Sand-shaped Cryolite Recovery From High Fluoride-containing Wastewater

A great deal of acidic high fluoride-containing wastewater was discharged from semiconductor manufacturing factories, glass production industries, metal processing industries, etc. The effluents from these industries are harmful to human, animal and natural environment. Many techniques of water defluorination have been developed over time, including chemical precipitation, flocculation, adsorption, and so on. Among these methods, liming precipitation has been widely used due to its low costs and simple procedure. However, the sludge with high moisture content prevents technical uses of precipitated fluoride. Mean while, as an important raw chemical material, fluoride mineral is limited in reserves but consumed in large quantities. The shortage of resources would be a challenge for the fluoride industries. Therefore, recover of fluoride as a valuable product from the wastewater is significant.A novel process of induced crystallization technology has been proposed and a "reaction and solid-liquid separation integrated device" has been designed for recovery of sandy cryolite from acid high fluoride-containing wastewater. Details are summarized as follows.The effects of addition sequence of reagents, feeding modes and seed sources on recovery efficiency of cryolite were investigated. The experimental results indicated that sodium aluminate and sodium hydroxide mixed solution was the best precipitant with high utilization ratio and fluorine recovery efficiency. Cryolite with regular morphology and dense surface could be obtained by "step by step" method. The precipitate could be used as seeds.The effects of reaction time, reaction temperature, fluoride concentration, and reaction pH on recovery efficiency of cryolite were investigated. The suitable coating conditions were as follows:reaction temperature was50℃, reaction pH was in the range of4.0to6.0. Cryolite would be easier formation in higher initial fluoride concentration.A "reaction and solid-liquid separation integrated device" was designed for experiments after the best coating technological conditions determined. Simulated wastewater and industrial wastewater were separately researched. Sand-shaped cryolite was recovered from the simulated wastewater with a fluorine recovery rate of80%in follow conditions:the initial fluoride concentration of4000mg/L, the reaction temperature of50℃, the reaction pH between4.0to6.0, the influent flow of40L/h. In these conditions, the "reaction and solid-liquid separation integrated device" running stably and the solid-liquid separation effect was ideal. Furthermore, the recovered sand-shaped cryolite in the experiments of treating industry wastewater was significantly better than in traditional precipitation method. The recovered fluoride from "reaction and solid-liquid separation integrated device" could be reused as raw material or additive to industrial applications.