Influence of calcite content on fluorine compound emissions during ceramic tile firing

The manufacture of traditional ceramic products (ceramic tiles, roof tiles and bricks) is often associated with the emission of fluorine compounds during the firing stage. According to the literature such emissions can be reduced by adding CaCO3 to the raw materials mixture used in fabricating these products. However, data available to the authors, indicate that this procedure, which has been successfully applied in manufacturing structural ceramics (roof files and bricks), is ineffective ill ceramic tile manufacture. The present study has sought to establish why the CaCO3 addition fails to reduce fluorine compound emissions during the ceramic tile firing stage. The study has thus determined the influence Of CaCO3 content on the evolution of the crystalline phases with firing temperature in a typical floor tile composition to which additions of CaF2 were made; additions of BaF2 and SrF2 were also made to this floor tile body, and the thermal stability of these three fluoride compounds was studied. The study shows that the effectiveness of CaCO3 in reducing fluorine compound emissions in roof tile and brick manufacture is due to the relatively low firing temperature (850-1000 degrees C) involved, which enables part of the CaF2 to be retained in the pieces, and to the formation of cuspidine, which is stable tip to 1050 degrees C. At higher temperatures (ceramic tiles are typically fired at temperatures of 1100-1200 degrees C), the fluorine-containing crystalline species (fluorite and cuspidine) are unstable, causing fluorine compound emissions to rise.