Sulfuric acid baking and water leaching of rare earth elements from coal tailings

• Effectiveness of acid baking investigated for REE recovery from coal tailings. • Light and Heavy REE in solid and solution analyzed to determine recovery. • Acid baking and water leaching achieved higher REE extraction than acid leaching. • Acid dosage was the most important process parameter affecting REE recovery. • During acid baking, partial decomposition of clays was observed. Sulfuric acid baking followed by water leaching (ABWL) is a conventional way to extract rare earth elements (REEs) from traditional REE ore deposits. The applicability of ABWL for enhancing REE-leaching recovery from coal tailings was investigated in this study. The effect of acid baking on the minerals present in the tailings was studied using X-ray diffraction (XRD) and scanning electron microscope (SEM). Process parameters such as acid dosages, bake temperature, bake time, water-leach temperature, and leaching time were also examined. It was found that ABWL has significantly improved REE extraction and reached around 80% REE recovery compared to direct acid-leaching recovery of only 29%. During the ABWL process, rare earth minerals such as monazite in the sample were transformed into easily soluble rare earth sulfates accompanied by a partial decomposition of clays and other acid- soluble carbonate minerals. The XRD analysis showed the formation of new sulfates such as hydronium aluminum sulfate, alunogen, steklite, melanterite, and pertlikite in the acid-baked sample, which were then dissolved during the water-leaching stage. The kinetic analysis showed that the process for extracting REEs from coal tailings using ABWL was diffusion-controlled with low activation energy.