Introduction
Waste will become the major resource in the future circular economy. In particular, E-waste is a major sector growing at an annual rate of about 2 million tonnes (Mt) with rising users of electrical and electronic items worldwide. This is a consequence of the versatility and affordability of technological innovation, thus resulting in massive sales and e-waste increases. Most end-users lack knowledge on proper recycling or reuse, often disposing of e-waste as domestic waste. Such improper disposals are threatening life and ecosystems because e-waste is rich in toxic metals and other pollutants.
Here we review e-waste generation, policies, and recycling methods. In 2019, the world e-waste production reached 53.6 Mt, including 24.9 Mt in Asia, 13.1 Mt in the USA, 12 Mt in Europe. In Asia, China (10.1 Mt), India (3.23 Mt), Japan (2.57 Mt), and Indonesia (1.62 Mt) are the largest producers contributing to about 70{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of the total world e-waste generated. Only 17.4{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} (9.3 Mt) of the world’s e-waste was recycled by formal means, and the remaining 82.6{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} (44.3 Mt) was left untreated or processed informally. As a consequence, most countries have framed policies to provide regulatory guidelines to producers, end-users, and recyclers. Yet the efficiency of these local policies is limited by the transfer of products across borders in a globalized world.
In recent decades, the electrical and electronic firms all over the countries have transformed the globe in such a way that these products are inevitable in everyone’s routine life. The electronic appliances include many domestic utilities such as refrigerators, television, washing machinery, smartphones, laptops, copiers, and iPad. As the applications of such items get increased among the inhabitants, the generation of their associated waste is unavoidable. E-waste refers to any discarded or non-functional electronic items which are no longer useful. About 42{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of the e-waste is from household appliances, 34{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of communication devices, 14{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of electronic items, and the rest 10{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} from accessories.
The global e-waste monitor 2020 reported that 53.6 million tonnes (Mt) of e-waste were generated in 2019 and it is likely to hit 74 Mt by 2030. Asia tops the list with the maximum waste generation of 24.9 Mt, followed by America (13.1 Mt) and Europe (12 Mt). In the Asian region, China (10.1 Mt), India (3.23 Mt), Japan (2.57 Mt), and Indonesia (1.62 Mt) are the largest producers of e-waste contributing to about 70.36{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of the total e-waste generation in 2019.
This enormous volume increase poses a great threat to the local community as it encompasses toxic metallic and non-metallic components that are capable of producing negative impacts on human health. The adverse effects include deoxyribonucleic acid damage resulting in mutation, cardiovascular problems; dermal disorders; cancer; hearing issues; neurological disorders, and adverse learning and respiratory effects. Hence, the aggravation of e-waste is the prime concern and their effective management is crucial to ensure human health and to attain the sustainability of the ecosystem.
An optimistic view of this huge e-waste is to consider them as additional resources of metals and so can be called non-natural ores. Given this fact, the Tokyo 2020 medal project was initiated in Japan and 78,985 tonnes of e-waste was collected across the country. An approximate amount of 32 kg of gold, 3500 kg of silver, and 2200 kg of bronze was recovered in this project to produce the Olympic and Paralympic medals.
Different classes of e-waste according to European Union classification
Of the total e-waste generated in 2019 (53.6 Mt), about 77{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of the waste (41.3 Mt) were from the classes of small equipment (17.4 Mt), large equipment (13.1 Mt) and heat exchanging devices (10.3 Mt). Technological advances, rivalry in the electronics market, reduced lifespan, availability of cheaper versions and accessibility to wide range of similar products are the prime reasons for these huge accumulations of e-waste across the globe. Mt—million tonnes.
It can be seen that the class of small equipment contributed a maximum of 17.4 Mt followed by large equipment (13.1 Mt) and heat exchange equipment (10.8 Mt). The prime reasons are the rivalry in the electronics market, scientific uplifts, diminished lifespan and cheaper market for electrical and electronic devices. With the rapid expansion of technology, new-fangled and versatile items in these classes resulting in the accretion of e-waste.
The beneficiary aspect of such a huge volume of e-waste is that these wastes are the secondary resources of precious metals and elements. In general, the e-waste comprises about 50{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of Fe and steel, 20{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of plastic materials. About 13{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of non-ferrous elements are the pool of Cu, Al, Au, Pb, Hg, Cd, Pd and Pt. The amount of Au, Cu, Ag, Pd in the e-waste is more than the amount existing in the natural reserves. Furthermore, the existing ore is deteriorating and the supplies from the ores are not sufficient to pace the total requirement. Further, the processing cost from the natural ore is about 10–160 times higher than the recycling process. Given these facts, the market size of e-waste recycling was tremendously increased and was at 41.97 billion US dollars in 2019 and is expected to reach 102.62 billion US dollars in 2027 which is growing at a Compound Annual Growth Rate (CAGR) of 11.9{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} from 2020 to 2027. With these huge market sizes, the e-waste recycling processes are turned into an emergent business across the globe.
