Efficient nickel extraction from nickel matte by combined atmospheric-oxygen pressure acid leaching: Thermodynamic analysis and sulfur conversion mechanism

Atmospheric acid leaching was prevalently employed in extraction nickel from nickel matte owing to its gentle effect, but the leaching efficiency of nickel still remains limited. In this paper, the efficient nickel leaching from nickel matte and sulfur conversion mechanism through the combined atmospheric-oxygen pressure acid leaching process were investigated. Factors affecting nickel leaching and sulfur conversion from nickel matte were systematically investigated on the basis of thermodynamic calculations. The results showed that the nickel leaching rate after atmospheric acid leaching could reach 79.91 % under the optimized conditions of H2SO4 concentration of 300 g/L, H2O2 dosage of 7 %, 80 C, liquid-solid ratio of 8 mL/g, and 3 h. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM-EDS) analyses indicated that the S8- coating produced during atmospheric acid leaching and the small amount of NiO passivation film on the surface of the nickel matte impeded the further leaching of nickel. The S-8-coating and NiO passivation film were converted to H2SO4 and water-soluble NiSO4, respectively, after the oxygen pressure leaching process. The Ni3S2 and NiS in the nickel matte were released to continue leaching, and the nickel extraction rate reached 99.11 %. The specific optimized process parameters were H2SO4 concentration of 90 g/L, operating pressure of 1.5 MPa, 180 C, L/S of 10 mL/g and 3 h. This provided a green economical and efficient method for the extraction of nickel from nickel matte.