Method for producing spherical potassium chloride

Abstract

The application provides a preparation method of spherical potassium chloride. The preparation method of the spherical potassium chloride comprises the following steps: preparing a first potassium chloride mother liquor; adding the first potassium chloride mother liquor into a crystallizer; heating the first potassium chloride mother liquor to a first preset temperature T1; in a nucleation stage, stirring the first potassium chloride mother liquor at a first stirring speed N1, and evaporating the first potassium chloride mother liquor in the crystallizer at a first evaporation rate E1; in a crystal growth stage, stirring the first potassium chloride mother liquor at a second stirring speed N2, and evaporating the first potassium chloride mother liquor in the crystallizer at a second evaporation rate E2; and discharging part of the first potassium chloride mother liquor containing crystals with a preset particle size from the crystallizer during the evaporation crystallization process of the first potassium chloride mother liquor in the crystallizer. The technical scheme of the application can solve the problems that the potassium chloride product particles generated by the existing potassium chloride evaporation crystallization production process are generally small, and the particle size distribution is extremely uneven.

Description

Technical Field

[0001] This invention relates to the field of chemical engineering industrial crystallization technology, and more specifically, to a method for preparing spherical potassium chloride. Background Technology

[0002] Potassium chloride is an important raw material for potash fertilizers, widely used in agricultural production and industry. The physical properties of the product, especially its particle morphology, size, and uniformity of distribution, directly determine its market value, transportation and storage performance, and utilization efficiency. However, existing potassium chloride evaporation and crystallization processes generally produce potassium chloride particles that are too small and have extremely uneven particle size distribution. Summary of the Invention

[0003] The main objective of this invention is to provide a method for preparing spherical potassium chloride, which can solve the problem that the potassium chloride products produced by the existing potassium chloride evaporation and crystallization production process are generally small in size and have extremely uneven particle size distribution.

[0004] To achieve the above objectives, the present invention provides a method for preparing spherical potassium chloride, comprising: preparing a first potassium chloride mother liquor; adding the first potassium chloride mother liquor into a crystallizer; heating the first potassium chloride mother liquor to a first preset temperature T1; in the nucleation stage, stirring the first potassium chloride mother liquor at a first stirring speed N1, and evaporating the first potassium chloride mother liquor in the crystallizer at a first evaporation rate E1; in the crystal growth stage, stirring the first potassium chloride mother liquor at a second stirring speed N2, and evaporating the first potassium chloride mother liquor in the crystallizer at a second evaporation rate E2, wherein the first stirring speed N1 is less than the second stirring speed N2, and the first evaporation rate E1 is less than the second evaporation rate E2; during the evaporation and crystallization process of the first potassium chloride mother liquor in the crystallizer, a portion of the first potassium chloride mother liquor containing crystals of a preset particle size is discharged from the crystallizer, and a potassium chloride solution with a second preset temperature T2 and a mass fraction greater than that of the first potassium chloride mother liquor is introduced into the crystallizer, wherein the second preset temperature T2 is greater than the first preset temperature T1.

[0005] Furthermore, the first preset temperature T1 has a range of 75℃≤T1≤85℃, and the first preset temperature T1 and the second preset temperature T2 satisfy the following condition: 5℃≤T2-T1≤20℃.

[0006] Furthermore, the mass fraction W1 of the potassium chloride solution is in the range of 33%≤W1≤35%, and the mass fraction W1 of the potassium chloride solution and the mass fraction W2 of the first potassium chloride mother liquor both satisfy: 1%≤W1-W2≤2%.

[0007] Furthermore, the range of the first stirring speed N1 is 300rpm≤N1≤500rpm, and the range of the second stirring speed N2 is 500rpm<N2≤700rpm.

[0008] Furthermore, the first evaporation rate E1 ranges from 50 ml / h to 70 ml / h, and the second evaporation rate E2 ranges from 70 ml / h to 100 ml / h; and / or, the vacuum degree of the crystallizer is 15 kPa to 30 kPa.

[0009] Furthermore, the preset particle size range is (0μm, 150μm).

[0010] Further, the steps for preparing the first potassium chloride mother liquor include: taking crude potassium solid, the crude potassium solid comprising potassium chloride, sodium chloride, magnesium chloride, calcium chloride and sand, wherein the mass fraction of potassium chloride ranges from 82% to 90%, the mass fraction of sodium chloride ranges from 6% to 12%, the mass fraction of magnesium chloride ranges from 3% to 6%, and the mass fraction of calcium chloride ranges from 0.1% to 0.5%; mixing the crude potassium solid with water to prepare a mixed solution; and filtering the mixed solution to remove the sand in the mixed solution to obtain the first potassium chloride mother liquor.

[0011] Further, during the evaporation and crystallization process of the first potassium chloride mother liquor in the crystallizer, the step of discharging a portion of the first potassium chloride mother liquor containing crystals of a predetermined particle size from the crystallizer and introducing a potassium chloride solution with a second predetermined temperature T2 and a mass fraction greater than that of the first potassium chloride mother liquor into the first potassium chloride mother liquor includes: discharging a portion of the first potassium chloride mother liquor containing crystals of a predetermined particle size from the crystallizer and then introducing it into a dissolving tank, wherein the dissolving tank contains a second potassium chloride mother liquor, the mass fraction of the second potassium chloride mother liquor is equal to the mass fraction of the first potassium chloride mother liquor, and the temperature T3 of the second potassium chloride mother liquor is greater than the first predetermined temperature T1; stirring the second potassium chloride mother liquor at a third stirring speed N3 to dissolve the crystals of the predetermined particle size in the second potassium chloride mother liquor to form a potassium chloride solution; and introducing the potassium chloride solution from the bottom of the crystallizer to mix it with the first potassium chloride mother liquor.

[0012] Furthermore, the temperature T3 of the second potassium chloride mother liquor is in the range of 90℃≤T3≤95℃, and the stirring speed N3 is in the range of 200rpm≤N3≤400rpm.

