SO2is an acid gas and thus the typicalsorbentslurries or other materials used to remove the SO2from the flue gases are alkaline. The reaction taking place in wet scrubbing using a CaCO3(limestone) slurry produces CaSO3(calcium sulfite) and can be expressed as: CaCO3(solid) + SO2(gas) → CaSO3(solid) + CO2(gas) When wet scrubbing with a Ca(OH)2(lime) slurry, the reaction also produces CaSO3(calciumsulphite): Ca(OH)2(solid) + SO2(gas) → CaSO3(solid) + H2O (liquid) A similar process is possible with magnesium hydroxide: Mg(OH)2(solid) + SO2(gas) → MgSO3(solid) + H2O (liquid) Some FGD systems go a step further and oxidize the CaSO3 (calciumsulphite) to produce marketable CaSO4· 2H2O (gypsum) CaSO3(solid) + ½O2(gas) + 2H2O (liquid) → CaSO4· 2H2O (solid)