The crystallization is an important technology for separating or purifying some materials, which can be widely used in a great variety of fields, such as chemical engineering, food engineering, and bioengineering.
What is Crystallization?
The crystallization is a process in which solid substances in crystalline state precipitates from steam, liquid or molten material. The crystallization process is to separate and purify the product that we need from a complex mixed system.
Industrial Applications of Crystallization
The crystallization can be used in the bioengineering for refining of amino acids, organic acids, antibiotics, vitamins and nucleic acids and other products. In the separation of chemical products, the crystallization separation method is often used when the final product with high purity is needed and the melting temperature and solubility of impurities and products are greatly different, such as the extraction of coal. Furthermore, the wastewater treatment can be treated by crystallization method with the addition of a small amount of precipitant when pollutants have a certain value and can be recycled and reused, such as phosphorus and heavy metals.
How does Crystallization Happen?
The happen of crystallization is mainly related to four factors, including the supersaturation, cooling (evaporation) rate, crystal seed, impurities and stirring. The supersaturation acts as the driving force in the crystallization process as well as the prerequisite for the production of crystal products. It is also the most important factor affecting the crystallization operation. Generally, the increase of supersaturation can cause the increase of rate of crystal growth. But at the same time, the solution viscosity can be increased and the crystallization rate can be retarded. There are generally three methods achieving the supersaturation of solution: cooling, evaporation, and chemical reaction. The rapid cooling and evaporation will make the solution reach the supersaturation state quickly, and even reach a relatively high supersaturation by directly going through the meta-stable zone to obtain a large quantity of tiny crystals. Otherwise, the slow cooling and evaporation can usually get large crystals. The main purpose of adding crystal seeds is to control the number of crystal nucleus in order to obtain large and uniform crystal products. Some impurities can inhibit the growth of crystals and some can promote them. Most of the crystallization equipment is equipped with a stirring device. The stirring can promote diffusion and accelerate the formation of crystals. Pay attention to the type and speed of stirring.
What is the Best Solvent for Crystallization?
- The selected solvent shouldn’t react with the chemical reagent to be purified.
- The selected solvent should be more soluble to the chemical reagent to be purified when they are heated, and less soluble to the chemical reagent at a low temperature.
- The selected solvent should be more soluble to the possible impurities in the chemical reagent so that the impurities can remain in the mother solution when the chemical reagent is crystallized or recrystallized and can’t be precipitated along with crystals. Or the solvent is less soluble to the possible impurities so that the impurities rarely dissolve in the hot solvent when the chemical reagent is heated to dissolve and the impurities can be removed during the thermal filtration.
- The boiling point of the solvent shouldn’t be too high for fear that the solvent attaches to the crystal surface during the crystallization or re-crystallization, which may be difficult to be completely removed.