Study of thiosulfate leaching of silver sulfide in the presence of EDTA and sodium citrate. Effect of NaOH and NH4OH
Introduction
The gold and silver thiosulfate leaching has been studied extensively during the last decades (Aylmore, 2001, Aylmore and Muir, 2001, Baláz et al., 2000, Breuer and Jeffrey, 2003, Fuentes-Aceituno et al., 2005, Lam and Dreisinger, 2003, Muir and Aylmore, 2005, Senanayake, 2005, Zipperian and Raghavan, 1988), and can be considered as an alternative green process to the conventional cyanidation (Abbruzzese et al., 1995). The last investigations have revealed the high efficiency of the thiosulfate–copper–ammonia leaching solutions: the precious metals can be recovered from sulfide ores in 48 h. However, the reaction mechanism in this leaching system has been proved to be very complicated. Basically, the next issues are known: the thiosulfate is employed as the precious metal complexing agent, cupric ions (Cu2 +) are the oxidizing agent for silver and gold, and ammonia stabilizes the cupric ions in solution avoiding their precipitation. On the other hand, according to Eqs. (1), (2), the silver sulfide leaching mechanism is by a chemical substitution reaction between cuprous or cupric ions and the silver contained in the sulfide matrix, producing a thiosulfate–silver complex (Aylmore and Muir, 2001):
Other authors (Alonso and Lapidus, 2009, Alonso et al., 2007, Feng and Van Deventer, 2010, Feng and Van Deventer, 2011, Fuentes-Aceituno et al., 2005, Puente-Siller et al., 2013) have proposed the EDTA as an additional complexing agent for copper, which has the ability to maintain the cupric ion species in solution for longer time. Despite the improvement found in the gold and silver leaching kinetics at low EDTA concentrations (Feng and Van Deventer, 2011, Puente-Siller et al., 2013), the EDTA has the disadvantage to produce many multimetallic complexes when it is in contact with the ore. Consequently, the EDTA consumption and the process cost are increased. Therefore, the search of new alternative complexing agents for copper is necessary; for example, the amino acids can be an interesting alternative, because they are cheaper, efficient and more selective than EDTA (Feng and Van Deventer, 2011).
Sodium citrate, in alkaline solutions, has the ability to produce strong metallic complexes with a wide range of heavy metals such as Cu (II), Ni(II), Co(II), Pb(II) and Fe(III); for that reason it is often used as an effective complexing agent (Gyliene et al., 2004).
Several studies have demonstrated the formation of Cu–citrate complexes (Hong et al., 2007). In this sense, alkaline citrate solutions (pH 8–11) at a temperature range from 30 to 50 °C can be employed to complex high concentration of cupric ions in the form of a stable Cu (II)–citrate species. The Cu (II)–citrate complex concentration is favored when the pH is increased (Eskhult et al., 2008).
Silva et al. (2012) have found a high silver leaching kinetics with the citrate–thiosulfate system at acid conditions; on the other hand, Deutsch and Dreisinger (2013) proposed the use of citrate for complexing the iron instead of copper in acid solutions; they found that the thiosulfate decomposition can be reduced in this alternative thiosulfate–citrate–iron system. Senanayake and Zhang (2012) demonstrated an increase in the thiosulfate leaching efficiency at an alkaline pH, when ammonium hydroxide is added to the leaching solution to increase the pH and to complex the copper, the silver leaching kinetics increases (Aylmore, 2001, Breuer and Jeffrey, 2000, Briones and Lapidus, 1998, Puente-Siller et al., 2013).
According to Aylmore and Muir (2001), the gold dissolution in the thiosulfate systems is passivated by a sulfide layer due to the absence of ammonium hydroxide and to the oxidative decomposition of thiosulfate. This suggests that the ammonium species are preferentially adsorbed on the gold surface avoiding its passivation. However, ammonium hydroxide has a negative impact in the environment; therefore Fuentes-Aceituno et al. (2005) tried to replace the ammonium hydroxide by caustic soda (NaOH) employing thiosulfate–copper–EDTA solutions. The authors observed slower silver leaching kinetics with NaOH than with NH4OH.
In this research, the silver sulfide leaching with thiosulfate–copper systems containing NaOH or NH4OH was evaluated. The effect of EDTA and sodium citrate in the silver sulfide leaching kinetics was also evaluated. This information is important in order to find suitable conditions to carry out the silver sulfide dissolution in a sustainable manner.
Section snippets
Materials and methods
For the silver sulfide leaching tests, different thiosulfate systems were evaluated employing ammonium or sodium hydroxide with EDTA or sodium citrate as cupric ion complexing agents. Species distribution diagrams were constructed for each system and a microstructural characterization of the solid residues obtained in the leaching tests was carried out.
Results and discussion
The results of this study are discussed in two parts; the first part presents the silver sulfide leaching with the thiosulfate–copper–EDTA and the thiosulfate–copper–citrate systems, both with ammonium hydroxide; and the second section shows a similar analysis, in which ammonium hydroxide was replaced by sodium hydroxide as the pH conditioner agent. The results are discussed from a thermodynamic, kinetic and microstructural view-point.
Conclusions
On the basis of the results obtained for the silver sulfide leaching tests, the following facts can be stated:
- 1.
The silver sulfide leaching with the thiosulfate–ammonium hydroxide system employing EDTA or sodium citrate as cupric ion complexing agents presented a higher silver leaching kinetics than the thiosulfate–NaOH systems, due to the presence of the cupric tretraamine complex in the ammoniacal leaching solutions.
- 2.
The thiosulfate–citrate–ammonia system had the highest silver dissolution
Acknowledgments
The authors are grateful to CONACYT (Mexico) for the postgraduate scholarship granted to Damaris M. Puente-Siller.
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2021, HydrometallurgyCitation Excerpt :However, according to Puente-Siller et al. (2014), all these species can be produced in the leaching system with different proportions as solid residues which also contain unreacted silver sulfide. Based on this consideration, it is probable that the mixture of copper species contained in the solid residues can be responsible to promote a massive decomposition of thiosulfate during the first silver sulfide leaching and therefore, when the liquor of the first leaching is used in a second leaching test, the silver recovery reaches only 27%, as shown in a leaching test performed by Puente-Siller et al. (2014). In order to evaluate this possibility, it was decided to study the effect of these solid residues on the stability of thiosulfate and silver recovery.