How does nanofiltration of (diluted or concentrated) sea water affect brine quality for the production of highly pure crystalline NaCl?

NaCl salt can be produced from zero-liquid discharge sea water desalination, where nanofiltration is applied to increase brine purity. However, during the development of the nanofiltration process the focus is usually on reducing
liquid waste discharge and increasing drinking water yield in reverse osmosis, rather than on producing salt of sufficient purity. This study shows that nanofiltration of (synthetic) sea water, or its marginally diluted or more
concentrated derivatives, is able to strongly reduce divalent ion concentrations in brine. However, the obtained selectivity for potassium over sodium (approximately 1) and for bromide over chloride (increasing from 1 to 1.2–1.3,
depending on the membrane type, as function of flux) lead to an increased monovalent impurity level in the purified brine as compared to brine obtained from solution mining, thereby requiring additional post treatment to produce
high quality crystalline salt. Furthermore, obtained sodium and chloride retentions between 0 and 40 % reduce the NaCl concentration in the brine leading to the need for additional brine concentration to reach saturation prior to the crystallization process. DSPM-DE model results, using membrane characteristics determined from characterization and salt solution experiments, predict the obtained potassium over sodium selectivity properly, but underpredicts the obtained bromide over chloride selectivity with a predicted selectivity of approximately 1 for all experimental conditions and evaluated membranes. Ranking commercial membranes based on performance results obtained over a large variety of experimental conditions is not straightforward when permeate quality is considered as well.