Method for improving grade of potassium chloride in process for preparing potassium chloride by using vacuum crystallizer

Abstract

The present application relates to a method for improving the grade of potassium chloride in the process of preparing potassium chloride by using a vacuum crystallizer, comprising a potassium chloride crystallization step (S1), a fine potassium mother liquor is introduced into a DTB vacuum crystallizer for crystallization, part of small particle nuclei enter the circulating channel, are dissolved and then returned to the DTB vacuum crystallizer, and a slurry containing large particle nuclei is discharged to obtain a large particle crystal slurry; a screening step (S2), the large particle crystal slurry is screened by a first screening device to obtain a first screen oversize and a first screen undersize, a second screening device is used to screen the first screen undersize to obtain a second screen oversize and a second screen undersize; a large particle size nucleus treatment step (S3), the first screen oversize is dissolved with fresh water to obtain a targeted mother liquor, and the targeted mother liquor is returned to the circulating channel and then into the DTB vacuum crystallizer for continuous crystallization; and a small particle size nucleus treatment step (S4), the second screen undersize is returned to the inside of the DTB vacuum crystallizer for continuous crystallization. The present application can improve the grade of potassium chloride.

Description

Technical Field

[0001] This invention relates to a method for improving the grade of potassium chloride in a process of preparing potassium chloride using a vacuum crystallizer. Background Technology

[0002] In the process of producing potassium chloride using the hot-melt crystallization method, a DTB vacuum crystallizer is typically used for crystallization. Its advantage lies in its ability to generate large potassium chloride crystal nuclei. High-temperature refined potassium mother liquor (a saturated potassium chloride solution) is passed into the DTB vacuum crystallizer. Under vacuum conditions, adiabatic cooling causes some of the water to evaporate into steam, while simultaneously bringing the refined potassium mother liquor to a supersaturated state, precipitating potassium chloride crystal nuclei. During crystallization, fine crystals are returned to the DTB vacuum crystallizer along with a portion of the slurry through a circulation channel, where they grow into larger crystal particles. These larger crystal particles settle to the bottom under gravity, are then removed, and undergo solid-liquid separation and drying to obtain the final potassium chloride crystal nuclei.

[0003] Because the mother liquor for potassium chloride contains impurities such as sodium chloride, magnesium chloride, and calcium sulfate in addition to potassium chloride, with sodium chloride being the most abundant, it is inevitable that these impurities will be introduced during the potassium chloride crystallization process, leading to a decrease in potassium chloride grade. While increasing the particle size of potassium chloride crystals can reduce impurities and improve the grade, the current process can only achieve a maximum grade of 95%. To obtain higher-grade potassium chloride, an additional impurity removal process is required, increasing the complexity and cost of the process.

[0004] The purpose of this invention is to improve the process of generating potassium chloride crystal nuclei using a DTB vacuum crystallizer, thereby reducing the content of impurities such as sodium chloride and increasing the grade of potassium chloride. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for improving the grade of potassium chloride in a potassium chloride preparation process using a vacuum crystallizer, comprising the following steps: Potassium chloride crystallization step S1: High-temperature refined potassium mother liquor is introduced into a DTB vacuum crystallizer 6 through a feed pipe 14. Crystallization occurs in an adiabatic and cooling environment to form a slurry containing potassium chloride crystal nuclei. Large crystal nuclei settle at the bottom, while the upper layer of slurry containing small crystal nuclei enters a circulation channel 18 to dissolve the small crystal nuclei before returning to the DTB vacuum crystallizer 6 for further crystallization. The slurry containing large crystal nuclei at the bottom of the DTB vacuum crystallizer 6 is discharged to obtain a crystal slurry; Screening step S2: The crystal slurry is screened using at least two screening devices. The aperture of the first screening device 21 is the upper screening value, screening out the first sieve oversize particles with a particle size greater than the upper screening value and the particles... The first undersize material with a particle size below the upper screening value is screened by the second screening device 22 with a pore size of the lower screening value, which is smaller than the upper screening value. The second screening device 22 screens the first undersize material to obtain the second oversize material with a particle size between the upper and lower screening values ​​and the second undersize material with a particle size below the lower screening value. The second oversize material is used to produce potassium chloride. In the large particle size crystal nucleus treatment step S3, the first oversize material with a particle size above the upper screening value is introduced into the targeted mother liquor tank 31 and dissolved in fresh water to obtain a potassium chloride saturated targeted mother liquor. The targeted mother liquor is returned to the DTB vacuum crystallizer 6 through the circulation channel 18 for recrystallization. In the small particle size crystal nucleus treatment step S4, the second undersize material with a particle size below the lower screening value is returned to the DTB vacuum crystallizer 6 through the feed pipe 14 as a seed crystal for crystallization.

