Membrane separation of ammonium bisulfate from ammonium sulfate in aqueous solutions for CO2 mineralisation

The separation of ammonium bisulfate (ABS) from ammonium sulfate (AS) in aqueous solutions by  monovalent ion selective membranes was studied. Optimized usage of these chemicals is both an important and challenging step towards a more efficient CO2 mineralisation process route developed at Åbo Akademi University (ÅA). The membranes were placed in a three-compartment electrodialysis stack and both silver and stainless steel electrodes were tested, of which stainless steel electrodes were found to be more suitable. Separation efficiencies up to 90% were reached based on ABS concentrations in the feed solution. The tests were performed on an initial voltage of either 10 or 20 V, but limitations in the electrical power supply equipment eventually resulted in a voltage drop as separation proceeded. Exergy calculations showthat the input exergy (electrical power) is many times higher than the reversible mixing exergy, which indicates that design modifications must be made but also that more accurate analysis methods should be developed. However, this study has shown that there is potential in the application of monovalent membranes to the mineral carbonation process described. Further work will focus on the possibilities to make the separation even more efficient and to develop the analysis methods.