Color control in industrial clay calcination

Abstract

Low grade clays often have iron in their composition. The iron hydroxides in original clay decompose at 300°C and, depending on the availability of oxygen, the iron oxides occur in different phases. For an oxygen rich environment, hematite formation prevails, but at around 650°C it converts to magnetite. In an oxygen free environment hematite does not form during calcination at any range of temperature. During cooling, the presence of oxygen can revert the formation of magnetite back to hematite at temperatures in the range 650-300°C, thus the resulting material can have a reddish color. Color is a quality parameter and it is a matter of concern for the cement producers. If hematite is the main iron phase, calcined clay has a reddish color, which could modify cement’s final color. This paper proposes a method to modify color during calcination by controlling the atmosphere during the cooling. The focus is the cooling phase, where the presence of oxygen is avoided, in order to keep magnetite as the main iron phase in the calcined product. The procedure to control color consists of injecting liquid fuel at the carcass of the kiln while the calcined material exits, so that it combusts and exhausts the oxygen available during the cooling process. The procedure was successfully implemented at a pilot kiln in India. Controlling the calcination atmosphere enabled the production of a black calcined clay, instead of a red material. The reactivity and properties of both red and black clay are very similar, and no side effects have impacted properties of LC3 cements produced with the treated clay. The reactivity and properties of both red and black clay are very similar; compressive tests testes were carried out with both clays and show that differences in strength of LC3 cement produced with the grey clay when compared with the red clay are not significant.