Reactions oxidation-reduction, molten salt

In general case, the reactions with the molten salt solvent should also be taken into consideration. One equilibrium or more, depending on the number of possible products of reduction (or oxidation) reactions with the solvent, should be added to the basic system (2.1). 

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

A production process has evolved from this original work, and is presently used for extracting americium from kilogram amounts of plutonium metal. This process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and the molten plutonium phase. The chemistry of this process is indicated by the following reactions ... 

Fig. 9.25. (a) Tafel plots of numerous reactions reduction of benzene, oxidation of Cu in solid electrolyte, reduction of NOj in molten salt, reduction of Fe CNJg,... 

Excision reactions are sometimes accompanied by redox chemistry. For example, dissolution of the 2D solid Na4Zr6BeCli6 in acetonitrile in the presence of an alkylammonium chloride salt results in simultaneous reduction of the cluster cores (144). Here, the oxidation product remains unidentified, but is presumably the solvent itself. As a means of preventing such redox activity, Hughbanks (6) developed the use of some room temperature molten salts as excision media, specifically with application to centered zirconium-halide cluster phases. A number of these solids have been shown to dissolve in l-ethyl-2-methylimidazolium chloride-aluminum chloride ionic liquids, providing an efficient route to molecular clusters with a full compliments of terminal chloride ligands. Such molten salts are also well suited for electrochemical studies. 

In acetonitrile electrochemical reduction gives iron(I) and iron(O) spedes.557b In this solvent ligand oxidation reactions are almost absent, but they are not completely avoided due to the presence of small amounts of water.558 This problem was overcome by using a totally anhydrous low temperature (25 °C) molten salt comprising aluminum chloride and ethylpyridinium bromide (2 1 mole ratio). In this medium, reversible one-electron electrochemical oxidations take place559 with [Fe(37a)3]2+, [Fe(37b)3]2+ and a number of related complexes.560 Here the iron(III) form is thermodynamically more stable than is iron(II), whereas in aqueous solutions the reverse is true. 

Voltaic cells produce electric current by means of oxidation and reduction reactions occurring at different locations. A complete circuit is necessary, including a wire to transport the electrons and a path for ions to migrate to maintain charge neutrality at each electrode. To allow for ion mobility, the reactants must be in a solution or a molten salt. (Section 17.1)... 

Robie and Hemingway [95ROB/HEM] used their accurate heat capacity measurements, combined with results from molten salt calorimetry, thermal decomposition of the Ni2Si04-olivine into its constituent oxides, and equilibrium studies, both by CO reduction and solid state electrochemical cell measurement for Reaction (V.121) [87NE1], and calculated the following standard molar enthalpy of formation of Ni2Si04-olivine (liebenbergite) A,// (298.15 K) = - (1396.5 3.0) kJ mol. ... 

In Chapter VI we defined oxidation as the loss of electrons and reduction as the gain of electrons, and we showed that oxidation-reduction reactions that involve ions can generally be made to produce an electric current. Chemical energy is thereby transformed into electrical energy, and the electromotive force of the cell is a measure of the free energy of the reaction. Conversely, we can make an electric current produce a chemical reaction. If a current is passed through an electrolyte—a conducting solution or molten salt—oxidation takes place at the anode, where electrons are withdrawn from the solution and reduction takes place simultaneously at the cathode, where the electrons enter. 

As the molten salt is electrolytic. Hot Corrosion processes involve electrochemical reactions like oxidation of the metal and reduction of melt components and dissolved gases. Hence, many of investigations of Hot Corrosion have been done by electrochemical techniques, mostly combined with conventional corrosion... 

SECTION 20.9 An electrolysis reaction, which is carried out in an electrolytic cell, employs an external source of electricity to drive a nonspontaneous electrochemical reaction. The current-carrying medium within an electrolytic ceU may be either a molten salt or an electrolyte solution. The products of electrolysis can generaUy be predicted by comparing the reduction potentials associated with possible oxidation and reduction processes. The electrodes in an electrolytic ceU can be active, meaning that the electrode can be involved in the electrolysis reaction. Active electrodes are important in electroplating and in metaUuigical processes. 

Apart from the most electropositive metals, most other metals extracted through molten salt routes are recovered as solids these include many important refractory and other transition metals, the lanthanides, and some actinides. Particularly interesting problems arise in the electrowinning of the refractory metals. Attempts to deposit these metals in a coherent, massive form of theoretical density usually meet with a number of difficulties. Deposits may be dendritic, for example, if electrodeposition proceeds under mass transfer control, or they may be powdery and nonadherent if secondary reactions, such as alkali metal deposition, followed by backreaction with the solute, occurs. Moreover, powdery deposits may also arise if low oxidation states, formed as intermediates during the reduction process, disproportionate in the metal-melt interphase. Charge-transfer-controlled electrodeposition or coupled chemical steps appear to be a prerequisite for obtaining dense, coherent, and adherent deposits. Such deposits have been obtained... 

An electrolytic cell consists of two electrodes in a molten salt or a solution. A battery or some other source of direct electrical current acts as an electron pump, pushing electrons into one electrode and pulling them from the other. Just as in voltaic cells, the electrode at which the reduction occurs is called the cathode, and the electrode at which oxidation occurs is called the anode. In the electrolysis of molten NaCl, shown in Figure 20.28 , Na ions pick up electrons and are reduced to Na at the cathode. As the Na ions near the cathode are depleted, additional Na ions migrate in. Similarly, there is net movement of Cl ions to the anode, where they are oxidized. The electrode reactions for the electrolysis of molten NaCl are summarized as follows ... 

It was mentioned in Section 6-9 that in the period from 1884 to 1887 Svante Arrhenius developed the theory that electrolytes (salts, acids, bases) in aqueous solution are dissociated into electrically charged atoms or groups of atoms, called cations and anions. The present chapter is devoted in part to the phenomena involved in the interaction of molten salts and ionic solutions with an electric current. It is found that the electron reactions that take place at electrodes can be described as involving oxidation or reduction of atoms or groups of atoms, and that the chemical reactions called oxidation-reduction reactions (sometimes shortened to redox reactions) can often be conveniently described in terms of two electrode reactions. 

Electrochemical cells can also be used to attempt to obtain data on the mechanisms of the salt-induced corrosion processes. Cyclic voltammetry has been used [78-22] to obtain information on the oxidation and reduction reactions that may occur during molten salt corrosion. Chronopotentio-metric investigations with platinum as the working electrode in cells can also be used to determine and control the compositions of molten salts, as well as to measure the solubilities of various oxidation products in melts [25-28,30]. 

The complexing role of fluoride ions in the reduction of refractory metals is now well known for the metal recovery [12]. The technology of extracting tantalum in molten salts is based on the formation of K2TaF7, obtained by reaction of HF on the oxide Ta205 extracted from raw materials [14] before the reduction of this compound by sodium in the liquid phase ... 

An electrolytic cell uses electrical energy to drive a nonspontaneous reaction. Oxidation occurs at the anode and reduction at the cathode, but the direction of electron flow and the charges of the electrodes are opposite those in voltaic cells. In electrolysis of a pure molten salt, the metal cation is reduced at the cathode, and the nonmetal anion is oxidized at the anode. 

---