Pyro‑metallurgy
Pyro-metallurgy is an energy-intensive process employed in the refining or extraction of non-ferrous metals from metallurgical materials at extreme temperatures. It includes incinerating, smelting, drossing, melting and roasting to extract precious metals, fundamental operations of the pyro-metallurgy and the selections of the operations are highly dependent on the nature and type of e-waste and their requirements on smelting operations. This method can recover the various metals like Cu, Ag, Au, Pd, Ni, Se, Zn, Pb from the different sources of e-waste. However, it is highly challenging in maintaining the operational parameters for the successful recovery of precious metals. For instance, to maintain elevated temperatures, the process must balance the heat and the materials under treatment. To accomplish this, in addition to the exothermic nature of the metallurgical process, fuel combustion must supplement the smelting process in which either air or pure oxygen can be used as fuel. If the combustion process utilizes air as a fuel, 79{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of N2 in the air emits the heat into the surroundings and thus reduces the thermal efficiency.
On the other hand, the use of pure oxygen might improve the efficiency and also reduce the flue gas generation. Also, it is quite complex to predict the mechanistic aspects of the recovery process because the metallurgical reactions attain equilibrium at a rapid rate and hence change their chemical and phase composition of the e-waste. During this pyrometallurgical process, the formation of slag, soot and flue gases are unavoidable which are composed of high-temperature dust, smoke, and toxic gases. The toxic gases include dibenzo-p-dioxin, biphenyl, anthracene, poly-brominated-dibenzofurans (PBDF) and poly-brominated-di-benzodioxins (PBDD) which are capable of affecting human health and the atmosphere. Besides, the slag formation strongly affects the recovery yield of the process as it might hinder the metal extraction process. Owing to these facts, limited numbers of recycling plants were functioning like a Ronnskar smelter in Sweden, Umicore in Belgium, Aurubis in Germany and Noranda in Canada Despite the method has certain limitations, researchers are continuously working on improving the yields with reduced emissions.
Hydro-Metallurgy
Another traditional approach to recover the metal from the e-waste is a hydro-metallurgical process. Low investment and the high recovery rate are the prime advantages and made them a superior technique than the former operation. Similar to the pyro-metallurgical process, it also requires a pre-treatment step in which mechanical shredding is followed by the sorting process so that the metallic fractions can be allowed to expose them into the chemical actions called the leaching process. The leaching is the process in which the solid materials are allowed to react with the chemical reagents namely extractant or lixiviant to extract the desired solute (metals) in the dispersion medium. This solute transfer process can be mediated by the formation of metal complexes with the lixiviants such as halide, thiourea, thiosulfate, cyanide, HCl, H2SO4, and HNO3. These classes of lixiviants dissolve the metallic components and leave them in the solution called leachate.
The leaching process is one of the enrichment operations as the metals from the natural ores or the solid waste can be enriched by the preferential dissolution into the lixiviants and concentrated in the liquid media. With the gravity settlement or by centrifugal effects, the desired metals are precipitated and can be progressed to the metal recovery process. As this is an intermediate process, the maximum dissolution of metals into the chemical extractants is highly required to prevent any loss.
To accomplish this, several design parameters such as particle size, nature of lixiviants, their concentration, operating temperature, lixiviant and sample ratio and leaching time must be considered. Further, the downstream of metals from this suspension can be accomplished with the recovery processes include solvent extraction, electrodeposition, ion-exchanging and adsorption. The selections of one or more methods in the downstream operations are highly dependent on the types of e-waste, metals to be recovered and their level of purity. The prime advantages in the use of hydro-metallurgical methods are high recovery rate, energy inexpensive, requires no extreme temperature, no secondary waste generation and zero-emission of toxic gases. However, this process is also associated with potential limitations that are highly affecting their use on the industrial scale.
A typical hydrometallurgical process comprises three vital phases including pre-treatment, leaching process and downstream recovery. The choice of the extractant or lixiviant is crucial in this process as it preferentially solubilizes the desired metal(s) in the solution. Further, the metal solubilized in the solution was concentrated and selectively removed by the downstream operations such as solvent extraction, electrodeposition, ion-exchanging and adsorption.
Five classes of lixiviants are used in the metal leach out process and the high partitioning behavior towards the metals is one of the key factors in their selection. Hence, the metals in the e-waste are translocated to the solution in the ionized form due to the concentration gradient existing among them. Further these ionized metals can be recovered in the solid form by means of the downstream operations such as solvent extraction, electrodeposition, ion-exchanging and adsorption.
