Refrigerant direct cooling forced freezing crystallization process for sulfate solution

Abstract

The invention relates to a refrigerant direct-cooling forced freezing crystallization process for a sulfate solution. The process comprises the following steps: primary cooling: directly mixing a high-temperature sulfate solution at 95-100 DEG C with a primary mother solution to form a cooled mother solution; the cooled mother liquor is divided into a cooled mother liquor C1 and a cooled mother liquor C2, the cooled mother liquor C1 is cooled by cooling water to form primary mother liquor, and the primary mother liquor is circulated to primary cooling; first-stage freezing: after cooling, performing first-stage freezing on the mother liquor C2, and mixing the mother liquor C2 with a low-temperature sulfate solution at 0-15 DEG C for crystallization to obtain a mixed material; freezing the mixed material through liquid Freon to form a low-temperature sulfate solution; the sulfate solution is a sodium sulfate solution or a potassium sulfate solution.

Description

[0001] This application is a divisional application of application number 202410990247.6, application date 2024.07.23, and the patent name is: "A refrigerant direct cooling forced freezing crystallization process". Technical Field

[0002] The invention belongs to the technical field of freezing crystallization, and in particular relates to a refrigerant direct cooling forced freezing crystallization process. Background Art

[0003] Freeze crystallization is a common technique used in the chemical and biopharmaceutical industries to purify substances from solutions or prepare crystals. This process exploits the reduced solubility of substances at low temperatures. By controlling the temperature and solute concentration in the solution, the target substance is crystallized and precipitated.

[0004] In the traditional freeze crystallization process, the high-temperature material needs to be quickly cooled to below the crystallization temperature, which usually requires a large amount of cooling energy. However, due to the large temperature difference, there is a situation of uneven energy transfer during the cooling process, resulting in some energy being wasted. In addition, due to the large temperature difference, more energy is required to maintain the stable operation of the cooling system, and the large temperature difference can easily lead to scaling and wall formation of heat exchange equipment, and poor system stability. In summary, the freeze crystallization process in the prior art has the problems of high energy consumption and low freezing efficiency. Summary of the Invention

[0005] The purpose of the present invention is to provide a refrigerant direct cooling forced freezing crystallization process, which directly mixes the cold and hot materials in the process steps with relatively large heat exchange, and can utilize the temperature difference between the materials for heat exchange, thereby realizing effective energy utilization and improving efficiency; for steps with relatively small heat exchange, cooling water, Freon and other refrigerants are used for freezing or cooling, which can more accurately control the temperature and ensure the stable operation of the process, effectively solving the problems raised in the background technology.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions: The present invention provides a refrigerant direct cooling forced freezing crystallization process, which includes a first-stage cooling and a first-stage freezing process. In the first-stage cooling process, a high-temperature material is directly mixed with a first-stage mother liquor to form a cooled mother liquor. A portion of the cooled mother liquor is cooled by cooling water to form a first-stage mother liquor, which is returned to the first-stage cooling process to cool the high-temperature material, and is recorded as the cooled mother liquor C1. Another portion of the cooled mother liquor enters the first-stage freezing process, which is recorded as the cooled mother liquor C2, and is directly mixed with a low-temperature material to precipitate crystals to obtain a mixed material. The mixed material is frozen by Freon to form a low-temperature material, and the cooled mother liquor C2 entering the first-stage freezing process is frozen. In the first-stage cooling, the temperature difference between the high-temperature material and the first-stage mother liquor is 40-60°C, the temperature difference between the high-temperature material and the cooled mother liquor is 40-60°C, and the temperature difference between the cooled mother liquor and the first-stage mother liquor is 1-3°C; In the primary freezing, the temperature difference between the cooled mother liquor C2 and the low-temperature material entering the primary freezing is 25-40°C, the temperature difference between the cooled mother liquor C2 and the mixed material entering the primary freezing is 25-40°C, and the temperature difference between the mixed material and the low-temperature material is 1-3°C; Among them, liquid Freon is used directly as a refrigerant, and the mixed material is frozen into a low-temperature material through the absorption of heat during the conversion of liquid Freon to gaseous Freon.

[0007] Furthermore, the temperature of the high-temperature material is 95-100°C, and the temperature of the mixed material is 0-15°C.

[0008] In the present invention, the high-temperature material is frozen until the mixed material precipitates crystals, and the temperature of the material needs to be reduced from 95~100℃ to 0~15℃. The temperature difference is too large. If the traditional freezing crystallization mode is adopted, not only a multi-stage heat exchanger is required, but also the heat exchange ratio of each stage is very large, which requires a heat exchanger with a larger heat transfer ratio and a larger heat exchange area. At the same time, due to the large temperature difference of the material, it will cause changes in the viscosity and density of the material, increase the heat transfer resistance, reduce the heat transfer efficiency, and face a greater risk of condensation.

[0009] In the present invention, in both the first-stage cooling and the first-stage freezing, a process step of directly mixing hot and cold materials is adopted, which avoids heat transfer between two materials with a large temperature difference in the heat exchanger, effectively improves the heat exchange efficiency, reduces material loss, and reduces the risk of condensation; and for materials with a small temperature difference, direct cooling or direct freezing is adopted by the heat exchanger, which improves the freezing crystallization efficiency while avoiding the use of a heat exchanger with a large heat exchange area, saving equipment costs.

