**3.3.1 Stable phase equilibria**

For the stable equilibrium study, the isothermal dissolution method was used in this study. The series of complexes of the quaternary system were loaded into clean polyethylene bottles and capped tightly. The bottles were placed in the thermostatic rotary shaker, whose temperature was controlled to (298.15 ± 0.1) K, and rotated at 120 rpm to accelerate the equilibrium of those complexes. A 5.0 cm3 sample of the clarified solution was taken from the liquid phase of each polyethylene bottle with a pipet at regular intervals and diluted to 50.0 cm3 final volumes in a volumetric flask filled with DDW. If the compositions of the liquid phase in the bottle became constant, then equilibrium was achieved. Generally, it takes about 50 days to come to equilibrium.

#### **3.3.2 Metastable phase equilibria**

The isothermal evaporation method was used in metastable phase equilibria study. According to phase equilibrium composition, the appropriate quantity of salts and DDW calculated were mixed together as a series of artificial synthesized brines and loaded into clean polyethylene containers (15 cm in diameter, 6 cm high), then the containers were put into the box for the isothermal evaporation at (308.15 ± 0.2) K. The experimental conditions with air flowing velocity of 3.5-4.0 m/s, relative humidity of 20-30%, and evaporation rate of 4-6 mm/d are presented, just like the climate of the Qaidam Basin. For

Stable and Metastable Phase Equilibria in the Salt-Water Systems 407

K2B4O7·4H2O

Borax

single polarized light (10×10) orthogonal polarized light (10×10) (b) Invariant point (KCl + Borax + K2B4O7·4H2O)

The metastable equilibria solid phases in the two invariant points are further confirmed with X-ray diffraction analysis, and listed in Figure 4, except in the invariant points (NaCl + KCl + Borax) in Figure 4a which shows that the minerals KCl, NaCl, Na2B4O7·10H2O and a minor Na2B4O7·5H2O are existed. The minor of Na2B4O7·5H2O maybe is formed due to the dehydration of Na2B4O7·10H2O in the processes of transfer operation and/or grinding.

Fig. 3. Identification of the invariant points for the solid phase in the reciprocal system (NaCl - KCl – Na2B4O7 - K2B4O7 - H2O) with a polarized microscopy using an oil-immersion method. (a), the invariant point (NaCl + KCl + Na2B4O7·10H2O); (b), the invariant point

No. Visible Ref. Code Chemical Formula Score Scale

KCl NaCl

B4O5(OH)4(Na2(H2O)8 Na2B4O7(H2O)5

(a), the X-ray diffraction photograph and the analytical data for the invariant point

Factor

0.369 0.732 0.030 0.007

Semi-Quant/%

(KCl + Na2B4O7·10H2O + K2B4O7·4H2O).

K2B4O7·4H2O KCl

Borax

True True True True

(NaCl + KCl + Borax)

01-075-0296 01-075-0296 01-075-0296 01-075-0296

metastable evaporation, the solutions were not stirred, and the crystal behavior of solid phase was observed periodically. When enough new solid phase appeared, the wet residue mixtures were taken from the solution. The solids were then approximately evaluated by the combined chemical analysis, of XP-300D Digital Polarizing Microscopy (Shanghai Caikon Optical Instrument Co,. Ltd., China) using an oil immersion, and further identification with X-ray diffraction (X'pert PRO, Spectris. Pte. Ltd., The Netherlands). Meanwhile, a 5.0 cm3 sample of the clarified solution was taken from the liquid phase of each polyethylene container through a filter pipette, and then diluted to a 250.0 cm3 final volume in a volumetric flask filled with DDW for the quantitative analysis of the compositions of the liquid phase. Some other filtrates were used to measure the relative physicochemical properties individually according to the analytical method. The remainder of the solution continued to be evaporated and reached a new metastable equilibrium.
