Multiscale Mechanisms of Selective Adsorption of Inorganic and Organic Depressants in Coal-Kaolinite Flotation

Clay-mineral interference severely limits the flotation of high-ash fine coal, making selective depression essential for improving the ash-recovery balance. Sodium silicate and starch were compared under identical direct-flotation conditions using an original kaolinite-containing high-ash fine-coal sample. Batch flotation, zeta-potential measurements, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations were combined to investigate their different interfacial effects. Flotation tests identified 2000 g/t as the dosage providing the best overall separation performance for both reagents. Compared with starch, sodium silicate produced lower clean-coal ash (10.84% vs 11.54%), higher combustible recovery (87.60% vs 84.80%), and a higher flotation perfection index (62.99% vs 59.23%), whereas excessive starch addition caused a marked recovery loss. Zeta- potential measurements showed that sodium silicate caused a pronounced negative shift for kaolinite, whereas starch produced a larger negative shift for coal. XPS further showed that starch markedly increased oxygen-containing carbon species, particularly C–O, on the coal surface, while sodium silicate caused comparatively limited modification. DFT calculations indicated that the starch model fragment (C6H12O6) formed multipoint hydrogen bonds with oxygen-containing coal functional groups. In contrast, the representative silicate species, Si(OH)4, interacted preferentially with kaolinite, particularly the hydroxylated (001) surface, while showing weaker interactions with the siloxane-terminated (001̅) surface. These results link molecular interactions with interfacial-property changes and flotation performance, providing mechanistic guidance for depressant selection and dosage optimization in high-ash fine-coal flotation.