APPROACH OF THE MOLTEN SALT CHEMISTRY FOR ALUMINIUM PRODUCTION: HIGH TEMPERATURE NMR MEASUREMENTS,
MOLECULAR DYNAMICS AND DFT CALCULATIONS Kelly Machado1, Didier Zanghi1, Vincent Sarou-Kanian1, Sylvian Cadars2, Mario Burbano3,
Mathieu Salanne3, Catherine Bessada1
1CEMHTI (Conditions Extrême Matériaux Haute Température et Irradiation) CNRS, Université Orléans;
1D Avenue de la Recherche Scientifique CS90055; Orléans, 45071, France
2IMN (Institut des Materiaux Jean Rouxel) CNRS, Université de Nantes;
2 Rue de la Houssinière BP32229; Nantes, 44322, France
3UPMC (University Pierre et Marie Curie), PHENIX CNRS;
4 place Jussieu, Paris, 75005, France
Keywords: NaF-AlF3; high temperature NMR; chemical shifts, anionic species; molecular dynamic; functional theory of density (DFT); Polarizable Ion Model (PIM).
Abstract
In aluminum production, the electrolyte is a molten fluorides mixture typically around 1000°C.
In order to have a better understanding of the industrial process, it is necessary to have a model which will describe the molten salts on a wide range of compositions and temperatures, to accurately cover all the combinations that may be encountered in an operating electrolysis vessel.
The aim of this study is to describe the speciation in the electrolyte in terms of anionic species in the bulk materials far from electrodes. To determine the speciation in situ at high temperature in the absence of an electrical field, we develop an original approach combining experimental methods such as Nuclear Magnetic Resonance spectroscopy (NMR) at high temperature with Molecular Dynamics (MD) simulation coupled with first principle calculations based on Density Functional Theory (DFT). This approach allows the calculation of NMR parameters and the comparison with the experimental ones. It will be provide an additional validation and constraint of the model used for MD. We test this approach on the model NaF-AlF3 system.
Introduction
In Hall-Héroult electrolytic process used for aluminum production, the industrial electrolyte is mainly composed of a mixture of NaF, AlF3 and Al2O3 for an operating temperature around 1000°C [1]. Due to its industrial interest, several studies have been made in order to improve the process, in particular around the electrolyte composition in order to lower the temperature and thus to minimize the operating costs. An important point necessary to improve the process is to know the structure of the molten salt used as an electrolyte.
Under the high temperature conditions, these electrolytes are corrosive and difficult to handle, making any experimental measurement particularly challenging. In situ Raman [4] or NMR [2,3]
spectroscopies at high temperature in NaF-AlF3 melts have succeeded in identifying and quantifying the anions present in the electrolyte, i.e. free F-, as well as different aluminum-
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
bearing fluorinated species, AlF63-
, AlF52-
, AlF4-
. Some questions still remain, however, regarding their nature, the evolution of their relative amounts with temperature and their lifetime.
To further extend our understanding of such systems, it is necessary to develop in parallel numeric tools that are capable of simulating the liquid phase. To fulfill these objectives, we propose to focus on the description of the speciation in NaF-AlF3 melts, using an original approach combining in situ NMR measurements at high temperature with simulation by molecular dynamics (MD) coupled with "first principle" calculations of NMR chemical shifts at the density functional level of theory (DFT). For molecular dynamics simulations we use the Ion Polarizable Model (PIM) which was initially developed by Madden and Wilson [5, 6]. This force field, specifically designed for the description of ionic liquids, is based on an interatomic potential containing four interactions: the electrostatic interaction between two formal charges, the overlap repulsion component, the dispersion component and the polarization, with parameters typically obtained by force-fitting to ab initio electronic structure calculations. The result of MD simulations gives the ion trajectories of each ion along the simulation time, which may then be analyzed to calculate several important macroscopic and molecular-level properties such as viscosity, conductivity, the radial distribution functions of each atomic pair and the coordination states of the aluminum.