[0013] Further, the step of discharging a portion of the first potassium chloride mother liquor containing crystals of a predetermined particle size from the crystallizer and then introducing it into the dissolving tank at a first flow rate Q1 includes: in the nucleation stage, discharging a portion of the first potassium chloride mother liquor containing crystals of particle size D1 from the crystallizer at a first flow rate Q1; in the crystal growth stage, discharging a portion of the first potassium chloride mother liquor containing crystals of particle size D2 from the crystallizer at a second flow rate Q2, wherein the value of D1 ranges from 0 μm < D1 ≤ 100 μm, the value of D2 ranges from 0 μm < D2 ≤ 150 μm, the value of the first flow rate Q1 ranges from 200 ml / min ≤ Q1 ≤ 250 ml / min, and the value of the second flow rate Q2... The value range of Q1 is 300 ml / min ≤ Q2 ≤ 380 ml / min; the step of introducing potassium chloride solution from the bottom of the crystallizer includes: in the nucleation stage, introducing potassium chloride solution from the bottom of the crystallizer at a third flow rate Q3; in the crystal growth stage, introducing potassium chloride solution from the bottom of the crystallizer at a fourth flow rate Q4; wherein, the value range of the third flow rate Q3 is 200 ml / min ≤ Q3 ≤ 250 ml / min, the value range of the fourth flow rate Q4 is 300 ml / min ≤ Q4 ≤ 380 ml / min, and the first flow rate Q1 is equal to the third flow rate Q3, and the second flow rate Q2 is equal to the fourth flow rate Q4.

[0014] The present invention provides a method for preparing a first potassium chloride mother liquor. This first potassium chloride mother liquor is then added to a crystallizer (DTB crystallizer, also known as a guide tube and baffle crystallizer). The first potassium chloride mother liquor is heated to a first preset temperature T1. During the nucleation stage, a lower first stirring speed N1 is used to stir the first potassium chloride mother liquor, and a lower first evaporation rate E1 is used for evaporation and crystallization. This aims to strictly control the supersaturation of the first potassium chloride mother liquor within the metastable region, effectively suppressing explosive spontaneous nucleation caused by excessive local supersaturation or excessive shear force. This significantly reduces the number of initial crystal nuclei, preventing the solute from being divided by a large number of tiny crystal nuclei, and reserving sufficient solute space for subsequent large particle growth. After entering the crystal growth stage, increasing the stirring speed to the second stirring speed N2 and the evaporation rate to the second evaporation rate E2 provides sufficient mass transfer driving force and supersaturation while maintaining crystal suspension, promoting the rapid directional growth of existing crystal nuclei. Furthermore, during the entire evaporation and crystallization process, discharging a portion of the mother liquor containing crystals of a preset particle size avoids small particles from contaminating the final product and eliminates interference from fine particles on the growth of larger particles. Simultaneously, a high-temperature potassium chloride solution with a higher mass fraction at a second preset temperature T2 is added to a lower-temperature crystallizer. Because the second preset temperature T2 is higher than the first preset temperature T1, the solubility of the high-temperature potassium chloride solution decreases upon entering the crystallizer, instantly generating controllable supersaturation. This supersaturation is not used to generate new crystal nuclei but is absorbed by the large crystal particles remaining in the crystallizer, causing them to continue growing. This significantly narrows the particle size distribution range, ultimately obtaining large-sized, uniformly distributed potassium chloride crystals, resulting in spherical large-particle potassium chloride products and effectively improving product quality.

[0015] The method for preparing spherical potassium chloride in this application has the following beneficial effects:

[0016] 1) The salt particles generated in the crystallizer during the evaporation process are classified by particle size. The fine crystals are taken out and dissolved, while the large particles are centrifuged and filtered to make the final product particles large and uniform, thereby improving the product's sales value.

[0017] 2) By classifying the salt particles by particle size, the vibrating sieving step of salt after centrifugal filtration can be eliminated, resulting in a significant reduction in the moisture content of the salt particles, thereby reducing the energy consumption of the drying process.

[0018] 3) It extends the crystallization residence time of large salt particles in the crystallizer, making the particle size of the final product much larger than that before the modification, and the strength of the salt is significantly improved due to the increase in aging time.

[0019] 4) The stirring step of this application uses a propeller-type agitator, which generates mechanical friction to produce spherical salt particles. The spherical salt particles are in point contact, and the salt in point contact is not easy to clump together, thus solving the problem of serious product loss during the re-slurry washing process.

[0020] 5) The final crystal product has a particle size of 900μm to 1200μm and a uniform particle size distribution. 50% of the potassium chloride crystals in the product have a particle size of more than 900μm, and the vast majority are spherical crystals. Attached Figure Description

[0021] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0022] Figure 1 A flowchart of a method for preparing spherical potassium chloride according to an embodiment of the present invention is shown;

[0023] Figure 2 An electron microscope image of potassium chloride crystals from Example 1 is shown, with a scale bar of 500 μm;

[0024] Figure 3 An electron microscope image of potassium chloride crystals from Example 2 is shown, with a scale bar of 500 μm;

[0025] Figure 4 An electron microscope image of potassium chloride crystals from Example 3 is shown, with a scale bar of 500 μm;

[0026] Figure 5 Electron microscope images of potassium chloride crystals from Comparative Example 1 are shown, with a scale bar of 500 μm.

[0027] Figure 6 Electron microscope images of potassium chloride crystals from Comparative Example 2 are shown, with a scale bar of 500 μm.

