TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Process: The gas which further includes carbon dioxide is introduced to the slurry present in the rakeless reactor in bubbles of controlled size.Preferred Aeration Means: The aeration means includes at least one diffuser which is tubular, dome-shaped, doughnut shaped and disc shaped and is placed adjacent to the base of the reactor. The aeration means is located in a pipeline communicating with at least one reactor.Preferred Bacteria: Thiobacillus ferrooxidans, Thiobacillus thiooxidans, Leptospirillum ferrooxidans, Sulfobacillus thermooxidans, Sulfolobus brierleyi, Sulfolobus BC or Sulfolobus acidocaldarius are used as the fermentative organisms.Preferred Reactor: The reactor of the reactor system is a tank or vat and includes a reservoir lined with a liquid impermeable barrier. The reactor has a feed and a discharge end with a base of the reactor having a descending slope towards the discharge end.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Metal: The metal to be recovered is a non-ferrous base metals, precious metal or rare earth metal. SPECIFIC MICROORGANISMS - Thiobacillus ferrooxidans, Thiobacillus thiooxidans, Leptospirillum ferrooxidans, Sulfobacillus thermooxidans, Sulfolobus brierleyi, Sulfolobus BC or Sulfolobus acidocaldarius are specifically claimed as microorganisms.    EXAMPLE - A sample of mixed polymetallic concentrate containing mixture of mineral sulfides was bio-oxidized to extract zinc, nickel, copper and cobalt. Nickel was present as pentlandite with a content (%) of 0.9, copper content (5.7) predominately in the form of chalcopyrite . Zinc content (0.21) as sphalerite. Cobalt was present with the nickel minerals and as mineral cobalt (0.06). The concentrate was ground so that 80% of particles were less than 15 mum in diameter. A slurry of the ground concentrate in water was prepared to which sulfuric acid was added to achieve a pH of 1.2. 0.5 liters of an inoculum slurry containing a mixture of metal concentrate, Thiobacillus ferroxidans and Thiobacillus thiooxidans was added to the slurry. Nutrients were added to the slurry mixture which was maintained at 35degreesC. The mixture was aerated. The metals were oxidized and released into solution. After completion of the reaction, the solid and liquid phases were separated and analyzed. The % extraction of copper, nickel, cobalt and zinc was 92,97,97 and 95 respectively.