[0006] Therefore, the potassium mother liquor crystallizes in the DTB vacuum crystallizer 6. After the slurry containing large crystal nuclei settles at the bottom and is discharged, it is screened by the first screening device 21 and the second screening device 22. The material on the second screen with a particle size within the range of the upper and lower screening values ​​is used to produce potassium chloride. Compared with simply increasing the particle size of potassium chloride crystal nuclei, this method can increase the potassium chloride content in the potassium chloride crystal nuclei and improve the potassium chloride grade.

[0007] Furthermore, during the process of dissolving the large-sized material on the first sieve into a potassium chloride-saturated targeted mother liquor in fresh water and returning it to the DTB vacuum crystallizer through the circulation channel, the small-particle crystal nuclei and sodium chloride crystals in the slurry circulating in the channel are dissolved, which helps to prevent the growth of sodium chloride crystals.

[0008] Preferably, the upper and lower screening values ​​are determined by the following method: the potassium mother liquor is introduced into the DTB vacuum crystallizer 6, and crystallization is carried out in an adiabatic and cooling environment to form a slurry containing potassium chloride crystal nuclei. The slurry at the bottom of the DTB vacuum crystallizer 6 is analyzed to calculate the particle size distribution and the corresponding potassium chloride crystal nuclei content, thereby obtaining the normal distribution of potassium chloride grade. The upper and lower screening values ​​are determined by the particle size range corresponding to the highest potassium chloride grade range in the normal distribution.

[0009] Preferably, the upper sieve value is 0.7 mm and the lower sieve value is 0.3 mm, and the second sieve oversize particles with a particle size in the range of 0.3 mm to 0.7 mm are screened out for use in the preparation of potassium chloride. Attached Figure Description

[0010] Figure 1 Schematic diagram of a vacuum crystallizer;

[0011] Figure 2 A schematic diagram of the equipment connections in a production system for preparing potassium chloride using a vacuum crystallizer;

[0012] Figure 3 Flowchart of a method for improving the grade of potassium chloride in the process of preparing potassium chloride using a vacuum crystallizer.

[0013] In the diagram, 1. Crystallization device, 11. Primary crystallizer, 12. Secondary crystallizer, 13. Tertiary crystallizer, 14. Feed pipe, 15. Primary axial flow pump, 16. Secondary axial flow pump, 17. Tertiary crystal slurry pump, 18. Circulation channel, 2. Screening device, 21. First screening equipment, 22. Second screening equipment, 3. Circulation device, 31. Targeted mother liquor tank, 32. Targeted mother liquor pump, 4. Heat exchange device, 41. Low temperature mother liquor tank, 42. Secondary hot well, 43. 44. Primary thermal well, 45. Cryogenic mother liquor pump, 46. Secondary thermal well pump, 47. Primary thermal well pump, 48. Tertiary thermal well, 5. Vacuum pump, 69. DTB vacuum crystallizer, 60. Shell, 611. Crystallization chamber, 612. Feed inlet, 613. Discharge outlet, 614. Circulation port, 615. Vacuum port, 616. Baffle zone, 62. Cooler, 621. Cooling inlet, 622. Cooling outlet, 63. Crystallization cylinder, 64. Baffle cylinder, 65. Stirring device. Detailed Implementation

[0014] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0015] like Figure 1 As shown, the DTB vacuum crystallizer 6 includes a shell 61, a cooler 62, a crystallization cylinder 63, a baffle cylinder 64, and a stirring device 65. The top of the shell 61 is provided with a vacuum port 615, the side wall of the shell 61 is provided with a circulation port 614, the bottom of the shell 61 is provided with a feed port 612 and a discharge port 613, and a crystallization chamber 611 is formed inside the shell 61. An evaporation arc plate is provided inside the crystallization chamber 611.

