The Bureau of Mines constructed a process research unit (pru) to demonstrate new technology for extracting zinc from sphalerite concentrates. Up to 9 kg/hr of sphalerite (zns) concentrate was reacted at ambient pressure in the pru with 80 pct sulfuric acid at 160 deg to 180 deg c to produce zinc sulfate and elemental sulfur. The pru consisted of a three-stage glass reactor system: a 4-liter premix stage in which feed was slurried with 78 to 82 pct h2so4, a 40-liter stirred reactor, and a 150-liter settling stage in which high reaction conversions were effected and excess h2so4 for recycle was separated from insoluble znso4 and sulfur. Evolution of sulfur- containing gases was minimized by subsurface addition of acid feed slurry to the stirred reactor and by exclusion of air from the reactor to prevent the oxidation of elemental sulfur. Conversion of zinc to zinc sulfate averaged over 99 pct at the optimum acid concentration of 80 pct. Total sulfur losses (primarily so2) were less than 2 pct of the sulfur in the sphalerite. Elemental sulfur remained in the insoluble portion of the settled reactor product after leaching with simulated spent zinc electrolyte containing sulfuric acid. The sulfur residue contained 75 to 89 pct sulfur. Neutral zinc sulfate, required for zinc electrowinning processes, was separated from the sulfuric acid leach solution by crystallization at -15 deg c as zinc sulfate heptahydrate (znso4.7H2o). The crystallization operation simultaneously effected a partial concentration of the recycled electrolyte solution by removing water.