THE NATURE OF REACTIONS BETWEEN SODIUM SULPHIDE SLAG AND CARBON-SATURATED-IRON AND -COPPER ALLOYS

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1 THE NATURE OF REACTIONS BETWEEN SODIUM SULPHIDE SLAG AND CARBON-SATURATED-IRON AND -COPPER ALLOYS

2 THE NATURE OF REACTIONS BETWEEN SODIUM SULPHIDE SLAG AND CARBON-SATURATED-IRON AND -COPPER ALLOYS By YAVUZ A. TOPKAYA, B.Met. A Thesis Submitted to the School of Graduate Studies in Partial Fulfilment of the Requirements for the Degree Doctor of Philosophy McMaster University October, 1974 < VUZ A.

3 DOCTOR OF PHILOSOPHY (1974) (Metallurgy) McMASTER UNIVERSITY Hamilton, Ontario TITLE: THE NATURE OF REACTIONS BETWEEN SODIUM SULPHIDE SLAG AND CARBON-SATURATED-IRON AND -COPPER ALLOYS AUTHOR: YAVUZ ALI TOPKAYA, B.Met. (Sheffield University) SUPERVISOR: Professor W-K. Lu NUMBER OF PAGES: (xiv), 216 SCOPE AND CONTENTS: The nature of reactions between sodium sulphide slag and carbonsaturated-iron and -copper alloys were studied to obtain a more clear understanding of slag/metal reactions as well as to enhance the knowledge with respect to decopperization and desulphurization reactions. A model has been developed for the kinetics of reactions, taking into consideration the chemical reactions at the gas/slag and metal/slag boundaries as well as diffusion of sodium vapour in the gas phase. The rate constants have been determined by comparing the experimental results with the proposed model of the system using a CDC 6400 computer. Some understanding of the thermodynamics of sulphide solutions, never studied before, was accomplished through calculations. Suggestions were made for the most efficient use of sulphide slags for removing copper and sulphur from carbon-saturated iron melts. It is hoped that this work will open a new area of research on the fundamentals of slag/metal reactions involving sulphides. Such investigations may be helpful in understanding the slag/metal systems involving oxides., 1 11

4 ACKNOWLEDGEMENTS The author wishes to express his sincere gratitude to his research supervisor, Dr. Wei-Kao Lu, whose direction and encouragement throughout different stages of the work led to a successful completion of the project. Special thanks are also due to Dr. M. Tokuda, whose help in the development of the kinetic theories was invaluable. The author would like to extend his thanks to the members of the staff and graduate students of the McMaster University. who through many discussions contributed to the solution of various problems. Finally, the author gratefully acknowledges the following groups for their financial support: the National Research Council of Canada, the Depar~ent of Metallurgy and Material Science and the Maden Tetkik ve Arama - EnstitusU of Turkey. iii

5 '--./ TABLE OF CONTENTS CHAPTER 1: INTRODUCTION CHAPTER 2: LITERATURE SURVEY Kinetics of Slag/Metal Reactions... Literature survey Related to use of Sodium Su1 phi de CHAPTER 3: GENERAL REVIEW OF SULPHIDES AND PROPERTIES OF COMPONENTS OF SLAG/METAL SYSTEM 3.1 General Review of Sulphides for Decopperization and Desulphurization Purposes Properties of Components of Slag/Metal System Properties of sodium sulphide Properties of the Fe - C - S, Cu - C - Sand Fe - Cu - C ternary systems CHAPTER 4: EXPERIMENTAL APPARATUS AND PROCEDURE Introduction... Furnace Assembly Temperature control Furnace Atmosphere Experimental Set-Up... Thermal stability of Na2S. Double crucible experiments.... Capillary experiments! 1} Metal diffusion... 11) Slag/metal reaction. ii1) Vacuum induction me ting unit for filling of capi1la es.. Closed-system experiments. Preparation of materials... Preparation of anhydrous so ium sulphide Preparation of carbon-satur ted alloys... Sampling of slag and metal Chemical Analysis Sulphur analysis Analysis of slag and metal elements (i) Wet chemical analysis of Na 2 S. (i1) Wet chemical analysis of iron in the slag phase (iii) Wet chemical analysis for copper in the slag phase PAGE, ,

