Crystallization is one of the basic and common processes in chemical production. Crystallization processes fall into three major categories: cooling crystallization, evaporation crystallization and vacuum crystallization. Cooling crystallization essentially separates the solute from saturated solution in the form of crystal by reducing the temperature.
This method does not remove the solvent, but the solution will be cooled into supersaturated solution. It is also applicable for substances whose solubility increases markedly as temperature increases. Cooling crystallization becomes a widely used industrial crystallization method.
2.Industry application and advantage of cooling crystallization technology
The cooling crystallization technology applied in the industry achieves crystallization by cooling or freezing the thermal saturated solution. Compared to evaporation crystallization, cooling crystallization is more widely applicable to substances whose solubility increases markedly as its temperature goes up. These substances include ammonium nitrate, potassium nitrate, ammonium chloride, sodium phosphate and Glauber's salt. The coefficient change of temperature and solubility is great.
When the temperature drops, the solubility of these substances also decreases and the supersaturated solution is formed. Because of its thermodynamic instability, the solute will crystallize out of the solution. Cooling crystallization method uses the difference of the solubility of each component in the solution as the temperature changes (See Figure 1) to achieve the purpose of material separation.
In industrial applications, cooling crystallization is often combined with concentration technology so that the solution is first evaporated and concentrated to form saturated solution. Then the saturated solution is cooled and crystallized to obtain the solute by centrifugal separation.
3.Application of cooling crystallization technology in wastewater treatment field
Industrial wastewater often contains a large amount of salt, and the composition of wastewater is complicated. The saturated concentration of each component is also different. Thus, traditional evaporation crystallization methods cannot separate the component salt in the crystal products. In other words, the resulting crystallized product is not available as a final product. It still costs money and manpower to deal with.
Below is a typical application of cooling crystallization technology in one chemical plant in China. The main component of the wastewater generated in this chemical plant is Na2SO4. According to the wastewater treatment requirement, it is necessary to extract the sodium sulfate from the solution.
For this objective, evaporation concentration technology and cooling crystallization technology were adopted to treat wastewater and obtain sodium sulfate crystal at the same time as value added by-product. The specific flow chart is shown in Figure 2.
After the saline Na2SO4 wastewater is preheated by condensation water from the evaporation process, it enters into the 1st and 2nd effect heater to evaporate and condensate. After reaching the saturation concentration, sodium sulfate decahydrate is separated and frozen through the freezing crystallization device. The mother liquid after centrifugal separation contains a small amount of sodium sulfate, which can be treated by other wastewater treatment methods.
The separated crystalline slurry is mainly composed of sodium sulfate decahydrate crystals, and it also contains small amounts of organic compounds and other impurities. This needs to be refined into anhydrous sodium sulfate. First of all, the sodium sulfate decahydrate enters a dissolving tank to get the sodium sulfate slurry. Then, it enters MVR evaporator for evaporation and crystallization.
Sodium sulfate crystal is produced due to high temperature. After solid-liquid separation by centrifugal action, the liquid is dried in a fluidized bed and sodium sulfate crystal is also produced. Finally, the sodium sulfate in accordance with the standards is obtained.
Cooling crystallization technology is applied reasonably in the above process, using the physical property that the solubility of sodium sulfate decreases with the decrease of temperature. By reasonable regulation of temperature, the separation of sodium sulfate and other impurities in wastewater is realized. The obtained product, sodium sulfate, can be sold as a product, which not only eliminates the cost of dealing with miscellaneous salt, but also produces additional value.
What’s more, as a common separation technology, cooling crystallization has many advantages, such as simple process principle, easy operation, and so on. It also has extensive application in industry. The refrigerant used for cooling crystallization is the solution of -10℃, which will go into the tube type heat exchanger to recycle.
The evaporation crystallization adopts MVR process, which has the advantages of low energy consumption and high operation efficiency. If these two process methods are combined, higher economic benefits will be produced.
Due to national policy and the enterprise's own demand, the importance of industrial wastewater treatment is increasing day by day. A large amount of salt contained in wastewater still has high economic benefits. But the traditional methods are often unable to obtain the pure industrial salt that can be used as a product. Therefore, the cooling crystallization method has great advantages and feasibility in dealing with industrial wastewater and realizing zero wastewater discharge, and it will be used more widely in the wastewater treatment field.