Multi-stage freezing crystallization salt separation system and multi-stage freezing crystallization salt separation method

Abstract

The present invention relates to a multi-stage freezing crystallization salt separation system and a multi-stage freezing crystallization salt separation method. The multi-stage freezing crystallization salt separation system includes a concentration device and a multi-stage salt separation device. Each stage of the salt separation device includes a freezing crystallization unit, a solid-liquid separation unit, and a nanofiltration separation and concentration unit. In each stage of the salt separation device, the solid-liquid separation unit is communicated with the freezing crystallization unit, and the inlet of the nanofiltration separation and concentration unit is communicated with the outlet of the solid-liquid separation unit; the freezing crystallization unit in the first-stage salt separation device is communicated with the outlet of the concentration device, and the outlet of the solid-liquid separation unit in the first-stage salt separation device is also communicated with the inlet of the concentration device; in two adjacent stages of the salt separation device, the freezing crystallization unit in the lower-stage salt separation device is communicated with the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit in the upper-stage salt separation device, and the outlet of the solid-liquid separation unit in the lower-stage salt separation device is also communicated with the inlet of the nanofiltration separation and concentration unit in the upper-stage salt separation device.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and particularly to a multi-stage freezing crystallization salt separation system and a multi-stage freezing crystallization salt separation method. Background Art

[0002] Wastewater generated in industries such as electric power and coal chemical industries has the characteristics of high hardness, high salt content, large water volume and great treatment difficulty. In order to meet the demand for protection and resource utilization, zero discharge of wastewater has become an industry trend. "Zero discharge" of wastewater is to clean the industrial wastewater, precipitate the pollutants in the wastewater in the form of solids, reduce the products in the wastewater to be infinitely close to zero, and make the produced solids have great utilization value.

[0003] At present, the mainstream method for salt treatment is salt separation by quality, and the main technologies are thermal crystallization and cold crystallization. However, there are problems such as low purity and low yield of the produced crystalline salts, and the produced miscellaneous salts are classified as solid hazardous wastes. The volume of miscellaneous salts is large, the resource utilization rate is low, and the treatment cost of miscellaneous salts is high. Therefore, it is urgent to develop process technologies that can improve the resource utilization rate and reduce the operating cost. Summary of the Invention

[0004] Based on this, it is necessary to provide a multi-stage freezing crystallization salt separation system and a multi-stage freezing crystallization salt separation method that can improve the resource utilization rate and reduce the operating cost.

[0005] A multi-stage freezing crystallization salt separation system includes:

[0006] A concentration device; and

[0007] A multi-stage salt separation device, each stage of the salt separation device includes a freezing crystallization unit, a solid-liquid separation unit and a nanofiltration separation and concentration unit. In each stage of the salt separation device, the solid-liquid separation unit is connected to the freezing crystallization unit to perform solid-liquid separation on the mixture after freezing crystallization, and the inlet of the nanofiltration separation and concentration unit is connected to the outlet of the solid-liquid separation unit; the freezing crystallization unit in the first-stage salt separation device is connected to the outlet of the concentration device, and the outlet of the solid-liquid separation unit in the first-stage salt separation device is also connected to the inlet of the concentration device; in adjacent two-stage salt separation devices, the freezing crystallization unit in the next-stage salt separation device is connected to the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit in the previous-stage salt separation device, and the outlet of the solid-liquid separation unit in the next-stage salt separation device is also connected to the inlet of the nanofiltration separation and concentration unit in the previous-stage salt separation device.

[0008] The above-mentioned multi-stage freezing crystallization salt separation system, in which the multi-stage salt separation device is multi-stage freezing separation crystallization, and hierarchical freezing crystallization can obtain hydrated crystal salts with different application values. Compared with the traditional process of circulating all the mother liquor in the freezing crystallization system, the above-mentioned multi-stage freezing crystallization salt separation system further processes the mother liquor separated from solid-liquid separation, further separates the residual divalent and higher-valent ions in the mother liquor, and performs freezing crystallization to precipitate crystal salts, improving the purity and yield of the crystal salts. In this way, it avoids the problems of low purity and no application value of the crystal salts produced during long-term operation due to the continuous accumulation of impurities such as inorganic salts and organic substances in the mother liquor of the traditional process; at the same time, the mother liquor is separated by nanofiltration in stages and discharged from the multi-stage salt separation device in the form of nanofiltration-produced water, and high-purity monovalent salts can be further prepared. At the same time, the volume of the final miscellaneous salts is greatly reduced, avoiding the problem of high miscellaneous salt treatment costs. The above-mentioned multi-stage freezing crystallization salt separation system improves the purity and yield of the crystal salts, greatly reduces the volume of the final miscellaneous salts, improves the resource utilization rate, does not need to adopt the method of adding medicine for crystallization, and reduces the operation cost.

[0009] In some embodiments, the multi-stage freezing crystallization salt separation system further includes a monovalent salt evaporation crystallization device, and the monovalent salt evaporation crystallization device is communicated with the nanofiltration-produced water outlets of the nanofiltration separation and concentration units of each stage of the salt separation device.

