Carbothermic reduction of zinc sulfide in the presence of calcium oxide

The carbothermic reduction of zinc sulfide in the presence of calcium oxide has been studied using X-ray diffractometry (XRD), scanning electron microscopy (SEM) and surface area measurements. The results of XRD indicated that zinc sulfide was first transformed from β type to α type, then reacted to give an intermediate product of zinc oxide before being reduced finally to zinc vapor. The sulfur of zinc sulfide was scavenged as calcium sulfide which remained in the solid, and carbon black formed carbon monoxide. SEM showed that the zinc containing particles and carbon grains shrank gradually; the calcium containing grains swelled and sintered during the reaction. The surface area of the solid sample decreased drastically in the initial stage and then increased with reaction time; the pore volume of the solid sample was also reduced much faster initially and then increased slowly. The average pore diameter, however, increased remarkably in the initial stage, decreased and then leveled off. These results were explained by considering the phase transformation of zinc sulfide, escape of zinc vapor, gasification of carbon and expansion and sintering of calcium sulfide. A reaction mechanism and model are proposed to explain the variations in chemical composition and physical properties of the solid sample during the reaction.