The impact of achieving a higher aggregate density on polymer-bridging flocculation

• Being near the optimal solid fraction is crucial to flocculation in thickeners.
• Calcite flocculation kinetics was measured in turbulent pipe flow with FBRM detection.
• Two flocculants were studied, with one giving a higher optimal solid fraction.
• Modelling of the results supports this being due to a higher aggregate density.
• Benefit greatest for applications requiring higher dosages and mass throughputs

The polymer-bridging flocculation processes that take place in hydrometallurgical thickening are very much affected by the solid concentration at the point of flocculant dosing, with the optimum concentrations maximising settling flux often well below that of the thickener feed. This is a consequence of the open, low density aggregate structures formed by flocculation. Achieving adequate solids dilution in full-scale applications can be problematic, and a flocculant that can produce a denser aggregate and thereby reduce the extent of solids dilution required may offer practical advantages. There is evidence that Rheomax® DR 1050 (BASF) has such potential, although detailed quantification of this has not been previously published.The kinetics of flocculation of fine calcite slurries in turbulent pipe flow by Rheomax DR 1050 was contrasted against a conventional acrylamide/acrylate copolymer over a range of solid concentrations and dosages. Focused beam reflectance measurement (FBRM) was used to provide in-line monitoring of aggregate size as a function of reaction time, with batch sampling also producing corresponding settling rates. These reaction profiles confirmed distinct responses to solid concentration for the two flocculants; the conventional flocculant was superior in the range 20–40 kg m− 3, while the optimum for Rheomax DR 1050 was near 60 kg m− 3. Fitting of a population balance model to this kinetic data indicated that this behaviour primarily reflected a higher fractal dimension from flocculation with Rheomax DR 1050. The experimental results and model predictions are used to explore the practical scenarios under which Rheomax DR 1050 would be expected to perform best, showing that even at high solids, the benefit may only be realised in applications requiring higher dosages and mass throughputs. The practical limitations of the experimental approach used here to measure flocculation kinetics are also discussed.