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High-pressure coal injection in the zinc slag fuming process Cockcroft, Steven Lee 1986

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HIGH-PRESSURE COAL INJECTION IN THE ZINC SLAG FUMING PROCESS by STEVEN LEE COCKCROFT B . S c , U n i v e r s i t y of B r i t i s h Columbia, 1980 B.A.Sc, U n i v e r s i t y of B r i t i s h Columbia, 1984 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of M e t a l l u r g i c a l E n g i n e e r i n g ) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA December 1986 (§) Steven Lee C o c k c r o f t , 1986 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of M Cf^LLUttGtI CfcL IAJ£S£/KT6I The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6C3/81) i i ABSTRACT Zinc slag fuming i s a k i n e t i c a l l y controlled process based on the d i r e c t reduction of slag by entrained coal. The kinetics of the process are governed primarily by two factors: (1) the f r a c t i o n of coal entrained in the slag, and (2) the rate of ferrous iron oxidation. A series of high-pressure coal i n j e c t i o n t r i a l s have been completed at Cominco's lead smelter in T r a i l , B r i t i s h Columbia in order to f a c i l i t a t e increased coal entrainment. Fuming rates were increased s u b s t a n t i a l l y and o v e r - a l l e f f i c i e n c i e s were improved. These results are in d i r e c t contradiction to the predictions of models based on equilibrium. The k i n e t i c s based mathematical model of the zinc slag - fuming process o r i g i n a l l y developed by Richards and Brimacombe*3 has been modified to include the reduction and removal of lead from the furnace. A lead 11 p r i l l " - slag reaction model has been formulated to account for the behaviour of l i q u i d metallic lead. Analyses of the t r i a l data with the modified model indicates that s i g n i f i c a n t improvements in entrainment were achieved with high-pressure coal i n j e c t i o n . i i i TABLE OF CONTENTS ABSTRACT i i TABLE OF CONTENTS i i i LIST OF TABLES v i LIST OF FIGURES v i i TABLE OF SYMBOLS xi ACKNOWLEDGEMENTS xiv CHAPTER 1 INTRODUCTION: SLAGS AND SLAG PROCESSING 1 1.1 Slag Processing 3 1.1.1 Slag Cleaning 4 1.2 The Zinc Slag Fuming Process 6 1.2.1 History . 11 CHAPTER II LITERATURE REVIEW 15 2.1 Pulverized Coal Transport 15 2.1.1 Pneumatic Conveying of Materials 16 2.1.1.1 Material-Into-Air Systems 17 2.1.1.2 Air-Mixing Systems 17 2.1.1.3 Air-Into-Material System 21 2.1.1.4 Selection of System 21 2.1.2 Non-Pneumatic Transport 21 2.1.3 Summary: Pneumatic Conveying 22 2.2 The Zinc Slag Fuming Process 22 2.2.1 Studies and Modelling 23 2.2.1.1 Equilibrium Models 23 2.2.1.2 Empirical Models 26 2.1.1.3 Industrial and Laboratory Studies 27 2.1.1.4 The Kinetic Model of the Process 29 v. i v CHAPTER I I I OBJECTIVES AND SCOPE OF THE RESEARCH 41 CHAPTER IV EXPERIMENTAL EQUIPMENT AND TECHNIQUES 43 4.1 I n d u s r i a l Equipment 43 4.1.1 High-Pressure Coal D e l i v e r y / I n j e c t i o n System 43 4.1.1.1 Design C r i t e r i a 43 4.1.1.2 Summary of Design C r i t e r i a 49 4.1.1.3 System S e l e c t i o n 50 4.1.1.4 The High-Pressure D e l i v e r y System 52 4.2 Experimental Techniques 57 4.2.1 I n d u s t r i a l Tests 57 4.2.1.1 S l a g Sampling 58 4.2.1.2 Operating Procedures 59 4.2.1.3 Summary: I n d u s t r i a l T e s t i n g Procedure . . . 61 4.2.2 Chemical A n a l y s i s of Sla g Samples 62 4.2.3 U n c e r t a i n t y 62 CHAPTER V EXPERIMENTAL RESULTS AND PRELIMINARY ANALYSIS 64 5.1 R e s u l t s 64 5.2 P r e l i m i n a r y A n a l y s i s and D i s c u s s i o n 64 5.2.1 High-Pressure I n j e c t i o n Dynamics 65 5.2.2 Normal Fuming P r a c t i c e at Cominco 68 5.2.3 High-Pressure Coal I n j e c t i o n P r e l i m i n a r y A n a l y s i s 70 5.3 Summary 91 CHAPTER VI MATHEMATICAL MODEL OF ZINC FUMING PROCESS AND DISCUSSION OF MODEL FITTING 93 6.1 M o d i f i c t i o n s to the Richards Model 95 6.1.1 The K i n e t i c C o n c e p t u a l i z a t i o n of the Process 95 6.1.1.1 The Coal P a r t i c l e - S l a g Reaction Model . . . . 95 6.1.1.1.1 Zinc Balance 103 6.1.1.1.2 Lead Balance 104 6.1.1.1.3 Carbon Balance 106 6.1.1.1.4 Oxygen Balance 107 6.1.1.1.5 Hydrogen Balance 108 6.1.1.1.5.1 E q u i l i b r i u m of the Water-Gas Reaction 108 6.1.1.1.6 N 2 Balance . 109 V 6.1.1.1.7 Bubble Radius 110 6.1.1.1.8 Char P a r t i c l e Radius 110 6.1.1.1.9 Char P a r t i c l e Weight I l l 6.1.1.1.10 Gas Volume I l l 6.1.1.1.11 I n i t i a l C o n d i t i o n s 112 6.1.1.1.12 Thermodynamic Data . 113 6.1.1.1.13 Mass T r a n s f e r 113 6.1.1.1.13.1 Mass T r a n s f e r of Pe2"*- and F e 3 * 116 6.1.1.1.13.2 Mass T r a n s f e r C o e f f i c i e n t s 120 6.1.1.1.14 Boudouard and Char-Steam Reactions . . . 121 6.1.1.1.15 Model S o l u t i o n 122 6.1.1.2 The K i n e t i c s of Lead Removal 122 6.1.1.2.1 The Lead P r i l l - S l a g R e action Model . . . 123 6.1.1.2.1.1 Mass Balance on L i q u i d Lead 124 6.1.1.2.1.2 Mass T r a n s f e r 124 6.1.1.2.1.3 Mass T r a n s f e r C o e f f i c i e n t s . 127 6.1.2 The Furnace Model 127 6.2 D i s c u s s i o n of Model F i t t i n g 128 6.2.1 R e s u l t s of F i t to Normal Operation 129 6.2.1.1 D i s c u s s i o n of F i t to Normal Oper a t i o n . . . . 134 6.2.2 R e s u l t s of F i t to High-Pressure Operation . . 136 6.2.2.1 D i s c u s s i o n of F i t to High-Pressure Operation 136 6.2.2.2 F i t to High-Pressure Operation: Entrainment F a c t o r s 151 CHAPTER VII SENSITIVITY ANALYSIS 153 7.1 S e n s i t i v i t y A n a l y s i s : R e s u l t s 153 7.1.1 R e s u l t s and D i s c u s s i o n 155 7.1.1.1 The Inf l u e n c e of Lead on K i n e t i c s 184 7.1.1.1.1 The K i n e t i c s of the Char P a r t i c l e - S l a g Model 184 7.1.1.1.2 The K i n e t i c s of Lead P r i l l O x i d a t i o n . . 186 7.2 S e n s i t i v i t y A n a l y s i s : D i s c u s s i o n 186 7.3 S e n s i t i v i t y A n a l y s i s : Summary 189 CHAPTER VIII SUMMARY AND CONCLUSIONS 190 8.1 Summary . 190 8.2 Sugguestions f o r Furth e r Work 191 REFERENCES 19 2 APPENDIX I FUMING SAMPLING DATA 196 LIST OF TABLES Table 1. 1 C l a s s i f i c a t i o n of Pneumatic Conveyors . 18 Table 4. 1 Estimated Uncertainty of Analysis . . . 63 Table 5. 1 High-Pressure Injection Dynamics . . . 66 Table 5. 2 Fuming Rates and E f f i c i e n c i e s 77 Table 6. 1 Thermodynamic Data for Reactions . . . . 114 Table 6. 2 Model F i t t i n g Parameters . 130 Table 7. 1 Standard Conditions for S e n s i t i v i t y v i i LIST OF FIGURES Figure 1.1 Schematic diagram showing the furnace in cross-section and the chemical reactions occurring in the process . . . 7 Figure 4.1 Schematic diagram showing the pneumatic conveyor, high-pressure injector and the standard low-pressure tuyere . . . . 48 Figure 4.2 Schematic diagram showing the pneumatic conveyor in d e t a i l 55 Figure 5.1 Zn and Pb p r o f i l e s for Run 1 72 Figure 5.2 Fe2"*- and Fe3 + p r o f i l e s for Run 1 . . . . 73 Figure 5.3 Temperature p r o f i l e for Run 1 74 Figure 5.4 Furnace operating conditions for Run 1 75 Figure 5.5 Zn and Pb p r o f i l e s for Run 2 80 Figure 5.6 Fe 2* and Fe3* p r o f i l e s for Run 2 . . . . 81 Figure 5.7 Temperature p r o f i l e for Run 2 82 Figure 5.8 Furnace operating conditions for Run 2 83 Figure 5.9 Zn an Pb p r o f i l e s for Run 3 87 Figure 5.10 Fe 3 + and F e 3 + p r o f i l e s for Run 3 . . . . 88 Figure 5.11 Temperature p r o f i l e for Run 3 89 Figure 5.12 Furnace operating conditions for Run 3 90 Figure 6.1 The char p a r t i c l e - s l a g reaction system 99 Figure 6.2 Photomicrograph of quenched and polished slag sample 100 Figure 6.3 The lead p r i l l - s l a g reaction system . . . 125 Figure 6.4 Industrial data and model f i t to the Zn and Pb p r o f i l e s for normal operation 131 v i i i F i g u r e 6.5 F i g u r e 6.6 F i g u r e 6.7 F i g u r e 6.8 F i g u r e 6.9 Fi g u r e 6.10 F i g u r e 6.11 F i g u r e 6.12 F i g u r e 6.13 Fi g u r e 6.14 F i g u r e 6.15 F i g u r e 6.16 F i g u r e 6.17 F i g u r e 6.18 F i g u r e 7.1 F i g u r e 7.2 F i g u r e 7.3 I n d u s t r i a l data and model f i t to the Fe2"*" and F e 3 * p r o f i l e s f o r normal o p e r a t i o n 132 I n d u s t r i a l data and model f i t to the temperature p r o f i l e f o r normal o p e r a t i o n 133 I n d u s t r i a l data and model f i t to the Zn and Pb p r o f i l e s f o r Run 1 137 I n d u s t r i a l data and model f i t to the F e 2 + and F e 3 + p r o f i l e s f o r Run 1 . . . . 138 I n d u s t r i a l data and model f i t to the temperature p r o f i l e f o r Run 1 . . . . . . 139 I n d u s t r i a l data and model f i t to the Zn and Pb p r o f i l e s f o r Run 2 . . . . . . 140 I n d u s t r i a l data and model f i t to the F e 2 * and F e 3 * p r o f i l e s f o r Run 2 . . . . 141 I n d u s t r i a l data and model f i t to the temperature p r o f i l e f o r Run 2 142 I n d u s t r i a l data and model f i t to the Zn and Pb p r o f i l e s f o r Run 3 143 I n d u s t r i a l data and f i t to the Fe2''" and F e 3 + p r o f i l e s f o r Run. 3 144 I n d u s t r i a l data and f i t t o the temperature p r o f i l e f o r Run 3 145 P r e d i c t e d oxygen u t i l i z a t i o n as a f u n c t i o n of bath temperature 148 F L P O C as a f u n c t i o n of bath temperature 149 F e > , y as a f u n c t i o n of bath temperature 150 The e f f e c t of Fs>sr. on the p r e d i c t e d Zn p r o f i l e 156 The e f f e c t of Fs»»t. on the p r e d i c t e d Pb p r o f i l e . 157 The e f f e c t of Fs»»r. on the p r e d i c t e d F e 2 * p r o f i l e 158 Figure 7.4 Figure 7.5 Figure 7.6 Figure 7.7 Figure 7.8 Figure 7.9 Figure 7.10 Figure 7.11 Figure 7.12 Figure 7.13 Figure 7.14 Figure 7.15 Figure 7.16 Figure 7.17 Figure 7.18 Figure 7.19 Pb The e f f e c t of r on the predicted Zn p r o f i l e . . ? 160 Pb The e f f e c t of r on the predicted Pb p r o f i l e . . ? 161 Pb The ef f e c t of r on the predicted F e 2 + p r o f i l e . ? 162 The e f f e c t of Dz„o/Ds>too on the predicted Zn p r o f i l e 163 The e f f e c t of D Z no/Dp bo on the predicted Pb p r o f i l e 165 The e f f e c t of Ozno /Dpto on the predicted F e a + p r o f i l e 166 The e f f e c t of D Zno/Dpbo on the predicted temperature p r o f i l e 167 The e f f e c t of V . on the slag predicted Zn p r o f i l e 168 The e f f e c t of V . on the slag predicted Fe 2 + p r o f i l e , 170 The e f f e c t of V . on the slag predicted Pb p r o f i l e 171 The eff e c t of V . on the slag predicted temperature p r o f i l e 172 The eff e c t of F O X 3 r on the predicted Zn p r o f i l e 173 The e f f e c t of Foxy on the predicted Pb p r o f i l e 175 The eff e c t of F O X y on the predicted Fe 2* p r o f i l e 176 The e f f e c t of Foxy- on the predicted temperature p r o f i l e 177 The e f f e c t of F e 2 0 3 on the predicted Zn p r o f i l e 178 X F i g u r e 7.20 The e f f e c t of F e a 0 3 on the p r e d i c t e d F e 2 * p r o f i l e 179 F i g u r e 7.21 The e f f e c t of D F e 0 / D F e 2 o 3 o n t h e p r e d i c t e d Zn p r o f i l e . . . . . 180 F i g u r e 7.22 The e f f e c t of D F e 0 ' D F e 2 0 3 o n t h e p r e d i c t e d Pb p r o f i l e 181 F i g u r e 7.23 The e f f e c t of D P e 0 ' ' D P e 2 0 3 o n t n e p r e d i c t e d F e 2 * p r o f i l e 182 F i g u r e 7.24 The e f f e c t of D F e 0 ^ D P e 2 0 3 o n t h e p r e d i c t e d temperature p r o f i l e 183 XI b ,s B LIST OF SYMBOLS Surface area of secondary bubble Boudouard Reaction Rate pre-exponential constant Char-Steam Reaction Rate pre-exponential contant Boudouard Reaction Rate m k Pa 1 S 1 k Pa 1 3 1 k Pa 1 S 1 A c t i v i t y of Species j Concentration of Species j in secondary bubble k Pa 1 S 1 kg mole m ,sl 'j ' , i D Concentration of species j in slag kg mole m Concentration of species j at bubble slag interface D i f f u s i v i t y of species j -3 kg mole m m2 S"1 -3 1-5 ,B Constants Activation energy Boudouard Reaction kJ kg mole -1 E Activation energy Char-Steam kJ kg mole -1 LPCE Fraction of low-pressure coal entrained HPCE Fraction of high-pressure coal entrained F LPCC Fraction of low-pressure coal combusted x i i xi i i f rate of reaction Sh^ Sherwood Number os species j -1 S^ Rate of Char-Steam Reaction k Pa s u Slag v e l o c i t y m s 1 A c t i v i t y c o e f f i c i e n t of species j xi v ACKNOWLEDGEMENTS I would l i k e to express my sincere indebtedness to Dr. Greg Richards for his untiring support and friendship throughout this project. I would also l i k e to thank Dr. G. W. Toop, E. T. DeGroot and G. Heney for their guidance and assistance during my stay at Cominco. The co-operation and f i n a n c i a l support of Cominco Ltd., T r a i l , B.C., was greatly appreciated. The f i n a n c i a l support of the Science Council of B.C. also was greatly appreciated. Many thanks are also in order to Dr. Keith Brimacombe for his encouragement throughout the entire project and his very helpful e d i t o r i a l suggestions during the writing of th i s thesis. It i s impossible to assess the contribution that the "boys" in the o f f i c e have made. Many thanks to Dave Tripp, Bob Adamic, and Barry Wiskel. F i n a l l y , I would l i k e to express my deepest thanks to my wife, Susan, for the many s a c r i f i c e s she has made and the support she has given me throughout this project. 1 CHAPTER 1 INTRODUCTION; SLAGS AND SLAG PROCESSING Slags play a v i t a l role in pyrometallurgical processes. The functions of slags include: the thermal insulation of melts, chemical insulation of melts from the surrounding atmosphere, r e s i s t i v e heat sources in melting processes, and the r e f i n i n g of metals through both the chemical and physical absorption of unwanted l i q u i d and s o l i d components and the supply of desired components. Yet in spite of their importance in the processing of metals, they remain poorly understood t h e o r e t i c a l l y and not exploited to the i r f u l l potential in industry. This s i t u a t i o n can be attributed to three main factors: f i r s t l y , the circumstances under which slags form in i n d u s t r i a l processes; secondly, the complexity of i n d u s t r i a l slag systems, and t h i r d l y , the preva i l i n g economics. The role these factors have played in determining the technological state of slag metallurgy i s interesting and worthy of further discussion. With regard to the f i r s t factor, slags generally form in pyrometallurgical processes through contributions from three main sources. These are: gangue material, which is introduced with the concentrate or ore; flux additions, which are de l i b e r a t e l y made during the smelting stage; and oxidation of the metal or 2 matte. A fourth and more minor contribution i s made through the chemical and/or mechanical erosion of r e f r a c t o r i e s where present. Of these four only one, the flux addition, i s the resu l t of a deliberate addition made in an attempt to optimize the physiochemical properties of a slag which in the majority of cases already exists via other contributions. H i s t o r i c a l l y , therefore, slags have been present as a consequence of the process, not through deliberate e f f o r t s . Optimization has been by t r i a l and error which has promoted the practice of slag metallurgy through t r a d i t i o n rather than science. The second factor, slag complexity, has acted to further perpetuate the t r a d i t i o n a l unimaginate approach. This complexity is a consequence of several factors which include: slag behaviour which ranges from that of ionic melts to that of polymeric or network forming melts; the fact that slags are solutions not dominated by any one species; and the r e a l t i e s of the i n d u s t r i a l environment - such as the natural v a r i a t i o n in gangue material, and the a v a i l a b i l i t y and changing economics of fluxes. These factors have a l l hindered slag investigation. Not sur p r i s i n g l y , therefore, there has been l i t t l e experimental work done r e s u l t i n g in a profound lack of fundamental thermodynamic, physiochemical and reaction k i n e t i c data on metallurgical slags. The two factors discussed so far have helped to keep slag metallurgy in a r e l a t i v e l y unenlightened state. The t h i r d and 3 debateably most important contributor to t h i s trend is economics. Not s u r p r i s i n g l y , i f there i s l i t t l e obvious monetary gain to be made through the exploitation of slags the driving force for their study i s limited. A c l a s s i c example of t h i s is the electro-slag remelting process (ESR) which produces expensive s p e c i a l t y tool s t e e l s . Due to t h i s interest there is considerably more physiochemical data available on the CaF2-Al 20 3 - CaO ternary on which ESR slags are based than on i n d u s t r i a l l y more prevalent slag systems. It i s these three factors which have acted, and continue to act, to slow the exploitation of slags in the i n d u s t r i a l work place. Recently, however, signs of interest have been shown both on the academic and i n d u s t r i a l fronts in the slag cleaning process. 1 . 1 Slag Processing An inherent problem associated with slags in pyrometallurgical processes is that they unavoidably contain metal values at the end of the process. The metal appears in the slag in two forms: one, as solution or chemical losses in which the metal i s dissolved into the slag as a r e s u l t of an exi s t i n g chemical p o t e n t i a l ; and second, as physical losses due to mechanical entrainment. In many cases, these metal values are discarded with the slag and l o s t , or stock p i l e d , with the slag to await an economic means of recovery. The dr i v i n g force for the development of an economic means of recovery may ari s e from 4 several sources: for example, an increase in the price of the contained metal brought about by an increase in demand and/or a decrease in the grade of ore bodies, or the development of a new technology requiring slag treatment as an integral part of the process. A good example of the l a t t e r are the Q.S.L. and flash smelting processes for the treatment of lead concentrates. These processes use continuous multi-stage single-vessel reactors and therefore require improved methods of slag treatment. The same can be said for developments in the smelting of t i n , copper and ni c k e l . 1.1.1 Slag Cleaning H i s t o r i c a l l y , slag processing, when practiced, has been a secondary step outside the primary flow chart of the smelter and mainly consists of slag cleaning. The term slag cleaning refers to those processes concerned with the removal of metals from slags. To date there have been three main approaches to this problem. The most predominant of these, pyrometallurgical reductive processes, involve the treatment of slag with a reducing agent, such as coal, natural gas or pyrites. Metal values are then recovered in metallic form or as sulphides. For example, in the zinc slag fuming process, coal is injected into molten lead blast furnace slag to produce a metallic vapour which is then oxidized and collected for subsequent treatment. 5 The second cleaning technique ' s e t t l i n g ' employs gravity to f a c i l i t a t e separation based on density differences. This method is p a r t i c u l a r l y e f f e c t i v e in those instances where the slag contains a large amount of mechanically entrained metal, for example, for the removal of copper droplets in copper slags. This technique is often used in conjunction with reductive treatment of the slag. In fact, a combination of s e t t l i n g and reductive treatment were applied in the developmental stage for the Kivcet process for the treatment of lead concentrates. In th i s process zinc and lead bearing slags are reacted in a r e l a t i v e l y quiescent e l e c t r i c arc furnace with coke to permit both s e t t l i n g and reduction. The t h i r d cleaning process i s m i l l i n g and f l o a t a t i o n . This involves grinding the cold slag followed by separation of any metallic or sulphide p a r t i c l e s in a standard rougher-cleaner c i r c u i t . There are both advantages and disadvantages to this process in comparison to the pyrometallurgical routes depending on the p a r t i c u l a r a p p l i c a t i o n . These w i l l not be discussed further as they are beyond the scope of this t h e s i s . The potential for improvement in the area of slag cleaning appears to be considerable. The factors alluded to previously have contributed to create a technology which is in a r e l a t i v e l y backward state and yet is poised to play a c r i t i c a l role in the 6 newer non-ferrous processes. A good example of t h i s is the zinc slag fuming process which is only just beginning to be understood mechanistically. Thermodynamic data, physiochemical data and a knowledge of process k i n e t i c s have been brought to bear to describe the process in d e t a i l for the f i r s t t i m e . l a , b The results of t h i s analysis could p o t e n t i a l l y have a great influence on slag processing in general. The application of high-pressure coal i n j e c t i o n to the zinc slag fuming process, the focus of t h i s t h esis, follows as a d i r e c t r e s u l t of the " k i n e t i c " conception of the process. However, before proceeding, i t is both necessary and i l l u s t r a t i v e to review the zinc slag fuming process in general, i t s history, and i t s potential role in the future of slag processing. 1.2 The Zinc Slag Fuming Process In the zinc slag fuming process, molten slag is treated reductively in order to recover contained zinc and lead. Zinc, the more prevalent metal, is present dissolved in the slag in oxidic form, whereas lead usually appears in both oxidic and metallic forms. The process is carried out on a batch basis in rectangular water-jacketed furnaces that are, on average, 6m long by 2.5m wide; see the schematic diagram in F i g . 1.1. The charge may be made up e n t i r e l y of l i q u i d slag or a combination of l i q u i d and s o l i d materials. T y p i c a l l y batches containing 50 tonnes are Primary air 05-33Nm 3 /s Cool: I-1-5 kq/s Secondary air 33-75 Nm3/s Punching Valve S & Tuyere C.CO — C 0 2 Zn, +1/2 0o — Z n Q % (g) 2 (s) Tertiary air (unregulated) Water-jacketed Walls Slag Fuming Process Figure 1 . 1 Schematic diagram showing the furnace in cross -sect ion and the chemical reactions occurring in the process 8 processed at a time. A reductant, usually coal, pulverized to approximately 80%-200 mesh (B.S.S.), is injected into the furnace through a row of tuyeres located near the bottom of each of the two long sides. The coal is injected into the tuyeres at overal l rates ranging from 50 - 75 kg/min. conveyed by "primary a i r " at 3 t y p i c a l o v e r a l l rates of approximately 30 Nm /min. The main or "secondary b l a s t " enters the tuyeres behind the c o a l / a i r mixture at o v e r a l l rates ranging from 300 - 400 Nm3 /min usually at ambient temperature. In some operations however, there are variations in furnace geometry and operating practice. For example, primary and secondary blast flow rates may be equal at about 200 Nm3 /min and secondary blast preheats of up to 700°C have been used. Removal of zinc from the bath is effected through reduction of dissolved zinc oxide to metallic zinc vapour which then exits v i a the surface of the bath. However, the vapour form in which lead exits the bath, be i t as metal, oxide or sulphide is less clear as these compounds are present predominantly as l i q u i d s at slag fuming temperatures. The characterization of lead removal from the bath i s an important aspect of t h i s thesis and w i l l be discussed in the sections that follow. After e x i t i n g the surface of the bath the metallic zinc vapour and any lead bearing species are then reoxidized by " t e r t i a r y " or leakage a i r . This mixture of metal oxides (fume), 9 combusted coal and t e r t i a r y a i r i s then drawn up through a flue and the fume i s coll e c t e d in a baghouse for subsequent processing. The exothermic nature of these oxidation reactions results in extremely hot off-gases. An attempt i s made to recoupe some of t h i s sensible heat by passing the gases through a steam boiler prior to entering the baghouse. Slag fuming is normally used to treat lead blast furnace slags. At some operations, however, i t has been applied successfully to the treatment of copper reverberatory slags. The only s i g n i f i c a n t difference in th i s case i s in the i n i t i a l zinc and lead concentrations: 13-18 wt% zinc and 1-3 wt% lead for the lead blast furnace slags, 8 wt% zinc and less than one weight percent lead for the copper reverberatory slags. These slags are then fumed down to between 1.5 and 2.5% zinc at which point the furnace is tapped. In both cases lead contents are t y p i c a l l y around 0.02% at th i s point. Complete fuming cycles take approximately 3 hours including 30 minutes for charging and tapping. The recovery of zinc is generally 85-90% with 1-2 kg of coal required per kg of zinc fumed. In contrast, lead recoveries tend to be higher, near 100%. In addition to zinc and lead, the fume from lead blast furnace slag contains traces of v o l a t i l e species such as sulphur, t i n , cadmium, indium, chlorine and fluor ine. 10 Under ideal conditions at some operations, the fuming cycle is broken down into two periods d i s t i n c t from those associated with charging and tapping the furnace. These may be described as an i n i t i a l "heating period" followed by a period of "proper fuming". In the heating period, the co a l - t o - a i r r a t i o i s reduced in order to approach stoichiometric combustion of the a i r / c o a l mixture providing the highest possible heat input per unit of coal. Throughout th i s period, the bath temperature r i s e s , zinc and lead fuming rates tend to be low, and the bath f e r r i c iron concentration increases r e f l e c t i n g the oxidizing conditions pre v a i l i n g in the furnace. There are several reasons for having an i n i t i a l heating period. These may include: the build up of a reserve of sensible heat which i s later consumed throughout the fuming period and/or, the a b i l i t y to process s o l i d crushed slag or ladle s k u l l . The heating period is generally terminated when o the bath temperature approaches approximately 1325 C . The onset of the fuming period is i n i t i a t e d by an increasing in the coal - t o - a i r r a t i o causing conditions in the furnace to become reducing. This period is characterized by a steady decline in bath temperature, high zinc and lead fuming rates, and a decrease in the bath concentration of f e r r i c iron. At other operations however, th i s two-stage fuming cycle i s not practiced. The entire fuming cycle is run at a constant co a l - t o - a i r r a t i o . At s t i l l other operations, a s i g n i f i c a n t l y d i f f e r e n t approach i s taken. Instead of a batch type operation, 11 slag fuming is carried out on a continuous basis in a geometrically smaller furnace using fuel o i l as the reductant. 