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Most natural argillaceous materials contain smaller amounts of fluorine. The release of fluorine gases during firing of clays for bricks and tiles may be an environmental hazard in the closer neighbourhood of brickworks. Investigations have shown that the fluorine emission into the environment can be eliminated or at least strongly reduced by adding powdered lime to the raw argillaceous material. In Switzerland argillaceous materials used in brickworks normally have a relatively high natural content of lime in form of Calcite and Dolomite. Normally, fluorine emission therefore is not a problem. The lime however, which is decomposed during firing, may be able to cause problems with the quality of the brick surface by producing harmful lime popping. It has been shown that fluorine emission from the kiln during firing of the clay may be reduced if the fluorine containing hot gases are transported into the preheating zone of the kiln - which is the case in modern kilns - where they are reacting with the incoming material before leaving the chimney. The reaction with the incoming material not only reduces the amount of fluorine into the environment but also increases the quality of the brick by reducing the harmful lime popping. With the aim of a better understanding of the reactions between fluorine containing gases and lime investigations in the laboratory using a combination of a thermal balance and a mass spectrometer have been carried out. The results show that the decomposition of lime in the clay takes place at lower temperature if alkalifluoride is added to the material. A certain amount of fluorine therefore seems to be of some advance in brickmaking using clays with relatively high lime content.
Authors:Kahr, Günter and Mumenthaler, W and Schmitt, H. W. and Madsen, Fritz
Index Terms:Clay; ClayGroup
Further Information:Date published: 1995