The traditional research contents of salt lake water-salt system include the physical and chemical properties of brine and the phase equilibrium of water-salt system. The physical properties of brine include the color, turbidity, density and stratification of brine, and the chemical properties of brine mainly study the composition of brine and the brine chemistry of solarization salt fields. It should be pointed out that none of the above research contents can avoid biological and biological effects and their influence on the physicochemical properties of brine.
1. Physical properties of brine
1. Color of salt lake water
There are various colors of salt lake water, including blue-green, reddish, pink and red. The same salt lake has different colors in different seasons. The ancients recorded the color of salt lake water for a long time. More than 2, years ago, Li Daoyuan of the Northern Wei Dynasty wrote in his Notes on Water Classics: "Today's pool water (Yuncheng Salt Lake, Shanxi Province) is 7 miles east and west, and 17 miles north and south, purple and clear, muddy but not flowing." In the Song Dynasty, Shen Kuo said in his Meng Xi Bi Tan: "The salt pond in Jiezhou ... is full of red brine, and under the Sakamoto Spring, it is called' Chiyou blood' in slang ...". Here, "purple", "red" and "Chiyou blood" all refer to the color of brine in Yuncheng Salt Lake, Shanxi.
Whether it is a salt lake or a salt drying pond, we can see that with the gradual increase of salinity, the color of brine gradually deepens, from green to brown to red. In the past, people thought that the color of brine was related to the ionic composition of brine, but until now, people have a clear understanding that the color is related to the number of bacteria and algae living in different salinities and different biological species. Li Daoyuan's "purple", Shen Kuo's "red" and "blood red" all indicate that there were many red halophilic bacteria, halophilic algae and red brine shrimp in Yuncheng Salt Lake at that time.
2. turbidity
the turbidity of salt lake brine changes with place and time. The turbidity is determined by algae, suspended salt minerals and non-salt mineral debris. Taking sodium sulfate salt lake as an example, when mirabilite dissolves in spring and newly hatched brine shrimp and brine fly larvae consume algae, the turbidity of brine drops rapidly. Brine shrimp and brine fly larvae living in salt lakes are natural "cleaners", which make the turbidity of brine smaller and smaller.
3. Density
The density of salt lake brine is a reverse effect with the seasonal, annual and long-term fluctuation of lake water level.
4. Stratification phenomenon
Generally, brine in salt lakes is divided into two distinct layers: the upper part is transparent and odorless, and the lower part is dense. For example, the South Bay of the Great Salt Lake in the United States is divided into upper and lower layers with a water depth of 6.8～6.99m m. The upper layer is odorless and transparent, while the lower layer has a sharp increase in density and heavy pollution. In 1979, the density was 1.89 ~ 1.11 g/cm3 at the depth of 3m, and 1.166 ~ 1.17 g/cm3 at the depth of 7.6m.
5. Brine temperature
The fluctuation of brine temperature can lead to the change of ion saturation degree, which will cause selective precipitation and dissolution of salts in brine.
the temperature of brine is changeable, and it changes with the change of day and night and seasons.
(1) Average brine temperature
When brine evaporates, some crystals formed on the surface of brine will decompose or dissolve once they sink to the bottom of the pond due to the change of conditions, and the decomposition rate is related to the temperature difference and concentration change of brine. The experiment shows that the actual temperature of brine in the pool is .6 ~ .8 times of the air temperature, and the upper and lower concentrations of brine in the same pool are also different, and many salt crystals are not in balance with the brine in which they are located. Therefore, the average temperature of brine is usually the best judgment when predicting the type of salt crystals at the bottom of the sedimentation tank.
(2) the diurnal temperature change period of brine
The diurnal temperature change of brine is obvious and regular. The temperature change of brine day and night directly affects the salt precipitation. For example, in the Great Salt Lake of the United States, when the daytime temperature is 35℃, it is favorable for the formation of magnesium sylvite (MgSO4·KCl·3H2O) under the natural salt field conditions, while at night, when the temperature drops to 15℃, it is favorable for the formation of Picromerite (MGSO 4 K2SO 4 6H2O). Because ions are sensitive to temperature, salts can be precipitated at a lower temperature. As a result, a mixture of two salts can be obtained in the same brine in a single tank. For example, in Balikun Salt Lake, Xinjiang, long columnar mirabilite crystals crystallized at night can be seen in summer morning, and the sufficient activation energy provided by the sun near noon can make mirabilite crystallized several hours ago completely dissolve again.
(3) Seasonal temperature changes
Seasonal temperature changes in salt lakes can affect the deposition of salts. For example, the salts deposited in the Great Salt Lake from June to August are transformed into other salts when it is cold and rainy in winter, and these salts may undergo comprehensive chemical changes. For example, magnesium sylvite can be transformed into hard potassium chloride and cathartic salt, but it can be transformed into Picromerite if it comes into contact with brine containing sulfate. On the contrary, mirabilite precipitates in winter and dissolves again in summer.
