High-salinity wastewater zero-discharge salt separation system and method

Abstract

The invention provides a high-salinity wastewater zero-discharge salt separation system and method. The method comprises the following steps: 1) high-salinity wastewater is subjected to nanofiltration concentration, nanofiltration concentrated water is subjected to evaporation and high-temperature crystallization, mixed salt is separated out, and residual mother liquor is subjected to evaporation drying to obtain carnallite; 2) performing reverse osmosis concentration on the nanofiltration produced water to obtain reverse osmosis concentrated water and reverse osmosis produced water; 3) re-dissolving the mixed salt with part of reverse osmosis concentrated water, separating out sodium sulfate decahydrate through low-temperature crystallization, re-dissolving, and performing high-temperature crystallization to obtain an anhydrous sodium sulfate finished product; 4) mixing mother liquor discharged through low-temperature crystallization with part of reverse osmosis concentrated water, performing nanofiltration secondary separation, and performing high-temperature crystallization on secondary nanofiltration produced water to prepare sodium chloride salt; and 5) dissolving the mixed salt by using a part of the secondary nanofiltration concentrated water, and combining the other part with the primary nanofiltration concentrated water to separate out the mixed salt. According to the system and the method, the design is reasonable and optimized, the process investment is low, the overall design evaporation capacity is smaller, large water quality fluctuation can be dealt with, and the operation is more stable and reliable.

Description

Technical Field

[0001] The present invention relates to the field of wastewater treatment, and in particular to a high-salinity wastewater zero-discharge salt separation system and method. Background Art

[0002] In recent years, there are more and more zero-discharge projects for industrial wastewater. As the environmental protection capacity of many regions in China has reached its limit, many new projects and existing projects have been required to undergo zero-discharge transformation. In the process of zero-discharge of wastewater, salt precipitation is inevitable. At present, national policies have made it clear that miscellaneous salts are "hazardous wastes". Therefore, in addition to zero-discharge, the salts in the wastewater need to be recycled, that is, one or several salts in the wastewater are separated and made into chemical finished raw materials that meet the requirements and re-enter the industrial field for use. At present, the main salts in industrial wastewater are NaCl, Na2SO4 and mixed salts of other impurities. Therefore, most of the current zero-discharge salt separation technologies focus on how to extract as much sodium chloride and sodium sulfate products as possible from wastewater that meet the requirements of industrial salt.

[0003] Early domestic high-salinity wastewater zero-discharge treatment projects did not consider the resource utilization of inorganic salts much. Generally, mixed salts were produced in the evaporation and crystallization stage. Crystallized salts are easily soluble in water and usually contain organic matter and even heavy metals. They are difficult to dispose of as ordinary solid waste. Even if they are disposed of as hazardous waste at a high cost, it is difficult for general hazardous waste disposal centers to absorb them due to their huge output. Therefore, the resource utilization of crystallized salts in the high-salinity wastewater zero-discharge treatment process is imperative.

[0004] At present, the separation objects of the salt separation and crystallization process in high-salinity wastewater are mainly sodium chloride and sodium sulfate. There are two main ideas for salt separation and crystallization process: one is to first perform nanofiltration on the high-salinity water, and then conduct secondary concentration of the nanofiltration water and concentrated water before crystallization.

[0005] One is to directly utilize the concentration difference and solubility difference of different inorganic salts in wastewater, and realize the separation of salt by controlling the appropriate operating temperature and concentration multiple during the crystallization process, which is commonly known as the thermal salt separation crystallization process. For example, CN111977882A discloses a simple and efficient high-salt wastewater zero-discharge quality separation crystallization device and process. The quality separation crystallization device includes a high-efficiency concentration device, an evaporation crystallization device, a freezing crystallization device, a nitrate recrystallization device and a mixed salt crystallization device. Each device is connected to a liquid pump and a solid conveyor through a pipeline; the treatment process of this application is used to achieve the purpose of zero discharge of high-salt wastewater with a salt-nitrate ratio (the mass ratio of sodium chloride to sodium sulfate) ≥10:1 or ≤1:1. Since this type of process is limited to a specific salt-nitrate ratio range, the scope of use is very limited.

[0006] The second method is to utilize the differences in ionic radius or charge characteristics of chloride ions and sulfate ions to separate or enrich different salts before crystallization through a membrane separation process, and then use a thermal crystallization process to obtain a solid, which is a membrane salt separation crystallization process.

