Structure of High-Temperature NaF−AlF₃−Al₂O₃ Melts: A Multinuclear NMR Study

Abstract

Fluoroaluminate molten salts are used in the Hall-Heroult industrial process for the production of aluminum by electrolysis. To better understand the mechanism of the dissolution of alumina (Al 2 O 3 ) in cryolitic melts, we have studied the structure of these liquids up to 1025 °C by in-situ NMR of 27 Al, 23 Na, 19 F, and 17 O, using a laser-heated experimental setup. From these experiments we can propose a quantification of the different aluminum-bearing species AlF 4 -, AlF 5 2-, and AlF 6 3in the liquid and give a direct experimental evidence of the existence of at least two different oxy-fluoride species, Al 2 OF 6 2and Al 2 O 2 F 4 2-, when dissolving alumina in the fluoride liquid. The obtained results are compared with other spectroscopic data obtained by in-situ Raman spectroscopy.

Evidence of dynamical local scale distribution heterogeneities in CsF-AF (A=Li, Na, K and Rb) molten salts

“…58,40,72 For this purpose, the chemical shift is measured for several crystalline compounds which structure is perfectly defined. 73,59 Unfortunately, crystalline compounds containing cesium and fluorine ions are scarce. In addition to pure CsF, two other compounds CsLiF 2 (C 1 2/c 1) 74 and Cs 3 SiF 7 (P 4/m b m) 75 are good candidates but were not available for this study.…”

“…83,69 Furthermore, the evidence of these effects must be higher at low The High Temperature NMR (HT NMR) spectra were recorded using a Bruker Avance WB 400MHz spectrometer, operating at 9.40 T. In situ HT NMR spectra were obtained by using the CO 2 laser heating system developed at CEMHTI-CNRS in Orléans (France) and previously described. 91,92 The samples were heated directly up to melts and the spectra were collected about 10 to 20K above respective liquidus temperature for each composition.…”

Structure and dynamics in yttrium-based molten rare earth alkali fluorides

“…Many efforts have been made towards this direction and several experimental techniques have been adapted to the study of molten salts. The development of new cells has unlocked the study of molten fluorides by EXAFS spectroscopy 19 , which, coupled to other spectroscopic methods such as nuclear magnetic resonnance (NMR) 20,21 or Raman 22,23 , is able to provide a view of the local structure in these melts. As for the dynamic properties, similar improvements have been made and the self-diffusion coefficients can be measured up to 1500 K by pulsed field gradient nuclear magnetic resonance (PFG-NMR) 24,25 .…”

Electrochemical Extraction of Rare Earth Metals in Molten Fluorides: Conversion of Rare Earth Oxides into Rare Earth Fluorides Using Fluoride Additives

“…Since the stability of these oxides is lower than that of NdF 3 , parts of Al 2 O 3 , ZnO, and FeO might dissolve in the fluoride melt and get subjected to the electrolysis process. The solubility of alumina in the molten fluorides is 6 mol% [17]. Based on the density of the LiF containing different concentrations of NdF 3 [18], an estimation shows that the density of LiF-5 mol% NdF 3 is 4 g/cm 3 (aluminum oxide density is 3.95 g/cm 3 ).…”