Recovery of SiO₂ and Al₂O₃ from coal fly ash
Most of the world's energy production is still mainly achieved by the combustion of coal in power stations. Coal fly ash is the inevitable waste product that accumulates to metric ton volumes each year. These vast volumes pose a problem in the disposal of the coal fly ash which conventionally is loaded onto ash dumps located near the coal power stations. Alternatives need to be investigated for the use of the coal fly ash in applications that would make the coal fly ash useful and thereby help to mitigate the environmental strain imposed by conventional ash dump disposal. This study focussed on investigating the extraction of Si and Al from CFA. The investigation into the removal of the magnetic iron oxide content and calcium content from coal fly ash was also carried out to enhance the extraction of the Si and Al from CFA e.g. the removal of calcium was attempted to promote the leaching of aluminium from the ash. The rationale for this process was that by removing and recovering these major constituent elements from the ash, it would be easier to concentrate and isolate the trace elements especially the rare earth elements that are present in the CFA. Coal fly ash sourced from Matla coal power station was characterised using x-ray diffraction to determine the mineral phases present in the raw coal fly ash and elemental composition determined by x-ray fluorescence and laser ablation ICP-MS. The main mineral phases in coal fly ash were determined to be quartz, mullite, magnetite and lime (CaO). Magnetic extraction was initially carried out on the coal fly ash to remove the iron rich magnetic material. Extraction tests were then performed on the coal fly ash using alkaline and acidic media namely; NaOH, HCl and H₂SO₄. The extraction tests were assessed and a sequential extraction experimental procedure developed to achieve the highest extraction yield for Si, Al, Fe, Ca, and Mg from the coal fly ash. Lastly the rare earth element content in coal fly ash was tracked from the beginning till the end of the sequential extraction procedure to ascertain whether the rare earth elements partitioned to the leachates or the solid residues. The total element recoveries for Al, Si, Ca Fe, Mg were 53.36 %, 39.96 %, 93.8 %, 25.6 % and 67.3 % respectively using the sequential extraction procedure developed in this study. The rare earth elements contents were not affected by the sequential extraction procedure and on the whole remained in the solid residues at the completion of the sequential extraction, resulting in a residue with enriched levels of recoverable or extractable REE content after the removal of the major oxides from the CFA. The lowest enrichment being approximately 5 % for Thulium and the highest being approximately 76 % for Erbium.