Coadsorption Phenomena in the Separation of Pyrite from Coal by Reserve Flotation

Abstract

The reverse flotation of pyrite from coal was studied to determine surface chemical aspects of this system. Included in the research was determination of the effect of dextrin on the adsorption of xanthate by pyrite, measurement of rest (mixed) potentials for crystal- and coal-pyrite electrodes and evaluation of the bench scale flotation response with respect to the understanding of the adsorption reactions. Although little dextrin is adsorbed by crystal pyrite in the absence of xanthate, significant adsorption of dextrin in the presence of xanthate occurs. These results further support the general hypothesis that dextrin adsorption involves a hydrophobic bonding mechanism, In this system hydrophobic bonding is thought to occur at the surface between the non-polar portion of the dextrin and the dixanthogen oil which forms on the crystal pyrite surface. On the other hand, the slightly hydrophobic character of coal pyrite (pyrite associated with locked coal) exhibits a higher adsorption potential for dextrin in the absence of xanthate. The coal impurities presumably provide hydrophobic sites for the adsorption process. As with crystal pyrite, the dextrin adsorption density is increased further when potassium amyl xanthate is added. Most importantly, the coadsorption of dextrin and xanthate does not affect the flotability of pyrite in the range of concentrations tested. From these results it was inferred that the dixanthogen either wets surface dextrin molecules or that the dixanthogen forms in surface patches, providing the necessary hydrophobic character to permit xanthate flotation of pyrite even in the presence of coadsorbed dextrin. Unlike crystal pyrite electrodes, the rest (mixed) potential of a locked coal-pyrite electrode was not a strong function of either the xanthate or dextrin concentration. In fact the rest (mixed) potential of the locked-electrode was very close to the potential of crystal pyrite in the absence of xanthate (0·42 V at pH 7). It seems that the coal portion of the locked electrode may act as the anodic half cell which determines the electrode potential. In any event, coal pyrite exhibits the necessary electrochemical characteristics for the discharge of xanthate to dixanthogen. Bench scale flotation of pyrite from a ROM Illinois No.6 coal was studied by factorially designed experiments. The separation effectiveness was evaluated as a function of xanthate addition, dextrin addition, and acidity by surface response methodology. The pyrite flotation behavior was explained to some extent in terms of the coadsorption of collector and depressant.