固体废物处理与处置 (2).pdf

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1、Journal of Hazardous Materials 311(2016)203209Contents lists available at ScienceDirectJournal of Hazardous Materialsj o ur nal ho me pa ge: characteristics after cryogenic modification and air tableseparation of discarded printed circuit boardsCuihong Zhoua,Yongtai Panb,Maxi Lub,Changshun Yanga,caD

2、epartment of Environmental Engineering,Beijing Institute of Petrochemical Technology,Beijing 102617,PR ChinabCollege of Chemical and Environmental Engineering,China University of Mining and Technology,Beijing 100038,PR ChinacCollege of Mechanical and Electrical Engineering,Beijing University of Chem

3、ical Technology,Beijing 100029,PR Chinah i g h l i g h t sThe PCBs were preferably crushed by impact and bending means.The power consumption had a strong dependence on cooling temperature.Cryogenic grinding obtained fine size reduction and good liberation.Air table effectively separated 2.80.5 mm gr

4、ade PCBs material.a r t i c l e i n f oArticle history:Received 1 September 2015Received in revised form 2 March 2016Accepted 4 March 2016Available online 8 March 2016Keywords:Printed circuit boardsCrushingCryogenic grindingPower consumptionSize distributionAir tablea b s t r a c tLiberating useful

5、materials from printed circuit boards(PCBs)is challenging because PCBs are flexible andcomplex in terms of materials and components.In this study,the crushing of PCBs at low-temperature wasinvestigated.The results indicated that when the temperature was decreased to approximately 20C,the strength of

6、 PCBs decreased and their brittleness increased,making them easier to crush.A doubleroll crusher was selected to crush the PCBs.The particle size distribution and power consumption werestudied under different working conditions.The results showed that the particle size of most of the lumpswas in the

7、 range 15 2025 20 mm,and that power consumption was minimal when the frequencyof the crusher was 4050 Hz.A small shredder was used for cryogenic grinding,and it was found thatits power consumption strongly depended on the cooling temperature.An orthogonal experiment wasconducted,which revealed that

8、a smaller cutter gap and higher rotational speed could achieve higheryield.Furthermore,the results indicated that the air table developed to liberate PCB materials couldeffectively separate 2.80.5 mm grade materials.Overall,the results of this study provide an experimentalfoundation for more effecti

9、vely recycling discarded PCBs.2016 Elsevier B.V.All rights reserved.1.IntroductionWith the continuing development of information and elec-tronic technology,the amount of electrical and electronic waste hasincreased.The recovery of useful materials from PCBs has drawnconsiderable attention because th

10、e printed circuit boards(PCBs)inpersonal computers(PCs)and mobile phones contain valuable met-als.These include precious metals such as gold,silver,palladium,and rhodium 15.PCBs also contain major metals and alloys,suchCorresponding author at:Department of Environmental Engineering,Instituteof Petro

11、chemical Technology,Qingyuan North Road 19,Daxing,Beijing 102617,PRChina.E-mail addresses:tide ,(C.Zhou).as ferromagnetic materials,aluminum,and copper,which are diffi-cult to liberate from PCBs at normal temperature or by using generalcrushing technology 68.Moreover,the treatment of PCBs is verycha

12、llenging because PCBs are diverse and complex in terms ofmaterials and components.Nowadays,new technological innova-tions continue to accelerate the replacement of electronic devices,leading to a significant increase in discarded PCBs 913.The process for recovering useful materials from PCBs include

13、sdismantling,crushing,and separation.Crushing is a crucial step forthe subsequent treatment of PCBs 1417.There are three primaryobjectives for size reduction by crushing:1)generation of particlesthat are more easily handled than bulky parts,2)generation of uni-formly sized and shaped particles that

14、can be separated effectivelyin downstream processes,and 3)liberation of dissimilar materialsfrom one another 18,19.http:/dx.doi.org/10.1016/j.jhazmat.2016.03.0080304-3894/2016 Elsevier B.V.All rights reserved.204 C.Zhou et al./Journal of Hazardous Materials 311(2016)203209Because traditional grindin

