Calcination of Kaolin Clay
It is essential to calcine kaolin clay to improve its brightness and abrasion properties. Kaolin clay calcination at elevated temperature will improve the whiteness of clay to a good extent which will make it highly usable in paint, rubber and plastic industries. In order to obtain the optimum whiteness, the kaolin clay has to be calcined in dry form. The calcination process should be done very carefully to prevent overburing of the clay. Kaolin Clay is generally calcined in rotary kiln in a temperature range of 2100F to 2400F. At this temperature the whiteness of the clay is improved better than at lower temperature. It is calcined by indirect heating methods, by bruning the fuel external to the calcinatory and passing the combustion gases in a counter current direction to the clay flow and thereby preventing direct exposure of flame on the clay. The Clay is calcined through the length of the kiln for a resident time of about half an hour to two hours. After calcining the clay can be easily pulverized to 325 mesh in hammer mills.
Properties of Clay before and after calcination
Whiteness – Kaolin with brightness of 87%, can be calcined and improved to 92%. That is approximately 5% increase in brightness due to calcination.
Abrasiveness- Clay with abrasive strength less than 15 can be increased to less than 150.