6. evaporation rate
evaporation takes place on the surface of brine, and the crystallization of salt also takes place there. The higher the evaporation rate, the greater the possibility of brine supersaturation and salt formation. When the density of salt crystals on the surface of brine is high, it will sink to the bottom of the pool (or lake).
It is a traditional understanding that the factors that affect the evaporation rate are sun drying and wind blowing. In recent years, through the study of halophilic organisms in brine, it is found that red halophilic algae and red brine shrimp can dye the whole brine red, and absorb sunlight, which not only increases the temperature of brine, but also increases the evaporation rate.
7. The depth of brine and the granularity of salt crystals
The depth of salt field is closely related to the granularity of salt crystals. For example, in the crystallization pool of salt field, the granularity of the crystallized halite is smaller than that of the crystallized halite with the depth between 7.6cm and 3.5cm when the depth is < 7.6 cm and > 3.5 cm.
ii. chemical properties of brine
the chemical properties of salt lake brine mainly study cations, trace ions and radioactive chemical components in brine.
Modern salt lakes can be divided into chloride type, sulfate type, carbonate type and nitrate type according to the composition of brine (lake surface brine, intergranular brine and silt brine) (Zheng Xiyu, 1993). The salts separated from different types of salt lake brine are different.
it should be pointed out that the types of salt lake brine mentioned above are not static, but change with the change of restrictive conditions. For example, the evolution of hydrochemical types in salt lakes in Xinjiang is a proof. The HCO 3-Cl-Ca Na type water in bedrock mountain area is transformed into HCO 3-SO 4-Ca Mg and SO 4-Cl-Na Mg type water, and then into SO 4-Na Mg, even Cl, with the chemical, thermodynamic and hydrodynamic factors, such as continuous leaching, migration, adsorption, exchange, oxidation, precipitation and diffusion of elements. The concentration and density of brine changed from small to large, and the pH value changed from weak acidity to weak alkalinity.
iii. Brine chemistry of sun-exposed salt pans
The practice of artificial salt pans shows that the chemical reactions in salt pans are influenced by natural conditions such as wind, rain and temperature changes. In the past, chemists and geologists thought that the main factors affecting the crystallization of salt field minerals were evaporation rate, brine temperature, brine depth, brine segregation, retention time, pond permeability and so on. After in-depth study, the author thinks that the most important factor of mineral crystallization is the role of biology, whether it is artificial salt pond crystallization or natural crystallization. Later chapters will describe it.
The production technology of solarization salt field is quite different from the research on the balance of water and salt system in the laboratory. In the following, the brine chemistry of solarization salt pans is illustrated by taking the production processes of Yuncheng Salt Lake in Shanxi and Great Salt Lake in the United States as examples.
1. Salt making process of Yuncheng Salt Lake in Shanxi
Yuncheng Salt Lake in Shanxi is the earliest salt lake developed and utilized in China, with a development history of about four or five thousand years. The salt lake has made outstanding contributions to the development of Chinese civilization. Yuncheng Salt Lake is a typical Na _ 2SO _ 4 subtype water, and the composition of lake water belongs to the quaternary system of Na+, Mg _ 2+/Cl-. The preparation of halite from Na2SO4 subtype water is a pioneering work of the ancients, who practiced the metastable phase diagram of Na+, Mg2+/Cl- system.
The early production methods of Yuncheng Salt Lake were: insolation, natural crystallization, and intensive fishing. Li Daoyuan, a famous geographer in the Northern Wei Dynasty, wrote in Notes on Water Classics: "The purple pavilion is clear, muddy but not flowing, and the salt in the water is naturally printed, and it will be repeated in the morning and evening, and it will never be impaired." However, after the production practice of Qin, Han, Wei, Jin, Southern and Northern Dynasties and Sui Dynasties, the working people worked hard to explore the method of cultivating and drying salt, and finally the method of cultivating and watering salt appeared in the Tang Dynasty in the heyday of feudal society in China. The preface to the Ode to the Lingqinggong Shrine in Yanchi, Hedong, Datang, records the technological method of "waterlogging with muddy flow, pouring with special sources, steaming with yin and yang, and clearing with pregnancy".
The author made an investigation in Yuncheng Salt Lake in the late 196s. Now, the salt-making situation in this salt lake is described in combination with the salt-drying process in this area in history.
There are three kinds of brine for drying salt in Yuncheng Salt Lake: one is "old beach water", that is, light lake water, generally 5 ~ 1 be ′; One is "torrential water", which is also lake water, but the concentration is relatively large, generally 15 ~ 2 be'; The third type is well water, that is, bittern water pumped from salt lakes. Its main component is NaCl, which is bittern water after a layer of rock salt buried tens of meters underground from the surface is dissolved by water injection.
