Review on Solvents for Extraction of Metals from Waste Electrical and Electronic Equipment (WEEE)

Abstract

Waste Electrical and Electronic Equipment (WEEE) are electrical and electronic gadgets known to have reached their end-of-life. They are made of printed circuit boards (PCBs) containing valuable metals like copper, gold, iron, lead nickel, silver, tin and zinc which can leach into the environment when landfilled. Current extraction methods for the recovery and recycling of metals from WEEE uses various solvents which are not selective, releases toxic gases and recover metals in a mixed form which requires further purification steps to recover individual metals. This paper has reviewed the use of inorganic acids, organic acids and designer solvents which do not release toxic gases when used for extraction of metals from WEEE. The results reveal that ionic liquids ae promising solvents for extracting metals.

Introduction

I. INTRODUCTION

When wastes have the potential to be recycled and converted to useful materials they are considered as secondary raw materials with great value. Rich ores for the refining of metals are getting scarce gradually and other materials such as low-grade ores and other secondary sources such as tailings, solid and liquid wastes are now considered as economical sources of metal by metallurgical industries. Urban mining is a term widely used to describe the recovery of metals from anthropogenic wastes. In other words, it is a process of recovering elements or compounds from wastes. Some examples of such wastes are Waste Electrical and Electronic Equipment (WEEE), electroplating wastes and tannery waste. The metals present in these mixed waste streams include both hazardous, transition and precious group metals (mercury, lead, lithium, antimony, beryllium, cadmium, copper, silver, platinum, gold, nickel, zinc). When WEEE and other metal bearing wastes are sent to landfills they may contaminate groundwater due to leaching of metals; they may also pollute soil and air due to fire outbreak or release of hazardous components; they can also fill up landfill volume (Sum, 1991; Spalvins, 2008; Adediran & Abdulkarim, 2012; Damodaran et al., 2013; He and Xu, 2014; Kumar et al., 2014; Letsrecycle, 2020; Zhang & Xu, 2016).

Wastes containing valuable or hazardous metals may need to be sorted for reuse or recycling. In WEEE, the mixture of metals and other materials is heterogenous and this has led to a serious challenge in selective recovery of desired materials. The recovery is low as metals are lost in slag and gaseous effluents is high. However, researchers have also combined complexants and oxidants to study the dissolution of metals. It has been reported by Kumar et al., 2014 that 100 % of zinc and copper were dissolved when a combination of 2 mol/L H2SO4 and 0.2 mol/L H2O2 were used to leach printed circuit board (PCB) at 85 0C for 8 hours. According to Dhawan et al., 2009, HNO3 is more effective than HCl and H2SO4 in leaching metals from PCB and these mineral acids are known to be really aggressive for metal recovery they are disadvantaged as they release toxic gases (NOx, Cl2 & SO2). Metal recovery through this process is non-selective as the metals are extracted in a mix and a further purification step is required (Sum, 1991; Oh et al, 2003; Veit et al., 2005; Dalrymple, 2007; Oishi et al 2007; Eswaraiah et al., 2008; Wu et al., 2008; Dhawan et al., 2009; He and Xu, 2014;  Kumar et al., 2014; Jadhav & Hocheng, 2015; Wang et al. 2016; Zhang & Xu, 2016). Various types of organic acids are considered as environmentally friendly leaching agents and have been used for the extraction of Pb and PbO from scrap battery paste by Sonmez & Kumar, 2009. Citric acid and acetic acid were used to leach large PCB but poor metal slubilization was observed as only 9.89 % and 19.57 % of copper respectively was recovered. However, 100 % Li and 90 % Co have been recovered using 1.25 M citric acid and 1 % vol H2O2 at 90 0C for 30 min and a solid to liquid ratio of 20 g/L. The combination of IL extractants have yielded more results than organic solvents in metal ion extraction (Seal et al., 2003; Chen et al., 2004; Chen & Tsai, 2004; Du et al., 2004; Pandija et al. 2007; Sonmez & Kumar, 2009; Li et al., 2010; Jadhav & Hocheng, 2015; Wang et al. 2016).

II. WASTE ELECTRICAL AND ELECTRONIC EQUIPMENT (WEEE)

Waste Electrical and Electronic Equipment (WEEE) is defined as equipment that require the use of electricity for its function and they are also regarded as equipment which works with electrical networks. Examples are cell phones, computers, televisions, batteries, refrigerators which have reached their end-of-life. These gadgets are made up of wires, printed circuit boards (PCBs), cathode ray tubes (CRT) and liquid crystal display (LCD). Some of the valuable materials WEEE is made up of are metals, plastics, glass and other materials (Figure 2.1 and Table 2.1). These materials contain precious group metals like gold and silver, heavy metals like iron, copper, nickel, lead, cobalt, chromium, cadmium along with other hazardous materials like brominated flame retardant (BFR). Table 2.2 shows some useful and hazardous components present in WEEE. Every year, between 34 - 54 million metric tons of Waste Electrical and electronic equipment (WEEE) are generated globally and this figure is growing yearly (Habashi, 1999; EU, 2003; Huang, 2009; Terada, 2012; Ogunseitan, 2013; He and Xu, 2014; Zhang & Xu, 2016; Statista, 2021). The research by Kiddee et al., 2013 showed that 500 million tons of computers were discarded in the United States while at the end of 2010 about 610 million tons of computers became outdated in Japan (Kiddee et al., 2013; Park et al., 2014; Hadi et al., 2015; Zhang & Xu, 2016). In Nigeria for instance, 60,000 – 71,000 tons of used equipment are imported yearly and the largest market for computers in Ikeja, Lagos and the Computer and Allied Product Dealers Association stated that about 75 % of the electronics shipped to the country are considered wastes from the source country; 19 % of these electronics do not function properly and are already near their end-of-life but they become useful to the Nigerian second-hand market as the level of poverty does not allow the people to afford new gadgets hence the reason for the large market of e-waste. The solid waste heap is a source for mixed metals (Adediran & Abdulkarim, 2012; Letsrecycle, 2020).