1.2.1 History The development of the zinc slag fuming process is rooted in the development of the sinter-lead blast furnace process for the smelting of lead sulphide ores in the 1870's. The presence of s i g n i f i c a n t amounts of zinc sulphide in association with the lead sulphide in the concentrate meant that the process had to effect separation between the two. This was achieved by taking advantage of the difference in s t a b i l i t y between lead oxide and zinc oxide. By c o n t r o l l i n g the oxygen potential in the blast furnace i t is possible to produce lead while holding the majority of the zinc as oxide in the slag phase. I n i t i a l l y , this slag was stock p i l e d and smelters the world over began to accumulate slag dumps containing up to 20% zinc. This provided the i n d u s t r i a l setting for the emergence of a process that could recover t h i s zinc. What was s t i l l required, was the economic incentive and technical expertise. The economic incentive was provided by a sharp increase in the demand for zinc which followed the outbreak of World War I, and continued through the 1920's as the i n d u s t r i a l potential of zinc was r e a l i z e d . The technical development began almost immediately. 12 Some of the e a r l i e s t work on the extraction of zinc from lead blast furnace slags was carried out by the Sulphide Corporation at Cockle Creek between 1906 and 1916. The i n i t i a l experiments in 1906 involved blowing compressed a i r through molten slag. This resulted in some fuming and rapid freezing of the slag. A second set of experiments was carried out using a c o a l - a i r mixture and proved to be successful enough to warrant patenting. The patent was issued to F.H. Evans and P.A. McKay in 1908. Other experiments included: the reverberatory furnace treatment of slag by f i r i n g with coke and limestone, and a blast furnace type treatment of slag briquettes and nodules. The only resu l t of any significance was that water-jacketed furnaces appeared to be necessary in any i n d u s t r i a l a p p l i c a t i o n . In the 1920's, investigation into the commercial application of the process lead to the f i r s t furnace being blown in at the Anaconda Copper Mining Company at East Helena, Mont."^, and to the i n s t a l l a t i o n of the second furnace by the Consolidated Mining and Smelting Company, T r a i l , B.C. in 1930. These two furnaces were s l i g h t l y d i f f e r e n t . The East Helena furnace was 2.44m (8ft) by 3.66m (12ft) and used a tuyere design borrowed from the burners in c o a l - f i r e d reverberatory furnaces, whereas the T r a i l furnace was s l i g h t l y wider at 3.5m (10ft), s i g n i f i c a n t l y longer at 7.9m (20ft), used newer double-inlet tuyeres, and incorporated a waste-heat b o i l e r . Apart from these differences, the furnaces were e s s e n t i a l l y the same and were both used in the treatment of 13 s o l i d and l i q u i d material in order to recover zinc from slag dumps. As the dump supply of zinc was exhausted, they were shif t e d to the treatment of l i q u i d lead blast furnace slag. Since the 1930's, there have been l i t t l e or no s i g n i f i c a n t changes in the process despite i t s adoption in v i r t u a l l y every lead smelter throughout the world. Individual modifications have included: the adoption of secondary blast preheat at the Broken 4 H i l l Associated Smelters of Port P i r i e in 1967 ; continuous 5 fuming in Bulgaria u t i l i z i n g fuel o i l instead of coal ; and the use of natural gas as a reductant in a Russian process.** As mentioned previously, the newer technologies for the smelting of lead concentrates require improved methods of slag treatment. These technologies a l l employ continuous smelting processes incorporating reductive slag cleaning stages within the main stream of the process. In the p i l o t QSL process for example, natural gas is injected through submerged tuyeres in a reductive zone. In the Kivcet process, coke additions are made to the slag surface in a r e l a t i v e l y quiescent e l e c t r i c arc holding furnace. Both of these processes require a fundamental understanding of the kin e t i c s of oxidic zinc and lead reduction, and metallic lead removal from these slags in order for them to be economically successful processes. The role played by the zinc slag fuming process in the near future of the non-ferrous metals industry must be that of a testing arena. It must be used 14 to investigate k i n e t i c phenomena essential to the success of the newer smelting technologies. In r e f l e c t i o n , the lack of consistency in zinc fuming practice, the non-universal adoption of process improvements, such as blast preheat and continuous operation, and the fact that the process remains e s s e n t i a l l y unchanged since the 1930's, a l l point to a t r a d i t i o n a l optimization approach to process improvement. The proposed slag cleaning stages in the newer smelting technologies also suggest the continuation of this trend in thinking. In both the QSL and Kivcet processes, the approaches taken suggest a lack of understanding of slag reduction k i n e t i c phenomena. The economic incentive for the elucidation of these phenomena now exists as they are paramount to the success of these processes. High-pressure coal i n j e c t i o n follows from an understanding of kinetics and could prove to be of considerable benefit to slag treatment technology. Taken one step further, the phenomena described elsewhere^" 3'^ and b u i l t upon in th i s thesis help forge the way to the use of slags as active tools in metals extraction. 15 CHAPTER II LITERATURE REVIEW This thesis deals with the application of high-pressure coal i n j e c t i o n to the i n d u s t r i a l zinc slag fuming process. In order to outline some of the options available for coal i n j e c t i o n systems, experimental techniques and data analysis, i t is necessary to review .the l i t e r a t u r e . The results of these reviews are presented in thi s chapter. 2.1 Pulverized Coal Transport 7-10 A general review of the l i t e r a t u r e indicates that there are several precautions which must be observed when handling and transporting pulverized coal. F i r s t l y , because in certain concentrations coal dust suspended in a i r constitutes a highly explosive mixture, extreme caution must be exercised in i t s handling, ensuring that work areas are well vented and clean. The second precaution is d i r e c t l y concerned with the transport of coal and therefore of d i r e c t a p p l i c a b i l i t y to this project. There is strong evidence to suggest that under certain conditions, such as during pneumatic transport in p l a s t i c pipes with l i t t l e or no moisture present, a s i g n i f i c a n t s t a t i c charge can be generated. The random uncontrolled grounding of this charge can resu l t in i g n i t i o n of the co a l / a i r mixture. Since the 16 conditions leading to th i s phenomena are not well documented, metallic pipes and/or grounding must be provided for pneumatic transport as a general precaution. F i n a l l y , i t is essential to maintain proper aeration to achieve f l u i d - l i k e behaviour when transporting pulverized coal. Sticky s o l i d - l i k e behaviour can resu l t from lack of aeration as well as from other factors: for example, high moisture contents and/or a build up of s t a t i c surface charge. 2.1.1 Pneumatic Conveying of Materials Pneumatic conveying systems have been c l a s s i f i e d h i s t o r i c a l l y into a broad range of categories including: gravity-feed systems, low-velocity systems, high-velocity systems, light-phase systems, dense-phase systems, vacuum systems, low-pressure systems, medium-pressure systems, high-7 8 9 pressure systems, etc. ' ' In certain instances these terms have been used loosely by manufacturers to c l a s s i f y d i f f e r e n t types of equipment. In the engineering l i t e r a t u r e , a quantitative c l a s s i f i c a t i o n system has been developed. For example, dense phase systems are considered to be those with mass flow r a t i o s of s o l i d to gas in the hundreds; l i g h t phase or lean systems, have rat i o s below about one hundred, with eighty frequently being the d i v i s i o n between the two. Additional c l a s s i f i c a t i o n s include: vacuum systems, considered to be those operating below atmospheric pressure; low-pressure systems, 17 between atmospheric and 82.73 kPa (12 psig); medium-pressure systems, up to 310.23 kPa (45 psig); and high-pressure systems, up to 861.75 kPa (125 psig). Unfortunately, there are no uni v e r s a l l y adopted standards for c l a s s i f y i n g these systems. It is clear, however, that for gas-solids mixtures pneumatic conveying systems e s s e n t i a l l y can be broken down into three main 7 types : mater i a l - i n t o - a i r , air-mixing and air-into-material systems. 2.1.1.1 Material-Into-Air Systems The material-into-air systems are c l a s s i f i e d as those systems in which material enters a stream of a i r under either negative or low pressure or, is induced into a stream by vacuum. This c l a s s i f i c a t i o n includes the vacuum systems and low-pressure systems mentioned e a r l i e r . Table 2.1 presents a summary of some of the advantages and disadvantages of this type of system. 2.1.1.2 Air-Mixing Systems In the air-mixing systems the material to be conveyed and a i r are intimately mixed in a special feeder prior to entering a conveying l i n e . These systems resemble the f i r s t type except that a denser stream, and, feeding into higher pressures are possible. These systems include the medium pressure systems. The feeders used include: rotary feeders, F u l l e r Kinyon pumps 18 TABLE 2.1 Classification of Pneumatic Conveyors Cluificatto Advantages Disadvantages Material-into-air Positive-pressure 1. Handles wides range of taterials,in-cluding irregular shapes. 1. Delivers material through a single pipe-line to several discharge points. 2. Uses smaller pipelines than comparable vacuum systems. 3. Air leakage is outward so that moisture can not be drawn into equipment. 1. Uses lower material-to-air ratios than other systems. 2. Requires larger equipment than other systeas-air movers, pipelines, dust filters, etc. 1. Requires an airlock feeder and blowback vent filter at each material entry point. 2. Requires large dust filter at each material discharge point. 3. Transport unloading requires use of fixed or portable airlock feeder installation. Negative-pressure 1. Transports easily unloaded using pickup hoppers or nozzles. 2. Material can enter a single pipeline from several sources of supply without venting 3. Material can enter line using airlock feeder, controlled-feed tank, or pickup hopper. 1. Material can be delivered only to a single discharge point. 2. Dust filter and airlock discharger required at the discharge point. 3. Moisture and outside air can be drawn into equipment. 4. Requires larger piping and equipment than positive-pressure systems, due to lower density air. 5. Air mover and receiver-filter requires lo-cation stop silos or building roof. Negative-pressure (vacuum)with dust-return loop 1. Delivers material to several discharge 1. points. 2. Dust is returned to a single, conveniently 2. located dust filter for re-entry into con-veying line or silo. Requires a collector and an airlock dis-charger at each discharge point. Requires diverter valves in the dust return and conveying lines at all but one dis-charge point. 1 9 Combination vacuum/ pressure 1. Permits pickup of materia) froi trans-ports by vacuui and simultaneous dis-charge to each of several discharge points by pressure. 2. Dust from vacuut-side receiver is de-livered into conveying line. 1. See above for respective portions of syste vacuui or pressure side. 2. Requires larger-capacity and horsepower air eover than for the vacuui or positive-pressure system. 3. If air eover breaks down, two plant operations are affected - unloading and reclaiming. Closed-loop, positive or negative pressure 1. Periits use of inert gas for conveying, with minimum uke-up requirement. 1. Same as for combination vacuum-pressure system. 2. Permits re-use of conveying air where air is dried or filtered to be free of con-tamination. 3. Eliminates the use of diverter valves on dust-return line at each discharge point. Air-into-Mftriai 1. Uses highest possible aaterial-to-air 1. ratio compared to other systems. Operation is intermittent. 2. Requires smaller equipment than other 2. systems - air movers, pipelines, dust filters, etc. 3. 3. Low velocity, dense mixture permits handling abrasive and friable materials. 4. High-pressure units and systems permit 4. delivery through very long pipelines. High pressure systems (over IS psig) re-quire ASHE-Code-constructed vessels. Surge hopper is reuired for rapid filling if auxiliary feed capacity is limited. A dust filter is required for venting surge hopper and blow tank. 5. Delivers material to several discharge 5. points using a single pipeline. 6. Each batch can be weighed before being conveyed. Haterial must be delivered to blow tanks at higher than pneuiatic conveying rates, due to 1iiited filling time between discharge cycles. Free-feed blow tank 1. Bottom-discharge units can handle lumpy, 1. non-fluidizable and fluidizable materials. Haterial-to-air ratio is variable - high at start and finish of each cycle. 2. Bottom-discharge units can be emptied 2. completely when handling sanitary materials. Top-discharge units leave residue in bot-tom of tank that may cause contamination of material. 20 Controlled-feed blov 1. Material-to-air ratio of discharge is uni- 1. Use is liaited to fluidizable, powdered fora. or aixed-granular aaterials. 2. Conveying-line velocity can be reduced to the slug flow liait. 3. Booster nozzles aay be installed in the conveying line to aaintain dense, low velocity flow. Air -aixiaf Feed-screw into air nozzles 1. Haterial-to-air ratio is high, but not as 1. high as in air-into-aaterial systems. 2. Conveying is continuous. 2. 3. Delivers aaterial to several discharge 3. points through a single pipeline. 4. High pressure peraits delivery through 4. long, saall-diaaeter pipelines 1. Requires saaller auxiliary conveyors than blow tanks. 2. Requires lower headrooa than blow tanks. 2. Use is liaited to fluidizable, powdered or aixed-granular aaterials. Can not handle friable aaterial. Haterial must be delivered by aetered auxiliary conveyors. Feed hopper aust be level-controlled and vented to dust filters. 5. Feed hoppers aust be aaintained full when handling abrasive aaterial. Subject to erosion and loss of capacity when handling abrasive aaterial. Requires skilled aaintenance, as for high-grade aachinery. Must run fully loaded when handling abrasive aaterial - wear increases when idling. 4. Requires high-prssure, sliding-vane or re-ciprocating air coapressor. 5. Coapressors require cooling water. Oil separators aust be installed ahead of air nozzles. Air-swept, double-entry rotary feeder i. Can be aade with cutting vane to handle tacky aaterial such as sugar or de-tergents. 1. Pressure liait is 20 psig. requiring low-voluae, positive-displaceaent blowers in tandea. 2. Capacity varies with head of aaterial in feed hopper. 21 and any other type of pump capable of de l i v e r i n g a dense material-air mixture into a pressurized a i r stream. The r e l a t i v e merits of th i s system are summarized in Table 2.1. 2.1.1.3 Air-Into-Material Systems The air - i n t o - m a t e r i a l systems are c l a s s i f i e d as those systems in which a i r enters a mass of material to i n i t i a t e flow. These include the high-pressure systems and dense phase systems which are more generally termed as "blow-tank" systems. The advantages and disadvantages of thi s type of system are described in Table 2.1. 2.1.1.4 Selection of System The type of system selected depends on such factors as economics, application, the desired volume and mass flow rates and the type of material to be conveyed. If for example, the c l a s s i f i c a t i o n i s narrowed down to include only those systems capable of conveying pulverized coal, as would be required for this project, a number of acceptable systems decreases. Unfortunately, no single best system is obvious. 2.1.2 Non-Pneumatic Transport In addition to a i r , water has also been used as a conveying 22 medium to inject pulverized coal successfully into an iron blast f u r n a c e . ^ There are several obvious advantages to injecting coal-water s l u r r i e s : reduction in explosion hazard, no gaseous oxygen i n j e c t i o n , and the p o s s i b i l i t y of achieving higher pressures. Furthermore, th i s brings to l i g h t other alternatives such as c o a l - o i l s l u r r i e s and coal-steam mixtures which might find future application in the zinc slag fuming process. 2.1.3 Summary: Pneumatic Conveying In summary, i t is evident that design c r i t e r i o n w i l l have to be established before the information in the l i t e r a t u r e can be applied to aid in system sele c t i o n . This endeavour w i l l be addressed in Chapter IV. 2.2 The Zinc Slag Fuming Process The conceptual idea for high-pressure coal injection f o l l o w s d i r e c t l y from the work of R ichards. "*"a As a r e s u l t , his o r i g i n a l work"*"3 and the later treatment by Richards, Brimacombe and Toop"'"^  are strongly recommended as preliminary reading to this thesis. To minimize r e p e t i t i o n , material previously covered by Richards w i l l only receive cursory treatment. Relevant material later than 1983 w i l l be covered in more d e t a i l . For a more detailed review of relevant pre-1983 material the reader is 23 referred to Richard's work. 2.2.1 Studies and Modelling Soon after the f i r s t furnaces were constructed two schools of thought emerged on the mechanism of reduction of zinc from slag. The f i r s t school postulated that coal entering the furnace was immediately combusted to a carbon monoxide-carbon dioxide mixture. The carbon monoxide in this gas stream then acted to reduce the dissolved zinc oxide and produce metallic zinc vapour 12 13 as the reducing gas ascended through the bath. ' In contrast the second school postulated that p a r t i c l e s of coal in contact 14 with the slag act as the sight for reduction. Surprisingly, these d i f f e r e n t concepts remain the source of debate today. 2.2.1.1 Equilibrium Models H i s t o r i c a l l y , the f i r s t school of thought received substantial support in 1954 from the c l a s s i c a l study of B e l l , 15 Turn and Peters. In their investigation, a simple, model based on mass balances and the assumption of internal equilibrium was developed to estimate the a c t i v i t y of zinc oxide in the slag for three i n d u s t r i a l runs during which fuming rates were measured as a function of bath zinc concentration. With th i s data, the authors found that their model accounted quite well for the ov e r a l l furnace heat balance. The model also was used to 24 evaluate the ef f e c t of various changes in operating practice. These included: the i n j e c t i o n of natural gas as a reductant and preheating the blast. Despite obvious shortcomings, such as the exclusion of the role of iron oxides and the lack of i n d i r e c t experimental evidence to support the fundamental assumption of equilibrium, the model reasonably accounted for observations with only minor discrepancies. In one instance however, th i s was not the case. During fuming with natural gas as the reductant, fuming rates predicted by the model were s i g n i f i c a n t l y higher than those observed in practice. This observation is of considerable significance because i t alludes to the importance of the role coal p a r t i c l e s play in slag reduction. Additional support for the equilibrium approach continued to build in 1956 when Kellogg conducted a "no-coal" test on No. 2 14 slag fuming furnace at Cominco Ltd., T r a i l , BC. In t h i s test the coal supply to the furnace was interrupted for a period of five minutes during which time changes in slag composition and temperature were recorded. The analysis of t h i s data suggested that there was s u f f i c i e n t gas stream-slag interface for i t to be the "seat of the fuming reaction". Based on the results of this experiment and on the previous work by B e l l et a l , Kellogg developed a comprehensive model of 25 the zinc slag fuming process in 1 9 6 7 . I n this model, the thermodynamics of the iron oxides as well as that of PbO and sulphur were included. In addition, the overall furnace heat balance was improved to incorporate the melting and freezing of slag on the water cooled walls and bottom, and the reactions occurring above the bath. As a re s u l t , the rates of change of iron, lead, sulphur and temperature could be predicted in addition to that of zinc. The model was f i t t e d to average i n d u s t r i a l data by adjusting slag a c t i v i t y c o e f f i c i e n t s and heat-transfer c o e f f i c i e n t s . This p o t e n t i a l l y could have raised concern but the re s u l t i n g values of the c o e f f i c i e n t s were reported to be within expected l i m i t s . The Kellogg"''*' model represented a s i g n i f i c a n t advance in the study of the zinc slag fuming process. It established conclusively that the assumption of equilibrium could be used qua n t i t a t i v e l y to account for the behaviour of zinc and q u a l i t a t i v e l y to account for the behaviour of iron, lead and bath temperature. In the early 1970's the Kellogg model was modified by Grant 17 and Barnett at the Broken H i l l Associated Smelters, Port P i r i e , A u s t r a l i a . The heat balance in the model was upgraded to include waste heat boilers and recuperators. Some improvements also were made in the handling of input parameters, such as changing coal 26 rates and variations in charge with respect to hot and cold additions. As with the o r i g i n a l Kellogg model ^ the modified model was f i t t e d to i n d u s t r i a l data by adjusting the a c t i v i t y c o e f f i c i e n t of zinc oxide in the slag. It i s interesting to note that Grant and Barnett needed a value 2.6 times greater than that used by Kellogg, and, that this increase was not substantiated by any independent thermodynamic data. This becomes a s i g n i f i c a n t point because i t means that the v a l i d i t y of the model rests on the a c t i v i t y of ZnO in the slag which is e f f e c t i v e l y a f i t t i n g parameter. From a process point of view there is a fundamental problem with the assumption of equilibrium - i t negates the consideration of process k i n e t i c s . In so doing, i t is impossible to assess the influence of k i n e t i c phenomena on furnace performance. For example, i t could prove more e f f i c i e n t with respect to zinc removal to operate the process in such a manner that equilibrium is not achieved. 2.2.1.2 Empirical Models H i s t o r i c a l l y , support for the second school of thought has been less s c i e n t i f i c a l l y based. The empirical models of the 18 — 20 21 22 process by Quarm , Sundstom and Ivanov are of l i t t l e value in elucidating fundamental thermodynamic and kin e t i c phenomena. Even though they have been shown to reproduce 27 observations quite well, p a r t i c u l a r l y for the furnace from which they have been derived, they lack broad a p p l i c a b i l i t y and therefore universal acceptance. Nevertheless, the empirical models can be useful at least for the i d e n t i f i c a t i o n of k i n e t i c and thermodynamic phenomena. In p a r t i c u l a r , some of the variables that have been i d e n t i f i e d by Sundstrom and Ivanov to be of c r i t i c a l importance to the process could be interpreted as k i n e t i c e f f e c t s . These include: the negative influence of bath weight, and the ef f e c t of the number of operating tuyeres. In the l a t t e r case, t h i s is strong evidence that i n j e c t i o n dynamics play a v i t a l role in the slag fuming process. Indeed, this also has been corroborated by the findings of Glinkov et a l . 