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Development of an integrated glycine-based process for base and precious metals recovery from waste printed circuit boards

https://doi.org/10.1016/j.resconrec.2022.106631Get rights and content

Abstract

The present study developed an integrated glycine-based process to recover metals from waste printed circuit boards at high solid contents (10–15%). A novel glycine-ammonia leaching was used to selectively extract >99% copper, >85% zinc and around 50% lead at room temperature. Ammonia played a key role, increasing the copper solubility in solution (>40 g/L), shortening leaching time from 72 to 24 h, and also acting as a pH modifier. The 2nd leaching step utilised cyanide-starved glycine where extractions of 70–90% for gold and silver, and >50% for palladium were obtained. The remaining metals from the previous steps were removed by ferric sulfuric acid leaching, with/without thiourea. In the presence of thiourea, 70–95% of remaining base and precious metals were removed. Overall, the metals with major economic value were all extracted at >99% for copper, >80% for gold, >90% for silver and >85% for palladium.

Introduction

Waste printed circuit board (PCB) is a core component of almost all the waste electric and electronic equipment (WEEE or e-waste) (Li et al., 2018). It contains various base (BMs) and precious (PMs) metals, indicating a significant economic value for recycling (Cui and Zhang, 2008). The toxic metals (lead, mercury, arsenic, cadmium etc.) and organic pollutants (e.g., brominated flame retardants) also need to be recycled or disposed of properly due to environmental risks (McGrath et al., 2018). According to Li et al. (2018), the primary metals of economic value in waste PCBs are copper and precious metals, which determines the economic feasibility of a recycling process. Copper, mostly laminated in different layers of PCB, occurs at the highest concentration (10–20 wt.%) of all the metals. Gold, silver and palladium show very high variability in concentration (e.g., 10–1000 ppm level) for different types of PCB. Mostly, the hydrometallurgical recovery of copper and precious metals has been made in different steps due to their content and chemical activity differences.

In recent years, glycine has been actively studied as a “green” lixiviant to leach base and precious metals from natural and secondary resources (Eksteen et al., 2017a; Oraby et al., 2019). Glycine (NH2CH2COOH, GlyH), the simplest amino acid, is non-toxic, non-volatile, of low cost, and available in mass production. It has been proven to be a suitable reagent at laboratory and commercial scales for gold, silver and copper recovery from ores, concentrates and tailings, an alternative to the traditional cyanidation (Eksteen et al., 2017b; Newton and Trask, 2017). Recently, the authors of present study have researched various glycine-based process steps and applied them to process e-waste. As shown in Fig. 1, the process includes two steps of glycine-based leaching, i.e.:

  • In the 1st step, alkaline glycine-only solution is used to selectively extract copper (>95%), zinc (>90%), and other base metals (lead, cobalt, etc.) over precious metals (Li et al., 2020; Oraby et al., 2020).

  • In the 2nd step, alkaline glycine solution with a starved level of cyanide is used to extract precious metals of gold (>90%), silver (>85%) and palladium (>70%) (Li et al., 2021a).

Although cyanide was introduced in the 2nd step, its concentration was reduced to a minimal level of 250 ppm, and no free cyanide was present in solution after 4 h. This significantly reduced the safety risks from traditional cyanidation. Both steps were operated at pH 10–11, room temperature and ambient pressure, indicating low requirements for equipment and operation.

After leaching, the subsequent recovery of metals from pregnant leaching solution (PLS) has also been explored. The authors of present study (Li et al., 2021b) used sulfide precipitation, chemical reduction and solvent extraction, respectively, to recover base metals from the 1st step leachate of waste PCBs. Deng et al. (2020a) investigated copper recovery from cyanide-starved glycine solutions using sulfide precipitation. Tanda et al. (2017) used solvent extraction to recover copper from glycine leachates. Tauetsile et al. (2019a, 2019b) reported in-depth studies on gold recovery using carbon adsorption from glycine-only and cyanide-starved glycine solutions. Schoeman et al. (2017a, 2017b) and Deng et al. (2020b) investigated the recovery of platinum, palladium or precious metals from cyanide or glycine-cyanide leachates using ion exchange resins. Snyders et al. (2015a, 2015b, 2014, 2013) and Mpinga et al. (2014) also investigated the behaviour of platinum group metals in cyanide leachates with respect to adsorption onto and elution from activated carbon.