[0028] Figure 7 Electron microscope images of potassium chloride crystals from Comparative Example 3 are shown, with a scale bar of 500 μm. Detailed Implementation

[0029] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0030] like Figure 1As shown, the present invention provides a method for preparing spherical potassium chloride. The method includes: preparing a first potassium chloride mother liquor; adding the first potassium chloride mother liquor into a crystallizer; heating the first potassium chloride mother liquor to a first preset temperature T1; in the nucleation stage, stirring the first potassium chloride mother liquor at a first stirring speed N1, and evaporating the first potassium chloride mother liquor in the crystallizer at a first evaporation rate E1; in the crystal growth stage, stirring the first potassium chloride mother liquor at a second stirring speed N2, and evaporating the first potassium chloride mother liquor in the crystallizer at a second evaporation rate E2, wherein the first stirring speed N1 is less than the second stirring speed N2, and the first evaporation rate E1 is less than the second evaporation rate E2; during the evaporation and crystallization process of the first potassium chloride mother liquor in the crystallizer, a portion of the first potassium chloride mother liquor containing crystals of a preset particle size is discharged from the crystallizer, and a potassium chloride solution with a second preset temperature T2 and a mass fraction greater than that of the first potassium chloride mother liquor is introduced into the crystallizer, wherein the second preset temperature T2 is greater than the first preset temperature T1.

[0031] In this embodiment, the process of potassium chloride forming crystals in the mother liquor is divided into a nucleation stage and a crystal growth stage. When the supersaturation S of the first potassium chloride mother liquor is greater than 1.012, it belongs to the nucleation stage. When the supersaturation S of the first potassium chloride mother liquor is equal to S1, it belongs to the crystal growth stage. The value of S1 is in the range of 1.005≤S1≤1.012.

[0032] A first potassium chloride mother liquor is prepared and then added to a crystallizer (DTB crystallizer, also known as a guide tube and baffle crystallizer). The first potassium chloride mother liquor is heated to a first preset temperature T1. During the nucleation stage, a lower first stirring speed N1 is used to stir the first potassium chloride mother liquor, and a lower first evaporation rate E1 is used for evaporation crystallization. This aims to strictly control the supersaturation of the first potassium chloride mother liquor within the metastable region, effectively suppressing explosive spontaneous nucleation caused by excessive local supersaturation or excessive shear force, thereby significantly reducing the number of initial crystal nuclei and preventing the solute from being divided by a large number of small crystal nuclei, reserving sufficient solute space for subsequent large particle growth. After entering the crystal growth stage, the stirring speed is increased to a second stirring speed N2 and the evaporation rate is increased to a second evaporation rate E2. This provides sufficient mass transfer driving force and supersaturation while maintaining crystal suspension, promoting the rapid directional growth of existing crystal nuclei. In addition, by discharging a portion of the mother liquor containing crystals of a preset particle size during the entire evaporation crystallization process, it is possible to avoid small particles from being mixed into the final product and to eliminate the interference of fine particles on the growth of large particles.

[0033] Simultaneously, a high-temperature potassium chloride solution with a higher mass fraction and a second preset temperature T2 is added to a crystallizer with a lower temperature. Since the second preset temperature T2 is greater than the first preset temperature T1, the solubility of the high-temperature potassium chloride solution decreases upon entering the crystallizer due to the lower temperature, instantly generating a controllable supersaturation. This supersaturation is not used to generate new crystal nuclei, but is absorbed by the large crystal particles remaining in the crystallizer, causing them to continue growing. This significantly narrows the particle size distribution range, ultimately obtaining potassium chloride crystals with large and uniform particle size, resulting in spherical large-particle potassium chloride products, effectively improving product quality. The spherical potassium chloride preparation method of this application can produce potassium chloride crystals with a particle size of 900μm~1200μm, with uniform particle size distribution. 50% of the potassium chloride crystal products have a particle size of 900μm or larger.

[0034] It should be noted that in the method for preparing spherical potassium chloride in this application, a propeller-type stirrer is used for the stirring operation.

[0035] The crystallizer used in the above steps is a DTB crystallizer, also known as a guide tube and baffle crystallizer. The guide tube of the DTB crystallizer houses the crystal growth zone, while the annular channel between the guide tube and the baffle forms the clarification zone. The guide tube separates the different functional areas within the crystallizer. Crystals in the clarification zone settle in the mother liquor and are discharged from the bottom of the crystallizer. The portion of the first potassium chloride mother liquor containing crystals of the preset particle size is discharged from the top of the clarification zone outside the baffle. A discharge valve is located at the bottom of the crystallizer; the extracted product slurry passes through it first. The potassium chloride solution enters the bottom of the crystallizer, allowing the crystals of the preset particle size to float back into the crystal growth zone. Large, qualified crystals are not swept up and settle at the bottom of the crystallizer, simultaneously washing the product. After evaporation, the crystals are finally separated from the outlet. This process ensures the quality and uniformity of the product crystals. DTB crystallizers produce large and uniform crystals, have high production intensity, and are less prone to crystallization buildup within the crystallizer.

[0036] In one embodiment of the present invention, the first preset temperature T1 is in the range of 75℃≤T1≤85℃, and the first preset temperature T1 and the second preset temperature T2 satisfy: 5℃≤T2-T1≤20℃.

[0037] In this embodiment, the first preset temperature T1 is controlled within the high-temperature range of 75°C to 85°C. This not only reduces the risk of spontaneous nucleation caused by temperature fluctuations, making the system more stable in the metastable region and allowing evaporation under high-temperature conditions, but also increases the crystal growth rate, which is beneficial for obtaining large-particle crystals. Secondly, the second preset temperature T2 is limited to be 5°C to 20°C higher than T1. On the one hand, this temperature difference is sufficient to produce a significant solubility difference, and when the high-temperature, high-concentration potassium chloride solution enters the low-temperature crystallizer, it can release sufficient supersaturation as a driving force for the growth of large-particle crystals, ensuring effective crystal growth. On the other hand, controlling the temperature difference within 20°C avoids the sudden increase in temperature in the crystallizer due to excessive temperature difference, which would cause boiling and generate a large number of crystal nuclei, resulting in smaller final product particles.

[0038] It should be noted that the first potassium chloride mother liquor was kept at 75℃~85℃ throughout the entire evaporation and crystallization process.

