, .0 100, .0  -  8..18  80. ,2 79,.0 98, .0  2.70 0.10  100.0 90..7 76, .7  Greece India Iran Italy Japan Korea.Republic of  5  kit, .80 it.,50  -  0.0K  1.0  i.kk  0.20 0.15  2.8 3.5  2.35 7.00 <».33  -  -  6.25 0.32 0.36  -  -  1.00 11.47 30.97  _  _  0.50 15.09  -  2.03 9.20  19.8 16.3  -  0.32 0.68  9.3 23.3  0.38  100.0  0.38 0.75 0.65  _  0.51 6.04 4.88  _  .  -  Portugal South Africa Spain  0..51 5..60 k..88  100. .0 92..7 100.0  O.kk-  Sweden Taiwan Turkey  1. 20 0.,91 .76  100. .0 82. 3 100. .0  -  78. 0  53.<» 7k. 5  1.20 1.28  -  -  100. ,0 100. ,0  20<t. 1»  1.8  6. 12 23.09  -  TOTAL  it.O  •  0..75 0..65  (2  -  0.20  Norway Pakistan Peru  7. 2 3 (7)6 Y,7 52. <»0 > 86. 9 3. 58 100. ,0 0..1)0 100. ,0  mmt(l)  _  -  0..48 3..12 2. 2ft  United Kingdom United States Yugoslavia Zimbabwe  Total  mmrtl)  _  Morocco Mexico Netherlands  It.  %  Independent  0.7 2.0  -  -  -  7.3  -  -  _  _  17.7  36.78  11.0  -  20.SI  0.48  3.kit 2.92  -  3.19  7.20  10.21 56.70 ».60  -  -  3.32 . 30.2 0.70*"" 1.2  o.«o  1.20 1.10 ft.76  -  U>  *. 1 11.9  -  11.00 60.30 3.58 0.30  8.0  261.76  (I (Millions of metric tons (2) Blast Furnace coke producers (3) Plant using Beehive Ovens (4) Found'-y coke producers  Table I.  Nominal Cokemaking Capacity of the Western World i n 1981 ( r e f .  1).  3 industry. As indicated in Table I , the vast majority of coke was produced for the steel industry where i t was used to make pig iron in the blast furnace. In pig iron production, coke, ore and fluxes are the ingredients charged  into a blast  furnace. Coke consumption in the blast furnace i s more than 500kg per ton of pig iron (2). A smaller type of blast furnace i s also normally used in producing lead from the roasted lead oxide. However, lower temperatures  (^1200°C in  the hearth) and a higher COj/CO r a t i o in the gas in the lead blast furnace, mean that coke consumption i s considerably less in these furnaces, than in the iron blast furnace. The coke used in blast furnaces i s derived from blends of low, medium and high v o l a t i l e bituminous c o a l s . Such coals which are used to produce blast furnace coke are often termed "metallurgical coals" and only metallurgical coals are suitable for blast furnace coke production.  1.1 ROLE OF COKE IN THE BLAST FURNACE Coke has three important functions: 1.  provides fuel for combustion and supplies heat to the charge,  2.  supplies reductant gases for smelting the iron ore, and  3.  physically supports the burden while i t descends down the blast furnace shaft.  1.1.1  COKE AS A FUEL Figure 1 i s a schematic representation of the  ironmaking blast furnace (2). Shown are the temperatures varying with height in the shaft and the p r i n c i p l e reactions that occur at each l e v e l . At the tuyere l e v e l , hot blast a i r between 900 and 1000°C i s injected and t h i s reacts with the coke to produce mainly carbon dioxide. The temperature in t h i s region i s between 1800 and 2000°C at which point, the only s o l i d phase present i s the coke. The carbon dioxide penetrates to the center of the furnace forming carbon monoxide v i a the Boudouard  reaction. The heat involved from  burning coke at the tuyeres i s needed to achieve the minimum temperature required in t h i s region (~1800°C). This temperature at the tuyere i s necessary to ensure that the burden higher up in the furnace i s heated s u f f i c i e n t l y .  1.1.2 COKE AS A REDUCING AGENT Both carbon dioxide and carbon monoxide are produced above the tuyere region of the blast furnace. The carbon monoxide i s the reducing agent needed to reduce the iron ore. The blast furnace can be divided into two separate reduction zones represented by the dotted l i n e in Figure 1: the d i r e c t reduction zone (the lower half of the furnace) and the indirect reduction zone (the upper h a l f ) . In the d i r e c t reduction zone, metal oxides are reduced by carbon monoxide to form carbon dioxide, however,the  Boudouard  reaction i s also occuring to produce carbon monoxide and  5  Ore + coke • fluxes  n r  Top gas  2Fej0  3Fe 0j • CO 2  Fe 0 3  • CO  4  = C0  2CO  Stack  FeO + C O  1000° C  , > 1800° C ,  Hearth I  1400° C  + CO  2  = CaO + C O j  COj + C FeO + C O  = 2CO ( = Fe • C O j f  2  = Mn + C O = & + 2CO  • 2C  COj + C  c+o  2  +C  CaCOj  Si0  Tuyeres l o o o o o o o l  2  Fe + C 0  MnO + C Bosh  4  = 3FeO + C 0  2  = 2CO \ = COj /  S + C a O + C = CaS + C O C  =  c  Slag Hot metal  Figure  1.  Schematic Blast  R e p r e s e n t a t i o n of t h e I r o n m a k i n g  Furnace  (ref. 2).  6  consuming coke at the same time. For example: FeO + CO = Fe + C0  2  + C =  C0  2  (A)  2CO  FeO + C = Fe + CO  The Boudouard reaction in [A] controls the CO2/CO ratio so the net reaction results in the consumption of coke. The  indirect reduction zone i s characterized by the  consumption of carbon monoxide to produce carbon dioxide. This zone exists above the 900°C isotherm  (dotted l i n e in  Figure 1). Below 900°C , the Boudouard reaction i s sluggish and therefore provides no s i g n i f i c a n t contribution to the reduction mechanisms. In the upper regions of the stack Fe 02 2  and F e 0 3  4  are reduced to FeO. It i s i n t e r e s t i n g to  note that a "sooting" reaction also occurs in the upper regions of the furnace forming carbon dioxide and carbon from carbon monoxide. This i s undesirable because i t consumes carbon monoxide needed for the the reduction of the ore, thereby increasing the need for a higher coke consumption rate.  1.1.3  COKE AS A PHYSICAL SUPPORT FOR THE BURDEN Perhaps the most important function of coke in the iron  blast furnace  i s i t s a b i l i t y to physically support the  burden as i t descends down the shaft. The blast furnace  is a  counter current reactor where, as mentioned before, hot reducing gases formed at the tuyere region must find their  7  way to the top of the shaft while the burden progresses downward. An evenly d i s t r i b u t e d upward gas flow i s necessary for stable blast furnace operation. This ensures an evenly reduced burden as well as even heat exchange between the hot gas and the descending burden. Figure 2 demonstrates how gas flow i s impeded by a decrease in the harmonic mean size of coke lumps (3). It i s , therefore, desirable to use a coke which degrades as l i t t l e as possible during i t s descent. This i s p a r t i c u l a r l y important in the lower half of the furnace where, between 1100°C and 1400°C, the burden fuses and melts leaving coke as the only s o l i d phase present. Figure 3 shows in some d e t a i l how the burden behaves in the blast furnace. The burden in the melting zone fuses into s t r a t i f i e d layers of coke, ore, and flux (4). The gases moving upward rely on the coke layers (coke s l i t s ) to allow these to escape into the lumpy zone at the top half of the furnace. Closer to the raceway, the phases present are l i q u i d metal, slag, and coke. In this region, the coke lumps are supporting the entire burden while allowing hot metal to percolate to the hearth and the combustion gases to penetrate to the center of the furnace. Coke fines near the raceway w i l l prevent adequate penetration of gases to the center of the furnace, therefore forcing the gases upward near the wall and trapping molten metal and slag from flowing downward resulting, in i n s u f f i c i e n t tapping volume and increasing tapping frequency. Coke fines suspended in  8  HARMONIC MEAN SIZE Sh(mm) FIGURE  2. E F F E C T OF COKE SIZE BULK  RESISTANCE  A N D UNIFORMITY  TO A I R F L O W  ON  9  C O K E  Figure 3.  Schematic Representation of Layers of Coke, Ore, and Flux as They Occur in The Blast Furnace (ref. 4 ) .  10 the slag and l i q u i d metal can r e s u l t in uneven tapping. It i s apparent efficiently  that the blast furnace w i l l operate more  i f the coke lumps maintain their i n t e g r i t y as  much as possible while physically supporting the burden and being attacked by carbon dioxide and a l k a l i vapours. For this reason, coke must be physically strong without  losing  i t s strength while being subjected to the harsh conditions in the lower part of the furnace.  1.2 COKE CHARACTERISTICS NECESSARY FOR THE BLAST FURNACE The carbon content of coke i s the primary agent, in addition to the small concentrations of hydrogen, for the reduction of iron ore. Therefore, i t i s desirable to reduce the impurity components in the coke such as moisture, ash, and sulfur as much as possible. A good coke should be reactive enough to supply the reducing gas required for a particular ore (5). It i s generally better, however, to have an ore which i s very reactive and a coke which has low  reactivity.  This lowers the coke consumption by reducing the effect of the Boudouard reaction. A coke with low r e a c t i v i t y i s more l i k e l y to maintain i t s mechanical  strength in the direct  reduction zone of the blast furnace. The impurities in the coke have detrimental effects other than lowering the coke-carbon content. A low sulfur content i s prefered since this impurity generally ends up in the pig iron which makes r e f i n i n g to steel more d i f f i c u l t . Moisture escaping from the coke in the upper regions of the  11 furnace w i l l disturb the reducing gases as they pass upward through the burden. Perhaps the most detrimental impurity in coke i s ash which has two negative e f f e c t s on the performance of coke: catalyzing the Boudouard reaction and reacting internally with the carbon in the matrix. The presence of minerals i s known to a f f e c t the r e a c t i v i t y of coke to C0  2  (6). In these  investigations various ash components commonly found in coal were separately added to a very low ash coal (less than 1.0%)  to determine  their effect on r e a c t i v i t y with CO^.  The  results show that r e a c t i v i t y increased with the following order of e f f e c t : K & Na > Ca > Fe > Mg > ( S i , A l , T i ) The a l k a l i oxides greatly increased the r e a c t i v i t y of coke with S i , A l , and T i having a much lower e f f e c t . Ash-carbon reactions were found to occur at temperatures and  between  1600  1800°C. It was shown that a l l the oxides in the minerals  (ash) contained in the coke were reduced producing carbon monoxide as a product (6). S i l i c o n was  found to react most  readily with the carbon producing s i l i c o n carbide and carbon monoxide. These reactions a l l contributed to carbon  loss  thereby weakening the coke matrix. Typical blast furnace coke c h a r a c t e r i s t i c s are given in Table II (5). The f i r s t two columns are strength indices from drum t e s t s . These indices w i l l be explained l a t e r . Coke c h a r a c t e r i s t i c s must be c l o s e l y controlled for optimum blast furnace operation.  12  Metallurgical Coke III  II  I  Property  > go mm  > 60 mm  > 40 mm  Tumbler strength (%)* Abradability, M„ (%)" Moisture (% of raw product) Ash (%) Sulfur (% of dry product) Volatile matter (% of dry product) Alkali (% of dry product)* Bulk density (kg/m*)  85 6 1.5 +0.5 8.5 +0.3 0.9 ±0.03 < 1  85 6 2.5 ±0.5 8.5 ±0.3 0.9 ±0.03 < 1  85 6 3.5 ±0.5 8.5 ±0.3 0.9 ±0.03 < 1  0.26  0.26  0.26 438  • According to DIN 51 717, conforming with ISO/R—1967. * Content of Na,0 and K,0 determined in different plants.  Table I I .  Typical Properties (ref.  5).  of Blast Furnace Coke  13 1.3 PRODUCTION OF BLAST FURNACE COKE Since the major f r a c t i o n of coke produced in the world i s made from metallurgical coal, i t i s important to know the coal c h a r a c t e r i s t i c s suitable for coke-making.  1.3.1  COAL CHARACTERISTICS 1.3.1.1 Coal Chemistry Coal i s composed of organic minerals containing  carbon, hydrogen, oxygen, and small amounts of sulfur and nitrogen. Studies have shown that coal i s made up of both aromatic and a l i p h a t i c molecules (7). Aromatic molecules consist of rings of six carbon atoms joined to form a hexagonal shape - the simplest type being benzene. Aliphatic molecules are chains of hydro-carbons - the simplest form i s methane. There i s a d e f i n i t e relationship between carbon content of the coal and the r e l a t i v e amounts of a l i p h a t i c and aromatic molecules. The carbon content of the coal increases with the aromatic/aliphatic  ratio.  1.3.1.2 Coal Rank There are several c l a s s i f i c a t i o n s of coal  depending  on their o r i g i n s . Figure 4 shows the stages of metamorphosis to form various types of coal (5). The metamorphosis of coal increases with increasing time and pressure inside the earth. The term most often used to represent the degree of metamorphosis i s rank, which  14  ANTHRACITE  Figure  4.  Schematic Representation of the Coali f ication Process ( r e f . 5 ) .  15 increases with increasing metamorphic change. In other words, anthracite has a high rank, whereas l i g n i t e i s a low rank c o a l . In general, rank increases with increasing carbon content, decreasing v o l a t i l e matter content, decreasing moisture content, and increasing c a l o r i f i c value. To form a high rank coal (low v o l a t i l e matter content), extreme depths and times of the order of  150 m i l l i o n years are needed. The best coals for  making metallurgical coke are generally a combination of high v o l a t i l e coal for low bulk density , and a medium or low v o l a t i l e coke for high strength (5). 1.3.1.3 Coal Minerals Coal i s not a homogeneous substance, but i s comprised of metamorphized organic minerals (from different  sources) c a l l e d macerals. The origins of  d i f f e r e n t macerals are l i s t e d in Figure 5 (5). During metamorphosis, the chemical composition and the percent aromatic structures changes but the proportion of each maceral  in the coal change very l i t t l e . The most common  maceral shown in Figure 5 i s v i t r i n i t e (5). This along with r e s i n i t e and exinite are termed "reactive" due to their a b i l i t y to become p l a s t i c and release v o l a t i l e s during p y r o l y s i s . V i t r i n i t e makes up 75 to 80 percent of coals in the bituminous range. Reactive macerals are noted for their high H/C r a t i o . The other important macerals are c a l l e d "inerts" which are high carbon macerals that release small  16  Source  Coal  Material  Peat Swamp  "Wood"  Humified — "Decomposed""Charred" -  Waxy Exines  Components  Incorporated  "Chelated"  Vitrinite *» Micrinite Fusinite fc  —  Incorporated -  Resins  (  /  r  Exinite  r  „ .  ^ , fc. Resmite  -  Inorganic Ions  Mineral Grains  Figure 5 .  Precipitated -  -fc- Minerals  Incorporated  -»>  Minerals  Origins of Different Coal Macerals (ref. 5)  17 amounts of v o l a t i l e compounds and do not become p l a s t i c during p y r o l y s i s . Macerals such as f u s i n i t e and m i c r i n i t e are c l a s s i f i e d under the name i n e r t i n i t e . These originate from charred or decomposed plant matters. 1.3.1.4 Reflectance Coal s c i e n t i s t s have found that a l l macerals exhibit distinguishing o p t i c a l properties when viewed under an o p t i c a l microscope. Each maceral r e f l e c t s and absorbs l i g h t d i f f e r e n t l y . A special microscope i s used in which the l i g h t r e f l e c t e d off the specimens can either to the eye-piece or to a photomultiplier (8,9). The  go  tube  incident l i g h t i s polarized at an angle of  45° and a f i l t e r with a peak transmittance of 546  nm  (monochromatic green) i s used at any position along the path of l i g h t . A drop of immersion o i l i s added on sample to provide more contrast between the  the  various  phases. The percent reflectance (in o i l ) of each phase i s measured by the photomultiplier  and  i s based on  glass  standards. Since 75-80% of the coal i s v i t r i n i t e , i t i s possible to represent the rank of the coal by i t s v i t r i n i t e reflectance as described  above. In  the higher rank materials have higher  general,  reflectance  values. The best coking coal blends have reflectance between 1.3 and  1.5  (percent) in o i l .  18  1.3.2  COAL PYROLYSIS Coal p y r o l y s i s  (or c a r b o n i z a t i o n ) i s the term used  to  d e s c r i b e the t r a n s f o r m a t i o n of c o a l t o coke. As c o a l i s heated  i n the absence of oxygen, many r e a c t i o n s occur up to  1000°C. Between 110°C  and  150°C, absorbed  r e l e a s e d . Gases, such as H S,  are r e l e a s e d between 150°C and  2  350°C. The c o a l i t s e l f  moisture i s  begins to decompose between 300°C and  350°C. Above 350°C, thermal decomposition  becomes r a p i d with  the r e l e a s e of primary v o l a t i l e gases and the s o l i d mass begins t o l i q u e f y  (mesophase). Between 500°C and  1000°C the  s o l i d mass undergoes a p o l y m e r i z a t i o n r e a c t i o n which the l i q u e f i e d c o a l t o s o l i d i f y , when d e n s i f c a t i o n  causes  and  s h r i n k a g e occur s i m u l t a n e o u s l y with the r e l e a s e of  secondary  v o l a t i l e components such as methane and carbon monoxide. At 1000°C, the c o k i n g p r o c e s s i s v i r t u a l l y complete with the product being coke c o n t a i n i n g m i n e r a l matter  (ash).  1.3.2.1 P y r o l y s i s R e a c t i o n The p y r o l y s i s r e a c t i o n has been r e p r e s e n t e d  by  t h r e e s i m p l i f i e d c o n s e c u t i v e r e a c t i o n s (10): K, P—\>M  K  (B)  2  M K,  R —>  •R  + G,  S + G  2  where P denotes the o r i g i n a l c o a l , M denotes the  (C)  (D)  19 mesophase (or l i q u i d phase) , R i s the semi-coke, S i s the f i n a l coke product, G, and G secondary v o l a t i l e  2  are the primary and  gases, respectively. These reactions  have been treated as being f i r s t order, but  in r e a l i t y  they are probably more complex since each reaction  [B-D]  represents a large number of separate reaction steps (7). It has been shown that the r e l a t i v e magnitude of each of these reaction rates determines whether or not a coal i s suitable to make metallurgical coke (10). A good coal should be fusable and  (good bonding c h a r a c t e r i s t i c s )  t h i s i s determined by the amount of mesophase  produced and by i t s f l u i d i t y . If there i s an accumulation of mesophase, the coal i s said to be caking and w i l l produce coke. This w i l l occur i f K^ larger than K .  If K  2  2  i s larger than K,,  i s much  a char  (non-fusible product) w i l l be formed. It i s important to r e a l i z e that a l l metallurgical coals are caking coals but not a l l caking coals w i l l produce metallurgical coke. 1.3.2.2 F l u i d i t y Fluidity  during  Pyrolysis  i s commonly measured using a Gieseler  plastometer (11). It e s s e n t i a l l y consists of a s t i r r e r , which has an e l e c t r i c  motor, and a hysteresis brake  c o n t r o l l i n g the torque. Each rotation i s counted p h o t o - e l e c t r i c a l l y and the f l u i d i t y i s measured by d i a l d i v i s i o n s per minute (100 d i a l d i v i s i o n s per  the  20  revolution). Another method of assessing the mesophase i s the Free Swelling Index. The Free Swelling Index i s measured by placing one gram of coal in a s i l i c a c r u c i b l e and v i s u a l l y comparing the coke button produced afterward with a set of standards (see Figure 6) (39). Good metallurgical coal has a Free Swelling Index between f i v e and seven.  