The SO₂ from the off-gas that arises from melting processes is converted into sulphuric acid in the double contact plant, which is at least 99.8 % effective, after almost all impurities except SO₂ are removed during the gas cleaning process. The SO₂ is oxidised into SO3 in the V₂O₅ catalyst (the so-called “contact”). This SO₃ is dissolved in concentrated sulphuric acid in the absorption process and forms H₂SO₄ (sulphuric acid) with the addition of H₂O. Since the catalysis/absorption process is carried out twice in a row, it is referred to as double catalysis and the whole plant is called a double contact plant. The intermediate SO₃ removal process allows for a high level of SO₂ conversion, so only very low quantities of SO₂ remain in the cleaned off-gas.

A facility used at Aurubis to remove mercury before producing sulphuric acid from the SO₂ containing off-gases in the RWO. The offgases are channelled through a tower filled with pumice pellets. DOWA is the name of the Japanese firm which developed the process and holds the patent.