Physicochemical parameters of a hydrochemical technology employing sodium chloride to obtain cryolite used in aluminium production

   The paper aims to study the physicochemical parameters of a hydrochemical technology employing hydrofluoric acid and local mineral resources (sodium chloride) to obtain cryolite used in the electrolysis of cryolite-alumina melts. In order to determine the elemental chemical and phase compositions of initial, intermediate, and final products, titration and X-ray diffraction analysis (using an upgraded Dron-2 unit) were employed. The conducted studies indicate that the proposed process of cryolite production from hydrofluoric acid at 28–30 % concentration using aluminium hydroxide and a concentrated sodium chloride solution occurs at 25 °С for 10–15 min. The yield of cryolite reaches 87.6 %, while ~12 % of cryolite remains dissolved in the hydrochloric acid solution. With the temperature rising from 25°С to 95°С, the cryolite yield is shown to decrease from 87.6 % to 69.3 % due to its higher solubility in the formed hydrochloric acid. The cryolite production process was validated via X-ray diffraction analysis. The analysed sample was found to be consistent with the cryolite reference, i. e., indicating an interaction between sodium chloride and fluoroaluminic acid. The conducted studies served as a basis for developing a process flow diagram of hydrochemical cryolite production using hydrofluoric acid, aluminium hydroxide, and sodium chloride. The conducted studies revealed that the technology of cryolite production employing sodium chloride is easy to implement and cost-effective due to the use of local mineral resources and low energy consumption.

1. Altenpohl D. G. Aluminum: technology, applications and environment. A Profile of a modern metal aluminum from within. 6th Edition. Tukwila: Wiley-TMS; 1998, 488 p.

2. Grjotheim K., Kvande H. Introduction to aluminium electrolysis. Dusseldorf: Aluminium Verlag; 1993, 260 р.

3. Thonstad J., Fellner P., Haarberg G. M., Hives J., Kvande H., Sterten A. Aluminium electrolysis - fundamentals of hall-heroult process. 3rd edition. Düsseldorf: Aluminium-Verlag; 2001, 372 p.

4. Mincis M. Ya., Polyakov P. V., Sirazutdinov G. A. Electrometallurgy of aluminium. Novosibirsk: Nauka; 2001, 368 р. (In Russ.).

5. Haupin W. E. Electrochemistry of the Hall-Heroult process for aluminum smelting. Journal of Chemical Education. 1983; 60 (4): 279-282. https://doi.org/10.1021/ed060p279.

6. Gunasegaram D. R., Molenaar D. Towards improved energy efficiency in the electrical connections of Hall-Héroult cells through finite element analysis (FEA) modeling. Journal of Cleaner Production. 2015; 93: 174-192. https://doi.org/10.1016/j.jclepro.2015.01.065.

7. Polyakov P. V., Klyuchantsev A. B., Yasinskiy A. S., Popov Y. N. Conception of “Dream Cell” in aluminium electrolysis. In: Williams E. (eds.). Light Metals. Cham: Springer; 2016, р. 283-288. URL: https://link.springer.com/chapter/10.1007/978-3-319-48251-4_47

8. Asadikiya M., Yang Songge, Zhang Yifan, Lemay C., Apelian D., Zhong Yu. A review of the design of high-entropy aluminum alloys: a pathway for novel Al alloys. Journal of Materials Science. 2021; 56: 12093-12110. https://doi.org/10.1007/s10853-021-06042-6.

9. Tyutrin А. А., Nemchinova N. V., Volodkina A. A. Effects of electrolysis parameters on the technical and economic performance indicators of OA-300M baths. Vestnik Irkutskogo gosudarstvennogo tehnicheskogo universiteta = Proceedings of Irkutsk State Technical University. 2020; 24 (4): 906-918. (In Russ.). https://doi.org/10.21285/1814-3520-2020-4-906-918.

10. Korenko M., Priščák J., Šimko F. Electrical conductivity of system based on Na3AlF6–SiO2 melt. Chemical Papers. 2013; 67 (10): 1350-1354. https://doi.org/10.2478/s11696-013-0393-x.

11. Khokhlov V. A. On the classification of molten salt electrolytes. Russian Metallurgy (Metally). 2010; 96-104. https://doi.org/10.1134/S0036029510020047.

12. Tarcy G. P., Tørklep K. Current efficiency in prebake and Søderberg cells. In: Bearne G., Dupuis M., Tarcy G. (eds.). Essential Readings in Light Metals: Aluminum Reduction Technology. Vol. 2. Cham: Springer; 2013, p. 211-216. https://doi.org/10.1002/9781118647851.ch30.

