Recent studies funded by the US Department of energy have shown that coal and coal byproducts contain elevated contents of Rare Earth Elements (REEs), making them a potential resource for these critical materials. The approach employed in this research focused on the concentration and extraction of REEs from fine coal refuse derived from various preparation plants in the Appalachian coal basin of the United States.
Initial efforts in this research focused on the identification and characterization of REEs in various fine coal refuse streams from nine distinct industrial preparation plants in Appalachia. The average REE content in these materials was determined to be approximately 200 ppm, but the REE content showed a strong correlation to the aluminum content, suggesting that the REEs are closely associated with the clay minerals present in the refuse.
Given the relatively low REE concentrations, initial efforts sought to concentrate the REEs through decarbonization and dispersive liberation steps. In these tests, high-shear agitation in the presence of a polyelectrolyte, followed by sedimentation was able to isolate the REE-enriched fine clay particles from siliceous gangue minerals. Following the dispersive liberation step, all samples were found to have an REE content greater than 300 ppm, a benchmark used for many initial exploratory studies. In one case, the REE content was increased by more than 125%.
Subsequent extraction tests initially utilized a direct ion-exchange leaching approach with ammonium sulfate as lixiviant. In all cases, the simple ion-exchange leaching process failed to recover significant quantities of rare earth elements, ultimately suggesting that the REEs in fine coal waste may be passivated or bound in a colloidal phase. To access this colloidal phase, several alternative approaches were evaluated, including leaching with alternative ion-exchange lixiviants, reductive leaching, gas-purged leaching, and others. The approach that showed the most promise was strong alkaline pretreatment, followed by ion-exchange leaching with ammonium sulfate at pH 4. A combination of strong alkali and high-temperatures treatment successfully liberated the REEs, converting them to a form amenable to ion-exchange leaching. The highest REE recovery achieved with this method was determined to be 39%.
Lastly, bench-scale solvent extraction tests were used to further concentrate REEs in the leach solution and demonstrate that mixed rare earth concentrates can be successfully produced from fine coal refuse. / Master of Science / Since the introduction of personal electronics, rare earth elements (REEs) have become essential raw materials for modern life. They are used in many common household goods such as cell phones, computers, and flat screen TVs. They are also vital components in various industrial, medical, and military applications. Currently, the majority of the world's supply is obtained from China, which has raised concerns on the vulnerability of the supply chain and the potential impacts of supply disruption on clean energy technologies. In light of this risk, the US Department of Energy has classified a number of REEs as critical elements and has subsequently funded research to investigate ways to diversify the supply chain through alternative resources.
The approach employed in this research seeks to extract and recover REEs from fine coal refuse. This industrial waste is a byproduct of the coal mining and beneficiation processes. Given the long legacy of coal mining in the Appalachian region, hundreds of millions of tons of fine waste are currently being stored in surface impoundments, and millions of tons of additional fine coal waste is being produced each year from active mining and beneficiation operations. By valorizing this waste material through REE recovery, mining companies will be incentivized to reprocess existing impoundments, ultimately promoting superior economic and environmental outcomes.
Despite their name, rare earths are not "rare" from the standpoint of raw abundance; however, their scarcity is derived from the complexity of the extraction and separation processes. In China, the majority of the heavy rare earth elements are produced from ion-exchangeable clays. These clays have REEs weakly attached to the surface, so that they can be readily recovered by washing them with a salt solution that remove the positively charged rare earth ions from surface. The technical approach employed in this project sought to replicate this process for the clay materials found in fine coal refuse. Additional steps were needed to properly concentrate, activate, and extract the REEs; however, the end-to-end processing tests confirmed that mixed rare earth concentrates can be produced from fine coal wastes consisting primarily clay minerals.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/109751 |
| Date | 02 November 2020 |
| Creators | MacCormac, Brendan Lloyd |
| Contributors | Mining Engineering, Noble, Christopher Aaron, Luttrell, Gerald H., Yoon, Roe-Hoan |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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