Dispersion or inhibition? A new insight into the mechanism of sodium hexametaphosphate on the improvement of flotation of clay-containing fine coal slime

Abstract

Sodium hexametaphosphate (SHMP) is a commonly used reagent to alleviate unwanted effect of gangue slime recovery (clay and other fine-grained minerals) during froth flotation. However, there is still controversy over whether its mechanism is inhibitory effect (depression) or dispersion effect. In this study, the effects and underlying mechanisms of SHMP-enhanced flotation separation of fine coal from kaolin were systematically examined using Zeta potential, X-ray photoelectron spectroscopy, atomic force microscope, scanning electron microscopy, coverage angle, induction time, as well as sedimentation and flotation experiments. SHMP molecules were adsorbed through chemical bonds formed between HPO₄^2-/H₂PO₄^- groups and the kaolin surface. This adsorption increased the surface charge of kaolin, promoting a shift in the interaction between kaolin and coal particles from weak attraction to strong repulsion, thereby preventing the coating of kaolin particles on the coal surface. Consequently, more hydrophobic regions were exposed on coal particles. The increased hydrophobicity of coal particles enhanced the adhesion force between coal particles and bubbles, shortened the induction time, and increased the coverage angle. These changes demonstrated that SHMP improved the adhesion speed, strength, and probability between coal particles and bubbles, thereby enhancing the flotation separation efficiency between coal particles and kaolin. This work demonstrated that the promoting mechanism of SHMP on clay-containing fine coal flotation should be its dispersion effect rather than inhibition effect.