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

• The relationships of the structures of alkyl phenol weak organic acids, with their acidities and sodium extraction capacities for Bayer liquor were investigated.
• We examine the effect the key parameters on the sodium extraction capability.
• The equilibrium data under the optimized parameters were used to calculate the extraction parameters.
• The two-stage countercurrent extraction experiments in laboratory scale verified the design parameters.

Enhanced separation of sodium and aluminum in Bayer spent liquor has been a challenge of the alumina industry for decades in its effort to obtain a stream of caustic solutions with high MR (molar ratio of Na2O to Al2O3) to treat red mud. The existing methods are too energy-intensive to use in practice. Solvent extraction technology has attracted considerable attention owing to its high selectivity and efficiency. However, the extraction capacities of extractants as reported in the literature are insufficiently high, requiring multiple-stage extraction and high organic-to-aqueous phase ratio. In this paper, the mechanisms for the extraction of sodium from Bayer liquor using alkyl phenol weak organic acids are investigated in terms of their structures and acidities, and an optimized extractant consisting of 2-tert-butyl-4-methylphenol dissolved in 1-octonal was selected due to its high sodium extraction capacities. The equilibrium relations between equilibrated organic sodium concentration and aqueous alkaline concentration for sodium extraction from sodium aluminate solutions under different concentrations of organic acid at various temperatures were systematically investigated. The equilibrium data obtained using 1 mol/L 2-tert-butyl-4-methylphenol diluted by 1-octonal at 40 °C for extraction and stripping were used to design the extraction parameters to achieve a sodium extraction efficiency of 50%. The results of the extraction of a two-stage countercurrent experiment in laboratory scale verified the design parameters. Sodium extraction efficiency of 40.4% was achieved. Concurrently near-pure alkaline solution with about 210 g/L NaOH was obtained from the stripping unit, which could be evaporated to treat red mud.