ReviewThe role of organic compounds in the recovery of valuable metals from primary and secondary sources: a mini-review
Graphical abstract
Introduction
Cobalt, manganese, lithium and nickel are ubiquitous to technological advancement with widespread applications in areas such as portable consumer electronics, batteries and accumulators, catalysts and alloys. (European Commission, 2017; Jowitt et al., 2018). Globalization, population growth and change in lifestyle have led to increased demand for these metals as measured by the increasing mine production (See Fig. 1) (United States Geological Survey, 2021). In fact, cobalt and manganese are now considered “critical”, that is, no sustainable substitute but vital to global economy (European Commission, 2017; Fortier et al., 2019). Therefore, the future of products with heavy reliance on batteries produced using these metals can be jeopardized in the event of a supply risk (Mo and Jeon, 2018).
With lithium ion batteries (LBs) commanding the highest market share of energy storage systems, LiBs have become a major consumer of these metals (Fan et al., 2020a)(Azevedo et al., 2018). Table 1 shows the chemical compositions of some common LiBs in the market. With a lifespan of 2–3 years in electronic devices and 8–10 years in electric vehicles (EVs), widespread usage of LiBs inevitably leads to a rise in quantity of end-of-life LiBs (Gao et al., 2018a; Wang et al., 2014). About 400 million tons of LiBs are estimated to reach end of life by 2020 (Gu et al., 2017) whereas global stockpile of EV batteries may exceed 3.4 million packs by 2025 (Stringer and Ma, 2018). In China alone, the volume of spent EV batteries is projected to be between 120,000–170,000 tons by 2020 (Tang et al., 2018). These statistics signify that spent LiBs are among the fastest growing waste streams that contain high levels of cobalt, nickel and manganese (Fan et al., 2020a). These metals, due to their high concentrations in the spent LiBs that exceeded the limits set by environmental protection agencies, are toxic and pose substantial threats to the environments if untreated (Po Kang et al., 2013).
To minimize the adverse environmental impacts from direct disposal of the spent LiBs, as well as to recover materials from them, a number of end-of-life management approaches have been developed. They are based on pyro-, bio- and hydro-metallurgical process principles and all aimed at achieving a high recovery efficiency and a lower environmental impact (Yang et al., 2021). Pyro-metallurgical processes require no pre-treatment of collected spent LiBs and recover the metals mostly as alloys or produce intermediates for hydrometallurgical processing (Velázquez-Martínez et al., 2019). However, high energy input, environmental pollution (via release of harmful gasses such as SOx, NOx, HF, Cl2, CO2 as flue gas) and high operational costs limits its sustainability (Zhang et al., 2018a). Bio-metallurgical approach, in comparison, is environmentally friendly but is yet to be commercialized due to its slow kinetics, time required to culture microbes, high sensitivity of microbes to pH and the need to adapt the process to suit different compositions of the feedstock (Naseri et al., 2019; Vanitha and Balasubramanian, 2013). Hydrometallurgical methods involve leaching with acids or alkalis, often in the presence of reducing agents (Sinha and Purcell, 2019; Zhang et al., 2018a) and subsequent separation and purification of leached metals (Zheng et al., 2018). Compared to pyrometallurgical approach, they are more flexible, can recover all valuable metals at high purity, and are more energy efficient and eco-friendly (Mayyas et al., 2018). Compared to bio-metallurgical processes, reaction kinetics of hydrometallurgical processes is also generally faster. As such, the majority of research and development efforts for material recovery from spent LiBs are focused on hydrometallurgy based processes.
Most eco-inspired research on green metal recovery processes focus on leaching agents. Organic acids, due to their biodegradability, offers advantages such as lower secondary pollution, high metal selectivity and recyclability, and have been used in the leaching of metals from low grade ores (Gao et al., 2018a,2017; Golmohammadzadeh et al., 2017). They can contribute hydrogen ions and form metal complexes; thus can function as a leaching agent, chelating agent and precipitant (Sun and Qiu, 2012; Verma et al., 2019). However, the efficiency of hydrometallurgical approach to recover transition metals depends on the stability of the metal ions in aqueous solutions. In spent LiBs and low grade mineral ores, cobalt, manganese and nickel exist in variable oxidation states, mostly in trivalent or tetravalent states (Meng et al., 2020; Sinha and Purcell, 2019) and are often insoluble or unstable in aqueous media. A common strategy to improve their leaching is by establishing a reducing environment in the solution via the use of reductants. (Lv et al., 2018; Meng et al., 2020).