[0010] In some embodiments, the multi-stage freezing crystallization salt separation system further includes a miscellaneous salt evaporation crystallization device and a miscellaneous salt solid-liquid separation device. The miscellaneous salt evaporation crystallization device is communicated with the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit in the last stage of the salt separation device, and the miscellaneous salt solid-liquid separation device is communicated with the miscellaneous salt evaporation crystallization device for solid-liquid separation of the miscellaneous salts after evaporation crystallization.

[0011] In some embodiments, the multi-stage freezing crystallization salt separation system further includes a miscellaneous salt mother liquor tank. The inlet of the miscellaneous salt mother liquor tank is communicated with the outlet of the miscellaneous salt solid-liquid separation device, and the outlet of the miscellaneous salt mother liquor tank is communicated with the inlet of the miscellaneous salt evaporation crystallization device.

[0012] In some embodiments, each stage of the salt separation device further includes a salt separation mother liquor tank. In each stage of the salt separation device, the inlet of the nanofiltration separation and concentration unit is communicated with the outlet of the solid-liquid separation unit through the salt separation mother liquor tank.

[0013] In some embodiments, the outlet of the salt separation mother liquor tank in the next stage of the salt separation device is further communicated with the inlet of the salt separation mother liquor tank in the previous stage of the salt separation device.

[0014] In some embodiments, each stage of the salt separation device further includes a melt crystallization unit. In each stage of the salt separation device, the melt crystallization unit is used to perform melt crystallization on the hydrated crystal salts separated by the solid-liquid separation unit.

[0015] A multi-stage freezing crystallization salt separation method includes the following steps:

[0016] Concentrate the brine to be treated to obtain concentrated brine;

[0017] Perform multi-stage salt separation treatment on the concentrated brine in sequence;

[0018] Among them, each stage of salt separation treatment includes:

[0019] Freezing crystallization treatment;

[0020] Perform solid-liquid separation treatment on the mixture after the freezing crystallization treatment to obtain hydrated crystalline salt and mother liquor; and

[0021] In the first-stage salt separation treatment, the mother liquor obtained from the solid-liquid separation treatment is shunted, and nanofiltration separation and concentration treatment are respectively performed to obtain nanofiltration product water and nanofiltration concentrated water and return to the concentration treatment; in other stages of salt separation treatment except the first-stage salt separation treatment, the mother liquor obtained from the solid-liquid separation treatment is shunted, and nanofiltration separation and concentration treatment and return to the previous-stage salt separation treatment for the nanofiltration separation and concentration treatment are respectively performed to obtain nanofiltration product water and nanofiltration concentrated water;

[0022] In adjacent two-stage salt separation treatment, the nanofiltration concentrated water after the nanofiltration separation and concentration treatment in the previous stage is transferred to the freezing crystallization treatment in the next stage.

[0023] In some embodiments, the multi-stage freezing crystallization salt separation method further includes the following steps:

[0024] Perform evaporation crystallization on the nanofiltration product water obtained from the nanofiltration separation and concentration treatment in each stage of salt separation treatment to obtain monovalent salt.

[0025] In some embodiments, the multi-stage freezing crystallization salt separation method further includes the following steps:

[0026] Perform melting crystallization on the hydrated crystalline salt obtained from the solid-liquid separation treatment in each stage of salt separation treatment to remove crystal water to obtain anhydrous crystalline salt.

[0027] In some embodiments, the multi-stage freezing crystallization salt separation method further includes the following steps:

[0028] Perform evaporation crystallization and solid-liquid separation on the nanofiltration concentrated water obtained from the nanofiltration separation and concentration treatment in the last stage of salt separation treatment to obtain miscellaneous salt.

[0029] In some embodiments, the multi-stage freezing crystallization salt separation method further includes the following steps:

[0030] The nanofiltration concentrated water obtained by nanofiltration separation and concentration treatment in the final-stage salt separation treatment is subjected to evaporation crystallization, and the mother liquor is also obtained in the solid-liquid separation step; and the mother liquor is returned to the evaporation crystallization step of the nanofiltration concentrated water.

[0031] In some embodiments, the number of stages of the salt separation treatment of the concentrated brine is 2;

[0032] When the crystallization rate of the freeze crystallization treatment in the first-stage salt separation treatment is 50% - 85%, the mother liquor obtained by the solid-liquid separation treatment in the first-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0033] When the crystallization rate of the freeze crystallization treatment in the first - second-stage salt separation treatment is 85% - 96%, the mother liquor obtained by the solid-liquid separation treatment in the second-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step.

[0034] In some embodiments, the number of stages of the salt separation treatment of the concentrated brine is 3;

[0035] When the crystallization rate of the freeze crystallization treatment in the first-stage salt separation treatment is 40% - 75%, the mother liquor obtained by the solid-liquid separation treatment in the first-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0036] When the crystallization rate of the freeze crystallization treatment in the first - second-stage salt separation treatment is 75% - 86%, the mother liquor obtained by the solid-liquid separation treatment in the second-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0037] When the crystallization rate of the freeze crystallization treatment in the first - third-stage salt separation treatment is 86% - 96%, the mother liquor obtained by the solid-liquid separation treatment in the third-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step.

