Alkali metavanadates

Metavanadates, R VOs or R 20.V205.—These salts are more stable than either the ortho- or pyro-vanadates. Solutions of the latter yield metavanadates on being evaporated or by treatment with carbon dioxide. The alkali metavanadates are prepared directly by dissolving vanadium pentoxide in the calculated quantity of alkali hydroxide. The metavanadates of other metals are prepared by fusing vanadium pentoxide with the oxide or carbonate of the metal in calculated quantity,11 or by the action of a soluble salt of the metal... 

The reaction may be assisted mechanically by mixing with kieselgiihr, or other finely divided material, and catalytically by the addition of iodine, or alkali metavanadate (U.S.P., 1915925) in the case of benzene, and of boric acid, mercury and mercury salts in the case of anthraquinone. 

Vanado-vanadates can also be obtained by fusing alkali vanadites with metavanadates, when no oxidation or loss of oxygen takes place. This process has furnished two salts,1 Na20.2V02.Va06 and K20. 2V02.V205. 

Metavanadates of the alkalis are white or colourless, and give colourless aqueous solutions which rapidly become yellow, and, on addition of acids, red or orange. These coloured solutions contain polyvanadates, the formation of which is comparable to that of the polychromates and other salts formed by condensation of weakly acid oxides of metals, e.g. molybdates and borates. Thus, under definite conditions of temperature and concentration, potassium metavanadate is converted into the acid salt 2K20.3Va05, in accordance with the equation ... 

Aluminium Metavanadate, AI(V03)3, may be prepared eleetro-lytieally or by the addition of an aluminium salt in solution to an alkali vanadate.2... 

Beryllium Metavanadate, Be(V03)2.4H20.—Addition of solutions of beryllium salts to alkali vanadates gives rise to basic vanadates of indefinite composition. The pure salt is obtained by boiling beryllium hydroxide and vanadium pentoxide in water in the required proportions. The solution is filtered, concentrated to a syrup and poured into alcohol, whereupon isometric cubes, modified by an octahedron, are obtained. The larger crystals polarise light. Density, 2-273. One gram dissolves in a litre of water at 25° C.s... 

Derivation (1) Alkali or acid extraction from vanadium minerals. (2) By igniting ammonium metavanadate. (3) From concentrated ferrophosphorus slag by roasting with sodium chloride, leaching with water and purification by solvent extraction followed by precipitation and heating. 

Method of purification Alkali solution, precipitation as ammonium metavanadate and ignition to... 

The alkali vanadates are soluble, NH4VO3 least (a non-cycUc metavanadate quantitatively precipitated by NH4CI with ethanol). Many d- or p-block vanadates... 

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). 

---