A zero-emission salt separation process and system with low impurity salt ratio and high salt and nitrate quality

Abstract

The present invention provides a salt separation zero-discharge process and system with low impure salt rate and high salt nitrate quality. The process includes step 1: membrane concentration, high-salt wastewater is subjected to a membrane concentration process to obtain a first concentrated water mainly composed of sodium sulfate and a second concentrated water mainly composed of sodium chloride; step 2: salt separation crystallization, including 1) the first concentrated water is subjected to a sodium sulfate evaporation crystallization process, a sodium sulfate freezing crystallization process, a sodium sulfate recrystallization process and a sodium sulfate drying process to obtain a high-purity sodium sulfate product; 2) the second concentrated water is subjected to a sodium chloride evaporation crystallization process, a sodium chloride salt washing process and a sodium chloride drying process to obtain a high-purity sodium chloride product; 3) the second mother liquor discharged from the sodium sulfate evaporation crystallization process and the third mother liquor discharged from the sodium chloride evaporation crystallization process are mixed through a mixed salt pretreatment process, and then subjected to a mixed salt evaporation crystallization process and an impure salt drying process to obtain impure salt. The process of the present invention can reduce the impure salt rate while producing high-quality sodium sulfate and sodium chloride products, thereby reducing the operating cost of the salt separation zero-discharge project.

Description

Technical Field

[0001] The invention relates to the technical field of sewage and wastewater treatment, and in particular to a salt separation zero-discharge process and system with low impurity salt rate and high salt and nitrate quality. Background Art

[0002] The soluble solids contained in the concentrated brine of industrial wastewater from coal chemical, coking, petrochemical, electric power and other industries and coal mine water salt zero discharge projects are mainly sodium chloride and sodium sulfate. The organic content in the concentrated brine is high and the water quality is complex. The amount of mother liquor discharged during the evaporation and crystallization process is the determining factor for the amount of impure salts. There are two factors to control the amount of evaporation and crystallization mother liquor discharge in wastewater zero discharge. One is the co-saturation point control, and the other is the organic content control in the mother liquor. Under normal circumstances, the organic content of wastewater zero discharge projects is very high, so most of the organic content in the mother liquor controls the discharge of evaporation and crystallization mother liquor. When the organic content in the mother liquor reaches the control value, the mother liquor must be discharged to ensure that the evaporation crystallization or freezing crystallization system can operate stably for a long time, which leads to a large discharge of mother liquor, resulting in a high impure salt rate.

[0003] At present, the high-salt wastewater salt zero-discharge process generally adopts the process of sodium chloride evaporation crystallization + centrifugal separation + drying to obtain sodium chloride products, and the sodium sulfate evaporation crystallization + centrifugal separation + drying process to obtain sodium sulfate products. It is difficult to obtain sodium chloride with a purity of more than 98.5% and sodium sulfate products with a purity of more than 98%. Chinese patent application CN109775939A discloses a coal chemical wastewater zero-discharge and salt separation crystallization system and method. The salt separation crystallization unit in the method includes a sodium sulfate crystallization device and a sodium chloride crystallization device to obtain sodium chloride, sodium sulfate crystal salt and miscellaneous salts. The purity guarantee values ​​of sodium chloride and sodium sulfate crystal salt in the patent are only 92% and 97%, respectively. The limitation of product purity leads to the difficulty in selling the output sodium chloride and sodium sulfate products and the low selling price.

[0004] In addition, it cannot be ignored that the rate of foreign salts produced in the high-salt wastewater salt zero-emission process is also an important limiting factor. Chinese patent application CN113213684A discloses a salt separation system for coking wastewater to improve the salt resource utilization rate. In the method, the coking wastewater is treated by a freezing crystallization unit, a sodium chloride evaporation crystallization unit, a sodium sulfate evaporation crystallization unit, and a mother liquor evaporation crystallization unit to obtain sodium chloride, sodium sulfate crystal salt and foreign salts. The foreign salt rate in the patent can reach up to 29.4%. Foreign salts are hazardous wastes, and their disposal costs are relatively high. Therefore, a high foreign salt rate is one of the important reasons for the high operating cost of the high-salt wastewater salt zero-emission project. Summary of the invention

[0005] In view of the above problems, the present invention discloses a zero-emission salt separation process and system with low impure salt rate and high salt nitrate quality, which can reduce the impure salt rate while producing high-quality sodium sulfate and sodium chloride products, thereby reducing the operating cost of the zero-emission salt separation project.

[0006] Specifically, on the one hand, the present invention provides a salt separation zero-discharge process with low impurity salt ratio and high salt nitrate quality, the process comprising the following steps:

[0007] Step 1: membrane concentration: the high-salt wastewater is concentrated through the membrane process to obtain the first concentrated water mainly composed of sodium sulfate and the second concentrated water mainly composed of sodium chloride;

[0008] Step 2: Salt crystallization, including

[0009] 1) The first concentrated water is subjected to a sodium sulfate evaporation crystallization step, a sodium sulfate freeze crystallization step, a sodium sulfate recrystallization step and a sodium sulfate drying step to obtain a high-purity sodium sulfate product;

[0010] 2) The second concentrated water is subjected to a sodium chloride evaporation and crystallization process, a sodium chloride salt washing process and a sodium chloride drying process to obtain a high-purity sodium chloride product;

[0011] 3) The second mother liquor discharged from the sodium sulfate evaporation and crystallization process and the third mother liquor discharged from the sodium chloride evaporation and crystallization process are mixed in a mixed salt pretreatment process, and then subjected to a mixed salt evaporation and crystallization process and a mixed salt drying process to obtain mixed salt.

[0012] In the above technical scheme, high-salt wastewater containing mainly sodium chloride and sodium sulfate as soluble solids is divided into two streams after primary nanofiltration treatment: first concentrated water and first produced water; wherein, the first concentrated water mainly contains sodium sulfate and a small amount of sodium chloride, and the first produced water mainly contains sodium chloride and a small amount of sodium sulfate.

[0013] The first produced water is further filtered to remove sodium sulfate, so that the amount of sodium sulfate contained in it is further reduced to obtain second produced water; the second produced water is concentrated by a high-pressure reverse osmosis process to produce a second concentrated water rich in salt, and the second concentrated water enters the sodium chloride evaporation and crystallization process.

[0014] In the above technical scheme, the first concentrated water enters the sodium sulfate evaporation crystallization process, and a nitrate slurry is produced through single-effect evaporation crystallization, multiple-effect evaporation crystallization, MVR evaporation crystallization or TVR evaporation crystallization. The nitrate slurry is separated into solid and liquid by a thickener and a centrifugal dehydrator or only by a centrifugal dehydrator to produce a first mother liquor and a solid first crude nitrate; wherein, the first crude nitrate enters the sodium sulfate recrystallization process, and the first mother liquor enters the sodium sulfate freezing crystallization process.

[0015] The sodium sulfate freeze crystallization process adopts forced circulation freeze crystallization of the first mother liquor, and then solid-liquid separation is carried out through a thickener and a centrifugal dehydrator or only through a centrifugal dehydrator to produce a second mother liquor and solid Glauber's salt; among them, the Glauber's salt enters the sodium sulfate recrystallization process, and the second mother liquor enters the mixed salt pretreatment process.

