Shear-induced floc structure changes for enhanced dewatering of coal preparation plant tailings

The handling and disposal of industrial waste slurries containing colloidal particulate matter, with particular reference to coal and mineral processing plant tailings with a high proportion of clay materials present significant challenge. To develop effective disposal methods requires an improved understanding of floc structure that promote better water recovery and higher sediment density during settling and consolidation of flocculated tailings. In this paper, the effect of floc structure on coal tailings dewatering and the impact of shear-induced modification of floc network structure on additional sediment consolidation and densification have been examined. Four real tailings and a high molecular weight anionic polymeric flocculant were used. Flocculation tests were conducted under reproducible hydrodynamic conditions in batch mode. Flocculated aggregates have complex structures with different sizes, shapes and voidages. The structures of the flocs were characterised by fractal analysis using light scattering technique and the dewatering characteristics of the flocs were investigated in terms of settling rates and sediment consolidation. The fractal analysis by laser scattering identified optimum mixing speed and flocculant dosage required for the production of flocs with compact structure of fractal dimensions greater than 2. It was determined that hydrodynamic conditions during flocculation have a profound impact on floc settling rates and to a lesser extent sediment consolidation. Low mixing speeds produced large flocs with high settling rates, while very high mixing speed during flocculation resulted in irreversible floc degradation leading to much reduced settling rates with some additional sediment consolidation. Application of shear to pre-sedimented flocs resulted in significant additional sediment consolidation for the tailings samples and flocculants studied. This approach would allow flocculation to be optimised for fast settling with subsequent application of shear to the settled sediment to promote the removal of inter and intra-floc liquor to improve consolidation and sediment density.

► Floc structure of flocculated tailings was characterised by fractal analysis.
► Hydrodynamic condition during flocculation has significant impact on floc structure and settling rates.
► Shear-induced modification of floc structure resulted in significant additional aggregate consolidation.
► Optimum mixing speeds and flocculant dosage to produce aggregates of compact structure with fractal dimension greater than 2 were identified.