A method for concentrating plant extracts based on gradient freezing and directional salt removal

Abstract

This application relates to a method for concentrating plant extracts based on gradient freezing and directional salt removal, belonging to the field of freeze concentration technology. The method includes: dispensing the extract into containers with a heat-conducting bottom wall and insulated side walls, with a liquid layer thickness of 60-150 mm; cooling the bottom of the container to a cold source temperature of -25°C to -15°C, causing ice crystals to grow unidirectionally from bottom to top, continuously displacing the solute to the top unfrozen liquid phase, forming a layered structure of lower ice crystals and upper concentrated liquid; after stopping freezing, separating the upper concentrated liquid and concentrating to the target concentration through multiple stages. This invention utilizes a vertical temperature gradient to achieve directional solute salt removal, with an ice crystal entrainment rate ≤2%, eliminating the need for ice crystal crushing, heating to dissolve, or fractional distillation for recovery. The equipment is extremely simple, and the retention rate of volatile aromatic components is ≥90%, fundamentally solving the technical problems of severe ice crystal entrainment and complex subsequent processing in freeze concentration.

Description

Technical Field

[0001] This application relates to the field of cryogenic concentration technology, and in particular to a method for concentrating plant extracts based on gradient freezing and directional salt removal. Background Technology

[0002] Freeze concentration is a non-thermal processing technique that utilizes the solid-liquid phase equilibrium principle between ice and aqueous solution to concentrate a solution by removing water in the form of solid ice. Compared with vacuum heating concentration, freeze concentration has significant advantages such as high retention rate of heat-sensitive components and low energy consumption (the latent heat of freezing of water is only about 1 / 7 of its latent heat of vaporization), and has broad application prospects in food, plant extracts, and biomedicine. Based on the different water crystallization methods, existing freeze concentration technologies are mainly divided into two categories: suspension crystallization and progressive freezing.

[0003] Suspension crystallization generates fine ice crystals on a cooled surface using a scraped surface heat exchanger. These ice crystals grow to several hundred micrometers in the crystallizer after several hours of Ostwald ripening, and are then separated and purified using a countercurrent washing column to achieve concentration. While suspension crystallization has advantages in heat transfer coefficient and ice production rate, its actual process is complex, involving multiple steps such as ice slurry production, ice crystal ripening, ice crystal bed formation, and washing. Each step is carried out in a separate unit operation, which not only complicates the process but also significantly increases equipment costs. Progressive freezing, on the other hand, forms an ice layer directly on the cooled surface. Its equipment cost is relatively low, and operation and management are simpler. However, due to the poor heat transfer of the ice layer and the tendency for the formed ice layer to entrain liquid sacs, significant solute loss occurs.

[0004] Both methods face a common technical challenge: solute loss due to ice crystal entrainment. During ice crystal formation and growth, solutes inevitably become trapped inside the ice crystals or adhere to their surface, leading to the loss of active ingredients. Studies have shown that solute entrainment is one of the key factors restricting the widespread application of freeze concentration technology. Even by reducing the freezing rate to decrease entrainment, the solute entrainment rate in progressive freezing methods remains around 5%-7%, meaning solute loss cannot be completely avoided.

[0005] In recent years, researchers have proposed a remedial solution: breaking up and heating ice crystals containing entrained solutes generated during freeze-thaw concentration, and then collecting the solution in stages to recover the solute. For example, patent document CN119034249A discloses a related technical solution. The core of this solution lies in utilizing the negative correlation between solute concentration and melting point to break up and heat-dissolve ice crystals, and then collecting the solution in stages according to the target concentration to recover the solute entrained in the ice crystals. This method reduces solute loss to some extent, but it still has the following inherent drawbacks: First, the equipment structure is complex. Taking the cryogenic concentration device disclosed in CN119034249A as an example, it requires a crushing and dissolving assembly, including a rotating chamber, a filtration mechanism along the inner wall of the rotating chamber, and a heat source. The dilute solution is first frozen into a solid or solid-liquid mixture in a cryogenic device, then crushed and loaded into the rotating chamber. While the rotating chamber rotates, heat is transferred to the crushed ice through the heat source to dissolve it, and the solution is collected in stages. The entire device involves multiple functional modules such as crushing, filtering, rotating, and heating, resulting in a high degree of system integration and significant equipment investment and daily maintenance costs.

