may have been insufficient to effectively suppress gangue flotation, and that higher concentrations should be evaluated in future tests to enhance selectivity. Among the depressants investigated, phytic acid exhibited a distinct behavior, promoting a reduction in barium content in the flotation concentrates from both flotation stages, along with a slight decrease in calcium and magnesium in the second stage. This trend suggests preferential depression of barite, leading to its enrichment in the underflow fraction, and partial depression of calcium-bearing gangue minerals, such as calcite, during the second stage. The performance of phytic acid is attributed to its strong chelating ability toward divalent metal ions, such as Ca²⁺ and Mg²⁺, resulting from its six phosphate groups linked to an inositol ring. This interaction favors its adsorption onto calcium-bearing mineral surfaces, as reported in the literature (Zhang et al., 2019), and the limited extent of gangue rejection observed in this study further indicates that higher depressant dosages should be evaluated to enhance flotation selectivity. In contrast, the use of quinic acid resulted in a reduction of barium only during the first flotation stage, while a decrease in aluminum content was also observed, indicating a more selective but stage-dependent interaction with gangue phases. No studies using this depressant in the flotation of REE were found in the literature. Sodium alginate exhibited a more pronounced depressant effect, with reductions in magnesium and barium contents in the flotation concentrates from both flotation stages, suggesting effective depression of barite and a possible interaction with magnesium-rich gangue phases, such as dolomite or other Mg-bearing carbonates. In addition, decreases in aluminum, strontium, potassium, and sodium contents were observed, particularly in the second flotation stage, indicating enhanced gangue rejection under lower solids concentration conditions. This behavior may be attributed to the physicochemical nature of sodium alginate, a linear anionic polymer rich in hydroxyl (–OH) and carboxylate (–COO⁻) functional groups, which enables strong complexation with divalent and multivalent metal ions, including Ca²⁺, Mg²⁺, Fe²⁺, and Pb²⁺, thereby promoting its adsorption onto calcium-bearing mineral surfaces and rendering them hydrophilic, as reported in the literature (Guo et al., 2024). In contrast, citric acid exhibited the least promising performance among the evaluated depressants, resulting only in reductions in magnesium, manganese, and strontium contents, while showing limited effectiveness in suppressing other gangue-associated elements. This behavior contrasts with literature reports describing citric acid as an efficient and highly selective depressant for the flotation separation of scheelite from fluorite, where its action is attributed to the selective dissolution of calcium ions from the gangue mineral surface (Dong et al., 2021). The discrepancy between the present results and previous findings may be associated with differences in operating conditions, particularly pH and depressant dosage, as optimal performance was achieved at higher pH values (≈9.5) and with citric acid dosages approximately four times greater than those applied in this study. More importantly, this divergence is likely related to fundamental mineralogical differences, since scheelite (CaWO₄) is a tungstate mineral with surface chemistry distinct from bastnäsite, parisite, synchysite, and monazite, which may limit the effectiveness of citric acid in the flotation system investigated.
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