Lixiviants in the hydro-metallurgical recovery of metals
Bio‑Hydrometallurgy
Biohydrometallurgy is a process in which microbes assist the solubilisation of the elements from the solid substances into the solutions which can be recovered later through separation operations. The copper recovery was successfully achieved through bio-hydrometallurgy and hence more emphasis is given in this context. Even from the pre-Roman period, this technique is adopted to recover elements like copper, silver and aluminium from Rio Tinto mines in the south-west part of Spain. But the commercial application was observed only ten years ago in a Thorsis mine. With the less energy-intensive and minimal use of chemicals made this process as the most promising and alternative strategy for the metal recovery process. Further, the industrialists are referring to the technology as clean and green as it meets the sustainable way of processes .It can be operated in two different ways, viz. single stage and double stage; in single-stage bio-hydrometallurgy, both microbes and metals are supplemented together along with the nutritional media. Subsequently, the microbial growth occurs and resulting in the dissolution of metals in the solution takes place. In most cases, the presence of a high concentration of metal ions might induce cellular toxicity and thereby limiting the growth and hence affecting the recovery efficiency.
On the contrary, the two-stage bio-hydrometallurgy processes cultivate microbes in the nutritional growth media until it attains the stationary phase of their growth period. Afterward, the waste material to be recycled is added aseptically to allow for the recovery process. Recently, the researchers are using the cell-free supernatant to extract the metal and in such methods, the cells are harvested once it attains the stationary phase, so that the spent medium contains all the active metabolites required for the separation of metals from e-waste. By doing so, it prevents the direct physical contact between the microbe and toxic metals and thus evades their associated inhibitory effects and hence, the recovery potential is comparatively higher than the former methods.
Acidobacillus thiooxidans, Acidobacillus ferrooxidans and Sulfolobus sp. are the widely used organisms in the leaching process. These groups of bacteria acquire the energy from the oxidation reaction of ferrous ions as well as from the reduction of sulphur containing compounds. Besides, the heterotrophic form of bacteria and fungal species possess the ability to translocate the precious metals from waste. In general, the likely mechanism in such metal transport might be facilitated through acidolysis, metal complexation, redox reaction or using bioaccumulation. The use of microbes to extract the metal from the e-waste. The study employed fungal culture of A. niger, P. simplicissimum and the mixed consortium of A. thiooxidans and A. ferrooxidans to recover the Cu, Zn, Ni and Al. Moreover, the study proposed that the secretion of inorganic and organic acids by the microbes facilitates the transfer across the phase boundary, more than 80{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} recovery process of Ni, Cu and Zn from e-waste using moderate thermophilic organisms, Sulfobacillus thermosulfdooxidans. Likewise, Prof. Lianget (2010) recovered more than 90{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of Cu, Ni, Zn and Pb using the consortium of A. thiooxidans and A.ferrooxidans. Prof Chen developed a column type of bio-hydrometallurgy process to recover the copper from the waste printed circuit boards (WPCBs) using A. ferrooxidans with an incubation period of 28 days and about 94.8{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of Cu was recovered. Prof Nithya (2018) reported the use of A.ferrooxidans cells free supernatant to effectively recover Cu from the 10 g of e-waste at a pH of 1.4 and the recovery was mediated according to the Eqs and fig given below.
Fe 2+ + O2 + 4H+ → Fe3+ + 2H2O
2Fe3+ + Cu → 2Fe2+ + Cu2+
Cu 2+(l) + Fe (s) → Fe2+(l) + Cu(s)
The use of biotechnological principles in metallurgical science might help in the successful recovery of precious metals. The microbial choice, surface area, pulp density, selection of precursor, temperature and pH are the prime factors influencing the efficiency of the biohydrometallurgy process.
Microbial choice
The selection of microbes is a significant factor in determining the efficiency of the bio-hydrometallurgy process. Under alkaline conditions, the cyanogenic group of organisms served best, whereas, in an acidic environment, the autotrophic microbes will be the better option. Also, in some cases, it required the pre-treatment step to provide an optimal ambience for the organism to survive and translocate the metal efficiently from the e-waste. Hence, the microbial selection must be decided upon the micro-environmental conditions provided by e-waste.
Surface area
The mass transfer on the solid–liquid interface determines the efficiency of the bio-hydrometallurgy process. The particle size of the solid waste material determines the contacting surfaces and thus the area for the transit of mass through the phase boundary. Moreover, the reduced particle size results in the increased surface area and thus increases the rate of mass transfer. From the literature, the optimal particle size is ranging from 40 – 200 μm for efficient metal recovery processes. It is evident that the size of e-waste, in turn the surface area, is one of the prominent factors in determining the efficiency of the metal recovery process.