[0039] In one embodiment of the present invention, the mass fraction W1 of the potassium chloride solution is in the range of 33%≤W1≤35%, and the mass fraction W1 of the potassium chloride solution and the mass fraction W2 of the first potassium chloride mother liquor satisfy: 1%≤W1-W2≤2%.

[0040] In this embodiment, the mass fraction difference between W1 and the first potassium chloride mother liquor W2 is limited to between 1% and 2%. On the one hand, this ensures that when the potassium chloride solution enters the low-temperature crystallizer, the supersaturation generated by the temperature drop and concentration superposition is sufficient to provide the driving force required for crystal growth, thus avoiding crystal growth stagnation. On the other hand, it effectively prevents the local supersaturation from spiked instantaneously due to excessive concentration difference, thereby avoiding the occurrence of new spontaneous nucleation or crystal breakage near the reflux inlet. This ensures that the supersaturation can be smoothly and uniformly transferred to the surface of the suspended large-particle crystals, promoting their uniform growth, and ultimately obtaining a potassium chloride product with a narrow particle size distribution, large particle size, and stable quality.

[0041] In one embodiment of the present invention, the first stirring speed N1 is in the range of 300rpm≤N1≤500rpm, and the second stirring speed N2 is in the range of 500rpm<N2≤700rpm.

[0042] In this embodiment, during the nucleation stage, the first stirring speed N1 is controlled in the lower range of 300~500 rpm to reduce the turbulence intensity and shear stress of the fluid, avoid crystal breakage caused by violent stirring and the resulting secondary nucleation, and ensure uniform mixing of the solution to maintain the consistency of supersaturation distribution. After entering the crystal growth stage, the stirring speed is increased to 500~700 rpm. On the one hand, it provides sufficient circulation and suspension power to prevent large crystal particles from accumulating or agglomerating due to gravity sedimentation, and ensures that the crystal and the mother liquor are in full contact to accelerate the mass transfer process. On the other hand, the moderate shear force helps the micro-wear and shaping of the crystal surface, promoting the formation of spherical particles. At the same time, this speed range is still within the controllable range of the metastable region, and no new significant nucleation will occur. Thus, while ensuring rapid and uniform crystal growth, the large size and uniform distribution of the product particle size are maintained.

[0043] In one embodiment of the present invention, the first evaporation rate E1 is in the range of 50 ml / h ≤ E1 < 70 ml / h, and the second evaporation rate E2 is in the range of 70 ml / h ≤ E2 ≤ 100 ml / h.

[0044] In this embodiment, during the nucleation stage, a lower first evaporation rate E1 is used, which allows the solvent to be removed slowly and the supersaturation of the solution to rise gradually. This avoids the supersaturation from instantly exceeding the metastable limit due to excessively rapid evaporation, thus preventing the generation of a large number of primary crystal nuclei and ensuring the controllability of the number of crystal nuclei. After entering the growth stage, the evaporation rate is increased to a second evaporation rate E2, which provides sufficient solute driving force while maintaining stable system operation, accelerates the diffusion and deposition of potassium chloride molecules to the crystal surface, and promotes rapid crystal growth.

[0045] In one embodiment of the present invention, the vacuum degree of the crystallizer is 15 kPa to 30 kPa. By coordinating the vacuum degree with the evaporation rate, the stability of supersaturation generation is ensured, and secondary nucleation or crystal breakage caused by excessive local supersaturation is prevented. Ultimately, the technical effect of large product particle size, uniform distribution and high sphericity is achieved.

[0046] It should be noted that the range of vacuum values ​​mentioned above includes the values ​​at both ends.

[0047] In one embodiment of the present invention, the preset particle size range is (0 μm, 150 μm).

[0048] In this embodiment, by removing fine crystals of a preset particle size, their competition for resources and nucleation interference with the growth of large particles can be eliminated, while retaining intermediate-sized crystals with certain growth potential. This avoids the problem of insufficient effective seed crystals and slow crystal growth caused by excessive removal, thereby ensuring that the fine crystals are completely removed to maintain particle size uniformity, while ensuring the efficient crystallization process and the continuous growth of large-particle crystals.

[0049] In one embodiment of the present invention, the step of preparing the first potassium chloride mother liquor includes: taking crude potassium solid, the crude potassium solid comprising potassium chloride, sodium chloride, magnesium chloride, calcium chloride and sand, wherein the mass fraction of potassium chloride ranges from 82% to 90%, the mass fraction of sodium chloride ranges from 6% to 12%, the mass fraction of magnesium chloride ranges from 3% to 6%, and the mass fraction of calcium chloride ranges from 0.1% to 0.5%; mixing the crude potassium solid with water to prepare a mixed solution; and filtering the mixed solution to remove the sand in the mixed solution to obtain the first potassium chloride mother liquor.

[0050] In this embodiment, the crude potassium contains 82% to 90% potassium chloride. After dissolving in water, insoluble impurities (sand) are removed by filtration, thus avoiding solid particles that adhere to the crystal surface and affect the purity of the product, and ensuring the cleanliness of the first potassium chloride mother liquor.

[0051] It should be noted that the above-mentioned crude potassium solid is crude potassium chloride product prepared by cold crystallization of low-sodium carnallite from salt lakes.

[0052] In one embodiment of the present invention, during the evaporation and crystallization process of the first potassium chloride mother liquor in the crystallizer, the step of discharging a portion of the first potassium chloride mother liquor containing crystals of a preset particle size from the crystallizer and introducing a potassium chloride solution with a second preset temperature T2 and a mass fraction greater than that of the first potassium chloride mother liquor into the first potassium chloride mother liquor includes: discharging a portion of the first potassium chloride mother liquor containing crystals of a preset particle size from the crystallizer and then introducing it into a dissolving tank, wherein the dissolving tank contains a second potassium chloride mother liquor, the mass fraction of the second potassium chloride mother liquor is equal to the mass fraction of the first potassium chloride mother liquor, and the temperature T3 of the second potassium chloride mother liquor is greater than that of the first preset temperature T1; stirring the second potassium chloride mother liquor at a third stirring speed N3 to dissolve the crystals of the preset particle size in the second potassium chloride mother liquor to form a potassium chloride solution; and introducing the potassium chloride solution from the bottom of the crystallizer to mix it with the first potassium chloride mother liquor.