1.3.3 CONVENTIONAL COKEMAKING PROCESS Figure 7a i s a schematic representation of the charge in a conventional coke-oven at some intermediate stage during the heat. The coke-oven consists of a v e r t i c a l retort approximately 6m high, 15m long, and 0.5m  wide. Coal i s  charged into the chamber between two externally heated refractory walls. The charge mass closest to the wall experiences a higher temperature than at the center. Near the wall, the mass softens and then s o l i d i f i e s into coke where the temperature i s above 600°C. The p l a s t i c zone moves toward the center of the coke-oven as the charge heats up and the p a r t i c u l a t e coal near the center l i q u e f i e s and decomposes. The large fissures shown in Figure 7a are a result of d i f f e r e n t i a l contraction during r e s o l i d i f i c a t i o n of the charge near the oven wall. The f i n a l coke product i s a highly porous, carbonaceous product containing large fissures throughout. Figure 7b shows the arrangement of coke lumps with respect to their position in a coke- oven  above 5 0 0 ° C  • 350* C i-500°C Coke Material  Refractory  wot  Figure 7a.  Coke  Ptoitk etoge  less than  350 C  Coal particles  #  Intcrstfctt  Ptxtkulou coal  Schematic Representation of the Charge in a Conventional Coke-oven at some Intermediate Stage During the Heat (ref 52).  Cfcarft centra  Figure 7b.  Coke Lumps as They Would Be Positioned i n a Coke-oven Battery  (ref.4).  24 battery(arrangements were suggested by CANMET). The coking chambers are arranged in batteries as i l l u s t r a t e d i n Figure 7c. Between two oven walls there i s a flue chamber which heats two oven walls simultaneously. The flue i s often fueled by the v o l a t i l e gases from the coking process i t s e l f . When the charge has reached the f i n a l temperature (1000-1200°C ), the mass i s "pushed" out of the slot into r a i l cars and subsequently water quenched (see Figure 7d). The coking process from charging to pushing can take up to twenty one hours.  1.4 TESTING METHODS FOR COKE Three c h a r a c t e r i s t i c s of coke are normally used to predict i t s performance in the blast furnace: 1.  porosity and true s p e c i f i c gravity  2.  r e a c t i v i t y to carbon dioxide, and  3.  tumbler strength t e s t s .  Porosity i s studied due to i t s contribution to the weakening of coke and to coke r e a c t i v i t y . Reactivity assessment i s important for minimizing the coke consumption rate. Tumbler tests are used as general information on the a b i l i t y of coke to r e s i s t volumetric breakage and abrasion.  1.4.1 POROSITY AND TRUE SPECIFIC GRAVITY In North America, coke porosity and true s p e c i f i c gravity are normally assessed in accordance with ASTM designation Dl67-73 (12). The apparent s p e c i f i c gravity i s  Figure  7d.  Cross-section  of a T y p i c a l Coke-oven B a t t e r y  the r a i l c a r )  (ref. 4 ) .  (showing  27 f i r s t measured by a water displacement method. The true s p e c i f i c gravity i s determined using pycnometry. The porosity i s then calculated using the measured true and apparent s p e c i f i c gravity values. Some workers have begun to explore ways of measuring porosity using computerized image analyzers. Such methods rely on the subjective way the operator determines the level of the  greyness (explained l a t e r ) the pore features appear on computer monitor. Since t h i s work i s s t i l l in the  development  stage, no standard has yet been presented.  1.4.2 TUMBLER TESTS Tumbler testing of coke i s the most common method used in  industry for determining coke q u a l i t y . There are many  accepted standards for measuring the drum strength of coke. Leeder, Price, and Gransden have made comparisons of d i f f e r e n t drum standards as given in Table III (13). From Table I I I , i t i s apparent that the four standards shown, vary s i g n i f i c a n t l y from one another. Variations include coke lump size, sample weight, drum dimensions, drum rotational speed, duration of the test, moisture content of the coke, and the number and widths of the l i f t s . L i f t s are s l a t s in the  drum which l i f t the coke lumps upward with the  revolution of the drum u n t i l the angle of the l i f t s  i s large  enough to cause lumps to f a l l . These variations have made i t d i f f i c u l t to make correlations between the d i f f e r e n t standard drum tests, and such correlations are unreliable  Strength  Test Method Coke Test  Designation S i z e  ASTM  D 294-64  JIS Drua 2151-72 Test  Drum Dimensions  Weight Moisture Width Dlam  L i f t e r s Width  Test Drum Duration T o t a l RPM Rev 8 Breakage (min)  3x2 In. 22 l b  dry  18 In. 36 In.  2  2 In.  24±1  58  +5 Omm  dry  1.5m  6  250mm  15  2  10 kg  1.5m  1400  30 150  10 Mlcua*  R 556  +60mm  50 kg  <5  1.0m  1.0m  4  100mm  25±1  100 4  (Not .e h a l f m:.cum IRSID*  R 1881  +20mm  50 kg  <3  t c! S t  1.0m  Indices  Abrasion  Z>1" Z>V S t a b i l i t y Hardness Factor Factor Z>15mm-DI 30  15 Z>15nm-DI 150 15 Z>40am, M*»o  Z<10ssi  us< >s h a l f i :he weight ar d a drum 0.5m : n width)  1.0m  4  100mm  25±1  20  500  X>40,20mm KlOmm  I«»0.l20  IlO  *Round-hole s i e v e s used - other t e s t s use square-hole sieves  Table I I I .  ASTM, JIS, and ISO Standard Coke Drum Tests ( r e f . 13).  w  29  (13). Two  types of breakage mechanisms occur during drum  t e s t s : volume breakage and surface abrasion. Peirce and Horton  (14,15) and Peirce et a l (16) have shown that the  volume breakage i s associated with the propagation of large fissures and that surface abrasion i s due to l o c a l i z e d stress at contact points between the lumps. Considering t h i s , as well as observing Table I I I , i t i s obvious that the ASTM drum test r e l i e s more on abrasion than on impact to degrade coke since i t only has two small l i f t s . In contrast, the JIS, Micum and IRID standards specify six or four wide l i f t s respectively, which i n f l i c t more impact breakage.  1.4.3  REACTIVITY TESTS There has been no ASTM standard proposed to date for  determining the r e a c t i v i t y of coke. Three methods are worth noting: the ECE test (European Commision for Europe) (6), the NSC  test (Nippon Steel Corporation) (17), and a test  used by Jeulin et a l (18). The ECE test requires 7 grams of -3+1mm p a r t i c l e s to be reacted in pure C0  2  for t h i r t y minutes. The r e a c t i v i t y i s  expressed as a reaction rate constant by monitoring the C0  2  in the off gas. The NSC test uses 200 grams of +l9-21mm coke reacted in pure C0  2  for 120 minutes at 1100°C. The r e a c t i v i t y i s  calculated as the weight present l o s t during the t e s t .  30  Jeulin et a l (18) used -30+20mm coke lumps which were g a s i f i e d in a gas mixture containing 10% C0 , 2  H, 2  20% CO,  10%  and 60% N . The sample was heated to 650°C under 2  nitrogen and from 650 to 1200°C at 200°C per hour in the reactive gas. The sample weight was continuously monitored during the experiment. The relevant parameter studied was the g a s i f i c a t i o n threshold temperature. This i s defined as the temperature where the rate of weight loss due to g a s i f i c a t i o n becomes s i g n i f i c a n t . The importance of the g a s i f i c a t i o n threshold temperature stems from the fact that the Boudouard  reaction consumes heat. Therefore, a low  g a s i f i c a t i o n threshold w i l l decrease the length of indirect reduction zone of the blast thermal p r o f i l e w i l l  furnace and a s h i f t in i t s  occur.  The ECE and the NSC tests are similar  in p r i n c i p l e but  the ECE test uses a small coke p a r t i c l e s i z e . The use of larger p a r t i c l e sizes in the NSC test distinguishes mass transfer rates between cokes as well as r e a c t i v i t y of the s o l i d i t s e l f . The method used by Jeulin et a l (18) helps to predict the effect the coke w i l l have on the thermal p r o f i l e of the blast  furnace.  1.4.4 COKE STRENGTH AFTER GASIFICATION A test which i s gaining increased popularity i s the coke strength after reaction test  (CSR). This involves  measuring the strength of -21+19mm coke after  i t has been  g a s i f i e d at 1100°C for two hours in 100% carbon dioxide.  31 Nippon Steel Corporation employs an I-drum for 600 revolutions at 20 revolutions per minute and t h i s has become the most commonly used CSR t e s t . The CSR test has been employed by Fellows and Wilmers (17), and by BCRA workers (19). Both groups found approximately negative linear relationships between r e a c t i v i t y and post reaction strength. Brown et a l (6) found that the CSR test exaggerates the extent of reaction *  occuring in the blast furnace. So the CSR developed using 10% C0  2  and 90% N  2  test was  which more closely  represented the extent of reaction of tuyere coke. It was *  found that l i t t l e correlation existed between CSR  and CSR  t e s t s . The results show a general weakening of coke after gasification. 1.4.5 SHATTER TESTS The drop shatter test for coke i s perhaps the most primitive of the coke quality assessment techniques. The ASTM standard (20) for measuring the shatter strength of coke requires that f i f t y pounds of coke be dropped from a height of six feet . The minus half inch i s removed from the sample and the procedure i s repeated three more times. A sieve analysis i s then performed on the coke sample. The shatter test has the disadvantage that i t only measures the coke's resistance to impact and not abrasion.  32 1.5 PREVIOUS STRENGTH TESTS AND ANALYSIS With the r e a l i z a t i o n that the quality of coke a f f e c t s the operation of the blast furnace , coke researchers have been turning towards a more fundamental approach to assessing coke q u a l i t y . High temperature drum tests are being used to see how  impact and abrasion resistance are affected by high  temperatures. Also, since drum tests induce very complex stress states, more simple states of stress such as uniaxial compressive and t e n s i l e stresses are being used to characterize coke strength. Microstructural techniques have recently been employed to determine the effect of microstructural variations on the strength of coke.  1.5.1  HIGH TEMPERATURE TUMBLER TESTS A comprehensive review made by Reeve, Price, and  Gransden  (21) produced several conclusions about the tumbler  strength of coke at high temperatures: coke strength i s lower at high temperatures than at ambient temperature, there i s no c o r r e l a t i o n between cold and hot strength tests, and the higher the cold Micum index value, the lower the reduction in hot Micum values at elevated temperatures. Patrick and Wilkinson (22) and other workers at the BCRA (19) found a general decrease in the drum strength indices as temperature increased. This suggests that resistance to volume and surface breakage i s reduced as temperature of testing i s increased.  33  1.5.2  FUNDAMENTAL STRENGTH TESTS The nature of stresses induced in coke during tumbler  tests i s d i f f i c u l t to assess. Therefore, more fundamental test methods such as uniaxial compressive and t e n s i l e strength tests are being more frequently used. Both these tests on coke specimens have been performed at ambient and elevated temperatures. 1.5.2.1 Ambient Temperature Tests Perhaps the f i r s t group to consider testing coke on a fundamental basis was Holoway and Squarcy (23). They performed compressive strength tests on 1.27cm cubical specimens and obtained strengths ranging from 4.41MPa to 31.83MPa for a l l cokes tested. However, only three tests were performed on each coke specimen, thereby r a i s i n g doubt as to the c r e d i b i l i t y of these values for representing the whole population. The BCRA researchers (19) performed t e n s i l e strength tests on 16 i n d u s t r i a l cokes using the diametral compression test. This test i s shown schematically in Figure 8a. The t e n s i l e stress i s calculated using the equation:  a  t  = 2w/(rrDt)  (1 )  where w i s the applied load, D i s the specimen diameter, and t i s the thickness of the disk. F i f t y specimens were used for each coke where mean t e n s i l e strengths ranged  34  lot  (cl  w  The Schematic Representation  of the Diametral  Compression Test Showing: a. The Disc Sample b. The Typical Fracture Pattern, and c. The Stress F i e l d Within The Disc ( r e f . 25).  35 between 1.62  and 6.0MPa. Similar tests were conducted by  Patrick and Wilkinson  (22) y i e l d i n g nearly the same  r e s u l t s . Klose and Suginobe (24) measured t e n s i l e strengths of i n d u s t r i a l cokes also using compression. Strengths ranged between 2.0  diametral and 7.0MPa.  There was a d e f i n i t e strength dependence on specimen size with larger samples tending towards being weaker. No s t a t i s t i c a l evidence showing an e f f e c t of cross-head speed ( i e . strain rate) was The diametral  found.  testing method has two  serious  drawbacks which a f f e c t the p r a c t i c a l i t y of i t s use for assessing coke q u a l i t y . The theory from which equation 1 was  derived, states that for the method to y i e l d the  correct t e n s i l e strength, the material being tested must be homogeneous and e l a s t i c (24). It i s well known that coke i s a very heterogeneous material. Samples which do not exhibit the fracture pattern in Figure 8b, can not be included in the test r e s u l t s . The  stress f i e l d shows  that the compressive stress at the points of loading tends toward i n f i n i t y  (Figure 8c), and that an uneven  t e n s i l e stress f i e l d i s induced in the specimen. Furthermore, the t e n s i l e strength i s measured by the fracture stress at the point of loading. In most cases, small tributary cracks occur at the point of loading which raises question as to the mode of fracture (25). Klose and Suginobe (24) found that only 25% of the coke discs tested produced the fracture pattern in Figure  8b.  36  Therefore, a very large number of tests are needed for s t a t i s t i c a l r e l i a b i l i t y of the data. Also, by discarding 75% of the tests made, there w i l l be a bias toward the homogeneous samples which may not be representative of the  coke as a whole.  1.5.2.2 High Temperature Tests High temperature compressive and t e n s i l e strengths of coke have been less widely studied than tests at ambient temperature due to the inherent d i f f i c u l t y of performing the large number of tests needed to characterize coke. The e a r l i e s t work encountered was that of Holoway and Squarcy (23). Compression tests were performed on 0.635cm (edge length) cubes at 1650°C. They found that the compressive strength was higher than that observed at room temperature. However, only three to six samples were tested for each coke on a d i f f e r e n t testing machine than the Baldwin t e n s i l e machine used at room temperature. The test sample size was smaller than those tested at room temperature. This i s not a good practice since coke has been shown to exhibit a strength dependence on specimen size (24). Patrick and Wilkinson (22),  BCRA workers  (19), and J e u l i n et a l (18), a l l  observed a strength increase as temperature increased. They attributed t h i s behaviour to the p o s s i b i l i t y that coke continued to graphitize after exceeding the f i n a l coking temperature. This i s in accordance with dilatometer tests performed by Golezka and Roberts  37 (26,27), Golezka et a l (3), and Fellows and Willmers (17).  These workers have shown that coke expands upon  heating to the f i n a l coking temperature and subsequently contracts as graphitization continues above t h i s temperature.  1.5.3  PORE STRUCTURE AND COKE STRENGTH With the a v a i l a b i l i t y of automated microscopes with  attached image a n a l y t i c a l f a c i l i t i e s , a new technique of coke quality assessment  i s being developed. This technique  can quickly examine the pore structure of a coke sample, and with the a i d of computerized image processing, can statistically  quantify such features as pore size,  shape,  area fractioning, and c e l l wall thickness. Klose and Suginobe  (24) used a Leitz TAS system to  determine the porosity and maximum pore size by examining half of each fractured t e n s i l e specimen. They found that coke t e n s i l e strength correlated well with the porosity and maximum pore size according to the semi-empirical r e l a t i o n :  a = (K//dp)[exp(-Be)]  (2)  where dp i s the maximum pore size, e i s the porosity (pore f r a c t i o n ) , and K and B are empirical constants. This equation i s a modified form of porosity dependence on strength equation developed by Knudsen (28) for porous alumina:  38  o = a  where o  0  0  exp (-be)  (3)  i s the strength of the non-porous body. The  K/Vdp i s derived from the G r i f f i t h  term  crack theory.  P i t t and Rumsey (30) claimed to have estimated the strength of the non-porous coke by extrapolation of strength-porosity relationships developed by previous workers. These values d i f f e r e d by an order of magnitude depending on the equation used, with values ranging between 16MPa and 524MPa for one type of coke tested. They also attempted  to measure the strength of non-porous coke by  crushing 0.13mm coke p a r t i c l e s between two s t e e l plates. The values obtained were within the range of values calculated by extrapolation, but were determined on the assumption  that  the coke p a r t i c l e s cruched were spherical. J e u l i n et a l (18) used a texture analyser to measure pore volume f r a c t i o n and s p e c i f i c pore surface area. They found that the best correlation between t e n s i l e strength and pore volume fraction was obeyed by equation 2. However, their derivation of t h i s equation was more t h e o r e t i c a l l y based than that of Klose and Suginobe (24). The most detailed studies made to date were those of Patrick et a l (31-36). These studies included a very detailed analysis of the porous structure of coke by employing  a Quantimet 720 Image Analyzer. The analysis  was  divided into two sections: f i e l d data (values describing each f i e l d of view as a whole), and feature data  (individual  39 features were examined separately). In an e a r l i e r work (32), pore and wall sizes were assessed by horizontal chord s i z i n g . The analysis also included the number of pores per f i e l d . Porosity was determined as the area f r a c t i o n of the pores. It was found that t h i r t y to forty f i e l d s of view were needed to characterize the pore structure of a p a r t i c u l a r coke type. Most pores were found to occur in the size range 1 urn to 120Aim and a d e f i n i t e p o s i t i v e skewness was  observed  in the pore size d i s t r i b u t i o n . Tensile strengths of some cokes were found to f i t the Knudsen equation (28), but variations from the relationship were found to occur when a variety of coke types were tested (33). Patrick (31), and Patrick and Stacey (35), found d e f i n i t e trends between coke structure and coking conditions. They concluded that preheating coal charges produced a greater number of smaller pores and decreased porosity which resulted in a stronger coke. Additions of up to 30% of petroleum coke decreased the porosity,  increased  the wall thickness, and increased the number of smaller pores which had the combined e f f e c t of increasing t e n s i l e strength. The strength.was found to be related to pore and wall size by equation 4:  a N = 10  5  (W/P ) - K 2  (4)  where o i s the t e n s i l e strength, N i s the number of pores per f i e l d , W i s the wall size, P i s the pore s i z e , and K i s  40 a constant. It i s clear that equation 4 has no theoretical basis, and i s therefore not r e l i a b l e for predicting the strength of cokes outside the range of cokes tested. Patrick and Stacey (35) and other workers at the BCRA (34) both realized the inadequacy of equation 4 and derived another equation which i s more t h e o r e t i c a l l y based. They u t i l i z e d an equation of the form proposed by Jeulin et a l (18),  and by Klose and Suginobe (24) which i s similar to  equation 2. The crack length (dp in equation 2) was assumed to be the mean maximum Feret's diameter of the pores. The Quantimet 720 Image Analyzer used i s capable of measuring the maximum and minimum c a l i p e r dimensions  (Feret's  diameter) of a feature (in t h i s case individual pores). The constant b (in equation 2) was assumed (34,35) to be the stress concentration factor: 2( Fmax / Fmin )  0  ,  5  of an e l i p t i c a l crack perpendicular to the applied stress. The resulting equation used was then:  a = 450 (Fmax)" * 0  5  exp[ -2( Fmax / Fmin )  0  ,  5  p ]  (5)  where Fmax and Fmin are the mean maximum and mean minimum Feret's diameters respectively, and p i s the porosity. This equation was found to predict the strength of various cokes with a standard error of =«10%. It should be noted that the feature analysis was only performed on pores greater than 2 0.015mm i n area, and i s therefore, biased toward the larger  41 pores. This i s t h e o r e t i c a l l y v a l i d since larger flaws (pores) are considered to be the origin of most f a i l u r e s .  