3 ^ 2.1.1.3 Industrial and Laboratory Studies Additional support for the ki n e t i c approach to process 2 3 2 4 modelling has been received from i n d u s t r i a l ' and labor-25-29 5 atory studies of slag fuming. In one instance , engineers at the Non-Ferrous Metal Works in Plovdiv, Bulgaria departed considerably from standard practice and developed a continuous fuming, furnace using fuel o i l as the reductant. The furnace, 2 which has a cross-sectional area of 5.85 m , received shaft smelting furnace slag v i a an intermediate e l e c t r i c arc s e t t l i n g furnace. Of pa r t i c u l a r interest, apart from the significance of 28 the approach taken, is that according to Abrashev, the fuming rate is 50% lower than that predicted by equilibrium. This statement is further corroborated by the fact that a reduction in 2 cross-sectional area from 8.9 to 5.95 m was reported to have brought about a two-fold increase in furnace capacity with o i l and a i r rates unchanged. This is strong evidence that the furnace is limited by k i n e t i c s . Industrial p i l o t - p l a n t studies ' have suggested that the coal-slag interaction may be of fundamental importance to slag 25-29 reduction. Other laboratory experiments have confirmed these findings and suggest that the reduction a c t u a l l y occurs via the gaseous intermediates, CO and CO^. These studies indicate that k i n e t i c phenomena, such as the rate of the Boudouard 2 6 - 2 9 reaction occurring on carbon and oxide d i f f u s i o n through the slag may be rate c o n t r o l l i n g depending on the stage of reduction. In a more recent study on the mechanisms of slag reduction by Malone, Floyd and Denholm^, the role of carbon is less clear. Based on a series of experiments which involved coke, CO and nitrogen i n j e c t i o n into zinc-bearing slags, with and without iron present. The authors proposed a mechanism by which ZnO is reduced from iron containing slags. Unfortunately however, there appear to be a number of d i f f i c u l t i e s in their interpretation of 29 the results p a r t i c u l a r l y for the case where fuming was achieved 2+ 3 + with only nitrogen i n j e c t i o n . In this experiment the Fe /Fe r a t i o was reported to have increased i n i t i a l l y and then remained constant during fuming which is c l e a r l y inconsistent with the proposal that FeO is the source of the reductant. 3" 1" One plausible explanation is that metallic iron was present prior to the addition of ZnO and the i n i t i a t i o n of nitrogen i n j e c t i o n . Substantial proof for the zinc fuming process as a non-equilibrium process was not provided u n t i l the work of R i c h a r d s 1 3 in 1983. In t h i s work, an exhaustive i n d u s t r i a l study of the process revealed coal p a r t i c l e s entrained in the slag negating the p o s s i b i l i t y of internal equilibrium. Analysis of tuyere in j e c t i o n dynamics was also used to further support these findings. 2.1.1.4 The Kinetic Model of the Process Based on i n d u s t r i a l findings, a mathematical model incorporating thermodynamic data, physiochemical data and k i n e t i c phenomena was developed by Richards"''3 based on coal p a r t i c l e s entrained in the slag as the s i t e for reduction. Mechanistically, the model is formulated around the concept of d i v i d i n g input coal into the following fracti o n s : that entrained in the slag, that combusted in the tuyere gas column and the 30 balance which is assumed to pass through the furnace unconsumed. In terms of model formulation, i t follows from this coal p a r t i t i o n i n g that the furnace has e f f e c t i v e l y been separated into two reaction zones. The f i r s t of these is a reduction zone in which entrained coal reacts d i r e c t l y with the slag. The second then is an oxidation zone in which the tuyere gas column reacts with injected coal and the surrounding slag. The overa l l furnace model can be broken down e s s e n t i a l l y into a series of heat and mass balances around the two reaction zones and the furnace as a unit. Of these balances, those associated with the reduction zone are of p a r t i c u l a r significance because they are formulated from fundamentally based k i n e t i c concepts. They w i l l be discussed in considerable d e t a i l . Within the model, the heat and mass balances on the reduction zone were formulated around an elaborate coal-gas-slag reaction system which represents a cluster of several coal p a r t i c l e s , entrained in the slag, after they have undergone pyrolysis and have become surrounded by an intermediate or "secondary bubble" containing the products of pyrolysis. This cluster or char was assumed by Richards to have an e f f e c t i v e radius of around 80 um. Within t h i s system, the following reactions were considered: 31 Z n 0 ( s l ) + C 0 ( g ) = Z n ( g ) + C°2 ( g ) ' ' • ( 2 ' 1 } Fe-O- + CO, . = 2FeO, ,,+ CO_ . . . (2.2) 2 3 ( s l ) (g) (si) 2 { g ) C ( s ) + C 0 2 _ - 2 C 0 ( g ) • • • ( 2 ' 3 ) (g) CO + H = CO + HO . . . (2.4) (g) (g) (g) (g) Kinetic phenomena considered included: the d i f f u s i o n of zinc oxide (ZnO) and hematite (Fe 20 3) in the slag to the secondary bubble-slag interface, d i f f u s i o n of wustite (FeO) from the bubble-slag interface to the bulk of the slag, and the rate of the Boudouard reaction (Eq. 2.3) on the char p a r t i c l e . Diffusion rates were estimated based on the dynamics of r i s e of the secondary bubble ascending through the slag. Mass transfer c o e f f i c i e n t s for d i f f u s i n g species were calculated from an empirically based equation, taken from the l i t e r a t u r e , which is v a l i d for r i g i d spheres in creeping flow. The v e l o c i t y of these bubbles r e l a t i v e to the slag was calculated assuming Stokes law. D i f f u s i v i t i e s for ZnO and FeO in slag systems approximating those found in slag fuming were taken from the l i t e r a t u r e . The d i f f u s i v i t y of Fe20^ was assumed to be one tenth that of FeO. The d i f f u s i o n of zinc oxide and f e r r i c iron to the bubble-slag interface is a consequence of the reducing conditions which 32 pr e v a i l in the secondary bubble. Upon a r r i v a l at the interface, zinc oxide and f e r r i c iron were assumed to be reduced via equations 2.1 and 2.3, respectively. These reduction reactions were assumed to be rapid and therefore at equilibrium. Equilibrium was also assumed within the secondary bubble as dictated by equation 2.4. Within the bubble, CO^ in the gas phase, generated by the reduction reactions, reacts with the char p a r t i c l e according to the Boudouard reaction. The rate of the Boudouard reaction was determined by an empirically based equation taken from the l i t e r a t u r e . Mass-transfer within the gas phase was assumed to be rapid and not rate l i m i t i n g . As a r e s u l t of these reactions, the p a r t i a l pressure of metallic zinc vapour in the secondary bubble increases as i t r i s e s through the bath. Concurrently, f e r r i c iron also is reduced. The rate at which these reduction reactions proceed is s o l e l y dependent on the r e l a t i v e rates of mass-transfer. The net resu l t is an influx of C0 9 and r^O, and an increase in the volume of the secondary bubble. The tendency for the oxygen potential to r i s e in the secondary bubble is negated to a certain extent by the Boudouard reaction occurring on the char p a r t i c l e s . However, because the rate of the Boudouard reaction is dependent on the amount of 33 carbon remaining in the char, the extent to which i t can maintain an overall reduction potential decreases as reduction proceeds. Consequently, the oxygen potential of the secondary bubble increases and moves towards equilibrium with that of the slag. Within the model, reaction was assumed to cease when the char-secondary bubble reaches the surface of the bath and the contents of the secondary bubble are released to the furnace atmosphere. Despite i t s inherent complexity, substantial evidence in support of th i s conceptualization of the process was found by Richards^ 3 from electron microprobe analysis of quenched slag samples. Zinc and iron concentration p r o f i l e s were observed around bubbles in these samples which is consistent with the bubbles a c t i v i l y reacting with the surrounding slag, and zinc oxide and f e r r i c iron as the d i f f u s i n g species. Thus the 2+ 3 + possible role of electron transfer between the Fe /Fe couple and oxygen d i f f u s i o n in iron reduction appears to be small. In order to complete the mathematical formulation of the reduction zone, Richards l a conducted a heat and mass balance on the char p a r t i c l e secondary bubble-slag reaction system over the residence time of the secondary bubble in the bath. The balances were constructed on the basis of a unit weight of the i n i t i a l coal p a r t i c l e . The residence time of the p a r t i c l e in the bath was calculated based on a path length and an assumed slag c i r c u l a t i o n v e l o c i t y . The path length of the p a r t i c l e , in turn, 34 was calculated based on estimates of the void f r a c t i o n of the tuyere gas column, slag porosity and slag c i r c u l a t i o n flow patterns. Multiplying the conservation equations by the t o t a l rate at which coal is entrained in the slag gives the instantaneous rates of change of zinc, f e r r i c and ferrous iron concentrations, and the rate of heat consumption in the reduction zone. By holding these rates constant over a given increment of time, the heat and mass balances are determined for the period in question. In comparison to the reduction zone, the balances associated with the tuyere gas column were formulated on a more empirical basis without invoking s p e c i f i c k i n e t i c mechanisms. The heat and mass balances for this zone were determined e s s e n t i a l l y by two externally set adjustable parameters: the fr a c t i o n of coal combusted, as previously described, and the f r a c t i o n of oxygen consumed by ferrous iron oxidation. The l a t t e r was defined by Richards"''3 as the f r a c t i o n of oxygen in the tuyere gas stream unconsumed by coal which reacts with ferrous iron. It was reasoned that ferrous iron would be oxidized to f e r r i c iron within the tuyere gas column because the p r e v a i l i n g conditions generally would be oxidizing. By specifying these two parameters, the heat and mass balances for the tuyere gas column were determined for a given o v e r a l l coal and a i r i n j e c t i o n rate and temperature, following assumptions were made: 35 The [i] the coal is assumed to be completely combusted to CO and H90 via equations 2.5 and 2.6, respectively; C + 0 2 = CO 2 (2.5) H 2 + 1/2 0 2 = H20 (2.6) [ i i ] any ash accompanying the combusted coal is assumed to enter the bath as s i l i c a ; [ i i i ] ferrous iron oxidation is assumed to proceed via equation [iv] the tuyere gas and any unburnt coal are assumed to exit the bath at the slag temperature. In addition to these balances, the fr a c t i o n of coal combusted and fr a c t i o n of oxygen reacted with ferrous iron were used by Richards^ 3 to predict oxygen u t i l i z a t i o n . It is with this number that the underlying kinetics of the tuyere gas column are evaluated. For example, a value less than 100% oxygen 2.7; 2Fe0 + 1/2 0 2 = F e 2 0 3 (2.7) and 36 u t i l i z a t i o n indicates a k i n e t i c l i m i t a t i o n to complete reaction of the input oxygen with coal and/or slag. The t h i r d and f i n a l set of heat and mass balances were associated with the furnace walls and bottom. Sim i l a r l y to Kellogg 1** , R i c h a r d s l a considered the melting and freezing of slag on the furnace walls and bottom. Heat-transfer c o e f f i c i e n t s , heat capacities and thermal conductivities were taken from the l i t e r a t u r e . Assumptions made included: a s p e c i f i c slag melting point based on averaged melting ranges taken from the l i t e r a t u r e , and a uniform composition through the thickness of the wall freeze layer. The composition of the freeze layer was determined by averaging a through thickness assay of actual frozen wall material taken from the wall of No. 2 furnace at Cominco. In order to close the furnace heat balance completely, Richards assumed that the oxidation reactions occurring above the bath do not contribute to the overall slag heat balance. This is d i f f e r e n t from the approach taken in the Kellog model which considers these reactions. The model was f i t t e d to i n d u s t r i a l data by adjusting the previously mentioned parameters (fraction of coal entrained, f r a c t i o n of coal combusted and f r a c t i o n of oxygen reacting with ferrous iron) u n t i l good agreement was obtained between measured 37 and predicted p r o f i l e s of bath composition and temperature. Of particular significance is that this f i t t i n g procedure was carried out on a cycle-to-cycle basis, and not on averaged plant data as was the case with the Kellogg'''^ model and later the Grant 17 and Barnett model. F i t t e d values of fractio n of coal entrained, f r a c t i o n of coal combusted, t o t a l furnace oxygen u t i l i z a t i o n , bath depth and non-stoichiometric factor, x, were obtained for five d i f f e r e n t furnaces and a t o t a l of eleven cycles. These results permitted a comparison to be made between cycles and furnaces so that trends could be delineated as a function of operating practice. Despite a lack of accurate fundamental thermodynamic, k i n e t i c and physical data, the model has been shown to predict the behaviour of zinc, ferrous and f e r r i c iron and, at least, to approach quantitative prediction of bath temperature. The model excludes lead, however this is not a problem provided that the fuming slags under consideration have a low lead content - less than approximately 1 wt%. Of pa r t i c u l a r significance is the consistency in the values of the f i t t i n g parameters. The overall ranges obtained for the fr a c t i o n of coal entrained was 0.29 to 0.39, for fraction of coal combusted, 0.45 to 0.60, and for furnace oxygen u t i l i z a t i o n , 0.67 to 0.92. In the case of the l a t t e r , i t was found that the oxygen u t i l i z a t i o n could be correlated to bath depth. 38 As a r g u e d by R i c h a r d s , t h e c o n s i s t e n c y i n t h e f r a c t i o n o f c o a l e n t r a i n e d i s t o be e x p e c t e d s i n c e a l l t h e f u r n a c e s have a p p r o x i m a t e l y t h e same l o w - p r e s s u r e i n j e c t i o n s y s t e m . The l a c k of s e n s i t i v i t y of t h e f i t t i n g p a r a m e t e r s t o o t h e r p r o c e s s v a r i a b l e s s u c h as c o a l t y p e , b l a s t p r e h e a t , c o a l r a t e , f u r n a c e d i m e n s i o n s , s l a g c o m p o s i t i o n and b l a s t f l o w r a t e , w hich v a r y f r o m f u r n a c e t o f u r n a c e , i s i n d i c a t i v e of t h e g e n e r a l a p p l i c a b i l i t y of t h e model and s u p p o r t s i t s t h e o r e t i c a l f o u n d a t i o n s . A l t h o u g h t h e r e s u l t s of t h e model a r e s e l f - c o n s i s t e n t , t h e r e i s c a u s e f o r c o n c e r n w i t h t h e p r e d i c t i o n s o f oxygen u t i l i z a t i o n f o r t h r e e f u r n a c e s , c ompanies A, B and E, f o r w h i c h t e m p e r a t u r e p r o f i l e s were n o t a v a i l a b l e . C l e a r l y , s i n c e t h e o n l y mechanisms of g e n e r a t i n g h e a t w i t h i n t h e model a r e t h r o u g h e i t h e r c o a l c o m b u s t i o n or f e r r o u s i r o n o x i d a t i o n , i t i s a n t i c i p a t e d t h a t t h e model p r e d i c t i o n s f o r f r a c t i o n of c o a l combusted and f r a c t i o n of oxygen consumed i n f e r r o u s i r o n o x i d a t i o n w i l l be h i g h l y s e n s i t i v e t o b a t h t e m p e r a t u r e p r o f i l e s . T h i s p l a c e s i n q u e s t i o n t h e a c c u r a c y of t h e model p r e d i c t i o n s w i t h r e s p e c t t o t h e s e two p a r a m e t e r s , and t h e r e f o r e , f u r n a c e oxygen u t i l i z a t i o n . U n f o r t u n a t e l y , t h e o f f e n d i n g c y c l e s r e p r e s e n t s e v e n t y p e r c e n t of t h e oxygen u t i l i z a t i o n d a t a . To e v a l u a t e some of t h e more q u e s t i o n a b l e a s s u m p t i o n s t h a t were n e c e s s a r y t o f o r m u l a t e t h e model, a s e n s i t i v i t y a n a l y s i s was 39 performed with respect to the effect of slag c i r c u l a t i o n v e l o c i t y , coal p a r t i c l e cluster si z e , wustite stoichiometric factor, d i f f u s i v i t i e s of wustite and zinc oxide, the wustite-to-hematite d i f f u s i v i t y r a t i o , bath slag porosity, slag density, and the void f r a c t i o n of the tuyere gas column on the fuming rate (%Zn/min) and fuming e f f i c i e n c y (kg Zn fumed/kg coal injected). Remarkably, the model was shown to be r e l a t i v e l y insensitive to these parameters. F i n a l l y the predictive c a p a b i l i t y of the k i n e t i c model was compared to that of the thermodynamic models of Kellogg"^ and, 17 Grant and Barnett. For these comparisons, the entrainment factor was held constant at 0.33 (the average of the eleven cycles investigated), and the f r a c t i o n of coal combusted and oxygen u t i l i z a t i o n were calculated within the model based on the correlations with bath depth. The comparison was made for the eleven cycles described e a r l i e r and shows that the k i n e t i c model is at least as accurate as the more widely accepted thermodynamic models. Having established the a b i l i t y of the model to simulate process behaviour, R i c h a r d s 1 3 turned to using the model as a predictive t o o l . This yielded the major finding of the work-the prediction of a simultaneous increase in both fuming rate and e f f i c i e n c y by increasing the f r a c t i o n of coal entrained. This is a p a r t i c u l a r l y s i g n i f i c a n t result because i t implies that a 40 substantial ,improvement in furnace performance can be obtained through manipulation of i n j e c t i o n dynamics which c l e a r l y cannot be predicted by an equilibrium based model. 2.1.1.5 Summary In summary, the investigation of Richards 1" has conclusively proved that the zinc slag fuming process does not operate at internal equilibrium. In addition, a fundamentals based k i n e t i c model of the process has been shown to predict process behaviour at least as well as the thermodynamics based models. The significance of these findings in terms of potential for process improvement and understanding slag reduction in general is profound. 41 CHAPTER III OBJECTIVES AND SCOPE OF THE RESEARCH It i s clear from the l i t e r a t u r e that the zinc slag fuming process i s governed by k i n e t i c factors. The work of Richards et a±la,b has established conclusively that the process does not operate at internal equilibrium and that coal p a r t i c l e s entrained in the slag are the major s i t e for reduction. The objective of thi s thesis was to study the eff e c t of increased coal entrainment on process k i n e t i c s in an i n d u s t r i a l furnace. This was f a c i l i t a t e d through the use of a "high-pressure" coal stream d i s t i n c t from the normal coal supply to the furnace. The data taken during the high-pressure t r i a l s was analyzed to esta b l i s h the e f f i c a c y of increased coal entrainment quantitatively. The ov e r a l l project can be broken down into three phases. Phase one involves the design and a c q u i s i t i o n of a high-pressure coal d e l i v e r y / i n j e c t o r system which was capable of meeting spec i f i e d performance c r i t e r i a for a series of i n d u s t r i a l t r i a l s . Phase two was comprised of the i n d u s t r i a l t r i a l s with the high-pressure system. During the high-pressure runs slag composition, bath temperature, furnace operating parameters and high-pressure 42 coal system operating parameters were recorded in order to measure furnace performance quantitatively. Phase three focused on data analysis and involved modifying the mathematical of Richards et a l l a ' ' 3 in order to process furnace data and calculate entrained factors for the high-pressure coal. It must be emphasized that the f i n a n c i a l and human resources available for th i s project, conducted in an i n d u s t r i a l setting, were limited. Thus the scope of the research and the number of t r i a l s undertaken were necessarily r e s t r i c t e d to that of a preliminary investigation. 43 CHAPTER IV EXPERIMENTAL EQUIPMENT AND TECHNIQUES 4 .1 Industrial Equipment The a c q u i s i t i o n and design of the high-pressure coal d e l i v e r y / i n j e c t i o n system constituted a major part of the overall project because i t s successful performance was c r i t i c a l to achieving increased coal entrainment. The following sections present the steps taken for the ac q u i s i t i o n and design of the system. 4.1.1 High-Pressure Coal Deliverv/In-iection System Because the underlying objective of t h i s series of t r i a l s was to increase coal entrainment in the slag s i g n i f i c a n t l y and to increase reduction rates, a minimal amount of c a r r i e r a i r was to be used to avoid oxidation of the slag. This was a basic consideration in establishing design c r i t e r i a . 4.1.1.1 Design C r i t e r i a At f i r s t glance, i t would appear that the c o a l - s l u r r y delivery systems would be superior to the pneumatic delivery 44 systems owing to a lack of gaseous oxygen in the c a r r i e r f l u i d . However, in order to isolate the e f f e c t of increased coal entrainment due to high-pressure i n j e c t i o n , i t was essential to maintain as much s i m i l a r i t y as possible to the o r i g i n a l work of Richards et a l l a / b on conventional low-pressure a i r i n j e c t i o n . Therefore considerations were r e s t r i c t e d to pneumatic delivery systems and to minimize slag oxidation by a i r . This established the f i r s t o v e r a l l system design c r i t e r i o n - maximization of coal-t o - a i r loading. Under normal operating conditions c o a l - t o - a i r loadings are about 0.15 (wt. coal/wt. a i r ) for the No. 2 slag fuming furnace of Cominco. Injection dynamics also figure importantly in the design. It has been established previously l a that in the zinc slag fuming process the gas discharges from the tuyeres as discrete bubbles. It follows then, that under conditions when a bubble is attached to the tuyere, the following events must occur in order for a coal p a r t i c l e exiting the tuyere to become entrained in the slag: [ i ] The coal p a r t i c l e s must traverse the bubble unconsumed. [ i i ] Upon impingement with the slag, the coal p a r t i c l e s must have s u f f i c i e n t momentum to overcome the surface tension at the bubble slag interface. The p r o b a b i l i t y of a coal p a r t i c l e surviving the traverse under 45 normal operating conditions has been discussed in d e t a i l by R i c h a r d s 1 3 and, w i l l not be repeated here. With respect to improving entrainment, however, i t is obvious that gains could be realized through a decrease in the p r o b a b i l i t y of coal p a r t i c l e combustion and/or an increase in the p r o b a b i l i t y of coal p a r t i c l e entrainment. For example, one possible method would be to increase p a r t i c l e exit v e l o c i t i e s which has the advantage of both decreasing traverse time and increasing p a r t i c l e momentum. Another approach would be to increase coal p a r t i c l e mass while maintaining v e l o c i t y . Again thi s has the advantage of greater survival p r o b a b i l i t i e s and higher p a r t i c l e momentums. As a t h i r d alternative some combination could be used. However, due to p r a c t i c a l constraints and the desire to avoid the introduction of an additional variable, i t was decided that increasing coal p a r t i c l e size would not be feasi b l e . Therefore, the only viable means of increasing coal entrainment in the study was through the use of high exit v e l o c i t i e s . This e s t a b l i s h e d the second o v e r a l l design c r i t e r i o n - maximization of tuyere exit v e l o c i t y . For reference, under normal operating conditions a i r volume flow rates and tuyere exit diameters place coal p a r t i c l e exit v e l o c i t i e s at between 40-50 m/sec in the No.2 furnace. This i s calculated based on the assumption that the 46 coal p a r t i c l e s are t r a v e l l i n g 100% of the gas v e l o c i t y . Considering these system design c r i t e r i a further, i t is re a d i l y apparent that the desire for high stream exit v e l o c i t i e s (high volume flow rates) is inconsistent with the desire for high stream c o a l - t o - a i r loadings (low volume flow r a t e s ) . The solution to t h i s dilemma is to employ injectors having a small cross-sectional area. Q u a l i t a t i v e l y , t h i s established the t h i r d design c r i t e r i o n - minimization of cross-sectional area of the inje c t o r . The standard tuyeres on the No.2 furnace have an I.D. of 38.1 mm 2 (1.53 in.) r e s u l t i n g in a cross-sectional area of 11 cm (1.73 in ) . Having established the design c r i t e r i a q u a l i t a t i v e l y for the injector and delivery system, i t is possible to arrive at some quantitative s p e c i f i c a t i o n s bearing in mind p r a c t i c a l l i m i t a t i o n s of f a c i l i t i e s and time. In the i n i t i a l high-pressure t r i a l s i t was decided to increase the coal entrainment rate of No.2 furnace by no more than 100%. From the analysis of the Cominco No. 2 furnace by Richards et a l l a/b^ t h g c o a-^ entrainment rate for low-pressure i n j e c t i o n i s roughly 23 kg/min (50 lbs/min). Thus the rated maximum of the high-pressure feed system should be approximately 46 kg/min (100 lbs/min) for continuous coal delivery. In addition, i f the furnace is not well mixed, i t would be 47 necessary to i n s t a l l several high-pressure injectors at d i f f e r e n t locations, eg. two on either side of the furnace. Hence the i n j e c t i o n system had to be s u f f i c i e n t l y f l e x i b l e to f a c i l i t a t e t h i s option. I n t u i t i v e l y the operating pressure of the delivery system has to be high in order to overcome the large li n e losses associated with the high i n j e c t i o n v e l o c i t i e s and high coal-to-2 a i r loadings. A p r a c t i c a l l i m i t was 690 kN/m (100 psig). Turning to injector design, the overriding p r a c t i c a l considerations were compatibility with the e x i s t i n g tuyere design (the injectors were to be inserted into the furnace through the existing tuyeres), s i m p l i c i t y of design and the a v a i l a b i l i t y of construction materials. These requirements were met by using a length of standard straight pipe as an i n j e c t o r , see F i g . 