Smelting of copper concentrates results in the discharge of millions of tons so2 to the atmosphere each year. Because of increasing concern with air pollution, the Bureau of Mines has developed a hydrometallurgical procedure to obtain elemental sulfur from chalcopyrite by means of the following reaction: cufes2 + 3fecl3 -> cucl + 4fecl2 + 2s. Under optimum conditions, 99.9 percent of the copper and 73.7 percent of the iron was extracted in 2 hr from a typical mill concentrate containing approximately 75 percent cufes2 and 15 percent fes2. At the same time, 70.5 percent of the sulfur was oxidized to the elemental form. The leach was highly selective; gangue minerals, including pyrite, were not attacked, thus accounting for the relativley low recoveries of iron and sulfur.