Iron Removal from Kaolins by Bacterial Leaching

Weathering of rocks generated large volumes of kaolin clays, quartz sands and micas, as well as other minerals, especially iron and titanium oxides. Kaolin is an economically important raw material often used in wide variety of ceramic applications, from high quality tableware and sanitaryware to electrical porcelain, tiles and glasses. There are some less common uses including glass fibre, white cement and refractory insulation bricks. However, oxyhydroxides of Fe are often deposited along with the kaolinite, contaminating and making much of the kaolin unusable for commercial applications due to insufficient whiteness. That is why the mined kaolins need to be beneficiated by removal of undesirable minerals mica constituents and other ironbearing minerals before they could be considered as a suitable raw material for some of the above-mentioned purposes. According to the mineralogical, chemical and granulometric composition and corresponding technological properties, three kaolin types are distinguished: The 1st class kaolin of the best quality is created by kaolinite and hydromica; kaolin of the 2nd class differs from the best one mainly by increased content of colouring oxides with the lower whiteness in the rough state; 3rd class kaolin has increased content of Fe2O3 and worst technological properties [1]. Although physico-chemical procedures, such as magnetic separation and flotation, could be used for beneficiation of kaolin, they are expensive. On the other hand, biological methods could be cheaper and less adverse impact. Field observations and laboratory experiments demonstrate that microbes can accelerate aluminosilicate mineral weathering reactions in direct contact with mineral surfaces, by producing organic and inorganic acids, creating metal-complexing ligands, changing redox conditions, or mediating formation of secondary mineral phases. Several strains of bacteria released by this way the cations from biotite (Si, Fe, Al), plagioclase, and feldspar (Si, Al) much more than abiotic procedures [2]. A process for biological removal of iron from quartz sands, kaolins and clays was developed as leaching at 90 oC with lixiviant produced as a result of the cultivation of acid-producing heterotrophic microorganisms, mainly strains of Aspergillus niger [3]. The iron content of different kaolins was lowered from 0.65 – 1.49 wt.% to 0.44 – 0.75 wt.% Fe2O3 and their whiteness was increased from 55 – 87 % to 86 – 92 % [3]. Original papers