PURPOSE: To reduce the formation of gypsum scale and to carry out continuous operation without a hitch by a method wherein a lime suspension is prepared by oxidizing a lime suspension containing absorbed SOx, separating liquid from solids and reducing the MgSO4 concentration of the recovered liquid.

CONSTITUTION: Gas containing SOx is absorbed by a lime suspension. The resulting slurry containing CaSO4 and/or Ca(HCO3)2 is supplied to an oxidation tower where the slurry is oxidized. After the oxidation, solid and liquid in the slurry are separated. Gypsum is recovered, and liquid containing from 1000W2500ppm of MgSO4 is also recovered. MgSO4 in the recovered liquid is removed by a chemical or physical treatment to reduce the concentration of MgSO4 to below 1500ppm. The resulting liquid is recycled and used for the preparation of the lime suspension.