Molten Salt Compositions

One of the many variations involves bubbling fluorine through fuel molten salt compositions to selectively remove uranium and protactinium. The main problems arise from the chemical reactivity of fluorine that requires special handling (NBA 2012). 

CorrApp modelling of molten salt composition after 2 min contact between Fe and KCI/ZnCl2 salt at 320°C. 

Oxidation Step. A review of mechanistic studies of partial oxidation of propylene has appeared (58). The oxidation process flow sheet (Fig. 2) shows equipment and typical operating conditions. The reactors are of the fixed-bed shell-and-tube type (about 3—5 mlong and 2.5 cm in diameter) with a molten salt coolant on the shell side. The tubes are packed with catalyst, a small amount of inert material at the top serving as a preheater section for the feed gases. Vaporized propylene is mixed with steam and ak and fed to the first-stage reactor. The feed composition is typically 5—7% propylene, 10—30%... 

The principal use of AIF. is as a makeup ingredient in the molten cryoflte, Na.. AIF AI2O2, bath used in aluminum reduction cells in the HaH-Haroult process and in the electrolytic process for refining of aluminum metal in the Hoopes cell. A typical composition of the molten salt bath is 80—85%... 

Thermodynamic data show that the stabilities of the caesium chloride-metal chloride complexes are greater than the conesponding sodium and potassium compounds, and tire fluorides form complexes more readily tlrair the chlorides, in the solid state. It would seem that tire stabilities of these compounds would transfer into tire liquid state. In fact, it has been possible to account for the heats of formation of molten salt mixtures by the assumption that molten complex salts contain complex as well as simple anions, so tlrat tire heat of formation of the liquid mixtures is tire mole fraction weighted product of the pure components and the complex. For example, in the CsCl-ZrCU system the heat of formation is given on each side of tire complex compound composition, the mole fraction of the compound... 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

Molten salts are characterized by the formation of discrete complex ions that are subjected to coordination phenomenon. Such complex ions have specific compositions that are related to the rearrangement of their electronic configuration and to the formation of partially covalent bonds. The life time of the coordinated ions is longer than the contact period of the individual ions [293]. 

Molten salt investigation methods can be divided into two classes thermodynamic and kinetic. In some cases, the analysis of melting diagrams and isotherms of physical-chemical properties such as density, surface tension, viscosity and electroconductivity enables the determination of the ionic composition of the melt. Direct investigation of the complex structure is performed using spectral methods [294]. 

Because of the interest in its use in elevated-temperature molten salt electrolyte batteries, one of the first binary alloy systems studied in detail was the lithium-aluminium system. As shown in Fig. 1, the potential-composition behavior shows a long plateau between the lithium-saturated terminal solid solution and the intermediate P phase "LiAl", and a shorter one between the composition limits of the P and y phases, as well as composition-dependent values in the single-phase regions [35], This is as expected for a binary system with complete equilibrium. The potential of the first plateau varies linearly with temperature, as shown in Fig. 2. 

The lithium-silicon system has also been of interest for use in the negative electrodes of elevated-temperature molten salt electrolyte lithium batteries. A composition containing 44 wt.% Li, where Li/Si=3.18, has been used in commercial... 

The composition of the electrolyte is quite important in controlling the electrolytic deposition of the pertinent metal, the chemical interaction of the deposit with the electrolyte, and the electrical conductivity of the electrolyte. In the case of molten salts, the solvent cations and the solvent anions influence the electrodeposition process through the formation of complexes. The stability of these complexes determines the extent of the reversibility of the overall electroreduction process and, hence, the type of the deposit formed. By selecting a suitable mixture of solvent cations to produce a chemically stable solution with strong solute cation-anion interactions, it is possible to optimize the stability of the complexes so as to obtain the best deposition kinetics. In the case of refractory and reactive metals, the presence of a reasonably stable complex is necessary in order to yield a coherent deposition rather than a dendritic type of deposition. 

The experiments were conducted batchwise in small stainless-steel pipe-bombs immersed in a molten-salt bath that was maintained at a desired, constant temperature. Pipe-bomb heat-up and quench times, on the order of 1 min each, were negligible compared with reaction times, which were on the order of 1 hr. The reagents used were obtained commercially all were of purity > 98% except for the A2-dialin which had a composition of ( 0)> 0 0 9 Q)) = 9 20, 64) mol%. The proportions of sub-... 

Relatively little attention has been devoted to the direct electrodeposition of transition metal-aluminum alloys in spite of the fact that isothermal electrodeposition leads to coatings with very uniform composition and structure and that the deposition current gives a direct measure of the deposition rate. Unfortunately, neither aluminum nor its alloys can be electrodeposited from aqueous solutions because hydrogen is evolved before aluminum is plated. Thus, it is necessary to employ nonaqueous solvents (both molecular and ionic) for this purpose. Among the solvents that have been used successfully to electrodeposit aluminum and its transition metal alloys are the chloroaluminate molten salts, which consist of inorganic or organic chloride salts combined with anhydrous aluminum chloride. An introduction to the chemical, electrochemical, and physical properties of the most commonly used chloroaluminate melts is given below. 

Reciprocal molten salt systems are those containing at least two cations and two anions. We shall deal with the simplest member of this class, that containing the ions A+, B+, X-, and Y-. The four constituents of the solution, AX, BX, AY, and BY, will be designated by 1, 2, 3, and 4 respectively. There are four ions in the system and one restriction of electroneutrality. Consequently, of the four constituents, there are only three which are independent components. In order to calculate the Helmholtz free energy of mixing conveniently, we must (arbitrarily) choose the three components. Here we choose BX, AY, and BY. This choice requires that in order to make mixtures of some compositions a negative quantity of BY must be used. This presents no difficulty in the theory and is thermodynamically self-consistent. One mole of some arbitrary composition (XA, XB, Yx, XY) can be made by mixing Arx moles of BX (component 2), XA moles of AY (component 3), and (XY — XA) moles of BY (component 4). ... 

So from electrical data, it is possible to get information on partial thermodynamic functions of the salt and then develop thermodynamic models for quantitative interpretation of the conductivity variation with composition. These models are not very different from those already developed for molten salt mixtures or metallic alloys. 

---