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A Thermodynamic Study of the Molybdenum-Sulfur System at 1100⁰C

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A Thermodynamic Study of the Molybdenum-Sulfur System at 1100⁰C Scholars' Mine Masters Theses Student Theses and Dissertations 1963 A thermodynamic study of the molybdenum-sulfur system at 1100⁰C Seth C. Schaefer Follow this and additional works at: https://scholarsmine.mst.edu/masters_theses Part of the Metallurgy Commons Department: Recommended Citation Schaefer, Seth C., "A thermodynamic study of the molybdenum-sulfur system at 1100⁰C" (1963). Masters Theses. 2858. https://scholarsmine.mst.edu/masters_theses/2858 This thesis is brought to you by Scholars' Mine, a service of the Missouri S&T Library and Learning Resources. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. A THERMODYNAMIC STUDY OF THE MOLYBDENUM-SULFUR SYSTEM AT 1100°C BY SETH C. SCHAEFER A t h e s i s submitted to the faculty of the SCHOOL OF MINES AND METALLURGY OF THE UNIVERSITY OF MISSOURI in partial fulfillment of the requirements for the Degree of MASTER OF SCIENCE, METALLURGY MAJOR Rolla, Missouri 1963 Approved by (advisor) ACKNOWLEDGEMENTS The author wishes to express his appreciation to Dr. A. H. Larson and Dr. A. W. Schlechten for their valuable guidance through­ out this investigation. The author also wishes to thank American Metals Climax for supplying the high purity molybdenum metal powder used in conducting this investigation. A letter from Dr. John Stubbles, formerly of the University of Manchester, England and now with Youngstown Sheet and Tube Co., Youngstown, Ohio, was helpful in the analysis of the complex X-ray pattern of the molybdenum sesquisulfide phase. ABSTRACT A static method was used to determine equilibrium sulfur pressures for molybdenum-sulfur mixtures at 1100°C. The results obtained were: (1) The molybdenum disulfide phase was stable in a sulfur pressure range of 1.023 x 10-5 to 6.026 x lO-1 atmospheres. A range of non-stoichiometry from MoS2.00 to MoS2.22 was determined under these conditions. (2) The upper limit of sulfur content of molybdenum sesqui­ sulfide (Mo S ) is nearly stoichiometric. 2 3 (3) The extent of solid solution of sulfur in molybdenum is approximately 1.5 atomic % (0.5 wt % ). (4) Molybdenum sesquisulfide (Mo2S3) formed at high tempera­ tures is stable at 600°C. Integral free energies of formation of equilibrium mixtures of Mo and S from solid Mo and diatomic sulfur vapor were calculated. TABLE OF CONTENTS PAGE LIST OF FIGURES ............................................ v LIST OF TABLES ............................................ vi CHAPTER I INTRODUCTION ...................................... 1 II REVIEW OF L I T E R A T U R E .............................. 4 III THEORETICAL CONSIDERATIONS ........................ 11 IV APPARATUS AND E QUI P M E N T ............................ 18 V EXPERIMENTAL PROCEDURE ............................ 25 Materials ........................................ 25 Preparation of MoS2 ............................... 26 Capsulation of Mo-S Mixtures ..................... 27 Equilibration of Mo-S M ixtures ................... 27 VI RESULTS AND C A L C U L A T I O N S .......................... 32 VII DISCUSSION OF RESULTS ............................ 44 VIII CONCLUSIONS ...................................... 55 B I B L I O G R A P H Y ...................................... 57 VITA .............................................. 59 LIST OF FIGURES FIGURE PAGE 1 4 1. Relationship of Log P vs. -o x 10 , showing effect of i T K. temperature on p(S), p(S2), p(S^), p(Sg) and p(Sg) .... 16 1/ 4 2. Relationship of Log pi vs x TK x10 , showing effect of temperature on p(S2) and p ( S ) ............................. 17 3. Sketch of Apparatus ...................................... 24 Equilibrium sulfur pressures for two phase regions at 1 1 0 0 ° C .................................................... 50 5. Variation of sulfur pressure with composition for the disulfide phase at 1100°C ............................... 51 6. Mo-S solid solution and Mo2S2 stoichiometry on 1100°C i s o t h e r m .................................................. 52 7(a). A F- v s . Atomic % S ....................................53 n2 7(b). A F° vs. Atomic % S ................................. 53 8(a). A .F vs. Atomic % S .................................54 S2 8(b). A F° vs. Atomic % S ....................................54 LIST OF TABLES TABLE PAGE I Computer Tabulation of p(S2) ........................ 12 II Tabulation of p(S2) and. p ( S ) ......................... 1^ 11(a) Tabulation of PT = p(S) + p(S2) + p(S^) + p(Sg) + p(Sg ) ................................................ 15 III 500°0 Isotherm (2 day tests) Charge: MoS2 + S . 33 IV 500°C Isotherm (3 day tests) Charge: MoS2 + S . 