Selective collection of fine particles by water drops

Abstract

This study was concerned with the interaction between a gaseous dispersion of fine particles travelling in the horizontal direction and discrete drops of water falling vertically through the dispersion. A simple analytical model of the particle–drop collision was developed to describe the particle recovery by the drops as a function of the water flux, covering two extremes of relative velocity between the particles and drops. The Discrete Element Method was used to validate the analytical model. Further validation of the model and insights were obtained through experimental studies. The physical process of wetting was observed to be important in influencing the tendency of particles to become engulfed by the drops of water, or to either adhere to the drops or by-pass the drops altogether. Hydrophilic particles were readily engulfed while hydrophobic particles, at best, adhered to the surface of the drop, or failed to attach. Moreover, the recovery of the hydrophilic silica particles was significantly higher than the recovery of hydrophobic coal particles, with the selectivity ratio approximately 1.5. Spherical ballotini particles were the most sensitive, with a notable increase in recovery when cleaned, and evidence of increased recovery with increasing particle size. The recovery of irregular shaped silica flour particles, however, was largely independent of the particle size. A similar result was observed for irregular coal particles, though the recoveries were all lower than relatively more hydrophilic ballotini or silica flour.