Recovery of cryolite with high molar ratio from high fluorine-containing wastewater

Abstract

The recovery of fluoride from wastewater has great economic and environmental significance for the fluorine industry. Cryolite is an optimal product for fluorine recovery from high fluorine-containing wastewater. The cryolite with a high molar ratio is more valuable and is extensively used in the aluminum electrolytic industry. The present study aims to recover cryolite with a high molar ratio from high fluorine-containing wastewater through the crystallization process. The crystallization conditions of cryolite with a high molar ratio were investigated by laboratory experiment and thermodynamic modeling. The results show that the composition of the cryolite products may be a mixture of Na3AlF6 and NaAlF4 for the Al3+–Na+–F−–H+–H2O system. Favorable conditions for the formation of AlF6 3− and Na3AlF6 include higher pH, higher influent concentration of fluorine, and lower reaction temperature. Cryolite products with a high molar ratio can be recovered from high-fluorine containing wastewater at a pH range of 3.69–8.39, a reaction temperature of 20–30 °C, and an α k of 3.11–4.44. The results can be used to predict and to guide the crystallization process of cryolite with a high molar ratio.