Recovery of Anhydrous Na₂SO₄ from SO₂-Scrubbing Liquor by Extractive Crystallization:  Liquid−Liquid Equilibria for Aqueous Solutions of Sodium Carbonate, Sulfate, and/or Sulfite Plus Acetone, 2-Propanol, or <i>tert</i>-Butyl Alcohol

Abstract

Sodium carbonate is a superior scrubbing agent for removing SO 2 from combustion gases, but the resulting sodium sulfate (or sulfite) must be recovered for environmental reasons. Recovery by evaporative crystallization is energy-intensive; extractive crystallization (described below) provides an attractive alternative when technically feasible. Liquid/liquid equilibrium data were determined for two-phase mixtures containing aqueous solutions of sodium carbonate, sulfate, or sulfite and a polar organic solvent: acetone, 2-propanol, and 2-methylpropan-2-ol ( i . e ., tert -butyl alcohol). In the salt-saturated two-phase region, data were obtained between the lower consolute temperature and 60 °C (50 °C for acetone). Data were also obtained at 35 °C for liquid/liquid systems that were subsaturated with their respective salts and for liquid/liquid systems with overall molar ratios of sodium sulfite/sodium sulfate fixed at 25/75, 50/50, and 75/25. In the latter systems, it was found that the sulfite/sulfate ratios in the organic and aqueous phases were the same; i . e ., there is no selectivity by these solvents for one salt relative to the other. The data show that any one of these solvents can be used to extract water from a concentrated solution of either sodium sulfite or sodium sulfate in a countercurrent extractor at 35 °C, causing the anhydrous salt to crystallize. The wet solvent can be dried for recycle in a similar countercurrent operation at 35 °C, using a saturated solution of Na 2 CO 3 as the drying agent. The number of moles of carbonate required for drying does not exceed the number of moles of sulfite-plus-sulfate precipitated. The process energy is about 10% of that required for single-stage evaporative crystallization of the same liquor.