Doctor of Philosophy / Rare earth elements (REEs) play a significant role in industrial production. Currently, much effort has been made to recover REEs from clays isolated from mining wastes such as coal byproducts. However, the concentrations of REEs in those clayey waste materials are too low, and they are not amenable to leaching even using strong acids. The developed extraction techniques usually need to be carried out at elevated temperatures (e.g., >100 oC) and consume substantial amounts of chemicals, which are not cost-effective and environmentally friendly. Given the issues, this study proposed a novel leaching technology that can recover REEs from clayey waste materials under mild conditions (<100 oC). Firstly, to simulate the recovery of REEs from coal-based clay materials, a monazite sample was pretreated with caustic soda (i.e., NaOH) at 80 oC for 24 h to convert the difficult-to-dissolve REEs (i.e., rare earth phosphate) into readily soluble forms (rare earth hydroxide), after which they were dissolved in 0.5 M ammonium sulfate solution at pH 4 and room temperature. A conceptual model was developed to explain the leaching mechanism of ammonium sulfate, which was found to be an ion exchange process. The proposed leaching process was also used to extract REEs from clay materials isolated from coal-based clay samples. A chelating ligand named ethylenediaminetetraacetic acid (i.e., EDTA) was added to the dilute NaOH solutions to reduce the alkali consumption during NaOH pretreatment. It was found that the presence of EDTA can improve the performance of NaOH pretreatment. Additionally, the content of REEs in a kaolinite waste material was physically upgraded to 10,765 ppm with ~72% recovery using a novel separation technique called hydrophobic-hydrophilic separation (HHS). The NaOH pretreatment and ammonium sulfate leaching process can also effectively recover REEs from the concentrate. The proposed leaching technology in this study can extract REEs from other low-grade clayey waste materials under mild conditions, which helps reduce wastewater generation and energy consumption. Furthermore, it will relieve the supply risk of REEs in the future.