Chlorination Recovery Of Indium In LCD With HCl Produced By PVC Pyrolysis

In this paper, chlorine metallurgy is utilized to recover indium in LCD by using HCl generated from PVC pyrolysis as chlorination agent. GC-MS, IC, ICP and XRD are employed to reveal the reaction mechanism of HCl production by PVC pyrolysis. A technology to recycle indium from wasted LCD with HCl generated from wasted PVC pyrolysis is constructed. The reaction mechanism and kinetics equations of HCl-In2O3 system are proposed.Indium, a kind of scare metal which is acid-resistant, abrasion-resistant and self-lubricating, is widely used in national defense, aerospace, electronic information and other high-tech fields. There is no independent ore deposit for indium in nature. Indium always exists as impurity in other metal ore with rather low content.At present, indium is mainly extracted from smelting dust, slag and other side products by chemical extraction with complex process and high cost. In addition, indium trichloride (InCl3) is the basic raw material for the production of indium tin oxide film (ITO) and â…¢-â…¤ semiconductor materials and the synthesizing of organic indium compound. About 75% of indium all over the world is utilized in the production of ITO (SnO2:In2O3=1:9) target, which is widely used in liquid crystal display. With the progress of science and technology, the electronic products renew at a higher speed, resulting in a much shorter service life and electronic products with LCD scrap volume increasing year by year. As the content of indium in these electronic wastes is higher than its content in nature, and indium is non renewable resource and its nature reserve is limited, the recovery of indium from indium-containing waste is particularly important.LCD is mainly composed of a glass substrate, liquid crystal and ITO, wherein, the glass substrate (non alkali boron silicon glass) is mainly used in the synthesis of building materials (brick and concrete), liquid crystal is a polymer mixture of organic polymer containing sulfonic acid group, carboxyl and other non covalent complexes, with potential toxicity and carcinogenicity. There are many methods for the treatment of LCD, for example, sponge indium could be obtained by dissolving LCD with hydrochloric acid and replacing it with zinc dust.In addition, liquid crystal is disposed by acetone extraction, ultrasonic cleaning and heat treatment. However, these methods rarely refer to the extraction of indium. As a kind of widely used engineering plastic, PVC contains nearly 60% chloride and dioxins generate when it is heated in the air. As studies have shown that it is feasible for PVC using as the release source of hydrogen chloride, this paper uses PVC as the chlorinating agent to recover the indium in LCD. Chlorinating metallurgy is used to extract metal from complex multiphase metal ore and low-grade ore, taking advantage of the low boiling point and high volatility of metal chlorides. As HCl is highly corrosive and pollutant, it has high requirements for equipment, and is difficult to transport and store. As a result, little research on HCl as chlorinating agent is found. Therefore, we should strengthen the research on the basic theory of this field.In this paper, PVC is used as the release source of HCl, which is an effective solution to the transport and store difficulty for HCl. Additionally, as the pyrolysis residue of PVC is low in chlorine, it can be used as the raw material of pyrolysis oil production and blast furnace blowing smelting, which effectively solve the problem of environmental pollution with chloride. A self-made quartz reactor is employed for HCl production by PVC pyrolysis. The effect of feeding amount, pyrolysis time and pyrolysis temperature on the yield of HCl is studied. The effect of pyrolysis temperature on tar and gas composition is researched by analyzing tar by GC-MS and detecting gas by gas chromatography. The reaction mechanism of the production of HCl by PVC pyrolysis is obtained. The thermodynamic properties of PVC-LCD are analyzed. By comparing the Gibbs free energy of PVC-LCD system without and with carbon, a conclusion can be drawn that the Gibbs free energy of the PVC-LCD reaction system with carbon decline dramatically, which promotes the indium chloride reaction and reduces chlorination temperature. However, the reaction Gibbs free energy of other oxides in LCD glass substrate reducesand makes the reaction system more complex. Indium monochloride generate in the main reaction system In2O3-C-HCl, while impurity AlC13, FeCl3, FeCl2 and SiCl4generate in Al2O3-C-HCl system and Fe2O3-SiO2-HCl system. In non-carbon adding system, the target product InCl3 without impurity can be abtained by controlling the oxygen partial pressure and chlorination temperature between 200℃ and 554.25℃. As a result, the recovery of indium by chlornation is conducted at 200℃-554.25℃ without carbon.A pyrolysis furnace with two stages is utilized to study the process of indium recovery by PVC-LCD chlorination.In2O3 and LCD glass substrate after dissolved by alkali soluble are used as raw materials. The effects of indium chloride ratio, temperature, reaction time and flow rate of carrier gas on the indium recovery rate are studied through the single factor experiments. The influence of multi factors on the recovery rate of indium is studied by Design-Expert software, and the optimum operation condition is obtained. In2O3 conversion ratesin HCl-In2O3 system at different temperature, different time and different HCl partial pressure conditions have been studied and the collected products have been analyzed by XRD. The reaction mechanism of HCl-In2O3 is revealed and kinetic equations are presented. The main research contents are as follows:In chapter 3, the products during the production of HCl by PVC pyrolysis are analyzed. The effects of pyrolysis temperature, feeding amount, pyrolysis time and flow rate of carrier gas on the yield of HCl are studied. The reaction mechanism of HCl production by PVC pyrolysis is revealed by detecting the products with GC-MS, GC and IC.In chapter 