Investigation of anodic dissolution and surface passivation of high-grade nickel matte in sulfuric acid solution

The anodic dissolution of the high-grade nickel matte in the sulfuric acid solution was investigated at the dynamic potential and potentiostatic polarizations, and the solid products on the surface of the matte at various polarized potentials were characterized by optical microscope, scanning electron microscope and Raman spectroscopy in order to understand the passivation mechanism for the anodic dissolution of nickel matte. The results show that the oxidation dissolution activity of nickel and copper sulfides existing in the matte during the anodic dissolution decreases in the order Cu₂S ​> ​Ni₃S₂ ​> ​NiS ​> ​CuS. When the anodic polarization potential is increasing to 1.2 ​V vs. SCE, the solid product for the oxidation of Cu₂S is composed of CuS, which forms a compact layer with fine cracks covering the surface and hinders the anodic dissolution further of Cu₂S underneath. While, at the same polarization potential, Ni₃S₂ in the matte can be anodically dissolved to form Ni²⁺ and elemental sulfur, and the solid product layer composing of sulfur and NiS is porous with coarse cracks, and has adverse effect on the anodic dissolution of Ni₃S₂ in the nickel matte.