Distillation of LiCl from the LiCl–Li2O molten salt of the electrolytic reduction process

Electrolytic reduction of the uranium oxide in LiCl–Li₂O molten salt for the treatment of spent nuclear fuel requires the separation of the residual salt from the reduced metal product, which contains about 20 wt% salt. In order to separate the residual salt and reuse it in the electrolytic reduction, a vacuum distillation process was developed. Lab-scale distillation equipment was designed and installed in an argon atmosphere glove box. The equipment consisted of an evaporator in which the reduced metal product was contained and exposed to a high temperature and reduced pressure; a receiver; and a vertically oriented condenser that operated at a temperature below the melting point of lithium chloride. We performed experiments with LiCl–Li₂O salt to evaluate the evaporation rate of LiCl salt and varied the operating temperature to discern its effect on the behavior of salt evaporation. Complete removal of the LiCl salt from the evaporator was accomplished by reducing the internal pressure to <100 mTorr and heating to 900 °C. We achieved evaporation efficiency as high as 100 %.     

Electrolytic reduction of spent oxide fuel in a molten LiCl-Li2O system

Summary The Advanced Spent Conditioning Process (ACP) developed by the KAERI is based on pyrometallurgy and the electrolytic reduction plays a central role in transforming spent oxide fuels into metals. The constituents of the spent fuels are distributed between a salt and a reduced metal phase during electrolysis. Lithium metal is produced in a molten LiCl-Li₂O cell and then it reacts with the metal oxides of the spent fuel producing Li₂O and reduced metals. By focusing on the activity of Li₂O and the electric potential, the electrolytic reduction process of the ACP is discussed. Thermodynamic considerations are defined and operation conditions are proposed including Li₂O activity and cell potential.     

Structural parameters of concentrated aqueous solutions of LiCl under extreme conditions,as calculated by the method of integral equations. Effect of temperature

Peculiarities of structure formation of aqueous LiCl solutions at different salt : water molar ratios (LiCl : n H₂O, n = 3.15, 8.05, 14.90) under conditions of isobaric heating (p = 100 bar, T = 298, 323—523 K, T = 50 K) were studied by the method of integral equations. Heating of LiCl : 14.90H₂O solution was found to lead to disappearance of tetrahedral ordering of solvent molecules, appreciable weakening of the coordination abilities of both ions, and to an increase of the number of contact ion pairs and a decrease of the number of solvent-separated ion pairs. For the LiCl : 8.05H₂O system, the tetrahedral structure of the solvent disappears at a lower temperature and heating has a less pronounced effect on the coordination and associative abilities of the ions. In the LiCl : 3.15H₂O solution, tetrahedral ordering of the solvent molecules disappears at 298 K and the number of contact ion pairs decreases as temperature increases. Other structural changes in this system upon heating are similar to those found for the LiCl : 14.90H₂O and LiCl : 8.05H₂O solutions.     

Electrochemical reduction of UO2 to U in a LiCl–KCl-Li2O molten salt

An electrochemical reduction of UO₂ to U in a LiCl–KCl-Li₂O molten salt has been investigated in this study. A diagram showing equilibrium potentials (relative to Cl₂/Cl^?) plotted versus the negative logarithms of oxide-ion activity (pO^2?) was constructed. The crushed UO₂ pellets in the cathode basket of an electrolytic reducer were successfully reduced to U. The reduction of UO₂ is proved to proceed mainly through chemical reaction with in situ generated Li and K at the cathode. The control of cathode potential is essential to prevent the deposition and subsequent vaporization of K metal at the cathode for the applications of a LiCl–KCl-Li₂O molten salt as an electrolyte for the metal production from its oxide sources.     

Phase equilibria in system LiCl–NaCl–H<Subscript>2</Subscript>O at 308 and 348 K

The solubilities and densities of the solutions in the ternary system LiCl–NaCl–H₂O at 308 and 348 K were determined by the method of isothermal dissolution equilibrium. There are one invariant point, two univariant isotherm dissolution curves, and two crystallization regions corresponding to monohydrate (LiCl · H₂O) and NaCl, respectively. This system at both temperatures belongs to hydrate type I, and neither double salt nor solid solution was found. A comparison of the phase diagram for the ternary system at 273–348 K shows that the area of crystallization region of LiCl · H₂O is decreased with the increasing of temperature, while that of NaCl is increased obviously. The solution density of the ternary system at two temperatures changes regularly with the increasing of LiCl concentration.     

