The effective separation of sodium chloride (NaCl) and sodium sulfate (Na2SO4) in wastewater can be achieved through various methods. The following are some commonly used separation techniques:

      1. Evaporation Crystallization Method

      one Preliminary evaporation concentration: Heat the wastewater and perform evaporation concentration. As the water gradually evaporates, the concentration of salt in the solution increases.

      two High temperature crystallization of sodium sulfate: Sodium sulfate forms its maximum solubility at 33~34 ℃. Before this temperature, the solubility increases with increasing temperature, and then decreases with increasing temperature. When the solution reaches a specific high temperature (usually higher than the saturation temperature of sodium chloride), sodium sulfate begins to crystallize and precipitate due to its relatively low solubility. By controlling the evaporation temperature and solution concentration, sodium sulfate can be induced to crystallize first and obtain sodium sulfate crystals through solid-liquid separation (such as filtration).

      three Crystallization of sodium chloride: After separating sodium sulfate, the mother liquor is further subjected to flash evaporation or cooling treatment. Under lower temperature conditions, sodium chloride begins to crystallize and precipitate, and the sodium chloride crystals are collected through solid-liquid separation.

      If the solubility difference between sodium sulfate and sodium chloride in wastewater is more significant at low temperatures, it can be considered to use freeze crystallization to preferentially crystallize sodium sulfate, and then recover it through solid-liquid separation.

      The specific steps of membrane salt separation are as follows:

      one Nanofiltration: By utilizing the high retention effect of nanofiltration membranes on divalent and higher valence ions, the separation of NaCl and Na2SO4 is achieved. The [Cl -]/[SO42-] in the produced water further increases, while the [Cl -]/[SO42-] in the concentrated water further decreases, all deviating from the co saturation curve as much as possible. The purity of the crystalline salt can meet the requirements of relevant standards.

      two Evaporation crystallization: The concentrated water after nanofiltration is sent to the evaporation crystallization system, and the salt in the wastewater is concentrated and crystallized through evaporation crystallization technology.

      However, the investment and operating costs of nanofiltration for salt separation are relatively high, and the nanofiltration membrane has problems such as rapid performance degradation and low recovery rate. As the operating time goes on, its salt separation effect will deteriorate.

      Fourth, MVR evaporation crystallization salt separation

      MVR evaporation crystallization salt separation technology adopts the principle of mechanical steam recompression to remove water from wastewater through evaporation process and achieve salt crystallization separation. Compared to traditional evaporation technology, MVR technology has higher thermal efficiency and lower energy consumption, making it excellent in treating high salinity wastewater.

      The specific steps of MVR evaporation crystallization salt separation method are as follows:

      one Evaporative crystallization: By utilizing the different solubility characteristics of sodium chloride and sodium sulfate, a closed loop system is designed, in which sodium chloride and sodium sulfate crystallize and precipitate in two evaporation processes. During the sodium sulfate evaporation crystallization process, sodium sulfate crystals are precipitated and sodium chloride is continuously concentrated. In the sodium chloride evaporation crystallization process, the nearly saturated nitrate mother liquor rich in sodium chloride from the sodium sulfate evaporation process is evaporated and crystallized to precipitate sodium chloride crystals. At the same time, the sodium sulfate content in the salt mother liquor increases but does not crystallize.

      two Recycling and Recycling: In the design of salt separation, there are recycled materials to improve the recovery rate of salt. In actual production, it may be possible to reduce the amount of recycling and improve product quality by discharging a portion of impurities.

      The advantages of MVR evaporation crystallization salt separation are simple process, strong operational reliability, low investment and operating costs, but the quality of crystallized salt is slightly lower, and the purity of the final product salt is significantly affected by the incoming materials.

      5、 Solvent extraction method

      Use solvents with selective solubility for sodium sulfate and sodium chloride, such as certain ionic liquids or organic solvents, to extract sodium sulfate first, and then recover sodium chloride through reverse extraction or evaporation. This method is suitable for wastewater under specific conditions, taking into account solvent selection, recovery, and potential environmental impacts.

      6、 Flotation Separation Method

      Under specific conditions, by adding appropriate surfactants or adjusting the pH value, sodium sulfate and sodium chloride can form different types of crystals, one of which may have better flotation performance. By flotation separation, the two can be effectively separated. This method requires high selection of operating conditions and additives.

      In practical applications, the effective separation of sodium sulfate and sodium chloride should be achieved by selecting or combining the above methods based on factors such as the specific composition, concentration, treatment scale, economic cost, and environmental requirements of the wastewater. At the same time, when designing separation processes, it is also necessary to consider the treatment of by-products, resource utilization, as well as the energy efficiency and environmental impact of the entire process.