recovery of rare and precious metals from urban mines a · pdf filerecovery of rare and...

FEATURE ARTICLE

Recovery of rare and precious metals from urban mines—Areview

Mengmeng Wang1, Quanyin Tan1, Joseph F. Chiang (✉)2, Jinhui Li (✉)1

1 Key Laboratory for Solid Waste Management and Environment Safety (Tsinghua University), Ministry of Education of China,School of Environment, Tsinghua University, Beijing 100084, China

2 Department of Chemistry and Biochemistry, State University of New York College at Oneonta, Oneonta, NY 13820, USA*

1 Introduction

Rare and precious metals (RPMs)— a category thatincludes both rare metals (RMs) and precious metals(PMs)— generally refer to metallic mineral resources thatare rare, widely scattered and difficult to extract, in the

✉ Corresponding authors

E-mail: [email protected] (Chiang J F), jinhui@tsinghua.

edu.cn (Li J H)

Special Issue—Recycling Materials from WEEE (Responsible Editors:

Jinhui Li & Eric David Williams)

Front. Environ. Sci. Eng. 2017, 11(5): 1DOI 10.1007/s11783-017-0963-1

H I G H L I G H T S

•Distribution characteristics of various RPMs inurban mines are summarized.

•Conventional and emerging RPM recyclingtechnologies are reviewed systematically.

•Advantages and shortcomings of various tech-nologies are discussed and highlighted.

A R T I C L E I N F O

Article history:Received 7 November 2016Revised 24 April 2017Accepted 26 April 2017Available online 8 July 2017

Keywords:Rare and precious metals (RPMs)Distribution characteristicsRecycling technologyEmerging technologySupercritical fluid

G R A P H I C A B S T R A C T

A B S T R A C T

Urban mining is essential for continued natural resource extraction. The recovery of rare and preciousmetals (RPMs) from urban mines has attracted increasing attention from both academic and industrialsectors, because of the broad application and high price of RPMs, and their low content in natural ores.This study summarizes the distribution characteristics of various RPMs in urban mines, and theadvantages and shortcomings of various technologies for RPM recovery from urban mines, includingboth conventional (pyrometallurgical, hydrometallurgical, and biometallurgical processing), andemerging (electrochemical, supercritical fluid, mechanochemical, and ionic liquids processing)technologies. Mechanical/physical technologies are commonly employed to separate RPMs fromnonmetallic components in a pre-treatment process. A pyrometallurgical process is often used for RPMrecovery, although the expensive equipment required has limited its use in small and medium-sizedenterprises. Hydrometallurgical processing is effective and easy to operate, with high selectivity oftarget metals and high recovery efficiency of RPMs, compared to pyrometallurgy. Biometallurgy,though, has shown the most promise for leaching RPMs from urban mines, because of its low cost andenvironmental friendliness. Newly developed technologies—electrochemical, supercritical fluid, ionicliquid, and mechanochemical—have offered new choices and achieved some success in laboratoryexperiments, especially as efficient and environmentally friendly methods of recycling RPMs. Withcontinuing advances in science and technology, more technologies will no doubt be developed in thisfield, and be able to contribute to the sustainability of RPM mining.

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2017

natural environment. Yet RPMs have shown broadprospective application in areas such as machinery, energy,aviation, and the chemical industry, because of theirexcellent physical and chemical properties, such as high-temperature oxidation resistance, corrosion resistance,good conductivity, high-temperature thermoelectricalproperties, and stability of the temperature coefficient ofresistance. RPMs are critical strategic resources affectingnational economic growth and the progress of science andtechnology. The classification and characteristics of RPMsare listed in Table 1.As can be seen from Table 1, precious metals (PMs)

include Au, Ag, platinum-group metals (PGMs), Ru, Rh,Pd, Os, Ir, and Pt. Rare metals (RMs) are divided into rarelight metals, rare refractory metals, rare scattered metals,rare earth metals and rare radiation metals. The reserves ofthe precious metals Au and Ag are abundant in China,ranking eighth and sixth in the world, respectively [1,2].The major application areas for PMs are listed in Table 2.Au, Ag, Pt, and Pd are mainly used in jewelry andelectronics. For example, these types of PMs are used inthe electronics industry as a mixed paste during theproduction process of printed circuit boards (PCBs) toimprove the electrical conductivity of joints [3,4]. Pt, Pd,and Rh in automobile catalytic converters are doped asactive ingredients to efficiently remove CO, hydrocarbonsand NOx, achieving a removal rate of 99% [5]. And PGMsare utilized in a variety of industrial products and chemicalreagents such as glass, ceramics, oil and pharmaceuticals,because of their unique physical and chemical properties[6]. However, the natural reserves of PGMs are scarce andwidely scattered: the total global reserves of PGMs areonly about 31000 tons. Of these, the total reserves of Pt areonly 14000 tons, which is not sufficient to meet current andanticipated demands [7]. The increasing demand butlimited availability of PMs has led to a steep rise in theirmarket prices.RM reserves are widely scattered, with low naturally

occurring content; most are contained within otherminerals and must be recovered from by-products.However, RMs play a major role in scientific andtechnological development. For example, Li, In, Ga, Ge,Mo, W, V, etc. are the indispensable components of high-

end, high-demand industrial products (such as LCDpanels, TVs, superconducting coils etc.), industrial man-ufactured materials (such as steel, atomic furnaces, etc.)and manufactured household goods (such as lighters,glassware, automobiles, etc.) [8]. RMs occur in higherconcentrations in urban mines than in traditionally minedores (Table 3), indicating a high exploitation and recoveryvalue for urban mines. For practical applications, Li iswidely used in batteries, especially by the electrical vehicle(EV) industry. The largest application area for In iselectronic materials; it has been widely applied in theindustrial production of LCD panels and plasma displays[9]. A series of compound semiconductors, electron opticalmaterials, special alloys, new functional materials andorganometallic compounds, made of dilute metallic Ga andGe, are critical for infrared imaging [10,11]. Mo, W, and Vdoped in hydrodesulfurization (HDS) catalysts can effec-tively promote the hydrodesulfurization reaction of dieseloil, achieving the purpose of exhaust gas purification, andwaste HDS catalysts generally consist of 10%–30% Moand 1%–12%V, indicating a huge economic value for RMsrecovered from scrap catalysts [12,13].In China, large amounts of municipal wastes have been

generated during the period of urbanization, and this wastecontains a large number of RPM resources, qualifying it asan urban mine [18]. Most of these RPMs are distributed inwaste electrical and electronic equipment (WEEE), spentautomobile catalytic converters, and some industrialwastes [19,20]. Compared with natural ore resources, theRPMs contained in urban mines have high grade and purecomposition, making it easy to control the recoveryprocesses. Reusing secondary resources rationally andrecycling valuable RPMs could greatly improve theutilization of existing resources in China, and achievesustainable recycling of waste materials.In this study, the distribution characteristics of various

RPMs are summarized, and the technologies for recover-ing them from urban mines are systematically reviewedand discussed, with emphasis on the recovery of preciousmetals (Au, Ag, and PGMs) and rare metals (Li, In, Ga,Ge, Mo, W, and V). Traditional processes— pyrometal-lurgy, hydrometallurgy, and biological metallurgy technol-ogies— for RPM recovery are reviewed in detail. In

Table 1 Classification and characteristics of RPMs

metals category metals characteristics

precious metals Au, AgPGMs: Pt, Pd, Rh, Os, Ir, Ru

low content in natural environment, high price; most recovered from by-products

rare light metals Li, Rb, Cs, Be small specific gravity and high chemical activity

rare and refractory metals Ti, Zr, Hf, V, Nb, Ta, Mo, W higher smelting point, generally obtained by reduction of compounds

rare and scattered metals Ga, In, Tl, Ge, Re, Se, Te low content in natural environment; most contained in other minerals; must be recovered from by-products

rare earth metals Sc, Y, Lanthanides similar in chemical characteristics; associated with other metals in naturally occurring minerals

rare radiation metals Fr, Ra, Po, Actinides radioactive; often associated with rare earth metals in naturally occurring minerals

2 Front. Environ. Sci. Eng. 2017, 11(5): 1

addition, the challenges and possible state-of-the-arttechniques related to the reuse of these secondaryresources, such as electrochemical, supercritical fluid,mechanochemical, and ionic liquids technology, are alsoanalyzed.

2 Distribution characteristics of RPMs intypical urban mines

Because of their wide use, most of the RPMs in urbanmines are encased in electronic wastes (e-wastes), wastecatalysts, tailings and slag. Mine tailings and slag,however, are of less importance because of their enormoussize and very low RPM content. The concentrations ofRPMs in e-waste and waste catalysts, on the other hand,are high, indicating a considerable economic benefit fromextracting them. Moreover, as science and technologycontinue to advance in China, the demand for RPMs willcontinue to increase. Therefore, recycling RPMs fromthese secondary resources is both essential and profitable.Figure 1 shows the various products containing RPMs

and the specific RPM resources included in urban mines.The primary PMs found in waste PCBs are Au, Ag, and Pd[21]; HDS catalysts contain large amounts of RMs such asMo, W, and V; and Li is combined with Co, in the form ofLiCoO2, in the cathode material of spent lithium-ionbatteries (LIBs), where it enhances the charging anddischarging processes [22].