LiCl-LiBr-LiVO3 and LiCl-LiBr-Li2MoO4 ternary systems

Phase equilibria in the LiCl-LiBr-LiVO₃ and LiCl-LiBr-Li₂MoO₄ ternary systems have been investigated by differential thermal analysis. The following compositions have been revealed (mol %): eutectic in the LiCl-LiBr-LiVO₃ system (18.0% LiCl, 72.0% LiBr, and 10.0% LiVO₃) with a melting point of 464°C and specific enthalpy of melting of 213 kJ/kg, and a minimum in the LiCl-LiBr-Li₂MoO₄ system (27.0% LiCl, 48.0% LiBr, and 25.0% Li₂MoO₄) with a melting point of 444°C. The investigation of ternary systems including salts of alkali metals is of practical interest for chemical industry and metallurgy, where salt mixtures are used as fused electrolytes and heat carriers. Original Russian Text ? T.V. Gubanova, E.I. Frolova, I.K. Garkushin, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 7, pp. 1220–1223.     

Characteristics of an integrated cathode assembly for the electrolytic reduction of uranium oxide in a LiCl-Li2O molten salt

Summary Electrochemical behavior of the reduction of uranium oxide was studied in a LiCl-Li₂O molten salt system with an integrated cathode assembly. The mechanism for the electrolytic reduction of uranium oxide was studied through cyclic voltammetry. By means of a chronopotentiometry, the effects of the thickness of the uranium oxide, the thickness of the MgO membrane and the material of the conductor of an integrated cathode assembly on the overpotential of the cathode were investigated. From the voltamograms, the reduction potential of the uranium oxide and Li₂O was obtained and the two mechanisms of the electrolytic reduction were considered with regard to the applied cathode potential. From the chronopotentiograms, the exchange current, the transfer coefficient and the maximum allowable current based on the Tafel behavior were obtained with regard to the thickness of the uranium oxide, and of the MgO membrane and the material of the conductor of an integrated cathode assembly.     

Study on a separation method of radionuclides (Ba, Sr) from LiCl salt wastes generated from the electroreduction process of spent nuclear fuel

In this paper, a separation method of radionuclides (Ba, Sr) from LiCl salt wastes generated from the electroreduction process of spent nuclear fuel was studied to recover pure LiCl salts and reduce radioactive wastes. The method consisted of chemical conversion process of BaCl₂ and SrCl₂ in LiCl molten salts by using lithium compounds and vacuum distillation process of LiCl salts. In the chemical conversion, BaCl₂ and SrCl₂ in LiCl molten salts were mainly converted into (Ba,Sr)CO₃ or (Ba,Sr)SO₄. Contents of Ba and Sr in LiCl salts recovered from the vacuum distillation process were equal to about 0.01 of initial concentrations of Ba and Sr in LiCl molten salts. These results will be utilized to recycle the LiCl salt wastes.     

Formation of binary salts in the system LiCl–CsCl–H<Subscript>2</Subscript>O

In the ternary system LiCl–CsCl–H₂O the binary salt LiCl·2CsCl·4H₂O was formed. Its structure was proved by the X-ray structural analysis. The binary compound formed in the aqueous solution by the structurally-forced insertion mechanism.     

Electrolytic production of metallic uranium from U3O8 in a 20-kg batch scale reactor</p> </p>

Summary The electrolytic reduction of U₃O₈ powder was carried out using LiCl-Li₂O molten salt in a 20-kg U₃O₈ batch cell to verify the feasibility of the process. As the current passes the cell, the decomposition of Li₂O and the reduction of U₃O₈ occur simultaneously in a cathode assembly and oxygen gas evolvs at the anode. The results from a 20-kg U₃O₈ scale cell were compared with data obtained from a bench scale cell. The results suggest a successful demonstration of this process, exhibiting a reduction conversion of U₃O₈ of more than 99% in a batch.</p> </p>     

Lithium Recovery from Radioactive Molten Salt Wastes by Electrolysis

In order to determine the operating conditions of an electrolyzer to recover lithium metal from molten salt wastes composed of LiCl, Li₂O, Cs₂O, and SrO, electrolytic reduction experiments have been carried out in a single compartment electrochemical reactor with a mono-polar connection. All the combinative experiments were conducted in an argon atmospheric glove box, and each applied potential-current value was synchronously measured and analyzed in aspects of the preferentially recovering probability of lithium in mixed phases. The effect of the electrode surface area on the current was also observed. Based on our experimental results compared with electrochemical thermodynamic evaluation, it is revealed that Li₂O can be preferentially reduced to lithium by controlled LiCl concentration and applied potential.     

