Reactive extraction of sodium hydroxide from alkali solutions for the separation of sodium and aluminum—Part II. Effect of diluent

Abstract

Enhancing the precipitation of Al(OH)3 from supersaturated sodium aluminate solution has been a challenge for the alumina industry for decades in its effort to increase the productivity. However, limited by the thermodynamic equilibrium of alumina solubility in caustic solutions, the processes that are available to date for promoting the precipitation of Al(OH)3 are unsatisfactory. An acid–base reactive solvent extraction method is proposed to enhance the separation of sodium hydroxide from sodium aluminate mother liquor by creating a condition for re-precipitation. The extraction process has been reported in Part I of this study by the present authors. In this paper the effects of diluents on sodium extraction when using 2-tert-butyl-4-methylphenol as the main organic extractant were systematically investigated. According to the classification of diluents, the capabilities for extracting sodium in different types of diluents were compared, and the protonic polar diluents were identified as being the most effective diluent. In the absence of alkyl phenol organic extractant, the sodium extraction equilibrium in 1-octanol and 2-ethyl hexanol, and the co-extraction of water in each of them were compared. In the presence of alkyl phenol, the extraction capabilities for sodium in different C8 alcohols, including 1-octanol, 2-ethyl hexanol and 2-octanol, were compared and analyzed. The results showed that 1-octanol is the most effective diluent among the three that are studied in this project. Furthermore, inert diluents including kerosene, octane and heptane, were used to dilute the pure 1-octanol solvent in different proportions in order to decrease the viscosity of the organic phase. The effects of the inert diluent on sodium extraction and water co-extraction were studied. Finally, the particle size distributions in the equilibrated organic phases and the Margules ternary 2-suffix equations were introduced to discuss the mechanisms of sodium extraction by 1-octanol and 2-ethyl hexanol. The results of this systematic research are expected to provide valuable guidance for the selection of diluents.