[0038] In some embodiments, the number of stages of the salt separation treatment of the concentrated brine is 4;

[0039] When the crystallization rate of the freeze crystallization treatment in the first-stage salt separation treatment is 35% - 60%, the mother liquor obtained by the solid-liquid separation treatment in the first-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0040] When the crystallization rate of the freeze crystallization treatment in the first - second-stage salt separation treatment is 60% - 75%, the mother liquor obtained by the solid-liquid separation treatment in the second-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0041] When the crystallization rate of the freeze crystallization treatment in the first - third-stage salt separation treatment is 75% - 85%, the mother liquor obtained by the solid-liquid separation treatment in the third-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0042] When the crystallization rate of the freeze crystallization treatment in the 1st to 4th stage salt separation treatment is 85% to 96%, the mother liquor obtained from the solid-liquid separation treatment in the 4th stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step.

[0043] In some embodiments, the number of stages of the salt separation treatment of the concentrated brine is 5;

[0044] When the crystallization rate of the freeze crystallization treatment in the 1st stage salt separation treatment is 30% to 50%, the mother liquor obtained from the solid-liquid separation treatment in the 1st stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0045] When the crystallization rate of the freeze crystallization treatment in the 1st to 2nd stage salt separation treatment is 50% to 65%, the mother liquor obtained from the solid-liquid separation treatment in the 2nd stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0046] When the crystallization rate of the freeze crystallization treatment in the 1st to 3rd stage salt separation treatment is 65% to 75%, the mother liquor obtained from the solid-liquid separation treatment in the 3rd stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0047] When the crystallization rate of the freeze crystallization treatment in the 1st to 4th stage salt separation treatment is 75% to 85%, the mother liquor obtained from the solid-liquid separation treatment in the 4th stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step;

[0048] When the crystallization rate of the freeze crystallization treatment in the 1st to 5th stage salt separation treatment is 85% to 96%, the mother liquor obtained from the solid-liquid separation treatment in the 5th stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. Description of the Drawings

[0049] Figure 1 It is a schematic structural diagram of a multi-stage freeze crystallization salt separation system of an embodiment. Detailed Embodiments

[0050] For the convenience of understanding the present invention, the present invention will be described more comprehensively below with reference to the relevant drawings. Preferred embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0051] It should be noted that when an element is referred to as being "fixed to" another element, it can be directly on the other element or there may also be an intermediate element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be an intermediate element at the same time.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this invention belongs. The terms used in the description of the invention herein are for the purpose of describing particular embodiments only and are not intended to limit the invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0053] Traditional salt separation by crystallization produces salt in one go as a whole, with poor purity and uneven particle size of the produced crystalline salt. Moreover, all the cryogenic crystallization mother liquor generated is recycled back into the system and circulated within the system. The organic matter content in the mother liquor exceeds the standard, and long-term operation will result in low purity of the precipitated crystalline salt and a large amount of impurity salts, reducing the economic value of the crystalline salt.

[0054] Based on this, an embodiment of the present invention provides a multi-stage cryogenic crystallization salt separation system. Correspondingly, an embodiment also provides a multi-stage cryogenic crystallization salt separation method, and this multi-stage cryogenic crystallization salt separation method can be carried out using the above multi-stage cryogenic crystallization salt separation system. The multi-stage cryogenic crystallization salt separation method will be introduced in more detail below in combination with the multi-stage cryogenic crystallization salt separation system.

[0055] Please refer to Figure 1 , an embodiment of the present invention provides a multi-stage cryogenic crystallization salt separation system 10, which can be used for multi-stage salt separation treatment of the brine to be treated. The above multi-stage cryogenic crystallization salt separation system 10 includes a concentration device 100 and a multi-stage salt separation device 200. The multi-stage salt separation device 200 includes at least two stages of salt separation devices 200.

[0056] Each stage of the salt separation device 200 (not all shown in the figure) includes a cryogenic crystallization unit 210, a solid-liquid separation unit 220, and a nanofiltration separation and concentration unit 230 connected in sequence. Within each stage of the salt separation device 200, the solid-liquid separation unit 220 is connected to the cryogenic crystallization unit 210 for solid-liquid separation of the mixture after cryogenic crystallization, and the liquid inlet of the nanofiltration separation and concentration unit 230 is connected to the liquid outlet of the solid-liquid separation unit 220.

[0057] The freezing crystallization unit 210 in the first-stage salt separation device 200 is communicated with the liquid outlet of the concentration device 100. In this way, after the brine is first concentrated by the concentration device 100, it enters the freezing crystallization unit 210 of the salt separation system 200 for freezing crystallization treatment, and then enters the solid-liquid separation unit 220 to perform solid-liquid separation on the frozen crystal mixture to obtain hydrated crystal salt and mother liquor; a part of the mother liquor enters the inlet of the nanofiltration separation and concentration unit 230 from the liquid outlet of the solid-liquid separation unit 220 for nanofiltration separation and concentration treatment. The liquid outlet of the solid-liquid separation unit 220 in the first-stage salt separation device 200 is also communicated with the liquid inlet of the concentration device 100, and another part of the mother liquor returns from the liquid outlet of the solid-liquid separation unit 220 to the concentration device 100 through the liquid inlet of the concentration device 100 for re-concentration treatment. In this way, the amount of mother liquor entering the nanofiltration separation and concentration unit 230 is reduced, and at the same time, the yield of the hydrated crystal salt obtained by solid-liquid separation is increased without affecting its purity.