[0016] In the sodium sulfate recrystallization process, single-effect evaporation crystallization, multi-effect evaporation crystallization, MVR evaporation crystallization or TVR evaporation crystallization is used to dissolve and recrystallize the first crude saltpeter and mirabilite, and most of the organic matter, impurity ions, etc. in the first crude saltpeter and mirabilite are transferred out to improve the purity and quality of the sodium sulfate product. The slurry produced by recrystallization is separated into solid and liquid by a thickener and a centrifugal dehydrator or only by a centrifugal dehydrator to obtain wet saltpeter with a moisture content of no more than 5%, which enters the sodium sulfate drying process.

[0017] The sodium sulfate drying process uses box dryers, fixed bed dryers, vibrating fluidized bed dryers, etc. to dry wet nitrate to obtain sodium sulfate products with a purity of more than 98%.

[0018] Optionally, the first concentrated water is subjected to an advanced oxidation treatment process for further treatment to reduce its chemical oxygen demand (COD), and then enters the sodium sulfate evaporation and crystallization process. Further optionally, the advanced oxidation treatment process uses an ozone catalytic oxidation unit, and ordinary technicians in this field can select other advanced oxidation treatment methods that can achieve the same technical effect through non-creative labor, and the technical solution formed thereby is within the scope of protection of the present invention.

[0019] In the above technical scheme, the second concentrated water is subjected to single-effect evaporation crystallization, multi-effect evaporation crystallization, MVR evaporation crystallization or TVR evaporation crystallization in the sodium chloride evaporation crystallization process to produce salt slurry, and the salt slurry is subjected to solid-liquid separation by a thickener and a centrifugal dehydrator or only by a centrifugal dehydrator to produce a third mother liquor and a solid first coarse salt; wherein, the third mother liquor enters the salt mixing pretreatment process, and the first coarse salt enters the sodium chloride salt washing process.

[0020] In the sodium chloride salt washing process, one or more of the evaporation condensate obtained in the sodium sulfate evaporation crystallization process, the sodium chloride evaporation crystallization process, the mixed salt evaporation crystallization process or the second concentrated water is used as a washing liquid for salt washing, and most of the organic matter, impurity ions, etc. adhered to the surface of the first crude salt are transferred to the washing liquid, and then the solid-liquid separation is carried out by a centrifugal dehydrator to obtain wet salt with a moisture content of no more than 5%, and the wet salt then enters the sodium chloride drying process.

[0021] The sodium chloride drying process uses a box dryer, fixed bed dryer or vibrating fluidized bed dryer to dry the wet salt to obtain a sodium chloride product with a purity of more than 98.5%.

[0022] In the above technical solution, the second mother liquor and the third mother liquor are mixed evenly in the mixed salt pretreatment process to obtain the fourth mother liquor. Optionally, a storage tank with a stirrer is used in the mixed salt pretreatment process. The fourth mother liquor enters the mixed salt evaporation crystallization process.

[0023] The fourth mother liquor is subjected to single-effect evaporation crystallization, multi-effect evaporation crystallization, MVR evaporation crystallization or TVR evaporation crystallization in the mixed salt evaporation crystallization process to produce a slurry of crude salt, and then solid-liquid separation is carried out through a thickener and a centrifugal dehydrator or only through a centrifugal dehydrator to obtain a fifth mother liquor and a solid second crude salt, wherein the water content of the second crude salt is not more than 5%; wherein, the second crude salt enters the sodium chloride salt washing process, and the fifth mother liquor enters the mixed salt drying process.

[0024] The mixed salt drying process uses a rake dryer, a drum scraper dryer or a spray dryer to produce mixed salt.

[0025] On the other hand, the present invention proposes a salt separation zero-emission system with low impure salt rate and high salt nitrate quality, which includes a membrane concentration unit, a sodium sulfate crystallization unit, a sodium chloride crystallization unit and an impure salt crystallization unit.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] A sodium sulfate recrystallization process is provided to dissolve the first crude saltpeter and the mirabilite and then recrystallize them, thereby transferring out most of the organic matter, impurity ions, etc., effectively reducing the content of organic matter and impurity ions in the output sodium sulfate, and obtaining a sodium sulfate product with a purity of more than 98%. The present invention also provides a sodium chloride salt washing process to wash the salt, and transfer most of the organic matter, impurity ions, etc. adhering to the surfaces of the first crude salt and the second crude salt to the washing liquid, thereby effectively reducing the content of organic matter and impurity ions in the sodium chloride, and obtaining a sodium chloride product with a purity of more than 98.5%.

[0028] In addition, by setting up a "sodium sulfate-sodium chloride-miscellaneous salt" combined salt separation and crystallization process, the mother liquor discharge in the system can be minimized; by setting up a mixed salt evaporation crystallization process and system, part of the sodium chloride in the discharged mother liquor is extracted, and then the sodium chloride product is obtained through refining, thereby increasing the yield of sodium chloride, reducing the impure salt rate, and reducing the operating cost of the overall process. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0030] Figure 1 This is a process flow chart of zero-discharge salt separation with low impurity salt ratio and high salt and nitrate quality according to the present invention;

[0031] Figure 2The invention discloses a salt separation zero-emission system with low impurity salt ratio and high salt and nitrate quality.

[0032] The above drawings include the following reference numerals:

[0033] 10-membrane concentration unit, 11-first nanofiltration, 111-first water production, 112-first concentrated water, 12-second nanofiltration, 121-second water production, 13-high-pressure reverse osmosis device, 131-second concentrated water; 20-sodium sulfate crystallization unit, 21-sodium sulfate evaporation crystallization device, 211-first crude saltpeter, 212-first mother liquor, 22-sodium sulfate freezing crystallization device, 221-glauber's salt, 222-second mother liquor, 23-sodium sulfate recrystallization device, 231-wet saltpeter, 2 4-sodium sulfate drying device; 30-sodium chloride crystallization unit, 31-sodium chloride evaporation crystallization device, 311-first crude salt, 312-third mother liquor, 32-sodium chloride salt washing device, 321-wet salt, 33-sodium chloride drying device; 40-miscellaneous salt crystallization unit, 41-mixed salt pretreatment device, 411-fourth mother liquor, 42-mixed salt evaporation crystallization device, 421-second crude salt, 422-fifth mother liquor, 43-miscellaneous salt drying device; 50-advanced oxidation treatment device. DETAILED DESCRIPTION

[0034] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively below, and preferred embodiments of the present invention are given. However, it should be understood that these embodiments are only used for more detailed description and should not be understood as limiting the present invention in any form, i.e., not intended to limit the scope of protection of the present invention.

[0035] Example 1

[0036] This embodiment is a salt separation zero-discharge process with low impurity salt ratio and high salt and nitrate quality. Figure 1 As shown, the process includes:

[0037] Step 1: membrane concentration, high-salinity wastewater is subjected to a membrane concentration process to obtain a first concentrated water 112 mainly composed of sodium sulfate and a second concentrated water 131 mainly composed of sodium chloride. Specifically, in the membrane concentration process, the high-salinity wastewater is separated into the first concentrated water 112 and the first produced water 111 through a first nanofiltration process; the first produced water 111 is subjected to a second nanofiltration process to obtain a second produced water 121, and the second produced water 121 is subjected to a high-pressure reverse osmosis process to obtain a second concentrated water 131.