[0016] The cooler 62 is fixedly installed on the top of the shell 61. The cooler 62 and the crystallization chamber 611 are not connected. The cooler 62 is provided with a cooling inlet 621 and a cooling outlet 622. The cooling medium enters the cooler 62 from the cooling inlet 621 and exchanges heat with the solution inside the crystallizer to cool it down. The cooled medium after heat exchange is discharged from the cooling outlet 622.

[0017] The crystallization cylinder 63 is cylindrical and is fixedly installed in the middle of the crystallization chamber 611. The evaporation arc plate is located above the crystallization cylinder 63. The evaporation arc plate can promote the evaporation of water in the solution and accelerate the heat dissipation of the solution.

[0018] The baffle 64 is cylindrical, with a diameter larger than that of the crystallizing cylinder 63. It is fitted around the outer periphery of the crystallizing cylinder 63, and its upper edge is folded outward to the inner wall of the shell 61. A baffle zone 616 is formed between the baffle 64 and the inner wall of the shell 61. The circulation port 614 is connected to the upper part of the baffle zone 616.

[0019] The stirring blades of the stirring device 65 are disposed inside the crystallization cylinder 63 to stir the solution inside the crystallization cylinder 63, so that the solution flows upward inside the crystallization cylinder 63.

[0020] The crystallization process in the DTB vacuum crystallizer 6: The potassium chloride mother liquor enters the bottom of the crystallization chamber 611 through the feed inlet 612. The solution inside the crystallization chamber 611 is cooled by the cooler 62. The vacuum pump 5 is connected to the vacuum port 615 at the top of the casing, and the vacuum pump 5 creates a vacuum inside the crystallization chamber 611, thereby promoting the evaporation, cooling, and concentration of the solution. Through cooling and evaporation concentration, a potassium chloride supersaturated region is formed in the upper layer of the solution, causing potassium chloride in the solution to crystallize and form a slurry containing potassium chloride crystal nuclei.

[0021] Under the stirring action of the stirring device 65, the slurry located at the bottom of the crystallization chamber 611 flows upward inside the crystallization cylinder 63, while the slurry in the upper layer flows downward to the bottom of the crystallization chamber 611 in the area between the crystallization cylinder 63 and the baffle cylinder 64. Large crystal nuclei settle at the bottom of the crystallization chamber 611 under the action of gravity. Some small crystal nuclei continue to rise from the crystallization cylinder 63 with the slurry to the upper layer of the slurry and continue to grow. Some small crystal nuclei enter the baffle zone 616 between the baffle cylinder 64 and the inner wall of the shell 61 with the slurry. The slurry in the upper part of the baffle zone 616 is discharged through the circulation port 614 and enters the circulation channel. After the small crystal nuclei in the slurry are dissolved by the targeted mother liquor in the circulation channel, part of the solution returns to the crystallization chamber 611 to continue crystallization, and part of the solution enters the subsequent equipment for processing.

[0022] Finally, when the large crystal nuclei meet the conditions, they are discharged from the outlet 613 along with the slurry located at the bottom of the crystallization chamber 611, thus obtaining crystal slurry.

[0023] The stirring action of the stirring device 65 can promote the upward flow of unsaturated slurry at the bottom of the crystallization chamber 611, forming near-good mixing conditions in the crystallization chamber 611, promoting the floating of small crystal nuclei to the upper layer of the slurry, and enabling the rapid growth of potassium chloride crystal nuclei. The small crystal nuclei in the slurry can be dissolved through the circulation channel, which can control the number of crystal nuclei and promote crystal growth.