[0058] Among two adjacent stages of salt separation devices 200, the freezing crystallization unit 210 in the lower-stage salt separation device 200 is communicated with the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit 230 in the upper-stage salt separation device 200. In this way, the nanofiltration concentrated water generated by the nanofiltration separation and concentration unit 230 in the upper-stage salt separation device 200 enters the lower-stage salt separation device 200 for further treatment. Each stage of salt separation device 200 respectively obtains hydrated crystal salt through the solid-liquid separation unit 220. Further, the hydrated crystal salt can be melted and crystallized to remove crystal water to obtain anhydrous crystal salt. At the same time, each stage of salt separation device 200 respectively obtains nanofiltration produced water through the nanofiltration separation and concentration unit 230, and the nanofiltration produced water can be further evaporated and crystallized to obtain monovalent salt.

[0059] Among two adjacent stages of salt separation devices 200, the liquid outlet of the solid-liquid separation unit 220 in the lower-stage salt separation device 200 is also communicated with the liquid inlet of the nanofiltration separation and concentration unit 230 in the upper-stage salt separation device 200. In this way, the mother liquor generated by the solid-liquid separation unit 220 in the lower-stage salt separation device 200 is returned to the nanofiltration separation and concentration unit 230 in the upper-stage salt separation device 200 for further separation and concentration, and the mother liquor is recycled. The monovalent salt is separated by the nanofiltration separation and concentration unit 230 as much as possible, and the hydrated crystal salt is further separated by freezing crystallization and solid-liquid separation. Therefore, after being treated by each stage of salt separation device 200, the yield of subsequent miscellaneous salts is reduced.

[0060] The above-mentioned multi-stage freezing crystallization salt separation system, in which the multi-stage salt separation device 200 is a multi-stage freezing separation crystallization device, and different hydrated crystallization salts with different application values can be obtained through fractional freezing crystallization. Compared with the traditional process of circulating all the mother liquor in the freezing crystallization system, the above-mentioned multi-stage freezing crystallization salt separation system further processes the mother liquor after solid-liquid separation, further separates the residual divalent and higher-valent ions in the mother liquor, and performs freezing crystallization to precipitate crystallization salts, thereby improving the purity and yield of the crystallization salts. In this way, it avoids the problem that the purity of the crystallization salts produced during long-term operation is low and has no application value due to the continuous accumulation of impurities such as inorganic salts and organic matters in the mother liquor of the traditional process; at the same time, the mother liquor is separated by nanofiltration in stages and discharged from the multi-stage salt separation device 200 in the form of nanofiltration-produced water, and high-purity monovalent salts can be further prepared. At the same time, the volume of the final miscellaneous salts is greatly reduced, avoiding the problem of high miscellaneous salt treatment costs. The above-mentioned multi-stage freezing crystallization salt separation system improves the purity and yield of the crystallization salts, greatly reduces the volume of the final miscellaneous salts, improves the resource utilization rate, does not need to adopt the method of adding medicine for crystallization, and reduces the operation cost.

[0061] In some embodiments, the concentration device 100 can be any one of an MVR evaporation device, a multi-effect evaporation device, a high-pressure reverse osmosis device, and an electrodialysis device, or a combination of multiple devices connected in series or in parallel to deeply concentrate the salt water to be treated.

[0062] In some embodiments, the freezing crystallization unit 210 can be any one of a freezing crystallizer, a freezing heat exchanger, a refrigeration unit, and a heat exchanger, or a combination of multiple devices connected in series or in parallel. Further, the freezing crystallizer can be selected from an OSLO crystallizer, a DTB crystallizer, and an FC crystallizer.

[0063] In some embodiments, the solid-liquid separation unit 220 includes a hydrocyclone, a thickener, a centrifuge, or a dryer connected in sequence. Specifically, the mother liquor outlet of the freezing crystallization unit 210 is connected to the inlet of the hydrocyclone, the outlet of the hydrocyclone is connected to the inlet of the thickener, the outlet of the thickener is connected to the inlet of the centrifuge, and the outlet of the centrifuge is connected to the inlet of the dryer.

[0064] Further, the centrifuge can be a pusher centrifuge, a scraper centrifuge, or a horizontal spiral centrifuge.

[0065] In some embodiments, the nanofiltration separation and concentration unit 230 can be any one of a spiral-wound nanofiltration and a high-pressure flat-sheet nanofiltration membrane, or a combination of multiple devices connected in series or in parallel. Among them, the operating pressure of the high-pressure flat-sheet nanofiltration membrane is 40 bar to 160 bar.

[0066] In some of these embodiments, the multi-stage freezing crystallization salt separation system 10 further includes a monovalent salt evaporation crystallization device 300, and the monovalent salt evaporation crystallization device 300 is communicated with the nanofiltration product water outlets of the nanofiltration separation and concentration units 230 of each salt separation device 200 at all levels.

[0067] Furthermore, the monovalent salt evaporation crystallization device 300 can be any one of a falling film evaporator, a forced circulation evaporator, and an MVR evaporator.

