Processing of Non-Bauxite Aluminum-Bearing Resources for Alumina Production: A Review

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.