The Bureau of Mines conducted a laboratory-scale investigation of a technique that produces zinc sulfate and elemental sulfur from sphalerite concentrates by reaction with sulfuric acid at 175 deg to 200 deg c and ambient pressure. The objective of the investigation was to develop a hydrometallurgical method of producing zinc from sulfide concentrates that does not generate sulfur dioxide pollutants. Reaction with 80- to 85-pct sulfuric acid produced 4 moles of sulfur for every 3 moles of sphalerite reacted, with minimal production of sulfur gases under optimum conditions. More than 98 pct of the zinc was recovered as zinc sulfate after 3 hours at 175 deg c. The reaction mixture was leached with simulated spent zinc electrolyte containing 13-pct sulfuric acid to recover zinc sulfate from the insoluble, high-sulfur residue. Neutral zinc sulfate, which is required for electrolysis, was separated from the sulfuric acid solution by crystallization as znso4.7H2o at -10 deg c. Up to 80 pct of the water in the recycled electrolysis solution was removed by the crystallization process; evaporation was used to further concentrate the sulfuric acid for reuse in leaching sphalerite. Advantages of the proposed method over conventional roasting and leaching include virtual elimination of sulfur dioxide, production of a solid sulfur byproduct, and higher zinc recoveries due to elimination of zinc ferrite losses.