Method for preparing calcium chloride dihydrate through high-temperature crystallization

Abstract

The invention relates to the technical field of calcium chloride production and preparation, in particular to a method for preparing calcium chloride dihydrate through high-temperature devitrification, which comprises the following steps: S1, concentrating an impurity-removed calcium chloride solution at high temperature under a reduced pressure condition until the content of calcium chloride is 60-65% to obtain a calcium chloride concentrated solution; s2, heating the calcium chloride concentrated solution to a first temperature, and adding a compound additive to obtain a calcium chloride solution containing the additive; s3, cooling, stirring and crystallizing the calcium chloride solution containing the aid in stages to obtain calcium chloride dihydrate crystal mush; s4, carrying out heat preservation and filtration on the calcium chloride dihydrate crystal mush to obtain a calcium chloride dihydrate filter cake; and S5, drying the calcium chloride dihydrate filter cake at low temperature under a vacuum condition to obtain a calcium chloride dihydrate finished product. According to the technical scheme, good separation of calcium chloride dihydrate and other hydrates is realized, and the calcium chloride dihydrate with relatively high purity and yield is prepared.

Description

Technical Field

[0001] This invention relates to the field of calcium chloride production and preparation technology, and in particular to a method for preparing calcium chloride dihydrate by high-temperature crystallization. Background Technology

[0002] Calcium chloride is an important inorganic chemical product widely used in food, medicine, and industry. Among them, calcium chloride dihydrate (CaCl2·2H2O) plays a crucial role in high-end applications such as pharmaceutical calcium supplements and food additives due to its stable crystalline morphology and excellent physicochemical properties. The key to preparing calcium chloride dihydrate lies in precisely controlling the temperature and concentration conditions during the crystallization process to obtain a product with high purity and uniform particle size.

[0003] Calcium chloride exists in various hydrate forms, including hexahydrate (CaCl2·6H2O), tetrahydrate (CaCl2·4H2O), dihydrate (CaCl2·2H2O), and monohydrate (CaCl2·H2O), with different stable temperature ranges. According to the CaCl2·H2O phase diagram, the stable existence conditions for dihydrate calcium chloride are approximately 45-175℃. In traditional production processes, calcium chloride solution often requires complex equipment such as spray granulators, fluidized bed dryers, or slicers to complete the conversion from concentrate to solid product after high-temperature concentration. For example, Chinese patent CN202322260282.6 (A calcium chloride dihydrate production system) achieves the concentration to drying process by sequentially connecting a multi-effect evaporation concentration system and a rotary fluidized bed granulator through pipelines. However, this process requires the preparation of high-temperature, high-concentration calcium chloride concentrate, resulting in high energy consumption and potential for excessive dust levels in the environment. Chinese patent CN202322117737.9 (A production apparatus for calcium chloride dihydrate) describes a method that involves introducing the concentrated solution into a slicer for slicing, followed by drying in a subsequent drying system. This method is energy-intensive. Chinese patent CN109534381B (A calcium chloride production process) describes a method that directly produces calcium chloride dihydrate from a calcium chloride solution through primary and secondary concentration devices. However, these methods not only involve large equipment investments and high energy consumption, but also the high drying temperature can easily lead to uncontrolled hydrate morphology, resulting in a mixture of dihydrate, tetrahydrate, and monohydrate hydrates in the product, making it difficult to guarantee purity and uniformity. Summary of the Invention

[0004] In view of this, the present invention proposes a method for preparing calcium chloride dihydrate by high-temperature crystallization, in order to solve the problems of existing separation and purification methods for calcium chloride dihydrate, which involve large equipment investment, high energy consumption, and excessively high drying temperature that can easily lead to uncontrolled hydrate morphology, resulting in a mixture of dihydrate, tetrahydrate, and monohydrate in the product, making it difficult to guarantee purity and uniformity.