Pulp density
The higher pulp density lowers the microbial growth as it hinders the nutrient dissolution and increases the mass transfer resistances across the phases. On the contrary, the metal recovery at low pulp density is not cost-effective in the commercial perspectives. Hence, an optimal level needs to be identified for the efficient metal translocation process. The maximum recovery of gold with the use of C. violaceum strain at 5 g/L. Likewise, Extracting metals using cyanogenic bacteria with 50–100 g/L and found that the maximum recovery was obtained with the pulp density of 10 g/L. Prof. Marra (2018) reported the highest bio-hydrometallurgy efficiency with 10 g/L using P. putida. Hence, from the results of the reported works, the optimal pulp density of 10 g/L can be utilized for the efficient process.
Selection of Precursor
Precursor molecules could be termed as enhancers and in most cases, glycine served the purpose. Faramarzi stated that the quantity of glycine more than 10 g/L results in retarded microbial growth and thus the extraction efficiency. On the contrary, at a reduced concentration of glycine, the cyanide production gets reduced and so an optimal amount of glycine needs to be investigated.
Temperature
The bacteria employed will be either of the meso or thermophilic group of organisms. Hence, it is imperative to reveal the optimal temperature for the rapid metal translocation process. In general, the ambient temperatures directly affect the intracellular mechanism and most of the studies employing cyanogenic bacteria were operated with a temperature of 30°C showed maximum recovery efficiency. The optimum temperature can be defined as the temperature in which the bacterial metabolism is highly active to carry out their anabolic or catabolic reactions.
pH
The pH is vital for the microbial growth as well as the metal dissociation into the solution. In the neutral or in the near-neutral (pH—8.0), the maximum bacterial growth was observed and in the high alkaline pH, the maintenance of cell viability is quite challenging as a result of intracellular cytotoxic effects.
Two-stage bio-hydrometallurgical processes were employed in the removal of Cu from the e-waste. PCBs were pre-processed and the bio-leaching was performed using the cell-free extract of A. ferrooxidans. The ferric containing extract was used to translocate Cu from the PCB to its ionized form in the solution. Further, an appropriate amount of iron powder was used to recover Cu as insoluble precipitate. Cu – copper, PCB – printed circuit board
Current Industrial Process for Metal Recovery
Industrial scale granulation machines are applied to grind materials down to 0.177 mm to 5 cm in size. These granulators are utilized for the recuperation of plastics, non-ferrous and heterogeneous materials (Fig. 5A). Fractionator plants are utilized to recoup metals from e-squander in a mechanical manner. Increasing speeds and decelerations instruments are utilized in the fractionator mills to recuperate materials as indicated by their physical properties (Fig. 5B). Recently industrial scale circuit board recycling machines have been introduced to separate Cu metals and resin powder, shown in Fig. 5C. High recovery rate and purity of metals (98{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78}) are the main strength of these machines. Drum type disassembling machines are utilized to separate metals and non-metals from printed circuit boards.
Capacity of these machines range from 200 -500 kg/h (Fig. 5D). Integration of heat with tunnel type dismantling machines is introduced to recover metals from e-waste. Dust casing part is incorporated in tunnel type dismantling machines to prevent environmental pollution (Fig. 5E). Effective separation of conductive and non-conductive fractions can be performed by rotor type corona electrostatic machines. No waste water and gaseous emission are generated by using these machines (Fig. 5F). Powerful magnetic fields connected with eddy current separators are used to separate non-ferrous metals from e-waste. Cu, Al and other nonferrous metals from industrial e-waste are separated by using eddy current separators (Fig. 5G). All the physical separators require basic apparatuses and low investment cost. In any case, the long detachment time, health and safety issues because of residue introduction are the fundamental problem of the physical separation process. Therefore, it is required to search for an environmentally cordial disassembling process. As of late, different modern procedures utilized internationally for recuperating specific parts from e-squander are the Umicore’s process, Muller Guttenbrunn Group (MGG)-Austria, Eldan recycling in Zaragoza –Spain, Daimler Benz in Ulm–Germany, NEC Group-Japan, LS-Nikko’s recycling facility Journal Pre-proof 8 Korea, Day’s patent and Aleksandrovich patent.
Current Methods
Recovery of Ruthenium (Ru), Rhodium (Rh), Platinum (Pt) and Palladium (Pd)
Ruthenium (Ru), Rhodium (Rh), Platinum (Pt) and Palladium (Pd) fractions were separated from e-waste using hydrometallurgical, pyrometallurgical, electrometallurgical processes. To enhance the recycling efficiency and reduce reactants consumption, pyrometallurgical processes could be a good option. Attention needs to be paid on the gas purification systems for dioxin, furans, disposal of waste gas and slags. It is not possible with any of the processes to possess simultaneously a high extraction efficiency, recyclability and inexpensive extraction process. Certainly, a compromise needs to be reached in each case. Chlorination and Cyanidation systems are used in the conventional hydrometallurgy process to separate the precious group metals. Although hydrometallurgical processes have some disadvantages, leaching processes, liquid–liquid extraction, Liquid-oxidant systems have been raised recently. In the leaching process, precious metals are dissolved in aqua regia (3HCl:HNO3), hydrogen peroxide, sulfuric acid, nitric acid, hydrochloric acid as a leaching agent to precipitate platinum group metals (PGM) from e-waste.
