The Enhancing Mechanism of Na2SO4 on Mullite Decomposition and Alumina Recovery During the Vacuum Carbothermic Reduction of Coal Fly Ash

The utilization of coal fly ash has been of great concern in recent years due to the growing awareness of waste recycling and environmental protection. Alumina recycling from coal fly ash is a good path to realize the state-of-the-art utilization of coal fly ash. The present work proposes a novel strategy for recovering alumina and producing Fe–Si alloy from coal fly ash employing Na₂SO₄ as the additive. The enhancing mechanism of the Na₂SO₄ addition on the mullite (Al₆Si₂O₁₃) decomposition and alumina extraction during vacuum carbothermic reduction of coal fly ash was systematically investigated. The thermodynamic calculation results show that the theoretical decomposition temperature of mullite can be decreased from 1048 to 683 K with the assistance of Na₂SO₄, which means that the addition of Na₂SO₄ can effectively eliminate the mullite phase. Furthermore, the aggregation of Fe–Si alloy particles was enhanced efficiently in the presence of Na₂SO₄, which was proved to be conducive to the alumina and Fe–Si alloy separation in the subsequent magnetic separation process. As the Na₂SO₄ addition amounts changed from 0 to 12 wt% at 1423 K for 4 h, the average size of the Fe–Si alloys particle exhibited an enlargement from 21.39 to 39.94 μm, by which the recovery efficiency of alumina increased from 61.03 to 75.26% correspondingly.

Graphical Abstract