INHIBITION OF MASS TRANSFER TO POROUS ADSORBENTS BY FINE PARTICLES AND ORGANICS

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Abstract
Activated carbon and ion-exchange resins are used industrially for the extraction of metal cyanides from slurries. This paper attempts to quantify the inhibition of mass transfer to these adsorbents owing to fouling by fine particles and organics usually present in leached pulps. In this study, resins and carbon were contacted with either solutions of organics, or dilute slurries of synthetic silica or alumina in batch stirred tanks. Silver cyanide was used as an adsorbate to detect changes in the mass transfer characteristics of the adsorbents, which were ashed to determine the loading of silica or alumina. The rate of intrusion of fine particles into the adsorbent matrix was relatively fast, and was dependent on the concentration of particles in the slurry. SEM mappings of Si and Al showed that the intruded particles were distributed fairly homogeneously throughout the adsorbent particles. It was found that the presence of fine inert particles and organics had no effect on the equilibrium loading of silver cyanide on either the resin or carbon. The reduction in the rate of adsorption appeared to be a kinetic influence, and could be attributed to two effects, viz.: (1) pore blocking, and therefore retarded diffusion into the resin bead or carbon granule, and (2) temporary blinding of the resin or carbon surface. By fitting a film transport-surface diffusion model to the profiles for the uptake of silver cyanide, it was found that a decrease in the intraparticle diffusivity was associated with pore blocking.