ARTICLE pubs.acs.org/IECR
Recycling of Waste Printed Circuit Boards by Microwave-Induced Pyrolysis and Featured Mechanical Processing Jing Sun,† Wenlong Wang,*,† Zhen Liu,† and Chunyuan Ma† †
National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Energy and Power Engineering School, Shandong University, 17923 Jingshi Road, Jinan 250061, PR China ABSTRACT: The combination of microwave-induced pyrolysis and mechanical processing is a promising way to recycle the waste printed circuit boards (WPCBs). In pyrolysis, WPCBs yield an average of 78.6 wt.% solid residues, 15.7 wt.% oil, and 5.7 wt.% gas. The solid residues are rich in metals; the oil is abundant with phenol and substituted phenols which can be reclaimed as chemicals or fuels; and the gas is combustible with a caloric value of 4504 kcal/m3. Our featured mechanical processing, including crushing and specially designed sink-float separation, is very suitable for metal reclamation from the pyrolysis residues. Over 99 wt.% of metals can be dissociated by the crushing step; the final recycling rate and grade of metals in the separation step can amount to 95 wt.% and 96.5%, respectively. The economic assessment reveals that the combined treatment is amazingly profitable and very promising to tackle the challenges posed by the electronic scraps.
1. INTRODUCTION With the rapid technology innovation, the upgrade and replacement of electrical and electronic equipments (EEE) have been immensely accelerated in the last two decades, resulting in the ever-increasing generation of waste electrical and electronic equipments (WEEE) or so-called e-waste. As indispensible components, printed circuit boards (PCBs) constitute about 6% of the total weight of e-waste and are the most difficult parts to dispose of.1 The typical PCBs contain nonmetals (i.e., plastic, resin, glass fiber, etc.) >70%, copper ∼16%, solder ∼4%, iron, ferrite ∼3%, nickel∼2%, silver ∼0.05%, and some other precious metals (PMs), etc.2 The presence of metals encourages the recovery of waste printed circuit boards (WPCBs). For instance, the concentration of gold in gold ores is commonly between 0.5 and 15 ppm,3,4 while its concentration in PCBs can be over 10 times higher (e.g., 150 ppm in expansion cards, over 10,000 ppm in central processing units). However, the basic materials of PCBs are glassfiber reinforced polymers, which are thermoset composite materials with low calorific value. And serious pollution may be generated if PCBs are not properly disposed of since the flame retardants therein may cause the formation of polybrominated dibenzo-p-dioxins (PBDDs) or polybrominated dibenzofurans (PBDFs). Additionally, the toxic ingredients, such as lead and cadmium, etc., may enter the aquatic or terrestrial ecosystems, which has resulted in the elevation of blood lead and cadmium levels of children in some typical neighborhoods like Guiyu of China.513 Pyrometallurgy and hydrometallurgy are the traditional recycling technologies for WPCBs. But these methods are metaloriented and just challenged by the steadily decreasing content of the precious metals used in electronics and the ever-growing environmental awareness. Thus, they are gradually phased out. Various mechanical or physical processing methods are becoming more favored and widely used in industrial practice.1421 During the mechanical processing, multilevel crushing is the first and crucial step; after that, a magnetic, electric, air classification, r 2011 American Chemical Society
or several combined separations are used to recover the metals and nonmetals separately. Commonly, good fineness of crushed material particles is required to guarantee a high degree of metal liberation. However, many problems, such as dust and noise pollution, serious abrasion of the cutting tools, etc., can be caused during the crushing process.22 Moreover, direct and continual crushing of PCBs could lead to the local high temperature where pyrolysis and release of toxic substances can easily occur.23 Zhao and Li reported that the surface temperature of WPCBs could reach 300350 °C for over 3-s crushing.23 Thus, measures should be taken to reduce the negative effects. Using thermal shock as a pretreatment method to change the interfacial impact and tensile strength has been proven to be a potential solution to reduce crushing difficulty and to cope with the above-mentioned deficiencies.24 Pyrolysis, which can be considered as a special kind of thermal shock, is an attractive route to pretreat WPCBs. In a pyrolysis process, the organic materials in WPCBs can be decomposed into oil and gas, which can be recycled as fuels or chemicals; the metals can be enriched in the solid residues and become convenient for separation by mechanical processing; in particular, the pollution control of exhaust gas is much easier than in incineration. Although a significant amount of research has been reported for the pyrolysis of WPCBs, most of them are finished in externally heated devices, such as thermal gravimetric analyzer, rotary kiln, fixed-bed reactor, etc.2529 These pyrolysis processes are featured by external heating via conduction, convection, and radiation. Being remarkably different, microwave heating shows selectivity to different materials and can be regarded as an internal and volumetric heating way. Just due to the selective Received: June 22, 2011 Accepted: September 8, 2011 Revised: September 3, 2011 Published: September 08, 2011 11763
dx.doi.org/10.1021/ie2013407 | Ind. Eng. Chem. Res. 2011, 50, 11763–11769
Industrial & Engineering Chemistry Research
Figure 1. Whole process of integrated recycling for WPCBs.
