Efficient recovery of valuable metals from waste printed circuit boards by microwave pyrolysis

Yubo Liu,Jialiang Zhang,4,*,Xu Yang,Wenguang Yang,Yongqiang Chen,4,Chengyan Wang,3,4,*

1 State Key Laboratory of Advanced Metallurgy,University of Science and Technology Beijing,Beijing 100083,China

2 School of Metallurgical and Ecological Engineering,University of Science and Technology Beijing,Beijing 100083,China

3 School of Metallurgy Engineering,JiangXi University of Science and Technology,Ganzhou 341000,China

4 Beijing Key Laboratory of Green Recycling and Extraction of Metals,Beijing 100083,China

Keywords:Recovery Waste printed circuit boards (WPCBs)Microwave pyrolysis Valuable metals Waste treatment

ABSTRACT The recycling of waste printed circuit board(WPCBs)is of great significance for saving resources and protecting the environment.In this study,the WPCBs were pyrolyzed by microwave and the contained valuable metals Cu,Sn and Pb were recovered from the pyrolyzed WPCBs.The effect of pyrolysis temperature and time on the recovery efficiency of valuable metals was investigated.Additionally,the characterization for morphology and surface elemental distribution of pyrolysis residues was carried out to investigate the pyrolysis mechanism.The plastic fiber boards turned into black carbides,and they can be easily separated from the metals by manual.The results indicate that 91.2%,96.1% and 94.4% of Cu,Sn and Pb can be recovered after microwave pyrolysis at 700°C for 60 minutes.After pyrolysis,about 79.8%(mass)solid products,11.9%(mass)oil and 8.3%(mass)gas were produced.These gas and oil can be used as fuel and raw materials of organic chemicals,respectively.This process provides an efficient and energy-saving technology for recovering valuable metals from WPCBs.

1.Introduction

Presently,with the wide application of electronic products,a large amount of e-waste has been generated [1-3].According to the statistics,the global e-waste generation amounts were 41.8 million [4],44.7 million [5] and 49.8 million tons [6] in 2014,2016 and 2018,respectively,and the estimated number will increase to 52.2 million tons in 2021[7].The printed circuit boards(PCBs),as the carrier of various types of electronic equipment and components,are important parts of electronics [8,9].With the massive production of e-waste,the number of waste printed circuit boards(WPCBs)increases rapidly as well[10].WPCBs usually contain a large proportion of rare and precious metals,which need to be recovered to avoid the excessive usage of mineral resources[11,12].In addition,WPCBs contain various harmful elements,including heavy metals and flame retardants (polybrominated diphenyl ethers),which can cause serious damage to ecosystems during traditional landfill and incineration processes [13-15].As a result,it is imperative and significant to develop an efficient and environmentally friendly method to recycle WPCBs.

There are four main recycling methods for WPCBs:the incineration [16-19],mechanical treatment [2,20-22],hydrometallurgical [23-27] and pyrolysis process [28-32].In the incineration process,WPCBs are burned with oxygen sufficiently in the incinerator and the incombustible valuable metals can be enriched.The process has been relatively mature,but various harmful gases(such as dioxins) are produced.In mechanical process,the metals and non-metallic compositions can be separated through dismantling,crushing,sorting and other physical processes.Since mechanical energy is employed instead of thermal energy,the enrichment of valuable metals in WPCBs can be accomplished without harsh conditions such as high temperature or high pressure[20].However,the metal loss is relatively high and the equipment investment is relatively high by using mechanical method.In hydrometallurgical process,metals in WPCBs can be dissolved by various leaching agents,and thereby be separated from the insoluble non-metal substances in WPCBs.However,the process consumes a large amount of chemicals and generates a lot of complex wastewater as well.WPCBs are heated in pyrolysis method under the condition of no or little oxygen.The organic matters can be thermally decomposed and be separated from metals.Metals are therefore enriched and the organic matters can be recovered as liquid oil and gas of high heat value.The amounts of toxic gas produced in the pyrolysis are much less than that in the incineration process.However,since it is performed at high temperature,high energy consumption and equipment requirements are inevitable.

