Study On The Crystallization Characteristics Of Calcium Fluorophosphate And Fluorine Removal By Chemiacl Crystallization Granulation Fluidized Bed

Fluorine is one of the essential trace elements in the human body,but the human body has a small demand for fluoride,and the fluoride that enters the human body almost remains in the bones and teeth.The presence of an appropriate amount of fluoride in drinking water has a positive effect on human health,such as preventing dental caries and treating osteoporosis.Excessive fluoride intake can cause various hazards to human health.Drinking high fluoride water for a long time can cause a series of diseases such as dental fluorosis and skeletal fluorosis.Compared with other methods,induced crystallization method has the characteristics of low cost,low pollution,and stable effluent quality.In this paper,the crystallization characteristics of calcium fluorophosphate and the fluidized bed crystallization technology for fluoride removal have been studied,and the following conclusions have been obtained:（1）The supersolubility and induction period of calcium fluorophosphate crystals were measured using the conductivity method and the laser method.The results showed that the induction period of the supersolubility of calcium fluorophosphate crystals was significantly reduced under the condition of using seed crystals.Using natural phosphate rock as seed crystals had the best effect.Using seed crystals with particle sizes of 0.1-0.155 mm,0.2-0.45 mm,0.5-1 mm,and 1-2 mm,it was found that the smaller the particle size,the larger the surface area of the solution contacting the seed,the smaller the saturation and induction period required for crystallization.Similarly,increasing the amount of seed crystal added can also reduce the required saturation and induction period of crystallization.An increase in p H is beneficial to the crystallization of calcium fluorophosphate,but when the p H is too high,the solution also generates calcium hydroxyphosphate and calcium phosphate.The presence of Mg²⁺and HCO₃^-can affect the crystallization of calcium fluorophosphate,which is inhibited dynamically by Mg²⁺.However,the presence of HCO₃^-can generate a new supersaturated system Ca CO₃,consuming Ca²⁺in the solution,thereby affecting the crystallization.（2）The crystal seed concentration is 0,10,20,30,and 40 g/L,the stirring rate is controlled at 400r/min,the initial F concentration is 4.75 mg/L,and Ca:P:F=10:4:1.As the crystal seed concentration increases,the reaction rate significantly increases.The results of reaction kinetics show that the pseudo first order kinetics can better describe the reaction when the seed concentration is low,while the pseudo second order kinetics are closer when the seed concentration is high,indicating that when the seed dosage is high,the reaction mainly focuses on secondary nucleation,which is conducive to the progress of the reaction.The increase of P/F has little effect on the reaction rate of the system,with a seed concentration of 40 g/L.The reaction rate constant obtained by fitting is 0.0085 mg/（L·min）when Ca:P:F=10:4:1,while it is 0.0078 mg/（L·min）when Ca:P:F=10:3:1.Under the conditions of initial solution fluoride ion concentration of4.75 mg/L,Ca:P:F=10:3:1,p H 7.0±0.2,seed size 0.1-0.2 mm,and seed concentration of 40 g/L,the crystallization rate of calcium fluorophosphate was 0.00033 mm/h,and new crystal centers were more easily formed on particles with slightly larger particle sizes.（3）Using a fluidized bed reactor,the effects of hydrochemical conditions on the effluent of the fluidized bed were investigated by changing p H,phosphate dosage,and P/F.The treatment effect was best when the p H was around 8.Comparing the two conditions of P:F=4:1 and 3:1,when P:F=3:1,the effluent F^-ups to the standard and the effluent P is controlled within 0.5mg/L,indicating good effluent quality.Under the conditions of p H=8,Ca:P:F=10:3:1,initial fluoride concentration of 4.75 mg/L,and flow rate of 7.8-13 m/h,the fluoride ion concentration in the effluent of the fluidized bed reactor can all reach below 1 mg/L.Under high alkalinity water conditions（300mg/L calculated as Ca CO₃）,adjusting Ca:P:F to 20:3:1 can achieve a effluent F concentration of 0.7-0.8 mg/L.Under the presence of Mg²⁺（50 mg/L）,the fluoride ion concentration in the effluent is around 2 mg/L,which cannot meet the standard.Based on Minteq’s calculation results and characterization analysis of crystal seeds,the main precipitate generated in the fluidized bed is Ca₅（PO₄）₃F.In addition,during the reaction process,there will be the formation of Ca₅（PO₄）₃OH and Ca₃（PO₄）₂,which crystallize together to achieve the purpose of fluoride removal and control the concentration of Ca and P in the effluent at a low level.