Water treatment process method for treating high-salinity wastewater through freezing eutectic crystallization method

Abstract

The freezing eutectic crystallization method for treating the high-salinity wastewater is a wastewater treatment process which comprises the following steps: cooling the high-salinity wastewater to a solution eutectic point, separating out ice crystals and salt crystals at the same time, and separating the ice crystals and the salt crystals through a three-phase separator so as to obtain solid salt and purified water. Compared with conventional cooling crystallization, the process has the advantages that salt and ice can be continuously produced, subsequent evaporation treatment is not needed, the purity of recycled salt is high, the salt content of purified water is low, compared with an evaporation process, the energy consumption is low, the manufacturing cost is low, and the water treatment cost is remarkably reduced. The method can be used in the fields of high-salinity wastewater treatment, inorganic salt recovery, seawater desalination and the like.

Description

1. Technical Field

[0001] The invention is mainly used in the field of environmental protection, sewage treatment, and treatment of high-salt wastewater. 2. Background Technology

[0002] 1. Research Background

[0003] Industrial high-salt wastewater is defined as wastewater containing a total dissolved solids (TDS) concentration higher than 3.5%. The wastewater usually contains soluble inorganic salts such as sodium chloride, sodium sulfate, and calcium chloride. Industrial high-salt wastewater is found in every factory. Water is one of the essential and most consumed solvents in the production process. Whether it is the solutes that need to be actively added for production or the solutes that are continuously accumulated by wastewater recycling, wastewater with high salt content will be formed. If high-salt wastewater is discharged directly without treatment, it will cause serious harm to water bodies, soil and related organisms. For example, if high-salt wastewater penetrates into the soil environment, it will poison soil layer organisms, and plant roots and stems will wither due to cell water loss, leading to the complete disorder and corruption of the soil ecosystem. If it is discharged into the water body, it will lead to the deterioration of the water ecological environment, the death of aquatic animals, and the acceleration of the eutrophication process of rivers and lakes.

[0004] 2. Background technology of high-salt wastewater treatment

[0005] At present, the main methods for treating industrial high-salt wastewater are evaporation and membrane separation.

[0006] 1) Evaporation method The evaporation method is a treatment method that heats the salt-containing wastewater with steam to reach the boiling point of the solution, turns the water into water vapor, separates the solvent water, and concentrates the solution. The evaporation method of high-salt wastewater is often used for high-concentration inorganic salt wastewater with a salt content greater than 40,000 mg·L-1. For low-concentration salt-containing wastewater with an inorganic salt content of less than 4%, the desalination efficiency of evaporation technology is too low and it is not suitable to use this technology. Thermal evaporation technology mainly includes multiple-effect evaporation technology and mechanical steam cycle compression technology. Multiple-effect evaporation technology refers to connecting three or more evaporators in series at the same time. The hot steam passes through several evaporators in turn, and the hot steam of the previous evaporator enters the next evaporator, evaporating step by step. The purpose of desalination of high-salt wastewater is achieved by multiple utilization of the heat source. Mechanical steam cycle compression technology is referred to as MVR technology. In this process, the steam compressor effectively utilizes the heat source, recompresses the heat source, and makes it reciprocate continuously to improve the thermal utilization efficiency of the steam.

[0007] Currently, this technology is widely used in the treatment of high-salt wastewater. However, the high energy consumption, high cost, easy corrosion and easy clogging of the evaporation method have always troubled the majority of users.

[0008] 2) Membrane separation method: Membrane is a material with selective permeability that can achieve the purpose of material purification, concentration and separation. The pore size of the membrane is generally in the micron level, and it is divided into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis from large to small. According to the difference in driving force, the membrane can be divided into pressure-driven and electric-driven membranes. Several membranes commonly used in industrial high-salt wastewater treatment are: electrodialysis membrane, reverse osmosis membrane, and nanofiltration membrane. Electrodialysis technology has been well developed and is widely used in seawater desalination and industrial wastewater treatment.

[0009] Due to the material characteristics of the membrane, the membrane system has high requirements for the quality of the influent water. Wastewater with severe pollution will clog the membrane pores and reduce the service life of the membrane. In addition, when treating saline wastewater of different concentrations, it corresponds to different system pressures, system costs, and operating costs. When treating high-salt wastewater, it is often unable to cope with the high system pressure and high cost, and the concentrate usually needs to be treated secondary through an evaporation process.