Recently,microwave has been widely used in various fields as a special energy source[33-35].Microwave technology is an energyefficient method and has the property of selective heating[36-38].As for WPCBs,the non-metallic components have better waveabsorption performance,so they will be heated selectively in the microwave field and achieve high temperature rapidly.After the non-metallic components are decomposed at high temperature,the rest part,valuable metals,are enriched and can be recovered by simple methods.In addition,compared with the traditional pyrolysis,the formation of dioxins,furans and other toxic substances can be further reduced in the microwave pyrolysis,due to its higher reaction speed.

Few available literatures have reported the microwave pyrolysis of WPCBs.Zhanget al.found that most bromine in WPCBs was transformed into the form of HBr,and a large proportion of liquid products containing phenolic compounds,bisphenol A and other aromatic compounds were produced in the microwave pyrolysis process [39].Suriapparaoet al.have studied the carbonaceous susceptor-assisted microwave processing of WPCBs,and analyzed the influence of the susceptor amount on the yield of products [40].Sunet al.used conventional thermogravimetric analyzer (TGA) to monitor the expulsion of volumetric volatiles,and using this data a kinetic study of the decomposition of WPCBs was conducted under microwave pyrolysis condition[41].In addition,mechanical featured processing,including crushing and specially designed sink-float separation,was proposed to recover metals from the pyrolysis residues [42].Generally,the available research mainly focused on the yield of pyrolysis products and the analysis of oil and gas products.But as another important aspect,the recovery of valuable metals from WPCBs by microwave pyrolysis still needs to be deeply investigated.

In this study,the efficiency of microwave pyrolysis and traditional pyrolysis was compared,and the feasibility for recovery of valuable metals from WPCBs after microwave pyrolysis was verified.The recovery efficiency of metals at different pyrolysis temperature and time was investigated.Particularly,in order to explain the results,we used SEM-EDS analysis to observe the morphology and surface elemental distribution of pyrolysis residues.Finally,the main components of recovered oil and gas were investigated by GC-MS and GC-TCD analysis.

2.Materials and Methods

2.1.Materials

In the present work,the waste printed circuit boards (WPCBs)were provided by local recycling companies.They compose of substrate,copper foil,solder and adhesive.The substrate is composed of plastic,glass fiber and resin,with thin strips of copper foil wire in its surface.WPCBs were first cut manually into small pieces to facilitate subsequent microwave pyrolysis.The contents of major valuable elements in the WPCBs are 16.2%(mass)Cu,9.92%(mass)Sn,and 6.29% (mass) Pb according to ICP-AES analysis (Optima 7000 DV,Perkin Elmer instruments,U.S.).The valuable metals are of relatively high contents,indicating a considerable economic benefit for the WPCBs recycling.

2.2.Microwave pyrolysis

First,a certain mass of raw WPCBs pieces were placed into a quartz crucible and then put them into the box-type microwave reactor (MW-HS-06,Kunming University of Science and Technology,China).Then,N2with flow rate of 120 ml·min-1was injected into the experimental device for 1 h to exhaust oxygen in it,avoiding the oxidation of WPCBs during the high temperature process.Then,WPCBs were heated to a certain temperature (500-900 °C)and kept the temperature for a certain time (30-90 min).After pyrolysis,the samples were naturally cooled to room temperature and N2was injected till complete cooling.The mass of samples before and after microwave pyrolysis were measured by digital electronic balance (ME104,METTLER TOLEDO,China),so that Eq.(1) can be used to calculate the weightlessness ηwof the samples after the experiment.

wheremb(g) andma(g) are the mass of WPCBs before and after microwave pyrolysis,respectively.

After microwave pyrolysis,the plastic fiber boards turned into black carbides.They can be easily separated from the metals manually.The contents of Cu,Pb and Sn in the separated residues and metals were measured by ICP-AES and the contents of other elements were determined by X-Ray Fluorescence Spectroscopy(XRF-1800,Shimadzu,Japan).The recovery rate of main metals ηican be calculated according to Eq.(2).

wheremr(g)and ωr,i(%)are the mass of the separated residues and the content of element‘‘i”in them,m0(g)and ωi(%)are the mass of the raw WPCBs and the content of element‘‘i”in them,respectively.The morphology and surface elemental distribution in residues were characterized by SEM (MLA250,FEI,US) and EDS (MLA250,FEI,US).

