Value addition of paper coating grade kaolins by the removal of ultrafine coloring impurities

Abstract

Two kaolin samples from the southern part of Kerala (A and B) have been characterised. The colouring impurities present were found to be carbonaceous and ferruginous, being also present in ultrafine size ranges. Modern beneficiation techniques such as ultra flotation and high gradient magnetic separation (HGMS) have been carried out to reduce the level of these contaminants and to increase the brightness of the product clays. The results showed that nearly 70% of the ultrafine carbonaceous matter in A, which originally contains 0.13% carbon (as fixed carbon), could be removed by shear floc-flotation. HGMS treatment could remove ∼ 45% of the iron minerals (feed assay 0.58% Fe2O3). Sample B contained 0.32% Fe2O3 and 0.19% carbon. The shear floc-flotation was found to be the only effective method by which ∼ 75% of the carbon could be floated out. This sample did not respond to HGMS which may be because of the ‘structural’ nature of the iron present. Both the kaolins, after processing, were found suitable for paper coating. Another important aspect of this work is the establishment of maximum permissible level for carbon in paper coating grade clays since the standard specifications do not give any limiting value.