The basic meaning of crystallization

The process of solute precipitation from solution can be divided into two stages: nucleation and crystal growth, and the driving force of both stages is supersaturation of solution (the concentration of solute in solution exceeds its saturated solubility). There are three forms of nucleation: homogeneous nucleation, heterogeneous nucleation and secondary nucleation. In supersaturated state, the process of spontaneous nucleation in solution is called homogeneous nucleation; The process of solution nucleation induced by foreign objects (such as dust in the atmosphere) is called a heterogeneous nucleation; The nucleation process in a solution containing solute crystals is called secondary nucleation. Secondary nucleation also belongs to heterogeneous nucleation process, which occurs under the induction of tiny grains between crystals or when crystals collide with other solids (walls, stirrers, etc.). ).

The requirement of crystallization operation is to prepare pure crystals with a certain particle size distribution. The particle size and distribution of crystal products mainly depend on the nucleation rate (the number of crystal nuclei produced per unit volume of solution per unit time), the crystal growth rate (the increase of a certain linear size of crystal per unit time) and the average residence time of crystal in the crystallizer. The supersaturation of solution is related to nucleation rate and crystal growth rate, so it has an important influence on the particle size and distribution of crystal products. In low supersaturation solution, the ratio of crystal growth rate to nucleation rate is large (see figure), so the obtained crystal is large and complete, but the crystallization rate is very slow. In industrial crystallizer, supersaturation is usually controlled in metastable region. At this time, the crystallizer has high production capacity and can obtain crystal products with a certain size. Complete crystallization.

The specific crystal form formed by a crystal under certain conditions is called crystal habit. Adding or removing a substance (called a crystal habit regulator) to the solution can change the crystal habit and make the obtained crystal have another shape. This has certain significance for industrial crystallization. Crystal habit changing agents are usually some surface active substances and metal or nonmetal ions.

The process of crystal formation in solution is called crystallization. There are generally two kinds of crystallization methods: one is evaporation solvent method, which is suitable for substances whose solubility is not affected by temperature. This method is used in "drying salt" in coastal areas. The other is the method of cooling the hot saturated solution. This method is suitable for substances whose solubility increases with temperature. For example, the salt lake in the northern region has high temperature in summer and no crystals appear on the lake; When the temperature drops every winter, substances such as caustic soda (Na2CO3 10H2O) and mirabilite (Na2SO4 10H2O) will be precipitated in the salt lake. In order to obtain large and complete crystals in the laboratory, the method of slowly lowering the temperature and slowing down the crystallization speed is often used.

People can't see the macroscopic phenomenon of simultaneous dissolution and crystallization of substances in solution, but there are actually two kinds of reversible movements of particles in solution. By changing the temperature or reducing the solvent, the crystallization rate of solute particles at a certain temperature can be greater than the dissolution rate, so that the solute can be crystallized from the solution. In the operation of purifying chemical reagents by crystallization and recrystallization, the selection of solvents is a key issue related to the purification quality and recovery rate. When choosing a suitable solvent, we should pay attention to the following issues:

The selected solvent should not react with the chemical reagent to be purified. For example, aliphatic halogenated hydrocarbon compounds should not be used as solvents for crystallization and recrystallization of basic compounds; Alcohol compounds should not be used as solvents for crystallization and recrystallization of ester compounds, nor should they be used as solvents for crystallization and recrystallization of amino acid hydrochloride.

The selected solvent should have a large solubility for the chemical reagent to be purified when it is hot, but the solubility of the chemical reagent to be purified will be greatly reduced at a lower temperature.

The selected solvent has great solubility for impurities that may exist in the chemical reagent to be purified, so that the impurities can remain in the mother liquor during the crystallization and recrystallization of the chemical reagent to be purified, and will not precipitate with the crystal during the crystallization and recrystallization; Or the solubility is very small, so that when the chemical reagent to be purified is dissolved by heating, impurities are rarely dissolved in the hot solvent, and are removed by hot filtration.

