- Title
- A characterisation of rare earth elements in by-products of coal preparation and utilisation
- Creator
- Palozzi, Jason
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2024
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Establishing an alternative source of rare earth elements (REEs) in coal by-products has been a topic of high interest for the past decade. Despite this, research that characterises the REEs in such materials produced in Australia is lacking. This thesis has analysed the REEs in coal by-products that represent a suite of commercially important coal basins in Australia. The studied samples included thickener underflows and coal combustion ashes, which represent by-products of coal preparation and utilisation, respectively. The concentration of REEs in the samples were determined by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). The REEs were hosted primarily within the inorganic ash components of the coal by-products. As a result, coal combustion ashes contained higher average REE concentrations than the thickener underflows. Coal ash derived from the utilisation of Collie Basin coals contained the highest total REE concentrations and were categorised as promising for industrial development. Various techniques including X-ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive X-rays (SEM-EDX), TESCAN Integrated Mineral Analysis (TIMA), and Synchrotron Micro-X-ray Fluorescence (μ-XRF) were adopted to determine modes of REE occurrence in tailings and coal ash samples of high economic potential. Fine-grained (avg. <5 μm) monazite was found to be the primary mode of EDX-detectable REEs in by-products from both waste streams. For Collie Basin-derived fly ash, there is evidence to suggest that monazite grains were refractory during high-temperature coal combustion. However, the monazite grains were typically bound to Al/Si-rich phases that represent the inert amorphous fraction of coal ash. Furthermore, a portion of the REEs is dispersed throughout these amorphous phases at relatively low concentrations and may require intensive pre-treatment to efficiently recover. The observed rates of REE extraction from coal tailings were consistently higher than those achieved from combustion ashes. The percentage of total REE extracted from calcined thickener underflows averaged almost 70%, whereas the average total REE extraction rate for the studied coal combustion ashes was 26%. It is postulated that the discrepancy in REE extraction rates between the two feed stock materials is directly related to the observed modes of REE occurrence. The amorphous fraction of coal ash is notoriously difficult to dissolve and its association with the REEs is consistent with the low levels of extraction achieved using aqua regia (~12 M HCl + 15 M HNO3). A substantial increase in the fraction of REEs extracted in acid solution was reported when fly ash was milled to an ultrafine particle size. This is due to the release of REE-bearing minerals from host phases that are resistant to acid attack and an overall increase in fly ash surface area, promoting the dissolution of REEs that are dispersed throughout the amorphous matrix. Dilute HNO3 (~1 M) achieved only marginally lower REE extraction rates than those measured after digestion in aqua regia. Additionally, the increased surface area of ultrafine fly ash may enhance the relative effectiveness of a less concentrated lixiviant. The current investigation has demonstrated that Australian coal by-products may represent a promising alternative source of REEs as the demand for these metals increases. Due to the high concentrations reported in this thesis, emphasis should be placed on fly ash generated during the utilisation of Collie Basin coals. Despite monazite grains appearing to have survived coal combustion, their ultrafine size and association with the amorphous fraction may prohibit their concentration by ash beneficiation. Direct chemical pre-treatment followed by acid leaching may be a more effective means of REE recovery. When attempting to extract the REEs from fly ash, the addition of a milling stage may improve the efficiency of subsequent chemical treatments without the use of any additives. However, the reduction in particle size necessary to achieve a significant increase in REE extraction may translate to a high energy requirement. It will be imperative to include such costs in future feasibility studies, in addition to potential added value from the reuse of ash remaining after extraction.
- Subject
- rare earth elements; coal by-products; fly ash; monazite grains
- Identifier
- http://hdl.handle.net/1959.13/1509963
- Identifier
- uon:56331
- Rights
- Copyright 2024 Jason Palozzi
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
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| View Details Download | ATTACHMENT01 | Thesis | 17 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 441 KB | Adobe Acrobat PDF | View Details Download |