Siberian Federal University, Krasnoyarsk, Russia:

N. V. Vasyunina, Assistant Professor of a Chair of Metallurgy of Non-Ferrous Metals, e-mail: Nvvasyunina@gmail.com
N. A. Sharypov, Senior Lecturer of a Chair of Automation of production processes in metallurgy

I. P. Vasyunina, Assistant Professor

LLC “RUSAL Engineering-Technical Center”, Krasnoyarsk, Russia:
S. K. Zhed, Manager

Cryolite-alumina crust is responsible for about 10% of the heat released in the cell. The permeability of the crust plays an important role when it comes to the emissions of gases from aluminum production. Thus, in order to ensure a stable energy balance and reduce emissions, under conditions of a tendency to increase the amperage on an aluminum cells, it is necessary to ensure the formation of a gas-tight, holistic, maximally thermal conductivity and sufficiently strength crust into the space of the side-anode. The influence of the granulometry of the cryolite-alumina mix on the thickness, mechanical strength, thermal conductivity, gas permeability and density of the crust formed from it is considered in the article. The formation of the crust from the cryolite-alumina mix was carried out in a steel pipe with the creation of a unidirectional heat flux with fixation of the temperature gradient over the thickness of the crust. The thermal conductivity was determined from the distribution of temperatures in the crust and loose coating. Crusts were formed from 5 different cryolite-alumina mixes: one for comparison from primary alumina, the content of the crushed bath material in the cryolite-alumina mix of the remaining was 50%. The strong decrease in the content of fines in the crushed bath material for thermal conductivity, mechanical strength and density of crusts, as well as the penetration rate of electrolyte in them and the thickness of the formed crusts were obtained. All the crusts formed from the studied mixes had a significantly higher thermal conductivity than the crust formed from the primary alumina. A large amount of fine fraction leads to the formation of thick, loose, unstable crusts that have a higher gas permeability, but the thermal conductivity of the crust increases with the content of the fine fraction of the crushed bath material in the mix.

1. Aljasmi A., Arkhipov A., Alzarooni A. Evaluation of anode cover heat loss. ICSOBA 33rd Conference and Exhibition in Dubai from 29 November to 1 December 2015.
2. Slaugenhaupt M. L., Bruggeman J. N., Tarcy G. P., Dando N. R. Effect of open holes in the crust on gaseous flouride evolution from pots. Light Metals. 2003. P. 199–204.
3. Allard F., Désilets M., LeBreux M., Blais A. Chemical characterization and thermodynamic investigation of anode crust used in aluminum electrolysis cells. Light Metals. 2015. P. 565–570.
4. Allard F., Désilets M., LeBreux M., Blais A. The impact of the cavity on the top heat losses in aluminum electrolysis cells. Light Metals. 2015. P. 289–294.
5. Rye K. Crust Formation in Cryolite Based Baths : Ph. D. thesis. — Trondheim, 1992.
6. Volberg A. A., Sukhanov E. L., Belyaev A. I. Structure and thermophysical properties of the surface of the crust formed in industrial reduction cells for aluminum production. News of the Academy of Sciences of the USSR. 1964. No. 5. P. 45–56.
7. Semenov V. S., Urda N. N. On the thermophysical properties of accretions and frozen crusts in aluminium electrolytic cells. Soviet Journal of Non-ferrous Metals. 1973. No. 14. P. 37–39.
8. Becker A. J., Hornack T. R., Steinback T. J. In-situ properties of crusts formed with five ores in a Soderberg smelting cell. Light Metals. 1987. P. 41–50.
9. Llavona M. A., Verdeja L. F., Zapico R., Alvarez F., Sancho J. P. Density, hardness and thermal conductivity of Hall-Heroult crusts. Light Metals. 1990. P. 429–438.
10. Liu X., Taylor M. P., George S. F. Crust formation and deterioration in industrial cells. Light Metals. 1992. P. 489–494.
11. Zhang Q., Taylor M. P., Chen J. J., Cotton D., Groutzo T., Yang X. Composition and thermal analysis of crust formed from industrial anode cover. Light Metals. 2013. P. 675–680.
12. Zhang Q., Taylor M. P., Chen J. J. The melting behaviour of aluminium smelter crust. Light Metals. 2014. P. 591–596.
13. Johnston T. J., Richards N. E. Correlation between alumina properties and crusts. Light Metals. 1983. P. 623–639.
14. Becker A. Properties of crust. 7^th Int. Course on process metallurgy of aluminium. Inst. of Inorg. Chemistry, the Norwegian Institute of Technology, May – June 1988.
15. Rye K. A., Thonstad J., Liu X. Heat transfer, thermal conductivity, and emissivity of Hall-Heroult top crust. Light Metals. 1995. P. 441–449.
16. Townsend D. W., Boxall L. G. Crusting behaviour of smelter aluminas. Light Metals. 1984. P. 649–665.
17. Less L. N. The crusting behaviour of smelter aluminas. Metallurgical Transactions 8B. 1977. P. 219–225.
18. Johnson A. R. Alumina crusting and dissolution in molten electrolyte. J. Metals. 1982. Vol. 34, No. 3. P. 63–68.
19. Gerlach J. Dissolution and interaction between alumina and cryolite melts. Proc. 1’st. Int. Symp. Molten Salts Chem. Tech. Kyoto, 1983. P. 89–92.
20. Woodfield D., Picot G., Harding M. Toward Optimum Anode Cover. – Eighth Australasian Aluminium Smelter Technology Conference and Workshops, 2004.
21. Llavona M. A., Verdeja L. F., Alvarez F., Garcia M. P., Sancho J. P. Formation and characterization of aluminium electrolysis crusts. Light Metals. 1990. P. 439–446.
22. Vasyunina N. V., Vasyunina I. P., Mikhalev Yu. G., Vinogradov A. M. Solubility and dissolution rate of alumina in acidic cryolite-alumina melts. Proceedings of Higher Schools. Nonferrous Metallurgy. 2009. No. 4. P. 24–29.
23. GOST 2409–95. Refractories. Method for determining apparent density, open and total porosity, water absorption. – 1997.01.01.
24. GOST 10180–90. Concretes. Methods for determining the strength of the control samples. — 1991.01.01.
25. Skybakmoen E., Solheim A., Sterten A. Phase diagram for the system Na₃AlF₆ – AlF₃ – Al₂O3. Part II: Alumina solubility. Light Metals. 1990. P. 317–323.