[0038] Step 2: Salt crystallization, including

[0039] 1) The first concentrated water 112 is subjected to the sodium sulfate evaporation crystallization process, the sodium sulfate freeze crystallization process, the sodium sulfate recrystallization process and the sodium sulfate drying process to obtain a high-purity sodium sulfate product. Specifically, the sodium sulfate evaporation crystallization process includes: crystallizing the first concentrated water 112, and separating the solid and liquid to obtain the first crude saltpeter 211 and the first mother liquor 212, the first crude saltpeter 211 enters the sodium sulfate recrystallization process, and the first mother liquor 212 enters the sodium sulfate freeze crystallization process; the sodium sulfate freeze crystallization process includes: forcing the first mother liquor 212 to undergo freeze crystallization and solid-liquid separation to obtain Glauber's salt 221 and the second mother liquor 222; Glauber's salt 221 enters the sodium sulfate recrystallization process; the sodium sulfate recrystallization process includes: dissolving and recrystallizing the first crude saltpeter 211 and Glauber's salt 221, and separating the solid and liquid to obtain wet saltpeter 231, and the wet saltpeter 231 enters the sodium sulfate drying process.

[0040] 2) The second concentrated water 131 undergoes a sodium chloride evaporation crystallization process, a sodium chloride salt washing process, and a sodium chloride drying process to obtain a high-purity sodium chloride product. Specifically, the sodium chloride evaporation crystallization process includes: crystallizing the second concentrated water 131 and performing solid-liquid separation to obtain a first crude salt 311 and a third mother liquor 312; the first crude salt 311 enters a sodium chloride salt washing process; the sodium chloride salt washing process includes: washing the first crude salt 311 with a washing liquid and then performing solid-liquid separation to obtain a wet salt 321, and the wet salt 321 enters a sodium chloride drying process.

[0041] 3) The second mother liquor 222 discharged from the sodium sulfate evaporation and crystallization process and the third mother liquor 312 discharged from the sodium chloride evaporation and crystallization process are mixed in a mixed salt pretreatment process, and then the mixed salt evaporation and crystallization process and the mixed salt drying process are carried out to obtain mixed salt. Specifically, the mixed salt pretreatment process includes: the second mother liquor 222 and the third mother liquor 312 are mixed evenly to obtain the fourth mother liquor 411; the mixed salt evaporation and crystallization process includes: the fourth mother liquor 411 is crystallized and the solid-liquid separation is carried out to obtain the second crude salt 421 and the fifth mother liquor 422; the second crude salt 421 enters the sodium chloride salt washing process, and the fifth mother liquor 422 enters the mixed salt drying process.

[0042] Optionally, the above process further includes performing an advanced oxidation treatment process on the first concentrated water 112 to reduce its COD before entering the sodium sulfate evaporation and crystallization process.

[0043] Example 2

[0044] This embodiment is a salt separation zero-discharge system with low impurity salt ratio and high salt and nitrate quality. Figure 2 As shown, the system includes a membrane concentration unit 10, a sodium sulfate crystallization unit 20, a sodium chloride crystallization unit 30 and a miscellaneous salt crystallization unit 40.

[0045] Specifically, the membrane concentration unit 10 includes a primary nanofiltration 11, a secondary nanofiltration 12 and a high-pressure reverse osmosis device 13 connected in sequence;

[0046] The sodium sulfate crystallization unit 20 includes a sodium sulfate evaporation crystallization device 21, a sodium sulfate freezing crystallization device 22, a sodium sulfate recrystallization device 23 and a sodium sulfate drying device 24 connected in sequence; the sodium sulfate evaporation crystallization device 21 is connected to the concentrated water outlet of the primary nanofiltration 11, and the sodium sulfate evaporation crystallization device 21 is also provided with a solid discharge port connected to the sodium sulfate recrystallization device 23; the sodium sulfate freezing crystallization device 22 is provided with a first discharge outlet;

[0047] The sodium chloride crystallization unit 30 includes a sodium chloride evaporation crystallization device 31, a sodium chloride salt washing device 32 and a sodium chloride drying device 33 connected in sequence; the sodium chloride evaporation crystallization device 31 is connected to the concentrated water outlet of the high-pressure reverse osmosis device 13; the sodium chloride evaporation crystallization device 31 is provided with a second discharge outlet;

[0048] The mixed salt crystallization unit 40 includes a mixed salt pretreatment device 41, a mixed salt evaporation crystallization device 42 and a mixed salt drying device 43 connected in sequence; the mixed salt pretreatment device 41 is connected to the first discharge outlet and the second discharge outlet; the solid discharge port of the mixed salt evaporation crystallization device 42 is connected to the sodium chloride salt washing device 32.

[0049] Optionally, an advanced oxidation treatment device 50 is provided between the membrane concentration unit 10 and the sodium sulfate crystallization unit 20 .

[0050] The embodiments are overviews of the technical solutions of the invention. In order to more clearly understand the technical means of the invention and implement them according to the contents of the specification, the preferred embodiments of the invention under specific working conditions are described in detail below.

[0051] Example 3

[0052] This embodiment adopts Figure 2 The zero-emission salt separation system with low impure salt rate and high salt nitrate quality is used to treat the reverse osmosis high-salt wastewater from the coal mine water wastewater reuse system. The soluble solids contained in the high-salt wastewater are mainly sodium chloride and sodium sulfate. The specific parameters are total dissolved solids: 23306 mg / L, chloride ion / sulfate: 0.54, impurity ions mainly composed of nitrate: 112 mg / L, COD: 99 mg / L.

[0053] After being treated by the primary nanofiltration 11, the concentrated brine is divided into two streams. The first produced water 111 is mainly sodium chloride and contains a small amount of sodium sulfate. The specific parameters are total dissolved solids: 9499 mg / L, chloride ion / sulfate: 13.5, impurity ions mainly composed of nitrate: 108 mg / L, COD: 39 mg / L; the first concentrated water 112 is mainly sodium sulfate and contains a small amount of sodium chloride. The specific parameters are total dissolved solids: 119222 mg / L, chloride ion / sulfate: 0.07, impurity ions mainly composed of nitrate: 143 mg / L, COD: 515 mg / L. The first produced water 111 enters the secondary nanofiltration 12, and the first concentrated water 112 enters the sodium sulfate evaporation crystallization device 21.

[0054] The sodium sulfate evaporation crystallization device 21 uses MVR evaporation crystallization to produce nitrate slurry, and the nitrate slurry is separated into solid and liquid by a thickener and a centrifugal dehydrator to produce a first crude nitrate 211 and a first mother liquor 212. The first crude nitrate 211 enters the sodium sulfate recrystallization device 23 through the crude nitrate outlet, and the first mother liquor 212 enters the sodium sulfate freezing crystallization device 22. Among them, the mass flow rate of the first mother liquor 212 is 2.9% of the mass flow rate of the first concentrated water 112 entering the sodium sulfate evaporation crystallization device 21, and the specific parameters of the first mother liquor 212 are total dissolved solids: 363968 mg / L, chloride ion / sulfate: 4.7, impurity ions mainly composed of nitrate: 4933 mg / L, COD: 17657 mg / L.

[0055] The sodium sulfate freeze crystallization device 22 uses forced circulation freeze crystallization, thickener and centrifugal dehydrator or only centrifugal dehydrator for solid-liquid separation to produce mirabilite 221 and second mother liquor 222. The mirabilite 221 enters the sodium sulfate recrystallization device 23, and the second mother liquor 222 enters the mixed salt pretreatment device 41. Among them, the volume flow rate of the second mother liquor 222 is basically the same as the volume flow rate of the first mother liquor 212 entering the sodium sulfate freeze crystallization device 22. The specific parameters of the second mother liquor 222 are total dissolved solids: 321072 mg / L, chloride ion / sulfate: 20, impurity ions mainly composed of nitrate: 4933 mg / L, COD: 17500 mg / L.

