A study into the behaviour of nickel, cobalt and metal impurities during partial neutralisation of synthetic nickel laterite pressure leach solutions and pulps

The behaviour of nickel (Ni), cobalt (Co) and major impurities such as aluminium (Al), trivalent chromium [Cr (III)] and ferric iron [Fe(III)] during the partial neutralisation of synthetic pressure acid leach (PAL) liquors and pulps was investigated. Ferric iron precipitation took place between pH(85 degrees C) 2 to 3, and up to 54% of Cr(III) and 42% of Al were co-precipitated with Fe(III). Haematite appeared to act as a seed and promote the removal of Fe (III) from solution at a pH(85 degrees C) as low as 2. Precipitation of the remaining Al and Cr(III) from solution did not commence until at or above pH(85 degrees C) 3.5, and virtually went to completion at pH(85 degrees C) 4 (96% precipitation extent). There was a linear correlation between the %Cr(III) precipitation and %Al precipitation from solution, which suggests that the behaviour of the two species is related and that Cr(III) is unlikely to behave independently in solution. Nickel and cobalt losses were strongly dependent on pulp pH, and valuable metal losses rapidly climbed above pH(85 degrees C) 3 and further increased after 1 h of ageing; this behaviour appears to be consistent with adsorption processes. Cobalt losses were found to increase significantly in the presence of haematite whilst Ni losses were unchanged, suggesting that there is partitioning of Ni and Co during partial neutralisation of PAL pulps with Co preferentially adsorbing onto haematite. Final Ni losses were on average 1.95% and were unaffected by the presence of haematite; final Co losses were 0.38% in the absence of haematite and increased to 3.30% in the presence of haematite.