Sulfur speciation analysis in cyanide leach solutions with liquid chromatography

During the cyanide leaching process of gold and silver, ores that come from the transition zones which typically contains low levels of sulfide may produce one or more of the following reduced sulfur species: sulfide (HS−), polysulfides (Sn 2−), thiocyanate (SCN−), thiosulfate (S2O3 2−), trithionate (S3O6 2−), tetrathionate (S4O6 2−), pentathionate (S5O6 2−) and hexathionate (S6O6 2−). The actual speciation of the reduced sulfur species in the circuits will primarily depend on the redox potential, pH, cyanide levels and existing sulfur species in recycled process water, as well as the presence of other reactive metal species. Development of an accurate analytical method for sulfur species in cyanide leaching units of gold ores where sulfur speciation occurs has an important role in explaining the metal recovery deviations. This study presents the speciation of sulfur species and development of an ion chromatography method to analyze the reduced sulfur species thiocyanate, thiosulfate, trithionate and tetrathionate in cyanide leaching circuits for gold ores. The method developed has resulted in the precise quantification of target sulfur species in leach solutions. Background During conditions typically observed when leaching gold and silver ores with cyanide, the presence of sulfidic ores can result in high levels of cyanide consumption, excessively increasing operating costs, and polythionates formed in leach solution represent a drawback to the recovery of gold and silver by cyanidation. Dissolved oxygen in leach solutions is the primary cause of the oxidation of the reduced sulfur species. Tetrathionate and higher polythionates react with cyanide in aqueous solutions producing thiosulfate, thiocyanate and sulfate. The main consumption of cyanide is due to its reaction with higher polythionates. Methods Sulfur species analysis. Agilent 1100 series high-performance liquid chromatography (HPLC) with ultraviolet (UV) absorbance detection was used to separate and quantify sulfur species in leach solutions. This system pumps the leach solution samples in the solvent through an adsorption column under high pressure. HPLC can separate and identify any sulfur species of interest that can be dissolved in a solvent in trace concentrations as low as parts per trillion. Substance detection in solution was accomplished by using UV detection. This detection method is based on quantifying the amount of UV light of known wavelength absorbed by the compounds of interest in leach solutions. The height of the peaks on the y-axis of the chromatography plot shows the milli absorbance unit (mAU), representing how much of the UV light was absorbed by each sulfur species, and the x-axis shows the retention time. HPLC identifies each compound by its retention time, which depends on the interaction be- tween the stationary phase, type of solvents used and type of compounds that are analyzed. WAD cyanide measurements. The analytical CNSolution 3202 Cyanide Analyzer was used to measure the available cyanide — weak acid dissociable (WAD) cyanide complexes plus free cyanide ions — in precious-metal leach solutions by employing the USEPA OIA-1677 method. CNSolution 3202 provided the applicability of gas-diffusion amperometry type analysis, which avoided interferences from copper and metallic sulfides. The detector response for each sample was displayed on the computer screen as a peak in real time during the leach solution analysis. General specifications of the equipment system are shown in Table 1. Results Initially, standards with known amount of each sulfur species were used for the calibration of the chromatograph. Following HPLC calibration, leach solutions were analyzed for their reduced sulfur species concentration. The developed chromatography method has resulted in definitive peaks for each of the reduced sulfur species of interest. As part of this paper, selected bottle roll test results are presented for the sulfur species thiocyanate, thiosulfate, trithionate, tetrathionate, pentathionate and hexathionate, with peaks obtained from the flotation tailings leach test solution samples. The sulfur species peaks obtained from the leach solution samples proved the precision of the measurement method and accuracy obtained by the HPLC equipment calibration procedure. Table 2 presents a summary of the chromatography method developed. Figure 1 shows the ion chromatogram of bottle roll test 24. The retention times of each species in the bottle roll leach solutions were in the range of ±0.1 seconds compared to the HPLC calibration standard. The peaks of each reduced sulfur species are shown on the chromatogram. Conclusion Analytical method development and its validation has a crucial role in understanding the solution chemistry affecting the metal recoveries at the processing plants. In addition to the method development, its validation is important to ensure quality of work done. Validation demonstrates the accuracy and repeatability of the method developed. Analytical method precision expresses the closeness of the values measured for the same samples with a dedicated measurement procedure. There have been a number of method development conclusions made in the industry without conducting proper validation processes, which has resulted in variance in results between two laboratories. Throughout this research study, the main goal of the method development and validation for the reduced sulfur species in cyanide leach solutions was for the identification and quantification of the species that affects the metal recoveries during the leaching process. At the end of the study, a HPLC method was developed for precise quantification of the species thiocyanate, thiosulfate, trithionate and tetrathionate. Even though the developed method quantified the species of trithionate, pentathionate and hexathionate, because the pure standards for these species were not commercially available at the time of the research, validations for these species quantifications were not completed and they were excluded from the conclusions of this paper. Fig. 1 Bottle roll samples ion chromatography peaks. References A list of all references is available in the full paper.

Sulfur speciation analysis in cyanide leach solutions with liquid chromatography

Authored by:
N. Duru, C. Nesbit

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