Abstract: A method for controlling the concentration of a slurry in an absorption tower in which a flue gas comprising SO.sub.2 is brought into contact with a slurry containing suspended Ca compounds therein to absorb the SO.sub.2 with the compounds. The method comprises withdrawing the slurry from the absorption tower in such a way that the slurry is divided into at least two groups with different concentrations, one group being a slurry having a higher concentration of the Ca compound, the other group being a slurry having a lower concentration of the Ca compound, and regulating rates of the slurries being withdrawn from the absorption tower whereby the concentration of the Ca compound in the slurry being contacted with the flue gas is controlled.

Abstract: A process for treating a waste gas containing SO.sub.2 and HCl which comprises determining the amount of HCl in said waste gas and supplying at least one magnesium salt selected from the group consisting of MgSO.sub.4, MgSO.sub.3, MgO, MgCO.sub.3 and Mg(HSO.sub.3).sub.2 in an amount corresponding to the amount of HCl additionally supplying CaCO.sub.3 or Ca(OH).sub.2 as an absorbent for SO.sub.2 to a column for treating said waste gas.