[0056] The sodium sulfate recrystallization device 23 adopts MVR evaporation crystallization to dissolve the first crude saltpeter 211 and the thenardite 221 and then recrystallize them, so as to transfer out most of the organic matter, impurity ions, etc. in the first crude saltpeter 211 and the thenardite 221, so as to reduce the content of organic matter and impurity ions in the sodium sulfate; the slurry produced by the recrystallization is separated into solid and liquid by a thickener and a centrifugal dehydrator to obtain wet saltpeter 231 with a moisture content of not more than 5%, and the wet saltpeter 231 enters the sodium sulfate drying device 24.

[0057] The sodium sulfate drying device 24 uses a vibrating fluidized bed dryer to dry the wet nitrate 231 to obtain a sodium sulfate product with a purity of more than 98%.

[0058] The first produced water 111 is further filtered through the secondary nanofiltration 12 to remove the sodium sulfate therein, so that the sodium sulfate content in the produced water is further reduced. The specific parameters of the second produced water 121 are total dissolved solids: 8407 mg / L, chloride ion / sulfate: 136.6, impurity ions mainly composed of nitrate: 97 mg / L, COD: 22 mg / L. The second produced water 121 enters the high-pressure reverse osmosis device 13. The second produced water 121, which is mainly composed of sodium chloride and contains only a small amount of sodium sulfate, is concentrated through the high-pressure reverse osmosis device 13 to produce a salt-rich second concentrated water 131, whose specific parameters are total dissolved solids: 103749 mg / L, chloride ion / sulfate: 136.6, impurity ions mainly composed of nitrate: 1198 mg / L, COD: 275 mg / L. The second concentrated water 131 enters the sodium chloride evaporation crystallization device 31.

[0059] The sodium chloride evaporation crystallization device 31 uses MVR evaporation crystallization to produce salt slurry, and the salt slurry is separated into solid and liquid by a thickener and a centrifugal dehydrator to produce a first crude salt 311 and a third mother liquor 312. The first crude salt 311 enters the sodium chloride salt washing device 32, and the third mother liquor 312 enters the mixed salt pretreatment device 41. The mass flow rate of the third mother liquor 312 is 1.18% of the mass flow rate of the second concentrated water 131 entering the sodium chloride evaporation crystallization device 31, and its specific parameters are total dissolved solids: 581224 mg / L, chloride ion / sulfate: 4.7, impurity ions mainly composed of nitrate: 101200 mg / L, COD: 23219 mg / L.

[0060] The sodium chloride salt washing device 32 uses evaporative crystallization condensate to wash the salt, and transfers most of the organic matter, impurity ions, etc. adhered to the surface of the first crude salt 311 to the washing liquid, so that the content of organic matter and impurity ions in the sodium chloride is reduced; and then the solid-liquid separation is carried out by a centrifugal dehydrator to obtain wet salt 321 with a moisture content of no more than 5%, and the wet salt 321 enters the sodium chloride drying device 33.

[0061] The sodium chloride drying device 33 uses a vibrating fluidized bed dryer or the like to dry the wet salt 321 to obtain a sodium chloride product with a purity of more than 98.5%.

[0062] The mixed salt pretreatment device 41 uses a storage tank with a stirrer to stir and homogenize the second mother liquor 222 and the third mother liquor 312 to obtain a fourth mother liquor 411, which enters the mixed salt evaporation crystallization device 42. The specific parameters of the fourth mother liquor 411 are total dissolved solids: 367111 mg / L, chloride ion / sulfate: 11.7, impurity ions mainly composed of nitrate: 21452 mg / L, COD: 18612 mg / L.

[0063] The mixed salt evaporation crystallization device 42 adopts MVR evaporation crystallization, and then undergoes solid-liquid separation through a thickener and a centrifugal dehydrator to obtain a second crude salt 421 with a moisture content of no more than 5% and a fifth mother liquor 422. The second crude salt 421 enters the sodium chloride salt washing device 32, and the fifth mother liquor 422 enters the impure salt drying device 43. The mass flow rate of the fifth mother liquor 422 is 48.5% of the mass flow rate of the fourth mother liquor 411 entering the mixed salt evaporation crystallization device 42, and its specific parameters are total dissolved solids: 426008 mg / L, chloride ion / sulfate: 5.4, impurity ions mainly composed of nitrate: 43945 mg / L, and COD: 38126 mg / L.

[0064] The mixed salt drying device 43 uses a rake dryer to produce mixed salt.

[0065] Example 4

[0066] This embodiment adopts Figure 2 The zero-emission salt separation system with low impure salt rate and high salt nitrate quality is used to treat the reverse osmosis high-salt wastewater from the coal chemical wastewater reuse system. The soluble solids contained in the high-salt wastewater are mainly sodium chloride and sodium sulfate. The specific parameters are total dissolved solids: 21800 mg / L, chloride ion / sulfate: 3.03, impurity ions mainly nitrate: 42 mg / L, COD: 144 mg / L.

[0067] After being treated by the primary nanofiltration 11, the brine is divided into two streams. The first produced water 111 is mainly sodium chloride and contains a small amount of sodium sulfate. The specific parameters are total dissolved solids: 17365 mg / L, chloride ion / sulfate: 30, impurity ions mainly composed of nitrate: 37.3 mg / L, COD: 72 mg / L; the first concentrated water 112 is mainly sodium sulfate and contains a small amount of sodium chloride. The specific parameters are total dissolved solids: 102978 mg / L, chloride ion / sulfate: 0.17, impurity ions mainly composed of nitrate: 120 mg / L, COD: 1511 mg / L. The first produced water 111 enters the secondary nanofiltration 12. After the first concentrated water 112 enters the advanced oxidation treatment device 50 for ozone catalytic oxidation, 30% of the COD is removed, and then it enters the sodium sulfate evaporation crystallization device 21.

[0068] The sodium sulfate evaporation crystallization device 21 uses MVR evaporation crystallization to produce nitrate slurry, and the nitrate slurry is separated into solid and liquid by a thickener and a centrifugal dehydrator to produce a first crude nitrate 211 and a first mother liquor 212. The first crude nitrate 211 enters the sodium sulfate recrystallization device 23 through the crude nitrate outlet, and the first mother liquor 212 enters the sodium sulfate freezing crystallization device 22. Among them, the mass flow rate of the first mother liquor 212 is 10.7% of the mass flow rate of the first concentrated water 112 entering the sodium sulfate evaporation crystallization device 21, and the specific parameters of the first mother liquor 212 are total dissolved solids: 221299 mg / L, chloride ion / sulfate: 2.47, impurity ions mainly composed of nitrate: 1126 mg / L, COD: 9912 mg / L.

[0069] The sodium sulfate freeze crystallization device 22 uses forced circulation freeze crystallization, thickener and centrifugal dehydrator or only centrifugal dehydrator for solid-liquid separation to produce mirabilite 221 and second mother liquor 222. The mirabilite 221 enters the sodium sulfate recrystallization device 23, and the second mother liquor 222 enters the mixed salt pretreatment device 41. Among them, the volume flow rate of the second mother liquor 222 is basically the same as the volume flow rate of the first mother liquor 212 entering the sodium sulfate freeze crystallization device 22. The specific parameters of the second mother liquor 222 are total dissolved solids: 181361 mg / L, chloride ion / sulfate: 8.5, impurity ions mainly composed of nitrate: 1126 mg / L, COD: 9850 mg / L.

