Post-Crystallization Purification for Alumina Recovery from Coal Fly Ash Through Thermal Decomposition and Stepwise Leaching

Residual Na, K, Mg, and Ca in crystallized aluminum chloride limit alumina purity during recovery from coal fly ash. This study investigated a purification route combining low-temperature thermal decomposition (250–450 °C), stepwise leaching (single-step leaching tests: 60–160 °C), and final calcination (950 °C). Steam enhanced dechlorination compared with argon. At comparable mass loss and residual chlorine levels, treatment at 350 °C for 360 min favored subsequent Mg and Ca removal over treatment at 400 °C for 240 min. The former yielded alumina containing 150 ppm MgO and 182 ppm CaO after leaching and calcination. K and Ca were readily removed under mild conditions, whereas Mg removal benefited from longer leaching times. Structural rearrangement during boehmite formation may facilitate impurity release. Two-step leaching with dilute hydrochloric acid, initially adjusted to pH 4, improved Mg removal. After calcination, alumina purity reached 99.73%, compared with 95.44% for the control. Simulated leachate reuse showed greater sensitivity of Mg removal to impurity accumulation. An experiment using 10 kg of aluminum chloride hexahydrate achieved removal efficiencies above 99.4% for Na, K, and Ca and 98.02% for Mg, with 95.29% Al recovery. These results support thermal decomposition coupled with stepwise leaching for selective impurity removal while retaining Al.