Separation of NaOH by Solvent Extraction Using Weak Hydroxy Acids

Abstract

Separation of sodium hydroxide from aqueous salt solutions at elevated pH may be accomplished by use of solvent extraction with weak hydroxy acids. A series of lipophilic weak hydroxy acids, including alkyl phenols and fluorinated alcohols, has been characterized with regard to their ability to extract sodium into 1‐octanol as a function of temperature and extractant concentration. Isotherms are presented at 10, 25, and 60°C, employing extractant concentrations up to 1 M and aqueous NaOH concentrations up to 7 M. Near stoichiometric loading of all compounds tested may be achieved in a process postulated to be simple cation exchange. Extraction strength increases with the expected acidity of the hydroxy acids. Strongest extraction was observed with two alkyl phenols and 4‐n‐octyl‐α,α‐bis(trifluoromethyl)benzyl alcohol. Essentially complete removal of free hydroxide from a solution containing sodium hydroxide, nitrate, and aluminate was accomplished using 1 M 4‐tert‐octylphenol in 1‐octanol at 60°C. Consequently, as the pH of the solution was lowered, aluminum was observed to precipitate as the Bayerite form of Al(OH)3. Contacting the loaded solvent with water released NaOH into the strip solution, and good selectivity for hydroxide vs. nitrate and aluminate was obtained. Since hydroxide equivalents rather than the hydroxide ion itself are believed to be extracted by the weak hydroxy acids, the process is referred to as pseudo‐hydroxide extraction. Application of this concept to sodium removal and caustic recycle in the treatment of alkaline nuclear waste appears feasible.