Electrochemical and Chemical Pathways for Resource Recovery Utilizing Seawater Resources

Abstract

Oceans are a largely underutilized resource for energy and mineral resources. Developing technology, methods, and materials to responsibly harvest and utilize the diverse resources of the ocean could aid our national security and boost domestic production of critical minerals. Work performed in this thesis explores direct extraction of minerals from seawater and the utilization of seawater produced materials for resource recovery. Direct mineral extraction from seawater encompassed two projects split into separate chapters including: (i) an investigation of how pulsed voltage profiles can be used to alter Ca/Mg composition of calcareous electrodeposits in seawater for applications in corrosion prevention and carbon mineralization; and (ii) survey and synthesis of sorbents for selective Sr recovery from seawater to support domestic production of SrFe magnets as replacement strategy for certain applications of rare-earth containing magnets. Seawater produced materials, namely a waste acid stream was used to:(iii) economically produce Ni-metal and Ni-alloy from domestically sourced olivine minerals; and (iv) to perform a proof-of-concept extraction of Al from anorthosite. In chapter 2, the investigation of pulsed voltage profiles on calcareous electrodeposits found that providing a pulsed voltage between -0.8V and -1.2 V vs. SCE between 1-100 Hz produced more CaCO3 relative to Mg(OH)2 than the baseline case of a constant voltage at -0.8V. The formed deposits were also denser and provided better coverage than the deposits formed at a constant voltage at -1.2V which primarily consisted of Mg(OH)2. The most CaCO3-rich deposits were obtained under 10 Hz frequency and 10% duty cycle conditions for the voltage window investigated. While pulsing the voltage increases the amount of CaCO3 deposited, the energy required per gram of CaCO3 is significantly higher (14.5x) when compared to the base case of applying a constant voltage of -0.8V vs SCE. Further optimization of pulse conditions and system configuration could improve selectivity for carbonate deposits without compromising precipitation rates. In chapter 3, the synthesis of a Sr selective ion-exchange sorbent composed of barium silicate is investigated to increase its performance. The synthesis optimizations include altering the total volume of reactants in the hydrothermal reactor and varying the physical form of barium introduced into the system. Initial experiments show that limiting reaction volume to 30 mL provides the best Sr/Ca selectivity while using a solid source of barium provides better absorption performance. Further experiments characterizing and assessing sorbent performance are ongoing. In chapter 4, a waste acid generated by bipolar membrane electrodialysis technology is used to leach olivine for the purposes of extracting nickel. The waste acid was shown to leach nickel from olivine at a 37% increased rate over equal strength commercial HCl at room temperature. Treating this leachate with alkalinity to increase the pH to remove dissolved Si and the majority of dissolved iron resulted in retaining the majority of dissolved Ni (65%) and Mg (84%). This enriched solution is used for Ni-metal recovery via electroplating and demonstrated that the formed deposits is a FeNi alloy with a 43.4 wt% of Ni. Preliminary assessment indicates an overall economic benefit from recovering nickel from olivine using the proposed method and may increase further upon process optimization and changes in supply and demand of nickel. In chapter 5, a process for the extraction of Al from anorthosite utilizing oxalate chemistry is developed and explored for its feasibility. We found that leaching the anorthosite sample in 0.4 M HCl removes >95% of the Al from the sample and that applying oxalate to the leachate allows for the selective extraction and recovery of ~70% of the original Al. Economic cost-benefit shows a production cost of $1.07/kg alumina, about 2.5-3.5x more expensive than the Bayer process. Future optimizations and process refinement can bring this cost down while providing an Al extraction pathway with significantly less waste.