Preparation Of Mullite From Kaolin Via A Wet Chemistry Method

Mullite has been widely used in many areas because of its several excellent properties such as high-temperature creep resistance, high-temperature strength, and low thermal expansion etc. Using kaolin as raw material to prepare mullite can reduce the production cost and increase the Kaolin's extra value and economic efficiency in a large scale. Although the preparation of mullite derived from kaolin has been studied extensively, the methods used are almost solid state sintering method. Using wet chemistry method to prepare mullite from kaolin provides a new approach, and the results show good theoretical and pratical application value. Meanwhile, a patent for invention has been obtained.In this study, inexpensive kaolin was used as the main starting material. A wet chemistry method was used to synthesize mullite instead of conventional solid-state reaction. The calcined kaolin was reacted with hydrochloric acid (HCl) to dissolve the aluminum in kaolin. The effects of reaction temperature, reaction time, concentration of HCl and the molar ration of HCl/Al were studied. After the acid reaction, The stoichiometric mole ratio of Si/Al (1/3) in the system was controlled by adding Al3+ solution, then Al3+ was precipitated by NH3·H2O. After filting, washing and drying, the mullite precursors were obtained. AlF₃ was added to the precursors in order to reduce mullitization temperature and synthesize acicular mullite. At the same time, the sintering properties of mullite were discussed. The powders and sinters were characterized by TG-DTA, XRD, SEM, ICP-AES, bending strength etc.According to large numbers of experiments, the results were shown as below. (1) After kaolin was calcined at 700℃, the metakaolinite was obtained, which was amorphous and had a high chemical activity. (2) For the acid reaction, the residual rate of kaolin could be reduced to the lowest as the following factors: the concentration of HCl was 20wt%, reaction temperature was 100℃, reaction time was 3h, and the molar ration of HCl/Al was 3.5. (3) When the AlF₃ was added into precursors, the mullite began to occur at 1100℃and completed at 1300℃. The beginning mullitization temperature of samples with AlF₃ was nearly 200℃lower than that without AlF₃. The acicular mullite grew with the increasing of AlF₃ addition, when 6.0wt% AlF₃ was added to the precursors, the length of mullite was 5-8μm and aspect ratio was 8-12. (4) For the samples with different amount AlF₃ sintered at 1500℃, the strength was reduced after thermal shock, and the maximum loss of strength was occurred after the first thermal shock. The sample with 6wt% AlF₃ had the highest rate of residual strength.