Table 2.1. Main components of WEEE

Adapted from: (He and Xu, 2014; Zhang & Xu, 2016)

Table 2.2. Useful and Hazardous Components in WEEE

Adapted from: (He and Xu, 2014)

The United States Environmental Protection Agency (USEPA) stated that e-waste of 80 – 85 % by weight ends up in landfills and when they are leached the soil gets contaminated along with groundwater in adjacent regions. The study by Splavins et al., 2008 showed that when e-waste is mixed with municipal wastes the leaching of lead increased due to more aggressive acidic conditions. It has been reported by Vehlow et al., 2000 that the incineration of e-waste with municipal waste will lead to the formation of brominated and chlorinated dioxins and furans which are extremely toxic chemicals. Therefore, the use of open indiscriminate dumping, burning (Figure 2.2 a-b), landfill and incineration for the management of WEEE are not environmentally friendly options and don’t comply with the sustainability principles (Vehlow et al., 2000; Splavins et al., 2008; Adediran & Abdulkarim, 2012; Tuncuk et al., 2012; Hadi et al., 2015; Wang and Xu, 2015).

There is high value of precious metals in e-wastes and PCBs contains larger portions of precious metals since microelectronic components like capacitors and semiconductor chips are mounted on them. Most electric and electronic equipment contain PCBs which make up 3 % of all WEEE produced. Nearly 160 – 210 kg of copper and 80 – 1,500 g of gold can be contained in one metric ton of circuit boards and, when compared to mining in the United States, the amount of gold is 40 – 800 times and the amount copper is 30 – 40 times the amounts present in the ores which are currently being processed. PCBs are the most economically attractive portion of WEEE but as they are mixed with both hazardous and precious materials such as metals, organics, fibre glass, plastics in small volumes it poses difficulty in extraction, recovery and recycling of materials of interest. Table 2.3 shows the composition of materials in a populated PCB. The separation techniques also limit the exploitation of the materials that can be recovered and the pre-treatment of PCBs is important in the recycling process (Grossman, 2007; Adediran & Abdulkarim, 2012; Tuncuk et al., 2012; Canal et al., 2013; He & Xu 2014; Hadi et al., 2015; Wang & Xu, 2015; Akbari, 2019). When the value of resale for a metal can cover the cost of recovery it is given a high priority (Yordanov, 1998).

Table 2.3 Composition of Materials in Populated PCBs

                     Adapted from: (Hadi et al., 2015)

III. PRE-TREATMENT OF PCBS

In recycling WEEE, a pre-treatment step is carried out and this is a physical process of mechanically breaking and then crushing the PCBs. At first, the electronic components are dismantled and are further concentrated based on their physical properties. The physical properties mainly considered are metals and non-metals and this is achieved by using one of the following methods: magnetic separation, eddy current separation, screening, pneumatic separation and electrostatic separation. To completely separate magnetic fractions of the crushed PCBs it is best to use the magnetic separation. However, some non-ferrous metals might be included as a result of insufficient liberation. Eddy-current is more suitable for aluminium fractions and sieving crushed PCBs is necessary as this will help to prevent the influence of different particle sizes during separation. The hazardous components on PCBs are selectively disassembled as this would reduce the amount of toxic elements ending up in the mainstream of the recycling process. During the crushing of PCBs the local temperature of the PCBs increase and some researchers have argued that this process may result in agglomeration and localized pyrolysis of plastics. However, Yuan et al. 2007 researched on using low temperature for crushing PCBs but the method involved high energy cost (Zhang et al., 2012; Havlik et al., 2014; Ghosh et al., 2015; Silvas, 2015). 

IV. HYDROMETALLURGY

Hydrometallurgy process uses a procedure known as leaching where a solution is used to solubilize the crushed metal waste. It also involves purification (solvent extraction or ion exchange) and metal recovery. Hydrometallurgy has one or more aqueous phase and it has been studied as an independent discipline from the alchemist’s days. Over the years researches have been conducted to determine hydrometallurgical processes with less setup costs which may be suitable for small scale with high metal yields. Hydrometallurgy is cheap since it requires less capital and high metal recovery but it is disadvantaged by the high level of toxic pollutants such as NOx, Cl2 & SO2. Hydrometallurgy is more suitable method of metal extraction as the primary ores needed for metal refining has become lower in grade (Conard, 1992; Salgado et al., 2003; Mooiman et al., 2005; Arens & Chernyak, 2008; Sepulveda, 2010; Jadhav & Hocheng, 2015; Jadhav et al., 2016). Hydrometallurgy involves the use of acid or base for leaching; it may be combined with electrowinning. Examples of these acidic and basic solvents used for hydrometallurgy are hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), thiosulphate (S2O32-), thiourea ((NH2)2CS), sodium cyanide (NaCN). They are known to have several effects on health and environment because the acids solubilize metals and the use of sulfates to remove metals from its solid phase produces harmful sulfate-rich by-product. A large-scale release of this by-product could affect drainage systems. The use of H2SO4, HCl and HNO3 or aqua-regia have been investigated by several researchers as very effective leaching agent for heavy metals but the gases released through this process comprises Cl2, SO2 and NOx which poses threat to the persons involved in the experiment and the environment. Some other hydrometallurgical problems include solid-liquid separation difficulty and the impact of impurities during purification, time it takes to recover high amount of metals as the process is done in lower temperatures (Li et al., 2010, Park et al., 2013, Park et al., 2014).