The interatomic potential is ultimately validated with a combination of MD and DFT calculation to obtain the NMR chemical shifts of each nucleus in the NaF-AlF3 system at a given temperature. The calculated parameters are thus compared with the experimental values and allow us to go further in the description of such melts.
The NMR measurements were done using the laser heating NMR setup developed in Orleans in the system NaF-AlF3, for each nucleus (23Na, 19F and 27Al) at different temperatures and compositions. In these melts, at high temperature, the spectrum obtained for all the observed nuclei, is a single, narrow line, characterized by its position: the chemical shift. Because of the rapid exchange existing between the different local environments around the observed nucleus, the chemical shift measured is the average of the chemical shifts of each individual anionic species, weighted by their respective atomic fraction.
Some computational simulations have been published for some compositions in the NaF-AlF3
system corresponding to the cryolite (Na3AlF6), chiolite (Na5Al3F14) and NaAlF4 [7, 8]. In this work, the challenge was to build an interatomic potential valid for a wide range of NaF-AlF3
compositions at high temperature.
Experimental
NMR experiments
Different NaF-AlF3 compositions have been prepared with 0 to 50 mol% of AlF3. The different mixtures were introduced in a glove box, in a high purity boron nitride crucible tightly closed by a screw cap. The NMR experiments were carried out using a Bruker DSX 400 NMR spectrometer operating at 9.4T. The laser heating NMR setup has been described in details in previous publications [2, 9, 10]. Two CO2 laser (λ= 10.6μm, 250 W) heat directly the top and the bottom of the crucible and allow heating up to 1500°C. The crucibles are protected from oxidation at high temperature by a continuous argon flow. The electronics and the radio frequency coil are cooled by an air flow at room temperature and protected by a zirconia thermal shield. The chemical shifts of all nuclei (27Al, 23Na, 19F) are referenced to 1M aqueous solutions
Al(NO3)3, NaCl, CFCl3, respectively. The spectra acquisition starts 5 minutes after reaching the desired temperature to insure thermal equilibrium and the three nuclei are observed sequentially.
Molecular Dynamics
Molecular dynamics is based on the resolution of the equation of motion for each particle interacting with its neighbors. It is therefore necessary to know the forces involved in the system to integrate this equation over time.We use the polarizable ion model [5, 6] (PIM) where the interatomic potential (VTotal) is a pairwise sum of four interactions: charge, dispersion, repulsion and polarization.
VTotal = Vcharge + Vdispersion + Vrepulsion + Vpolarisation
(1) The parameters involved in such interaction potential were obtained by force matching with ab initio calculations. These calculations were performed using a plane wave electronic structure code, the Vienna Ab initio simulation package (VASP) and projector augmented wave (PAW- PBE) pseudopotential was used. The target function involves the forces and dipoles of each ion, along with the stress of each configuration. Several cubic boxes were prepared representing the full range compositions up to 50 mol% of AlF3 in the NaF-AlF3 system.
MD simulations were performed for a cubic box of length approximately of 15.4Å with nearly 200 atoms. For all simulations, the systems were equilibrated constant temperature and fixed pressure at 0 GPa (NPT ensemble) and the lengths of the cubic box were obtained. The Nosé- Hoover thermostat algorithm was used [11]. Atoms trajectories were obtained at constant temperature and volume (NVT ensemble) for 1ns. Then a statistical analysis of the given trajectories provides access to different physical properties.
Functional theory of density
Density functional theory (DFT) is a computational quantum mechanical method used to investigate the electronic structure of a system. To calculate the NMR chemical shifts, using DFT calculations, snapshots of the MD simulations are extracted at different times (sufficiently far apart to avoid correlation), and used without further optimization to calculate electronic properties with the CASTEP code [12, 13], which is based on DFT with a plane-wave basis set and pseudopotentials. The generalized gradient approximation (GGA) was used with the exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE), a cutoff energy of 610 eV and a 1×1×1 Monkhorst-Pack [14] grid of k-points (yielding NMR shielding converged within 1 ppm or less). Chemical shift are referenced on the basis of series of calculations conducted on simple crystalline systems [15].