[0024] like Figure 2 As shown, the production system for preparing potassium chloride using a vacuum crystallizer includes a crystallization device 1, a screening device 2, a circulation device 3, a heat exchange device 4, and a vacuum pump 5.

[0025] The crystallization device 1 includes a primary crystallizer 11, a secondary crystallizer 12 and a tertiary crystallizer 13 connected in series. The primary crystallizer 11, the secondary crystallizer 12 and the tertiary crystallizer 13 are all DTB vacuum crystallizers 6.

[0026] The feed inlet of the primary crystallizer 11 is connected to the feed pipe 14, the feed inlet of the secondary crystallizer 12 is connected to the discharge outlet of the primary crystallizer 11, and the feed inlet of the tertiary crystallizer 13 is connected to the discharge outlet of the secondary crystallizer 12.

[0027] During operation, the high-temperature potassium chloride mother liquor first enters the primary crystallizer 11 through the feed pipe 14. The temperature inside the primary crystallizer 11 is maintained at around 80°C. After the solution in the primary crystallizer 11 cools down, the potassium chloride in the solution crystallizes to form a slurry containing potassium chloride crystal nuclei. The crystal nuclei gradually grow. Under the stirring action of the stirring device, small crystal nuclei float to the upper layer of the slurry and the baffle zone, while large crystal nuclei sink to the bottom of the primary crystallizer 11. The slurry at the bottom of the primary crystallizer 11 is discharged from the outlet to obtain the primary crystal slurry. The primary crystal slurry is then pumped to the secondary crystallizer 12.

[0028] The slurry located in the upper part of the baffle zone of the primary crystallizer 11 is discharged from the circulation port to obtain the primary mother liquor. The primary mother liquor enters the circulation channel 18 and mixes with the target mother liquor. The small crystal nuclei in the primary mother liquor are dissolved by the target mother liquor in the circulation channel 18. Part of the primary mother liquor is returned to the primary crystallizer 11 by the primary axial flow pump 15 to continue crystallization, and the remaining primary mother liquor is transported to the secondary crystallizer 12.

[0029] The temperature inside the secondary crystallizer 12 is maintained at around 60°C. The primary crystal slurry and the primary mother liquor enter the secondary crystallizer 12 for mixing. The crystal nuclei in the slurry inside the secondary crystallizer 12 continue to grow. Under the stirring action of the stirring device, small crystal nuclei float to the upper layer of the slurry and the baffle zone, while large crystal nuclei fall to the bottom of the secondary crystallizer 12. The slurry at the bottom of the secondary crystallizer 12 is discharged from the outlet to obtain the secondary crystal slurry. The secondary crystal slurry is then pumped to the tertiary crystallizer 13.

[0030] The slurry located in the upper part of the baffle zone of the secondary crystallizer 12 is discharged from the circulation port to obtain the secondary mother liquor. The secondary mother liquor enters the circulation channel 18 and mixes with the target mother liquor. The small crystal nuclei in the secondary mother liquor are dissolved by the target mother liquor in the circulation channel 18. Part of the secondary mother liquor is returned to the secondary crystallizer 12 for continued crystallization by the secondary axial flow pump 16, and the remaining secondary mother liquor is transported to the tertiary crystallizer 13.

[0031] The temperature inside the tertiary crystallizer 13 is maintained at around 35°C. The secondary crystal slurry and the secondary mother liquor are mixed inside. The crystal nuclei in the slurry inside the tertiary crystallizer 13 grow further. Under the stirring action of the stirring device, the small crystal nuclei float to the upper layer of the slurry and the baffle zone. The small crystal nuclei sink to the bottom of the tertiary crystallizer 13. The slurry at the bottom of the tertiary crystallizer 13 is discharged from the outlet to obtain large-particle crystal slurry. The large-particle crystal slurry is transported to the screening device 2 for screening by the tertiary crystal slurry pump 17.

[0032] The slurry located in the upper part of the baffle zone of the three-stage crystallizer 13 is discharged from the circulation port to obtain crystallization mother liquor, which is then transported to the low-temperature mother liquor tank 41 by the three-stage mother liquor pump.