[0070] The sodium sulfate recrystallization device 23 adopts MVR evaporation crystallization to dissolve the first crude saltpeter 211 and the thenardite 221 and then recrystallize them, so as to transfer out most of the organic matter, impurity ions, etc. in the first crude saltpeter 211 and the thenardite 221, so as to reduce the content of organic matter and impurity ions in the sodium sulfate; the slurry produced by the recrystallization is separated into solid and liquid by a thickener and a centrifugal dehydrator to obtain wet saltpeter 231 with a moisture content of not more than 5%, and the wet saltpeter 231 enters the sodium sulfate drying device 24.

[0071] The sodium sulfate drying device 24 uses a vibrating fluidized bed dryer to dry the wet nitrate 231 to obtain a sodium sulfate product with a purity of more than 98%.

[0072] The first produced water 111 is further filtered through the secondary nanofiltration 12 to remove the sodium sulfate therein, so that the sodium sulfate content in the produced water is further reduced. The specific parameters of the second produced water 121 are total dissolved solids: 16889 mg / L, chloride ions / sulfate: 303, impurity ions mainly composed of nitrate: 33.6 mg / L, COD: 54 mg / L. The second produced water 121 enters the high-pressure reverse osmosis device 13. The second produced water 121, which is mainly composed of sodium chloride and contains only a small amount of sodium sulfate, is concentrated through the high-pressure reverse osmosis device 13 to produce a salt-rich second concentrated water 131, whose specific parameters are total dissolved solids: 80869 mg / L, chloride ions / sulfate: 303, impurity ions mainly composed of nitrate: 37.3 mg / L, COD: 72 mg / L. The second concentrated water 131 enters the sodium chloride evaporation crystallization device 31.

[0073] The sodium chloride evaporation crystallization device 31 uses multiple-effect evaporation crystallization to produce salt slurry, which is separated into solid and liquid by a thickener and a centrifugal dehydrator to produce a first crude salt 311 and a third mother liquor 312. The first crude salt 311 enters the sodium chloride salt washing device 32, and the third mother liquor 312 enters the mixed salt pretreatment device 41. The mass flow rate of the third mother liquor 312 is 2.2% of the mass flow rate of the second concentrated water 131 entering the sodium chloride evaporation crystallization device 31, and its specific parameters are total dissolved solids: 378668 mg / L, chloride ion / sulfate: 24.53, impurity ions mainly composed of nitrate: 7377 mg / L, COD: 11859 mg / L.

[0074] The sodium chloride salt washing device 32 uses evaporative crystallization condensate to wash the salt, and transfers most of the organic matter, impurity ions, etc. adhered to the surface of the first crude salt 311 to the washing liquid, so that the content of organic matter and impurity ions in the sodium chloride is reduced; and then the solid-liquid separation is carried out by a centrifugal dehydrator to obtain wet salt 321 with a moisture content of no more than 5%, and the wet salt 321 enters the sodium chloride drying device 33.

[0075] The sodium chloride drying device 33 uses a vibrating fluidized bed dryer or the like to dry the wet salt 321 to obtain a sodium chloride product with a purity of more than 98.5%.

[0076] The mixed salt pretreatment device 41 uses a storage tank with a stirrer to stir and homogenize the second mother liquor 222 and the third mother liquor 312 to obtain a fourth mother liquor 411, and the fourth mother liquor 411 enters the mixed salt evaporation crystallization device 42. The specific parameters of the fourth mother liquor 411 are total dissolved solids: 263551 mg / L, chloride ion / sulfate: 12.3, impurity ions mainly composed of nitrate: 3700 mg / L, COD: 10713 mg / L.

[0077] The mixed salt evaporation crystallization device 42 adopts MVR evaporation crystallization, and then undergoes solid-liquid separation through a thickener and a centrifugal dehydrator to obtain a second crude salt 421 with a moisture content of no more than 5% and a fifth mother liquor 422. The second crude salt 421 enters the sodium chloride salt washing device 32, and the fifth mother liquor 422 enters the impure salt drying device 43. The mass flow rate of the fifth mother liquor 422 is 10.7% of the mass flow rate of the fourth mother liquor 411 entering the mixed salt evaporation crystallization device 42, and its specific parameters are total dissolved solids: 221299 mg / L, chloride ion / sulfate: 0.44, impurity ions mainly composed of nitrate: 8387 mg / L, and COD: 24285 mg / L.

[0078] The mixed salt drying device 43 uses a spray dryer to produce mixed salt.

[0079] Example 5

[0080] This embodiment adopts Figure 2 The zero-emission salt separation system with low impure salt rate and high salt nitrate quality is used to treat the reverse osmosis high-salt wastewater of the coal chemical wastewater reuse system. The soluble solids contained in the high-salt wastewater are mainly sodium chloride and sodium sulfate. The specific parameters are total dissolved solids: 70604 mg / L, chloride ion / sulfate: 1.88, impurity ions mainly composed of nitrate: 1743 mg / L, COD: 837 mg / L.

[0081] After being treated by the primary nanofiltration 11, the brine is divided into two streams. The first produced water 111 is mainly sodium chloride and contains a small amount of sodium sulfate. The specific parameters are total dissolved solids: 50654 mg / L, chloride ion / sulfate: 18.8, impurity ions mainly composed of nitrate: 1478 mg / L, COD: 293 mg / L; the first concentrated water 112 is mainly sodium sulfate and contains a small amount of sodium chloride. The specific parameters are total dissolved solids: 154448 mg / L, chloride ion / sulfate: 0.39, impurity ions mainly composed of nitrate: 2342 mg / L, COD: 3156 mg / L. The first produced water 111 enters the secondary nanofiltration 12. After the first concentrated water 112 enters the advanced oxidation treatment device 50 for ozone catalytic oxidation, 30% of the COD is removed, and then it enters the sodium sulfate evaporation crystallization device 21.

[0082] The sodium sulfate evaporation crystallization device 21 uses multiple-effect evaporation crystallization to produce nitrate slurry, and the nitrate slurry is separated into solid and liquid by a thickener and a centrifugal dehydrator to produce a first crude nitrate 211 and a first mother liquor 212. The first crude nitrate 211 enters the sodium sulfate recrystallization device 23 through the crude nitrate outlet, and the first mother liquor 212 enters the sodium sulfate freezing crystallization device 22. Among them, the mass flow rate of the first mother liquor 212 is 10.7% of the mass flow rate of the first concentrated water 112 entering the sodium sulfate evaporation crystallization device 21, and the specific parameters of the first mother liquor 212 are total dissolved solids: 348239 mg / L, chloride ion / sulfate: 2.07, impurity ions mainly composed of nitrate: 11759 mg / L, COD: 11094 mg / L.

[0083] The sodium sulfate freeze crystallization device 22 uses forced circulation freeze crystallization, thickener and centrifugal dehydrator or only centrifugal dehydrator for solid-liquid separation to produce mirabilite 221 and second mother liquor 222. The mirabilite 221 enters the sodium sulfate recrystallization device 23, and the second mother liquor 222 enters the mixed salt pretreatment device 41. Among them, the volume flow rate of the second mother liquor 222 is basically the same as the volume flow rate of the first mother liquor 212 entering the sodium sulfate freeze crystallization device 22. The specific parameters of the second mother liquor 222 are total dissolved solids: 262448 mg / L, chloride ion / sulfate: 15.41, impurity ions mainly composed of nitrate: 11759.5 mg / L, COD: 11000 mg / L.

