Metal speciation in chloride melts
File(s)
Author(s)
Watson, Ken Leo
Type
Thesis
Abstract
The pyroprocess is used to separate and extract metals from a complex mixture in an electrorefiner, using molten chloride salts as the electrolyte. The solvation of metal ions within the molten salt is an area that requires further study, as there is currently limited understanding of the solvation of heavy metals in molten salts beyond the first solvation shell, important for improvements in not only the pyroprocess but also developments in battery recycling and coolants in nuclear reactors.
In the pyroprocess of interest, two molten salt systems are used, eutectic NaCl-KCl and CaCl2 which yield difference amounts of heavy metal. In the molten salt, solvation shells form around a metal ion, the first solvation shell consists of chloride ions with the salt counter cations, [Na]+, [K]+, and [Ca]2+, in the second solvation shell of the heavy metal ion. By including different salt cations in the second solvation shell, the impact of each salt system on the speciation, coordination number and charge, has been calculated for various heavy metal ions in gas phase calculations. The dominant bonding interaction within clusters has been determined by analysis of ion charges, molecular orbitals, and the bond critical point values.
To represent the large scale solvation of metal ions different solvation models have been used, the solvent model density (SMD) continuum model, molecular dynamics, and ChemShell - a QM/MM method. Using three different methods allows different aspects of solvation to studied, comparisons between methods to be made, and validation of new methods.
The SMD model accounts for average interactions between the cluster and the solvent ions.
Molecular dynamics allows the motion of ions within the solvation shells of the heavy metal ion to be observed and the interactions with bulk salt ions to be considered.
Quantum Mechanics/Molecular Mechanics (QM/MM) calculations allow explicit solute-solvent interactions to be determined between a QM cluster and the bulk molten salt, MM ions.
Solvation of larger metal chloride clusters that contain multiple heavy metal ions have been determined using gas phase and SMD calculations. A dimer cluster, not reported within the literature for the heavy metals studied, has been identified for the Y2Cl6 and La2Cl6 clusters. The location of the salt cation in the second solvation shell has also been determined for these larger clusters, a rarely studied area throughout molten salt research.
By combining the results from gas phase calculations and results from the solvation methods the potentials cause of the difference in heavy metal yield has been determined and suggestions on how to improve the efficiency of the pyroprocess have been made.
In the pyroprocess of interest, two molten salt systems are used, eutectic NaCl-KCl and CaCl2 which yield difference amounts of heavy metal. In the molten salt, solvation shells form around a metal ion, the first solvation shell consists of chloride ions with the salt counter cations, [Na]+, [K]+, and [Ca]2+, in the second solvation shell of the heavy metal ion. By including different salt cations in the second solvation shell, the impact of each salt system on the speciation, coordination number and charge, has been calculated for various heavy metal ions in gas phase calculations. The dominant bonding interaction within clusters has been determined by analysis of ion charges, molecular orbitals, and the bond critical point values.
To represent the large scale solvation of metal ions different solvation models have been used, the solvent model density (SMD) continuum model, molecular dynamics, and ChemShell - a QM/MM method. Using three different methods allows different aspects of solvation to studied, comparisons between methods to be made, and validation of new methods.
The SMD model accounts for average interactions between the cluster and the solvent ions.
Molecular dynamics allows the motion of ions within the solvation shells of the heavy metal ion to be observed and the interactions with bulk salt ions to be considered.
Quantum Mechanics/Molecular Mechanics (QM/MM) calculations allow explicit solute-solvent interactions to be determined between a QM cluster and the bulk molten salt, MM ions.
Solvation of larger metal chloride clusters that contain multiple heavy metal ions have been determined using gas phase and SMD calculations. A dimer cluster, not reported within the literature for the heavy metals studied, has been identified for the Y2Cl6 and La2Cl6 clusters. The location of the salt cation in the second solvation shell has also been determined for these larger clusters, a rarely studied area throughout molten salt research.
By combining the results from gas phase calculations and results from the solvation methods the potentials cause of the difference in heavy metal yield has been determined and suggestions on how to improve the efficiency of the pyroprocess have been made.
Version
Open Access
Date Issued
2019-05
Date Awarded
2020-02
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Hunt, Patricia
Proud, William Graham
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)