REFINEMENT OF LOW-GRADE CLAY BY MICROBIAL REMOVAL OF SULFUR AND IRON COMPOUNDS USING THIOBACILLUS-FERROOXIDANS

The refinement of low-grade clay, of which impurities are mainly sulfu r and iron compounds, is required because of the recent shortage of hi gh-grade clay for manufacturing of structural ceramics. The major impu rity compound contained in the low-grade clay we treated was identifie d as pyrite by X-ray powder diffraction and inductively coupled plasma analyses. The well-formed crystals of pyrite had a framboidal form of 1 mu m-20 mu m diameter. The microbial removal of pyrite from the low -grade clay was investigated by using a sulfur and iron-oxidizing bact erium, Thiobacillus ferrooxidans. About 82-90% of the pyrite was remov ed in 5-12 d for pulp densities up to 70% (w/v). The removal rate of p yrite ranged from 270 to 914 mg-pyritic sulfur/l . d depending upon cl ay pulp density. The rate of pyrite removal (r) could be expressed as a function of pyritic sulfur concentration (S): r (mg-pyritic sulfur/l . h)=1.96 x 10(-2) S (mg-pyritic sulfur/l). The logarithm of the amou nt of oxidized pyrite per unit volume and the final pH in the reaction medium were found to have a linear relationship which could be expres sed as pH=2.43-0.55 log [FeS2 (mM)]. With the refined clay no red colo r due to the presence of pyrite was developed after firing, and its wh iteness was similar to that of a high-grade clay.