Oxidation of cobalt sulfide
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I.
INTRODUCTION
II,
OXIDATION kinetics of sulfide minerals is important for the studies of various nonferrous metallurgical processes. The concentrates are usually the mixed sulfides: for example, iron is preferentially oxidized in the oxidation of copper concentrate, and in the roasting of zinc sulfide concentrate, zinc and iron are simultaneously oxidized, and zinc ferrite is formed. Thus the oxidation mechanism of sulfide concentrate is complicated, and it is of basic importance at the first step of investigation to clarify the oxidation mechanism of pure metal sulfide. In the previous papers,~'2 the authors studied the oxidation of stoichiometric iron sulfide and nickel sulfide of the atomic fraction of sulfur between 0.40 and 0.44. Both iron and nickel sulfides of nonstoichiometric composition are stable at higher temperature. It is known that the cations diffuse outward to the surface in the oxidation of sulfide of lower sulfur activity, and a dense oxide layer is formed without the evolution of SO2 gas. In the oxidation of sulfide having higher sulfur activity, on the other hand, the oxidation proceeds accompanying the evolution of SO2 gas, and a porous oxide is formed. In succession to the investigations on the oxidation of iron and nickel sulfides, the oxidation of cobalt sulfide was studied in the present work. Five sulfide phases of C04S3• C09S8, Co~_,.S, Co354, and CoS2 are known to be stable in the system Co-S. 3 Pannetier et al. 4 measured the oxidation rate of CogSs at temperatures of 483 to 513 K and reported that no SO2 gas was evolved and the reaction was written as C0958 W 2 0 2 = 2C0384 Jr C 0 3 0 4
[1]
Lenchev et al. s oxidized CoS particles in a fluidized bed at 873 to 1123 K and found that the oxidation rate was markedly increased at higher temperature. However, the detailed mechanism of oxidation was not discussed. In the present work, dense plate of Co~_, S was oxidized, and the weight change of the sample and the amount of evolved SO2 gas were continuously measured during the oxidation to pursue the oxidation mechanism. Z. ASAKI, Associate Professor, T. TANABE, Research Associate, and Y. KONDO, Professor, are with the Department of Metallurgy, Kyoto University, Kyoto 606 Japan. M. NITTA, former Graduate Student, Kyoto University, is with Kobe Steel Ltd., Kobe 651, Japan. Manuscript submitted July 8, 1985.
METALLURGICALTRANSACTIONSB
EXPERIMENTAL
A. Preparation of Sample
Cobalt sulfide was prepared from cobalt powder of 99.9 pct purity and distilled sulfur powder of 99.99 pet purity. Weighed powder mixture was heated in an evacuated quartz tube at 520 K for 24 hours and at 1520 K for one hour. Subsequently, the temperature was lowered to 970 K where solid Co~_yS is stable. After the quartz tube was maintained at this temperature for 5 hours, it was quenched in water. The resulting lump of cobalt sulfide was cut to rectangular plates of about 7 mm long, 4 mm wide, and 2 mm thick. Atomic fraction of sulfur, Xs, of the cobalt sulfide was chosen at 0.505, 0.515, and 0.525. From the powder
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