2.3.Collection and analysis of oil and gas

A condensing device and a gas purification and collection device were connected in sequence to the gas outlet of the box-type microwave reactor.The oil was collected by the condensing device and the gas was collected by a gas sampling bag after purification.Acidic gas (mainly HBr) can be adsorbed by NaOH solution in the purification process.Since part of the produced oil was attached on the inner wall of the quartz protective tube,dichloromethane was used as the solvent to collect this oil.Considering the melting point of dichloromethane is 39.75 °C,the mixed organics were heated to 41°C to evaporate the dichloromethane and the pyrolysis oil was then obtained.The main components of collected oil and the purified gas were analyzed by GC-MS (QP2010,SHIMADZU,Japan),and GC-TCD (QP2010,SHIMADZU,Japan),respectively.

3.Results and Discussion

3.1.Apparent features for the WPCBs before and after pyrolysis

Fig.1. (a,b) WPCBs pieces before and after microwave pyrolysis;(c,d) manually sorted residues and metals.

Fig.1 shows the photographs of WPCBs before and after microwave pyrolysis.From Fig.1(a),it is clearly observed that the copper wires were distributed on the circuit substrate in a sheet or thin strip shape.The solder was located at the welding spot and connected with the copper wire,forming a small electronic circuit.The substrate was made up of a layer of green plastic fiberboards.Fig.1(b)shows that after microwave pyrolysis,the plastic fiberboards have turned into black carbides.The unreacted copper was distributed on the surface of the fiberboards as the form of copper sheets or thin strips.Metals with lower melting points such as tin and lead were melted into individual balls of different sizes and embedded on the fiberboards.Non-metallic residues and metals could be easily manually separated and the products after sorting are shown in Fig.1(c) and (d).The residue mainly contained flaky carbon fiber and glass fiber cloth and no visible metals were observed in them.For the sorted metals,copper mainly existed in the form of copper sheet and copper wire,while lead,tin,and other metallic elements mainly existed in the form of pellets.Nonmetallic substances were rarely mixed in the metals.

We compared the effects of microwave pyrolysis and conventional pyrolysis for WPCBs.The weightlessness rates were 31.7%and 26.5% after microwave and conventional pyrolysis under the condition of 700 °C and 60 minutes.Under the same temperature and time,the weightlessness rate of WPCBs after microwave pyrolysis was higher than that of conventional pyrolysis.The higher weightlessness rate obtained by microwave pyrolysis indicates that more components in raw materials became oil or gas phase,the quality of residues was correspondingly smaller.Likewise,the decomposition of organic substances in WPCBs was more thorough and the yield of valued oil and gas was higher.Accordingly,the recovery of metals would be more efficient due to the destruction of plastic fiberboards.The electrical power and energy consumption during microwave and conventional pyrolysis are listed inTable S1.The results show that the energy consumption of conventional pyrolysis was about three times that of microwave pyrolysis.It should be noted that the heating rate of microwave pyrolysis was controlled to be the same as conventional pyrolysis(25 °C·min-1).Otherwise,the heating rate of microwave pyrolysis can be faster and the energy consumption will be further reduced.

3.2.Effect of microwave pyrolysis temperature

Fig.2(a)shows the effect of temperature on the microwave pyrolysis of WPCBs.The pyrolysis degree was characterized by weightlessness rate.As the temperature increased from 500 °C to 700 °C,the weightlessness rate gradually increased and reached a maximum value of 31.7% at 700 °C.The weightlessness rate exhibited a slight decrease when the temperature continued to rise.

Table S2shows the compositions of manually separated residues at different pyrolysis temperature.The main compositions of the residues were Si and Ca.The Br content was an important index to represent the harmfulness of solid waste.When the temperature exceeded 500 °C,the content of Br in residues reached a lower level,indicating that most of Br was volatilized from the solid WPCBs,and absorbed by NaOH solution.This significantly reduced the difficulty of the following hazard-free treatment for pyrolyzed residues.As for the valuable metals,the contents of Cu,Sn and Pb in the manually separated residue reached the lowest value of 3.72%,0.69%,and 0.48% when pyrolysis temperature was 700 °C.The residue metals could be recovered by smelting simultaneously with copper minerals [43].Fig.2(b)shows the effect of temperature on the recovery rates of main metals.It shows that the recovery rates of Cu,Pb and Sn fluctuated with the increase of temperature and the tendency was basically same for the three metals.The highest recovery rates for them were all obtained at 700 °C,which are 88.1%,96.1% and 96.4% for Cu,Pb and Sn,respectively.As pyrolysis temperature continued to increase,the recovery rates of the three metals decreased,and the decreased degree was more prominent for copper.Hence,700 °C was taken as the optimal pyrolysis temperature.