The boiling point of the selected solvent should not be too high, so as to avoid the solvent adhering to the crystal surface during crystallization and recrystallization, which is not easy to remove.

Commonly used solvents for crystallization and recrystallization are: water, methanol, ethanol, isopropanol, acetone, ethyl acetate, chloroform, glacial acetic acid, dioxane, carbon tetrachloride, benzene, petroleum ether, etc. In addition, toluene, nitromethane, ether, dimethylformamide, dimethyl sulfoxide, etc. It is also often used. Dimethylformamide and dimethyl sulfoxide have great solubility, so they can be tried when other suitable solvents cannot be found. However, it is often difficult to separate the crystal from the solvent, and the solvent adsorbed on the crystal is not easy to remove because of its high boiling point, which is its disadvantage. Although ether is a commonly used solvent, if there are other suitable solvents, it is best not to use ether, because on the one hand, ether is flammable and explosive, and it is particularly dangerous to use, so be especially careful; On the other hand, the chemical reagent to be purified is precipitated on the bottle wall because ether is easy to creep and volatilize along the bottle wall, which affects the purity of crystallization.

When choosing a solvent, we must know the structure of the chemical reagent to be purified, because solutes are often easily soluble in solvents with similar structures-the principle of "similar mutual solubility". Polar substances are easy to dissolve in polar solvents and difficult to dissolve in nonpolar solvents; On the contrary, nonpolar substances are easy to dissolve in nonpolar solvents and difficult to dissolve in polar solvents. This solubility law has a certain guiding role for experimental work. For example, the chemical reagent to be purified is a nonpolar compound, and it is known in experiments that its solubility in isopropanol is too small, so isopropanol is not suitable for its crystallization and recrystallization. At this time, it is generally not necessary to carry out experiments with solvents with larger polarity, such as methanol and water, but with solvents with smaller polarity, such as acetone, dioxane, benzene and petroleum ether. The final choice of suitable solvent can only be determined by experiments.

As for the precipitation of crystals, crystals will precipitate after the filtrate obtained by filtration is cooled. When the solution is rapidly cooled and stirred with cold water or ice water, very small crystals can be obtained, while when the hot solution is left standing at room temperature and slowly cooled, even large crystals can be obtained.

If the solution does not crystallize after cooling, glass can be used to rub the container wall under the liquid surface, or seed crystals can be added, or the temperature of the solution can be further reduced (cooled with ice water or other frozen solutions). If the solution does not precipitate crystals and get oil after cooling, it can be reheated until a clear hot solution is formed and allowed to cool itself. The solution is constantly stirred with a glass rod and rubbed or inoculated on the wall of the grinder to accelerate the precipitation of crystals. If the oil still starts to precipitate, it should be stirred vigorously immediately to disperse the oil droplets. The process of crystal formation in solution is called crystallization. There are generally two kinds of crystallization methods: one is evaporation solvent method, which is suitable for substances whose solubility is not affected by temperature. This method is used in "drying salt" in coastal areas. The other is the method of cooling the hot saturated solution. This method is suitable for substances whose solubility increases with temperature. For example, the salt lake in the northern region has high temperature in summer and no crystals appear on the lake; When the temperature drops every winter, substances such as caustic soda (Na2CO3 10H2O) and mirabilite (Na2SO4 10H2O) will be precipitated in the salt lake. In order to obtain large and complete crystals in the laboratory, the method of slowly lowering the temperature and slowing down the crystallization speed is often used.

Only when the solution is supersaturated will the crystals precipitate.

Generally, there are two crystallization methods, one is evaporation crystallization and the other is cooling crystallization.

Evaporative crystallization

principle

Evaporative crystallization: the evaporation of the solvent changes the solution from unsaturated to saturated. If it continues to evaporate, excess solute will precipitate in the form of crystals, which is called evaporative crystallization. For example, when the mixture of NaCl and KNO _ 3 contains more NaCl and less KNO _ 3, NaCl can be separated first and then KNO _ 3 can be separated by this method.

