Methods for Improving Silica and Alumina Contents of Kaolin

This study focus on chemical activation of kaolin under acid and alkaline conditions. For this purpose the metakaolin were prepared by calcination of a natural kaolin at 800 and 900 °C. This metakaolin was more reactive than the initial kaolin and then was utilized for acid and alkaline activation. The physical and chemical features of kaolin is related on external layer. Usually two methods are applied for improvement of kaolin which included physical and chemical methods. In physical method hydrothermal reaction is used [1]. In chemical method acids (HNO3 ،HCl ،H3PO4 ،CH3COOH) and alkaline (NaOH, KOH) are utilized [2]. In this study the extraction of aluminum and silicon was done by HF and NaOH. The kaolin contains Al2O3 (38%) and SiO2 (45%). After calcination, the percentages of alumina and silica became, Al2O3 (41%), SiO2 (54%). For reducing silica content and producing high alumina materials, metakaolin was leached by caustic soda (NaOH) solution. Silica solubility increased in high temperature so the reaction temperature increased to 100 °C. After this process (20 min) percentages of alumina and silica became, Al2O3 (46%), SiO2 (48%). In order to increase the percentage of silica in kaolin, leaching process was done by HF diluted solution in a room temperature for 20 min on metakaolin, after that percentages of alumina and silica became, Al2O3 (35%), SiO2 (61%). The XRD patterns of kaolin, metakaolin and leaching with NaOH were shown in Fig 1. a-c.