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.