[0068] In some of these embodiments, the multi-stage freezing crystallization salt separation system 10 further includes a miscellaneous salt evaporation crystallization device 400 and a miscellaneous salt solid-liquid separation device 500. The miscellaneous salt evaporation crystallization device 400 is communicated with the nanofiltration concentrate water outlet of the nanofiltration separation and concentration unit 230 in the last-stage salt separation device 200, and the miscellaneous salt solid-liquid separation device 500 is communicated with the miscellaneous salt evaporation crystallization device 400 for solid-liquid separation of the evaporated and crystallized miscellaneous salt.

[0069] Furthermore, the miscellaneous salt evaporation crystallization device 400 includes a crystallizer, a heat exchanger, and a circulation pump. The heat exchanger is used for heat exchange of the crystallizer, and the circulation pump is used for circulating the heat exchange medium in the heat exchanger.

[0070] In some of these embodiments, the multi-stage freezing crystallization salt separation system 10 further includes a miscellaneous salt mother liquor tank 600. The inlet of the miscellaneous salt mother liquor tank 600 is communicated with the outlet of the miscellaneous salt solid-liquid separation device 500, and the outlet of the miscellaneous salt mother liquor tank 600 is communicated with the inlet of the miscellaneous salt evaporation crystallization device 400.

[0071] Furthermore, the miscellaneous salt solid-liquid separation device 500 can be any one of a hydrocyclone, a freezing centrifuge, a thickener, and a dryer, or a combination of multiple ones in series or parallel.

[0072] In some of these embodiments, each salt separation device 200 further includes a salt separation mother liquor tank 240. Inside each salt separation device 200 at all levels, the inlet of the nanofiltration separation and concentration unit 230 is communicated with the outlet of the solid-liquid separation unit 220 through the salt separation mother liquor tank 240. In other words, the inlet of the salt separation mother liquor tank 240 is communicated with the outlet of the solid-liquid separation unit 220, and the outlet of the salt separation mother liquor tank 240 is communicated with the inlet of the nanofiltration separation and concentration unit 230.

[0073] Further, the liquid outlet of the mother liquor tank 240 in the next-stage salt separation device 200 is also connected to the liquid inlet of the mother liquor tank 240 in the previous-stage salt separation device 200, so that the liquid outlet of the solid-liquid separation unit 220 in the next-stage salt separation device 200 is connected to the liquid inlet of the nanofiltration separation and concentration unit 230 in the previous-stage salt separation device 200, so as to return the mother liquor generated by the solid-liquid separation unit 220 in the next-stage salt separation device 200 to the mother liquor tank 240 of the previous stage first, and then return it to the nanofiltration separation and concentration unit 230 in the previous-stage salt separation device 200 through the mother liquor tank 240 for further separation and concentration.

[0074] In some embodiments, each stage of the salt separation device 200 further includes a melt crystallization unit 250. In each stage of the salt separation device 200, the melt crystallization unit 250 is used to perform melt crystallization on the hydrated crystalline salt separated by the solid-liquid separation unit 220. Specifically, by further controlling the concentration of the feed and selecting different freeze crystallization equipment in the freeze crystallization unit 210, crystalline salts with different purities and different particle sizes can be obtained. The purity of the crystalline salts gradually decreases from top to bottom, and crystalline salts with different purities have different application values.

[0075] Further, the melt crystallization unit 250 can be any one of a hot melt crystallizer, an evaporation crystallizer, and a hot melt heat exchanger, or a combination of multiple units connected in series or parallel.

[0076] It can be understood that in the specific example as Figure 1 shown, in the direction of material flow, the multi-stage freeze crystallization and salt separation system 10 includes N stages of salt separation devices, where N can be any integer greater than or equal to 2. In this specific example, N is an integer greater than 3, such as N = 4, 5, 6, etc.

[0077] Specifically, the first-stage salt separation device 200 includes a freeze crystallization unit 1, a solid-liquid separation unit 1, a mother liquor tank 1, a nanofiltration separation and concentration unit 1, and a melt crystallization unit 1. The freeze crystallization unit 1 is connected to the liquid outlet of the concentration device. The solid-liquid separation unit 1 is connected to the freeze crystallization unit 1. The liquid inlet of the mother liquor tank 1 is connected to the liquid outlet of the solid-liquid separation unit 1 of the solid-liquid separation unit 1, and the liquid outlet of the mother liquor tank 1 is respectively connected to the liquid inlet of the nanofiltration separation and concentration unit 1 and the liquid inlet of the concentration device. The melt crystallization unit 1 is used to perform melt crystallization on the hydrated crystalline salt separated by the solid-liquid separation unit 1.

[0078] The second-stage salt separation system 200 includes a freezing crystallization unit 2, a solid-liquid separation unit 2, a nanofiltration separation and concentration unit 2, and a melting crystallization unit 2. The freezing crystallization unit 2 is communicated with the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit 1. The solid-liquid separation unit 2 is communicated with the freezing crystallization unit 2. The liquid inlet of the salt separation mother liquor tank 2 is communicated with the liquid outlet of the solid-liquid separation unit 2 of the solid-liquid separation unit 2. The liquid outlet of the salt separation mother liquor tank 2 is respectively communicated with the liquid inlet of the nanofiltration separation and concentration unit 2 and the liquid inlet of the salt separation mother liquor tank 1. The melting crystallization unit 2 is used for melting and crystallizing the hydrated crystal salt separated by the solid-liquid separation unit 2.

