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 remains a challenge due to their complex mineralogy. In this study, we investigated the impact of alkali pretreatment on aluminosilicate structures in coal tailings and the subsequent implications for REE recovery during acid digestion. Pre-combustion 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 minutes), including a multi-step (five cycles) approach under microwave conditions. At 180◦C, XRD, 29Si NMR Spectroscopy and thermodynamic modeling show kaolinite transformation to hydrosodalite under high S/L ratios, while kaolinite completely dissolved under low S/L ratios. Conversely, quartz maintained crystallinity under both S/L ratios but slowly converted to amorphous silica over longer alkaline treatments. These alkaline pre-treatments resulted in greater REE extraction under acidic conditions. Light REE extraction was enhanced by a factor of ∼3 when kaolinite dissolved, but by a factor of ∼2 when kaolinite was converted to hydrosodalite; the heavy REEs (including Y) extractions were enhanced by ∼2 and ∼1.5, respectively, under the same conditions. Extending pre-treatment time from 5 to 15 minutes led to minimal enhancement in REE extraction, therefore kaolinite solubility due to low S/L ratio appears to be the primary factor in REE release. This alkaline pre-treatment also led to uranium removal prior to acid digestion, while correlation analyses showed that REE recovery was closely linked to simultaneous Mg, Ca, Fe, and Ti recovery. These results provide plausible routes to co-production of REE and other elements on both the United States and European Union’s critical materials list.