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The floatability of coal and other inherently hydrophobic solids in relation to the surface tension of aqueous methanol solutions Hornsby, David Theodore Burt

Abstract

The relative floatability of coal and other inherently hydrophobic solids in aqueous solutions of short-chain n-alcohols was studied. An analysis of wettability data from the literature for such systems indicated that, in general, a linear relationship existed between the solution surface tension, Y1v» and the adhesion tension, Y1v COS Ɵ (where Ɵ is the contact angle). Except for very non-polar solids, the critical surface tension of wetting, Yc (Ɵ= 0° intercept), determined with the alcohol solutions, was relatively constant and independent of the type of solid. The parameter which reflected the relative wettability of a solid was the slope of its Y1v cos Ɵ vs. Y1v line. Since wettability and floatability are not necessarily synonymous, a concept of critical surface tension of floatability, Ycf, was developed to characterize the threshold floatability condition where successful bubble-particle attachment was controlled by either adhesion or aggregate stability. In both cases it was concluded that Ycf would depend on the surface and the physical properties of a particle for a given set of flotation conditions, and that Ycf > Yc . Under certain circumstances it also appeared that selective flotation separation between particles of two inherently hydrophobic solids, A and B, would be feasible in an aqueous alcohol solution if Ycf A < Y1v ≤ Ycf B even when the two solids had the same Yc value. Small-scale flotation tests in aqueous methanol solutions were performed on narrow size fractions of several inherently hydrophobic solids. In most cases the wt.% floated decreased from a maximum to a minimum over a discrete range of Y1v, implying that the particles in a sample had a range of Ycf values. The relative floatability of a sample was given by the position of its Ycf band along the Y1v axis. For the same size fraction, floatability decreased in the order: polytetrafluoroethylene (PTFE), sulphur, medium volatile bituminous coal, molybdenite. Almost complete selective separation by flotation was achieved between samples whose Ycf bands did not overlap. Because coal is a very heterogeneous material, it was expected that a sample of fine coal particles would display a wide range of wettability and floatability characteristics, with a corresponding broad range of Ycf values. This was verified in aqueous methanol solutions for narrow particle size fractions of several high rank coal samples from the East Kootenay coalfield, British Columbia. Incremental ash content/floatability relationships were developed which could be roughly divided into two main regions'. In one, the coal particle ash content was low and independent of floatability, while in the other they were inversely related. The effects of physical and compositional factors other than ash content on the distribution of floatability within a coal sample have been discussed. For a readily floatable coal sample the yield-ash content relationships for the flotation products were independent of the flotation time and conditioning time, and were considered to approximate the ideal flotation wash-ability characteristics of the sample. Flotation washability was found to be linearly related to feed ash content. The shape and position of the incremental ash content/floatability curves for different coal samples allowed the relative wettabilities of their organic matter to be assessed in terms of degree of oxidation, rank and petro-graphic composition. Coal sample floatability in aqueous methanol solutions was found to be sensitive to small differences in surface properties which were not reflected by bulk compositional parameters.

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