**1.2.2 The metastable phase equilibria of salt-water systems**

However, the phenomena of super-saturation of brines containing magnesium sulfate, borate is often found both in natural salt lakes and solar ponds around the world. Especially for salt lake brine and seawater systems, the natural evaporation is in a autogenetic process with the exchange of energy and substances in the open-ended system , and it is controlled by the radiant supply of solar energy with temperature difference, relative humidity, and air current, etc. In other word, it is impossible to reach the thermodynamic stable equilibrium, and it is in the status of thermodynamic non-equilibrium.

For the thermodynamic non-equilibrium phase diagram of the sea water system as called "solar phase diagram" in the first, N.S. Kurnakov (1938) was in the first to report the experimental diagrams based on the natural brine evaporation, and further called

Stable and Metastable Phase Equilibria in the Salt-Water Systems 403

temperature. Under low temperature, the refrigerator or freezing saline bath is commonly used. Figure 1 shows the common used equalizer pipe with a stirrer. The artificial synthesis complex put in the pipe to gradually reach equilibria under vigorous stirring. In order to avoid the evaporation of water, the fluid seal installing is needed, and the sampling branch pipe is also needed to seal. Usually, for aqueous quaternary system study, a series of artificial synthesis complex, normally no less than 30, was needed to be done one by one the experimental time consume is equivalence large. At present, a thermostatic shaker whose temperature could controlled with temperature precision of ± 0.1 K can be used for the measurement of stable phase equilibrium (Deng et al., 2002; Deng, 2004). The advantage is that a series artificial synthesis complexes which is loading in each sealed bottle can be put

In this study, the stable phase equilibria system (NaCl - KCl – Na2B4O7 - K2B4O7 - H2O) at 298.15 K, a thermostatic shaker (model HZQ-C) whose temperature was controlled within

The isothermal evaporation method was commonly used, and Figure 2 is our designed isothermal evaporation device in our laboratory (Guo et al., 2010). The isothermal evaporation chamber was consisted of evaporating container, precise thermometer to keep the evaporating temperature as a constant and electric fan to simulate the wind in situ, and the solar energy simulating system with electrical contact thermograph, electric relay and heating lamp. The temperature controlling apparatus is made up of an electric relay, an

Fig. 1. Apparatus of equalizer pipe. 1, thermostatic water-circulator bath; 2, pipe body;

In this example of the metastable phase equilibria system (NaCl - KCl – Na2B4O7 - K2B4O7 - H2O) at 308.15 K, the isothermal evaporation box was used. In an air-conditioned laboratory, a thermal insulation material box (70 cm long, 65 cm wide, 60 cm high) with an apparatus to

3, assay; 4, stirrer; 5, fluid seal; 6, rubber seal lock; 7, sampling branch pipe.

**2.2 Apparatus for the metastable phase equilibria in the salt-water system** 

in and vigorous shaking together.

0.1 K was used for the measurement of phase equilibrium.

electrical contact thermograph and heating lamps.

"metastable phase diagram" for the same system (Na – K – Mg – Cl – SO4 – H2O) at (288.15, 298.15, and 308.15) K was reported on the basis of isothermal evaporation method (Jin, et al., 1980, 2001, 2002; Sun, 1992). Therefore, the metastable phase equilibria research is essential to predict the crystallized path of evaporation of the salt lake brine.

The isothermal evaporation phase diagrams of the sea water system at different temperature show a large difference with Vant'hoff stable phase diagram. The crystallization fields of leonite (MgSO4.K2SO4.4H2O), and kainite (KCl.MgSO4.3H2O) are all disappear whereas the crystallization field of picromerite (MgSO4.K2SO4.6H2O) increases by 20-fold, which is of great importance for producing potassium sulfate or potassium-magnesium fertilizer.

Therefore, in order to separate and utilize the mixture salts effectively by salt-field engineering or solar ponds in Qaidam Basin, studies on the phase equilibria of salt-water systems are focused on the metastable phase equilibria and phase diagrams at present years (Deng et al., 2011; Deng et al., 2008a-g; Deng, et al., 2009a-c; Wang & Deng, 2008, 2010; Li & Deng, 2009; Li et al., 2010; Liu et al., 2011; Meng & Deng, 2011; Guo et al., 2010; Gao & Deng, 2011a-b; Wang et al., 2011a-b).
