High pressure oxidative acid leaching (HPOXAL) was successfully applied to slow-cooled converter slags from Vale's operations in Sudbury (Ontario, Canada). Extractions of Ni, Co and Cu exceeded 90% within 15-20 min and levelled at 95-97% after 45 min at 250 degrees C, 90 psi O(2) overpressure and 70 g/L. initial H(2)SO(4). Pyrrhotite tailings with similar to 0.6% Ni content were also tested as a source of sulphuric acid in high pressure oxidation. Co-leaching of pyrrhotite tailings with converter slags at the same temperature, oxygen partial pressure and equivalent stoichiometric H(2)SO(4) was found to have kinetics similar to that of leaching with sulphuric acid. Lowering the addition of pyrrhotite tailings (and hence, the acidity) was found to have a detrimental effect on the kinetics of leaching and final extractions (especially at 250 degrees C), and cause precipitation of metal sulphates. Continuous on-line acidity measurements were facilitated in experiments with an electrodeless conductivity sensor. It was shown that acid plays a major role in the conversion of fayalite to hematite and silica, and the dissolution of the base metals, while oxygen overpressure (or dispersion efficiency) determines the rate of acid generation and re-generation. (C) 2011 Elsevier B.V. All rights reserved.