[0007] The main disadvantage of the existing technology is that the salt-nitrate ratio of the incoming water needs to be stable within a narrow range. The system's processing capacity is difficult to cope with large water quality fluctuations, and its ability to resist water quality shocks is poor. When the salt-nitrate ratio fluctuates greatly, the system is difficult to adapt, and there have even been situations where the design water quality and the actual operating water quality deviate greatly, resulting in the system being unable to handle the problem. In order to cope with this situation in the design, it is necessary to consider the fluctuation of the salt-nitrate ratio within a certain range and recalculate the system, which ultimately leads to a large system design scale. Many evaporation and crystallization equipment can only operate at a low load level, resulting in reduced operating efficiency, resulting in waste of investment and increased operating costs.

[0008] The purpose of the present invention is to minimize the impact of water quality fluctuations on the salt separation system through reasonable optimization design of the system, overcome the disadvantages of the existing zero-emission salt separation process that it is difficult to balance investment and operation effects, and achieve maximum system adaptability at the lowest investment and operation cost. Summary of the invention

[0009] In order to solve the above technical problems, the first aspect of the present invention provides a method for zero-discharge salt separation of high-salinity wastewater, comprising the following steps:

[0010] 1) The high-salt wastewater is concentrated by a nanofiltration process to obtain a primary nanofiltration concentrated water and a primary nanofiltration produced water (the nanofiltration concentrated water is a solution obtained by nanofiltration interception and concentration on the nanofiltration concentrated water side, and the nanofiltration produced water is a solution produced in addition to the nanofiltration concentrated water during the nanofiltration process). The primary nanofiltration concentrated water is evaporated and crystallized at high temperature to precipitate a mixed salt of sodium chloride and sodium sulfate. The residual mother liquor after crystallization is evaporated and dried to obtain impurity salts;

[0011] 2) The primary nanofiltration water produced in step 1) is concentrated by reverse osmosis to obtain reverse osmosis concentrated water and reverse osmosis produced water (the reverse osmosis concentrated water is a solution obtained by reverse osmosis interception and concentration, and the reverse osmosis produced water is a solution produced in addition to the concentrated water during the reverse osmosis process), and the reverse osmosis produced water is used as recycled water;

[0012] 3) using part of the reverse osmosis concentrated water in step 2) to re-dissolve the mixed salt in step 1), refining the salt water to remove organic matter and hardness, treating at low temperature to precipitate sodium sulfate decahydrate, re-dissolving the precipitated sodium sulfate decahydrate with evaporation condensate, and crystallizing at high temperature to obtain anhydrous sodium sulfate finished product;

[0013] 4) In step 3), the remaining mother liquor discharged from the precipitation of sodium sulfate decahydrate is mixed with part of the reverse osmosis concentrated water in step 2), and then subjected to secondary nanofiltration separation to obtain secondary nanofiltration concentrated water and secondary nanofiltration produced water, and the secondary nanofiltration produced water is evaporated and concentrated and then crystallized at high temperature to prepare sodium chloride salt;

[0014] 5) In step 4), part of the secondary nanofiltration concentrate and part of the reverse osmosis concentrate are used to redissolve the mixed salt in step 1), and the remaining part of the secondary nanofiltration concentrate and the primary nanofiltration concentrate in step 1) are sent to the mixed salt evaporation crystallization together.

[0015] In some embodiments, in step 1), the high-salinity wastewater is initially subjected to removal of hardness and organic matter before being concentrated by nanofiltration.

[0016] In some embodiments, in step 3), the brine is refined by filtering through a tubular membrane with powdered carbon added and refining with a chelating resin after the tubular membrane.

[0017] In some embodiments, in step 3), the temperature of the low temperature treatment is 0-10°C.

[0018] In some embodiments, in step 4), the mother liquor remaining from the high temperature crystallization is mixed with the concentrated water from the primary nanofiltration in step 1) to be evaporated and high temperature crystallized.

[0019] The present invention also provides a high-salinity wastewater zero-discharge salt separation system, such as Figure 1 As shown, including:

[0020] A first nanofilter 1, the first nanofilter 1 is connected to a first evaporator 2, and the primary nanofiltration concentrated water obtained by concentrating the first nanofilter 1 can be transported to the first evaporator 2;

[0021] The first evaporator 2 is also connected to the first high-temperature crystallizer 3, and the primary nanofiltration concentrated water can be transported to the first high-temperature crystallizer 3 after evaporation in the first evaporator 2;

[0022] The high-temperature crystallizer 3 is also connected to the mother liquor dryer 4 and the mixed salt return dissolver 6 respectively. The mixed salt of sodium chloride and sodium sulfate precipitated from the high-temperature crystallizer 3 can be transported to the mixed salt return dissolver 6. The residual mother liquor crystallized from the high-temperature crystallizer 3 can be transported to the mother liquor dryer 4. The mother liquor dryer 4 can evaporate and dry the mother liquor to obtain impurity salt.

