Coagulation and flocculation of laterite suspensions with low levels of aluminium chloride and polyacrylamids

Laterite particles in suspension undergo auto-flocculation as the concentration increases from 160 NTU, and therefore, coagulation and flocculation properties are affected. The critical coagulant concentration of laterite by aluminium chloride increases when the initial turbidity is less than 160 NTU, but decreases with the initial turbidity for more turbid ones. The maximum concentration is fourth of the standards for potable water. In all cases, the critical Zeta potential for coagulation equals ca. −20 mV. The Zeta potential appears to be a more relevant parameter to study coagulation than the turbidity of the supernatant. Flocculation either by non-ionic (PAM-N), cationic (PAM-C), or anionic (PAM-A) high molecular weight polyacrylamids promotes turbidity reduction of pre-coagulated laterite suspensions. This turbidity reduction is independent on the amount of polymer added when the suspension is coagulated at the CCC. In the other cases, turbidity reduction depends on polymer concentration. For suspensions of high turbidity, flocculation does not improve significantly the efficiency as compared to coagulation. At low concentration, PAM-N and PAM-A do not significantly modify the Zeta potential of the particles, enabling it to remain a relevant parameter to monitor destabilisation by combined coagulation and flocculation. Laterite particles are very sensitive to the presence of PAM-C, which induces charge reversal even at very low concentration. The critical concentration for flocculation is lower than 0.1 mg/L.