According to A Survey of Hedong Salt Law, written by Jiang Chunfang in Qing Dynasty, the brine used for salt making is different, so the salt making process is different. For example, the process of using well water is:
On the biogenesis of evaporite
The "Luo" mentioned here is the pool for storing brine to be evaporated. "Toudaoluo" is the first evaporation pool, and "Erdaoluo" is the second evaporation pool.
For another example, the salt-making process with "torrential water" is:
On the biogenesis of evaporite
The "whipping boy" here refers to an evaporation pool with a nitrate plate at the bottom of the pool. What is a nitrate board? Nitrate plate refers to the salt layer mainly composed of white sodium magnesium alum crystallized from the residual liquid after sun-drying. Due to the accumulation of time, the thickness of nitrate plate accumulated for thousands of years is several meters. The main purpose of laying nitrate board at the bottom of evaporation pool is to remove the "porridge paste" sulfate impurities crystallized in the process of salt making.
Low-concentration "Laotan water" is the most complicated salt-making process, and it was perfected as early as 14 years ago, which is a great contribution of ancient salt people in Yuncheng Salt Lake to the history of salt industry science and technology. The salt-making process is as follows:
On the biogenesis of evaporite
The "circle" here refers to the evaporation pool with no nitrate plate at the bottom, but only the mud bottom.
To understand the production process summarized by the ancients in modern physical and chemical language, the low-concentration "Laotan water" evaporates through the first, second and third circle evaporation pools, completing the primary stage of brine evaporation concentration and reaching the "whipping boy" evaporation pool. At this time, (Na, Mg)SO4 appears in the brine, so it begins to have porridge paste (the concentration is about 24 ~ 25 be'). This is the difference between seawater salt making and Na2SO4 subtype water salt making. The latter has an additional sulfate crystallization stage. How to dispose of sulfate in the process of brine evaporation, that is, the porridge paste of (Na, Mg)SO4, is the core problem of this process. In ancient times, the process of salt people's treatment of "porridge paste" sulfate substances was as follows: when the "whipping boy" evaporation pool was supersaturated (porridge paste), fresh water was added to eliminate the supersaturated state. At this time, the brine was called "sunny water", and a small trench (called a side trench) was dug at the appropriate edge of each pool, which was deeper than the depth of the pool. The side trench is not connected with the brine in the pool, so it needs to be separated by dikes. The brine in the pond is leached by the bottom nitrate plate (commonly known as the reed). The essence of leaching is that MgSO _ 4 in the sun water reacts with Na2SO _ 4 in the gypsum board to form white sodium magnesium vanadium (Na2SO _ 4 MgSO _ 4 4H2O), Na2SO _ 4 reacts in the gypsum board to form anhydrous mirabilite (Na2SO4), and calcium-containing sulfate reacts in the gypsum board to form glauberite (Na2CA (SO4) _ 2). When the author studied the composition of nitrate plate under microscope, it was found that the main mineral of nitrate plate was glauberite, followed by anhydrous mirabilite and glauberite, which was consistent with the result of brine chemical reaction in salt making. When the positive water flows through the honeycomb pores of the nitrate plate, it removes sulfate substances and flows to the side ditch. At this time, the water is called yin water, which is then sent to the crystallization pool by gravity or pumping, and finally pure salt is made.
to sum up, the metastable phase diagram of Na+, Mg2+/Cl- system can explain the technological principle of "steaming in the yin and yang phase, clearing and pregnant with each other" of sodium sulfate subtype water in Yuncheng Salt Lake, Shanxi Province, which was summarized by the ancients.
It should be pointed out that the research under the microscope of nitrate plate shows that minerals such as bischofite are composed of brine shrimp and brine fly fossils, which shows that organisms play an important role in the process of salt making.
After the founding of New China, Yuncheng Salt Lake not only produces salt, but also produces Sodium Sulfate, and it is the main export base of China Sodium Sulfate.
2. Salt-making process of the Great American Salt Lake
The Great American Salt Lake is the largest salt lake in North America, and its material composition is shown in Table 1-1.
Table 1-1 Chemical Composition of Brine in North Bay of Great Salt Lake
(According to F.J.Post, 1975)
A single pool for drying salt in Great Salt Lake and several salt pools are connected in series to form a pool group.
salt drying in a single pond: salt lake brine is used as raw material, and the content of Mg is kept at 7%, so that halite, carnallite and bischofite are crystallized.
Four ponds are connected in series to sun-salt: cathartic salt, Picromerite, potassium magnesium alum and potassium magnesium alum are generated.
In the salt drying pond, some salts take longer to crystallize than others. If they are not crystallized in brine, they will be transferred to another brine pond with different concentrations, and different salt groups will be deposited in the brine. For example, if a supersaturated brine from which magnesium sylvite, catharsis salt and halite are crystallized is transferred to another tank, the obtained mixed brine is beneficial to the crystallization of carnallite, while magnesium sylvite will disappear. The above can be expressed by two chemical reactions:
reaction 1:
biogenesis of evaporite
reaction 2:
biogenesis of evaporite.