Influence of flocculant charge on the dewaterability of flocculated clay suspensions

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, F. For harbour sediment, a dewatering index, I_D, of 80 was achieved with the highly charged PTCA 3 (F=40%) at 30% lower flocculant dose than with the weakly cationic PTCA 1 (F=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 (d₅₀=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.