Sulfate and sulfide reactions in the Mn-S-0 system
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studies involving sulfiding and sulfating of manganese oxides, inconsistencies were observed with some of the available equilibrium data for the Mn-S-O system. For example, our initial experiments indicated that MnS is more stable than suggested by the available thermochemical data. Also according to the work of Chao, e t al, 1 u s i n gX - r a y techniques, the solubility limit of MnO in MnS near the eutectic temperature of 1232°C is about 1.7 pct, and the solubility limit of MnS in MnO is about 1.8 pct. Yet, in the CaO-CaS and FeO-FeS systems, the sulfide solubility at saturation is below 0.03 p c t .2"~ In addition, the equilibrium gas partial pressures of SO2 and O2 for the Mn304-MnSO4 system have been measured by Ingraham 5 with the manometric technique, and by Skeaff and Espelund6 with an emf method. T h e i r results are in reasonable accord. However, in some of our tests on the sulfation of mang a n e s e oxides, it was noted that some MnSO4 f o r m e d at SO2 and 02 concentrations below those indicated by the equilibrium constant PSO2 (P02)1/3 obtained by the above investigators. This would indicate the possibility of some sulfate solid solution in manganese oxide, s i m i l a r to the sulfate solid solution in CaO and MgO found in our previous studies.2,7 To resolve some of the inconsistencies in the t h e r modynamics of the Mn-S-O system, the present work was undertaken t o m e a s u r e the gas partial pressure ratios PH20/PH2s for the MnO/MnS equilibrium and t o determine the sulfide and sulfate solid solubility in manganese oxides. E X P E R I M E N T A L
A high-purity manganese dioxide used in the sulfating and sulfiding experiments had the following m a j o r impurities: 0.003 pct (Ca + Mg), 0.01 pct (Na + K) and 0.01 pct Fe. Since the alkali and alkaline-earth sulfates are more stable than the manganE. T. TURKDOGAN, R. G. OLSSON, and J. V. VINTERS are Senior Research Consultant, Associate Research Consultant, and Assistant Scientist(retired), respectively, U. S. Steel Corporation, Research Laboratory, Monroeville, PA 15146. Manuscript submitted May 21, 1976. METALLURGICAL TRANSACTIONS B"
ese sulfate, a high-purity manganese oxide is a prerequisite for this type of equilibrium s t u d y . In the sulfation experiments about 50 mg of manganese oxide was placed on a s m a l l platinum gauze, suspended with a platinum wire in the uniform and constant temperature zone of a vertical furnace tube, and reacted with a desired mixture of SO2 + 02 + N2 at atmospheric pressure. Particular attention had t o be paid t o the quenching of the sample at the end of the experiment. Although the sample was readily quenched by rapidly pulling it to the top of the furnace tube, t h e r e was sulfur pickup from SO2 in the reaction tube. If the sample were pulled out of the reaction tube, t h e r e was sulfur loss from the sample due t o reoxidation in air during cooling. These difficulties were circumvented by placing a cooling chamber at the top of the reaction tube and flushing the chamber with helium p r i o r t
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