Sustainable process for valuable-metal recovery from circulating fluidized bed fly ash through nitric acid pressure leaching

Circulating fluidized bed fly ash (CFBFA) is receiving widespread attention due to its threat to the environment and its high content of alumina (30–50%), gallium (30–100 g/t), etc. In this research, nitric acid pressure leaching technology was proposed to selectively extract aluminum and gallium from CFBFA. The E-pH diagram of the Al–Fe–H2O system and experiments proved that the leachate ranged within pH −0.5 to 1, which was not favorable for the generation of Al(OH)3 colloids. Moreover, Al entered the leachate as Al3+, and the divalent iron was initially oxidized by NO3− and then hydrolyzed to form hematite. The lg cFe3+ at equilibrium of Fe3+ hydrolysis had a certain relationship with pH. Thus, at high acidity with increased temperature, more Al entered the solution and more Fe entered the residue. Based on experiments, the optimum conditions were found to be as follows: leaching temperature of 220 °C, liquid-to-solid ratio of 4:1 mL/g, HNO3 concentration of 340 g/L, and leaching duration of 2 h. The leaching rate of Al, Fe, and Ga were approximately 80%, 6%, and 78% respectively. The basic separation of aluminum and iron could be realized during leaching. In the evaporation, concentration and crystallization process of the leachate, aluminum nitrate precipitated out at room temperature when leachate density exceeded 1.38 g/cm3. A greater density of the leachate was concentrated, and crystallization benefited the formation of aluminum nitrate crystals with less crystal water.