[0005] The technical solution of this invention is achieved as follows: This invention provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, comprising the following steps: S1. The purified calcium chloride solution is concentrated at high temperature under reduced pressure until the calcium chloride content is 60-65% to obtain calcium chloride concentrate. S2. Heat the calcium chloride concentrate to the first temperature, add the composite additive, and obtain a calcium chloride solution containing the additive; S3. The calcium chloride solution containing the additive is cooled and stirred in stages to crystallize, resulting in calcium chloride dihydrate slurry. S4. The calcium chloride dihydrate slurry is kept warm and filtered to obtain calcium chloride dihydrate filter cake; S5. Dry the calcium chloride dihydrate filter cake under vacuum conditions at low temperature to obtain the finished calcium chloride dihydrate product.

[0006] Specifically, firstly, high-temperature and low-pressure concentration is used to achieve sufficient supersaturation in the calcium chloride solution to drive crystallization. Then, a composite additive is added to improve the selectivity of subsequent calcium chloride dihydrate precipitation and inhibit the formation of other hydrates. Finally, a staged cooling and stirring crystallization strategy is employed. First, a rapid cooling rate is used to quickly cross the unstable region of the tetrahydrate. Then, a slower cooling rate is used to precisely control the supersaturation, allowing calcium chloride dihydrate to precipitate slowly and grow sufficiently. Finally, constant-temperature crystal growth in a low-temperature region dissolves small crystals, which then re-precipitate on the surface of large crystals. This staged cooling strategy is designed based on the stable temperature ranges of different hydrates in the calcium chloride hydrate phase diagram, ensuring that the crystallization process remains within the stable region of the dihydrate throughout, fundamentally avoiding the mixing of tetrahydrate, hexahydrate, or monohydrate. After insulated filtration and vacuum drying, a high-purity, uniformly sized, and precisely controlled water of crystallization dihydrate product is obtained.

[0007] Based on the above technical solutions, preferably, in step S1, the temperature of high-temperature vacuum concentration is 80~90℃ and the pressure is ≤-0.09Mpa.

[0008] Based on the above technical solutions, preferably, in step S2, the first temperature is 110~115℃, and the composite additives include sodium chloride and calcium chloride dihydrate seed crystals.

[0009] Based on the above technical solutions, preferably, in the composite additive, the amount of sodium chloride added is 1-2% of the mass of calcium chloride after impurity removal, and the amount of calcium chloride dihydrate seed crystals added is 0.8-1.2% of the mass of calcium chloride after impurity removal.

[0010] Specifically, the calcium chloride concentrate is heated to 110-115℃ for dissolution. This temperature range is much higher than the upper stability limit temperature of calcium chloride tetrahydrate (45.3℃) and is in the center of the stability region of calcium chloride dihydrate. This ensures complete dissolution and uniform dispersion of the composite additive, while eliminating any possible microcrystal nuclei and localized concentration inconsistencies in the concentrate, providing a homogenized initial solution environment for subsequent controllable crystallization. In the composite additive, the amount of sodium chloride added is controlled at 1-2% of the mass of calcium chloride after impurity removal. This promotes the reduction of the solubility of calcium chloride tetrahydrate and hexahydrate by sodium and chloride ions through common ion effects and salting-out, while also increasing the ionic strength of the solution, thus increasing the supersaturation of calcium chloride dihydrate and enhancing the driving force for dihydrate precipitation. The amount of calcium chloride dihydrate seed crystals added is 0.8-1.2%. This provides a sufficient number of heterogeneous nucleation sites to induce calcium and chloride ions in the solution to preferentially arrange themselves on the seed crystal surface to form calcium chloride dihydrate crystals, while avoiding excessive seed crystals that would lead to overly dense nucleation sites and excessively small crystal size.

[0011] Based on the above technical solutions, preferably, step S3 specifically includes: S31. The calcium chloride solution containing the additive is rapidly cooled to the second temperature at the first cooling rate, while maintaining a stirring rate of 200-300 rpm during the process. The cooling time is controlled at 10-15 min to obtain the first solution. S32. Then, the first solution is slowly cooled to the third temperature at the second cooling rate, while maintaining a stirring rate of 200-250 rpm during the process. The cooling time is controlled at 15-20 min to obtain a preliminary crystal slurry. The first cooling rate is greater than the second cooling rate. S33. The initial crystal slurry is kept at a constant temperature under a third temperature, and the stirring rate is reduced to 150-180 rpm to obtain calcium chloride dihydrate crystal slurry.

