Syntheses of anion exchange resins selective for gold and silver cyanide complexes

Deals with the preparation, characterisation and testing of a series of new and improved synthetic anion exchange resins for selective absorption of the precious metals gold and silver, over the base metals, iron, copper, cobalt, zinc, nickel and aluminium, from an industrial leach solution.. The major method employed by industry, over the last thirty years, to abstract gold from low grade ore or tailings has been cyanide leaching followed by activated carbon concentration. The inherent problems with carbon prompted investigations into alternative methods for the concentration and abstraction of the metal cyano complexes. Ion exchange resins were employed for this purpose due to their versatility towards derivatisation and inertness to chemical and physical attack. The aims of this study were three-fold. The first aim was to design, synthesise and characterise a series of precious metal selective, strong and mixed base, anion exchange resins, where the cationic active site of each is sterically hindered to the approach of the base metal complexes. This aim also included development of the synthetic procedure to yield a resin with the highest degree of substitution possible. The second aim involved the synthesis of each resin on a variety of solid supports, from Merrifield (gel) type resins with different percentage crosslinking to highly crosslinked macroreticular (macroporous) matrices, and comparison of the degree of substitution, precious metal selectivity, equilibrium loading and loading rate achieved with each. This would then allow the identification and use of the most suitable resin base for this study. The final objective of this research involved an investigation into the performance of each resin employing both synthetic and industrial metal cyanide solutions. These investigations were performed under varying chemical and physical conditions (pH, temperature and concentration, etc.) and various kinetic models applied to each resin to mimic its loading characteristics. The selectivity and loading properties of the prepared resins were compared to those of commercial anion exchange resins, industrial activated carbon and other specific gold selective resins. Twenty-four resins were prepared by attaching tertiary amines to blank, chloromethylated poly-styrene/divinylbenzene copolymer resin matrices via seven day reactions in DMF at temperatures of 110°C, thus forming the strong base, quaternary ammonium anion exchanging active site. If required, the resins were then further functionalised to form the respective gold selective species. The degrees of substitution for the novel resins ranged from 0.2 meq/g to 2.5 meq/g depending upon the nature of the starting amine. As some resins are mixed base anion exchange, having one strong base site and one or more (up to four) weak/moderate base sites, gold loadings were obtained in the range 40 000 g Au/ton resin to 1 110 000 g Au/ton resin, or 4 % Au w/w to 110 % Au w/w. No obvious differences in substitution were noted between the different resin matrices trialed. The structure of these function-alised groups on the synthesised resins were characterised by microanalysis, difference infra-red and solid state nuclear magnetic resonance spectroscopy, atomic absorption spectrophotometry, and by comparison with model compounds. Similarities between the NMR spectral properties of the resins and model compounds as well as differences in these properties between the unattached amines and the resins were sought and inferences made as to the success of either the attachment or derivatisation reactions. Microanalysis and anion exchange were also employed to confirm the existence of each functionality and the type of novel resin formed. All the resins prepared show good to excellent selectivity for the precious metal cyano complexes, with four - NOTREN, TIPA, TIPAAPS and TEA-BE resins - displaying extra-ordinary discrimination at all pH values. The pH of the solution was observed to have no effect on the loading of strong base resins, however the equilibrium loading on the mixed base resins was limited by the acidity of the solution. Variation in solution temperature results in an almost linear increase in initial loading rate, indicating a first order process, with an average rate constant of 2 x 10-³sec-¹and activation energy of 55 kJ/mol. At 30 C the reactions were complete in about six hours, while at 80 C the resins were fully loaded in approximately 45 minutes. An increase in temperature also resulted in a linear decrease in equilibrium loading, indicating an exothermic process, with an average Hextraction of - 35 kJ/mol. Change in ionic strength of the solution resulted in a decrease in the final loading due to increased competition for the active site. Variation of resin matrix had little effect on the gold selectivity or loading of the respective resin. Each of the resins tested with the metal cyanide solutions (both synthetic and industrial) could be easily regenerated to their full capacity. The thiocyanate and thiourea elution procedures proved to be the most efficient for recovery of the precious metals with these selective resins. Rate constants of approximately 5 x 10-³sec-¹(first order process) for the thiourea method and 1.55 L/mol/sec (second order process) for the thiocyanate method were measured. The resins were recycled many times through loading, elution and regeneration cycles employing industrial leachate solutions for loading without any loss of selectivity or gold loading ability. There was also no evidence of the resins being poisoned by non-eluting metal cyano complexes during this trial. This reported research has thus led to the preparation of a series of resins which display a high selectivity for the precious metal cyano complexes while limiting the abstraction of the base metal cyano complexes. The performance of these resins compares favourably with that of other gold selective resins (some of which are being employed commercially as gold selective extractants ) and as a result show enormous potential to industry for use as selective auro- and argento-cyanide concentrators from cyanidation leachates.