Investigation of rare earths extraction potential from various solid fossil fuel wastes

Date

2024-6-26

Author

Büyüktanır Aktar, Karden

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Solid fossil fuels and -wastes are the important sources of rare earth elements. Rare Earth Elements have strategic importance because they find numerous applications in various sectors of the global economy. Furthermore, solid fossil fuels are resources primarily used for electric power generation and the residues should be utilized. This study aimed at examining extraction methods of rare earth elements (Scandium, Yttrium and Lanthanides) in solid fossil fuels, such as lignite, hard coal, asphaltite by products, mainly fly ash. Within the scope of the study, chemical characterization was first carried out. Total rare earth element contents of lignite, hard coal, and asphaltite fly ash samples were determined (365 ppm, 428 ppm and 439 ppm, respectively). The major impurities of the samples were found to be Ca, Al, Si, and Fe. Then, hydrometallurgical leaching experiments were conducted. Fly ash samples were treated with two inorganic, and one organic acid leach containing sulfuric acid, hydrochloric acid and citric acid. The best leaching conditions were determined as 24 hours of leaching at 90°C at 30% acid concentration for all the direct leaching experiments. As a result of the statistical analysis, the effects of the parameters were examined and the post-leaching state of the materials was examined with XRD and SEM analysis. Later, since it was seen that sequential leaching would increase the leaching efficiency, inorganic sequential leaching was performed. The first hydrometallurgical leaching was done using sulfuric acid. The leach sediment was subjected to a second leaching process involving hydrochloric acid. The total rare earth element productivity of sequential leaching of fly ashes was determined as 86.05% for lignite fly ash, 96.93% for asphaltite fly ash, and 73.45% for hard coal fly ash, respectively. As the last stage, neutralization - selective precipitation experiments were carried out on the pregnant leach solution whose efficiency was increased by sequential leaching. The pH values of the obtained pregnant leach solutions were first increased to 4.5 and then to 8.5 using 10 M NaOH solution. Precipitation processes were carried out in two stages in order to purify it from impurities. In the first stage, Al and Fe+3 impurities were precipitated, and in the second stage, pH was increased to 8.5 and rare earth elements were precipitated. Fly ash materials responded positively to hydrometallurgical leaching and neutralization - selective precipitation processes, and sediments containing high grade total rare earth elements (2830 ppm (lignite fly ash), 2765 ppm (asphaltite fly ash) and 2232 ppm (hard coal fly ash)) were obtained.

Subject Keywords

Leaching, Hydrometallurgy, Precipitation, Rare Earth Elements, Fly Ash

URI

https://hdl.handle.net/11511/110373

Collections

Graduate School of Natural and Applied Sciences, Thesis