6 CHAPTER 5: PAGE (iv) Wet chemical analysis for silicon in iron Precautions and difficulties related to chemical analysis Experimental Procedure.. 1/ Thermal stability of Na S Double crucible experim~nts Filling of capillaries..... Diffusion of sulphur from the capillary to the metal pool Reaction of capillary metal with slag Closed-system experiments EXPERIMENTAL RESULTS CHAPTER 6: DISCUSSION OF RESULTS Introduction Thermal Stabiiity of Sodium Sulpp1de. Double Crucible Experiments..(. Preliminary experiments with large crucibles Experiments with smaller crucibles Ii) Effect of II na ture of the metal phase" 11) Effect of "temperature of reaction ll iii) Effect of II stirring rate ll iv) Effect of ligas/slag and/or slag/metal interfacial areas ll (v) Effect of IIl ength of graphite-gas-slag contact line ll!vi) Effect of liargon flow rate". vii) Effect of "crucible material". Viii) Effect of II slag and metal weights ll ix) Effect of "extremely long times of reaction and high temperature of reaction"... (x) Effect of "initial excess sulphur in the slag phase"..... (xi) Effect of II vacuum ll Capillary Experiments Metal capillary;neta1 pool. Slag pool-metal capillary Closed-System Experiments Introduction Thermal Stability of Sodium Sulpnide.. Thermodynamic Calculations... For the case of carbon-saturated iron For the case of carbon-saturated copper v

7 PAGE Electrochemical Explanation of Slag/Metal Reactions Reversion of SUlphur Transfer.... Theoreti ca 1 Cons i derat ions..... Rate-Controlling Steps..... Experimental Findings i) Effect of "stirring rate"... i1) Effect pf "temperature" iii) Effect of "sl ag -to-metal ratio" iv) Effect of "crucible material II v) Effect of "gas flow rate". vi) Effect of "changes in graphite-gas-slag contact line".... (vii) Calculated partial pressure of sodium vapour (viii) Effect of IIcrucible geometr/ I (ix) Effect of IIchemica1 composition of metal phase" (x) Effect of "initial excess sulphur in slag phase". (xi) Effect of "vacuum" Conclusion from experimental findings. Mixed-Control Moae1s. Mixed-control model for cases of carbonsaturated-iron and -copper alloys Mixed-control model for iron-carbon-copper alloy Accuracy of calculated rate constants... Discussions of Assumptions made in the Calculations and Development of Mixed Control Model Discussions of the Results of the Closed- System experiments Theoretical considerations... Discussion of experimental results Capillary Experiments. Principle of measurements. Macrosegregation on solidification of capillary metal Concentration profi)~n capillary metal Experimental diffusion Joefficient obtained... I'.. Capillary Slag/Metal Reactions " T! vi

8 PAGE CHAPTER 7: SUMMARY AND CONCLUSIONS. APPENDIX I: II: III: IV: V'. VI: VII: VI II : REFERENCES Partial Pressure 0) Sodium and Sulphur in Equilibrium with Pure Na2S at 1250 C Expected Weight Loss in Vacuum from Knudsen's Equation. Solubility of Sulphur in Fe - C Melts at 1250 C Calculation of the Activities in the Ter~ary FeS - Cu?S - Na2S System Calculation of the Activities in Ternary FeS - MnS - Na2S System. Rosenbrock's Optimizatlon Search Technique. 1. Sensitivity of Calculated Rate Constants 2. Sources of Experimental Error Alternate Mechanisms of Reaction vii

