Saint Petersburg Mining University, Saint Petersburg, Russia:

V. N. Brichkin, Head of Metallurgy Department, Doctor of Technical Sciences, e-mail: Brichkin_VN@ pers.spmi.ru
A. T. Fedorov, Postgraduate Student at the Metallurgy Department, e-mail: s185005@stud.spmi.ru

The structure and ionic composition of aluminate solutions are the key problems in the theory of alkali-based alumina production. Good understanding of these problems determine how adequately the processes will be evaluated that take place at different stages of aluminium material processing and if modern digital twins of processing plants can be enhanced with flexible thermodynamic and kinetic models. Most researchers recognize two forms of complex alumi nium anions for the process critical region of the Na₂O – Al₂O₃– H₂O and K₂O – Al₂O₃ – H₂O systems. They exist in the form of hydrated monomers and dimers of meta-aluminate ion. Based on the analysis of phase equilibria in the Na₂O – Al₂O₃ – H₂O system, it was established that the degree of nonlinearity of solubility isotherms can serve as an indicator of a more complex composition of aluminate solutions with dimers of tetrahydroxo complexes of meta-aluminate ions, and helps calculate the ionic composition for an isothermal model with two ionic forms of aluminium. It is shown that, with the participation of dimers, the region of the corresponding equilibria can be determined by building solubility isotherms in the form of the following function: [Al₂O₃] = f ([Na₂O] – [Al₂O₃]). It helps narrow the range of model representation of coexistence between monomers, dimers and dehydrated meta-aluminate ions in the system Na₂O – Al₂O₃ – H₂O. The modelling results show that, with the participation of dimers of tetrahydroxo complexes of meta-aluminate ions, the region of phase equilibria tends to expand significantly as the temperature rises and, with the participation of dimers, the equilibrium maximum tends to shift into the region of low-alkaline solutions. Thus, the equilibrium isotherms in the Na₂O – Al₂O₃ – H₂O system at the temperatures of 30, 60, and 95oC include phase equilibria involving one, two or three ionic forms of aluminium, the fraction and the region of which are determined by the concentration of alkaline component and the temperature.
This research was funded by the Russian Science Foundation under Agreement No. 18-19-00577-П dated April 28, 2021 on the Provision of a Grant for Conducting Fundamental and Exploratory Research Studies.

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