Tungsten Trioxide, WO₃

Tungsten Trioxide, Tungstic Anhydride, or "tungstic acid," WO₃, occurs naturally as tungstite, but it is usually obtained from the commoner ores, scheelite and wolframite. Scheelite is readily decomposed by means of hydrochloric acid or nitric acid, and on washing and igniting the residue, tungsten trioxide results. Wolframite is more resistant to acid attack, but by treating the finely powdered ore first with hydrochloric acid and then with aqua regia, the iron and manganese may be dissolved out, and the residue on addition of ammonia will yield a solution from which crystals of ammonium tungstate can be obtained on concentration, and these on heating yield the trioxide.

It is more usual, however, to effect the decomposition of wolframite by fusion with alkali according to one of the following methods:

1. The ore is fused with twice its weight of potassium carbonate and the resulting mass digested with water. Ammonium chloride is added to the aqueous extract and the solution evaporated to dryness, the residue being ignited to drive off the ammonium salt; potassium chloride is next removed by washing with hot water, and any acid potassium tungstate remaining is removed by boiling with dilute potassium hydroxide, leaving a residue of tungsten dioxide which after thorough washing with water is converted to the trioxide by ignition in an open crucible.
2. Powdered wolframite is fused with sodium carbonate and sodium nitrate, and after cooling, sodium tungstate is extracted from the mass with water, leaving a residue containing iron, manganese, calcium, and any columbium, tantalum, or tin that may have been present in the mineral. Crystallisation of the solution yields the dihydrate, Na₂WO₄.2H₂O, or if the hot solution is first nearly neutralised by means of nitric acid or hydrochloric acid, sodium paratungstate can be crystallised out. The oxide may then be obtained directly from either of these compounds.
3. An intimate mixture of the ore with chalk and sodium chloride or calcium chloride, or with calcium chloride alone, is heated to about. 600° to 700° C.; or the ore may be first fused with sodium hydrogen sulphate, and then with lime or a calcium salt. The residue is powdered and treated with boiling concentrated hydrochloric acid, which decomposes the tungstate with precipitation of tungsten trioxide.

Other methods of extraction are applicable on a smaller scale. When a mixture of powdered ore and quartz is heated in a stream of carbon tetrachloride vapour, a distillate is obtained of tungsten chloride, which may be decomposed by acid. Solutions of tungstates, such as are obtained as a by-product from zinc minerals containing tungsten, may be decomposed with hydrochloric acid.

The oxide prepared by any of the above processes is always impure, the nature of the impurities depending on the composition of the ore and on the materials employed in the process. If alkali has been used, the presence of sodium, potassium, or calcium tends to give the product a greenish appearance. Iron, manganese, silica, phosphorus, tin, molybdenum, vanadium, and columbium may all be present, and since tungsten is prone to form complex compounds with many of these, the purification of the oxide is not easily accomplished.

Probably the most satisfactory method is that of Smith and Exner, which consists in digesting ammonium tungstate with nitric acid (1:1) and a little hydrochloric acid, and after thoroughly washing the tungsten trioxide produced, dissolving it in ammonia, and allowing the para- tungstate to crystallise out. After repeating the process several times pure paratungstate is obtained. This salt, on further digesting with nitric acid and then evaporating to complete dryness, yields pure tungsten trioxide.

Silica may be removed from impure tungstic anhydride by fusion with potassium hydrogen sulphate and extraction of the alkali tungstate from the cooled mass with water.

Molybdenum may be separated as sulphide by passing hydrogen sulphide into a solution of the oxide in hydrochloric acid containing tartaric acid. This method is not effective with large quantities, and it is better to convert the trioxide to sodium tungstate, dissolve in water, and nearly neutralise with hydrochloric acid, when the paratungstate can be crystallised out. One half of it is dissolved in boiling water and the trioxide precipitated by the addition of hydrochloric and a little nitric acid. The precipitate is then added to a boiling solution of the other half of the paratungstate and the boiling continued until complete transformation into the metatungstate occurs and hydrochloric acid no longer gives a precipitate. The acidified solution is then treated with hydrogen sulphide, when all the molybdenum is precipitated.

