Treatment Of High-concentration Fluoride Wastewater By Fluidized Bed Crystallization Method

In production process of fluoride salt massive high-concentration fluoride wastewater is generated. At present, in the treatment methods of high-concentration fluoride wastewater, the most widely used is lime precipitation. The method has advantages of simple process, easy operation and low costs. However, moisture content of sludge produced is high, the settlement is poor and CaF2 can not be recovered, and at the mean time, the F concentration in effluent is high, causing that the effluent can not meet discharge standards.In this paper, based on the principle of induced crystallization and fluidized bed, fluidized bed crystallization method is proposed for treating high fluoride-containing wastewater. According to the method, after CaF2 seed is added to the fluidized bed, wastewater and lime solution are put into the fluidized bed, and then fluoride and calcium ions are precipitated on the CaF2 seed by heterogeneous nucleation. The main conclusions was as follows:By using river sand, quartz sand and CaF2 particles as filling materials, fluidization curve of particles with different size were measured. The minimum fluidization velocity increased with the increasing of particl size, no correlation with material mass.By treating simulated fluoride-containing wastewater, various impact factors on fluidized bed treatment effect had been systematically studied, and the optimum conditions for fluidized bed treatment of 1000mg/L wastewater were determined. At the processing capacity of 16L/h, when nCa/nF was 0.6 and CaCl2 ratio in calcium solution was 10%, the effluent fluoride concentration could be reduced to lower than 1 Omg/L, the effluent pH was between 6 and 7, the effluent turbidity was less than 60NTU (nephelometric turbidity units), the effluent could meet discharge standards. At the same time, the moisture content of sludge produced was only 24.7%, the CaF2 purity of crystal products was 98%, the average size of the CaF2 particle was 378μm, and so it could be directly recycled.On this basis, by treating the fluorine-containing wastewater from a Fluorine-Chemistry Industry Ltd in Hunan, the pilot-scale study of fluidized bed process was carried out with the processing capacity of 0.5m3/h. After two weeks test, the running effect had been basically stable. In the effluent, pH was between 6 and 8, the turbidity was controlled less than 50NTU. The recovery rate of fluorine exceeded 96%, the average size of the CaF2 product was 100μm, and so it could be used as fluorite-class product.The results indicate that by using the fluidized bed crystallization method to treat high-concentration fluoride wastewater, the effluent effect is stable, sludge has low moisture content and CaF2 products can be directly recycled. The process can not only handle high-concentration fluoride wastewater effectively, but also brings economic benefits.