Short-time tracer dispersion in a two-dimensional rising froth

Experimental results on the dispersion of a fluorescein tracer in a rising froth of water/glycerol mixture with SDS as the frother are described. These experiments illuminate the physical phenomena that are important to the application of washwater in flotation processes. Two-dimensional numerical solutions of foam drainage in rising systems are also presented at values of background liquid hold-up relevant to the flotation process. It is shown that, at short times, the dominant mechanism for tracer dispersion is due to liquid drainage in the froth rather than due to packed-bed type behaviour as liquid percolates through a network of Plateau borders in between the bubbles. Two foam drainage models, channel-dominated drainage and node-dominated drainage, have been previously described in the literature. It is shown that neither can adequately predict the phenomena of tracer dispersion in a rising froth. The reality appears to lie somewhere in between the two extremes. However, it has been observed that channel-dominated behaviour is approached as liquid disperses and local volumetric liquid hold-up decreases. Finally experimental results are presented for a tracer of very concentrated fluorescein solution that show dispersion behaviour quite unlike that described by the two established foam drainage models.