Cascade crystallizer

Abstract

The utility model relates to a cascade crystallizer which comprises a first-stage crystallizer (1) and a second-stage crystallizer (2), and materials discharged from the bottom of the first-stage crystallizer (1) can be fed into the second-stage crystallizer (2). The bottom end of a first guide cylinder (12) arranged in the primary crystallizer (1) is sealed by a circular seal head, a dissolving crystallization space (120) is formed in the first guide cylinder (12), a settling space (110) is formed on the outer side of the first guide cylinder (12), a plurality of holes (18) are formed in the lower part of the first guide cylinder (12), and the dissolving crystallization space (120) is communicated with the settling space (110); a second overflow weir (26) is arranged on the upper side of the secondary crystallizer (2), and overflow liquid of the secondary crystallizer (2) can be guided out and fed into the primary crystallizer (1) through a pipeline. According to the utility model, the problems that the crystallizer for producing potassium chloride is easy to explode and nucleate and partial carnallite is not completely decomposed can be solved, and the purity, the yield and the particle size of a potassium chloride product are improved.

Description

Technical Field

[0001] The utility model relates to potassium chloride production equipment, in particular to a cascade crystallizer for producing potassium chloride using carnallite ore as a raw material. Background Art

[0002] The industrial production of potassium chloride is a process of removing sodium and magnesium from carnallite ore by chemical means. The removal of magnesium is the process of dissolving and recrystallizing carnallite under the action of water. This process is usually carried out at room temperature, which is different from the high-temperature hot melt crystallization commonly used in this field. Therefore, it is called the "cold crystallization" process. The crystallizer traditionally used for removing magnesium from carnallite ore is a single crystallizer, which is mostly a fully mixed crystallizer with good mixing performance.

[0003] Potassium chloride products are preferably high-purity and large-particle products. The technical problems that lead to product quality and crystal particle size in the existing cold crystallization process are mainly reflected in the following two aspects:

[0004] 1. During the cold crystallization process, the rate of dissolution and crystallization is difficult to control, resulting in excessive local supersaturation and easy nucleation, which leads to small particle size of potassium chloride crystals.

[0005] Second, some carnallite ore settles at the outlet of the crystallizer before being decomposed, resulting in a decrease in the purity of the potassium chloride product. The back-mixing theory of chemical engineering found that a considerable portion of the solid particles that settle at the outlet of the crystallizer have a short residence time in the crystallizer. In actual production, the slurry discharged from the crystallizer does contain undecomposed carnallite ore that matches the theoretical amount, which can support the above theory.

[0006] The existing potassium chloride crystallizer technologies for the above technical problems are:

[0007] The utility model patent with the publication number CN215232225U and the name of "a potassium chloride crystallization device" has improved the problem of explosive nucleation in traditional crystallizers. The device consists of a crystallizer and a fine crystal tank. Overflow ports of varying heights are set on the top of the crystallizer, which are combined with a filter screen to classify and overflow impurities such as fine crystals and foam, which can reduce the number of seed crystals in the crystallizer so that the crystals can grow. However, the device cannot fundamentally eliminate the problem of insufficient decomposition of carnallite and its discharge from the crystallizer caused by the back-mixing phenomenon.

[0008] The utility model patent with the publication number CN 218339021U and the name "Crystallizer for Potassium Chloride Production" has made improvements to the problem that the traditional crystallizer cannot completely dissolve the carnallite ore. A decomposition tank is provided upstream of the crystallizer, and the carnallite ore forms potassium chloride crystals and a saturated magnesium chloride solution in the decomposition tank. The carnallite ore is completely decomposed and recrystallized by controlling the angle of the conveying pipeline. After that, the slurry is input into the crystallizer through the conveying pipeline, and the sodium chloride is dissolved by fresh water to obtain potassium chloride crystals with a purity of 95%. However, in this technology, the dissolution of sodium chloride is mainly carried out in the crystallization tank, and the crystallization growth process of carnallite is carried out in the decomposition tank, which still cannot solve the problem of the difficulty in regulating the decomposition and crystallization process of carnallite. Utility Model Content

[0009] The first purpose of the utility model is to solve the problem that the existing crystallizer for producing potassium chloride is prone to explosive nucleation, resulting in a small particle size of potassium chloride crystals; the second purpose is to solve the problem that the residence time of some carnallite ores in the existing crystallizer is short and the decomposition is incomplete, resulting in low purity of potassium chloride products.

