Semester

Spring

Date of Graduation

2023

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mining Engineering

Committee Chair

Qingqing Huang

Committee Member

Hassan Amini

Committee Member

Ihsan B. Tulu

Committee Member

Deniz Talan

Abstract

Rare earth elements (REEs) are critical in emerging clean energy, advanced technologies, and military defense applications due to their unique physical, chemical, optical, and luminescent properties. Given the potential supply chain restrictions imposed by one country's dominance in the global REE market and the consistently growing demand for REEs, developing the US domestic REE supply chain is imperative. Thus, alternative sources are crucial to supply the demand and prevent interruption in manufacturing products that consume REEs. Concurrently, acid mine drainage (AMD) is a long-standing and widespread global challenge the mining industry encounters. AMD is generated in large volumes and continuously when the sulfidebearing minerals, typically pyrite, are exposed to air and water and then oxidized. The highly acidic nature of AMD dissolves high concentrations of heavy metals, which imposes severe risks to receiving waters and soil due to its high acidity and elevated concentrations of metals.

Past and ongoing research has suggested that AMD and its treatment sludge are promising sources of critical minerals, including rare earth elements (REEs). Therefore, by developing a new AMD processing strategy, AMD and its treatment byproducts can be turned from mine waste to a feedstock of strategic, critical elements. This study characterized various coal-based AMD and their treatment sludge samples for REEs and major metal elements. Characterization study results indicated that several AMD sludge materials contain a significantly high content of critical minerals. For example, around 356.76 ppm of REEs, 226.01 ppm of cobalt, 2.63% aluminum, 1.11% magnesium, and 0.55% manganese were detected on a dry basis in one sludge sample collected from a coal-based AMD treatment facility.

Following identifying promising feedstock, a separation process was developed to recover REEs. The main challenge facing AMD sludge processing was the presence of significant amount of impurities, such as aluminum, iron, calcium, and silicon, compared to a low REE content. Firstly, parametric leaching tests were conducted to determine the optimum leaching recovery. Then, stage-wise precipitation tests were performed to study the precipitation behaviors of various groups of elements, including REEs and other major metals, at different pH regions and the feasibility of selectively recovering REEs by targeting one particular pH region. The results revealed that selectively recovering REEs by targeting one specific pH region was not feasible for the studied AMD sludge feedstock. However, the impurities, specifically iron and silicon, could be selectively removed from REEs at a pH set-point of 3.5.

Afterward, separation approaches were investigated to further remove the impurities while selectively extracting REEs. First, the effect of different organic-to-aqueous (O:A) ratios on solvent extraction was studied, and the O:A ratio of 1 was found to be optimum. Then, three different solvent extraction methodologies, including regular solvent extraction with DEHPA, solvent extraction with a phase modifier of TBP, and post-oxidation solvent extraction, were evaluated to enhance the purity of produced rare earth oxides (REO). The final process selected for REE recovery consists of (a) sulfuric acid leaching at a pH of 0.5 with a leaching time of 8 minutes, (b) selective oxidation precipitation at pH 3.5 using hydrogen peroxide, (c) post-oxidation solvent extraction with 0.5 M DEHPA at an O:A ratio of 1, (d) two-stage stripping with hydrochloric acid, (e) oxalic acid precipitation at a pH of 1.5 to produce REE oxalate, (f) roasting to convert REE oxalates to oxides, and finally (g) washing. As a result of the developed process, a mixed REO product with a purity up to 80% by weight was produced from the AMD treatment byproduct.

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