Dissolution of nickel (74%)-copper (4%)-sulphide matte anode in acidic chloride solutions

A comparative study was conducted to assess the performance of 1 N solutions of hydrochloric acid, ferrous chloride and ferric chloride solutions to dissolve the 74% Ni-4% Cu-20% S nickel sulphide matte anodes. The essential variables considered here were: temperature (25° and 75° C), oxygen and controlled potential. The relative aggressiveness as estimated by the potentiokinetic polarization curves and dissolution experiments with open circuit can be arranged in the following sequence: Fe³⁺>Fe²⁺>H⁺. At +400 mV versus SCE, the aggressiveness reversed in the following sequence: H⁺>Fe²⁺>Fe³⁺. The experimental values of dissolution rates with open circuit and at constant potential were higher than those calculated from the dissolution current densities (potentiokinetic polarization curves), and from the amount of charge passed (at +400 mV versus SCE).Attack by intergranular dissolution and pitting was observed throughout these experiments. The formation of -NiS by the reaction Ni₃S₂ Ni²⁺+2-NiS+ 2e was confirmed by X-ray analysis. The above results are interpreted in the light of the possible electrochemical mechanisms.