Several studies have shown that reductant-assisted leaching (reductive leaching) offers the advantage of concurrent occurrence of reduction and leaching in one processing step (Sinha and Purcell, 2019). Reducing agents that have been investigated include inorganic compounds (such as iron, hydrogen peroxide, sulfur dioxide, sodium sulfite etc.) and organic compounds (such as glucose, sucrose cellulose, ascorbic acid, oxalic acid, molasses etc.) (Sinha and Purcell, 2019; Zhang et al., 2018a). Inorganic reductants are effective but can be toxic (e.g. sulfur dioxide) (Sinha and Purcell, 2019) or unstable (e.g. hydrogen peroxide) (Zhang et al., 2018) . In contrast, organic compounds are renewable, cheap and ecofriendly with proven reductive abilities, especially in ore processing (Hariprasad et al., 2007; Sinha and Purcell, 2019; Tian et al., 2010). The presence of hydrolysable compounds, organic acids and phytochemicals in agricultural wastes, which are beneficial to the leaching process, makes them excellent reductants as their use reduces the total environmental impacts of metal recovery process as well as provides a better disposal option for the large quantities of agricultural wastes (Chen et al., 2015; Zhang et al., 2018; Sinha and Purcell, 2019).
A number of review articles have examined the use of organics especially organic acids in the recovery of metals from spent LiBs (Golmohammadzadeh et al., 2018; Meshram et al., 2020; Zheng et al., 2018). However, the focus was mostly on the performance of the organic acids as either leaching or reducing agents with little insight into the underlining fundamentals and the role of other organic compounds such as biomass in the recovery of metals. For instance, there is limited information on the influence of possible oxidation products of reducing agents on the leaching process. In comparison, much more in-depth research has been conducted on the recovery of transition metals from low-grade mineral ores using organic-based compounds with high recovery efficiencies (Astuti et al., 2016; Baral et al., 2015; Ghosh et al., 2008; Lu et al., 2015; Tzeferis and Agatzini-Leonardou, 1994). The fact that some transition metals in the low-grade ores exist in multiple charge states makes these research results relevant to the recovery of metals from spent LiBs (Safarzadeh et al., 2011; Wang et al., 2017).
This review will critically analyze recent progress in the use of organic-based substances as lixiviants and reductants in the reductive leaching of metals. While the focus is on the recovery of metals from spent LiBs using organic substances, utilization of organics in the processing of relevant low-grade ores will also be examined in details.
Section snippets
Thermodynamic aspects of reductive acid leaching
Transition metals can exist in variable oxidation states with the stability of a particular oxidation state dependent on the pH and redox potential of the species present in the environment. In a solution environment, the reductive ability of a reducing agent can be inferred from its redox potential, and the energy of highest occupied molecular orbital (HOMO) (Sobianowska-Turek et al., 2014; Zhang et al., 2002). Table 2 shows the standard redox potential measured in electron activity (pE0) of
Organics assisted reductive leaching processes
In the hydrometallurgical approach to the recovery of metals from polymetallic materials such as spent LiBs, low grade manganese ores etc., reductive leaching is a critical process step that determines the overall metal recovery efficiency. The search for environmentally benign approaches has led to increased attention towards the use of biodegradable organics as the reductant and lixiviant in the processing of transition metals from spent batteries and low grade mineral ores.
Challenges and future perspective
Reductive leaching merges the traditional route of reductive roasting prior to acid leaching into a single step thus has the potential to reduce the emissions and effluents. The environmental impact of reductive leaching depends on the choice of reagents. Biodegradable organic reagents are more environmentally friendly, and potentially cheaper if tighter environmental impact control and sustainability standards are applied. While reductive leaching of low-grade mineral ores using organics has
Conclusion
The fundamental reason for recycling is to minimize environmental impacts of wastes. Secondary motivations include mitigating the conversion of useful lands to landfills and closing the loop in material flow and ensuring sustainability for future generation. There have already been a number of excellent review articles on the recycling of spent LiBs, highlighting the importance of minimizing the secondary environmental impacts, or the adverse impacts from the recycling process itself. In this
Declaration of Competing Interest
The authors declare that they have no competing interest.
Acknowledgments
The first author would like to thank James Cook University, Australia for the postgraduate research scholarship.
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