The use of oxygen in high pressure acid leaching of nickel laterites

Preliminary leaching tests on a sample of limonitic laterite at 220 °C, gave satisfactory recoveries of nickel. It was noted that a significant proportion of the iron remained in solution (about 11 g/l) due to its presence as divalent iron. This was equivalent to about 55 kg per ton of ore or 14% of the iron. The use of oxygen in the leach was investigated as a means of precipitating this iron as hematite and a few tests with oxygen were performed. Reaction with manganese dioxide was used subsequently, as a means of quantifying oxygen consumption and the ferrous content of the ore. A series of tests at various acid additions was used to test the benefit of oxidizing conditions. The concentration of iron in solution was reduced to about 1 g/l when oxidizing conditions were used, resulting in a calculated saving in acid consumption of about 88 kg/t. The data confirmed that the saving in acid consumption was of this order. The oxygen consumption of the ore, based on conversion of manganese dioxide, was very variable, with an average value of about 15 kg/t. This is about twice the calculated value (of 7 kg/t) based on reduction of iron in solution. This could imply that a similar proportion of ferrous iron was present in the residue. e of a high temperature (270–280 °C), for precipitation of ferrous compounds, may not necessarily reduce acid consumption, if these compounds contain sulphate or if they re-dissolve on cooling, as filtration at high pressure and temperature is required. The use of oxygen in the leach offers the potential of reducing acid consumption and eliminating unstable precipitates associated with ferrous compounds.