THE DISTRIBUTION OF GOLD IN ACTIVATED CARBON DURING ADSORPTION FROM CYANIDE SOLUTIONS

The kinetics of adsorption of gold cyanide on activated carbon have been extensively investigated recently and a number of kinetic models have been proposed. In these studies it was necessary to make assumptions on the important rate step(s) occurring in the carbon particle. The validity of these models were evaluated by measuring the bulk concentrations of gold in solution and on the carbon. In this study the distribution of gold in the carbon particle during adsorption was investigated. A scanning electron microscope with a dispersed X-ray analysis system was used to measure the gold concentration in activated carbon particles. This made the direct investigation of equilibrium and kinetic phenomena in individual particles possible. During adsorption, the gold cyanide penetrated the carbon with a reasonably sharp front moving into the particle with increasing time. After about 2 d of adsorption, a uniform distribution of gold was found. These findings correspond to mixed-rate control, by film mass transfer in the solution and homogeneous solid diffusion in the carbon particle, and it was modelled as such. The equilibrium was modelled by a Freundlich isotherm. Individual carbon particles exhibited fairly uniform intraparticle mass transfer behaviour throughout their volume but a significant variation existed between the kinetic behaviour of different carbon particles under identical experimental conditions. The intraparticle mass transfer characteristics of each activated carbon are important and determine its rate-limiting mechanism. A distribution of intraparticle diffusivities would, therefore, exist in carbon-in-pulp circuits.