4.1. The upper l i m i t to the inside diameter of the injector was 20.92 mm (0.824 in) based on the maximum diameter pipe which is accepted by the b a l l valve on the end of the tuyere (3/4 inch nominal, schedule 40S, s t e e l pipe, I.D. = 0.824 in, O.D. = 1.050 i n ) . The lower l i m i t to the injector I.D. was 6.83 mm (0.269 in) which was determined by the smallest pipe available (1/8 inch nominal, schedule 40S, s t e e l pipe, I.D. = 0.269 in, O.D. = 0.405 i n ) . From arguments presented e a r l i e r on injector cross-sectional area, however, the smallest diameter injector was needed. From storage hopper Upper f luidizer Continuous f luidizer Primary a i r -coa l Secondary V air 1 High - pressure injector / Standard tuyere / box / / /X) ) ) ) ) ! } I / 1 7 T J / / I * I 1  1 1 1 } 1 1 1 Standard tuyere Figure 4.1 Schematic diagram showing the pneumatic conveyor, the high-pressure injector and the standard low-pressure tuyere. ^ OB 49 F i n a l l y , because the e f f i c a c y of increased coal entrainment was to be established, i t was essential that the operating parameters, p a r t i c u l a r l y coal rates and a i r rates, of the high-pressure system be both measured and independently varied. It follows that the operating pressure and/or pressure d i f f e r e n t i a l s necessary to achieve these rates also had to be measured. 4.1.1.2 Summary of Design C r i t e r i a In summary, the design and a c q u i s i t i o n of the high-pressure coal d e l i v e r y / i n j e c t o r system was based on the following q u a l i t a t i v e design c r i t e r i o n . [ i ] Maximization of stream coal to a i r loading -s i g n i f i c a n t l y greater than 0.15 (wt coal/wt a i r ) , [ i i ] Maximization of injector exit v e l o c i t i e s -s i g n i f i c a n t l y greater than 40-50 m/sec. These c r i t e r i a could be met, within the framework of l o c a l constraints by the following s p e c i f i c a t i o n s for the coal d e l i v e r y / i n j e c t i o n system: [i ] coal feed rate - 45.5 kg/min. [ i i ] multiple injector c a p a b i l i t y 50 [ i i i ] high-pressure a i r flow - upper l i m i t approximately 690 kN/m2 [iv] small injector cross-sectional areas - i n i t i a l t r i a l s with 6.83 mm I.D. pipe [v] variable operating parameters - coal rates, a i r rates, and pressures [vi] measurement of operating parameters - coal rates, a i r rates, and pressures 4.1.1.3 System Selection By comparing the design c r i t e r i a with the l i t e r a t u r e on soli d s i n j e c t i o n systems (see Chapter II pgs. , p a r t i c u l a r l y Table 2.1), the choice of coal delivery system for the high-pressure t r i a l s becomes obvious. It is clear that the blow-tank or a i r - i n t o - m a t e r i a l systems are best suited to meet the requirements of thi s project because of the highest operating pressures and co a l - t o - a i r loadings. Unfortunately, before this project was i n i t i a t e d , the system selection was based more on the overriding economics and a p e r i s t a l t i c - t y p e hose pump was purchased by Cominco Ltd. As an anticipated, the system performance was shown to compromise design objectives to such an extent that i t proved unacceptable for the high-pressure t r i a l s . Nevertheless, in spite of the ove r a l l f a i l u r e of the hose pump tests, some valuable experience on the transporting of pulverized coal was gained. 51 Having established that the general c l a s s i f i c a t i o n of pneumatic delivery system would have to be a blow-tank system, i t now became a question of s p e c i f i c a t i o n s verses economics. Again compromises had to be made. I n i t i a l l y there was considerable incentive to build a blow-tank system using surplus equipment and on s i t e personnel. However, because of the lack of suitable pressure vessels and the regulations governing modifications to pressure vessels, i t was decided that t h i s route would be too c o s t l y and time consuming. It was determined that the best solution under the circumstances was rental of a blow-tank system, and i n s t a l l a t i o n with on s i t e personnel and equipment. Again t h i s was primarily a f i n a n c i a l decision. Not s u r p r i s i n g l y , the rental system compromised some of the previously established s p e c i f i c a t i o n s . Of p a r t i c u l a r concern, for example, was the absence of control over c o a l - t o - a i r loading, and the lack of a b i l i t y to measure coal and a i r delivery rates. In general however, the system, according to s p e c i f i c a t i o n s was capable of meeting the more c r i t i c a l c r i t e r i a of delivery capacity and operating pressures. The performance of the high-pressure pneumatic conveyor e s s e n t i a l l y dictated the duration and scope of the high-pressure t r i a l s . Thus i t is f e l t that some detailed discussion of the operation of t h i s system would be valuable. 52 4.1.1.4 The High-Pressure Delivery System; Operation The pneumatic conveyor e s s e n t i a l l y consisted of two pressure vessels mounted v e r t i c a l l y , one on top of the other, see F i g . 4.2. The lower pressure vessel or continuous f l u i d i z e r , continuously delivered material at a r e l a t i v e l y constant pressure. Continuous operation is achieved by p e r i o d i c a l l y c y c l i n g the upper f l u i d i z e r between atmospheric pressure (where i t i s charged with the material to be conveyed) and a pressure s l i g h t l y above that of the continuous f l u i d i z e r (where i t charges the continuous f l u i d i z e r ) . By adjusting the rate at which the upper f l u i d i z e r cycles according to the rate of which material is being conveyed, i t is possible to maintain continuous delivery. Starting at time t=0 with the delivery system empty and at atmospheric pressure, i t is possible to break down a complete cycle into the following steps (see Fi g . 4.2 for reference): [ i ] t=0; The a i r to the upper f l u i d i z e r is switched on (Valve No. 6) and at the same time the dome valve (No. 5) moves up and i s closed. The pressure in the lower f l u i d i z e r begins to increase u n t i l i t reaches P^, the delivery pressure - a i r is being conveyed. 53 [ i i ] t=t^; The b u t t e r f l y valve (No. 1) opens and pulverized coal f a l l s under gravity from a storage hopper through the conveyor feed funnel into the upper f l u i d i z e r . [ i i i ] t=t 2; The b u t t e r f l y valve closes - the upper f l u i d i z e r is f u l l . [iv] t=t^; The a i r to the upper f l u i d i z e r i s switched on (Valve No. 3) and simultaneously the dome valve (No. 2) closes - the pressure in the upper f l u i d i z e r begins to increase. When the pressure has reached approximately the dome value (No. 5) opens. The pressure in the upper f l u i d i z e r continues to r i s e u n t i l i t reaches P 2 (several psi greater than P^) - material is being conveyed to the lower f l u i d i z e r . At approximately the same time material begins to be conveyed from the lower f l u i d i z e r . [v] t=t^; The a i r to the upper f l u i d i z e r i s switched off (Valve No. 3) and simultaneously the vent valve (No. 4) opens. Almost immediately dome valve (No. 5) closes. The pressure in the upper f l u i d i z e r drops to atmospheric and the dome valve (No. 2) 54 opens. The continuous f l u i d i z e r is now approximately half f u l l and continues to convey material at P^. [vi] t=t(.; The b u t t e r f l y valve (No.l) opens and the cycle is repeated as from time t=t^. From a l o c a l i z e d control panel the operator has control over upper and continuous f l u i d i z e r pressures, P 2 and P^ respectively, the timers which dictate t^- t^ ( b u t t e r f l y valve open or charge time), t ^ - t^ (upper f l u i d i z e r a i r on time) and, t ^ - t^ (time to cycle repeat). With respect to i n j e c t i o n dynamics, i t is interesting to note that there is a s l i g h t v a r i a t i o n in conveying pressure and co a l - t o - a i r loading over the course of a complete cycle. This behaviour is a consequence of the mechanics of the cycle. In p a r t i c u l a r , the v a r i a t i o n in conveying pressure and coal-to-air loading that are associated with switching the conveying a i r from P^to P 2 during those periods when the upper f l u i d i z e r a i r is on and is e f f e c t i v e l y the source of conveying a i r . In order to measure high-pressure a i r and coal rates, which is fundamental to the objectives of t h i s project, i t was necessary to instrument the rental d e l i v e r y system without modifying i t . With respect to coal rate measurement, th i s was achieved by placing a small intermediate feed hopper on a 0 -55 From feed hopper # 4 ) Vent valve upper f lu id izer to vent Butterfly valve ( # |) Dome valve ( # 2 ) lower f lu id izer Upper fluidizer ( # 3 ) air Dome valve ( # 5 ) continuous fluidizer Continuous fluidizer air ( # 6 ) to H.R injector F i g u r e 4 .2 Schematic diagram showing the pneumatic conveyor i n d e t a i l 56 227.3 kg (0-500 l b ) t e n s i o n load c e l l to measure the c o a l charged each c y c l e ( t j - t ^ ) . Coal vented from the upper f l u i d i z e r (at t,.) was captured i n a mod i f i e d f o r t y - f i v e g a l l o n drum. T h i s was necessary because the r e n t a l system had a tendency to vent p u l v e r i z e d c o a l through v a l v e No. 4, see F i g . 4.2, when the upper f l u i d i z e r was d e p r e s s u r i z e d . By measuring the amount of c o a l charged t o the d e l i v e r y system and the amount of c o a l vented, the net amount r e p o r t i n g t o the furnace c o u l d be determined. The t o t a l h i g h - p r e s s u r e a i r flow r a t e to the d e l i v e r y system was measured with a 0 - 2.83 Nm3 /min (0 - 100 Scfm) o r i f i c e flow meter (Or i - F l o w Meter, Tokyo Ke i s o L t d . ) . High-pressure a i r t o the d e l i v e r y system was s u p p l i e d by a 11.3 Nm3/min (400 Scfm) p o r t a b l e d i e s e l compressor. T h i s proved 2 to be the o n l y means of a t t a i n i n g a r e l i a b l e source of 690 kN/m (100 p s i g ) a i r . U n f o r t u n a t e l y , moisture l e v e l s i n the a i r supply were high and e v e n t u a l l y n e c e s s i t a t e d the use of a 11.3 Nm3/min (400 Scfm) i n l i n e water t r a p . The p u l v e r i z e d c o a l from the Cominco storage hopper was passed through a 2 ram (0.08 inch) screen p r i o r t o e n t e r i n g the int e r m e d i a t e feed hopper and c h a r g i n g t o the d e l i v e r y system. T h i s was necessary due to the presence of l a r g e lumps i n the p u l v e r i z e d c o a l which c o u l d not be t o l e r a t e d by the sm a l l diameter h i g h - p r e s s u r e i n j e c t o r . 57 The transport pipe consisted of two sections of 31.75 mm (1.25 inch) I.D. drawn-over-mandrel steel tubing, approximately 8 m (26 feet) in t o t a l length, and a small section of f l e x i b l e 25.4 mm (1 inch) I.D. rubber hose, approximately 1.2 m (4 feet) in length. In addition, three 90°, high-wear, material-on-material elbows were employed. The f l e x i b l e rubber hose was used prior to coupling with the high pressure injector to f a c i l i t a t e rapid i n s t a l l a t i o n and removal of the injector assembly. 4.2 Experimental Techniques As discussed, the underlying objective of this project was to establish the e f f i c a c y of increased coal entrainment qua n t i t a t i v e l y . In order to achieve t h i s , s p e c i f i c information on furnace behaviour as function of operating conditions during the high-pressure t r i a l s must be obtained. The following sections discuss the experimental techniques used to obtain this i n d u s t r i a l data. 4.2.1 Industrial Tests It was decided that in the i n i t i a l t r i a l s , the high-pressure coal would be introduced part of the way through a "normal fuming cycle", and that simultaneously with the onset of high-pressure i n j e c t i o n , the normal low-pressure coal rate would be reduced by 58 an amount approximately equivalent to the high-pressure coal i n j e c t i o n rate. It was reasoned that t h i s procedure would best reveal the e f f e c t of increased coal entrainment. Furthermore, i t was also decided that only one high-pressure injector would be used in these t r i a l s because th i s would allow valuable experience to be gained, both in terms of furnace behaviour and high-pressure system operation. Since the zinc slag fuming process operates on a batch basis, the e f f e c t of increased coal entrainment on furnace behaviour can be determined from the manner in which the slag composition changes as a function of the high and low-pressure coal and a i r rates. 4.2.1.1 Slag Sampling Of pa r t i c u l a r interest in the t r i a l s were the rates of change of zinc, lead, ferrous and f e r r i c iron composition and bath temperature. To obtain information on slag composition, a technique employed previously by Richards l a was adopted. A 1.5 m (4.95 f t ) bar was inserted into the slag through a tuyere then was withdrawn 3 - 4 seconds later and quenched with water. To ascertain the degree of longitudinal bath mixing, the ef f e c t of in j e c t i n g the high-pressure coal in a lo c a l i z e d area was measured by taking two slag samples simultaneously; one in 59 the v i c i n i t y of the high-pressure injector and the other as far removed from i t as possible. This procedure was followed only during the i n i t i a l run. In subsequent te s t s , a l l slag samples were taken from tuyeres located d i r e c t l y opposite from the high-pressure i n j e c t o r . Under normal operating conditions the temperature in Cominco Ltd.'s No. 2 slag furnace is monitored continuously with a thermocouple placed in the slag through a non-operational tuyere. For the high-pressure coal runs t h i s temperature data was retrieved from the charts on which i t was routinely recorded. Furnace charge d e t a i l s , in p a r t i c u l a r t o t a l weight and constit u t i o n with respect to hot and cold additions, were obtained from plant personnel. 4.2.1.2 Operating Procedures Two sets of operating parameters had to be recorded over the course of a high-pressure run. The f i r s t group encompassed the measurement of the standard furnace operating parameters which include the normal low-pressure coal rate and the low-pressure a i r - b l a s t volume flow rates. In order to measure the normal coal rate, the No. 2 furnace Omega coal feeder had to be ca l i b r a t e d . This was achieved by recording feeder belt current (measured in percent) over measured time intervals s t a r t i n g with a f u l l 60 storage hopper. The amount of coal needed to r e f i l l the hopper then was measured. Once cali b r a t e d , a record was kept of No. 2 Omega feeder settings for the high-pressure runs and these were converted to coal rates. Blast volume flow rates are recorded under normal operating conditions. The second set of operating conditions pertained to the high-pressure coal d e l i v e r y system which included coal and a i r inj e c t i o n rates and pressures. High-pressure coal rates, as mentioned, were determined by measuring the amount of material charged to the delivery system and the amount of coal vented from i t as described e a r l i e r . The instrumented intermediate hopper was f i l l e d from the larger Cominco storage hopper during those periods when the delivery system was not charging. After f i l l i n g , the intermediate hopper then was used to charge the deli v e r y system. This procedure was repeated for every cycle. The output from the load c e l l on the intermediate hopper was recorded on a s t r i p chart recorder. The amount of coal vented from the delivery system per cycle was collected and also recorded. From the load c e l l output the t o t a l amount charged to the delivery system over the course of high-pressure run could be determined. The difference between th i s amount and the amount vented then could be divided by the length of the run to determine the average high-pressure coal i n j e c t i o n rate. The average amount of high-pressure conveying a i r injected into the furnace with the coal was assumed to be equal to the high-61 pressure a i r flow rate measured during a period when only the continuous f l u i d i z e r a i r supply was on ( t ^ - t ^ ) , see Section 4.1.1.4. 4.2.1.3 Summary: Industrial Testing Procedure The procedure during a high-pressure coal run can be summarized as follows: [ i ] After the furnace had been f u l l y charged and allowed to reach a temperature of 1280-1300° C, normal fuming operation was started. Slag sampling was carried out every 5-10 minutes. [ i i ] After 15-20 minutes of normal fuming, high-pressure coal was injected. Simultaneously, the low-pressure coal rate was reduced and the slag subsequently sampled every 5 minutes. [ i i i ] Following the termination of high-pressure coal i n j e c t i o n , the slag was sampled for a period of 10-15 minutes. [iv] A l l pertinent operating data was co l l e c t e d . Unfortunately, f a i l u r e of the high-pressure coal delivery system generally dictated the length of the high-pressure runs. 4.2.2 Chemical Analysis of Slag Samples 62 Chemical analysis of the slag samples was performed by the assay labs of Cominco. Assays for a l l metals (Zn, Pb 7 Fe), oxides (CaO, s i 0 2 ' A 1 2 ° 3 ^ a n d n o n ~ m e t a l (S) were done by X-ray emission spectrography. Analysis for ferrous iron was carried out by a wet chemical oxidative technique using potassium dicro-mate. The procedure i s summarized elsewhere. 1 3 F e r r i c iron reported in this thesis is the difference between t o t a l iron determined by X-ray analysis and ferrous iron determined by wet chemistry. The e f f e c t of slag sulphide iron on the ferrous iron assay has been investigated elsewhere. 1 3 Ferrous iron assays reported in this thesis are assumed not to require correction for sulphur. 4.2.3 Uncertainty The uncertainty in slag assays has been estimated by R i c h a r d s 1 3 through a process of submitting sixteen duplicate samples for analysis at the Cominco assay laboratory. It is assumed that laboratory procedures and equipment have not changed appreciably and therefore that the uncertainties for the assays presented in t h i s thesis are the same as those found by Richards, see Table 4.1. TABLE 4.1 Estimated U n c e r t a i n t y of A n a l y s i s Average Absolute D i f f e r e n c e Average R e l a t i v e D i f f e r e n c e Est imated U n c e r t a i n t y Zn SiO, CaO Pe Pe Fe 2 + 3 + 0.25 pet 3.00 pet 0.7 4 pet 0.74 pet 0.39 pet 0.71 pet 7.8 pet 10.0 pet 4.1 pet 2.6 pet 1.9 pet 34.0 pet ± 4 pet pet pet i 1.3 pet pet ± 5 ± 2 i l - 17 pet CHAPTER V 64 EXPERIMENTAL RESULTS AND PRELIMINARY ANALYSIS Problems with the high-pressure delivery system limited the number of t r i a l s to three runs with a single injector. The results from these runs w i l l be discussed in the following chapter. 5.1 Results Assays of the slag samples and operating conditions for the three high-pressure runs are presented in Figs. 5.1 - 5.12 and are tabulated in Appendix I. It should be noted that problems with the high-pressure coal supply were encountered in two of the three t r i a l s . The length of the period of high-pressure i n j e c t i o n in Runs 2 and 3 was cut short by delivery system f a i l u r e . Fortunately, operating parameters remained within acceptable l i m i t s u n t i l f a i l u r e occurred. 5.2 Preliminary Analysis and Discussion Before the eff e c t of high-pressure coal i n j e c t i o n on furnace performance can be discussed i t is necessary to evaluate the 65 high-pressure i n j e c t i o n dynamics to estab l i s h i f improvements in key parameters have in fact been achieved. Of primary importance are the magnitudes of high-pressure injector exit v e l o c i t y and stream co a l - t o - a i r loading. 5.2.1 High-Pressure Injection Dynamics High-pressure co a l - t o - a i r loadings and stream exit v e l o c i t i e s have been estimated for three high-pressure runs and are presented in Table 5.1. Loadings and v e l o c i t i e s t y p i c a l of the standard low-pressure tuyeres used in the Cominco No. 2 fuming furnace also are presented in Table 5.1 for comparison. High-pressure injector loadings are expressed in terms of weight of coal conveyed per weight of conveying a i r . Stream exit v e l o c i t i e s were calculated assuming the coal to be t r a v e l l i n g at the a i r v e l o c i t y . The r e s u l t i n g increases in stream exit momentum per unit area for the high-pressure tuyere r e l a t i v e to the standard low-pressure tuyeres have been calculated for the three high-pressure runs and are included in Table 5.1. Based on the magnitude of the momentum increases (factor of 171 for Run 1, 249 for Runs 2 and 222 for Run 3), i t i s evident that a considerable gain in the drivi n g force for slag entrainment (momentum) has been achieved with the high-pressure i n j e c t o r . 66 TABLE 5.1  High-Pressure I n j e c t i o n Dynamics Run No. Coal r a t e (kg/min) A i r r a t e (NraVmin) Coal to (wt./wt.) Estimated c o a l e x i t v e l o c i t y (m/s) "Estimated momentum inc r e a s e Run 1 12 0.37 25 ~114 ~171x Run 2 18 0.37 36 ~116 ~249x Run 3 16 0.37 32 ~115 ~222x Normal Operation (low p r e s s u r e tuyere) / / 0.16 ~42 / "These numbers r e p r e s e n t an estimate f o r the i n c r e a s e i n momentum per u n i t area f o r the high-pressure i n j e c t o r r e l a t i v e t o the standard low-pressure t u y e r e s . 67 Having established that the in j e c t i o n dynamics are consistent with the o r i g i n a l objectives, the question of assessing the ef f e c t of high-pressure i n j e c t i o n on furnace performance can now be addressed. At the onset of the project i t was rea l i z e d that this would not be an easy task. In pa r t i c u l a r , there is a problem associated with defining "normal" furnace performance against which the high-pressure runs can be compared. O r i g i n a l l y , t h i s problem was to be avoided by introducing the high-pressure coal part of the way through the proper fuming stage of the fuming cycle (see Chapter IV for outline of thi s procedure). By adopting this procedure i t was reasoned that a di r e c t comparison between furnace performance during normal operation and high-pressure i n j e c t i o n could be made on a run by run basis. The comparison simply would be an exercise of cal c u l a t i n g the corresponding fuming rates (% Zn per min) and e f f i c i e n c i e s (kg zinc fumed per kg coal injected) which are normally used to measure furnace performance. Unfortunately, p r a c t i c a l constrains prevented th i s procedure from being f u l l y implemented. As a r e s u l t , normal fuming operation and performance must be c a r e f u l l y defined to permit a v a l i d comparison to be made. 68 5.2.2 Normal Fuming Practice at Cominco At the T r a i l Smelter of Cominco under "normal" conditions ( see Appendix I, Table I for approximate slag composition with 2+ 3 + respect to Pb, Fe , Fe , CaO and S i 0 2 ) , the fuming cycle can be broken down into two stages, as previously described in Chapter I. During Stage 1, the heating period, the secondary blast rate and coal rate are generally in the range of 340-400 Nm3/min (12,000-14,000 Scfm), and 46-55 kg/min (100-120 lbs/min), respectively. T y p i c a l l y , this stage ends when the bath temper-ature reaches 1300-1325°C. Not s u r p r i s i n g l y , the length of the heating period is very dependent on the charge constitution with respect to hot and cold additions. For example, under conditions when there is a 100% hot charge, th i s period may last 15-20 minutes, whereas for a 50:50 hot-cold charge, the period may l a s t upward of 75 minutes-half a normal fuming cycle. In thi s s i t u a t i o n , cold granulated slag may be continually charged to the furnace over the course of the heating period. Bath zinc concentrations at the end of the heating period are t y p i c a l l y 16-14 wt%. However, when the heating period is long, concentrations may f a l l to 8-10 wt%. At the onset of the second stage, proper fuming, the secondary blast rate is fixed at 340 Nm3/min (12,000 Scfm) and the coal rate i s raised to between 64 and 68 kg/min (140-150 69 lbs/rain). During t h i s stage, for a 100% hot charge, fuming rates are t y p i c a l l y about 0.082 % Zn/min. and e f f i c i e n c i e s are approximately 0.7 kg Zn/kg coal (values are an average based on one month of operation for 100% hot charges only, and were calculated from the zinc assays; zinc fuming e f f i c i e n c i e s were calculated assuming a constant i n i t i a l bath weight). It should be noted that fuming rates and e f f i c i e n c i e s calculated in this manner appear to be sensitive to the temperature of the charge. For example, for charges consisting of 30% and 50% cold material, the fuming rate f a l l s to 0.067 and 0.055 % Zn/min. and e f f i c i e n c i e s f a l l to 0.55 and 0.48 kg zinc/kg coal respectively. As the proportion of cold material charged to the furnace increases, the calculated fuming rate and e f f i c i e n c y decrease. This e f f e c t is believed to be due to the manner in which these quantities are calculated. Clearly, i f any s o l i d (cold) z i n c - r i c h material melts during the fuming period, i t w i l l tend to influence the slag assay because i t s zinc content is greater than that of the bulk of the bath. The net result w i l l be an increase in the zinc concentration of the bath and a decrease in the fuming rate and e f f i c i e n c y based on zinc assays. The same should also hold for lead i f the melting material is r i c h in lead. Taken one step further, t h i s implies that the actual fuming rate (kg zinc reporting to fume) and fuming e f f i c i e n c y (kg zinc 70 reporting to fume/kg coal injected) during the period of proper fuming should be insensitive to charge temperature for a given set of operating conditions. This behaviour is more in accordance with what would be expected. As a f i n a l note, based on the preceding discussion i t appears necessary to include information on charge temperature prior to quantifying normal furnace performance. Having established normal operation and performance for the Cominco No. 2 fuming furnace, i t is possible to proceed with the preliminary analysis of the high-pressure runs. 5.2.3 High-Pressure Coal Injection Preliminary Analysis In Run 1, the charge to the furnace was 50,909 kg (56 tons), and consisted of 50% hot slag and 50% cold crushed material. As anticipated, t h i s resulted in a prolonged heating period and a low bath zinc concentration (approximately 9 wt %) at the onset of the proper or normal fuming period. Of par t i c u l a r concern, however, is the eff e c t that the charge temperature had on fuming rate and e f f i c i e n c y . This must be considered when interpreting the r e s u l t s . Analysis of Run 1 is complicated further by d i f f i c u l t i e s which were experienced with charging the high-pressure delivery 71 system in the i n i t i a l stages of high-pressure i n j e c t i o n . As a re s u l t , from 15 to 25 minutes elapsed time, the t o t a l coal rate to the furnace (low-pressure plus high-pressure) ranged between 4.5 and 9 kg/min. (10-20 lbs/min) less than normal for the fuming stage, see F i g . 