characteristic, the microwave-induced pyrolysis of WPCBs is quite possible to work in an energy-efficient way. For instance, with the assistance of certain strong microwave absorbers, a rapid heating rate and pyrolysis process can be obtained.30,31 And the corona discharge, or even intense arc discharge caused by the metals with tips or corners may lead to local high temperature and accelerate the pyrolysis process. The coupling of microwave heating and corona (arc) discharge can help to achieve rapid pyrolysis and good energy utilization efficiency. Thus, the selective and internal heating style is very suitable for the disposal of WPCBs. Our working group has already carried out some research on the microwave-induced pyrolysis and proven it to be a potential way to deal with WPCBs.30,32 In this work, a combination of microwave-induced pyrolysis and mechanical processing is developed to recycle WPCBs and tackle the previous environment-harming or inefficient deficiencies. The organic components in WPCBs are first recycled by the microwave-induced pyrolysis. Subsequently, the less hazardous and more fragile pyrolysis residues are crushed by a high-speed shearing machine to liberate the metals from the base plate, and then the crushed pyrolysis residues are separated into light nonmetals and heavy metals by a special kind of sink-float separation. This work aims to recycle WPCBs comprehensively in an energy-efficient and environment-friendly way.
2. MATERIALS AND METHODS 2.1. Materials. Some obsolete integrated computer circuit boards were collected from a local electronic products market for the experiments. The electronic components, such as capacitors and metal blocks, were removed first, but there were still some metal wires and electronic chips on the epoxy base plates due to the dismantlement difficulty. 2.2. Methods. The recycling process was comprised of microwave-induced pyrolysis and mechanical processing. A flowchart is shown in Figure 1 for the process. At the first stage of this work, approximately 3 kg of WPCBs were first cut into fragments of about 2 cm 2 cm using a self-made shredder. The shredded WPCBs were then pyrolyzed by microwave irradiation in a self-made quartz reactor. At the second stage, the pyrolysis residues were tested to separate metals and nonmetals by mechanical methods such as crushing, screening and sink-float
ARTICLE
separation. A kilowatt-hour meter was used to record the electric energy consumed during each process. Based on the assessment of the electric energy consumed and the value of the recycled useful products, the economic benefits can be assessed. 2.2.1. Microwave-Induced Pyrolysis. The microwave-induced pyrolysis was carried out in a reconstructed household microwave oven (Galanz P7021TP-6). Figure 2 shows a schematic diagram of the experimental setup. In each pyrolysis experiment, 250 g WPCB samples were placed in a self-made quartz device, which was sealed by a set of PTFE flanges with silicone gaskets. The inlet and outlet of the quartz device were connected with nitrogen cylinder and condensing system respectively by silicone tubes. At the beginning, the entire system was purged with pure nitrogen (99.999%) at a flow rate of 100 mL/min for 30 min to expel oxygen. Then the microwave irradiation was imposed on the sample with an output microwave power of 700 W, and simultaneously the nitrogen flow was cut off. The temperature was monitored online by a thermocouple which was centered in the sample. The volatiles passed through a two-stage condensing unit which was cooled by ice water (less than 4 °C), and then a glass wool trap which was used to remove any oil that was not trapped by the condensers, and finally a gas-washing bottle which contained deionized water to absorb the soluble gases. The final gas was sampled with a gas bag, and the condensable liquids were collected by the condenser. The microwave oven was turned off when no further significant release of gas was observed. The yields of solid and liquid products were measured by weighing. According to the mass balance, the yield of gaseous products was obtained by difference. The data of kilowatt-hour meter were recorded at the beginning and the end of the pyrolysis process to calculate the electric energy consumption. 2.2.2. Crushing, Size Classification, and Liberation Degree Determination. In order to strip metals from the glass fiber reinforced base plates, the pyrolysis residues of WPCBs were shredded roughly by a crusher with high-speed rotating blades. In order to release metals completely, woven glass fiber cloth need be shredded into hairy one as more as possible. Here, the transformation of the presence of glass fiber was defined as TGF ¼ mhairy =mtotal
ð1Þ
where TGF is the transformation rate, mhairy is the mass of hairy glass fiber, and mtotal is the mass of the total glass fiber. The crushed materials were classified into grades of >0.9, 0.450.9, 0.280.45, and 0.9
78.27
99.2
acetone tetrahydrofuran
0.62 2-methylbenzofuran 0.55 2-ethylphenol
1.91 0.85
0.450.9
5.10
100
0.280.45
6.38
100
benzene
0.97 24-dimethylphenol
0.68
95%) which ensured the effective liberation of metals. The residues coming out of the crusher were classified into four size grades: >0.9 mm, 0.450.9 mm, 0.280.45 mm, and 0.9 mm), while only a small amount of products were distributed in the fine fractions. More than 99 wt.% of metals were released from the base plate by crushing. The distributions of metallic and nonmetallic components in each size group are shown in Figure 5. Noticeably, most metals and nonmetals fell into the coarse fraction of >0.9 mm. In stark contrast, more than 15 wt.% of nonmetals were distributed in the finest fraction (
