Editing Contact process (section)

== Purification unit ==
This includes the dusting tower, cooling pipes, scrubbers, drying tower, arsenic purifier and testing box. Sulfur dioxide has many impurities such as vapours, dust particles and [[arsenous oxide]]. Therefore, it must be purified to avoid catalyst poisoning (i.e.: destroying catalytic activity and loss of efficiency). In this process, the gas is washed with water, and dried by sulfuric acid. In the dusting tower, the sulfur dioxide is exposed to a steam which removes the dust particles. After the gas is cooled, the sulfur dioxide enters the washing tower where it is sprayed by water to remove any soluble impurities. In the drying tower, sulfuric acid is sprayed on the gas to remove the moisture from it. Finally, the [[arsenic oxide]] is removed when the gas is exposed to [[ferric hydroxide]].