Abstract: A double alkali process employs limestone to regenerate spent sodium sulfite and then recirculates the unreacted limestone to attain a high internal stoichiometry and high limestone utilization. The regeneration of the sodium sulfite is carried out in a plurality of reactors and separation of the slurry from the reactors is carried out in at least one hydroclone. The at least one hydroclone has two primary functions: one to promote the reaction of Na.sub.2 SO.sub.3, which reduces the reactor volumes and reduces the temperature of conversion; and two to reduce the concentration of the limestone overflow, thus increasing the limestone utilization.