Atmospheric acid leaching mechanisms and kinetics and rheological studies of a low grade saprolitic nickel laterite ore

• The impact of time and temperature on leaching mechanisms and kinetics
• Acid consumption capacity of the ore and its impact on leaching economics
• The effect of pulp chemistry changes on slurry rheological behaviour
• The impact of agitation/shear on leaching kinetics
• The controlling mechanism underpinning the leaching of the saprolitic ore

Improved atmospheric acid leaching (AL) of complex, low grade nickel (Ni) laterite ores warrants greater knowledge of the exact processes underpinning Ni and pay metal cobalt (Co) extraction rates, acid consumption and pulp handleability. In this study, the influence of agitation rate (600–1000 rpm) and temperature (70 and 95 °C) on the isothermal, batch acid leaching and rheological behaviour of saprolitic Ni laterite slurry (40 wt.% solid) was investigated over 4 h at pH 1. The leaching behaviour was distinctly incongruent and reflected strong temperature-dependent Ni/Co extraction, acid consumption and the proliferation of gangue minerals' constituent elements (e.g., Na, Mg, Al, Fe). Whilst the total mass of acid consumed per ton of dry ore processed was greater at higher temperature, the total kg acid consumed per kg Ni and Co extracted was markedly lower. In all cases, the slurries displayed time-dependent, non-Newtonian, shear thinning rheological behaviour. The pulp viscosities and shear yield stresses were generally greater at lower than at higher temperature, with both increasing dramatically in the course of 4 h leaching. Agitation rate in the range 600–1000 rpm had no noticeable impact on Ni and Co leaching rates, confirming the insignificance of volume diffusion limitation. Although high pulp shear viscosities in the range 37–120 mPa s were observed in the course of leaching, they did not have an impact on Ni and Co leaching mechanisms and kinetics from the saprolitic laterite ore. The mechanism of saprolitic laterite ore leaching appears to follow a chemical reaction controlled, shrinking core model with apparent activation energies of 75.5 ± 3.8 and 81.2 ± 4.1 kJ/mol, respectively, for the release of Ni and Co.