An Equilibrium Model of Pseudo-Hydroxide Extraction in the Separation of Sodium Hydroxide from Aqueous Solutions using Lipophilic Fluorinated Alcohols and Phenols

Abstract:

So-called pseudo-hydroxide extraction has been proposed as a method for separating alkali metal hydroxide from alkaline salt solutions by use of weak-acid cation exchangers such as lipophilic phenols or fluorinated alcohols. Under extraction conditions, these lipophilic weak acids (HAs) are converted to their salt forms when dissolved in a suitable water-immiscible polar diluent like 1-octanol. Upon contact with water, the loaded solvents release alkali metal hydroxide to the aqueous phase by hydrolysis, thereby regenerating the extractant. This type of cycle has been demonstrated in previous work, wherein it was shown that bulk concentrations of sodium hydroxide could be selectively separated from alkaline high-level waste simulants. In the present work, it has been the aim to identify the controlling equilibria and predominant species involved in the extraction process within the framework of a thermochemical model. Using 1-octanol as the diluent, distribution ratios for sodium (Z)_Na) extraction were measured as a function of organic-phase HA and aqueous-phase NaOH molarity at 25°C. The tested weak acids, 3,5-di-rert-butylphenol (HA-1) and 1-trifluoromethyl-2-(3,5-di-rerr-butylphenoxy)ethanol (HA-2), significantly enhanced the extraction of NaOH over the minor amount that is extracted by 1-octanol alone. The extraction efficiency of HA-1 was approximately 10-fold higher than that of HA-2. By use of the equilibrium modeling program SXLSQf, it was shown that a simple model is sufficient to quantitatively explain the observed extraction behavior based upon (a) ion-pair extraction to give Na⁺OH⁻ ion pairs and corresponding free ions in 1-octanol and (b) cation exchange by monomeric HA molecules to form monomeric organic-phase Na⁺A⁻ ion pairs and corresponding free organic-phase ions.