Siberian Federal University, Krasnoyarsk, Russia:

Yu. G. Mikhalev, Professor at the Department of Physical and Inorganic Chemistry, e-mail: y.mihalev@bk.ru
N. Yu. Zharinova, Associate Professor at the Department of Geography

Due to mobility of electrode/electrolyte interface, the polarization of liquid metal electrodes in molten salts enables a variety of mass transfer conditions (regimes). As a result, the current efficiency value in fused electrolysis with liquid metal electrodes can have a wide range of variability. In electrowinning of metals in molten salts at given current density or potential, the current efficiency is mainly dictated by metal solubility and the flows of dissolved metals (metal sub-ions) from cathode to anode. The flow of sub-ions is dictated by the intensity of mass transfer at the cathode surface characterized with the mass transfer coefficient, which can vary by orders of magnitude depending on the mass transfer conditions or regimes at the electrode/electrolyte interface. Equations have been deduced that show a relationship between the current efficiency in electrowinning and the mass transfer intensity for potentiostatic and galvanostatic electrolysis conditions. With the help of these equations one can estimate the current efficiency for different mass transfer coefficients at given cathode current densities or overvoltage values and solubility of the target metal and the background electrolyte metal. Analysis of the resultant equations and the current efficiency suggested by them shows that in potentiostatic conditions the metal losses do not depend on the mass transfer intensity but are rather governed by overvoltage and the concentrations of the sub-ions of the target metal and the background electrolyte metal at the electrode/melt interface. In galvanostatic conditions the current efficiency is dictated by the mass transfer regime at the electrode/ electrolyte interface. Thus, it decreases as the mass transfer intensity rises.

1. Mikhalev Yu. G. Dissipative structures and mass transfer in high-temperature electrochemical kinetics: Extended abstract of doctoral dissertation. Ekaterinburg, 2000. 43 p.
2. Solheim А., Gudbrandsen H., Osen K. S., Kongstein O. E., Skybakmoen E. Current efficiency in Hall-Héroult cells: the role of mass transfer at the cathode. Light Metals. 2018. pp. 605–609.
3. Solheim A. Polyvalent impurities and current efficiency in aluminium cells: a model concerning electrochemical short circuiting. Light Metals. 2016. pp. 371–376.
4. McIntosh G. J., Metson J. B., Lavoie P., Niesenhaus T., Reek T., Peran der L. The impact of alumina quality on current efficiency and energy efficiency in aluminum reduction. Light Metals. 2016. pp. 417–422.
5. Al-Mejali J. A., Haarberg G. M., Bensalah N., Ben-Aissa Benkahla, Lange H. P. The role of key impurity elements on the performance of aluminium electrolysis – current efficiency and metal quality. Light Metals. 2016. pp. 389–394.
6. Côté P., Martin O., Allano B., Dassylva-Raymond V. Predicting instability and current efficiency of industrial cells. Light Metals. 2017. pp. 623–629.
7. Solheim A. Sodium in aluminium as a cell performance indicator: a quantitative framework. Light Metals. 2017. pp. 633–639.
8. McIntosh G. J., Wijayaratne H., Agbenyegah G. E. K., Hyland M. M., Metson J. B. Impacts of sodium on alumina quality and consequences for current efficiency. Light Metals. 2018. pp. 533–539.
9. Jun-qing Wang, Chang-lin Li, Deng-peng Chai, Yun-feng Zhou, Bin Fang, Qiang Li. Relationship between aluminium electrolysis current efficiency and operating condition in electrolyte containing high concentration of Li and K. Light Metals. 2018. pp. 621–626.
10. Grjotheim K., Kvande H. Introduction to aluminium electrolysis. Understanding the Hall – Heroult process. Dusseldorf : Aluminium-Verlag, 1993. 260 p.
11. Lebedev O. A. Production of magnesium by electrowinning. Moscow : Metallurgiya, 1988. 286 p.
12. Solheim A. Current efficiency in aluminium reduction cells: theories, models, concepts, and speculations. Light Metals. 2014. pp. 753–758.
13. Vetyukov M. M., Tsyplakov A. M., Shkolnikov S. N. Electrometallurgy of aluminium and magnesium. Moscow : Metallurgiya, 1987. 320 p.
14. Levich V. G. Physicochemical hydrodynamics. Moscow : Gostekhteorizdat, 1959. 699 p.
15. Bukhbinder A. I. The theory of flows. Leningrad : LPI, 1973. 218 p.
16. Smirnov M. V. Electrode potentials in molten chlorides. Moscow : Nauka, 1973. 247 p.
17. Delimarskiy Yu. K., Zarubitskiy O. G. Electrolytic refining of heavy metals in ionic melts. Moscow : Metallurgiya, 1975. 248 p.
18. Wang X., Peterson R. D., Richards N. E. Dissolved metals in cryolitic melts. Light Metals. 1991. pp. 323–330.