Sulfur Dioxide

There are two major processes (lead chamber and contact) for production of sulphuric acid, and it is available commercially in a number of grades and concentrations. The lead chamber process, the older of the two processes, is used to produce much of the acid used to make fertilizers; it produces a relatively dilute acid (62%–78% H2SO4). The contact process produces a purer, more concentrated acid but requires purer raw materials and the use of expensive catalysts. In both processes sulphur dioxide is oxidized and dissolved in water. The sulphur dioxide is obtained by burning sulphur, by burning pyrites (iron sulphides), by roasting nonferrous sulphide ores preparatory to smelting, or by burning hydrogen sulphide gas. Some sulphuric acid is also made from ferrous sulphate waste solutions from pickling iron and steel and from waste acid sludge from oil refineries.