Polyvanadates

For direct precipitation of vanadium from the salt-roast leach Hquor, acidulation to ca pH 1 without the addition of ammonia salts yields an impure vanadic acid when ammonium salts are added, ammonium polyvanadate precipitates. The impure vanadic acid ordinarily is redissolved in sodium carbonate solution, and ammonium metavanadate precipitates upon addition of ammonium salts. Fusion of the directly precipitated ammonium salts can yield high purity V20 for the chemical industry. Amine solvent extraction is sometimes used to recover 1—3 g/L of residual V20 from the directly precipitated tail Hquors. 

The usual extraction procedure is to roast the crushed ore, or vanadium residue, with NaCl or Na2C03 at 850°C. This produces sodium vanadate, NaV03, which is leached out with water. Acidification with sulfuric acid to pH 2-3 precipitates red cake , a polyvanadate which, on fusing at 700°C, gives a black, technical grade vanadium pentoxide. Reduction is then necessary to obtain the metal, but, since about 80% of vanadium produced is used as an additive to steel, it is usual to effect the reduction in an electric furnace in the presence of iron or iron ore to produce ferrovanadium, which can then be used without further refinement. Carbon was formerly used as the reductant, but it is difficult to avoid the formation of an intractable carbide, and so it has been superseded by aluminium or, more commonly, ferrosilicon (p. 330) in which case lime is also added to remove the silica as a slag of calcium silicate. If pure vanadium metal is required it can... 

Certain vertebrates have an astonishing ability to accumulate vanadium in their blood. For example, the ascidian seaworm Phallusia mammilata has a blood concentration of V up to 1900 ppm, which represents more than a millionfold concentration with respect to the sea-water in which it lives. The related organism Ascidia nigra has an even more spectacular accumulation with concentrations up to 1.45% V (i.e. 14 500 ppm) in its blood cells, which also contain considerable concentrations of sulfuric acid (pH 0). One possibility that has been mooted is that the ascidia accumulates vanadate and polyvanadate ions in mistake for phosphate and polyphosphates (p. 528). 

The equilibria among the species are dependent on the pH of the solution in much the same way as the equilibria among the various phosphate species (see Chapter 14). Not only is the formation of polyvanadates reminiscent of phosphorus chemistry, so is the fact that vanadium forms oxyhalides such as OVX3 and V02X. 

Vanadium is stripped from the organic solution with 1.5 mol dm sulfuric acid to a concentration of about 50gdm V. After an iron removal step with strong sulfuric acid, the organic solution is washed with water and recycled. From the vanadium strip liquor, ammonium polyvanadate (APV) is precipitated by oxidation and addition of ammonia. The APV slurry is thickened and pumped to a vacuum belt filter, where the APV cake is carefully washed with fresh water. The APV filter cake is dried and then calcined to vanadium pentoxide. 

Vanadium usually is recovered from its ores by one of two processes, (1) leaching raw mineral with hot dilute sulfuric acid, and (2) roasting ore with common salt to convert vanadium into water soluble sodium vanadates. In the sulfuric acid leaching process, vanadium is extracted from acid leach liquors by solvent extraction with an aliphatic amine or an alkyl phosphoric acid in kerosene. The organic solvent extract then is treated with an aqueous solution of ammonia in the presence of ammonium chloride to convert vanadium into ammonium metavanadate. Alternatively, the organic extract is treated with dilute sulfuric acid or an aqueous solution of soda ash under controlled conditions of pH. Vanadium is precipitated from this solution as a red cake of sodium polyvanadate. 

Alternatively, ore is roasted with common salt and the residue leached with water or sodium carbonate solution. To this aqueous solution of sodium vanadates, sulfuric acid is added and pH is adjusted between 2 and 3. Vanadate precipitates as a red cake of sodium polyvanadate. 

The sodium polyvanadate obtained above by either method is decomposed thermally at 700°C producing a melt of vanadium pentoxide, V2O5. Pentoxide obtained at this stage is in impure form. Purified vanadium pentoxide is obtained by dissolving the red cake in sodium carbonate solution to precipitate ammonium metavanadate. The metavanadate is decomposed at 320 to 430°C to form highly purified vanadium pentoxide. 

Vanadium pentoxide is an intermediate in recovering vanadium from minerals (See Vanadium). Sodium polyvanadate, obtained as a red cake in one of the steps in extracting vanadium from its ores is calcined at 700°C in air to form a melt of vanadium pentoxide. Pentoxide is prepared in purified form by dissolving red cake in sodium carbonate solution followed by addition of an aqueous solution of ammonia and ammonium chloride. Ammonium metavanadate is precipitated which on decomposition at 320 to 430°C forms vanadium pentoxide. 

For example, the oxidative dehydrogenation of ethane and propane was examined via UV-visible and Raman spectra. The study investigated the catalytic properties vanadia formulations that possessed a range of VO surface species density (1.4—34.2 V/nm ) on an AI2O3 support. The observations showed increased surface densities, greater than 2.3 V/nm , favored two-dimensional polyvanadates. At lower surface densities, ca. 2.3 V/nm , predominately isolated monovanadate species were observed. Further increasing surface densities to more than 7.0 V/nm yielded V2O5 crystallites. ... 

NaV03 is leached out with water. Then the solution is treated with sulfuric acid to precipitate a red polyvanadate compound, which is fused to V2O5. Reduction of the V2O5 with Ca or A1 gives the pure metal. 

With some ions the extent of polymer formation is high, even in acidic solution, e.g. vanadium(V) can exist as a polyvanadate ion, Vio0286. ... 

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

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 ... 

Aminonium Polyvanadates.—The compound (NH4)20.3V205 appears to be the most stable of the arid ammonium vanadates. It is formed by addition of a 10 per cent, solution of acetic acid to a solution of ammonium metavanadate, or by boiling an aqueous solution of any other ammonium polyvanadate. It forms thin, transparent, yellow, octagonal plates, or golden, microscopic, rhombic plates which become temporarily vermilion on being heated, and which are only slightly soluble in water. It has also been obtained combined with two, five, or six molecules of water, according to the temperature of crystallisation or extent of dehydration.2... 

Beryllium Polyvanadate, 2Be0.3V205.IlH20, forms dark red, hygroscopic crystals which are obtained by the action of beryllium sulphate on barium hexavanadate.10... 

Cadmium Polyvanadates.—Two are known, 2Cd0.3V20s.15H20 and Cd0.3Vs08.2HgO. The former is obtained by the action of cadmium sulphate on the corresponding barium salt,1 and the latter by the action of cadmium nitrate on ammonium metavanadate in the presence of acetic acid.2... 

Cerous Polyvanadate, Ce203.5V20s.27H20, has been prepared by evaporating a solution containing ammonium metavanadate and cerous sulphate. ... 

Cobalt Polyvanadate, 2Co0.3V2Os.15H20, forms large, six-sided brown leaves which effloresce readily in air. It is obtained by the action of cobalt sulphate on the corresponding barium salt.6... 

Copper Polyvanadate, 3Cu0.5Va08.22H20, is prepared by the action of copper sulphate on barium hexavanadate.7 It forms thin, iridescent spangles. 

Didymium Polyvanadate, Di 203.5V205.28H20, is obtained by mixing solutions of didymium nitrate and the acid sodium salt, Naa0.2Va0g. ... 

Gadolinium Polyvanadate, Gd20s.5V205.26H20,9 and Lanthanum Vanadate10 are known. 

Lead Polyvanadate, Pb0.2Va06, can be obtained as yellow, transparent needles by fusing a mixture of vanadium pentoxide, sodium iodide, and lead iodide.11... 

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