[0010] 3) In practical applications, two or more methods are often used in combination, such as first using membrane concentration to reduce the amount of wastewater, then using evaporation concentration to make the solute reach saturation concentration, and then cooling and crystallizing to produce salt particles.

[0011] The energy consumption of evaporation is high, the amount of water is large, and the cost of directly using evaporation for low-concentration salt-containing wastewater is expensive. It is necessary to reduce the amount of water to be treated through pre-concentration.

[0012] Membrane systems can play the role of concentration, but the concentrate must eventually be processed by evaporation, so they generally cannot be used alone. III. Summary of the Invention

[0013] Frozen eutectic crystallization

[0014] 1) Concept of frozen eutectic crystallization

[0015] Frozen eutectic crystallization is a new technology for treating high-salt wastewater. This technology cools high-salt wastewater to the eutectic point temperature. When the solution reaches the eutectic state, ice and salt coexist in the crystallizer. Due to the density difference between ice and salt, ice will float on the top of the saturated solution, while salt will sink to the bottom of the crystallizer. The two solid phases can be easily separated, achieving the purpose of solid-liquid separation and recovery of ice and inorganic salts. Then, the ice and inorganic salts are subjected to simple process treatments respectively, and finally purified water and salt crystals are obtained. Figure 1

[0016] 2) Basic principles of frozen eutectic crystallization

[0017] like Figure 2 The basic principle and basis of frozen eutectic crystallization technology can be explained by a simple binary water-salt system phase diagram. The initial state of the solution is at point X, which is an unsaturated state, with a mass concentration of W. X , temperature is TX , by cooling down the system, the state moves from point X to point A. When the temperature drops to A, ice crystals begin to form in the system. Continue to cool down the system so that point A moves along the AC direction. During the movement to point C, the number of ice crystals continues to grow, and the concentration of salt in the remaining solution also increases. When the system is at point C, the three phases of concentrated solution, ice crystals, and salt crystals are in equilibrium. If the temperature continues to drop, ice crystals and salt will precipitate at the same time. Point C is called the eutectic point of this solution system. Similarly, the initial state of the solution is at point Y, which is an unsaturated state, and the mass concentration is W. Y , temperature is T Y , the state moves from point Y to point A1 by cooling. When the temperature drops to A1, salt crystals begin to form in the system. Continuously cooling the system causes point A1 to move in the direction of A1C. During the movement to point C, the number of salt crystals continues to increase, and the concentration of salt in the remaining solution also decreases due to the precipitation of salt crystals. When the system is at point C, the three phases of the concentrate, salt crystals, and ice crystals reach equilibrium. If the temperature continues to drop, ice crystals and salt crystals coexist in the system at the same time, which is the eutectic point C. The frozen eutectic crystallization technology uses the principle of the eutectic point. The ice crystals and salt in the wastewater precipitate and coexist in the crystallizer at the same time. The difference in density is then used to achieve the purpose of separating salt and ice in the wastewater. Through subsequent treatment, high-purity salt and purified water can be obtained.

[0018] 3) Characteristics and advantages of frozen eutectic crystallization

[0019] As a new technology for high-salinity wastewater treatment, the frozen eutectic crystallization process can effectively reuse the useful inorganic salts in the wastewater and obtain high-purity water by freezing the high-salinity wastewater until the temperature reaches the eutectic point of the system. Compared with traditional high-salinity wastewater treatment technology, this method has the following advantages:

[0020] 3.1) Low energy consumption for frozen eutectic crystallization

[0021] The heat of fusion of water (at 0°C) is 6.00 kJ / mol, while the heat of evaporation (at 100°C) is 40.65 kJ / mol. The energy required for the phase change from water to ice is only about 15% of the energy required for the corresponding phase change from water to steam. In practical applications, combined with the mechanical energy consumption of other auxiliary equipment, the total system energy consumption is only about 30% of that of multi-effect evaporation under the same processing capacity.

[0022] 3.2) Low cost of frozen eutectic crystallization

[0023] Since frozen eutectic crystallization operates in a low-temperature environment and at normal pressure, the corrosiveness of the brine is significantly reduced, so relatively common materials can be used to produce equipment, such as 316L, and the service life is greatly increased. However, due to the high temperature operation of evaporation equipment, the brine is highly corrosive, and expensive materials such as titanium are often required, and the overall service life is still very low.