[0053] In this embodiment, the first potassium chloride mother liquor containing fine crystals of a preset particle size is discharged and fed into a dissolving tank containing a high-temperature second potassium chloride mother liquor. The high temperature and high solubility characteristics are used to quickly dissolve the fine crystals, eliminating the ineffective consumption of the supersaturation of the system by the fine crystals and the risk of them acting as secondary nuclei to trigger explosive nucleation. Secondly, the fine crystals are dissolved in the second potassium chloride mother liquor and then fed into the crystallizer. Because its temperature is higher than that of the first potassium chloride mother liquor in the crystallizer, it can provide a suitable supersaturation for growth. This reflux method not only realizes the resource utilization of the fine crystals (converting them into nutrients for the growth of large particles), but also promotes the fluid circulation in the crystallizer through bottom injection, ensuring that the potassium chloride crystals in the crystallizer are in a suspended state, thereby effectively promoting the continuous and uniform growth of large particle crystals.

[0054] In one embodiment of the present invention, the temperature T3 of the second potassium chloride mother liquor is in the range of 90℃≤T3≤95℃, and the stirring speed N3 is in the range of 200rpm≤N3≤400rpm.

[0055] In this embodiment, the dissolution temperature T3 is controlled in the high-temperature range of 90~95℃. This utilizes the characteristic that the solubility of potassium chloride increases significantly with increasing temperature, which greatly improves the solvent's ability to dissolve fine crystals (≤150μm). This ensures that the discharged fine crystals can be quickly and completely dissolved in the second potassium chloride mother liquor, thereby eliminating the resource competition and nucleation interference of fine crystals on the growth of large particles. Secondly, a third stirring speed N3 of 200~400rpm is used to provide sufficient turbulent mixing to accelerate heat transfer and dissolution mass transfer, ensuring uniform temperature and rapid dissolution of fine crystals.

[0056] In one embodiment of the present invention, the step of discharging a portion of the first potassium chloride mother liquor containing crystals of a predetermined particle size from the crystallizer and then introducing it into a dissolving tank at a first flow rate Q1 includes: in the nucleation stage, discharging a portion of the first potassium chloride mother liquor containing crystals of particle size D1 from the crystallizer at a first flow rate Q1; and in the crystal growth stage, discharging a portion of the first potassium chloride mother liquor containing crystals of particle size D2 from the crystallizer at a second flow rate Q2, wherein the value of D1 ranges from 0 μm < D1 ≤ 100 μm, the value of D2 ranges from 0 μm < D2 ≤ 150 μm, the value of the first flow rate Q1 ranges from 200 ml / min ≤ Q1 ≤ 250 ml / min, and the value of the second flow rate Q2 ranges from 200 ml / min ≤ Q1 ≤ 250 ml / min. The value of flow rate Q2 is in the range of 300 ml / min ≤ Q2 ≤ 380 ml / min; the step of introducing potassium chloride solution from the bottom of the crystallizer includes: in the nucleation stage, introducing potassium chloride solution from the bottom of the crystallizer at a third flow rate Q3; in the crystal growth stage, introducing potassium chloride solution from the bottom of the crystallizer at a fourth flow rate Q4; wherein, the value of the third flow rate Q3 is in the range of 200 ml / min ≤ Q3 ≤ 250 ml / min, the value of the fourth flow rate Q4 is in the range of 300 ml / min ≤ Q4 ≤ 380 ml / min, and the first flow rate Q1 is equal to the third flow rate Q3, and the second flow rate Q2 is equal to the fourth flow rate Q4.

[0057] In this embodiment, discharging a portion of the first potassium chloride mother liquor containing crystals with a particle size of D1 from the crystallizer at a first flow rate of Q1 means discharging a portion of the first potassium chloride mother liquor containing crystals with a particle size of D1 from the crystallizer at a first flow rate of Q1. Discharging a portion of the first potassium chloride mother liquor containing crystals with a particle size of D2 from the crystallizer at a second flow rate of Q2 means discharging a portion of the first potassium chloride mother liquor containing crystals with a particle size of D2 from the crystallizer at a second flow rate of Q2.

[0058] During the nucleation stage, a smaller initial flow rate Q1 (200~250 ml / min) and a stricter particle size limit D1 (≤100 μm) are used to remove extremely fine crystals at a lower rate. This avoids a sharp decrease in the number of effective seed crystals in the crystallizer due to rapid removal of fine crystals, which would affect the nucleation stability. It also prevents material balance fluctuations caused by excessive discharge of the first potassium chloride mother liquor, ensuring a smooth start-up of the nucleation process under low supersaturation. After entering the crystal growth stage, the flow rate is increased to a second flow rate Q2 (300~380 ml / min) and the upper limit of the particle size is relaxed to D1. 2 (≤150μm) can more efficiently remove medium-sized fine crystals generated during the growth process, eliminating their competition for the growth of large particles. At the same time, by simultaneously increasing the reflux flow rate Q4 bottom injection, not only is the material and heat balance in the system maintained, but the fluid circulation at the bottom of the crystallizer is also enhanced, promoting the uniform suspension and contact growth of large crystal particles. By combining the real-time balance of inlet and outlet flow rates, burst nucleation is effectively suppressed, the residence time of large crystal particles is extended, and the product particle size, uniformity and sphericity are significantly improved, achieving a unity of high yield and high quality.

[0059] In one embodiment, the dissolution process of crystals with a preset particle size in the dissolution vessel is carried out under atmospheric pressure stirring, and the third stirring speed N3 is 200 rpm to 300 rpm.

[0060] In one embodiment, the method for preparing spherical potassium chloride further includes solid-liquid separation of the crystallized product. The solid-liquid separation can be performed using various methods commonly used in the art for separating liquids and solids, such as filtration or centrifugation.