Fig. 1 Products containing RPMs and resources included in urban mines

Table 3 Comparative contents of some RMs in traditional ore mining and urban mining, and their application areas [14–17]

rare metals traditionally mined ores/% urban mines/% application areas known reserves/t

Li 6.5�10–3 1–6 ceramics, glass, batteries, nuclear industry 39.78 million

In 1�10–5 0.5–5 LCD television and monitors, laptops, and PV cells 5 million

Ga 1.5�10–3 – semiconductors, photovoltaics, fuel cells, and alloys 1 million

Ge 7�10–5 – optic fiber, infrared fiber, polymerization catalysts 8600

Mo 1.1�10–4 10–30 electrical and electronic products, pharmaceuticals, HDS catalysts 19.4 million

W 1�10–3 – cemented carbide, chemical catalysts 3.3 million

V 1.8 1–12 steel industry, aviation, HDS catalysts 63 million

Table 2 Some of the major applications for PMs

application areas Au Ag Pt Pd Rh Ir Ru

jewelry √ √ √ √

catalysts √ √ √ √ √ √

chemistry √ √ √

electronics √ √ √ √ √ √ √

fuel cells √ √ √ √

glass, ceramics √ √

medical/dental √ √ √ √ √

oil √ √

pharmaceuticals √ √ √ √

photovoltaics √ √

superalloys √ √ √

Mengmeng Wang et al. Recovery of rare and precious metals from urban mines—A review 3

The traditional analytical methods used for the detectionof RPM ions include various instrumental techniques suchas graphite furnace atomic absorption spectrometry(GFAAS), flame atomic absorption spectrometry (FAAS),inductively coupled plasma atomic emission spectrometry(ICP-AES), and inductively coupled plasma mass spectro-metry (ICP-MS) [23]. The detection of trace levels ofvarious gold species can also be carried out by spectro-scopic (atomic absorption/emission spectroscopy withvarious atomizers), mass, and electrochemical analysismethods. In addition, fluorescent and colorimetric chemo-sensors are powerful tools for monitoring biologicallyrelevant species at the ng or sub-ng level in vitro and/or invivo, because of their simplicity and high sensitivity [24].

2.1 RPMs in e-waste

RPMs have been widely used in electrical and electronicequipment (EEE) such as household appliances, mobilephones, computers, and other manufactured goods, byvirtue of their excellent conductivity. This developmenthas resulted in a large increase in e-wastes. The highcontent of RPMs present in various types of e-waste makesthis waste a potential source of valuable metals recovery.Table 4 shows that e-waste such as TV board scrap, PCBscrap, mobile phone scrap, spent LIBs and LCD panelscontain high levels of RPMs. The concentrations of RPMsin WEEE are around 10 g$t–1–10 kg$t–1, accounting for40%–70% of the total economic value of this waste [25].

LIBs have been widely used in EEE since the 1990sbecause of their superior characteristics of high workingvoltage, large capacity, long circle-life and non-memoryeffect [30,31]. Furthermore, the continued growth in thedemand for EEE and EV, driven partly by frequentupgrades and replacements of electronic devices, is greatlyincreasing battery consumption, and massive amounts ofspent LIBs are generated worldwide every year, with noproper large-scale disposal. In China, for example, the totalquantity and weight of spent LIBs are estimated to reach 25billion units and 500 thousand tons, respectively, by 2020[32]. The most widely used rare metal in cathode material,Li, occurs at a concentration of about 5%–7% in thebatteries for these electronic products—much higher than

that in natural ore deposits, indicating great recovery valuefrom this type of e-waste [33–35].LCD panels have been used in notebook computers, and

are becoming increasingly popular in desktop computermonitors and televisions. Over 70% of the total consump-tion of indium can be attributed to LCD panel production[36]. Indium-tin oxide (ITO) is applied as a transparentconductive film in LCDs in the form of a solid solution ofIn (III) oxide (In2O3), because of its excellent electricalconductivity and optical transparency [17,37]. Since LCDstypically have a lifespan of 3–5 years, larger and largerquantities of LCDs are entering their end-of-life stage andbecoming e-waste. Thus, a great deal of reusable Incontained in LCDs could be recovered.

2.2 RPMs in scrap automobile catalytic converters

The automobile industry is the biggest consumer marketfor Pt, Pd, and Rh, and the demands from this industryaccount for 35%–60% of the Pt and Pd, and about 95% ofthe Rh [5]. These three precious metals in automobilecatalytic converters have high catalytic activity, stability,and selectivity, and can reduce 95% of the harmful gasemissions from automobile exhaust. PGMs are an integralpart of automobile catalytic converters, and conventionalconverters generally contain 0.08% Pt, 0.04% Pd and0.005%–0.007% Rh [38]. Recovering PGMs from spentautomobile catalytic converters is well worth the effort,because these PGMs are expensive and their concentra-tions in the converters are higher than in natural oredeposits. With the growing number of end-of-life vehicles,waste catalytic converters have become the most valuableresource for PGM recovery.The RPMs Pt, Pd, Ag, and Mo are widely used in

hydrogenation, oxidation, dehydrogenation and synthesisof the catalysts as well. For example, Co-Mo catalyst isapplied in chemical fertilizer plants and oil refiningdesulfurization, and it contains 1.02% Co and 4.92% Mo[39]. Platinum reforming catalyst is used in the catalyticreforming process of the catalyst, and usually contains0.2% Pt, which is equivalent to the platinum content inspent automobile catalytic converters [40].

2.3 RPMs in mine tailings and various types of slag

Tailings and smelting slag from metal smelting furnacesare a valuable source of RPMs, and if left untreated mayconstitute an ecological threat to the surrounding environ-ment. Tailings, a valuable secondary mineral resource,contain a variety of colored, black, rare, rare-earth andnonmetallic minerals, and require further development ofrecycling techniques. For example, Cu, Au, Ag, Fe, sulfur,fluorite, wollastonite, barite and other useful ingredientscan be extracted from copper tailings; and lead, zinc,antimony, silver and other metal elements could berecovered from Sn tailings.

Table 4 Various types of e-waste and their RPMs content [26–29]

e-wasteRPMs weight /ppm

Au Ag Pd In

TV board scrap 20 280 10 0

PCB scrap 250 1000 110 0

mobile phone scrap 30 2000 1700 1102

laptop PCs 32 190 19 140

CRTs 46 207 18.4 0

LCD panels 60 300 25 40


Smelting slag is a type of formed on the surface ofmolten metal in the pyrometallurgical processing of ore ormetal-containing waste [41]. Its composition is mainlyoxides (silica, alumina, calcium oxide, magnesia). Thecomposition of the various ores and waste sources has asignificant influence on the type of RPMs in the slag. Thistype of slag is usually discarded because recycling theRPMs in the slag is not economically feasible. However,the accumulation of large amounts of this slag is not only aserious environmental problem but also an overlookedsource of RPMs.

3 Traditional technologies for RPMrecovery

Traditional methods of recycling RPMs include pyrome-tallurgical, hydrometallurgical and biometallurgical tech-nologies. Mechanical/physical technologies are commonlyemployed first, as a pre-treatment process, to separateRPMs from nonmetallic components. Figure 2 shows thetraditional process of recovering RPMs from e-waste andspent catalysts. One single technology, however, can notseparate and purify RPMs completely; therefore, two ormore techniques are combined to achieve RPM recovery.

3.1 Mechanical/physical technologies

It is hard to separate or recover RPMs directly because theyare usually covered with, bonded with, or encapsulated invarious nonmetallic components, such as plastic or ceramicmaterials. Furthermore, their recovery may cause environ-mental problems during the waste management phase if thewaste is not properly pre-treated. Mechanical/physical

technologies are therefore used to separate RPMs fromnonmetallic components, as a pre-treatment process [42].These technologies include manual disassembly, crushing,sorting, and similar processing methods. For example, torecycle RPMs from waste PCBs, the first step is todismantle the electronic components on the surface of thedial. Then various further treatments are employed,depending on the materials’ physical characteristics,including density, magnetic susceptibilities, and electricalconductivity [43].Huang et al. established an integrated recycling process

for waste PCBs, which included two crushing steps,corona electrostatic separation, and metallic materialsrecovery [3]. Waste PCBs crushed first coarsely and thenfinely are converted into fine particles. The resultsindicated that the application of corona electrostaticseparation could efficiently separate metals from non-metals with particle sizes between 0.6 and 1.2 mm. Theresulting granules consisted of about 30% metallicmaterials after separation. The metals in these materialsare still difficult to recover, because the materials obtainedfrom the process are a mixture of various metals, such asCu, Al, Pb, Zn, and PMs.Similar to waste PCBs, spent LIBs must be dismantled

and separated through manual or mechanical processesbefore further treatment [28]. Mechanical operations areindispensable because Li and Co are covered with plasticand encapsulated in an iron shell. After the removal ofplastic and steel cases from the cells, the cathode,containing lithium cobalt oxide (LiCoO2), can be extractedand used as a raw material.Actually, in practical application, the mechanical/

physical technologies can effectively enrich RPMs fromvarious wastes to enhance their recovery rates. Another

Fig. 2 Flowchart of traditional process of recovering RPMs from e-waste and spent catalysts


advantage of this method is that it causes less secondarypollution during the recycling process. However, thisapproach is insufficient to disperse RPMs, and conse-quently this technology is proposed as a pre-treatmentphase in the process of RPM recovery from wastes.

3.2 Pyrometallurgical technology

Pyrometallurgical technology refers to treating variouswastes containing RPMs under high-temperature condi-tions and recycling them via high-temperature chemicalreactions. It is difficult to recover RPMs directly because oftheir high dispersion and low concentration, and becausethey are mixed with various types of nonmetalliccomponents and other base metals. However, extractioncan be achieved by using different high-temperaturerecovery technologies for various dispersion characteris-tics of RPMs in waste materials [44,45]. Currently,pyrometallurgy processing for RPM recovery includesmainly high-temperature incineration, vacuum carbon-thermal reduction and chlorination volatilization methods.Table S1 in the Supplementary Material summarizes therelated researches for RPM recovery from urban minesusing pyrometallurgical technology.

