Crystallization refers to the process of forming solid particles from a homogeneous phase, including the transformation from vapor to solid, the solidification of liquid melts, and the crystallization of liquids. In the food industry, the crystallization process includes: crystallization from an aqueous solution to obtain a solid product, freezing of water from a dilute solution to concentrate the solute, that is, freeze concentration, and then controlling the crystallization operation to obtain certain rheological properties.
Next, the editor will introduce several technologies and developments for the production of crystals:
1. Freeze crystallization method: the solution is cooled to reach saturation to produce crystallization.
This method is used for substances whose solubility decreases with temperature drop, such as: ammonium nitrate, potassium nitrate, ammonium chloride, sodium phosphate, Glauber’s salt, etc. The temperature coefficient of solubility of these substances changes greatly. When the temperature drops, the solubility of these substances Decrease, forming a supersaturated solution, in a thermodynamically unstable state, the solute will crystallize out from the solution, chemical substances are especially suitable for separation by freezing crystallization.
2. Evaporate and concentrate the solution to make the solution saturated and crystallize: it is often used for substances with little change in solubility. For example, salt field sun-dried salt (sodium chloride).
Seawater or brine is introduced into the salt field, and the water is evaporated and concentrated by wind and sun to crystallize table salt. "Tiangong Kaiwu" records the fact that our ancestors adopted this method to produce salt.
3. Fractional crystallization method: This method is suitable for the difference in the solubility of certain similar salts.
Due to this difference, when the mixture salts are distributed between the solid phase and the solution phase, the components with low solubility will be enriched in the solid phase, and those with high solubility will remain in the liquid phase. This method is widely used in the crystallization of various substances. separate. For example, the separation of double salts of rare earth elements can also be used to remove impurity components. The fractional crystallization process usually uses evaporative crystallization or freezing (cooling) crystallization.
After step-by-step operation, some insoluble components and easily soluble components will be enriched in the first and last parts of the process respectively to form products with higher purity. In the atomic energy industry, zirconium containing less than 0.01% hafnium is required, which is obtained by the fractional crystallization method of fluorine complexes. The advantage of this method is that it is easy to operate and does not consume reagents. The disadvantage is that it is difficult to achieve continuous production.
4. Chemical reaction crystallization: This is a commonly used method in industry. The precipitation (crystallization) of uranium concentrate in the uranium hydrometallurgy process is a typical chemical reaction crystallization process.
The degree of supersaturation of the solution, the stirring speed, the nature of the solvent, the composition of the solution and the pH value are all factors that directly or indirectly affect the crystallization. There are many factors affecting the crystallization process. When the process conditions are optimal, the key to realize industrial production is to design an excellent reaction equipment. Crystallization equipment can be divided into three categories: cooling crystallizer, evaporative crystallizer and evaporative cooling crystallizer according to the method of establishing supersaturation.
Each category can be subdivided according to its internal circulation, stirring and grading devices. Such as cooling crystallizer can be divided into natural circulation cooling crystallizer and forced circulation cooling crystallizer. The latter can be divided into three types: using agitator, using rotating scraper and using forced circulation pump.
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