[0006] Secondly, the recovery process is cumbersome and inefficient. In practice, this scheme is not simply a matter of heating and dissolving. Studies have shown that the solute, before migration occurs, may be blocked by the ice crystal structure, leading to poor recovery results. To achieve good recovery, precise temperature control is required, sometimes even requiring slow operation at around 0°C, with a single processing time of up to approximately 4 hours. Furthermore, relevant literature indicates that in conventional heating and dissolving processes, the concentration difference between the received solutions is not significant enough, resulting in low recovery efficiency. This means that precise temperature control and time-consuming operations are necessary to achieve the desired results, significantly increasing the difficulty of process control and extending the production cycle.

[0007] Third, the concentration of the recovered solution is insufficient, requiring further recycling. In this scheme, solutions that do not meet the target concentration after fractional collection need to be sent back to the cryogenic concentration unit for further processing. This means that solute recovery is not a one-step process; the concentration of the recovered intermediate fraction is still insufficient, requiring multiple cycles within the system, further increasing process complexity and energy consumption. The fundamental reason is that the upper limit of the recovery efficiency of this technical route itself restricts its economic viability for industrial application.

[0008] It is evident that existing remedial solutions attempt to recover entrained solutes by breaking ice crystals and then heating and dissolving them. However, due to the inherent structural complexity, cumbersome operation, and limited efficiency of the breaking and heating dissolving processes, the overall concentration effect and industrial feasibility remain unsatisfactory.

[0009] Therefore, there remains an urgent technological need to develop a cryogenic concentration method that can fundamentally reduce ice crystal entrainment at the source and eliminate the need for subsequent crushing and fractionation recovery. Summary of the Invention

[0010] The purpose of this invention is to provide a method for concentrating plant extracts based on gradient freezing and directional salt removal, thereby solving the technical problems of severe ice crystal entrainment and the need for complex subsequent recovery processes in existing freeze concentration technologies. This invention alters the temperature field distribution during the freezing process, causing the solute to actively migrate to a designated area before ice crystal formation, thus obtaining a high-purity ice crystal region without the need for subsequent crushing and fractionation recovery.

[0011] To achieve the above objectives, the present invention provides the following technical solution: A method for concentrating plant extracts based on gradient freezing and directional salt removal, characterized by comprising the following steps: Step 1: Pretreatment and dispensing of the concentrate Plant raw materials are extracted with water or alcohol to obtain a plant extract stock solution. Suspended particulate matter is removed by microfiltration or centrifugation to obtain a clear plant extract. The extract is dispensed into containers with heat-conducting bottom walls and heat-insulating side walls. The bottom of the container is in contact with a cold source, and the side walls and top are insulated to ensure that heat transfer occurs primarily in the vertical direction.

[0012] The height (i.e., the thickness of the liquid layer) of the container is 60-150 mm, the bottom is made of thermally conductive material (such as stainless steel or aluminum), and the side walls are made of thermally insulating material (such as polyurethane foam or vacuum insulation board).

[0013] Step 2: Vertical gradient directional freezing Place the container containing the extract on a freezing platform and turn on the cold source to cool the bottom of the container. Set the cold source temperature to -25°C to -15°C and the freezing time to 4-12 hours. Do not stir or apply any physical field during the freezing process.

[0014] Due to the thermal insulation of the sidewalls and the thermal conductivity of the bottom, a vertical temperature gradient is formed in the extract from bottom to top: the bottom temperature is the lowest, and the top temperature is the highest (close to ambient temperature or pre-cooling temperature). Ice crystals preferentially nucleate at the bottom and grow unidirectionally from bottom to top. During the ice crystal growth process, the solute is continuously displaced into the liquid phase. Due to the stable temperature gradient and the unidirectional growth direction, the solute is continuously "pushed" towards the unfrozen liquid phase at the top, forming a layered structure with a pure ice layer at the bottom and a concentrated liquid layer at the top.

