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, a t 30°C in a nutrient mediom 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 n 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 t o 5 hours. The iron content of some sands treated by this method was lowered from 0.035-0.088 t o below 0.012~o Fe,O, making them suitable for the preparation of highquality glass. The iron content of different kaolins was lowered from 0.65-1.49 to 0.44-0.55y0 Fe,O, 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% Fe,O, and this increased the fireproofness of the clay from 1670 to 175O'C. Similar process was used for leaching of aluminium from alnniinosilicates, 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 a t 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 t o the species Bacillus circulans and B. mucilaginoaus were used to leach silicon from low-grade baurite ores containing aliiminosilicates as impurities. The bacterial action was connected with the formation of mucilaginous capsules consisting of exopolysaccharides. 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. Different heterotrophic microorganisms are widely used under laboratory conditions for releasing metals from various mineral raw materials. I n contrast to the autotrophic bacteria, the heterotrophic bacteria and fungi require organic carbon for growth and energy-generation and do not derive any benefit from the degradation of the minerals which occurs because of their presence. This makes the heterotrophs less amenable for commercial operations than the autotrophs. However, some leaching processes connected with the activity of different het.erotrophic microorganisms seem promising for a real application. In this paper some of these processes are presented. 300 Bcta Biotechnol. 7 (1987) 4 Microbial Removal of Iron from Mineral Raw MaterialsThe iron oxides contained as impurities in quartz sands, kaolins and clays can greatly lower the...

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