[0010] In order to achieve the above-mentioned purpose, the utility model provides a cascade crystallizer, comprising a primary crystallizer 1 and a secondary crystallizer 2.

[0011] The upper portion of the primary crystallizer 1 is provided with a feed port 19A for feeding carnallite, and is also provided with a first feed pipe 19B for feeding the first mother liquor. The lower portion of the primary crystallizer 1 is narrowed to form a first discharge cone 11.

[0012] A second feed pipe 28A is provided on the upper part of the secondary crystallizer 2 for liquid inlet, and the outlet of the first discharge cone 11 is connected to the second feed pipe 28A through the first discharge pipe 150, so that the bottom discharge of the primary crystallizer 1 can be fed into the secondary crystallizer 2.

[0013] A first guide tube 12, a first deflector tube 13 and a first stirring device 14 are arranged inside the primary crystallizer 1. The first guide tube 12 is a hollow straight cylindrical shape, the upper end of which is connected to the upper inner wall of the primary crystallizer 1 and the bottom end is sealed by a circular head.

[0014] A dissolution and crystallization space 120 is formed inside the first guide tube 12 , and a sedimentation space 110 is formed outside the first guide tube 12 . A plurality of holes 18 are provided at the lower portion of the first guide tube 12 to connect the dissolution and crystallization space 120 with the sedimentation space 110 .

[0015] The first baffle cylinder 13 is located in the dissolution and crystallization space 120 , and divides the dissolution and crystallization space 120 into baffle channels. Under the stirring action of the first stirring device 14 , the slurry can baffle back and forth along the baffle channels.

[0016] A second overflow weir 26 is arranged inside the secondary crystallizer 2 . The second overflow weir 26 is arranged around the upper side of the interior of the secondary crystallizer 2 . A second overflow discharge port 27 facing the second overflow weir 26 is arranged on the side wall of the secondary crystallizer 2 .

[0017] The second overflow hole 27 is connected to the first feed pipe 19B through the second overflow pipe 270, so that the overflow liquid of the secondary crystallizer 2 can be fed into the primary crystallizer 1.

[0018] Therefore, by using a structure in which two crystallizers are connected in series, that is, using the secondary crystallizer 2 to continue to dissolve and crystallize the undissolved carnallite in the primary crystallizer 1, the residence time of the carnallite in the crystallizer can be significantly increased, and the incomplete decomposition of part of the carnallite ore due to backmixing can be avoided. Large particles of carnallite can be effectively prevented from directly penetrating the crystallizer and being discharged, which can significantly improve the yield and purity of potassium chloride.

[0019] The first guide tube 12 with holes 18 disposed in the primary crystallizer 1 is used to screen the particle size of small particles in the slurry, thereby ensuring that the low-sodium carnallite has been decomposed and preventing large particles of low-sodium carnallite from leaving the primary crystallizer 1 when the decomposition is incomplete.

[0020] By using the second overflow pipe 270 to return the overflow liquid containing potassium chloride crystals produced in the secondary crystallizer to the primary crystallizer 1 as a seed slurry, the explosive nucleation phenomenon of potassium chloride in the primary crystallizer 1 can be effectively avoided or reduced, which is beneficial to the growth of potassium chloride crystals. At the same time, it avoids the aggregation of small potassium chloride crystals to wrap impurities, and the potassium chloride crystals grow more completely, which can significantly increase the particle size of potassium chloride crystals.

[0021] The first overflow liquid is processed into unsaturated brine without small potassium chloride crystal seeds by using the regulating device N, which is beneficial to reducing the potassium chloride content in the mother liquor, thereby reducing the potassium chloride loss in the discharged brine and improving the recovery rate of potassium chloride.

[0022] Preferably, the cascade crystallizer also includes an adjusting device N, a first overflow weir 16 is arranged inside the primary crystallizer 1, the first overflow weir 16 is arranged around the upper side of the interior of the primary crystallizer 1, and a first overflow liquid discharge port 17 is arranged at the lowest point of the first overflow weir 16 on the side wall of the primary crystallizer 1.

