Microbial refinement of kaolin by iron-reducing bacteria

Low-grade kaolin contains Fe(III) impurities, which cause the detraction of refractoriness and whiteness of porcelain and pottery. Microbial refinement of low-grade kaolin to remove Fe(III) by iron-reducing bacteria was investigated. The removal of Fe(III) impurities could be performed by the indigenous microorganisms in kaolin, but could be enhanced by the inoculation of iron-reducing bacteria. Maltose, sucrose, and glucose could be used as substrates. The removal efficiency of Fe(III) increased with increasing sugar concentrations in the range of 1–5% (w/w, sugar/clay). After the microbial refinement, the whiteness of the kaolin increased from 64.49 to 71.50, and the redness decreased remarkably from 7.41 to 2.55. Fe(III) impurities were selectively removed from the kaolin without changing or losing the mineralogical composition of the kaolin by the microbial refinement.