Leaching mechanism and behavior of rare earth elements from roasted coal gangue using citric acid

The recovery of rare earth elements (lanthanides and yttrium, REY) from coal gangue offers a potential approach for the resource utilization of this industrial waste. In this work, the effect of citric acid on the leaching of REY from roasted coal gangue was systematically investigated. The leaching mechanism of REY was elucidated by comparing the leaching behavior of REY in different acid systems, combined with zeta potential analysis and solution chemical equilibrium calculations. The maximum leaching efficiencies of REY, light REY (LREY), and heavy REY (HREY) were 48.6%, 59.1%, and 17.2%, respectively, achieved at 0.8 mol/L citric acid, a liquid-to-solid ratio of 25 mL/g, 150 min, and 90 °C. Acid leaching experiments at a fixed concentration of 0.8 mol/L showed that hydrochloric acid leached REY more rapidly than citric acid. At the same initial pH (approximately 1.5), citric acid achieved a higher REY leaching efficiency, and the pH change of the solution during leaching was the smallest, at 0.43. The citric acid leaching reaction mainly occurs in the pH range of 1.5-2.0. Meanwhile, the Al leaching efficiency was 51.0% under the condition of 0.8 mol/L citric acid. SEM-EDS analysis reveals that the Si/Al atomic ratio on the solid surface increased from an initial value of 1.0 to 2.6 and 3.0 after leaching with 0.8 mol/L citric acid and hydrochloric acid, respectively. Zeta potential analysis and solution chemical equilibrium calculations indicate that the complexation between citrate ions and rare earth ions is weak during the leaching stage. Therefore, the leaching of REY from roasted coal gangue by citric acid is mainly due to the effect of H⁺. These findings can provide support for improving sustainable recovery strategies of REY from coal gangue.