Methods and apparatus are disclosed for enhanced purification of caustic soda solutions. In place of conventional mechanical separators, flotation separation methods are taught utilizing sequential multi-temperature sparging gas treatments. A first stage treatment, at about 70.degree. F.-90.degree. F., provides separation of sodium chloride and a subsequent stage, higher temperature (about 90.degree. F.-100.degree. F.) treatment provides enhanced removal of suspended sodium sulfate. Sensible heat is added to sparging gas to offset the cooling effect of gas expansion in a treatment tank and avoid crystallization of dissolved impurities from saturated solutions on sparging gas nozzles.