5.4. In addition, because the bath temperature leveled off during t h i s period, the low-pressure coal rate was raised to 61.4 kg/min (135 lbs/min) for approximately five minutes. The simultaneous ef f e c t of a l l of these factors complicates interpretation of the results in Run 1. Q u a l i t a t i v e l y , there does not appear to be any consistent increase in either the zinc or lead elimination rates with the onset of high-pressure i n j e c t i o n , see F i g . 5.1. It i s worth noting, however, that the maximum rate of decrease of zinc concentration coincides exactly with the point at which steady state high-pressure inje c t i o n was achieved. If the f e r r i c and ferrous iron assays are considered, see F i g . 5.2, the ef f e c t of high-pressure coal i n j e c t i o n is more c l e a r l y seen. There is a general increase in the ferrous iron l e v e l and a corresponding decrease in f e r r i c l e v e l coincident with steady high-pressure i n j e c t i o n . This behaviour is consistent with increased coal entrainment and more reducing conditions in the slag. Figure 5.1 Zn and Pb p r o f i l e s for Run 1 WT. P E R C E N T F E 2 + , F E 3 + JO vj c o -3 b CO o -b to J p b • • • (B n • • • • -B- -B- • • LEGEND • = NORMAL COAL(LBS/MIN) 0=HIGH P.COAL(LBS/MIN) A = 0.1*SECOND. BLAST. VOL.(NM3/MIN) + =O.WEMP. SECBLAST(C) X= OXY.ENR ICHMENT A A-o • IIIIII n * i • 11 -©-• A • O •X 0.0 10.0 20.0 30.0 40.0 50.0 60.0 ELAPSED TIME(MIN) 70.0 80.0 90 Figure 5 . 4 Furnace operating conditions for Run 1 76 The temperature p r o f i l e , see F i g . 5.3, exhibits a roughly continuous decline during high-pressure i n j e c t i o n . It should be noted that the maximum rate of temperature drop coincided with the short term increase in the low-pressure coal rate and not with the period of maximum zinc decrease. In quantitative terms, a fuming rate and e f f i c i e n c y have been calculated for the period of steady-state, high-pressure i n j e c t i o n (30-65 minutes elapsed time) in Run 1, see Table 5.2. These values can be compared with the average fuming rate and e f f i c i e n c y for a 50:50 hot-cold charge - also shown in Table 5.2. From this comparison i t is clear that substantial improvements have been achieved with high-pressure i n j e c t i o n (factor of 1.69 increase in fuming rate and 1.68 increase in fuming e f f i c i e n c y ) . It should be noted that the results of the double sample tests (procedure described e a r l i e r in Chapter IV) indicate that there is a high degree of longitudinal bath mixing. The differences in the zinc concentrations of the double samples (see Appendix I, Table 1) are within -0.04 weight percent. Therefore, i t can be concluded that the slag samples taken in the v i c i n i t y of the high-pressure tuyere are representative of the entire bath composition. It i s worth mentioning that in F i g . 5.1 there is some evidence to suggest that the melting of zinc a lead-rich 77 TABLE 5.2  Fuming Rates and E f f i c i e n c i e s Run No. Fuming r a t e %Zn Fuming e f f i c i e n c y kg. Zn *Estimated i n c r e a s e i n fuming r a t e *Estimated i n c r e a s e i n fuming e f f i c i e n c y P r e d i c t e d fuming e f f i c i e n c y kg.Zn+ka.Pb min kg.coal kg.coal Run 1 0 .10 0.635 ~1.69x ~1.68x 0.77 **0.92 Run 2 0.124 1.02 ~1.85x ~1.86x 1.2 Run 3 0.150 1.27 ~1.85x ~1.89x 1.4 * These numbers r e p r e s e n t i n c r e a s e s i n fuming r a t e r e l a t i v e to normal furnace performance. ** T h i s value was obtained from the model u s i n g a term to account f o r the m e l t i n g of z i n c - r i c h m a t e r i a l . 78 m a t e r i a l was o c c u r r i n g throughout the run. In p a r t i c u l a r , there i s a n o t i c e a b l e decrease i n the z i n c and le a d e l i m i n a t i o n r a t e s toward the end of the run which was unaccompanied by a change i n the furnace o p e r a t i n g parameters. With regard to the behaviour of l e a d , there i s some j u s t i f i c a t i o n f o r q u e s t i o n i n g the v a l i d i t y of t h i s r e s u l t i n l i g h t of the r e l a t i v e u n c e r t a i n t i e s i n assay r e s u l t s a t the low c o n c e n t r a t i o n s i n v o l v e d . N e v e r t h e l e s s , both of these o b s e r v a t i o n s are c o n s i s t e n t with the a d d i t i o n of z i n c and lead to the bulk of the bath, and t h e r e f o r e , the melting of z i n c and l e a d - r i c h m a t e r i a l . As a f i n a l note, steam g e n e r a t i o n i n No. 2 b o i l e r i n c r e a s e d s u b s t a n t i a l l y d u r i n g h i g h - p r e s s u r e i n j e c t i o n i n Run 1. In f a c t , l e v e l s i n c r e a s e d t o the p o i n t where they exceeded the r a t e d maximum of the b o i l e r . I t was p o s t u l a t e d t h a t t h i s was due to the high-fuming r a t e s and/or e x c e s s i v e amounts of high-pressure c o a l s t r i p p i n g through the bath and combusting i n the b o i l e r . T h e r e f o r e , i t was decided t h a t low-pressure c o a l r a t e s would have to be lower d u r i n g h i g h - p r e s s u r e i n j e c t i o n i n subsequent runs. In Run 2, the charge to the furnace was 52,727 kg. (58 tons) and c o n s i s t e d of 70% hot s l a g and 30% hot pot s h e l l . Not s u r p r i s i n g l y , t h i s charge r e q u i r e d a r e l a t i v e l y s h o r t h e a t i n g p e r i o d and as a r e s u l t , bath z i n c c o n c e n t r a t i o n s were r e l a t i v e l y high - approximately 16 wt% a t the onset of the fuming p e r i o d . T h i s made Run 2 p a r t i c u l a r l y w e l l s u i t e d f o r t e s t i n g the high-79 pressure i n j e c t i o n . U n f o r t u n a t e l y , problems with i n t e r p r e t a t i o n of the r e s u l t s s t i l l p r e v a i l e d . In t h i s run, the problem was a s s o c i a t e d with the magnitude of the c o a l r a t e d u r i n g the p e r i o d of normal fuming pr e c e d i n g the s t a r t of high-pressure i n j e c t i o n , see P i g . 5.8. Although not c o n s i d e r e d to be o x i d i z i n g , the low-pressure c o a l r a t e was d e f i n i t e l y low i n comparison with those more r e p r e s e n t a t i v e of normal fuming p r a c t i c e . As a r e s u l t , furnace performance d u r i n g high-pressure i n j e c t i o n c o u l d not be compared q u a n t i t a t i v e l y to the i n i t i a l p e r i o d of low-pressure fuming. I t i s i n t e r e s t i n g to note that t h i s problem was a d i r e c t r e s u l t of the more c o n s e r v a t i v e approach taken by p l a n t operators i n response to the problems with e x c e s s i v e steam g e n e r a t i o n i n Run 1. Q u a l i t a t i v e l y , i n Run 2 there was a c o n s i d e r a b l e i n c r e a s e i n the e l i m i n a t i o n r a t e s of both z i n c and lead with the onset of high- p r e s s u r e c o a l i n j e c t i o n , see F i g . 5.5. T h i s i s p a r t i c u l a r l y e v i d e n t with r e s p e c t to the behaviour of l e a d . In c o n t r a s t , i f the behaviour of the f e r r i c - f e r r o u s couple i s c o n s i d e r e d , see F i g . 5 .6, there appears to be a s l i g h t decrease i n the g e n e r a t i o n r a t e of f e r r o u s i r o n with h i g h - p r e s s u r e i n j e c t i o n . T h i s i s i n d i r e c t c o n t r a d i c t i o n with the behaviour observed i n Run 1, and appears to be i n c o n s i s t e n t with c o n d i t i o n s becoming more reducing i n the s l a g . I n t e r e s t i n g l y , the d i f f e r e n c e i n low-pressure c o a l WT. P E R C E N T ZN,PB 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 p b 08 WT. PERCENT FE2+,FE3+ p J b T8 Z8 o d IT) o d o ID o _ -e--e- -B- -s-•e-r -s- -B LEGEND • = NORMAL C0AL(LBS/MIN) 0=HIGH P.COALrLBS/MIN) A=0.1*SEC0ND. BLAST. V0L.(NM3/MIN) + = 0.1*TEMP. SECBLAST(C) X= OXY.ENRICHMENT 10.0 30.0 40.0 50.0 60.0 ELAPSED TIME(MIN) 90.0 Figure 5.8 Furnace operating conditions for Run 2 oo 84 r a t e s between Runs 1 and 2 pr o v i d e s some i n s i g h t i n t o t h i s seemingly c o n t r a d i c t o r y behaviour. T h i s i s worthy of f u r t h e r d i s c u s s i o n before proceeding with a n a l y s i s of the r e s u l t s . F o l l o w i n g from the mathematical model of Richards et a l l a , b the mechanism f o r g e n e r a t i o n of f e r r o u s i r o n i s r e d u c t i o n of f e r r i c i r o n by e n t r a i n e d c o a l , and the mechanism f o r consumption i s o x i d a t i o n by the tuyere gas column. Under normal o p e r a t i n g p r a c t i c e , the r a t e s a t which r e d u c t i o n and o x i d a t i o n of f e r r o u s i r o n proceed are i n v e r s e l y r e l a t e d . I t i s im p o s s i b l e t o envisage a s i t u a t i o n where one i s not i n c r e a s e d a t the expense of the other. However, i f the model i s c o r r e c t , with a second h i g h -pressure c o a l source, t h i s should no longer be the case. There i s evidence t o support t h i s i n f e r e n c e i n Run 2. In t h i s case, r e d u c t i o n was i n c r e a s e d d u r i n g high-pressure i n j e c t i o n ( i n c r e a s e d z i n c and le a d e l i m i n a t i o n r a t e s ) and s i m u l t a n e o u s l y , o x i d a t i o n was i n c r e a s e d i n the tuyere gas column ( i n c r e a s e i n the r a t e of ge n e r a t i o n of f e r r i c i r o n ) . The f a c t t h a t t h i s behaviour was not observed i n Run 1 i s probably due to the d i f f e r e n c e i n the low-pressure c o a l r a t e d u r i n g fuming. Moreover, by re d u c i n g the low-pressure c o a l r a t e from 54.5 kg/min (120 lbs/min) i n Run 1 to 47.7 kg/,om (105 lbs/min) i n Run 2, the r a t e of o x i d a t i o n of f e r r o u s i r o n was in c r e a s e d by an amount which was not completely o f f - s e t by the h i g h e r - r e d u c t i o n r a t e . T h i s w i l l be borne out by the m o d e l l i n g a n a l y s i s presented i n a l a t e r Chapter. 85 Additional evidence for these mechanisms comes from the temperature p r o f i l e , see F i g . 5.7. Q u a l i t a t i v e l y , in Fig . 5.7 i t is clear that there i s a net decrease in the bath temperature during high-pressure i n j e c t i o n . This trend is in i t s e l f consistent with normal fuming practice. However, the consistency and magnitude of the temperature drop are less than that expected 4 4 with an equivalent fuming rate obtained under normal operation. This behaviour, in part i c u l a r the magnitude of the drop, is consistent mechanistically with increased ferrous iron oxidation which is a source of heat. The results of a quantitative analysis of furnace performance in Run 2 are presented in Table 5.2. The high-pressure fuming rate and e f f i c i e n c y obtained in Run 2 can be compared with average values representative of low-pressure operation for a 70:30 hot-cold charge, shown also in Table 5.2. Based on thi s comparison, substantial gains have again been realized with high-pressure i n j e c t i o n (factor of 1.85 increase in fuming rate and 1.86 increase in e f f i c i e n c y ) . In Run 3, the charge to the furnace was 58180 kg. (64 tons) and consisted of 100% hot slag. In t h i s p a r t i c u l a r run an attempt was made to avoid excessive generation of f e r r i c iron during the heating period (as in the case of Run 2) by maintain-ing a secondary blast rate of 340 Nm3/min (12,000 Scfm), see F i g . 5.12. As a r e s u l t , the heating period was a l i t t l e longer in 86 comparison with Run 2. The zinc concentration at the onset of normal fuming was approximately 13 wt.%, see F i g . 5.9.. Problems with interpretation of the results are again encountered because of the r e l a t i v e l y low low-pressure coal rate during the period of fuming prior to high-pressure i n j e c t i o n , see F i g . 5.12. Q u a l i t a t i v e l y , from Fig 5.9, a considerable increase in the elimination rates of both zinc and lead was observed at the i n i t i a t i o n of high-pressure i n j e c t i o n at 30 mins. elapsed time. The behaviour of the ferrous - f e r r i c couple, see F i g . 5.10, was consistent with observations in Run 2. The rate of increase in ferrous iron concentration was slowed during high-pressure i n j e c t i o n in comparison with the preceding period of low-pressure fuming. If the temperature p r o f i l e i s considered, see F i g . 5.11, the res u l t s are again consistent with expectations of a net temperature drop over the period of high-pressure i n j e c t i o n . Moreover, the magnitude of the drop, 50°C i s similar to that observed in Run 2 - small for the observed fuming rate. Both of these observations, the behaviour of the f e r r o u s - f e r r i c couple and bath temperature, give further support to the mechanism proposed by Richards et a l . 1 3 ' * 3 The re s u l t s of a quantitative analysis are summarized in Table 5.2. These can be compared with average values Figure 5.11 Temperature p r o f i l e for Run 3 oo o O O d o CO o_| m o d •e- -B-A A A A -A- A-)( )( X - )( )( —X-a—6—a—r- ,a—a--e- -B- •e-LEGEND • = NORMAL C 0 A L ( L B S / M I N ) 0 = H I G H P . C 0 A L ( L B S / M I N ) A= 0 .1*SEC0ND. B L A S T . V 0 L . ( N M 3 / M I N ) + =0.1»TEMP. S E C . B L A S T ( C ) X = OXY.ENR ICHMENT T —T 70.0 H—i!r"rt"6-80.0 0.0 10.0 20.0 T 30.0 40.0 50.0 60.0 ELAPSED TIME(MIN) 90.0 Figure 5 . 1 2 Furnace operating conditions for Run 3 10 o 91 representative of low-pressure fuming for a 100% hot charge, also presented in Table 5.2. Again, as with the two previous t r i a l s , substantial improvements in furnace performance have been achieved with high-pressure i n j e c t i o n (a factor of 1.83 increase in fuming rate and a 1.89 increase in fuming e f f i c i e n c y ) . Before proceeding with a summary of the preliminary analysis, i t must be emphasized that the quantitative comparisons made with normal low-pressure operation are based on average plant data which is assumed to best represent normal operation. Emphasis has been placed on maintaining as much s i m i l a r i t y between operating conditions pre v a i l i n g during high-pressure i n j e c t i o n and those runs used to establish normal operations. In pa r t i c u l a r , the importance of charge temperature has been considered. It is f e l t that in terms of a preliminary analysis, th i s is the only v a l i d approach possible. 5.3 Summary In summary, the preliminary analysis of the results in Runs 1 to 3 reveals that considerable improvement in both fuming rates and e f f i c i e n c i e s have been r e a l i z e d with high-pressure i n j e c t i o n . In Run 1, the zinc fuming rate and e f f i c i e n c y were increased by factors of 1.69 and 1.68 over normal operation, respectively. In Run 2, the increase was a factor of 1.85 and 1.86, while, in Run 3, the factors were 1.83 and 1.89, respectively. These results 92 are in d i r e c t contradiction to any predictions based on an equilibrium model. It remains to be established i f a model based on ki n e t i c s can account for these r e s u l t s . 93 CHAPTER VI MATHEMATICAL MODEL OF ZINC FUMING PROCESS AND  DISCUSSION OF MODEL FITTING As mentioned p r e v i o u s l y , the concept of high-pressure c o a l i n j e c t i o n i n the z i n c s l a g fuming process f o l l o w e d from the work of Richards e t a l l a , b and i n p a r t i c u l a r , t h e i r mathematical model of the proc e s s . I t i s l o g i c a l t h e r e f o r e , t h a t the Richards k i n e t i c model be used to f u r t h e r analyze the r e s u l t s of the three h i g h - p r e s s u r e runs i n an attempt t o p r e d i c t entrainment f a c t o r s f o r the high-pressure c o a l . In i t s present form the k i n e t i c model i s f u l l y capable of doing t h i s . The procedure would i n v o l v e c a l c u l a t i o n of entrainment f a c t o r s f o r the t o t a l combined (low-pressure and high-pressure) c o a l s upply to the furnace f o r each of the three high-pressure runs. T h i s i n f o r m a t i o n then would be used i n c o n j u n c t i o n with the i n d i v i d u a l c o a l r a t e s and an assumed low-pressure entrainment f a c t o r t o estimate the f a c t o r of h i g h - p r e s s u r e c o a l e n t r a i n e d . The low-pressure entrainment f a c t o r c o u l d be obtained by f i t t i n g the model t o data taken from the Cominco No. 2 furnace under normal low-pressure o p e r a t i n g c o n d i t i o n s . At the o u t s e t , however, i t was decided to modify the e x i s t i n g program by i n c l u d i n g a separate high-pressure c o a l supply d i s t i n c t from the normal low-pressure c o a l . I t was f e l t t h a t t h i s s e p a r a t i o n would s i m p l i f y the f i t t i n g procedure. Moreover, i t would serve to emphasize the d i f f e r e n t r o l e s of the two c o a l s u p p l i e s w i t h i n the furnace: one a c t i n g as a source of r e d u c t a n t , and the other as a source of heat. T h i s m o d i f i c a t i o n was l a r g e l y a book keeping e x e r c i s e and does not warrant f u r t h e r d i s c u s s i o n . The o p p o r t u n i t y d i d a r i s e however, to modify the Richards model i n a more fundamental manner. The area f o r p o t e n t i a l improvement was a s s o c i a t e d with the behaviour of l e a d i n the bath. The d e s i r e to i n c l u d e lead r e d u c t i o n i n a k i n e t i c model of the process was l a r g e l y p r e c i p i t a t e d by two f a c t o r s : f i r s t l y , the u n u s u a l l y high lead c o n c e n t r a t i o n a t the s t a r t of Run 3, and secondly, the d e s i r e to e l u c i d a t e the mechanisms of l e a d r e d u c t i o n i n g e n e r a l . With regard to the former, i t was f e l t t h a t a t a c o n c e n t r a t i o n of 3 wt%, the i n f l u e n c e of l e a d on z i n c fuming r a t e s and e f f i c i e n c i e s c o u l d no longer be n e g l e c t e d . T h e r e f o r e , i n order to complete a v a l i d a n a l y s i s of one t h i r d of the h i g h -pressure data (Run 3) the e x i s t i n g model would have to be modified to i n c l u d e lead r e d u c t i o n . The l a t t e r reason f o l l o w s from the emergence of the newer t e c h n o l o g i e s f o r the s m e l t i n g of l e a d c o n c e n t r a t e s . These t e c h n o l o g i e s , as i n d i c a t e d p r e v i o u s l y , r e l y h e a v i l y on s l a g c l e a n i n g stages and i n p a r t i c u l a r the removal of l e a d from high l e a d content s l a g s . I t i s e s s e n t i a l , t h e r e f o r e , t h a t the mechanisms surrounding the r e d u c t i o n and removal of lead be w e l l understood i n these p r o c e s s e s . With t h i s i n mind, i t was f e l t t h a t the i n c o r p o r a t i o n of lead r e d u c t i o n i n the z i n c fuming model would a i d i n e l u c i d a t i n g the mechanisms of l e a d r e d u c t i o n and removal i n g e n e r a l . 6.1 M o d i f i c a t i o n s to the Richards Model The f o l l o w i n g s e c t i o n s d i s c u s s the m o d i f i c a t i o n s made to the R i c h a r d s 1 3 k i n e t i c model of the p r o c e s s . 6.1.1. The K i n e t i c C o n c e p t u a l i z a t i o n of the Process As d i s c u s s e d p r e v i o u s l y , the key to R i c h a r d s 1 3 m e c h a n i s t i c breakdown of the z i n c fuming process i s the s e p a r a t i o n of the furnace i n t o two r e a c t i o n zones: the s l a g bath or r e d u c t i o n zone, and the tuyere gas column or o x i d a t i o n zone. Thus i t i s obvious t h a t the m o d i f i c a t i o n surrounding l e a d r e d u c t i o n w i l l be l i m i t e d t o the r e d u c t i o n zone and i n p a r t i c u l a r the c o a l p a r t i c l e - s l a g r e a c t i o n system developed by Richards et a l . * a'k 6.1.1.1 The Coal P a r t i c l e - S l a g Reaction Model At the i n s t a n t a c o a l p a r t i c l e becomes e n t r a i n e d i n the s l a g i t i s s u b j e c t e d to v e r y r a p i d h e a t i n g due to d i r e c t c o n t a c t with the s l a g . Under these c o n d i t i o n s , a c c o r d i n g to R i c h a r d s , p y r o l y s i s occurs v i r t u a l l y i n s t a n t a n e o u s l y . The f o l l o w i n g assumptions were made i n the o r i g i n a l model i n an attempt to c h a r a c t e r i z e the e x c e e d i n g l y complex p y r o l y s i s p r o c e s s . (a) a l l proximate v o l a t i l e hydrogen i s r e l e a s e d as H 2, (b) a l l proximate v o l a t i l e n i t r o g e n i s r e l e a s e d as N ^ , (c) a l l proximate v o l a t i l e oxygen r e a c t s with v o l a t i l e carbon to form CO, and (d) the remaining proximate v o l a t i l e carbon p r e c i p i t a t e s on the char p a r t i c l e . F o l l o w i n g p y r o l y s i s i t i s assumed t h a t the remaining char p a r t i c l e r e s i d e s i n the s l a g surrounded by an atmosphere of , N 2 and CO. T h i s char p a r t i c l e - secondary bubble then begins r e a c t i o n with the s l a g . The r e d u c i n g c o n d i t i o n s p r e v a i l i n g i n the secondary bubble i n i t i a t e the d i f f u s i o n of ZnO, PbO and P e 2 0 3 to the b u b b l e - s l a g i n t e r f a c e ( f o r m e r l y o n l y ZnO and FSjO^ were c o n s i d e r e d ) . At the b u b b l e - s l a g i n t e r f a c e these s p e c i e s then are reduced by CO or H 2 v i a the f o l l o w i n g r e a c t i o n s : + CO . . . (6.1) (g) + CO- . . . (6 . 2 ) (g) ZnO ( s i ) + CO (g) = Zn (g) PbO ( s i ) + CO (g) Pb (v) 97 Fe,0 + CO. = 2FeO, + CO- . . . (6.3) 1 J ( s l ) { q ) l S A ' Z ( g ) and ZnO. ,. + H- = Zn + H_0 . . . (6.4) { s l ) 2 ( g ) t g ' 2 (g) p b 0 / = i \ + H o = Pb + H„0 . . . (6.5) ( s l ' 2 ( g ) 2 (v) F e 9 0 ^ + H, = 2FeO, + H„0 . . . (6.6) Z J ( s l ) ^(g) { S X 1 1 (g) The C0 2 and H 20 produced v i a Reactions (6.1) - (6.6) then d i f f u s e s through the gas phase to r e a c t with the char p a r t i c l e v i a the f o l l o w i n g r e a c t i o n s : Boudouard C ( r h a r . + CO, = 2C0. . . . (6.7) (char) 2 ( q ) ( g ) Char - Steam C(^=>T\ + H ? ° = H» • t CO. . . . (6.8) (char) 2 ( g ) 2 ( g ) (g) T h i s marks the second divergence from the o r i g i n a l R i c h a r d s 1 3 r e a c t i o n model which o n l y c o n s i d e r s the Boudouard r e a c t i o n . In the i n i t i a l stages as t h i s r e a c t i o n system r i s e s through the s l a g , the vapour pre s s u r e s of Zn and Pb i n c r e a s e , and the char p a r t i c l e g r a d u a l l y s h r i n k s as carbon i s consumed by the Boudouard and Char - Steam r e a c t i o n s . The r a t e of i n c r e a s e i n the p a r t i a l pressure of CO^ and H^O are a f u n c t i o n of the r a t e s 98 a t which they are generated v i a Reactions (6.1)-(6.6), and the r a t e s a t which they are consumed by Reactions (6.7)-(6.8). An i n t e r e s t i n g problem with r e s p e c t t o model f o r m u l a t i o n a r i s e s as the char p a r t i c l e - secondary bubble c o n t i n u e s to r e a c t with the s l a g . The problem i s a s s o c i a t e d with the form of m e t a l l i c l e a d . At slag; fuming temperatures, 1200-1325° C, l e a d i s s t a b l e as a l i q u i d (m.pt. 327.50, b.pt. 1740° C) u n l i k e m e t a l l i c z i n c which i s a vapour (m.pt. 419.58, b.pt? 907 C). M e c h a n i s t i c a l l y , i t can be argued t h a t the vapour pressure of l e a d w i l l c ontinue to r i s e i n the secondary bubble u n t i l the p a r t i a l pressure reaches t h a t i n e q u i l i b r i u m with l i q u i d m e t a l l i c l e a d at the temperature of the s l a g bath, i e . P b ( v ) = P b ( 1 ) . . . (6.9) From t h i s p o i n t on, as l e a d vapour c o n t i n u e s to be produced at the bubble - s l a g i n t e r f a c e , l i q u i d l e a d i s s i m u l t a n e o u s l y p r e c i p i t a t e d a t such a r a t e so as to maintain Reaction (6.9) a t e q u i l i b r i u m . For the purpose of m o d e l l i n g i t i s assumed t h a t the l i q u i d l e a d produced c o l l e c t s i n the bottom of the secondary bubble. The r e a c t i o n system which develops i s s c h e m a t i c a l l y i l l u s t r a t e d i n F i g . 6.1. L i q u i d l e a d Figure 6.1 The char p a r t i c l e - s l a g r e a c t i o n system *° 100 As d i s c u s s e d p r e v i o u s l y i n Chapter I I , s u b s t a n t i a l evidence i n support of t h i s r e a c t i o n was p r o v i d e d by Richards^" 3 S i m i l a r evidence f o r the lead r e d u c t i o n m o d i f i c a t i o n i s a v a i l a b l e from aphotomicrograph of a quenched and p o l i s h e d s l a g sample of Cominco lead b l a s t furnace s l a g . F i g 6 . 2 shows a white m e t a l l i c i n c l u s i o n , or le a d p r i l l , i n c o n t a c t with black gas pores (secondary bubbles) as p o s t u l a t e d . Note: Course stubby i n c l u s i o n s are more Fe r i c h than s l a g . M e t a l l i c i n c l u s i o n c o n t a i n s a f a i r l y pure Pb core with a rimming of Cu, S and minor Fe. The s l a g and stubby i n c l u s i o n s a re Zn and S r i c h . R e f l e c t e d l i g h t , x 160 m a g n i f i c a t i o n s . F i g 6 . 