Synchronous extraction of nickel and copper from a mixed oxide-sulfide nickel ore in a low-temperature roasting system

Mixed oxide-sulfide nickel ore is becoming the major source of nickel produced in the nickel smelting enterprise due to the increasing global demand of nickel and the depletion of high-grade nickel sulfide ore. Low temperature roasting technique by ammonium sulfate is a potential cleaner production process with high metal recovery, high reaction selectivity and low energy consumption. In this work, Chinese mixed oxide-sulfide nickel ores were roasted by ammonium sulfate at low temperatures of 300-500 °C to extract valuable metals directly and synchronously. Some influence factors including roasting temperature, ammonium-to-ore mole ratio, roasting time and particle size of the ore samples were studied in the roasting process. Sodium sulfate was added to improve the extraction of nickel by converting into Na-pyrosulfate. The results showed that more than 97% Cu, 97% Ni and 82% Fe are extracted, under the conditions of roasting temperature 450 °C, roasting time 4 h, ammonium-to-ore mole ratio 2.2:1, particle size −160 + 180 mesh, sodium sulfate dosage 3 g. The evolution behavior and mechanism of various minerals were elucidated using X-ray Diffraction, Thermogravimetric Analysis, Differential Thermal Analysis, Scanning Electron Microscope and Energy Dispersive Spectrometer. It is found that pyrite (FeS2), pyrrhotite (Fe7S8), pentlandite ((Fe,Ni)9S8) and chalcopyrite (CuFeS2) can react sufficiently with ammonium sulfate to form the corresponding metallic ammonium sulfate salts. Ammonium ferric sulfate can be decomposed to produce insoluble iron oxide by increasing the roasting temperature to higher than 500 °C to realize the separation from copper and nickel in the leaching liquid. The ammonium sulfate roasting-water leaching technique provides an effective approach for the extraction of nickel and copper from Chinese low grade mixed oxide-sulfide nickel ores.