Enhanced Rare Earth Element Recovery from NaOH-Roasted Coal Fly Ash by Citric Acid Leaching

Abstract

Coal fly ash (CFA) represents a promising secondary source of rare-earth elements (REEs), yet their entrapment within the aluminosilicate glass matrix makes extraction difficult. Herein, we introduce a strategy that combines NaOH roasting CFA pretreatment with subsequent citric acid leaching for high REE recovery under operationally benign conditions. NaOH roasting improves REE accessibility through partial aluminosilicate network depolymerization, increasing surface area, generating mesoporosity, enhancing surface wettability, and creating active sites for sodium aluminosilicate hydrate (N–A–S–H) gel formation during acid leaching. Benchmarking against widely used mineral acids (H₂SO₄, HNO₃, HCl, H₃PO₄) revealed that roasting benefits are highly acid-dependent, with citric acid exhibiting the greatest synergy. Under optimized conditions, 2 M citric acid achieved 83% aggregate REE extraction, more than double that of equimolar H₂SO₄ (39%), the next best lixiviant. Initial mechanistic investigations indicate that this enhanced performance stems from the distinct advantages citric acid confers on the underlying sol–gel reaction. Specifically, it produces a hierarchically meso-/microporous gel with a higher surface area that can facilitate lixiviant–substrate interactions. In combination with citric acid’s chelating capacity, this dual benefit establishes a driving force for REE sequestration that appears absent in mineral acids. Overall, this work highlights citric acid leaching as an effective route for REE recovery from NaOH-roasted CFA.