Paste Supplement Article by Phillip Fawell, Alton Grabsch, Ismael Mola and Claudia Castillo, CSIRO Mineral Resources, Australia

Settling tests are ubiquitous in the study of fine particle aggregation and subsequent dewatering by sedimentation. They play a valuable role in process optimisation, reagent selection and in the design of industrial unit operations. While evidence of such testing may go back centuries, arguably the earliest general study of aggregated suspension settling to inform design can be credited to Coe & Clevenger (1916), with ready quantification from visual observations and sampling making it a fertile area for research.
That a test can be easily performed does not then mean it is routinely done well, and critically, procedures optimised for one industry will not be so when applied in another. Most settling tests originate in waste water clarification, where solids are predominantly fine and of low density, their concentrations dilute, required settling rates no more than moderate, and the main performance target being the lowest possible supernatant solids concentration. Aggregation is usually through changing surface charge (coagulation), and the need to promote particle collisions at low solids generally means prolonged mixing measured in minutes (Gregory 1989). Impeller-stirred beakers (jar tests) are, therefore, the norm, with settling rates (often on transfer to a cylinder) and clarity then measured after mixing is stopped. Alternatively, graduated cylinders can be used directly; mixing then being achieved by inversions or plunger strokes. Amarjargal & Taşdemir (2023) recently contrasted both approaches in waste water studies, noting the mixing in jar tests was more conducive to fines capture, while cylinder testing gave more accurate settling rates.
It is a very different situation with mineral tailings – solids are coarser, denser and at higher concentrations, while aggregation is through polymer bridging by high molecular weight flocculants that give larger aggregates and the potential for very fast settling. Optimal aggregation times are measured in seconds instead of minutes, and with polymer-bridged aggregates broken irreversibly, performance is adversely affected by prolonged mixing. Jar tests become problematic – shear rates near the tips of impeller blades can be an order of magnitude higher than the mean value across the vessel volume (Glasgow & Kim 1986). Most aggregation takes place near the impeller in low solids coagulation, but aggregation proceeds throughout the vessel for tailings flocculation, making it difficult to define the shear regime or to prevent over-mixing. Jar tests can be relevant when more concerned with downstream behaviour, but are still used widely to compare feedwell flocculants despite the likelihood of missing key trends.
Cylinder tests are also far from ideal for research, in that the applied shear rates will still be broad and ill-defined. However, they do offer greater scope for shorter mixing durations through simple variation of the number of applied inversions or plunger strokes, and settling rates can then be measured directly without additional shear from transfer to cylinders. Settling rate results are known to depend upon the cylinder size and volume, which has been attributed to wall effects, but was clearly shown by Farrow & Swift (1996) to reflect distinct mixing regimes, obtaining identical initial settling rates when flocculating externally and filling cylinders of varying diameter. Wall effects are significant for subsequent consolidation (Lester et al. 2014), and, therefore, affect final settled volumes.
Recommendations on conducting cylinder tests for effective comparison of flocculants have been provided (Fawell et al. 2015), but there is also considerable scope to maximise the information that can be derived from such tests for modelling and design purposes. Several examples will now be given.