Abstract: A multi-stage leaching process is provided for leaching and refining nickel-copper-sulfur concentrates, a substantial portion of the total nickel present being in an acid-soluble metallic form. The process employs a first stage atmospheric leach in combination with two serially coupled second and third stage high pressure oxidation leaching steps in which process streams are recycled within the overall process, such that a purified nickel solution is continually drawn from atmospheric leach and a purified copper product recovered from a pregnant solution obtained in the third stage leach, a spent copper-sulfuric acid being formed which is recycled back into the process for the recovery of metal values therefrom.