15、g is not suitable for PCB recovery,a wet impact crusher and shear hammer mills are generally usedto obtain coarse size reduction and liberation of materials 10,20.To develop a low-cost recycling process for treating PCBs fromscrap electronics,selective grinding for ductile and stiff materialswas inv

16、estigated with a PC-assisted impact grinding system thatcan automatically control various operating conditions in real time21,22.The cryogenic grinding process has advantages,such as lowerenergy costs compared to traditional grinding,and can help real-ize a more uniform particle size.However,studies

17、 are required toascertain the economic value of this process.This technology hasbeen adopted for crushing rubber,thermoplastics,spices and otherfood products,pharmaceuticals,waxes,hot melt adhesives,pig-ments,powder coating materials,and electronic wastes 2325.Several studies have been conducted for

18、 embrittlement of mate-rials by pretreatment at low temperatures(20C or less)orhigh temperatures(250C)utilizing selective grinding in wasteprocessing.2628.The results show that the metal and non-metal particles of PCBs dissociate completely at the crushingsize of 0.6 mm and that metal is mainly enri

19、ched in four sizefractions between 0.15 and 1.25 mm 29,30.By using cryogenicgrinding technology,the material properties are changed at lowtemperatures,so materials that cannot be ground finely at roomtemperature can be ground when brittle at lower temperatures.Before the cryogenic modification exper

20、iment,we crushed thePCBs using a hammer mill,and the experiment failed.The mate-rials did not disintegrate,the quantity of qualified material wassmall,and the separation requirements were not satisfied.Hence,we improved the crusher and adopted cryogenic technology.Theobjective of this study was to i

21、nvestigate the mechanical propertiesof discarded PCBs at low temperatures,including impact,bend-ing,and shear strengths below a critical temperature.These resultswould be beneficial for the selection of effective crushers andgrinders for use with waste PCBs.The influences of temperature andcrushing

22、time on the crushing characteristics and power consump-tion were also investigated.Finally,an air table was adopted,whichFig.1.Diagram of the air table.1Angle adjusting nut of table,2Damper spring,3-Frame,4Steel wire rope,5Vibration motor,6Discharge port,7Backboard,8Bed,9Lattice bar,10Discharging Pl

23、ate,11Air chamber,12Air valve,13Airhose,14Discharge port.uses air as a medium to separate metals and nonmetals throughasymmetric reciprocating movement.The separation mechanismwas analyzed,and the separation of PCB materials was demon-strated.2.Experimental method2.1.Materials and devicesThe PCBs we

24、re collected from discarded electronics.Liquidnitrogen was selected as the refrigerating medium because it isa non-flammable and non-toxic inert gas.The double roll crusherused for the PCBs was improved by adding spirally arranged teeth.The particles are crushed by the relative motion of the two rol

25、lersas the grid rotates.The broken tooth space and the cutter gapof the top gear will determine the size of the crushing material,Fig.2.Crushing and separation procedures.C.Zhou et al./Journal of Hazardous Materials 311(2016)203209 205Table 1Power consumption for crushing PCBs.Frequency(Hz)Length of

26、 roll(mm)Material mass(g)Crushing time(s)Power(Wh)Power consumptionSingle value(Wh/kg)Average(Wh/kg)20145 69.2 1.11 531.0 2.372.16103 47.2 1 326.0 1.92121 52.5 1.06 388.0 2.1830100 48.50.79461.02.092.08120 60.2 0.96 233.0 1.03100.9 58.8 1.06 623.0 3.1240108 58.6 0.72 718.5 2.451.90114 55.6 0.94 225.