1.5.4  COKE MECHANICAL PROPERTIES IN RELATION TO POSITION IN COKE OVEN Nishioka and Yoshida ( 3 7 ) performed experiments to  evaluate the effect of position with respect to the oven width on coke mechanical properties. The coke was made in a 250kg coke-oven with a width of 0.450m. Temperature measurements were made at ten positions across the oven width. The general trend was a decrease in t e n s i l e strength of coke toward the center of the coke-oven. Porosity reached a minimum at the 1/4 width position in the oven, but increased to a maximum at the oven center. There appears to be no information available on the e f f e c t of coke quality as a function of position for commercial coke-ovens. Furthermore, no information on the coke q u a l i t y as a function of v e r t i c a l position in the coke-oven has been reported in the l i t e r a t u r e .  1.6 OBJECTIVES OF THIS RESEARCH PROJECT It i s clear from the discussion above that coke i s a very complex material. This complexity and d i v e r s i t y arise for several reasons: 1.  Blendings of High Volatile,Medium V o l a t i l e and Low V o l a t i l e coals are used, which determine the pore volume content and pore size d i s t r i b u t i o n s .  42 2.  Cokes at the bottom of the coke-oven batteries are denser (less porous) because of the s t a t i c load (burden) present during coking on these cokes.  3.  Cokes nearer the refractory walls are subjected to higher temperatures than those which are present at the center of the battery.  Thus, a large number of coke samples for commercial cokes have to be tested to get a representive value of any property. The properties of warf coke (aggregated commercial coke) vary widely from sample to sample. On the other hand, most coke quality studies have been performed to date on "test-oven" cokes which generally exhibit r e l a t i v e l y uniform properties throughout. In t h i s research program, experiments have been performed to f u l f i l the following objectives: 1.  The e f f e c t of coking conditions on the microstructure of coke was assessed using an image analyzer. The analysis was performed on i n d u s t r i a l coke batches extracted from three d i f f e r e n t heights in an Algoma 5m coke-oven. These microstructural c h a r a c t e r i s t i c s were compared with those obtained from coke batches produced using the same coal blend, but under various conditions in a 460mm test-oven at CANMET Laboratories in Ottawa.  2.  The mean strength values of the aforementioned seven coke batches were determined both at ambient temperature and at 1400°C. These results were treated s t a t i s t i c a l y and were correlated with microstructural parameters and  43  coking conditions. 3.  The p l a s t i c behavior of some coke samples have also been q u a l i t a t i v e l y evaluated as this property may have s i g n i f i c a n t influence on the coke performance in a blast furnace.  2. EXPERIMENTAL The coke samples used in t h i s study were supplied by the Energy Research Laboratory  (CANMET) i n Ottawa, Ontario.  The samples originated from two sources: CANMET-a 460mm test-oven, and an Algoma 5m i n d u s t r i a l oven at Sault Ste. Marie. The test-oven coke was produced from the same coal blend used in the 5m oven at Algoma. The blend consisted of 35% low v o l a t i l e (LV) coal and 65% high v o l a t i l e (HV) coal. The conditions of coking are l i s t e d in Table IV for the test-oven cokes.  Sample  CPR-264  CPR-265  CPR-266  Coal Moisture, %  5.9  4.1  1.2  Coal Bulk Density, gm/cm  0.578  0.863  0.976  Coking Time, h  17.2  17.15  18.1  Oven Wall Pressure, kPa  3.2  5.9  17.8  Table IV. Coking Conditions For Test-oven Cokes  The as-received test-oven cokes have the standard ASTM characteristics  as shown in Table V (values determined at  CANMET) (38). The coke samples produced in the 5m i n d u s t r i a l oven were sampled from  44  45  Sample  CPR-264  CPR-265  CPR-266  Apparent Specific Gravity  0.781  0.903  0.936  Stability  59.5  59.2  57.4  Hardness  69.3  70.2  71.1  Table V. C h a r a c t e r i s t i c s of the As-received Test-oven Cokes.  s t e e l mesh cages (28x28x35.6cm) that had been lowered into the  oven through the center charging hole as i l l u s t r a t e d in  Figure 9a(38). On the f i r s t day of testing, a basket was lowered to the bottom of the empty coke-oven before charging. On the second day, charging was interrupted after the  oven was half f u l l and another basket was placed in the  oven. F i n a l l y , on the t h i r d day, a t h i r d basket was placed in the coke-oven after 80% charging. Figures 9(b-d) show the variation in coke properties as a function of height as measured by CANMET. The as-received coke properties are summarized (ie.  in Table VI. The sample CPR-270 i s wharf coke  from the same battery in which cages were positioned).  Wharf coke i s refered to as the representative  46  a) Approximate Location of Cages  b) C)  73-7 0-798 634 0-888 64*3 0-947 %*90mm Coka,Wharf - 68 2 Apparant Specific Gravity,Wharf - 0-  46 4 37-2 48 2 63-3 S3 6 71-2 ASTM Stability .Wharf - 58 5 ASTM Hafdness.Wharf - 69 8  d) F i g u r e 9.  34 6 43-1 32 3 48-9 28 3 59-5 Reactivity, Wharf - 23-7 Stranffh aftar Reaction,Wharf .  64-  The O r i g i n and Corresponding ASTM P r o p e r t i e s of the Algoma Coke Batches.  47  Sample  Apparent  Specific Gravity  CPR -267 CPR -268 CPR -269 CPR-270  0.947  0.880  0.798  0.888  ASTM S t a b i l i t y  53 .6  48 .2  46 .4  58 .5  ASTM Hardness  71 .2  63 .3  57 .2  69 .8  5.0  3.3  0.8  Height Below Coal Line, m  Table VI. As-received Algoma coke properties.  coke for the whole oven. The parameters l i s t e d in Table IV,V,and VI were considered to be independent variables to be used in t h i s study. The dependent variables (to be determined) in this study are: 1.  Bulk density (apparent s p e c i f i c gravity)  2.  True density (true s p e c i f i c gravity)  3.  Porosity  4.  Ambient temperature compressive strength  5.  High temperature compressive strength, and  6.  Microstructure  Variables (1-3) and (6) were also used as independent variables to assess their influence on ambient and high temperature compressive strength of the cokes used in this study. The procedures used for sample preparation and for determining properties (1-6) are described in the following sections of t h i s chapter. These methods were necessary to  48 minimize inconsistencies attributed to variations in experiental procedure.  2.1 SAMPLE IDENTIFICATION A systematic procedure for identifying the seven coke packages received from CANMET has been developed . The method involved four steps: 1.  Positioning the coke lumps according to their approximate position along the width of the oven  2.  Selection of suitable lumps to be used  3.  Sectioning each of the selected test lumps by a diamond saw, and then  4.  C o r e - d r i l l i n g specimens from each lump section.  2.1.1  POSITIONING OF COKE LUMPS Each of the seven coke packages were examined and each  lump was positioned according to i t s approximate location across the coke-oven width. Positions were determined by the v i s u a l appearance of each lump. For example,  lumps with a  cauliflower type structure at one end were located at the wall of the coke-oven as shown in Figure 10. This was e a r l i e r suggested by Dr. W.R.  Leeder of Denison Mines. In  contrast, lumps with darker coloring were considered to be "coked" to a lesser degree, and therefore, were assumed to have originated close to the center of the oven. The approximate position of the coke lumps of the seven coke batches are shown in photographs (Figure 10a-g). These lumps  F i g u r e 10.  The p o s i t i o n s of Coke Lumps as they were Assumed t o be i n the Coke-oven.  Figure  10.  Continued.  51 were numbered and photographed for further reference. The lumps considered  suitable for further study were selected  from each of the seven coke batches.  2.1.2  SELECTION OF TEST LUMPS The selection of test lumps was based mainly on their  s i z e , p o s i t i o n , and their as-received condition. Larger lumps which exhibited the cauliflower structure were preferred because the s t r u c t u r a l v a r i a t i o n according to distance from the oven wall can be more r e a d i l y compared in large lumps. The position in the oven of smaller lumps not exhibiting the cauliflower structure i s less c e r t a i n . Coke lumps which contained  a large number of f i s s u r e s were  generally not used due to the d i f f i c u l t y of sectioning them. It was also desirable to, wherever possible, select an equivalent  number of each wall and center coke lumps.  The best example of large lump selections are those of CPR-267 as shown in Figure  11. These lumps contain both the  cauliflower structure and darker center regions. enabled c l a s s i f i c a t i o n , according  to distance  This  from the wall,  to be more e a s i l y performed.  2.1.3  SECTIONING OF TEST LUMPS Each selected coke lump was sectioned  into several 2cm  thick sections. Attempts were made to cut the coke lumps in such a way that the faces of each section were approximately p a r a l l e l to the coke-oven wall. Each section was numbered  52  Figure  11.  An Example of Coke Lumps S e l e c t e d f o r Further Study.  53  and the p o s i t i o n with respect to the coke-oven wall (or oven center for some lumps) was  recorded. Figure 12 i s a  schematic diagram of the sectioning procedure used for a l l selected test lumps.  2.1.4  CORE DRILLING OF COKE SPECIMENS Individual test samples were made by c o r e - d r i l l i n g  specimens from each of the sections described above. This is i l l u s t r a t e d in Figure 12. The d r i l l cores were 1.3cm  approximately  in diameter. The variation in the properties of  d r i l l - c o r e d specimens i s s u f f i c i e n t l y large, so the exact position of the specimen within the s l i c e was  not taken into  consideration.  2.2 PREPARATION OF TEST SPECIMENS The core d r i l l e d c y l i n d r i c a l specimens were each inserted into a hardened steel die with a 1.3cm ground and polished down to 1.3cm  diameter hole, then  in length using polishing  wheels between 80 and 600 g r i t . This method ensured that each specimen had smooth p a r a l l e l faces at both ends of the coke c y l i n d e r . The p a r a l l e l faces are e s s e n t i a l for good alignment during compression t e s t i n g .  2.3 BULK DENSITY MEASUREMENT Since the polished c o r e - d r i l l e d specimens were near perfect cylinders, the volume could e a s i l y be determined from the dimensions (measured by a c a l i p e r ) . Each recorded sample  54  Figure 1 2 .  Schematic Representation of the Procedure for Sectioning Coke Lumps ( r e f . 4).  55  diameter was the mean of three such measurements and the recorded height was the average of two measurements. The separate measurements rarely d i f f e r e d by more than 0.025mm (0.001 i n ) . The bulk density was e a s i l y determined by d i v i d i n g the sample weight by i t s measured volume.  2.4 TEST SAMPLE SELECTION The samples used for compression testing, creep testing, and microstructural analysis were grouped on the basis of their bulk density. The samples selected from each of the seven batches f e l l inside the range of bulk densities described by: M  where  pb " "pb  ? "pb * pb M  +  a  pb  i s the mean bulk density and a ^  ( 6 )  i s the sample  standard deviation. This method was used in order to minimize the effect of bulk density variation on coke strength amongst test samples in each of the seven coke batches studied.  2.5 MICROSTRUCTURAL ANALYSIS The microstructure of the seven coke batches was analyzed using a L e i t z TAS PLUS computerized image analyzer. This image analyzer has the c a p a b i l i t y of quantitatively measuring textural features of o p t i c a l images and determining their quantity, size d i s t r i b u t i o n , area, form, and other o p t i c a l l y distinguishable parameters. Figure 13  56  Storage Dts* Lint Printer Imagt  POP-II  Memories  Computer External Main-Frame Computer  Grty  Ltvtl  TV  Discriminator  Monitor  High Resolution TV  Operator Input  Camera  Keyboard  Photograph Macrostand  Coke Photomicrograph  Figure  13.  Schematic PLUS Image  Representation Analyzer.  of t h e L e i t z  TAS  57  schematically i l l u s t r a t e s the L e i t z TAS PLUS image analyzer. The system at U.B.C. has both an o p t i c a l microscope and a macrostand for analyzing photographs. The macrostand  was  used in t h i s study to analyze photomicrographs of the coke texture. This method was chosen due to the ease with which the photographs can be used with the image analyzer. Also, specimens can be re-examined without additional polishing and after the specimens have undergone compression testing.  2.5.1  PHOTOMICROGRAPH PREPARATION Each of the coke specimens selected for further study  based on their bulk density, were photographed for quantitative examination using the image analyzer. The samples were prepared by f i r s t l i g h t l y dry polishing each end using Carborundum 3/0 Flexbac polishing paper. The surface pores were then coated with soot from a luminous candle flame to eliminate any internal r e f l e c t i o n (from the shallow pore channels in the coke) which may a f f e c t the accuracy of the texture analysis. Each end of the coke cylinder was then photographed at a magnification of  12.8X  on polaroid films with an exposure time of two minutes for high contrast between pore walls and pore channels. An example of the excellent contrast achievable using this method i s shown in Figure 14 in which the dark regions are pores and the l i g h t regions are pore walls  58  Figure  14.  A Typical  Coke  Microstructure.  magnification  (12.82)  59  2.5.2  IMAGE ANALYSIS The L e i t z image analyzer i s e a s i l y automated using  software to analyze the porous structure of coke. This software has been developed indigenously to measure seven parameters of coke structure. The parameters that are considered important  for characterizing the porous  structures are pore (channel) perimeter, c e l l wall thickness, pore area, equivalent diameter, mean chord length, form factor, maximum and minimum Feret's  diameters.  Mean chord length (also used to determine c e l l wall thickness) was  determined by averaging the chord lengths of  the features at three orientations (0°, 60° and  120° to the  h o r i z o n t a l ) . Form factor i s used to characterize shape using area and perimeter measurements ( i e . form 2  factor=47r(area)/perimeter  ). Maximum and minimum Feret's  diameters were determined by measuring the c a l i p e r dimensions at 12 orientaions (15° intervals) and taking the maximum and minimum values of these. A l l of these measurements were performed on each individual pore in the photographs. A flow chart of this program i s presented  in Figure 15. A second software program  has been developed to do field-based mean chord measurements for  c e l l walls. The image analyzer detects features by t h e i r  grey-levels on a black and white TV monitor. The TAS PLUS image analyzer has a grey l e v e l range between 1 (white) and 100  (black). Measurements are performed on objects which  60  Mtatoro Volumo of Porotlty  Ellmlneto Small Poroi Deflno Porform 7 Moowromontt on toch Port  Arroyt  Input Calibration Factor  Input Numbor of Flo Idi  Anion Output Flit  Dofl no HJttog rami  Figure 15.  A Flow Chart of the Software Used to Quant i fy Coke Microstructure.  61 f a l l inside the specified grey-levels. This method i s c a l l e d threshold detection. However, another method of detection (edge detection) was found to be more suitable for analyzing coke microstructure. This method u t i l i z e s the sharp contrast between pore and wall grey l e v e l s and detects pores by these abrupt changes. Edge detection eliminates errors associated with judging grey levels of features by an operator. The image analyzer makes measurements on features by counting hexagonal p i x e l points. Very small features are therefore inaccurately measured. For t h i s reason, features with dimensions less than  12MITI  in diameter were not included  in these measurements. This was done by a series of image erosions followed by a series of d i l a t i o n s to reconstruct the remaining larger pores. The seven parameters measured were c l a s s i f i e d into seven d i f f e r e n t histograms describing their size d i s t r i b u t i o n . It should be noted that measurements made on i n d i v i d u a l features of the porous structure, may not be absolute but can be regarded as being r e l a t i v e because image analyzer results are resolution dependent (section 4.2).  2.5.3 TRUE DENSITY The true density of each of the seven coke types was measured to enable porosity c a l c u l a t i o n s based on bulk density to be made. This was done to v e r i f y the porosity measurements made using the image analyzer. The true density was determined using the pycnometric method.  