[0084] The sodium sulfate recrystallization device 23 adopts single-effect evaporation crystallization to dissolve the first crude saltpeter 211 and the thenardite 221 and then recrystallize them, so as to transfer out most of the organic matter, impurity ions, etc. in the first crude saltpeter 211 and the thenardite 221, so as to reduce the content of organic matter and impurity ions in the sodium sulfate; the slurry produced by the recrystallization is separated into solid and liquid by a thickener and a centrifugal dehydrator to obtain wet saltpeter 231 with a moisture content of not more than 5%, and the wet saltpeter 231 enters the sodium sulfate drying device 24.

[0085] The sodium sulfate drying device 24 uses a fixed bed dryer or the like to dry the wet nitrate 231 to obtain a sodium sulfate product with a purity of more than 98%.

[0086] The first produced water 111 is further filtered through the secondary nanofiltration 12 to remove the sodium sulfate therein, so that the sodium sulfate content in the produced water is further reduced. The specific parameters of the second produced water 121 are total dissolved solids: 48396 mg / L, chloride ion / sulfate: 188, impurity ions mainly composed of nitrate: 1329 mg / L, COD: 205 mg / L. The second produced water 121 enters the high-pressure reverse osmosis device 13. The second produced water 121, which is mainly composed of sodium chloride and contains only a small amount of sodium sulfate, is concentrated through the high-pressure reverse osmosis device 13 to produce a salt-rich second concentrated water 131, whose specific parameters are total dissolved solids: 86005 mg / L, chloride ion / sulfate: 188, impurity ions mainly composed of nitrate: 2363 mg / L, COD: 364 mg / L. The second concentrated water 131 enters the sodium chloride evaporation crystallization device 31.

[0087] The sodium chloride evaporation crystallization device 31 uses multiple-effect evaporation crystallization to produce salt slurry, which is separated into solid and liquid by a thickener and a centrifugal dehydrator to produce a first crude salt 311 and a third mother liquor 312. The first crude salt 311 enters the sodium chloride salt washing device 32, and the third mother liquor 312 enters the mixed salt pretreatment device 41. The mass flow rate of the third mother liquor 312 is 2.2% of the mass flow rate of the second concentrated water 131 entering the sodium chloride evaporation crystallization device 31, and its specific parameters are total dissolved solids: 550777 mg / L, chloride ion / sulfate: 11.96, impurity ions mainly composed of nitrate: 134489 mg / L, COD: 20736 mg / L.

[0088] The sodium chloride salt washing device 32 uses evaporative crystallization condensate to wash the salt, and transfers most of the organic matter, impurity ions, etc. adhered to the surface of the first crude salt 311 to the washing liquid, so that the content of organic matter and impurity ions in the sodium chloride is reduced; and then the solid-liquid separation is carried out by a centrifugal dehydrator to obtain wet salt 321 with a moisture content of no more than 5%, and the wet salt 321 enters the sodium chloride drying device 33.

[0089] The sodium chloride drying device 33 uses a fixed bed dryer or the like to dry the wet salt 321 to obtain a sodium chloride product with a purity of more than 98.5%.

[0090] The mixed salt pretreatment device 41 uses a storage tank with a stirrer to stir and homogenize the second mother liquor 222 and the third mother liquor 312 to obtain a fourth mother liquor 411, which enters the mixed salt evaporation crystallization device 42. The specific parameters of the fourth mother liquor 411 are total dissolved solids: 309699 mg / L, chloride ion / sulfate: 12.1, impurity ions mainly composed of nitrate: 31027 mg / L, COD: 12608 mg / L.

[0091] The mixed salt evaporation crystallization device 42 adopts single-effect evaporation crystallization, and then undergoes solid-liquid separation through a thickener and a centrifugal dehydrator to obtain a second crude salt 421 with a moisture content of no more than 5% and a fifth mother liquor 422. The second crude salt 421 enters the sodium chloride salt washing device 32, and the fifth mother liquor 422 enters the impure salt drying device 43. The mass flow rate of the fifth mother liquor 422 is 46% of the mass flow rate of the fourth mother liquor 411 entering the mixed salt evaporation crystallization device 42, and its specific parameters are total dissolved solids: 447776 mg / L, chloride ion / sulfate: 6.88, impurity ions mainly composed of nitrate: 67341 mg / L, and COD: 27364 mg / L.

[0092] The mixed salt drying device 43 uses a drum scraper dryer to produce mixed salt.

[0093] Example 6

[0094] This embodiment adopts Figure 2 The zero-discharge salt separation system with low impure salt rate and high salt and nitrate quality is used to treat the reverse osmosis high-salt wastewater from the coking wastewater reuse system. The soluble solids contained in the high-salt wastewater are mainly sodium chloride and sodium sulfate. The specific parameters are total dissolved solids: 72253 mg / L, chloride ion / sulfate: 1.35, impurity ions mainly nitrate: 2385 mg / L, COD: 900 mg / L.

[0095] After being treated by the primary nanofiltration 11, the brine is divided into two streams. The first produced water 111 is mainly sodium chloride and contains a small amount of sodium sulfate. The specific parameters are total dissolved solids: 47487 mg / L, chloride ion / sulfate: 13.5, impurity ions mainly composed of nitrate: 2035 mg / L, COD: 315 mg / L; the first concentrated water 112 is mainly sodium sulfate and contains a small amount of sodium chloride. The specific parameters are total dissolved solids: 145803 mg / L, chloride ion / sulfate: 0.36, impurity ions mainly composed of nitrate: 2939 mg / L, COD: 2655 mg / L. The first produced water 111 enters the secondary nanofiltration 12. After the first concentrated water 112 enters the advanced oxidation treatment device 50 for ozone catalytic oxidation, 30% of the COD is removed, and then it enters the sodium sulfate evaporation crystallization device 21.

[0096] The sodium sulfate evaporation crystallization device 21 uses multiple-effect evaporation crystallization to produce nitrate slurry, and the nitrate slurry is separated into solid and liquid by a thickener and a centrifugal dehydrator to produce a first crude nitrate 211 and a first mother liquor 212. The first crude nitrate 211 enters the sodium sulfate recrystallization device 23 through the crude nitrate outlet, and the first mother liquor 212 enters the sodium sulfate freezing crystallization device 22. Among them, the mass flow rate of the first mother liquor 212 is 10.7% of the mass flow rate of the first concentrated water 112 entering the sodium sulfate evaporation crystallization device 21, and the specific parameters of the first mother liquor 212 are total dissolved solids: 393270 mg / L, chloride ion / sulfate: 4.75, impurity ions mainly composed of nitrate: 21537 mg / L, COD: 13618 mg / L.

[0097] The sodium sulfate freeze crystallization device 22 uses forced circulation freeze crystallization, thickener and centrifugal dehydrator or only centrifugal dehydrator for solid-liquid separation to produce mirabilite 221 and second mother liquor 222. The mirabilite 221 enters the sodium sulfate recrystallization device 23, and the second mother liquor 222 enters the mixed salt pretreatment device 41. Among them, the volume flow rate of the second mother liquor 222 is basically the same as the volume flow rate of the first mother liquor 212 entering the sodium sulfate freeze crystallization device 22. The specific parameters of the second mother liquor 222 are total dissolved solids: 350374 mg / L, chloride ion / sulfate: 20, impurity ions mainly composed of nitrate: 21537 mg / L, COD: 13486 mg / L.

