Co-pyrolysis of waste printed circuit boards with iron compounds for Br-fixing and material recovery

Waste printed circuit boards (WPCBs) were co-pyrolyzed with iron oxides and iron salts. Solid, liquid, and gaseous products were collected and characterized. Co-pyrolysis with FeCl2, FeCl3, or FeSO4 was able to increase the yield of liquid product which was rich in phenol and its homologues. Also, the addition of co-pyrolysis reagents reduced the release of brominated organics to liquid as Br was either fixed as FeBr3 in solids or released as HBr. In particular, FeCl2 showed the best ability to reduce the release of Br-containing organics to liquid compared with FeCl3 and FeSO4. Solid residuals were rich in iron oxides, glass fibers, and charred organics with surface areas of 20.6–26.5 m2/g. CO2 together with a small amount of CH4 and H2 were detected in the gaseous products. Overall, co-pyrolysis could improve the quantity and quality of liquid oil which could be reused as chemical or energy sources. Pyrolysis of waste printed circuit board was promising as a method for recycling.


Introduction
Generation of electronic waste worldwide has been growing consistently in recent years due to rapid technological advancements and the short life span of electrical and electronic equipment (Zhu et al. 2020). As an important part of electrical and electronic equipment, printed circuit boards contained resources of high value including copper, tin, gold, silver, etc. In general, it contained 20-30% metals and about 20% plastics, while ceramics and glass fiber make up the rest (Nekouei et al. 2020).
Traditionally, after electronics reached their life span, the waste printed circuit boards (WPCBs) were separated and underwent a rough recovery of profitable metals (Cu, Au, etc.) before they were disposed by landfill or incineration which could lead to serious contamination of air, soil, and water (Mdlovu et al. 2018). In recent years, China has tightened its regulations on WPCB disposal after many reports of pollution by toxic metals and hazardous organic substances remained in WPCB. Effective recovery of metal resources (such as iron and copper) and non-metal fractions (such as resins) could reduce not only the environmental impact of WPCB but also the demand on natural ores (Hao et al. 2020).
One of the difficulties in WPCB recycling was its complex composition. It contained a wide range of organics and inorganics which varied according to device type, manufacturers, etc. Bazargan et al. (2014) reported that WPCB could contain up to 40 kinds of metals (e.g., Cu, Fe, Al, Sn, Ag, Au) and 10 kinds of non-metals. Some of the metal contents were even higher than mineral ores. Therefore, WPCB had great economic potential. Recovery of metals was usually more attractive due to their high added value. With advancement in research, the less valuable nonmetallic fraction could also be recycled as a source of materials and energy (Ghosh et al. 2015;Shen et al. 2018a, b).
For recycle, WPCB was first disassembled manually or automatically to remove batteries and capacitor components. Then, it was shredded and crushed or ground/pulverized before physical, chemical, and biological processes to further separate and enrich metals from resin, fiberglass, and plastics (Shen et al. 2018a, b). Physical separation methods could be shape-, density-, conductivity-, or magnetism-based (Salbidegoitia et al. 2015;Das et al. 2021). Other processes Responsible Editor: Santiago V. Luis investigated extensively to recycle metals or non-metals included gasification (Shen et al. 2018a, b), supercritical fluid treatment (Golzary and Abdoli 2020), dissolution in organic solvents (Yousef et al. 2018), hydrometallurgical and pyrometallurgical techniques (Zhou et al. 2011;Qiu et al. 2020), pyrolysis (Kim et al. 2018), and hydrothermal treatment (Yin et al. 2011).
As a waste disposal method for WPCB, pyrolysis was able to recycle both metals and organic matter. It was carried out under a non-oxygen atmosphere and organics were decomposed into small molecular products. Metals were not oxidized under an inert atmosphere and became easier to be liberated after pyrolysis while organics were converted to fuel or other valuable chemicals (Liu et al. 2021). Kim et al. (2015) reported that phenols and bisphenol A are the dominant compounds in the liquid-phase products in pyrolysis of epoxy-PCB and paper-laminated phenolic printed board. Rajagopal et al. (2016) used physical activation on char obtained from pyrolysis and obtained activated carbon with a surface area as high as 700 m 2 /g. Also, pyrolysis was carried out under a relatively low temperature and normal pressure which means low energy consumption and cost (Shen et al. 2018).