[0033] The screening device 2 includes a first screening device 21 and a second screening device 22 connected to each other. The aperture of the screen of the first screening device 21 (i.e., the first screening value) is 0.7 mm, and the aperture of the screen of the second screening device 22 (i.e., the second screening value) is 0.3 mm.

[0034] Large-particle crystal slurry first enters the first screening device 21 for screening, obtaining the first oversize material with a particle size greater than 0.7 mm and the first undersize material with a particle size less than 0.7 mm. The first oversize material enters the circulation device 3.

[0035] The first undersize material enters the second screening device 22 for screening, resulting in a second oversize material with a particle size between 0.3 mm and 0.7 mm and a second undersize material with a particle size less than 0.3 mm.

[0036] The material from the second screen enters the downstream equipment to produce potassium chloride, while the material from the second screen enters the primary crystallizer 11 through the feed pipe 14 for further crystallization.

[0037] Thermometers are installed inside the primary crystallizer 11, the secondary crystallizer 12, and the tertiary crystallizer 13.

[0038] The circulation device 3 includes a targeted mother liquor tank 31 and a targeted mother liquor pump 32. The first sieve residue and fresh water are added to the targeted mother liquor tank 31, where the fresh water dissolves the first sieve residue to obtain the targeted mother liquor. The targeted mother liquor is then pumped by the targeted mother liquor pump 32 to the circulation channel 18 to dissolve the small crystal nuclei and sodium chloride crystals in the circulation channel 18. The solution is then transported through the circulation channel 18 to the primary crystallizer 11, the secondary crystallizer 12, and the tertiary crystallizer 13 for crystallization.

[0039] The targeted mother liquor is conveyed to circulation channel 18, where it mixes with the primary mother liquor and returns to the primary crystallizer or enters the secondary crystallizer; alternatively, it mixes with the secondary mother liquor and returns to the secondary crystallizer or enters the tertiary crystallizer. The targeted mother liquor dissolves small crystal nuclei and sodium chloride crystals, reducing the sodium chloride content in the finished product and improving its grade. The amount of targeted mother liquor entering each crystallizer stage can be adjusted according to production needs, ensuring that sodium chloride remains unsaturated or nearly saturated during potassium chloride crystallization. This inhibits sodium chloride precipitation, further reducing the sodium chloride content in the finished product and improving its grade.

[0040] The heat exchange device 4 includes a three-stage hot well 47, a cryogenic mother liquor tank 41, a secondary hot well 42, a primary hot well 43, a cryogenic mother liquor pump 44, a secondary hot well pump 45, and a primary hot well pump 46.

[0041] The low-temperature mother liquor tank 41 is connected to the circulation port of the tertiary crystallizer 13 and the cooler of the secondary crystallizer 12, respectively, to receive the crystallization mother liquor discharged from the tertiary crystallizer 13. The temperature inside the low-temperature mother liquor tank 41 is approximately 30°C. The crystallization mother liquor in the low-temperature mother liquor tank 41 serves as the cooling medium for the secondary crystallizer 12, and is pumped to the cooler 62 of the secondary crystallizer 12 by the low-temperature mother liquor pump 44 to exchange heat and cool the solution inside the secondary crystallizer 12.

[0042] The secondary hot well 42 is connected to both the cooler of the secondary crystallizer 12 and the cooler of the primary crystallizer 11, receiving the mother liquor after heat exchange in the secondary crystallizer 12. The temperature inside the secondary hot well 42 is approximately 55°C. The mother liquor in the secondary hot well 42 serves as the cooling medium for the primary crystallizer 11, and is pumped to the cooler of the primary crystallizer 11 via the secondary hot well pump 45 to exchange heat and cool the solution inside the primary crystallizer 11.

[0043] The primary hot well 43 is connected to the cooler 62 of the primary crystallizer 11 and receives the mother liquor after heat exchange in the primary crystallizer 11. The temperature inside the primary hot well 43 is about 75°C. The mother liquor inside the primary hot well 43 is used as a solution for potassium salt and is transported to the hot dissolving tank through the primary hot well pump 46 to dissolve the potassium salt.