[0079] The third-stage salt separation system 200 includes a freezing crystallization unit 3, a solid-liquid separation unit 3, a nanofiltration separation and concentration unit 3, and a melting crystallization unit 3. The freezing crystallization unit 3 is communicated with the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit 2. The solid-liquid separation unit 3 is communicated with the freezing crystallization unit 3. The liquid inlet of the salt separation mother liquor tank 3 is communicated with the liquid outlet of the solid-liquid separation unit 3 of the solid-liquid separation unit 3. The liquid outlet of the salt separation mother liquor tank 3 is respectively communicated with the liquid inlet of the nanofiltration separation and concentration unit 3 and the liquid inlet of the salt separation mother liquor tank 2. The melting crystallization unit 3 is used for melting and crystallizing the hydrated crystal salt separated by the solid-liquid separation unit 3.

[0080] By analogy, the Nth-stage salt separation system includes a freezing crystallization unit N, a solid-liquid separation unit N, a nanofiltration separation and concentration unit N, and a melting crystallization unit N. The freezing crystallization unit N is communicated with the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit N-1. The solid-liquid separation unit N is communicated with the freezing crystallization unit N. The liquid inlet of the salt separation mother liquor tank N is communicated with the liquid outlet of the solid-liquid separation unit N of the solid-liquid separation unit N. The liquid outlet of the salt separation mother liquor tank N is respectively communicated with the liquid inlet of the nanofiltration separation and concentration unit N and the liquid inlet of the salt separation mother liquor tank N-1. The melting crystallization unit N is used for melting and crystallizing the hydrated crystal salt separated by the solid-liquid separation unit N.

[0081] In this specific example, there is one monovalent salt evaporation crystallization device 300 for evaporating and crystallizing the nanofiltration produced water of the nanofiltration separation and concentration units 1 to N. It can be understood that in other embodiments, there may be multiple monovalent salt evaporation crystallization devices 300, for example, evaporating and crystallizing the nanofiltration produced water corresponding to each nanofiltration separation and concentration unit 230 one by one.

[0082] A multi-stage freezing crystallization salt separation method according to an embodiment of the present invention can be carried out by using any of the above multi-stage freezing crystallization salt separation systems, and includes the following steps S10 to S20:

[0083] Step S10: Concentrate the brine to be treated to obtain concentrated brine.

[0084] Step S20: Perform multi-stage salt separation treatment on the concentrated brine in sequence.

[0085] Among them, the salt separation treatments at all levels in step S20 include steps S21 to S23 carried out in sequence:

[0086] Step S21, freeze crystallization treatment;

[0087] Step S22, subject the mixture after freeze crystallization treatment to solid-liquid separation treatment to obtain hydrated crystal salts and mother liquor; and

[0088] Step S23, in the first-stage salt separation treatment, the mother liquor obtained from the solid-liquid separation treatment is shunted, and nanofiltration separation and concentration treatments are respectively carried out to obtain nanofiltration produced water and nanofiltration concentrated water and return them to the concentration treatment; in the salt separation treatments at other levels except the first-stage salt separation treatment, the mother liquor obtained from the solid-liquid separation treatment is shunted, and nanofiltration separation and concentration treatments and return to the nanofiltration separation and concentration treatment in the previous-stage salt separation treatment are respectively carried out to obtain nanofiltration produced water and nanofiltration concentrated water;

[0089] In adjacent two-stage salt separation treatments, the nanofiltration concentrated water after the nanofiltration separation and concentration treatment in the previous stage is transferred to the freeze crystallization treatment in the next stage, and thus steps S21 to S23 are repeated.

[0090] It can be understood that the nanofiltration concentrated water obtained in step S23 is water containing divalent and higher-valent ions, such as containing sodium sulfate and / or sodium carbonate. The nanofiltration concentrated water is transferred to the freeze crystallization treatment in the next stage, and the hydrated crystal salts obtained from the freeze crystallization treatment can be further subjected to subsequent melting crystallization treatment to remove crystal water to obtain anhydrous crystal salts, such as sodium sulfate and / or sodium carbonate.

[0091] The above-mentioned hierarchical salt separation method improves the purity and yield of crystal salts, greatly reduces the volume of final miscellaneous salts, improves the resource utilization rate, does not need to adopt the method of adding medicine for crystallization, and reduces the operation cost.

[0092] In some of the embodiments, the multi-stage freeze crystallization salt separation method further includes the following step S30: subject the nanofiltration produced water obtained from the nanofiltration separation and concentration treatment in each stage of salt separation treatment to evaporation crystallization to obtain monovalent salts. Among them, the nanofiltration produced water in step S30 is water containing monovalent ions, enters evaporation crystallization, and obtains high-purity monovalent salts. Generally, the monovalent salts are sodium chloride.

[0093] In some of the embodiments, the multi-stage freeze crystallization salt separation method further includes the following step S40: subject the hydrated crystal salts obtained from the solid-liquid separation treatment in each stage of salt separation treatment to melting crystallization to remove crystal water to obtain anhydrous crystal salts.