[0012] Based on the above technical solution, preferably, in step S31, the first cooling rate is 2-3℃ / min and the second temperature is 75-85℃.

[0013] Based on the above technical solution, preferably, in step S32, the second cooling rate is 0.5-1.0℃ / min, and the third temperature is 65-70℃.

[0014] Based on the above technical solutions, preferably, magnesium chloride hexahydrate is added within 3-5 minutes of cooling, and the amount of magnesium chloride hexahydrate added is 0.3-0.5% of the mass of calcium chloride after impurity removal.

[0015] Based on the above technical solutions, preferably, in step S33, the constant temperature crystal growth time is 60-90 min.

[0016] In step S3, a three-stage cooling strategy of "rapid cooling - slow cooling - constant temperature crystal growth" is adopted. The purpose of rapid cooling in step S31 is to quickly cross the unstable region of calcium chloride tetrahydrate, avoid local overcooling leading to tetrahydrate nucleation, and establish the appropriate supersaturation required for calcium chloride dihydrate without triggering explosive nucleation. In step S32, the temperature is slowly lowered, and magnesium chloride hexahydrate is added at the beginning of the cooling process. At this time, after the rapid cooling in S31, the calcium chloride dihydrate crystal nuclei have just formed but have not yet grown in large quantities. Magnesium ions can be adsorbed on the freshly grown crystal surface in time to play a role in morphology regulation. Moreover, the adsorption of magnesium ions on the crystal surface is more stable in the temperature range of 65-75℃. At the same time, it avoids the partial dehydration of magnesium chloride hexahydrate that may be caused by prolonged exposure at high temperature of 110-115℃. Magnesium ions and calcium ions have similar ionic radii and chemical properties, and can selectively adsorb on calcium chloride dihydrate crystals, regulating and promoting the formation of large crystals with uniform particles. At the same time, the adsorption of magnesium ions can inhibit the explosive nucleation phenomenon, making the crystallization process more stable and controllable. In addition, the slow cooling process precisely controls the rate of increase of supersaturation of the solution, so that calcium chloride dihydrate precipitates and grows slowly and orderly under the regulation of magnesium ions. The gradual increase of supersaturation avoids the generation of a large number of new crystal nuclei, which is conducive to maintaining the integrity of the crystal morphology. In step S33, the isothermal crystal growth process is carried out at the center of the stable region of calcium chloride dihydrate. Under isothermal conditions, the tiny crystals in the solution gradually dissolve due to their relatively high solubility. The dissolved calcium ions and chloride ions migrate to the surface of the large crystals and re-precipitate under the drive of the concentration gradient. This Austeryl ripening process realizes the mass transfer of "small crystal dissolution - large crystal growth", which significantly improves the uniformity of crystal particle size distribution.

[0017] Based on the above technical solutions, preferably, in step S4, the heat preservation and filtration temperature is 65-70℃.

[0018] Based on the above technical solutions, preferably, in step S5, the low-temperature vacuum drying conditions are a drying temperature of 50-60℃, a vacuum degree of ≤-0.09 MPa, and a drying time of 10-20h.

