Vanadium vanadates

Other enzymes such as the cyclic AMP-dependent protein kinase are stimulated by vanadium. Vanadate seems to inhibit most strongly those enzymes that form a phosphoenzyme intermediate. This inhibition may be diminished within cells because vanadate is readily reduced by glutathione and other intracellular reductants. The resulting vanadyl ion is a much weaker inhibitor and also stimulates several metabolic processes. ... 

SynoTiycis Vanadium 51 Vanadic anhydride divanadium penta-ox de divenadium pantoxide vanadium oxide vanadic acid vanadic acid anhydride Vanadium oxysulfate xysulfato vanadium vanadium oxide sulfate vanadium oxosulfate oxo (sulfet (2-)-0)-vanadium Vanadic acid monosodiutn salt ... 

Vanadium dioxide, VO2 is dark blue (V2O5 plus SO2) but is readily reduced further to Vo.i86-V,.6a. VO2 gives the (VO) ion with acids and vanadates(IV) with alkalis and as mixed metal oxides. 

Vanadium pentoxide, vanadium(V) oxide, V2O5, is the most important compound in this oxidation state. It is a coloured solid (colour due to charge transfer, p. 60), the colour varying somewhat (red -> brown) with the state of subdivision it is formed when vanadium (or some of its compounds) is completely oxidised, and also by heating ammonium vanadate)V) ... 

The colour sequence already described, for the reduction of van-adium(V) to vanadium(II) by zinc and acid, gives a very characteristic test for vanadium. Addition of a few drops of hydrogen peroxide to a vanadate V) gives a red colour (formation of a peroxo-complex) (cf. titanium, which gives an orange-yellow colour). 

The vanadium pentoxide catalyst Is prepared as follows Suspend 5 g. of pure ammonium vanadate in 50 ml. of water and add slowly 7 5 ml. of pure concentrated hydrochloric acid. Allow the reddish-brown, semi-colloidal precipitate to settle (preferably overnight), decant the supernatant solution, and wash the precipitate several times by decantation. Finally, suspend the precipitate in 76 ml. of water and allow it to stand for 3 days. This treatment renders the precipitate granular and easy to 6lter. Filter the precipitate with suction, wash it several times with cold 5 p>er cent, sodium chloride solution to remove hydrochloric acid. Dry the product at 120° for 12 hours, grind it in a mortar to a fine powder, and heat again at 120° for 12 hours. The yield of catalyst is about 3 - 5 g. 

Several forms of magnesium vanadates have been characterized. Some physical properties ate summarized in Table 27 (28—30) (see also Vanadium AND VANADIUM ADLOYS). 

The recovery of vanadium from these slags is of commercial interest because of the depletion of easily accessible ores and the comparatively low concentrations (ranging from less than 100 ppm to 500 ppm) of vanadium in natural deposits (147,148). In the LILCO appHcations the total ash contained up to 36% 20 (147). Vanadium is of value in the manufacture of high strength steels and specialized titanium alloys used in the aerospace industry (148,149). Magnesium vanadates allow the recovery of vanadium as a significant by-product of fuel use by electric utiUties (see Recycling, nonferrous LffiTALS). 

Magnesium vanadates, as vanadium compounds in general, are known irritants of the respiratory tract and conjunctiva. The threshold limit value (TLV) for vanadium compounds in air recommended by the National Institute of Occupational Safety and Health is 0.05 mg/m based on a typical 8-h workday and 40-h workweek (7,147). Chronic inhalation can lead to lung diseases such as bronchitis, bronchopneumonia, and lobar pneumonia. These dust-related effects can be avoided by use of individual respirators in areas where exposure is likely. 

In the calciaation process, a mixture of corresponding oxides and an optional modifier, eg, molybdic acid, are milled together to achieve a homogenous mixture. The mixture is calciaed at 750—950°C and milled to a desired particle size. Wet milling ia an alkaline medium is recommended to remove any unreacted vanadium salts that ate beheved to degrade the pigmentary properties of bismuth vanadate (39). 

The ammonium salts of vanadic acid and vanadium pentoxide have been Hsted as toxic constituents in soHd wastes under the Resource Conservation and Recovery Act (27). 

