FLUID FLOW AND NATURAL DILUTION IN OPEN-TYPE THICKENER FEED WELLS

Feedwells which may be classed as open (ie. with low aspect ratio and few baffles) are commonly employed in Bayer process plants. These feedwells are prone to a natural recycle from the clarification zone. This gives rise to an automatic dilution which has been reported to be essential for flocculation in, for example, a Bayer process washer.

A computational fluid dynamics model, validated using tracer measurements on a number of full scale plants, has been used to determine the fluid flow patterns in a series of thickener feedwells. Fifteen cases of different dimensions, flow rates, and feed solids concentrations were modelled. The feed pipe was bifurcated, the twin arms entering the feedwell tangentially. The flow patterns were complex, being governed by the feed flow rate and the density difference between the feed stream and the material already in the feedwell. The flow between the feedwell and the main body of the thickener was strongly coupled, and the concentration of suspended solids was far from uniform.

The data are presented in terms of a recycle ratio of volumes flowing in and out of the feedwell, a solids dilution ratio based on mean fluxes in and out, and contoured sections of the concentration of suspended solids. This type of information is essential for an informed consideration of thickener and feedwell design.