Abstract: Sulfur oxides (SO.sub.x) are scrubbed from combustion effluents with aqueous limestone slurries with greater efficiency and with economies in capital and operating costs. In the preferred embodiment of a single-loop, open-tower countercurrent limestone wet scrubber, effluent flow rates are greatly increased while L/G values and reaction tank residence times are decreased. Improved entrainment eliminator design, novel nozzle placement and spacing, and the use of a hydrocyclone to separate and recycle smaller particles of limestone from the byproduct gypsum, facilitate these advantages. The limestone is reduced to very fine particles, e.g. about 8.mu. or less with more than 99% of the particle by weight less than 44.mu., and introduced into a scrubbing slurry which is contacted with SO.sub.x -laden effluent.