3.2.1 High-temperature incineration method

The high-temperature incineration method refers to burn-ing wastes in an incinerator, where the flammablenonmetallic components are converted to recoverablecalorific value after incineration, and the RPMs areenriched in the disposed residues. The incineration isfollowed by other refining processes, to extract the puremetals [20]. At present, this technology is mainly used forenriching and recovering the RPMs Au, Ag, Pt, Pd, Rh, Inand Co from e-waste and spent automobile catalyticconverters. RPMs in e-waste and spent catalytic convertersare scattered throughout the material at low concentrations,and the recovery rates are low when using directhydrometallurgical leaching or extraction. High-tempera-ture incineration, however, can separate metals from non-metals, and the RPMs are enriched in the residues. Themetals can then obtained with electrolytic or fire refining.One application of a high-temperature incineration

process was the Umicore integrated smelter and refinery.Their process focused on the recovery of RPMs fromWEEE, including the PMs Ag, Au, Pt, Pd, Rh, Ru, Ir,and the RMs In, Se, and Te [46]. In the first step, theWEEE was pre-treated (i.e. dismantling, shredding andphysical processing) and then the waste materials weresmelted in the Isa Smelt furnace. Almost all the RPMswere concentrated in the slag after smelting, and then theslag was further refined to gain high-purity RPMs, step bystep.

3.2.2 Vacuum carbon-thermal reduction method

RPM recovery using the vacuum reduction method refersto the process of reducing the RPM oxides to metals withreducing agents under high-temperature conditions. RPMscontained in the waste are generally in the form of oxides(such as LiCoO2 in spent LIBs, indium oxides in LCDscreens, germanium oxide in fly ash, etc.), and these aresingle oxides at high concentrations, allowing RPMs to berecovered directly with this method. An environmentallyfriendly oxygen-free roasting and wet magnetic separationtechnology for in suit recycling of Co, lithium carbonateand graphite from mixed electrode materials was presentedby Li et al. [47]. The reaction was complete in the processat below 1000°C for 30 min, and the recovery rates of Co,Li, and graphite were 95.72%, 98.93% and 91.05%,respectively. The results indicated that, through thetechnologies of oxygen-free roasting and wet magneticseparation, the materials mixture— consisting of LiCoO2

and graphite powders—was transformed to the individualproducts of cobalt, lithium carbonate and graphite.Furthermore, no chemical solution was added in theprocess, obviating the necessity of disposing of secondarypollutants.

3.2.3 Chlorination volatilization method

The chlorination volatilization method refers to heatingraw materials containing RPMs and chlorination agentstogether, then chloridizing the metals to generate volatilechlorides trapped in soot and lotion. This is followed by ahydrometallurgical process for metal recovery. Thechlorination volatilization method was first used to extractAu from gold mines, and at present this approach has someapplication in the extraction of RPMs from e-waste.Studies conducted by researchers from Shanghai Jiao TongUniversity developed an efficient rough vacuum-chlori-nated separation method for In recovery from waste LCDpanels by using NH4Cl as the chlorinating agent [48]. Highpurity In2O3 and SnO2 were first investigated in a roughvacuum, in an air/nitrogen atmosphere. The resultsindicated that the rough vacuum atmosphere couldsimultaneously increase the recovery ratio of In and reducethe influence of Sn. Moreover, the indium chloride andNH4Cl can be selectively recovered under differentcondensing temperatures, and the thermodynamic princi-ple of the recovery method was analyzed. Conditions of400°C for 10 min, under a rough vacuum atmosphere (0.09MPa) and sufficient NH4Cl (molar Cl/In ratio of 6) wereconfirmed as the optimal conditions for real substancerecovery. The weight ratio of NH4Cl to glass powder andthe optimum particle size were established as 1:2 and lessthan 0.13 mm, respectively. Pure indium chloride wassuccessfully recovered from the waste LCD panels via this


method, and the recovery rates of In and the purity ofindium chloride were 98.02% and 99.50%, respectively.Kakumazaki et al. developed a process of selective

separation of Au using a dry chlorination process, andinvestigated the release behavior of Au from incineratedash [49]. The results demonstrated that the addition ofcarbon to incinerated sewage sludge ash reduced thetemperature at which gold was volatilized by chlorine,from 700°C to 300°C. The released gold was adequatelycaptured by solid carbon, with the proportion recovered inthis way increasing with temperature. In this way, it ispossible for all the Au contained in the incinerated ash tobe recovered by solid carbon, by heating the ash to 800°Cin a nitrogen gas stream and then holding it at thattemperature for 1 h in a chlorine gas stream. The goldcaptured by the carbon is reduced to form fine metallicparticles, evenly distributed over the carbon surface.In summary, the pyrometallurgical process is a good pre-

concentration method for dispersed RPMs in wastematerials, and other subsequent refining processes can beincluded for further metal recovery. The process is simple,and has a large capacity and wide range of applications,and also allows the calorific value of the nonmetalliccomponents of urban mines to be recycled. Thistechnology has therefore been in practice for years, forrecovering RPMs from waste. However, it is liable toproduce a series of environmental problems, such as theemission of harmful volatile organic compounds and toxicsmoke, due to the presence of halogenated flameretardants. Moreover, it has poor selectivity in recyclingany single metal from RPMs. More importantly, theequipment is expensive, requiring significant investment.Therefore, it is not suitable for small and medium-sizedenterprises.

3.3 Hydrometallurgical technology

Currently, conventional recycling technologies for RPMrecovery from urban mines are divided into two majorsteps: 1) enriching the metal content by mechanical-physical pre-treatment, and 2) extracting and refining torecover the RPMs by pyro-/hydro/bio-metallurgical meth-ods. In the pyrometallurgical process, it is difficult torecover most RPMs due to the high dispersion degree andlow metal concentrations. In the last few decades, there-fore, hydrometallurgical processes have been givenconsiderable attention, for recovering PMs as well asRMs from waste materials [50].Hydrometallurgy refers to selecting an appropriate

leaching agent to dissolve the RPMs and base metals.The metal ions in the solutions are further separated andpurified by a replacement precipitation method, an ionadsorption method, a solvent extraction method, an ionexchange method, electrochemical reduction or otherprocesses, to separate the base metals and enrich the

precious metals [43]. In general, hydrometallurgicaltechnology for RPM recovery consists of two mainprocesses, namely dissolution and leaching of RPMs,and separation and purification of the RPMs.

3.3.1 Dissolution and leaching of RPMs

The initial step of hydrometallurgy is the leaching of RPMsfrom wastes. It must be pointed out that it is harder to leachout PMs than RMs, because of their inertia (metal activitylags behind that of hydrogen). Aqua regia can effectivelydissolve the PMs in waste materials, but it produces toxicnitrogen oxide fumes. Cyanide leaching for PMs has beenused for over a century, due to the selectivity and stabilityof the precious metal complex. However, the high toxicityof cyanide may cause serious environmental and humansafety problems. In recent years, some green leachingsystems, with high leaching speed and high efficiency,have been proposed and have gained considerableprogress, such as halide, thiourea, and thiosulfate leaching.Table S2 in the Supplementary Material shows someresearch emphasis on the leaching of RPMs from urbanmines. As can be seen from Table S2, most studies focusedon leaching PMs (mainly Au, Ag, and Pt) from e-waste andautomobile catalytic converters, and RMs from spent LIBs,LCD panels and HDS catalysts.RMs such as Li and In have higher metal activity

(although still lower than hydrogen); therefore inorganicacid or even weak organic acid could be used as leachingreagents: for example, organic oxalate [51], succinic acid[52], oxalic acid [53], etc. Compared with strong inorganicacid, sulfuric acid, hydrochloric acid, and nitric acid, weakorganic acids could substantially reduce the environmentalloads under the same conditions of high leaching rates andefficiency, while being more environmentally friendly[54].

3.3.2 Separation and purification of RPMs

RPMs contained in waste materials can be converted intoionic form and enter the leaching solution after dissolutionand leaching. However, the RPM ions are often mixed witha variety of impure metal ions in the solution, whichrequire further separation and purification. To that end, avariety of separation and purification methods— includingsolvent extraction, adsorption on activated carbon, ionexchange, precipitation, cementation, and electrolysis—have been developed. The appropriate selection of down-stream RPM separation and purification processes isprimarily based on factors such as the leaching reagentsystem, and the concentrations of metals and impurities.Solvent extraction is the conventional method for

separating RPMs from leaching solutions, and it hasbecome a mature technology. Based on the characteristicsof the variable valence and the spontaneous formation of


complexes of PMs, it is feasible to complex them byselecting appropriate extraction agents from base metalions [55]. Solvent extraction includes extraction and backextraction, and the extracting agent has an important effecton the separation and extraction efficiency. Consequently,selecting an appropriate extracting agent often determinesthe success of an extraction process. Many extractionsolvents have been investigated, including organopho-sphorus derivatives, guanidine derivations, and mixtures ofamines-organophosphorus derivatives.Zhang et al. successfully developed an environmentally

