Microwave-Assisted Alkaline Leaching of Aluminum from Coal Fly Ash Using Amorphous Graphite: Experimental Study and Kinetic Analysis

This study investigated the extraction of aluminum from aluminum silicate-rich coal ash from the ash-slag waste of the Almaty CHP-2 power station using microwave-assisted alkaline leaching. The high chemical stability of the quartz and mullite phases in the ash leads to high energy consumption during conventional acid–base treatment. To improve the kinetic parameters of the leaching process, amorphous graphite was therefore used as an active additive, which effectively absorbs microwave energy. The experiments were conducted in the temperature range of 50–200 °C, in 1–6 M NaOH solution, and over a period of 5–30 min. The amount of amorphous graphite varied between 5 and 20 wt%. The proportion of amorphous graphite varied between 5 and 20 wt%. Upon microwave irradiation, the graphite-free ash reached a temperature of 200 °C within approximately 12 min, whereas this temperature was reached in the system with 15% amorphous graphite after only 8–9 min. At low alkali concentrations (1–2 M NaOH), the aluminum transfer into solution in the graphite-free system was approximately 18%–35%. With increasing NaOH concentrations to 3–4 M, the aluminum removal efficiency increased to 38%–58%. Under the same temperature conditions, the leaching process was significantly accelerated by the addition of amorphous graphite; thus, at temperatures near 200 °C and in a 5–6 M NaOH solution, 70%–72% of aluminum was removed. The leaching kinetics were analyzed using the shrinking core model. The results showed that the apparent activation energy of the reaction decreased from 54 kJ/mol to 32 kJ/mol in the presence of graphite. These results suggest that microwave-assisted alkaline leaching in the presence of amorphous graphite is an energy-efficient and promising method for aluminum recovery from coal ash.