Cobalt and nickel recoveries from laterite tailings by organic and inorganic bio-acids

Cuban serpentines are known as one of the richest deposits of Ni and Co in the world. These ores are usually treated by pyrometallurgy or by hydrometallurgy. These processes generate huge volumes of mining residues, which still contain high amounts of Ni (0.25%) and Co (0.09%). Since metals are partially oxidized, the chance to use sulphur-oxidizing bacteria (Acidithiobacillus thiooxidans) which are able to generate sulphuric acid to leach the residues from CARON process has been evaluated. Thus, in this work, inorganic and organic acids produced by cultures of A thiooxidans or Aspergillus niger respectively were used to study the extraction of Co and Ni from laterite tailings. The results are compared with those obtained in leaching experiments with direct inoculation of those microorganisms. Using A. niger and sucrose as carbon source, 7% Ni and 16% Co were leached after 3 days although there was a significant metal precipitation later. In one-stage batch experiments using A. thiooxidans and elemental sulphur as energy source higher percentages of metal solubilization (100% Ni and 80% Co) were reached after 15 days. In two-stage batch experiments with sulphuric bio-acid the metal recoveries were much higher (79% Ni and 55% Co) than those obtained with citric bio-acid but lower than those reached in one-stage batch experiments. Since the mineral composition of raw material is a crucial parameter to select the suitable leaching agent for oxide ore, the treatment of residual nickel-ferrous material from the CARON process with bio-sulphuric acid produced by cultures of A. thiooxidans could be an attractive alternative in the development of a sustainable technology for the Cuban mining-metallurgy industry. (C) 2008 Elsevier B.V. All rights reserved.