Influence of flocculant charge on the dewaterability of flocculated clay suspensions

Abstract

The influence of charged components of cationic polyelectrolytes on the dewatering of clay-containing suspensions was investigated with a view to better predicting the efficiency of flocculating agents. In flocculation and dewatering experiments on suspensions of harbour sediment and gravel washings, flocculating agents of the polyacrylamide-co-(trimethylammoniumpropyl chloride) (PTCA) type exhibited different dewatering efficiencies depending on the degree of cationicity, τ. For harbour sediment, a dewatering index, I D, of 80 was achieved with the highly charged PTCA 3 (τ=40%) at 30% lower flocculant dose than with the weakly cationic PTCA 1 (τ=3%). However, for gravel washings PTCA 1 proved to be more effective: for comparable degrees of dewatering (I D=80) approximately 40% less flocculant was required than for PTCA 3. In shear experiments on gravel washings and model suspensions with particles of differing size (d 50=0.5 und 5.7 µm) weakly cationic PTCA 1 exhibited an increased floc stability at lower concentrations than is necessary to achieve maximum I D values. For harbour sediments and model suspensions with unimodal particle size distributions this stability did not occur until the doses used were higher than the concentrations needed to achieve maximum I D values.