Rare Earth Extraction from Clayey Waste Materials by Alkali Pretreatment

The increasing demand for rare earth elements (REEs) and the depletion of conventional rare earth deposits have enabled secondary REE resources to be promising feedstocks for REEs. Studies have been conducted in developing technologies that can physically preconcentrate and/or chemically extract REEs from low-REE-grade clayey waste materials (e.g., coal-based clays). However, the low REE grades and poor leachability of REE-bearing species still make the recovery of REEs from coal-based clays challenging. The primary objective of this study is to develop leaching technologies that can extract REEs from clayey waste materials under mild conditions (<100 ^oC).

In the first part of this work, a novel leaching process consisting of NaOH pretreatment followed by ammonium sulfate leaching has been proposed to recover REEs from monazite, which served as a proxy for the rare earth phosphates identified in coal-based clays. In this process, monazite can be decomposed at 80 oC. The following ammonium sulfate leaching was conducted under less aggressive conditions (i.e., pH 4 and room temperature) to recover REEs. After releasing RE3+ ions from RE(OH)3(s) by acid, the role of ammonium sulfate in the leaching process may be explained by an ion exchange mechanism. Sulfate ions also benefit the leaching process by complexing with RE3+ ions.

The influences of temperature and particle size on the leaching kinetics of REEs from the NaOH-treated monazite by ammonium sulfate were also investigated based on the shrinking core model. It was found that the leaching process is controlled by a chemical reaction with an activation energy of 61.28 kJ/mol. Besides ammonium sulfate, ammonium formate is a promising lixiviant for NaOH-treated monazite. However, other carboxylate ligands tested were inefficient at room temperature, mainly due to the slow dissolution kinetics of RE(OH)3(s) resulting from the passivation of the binuclear surface complexes.

Subsequently, the feasibility of decomposing rare earth phosphates by NaOH in the presence of ethylenediaminetetraacetic acid (EDTA) was explored by constructing the stability diagrams for La-, Nd-, and Y-PO4-H2O systems, respectively. The simulation results were validated using three coal-based clay samples. The leaching results of both HCl and ammonium sulfate indicated that the pretreatment conducted by combining EDTA with dilute NaOH solutions (5-10%) could significantly enhance the REE leachability of the clay samples, with the light REEs (LREEs) being preferentially extracted compared to heavy REEs (HREEs). Under optimal conditions, the co-extraction of Al and Si can be significantly reduced.

Besides liberated phosphate mineral particles, X-Ray photoelectron spectroscopy (XPS) analyses conducted on the synthetic ion adsorption clay samples revealed that phosphate could also passivate the REEs adsorbing on the surface of clay minerals in the form of “clay-RE-PO4”. This finding may partially explain the poor ion exchangeability of REEs in coal-based clays. After subjecting to the proposed NaOH pretreatment technique, the passivated REEs on the surface of clay can be effectively removed.

Lastly, the possibility of preconcentrating REEs from a kaolinite flotation reject material was explored by froth flotation and the hydrophobic-hydrophilic separation (HHS). A final concentrate assaying 10,765 ppm of REEs and 71% of recovery was obtained by the HHS process, which was superior to flotation in dealing with ultrafine particles. The microscopic characterization of the concentrate revealed that rare earth phosphates were the major REE-bearing species. The leaching results showed that the proposed NaOH pretreatment followed by ammonium sulfate leaching was also an effective method to recover REEs from the upgraded clayey waste material under mild conditions (<100 ^oC).