[0023] The first recovery pipe 19C and the second recovery pipe 28B are respectively provided at the upper part connecting the primary crystallizer 1 and the secondary crystallizer 2. The first overflow liquid discharge port 17 is connected to the regulating device N through the first overflow pipe 170. The regulating device N includes a thickener and a fresh water mixer to adjust the overflow liquid of the primary crystallizer 1 to unsaturated brine.

[0024] The outlet end of the regulating device N is connected to the first recovery pipe 19C and the second recovery pipe 28B through recovery pipes 010 and 020, respectively, so that the unsaturated brine is respectively sent to the primary crystallizer 1 and the secondary crystallizer 2.

[0025] The regulating device N can be used to eliminate fine crystals in the overflow liquid regulation of the primary crystallizer 1 and convert it into unsaturated brine that can be used, thereby increasing the yield of potassium chloride.

[0026] Preferably, a second guide tube 22 , a second deflector tube 23 , and a second stirring device 24 are provided inside the secondary crystallizer 2 .

[0027] The second guide tube 22 is a hollow straight cylindrical shape, and its upper end is connected to the upper inner wall of the primary crystallizer 1. The second deflector tube 23 is located inside the second guide tube 22. Under the stirring action of the second stirring device 24, the slurry can bend back and forth around the second guide tube 22.

[0028] In the secondary crystallizer 2, the carnallite is completely dissolved and the potassium chloride continues to grow.

[0029] Preferably, the first stirring device 14 is provided with an upper stirring blade 14A and a lower stirring blade 14B, the upper stirring blade 14A is located at the lower inner part of the first baffle cylinder 13, and the lower stirring blade 14B is close to the bottom of the first guide cylinder 12. Therefore, the upper stirring blade 14A can provide the slurry with a power for baffled flow, and the lower stirring blade 14B can lift the slurry at the bottom of the dissolving and crystallizing space 120 to prevent the particles from settling and accumulating.

[0030] Preferably, the feed port 19A, the first feed pipe 19B, and the first recovery pipe 19C are facing the inner side of the first deflector cylinder 13, and the second feed pipe 28A and the second recovery pipe 28B are facing the inner side of the second deflector cylinder 23. This can extend the movement path of these feeds and further reduce the phenomenon of insufficient dissolution.

[0031] Preferably, the upper stirring blade 14A is a spiral stirring paddle, the lower stirring blade 14B is a pitched blade paddle, and the second stirring blade 24A of the second stirring device 24 is an axial flow propulsion spiral stirring paddle.

[0032] Preferably, the ratio of the cross-sectional circular area of ​​the first guide cylinder 12 to the cross-sectional annular area formed between the first guide cylinder 12 and the first deflector cylinder 13 is 1:1-1:1.4.

[0033] Preferably, the ratio of the cross-sectional circular area of ​​the second guide cylinder 22 to the cross-sectional annular area formed between the second guide cylinder 22 and the second deflector cylinder 23 is 1:1-1:1.4.

[0034] Preferably, the plurality of holes 18 are evenly distributed in the annular space at the lower portion of the first guide tube 12 and are farther away from the feed point, which is more conducive to the full dissolution of the carnallite.

[0035] Preferably, the aperture and number of the holes 18 satisfy the average velocity of the slurry fluid flowing through the holes at 0.1-0.2 m / s, which can effectively prevent large particles of carnallite from short-circuiting and being discharged from the crystallizer, thereby increasing the decomposition time of the carnallite and ensuring complete decomposition of the carnallite. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a structural illustration of the cascade crystallizer;

[0037] Figure 2 This is a diagram illustrating the internal structure of the primary crystallizer;

[0038] Figure 3 This is a diagram illustrating the internal structure of the secondary crystallizer.

