TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Materials: The feed gas contains in excess of 85% oxygen by volume.The microorganisms are selected from Acidithiobacillus; Thiobacillus; Leptosprillum; Ferromicrobium and Acidiphilium, especially A. caldus (Thiobacillus caldus), A. thiooxidans (T. thiooxidans); A. ferrooxidans (T. ferrooxidans), A. acidophilus (T. acidophilus), T. prosperus, L. ferrooxidans, F. acidophilus and Acidiphilium cryptum.Bioleaching the slurry at 45-60 degreesC using moderate thermophile microorganisms selected from Acidithiobacillus (formerly Thiobacillus), Acidimicrobium, Sulfobacillus, Ferroplasma (Ferriplasma) and Alicyclobacillus, especially S. acidophillus, S. disulfidooxidans, S. thermosulfidooxidans, Ferroplasma acidarmanus, Thermoplasma acidophilum and Alicyclobacillus acidocaldrius. Bioleaching the slurry at 60-85 degreesC using thermophilic microorganisms selected from Acidothermus, Sulfolobus, Metallosphaera, Acidianus, Ferroplasma (Ferriplasma), Thermoplasma and Picrophilus, especially S. metallicus, S. acidocaldarius, S. thermosulfidooxidans, Acidianus infemus, Metallosphaera sedula, Ferroplasma acidarmanus, T. acidophilum, T. volcanium and Picrophilus oshimae.The slurry is leached in a vessel which is substantially closed.Preferred Method: Copper and cobalt removed before the nickel is recovered from the residue. Iron is precipitated from the bioleach residue by adding limestone. The nickel recovery process comprises solvent extraction and zinc electrowinning. The dissolved oxygen concentration is maintained in the slurry within a desired range of 0.2-10 x 10-3 kg /m3 by controlling the oxygen content of the feed gas, controlling the supply of feed gas to the slurry, and controlling the rate of feed of slurry to a reactor. The dissolved oxygen concentration is determined by direct measurement of the dissolved oxygen concentration in the slurry, by measurement of the oxygen in a gas above the slurry, and indirectly, by measurement of the oxygen content in off-gas from the slurry. The carbon content of the slurry is controlled by the addition of carbon dioxide gas to the slurry, and the addition of other carbonaceous materials the slurry. The carbon dioxide content is at 0.5-5.0% by volume. The bioleaching is carried out at at least 40 (especially 60-85) degreesC.