Concentrated slurry formation via drawdown and incorporation of wettable solids in a mechanically agitated vessel

Abstract

This article describes the effect of vessel configurations on the drawdown and incorporation of floating solids to prepare concentrated alumina slurries in stirred tanks. The impeller speed and power draw required to incorporate all dry powder within four seconds, N JI and P JI , are used to evaluate incorporation performance. The effect of impeller type is assessed, with pitched blade impellers proving to be the most effective across the full range of solid contents considered. At higher solids content the energy demand is shown to increase dramatically, with a 100-fold increase in energy required to add 1% w/w more solid at 50% by weight compared to 1% by weight. Analysis of impeller power numbers show this coincides with a transition from constant power number to a region where power number increases linearly with decreasing Reynolds number. Contrary to studies at low solids content, the presence of baffles is shown to inhibit drawdown.

110th Anniversary: Slurryability: What Makes a Powder Hard To Incorporate into a Slurry?

“…All three measures show a significant reduction in the slurryability of the powders considered as the solid content is increased, as seen by a turning point in all curves. These turning points align with the step change in incorporation performance that occurs as the solid concentration increases and the mechanism of incorporation changes as noted previously 5,6 . Table 2 shows the slurryability measurements for each of the thirteen initial powders.…”

“…This gives a temporarily higher solid phase volume concentration, increasing the slurry apparent viscosity and making it more difficult to add additional powder until the air inside the pores has been displaced. This effect is described by Wood et al 5 .…”

“…Slurryability was measured in a 5 L cylindrical stirred vessel with the geometric configuration that was found previously to incorporate powder most effectively into high-concentration slurries. 5,6 This geometry is shown in Figure 5 Slurryability was assessed by measuring the time taken to add 50 wt % powder to a given vessel. Prior to the start of the experiment the vessel with filled with 3.86 L of water and 3.86 kg of powder was weighed out into 50 ×77 g aliquots using a KTron KT20 loss in weight powder feeder configured to deliver a constant total mass.…”

“…Slurryability was measured in a 5 L cylindrical stirred vessel with the geometric configuration that was found previously to incorporate powder most effectively into high concentration slurries 5,6 . This geometry is shown in Figure 5 Slurryability was assessed by measuring the time taken to add 50 wt% powder to a given vessel.…”

“…The consequence of this is that as the powder hits the liquid surface and begins to wet, clustered particles are more likely to form semi-wet agglomerates on the free surface. Wood et al 5 showed that formation of large agglomerates on the free surface was one of the main hindrances to increasing the solid content of a concentrated slurry. Wide size distributions will lead to lower voidages while increasing amounts of fines will lead to higher surface area; both of which contribute to an increase in pressure drop.…”