Flocculation of clay suspensions by an anionic soil conditioner

Abstract

Ca- and Na-montmorillonite and Na-kaolinite were flocculated from suspension by two anionic soluble polymers. Light absorbance at 280 nm served as degree of flocculation measure. A prerequisite for flocculation was that salt concentration in clay suspension should exceed 1.0 and 0.2 meq. l −1 for Na- and Ca-montmorillonite, respectively. Furthermore it was shown that the clay had to adsorb a minimum amount of polymer: 0.8 mg/g for a 0.735 g l −1 Na-montmorillonite suspension and 1.0 mg/g for a 0.436 g l −1 Ca-montmorillonite suspension. Saturation adsorptions of polymers on Na- and Ca-clays were estimated to be 4.6 and 7.0 mg/g. These levels of adsorption stabilized the suspension and prevented flocculation. Na-polymetaphosphate (Calgon) added to a Na-montmorillonite suspension after the polymer addition retarded flocculation whereas its addition at a level >2.7 mg/g prior to the polymer addition prevented flocculation. Increasing the pH of the Na-kaolinite suspension from 6.5 to 8.2 considerably reduced its flocculation by the polymer. The effects of both pH and Na-polymetaphosphate indicate the importance of the positively charged sites on the clay in adsorption of anionic polymers and contribute to understanding of flocculation mechanisms.