Selective aluminium leaching from fly ash by water leaching: Combined low-temperature NaHSO4 molten salt-phase transformation of mullite

Leaching Al from fly ash (FA) represents a critical pathway for its resource recovery. Some Al is imprinted in the stable mullite structure, making it difficult to leach out. Using the conventional strong acid or alkali methods to dissolve mullite has the disadvantages of environmental hazard and inefficient Al-Si separation. This study created a low-temperature NaHSO 4 molten salt process that converts mullite into water-soluble NaAl(SO 4 ) 2 and insoluble SiO 2 , followed by the separation of Al and Si through water leaching. First, TG was used to define the temperature. Subsequently, the influence of calcination temperature, time, and the NaHSO 4 dosage was explored. Finally, the leaching behavior of Al was investigated, and the leaching kinetics were analyzed. The conversion of mullite to NaAl(SO 4 ) 2 was confirmed to be best when calcined at 350 °C for 3 h, while the molar ratio of NaHSO 4 /Al 2 O 3 was 10. Sequentially, leaching at 70 °C for 90 min with water can leach 85.28% and 0.60% of Al and Si. The leaching process conformed to the Avrami model and belonged to the interfacial reaction.