Oxidation of Molten Nickel and Cobalt Sulfides

An amount of 100 mg of nickel or cobalt sulfide was put in a slender alumina tube of 3 mm ID and 25 mm length and oxidized at 1473 K in a mixed O2–Ar gas stream. Sulfur atomic fraction, xs, was 0.30 to 0.40 for nickel sulfide and 0.400 to 0.446 for cobalt sulfide. When the xs was lower, the NiO or CoO layer was formed on the melt surface without the evolution of SO2 gas and the growth rate of the oxide layer was controlled initially by gas diffusion and subsequently by cation diffusion through the oxide layer. When the xs was higher, the desulfurization took place which resulted in lowering of sulfur potential of the melt. When the pSO2 in equilibrium with the melt and the oxide becomes lower than the atmospheric pressure, the formation of oxide starts to occur. The xs-value at the transition from desulfurization to the oxide formation was 0.30 to 0.33 for nickel sulfide and 0.41 to 0.43 for cobalt sulfide.