Coal flotation in inorganic salt solutions

Abstract

In the present work, the salt flotation technique has been tested with 27 different inorganic electrolytes on two high ash coal samples. The performance of various salt solutions were compared in terms of the separation efficiency. At an arbitrary concentration of 0.07 moles/liter, the best results were obtained with CuSO/sub 4/ at 68 percent separation efficiency. This may be compared with the separation efficiency of 61 percent obtained by the conventional flotation technique using both a frother and a collector. Other advantages of the salt flotation process are found in its improved sulfur rejection potential and its fast flotation kinetics. The frothability of salt solutions has been investigated as a function of concentration using double-distilled water. In general, the frothability of salt solutions increases with increasing surface tension of the solutions, contrary to what one might expect. A possible explanation for this is that bubble coalescence is hindered in the presence of electrolytes. The electrophoretic mobility of a bituminous coal sample has been measured as a function of pH in various salt solutions. It has been found that at 0.07 moles/liter, salts of trivalent cations reverse the mobility of coal from negative to positive, while salts of monovalent cations merely reduce the mobility due to double-layer compression. The most intriguing results have been obtained with salts of divalent cations; the coal samples exhibit zero mobility over wide ranges of pH. At the same time, the kinetics of salt flotation is best when these divalent salts are used. Contact angle and heat of immersion measurements have shown that the hydrophobicity of coal is not increased by the presence of electrolytes. The major role of the electrolytes is perhaps to reduce the zeta-potential around the coal particles, which facilitates the bubble-particle adhesion process. 33 figures, 10 tables.