However, WPCB contained many halogenated compounds as flame retardant. Among them, the most common is brominated compounds. The degradation of these compounds was found to produce toxic products such as dibenzo-p-dioxin, dibenzo-furan, 2-bromophenol, 2,6-dibromophenol, methyl bromide, etc. (Ning et al. 2017). Co-pyrolysis with various additives were investigated to minimize the contents of brominated compounds in pyrolysis oil and gas (Ma et al. 2018). KOH, NaOH, K 2 CO 3 , NaCO 3 , CaCO 3 , La 2 O 3 , MgO, CuO, CaO, Al 2 O 3 , zeolite, iron oxides, and carbon powder have all been used as debrominated agents (Evangelopoulos et al. 2015). Shen (2020) reported that pretreatment of WPCB by alkali was better in Br-fixing than alkali-salt. Br-fixing efficiency reached 53.6% by NaOH as Br released was adsorbed by NaOH to form NaBr and was retained in chars. Terakado et al. (2011) compared the debromination effects of metal oxides. The ability of oxides on HBr emission suppression varied. La 2 O 3 and CaO were more effective with a suppression efficiency of about 90%. Br-fixing was achieved by bromination of oxides as LaOBr, CuBr 2 , CaBr 2 , and FeBr 2 were found in the solid residues of pyrolysis. Many studies have shown that by choosing the right reagent, co-pyrolysis could not only reduce Br release into pyrolysis products but also induce a higher yield of valuable oil or change the properties of solid residues (Kakria et al. 2020).
This research attempted to study the feasibility of WPCB recycling by co-pyrolysis with iron oxides and iron salts. Studies have shown that iron is a good catalyst for the graphitization of carbonaceous compounds. Wu et al. (2018) reported that Fe 2+ or Fe 3+ salts can be used as template and catalyst in their pyrolysis of phenol formaldehyde resin and ethylene glycol. Activation and pyrolysis worked together when FeCl 2 and FeCl 3 were added, leading to an increase in yield and complete carbonization.
The WPCB herein was from Jiangsu Province, China. It was already disassembled, crushed, and shredded with most of the electronic components and metals recovered. The residual WPCB, mainly non-metallic, was currently used as an additive in brick making. However, the amount that could be consumed this way was limited by the demand for bricks. An additional outlet for the WPCB was urgently needed as landfill was no longer an option while incineration was problematic in fear of dioxin emission.
This research focused on co-pyrolysis of WPCB with iron oxides and iron salts for the possibility of energy or materials recovery. Conditions of pyrolysis (temperature, mass ratio, etc.) on the pyrolysis products were investigated in detail. In particular, the effects of different reagents (Fe 3 O 4 , FeCl 2 , FeCl 3 , and FeSO 4 ) on liquid yield and characteristics of residual solids and liquid products were compared. The behavior of WPCB and reagents during pyrolysis were studied via the monitoring of crystalline phases, morphology, surface chemistry, and elemental compositions. In addition, compositions of liquid products were investigated to study the Br-fixing behavior of different reagents.

Materials
The waste printed circuit board (WPCB) used in this study was from a printed circuit board recycling company (Qiuli Environmental Technology Ltd.) in Kunshan City, Jiangsu Province, China. Electronic components were disassembled first and a rough metal recovery was conducted by crushing and sorting. Therefore, the raw material in this study was mainly non-metallic with a residual metal content of 15.51%. Proximate analysis showed that WPCB contained 29.62% volatile carbon, 4.52% fixed carbon, and 65.86% ash. Figure 1 is a SEM image of the WPCB. It consists of particles and cylinder-shaped glass fibers. Particle sizes of the WPCB were mostly in the range of 10-200 μm.

