Synthesis and characterisation of aurothiosulfate-selective magnetic ion exchange resins

A series of novel ion-exchange resins was prepared based upon the commercial Magnetic Ion Exchange (MIEXTm) substrate produced by Orica Watercare for the selective adsorption and removal of aurothiosulfate from ammoniacal thiosulfate leach liquors. Gold adsorption by these resins was ascertained using bottle-roll tests both from water and from a synthetic thiosulfate leach solution. Elution of aurothiosulfate from these resins was examined using a variety of eluents and used to narrow the set of prospective resins. A common synthetic method was utilised, based upon amination using primary, secondary and tertiary amines of the terminal epoxide group present on the MIEXTM substrate using an aqueous solvent. The weak-base resins formed were then dried and solvated in dimethylformamide, followed by alkylation employing a variety of alkyl halides to form quaternary ammonium moieties. Several structurally-related derivatives of imidazole, piperidine, piperazine and diethylamine were prepared with ion-exchange capacities ranging between 0.5 and 1.5mmol/g, dependent on the steric nature of the amine. The strong-base functionalised resins formed by initial amination and alkylation of weak-base groups were then characterised by ion-exchange capacity determination, elemental analysis and attenuated-total-reflectance infrared spectroscopy with results from each of the techniques being comparable. The resin substrate was also characterised using BET surface area analysis and scanning electron microscopy, with surface areas ranging between 40-47m2/g. The performance of the resins was compared using a synthetic thiosulfate leach solution (pH-10.5) containing gold (20ppm), copper (200ppm), ammonia (1.3M) and thiosulfate (0.1M), with resin gold loadings of aurothiosulfate ranging between 14 and 124g/kg. Elution studies were performed using acidic thiourea, basic thiocyanate and concentrated nitrate solutions, with all resins showing efficient elution of aurothiosulfate utilising one or more of the elution regimes, with typical efficiencies in the range of 30 to 75% elution. The performance of a subset of resins was assessed by the sequential addition of small amounts of trithionate to assess the selective nature of the resins against a significant competitor ion. The best performing resins retained over 75% of the previously adsorbed aurothiosulfate when exposed to 10mM trithionate. Resins were also assessed for their permanent binding of the aurothiosulfate complex by repeated uptake/elution/regeneration studies utilising basic thiocyanate elution, followed by regeneration with ferric nitrate. The optimal resins tested eluted 63-78% aurothiosulfate over 5 cycles. Resins completing this testing were then destructively analysed for aurothiosulfate retained between elution cycles, with optimum resins retaining less than lmg Au over 5 cycles. The following alkylamine functionalised ion-exchange resins were determined to be viable industrial candidates, including N-methyl-piperidine, N,N-dimethyl-piperazine, methyl-diethylamine and piperidine-ethanol-based resins. Of these, N-methyl-piperidine showed the most selective uptake of aurothiosulfate from leach solution, coupled with the most efficient elution using the regime tested and is the most likely candidate for further study.