Solution-processed metal oxides vanadium oxide

Several liquid redox processes have been developed for smaller-scale (ca. 0.25-20 t sulfur day ) applications, achieving 99.9-t- % recovery. For example, SuIFerox and ARI-LO-CAT use complexed Fe species to oxidize absorbed H2S in acidic solutions, and Stretford, Unisulf, and Sulfolin use vanadium (V) in slightly alkaline solutions as the oxidant for absorbed HS ions. Atmospheric oxygen is the ultimate oxidant in all such processes, as it reoxidizes the reduced form of the dissolved metal species, the concentrations of which are not so limited as that of dissolved oxygen ... 

The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex... 

Figures 3 and 4 show diagrams for the transition metals vanadium and chromium and for the nonmetal nitrogen, all in l.OM aqueous acid solution. The slopes of lines joining redox couples represent the potentials for the halfreactions in question, a positive slope representing a reduction potential and a negative slope an oxidation potential. An intermediate state above tie lines joining higher and lower states will be unstable to disproportionation [e.g., Cr(V) and NO2] while species below such tie lines are stable [e.g., Cr(III) and N2]. The diagrams are a convenient way to postulate the feasibility of various possible one-and two-electron pathways in multistep redox processes involving a particular element.

In commercial units today, it is preferred to use the alkali metal metavanadates, particularly potassium metavanadate (KVO3). If de.sired, vanadium pentoxide (V2O5) can be added directly to the potassium carbonate solution, forming potassium metavanadate by reaction with potassium carbonate. For the corrosion inhibitor to be effective, the plant must be designed so that exposed carbon steel surfaces are wetted by the inhibited solution. In order to maintain the metavanadate in its fully oxidized (pentavalent) state, the licensors of the Benfield process recommend that potassium nitrite be injected into the solution on a carefully controlled basis (Sorell, 1990). 

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