Molten salts reactive

Fused Salt Electrolysis. Only light RE metals (La to Nd) can be produced by molten salt electrolysis because these have a relatively low melting point compared to those of medium and heavy RE metals. Deposition of an alloy with another metal, Zn for example, is an alternative. The feed is a mixture of anhydrous RE chlorides and fluorides. The materials from which the electrolysis cell is constmcted are of great importance because of the high reactivity of the rare-earth metals. Molybdenum, tungsten, tantalum, or alternatively iron with ceramic or graphite linings are used as cmcible materials. Carbon is frequently used as an anode material. 

Each type of metallic coating process has some sort of hazard, whether it is thermal energy, the reactivity of molten salt or metal baths, particulates in the air from spray processes, poisonous gases from pack cementation and diffusion, or electrical hazards associated with arc spray or ion implantation. 

Friedel-Crafts acylation reactions usually involve the interaction of an aromatic compound with an acyl halide or anhydride in the presence of a catalyst, to form a carbon-carbon bond [74, 75]. As the product of an acylation reaction is less reactive than its starting material, monoacylation usually occurs. The catalyst in the reaction is not a true catalyst, as it is often (but not always) required in stoichiometric quantities. For Friedel-Crafts acylation reactions in chloroaluminate(III) ionic liquids or molten salts, the ketone product of an acylation reaction forms a strong complex with the ionic liquid, and separation of the product from the ionic liquid can be extremely difficult. The products are usually isolated by quenching the ionic liquid in water. Current research is moving towards finding genuine catalysts for this reaction, some of which are described in this section. 

The very chemically reactive plutonium hydride is usually decomposed in a vacuum-tight furnace capable of attaining a temperature of 700°C. Plutonium hydride that is decomposed under vacuum at temperatures below 400°C forms a very fine (<20y) metallic powder above 500°C the powder begins to sinter into a porous frit which melts at 640°C to form a consolidated metal ingot. This metal typically contains significant oxide slag but is suitable for feed to either molten salt extraction or electrorefining. 

The alkali metals are the most violently reactive of all the metals. They are too easily oxidized to be found in the free state in nature and cannot be extracted from their compounds by ordinary chemical reducing agents. The pure metals are obtained by electrolysis of their molten salts, as in the electrolytic Downs process (Section 12.13) or, in the case of potassium, by exposing molten potassium chloride to sodium vapor ... 

Salt-inclusion solids described herein were synthesized at high temperature (>500°C) in the presence of reactive alkali and alkaline-earth metal halide salt media. For single crystal growth, an extra amount of molten salt is used, typically 3 5 times by weight of oxides. The reaction mixtures were placed in a carbon-coated silica ampoule, which was then sealed under vacuum. The reaction temperature was typically set at 100-150 °C above the melting point of employed salt. As shown in the schematic drawing in Fig. 16.2, the corresponding metal oxides were first dissolved conceivably via decomposition because of cor-... 

It has been pointed out that metals residing below the position held by manganese (and, therefore, much below hydrogen) in the electrochemical series (Table 6.11) cannot be electrodeposited from aqueous solutions of their salts. These metals are called base metals or reactive metals and can be electrodeposited only from nonaqueous electrolytes such as solutions in organic solvents and molten salts. As with an aqueous electrolyte, there is a minimum voltage which is required to bring about the electrolysis of a molten salt. 

Although the electrolysis of molten salts does not in principle differ from that of aqueous solutions, additional complications are encountered here owing to the problems related to the higher temperatures of operation, the resultant high reactivities of the components, the thermoelectric forces, and the stability of the deposited metals in the molten electrolyte. As a result of this, processes taking place in the melts and at the electrodes cannot be controlled to the same extent as in aqueous or other types of solutions. Considerations pertaining to Faraday s laws have indicated that it would be difficult to prove their applicability to the electrolysis of molten salts, since the current efficiencies obtained are generally too small in such cases. 

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. 

Electrolytic cells are constructed of materials that can withstand the action of the electrolytes and of the electrode products. The cell may be of the open type or may be partially or fully closed, depending on the requirement of handling the electrode products. Some of these cells will be described while dealing with the production of specific metals. Very stringent requirements are imposed when considering the design of electrolytic cells for the deposition of refractory and reactive metals. Most of such metals are produced by using molten salt electrolytes. These metals are prone to atmospheric contamination at the electrolysis temperature, and it is thus necessary to operate the cell under an inert atmosphere. 

Electrorefining has been used for the purification of many common as well as reactive metals. It has been seen that the emf or the potential required for such a process is usually small because the energy needed for the reduction of the ionic species at the cathode is almost equal to that released by the oxidation of the crude metal at the anode. Some metals, such as copper, nickel, lead, silver, gold, etc., are refined by using aqueous electrolytes whereas molten salt electrolytes are necessary for the refining of reactive metals such as aluminum,... 

Recently, there has been considerable interest in developing molten salts that are less air and moisture sensitive. Melts such as l-methyl-3-butylimidazolium hexa-fluorophosphate [211], l-ethyl-3-methylimidazolium trifluoromethanesulfonate [212], and l-ethyl-3-methylimidazolium tetrafluoroborate [213] are reported to be hydro-phobic and stable under environmental conditions. In some cases, metal deposition from these electrolytes has been explored [214]. They possess a wide potential window and sufficient ionic conductivity to be considered for many electrochemical applications. Of course if one wishes to take advantage of their potential air stability, one loses the opportunity to work with the alkali and reactive metals. Further, since these ionic liquids are neutral and lack the adjustable Lewis acidity common to the chloroaluminates, the solubility of transition metal salts into these electrolytes may be limited. On a positive note, these electrolytes are significantly different from the chloroaluminates in that the supporting electrolyte is not intended to be electroactive. 

