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

ABSTRACT 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 oC for digestion temperature and 50 min for digestion time, the phosphorus concentrates digested from the calcined phosphate rock of 1000 °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 of 1.61×104-3.82×104 g/t at 900-1100 °C. Cl2, HCl and Cl- were mainly released at 200-500 °C, less than 800 oC and more than 800 oC, which release amount were 9.40×10-2.54×102 g/t, 2.10×102-1.53×104 g/t and 2.24×102-5.61×102 g/t, respectively. By digestion with water for the calcined phosphate rock of 900-1100 oC, 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.