[0039] Figure numerals: primary crystallizer 1; first discharge cone 11; sedimentation space 110; first guide tube 12; dissolution crystallization space 120; first deflector tube 13; first stirring device 14; upper stirring blade 14A; lower stirring blade 14B; first discharge port 15; first discharge pipe 150; first overflow weir 16; first overflow liquid discharge port 17; hole 18; feed port 19A; first feed pipe 19B, first recovery pipe 19C; secondary crystallizer 2; second discharge cone 21; second guide tube 22; second deflector tube 23; second stirring device 24; second stirring blade 24A; second discharge port 25; second overflow weir 26; second overflow liquid discharge port 27; second overflow pipe 270; second feed pipe 28A, second recovery pipe 28B; regulating device N; thickener 3; static mixer 4; recovery pipes 010, 020. DETAILED DESCRIPTION

[0040] The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. The following description is a concretization of the claims of the present invention. Example

[0041] Figure 1 It is a structural illustration of the cascade crystallizer; Figure 2 This is a diagram illustrating the internal structure of the primary crystallizer; Figure 3 This is a diagram illustrating the internal structure of the secondary crystallizer.

[0042] like Figure 1 As shown, the cascade crystallizer includes a primary crystallizer 1, a secondary crystallizer 2, and a regulating device N.

[0043] The following combination Figure 2 The structure of the primary crystallizer 1 is described as follows:

[0044] like Figure 2 As shown, the lower part of the primary crystallizer 1 is narrowed to form a first discharge cone 11 , and the bottom of the first discharge cone 11 forms a first discharge port 15 , at which a discharge valve is provided.

[0045] The primary crystallizer 1 is a hollow open container, and a feed port 19A for feeding carnallites is provided at the top. During production, a belt conveyor can be used to feed the low-sodium carnallites raw material into the primary crystallizer 1 from the feed port 19A. The primary crystallizer 1 contains a first mother liquor, and the first mother liquor is mixed with the low-sodium carnallites in the primary crystallizer 1 to form a slurry.

[0046] A first feed pipe 19B and a first recovery pipe 19C for feeding liquid are arranged at the top of the primary crystallizer 1.

[0047] A first guide tube 12 , a first deflector tube 13 , and a first stirring device 14 are arranged inside the primary crystallizer 1 .

[0048] The first guide tube 12 is in the shape of a hollow straight cylinder, the upper end of which is connected to the upper inner wall of the primary crystallizer 1 and the bottom end of which is sealed by a circular head.

[0049] A dissolution and crystallization space 120 is formed inside the first guide tube 12 , and a sedimentation space 110 is formed outside the first guide tube 12 . A plurality of holes 18 (600 μm) are provided at the lower portion of the first guide tube 12 to connect the dissolution and crystallization space 120 with the sedimentation space 110 .

[0050] The first baffle cylinder 13 is located in the dissolution and crystallization space 120, and divides the dissolution and crystallization space 120 into baffle channels. Under the stirring action of the first stirring device 14, the slurry can baffle back and forth along the baffle channels. In this process, the low-sodium carnallite in the slurry dissolves and the potassium chloride crystallizes, making the particles in the slurry smaller. Among them, small particles with a particle size smaller than the hole 18 will pass through the hole 18 and come to the sedimentation space 110.

[0051] A first overflow weir 16 is arranged inside the primary crystallizer 1 . The first overflow weir 16 is arranged around the upper side of the primary crystallizer 1 . A first overflow liquid discharge port 17 facing the first overflow weir 16 is arranged on the side wall of the primary crystallizer 1 .

[0052] The following combination Figure 3 The structure of the secondary crystallizer 2 is described as follows:

[0053] like Figure 3 As shown, the primary crystallizer 1 is a hollow container, and a second feed pipe 28A and a second recovery pipe 28B are provided on the upper part for liquid inlet.

[0054] A second guide tube 22 , a second baffle tube 23 , and a second stirring device 24 are arranged inside the secondary crystallizer 2 . A second feed pipe 28A and a second recovery pipe 28B are directly opposite to the inner side of the second baffle tube 23 .

[0055] The second guide tube 22 is a hollow straight cylindrical shape, and its upper end is connected to the upper inner wall of the primary crystallizer 1. The second deflector tube 23 is located inside the second guide tube 22. Under the stirring action of the second stirring device 24, the slurry can bend back and forth around the second guide tube 22.

[0056] The second stirring blade 24A of the second stirring device 24 is an axial flow propulsion type screw stirring blade.