[0023] The mixed salt return dissolver 6 is also connected to the brine refining unit 7 and the low-temperature crystallizer 8 in sequence, and the mixed salt dissolved in the mixed salt return dissolver 6 can be transported to the brine refining unit 7 and the low-temperature crystallizer 8 in sequence;

[0024] The low-temperature crystallizer 8 is also connected to the second nanofilter 9 and the mirabilite re-dissolving device 12 respectively. The low-temperature crystallizer 8 processes the dissolved mixed salt after being treated by the brine refining unit 7 at a low temperature, and the precipitated sodium sulfate decahydrate can be transported to the mirabilite re-dissolving device 12. The remaining discharged mother liquor after precipitation in the low-temperature crystallizer 8 can be transported to the second nanofilter 9;

[0025] The mirabilite re-dissolving device 12 is also connected to the second high-temperature crystallizer 13, and the dissolved sodium sulfate decahydrate can be transported to the second high-temperature crystallizer 13 for high-temperature treatment to obtain sodium sulfate;

[0026] The second nanofilter 9 is also connected to the second evaporator 10, the mixed salt return dissolver 6, and the first evaporator 2 respectively. The secondary nanofiltration water produced by the second nanofilter 9 can be transported to the second evaporator 10 for evaporation and concentration, and the secondary nanofiltration concentrated water of the second nanofilter 9 can be transported to the mixed salt return dissolver 6 and the first evaporator 2 respectively.

[0027] The second evaporator 10 is also connected to the second high-temperature crystallizer 11, and the evaporated and concentrated solution can be transported to the second high-temperature crystallizer 11 for crystallization to prepare sodium chloride salt;

[0028] The second high-temperature crystallizer 11 is also connected to the first evaporator 2, and the sodium chloride crystallization mother liquor in the second high-temperature crystallizer 11 can be transported to the first evaporator 2;

[0029] The third high-temperature crystallizer 13 is also connected to the first evaporator 2, and the sodium sulfate crystallization mother liquor in the third high-temperature crystallizer 13 can be transported to the first evaporator 2;

[0030] The first nanofilter 1 is also connected to the reverse osmosis device 5, and the primary nanofiltration product water in the first nanofilter 1 can be transported to the reverse osmosis device 5;

[0031] The reverse osmosis device 5 is also connected to the mixed salt dissolving device 6 and the low temperature crystallizer 8 respectively. The reverse osmosis concentrated water in the reverse osmosis device 5 can be transported to the mixed salt dissolving device 6 and the low temperature crystallizer 8. The reverse osmosis water production system in the reverse osmosis device 5 is output.

[0032] In some embodiments, the brine refining unit 7 includes a tubular membrane to which powdered carbon is added and a hardness-removing chelating resin after the tubular membrane.

[0033] The present invention also provides a method for zero-discharge salt separation of high-salinity wastewater using the system of the present invention, comprising:

[0034] 1. After the high-salt wastewater is concentrated by the first nanofilter 1, primary nanofiltration concentrated water and primary nanofiltration produced water are obtained. The primary nanofiltration concentrated water passes through the first evaporator 2 and the first high-temperature crystallizer 3 in sequence. After crystallization in the first high-temperature crystallizer 3, a mixed salt of sodium chloride and sodium sulfate is precipitated. The residual mother liquor after crystallization is evaporated by the mother liquor dryer 4 to obtain impure salts.

