What Is The Contact Process?

          The large scale manufacture of sulphuric acid is extremely important as it is the most common acid used in industry.  Annually, over 1000 million metric tons of acid is produced. The process by which it is manufactured is called the Contact Process.

          Stage 1:
The raw materials are sulphur and air (oxygen). Sulphur dioxide is produce either by burning sulphur or ores containing sulphur.

          Stage 2:
The gas sulphur dioxide is then purified. Impurities like arsenic compounds are removed. It is then passed through an electrostatic dust precipitator that charges dust particles. These particles are removed by being attracted to oppositely charged plate.

          Stage 3:
Sulphur dioxide and air are then washed, dried and passed over Vanadium (V) oxide catalyst at 450 degrees Celsius and 2-3 atmospheres. At this temperature and pressure, 98% of conversion to sulphur trioxide is achieved. Gases which are not converted are removed and recycled.    

          Stage 4:
Concentrated sulphuric acid is then added to sulphur trioxide to form fuming sulphuric acid (oleum).

          Stage 5:
This oleum is then diluted with water to get the acid of the required strength.

          This may although seem a round about method to form an acid, but is necessary. As sulphur trioxide reacts too violently with water, therefore it is not directly dissolved in it.