[0094] In some of the embodiments, the multi-stage freeze crystallization salt separation method further includes the following step S52: the multi-stage freeze crystallization salt separation method further includes the following step: subject the nanofiltration concentrated water obtained from the nanofiltration separation and concentration treatment in the last-stage salt separation treatment to evaporation crystallization and solid-liquid separation to obtain miscellaneous salts.

[0095] Further, the multi-stage freezing crystallization salt separation method further includes the following step S54: subjecting the nanofiltration concentrated water obtained by nanofiltration separation and concentration treatment in the last-stage salt separation treatment to evaporation crystallization, and the solid-liquid separation step further obtains mother liquor; and returning the mother liquor to the evaporation crystallization step of the nanofiltration concentrated water.

[0096] In a specific example, step S20 performs two-stage salt separation treatment on the concentrated brine in sequence, and specifically, the multi-stage freezing crystallization salt separation system 10 with a two-stage salt separation device described above can be used. Further, when the crystallization rate of the freezing crystallization treatment in the first-stage salt separation treatment is 50% to 85%, the mother liquor obtained by the solid-liquid separation treatment in the first-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to second-stage salt separation treatments is 85% to 96%, the mother liquor obtained by the solid-liquid separation treatment in the second-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. Herein, the crystallization rate refers to the percentage of the mass of the anhydrous crystalline salt in the total salt mass of the treated brine, and the same applies hereinafter.

[0097] In another specific example, step S20 performs three-stage salt separation treatment on the concentrated brine in sequence, and specifically, the multi-stage freezing crystallization salt separation system 10 with a three-stage salt separation device described above can be used. Further, when the mass of the hydrated crystalline salt in the freezing crystallization treatment in the first-stage salt separation treatment is 40% to 75% of the total salt mass of the treated brine, the mother liquor obtained by the solid-liquid separation treatment in the first-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to second-stage salt separation treatments is 75% to 86%, the mother liquor obtained by the solid-liquid separation treatment in the second-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to third-stage salt separation treatments is 86% to 96%, the mother liquor obtained by the solid-liquid separation treatment in the third-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step.

[0098] In another specific example, in step S20, the concentrated brine is subjected to four-stage salt separation treatment in sequence, and specifically, the multi-stage freezing crystallization salt separation system 10 with a four-stage salt separation device described above can be used. Further, when the mass of the hydrated crystalline salt obtained by the freezing crystallization treatment in the first-stage salt separation treatment is 35% - 60% of the total salt mass of the treated brine, the mother liquor obtained by the solid-liquid separation treatment in the first-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to second-stage salt separation treatments is 60% - 75%, the mother liquor obtained by the solid-liquid separation treatment in the second-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to third-stage salt separation treatments is 75% - 85%, the mother liquor obtained by the solid-liquid separation treatment in the third-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to fourth-stage salt separation treatments is 85% - 96%, the mother liquor obtained by the solid-liquid separation treatment in the fourth-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step.

[0099] In another specific example, in step S20, the concentrated brine is subjected to five-stage salt separation treatment in sequence, and specifically, the multi-stage freezing crystallization salt separation system 10 with a five-stage salt separation device described above can be used. Further, when the mass of the hydrated crystalline salt obtained by the freezing crystallization treatment in the first-stage salt separation treatment is 30% - 50% of the total salt mass of the treated brine, the mother liquor obtained by the solid-liquid separation treatment in the first-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to second-stage salt separation treatments is 50% - 65%, the mother liquor obtained by the solid-liquid separation treatment in the second-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to third-stage salt separation treatments is 65% - 75%, the mother liquor obtained by the solid-liquid separation treatment in the third-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to fourth-stage salt separation treatments is 75% - 85%, the mother liquor obtained by the solid-liquid separation treatment in the fourth-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step. When the crystallization rate of the freezing crystallization treatment in the first to fifth-stage salt separation treatments is 85% - 96%, the mother liquor obtained by the solid-liquid separation treatment in the fifth-stage salt separation treatment is transferred to the nanofiltration separation and concentration treatment step.

[0100] Further, in this specific example of the five-stage salt separation treatment, the purity of the crystalline salt obtained by the freezing crystallization treatment in the first-stage salt separation treatment is 99.5% - 99.8%, the purity of the crystalline salt obtained by the freezing crystallization treatment in the second-stage salt separation treatment is 99.2% - 99.7%, the purity of the crystalline salt obtained by the freezing crystallization treatment in the third-stage salt separation treatment is 99% - 99.5%, the purity of the crystalline salt obtained by the freezing crystallization treatment in the fourth-stage salt separation treatment is 98.6 - 99.2%, and the purity of the crystalline salt obtained by the freezing crystallization treatment in the fifth-stage salt separation treatment is 98.5 - 99%.

