Author ORCID Identifier

Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation




Mining Engineering

First Advisor

Dr. Rick Honaker


Rare earth elements (REEs) are critical components in advanced electronics, clean energy technologies, and national energy and economic security. The global supply situation of these strategically valuable elements remains critical as the market is largely monopolistic. Thus, to alleviate the dependence on global imports, investments are being made in the research and development of innovative and cost-effective technologies for the indigenous processing and recovery of REEs from alternate sources that are cheap and abundant. Numerous research findings have identified potential benefits and proven the technological feasibility of extracting and selectively concentrating REEs from coal byproducts such as preparation plant rejects and acid mine drainage among other sources.

Given the relatively low grades, the chemical cost associated with hydrometallurgical extraction of REEs from coal-derived feedstocks is excessively high for commercial extraction. Recent studies have been successful in increasing REE recovery and leaching kinetics by oxidation pretreatment of the coal-based feed. However, the reasons behind the positive oxidation effects have yet to be systematically studied to provide an understanding of the mineralogical changes and to utilize the knowledge for process development. Therefore, the current investigation aimed to comprehensively study the effect of oxidation parameters on the leaching characteristics of REEs contained in coal-based feedstocks. Low and high-temperature oxidation methods were evaluated and assessed to quantify their impacts on the recovery of REEs as well as the extraction of common contaminant elements (aluminum, calcium, and iron) found in a coal-derived pregnant leach solution.

Initially, the effect of low-temperature plasma (LTP) was analyzed for improving the REE leaching recovery from coal coarse rejects. The refuse material originating from the West Kentucky No.13 (WK-13) and Fire Clay coal seams were oxidized by LTP and leached separately using different lixiviants, i.e., deionized water, 0.1M ammonium sulfate, and 1.2M sulfuric acid, at 75°C in a sequential manner. LTP pretreatment was found to significantly enhance REE leaching characteristics for the low ash content, light density fractions of the WK-13 with selectivity towards the heavy REEs that are of critical importance. High heavy REE recovery values from the 1.60 specific gravity (SG) float fraction were positively correlated with a decrease in organic content which was achieved by an increase in LTP treatment time and using both a mild 0.1M ammonium sulfate solution and 1.2M sulfuric acid as lixiviants. This finding indicated a strong affinity of the heavy REEs with the dispersed mineral matter existing within the organic material, which was partially liberated during the LTP treatment. The improvement in REE leachability can be attributed to increased porosity and surface area by LTP treatment as confirmed from surface area analysis and scanning electron micrographs. However, LTP had a limited effect on the high-ash content, heavy-density fractions and provided no significant improvement in leaching characteristics. Similarly, the LTP treatment of the Fire Clay provided no significant improvement in high ash density fractions and marginal benefits for the 1.60 float fraction, which suggests differences in the mode of occurrence of the REEs between the two coal sources.

Secondly, calcination at 600-800°C for two hours significantly improved REE leachability for all density fractions in both coals under standard acid leaching conditions. The majority of the heavy REEs were found to be marginally affected by thermal treatment. Additionally, a strong correlation between the light REEs and aluminum suggested a possible association of light REEs with clays. Test results from the parametric program performed based on a Box-Behnken design were used for modeling and optimization of three operating parameters associated with the lab-scale calcination process (temperature (400-800°C), ramp rate (2-10°C/min) and hold time (0-120 min)). The calcination temperatures required for selectively improving REE recovery while minimizing the extraction of contaminant ions were found to be dependent on the mineral composition of the feed.

Subsequently, the potential of iron contamination removal from the hydrometallurgical process by calcination and magnetic separation was investigated. The calcined products at 400-500°C of the pyrite-rich 2.20 SG sink fraction of the WK-13 seam provided an opportunity for REE enrichment in the tailings by magnetic separation. The magnetic fraction had a higher recovery of heavy REEs and iron as compared to the feed and the nonmagnetic fraction. The improvements in the heavy REE leaching characteristics resulted from the transformation of pyrites to intermediate hematite at high temperatures under the presence of oxygen. A novel flowsheet was proposed by integrating calcination and magnetic separation with a typical hydrometallurgical process for efficient extraction of REEs from a pyrite-rich feedstock.

Finally, the effect of calcination atmosphere (oxidizing/inert) on enhancing the REE leachability was evaluated at high temperatures. The REE recovery for high ash fractions (>2.00 SG) of a Fire Clay seam source was significantly improved at 1000°C under an inert atmosphere, which contrasted with the poor recovery values achieved under oxygen due to sintering of the clay surfaces. The benefits of the heavy REE leaching from the low ash density fractions were realized only when the organic matter was removed under an oxidizing environment suggesting the association of heavy REEs with the organic material. As a means of further identifying modes of occurrence, an excellent correlation was discovered between the leaching behavior of the bituminous coal sources and natural clay minerals in relation to the recovery of the heavy REEs and overall REE leaching kinetics. A characteristic exothermic peak for the phase transformation of meta kaolinite to mullite was observed for calcination under an inert environment for the density fractions of the two coal sources. Selectivity towards REEs associated with the clay samples was obtained by calcination at 500-800°C and using a short leaching time. Fe contamination can be controlled by converting the pyrite to crystalline hematite at temperatures >600°C using standard air and by calcination at lower temperatures (400-500°C) for Fe that is associated with clay minerals.

Digital Object Identifier (DOI)

Funding Information

US Department of Energy (2018-2021).

Award Number DE-FE0031525.