Some researchers have considered microwave-assisted leaching as a way of improving metal extraction yield and also reducing the process time. Connel & Moe 1966 were the earliest to research the use of microwave energy for heating minerals. Microwave technology is a type of electromagnetic energy which moves through high frequency waves. The microwave energy is a non-contact form of heating also known in the family of electro-heat techniques like infrared heating, radio frequency, direct resistance and induction. It is a very selective heating that is rapid with very efficient energy transfer. It initiates quickly and also terminates quickly. Microwave energy operates within a wavelength of 1 mm to 1 m and a frequency between 300 MHz to 300 GHz. The frequency used the most for the purpose of heating is 915 MHz to 2.45 GHz. In the dissolution stage, metals can be separated if one metal is insoluble in the solvent used. To remove components from the mixture selectively the leaching process usually follows the separation process (Connell & Moe, 1966; Fleming, 1992; Mooiman et al., 2005; Demim et al, 2013a; Demim et al, 2013b; Demim et al, 2014; Leyma et al, 2016; Yang et al., 2016).

A. Solvents for Leaching

Common solvents for recovery of metals as reported by other researchers are HNO3, HCl, H2SO4 and aqua-regia. Other solvents which are reported in the literature for the extraction of different metals consist of ionic liquids (ILs), cyanide (CN), thiosulfates, thiourea, and Ethylenediaminetetraacetic acid (EDTA). The rate of leaching can be affected by adjusting factors like temperature, stirring intensity, granularity, solid to solution ratio, acid-adding method and acid concentration. A typical leaching setup is shown in Figure 2.3. Recycling cost would benefit from reducing the sizes of the PCB as this was reported in the research by Dalrymple, 2007. (Dalrymple, 2007; Oishi et al 2007; Dhawan et al., 2009; Jadhav & Hocheng, 2015). Acid, base and salts may be combined in varying proportions to enhance the leaching efficiency of various types of leaching agents. Consequently, the addition of reducing agents such as SO2 & Fe2+ and oxidizing agents like HClO, H2O2, NaClO & Cl2 can also be used to enhance leaching process. One of the most widely used oxidizing agents is H2O2 because of its good properties; it is relatively versatile, powerful and safe. It is known to be stronger than potassium permanganate, chlorine and chlorine dioxide (Oh et al., 2003; Havlik et al, 2005; Oustadakis et al., 2010; Shawabkeh, 2010; Jadhav & Hocheng, 2015; Wu et al., 2015).

???????B. Traditional Solvents - Mineral Acids

Mineral acids are formed from inorganic compounds and when dissolved in water they form conjugate base and hydrogen ion. Dhawan et al., 2009’s report suggested that HNO3 is more effective than HCl and H2SO4 in leaching metals from PCB. Oishi et al 2007 reported that a leach liquor of 45 – 50 g/L was obtained when ammonium sulphate and chloride solutions with oxidizing agent (copper ammine complexes) were used to leach PCB. Almost 100 % of zinc and copper were dissolved when a combination of 2 mol/L H2SO4 and 0.2 mol/L H2O2 were used to leach PCB at 85 0C for 8 hours. Although Habbache et al, 2009 reported nitrate, chloride and sulfate ions as really aggressive for metal recovery but these inorganic acids are disadvantaged as they release toxic gases (NOx, Cl2 & SO2) and further purification steps are required. Metal recovery through this process is non-selective as the metals are extracted as mixed metal solutions and a further purification step is required to separate individual metals. When PCBs were shredded to less than 1mm, 95 % aluminium, iron and nickel were leached within 12 hours. Ground PCBs have also been researched by Veit et al., 2005, Wu et al., 2008. Other researchers have also combined complexants and oxidants to study the dissolution of copper (Oh et al, 2003; Du, 2004; Veit et al., 2005; Dalrymple, 2007; Oishi et al 2007; Wu et al., 2008; Dhawan et al., 2009; Kumar et al., 2014; Jadhav & Hocheng, 2015; Wang et al. 2016).

C.  Traditional Solvents - Organic Acids

Organic acids are compounds composed mainly of carbon and hydrogen atoms. Examples of organic acids used for leaching are citric acid, malic, tartaric acid, and EDTA. Researchers have studied the dissolution behaviour of copper in citric acid using H2O2 as oxidizer. Also, glycine as complexing agent and H2O2 as oxidizing agent has been used for copper dissolution; copper dissolution was also studied using oxalic acid and H2O2 by Pandija et al. 2007. Organic acids are considered environmentally friendly and are attracting attention lately since they are biodegradable. The recovery of Pb and PbO from scrap battery paste has been studied by Sonmez & Kumar, 2009 using citric acid monohydrate; 4 M of citric acid monohydrate with H2O2 was sufficient to leach PbO2. Citric acid is a natural acid, biodegradable and can dissolve in water with ease; it degrades in both aerobic and anaerobic conditions as it has also been used for the recovery of metals from sewage sludge. Jadhav et al., 2016 research showed that citric acid and acetic acid were used to leach large PCB for 96 hours and 364 hours but poor metal slubilization was observed as only 9.89 % and 19.57 % of copper respectively was recovered. However, Li et al., 2010 research recovered 100 % Li and 90 % Co using 1.25 M citric acid and 1 % vol H2O2  at 90 0C for 30 min and a solid to liquid ratio of 20 g/L (Seal et al., 2003; Chen et al., 2004; Chen & Tsai, 2004; Du et al., 2004; Pandija et al. 2007; Sonmez & Kumar, 2009; Li et al., 2010; Jadhav & Hocheng, 2015; Wang et al. 2016).

Biswas & Mulaba-Bafubiandi, 2016 leached oxidized ores using citric acid, oxalic and gluconic acid, 83.6 % Cu and 77.2 % Co were obtained with 0.15 M citric acid at 80 0C. Other organic acids that have been studied with H2O2 as reducing agent are oxalic acid by Sun & Qui, 2012 and malic acid. Ascorbic acid has been used as reducing agent along with citric acid and in the research sintered metal oxides (Cr-substituted hematites) were dissolved. Table 4.1 shows the best leaching conditions of different metals in organic acids (Manjanna et al., 2001; Li et al., 2012; Li et al., 2013; Sun & Qui, 2012; Biswas & Mulaba-Bafubiandi, 2016).