benign, non-acid process for selective recovery of Pd fromwaste PCBs [56]. After enriching Pd with a solution ofCuSO4 and NaCl, the dissolved Pd was extracted bydiisoamyl sulfide (S201). In the overall extraction process,the influence of base metals was negligible, because of therelatively weak nucleophilic substitution of S201 with basemetal irons and the strong steric hindrance of S201molecules. About 99.5% of the extracted Pd (II) could bestripped from S201/dodecane with 0.1 mol$L–1 NH3 after atwo-stage stripping at an aqueous-to-organic ratio (A/O) of1. The total recovery rate of Pd was 96.9% during thedissolution-extraction-stripping process. Lee et al. usedsolvent extraction and precipitation methods to develop aprocess to separate Pt and Rh from a chloride leach liquorof spent automobile catalytic converters containing basemetals such as Al, Mg, Fe [57]. The selective separation ofPt was achieved with 0.01 mol$L–1 Aliquot 336 (aquaternary ammonium salt) at an A/O of 3.3 in twostages. Stripping of Pt from loaded organic at an A/O of 6with 0.5 mol$L–1 thiourea and HCl indicated a 99.9%stripping efficiency. In stripping studies, needle-likecrystals of Pt were found and identified as tetrakis(thiourea) Pt (II) chloride ([Pt (tu)4]Cl2). The selectiveprecipitation of Rh was performed using (NH4)2S from Ptfree raffinate, with a recovery of more than 99%.The separation of rare metal ions is easier than that of

precious metal ions. Because the composition of a leachingsolution including rare metals is relatively simple, theselection of the appropriate method for recovering RPMs,like the selection of the precipitation or extracting agent, israther easy. Chen et al. developed a solvent extractionprocess for the separation and recovery of Cu, Mn, Co, Ni,and Li from leaching liquor of spent LIBs [58]. First,copper ions were selectively extracted using Mextral5640H as the extraction reagent after the removal ofimpurity ions. Manganese ions were then selectivelyseparated and precipitated using KMnO4 solution, andabout 99.2% manganese was removed and precipitated asMnO2 and Mn2O3. Subsequently, nickel-loaded Mextral272P was used as a new extraction reagent to separate andrecover cobalt from the leaching liquor. Finally, Ni and Liwere recovered in the form of Ni(OH)2 and Li3PO4 afterfiltration and drying by precipitation using NaOH andNa3PO3 solutions. Recovery efficiencies were 100% for

Cu, 99.2% for Mn, 97.8% for Co and 95.8% for Li, underthe optimized experimental conditions. This study indi-cated that solvent extraction was a candidate for theefficient separation and complete recovery of all metalsfrom the leaching liquor.Banda et al. investigated the separation and recovery of

Mo and Co from synthetic chloride leach liquor (Mo 394mg$L–1, Al 1782 mg$L–1 and Co 119 mg$L–1 in 3 mol$L–1

HCl) from a petroleum refining catalyst, employing TOPOand Alamine 308 as extractants [59]. The separation of Mofrom Co and Al was achieved with 0.05 mol$L–1 TOPO inEscaid 110, and complete stripping of Mo was attainedwith a combination of 0.1 mol$L–1 NH4OH and 0.05 mol$L–1 (NH4)2CO3. After separation of Mo, Co can beselectively extracted by Alanine 308 from Mo-freeraffinate, after adjusting the concentration of chlorideions to 5 mol$L–1 by adding AlCl3. The back-extraction ofcobalt was obtained quickly from loading Alanine 308with acidified water (pH = 1.0). And finally, Mo and Corecovery rates of 99.4% and 99.1% were obtained,respectively.Solvent extraction has been applied widely for separa-

tion and purification of RPMs from low-concentrationsources because it has the advantages of high selectivity,high RPM purification, and effective recovery rates ofmetals, using multi-stage extraction. Compared with theother separation and purification technologies, solventextraction reduces the need for excessive stages, short-ening the refining times significantly and lowering theproduction costs. However, its development depends onthe successful production and rational use of extractionagents with high efficiency, easy availability and low cost.Since the middle 1980s, hydrometallurgical processing

has been the predominant method of metals recycling.Compared with pyrometallurgy, hydrometallurgy hasmany advantages, such as low equipment investment,easy operation, high recovery efficiency of RPMs, goodselectivity of target elements, and mild leaching condi-tions. However, it still has some defects and shortages,such as the generation of wastewater and corrosion ofequipment. In the past decades some modified and novelhydrometallurgical technologies have been developed,such as mild extraction, to solve the problem of equipmentcorrosion and environmental pollution. And soon, hydro-metallurgy technology will be gradually applied inindustry for the recovery of RPMs from urban mines.

3.4 Biometallurgical technology

The RPM-recycling technologies discussed above— pyr-ometallurgical and hydrometallurgical methods— causesecondary pollution such as dioxin and furan emissions,and wastewater generation. In the last decade, however,biometallurgical technology has been developed as analternative technique for recovering RPMs from very


low-grade urban minerals. This technology includes twomain approaches: bioleaching and biosorption. Table S3summarizes the studies of RPM recovery from urbanmines using these processes, with emphasis on theoperating conditions and final results.

3.4.1 Bioleaching

Bioleaching refers to using specific microorganisms,bacteria, fungi, algae, or their metabolites to interact withRPMs via microbial and metal oxidation-reduction reac-tions [60,61]. Bioleaching of RPMs has been performed bya diverse group of microorganisms. As shown in Table S3,bioleaching has been applied widely for RPM recoveryfrom waste PCBs, spent LIBs, and scrap catalysts;however, the recovery efficiency is relatively small ascompared with hydrometallurgical leaching. Moreover,most of the applications of this method are still at thelaboratory scale.For bioleaching, Chromobacterium violaceum (C.

violaceum) has been widely applied for RPM recoveryfrom e-waste. For instance, Faramarzi et al. investigatedAu recovery from waste PCBs using a C. violaceumbioleaching process [62]. First, waste PCBs were crushedinto pieces (5 mm � 10 mm) manually, with each piececontaining approximately 10 mg of Au. Then, the pieceswere subjected to extraction by C. violaceum. The resultsdemonstrated that Au can be microbially solubilized fromPCBs, and the maximum dicyanoaurate [Au (CN)2

–]measured corresponds to a 14.9% dissolution of the goldinitially added. Brandl and his research group alsoevaluated the potential of cyanogenic microorganismssuch as C. violaceum, P. fluorescens, and P. plecoglossi-cida to solubilize metals from metal-containing solids: inparticular, the biological mobilization of Ag, Au, and Pt ascyano complexes which were identified by high-pressureliquid chromatography [63,64]. Finally, the findingsindicated the potential of microbial mobilization of metalsas cyanide compound from solid materials, and represent anovel type of microbial metal mobilization (termed“biocyanidation”) that might find industrial application.

3.4.2 Biosorption

Biosorption refers to the passive sorption and/or com-plexation of RPM ions by biological materials. Themechanisms of biosorption are based on physico-chemicalinteractions between RPM ions and the functional groupspresent on the biological material surface, such ascomplexation, ion exchange, and metal ion chelation orelectrostatic interactions [65,66]. Biosorbents, includingvarious microorganisms, algae, bacteria, yeasts, fungi, andeven biowaste materials, have been developed to accumu-late RPMs efficiently [67]. For example, Mata et al.successfully recovered Au (III) using dead biomass of the

brown alga Fucus vesiculous as metallic gold nanoparticles[68]. As can be seen from Table S3, the results showed thatbrown alga reduced Au (III) to Au (0) under the optimumpH of 7. In recent years, studies reported by researchers inthe Republic of Korea showed the feasibility of biosor-bents for recovery of PMs Au, Ag and Pt from e-waste andwaste catalysts [69–71]. For instance, Won et al. reportedthe recovery of Pt with a high-performance biosorbent,poly-ethylenimine (PEI)-modified biomass, prepared byattaching PEI onto the surface of inactive Escherichia colibiomass [69]. Wastewater collected from an industriallaboratory for ICP using PEI-modified biomass wasstudied, and the results showed that the maximumplatinum uptake of PEI-modified biomass was enhancedup to 108.8 mg$g–1, compared to 21.4 mg$g–1 in the rawbiomass.Bisorption belongs to a subsequent purification process

after RPM leaching, and a low-cost option for RPM ionrecovery from an aqueous phase. The biosorption-basedprocess offers many advantages when compared to theconventional methods used: low operating cost, minimiza-tion of the volume of chemical and/or biological sludge tobe handled, and high efficiency in detoxifying effluents.The biometallurgical process has the advantages of

simplicity, strong maneuverability, low cost and littleenvironmental pollution. However, most of the applica-tions of biometallurgy for recovering RPMs are still at thelaboratory scale because of low leaching rates and longoperating times. Moreover, in the bioleaching process, themicroorganisms can be poisoned by toxic elements such asarsenic and chromium leached from the e-waste. In mostcases, relying on biological leaching alone is inadequatefor thorough recovery of RPMs. Therefore, there is scantyprecedent for industrial application of microbiologicalmethods for RPM recovery from urban mines.In summary, as a pre-treatment process, mechanical or

physical technology is necessary for all e-wastes, since itcan effectively enrich RPMs for further metal leaching.Pyrometallurgical technology is simple, and has a largecapacity and wide range of applications. It has beenapplied for the effective recovery of RPMs from wastes foryears. Compared with pyrometallurgy, hydrometallurgy iseasy to operate and has shown high metal recovery rates.Hydrometallurgical technology is applied after high-temperature melting enrichment. Biometallurgical pro-cesses are the most environmentally friendly methodcompared with the above techniques, because no waste-water or toxic gases are generated. However, the smallleaching rates and long operating times limit its applica-tion. Therefore, two or three methods are required for RPMrecovery with high recovery rates and low pollution.

4 Emerging technology

In addition to the above traditional technologies, electro-


chemical technology, supercritical fluid technology, ionicliquid technology, and mechanochemical technology havebeen increasingly developed for RPM recovery, in recentdecades. Following is a brief introduction to severalemerging technologies.