[0101] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0102] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A multi-stage freezing crystallization salt separation system, characterized in that, it includes: a concentration device; and a multi-stage salt separation device, each stage of the salt separation device includes a freezing crystallization unit, a solid-liquid separation unit and a nanofiltration separation and concentration unit. In each stage of the salt separation device, the solid-liquid separation unit is connected to the freezing crystallization unit to separate the solid and liquid of the mixture after freezing crystallization, and the inlet of the nanofiltration separation and concentration unit is connected to the outlet of the solid-liquid separation unit; the freezing crystallization unit in the first-stage salt separation device is connected to the outlet of the concentration device, and the outlet of the solid-liquid separation unit in the first-stage salt separation device is also connected to the inlet of the concentration device; in two adjacent stages of salt separation devices, the freezing crystallization unit in the lower-stage salt separation device is connected to the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit in the upper-stage salt separation device, and the outlet of the solid-liquid separation unit in the lower-stage salt separation device is also connected to the inlet of the nanofiltration separation and concentration unit in the upper-stage salt separation device; the multi-stage freezing crystallization salt separation system further includes a monovalent salt evaporation crystallization device, and the monovalent salt evaporation crystallization device is connected to the nanofiltration water production outlets of the nanofiltration separation and concentration units of each stage of the salt separation device; the multi-stage freezing crystallization salt separation system further includes a miscellaneous salt evaporation crystallization device and a miscellaneous salt solid-liquid separation device, the miscellaneous salt evaporation crystallization device is connected to the nanofiltration concentrated water outlet of the nanofiltration separation and concentration unit in the last-stage salt separation device, and the miscellaneous salt solid-liquid separation device is connected to the miscellaneous salt evaporation crystallization device to separate the solid and liquid of the miscellaneous salt after evaporation crystallization.

2. The multi-stage freezing crystallization salt separation system according to claim 1, characterized in that, the multi-stage freezing crystallization salt separation system further includes a miscellaneous salt mother liquor tank, the inlet of the miscellaneous salt mother liquor tank is connected to the outlet of the miscellaneous salt solid-liquid separation device, and the outlet of the miscellaneous salt mother liquor tank is connected to the inlet of the miscellaneous salt evaporation crystallization device.

3. The multi-stage freezing crystallization salt separation system according to claim 1, characterized in that, each stage of the salt separation device further includes a salt separation mother liquor tank. In each stage of the salt separation device, the inlet of the nanofiltration separation and concentration unit is connected to the outlet of the solid-liquid separation unit through the salt separation mother liquor tank.

4. The multi-stage freezing crystallization salt separation system according to claim 3, characterized in that, the outlet of the salt separation mother liquor tank in the lower-stage salt separation device is also connected to the inlet of the salt separation mother liquor tank in the upper-stage salt separation device.

5. The multi-stage freezing crystallization salt separation system according to any one of claims 1 to 4, characterized in that, each stage of the salt separation device further includes a melting crystallization unit. In each stage of the salt separation device, the melting crystallization unit is used to melt and crystallize the hydrated crystal salt separated by the solid-liquid separation unit.

6. A multi-stage freezing crystallization salt separation method, characterized in that, it is carried out by using the multi-stage freezing crystallization salt separation system according to any one of claims 1 to 5, and the method includes the following steps: Concentrating the saline water to be treated to obtain concentrated saline water; Carrying out multi-stage salt separation treatment on the concentrated saline water in sequence; wherein, each stage of salt separation treatment includes: Freezing crystallization treatment; Perform solid-liquid separation on the mixture after the freeze crystallization treatment to obtain hydrated crystalline salt and mother liquor; and In the first-stage salt separation treatment, the mother liquor obtained from the solid-liquid separation treatment is split, and nanofiltration separation and concentration treatments are respectively performed to obtain nanofiltration product water and nanofiltration concentrate and return to the concentration treatment; in other stages of salt separation treatment except the first-stage salt separation treatment, the mother liquor obtained from the solid-liquid separation treatment is split, and nanofiltration separation and concentration treatments are respectively performed and returned to the previous stage of salt separation treatment for the nanofiltration separation and concentration treatment to obtain nanofiltration product water and nanofiltration concentrate; In adjacent two-stage salt separation treatments, the nanofiltration concentrate after the nanofiltration separation and concentration treatment in the previous stage is transferred to the freeze crystallization treatment in the next stage; The multi-stage freeze crystallization and salt separation method further includes the following steps: Perform evaporation crystallization on the nanofiltration product water obtained from the nanofiltration separation and concentration treatment in each stage of salt separation treatment to obtain monovalent salt; The multi-stage freeze crystallization and salt separation method further includes the following steps: Perform evaporation crystallization on the nanofiltration concentrate obtained from the nanofiltration separation and concentration treatment in the last stage of salt separation treatment, and perform solid-liquid separation to obtain miscellaneous salt.

7. The multi-stage freeze crystallization and salt separation method according to claim 6, wherein, The multi-stage freeze crystallization and salt separation method further includes the following steps: Perform melting crystallization on the hydrated crystalline salt obtained from the solid-liquid separation treatment in each stage of salt separation treatment to remove crystal water to obtain anhydrous crystalline salt.

8. The multi-stage freeze crystallization and salt separation method according to claim 6, wherein, The multi-stage freeze crystallization and salt separation method further includes the following steps: Performing evaporation crystallization on the nanofiltration concentrate obtained from the nanofiltration separation and concentration treatment in the last stage of salt separation treatment, the step of solid-liquid separation also obtains mother liquor; and returning the mother liquor to the evaporation crystallization step of the nanofiltration concentrate.

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