Molten salts redox process

Solution processes for removal of SO2 from effluent gas streams normally require lower absorption temperatures than do gas phase techniques. Thus, of the four listed in Table IV only the Molten Salt Method has the capability of accepting high temperature flue gas without cooling. In all but one of the cases, elemental sulphur is the end product and in the single instance of the Ammoniacal Solution Process, sulphur is a coproduct with ammonium sulphate. This process has been extensively examined and developed in a number of countries, and is chemically interesting because of the unusual redox reaction that is suspected to take place between the products of air oxidation of SO2 absorbed in ammonia solution. Both products, sulphur and ammonium sulphate, are normally saleable commodities. 

A positive standard cell potential tells you that the cathode is at a higher potential than the anode, and the reaction is therefore spontaneous. What do you do with a cell that has a negative " gii Electrochemical cells that rely on such nonspontaneous reactions cire called electrolytic cells. The redox reactions in electroljdic cells rely on a process called electrolysis. These reactions require that a current be passed through the solution, forcing it to split into components that then fuel the redox reaction. Such cells are created by applying a current source, such as a battery, to electrodes placed in a solution of molten salt, or salt heated until it melts. This splits the ions that make up the salt. 

Among the pentavalent elements, the most important are niobium and tantalum. Niobium is an excellent material for surface treatment of steel materials for chemical industry due to its high hardness and corrosion-resistance in wet acidic conditions. Nowadays, niobium is also used for the preparation of superconductor tapes and it is used in other branches of industry, for instance in nuclear technology and metallurgy. Tantalum is also of similar importance. For these applications, it is necessary to prepare high purity metal. Molten salt electrolysis, as an alternative process to classical thermal reduction, provides niobium and tantalum with required quality. In order to optimize these processes, it is necessary to know details of both complex formation and redox chemistry of the species present in the melts. 

A number of unique difficulties pertain to oxidation states of metal ions encountered in molten salt solutions. For example, for first-row transition metals, the highest oxidation state prevailing is often +3, as in the case of Fe and Cr. Frequently, for chlorides in particular, the +3 state compounds are volatile at suitable operating temperatures and, hence, their solutions are thermally unstable.Other problems encountered include rapidly dispropor-tionating states, the formation of oxyhalides, and precipitation of complexes by reaction with the melt. While redox reactions per se involve very fast charge transfer steps, these may occur at the extremes of the range of electrochemical stability, thus leading to concomitant solvent melt decomposition. Nevertheless, suitable processes such as Fe /Fe on vitreous carbon in chloride melts can be employed to determine the effective electrochemical areas of electrodes where diffusion coefficients are accurately known. ... 

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