4.1 Electrochemical technology

Electrochemical processing is an emerging technology andoffers an alternative method for recovering RPMs fromurban mines, due to its high energy efficiency, highenvironmental compatibility and minimal chemical usage.Electrochemical technology is essentially a deformation ofhydrometallurgy. Electrochemistry-based RPM recoveryschemes could be described as follows: 1) anodicgeneration of oxidizers to speed dissolution of RPMsfrom solid materials or 2) cathodic electro-winning ofleached RPMs for separation and recovery. Given properconsideration, the two processes may be performed in asingle cell [20,72]. Compared with RMs, PMs are moredifficult to dissolve due to their high inertness. The currentelectrochemical process has only been employed for thedissolution and leaching of PMs. And the dissolution ofPMs also requires the presence of oxidants and complexingagents. Some researchers have reported the use of theelectrochemical process to recover metals from wastematerials, while scanty research has focused on RPMrecovery [73,74].An electro-recycling system for a mobile electronic

recycling scheme using two-stage dissolution was reportedby Lister et al. [72]. An electrochemical cell used a troughconfiguration that also held a cathode solution. Thecathode, consisting of a 9-cm X 10-cm Cu foil, wassuspended into the bulk of the solution for ease ofreplacement as product growth occurred. The first stageused electro-generated Fe3+ in acidic sulfate media todissolve Fe, Ni, Sn, Cu, Ag, and REEs. The PM Ag waspreferentially reduced at the cathode, and Cu depositionhas been well established in an acidic sulfate solution.Kim and his research group from University Park carried

out the leaching of metals from waste PCBs by utilizingelectro-generated chlorine [75,76]. For instance, theleaching of Au from mobile phone PCBs could beachieved using electro-generated chlorine and enriching aconcentration of the purified gold solution from thehydrochloric acid media with an ion exchange procedure[77]. The separate reactor contained an electrolytic cell forchlorine generation and a separate vessel (reactor) for themetal leaching. In the first-stage leaching process, selectivedissolution of copper and gold using electro-generatedchlorine was achieved. The experimental results clearlydemonstrated that copper and gold can be mostly separatedduring the leaching stage itself, by adjusting the solutionacidity and chlorine concentration. The leaching efficiencyof gold increased with increasing temperature, and

dissolved the initial chlorine, besides lowering the amountof acid in a fixed chloride concentration. Subsequently,gold was recovered from the solution in the second stage ofchlorine leaching by ion exchange using Amberlite XAD-7HP. Researchers concluded that the electrochemicalprocess has the advantage of leaching precious as well asbase metals from waste PCBs, because it acquires highoxidation potential. At the same time, it is consideredenvironmentally friendly because the electro-generated Cl2leaching can operate in a closed system and chlorine can bereduced during leaching. In another study, Myoung et al.applied an electrochemical deposition and appropriatethermal treatment to prepare cobalt oxide from Co (III) inwaste LiCoO2 cathodes [78]. Under appropriate pHconditions, island-shaped cobalt hydroxide was precipi-tated on the titanium substrate, and heat treatment of thecobalt hydroxide resulted in the formation of cobalt oxide.The electrochemical process can utilize minimal chemi-

cal input to dissolve RPMs and recover them on a cathodefor further processing. The advantage of this method is thatit uses a single electrochemical cell to maximize energyefficiency. The electro-recycling process can generateoxidizing agents at the anode to dissolve metals from thescrap matrix, and in the meantime the dissolved metals arereduced at the cathode [72]. This emerging techniqueoffers a means for environmentally friendly and fastrecovery of RPMs from waste materials.

4.2 Supercritical fluid technology

In recent decades, supercritical fluid (SC) technology hasbeen introduced as an environmentally friendly method todecompose organic polymers and recycle RPMs. Thismethod has advantages such as low viscosity, a high masstransport coefficient, high diffusivity, and high solubility oforganics. SC refers to the fluids utilized; the temperatureand pressure are increased to the supercritical point. In thevicinity of the critical point, the physical properties(density, viscosity, solubility, heat capacity, dielectricconstant, etc.) of the fluid change abruptly. The interfacebetween the liquid and the gas will disappear and the fluidwill be converted into non-condensable gas. The super-critical fluid is sufficient for the degradation of nonmetallicpolymers and the recovery of metals in urban mines, underthe characteristics of the fluid when it is near thesupercritical point [79].Currently, supercritical carbon dioxide (Sc-CO2) extrac-

tion is the most common supercritical fluid used for metalrecovery using this technology. Sc-CO2 is a good low-costsolvent, with no secondary pollution and no residueextraction. However, in the actual extraction process itself,carbon dioxide is a non-polar molecule. The solute-solventinteraction between the metal ions and the supercriticalcarbon dioxide molecules is very feeble, and the metal ionsare difficult to dissolve into the Sc-CO2: hence the direct


extraction efficiency of the metal ion by Sc-CO2 is verylow [80,81]. During the extraction process, soft and hardacid-base coordination theory is used to select theappropriate chelating or complexing agent, and thecharged metal ions, via the coordination bond, generate aneutral compound that is readily soluble in Sc-CO2. Theneutral complex can therefore be separated from the matrixby mass transfer in the supercritical fluid phase [79]. Atpresent, RPM ions, including the PGMs In3+, Ga3+, Li+,and Co2+, have been extracted using supercritical carbondioxide. Table S4 shows the researches on RPM recoveryusing Sc-CO2 extraction technology.Liu et al. developed an environmentally benign process

for direct recovery of Pd and Ag from waste PCB powder;Fig. 3(a) shows the schematic diagram of this Sc-CO2

extraction apparatus [82]. The process combined super-critical water oxidation (SCWO) and Sc-CO2 extractiontechniques. SCWO treatment could effectively enrich Pdand Ag by degrading the nonmetallic components. In thesecond stage, more than 93.7% of the Pd and 96.4% of theAg could be extracted from the PMC by Sc-CO2 modifiedwith acetone and KI-I2 under optimum conditions. Furtherstudy of the mechanism has indicated that Pd and Agextraction by Sc-CO2 is a complicated physiochemicalprocess, involving oxidation, complexation, anionexchange, mass transfer and migration approaches. Thecombination of SCWO and Sc-CO2, however, establisheda benign and efficient process for selective recovery ofdispersed PMs from waste materials.Other supercritical fluids, such as supercritical water,

also have some application in RM recovery [83]. Liu et al.reported an effective and environmentally friendly processfor the recovery of Co and Li from spent LIBs, andsimultaneous detoxification of polyvinyl chloride (PVC) insubcritical water [84]. The entire co-treatment process isshown in Fig. 3(b). LiCoO2 powder from spent LIBs andPVC was co-treated by SCWO, where PVC served as ahydrochloric acid source to promote the leaching of metals.The results showed that the dechlorination of PVC and theleaching of metals were achieved simultaneously underSCWO. Assessment of economic and environmentalimpacts revealed that the PVC and LiCoO2 subcriticalco-treatment process had significant technical, economicand environmental benefits compared with the traditionalhydrometallurgy and pyrometallurgy processes.In the recycling of urban mine resources, supercritical

fluid technology has shown vast technological superiorityand potential application in the degradation of polymersand the detoxification of halogen-containing pollutants.However, in the field of RPM recovery, supercritical fluidtechnology is still constrained by its technical disadvan-tages, mainly in the following ways: the economicinvestment is high, especially for equipment, and RPMsare easily deposited in the metal pipe wall [85,86]. Despitethese drawbacks, supercritical fluid oxidation is anexcellent metal enrichment technique for dispersedRPMs mixed with organic polymers in urban mines. Sc-CO2 extraction with a suitable extraction agent could alsoefficiently achieve the extraction of a small amount oftarget metals from solid waste materials. Although the

Fig. 3 Supercritical fluid technology for RPM recovery [82,84]: (a) the schematic diagram of Sc-CO2 extraction apparatus, (b) the co-treatment process of LIB and PVC


application of supercritical fluid technology in RPMrecovery is scanty, the technology has shown potentialfor greater application as it progresses.

4.3 Mechanochemical technology

Mechanochemistry, commonly known as ball milling,refers to the study of physico-chemical transformationgenerated by mechanical force. It has been reported thatmechanical forces such as shear, impact and squeezingexerted by ball milling will transmit energy to the resultingpowder, reduce the size of the powder particles, anddestroy the crystal structures. Therefore, the ball millingprocess can reduce the specific surface energy of reactionproducts so as to promote the impossible solid-solidreaction and enhance the reaction efficiency [87,88].Moreover, unlike normal thermochemical reactions,mechanochemical reactions use mechanical energy ratherthan thermal energy, so that the reaction could becompleted without high temperature, high pressure, orother harsh conditions [89].In recent decades, considerable research has been

focused on RPM recovery by mechanochemical methods[90,91]. For example, Wang et al. developed a process forrecovering Co and Li from spent LIBs using a mechan-ochemical approach [92]. In the proposed process, LiCoO2

was co-ground with various additives in a hermetic ball-milling system, followed by a water leaching procedure, sothat neither corrosive acid nor strong oxidant needed to beapplied. It was found that EDTAwas the most suitable co-grinding reagent, and 98% of the Co and 99% of the Liwere recovered under optimum conditions. The mechan-isms of the study implied that a lone pair of electronsprovided by two nitrogen atoms and four hydroxyl oxygenatoms of EDTA could enter the empty orbit of Co and Livia a solid-solid reaction, thus forming the stable andwater-soluble metal chelates Li-EDTA and Co-EDTA.With a non-thermal process, Kano et al. recovered Inthrough mechanochemical reduction of In2O3/ITO bymilling with Li3N under a non-oxidative state of NH3

and/or N2 gas environment [93]. In addition, Lee et al.recovered In from waste LCD panels using high-energyball milling assisted with acid leaching [37]. Zhang et al.developed a process for metal recovery from alloy wastesand LiCoO2 powder by co-grinding with polyvinylchloride (PVC) [90].Mechanochemistry has demonstrated the ability to

modify and enhance the leaching process significantly.Tan and Li provided a systematic review of the utilizationof mechanochemistry in metal recycling of wastes. Theydiscussed the effect of mechanochemical methods onphysicochemical changes and the reactions that occurredduring these processes, and analyzed the mechanismsinvolved [94]. This promising technology can be used as ameans of pre-treatment, and, conjoined with hydrometal-

lurgical technology, to recover RPMs. In this way, therecovery rates of RPMs could be significantly higher thanwith ordinary hydrometallurgy.

