Analysis And Control Of Gaseous Products From Vacuum Pyrolysis Of Waste Printed Circuit Board

With the development of science and technology, electronic waste becomes the most dangerous and fastest increasing solid waste. It is an urgent and difficult research problem to find a clean, efficient and safe treatment and resource utilization of electronic waste. Vacuum pyrolysis is an effective method to deal with the waste printed circuit boards, and to realize resource utilization and harmless disposal. Because the residence time of vacuum pyrolysis products in the high-temperature reaction zone was shorten in vacuum condition, the probability of secondary reactions were reduced and the generation of substances like hydrogen bromide and brominated dioxins were also decreased. Meanwhile the recovery rate of pyrolysis oil was improved. But the pollution by-products existed in vacuum pyrolyzed gas phase products have hindered the popularization and application of vacuum pyrolysis_technology. This study made a deep research on the composition, compose characteristics, migration and transformation rule and pollution control technology of vacuum pyrolysis gas phase products of Waste Printed Circuit Board （WPCB） providing scientific basis for the high-valued and all-component resource utilization of electronic waste1. Thermal gravimetric analysis （TGA） was used to test the pyrolysis characteristics and reaction kinetics of WPCB powder. The results showed that the pyrolysis process of WPCB powder was divided into three stages:the first stage （<280℃） was evaporation of water; the second stage （from 280 ℃ to 500℃） was decomposition of organic matter; the third stage （>500℃） was stable one. The second stage was the main weight loss stage, including volatilization and decomposition of organic matter （330-380 ℃） and volatilization and decomposition of fixed carbon （460-500℃）. The calculations with the aids of kinetic equation Coats-Redfern showed that the apparent activation energy was 250.74 kJ·mol-1 and 23.58 kJ·mol-1 separately.2. Using test method of Py-GC/MS and TG/MS, vacuum pyrolysis gas phase products were studied. The results showed that after condensation, the main components of the gas phase products were volatile organics like phenol, benzene series and so on. The main inorganic components were HBr, NO₂, SO₂. Based on the theory of polymer chemistry structure, combined with the principle of chemical reaction of organic compounds in vacuum pyrolysis process, the mechanism of vacuum pyrolysis of WPCB and the formation process of gas phase products were discussed. Intramolecular dehydration, cyclization, molecular rearrangement, radical substitution and chemical bond rupture reacted in vacuum pyrolysis process, from which deduced the forming process of main gas phase products.3. Small-scale experimental devices for pyrolysing WPCB and collecting gas phase products under vacuum condition were constructed. Simultaneously the content of inorganic （HBr, NO₂ and SO₂） in gas phase were determined by ion chromatography. When sampling volume was 30L in the 50mL absorption liquid, the detection limit of HBr, NO₂ and SO₂ was 0.000005 7 mg·L^-1,0.000 0034 mg·L^-1 and 0.000 002 9 mg·L^-1 respectively. The recovery rate of this method was 95.7%～104.8%, which can fully meet the simultaneous determination requirements of HBr, NO₂ and SO₂ in gas phase. When using this method and the national standard method to determine the content of HBr, NO₂ and SO₂ in ambient air, and then carrying out F test and T test on the determination of data by Excel analysis tools, there was no significant difference between the two measurement methods, which showed that absorption - ion chromatography method established in this experiment is simple, rapid, accurate and reliable.4. The distribution characteristics of vacuum pyrolysis gas phase products of WPCB were studied by controlling different process conditions. It was found that with the increase of vacuum degree, gaseous productive rate was reduced and the content of benzene series and HBr got increased, while the content of phenol, NO₂ and SO₂ was reduced. The acceleration of heating rate resulted in the decrease of gaseous production rate, the increment of the content of benzene series and the reduction of the content of phenol. Heightening the final temperature caused the content reduction of phenol and benzene series, the content increment of HBr, NO₂ and SO₂. As for synthetical consideration, the more appropriate pyrolysis conditions are vacuum degree of 0.09 MPa, the heating rate of 10℃/min and the final temperature of 500 ℃. Under this condition, the content highest average reduction ratio of the main gas phase products of benzene, toluene, ethylbenzene, hydrogen bromide, nitrogen dioxide and sulfur dioxide was 72.3%,32.6%,23.5%,24.9%,39.8% and 60.3%.5. The effect of the 6 additives （CaCO₃, HZSM-5, CaO, Al₂O₃, FeOOH and Ca（OH）₂） on the migration, transformation and escaping behaviors of vacuum pyrolysis gas phase pollutants of WPCB was studied. The results showed that the additive Al₂O₃, CaO and Ca（OH）₂, FeOOH could reduce the yield of gas phase. The removal rate of pollutants such as benzene, toluene, ethyl benzene, phenol, p-xylene, HBr, NO₂and SO₂ in gas phase products changed with the increasing proportion of the 6 above additives. Judging from the control of gas phase pollutants discharge, Ca（OH）₂ has the best inhibition effect. Increasing the pyrolysis temperature and vacuum degree could weaken Ca（OH）₂ inhibition effect of organic pollutants, but it improves the removal rate of SO₂. Under the condition of 1/5 adding proportion,500℃ pyrolysis temperature, and 0.09MPa vacuum degree, the average removal rate of the gas phase pollutants by Ca（OH）₂ could reach 66.4%.6. Simulating benzene series pollutants in vacuum pyrolysis gas phase products of-WPCB, the removal experiment for benzene series was carried out by Fenton reagent. The results showed that the removal process of Fenton reagent to benzene, toluene, ethylbenzene and xylene conformed to the first-order reaction kinetics model. When pH=2, H₂O₂/Fe²⁺ was 5, the reaction temperature was 40℃, the removal rate of benzene series was:p-Xylene> Benzene> Toluen> Ethylbenzene. The optimal initial pH was 2 to 3and the appropriate temperature was 40～50 ℃. When the proportion of H₂O₂/Fe²⁺was from 4 to 6, the removal rate of benzene series went to the highest point.