Recovery of Na2CO3 and Na2SO4 from mixed solutions by membrane crystallization

Abstract Membrane crystallization is applied in this work for the separation and recovery of Na2CO3 and Na2SO4 as high purity crystals from aqueous solutions. These mixed solutions are for example produced when CO2 and SO2 are removed from flue gases by absorption in an alkaline solution containing NaOH. The application of membrane crystallization leads to a reusable product (i.e., Na2CO3 and Na2SO4) of industrial interest. A novel approach is proposed to avoid co-crystallization of both salts, in which a precipitation step is included to remove residual CO32− ions in the mixture. The effect of the feed composition on the overall mass transfer coefficient was studied. It was found that the mass transfer coefficient is independent of the feed concentration with an average value 7.06 × 10−11 m/Pa s. The characterization of the obtained crystals shows that the decahydrated forms of the salts, i.e. Na2CO3·10H2O and Na2SO4·10H2O, were obtained. Thus, membrane crystallization technology can be used for salt separation processes, avoiding co-crystallization, when operated under appropriate conditions.