[0035] 2. After the primary nanofiltration water produced in step 1 is concentrated by the reverse osmosis device 5, reverse osmosis concentrated water and reverse osmosis produced water are obtained. The reverse osmosis produced water is used as the recycled water output system, a part of the reverse osmosis concentrated water is used as the dissolved water of the mixed salt return dissolver 6, and the rest of the reverse osmosis concentrated water is mixed with the crystallization mother liquor of the low-temperature crystallizer 8;

[0036] 3. The mixed salt in step 1 is dissolved in the mixed salt re-dissolver 6 and then enters the brine refining unit 7. After filtering and removing the hardness, it enters the low-temperature crystallizer 8. At a low temperature of 0 to 10°C, the sodium sulfate salt is precipitated in the form of sodium sulfate decahydrate. The sodium sulfate decahydrate and the evaporated condensate are dissolved in the mirabilite re-dissolver 12, and then crystallized at high temperature in the second high-temperature crystallizer 13 to obtain anhydrous sodium sulfate product;

[0037] 4. The mother liquor remaining after precipitation in the low-temperature crystallizer 8 is mixed with part of the reverse osmosis concentrated water in step 1, and is subjected to secondary nanofiltration through the second nanofilter 9 to perform secondary nanofiltration to separate the sulfate and chloride in the mother liquor. The secondary nanofiltration product water is evaporated and concentrated through the second evaporator 10 and then enters the second high-temperature crystallizer 11 for crystallization to prepare sodium chloride salt. The sodium chloride crystallization mother liquor returns to the first evaporator 2, a part of the secondary nanofiltration concentrated water returns to the mixed salt return dissolver 6, and the rest is sent to the first evaporator 2 for circulation treatment.

[0038] The working principle of the system and method of the present invention is as follows:

[0039] In step 1, most of the sodium chloride and sodium sulfate are precipitated as mixed salt in the first high-temperature crystallizer 3, while most of the nitrate and organic impurities are basically not precipitated or only precipitated very little due to their greater solubility and higher boiling point through the control of the crystallization temperature, but enter the mixed salt evaporation crystallization mother liquor, and the mother liquor is finally processed by the mother liquor dryer 4 to obtain mixed salt. In this technical specification, the unseparated mixed salt of sodium sulfate and sodium chloride is called "mixed salt", and the remaining impurities, including nitrates / organic matter, are called "mixed salt".

[0040] In step 2, part of the reverse osmosis concentrated water is used as dissolving water in the mixed salt return dissolver 6 to configure the mixed salt into saturated or nearly saturated brine, and the excess reverse osmosis concentrated water is mixed with the mother liquor of the low-temperature crystallizer 8 to dissolve the supersaturated sodium sulfate decahydrate crystals (glauber's salt) that have not been removed in the frozen mother liquor to eliminate its supersaturation.

[0041] In step three, tubular membrane filtration with powdered carbon is used to reduce the residual color in the brine, and hardness-removing chelating resin is used to remove the residual hardness in the wastewater; in this way, the purity of sodium sulfate can reach more than 99%, which can meet the quality requirements of most sodium sulfate salts used in chemical industry.

[0042] Compared with the prior art, the beneficial effects of the present invention include at least:

[0043] 1. Compared with the currently existing nanofiltration / reverse osmosis + high temperature / low temperature crystallization coupled salt separation process, the present invention does not send the concentrated water from the nanofiltration directly to the low temperature crystallization unit, but sends it to the mixed salt evaporation crystallization unit to prepare mixed salt, thereby avoiding the introduction of excessive impurities such as nitrates / organic matter into the low temperature crystallization unit. The effect of the low temperature crystallization unit is more guaranteed, and the difficulty of handling the mother liquor of the low temperature crystallizer is reduced, thereby improving the operational reliability of the nanofiltration for treating the low temperature crystallization mother liquor.

[0044] 2. Compared with the high-temperature / low-temperature crystallization + nanofiltration coupled salt separation process, the present invention retains the nanofiltration and reverse osmosis parts, recovers part of the water through reverse osmosis, reduces the evaporation amount of the evaporator, and also uses the reverse osmosis concentrated water as the mixed salt dissolving liquid and the diluent of the low-temperature crystallization mother liquor, avoiding the disadvantage of increased evaporation caused by the addition of condensate.

[0045] 3. For the nanofiltration water, after being concentrated by reverse osmosis, it is not directly put into the sodium chloride evaporation crystallization, but used as the dissolving water of the mixed salt, and the concentrated water of reverse osmosis and part of the concentrated water of secondary nanofiltration are used as the mixed salt dilution water, so that the concentration of sodium chloride in the mixed salt after dilution and dissolution is improved, and the precipitation ability of sodium sulfate at low temperature is increased, which can increase the feed concentration of the sodium chloride evaporation crystallization unit (generally up to about 10-15%), and reduce the fluctuation of evaporation amount.