Table 4.1 Best leaching conditions of metals in different organic acids

Adapted from: (Li et al., 2012; Sun and Qiu 2012; Li et al., 2013; Li et al., 2015; Nayaka et al., 2015; Nayaka et al., 2016)

???????D. Designer Solvents - Ionic Liquids (ILs)

Ionic liquid (IL) is a molten salt made of cations and anions with melting points below 100 0C.  ILs with below 25 0C is regarded as a room-temperature ionic liquid (RTIL). ILs have non-flammable property and a negligible low vapour pressure. They also have properties that can be tuned to suit specific purpose or design. ILs also have distribution coefficients that is enhanced. Ionic liquids have specific properties that make them unique and some of these properties are miscibility with water and organic solvents, high solubility which means they only need small reactor volumes; they have good extractability for metal ions and organic compounds, good thermal stability up to 200 0C, large spectra transparency and good solvating properties; they are easy to prepare and relatively cheap. Other properties of ILs include exhibition of Franklin acidity, Bronsted Lewis and super-acidity. ILs are very good solvents as they do not dissolve glass or polythene and are good for organic, inorganic and polymeric materials. A researcher like Kenneth R Seddon earlier predicted that IL has the potentials to revolutionize activities where liquids are used. It is a task-specific method which is faster than using traditional solvents and the process does not release toxic gases (Seddon, K.R., 1997; Suarez et al, 1998; Welton,1999; Visser et al, 2001a; Berthod et al, 2008; Domanska & Rekawek, 2009; Leyma et al, 2016).

The most common ionic liquids used for the separation of metal ions is 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6], 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6], or 1-hexyl-3-methylimidazolium hexafluorophosphate, [HMIM][PF6] or other ILs with hexafluorophosphate, PF−6 , or bis[(trifluoromethyl)sulfonyl]imide, NTf−2, anions. Ionic liquids have a character that is hydrophobic and this enables them to extract compounds that are also hydrophilic in biphase separation. However, in a system that is biphasic the extractant would remain in the hydrophobic phase to enable the removal of the metal ion from the aqueous phase. There is a challenge of determining new classes of solvents for traditional separation with the ability to partition the solvent phase quantitatively and can also complex target metal ions readily or even identifying suitable conditions for which specific metal ion species could be extracted selectively from aqueous streams which contains complexing ions that are inorganic (Seddon, K.R., 1997; Visser et al, 2001a; Domanska & Rekawek, 2009).

The combination of IL extractants have yielded improved results than molecular organic solvents in metal ion extraction. Due to the unique physical and chemical properties of ionic liquids they have been used as alternatives for solvents in various applications. ILs are also used particularly in clean technology in place of conventional solvents. The most common formulation of ILs are the imidazolium or pyridilium cations which bears a simple appendage of the alkyl cation group. The physical properties of these ILs have been achieved by altering length of the alkyl group in the ring. This allows modifying the melting points, viscosity and hydrophobicity (Gordon et al., 1998; Visser et al., 2001a; Visser et al., 2001b; Domanska & Rekawek, 2009).

Other aspects of ILs formulation include appendage of side chains like ions and heterocyclic aromatic molecules with complex functions and structures. However, some researches show that the use of ILs is faced with the challenge of recovering the extracted metal ions and solubilization losses. ILs can be prepared by combining bulky organic cations like 1-butyl-3-methylimidazolium or 1-butylpyridinium with a wide variety of anions with different types of anions. The variation in the anions and cations of ILs are used to control the properties of ILs. Altering the anion of ILs has been well investigated and the effect has been understood. An example is the preparation of salts based on aluminium (III) chloride which are basic, acidic and very water sensitive while those anions based like triflate and hexafluorophosphate are highly hydrophobic and neutral. However, chemical modification effect of cations in ILs has been less studied and not well understood. An increase in cation chain length is expected to alter the melting point as well as the hydrophobicity and viscosity of the liquids. Liquid crystalline phase on melting point could be formed as a further effect. 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6 ] has found an increasing use as it is easy to produce and almost insoluble in water (Gordon et al., 1998; Visser et al., 2001a; Visser et al., 2001b; Domanska & Rekawek, 2009).

Table 4.2 and 4.3 shows the generic structure of common ILs and some selected properties. The choice of anion and the length of the alkyl chain in the cation determines the properties of alkylimidazolium ILs. The anion chosen determines the water miscibility of the IL which in turn affects the property of the liquid. Increasing the length of the cation chain from butyl to hexyl or butyl to octyl increases the viscosity and hydrophobicity of the ILs while the surface tension and density is decreased (Stepnowski, 2006).

Table 4.2 Common ionic liquids

Where Rx: -CH3 to -C9H19;  X-: AlCl4-, SbF6-, CF3SO3-, PF6-, (CF3SO2)2N-

Source: (Stepnowski, 2006)

Table 4.3 Selected properties of 1-butyl- and 1-hexyl-3-methylimidazolium chlorides, tetrafluoroborates and hexafluorophosphates

Where S: Soluble, NS: non-soluble, PS: Partly soluble

Source: (Stepnowski, 2006)