4.4 Ionic liquid technology

Recently, ionic liquids (ILs) have caused intense interest asalternatives to conventional organic solvents, because inconventional solvent extraction, the use of large amountsof volatile organic solvents has been problematic [95]. ILshave unique physicochemical properties: for example, theycan be composed of various cations and anions that changethe properties and phase behaviors of liquids. Thereforethey have been applied mainly in separation methods, suchas ionic liquid-supported membranes, mobile phaseadditives, etc [96,97]. Their application in RPM extractionhas been expanded recently by researchers’ efforts in thedevelopment of extractants compatible with hydrophobicILs, because their extraction ability and selectivity forRPMs have been found to increase when using hydro-phobic ionic liquids, which have potential applications inliquid-liquid extraction processes.Yang studied the recycling of rare earth metals (REEs)

from phosphor powders in waste fluorescent lamps bysolvent extraction using ILs [98]. After acid leaching theREEs from the waste phosphor powder, ILs were used asan extraction-phase alternative to the conventional organicdiluent. An effective recovery of the REEs Y, Eu, La, andCe, from impure metal samples of Fe, Al and Zn, wasachieved from an acidic leaching solution of phosphorpowders using an innovative ILs-containing extractant (N,N-dioctyldiglycol amic acid). Papaiconomou et al. studiedtask-specific ILs for Ag (I) and Pd (II) extraction fromwater solutions [99]. The research results found that thesynthesized ILs containing a nitrile functional group wereefficient and selective for extracting Ag and Pd ions fromwater. No other metal ions were significantly extracted byany of these ILs. This research proved that the selectiveextraction of precious-metal ions could be achieved byusing task-specific ILs with specific functional groups. Inaddition to the functional groups, the cation ring as well asthe anion of an ionic liquid also has an influence ondistribution coefficients.ILs can contribute significantly to the development of

novel and highly efficient separation processes for RPMs.However, the ionic liquids are more expensive thantraditional solvents— a major factor limiting their usagein the practical production process, especially when largequantities of solvent are required [100]. With thedevelopment of more cost-effective ILs, it would beappropriate to experiment with their application in RPMrecovery. Furthermore, the ability of ILs to be recycled isan important advantage for their implementation in acommercial environment, as the recyclability saves moneyand offers other benefits.


The comparisons among the four emerging technologiesare as follows: the electrochemical process is suitable forPM recovery, with good selectivity and effective recoveryrates; supercritical fluid oxidation is an excellent metalsenrichment technique for dispersed RPMs, and Sc-CO2

extraction can also efficiently separate and recover a smallamount of target RPMs from solutions or solid wastematerials; the mechanochemical method is used to recovermore active RPMs, like Li and Co, with excellent recoveryrates; ILs technology is a novel and excellent separationprocess, and it can be used for all RPM recovery withsuperior selectivity.

5 Conclusions and prospects

In summary, all types of urban mines, such as waste PCBs,scrap catalysts, and spent LIBs, contain a variety ofvaluable RPM resources. High concentrations of RPMspresent in urban mines make them great secondaryresources for the sustainable utilization of renewableresources. RPM recovery can be regarded as a win-winsituation for both the environment and the economy.In the past several decades, extensive research has been

carried out, and applications have been developed torecover RPMs from urban mines. Pyrometallurgicaltechnology has been applied in practice for years forRPM recovery, but the expensive equipment required haslimited its use in small and medium-sized enterprises.Compared with pyrometallurgy, hydrometallurgy hasmany advantages, such as low cost of equipmentinvestment, easy operation, good selectivity of targetelements, and high recovery efficiency of RPMs. However,it presents some challenges from the environmentalperspective. The biometallurgical process has been oneof the most promising technologies for the bioleaching ofRPMs from urban mines because of its low cost andenvironmental friendliness. Finally, some alternative newtechnologies, such as electrochemical, supercritical fluid,mechanochemical and ionic liquid, have received seriousattention and achieved some successful laboratory applica-tion examples.Researchers have made extraordinary efforts, but there is

no a single process that can achieve the goal of recyclingRPMs completely, due to the complexity of urban minewastes. Hence, in the future, some combined andintegrated technologies should be applied to recovermetals from urban mines. For example, the pyrometallur-gical process could be a significant pre-concentrationmethod for RPMs dispersed in waste materials, if theemissions of harmful volatile organic compounds and toxicsmoke can be well controlled. We predict that futuretechnologies will be more efficient, cheaper, and envir-onmentally friendly, and will combine various technolo-gies, such as chemical, biological, and so on.

Acknowledgements This study was financially supported by the NationalKey Technology R&D Program of China (No. 2014BAC03B04).

Electronic Supplementary Material Supplementary material is availablein the online version of this article at http://dx.doi.org/10.1007/s11783-017-0963-1 and is accessible for authorized users.

References

1. Zhou T, Goldfarb R J, Phillips N G. Tectonics and distribution of

gold deposits in China: an overview. Mineralium Deposita, 2002,

37(3): 249–282

2. Pirajno F, Bagas L. Gold and silver metallogeny of the South

China Fold Belt: a consequence of multiple mineralizing events?

Ore Geology Reviews, 2002, 20(3): 109–126

3. Huang K, Guo J, Xu Z. Recycling of waste printed circuit boards: a

review of current technologies and treatment status in China.

Journal of Hazardous Materials, 2009, 164(2–3): 399–408

4. Akcil A, Erust C, Gahan C S, Ozgun M, Sahin M, Tuncuk A.

Precious metal recovery from waste printed circuit boards using

cyanide and non-cyanide lixiviants: a review. Waste Management

(New York, N.Y.), 2015, 45: 258–271

5. Kim C H, Woo S I, Jeon S H. Recovery of platinum-group metals

from recycled automotive catalytic converters by carbochlorina-

tion. Industrial & Engineering Chemistry Research, 2000, 39(5):

1185–1192

6. Glaister B J, Mudd G M. The environmental costs of platinum–

PGM mining and sustainability: Is the glass half-full or half-

empty? Minerals Engineering, 2010, 23(5): 438–450

7. Xiao Z, Laplante A R. Characterizing and recovering the platinum

group minerals: a review. Minerals Engineering, 2004, 17(9–10):

961–979

8. Muchova L, Bakker E, Rem P. Precious metals in municipal solid

waste incineration bottom ash. Water Air and Soil Pollution Focus,

2008, 9(1–2): 107–116

9. Wei S, Liu J, Zhang S, Chen X, Liu Q, Zhu L, Guo L, Liu X.

Stoichiometry, isotherms and kinetics of adsorption of In(III) on

Cyanex 923 impregnated HZ830 resin from hydrochloric acid

solutions. Hydrometallurgy, 2016, 164: 219–227

10. Font O, Querol X, Juan R, Casado R, Ruiz C R, Lopez-Soler A,

Coca P, Garcia Pena F. Recovery of gallium and vanadium from

gasification fly ash. Journal of Hazardous Materials, 2007, 139(3):

413–423

11. Gupta B, Mudhar N, Begum Z, Singh I. Extraction and recovery of

Ga(III) from waste material using Cyanex 923. Hydrometallurgy,

2007, 87(1–2): 18–26

12. Wang A, Wang Y, Kabe T, Chen Y, Ishihara A, Qian W.

Hydrodesulfurization of dibenzothiophene over siliceous MCM-

41-supported catalysts. Journal of Catalysis, 2001, 199(1): 19–29

13. Song C, Reddy K M. Mesoporous molecular sieve MCM-41

supported Co–Mo catalyst for hydrodesulfurization of diben-

zothiophene in distillate fuels. Applied Catalysis A, General, 1999,

176(1): 1–10

14. Duan H, Wang J, Liu L, Huang Q, Li J. Rethinking China’s

strategic mineral policy on indium: implication for the flat screens


and photovoltaic industries. Progress in Photovoltaics: Research

and Applications, 2016, 24(1): 83–93

15. Scrosati B, Garche J. Lithium batteries: status, prospects and

future. Journal of Power Sources, 2010, 195(9): 2419–2430

16. Guo X, Liu J, Qin H, Liu Y, Tian Q, Li D. Recovery of metal

values from waste printed circuit boards using an alkali fusion–

leaching–separation process. Hydrometallurgy, 2015, 156: 199–

205

17. Alfantazi A M, Moskalyk R R. Processing of indium: a review.

Minerals Engineering, 2003, 16(8): 687–694

18. Li J. Wastes could be resources and cities could be mines. Waste

Management & Research, 2015, 33(4): 301–302

19. Binnemans K, Jones P T, Blanpain B, Van Gerven T, Yang Y,

Walton A, Buchert M. Recycling of rare earths: a critical review.