[0019] The high-temperature crystallization method for preparing calcium chloride dihydrate of the present invention has the following advantages over the prior art: (1) This invention achieves highly selective preparation of calcium chloride dihydrate through the synergistic effect of composite additives and staged cooling. Sodium chloride and calcium chloride dihydrate seed crystals establish the thermodynamic and kinetic basis of "salting out-induced nucleation" at high temperature. Then, rapid cooling crosses the unstable region of tetrahydrate. Magnesium chloride hexahydrate added at the beginning of slow cooling captures the golden period of crystal growth in time. It controls the crystal morphology and inhibits explosive nucleation through selective adsorption on crystal faces. The three work together to make full use of the timing of different additives at different stages of crystallization. Sodium chloride and seed crystals ensure the selectivity and timeliness of nucleation. Magnesium chloride hexahydrate intervenes in the crystal growth stage to precisely control the crystal quality. Staged cooling provides the best temperature window for the action of each additive. The synergistic effect significantly improves the precipitation selectivity and crystal uniformity of calcium chloride dihydrate, avoids the mixing of other hydrates, and obtains calcium chloride dihydrate products with high purity and uniform particle size.

[0020] (2) In the composite additive, sodium chloride first establishes a high supersaturation thermodynamic drive through the salting-out effect, creating sufficient conditions for the precipitation of calcium chloride dihydrate; the calcium chloride dihydrate seed crystals rapidly exert heterogeneous nucleation induction in the high supersaturation solution, guiding calcium ions and chloride ions to arrange themselves in an orderly manner on the seed crystal surface according to the crystal lattice structure of the dihydrate, avoiding the mixing of multiple hydrates caused by random primary nucleation in the solution. The two form a two-level synergistic regulation system of "salting-out driving force - seed crystal direction setting", which lays the foundation for the selective precipitation of calcium chloride dihydrate during the subsequent staged cooling crystallization process.

[0021] (3) During the phased cooling process, the three stages work together to achieve a progressive crystallization control from "rapid cross-region avoidance of impurities" to "magnesium ion guidance + slow and controllable growth" and then to "constant temperature crystal growth for better uniformity". Together with the previous sodium chloride salting-out and seed induction, a complete five-level synergistic control system of "salting-out driving force - seed orientation - rapid cross-region - magnesium ion morphology control - constant temperature crystal growth" is formed. This effectively solves the problems of calcium chloride dihydrate mixed with other hydrates, small crystal size and uneven distribution in the traditional single cooling process, and obtains high purity, large particles and uniform particle size calcium chloride dihydrate crystals. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a flowchart of the method for preparing calcium chloride dihydrate by high-temperature crystallization according to the present invention; Figure 2The CaCl2-H2O phase diagram of this invention. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 Depend on Figure 1 As can be seen, this embodiment provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, including the following steps: S1. The purified calcium chloride solution (containing 50g of calcium chloride) is concentrated to a calcium chloride concentrate with a mass fraction of 63% under conditions of -0.09 MPa and 85℃. S2. Heat the concentrated calcium chloride solution to 113℃ and stir until dissolved. Then add 1.5% sodium chloride and 1.0% calcium chloride dihydrate seed crystals (particle size 200-400 mesh) based on the mass of calcium chloride. Stir continuously at 350 rpm for 37 minutes to fully dissolve and disperse the composite additives evenly, and obtain a calcium chloride solution containing the additives. S3. The calcium chloride solution containing the additive is rapidly cooled to 80°C at a cooling rate of 2.5°C / min, while maintaining a stirring rate of 250 rpm. The cooling time is controlled within 13 min. Then, the solution is slowly cooled to 68°C at a cooling rate of 0.8°C / min, while maintaining a stirring rate of 230 rpm. The cooling time is controlled within 18 min. Magnesium chloride hexahydrate is added 4 min after cooling. The amount of magnesium chloride hexahydrate added is 0.4% of the mass of calcium chloride. Finally, the solution is kept at 68°C for 75 min, and the stirring rate is reduced to 160 rpm to obtain calcium chloride dihydrate slurry. S4. The calcium chloride dihydrate slurry was vacuum filtered at 68°C for 20 minutes to obtain calcium chloride dihydrate filter cake. S5. The calcium chloride dihydrate filter cake is dried at 58℃ and vacuum degree ≤ -0.09 MPa for 15h to obtain the finished calcium chloride dihydrate product.

