The discharge technology of high salt wastewater separation and crystallization process is an efficient method for treating high salt wastewater. The following is a detailed analysis of this technology:
1、 Overview of High Salt Wastewater
High salt wastewater generally refers to difficult to degrade wastewater containing Na+, Ca2+, Mg2+, K+, Cl -, SO42-, NO3-, HCO3-, heavy metal ions, with a concentration greater than 1%, and TDS (total dissolved solids) in the range of 10000~25000mg/L. These wastewater mainly come from industrial fields such as petrochemicals, coal chemicals, pharmaceuticals, and pesticides. If discharged directly without treatment, it will cause harm to water bodies such as eutrophication and increased salt content, thereby affecting aquatic animals, plants, and human health.
II. Principle of Salt Separation Crystallization Process
The salt crystallization process mainly controls temperature and pressure conditions to precipitate salt in the form of crystals in wastewater, thereby achieving purification of wastewater and resource utilization of salt. This process mainly includes steps such as pretreatment, concentration and desalination, crystallization and solidification.
1. Pre treatment: Remove suspended solids, organic matter, oils, and some ions from wastewater, reduce wastewater hardness, and provide favorable conditions for subsequent treatment.
2. Concentrated desalination: By means of evaporation concentration and other methods, the salt in the wastewater is concentrated to a certain concentration, providing a basis for subsequent crystallization treatment.
3. Crystallization solidification: The salt in the concentrated wastewater is precipitated in the form of solid salt, achieving the purification of wastewater and the resource utilization of salt.
III. Main Technology Types
1. Mechanical vapor recompression (MVR) technology
Principle: The secondary steam compressed by the steam compressor is introduced into the thermal cycle of the original system to treat high salt wastewater and reduce the need for external heating.
Advantages: Significant energy-saving effect, small footprint, and simple structure.
Evaporator types: including forced circulation evaporator, rising film evaporator, plate evaporator, falling film evaporator, etc., each with its own characteristics, suitable for different materials and processing needs.
2. Direct evaporation crystallization process
Principle: The water in the wastewater is evaporated by heating to achieve saturation of solute concentration, and then the solute is crystallized and precipitated by cooling or other methods.
Advantages: The process is simple and easy to operate.
Disadvantages: High energy consumption and easy to cause secondary pollution.
3. Salt nitrate co production and salt crystallization process
Principle: By utilizing the temperature dependent solubility of different solutes in high salt wastewater, different salts can be separated by controlling the temperature and crystallization time.
Advantages: High product purity, high recovery rate, and low energy consumption.
Disadvantages: Equipment investment and operating costs are relatively high.
4. Membrane separation and crystallization combined process
Principle: Through membrane separation technology (such as reverse osmosis and nanofiltration), wastewater is pretreated to remove some salt and impurities, and then the remaining salt is completely precipitated through evaporation crystallization technology.
Advantages: Good treatment effect, suitable for complex wastewater treatment.
Fourth, Practical Application Cases
Taking the high salt water separation and crystallization device of Guoneng Baotou Coal Chemical Co., Ltd. as an example, the device adopts the process of "pretreatment membrane concentration nanofiltration salt separation freezing nitrate evaporation salt production" to achieve the complete reuse of water after desalination treatment of high salt wastewater, and the maximum resource recovery and recycling of inorganic salts. As of September 20, 2023, the device has been operating continuously for 754 days, constantly breaking new records for long-term operation of similar devices in China.
5、 Notes
1. Raw material preparation: Ensure the appropriate ratio of solute and solvent, and pre treat impurities in the raw materials.
2. Temperature control: Increasing the temperature within a certain range can accelerate the crystallization rate, but excessively high temperatures can cause the growth rate of crystals to be too fast, affecting their shape and size.
3. Mixing speed: Proper mixing can promote the mixing and mass transfer of the solution, but excessive mixing can lead to crystal breakage and aggregation.
4. Safety and environmental protection: During the evaporation crystallization and salt separation process, attention should be paid to safety issues such as wastewater leakage and exhaust emissions to prevent environmental pollution.
The discharge technology of high salt wastewater separation and crystallization process is an efficient and environmentally friendly wastewater treatment method with broad application prospects. In practical applications, it is necessary to select appropriate processes and equipment based on the characteristics of wastewater and treatment goals to achieve the best treatment effect.
