Pyrolysis Mechanism Of Organic Materials Of Waste Liquid Crystal Display Panels And Extraction Of Indium

Liquid crystal display(LCD) panel contains organics, metals and inorganic nonmetal. The products, yields and parameters of oxygen-free atmosphere pyrolysis(nitrogen pyrolysis) and vacuum pyrolysis were studied, and the vacuum pyrolysis was selected to recover the organics of the LCD panels. The thermal decomposition kinetics were studied based on the vacuum thermogravimetric analysis. The optimal parameters of pyrolysis were obtained by single factor experiments and response surface methodology. Based on the analysis of pyrolysis products and AM1 semi-empirical method, the vacuum pyrolysis mechanism was studied. The products, yields, residues and parameters of vacuum carbon reduction metallurgy and vacuum chloridizing metallurgy were studied, and the vacuum chloridizing metallurgy was selected to recover the indium of the LCD panels. The optimal parameters of chlorination were obtained by single factor experiments and response surface methodology. Based on the calculation of gibbs free energy and the analysis of phase stability diagram, the vacuum chloridizing mechanism was studied. The combined recovery process was established and the pilot-scale equipment was fabricated on the basis of the theoretical research. The whole process was also been evaluated in environmental and economic aspects.The vacuum thermogravimetric analysis under different heating rates indicated that the heating rate is proportional to the pyrolysis oil yield and inversely proportional to the residual carbon yield. The 30 °C/min can make sure the maximum pyrolysis oil yield with the low activation energy on the basis of the thermal decomposition kinetics. The system pressure and pyrolysis temperature were also investigated for the influence of the yield. The single factor experiments about the different system pressures indicated that the pyrolysis oil yield increased with the increase of system pressure; the pyrolysis gas yield increased rapidly when the system pressure is higher than 3000 Pa; the residual carbon has an inflection point at 3000 Pa. The single factor experiments about the different pyrolysis temperatures indicated that the pyrolysis oil yield can reach 80 wt. % when the temperature reached 200 °C, and the pyrolysis oil yield does not have a rising trend after 200 °C. The vacuum pyrolysis optimal parameters are: 300 °C, 50 Pa and 35 min, which were obtained on the basis of single factor experiments and response surface methodology. The recovery tests were proceeded with the optimal parameters, and the recovery rates of pyrolysis oil, pyrolysis gas and residual carbon are 80.38 wt. %, 5.64 wt. % and 13.98 wt. %, respectively. The analysis of vacuum pyrolysis products and the inference of pyrolysis path indicated that the triphenyl phosphate in polarizing film will not pyrolysis under vacuum heating condition and the C-O bonds of cellulose triacetate will break down and recombine in backbone and branched chain of monoacetate side. Therefore, the vacuum pyrolsyis reactions are simple, and the product categories are concentrated.Ammonium chloride was chosen as the chlorinating agent, and the pure indium oxide and tin oxide were used in the initial single factor experiments. The single factor experiments about the different adding amounts of chlorinating agent indicated that the indium recovery rate can reach 99.01 % when the Cl/In molar ratio is 6. So the excess ammonium chloride is necessary for the high recovery rate of indium. The single factor experiments about the different heating temperatures indicated that the indium recovery rate can reach 95.01 % when the heating temperature reached 350 °C, and the indium recovery rate does not have a rising trend after 350 °C. The single factor experiments about the different heating times indicated that the decomposition rate of ammonium chloride can be speed up under vauum condition. The single factor experiments about the different atmospheres indicated that the vacuum condition will increases the recovery rate of indium and confines the volatilization of tin. The recovery rates of indium and tin are 99.01 % and 20.36 %, respectively. Through the observation, we found that the indium chloride and ammonia chloride are condensed in the different temperature zones under vacuun condition. Therefore, the indium chloride and ammonia chloride can be gathered separately. The single factor experiments and the response surface methodology were proceeded on the glass powder too, and the optimal parameters are: 52.22 % of mass ratio ammonium chloride to glass powder; 0.19 mm of partical size; 416 °C. The recovery tests were proceeded with the optimal parameters, and the recovery rates of indium can reach 99.98 %. The analysis of vacuum chloridizing mechanism indicated that the indium oxide will reacts to form indium chloride with the hydrogen chloride discharged from the ammonium chloride, but the tin oxide can not be generated. The indium can be recovered from the LCD panels,and the tin will stays at the glass powder. Therefore, the indium can be separated from the tin.Based on the theoretical research, the combined process of vacuum pyrolysis and vacuum chloridizing metallurgy was established. The pilot tests were proceeded using the self-designed and built pilot-scale equipments. 14.42 kg LCD panels were used in the pilot tests, and 1460 g pyrolysis oil, 54 g pyrolysis gas, 331 g residual carbon, 6.95 g indium oxide and 12474 g glass powder were obtained. The purity of the indium oxide extracted from the LCD panels is 99.0 %. The analysis of environmental and economic benefits indicated that the combined process is environmentally friendly and has significant economic benefits. This process can realize the harmless and recycling of waste LCD panels and has the industrialization prospect.