Pyrolysis
Pyrolysis of WPCB was carried out in a laboratory-scale tube furnace (OTF-1200X, Kejing, China) under nitrogen atmosphere. The reaction tube is 1000 mm in length and 54 mm in diameter. Fe 3 O 4 , FeSO 4 , FeCl 2 , and FeCl 3 were selected as reagents for co-pyrolysis. Co-pyrolysis of WPCB was carried out at temperatures ranging from 350 to 750°C. Fe 3 O 4 in its powder form was mixed directly with WPCB. For copyrolysis with FeCl 2 , FeCl 3 , and FeSO 4 , 10 g of WPCB was added to different volumes of solutions of 5 g/L then freezedried to remove water. The FeCl 2 , FeCl 3 , and FeSO 4 to WPCB mass ratios were set at 0.25, 0.50, and 1.0. Before heating, the furnace was purged for air with N 2 at a rate of 10.0 mL/min for 30 min. Temperature was raised at a rate of 10°C/min. Time of pyrolysis was kept constant at 60 min. The outlet of the tube furnace was connected to a condenser with a cooling system to collect liquid oil. The remaining gas passed through dilute NaOH solution before exhaust was collected. All pyrolysis samples were done in triplets. Results given were the averages.
Liquid yield (in percentage) was calculated as the mass of liquid collected divided by the mass of the initial WPCB. The co-pyrolyzed sample was named as WPCB/reagent-temperature, such as WPCB/FeCl 2 -650.

Characterization of pyrolysis products
Thermogravimetric analysis of WPCB was conducted via a thermal gravimetric analyzer (TGA, STA-8000, PerkinElmer, USA). X-ray diffraction analysis was carried out by an X-ray diffractometer (Rigaku UItima IV, Japan) with a Cu-Kα radiation at 40 kV and 30 mA as 2θ ranged from 10 to 80°. FTIR (NICOLET iS10, ThermoScientific, Germany) analysis was employed to characterize functional groups of solids. Morphologies of solid particles were analyzed by scanning electron microscopes (Nova Nano 450, FEI and ZEISS Gemini 300, Germany). Proximate analysis of WPCB was carried out according to ASTM standard method E1131-08.
The composition of the pyrolysis liquid was analyzed via gas chromatography-mass spectrometry (GC-MS, 7890/5975C, Agilent, USA). GC-MS was operated with a HP-5MS capillary column (30 m × 0.32 mm × 0.25 m). The initial temperature of the oven was 40°C which was kept for 5 min before it was raised to 260°C at 5°C/min and kept at 260°C for 20 min. The carrier gas was helium with a constant flow of 1.0 mL/min.

Results and discussion
Screening of co-pyrolysis reagent by liquid yield In this research, WPCB and WPCB mixed with iron compounds were pyrolyzed at high temperature to investigate the effects of these additives. The compounds selected were Fe 3 O 4 , FeCl 2 , FeCl 3 , and FeSO 4 . Figure 2 shows the change in liquid yield of pyrolysis with the change of temperature and iron compound/WPCB ratio.
Studies have shown that liquid products from WPCB pyrolysis could be a valuable source of chemicals or energy as it contained products such as phenols and bisphenol A, etc. (Quan et al. 2010;Gao et al. 2020). Figure 2a shows how liquid yield varied with temperature when different iron compounds were used. The iron compound/WPCB ratio was kept constant at 0.5. The liquid yield peaked at 550°C for WPCB. The addition of Fe 3 O 4 actually led to a slight drop in yield per unit of WPCB. In contrast, the addition of FeCl 2 , FeCl 3 , and FeSO 4 was beneficial probably due to these iron salts' ability to catalyze graphitization of carbonaceous materials. The highest yield increased from 13% to around 18-20%. The optimal pyrolysis temperature was 550-650°C.
The iron compound/WPCB ratio also affected liquid generation (Fig. 2b). The pyrolysis temperature was set at 650°C for FeCl 2 and 550°C for the rest. There is a general trend of increase in yield with the increase of mass ratio from 0.25 to 0.5. A higher ratio means more iron mass per unit of WPCB. The effects of mass leveled off after the ratio reached beyond 0.5 for FeCl 2 , FeCl 3 , and FeSO 4 . This is in accordance with results from Gao et al. (2019). They also found there is an optimal mass ratio between iron oxide and WPCB for liquid yield from pyrolysis.
