A cacl2-o2 leaching procedure and metal-recovery method for treating an arsenical cu-co sulfide concentrate was developed by the Bureau of Mines as an alternative to smelting, which is no longer considered an environmentally acceptable method for treating arsenic- containing ores. The procedure involved leaching in a cacl2 solution with caco3 for 96 h at 26 pct solids (concentrate and caco3), 115 deg c, and a pressure of 50 psig (345 kpa) with oxygen. More than 98 pct of the cobalt and 97 pct of the copper were extracted, and 99.98 pct of the arsenic and 99.9 pct of the iron remained in the leached residue. A filtration rate of 2.9 Gal/(ft2.H)[118 l/(m2.H)] and a cake formation rate of 36 lb/(ft2.H) [172 kg/(m2.H)] were obtained by filtering reacted pulp at 100 deg c. More than 99 pct of the copper in the pregnant solution was extracted as cucl2 by solvent extraction with a new copper extractant. Copper was stripped from the loaded organic with a sulfate-chloride solution, which was used as the anolyte for electrowinning of copper in a membrane cell. Copper plate produced was more than 99.99 pct pure and required 0.81 (Kw.H)/lb cu [1.78 (Kw.H)/kg cu]. Cobalt sulfide was precipitated at ph 3 with ca(hs)2.