GOLD ADSORPTION ON ACTIVATED CARBON AND THE EFFECT OF SUSPENDED SOLIDS AND DISSOLVED SILICON DIOXIDE

Abstract

The use of gold adsorption by activated carbon has recently become widespread on South African gold mines. This has caused a great deal of interest in the kinetics of gold adsorption onto activated carbon. However, almost all previous work has been done using clear solutions. In practice the gold adsorption occurs from a slurry containing fine ground rock particles and possibly a number of other metals in solution as well as organics which may poison the carbon. As the next step in the understanding of the gold adsorption process, this work assesses the impact of a fine solid suspension on the gold adsorption kinetics. This is found to significantly reduce the adsorption rate of gold cyanide (Au(CN)2¯) by G210 activated carbon in completely mixed batch adsorbers. The relative effects of solid particle size and mixing intensity on the adsorption rate are investigated. Batch adsorption rate experiments are performed with clear solutions, and with solutions containing 20% (m/m) fine suspended solid material, agitated at impeller speeds of 200 and 300 r.p.m. with an initial gold cyanide concentration of = 10 p.p.m. Equilibrium isotherms for gold cyanide, gold cyanide/glass particles and dissolved silicon on G210 activated carbon were determined. The batch adsorption kinetics of gold cyanide from clear solutions were successfully modelled using the Homogeneous Surface Diffusion Model of Crittenden & Weber (1978 Crittenden , J.C. , and Weber , W.J. , Jr. , Predictive Model for Design of Fixed-Bed Adsorbers parameter estimation and model development , J. Envir, Engng. Div. Am. Sac. Civ. Engnrs. , 104 ( EE2 ), 185 ( 1978 ). [Google Scholar]). The adsorption of gold cyanide from clear solutions was found to be characterized by intra-particle diffusion rate control, limited by a finite film mass transfer rate at low gold loadings. When fine suspended solids are present it is shown that blinding of the carbon macropores is the likely cause of the reduced adsorption rate. Although the solids used were artificial in the sense that these would not be present in this form in an industrial operation, it was felt that using solids from an operating mine would introduce complications in addition to the specific effects this paper addresses.