[0057] A second overflow weir 26 is arranged inside the secondary crystallizer 2. The second overflow weir 26 is arranged around the upper side of the interior of the secondary crystallizer 2. A second overflow liquid discharge port 27 facing the second overflow weir 26 is arranged on the side wall of the secondary crystallizer 2.

[0058] The following combination Figure 1 , 2 3. The connection relationship between the primary crystallizer 1, the secondary crystallizer 2, and the regulating device N is described:

[0059] like Figure 1 As shown, the primary crystallizer 1 and the secondary crystallizer 2 are in a series relationship. That is, the outlet of the first discharge cone 11 is connected to the second feed pipe 28A through the first discharge pipe 150, so that the bottom discharge of the primary crystallizer 1 can be fed into the secondary crystallizer 2.

[0060] like Figure 1 As shown, the overflow liquid of the secondary crystallizer 2 is sent back to the primary crystallizer 1. That is, the second overflow liquid outlet 27 is connected to the first feed pipe 19B through the second overflow pipe 270.

[0061] like Figure 1 As shown, the overflow liquid of the primary crystallizer 1 is processed by the regulating device N and then sent to the primary crystallizer 1 and the secondary crystallizer 2 respectively. That is, the first overflow liquid discharge port 17 is connected to the regulating device N through the first overflow pipe 170, and the outlet end of the regulating device N is connected to the first recovery pipe 19C and the second recovery pipe 28B through the recovery pipes 010 and 020 respectively, so that the regulating device N can regulate the overflow liquid of the primary crystallizer 1 into unsaturated brine, and the unsaturated brine is recovered to the primary crystallizer 1 and the secondary crystallizer 2 respectively.

[0062] The regulating device N comprises a thickener 3 and a static mixer 4, which can thicken small particles in the overflow liquid of the primary crystallizer, and the static mixer 3 can mix the thickened bottom flow into fresh water to form unsaturated brine.

[0063] When in use, under normal temperature environment, low sodium carnallite is continuously and stably fed into the primary crystallizer 1 from the feed port 19A by a belt conveyor, mixed with the first mother liquor in the primary crystallizer 1 (the first mother liquor is initially the brine at point E, and after stable operation it is a mixture of unsaturated brine and overflow liquid of the secondary crystallizer 2), and the first stirring device 14 is turned on for stirring.

[0064] In the primary crystallizer 1, low-sodium carnallite gradually dissolves, the particle size becomes smaller, potassium chloride crystals grow in the slurry, and small particles (low-sodium carnallite or potassium chloride) with a particle size less than 600 μm pass through the hole 18 and enter the sedimentation space 110, where a dense classification phenomenon occurs, that is, most of the small particles are located in the first bottom slurry, and a small part of the small particles are located in the overflow liquid of the primary crystallizer 1. Particles larger than 600 μm are mainly undecomposed carnallite, which continue to stay in the dissolution and crystallization zone 120 and continue to dissolve and crystallize.

[0065] The bottom slurry of the primary crystallizer 1 is sent to the secondary crystallizer 2 for further crystallization. In the secondary crystallizer 2, the carnallite is completely dissolved and the potassium chloride crystals continue to grow. The bottom discharge of the secondary crystallizer 2 is the crude potassium slurry, which can be transported to the next process to produce potassium chloride products. The overflow liquid formed at the top includes a small amount of potassium chloride crystals, which are sent to the primary crystallizer 1 as seed slurry for recovery and mixing with the first mother liquor.

[0066] The overflow liquid formed on the upper part of the primary crystallizer 1 is adjusted to unsaturated brine by the regulating device N. The unsaturated brine does not contain impurities, part of which is sent back to the primary crystallizer 1 to mix with the first mother liquor, and part of which is sent to the secondary crystallizer 2 to participate in the dissolution and crystallization.

[0067] Therefore, in the above process, the structure of two crystallizers connected in series is used, that is, the secondary crystallizer 2 is used to continue to dissolve and crystallize the undissolved carnallite in the primary crystallizer 1, which can significantly increase the residence time of the carnallite in the crystallizer, can avoid the incomplete decomposition of part of the carnallite ore leaving the crystallizer quickly due to backmixing, can effectively avoid large particles of carnallite directly penetrating the crystallizer and being discharged, and can significantly improve the yield and purity of potassium chloride.

