The effect of clay slimes on coal flotation, part I: The nature of the clay

The role of slimes in coal flotation has been the subject of controversy in the literature, and the present study is designed to determine the effect of the nature of the clay on coal flotation. Kaolinite and illite, which do not significantly depress coal flotation, contaminate the floated clean coal largely by carry-over with the froth, though electrostatic attachment to the coal contributes in a lesser way to the ash content of the froth. Bentonite greatly depresses all but the most hydrophobic of coals by armour-coating of the bubbles, preventing coarse particle attachment, and increasing slime coatings, all because of its high surface area, charged sites, and ion-exchange-capacity. Common clay dispersants were found to depress coal flotation. Fuel oil improves coal recovery in the presence of clay slimes, but the ash and pyrite content of the floated coal also increases.