[0010] The temperature difference between the high-temperature material and the first-stage mother liquor in the first-stage cooling provided by the present invention is 40-60° C., which can make the material close to a saturated solution after cooling, which is conducive to the cooling and crystal precipitation of the first-stage freezing, and improves the crystal yield; the temperature difference between the cooled mother liquor entering the first-stage freezing and the low-temperature material is set to 25-40° C., which can also make the concentrated liquid close to saturation directly cooled to a low temperature that is conducive to crystal precipitation, and most salts obtained by freezing crystallization have a large solubility change within this temperature range, which is conducive to crystal precipitation and improves the crystal yield.

[0011] Furthermore, the temperature difference between liquid Freon and gaseous Freon is 0°C.

[0012] Furthermore, the gaseous Freon is converted into liquid Freon through evaporative cooling and returned to the tertiary refrigeration for use as a refrigerant.

[0013] In the first-stage freezing provided by the present invention, Freon is used as a refrigerant to directly freeze the mixed material into a low-temperature material. In this process, Freon only undergoes a phase change without a temperature change, making the heat transfer ratio of the heat exchanger easier to control and preventing the low-temperature material from crystallizing in the heat exchanger, thereby avoiding the heat exchanger structure from affecting the heat exchange efficiency.

[0014] Furthermore, the flow ratio of the cooled mother liquor C1 to the cooled mother liquor is a / (a+b), where a is the temperature difference between the high-temperature material and the cooled mother liquor, in degrees Celsius; and b is the temperature difference between the cooled mother liquor and the primary mother liquor, in degrees Celsius. In the present invention, the cooled mother liquor produced by the primary cooling process is divided into two parts. One part, after cooling, serves as a refrigerant to freeze the high-temperature material, while the other part, as the mother liquor, enters the primary cooling process and continues to cool until crystals precipitate. This not only avoids material loss but also achieves direct contact between the hot and cold materials.

[0015] The present invention also provides a proportional relationship between the volume of the cooled mother liquor C1 formed after cooling with cooling water and the volume of the cooled mother liquor through a formula. This proportional relationship is affected by the temperature difference a between the high-temperature material and the cooled mother liquor and the temperature difference b between the temperature difference between the cooled mother liquor and the primary mother liquor. Through this proportional relationship, the ratio between the volume of the primary mother liquor formed after the cooled mother liquor C1 is cooled with cooling water and the volume of the cooled mother liquor can be calculated under given temperature difference conditions. This ratio can quantify and control the cooling process, ensuring that the primary mother liquor formed after the cooled mother liquor C1 is cooled with cooling water can meet the requirements of the primary cooling while minimizing material loss. In addition, this ratio can be used to optimize equipment design and operating parameters to achieve a more efficient production process.

[0016] Furthermore, in the primary freezing, the flow ratio of the primary concentrate entering the primary freezing to the low-temperature material is c / d, wherein c is the temperature difference between the mixed material and the low-temperature material, in °C, and d is the temperature difference between the cooled mother liquor C2 entering the secondary freezing and the mixed material, in °C.

[0017] In the present invention, the above formula is intended to describe the volume ratio of the cooled mother liquor C2 entering the primary freezer to the low-temperature material, and to quantify this proportional relationship using the temperature differences c and d. Specifically, c represents the temperature difference between the mixed material and the low-temperature material, i.e., the temperature difference between the mixed material and the low-temperature material; d represents the temperature difference between the cooled mother liquor C2 entering the primary freezer and the mixed material, i.e., the temperature difference between the cooled mother liquor C2 and the mixed material.

[0018] The purpose of this formula is to help optimize the primary freezing process and ensure that the ratio of cooled mother liquor C2 and low-temperature material entering the primary freezing process reaches the optimal state. By setting the volume ratio, the volume ratio of cooled mother liquor C2 and low-temperature material entering the primary freezing process can be accurately controlled to ensure that the ratio of the mixed material in the primary freezing process reaches the expected level, thereby maintaining production stability and consistency, while avoiding excessive or insufficient cooled mother liquor C2 entering the primary freezing process, thereby minimizing material waste and reducing production costs.

[0019] Furthermore, the present invention also provides a system for implementing the above-mentioned freezing crystallization process, including a primary cooling device and a secondary freezing device; wherein, the primary cooling device includes a heat exchanger and a continuous pre-cooling kettle, and the primary freezing device includes a continuous freezing crystallization kettle and a shell and tube heat exchanger and its components.

[0020] Furthermore, in the first stage of freezing, a shell and tube heat exchanger is used to freeze the mixed material; wherein the shell side of the shell and tube heat exchanger is liquid Freon, and the tube side is the mixed material.

[0021] Furthermore, in a shell and tube heat exchanger, the cross-sectional area of ​​the heat transfer tube bundle in the direction perpendicular to the length is 1 / 2 to 2 / 3 of the cross-sectional area of ​​the shell.

[0022] In the present invention, Freon expands in volume as it transforms from a liquid to a gaseous state within the heat exchanger. This transformation occurs when the Freon evaporates into a gas upon heating, completing the heat transfer process. Because gaseous Freon is larger than liquid Freon, sufficient space must be reserved in the shell-and-tube heat exchanger design to accommodate the gaseous Freon. This prevents excessive pressure and other adverse effects, ensuring safe and stable operation of the heat exchanger.