The sorted residues were characterized by SEM-EDS,to investigate the morphology and metal distribution after microwave pyrolysis at different temperature.As shown in Fig.3,when the temperature was 500 °C (Fig.3(a)),the fiber structure was regularly bundled in the vertical and horizontal directions,arranged in an orderly manner,and the tissue preservation was very complete.Some spherical particles were randomly scattered on the surface of the fiberboards.When the temperature was 700 °C(Fig.3(b)),the change of the fiberboards could be clearly observed:the fibers on the surface of fiberboards were completely destroyed and carbonized.Some holes of different sizes were formed which might be ascribed to the release of inner gas.When the temperature increased to 900 °C (Fig.3(c)),the surface fiberboards were melted due to the higher temperature.No pores were observed on the surface of fiberboards,which prevented the internal gas discharging.This suppressed the pyrolysis to a certain extent,as the reduced weightlessness rate at 900 °C shown in Fig.2(a).Furthermore,some metal particles were wrapped by the melted fiberboards and became difficult to be manually separated.

Fig.2. Effect of temperature on the (a) microwave pyrolysis of WPCBs and (b) recovery rates of main metals (time:60 min).

Fig.3. SEM and EDS mapping for the manually sorted residues after microwave pyrolysis at different temperature (a) 500 °C,(b,d) 700 °C and (c,e) 900 °C.

Fig.4. Effect of time on the (a) microwave pyrolysis of WPCBs and (b) recovery rates of main metals (temperature:700 °C).

Fig.5. SEM and EDS mapping for the manually sorted residues after microwave pyrolysis at different time (a) 30 min,(b,e) 45 min,(c,f) 60 min and (d,g) 90 min.

EDS mapping shows the distribution of Cu,Pb and Sn in the residues after pyrolysis at 700 °C was uniform in the entire sample(Fig.3(d)).Meanwhile,few metallic elements were aggregated,and the spherical metal particles were pretty small.However,when the temperature was 900°C,as shown in Fig.3(e),part of Cu existed in an aggregate state,and the aggregation regions were corresponded to the particles in the SEM image.So,the reductions of recovery rates,especially for Cu,were ascribed to the wrapping by the melted fiberboards.The SEM-EDS analysis well explained the reason for the variation of weightlessness and recovery rates and further indicated 700°C was the optimal microwave pyrolysis temperature for the recovery of valuable metals,oil and gas.

Fig.6. The main components of liquid products after microwave pyrolysis at 700°C for 60 minutes.

3.3.Effect of microwave pyrolysis time

Fig.4(a)shows the effect of time on the microwave pyrolysis of WPCBs.As the time increased from 30 to 60 min,the weightlessness rate gradually increased and reached a maximum value of 31.7%,then the weightlessness rate exhibited a slight decrease when the time continued to prolong.So,60 min was the most suitable time for the recovery of oil and gas.

Table S3shows the compositions of manually separated residues at different pyrolysis time.The contents of Cu,Sn and Pb were all reached the lowest at 60 min,which were 2.75%,0.74% and 0.68% respectively.The contents were relatively low,indicating that most of the valuable metals could be recovered as the metal mixture via microwave pyrolysis and manually separation.The effect of time on the recovery rates of main metals is shown in Fig.4(b).The highest recovery rates for Cu,Sn and Pb were all obtained at 60 min,which were 91.2%,96.1% and 94.4%,respectively.However,the recovery rates of Sn and Pb decreased significantly when pyrolysis time was longer than 60 minutes.

The SEM-EDS analysis of the separated residues at pyrolysis time of 30,45,60,and 90 minutes are shown in Fig.5.When the pyrolysis time was 30 and 45 minutes(Fig.5(a,b)),the fiber structures in the residues did not change obviously:the glass fibers were still relatively complete,the arrangement was neat and the order was regular.It is evident that some spherical metal particles were attached to the fiberboards.When the pyrolysis time was 60 minutes (Fig.5(c)),the original bundles arrangement and interlaced fiberboards surfaces were gradually destroyed.Many holes of different sizes appeared,and only some small metal particles could be observed.When the pyrolysis time prolonged to 90 minutes(Fig.5(d)),it could be found that the surface layer of the fiberboards is completely destroyed.Compared with the time of 60 minutes,the holes on the surface flattened with the prolonging of the pyrolysis time due to the melting of fiberboards.Meanwhile,many irregular particles were adhered on the surface of fiberboards.

