Recovery And Removal Of Fluoride From Fluorine Industrial Wastewater By Crystallization Process

Abstract:China is one of the largest producers of fluoride salts in the world. Large amount of high-concentration fluorine-containing wastewater are produced in fluorine industry, causing serious hazardous effects to the propagations and human-beings. The traditional methods for fluoride removal generally involve chemical precipitation and coagulation. The process generates large amounts of sludge with high water content, low quality, and non-reusability, which has to be disposed with increasing costs. Meanwhile, as precious non-metallic ore resources, fluorites are suffering a shortage crisis worldwide. Hence, the recovery of fluoride from industrial wastewater has great economic and environmental significance from the perspective of resource circulation and sustainable development.In this paper, the treatment of fluorine-containing industrial wastewater from a fluorine salts factory in Hunan Province has been investigated. The aim of the study is that the fluorine of wastewater can be recovered as resource, and that the effluent water quality can meet the discharge standard. A novel process, including recovery of sandy cryolite by crystallization and recovery of sandy calcium fluoride by crystallization, has been proposed. A noval reaction-separation integrated reactor and a fluidized bed reactor are designed, respectively. The crystallization technologies for recovery of sandy cryolite and sandy calcium fluoride are investigated. The problems that the cryoliye products and calcium fluoride products with small particle size and high water content cannot be reused have been solved by the novel process. The main results are as follows:The solid-liquid equilibrium conditions for the cryolite system and for the calcium fluoride system were investigated, respectively.(1) The distribution of dissolved Al and F complexes, and the stable zones of Na3AlF6, NaAlF4and A1(OH)3for Al3+-Na+-F--H+-H2O system were obtained. The results showed that the stable solids were Na3AlF6, NaAlF4or A1(OH)3, indicating that the cryolite with Na/Al molar ratio of3cannot be precipitated in wastewater. The solid of Na3AlF6with higher Na/Al molar ratio was favored to precipitate with increasing total concentration of fluoride.(2) The relationship between the solubility and particle size of calcium fluoride, and the metastable zones of calcium fluoride for Ca2+-F--H+-Al3+-H2O system were studied. The results showed that the solubility of CaF2and the equilibrium fluoride concentration decreased with increasing particle size of CaF2. The removal efficiency of fluoride increased with increasing particle size of calcium fluoride. The solubility curve and supersolubility curve divided the diagram into undersaturated region, metastable region, and labile region, respectively. The diagram of metastable region can be used to predict the nucleation process of calcium fluoride.The technological conditions for recovery of cryolite and recovery calcium fluoride were investigated, respectively.(1) The influence of several variables on the particle size and the molar ratio of cryolite products were studied. The results showed that the sandy cryolite products with low water content can be obtained with increasing seed retention time. The molar ratio of cryolite products can be maintained between2.4to3.0with increasing molar ratio of sodium aluminate.(2) The fluidized state of quartz sand in the fluidized bed (FBR) reactor was characterized. The results showed that the fluidization velocity and terminal velocity increased with increasing seed size, leading to a wide range of controlling conditions. The pressure difference can be used to predict the operation state and the particle size of pellets in the FBR.(3) The growth kinetics of calcium fluoride at high supersaturation was studied in the FBR, and the kinetics model of crystal growth was determined. The results showed that the calcium fluoride and fine particles grew in size with a high linear growth rate by adsorption and aggregation at high supersaturation.(4) The removal efficiency of fluoride by precipitation of calcium fluoride in the FBR was studied at high supersaturation. The synthetic HF wastewater was treated by the suspension of Ca(OH)2. The results showed that the fluoride concentration of the effluent was20to80mg/L at an influent flow of11to32L/h, an influent concentration of1000mg/L, Ca/F molar ratio above 1, and pH of6.0to11.0.(5) The deep removal efficiency of fluoride by flocculation was studied. The results showed that the fluoride concentration of the effluent was below10mg/L at a polyaluminium chloride dosage of100to400mg/L, and pH of6.5to8.0.Two crystallization reactors were designed and used for recovery and treatment of fluorine-containing industrial wastewater by steps.(1) A novel reaction-separation integrated reactor was designed. The seed retention time can be prolonged to strengthen the induced crystallization process in the integrated reactor. The fluorine-containing industrial wastewater was treated by a pilot-scale integrated reactor (capacity80L/h). The recovery rate of cryolite was above70%at a reaction temperature of35?to50?, a reaction pH of4.0to7.0, and a stirring reaction time of14min. The quality of cryolite products with large size and low water content (<20%) can meet the national standard of GB/T4291-2007.(2) The industrial complex wastewater with a fluoride concentration of300to1000mg/L was treated by a pilot-scale fluidized bed reactor (capacity0.49m3/h). The suspension of Ca(OH)2was used as the precipitant. The results showed that the fluoride concentration of the effluent was15to40mg/L at pH of7.0to10.0, and Ca/F molar ratio above1. After50h operation, the particle size and water content of calcium fluoride on bottom of the FBR were159.0?m and27.0%, respectively. The qualities of calcium fluoride products can meet the national standard of comminuted fluorspar (GB19321-88) in China. The calcium fluoride products could be used as raw materials in the ceramics and cement industry.(3) The mother liquor of calcium fluoride with a fluoride concentration of20to40mg/L was treated by flocculation process. The influent flow of the wastewater was40L/h, and the reaction pH of wastewater was adjusted at7.0to8.0. The dosage of polyaluminium chloride was200mg/L. The turbidity and fluoride concentration of effluent were less than30NTU and lOmg/L, respectively. The qualities of effluent can meet the national discharge standard (GB8978-1996).Based on the pilot-scale study, the system with a production capacity of100m3/h was designed for recovery of cryolite. The reaction-separation integrated reactor was the key equipment of the stystem. One of the integrated reactors with a production capacity of25m3/h was tested. The cryolite products with large size and high quality (GB/T4291-2007) can be precipitated from industrial wastewater. The system for recovery of cryolite has been successfully applied to industry.