[0098] The sodium sulfate recrystallization device 23 adopts multi-effect evaporation crystallization to dissolve the first crude saltpeter 211 and the thenardite 221 and then recrystallize them, so as to transfer out most of the organic matter, impurity ions, etc. in the first crude saltpeter 211 and the thenardite 221, so as to reduce the content of organic matter and impurity ions in the sodium sulfate; the slurry produced by the recrystallization is separated into solid and liquid by a thickener and a centrifugal dehydrator to obtain wet saltpeter 231 with a moisture content of not more than 5%, and the wet saltpeter 231 enters the sodium sulfate drying device 24.

[0099] The sodium sulfate drying device 24 uses a fixed bed dryer or the like to dry the wet nitrate 231 to obtain a sodium sulfate product with a purity of more than 98%.

[0100] The first produced water 111 is further filtered through the secondary nanofiltration 12 to remove the sodium sulfate therein, so that the sodium sulfate content in the produced water is further reduced. The specific parameters of the second produced water 121 are total dissolved solids: 44638 mg / L, chloride ion / sulfate: 135, impurity ions mainly composed of nitrate: 1831 mg / L, COD: 220 mg / L. The second produced water 121 enters the high-pressure reverse osmosis device 13. The second produced water 121, which is mainly composed of sodium chloride and contains only a small amount of sodium sulfate, is concentrated through the high-pressure reverse osmosis device 13 to produce the second salt-rich concentrated water 131, whose specific parameters are total dissolved solids: total dissolved solids: 98589 mg / L, chloride ion / sulfate: 135, impurity ions mainly composed of nitrate: 4046 mg / L, COD: 487 mg / L. The second concentrated water 131 enters the sodium chloride evaporation crystallization device 31.

[0101] The sodium chloride evaporation crystallization device 31 uses multiple-effect evaporation crystallization to produce salt slurry, which is separated into solid and liquid by a thickener and a centrifugal dehydrator to produce a first crude salt 311 and a third mother liquor 312. The first crude salt 311 enters the sodium chloride salt washing device 32, and the third mother liquor 312 enters the mixed salt pretreatment device 41. The mass flow rate of the third mother liquor 312 is 2.2% of the mass flow rate of the second concentrated water 131 entering the sodium chloride evaporation crystallization device 31, and its specific parameters are total dissolved solids: 624355 mg / L, chloride ion / sulfate: 10.8, impurity ions mainly composed of nitrate: 166082 mg / L, COD: 19997 mg / L.

[0102] The sodium chloride salt washing device 32 uses evaporative crystallization condensate to wash the salt, and transfers most of the organic matter, impurity ions, etc. adhered to the surface of the first crude salt 311 to the washing liquid, so that the content of organic matter and impurity ions in the sodium chloride is reduced; and then the solid-liquid separation is carried out by a centrifugal dehydrator to obtain wet salt 321 with a moisture content of no more than 5%, and the wet salt 321 enters the sodium chloride drying device 33.

[0103] The sodium chloride drying device 33 uses a fixed bed dryer or the like to dry the wet salt 321 to obtain a sodium chloride product with a purity of more than 98.5%.

[0104] The mixed salt pretreatment device 41 uses a storage tank with an agitator to stir and homogenize the second mother liquor 222 and the third mother liquor 312 to obtain a fourth mother liquor 411, which enters the mixed salt evaporation crystallization device 42. The specific parameters of the fourth mother liquor 411 are total dissolved solids: 399339 mg / L, chloride ion / sulfate: 15.9, impurity ions mainly composed of nitrate: 46825 mg / L, COD: 14734 mg / L.

[0105] The mixed salt evaporation crystallization device 42 adopts single-effect evaporation crystallization, and then undergoes solid-liquid separation through a thickener and a centrifugal dehydrator to obtain a second crude salt 421 with a moisture content of no more than 5% and a fifth mother liquor 422. The second crude salt 421 enters the sodium chloride salt washing device 32, and the fifth mother liquor 422 enters the impure salt drying device 43. The mass flow rate of the fifth mother liquor 422 is 45.7% of the mass flow rate of the fourth mother liquor 411 entering the mixed salt evaporation crystallization device 42, and its specific parameters are total dissolved solids: 516840 mg / L, chloride ion / sulfate: 7.17, impurity ions mainly composed of nitrate: 102404 mg / L, and COD: 32224 mg / L.

[0106] The mixed salt drying device 43 uses a drum scraper dryer to produce mixed salt.

[0107] Comparative Example

[0108] This comparative example is the currently commonly used "sodium chloride evaporation crystallization + centrifugal separation + drying" process and "sodium sulfate evaporation crystallization + centrifugal separation + drying" process. Compared with the technical scheme used in Example 3 of the present invention, the treatment system used does not include the secondary nanofiltration 12, the sodium chloride salt washing device 32, the sodium sulfate freezing crystallization device 22, the sodium sulfate recrystallization device 23 and the mixed salt evaporation crystallization device 42.

[0109] The concentrated brine treated in this comparative example is the reverse osmosis high-salt wastewater from the coking wastewater reuse system. The soluble solids contained in the high-salt wastewater are mainly sodium chloride and sodium sulfate. The specific parameters are total dissolved solids: 31201 mg / L, chloride ion / sulfate: 0.77, impurity ions mainly composed of nitrate: 832 mg / L, COD: 196 mg / L. After membrane concentration and the steps of sodium sulfate, sodium chloride and miscellaneous salts separation and crystallization, the corresponding products of sodium sulfate, sodium chloride and miscellaneous salts are obtained.

[0110] The sodium sulfate and sodium chloride products and the impure salt ratios obtained by the zero-emission salt separation process of Examples 3-6 and the comparative example are shown in Table 1; the purity of sodium sulfate and sodium chloride is measured according to the method shown in the corresponding product standards.

[0111] Table 1

[0112] Serial number Example 3 Example 4 Example 5 Example 6 Comparative Example Sodium sulfate product purity 99.1% 98.5% 98% 98.3% 97% Sodium sulfate product standard GB / T6009-2014 "Industrial Anhydrous Sodium Sulfate" Class I First Grade Product GB / T6009-2014 "Industrial Anhydrous Sodium Sulfate" Class II First Grade Product GB / T6009-2014 "Industrial Anhydrous Sodium Sulfate" Class II First Grade Product GB / T6009-2014 "Industrial Anhydrous Sodium Sulfate" Class II First Grade Product GB / T6009-2014 "Industrial Anhydrous Sodium Sulfate" Class II qualified products Salt content 6.23% 8.59% 13.46% 13.11% 27% Sodium chloride product purity 98.9% 98.7% 98.5% 98.6% 94.9% Sodium Chloride Product Standard GB / T5462-2015 "Industrial Salt" Industrial Dry Salt Grade 1 GB / T5462-2015 "Industrial Salt" Industrial Dry Salt Grade 1 GB / T5462-2015 "Industrial Salt" Industrial Dry Salt Grade 1 GB / T5462-2015 "Industrial Salt" Industrial Dry Salt Grade 1 GB / T5462-2015 "Industrial Salt" Sun-dried Salt Grade 1

[0113] It can be verified from Table 1 that the purity of the sodium sulfate obtained in Examples 3-6 reached more than 98%, and that in Example 3 reached 99.1%, which meets the requirements of Class I first-class products in "Industrial Anhydrous Sodium Sulfate" (GB / T6009-2014), while the sodium sulfate obtained in the comparative example only meets the requirements of Class II qualified products, and is of lower value than the sodium sulfate obtained in Examples 3-6.

