Sedimentation and consolidation of different density aggregates formed by polymer-bridging flocculation

Abstract

There are numerous models of sedimentation in fine particle suspensions, derived from or validated with physical measurements. Such models could be applied to optimise and control gravity thickeners used for solid-liquid separation in mineral processing. However, these applications rely upon particle bridging by high molecular weight polymers to form large, low density and fragile aggregates. The evolution and refinement of sedimentation modelling for flocculated systems is restricted by a lack of meaningful control of flocculation conditions and inadequate detail in the experimental sedimentation data used for validation.To address this problem, an experimental system was built to give high fidelity sedimentation data, its use demonstrated for flocculated calcium carbonate suspensions. Turbulent pipe flow flocculation offers tight control of dosage, mean shear rate and reaction time, with aggregate size monitored in-line. The pipe discharges into the base of a wide (190 mm ID) transparent column for which the contents can be isolated, with mudlines then determined from image capture and bed profiles by γ-attenuation. The use of wide columns minimised wall effects that can limit consolidation, while simultaneous efforts to maximise measurement reproducibility and sensitivity led to a greater ability to distinguish subtle impacts from variations in flocculation. Duplicate columns with detachable lower sections enable direct vane yield stress measurement at known bed depths and thereby plots of yield stress vs. solids fraction.Modelling of such data will be described in subsequent publications, with the present study focusing on key experimental requirements and limitations, the form of data outputs and insights into flocculation impacts. For the latter, off-line determination of individual aggregate settling rate vs. size after bed sampling confirmed that higher solids volume fractions low within beds occurred through removal of both inter- and intra-aggregate liquor, i.e. there is a contribution from aggregate densification. Bed profiles and yield stress responses also indicate flocculant functional chemistry can alter how aggregation limits the solids volume fractions attained.