However, it has been shown that leaching in alkaline glycine-only solution presented slow kinetics compared with conventional mineral acid leaching (Yazici and Deveci, 2013). The 1st step, glycine-only leaching of waste PCBs, took 3 days to achieve copper extractions in excess of 90%. This will impact capital costs as more agitated tanks would be required to provide adequate residence time, especially given the high copper content in waste PCBs (10–20%). Moreover, due to the precipitation of cupric glycinate at high concentrations and the risks of glycine decomposition, the solid content of the glycine-only leaching was restricted to 2–5% (Li et al., 2020; O'Connor et al., 2018). Such a low solid content is considered impractical in industry. According to Delft et al. (1979), the cupric glycinate (bis glycinato copper (II)) can form a number of structural isomers and hydrates which all have different solubilities in water, with the cis- and trans- structures having different thermodynamic stabilities, and kinetic preferences, although the kinetically favoured form has not yet been characterised for metal leachates. The formation of the trans-form of the bis glycinate copper (II) chelate during copper leaching from waste PCBs is represented by reaction R1–2 (Li et al., 2020).2Cu (s) + O2 (aq) → 2CuO (s), ΔGo (25 ℃) = −271.3kJ/molCuO (s) + H2O (l) + 2NH2CH2COO → Cu(NH2CH2COO)2 (aq) + 2OH (aq), ΔGo (25 ℃) = 26.2kJ/mol

In addition to glycine-only leaching, all the other steps in previous studies were also conducted in lab-scale and batch tests at a low solids content (mostly 2%). The recovery of metals from leachate was done in preliminary or theoretical manner and was separated with the upstream leaching steps. Finally, the removal of impurity metals after the two steps of glycine-based leaching has not been studied yet.

Therefore, aiming to solve the issues and bring the process forward towards industry, the present study reports an integrated glycine-based process to recover base and precious metals from waste PCBs. During the 1st leaching step, ammonia was introduced into alkaline glycine solution. This allowed the acceleration of the leaching kinetics and allowed working with higher solid content. After the alkaline glycine-ammonia leaching, alkaline cyanide-starved glycine leaching, coupled with carbon-in-leach (CIL), was performed to recover precious metals in a second step. Finally, two acidic leaching methods were evaluated to remove impurity metals from the second-step residue. All the leaching steps in this study were done at 10–15% solids, which reflect far more practical and economic solid concentrations (than 2% used in the initial research). It is hoped that the present study could support the design of industrial glycine-based process and pave a way for the next step of commercialisation.

Section snippets

Preparation of waste PCB samples

The waste PCB sample used for the first glycine-ammonia leaching step was obtained by pre-treating about 100 kg of waste computer motherboards to 100% <2 mm. The high-value CPUs and RAMs from the original waste PCBs (supplied by Total Green Recycling Pty Ltd, Perth, Australia) were removed for separate recycling. The pre-treated sample was split using a laboratory splitter to prepare sub-samples at around 100.0 g each. The detailed pre-treatments have been reported in our previous study (

Glycine-ammonia leaching

As it is the first time introducing glycine-ammonia leaching system, the theoretical considerations are discussed in this section. Since copper has the highest content in waste PCBs (Table 1) and represents the second major economic value, it is considered the top priority among other metals during this leaching step. Therefore, the effects of variables on copper extraction were also investigated.

Conclusions

The present study has integrated glycine-ammonia, cyanide-starved glycine, and acidic H2SO4 leaching steps as a whole leaching process to recover most of the base and precious metals from waste PCBs. The study has demonstrated that the process could be effectively operated at industrially viable solids percentage (10–15%), room temperature and ambient pressure. Overall, the process was able to extract most of the metals with high recoveries. The metals, representing majority of economic value,

CRediT authorship contribution statement

H. Li: Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing – original draft. E.A. Oraby: Conceptualization, Formal analysis, Funding acquisition, Methodology, Project administration, Supervision, Validation, Writing – review & editing. J.J. Eksteen: Conceptualization, Resources, Writing – review & editing.

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.

Acknowledgement

This study received funding from the Western Australian Government's Department of Jobs, Tourism, Science and Innovation as part of the New Industries Fund “WasteSorted” Grant (e-waste 2020–2021). The views expressed herein are not necessarily the views of the Western Australian Government, and the Western Australian Government does not accept responsibility for any information or advice contained herein. The Western Australian Government does not endorse any information, product, process or

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