Potassium nitrate synthesis by electrodialysis-metathesis: The effect of membrane type

The applicability of four commercially available ion exchange membranes: (Ralex AM-PP, CM-PP; Neosepta AMX, CMX; Selemion AMV, CMV; Ionsep AM-A, AM-C) for potassium nitrate synthesis by electrodialysis-metathesis was investigated. Solute back-diffusion, solvent transport, and potassium nitrate synthesis performance (the product purity, concentration and energy consumption) were evaluated for each IEM. The ED-M synthesis efficiency indicators: potassium nitrate concentration, purity, and current efficiency decreased with membrane type in the following order: Seleminon > Neosepta > Ionsep > Ralex. A Selemion AMV-CMV membrane pair performed the best among the membranes tested. Heterogeneous membranes were found to constitute a more efficient barrier for water transport by osmosis than homogeneous ones. Water transport along with ions was found to be the main source of water transport to products, leading to an undesired dilution of product solutions. All of these: back-diffusion, osmosis and water electrotransport can, however, be diminished by the selection of favorable ED-M conditions. The high performance of ED-M synthesis can be provided by using membranes with high fixed ion concentration and low water content. High selectivity, high conductivity and low water electrotransport ability are the most important membrane properties in terms of providing concentrated product under low energy consumption through ED-M.