[0026] Example 2 This embodiment provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, including the following steps: S1. The purified calcium chloride solution (containing 50g of calcium chloride) is concentrated to a calcium chloride concentrate with a mass fraction of 60% under conditions of -0.08 MPa and 80℃. S2. Heat the concentrated calcium chloride solution to 110℃ and stir until dissolved. Then add 1% sodium chloride and 0.8% calcium chloride dihydrate seed crystals (particle size 200-400 mesh) based on the mass of calcium chloride. Stir continuously at 300 rpm for 45 minutes to fully dissolve and disperse the composite additives evenly, and obtain a calcium chloride solution containing the additives. S3. The calcium chloride solution containing the additive is rapidly cooled to 75°C at a cooling rate of 2°C / min, while maintaining a stirring rate of 200 rpm during this process, and the cooling time is controlled within 15 min. Then, it is slowly cooled to 65°C at a cooling rate of 0.5°C / min, while maintaining a stirring rate of 200 rpm during this process, and the cooling time is controlled within 20 min. Magnesium chloride hexahydrate is added 3 min after cooling, and the amount of magnesium chloride hexahydrate added is 0.3% of the mass of calcium chloride. Finally, the temperature is kept constant at 65°C for 90 min, and the stirring rate is reduced to 150 rpm to obtain calcium chloride dihydrate slurry. S4. The calcium chloride dihydrate slurry was vacuum filtered at 65°C for 25 minutes to obtain calcium chloride dihydrate filter cake. S5. The calcium chloride dihydrate filter cake is dried at 55℃ and vacuum degree ≤ -0.09 MPa for 18h to obtain the finished calcium chloride dihydrate product.

[0027] Example 3 This embodiment provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, including the following steps: S1. The purified calcium chloride solution (containing 50g of calcium chloride) is concentrated to a calcium chloride concentrate with a mass fraction of 65% under conditions of -0.07 MPa and 90℃. S2. Heat the concentrated calcium chloride solution to 115℃ and stir until dissolved. Then add 2% sodium chloride and 1.2% calcium chloride dihydrate seed crystals (400 mesh) based on the mass of calcium chloride. Stir continuously at 400 rpm for 30 minutes to fully dissolve and disperse the composite additives evenly, and obtain a calcium chloride solution containing the additives. S3. The calcium chloride solution containing the additive is rapidly cooled to 85°C at a cooling rate of 3°C / min, while maintaining a stirring rate of 300 rpm during this process, and the cooling time is controlled within 10 min. Then, it is slowly cooled to 70°C at a cooling rate of 1.0°C / min, while maintaining a stirring rate of 250 rpm during this process, and the cooling time is controlled within 15 min. Magnesium chloride hexahydrate is added within 5 min of cooling, and the amount of magnesium chloride hexahydrate added is 0.5% of the mass of calcium chloride. Finally, the temperature is kept constant at 70°C for 60 min, and the stirring rate is reduced to 180 rpm to obtain calcium chloride dihydrate slurry. S4. The calcium chloride dihydrate slurry was vacuum filtered at 70°C for 15 minutes to obtain calcium chloride dihydrate filter cake. S5. The calcium chloride dihydrate filter cake is dried at 60℃ and vacuum degree ≤ -0.09 MPa for 12h to obtain the finished calcium chloride dihydrate product.

[0028] Comparative Example 1 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, the method being the same as in Example 1, except that sodium chloride is not added in step S2, i.e. S2. Heat the concentrated calcium chloride solution to 113℃ and stir until dissolved. Then add 5% (by weight of calcium chloride) of dihydrate calcium chloride seed crystals (particle size 200-400 mesh) and stir continuously at 350 rpm for 37 minutes to fully dissolve and disperse the composite additive, thus obtaining a calcium chloride solution containing the additive.

[0029] Comparative Example 2 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, which is the same as in Example 1, except that the amount of sodium chloride added in step S2 is 2.5%.

[0030] Comparative Example 3 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, the method being the same as in Example 1, except that calcium chloride dihydrate seeds are not added in step S2, i.e. S2. Heat the concentrated calcium chloride solution to 113℃ and stir until dissolved. Then add sodium chloride at 1.5% of the mass of calcium chloride and stir continuously at 350 rpm for 37 minutes to fully dissolve and disperse the composite additive, thus obtaining a calcium chloride solution containing the additive.

