Flotation wastes precalcined in air with (i) increased or (ii) not increased H2O content in oven gases, to contain (CaO+MgO) similar to87% (H2O-leachable CaO and MgO 38.9 and 35%) and, for comparison, (iii) a min 92%CaO-max. 2.5% MgO quicklime, 35.9, 31.9 and 17.8 mum median diam., resp. were used to prep. (1:2 w/w) suspensions in H2O. These mixed 40 min at 80degreesC and sieved (0.1 mm), were used, 1:1-1:2.5 kg sorbent/kg SO2, to sorb SO2 from 500-1000 ppm SO2 flue gases, 190 m(3)(S.T.P.)/h, 130degreesC, humidity 0.048 kg H2O/kg dry flue, by the spray-dry technique in a pilot plant. Suspensions (i) and (ii) were similar to20% less effective than (iii). Contact time was 2 s. Twice as much of (i) or (H) would allow to achieve the efficiency of (iii). Low-SO2 flue gases could thus be desulfurized at high temperatures with no recycle of partially converted sorbent.