[0023] Furthermore, in the primary refrigeration, an independent cavity is provided between the shell side of the shell and tube heat exchanger and the evaporative cooling device, and the independent cavity is used to store gaseous Freon.

[0024] In the present invention, the design of the shell and tube heat exchanger further considers the storage requirements of Freon in a gaseous state. Therefore, an independent cavity is set between the shell side and the evaporative cooling equipment. This independent cavity is specifically used to store gaseous Freon to ensure that the gaseous Freon can be fully stored and released during the operation of the heat exchanger, thereby achieving precise control of the Freon storage and release process, thereby ensuring the normal operation and efficiency of Freon during the heat exchange process.

[0025] Furthermore, the first-stage freezing adopts a continuous freezing crystallization kettle, which is divided into a heat exchange crystallization zone, a crystal growth zone and a discharge zone from top to bottom, and the cross-sectional area of ​​the heat exchange crystallization zone is greater than the cross-sectional area of ​​the crystal growth zone and the cross-sectional area of ​​the discharge zone.

[0026] The crystallization kettle provided by the present invention has the advantage of utilizing different parts of the kettle structure to achieve different functions, thereby improving the quality and yield of crystals. The heat exchange and crystallization zone located at the top provides ample space and effective heat exchange conditions, allowing the material to fully contact the cooling medium, thereby achieving efficient heat exchange. Due to the slow flow rate, the material is more easily crystallized here, resulting in larger crystals, which is beneficial to improving product quality and yield.

[0027] The crystal growth zone, located at the bottom, is designed with a smaller structure to facilitate crystal growth. In this area, large crystals sink, while small crystals remain suspended and grow in the process. This partitioning design allows for effective separation and growth of crystals within different zones, helping to control crystal size and morphology, further improving product quality and purity. Furthermore, the design of the crystal growth zone helps reduce collisions between crystals, lowering the breakage rate and ultimately increasing product yield and production.

[0028] Furthermore, the cross-sectional area of ​​the discharge zone is the ratio of the flow rate of the low-temperature material to the flow rate of the low-temperature material + the mother liquor C2 after cooling, that is, S 出料区 :S 换热析晶区的横截面积 =V 低温物料 / (V 低温物料 +V 冷却后母液C2 ).

[0029] In the present invention, by adjusting the cross-sectional area ratio of the heat exchange crystallization zone and the discharge zone, the temperature distribution and temperature gradient during the crystallization process can be effectively controlled. This helps control the crystal growth rate, avoiding excessively fast or slow crystallization rates, thereby ensuring the quality and morphology of the crystals.

[0030] At the same time, a reasonable design of the cross-sectional area ratio of the heat exchange crystallization zone and the discharge zone can ensure that sufficient heat and heat exchange surface are provided in the heat exchange crystallization zone to promote the dissolution of the solute and the formation of crystals. At the same time, sufficient space is provided in the discharge zone to ensure that the crystals can be smoothly discharged from the crystallization kettle to avoid accumulation and blockage of the crystallized material.

[0031] Furthermore, the shell side of the shell and tube heat exchanger is provided with guide plates on both sides of the feed port and the discharge port to balance the flow rates of liquid Freon and gaseous Freon.

[0032] In the present invention, the presence of a guide plate at the feed inlet effectively guides the flow of liquid Freon, preventing it from directly impacting the heat exchange tube bundle. This helps reduce the risk of liquid Freon boiling and ensures the safety of the heat exchanger. Furthermore, the presence of the guide plate promotes uniform distribution of liquid Freon within the shell-and-tube heat exchanger, improving heat exchange efficiency.

[0033] On the other hand, the guide plate at the discharge port can effectively control the flow rate of gaseous Freon; when liquid Freon is heated and vaporized in the shell side, gaseous Freon will be formed, and its flow rate will often be faster. Too fast a flow rate will affect the heat exchange efficiency and stability of the heat exchanger. Therefore, the present invention can effectively balance the flow rate of gaseous Freon by setting a guide plate, reduce its impact on the tube bundle, and ensure the safe operation of the heat exchanger.

[0034] In summary, the present invention has the following beneficial effects: The present invention provides a refrigerant direct cooling forced freezing crystallization process, which directly mixes the hot and cold materials in the process steps with relatively large heat exchange, and utilizes the temperature difference between the materials to exchange heat, thereby realizing effective energy utilization and improving energy efficiency; for the steps with relatively small heat exchange, cooling water, Freon and other refrigerants are used for freezing or cooling, which can more accurately control the temperature, ensure the stable operation of the process, and achieve the purpose of continuous production while improving the crystal yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a process flow chart of the present invention; Figure 2 This is a detailed diagram of the continuous heat exchange crystallization kettle of the present invention; Figure 3 It is a cross-sectional view of the shell and tube heat exchanger of the present invention.

[0036] Reference numerals 1. Heat exchanger, 2. Continuous pre-cooling kettle, 3. Continuous freezing crystallization kettle, 4. Shell and tube heat exchanger, 21. Heat exchange crystallization zone, 22. Crystal growth zone, 23. Discharge zone, 41. Heat transfer tube bundle, 42. Guide plate. DETAILED DESCRIPTION

[0037] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined purpose of the invention, a refrigerant direct cooling forced freezing crystallization process proposed in accordance with the present invention, its specific implementation method, characteristics and effects are described in detail as follows.

