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An integrated bio- and hydrometallurgical approach for the extraction and recovery of metals from printed circuit boards

Van Yken, Jonovan (2022) An integrated bio- and hydrometallurgical approach for the extraction and recovery of metals from printed circuit boards. PhD thesis, Murdoch University.

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Electronic waste (e-waste) is one of the fastest-growing waste streams globally. In 2019, 53.6 million tons of e-waste were generated globally. However, despite being projected to increase to 65.3 million tons by 2025, currently, only 17.4% of e-waste was recycled globally in 2019. In Oceania, the recycling rate is only 8.8%, despite generating 16.1 kg e-waste per capita. This is of environmental and economic concern. E-waste contains a range of heavy metals and other hazardous substances that can cause land, air and water contamination if improperly handled or sent to landfills, and thus risk to human and environmental health. Moreover, the value of raw materials in global e-waste was estimated to be USD 57 billion in 2019. Hence, there are significant environmental, economic, and social incentive to recycle e-waste. In fact, e-waste has been increasingly recognised as an urban mine. Given that the majority (3-6%) of value in e-waste is typically associated with end-of-life printed circuit boards (PCBs), which contain a range of valuable base and precious metals that can be recovered, the overarching goal of this PhD thesis was to develop new technology and knowledge useful for recovering valuable metals from PCBs.

The first objective of this thesis was to investigate an integrated biohydrometallurgical approach for the extraction of base metals from PCBs. Once the metals were extracted, the second objective of the thesis was to investigate the recovery of these metals using biogenic hydrogen sulfide. The third objective of this thesis was to develop a process flow sheet and conduct a techno-economic analysis (TEA) to determine the economic viability of the process.

The growing e-waste problem in Oceania is unique. Despite generating only 0.7 million tons of e-waste in 2019, it is one of the largest per capita generators of e-waste. A range of political, economic, legal, social, technological, and environmental factors affect the recycling of e-waste in this region. This thesis investigated these factors and identified various barriers and enablers for recycling e-waste in the region (Chapter 1). It also investigated the resource recovery technologies available that can be applied and identified the advantages and disadvantages associated with each of these technologies. This information is critical to understanding the incentives to recycling e-waste in the region and the challenges any future industrial process can face.

Due to their unique and highly heterogeneous composition, PCBs are challenging to be characterised. Currently, there is no standardised method for characterising the metal content of PCBs. Various analytical methods can have significantly different results, hindering the accurate tracking of metals throughout the value recovery process. To address this, various pre-treatment and analytical methods were compared to determine the most precise and accurate method for characterising base metals in PCBs (Chapter 2). Using a mixed metal standard, the proposed methods' accuracy was determined. Of these methods, microwave-assisted multi-acid digestion offered the most precision and accuracy. By determining the optimal method for quantifying base metals in PCBs, the accurate determination of base metal leaching efficiencies was greatly improved.

To facilitate base metals leaching from PCBs in biohydrometallurgical processing, a novel two-step biological lixiviant generation process was proposed and validated (Chapters 3 and 4). In the first step, crushed waste sulfur pastilles were biologically oxidised to produce biogenic H2SO4. The acidified effluent containing biogenic H2SO4 was then augmented with ferrous iron (20 g Fe L-1) in the second step, where it was biologically oxidised to produce ferric iron. The produced bio-lixiviant was then used for leaching base metals from PCBs.

The leaching of base metals from PCBs using the biogenic lixiviant was investigated at various experimental conditions to determine the effects of the process parameters on leaching efficiency. Leaching parameters tested in this study included temperature (25-45oC), aeration, PCB pulp density (PD, 1 and 5%), ferric iron concentration, solution pH, pH control, leaching reactor configuration, and pre-treatment of PCBs by ashing. Using optimised conditions, near-complete metal extraction was achieved for 1% PD. By controlling pH to a set point of pH 1.4 with feedback-dosing of 2 M H2SO4, high leaching efficiencies were also achieved at 5% PD compared to leaching without pH control.

By separating the unit processes for biogenic lixiviant generation from the leaching process, the optimisation of biogenic lixiviant production and PCB leaching parameters could be effectively achieved without compromising the growth and activity of the microorganisms.

Once in solution, the base metals from the leached PCBs were recovered through precipitation with biogenic hydrogen sulfide produced by sulfate-reducing bacteria (SRB) in an anaerobic fluidised bed reactor. The use of both lactate and glycerol as a carbon source for SRB was evaluated (Chapter 5). High metal precipitation efficiencies were achieved for base metals using biogenic sulfide and NaOH. This process facilitated the recovery of base metals as a high value mixed metal precipitate.

After the technical feasibility of the proposed biohydrometallurgical process was proven, a preliminary TEA was conducted to examine the potential capital and processing costs of establishing a plant capable of treating 1000 t/pa of PCBs in Australia (Chapter 6). The key factors that influence the economic feasibility were identified, and the effects of process efficiencies and market conditions on plant profitably were evaluated. The assessment provided insight into the economic aspects of the proposed process and helped to identify the critical research pathways for implementing the developed technology and opportunities for further optimisation in future work. Based on this analysis, the proposed process is profitable under optimised conditions.

Overall, a new biohydrometallurgical processing concept for extracting and recovery base metals from PCB waste was proposed and successfully validated in this thesis. The thesis also showed that the new process is an economically viable option that can potentially address some key challenges associated with e-waste processing in Australia.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Engineering and Energy
United Nations SDGs: Goal 12: Responsible Consumption and Production
Supervisor(s): Nikoloski, Aleksandar, Moheimani, Navid, Kaksonen, A., Cheng, Kayu and Boxall, N.
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