Chromatographic and electromigrative determination of sulfur-oxygen anions in gold thiosulfate leach solutions

This work presents a series of investigations into the use of chromatographic and electromigrative techniques for the analysis of gold thiosulfate leach solutions. The focus of the project was determination of the gold thiosulfate complex (Au(S203)23-), thiosulfate (S2O32-) , the polythionates (SxO62-, x = 3 to 5) and sulfate (So42-) in these liquors. The fundamental behaviour of the gold thiosulfate complex was studied in an ion interaction chromatographic system. Partial dissociation or decomposition of the gold complex occurred on-column in standards, although this was minimised through adding thiosulfate to the eluent. Addition of the matrix ions, thiosulfate, trithionate, tetrathionate or the leach matrix to gold thiosulfate samples further complicated the chromatography, with the gold peak area dependent on the concentrations of these species in solution. Broadening of the gold peak occurred in solutions containing high concentrations of thiosulfate or the leach matrix that was in part attributed to a self-elution effect. Other mechanisms were also thought to affect the chromatography, such as the type of stationary phase. These problems prevented the successful determination of gold thiosulfate in the leach matrix. Ion-interaction chromatography was successfully applied to the determination of trithionate, tetrathionate, and pentathionate in undiluted leach liquors. A total analysis time of 18 min was required for the developed method using a Dionex NS 1-5 µ column with guard and an eluent comprising an acetonitrile step gradient at injection from 15% to 28% v/v, 3 mM tetrabutylammonium hydroxide and 2.5 mM sodium carbonate. Detection limits for polythionates using a 10 µL injection volume ranged between 5-23 µM for conductivity and 4-68 µM for UV detection based on a signal to noise ratio of 2. The electromigrative methods, capillary electrophoresis, isotachophoresis and mixed mode isotachophoresis/capillary electrophoresis were also investigated for their applicability to the determination of sulfur-oxygen species in thiosulfate leach liquors. Using capillary electrophoresis a method was developed that allowed the separation of thiosulfate, polythionates and the gold thiosulfate complex. The method separated the five species in under 3 min with a total analysis time of 8 min, using an electrolyte containing 25 mM bis-tris adjusted to pH 6.0 with sulfuric acid and an applied voltage of-30 kV. Quantification of the gold thiosulfate complex was not possible by this technique due to inconsistent peak areas and peak splitting effects induced by the presence of other sulfur oxygen species in the sample. Detection limits of the method ranged between 0.5 2 µM. The teclmique was applied successfully to a thiosulfate leach liquor diluted 1:100. Using isotachophoresis, simultaneous determination of thiosulfate and sulfate, in less than 30 minutes, was possible for a synthetic thiosulfate leach liquor requiring a dilution factor of only 2:5. Detection limits of the developed method were 1.3 mM for sulfate and 2.1 mM for thiosulfate. The method also showed promise for the simultaneous determination of thiosulfate, sulfate, trithionate and tetrathionate in these leach solutions. The concept of single capillary isotachophoresis/capillary electrophoresis for these sulfur ions was also demonstrated, however problems with reproducible quantitation prevented· the development of a working method.