INFLUENCE OF RETENTATE FLOW ON MEMBRANE SEPARATION OF BINARY SOLUTIONS OF SODIUM, MAGNESIUM AND CALCIUM CHLORIDES

The effect of the retentate consumption on the formation of flows of water and sodium, magnesium and calcium chlorides during their transfer through a Vontron VNF nanofiltration membrane from individual and mixed solutions at a constant driving force of the solvent transfer was studied. The constancy of the driving force was ensured by a constant concentration of osmotically active particles M +/-=100 mmol/dm 3 . It has been shown that the rejection of salts by the membrane in their individual solutions increases with increasing retentate consumption. The permeability coefficient and conversion increases, as expected, several times, and the density of the molar flux of sodium chloride is 2-6 times higher than for magnesium and calcium chlorides. The obtained experimental data are satisfactorily described in the framework of the Spiegler-Kedem-Katchalsky model. It is noted that the transmembrane transfer of solution components in the Vontron VNF membrane does not stop even at an operating pressure below the osmotic pressure, which makes them promising for solving the problems of energy-saving separation and separation of salts of different valence metals. The presence of small amounts of doubly charged metal salts in the solution leads to a sharp increase in the molar flux of sodium chloride through the membrane. When the mole fraction of Na-Mg and Na-Ca is 0.2, the separation factor increases 5 times compared to individual solutions. The transfer of magnesium chloride through the membrane in the studied range does not depend on the presence of sodium chloride in the solution, but a slight increase in the molar flux density is observed for calcium. An increase in the molar flux of NaCl in the presence of small fractions of a divalent metal may be due to the manifestation of Donnan exclusion in the near-membrane layer. It is shown that the maximum ratio of the values of the molar flux densities of the separated components is achieved in the range of small values of the retentate consumption. In turn, the isoselective rejection of salts in the retentate provides a high purge rate of the membrane apparatus.