Imidazolium cations commonly used for the preparation of ILs can be made to possess functions that is task-specific. An example is metal legating group which could enhance the partitioning of target metal ions from water into the ILs phase. When imidazolium cations are used in less expensive ILs it also improves the partitioning of target metals. Table 4.4 shows some physicochemical properties of selected ionic liquids (Gordon et al., 1998; Visser et al., 2001a; Visser et al., 2001b; Domanska & Rekawek, 2009). Due to the unique properties like negative vapour pressure and flame resistance possessed by ionic liquids they have been highlighted as green solvents which are also alternative to conventional solvents. By combining the ion pairs of ionic liquids their physicochemical properties can be readily tunable. Also, the development of extractant compatibility with hydrophobic ILs have been developed by researchers. However, difficulty has been encountered in discovering a commercial extractant that would be soluble in ILs or in stripping metals ions from extracting ILs phase. Typical examples are the organophosphorus extractants like the di(2-ehtylhexyl) phosphoric acid (D2EHPA) and PC-88A (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) are used as extractant which separates and purifies rare earth metals since they exhibit high extraction and selectivity for rare earth metals but the solubility of these extractants in ionic liquids are still poor. However, N, N-dioctyldiglycol amic acid (DODGAA) is a phosphorus-free green extractant which was developed recently. This new extractant works well as a carrier for an IL-based membrane or as an extractant of rare earth metal in ILs. Researchers like Yang et al., 2012 previously confirmed that DODGAA should be suitable for extracting rare earth metals using imidazolium based IL (Freemantle, 1998; Renner, 2001; Binnemans, 2005; Sun et al., 2012; Baba et al., 2011; Naganawa, et al., 2007; Shimojo et al., 2007; Kubota, et al., 2010; Kubota et al., 2011; Yang et al., 2012; Yang et al, 2013). 

Alternative leaching agents which do not release toxic substances are gaining more attention. Ionic liquids (ILs) and organic solvents are some of the environment-friendly solvents useful for hydrometallurgy. There are a good number of literatures on the study of aprotic ILs but there are limited literatures on the ionicity of protic ILs. Rogers and Coworkers used 1-butyl-3-methylimidazolium hexafluorophosphate [C4mim][PF6] as extractant for Hg2+ and Cd2+ (Wei et al., 2003; Li et al., 2010, Park et al., 2013, Park et al., 2014).

Table 4.4 Physicochemical properties of selected ionic liquids

Where S: soluble, N: non-soluble, Tfo: triflate anion or trifluoromethyl sulfate

Adapted from: (Welton,1999; Liu et al, 2005; Stepnowski, 2006; Berthod et al, 2008)

V. ACKNOWLEDGEMENTS

Special thanks to the sponsor of this work, Petroleum Development Trust Fund (PTDF) Nigeria

Conclusion

This paper summarized the extraction of metals from waste electrical and electronic equipment (WEEE) using traditional and designer solvents. About 54 million metric tons of WEEE are generated globally every year and the metals present in these WEEE are of high value and could be recycled instead of sending them to landfill. The initial recycling steps involve pre-treatment of WEEE such as breaking and crushing of the PCBs. Leaching is then carried out using suitable solvents. The hotplate and microwave methods could be used. The solvents most suitable to effectively extract metals are the inorganic acids but as they release toxic gases, they are not environmentally friendly. Leaching is improved with the addition of an oxidizing agent such as H2O2 which is safe. Organic acids are biodegradable and do not release toxic gases hence they are environmentally friendly. Leaching is also improved with the addition of an oxidizing agent such as H2O2 which is safe. Ionic liquids have good extraction ability for metal ions and they do not release toxic gases. Ionic liquids can be tuned to suit meet specific needs which makes it task specific. Combination of IL extractants have yielded improved results than organic acids. More research is needed the use of ionic liquids as they do not release toxic gases and they have shown promising features in extracting metals.

