Date of Award

12-17-2024

Document Type

Thesis

Abstract

The increasing demand for rare earth elements (REEs) as critical components in modern technologies has led to growing interest in their efficient recovery from alternative sources. Coal ash, a waste product generated from coal combustion, has been identified as a potential reservoir of valuable REEs with reported REE concentrations varying between 270 and 1480 ppm. In this research paper, we investigate the recovery of REEs from three coal ashes: anthracite, bituminous, sub-bituminous using environmentally benign supercritical fluid (SCF) carbon dioxide (CO2). Additionally, the effect of tributyl phosphate (TBP) and nitric acid (HNO3) as complexing agents is explored to enhance the extraction efficiency. The advantage of this option over conventional solvent extraction methods includes minimization of liquid waste generation, solute separation, and rapid reaction rates. Supercritical fluids (SCFs) can penetrate and transport solutes from different matrices due to its high diffusivity, low viscosity, and liquid-like solvating. CO2 provides a good option as an efficient solvent since it has the benefit of being easy to obtain and has a medium critical constant (Tc = 31.1oC and Pc = 7.38 MPa), as compared to other solvents. Additionally, CO2 is inert and stable (chemically and radio chemically), inexpensive, easy to supply at high purity, and it is environmentally friendly and widely used. The experimental work involved the optimization of process parameters, including temperature, pressure, and solvent-to-solid ratio, to ensure maximum REE recovery while minimizing environmental impact. The optimum extraction conditions for anthracite ash were determined to be 60°C, 1100 psi, 120 minutes residence time, 250 rpm agitation rate, solid to chelating agent ratio 1:10 and TBP to HNO3 ratio 1:1, with corresponding 80% extraction efficiency which is 230 ppm. The optimum extraction conditions for bituminous ash were determined to be 60°C, 1100 psi, 120 minutes residence time, 250 rpm agitation rate, solid to chelating agent ratio 1:10 and TBP to HNO3 ratio 1:1, with corresponding 49% extraction efficiency which is 290 ppm. The optimum extraction conditions for sub-bituminous ash were determined to be 60°C, 1835 psi, 120 minutes residence time, 250 rpm agitation rate, solid to chelating agent ratio 1:10 and TBP to HNO3 ratio 1:2, with corresponding 58% extraction efficiency which is 149 ppm.

Handle

http://hdl.handle.net/11122/15693

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