EDS elemental mappings show that when the pyrolysis time is 45 minutes (Fig.5(e)),it was found that most visible metal particles on the fiberboards were larger particles of Cu.Since the pyrolysis process was not complete,part of the copper sheet was difficult to separate from the fiberboards.When the time was 60 minutes (Fig.5(f)),the distribution of Cu,Pb and Sn was uniform in the entire sample after pyrolysis.The fiberboards were broken but not melted,and no large metal particles were observed on them.When the time was 90 minutes(Fig.5(g)),due to the severe melting of fiberboards,some large particles of Pb and Sn were adhered to them,making them difficult to separate by manual.This could explain the reduction of recovery rates of Pb and Sn at 90 minutes in Fig.4.Therefore,it further proved that 60 minutes was the best pyrolysis time under microwave conditions.

3.4.Oil and gas recovery

The mass of various products after microwave pyrolysis of WPCBs at 700 °C for 60 minutes was analyzed.The mass of raw WPCBs,solid products,oil products were 30.66 g,24.45 g and 3.65 g,respectively.The mass of gas products was calculated to be 2.56 g by subtraction method.So,the pyrolysis products accounted for 79.75% (mass) solid,11.90% (mass) oil and 8.35%(mass) gas which were similar to the results of Sun [42].

GC-TCD analysis of the gas that passed through the condensing device and purified by NaOH solution is shown inTable S4.It indicates that the gas produced by microwave pyrolysis was mainly H2,CO,CO2,CH4and organic hydrocarbons.Except for CO2,they were all flammable gas,and the volume proportion of them was over 90 vol%.According to other relevant researches,the remaining gas might be mainly C1-C4 hydrocarbons,ethylene,propylene and organic bromide such as methyl bromide and ethyl bromide[39,42].As a result,the pyrolysis gas was a secondary energy source which had high heat energy [44].

GC-MS analysis of the oil product (Table S5) indicates that it was mainly composed of C-H-O compounds,followed by a small amount of C-H compounds,bromides and nitrides.According to the results of GC-MS,the components of oil were classified into six categories:phenols,ketones,phenyl groups,furans,brominecontaining substances and others,as shown in Fig.6.Among them,phenolic organic compounds accounted for the highest proportion,up to 57.4%.They could be used to synthesize organic chemicals[45,46].

4.Conclusions

A method combining with the processes of microwave pyrolysis and manual separation was presented to recover valuable metals from WPCBs.Compared with the conventional pyrolysis,the weightlessness rate is higher by microwave pyrolysis and it indicates that the decomposition of organic substances in WPCBs is more thorough and the yield of valued oil and gas is higher.The maximum recovery efficiencies of Cu,Sn and Pb are all obtained under the optimum conditions of pyrolysis temperature of 700 °C and pyrolysis time of 60 min with recovery rates of 91.2%,96.1%and 94.4%,respectively.The contents of Cu,Sn and Pb in residues can reach the lowest value,which are 2.75%,0.74%,and 0.68%.SEM-EDS analysis indicates that when the temperature and time are not sufficient,the pyrolysis process is not complete,and the separation of metals and residues is difficult.In addition,the excessively high temperature and long time lead to the metal adhesion and low recovery rates due to the severe melting of fiberboards.Over 90%of the gas produced is flammable gas,and it can be used as fuel with high value.The oil produced is mainly composed of CH-O compounds,which can be used as the raw material of organic chemicals.This process provides an efficient and energy-saving technology for recovering valuable metals from WPCBs.As largescale equipment of microwave has been maturely applied in the metallurgical and chemical industry,this method has great potential for the industrial-scale recycling of WPCBs.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2019YFC1908404),the National Natural Science Foundation of China (Nos.51834008,51874040,52034002),the Guangxi Innovation-Driven Development Project(AA18242042-1) and the Fundamental Research Funds for the Central Universities (FRF-TP-18-020A3).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.11.008.