Animation showing the contact process, the industrial process that makes sulfuric acid from sulfur. Sulfur enters a roasting tower on a conveyor belt. The air supply from bottom produces sulfur dioxide, which is piped to the oxidising tower. Catalytic oxidation (450 degrees Celsius and 1-2 atmospheres of pressure) occurs in the presence of vanadium pentoxide to produce sulfur trioxide gas, with more air supplied from bottom. This is then pumped into a tank of concentrated sulfuric acid (final stage, right). The sulfur trioxide dissolves in the sulfuric acid to form oleum (fuming sulfuric acid), which is diluted with water to form sulfuric acid.