Application of depletion flocculation for prevention of formation of dilatant sediments

Abstract

The flocculation of a pesticidal (ethirimol) suspension concentrate (stabilised using a graft copolymer) obtained by addition of free (non-adsorbing) polymer (hydroxyethylcellulose) was investigated. Three molecular weight polymers, namely Natrosol 250 LR ( M w=70 000), Natrosol 250 JR ( M w=124 000) and Natrosol 250 GR ( M w=223 000) were investigated. The volume fraction of the suspension was also varied (from 0.1 to 0.6). Flocculation was studied using three rheological techniques, namely steady state shear stress (τ) -shear rate (⋗g), shear wave and oscillatory measurements. This allowed one to obtain the Bingham yield stress τ β, Casson yield stress τ c; shear modulus G∞, complex modulus G*, storage modulus, G and loss modulus G″ as a function of φ p (the free polymer volume fraction). All results showed a rapid increase in rheological parameters above a critical φ p value, to be denoted φ + p. The latter depended on the polymer molecular weight and suspension volume fraction, φ s. For example, at δ s=0.4, φ p was found to be 0.0075; 0.0024 and 0.0018 for M w=70 000, 124 000 and 223 000 respectively. Moreover, for any given M w, φ p was found to decrease with increase in φ s as predicted theoretically. Sediment volume experiments showed weak flocculation of the suspension above a critical φ p value, resulting in the formation of a “loose” sediment with a clear supernatant liquid. With increase in φ s, the relative sediment volume increases, reaching values approaching 100% when φ s was increased above 0.3 and at high φ p value, such sediments can be easily redispersed and hence the phenomenon of depletion flocculation can be applied for prevention of formation of dilatant sediments on separation of the suspension concentrates. Optical photomicrographs showed a gradual change in the floc structure, with increase in φ p (below φ + p) but above ø + p there was a tendency of aggregation of the primary flocs, forming an interconnected structure which fills the whole volume. This is consistent with the rheological and sediment volume experiments which show a rapid increase with increase in φ p above φ + p.