The Graphical Calculatin of the Isothermal Evaporation Process of Ion-Exchange Membrane Brines at 25°C

Studies on the Concentration of Ion-Exchange Membrane Brine (Part 2)

Abstract

In their previous paper (This Journal, 19, 164-7 (1965)), the authors reported that the composition of ion-exchange membrane brines could be roughly divided into two 6-component systems; a-type brine belonging to NaCl-KCl-MgCl₂-CaCl₂-CaSO₄-H₂O and b-type brine belonging to NaCl-KCl-MgCl₂-MgSO₄-CaSO₄-H₂O.
When the authors considered the process of concentration of ion-exchange membrane brines by phase rule, they omitted CaSO₄. from the consideration on the assumption that CaSO₄ contained in these brines is entirely precipitated out prior to the precipitation of NaCl. Also, they simplified the equilibrium diagram on the assumption that the equilibrium system is always saturated with NaCl, just as in the case of sea-water concentration.
In the present paper, the authors determined the kinds of precipitated crystals and the changes of liquid phase components during the process of thermally evaporating the brine which was supposed to have representative composition of a-type and b-type brines at the temperature of 25°C. In this case, they used for the a-type brine the equilibrium diagram on the triangular coordinate, and for the b-type brine the equilibrium diagram on the stereo-geometric axis for graphicalcal culation. Also, the composition of the liquid phase obtained when NaCl crystals begin to precipitate was graphically calculated from the NaCl solubility and the NaCl concentration obtained when KCl crystals begin to precipitate.
As is shown by the results of the calculation of the a-type and the b-type brines, about 97% of NaCl content was precipitated as pure crystals before the volume of mother liquor was reduced to approximately 1/10 of the original brine. Then, the precipitation took place in the order of KCl, MgCl₂·KCl·6H₂O. The compositions of precipitated crystals in each stage of concentration were also approximately the same, and the liquid-phase compositions were analogous except for the difference between CaCl₂ and MgSO₄·From these facts, the authors reached the conclusion that in so far as the concentration is not so much advanced, the properties of brine such as the boiling point, specific heat, etc. may be of nearly the same value.