You can observe the solubility curve. When the solubility increases obviously with the increase of temperature, this solute is called steep rise type, and vice versa.

When the steep-rising solution is mixed with the slow-rising solution, if the steep-rising solution is to be separated, it can be separated by cooling crystallization, and if the slow-rising solute is to be separated, it can be separated by evaporation crystallization, that is, the evaporation crystallization method is suitable for substances whose solubility does not change much with temperature, such as sodium chloride.

For example, potassium nitrate belongs to the steep rising type and sodium chloride belongs to the slow rising type, so sodium chloride can be separated by evaporation crystallization or by cooling crystallization.

Experimental process

It is carried out in an evaporating dish. The evaporating dish is placed on the iron ring of the iron frame, and the poured liquid does not exceed 2/3 of the volume of the evaporating dish. In the process of evaporation, the liquid is constantly stirred with a glass rod to prevent the liquid from uneven heating and splashing. When a large amount of solid precipitates or only a small amount of liquid remains, stop heating and use the waste heat to evaporate the liquid.

filter

Evaporation is usually accompanied by filtration. Here are some common filtering methods:

1 normal pressure filtration, using instruments such as glass funnel, small beaker, glass rod, iron frame, etc. Attention should be paid to the following problems: when stacking filter paper, try to make it close to the inner wall of glass funnel, which will form continuous water droplets and speed up filtration. This is not obviously different from the slow filtration in general, but it is very different when hot filtration is needed. For example, when preparing KNO3, if your speed is too slow, it will be cooled in the funnel, and part of KNO3 will precipitate and block the funnel mouth, and the experimental effect will not be ideal.

2 Decompression filtration, using instruments such as Buchner funnel, bottle suction, filter paper, bottle washing, glass rod, circulating vacuum pump, etc. Attention should be paid to the following issues: when choosing filter paper, it should be moderate. When the suction bottle is connected to the circulating vacuum pump, the periphery of the filter paper should be wetted with a washing bottle, and then the product to be filtered should be transferred to it (if there is a solution part, it should be drained with a glass rod). Cooling crystallization

Cooling crystallization

Evaporation concentration cooling crystallization

brief introduction

First, the solution is heated to evaporate the solvent into a saturated solution. At this time, the temperature of the hot saturated solution decreases, and the solute whose solubility changes greatly with the temperature will precipitate crystals, which is called cooling crystallization. For example, when the mixture of NaCl and KNO3 contains more KNO3 but less NaCl, this method can be used to separate KNO3 first and then NaCl.

After cooling and crystallization, the solute mass becomes smaller.

The quality of the solvent remains unchanged, and the quality of the solution becomes smaller.

The mass fraction of solute becomes smaller.

The solution is saturated.

Crystallization is usually accompanied by filtration. This paper introduces several common filtration methods: 1 normal pressure filtration, and the instruments used are: glass funnel, small beaker, glass rod, iron frame, etc. Attention should be paid to the following problems: when stacking filter paper, try to make it close to the inner wall of glass funnel, which will form continuous water droplets and speed up filtration. This is not obviously different from the slow filtration in general, but it is very different when hot filtration is needed. For example, when preparing KNO3, if your speed is too slow, it will be cooled in the funnel, and part of KNO3 will precipitate and block the funnel mouth, and the experimental effect will not be ideal. Decompression filtration, the instruments used are: Buchner funnel, bottle suction, filter paper, bottle washing, glass rod, circulating vacuum pump, etc. Attention should be paid to the following issues: filter paper should be selected properly. When the suction bottle is connected to the circulating vacuum pump, the periphery of the filter paper should be wetted by washing the bottle, and then the product to be filtered should be transferred to it (if there is a solution, it should be sucked dry with a glass rod).

principle

1. The principle of cooling crystallization is that the temperature decreases, the solubility of the substance decreases, the solution reaches saturation, and excess insoluble solute will be precipitated. The principle of evaporative crystallization is that at constant temperature or before and after evaporation, the temperature remains unchanged, the solubility remains unchanged, and the water content decreases. When the solution reaches saturation, the excess solute will precipitate. For example, the saline-alkali lake basks in salt in summer and catches alkali in winter, which is the reason.

