Due to public and environmental concerns over the use of cyanide in gold recovery from ores, renewed interests in thiosulfate leaching as an alternative have started growing. At the research stage on thiosulfate leaching, recovering gold has been the main focus for the researchers and recovering silver has not been focused on as seriously as gold, possibly due to the market value for gold being significantly higher than silver. Therefore, knowledge is still limited in recovering silver by the thiosulfate technique, particularly for recovering it from mine tailings.
The application of the thiosulfate leaching system for recovering silver from the Cannington mine tailings was studied in this project. The studies covered the tests to determine the optimum leach parameters such as concentration of thiosulfate, copper sulfate and ammonia, time temperature and particle size. The recovery of silver from pregnant thiosulfate leach
liquors by ion exchange and cementation processes were also investigated.
The studies on the effect of concentrations of ammonia, thiosulfate, and copper sulfate on thiosulfate leaching of Cannington tailings showed that the optimum leaching conditions (with respect to the highest silver recovery) were a thiosulfate concentration of 0.15M, and a copper sulfate concentration of 0.076M in the absence of ammonia and a leaching time of two hours resulting in a recovery of silver up to 60%.
It was found that silver can be leached from Cannington tailings at an appreciable rate in the absence of ammonia. Mechanisms of leaching silver by thiosulfate can be oxidative, or by substitution depending on the form of silver. The oxidative mechanism requires the presence of ammonia whereas the substitution mechanism can proceed in the absence of ammonia. This was demonstrated by the findings that the oxidation of pure silver metal powder by thiosulfate
solution with copper required ammonia whereas thiosulfate leaching of silver from its oxidized forms of silver sulfide and silver oxide yielded similar results without ammonia. Therefore, it was concluded that the thiosulfate extraction of silver from Cannington tailings, which contain silver mostly in its oxidised forms, followed the substitution mechanism as the leaching could be performed in the absence of ammonia.
A simple cost analysis (profit value = recovered silver value reagents cost) on thiosulfate leaching of silver from Cannington tailings showed that the leaching process was profitable at AUD 5.85 per tonne of tailings under conditions of a thiosulfate concentration of 0.15M, and a copper sulfate concentration of 0.003M in the absence of ammonia. Therefore, thiosulfate leaching of silver from Cannington tailings may possibly be an economically viable leaching process for the Cannington mine. However, a more thorough cost analysis must be performed
that includes the costs involved in a possible recovery process after leaching.
The mineralogy of silver is more complicated than the mineralogy of gold because silver can be distributed in at least 200 silver bearing minerals whereas gold appears mostly as native gold in gold bearing ores. Mineralogical studies by Mineral Liberation Analysis (MLA) on Cannington tailings showed that silver was distributed in pyrargyrite, native silver, acanthite, freibergite, jamesonite, and galena. The MLA studies on the leach residue from the thiosulfate leach of Cannington tailings also showed that silver remained undissolved mostly in freibergite and galena minerals and silver mineral grains in the leach residue were mostly smaller than 10µm. It was expected that, on reduction to particle sizes less than 10µn by further grinding, silver recovery would increase due to the liberation of silver mineral grains and the decrease in the crystallinity of the
complex mineral structure of the liberated minerals. This was proved by the experimental results in which silver leach recovery increased with a finer size reduction by wet grinding and silver recovery up to 81.5% was obtained for size reduction to a particle size of P8o= 2 µm. The unrecoverable silver (18.5%) remained in galena (12.4%) and freibergite (6.1%).
Following the thiosulfate leaching studies, experiments were carried out on the ion-exchange resin process to recover silver from the thiosulfate leach liquors. It was found that the presence of ammonia had a negative effect on leaching of silver from Cannington tailings by thiosulfate, but had a positive effect on adsorption and elution processes in ion exchange. In contrast, the presence of copper had a positive effect on leaching and a negative effect on the ion exchange process.
It has been found that complete adsorption of silver from leach
liquors of various pH values and different concentrations of thiosulfate, ammonia, and copper sulfate onto the commercial resin Amberjet 4200 was achievable in three hours. The elution of silver was better achievable with 80% efficiency by 5M NaCl eluant solution than by 2M NaNO3 eluant solution. The overall ion exchange resin process can concentrate leach liquor with a silver concentration of around 20 mg/L to a silver concentration of 100 mg/L in the eluate (5 times more concentrated), which may be suitable for electrowinning processing.
In addition to the ion exchange process followed by electrowinning, the cementation process by zinc powder can be an alternative to recover silver from the thiosulfate leach liquors. The cementation studies were performed on four types of thiosulfate leach liquors with the compositions summarised as follows:
1. Dilute leach liquor from thiosulfate
leach of the original tailings under optimum
conditions in the presence of copper at a solid/liquid ratio of 1/15.
2. Dilute leach liquor from 24 hrs extended thiosulfate leach of the original tailings in
the absence of copper at a solid/liquid ratio of 1/1
3. Concentrated leach liquor from thiosulfate leach of the original tailings under
optimum conditions in the presence of copper at a solid/liquid ratio of 1/1.
4. Concentrated leach liquor from 24 hrs extended thiosulfate leach of the original
tailings in the absence of copper at a solid/liquid ratio of 1/1.
Studies on the effect of the mass of zinc powder on the cementation of silver from the thiosulfate leach liquors led to the proposal of two reaction mechanisms: the reaction mechanism with the stoichiometry of one mole of zinc precipitating one mole of silver and one mole of copper, and the reaction mechanism with the stoichiometry of one mole of zinc precipitating two moles of silver in the absence of copper.
Thiosulfate leaching of Cannington tailings under optimum leaching conditions with solid/ liquid ratios of 1/15 and 1/1 in the presence of copper resulted in leach liquors of relatively lower and higher silver contents, respectively. The cementation of silver from these leach liquors containing copper was found to follow the reaction mechanism with the stoichiometry of one mole of zinc
precipitating one mole of silver and one mole of copper. The cementation of silver was better achieved in terms of utilisation of zinc from the more concentrated leach liquor.
The effect of temperature was not significant on the cementation of silver from the leach liquors for the use of less than the stoichiometric amount of zinc powder. The cementation was also unaffected by temperature for the use of an excess amount of zinc powder.
The cementation of silver and copper were independent of time after 60 minutes and the kinetics studies showed that the cementation was best performed at 60 minutes.
Thiosulfate leaching of the original tailings with solid/liquid ratios of 1/15 and 1/1 in the absence of copper under 24 hrs extended leaching conditions resulted in the leach liquors having relatively lower and higher silver contents, respectively. The cementation of silver from these leach liquors was found to follow
the reaction mechanism with the stoichiometry of one mole of zinc precipitating two moles of silver. A doubled amount of silver per mole of zinc was precipitated from these leach liquors in the absence of copper as compared to the leach liquors containing copper.
As far as cost was concerned, it was not economic to recover silver by cementation from the dilute leach liquors obtained from thiosulfate leaching of the Cannington tailings with a solid /liquid ratio of 1/15 in the presence and absence of copper. The cementation of silver was potentially cost effective from the concentrated leach liquors from thiosulfate leaching of the Cannington tailings with solid /liquid ratio of 1/1 in the presence and absence of copper. The cost was slightly higher for the cementation of silver from the leach liquor in the presence of copper and this may be compensated by the benefits of higher silver leach recovery and fast leaching time from thiosulfate leaching in the
presence of copper.
It was shown that thiosulfate leaching followed by the cementation process is a viable processing route to recover silver from Cannington mine tailings. However, a detailed cost analysis is necessary before any decision to proceed can be made.