[0044] The cooling medium of the cooler in the three-stage crystallizer 13 is cooling water. After exchanging heat with the solution in the three-stage crystallizer 13, the cooling water is discharged into the three-stage hot well 47 and then discharged.

[0045] Vacuum pump 5 is connected to the vacuum ports of primary crystallizer 11, secondary crystallizer 12 and tertiary crystallizer 13 respectively, and evacuates the vapor generated in the crystallization chambers of primary crystallizer 11, secondary crystallizer 12 and tertiary crystallizer 13, thereby reducing the gas pressure in the crystallization chamber, accelerating the evaporation rate, improving heat dissipation efficiency and promoting potassium chloride crystallization.

[0046] like Figure 3 As shown, a method for improving the grade of potassium chloride in the process of preparing potassium chloride using a vacuum crystallizer includes a potassium chloride crystallization step S1, in which a high-temperature refined potassium mother liquor is introduced into the crystallization device through a feed pipe 14 and crystallized in an adiabatic and cooling environment to obtain a crystallization mother liquor and large-particle crystal slurry.

[0047] The crystallization apparatus includes three DTB vacuum crystallizers, which are connected in series to form a primary crystallizer 11, a secondary crystallizer 12, and a tertiary crystallizer 13.

[0048] The potassium chloride crystallization step S1 includes a crystal nucleus formation step S11, in which high-temperature refined potassium mother liquor enters the primary crystallizer 11 through the feed pipe 14, the solution inside the crystallization chamber is cooled by the cooler, and the crystallization chamber is evacuated by the vacuum pump 5, so that the solution in the primary crystallizer 11 evaporates, cools down and concentrates, forming a supersaturated region on the upper layer of the solution, and potassium chloride in the solution crystallizes to form a slurry containing potassium chloride crystal nuclei, which gradually grows.

[0049] In step S12, which separates large and small crystal nuclei, the large crystal nuclei, due to their greater weight, gradually sink to the bottom of the primary crystallizer 11, while the small crystal nuclei, due to their smaller weight, gradually float to the upper layer of the slurry and the baffle zone. The slurry at the bottom of the primary crystallizer 11 is discharged from the outlet to obtain the primary crystal slurry, which then enters the secondary crystallizer 12 for further crystallization.

[0050] The slurry containing small crystal nuclei in the upper part of the baffle zone of the primary crystallizer 11 is discharged from the circulation port to obtain the primary mother liquor. After the small crystal nuclei in the primary mother liquor are dissolved by the targeted mother liquor in the circulation channel, part of it returns to the primary crystallizer 11, and the rest enters the secondary crystallizer 12 to continue crystallization.

[0051] The crystallization process in the secondary crystallizer 12 is the same as that in the primary crystallizer 11. The slurry at the bottom of the secondary crystallizer 12 is discharged from the outlet to obtain secondary crystal slurry, which then enters the tertiary crystallizer 13 for further crystallization. The slurry containing small crystal nuclei in the baffle zone of the secondary crystallizer 12 exits through the circulation port to obtain secondary mother liquor. The small crystal nuclei in the secondary mother liquor are dissolved by the targeted mother liquor in the circulation channel, with a portion returning to the secondary crystallizer 12 and the remainder entering the tertiary crystallizer 13.

[0052] The crystallization process of the third-stage crystallizer 13 is the same as that of the first-stage crystallizer 11. The slurry at the bottom of the third-stage crystallizer 13 is discharged from the outlet to obtain large-particle crystal slurry. The slurry containing small-particle crystal nuclei in the upper part of the baffle zone of the third-stage crystallizer 13 is discharged from the circulation port to obtain crystallization mother liquor. After the crystallization mother liquor is discharged into the low-temperature mother liquor tank, it is circulated as a cooling medium.

