Recovery of gold from thiosulfate solutions and pulps with anion-exchange resins

Glen Peter O'Malley

With growing environmental and occupational safety concerns over the use of cyanide in gold processing, more acceptable alternatives are receiving increased interest. The most promising of the possible alternatives is thiosulfate. However, as activated carbon is not an effective substrate for the adsorption of the gold thiosulfate complex, the thiosulfate process lacks a proven in-pulp method for recovering dissolved gold. Anion exchange resins offer a possible route for in-pulp recovery. This thesis describes work aimed at evaluating the effectiveness of commercially available anion exchange resins for the recovery of gold from thiosulfate leach liquors and pulps. It was found that Strong-base resins are superior at accommodating the gold thiosulfate complex compared to Weak-base resins, which means Strong-base resins have a greater capacity to compete with other anions in leach solutions. Strong-base resins were therefore the preferred choice of resin for recovery of gold from thiosulfate leach solutions and pulps. Work with a selected commercial Strong-base resin showed that competing polythionates (particularly tri- and tetrathionate) lower the maximum possible loading of gold but that gold is selectively recovered over other base-metal anions in typical leach solutions. From kinetic experiments, it was found that competing polythionates did not affect the initial rate of loading of gold but displaced the loaded gold at long times. Thus it would be important to minimise the contact time of the resin with the pulp. Equilibrium loading isotherms of gold in the presence of competing anions could be analysed by treating the ion exchange reaction as a simple chemical reaction. However, a stoichiometry and equilibrium quotient which does not follow that normally used for anion exchange, was required to describe the experimental data. A single value for the equilibrium constant also cannot be used to describe the data over the range of concentrations for a given competing anion. The order of selectivity of the anions for the anion exchange resin could be explained by the difference in structure and the charge of each anion. The rate of loading of gold is controlled by mass transport in the aqueous phase in the presence of weakly competing anions such as sulfate and thiosulfate. An attempt was made to describe the more complex loading curves obtained in the presence of stronger competing anions such as sulfite, trithionate and tetrathionate in which it was found that the loading of gold increased to a maximum before declining to a lower equilibrium value. The difference in the rate of loading between the macroporous and gel anion exchange resins was explained by the difference in the location of their functional groups. Operation of a small-scale resin-in-pulp plant showed that gold could be recovered from a leach pulp to yield loadings of gold of up to 6000 mg L-1 and loadings of copper below 100 mg L-1. Under ideal conditions, the gold concentration in the barren pulp could contain less than 0.01 mg L-1. Throughout the trial it was shown that loaded copper would be displaced by gold which would result in the loading of copper falling from 2000 mg L-1 in the last stage to lower than 100 mg L-1 on the resin in the first stage. It was observed that some of the dissolved gold precipitated or adsorbed on the solids during leaching. Some of this adsorbed gold was found to be recovered by the anion exchange resins that would have reported to the tails if a solid/liquid separation method was employed. Gold was efficiently eluted with a nitrate solution and the two-step elution process using aerated ammonia followed by nitrate effectively stripped all the copper and gold from the resin. This process was found not to materially affect the equilibrium gold concentration on the resin after eight cycles, thus allowing the resin to be recycled without the need for regeneration. Electrochemical studies showed that the gold thiosulfate complex was reduced on stainless steel from a nitrate solution. Conventional electrowinning could therefore be used to recover the gold from the eluant.

Recovery of gold from thiosulfate solutions and pulps with anion-exchange resins

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