Enhancement of flotation response of fine low-rank coal using positively charged microbubbles

Abstract

This study investigates the possibility of using positively charged (PC) microbubbles in pulp conditioning to improve the flotation response of fine low-rank coal. The surface properties of the coal are characterized by means of a zeta potential analyzer, scanning electron microscopy, and X-ray photoelectron spectroscopy. Batch flotation tests are conducted with the use of PC microbubbles, cetyltrimethylammonium bromide (CTAB) solution, and tap water in pulp conditioning. The roles of PC microbubbles in the coal flotation are illustrated by combining observations of particle–particle and particle–bubble interactions and extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory calculations. The results indicate that fine low-rank coal is negatively charged and contains a large amount of oxygen. In addition, the rough structure and the presence of numerous impurity mineral particles on the surface of low-rank coal afford adsorption sites for water molecules. These features of the coal render traditional collectors ineffective, whereas the presence of PC microbubbles significantly enhances flotation recovery. However, increasing CTAB concentration cannot continuously improve flotation performance with the use of PC microbubbles, possibly because the excess CTAB molecules can form bilayer or micelle adsorption structures on the coal surface, particularly on the mineral surface. The presence of PC microbubbles promotes particle–particle agglomeration and particle–bubble attachment by providing an additional attractive electrical double-layer force and enhancing hydrophobic interactions. The extended DLVO theory calculations agree well with the experimental results.