Chromite beneficiation tailings (CBTs) represent a significant environmental challenge, while simultaneously containing valuable metals that remain largely unrecovered. In this study, a sequential thermochemical–hydrometallurgical route was investigated for selective chromium extraction and the enrichment of platinum group metals (PGMs) from CBTs generated at the Donskoy Mining and Processing Plant. Alkaline sintering with Na₂CO₃ at 1000 °C followed by aqueous leaching enabled the transfer of up to 98%–99% of chromium into solution. The resulting residue was enriched in non-ferrous metals, rare earth elements, and PGMs. Subsequent sulfation roasting and water leaching promoted the dissolution of magnesium, nickel, and rare earth elements, while platinum and palladium remained predominantly in the solid phase, due to their low solubility under the applied conditions. Microstructural analysis using SEM–EPMA revealed that PGMs are selectively concentrated in Ni-bearing micro-inclusions, with local platinum content reaching up to 3.8 wt.% in Ni-rich regions. The proposed sequential processing strategy enables efficient chromium recovery and significant PGM enrichment in the residual phase, demonstrating the potential of CBTs as a secondary resource for integrated metal recovery.

The efficient processing of nepheline ores and red mud requires mineralogical preconditioning strategies that enhance aluminum recovery while controlling the potassium behavior in alkaline systems. In this study, carbonate–hydrothermal activation of a nepheline ore–red mud mixture (1:1 mass ratio) was investigated as a targeted approach for phase redistribution prior to alkaline leaching. Thermochemical activation in NaHCO₃ solution at 260 °C resulted in the partial destabilization of primary aluminosilicate phases and the formation of secondary carbonate-bearing and hydroxide phases. Quantitative phase analysis revealed the formation of cancrinite (~19 wt.%) and boehmite (~2.8 wt.%), indicating dissolution–reprecipitation processes accompanied by aluminum redistribution and carbonate incorporation. A two-stage alkaline leaching process with controlled calcium addition was applied to evaluate the effect of phase transformation on the extraction performance. The activated material exhibited enhanced reactivity, resulting in an alumina recovery of 92%–94% and selective potassium removal of 82%–87%. The results demonstrate that carbonate–hydrothermal activation acts as an effective mineralogical preconditioning strategy, transforming structurally stable aluminosilicates into more reactive phases. The proposed approach provides a basis for improving the efficiency and selectivity of integrated alumina recovery from complex aluminosilicate raw materials and industrial waste.

Carbonate thermochemical activation is a promising approach for modifying the mineralogical structure of nepheline–red mud mixtures prior to subsequent alkaline processing. However, quantitative statistical relationships between activation parameters and phase transformation remain insufficiently explored. In this study, Response Surface Methodology (RSM) combined with a Central Composite Design (CCD) was employed to develop and experimentally validate a quadratic regression model describing the influence of activation temperature, activation time, and NaHCO₃ concentration on cancrinite formation. Quantitatively determined cancrinite content obtained by X-ray diffraction (XRD) phase analysis was selected as the mineralogical response variable, enabling direct statistical evaluation of cancrinite-forming transformation. The developed quadratic model was statistically significant (F = 21.24, p < 0.0001) and explained 95.5% of the variability in the measured response (R² = 0.9550). Activation time exerted the strongest statistical influence on cancrinite formation, while activation temperature and NaHCO₃ concentration also showed statistically significant effects; the investigated interaction terms were not statistically significant. Numerical optimization identified a favorable region for cancrinite formation, with a predicted maximum response of 22.75 wt.%. Three independent validation experiments performed at practically selected operating conditions of 260 °C, 4 h, and 110 g/L NaHCO₃ yielded an average cancrinite content of 20.0 ± 1.0 wt.%. The principal novelty of the study lies in the use of quantitatively determined cancrinite content as the response variable in RSM, providing a direct statistical description of mineralogical transformation rather than optimization based solely on conventional technological responses. The developed model provides a quantitative framework for evaluating factor effects and identifying favorable activation conditions within the investigated experimental domain.

The growing demand for alumina, regional limitations in the availability of high-quality bauxite, and the accumulation of aluminum-bearing industrial residues have increased interest in alternative raw materials. In this review, we examine natural and technogenic non-bauxite aluminum-bearing resources, including nepheline ores, kaolin and kaolinitic clays, alunite ores, coal fly ash, and bauxite residue. Their geographical occurrence, chemical composition, aluminum content, major mineral phases, and associated valuable components are summarized and compared. Particular attention is paid to the mineralogical occurrence of aluminum, which largely determines the technological value of a resource. In nepheline ores, aluminum is incorporated into a stable framework aluminosilicate structure. Kaolinite is also relatively resistant, but its transformation into metakaolin substantially increases aluminum reactivity. Alunite represents a distinct sulfate-bearing resource that may provide aluminum together with potassium- and sulfur-containing products. In coal fly ash, aluminum is distributed between reactive amorphous aluminosilicate material and refractory mullite, whereas bauxite residue contains aluminum in several residual and secondary mineral phases associated with Fe, Ti, Si, Ca, and Na. Our comparison demonstrates that the suitability of alternative resources for alumina production cannot be evaluated from the total Al₂O₃ content alone: mineralogy, silica reactivity, impurity composition, resource availability, and the potential recovery of co-products must also be considered. Processing routes are therefore discussed in relation to the specific characteristics of each feedstock.