The salt crystallization method for treating high salt wastewater is an efficient and environmentally friendly wastewater treatment technology, especially suitable for high salt wastewater generated in industries such as coal chemical industry. The following is a detailed introduction to the process:

        1、 Process Overview

        Salt crystallization method refers to the separation of different salt components (such as NaCl, Na2SO4, KCl, etc.) in industrial wastewater through thermal or membrane methods, and then the separation of water and elemental salts is achieved through crystallization. The condensed water in this process can be reused, and salt crystals can be used as raw materials for other industries, thereby achieving the goal of reducing the amount of new water and zero discharge of wastewater.

        2、 Main technical route

        There are two main ideas for salt crystallization process:

        1. Thermal salt crystallization process:

        Direct evaporation crystallization process: When a certain salt content has a significant advantage in high salt wastewater, this process can be considered. The pre treated high salt wastewater is first further concentrated and reduced through an evaporator to make the dominant salt components close to saturation, and then enters a pure salt crystallizer to extract most of the sodium chloride or sodium sulfate. However, this process is highly dependent on the characteristics of inorganic salt components in raw water, and may result in low purity and poor whiteness of crude salt products due to the influence of organic matter and impurity salt components.

        Salt nitrate co production and salt crystallization process: This process mainly utilizes the temperature dependent difference in solubility between sodium chloride and sodium sulfate. By controlling the evaporation crystallization temperature, sodium sulfate and sodium chloride can be obtained separately. This process is widely used in industry, but when applied in the wastewater industry, the impact of impurities such as organic matter needs to be considered.

        Low temperature crystallization process: This process utilizes the large difference in solubility between sodium sulfate and sodium chloride at low temperatures to achieve salt separation. Concentrate high salt wastewater containing a mixture of sodium sulfate and sodium chloride at a high temperature to a certain extent, and then rapidly cool it down, which can crystallize and precipitate a large amount of solid sodium sulfate (saltpeter) decahydrate. In order to obtain sodium chloride, it also needs to be combined with high-temperature crystallization process. This process can achieve high recovery rates of sodium sulfate and sodium chloride, and the purity of crystalline salts is easy to control.

     2. Membrane salt crystallization process:

        Nanofiltration salt separation process: mainly utilizing the selective retention characteristics of nanofiltration membranes for divalent salts, to achieve the separation of monovalent salt sodium chloride and divalent salt sodium sulfate in the liquid phase. By separately crystallizing the nanofiltration permeate and concentrate, the recovery of sodium chloride and sodium sulfate crystalline salts is ultimately achieved. This process has a high recovery rate and purity of crystalline salts.

        Electrodialysis desalination process: An electrodialysis system consisting of a monovalent selective anion exchange membrane and a common cation exchange membrane is used to separate sodium chloride and sodium sulfate. Under the action of a direct current electric field, chloride ions and sodium ions in the raw water enter the concentration chamber through the monovalent selective anion exchange membrane and cation exchange membrane, respectively, to obtain concentrated sodium chloride solution. The raw water in the dilution chamber is separated from sodium sulfate due to the relative increase in sodium sulfate content caused by the decrease in sodium chloride concentration.

        3、 Technological advantages and challenges

        1. Advantages:

        Zero discharge and resource utilization of high salt wastewater can be achieved.

        The obtained crystalline salt can be used as a raw material for other industries and has high economic value.

        The membrane based salt crystallization process has a high recovery rate and purity of crystalline salts.

        2. Challenge:

        The thermal salt crystallization process is highly dependent on the characteristics of inorganic salt components in raw water and may be affected by organic matter and impurity salt components.

        The membrane based salt crystallization process requires the selection of appropriate membrane materials and operating conditions to ensure separation efficiency and membrane lifespan.

        The entire process requires precise control and monitoring to ensure stable operation and product quality.

        4、 Application Case

        In the field of high salt wastewater treatment such as coal chemical industry, the salt crystallization method has been widely used. For example, some coal chemical enterprises have adopted the nanofiltration low-temperature crystallization membrane salt separation process to treat high salt wastewater, successfully achieving zero discharge of wastewater and resource utilization of crystallized salt. This process not only improves the recovery rate and purity of crystalline salt products, but also reduces the overall processing cost.