The flocculation of quartz and other suspensions with gelatine

Abstract

When mixed mineral suspensions are flocculated with gelatine, complex and varied pH-dependence effects are observed. With pure suspensions of insoluble materials of widely different character such as quartz, kaolin, and silicon carbide, a uniform pattern of results is obtained. A detailed study has been made of the flocculation of quartz with gelatine with relation to the amount of gelatine adsorbed under various conditions, and to the electrophoretic mobility of the particles. With maximum adsorption, the thickness of the gelatine layer on the surface of the particles is greater than that of a unimolecular layer. Under these conditions there is little or no flocculation. The best flocculations are obtained when one-third this amount of gelatine has been adsorbed. Under the optimum conditions for flocculation more than 99.9% of the gelatine added is adsorbed. The efficiency of flocculation, but not the degree of adsorption, is very pH-dependent. With a negatively charged suspension, the best flocculation is obtained under pH conditions where the gelatine molecules carry a small positive charge, which leads to only a small decrease in electrophoretic mobility on adsorption, so that after flocculation, particles remain separated by a distance probably of the order of 500 A. Under pH conditions where the gelatine molecules are more highly charged, so that the electrophoretic mobility is reduced to near zero on adsorption, a poorer flocculation is obtained. In the presence of excess gelatine, when there is no flocculation, the quartz particles have the same isoelectric point as the gelatine but the electrophoretic mobility is not entirely governed by the adsorbed gelatine layer. Agitation has an irreversible, adverse effect on flocculation. It is postulated that polymeric flocculating agents are not adsorbed initially in ordered monolayers and that a disordered structure in which molecular chains project from the particles is first formed. These enable polymer bridges that span particles to be formed on collision. When these links are disrupted by agitation, a slow rearrangement of the surface layer involving adsorption of free chains leads to poorer flocculation. The pH dependence of flocculation is due primarily to the influence of molecular charge on the configuration of the adsorbed layer rather than to its effect on the net charge on the particles.