[0053] Screening step S2 includes a first screening device screening step S21, in which the first screening device 21 is used to screen the large particle crystal slurry. The aperture of the screen of the first screening device 21 (i.e. the first screening value) is 0.7 mm, and the first oversize material with a particle size greater than 0.7 mm and the first undersize material with a particle size less than 0.7 mm are screened out.

[0054] In the second screening step S22, the first undersize material is screened using the second screening device 22. The aperture of the screen of the second screening device 22 (i.e., the second screening value) is 0.3 mm. The second oversize material with a particle size between 0.3 mm and 0.7 mm is screened out, and the second undersize material with a particle size less than 0.3 mm is screened out. The second oversize material is used to produce potassium chloride.

[0055] In the large-particle crystal nucleus processing step S3, the first sieve material with a particle size above the upper screening value is introduced into the targeted mother liquor tank 31, dissolved in fresh water to obtain a potassium chloride saturated targeted mother liquor, and the targeted mother liquor is transported to the circulation channel 18 through the targeted mother liquor pump 32 to dissolve the small-particle crystal nuclei and sodium chloride crystals.

[0056] In the small-size crystal nucleus processing step S4, the second sieve undersize material with a particle size below the lower screening value is returned to the primary crystallizer through the feed pipe 14 as crystal seeds to continue crystallization.

[0057] In this invention, the aperture of the first screening device 21 and the aperture of the second screening device 22 are determined by the following method: the potassium mother liquor is introduced into the DTB vacuum crystallizer 6, and crystallization is carried out in an adiabatic and cooling environment to form a slurry containing potassium chloride crystals. The slurry located at the bottom of the crystallization chamber 611 of the DTB vacuum crystallizer 6 is analyzed to calculate the particle size distribution and the corresponding potassium chloride crystal content, thereby obtaining the normal distribution of potassium chloride grade. The upper screening value of the first screening device 21 and the lower screening value of the second screening device 22 are determined by the particle size range corresponding to the highest potassium chloride grade range in the normal distribution.

[0058] By screening the potassium chloride product produced by the existing process, potassium chloride products of different particle sizes were obtained. The composition of the potassium chloride products of different particle sizes was detected and the results were analyzed below.

[0059] Table 1 Comparison of the content of various substances in potassium chloride products with different particle sizes

[0060]

[0061]

[0062] Table 1 shows that the potassium chloride content is highest, indicating the highest grade, when the particle size of the finished potassium chloride product is in the range of 300 μm to 600 μm. When the particle size of the finished potassium chloride product is less than 300 μm, the potassium chloride content decreases with decreasing particle size, and the grade of potassium chloride gradually declines. When the particle size of the finished potassium chloride product is greater than 700 μm, the potassium chloride content decreases with increasing particle size, and the grade of potassium chloride gradually declines.

[0063] Therefore, this invention controls the particle size of the finished potassium chloride product within the range of 0.3–0.7 mm, eliminating low-grade finished products containing fine particles smaller than 0.3 mm and some large particles larger than 0.7 mm, thus significantly improving the quality of the potassium chloride product. Controlling the particle size of the finished potassium chloride product within the 0.3–0.7 mm range, removing both larger and smaller particles, and ensuring a uniform particle size distribution reduces the amount of mother liquor entrained in the finished product, as well as impurities such as calcium sludge attached to the mother liquor, thereby purifying the product.

[0064] Therefore, the potassium mother liquor crystallizes in the DTB vacuum crystallizer 6. After the slurry containing large crystal nuclei settles at the bottom and is discharged, it is screened by the first screening device 21 and the second screening device 22. The material on the second screen with a particle size within the range of the upper and lower screening values ​​is used to produce potassium chloride. Compared with simply increasing the particle size of potassium chloride crystals, this method can increase the potassium chloride content in the potassium chloride crystals and improve the potassium chloride grade.

[0065] Furthermore, during the process of dissolving the large-sized material on the first sieve into a potassium chloride-saturated targeted mother liquor in fresh water and returning it to the DTB vacuum crystallizer 6 through the circulation channel 18, the small-particle crystal nuclei and sodium chloride crystals in the slurry circulating in the channel are dissolved, thereby preventing the growth of sodium chloride crystals.