[0061] In one embodiment, the solid-liquid separation is achieved by vacuum filtration. The method for preparing spherical potassium chloride further includes obtaining potassium chloride crystals by vacuum filtration when the volume of crystals precipitated in the crystallizer accounts for 10% to 20% of the volume of the first potassium chloride mother liquor.

[0062] In one embodiment, the method for preparing spherical potassium chloride further includes drying the crystallized product at a temperature of 30°C to 60°C for a time of 18 to 60 hours.

[0063] In one embodiment, the composition of the prepared potassium chloride crystals is as follows: potassium chloride mass fraction of 98.59%, sodium chloride mass fraction of 1.22%, magnesium chloride mass fraction of 0.14%, and calcium chloride mass fraction of 0.05%. The purity of the potassium chloride product determined above meets the technical requirement of 95% purity.

[0064] In one embodiment, a portion of the first potassium chloride mother liquor containing crystals of a predetermined particle size is pumped to a dissolving tank via a circulation pump. The crystals of the predetermined particle size are then melted in a second potassium chloride mother liquor within the dissolving tank and pumped to a crystallizer via another circulation pump. The flow rates of the two circulation pumps are 300 ml / min to 380 ml / min.

[0065] In one embodiment, during the nucleation stage, the first evaporation rate E1 is 50 ml / h to 70 ml / h, and the above evaporation rate is maintained for 5 min to 30 min. Then, the first potassium chloride mother liquor is stirred at the first stirring speed N1 to ensure that the crystals in the crystallizer are suspended in the mother liquor. During the crystal growth stage, the circulating pump discharges the fine crystals from the top of the clarification zone outside the crystallizer baffle and continuously enters the dissolving tank for dissolution. The dissolved high-temperature potassium chloride solution is pumped into the bottom of the crystallizer by another internal circulating pump. The pumped potassium chloride solution provides a certain supersaturation for the crystals at the bottom of the crystallizer, allowing the crystals at the bottom of the crystallizer to grow slowly. During the evaporation process, the large crystal particles generated and remaining inside the crystallizer are mechanically worn down by the high-speed stirring blades, gradually grinding the crystals into spherical shapes. When the total volume of the precipitated crystals in the crystallizer accounts for 10% to 20% of the liquid volume (when the volume ratio is low, the crystals have not yet grown; when the volume ratio is high, too many crystals lead to excessive mechanical wear between the crystals, resulting in too many crystal nuclei and generating many small crystals, ultimately leading to uneven crystal size distribution), evaporation is stopped, and finally, large spherical potassium chloride crystals are obtained by filtration.

[0066] Preferably, the concentration of potassium chloride in the potassium chloride feed solution is 20wt%~30wt% (e.g., 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, etc.), more preferably 20wt%~25wt%.

[0067] Example 1:

[0068] Under the conditions of a vacuum degree of 30 kPa in the crystallizer, a first preset temperature T1 of 75℃, and a first stirring speed N1 of 300 rpm during the nucleation stage, the first potassium chloride mother liquor in the crystallizer is stirred. The dissolving tank is at atmospheric pressure, the temperature of the potassium chloride solution T2 is 90℃, the first flow rate Q1 is 200 ml / min, and the first evaporation rate E1 is 60 ml / h. The first potassium chloride mother liquor with a concentration of 25 wt% is subjected to vacuum evaporation crystallization. After crystals begin to form in the crystallizer, the above evaporation rate and stirring rate are maintained for about 10 minutes. During the crystal growth stage, the second stirring speed N2 is 600 rpm, and the second evaporation rate E2 is 70 ml / h. The crystallizer can classify the crystals inside the crystallizer by particle size. The portion of the first potassium chloride mother liquor containing crystals of the preset particle size is discharged from the crystallizer and can be pumped to the dissolving tank for dissolution to obtain a potassium chloride solution. Then, the potassium chloride solution is pumped back to the crystallizer for further evaporation. When the volume of precipitated crystals in the crystallizer accounts for 10~20% of the volume of the liquid, potassium chloride crystals are obtained by filtration. Optical microscope images of the product, such as Figure 2As shown in the figure, the spherical particles are potassium chloride particles. The particle size of the crystal product is 850μm~1000μm, and 50% of the particles in the crystal product have a particle size greater than 900μm.

[0069] Example 2:

[0070] Under the conditions of a crystallizer vacuum of 20 kPa, a first preset temperature T1 of 80℃, and during the nucleation stage, the first potassium chloride mother liquor in the crystallizer is stirred at a first stirring speed N1 of 300 rpm. The dissolving tank is at atmospheric pressure and a temperature of 93℃. The first flow rate Q1 is 200 ml / min, and the first evaporation rate E1 is 60 ml / h. Vacuum evaporation crystallization is performed on the first potassium chloride raw material solution with a concentration of 25 wt%. After crystals begin to form in the crystallizer, the above evaporation and stirring rates are maintained for approximately 10 minutes. During the crystal growth stage, the second stirring speed N2 is 600 rpm, and the second evaporation rate E2 is 80 ml / h. The crystallizer allows for particle size classification of the crystals inside. The portion of the first potassium chloride mother liquor containing crystals of a preset particle size is discharged from the crystallizer and pumped to the dissolving tank for dissolution to obtain a potassium chloride solution. The potassium chloride solution is pumped to the bottom of the crystallizer for further evaporation. When the volume of precipitated crystals in the crystallizer accounts for 10-20% of the liquid volume, potassium chloride crystals are obtained by filtration. An optical microscope image of the product is shown below. Figure 3 As shown in the figure, the spherical particles are potassium chloride particles. The particle size of the crystal product is 950μm~1200μm, and 50% of the particles in the crystal product have a particle size greater than 1000μm.