62  The coke s l i c e s ,  sectioned 2cm from the c a u l i f l o u r end,  were ground to minus 75jxm in a Spex Mixer M i l l . Approximately 2gm of t h i s powder was used for true density determination. The fine powder was needed to eliminate any closed pores that might be present in the coke. The 2gm sample was placed in a 25ml pycnometric flask and evacuated or boiled to remove trapped a i r . Both d i s t i l l e d water and methanol were used as the measuring f l u i d s for a l l seven coke batches. The true density of the coke powders was calculated using the following equation: W - W, Density = (W.-W,)-(w7~W) 2  2  Pi  (7)  Pi  where W, = Weight of Pycnometer bottle, W  = Weight of Pycnometer bottle + sample,  W  = Weight of Pycnometer + sample + f l u i d ,  2  3  W„ = Weight of Pycnometer f i l l e d with f l u i d alone,and p,= Density of f l u i d . The porosity was calculated from the r e l a t i v e density, which was obtained by d i v i d i n g the average bulk density by the  measured true density.  2.6 MECHANICAL TESTS A l l mechanical tests on coke cylinders were performed under compression. Compression strengths were determined both at ambient temperatures and at 1400°C. Some tests were also c a r r i e d out at 1000°C for three of the seven coke batches.  63  Constant load tests were performed on a l l four of the Algoma coke samples.  2.6.1  COMPRESSION TEST APPARATUS These studies were made using the apparatus i l l u s t r a t e d  schematically  in Figure  16. The coke sample was positioned  between two graphite plungers which acted as suseptors in the induction c o i l . The graphite plungers and the specimen were enclosed  in a quartz tube with two water cooled copper  disks at each of the top and bottom ends of the tube. The system was flushed with argon to prevent oxidation of the coke specimen and the graphite plungers.  I t i s important  that the oxidation of the specimens was kept to a minimum so that the pore structure of the coke remained unchanged during the t e s t . The temperature of the specimen was monitored using a Pt-lO%RdPt thermocouple inserted through the bottom plunger to an approximate distance of 2mm away from the specimen.  2.6.2  PROCEDURE OF TESTING The load was applied using a hydraulic system. The  loading rate can be controlled using a leak valve which allows a l i n e a r build-up of pressure  i n the piston chamber.  The loading rate used on a l l strength and constant  load  tests was approximately 1.2MPa/sec. The load was recorded using a load c e l l made by A.L. Design Inc. of Tonawanda, N.Y. situated at the bottom of the assembly. The load c e l l  64  Meehonical To Chart Rtcordtr  Dial Indicator  Tronsductr  -Air Inlet Valvt  Argon Grophite Plungers'  Coke  In te t  Specimen  Induction Coil  Argon Outlet-  • Load  Washer  -Thermocouple  To Load Rtcordtr  Te T e m p e r a t u r e Controller  Figure  16.  A Schematic R e p r e s e n t a t i o n of the Compression T e s t i n g Apparatus.  65 was connected to a Kipp and Zonen BD-41  chart recorder set  to an 800 pound f u l l scale load. The c e l l was calibrated using an Instron testing machine with an FR tension-compression c e l l . The linear dimensional change was monitored by an LVDT mechanically connected to the top of the plunger. The transducer was connected to a second pen on the same chart recorder. The heating rate was a r b i t r a r i l y set at =*200°C/min. and used in a l l experiments. For both high temperature strength and constant load tests, specimens were heated to the desired temperature and maintained at that temperature for ten  minutes before the load was applied. The constant load  tests were carried out normally between t h i r t y and sixty minutes.  2.6.3  EXPERIMENTAL PARAMETERS The fixed experimental parameters used for a l l  compression and constant load tests are summarized in Table VII.  Room temperature compression strength tests were also  performed in accordance with Table VII except without the argon atmosphere  which was unnecessary.  2.7 GRAPHITIZATION TESTS To determine the extent of coke shrinkage during heating, graphitization tests were also performed. These tests involved placing a sample in the strength testing apparatus,  66  Parameter  Value  Sample Diameter  1  Sample Length  1 . 3cm  Loading Rate  1.2MPa/sec  Heating Rate  ~200°C/min.  Time at Temperature  10 min.  . 3cm  (before testing) Atmosphere  Argon  Number of Compression  12-25  Tests per Condition  Table VII. Values of Fixed Compression Test Parameters.  and heating to 1400°C for t h i r t y minutes without applying any load to the coke sample. The dimensions of the sample were measured after cooling.  3. RESULTS AND  3.1 It  ANALYSIS  GENERAL COMMENTS s h o u l d be noted that because of the wide v a r i a t i o n i n  m i c r o s t r u c t u r e and other p r o p e r t i e s of coke even w i t h i n a s i n g l e lump, a l l p r o p e r t i e s  ( p h y s i c a l and mechanical) should  be t r e a t e d s t a t i s t i c a l l y . T h i s wide v a r i a t i o n of p r o p e r t i e s arises  from the method of coke p r o d u c t i o n where temperature  v a r i a t i o n s of =«200 C e x i s t s from the c e n t e r of the b a t t e r i e s o  to the r e f r a c t o r y w a l l , and from the top of the b a t t e r y t o the s o l e  (M a<300 C). Furthermore, the coke at the top of the |  o  b a t t e r i e s i s not s u b j e c t e d to as much a s t a t i c coke at the bottom  (ie. significant  l o a d as the  differences  i n oven  p r e s s u r e are encountered). In a d d i t i o n to t h e s e , the m a t e r i a l used i n coke-making heterogeneous. So,  i t i s not s u r p r i s i n g t h a t coke has such a i n p r o p e r t i e s . In s p i t e  of t h i s f a c t , very few r e s u l t s r e p o r t e d are t r e a t e d s t a t i s t i c a l l y .  3.2  raw  ( c o a l ) i s extremely  complex t e x t u r e with wide v a r i a t i o n s  results  wall  i n the  literature  In t h i s p r o j e c t , almost a l l  (except the c r e e p data) are t r e a t e d  statistically.  DENSITY  3.2.1  BULK DENSITY Bulk d e n s i t y was  used as an i n i t i a l c r i t e r i o n f o r  c h a r a c t e r i z i n g the seven d i f f e r e n t coke batches. The bulk d e n s i t i e s were c l a s s i f i e d  i n t o histograms f o r each coke type  67  68 from which their mean bulk densities and standard deviations were obtained. Figure 17 i s the d i s t r i b u t i o n of bulk densities for a l l seven coke batches (CPR-264-CPR-270). The histograms for the individual coke batches are included in the Appendix A. Figure 18a i s a summary of the mean bulk density as a function of coke type. The error bars correspond to the standard deviations. The mean bulk densities range between 0.779gm/cm 0.947gm/cm  (CPR-269) and  (CPR-266). The variation between oven wall  pressure and bulk density for the test-oven cokes i s i l l u s t r a t e d in Figure 18b. There i s a strong c o r r e l a t i o n between bulk density and oven wall pressure when plotted in a semi-logarithmic scale. Also included are the bulk density values of three i n d u s t r i a l cokes on the straight  line  obtained from the test-oven cokes. The best f i t curve which describes the relationship between oven pressure and bulk density of the test-oven coke i s :  P (gm/cm )=0.11[log P(KPa)1+0.808 3  b  lQ  r=0.997  (8)  where p^ i s the bulk density and P i s the oven wall pressure. Table VIII shows the relationship between estimatedCfrom Figure 18b) s t a t i c load pressure and depth below the coal l i n e for the i n d u s t r i a l cokes. As expected, there i s an increase of s t a t i c load with increasing depth below the coal l i n e , resulting in a higher coke bulk density.  69  Coke Type * All Coke Samples Density  (gm/cc) •643 •654 •665 •676 •686 •698 •709 •720 •731 •742 •753 •764 •770 •776 •787 -798 •809 •820 •831 •841 •853 •864 •875 •886 •897 •908 •9 I 9 •930 •941 •952 •963 •973 •984 •995 1006 1018 1029 I 040 10 51 1062  Percent  3  3  •  3  Mean* -861 gm/cc Standard Deviation Figure 17.  4-9 J 6-4 4-3  3-8 3-6  083 gm/cc  The D i s t r i b u t i o n of Bulk D e n s i t y Values f o r all  Seven Coke Batches.  70  1  1  r  -Test Furnace-  Coke  I 00  1 i r r Commercial Oven—^ Coke I  to  E o ^ 0 90 o> o  OA  to c  v  0)  O  0 80  CD  0 70  Figure 18a.  264  265  266  CPR  Coke  267  268  ±  269  270  Type  A Summary of Mean Bulk Density Values for a l l Seven Coke Batches.  71  1-0  I  I  I  I  I  1  1—I—I  I I I I  266. 267.  E o \  E 0-9 268 /6 A  w  264  >*  *—  s  «/>  c Q)  Bu!  a  c o  0-8  "l 269,  '  Correlation Coef.  s A K  O A  0)  2  0-7' * 0-5  1  »  1  Test Oven C o k e s Industrial Oven Cokes  J  1  Oven Figure  18b.  r-0.997  L  I  5 Pressure  t  I  I .  I  10 (kPa)  Bulk D e n s i t y versus Oven Wall  Pressure  ( P o i n t s f o r i n d u s t r i a l oven cokes estimated the u s i n g l i n e a r values).  l i n e and bulk d e n s i t y  20  72 3.2.2 TRUE DENSITY The true densities of a l l seven coke batches have been determined. These values are presented  in Table IX. There i s  no apparent c o r r e l a t i o n between the true density and the oven wall pressure  (or s t a t i c load). The true density  of a l l seven cokes vary within five percent  values  of each other.  These values are lower than that of graphite (2.266 gm/cm  3  (40)) which indicate that these cokes are not well graphitized. Since CPR-270 i s the wharf coke, i t would require a large number of true density measurements to get a representative value for a l l wharf coke, so i t s true density was calculated from the average of CPR-267, CPR-268 and CPR-269 true density values.  3.3 MICROSTRUCTURE The microstructure of the seven coke batches used in t h i s project was examined q u a n t i t a t i v e l y using a L e i t z TAS PLUS image analyzer, as discussed in section 2.5.2. Since t h i s analysis was performed on two dimensional  photographs with  the intention of representing a three dimensional e f f o r t was made toward improving the accuracy employed. This includes determining  system, an  of the method  the s e n s i t i v i t y of the  results towards the number of f i e l d s (photographs of c i r c u l a r faces) analyzed and assessing the e f f e c t of large pores crossing the outside boundaries of the f i e l d s . The analysis of the porous structure proceeded in the following manner: 1) the s e n s i t i v i t y analysis was performed to  Coke  Depth Below  Type  Coal L i n e  Est imated Static  CPR-  Mean Bulk  Load  Density  Pressure (m)  gm/cm'  (kPa)  267  5.0  10.2  0.919  26B  3.3  4.9  0.884  269  0.8  Table VIII .  5.4(10"')  0.779  Estimated V a r i a t i o n of Mean Bulk D e n s i t y and Oven Wall Pressure with Depth Below The Coal  Line.  Coke  Depth  Oven Wall  True  Standard  Type  Below  Pressure  Density  Deviation  CPR-  Coal Line (gm/cm )  (gm/cm )  (kPa)  <Ol)  3  1  264  -  3.2  1 .9672  0.0784  265  -  5.9  2.0229  0.0918  266  -  17.8  1.9009  0.0729  267  5.0  10.2*  1.9167  0.0326  2.0163  0.0216  268  3.3  269  0.8  270  Table IX.  * 4.9 5.4(10"')*  -  Warf  1.9194  0.0640 **  1 .9470  0.0621  V a r i a t i o n of T r u e Density as a Function of Coking Condi t i o n s . * Calculated.  •*  Average of true d e n s i t i e s of CPR-267,268 and 269.  74 determine  the minimum number of f i e l d s required to represent  the whole microstructure; 2) the effect of pores situated on the borders of the f i e l d was assessed; 3) d i s t r i b u t i o n s of pore area, equivalent diameter, mean chord length, form factor (4jr(area)/(perimeter) ), pore perimeter, maximum and minimum Feret's diameters along with a mean chord sizing of coke c e l l walls were determined; and 4) a s t a t i s t i c a l analysis was performed on the r e s u l t s .  3.3.1 SENSITIVITY Table X shows the measured parameters obtained from measuring 4,10,20 and 40 f i e l d s (photographs) of a batch of coke. As shown, some variables are more sensitive to the number of f i e l d s analyzed than others with pore perimeter being the most sensitive, while the mean chord length (averaged over three orientations) varying the least. The values appear to converge when more than twenty f i e l d s of view have been analyzed. Therefore, i t was decided that an analysis of t h i r t y or more f i e l d s of view would represent each coke batch adequately. The porosity was also measured using the same software. These values are compared in Table XI, with those calculated from the measured bulk and true density values. The values obtained from the image analyzer were consistently higher  Number  pore  Equivalent  Mean Chord  Form  of  Area  Diameter  Length  Factor  Maximum  Minimum  Feret'i  Feret' •  Diameter  Diameter  (mm)  (mm)  <->  Pore Per imeter  Fields Analyzed  (mm )  (mm)  (mm)  4  0. 174  0.298  0. 178  0.858  1 .483  0.427  0.273  0.178  0.847  1.596  0.449  0.283  1  10  0. 180  0.310  20  0. 133  0.261  0. 176  0.879  l .251  0.403  0.250  40  0.130  0.285  0.177  0.877  1.248  0.404  0.253  Table  Variation of The Arithmetic Mean Values of The Pore-parameters Measured Using The L e i t z TAS PLUS Image Analyzer for CPR -264 as aFunct ion of Number of F i e l d s Analyzed.  Coke  Number  Porosity  Porosity  Type  of Frames  f rom  from  CPR-  Analyzed  Image Analyzer  Pycnometry  ( % )  ( % )  264  40  60.54  55.50  265  33  59.29  54.32  266  40  56.05  51.34  267  40  57. 12  52.05  268  40  57.91  56. 16  269  40  59.03  59.41  270  40  57.54  55.88  Table XI  Comparison of Porosity Values as Measured using the Image Analyzer  with Those Values Obtained From Pycnometry.  77  than those found by pycnometry. The differences between the two methods follow the best f i t r e l a t i o n s h i p :  e= p  where  and  0.944ej- 0.023  r=0.994  (9)  C j are the p o r o s i t i e s (in percent) measured by  pycnometry and by the image analyzer, respectively. In order to calculate the p o r o s i t i e s of the test-oven cokes (CPR-264 to CPR-266) a single averaged true density value was used. This average true density was obtained from eighteen d i f f e r e n t true density measurements of a l l three cokes. The j u s t i f i c a t i o n in using such a method i s that the oven wall pressure should not a f f e c t the true density of the coke (5).  3.3.2  EDGE EFFECTS One  form of inaccuracy, that a f f e c t s the o p t i c a l  methods of q u a n t i t a t i v e l y characterizing microstructure, i s the effect of edges. Often, large features cross over the boundaries of the photographs. This allows only a fraction of their size to be analyzed. The Leitz TAS PLUS image analyzer has the c a p a b i l i t y of eliminating features which cross the boundaries of the frame being analyzed. This e f f e c t was examined by analyzing four and twenty f i e l d s .  The  arithmetic mean values of seven parameters are l i s t e d in Table XII. The mean values for a l l seven microstructural parameters are consistently lower for the data obtained in the absence of features bordering the f i e l d edges as  Parameter  20  Fields  4 Fields  4 Fields  Edge Pores  Containing  Edge Pores  Containing  Eliminated  Edge Pores  Eliminated  Edge Pores  20  Fields  Pore Area 0.085  0.174  0.073  0.133  0.25B  0.298  0.241  0.281  Length (mm)  0.170  0.178  0.162  0.175  Form F a c t o r  0.894  0.858  0.913  0.879  1 .016  1 .483  0.925  1.251  0.352  0.427  0.330  0.403  0. 174  0.273  0.178  0.250  (mm) Equivalent D i a . (mm) Mean Chord  Pore Perimeter (mm) Max. F e r e t ' s Dia (mm) Min.  Feret's  Dia (mm)  Table X I I .  The E f f e c t CPR-264.  of Edges on M i c r o s t r u c t u r a l Parameters of  79  compared to those values obtained when edge features are included. This i s not surprizing as the larger features are expected to be more frequently intercepted by the f i e l d boundaries than the smaller ones. Since larger pores are considered l i k e l y to be the cause for mechanical f a i l u r e in coke, i t was decided that further microstructural analysis would be carried out including a l l the edge pores. This decision i s based on the assumption that edge effects w i l l be similar for a l l seven coke batches and, therefore, the results produced by the image analyzer w i l l be r e l a t i v e l y consistent.  3.3.3 CELL WALL SIZE The thickness of the c e l l wall was determined using the image analyzer to perform intercept s i z i n g . This method i s similar to the manual technique but i s performed on a series of l i n e s oriented at 0°, 60°, and 120° r e l a t i v e to the horizontal axis. The mean chord length i s found by counting the number of intercepts of each l i n e and d i v i d i n g the length of each l i n e by the number of intercepts for each of the three angles. The f i n a l value results from an average of the chord s i z i n g of the three orientations. The wall size values of the seven coke batches are l i s t e d i n Table XIII. The c e l l wall width of the cokes produced in 460mm test-oven are thinner than those for the cokes produced i n the 5m i n d u s t r i a l coke-oven.  coke  CPR-264  CPR-265  CPR-266  CPR-267  CPR-268  CPR-269  CPR-270  0.131  0.122  0.131  0.154  0.152  0.168  0.169  Type  Wall Size (mm)  Table XIII.  Comparison of C e l l Wall Size Values for D i f f e r e n t Cokes.  CD  81  3.3.4  PORE SIZE AND GEOMETRY The analysis of pore size and geometry was c a r r i e d out  by measuring a l l seven parameters of each pore on a l l photographs. The number of pores analyzed per coke batch ranged between 3500 and 5500 depending on the coke being analyzed. The seven pore-parameters which were expected to have an influence on coke mechanical properties are l i s t e d in section 2.5.2. Each pore measurement has been c l a s s i f i e d into histograms and the d i s t r i b u t i o n s t a t i s t i c s were calculated using the PDP-11 computer attached with the image analyzer. In addition, a l l pore measurements were transferred to the main AMDAHL computer  for further  s t a t i s t i c a l analysis. The calculated mean values and their corresponding standard deviations are l i s t e d on Table XIV and the histograms for CPR-264 are included in Appendix B as an example. There i s a general decrease in the pore dimension values from CPR-264 to CPR-266 for the test-oven cokes while an increase in pore dimensions i s observed from CPR-267 to CPR-269. The mean pore size (as described by equivalent diameter, mean chord, maximum and minimum Feret's diameters) i s found to be larger for the commercially produced coke than that for the test-oven cokes. The standard deviations of each parameter i s shown to be larger than the arithmetic mean values. This i s c h a r a c t e r i s t i c of heavily skewed d i s t r i b u t i o n s . The histograms shown in the appendix are seen to have a p o s i t i v e skewness (there i s a very large number of  Coke  Pore Area  Equivalent  CPR-  Mean Chord Length  Form  Diameter (mm') u  Factor  (mm) o  Pore Perimeter  o  Feret's  (mm)  (mm)  u  Maximum  u  o  u  o  u  Minimum  Diameter  Feret's  (mm) o  Diameter  (mm)  u  o  u  o  264  0.130  0.517  0.285  0.290.  