The development of resin sorbents selective for gold in ammoniacal thiosulfate leach liquors

A set of 48 novel ion-exchange (1EX) resins bearing a range of amine and quaternary ammonium moieties were prepared on a polystyrene-divinylbenzene substrate. These were tested in Bottle-Roll (B/R) tests using a simulated ammoniacal thiosulfate leach solution. The relative gold sorption capacities of these resins were compared and assessed in terms of common structural features, with a view to developing a gold-selective resin for use with thiosulfate leachates. Elution of the gold-loaded resins was also examined, and a shortlist of promising structural features was derived from the combined results. A common reaction scheme was employed for most preparations, via treatment of the exhaustively chloromethylated resin substrate with excess amine in dimethylformamide. The subsequent functionalised resins were characterised using infra-red spectroscopy, solid state 13C-NMR spectroscopy, ion-exchange testing and elemental analysis. The weak-base resins prepared in this manner were then exhaustively allcylated with either benzyl chloride or iodomethane, to provide a range of structurally related moieties. These included alkylated derivatives of ethanolamine, diethanolamine, piperidine, morpholine, piperazine, imidazole, and tetramethy1-1,2-diaminoethane. The resin moieties were examined by computational techniques to determine approximate pKavalues, and by 13C-NMR spectroscopy. Minimum-energy structural conformations were obtained using molecular modelling (MM+ parameters). The gold and copper thiosulfate complexes, along with the thiosulfate, trithionate and tetrathionate anions were modelled using ab-initio density functional theory (DFT). The performance of the resins was compared in B/R tests using an artificial leach liquor (pH ‚ÄövÑvÆ10.2) containing gold (20 ppm), copper (200 ppm), ammonia (1.3 M) and thiosulfate (0.1 M). Gold sorption by the resin was followed by periodic sampling of the liquor and AAS analysis, over a 24 hour period. Relative gold affinity for each resin was assessed using the ratio of maximum gold sorption to LEX capacity (ie. [Au]R/Cm), with values ranging up to 16.2 mmol Au per mol of EEX sites (resin BIM1; N'-benzylimidazole). Comparison between the various methyl and benzyl- derivatives showed that gold loading was relatively insensitive to the steric bulk of the functional group. However, gold affinity was apparently enhanced by 2-hydroxyethyl and imidazolium moieties. A set of promising moieties (piperidineethanol, quinuclidine, TMEDA, and methyl derivatives of diethanolamine, imidazole, morpholine and piperazine) had similar [Au]R/Cm values to the commercial IEX resins Dowex-1, IRA-410 and Amberlite A26 (-10 mmol/mol). These resins were examined in controlled B/R tests spiked with 10 ppm trithionate, with the trithionate and tetrathionate concentrations followed by ion chromatography (IC). The novel resins loaded more slowly and were more perturbed by the presence of an initial 10 ppm trithionate, while the commercial resins were more rapidly fouled by trithionate. The quinuclidine resin was subjected to four cycles of B/R gold loading and elution. The resin retained 89% of the initial gold sorption capacity, and 99% of the adsorbed gold was eluted with a modified nitrate eluent.