Results
Simulations and NMR experience were done 20°C above the melting point for the different NaF-AlF3 compositions. Molecular dynamics reproduce the ionic motion for the system NaF- AlF3 making it possible to identify, quantify, and track the different ionic species present: AlF63-
, AlF52-
, AlF4-
and free F-, (figure 1).
Figure 1 - Simulation cell with the atomic model for NaF-AlF3 mixture derived from molecular dynamics at 1305K. (Blue areas: AlF63-
, AlF52-
or AlF4-
)
The chemical shifts evolution of 23Na, 19F and 27Al were measured by NMR and calculated with CASTEP for all configurations (up to 50 mol% of AlF3). In figure 2 we reported the experimental and calculated chemical shifts for all three nuclei as open and: the empty markers are measured experimentally and the full markers are the calculated chemical shifts.
Figure 2 - Comparison of calculated (full markers) and experimental [3] (empty markers) NMR
chemical shifts vs AlF3 content at liquid temperature.
For 23Na, 19F and 27Al nuclei, not only the trends but also the absolute values of the calculated average chemical shifts are in excellent agreement with the experimental values measured at high temperature. Given the extreme sensitivity of NMR chemical shifts to both the fine details of the local structure around atoms [16] and to the speciation in molten salts [2,3] these results are a solid proof that the force field used to describe inter-atomic interactions in the classical MD
-230 -210 -190 -170 -150 -130 -110
-80 -60 -40 -20 0 20 40
0 20 40 60
d19F (ppm) d 23Na et 27Al (ppm)
AlF3 (mol%)
19F
23Na
27Al
Sodium Fluorine Aluminium
simulations. Such a validation is all the more valuable that very few physic-chemical parameters can be reliably measured under such extreme temperature and corrosive conditions.
The 19F chemical shift in molten NaF, measured in the melt at 995 °C is -232 ± 5 ppm and can be associated to free F-. As AlF3 is added to NaF, the contribution of free fluorine decreases due the formation of anionic complexes AlFx3-x
, the relative amounts of which appear to be accurately reproduced by the calculation, as well as their evolution as a function of composition.
Molecular dynamics provides detailed information on the structure, dynamic and thermodynamic properties of the melts. And the validation of the inter-atomic potential for the system NaF-AlF3
now allows us to conduct more extensive dynamic studies with systems containing larger numbers of atoms, from which important macroscopic properties such as conductivity and viscosity can be evaluated. To improve the electrolyte used in aluminum production is important to have a detailed understanding of chemical and physical phenomena occurring in these melts.
Because inter-atomic models are pair-wise interactions, this validation on the binary NaF-AlF3
system was also a prerequisite to then explore more complex compositions, and in particular those involving Al2O3, the Al precursor in the electrochemical syntheses of aluminum.
Conclusion
The main idea of this work is to build a polarizable interaction potential that reproduce the structure of the molten mixtures of the binary NaF-AlF3 at high temperature in a wide range of composition (0 – 50 mol. % ). In challenging high-temperature and corrosive conditions, where reliable experimental data are particularly challenging to obtain, the comparison of experimental NMR results with data obtained from the combination of MD and DFT simulations provides an essential and particularly solid approach for the evaluation of the ionic potential of atomic interactions used in our simulations. Now the force field can be transferred to more complex systems. Analyzing the radial distribution function, the coordination number of aluminum ions was also calculated from MD simulation and confirms the presence of the ionic complex AlF63-
, AlF52-
, AlF4-
and of free F-.
An important point of this study is to describe the speciation in the electrolyte to obtain input data for modeling electrokinetic transport phenomena during the process of aluminum production. The corrosion behavior and the interactions between the molten electrolyte and the anode are not completely understood. These models should identify forward, the species present on the surface of the electrode and to better understand the mechanisms of corrosion observed on the anodes.
