Use of heterotrophic microorganisms in mineral biotechnology

A process for biological removal of iron from quartz sands, kaolins and clays was developed in which these industrial minerals were leached at 90°C with lixiviant produced as a result of the cultivation of acid-producing heterotrophic microorganisms, mainly strains of Aspergillus niger, at 30°C in a nutrient medium containing molasses as a source of carbon and energy. The lixiviant, i.e. the fermentation fluid, contained oxalic and citric acids as main components and after the cultivation was acidified to a pH of 0.5 by means of hydrochloric acid. The leaching was carried out in mechanically stirred acid-resistant vats for a period of from 1 to 5 hours. The iron content of some sands treated by this method was lowered from 0.035–0.088 to below 0.012% Fe₂O₃ making them suitable for the preparation of high quality glass. The iron content of different kaolins was lowered from 0.65–1.49 to 0.44–0.75% Fe₂O₃ and as a result of this their whiteness was increased from 55–87 to 86–92%. The iron content of a clay was lowered from 6.25 to 1.85% Fe₂O₃ and this increased the fireproofness of the clay from 1 670 to 1 750°C. Similar process was used for leaching of aluminium from aluminosilicates, mainly clays and kaolins. However, after the cultivation the fermentation fluid was acidified either by means of sulfuric or hydrochloric acid or by means of different mixtures of inorganic acids. For enhancing aluminium solubilization the aluminosilicates were heated before leaching at 600–650°C for 1–2 hours. Over 90% of the aluminium present in different clays and kaolins was leached within 3–6 hours in this way. “Silicate” bacteria related to the species Bacillus circulans and B. mucilaginosus were used to leach silicon from low-grade bauxite ores containing aluminosilicates as impurities. The bacterial action was connected with the formation of mucilaginous capsules consisting of expolysaccharides. The solid residues after leaching were characterized by higher values of alumina content and were suitable for processing by means of the BAYER process for recovering aluminium. Heterotrophic bacteria were used to leach manganese from oxide ores using different organic compounds as reducing agents.