Influence of mineral impurities on the properties of kaolin and its thermally treated products

Kaolin or china clay is widely used in a number of industries due to its special properties like fine particle size, brightness and whiteness, chemical inertness, platy structure, etc. Raw kaolin is beneficiated to remove mineral impurities that affect these special properties. Kaolin—when thermally treated at specific temperatures—gives products of industrial significance called “calcined kaolin” with characteristic properties such as high brightness, hydrophobicity, abrasive nature and improved electrical properties. The present paper deals with the study of two Indian kaolins of different genesis. One of them is from Thiruvananthapuram district, Kerala, and the other from Kutch district, Gujarat. The latter one is of inferior quality due to the presence of ferruginous and titanoferrous impurities and, therefore, subjected to high gradient magnetic separation. The beneficiated clay was also studied. The three samples were thermally treated under identical conditions at various temperatures. Calcined samples were analysed for their optical properties, pH, true and apparent densities and particle size distribution. The influence of iron and titanium containing impurities on the properties of the kaolins and their calcined products is discussed. The beneficiated Gujarat clay is found to be comparable with the Kerala clay. Calcination at 1100 °C gives products of similar optical properties.