Theoretical Study On The Ion Aggregationin Characteristics And The Mechanism Of Double Salt Formation In Saline Solutions

The development of battery,glass,ceramics,optoelectronic industry has resulted in an enormous demand for lithium.The raw materials for lithium resources are mainly from salt lakes,seawater and underground brine.However,it is very difficult to extract lithium from these salt lakes because of the similar properties between lithium and magnesium,especially for the high Mg/Li ratio salt lakes.Therefore,it is of great significance for the economic and effective separation of magnesium and lithium.The formation of double salt may be one possible way to improve the efficiency of Mg/Li separation for the high Mg/Li ratio salt lakes.In order to understand the microscopic characteristics of double salt and the nucleation mechanism in aqueous solution,the ion aggregation characteristics in MgCl₂-LiCl aqueous solution was compared with that of MgCl₂ aqueous solution based on density functional theory?DFT?calculations and classical molecular dynamics?MD?simulations in this work.The nucleation and crystallization mechanism of LiCl·MgCl₂·7H₂O double salt was revealed preliminarily under a microscopic level.The results show that solvent-separated and contact-associated Mg-Cl species can coexist in MgCl₂ and MgCl₂-LiCl aqueous solutions.When the concentration of solution increases,the aggregation trend of solvent-separated Mg-Cl cluster is very obvious.In concentrated solution,the characteristics of some SS Mg-Cl clusters are similar to the block unit of MgCl₂·6H₂O crystal.However,there is also no obivous aggregation of contact associated Mg-Cl species,which may lead to the metastable solution or supersaturated solution.Since the ion exchange in solvent-separated Mg-Cl cluster is more frequent,it can be inferred microscopically that the nucleation and crystallization of MgCl₂ hydrate proceed by a classical mechanism.In the MgCl₂-LiCl mixed solution with very low Mg/Li ratio,the Li-Cl association began to play a dominant role and the aggregation trend of CA Li-Cl clusters is quite obvious.Furthermore,the distribution of CA Mg-Cl species can hinder the formation of MgCl₂·6H₂O crystals.Therefore,the characteristic of ion aggregation in MgCl₂-LiCl aqueous solutions with very low Mg/Li ratio will benefit the nucleation of LiCl solution.The evolution of Mg-Cl mixed clusters show that all solvent-separated Mg-Cl mixed clusters did not diassociate in the solution.It is found that the Mg²⁺in all solvent-separated Mg-Cl mixed clusters are directly coordinated with six water molecules,the Cl^-ions in the second shell,and Li⁺are directly associated with such Cl^-,which is more analogous to the LiCl·MgCl₂·7H₂O doube salt crystal units.In general,the association tendency of Mg-Cl is stronger than that of contact-associated Li-Cl in MgCl₂-LiCl solution with higher Mg/Li ratio,thus the MgCl₂·6H₂O crystals are salting out.The association trend of solvent-separated Mg-Cl is consistent with that of contact-associated Li-Cl in the relatively low Mg/Li ratio salt lake.This may be very important for the formation of double salt.Calcium carbonate plays an important role in geological science and biomineralization.The results show that the association of Ca-OC in CaCO₃ is mainly due to the electrostatic interaction,and the charge transfer has little effect on the association.Further aggregation is likely to be driven by the electrostatic interaction.The dissociation barrier of CaCO₃ is not large,and solvent recombination occurs during the transform of association and dissociation structure.Our results show that the organization of water molecule and hydrogen bond around ions or ion associated structures play an important role in the further aggregation process in the aqueous solution and monodentate association species are dominant.The hydration characteristics of CaCO₃ are probably similar to those of Ca²⁺,in which the residence time of the surrounding water molecules is relatively short,and the hydration shell is relatively loose,it may be beneficial for the formation of amorphous calcium carbonate.Our results indicate that the controlling of the hydration behavior around ions or ion associated structures may be one of the key factors for the formation of compact CaCO₃ biominerazation materials in the solution.