Carbothermic reduction of mechanically activated celestite

Effects of dry milling on the carbothermic reduction of celestite were examined. Celestite and coke mixture was milled up to 120 h in a planetary ball mill. Unmilled and milled mixtures and their black ashes from carbothermic reduction were characterized by a combination of X-ray diffraction (XRD) analysis, scanning electron microscope analysis. thermogravimetric analysis, particle size analysis and leaching tests. XRD diffraction peaks for celestite in the milled mixtures are lower and broader than those for the unmilled mixture, mainly due to a disordering of celestite crystal structure. Size of the particles significantly decreased by 1 h of milling, so that d(50) decreased from 79.4 to 6.1 mu m. But, milling for longer periods shifted the particle size distribution to coarse size region. mainly due to the agglomeration of fine particles. By milling the celestite-coke mixture for 1 h, formation temperature of SrS decreased from 957 degrees to 900 degrees C, whereas 24, 72 and 120 h of milling decreased nearly to 700 degrees C. At low roasting temperatures, the mass loss by reduction increased with extending of the milling and the milled mixtures gave a high degree of soluble strontium in the black ash. However, sintering process prevailing at higher temperatures during the roasting of the milled mixtures became active. and the effects of disordering of the celestite structure vanished and subsequently the reduction reaction was retarded. It was concluded that although milling in a planetary ball mill produces more X-ray amorphous SrS, together with SrC2, carbothermic reduction reactions occur at lower temperatures as the milling time increases. (C) 2009 Elsevier B.V. All rights reserved.