[0031] Comparative Example 4 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, which is the same as in Example 1, except that the amount of calcium chloride dihydrate seed crystals added in step S2 is 1.4%.

[0032] Comparative Example 5 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, which is the same as in Example 1, except that magnesium chloride hexahydrate is not added in step S32.

[0033] Comparative Example 6 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, the method being the same as in Example 1, except that magnesium chloride hexahydrate is added in step S2, i.e. S2. Heat the concentrated calcium chloride solution to 113℃ and stir until dissolved. Then, add 1.5% sodium chloride, 1.0% calcium chloride dihydrate seed crystals (particle size 200-400 mesh), and 0.4% magnesium chloride hexahydrate by weight of calcium chloride in sequence. Stir continuously at 350 rpm for 37 minutes to fully dissolve and disperse the composite additives evenly, and obtain a calcium chloride solution containing the additives.

[0034] Comparative Example 7 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, which is the same as in Example 1, except that the amount of magnesium chloride hexahydrate added in step S32 is 0.6%.

[0035] Comparative Example 8 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, the method being the same as in Example 1, except that segmented cooling is not performed in step S3, i.e. S3. The calcium chloride solution containing the additive is rapidly cooled to 68°C at a cooling rate of 2.5°C / min, while maintaining a stirring rate of 250 rpm. The cooling time is 31 min. Magnesium chloride hexahydrate is added 4 min after cooling. The amount of magnesium chloride hexahydrate added is 0.4% of the mass of calcium chloride. Finally, the temperature is kept constant at 68°C for 75 min, and the stirring rate is reduced to 160 rpm to obtain calcium chloride dihydrate slurry.

[0036] Comparative Example 9 This comparative example provides a method for preparing calcium chloride dihydrate by high-temperature crystallization, the method being the same as in Example 1, except that segmented cooling is not performed in step S3, i.e. S3. The calcium chloride solution containing the additive is rapidly cooled to 68°C at a cooling rate of 0.8°C / min, while maintaining a stirring rate of 230 rpm during the process. The cooling time is 106 min. Magnesium chloride hexahydrate is added 4 min after cooling. The amount of magnesium chloride hexahydrate added is 0.4% of the mass of calcium chloride, to obtain calcium chloride dihydrate slurry.

[0037] Performance testing The content of calcium chloride dihydrate in the finished product was tested according to the 2020 edition (11th edition) of the Chinese Pharmacopoeia, and the contents of magnesium salts and alkali metal salts in the finished product were also tested. The test results are shown in Table 1.

[0038] Table 1 Performance Test Results

[0039] Calcium chloride forms various types of bound water, with the following compositions: hexahydrate CaCl₂·6H₂O; tetrahydrate CaCl₂·4H₂O; dihydrate CaCl₂·2H₂O; and monohydrate CaCl₂·H₂O. The formation of bound water is determined by temperature. For example... Figure 2As shown in the CaCl2-H2O phase diagram, it can be seen from the phase diagram that the conditions for the existence of dihydrate calcium chloride in the CaCl2-H2O binary compound with a calcium chloride weight of 58%~75% are approximately 50~175℃. If the temperature is below 50℃ or the calcium chloride concentration is below 58%, calcium chloride tetrahydrate will crystallize and precipitate. If the temperature is above 175℃ or the concentration is greater than 75%, calcium chloride monohydrate will also crystallize and precipitate.