Efforts were concentrated on the precipitation and chromatographic separation of Pd, Ru and Rh from model solution using phosphonium quaternary salts. An efficient precipitation of Ru was reported using quaternary ammonium salt in chloride and Journal Pre-proof 9 polymer medium. Chromatographic separation was used to separate Pd and Pt of 99.99{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} purity. However, high cost and difficulties of polymer recovery using chromatographic separation process limits their industrial application. The precipitation of ruthenium complex salt by using changing pH was reported. Palladium (II) was separated from Platinum (IV) and rhodium (IV) at pH 4.47 using selective adsorption process. A Pt ion from Palladium (II) and Rhodium (IlI) was separated at pH 1.00 by selective photocatalytic adsorption process. The multiple steps of the recovery process and high amount of expensive organic solvents consumption are required in this process. Several research groups have focused on developing benign oxidizing agents to recover Ru efficiently from waste. Ruthenium might preferentially be separated from a solution containing platinum group metals (PGM) by the oxidation–distillation method. Overall Ru retrieval of 86{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} was reported by Blicharska (2013) using H5IO6 as the oxidizing agent. The formation of CCl4 during the oxidation process causes a high radiation field and thus requires a more benign process with well-suited diluents.
Effective ruthenium tetroxide was extracted by adsorption into polymer beads through an oxidation process from nitric acid media. Conversion of Ru into tetraoxide (RuO4) by oxidation and recovering from organic solvents were the key method of Ru separation from waste. However, the formation of RuO4 species during the recovery process exhibits explosive nature over 180C and thus limits its industrial application. The complex formation of Ru species could be eliminated by co-precipitation with ferrocyanide with excess Cu2+ at pH 5 ± 1, as follows. Ru (NO)3+ + Fe (CN)6 4- Ru(CN)6 4 The separation of Pd(II) and Pt(IV) has been achieved by solvent extraction, liquid– liquid extraction with an ionic liquid, Poly (styrene sulfonic acid)-impregnated alginate capsule, 2-Octyl Aminopyridine assisted solvent extraction system or polymer inclusion membrane (PIM) based separation. Solvents were used as organic phase of extraction and the specific solvents of platinum group metals and its related chemical reaction. Aqueous-organic solution system was suggested by many researchers as an effective technique to separate Pt and Pd from spent catalysts. Chloride-thiodiglycol amide solution system was introduced to separate platinum and palladium.
A selective stripping in alamine 336-NaOH-HCl solution system was reported to be an effective process for Pd separation. Similarly, the selective separation of Pt, Pd and Rh from phosphonium-chloride solution. However, most of the PGMs separation proceeds in the presence of reducing agents. The use of N,N’-dimethyl-N,N’- dicyclohexyl tetradecyl malonamide (DMDC TDMA) with no added reducing agents was found to be efficient for the separation of PGMs. It was explored that calixarene-based effective extractants for the separation of Pd and Pt from automotive catalysts. Recently, a 4-alkyl anilines-HCl solutions system was found to be promising for selective recovery of Rh (III). The highly efficient method for the separation of Rh could be due to the high stability of Rh/4-alkyl aniline ion pairs. However, low boiling points of alkyl aniline ion pairs and environment impact have to be taken into consideration for practical application.
Recovery of gold (Au).
The MacArthur-Forrest process or cyanidation was fairly established to extract gold for ores. Adequate quantity of oxygen was required to progress the reaction. Generally, extraction of gold was performed by using thiosulfate, cyanide, halide and thiourea. Alkaline conditions were used to leach out the cyanide and thiosulfate while acidic conditions were used for thiourea and halide leaching. The toxicity of cyanide may cause the long-term effect to the environment. Thiosulphate was recognized as an alternative to cyanide for extraction of gold from ores. The formation of copper-tetramine during the thiosulphate leaching process is responsible for gold dissolution. Non-toxicity and cheap are the main strengths of thiosulphate for large scale application. The thionate and oxygen-sulfur complexes were formed during the industrial thiosulfate leaching process. This process is particularly applicable to the carbonaceous ores. The extraction of gold by sodium bisulfite could be used as a replacement of thiosulphate.