Finely powdered sodium tungstate or paratungstate, or a concentrated solution of the latter, when treated with a large excess of boiling hydrochloric acid (1:1) containing a little nitric acid, yields a voluminous orange-coloured mass from which the trioxide may be obtained by ignition. A similar product remains when the tungstate or paratungstate is heated with concentrated sulphuric acid in a porcelain dish until the acid fumes strongly, the mixture after cooling being diluted with water and washed by decantation.

The precipitation from dilute solutions of sodium tungstate by the addition of mineral acids has been investigated by optical methods by Lottermoser, whose results indicate that the process is auto-catalytic. The velocity of the reaction depends upon the hydrogen-ion concentration, the change taking place more rapidly with hydrochloric acid than with sulphuric acid, whilst acetic acid does not cause precipitation.

As obtained by any of the above methods, tungsten trioxide is a bright canary-yellow coloured amorphous powder, which becomes dark orange when heated, but regains its bright yellow colour on cooling. It has also been obtained in the crystalline form: (1) by strongly heating amorphous tungsten trioxide; (2) by fusion of hydrated tungsten trioxide with borax; (3) by passing hydrogen chloride over tungsten trioxide heated to bright redness; or (4) by strongly heating a mixture of sodium tungstate and sodium carbonate in a stream of hydrogen chloride. According to Nordenskjold, the crystals obtained were small transparent prisms of the orthorhombic system:

a:b:c = 0.6966:1:0.4026;

those obtained by Debray were octahedral, some yellow and translucent, others dark green and opaque.

Tungsten trioxide may also be obtained by direct synthesis. When a tungsten wire is heated in oxygen at low pressure, the trioxide is formed at about 800° Abs.; above this temperature the oxide volatilises and the metal is left clean and bright at 1200° Abs.

The density of amorphous tungsten trioxide at 17°C. is 7.16; whilst that of the crystalline variety at the same temperature is 7.23. The specific heat of the trioxide between 8° C. and 98° C. was determined by Regnault to be 0.0798. The following values for lower temperatures have been determined more recently:

Temperature Range, ° C.	Specific Heat.	Molecular Heat.
–189.0 to –80.9	0.0442	10.25
–75.8 to 0.0	0.0678	15.73
+2.3 to +46.6	0.0783	18.16

When heated, tungsten trioxide fuses at a temperature between 1300° and 1400° C., but, unlike the trioxide of uranium, no change in composition occurs up to 1750° C. It is more strongly magnetic than metallic tungsten, its magnetic susceptibility at 15° C. being 0.808. The compound readily undergoes reduction; the greenish tinge which the powder sometimes possesses, if not due to metallic impurity, is due to reduction at ordinary temperatures by traces of organic matter, lower oxides being formed; the yellow colour may be restored by heating in a current of oxygen. When heated with carbon, tungsten trioxide yields the blue oxide between 650° and 850° C., a dark brown mixture of oxides between 900° and 1050° C., and metallic tungsten above 1050° C.; if hydrogen is used as the reducer, the mixture of brown oxides is obtained at 800° to 900° C., and at 1080° C. a deposit of pure tungsten (99.4 per cent.) results. The metal is also produced when the oxide is heated with aluminium or zinc.

If hydrogen is passed through water at 85° C. and the mixture of hydrogen and water vapour then passed over tungstic anhydride at 900° C., the latter is reduced to the dioxide, WO₂. The reaction is influenced by the amount of water vapour present, and the pentoxide, W₂O₅, results when the temperature of the water is maintained at 97° C.

Tungsten trioxide is insoluble in water and in most acids, including aqua regia, but is dissolved by hydrofluoric acid. It is also soluble in solutions of alkali hydroxides and carbonates yielding tungstates.

It is acted upon by chlorine, the yellow oxychloride, WO₂Cl₂, being formed; hydrogen sulphide gives the sulphide; gaseous ammonia reacts to form the oxy-amidonitride.

The trioxide remains unchanged when heated in a current of nitric oxide; it is reduced to lower oxides by ethylene or acetylene at red heat, by methane at a higher temperature, and by phosphine at 125° to 150° C. Heated with phosphorus pentachloride in an atmosphere of carbon dioxide it yields a red-brown product which consists of a mixture of tungsten chlorides and oxychlorides.

Tungsten trioxide may be used as a yellow colouring matter in the ceramic industry, since permanent yellow glazes can be produced by fusion at 800° C. with lead silicate, with bismuth oxide, or with a mixture of zinc borate and silicate.