[0071] Example 3:

[0072] Under the conditions of a crystallizer vacuum of 15 kPa, a first preset temperature T1 of 85℃, and during the nucleation stage, stirring of the first potassium chloride mother liquor in the crystallizer at a first stirring speed N1 of 300 rpm, with the dissolving tank at atmospheric pressure and a temperature of 95℃, a first flow rate Q1 of 200 ml / min, and a first evaporation rate E1 of 60 ml / h, vacuum evaporation crystallization is performed on a 25 wt% first potassium chloride mother liquor. After crystals begin to form in the crystallizer, the above evaporation and stirring rates are maintained for approximately 10 minutes. During the crystal growth stage, a second stirring speed N2 of 600 rpm and a second evaporation rate E2 of 90 ml / h are used. The crystallizer allows for particle size classification of the crystals inside. The portion of the first potassium chloride mother liquor containing crystals of a preset particle size is discharged from the crystallizer and pumped to the dissolving tank for dissolution to obtain a potassium chloride solution. The potassium chloride solution is then pumped to the bottom of the crystallizer for further evaporation. When the volume of precipitated crystals in the crystallizer reaches 10-20% of the liquid volume, potassium chloride crystals are obtained by filtration. An optical microscope image of the product is shown below. Figure 4As shown in the figure, the spherical particles are potassium chloride particles. The particle size of the crystal product is 950μm~1200μm, and 50% of the particles in the crystal product have a particle size greater than 1000μm.

[0073] Comparative Example 1:

[0074] The crystallizer was operated at a vacuum of 30 kPa and a temperature of 75°C. The first potassium chloride mother liquor was stirred at 300 rpm, and the evaporation rate within the crystallizer was 60 ml / h. Vacuum evaporation crystallization was performed on a 25 wt% potassium chloride mother liquor. When the volume of precipitated crystals in the crystallizer reached 10-20% of the liquid volume, potassium chloride crystals were obtained by filtration. An optical microscope image of the product is shown below. Figure 5 As shown, the particle size of the crystal product ranges from 300μm to 900μm, with 50% of the particles having a diameter greater than 500μm. The spherical particles in the figure are potassium chloride particles, and the particle size distribution is uneven.

[0075] Comparative Example 2:

[0076] Under the conditions of a crystallizer vacuum of 20 kPa, a crystallizer temperature of 80℃, and stirring of the first potassium chloride mother liquor at 300 rpm, with an evaporation rate of 60 ml / h within the crystallizer, a 25 wt% first potassium chloride mother liquor was subjected to vacuum evaporation crystallization. When the volume of precipitated crystals in the crystallizer reached 10-20% of the liquid volume, potassium chloride crystals were obtained by filtration. An optical microscope image of the product is shown below. Figure 6 As shown, the particle size of the crystal product ranges from 400μm to 1100μm, with 50% of the particles having a diameter greater than 600μm. The spherical particles in the figure are potassium chloride particles, and the particle size distribution is uneven.

[0077] Comparative Example 3: Under the conditions of a crystallizer vacuum of 15 kPa, a crystallizer temperature of 85℃, and stirring of the first potassium chloride mother liquor at 300 rpm, and an evaporation rate of 60 ml / h in the crystallizer, a 25 wt% first potassium chloride mother liquor was subjected to vacuum evaporation crystallization. When the volume of precipitated crystals in the crystallizer accounted for 10-20% of the liquid volume, potassium chloride crystals were obtained by filtration. An optical microscope image of the product is shown below. Figure 7 As shown, the particle size of the crystal product is 500μm~800μm, and 50% of the particles in the crystal product have a particle size greater than 550μm. The spherical particles in the figure are potassium chloride particles, and the particle size distribution is uneven.

[0078] As can be seen from the above, the crystal products obtained by the spherical potassium chloride preparation method of this application have larger particle size and more uniform particle size.

[0079] As can be seen from the above description, the above embodiments of the present invention achieve the following technical effects: a first potassium chloride mother liquor is prepared, and then the first potassium chloride mother liquor is added into a crystallizer (DTB crystallizer, also known as a guide tube and baffle crystallizer). The first potassium chloride mother liquor is heated to a first preset temperature T1, and a lower first stirring speed N1 is used to stir the first potassium chloride mother liquor and a lower first evaporation rate E1 is used for evaporation and crystallization during the nucleation stage. The aim is to strictly control the supersaturation of the first potassium chloride mother liquor within the metastable region, effectively suppress the explosive spontaneous nucleation caused by excessive local supersaturation or excessive shear force, thereby significantly reducing the number of initial crystal nuclei, avoiding the solute being divided by a large number of small crystal nuclei, and reserving sufficient solute space for subsequent large particle growth. After entering the crystal growth stage, increasing the stirring speed to the second stirring speed N2 and the evaporation rate to the second evaporation rate E2 provides sufficient mass transfer driving force and supersaturation while maintaining crystal suspension, promoting the rapid directional growth of existing crystal nuclei. Furthermore, during the entire evaporation and crystallization process, discharging a portion of the mother liquor containing crystals of a preset particle size avoids small particles from contaminating the final product and eliminates interference from fine particles on the growth of larger particles. Simultaneously, a high-temperature potassium chloride solution with a higher mass fraction at a second preset temperature T2 is added to a lower-temperature crystallizer. Because the second preset temperature T2 is higher than the first preset temperature T1, the solubility of the high-temperature potassium chloride solution decreases upon entering the crystallizer, instantly generating controllable supersaturation. This supersaturation is not used to generate new crystal nuclei but is absorbed by the large crystal particles remaining in the crystallizer, causing them to continue growing. This significantly narrows the particle size distribution range, ultimately obtaining large-sized, uniformly distributed potassium chloride crystals, resulting in spherical large-particle potassium chloride products and effectively improving product quality.