[0024] 3.3) The operation of frozen eutectic crystallization is more stable

[0025] Since the frozen eutectic crystallization is carried out when the solution reaches the eutectic point, the solution concentration remains unchanged and will not continue to concentrate. The excess salt will be supersaturated and precipitated. Therefore, the operating conditions of the same solution are relatively simple and stable. However, the final concentration point must be manually controlled during the evaporation method. Otherwise, the evaporation tube will be blocked and the system will have serious failures.

[0026] 3.4) Frozen eutectic crystallization produces high purity salt and high water desalination rate

[0027] When there is more than one solute in the solution, due to the different solubilities of different solutes and the corresponding eutectic points, the salt crystals will not precipitate at the same time, ensuring the purity of the salt. Usually the purity of solid salt is above 99%.

[0028] Solvent water has natural exclusivity when it freezes. Whether it is inorganic salt or organic matter, the removal rate is above 90%. After subsequent treatment measures, the desalination rate can reach more than 95%. IV. Description of the drawings

[0029] 1, Figure 1 This is a frozen eutectic crystallization flow chart, which mainly introduces the main components of the system and related equipment

[0030] 2, Figure 2 This is a phase diagram of a binary water-salt system, which mainly introduces the three freezing processes of the binary water-salt system. V. Specific Implementation Methods

[0031] The specific implementation methods of the present invention are as follows:

[0032] Take 10% sodium sulfate solution as an example

[0033] 3. Through data calculation and experimental tests, we know that the eutectic point of the solution is -2℃~-3℃, so the appropriate choice is the refrigeration system.

[0034] 4. Design the appropriate size of the ice crystal generator and auxiliary facilities such as the three-phase separator based on the specific water volume.

[0035] 5. The refrigeration system, separation system and post-processing system are integrated into a whole, such as Figure 1 , starting the frozen eutectic crystallization.

[0036] 6. Pump the wastewater into the raw water pool (the liquid level will decrease after ice crystals are generated and separated), and automatically replenish the liquid through liquid level control to keep the liquid level constant.

[0037] 7. Pump the wastewater in the raw water tank into the ice crystal generator and then into the three-phase separator. When the liquid level in the three-phase separator reaches the specified level, open the connecting valve between the three-phase separator and the raw water tank and keep it consistent with the inlet flow rate. At this time, the liquid levels of the raw water tank and the three-phase separator reach a dynamic balance.

[0038] 8. Turn on the refrigeration system to cool the wastewater. As the temperature drops, the solubility of the solute decreases, the solution reaches saturation, and salt crystals begin to precipitate. When the temperature continues to drop to the eutectic point (the solution concentration is 4% to 5%), ice crystals and salt crystals precipitate at the same time and are separated by a three-phase separator. The ice crystals float on the water surface, while the salt crystals sink to the bottom.

[0039] 9. Open the ice scraping system on the top of the separator, scrape the floating ice to the washing tower, wash away the salt crystals on the surface of the ice crystals, and then send the ice crystals to the centrifugal device through the conveyor belt and filtration system, and enter the ice melting pool after centrifugation.

[0040] 10. When a certain amount of salt crystals accumulate at the bottom of the separator, open the bottom salt discharge valve and send the salt crystals into the centrifugal device for dehydration to obtain solid salt. This solid salt is sodium sulfate decahydrate crystals. After drying to remove water, it becomes an anhydrous sodium sulfate product with a purity of 99%, which can be sold or reused.

[0041] 11. The ice is melted in the ice melting pool by introducing hot air from the compressor and precooling it with the raw water through heat exchange, so as to obtain ice-melted water with a concentration of 0.3%. According to the specific requirements of the owner, it can be directly discharged or reused after being filtered through a low-pressure membrane.

Claims

1. Frozen eutectic crystallization is a new technology for treating high-salt wastewater. This technology cools high-salt wastewater to the eutectic point temperature. When the solution reaches the eutectic state, ice and salt coexist in the crystallizer. Due to the density difference between ice and salt, ice will be suspended at the top of the saturated solution, while salt will sink to the bottom of the crystallizer. The two solid phases can be easily separated, achieving the purpose of solid-liquid separation and recovery of ice and inorganic salts. Then the ice and inorganic salts are subjected to simple process treatments respectively, and finally purified water and salt crystals are obtained.

Images