A cleaner approach for recovering Al and Ti from coal fly ash via microwave-assisted baking, leaching, and precipitation

Coal fly ash (CFA) is a potential mineral resource from which to recover Al and other valuable metals. In this study, a new processing technology for the recovery of Al and Ti from CFA has been developed and comprehensively investigated. The baking process applied in previous work has been improved by using microwave heating and a mixture of H2SO4 + NH4HSO4 as the extractant. This method enhanced the Al and Ti extraction efficiencies, while decreasing energy consumption and gas emissions relative to other acidic baking processes. When employing the optimized baking and leaching parameters (baking conditions: 280 °C, 1.2 times the theoretical amount of reagents, 60 min; leaching conditions: 60 °C, L/S: 5 g water to 1 g baked ash, 30 min) 82.4% Al and 55.6% Ti could be extracted. Scanning electron microscopy images and X-ray diffraction analysis indicated that most of the mullite (3Al2O3.2SiO2) in the CFA was transformed into godovikovite (NH4Al(SO4)2) and quartz (SiO2) after microwave-assisted baking. The soluble salts were then leached into solution, while the quartz remained in the residue. Precipitation allowed for the recovery and separation of Al and Ti from the leach solution. Al was selectively recovered via NH4Al(SO4)2·12H2O precipitation after maintaining the solution at 0 °C for 10 h. A high-quality product of alumina was obtained from the NH4Al(SO4)2·12H2O. After reducing the iron in the solution from Fe3+ to Fe2+, Ti was recovered via hydrolysis after increasing the pH to 3.1. The Ti precipitate contained 44.2% Ti with a small amount of impurity. The developed approach was cleaner and more efficient than those reported to date for the recovery of Al and Ti from stable CFA.