Effect of Shear on Particulate Suspension Dewatering

Characterization of Multiple Hindered Settling Regimes in Aggregated Mineral Suspensions

“…The scale of the challenge is further complicated by the fact that the size, shape and effective density of these aggregates in many industrial effluents are often poorly characterised 16 and can be highly dependent on the chemistry of the continuous phase 10 and on the shear environment of the suspension [17][18][19] . There is also evidence that larger aggregates are more fractal and less dense than smaller aggregates, due to the incorporation of large ratios of intra-aggregate fluid 20,21 , with the largest aggregates (>100 µm) often only marginally denser than the continuous phase [21][22][23] .…”

Aggregate densification and batch settling

“…Such discrepancies have been attributed to various shear and compression induced effects, whereby the flocculated aggregate structure is modified [10][11][12]. Laboratory experiments [11] have demonstrated that the shear rates required to induce improvement in the rate of dewatering are mild (<6 s −1 ). The steady-state thickener model described by Usher and Scales [8] uses functional forms of the compressive yield stress and hindered settling function as inputs.…”

“…These shear effects are small or absent in batch settling tests and as such, the shear conditions are not representative of those found in the scenarios which are to be modelled. Experimental studies [11,13,14] have demonstrated the effect of shear on the hindered settling function and the compressibility of a flocculated suspension, from which it was postulated that increased dewatering occurs due to the densification of aggregates settling under shear. There are various ways in which shear can be imparted.…”

“…However, in reality this may not be the case as shear rates increase. For example, it has been observed that an optimum shear rate exists, beyond which aggregate breakage is detrimental to dewatering [11].…”

Mathematical modelling of batch sedimentation subject to slow aggregate densification

“…the solids in the aggregate bind together more tightly by expelling liquid (Farrow et al, 2000). Shear can therefore have a very significant effect on suspension dewatering (Gladman, 2004;Gladman et al, 2005Gladman et al, , 2010.…”