Huge quantities of slag, a waste solid product of pyrometallurgical operations by the metals industry are dumped continuously around the world, posing a potential environmental threat due to entrained values of base metals and sulfur. High temperature pressure oxidative acid leaching of nickel smelter slags was investigated as a process to facilitate slag cleaning and selective dissolution of base metals for economic recovery. Five key parameters, namely temperature, acid addition, oxygen overpressure, solids loading and particle size, were examined on the process performance. Base metal recoveries, acid and oxygen consumptions were accurately measured, and ferrous/ferric iron concentrations were also determined. A highly selective leaching of valuable metals with extractions of > 99% for nickel and cobalt, > 97% for copper, > 91% for zinc and < 2.2% for iron was successfully achieved for 20 wt.% acid addition and 25% solids loading at 200-300 kPa O-2 overpressure at 250 degrees C in 2 h. The acid consumption was measured to be 38.5 kg H2SO4/t slag and the oxygen consumption was determined as 84 kg O-2/t slag which is consistent with the estimated theoretical oxygen consumption. The as-produced residue containing less than 0.01% of base metals, hematite and virtually zero sulfidic sulfur seems to be suitable for safe disposal. The process seems to be able to claim economic recovery of base metals from slags and is reliable and feasible. (c) 2007 Elsevier B.V. All rights reserved.