Catalytic Pyrolysis of Waste-Printed Circuit Boards Using a Cu/Fe Bimetal Synergistic Effect to Enhance Debromination

Waste electrical and electronic equipment (WEEE) is a significant contaminant because of the large amounts produced, their complex components, and their high pollution levels. The global production of WEEE is considered to be more than 50 million tons, and the yield continuously increases by 10% every year [1]. Waste-printed circuit boards (WPCBs) are the most important constituent of WEEE and have dramatically increased with the production of WEEE [2]. WPCBs can comprise various metals (such as gold, silver, copper, iron, aluminum, etc.), organic matter (epoxy resin, plastic, phenolic resin, etc.), and hazardous substances (heavy metals, binders, and brominated flame retardants (BRFs)), all of which are valuable for “urban mines” [3]. Therefore, some researchers are committed to recycling valuable metals and have made significant achievements in this regard [4,5]. However, the safe disposal and recovery of residual nonmetals in WPCBs (NM-WPCBs) has been neglected. NM-WPCBs are a typical form of hazardous waste that contains a large number of BRFs [6]. Improper disposal methods such as landfilling and incineration can release polybrominated diphenyl ethers (PBDEs), Tetrabromobisphenol A (TBBPA), polycyclic aromatic hydrocarbons (PAHs), and brominated aromatic hydrocarbons, which can accumulate in human bodies and cause serious diseases [7]. Therefore, the debromination of NM-WPCBs is necessary.

A few debromination technologies (such as supercritical fluid treatment, subcritical fluid method, in situ pyrolysis, the mechanochemical method, and the microbial method) have been adopted to remove hazardous substances and recycle valuable materials from NM-WPCBs [8,9,10]. Of these, in situ pyrolysis is one of the most promising technologies for bromine removal and sustainable resource recovery because of its obvious advantages. Firstly, toxic components such as PBDEs, TBBPA, and PAHs are not produced by in situ pyrolysis because of its oxygen-deficient atmosphere, and other bromine-containing substances such as bromophenol and hydrogen bromide can be simultaneously removed via the reactions of debromination agents. Secondly, NM-WPCBs can be decomposed into small molecular organics during the process, which can then be recycled as an alternative chemical feedstock. Thirdly, compared with supercritical fluid treatment, the subcritical fluid method, the mechanochemical method, and the microbial method, in situ pyrolysis is more efficient and is usable at relatively low temperatures and pressures, which can reduce energy consumption and costs. Considering the importance of environmental protection and resource recycling, researchers have devoted themselves to investigating the effectiveness of debromination using the in situ pyrolysis method [11,12,13]. Zhu et al. accurately calculated the capture capacity and mechanisms of CaCO₃ for bromides generated when using the pyrolysis process on brominated resin but found that the capture capacity of CaCO₃ for CH₃Br was only 2.37 × 10⁻⁹ mol/g. Gao et al. removed bromides from WPCBs via the in situ pyrolysis of Ca(OH)₂, and the results showed that all HBr species and nearly 75% of organic bromide species (such as bromophenol, 4-bromophenol, and 2,4-dibromophenol) were successfully removed. Bhaskar et al. used a calcium hydroxide carbon composite as a debromination agent to remove bromide from Br-containing plastic, and they found that most of the bromides were absorbed and removed [14,15]. Ma et al. enhanced debromination performance by adding Fe particles to ZSM-5 and Ni/SiO₂-Al₂O₃ catalysts, and the results showed that the bromine content in the oils was reduced by between 8.74 wt% and 5.96 wt% and between 8.74 wt% and 5.46 wt%, respectively [16,17]. However, removing organic bromide species generated by pyrolysis is difficult. Taking into consideration the damage caused by organic bromides, the deep and complete debromination of WPCBs is necessary.

Metal catalysts are one of the agents used for WPCB debromination [7,18]. Mono-metals such as iron, zinc, nickel, and their oxides are used to remove Br from WPCBs. Studies suggest that most inorganic bromides, such as HBr and Br₂, can be efficiently removed. However, the removal efficiency for organic bromides such as bromomethane and bromophenol is low. However, bimetal catalysts have several apparent advantages and thus have the potential to remove Br from WPCBs. Bimetal agents can promote pyrolysis reactions by facilitating heat transfer. Furthermore, bimetal debromination agents can simultaneously remove organic and inorganic bromides via chemical and physical reactions. Therefore, some studies on debromination via bimetals have been carried out. Ma et al. found that zero-valent Fe and Ni can capture evolved bromides from WPCBs via two-stage catalytic pyrolysis. Terakado et al. investigated the debromination ability of metal oxides such as Fe₂O₃ and ZnO and found that these catalysts have obvious debromination efficiency with HBr and organic bromine compounds. However, previous studies have mainly focused on debromination efficiency. The synergistic debromination mechanism of bimetal catalysts is unclear [19,20], and the removal of organic bromides needs to be improved. Our previous study found that copper has excellent removal efficiency with organic bromides and Fe has advantages in removing Br₂ and HBr. Therefore, Cu/Fe has great potential regarding WPCB deep debromination.