Use of polyethylenimine functionalised magnetic nanoparticles for gold thiosulfate recovery
Graphical abstract
Introduction
For more than a century, cyanidation has been the preferred method for leaching gold from different types of ores. However, cyanidation of complex refractory ores, specifically carbonaceous and copper- rich ores, can be ineffective (Hiskey and Atluri, 2007). For example, the gold-cyanide complex readily adsorbs onto carbonaceous materials, such as humic acid, graphite, bitumins and asphaltic compounds in the ore itself, which is known as preg-robbing. This leads to higher reagent consumption and gold.
losses to tailings (Aylmore and Muir, 2001; Ji et al., 2013). In addition to this, there are restrictions on the use of cyanide for gold leaching in some areas of the world, owing to the adverse enviro`nmental impact (Oraby, 2009). This has prompted the development of alternate lixiviants to cyanide (Sparrow and Woodcock, 1995).
From an environmental perspective, the use of thiosulfate as an alternative lixiviant to cyanide for gold leaching shows considerable promise. The gold-thiosulfate complex does not show a good affinity for carbon, which makes it a viable alternative lixiviant for the leaching of refractory carbonaceous ores. Compared with different cyanide alternatives, such as halides (Qi and Hiskey, 1991), ammonia (Han and Meng, 1994), thiourea (Murthy et al., 2003), thiocyanate (Kholmogorov et al., 2002) and glycine (Eksteen and Oraby, 2015; Oraby and Eksteen, 2015a; Oraby and Eksteen, 2015b), the thiosulfate system is the most widely studied cyanide alternative. Furthermore, mixed cyanide and another lixiviant that minimises cyanide usage have also been proposed (Muir, 2011; Oraby et al., 2017).
Ammoniacal thiosulfate leaching systems have received the most attention over the last two decades (Zhao et al., 1998; Breuer and Jeffrey, 2002; Feng and van Deventer, 2011; Ji et al., 2013). Copper acts as a catalyst and oxidant for gold oxidation, while ammonia stabilises the copper by forming a cupric ammine complex and alternatively prevents gold passivation (Aylmore and Muir, 2001). However both cationic copper and ammonia are hazardous to the environment and toxic to aquatic life (Ji et al., 2013). Despite the considerable increase in gold oxidation rate, copper-ammonia pair can cause unfavourable effects on subsequent gold recovery from leach or pulp. In the presence of reducing sulphide minerals, copper can precipitate as copper sulphide. This precipitate can passivate gold, or cause rapid decomposition of thiosulfate to form polythionates in the solution that can dramatically reduce gold recovery. In addition, ammonia can be lost by volatilisation in most open vessels or heap leach systems at a pH > 9.2. Higher temperatures can further exacerbate ammonia consumption (Grosse et al., 2003).
The co-existence of different cations and anions in solution makes the solution chemistry more complex (Zhang and Senanayake, 2016). In order to achieve an acceptable level of gold leaching and recovery, this system should be operated in a narrow pH and Eh range. The use of ammoniacal thiosulfate leaching systems as an environmental friendly alternative to cyanidation is therefore questionable. Ideally, there is a need for a system that eliminates or minimises the use of either copper or ammonia, or both if possible.
A non-ammoniacal leaching process which uses calcium thiosulfate (CaS2O3) with a much lower concentration of copper and a moderate amount of air at Barrick Gold's Goldstrike operation in Nevada, USA is an industrially proven alternative process for treating double refractory gold ores, after pre- oxidation (e.g. by pressure oxidation in an autoclave) (Choi, 2013). The resin-in-leach (RIL) operating conditions of this thiosulfate-air leaching system include a CaS2O3 concentration of approximately 0.1 M, with low levels of copper and moderate air addition at 50 °C (Choi, 2016). Currently, strong base anion exchange resins are used to recover gold from pregnant leach solutions. This system provides a number of benefits over conventional thiosulfate leaching systems by using less copper and completely eliminating the use of harmful ammonia in the system. The reduced reagent cost, reduction of the environmental impact, as well as increased solution stability as a result of operating at a pH of approximately 8 are all major benefits.
In a recent study, the use of the aforementioned calcium thiosulfate-air leaching system was attempted for various gold ores, such as pyrite, pressure oxidised residues, calcine concentrates, CuAu concentrates and gold-quartz ores (Dai et al., 2013). Furthermore, a few other oxidant and stabiliser pairs have also been reported. This includes the use of nickel (II) with ammonia (Arima et al., 2004), iron (III) with EDTA (Zhang et al., 2005) and ferric oxalate with thiourea as the catalyst (Chandra and Jeffrey, 2005). All of these alternative systems highlighted the lesser thiosulfate consumption compared to the copper ammonia system.
