Contact Process: The contact process is the most widely used method of manufacturing tetraoxosulphate(IV) acid, otherwise called sulphuric acid on a commercial scale. The main reaction in the manufacturing process of this very important compound is the catalytic combination of sulphur(IV) oxide and oxygen to form sulphur(VI) oxide. Sulphur(IV) oxide is made by burning sulphur in dry air or by roasting sulphide ores.
2SO2(g) + O2(g) 2SO3(g) + heat
S(s) + O2(g) SO2(g)
Sulphur(IV) oxide is mixed with excess air and passed through an electric chamber in order to remove impurities and dust which might poison the catalyst. The gaseous mixture is then passed through concentrated tetraoxosulphate(VI) acid to be dried before it is taken to the reaction chamber or contact tower. Here, the sulphur(IV) oxide combines with oxygen in the presence of pellets of a catalyst, vanadium(V) oxide to yield sulphur(VI) oxide.
The contact process is carried out at an atmospheric pressure and at a temperature of about 450 -500oC with excess air or oxygen to ensure that all the sulphur(IV) oxide has reacted. Around 98% of the possible yield of sulphur(VI) oxide is obtained.
The sulphur(VI) oxide is cooled and passed into an absorption tower where it dissolves in concentrated tetraoxosulphste(VI) acid to produce a very thick liquid called oleum. Note that the oxide, sulphur(VI) oxide, is not dissolved directly in water because the heat evolved during the process would cause the acid solution to boil and producing a mist of acid droplets which could spread throughout the factory.
SO3(g) + H2SO4(aq) → H2S2O7(aq)
The Oleum, H2S2O4 is then diluted with appropriate amounts of water to produce the 98% tetraoxosulphate(VI) acid often used in the laboratory and also into other desired concentrations.