Author ORCID Identifier

https://orcid.org/0000-0002-2757-398X

Date Available

8-2-2023

Year of Publication

2023

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Mining Engineering

First Advisor

Josh Werner

Second Advisor

Jack Groppo

Abstract

The growing quantity of waste electrical and electronic equipment (WEEE), also known as electronic waste (E-waste), has been an area of growing public concern. The abundance of valuable metals contained in waste printed circuit boards (WPCBs) has made it a promising secondary resource, especially for Cu and Au. Although the recovery of metals from WPCBs via hydrometallurgical routes has been studied extensively over the past 20 years, most of the research has been limited in the laboratory. In current hydrometallurgical processes, strong acids and expensive oxidizers are often used to ensure a high recovery of metals without considering the sustainability aspects of the environment and economics. To improve upon current hydrometallurgical offerings, the current study seeks to develop an energy-saving, environmentally friendly, economic and sustainable process to efficiently recover the valuable metals from real-world end-of-life WPCBs. The new contributions presented in this study are 1) design and evaluation of a comprehensive hydrometallurgical flowsheet; 2) employment of real end-of-life PCBs as feed materials in an investigation on Cu-NH3 leaching kinetics; 3) further application of kinetic model on a counter flow process simulation; and 4) evaluation of the influences by co-existing metals in Au-S2O3 leaching and recommendation for favorable leaching conditions.

A novel processing flowsheet was developed in the presented study, using ammonia-based hydrometallurgical methods to recover Cu and Au from WPCB materials. The proposed process includes a preliminary physical process for metal enrichment and material characterization, and multiple chemical processes for Cu and Au extraction. The chemical processes consisted of two major parts, the extraction of Cu by ammoniacal sulfate solution and the extraction of Au by oxidative thiosulfate leaching. The materials studied in the physical process were a mixture of general WPCBs, obtained from various sources, to provide a broader application using the developed physical method. Waste RAM chips, with a relative higher content of Cu and Au, were used specifically in the chemical processes. SEM-EDS results on the feed materials proved the existence of Au-Ni-Cu interlayers, which could hypothetically reduce the efficiency of Au recovery in leaching.

The investigation on physical beneficiation by different size and density fractions revealed that sufficient metal liberation providing an effective metal concentration occurred at a particle size less than 2 mm and specific gravity (S.G.) over 2.67. Further investigation on thermogravimetric properties provided additional information on the thermal processing that were useful for analytical methods and, although not implemented here, a pretreatment for hydrometallurgical processing.

In Cu-ammoniacal leaching process, an in-depth study on the kinetic characteristics was conducted under an anaerobic environment. In comparison to the conventional ammoniacal leaching with assistance of oxygen, where Cu was oxidized all the way to Cu(II), the anaerobic ammoniacal leaching allowed partial oxidation of Cu to Cu(I). The anaerobic ammoniacal leaching was designed under assumption of a potentially coupled leaching-electrowinning circuit, to produce Cu from a leaching solution replenished of Cu(I) and depleted in Cu(II). A feasible kinetic model was obtained using real end-of-life RAM chips, by assessing the effect of stirring rate, particle size, Cu(II) concentration, and temperature. Results on the kinetic study revealed that particle size and initial Cu(II) concentration were the two most important factors affecting the leach efficiency. Cu recovery of 96% was achieved using 40 g/L Cu(II) as initial oxidizer concentration, 1.2 mm as top particle size, under ambient temperature. Kinetic models which approximate changing reactant concentration and mixed diffusion mechanism showed a reasonable fitting of the leach data, with average R2 of 0.97 and 0.96, respectively. The estimated activation energy was 4 kcal/mol, indicating that the anaerobic Cu-ammoniacal leaching was governed by diffusion-controlled mechanism. Furthermore, the kinetic data was adopted in modeling of a counter-current leaching circuit to optimize the leaching performance and to promote the EW current efficiency.

Dissolution behavior of Au in oxidative thiosulfate leaching system using Cu(II)-NH3 as the catalyst was studied. Under influences by the co-existing metals, the co-extraction of Ni was found to have an inverse impact on the Au recovery, as a result of chemical interactions within the Au-Ni-Cu interlayer. Decopperization as a pretreatment was found necessary, especially when using complex PCB materials, to removal the pre-existing Cu and prevent Au deposition during leaching. Investigation on leaching parameters indicated the importance of the initial Cu(II) concentration, the controlled aeration, and the suitable range of thiosulfate-to-ammonia ratio on achieving a satisfactory Au recovery up to 98%. Suggested by the leaching result, a maintained solution Eh value around 0.15 V was favorable to the Au leaching by preventing the formation of sulfur species.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2023.034

Funding Information

1. Honaker, R., (PI), Werner, J., and Zhang, W., “Recovery of Rare Earth Elements from Coal Byproducts: Characterization and Laboratory-Scale Separation Tests,” Virginia Tech (U.S. DOE/NETL subaward, Project No. DE-FE0029900), August 1, 2017 – May 31, 2019, Total Project Value = $421,052; Agency Share = $400,000; 25% involvement.

2. Werner, J. (PI), Groppo, J., “SMaRT (Sustainable Materials and Recovery Technologies) for Lexmark Business Development” Lexmark International, March 2020 – November 2020, Total Project Value = $ 72,310; 50% Involvement.

3. Werner, J. (PI), Groppo, J., “Sustainable Campus Electronics Recycling Program at the University of Kentucky” University of Kentucky, May 2020 – May 2021, Total Project Value = $ 32,703; 50% Involvement.

4. Werner, J. (PI), Groppo, J., Badurdeen, F., Hubert, K., “PFI-RP: Development of a SMART (Sustainable Materials and Recovery Technology) Process for the Recovery of High-Value Metals from Electronic Waste” NSF, August 2021 – July 2024, Total Project Value = $ 549,986; 75% Involvement.

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