[0046] 4. The design of nanofiltration + reverse osmosis avoids the pursuit of concentration limits. The sulfate retention rate of the nanofiltration system does not require strict requirements, and the reverse osmosis system only needs its concentrated water to meet the amount of water required for mixed salt dissolution; it greatly reduces its operating requirements, reduces the complexity of its design and the difficulty of operation.

[0047] 5. When the salt-nitrogen ratio fluctuates greatly, nanofiltration (NF1) and reverse osmosis (RO) can be used to adjust the flow matching on both sides. The reflux rate of the low-temperature crystallization mother liquor can also be controlled to adjust the feed rate of the mixed salt evaporation crystallization and the sodium chloride evaporation crystallization to avoid excessive fluctuations in the evaporation amount of the evaporation crystallization unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 The invention discloses a high-salinity wastewater zero-discharge salt separation system and process route.

[0049] In the figure: 1, first nanofilter; 2, first evaporator; 3, first high-temperature crystallizer; 4, mother liquor dryer; 5, reverse osmosis; 6, mixed salt re-dissolver; 7, brine refining unit; 8, low-temperature crystallizer; 9, second nanofilter; 10, second evaporator; 11, second high-temperature crystallizer; 12, mirabilite re-dissolver; 13, second high-temperature crystallizer DETAILED DESCRIPTION

[0050] In order to make the purpose, technical scheme and beneficial effects of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. Examples of the embodiments are shown in the accompanying drawings. It should be understood that the specific embodiments described in the following embodiments of the present invention are only exemplary descriptions of the specific embodiments of the present invention, and are intended to be used to explain the present invention, but do not constitute a limitation of the present invention.

[0051] The endpoints of ranges and any values ​​disclosed herein are not limited to the exact range or value, and these ranges or values ​​should be understood to include approximations to these ranges.

[0052] Example 1 A high-salinity wastewater zero-discharge salt separation system and method

[0053] Systems and processes such as Figure 1 shown.

[0054] A high-salinity wastewater zero-discharge salt separation system, comprising:

[0055] A first nanofilter 1, the first nanofilter 1 is connected to a first evaporator 2, and the primary nanofiltration concentrated water obtained by concentrating the first nanofilter 1 can be transported to the first evaporator 2;

[0056] The first evaporator 2 is also connected to the first high-temperature crystallizer 3, and the primary nanofiltration concentrated water can be transported to the first high-temperature crystallizer 3 after evaporation in the first evaporator 2;

[0057] The high-temperature crystallizer 3 is also connected to the mother liquor dryer 4 and the mixed salt return dissolver 6 respectively. The mixed salt of sodium chloride and sodium sulfate precipitated from the high-temperature crystallizer 3 can be transported to the mixed salt return dissolver 6. The residual mother liquor crystallized from the high-temperature crystallizer 3 can be transported to the mother liquor dryer 4. The mother liquor dryer 4 can evaporate and dry the mother liquor to obtain impurity salt.

[0058] The mixed salt return dissolver 6 is also connected to the brine refining unit 7 and the low-temperature crystallizer 8 in sequence, and the mixed salt dissolved in the mixed salt return dissolver 6 can be transported to the brine refining unit 7 and the low-temperature crystallizer 8 in sequence;

[0059] The low-temperature crystallizer 8 is also connected to the second nanofilter 9 and the mirabilite re-dissolving device 12 respectively. The low-temperature crystallizer 8 processes the dissolved mixed salt after being treated by the brine refining unit 7 at a low temperature, and the precipitated sodium sulfate decahydrate can be transported to the mirabilite re-dissolving device 12. The remaining discharged mother liquor after precipitation in the low-temperature crystallizer 8 can be transported to the second nanofilter 9;

[0060] The mirabilite re-dissolving device 12 is also connected to the second high-temperature crystallizer 13, and the dissolved sodium sulfate decahydrate can be transported to the second high-temperature crystallizer 13 for high-temperature treatment to obtain sodium sulfate;

[0061] The second nanofilter 9 is also connected to the second evaporator 10, the mixed salt return dissolver 6, and the first evaporator 2 respectively. The secondary nanofiltration water produced by the second nanofilter 9 can be transported to the second evaporator 10 for evaporation and concentration, and the secondary nanofiltration concentrated water of the second nanofilter 9 can be transported to the mixed salt return dissolver 6 and the first evaporator 2 respectively.

