OFR-71-83 Zinc Retorting Without Pollution

The direct retorting of zinc sulfide concentrates in the presence of lime and carbon was examined on a laboratory scale with ten-gram charges of loose mix or fifteen-gram ones of pellets. A slow stream of nitrogen swept through the furnace tube over the boat of reactants, carrying with it the zinc vapor, which condensed on a rolled-up steel sheet near the exit, permitting weight of zinc produced to be obtained after completion of a run. Variables explored were percent of theoretical reagents, temperature, and time. During the runs gas samples were analysed periodically, and the content of CO2, CO and nitrogen were obtained. The quantities of the carbon gases could be calculated because the nitogen flow rate was measured. The gases were also examined for sulfur species, but neither COS nor SO2 could be detected at maximum sensitivity. Zinc recovery increased with excess carbon and lime and with temperature. It ranged from 77% at a ten percent deficiency to 90% at a ten percent excess at 1100°C. Going to 1200°C and theoretical reagents gave 93% recovery. Increasing time from 200 minutes to 300 minutes increased zinc yield by up to 10%. Results from sphalerite and a commercial concentrate from AMAX gave generally similar results. The reaction rate of zinc sulfide as sphalerite and of calcined sphalerite indicated that the latter reacted almost twice as fast, and recovery of zinc was five percent higher. Though the retorting step using sulfide, lime, and carbon proved to be non-polluting, it was feared that on storage the calcium sulfide in the residues would decompose, giving off noxious gases. Consequently the stabilization of the residues by oxidation was essayed. Because of the presence of unreacted zinc sulfide, the first product of oxidation was S02. Reaction of CaS to CaSO4 was incomplete, going from 20% at 800°C to 90% at 1200°C. Thus the non-polluting nature of the process as a whole is in doubt, and its economics are likely to be unfavorable compared to the traditional way of calcining the concentrates, followed by retorting of the oxide with carbon.