Study On The Treatment Of High Concentration Fluoride Wastewater And Calcium Fluoride Recovery

With the development of fluorine chemical industry, emissions of wastewater containing fluoride and fluorine-containing residue increased, the damage to the environment increased year by year. Currently, in terms of treatment of fluorine-containing waste water, chemical sedimentation-flocculation sedimentation section technology for removing fluoride is widely used because of its simple operation, lime becomes the most common precipitant for its low cost. But the process has the following disadvantages:lime is difficult to react adequately, the treated sediment volume is large, the sludge’s settleability is poor for its high moisture, which resulted in that a lot of fluoride in sediment cannot be reclaimed.Besides, the fluorine-containing residue not only takes up much land, but also leads to a secondary pollution.With respect to the deficiencies of traditional two-stage fluoride removal technology, this study put forward the CaCl2-PAC two-stage fluoride removal technology. In this process, lime was replaced by "calcium chloride" and the waste water was obtained from Hunan Nonferrous Chemistry CO. Ltd. In this study, the optimum process conditions were determined by single factors experiment and orthogonal test.; comparative studies were carried out on the fluoride-removal effect as well as the moisture content and the CaF2 purity of the sediment, with or without adding the seed crystal of CaF2; the equation relationship between the sludge moisture content and its influencing factors was matched; the technology using sludge as seed crystal was studied and the moisture content and the CaF2 purity of the sediment were tested, in the end the treatment process of CaCl2-PAC two-stage fluoride removal method was designed. The main conclusions are as follows:Results showed that (i) in the first stage of precipitation with calcium salt, the optimum conditions were as follows:the pH was adjusted around 9 using a calcium hydroxide solution, and the dosage of the 5% calcium chloride solution was determined according to the mole number of CaF2 (herein, n=0.75), stirring for 45 min and precipitating for 90 min, and then (ⅱ) in the second stage of flocculation with PAC, the optimum conditions were as follows:the pH was around 7.5 and the dosage of PAC was 350 mg/L. Then, the residual mass concentration of fluoride ion in the wastewater could be controlled at 7 mg/L, meeting the first class of Integrated Wastewater Discharge Standard (GB-8978-1996). While the moisture content of sludge was too high to be recycled. On the basis of the process above, CaF2 was added to act as seed crystal and its optimum dosage was 1.2g. After its addition, the residual mass concentration of fluoride ion in the wastewater could be controlled under 6 mg/L, the CaF2 purity increased up to 85% while the moisture content was found to decrease by 20%, which indicates a better fluoride-removal effect and benefits for CaF2 recovery and reuse. The multiple regression analysis equation was established through SPSS:Y= 0.820-0.188X1-0.104 X2+0.005 X3-0.272 X4, the equation showed that sludge moisture content had significant linear relationship with its influencing factors.The study using sludge to act as seed crystal showed that, sludge could also had a good effect on fluoride removal, after adding the sludge, the residual mass concentration of fluoride ion in the wastewater was better than the first class of discharge standard and the moisture content of sediment was low enough to be recycled, which leaded to a lower cost and a good use of waste materials. In the end, treatment process of CaCl2-PAC two-stage fluoride removal method was designed, it mainly consisted of regulation poor, the primary sedimentation tank, the flocculation reaction pool, the secondary sedimentation tank and drain tank. The process was benefit to continuous injections, the effluent was discharged directly with partial reflux. The process was easy to control and its treatment efficiency was excellent.The conclusion could be reached that the residual mass concentration of fluoride ion in the wastewater could be reached the emission standard and fluorine resources could be reused by CaCl2-PAC together seed crystal adding process. The study owns a good social and economic benefit.