References

[1] Adediran Y.A. & Abdulkarim A. 2012. Challenges of electronic wastes management in Nigeria. International Journal of Advances of Engineering and Technology. 4(1): 641 – 648. [2] Akbari, S. & Ahmadi A. 2019. Recovery of copper from a mixture of printed circuit boards (PCBs) and sulphidic tailings using bioleaching and solvent extraction processes. Chemical Engineering & Processing: Process Intensification. 142: 107584. [3] Arens V.Z. & Chernyak S.A., 2008. Hydrometallurgy in the mining industry. Metallurgist. 52: 3–10. [4] Baba Y., Kubota F., Kamiya N. & Goto M., 2011. Recent advances in extraction and separation of rare earth metals using ionic liquids. Journal of Chemical Engineering. 44(10): 679–685. [5] Berthod A., Angel M.J.R. & Broch S.C. 2008. Ionic liquids in separation techniques. Journal of Chromatography A, 1184: 6–18. [6] Binnemans, K., 2005. Ionic liquid crystals. Chemical Review. 105: 4148–4204. [7] Biswas S. & Mulaba-Bafubiandi A.F., 2016. Extraction of Copper and Cobalt from Oxidized Ore using Organic Acids. Hydrometallurgy Conference 2016: Sustainable Hydrometallurgical Extraction of Metal, Cape Town, Southern African Institute of Mining and Metallurgy. [8] Canal M.A., Cabrera J.-M. & Malfatti C.D.F., 2013. Printed circuit boards: a review on the perspective of sustainability. Journal of Environmental Management. 131: 298–306. [9] Chen J.C., Lin S.R. & Tsai W.T., 2004. Effects of oxidizing agent and hydrodynamic condition on copper dissolution in chemical mechanical polishing electrolytes. Applied Surface Science. 233: 80-90. [10] Chen J.C. & Tsai W.T., 2004. Effects of hydrogen peroxide and alumina on surface characteristics of copper chemical-mechanical polishing in citric acid slurries. Materials Chemistry & Physics. 87: 387-393. [11] Conard B.R., 1992. The role of hydrometallurgy in achieving sustainable development. Hydrometallurgy. 30: 1–28. [12] Connell, L.H. & Moe, L.A., 1966. Apparatus for treatment of ore, USPatent No. 3261959. [13] Dalrymple I., Wright N., Bains N., Geraghty K., Goosey M. & Lightfoot L., 2007. An integrated approach to electronic waste (WEEE) recycling. Circuit World. 33: 52–58. [14] Demim S., Drouiche N., Aouabed A., Benayad T., Dendene-Badache O., Semsari S., 2013a. Cadmium & nickel: Assessment of the Physiological Effects and Heavy Metal Removal using a Response Surface approach by L. Gibba. Ecological Engineering. 61: 426–435. [15] Demim S., Drouiche N., Aouabed A. & Semsari S., 2013b. CCD Study on The Ecophysiological Effects of Heavy Metals on Lemna Gibba. Ecological Engineering. 57: 302–313. [16] Demim S., Drouiche N., Aouabed A., Benayad T., Couderchet M. & Semsari S., 2014. Study of Heavy Metal Removal from Heavy Metal Mixture Using the Ccd Method. Journal of Industrial & Engineering Chemistry. 20: 512–520. [17] Dhawan N., Wadhwa M., Kumar V., Kumar M., 2009. Recovery of Metals from Electronic Scrap by Hydrometallurgical. Route, In: Proc. Annual Meeting & Exhibition, TMS 2009, Stanley M.H. (Ed.), EPD Congress, San Francisco, California, USA. 1107–1109. [18] Domanska U. & Rekawek A. 2009. Extraction of Metal Ions from Aqueous Solutions Using Imidazolium Based Ionic Liquids. Journal of Solution Chemistry. 38: 739–751. [19] Du T., Vijayakumar A. & Desai V., 2004. Effect of hydrogen peroxide on oxidation of copper in CMP slurries containing glycine and Cu ions. Electrochimica Acta; 49: 4505-4512. [20] Eswaraiah C., Kavitha T., Vidyasagar S. & Narayanan S.S., 2008. Classification of metals and plastics from printed circuit boards (PCB) using air classifier. Chemical Engineering and Processing: Process Intensification. 47: 565–576. [21] EU, 2003. Directive 2002/96/EC of the European Parliament and of the council of 27 January 2003 on waste electrical and electronic equipment (WEEE). Official Journal of the European Union. 37: 24 – 38. [22] Fleming, C.A., 1992. Hydrometallurgy of precious metals recovery. Hydrometallurgy. 30:127 – 162. [23] Freemantle M., 1998. Ionic liquids show promise for clean separation technology. Chemical Engineering News. 76: 12. [24] Ghosh B., Ghosh M.K., Parhi P., Mukherjee P.S. & Mishra B.K., 2015. Waste Printed Circuit Boards recycling: an extensive assessment of current status. Journal of Cleaner Production. 94: 5 -19. [25] Gordon C. M., Holbrey J.D., Kennedy A. R. & Seddon K. R., 1998. Ionic liquid crystals: hexafluorophosphate salts. Journal of Materials Chemistry. 8: 2627–2636. [26] Grossman E., 2007. High Tech Trash: Digital Devices, Hidden Toxics, and Human Health. [27] Habashi F., 1999. A Textbook of hydrometallurgy, 2nd edition. Métallurgie Extractive Québec. [28] Habbache N., Alane N., Djerad S. & Tifouti L., 2009. Leaching of copper oxide with different acid solutions. Chemical Engineering Journal. 152: 503–508 [29] Hadi P., Xu M., Lin C.S.K., Hui C.-W. & McKay G., 2015. Waste printed circuit board recycling techniques and product utilization. Journal of Hazardous Materials. 283: 234 - 243. [30] Haqshenas M.A., 2018. Faced with e-waste. Tehran Times. Available at: https://www.tehrantimes.com/news/421667/Faced-with-e-waste (Accessed on 15th July 2020) [31] He Y. & Xu Z., 2014. The status and development of treatment techniques of typical waste electrical and electronic equipment in China: a review. Waste Management Resources. 32: 254 - 269. [32] Huang K., Guo J. & Xu Z., 2009. Recycling of waste printed circuit boards: a review of current technologies and treatment status in China. Journal of Hazardous Materials. 164: 399-408. [33] Havlik T., Orac D., Berwanger M. & Anja M., 2014. The effect of mechanical–physical pretreatment on hydrometallurgical extraction of copper and tin in residue from printed circuit boards from used consumer equipment. Minerals Engineering 65: 163–171. [34] Jadhav U. & Hocheng H., 2015. Hydrometallurgical Recovery of Metals from Large Printed Circuit Board Pieces. Scientific Reports. 