Pseudohydroxide extraction from aqueous sodium hydroxide solutions with 3,5-di-tert-butylphenol in Isopar (R) L modified with 1-octanol

Pseudohydroxide extraction (PHE) was investigated for recovering sodium hydroxide (NaOH) from alkaline process solutions. PHE relies on the deprotonation of a lipophilic weak acid by hydroxide ion with concomitant transfer of sodium ion into an organic phase. Contact of the sodium-loaded organic phase with water results in the reconstitution of the extractant in the organic phase and NaOH in the aqueous phase, thus leading to a process in which NaOH equivalents are transferred from an alkaline feed solution to an aqueous stripping solution. In this work, we investigated PHE using a process-friendly diluent-Isopar trademark L. The lipophilic cation exchanger 3,5-di-tert-butylphenol (35-DTBP) was used as the extractant. The Isopar trademark L diluent was modified with 1-octanol to improve its solvation properties and the solubility of 35-DTBP so that practical Na+ concentrations could be achieved in the process solvent. The PHE mechanism at process-relevant conditions was explored by Raman and Fourier transform infrared spectroscopic measurements. Complementary electrospray ionization mass spectrometry studies were also performed. Equilibrium computer modeling suggested that the Na+ extraction behavior can be largely explained by the formation of 1:1 and 1:2 Na/35-DTBP species in the organic phase. Extraction isotherms obtained using simulated caustic leaching solutions indicate the potential utility of this approach for recycling NaOH from complex alkaline mixtures.