Phase diagram for the ternary system LiClCaCl2CaCrO4

The phase diagram for the system LiClCaCl₂CaCrO₄ has been studied using differential thermal analysis. LiClCaCl₂CaCrO₄ has been shown by X-ray diffraction to be a stable, diagonal section of the Li, Ca//Cl, CrO₄ reciprocal ternary system. The three binary systems are: LiClCaCl₂ which exhibits a double salt (LiCaCl₃), which decomposes without melting at 439°C and a eutectic at 36.3 mole % CaCl₂ (m.p. 487°C); CaCl₂CaCrO₄ which shows a eutectic at 23.4 mole % CaCrO₄ (m.p. 660°C); and LiClCaCrO₄ with a eutectic at 14.3 mole % CaCrO₄ (m.p. 538°C).In the ternary system, a eutectic exists at 63.2 mole % LiCl32.9% CaCl₂3.9% CaCrO₄ (m.p. 479°C). In addition, a four-phase equilibrium, involving all solid phases, exists at nearly all compositions at 435°C.Isotherms are shown for the liquidus surface (primary crystallization) and for the secondary crystallization surface. Isothermal and vertical sections through the ternary phase diagram are shown.     

Thermokinetic characteristics of lithium chloride

The characterisation of the ionic compound of lithium chloride, LiCl, through XRD, SEM, DSC, TG, DTG and TG-MS analysis is reported. The results show that nominally anhydrous LiCl particles can readily absorb water from the ambient atmosphere to form a surface layer of lithium chloride mono-hydrate, LiCl·H₂O. Solid surface-hydrated LiCl is de-dehydrated via a two-stage mechanism at low heating rates and via a single-stage mechanism at high heating rates. Molten LiCl exhibits substantial evaporation at temperatures below its nominal boiling point, with the rate of evaporation increasing significantly before complete evaporation occurs. The melting process of de-hydrated LiCl is marginally affected by the heating rate; whilst the evaporation process is strongly affected by the heating rate and also dependent on the quantity of material used and the flow rate of the gas passed over it. Heating of surface-hydrated LiCl up to the point of evaporation under a flow of argon and under a flow of ambient air gives identical results, proposing the possibility of performing LiCl-based processes in an air environment. The enthalpies and activation energies for the processes of surface de-hydration, melting, and high-temperature evaporation are determined. The results are consistent with the following thermal phase evolution:

$ [{\text{LiCl + LiCl}} \cdot {\text{H}}_{{\text{2}}} {\text{O}}]_{{{\text{solid}}}} \to [{\text{LiCl}}]_{{{\text{solid}}}} \to [{\text{LiCl}}]_{{{\text{liquid}}}} \mathop\rightarrow\limits^{{{{\text{H}}_{{\text{2}}} {\text{O}} \downarrow {\text{ HCl}} \uparrow}}}[{\text{LiCl-LiOH}}]_{{{\text{liquid}}}} \mathop\rightarrow\limits^{{{{\text{H}}_{{\text{2}}} {\text{O}} \uparrow}}}[{\text{LiCl-Li}}_{{\text{2}}} {\text{O}}]_{{{\text{liquid}}}} \to {\text{Gas}} $

     

离子相互作用模型在HCl-LiCl-MgCl2-H2O体系20˚C时溶解度预测中的应用

Component solubility in HCl-LiCl-MgCl2-H2O system of high ionic strength at 20℃ was predicted by using the Pitzer＇s ion-interaction model. The results indicated that the model supplied a very good prediction of the component solubility of the system mentioned above. The values of parameters of β^0, β^1 and C^＊ of HCl, LiCl and MgCl2 were obtained from optimization of literature data, while those of θMN and ψMNX were calculated from a least-squares optimization procedure to couple activity coefficient with solubility data. According to the ion-interaction model, no additional parameters need to be determined for more complex systems. The study provided theoretical basis for the manufacture process, which was proposed by Gao and employed to extract LiCl and MgCl2·6H2O from salt lake brine.     

Kinetics and equilibria associated with the absorption and desorption of water and lithium chloride in an ethylene—vinyl alcohol copolymer

Sorption and desorption equilibria and kinetics for LiCl and H₂O in an ethylene—vinyl alcohol copolymer film containing 70 mole percent vinyl alcohol were investigated at 25°C. The swelling behavior of water in the polymer was characterized by vapor and liquid sorption experiments over a range of water activities. p]The effects of LiCl content on the water sorption kinetics and equilibria in the films are presented and discussed. The kinetics and mechanism of LiCl sorption have also been studied. The amount of salt sorbed into the polymeric films increases linearly with the salt concentration in the external aqueous solutions. Both the rate and the amount of sorbed water increase significantly as the LiCl content increases. p]The desorption of LiCl, previously sorbed into the polymer, was characterized for different salt loadings. The rate of fractional salt release is independent of LiCl concentration in the film. Initially, the salt release is controlled by the nearly constant-rate absorption of water. The salt release, at long times, lags behind the swelling-controlled water uptake, indicating that the salt release is not completely controlled by the water sorption and that diffusion in the swollen polymer matrix contributes significantly to the long term elution of LiCl. Independent thermal analysis experiments suggest the formation of a metal salt—poly(ethylene—vinyl alcohol) complex.     