27、5 1.0687 44 0.88 395.5 2.2050120 72.50.91461.31.611.76102 55 0.99522.32.61104 59.7 0.98 235.3 1.0760158.5 94.4 0.92 306.8 0.831.00176 105.5 0.79 461.8 0.96175.5 107 0.97 480.8 1.21while minimizing over crushing and excess power consumption.The parameters were as follows:(1)Feeding granularity:300 20

28、0 3 mm,(2)Product granularity:20 20 mm,(3)Dimensions:1336 466 485 mm,(4)Current:6.8 A,(5)Power:3 kW,and(6)Speed:1420 r/min.We selected an R-type small shredder with a vertical axis.Itconsisted of a motor body,cover,rotary knife,fixed knife,mesh,androtation-tool post occupied by three vertical cutter

29、s.The particlesize was determined by the gap of the knife,the discharge size,andthe speed of the rotary knife.The air table used in the experiment included a feeding mech-anism,bed body,control mechanism,and framework,as shown inFig.1.On the bed,there is a sieve with 0.1-mm apertures.The sur-face of

30、 the bed,from the feeding end to the discharging end,wasdivided into three chambers.The separation effect of the separatoron waste circuit boards,and the separation effect of the processparameters on the waste circuit boards,could be managed sepa-rately.The related parameters were set as follows:(1)

31、Air volume:824 m3/h,(2)Frequency:12.5 Hz,(3)Current:1.0 A,(4)Separation time:10 min,(5)Longitudinal angle of bed:8,and(6)Horizontal angle for bed:10.2.2.MethodFirst,a double roll crusher was used to crush the PCBs into20-mm particles,after which these were placed in a freezingdevice.The temperature

32、was controlled by directly injecting liquidnitrogen with ethanol.After cooling,the PCBs were ground into par-ticles of less than 3.5 mm.During the crushing process,the powerconsumption was measured every 30 s.The relationship betweenpower diversification and size distribution was investigated.Fig.2s

33、hows the crushing and separation procedures.After the separationexperiment conducted using the air table,the separation character-istics of the PCB materials were studied.3.Results and discussion3.1.Mechanical propertiesWith lower temperatures,the brittleness of the PCBs increased(Fig.3 shows the me

34、chanical properties of PCBs at different cryo-genic temperatures).When we tested the mechanical propertiesfrom room temperature to 190C,as shown in Fig.3(a),we foundFig.3.Mechanical properties of PCBs at different cryogenic temperatures:(a)Change Curves,and(b)Impact test results at 20C to 30C.206 C.

35、Zhou et al./Journal of Hazardous Materials 311(2016)203209Table 2Negative cumulative yield of PCBs(%)at different frequencies.Particle size(mm)Negative cumulative yield(%)at 20 Hz at 30 Hz at 40 Hz at 50 Hz at 60 Hz30 100 100 100 100 1002530 79.54 63.78 87.01 79.42 73.442025 69.72 55.75 83.72 70.06

36、62.68152027.3329.5058.72 44.16 37.541015 15.06 12.37 19.74 13.68 13.8130,2530,2025,1520,1015,and 3.5,3.53.0,3.02.26,2.261.4,1.41.0,1.00.5,0.50.3,0.30.2,0.20.125,0.1250.074,and Frequency Cutter gap Discharge size,andPower consumption:Frequency Cutter gap Dischargesize Crushing time.Table 6Factors and

37、 levels of orthogonal experiments.Level Discharge size A(mm)Cutter gap B(mm)Frequency C(Hz)Crushing time D(min)1 2 2 30 52 3 6 40 103 4 4 50 15Table 7Results of orthogonal experiments.No Discharge size(mm)Cutter gap(mm)Frequency(Hz)Processing time(min)Yield(%)Power consumption(Wh/kg)1 2 2 30 5 14.7

38、0.572 2 6 40 10 28.2 0.293 2 4 50 15 46.2 0.374 3 2 40 15 35.5 0.375 3 6 50 5 26.9 0.196 3 4 30 10 35.9 0.477 4 2 50 10 40.6 0.218 4 6 30 15 29.8 0.419 4 4 40 5 22.7 0.34Table 8Calculated results of orthogonal experiments.K Yield(%)Power consumption(Wh/kg)Discharge size Cutter gap Frequency Processi

