Leaching kinetics of rare earth elements from NaOH-treated monazite by ammonium sulfate and carboxylate ligands

Liu et al. (2022) found that the leaching performance of rare earth elements (REEs) from NaOH-treated monazite by (NH₄)₂SO₄ was significantly influenced by particle size and temperature. As a continuation of this work, this study is to re-examine the leaching data from a kinetic aspect based on the shrinking core model. Moreover, the potential of using carboxylate ligands (i.e., formate, acetate, citrate, EDTA, and DTPA) as alternative lixiviants for NaOH-treated monazite was also assessed by performing thermodynamic calculation and leaching tests. Additionally, the leaching mechanisms of the selected ligands were explored by leaching kinetic study and Fourier transform infrared spectroscopy (FTIR) characterization.

The kinetic study showed that the (NH₄)₂SO₄ leaching is a chemical reaction-controlled process with an activation energy (E_a) of 57.34 kJ/mol. The process can be described by the empirical equation: $1-{\left(1-x\right)}^{\frac{1}{3}}=e^{21.41}\bullet r_0^{-2}\bullet e^{-\frac{57340}{\mathrm{RT}}}\bullet t$. Except for formate, poor total REE (TREE) recoveries (<2.0 %) were obtained at pH 4.0 by other carboxylate ligands at room temperature. It was opposite to the thermodynamic prediction due to the slow leaching kinetics of REEs in the presence of these ligands. The TREE recoveries of leaching La(OH)₃(s) (a substitute of NaOH-treated monazite) with 0.5 M ammonium acetate and 0.1 M EDTA increased to >60 % as the temperature increased to 70 and 80 °C, respectively. Kinetic analysis indicated that both leaching processes were controlled by inner diffusion with high E_a values (77.95 kJ/mol for ammonium acetate and 101.87 kJ/mol for EDTA). FTIR analysis revealed that inner-sphere surface complexes (mainly in binuclear form) may have formed on RE(OH)₃, significantly reducing the RE dissolution kinetics. Findings from this study indicate that, other than formate, the carboxyl ligands tested may not be effective lixiviants for NaOH-treated monazite at room temperature.