BIOLEACHING AND BIOPRECIPITATION OF NICKEL AND IRON FROM LATERITES

Leaching of silicate ores, particularly nickel laterites, with the aid of heterotrophic organisms has been briefly reviewed. Samples of late rite ores from Greece were characterised mineralogically and a number of microorganisms isolated from them. One of these organisms (code F1) was successfully acclimatized to 6400 ppm nickel. Samples of the high -grade Greek Kastoria nickel laterite were leached with sulphuric acid and a number of organic acids. Sulphuric and citric acids extracted o ver 60 and 40% of the contained nickel, respectively, but the other ac ids employed were less efficient leachants. Oxalic acid precipitated n ickel oxalate. Roughly the same extraction of iron was observed. The m ain leaching parameter was confirmed to be hydrogen ion concentration, although complexation with organic anions was a contributor. Organism F1 (a strain of Penicillium) was used in comparison with organisms fr om various culture collections to bioleach nickel from samples of the low-grade Greek Litharakia nickel laterite. The organisms were cultiva ted in a mixture of a sugar-based nutrient mineral medium and finely g round ore. Several penicillia and aspergilli leached 55-60% of the con tained nickel and cobalt, and 25-35% of the iron when sucrose was the carbon source, but Fl was not efficient. However, in molasses medium, Fl extracted nearly 40% of the nickel. Biosorption and bioprecipitatio n reactions were observed. The mechanism of bioleaching or in situ lea ching is discussed in terms of close physical and chemical association between the fungal hyphae and mineral phases in the ore. This account ed for the low overall hydrogen ion concentration observed during biol eaching.