The Bureau of Mines conducted bench-scale leaching tests with recycled spent copper electrolytes and copper-nickel mattes produced from the Minnesota Duluth Complex concentrates. This report describes the first stage of an overall two-stage leach operation. The objective of this research was to determine the matte-leach technology required to convert spent copper electrolyte into an enriched nickel liquor suitable for a nickel refinery and therefore essentially free of copper, iron, and sulfuric acid. The experimental variables investigated were matte composition, particle size, spent electrolyte content, pulp concentration, oxygenation, temperature, and time. The time required to produce an enriched purified nickel liquor decreased with increasing pulp concentration, temperature, and oxygenation, but increased with increasing particle size and electrolyte copper and acid content. Nickel-rich (about 50 g/l) liquors at a ph of about 4 and containing less than 1 g/l of copper and iron were produced at 70 deg. C by leaching minus 325- mesh low-iron (less than 5 pct) matte for 8 hours with spent electrolyte containing 22.4 G/l cu, 28.9 G/l ni, less than 0.1 G/l fe, and 50 g/l h2so4 at a pulp concentration of 1 kg of matte per liter of spent electrolyte. Varying the nickel content of the spent electrolyte from 11 to 101 g/l appeared to have no appreciable influence on the matte leaching characteristics.