Study of thiosulfate leaching of silver sulfide in the presence of EDTA and sodium citrate. Effect of NaOH and NH₄OH

Highlights

• Porous layers of copper species are formed in the EDTA systems with NaOH and NH₄OH.

• Porous and compact layers of copper species are formed in the NaOH citrate systems.

• Silver leaching is inhibited in the citrate and EDTA systems at 5 h.

• Thiosulfate is partially decomposed in the ammoniacal–citrate systems.

• Ammoniacal EDTA and citrate systems are more efficient than NaOH systems.

Abstract

In this research, an analysis of the effect of NaOH and NH₄OH on the silver sulfide leaching kinetics was performed using thiosulfate–copper solutions in the presence of EDTA and sodium citrate, in order to develop an alternative green hydrometallurgical process for the sustainable recovery of precious metals. For that purpose, silver sulfide leaching experiments were carried out with different thiosulfate–copper solutions at room temperature. The different copper species present in the leaching systems were elucidated with the aid of species distribution thermodynamic diagrams. The solid residues of all leaching experiments were morphologically and chemically characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive X Ray Spectroscopy (EDXS), in order to identify the solid products formed, and to elucidate the effect of the morphology of the solid and its composition on the silver sulfide leaching kinetics. The results of this study showed that the cupric tetraamine complex (Cu(NH₃)₄^2 +), generated by the NH₄OH presence in the leaching solutions containing EDTA or citrate, enhances the silver leaching kinetics. The ammoniacal–citrate and ammoniacal–EDTA system showed the formation of a porous layer composed by copper oxides and copper sulfides on the silver sulfide particle. The ammoniacal–citrate system presented a degradation of thiosulfate which inhibits the silver sulfide leaching kinetics. On the other hand, when the sodium hydroxide is employed, the complexes Cu(EDTA)OH^3 − and Cu₂(cit)₂OH^3 − are formed and play a similar role to that of the Cu(NH₃)₄^2 + in systems containing ammonium hydroxide. The solid residues presented a porous layer composed by copper oxides and copper sulfides through which the fluid reactants have to diffuse in order to leach the silver, with the NaOH–EDTA and NaOH–citrate systems. This research also reveals that the thiosulfate–copper–NaOH–citrate system could be a promising alternative process to leach silver from silver sulfide.

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 (Cu^2 +) 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):

$$2{\mathrm{Cu}}^{+}+{\mathrm{Ag}}_2\mathrm{S}\to 2{\mathrm{Ag}}^{+}+{\mathrm{Cu}}_2\mathrm{S}\left(\mathrm{Chalcocite}\right)+4{\mathrm{S}}_2{\mathrm{O}}_3^{2-}\to 2\mathrm{Ag}{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2^{3-}$$

$${\mathrm{Cu}}^{2+}+{\mathrm{Ag}}_2\mathrm{S}\to 2{\mathrm{Ag}}^{+}+\mathrm{CuS}\left(\mathrm{Covelite}\right)+4{\mathrm{S}}_2{\mathrm{O}}_3^{2-}\to 2\mathrm{Ag}{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2^{3-}.$$

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.