Extraction of aluminum values from fly ash

Abstract

Coal fly ash is an incombustible residue produced by burning coal in thermal power plants. Indian thermal power plants generate around 1-million ton of fly ash which contains around 25-35% Al2O3 and 50-60% SiO2. Typically, Aluminum (Al) extraction from fly ash is carried out by sintering and roasting followed by leaching. Al is extracted from fly ash collected from the nearby power plant which contains cenospheres of size 10-15 micron and consists of quartz, mullite and sillimanite phase. Extraction of Al from fly ash has been carried out on as such as well as mechanically activated samples by direct acid leaching, heat treatment using conventional and microwave heating followed by leaching. Sintering with Na2CO3 and NaCl followed by nitric acid leaching extracted around 75% Al whereas sintering with CaO extracted around 60% with conventional heating and 50% with microwave heating. Sintering with CaCl2 followed by sulfuric acid leaching extracted 85% Al whereas leaching with fluoro-silicic acid extracted around 87% Al, whereas with other inorganic acids it was ∼10%. Surprisingly, it was seen that the mechanical activation has less effect on Al extraction with easier breakage of cenospheres and change in morphology. The high Al extraction in case of fluorosilicic acid is due to solubilization of mullite phase by fluoride ion and liberating the alumina values as confirmed by quantitative phase analysis. Sintering with CaCl2 resulted into formation of calcium aluminum silicate from which 87% Al was extracted by sulfuric acid leaching with calcium sulfate as leach residue. Finally, it can be concluded that the leaching condition is suitable for downstream operations to precipitate alumina values.