[0080] Obviously, the embodiments described above are merely some, not all, embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0081] The endpoints and any values ​​of the ranges disclosed in this application are not limited to the precise ranges or values, and such ranges or values ​​should be understood to include values ​​close to such ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0082] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0083] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing spherical potassium chloride, characterized in that, include: Preparation of the first potassium chloride mother liquor; Add the first potassium chloride mother liquor to the crystallizer; The first potassium chloride mother liquor is heated to a first preset temperature T1; During the nucleation stage, the first potassium chloride mother liquor is stirred at a first stirring speed N1, and the crystallizer evaporates the first potassium chloride mother liquor at a first evaporation rate E1. During the crystal growth stage, the first potassium chloride mother liquor is stirred at a second stirring speed N2, and the crystallizer evaporates the first potassium chloride mother liquor at a second evaporation rate E2, wherein the first stirring speed N1 is less than the second stirring speed N2, and the first evaporation rate E1 is less than the second evaporation rate E2. During the evaporation and crystallization process of the first potassium chloride mother liquor in the crystallizer, a portion of the first potassium chloride mother liquor containing crystals of a preset particle size is discharged from the crystallizer, and a potassium chloride solution with a second preset temperature T2 and a mass fraction greater than that of the first potassium chloride mother liquor is introduced into the crystallizer, wherein the second preset temperature T2 is greater than the first preset temperature T1.

2. The method for preparing spherical potassium chloride according to claim 1, characterized in that, The first preset temperature T1 has a value range of 75℃≤T1≤85℃, and the first preset temperature T1 and the second preset temperature T2 both satisfy: 5℃≤T2-T1≤20℃.

3. The method for preparing spherical potassium chloride according to claim 1, characterized in that, The mass fraction W1 of the potassium chloride solution is in the range of 33%≤W1≤35%, and the mass fraction W1 of the potassium chloride solution and the mass fraction W2 of the first potassium chloride mother liquor satisfy the following: 1%≤W1-W2≤2%.

4. The method for preparing spherical potassium chloride according to any one of claims 1 to 3, characterized in that, The first stirring speed N1 has a value range of 300rpm≤N1≤500rpm, and the second stirring speed N2 has a value range of 500rpm<N2≤700rpm.

5. The method for preparing spherical potassium chloride according to any one of claims 1 to 3, characterized in that, The first evaporation rate E1 is in the range of 50 ml / h ≤ E1 < 70 ml / h, the second evaporation rate E2 is in the range of 70 ml / h ≤ E2 ≤ 100 ml / h; and / or, the vacuum degree of the crystallizer is 15 kPa to 30 kPa.

6. The method for preparing spherical potassium chloride according to any one of claims 1 to 3, characterized in that, The preset particle size range is (0μm, 150μm).

7. The method for preparing spherical potassium chloride according to any one of claims 1 to 3, characterized in that, The steps for preparing the first potassium chloride mother liquor include: Take crude potassium solid, the composition of which includes potassium chloride, sodium chloride, magnesium chloride, calcium chloride, and sand, wherein the mass fraction of potassium chloride ranges from 82% to 90%, the mass fraction of sodium chloride ranges from 6% to 12%, the mass fraction of magnesium chloride ranges from 3% to 6%, and the mass fraction of calcium chloride ranges from 0.1% to 0.5%. The crude potassium solid was mixed with water to prepare a mixed solution; The mixed solution is filtered to remove the sand and soil, in order to obtain the first potassium chloride mother liquor.

8. The method for preparing spherical potassium chloride according to any one of claims 1 to 3, characterized in that, The steps of evaporating and crystallizing the first potassium chloride mother liquor in the crystallizer, discharging a portion of the first potassium chloride mother liquor containing crystals of a preset particle size from the crystallizer, and introducing a potassium chloride solution with a second preset temperature T2 and a mass fraction greater than that of the first potassium chloride mother liquor into the first potassium chloride mother liquor include: After the portion of the first potassium chloride mother liquor containing crystals of the preset particle size is discharged from the crystallizer, it is introduced into a dissolving tank. The dissolving tank contains a second potassium chloride mother liquor, the mass fraction of the second potassium chloride mother liquor is equal to the mass fraction of the first potassium chloride mother liquor, and the temperature T3 of the second potassium chloride mother liquor is greater than the first preset temperature T1. The second potassium chloride mother liquor is stirred at a third stirring speed N3 to dissolve the crystals of the preset particle size in the second potassium chloride mother liquor to form the potassium chloride solution. The potassium chloride solution is introduced from the bottom of the crystallizer to mix with the first potassium chloride mother liquor.

9. The method for preparing spherical potassium chloride according to claim 8, characterized in that, The temperature T3 of the second potassium chloride mother liquor is in the range of 90℃≤T3≤95℃, and the stirring speed N3 is in the range of 200rpm≤N3≤400rpm.

10. The method for preparing spherical potassium chloride according to claim 8, characterized in that, The step of discharging the portion of the first potassium chloride mother liquor containing crystals of a predetermined particle size from the crystallizer and then introducing it into the dissolving tank at a first flow rate Q1 includes: During the nucleation stage, a portion of the first potassium chloride mother liquor containing crystals with a particle size of D1 is discharged from the crystallizer at a first flow rate of Q1; During the crystal growth stage, a portion of the first potassium chloride mother liquor containing crystals with a particle size of D2 is discharged from the crystallizer at a second flow rate Q2, wherein the value of D1 ranges from 0 μm to 100 μm, the value of D2 ranges from 0 μm to 150 μm, the value of the first flow rate Q1 ranges from 200 ml / min to 250 ml / min, and the value of the second flow rate Q2 ranges from 300 ml / min to 380 ml / min. The step of introducing the potassium chloride solution from the bottom of the crystallizer includes: During the nucleation stage, the potassium chloride solution is introduced from the bottom of the crystallizer at a third flow rate Q3; During the crystal growth stage, the potassium chloride solution is introduced from the bottom of the crystallizer at a fourth flow rate Q4; The value range of the third flow rate Q3 is 200ml / min≤Q3≤250ml / min, the value range of the fourth flow rate Q4 is 300ml / min≤Q4≤380ml / min, and the first flow rate Q1 is equal to the third flow rate Q3, and the second flow rate Q2 is equal to the fourth flow rate Q4.

Images