9 LIST OF FIGURES Figure Title PAGE 1 The free energy-tew.perature diagram of sulphides Effect of sulphur on solubility of graphite in molten iron 24 3 Composition-temperature relationship in carbon-rich layer 4 of two immiscible liquids in Fe - C - S system Binary phase diagram of Cu - C system Phase diagram of Cu - S system Phase boundaries in the Fe - Cu - S system at 1550~C The furnace and crucible assembly Temperature profile of the resistance furnace at l250 C Crucible assembly for studying the thermal stability of dehydrated NaZS ~ Crucible assembly for the double crucible s~ag/metal experiments Crucible assembly for the capillary experiments: metal <' capill ary-meta 1 pool sys tern Crucible assembly for the capil)ary 'exp~riments: slag/ metal system '".. t' Vacuum induction melting unjt for filling capillaries Crucible assembly for ~he closed-system experiments. Diagram.of the apparatus used f~r the dehydration ~f Na 2 S. 9H Z O.~.... Equilibrium ~iagram ~f the NaZS - H 2 0 system '" viii....'

10 Figure Title PAGE 17 Weight lost from dehydrated Na 2 S against time, at 1250 C wt%~ 1n the metal phase against time Variation of compositions of slag and met~ with time For Fe - C - O.005wt:tS with Na S em ~ crucible Variation of compositions of slag and metal wlth time For Fe - C - O.005wt%S with NdzS. 3.0 em ~ crucible Variation of compositions of slag and metal with time For Fe - C wt%S with N~St 3.0/1.5 ern ' crucible Variation of compositions of slag and metal with time For Fe - C - O.7wttS with Na 2 S, 1.5 em ~ crucible Variation of compositions of slag and metal with time For Fe - C - O.9wt%Cu - O.005wt%S with Na S, em ~ crucible Variation of compositions of slag and metal with time For Fe - C wt%Hn - O.OO5wt1S with NdZS. 1.5 em ~ 25 crucible. Variation of compositions of slag and metal with time 78, \ For Cu - O.OOOlwt%C - O.OOZwttS with NaZS. 1.5 em ~ c ruei b1e a... ~ Variation of,compositions of slag and metal wlth time For. ell - O.OOOlwt%C'- O.002wttS with HaZS, 3.0 em.p c ruei b1e :.. Di~~nsions of cruc;b1~s ~sed in 1nveS~~1ons of effect of interfac.ial areas x

11 Title 28 Dimensions of crucibles used 1n investigations of effect of length of graphite-gas-s1ag contact line 29 System NaZS - FeS System CUZS - Na 2 S 31 II FeS" - Na,.,S Ac tivi ty diagram at 1250 C L 32 I SO-(\C t i vi ty curves for iron and carbon in the liquid and the y-phasc regions of the stable iron-graphite system PAGE CU 2 S - NaZS Activ1ty diagram at 1250 C... Change of partial pressure of sodium vapour with time Sodium lost from slag (in grams) against time (hours) Equivalent transferred or evolued against time, 37 (For Fe - C - O.005wt%$ with Na2S~ 1.5 em ep crucible) Equivalent transferred or evolved against time (For Fe - C wt%S with Na2S~ 3.0 cm ~ crucible) Equivalent transferred or evolved against time (For Fe - C - O.OO5wt%S with Ha2S~ 3.0/1.5 cm ep crucible) Equivalent transferred or evolved against time (For Fe - C - 0.7wt%S with NaZS. 1.5 cm ~ crucible) Equivalent transferred or evolved against time (For Fe - C - O.9wt%Cu - O.OOSwt%S '. with Na S. 1.5 cm ~ 2 crucible) Equivalent transferred or evolved against time (For Fe - C wt%Mn - O.OO5wt%S with Na 2 S, 1.5 cm ep crucible) x

12 Fig.ure Title PAGE 42 Equivalent transferred or evolved against time (For Cu - O.OOOlwt%C - O~~02wt%S with Na 2 S, 1.5 cm ~ cruci ble) Equivalent transfered or evolved against time (For Cu - O.OOOlwt%C - O.002wt%S with Na 2 S, 3.0 cm - crucible) wt%i in the metal phase against time 1-1 -log (rate) against T ok Effect of "crucible material" Two-dimensional arrangement for expected non-stoichiometry Concentration of sulphur (wt%) against distance from interface (em): Metal capillary-metal pool Concentration of sulphur (wt%) against distance from inter- face (on): Metal capillary-slag pool System Cu 2 S - Na 2 S - FeS Rosenbrock's search pattern Sum of Squares of Dif~rences against k**pb Sum of Squares of Differences against k xi

