Sulphuric acid production. Schematic diagram of the Contact Process to make sulphuric acid from sulphur. Sulphur (yellow) enters a roasting tower on a conveyor belt (far left). The air supply from bottom produces sulphur dioxide (orange), which is piped to the oxidising tower (centre). Catalytic oxidation (450 degrees Celsius and 1-2 atmospheres of pressure) occurs in the presence of vanadium pentoxide (red) to produce sulphur trioxide gas (yellow, more air supplied from bottom). This is then pumped into a tank of concentrated sulphuric acid (green, final stage, right). The sulphur trioxide dissolves in the sulphuric acid to form oleum (fuming sulphuric acid, dark green), which is diluted to form sulphuric acid (bottom right).