[0068] The first guide tube 12 with holes 18 disposed in the primary crystallizer 1 is used to screen the particle size of small particles in the slurry, thereby ensuring that the low-sodium carnallite has been decomposed and preventing large particles of low-sodium carnallite from leaving the primary crystallizer 1 when the decomposition is incomplete.

[0069] By using the second overflow pipe 270 to return the overflow liquid containing potassium chloride crystals produced in the secondary crystallizer to the primary crystallizer 1 as a seed slurry, the explosive nucleation phenomenon of potassium chloride in the primary crystallizer 1 can be effectively avoided or reduced, which is beneficial to the growth of potassium chloride crystals. At the same time, it avoids the aggregation of small potassium chloride crystals to wrap impurities, and the potassium chloride crystals grow more completely, which can significantly increase the particle size of potassium chloride crystals.

[0070] The first overflow liquid is processed into unsaturated brine without small potassium chloride crystal seeds by using the regulating device N, which is beneficial to reducing the potassium chloride content in the mother liquor, thereby reducing the potassium chloride loss in the discharged brine and improving the recovery rate of potassium chloride.

[0071] It is worth noting that the ratio of the cross-sectional circular area of ​​the first guide tube 12 to the cross-sectional annular area formed between the first guide tube 12 and the first baffle tube 13, and the ratio of the cross-sectional circular area of ​​the second guide tube 22 to the cross-sectional annular area formed between the second guide tube 22 and the second baffle tube 23 are both between 1:1 and 1:1.4, preferably 1:1.2. The fluid velocity at the annular interface between the crystallizer shell and the guide tube is 0.001-0.004 m / s, preferably 0.0015 m / s.

[0072] The feed port 19A, the first feed pipe 19B, and the first recovery pipe 19C of the primary crystallizer are directly opposite to the inner side of the first baffle cylinder 13, that is, the feed of the primary crystallizer 1 will enter the inner side of the first baffle cylinder 13, which can extend the movement path of these feeds and further reduce the phenomenon of insufficient dissolution. In addition, the holes 18 are evenly distributed in the annular space with a height of 1m at the lower part of the first guide cylinder 12, and are far away from the feed point, which is more conducive to the full dissolution of carnallite.

[0073] The first stirring device 14 is provided with an upper stirring blade 14A (spiral stirring paddle) and a lower stirring blade 14B (inclined blade paddle). The upper stirring blade 14A is located at the lower inner side of the first deflector tube 13, and can provide the slurry with a reciprocating power for the deflection. The lower stirring blade 14B is close to the bottom of the first guide tube 12, and can lift the slurry at the bottom of the dissolution crystallization space 120 to avoid particle sedimentation and accumulation.

[0074] The aperture and number of the holes 18 should be set according to the processing volume. The average velocity of the slurry fluid flowing through the holes is 0.1-0.2 m / s, which can effectively prevent large particles of carnallite from short-circuiting and being discharged from the crystallizer, thereby increasing the decomposition time of the carnallite and ensuring complete decomposition of the carnallite.

Claims

1. A cascade crystallizer, characterized in that: It comprises a primary crystallizer (1) and a secondary crystallizer (2). The upper portion of the primary crystallizer (1) is provided with a feed port (19A) for feeding carnallite, and contains a first mother liquor. The lower portion of the primary crystallizer (1) is narrowed to form a first feed cone (11). A second feed pipe (28A) is provided at the upper portion of the secondary crystallizer (2) for feeding liquid, and the outlet of the first discharge cone (11) is connected to the second feed pipe (28A) via a first discharge pipe (150). The first crystallizer (1) is provided with a first guide tube (12), a first deflector tube (13), and a first stirring device (14) inside. The first guide tube (12) is in the shape of a hollow straight cylinder, the upper end of which is connected to the upper inner wall of the first crystallizer (1), and the bottom end is sealed by a circular head. The first guide tube (12) has a dissolution and crystallization space (120) formed inside and a sedimentation space (110) formed outside. A plurality of holes (18) are provided at the bottom of the first guide tube (12) to connect the dissolution and crystallization space (120) with the sedimentation space (110). The first baffle cylinder (13) is located in the dissolution and crystallization space (120), and divides the dissolution and crystallization space (120) into baffle channels. Under the stirring action of the first stirring device (14), the slurry can baffle back and forth along the baffle channels. A second overflow weir (26) is arranged inside the secondary crystallizer (2). The second overflow weir (26) is arranged around the upper side of the interior of the secondary crystallizer (2). A second overflow liquid discharge port (27) is arranged on the side wall of the secondary crystallizer (2) at the lowest point of the overflow groove of the second overflow weir (26). The second overflow liquid discharge port (27) is connected to the first feed pipe (19B) arranged on the upper side of the primary crystallizer (1) through the second overflow pipe (270), so that the overflow liquid of the secondary crystallizer (2) can be fed into the primary crystallizer (1).