13 Table LIST OF Title TAB4ES I Probable volatility and stability relationship of metallic sulphides II Free energy change for the formation of Na 2 5. III Chemical composition of Na 2 5.9H 2 0 crystals IV Chemical composition of electrolytic iron V Chemical composition of copper metal shots.. VI Accuracy of atomic absorption spectrophotometer It VII Thenmal ~tabi1ity of sodium sulphide.... h' VIII Desulphurization of Fe - C - S alloys: Large double PAGE crucible experiments ,, - - alloys IX Desu1phurization of Fe - C - Mn 5 and,fe C 51 S XI XII XIII XIV X Change of slag and metal compositions with time, For Fe - C - O.005wt%S with NaZS, 1.5 cm ~ crucible 67 Change of slag and metal compositions with time For Fe - C - O.OOSwt%S with Na 2 S, 3.0 cm ~ Change of slag and metal compositions with time For Fe - C - O.005wt%S with Na2S, 3.0/1.5 em ~ Change of slag and metal compositions with time crucible crucible For Fe - C - O.7wt%S with NaZS, 1.5 cm ~ crucible.. 73 Change of slag and metal compositions with time For Fe - C - O.9wt%Cu wt%S with NaZS, 1.5 cruei bl e xii, /' "' " I \., ~

14 Table Title PAGE XV Change of slag and metal compositions with time for Fe - C wt%Mn wt%s with Na 2 S, 1.5 cm ~ crucible XVI Change of slag and metal compositions with time, XVII For Cu - O.OOOlwt%C - O.OO2wt%S with Na 2 S, 1.5 em crucible Change of slag and metal compositions with time For Cu - O.OOOlwt%C - O.OO2wt%S with Na 2 S, 3.0 cm crucible XVIII Effect of llreaction temperature". 83 XIX Effect of llstirring rate of slag/metal system ll. 85 XX Effect of ligas/slag and slag/metal interfacial areas" 87 XXI Effect of III ength of graphite-gas-slag contact 1i ne". 88 XXII Effect of "argon flow rate'" XXII I( Effect of llcrud b1e rna teri a1" XXIV XXV XXVI Effect of "slag and metal weights ll Effect of "excess sulphur in the slag phase ll. Effect of "vacuum ll... XXVII Diffusion of sulphur in carbon-saturated iron at 1250 C 96 XXVIII Capillary experiments: slag/metal reactions. XXIX Closed-system experiments XXX Na 2 S weight loss from a 1.5 cm ~ graphite crucible XXXI Calculated thermodynamic parameters for "FeS li - Na 2 S XXXII Calculated thermodynamic parameters for Cu 2 S - Na 2 S xiii

15 Table Title PAGE XXXIII XXXIV XXXV XXXVI The difference between calculated and observed final slag weights... The effect of "temperaturell... Calculated rate constants for Fe - C - O.005wt%S - NaZS system, 1.5 cm +crucible Calculated rate constants for Fe - C - O.005wt%S - NaZS system, 3.0 cm crucible XXXVII Calculated rate constants for Cu - O.OOOlwt%C - O.OOZwt%S - Na 2 S system. 1:5 cm crucible XXXVIII Calculated rate constants for Cu - O.OOOlwt%C - O.002wt%S - Ka 2 S system, 3.0 cm crucible XXXIX Calculated rate constants for Fe - C - O.9wt%Cu - O.005wt%S - NaZS system, 1.5 cm crucible XXtX Calculations for closed-system experiments. 167 XXXXI Interdiffus;vity of sulphur in carbon-saturated iron at 1250 C, estimated by extrapolation of published data to lower temperature range

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