| With the progress of display technology,the liquid crystal display?LCD?has already occupied the main display market.However,the discard amount of LCD panels has been increasing from year to year,and thus the safe disposal of LCD has become an important issue.Waste LCD mainly consists of glass substrate with indium,polarizing film and liquid crystal materials.Based on the composition of LCD panels,this research studied nitrogen pyrolysis and sub/supercritical water treatments to degrade organic matters?polarizing film and liquid crystal?and a new method of using pyrolysis residue in vacuum carbon reduction to recover indium is proposed after analysis of pyrolysis residue.At the same time,the mechanism of the degradation of organic matters is studied.Reaction condition,product composition,treatment efficiency and reaction mechanism were compared and analysed.At last,the nitrogen pyrolysis combined vacuum carbon reduction disposal process was established to recycle the organic matters and indium from waste LCD panels.The pyrolysis process mainly generated oil,gas and residue.Acetic acid and triphenyl phosphate were the main oil products,which verified the mechanism.Besides,hydrocarbon compounds were the main gas products,and the residue mainly consisted of carbon.Meanwhile,pyrolysis characteristics were studied by thermo gravimetric analysis combined with online mass spectrometry?TG-MS?.The mechanism of pyrolysis is proposed:CTA,the main component of polarizing film,after heating will generate active CTA with a low polymerization,and then decompose into triacetyl-d-glucose;some triacetyl-d-glucose generated triacetyl-d-mannosan and its isomers through an intramolecular dehydration,while most triacetyl-d-glucose generate the main oil product,namely acetic acid,through a six-member cyclic transition state.Furthermore,single factor experiment and response surface methodology?RSM?were used to design and optimize the experiment parameter,the oil yield could reach 70.53%and the residue rate could reach 14.05%when the pyrolysis conditions were 570°C,6 L·min-1,10 min and0.5 mm.Furthermore,the pyrolysis products of liquid crystal materials were analyzed,the pyrolysis oil were mainly bicyclohexyl and biphenyl compounds at 300°C and 500°C,while the pyrolysis oil were mainly polycyclic aromatic hydrocarbon?PAH?compounds at800°C.The pyrolysis gas was mainly water vapor at 300°C,while the pyrolysis gases were mainly hydrocarbon compounds at 500°C and 800°C.After product analysis,the relations between liquid crystal removal rate and reaction parameters were studied.The result turns out that the liquid crystal removal rate was over 90%when the reaction temperature was over 500°C and nitrogen flow was 2 L·min-1,and the reaction time has little impact on the liquid crystal removal rate.Considering the results and parameters in former experiments,the LCD panels were shattered and pyrolyzed at 570°C,6 L·min-1,10 min and 0.5 mm.71.89%of pyrolysis oil,14.27%of pyrolytic carbon residue and13.84%of pyrolysis gas are obtained from the pyrolysis process.The oil was then collected and separated by reduced pressure distillation to get acetic acid and TPP.Through sub/supercritical process,the organic matters mainly generate acetic acid and phenol.Based on product analysis,the reaction mechanisms were proposed:the ethyl ester in CTA could generate acetic acid with H+or OH-as the catalyst which came from the sub/supercritical water,and the process would go through nucleophilic addition,tetrahedral intermediate and electron transfer.Meanwhile,the glucose ring could generate acetic acid,and the process would go through a series of isomerization,retrograde aldol condensation and decarbonylation which generated intermediate products such as lactic acid,5-HMF and glucose,while TPP can generate phenol in the process.Futhermore,the experiment was optimized:99.77%of organic matters were removed,and 78.23%of acetic acid was recycled at 400°C,23 MPa and 5 min.Compared with sub/supercritical water treatment,nitrogen pyrolysis was selectd to recycle the organic matters of LCD panels for the superiority of simple device,mild reaction conditions and product easy to separate.The pyrolysis residue was used as a reductant to recycl indium from In2O3.Single factor experiment and RSM were used to design and optimize the experiment parameter.The conversion rate of indium could reach 97.89%when the reaction temperature is935°C,pressure is 5 Pa,pyrolytic carbon loading is 38%,and reaction time is 30 min.In contrast,the conversion rate of using coke as a reductant is 93%under the condition of950°C,1 Pa,30min,30 wt%.The comparison indicates that the reaction effects of pyrolytic carbon were much better than coke since the structure of pyrolytic carbon is porous which would make the reactants contact better,and promote the reaction efficiency.What's more,elemental analysis showed that pyrolysis residue contained more hydrogen than coke and hydrogen is believed to help promote reducibility.Meanwhile,recycling indium from waste LCD glass was conducted.Effects of reaction time,pyrolytic carbon loading and particle size of LCD glasses on indium conversion rate were studied.The result showed that the pyrolytic carbon residue is sufficient for the indium reduction without adding extra reductant.Indium convertion rate is 99.08%under the condition of935°C,5 Pa,2.5 wt%,30 min,<0.3mm.Based on the above studies,the nitrogen pyrolysis combined vacuum carbon reduction disposal process is established and the overall recovery process was conducted.Mass balancing analysis showed that 306.8 g LCD panel could generate 31.62 g of pyrolysis oils,6.02 g of pyrolysis gas,257.6 g of foam glass and 77.64 mg of indium.Meanwhile,the recovery rate of pyrolysis oil,pysolysis gas and indium could reach71.70%,13.66%and 99.03%.The combined process could efficiently degrade the organic matters,and the pyrolytic carbon can be reused to recycle indium from the glass substrate of waste LCD panels.Furthermore,the recovey process could integrate nitrogen pyrolysis and vacuum carbon reduction,simplify the recycle process and improve the indium recovery.Above studies achievements provide theoretical foundation for the environmental friendly treatment and resource utilization of waste LCD panels and supported the industrialization of recycling technologies and equipments. |
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