[0114] The purity of the sodium chloride obtained in Example 3-6 reaches more than 98.5%, which meets the first-grade industrial dry salt standard in the refined industrial salt of "Industrial Salt" (GB / T5462-2015), and its value is relatively high, and has broad market application prospects; the sodium chloride obtained in the comparative example contains a certain amount of impurities, and its purity is only 94.9%, which meets the first-grade standard of solar-dried industrial salt, and its value is lower than the first-grade refined industrial salt.

[0115] The impure salt rate of the zero-emission salt separation process with low impure salt rate and high salt nitrate quality in Example 3-6 is controlled within 13.46%. Compared with the impure salt rate of up to 27% in the control example, the technical solution of the present invention can significantly reduce the impure salt rate. Referring to the current processing cost of about 3,000 yuan per ton of crystalline impure salt, the process of the present invention can save about 50% or more of the impure salt processing cost and reduce the overall operating cost.

[0116] It should be noted that the above contents are further detailed descriptions of the present invention in combination with specific implementation methods, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions; the dimensional data of this embodiment does not necessarily limit the technical solution, but only shows one of the specific working conditions. For ordinary technicians in the technical field to which the present invention belongs, several simple improvements and modifications can be made without departing from the concept of the present invention, which should be regarded as falling within the scope of protection of the present invention.

Claims

1. A zero-discharge process for salt separation with low impurity salt ratio and high salt and nitrate quality, characterized in that: The following steps are involved: Step 1: membrane concentration, the high-salinity wastewater is subjected to a membrane concentration process to obtain a first concentrated water mainly composed of sodium sulfate and a second concentrated water mainly composed of sodium chloride; in the membrane concentration process, the high-salinity wastewater is separated into a first concentrated water and a first produced water through a first nanofiltration process; the first produced water is subjected to a second nanofiltration process and a high-pressure reverse osmosis process to obtain a second concentrated water; Step 2: Salt crystallization, including 1) The first concentrated water is subjected to a sodium sulfate evaporation crystallization process, a sodium sulfate freeze crystallization process, a sodium sulfate recrystallization process and a sodium sulfate drying process to obtain a high-purity sodium sulfate product; the sodium sulfate evaporation crystallization process comprises: crystallizing the first concentrated water and performing solid-liquid separation to obtain a first crude saltpeter and a first mother liquor; the first crude saltpeter enters a sodium sulfate recrystallization process, and the first mother liquor enters a sodium sulfate freeze crystallization process; The sodium sulfate freezing crystallization process comprises: performing forced circulation freezing crystallization on the first mother liquor, and obtaining mirabilite and the second mother liquor after solid-liquid separation; the mirabilite enters the sodium sulfate recrystallization process; The sodium sulfate recrystallization process comprises: dissolving and recrystallizing the first crude saltpeter and mirabilite, and obtaining wet saltpeter after solid-liquid separation, and the wet saltpeter enters; 2) The second concentrated water is subjected to a sodium chloride evaporation and crystallization process, a sodium chloride salt washing process and a sodium chloride drying process to obtain a high-purity sodium chloride product; the sodium chloride evaporation and crystallization process comprises: crystallizing the second concentrated water and separating the solid from the liquid to obtain a first crude salt and a third mother liquor; the first crude salt enters a sodium chloride salt washing process; 3) The second mother liquor discharged from the sodium sulfate evaporation and crystallization process and the third mother liquor discharged from the sodium chloride evaporation and crystallization process are mixed in a mixed salt pretreatment process, and then subjected to a mixed salt evaporation and crystallization process and a mixed salt drying process to obtain mixed salt; The mixed salt pretreatment process comprises: uniformly mixing the second mother liquor and the third mother liquor to obtain a fourth mother liquor, and the fourth mother liquor enters the mixed salt evaporation crystallization process; The mixed salt evaporation crystallization process comprises: crystallizing the fourth mother liquor and separating the solid and the liquid to obtain the second crude salt and the fifth mother liquor; the second crude salt enters the sodium chloride salt washing process, and the fifth mother liquor enters the mixed salt drying process.

2. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1 is characterized in that: The moisture content of the wet nitrate is not more than 5%.

3. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1 is characterized in that: The sodium chloride salt washing process comprises: washing the first crude salt with a washing liquid, and separating the solid from the liquid to obtain wet salt, and the wet salt enters the sodium chloride drying process.

4. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1 is characterized in that: In the sodium chloride salt washing process, one or more of the evaporation condensate obtained in the sodium sulfate evaporation crystallization process, the sodium chloride evaporation crystallization process, the mixed salt evaporation crystallization process and the second concentrated water is used as the washing liquid.

5. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1 is characterized in that: The sodium chloride salt washing process comprises: washing the first crude salt with a washing liquid, and obtaining wet salt by solid-liquid separation, wherein the moisture content of the wet salt is not greater than 5%.

6. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1 is characterized in that: The moisture content of the second crude salt is not more than 5%.

7. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1 is characterized in that: Any crystallization step in the process adopts one of single-effect evaporation crystallization, multi-effect evaporation crystallization, MVR evaporation crystallization or TVR evaporation crystallization.

8. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1 is characterized in that: Any drying process in the process adopts one of a box dryer, a fixed bed dryer or a vibrating fluidized bed dryer.

9. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1, characterized in that: The mixed salt drying process adopts one of a rake dryer, a drum scraper dryer or a spray dryer.

10. The zero-discharge salt separation process with low impurity salt ratio and high salt and nitrate quality according to claim 1, characterized in that: The first concentrated water is subjected to an advanced oxidation treatment process to reduce its COD before entering the sodium sulfate evaporation and crystallization process.

11. A salt separation zero-discharge system with low impurity salt ratio and high salt and nitrate quality, characterized in that: It includes a membrane concentration unit, a sodium sulfate crystallization unit, a sodium chloride crystallization unit and a miscellaneous salt crystallization unit, wherein The membrane concentration unit comprises a primary nanofiltration device, a secondary nanofiltration device and a high-pressure reverse osmosis device connected in sequence; The sodium sulfate crystallization unit comprises a sodium sulfate evaporation crystallization device, a sodium sulfate freezing crystallization device, a sodium sulfate recrystallization device and a sodium sulfate drying device connected in sequence; the sodium sulfate evaporation crystallization device is connected to the concentrated water outlet of the primary nanofiltration, and the sodium sulfate evaporation crystallization device is also provided with a solid discharge port connected to the sodium sulfate recrystallization device; the sodium sulfate freezing crystallization device is provided with a first discharge outlet; The sodium chloride crystallization unit comprises a sodium chloride evaporation crystallization device, a sodium chloride salt washing device and a sodium chloride drying device connected in sequence; the sodium chloride evaporation crystallization device is connected to the concentrated water outlet of the high-pressure reverse osmosis device; the sodium chloride evaporation crystallization device is provided with a second discharge outlet; The mixed salt crystallization unit comprises a mixed salt pretreatment device, a mixed salt evaporation crystallization device and a mixed salt drying device which are connected in sequence; the mixed salt pretreatment device is connected to a first discharge outlet and a second discharge outlet; and the solid discharge port of the mixed salt evaporation crystallization device is connected to the sodium chloride salt washing device.

12. The zero-emission salt separation system with low impurity salt ratio and high salt and nitrate quality according to claim 11, characterized in that: An advanced oxidation treatment device is provided between the membrane concentration unit and the sodium sulfate crystallization unit.

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