Reversion of calcined calcium aluminium phosphate fertilizers due to rehydroxylation of crandallite

Abstract

Calcium aluminium phosphate fertilizers prepared by calcination of Christmas Island C-grade ore at temperatures between 400°C and 600°C are amorphous and soluble in neutral ammonium citrate. Hydrothermal treatment of calcines at temperatures between 60°C and 150°C for periods of up to one year resulted in various amounts of reversion to an insoluble form due to rehydroxylation and recrystalization of crandalite. The rate at which reversion occurs increases with increasing temperature of hydrothermal treatment. Reversion did not occur within one year at 45°C, and is therefore unlikeley to affect fertilizer performance in the field.