The fundamentals of unit operations in CIP plants: A progress report
During the past fifteen years significant progress has been made on carbon-in-pulp (CIP) technology for the extraction of gold. While most research on the mechanisms and kinetics of CIP sub-processes has focused on the adsorption step only, auxiliary unit operations such as elution, acid washing and the thermal regeneration of spent carbon have received less attention. The mathematical models which have been developed for these unit operations at the University of Stellenbosch can be used for design as well as process optimisation by incorporation into an overall CIP simulation program. It is the aim of this paper to review some of these models, as well as recent progress on understanding the dynamics of elution and acid washing. An equilibrium model can be used for the elution of gold activated carbon in which the equilibrium isotherm shifts as cyanide and spectator cations are removed, and the surface of the carbon is reactivated by removal of the cyanide degradation products during an elution run. Equilibrium conditions may be assumed in an elution column when adsorption is weak, while diffusional phenomena become rate controlling when desorption is weak towards the end of an elution cycle. An intraparticle diffusion model for elution may approach the equilibrium model when the diffusivities are high. The removal of calcium carbonate precipitate from carbon during washing by hydrochloric acid appears to be controlled by both diffusion and equilibrium considerations. The kinetics of acid washing are slower when the radial distribution of calcium is more even, i.e. when calcium has been loaded over an extended period. A heat transfer model for a rotary regeneration kiln has been used in conjunction with the kinetics of drying, pyrolysis and the steam gasification of spent carbon in order to predict the mass loss, and hence the adsorptivity of carbon due to regeneration. Thermogravimetric analysis (TGA) can be used to determine the kinetics of drying, pyrolysis and gasification. © 1994.