The role of viscosity in the density fractionation of particles in a laboratory-scale Reflux Classifier

It is common practice in the coal industry to use heavy organic liquids to fractionate coal samples on the basis of density. However, concerns over worker health and the influence of these liquids on coal carbonisation properties are prompting the search for alternative water-based methods. Previous work has already shown that 0.038-0.25 mm samples can be very effectively separated using pure water in a Reflux Classifier with narrow 1.7 mm channels. Narrow channels give laminar flow with high shear rates which promotes density-based separation. Processing coarser particles requires wider channels and the laminar flow condition is lost, reducing performance. This work tested whether using viscous glycerol solutions to restore the laminar flow condition could improve the separation performance of the laboratory Reflux Classifier for larger particles. For 0.25-2.0 mm coal particles, using 50 wt.% glycerol solution in 6 mm channels, the Reflux Classifier was able to match the float-sink yield-ash curve across the entire yield range. For 2.0-16 mm coal, using 70 wt.% glycerol solution in 24 mm channels, the Reflux Classifier gave results which were at worst only 1.0 wt.% ash units off the float-sink curve. Hence the Reflux Classifier can potentially replace the float-sink method for measuring the washability of small bore core samples and producing clean coal composites.