AbstractCoal fly ash is an industrially useful by-product of coal combustion, with millions of tonnes held in repositories globally. This fine and cheaply-available waste material, is proven to sometimes be enriched in valuable elements/compounds, such as rare earth element (REE) minerals and actinides, alongside heavy toxic elements such as Cr, Pb, As and Cd, making coal fly ash both a potential unconventional source of valuable minerals and a hazardous material depending on the elemental trace chemistry of the parent coal body. The importance of REE and actinides for use in advanced electronics technologies and the nuclear industry, alongside concerns over geopolitics and instability in the global supply market, means there is now a renewed incentive for countries to secure native sources of economically-sustainable rare earth elements through research and development efforts. Unconventional sources of REE, such as coal fly ash, represent a potentially interesting value proposition, especially for developing countries.
The work presented in this thesis provides a study of rare earth elements and actinides in Nigerian simulant coal fly ash from the perspective of resource recovery and environmental protection. A suite of advanced analytical laboratory techniques and novel synchrotron radiation techniques, alongside micromanipulation methods, were used in this study to understand the amounts and mineralogical association of REE and actinides in 3 different Nigerian coal deposits.
Bulk and micro mineralogical analyses were first performed on the simulant coal fly ash using x-ray diffraction and scanning electron microscopy / with energy dispersive x-ray fluorescence and spectroscopy used for elemental analysis. It was observed that the Nigerian simulant coal fly ash samples were less complex in mineralogy than conventional rare earth ores (with quartz, mullite, haematite and cristobalite as the major mineral phases), composed of discrete micron-scale particles of rare earth mineral monazite ([Ce,La,Nd,Th]PO4), xenotime (YPO4) and zircon (ZrSiO4), alongside uraninite (UO2) and thorite (ThSiO4). Subsequent bulk elemental analysis performed using inductively-coupled plasma mass spectrometry, showed the simulant coal fly ash to be enriched in the rare earth elements, being more enriched in the higher-valued critical rare earth elements Nd, Eu, Tb, Dy, Y and Er, compared to conventional rare earth ores. Sequential extraction analysis of the simulant fly ash materials showed significant recoverability of rare earth elements in the acid-soluble fraction using cheap and environmentally-friendly ethanoic acid; the toxic heavy metals Cd and As were also significantly recovered in the acid-soluble fraction.
Synchrotron radiation analysis of individual micro-particles of monazite and uraninite using micro-x-ray fluorescence mapping, micro-x-ray fluorescence tomography, micro-x-ray absorption near edge spectroscopy and micro-x-ray diffraction showed a zonation pattern in the monazite particles, with the rims rich in the rare earth elements, and the core rich in U and Th but depleted in the rare earth elements. In the uraninite particles, U was homogeneously-distributed, existing in the chemically reduced IV oxidation state.
Gamma-ray spectrometry analysis of the bulk coal and simulant coal fly ash was also performed using a high-purity Ge gamma-ray detector. Compared to World mean levels, the radioactivity and associated hazard associated with the parent coal samples were insignificantly-low. However, the equivalent activity concentration and radiological hazard levels associated with the simulant coal fly ashes were significantly-high, above United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) World mean and recommended levels.
The results from this study are valuable for the development of customised methods for rare earth element and actinide recovery from Nigerian coal fly ash and coal fly ash in repositories globally, including the derivation of methodologies for general management and recycling of coal fly ash to extract best economic value. If Nigerian coal is substantially consumed by power generating processes over the coming decades, the REE content of the amassed residual fly ash materials presents a potentially exciting economic prospect for the country. Regardless of REE exploitation, the high residual metal, REE and actinide content of the fly ashes dictates that they should be responsibly managed to prevent environmental or human harm
|Date of Award||24 Jun 2021|
|Supervisor||Thomas Bligh Scott (Supervisor)|