2 Photomicrograph of quenched and p o l i s h e d s l a g sample 101 In t h i s system, as with R i c h a r d s , i t i s assumed that e l e c t r o n t r a n s f e r v i a the f e r r i c - f e r r o u s couple does not p l a y a s i g n i f i c a n t r o l e i n the r e d u c t i o n of f e r r i c i r o n (Fe 3 + ) . Evidence to t h i s e f f e c t has been d i s c u s s e d p r e v i o u s l y i n Chapter I I . The treatment of d i f f u s i o n processes i n and around the r e a c t i o n system i s i d e n t i c a l to t h a t of Richards except f o r the i n c l u s i o n of b e t t e r r h e o l o g i c a l d ata. E q u i l i b r i u m i s assumed w i t h i n the gas phase of the secondary bubble and l o c a l l y a t the bu b b l e - s l a g i n t e r f a c e . T h i s assumption was j u s t i f i e d 1 3 from an assessment of the r e l a t i v e r a t e s of mass-transfer w i t h i n the secondary bubble and surrounding s l a g . M a s s - t r a n s f e r c o e f f i c i e n t s were estimated as f o l l o w s . The t e r m i n a l r i s e v e l o c i t y of the secondary bubble a c c o r d i n g to Stoke's Law i s : t e r m i n a l = ^ A p / l S ^ • • • (6.10) 35 where a c c o r d i n g to Altman et a l , f o r le a d b l a s t furnace s l a g c o n - j o l o g p = 0.0160 l o g (CR) + y*^ 0 ' 3 9 8 8 1 • • • ( 6 - H ) where, % SiO + % A l 0 + % MgO CR = 1 . . . (6.12) % CaO + % FeO + % ZnO + % S (percentages are i n weight p e r c e n t ) . 102 36 From C l i f t e t a l , f o r r i g i d spheres i n c r e e p i n g flow, f o r s p e c i e s i : Shj * 1 + ( 1 + P e j ) 1 7 3 . . . (6.13) where Pe i s the P e c l e t Number; Pei - Z r ^ / D j . . . (6.14) and, Sh i s the Sherwood Number; Sh. = 2k.r./D, . . . (6.15) i i b i For the purpose of m o d e l l i n g , a d d i t i o n a l assumptions a l s o must be made. F o l l o w i n g from R i c h a r d s 1 3 , these a r e : (a) the char p a r t i c l e i s s p h e r i c a l , (b) the char p a r t i c l e r e a c t s o n l y with CO^ and H^O, and (c) the system i s i s o t h e r m a l a t bath temperature. A sample model of the o r i g i n a l r e a c t i o n system has been developed by R i c h a r d s . E i g h t unknown time-dependent v a r i a b l e s , C Z n ' CCO* C £ o 2 ' CH*2' C H 2 0 ' CS2' r p a n d r b ' i n a d d i t i o n to t h r e e time-dependent m o d e l l i n g parameters, W ^ , V g and M^, have been c o n s i d e r e d where , i s the c o n c e n t r a t i o n of s p e c i e s i i n the secondary bubble, r p i s the r a d i u s of the secondary bubble, and M c i s the molar amount of carbon i n the char p a r t i c l e . The c i n c o r p o r a t i o n of l e a d r e d u c t i o n adds two more v a r i a b l e s : C p f a v 103 and M p b , the c o n c e n t r a t i o n of lead vapour i n the secondary bubble and the molar amount of l i q u i d l e a d r e s p e c t i v e l y . T h i s makes a t o t a l of t h i r t e e n time dependent q u a n t i t i e s . T h i r t e e n equations must be developed. The development of these equations i s s u f f i c i e n t l y changed by the c o n s i d e r a t i o n of lead r e d u c t i o n to warrant a repeat of t h e i r d e r i v a t i o n here. 6.1.1.1.1 Zinc Balance The mass-transfer of z i n c oxide to the secondary bubble can be c h a r a c t e r i z e d by the e m p i r i c a l e quation: fx ZnO - A. k ZnO (C SI ZnO - ci ZnO (6.16) A z i n c mass balance on the secondary bubble y i e l d s : n ZnO Thus (6.18) 104 6.1.1.1.2 Lead Balance The mass-transfer of le a d oxide to the secondary bubble can be c h a r a c t e r i z e d by the e m p i r i c a l e q u a t i o n : A = A k i c s l - c 1 ) nPbO A b K PbCT^PbO ^PbO ' (6.19) For c o n d i t i o n s when the p a r t i a l pressure of P b v i n the secondary bubble i s l e s s than t h a t i n e q u i l i b r i u m with l i q u i d l e a d , a mass balance on the gas phase y i e l d s : n PbO d V cPb v V = vg dT < cPb v> + c P b v fe<V ^ ( C b ) = 1 dt ( C P b v > vg PbO Cb ~ (V ) u P b d t ( V v . . (6.20) . . . (6.21) For c o n d i t i o n s when the p a r t i a l pressure of le a d vapour equals t h a t i n e q u i l i b r i u m with l i q u i d l e a d , a p o r t i o n of le a d reduced a t the b u b b l e - s l a g i n t e r f a c e ends up i n the gas phase. T h i s p o r t i o n s e r v e s t o maintain the p a r t i a l pressure of le a d a t e q u i l i b r i u m with l i q u i d l e a d . S e p a r a t i n g the t o t a l f l u x of PbO i n t o two components: n^.o. = p o r t i o n of t o t a l PbO f l u x r e p o r t i n g to gas phase, PbO v nPb0 = P o r t i o n o t t o t a l p b 0 f l u x r e p o r t i n g to l i q u i d . From Equation (6.9), assuming one atmosphere t o t a l p r e s s u r e , i t f o l l o w s t h a t 105 K6.9 " PPb " ^ • ' ' ( 6 ' 2 2 ) v p g Rearranging Equation (6.22) CPb " K6.9 Pg • ' ' ( 6 ' 2 3 ) and d i f f e r e n t i a t i n g gives dt < CPb > v d m dt <K6.9 D g > " 0 (6.24) A mass balance for lead in the gas phase y i e l d s : n PbO — (Cb V ) dt * Pb g' v ^ . (6.25) Expanding and rearranging, (C° ) dt ltW v 1 Vg ™b d , „ . n PbO " CPb d t ( V v v ^ (6.26) Substituting Equations (6.23) and (6.24) into Equation (6.26), and rearranging, • u n\ d . . "PbO = K6.9 Pg d t ( V v * * (6.27) 106 T h i s then i s the p o r t i o n of the t o t a l f l u x of PbO which ente r s the gas phase. The p o r t i o n of the t o t a l f l u x of PbO r e p o r t i n g to l i q u i d lead i s simply A P b 0 l - dV ^ P b ^ • • • < 6 - 2 8 ) F i n a l l y , a mass balance on the t o t a l lead (vapour and l i q u i d ) in the system y i e l d s : *PbO = "PbO v + nPbO x ' ' ' ( 6 > 3 0 ) S u b s t i t u t i n g Equations (6.27) and (6.28) i n t o (6.29) g i v e s : APbO - K6.9 Pg dt< Vg> * d t + ( M Pb,l> ' ' * ( 6 ' 3 0 ) 6.1.1.1.3 Carbon Balance Carbon does not leave the secondary bubble. Carbon enters the gas 1 phase by the Boudouard and char-steam r e a c t i o n s , Equations (6.7) and (6.8) r e s p e c t i v e l y . Thus f • B + S . . . (6.31) c i n p u t r r where &r and*? are the r a t e s of the Boudouard and char-steam r e a c t i o n s r e s p e c t i v e l y . A mass balance f o r carbon i n the gas 107 phase yie l d s V , fe « C CO» * fe <CCO > I fe <V cb + c b . (6.32) r e a r r a n g i n g , dt ( C C O ) + dt ( C C 0 2 } dt * v g ' CO CO, (6.33) 6.1.1.1.4 Oxygen Balance Oxygen e n t e r s the gas phase v i a R e a c t i o n s (6.1), (6.2) and (6.3). There i s no g e n e r a t i o n or consumption. Thus: 2 input 1 . • 1 * + 1 A 2 nZnO r 2 nPbO T 2 F e 2 0 3 . . . (6.34) where n p e Q i s the rate o£ mass transfer of F e 2 0 3 to the secondary bubble. A mass balance f o r oxygen i n the gas phase y i e l d s 2, input 1 d_ (_b „ . d_ ( c b v . 1 d_ ( C b v . 2 dt l U C 0 g' + dt t u C 0 9 g' 2 dt l u H 9 0 g' . (6.35) 108 r e a r r a n g i n g , AZnO + nPbO + n F e 2 0 3 St <V Cb + 2C b + C b C C O + ^ C C 0 2 + L H 2 0 d_ c b d_ b i _ b dt uCO + *^dt U C 0 2 dt U H 2 0 (6.36) 6.1.1.1.5 Hydrogen Balance There i s no net inpu t , output, g e n e r a t i o n or consumption of hydrogen i n the secondary bubble. Thus a mass balance f o r hydrogen i n the secondary bubble y i e l d s d V<% cH 2 + ^ ( V g c H , O (6.37) Rearranging, 0 = T T (V ) at g Cb + C b ) CH 20 + C H 2 J + V 3- c b + c b dt U H 2 dt ^H 20 (6.38) 6.1.1.1.5.1 E q u i l i b r i u m of the Water-Gas Re a c t i o n F o l l o w i n g from the assumption of e q u i l i b r i u m w i t h i n the secondary bubble a f o u r t h e quation i n ( C b Q ) , ^ ( C b Q ^ ( C b ) and ( C b Q ), can be developed. From the e q u i l i b r i u m of the water-gas r e a c t i o n : CO + H 20 = H 2 + C0 2 . (6.39) i t f o l l o w s t h a t ; K H 6.39 2_ CO, P P CO H 20 c b C b c b c b CO H 20 (6.40) 109 D i f f e r e n t i a t i n g and r e a r r a n g i n g k6.32 c b d_ b d_ b ''CO dt H 20 dt CO b d_ _b b d r b = C C 0 2 dt C H 2 + H 2 dt C C 0 2 (6.41) Combining Equations (6.33), (6.36) and (6.38) with the Water-Gas e q u i l i b r i u m , Equation (6.41) g i v e s four equations i n four unknowns; A h K6.32 C C O ( D i - D'> + K6.32 CH^O D> " CCO < D* " D * + D'> Q—ir" \ 2 2 d t ^ c o j " dt^co' C H 2 + C C 0 2 + K6.32 C H 2 0 + K6.32 CC0 D 3 " fe<CSo> . . . (6.42) . . . (6.43) ( cb > dt ( CH 2> and, D i - D 2 + D 3 + ar <cco2> (6.44) dt U H 2 0 dt UC0, . . . (6.45) where D„ and D_ 1 * - (V ) V g dt 1 V 1 d V d t 9 n Z n O + n n. + Pbo 'H, A F e 2 o ; St <V (6.46) C D + 2C b + C b CCO + ^ C C 0 + L H 2 0 B + S fe <V cco + 'CO (6.47) . . (6.48) 6.1.1.1.6 Na Balance There i s no inpu t , output, g e n e r a t i o n or consumption of n i t r o g e n . Thus: 110 d_ dt N 2 g (6.49) -C S i - ( C b ) dt ( CN 2> — - — (V ) V dt K q' (6.50) 6.1.1.1.7 Bubble Radius F o l l o w i n g from the assumption of s p h e r i c a l geometry, the gas volume can be w r i t t e n as 4 3 4 3 — r 3 P (6.51) D i f f e r e n t i a t i n g and r e a r r a n g i n g d_ 7 J r ( V ) + 2 r 4 dt g r dt j (6.52) 6.1.1.1.8 Char P a r t i c l e Radius R e l a t i n g the r a d i u s of the char p a r t i c l e to i t s weight, i t f o l l o w s t h a t : . . . (6.53) I t i s assumed the d e n s i t y of the char p a r t i c l e i s equal to that of the o r i g i n a l c o a l . D i f f e r e n t i a t i n g d_ , 1 2 1 d_ . dt t r p ' 4 r p pc d t l p' (6.54) 6.1.1.1.9 Char P a r t i c l e Weight 111 Assuming t h a t the char p a r t i c l e l o s e s weight i n p r o p o r t i o n to the r a t e of the Boudouard and char-steam r e a c t i o n s i n f o l l o w s t h a t : W TZiV) = -(B„ + S ) , C A P * 12.01 . . . (6.55) a t p r r Jpwp where = the weight of carbon remaining i n the char a f t e r p y r o l y s i s , = the i n i t i a l p a r t i c l e weight a f t e r p y r o l y s i s (carbon plu s a s h ) . 12.01 i s the molecular weight of carbon i n kg/kg.mole. 6.1.1.1.10 Gas Volume The secondary bubble gas volume changes as a f u n c t i o n of the r e d u c t i o n r e a c t i o n s and, the Boudouard and char-steam r e a c t i o n s . For c o n d i t i o n s when the p a r t i a l pressure of le a d i n the secondary bubble i s below t h a t i n e q u i l i b r i u m with l i q u i d l e a d : CAP JPWT The f a c t o r ''gen 2 AZnO + 2 APbO 2B + r 2S. (6.56) 112 g, ZnO PbO r r " c o n s (6.57) A mass balance f o r the secondary bubble y i e l d s : ZnO PbO r r dt g g (6.58) Rearranging, dV<V = £• g n„ _ + n„._ + B + S ZnO PbO r r (6.59) where P g i s the molar gas d e n s i t y a t temperature. For c o n d i t i o n s when the p a r t i a l pressure of lead equals t h a t i n e q u i l i b r i u m with l i q u i d l e a d , a mass balance f o r the secondary bubble y i e l d s n„ _ + n n. n + B + S ZnO PbO r r v It ( P a V dt g g . . . (6.60) S u b s t i t u t i n g Equation (6.27) i n t o E quation (6.60) and r e a r r a n g i n g g i v e s dT(V = m ZnO r r . . (6.61) 6.1.1.1.11 I n i t i a l C o n d i t i o n s The i n i t i a l gas volume and composition are c a l c u l a t e d u s i n g the o r i g i n a l procedure of Richards l a Equations (6.7) and (6.8) 113 are assumed to be at e q u i l i b r i u m and a mass balance i s conducted on the v o l a t i l e c o n s t i t u e n t s , oxygen, n i t r o g e n and hydrogen. T h i s y i e l d s f i v e equations and f i v e unknowns, C b , C b , C„ L.U ^ ^ 9 " 9 b b C„ rt# C„ . The z i n c and l e a d vapour c o n c e n t r a t i o n s are H 2 ° N 2 assumed to be z e r o . Having e s t a b l i s h e d the gas c o n c e n t r a t i o n , the i n i t i a l gas volume can be found. The i n i t i a l charge p a r t i c l e weight and carbon content f o l l o w from a mass balance. The i n i t i a l v alues of r p and r ^ a l s o may be c a l c u l a t e d . F i n a l l y , the i n i t i a l amount of l i q u i d lead i s assumed to be z e r o . 6.1.1.1.12 Thermodynamic Data The necessary thermodynamic data together with r e f e r e n c e s are shown i n Table 6.1. 6.1.1.1.13 Mass T r a n s f e r The c a l c u l a t i o n of the mass-transfer r a t e s of ZnO and PbO i s a r e l a t i v e l y s t r a i g h t forward e x e r c i s e . C a l c u l a t i o n of the mass-t r a n s f e r r a t e s of FeO and F e 2 ° 3 l s m o r e complex. A review of the l i t e r a t u r e by Richards l a h a s i n d i c a t e d t h a t a s i g n i f i c a n t p o r t i o n of the f e r r i c i r o n present i n z i n c fuming s l a g s may be a s s o c i a t e d with n o n - s t o i c h i o m e t r i c w u s t i t e ( F e x 0 ) . T h i s behaviour was accounted f o r i n the o r i g i n a l R i c h a r d s 1 3 model 114 TABLE 6.1 Thermodynamic Data f o r R e a c t i o n s . AG° = & H ° - T A s ° H° S° Rea c t i o n (J) (JK ) Reference Z n<9) + 1 2 °2 = Z n 0 ( s ) -460240 -198.3 36 + 1 2 °2 = P b 0 ( l ) -181167 -68.03 36 c + °2 = C 0 2 ( g , -395350 0.544 36 c + 1 2 °2 = c o ( g ) -114390 85.75 36 H2 + 1 2 °2 = H 2 ° ( g ) -247484 -55.86 36 P e ( s , + 1 2 °2 3 FeO. . (s) -264889 -65.35 42 2 P e ( s ) + 3 2 °2 = P e 2 ° 3 ( s ) -814123 -250.66 36 'Standard s t a t e s f o r gases are the pure gas at 1 a tin at temperature, f o r the l i q u i d s , the pure l i q u i d a t temperature and f o r the s o l i d s , the pure s o l i d a t temperature. 115 TABLE 6.1 CONT'D Activity Coefficient of Slag Species Species Activity Coefficient Reference ZnO I n V 7 n - 1 6 4 0 0 (S"1 " 1 2 0 0 0 - 10.75 (S-) • 8.8 la.37,43 PoO In^nun 3 ^ ? I" (XCaO/XSiO,) - .37<XFe/Hi0J • 0.27 39 PbO (1) T 2 2 ^ V F e Q andVp e^ 0^ have been calculated usLng the three-suffix Margules model 44 developed by Goel et a l . Fe203 where S* = the eolar ratio of Ca0/Si02 •Standard states for liquids are the pure liquid at teiperature and for solids, the pure solid at tenperature. 116 by r e w r i t i n g Equation (6.3) i n the form: P e 2 0 3 + (3 - 2/X)CO = (2/X)Pe x0 + (3 - 2/X)C0 2 (6.62) In a d d i t i o n t o making mass-transfer c a l c u l a t i o n s more cumbersome, there are some fundamental problems with t h i s approach. One problem, as i n d i c a t e d by R i c h a r d s , i s the i m p l i c a t i o n t h a t 2+ d i f f u s i o n of Pe i s o c c u r r i n g a g a i n s t a c o n c e n t r a t i o n g r a d i e n t -c l e a r l y not p o s s i b l e . A second problem, not c o n s i d e r e d i s the i n f l u e n c e t h a t the n o n - s t o i c h i o r o e t r i c f a c t o r has on chemical p o t e n t i a l s . For example, as x approaches 0.667 the d r i v i n g f o r c e f o r R e a c t i o n (6.64) c l e a r l y tends to zero . A d i f f e r e n t approach i s necessary to r e s o l v e these problems. The f o l l o w i n g s e c t i o n d i s c u s s e s the method fo l l o w e d i n the mod i f i e d model. 6.1.1.1.13.1 Mass T r a n s f e r of F e 2 * and F e 3 * To access the i n f l u e n c e of the n o n - s t o i c h i o m e t r i c f a c t o r x on the mass-transfer of F e 2 0 3 and FeO, R e a c t i o n (6.3) must be broken down i n t o the f o l l o w i n g more fundamental r e a c t i o n s : 2Fe 3+ + 2e 2Fe 2 + (6.63) ( s i ) ( s i ) ° 2 " < s i r 1/2 0, + 2e (6.64) 117 and, CO. . + 1/2 0 o = CO,. . . . (6.65) ( g ) 2 ( g ) 2 ( g ) On the b a s i s of t h i s breakdown i t i s c l e a r t h a t the o v e r a l l r e a c t i o n e s s e n t i a l l y i s an oxygen exchange r e a c t i o n . Moreover, the d r i v i n g f o r c e i s simply the d i f f e r e n c e i n oxygen p o t e n t i a l between the s l a g and the secondary bubble. The former i s 3+ 2+ e s t a b l i s h e d by the Fe /Fe r a t i o i n the s l a g and the l a t t e r by the CO2/CO r a t i o i n the secondary bubble. From t h i s s i m p l i s t i c a n a l y s i s s e v e r a l comments can be made. F i r s t l y , i f the s l a g composition i s expressed i n terms of F e 3 + 2+ and Fe , then c l e a r l y the s t o i c h i o m e t r y of R e a c t i o n (6.3) would a p p l y and would be independent of x. Since t h i s i s i n f a c t the case, R e a c t i o n (6.3) i s a p p l i c a b l e f o r mass-transfer c a l c u l a t i o n s . Secondly, i t i s e v i d e n t t h a t the oxygen p o t e n t i a l of the s l a g must be c a l c u l a t e d i n order to determine mass-t r a n s f e r r a t e s . For t h i s purpose, the f o l l o w i n g i n f o r m a t i o n i s need f o r t y p i c a l z i n c fuming s l a g s : [ i ] the s l a g composition, [ i i ] the a c t i v i t y c o e f f i c i e n t of Fe xO, f i i i ] the a c t i v i t y c o e f f i c e n t of F e 2 0 3 , [ i v ] the e q u i l i b r i u m constant f o r the r e a c t i o n : 118 Pe xO = 1/2 0 2 + xPe . . . (6.66) and, [v] the e q u i l i b r i u m constant f o r the r e a c t i o n : F e 2 0 3 • 2Pe + 3/2 0 2 . . . (6.67) C l e a r l y two of these q u a n t i t i e s are dependent on x: the a c t i v i t y c o e f f i c i e n t of F e x ° / a n d the e q u i l i b r i u m c o n s t a n t f o r Rea c t i o n (6.65). U n f o r t u n a t e l y , these values are not a v a i l a b l e i n the l i t e r a t u r e . The best r e p o r t e d data f o r the a c t i v i t y c o e f f i c i e n t of w u s t i t e i n s l a g s approximating z i n c fuming s l a g s (see Table 6.1), are g i v e n r e l a t i v e to FeO , .as the standar d s t a t e . T h i s ' * (s) being the case, the e q u i l i b r i u m constant f o r R e a c t i o n (6.66) a l s o must be e v a l u a t e d r e l a t i v e to FeO . . (x=l) i n order to be (s) c o n s i s t e n t thermodynamically. T h e r e f o r e , there i s l i t t l e a l t e r n a t i v e other than to s e t x=l f o r mass-transfer c a l c u l a t i o n s . The mass t r a n s f e r of FeO and F e 2 0 3 through the s l a g to the bubble s l a g i n t e r f a c e may be c h a r a c t e r i z e d e m p i r i c a l l y as " F e 2 ° 3 = A b k p e 2 ° 3 n F e 0 " A b k F e 2 0 3 , s l - F e 2 0 3 ' F e 2 ° 3 'FeO , s l 'FeO . . . (6.68) . . . (6.69) From the s t o i c h i o m e t r y of R e a c t i o n (6.3) and assuming the p o s i t i v e d i r e c t i o n t o be away from the bubble s l a g i n t e r f a c e , i t 119 f o l l o w s t h a t AFeO " 2 A P e 2 0 3 . . . (6.70) S u b s t i t u t i n g Equations (6.68) and (6.69) wi t h Equation (6.70) y i e l d s 2k 'FeO - ^ 3 - ( Cs l - C 1 ) + C s l kFeO P e 2 ° 3 F e 2 ° 3 F e 0 (6.71) Since Equation (6.3) i s a t e q u i l i b r i u m a t the b u b b l e - s l a g i n t e r f a c e 2 P 3 FeO C 0 2 (6.72) K6.6 a p e 2 ° 3 P c 0 Combining Equations (6.71) and (6.72), and s o l v i n g f o r C_ _ F e 2 ° 3 y i e l d s 0 - " D 6 " D 5 ( C F e 2 0 3 ) (6.73) where D. = K, _ P* * F e 2 ° 3 P C 0 4 b. o s i H FeO x PCO, (6.74) 2k 5 k-F e 2 0 3 FeO . . . (6.75) D 6 " D S C F e 2 0 3 + CFeO (6.76) 120 Equation (6.69) can be s o l v e d a n a l y t i c a l l y or n u m e r i c a l l y f o r C * _ and then s u b s t i t u t e d back i n t o E quation (6.68) to F e 2 ° 3 c a l c u l a t e n„ rt  F e 2 ° 3 6.1.1.1.13.2 Mass T r a n s f e r C o e f f i c i e n t s In order to c a l c u l a t e A Z n Q , Ap b 0# and A p e Q , i t i s necessary to c a l c u l a t e the mass-transfer c o e f f i c i e n t s : kznO' k..,,, k_ _ . The method of c a l c u l a t i o n i s i d e n t i c a l t o t h a t of PbO Fe_0^ Ia R i c h a r d s . S i n c e , as d i s c u s s e d p r e v i o u s l y , the secondary bubble behaves as a r i g i d sphere, the mass-transfer c o e f f i c i e n t s are c h a r a c t e r i z e d by Equations (6.13)-(6.15). The necessary p h y s i c o -chemical data can be c a l c u l a t e d from Equations (6.10)-(6.12). The d i f f u s i v i t i e s are those used by R i c h a r d s . Due to a l a c k of d a t a , the f o l l o w i n g assumptions have been made: DZn0 " DFeO ' ' ' ( 6 ' 7 7 ) DPb0 * D F e 0 ' ' • ( 6 ' 7 8 ) D F e 2 0 3 * 0 ' 1 D F e O ' ( 6 ' 7 9 ) D p e 0 i s assumed t o be e m p i r i c a l l y c h a r a c t e r i z e d by the f o l l o w i n g equation f o r the s e l f - d i f f u s i v i t y of i r o n i n a CaFeSiO^ melt over a temperature range of 1250 to 1 5 4 0 ° C . 3 6 121 l o g 2 + = l o g D p e Q » 5450^t- $20. 5 „ _+ 0 >37 ( 6.80) 2 where D 9 + i s expressed i n u n i t s of m / s . Fe^ 6.1.1.1.14 Boudouard and Char-Steam Reactions The l a s t terms t o e v a l u a t e d i n the char p a r t i c l e - s l a g r e a c t i o n model are the r a t e s of the Boudouard and char-steam r e a c t i o n s o c c u r r i n g on the char p a r t i c l e . From Skinner and 37 Sraoot , f o r a p u l v e r i z e d bituminous c o a l char (70% through -200 mesh), the r a t e of the Boudouard r e a c t i o n i s f i r s t order i n the q u a n t i t y of carbon l e f t unreacted and the p a r t i a l pressure of C 0 2 as f o l l o w s B r = A® exp (E® /RT) . . . (6.81) where A B = 3.13 x 10 6 (kg mole kg mole" 1 k P a - 1 S - 1 ) o E® = 1*6200 (kJ kg m o l e - 1 ) As an a d d i t i o n t o R i c h a r d s 1 3 model, the char-steam r e a c t i o n has 37 been i n c l u d e d . Again from Skinner and Smoot , f o r a p u l v e r i z e d bituminous c o a l char, the r a t e of the char-steam r e a c t i o n i s f i r s t order i n the q u a n t i t y of carbon l e f t unreacted and the 122 p a r t i a l pressure of K^O: S = A* exp (E* /RT) P„ ft . . . (6.82) o a 2 where A* = 1.0 x 10 6 (kg mole kg m o l e - 1 k P a - 1 S _ 1 ) E s = 183739 (kJ kg. mole" 1) £1 6.1.1.1.15 Model S o l u t i o n The r e s u l t i n g model i s an i n i t i a l - v a l u e problem i n a system of t h i r t e e n o r d i n a r y d i f f e r e n t i a l e q u a t i o n s . These were s o l v e d u s i n g a f o u r t h order Runga-Kutta technique i n double p e r c i s i o n a v a i l a b l e through the UBC Computing Centre. 6.1.1.2 The K i n e t i c s of Lead Removal Having completed the a d d i t i o n of l e a d r e d u c t i o n to the char p a r t i c l e - s l a g r e a c t i o n model, an i n t e r e s t i n g problem a r i s e s s i n c e mass balances on o p e r a t i n g furnaces i n d i c a t e t h a t a l l of the l e a d 3 8 r e p o r t s to fume. The q u e s t i o n i s how i s the l i q u i d l e a d removed from the furnace? 123 W i t h i n the context of the present model the l i q u i d l e a d w i l l be l e f t behind a f t e r the gaseous contents of the secondary bubble are r e l e a s e d t o the furnace atmosphere. The r e s u l t i n g " l e a d p r i l l " then w i l l be c a r r i e d back down i n t o the bulk of the bath by s l a g motion. Thermodynamically the m e t a l l i c l e a d w i l l be unstable i n the s l a g . I t i s proposed, t h e r e f o r e , t h a t the lead p r i l l w i l l r e a c t with the s l a g v i a the f o l l o w i n g r e a c t i o n : P e o 0 , + Pb.,. = 2PeO. + PbO. . . . (6.83) ^ ^ ( s i ) ' ' ( s i ) ( s i ) By t h i s mechanism, the m e t a l l i c l e a d p r i l l i s o x i d i z e d i n t o the bulk of the s l a g where i t can be re-reduced and removed, i n p a r t , as vapour. Obv i o u s l y , as with the secondary bubble s l a g r e a c t i o n s , k i n e t i c s w i l l p l a y an important r o l e i n determining the r a t e a t which o x i d a t i o n w i l l proceed. A lead p r i l l - s l a g r e a c t i o n model has been developed to account f o r these k i n e t i c s . 6.1.1.2.1 The Lead P r i l l - S l a g R e a c t i o n Model In order to s i m p l i f y the mathematics i n v o l v e d , i t i s assumed t h a t the m e t a l l i c lead present i n the furnace a t any given time i s i n the form of s p h e r i c a l l e a d p r i l l s with an average r a d i u s , 124 D i f f u s i o n i n the s l a g i s assumed to be the r a t e - l i m i t i n g s t e p . T h i s i s not unreasonable s i n c e no mass-transfer need take p l a c e w i t h i n the p r i l l i t s e l f and, R eaction (6.83) should proceed r a p i d l y a t s l a g fuming temperatures. The r e s u l t i n g r e a c t i o n system i s shown s c h e m a t i c a l l y i n P i g . 6.3. 6.1.1.2.1.1 Mass Balance on L i q u i d Lead L i q u i d l e a d i n the p r i l l i s consumed by o x i d a t i o n a t a r a t e equal to the mass - t r a n s f e r r a t e of F e 2 0 3 i n the s l a g . T h i s e s t a b l i s h e s the r a t e of back o x i d a t i o n of l i q u i d lead per p r i l l and, t h e r e f o r e , can be used to determine the o v e r a l l r a t e of m e t a l l i c l e a d o x i d a t i o n w i t h i n the furnace. 6.1.1.2.1.2 Mass T r a n s f e r The mass-transfer r a t e s of PbO, F e 2 ° 3 and PeO can be c h a r a c t e r i z e d e m p i r i c a l l y by the f o l l o w i n g e q u a t i o n s : ft = k A_ [ CifPh - C s l 1 nPbO KPbO,Pb *Pb 1 cPbO uPbO J * ' ' n F e 2 0 3 k P e 2 0 3 , Pb CFJ g " Ci£$ 1 • • • nPeO " KFeO,Pb T>b 1 uPeO u F e O J * ' * 126 From the s t o i c h i o m e t r y of Equ a t i o n (6.83) i t f o l l o w s t h a t : n F e 2 0 3 " APbO = " nFeO * * * < 6- 8 7> and, from the assumption- of l o c a l e q u i l i b r i u m a t the p r i l l - s l a g i n t e r f a c e , ( C F g Q P b ^ °PbQ b = K6.78 ^ P e 2 ° 3 ( P " j ) 2 . . (6.88) Combining Equations (6.84) - (6.88) and s o l v i n g f o r C„ n i t can r e 2 u 3 be shown t h a t o = i D 9 - ( D 7 ) c*;*^ , 2 i D 1 0 - D 8 (c*;^)i - 4e^Dliy . . . (6.89) where 2 k F e O D ? - 2"3 . . . (6.90) kFeO D g = k F e 2 ° 3 . . . (6.91) kPbO D 9 - D 7 ( C ? e 2 0 3 ) + CFeO ' * * ( 6 * 9 2 ) D10 " D 8 < CFe 20 3> + CP*0 ' « ' < 6 ' > 3 ) # 2 Fe O o*PbO D X 1 = "6.78 " 2 " 3 ^ s l ' . . . (6.94) Equation (6.89) can be s o l v e d a n a l y t i c a l l y or n u m e r i c a l l y f o r c i n . The r e s u l t i n g value of C* rt then can be s u b s t i t u t e d F e 2 0 3 F e 2 0 3 i n t o Equation (6.85). Only the mass-transfer c o e f f i c i e n t s remain be determined. 6.1.1.2.1.3 Mass-Transfer C o e f f i c i e n t s On the b a s i s t h a t the lead p r i l l s behave as r i g i d spheres i n c r e e p i n g flow, the mass-transfer c o e f f i c i e n t s , k p b 0 * kFeO' a n d k , are c h a r a c t e r i z e d e m p i r i c a l l y by Equations (6.13)-(6.15). F e 2 ° 3 The necessary p h y s i c o - c h e m i c a l data can be c a l c u l a t e d from Equations (6.10) - (6.12). D i f f u s i v i t i e s are the same as those used i n the c h a r - p a r t i c l e s l a g r e a c t i o n model. 