[0040] As shown in Table 1, the calcium chloride dihydrate prepared by the technical solution of this invention has high purity and yield, achieving good separation of calcium chloride dihydrate from tetrahydrate, hexahydrate, and monohydrate. As shown in Example 1 and Comparative Examples 1-2, the lack of sodium chloride salting-out effect weakens the precipitation driving force, resulting in some tetrahydrate and hexahydrate contaminating the product and reducing the purity and selectivity of calcium chloride dihydrate. Excessive sodium chloride addition leads to excessive salting-out, causing excessive supersaturation of the solution, decreasing the purity of calcium chloride dihydrate, and excessive microcrystals reduce filtration efficiency and yield. As shown in Example 1 and Comparative Examples 3-4, the lack of heterogeneous nucleation centers or excessive seed crystal addition both affect the product yield and purity. As shown in Example 1 and Comparative Examples 5-7, the lack of magnesium ion crystal facet selective adsorption regulation or excessive magnesium ion addition also affects purification efficiency and yield. Prolonged exposure of magnesium ions at 110-115℃ reduces the effective magnesium ion concentration, weakening its crystal form regulation effect, resulting in lower purity and yield than in Example 1. As can be seen from Example 1 and Comparative Examples 8-9, both rapid and slow cooling throughout the process will result in the partial mixing of tetrahydrate and hexahydrate, leading to a significant decrease in purity and yield.

[0041] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing calcium chloride dihydrate by high-temperature crystallization, characterized in that, Includes the following steps: S1. The purified calcium chloride solution is concentrated at high temperature under reduced pressure until the calcium chloride content is 60-65% to obtain calcium chloride concentrate. S2. Heat the calcium chloride concentrate to the first temperature, add the composite additive, and obtain a calcium chloride solution containing the additive; S3. The calcium chloride solution containing the additive is cooled and stirred in stages to crystallize, resulting in calcium chloride dihydrate slurry. S4. The calcium chloride dihydrate slurry is kept warm and filtered to obtain calcium chloride dihydrate filter cake; S5. Dry the calcium chloride dihydrate filter cake under vacuum conditions at low temperature to obtain the finished calcium chloride dihydrate product.

2. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 1, characterized in that: In step S1, the high-temperature vacuum concentration is performed at a temperature of 80~90℃ and a pressure of ≤-0.09Mpa.

3. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 1, characterized in that: In step S2, the first temperature is 110~115℃, and the composite additives include sodium chloride and calcium chloride dihydrate seed crystals.

4. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 3, characterized in that: In the composite additive, the amount of sodium chloride added is 1-2% of the mass of calcium chloride after impurity removal, and the amount of calcium chloride dihydrate seed crystals added is 0.8-1.2% of the mass of calcium chloride after impurity removal.

5. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 1, characterized in that: Step S3 specifically includes: S31. The calcium chloride solution containing the auxiliary agent is rapidly cooled to the second temperature at the first cooling rate, while maintaining a stirring rate of 200-300 rpm during the process. The cooling time is controlled at 10-15 min to obtain the first solution. S32. Then, the first solution is slowly cooled to the third temperature at the second cooling rate, while maintaining a stirring rate of 200-250 rpm during the process. The cooling time is controlled at 15-20 min to obtain a preliminary crystal slurry. The first cooling rate is greater than the second cooling rate. S33. The initial crystal slurry is kept at a constant temperature for 60-90 minutes at the third temperature, and the stirring rate is reduced to 150-180 rpm to obtain calcium chloride dihydrate crystal slurry.

6. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 5, characterized in that: In step S31, the first cooling rate is 2-3℃ / min, and the second temperature is 75-85℃.

7. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 5, characterized in that: In step S32, the second cooling rate is 0.5-1.0℃ / min, and the third temperature is 65-70℃.

8. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 5, characterized in that: Step S32 also includes: adding magnesium chloride hexahydrate within 3-5 minutes of cooling, wherein the amount of magnesium chloride hexahydrate added is 0.3-0.5% of the mass of calcium chloride after impurity removal.

9. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 1, characterized in that: In step S4, the heat preservation and filtration temperature is 65-70℃.

10. The method for preparing calcium chloride dihydrate by high-temperature crystallization as described in claim 1, characterized in that: In step S5, the low-temperature drying conditions are a drying temperature of 50-60℃, a vacuum degree of ≤-0.09 MPa, and a drying time of 10-20h.

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