[0038] This specific embodiment provides a refrigerant direct cooling forced freezing crystallization process, which includes a first stage cooling and a first stage freezing. In the first stage cooling, a high-temperature material is directly mixed with a first stage mother liquor to form a cooled mother liquor. A portion of the cooled mother liquor is cooled by cooling water to form a first stage mother liquor, which is returned to the first stage cooling to cool the high-temperature material, and is recorded as the cooled mother liquor C1. Another portion of the cooled mother liquor enters the first stage freezing, which is recorded as the cooled mother liquor C2, and is directly mixed with a low-temperature material to precipitate crystals to obtain a mixed material. The mixed material is frozen by Freon to form a low-temperature material, which is then used to freeze the cooled mother liquor C2 entering the first stage freezing. In the first-stage cooling, the temperature difference between the high-temperature material and the first-stage mother liquor is 40-50°C, the temperature difference between the high-temperature material and the cooled mother liquor is 40-60°C, and the temperature difference between the cooled mother liquor and the first-stage mother liquor is 1-3°C; In the primary freezing, the temperature difference between the cooled mother liquor C2 and the low-temperature material entering the primary freezing is 25-40°C, the temperature difference between the cooled mother liquor C2 and the mixed material entering the primary freezing is 25-40°C, and the temperature difference between the mixed material and the low-temperature material is 1-3°C; Among them, liquid Freon is used directly as a refrigerant, and the mixed material is frozen into a low-temperature material through the absorption of heat during the conversion of liquid Freon to gaseous Freon.

[0039] Specifically, such as Figure 1 As shown, the high-temperature material enters the primary cooling system, which includes a continuous pre-cooling kettle 2 and a heat exchanger 1. The material flows through the tube side of the heat exchanger 1, and the cooling circulating water flows through the shell side. The high-temperature material is cooled by the primary cooling system to form a cooled mother liquor; the cooled mother liquor is pumped to the primary refrigeration system, which includes a continuous freezing crystallization kettle 3 and a shell and tube heat exchanger 4. The material flows through the tube side of the shell and tube heat exchanger 4, and the refrigerant medium flows through the shell side. The cooled mother liquor is cooled by the primary refrigeration system to form a mixed material and a crystal mixture.

[0040] In this specific embodiment, the temperature of the high-temperature material is 95-100°C, and the temperature of the mixed material is 0-15°C. The freeze crystallization process provided by this specific embodiment is suitable for materials that have high solubility in high-temperature solutions and reduced solubility in low-temperature solutions, and whose solubility can be reduced by cooling, thereby achieving the precipitation and crystallization of solid materials, including but not limited to sodium sulfate, magnesium sulfate, potassium sulfate, potassium aluminum sulfate, copper sulfate, ammonium sulfate, etc. These materials have high solubility in solutions at 95-100°C and low solubility in solutions at 0-15°C. The method provided by this specific embodiment can be used to separate the precipitated crystals from the solution. Therefore, the freeze crystallization process provided by this specific embodiment can be used for both crystal purification and impurity removal of other materials.

[0041] However, in order to freeze high-temperature materials until crystals precipitate from the mixed materials, the temperature of the materials needs to be reduced from 95~100℃ to 0~15℃. The temperature difference is too large. If the traditional freezing crystallization mode is used, not only multi-stage heat exchangers are required, but also the heat transfer ratio of each stage is very large, which requires a heat exchanger with a larger heat transfer ratio and a larger heat transfer area. At the same time, due to the large temperature difference of the materials, the viscosity and density of the materials will change, which will increase the heat transfer resistance, reduce the heat transfer efficiency, and face a greater risk of condensation.

[0042] Therefore, in this specific embodiment, in both the first-stage cooling and the first-stage freezing, the process steps of direct contact between hot and cold materials are adopted, which avoids heat transfer between two materials with a large temperature difference in the heat exchanger, effectively improves the heat exchange efficiency, reduces material loss, and reduces the risk of condensation; and for materials with a small temperature difference, direct cooling or direct freezing is adopted by the heat exchanger, which improves the freezing crystallization efficiency while avoiding the use of a heat exchanger with a large heat exchange area, saving equipment costs.

[0043] Specifically, in the first stage cooling, the temperature difference between the high-temperature material and the first stage mother liquor is 40-60°C, so the high-temperature material is in direct contact with the first stage mother liquor; the temperature difference between the cooled mother liquor C1 and the first stage mother liquor is 1-3°C, so cooling water is used to cool the first stage concentrate; In the primary freezing, the temperature difference between the cooled mother liquor C2 entering the primary freezing and the low-temperature material is 25~40℃. The low-temperature material is directly mixed with the cooled mother liquor C2 entering the secondary freezing to obtain a mixed material. The temperature difference between the mixed material and the low-temperature material is 1~3℃. Liquid Freon is used to directly freeze the mixed material.

[0044] In the primary refrigeration provided in this embodiment, the temperature difference between liquid Freon and gaseous Freon is 0°C, and the gaseous Freon is converted into liquid Freon through evaporative cooling and returned to the primary refrigeration for use as a refrigerant.

