2564 compared to Refinery A and Refinery C. It may be possible due to variations in the mineralogy of the bauxite feedstock or differences in the alumina refining process. The second most common oxide found in all the red mud samples is aluminum oxide (Al2O3). Refinery A and Refinery C have significant changes in Al2O3 (> 22 wt.%) but a significantly lower concentration of the substance can be observed in Refinery B. Such discrepancy is a sign of different levels of recovery of alumina in the process of refining. The amount of alumina that remains in the red mud is also of significance as alumina is a highly reactive solid and thus it might affect the reactivity of the substance and its possible further use. There is a significant difference in the silicon dioxide (SiO2) concentration in the red mud samples. Refinery A has the highest concentration of SiO2 and Refinery B has the least concentration. This difference can be explained by the nature of bauxite input into the refinery and level of desilication attained during the refining process. SiO2 in high concentrations may influence the mineral phases and processing behaviors. In the red mud samples, titanium dioxide (TiO2) is found in moderate and constant amounts. Refinery C presents a higher concentration of TiO2 when compared to the other two refineries. This observation can indicate that there are more titanium bearing minerals, and this can be used to further suggest that titanium and other useful metals can be recovered by the red mud. Refinery A has the highest Na₂O concentration. The variation in Na₂O concentration in the red mud samples reflects differences in the retention of caustic soda and washing efficiencies during the refining process. The high Na₂O concentration is an important factor that contributes to the environmental behaviors of the red mud and the technical difficulties encountered during the processing. The calcium oxide (CaO) concentration in all the red mud samples is low. There is a slight increase in the CaO concentration in Refinery C. This may be indicative of additional lime additions made during the refining process. The minor oxide components such as potassium oxide (K₂O), manganese oxide (MnO), phosphorus pentoxide (P₂O₅) and magnesium oxide (MgO) are present in trace amounts. Even though these minor oxide components are present in small quantities, they can still influence the formation of mineral phases and the behaviors of the red mud. Overall, the major oxide composition of the red mud samples provides evidence of the source dependent nature. The differences in the compositions of the samples from the three refineries reflect differences in the types of bauxite feedstocks and the operating conditions of the refineries. These compositional differences provide important information regarding the technical feasibility of using the red mud for the recovery of critical minerals. Figure 1. Major oxide composition (wt%) of red mud samples from three refineries. 3.2 Trace, Critical, and Toxic Element Distribution As per the visual representation of Figure 2 and 3, scandium shows the largest enrichment of all the collected samples, especially in Refinery B; whilst Refinery A and Refinery C contain elevated levels of
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