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
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