THE FUNDAMENTALS OF UNIT OPERATIONS IN CIP PLANTS - A PROGRESS REPORT

During the past fifteen years significant progress has been made on ca rbon-in-pulp (CIP) technology for the extraction of gold. While most r esearch on the mechanisms and kinetics of CIP sub-processes has focuse d on the adsorption step only, auxiliary unit operations such as eluti on, acid washing and the thermal regeneration of spent carbon have rec eived less attention. The mathematical models which have been develope d for these unit operations at the University of Stellenbosch can be u sed for design as well as process optimisation by incorporation into a n overall CIP simulation program. It is the aim of this paper to revie w some of these models, as well as recent progress on understanding th e dynamics of elution and acid washing. An equilibrium model can be us ed for the elution of gold from activated carbon in which the equilibr ium isotherm shifts as cyanide and spectator cations are removed, and the surface of the carbon is reactivated by removal of the cyanide deg radation products during an elution run. Equilibrium conditions may be assumed in an elution column wen adsorption is weak, while diffusiona l phenomena become rate controlling wen 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. T he removal of calcium carbonate precipitate from carbon during washing by hydrochloric acid appears to be controlled by both diffusion and e quilibrium considerations. The kinetics of acid washing are slower whe n the radial distribution of calcium is more even, i.e. when calcium h as been loaded over an extended period. A heat transfer model for a ro tary regeneration kiln has been used in conjunction with the kinetics of drying, pyrolysis and the stem gasification of spent carbon in orde r to predict the mass loss, and hence the adsorptivity of carbon due t o regeneration. Thermogravimetric analysis (TGA) can be used to determ ine the kinetics of drying, pyrolysis and gasification.