2. If two soluble substances are mixed and separated or purified, whoever is easy to reach saturation will use crystallization. For example, if sodium chloride contains a small amount of sodium carbonate impurities, the crystallization method of sodium chloride will be used, that is, evaporation crystallization, and vice versa.

3. Of course it matters. For substances whose solubility curve shows an obvious upward trend, the solubility changes greatly with temperature, and for substances whose solubility curve is slightly flat, they are generally crystallized by cooling, but their solubility changes little with temperature, and they are generally crystallized by evaporation.

4. Supplementary explanation: "Who is easy to reach saturation" refers to which of the two soluble substances has a large content, then it will reach saturation first. It is easy to precipitate at this time, so we adopt its crystallization method.

5. Except for calcium hydroxide and gas, the solubility curve decreases with the increase of temperature, so the temperature should be reduced when cooling the hot saturated solution, and the other methods are the same. Crystallization is indeed a science, and domestic crystallization experts first recommend Academician Wang Jingkang from the School of Chemical Engineering of Tianjin University. There are many theoretical books about this, but when it comes to every substance or every substance, they are not exactly the same. Common things may be theoretical, and the discussion about the crystallization process of each kind of compound may be most helpful to everyone. Selection of solvent (single or compound), crystallization temperature, stirring speed, stirring mode, selection of supersaturation, crystal growth time, way and speed of solvent dropping, etc. In addition, in the process of dissolution, crystallization and crystal growth, the above temperature, stirring speed, time, adding method and speed are not exactly the same. So many factors are superimposed, which is even more difficult. Generally speaking, the main conditions should be selected first, so that the crystallization process can be continued and crystals can be obtained, and then the above conditions can be optimized. When the conditions are ripe, pilot test and production can be carried out. If it is theoretical study, the emphasis may be different. If it is applied research, the solvent is relatively easy to choose. The key is whether the supersaturation point can be found with this solvent and whether the supersaturation point interval is well controlled. If the supersaturation point is not easy to choose, or the supersaturation is not enough, it is difficult to crystallize, let alone grow crystals. At this time, it may be necessary to consider the composite solvent and adjust the supersaturation interval. So I think the crystallization process is the crystallization process, and the control of various conditions is the most important at this time. The crystallization process is well controlled, and the crystallization process is about 60% complete.

The crystal growth process is relatively easy to control, mainly by optimizing parameters and control conditions. Generally, there is no problem, and the amplification process is basically no problem. If you do basic research, the physical properties are not very clear, and the study of crystallization process may take a lot of time and energy. But once you understand the whole process, it is still very valuable. Precipitation of crystalline substances from liquid (solution or melt) or gas raw materials is a unit operation in the process of heat and mass transfer. The process of crystal precipitation from melt is used for single crystal preparation, and the process of crystal precipitation from gas is used for vacuum coating. However, in chemical production, crystals are often precipitated from the solution. According to the characteristics of liquid-solid equilibrium, crystallization operation can not only obtain solid solute from solution, but also separate solute from impurities, thus improving the purity of products. As early as 5000 years ago, people began to use solar energy to evaporate seawater to make salt. Crystallization has developed into an economical and effective operation to prepare pure solid products from impure solutions. Many chemical products (such as dyes, paints, drugs and various salts) can be prepared by crystallization. The obtained crystal product not only has certain purity, but also has beautiful appearance, which is convenient for packaging, transportation, storage and application.