Journal of Cleaner Production, 2013, 51: 1–22

20. Zhang L, Xu Z. A review of current progress of recycling

technologies for metals from waste electrical and electronic

equipment. Journal of Cleaner Production, 2016, 127: 19–36

21. Zeng X, Zheng L, Xie H, Lu B, Xia K, Chao K, Li W, Yang J, Lin

S, Li J. Current status and future perspective of waste printed

circuit boards recycling. Procedia Environmental Sciences, 2012,

16: 590–597

22. Goodenough J B, Park K S. The Li-ion rechargeable battery: a

perspective. Journal of the American Chemical Society, 2013, 135

(4): 1167–1176

23. Reddi G S, Rao C R M. Analytical techniques for the

determination of precious metals in geological and related

materials. Analyst (London), 1999, 124(11): 1531–1540

24. Zhang J F, Zhou Y, Yoon J, Kim J S. Recent progress in fluorescent

and colorimetric chemosensors for detection of precious metal ions

(silver, gold and platinum ions). Chemical Society Reviews, 2011,

40(7): 3416–3429

25. Kumar V, Lee J C, Jeong J, Jha M K, Kim B S, Singh R. Recycling

of printed circuit boards (PCBs) to generate enriched rare metal

concentrate. Journal of Industrial and Engineering Chemistry,

2015, 21: 805–813

26. Cui J, Zhang L. Metallurgical recovery of metals from electronic

waste: A review. Journal of Hazardous Materials, 2008, 158(2–3):

228–256

27. Zeng X,Wang F, Sun X, Li J. Recycling indium from scraped glass

of liquid crystal display: process optimizing and mechanism

exploring. ACS Sustainable Chemistry & Engineering, 2015, 3(7):

1306–1312

28. Zeng X, Li J, Singh N. Recycling of spent lithium-ion battery: a

critical review. Critical Reviews in Environmental Science and

Technology, 2014, 44(10): 1129–1165

29. Zeng X, Gong R, Chen W Q, Li J. Uncovering the recycling

potential of “new” WEEE in China. Environmental Science &

Technology, 2016, 50(3): 1347–1358

30. Zeng X, Li J, Liu L. Solving spent lithium-ion battery problems in

China: opportunities and challenges. Renewable & Sustainable

Energy Reviews, 2015, 52: 1759–1767

31. Li J, Shi P, Wang Z, Chen Y, Chang C C. A combined recovery

process of metals in spent lithium-ion batteries. Chemosphere,

2009, 77(8): 1132–1136

32. Zeng X, Li J, Ren Y.Prediction of various discarded lithium

batteries in China. In: 2012 IEEE International Symposium on

Sustainable Systems and Technology (ISSST). Beijing: IEEE,

2012,1–4

33. Xu J, Thomas H R, Francis R W, Lum K R, Wang J, Liang B. A

review of processes and technologies for the recycling of lithium-

ion secondary batteries. Journal of Power Sources, 2008, 177(2):

512–527

34. Stevels A, Huisman J, Wang F, Li J, Li B, Duan H. Take back and

treatment of discarded electronics: a scientific update. Frontiers of

Environmental Science & Engineering, 2013, 7(4): 475-482

35. Zeng X, Li J. Implications for the carrying capacity of lithium

reserve in China. Resources, Conservation and Recycling, 2013,

80: 58–63

36. Jian C, Jisheng Y, Youyuan Z, Zhifei C, Xi W, Junwu H.Recovery

indium from waster ITO target. Chinese Journal of Rare Metals,

2003, 1: 023 (in Chinese)

37. Lee C H, Jeong MK, Fatih Kilicaslan M, Lee J H, Hong H S, Hong

S J. Recovery of indium from used LCD panel by a time efficient

and environmentally sound method assisted HEBM. Waste

Management (New York, N.Y.), 2013, 33(3): 730–734

38. Ivanovic S Z, Trujuc V K, Gorgievski M D, Misic L D, Bozic D S.

Removal of platinum group metals (PGMs) from the spent

automobile catalyst by the pyrometallurgical process. In: Ekinovi

S, Calvet J V, Tacer E, eds. Trends in the Development of

Machinery and Associated Technology. Prague: TMT2011, 2011,

701

39. Sun F, Wu W, Wu Z, Guo J, Wei Z, Yang Y, Jiang Z, Tian F, Li C.

Dibenzothiophene hydrodesulfurization activity and surface sites

of silica-supported MoP, Ni2P, and NiMoP catalysts. Journal of

Catalysis, 2004, 228(2): 298–310

40. Shabaker J. Aqueous-phase reforming of methanol and ethylene

glycol over alumina-supported platinum catalysts. Journal of

Catalysis, 2003, 215(2): 344–352

41. Yang Z, Rui-lin M, Wang-dong N, Hui W. Selective leaching of

base metals from copper smelter slag. Hydrometallurgy, 2010, 103

(1–4): 25–29

42. Cui J, Forssberg E. Mechanical recycling of waste electric and

electronic equipment: a review. Journal of Hazardous Materials,

2003, 99(3): 243–263

43. Yoo J M, Jeong J, Yoo K, Lee J, Kim W. Enrichment of the

metallic components from waste printed circuit boards by a

mechanical separation process using a stamp mill. Waste Manage-

ment (New York, N.Y.), 2009, 29(3): 1132–1137

44. Lee J C, Song H T, Yoo J M. Present status of the recycling of

waste electrical and electronic equipment in Korea. Resources,

Conservation and Recycling, 2007, 50(4): 380–397

45. Zhou Y, Qiu K. A new technology for recycling materials from

waste printed circuit boards. Journal of Hazardous Materials, 2010,

175(1–3): 823–828

46. Hagelüken C, Corti C W. Recycling of gold from electronics: cost-

effective use through “Design for Recycling”. Gold Bulletin, 2010,

43(3): 209–220

47. Li J, Wang G, Xu Z. Environmentally-friendly oxygen-free

roasting/wet magnetic separation technology for in situ recycling

cobalt, lithium carbonate and graphite from spent LiCoO2/graphite

lithium batteries. Journal of Hazardous Materials, 2016, 302: 97–


104

48. Ma E, Lu R, Xu Z. An efficient rough vacuum-chlorinated

separation method for the recovery of indium from waste liquid

crystal display panels. Green Chemistry, 2012, 14(12): 3395

49. Kakumazaki J, Kato T, Sugawara K. Recovery of gold from

incinerated sewage sludge ash by chlorination. ACS Sustainable

Chemistry & Engineering, 2014, 2(10): 2297–2300

50. Tuncuk A, Stazi V, Akcil A, Yazici E Y, Deveci H. Aqueous metal

recovery techniques from e-scrap: Hydrometallurgy in recycling.

Minerals Engineering, 2012, 25(1): 28–37

51. Sun L, Qiu K. Organic oxalate as leachant and precipitant for the

recovery of valuable metals from spent lithium-ion batteries. Waste

Management (New York, N.Y.), 2012, 32(8): 1575–1582

52. Li L, Qu W, Zhang X, Lu J, Chen R, Wu F, Amine K. Succinic

acid-based leaching system: a sustainable process for recovery of

valuable metals from spent Li-ion batteries. Journal of Power

Sources, 2015, 282: 544–551

53. Zeng X, Li J, Shen B. Novel approach to recover cobalt and lithium

from spent lithium-ion battery using oxalic acid. Journal of

Hazardous Materials, 2015, 295: 112–118

54. Nayaka G P, Pai K V, Santhosh G, Manjanna J. Dissolution of

cathode active material of spent Li-ion batteries using tartaric acid

and ascorbic acid mixture to recover Co. Hydrometallurgy, 2016,

161: 54–57

55. Nguyen T H, Sonu C H, Lee M S. Separation of Pt(IV), Pd(II), Rh

(III) and Ir(IV) from concentrated hydrochloric acid solutions by

solvent extraction. Hydrometallurgy, 2016, 164: 71–77

56. Zhang Z, Zhang F S. Selective recovery of palladium from waste

printed circuit boards by a novel non-acid process. Journal of

Hazardous Materials, 2014, 279: 46–51

57. Lee J Y, Raju B, Kumar B N, Kumar J R, Park H K, Reddy B R.

Solvent extraction separation and recovery of palladium and

platinum from chloride leach liquors of spent automobile catalyst.

Separation and Purification Technology, 2010, 73(2): 213–218

58. Chen X, Xu B, Zhou T, Liu D, Hu H, Fan S. Separation and

recovery of metal values from leaching liquor of mixed-type of

spent lithium-ion batteries. Separation and Purification Technol-

ogy, 2015, 144: 197–205

59. Banda R, Sohn S H, Lee M S. Process development for the

separation and recovery of Mo and Co from chloride leach liquors

of petroleum refining catalyst by solvent extraction. Journal of

Hazardous Materials, 2012, 213–214: 1–6

60. Watling H R. The bioleaching of sulphide minerals with emphasis

on copper sulphides: a review. Hydrometallurgy, 2006, 84(1–2):

81–108

61. Zhao L, Wang L, Yang D, Zhu N. Bioleaching of spent Ni-Cd

batteries and phylogenetic analysis of an acidophilic strain in

acidified sludge. Frontiers of Environmental Science & Engineer-

ing in China, 2007, 1(4): 459–465

62. Faramarzi M A, Stagars M, Pensini E, Krebs W, Brandl H. Metal

solubilization from metal-containing solid materials by cyanogenic

Chromobacterium violaceum. Journal of Biotechnology, 2004, 113

(1–3): 321–326

63. Brandl H, Bosshard R, Wegmann M. Computer-munching

microbes: metal leaching from electronic scrap by bacteria and

fungi. Hydrometallurgy, 2001, 59(2–3): 319–326

64. Brandl H, Lehmann S, Faramarzi M A, Martinelli D. Biomobiliza-

tion of silver, gold, and platinum from solid waste materials by

HCN-forming microorganisms. Hydrometallurgy, 2008, 94(1–4):

14–17

65. Gadd G M. Biosorption: critical review of scientific rationale,

environmental importance and significance for pollution treatment.

