Because state and federal agencies have limited the amount of sulfur oxides that can be discharged to the atmosphere, most copper smelters plan to convert the so2 in offgases to sulfuric acid. In some cases, however, it might be preferable to use a nonsmelting method to fix the sulfur in the form of a discardable product. Research by the federal Bureau of Mines has shown that sulfur in copper concentrate can be converted into insoluble anhydrite (caso4) by roasting the concentrate with lime (cao). After leaching the calcine to recover metal values, the residue may be discarded without harm to the environment. The process can use a rougher flotation product, thus reducing separation costs and saving valuable minerals (for example, molybdenite) that are now lost in making cleaner concentrate. In tests, rougher concentrate was pelletized with the theoretical amount of lime and roasted for 1 hour in a 6 1/2-inch-id by 11-foot-long rotary kiln at 700 deg c. Sulfur retention was about 97 percent. Leaching the calcine with 20 percent hcl extracted approximately 99 percent of the copper, 73 percent of the iron, and 91 percent of the molybdenum. After the molybdenum was removed from the leach solution by adsorption on activated carbon, copper with a purity in excess of 99 percent was obtained by cementation on sponge iron. The strip solution then could be spray-roasted at 500 deg c to recover the hcl for recycle and obtain fe2o3 as a byproduct.