KINETICS OF COAGULATION WITH HYDROLYZED AL (3) - RATE-DETERMINING STEP

The kinetics of agglomeration of silica dispersions coagulated with hydrolyzed AI (III) have been studied with respect both to physical parameters, such as diameter and concentration of dispersed colloids and extent of agitation of the medium, and to chemical parameters of the solution, such as pH and AI(III) concentration relative to surface concentration of the dispersed phase. The rate of colloid destabilization, i.e., the rate of surface coverage of the colloid with adsorbed aluminum hydroxo complexes, was evaluated on the basis of results from adsorption studies. The rate of collision between colloids was determined for Ludox colloids (15 mμ) from light adsorbance measurements and for Min-U-Sil colloids (1.1μ) from microscopic counts. A comparison of the rates of destabilization and particle collision shows that the latter reaction is rate determining in the coagulation of colloidal silica with hydrolyzed AI(III). The factor of collision efficience reflecting the relative colloid stability is found as the ratio of actual coagulation rates and theoretical collision rates at various concentrations of coagulant. The observed relationship between colloid stability and coagulant concentration confirms the assumed model of particle destabilization and restabilization: specific adsorption of isopolycations causes a decrease of the surface potential and, at sufficiently high coagulant concentrations, reversal of sign of the surface potential. The over-all coagulation rate is determined by physical parameters characterizing the collision frequency and by chemical solution parameters describing the factor of collision efficiency. © 1968.