Abstract
The thermal reactions in the mixtures of hydroxylapatite or fluorapatite and (NH4)2SO4up to 500°C were studied with the purpose of elaborating the conditions of obtaining calcium–ammonium cyclophosphate that could be used as fertilizer. Thermal analysis with a simultaneous FTIR analysis of the evolved gases as well as the analyses of chemical and phase composition of solid products were performed. The thermal changes in the mixtures could be divided into three steps: (1) decomposition of (NH4)2SO4and reactions of apatite with these products at 250–420°C, (2) calcium ammonium polyphosphate formation at 290–450°C, and (3) reaction of CaSO4with CaNH4P3O9at 320–500°C. Higher concentrations of NH3in the gas phase promote the formation of CaNH4P3O9and increase its stability. Calcination at temperatures above 350°C causes decomposition of CaNH4P3O9with a decrease in the content of water-soluble phosphorus and evolvement of SO2.