Using turbulent pipe flow to study the factors affecting polymer-bridging flocculation of mineral systems

Batch flocculation tests in cylinders (jars) or stirred vessels suffer from poor control over the reaction time and involve broad shear rate distributions. The use of a linear pipe reactor provides continuous turbulent mixing of flocculant and slurry at well-defined mean shear rates. Combined with in-line, real-time monitoring of the aggregation state with a focused beam reflectance measurement (FBRM) probe, reaction times can be determined to within a fraction of a second. The resultant reaction profiles of aggregate growth and breakage obtained while varying different conditions (dosage, solids concentration, shear rate, split dosing) provide new insights into flocculation kinetics that are more relevant to mineral processing applications. In this study, the reaction profiles are examined in terms of the effects of flocculant adsorption, applied shear and solids dilution on the aggregate structures formed. Also presented is the first definitive evidence of “post-aggregation”, where surface adsorbed flocculant remains active at short reaction times (up to the peak in aggregate size in the measured profiles), allowing additional aggregate growth under subsequent reduced shear. At reaction times beyond the peak, little or no additional growth is observed. This behaviour explains the observed discrepancy between size and settling rate reaction profiles, and highlights the importance of optimising hydrodynamics in industrial flocculation applications to maximise settling flux.