Study On Recycling Mechanisms And Methods Of Key Units In Liquid Crystal Display

With the rapid development of IT industry and the continuous improvement of the people’s living standards, the use cycle of the liquid crystal display is getting shorter and shorter. LCD has become one of the fastest growing waste electrical and electronic products. When recycling liquid crystal displays, we should focus on the LCD panels and the printed circuit boards (PCBs), the other parts such as frames and bases, are metallic or nonmetallic structures and can be recycled as materials in sorts. There are valuable liquid crystal, rare and precious metal indium, high quality ITO glass and so on in LCD panels. On one hand the LCD printed circuit boards contain heavy metals such as Pb, Hg, Cr and Cd, as well as toxic and harmful substances such as multi-bromine association phenylate, multi-bromine biphenyl and multi-chlorine biphenyl which damage the human health, and on the other hand the LCD printed circuit boards also contain a number of functional electronic components and various rare and precious metals such as Au, Ag, Pd, Sn and Cu. If the abandoned liquid crystal displays cannot be recycled effectively, the soil and the ground water will be polluted seriously and the resources will be wasted enormously. Therefore, it has great significance for developing circulation economy and protecting environment to seek suitable methods to recycle available materials and process toxic and harmful ingredients effectively in LCDs.On the basis of the analysis and summary of the wasted LCD recycling technologies at home and abroad, and supported by the National Natural Science Foundation Project and the "12th Five Year" National Science and Technology Supporting Project, the recycling mechanisms and methods of the key units—— LCD panels and PCBs of the wasted liquid crystal displays were studied mainly in the dissertation. The main research work is as follows:1) The recycling mechanism of the liquid crystal was studied and a recycling method of using the hydrocarbon organic solvent with the ultrasonic wave dissolving LCD screens was proposed. In this method, liquid crystals were collected through membrane filtration and low temperature vacuum distillation. Detection of photoelectric properties and analysis of phase transition for liquid crystals were conducted. Meanwhile, the potential value of the recycled liquid crystals was also evaluated. The results showed that the recovery rate of the liquid crystal could reach 74% using this method, the collection has evident characteristics of liquid crystal and can satisfy the requirements of the standard AM-LCD parameters.2) In view of the ITO glass after extracting liquid crystal, the recovery mechanism and extracting method of indium and the method of recycling the glass substrate were studied. First, indium rich solution and glass substrate without impurities were obtained by using hydrofluoric acid solution to soak ITO glass. Then, indium rich material was got after evaporating and concentrating the solution containing indium. Finally, the indium rich material was dissolved in sulfate solution to get solution containing indium ion, the sponge indium was obtained by the replacement of aluminum sheet, and the elemental indium was got by using sodium hydroxide to melt and purify the sponge indium. The glass substrate was used as the raw material for the production of the recycled glass, the performances of the recycled glass was detected, and the experimental data were provided for the reuse of the glass without indium.3) In order to realize the high efficiency and green disassembly of the electronic components on PCBs of wasted LCDs, firstly the disassembling mechanism of the electronic components on PCBs was studied, the threshold value of the minimum disassembly force was determined by disassembling experiments. Then the automatic disassembly device for electronic components on PCBs was designed, and the automation of the process was realized. The dynamic simulation of the mechanical system was carried out, which provided the technical support for the selection of the key parts of the disassembling device. Also the main components of the gas emissions were studied, which generated in thermal decomposition process of PCBs. A comprehensive control method of gas emissions was put forward that provided experimental basis for the secondary pollution control of PCBs. Finally, a series of disassembling experiments were carried out on various PCBs by using a self-made disassembly device. The results showed that the device was effective, safe, reliable, energy saving, environmental protection and low damage rate of the components on PCBs.4) Aiming at the recycling problems of the blank PCBs, the high voltage electrostatic separating technology of the metal and nonmetal mixed particles after crushing of PCBs was studied. The parameters of the electromagnetic vibrating feeder on electrostatic separating device were optimized in order to ensure the stability and speed of the feeding particles. The interior electric field intensity distribution inside the high voltage electrostatic separator and the surface electric field intensity distribution of the grounding roller electrode were studied by numerical simulation respectively, providing theoretical basis for the parameters setting of the roller type high voltage electrostatic separator. The dynamic characteristics of PCB particles under high voltage electrostatic field were studied. The influences of the electric field strength and the grounding roller rotating speed on the results of the particles’separation were analyzed. The mechanism of agglomeration of PCB particles was explained and the measures to overcome the agglomeration were presented. The simulation and analysis were verified by the high voltage electrostatic separating experiments of PCB particles, and the separating results of the mixed particles achieved the expected purpose.