[0045] In this embodiment, directly freezing the mixed material with liquid Freon can achieve high freezing efficiency. However, if the heat exchange is high (i.e., if the heat exchanger needs to process a large amount of heat), the temperature of the liquid Freon during the heat exchange process is difficult to control. This can lead to large temperature fluctuations in the liquid Freon, making the output temperature unstable, affecting the freezing effect and product quality. In this embodiment, the heat exchange is relatively low, and the liquid Freon only undergoes phase change without temperature change during the heat exchange process, achieving the desired freezing effect. This ensures that the output temperature remains constant, thus maintaining the heat exchange efficiency of the heat exchanger.

[0046] In this specific embodiment, the volume ratio of the cooled mother liquor C1 to the cooled mother liquor is a / (a+b), wherein a is the temperature difference between the high-temperature material and the cooled mother liquor, in °C; b is the temperature difference between the cooled mother liquor and the first-stage mother liquor, in °C.

[0047] In the primary freezing, the volume ratio of the cooled mother liquor C2 entering the primary freezing to the low-temperature material is c / d, where c is the temperature difference between the mixed material and the low-temperature material, in °C, and d is the temperature difference between the cooled mother liquor C2 entering the secondary freezing and the mixed material, in °C.

[0048] For the freeze-crystallization process provided in this embodiment, the above formula establishes a relationship between the volume ratio of the materials and the temperature difference, based on the heat transfer during the mixing of two identical materials at different temperatures. This relationship allows for a more precise understanding of the effect of temperature changes on material volume, thereby enabling better control of the temperature and material volume ratio during the freeze-crystallization process. Compared to complex thermodynamic calculations, the use of these formulas simplifies the process of determining material volume, improving the operability and controllability of the process.

[0049] This specific embodiment also provides a system for implementing the above-mentioned freezing crystallization process, including a primary cooling device and a secondary freezing device; wherein, the primary cooling device includes a heat exchanger 1 and a continuous pre-cooling kettle 2, and the primary freezing device includes a continuous freezing crystallization kettle 3 and a shell and tube heat exchanger 4 and its components.

[0050] like Figure 3 As shown, in the primary refrigeration provided in this specific embodiment, a shell and tube heat exchanger is used to freeze the mixed material; wherein, the shell side of the shell and tube heat exchanger is liquid Freon, and the tube side is the mixed material; more preferably, in the shell and tube heat exchanger, the cross-sectional area of ​​the heat transfer tube bundle 41 in the direction perpendicular to the length is 1 / 2 to 2 / 3 of the cross-sectional area of ​​the shell.

[0051] like Figure 3 As shown, in the primary refrigeration provided in this specific embodiment, an air bag 42 is further provided between the shell side of the shell and tube heat exchanger and the evaporative cooling device, and the air bag 42 is used to store gaseous Freon.

[0052] In the shell side of the shell and tube heat exchanger provided in this specific embodiment, guide plates are provided on both sides of the feed inlet and the discharge port to balance the flow rates of liquid Freon and gaseous Freon.

[0053] like Figure 2 As shown, the first-stage freezing provided in this specific embodiment adopts a continuous freezing crystallization kettle, which is divided into a heat exchange crystallization zone 21, a crystal growth zone 22 and a discharge zone 23 from top to bottom. The cross-sectional area of ​​the heat exchange crystallization zone is greater than the cross-sectional area of ​​the crystal growth zone and the cross-sectional area of ​​the discharge zone.

[0054] In this embodiment, the cross-sectional area of ​​the discharge zone is also defined: the cross-sectional area of ​​the heat exchange crystallization zone is the ratio of the flow rate of the low-temperature material to the flow rate of the low-temperature material + the mother liquor C2 after cooling, that is, S 出料区 :S 换热析晶区的横截面积 =V 低温物料 / (V 低温物料 +V 冷却后母液C2 ).

[0055] By adjusting the cross-sectional area ratio of the heat exchange crystallization zone and the discharge zone, the temperature distribution and temperature gradient during the crystallization process can be effectively controlled. This helps control the crystal growth rate, avoiding excessively fast or slow crystallization rates, and thus ensuring the quality and morphology of the crystals.

[0056] At the same time, a reasonable design of the cross-sectional area ratio of the heat exchange crystallization zone and the discharge zone can ensure that sufficient heat and heat exchange surface are provided in the heat exchange crystallization zone to promote the dissolution of the solute and the formation of crystals. At the same time, sufficient space is provided in the discharge zone to ensure that the crystals can be smoothly discharged from the crystallization kettle to avoid accumulation and blockage of the crystallized material.

[0057] Example 1 In this embodiment, a sodium sulfate aqueous solution with a mass concentration of 30% is used as a raw material, and a refrigerant direct cooling forced freezing crystallization process provided by the present invention is adopted to prepare pure sodium sulfate. The specific process steps are as follows: It includes primary cooling and primary freezing; wherein, the high-temperature material is directly mixed with the primary mother liquor in the primary cooling to form a cooled mother liquor; part of the cooled mother liquor is cooled by cooling water to form a primary mother liquor, which returns to the primary cooling to cool the high-temperature material, recorded as cooled mother liquor C1; another part of the cooled mother liquor enters the primary freezing, recorded as cooled mother liquor C2, and is directly mixed with the low-temperature material to obtain a mixed material after crystal precipitation; the mixed material is frozen by Freon to form a low-temperature material, and the cooled mother liquor C2 entering the primary freezing is frozen.

[0058] Among them, liquid Freon is used directly as a refrigerant, and the mixed material is frozen into a low-temperature material through the absorption of heat during the conversion of liquid Freon to gaseous Freon.

