Effect of calcium fluoride addition to molten NaF-AlF3 mixtures: A study by Raman spectroscopy

Robert, E.; Materne, T.; Tixhon, E.; Gilbert, Bernard

doi:10.1016/0924-2031(93)87023-m

Cited by 18 publications

(10 citation statements)

References 8 publications

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“…Such an ion had also been predicted by Dewing based on thermodynamic arguments. Further Raman work by Gilbert et al on the NaF−AlF 3 −CaF 2 and the KF−AlF 3 liquid systems strengthen the hypothesis of the formation of three ionic complexes AlF 4 - , AlF 5 2- , and AlF 6 3- in these melts.…”

Section: Introductionmentioning

confidence: 77%

“…Because of the difficulty involved in the precise reproduction of the molten drop shape and size in the windowless cell, only relative intensity measurements can be made. The general procedure for the extraction of the intensity ratio of the bands has been described in previous papers. , The various computer techniques used were very similar to the one described by Irish et al Briefly, the procedure consists of five steps: digital subtraction of the furnace blackbody emission if necessary (this emission background can be easily measured by blocking the laser beam and recording the spectrum again); flattening of the Rayleigh light by an exponential quadratic function; subtraction of the 215 cm -1 , 320 cm -1 and 345 cm -1 bands by visual adjustment on the screen, if necessary; initial generation of a spectrum made of three components in order to obtain rough numbers for the height, width and position of the three remaining main bands; quantitative decomposition of the overlapping bands by a least square and simplex procedure. This procedure leads to the intensity ratios I 510 / I AlF 4− and I 560 / I AlF 4− .…”

Section: Resultsmentioning

confidence: 99%

“…AlF 5 2-Complex (d). In previous papers 11,13,14 the 560 cm -1 band was assigned to AlF 5 2ions, and the 510 cm -1 band to AlF 6 3-. This model allows a consistent interpretation of the Raman data in the whole composition and temperature range.…”

Section: Discussionmentioning

confidence: 98%

“…For high CaF 2 additions (>15-20 mol %), some deformations of the AlF 5 2band can be observed (broadening, shift of frequency). A detailed study of these spectra has been described in another paper, 13 and we will concentrate on the comparison between the Raman data and the vapor pressure results only. Since CaF 2 shifts equilibria 1 and 2 to the left, it means that it has to dissociate in some way to liberate F -.…”

Section: Discussionmentioning

confidence: 99%

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Structure and Thermodynamics of NaF−AlF₃ Melts with Addition of CaF₂ and MgF₂

et al. 1996

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The NaF-AlF(3) system with additions of CaF(2) and MgF(2) has been studied with Raman and vapor pressure measurements for 3 >/= CR (NaF/AlF(3) molar ratio) >/= 1 and up to 50 mol % additive. The results show that the binary melt can be described using the two equilibria AlF(6)(3)(-) = AlF(6)(2)(-) + F(-) and AlF(5)(2)(-) = AlF(4)(-) + F(-) with equilibrium constants 0.25 and 0.05, respectively, at 1293 K. Both reactions have positive reaction enthalpies. The first equilibrium is strongly shifted to the right resulting in a melt mixture with very low AlF(6)(3)(-) concentrations even at the Na(3)AlF(6) composition. Evidence for nonideal mixing of anions was found. For the ternaries, models based on Raman data are presented and compared with vapor pressure measurements. Good agreement is observed when association between the additives, CaF(2) or MgF(2), with the AlF(5)(2)(-) ions in the melt was considered. This association could be experimentally observed through a band broadening and a slight shift in the AlF(5)(2)(-) band frequency. Our vapor pressures and Raman data both indicate that MgF(2) clearly acts as an acid when added to NaF-AlF(3) melts of any composition. When CaF(2) is added, a slight decrease of vapor pressure occurs. Raman data indicate a decrease of AlF(4)(-) concentration, corresponding to a dissociation of CaF(2) with liberation of F(-) ions. All these results are, however, very much dependent on the initial melt composition. These data are explained in terms of acid-base, dilution, and association reactions of the solute with the solvent.