4, thermodynamic analysis of chlorination reaction of PVC-LCD system is conducted. The Gibbs free energy change of the system at different chlorination temperaturesis studied to judge possible reactions preliminarily. The appropriate chlorination system has been chosen by comparingthe Gibbs free energy change situation of PVC-LCD system with and without carbon. The influence of H2, CH4, C2H4, C2H6 and other organic gases on the chlorination reaction has also been illustrated.In chapter 5, the removal of silicon aluminum oxide by alkaline solution at low temperature is conducted. The influence alkalinity, liquid-solid ratio, temperature and reaction time analysis of on the removal rate of silicon aluminum oxide are studied. The reaction residue has been analyzed by XRD. The change regulation of silicon aluminum oxide removal rate is revealed and the optimal operating condition is obtained by orthogonal experiment. The effect of chlorination temperature, chloride indium ratio, chlorination time and flow rate of carrier gas on recovery of indium in PVC-In2O3/LCD system are researched. The influence regulation of multi factors on indium recovery rate and the optimum operating conditions are obtained by orthogonal experiments designed by the Design Expert software.In chapter 6, the kinetics of HCl-In2O3 chlorination system is studied and the reaction mechanism is revealed.The following conclusions can be drawn from this research:(1)The grade of In2O3 in LCD glass substrate powder dissolved in alkali is 5 times more than that of the untreated raw material. In the recovery of indium by chlornation, the indium recovery rates in the experiments using the treated LCD as reactants are generally higher than the untreated LCD, up to 32.41%.(2) The removal rate of silicon aluminum oxide becomes higher with the increasing of alkalinity and the variation range is 47.93%-74.65%; the total removal rate of silicon and aluminum oxides increased with reaction time and showed a trend of increase, the change range of 46.73%-58.01%. When the reaction temperature rises, the silicon aluminum oxide removal rate first increases from 45.50% to 57.60% and then reduces to 54.95%. With the increasing of liquid-solid ratio, the total removal rate of silicon and aluminum oxides increases from 61.99% to 74.32%. The optimum condition is:liquid-solid ratio 90:2 (ml/g), reaction temperature 95℃, basicity 0.56 (g/ml), reaction time 2.0h.(3) By comparing the Gibbs free energy of PVC-LCD system without and with carbon, a conclusion can be drawn that the Gibbs free energy of the PVC-LCD reaction system with carbon decline dramatically, which promotes the indium chloride reaction and reduces chlorination temperature. However, the reaction Gibbs free energy of other oxides in LCD glass substrate reducesand makes the reaction system more complex. As a result, the recovery of indium by chlornation is conducted at 200℃-554.25℃ without carbon.(4) With temperature increasing, chlorine conversion, tar yield and hydrogen chloride yield raise, while char yield decreases and gas yield first declines and then increases. H2, C2H4 and C3H6 were detected in the gas product when the temperature was below 400℃, while CH4, C2H6 and C3H8 were also found when the temperature was higher than 400℃. The large ring to small ring ratio decreased significantly when the pyrolysis temperature increased. A possible mechanism for HCl production by PVC pyrolysis was as followings:in the first thermal decomposing stage, chain reaction for the removal of HCl accompanied with the generation of small amount of benzene and other aromatics and cycloalkanewas the main reaction, while in the second stage, the main reactions were tar restructuration, isomerization and aromatization.(5) For the recovery of indium by chlornation in PVC-In2O3 system, with the increase of chloride-indium ratio, the recovery rate of indium increases from 40.68% to 99%, while the utilization rate of HC1 first increases from 41.25% to 69.68% and then decreases to 35.7%. There is few effect of chlornation temperature on the recovery rate of indium and the utilization rate of HCl, and the values of the two parameters are 99.00% and 50% respectively. With the increase of carrier gas flow rate, the recovery rate of indium variation range is 90.38%-99.36%, and the HC1 utilization rate reduces from 82.71% to 34.82%. With the lengthen of reaction time, the rate of indium recovery and HCl utilization increase by 53.42% and 20.03 to 98.83% and 72.80%, respectively.(6) For the recovery of indium by chlornation in PVC-LCD system, when the chloride-indium ratio increases, the recovery rate of indium increases from 26.57% to 97.50%. The influence of chlorination temperature on the recovery rate of indium is obvious, increasing from 17.44% to 95.92% with temperature raises. With the prolonging of reaction time, the recovery rate of indium first increases from 50.20% to 97.89% and then decreases to 84.56%, reaching a maximum at 45min. Along with the raising of PVC pysolysis temperature, the recovery rate of indium first increases and then decreases, reaching the maximum value of 97.50% at 400℃.(7) Design Expert 8.0.5b is employed to design the experiments of indium recovery by chlornation and the optimal operating condition is obtained as follows: PVC pyrolysis temperature 400℃, carrier gas flow rate 100ml/min, chlornation temperature 500 C, chlornation time 30min and chloride-indium ratio 11:1.(8) By analyzing the products of non isothermal experiments by XRD, the reaction mechanism of HCl-In2O3 chlornation is speculated as follows. When temperature is lower than 300℃, the reaction is HCl adsorptionâ†'InCl3 +In4Cl6 â†' In5Cl9, while when the temperature is higher than 300℃ chloride, the reaction is In5Cl9 â†' InCl3 +InCl â†'InOClâ†' InCl3.(9) Dynamics of the reaction of hydrogen chloride and indium oxide are solved with empirical mode method, and the dynamic equation is with the reaction order 1.0384 and fitting degree 0.9745. While the dynamic equation obtained by double logarithm method is with the reaction order 1.2942 and fitting degree 0.9208.