0.177  0.106  0.877  0.243  1.248  2.642  0.404  0.506  0.253  0.299  265  0.105  0.356  0.270  0.248  0.173  0.096  0.885  0.236  1.116  1.845  0.376  0.423  0.240  0.252  266  0.100  0.301  0.269  0.234  0.170  0.086  0.874  0.243  1.140  1.864  0.378  0.408  0.239  0.241  267  0.128  0.478  0.287  0.284  0.180  0.106  0.877  0.244  1.244  2.422  0.407  0.498  0.255  0.291  268  0.150  0.493  0.307  0.311  0.189  0.120  0.867  0.249  1.350  2.451  0.435  0.535  0.274  0.315  269  0.184  0.751  0.323  0.361  0.196  0.140  0.862  0.248  1.446  2.929  0.463  0.596  0.289  0.368  270  0.164  0.595  0.314  0.332  0.193  0.125  0.866  0.243  1.380  2.672  0.440  0.545  0.278  0.333  Table XIV  Mean (y)and Standard Deviations (a)of The Coke Porous Parameters for a l l Seven Coke Batches.  00  83 small pores). This raises doubt as to the v a l i d i t y of using the arithmetic mean as a representative value.  3.3.5 REPRESENTATION OF PORE SIZE AND GEOMETRY The s t a t i s t i c a l problems (associated with skewed d i s t r i b u t i o n s ) mentioned in section 3.3.4  can be avoided by  considering cummulative d i s t r i b u t i o n s . For the purpose of t h i s project, the seven parameters used to characterize coke structures were represented a r b i t r a r i l y by the cummulative 80% finer values (values at which 80% are smaller). The cummulative 80% finer values at the 95% confidence l i m i t are shown in Table XV. The values shown for Form Factor a l l exceed the t h e o r e t i c a l maximum of 1.0 (for a c i r c l e ) and therefore can not be r e l i e d upon. With t h i s in mind, the r a t i o of maximum Feret's diameter to minimum Feret's diameter was used to characterize pore shape as previously suggested by other workers (32-36). CPR-265 exhibits the smallest pore dimensions of the test oven cokes, whereas CPR-267 exhibits the same(ie. smallest pore dimensions) for the cokes produced in the 5m i n d u s t r i a l coke-oven. Another parameter which has been considered important i s the number of pores per f i e l d (see Table XV). It can be seen the test-oven cokes contain a larger number of smaller pores when compared with cokes produced at Algoma.  Coke  Number of  Pore  Equivalent  Hean Chord  Form  Pore  Maximum  Minimum  R a t i o of Mai  Type  Pores  Area  Diameter  Length  Factor  Perimeter  Feret's  Feret's  t o Min  Diameter  Diameter  (mm)  (mm)  (-">  CPR-  per  Field  (mm)  (mm)  (mm)  1  Feret's Diameter  264  121  0 104  0.363  0.229  1.10  1.378  0.518  0.310  1.771  266  140  0 093  0.343  0.222  1. 10  1.290  0.482  0.301  1.7)4  266  126  0 098  0.353  0.224  1.10  1 .345  0.506  0.310  1.755  267  115  0 105  0.365  0.231  1.10  1.410  0.530  0.310  1.749  268  101  0 105  0.365  0.231  1.10  1.574  0.573  0.352  1.754  269  89  0  134  0.413  0.255  1.10  1.629  0.603  0.352  1.766  270  97  0  I  25  0.399  0.251  1.09  1 .531  0.566  0.350  1.766  T a b l e XV  Cummulat i v e 8 0 % Parameters  F i n e r Values o f the  f o r t h e S e v e n Coke  Seven Pore  Batches.  CO  85  3.4 COMPRESSIVE Table the  STRENGTH OF COKE BATCHES  X V I i n c l u d e s t h e mean c o m p r e s s i v e s t r e n g t h v a l u e s o f  seven coke batches,  b o t h a t ambient t e m p e r a t u r e and a t  1400°C. A l s o g i v e n a r e t h e i r  standard  deviation values.  S e v e r a l c o n c l u s i o n s c a n be drawn f r o m t h i s 1.  Coke i s s t r o n g e r a t h i g h  2.  Test-oven cokes  data:  temperature,  (CPR-264 t o CPR-266) a r e s t r o n g e r  than  the Algoma cokes,and 3.  The s t a n d a r d large  d e v i a t i o n f o r e a c h mean s t r e n g t h v a l u e i s  ( u p t o 50% o f t h e mean s t r e n g t h ) .  The l a r g e s t a n d a r d  d e v i a t i o n s show t h e l a r g e v a r i a t i o n i n  coke s t r e n g t h v a l u e s w i t h i n each coke batch. T h i s s c a t t e r i n the data  will  be d e s c r i b e d  i n more d e t a i l  i n s e c t i o n 3.4.5..  3.4.1 COKE STRENGTH AS A FUNCTION OF V A R I A B L E S The i n d e p e n d e n t v a r i a b l e s u s e d t o c h a r a c t e r i z e a m b i e n t and  h i g h t e m p e r a t u r e coke s t r e n g t h have been l i s t e d i n  section are  2.0. The e f f e c t s o f t h e s e  illustrated  i n Figures  v a r i a b l e s on c o k e  19-21 ( a - g ) . A l l s t r e n g t h  strength results  show a s i g n i f i c a n t amount o f s c a t t e r w h i c h i s a s s o c i a t e d w i t h the heterogenous nature  of c o k e .  3.4.2 BULK DENSITY The p o s i t i o n o f c o k e i n t h e A l g o m a c o k e - o v e n respect the  t o h e i g h t was f o u n d t o a f f e c t  final  coke product  with  the bulk d e n s i t y of  ( s e c t i o n 3 . 2 . 1 ) . The r e s u l t s  that bulk d e n s i t y r i s e s with i n c r e a s i n g depth i n the  show  Coke Type  CPR-  Mean S t r e n g t h  Standard  at Ambient  Deviation  Mean S t r e n g t h at  1400°C  Standard D e v i a t ion  Temperature (MPa)  (MPa)  (MPa)  (MPa)  264  12.76  4.09  16.41  5.06  265  14.34  6.26  17.81  7.17  266  17.57  6.90  18.72  7.71  267  10.72  4.74  13.32  5.41  268  10.64  5.95  13.84  7.55  269  8.16  4.06  6.29  2.40  270  9.93  3.56  11 .35  5.78  T a b l e XVI.  Mean Compressive S t r e n g t h V a l u e s f o r theSeven Coke Batches a t Both Ambient  Temperature and a t 1400°C.  87 coke-oven. Figure 19a shows the effect of mean bulk density on mean compressive strength. There was no c o r r e l a t i o n found between the strength of i n d i v i d u a l specimens, within a coke batch, and their respective bulk densities as shown in Figure 19b. Figure 19b shows the strength versus density for the range of bulk density values  bulk  (mean ±  standard deviation) chosen for t h i s strength study. This further confirms  the need for a s t a t i s t i c a l approach to the  treatment of coke strength data. The error bar in Figure 19a represents the 95% confidence  l i m i t for estimating the mean  strength. Despite the large scatter, d e f i n i t e trends are noticable as indicated by the "best f i t " l i n e s . The r e s u l t s suggest a linear r e l a t i o n s h i p between bulk density and strength for the range of cokes studied. The coke strength appears to be greater for cokes extracted from the lower regions of the Algoma coke-oven than those sampled from the top of the oven.  3.4.3  POROSITY  Porosity has previously been reported to affect the strength of materials (28). Figure 20 i s the r e l a t i o n s h i p between strength and porosity. The strength was found to decrease with increasing porosity. Two empirical relationships, as postulated previously, were used to test the data: = ~  a  and  a - o  0  n c  p  +  D  exp(-be )  (ref 28)  »  *  i—f—i—i  i  |  i  i  i  i  i  i  —o 2 2 Q.  5  S c  TEST OVEN  20 1  A  - o-  8  ' '• ' I  ALGOMA  1400 C  A  I400*C  R.T  •  R.T  #  I  £ \6  g  in  14  o> 2  O Q)  8 i  075  0 79  083  Mean Bulk Figure 19a.  I  i  i  i  0*91 3 (g/cm )  0-87  Density  Mean Compression Strength versus Mean Bulk Density.  i  0-95  89  o CL  20  o 0  5  c  1  o  g 15  -  °  to  o o  o  o  >  1  o  V)  </> 0) k_ CL  o 0 0  o  O  o  o  .  Bulk Density  Figure  19b.  Compressive S t r e n g t h vs Bulk D e n s i t y f o r CPR-264 at Ambient  Temperature.  90  1  22  1  I400*C R.T.  ~  —-r-  TEST OVEN  ALGOMA  •A  -o  18  c 16 0)  _> to  12  0)  o. 10 E o  u  s  o  6 -  5  50  F i g u r e 20.  52  _L  54  56  Porosity  (%)  56  Mean Compressive S t r e n g t h versus Coke Porosity.  60  91 i s t h e p o r o s i t y , a i s t h e c o m p r e s s i v e s t r e n g t h and  where o  0  i s an e m p i r i c a l c o n s t a n t .  these equations  The e m p i r i c a l p a r a m e t e r s o f  a r e shown i n T a b l e s X V I I a n d X V I I I ,  r e s p e c t i v e l y . J u d g i n g by t h e v a l u e s coefficients, described  the l i n e a r  of the c o r r e l a t i o n  r e l a t i o n s h i p more  accurately  t h e v a r i a t i o n of c o m p r e s s i v e s t r e n g t h  p o r o s i t y than the exponential  equation  with  p r o p o s e d by Knudsen  (28).  3.4.4 MICROSTRUCTURE It  i s o b v i o u s f r o m s e c t i o n 3.4.3 t h a t t h e c o r r e l a t i o n  between c o m p r e s s i v e s t r e n g t h and p o r o s i t y i s s i g n i f i c a n t but not  very  s a t i s f a c t o r y ( a s i n d i c a t e d by t h e c o r r e l a t i o n  coefficients). the  So, t o o b t a i n a b e t t e r c o r r e l a t i o n between  s t r e n g t h a n d some m i c r o s t r u c t u r a l f e a t u r e s ,  attempts  h a v e been made t o t e s t t h e d e p e n d e n c e o f t h e s t r e n g t h on t h e p a r a m e t e r v a l u e s 3.4.4.1 C e l l W a l l  determined using  analyzer.  Size  The e f f e c t o f c e l l strength value  t h e image  data  w a l l s i z e on t h e c o m p r e s s i v e  i s shown i n F i g u r e  21a, f o r t h e data  both  a t a m b i e n t t e m p e r a t u r e a n d a t 1400°C. E r r o r b a r s a r e n o t shown s i n c e t h e 9 5 % c o n f i d e n c e  limits  f o r mean  c o m p r e s s i v e s t r e n g t h a r e t h e same a s i n F i g u r e s 20.  There i s a s i g n i f i c a n t negative  the  s t r e n g t h and t h e c e l l  calculated  linear  19 a n d  c o r r e l a t i o n between  w a l l s i z e a s shown by t h e  regression  lines.  These r e s u l t s a r e i n  d i s a g r e e m e n t w i t h P a t r i c k e t a l ( 3 2 ) who  found a  Testing  Condition  Test Oven Cokes,  Ambient Temp.  Test Oven Cokes, Algoma Cokes,  Ambient Temp.  Algoma Cokes,  Table XVII  1400°C  1400°C  n  b  (MPa)  (MPa)  Coeff.  1. 14  76 .25  -0.999  0. 51  44 .92  -0.934  0. 33  28 .50  -0.845  0. 91  61 .94  -0.794  Parameters for the Equation:  Testing  Condition  o  o»-ne*b.  b  0  (MPa)  Test Oven Cokes,  Ambient Temp.  Test Oven Cokes, Algoma Cokes,  Ambient Temp.  Algoma Cokes,  Table XVI11  1400°C  1400°C  Corr.  Corr. Coeff.  837. 98  0 .075  -0 .996  82. 85  0 .029  -0 .927  71 . 45  0 .036  -0 .842  2435. 73  0 .097  -0 .801  Parameters for the Equation:  a*o exp(-b< ) . 0  Mean  Compressive Strength ( M P a ) — « — — — ro to  o a * a > o r o - ( * 0 > a > o r o  £6  94 p o s i t i v e c o r r e l a t i o n between c e l l tensile  w a l l t h i c k n e s s and  strength.  3.4.4.2 P o r e  Size  The p o r e s t r u c t u r e was r e p r e s e n t e d by  five  d i f f e r e n t parameters: pore a r e a , e q u i v a l e n t diameter, mean c h o r d l e n g t h , a n d maximum a n d minimum F e r e t ' s d i a m e t e r s . P l o t s of compressive s t r e n g t h v e r s u s these parameters  a r e shown i n F i g u r e s 2 l ( b - e ) . The e f f e c t o f  minimum F e r e t ' s d i a m e t e r on s t r e n g t h was n o t p l o t t e d due t o t h e s m a l l v a r i a t i o n o f v a l u e s between t h e seven  coke  batches. In F i g u r e s 21(b-e), e r r o r bars representing the 95% c o n f i d e n c e i n t e r v a l of t h e 80% c u m m u l a t i v e v a l u e s a r e o n l y shown f o r t h e a m b i e n t  finer  temperature  points  s i n c e t h e s e i n t e r v a l s a r e t h e same f o r t h e h i g h t e m p e r a t u r e d a t a a s w e l l . The " b e s t f i t "  l i n e s represent  t h e g e n e r a l t r e n d s o f t h e d a t a . From t h e s e p l o t s , apparent  that the strength decreased with  p o r e a r e a and p o r e s i z e , but t h e l i n e a r obtained  increasing  correlations  i n a l l four p l o t s are poor. S i m i l a r  been o b s e r v e d  i n previous studies  i t is  t r e n d s have  (32-36)  3.4.4.3 Number o f P o r e s L a r g e r p o r e a n d w a l l s i z e h a v e been f o u n d t o decrease the compressive apparent, therefore,  s t r e n g t h of coke. I t i s  t h a t t h e number o f p o r e s p e r u n i t  volume might a l s o c o r r e l a t e w i t h s t r e n g t h . T h i s r e l a t i o n s h i p i s shown i n F i g u r e 2 1 f . I n t h i s  f i g u r e , the  96  22  T  T  CL20-I  A  o  18  I400°C  (r--0-859)  RT.  (r--0-821)  T  c 16 0)  55 > vt  <> /  14 I2  0)  ciio E o  <~> 8 0)  O  033  0-35  0-37  Equivalent Figure 21c.  0-39  0-41  Diameter (mm)  Mean Compressive Strength versus Pore Equivalent Diameter.  0-43  -0-917) -0849)  0 21  0-22  0-23  Mean F i g u r e 21d.  Chord  0-24  0-25  0-26  Length (mm)  Mean C o m p r e s s i v e S t r e n g t h v e r s u s P o r e Mean Chord  Length.  to  - i — — i  6  -  0-48  0-50 Figure  21e.  r  «  r  A  l 4 0 0 ° C ( r - - 0 906)  o  R.T.  1  1  0-52  0-54  Maximum  1  Feret's  (r-0-833)  11  0-56  Diameter  0-58  (mm)  Mean C o m p r e s s i v e S t r e n g t h v e r s u s P o r e Maximum Feret's  Diameter.  0-60  o  ^  I  8 0  I  -J  100  9 0  I  110  I  1 2 0  I  1 3 0  L.  1 4 0  Number of Pores per Field Figure 2 1 f .  Mean Compressive Strength versus Number of Pores per Field. vo VO  100 coke strength appears to r i s e with increasing number of pores per f i e l d . This result agrees with previous observations  (32-36).  3.4.4.4 Pore Perimeter The compressive  strength has been shown in the past  (32-36) to be a function of the pore perimeter. This parameter i s thought to represent the pore surface area. P l o t t i n g mean compressive  strength versus pore perimeter  (see Figure 2lg), yields a negative c o r r e l a t i o n indicating that strength decreased with increasing pore perimeter. However, t h i s correlation does not describe the coke strength more accurately than other correlations shown i n Figures 2 l ( a - f ) .  3.4.5 STATISTICAL ASPECTS OF COKE FRACTURE The Weibull s t a t i s t i c s have provided a useful tool to test the strength data of b r i t t l e s o l i d s . An attempt has been made to assess the v a r i a b i l i t y of compressive values of coke (that has been encountered  strength  in t h i s study for  d i f f e r e n t coke batches) using the Weibull model of f a i l u r e . The f a i l u r e behaviour  i s represented by a p r o b a b i l i t y curve,  which i s expressed mathematically as:  P U ) = 1 - exp[-Ko ] m  where P i s the p r o b a b i l i t y of f a i l u r e , a i s the f a i l u r e stress, K i s an empirical constant and m i s the Weibull  (10)  Pore Perimeter F i g u r e 21g.  (mm)  Mean Compressive S t r e n g t h v e r s u s Perimeter.  Pore  102 m o d u l u s ( a l s o an e m p i r i c a l c o n s t a n t ) . Theoretically, weakest l i n k  t h i s equation  theory  (18)  and,  was  developed  t h e r e f o r e , was  from  not  the  truely  meant t o be a p p l i e d t o c o m p r e s s i v e s t r e n g t h d a t a o f where t h e of  f a i l u r e m e c h a n i s m may  f l a w s . However, e q u a t i o n  distribution failure  be a p r o g r e s s i v e  10 c a n  be u s e d a s a  f u n c t i o n of t h e p r o b a b i l i t y o f  the W e i b u l l d i s t r i b u t i o n  at high temperatures.  i s l a r g e and  The  at  distribution  of  s o l i d s ) . The  order  i s the W e i b u l l  coke  of  parameter of  W e i b u l l f u n c t i o n w h i c h most a c c u r a t e l y d e s c r i b e s i n strength values  p l o t s of  both  s o m e t i m e s v a r i e s by an  magnitude(normal for b r i t t l e  variation  versus  strength  c o m p r e s s i v e s t r e n g t h f o r a l l seven coke types  strength  cummulative  specimens.  F i g u r e s 22(a-g) are  ambient and  cooperation  failure  s t r e s s to d e s c r i b e the v a r i a t i o n s of  between coke  coke  the  the  modulus(m)  ( s l o p e of the W e i b u l l p l o t ) . These v a l u e s were c a l c u l a t e d using linear  r e g r e s s i o n and  a r e compared i n T a b l e  t e s t s at ambient temperature, the t e s t - o v e n higher  than  cokes  those  more u n i f o r m than  those  not a p p l y  the W e i b u l l modulus v a l u e s  found f o r the Algoma oven  test-oven  c o k e s a p p e a r t o be  i n s t r e n g t h p r o p e r t i e s at ambient  of the Algoma c o k e s . t o the data at  literature  of  (18).  cokes.  slightly  temperature  T h i s p a t t e r n , however, does  1400°C. A l l v a l u e s o f t h e  modulus found i n t h i s p r o j e c t f a l l i n the  For  (CPR-264 t o CPR-266) a r e c o n s i s t e n t l y  values  T h i s shows t h a t t h e  XIX.  w i t h i n the range  Weibull reported  103  0 98 0-95 0-90  T—r—r A  l 4 0 0 ° C ( r - 0 958)  o  R.T.  (r-0987)  0 80  0-60  S 0-40 o JO  o  OL <D  0-20  o LL  010 0 08 0 06 004  1  J_J.  2  3 4 5 Failure  F i g u r e 22a.  10  20  Stress  Weibull Distribution Plot Compressive S t r e n g t h  30  50  (MPa)  o f CPR-264 Mean  Values.  104  I  2  3  4  5  Failure  F i g u r e 22b.  10  20 SO 50  Stress (MPa)  Weibull D i s t r i b u t i o n  P l o t of CPR-265 Mean  Compressive S t r e n g t h  Values.  105  0 9 8 0 9 5 09 0  —  i  —  r  ~  n  A - — | 4 0 0 ° C  o  1  ( r - 0-961)  R T . (r-0 992)  0-80 0-60  o o  n  0)  ix  0-20  0 1 0  0 0 5 0 0 4  F i g u r e 22c.  1  2  Weibull  3 4 5  10  2 03 0  Failure  Stress  (MPa)  Distribution  Compressive Strength  Plot  o f CPR-266 Mean  Values.  5 0  106  0 98 0 95  - 1 1 1  1 -  —  0- 90  I 4 0 0 C (r-0955) (r-0 974)  1  °/r  #  R.T.  0  1  1  -  c  o/ij  0- 80  ft  O  0 60  A  A*' a  •  E  o  /°  0-40  /OA  i  / o f  o  ol  A /  /  0. O  7/  /  A  / A  /  /  /  1  O  010 0- 08  /  Ol  0- 20  ^  j  _  0- 06  /  /  A  / / / / / / / / O  /  — «  —  A  •  »  0- 04  •  2  I I  3 4 5 Failure  Figure  22d.  1 10  1 1 1 ,. 20 30 50  Stress (MPa)  W e i b u l l D i s t r i b u t i o n P l o t o f CPR-267 Mean Compressive Strength  Values.  .  107  0  98  0 95 0-90 0  n—r-r—i  —  1  I400°C (r-0  990)  /<*/  -  0  80  0- 60  0- 40 JD O  2 O  0  20  0  I 0  0)  O Li.  0- 08 006 0  04  J  I  I  I  2  3 4 5  Failure F i g u r e 22e.  1  J  10  20  30  Stress  Weibull Distribution Plot Compressive Strength  1  (MPa)  o f CPR-268 Mean  Values.  L  50  108  0  98  0  95  A  0-90 o  n —r — i — r  i iI  I « . . .  l400°C(r-0-979)* R.T.  (r-0  0 • 80  /  (  ;1  977) y A  0 • 60  o .o  0  -40  o  0. o>  0-20  O  Li.  0  10  0 • 08 0 • 06 0 • 04  J  3 4 5 Failure  Figure 22f.  10 Stress  Weibull Distribution Plot Compressive Strength  I  20 30 (MPa)  o f CPR-269 Mean  Values.  L  50  109  0  98  0 95 0 90  1 TTT 1 A--~I400*C (r-0 987)  1  r-T  /  0-80  0-60  o  0-40  .O  o  CL 0)  0-20  o  LL.  01 0 008 006 0  04  '\  1  J  I  2  3 4 5 Failure  F i g u r e 22g.  1  J  10  20  Stress  Weibull Distribution  Plot  Compressive Strength  Values.  l_L  30  (MPa)  o f CPR-270 Mean  50  Batch  264  265  266  267  268  269  270  m, A.T.  2.881  2.039  2.184  2.008  1.709  1.997  2.630  3.499  2.018  2.310  2.607  1.519  2.752  1.714  m,  1400°C  T a b l e XIX  W e i b u l l Modulus Values of Seven Coke B a t c h e s a t Ambient  Temperature and a t 1400°C.  111 3.4.6 THEORETICAL ASPECTS OF COKE FRACTURE The e m p i r i c a l c o r r e l a t i o n s b e t w e e n c o k e s t r e n g t h a n d a number o f m i c r o s t r u c t u r a l p a r a m e t e r s h a v e been d i s c u s s e d a b o v e . The t r e n d s show t h a t a number parameters  t o g e t h e r may  of  structural  d e t e r m i n e t h e coke  strength.  P a t r i c k e t a l ( 3 5 ) a n d o t h e r w o r k e r s a t t h e BCRA ( 3 4 ) p r o p o s e d an e q u a t i o n ( e q u a t i o n 5 ) : a=450(Fmax)"°' exp[-2 ( F m a x / F m i n ) * e ] 5  0  (5)  5  p  where Fmax i s t h e mean maximum F e r e t ' s d i a m e t e r w h i c h i s t r e a t e d as t h e c r i t i c a l  f l a w s i z e , and t h e term  0 5 2(Fmax/Fmin) * the  i s the s t r e s s concentration  data generated i n t h i s  factor. Using  s t u d y , e q u a t i o n 5 was t e s t e d ( s e e  F i g u r e 23) where Fmax a n d (Fmax/Fmin) a r e t h e 80 values.  