**1.1 Salt lakes and the classification of hydrochemistry**

Salt lakes are widely distributed in the world, and some famous salt lake resources are shown in Tables 1 and 2. In China, salt lakes are mainly located in the area of the Qinghai-Xizang (Tibet) Plateau, and the Autonomous Regions of Xinjiang and Inner Mongolia (M.P. Zheng et al., 1989). The composition of salt lake brines can be summarized to the complex salt-water multicomponent system (Li - Na – K – Ca - Mg - H - Cl – SO4 – B4O7 – OH- HCO3 – CO3 - H2O).

According to the chemical type of salt lake brines, it can be divided into five types, i.e. chloride type, sulphate type, carbonate type, nitrite type, and borate type among those salt lake resources in the world (Gao et al., 2007).

Chloride type: the component of brines in Death Sea, Mideast and Caerhan Salt Lake in China belongs to the system of chloride type (Na – K – Mg - Cl - H2O), and the main precipitation of salts are halite (NaCl), sylvite (KCl), carnallite (KCl•MgCl2•7H2O), and bischofite (MgCl2•6H2O).

Sulphate type: this kind of salt lake resources is similar with the sea water system (Na – K – Mg – Cl – SO4 – H2O), and it can be divided into two kinds of hypotypes i.e. sodium sulphate and magnesium sulphate. As to sodium sulphate hypotype, the Great Salt Lake in America, the gulf of Kara-Bogaz-Golin Urkmenistan, and Da-Xiao Qaidan in China belong to this hypotype with the main deposit of glauberite (Na2SO4•CaSO4), glauber salt (Na2SO4·10H2O), halite, galserite (Na2SO4•3K2SO4), schonenite (K2SO4•MgSO4•6H2O), and so on. As to magnesium sulphate hypotype, there are Yunchen Salt Lake in Shanxi Provine and Chaka Salt Lakes in Qinghai Province, China, especially Salt Lakes of the Qaidam Basin in Qinghai Province are a sub-type of magnesium sulphate brines famous for their

Stable and Metastable Phase Equilibria in the Salt-Water Systems 401

Caerhan and Caida, China 10 million tons Kata Baca, Argentina Sever million tons

America 6.34 million tons Chile 4.3 million tons Canada 6.6 million tons Greenbusbse, Australian 6 million tons Table 2. Statistical distribution of the lithium reserves in the world (Song, 2000; Zhao, 2003)

It is essential to study the stable and metastable phase equilibria in multi-component systems at different temperatures for its application in the fields of chemical, chemical engineering such as dissolution, crystallization, distillation, extraction and separation.

The research method for the stable phase equilibria of salt-water system is isothermal dissolution method. It is worthy of pointing out that the status of the stable phase equilibrium of salt-water system is the in a sealed condition under stirring sufficiently, and the speed of dissolution and crystallization of equilibrium solid phase is completely equal with the marker of no change for the liquid phase composition. As to the thermodynamic stable equilibrium studies aiming at sea water system (Na – K – Mg – Cl – SO4 – H2O), J.H. Vant'hollf (1912) was in the earliest to report the stable phase diagram at 293.15 K with

In order to accelerate the exploiting of Qaidam Basin, China, a number of the stable phase equilibria of salt-water systems were published at recent decades (Li et al., 2006; Song, 1998,

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,

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

Country and section Lithium storage capacity,

Uyuni, Bolivia More than 19 million tons Silver and Searles, US More than 10 million tons

(Li2O)

Type of lithium resources

Salt lakes

Type of crystalline rocks

**1.2 Phase equilibria of salt-water systems** 

isothermal dissolution method.

2000; Song & Du, 1986; Song & Yao, 2001, 2003).

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

and it is in the status of thermodynamic non-equilibrium.

**1.2.1 The stable phase equilibria of salt-water systems** 

abundance of lithium, potassium, magnesium and boron resources (Zheng et al., 1989). The main precipitation of salts are halite, glauber salt, blodite (Na2SO4•MgSO4•7H2O), and epsom salt (MgSO4•7H2O).

Carbonate type: this type belongs to the system (Na – K – Cl – CO3 - SO4 – H2O), and Atacama Salt Lake in Chile and Zabuye Salt Lake in Tibet are the famous carbonate type of salt lake. The main precipitated minerals are thermonatrite (Na2CO3•10H2O), baking soda (NaHCO3), natron (Na2CO3•10H2O), glauber salt, and halite.

Nitrite type: the brine composition of this type salt lake can be summarized as the system (Na – K – Mg - Cl – NO3 - SO4 – H2O). The type salt lake main locates in the salt lake area in the northern of Chile among the salt lake group of Andes in the South-America, semi and dry salts in Luobubo and Wuzunbulake Lakes in Xinjiang, the northern of China. There are natratime saltier (NaNO3), niter (KNO3), darapskite (NaNO3•Na2SO4·H2O), POTASSIUMdarapskite (KNO3•K2SO4•H2O), humberstonite (NaNO3•Na2SO4•2MgSO4•6H2O).