[0062] The second evaporator 10 is also connected to the second high-temperature crystallizer 11, and the evaporated and concentrated solution can be transported to the second high-temperature crystallizer 11 for crystallization to prepare sodium chloride salt;

[0063] The second high-temperature crystallizer 11 is also connected to the first evaporator 2, and the sodium chloride crystallization mother liquor in the second high-temperature crystallizer 11 can be transported to the first evaporator 2;

[0064] The first nanofilter 1 is also connected to the reverse osmosis device 5, and the nanofiltration product water in the first nanofilter 1 can be transported to the reverse osmosis device 5;

[0065] The reverse osmosis device 5 is also connected to the mixed salt return dissolver 6 and the low-temperature crystallizer 8 respectively, and the reverse osmosis concentrated water in the reverse osmosis device 5 can be transported to the mixed salt return dissolver 6 and the low-temperature crystallizer 8, and the reverse osmosis water output system in the reverse osmosis device 5;

[0066] The brine refining unit 7 includes a tubular membrane to which powdered carbon is added and a hardness-removing chelating resin behind the tubular membrane.

[0067] A method for zero-discharge salt separation of high-salinity wastewater using the above system comprises:

[0068] 1. After the high-salt wastewater is concentrated by the first nanofilter 1, primary nanofiltration concentrated water and primary nanofiltration produced water are obtained. The primary nanofiltration concentrated water passes through the first evaporator 2 and the first high-temperature crystallizer 3 in sequence. After crystallization in the first high-temperature crystallizer 3, a mixed salt of sodium chloride and sodium sulfate is precipitated. The residual mother liquor after crystallization is evaporated and dried by the mother liquor dryer 4 to obtain impure salts.

[0069] 2. After the primary nanofiltration water produced in step 1 is concentrated by the reverse osmosis device 5, reverse osmosis concentrated water and reverse osmosis produced water are obtained. The reverse osmosis produced water is used as the recycled water output system, a part of the reverse osmosis concentrated water is used as the dissolved water of the mixed salt return dissolver 6, and a part of the reverse osmosis concentrated water is mixed with the crystallization mother liquor of the low-temperature crystallizer 8;

[0070] 3. The mixed salt in step 1 is dissolved in the mixed salt re-dissolver 6 and then enters the brine refining unit 7. After filtering and removing the hardness, it enters the low-temperature crystallizer 8. At a low temperature of 0 to 10°C, the sodium sulfate salt is precipitated in the form of sodium sulfate decahydrate. The sodium sulfate decahydrate and the evaporated condensate are dissolved in the mirabilite re-dissolver 12, and then crystallized at high temperature in the second high-temperature crystallizer 13 to obtain anhydrous sodium sulfate product;

[0071] 4. The mother liquor remaining after precipitation in the low-temperature crystallizer 8 is mixed with part of the reverse osmosis concentrated water in step 1, and is subjected to secondary nanofiltration through the second nanofilter 9 to perform secondary nanofiltration to separate the sulfate and chloride in the mother liquor. The secondary nanofiltration product water is evaporated and concentrated through the second evaporator 10 and then enters the second high-temperature crystallizer 11 for crystallization to prepare sodium chloride salt. The sodium chloride crystallization mother liquor returns to the first evaporator 2, a part of the secondary nanofiltration concentrated water returns to the mixed salt return dissolver 6, and the rest is sent to the first evaporator 2 for circulation treatment.

[0072] Example 2 Performance Evaluation

[0073] Using the method of Example 2, 20 m 3 / h as an example, when the Cl / SO4 ratio in water changes from 3:1 to 1:3, the processing capacity of each device changes, as shown in Table 1 below. The smaller the change in processing capacity, the smaller the impact of water quality fluctuations on the salt separation system.

[0074] Table 1 Changes in equipment processing capacity

[0075]

[0076] Note: The membrane system uses its water output as its design scale, while the evaporation capacity refers to the amount of water evaporated during the evaporation process.

[0077] In order to cope with the fluctuation of water quality, the equipment processing capacity needs to cover the working conditions of its maximum processing capacity. At this time, the scale of each unit system is shown in Table 2. In order to cope with the fluctuation of high salt-nitrogen ratio, the specifications of low-temperature crystallization and sodium chloride evaporation crystallization of nanofiltration / reverse osmosis + low-temperature / high-temperature crystallization coupled salt separation process are much larger than those of the other two schemes, and the designed evaporation capacity of mixed salt in the low-temperature / high-temperature crystallization + nanofiltration coupled salt separation process is much larger than that of other processes.

