Nickel laterite contains metal values but is not capable of participating in the primary chemolithotrophic bacterial oxidation because it contains neither Fe2+ iron nor substantial amount of reduced sulphur. Its metal value can, however, be recovered by allowing the primary oxidation of FeS2, or similar iron/sulphur minerals to provide H2SO4 acid solutions, which solubilise the metal content. This study investigated the possibility of treating nickel laterites using chemolithotrophic microorganisms. Preliminary studies conducted using H2SO4 acid, citric acid and acidified Fe2(SO4)3 gave an insight on the use of chemolithotrophic bacteria in this process,. Results showed that H2SO4 acid performed better, in terms of nickel recovery, than citric acid or acidified Fe2(SO4)3. In the bacterial leaching test works, mixed cultures of Acidithiobacillus ferrooxidans, Acidithiobacillus caldus and Leptospirillum ferrooxidans were used in the presence of elemental sulphur and FeS2 as energy sources. The sulphur substrate exhibited better effects in terms of bacterial growth, acidification and nickel recovery than the FeS2 substrate. Using response surface methodology, the theoretical optimum conditions for maximum nickel recovery (79.8%) within the conditions studied was an initial pH of 2.0, 63μm particle size and 2.6% pulp density.
