RUSAL Engineering-Technical Center, Saint Petersburg, Russia:

V. O. Golubev, Head of the Mathematical Modelling Department, e-mail:
Vladimir.Golubev2@rusal.com
D. G. Chistyakov, Leading Engineer at the Mathematical Modelling Department, e-mail: Dmitriy.Chistiakov@rusal.com

Saint Petersburg Mining University, Saint Petersburg, Russia:
V. N. Brichkin, Head of the Department of Metallurgy, e-mail: Brichkin_VN@pers.spmi.ru
M. F. Postika, Undergraduate Student, e-mail: kafmet@spmi.ru

Population balance modelling is the most developed technique for modelling the process of aluminate solution decomposition induced by aluminium hydroxide, and it is of great scientific and practical relevance. Through solving the population balance equation one can track how the process conditions influence the decomposition parameters, which include: degree of decomposition, solids in the slurry, alumina/caustic ratio, particle size distribution of the deposit, etc. Periodic and half-periodic experiments are applied to set up the functional relationships that dictate the growth rate, as well as the nucleation and granulation intensities in that equation. Using physical experiment and mathematical modelling, the authors demonstrate that agitation in a big lab reactor (5 litres) is far from being ideal and is characterized with significant fluctuations in the slurry density. This effect causes significant deviations from the population balance provisions and may affect the accuracy of predicted decomposition parameters. The authors conducted a laboratory study that looked at the decomposition process in an environment simulating the industrial regime of decomposing high-modulus aluminate solutions adopted by in-country plants that rely on Bayer process. The authors found a huge difference between the process in view and a layer-by-layer growth of the seed, a low relevance of the granulation stage and a high rate of secondary nucleation. A big discrepancy was found between the experimental data and the model analysis data at the initial stage of an hour-long decomposition process after the seed had been charged. Such discrepancy is due to an induction period. All the other simulation results proved to be acceptable in terms of accuracy. The method of physical modelling integrated with population balance modelling proved to be generally acceptable for experimental data description and analysis regardless of the original state of the system. At the same time the method requires further development so that it could produce consistent descriptions of all the stages and regimes of decomposition.
This research was funded by the Russian Science Foundation under the Agreement No. 18-19-00577 dated April 26, 2018 on granting funds for basic and exploratory research.

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