Effect of thiosulfate, sulfide, copper(II), cobalt(II)/(III) and iron oxides on the ammoniacal carbonate leaching of nickel and ferronickel in the Caron process

Previous studies have related the low recovery of nickel and cobalt in the Caron roast-leach process to the formation of less reactive sulfides during roasting and/or the passivation or surface blockage of ferronickel by oxides and/or sulfides. This study examines the different types of reactions and background reagents which may affect the dissolution of nickel and ferronickel alloy in oxygenated NH3/NH4+/HCO3− solutions at 45 °C based on equilibrium constants and measured leach results at a low solid/liquid ratio of 1 g/dm3. Some of the additives tested in the present study represent interim leach products. Initial leaching rates of nickel during oxygenation of presoaked Fe–Ni alloys decreases with increasing iron mole fraction. The Fe–Ni(45%) alloy continues to react and dissolve about 90% Ni over the first 15–40 min, depending upon the additives. In contrast, iron leaching reaches a broad maximum of ~ 10% over 20–35 min, or a sharp maximum of 6% after 5 min in the absence or presence of additives, respectively. This is followed by a decrease in iron extraction to ≤ 2% after 45–60 min due to the precipitation of red/black oxides and sulfides. Direct involvement of S2O32− and redox mediation by Cu(II) or Co(III) is evident from the enhanced initial rates of nickel leaching from Fe–Ni(45%) alloy in the order: O2/HS− ≪ O2 < O2/S2O32− < O2/Co(III)/S2O32− < O2/Cu(II)/S2O32−. While the added S2O32− has a detrimental effect on iron leaching, HS− retards the leaching of both iron and nickel from Fe-Ni(45%) alloy to < 1%. The final leaching of 95% Ni from Fe–Ni(45%) alloy after 3 h is unaffected by Fe2O3 and Fe3O4, but FeOOH causes about 5% decrease in nickel leaching. Thermodynamics predict the passivation of nickel and ferronickel by M(OH)3–MOOH as well as the formation of MFe2O4.