Effect of sodium fluorosilicate on the flotation preparation of ultra-low ash coal: selective separation of coal and kaolinite

The comprehensive utilisation value of ultra-low ash coal has been widely recognised, and ultra-fine coal flotation as an effective method for the preparation of ultra-low-ash coal products, the efficient separation of ultra-fine coal and gangue minerals in the sorting process is crucial. In the present research work, the inhibitor sodium fluosilicate (SSF) was used for ash removal in flotation of ultrafine anthracite using sodium oleate (NaOL) as a collector. Flotation results show that the addition of SSF increased the concentrate yield from 17.68% to 29.34% and reduced the ash content from 2.10% to 1.93%. SSF acted on the kaolinite surface by both physical adsorption and chemical adsorption, and the competitive adsorption hindered NaOL adsorption on the kaolinite surface, which further enlarged the difference in hydrophobicity between the coal and kaolinite surfaces, and the difference in the contact angle between coal and kaolinite increased from 70.1° to 79.6°. And SEM-EDS verified that the addition of SSF did attenuate the adhesion of high ash fine mud gangue minerals on the surface of coal particles. XPS analysis showed that while SSF acted on the surface of kaolinite by hydrogen bonding, its hydrolysis products Si(OH)₃^- and SiO(OH)₂^2– could be chemisorbed on the new Al active sites exposed by kaolinite due to ultrafine grinding. The stable adsorption behavior of SSF enlarges the surface interface difference between ultrafine-grained coal and kaolinite, which is conducive to the preparation of ultra-low-ash refined coal products by flotation.