The Hydrothermal Synthesis of Kaolinite Up to 350°C
Hydrothermal synthesis of kaolinite using amorphous alumina and silica as a starting material has been studied in the temperature range from 250 to 350°C and the corresponding saturated water vapor pressure. The pH varied from 1 to 5, the time of treatment from 3 to 70 days, and the percentage of autoclave water from 36% to 72%. The aim of the work is to demonstrate the factors influencing synthesis and the conditions under which the best crystallized product can be obtained. It was found that pH 3, 300°C, and the time interval starting with 5 days produced the best ordered crystals. At higher temperatures (up to 350°C) pyrophyllite was formed along with kaolinite. At lower temperature or shorter time of treatment, poorly crystallized products appeared. The lowering of pH to the value of 1 and 2 with sulphuric acid resulted in formation of better crystallized kaolinite, but alunite was formed at the same time.
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Kaolin is an important industrial mineral in several world markets including uses in paper coating and filling, ceramics, paint, plastics, rubber, ink, fiberglass, cracking catalysts and many other uses (Murray, 1991). The kaolin minerals kaolinite, halloysite, dickite, and nacrite have essentially similar chemical composition but each has important structural and stacking differences. The most common kaolin mineral and the one that is the most important industrially is kaolinite [Al2Si205(OH)4]. Kaolinite can be formed as a residual weathering product, by hydrothermal alteration, and as an authigenic sedimentary mineral. The residual and hydrothermal occurrences are classed as primary and the sedimentary occurrences as secondary. Primary kaolins are those that have formed in situ usually by the alteration of crystalline rocks such as granites and rhyolites. The alteration results from surface weathering, groundwater movement below the surface or action of hydrothermal fluids. Secondary kaolins are sedimentary which were eroded, transported and deposited as beds or lenses associated with other sedimentary rocks. Most kaolin deposits of secondary origin were formed by the deposition of kaolinite which had been formed elsewhere. Some secondary deposits were formed from arkosic sediments that were altered after deposition, primarily by groundwater. There are far more deposits of primary kaolins in the world than secondary kaolin deposits because special geologic conditions are necessary for both the deposition and preservation of secondary kaolins.