Feasibility Study of Multi-Critical Element Extraction from Pennsylvania Mercer Clay

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- Author:
- Gill, Jesse
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 05, 2024
- Committee Members:
- Sarma V Pisupati, Thesis Advisor/Co-Advisor
Mohammad Rezaee, Committee Member
Jeremy Gernand, Program Head/Chair
Barbara J Arnold, Committee Member - Keywords:
- Critical Minerals
Critical Elements
Lithium
Rare Earth Elements
Mercer Clay
Low-grade resource
Secondary Resource
Recovery - Abstract:
- The objective of this thesis was to determine the feasibility of multi-critical element extraction from Mercer Clay, a promising low-grade, polymetallic resource due to its elevated concentrations of lithium and rare earth elements. Furthermore, Mercer Clay has been cited as a resource of aluminum and potentially titanium due to anatase occurrence in the clay. Mercer Clay is the underclay of the Mercer Coal seam and is abundant in secondary resources such as coal refuse piles. The US is import-reliant on many critical elements, especially the REEs. Given these elements' lack of domestic primary resources, secondary and low-grade resource exploitation and domestic production are required to maintain a reliable supply. Two Mercer Clay samples taken from different locations in Pennsylvania were characterized using standard techniques, such as XRD, TGA/DSC, and modes of occurrence analysis. It was determined that Mercer Clay primarily consists of kaolinite, muscovite/illite, and quartz with trace concentrations of anatase and, likely, chlorite group minerals. Modes of occurrence analysis through sequential chemical extraction techniques determined that lithium and REEs primarily existed in the insoluble silicate phase, indicating the need for an aggressive extraction method to break the strong silicate structure and solubilize the critical minerals. Multiple commercial and promising lab-phase extraction methods were performed on the samples. It was determined that heat treatment followed by sulfuric acid baking and water leaching of the clay provided the best recovery of multiple critical elements, particularly lithium, REEs, and aluminum. A sequential optimization procedure was used to determine the ideal conditions for multi-critical element extraction from Mercer Clay while minimizing energy and chemical consumption and cost. Through optimization, a two-stage acid bake water leach (ABWL) method can extract 96-99% and 91-94% of lithium and total rare earth elements (TREE), respectively. Furthermore, 98-99% of aluminum is extracted, and titanium is concentrated in the residue to a concentration as high as 2%. Dissolution kinetic analysis of the first stage of water leaching determined that surface chemical reactions fit a shrinking core model and were likely the driving mechanism. The entire ABWL process can be modeled with a diffusion-controlled shrinking core model. The leach solution from the extraction process was purified using a fractional precipitation process developed by the Penn State Center for Critical Minerals for acid mine drainage (AMD) purification. This proved to be rather difficult to process, given the large quantities of chemicals, such as sodium carbonate, required to increase the pH of the resultant solution. Only 5.5% of available lithium in solution could be recovered from the PLS, totaling less than 2% yield. All the REEs are removed from the solution but are mixed into the iron and aluminum products. Seventy percent of the TREE is lost to the iron product, an additional 10% to the aluminum, and the rest is precipitated at pH 7. Current conventional processes are unable to produce high-purity products from this extraction solution. Ultrasonic irradiation has shown promise in the literature in reducing energy consumption and chemical requirements and increasing reaction rates, mass transfer, and extraction percentages. An ultrasonic probe was used to apply ultrasonic waves to both the leaching and precipitation of critical minerals. It was determined that ultrasonic irradiation of the leaching process does not provide enough benefit to warrant further research. It does appear to provide possible benefits for lithium leaching, but it negatively impacts REE leaching by about 10% reduced recovery. Ultrasonic precipitation results were also lackluster but should not be abandoned as a potential improvement to the process. Critical element concentrations in solution were too low to precipitate. Presently, the feasibility of multi-critical element extraction from Mercer Clay is apparent. High recoveries are achievable for many critical elements, with the bonus of concentrating titanium in the residue. However, the purification of the extraction solution requires more research. The conclusion of this thesis highlights areas of focus and future research.
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