ROLE OF ELECTRICAL DOUBLE-LAYER FORCES AND HYDROPHOBICITY IN COAL FLOTATION IN NACL SOLUTIONS

It is well-known that coal floatability can be enhanced by the addition of inorganic salts. Although considerable effort has been made in the past to investigate the role of these salts, their function in the coal flotation system is not yet clear. In this study, the floatability of a bituminous coal in NaCl solutions were evaluated using a modified Hallimond tube to delineate the role of the electrostatic interaction between bubbles and particles and the coal hydrophobicity. Zeta potential measurements showed that bubbles are negatively charged in the entire pH range tested, while coal particles are negatively charged only above pH 6.0. Also, both sodium and chloride ions were confirmed to act as indifferent ions for both coal particles and bubbles. Floatability of coal measured as a function of pH exhibited a maximum and minimum at low salt concentrations which is shown to be determined mainly by the coal hydrophobicity as measured by a new film levitation technique. At high salt concentrations (above 0.1 mol/dm3), the floatability without such maximum or minimum is controlled by the electrostatic interaction between the bubble and the particle.