Resins have a significant influence on the adsorption of gold. A compressive study was performed by Prof. Dong to investigate the gold adsorption ability of resin. The improved ability of gold adsorption was observed at pH lower than 7. Hence, the alkaline thiosulfate solution was not possible to apply for gold recovery. Staying the free electron in a polymer matrix at alkaline condition could be associated with the loss of ion exchange ability of resin. The high gold adsorption capacity of polyfunctionality anion exchangers at a wide range of pH. The highly effective 1- ethylenediamine (1EDA) and 1-(2-aminoethyl) piperazine resin was reported for gold recovery. However, the degeneration and structural blockage of resin under acidic condition should be taken into account to improve the adsorption ability of resin. High purity gold(III) (>98{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78}) was extracted by using aqueous polyethylene glycol (PEG1500)–(NH4)2SO4 two-phase system with chloride ions as extracting agents. The enhancement of gold(III) extraction efficiency was reported to be increased by increasing chloride concentration, which formed with gold stable anionic complexes (AuCl4), with lower hydration degree. In the case of gold separation and reported that the gold and copper could effectively be separated by using a two step stripping process as shown below. Still, high cost and non-recyclability of extractant limit its large scale application although it has high selectivity and superb extraction efficiency.
Au (s) + [Cu(NH3)4] (2+) + 3(S2O3 ) (2-) = [Au (NH3)2] (2+) + [Cu (S2O3)3] (5-)+ 2NH3
Pyrometallurgy process requires high temperatures to burn the gold off. This process releases dangerous gases, like dioxins during pyrometallurgy processes that affect the environment in a negative way. Moreover, the process uses cyanide solution or aqua regia to dissolve the gold which is expensive, very toxic and completely non-recyclable. Moradi et al., (2014) developed an eco friendly process to extract gold efficiently and effectively without any of the downfalls of current industry practices. It was reported that a minute amount of an acid and an oxidant is required to finish the reaction. The contact layer of gold leached out from circuits in about 10 seconds leaving behind the other metals intact under very mild conditions. It was highlighted that the cost to extract one kilogram of gold using aqua regia is $1,520 and results in 5,000 litres of waste. The cost could reduce to $66 to produce one kilogram of gold using a developed process and results in 100 litres of waste. Adsorption of gold using low concentration amino acids (glycine) and hydrogen peroxide. This process has the advantages of low cost and elimination of toxic waste disposal problems. It could be mentioned that the dissolution of gold can be enhanced by using other amino acids, but more toxicity of other amino acids compared to glycine limits their industrial application.
Recovery of Silver (Ag), Cupper (Cu), Tin (Sn) and Nickel (Ni)
Silver is commonly electro-refined using similar electrolytic cells. Pure silver powder is harvested from the cathode manually or using mechanical wipers that form on the cathode, before being dewatered and washed. Usually, acid leaching procedures for the treatment of e-waste, trailed by the separation of Ag and Cu, Sn were performed using various lixiviants. A conclusion was that the utilization of solvent extraction or ion exchange process for Ag separation from e-waste was not efficient. The Cement process was reported to be more efficient for silver recovery from e-waste among metal separation processes. The reactions during the cementation are shown below.
2AgCl(2-) + Cu = [CuCl4]2- + 2Ag
Cu + [CuCl4]2- = CuCl2
Recently, a combined pyro- and hydrometallurgy based anode slimes was proposed for efficient recovery of metal. Cu was separated from Cu and Ni contained anode slimes using LIX 63 solvent. However, multi-stage leaching and high energy consumption limits its industrial application. The involvement of the electrochemical process in the metal recovery process could reduce energy consumption and negative environmental effects. A potentially useful source of tin is e-waste which includes a broad range of electronic devices from computer to hand-held cellular phones, stereos, and consumer electronics. Overall electrochemical recovery of tin consists of two individual processes
(1) Tin leaching in either (a) H2SiF5 + H2SO4 + H2O2 solution or (b) HNO3 solution and subsequently leaching in HCl solution. The most efficient recovery of tin was the observed recovery rate of 93.2{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78}.
Sn(s) + 4 HCl(aq) + HNO3 (aq) →SnCl2 (aq) + NOCl (aq) + 2H2O (aq)
SnCl2 (aq) + H2O (l) →Sn(OH) Cl (s) + HCl (g)
A significant amount of Ni is contained in printed circuit boards (PCBs) of cathode ray tubes. About 96{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} recovery of Ni obtained by using hydrochloric acid was reported by Sutherland, (2002). The efficiency of Ni recovery from printed circuit boards using sulphuric acid was reported to be higher than that of hydrochloric acid. Further, ultrasonic agitation during Ni recovery from a printed circuit board may improve the leaching efficiency. However, nickel exposure related health risk and environmental concerns should be considered during recovery of nickel. Therefore, a relatively low temperature leaching process using organic citric acid could be more efficient for Ni recovery from printed circuit boards. Biological processes may reduce energy consumption and negative environmental influence for the metal recovery from waste.
Biological Process for Metal Recovery
Bioleaching is a natural phenomenon in which a diverse group of microorganisms including chemolithotrophic, heterotrophic bacteria and fungi are involved in dissolving metals from e waste. Biosorption including precipitation, complexation, adsorption and ion exchange are metabolically independent processes. Separation of materials from substances is initiated by an autotrophic, heterotrophic or hybrid leaching process. Multistep recycling strategies and Jarosite formation due to the consequence of the increase in pH involved in the autotrophic leaching process.
