Reductive atmospheric acid leaching of lateritic smectite/nontronite ores in H2SO4/Cu(II)/SO2 solutions

Despite the success of reductive atmospheric acid leaching (RAAL) of limonitic nickel laterite ores in recent studies limited attempt has been made to apply this method to smectite/nontronite ores of different mineralogies. A comparative study of four smectite/nontronite ores in this study showed that the use of 700 kg H2SO4/ton dry ore leaches only 74–86% Ni, 37–76% Co, 47–58% Fe and 24–66% Mn at 90 °C from slurries of 20–35% (w/w) pulp density even after 10 h, depending upon the mineralogy. These values increased to 90–97% Ni, 94–97% Co, 92–98% Mn and 72–85% Fe in the presence of Cu(II)/SO2. The first order dependence of initial fraction of iron, aluminium and nickel leached from a typical smectite ore in the first 0.5 h on the initial acid concentration provides evidence for the involvement of hydrogen ions in the surface reaction. Low activation energy of 10 kJ/mol based on the fraction of nickel leached in the first 0.5 h indicates a diffusion controlled reaction. This is supported by the applicability of a shrinking core kinetic model for metal dissolution over the first 2 h, with different apparent rate constants (kap) depending upon the iron oxide content, mineralogy and porosity. A log–log plot of kap for ores with high iron content as a function of acid concentration agrees reasonably well with the correlation already established for the leaching of nickel from limonitic laterite and manganese nodules. Thus, initial fast leaching can be related to the higher porosity and a rate controlling step which involves the diffusion of H+ through a thickening solid layer. The slow leaching at latter stages is a result of low remnant acid, thickening solid layer and changes in mineral composition.