Ceramics-Silikáty 47, (1) 20 - 26 (2003)

Dissolution of aluminosilicates and Fe-oxyhydroxide minerals from granitic eluvium using bacteria of Bacillus genus was monitored with solution chemistry, granulometric and X-ray analyses as well as microscopic techniques to determine the effects of these bacteria on crystal surface and releasing mechanism of K, Si, Fe, and Al from minerals. Feldspars, quartz and micas are dominant minerals in granitic eluvium (GE). Oxyhydroxides of Fe are found in the intergranular spaces of minerals, contaminating and making most feldspar raw materials unsuitable for commercial applications. Bacteria of Bacillus spp. decrease pH of leaching medium by production of organic acids. These organic acids are directed by glycocalyx of adherent bacteria to specific sites on mineral surface (e.g. to crystal defects). The impregnated iron minerals are released by bacterial destruction of intergranular and cleavage spaces of silicate grains. This bacterial activity results in the release of Fe, Si, and K from feldspar and Fe oxyhydroxides. However, pH of leaching medium is adjusted to neutral value in regular intervals during bioleaching to maintain bacterial activity. The decrease of fine-grained fraction is the result of bacterial destruction of GE. Despite the impoverishment of the distribution of fine-grained fraction, there was observed the increase of the surface area of feldspar grains from the value 3.65 m²/g to value 4.82 m²/g. This fact confirms the activity of bacteria of Bacillus genus together with hydrolysis in point corrosion of mineral grains. After 120 days of bioleaching, 31 % Fe extraction from granitic eluvium was observed. It was also possible to accelerate this process by using 0.1 M oxalic acid after 1 month's bacterial pretreatment. Moreover, the bacterial pretreatment facilitated the access of oxalic acid to Fe-bearing minerals and showed a possibility to use the oxalic acid in lower concentration. This fact is important especially from the view of producing less-agressive effluent to the environment and decreasing of costs in subsequent recyclation of oxalic acid.