The structure and strength of depletion force induced particle aggregates

Aggregating fine particulate matter is common practice in many industrial s olid-liquid separation processes. Data obtained in this work on dilute aque ous dispersions of model colloidal polystyrene latex spheres indicate that depletion flocculation, which uses non-adsorbing polymer, can yield very co mpact aggregates. Flocculation of the negatively charged latex particles wa s induced by the addition of a poly(acrylic acid) at pK 10. The structural compactness of the latex flocs formed in the dilute dispersions was charact erised using small-angle static light scattering in terms of mass fractal d imensions. Rheological measurements on the concentrated latex dispersions i n the presence of the non-adsorbing polyacid showed Bingham yield stress be haviour. Both the compactness and strength of the latex flocs were found to be significantly dependent upon the level of the polyacid, as well as the concentration of the initial particles. In particular, as the level of the polyacid was raised the floc compactness decreased, whereas its strength in creased. They were both seen to level off at high polymer concentrations. A tomic force microscopy measurements were made at varying concentrations of the polyacid to provide a qualitative explanation of the observed floc stru ctural behaviour of the dilute dispersions. By combining the fractal dimens ion and the Bingham yield stress we were also able to estimate the energy r equired to separate the Aocs into single units in the concentrated dispersi ons. It was concluded that the interparticle interaction energy is the key to understanding the dependence of both the floc structure and strength on the polymer concentration. (C) 2000 Elsevier Science B.V. All rights reserv ed.