Alkali Treatment Implications for Microwave-Assisted Rare Earth Elements Extraction from Coal Mine Tailings

Coal tailings represent a promising secondary resource of rare earth elements (REEs), yet efficient extraction is limited by their complex mineralogy. This study investigated the impact of alkali pretreatment on aluminosilicate structures in coal tailings and its implications for REE extraction via acid digestion. Precombustion coal refuse was treated with 5 M NaOH at varying solid-to-liquid (S/L) ratios (5, 50 g/L) and reaction times (5, 15 min), including a multistep (five-cycle) treatment under microwave conditions. At 180 °C, XRD, ²⁹Si NMR, and thermodynamic modeling showed kaolinite transformed to hydrosodalite at high S/L ratios, while kaolinite completely dissolved at low S/L ratios. Quartz maintained crystallinity but slowly transformed to amorphous silica during prolonged alkaline treatment. Compared to untreated tailings, light REE extraction was enhanced by a factor of ∼3 when kaolinite dissolved and by ∼2 when it converted to hydrosodalite; heavy REE extraction increased by ∼2 and ∼1.5, respectively. Extending pretreatment time produced minimal additional enhancement, indicating that under microwave conditions, kaolinite concentration in alkaline solutions and hydrosodalite solubility in acidic solutions are the primary factors controlling REE release. Alkali pretreatment also promoted uranium removal prior to acid digestion, while REE extraction correlated strongly with Mg, Ca, Fe, and Ti release.