Solubility of Na2CO3 and NaHCO3 in Aqueous Sodium Sulfate Solutions and Its Application to Separating Na2CO3 and Na2SO4 Salt Mixtures

To separate a salt mixture (mainly consisting of Na2CO3 and Na2SO4) which is formed from the 4,4′-diaminostilbene-2,2′-disulfonic acid wastewater treatment, the solubility data of Na2CO3 and NaHCO3 in Na2SO4–water solvent mixtures were measured using a dynamic method over the 275–335 K temperature range. The solute-free mass fraction of Na2SO4 in the solvent mixtures extended from 0 to 0.25. The solubility of Na2CO3 and NaHCO3 in Na2SO4–water solvent mixtures decreased with addition of Na2SO4. With rising temperature, the solubility of NaHCO3 increased while the solubility of Na2CO3 increased at first and then decreased after transition points. The experimental data were correlated with the electrolyte nonrandom two-liquid model. The root-mean-square deviations of solubility temperature varied from 0.35 to 1.50 K. Based on the solubility data of this work, a new strategy for separating the Na2CO3 and Na2SO4 mixture was proposed theoretically.