Evolution of fluorine emissions during the fast firing of ceramic tile

The present paper examines the emission-adsorption phenomena of fluorine compounds in various ceramic tile body compositions with different fluorine and calcium contents, processed by fast firing cycles in an electric laboratory batch kiln and in continuous fast-cycle industrial combustion kilns (roller kilns). In the roller kilns, fluorine emission was monitored by analysing the fluorine content in both solid and gas samples throughout the kilns. It was found that fluorine emission during firing in air atmosphere in the laboratory batch kiln began at temperatures above 800 degrees C and increased progressively up to peak firing temperature. In the roller kilns, however, the emissions started at lower temperatures and decreased in certain kiln zones. This decrease indicates that there are regions in these kilns in which fluorine is adsorbed rather than emitted by the material being processed. A fluorine adsorption process is thus shown to occur in the preheating stage in the roller kilns, despite the rapid heating rate. However, encouraging this adsorption process in order to foster the formation of crystalline phases is ineffective in abating fluorine emissions when ceramic tiles are fired at peak temperatures exceeding 1100 degrees C. (C) 2007 Elsevier B.V. All rights reserved.