I t was a s s u m e d t h a t a l l f a i l u r e was  during the compression t e s t s  percentile  i n tension  (24) w h i c h j u s t i f i e s  t h e use of  e q u a t i o n 5 f o r t h e s e t e s t s . The s l o p e o f t h e p l o t s a r e 2385 and  3331 MPa  f o r room t e m p e r a t u r e a n d 1400°C,  w h i c h i s h i g h e r t h a n 450MPa, It  respectively,  r e p o r t e d by P a t r i c k  et a l (35).  s h o u l d be r e a l i z e d , h o w e v e r , t h a t t h e n o m i n a l  compressive  strength of b r i t t l e tensile  strength  s o l i d s are always higher than the  (theoretically  8 times greater  (24)).  A l t h o u g h P a t r i c k ' s e q u a t i o n d i d n o t c o r r e l a t e any than p l o t t i n g  better  s t r e n g t h vs i n d i v i d u a l porous parameters, i t  h a s t h e m e r i t o f p r e d i c t i n g t h e o b s e r v e d o r d e r o f mean s t r e n g t h v a l u e s f o r the coke b a t c h e s . A c c o r d i n g t o the theory of f r a c t u r e mechanics, the failure  s t r e s s of a m a t e r i a l  i s g o v e r n e d by i t s c r i t i c a l  "1  1  1  1  1  l400°C(<r-333IF-20) R.T  )084  (cr-2385F-l2)  00090  0-0096  I—  (r- 0 846) (r-0-821)  0-0102  0-0108  0-5 F  = ( F max) Figure 23.  0-0M4  0-5  e x p ( - 2 ( F max/Fmin) Mean Compressive S t r e n g t h {Fmax  -0,5  versus  exp[-2(Fmax/Fmin)  0,5  p]}.  p )  113 flaw  s i z e , and hence,  ( 4 1 ) . The  stress  itscritical  critical  stress  N i e d and A r i n treated  small  intensity factor  intensity factor K i s a function  nominal s t r e s s a p p l i e d the  stress  and  the flaw  of  s i z e , a n d when K  intensity factor, K ,  the reaches  f a i l u r e occurs.  I C  (42) d e v e l o p e d a m o d e l f o r c e r a m i c s  p o r e s as s p h e r i c a l  f l a w s and d e v e l o p e d  r e l a t i o n s h i p t o c a l c u l a t e the s t r e s s  which  a  intensity factors  by  a c c o u n t i n g f o r the i n t e r a c t i o n of s t r e s s t r a j e c t o r i e s i n three  dimensions: K = 2/rr { 1 / V  f  tan[ (*/2)V ] }  where a i s t h e p o r e r a d i u s , porosity, stress are  and  o  0  (  K If  (11)  0  s t r e s s . The' c r i t i c a l  ^) f o r the seven coke  shown i n T a b l e XX a s c a l c u l a t e d J C  a /(2a)  i s t h e volume f r a c t i o n of  i s the nominal a p p l i e d  intensity factors  v a l u e s of K  1/ 2  f  f o r cokes produced  batches  u s i n g e q u a t i o n 11.  The  i n the e x p e r i m e n t a l  t e s t - o v e n a r e h i g h e r t h a n t h o s e f o r A l g o m a c o k e s . I t has been s u g g e s t e d p r e v i o u s l y stress  intensity factor  account difficult  f o r flaw  the  the  ( 3 2 - 3 6 ) , b u t e q u a t i o n 11 makes no  s h a p e . The  flaw  shape,  however, i s  in a material,  such  as  t o the complex system of p o r e s .  i s i n t e r e s t i n g to note that  critical  stress  values calculated  the c a l c u l a t e d v a l u e s of  i n t e n s i t y f a c t o r s are s i m i l a r to those  f o r coke  b a t c h e s o f t h e same o r i g i n ( i e .  t e s t - o v e n cokes have c r i t i c a l close  p o r e shape i n f l u e n c e s  to assess s t a t i s t i c a l l y  coke, owing It  that  stress  intensity factor values  t o one a n o t h e r a s do t h o s e o f A l g o m a c o k e s ) . T h i s  Coke Type  Porosity V  f  Max. F e r e t ' s  K  IC  (MPa/mm)  D i a , 2a (mm)  A.T.  1400°C  CPR-264  0.555  0.518  8.560  11.006  CPR-265  0.543  0.482  9.205  11.431  CPR-266  0.513  0.506  11.341  12.084  CPR-267  0.521  0.530  7.112  8.835  CPR-268  0.562  0.573  7.542  9.810  CPR-269  0.594  0.603  6.081  4.686  CPR-270  0.559  0.566  6.980  7.982  T a b l e XX.  Calculated C r i t i c a l M u l t i p l e Flaw Model.  Stress Intensity  F a c t o r s Using a  115 suggests that the t e x t u r a l c h a r a c t e r i s t i c s a significant (see  section  (pore s i z e )  have  i n f l u e n c e on t h e m e c h a n i c a l b e h a v i o u r o f c o k e 4.5.2).  F i g u r e s 2 4 ( a & b) i l l u s t r a t e t h e  v a r i a t i o n o f t h e c a l c u l a t e d K j ^ v a l u e s amongst t h e c o k e b a t c h e s t e s t e d a t room t e m p e r a t u r e The  a n d 1400°C, r e s p e c t i v e l y .  e r r o r b a r s r e p r e s e n t t h e range o f K , v a l u e s t h a t J(  would  o c c u r f r o m t h e mean s t r e s s r a n g e c a l c u l a t e d p r e v i o u s l y a t t h e 9 5 % c o n f i d e n c e l i m i t . The t e s t - o v e n c o k e K j ^ . v a l u e s i n c r e a s e w i t h i n c r e a s i n g oven p r e s s u r e , b u t t h i s n o t be f o u n d  f o r t h e A l g o m a c o k e s . CPR-269 r e c o r d e d t h e  lowest K , values a t both I(  3.4.7  trend could  temperatures.  S T A B I L I T Y AND HARDNESS Coke s t r e n g t h i s t r a d i t i o n a l l y a s s e s s e d i n N o r t h  America  i n accordance  standard  w i t h t h e ASTM s t a b i l i t y  and hardness  ( 4 3 ) . T h i s s t a n d a r d has been p r e s e n t e d  previously  in Table I I I (13). I t i s t h e r e f o r e d e s i r a b l e t o attempt t o develop a r e l a t i o n s h i p describing using compressive  stability  and hardness  strength values.  F i g u r e 25 shows s t a b i l i t y  and hardness  CANMET) v e r s u s t h e mean c o m p r e s s i v e  (measured  at  s t r e n g t h v a l u e s a t room  t e m p e r a t u r e . No c o r r e l a t i o n c o u l d be o b s e r v e d b e t w e e n t h e stability obtained  and hardness in this  and t h e c o m p r e s s i v e  project.  strength data  116  15  U*—Algoma  Coke-eJ  13  o u  o  ov.  E E o 0.  u 6 -  Test-oven "  264  Coke  "  265  266  CPR  F i g u r e 24a.  _L  267  Coke  Calculated C r i t i c a l  268  269  270  Type  Stress Intensity  u s i n g Ambient Temperature Compressive Strength Data.  Factors  117  15  n  1—rr  1  r * - A l g o m a Coke-#»»i  13  II  O  E E o  9  I  Test-oven. Coke  264  F i g u r e 24b.  I  265 266 267 268 269 CPR Coke T y p e  Calculated C r i t i c a l  Stress Intensity  ±  270  Factors  u s i n g Compressive S t r e n g t h Data Obtained a t 1400°C.  1  1  -  1  1  1 A  A A  A  A  A  O  O  O  A  O  0  O  o  Stability  A  Hardness  o  1 8  6  Mean Figure  25.  1 10  1 12  Compressive Stability Strength.  l 14 Strength  i 16 (MPa)  a n d H a r d n e s s v e r s u s Mean  Compressive  IC  119 3.5 HIGH TEMPERATURE CREEP A literature  s u r v e y r e v e a l e d no r e f e r e n c e on t h e p l a s t i c  behaviour of b l a s t  f u r n a c e c o k e . However, because  e x i s t e n c e of a s h h a v i n g low m e l t i n g temperature such as NajO, S i 0 , 2  plastic  constituents  1^0, e t c . ( a n d t h e i r e u t e c t i c s ) ;  some  d e f o r m a t i o n c a n be e x p e c t e d t o o c c u r when t h e  specimens  a r e heated t o temperatures greater than  subjected to a load. In t h i s project, constant e x p e r i m e n t s were p e r f o r m e d than  of the  load  at varying temperatures  1000°C) a n d p r e s s u r e s t o d e t e r m i n e  high temperature p l a s t i c i t y . be d i f f i c u l t  1000°C a n d  (greater  i f coke e x h i b i t s any  Such e x p e r i m e n t s were f o u n d t o  t o p e r f o r m a s most o f t h e s p e c i m e n s  failed  (collapsed) during loading to the desired stress f o r testing. H o w e v e r , a few c r e e p e x p e r i m e n t s were c a r r i e d o u t u n d e r a c o n s t a n t l o a d and i s o t h e r m a l c o n d i t i o n s . I n o r d e r t o e s t i m a t e t h e d i m e n s i o n a l change, which specimens due  t o c o n t i n u e d g r a p h i t i z a t i o n above t h e f i n a l  temperature, g r a p h i t i z a t i o n  may u n d e r g o coking  t e s t s were a l s o p e r f o r m e d .  This  was n e c e s s a r y t o e n s u r e t h a t d i m e n s i o n a l c h a n g e d u r i n g c r e e p t e s t i n g was n o t c a u s e d by g r a p h i t i z a t i o n .  3.5.1  GRAPHITIZATION The  coke-oven  maximum t e m p e r a t u r e e n c o u n t e r e d  in a  normal  i s i n t h e o r d e r o f 1250°C, s o i t i s q u i t e  t h a t when c o k e  natural  i s s u b j e c t e d t o a temperature h i g h e r than  t h i s and f o r a p r o l o n g e d p e r i o d of t i m e , t h e specimens  would  120 graphitize  resulting  i n an i n c r e a s e i n d e n s i t y w i t h an  associated shrinkage. I n o r d e r t o d e t e r m i n e t h e e x t e n t o f s h r i n k a g e due t o g r a p h i t i z a t i o n , coke specimens as d e s c r i b e d  i n section  w e r e h e a t t r e a t e d a t 1400°C  2.7. The maximum l i n e a r s h r i n k a g e  e n c o u n t e r e d i n t h e s e e x p e r i m e n t s was i n t h e r a n g e 0.96 t o 1.33%, w h i c h a g r e e s w i t h d i l a t o m e t e r r e s u l t s o b t a i n e d by other workers  ( 3 , 2 3 , 2 6 , 2 7 ) , who r e p o r t e d v a l u e s i n t h e r a n g e  o f 0.70 t o 1.0% a t t e m p e r a t u r e s r e a c h i n g 1400°C.  3.5.2 ISOTHERMAL CONSTANT LOAD TESTS The  c o n s t a n t l o a d t e s t s were p e r f o r m e d a t t e m p e r a t u r e s  r a n g i n g b e t w e e n 1000 a n d 1600°C w i t h s t r e s s e s r a n g i n g f r o m 5.0 t o 13.0MPa on t h e f o u r Algoma c o k e s CPR-270). Such  t e s t s c o u l d n o t be c a r r i e d o u t on t h e  t e s t - o v e n cokes as t h e i r  sample  s i z e s were n o t s u f f i c i e n t l y  l a r g e t o p r o d u c e enough c y l i n d r i c a l s t r e n g t h and c r e e p The  specimens  t o do b o t h  studies.  c o n s t a n t l o a d t e s t s were p e r f o r m e d u s i n g t h e  h e a t i n g and l o a d i n g c o n d i t i o n s d e s c r i b e d The  (CPR-267 t o  i n section  2.6.2..  d e f l e c t i o n was r e c o r d e d a s a f u n c t i o n o f t i m e a n d was  corrected for elastic  strain,  o b t a i n e d by r e m o v i n g  the load  a t t h e end o f t h e c r e e p e x p e r i m e n t s and n o t i n g t h e magnitude of t h e e l a s t i c The  first  "spring  back".  s e t o f e x p e r i m e n t s p e r f o r m e d was t o d e t e r m i n e  the r e p r o d u c i b i l i t y o f t h e c r e e p c u r v e s . Three similar  specimens  f r o m one b a t c h o f c o k e  "so-called"  (CPR-269) were  121 c r e e p t e s t e d u n d e r i s o t h e r m a l conditions(1500°C) a n d pressure  ( 6 . 0 M P a ) . The  n o r m a l i z e d c r e e p c u r v e s a r e shown i n  F i g u r e 26. U n f o r t u n a t e l y , t h e c r e e p c u r v e s a r e n o t v e r y reproducible. Similar encountered  randomness i n c r e e p b e h a v i o r  was  i n a l m o s t a l l o t h e r c a s e s . Because of the  of a v a i l a b i l i t y  o f a l a r g e number o f s p e c i m e n s  lack  having  i d e n t i c a l b u l k d e n s i t y a n d p r o p e r t i e s , no e x t e n s i v e c r e e p t e s t i n g c o u l d be c a r r i e d o u t The  in this  t e s t c o n d i t i o n s and  experiments F i g u r e 26)  are l i s t e d that  were u n d e r g o i n g  deformation at temperatures  (not t a k i n g  strain  for a l l creep  i n Table XXI. I t i s obvious  specimens  s t r a i n t h a t c o u l d be  total  program.  >1000°C and  i n t r o d u c e d was  plastic  the t o t a l  to  graphitization  c o n s i d e r e d unnessary  to account  measure d u r i n g t h e c r e e p e x p e r i m e n t s . Most of t h e  w h i c h were s e l e c t e d  for further  creep  analysis. creep  behavior w i t h respect to temperature  and  2 7 ( a - b ) ) . I n c r e a s i n g the temperature  and p r e s s u r e o f  a higher t o t a l  to  C, e x c e p t t h o s e o f CPR-270,  c r e e p c u r v e s o f CPR-270 showed n o r m a l  should produce  the  for  s h r i n k a g e w h i c h c o u l d be v e r y d i f f i c u l t  c u r v e s a r e shown i n A p p e n d i x  8%  graphization  t o t h e q u a l i t a t i v e n a t u r e of t h i s p a r t of  s t u d y , i t was  The  maximum  i n the o r d e r of 6 t o  i n t o c o n s i d e r a t i o n s h r i n k a g e due  of ^ 1 % ) . Due  (from  strain,  stress (Figures testing  i f the assumption  the m e l t i n g of ash c o n t r o l s the p l a s t i c  that  b e h a v i o r of coke i s  c o r r e c t . B o t h the c o n c e n t r a t i o n of the g l a s s y phase  (ash)  and  and  i t s viscosity  s h o u l d be a f f e c t e d by t e m p e r a t u r e  qure  26.  Normalized Creep Curves Produced at and 6.0MPa.  1500  123  Coke  Temperature  Type  of  Applied  Tests  Stress  During  Total Strain  Tests (MPa)  (°C>  CPR-267  CPR-268  CPR-269  CPR-270  Table XXI.  (%)  1000  12.6  6.55  1600  12.6  8.52  1600  12.6  6.42  1200  9.69  1.81  1400  9.69  6.08  1600  9.69  5.74  1600  6.0  3.22  1600  13.0  4.04  1  500  6.0  4.90  1 500  6.0  2.09  1500  6.0  2.30  1600 •  9.69  4,74  1500  5.0  1.71  1 100  9.29  1 .69  1 300  9.29  1 .86  1  500  9.29  3.12  1  500  6.0  1 .98  1  500  13.0  4.03  Conditions  f o r Creep  Tests.  F i g u r e 27a.  C r e e p o f CPR-270 a t 1500°C U n d e r Conditions  of Applied  Stress.  Varying  125  20 Time Figure  27b.  •  1500 C  O  1300  A  II00*C  30  40  -  #  C  C r e e p o f CPR-270 a t 9.29MPa Conditions  50  60  (min)  of Temperature.  Under  Varying  126 pressure. R a i s i n g the temperature effect  of i n c r e a s i n g t h e c o n c e n t r a t i o n of g l a s s y phase and  decreasing  i t s v i s c o s i t y . D e v i a t i o n s from t h i s b e h a v i o r can  be a t t r i b u t e d (large  t o t h e v e r y complex n a t u r e of coke t e x t u r e  variations  An  were e n c o u n t e r e d  interesting  these creep curves affect  e v e n i n t h e same b a t c h ) .  o b s e r v a t i o n t h a t c o u l d be made i s that the temperature  from  was f o u n d t o  t h e shape of t h e c u r v e s . C r e e p c u r v e s g e n e r a t e d a t  lower temperatures final  o f coke s h o u l d have t h e  s t r a i n values  produced  (1000-1200°C) were f o u n d t o r e a c h (leveled  o f f ) more q u i c k l y  at higher temperatures.  than  their those  This indicates a viscous  type flow ( s i m i l a r t o g l a s s ) at higher temperatures  which  supports the c o n t e n t i o n that i t i s the m e l t i n g of ash components which  produced  the p l a s t i c  strain.  3.5.3 CREEP BEHAVIOUR OF COKE The behaviour and  d e t e r m i n a t i o n o f t h e mechanisms o f t h e c r e e p of coke i s beyond t h e scope of t h i s  investigation  r e q u i r e s more e x t e n s i v e s t u d y . H o w e v e r , t h e shape o f t h e  c r e e p c u r v e s was f o u n d t o be s i m i l a r refractories The  t o creep curves of  (44,45).  c r e e p behavior of r e f r a c t o r i e s  are often analyzed  by v i s c o - e l a s t i c m o d e l s a s t h e s y s t e m s a r e t o o c o m p l e x t o be a n a l y z e d u s i n g any a t o m i s t i c mechanisms. I n a l m o s t a l l cases, the creep of r e f r a c t o r i e s a g l a s s y phase produced constituents  i s due t o t h e f o r m a t i o n o f  by l o w m e l t i n g  (45). Furthermore,  temperature  i t was r e p o r t e d t h a t an  127 increase  i n p o r o s i t y reduces t h e a b i l i t y of r e f r a c t o r i e s t o  r e s i s t c r e e p (45). Since coke i s a h i g h l y porous containing  low m e l t i n g  constituents), analyze Also,  i t i s n a t u r a l t h a t a n a t t e m p t be made t o  the creep data  of coke u s i n g a v i s c o - e l a s t i c  model.  of t h e coke s t r u c t u r e does not  any i n t e r p r e t a t i o n o f t h e c r e e p o f coke u s i n g any  atomistic  3.5.4  temperature phases ( a s h  t h e complex nature  permit  solid  model.  V I S C O - E L A S T I C MODELS Three models a r e c o n s i d e r e d  creep behavior (Figure 28a),  f o rthe a n a l y s i s of the  o f coke. These a r e a s i n g l e - K e l v i n u n i t an i n t e r a c t i v e d o u b l e - K e l v i n  element u n i t  ( 4 6 , 4 7 ) ( F i g u r e 28b) a n d t h r e e - n o n - i n t e r a c t i v e - K e l v i n u n i t s (48)  ( F i g u r e 2 8 c ) . Of t h e s e  interactive double-Kelvin adequately load  3.5.5  three models, only t h e  element model i s found t o  d e s c r i b e t h e response of coke s u b j e c t  t o constant  tests.  ANALYSIS OF CREEP DATA The  a n a l y s i s of t h e creep data  visco-elastic hot-pressing  was c a r r i e d o u t u s i n g a  model d e r i v e d p r e v i o u s l y (46,47) f o r of ceramic powders. In F i g u r e s  27(a & b), the  128  F i g u r e 28a.  Single-Kelvin  Unit.  /  /  F i g u r e 28b.  //)///////////////////////  Interactive-double-Kelvin  Unit.  130  •  M,  M  •  2  • unnri u Figure 28c.  mn  Three-non-interactive-Kelvin Units,  131  points correspond  t o e x p e r i m e n t a l d a t a and  d r a w n c a n be r e p r e s e n t e d by an e q u a t i o n a s  e=AL/L =K(1-Ae~ -Be" a t  / 3 t  o  The  parameters  K,A,B,a a n d  e x p e r i m e n t a l c u r v e s and data to equation curves  0 can  )  (12)  by  digitizing  then, using a computer, f i t t i n g  12. I t i s a l s o p o s s i b l e t o f i t t h e  e q u a t i o n would not l e n d  The  as  follows:  be d e t e r m i n e d  to a s i x t h order polynomial  visco-elastic  the l i n e s  itself  (46), but  such  the  creep  an  t o the development of a  model.  problem  can  be  represented using system  (47) w h e r e a s t e p i n p u t  dynamics  stress: o = a U (t) 0  (13)  c  where U (t)=  0 for t < 0  (14)  c  1 for t > 0  produces a s t r a i n output s t r a i n c a n be system  ( e q u a t i o n 12). T h e r e f o r e ,  r e p r e s e n t e d by  transfer  the product  the  o f s t r e s s and  the  function:  e=xo  where x i s a t r a n s f e r  function.  (15)  132  A differential  equation  applying the Laplace  f o r t h e s y s t e m c a n be f o u n d by  transform to equation  ( r e p r e s e n t e d by c a p i t a l  15 ( 5 1 )  letters): E = X L  (16)  where  «-*(.<t» -  f - J b ! - H K(s+a)(s+g)-KAs(s+fl)-KBs(s+g) s(s+a)(s+0)  and  I = £(U ( t ) ) • 1/s  Therefore, found  the Laplace  t o be ( t a k i n g  transform of the t r a n s f e r  function i s  i n t o c o n s i d e r a t i o n t h a t a t t=0, e ( t ) =  0, t h e r e f o r e , A+B=1):  E _  x  _ sK(Aa+B/3)+Kag s +(g+0)s+g0  Z  w h i c h c a n be a r r a n g e d E[s  Taking  2  to give:  + (g+0)s+g/3] = I[sK(Ag+B/3)+Kg/3]  the inverse Laplace  the d i f f e r e n t i a l  +  dt  2  This d i f f e r e n t i a l  (17)  2  transform of equation  (18)  18 p r o d u c e s  equation:  (a+p)—  + g/3c = K ( A g + B / 3 ) ^ + g/3a dt dt  equation  d  K  i s similar  t o t h a t which  (19a)  133  d e s c r i b e s t h e b e h a v i o r o f t h e two i n t e r a c t i v e elements  Kelvin  shown i n F i g u r e 28b ( 4 6 , 4 7 ) :  + [ ^ ( M i + M a i + M a n , ] de  i l l dt  T7,T7  2  +  The s p r i n g c o n s t a n t s a n d t h e d a s h p o t  [M,M ] a  £  [T7,7? ]  dt  2  _1_d£ . [MjMx]  =  +  2  0  {  l  9  b  )  v i s c o s i t i e s h a v e been  shown t o be r e l a t e d t o K, A, B, a, a n d 0 b y :  T) M  = l/K[Aa+B0]  2  (20)  = 7 7 [a+p -Ka/377 2 ]  (21)  ,  2  2  TJ 1 = M / T ? [ K a / 3 M - a / 3 ] 2  2  2  M i » nyr\ a$M 2  T h i s mechanical analogue  (22)  2  (23)  2  was u s e d t o d e s c r i b e t h e c o m p o n e n t s  of c r e e p s t r a i n  f o r t h e coke  samples  t e s t e d under  l o a d , and under  isothermal conditions.  