Chlorellestadite in the preheater system of cement kilns as an indicator of HCl formation

The conditions leading to the formation of water-insoluble chlorellestadite, ideal formula (unit cell content) Ca-10(SiO4)(3)O-2, instead of water-soluble KCl and NaCl in Portland cement kiln systems were investigated by quantitative X-ray phase analysis of plant samples, preparation of pure chlorellestadite, and thermodynamic calculations. The results support the view that water-insoluble chloride is present as chlorellestadite. A reason for its occurrence could be poor contact between the hot kiln gases and lime-containing dust causing formation of gaseous HCl, which then produces chlorellestadite in contact with the hot meal in the preheater unit. The structure model for synthetic chlorellestadite, used in the quantitative X-ray phase analysis, was refined by the full profile Rietveld method using the powder X-ray diffraction spectrum (CuKalpha 1). Crystal data: hexagonal, space group P6(3)/M, a = 9.6773(3) angstrom, c = 6.8585(1) angstrom, Z = 1. A disordered S/Si distribution was observed. (c) 2004 Elsevier Ltd. All rights reserved.