Structure of the solubility diagram in the Na2SO4-Na2CO3-NaHCO3-H2O system at 0, 25 and 50

Introduction Four-component system Na2SO4Na2CO3-NaHCO3-H2O is a part of more complex six-component system Na, Са// SO4, CO3, HCO3, F-H2O. Equilibria in the latter determine the conditions of aluminium production liquid waste disposal. The waste water of cryolite recycling plants contains fluorides, carbonates, bicarbonates and sulphates of sodium and calcium [1, 2]. Crystallization and dissolution processes in such waste water are governed by the phase equilibria both in six-component system Na, Са//SO4, CO3, HCO3, F-H2O and in its constituents, fiveand four-component systems. In our earlier studies [3, 4] the phase diagrams in similar systems were constructed. This study presents the results of investigation of Na2SO4-Na2CO3-NaHCO3-H2O system at 0, 25 and 50 °C using solubility method. The main goal of this work was to establish the concentration parameters of geometrical images and separation of the crystallization fields of individual equilibrium solids in the phase diagrams.


Introduction
Four-component system Na 2 SO 4 -Na 2 CO 3 -NaHCO 3 -H 2 O is a part of more complex six-component system Na, Са// SO 4 , CO 3 , HCO 3 , F-H 2 O. Equilibria in the latter determine the conditions of aluminium production liquid waste disposal.The waste water of cryolite recycling plants contains fluorides, carbonates, bicarbonates and sulphates of sodium and calcium [1,2].Crystallization and dissolution processes in such waste water are governed by the phase equilibria both in six-component system Na, Са//SO 4 , CO 3 , HCO 3 , F-H 2 O and in its constituents, five-and four-component systems.
In our earlier studies [3,4] the phase diagrams in similar systems were constructed.This study presents the results of investigation of Na 2 SO 4 -Na 2 CO 3 -NaHCO 3 -H 2 O system at 0, 25 and 50 °C using solubility method.The main goal of this work was to establish the concentration parameters of geometrical images and separation of the crystallization fields of individual equilibrium solids in the phase diagrams.
Based on the data available [5,6], we prepared the mixtures of precipitates with saturated solutions according to the invari-  Then, transferring the non-variant points from the three-component section to the four-component section [3,4], the saturated solutions prepared had been kept in a thermostat at a given temperature until the equilibrium was reached.
Thermostating was carried out in U-8 ultra-thermostat.Stirring was performed using a PD-09 magnetic stirrer for 50-120 h.Temperature was maintained with 0.1 °C accuracy using a contact thermo-meter.Crystallization of solid phases was observed with a POLAM-R 311 microscope.After the equilibrium in a given system was achieved, the solid phases were photographed with a Sony-DSC-S500 digital camera.Equilibrium was assumed to be attained when the phase composition of the precipitates was constant.
A Buchner funnel with an ash-free filter paper (Blue Band) connected to a vacuum pump has been used for separation of the liquid phase and solid phase.The precipitate after filtration was washed with 96 % ethanol and then dried at 120 °С.The Results of the crystallooptical analysis [11] of equilibrium solid phases (microphoto) are presented in Fig. 1, and the results of the chemical analysis of the saturated solutions are given in Table 1.
On the basis of the data obtained, the diagrams of solubility in the Na 2 SO 4 -Na 2 CO 3 -NaHCO 3 -H 2 O system at 0, 25 and 50 °C were constructed.Salt parts of these diagrams are shown in Fig. 2. The location of non-variant points on the diagrams were determined by the center of mass method [12].