Enhanced electrochemical reduction of rare earth oxides in simulated oxide fuel via co-reduction of NiO in Li2O–LiCl salt

Rare earth oxides in spent oxide fuel from nuclear plants have poor reducibility in the electrochemical reduction process due to their high oxygen affinity and thermodynamic stability. Here, we demonstrate that the extent of their reduction can be enhanced via co-reduction of NiO in a Li₂O–LiCl electrolyte for the electrochemical reduction of a simulated oxide fuel (simfuel). First, the electrochemical behaviors of Nd₂O₃, NiO, and Nd₂O₃–NiO were studied by cyclic voltammetry and voltage control electrolysis. Then, the electrochemical reduction of the simfuel containing UO₂ and rare earth oxides (Nd₂O₃, La₂O₃, and CeO₂) was conducted in molten LiCl salt with 1 wt.% Li₂O via the co-reduction of NiO. The extent of reduction of the rare earth oxides was found to be significantly improved.     

Electrochemical response of various metals to oxygen gas bubbling in molten LiCl–Li2O melt

The electrochemical response of several alloys (stainless steel 316, Hastelloy C276, Inconel 600, and tantalum) was investigated in molten LiCl–Li₂O (1 wt%) at 923 K while bubbling oxygen gas into the molten salt. Tafel and zero resistance ammeter (ZRA) electrochemical methods were used to measure electrochemical effects of oxidation processes at the surface of each alloy. The Tafel method required approximately 15 min and was, thus, applied only in intervals between periods of oxygen bubbling in the salt. ZRA measurements were made in real time, while the O₂ was actively being bubbled into the salt. This method recorded both open circuit potential of the alloy relative to a Ni/NiO reference electrode and current between the alloy and the galvanically coupled platinum plate that served as the counter electrode. Both open circuit potential and galvanic oxidation current started to increase at the initiation of oxygen flow. Based on the observed oxidation current trend, it was inferred that the metals in order of increasing resistance to oxidation in molten LiCl–Li₂O are as follows: tantalum < SS-316 < Inconel 600 < Haynes C276. Scanning electron microscopy images indicated formation of an oxide layer of thickness 560–3370 nm that correlates with the galvanic oxidation current measurements.

     

A new approach to the structure of concentrated aqueous electrolyte solutions using pulsed NMR methods

It is shown that selective measurements of the magnetic dipole—dipole interactions between specific pairs of nuclear spins in glasses of concentrated aqueous electrolytes can provide detailed structural information of relevance to the liquid. The validity of the approach is demonstrated by selective measurements of the inter- and intra-molecular (H₂O) proton—proton interactions in LiCl and LiBr solutions at 100 K. For LiCl,4H₂O, these measurements are consistent with a short range (≈0.8 nm) distorted sodium chloride structure with Cl^? and Li(H₂O)⁺₄ as basic units: each Cl^? ion is octahedrally coordinated to six OH bonds. For LiCl,RH₂O solutions in the composition range 4 <R < 10, the excess H₂O is packed interstitially at the interfaces between clusters of LiCl,4H₂O. This structure is consistent with various properties of the solution at room temperature. LiBr solutions have similar structures.     

Thermodynamic characteristics of double salts crystallizing in LiCl-RbCl-H<Subscript>2</Subscript>O system at 298.15 K

The Pitzer ion-interaction model has been used for calculations of thermodynamic characteristics of double salts 3RbCl · LiCl · 2H₂O and RbCl · 2LiCl · 4H₂O in the ternary system LiCl-RbCl-H₂O at 298.15 K. The standard molar Gibbs energy of formation of the two double salts from the corresponding simple salts LiCl · H₂O and RbCl, as well as the standard molar Gibbs energy of formation have been determined.     

Salt effects in the aqueous alkaline hydrolysis of N-hydroxyphthalimide. Kinetic evidence for the ion-pair formation

The effects of the concentrations of LiCl, NaCl, KCl, CsCl, Na₂CO₃, BaCl₂, and Me₄NCl on the rates of reactions of hydroxide ion with ionized N-hydroxyphthalimide (NHP) at 30°C and in a H₂O–MeCN solvent containing 98%, v/v, H₂O reveal a nonlinear increase in observed rate constants with increase in salt concentrations. The observed rate constants are highly sensitive to the valence state of cations and almost insensitive to the valence state of anions of the salts. These observations are explained in terms of ion-pair formation between cations and NHP^?.