Pyrolysis Characteristics Of Printed Circuit Board Waste And Recovery Of Its Pyrolysis Oil

Printed circuit board (PCB) waste is a major constituent in electronic waste, which is very difficult to treat compared with other components. Mechanical treatment which employs crushing and separation processes for PCB recovery is becoming more popular and widely used in industrial practice. However, the characteristics of PCB with high hardness and tenacity, compact cohesion between metal and non-metal, could lead to secondary pollution, low efficiency, high-energy consumption and over-pulverization during mechanical crushing process.PCB waste mainly contains organic materials, metals and glass fiber. Pyrolysis is considered as a promising technique to recycle PCB waste. In pyrolysis process, the organic resin in PCB was decomposed into gas and liquid, thus weakening the binding force between the metal and non-metal layer and achieving efficient dissociation of metal and non-metal. Addtionally, the liquid and gas produced during pyrolysis could be reused as chemical feedstock or fuel. Based on the advantages of PCB pyrolysis listed above, a new process for recycling PCB waste defined as’pre-pyrolysis method’was proposed in this study, that is PCB waste was pyrolyzed, and then the metal separation occurred, with the aim to improve the liberation efficiency of metal and non-metal and reduce the energy consumption. The following works are carried out and the main conclusions are as follows in this dissertation:(1) The mechanical liberation characteristics of PCB waste were studied. A two-step crushing process which combined with a coarse-crushing step and a fine-pulverizing step was adopted. The liberation situation of the crushed products was observed. Properties of the crushed products, such as morphology, particle size distribution were determined. The results indicated that the crushing process of PCB could be divided into two stages, including metal dissociation and particle size adjustment. The suitable particle size for mechanical dissociation of PCB should be controlled in the range of0.59-0.15mm. The major elements in PCB include Si, Ca, Cu, Br except C, H, O. Copper was the most dominating metal in PCB and reached its maximum content in0.42-0.25mm particle size, about32.32%. The calorific values of PCB in different size changed in the range of8.07-11.69MJ/kg. The ash content was between53.87%and80.45%.(2) Thermogravimetric analysis on a self-designed laboratory scale thermo-balance (called Macro-TG) with more sample loading was carried out. The pyrolysis characteristics of typical components of e-waste, wire, keyboard and PCB waste, were investigated. The results were compared with those obtained in common thermo-gravimetric measurements (called common TG). The kinetic mechanism function of samples both in Macro-TG and common TG was determined. Additionally, the corresponding kinetic parameters of each sample were also obtained. The results indicated that large particles existed a pyrolysis reaction retardarce compared to fine one. And smaller activation energy were found in pyrolysis reaction occurred in Macro-TG. Distributed activation energy model analysis revealed that activation energy did not show a constant value during the pyrolysis process, and it changed with the rate of weight loss. Due to the composition difference, pyrolysis behaviors of wire, keyboard and PCB waste were different. In the same pyrolysis conditions, the sequence of thermal stability for the three studied species is described as:wire<keyboard<PCB. Heating rate and particle size are two important variables that influence the overall reaction rate of PCB. The TG and differential thermogravimetry curves were shifted to higher temperature when increasing the heating rate. The increase of particle size inhibits the intraparticle pyrolysis reaction.(3) A combination of thermogravimetry-fourier transform infrared spectrum (TG-FTIR) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) techniques was employed to investigate the pyrolysis characteristics and gas product properties of PCB waste. The obtained information is used to explain the possible mechanism of PCB pyrolysis. The results indicated that gaseous products from PCB waste pyrolysis mainly evolved at270-400℃, which is in agreement with the observation obtained from the thermoanalysis. PCB waste degradation could be separated into three stages. The main products in the first stage (<293℃) are H2O, CH4, HBr, CO2, CH3COCH3. High-molecular-weight organic species, including phenolic compounds, aldehyde, ketone, etc., mainly evolved in the second stage. In the last stage, at temperatures above400℃, carbonization and char formation occured. The cleavage of OH-C, O-CH2, C-C(phenyl), CH2-O-phenyl and C(phenyl)-Br bonds occurs in the pyrolysis process of PCB, and the cleavage of the O-CH2bonds was a prevailing cracking reaction. About27kinds of organic matters were produced during PCB pyrolysis. Phenol, p-isopropenyl phenol and bisphenol A are the most prominent products.(4) Batch pyrolysis of PCB waste was conducted on a laboratory scale tubular furnace reactor, the influences of final pyrolysis temperature (FPT) and heating ways on the product yields and product property were investigated. The qualitative analysis of the pyrolysis products were carried out using GC, oxygen bomb calorimeter, FTIR,1H-NMR, and GC-MS. The results indicated that about76.13%solid product,14%pyrolysis liquid and9.86%pyrolysis gas were obtained at600℃. When the FPT above600℃, the char yields reach a constant value. The rise of FPT lead to the change of the ratio of the oil to gas. The yield of pyrolysis liquid obtained at fast pyrolysis was higher than that in slow pyrolysis. The decomposition of the resin binder weaken the binding force between the metal and nonmetal layer, and the metal and nonmetal in PCB achieve100%dissociation at coarse particle size after pyrolysis. The liberated glass fiber was in the form of flaky rather than pulverous. Additionally, the gas products contained combustible components like H2, CO, CH4, heating values of the gases were in the range of11.24-15.21MJ/Nm3. The pyrolysis liquids contained high proportion of phenolic compounds like phenol, isopropylphenol whose heating values were in the range of24.5-27.5MJ/kg.(5) The reutilization methods of the PCB pyrolysis oil were discussed. As the pyrolysis oil contained high concentration of phenol and phenol derivatives, pyrolysis oil was substituted for phenol as the raw material for preparing pyrolysis oil-based resin, which was then used as a precursor to prepare carbon nanotubes (CNTs) and porous carbon. The CNTs was prepared by catalytic pyrolysis of the oil-based resin with ferrocene at900℃. The porous carbon was prepared from carbonization of KOH-treated resin at700℃. Morphologies and structures of the resulting products were characterized by FTIR,1H-NMR, TG, XRD, SEM, TEM, N2-adsorption isotherm techniques. The results indicated that the oil could be polymerized into pyrolysis oil-based resin with formaldehyde. The aromatic nuclei in the oil were linked by-CH2-O-CH2-or-CH2-after polymerization. The yield of CNTs was56.82%. The products were hollow-centered CNTs with outer diameter of～338nm and wall thickness of-86nm. The length of the prepared tubules reached several micrometers. The prepared resin carbonized to porous carbons at yield of38.05%and its external surface covered cavities. All existing pores were predominance of microporosity. A type I isotherm was observed for the prepared porous carbon. The BET surface area, micropore volume and total pore volume of the porous carbons were1214m2/g,0.41cm3/g and0.64cm3/g, respectively.