Environment-friendly flotation technology of waste printed circuit boards assisted by pyrolysis pretreatment
Graphical abstract
Introduction
The disposal of solid waste from human activities is a great challenge (Yang et al., 2021; Guo et al., 2020). With the dependence of daily life on electronic products, huge amounts of waste electronic products have become a potential environmental threat (Abdelbasir et al., 2018; Tatariants et al., 2018). Recycling of WPCBs has attracted widespread attention due to the remarkable economic value (Chen et al., 2019; Kaya, 2016). A variety of metallic elements are concentrated in the WPCBs, along with harmful substances and heavy metals. Compared with the natural metal ore, WPCBs has the greater metal grade, and most of the metals are in simple substance, which is attractive for recycling (Birloaga et al., 2013). It is reported that the content of metal and alloy accounts for 40 % of the weight of WPCBs, and the content of non-metal components represented by glass fiber and organic matter is 60 % (Tiwary et al., 2017). 8% of the brominated flame retardants are also included in the WPCBs, which are hazardous to humans and the environment (Zhu et al., 2019a). Therefore, WPCBs are considered to be secondary sources of metals but are also considered environmental pollutants. In subsequent studies, the research on the deep utilization of different components in the WPCBs is also carried out. Pyrolysis and catalytic pyrolysis of WPCBs was studied by TG-FT-IR (Zhao et al., 2017). The organic matter in the WPCBs is converted into pyrolysis oil by debromination pyrolysis, and then the oil-based resin is synthesized (Gao et al., 2018; Quan et al., 2010). Studies on degradation of organic compounds using WPCBs as catalysts have also been reported (Wang et al., 2019). The Cu in the WPCBs is synthesized into nanoparticles with size of 5−32 nm (El-Nasr et al., 2020).
In addition to hydrometallurgy, pyrometallurgy and biometallurgy, environmentally friendly physical separation methods have been widely used to recover metals from WPCBs. The representative methods are magnetic separation (Veit et al., 2005), electric separation (Zhang et al., 2017), gravity separation (Zhang et al., 2018a; Nie et al., 2021) and flotation (Yu et al., 2017). Flotation is an effective method for separation of materials with similar magnetic, electrical properties or density (Zhu et al., 2021a; Yu et al., 2020; Huang et al., 2020a; Cheng et al., 2020), especially for ultrafine particles (Huang et al., 2020b; Zhang et al., 2021a, b). Using the difference in hydrophobicity between the metal particles and the non-metal particles, flotation is successfully used for the beneficiation of the WPCB powder. In order to increase the difference in hydrophobicity, collectors are usually employed, and the results show that kerosene can be used as non-ionizing collector to enhance flotation efficiency (He and Duan, 2017). Using the natural floatability of WPCB powder, the flotation process using only frother has also been reported (Vidyadhar and Das, 2013). In addition, inhibitors are also utilized in flotation of WPCB powder (Wang et al., 2014). Further, ionic collectors prepared from waste oils have proved to be effective in the flotation of WPCBs (Zhu et al., 2021b). Because of the existence of polar and nonpolar groups, ionic collectors also have foaming effect (Zhu et al., 2020a). In previous studies, the flotation test with ionic collector shows that the WPCBs has ideal flotation characteristics, and the metal and non-metal components can be effectively separated (Zhu et al., 2020b). Most of the flotation studies aim to improve flotation efficiency. Pyrolysis has been reported to improve the flotation efficiency of e-waste. The improvement of flotation efficiency of electrode materials in waste lithium-ion batteries by pyrolysis-assisted has been proved to be effective (Zhang et al., 2019, 2018b).
However, the adaptability of pyrolysis to WPCB flotation still needs to be verified. Moreover, the effect of pyrolysis on the floatability of different components in WPCB also needs to be studied in detail. Therefore, in order to further improve the accuracy of WPCB flotation, pyrolysis pretreatment was proposed to verify the feasibility in this study. In addition, the effects of pyrolysis pretreatment on the phase, morphology and functional groups of different components in WPCBs were verified. Further, the influence of pyrolysis pretreatment on separation efficiency was analyzed by laboratory-scale flotation tests.
Section snippets
Materials
The WPCBs were crushed by impact crusher and shear crusher, and the crushed products were screened. The 1−0.5 mm particles in the dissociation products of the WPCBs were used as the test sample. The morphology and elemental distribution analysis of the particles were tested by SEM and EDS, respectively. The results are shown in Fig. 1.
Combined with the results of EDS element analysis, it can be inferred that the long strip particles rich in Si, Al and O elements in SEM images are glass fiber
Effect of pyrolysis on group composition
The FT-IR results of the samples before and after pyrolysis are shown in Fig. 4.
The FT-IR results presented in Fig. 4 illustrate that WPCBs samples before and after pyrolysis have similar chemical characteristics. Peaks at 3400 cm−1 represent stretching and bending vibrations of the OH groups (Gao and Xu, 2019; Chen et al., 2018). CH3 is characterized by peaks of 2967, 2930, 2870 and 1456 cm−1 (Xia et al., 2017; Han et al., 2018). Peaks at 1607 cm−1 and 1506 cm−1 representing CC. Peaks of 1241 cm−1
Conclusion
The metal in the WPCBs is an excellent secondary metal resource. Flotation is an effective technology for metal recycling. The feasibility of improving the floatability of WPCBs by low temperature pyrolysis pretreatment was studied in this study.
The pyrolysis characteristics of the four components in the WPCBs were analyzed, and the pyrolysis temperature was determined by thermogravimetric analysis. Pyrolysis does not significantly change the morphology and phase composition of particles. FT-IR
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by Natural Science Foundation of Shandong Province (ZR2019BEE055) and supported by Qingdao Postdoctoral Application Research Project Funding. Jun-xiang Wang is the first corresponding author of this paper.
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All the authors belong to the same research team.