High-Temperature Reaction of Kaolin with Ammonium Sulfate

Reactions between kaolin and inorganic salts, such as sodium hydrogen sulfate and ammonium sulfate, or inorganic acids, such as hydrochloric acid and sulfuric acid, can be focused either on the production of Al or on the adjustment of the silica-to-alumina ratio of the kaolin. On the basis of adjusting the silica-to-alumina ratio, kaolin and ammonium sulfate [(NH4)2SO4] are mixed and heated in a furnace at temperatures between 200 and 1000 °C. The parameters studied were the reaction temperature and time, the proton-toalumina molar ratio, the calcination temperature and time, and the reaction atmosphere. The products of the reaction between kaolin and (NH4)2SO4 were characterized by means of X-ray diffraction (XRD). The Al reaction yield grew until reaching a maximum at 600 °C and started to decrease sharply beyond this temperature. The Ti reaction yield values were lower than the Al reaction yield values, showing a maximum at 600 °C and decreasing smoothly as the reaction temperature was increased. The Fe reaction yield exhibited the same tendencies as the Al reaction yield. With regard to the proton-to-alumina molar ratio, a maximum in Al, Ti, and Fe reaction yields was observed at rH = 3. Precalcination of kaolin produced only slight decreases in the Al and Ti reaction yields and a slight increase in the Fe reaction yield. The products of the reaction were triammonium hydrogen disulfate [(NH4)3H(SO4)2], ammonium aluminum sulfate [NH4Al(SO4)2], aluminum sulfate [Al2(SO4)3], and alumina [Al2O3]. Brunauer-Emmett-Teller (BET) specific surface area tests indicated an increase of the BET specific surface area for short reaction times, reaching values above 200 m^2/g.