Under specific controlled conditions. the addition Of SO2 to oxygen or air produces the peroxy-monosulphate free radical in solution, which is a stronger oxidant than oxygen alone. In this study, the practical strategies required to optimise the oxidation of Fe(II) with SO2/air was investigated at 75 degrees C as part of a process to remove iron as Fe(III) oxides from a synthetic nickel laterite high pressure acid leach solution containing 5 g/L Fe(II), 1 g/L Fe(III), 8 g/L Ni, 30 g/L Mg in sea water at pH about 2. The rate of Fe(II) oxidation was optimised in the pH range of 1.2-2.0 with respect to SO2/air ratio and gas flow rates for minimum production of H2SO4 and maximum utilisation of SO2. In order to minimise the air flow rates into the reactor vessel, the maximum rate of SO2 addition that could be employed with air was established whilst maintaining oxidising conditions. The results provide strategies for commercial applications of the SO2/air oxidising system and indicate important factors for reactor design. (C) 2009 Elsevier Ltd. All rights reserved.