A new procedure is described in which kaolin and a small amount of concentrated sulfuric acid are heated in a furnace at temperatures between 150 and 1000 °C. Parameters studied were reaction temperature and time, proton to alumina molar ratio, calcination temperature and time, and amount of water in the reaction medium. Al yield grew sharply until reaching a maximum at a reaction temperature of 700 °C and decreasing sharply beyond this temperature. Ti reaction yield values were lower than the Al reaction yield, showing a maximum at 200 °C and decreasing steadily as the reaction temperature was increased. Fe reaction yield showed a maximum in the same range of reaction temperature as Al. Previous calcination of kaolin produced only slight increases in reaction yields. Brunauer−Emmett−Teller (BET) surface area tests indicated an increase of the BET surface area for short reaction times, reaching values above 100 m 2 /g. Products of the reaction were mostly Al 2 (SO 4 ) 3 .
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Colina et al. (2002) studied this question.
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