**2.1 Apparatus for the stable phase equilibria in the salt-water system**

Stable phase equilibria are the thermodynamic equilibria. In order to reach the isothermal dissolve equilibrium, the apparatus mainly contains two parts i.e. constant temperature installing and equilibrator. Therefore, experimental apparatus depends on the target of temperature. Generally, thermostatic water-circulator bath is used under normal atmospheric temperature, and thermostatic oil-circulator bath is chosen at higher level

"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

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

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,

Pitzer and co-workers have developed an ion interaction model and published a series of papers (Pitzer, 1973a-b, 1974a-b, 1975, 1977, 1995, 2000; Pabalan & Pitzer, 1987) which gave a set of expressions for osmotic coefficients of the solution and mean activity coefficient of electrolytes in the solution. Expressions of the chemical equilibrium model for conventional single ion activity coefficients derived are more convenient to use in solubility calculations (Harvie & Weare, 1980; Harvie et al.1984; Felmy & Weare, 1986;

In this chapter, as an example, the stable and metastable phase equilibria in the salt-water system (NaCl - KCl – Na2B4O7 - K2B4O7 - H2O), which is of great importance to describe the metastable behavior in order to separate and purify the mixture salts of borax and halosylvite were introduced in detail. The stable phase diagrams of the sub-ternary systems (NaCl - Na2B4O7 - H2O), (KCl –K2B4O7 - H2O), (Na2B4O7 - K2B4O7 - H2O) at 298.15 K and the metastable phase diagrams of the sub-ternary systems (NaCl - Na2B4O7 - H2O) at 308.15 K for the mentioned reciprocal quaternary system were systematically studied on our previous researches under several scientific funding supports. The theoretical prediction for the stable solubility of this reciprocal quaternary system was also briefly introduced based

Stable phase equilibria are the thermodynamic equilibria. In order to reach the isothermal dissolve equilibrium, the apparatus mainly contains two parts i.e. constant temperature installing and equilibrator. Therefore, experimental apparatus depends on the target of temperature. Generally, thermostatic water-circulator bath is used under normal atmospheric temperature, and thermostatic oil-circulator bath is chosen at higher level

**1.2.3 Solubility prediction for the phase equilibria of salt-water systems** 

**2.1 Apparatus for the stable phase equilibria in the salt-water system** 

to predict the crystallized path of evaporation of the salt lake brine.

potassium-magnesium fertilizer.

2011a-b; Wang et al., 2011a-b).

Donad & Kean, 1985).

on the ion-interaction model.

**2. Apparatus** 

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 and vigorous shaking together.

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 0.1 K was used for the measurement of phase equilibrium.