6.1.2 The Furnace Model The c a l c u l a t i o n of the secondary bubble r e s i d e n c e time, the k i n e t i c s of the tuyere gas column, and the c h a r a c t e r i z a t i o n of the m e l t i n g and f r e e z i n g of s l a g on the furnace w a l l s and bottom are unchanged by the i n c o r p o r a t i o n of lead r e d u c t i o n . Thus the i n t e r e s t e d reader i s r e f e r r e d to the o r i g i n a l mathematical model of R i c h a r d s 1 3 f o r the treatment of t h i s m a t e r i a l . The o v e r a l l furnace heat and mass balances must be modified to account f o r the r e d u c t i o n of l e a d . These m o d i f i c a t i o n s are l a r g e l y a book keeping e x e r c i s e and w i l l not be d i s c u s s e d . For a g e n e r a l review of the mathematics i n v o l v e d i n the o v e r a l l furnace 128 heat and mass balances the reader i s r e f e r r e d t o the o r i g i n a l treatment of R i c h a r d s . 6.2 D i s c u s s i o n of Model F i t t i n g Before the model c o u l d be a p p l i e d t o analyze the h i g h -pressure c o a l i n j e c t i o n t r i a l s , i t was necessary to f i t i t to a run t y p i c a l of normal fuming p r a c t i c e f o r the Cominco No. 2 furnace. There were two main reasons f o r t h i s : f i r s t l y , t o e l u c i d a t e the e f f e c t t h a t the a d d i t i o n of l e a d r e d u c t i o n has on model p r e d i c t i o n s , and secondly, to determine the f r a c t i o n of low-pressure c o a l e n t r a i n e d i n the s l a g . The l a t t e r was necessary i n order to p r o v i d e a b a s i s f o r the c a l c u l a t i o n of entrainment f a c t o r s f o r the h i g h - p r e s s u r e c o a l . The procedure adopted was to a d j u s t P L P C E / F L P C C a n d F oxy u n t i l good agreement was obtained between measured and p r e d i c t e d p r o f i l e s of bath composition and temperature. F L P C E I S A E F I N E A as the f r a c t i o n of low-pressure c o a l e n t r a i n e d , f L P C C I S T N E f r a c t i o n of low-pressure c o a l combusted; and F i s the f r a c t i o n oxy of remaining oxygen which o x i d i z e s f e r r o u s i r o n (the remaining oxygen i s d e f i n e d as t h a t p o r t i o n of the t o t a l input oxygen which i s unconsuroed by c o a l combustion.) Within the computer program, val u e s f o r p L P C E ' F L P C C ' A N D F can be input f o r each s e t of o p e r a t i n g c o n d i t i o n s ( c o a l r a t e oxy and secondary b l a s t r a t e ) . T h e r e f o r e , f o r those runs where there i s a v a r i a t i o n i n operating, c o n d i t i o n s , i t i s p o s s i b l e to have a c o r r e s p o n d i n g v a r i a t i o n i n the primary f i t t i n g parameters. In t h i s way, the best p o s s i b l e f i t f o r a g i v e n s e t of o p e r a t i n g c o n d i t i o n s can be o b t a i n e d . Three low-pressure runs, two of which have been p r e v i o u s l y analyzed by Richards l a were a v a i l a b l e f o r a n a l y s i s . The model was f i t t e d to a l l three runs; however, o n l y the r e s u l t s f o r the run not p r e v i o u s l y analyzed by R i c h a r d s 1 3 w i l l be presented f o r d i s c u s s i o n . I t i s f e l t t h a t the other two runs were not i n d i c a t i v e of normal o p e r a t i o n and, t h e r e f o r e , do not warrant d e t a i l e d d i s c u s s i o n . 6.2.1 R e s u l t s of F i t to Normal Operation The r e s u l t s of f i t t i n g the model to a "normal" run are presented i n F i g s . 6 .4-6.6 The p r e d i c t e d values f o r the primary f i t t i n g parameters, ^ p c g / * " L P C C 3 n d F o x y a r e s u m r a a r i z e d together with the p r e d i c t e d furnace oxygen u t i l i z a t i o n i n Table 6.2. For the purpose of comparison, the r e s u l t s of the o r i g i n a l a n a l y s i s of the process by Richards a l s o have been presented i n Table 6.2. I t should be mentioned t h a t the i n c l u s i o n of lead r e d u c t i o n has n e c e s s i t a t e d the use of two a d d i t i o n a l f i t t i n g parameters: Pb the average r a d i u s of the lead p r i l l s (r ), and the i n i t i a l TABLE 6.2 130 Model Fitting Parameters Normal Operation Run 1 Run 2 Run 3 Fraction of low-pressure coal entrained (Richards) 0.2-0.32 (0.29) 0.27 0.27 0.27 Fraction of high-pressure coal entrained - 0.75* 0.65 0.90 Fraction of low-pressure coal combusted ( R i c h a r d s ) 0.40-0.65 (0.54) 0.50-0.54 0.70-0.50 0.70 Fraction of high-pressure coal combusted - 0 0 0 Fraction of uncombusted oxygen to ferrous iron oxidation (Richards) 0.03-0.30 (0.04) 0.15-0.50 0.75-0.10 0.55-0.30 Radius of Pb p r i l l (ra) 1 x 10" 3 1 x 10~ 3 1 x 10~ 3 1 x 10~ 3 I n i t i a l f r a c t i o n of Pb as metal 0.10 0.10 0.90 0.50 Slag c i r c u l a t i o n v e l o c i t y (m/sec) (Richards) 3 (1) 3 3 3 Predicted oxygen u t i l i z a t i o n 0.55-0.75 0.62-0.85 0.65-0.95 0.78-0.92 *This value was obtained using a term to account for the melting of z i n c - r i c h material at a rate of 0.3 x 10 kg mole Zn/s Figure 6.4 Industrial data and model f i t to the Zn and Pb p r o f i l e s for normal operation q to CU UJ . b q IT) q d F E 2 » / F E 3 « L 1 N E S - M O O E L POI NT S = A S S O T i i i i i i I I I I I I I 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 E L A P S E D T I M E ( M I N ) Figure 6.5 I n d u s t r i a l data and model f i t to the Fe p r o f i l e s for normal o p e r a t i o n 2 + and Fe 3 + L I N E S = M O O E L P O I NT = o s s n r o X L -© e-0~ O <) T T T T -r T 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 E L A P S E D T I M E ( M I N ) 100.0 110.0 120.0 130.0 140 Figure 6.6 I n d u s t r i a l data and model f i t to the temperature p r o f i l e f o r normal o p e r a t i o n 134 f r a c t i o n of m e t a l l i c l e a d Fpg^ • The l a t t e r of these was deemed nece s s a r y s i n c e a s i g n i f i c a n t p o r t i o n of the l e a d charged to the 39 furnace may be present i n m e t a l l i c form. The v a l u e s f o r Pb T p and F p B k are a l s o p r o v i d e d i n Table 6 . 2 . 6 . 2 . 1 . 1 D i s c u s s i o n of F i t t o Normal O p e r a t i o n The f i t of model p r e d i c t i o n t o measurement i s r e a s o n a b l y good, see F i g s . 6 . 4 - 6 . 6 . I t should be noted, however, t h a t a l l three primary f i t t i n g parameters have had t o be v a r i e d over the course of the run ( 0 . 2 3 to 0 .37 F L P C E ' 0 , 3 8 t o 0 • 6 5 P L P C C a n d 0.03 t o 0 .32 F Q x y ) , see Table 6 . 2 . U n f o r t u n a t e l y , on the b a s i s of one run, i t i s d i f f i c u l t t o s p e c u l a t e on the i m p l i c a t i o n s of these v a r i a t i o n s . F o r example, the higher entrainment f a c t o r of 0.37 (0 -20 mine, ela p s e d time} c o u l d i n d i c a t e t h a t the model i s u n d e r e s t i m a t i n g z i n c fuming r a t e s under o x i d i z i n g c o n d i t i o n s ; and/or the lower entrainment f a c t o r of 0 .23 (20-40 rains, elapsed time) c o u l d suggest the model i s o v e r e s t i m a t i n g fuming r a t e s under r e d u c i n g c o n d i t i o n s . Moreover, s i n c e the p r e d i c t e d changes i n F L p C K a r e c o i n c i d e n t with changes i n c o a l r a t e i t c o u l d a l s o be a r e a l e f f e c t . Not s u r p r i s i n g l y , the same holds f o r the v a r i a t i o n s i n F T „ „ ^ and F - there i s the p o s s i b i l i t y t h a t u r L L O X y t h e i r behaviour c o u l d be the r e s u l t of weaknesses w i t h i n the model, r e a l e f f e c t s , or some combination of the two. 135 Having s t a t e d t h i s , i t appears t h a t t h e r e i s l i t t l e a l t e r n a t i v e other than t o assume an average v a l u e f o r P L p C B which can be adopted i n the a n a l y s i s of the hi g h - p r e s s u r e d a t a . There-f o r e , f o r a l l subsequent a n a l y s i s ^ P C B 1 3 s e t e g u a A to 0.30. I f the r e s u l t s of t h i s f i t t i n g e x e r c i s e are compared t o t h e pr e v i o u s f i n d i n g of R i c h a r d s 1 9 (see Tab l e 6.2) some i n t e r e s t i n g o b s e r v a t i o n s can be made. F i r s t l y , i t i s immediately obvious t h a t there i s a s i g n i f i c a n t d i f f e r e n c e i n the s l a g c i r c u l a t i o n v e l o c i t i e s employed i n the two models - 3m/s i n the present model as compared t o lm/s i n the Richards model. C l e a r l y , t h i s i s a s i g n i f i c a n t d i f f e r e n c e and as such warrants some j u s t i f i c a t i o n . T h i s w i l l be postponed u n t i l a s e n s i t i v i t y a n a l y s i s can be completed. Secondly, t h e r e i s a remarkable c o n s i s t e n c y between the v a l u e s p r e d i c t e d f o r c o a l u t i l i z a t i o n ( F L p c E and p L p C C ) with the two models, see Table 6.2. T h i s c o n t i n u e s to lend soundness to the k i n e t i c foundations on which both models are based, i e . the p a r t i t i o n i n g of c o a l i n t o reductant and source of heat. Having completed f i t t i n g ; the model t o normal o p e r a t i o n i t i s now p o s s i b l e t o f i t the model t o the hi g h - p r e s s u r e t r i a l s . As mentioned p r e v i o u s l y , the value of F L P C E i s s e t equal to 0.30 f o r t h i s e x e r c i s e . 136 6.2.2 R e s o l t a of P i t t o High-Pressure O p e r a t i o n The r e s u l t s of f i t t i n g - the model to the high-pressure o p e r a t i o n , Runs 1-3, are presented i n P i g s . 6.8-6.19. The p r e d i c t e d v a l u e s f o r the primary f i t t i n g parameters P L p c g , F^pcE* P r n__ and P,„ are summarized together with the p r e d i c t e d oxygen u r C L O X y u t i l i z a t i o n i n Table 6.2 ( p H P C E i s d e f i n e d as the f r a c t i o n of high- p r e s s u r e c o a l e n t r a i n e d i n t h e s l a g ) . A summary of the Pb v a l u e s f o r the secondary f i t t i n g parameters r p and F p f i L a l s o are presented i n Table 6.2. Before proceeding with an assessment of the i n f l u e n c e of hig h - p r e s s u r e i n j e c t i o n on c o a l entrainment, i t i s necessary t o d i s c u s s the a b i l i t y of the model to be f i t t e d t o each of the thr e e h i g h - p r e s s u r e t r i a l s . T h i s w i l l h e l p i n e s t a b l i s h i n g the v a l i d i t y o f the r e s u l t s of the model a n a l y s i s . 6.2.2.1 D i s c u s s i o n of P i t t o High-Pressure O p e r a t i o n In a l l three runs the model p r e d i c t i o n s are seen t o f i t the measurements q u i t e w e l l , see P i g s . 6.8-6.19. The z i n c c o n c e n t r a t i o n p r o f i l e s , however, are the l e a s t s a t i s f a c t o r y . In Run 1, the model was i n i t i a l l y unable t o account f o r the low fuming r a t e . The most l i k e l y e x p l a n a t i o n i s t h a t z i n c r i c h m a t e r i a l was m e l t i n g i n the furnace throughout the run. T h i s i s o L I N E S = M O D E L PO I N T S = flSSAY I 1 1 1 1 1 1 1 1 1 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90 E L A P S E D T I M E ( M I N S ) Figure 6.7 I n d u s t r i a l data and model f i t to the Zn and Pb p r o f i l e s f o r Run 1 " A * B 2 ^ A i - - - , F E R R O U S L I N E S ^ M O D E L PO I NT S = flSSPY F E R R I C 0.0 10.0 20.0 30.0 4-0.0 50.0 60.0 E L A P S E D T I M E ( M I N S ) 70.0 80.0 90.0 F i g u r e 6.8 2+ 3 + I n d u s t r i a l data and model f i t to the Fe and Fe p r o f i l e s f o r Run 1 Figure 6 . 9 I n d u s t r i a l data and model f i t to the temperature p r o f i l e f o r Run 1 F i g u r e 6.10 I n d u s t r i a l data and model f i t to the Zn and Pb p r o f i l e s f o r Run 2 2+ 3 + Fig u r e 6.11 I n d u s t r i a l data and model f i t to the Fe and Fe p r o f i l e s for Run 2 q d o F i g u r e 6.12 I n d u s t r i a l data and model f i t to the temperature p r o f i l e f o r Run 2 Figure 6.14 I n d u s t r i a l data and model f i t t o the Fe and Fe p r o f i l e s for Run 3 F i g u r e 6.15 I n d u s t r i a l data and model f i t to the temperature p r o f i l e f o r Run 3 c o n s i s t e n t with both the charge make-up, 50:50 h o t - c o l d , and averaged p l a n t data which i n d i c a t e s a decrease i n the measured fuming r a t e f o r c o l d charges (see Chapter V). Assuming t h i s e x p l a n a t i o n to be c o r r e c t , a term was added to the model to account f o r the i n t r o d u c t i o n of z i n c i n t o the bath. A value of 0.75 f o r P H p c E was assumed f o r Run 1 (value a r r i v e d a t from the r e s u l t s of model a n a l y s i s of Runs 2 and 3) and the model was f i t t e d to the z i n c p r o f i l e , see P i g . 6.7, by a d j u s t i n g the melt r a t e of z i n c i n t o the bath. J u s t i f i c a t i o n f o r the i n t r o d u c t i o n of t h i s term i s t h a t the average p r e d i c t e d fuming e f f i c i e n c y i s i n c r e a s e d from 0.77 to 0.92 (Kg Zn + Kg Pb)/Kg c o a l which i s more i n l i n e with t h a t p r e d i c t e d f o r Runs 2 and 3, 1.2 and 1.4 (Kg Zn + Kg Pb)/Kg c o a l r e s p e c t i v e l y . T h i s i s c o n s i s t e n t with the t h e o r y o r i g i n a l l y proposed i n Chapter V t h a t the act fuming e f f i c i e n c y ( e q u i v a l e n t t o p r e d i c t e d ) and the apparent fuming e f f i c i e n c y ( t h a t based on assays) w i l l be d i f f e r e n t under c o n d i t i o n s when the m e l t i n g of z i n c - r i c h m a t e r i a l i s o c c u r r i n g . I f the z i n c p r o f i l e s of Runs 2 and 3 are c o n s i d e r e d , a d i f f e r e n t weakness i n the model i s e v i d e n t , see P i g s . 6.10 and 6.13, r e s p e c t i v e l y . The model i s unable to account f o r the observed fuming r a t e s d u r i n g p e r i o d s of t r a n s i t i o n from o x i d i z i n g to r e d u c i n g c o n d i t i o n s (0-30 minutes elapsed time i n both r u n s ) . T h i s behaviour i s c o n s i s t e n t with what was observed i n f i t t i n g the model to normal o p e r a t i o n , see P i g . 6.4. T h e r e f o r e , i t appears t h a t the model, i n g e n e r a l , has a tendency to underestimate fuming r a t e s (by approximately 1 wt% i n Runs 2 and 3) under c o n d i t i o n s when the bath c o n c e n t r a t i o n of f e r r i c i r o n i s r e l a t i v e l y h i g h . There are s e v e r a l p o s s i b l e e x p l a n a t i o n s f o r t h i s behaviour i n c l u d i n g f o r example, an i n c o r r e c t a c t i v i t y c o e f f i c i e n t of FejO^ i n the s l a g . I t i s d i f f i c u l t t o assess the v a l i d i t y of t h i s , o r any other e x p l a n a t i o n , owing to the c o m p l e x i t y of the mathematical model. T h e r e f o r e , d i s c u s s i o n w i l l have to be l e f t u n t i l a s e n s i t i v i t y a n a l y s i s can be completed. I t should be noted t h a t both P T n „ „ and ' have had to be Lifoo oxy v a r i e d over the course of the three runs i n order to f i t the 2+ 3 + Pe /Fe and temperature p r o f i l e s ( p L p c c : ° * 5 R U n 1 ' ° ' 5 ~ ° " 7 Run 2 and 0.68 t o 0.70 Run 3; P Q x y : 0.15-0.5 Run 1, 0.1-0.75 Run 2 and 0.30-0.50 Run 3), see Table 6.2. T h i s behaviour i s c o n s i s t e n t a l s o with the r e s u l t s of f i t t i n g the model to normal o p e r a t i o n . Moreover, i t i s i n t e r e s t i n g t o note t h a t the r e s u l t i n g values f o r oxygen u t i l i z a t i o n decrease with d e c r e a s i n g bath temperature. To assess the c o r r e l a t i o n on a run-to-run b a s i s , a p l o t of p r e d i c t e d oxygen u t i l i z a t i o n versus temperature was made, see F i g . 6.16. L i n e a r r e g r e s s i o n of the data f o r the three h i g h - p r e s s u r e runs and the normal o p e r a t i o n run y i e l d s a c o r r e l a t i o n c o e f f i c i e n t of r=0.85. To determine which of the two parameters, P L p c c a n d / o r F Q x y , are r e s p o n s i b l e f o r t h i s t r e n d , p l o t s of both P L p c c and F o x y versus temperature were made, see P i g s . 6.17 and 6.18, r e s p e c t i v e l y . Based on t h i s , i t appears 1.0 § 0.9 Z 0.5 A Normal • Run I O Run 2 O Run 3 operation 0.4 0.3' 1 1 1200 1250 1300 Bath temperature (°C ) 1350 F i g u r e 6.16 P r e d i c t e d oxygen u t i l i z a t i o n as a f u n c t i o n of bath temperature (J u Q_ 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 i r i T o o • A O A Normal • Run I O " 2 O " 3 operation J I I L 1200 1250 1300 1350 Bath temperature ( °C) F i g u r e 6.17 P, D f, r as a f u n c t i o n of bath temperature 0.8 0.7 0.6 0.5 > x o L L - 0.4 0.3 0.2 0.1 1 1 1 1 1 O o O o y o / • — — o An /u — — A , 0 0 — A / A N o r m a l o p e r a t i o n C K • R u n 1 O R u n 2 / o O R u n 3 1 1 1 1 1 1200 1250 1300 Bath temperature (°C) 1350 F i g u r e 6.18 F as a f u n c t i o n of bath temperature oxy 151 t h a t the r a t e of o x i d a t i o n of f e r r o u s i r o n ( e s t a b l i s h e d by F ) oxy i s s e n s i t i v e t o bath temperature. One p o s s i b l e e x p l a n a t i o n i s t h a t the k i n e t i c s of t h i s r e a c t i o n are s e n s i t i v e to bath temperature. Por example, an i n c r e a s e i n bath v i s c o s i t y r e s u l t i n g from a decrease i n bath temperature c o u l d cause an i n c r e a s e i n the average diameter of the bubbles i n the tuyere gas column and, t h e r e f o r e , a r e d u c t i o n i n m a s s - t r a n s f e r . I f t h i s i s the case, the k i n e t i c s of tuyere gas o x i d a t i o n of f e r r o u s i r o n a l s o should be s e n s i t i v e t o bath depth. T h i s t r e n d was observed i n the o r i g i n a l i n v e s t i g a t i o n of the p r o c e s s by R i c h a r d e t l a b a l * ' U n f o r t u n a t e l y , t h e r e i s a lack of enough experimental data t o e i t h e r c o n f i r m or r e f u t e t h e i r f i n d i n g s . Having completed a d i s c u s s i o n of the model a n a l y s i s of the h i g h - p r e s s u r e t r i a l s d ata, i t i s now p o s s i b l e to assess the i n f l u e n c e of h i g h - p r e s s u r e i n j e c t i o n on c o a l entrainment - the essence of t h i s study. 6.2.2.2 P i t t o High-Pressure O p e r a t i o n ; E n t r a p m e n t F a c t o r s P r e d i c t i o n s f o r the high-pressure c o a l entrainment f a c t o r s i n d i c a t e a s i g n i f i c a n t improvement i n entrainment, see Table 6.2. P r e d i c t e d v a l u e s f o r P H P C E a r e 0.75, 0.70 and 0.90 f o r Runs 1,2 and 3 r e s p e c t i v e l y . I t i s f e l t t h a t these v a l v e s are reasonable s i n c e p r e d i c t i o n s f o r fuming r a t e s d u r i n g those p e r i o d s of uniform h i g h - p r e s s u r e i n j e c t i o n agree w e l l with furnace behaviour. The value f o r Run 1, however, i s suspect i n l i g h t of 152 the a d d i t i o n of a t e r n t o account f o r the m e l t i n g of z i n c - r i c h m a t e r i a l . The d i f f e r e n c e i n the valu e s p r e d i c t e d f o r Runs 2 and 3 i s b e l i e v e d t o be due to a combination of the g r e a t e r bath depth (mass) and the lower h i g h - p r e s s u r e c o a l r a t e i n Run 3. I t i s reasoned t h a t t h i s combination acted t o reduce the p o r t i o n of hig h - p r e s s u r e c o a l s t r i p p i n g through the bath unconsumed. 153 CHAPTER VII SENSITIVITY ANALYSIS For reasons s t a t e d p r e v i o u s l y , i t was necessary t o modify the R i c h a r d s 1 3 k i n e t i c model t o i n c l u d e the r e d u c t i o n and removal of l e a d p r i o r t o the model being f i t t e d t o the i n d u s t r i a l data from the high - p r e s s u r e t r i a l s . T h i s m o d i f i c a t i o n has n e c e s s i t a t e d the use of parameters whose valu e s can o n l y be esti m a t e d . Consequently, i t was necessary t o e s t a b l i s h the s e n s i t i v i t y of the model t o these parameters. In a d d i t i o n , the s e n s i t i v i t y a n a l y s i s was necessary t o a i d i n i d e n t i f y i n g any p o t e n t i a l l y s i g n i f i c a n t k i n e t i c phenomena w i t h i n the pro c e s s . To c a r r y out the s e n s i t i v i t y a n a l y s i s , o p e r a t i o n a l data from the normal low-pressure run p r e v i o u s l y analyzed i n Chapter VI was used as a st a n d a r d . The data from t h i s run are presented i n Table 7.1 7.1 S e n s i t i v i t y A n a l y s i s ; R e s u l t s The s e n s i t i v i t y of the o r i g i n a l model t o parameters c e n t r a l to the k i n e t i c c o n c e p t i o n of the process has been demonstrated p r e v i o u s l y . l a ' D T h e r e f o r e , the s e n s i t i v i t y a n a l y s i s of the m o d i f i e d model focussed o n l y on the parameters r e l a t e d to the k i n e t i c s of l e a d r e d u c t i o n and removal from the bath. These TABLE 7.1 Standard Conditions for Sensitivity Analysis Operating Parameters: Slag Data: Coal Data: F L p c E - 0.23 - 0.37 FLPCC = 0 - 4 0 - ° - 6 5 Initial 15.01 Zn Composition LSI Pb Assay 50! Fixed Carbon (wt.Z) 25X Vol at lies 11 Moisture F0 Xv =0.03-0.32 FPBL = ( U 0 2+ 21.51 Fe 7.01 Fe3* Volatile 501 Fixed Carbon Coip. 20Z Hydrogen (vt.X) 10Z Oxygen rP b M x i o ' 3 . P 11.71 CaO 23.21 Si02 * 4.01 A1203  % 1.0Z S Furnace Dimensions: L = 8.00 • U = 3.10 • Furnace Operating Conditions (see Appendix I - Normal Operation) Injection Dynamics: Initial Teaperature = 1250°C Slag Circulation Velocity = 3.0 i/s Initial Height = 50,000 kg. Porosity = 30X * 74 No. Tuyeres Bubble Frequency Tuyere Column Vord Fr. = 0.600 ii1 -3 Density Liquid Slag = 3600 kg.m_3 Density Solid Slag = 3900 kg.m" DFE0 / 0 F E * 10.0 Boudouard Reaction: 6 -1 -1 A = 3.13 x 10 kPa S o E = 196200 a kJ kg.mole -1 Char-steam Reaction: 6 A = 1.0 x 10 o E = 183739 a k P a V 1 kJ kg.mole -1 155 parameters a r e : the i n i t i a l f r a c t i o n of lead as metal, F p f i L , the Pb r a d i u s of the m e t a l l i c lead p r i l l , r p and the magnitude of the d i f f u s i v i t y of PbO i n the s l a g r e l a t i v e to t h a t of ZnO, D Z n C / D P b o However, i n l i g h t of the q u e s t i o n r a i s e d i n Chapter VI, the a n a l y s i s has been expanded to i n c l u d e other parameters: l i j the c i r c u l a t i o n v e l o c i t y of the s l a g , V s j a g ( i 1 ] the f r a c t i o n of remaining oxygen consumed by f e r r o u s i r o n o x i d a t i o n , P Q x y / [ i i i ] the a c t i v i t y c o e f f i c i e n t of P e 2 0 j i n the s l a g , and, [ i v ] the magnitude of the d i f f u s i v i t y of Fe^O^ to FeO, ZnO and PbO ( D p e Q / D p . ^ -7.1.1 R e s u l t s and D i s c u s s i o n The s e n s i t i v i t y of the model to F p f i L i s i l l u s t r a t e d i n F i g s.7.1 - 7.3. A decrease i n F p B L f r o m the standard value of 0.5 to 0.1 has no obvious e f f e c t on the p r e d i c t e d lead p r o f i l e , see F i g 7.2. Mo obvious e f f e c t a l s o was observed on the bath temperature p r o f i l e , not shown. There was however, a n o t i c e a b l e i n f l u e n c e on both the z i n c and f e r r o u s i r o n p r o f i l e s , see F i g s . 7.1 and 7.3. A decrease i n F p f i L brought about a s l i g h t decrease i n the i n i t i a l z i n c fuming r a t e and s i m u l t a n e o u s l y a s l i g h t decrease i n the i n i t i a l r a t e of g e n e r a t i o n of f e r r o u s i r o n . The e f f e c t i s o p posite f o r an i n c r e a s e i n F D n r to 0.9. I t i s worth Figure 7.1 The effect of F D n r on the predicted Zn p r o f i l e LEGEND • = INT. FRAC. PB AS METL.=0.50(STD.) o = STD.*0.2 A = STD.*1.8 K ) -in <N -CD | 1 1 1 1 1 I I I " | U IJi 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100 TIME(MINS) F i g u r e 7.2 The e f f e c t of F on the p r e d i c t e d Pb p r o f i l e 2 + F i g u r e 7.3 The e f f e c t of F p B L on the p r e d i c t e d Fe p r o f i l e 159 n o t i n g t h a t the i n f l u e n c e on the c o n c e n t r a t i o n of f e r r o u s i r o n i s o n l y s h o r t term. T h i s behaviour i s not unexpected s i n c e the model should tend t o move toward an e q u i l i b r i u m amount of m e t a l l i c l e a d i n the furnace f o r a g i v e n s l a g c o n c e n t r a t i o n and s e t of o p e r a t i n g c o n d i t i o n s . F o r example, i f the i n i t i a l value s e l e c t e d f o r F p B L i s h i g h , the r a t e of o x i d a t i o n of m e t a l l i c l e a d w i l l be e l e v a t e d r e l a t i v e t o the r a t e of r e d u c t i o n and, t h e r e f o r e , the r a t e of g e n e r a t i o n o f f e r r o u s i r o n v i a o x i d a t i o n of the l e a d p r i l l a l s o w i l l be e l e v a t e d . Since t h i s w i l l r e s u l t in a s h o r t term r a p i d decrease i n the amount of m e t a l l i c l e a d , the i n c r e a s e i n the f e r r o u s i r o n g e n e r a t i o n r a t e should o n l y be s h o r t l i v e d . I t i s i n t e r e s t i n g to note t h a t the z i n c and f e r r o u s i r o n p r o f i l e s f o r * p B L - ft.l and 0.9 bound t h a t f o r F p B L = 0.5. T h i s i n d i c a t e s t h a t F p B L = 0.5 i s c l o s e r to the e q u i l i b r i u m amount of l i q u i d l e a d . Pb Model s e n s i t i v i t y t o r i s i l l u s t r a t e d i n F i g s . 7 . 4 - 7.6. P Both the l e a d and f e r r o u s i r o n p r o f i l e s show a moderate s e n s i t i v i t y t o t h i s parameter, F i g s . 7.5 and 7.6, r e s p e c t i v e l y . In the case of the model p r e d i c t i o n s f o r l e a d removal, there i s a marked decrease i n the removal r a t e a t bath l e a d contents below approximately 0.75 wt% f o r a f a c t o r o f ten i n c r e a s e i n r P b • The f e r r o u s i r o n p r o f i l e s i n d i c a t e an i n i t i a l decrease i n the r a t e of Pb g e n e r a t i o n f o r an i n c r e a s e i n r . The long term e f f e c t , Figure 7 . 5 The effect of r on the predicted Pb profile F i g u r e 7.6 The e f f e c t of r^b on the p r e d i c t e d F e 2 + p r o f i l e Figure 7.7 The effect of D_Mrt /D___ on the predicted Zn p r o f i l e 164 however, Is o n l y minimal. The z i n c p r o f i l e s , P i g . 