The chemistry of vanadium compounds is related to the oxidation state of the vanadium. Thus, V20 is acidic and weaMy basic, VO2 is basic and weaMy acidic, and V2O2 and VO are basic. Vanadium in an aqueous solution of vanadate salt occurs as the anion, eg, (VO ) or (V O ) , but in strongly acid solution, the cation (V02) prevails. Vanadium(IV) forms both oxyanions ((V O ) and oxycations (VCompounds of vanadium(III) and (II) in solution contain the hydrated ions [V(H20)g] and [V(H20)g], respectively. 

Coordination compounds of vanadium are mainly based on six coordination, in which vanadium has a pseudooctahedral stmcture. Coordination number four is typical of many vanadates. Coordination numbers five and eight also are known for vanadium compounds, but numbers less than four have not been reported. The coordination chemistry of vanadium has been extensively reviewed (8—12) (see Coordination compounds). 

Vanadium(IV) Oxide. Vanadium(IV) oxide (vanadium dioxide, VO2) is a blue-black solid, having a distorted mtile (Ti02) stmcture. It can be prepared from the reaction of V20 at the melting point with sulfur or carbonaceous reductants such as sugar or oxaUc acid. The dioxide slowly oxidizes in air. Vanadium dioxide dissolves in acids to give the stable (VO) " ions and in hot alkaUes to yield vanadate(IV) species, eg, (HV20 ) . 

The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidising conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of Hquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous siUcates, which often are present in the ore feed, yields HCl gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

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. 

Imports and Exports. The United States has long been a significant importer of vanadium slags, but imports of pentoxide were negligible until they rose quickly to 850 metric tons ia 1974, and 2000 t ia 1975 (mostly from the Repubhc of South Africa). Peatoxide imports thea decliaed to 1400 t ia 1980 with Finland being the maia and South Africa the minor suppHers. In recent years, U.S. imports of ammonium and potassium vanadates and of other vanadium compounds have been 100—200 t/yr, mainly from the U.K., Germany, and the RepubHc of South Africa. 

Conversion of fused pentoxide to alloy additives is by far the largest use of vanadium compounds. Air-dried pentoxide, ammonium vanadate, and some fused pentoxide, representing ca 10% of primary vanadium production, are used as such, purified, or converted to other forms for catalytic, chemical, ceramic, or specialty appHcations. The dominant single use of vanadium chemicals is in catalysts (see Catalysis). Much less is consumed in ceramics and electronic gear, which are the other significant uses (see Batteries). Many of the numerous uses reported in the Hterature are speculative, proposed. 

Minor uses of vanadium chemicals are preparation of vanadium metal from refined pentoxide or vanadium tetrachloride Hquid-phase organic oxidation reactions, eg, production of aniline black dyes for textile use and printing inks color modifiers in mercury-vapor lamps vanadyl fatty acids as driers in paints and varnish and ammonium or sodium vanadates as corrosion inhibitors in flue-gas scmbbers. 

Vanadium Carbide. Vanadium pentoxide [1314-62-17, V2O5, or vanadium trioxide [1314-34-7] VO3, are the most satisfactory oxides for the preparation of VC. Vanadium pentoxide is best prepared by igniting chemically pure ammonium vanadate [7803-55-6] NH VO, in the presence of moist oxygen to avoid reaction with nitrogen V2O3 is obtained by reduction of V2O3 with hydrogen (see Vanadium compounds). 

The active phase, which is soHd at room temperature, is comprised of mixed potassium and sodium vanadates and pyrosulfates, whereas the support is macroporous siUca, usually in the form of 6—12 mm diameter rings or pellets. The patent Hterature describes a number of ways to prepare the catalyst a typical example contains 7 wt % vanadium pentoxide, 8% potassium added as potassium hydroxide or carbonate, 1% sodium, and 78 wt % siUca, added as diatomaceous earth or siUca gel, formed into rings, and calcined in the presence of sulfur dioxide or sulfur trioxide to convert a portion of the alkah metal salts into various pyrosulfates (81,82). 

Vanadium-Sodium Compounds Most Corrosive. Physical property data for vanadates, phase diagrams, laboratory experiments, and numerous field investigations have shown that the sodium vanadates are the lowest melting compounds and are the most corrosive to metals and refractories. These compounds are thought to form by either the vapor phase reaction of NaCI and V2O5 or by the combination of fine droplets of these materials upon the cooler parts of combustion equipment. 

Chemical Designations - Synonyms Vanadic anhydride. Vanadium pentaoxide Chemical Formula ... 

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... 

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