Next, the pyrolysis behavior of WPCB with FeCl 2 , FeCl 3 , and FeSO 4 was investigated in detail. Figure 3 shows the mass loss of WPCB, FeCl 2 , and WPCB/FeCl 2 during pyrolysis at different temperature and XRD, FTIR, and SEM images of the residual solids. The FeCl 2 /WPCB ratio was set at 0.5. Figure 3a shows the mass loss during pyrolysis. 20.8% of the mass was lost when WPCB was pyrolyzed at 350°C for 1 h. As the pyrolysis temperature was raised from 450 to 750°C, mass loss increased only slightly from 22.6 to 24.7%.

Pyrolysis with FeCl 2
For comparison, mass loss of FeCl 2 in N 2 was also recorded in Fig. 3. There is a sharp mass loss of FeCl 2 between 350 and 550°C. Calculated mass losses in Fig. 3 were obtained  assuming the mass changes of WPCB and FeCl 2 in WPCB/FeCl 2 were independent of each other. Mass losses labeled as WPCB/FeCl 2 were the real values of the WPCB/FeCl 2 mixture. The total mass loss ranged from 10.9 to 33.6% as compared to the calculated loss of 7.4-29.9%. Real mass losses were slightly higher than the calculated ones. The higher mass loss could be due to interactions between WPCB and FeCl 2 .
In their study on FeCl 2 as co-pyrolysis reagent to pyrolyze polyester fabric, Xu et al. (2019) believed that FeCl 2 was converted to FeOOH and Fe 2 O 3 at temperatures between 120 and 490°C while Cl was released as HCl. Fu et al. (2017) reported that Fe 2 O 3 from FeCl 2 decomposition could be reduced by charred organics to produce Fe 3 O 4 or even Fe 0 (reactions (1) and (2)) when FeCl 2 was used as activating agent to recycle tomato stem waste.
XRD, FTIR patterns, and SEM images of WPCB/FeCl 2 residual solids are shown in Fig. 3b-d. The XRD pattern (Fig. 3b) showed Cu crystals in the raw WPCB. Fe 2 O 3 , FeOOH, or Fe 3 O 4 was found in pyrolysis residues from FeCl 2 and WPCB/FeCl 2 mixture. Fe 3 O 4 could be the result of Fe 2 O 3 being reduced when WPCB was present. These results showed that various reactions occurred during pyrolysis. It is possible that some of the ferrous iron was oxidized to ferric iron first. However, XRD results were qualitative and the finding of ferric iron structures by XRD did not exclude the possible presence of ferrous iron. Further studies on iron mass balance and iron speciation were needed for a clearer picture on how FeCl 2 reacted with WPCB under high temperature.
Peaks at 2θ of 53°and 57°were believed to be those of FeBr 3 . This means that Br from WPCB was fixed by FeCl 2 decomposition products. However, the absence of FeBr 3 in XRD patterns of 450°C did not indicate no Br-fixing occurred. It only means the resultant FeBr 3 may not be crystalline. Figure 3c shows the FTIR patterns. Peaks around 3440-3407 cm −1 were assigned to the vibration of O-H stretching (Ng et al. 2009). Bands at about 1400-1600 cm −1 were the C=C vibration in the benzene ring skeleton while those at 1040 −1 were attributed to C-O stretching (Zhang et al. 2015). In addition, C-Br peaks were found in all samples at 459-471 cm −1 as brominated epoxy was commonly employed in WPCB as fire retardant while Fe-O peaks at 603-620 cm −1 were found only in WPCB/FeCl 2 pyrolyzed samples. Figure 3d shows the SEM image of WPCB/FeCl 2 pyrolyzed at 650°C (WPCB/FeCl 2 -650). WPCB consist mainly of cylindrical glass fibers and small particles. After pyrolysis with FeCl 2 , glass fibers disintegrated due to the corrosive effects of HCl from FeCl 2 decomposition Pyrolysis with FeCl 3 Figure 4 shows the mass loss, XRD, FTIR patterns, and SEM image of WPCB pyrolyzed with FeCl 3 .
