Changes in physical structure and chemical composition of phosphate rock during calcination in fluidized and fixed beds

The present study deals with the effects of important variables on the calcination of phosphate rock in fluidized and fixed beds. Further, the calcined samples were characterized for variation in physical structure and chemical composition as compared to the raw ore by X-ray diffractometry (XRD), scanning electron microscopy (SEM), mercury porosimetry as well as BET surface area techniques. Calcination extent was found to increase slightly with gas flow rate. In studying the effect of particle size on calcination in the fluidized bed, the calcination extent increased with decreasing particle size. Calcination in the fluidized bed was found to be faster than that in the fixed bed. The XRD patterns showed that the chief mineral constituents of calcined samples were calcium oxide and fluorapatite, while those in the raw ore were calcite, fluorapatite and carbonate-fluorapatite. The carbonate-fluorapatite changed to fluorapatite during calcination. SEM micrographs of the calcined samples showed that macro pores occur in the carbonate during calcination. In addition, shrinkage and cracks in the surrounding parts of the phosphate due to sintering were observed. The pore size distribution indicated an increase in macro pores and a decrease in micro pores upon calcinations.