Recent developments for reactive C02 sorbents include sterically hindered amines such as 2-amino-2-methyl-l-propanol (AMP) and 1,8-p-methanediamine (MDA) and 2-piperidine ethanol (PE), which are claimed to have good reversible C02 capacity (Veawab et al., 1998) and low-temperature molten salts called ionic liquids (Bates et al., 2002). Ionic liquids are attractive due to their negligible vapor pressure up to their decomposition at... 

Protection of the molten metals from air and moisture The protection of the molten metals has always been an essential point. Fusion under vacuum or an inert atmosphere (pure He or Ar, possibly gettered) is systematically used. In the past, also for small scale laboratory preparations, fusion under a protective layer of molten non-reactive salts was often used. Low density salt mixtures having low-melting point and high-boiling point were generally employed (for instance eutectic mixtures of anhydrous stable alkali halides). 

Dense ionic fluids are not all that new if one examines the many applications of molten salt use in chemistry to date. A good deal of the work is in electrochemistry where the relatively high temperatures are less of a limitation but the relation between low-temperature molten salts and ionic fluids certainly exists. It would be wise neither to completely depend on nor to completely ignore all that has been learned with molten salts and molten salt chemistry. Some highly reactive, easily oxidized metals are readily purified in molten salt solvent systems without the problems with oxygen or the decomposition of water with release of hydrogen. 

Since chemical reduction means gain of electrons, electrolysis is the most direct way of recovering a metal from its ores, as long as these can be handled in a fluid state.6 Consideration of E° values for reactive metal halfcells such as Na+(aq)/Na(s), Mg2+(aq)/Mg(s), and Al3+(aq)/Al(s) (-2.71, -2.36, and -1.67 V, respectively) shows that these metals can never be obtained by electrolysis of aqueous solutions of their salts, as H2 would be produced instead, but they can often be obtained by electrolysis of suitable molten salts such as NaCl and MgCl2 ... 

In terms of disadvantages, most inorganic molten salts are liquid at temperatures considerably above room temperature, necessitating that provisions be made for heating and thermostatting the electrochemical cell. Many exhibit rather limited electrochemical windows. In addition, some melts are corrosive to the usual cell and electrode materials and reactive with atmospheric moisture. The former attribute... 

Between 1980 and about 2000 most of the studies on the electrodeposition in ionic liquids were performed in the first generation of ionic liquids, formerly called room-temperature molten salts or ambient temperature molten salts . These liquids are comparatively easy to synthesize from AICI3 and organic halides such as Tethyl-3-methylimidazolium chloride. Aluminum can be quite easily be electrode-posited in these liquids as well as many relatively noble elements such as silver, copper, palladium and others. Furthermore, technically important alloys such as Al-Mg, Al-Cr and others can be made by electrochemical means. The major disadvantage of these liquids is their extreme sensitivity to moisture which requires handling under a controlled inert gas atmosphere. Furthermore, A1 is relatively noble so that silicon, tantalum, lithium and other reactive elements cannot be deposited without A1 codeposition. Section 4.1 gives an introduction to electrodeposition in these first generation ionic liquids. 

It was John Wilkes who realized that room-temperature molten salts would only experience a widespread interest and uptake if they were stable under environmental conditions. Wilkes group published details of the first such liquid in 1992 using the BF]j" and the PFj anions, the latter showing a miscibility gap with water. Thus these liquids could, in principle, be made water free. (Today we know that ionic liquids containing BFJ and PF are subject to decomposition in the presence of water.) Electrochemical studies showed that even these early ionic liquids had wide electrochemical windows of about 4 V with cathodic limits of-2 to -2.5 V. vs. NHE. This cathodic limit should, from the thermodynamic point of view, be wide enough to electrodeposit many reactive elements. 

Both reactive and nonreactive molten salts can be used in nontopochemical routes. An example of a nontopochemical route to inorganic materials utilizing reactive molten salts is when a metallic element is reduced in a low-melting alkali metal polychalcogenide (hiQn, where Q = O, S, Se, Te) to form a ternary metal chalcogenide. Potassium bismuth sulfide (KBi3Ss) has been prepared in... 

Like fuel cells, batteries using molten salt electrolytes offer high performance. Molten salts have very high electrical conductivity, which permits the use of high current densities. Likewise, molten salts permit the use of highly reactive electrode materials, which cannot be used in aqueous electrolytes. For these reasons, batteries with molten salts offer very high specific energy (>100 Wh/kg). To... 

Electrowinning of metals in aqueous solutions is applicable to those metals that possess high electrochemical reduction potentials, such as silver, copper, cadmium, and zinc. Magnesium, aluminum, and sodium, like other reactive metals, are electro-produced from molten salt baths, such as NaCl/CaCh mixture at ca. 600 °C for sodium and MgCl2/NaCl/CaCl2 eutectic mixture at ca. 750 °C for magnesium. 

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