a constant  The v a l u e s o f t h e s e m e c h a n i c a l p a r a m e t e r s a r e c a l c u l a t e d u s i n g t h e v a l u e s o f K, A, B, a , 0 o f t h e e x p e r i m e n t a l c u r v e s a n d a r e shown i n t h e A p p e n d i x  C. The  t e m p e r a t u r e a n d s t r e s s d e p e n d e n c e o f T J , , T ? , M, , M  2  2  illustrated figures,  i n F i g u r e s 29 ( a & b ) , r e s p e c t i v e l y .  t h e v i s c o s i t y parameter  t h a n t h o s e o f T J , M,, a n d M . 2  appears  2  In both  77, h a s much h i g h e r v a l u e s  The p a r a m e t e r ,  TJ,,  also  t o be b o t h t e m p e r a t u r e a n d s t r e s s s e n s i t i v e  the other three parameters  are  are relatively  whereas  i n s e n s i t i v e . The  s i g n i f i c a n c e of t h i s behaviour with respect t o temperature  134  Figure  29a.  Variation  in Visco-Elastic  Model  Mechanical  Parameters as a F u n c t i o n of Temperature f o r CPR-270.  135  20001  o  1  1  1  29b.  1  r-  1  1  r  1  1  r~  10  Applied Stress  Variation Parameters for  1  6  6  Figure  r  12  (MPa)  in Visco-Elastic  Mechanical  as a F u n c t i o n of A p p l i e d P r e s s u r e  CPR-270.  136 and  pressure  c a n n o t be  resistance to flow s y s t e m and may  be  the  explained at present.  implies that very  r e d u c t i o n of v a l u e s  a s s o c i a t e d w i t h the  of  However,  high v i s c o s i t y TJ, w i t h  in  any the  temperature  i n c r e a s i n g f l o w p r o p e r t i e s of  4. DISCUSSION In  the previous chapter, the results obtained  in this  s t u d y h a v e been a n a l y z e d . H o w e v e r , i t i s w o r t h w h i l e t o critically  e v a l u a t e some o f t h e a t t e m p t e d  a l r e a d y made. B e f o r e efforts will As study  doing t h i s e v a l u a t i o n , the t o t a l  be s u m m a r i z e d  outlined  correlations  first.  i n s e c t i o n 1 . 6 , an e f f o r t  h a s been made t o  t h e v a r i a t i o n o f coke p r o p e r t i e s ( m i c r o s t r u c t u r e and  s t r e n g t h both a t ambient and h i g h temperatures) f u n c t i o n of p o s i t i o n Secondly,  as a  ( h e i g h t ) i n a coke-oven b a t t e r y .  a wharf coke sample t o r e p r e s e n t  t h e oven-coke, as  a w h o l e , was a l s o u s e d a s t h e c o n t r o l s a m p l e . T h i s was done in  order  t o compare t h i s wharf coke w i t h c o k e s e x t r a c t e d  from v a r i o u s h e i g h t s three test-oven prepared for  i n A l g o m a ' s 5m c o k e - o v e n .  Furthermore,  s a m p l e s made f r o m t h e same c o a l b l e n d b u t  under v a r y i n g c o k i n g c o n d i t i o n s were a l s o t e s t e d  f u r t h e r comparison with the commercially  produced  samples. The  experimental  program i n v o l v e d q u a n t i f y i n g the  m i c r o s t r u c t u r e o f t h e c o k e u s i n g an a u t o m a t e d and  correlating  compressive  the m i c r o s t r u c t u r a l parameters with  s t r e n g t h v a l u e s o b t a i n e d both a t ambient and  high temperatures.  Lastly,  the p l a s t i c  behaviour  i n d u s t r i a l c o k e s a m p l e s was q u a l i t a t i v e l y temperatures  image a n a l y z e r  above  1000°C.  137  of the  evaluated at  138 4.1  BULK DENSITY  Initially,  t h e b u l k d e n s i t y of the coke samples  was  c o n s i d e r e d as a major p a r a m e t e r f o r d i s t i n g u i s h i n g each of the  s e v e n c o k e b a t c h e s (CPR-264 t o C P R - 2 7 0 ) . T h e b u l k d e n s i t y  of coke depends product,  on i t s a s h c o n t e n t ,  i t s t r u e d e n s i t y , and  the carbonaceous  i t s p o r o s i t y . Since  these  c o k e s a m p l e s w e r e p r o d u c e d f r o m t h e same c o a l b l e n d , ash content reason,  s h o u l d be s i m i l a r  t h e b u l k d e n s i t y was  distinguishing  4.1.1  the coke  R E L I A B I L I T Y OF According  i f n o t t h e same. F o r  this  c o n s i d e r e d a good parameter f o r  batches.  BULK DENSITY RESULTS  t o t h e ASTM s t a n d a r d s  ( 5 7 ) , a 50 pound  o f w h a r f c o k e must be u s e d when d e t e r m i n i n g d e n s i t y . Such a l a r g e sample necessary  their  size  sample  t h e sample  i s normally  bulk  considered  t o g e t a r e p r e s e n t a t i v e v a l u e . However, t h i s  was  not p o s s i b l e f o r the purpose of t h i s p r o j e c t s i n c e the l a r g e sample  s i z e s needed and t h e n e c e s s a r y  a v a i l a b l e . The  b u l k d e n s i t y was  m e a s u r i n g t h e volume and t h e i r  of n e a r - p e r f e c t d r i l l - c o r e  respective weights.  (standard  The  results  the  specimens  showed  i n Figure  18a by t h e  deviation).  The number o f s p e c i m e n s m e a s u r e d between  were n o t  t h e r e f o r e d e t e r m i n e d by  c o n s i d e r a b l e s c a t t e r as r e p r e s e n t e d e r r o r bars  equipment  32(CPR-265) and  131(CPR-269),  i n t h i s way w h i c h was  ranged  g o v e r n e d by  s i z e o f t h e s a m p l e b a t c h e s s u p p l i e d by CANMET. T h e r e i s  a c o n s i d e r a b l e " o v e r l a p " of the bulk d e n s i t y ranges  (error  1 39 b a r s ) of t h e seven coke b a t c h e s . the p o s s i b i l i t y  This r a i s e d concern  about  t h a t p o p u l a t i o n mean b u l k d e n s i t i e s c o u l d be  t h e same. An a n a l y s i s o f v a r i a n c e o f t h e b u l k d e n s i t y u s i n g t h e One-Way C l a s s i f i c a t i o n a t 99% c o n f i d e n c e  to test  showed t h a t a t l e a s t  this possibility.  analysis  XXII  f r o m one a n o t h e r  shows t h e p r o b a b i l i t y  indicate that there two  coke batches.  and  CPR-270 p o s s e s s  i n most  that individual  cases.  comparisons  spaces i n the t a b l e  i s no s i g n i f i c a n t d i f f e r e n c e b e t w e e n t h e  The c o k e b a t c h e s  CPR-264, CPR-265, CPR-268  s i m i l a r mean b u l k d e n s i t y v a l u e s  compared a t a l e v e l  o f s i g n i f i c a n c e o f 0.01. An  when  interesting  o f t h i s a n a l y s i s i s t h a t t h e mean b u l k d e n s i t y  o f CPR-270  (wharf  coke) i s r e p r e s e n t a t i v e of the  of b u l k d e n s i t y v a l u e s o b t a i n e d CPR-269  results  (single-degree-of-freedom  h a v e t h e same mean b u l k d e n s i t y . B l a n k  result  The  ( 2 9 ) ) r e v e a l e d t h a t t h e i n d i v i d u a l mean b u l k  d e n s i t i e s do i n f a c t d i f f e r Table  ( 2 9 ) was e m p l o y e d  two s a m p l e mean b u l k d e n s i t y v a l u e s  d i f f e r e d . A more i n d e p t h comparisons  technique  values  value  combination  f r o m CPR-267, CPR-268, a n d  (Algoma c o k e s ) b u t i s n o t r e p r e s e n t a t i v e o f t h e  test-oven cokes.  Furthermore,  t h e mean b u l k d e n s i t y  values  o f CPR-264, CPR-265, a n d CPR-266 a s a g r o u p do n o t r e p r e s e n t t h o s e o f CPR-267,CPR-268, a n d CPR-269 w h i c h t h e y initially  4.1.2 OVEN  intended  were  t o do ( s e e A p p e n d i x D ) .  PRESSURE  1  B u l k d e n s i t y was shown t o be a f f e c t e d by t h e p r e s s u r e ' A l l pressures e x i s t i n g i n a battery (static r e f e r r e d t o a s oven p r e s s u r e s .  or other) are  264  264  265  —  265  —  266  0 .01  0.01  267  0 .01  0.05  268 269 270  T a b l e XXII •  0 .01  266  267  0.01  0.01  0.01  0.01  0.05  0.01  0.05  0.01  0.01  0.01  0.01  0.01  0.01  0.01  0.05  —  0.05  0.05 —  268  0.01  0.01  0.01  0.01  0.01  0.01  0.05  0.01  0.01  0.05  P r o b a b i 1 i t y that Densities Batches.  —  269  —  270  0.01  0.01  —  The Assupmtion of Di f f e r ing Bulk  i s Erroneous f o r Comparisons Between Coke  141 of t h e oven w a l l s d u r i n g c o k i n g 18b).  i n the test-ovens  (Figure  The r e l a t i o n s h i p b e t w e e n t h e b u l k d e n s i t y a n d t h e o v e n  wall pressure  existing  i n t h e c o k e - o v e n was f o u n d t o b e :  P (gm/cm ) = 0 . 1 1 [ l o g P ( K P ) ] + 0 . 8 0 8 3  b  This equation, points  (from  1 Q  h o w e v e r , was d e r i v e d u s i n g o n l y t h r e e  Shown a l s o i n F i g u r e  i n an o p e r a t i n g  It  i n coking pressure  be a t t r i b u t e d should  industrial  18b a r e t h e e s t i m a t e d  t h r e e A l g o m a c o k e s b a s e d on t h e i r  can  points f o r the  b u l k d e n s i t y v a l u e s . The  i n the i n d u s t r i a l  to the s t a t i c  coke-oven.  load existing  coke-oven  on t h e c h a r g e .  be m e n t i o n e d , h o w e v e r , t h a t t h e r e may be o t h e r  variables  (such as v e r t i c a l  the  b u l k d e n s i t y o f t h e A l g o m a c o k e . F i g u r e 30  final  illustrates time  data  t e s t - o v e n c o k e s ) s i n c e i t was i m p o s s i b l e t o  determine the pressure  difference  a  temperature gradients)  affecting  the v a r i a t i o n of temperatures as a f u n c t i o n of  i n t h e A l g o m a No.9 b a t t e r y f o r t h e t h r e e p o s i t i o n s f r o m  where c o k e s C P R - 2 6 7 ( s o l e ) t o C P R - 2 6 9 ( t o p ) were s a m p l e d ( 4 9 ) . The  temperature h i s t o r i e s of the three p o s i t i o n s a r e  somewhat v a r i e d w i t h f i n a l and  temperatures being  1262°C f o r t h e t o p , m i d d l e ,  and s o l e  r e s p e c t i v e l y . The e f f e c t o f f i n a l  935°C,  1195°C,  positions,  c o k i n g t e m p e r a t u r e on t h e  coke b u l k d e n s i t y i s u n c e r t a i n but i t i s p o s s i b l e t h a t the t e m p e r a t u r e e f f e c t may be  significant.  4.1.3 TRUE DENSITY The  t r u e d e n s i t y o f a l l s e v e n c o k e b a t c h e s was  d e t e r m i n e d from t h e samples taken  2cm f r o m t h e c a u l i f l o u r  142  16001 — T — i — i — i — i — r Top 0-5m below Coal Line  1400 O  e  1200  Mid-  —  2-5m  below  Cool  Line  Sole 4-5m below  1000  5 800 o S. 600  E £ 400 200 0  Figure  30.  ±  4 8 Time After Charge  12 16 (hours)  Temperature H i s t o r i e s Measured No. Sole  9 Coke-oven Positions.  20  i n t h e Algoma  B a t t e r y a t t h e Top,Middle and  143 e d g e . T h i s was  n e c e s s a r y as a p r e c a u t i o n  against  possible  t r u e d e n s i t y v a r i a t i o n as a f u n c t i o n of p o s i t i o n a l o n g oven w i d t h .  Two  determination each of  the  deviation  f l u i d s were u s e d i n t h e  pycnometric  of t r u e d e n s i t y g i v i n g s i x m e a s u r e m e n t s f o r  seven coke t y p e s .  ranged between  v a r i a t i o n ( i e . standard  was  s e e n t o be  greater  From T a b l e IX,  1.1%  The  (CPR-268) and  f o r the  f o l l o w e d no  particular  coke should  a l l have s i m i l a r  the  4.5%(CPR-265).  test-oven  cokes than  true density  t r e n d . T h e o r e t i c a l l y , the true density values  were p r o d u c e d u n d e r s i m i l a r c o n d i t i o n s o f c o k i n g ( 5 ) . A l g o m a c o k e was  true density values t e m p e r a t u r e was (CPR-270), the  toward the  higher  in this  s o l e of  extremely  values test-oven  since time  they and  the coke-oven as  r e g i o n . For  a v e r a g e o f CPR-267, CPR-268 and  those  e x p e c t e d t o show i n c r e a s i n g  true density value  required to achieve  standard  d e v i a t i o n ) between measurements  e n c o u n t e r e d f o r t h e A l g o m a c o k e s . The  temperature  the  was  the wharf coke  c a l c u l a t e d as  CPR-269, s i n c e t h e  a representative value  the  an sample  w o u l d have t o  be  large.  4.2  MICROSTRUCTURE  Two  s o u r c e s o f e r r o r a r e p o s s i b l e when m a k i n g q u a n t i t a t i v e  m i c r o s t r u c t u r a l a n a l y s i s from p h o t o g r a p h s : 1.  Inaccuracies  associated with  representing  a  three  144  d i m e n s i o n a l system u s i n g a two d i m e n s i o n a l p h o t o g r a p h s , and 2.  The e f f e c t o f l a r g e p o r e s c r o s s i n g measurable  The  first  t h e boundaries of t h e  area.  problem  area of a n a l y s i s  i s easily  t a k e n c a r e o f by i n c r e a s i n g t h e  ( i e . increasing  However, t h e problem of t h e measurable  t h e number o f p h o t o g r a p h s ) .  of large pores crossing  area i s less e a s i l y  the boundaries  solved.  T a b l e XII shows t h e d i f f e r e n c e s b e t w e e n t h e mean v a l u e s of seven m i c r o s t r u c t u r a l parameters  w i t h a n d w i t h o u t "edge"  p o r e s . The r e s u l t s show a s i g n i f i c a n t d e c r e a s e  i n mean  v a l u e s when edge p o r e s a r e e l i m i n a t e d . D e c r e a s i n g t h e m a g n i f i c a t i o n of the photographs  could  reduce t h i s  effect  but would d e c r e a s e t h e a c c u r a c y of such measurements, t h e measurments a r e o n l y a s a c c u r a t e a s t h e s m a l l e s t p o i n t " on t h e T.V. m o n i t o r , a n d a r e , t h e r e f o r e , dependent. Since small pores a r e d i f f i c u l t m a g n i f i c a t i o n of each photograph addition, included  since "pixel  resolution  t o resolve, the  was s e t a t 12X. I n  i t was d e c i d e d t h a t e d g e p o r e s w o u l d  a l s o be  i n t h e a n a l y s i s because i t i s t h e l a r g e s t pore ( o r  pores) which  i s e x p e c t e d t o have t h e g r e a t e s t  i n f l u e n c e on  coke mechanical behaviour. Comparison  between p o r o s i t y v a l u e s measured u s i n g b o t h  p y c n o m e t r y a n d image a n a l y s i s a n a l y z e r c o n s i s t e n t l y produced t h o s e o b t a i n e d by p y c n o m e t r y .  i s made i n T a b l e XI. The image h i g h e r p o r o s i t y v a l u e s than T h i s c a n be a t t r i b u t e d t o d a r k  a r e a s p r e s e n t i n t h e p o r e w a l l s c a u s e d by  insufficient  145 polishing  ( i e . areas w i t h rough s u r f a c e ) .  Although and  those  the p o r o s i t y v a l u e s o b t a i n e d u s i n g pycnometry  o b t a i n e d by t h e  show e s s e n t i a l l y  3 . 3 . 1 ) . The  and  this  justified  i s shown by e q u a t i o n  values determined  true density varied u s i n g the  the assumption t h a t the  another.  This i s important,  d i r e c t e d more t o w a r d e f f e c t on  the 9 the  linearly  image a n a l y z e r . T h i s  image a n a l y z e r d e s c r i b e d  the m i c r o s t r u c t u r e s of the v a r i o u s coke batches one  do  porosity values calculated using  r e s u l t s o f b u l k d e n s i t y and with those  they  t h e same r e l a t i v e v a r i a t i o n b e t w e e n  d i f f e r e n t coke batches (section  image a n a l y z e r d i f f e r ,  relative  to  s i n c e most o f t h e e f f o r t  was  v a r i a t i o n b e t w e e n c o k e t e x t u r e and i t s  the compressive  s t r e n g t h t h a n on t h e  acquisition  of a b s o l u t e v a l u e s of m i c r o s t r u c t u r a l parameters.  4.3  EFFECT OF  COKING CONDITIONS ON  The  c o k e s a m p l e s u s e d i n t h i s s t u d y were a l l p r o d u c e d  t h e same c o a l b l e n d , b u t c o n d i t i o n s . The  MICROSTRUCTURE  under a v a r i e t y of  e f f e c t s of d i f f e r e n t  from  coking  c o k i n g c o n d i t i o n s on  the pore s t r u c t u r e of coke are d e s c r i b e d below.  4.3.1  C E L L WALL THICKNESS Table XIII  all  summarizes the measured c e l l  seven coke batches.  cokes produced w a l l s than  I t i s immediately  wall size  apparent  i n t h e 460mm t e s t - o v e n p o s s e s s  of  that  thinner  cell  those p r o d u c e d i n Algoma. T h i s i s c o n t r a r y t o  r e s u l t s o f t h e BCRA ( 3 1 ) , where t h e y c l a i m e d t o f i n d  the  no  the  146 s i g n i f i c a n t e f f e c t o f o v e n s i z e on t h e c e l l However, t h e i r  r e s u l t s showed c e l l  wall thickness.  w a l l s i z e s o f 0.125,  0.134, a n d 0.137mm f o r t h e 2 5 0 k g , 3 5 0 k g , a n d 17 t o n n e s o v e n s , r e s p e c t i v e l y . T h i s may i n d i c a t e t h a t t h e r e was a n oven-size effect The  in their  variation  i n the f i n a l  f u n c t i o n of height illustrated  i n F i g u r e 3 0 . As c a n be s e e n i n T a b l e  coking temperature wall  temperature  coking temperature  as a  i n a n A l g o m a c o k e - o v e n , h a s been  d i r e c t dependence of c e l l  that c e l l  results.  wall  s i z e on e s t i m a t e d  was o b s e r v e d .  Perch  X X I I I , no final  (5), also reported  s i z e was i n d e p e n d e n t o f f i n a l c a r b o n i z i n g  f o r c o a l s c a r b o n i z e d above t h e r e s o l i d i f i c a t i o n  temperature. Coke-oven p r e s s u r e and c o a l m o i s t u r e apparent  e f f e c t on c e l l  contrast,  wall  (31,50).  4.3.2 PORE S I Z E AND  GEOMETRY  batches  preheating  t o c h a r g i n g has r e s u l t e d  pore-wall thickness  showed no  s i z e of t h e cokes examined. I n  i t h a s been w e l l d o c u m e n t e d t h a t  (drying) coal prior  The  content  ina  finer  pore s i z e and geometry v a l u e s of t h e seven coke with their  summerized i n Table  respective coking conditions are XXIV. F o r t h e purpose of t h i s  d i s c u s s i o n , pore s i z e f i n e r mean c h o r d  i s represented as the 80% cummulative  l e n g t h , and geometry i s r e p r e s e n t e d as t h e  r a t i o o f t h e maximum t o minimum F e r e t ' s  diameters.  Coke  Oven  Est imated  Charge  Cell  Sample  Pressure  Coking  Moisture  Wall  Content  Size  Temperature CPR-  kPa  <°C)  264  3.2  1250  5.9  0.131  265  5.9  1250  4.1  0.122  266  17.8  1250  1.2  0.131  267  10.2  1260  N/A  0.154  268  4.9  1200  N/A  0.152  269  0.54  940  N/A  0.168  N/A  0.169  270  Table XXIII.  —  —  (  % )  C e l l Wall S i z e Values as a F u n c t i o n of Coking Condit ions (Oven pressure e s t i m a t e d f o r Algoma  Coke).  (mm)  Coke Batch CPR-  Oven  Estimated  Coal  Mean  Fmax  Number  Chord  Fmin  of  Pressure  Coking  Charge  (kPa)  Temperature  Moisture  Length  Pores per  (°C)  ( % )  (mm)  Field  264  3.2  1250  5.9  0.229  1 .771  121  265  5.9  1250  4.1  0.222  1.714  140  266  17.8  1250  1.2  0.224  1 .755  126  267  10.2  1260  N/A  0.231  1 .749  115  268  4.9  1200  N/A  0.243  1 .754  101  269  0.54  940  N/A  0.255  1 .766  89  N/A  0.251  1 .766  97  270  —  T a b l e XXIV.  —  P o r e S i z e and Geometry Conditions.  V a l u e s as a F u n c t i o n of Coking  149 F i g u r e s 31a and b i l l u s t r a t e the r e l a t i o n s h i p between the pore s i z e as a f u n c t i o n of the estimated coking temperature  and oven p r e s s u r e , r e s p e c t i v e l y . I t appears  pore s i z e decreases with i n c r e a s i n g temperature  that  and oven  pressure. The  e x i s t a n c e of oven w a l l p r e s s u r e d u r i n g the  p r o d u c t i o n of the test-oven cokes seems to be  positively  c o r r e l a t e d t o the c o a l - c h a r g e bulk d e n s i t y (see t a b l e I V ) . T h i s agrees with the obsevations of other workers (5,31) have observed  a s i m i l a r decrease  i n the pore s i z e  from an i n c r e a s e i n the c o a l charge The  resulting  bulk d e n s i t y .  