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Vanadium speciation in a variety of alkali chloride based melts was studied in situ using high temperature electronic absorption spectroscopy. The experimental spectra were resolved into constituent Gaussian bands. Octahedral VCl63- complex ions is the only soluble species of V(III). Oxidation of VCl63- complexes or chlorination of V2O5, V2O4 or V2O3 by hydrogen chloride produces soluble vanadyl-based complexes. The results obtained indicate that the coordination environment of vanadyl-species in NaCl-2CsCl melts is different from the melts containing smaller cations. Anodic dissolution of metallic vanadium in chloride melts produces vanadium(II) ions. Increasing temperature and the mean radius of the salt-solvent cation resulted in changes of the electronic spectra as a result of shifting VCl64--VCl42- equilibrium towards the tetrahedral complexes.

A variety of electrochemical techniques (polarization curves measurements, chronopotentiometry, chronoamperometry, linear, cyclic and square-wave voltammetry, impedance spectroscopy) were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. Information about niobium speciation was obtained by combining electrochemical and spectroscopic (measuring electronic absorption spectra) methods. The major product of niobium anodic dissolution in chloride melts at current densities up to 40 mA/cm2 were niobium(III) species, NbCl63-. Soluble niobium(V) species, NbCl6-, represented the highest oxidation state of niobium in NaCl-KCl melts. It was shown that oxidation of Nb(IV) ions, NbCl62-, proceeds in a single step. Cathodic reduction of niobium(IV)-containing melts on tungsten electrode at -1.55/-1.6 V (vs. Cl-/Cl2) resulted in formation of niobium metal. Electrochemical and spectroelectrochemical data indicate that intermediate niobium(III) species can be formed during elecroreduction of NaCl-KCl-NbCln (n=3.8-4.1) melts.

The behavior of uranium(III) and (IV) chloro-species was studied in LiCl-KCl-CsCl eutectic melt between 573 and 1073 K by potentiometry, cyclic voltammetry and chronopotentiometry. Uranium formal standard electrode potentials and U(III)/U(IV) formal red-ox potentials in the given melt were determined and thermodynamic properties of uranium chlorides calculated. Diffusion coefficients of U(III) and U(IV) ions were determined employing cyclic voltammetry and chronopotentiometry measurements.

The reaction between 15 rare earth chlorides (yttrium, lanthanum and all the lanthanides except for promethium) and sodium orthophosphate was studied in NaCl-2CsCl eutectic based melts between 550 and 750 oC under an inert atmosphere. Rare earth phosphates were precipitated from the melt and their composition determined from chemical analysis, vibrational spectroscopy and X-ray powder diffraction measurements. The effects of temperature and the initial phosphate-to-rare earth mole ratio in the melt on the phase composition of the precipitated phosphates was investigated Over 99.9% precipitation could be achieved and the mole ratio required varied between 2 and 6. The minimum excess of phosphate precipitant required for complete precipitation from the melt was determined.

Physico-chemical behavior of vanadium in chloride melts was studied using high temperature electronic absorption spectroscopy and spectroelectrochemistry. Dissolution of VCl3 in molten chloride leads to the formation V(III) species, VCl63-. Vanadium(II) complex ions VCl64- are formed during electroreduction of vanadium(III)-containing melts and vanadium metal anodic dissolution. V(II) chloro-species slowly react with silica to form an oxygen-containing insoluble phase. Due to high volatility of VCl4 oxygen-containing vanadyl species, VOCl42-, is the only stable form of V(IV) in fused chlorides. It is formed when V2O5, V2O4 or V2O3 react with HCl. Electroreduction of vanadyl complexes leads to the formation of insoluble oxygen-containing vanadium(III)species, whereas electrochemical oxidation has no effect on vanadium speciation, VO3+/VO2+ red-ox potential is more positive than Cl2/Cl-. Further oxidation VO2+ species gives vanadate anions. The electronic absorption spectra of soluble vanadium species were measured in 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl mixtures at 450-750 0C.

Speciation of tungsten and molybdenum in alkali chloride melts (based on NaCl-2CsCl, NaCl-KCl and 3LiCl-2KCl mixtures) was studied between 450 and 750 oC using electronic absorption spectroscopy. Only W(IV) and W(V) chloro- and oxychlorospecies can be stabilised under conditions studied. Tungsten(IV) chloride ions are very sensitive to oxide/hydroxide impurities present in the melt. Anodic dissolution of W metal at anodic current densities in the range of 0.005-0.1 A/cm2 produces only WCl6 2- ions that can be electrochemically oxidised to WCl6 -. Small amounts of oxide ions present in the melt result in a gradual conversion of W(IV) chloro-species into W(IV) oxychloro-species. In the presence of O2 WCl6 2- is oxidised into tungsten(V) species WOCl5 2-. Molybdenum forms Mo(III), (IV) and (V) species. MoCl6 2- complex ions in chloride melts can be obtained by reacting MoO2 with HCl, or by electrochemical oxidation of Mo(III) (MoCl6 3- → MoCl6 2-) on a glassy carbon electrode.