[0059] Among them, the high-temperature material is a sodium sulfate aqueous solution A with a concentration of 30% and a temperature of 96°C; the first-level mother liquor is a sodium sulfate aqueous solution B at 39°C; the cooled mother liquor is a sodium sulfate aqueous solution C at 40°C; the low-temperature material is a sodium sulfate aqueous solution D at 3°C; the mixed material is a sodium sulfate aqueous solution E at 4°C, forming a first-level mother liquor. The cooled mother liquor C2 that returns to the first-level cooling is a sodium sulfate aqueous solution C1, and the partially cooled mother liquor C2 that enters the first-level freezing is a sodium sulfate aqueous solution C2.

[0060] In this embodiment, the flow rate of sodium sulfate aqueous solution A is 10.35m 3 / h; then according to the formula provided by the present invention, the volume ratio of the sodium sulfate aqueous solution C1 to the cooled mother liquor after cooling by cooling water is a / (a+b), wherein a is the temperature difference between the sodium sulfate aqueous solution A and the cooled mother liquor, in °C; b is the temperature difference between the cooled mother liquor and the primary mother liquor (the cooled mother liquor returned to the primary cooling, the sodium sulfate aqueous solution C1), in °C, and the flow rate of the sodium sulfate aqueous solution C1 is 580m 3 / h, the flow rate of sodium sulfate aqueous solution C2 is 10.35m 3 / h.

[0061] According to the present invention, in the first-stage freezing, the volume ratio of (sodium sulfate aqueous solution C2) to the low-temperature material (sodium sulfate aqueous solution D) is c / d, wherein c is the temperature difference between the mixed material and the low-temperature material, in °C, and d is the temperature difference between the mother liquor after cooling and the mixed material entering the first-stage freezing, in °C. It can be calculated that the flow rate of the sodium sulfate aqueous solution D is 379.98m 3 / h, the flow rate of sodium sulfate aqueous solution E is 379.98m 3 / h.

[0062] Based on the above calculations, the specific process steps for preparing pure sodium sulfate by the refrigerant direct cooling forced freezing crystallization process provided in this embodiment are as follows: S1, sodium sulfate aqueous solution A with a concentration of 30% and a temperature of 96°C, with a 10.35m 3 / h flow into the primary cooling, and at the same time, 39 ° C sodium sulfate aqueous solution B is added to the primary cooling, with a flow rate of 579.6m 3 / h, after the two are mixed, a sodium sulfate aqueous solution C with a temperature of 40°C is obtained; S2, the sodium sulfate aqueous solution C is divided into two parts, the sodium sulfate aqueous solution C1 is cooled by cooling water, the temperature is reduced to 39 ° C, and it enters the primary cooling again as the sodium sulfate aqueous solution B; S3, sodium sulfate aqueous solution C2 at 10.35m 3 / h flow into the primary refrigeration, and mixed with sodium sulfate aqueous solution D at a temperature of 3°C to obtain sodium sulfate decahydrate crystals and sodium sulfate aqueous solution E at a temperature of 4°C, wherein the flow rate of sodium sulfate aqueous solution D at a temperature of 3°C is 379.6m 3 / h; S4. The sodium sulfate aqueous solution E is directly cooled with Freon to reduce the temperature to 3°C and is returned to the primary refrigeration as the sodium sulfate aqueous solution D.

[0063] The volume of the sodium sulfate aqueous solution C1 accounts for 98.2% of the volume of the sodium sulfate aqueous solution C.

[0064] In this embodiment, the first-stage freezing adopts a continuous freezing crystallization kettle, which is divided into a heat exchange crystallization zone, a crystal growth zone and a discharge zone from top to bottom. The cross-sectional area of ​​the heat exchange crystallization zone is greater than the cross-sectional area of ​​the crystal growth zone and the cross-sectional area of ​​the discharge zone.

[0065] Among them, through the formula S 出料区 :S 换热析晶区的横截面积区 =V 低温物料 / (V 低温物料 +V 冷却后母液C2 ) can be calculated S 出料区 :S 换热析晶区的横截面积区 The ratio is (379.6:389.9).

[0066] In this embodiment, the first-stage refrigeration uses a shell and tube heat exchanger to freeze the mixed material; wherein, the shell side of the shell and tube heat exchanger is liquid Freon, and the tube side is the mixed material; the cross-sectional area of ​​the heat transfer tube bundle of the shell and tube heat exchanger perpendicular to the length direction is 1 / 2 of the cross-sectional area of ​​the shell, and an independent cavity is also provided between the shell side of the shell and tube heat exchanger and the evaporative cooling equipment, and the independent cavity is used to store gaseous Freon; the shell side is provided with guide plates on both sides of the feed port and the discharge port to balance the flow rate of liquid Freon and gaseous Freon.

[0067] Among them, the conversion of liquid Freon into gaseous Freon in the shell side absorbs heat, freezing the sodium sulfate aqueous solution E into sodium sulfate aqueous solution D, and the temperature difference between liquid Freon and gaseous Freon is 0°C. The gaseous Freon in the shell side is converted into liquid Freon through evaporative cooling and returns to the primary refrigeration for use as a refrigerant.

[0068] In this embodiment, the final yield of sodium sulfate decahydrate is 98.6% and the purity is 96.7%.

