Pesquisa | SciELO

Special clays are a group of clays different from the large volume of clay mineral products named "Industrial Clays": kaolins, ball clays, refractory clays, bentonites, fuller's earths, common clays. Two groups of special clays exist: rare, as in the case of hectorite and sepiolite and restricted areas, as in the case of white bentonite, halloysite and palygorskite (attapulgite). A review is given of the most important producers of the special clays and their properties in the Western World, as well as a discussion of the occurrence of these types of clays in Brazil.

Crystalline aluminium hydroxiacetate was prepared by reaction between aluminium powder (ALCOA 123) and aqueous solution of acetic acid at 96ºC ±1ºC. The white powder of Al(OH)(CH3COO)2 is constituted by agglomerates of crystalline plates, having size about 10mum. The crystals were fired from 200ºC to 1550ºC, in oxidizing atmosphere and the products characterized by X-ray diffraction, scanning electron microscopy and surface area measurements by BET-nitrogen method. Transition aluminas are formed from heating at the following temperatures: gamma (300ºC); delta (750ºC); alpha (1050ºC). The aluminas maintain the original morphology of the Al(OH)Ac2 crystal agglomerates, up to 1050ºC, when sintering and coalescence of the alpha-alumina crystals start and proceed up to 1550ºC. High surface area aluminas are formed in the temperature range of 700ºC to 1100ºC; the maximum value of 198m²/g is obtained at 900ºC, with delta-alumina structure. The formation sequence of transition aluminas is similar to the sequence from well ordered boehmite, but with differences in the transition temperatures and in the development of high surface areas. It is suggested that the causes for these diversities between the two sequences from Al(OH) Ac2 and boehmite are due to the different particle sizes, shapes and textures of the gamma-Al2O3 which acts as precursor for the sequence gamma- to alpha-Al2O3.

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