The effect of flocculant solution transport and addition conditions on feedwell performance in gravity thickeners

Solid–liquid separation of tailings slurries in gravity thickeners relies on the efficient mixing of slurry and dilute polymer flocculant solutions within the feedwell. Computational fluid dynamics (CFD) can provide predictions of solids distribution, liquor velocity and shear rate within a feedwell, and with the incorporation of an adsorption model, can also assess the effectiveness of flocculant mixing. This study presents the first use of feedwell CFD to examine the effect of the flocculant inlet direction and velocity on the subsequent distribution and adsorption of flocculant. When flocculant is injected inside the feed stream, a high inlet nozzle velocity will maximise adsorption, with injection preferably vertically upward or towards the feedwell walls. For injection inside the dilution stream (vertical upflow of liquor within the feedwell), the flocculant should be directed either upwards or inwards away from the strong downward flow of the feed stream, with the velocity not critical.At flocculant inlet velocities predicted by CFD to enhance mixing and adsorption, the shear rate experienced within the injection pipe exceeds that in the feedwell, and the duration under higher shear may be greater. Pipe flow studies for several flocculants have confirmed reductions in activity at a solution concentration of 0.025 wt.%; this effect diminishes with greater dilution. Much of this lost activity is recovered on standing, indicating that the applied shear leads to a mixture of chain scission (irreversible) and entanglement (reversible). Minimising the duration of such shear effects on flocculant solution transport to the feedwell is essential, as the potential for increased flocculant demand and reduced flocculation efficiency can easily exceed any benefit from improved feedwell mixing.