Journal of Chemical Technology and Biotechnology (Oxford,

Oxfordshire), 2009, 84(1): 13–28

66. Gadd G M, Yao Q, Zhang H, Wu J, Shao L, He P. Biosorption of

Cr(III) from aqueous solution by freeze-dried activated sludge:

Equilibrium, kinetic and thermodynamic studies. Frontiers of

Environmental Science & Engineering in China, 2010, 4(3): 286–

294

67. Das N. Recovery of precious metals through biosorption — A

review. Hydrometallurgy, 2010, 103(1–4): 180–189

68. Mata Y N, Torres E, Blazquez M L, Ballester A, Gonzalez F,

Munoz J A. Gold(III) biosorption and bioreduction with the brown

alga Fucus vesiculosus. Journal of Hazardous Materials, 2009, 166

(2–3): 612–618

69. Won S W, Mao J, Kwak I S, Sathishkumar M, Yun Y S. Platinum

recovery from ICP wastewater by a combined method of

biosorption and incineration. Bioresource Technology, 2010, 101

(4): 1135–1140

70. Won S W, Kotte P, Wei W, Lim A, Yun Y S. Biosorbents for

recovery of precious metals. Bioresource Technology, 2014, 160:

203–212

71. Won S W, Kwak I S, Yun Y S. The role of biomass in

polyethylenimine-coated chitosan/bacterial biomass composite

biosorbent fiber for removal of Ru from acetic acid waste solution.

Bioresource Technology, 2014, 160: 93–97

72. Lister T E, Wang P, Anderko A. Recovery of critical and value

metals from mobile electronics enabled by electrochemical

processing. Hydrometallurgy, 2014, 149: 228–237

73. Oishi T, Koyama K, Alam S, Tanaka M, Lee J C. Recovery of high

purity copper cathode from printed circuit boards using ammonia-

cal sulfate or chloride solutions. Hydrometallurgy, 2007, 89(1–2):

82–88

74. Oishi T, Yaguchi M, Koyama K, Tanaka M, Lee J C.

Hydrometallurgical process for the recycling of copper using

anodic oxidation of cuprous ammine complexes and flow-through

electrolysis. Electrochimica Acta, 2008, 53(5): 2585–2592

75. Kim E Y, Kim M, Lee J, Jha M K, Yoo K, Jeong J. Effect of

cuprous ions on Cu leaching in the recycling of waste PCBs, using

electro-generated chlorine in hydrochloric acid solution. Minerals

Engineering, 2008, 21(1): 121–128

76. Kim E Y, Kim M, Lee J, Yoo K, Jeong J. Leaching behavior of

copper using electro-generated chlorine in hydrochloric acid

solution. Hydrometallurgy, 2010, 100(3–4): 95–102

77. Kim E Y, Kim M, Lee J, Pandey B D. Selective recovery of gold

from waste mobile phone PCBs by hydrometallurgical process.

Journal of Hazardous Materials, 2011, 198: 206–215

78. Myoung J, Jung Y, Lee J, Tak Y. Cobalt oxide preparation from

waste LiCoO2 by electrochemical–hydrothermal method. Journal

of Power Sources, 2002, 112(2): 639–642

79. Teoh W H, Mammucari R, Foster N R. Solubility of organome-

tallic complexes in supercritical carbon dioxide: a review. Journal


of Organometallic Chemistry, 2013, 724: 102–116

80. Herrero M, Mendiola J A, Cifuentes A, Ibáñez E. Supercritical

fluid extraction: recent advances and applications. Journal of

Chromatography. A, 2010, 1217(16): 2495–2511

81. Erkey C. Supercritical carbon dioxide extraction of metals from

aqueous solutions: a review. Journal of Supercritical Fluids, 2000,

17(3): 259–287

82. Liu K, Zhang Z, Zhang F S. Direct extraction of palladium and

silver from waste printed circuit boards powder by supercritical

fluids oxidation-extraction process. Journal of Hazardous Materi-

als, 2016, 318: 216–223

83. Xiu F R, Qi Y, Zhang F S. Leaching of Au, Ag, and Pd from waste

printed circuit boards of mobile phone by iodide lixiviant after

supercritical water pre-treatment. Waste Management (New York,

N.Y.), 2015, 41: 134–141

84. Liu K, Zhang F S. Innovative leaching of cobalt and lithium from

spent lithium-ion batteries and simultaneous dechlorination of

polyvinyl chloride in subcritical water. Journal of Hazardous

Materials, 2016, 316: 19–25

85. Xiu F R, Qi Y, Zhang F S. Recovery of metals from waste printed

circuit boards by supercritical water pre-treatment combined with

acid leaching process. Waste Management (New York, N.Y.),

2013, 33(5): 1251–1257

86. Xing M, Zhang F S. Degradation of brominated epoxy resin and

metal recovery from waste printed circuit boards through batch

sub/supercritical water treatments. Chemical Engineering Journal,

2013, 219: 131–136

87. Nasser A, Mingelgrin U. Mechanochemistry: a review of surface

reactions and environmental applications. Applied Clay Science,

2012, 67–68: 141–150

88. Friščić T. New opportunities for materials synthesis using

mechanochemistry. Journal of Materials Chemistry, 2010, 20

(36): 7599

89. James S L, Adams C J, Bolm C, Braga D, Collier P, Friscic T,

Grepioni F, Harris K D, Hyett G, Jones W, Krebs A, Mack J, Maini

L, Orpen A G, Parkin I P, Shearouse WC, Steed JW,Waddell D C.

Mechanochemistry: opportunities for new and cleaner synthesis.

Chemical Society Reviews, 2012, 41(1): 413–447

90. Zhang Q, Saeki S, Tanaka Y, Kano J, Saito F. A soft-solution

process for recovering rare metals from metal/alloy-wastes by

grinding and washing with water. Journal of Hazardous Materials,

2007, 139(3): 438–442

91. Yuan W, Li J, Zhang Q, Saito F. Innovated application of

mechanical activation to separate lead from scrap cathode ray tube

funnel glass. Environmental Science & Technology, 2012, 46(7):

4109–4114

92. Wang M M, Zhang C C, Zhang F S. An environmental benign

process for cobalt and lithium recovery from spent lithium-ion

batteries by mechanochemical approach. Waste Management (New

York, N.Y.), 2016, 51: 239–244

93. Kano J, Kobayashi E, Tongamp W, Miyagi S, Saito F. Non-

thermal reduction of indium oxide and indium tin oxide by

mechanochemical method. Journal of Alloys and Compounds,

2009, 484(1–2): 422–425

94. Tan Q, Li J. Recycling metals from wastes: a novel application of

mechanochemistry. Environmental Science & Technology, 2015,

49(10): 5849–5861

95. Whitehead J A, Lawrance G A, McCluskey A. “Green” leaching:

recyclable and selective leaching of gold-bearing ore in an ionic

liquid. Green Chemistry, 2004, 6(7): 313–315

96. Han D, Row KH. Recent applications of ionic liquids in separation

technology. Molecules (Basel, Switzerland), 2010, 15(4): 2405–

2426

97. Fischer L, Falta T, Koellensperger G, Stojanovic A, Kogelnig D,

Galanski M, Krachler R, Keppler B K, Hann S. Ionic liquids for

extraction of metals and metal containing compounds from

communal and industrial waste water. Water Research, 2011, 45

(15): 4601–4614

98. Yang F, Kubota F, Baba Y, Kamiya N, Goto M. Selective

extraction and recovery of rare earth metals from phosphor

powders in waste fluorescent lamps using an ionic liquid system.

Journal of Hazardous Materials, 2013, 254–255: 79–88

99. Papaiconomou N, Lee J M, Salminen J, von Stosch M, Prausnitz J

M. Selective extraction of copper, mercury, silver, and palladium

ions from water using hydrophobic ionic liquids. Industrial &

Engineering Chemistry Research, 2007, 47(15): 5080–5086

100. Zeng X, Li J. Innovative application of ionic liquid to separate Al

and cathode materials from spent high-power lithium-ion batteries.

Journal of Hazardous Materials, 2014, 271: 50–56

Author Biographies

Joseph F. Chiang, professor of Chemistry/Biochemistry State University of New YorkCollege. His research includes the follow-ing fields: Gas phase electron diffraction,X-ray crystallography, quantum mechanicalcalculations, Studies of ceramic glass fromfly ash/waste utilization and treatment,Micro- and Nano-Therapeutics devices,and solar cells research.

In 2005, he was appointed as Distinguished Visiting Professor byTsinghua University, Beijing, China. He was also appointed byShanghai University and Beijing University of Chemical Technol-ogy as Visiting Professor. Professor Chiang is a Life Member ofAmerican Physical Society, and Chinese American ChemicalSociety, a member of American Chemical Society, MaterialsResearch Society, and AAAS. He is now the Associate Editor ofthe journal Frontiers of Environmental Science and Engineering.

Professor Li Jinhui, doctoral tutor, Divi-sion of Solid Waste Management, School ofEnvironment, Tsinghua University. Hisresearch includes hazardous waste manage-ment, environmental risk analysis, eco-design and management of electronicproducts, disposal engineering of hazar-dous waste and frontiers of environmentalscience & engineering.


Prof. Li is now the Chief Scientist of circular economy and urbanmining research team in Tsinghua University, while he has taken theposition of Executive Director in Basel Convention Regional Centrefor Asia and the Pacific since 2002. He has been engaged in theresearches including circular economy and chemicals and solidwaste management for nearly twenty years, undertake and completed

national, provincial and ministerial projects more than 100 projectssuch as national scientific and technological support, national "863",international cooperation and so on, depending on which he has wonthe title of national leading talent on environmental protection,national science & technology award as well as plenty of otherscientific awards.


recovery of rare and precious metals from urban mines a · pdf filerecovery of rare and...

Documents