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Section: Introductionmentioning

confidence: 77%

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 99%

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Structure and Thermodynamics of NaF−AlF₃ Melts with Addition of CaF₂ and MgF₂

et al. 1996

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“…Spectroscopic techniques are powerful tools to study the molten salts structure at the molecular level. In recent years, the use of Raman spectroscopy with a windowless cell allowed quantitative information to be obtained about the concentration of ionic complexes present in corrosive melts such as fluorides. − Recently, a reliable model was proposed, combining results from Raman spectroscopy and thermodynamic data . This model involves three fluoroaluminate species (AlF 4 - , AlF 5 2- , AlF 6 3- ) together with free fluoride ions F - .…”

Section: Introductionmentioning

confidence: 99%

Study of NaF−AlF₃ Melts by High-Temperature ²⁷Al NMR Spectroscopy: Comparison with Results from Raman Spectroscopy

et al. 1999

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The molten NaF−AlF3 system has been studied by 27Al NMR over the composition range 0.2 < X°(AlF3) < 0.5, using a high-temperature laser heating system. The experimental chemical shifts have been compared to the shifts calculated on the basis of a model obtained by the combination of Raman spectroscopy and thermodynamics results. This model involves three fluoroaluminate complexes (AlF4 -, AlF5 2-, AlF6 3-). The agreement between experimental and calculated shifts strengthens the reliability of the suggested model.

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Computational and analytical chemistry: Methodology to study chemical reactions between sodium, calcium, and aluminum fluorides in molten cryolite

Bouyer

Picard

Legendre

1997

Int. J. Quant. Chem.

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Reaction constants and composition profiles in molten cryolite have been theoretically investigated. Hartree᎐Fock and density functional calculations were applied to determine Ž . the exact nature structure and energetic of complexes existing in molten cryolite. The aim of this work was thus the understanding of chemical processes occurring in the electrowinning of aluminum and was a demonstration of how computational chemistry Ž . based on density functional theory can help us to determine structures and reaction energies in particularly complex medium such as cryolite. An analytical study, based on mass balance and equilibrium constants has been undertaken. This was performed on molecular liquid entities taking into account the four-, five-and sixfold coordinated aluminum complexes of the AlF ᎐3NaF melt system. Moreover, the effect of calcium has 3 been studied by substituting two sodium atoms with one calcium atom, thus leading to Ž . the CaNaAlF system. Two conformers instead of three for Na were obtained for this 6 system. They can be described as representing the four-and fivefold coordinated aluminum complexes in molten cryolite. The structurizing effect of calcium was clearly *To whom correspondence should be addressed.

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Effect of calcium fluoride addition to molten NaF-AlF3 mixtures: A study by Raman spectroscopy

Cited by 18 publications

References 8 publications

Structure and Thermodynamics of NaF−AlF₃ Melts with Addition of CaF₂ and MgF₂

Structure and Thermodynamics of NaF−AlF₃ Melts with Addition of CaF₂ and MgF₂

Study of NaF−AlF₃ Melts by High-Temperature ²⁷Al NMR Spectroscopy: Comparison with Results from Raman Spectroscopy

Computational and analytical chemistry: Methodology to study chemical reactions between sodium, calcium, and aluminum fluorides in molten cryolite

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Effect of calcium fluoride addition to molten NaF-AlF3 mixtures: A study by Raman spectroscopy

Cited by 18 publications

References 8 publications

Structure and Thermodynamics of NaF−AlF3 Melts with Addition of CaF2 and MgF2

Structure and Thermodynamics of NaF−AlF3 Melts with Addition of CaF2 and MgF2

Study of NaF−AlF3 Melts by High-Temperature 27Al NMR Spectroscopy: Comparison with Results from Raman Spectroscopy

Computational and analytical chemistry: Methodology to study chemical reactions between sodium, calcium, and aluminum fluorides in molten cryolite

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Product

Resources

About

Structure and Thermodynamics of NaF−AlF₃ Melts with Addition of CaF₂ and MgF₂

Structure and Thermodynamics of NaF−AlF₃ Melts with Addition of CaF₂ and MgF₂

Study of NaF−AlF₃ Melts by High-Temperature ²⁷Al NMR Spectroscopy: Comparison with Results from Raman Spectroscopy