Although the chemistry of gold leaching with thiosulfate has been investigated extensively and is reasonably well understood, comparatively little attention has been paid to the development of processes for gold recovery from such systems (Fotoohi and Mercier, 2014; Dong et al., 2017). Studies have been reported on the use of ion exchange (IX) resins, activated carbon (AC) in its pristine form and surface modified versions and mesoporous silica for gold thiosulfate adsorption. However AC does not adsorb gold thiosulfate well, possibly due to the high ionic charge density and larger size of the complex compared to the gold cyanide complex. Cyanocuprate loaded AC (Parker et al., 2008) and silver ferro cyanide impregnated AC (Yu et al., 2018) were attempted to recover gold thiosulfate, however these adsorbents added cyanide to the recovery solution. The maximum adsorption capacity of the latter adsorbent was only 3.55 kg/ton. Although the adsorption capacity of mesoporous silica was high in a near neutral pH range (6.5–7.5), it dramatically reduced when the pH was above 8. Use of this adsorbent in near neutral pH solutions (pH 7.5) is recommended for a longer life cycle. Moreover, the fine size of the adsorbent makes the separation from a pulp difficult (Fotoohi and Mercier, 2014).
Conventional methods such as solvent extraction and precipitation have been proposed, but these are only applicable to clarified solutions. Zinc cementation consumes large quantities of reagent. Despite the competitive adsorption of polythionates and copper, some strong base anion exchange resins are capable of gold recovery from thiosulfate solutions (Zhang and Dreisinger, 2004; Daenzer et al., 2016). However, gold elution associated with IX resins requires high reagent concentrations and the resin regeneration process is complex (O'Malley, 2002; Fleming et al., 2003; Jeffrey et al., 2010).
Functionalised magnetic nanomaterials have been reported widely for metal adsorption from aqueous media. Different studies have been reported for heavy metal adsorption (Hu et al., 2006; Ge et al., 2012; Hua et al., 2012) and precious metal adsorption (Alorro et al., 2010; Ranjbar et al., 2014). The ease of solid-liquid separation (Yavuz et al., 2006), fast adsorption kinetics (Hu et al., 2005) and high adsorption.
capacities associated with high specific surface areas (Ahmed et al., 2013) make the use of magnetic nanoparticles (MNPs) attractive. Gold recovery from different aqueous solutions with nanosized iron oxide based magnetic adsorbents were attempted, whereas in most instances gold was in a +3 oxidation state (Au(III)) as AuCl4− in acidic pH solutions (Zhang et al., 2013c; Roto et al., 2016; Abd Razak et al., 2018). The adsorption mechanism was concluded to be chemisorption through kinetic model fitting and thermodynamic approaches. The favourable solution pH was in the range of 3–5, while a decline in adsorption efficiency could be observed in alkaline solutions. Therefore in order to use iron oxide nanoparticles in alkaline solutions, a suitable surface coating agent appears to be necessary.
Polyethylenimine (PEI) is a water soluble amine rich polycation that consists of primary, secondary and tertiary amine groups (Pandey and Sawant, 2016) and frequently used as a coating agent for magnetic cores. PEI coated magnetic nanoparticles have been used in biological and medical applications, such as cancer cell separation (Lu et al., 2014), DNA transfection and gene delivery (Arsianti et al., 2010), as well as for removal of heavy metals from the environment (Larraza et al., 2012; Sui et al., 2015). PEI coated magnetite nanoparticles demonstrated good adsorption of gold nanoparticles in aqueous media at pH 7. The increase in PEI coating density increased the gold adsorption and full coverage of the nanoparticle surface (Goon et al., 2009).
Magnetic separation of solids from a liquid is relatively simple and hence beneficial over other conventional screening methods. Some studies have been reported on using magnetic separation for gold recovery with different magnetic based adsorbents. Aurothiosulfate selective magnetic IX resins were used in synthetic leach solutions which facilitates the use of smaller size resin particles. Different IX functional groups were attached to magnetic ion exchange (MIEX®) resin base for use in RIL, subsequently separated with an external magnetic field. This eliminates the need for finer screens to separate resin beads and hence fine grinding of the feedstock (Marshall, 2010). Powdered magnetic activated carbon has been used for gold recovery from cyanide solutions which eliminated the use of screens for AC separation (Miller et al., 2004; Kahani et al., 2007). Recovery of Au(III) by chitosan coated magnetic nanosorbents was also reported. Magnetite was used as the magnetic core for the composite preparation (Chang and Chen, 2006; Donia et al., 2007).