Acknowledgments
This study was financially supported by the ANR MIMINELA project of the French National Agency of Research. For the MD and DFT calculations, we thank the “Centre de Calcul Scientifique en Région Centre” (CCSC, Orléans, France).
References
1. J.Thonstadt et al., Aluminium Electrolysis Fundamentals of the Hall Héroult Process (Aluminium-Verlag: Marketing Kommunication GmbH, 3rd edition, 2001).
2. V.Lacassagne, C.Bessada, P.Florian, S.Bouvet, B.Ollivier, J.P.Coutures, D.Massiot,
‘Structure of High-Temperature NaF-AlF3-Al2O3 Melts: A Multinuclear NMR study’, J. Phys.
Chem. B, 106 (2002), 1862-1868.
3. I.Nuta, E.Véron, G.Matzen, C.Bessada, ‘High Temperature NMR Study of Aluminum Metal Influence on Speciation in Molten NaF-AlF3 Fluorides’, Inorg. Chem, 50 (8) (2011), 3304 – 3312.
4. E.Robert, V.Lacassagne, C.Bessada, D.Massiot, B.Gilbert, J.P.Coutures, ‘Study of NaF-AlF3 Melts by High-Temperature 27Al NMR Spectroscopy: Comparison with Results from Raman Spectroscopy’, Inorg. Chem, 38 (1999), 214-217.
5. P.Madden, M.Wilson, ‘‘Covalent’ effects in ‘ionic’ systems’, Chem. Soc. Rev, 25 (1996), 339-350.
6. F. Hutchinson, M. Wilson, P.A. Madden, ‘A unified description of MCl3 systems with a polarizable ion simulation model’, Molecular Phys, 99 (2001), 811.
7. Z. Akdeniz and P. A. Madden, ‘Raman Spectra of Ionic Liquids: A Simulation Study of AlF3
and Its Mixtures with NaF’, J. Phys. Chem. B, 110 (13) (2006), 6683–6691.
8. S. Cikit, Z. Akdeniz, and P. A. Madden, ‘Structure and Raman Spectra in Cryolitic Melts:
Simulations with an ab Initio Interaction Potential’, J. Phys. Chem. B, 118 (4) (2014), 1064–
1070.
9. D. Massiot, C. Bessada, P. Echegut., J.P. Coutures, Solid State Ionics, 37 (1990), 223-229.
10. A. L. Rollet, V. Sarou-Kanian, C. Bessada, ‘Self-diffusion coefficients measurement at high temperature by PFG NMR’, Comptes Rendus Chimie, 13 (2010), 399-404.
11. S. Nosé, ‘A unified formulation of the constant temperature molecular dynamics methods’, J. Chem. Phys., 81 (1) (1984), 511–519.
12. M.D.Segall, P.J.D.Lindan, M.J.Probert, C.J.Pickard, P.J.Hasnip, S.J.Clark, M.C.Payne, J.
Phys.: Condens. Matter, 14 (2002), 2717-2744.
13. S. J. Clark, M. D. Segall, C. J. Pickard, P. J. Hasnip, M. J. Probert, K. Refson, M. C. Payne, Zeitschrift fuer Kristallographie, 220 (5-6) (2005) , 567-570.
14. H. J. Monkhorst and J. D. Pack, Phys. Rev. B, 1976, 13, 5188–5192.
15. A. Sadoc, M. Biswal, M. Body, C. Legein, F. Boucher, D. Massiot and F. Fayon, “NMR parameters in column 13 metal fluoride compounds (AlF3, GaF3, InF3 and TlF) from first principle calculations”, Solid State Nuclear Magnetic Resonance, 59-60 (2014), 1–7.
16. C. Bonhomme, C. Gervais, F. Babonneau, C. Coelho, F. Pourpoint, T. Azais, S. E. Ashbrook, J. M. Griffin, J. R. Yates, F. Mauri and C. J. Pickard, “First-principles calculation of NMR parameters using the gauge including projector augmented wave method: a chemist’s point of view”, Chemical Reviews, 112 (2012), 5733–5779.