[0069] Example 2 In this example, a potassium sulfate aqueous solution with a mass concentration of 35% was used as a raw material, and a refrigerant direct cooling forced freezing crystallization process provided by the present invention was used to prepare pure potassium sulfate. The specific process steps are as follows: It includes primary cooling and primary freezing; wherein, the high-temperature material is directly mixed with the primary mother liquor in the primary cooling to form a cooled mother liquor; part of the cooled mother liquor is cooled by cooling water to form a primary mother liquor, which returns to the primary cooling to cool the high-temperature material, recorded as cooled mother liquor C1; another part of the cooled mother liquor enters the primary freezing, recorded as cooled mother liquor C2, and is directly mixed with the low-temperature material to obtain a mixed material after crystal precipitation; the mixed material is frozen by Freon to form a low-temperature material, and the cooled mother liquor C2 entering the primary freezing is frozen.

[0070] Among them, the high-temperature material is potassium sulfate aqueous solution A with a concentration of 35% and a temperature of 98°C; the first-level mother liquor is potassium sulfate aqueous solution B at 46°C; the cooled mother liquor is potassium sulfate aqueous solution C at 48°C; the low-temperature material is potassium sulfate aqueous solution D at 5°C; the mixed material is potassium sulfate aqueous solution E at 6°C, forming a first-level mother liquor. The cooled mother liquor returning to the first-level cooling is sodium sulfate aqueous solution C1, and the partially cooled mother liquor entering the first-level freezing is sodium sulfate aqueous solution C2.

[0071] In this embodiment, the flow rate of sodium sulfate aqueous solution A is 12.0m 3 / h; then according to the formula provided by the present invention, the volume ratio of the cooled mother liquor (potassium sulfate aqueous solution C1) to the cooled mother liquor (potassium sulfate aqueous solution C) is a / (a+b), wherein a is the temperature difference between the high-temperature material (potassium sulfate aqueous solution A) and the cooled mother liquor (potassium sulfate aqueous solution C), in °C; b is the temperature difference between the cooled mother liquor (potassium sulfate aqueous solution C) and the primary mother liquor, in °C. The flow rate of the potassium sulfate aqueous solution C1 is 300m 3 / h, the flow rate of potassium sulfate aqueous solution C2 is 12m 3 / h.

[0072] According to the present invention, in the first-stage freezing, the volume ratio of the cooled mother liquor to the low-temperature material entering the first-stage freezing is c / d, wherein c is the temperature difference between the mixed material and the low-temperature material, in ° C, and d is the temperature difference between the mother liquor and the mixed material after entering the cooling, in ° C. The flow rate of the potassium sulfate aqueous solution D is 504m 3 / h, the flow rate of potassium sulfate aqueous solution E is 504m 3 / h.

[0073] Based on the above calculations, the specific process steps for preparing pure sodium sulfate by the refrigerant direct cooling forced freezing crystallization process provided in this embodiment are as follows: S1, potassium sulfate aqueous solution A with a concentration of 35% and a temperature of 98°C, is heated to 12m 3 / h flow into the primary cooling, and at the same time, a 46 ° C potassium sulfate aqueous solution B is added to the primary cooling, with a flow rate of 300m 3 / h, after the two are mixed, a potassium sulfate aqueous solution C with a temperature of 48°C is obtained; S2, the potassium sulfate aqueous solution C is divided into two parts, the potassium sulfate aqueous solution C1 is cooled by cooling water, the temperature is reduced to 46 ° C, and it enters the primary cooling again as the potassium sulfate aqueous solution B; S3, potassium sulfate aqueous solution C2 with 12m 3 / h flow into the primary refrigeration, and mixed with potassium sulfate aqueous solution D at a temperature of 5°C to obtain potassium sulfate crystals and potassium sulfate aqueous solution E at a temperature of 6°C, wherein the flow rate of potassium sulfate aqueous solution D at a temperature of 5°C is 504m 3 / h; S4. The potassium sulfate aqueous solution E is directly cooled with Freon to reduce the temperature to 5°C and is returned to the primary refrigeration as the potassium sulfate aqueous solution D.

[0074] The volume of the potassium sulfate aqueous solution C1 accounts for 96.1% of the volume of the potassium sulfate aqueous solution C.

[0075] In this embodiment, the secondary freezing adopts a continuous freezing crystallization kettle, which is divided into a heat exchange crystallization zone, a crystal growth zone and a discharge zone from top to bottom. The cross-sectional area of ​​the heat exchange crystallization zone is greater than the cross-sectional area of ​​the crystal growth zone and the cross-sectional area of ​​the discharge zone.

[0076] Among them, through the formula S 出料区 :S 换热析晶区的横截面积区 =V 低温物料 / (V 低温物料 +V 冷却后母液C2 ) can be calculated S 出料区 :S 换热析晶区的横截面积区 The ratio is (504:516).

[0077] In this embodiment, the first-stage refrigeration uses a shell and tube heat exchanger to freeze the mixed material; wherein, the shell side of the shell and tube heat exchanger is liquid Freon, and the tube side is the mixed material; the cross-sectional area of ​​the heat transfer tube bundle of the shell and tube heat exchanger perpendicular to the length direction is 1 / 2 of the cross-sectional area of ​​the shell, and an independent cavity is also provided between the shell side of the shell and tube heat exchanger and the evaporative cooling equipment, and the independent cavity is used to store gaseous Freon; the shell side is provided with guide plates on both sides of the feed port and the discharge port to balance the flow rate of liquid Freon and gaseous Freon.

