The natural phosphate rocks were the raw materials in the production of phosphatic fertilizers. In technical point of view

for the evaluation of phosphate rock was not only considered the content of calcium phosphate

but also the amount of harmful minerals containing in it. The mineral sesquioxides were the most harmful substances in the manufacture of phosphate fertilizers by the acid decomposition processes. Solubilities of mineral sesquioxides of phosphate rock in nitric

hydrochloric and phosphoric acids would depressed significantly after the calcination of the rock at 900℃ for more than one hour. Hence

it was possible to direct use the phosphate rock containing high in mineral sesquioxides for the manufacture of phosphate fertilizers without previous treatment.