New insights into the interaction of sodium hexametaphosphate with coal slime and its application in the purification of kaolin

Coal slime is a solid waste generated during the coal washing process, and the high value-added clay minerals it contains are critical to reducing resource extraction and proactively adapting to changing policy requirements. In this work, sodium hexametaphosphate (SHMP) was selected to form thermodynamically stable complexes (i.e., Fe7P6O24, Fe10P6O26, Mg3P2O8, NaMgPO4, Na12Mg4P10O35, and Ca5HO13P3) with exposed Fe3+, Mg2+ and Ca2+ ion sites on the surface of coal slime. The adsorption of SHMP effectively enhanced the zeta potential of the coal slime, and the energy potential increased from 5 x 10-18 J to 17 x 10-18 J, which overcame the van der Waals force at a short distance of 1-2 nm. It promoted the formation of the solvent hydration layer and achieved stable dispersion of the slurry. Meanwhile, through the magnetic difference between minerals, the kaolin in the system was purified by combined overflow filtration superconducting magnetic separation technology. Finally, under 3T/5T tandem magnetic separation conditions, the removal rates for Fe and Ti were approximately 94.96 % and 85.86 % respectively. The purified kaolin complies with Chinese kaolin categorization requirements and shares a similar composition to commercial kaolin.