Using focused beam reflectance measurement (FBRM) to monitor aggregate structures formed in flocculated clay suspensions

Abstract

The shear sensitivity of fragile, low-density aggregates formed in particle suspensions by polymer-bridging flocculation makes characterisation methods needing sampling or dilution problematic. Focused beam reflectance measurement (FBRM) probes can readily monitor such structures in real-time, but interpretation of resulting chord length data can be superficial, often missing detail within the distributions. However, the potential for deeper insights can’t be realised if flocculating with poor control of applied shear conditions (intensity and duration). In this study, continuous flocculation was achieved in a Couette mixing device with a vertical flow-through arrangement, acquiring operator-independent settling rate and FBRM chord data. The performance of a series of anionic flocculants (same charge density but different molecular weights) was investigated when applied to kaolin suspensions, with the introduction of 10 or 20% bentonite used to induce ‘bad’ flocculation responses. Size-density relationships in selected samples were determined separately by image analysis of free-settling aggregates.Following well controlled flocculation, distinct relationships between settling rate and mean volume-weighted chord length were attributed to variations in both substrate mineralogy and applied mixing intensity impacting on aggregate density. No such distinctions could be drawn from variations in flocculant molecular weight. Unweighted chord length distributions offered sensitivity to inefficiencies in fines capture and aggregate breakage under higher shear, which were both impacted by flocculant molecular weight. The unweighted chord counts <10 µm for the unflocculated clay slurries better anticipated dosage demand than conventional (volume-based) particle sizing, showing the potential of FBRM feed monitoring for flocculation control.