Ammonium 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. 

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. 

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... 

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. 

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. 

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. ... 

Two metastable aqueous polyvanadates have been identified. Acidification of neutral vanadate solutions yields a solution with the expected V and O NMR spectra of the decavanadate anion and with additional resonances (two V, 1 12 three O, 1 1 1) that decay with a half-life of 80 min at 25 °C. The spectra are consistent with either a protonated a-Keggin anion or its e-isomer, [II12V13O40]3, 8 9 Reaction of V02+ with 50% H202 and subsequent treatment with tetramethyl-ammonium ion leads to black (NMe -jI I6[V, 5O4J in 36% yield. The anion has a trans bicapped Keggin structure analogous to that of [PV 4042]9 (see below) and has the expected four-line 51V NMR spectrum (1 2 8 4). At pH 3.5, solutions of the anion are stable for a week at room temperature, and exhibit an initial reduction potential of 0.23 V vs. Ag/AgCl. Acidification leads irreversibly to VO2+.X0... 

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