Borate type: it can be divided into carbonate-borate hypotype and sulphate-borate hypotype. Searles Salt Lake in America, Banguo Lake and Zabuye Salt Lakes in Tibet, China belong to the former, and the brines mostly belong to the system (Na - K - Cl - B4O7 - CO3 - HCO3 - SO4 - H2O). In order to prove the industrial development of Searles Lake brines, Teeple (1929) published a monograph after a series of salt-water equilibrium data on Searles lake brine containing carbonate and borate systems. The latter includes Dong-xi-tai Lake, Da-xiao-chaidan Lake and Yiliping Lake in Qinghai Province, Zhachangchaka Lake in Tibet, China. In those lake area, the natural borate minerals of raphite (NaO•CaO•3B2O3•16H2O), pinnoite (MgO•B2O3•3H2O), chloropinnoite (2MgO•2B2O3•MgCl2•14H2O), inderite (2MgO•3B2O3•15H2O), hungchanoite (MgO•2B2O3•9H2O), mcallisterite (MgO•3B2O3•7.5H2O), kurnakovite (2MgO•3B2O3•15H2O) and hydroborate were precipitated (Zheng et al., 1988; Gao et al., 2007). In addition, the concentration of lithium ion exists in the surface brine of salt lakes.


Table 1. Basic data of salt lakes and their salt reserves in the world. unit, /t (Song, 2000)

abundance of lithium, potassium, magnesium and boron resources (Zheng et al., 1989). The main precipitation of salts are halite, glauber salt, blodite (Na2SO4•MgSO4•7H2O), and

Carbonate type: this type belongs to the system (Na – K – Cl – CO3 - SO4 – H2O), and Atacama Salt Lake in Chile and Zabuye Salt Lake in Tibet are the famous carbonate type of salt lake. The main precipitated minerals are thermonatrite (Na2CO3•10H2O), baking soda

Nitrite type: the brine composition of this type salt lake can be summarized as the system (Na – K – Mg - Cl – NO3 - SO4 – H2O). The type salt lake main locates in the salt lake area in the northern of Chile among the salt lake group of Andes in the South-America, semi and dry salts in Luobubo and Wuzunbulake Lakes in Xinjiang, the northern of China. There are natratime saltier (NaNO3), niter (KNO3), darapskite (NaNO3•Na2SO4·H2O), POTASSIUM-

Borate type: it can be divided into carbonate-borate hypotype and sulphate-borate hypotype. Searles Salt Lake in America, Banguo Lake and Zabuye Salt Lakes in Tibet, China belong to the former, and the brines mostly belong to the system (Na - K - Cl - B4O7 - CO3 - HCO3 - SO4 - H2O). In order to prove the industrial development of Searles Lake brines, Teeple (1929) published a monograph after a series of salt-water equilibrium data on Searles lake brine containing carbonate and borate systems. The latter includes Dong-xi-tai Lake, Da-xiao-chaidan Lake and Yiliping Lake in Qinghai Province, Zhachangchaka Lake in Tibet, China. In those lake area, the natural borate minerals of raphite (NaO•CaO•3B2O3•16H2O), pinnoite (MgO•B2O3•3H2O), chloropinnoite (2MgO•2B2O3•MgCl2•14H2O), inderite (2MgO•3B2O3•15H2O), hungchanoite (MgO•2B2O3•9H2O), mcallisterite (MgO•3B2O3•7.5H2O), kurnakovite (2MgO•3B2O3•15H2O) and hydroborate were precipitated (Zheng et al., 1988; Gao et al., 2007). In addition, the concentration of lithium

darapskite (KNO3•K2SO4•H2O), humberstonite (NaNO3•Na2SO4•2MgSO4•6H2O).

Searles lake,

Intragranular

Table 1. Basic data of salt lakes and their salt reserves in the world. unit, /t (Song, 2000)

2300 1400

brine

1.1×10<sup>8</sup> — 1.2×10<sup>8</sup> — 2.8×106 — — — 1.6×107 —

Intragranular

Atacama, Chile Caerhan, China Zabuye,

Intragranular

2900 5882

brine

3×10<sup>8</sup> 4.3×1010 2.7×10<sup>9</sup> — — — — 3.4×105 5.5×106 —

Tibet

2677 120 ~3

6.6×107 2×10<sup>8</sup> 5.7×10<sup>8</sup> — — — — — 1.8×106 —

US

512 1000

brine

2.8×106 — — — 2.7×106 — — — 3×107 7.5×10<sup>4</sup>

(NaHCO3), natron (Na2CO3•10H2O), glauber salt, and halite.

ion exists in the surface brine of salt lakes.

lake, US

1280 3600 ~5

1×10<sup>8</sup> 3.2×10<sup>9</sup> 1.2×10<sup>9</sup> 1.7×107 3.2×106 — — — 1.9×106 —

Salt lakes Death Sea Great


2×10<sup>9</sup> 1.2×1010 2.2×1010 — 1.7×107 6×10<sup>9</sup> 1×10<sup>8</sup> 1×10<sup>9</sup> — —

Altitude, /m Area, /km2 Dept, m

KCl NaCl MgCl<sup>2</sup> MgSO<sup>4</sup> LiCl CaCl2 CaSO4 MgBr<sup>2</sup> B2O3 WO3

epsom salt (MgSO4•7H2O).


Table 2. Statistical distribution of the lithium reserves in the world (Song, 2000; Zhao, 2003)
