466773 Process Design and Economics of a Novel Process for the Extraction of Rare Earth Elements from Coal Ash
Rare earth elements are used extensively in military hardware, high-end aircraft equipment, refinery catalysts and the clean energy industry. However, the rare earth mineral resources in the U.S. are limited, with 95% of their supply coming from foreign imports. In this context, coal ash is an abundant waste material from coal power plants with high content in rare earth elements (REE), particularly in heavier REE (e.g., yttrium, dysprosium, and others) important in high performance magnets, flat panel displays, compact fluorescent lighting, LEDs, and lasers. Coal ash can be a promising, low-cost resource of heavy and light rare earths. Moreover, coal ash contains other valuable components such as carbon and magnetite, and is widely utilized as cement substitute in concrete and construction materials. Therefore, it is useful but also challenging to design a chemical plant to process coal ash for recovering the REE, as well as other valuable byproducts.
Figure 1: Simplified diagram of physical and chemical enrichment of REE from coal ash.
Our process for REE production from coal ash involves two main stages: a physical enrichment step and a chemical enrichment/extraction step. The process comprises several key steps, shown in Figure 1, including screening, flotation, magnetic separation, size separation, aqueous washing, nitric acid digestion, and solvent extraction. Batch laboratory-scale tests of each of these steps, as well as continuous flow test for key steps, were first carried out. Then, a steady state model of a continuous ash processing plant was developed in ASPEN Plus. The model was used for the simulation of the production capacity and efficiency of REE extraction, as well as the estimation of the streams of coproducts produced, such as carbon, magnetite, and the ashcake as cement substitute. The model was first matched with the laboratory results and then scaled up to a high throughput plant. Subsequently, economic analysis was performed to examine the commercial feasibility of the process and identify the most cost-sensitive and challenging steps that need to be researched further. The analysis showed that the process is profitable with sufficient recycling of the acid and organic solutions. Several case studies were explored for the process economics, by varying the price of the REE concentrate and the cost of recycling the reagents to assess the product/coproduct pricing and profitability space.
This work was supported by the Office of the Secretary of Defense (OSD) with Dr. David Shifler of the Office of Naval Research (ONR) as the Program Manager under Contract N00014-15-C-0039.
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