value of (Fmax/Fmin) d i d not show much v a r i a t i o n  between coke batches with values ranging between 1.7 1.766. T h i s compares with values between 1.7 r e p o r t e d by P a t r i c k  and  and  1.9  (31-36) and other BCRA workers. T h e i r  s t u d i e s showed that the value of the shape f a c t o r was to  who  found  be dependent on the number of o r i e n t a t i o n s performed  on  each measurement. The BCRA (34) s t u d i e s showed that the e r r o r f o r measuring maximum F e r e t ' s diameter 8.3%  f o r four o r i e n t a t i o n s , and  0.5%  ranged  between  for sixteen  o r i e n t a t i o n s . In t h i s study, twelve measurements at  15°  i n t e r v a l s were made to o b t a i n the maximum and minimum F e r e t ' s diameters  s i n c e t h i s was  the maximum number of  o r i e n t a t i o n s that c o u l d be performed PLUS image a n a l y z e r . A s t a t i s t i c a l  u s i n g the L e i t z  TAS  a n a l y s i s performed  by  BCRA (34) showed that the probable e r r o r of estimate f o r the d e t e r m i n a t i o n of maximum F e r e t ' s diameter  using  twelve  150  E 0-25 E  I?024 o>  TJ k. O O  o-23  o  Test-Oven  A  Algoma  Coke A  Coke  O  c o  CD  o o  0-22.  900  K>00  1100  1200  Estimated Coking Temperature  F i g u r e 31a.  1400  ( °C)  Pore S i z e as a F u n c t i o n of E s t i m a t e d Coking Temperature.  151  -r—r  TT  0-25  E E  A  Test-oven  O  Algoma  Coke  Coke  0-24 a>  _) o JC  O c D  IE  023  A  0-22  0-5  I  Estimated  Figure  31b.  J_l_ J_J_L 2 5 10 Oven Pressure (kPa)  Pore S i z e as a F u n c t i o n  20  o f E s t i m a t e d Oven  Pressure. y  152 o r i e n t a t i o n s was approximately 1%. The average number of pores per f i e l d of view i s a l s o listed  i n Table XXIV f o r each of the seven coke batches  s t u d i e d . In g e n e r a l , cokes with s m a l l e r pores had a l a r g e r t o t a l number of pores than cokes c o n t a i n i n g l a r g e r pores. Thus, these v a l u e s appear to be r e l a t e d to c o k i n g c o n d i t i o n s where the number of pores i s found to i n c r e a s e with coking temperature and oven p r e s s u r e .  4.3.3  SUMMARY OF MICROSTRUCTURAL DEPENDENCE ON COKING CONDITIONS . It can be concluded from the above,  t h i c k n e s s i s not g r e a t l y dependent  wall  on the c o n d i t i o n s of  c o k i n g . However, c e l l w a l l t h i c k n e s s was for  t h a t the c e l l  found to be g r e a t e r  cokes produced i n the Algoma coke-oven  than those of the  t e s t - o v e n cokes. In  c o n t r a s t , the pore s i z e was  lower f i n a l pressure  shown to i n c r e a s e at a  c a r b o n i z a t i o n temperature and lower  (or lower c o a l charge bulk d e n s i t y ) .  oven  The  temperature of c o k i n g and the e x i s t i n g oven p r e s s u r e are lower near the c o a l l i n e of" a commercial coke-oven  which  r e s u l t s i n coke products e x h i b i t i n g l a r g e r p o r e s .  4.4  COKE STRENGTH  The  importance of coke s t r e n g t h on the e f f i c i e n c y of a b l a s t  furnace has a l r e a d y been o u t l i n e d . One o b j e c t i v e s of t h i s p r o j e c t  of the main  i s t o determine the f a c t o r s  which  153 i n f l u e n c e t h e coke s t r e n g t h and t o r e l a t e t h e s e t o c u r r e n t coke p r o d u c t i o n p r a c t i c e s .  4.4.1 EFFECT OF  TEMPERATURE  T a b l e XVI i n c l u d e s t h e mean c o m p r e s s i v e s t r e n g t h f o r t h e seven coke b a t c h e s both a t ambient at  1400°C. The c o k e  strength increased  values  t e m p e r a t u r e and  i n every case  except  w i t h t h e c o k e b a t c h CPR-269 when t e s t e d a t 1400°C. The c a u s e of  t h i s discrepancy  (CPR-269) c a n n o t be e x p l a i n e d  be due t o a n o n - r e p r e s e n t a t i v e  sample used  b u t may  in this  investigation. A similar  effect  of i n c r e a s i n g strength a t high  t e m p e r a t u r e h a s a l s o b e e n o b s e r v e d by p r e v i o u s s t u d y i n g coke  (19) and g r a p h i t e  workers  ( 4 0 ) . The i n c r e a s e  i n coke  s t r e n g t h when h e a t e d a b o v e t h e c a r b o n i z a t i o n t e m p e r a t u r e i s explained  by i t s c o n t i n u e d  g r a p h i t i z a t i o n above t h e c o k i n g  temperatures. In g r a p h i t e , t h i s s t r e n g t h  increase at higher  t e m p e r a t u r e s h a s been a t t r i b u t e d t o r e l i e v i n g stresses  formed  subsequent The  during  the i n i t i a l  high temperature  probable that the increase non-existent  atmosphere,  g r a p h i t i z a t i o n - and  cooling (40).  p r o j e c t were o n l y c a r r i e d  even  the residual  strength t e s t s performed  in this  o u t i n an i n e r t g a s ( A r ) . I t i s i n s t r e n g t h would  be a f f e c t e d o r  i f t h e s e t e s t s were p e r f o r m e d  i n a C0  2  s i n c e t h e p o r e w a l l s w o u l d be s u b j e c t e d t o  a t t a c k by t h e B o u d o u a r d r e a c t i o n . No s u c h e x p e r i m e n t s h a v e y e t been r e p o r t e d  i n the l i t e r a t u r e .  154 4.4.2 EFFECT OF MICROSTRUCTURE As w i t h most porous to  s o l i d s , the s t r e n g t h has been shown  be reduced with i n c r e a s i n g p o r o s i t y  ( F i g u r e 20) and  c o n v e r s e l y the s t r e n g t h i n c r e a s e d l i n e a r l y with bulk d e n s i t y ( F i g u r e 19a). These trends have been e x t e n s i v e l y s t u d i e d i n the past, p a r t i c u l a r l y these parameters  i n the f i e l d  of ceramics. Though  ( p o r o s i t y and bulk d e n s i t y ) g e n e r a l l y have  p r o v i d e d a q u a l i t a t i v e c o r r e l a t i o n with coke s t r e n g t h , i t i s important  t o explore the cause of these observed  in coke s t r e n g t h . I t has been demonstrated  variations  i n s e c t i o n 4.1.1  that the coke batches CPR-264, CPR-265, CPR-268, and CPR-270 have s i m i l a r bulk d e n s i t i e s comparisons)  (using  one-degree-of-freedom  but they vary i n compressive  ambient and h i g h temperatures.  s t r e n g t h both at  These d i f f e r e n c e s may be  caused by v a r i a t i o n s i n pore s i z e d i s t r i b u t i o n s and pore geometry. F i g u r e s 22(a-g) show that a r i s e  i n compressive  s t r e n g t h c o r r e l a t e s with d e c r e a s i n g c e l l  wall size,  pore  s i z e , and with i n c r e a s i n g number of pores. I t i s s u r p r i s i n g that s t r e n g t h r i s e s with d e c r e a s i n g c e l l w a l l s i z e and i n c r e a s i n g number of pores, but t h i s e f f e c t can be e x p l a i n e d by c o n s i d e r i n g the pore s i z e t o be the dominant  factor  a f f e c t i n g the s t r e n g t h . For example, i f two cokes have the same p o r o s i t y , the coke with l a r g e r pores w i l l c o n t a i n a s m a l l e r number of pores which a r e , on average, more widely spaced than a coke with s m a l l e r pores. F i g u r e 32 i l l u s t r a t e s how the c e l l  w a l l s i z e and the number of pores per f i e l d are  155  Figure  32.  C e l l Wall S i z e and Number of Pores per as a F u n c t i o n of Pore  Size.  Field  1 56 j u s t a consequence of the pore s i z e . T h i s o b s e r v a t i o n d i s a g r e e s with the i m p l i c a t i o n s of P a t r i c k ' s equation (equation 4, s e c t i o n and  the number of pores per f i e l d  another pore  1.5.3) that pore s i z e , c e l l  (37,42).  wall size  are independent  I t can be concluded,  of  one  t h e r e f o r e , that the  s i z e has perhaps the g r e a t e s t e f f e c t on coke s t r e n g t h  compared with a l l other pore parameters. arguement that the c r i t i c a l be r e l a t e d to pore  T h i s supports  flaw s i z e governing  failure  may  size.  P a t r i c k et a l (35) a l s o proposed  an equation which  suggests t h a t there i s a s t r e n g t h dependence on the shape f a c t o r  (equation 5 ) . T h i s equation was  present data  ( F i g u r e 23) and,  c o r r e l a t i o n was  the  as can be seen,  poor. T h i s may  be due  pore  t e s t e d with the the  to the d i f f i c u l t y  of  s t a t i s t i c a l l y d e s c r i b i n g the shape of the pores. Furthermore,  no i n t e r a c t i o n of s t r e s s around pores  was  c o n s i d e r e d i n the d e r i v a t i o n equation 5. For t h i s reason, i t was  d e c i d e d to apply an equation developed  by Neid and  (42) which i s a m u l t i p l e flaw model that can p r e d i c t critical  stress intensity  factor  (K ) J C  f°  r  coke  IC  the  porous m a t e r i a l s .  Using p o r o s i t y v a l u e s and maximum F e r e t ' s diameter flaw s i z e , the v a l u e s of R  Arin  as the  were c a l c u l a t e d f o r a l l seven  batches. Judging by the compressive  s t r e n g t h v a l u e s of coke,  q u a l i t y of coke i n the Algoma coke-oven i s b e t t e r f o r samples e x t r a c t e d from the s o l e r e g i o n of the oven than those o b t a i n e d at h i g h e r p o s i t i o n s . T h i s may  be due  to both  157 the e f f e c t  of a l e s s porous p r o d u c t , and s m a l l e r pores i n  cokes produced at the s o l e l e v e l . P o r o s i t y and pore  size  appear to be a f f e c t e d by the f o l l o w i n g f a c t o r s : s t a t i c  load  (oven p r e s s u r e ) , f i n a l coking temperature and oven s i z e . To produce a stronger coke, i t i s best to i n c r e a s e the s t a t i c l o a d on the charge and i n c r e a s e the f i n a l  coking  temperature.  4.5 CREEP OF COKE A p r e l i m i n a r y study on the p l a s t i c  flow behaviour of coke  above 1000°C has been c a r r i e d out. From the knowledge of the f a c t that ash i s made up of low m e l t i n g p o i n t which are l i k e l y  constituents  to melt at temperatures encountered at the  tuyere region of the b l a s t  furnace, i t i s expected that coke  should deform p l a s t i c a l l y , as do r e f r a c t o r i e s . Thus, the formation of a g l a s s y phase should decrease the l o a d bearing c a p a c i t y of coke, which may The complex  result  in creep.  s t r u c t u r e of coke p r o h i b i t s any  i n t e r p r e t a t i o n of the creep data i n any fundamental way.  For  t h i s reason, a v i s o - e l a s t i c model was used to d e s c r i b e the creep behaviour of coke above 1000°C. I t was  found that an  i n t e r a c t i v e double-Kelvin-element model ( F i g u r e  28b)  d e s c r i b e d the behaviour most a c c u r a t e l y . F i g u r e s 29(a-b)  i l l u s t r a t e s the e f f e c t  of temperature  and a p p l i e d p r e s s u r e on the mechanical parameters T j 772,  M  2 of the two  1 r  M, ,  i n t e r a c t i v e K e l v i n element model. Only the  parameter, TJ,, v a r i e d with temperature and p r e s s u r e and i t  158  had very h i g h v a l u e s when compared with the other  three  parameters: M, , T J and M . I f i t i s c o n s i d e r e d that the 2  2  r e s i s t a n c e t o deformation  can be a t t r i b u t e d  values of the v i s c o u s parameter, r j  1 f  t o the very high  (as the other  parameters have low values) the temperature e f f e c t parameter should be the c o n t r o l l i n g v i s c o u s deformation  factor  d e s c r i b i n g the  of coke a t h i g h temperatures  Thus, Tj 1, may represent the e f f e c t i v e v i s c o s i t y g l a s s y phase (ash) c o n t r o l l i n g  on t h i s  (>1000°C) of the  the flow behaviour  of the  system. The parameters, M, , M , and T J , may somehow be 2  related response  with the carbon  2  s t r u c t u r e and/or the machine  which e x p l a i n s t h e i r  r e l a t i v e i n s e n s i t i v i t y to  temperature and a p p l i e d p r e s s u r e . As expected,  the v i s c o s i t y  77 , i s lowered 1  when the  temperature r i s e s . T h i s i s t y p i c a l of most g l a s s y m a t e r i a l s and  i t should r e s u l t  i n a decrease  i n the l o a d bearing  c a p a b i l i t i e s of coke a t higher temperatures. d e f i n i t e dependence of the v i s c o s i t y , pressure which i s shown to decrease  rj  1 r  There i s a l s o a  on the a p p l i e d  with a r i s e i n the  applied stress. This i s characteristic  of a non-Newtonian  fluid. In summary, a time dependent s t r a i n has been at temperatures  g r e a t e r than  observed  1000°C. T h i s behaviour  has been  d e s c r i b e d u s i n g an i n t e r a c t i v e - d o u b l e - K e l v i n element visco-elastic  model.  I t should be noted that some of the ash c o n s t i t u e n t s , such as N a , 0 , K , 0 , and SiO,, should a l s o v a p o r i z e i n the  159 highly (6).  reducing atmosphere e x i s t i n g  furnace  T h i s may somehow c o u n t e r a c t the weakening of coke due  t o m e l t i n g of a s h . In a d d i t i o n , by  i n the b l a s t  the Boudouard r e a c t i o n  the l o s s of carbon from coke  should be taken i n t o  when determining the s t r e n g t h p r o p e r t i e s would r e l a t e more c l o s e l y t o b l a s t  consideration  of coke, as that  furnace  conditions.  5. CONCLUSION In t h i s p r o j e c t , seven coke batches s p e c i a l l y prepared by CANMET were s t u d i e d . These samples o r i g i n a t e d  from three  sources: 1.  Three coke batches were prepared i n a 460mm t e s t - o v e n , each with d i f f e r e n t coke-oven  2.  pressures.  Three coke batches were sampled  from three  different  p o s i t i o n s , with r e s p e c t t o h e i g h t , i n a 5m coke-oven at Algoma S t e e l 3.  Corporation.  A warf coke sample was prepared t o be compared with the other s i x batches.  The experimental program  was c a r r i e d out i n four  major  steps: 1.  Bulk and t r u e d e n s i t y d e t e r m i n a t i o n s ,  2.  Quantitative microstructural  3.  Compressive  analyses,  s t r e n g t h value d e t e r m i n a t i o n s f o r a l l seven  coke batches at ambient  temperature and at 1400°C  (these  v a l u e s were then r e l a t e d to coking c o n d i t i o n s and to the m i c r o s t r u c t u r a l r e s u l t s ) , and 4.  Creep t e s t experiments above The  1.  1000°C.  f o l l o w i n g c o n c l u s i o n s can be made from t h i s study:  The bulk d e n s i t y of the Algoma cokes was h i g h e r f o r cokes e x t r a c t e d from the bottom of the coke-oven than those sampled c l o s e t o the top. T h i s i n c r e a s e i n d e n s i t y can be a t t r i b u t e d to an i n c r e a s e i n s t a t i c  load, due to  the weight of the burden, which was g r e a t e s t at the oven s o l e . T h i s e f f e c t may a l s o have been a i d e d by the higher  160  161 final  c o k i n g temperatures near the bottom of the  coke-oven,  but there i s i n s u f f i c i e n t evidence to make a  d e f i n i t e comment. Coke pore and w a l l s i z e were a l s o found to be a f f e c t e d by c o k i n g c o n d i t i o n s . The pore and w a l l s i z e were s m a l l e r i n t e s t - o v e n cokes than those i n Algoma cokes. T h i s i m p l i e s that there i s an oven s i z e a f f e c t on the m i c r o s t r u c t u r e of coke. Furthermore, an i n c r e a s e i n pore s i z e was  observed with height i n the Algoma  coke-oven.  No apparent trend c o u l d be e s t a b l i s h e d between c e l l  wall  s i z e and h e i g h t . The coke compressive s t r e n g t h was for  the specimens  found t o be g r e a t e r  o b t a i n e d from r e g i o n s c l o s e to the  s o l e of the 5m Algoma oven. Cokes produced a t the s o l e r e g i o n e x h i b i t e d lower p o r o s i t y v a l u e s and smaller pores than the cokes s i t u a t e d h i g h e r i n the oven. T h i s resulted the  i n a s t r o n g e r coke f o r the samples produced at  s o l e of the coke-oven. Coke batches produced i n the  t e s t - o v e n s were always s t r o n g e r than the Algoma cokes and were a l s o more uniform i n p r o p e r t i e s W e i b u l l modulus v a l u e s ) at ambient s t r e n g t h was  ( i e . higher  temperature. Coke  shown to be higher at 1400°C than at room  temperature f o r s i x of the seven coke batches. T h i s  was  thought t o be due to c o n t i n u e d g r a p h i t i z a t i o n a f t e r the coke exceeded The c r i t i c a l  i t s final  c o k i n g temperature.  stress intensity  factor values ( K ) I C  of the  t e s t - o v e n coke batches were shown to i n c r e a s e w i t h oven  162 w a l l p r e s s u r e , but t h i s t r e n d was not so apparent f o r v a l u e s of K j ^ o b t a i n e d from the s t r e n g t h data of the Algoma coke batches. I t was concluded that s i n c e K  J C  is  m a t e r i a l p r o p e r t y , the d i f f e r e n c e s i n these v a l u e s may be due t o v a r i a t i o n s  i n the mechanical p r o p e r t i e s of the  s o l i d coke ( i e . pore w a l l s ) and to l a r g e hidden  fissures  that may have been present i n s i d e some of the samples. However, these d i f f e r e n c e s were s l i g h t and, t h e r e f o r e , it  i s probable the coke mechanical behaviour  governed  by the porous  i s largely  structure.  The coke e x h i b i t e d p l a s t i c  flow behaviour when s u b j e c t  to a constant load at temperatures  g r e a t e r than  1000°C.  The c r e e p data were i n t e r p r e t e d by u s i n g an i n t e r a c t i v e - d o u b l e - K e l v i n element The value of the dashpot  visco-elastic  viscosity, 7 j  1 f  model.  was decreased  with i n c r e a s i n g temperature. The v i s c o s i t y of t h i s dashpot  i s thought  to be r e l a t e d with the presence of a  g l a s s y phase (ash) i n coke which may govern behavior.  i t s creep  6. REFERENCES  1.  "Western World CokemakingtCapacity and Operating P r a c t i c e s " ; Commission on Raw M a t e r i a l s and Technology B r u s s e l s , 1 9 8 3 ; I n t e r n a t i o n a l Iron and S t e e l I n s t i t u t e , 1 983  2.  T e r k e l R o s e n q u i s t ; " P r i n c i p l e s of E x t r a c t i v e M e t a l l u r g y " .'Copyright 1984 by McGraw-Hill Inc. 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Burgess, Personal Communication on the temperature h i s t o r y of the c o a l charge d u r i n g e x p e r i m e n t a l measurements of a coke oven at three d i f f e r e n t h e i g h t s at Algoma S t e e l Corp. 50., J.W. P a t r i c k , " S t u d i e s of the F a c t o r s C o n t r o l l i n g the Formation and Development of the Porous S t r u c t u r e of Coke",BCRA,Information Symposium on Coke Oven Techniques,1981;Luxembourg,Belgium  168 51. D.R. Coughanowr a n d L.R. K o p p e l . P r o c e s s S y s t e m s A n a l y s i s and C o n t r o l . M c G r a w - H i l l Book Company, 1965. 52. D. M e r r i c k , " M a t h e m a t i c a l M o d e l s o f The T h e r m a l D e c o m p o s i t i o n o f C o a l , P a r t 3", F u e l , 1983, V o l 62, May, pp547-552.  

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