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

Vinh Ya; Yi-Chieh Chen; Yi-Hsuan Chou; Kwang-Ho Choo; Jhy-Chern Liu; Naresh Mameda; Chi-Wang Li

doi:10.1016/j.jhazmat.2019.01.033

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

J Hazard Mater. 2019 Apr 15:368:90-96. doi: 10.1016/j.jhazmat.2019.01.033. Epub 2019 Jan 15.

Authors

Vinh Ya¹, Yi-Chieh Chen², Yi-Hsuan Chou², Kwang-Ho Choo³, Jhy-Chern Liu⁴, Naresh Mameda³, Chi-Wang Li⁵

Affiliations

¹ Department of Water Resources and Environmental Engineering, Tamkang University, 151 Yingzhuan Road, Tamsui District, New Taipei City 25137, Taiwan; Faculty of Environment and Natural Resources, Dalat University, Dalat, Viet Nam.
² Department of Water Resources and Environmental Engineering, Tamkang University, 151 Yingzhuan Road, Tamsui District, New Taipei City 25137, Taiwan.
³ Department of Environmental Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu, 702-701, Republic of Korea.
⁴ Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Rd., Section 4, Taipei 106, Taiwan.
⁵ Department of Water Resources and Environmental Engineering, Tamkang University, 151 Yingzhuan Road, Tamsui District, New Taipei City 25137, Taiwan. Electronic address: chiwang@mail.tku.edu.tw.

PMID: 30665112
DOI: 10.1016/j.jhazmat.2019.01.033

Abstract

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 AlF_n^3-n species. The adsorption of AlF_n^3-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.

Keywords: Chemical equilibrium modeling; Cryolite; Electro-crystallization; Fluoride; Recovery.