13. Khramov A. P., Shurov N. I. Modern views on the composition of anionic oxy-fluoride complexes of aluminium and their rearrangement during the electrolysis of cryolite-alumina melts. Russian Metallurgy (Metally). 2014; 2014: 581-592. https://doi.org/10.1134/S0036029514080059.

14. Kirik S. D., Zaitseva Yu. N., Leshok D. Yu., Samoilo A. S., Dubinin P. S., Yakimov I. S., et al. NaF-KF-AlF₃ system: phase transition in K₂NaAl₃F₁₂ ternary fluoride. Inorganic Chemistry. 2015; 54 (12): 5960-5969. https://doi.org/10.1021/acs.inorgchem.5b00772.

15. Samoilo A. S., Zaitseva Yu. N., Dubinin P. S., Piksina O. E., Ruzhnikov S. G., Yakimov I. S., et al. Structural aspects of the formation of solid solutions in the NaF-KF-AlF₃ system. Journal of Solid State Chemistry. 2017; 252: 1-7. https://doi.org/10.1016/j.jssc.2017.04.037.

16. Padamata S. K., Yasinskiy A. S., Polyakov P. V. Progress of inert anodes in aluminium industry: review. Journal Siberian Federal University. Chemistry. 2018; 11 (1): 18-30. https://doi.org/10.17516/1998-2836-0055.

17. Wang Jun-qing, Li Chang-lin, Chai Deng-peng, Zhou Yun-feng, Fang Bin, Li Qiang. Relationship between aluminium electrolysis current efficiency and operating condition in electrolyte containing high concentration of Li and K. In: Martin O. (eds.). Light metals. Cham: Springer; 2018, р. 621-626. https://doi.org/10.1007/978-3-319-72284-9_80.

18. Korenko M., Vaskova Z., Šimko F., Šimurda M., Ambrova M., Shi Zhong-ning. Electrical conductivity and viscosity of cryolite electrolytes for solar grade silicon (Si-SoG) electrowinning. Transactions Nonferrous Metals Society China. 2014; 24 (12): 3944-3948. https://doi.org/10.1016/S1003-6326(14)63554-8.

19. Vasyunina N. V., Vasyunina I. P., Polyakov P. V., Mihalev Yu. G. Solubility of aluminum in cryolite-alumina electrolytes. Izvestiya Vuzov. Tsvetnaya Metallurgiya = Izvestiya. Non-Ferrous Metallurgy. 2011; 52 (4): 360-363. (In Russ.).

20. Vasyunina N. V., Vasyunina I. P., Mihalev Yu. G., Vinogradov A. M. Alumina solubility and dissolution rate in acidic alumina-cryolite melts. Izvestiya Vuzov. Tsvetnaya Metallurgiya = Izvestiya. Non-Ferrous Metallurgy. 2009; 50 (4): 338-342. (In Russ.).

21. Pozin M. E. Technology of mineral salts (fertilizers, pesticides, industrial salts, oxides and acids): monograph. Part 2; 4th ed., amended. Leningrad: Chemistry; 1974, 768 p. (In Russ.).

22. Guz' S. Yu., Baranovskaya R. G. Production of cryolite, aluminum fluoride and sodium fluoride. Moscow: Metallurgiya; 1964, 238 р. (In Russ.).

23. Wan Bingbing, Li Wenfang, Sun Wanting, Liu Fang-fang, Chen Bin, Xu Shiyao, et al. Synthesis of cryolite (Na₃AlF₆) from secondary aluminum dross generated in the aluminum recycling process. Materials. 2020; 13 (17): 3871. https://doi.org/10.3390/ma13173871.

24. Wang Liangshi, Wang Chunmei, Yu Ying, Huang Xiaowei, Long Zhiqi, Hou Yongke, et al. Recovery of fluorine from bastnasite as synthetic cryolite by-product. Journal of Hazardous Materials. 2012; 209-210: 77-83. https://doi.org/10.1016/j.jhazmat.2011.12.069.

25. Chen Jie-Yuan, Lin Chih-Wei, Lin Po-Han, Li Chi-Wang, Liang Yang-Min, Liu Jhy-Chern, et al. Fluoride recovery from spent fluoride etching solution through crystallization of Na₃AlF₆ (synthetic cryolite). Separation and Purification Technology. 2014; 137: 53-58. https://doi.org/10.1016/j.seppur.2014.09.019.

26. Sattarov I. The national treasure of Tajikistan. To the 45th anniversary of the Tajik aluminum company. 2020. Available from: https://asiaplustj.info/ru/news/tajikistan/society/20200331/natsionalnoe-dostoyanie-tadzhikistana-k-45-letiyu-tadzhikskoi-alyuminievoi-kompanii [Accessed 23th September 2021]. (In Russ.).