[0078] Table 2 System scale of each unit

[0079]

[0080] The comprehensive scale investment is shown in Table 3. The process investment of the present invention is far more advantageous than the other two processes, the overall design evaporation volume is smaller, and under different operating conditions, the overall fluctuation of the processing load of each unit is smaller than that of the nanofiltration / reverse osmosis + low temperature / high temperature crystallization coupled salt separation process, and the operation is more stable and reliable.

[0081] Table 3 Comprehensive scale investment

[0082]

[0083]

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and do not constitute a limitation on the content of the present invention. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for zero-discharge salt separation of high-salinity wastewater, characterized in that: The steps include: 1) The high-salt wastewater is concentrated by a nanofiltration to obtain a primary nanofiltration concentrated water and a primary nanofiltration produced water. The primary nanofiltration concentrated water is evaporated and crystallized at high temperature to precipitate a mixed salt of sodium chloride and sodium sulfate. The residual mother liquor after crystallization is evaporated and dried to obtain impurity salts; 2) The primary nanofiltration water produced in step 1) is concentrated by reverse osmosis to obtain reverse osmosis concentrated water and reverse osmosis produced water, and the reverse osmosis produced water is used as recycled water; 3) using part of the reverse osmosis concentrated water in step 2) to re-dissolve the mixed salt in step 1), refining the salt water to remove organic matter and hardness, and performing low-temperature crystallization to precipitate sodium sulfate decahydrate, and re-dissolving the precipitated sodium sulfate decahydrate with evaporation condensate, and performing high-temperature crystallization to obtain anhydrous sodium sulfate finished product; 4) In step 3), the remaining mother liquor discharged from the precipitation of sodium sulfate decahydrate is mixed with part of the reverse osmosis concentrated water in step 2), and then subjected to secondary nanofiltration separation to obtain secondary nanofiltration concentrated water and secondary nanofiltration produced water, and the secondary nanofiltration produced water is evaporated and concentrated and then crystallized at high temperature to prepare sodium chloride salt; 5) In step 4), part of the secondary nanofiltration concentrate and part of the reverse osmosis concentrate are used to redissolve the mixed salt in step 1), and the remaining part of the secondary nanofiltration concentrate and the primary nanofiltration concentrate in step 1) are sent to the mixed salt evaporation crystallization together.

2. A high-salinity wastewater zero-discharge salt separation method as claimed in claim 1, characterized in that: In the step 1), the high-salinity wastewater is initially subjected to removal of hardness and organic matter before being concentrated by nanofiltration.

3. A high-salinity wastewater zero-discharge salt separation method as claimed in claim 1, characterized in that: In the step 3), the specific operation of refining the brine is: refining by tubular membrane filtration with powdered carbon added and hardness-removing chelating resin after the tubular membrane.

4. A high-salinity wastewater zero-discharge salt separation method as claimed in claim 1, characterized in that: In the step 3), the temperature of the low temperature treatment is 0-10°C.

5. A high-salinity wastewater zero-discharge salt separation method as claimed in claim 1, characterized in that: In the step 4), the mother liquor remaining from the high temperature crystallization is mixed with the concentrated water from the primary nanofiltration in the step 1) to be evaporated and high temperature crystallized.

