Alternative techniques for extracting alumina from coal fly ash

The extraction of aluminium from coal fly ash (CFA) has been carried out using two extraction techniques, acid leaching with an inorganic acid and gas phase extraction with acetylacetone, a beta-diketone. The acid leaching experiments were carried out at varying factor values of temperature, time, solid to liquid ratio, and acid concentration. Factors investigated for gas phase extraction included acetylacetone flow rate, extraction temperature and time, and aluminium (III) oxide source. Direct leaching of fly ash in 6 M sulphuric acid at a solid to liquid ratio of 1:4 gave an extraction of 23.5% at 75 °C after 8 h 45 min of leaching time. With gas phase extraction, 17.9% aluminium extraction was achieved at 250 °C, 6 mL/min acetylacetone flow rate, after 6 h. This represents 85% and 64% extraction from the CFA amorphous phase respectively. It was also found that when a synthetic aluminium (III) oxide mixture with a similar content to CFA (31 wt%) was subjected to gas phase extraction, 46.7% extraction could be achieved after 6 h at 250 °C. The difference in extraction levels could be explained by the fact that the alumina in CFA occurs in both mullite and amorphous phase compared with only the amorphous phase in synthetic fly ash, an indication that mineralogy has a significant effect on the extraction. Therefore the lower extraction from CFA by acetylacetone is possibly due to the non-amenability of the mullite phase to the acetylacetone beta-diketone. The results presented in this paper show that extractions using acetylacetone in the gas phase are comparatively lower than acid leaching but have advantages of reduced extraction times and the possibility of recycling the unreacted acetylacetone.