39、ng time Discharge size Cutter gap Frequency Processing timeK189.1 90.8 80.4 64.3 1.2 1.1 1.5 1.1K298.3 104.8 86.4 104.7 1.0 1.2 1.0 1.0K393.1 84.9 113.7 111.5 1.0 0.9 0.8 1.1R 9.2 19.9 33.3 47.2 0.3 0.3 0.7 0.2k129.7 30.3 26.8 21.4 0.4 0.4 0.5 0.4k232.8 34.9 28.8 34.9 0.3 0.4 0.3 0.3k331.0 28.3 37.9

40、 37.2 0.3 0.3 0.3 0.4R 3.1 6.6 11.1 15.7 0.1 0.1 0.2 0.1208 C.Zhou et al./Journal of Hazardous Materials 311(2016)203209Table 9Grade and recovery rate of metal in heavy product(%).Category 2.81.6 mm 1.61.2 mm 1.20.5 mm 2.80.25 mmGrade Recovery rate Grade Recovery rate Grade Recovery rate Grade Recov

41、ery rate1 36.2 95.7 56.5 91.6 64.0 77.6 72.9 41.12 62.6 64.5 59.1 85.5 84.9 94.6 59.5 77.9Table 10Grade and recovery rate of metal in light product(%).Category 2.81.6 mm 1.61.2 mm 1.20.5 mm 2.80.25 mmGrade Recovery rateGrade Recovery rateGrade Recovery rateGrade Recovery rate1 91.6 49.788.5 61.5 67.

42、3 51.3 79.0 95.92 80.5 86.7 58.0 29.4 94.0 83.3 85.5 78.64.Separation experiments4.1.Analysis of the separation mechanismThe motion of the particles on the bed is related to factors suchas air velocity,particle size,air gap rate,and operation parame-ters.During the separation process,the airflow req

43、uired for theloose bed layer need not be strong enough to suspend the material.Therefore,the airflow is lower than that of a traditional air table,the influence of the particle size is small,and the bed force on par-ticles is mainly related to the density of the bed.Thus,separationby the air table i

44、s mainly influenced by the density difference of thematerials separated under the operating parameters.Therefore,the air table developed in this study can sort mate-rials by density,reduce the influence of particle size and otherfactors,and improve fine-grade separation.During separation,par-ticles

45、under the influence of electromagnetic vibration,airflow,anda medium effect loosen the particles of different densities movingalong the bed.The bed-surface shape and the interaction of thematerials,such as spiral motion,scattered crumbs,and density-stratified sampling,influence particle separation.4

46、.2.Separation resultsConsidering the material separation results and the observedmaterial stratification in the bed,a separation experiment wasconducted with 50 g of waste mainboard(No.1)and mixed-gradematerial(No.2).The purpose of the experiment was to achieve theseparation of metals and nonmetals.

47、During the separation process,the stratification of material wasobserved.After separation,the light and heavy products were sep-arately collected,weighed,and analyzed.We conducted a detailedanalysis after separating the two types of material.First,material No.1(waste mainboard)and material No.2(mixe

48、d material)were separated into different size grades(2.81.6 mm,1.61.2 mm,1.20.5 mm).The weight of the productin the metal grade and the recovery rate are shown in Table 9.Formaterial No.1,the separation effect was best for the 1.61.2 mmgrade,for which the metal content in the heavy product was 56.4%

49、and the metal recovery rate was 91.6%.For material No.2,the sepa-ration effect was best for the 1.20.5 mm grade,for which the metalcontent in the heavy product was 84.9%and metal recovery rate was94.6%.The grades and recovery rates of light products such as resinand plastic are shown in Table 10.For

50、 material No.1,the nonmetalcontent in the light product was 88.50%,and the non-metal recov-ery rate was 61.5%.For material No.2,the separation effect was bestfor the 1.20.5 mm grade,for which the nonmetal content in thelight product was 94.0%and the nonmetal recovery rate was 83.4%.According to the