6. A high-salinity wastewater zero-discharge salt separation system, characterized in that: include: A first nanofilter (1), the first nanofilter (1) is connected to a first evaporator (2), and primary nanofiltration concentrated water obtained by concentration in the first nanofilter (1) can be transported to the first evaporator (2); The first evaporator (2) is also connected to the first high-temperature crystallizer (3), and the concentrated water from the primary nanofiltration can be transported to the first high-temperature crystallizer (3) after evaporation in the first evaporator (2); The high-temperature crystallizer (3) is also connected to the mother liquor dryer (4) and the mixed salt re-dissolving device (6) respectively. The mixed salt of sodium chloride and sodium sulfate precipitated from the high-temperature crystallizer (3) can be transported to the mixed salt re-dissolving device (6). The residual mother liquor precipitated from the high-temperature crystallizer (3) can be transported to the mother liquor dryer (4). The mother liquor dryer (4) can evaporate and dry the mother liquor to obtain impurity salt. The mixed salt return dissolving device (6) is also connected to the brine refining unit (7) and the low-temperature crystallizer (8) in sequence, and the mixed salt dissolved in the mixed salt return dissolving device (6) can be transported to the brine refining unit (7) and the low-temperature crystallizer (8) in sequence; The low-temperature crystallizer (8) is also connected to the second nanofilter (9) and the mirabilite re-dissolving device (12) respectively. The low-temperature crystallizer (8) processes the mixed salt dissolved after being treated by the brine refining unit (7) at a low temperature, and the precipitated sodium sulfate decahydrate can be transported to the mirabilite re-dissolving device (12). The mother liquor remaining after precipitation in the low-temperature crystallizer (8) can be transported to the second nanofilter (9); The mirabilite re-dissolving device (12) is also connected to the second high-temperature crystallizer (13), and the dissolved sodium sulfate decahydrate can be transported to the second high-temperature crystallizer (13) to obtain sodium sulfate after high-temperature treatment; The second nanofilter (9) is also connected to the second evaporator (10), the mixed salt return dissolver (6), and the first evaporator (2) respectively. The secondary nanofiltration water produced by the second nanofilter (9) can be transported to the second evaporator (10) for evaporation and concentration. The secondary nanofiltration concentrated water of the second nanofilter (9) can be transported to the mixed salt return dissolver (6) and the first evaporator (2) respectively. The second evaporator (10) is also connected to the second high-temperature crystallizer (11), and the evaporated and concentrated solution can be transported to the second high-temperature crystallizer (11) for crystallization to prepare sodium chloride salt; The second high-temperature crystallizer (11) is also connected to the first evaporator (2), and the sodium chloride crystallization mother liquor in the second high-temperature crystallizer (11) can be transported to the first evaporator (2); The third high-temperature crystallizer (13) is also connected to the first evaporator (2), and the sodium sulfate crystallization mother liquor in the third high-temperature crystallizer (13) can be transported to the first evaporator (2); The first nanofilter (1) is also connected to the reverse osmosis device (5), and the primary nanofiltration product water in the first nanofilter (1) can be transported to the reverse osmosis device (5); The reverse osmosis device (5) is also connected to the mixed salt return dissolver (6) and the low-temperature crystallizer (8) respectively. The reverse osmosis concentrated water in the reverse osmosis device (5) can be transported to the mixed salt return dissolver (6) and the low-temperature crystallizer (8). The reverse osmosis water output system in the reverse osmosis device (5) 7. A method for zero-discharge salt separation of high-salinity wastewater using a high-salinity wastewater zero-discharge salt separation system according to claim 6, comprising:

1. After the high-salt wastewater is concentrated by the first nanofilter (1), primary nanofiltration concentrated water and primary nanofiltration produced water are obtained. The primary nanofiltration concentrated water is sequentially passed through the first evaporator (2) and the first high-temperature crystallizer (3). After crystallization in the first high-temperature crystallizer (3), a mixed salt of sodium chloride and sodium sulfate is precipitated. The residual mother liquor after crystallization is evaporated by the mother liquor dryer (4) to obtain impure salts.

2. The primary nanofiltration water produced in step 1 is concentrated by a reverse osmosis device (5) to obtain reverse osmosis concentrated water and reverse osmosis produced water. The reverse osmosis produced water is output as a recycling water system, a part of the reverse osmosis concentrated water is used as dissolving water for the mixed salt return dissolving device (6), and the rest of the reverse osmosis concentrated water is mixed with the crystallization mother liquor of the low-temperature crystallizer (8); 3. The mixed salt in step 1 is dissolved in the mixed salt re-dissolving device (6) and then enters the brine refining unit (7). After filtering and hardness removal, it enters the low-temperature crystallizer (8). At a low temperature of 0 to 10° C., the sodium sulfate salt is precipitated in the form of sodium sulfate decahydrate. The sodium sulfate decahydrate and the evaporation condensate are dissolved in the mirabilite re-dissolving device (12). After high-temperature crystallization in the second high-temperature crystallizer (13), anhydrous sodium sulfate product is obtained. Fourth, the mother liquor remaining after precipitation in the low-temperature crystallizer (8) is mixed with part of the reverse osmosis concentrated water in step 1, and is subjected to secondary nanofiltration through a second nanofilter (9) to perform secondary nanofiltration to separate sulfate and chloride in the mother liquor. The secondary nanofiltration product water is evaporated and concentrated in a second evaporator (10) and then enters a second high-temperature crystallizer (11) for crystallization to prepare sodium chloride salt. The sodium chloride crystallization mother liquor is returned to the first evaporator (2), a part of the secondary nanofiltration concentrated water is returned to the mixed salt return dissolver (6), and the rest is sent to the first evaporator (2) for circulation treatment.

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