Y mainly exists in ionic rare-earth resources. During rare-earth carbonate precipitation, rare-earth ion loss in the precipitated rare-earth mother liquor often occurs due to CO₃ ²⁻ coordination and Y(III) hydration. Microscopic information on the coordination and hydration of CO₃ ²⁻ and H₂O to Y(III) has not yet been elucidated. Therefore, in this study, the macroscopic dissolution of Y(III) in different aqueous solutions of Na₂CO₃ was studied. The radial distribution function and coordination number of Y(III) by CO₃ ²⁻ and H₂O were systematically analyzed using molecular dynamics (MD) simulations to obtain the complex ion form of Y(III) in carbonate solutions. Density functional theory (DFT) was used to geometrically optimize and calculate the UV spectrum of Y(III) complex ions. This spectrum was then analyzed and compared with experimentally determined ultraviolet-visible spectra to verify the reliability of the MD simulation results. Results showed that Y(III) in aqueous solution exists in the form of [Y·3H₂O]³⁺ and that CO₃ ²⁻ is present in the bidentate coordination form. In 0–0.8 mol L⁻¹ CO₃ ²⁻ solutions, Y(III) was mainly present as the 5-coordinated complex [YCO₃·3H₂O]⁺. When the concentration of CO₃ ²⁻ was increased to 1.2 mol L⁻¹, [YCO₃·3H₂O]⁺ was converted into a 6-coordinated complex [Y(CO₃)₂·2H₂O]⁻. Further increases in CO₃ ²⁻ concentration promoted Y(III) dissolution in solution in the form of complex ions. These findings can be used to explain the problem of incomplete precipitation of rare earths in carbonate solutions.

Yb (III) shows complex behavior of coordination dissolution and precipitation in carbonate solutions, but the properties of ${\mathrm{C}\mathrm{O}}_3^{2-}$ coordination and hydration to Yb (III) in the solution have not been explicated. In this work, the dissolution rule of Yb (III) with ${\mathrm{C}\mathrm{O}}_3^{2-}$ concentration has been studied. The radial distribution function and the coordination number of ${\mathrm{C}\mathrm{O}}_3^{2-}$ and ${\mathrm{H}}_2\mathrm{O}$ to Yb (III) were calculated by molecular dynamics simulation, and the complex ion form of Yb was obtained. The ultraviolet–visible spectrum and the ionic structures of Yb (III) complex ions were geometrically optimized and calculated by using density functional theory. Then, the experimental ultraviolet–visible spectra and density functional theory results were combined to verify the molecular dynamics calculations. The results indicate that Yb (III) undergoes precipitation in low-concentration carbonate solution, but, in high-concentration carbonate solution, Yb (III)’s carbonates will undergo dissolution. The main reason for the dissolution of Yb (III)’s carbonates is the coordination effect of ${\mathrm{C}\mathrm{O}}_3^{2-}$ on Yb (III); the coordination of carbonate on Yb (III) occurs with a ${\mathrm{C}\mathrm{O}}_3^{2-}$ concentration range of 0.4~2.0 mol·${\mathrm{L}}^{-1}$. Yb (III) mainly exhibits ${[\mathrm{Y}\mathrm{b}·9{\mathrm{H}}_2\mathrm{O}]}^{3+}$ hydrated form in the aqueous solution, while, in the carbonate solution, ${[\mathrm{Y}\mathrm{b}·9{\mathrm{H}}_2\mathrm{O}]}^{3+}$ is converted into ${[\mathrm{Y}{\mathrm{b}(\mathrm{C}\mathrm{O}}_3{)}_2·5{\mathrm{H}}_2\mathrm{O}]}^{-}$ complex. ${[\mathrm{Y}{\mathrm{b}(\mathrm{C}\mathrm{O}}_3{)}_2·5{\mathrm{H}}_2\mathrm{O}]}^{-}$ complex is the main ionic form in high-concentration carbonate solutions. The analysis method in this work provides guidance for understanding the coordination and hydration characteristics of oxyacid radicals to rare earth elements.

Rare earths dissolved in carbonate solutions exhibit a metastable state. During the period of metastability, rare earths dissolve stably without precipitation. In this paper, neodymium was chosen as a representative rare earth element. The effects of additional NaCl and CO₂ on the metastable state were investigated. The metastable state can be controlled by adding NaCl to the Na₂CO₃ solution. Molecular dynamics studies indicated that the Cl⁻ provided by the additional NaCl partially occupied the coordination layer of Nd³⁺, causing the delayed formation of neodymium carbonate precipitation. In addition, the additional NaCl decreased the concentration of free carbonate in the solution, thereby reducing the behavior of free contact between carbonate and Nd, as well as resulting in the delay of Nd precipitate formation. Consequently, the period of the metastable state was prolonged in the case of introduction of NaCl. However, changing the solution environment by introducing CO₂ can destroy the metastable state rapidly. Introduction of CO₂ gas significantly decreased the CO₃²⁻ content in the solution and increased its activity, resulting in an increase of the free CO₃²⁻ concentration of the solution in the opposite direction. As a result, the precipitation process was accelerated and the metastable state was destroyed. It was possible to obtain a large amount of rare earth carbonate precipitation in a short term by introducing CO₂ into the solution with dissolved rare earths in the metastable state to achieve rapid separation of rare earths without introducing other precipitants during the process.

SmF₃ cannot be reduced to metallic samarium by aluminum due to variable valence states of Sm. This study investigates the reduction products of SmF₃ via an aluminothermic reduction. The effect of molar ratios of Al/SmF₃ on the morphology, elemental distribution, crystal structure, and chemical valence of the samples were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The thermodynamic results show that it is feasible for SmF₃ reduction by Al to form SmF₂ in 933~1356 K. SmF_2.413, AlF₃, and Sm(AlF)₅ are obtained under the condition of the molar ratio of Al to SmF₃ at 1:3, 2:3, 3:3, 4:3, and 5:3. The samarium of the reduction products exhibits mixed valence states of Sm³⁺ and Sm²⁺, with the ratio δ of F to Sm determined by a(δ) = −0.1794δ + 5.819 (0 ≤ δ ≤ 0.4615). The presence of adsorbed oxygen in the products facilitates the oxidation process from Sm²⁺ to Sm³⁺. These findings may provide a theoretical basis on the development of valence states for other rare earth elements in aluminothermic reduction.