Although the potential use of magnetic nanoparticles in the gold mining industry has been demonstrated recently (Feng et al., 2017b), only a few studies have been reported on the use of magnetic nanoparticles for gold recovery from different leaching systems. Gold recovery from copper anode slime by means of magnetite nanoparticles was reported when thiourea is used as the lixiviant for leaching (Ranjbar et al., 2014). Some attempts have been made to recover gold and other precious metals from chloride solutions using nanosized magnetite powder (Alorro et al., 2010; Aghaei et al., 2017). In a very recent study, PEI coated MNPs has been attempted for gold recovery from ammoniacal thiosulfate leaching solutions (Betancur et al., 2019).
Taking into consideration the importance of developing such a system which could concentrate precious metals such as gold from alkaline thiosulfate leaching solutions, an attempt was made to use functionalised magnetic nanoparticle as a potential adsorbent. In this manner, solid and liquid separation could be achieved by taking advantage of magnetic separation. Physical adsorption of gold rather than chemisorption was preferred which would facilitate easy metal elution at the end of the process. In this study, iron oxide magnetic nanoparticles functionalised with PEI were used for the adsorption of gold from a thiosulfate leaching system that contained calcium thiosulfate, gold and copper at a pH of 8 and at different temperatures. PEI was selected as a coating material, owing to its high cationic charge density, together with a higher pHPZC (pH at point of zero charge) value. In addition, it had no adverse effect on the magnetic separation of the particles.
In a previous study conducted by the authors, the efficacy of PEI coated iron oxide nanoparticles as a potential adsorbent to recover gold thiosulfate from an actual ore leachate has been demonstrated (Ilankoon et al., 2019). In this paper, the main focus is on better understanding of adsorption isotherms, adsorption kinetics and thermodynamics of the adsorption system using synthetic leaching solutions. The process conditions associated with aforementioned calcium thiosulfate-air leaching system was mainly considered. The adsorbent was characterised at different stages of the process. Modelling of the adsorption kinetics and isotherms using conventional model fitting approaches was also accomplished. A thermodynamic approach was used to determine the adsorption mechanism. Moreover, a simple two- step metal elution process requiring low eluant concentrations was used to regenerate the gold loaded nanoparticles. The competitive adsorption of copper and sulfur species onto the adsorbent was also discussed. Care was taken to use environmentally benign reagents and to avoid or minimise the use of harmful chemicals throughout the process. It is foreseen that the magnetic nature of the suspended MNPs will facilitate separation and recovery of them from a pulp with an external magnetic field.
Section snippets
Reagents
All the chemicals used were analytical reagent grade. Synthetic leaching solutions prepared by dissolving the required amounts of reagents in deionised (DI) water were used for the adsorption experiments. Iron oxide magnetic nanoparticles were supplied by US Research Nanomaterials, Inc., USA and polyethylenimine (branched, average Mw ~ 25,000, Mn ~ 10,000) was purchased from Sigma Aldrich, Australia. Sodium aurothiosulfate (I) powder from Surepure Chemicals, USA was used as the gold source and
Functionalisation of nanoparticles
Bare MNPs were first surface functionalised using trisodium citrate. The purpose of the intermediate citrate coating was to impart a higher negative surface charge to the nanoparticle surface. Bare iron oxide MNPs, both magnetite and maghemite, hold a negative surface charge above pHPZC. However, citrate groups can dramatically increase the charge density (amount of anionic groups in a specific surface area) of MNP surfaces. The efficacy of citrate coating at elevated temperatures, around
Conclusion
This study investigated the ability of polyethylenimine functionalised magnetic nanoparticles to recover anionic gold complexes from alkaline calcium thiosulfate leaching solutions. The particle characterisation results demonstrated the success of the surface functionalisation process and gold adsorption.
Adsorption of gold thiosulfate complex onto PEI-MNPs was evaluated in the absence and presence of free thiosulfate and copper in the solution. An increase in the adsorbent dosage, significantly
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge the contribution of an Australian Government Research Training Program (RTP) Scholarship in supporting this research and Curtin University's John de Laeter (JDL) Centre and CSIRO Mineral Resources, Waterford for sample analysis services. A special thanks to Ms. Danielle Hewitt in CSIRO, Waterford for the support given on sulfur species analysis using HPLC and to Dr. Jean-Pierre Veder (worked in JDL Centre at Curtin University) for the support given in XPS
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