Extraction of rare earth elements from coal ash using supercritical CO₂

Veerla, Uthej

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Author

Veerla, Uthej

Chair

Fan, Long

Committee

Ghosh, Tathagata
Arya, Sampurna

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.

Description

Thesis (M.S.) University of Alaska Fairbanks, 2024

Chapter 1: Introduction. -- 1.2 Physical and chemical properties of rare earth elements -- 1.2.1 Crystal structures -- 1.2.2 Ionic radius -- 1.2.3 Melting point -- 1.2.4 Boiling point -- 1.2.5 Magnetic properties -- 1.3 Applications of rare earth elements -- 1.4 Global statistics in rare earth elements -- 1.5 Geological abundance of rare earth elements -- 1.6 Source of rare earth elements -- 1.7 Demand and supply chain of the REEs -- 1.8 Global supply chain -- 1.8.1 China's rare earth element statistics -- 1.9 Research motivation -- 1.10 Research objectives -- 1.11 Thesis outline. Chapter 2: Literature review -- 2.1 REEs in coal -- 2.2.1 REEs distribution patterns in coal -- 2.2.2 Mode of occurrence of REEs in coal -- 2.3 World abundance of REEs in coal -- 2.4 Prospects of coal as alternative resources for REE in the United States -- 2.5 Source and occurrence of REEs in coal and coal disbands -- 2.6 REE analyzers in geological material -- 2.7 Rare earth elements processing -- 2.7.1 Conventional method of REEs extraction from coal and advantages and disadvantages -- 2.8 Physical beneficiation of REE from coal -- 2.8.1 Mult gravity separator -- 2.8.2 Froth flotation -- 2.8.3 Magnetic separators -- 2.8.4 Electrostatic separation -- 2.9 Chemical beneficiation of REE from coal and coal disbands -- 2.9.1 Roasting -- 2.9.2 Alkali leaching -- 2.9.3 Acid leaching -- 2.9.4 Bio-leaching -- 2.9.6 Precipitation -- 2.9.7 Ion exchange. Chapter 3: Supercritical fluid extraction -- 3.1 Introduction -- 3.2 Supercritical fluid properties -- 3.2.1 Density -- 3.2.2 Viscosity -- 3.2.3 Diffusivity -- 3.3 Selection of CO₂ as supercritical fluid -- 3.4 Background of supercritical fluid extraction -- 3.5.1 Previous studies in extraction metal ions using supercritical fluid extraction -- 3.6 Factors affecting supercritical fluid extraction -- 3.6.1 Solubility of solute in SC-CO₂ -- 3.6.2 Effect of temperature --3.6.3 Effect of pressure -- 3.6.4 Effect of extraction time -- 3.6.5 Effect of co-solvent modifiers -- 3.6.6 Chelating agent -- 3.7 The chemical complex of TBP and HNO₃ -- 3.8 The mechanism of REE extraction using SC-CO₂. Chapter 4: Experimental -- 4.1 Experimental design -- 4.2 Sampling and sample preparation -- 4.2.1 Crushing of coal samples -- 4.2.2 Proximate analysis -- 4.3 Analytical methods -- 4.3.1 Inductively Couples Plasma Mass Spectrometry (ICP-MS) analysis -- 4.3.2 Scanning Electron Microscopy (SEM) analysis -- 4.3.3 X-Ray Fluorescence (XRF) analysis -- 4.4 SCFE laboratory setup -- 4.5 Supercritical fluid extraction procedure -- 4.5.1 Chelating agent preparation -- 4.5.2 Extraction phase -- 4.5.3 Multistage stripping process. Chapter 5: Results and discussions -- 5.1 Characterization results -- 5.1.1 Proximate analysis results of coal -- 5.1.2 Inductively Couples Plasma Mass Spectrometry (ICP-MS) analysis results -- 5.1.3 Scanning Electron Microscope analysis -- 5.1.4 X-Ray Fluorescence (XRF) results -- 5.1 Extraction of REEs from coal ash -- 5.2.1 Chemical complex mechanism validation -- 5.2.2 Effect of temperature on SCFE -- 5.2.3 Effect of TBP and HNO₃ mixture at different ratios on SCFE -- 5.2.4 Effect of pressure on SCFE -- 5.2.5 Comparison of individual ranks of coal ash at different test conditions -- 5.2.6 Extraction efficiency of different ranks of coal ash at same test condition -- 5.2.7 Extraction efficiency of selective elements at different test conditions -- 5.2.8 Optimal test conditions to extract REEs from coal ash -- 5.3 Comparison of bio-leaching and supercritical CO₂ extraction efficiency -- 5.4 Impurities in anthracite ash -- 5.5 Impurities in bituminous ash -- 5.6 Impurities in sub-bituminous ash. Chapter 6: Conclusions and recommendations -- 6.1 Conclusions -- 6.2 Recommendations.

Date

2024-12

Type

Thesis

Collections

Engineering