Study On The Metastsble State Properties And Molecular Dynamics Simulations Of RECl₃-Na₂CO₃ Solution

Many rare earth?RE?insoluble oxygen-containing inorganic acid salts exhibit greater solubility in the corresponding anion salt solution,such as rare earth carbonates,rare earth oxalates,resulting in loss of rare earth and environmental protection issues during the rare earth precipitation process.The reason for this phenomenon is the coordination between the rare earth element and the anion.As a typical precipitation product,rare earth carbonates present metastable state characteristics and solubility in carbonate solutions of alkali metals or ammonium.Current research is mainly focused on the crystalline morphology regulation methods of rare earth carbonates,but there is a lack of in-depth systematic research on the coordination,hydration laws and metastable state properties between rare earth ions and oxygenate in the solution with rare earth and alkali metal carbonate coexisting.In this paper,taking the dissolution behavior of rare earth ions in sodium carbonate solution as an example,on the basis of experimental testing,combined with molecular dynamics?MD?,density functional theory?DFT?calculation and UV-vis spectroscopy analysis,the dissolution law of RE???in sodium carbonate solution,the maintenance and destruction mechanism of the RECl₃-Na₂CO₃metastable solution were studied;the metastable ion structure of RE???and the interaction between solution ions are analyzed in sodium carbonate solution.The results provide theoretical guidance for understanding the metastable properties of oxygen-containing rare earth inorganic acid salts,and also optimizing the process of rare earth precipitation.The obtained main conclusions are listed as follow.?1?The dissolutions experiments of 15 RE elements in different concentration Na₂CO₃ solutions were performed.The RE instantaneous saturation dissolved concentrations with the extension of aging time in various metastable RECl₃-Na₂CO₃ solution were verified.RE instantaneous saturation dissolved concentrations increase with the increase of CO₃^2-concentration and the RE atomic number?except Tb,Y?in solution,the metastable time of the solution also increases with the increase of rare earth atomic number.The different RE???can keep stable within the metastable time.While the RE???concentration is lower than the instantaneous saturation dissolved concentration,the solution metastable time will be greatly extended with the increase of RE atomic number.In addition,the metastable state of the solution can be changed in the diluted process,while the RECl₃-Na₂CO₃solutions with larger RE atomic number need more H₂O to be diluted.?2?Based on the RE???concentrations obtained in 15 RECl₃-Na₂CO₃ solutions,molecular dynamics?MD?simulation studies of various systems were carried out,respectively.The main composition of each RE???complex ion in concentrated sodium carbonate solution and the relationship between CO₃^2-coordination and atomic number were determined by the radial distribution function and average distribution of each ion pair.The number of rare earth ions coordinated with CO₃^2-decreases with the increase of RE atomic number,while the number of H₂O that coordinates with light rare earth ions and heavy rare earth ions is 1² and 2³,respectively.MD simulations reveals that the CO₃^2-outside of first short-range can not only coordinated with RE???,but also cluster with Na⁺in concentrated Na₂CO₃ solution,leading to the high RE???concentration and preventing the precipitation of RE???reacting with CO₃^2-,resulting in RE???complex ion dissolving in high concentration Na₂CO₃ solution.The RE???precipitates can be formed instantly with the CO₃^2-concentration lower than 0.8 mol·L^-1(more than 32%CO₃^2-outside the second layer of Na⁺Short-range order),and the RECl₃-Na₂CO₃ solution no longer dissolves rare earth elements steadily.?3?Based on the MD calculation results,the density functional theory?DFT?calculations of Y???and Yb???complex ions were performed to obtain the geometry optimized Y???and Yb???ion structure and theoretical UV spectrum.The characteristic peaks and bond length information of theoretical UV spectrum calculated by DFT are consistent with the experimental UV-vis,and the obtained bond length information is also consistent with the peak position of the first peak of RDF calculated by MD,which further demonstrates the simulation results of rare earth ion coordination and hydration laws.Clarified the evolution process and hydration behavior of Y???and Yb???complex ions at different concentrations of CO₃^2-.?4?The effects of adding NaCl on the instantaneous saturation dissolution of 15 rare earth elements and the metastable time in 2 mol·L^-1 Na₂CO₃ solution was studied.MD calculations of RE?La,Sm,Dy,Yb,Y?Cl₃-Na₂CO₃ solution were conducted to clarify the ionic behavior and RE???complex ions structure in the solution.With the increase of NaCl dosage,the number of interacting Cl^-and Na⁺increases,resulting in weak interaction of CO₃^2-on Na⁺in the first ordered layer.For RE???,the number of coordinated CO₃^2-increases,which results in the increase of the RE instantaneous saturation dissolved concentration and the extension of the solution's metastable time.When the concentration of Cl^-is further increased,it will not affect the coordination behavior of CO₃^2-to RE???,only squeezing out RE???'s short-range order layer H₂O molecules.?5?The conditions and mechanism of metastable state rapid destruction through CO₂ in RECl₃-Na₂CO₃ solution was clarified.The variation of pH value and 15 RE???instantaneous dissolution concentration of the solution were obtained during the CO₂ pumping process.Compared with heavy rare earths,RECl₃-Na₂CO₃ metastable solutions of light rare earth are more easily destroyed.Based on the C equilibrium calculation and CO₃^2-/HCO₃^-ratio in the solution system,the behavior of each ion pair and the Yb???complex ion structure were discussed using MD calculation.CO₂ can promote the precipitation process rapidly in RECl₃-Na₂CO₃ solutions,owing to the changed CO₃^2-/HCO₃^-ratio and decreased coordinate number of CO₃^2-to Yb???in the solution.When the pH value drops to9.35 and below,the solution could not provide enough coordinating CO₃^2-,directly leading to the rapid precipitation of Yb???in form of rare earth carbonates.This is the mechanism of rapidly destroying the metastable state of YbCl₃-Na₂CO₃ solution by passing CO₂.