Date of Award

January 2021

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

First Advisor

Michael Mann

Abstract

The rare earth elements (REEs) are irreplaceable in modern technology as theyplay a central role in catalysts, fiber optics, permanent magnets, and a myriad of other final products. These products go on to serve a variety of industries, including medical research and renewable energy. Crustal deposits of rare earths that are economically exploitable are very uncommon, with the largest known deposits occurring in China. During trade disputes, China has shown they are willing to exploit this natural monopoly over REEs, causing turmoil and large price fluctuations in the market for these critical minerals. This uncertainty led many countries to explore secondary sources as a supply of REEs, and the U.S. Department of Energy has funded research into extracting these metals from coal and coal byproducts. The Institute for Energy Studies (IES) at the University of North Dakota is currently developing technology to generate a mixed rare earth oxalate product from raw lignite coal. Unfortunately, calcium is a significant contaminant and the current product is only 10 – 15% rare earths on an oxide basis. In the present study, a two-step process was investigated to increase the purity of our rare earth product to greater than 90%. First, a selective dissolution brings the calcium and high-value rare earths into solution. Second, a chelating ion exchange resin selectively adsorbs the rare earths while the calcium is largely unaffected. A fractional factorial design with several follow-up runs found that by calcining the oxalate mixture at 1100 °C and dissolving with HCl at pH 3.5, the high-value rare xiv earths dissolved at 84 – 100% efficiency while the low-value cerium was less than 5% extracted. After acidifying the REE filtrate to pH 2.5, a chelating ion exchange resin with iminodiacetic acid functional groups in the sodium form is used to selectively adsorb the rare earths. The operating capacity for trivalent metals under these conditions is 0.774 mEq/mL resin. To recover the rare earths, the resin is stripped with 5% HCl until the pH turns negative and stabilizes. When deemed necessary, a secondary regeneration with 10% H2SO4 is carried out to prevent lead and zinc from accumulating on the resin. After conditioning with a 4% NaOH solution the resin is ready to be loaded in the next cycle. Before scaling this technology up to the pilot scale, the following tests are recommended: calcining the oxalate product at lower temperatures, measuring the kinetics of dissolution, testing different pH values for the ion exchange feed, and determining the combination of base and pH that achieves the highest purity rare earth product.

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