Vanadium Speciation in Fused Alkali Chlorides

Abstract

Electronic absorption spectra of vanadium ions were recorded between 450 and 750 • C in 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl based melts. Analysis of the spectra showed that vanadium in the oxidation states of +2 and +3 is present in the melts as complex chloro-ions. Vanadium(II) can form the complex ions in octahedral and tetrahedral coordination, the latter being favored by higher temperatures. Only six-coordinated V(III) ions were observed. Vanadium in the oxidation state of +4 can be present in chloride melts only in the form of vanadyl-based oxochloro-species. Main spectroscopic parameters of vanadium complex ions were determined. Electronic absorption spectroscopy was also applied for studying a variety of the red-ox processes in vanadium containing melts including electrochemical oxidation and reduction of various species, and reaction of vanadium containing melts with oxygen. Vanadium has many attractive properties including high creep strength, excellent strength-to-weight ratio, resistance to corrosion, low neutron cross-section, high thermal conductivity, etc.1 Due to these unique set of properties vanadium is used in non-ferrous and ferrous metallurgy as a component of titanium-based alloys for the aerospace industry and for manufacturing special steels. 2,3High-purity vanadium metal is required for manufacturing alloys for nuclear fusion applications, superconducting and hydrogen-storage materials.4-6 Vanadium on industrial scale is produced by aluminum or carbon reduction of oxides or magnesium reduction of vanadium chlorides. [7][8][9] To obtain the high-purity metal, the crude material is refined by vacuum melting or electrolysis in molten chlorides.10 Electrolytic refining is effective in removing most of metallic and nonmetallic impurities, i.e. aluminum, magnesium, iron, carbon, nitrogen, oxygen, silicon, etc. Understanding vanadium speciation in chloride melts is essential for designing or optimizing an industrial electrochemical process.Vanadium is a polyvalent metal and, as for many d-element ions with partially filled d-orbitals, electronic absorption spectroscopy was employed for characterization of vanadium species in fused salts. In molten salts vanadium is capable of forming the ions containing the element in oxidation states +2, +3, +4 and +5. First electronic absorption spectra (EAS) of vanadium-containing melts were recorded over a half of a century ago in LiCl-KCl eutectic, KAlCl 4 and Cs 2 ZnCl 4 , [11][12][13][14] followed by KSCN based melts. 15,16 Subsequent studies included other alkali chloride mixtures. [17][18][19][20] Analysis of the data present in the literature showed that there is a considerable controversy about the stable oxidation states of vanadium and the coordination geometry of its complex ions in the lower oxidation states in molten alkali chlorides. Vanadium in the oxidation states +2 and +3 was said to form complex ions where the metal is coordinated to six or four anions in octahedral or tetrahedral geometry, respectively. In the higher oxidation states, +4 a...

Electrodeposition of Aluminium-Vanadium Alloys from Chloroaluminate Based Molten Salt Containing Vanadium Ions

“…The recorded calculated and peak potentials were more than one volt anodic to the equilibrium potential of aluminium in the electrolyte used and therefore had to be attributed to the reversible reactions of the remaining constituents of the electrolyte. Chlorine evolution was found to start at potentials more positive than 2.000 V vs. Al and it is known that sodium equilibrium potential in chloride melts is substantially more negative than that of aluminium [15,16]. The recorded equilibrium and peak potentials, therefore, had to be attributed to vanadium species in metallic and ionized forms.…”

“…Vanadium ions were added to the equimolar electrolyte by dissolution of the vanadium working electrode. There are several studies on the vanadium ions being present in the chloride melts (NaCl + KCl at 690 to 860 °C) [15,16]. The equilibrium potentials of the reactions: V(II) ↔ V(0), V(III) ↔ V(II), Cl − + 2e − ↔ Cl2, were established [15,16].…”

“…There are several studies on the vanadium ions being present in the chloride melts (NaCl + KCl at 690 to 860 °C) [15,16]. The equilibrium potentials of the reactions: V(II) ↔ V(0), V(III) ↔ V(II), Cl − + 2e − ↔ Cl2, were established [15,16]. In our study, the results suggested that in the presence of aluminium ions in equimolar chloroaluminate electrolyte (0.19 mol AlCl3 + 0.19 mol NaCl at 200 to 300 °C) vanadium behaved very similarly.…”

“…Various molten salt electrolytes have been tested and proposed for Al-V alloy electrodeposition, including inorganic (halide [10,[15][16][17][18] and chloroaluminate [8,9,19]) and organic (ionic liquids [20]) electrolytes. Recently, several studies were reported on the deposition of Al-V alloys from inorganic molten salts, mostly from halide-based electrolytes [10,16].…”

“…Meanwhile, electrodeposition of Al-V from chloroaluminates has been rarely investigated [8,9,19]. A number of publication devoted to the electrochemistry of vanadium species in halide melts showed that the valence of vanadium species in halide electrolytes depends greatly on the experimental conditions, such as the electrolyte temperature and the source of the vanadium ions being introduced into the electrolyte [10,[15][16][17].…”