The Color of Liquid Caustic Soda Produced by the Ammonia-Soda Process

Abstract It is found that the coloring of caustic liquor produced by the ammonia-soda process is due to chromate in the milk of lime given to the sodium carbonate solution for causticization. But the solid caustic soda, even produced by the ammonia-soda process, contains a little chromate. The coloring of caustic liquor by chromate is found in products of all ammonia process makers in Japan. The existence of chromate not only gives the yellow color to the product, but also has an undesirable effect on viscose production in the rayon industry. Yellow-colored caustic liquor can be discolored by the addition of the reducing reagent. Refining by anion exchange resin is also effective, but there is a risk of the corrosion of the resin by the caustic liquor. As the chromate was identified in the caustic liquor, it was concluded that the abnormal wave of the polarogram caustic liquor was probably due to the hydrolysis current. By measuring the absorbancy of caustic liquor at the wavelength of 370 mμ, the chromate content can be nearly exactly determined. As much limestone contains a considerable impurities, it is not desirable to use the milk of lime for the purification of the brine in the electrolytic-soda process also.