The adsorption of gold from iodide solutions

Abstract

With the increasing awareness ofpreserving the environment, the need for other lixiviants in leaching processes has escalated. The investigation for lixiviants, especially for gold, has entailed mostly the halogen group. From halogens, iodine forms the most stable gold complex. The aim ofthis study was to investigate the adsorption characteristics of gold-iodide onto a coconut shell based activated carbon. In the first part of this study, the factors influencing the adsorption kinetics and equilibrium were investigated. Mixing experiments and interruption tests indicated clearly that the principal adsorption mechanism is by first order film diffusion. When the initial gold concentration was changed, the adsorption kinetics and equilibrium decrease viith an increase in gold concentration. This is due to the shielding effect by the adsorbed gold as it is partly reduced on the carbon surface. The concentration of iodide increased the adsorption kinetics and equilibrium. The increase in concentration of iodine leads to an increase in concentration ofthe principal oxidising agent, tri-iodide, thereby decreasing the adsorption parameters as the gold gets redissolved from the carbon surface. Furthermore, iodine and trio-iodide adsorbs competitively, thereby decreasing the rate ofadsorption ofthe gold complex. This is further iterated by experiments where the carbon was pre-treated v.ith a iodine/iodide solution. In acidic solutions, none, or a minimal amount ofthe gold is red11ced on the carbon surface, adsorption is only in the complex form, while in a alkaline solution, the gold is partly adsorbed and partly reduced. In the second section, the adsorption is approached from a basic electrochemical perspective. Here, depending on the pH ofthe solution, it is shown that the potential ofthe solution dictates the species of iodine and gold-iodide complex in solution, it affects the adsorption characteristics ofthe activated carbon.