[0078] Among them, the conversion of liquid Freon into gaseous Freon in the shell side absorbs heat, freezing the potassium sulfate aqueous solution E into potassium sulfate aqueous solution D, and the temperature difference between liquid Freon and gaseous Freon is 0°C. The gaseous Freon in the shell side is converted into liquid Freon through evaporative cooling and returns to the primary refrigeration for use as a refrigerant.

[0079] In this embodiment, the final yield of potassium sulfate is 97.3% and the purity is 98.1%.

[0080] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been presented as a preferred embodiment, it is not intended to limit the present invention. Any technician familiar with the present profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A refrigerant direct cooling forced freezing crystallization process for a sulfate solution, characterized in that: include: Primary cooling: The high-temperature sulfate solution at 95-100°C is directly mixed with the primary mother liquor to form a cooled mother liquor; Splitting the cooled mother liquor: the cooled mother liquor is divided into cooled mother liquor C1 and cooled mother liquor C2, the cooled mother liquor C1 is cooled by cooling water to form the primary mother liquor, and circulated to the primary cooling; Primary freezing: After cooling, the mother liquor C2 enters the primary freezing stage and is mixed with a low-temperature sulfate solution at 0-15°C for crystallization to obtain a mixed material; the mixed material is frozen with liquid Freon to form the low-temperature sulfate solution; The sulfate solution is a sodium sulfate solution or a potassium sulfate solution.

2. The refrigerant direct cooling forced freezing crystallization process of a sulfate solution according to claim 1, characterized in that: In the first-stage cooling, the temperature difference between the high-temperature sulfate solution and the first-stage mother liquor is 40-60°C, the temperature difference between the high-temperature sulfate solution and the cooled mother liquor is 40-60°C, and the temperature difference between the cooled mother liquor and the first-stage mother liquor is 1-3°C; In the primary freezing, the temperature difference between the cooled mother liquor C2 and the low-temperature sulfate solution is 25-40°C, the temperature difference between the cooled mother liquor C2 and the mixed material is 25-40°C, and the temperature difference between the mixed material and the low-temperature sulfate solution is 1-3°C.

3. The refrigerant direct cooling forced freezing crystallization process of a sulfate solution according to claim 1, characterized in that: The sodium sulfate solution has an initial concentration of 30% and an initial temperature of 96° C. It is subjected to forced freezing crystallization by direct cooling with a refrigerant to obtain sodium sulfate decahydrate crystals. The mixture after crystallization is frozen with liquid Freon to form a low-temperature sodium sulfate solution and then returned to the primary freezing.

4. The refrigerant direct cooling forced freezing crystallization process of a sulfate solution according to claim 3, characterized in that: In the refrigerant direct cooling forced freezing crystallization process of the sodium sulfate solution, the volume of the cooled mother liquor C1 accounts for 98.2% of the volume of the cooled mother liquor.

5. The refrigerant direct cooling forced freezing crystallization process of a sulfate solution according to claim 4, characterized in that: The 96°C sodium sulfate solution and the 39°C primary mother liquor are mixed in the primary cooling to form a 40°C cooled mother liquor. The 40°C cooled mother liquor C2 enters the primary freezing and is mixed with the 3°C sodium sulfate solution for heat exchange to precipitate sodium sulfate decahydrate crystals. The mixed material becomes 4°C, and the 4°C mixed material is frozen to 3°C by liquid Freon.

6. The refrigerant direct cooling forced freezing crystallization process of a sulfate solution according to claim 1, characterized in that: The potassium sulfate solution has an initial concentration of 35% and an initial temperature of 98° C. Potassium sulfate crystals are obtained by forced freezing and crystallization through direct cooling with a refrigerant. The mixture after crystallization is frozen with liquid Freon to form a low-temperature potassium sulfate solution and then returned to the primary freezing.

7. The refrigerant direct cooling forced freezing crystallization process of a sulfate solution according to claim 6, characterized in that: In the refrigerant direct cooling forced freezing crystallization process of the potassium sulfate solution, the volume of the cooled mother liquor C1 accounts for 96.1% of the volume of the cooled mother liquor.

8. The refrigerant direct cooling forced freezing crystallization process for a sulfate solution according to claim 7, characterized in that: The 98°C sodium sulfate solution and the 46°C primary mother liquor are mixed in the primary cooling to form a 48°C cooled mother liquor. The 48°C cooled mother liquor C2 enters the primary freezing and is mixed with the 5°C sodium sulfate solution for heat exchange to precipitate potassium sulfate crystals. The mixed material becomes 6°C, and the 6°C mixed material is frozen to 5°C by liquid Freon.

9. The refrigerant direct cooling forced freezing crystallization process for a sulfate solution according to any one of claims 3 to 8, characterized in that: The liquid Freon is converted into gaseous Freon through phase change to absorb heat and refrigerate, and the gaseous Freon is converted into liquid Freon through evaporative cooling. The temperature difference between the liquid Freon and the gaseous Freon is 0°C.

10. The refrigerant direct cooling forced freezing crystallization process of a sulfate solution according to claim 9, characterized in that: The first-stage refrigeration uses a shell and tube heat exchanger to freeze the mixed material; wherein the shell side of the shell and tube heat exchanger is liquid Freon, and the tube side is the mixed material.

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