Lignosulfonate improved the flotation of the problematic coal
containing clay minerals in deionised water at a low concentration but depressed the flotation in saline water. The different
roles of lignosulfonate were explored by conducting flotation of
pure coal in the absence of clay minerals, adsorption measurements and AFM tests. It was identified that different dispersion
mechanisms of lignosulfonate applied in water of different ionic
strength, leading to various flotation results. In deionised water,
electrostatic repulsion was enhanced by a small amount of lignosulfonate between coal and clay minerals, which mitigated clay
slime coatings from coal surfaces and improved coal flotation.
With increasing the lignosulfonate concentration, more lignosulfonate was adsorbed on coal surfaces and rendered them hydrophilic resulting in the depression of coal flotation. In saline
water, a low amount of lignosulfonate could not generate enough
steric repulsion to separate clay particles from coal surfaces.
Meanwhile, the adsorbed lignosulfonate made coal surfaces more
hydrophilic, exacerbating the depression of coal flotation.
Although a large amount of lignosulfonate may produce strong
repulsion between coal and clay surfaces, the reduced coal
surface hydrophobicity depressed coal flotation even when clay
coatings were removed from coal surfaces