| Summary: | The increasing concentration of CO2 in the atmosphere and the rapidly growing amount of waste (industrial and electronic) are two major environmental challenges faced by humanity today. Carbon capture, storage, and utilization (CCUS) aims to address the CO2 challenge and has been shown to be a promising means of CO2 mitigation. For carbon capture, amine scrubbing is an example of an effective means to separate CO2 from other gases, particularly natural gas and hydrogen. Carbon storage entails the injection of CO2 into natural geologic formations, such as basalt, to form permanent, harmless carbonates. Lastly, carbon utilization involves conversion of carbon to chemicals and fuels through a variety of pathways, such as carbon mineralization. Many large-scale projects on CCUS have been conducted, with ongoing research in the aforementioned areas of CCUS. The first half of this dissertation addresses carbon storage and utilization, specifically focusing on carbon mineralization, in order to evaluate the potential for CO2 storage in basalt and CO2 utilization in the transformation of industrial waste to valuable carbonates.
The mounting amount of electronic waste (e-waste) presents a significant challenge in the flow of valuable elements, especially as it relates to the materials cycle. E-waste contains valuable metals, such as copper, gold, silver, iron, and nickel, and contains much higher amounts of these metals than the amounts found in ores. Thus, the recycling of metals from e-waste is favorable and has gained attention over the last few years. E-waste is a complex mixture of metals, plastics, and refractory materials. The brominated flame retardants in the e-waste are of particular concern as they become hazardous when burned. Lead is also often found in the solder material of e-waste.
The risks associated with the toxic and hazardous components of e-waste, along with the heterogeneity in composition, challenge the development of recycling and processing methods for e-waste. While recent developments, such as hydrometallurgy i.e. chemical leaching, have lessened the hazards during processing, pyrometallurgical techniques, which involve smelting, remain the most commonly used treatment. Metal extraction and recovery processes are multi-step techniques that usually involve energy-intensive mechanical processing, and depending on the type of waste, the selectivity of metal separation processes can be quite low. Specifically, for Lithium-ion batteries (LIB), the majority of recycling techniques cannot recover Co and Ni simultaneously. The latter half of this dissertation explores new, sustainable separation processes for the recovery of metals from e-waste, Printed circuit boards (PCB) and LIB, via morphological changes induced by supercritical CO2 and via electrochemical techniques.
Chapter 2 presents an evaluation of the potential of sub-seafloor basalt in the Cascadia Basin offshore Washington State and British Columbia for CO2 storage. Basalt samples from the Cascadia Basin were tested for the extraction of Ca, Mg, and Fe to assess the ability of the basalt to form carbonates under the experimental conditions of injection with CO2. Combining laboratory results with modeling studies from collaborators, and comparisons to existing data on the reactivity of oceanic basalt demonstrated that the basalt formations in the Cascadia Basin are a feasible option for large-scale, permanent CO2 storage. In Chapter 3, the reaction of CO2 and industrial waste for Ca and Mg extraction, is investigated in greater detail in the tailored synthesis of high purity precipitate calcium carbonate (PCC) from slag. Different ligands were studied for the extraction of Ca and Mg and various experimental conditions, such as heating, controlling the pH, and bubbling with air vs. CO2 were studied for the formation of calcium carbonates from the steel slag. A novel synthesis method involving the dissolution of the slag using ligands, heating, and precipitation via bubbling with air or CO2 using the Ca-rich solution derived from dissolution, was developed. High purity PCC was successfully produced, making the proposed synthesis process a promising pathway for carbon management and sustainable waste transformation.
In Chapter 4, a critical review of current metal extraction and recovery techniques for the treatment and processing of electronic waste is presented. The complexity of e-waste requires the development of new metallurgical processes that can separate and extract metals from unconventional components such as plastics and a wide range of metals. This chapter focuses on the science and engineering of both conventional and innovative separation and recovery technologies for e-waste with special attention given to the overall sustainability. Physical separation processes, including disassembly and magnetic separation, as well as thermal treatment of the polymeric component, such as pyrolysis, are discussed for the separation of metals and non- metals from e-waste. The subsequent metal recovery processes through pyrometallurgy, hydrometallurgy, and biometallurgy are also discussed in depth. Finally, insights on future research towards sustainable treatment and recovery of e-waste are highlighted, including the use of supercritical CO2.
Chapter 5 investigates the use of supercritical CO2 for the extraction of metals from electronic waste, specifically Printed circuit boards (PCB). The complexity of PCB was first simplified by synthesizing laminate polymer and metal “model PCB” samples, where the polymer component was polycarbonate (PC) and the metal component was Cu foil. Through controlled studies of the effect of supercritical CO2 (scCO2) and sulfuric acid on the model PCB samples, a thorough understanding of the role of CO2 in the supercritical CO2/co-solvent system was developed. The scCO2/co-solvent system was found to induce permanent, morphological changes in the samples in just 30 minutes. Building on these results, a two-step metal extraction process for waste PCB was proposed. First, the pre-treatment of small pieces of waste PCB with scCO2 and sulfuric acid, and second, chemical leaching at ambient temperature and pressure in a sulfuric acid and hydrogen peroxide solution. This process was demonstrated to yield ~80% Cu extraction in under four hours, without the need for vigorous and energy-intensive mechanical processing, as the starting materials were small pieces of waste PCB, neither shredded nor crushed.
The final part of the thesis presents a study on the electrochemical recovery of Co and Ni from spent Lithium-ion batteries (LIB). Galvanostatic deposition and stripping of the metals were performed using a sulfuric acid-based electrolyte with concentrations of Co and Ni based upon waste LIB solution. A complexing agent, specifically EDTA, was introduced into the electrolyte to selectively deposit one metal over the other. The concentration of EDTA was maintained at the concentration of Co and Ni in the solution, and the pH values of the solution were varied to study the effect of pH on the ratio of Co/Ni in the deposit. In the presence of EDTA, the pH of the solution had a significant impact on the ratio of Co/Ni, making the electrochemical process presented in this study an effective, sustainable approach to simultaneous and tunable metal recovery from waste LIBs.
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