5: 14574. [35] Jadhav U., Su C. & Hocheng H., 2016. Leaching of metals from large pieces of printed circuit boards using citric acid and hydrogen peroxide. Environmental Science Pollution Research. 23: 24384–24392. [36] Kiddee, P., Naidu, R. & Wong, M.H. 2013. Electronic waste management approaches: an overview. Waste Management. 33: 1237-1250. [37] Kubota, F., Shimobori, Y., Baba, Y., Koyanagi, Y. Shimojo, K., Kamiya, N., Goto, M., 2011. Application of ionic liquids to extraction separation of rare earth metals with an effective diglycol amic acid extractant. Journal of Chemical Engineering. 44: 307–312. [38] Kubota, F., Shimobori, Y., Koyanagi, Y., Shimojo, K., Kamiya, N., Goto, M., 2010. Uphill transport of rare earth metals through highly stable supported liquid membrane based on ionic liquid. Analytical Sciences. 26: 289–290. [39] Kumar M., Lee J-C., Kim M.S., Jeong J. & Yoo, K., 2014. Leaching of metals from waste printed circuit Boards (WPCBs) using sulfuric and nitric acids. Environmental Engineering and Management Journal. 13(10): 2601-2607. [40] Leyma R., Platzer S., Jirsa F., Kandiolla W., Krachler R. & Keppler B.K. 2016. Novel thiosalicylate-based ionic liquids for heavy metal extractions. Journal of Hazardous Materials. 314: 164 – 171. [41] Letsrecycle, 2020. UK the ‘second largest producer’ of WEEE. Available at: https://www.letsrecycle.com/news/latest-news/uk-the-second-largest-producer-of-weee/ (Accessed: 2nd April 2021). [42] Li L., Dunn J.B., Zhang X.X., Gaines L., Chen R.J., Wu F. & Amine K., 2013. Recovery of metals from spent lithium-ion batteries with organic acids as leaching reagents and environmental assessment. Journal of Power Sources. 233: 180–189. [43] Li L., Ge J., Wu F., Chen R., Chen S. & Wu B., 2010. Recovery of Cobalt and Lithium from spent lithium ion batteries using organic citric acid as leachant. Journal of Hazardous Materials. 176: 288 – 293. [44] Li L., Lu J., Ren Y., Zhang X.X., Chen R.J., Wu F. & Amine K., 2012. Ascorbic-acid-assisted recovery of cobalt and lithium from spent Li-ion batteries. Journal of Power Sources. 218: 21–27. [45] Liu J., Jonsson J.A. & Jiang G., 2005. Application of Ionic Liquids in Analytical Chemistry. Trends in Analytical Chemistry. 24 (1). [46] Manjanna J., Venkateswaran G., Sherigara B.S. & Nayak P.V., 2001. Dissolution studies of chromium substituted iron oxides in reductive-complexing agent mixtures. Hydrometallurgy. 60: 155–165. [47] Mooiman M.B., Sole K.C., Kinneberg D.J., 2005. Challenging the traditional hydrometallurgy curriculum—an industry perspective. Hydrometallurgy. 79: 80–88. [48] Naganawa H., Shimojo K., Mitamura H., Sugo Y., Noro J. & Goto M., 2007. A new Green Extractant of the diglycol amic acid type for lanthanides. Solvent Extraction Resource Development. 14: 151–160. [49] Nayaka G.P., Manjanna J., Pai K.V., Vadavi R., Keny J. & Tripathi V.S., 2015. Recovery of valuable metal ions from the spent lithium-ion battery using aqueous mixture of mild organic acids as alternative to mineral acids. Hydrometallurgy. 151: 73–77. [50] Nayaka G.P., Pai K.V., Manjanna J. & Keny S.J 2016. Use of mild organic acid reagents to recover the Co and Li from spent Li-ion batteries. Waste Management. 51: 234–238 [51] Ogunseitan O.A., 2013. The Basel Convention and e-waste: translation of scientific uncertainty to protective policy. Lancet Global Health 1: 313-314. [52] Oh C.J., Lee S.O., Yang H.S., Ha T.J. & Kim M.J., 2003. Selective Leaching of Valuable Metals from Waste Printed Circuit Boards. Journal of the Air & Waste Management Association. 53(7): 897-902. [53] Oishi T., Koyama K., Alam S., Tanaka M. & Lee J-C, 2007. Recovery of high purity copper cathode from printed circuit boards using ammoniacal sulfate or chloride solutions. Hydrometallurgy. 89: 82–88. [54] Oustadakis P., Tsakiridis P.E., Katsiapi A. & Agatzini-Leonardou S., 2010. Hydrometallurgical process for zinc recovery from electric arc furnace dust (EAFD). Part I: Characterization and leaching by diluted sulphuric acid. Journal of Hazardous Materials. 179: 1–7. [55] Pandija S., Roy D. & Babu SV., 2007. Chemical mechanical planarization of copper using abrasive-free solutions of oxalic acid and hydrogen peroxide. Materials Chemistry & Physics. 102: 144-151. [56] Park H., Jung K., Alorro R.D. & Yoo K., 2013. Leaching Behavior of Copper, Zinc and Lead from Contaminated Soil with Citric Acid. Materials Transactions. 54(7): 1220 – 1223. [57] Park J., Jung Y., Kusumah P., Lee J., Kwon K. & Lee C.K. 2014. Application of Ionic Liquids in Hydrometallurgy. International Journal of Molecular Sciences. 15: 15320-15343. [58] Renner R., 2001. Ionic liquids: An industrial clean up solution. Environmental Science Technology. 35: 410A–413A. [59] Salgado A.L., Veloso A.M.O., Pereira D.D., Gontijo G.S., Salum A. & Mansur M.B., 2003. Recovery of zinc and manganese from spent alkaline batteries by liquid–liquid extraction with Cyanex 272. Journal of Power Sources. 115: 367–373. [60] Seal S., Kuiry S.C. & Heinmen B., 2003. Effect of glycine and hydrogen peroxide on chemical-mechanical planarization of copper. Thin Solid Films. 423: 243-251. [61] Seddon K.R., 1997. Ionic Liquids for Clean Technology- Review. Journal of Chemical Technology & Biotechnology. 68: 351 – 356. [62] Sepulveda A., Schluep M., Renaud F.G., 2010. A review of the environmental fate and effects of hazardous substances released from electrical and electronic equipments during recycling: examples from China and India. Environmental Impact Assessment Review. 30(1): 28–41. [63] Shawabkeh RA., 2010. Hydrometallurgical extraction of zinc from Jordanian electric arc furnace dust. Hydrometallurgy. 