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 120
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