33 V 900°C Isotherm (2 day tests) Charge: MoS2 + S . 3^ VI 900°C Isotherm (2 day tests) Charge: Mo + S ........ VTI 1100°C Isotherm (2 day tests) Charge: MoS2 + S . 35 VIII 1100°C Isotherm Data (2 day tests) Charge: Mo + S . 35 IX 1100°C Isotherm Data (Mo S2 Stoichiometry ) .......... J>6 X 1100°C Isotherm - Mo-S Solid Solution D a t a .......... 37 XI 1100°C Isotherm Data (MOgS^ Stoichiometry).......... 38 XII Miscellaneous Low p(S2) Data at 9 0 0 ° C ......... 38 XIII Miscellaneous Low p(S2) Data at 1200°C ......... 39 XIV MOgS^ Preparation D a t a .............................. 39 XV M o ^ ^ Stability D a t a ................................. 39 XVI Miscellaneous Mo Solid Solution Data . ... .......... ^0 XVII Miscellaneous Mo,^ Stoichiometry D a t a .............. ^0 XVIII X-Ray Diffraction Data of Mc^S^ (Results of Stubbles and Richardson Investigation) ............................. ^1 XIX X-Ray Diffraction Data of M o ^ ^ (Results of this Investigation) .......................................... ^2 XX Calculations of Free Energy of Mo-S System .......... ^3 CHAPTER I INTRODUCTION A static method was used to study the equilibria which exist between molybdenum and sulfur at temperatures of 500°, 900°, and 1100°C. However, equilibrium was attained only at 1100°C. The products were analyzed by chemical analysis and X-ray diffraction. The study of the molybdenum-sulfur system was selected to deter­ mine the stability of existent molybdenum sulfide phases and to deter­ mine relevant thermodynamic data. The data available in the literature are controversial and no phase diagram exists for this binary system. Molybdenum, molybdenum alloys, and molybdenum sulfides have a variety of industrial applications. Molybdenum and its alloys are especially suitable for high temperature and corrosion resistant applications. Molybdenum disulfide has excellent lubricating properties because of its molecular structure.'*' As a possible explanation of the lubricating properties of MoSg, the generally accepted theory holds that the strong bond between the sulfur to molybdenum atoms and the weak bonds between the sulfur to sulfur atoms provides a low-shear plane between sulfur to sulfur atoms. A more recent suggestion is that an adsorbed layer of amorphous sulfur may also exist and this may actually be necessary to provide the low shear phenomena. Molybdenum disulfide retains good lubricating properties from -300° to 750°F in air. In the absence of air it is stable at temperatures above 2000°F. 2 Molybdenum disulfide and trisulfide are used as catalysts in a variety of hydrogenation-dehydrogenation reactions involving complex 2 hydrocarbon mixtures such as petroleum and coal tars. The chief minerals of molybdenum are: Molybdenite - MoS^ Wulfenite - Pb MoO^ Molybdite - F e ^ , 3Mo03, 7 ^ 0 Of these, molybdenite is the most important. ^Molybdenite is a soft gray mineral containing 59*5 percent molybdenum and 40.5 percent sulfur. It has a specific gravity of 4.7 to 4.8. The largest single deposit, which is being mined commercially in the United States, is located at Climax, Colorado. The ore averages one percent molybdenite; however, this can readily be concentrated by flotation to a product containing 60 to 70 percent molybdenite. A second source of molybdenite is from copper mineral concentra­ tion operations. In 1940, this source contributed 30 percent of the 4 United States output. Molybdenite concentrates may be roasted to molybdic oxide. A "technical grade" molybdic oxide is produced by keeping the roasting temperature below 1100°F. The pure molybdic oxide is prepared by sublimation of the "technical grade" oxide at 1800° to 1900°F in a current of air."1 Calcium molybdate is produced by mixing uncalcined pulverized limestone with the roasted concentrates. This is a suitable product for direct addition to steel furnaces. 3 Ferromolybdenum is produced from molybdic oxide by the Thermit process in which aluminum is used as the reducing agent. Metallic molybdenum may be produced by reducing molybdic oxide with aluminum powder or by heating the trioxide or one of the chlorides 7 in a hydrogen atmosphere. Recently, American Metals Climax has been developing a process for production of metallic molybdenum powder by direct thermal decom- g position of molybdenite. CHAPTER II REVIEW OF THE LITERATURE Despite the widespread use and extensive applied research of molybdenum and molybdenum sulfides, very little has been done to develop fundamental thermodynamic data for the molybdenum-sulfur system. Investigations by M. Guichard, during I896 to 1901, concerning the thermal decomposition of molybdenite in an electric arc furnace, q led him to conclude that the chemical reactions are: 2 MoS2 = Mo2S3 + 1/2 S2 and with further heating: M o 2S3 = 2 Mo + 3/2 S2
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