104:61–65. [64] Shimojo K., Naganawa H., Noro J., Kubota F. & Goto M., 2007. Extraction behavior and separation of lanthanides with a diglycol amic acid derivative and a nitrogendonor ligand. Analytical Sciences. 23: 1427–1430. [65] Silvas F.P.C., Correa M.M. J., Caldas M.P.K., de Moraes V.T., Espinosa D.C.R. & Tenório J.A.S., 2015. Printed circuit board recycling: Physical processing and copper extraction by selective leaching. Waste Management. 46: 503–510 [66] Sonmez M.S. & Kumar R.V., 2009. Leaching of waste battery paste components. Part 1: Lead citrate synthesis from PbO and PbO2. Hydrometallurgy. 95 53–60. [67] Spalvins E., Dubey B. & Townsend T., 2008. Impact of electronic waste disposal on lead concentrations in landfill leachate. Environmental Science & Technology. 42: 7452–7458. [68] Statista, 2021. Electronic waste generated worldwide from 2010 to 2019 (in million metric tons). Available at: https://www.statista.com/statistics/499891/projectionewaste-generationworldwide/#:~:text=Since%202010%2C%20the%20volume%20of,be%20collected%20and%20properly%20recycled (Accessed: 2nd April 2021). [69] Stepnowski P., 2006. Review: Application of Chromatographic and Electrophoretic Methods for the Analysis of Imidazolium and Pyridinium Cations as Used in Ionic Liquids. International Journal of Molecular Science. 7: 497-509. [70] Suarez P.A.Z., Einloft S., Dullius J.E.L., Souza R.F. & Dupont J., 1998. Synthesis and physical-chemical properties of ionic liquids based on l -n-butyl-3-methylimidazolium cation. Journal de Chimie Physique. 95: 1626 – 1639. [71] Sun X., Luo S. & Dai S., 2012. Ionic liquids-based extraction: a promising strategy for the advanced nuclear fuel cycle. Chemical Review. 112: 2100–2128. [72] Sun L. & Qiu K., 2012. Organic oxalate as leachant and precipitant for the recovery of valuable metals from spent lithium-ion batteries. Waste Management. 32: 1575–1582. [73] Sum E.Y.L., 1991. The recovery of metals from electronic scrap. Journal of the Minerals, Metals and Materials Society. 43: 53–61. [74] Tribuneonlineng, 2018. Open Burning Of E-Waste: Recyclers, Residents At Risk Of Cancer, Study Shows. Nigerian Tribune. Available at: https://tribuneonlineng.com/open-burning-e-waste-recyclers-residents-risk-cancer-study-shows/ (Accessed on 15th July 2020) [75] Tuncuk A., Stazi V., Akcil A., Yazici E.Y. & Deveci H., 2012. Aqueous metal recovery techniques from e-scrap: Hydrometallurgy in recycling. Minerals Engineering. 25: 28 – 37. [76] Visser A.E., Swatlowski R.P., Griffin S.T., Hartman D.H. & Rogers R.D., 2001a. Liquid/liquid extraction of metal ions in room temperature ionic liquids. Separation Science & Technology. 36: 785–804. [77] Visser A.E., Swatlowski R.P., Reichert W.M., Mayton R., Sheff S., Wierzbicki A., Davis J.H. Jr. & Rogers R.D. 2001b. Task-specific ionic liquids for the extraction of metal ions aqueous solutions. Chemical Communications. 135–136. [78] Vehlow J., Bergfeldt B., Jay K., Seifert H., Wanke T. & Mark F.E., 2000. Thermal treatment of electrical and electronic waste plastics. Waste Management Resources. 18: 131–140. [79] Veit H. M., Diehl T.R., Salami A.P., Rodrigues J.S., Bernardes A. M. & Tenorio J. A. S., 2005. Utilisation of magnetic and electrostatic separation in the recycling of printed circuit boards scrap. Waste Management. 25: 67–74. [80] Wang J. & Xu Z., 2015. Disposing and recycling waste printed circuit boards: disconnecting, resource recovery, and pollution control. Environmental Science & Technology. 49: 721-733. [81] Wang Z., Guo S. & Ye C., 2016. Leaching of copper from metal powders mechanically separated from waste printed circuit boards in chloride media using hydrogen peroxide as oxidant. The Tenth International Conference on Waste Management and Technology (ICWMT). Procedia Environmental Sciences. 31: 917 – 924. [82] Welton T.,1999. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chemical Review. 99 (8): 2071–2084. [83] Wei G., Yang Z. & Chen C. 2003. Room temperature ionic liquid as a novel medium for liquid/liquid extraction of metal ions. Analytical Chimica Acta. 488: 183 – 192. [84] Wu J., Li J. & Xu Z., 2008. Electrostatic separation for multisize granule of crushed printed circuit board waste using two-roll separator. Journal of Hazardous Materials. 159: 230–234. [85] Wu J-Y., Chang F., Wang H.P., Tsai M.J., Ko C. & Chen C., 2015. Selective leaching process for the recovery of copper and zinc oxide from copper-containing dust. Environmental Technology. 36(23): 2952-2958. [86] Yang F., Baba Y., Kubota F., Kamiya N. & Goto M., 2012. Extraction and separation of rare earth metal ions with DODGAA in ionic liquids. Solvent Extraction Resource Development Japan. 19: 69–76. [87] Yang F., Kubota F., Baba Y., Kamiya N. & Goto M., 2013. Selective extraction and recovery of rare earth metals from phosphor powders in waste fluorescent lamps using an ionic liquid system. Journal of Hazardous Materials. 254– 255: 79– 88. [88] Yang K., Li S., Zhang L., Peng G., Chen W., Xie F. & Ma A., 2016. Microwave roasting and leaching of an oxide-sulphide zinc ore. Hydrometallurgy. 166: 243 -251. [89] Yordanov A.T. & Roundhill D.M., 1998. \"Solution extraction of transition and post-transition heavy and precious metals by chelate and macrocyclic ligands\". Coordination Chemistry Reviews. 170(1): 93-124. [90] Zhang Y., Liu S., Xie H., Zeng X. & Li J., 2012. Current status on leaching precious metals from waste printed circuit boards. Procedia Environmental Sciences. 16: 560 – 568. [91] Zhang L. & Xu Z., 2016. A review of current progress of recycling technologies for metals from waste electrical and electronic equipment. Journal of Cleaner Production. 127: 19 – 36.

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