The effect of cations on the activity of anionic polyacrylamide flocculant solutions

The conformation of acrylamide/acrylate copolymers' aqueous solutions is known to be affected by the presence of cations to varying degrees. Such effects are expected to be reflected in the flocculation activity of the polymer solutions, but the published literature in this area is far from convincing, particularly on the influence of divalent cations. Part of the problem was believed to stem from the inevitable poor control of the applied shear conditions (both intensity and duration) in conventional batch cylinder or jar tests. To address this, continuous flocculation was achieved in a Couette mixing device that allowed operator-independent settling rate measurements to be made, with much greater control over the residence time under shear and its intensity. A standard calcite (calcium carbonate) slurry was then used as the substrate for comparison between flocculants of increasing anionic character in the presence of selected cations at various concentrations. Rather than shifting the dosage response curve, the divalent cations Ca2+ and Mg2+ were found to limit the settling rates that could be achieved at higher dosages, but this effect could be masked under stronger applied shear conditions that favour some degree of aggregate breakage and also limit settling rates. Such effects increased with the flocculant's anionic character and the cation concentration. In contrast to the conventional anionic products, a flocculant that contained an alternative functional chemistry (BASF Rheomax® 1050) was found to be far less affected by the presence of these cations. Low levels of iron (either as Fe2+ or Fe3+) were sufficient to deactivate dilute flocculant solutions, but had reduced effects when present in stock solutions. The mechanisms by which the different cations affect flocculant solutions are discussed, together with the practical implications for flocculant make-up and application within gravity thickening as practiced in mineral processing.

► Multivalent cations affect flocculant conformation, but not always activity.
► This may be due to poor control of mixing in conventional batch cylinder/jar tests.
► Couette flocculation provided greater control of residence time and mixing.
► Divalent cations limit the aggregate size attained, an effect masked at high shear.
► Implications for flocculant make-up and dosing in gravity thickening are discussed.