Cryolite (Na3AlF6) crystallization for fluoride recovery using an electrolytic process equipped with a sacrificial aluminum anode

• Published in: Journal of hazardous materials 2019-04, Vol.368, p.90-96
• Main Authors: Ya, Vinh, Chen, Yi-Chieh, Chou, Yi-Hsuan, Choo, Kwang-Ho, Liu, Jhy-Chern, Mameda, Naresh, Li, Chi-Wang
• Format: Article
• Language: English
• Subjects: Chemical equilibrium modeling; Cryolite; Electro-crystallization; Fluoride; Recovery
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/j.jhazmat.2019.01.033

•Electro-crystallization process for fluoride removal was developed.•Effects of pH and Al/F molar ratio on F removal were studied.•The optimum condition for generating cryolite was identified.•Cryolite coating on anode was abated with chloride ions.•Formation of AlFn3−n decreases F removal at the Al/F molar ratio of >1/6. An electro-crystallization process equipped with a sacrificial aluminum anode was operated under an optimum condition to promote the formation of crystalline cryolite for the recovery of fluoride from synthetic F-containing wastewater. The effects of pH, Al/F molar ratio, initial F concentration, and electrolytes were investigated experimentally, and the results were compared with data obtained from chemical equilibrium modeling. Cryolite was successfully produced under optimum pH values of 5 to 6 and Al/F molar ratios of less than 1/6. The F removal increased with increasing Al/F molar ratio until reaching the molar ratio of 1/6 and decreased thereafter due to the formation of AlFn3−n species. The adsorption of AlFn3−n by Al(OH)3 precipitates contributed part of F removal. The removal efficiency reached 100% when the initial fluoride concentration was high while it was around 90% with the low initial fluoride concentration. XRD and SEM/EDX analysis showed that the obtained solids matched well to the commercial cryolite. Finally, the operating costs of chemical-crystallization (the process with Al ions added chemically) and electro-crystallization were compared, and the cost of the former was less than the latter. Energy consumption was the main contributor to the operating cost of the electro-crystallization process.