FeCl 3 was converted to FeOCl or FeOOH under high temperature (reaction (3)) which could then undergo further reaction (4) to form other iron oxides (Kang et al. 2010).
According to mass loss results in Fig. 4a, FeCl 3 suffered continuous mass loss as temperature rose from 350 to 650°C then plateaued. Fifty-five percent of the original FeCl 3 was lost during thermal treatment. Mass losses of the WPCB/FeCl 3 mixture varied from 20% to about 39% which was also slightly higher than the calculated mass losses. This could be due to the interaction between iron oxides and carbon produced from WPCB pyrolysis as shown in reactions (1) and (2).
The reduction of Fe 2 O 3 to Fe 3 O 4 was proven by XRD patterns in Fig. 4b. Fe 3 O 4 were detected in WPCB/FeCl 3 pyrolyzed at 350°C and 550°C. As comparison, the dominant crystalline phases detected in FeCl 3 treated under N 2 at 550°C were FeOOH and Fe 2 O 3 . Peaks at 2θ of 24.8°and 48.9°were ascribed to FeOCl·H 2 O. These were found in pyrolysis samples of FeCl 3 and WPCB/FeCl 3 treated at 350°C but not samples treated at 550°C. This means that FeOCl·H 2 O was an intermediate during decomposition which was converted at high temperature. In addition, FeBr 3 crystals were found in the solid residuals treated at temperature 550°C probably due to the reaction between Br from WPCB and iron oxides. Figure 4c shows the FTIR patterns of original WPCB and two pyrolyzed WPCB/FeCl 3 samples. Peaks of O-H, C=C, C-O, and C-Br structures were detected in all samples together with Fe-O structures. The SEM image of WPCB/FeCl 3 treated at 550°C showed glass fibers in cylindrical shapes, but cavities were observed on the surface of the glass fibers indicating also that the addition of FeCl 3 led to the corrosion of the glass fibers.
Pyrolysis with FeSO 4 Figure 5 shows the mass loss, XRD, FTIR, and SEM analysis results of WPCB pyrolyzed with FeSO 4 .
In their study on thermal decomposition of FeSO 4 ·6H 2 O, Masset et al. (2006) (6)). FeSO 4 completely converted to Fe 2 (SO 4 ) 3 and Fe 2 O 3 at 550°C, and Fe 2 (SO 4 ) 3 was found to be the only intermediate compound during the decomposition. The mass loss by FeSO 4 under N 2 shown in Fig. 5a in this research correlated well with Masset et al.'s findings. Sharp mass loss of FeSO 4 was recorded only when the pyrolysis temperature was greater than 550°C.
X-ray diffraction was conducted to identify the crystalline phases formed during pyrolysis. The XRD pattern of FeSO 4 treated at 550°C for 1 h in Fig. 5b showed the presence of both Fe 2 (SO 4 ) 3 and Fe 2 O 3 . These two phases were also detected with WPCB/FeSO 4 treated at 350°C. In contrast, Fe 2 (SO 4 ) 3 was not found in WPCB/FeSO 4 treated at 550°C indicating a complete conversion to Fe 2 O 3 at high temperature. FeBr 3 was present at WPCB co-pyrolyzed with FeSO 4 at 550°C but not at sample of 350°C.
FTIR patterns were quite similar to those co-pyrolyzed with FeCl 2 and FeCl 3 . Peaks at 3420-3450 cm −1 , 1510 cm −1 , 1040-1070 cm −1 , and 460-480 cm −1 were due to the vibration of O-H, C=C, C-O, and C-Br structures, respectively. Fe-O vibrations were detected around 590 cm −1 . The SEM image of WPCB/FeSO 4 treated at 550°C showed glass fibers remaining cylindrical with very little disintegration. The corrosive effects of FeSO 4 were much less than those of FeCl 2 and FeCl 3 .