Using the advantages of heterotrophic, autotrophic leaching or chemical leaching might be suitable opportunities for metals from e-waste focused on the use of sulphate reducing bacteria to recover metals from e-waste. The precipitation of metal occurred through the formation of insoluble metal sulphidic precipitates. Releasing compounds from metabolized bacteria was used in the precipitation process to recover metals. Several researchers demonstrated the recovery of precious metal (e.g., Ag, Au, Co) as well as base metals (e.g., Cu, Ni, Fe, Zn) have been demonstrated. The A. niger MXPE6 or Acidithiobacillus thiooxidans is suitable for Au recovery while the Aspergillus niger or Aspergillus nomius is the most promising microorganism for Cu recovery. The mobilization of metals was caused through the formation of organic acid and inorganic acid. Two-step leaching process was reported to be efficient for metal mobilization. However, low bioleaching efficiency and long bioleaching time are the main drawbacks of biological methods compared to the chemical one. Hence, the use of the optimum conditions, the right mixed culture of bacteria, and the proper method could improve the metal recovery and efficiency in future. Auxo Autotrophs and heterotrophs are the two microorganisms used in bioleaching. Auxo Autotroph species include Sulfobacillus, Acidianus, Acidiphilium Thiobacillus and Leptospirillum ferrooxidans, of which the Acidithiobacillus thiooxidans, L. ferrooxidans and Acidithiobacillus ferrooxidans are widely used in practical application.
Separation of metals from e-waste using biological process occurs through acidolysis, complexation. Organic substrates (bacteria and fungi) containing auxo heterotrophs could also be used to leach metals from e-waste. Organic acid (acetic acid and Carbonic acid) and inorganic acid (sulfuric acid, nitric acid, formic acid) are the metabolites of bacteria. Aspergillus and penicillium can produce metabolites such as oxalic acid, citric acid, and gluconic acid, so they are the most important fungi. Precious metals can only extract base metals from E-waste with A. ferrooxidans but extraction of this bacterium is difficult. Bioleaching base metals recovery was performed by A. ferrooxidans under optimized condition, however, the bioleach efficiency of A. ferrooxidans is low. Comparing with A. ferrooxidans. L. ferrooxidans have higher affinity for Fe2+. A conclusion has been reached by the researchers that fungi can extract precious metals (Ag, Au, and Pd) from E-waste through bioleaching.
Mechanisms of biological method
Growth of microorganism without addition of e-waste and the addition of e-waste powder to the reactor are the two main stages in the biological process. The reaction of organic sugar and acid with e-waste powder arises at the first stage and the growth of microorganism is initiated in the second stage. The heavy metals can selectively be extracted from electronic waste through these processes. Fungi and bacteria are commonly used for noble metal recovery from e-waste through the bioleaching process. Bioleaching is emphatically affected by the chemical composition of culture media, the particle size of the milled PCBs, and the pH of the system. Two fundamental sorts of culture are ferrooxidans and thiooxidans, and these microscopic organisms can solubilize Cu, Al, Zn, and Ni.
Mechanism for ferrooxidans solubilized Cu in culture medium is shown.
It was reported that ideal pH of A. ferrooxidans for the bioleaching of Cu from waste PCBs was in the range of 1.8-2.5. The Cu extraction of 99{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} was achieved due to the higher concentration of Fe3+ present in the culture medium. The amount of dissolved oxygen has a deciding impact on the metal extraction by cyanogenic bacteria as oxygen required for bacterial respiration. The co-existence of oxidizing chemolithoautotrophs in the medium favored the mobilization of copper.
The utilization of the ideal conditions, the privileged mixed culture of bacteria and the correct strategy could improve the metal recovery and efficiency in future.
The consumption of Fe3+ due to the mobilization of Cu ions favors the bioleaching with ferrooxidans, as reported in the following reactions.
4Fe2+ + O2 + 2H+ →4Fe3+ + 2OH
Cuo + 2Fe3+ →Cu2+ +2Fe2+
Bioleaching mechanisms of bacteria
Bioleaching reaction mechanisms of bacteria and electronic-waste have been investigated by the researchers using different states of A. ferrooxidans and A. thiooxidans. No contact between E-Waste and bacteria is required to proceed the reaction in the bioleaching process. The oxidation of Fe2+ and Fe3+ could be performed in the bioleaching process using inorganic acid and enzymes. The speeding up of bioleaching reaction process is initiated by the production of H+ during the oxidation of metallic sulfide to sulfate by Fe3+. It might be noted that the production of Fe3+ from A. ferrooxidans and A. thiooxidans accelerates the bioleaching process. A direct bioleaching of noble metals from e-waste using A. ferrooxidans and A. thiooxidans. It was concluded that the bioleaching efficiency of metals was dependent on the oxidation rate of Fe2+ to Fe3+, pH and Fe2+ concentration.
