Refinement of industrial kaolin by removal of iron-bearing impurities using thiourea dioxide under mechanical activation

This study reports a series of experiments conducted to evaluate the suitability of thiourea dioxide (TD) under mechanical activation, in bleaching iron-bearing impurities from raw kaolin. The optimum conditions for the maximum whiteness of 76.32% were determined as follows: rotation speed of ball mill, 400 rpm; activation time, 1.5 h; TD dosage, 0.8% of kaolin mass; solid-to-liquid ratio, 1:2; pH, 8.0; room temperature. The effect of grinding causes the partial surface amorphization of kaolin particles and a decrease in the dehydroxylation temperature. The assistance of grinding leads to a higher iron removal when compared with the conventional bleaching method using sodium hydrosulfite.