Acid leach process for treating magnetic and non-magnetic nickel-copper mattes

Magnetic and non-magnetic nickel- copper mattes are treated by an acid pressure leach in which the initial phase is leached out under oxidizing conditions until up to 70% of nickel values are extracted, at which time the leach is continued under non-oxidizing conditions. Copper values are converted from a basic copper sulphate formed during the oxidizing phase to a less acid-soluble form as digenite such that a resulting nickel-rich leach solution has reduced copper values permitting purification of the said leach solution by an atmospheric leach in which non- magnetic as well as magnetic nickel- copper mattes can be added for precipitation of dissolved copper and iron values. A further acid pressure leach is effected under oxidising conditions on the Cu-containing residue, copper recovered from this leach solution and the latter recycled to the first stage pressure leach to maintain the pH therein at 1.8 to 5.0.