*2.2. Reagents*

All reagents used in this study were of analytical purity. Distilled water was obtained using a GFL distiller (GFL mbH, Burgwedel, Germany). A model alkali liquor obtained was obtained by dissolving NaOH in distilled water. Then, the necessary amount of Al(OH)3 was dissolved in the resulting hot caustic alkali liquor to achieve the desired ratio of NaOH and Al2O3 in the model liquor. After complete dissolution of aluminum hydroxide, the liquor was brought to the required concentration in caustic alkali. The aluminate liquor had the following chemical composition: Na2Ok (caustic alkali) = 240 g/dm3, Al2O3 = 120 g/dm3. The sinter and ESP dust were obtained from the RUSAL-Kamensk-Uralsky alumina refinery (56.304530, 61.980334; Kamensk-Uralsky, Russia) where low-grade bauxite is processed via combined Bayer-Sintering process. Bauxite with the high silica content is sent in rotary kiln together with sodium carbonate to obtain water-soluble sinter at 1150 ◦C. The chemical and phase composition of the ESP dust and the sinter are different, since dust passes through the hot zone of the sintering kiln faster and many processes fail to complete; also, the difference may be caused by the difference in the distribution of minerals into different fractional classes. Table 1 shows the chemical composition of the ESP dust of the Kamensk-Uralsky alumina refinery, against the chemical composition of the bauxite sinter.


**Table 1.** The chemical composition of electrostatic precipitator (ESP) dust and bauxite sinter, wt. %.

The results of X-ray diffraction analysis of the electrostatic precipitator dust of the RUSAL-Kamensk-Uralsky alumina refinery is shown in Figure 2.

**Figure 2.** X-ray diffraction analysis of the electrostatic precipitator dust of the RUSAL-Kamensk-Uralsky alumina refinery in Russia.