[0066] In addition, existing technology involves directly adding fresh water to the crystallizer to eliminate sodium chloride crystal nuclei. However, this method is not targeted and precise, and while eliminating sodium chloride crystal nuclei, it also eliminates a large number of potassium chloride crystal nuclei.

[0067] In this invention, the targeted mother liquor can replace the fresh water added in the prior art. The targeted mother liquor, which enters the crystallizer through the circulation channel, can dissolve small crystal nuclei and eliminate sodium chloride crystals precipitated during the adiabatic crystallization process. The targeted mother liquor, after being mixed with potassium chloride mother liquor through the feed pipe, can increase the unsaturation of sodium chloride in the solution and inhibit its crystallization. In this way, the targeted mother liquor plays a dual role in reducing the sodium chloride content, effectively reducing the sodium chloride content in the finished product and further improving product quality.

[0068] It should be noted that the above embodiments are illustrative of the present invention and not restrictive of the present invention, and that those skilled in the art can devise alternative embodiments without departing from the scope of the appended claims.

Claims

1. A method for improving the grade of potassium chloride in a process of preparing potassium chloride using a vacuum crystallizer, characterized in that, Includes the following steps: In the potassium chloride crystallization step (S1), the high-temperature refined potassium mother liquor is introduced into the DTB vacuum crystallizer (6) through the feed pipe (14). In the adiabatic and cooling environment, crystallization is carried out to form a slurry containing potassium chloride crystal nuclei. Large crystal nuclei settle at the bottom, and the slurry containing small crystal nuclei in the upper layer enters the circulation channel (18) to dissolve the small crystal nuclei and then returns to the DTB vacuum crystallizer (6) to continue crystallization. The slurry containing large crystal nuclei at the bottom of the DTB vacuum crystallizer (6) is discharged to obtain large crystal slurry. Screening step (S2): The large particle slurry is screened using at least two screening devices. The aperture of the first screening device (21) is the upper screening value, which screens out the first oversize material with a particle size above the upper screening value and the first undersize material with a particle size below the upper screening value. The aperture of the second screening device (22) is the lower screening value, which is smaller than the upper screening value. The first undersize material is screened by the second screening device (22), which screens out the second oversize material with a particle size between the upper screening value and the lower screening value and the second undersize material with a particle size below the lower screening value. The second oversize material is used to produce the potassium chloride. In the large-size crystal nucleus processing step (S3), the first sieve material with a particle size above the upper screening value is introduced into the targeted mother liquor tank (31) and dissolved in fresh water to obtain a potassium chloride saturated targeted mother liquor. The targeted mother liquor is returned to the DTB vacuum crystallizer (6) through the circulation channel (18) for recrystallization. In the small-size crystal nucleus processing step (S4), the second undersize material with a particle size below the lower screening value is returned to the DTB vacuum crystallizer (6) through the feed pipe (14) as crystal seeds for crystallization.

2. The method for improving the grade of potassium chloride in the process of preparing potassium chloride using a vacuum crystallizer according to claim 1, characterized in that, The upper and lower filter values ​​are determined by the following method. The potassium mother liquor is introduced into the DTB vacuum crystallizer (6) and crystallized in an adiabatic and cooling environment to form a slurry containing potassium chloride crystal nuclei. The slurry at the bottom of the DTB vacuum crystallizer (6) is analyzed to calculate the particle size distribution and the corresponding potassium chloride crystal nuclei content, and the normal distribution of potassium chloride grade is obtained. The upper screening value and the lower screening value are determined by the particle size range corresponding to the highest potassium chloride grade range in the normal distribution.

3. The method for improving the grade of potassium chloride in the process of preparing potassium chloride using a vacuum crystallizer according to claim 2, characterized in that, The upper sieve value is 0.7 mm, and the lower sieve value is 0.3 mm. The second sieve oversize particles with a particle size in the range of 0.3 mm to 0.7 mm are screened out and used to produce the potassium chloride.

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