The possible mechanisms of bioleaching are provided as follows.
2So +3O2 +2H2O → 4H+ +2SO4 2-(Action of bacteria)
2Cuo +4H+ +2SO4 2- + O2→ 2Cu2+ + 2SO4 2- +2H2O
An indirect and direct leaching mechanism exists in the bioleaching base metal extraction process extracted 68.5{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} and 85{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of gold by using C. violaceum and L. acidophilus bacteria, respectively. Klebsiella pneumoniae bacteria was used to extract 99{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} of Ag, Pd. However, the factors that influenced the process and mechanism of bioleaching should be studied in detail to make the process sustainable.
Bioleaching mechanisms of fungi
The reaction of precious metals recovered from e-waste in biological methods is initiated by the bacteria-produced cyanide or fungi-produced organic acids. Numbers of organic complexing agents including the citric acid, tartaric acid, and oxalic acid are required to recover precious metals from e-waste. Aspergillus niger is the common fungi used for the extraction of Au and Ag from e-waste. It might be mentioned that the leaching agents produced by the Aspergillus niger are the gluconic acid and citric acid.
The theoretical equation for the generation of energy and acceleration of reaction to extract precious metals from e-waste as follows:
C6H12O6 + 1.5O2 →C6H8O7 + 2H2O (Action of A. niger)
The adsorption of metal ions in the biosorption process was initiated by the microorganism generated ligands through the chelation process. Metal can be recovered from e-waste by using A. niger as an adsorbent. Microbial leaching of metals is accompanied in both aerobic and anaerobic conditions. The process could be affected by the solid-liquid ratio, temperature or the pH of the solution. A different kind of microorganisms used in extraction metals from electronic waste. Microorganisms are adapted to environmental friendly living conditions compared with the conventional techniques. However, the metal dissolving speed with bacteria is significantly slow.
Limitation of PyroMetallurgy Process
Limitation of HydroMetallurgy Process
Challenges in Bio- HydroMetallurgy
The prime challenges in the bio-hydrometallurgy process are the extended time of operation and toxicity effects on the microbes and hence viewed as the rising concerns in the scaling up of the process to commercialization. The metal recovery efficiency up to 99{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} with 3 days of operation at moderately higher pulp density (10{828d3c3f61672f5f3f915e8276d5ef35c5e0117d26b89f98a9d5e22a85a2eb78} w/v). The possible use of adaptive microorganisms towards the harsh micro-environmental conditions such as temperature, salinity and toxicity are the promising option to enhance their applicability to the wide range of waste.
Further, the elucidation of intrinsic mechanisms of bio-hydrometallurgy still needs to be envisaged more precisely in the atomic sense. Previously, the authors reported the recovery of copper from the WPCBs with this route using A. ferrooxidans culture-free supernatant and validated the role of Fe3+ ions in the leaching process. Besides, very few reports were made with the successful use of continuous or modified reactors and hence, systematic investigations on the effects of different types of reactors in the e-waste recovery process can be studied in the future.
E- Waste Scenario in the Region
Digitalization in this region is the main driving force which increases the consumption of electronic components and e-waste over the last decade. Different categories of e-waste include television sets, mobile phones, dental waste and household electrical appliances are available in this part of the world.
The high amount of e-waste per year is harmful for the environment due to its unconscious dumping practice. For proper management of e-waste can be maintained in four steps like: Consume, Collect, Function for recovery, material and energy for recovery. Open disposal of electronic waste, households, private enterprises and public bodies are the main sources of consuming e-waste. In the collection step includes the traders of electronic waste, informal collection, sales and formal collection. A number of models were developed for the environment and health hazards.
The equation for formal recycle of e-waste as follows:
E=MN/L
Where,
E= generated e-waste (million metric ton/ per year);
M=mass of the product in metric ton,
N=average number of used products
L= lifespan in years.
The balance analysis model can be used to calculate the fund required for e-waste processing.
The surplus or deficit fund and sustainability of e-waste was measured by using the fund balance analysis model.
Moreover, Government may take initiatives to manage e-waste and protect the environment. Currently there is no regulation and particular policy for e-waste management (?). A national 3R (Reduce, Reuse, Recycle) should be prepared by the Government. For proper e-waste management, the Government should take some steps as shown. Moreover, the ability of implementation for the policies based on extended producer responsibility policies. After all, the requirement of strong incentive for the researchers is imperative for a sustainable solution to extract noble metals from e-waste.
E-Waste Management
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By Sakkir Hussain, Partner at Theia New Consultancy based out of Kingdom of Saudi Arabia