Overall, co-pyrolysis of WPCB with FeCl 2 , FeCl 3 , and FeSO 4 generated solids with different morphologies and compositions. Table 1 further lists the elemental composition, pore volume, and surface areas of these solids. About half of the carbon in raw WPCB was lost during pyrolysis. The addition of FeSO 4 had resulted in an obvious increase in S content. The solid manifested a certain porosity with surface areas about 20.6-26.5 m 2 /g. Pyrolysis changed physical and chemical properties of WPCB. Based on liquid yield, it is optimal to conduct . Part of Br was fixed as FeBr 3 crystals at these temperatures. The resultant solids consist of chars, iron oxides, and metals from raw WPCB and added reagents. Research on the recovering of residual metals (e.g., Cu), toxicity leaching, and potential reuse of the pyrolysis solids is currently underway. It could be a potential source of building materials. Gao et al. (2019) used pyrolysis products as carbon and iron source to synthesize magnetic carbon fibers.

Characterization of liquid and gas products
In addition to the characterization of solids, the composition of liquid products was determined by GC-MS as shown in Fig. 6. Liquids were collected from pyrolysis at a mass ratio of 0.5 and temperature of 650°C for WPCB/FeCl 2 and 550°C for WPCB, WPCB/FeCl 3 , and WPCB/FeSO 4 . Phenol and its derivatives were identified as the main products. Monitoring of liquid composition showed varied presence of brominated  organics with the addition of the co-pyrolysis reagent. Pyrolysis of WPCB generated a wealth of brominated organics incl uding 2,4-dibromophenol and 2,4,6tribromophenol, 2-bromo-4-methylphenol. In contrast, liquid products of FeCl 2 were dominated by phenol and no brominated organics compounds were detected. Chloride and bromide were released as HCl and HBr into the liquid phase as shown in Fig. 6b. Brominated organics were also found when FeCl 3 and FeSO 4 were used. Particularly, co-pyrolysis with FeCl 3 actually led to the generation of chlorinated compound, i.e., 4-chloro-2-(1-methylethyl)phenol in Fig. 6c. Compared with FeCl 3 and FeSO 4 , conversion of FeCl 2 started at a lower temperature (120-200°C). It is possible that the lack of chlorinated products was due to the fact that Cl in FeCl 2 was quickly released as HCl at low temperature which prevented the formation of chlorinated products. Iron oxides generated could act as Br-fixing agent when Br-containing fire retardant started to degrade. Also, the addition of FeCl 2 suppressed the bromination of organics as Br released from fire retardant was either released as HBr or fixed as FeBr 3 . The mechanism of Br-fixing will be further clarified by studying the mass balance of Br in future research. Based on the quantity and quality of liquid products, FeCl 2 could be recommended as a co-pyrolysis reagent.
In addition, pyrolysis exhaust was collected and analyzed. CO 2 , CH 4 , and H 2 were the main compounds detected together with trace amounts of CO. It appeared that no toxic gas was generated during pyrolysis.

Conclusions
WPCB was co-pyrolyzed with Fe 3 O 4 , FeCl 2 , FeCl 3 , and FeSO 4 . Liquid yield was used as the screening factor which showed that the addition of FeCl 2 , FeCl 3 , and FeSO 4 was conducive to liquid production. The optimal mass ratio and temperatures were 0.5 and 550-650°C for liquid yields which ranged from 18 to 20%. About half of the C in raw WPCB remained in solid phase and was present as chars. Iron compounds were decomposed to iron oxides at high temperature. Characterization of liquid by GC-MS revealed that phenol and its homologues are the main products which could be a valuable chemical source. FeCl 2 , FeCl 3 , and FeSO 4 showed different Br-fixing capacities. No brominated or chlorinated organics were found in liquid products from co-pyrolysis with FeCl 2 . The addition of FeCl 2 suppressed the generation of brominated organics. Br was either fixed as FeBr 3 in solids or released as HBr to liquid. In summary, the roles of the copyrolysis reagent such as FeCl 2 were twofold. First, it could improve the decomposition of macro-molecules in WPCB and increase liquid yield. Secondly, it reduced the bromination of organics and prevented excessive release of brominated organics.