Purifn of sodium hydroxide solns - by cooling to supersaln, pptg crystals, classifying and removing impure crystals with classifying soln

摘要

Method esp. for reducing diaphragm cell effluent impurities (partic NaCl) to 0.01-0.001 %, involves (a) cooling soln to NaOH hydrate supersatn. temp. (b) forming from this soln a slurry of large NaOH hydrate crystals and finer impurity crystals (e) separating the two by countercurrent contact with a classifying soln such that the fine impurity crystals are carried with the classifying soln and the larger NaOH hydrate crystals settle to form a bed (d) separating the classifying soln. (e) removing the fine impurity crystals from the classifying soln (f) adding NaOH soln to the classifying soln and recycling to step (a), and (g) removing and washing the NaOH hydrate crystals.