7.4, i n d i c a t e Pb l i t t l e s e n s i t i v i t y t o r p . Temperature s e n s i t i v i t y a l s o was i n v e s t i g a t e d (not shown) and was found t o be minimal. I t i s worth n o t i n g t h a t i n a l l three cases, z i n c , lead and f e r r o u s Pb i r o n , there i s no e f f e c t observed f o r a decrease i n r The s e n s i t i v i t y of the model t o the r e l a t i v e magnitude of D p B 0 i s shown i n F i g s . 7.7 - 7.10. The z i n c , l e a d and temperature p r o f i l e s are r e l a t i v e l y i n s e n s i t i v e t o D Z N Q / D p g 0 ( D Z N O ) c o n s t a n t , see F i g s . 7.7, 7.8 and 7.10, r e s p e c t i v e l y . For example, a f a c t o r of four change i n B p B 0 r e s u l t s i n on l y a 0.5 wt% change i n bath z i n c c o n c e n t r a t i o n . The f e r r o u s i r o n p r o f i l e , F i g . 7.9 shows a moderate s e n s i t i v i t y to D p B Q . An in c r e a s e i n DZMO ^ DPBO r e s u l f - s * n a n i n c r e a s e i n the p r e d i c t e d f e r r o u s i r o n g e n e r a t i o n r a t e . As mentioned p r e v i o u s l y , the quest i o n s r a i s e d i n Chapter VI have n e c e s s i t a t e d an i n v e s t i g a t i o n of the models s e n s i t i v i t y t o s e v e r a l parameters not d i r e c t l y a s s o c i a t e d with lead r e d u c t i o n . Model s e n s i t i v i t y to these parameters i s d e s c r i b e d below. One of the more obvious changes brought about by the i n c l u s i o n of l e a d r e d u c t i o n was a s u b s t a n t i a l i n c r e a s e i n s l a g c i r c u l a t i o n v e l o c i t y - from Im/sec p r e v i o u s l y assumed by R i c h a r d s 1 3 t o 3m/sec i n the present model. Model s e n s i t i v i t y to "1 LEGEND t n ~ • = DZNO/DPBO=1.0(STD.) 0 = DZNO/DPBO=0.5 Q |A = DZN0/DPB0=2.0 in CN -q CN -T I M E ( M I N S ) Figure 7.8 The e f f e c t of D /DDnr. on the predicted Pb p r o f i l q d o o t o O Q o o CN O o LEGEND • = DZNO/DPBO=1.0(STD.) C = DZNO/DPBO=0.5 A = DZNO/DPBO=2.0 o o o 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 TIME(MINS) F i g u r e 7 . 1 0 The e f f e c t of D...,- / D D n A on the p r e d i c t e d temperature p r o f i l e cri - J Figure 7.11 The e f f e c t of V , on the p r e d i c t e d Zn p r o f i l e 169 the parameter i s i l l u s t r a t e d i n P i g s . 7.11-7.14. I t should be mentioned t h a t , w i t h i n the context of the model, s l a g c i r c u l a t i o n v e l o c i t y i s e f f e c t i v e l y a measure of char p a r t i c l e r e s i d e n c e time i n the bath. With t h i s i n mind, the z i n c p r o f i l e , F i g . 7.11, can be seen to be moderately s e n s i t i v e to t h i s parameter. There i s a s i g n i f i c a n t decrease i n the z i n c fuming r a t e , 0 - 20 mins elapsed time, f o r a lm/sec decrease i n V , . I t s hould be noted t h a t ' s l a g the e f f e c t of a 2ro/sec v a r i a t i o n i n V , becomes minimal f o r s l a g e l apsed times g r e a t e r than approximately 80 mins. T h e r e f o r e , f o r p r e d i c t i o n s of o v e r a l l furnace c y c l e times the e f f e c t should be minimal. Ferrous i r o n p r e d i c t i o n s , see F i g . 7.12, are c o n s i d e r a b l y more s e n s i t i v e to V , than p r e d i c t i o n s f o r z i n c c o n c e n t r a t i o n s l a g i n the bath. The r a t e of g e n e r a t i o n of f e r r o u s i r o n i s reduced by an i n c r e a s e i n s l a g c i r c u l a t i o n v e l o c i t y . T h i s behaviour i s o p p o s i t e to t h a t of z i n c . The l e a d and temperature p r o f i l e s , F i g s . 7.13 and 7.14, r e s p e c t i v e l y , show no obvious s e n s i t i v i t y to the s l a g c i r c u l a t i o n v e l o c i t y . Model s e n s i t i v i t y t o F i s i l l u s t r a t e d i n F i g s . 7.15-7.18. J oxy The p r o f i l e s of z i n c , l e a d , f e r r o u s i r o n and temperature a l l show a high degree of s e n s i t i v i t y to F Q x y . The s e n s i t i v i t y of l e a d i s l e s s , approximately 0.5 wt%, and o p p o s i t e . An i n c r e a s e i n F 2 + Figure 7.12 The e f f e c t of Vslaq o n t n e p r e d i c t e d Fe p r o f i l e ZINC SLAG FURNACE MODEL LEGEND • = SLAG CIR. VEL.=3.0(M/S) o = SLAG CIR. VEL.=2.0 A = SLAG CIR. VEL.=4.0 T I M E ( M I N S ) Figure 7.13 The e f f e c t of V on the p r e d i c t e d Pb p r o f i l e q d o q d o -o O H o o o LEGEND • = SLAG CIR. VEL.=3.0 (M /S) o = SLAG CIR. VEL.=2.0 A = SLAG CIR. VEL.=4.0 I I I I I I I I I 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 TIME(MINS) F i g u r e 7.14 The e f f e c t of V s l a g on the p r e d i c t e d temperature p r o f i l e Figure 7.15 The e f f e c t of F on the p r e d i c t e d Zn p r o f i l e oxy Ul 174 r e s u l t s i n an i n c r e a s e i n the fuming r a t e of Pb, see P i g . 7.16. An i n c r e a s e i n F o x y a l s o g i v e s r i s e t o a s i g n i f i c a n t decrease i n the p r e d i c t e d f e r r o u s i r o n c o n c e n t r a t i o n , and an inc r e a s e i n the p r e d i c t e d bath temperature, see P i g s . 7.17 and 7.18, r e s p e c t i v e l y . Model s e n s i t i v i t y t o the s l a g a c t i v i t y c o e f f i c i e n t of P e 2 ° 3 i s i l l u s t r a t e d i n F i g s . 7.19 and 7.20. As can be seen, there i s l i t t l e or no obvious e f f e c t on the z i n c or f e r r o u s i r o n p r o f i l e s , see F i g s . 7.19 and 7.20 r e s p e c t i v e l y . The s e n s i t i v i t y o f l e a d and temperature p r e d i c t i o n s t o P e 2 ° 3 ' w e r e a l s o i n v e s t i g a t e d (not shown) and found t o be minimal. F i n a l l y , the s e n s i t i v i t y of the model t o the r e l a t i v e magnitude of D „ n was i n v e s t i g a t e d . The r e s u l t s are r e 2 3 i l l u s t r a t e d i n F i g s . 7.21-7.24. Both the p r e d i c t e d z i n c and f e r r o u s i r o n p r o f i l e s show a moderate s e n s i t i v i t y . In the case of the z i n c p r o f i l e s , F i g . 7.21, a f a c t o r of four i n c r e a s e i n D„ n r e s u l t s i n a 1 wt% decrease i n p r e d i c t e d bath c o n c e n t r a -F e 2 0 3 t i o n a t about 35 minutes elapsed time. At 100 minutes elapsed time the v a r i a t i o n i s down t o 0.5 wt%. In the case of f e r r o u s i r o n , d e c r e a s i n g D „ rt by a f a c t o r of f o u r r e s u l t s i n a 2 wt P e 2 ° 3 decrease i n the f e r r o u s i r o n c o n c e n t r a t i o n , see F i g 7.23. The p r e d i c t e d l e a d p r o f i l e , F i g . 7.22, can be seen t o be r e l a t i v e l y i n s e n s i t i v e , and the temperature p r o f i l e , F i g . 7.24, completely i n s e n s i t i v e t o D_ rt . P e 2 ° 3 LEGEND • = FRAC. 02 TO FE2O3=0.20(STD.) 0 = STD.«0.5 A = STD.»2.0 TIME(MINS) Figure 7.16 The e f f e c t of F on the p r e d i c t e d Pb p r o f i l e Figure 7.17 The effect of F on oxy 2 + the predicted Fe p r o f i l e q d o —i q d o Q o o <N q d o o o o 0.0 10.0 LEGEND • = FRAC. 02 TO FE2O3=0.2(STD.) 0 = STD.»0.5 A = STD.*2.0 ^ 1 1 I 1 I I 1 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 T I M E ( M I N S ) F i g u r e 7.18 The e f f e c t of F Q x y on the p r e d i c t e d temperature p r o f i l e -4 to - i 100 T I M E ( M I N S ) F i g u r e 7.19 The e f f e c t of XFe Q on the p r e d i c t e d Zn p r o f i l e IT) . rO rO . rO O ro O S or cn z> o or q iri. CN q ro . q CN ' o LEGEND • = ACT. COEFF. FE203=STD. 0 = STD.»0.5 A = STD.*2.0 q q 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 T I M E ( M I N S ) 80.0 90.0 100 Figure 7.20 The effect of ^ P e 0 on the predicted F e 2 + p r o f i l Figure 7.21 The e f f e c t of D_ /D_, n on the predicted Zn p r o f i l e CO O LEGEND • = DFEO/DFE2O3=10.0(STD.) 0 = DFEO/DFE2O3=5.0 A = DFEO/DFE2O3=20.0 - T « -90.0 100.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 T I M E ( M I N S ) 70.0 80.0 figure 7 . 2 2 The effect of D Feo / D F e 2 0 3 on the predicted Pb p r o f i l e 2 + F i g u r e 7.23 The e f f e c t of D p e Q /D p e Q on the p r e d i c t e d Fe p r o f i l * o LEGEND • = DFEO/DFE2O3=10.0(STD.) 0 = DFEO/DFE2O3=5.0 A = DFEO/DFE2O3=20.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 T I M E ( M I N S ) F i g u r e 7.24 The e f f e c t of n p e Q /D p e Q on the p r e d i c t e d temperature p r o f i 184 Having presented the r e s u l t s of the s e n s i t i v i t y a n a l y s i s , i t i s worthwhile t o assess the i n f l u e n c e t h a t the i n c l u s i o n of l e a d r e d u c t i o n has had on the p r e d i c t i o n of process k i n e t i c s before proceeding with a d i s c u s s i o n of the r e s u l t s . 7.1.1.1 The I n f l u e n c e of Lead on K i n e t i c s As d e s c r i b e d p r e v i o u s l y , w i t h i n the o v e r a l l furnace model, there are two d i s t i n c t k i n e t i c models which i n v o l v e l e a d : the char p a r t i c l e - s l a g r e a c t i o n model, and the l e a d p r i l l - s l a g r e a c t i o n model. T h e r e f o r e , t o assess the i n f l u e n c e of l e a d i n the o v e r a l l model, the e f f e c t of l e a d on k i n e t i c s of the two r e a c t i o n models f i r s t must be e s t a b l i s h e d . 7.1.1.1.1 The K i n e t i c s of the Char P a r t i c l e - S l a g Model At the i n i t i a l i z a t i o n of char p a r t i c l e - secondary bubble r e a c t i o n with the s l a g , the r a t e s of Boudouard and char-steam r e a c t i o n s a r e a t a maximum. I f the combined r a t e s of r e a c t i o n are s u f f i c i e n t l y high so as to f i x the secondary bubble oxygen p o t e n t i a l , then r e d u c t i o n of ZnO, PbO and P e 2 0 3 w i l 1 proceed a t a r a t e l i m i t e d by mass-transfer of these s p e c i e s i n the s l a g . The r e l a t i v e r a t e s of mass-transfer or r e d u c t i o n w i l l be dependent on the p a r t i c u l a r oxides m o b i l i t y and thermodynamic s t a b i l i t y i n the s l a g . 185 As r e a c t i o n proceeds and carbon w i t h i n the c o a l char i s consumed, the combined r a t e of Boudouard and char-steam r e a c t i o n s begin to slow. E v e n t u a l l y a p o i n t i s reached where these r e a c t i o n s can no longer "keep up" with the r a t e a t which r e d u c t i o n i s o c c u r r i n g , and t h e r e f o r e , the oxygen p o t e n t i a l begins to r i s e . T h i s r e s u l t s i n a decrease i n the r a t e s of r e d u c t i o n and a s h i f t i n k i n e t i c c o n t r o l to the combined r a t e of the Boudouard and char-steam r e a c t i o n s . As the oxygen p o t e n t i a l c o n t i n u e s to r i s e , a p o i n t i s r e a c t e d e v e n t u a l l y where the r e d u c t i o n of ZnO i s no longer thermodynamically p o s s i b l e . Prom t h i s time on, the m e t a l l i c z i n c vapour e f f e c t i v e l y becomes the source of r e d u ctant and i s r e - o x i d i z e d back i n t o the s l a g . K i n e t i c c o n t r o l i s s h i f t e d back to s l a g d i f f u s i o n as both PbO and Fe^O^ continue to be reduced. As s t i l l longer r e a c t i o n times ( s l a g r e s i d e n c e times) the r e d u c t i o n of PbO a l s o i s no longer p o s s i b l e . From t h i s time on, the l a s t stages of r e a c t i o n proceed with the back o x i d a t i o n of metal z i n c and l e a d , and the r e d u c t i o n of F e 2 0 3 u n t i l , i f r e s i d e n c e time permits, e q u i l i b r i u m with the s l a g i n e v e n t u a l l y a c h i e v e d . I t i s worth n o t i n g t h a t the extent of char p a r t i c l e - s e c o n d a r y bubble r e a c t i o n with the s l a g , and t h e r e f o r e , the amount of z i n c and l e a d p r e d i c t e d to be reduced are s o l e l y dependent on the r e s i d e n c e time of the char p a r t i c l e i n the s l a g f o r a g i v e n s e t of r e a c t i o n k i n e t i c s . In summary, from t h i s d e s c r i p t i o n , the i n f l u e n c e of PbO on the k i n e t i c s of the char p a r t i c l e / s a g model are c l e a r . In 186 essence, PbO competes f o r r e d u c t a n t with both ZnO and Pe^O^ and i n so doing, e f f e c t i v e l y decreases the amount of each reduced f o r a g i v e n c h a r - p a r t i c l e s l a g r e s i d e n c e time. 7.1.1.1.12 The K i n e t i c s of Lead P r i l l O x i d a t i o n In comparison, the k i n e t i c s of l e a d p r i l l o x i d a t i o n are r e l a t i v e l y s t r a i g h t forward. The r a t e of back o x i d a t i o n of l e a d i s s o l e l y dependent on the mass-transfer r a t e s of P e 2 0 3 , PbO and FeO ( F e 2 0 3 t o the p r i l l - s l a g i n t e r f a c e and PbO and FeO away from the i n t e r f a c e ) . The r e l a t i v e r a t e s of mass-transfer are dependent on s e v e r a l q u a n t i t i e s i n c l u d i n g the r e l a t i v e m o b i l i t y and thermodynamic s t a b i l i t y of the p a r t i c u l a r o x i d e . The o v e r a l l mass t r a n s f e r r a t e w i l l be a f u n c t i o n of the r a d i u s of the l e a d p r i l l . Having completed both a s e n s i t i v i t y a n a l y s i s and a d e s c r i p t i o n of r e a c t i o n k i n e t i c s , i t i s now p o s s i b l e to d i s c u s s model s e n s i t i v i t y . 7.2 S e n s i t i v i t y A n a l y s i s : D i s c u s s i o n The model has been shown t o be r e l a t i v e l y i n s e n s i t i v e to those parameters used i n the f o r m u l a t i o n of lead r e d u c t i o n and Pb removal w i t h i n the model. T h e r e f o r e , the v a l u e s used f o r r p F_ n_ and D i n the model may be assumed to be reasonable PBL PBO approximations. 187 I t i s i n t e r e s t i n g to note t h a t i n d u s t r i a l l e a d p r o f i l e s f o r the normal low-pressure run and three h i g h - p r e s s u r e runs, see F i g s . 6.4, 6.8, 6.12, and 6.16, r e s p e c t i v e l y , more c l o s e l y resemble the p r o f i l e p r e d i c t e d with the model u s i n g the l a r g e r P R l e a d p r i l l s i z e ( r " = 80 E - 04 M), See F i g . 7.2. T h i s would seem to i n d i c a t e d t h a t there i s some k i n e t i c phenomenon which l i m i t s the r a t e of l e a d e l i m i n a t i o n a t lower bath l e a d l e v e l s . W i t h i n the model, t h i s behaviour i s a r e s u l t of d e c r e a s i n g the Pb r a t e of o x i d a t i o n of the l e a d p r i l l ( i n c r e a s i n g r ) to such an P extent t h a t i t e v e n t u a l l y becomes the r a t e l i m i t i n g s t e p i n the removal of l e a d . V i r t u a l l y a l l the o x i d i c l e a d i s reduced to m e t a l l i c l e a d w i t h i n the furnace and, hence, must await r e -o x i d a t i o n i n order to be re-reduced and removed from the bath. If t h i s i s i n f a c t the case, i t p o i n t s to the need f o r proper r e a c t o r d e s i g n i n order to remove the l e a d i n l i q u i d form, eg. c o l l e c t i o n on the bottom, and a v o i d the r e c y c l i n g of l i q u i d lead and waste of r e d u c t a n t . One of the more troublesome q u e s t i o n s r a i s e d i n Chapter VI i s the j u s t i f i c a t i o n f o r i n c r e a s i n g s l a g c i r c u l a t i o n v e l o c i t y from lm/sec, assumed i n R i c h a r d s 1 3 o r i g i n a l model, to 3m/sec i n the m o d i f i e d model. From the p r eceding d i s c u s s i o n on the k i n e t i c s of the char p a r t i c l e r e a c t i o n system, the reason f o r the i n c r e a s e becomes c l e a r - i t i s necessary to reduce the char 188 p a r t i c l e r e s i d e n c e time to o f f s e t the a b i l i t y of l e a d to d i s p l a c e z i n c from the secondary bubble. I t remains however, to j u s t i f y the value of 3m/sec. U n f o r t u n a t e l y , there i s no d i r e c t method of doing t h i s . T h e r e f o r e , there i s l i t t l e a l t e r n a t i v e other than to adopt an i n d i r e c t approach. S t a r t i n g with the heat balance, i t i s c l e a r from the s e n s i t i v i t y a n a l y s i s and p r e v i o u s work of R i c h a r d s 1 3 t h a t the model temperature p r o f i l e i s o n l y s e n s i t i v e t o two parameters-F T D _ _ and Fn . On t h i s b a s i s , t h e v a l u e s used f o r F_„„ and u E r ww O X Y OXj f PLPCC most a t l e a s t be c l o s e t o c o r r e c t . By n e c e s s i t y , i t f o l l o w s t h a t the r a t e of r e d u c t i o n of Fe 0 must a l s o be c o r r e c t 2 3 s i n c e the f e r r o u s and f e r r i c i r o n p r o f i l e s e x h i b i t a reasonable f i t . To f i t the remaining z i n c p r o f i l e , i t simply becomes a q u e s t i o n of o b t a i n i n g a c o r r e c t z i n c r e d u c t i o n r a t e r e l a t i v e to the r e d u c t i o n r a t e s of F e 2 ° 3 a n d PbO, w i t h i n the model, f o r a g i v e n s e t of k i n e t i c and thermodynamic d a t a . The o n l y means of doing t h i s i s through adjustment of the c o a l p a r t i c l e r e s i d e n c e time or s l a g c i r c u l a t i o n v e l o c i t y . To r e i t e r a t e , t o f i t a l l t h r e e p r o f i l e s - temperature, z i n c and f e r r o u s i r o n - the r e l a t i v e r a t e ? of r e d u c t i o n must be c o r r e c t . A d d i t i o n a l support f o r t h i s argument comes from the f a c t t h a t a l l of the runs can be f i t w i t h a c o n s t a n t s l a g c i r c u l a t i o n v e l o c i t y once the c o r r e c t v e l o c i t y has been found. T h i s was a l s o the case with the o r i g i n a l R i c h a r d s 1 3 model. 189 F i n a l l y , the q u e s t i o n r a i s e d i n Chapter VI with r e s p e c t to the apparent temperature dependence of F Q x y can be addressed. Based on the s e n s i t i v i t y of the model to F e 2 0 3 , i t i s u n j u s t -i f i a b l e t o assume t h a t the observed e f f e c t i s a s s o c i a t e d with i n c o r r e c t thermodynamic d a t a . 7.3 S e n s i t i v i t y A n a l y s i s : Summary To summarize, the m o d i f i e d model i s a t l e a s t as v a l i d as the o r i g i n a l R i c h a r d s model. A g e n e r a l lack of s e n s i t i v i t y t o F p B L / Pb r and D„„_./D_._lP, i n d i c a t e s t h a t the val u e s used i n the model p ZNO PBO are reasonable approximations. The f a c t t h a t the k i n e t i c s based models have been f i t t e d to a wide range of o p e r a t i n g c o n d i t i o n s i n c l u d i n g h i g h - p r e s s u r e c o a l i n j e c t i o n b u i l d s support f o r t h e i r g e n e r a l v a l i d i t y . 190 CHAPTER VIII  SUMMARY AND CONCLUSIONS 8.1 Summary The r e s u l t s of th r e e runs with a s i n g l e h igh-pressure i n j e c t o r on the Cominco No. 2 z i n c fuming furnace has demonstrated t h a t s u b s t a n t i a l improvements i n both fuming r a t e and e f f i c i e n c y can be achieved with h i g h - p r e s s u r e i n j e c t i o n . T h i s i s i n d i r e c t c o n t r a d i c t i o n t o the p r e d i c t i o n s of the e q u i l i b r i u m based models. The mathematical model by Richards e t a l l a ' b has been m o d i f i e d to i n c l u d e the r e d u c t i o n and removal of le a d from the s l a g bath. A model of d i r e c t " l e a d p r i l l " - s l a g r e a c t i o n has been developed assuming mass t r a n s f e r c o n t r o l i n the s l a g phase. T h i s model has been i n c o r p o r a t e d i n t o the o v e r a l l furnace model. P i t t i n g of t h i s model t o the i n d u s t r i a l h i g h - p r e s s u r e t r i a l data has confirmed t h a t s u b s t a n t i a l improvements i n c o a l entrainment were achieved with h i g h - p r e s s u r e i n j e c t i o n . Roughly 80% of the high - p r e s s u r e c o a l was p r e d i c t e d t o have been e n t r a i n e d i n the s l a g . The c y c l e behaviour of m e t a l l i c l e a d i n the bath p o i n t s t o the need f o r proper r e a c t o r d e s i g n . 191 8.2 Suggestions f o r F u r t h e r Work A t o t a l of three runs with a s i n g l e i n j e c t o r r e p r e s e n t s o n l y a p r e l i m i n a r y study of high- p r e s s u r e c o a l i n j e c t i o n . 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Altman, G. S t a v r o s p o u l o s , K. Parameswaran, R.P. Goel, " V i s c o s i t y Measurements of I n d u s t r i a l B l a s t Furnace S l a g s " , P h y s i c a l Chemistry of E x t r a c t i v e M e t a l l u r g y . Y. Kudryk and Y.K. Rao, ed., Proc.Conf. AIME, Feb. 24-28, 1985, New York, N.Y. 195 39) A.J. Taskinen et a t , "Thermodynamics of Slags i n D i r e c t Lead Smelting", 2nd I n t e r n a t i o n a l Symposium on M e t a l l u r g i c a l  Slaos and F l u x e s . H.A. F i n e , D.R. G a s k e l l , ed., AIME, Nov. 11-14, 1984, Lk. Tahoc, Nev., pp. 741-756 40) R.A. Reyes and D.R. G a s k e l l , "The Thermodynamic A c t i v i t y of ZnO i n S i l i c a t e M e l t s " , Met .Trans .B., V o l 14B. D e c , 1985, pp. 725-731 41) M. Timucin, A.E. M o r r i s , "Phase E q u i l b r i a and Thermodynamic St u d i e s i n the System CaO-FeO-Fe a0 3-SiO*", Metl.Trans., Vol.1, Nov., 1970, pp. 3193-3201 42) 0. Kubaschewski and C B . A l c o c k , M e t a l l u r g i c a l  Thermochemistry. 5ed., Pergamon, New York, 1979 43) K. Azuma, S. Goto, and A. Ogawa, "Thermodynamic S t u d i e s on Zinc Oxide S l a g s " , J.Pac.Eng.Univ.Tokyo, Ser.A, 5.(1), 1967, pp. 54-55 44) R.P. Goel, H.H. Kellogg, and L. L a r r a i n : M e t a l l . Trans. B, Vol. 11B, March 1980, pp. 107-17. APPENDIX 1 FUMING SAMPLING DATA Normal o p e r a t i o n Bath Weight: Coa l Compos i t ion: 50,000 Height Fixed i kg. F r a c t i o n Carbon: 0 .500 V o l a t i l e a : 0.250 M o i s t u r e : 0.01 C o a l V o l a t i l e s : Height Carbon F r a c t i o n : 0.500 Hydrogen: 0.200 Oxygen: 0.100 N i t r o g e n : 0.0 Pr imary Secondary B l a s t B l a s t C o a l ra te H.P - C o a l r a t e Time (min) Temp C C ) Zn % Pb % F e * » % F e a * \ CaO SiOa \ % A 1 , 0 , \ 3 \ tmi mi min min lbs min l b s min 0.0 1250 15.0 1.90 21.5 7.0 11.7 23.2 4.0 0.1 40 330 104 0 10.0 1254 14.7 1.50 22.7 6.1 11.3 22.9 20.0 1260 14.1 1.12 23.7 5.3 11.6 23.0 140 30.0 1250 13.2 0.80 24.6 4.7 11.9 23.1 40.0 1246 12.4 0.53 25.4 4.3 12.0 23.4 50.0 1243 11.5 0.43 26.1 4.0 12.1 23.7 60.0 1240 10.3 0.30 26.8 3.7 12.5 24.3 *E »E -70.0 1237 9.5 0.23 27.8 3.4 12.4 24.9 3 3 154 80.0 1228 8.4 0.16 28.7 3.0 12.5 25.2 T T 90.0 1214 7.4 0.10 29.2 2.7 12.6 25.5 I I 100.0 1201 6.5 0.10 29.8 2.5 12.6 25.5 M M 110.0 1195 5.7 30.4 2.3 12.7 25.4 A A 120.0 1191 4.9 30.9 2.0 12.9 25.9 T T 130.0 1190 4.3 31.4 1.7 13.1 26.4 E E 140.0 U?Q 3,7 31.7 1.5 13.2 26.7 1^ Bath Composi t ion: 50909 kg . Coa l Compos i t ion: Weight F r a c t i o n Fixed Carbon: 0.500 V o l a t i l e s : 0.250 M o i s t u r e : 0.01 Coa l V o l a t i l e s : Weight F r a c t i o n Carbon: 0.500 Hydrogen: 0.200 Oxygen: 0.100 N i t r o g e n : 0.0 Pr imary Secondary Coal H . P . - C o a l B l a s t B l a s t ra te r a t e Time Temp Zn Pb F e a * Fe"* CaO S i O a A 1 2 0 * S mi HM J ibjL l b B (min) (-C) % % % % % % % min min min min 0.0 1330 8.7 0.21 28.7 3.1 12.8 25.0 4.0 0.1 40.0 330 150 0 5.0 1325 8.3 0.12 28.0 3.6 13.0 26.0 40.0 10.0 1320 -7 .6 -7 .9 0.12 29.0 3.0 13.0 26.0 15.0 1305 7.2 0.09 28.6 3.4 12.0 26.2 16 . 17.0 120 16 20.0 1300 6.8 0.08 27.7 4.3 13.0 26.6 120 20 25.0 1300 "6.3-6.5 0.07 28.4 3.6 13.1 27.0 -E - E 135 26 30.0 1280 6.1 0.05 28.2 4.0 13.2 27.2 S S 120 35.0 1260 5.3 0.04 29.5 3.0 13.3 27 .6 T T 40.0 1255 -4 .8 -4 .9 0.03 29 .6 2.7 13.5 28.4 I I 45.0 1250 4.4 0.02 29.6 2.9 13.6 28.6 M M 50.0 1240 3.9 0.02 30.6 2.4 13.4 27.8 A A 55.0 1230 "3.5-3.5 0.02 31.6 1.0 13.6 29.1 T T 60.0 1225 3.1 0.02 30.7 2.0 13.7 29.5 E E 65.0 1225 3.Q 0.07 30.1 2.7 13.7 29.5 1 1 *note: the two numbers represent the r e s u l t s of the double sampling procedure (procedure i s e x p l a i n e d i n text ) Run 2 Bath Weight: 52727 kg . Coal Compos i t ion: Weight F r a c t i o n Fixed Carbon: 0.500 V o l a t i l e s : 0.250 M o i s t u r e : 0.01 Coal V o l a t i l e s : Weight F r a c t i o n Carbon: 0.500 Hydrogen: 0.200 Oxygen: 0.100 N i t r o g e n : 0.0 P r i m a r y Secondary C o a l H . P . - Coal B l a s t B l a s t r a t e r a t e Time Temp Zn Pb F e a - F e a - CaO SiO* A l a O , S "ro* Nm3 lbs. IhS. (min) (*C) % % % % % % \ % min min min min 0.0 1275 16.0 JL.9J 2 L i 6 J J L U 22^ 2 U J JLJ 15 liLQ Lft5 Q - i u f l 112 10.0 1310 15.7 JL-9JI 1^2 fL& L L J 2JLJ : 115 UL2J !L_8_5 2 U . L J , LUfi 2 U 20.0 1325 15.2 fUM 2.LS L J 11*5 2AJ. 25.0 1310 14.8 0x15 22x4 L J m 2 2 J ; UL5 ip__ 30,0 1215 11x2 fLM 2 2 J 5_J 1 2 J 22x2 35.0 1285 13.6 0.50 25.4 4.0 12.5 U J 1Q.Q 1285 13.0 0.40 25.2 4.3 12.4 21x2 15_J2 1215 12.4 0.35 25.7 L i 12x5 25x2 "_£ mJ£ 5JL-Q 1215 11.7 0.30 26.2 4.8 12.6 2^ _5 3 i 51x3 1255 11.0 0.21 26.2 4.0 12.6 25.5 I J. 60.0 1265 10.5 0.25 26.6 3.9 12.6 25.4 I J UQ 2fi_ iS^O 127J} 9_J 0-15 27.1 3-J) 1 2 J 2 i J t) M I I U 1215 9.3 0.13 28.0 3J3 12.7 26.4 & A. U 5 <L_ 75.Q 1212 8.6 0.10 28.2 3.1 12.9 26.7 J T 80.0 1255 7.6 0.12 28.4 3.4 13.1 27.0 g g 85.0 1240 6.9 0.03 29.4 2 . 2 13.2 27.2 :  10 10 Run 3 Bath Weight: 58182 kg. Coa l Composi t ion: Weight P r a c t i o n F ixed Carbon: 0.500 V o l a t i l e s : 0.250 M o i s t u r e : 0.01 C o a l V o l a t i l e s : Weight Carbon ; F r a c t i o n >: 0.500 Hydrogen: 0.200 Oxygen: 0.100 N i t r o g e n : 0.0 Primary B l a s t Secondary B l a s t Coa l r a t e H . P . - C o a l r a t e Time (min) Temp C O Zn % Pb % % Fe»«-% CaO % S i O a A l a o 3 S % Nm» min ML min lbs min Iks. min 0.0 J.265 13.5 3.30 21.9 6.8 10.6 23.1 4.0 .01 40 330 105 0 5.0 1270 13.4 3,10 21.1 7.8 10.6 23.2 40 330 105 0 10.0 J.272 13.2 2.90 21.2 7.8 10.7 23.3 15.0 1275 13.0 2.70 21.6 7.8 10.7 23.3 20.0 1280 12.9 2.70 20.3 8.8 10.8 23.3 25.0 1290 12.5 2.50 22.1 7.6 10.8 23.2 30.0 1300 12.4 2,10 24.2 5.9 10.8 23.6 120 35 35.0 1290 11.8 1.70 24.9 5.4 11, i 24.0 120 38.0 105 40.0 1275 11.0 1.20 26.5 4.3 11.3 24.4 45.0 1265 10.5 1.00 26.1 4.9 11.5 25.0 "E "P 50.0 1255 10.0 0.85 26.7 4,6 11.6 25.5 S s 55.0 1252 9.3 0.62 28.1 3.6 11.8 26.0 T T 60.0 1250 8.4 0.40 28.0 3.8 11.9 26.4 I J 65.0 1255 7.9 0.33 28.5 3.5 12.0 26.9 M M 70.0 1255 7.1 0.25 28.4 3.7 12.1 27.4 A A 72.0 T T 0 75.0 1255 6.8 0.21 28.5 3.9 12.15 27.6 E e 76.0 12P 60.0 1260 6.3 0.15 29.6 3.0 12.2 27.7 120 81.0 135 85.0 126p 5.7 P-12 29-8 3.2 12.2 27.8 135 O o 

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