2. The cascade crystallizer according to claim 1, characterized in that: Also includes a regulating device (N), A first overflow weir (16) is arranged inside the primary crystallizer (1), the first overflow weir (16) is arranged around the upper side of the primary crystallizer (1), and a first overflow liquid discharge port (17) is arranged on the side wall of the primary crystallizer (1) at the lowest point of the overflow groove of the first overflow weir (16). A first recovery pipe (19C) and a second recovery pipe (28B) are respectively provided at the upper part connecting the primary crystallizer (1) and the secondary crystallizer (2); the first overflow liquid discharge port (17) is connected to the regulating device (N) via a first overflow pipe (170); the regulating device (N) comprises a thickener and a fresh water mixer, and regulates the overflow liquid of the primary crystallizer (1) into unsaturated brine. The outlet end of the regulating device (N) is connected to the first recovery pipe (19C) and the second recovery pipe (28B) through recovery pipes (010, 020), respectively, so that the unsaturated brine is respectively sent to the primary crystallizer (1) and the secondary crystallizer (2).

3. The cascade crystallizer according to claim 2, characterized in that: The secondary crystallizer (2) is provided with a second guide tube (22), a second baffle tube (23), and a second stirring device (24). The second guide tube (22) is in the shape of a hollow straight cylinder, the upper end of which is connected to the upper inner wall of the primary crystallizer (1). The second deflector tube (23) is located inside the second guide tube (22). Under the stirring action of the second stirring device (24), the slurry can be deflected back and forth around the second guide tube (22).

4. The cascade crystallizer according to claim 3, characterized in that: The first stirring device (14) is provided with an upper stirring blade (14A) and a lower stirring blade (14B), The upper stirring blade (14A) is located at the lower inner part of the first deflector tube (13), and the lower stirring blade (14B) is close to the bottom of the first guide tube (12).

5. The cascade crystallizer according to claim 4, characterized in that: The feed port (19A), the first feed pipe (19B), and the first recovery pipe (19C) are directly opposite to the inner side of the first deflector cylinder (13); the second feed pipe (28A) and the second recovery pipe (28B) are directly opposite to the inner side of the second deflector cylinder (23).

6. The cascade crystallizer according to claim 5, characterized in that: The upper stirring blade (14A) is a spiral stirring blade, the lower stirring blade (14B) is a pitched blade, and the second stirring blade (24A) of the second stirring device (24) is an axial flow propulsion spiral stirring blade.

7. The cascade crystallizer according to any one of claims 1 to 6, characterized in that: The ratio of the cross-sectional circular area of ​​the first flow guide cylinder (12) to the cross-sectional annular area formed between the first flow guide cylinder (12) and the first deflector cylinder (13) is 1:1 to 1:1.

4.

8. The cascade crystallizer according to any one of claims 3 to 6, characterized in that: The ratio of the cross-sectional circular area of ​​the second flow guide cylinder (22) to the cross-sectional annular area formed between the second flow guide cylinder (22) and the second deflector cylinder (23) is 1:1 to 1:1.

4.

9. The cascade crystallizer according to any one of claims 1 to 6, characterized in that: The plurality of holes (18) are evenly distributed in the annular space at the bottom of the first guide tube (12).

10. The cascade crystallizer according to claim 9, characterized in that: The aperture and number of the holes (18) are such that the average velocity of the slurry fluid flowing through the holes is 0.1-0.2 m / s.

Images