Release of fluorine and chlorine during increase of phosphate rock grade by calcination and digestion

A series of experiments for calcination and subsequent digestion to increase the grade of phosphate rock were performed. Fluorine and chlorine released by calcination and digestion were investigated. The forms of both elements were studied by sequential chemical extraction. The grades of raw and calcined phosphate rocks, and their phosphorus concentrates were confirmed according to the value of P2O5%. The results showed that fluorine and chlorine in phosphate rock mainly existed in carbonate-bound and residual forms. The grade of phosphate rock can be well increased by calcination and subsequent digestion. By digestion at the optimal condition of 2.5:1 for liquid-solid ratio, 60 degrees C for digestion temperature and 50 min for digestion time, the phosphorus concentrates digested from the calcined phosphate rock of 1000 degrees C reached the highest grade (P2O5% = 33.24%). By calcination, the released fluorine was mainly HF at a low temperature and SiF4 at a high temperature, with fluorine release amount degrees C, less than 800 degrees C and more than 800 degrees C, which release amount were 9.40 x 10-2.54 x 10(2) g/t, 2.10 x 10(2)-1.53 x 10(4) g/t and 2.24 x 10(2)-5.61 x 102 g/t, respectively. By digestion with water for the calcined phosphate rock of 900-1100 degrees C, the concentrations of fluorine and chlorine in effluent were respectively 77.2-160 mg/L and 7.99-19.6 mg/L. It can be concluded that fluorine and chlorine released by calcination for phosphate rock contribute greatly to atmospheric acid rain and equipment corrosion; by digestion, the discharge of fluorine seriously exceeds the standard and may cause greater pollution to the water body. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The experiments focused on calcination and digestion to increase the grade of phosphate rock, investigating the release of fluorine and chlorine in the process. Fluorine and chlorine mainly existed in carbonate-bound and residual forms in phosphate rock. Optimal digestion conditions were found to be a liquid-solid ratio of 2.5:1, digestion temperature of 60 degrees C, and digestion time of 50 minutes.