[0015] Unlike existing gradual freezing methods, this method does not pursue extremely slow freezing rates to reduce entrainment. Instead, it actively utilizes a relatively fast freezing rate (-25℃ to -15℃) to create a steep temperature gradient, enhancing the directional migration and displacement effect of the solute. Experiments show that under vertical gradient freezing conditions, even with a faster freezing rate, the purity of ice crystals is actually higher than that of slow isothermal freezing because the solute migration path is clear and unaffected by lateral interference.

[0016] Step 3: Separation of ice layer from concentrate When the volume of the unfrozen concentrate at the top decreases to 20%-50% of its initial volume (or when the freezing time reaches the preset value), freezing is stopped. At this point, a two-phase stratified structure is formed in the container: a lower layer of ice crystals and an upper layer of concentrate. Because the ice crystals are dense, blocky, and continuously distributed, and have a clear interface with the concentrate, they can be directly separated using one of the following methods: Method A (pouring method): Pour or pump out the upper layer of concentrate directly, leaving the lower layer of ice crystals in the container; Method B (overflow method): An overflow port is set on the side wall of the container, and the concentrate will automatically flow out after reaching the overflow height; Method C (Inverting Method): Invert the container, the concentrate flows out, and ice crystals adhere to the bottom of the container.

[0017] The concentrate obtained after separation is the first-stage concentrate. The lower layer of ice crystals is high-purity ice (entrainment rate ≤2%), which can be used as process water or discarded directly after melting.

[0018] Step 4: Multi-stage gradient concentration Using the primary concentrate as raw material, repeat steps one through three 2-4 times. Each time, adjust the cold source temperature by 1-3°C based on the decrease in the freezing point of the concentrate to maintain sufficient subcooling to drive gradient freezing. Once the concentrate reaches the target concentration, the concentration process is complete.

[0019] Step 5 (optional): Concentrate purification After the final stage of concentration is completed, the concentrate is filtered through a 0.22 μm low-temperature microfiltration membrane to remove any trace ice crystal nuclei that may remain, thus obtaining the final product.

[0020] The core invention of this application lies in the following: by using a vertically insulated and heat-conducting container to construct a unidirectional temperature gradient, ice crystals grow directionally from bottom to top, and solute is continuously squeezed into the top liquid phase, forming a clear stratification of "pure ice layer - concentrated liquid layer", thereby completing crystallization and separation simultaneously in a one-step freezing process without the need for subsequent crushing, dissolving, fractionation and recovery operations. Beneficial effects

[0021] 1. Extremely low ice crystal entrainment and almost no solute loss: This invention utilizes vertical gradient directional salt removal, with an ice crystal entrainment rate of ≤2% at the bottom layer. Compared with conventional static freezing methods (3%-5%) and progressive freezing methods (4%-8%), it solves the core entrainment loss problem of freeze concentration from the source.

[0022] 2. Existing technologies still require the crushing, heating, dissolving, and fractional distillation of ice crystals containing solutes after freezing, which involves complex equipment, cumbersome operation, and is time-consuming and energy-intensive. This invention completely eliminates the post-processing of ice crystals, requiring no post-processing and greatly simplifying the process.

[0023] 3. No heating is required throughout the process, the retention rate of volatile aromatic components is ≥90%, the retention rate of active ingredients such as polyphenols and flavonoids is ≥95%, there is no cooking odor, and the product quality is significantly better than that of vacuum heating concentration. Detailed Implementation

[0024] The embodiments of this application will now be described in more detail with reference to the examples. While embodiments of this application are shown in the examples, it should be understood that this application can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make this application more thorough and complete, and to fully convey the scope of this application to those skilled in the art.

[0025] Example 1: Gradient-directed freeze concentration of peppermint extract Preparation of extract: 500 g of fresh peppermint leaves were added to 5 L of pure water at 50℃, and the extract was extracted with ultrasonic assistance for 30 min. After filtration and clarification by 0.45 μm microfiltration, 4.5 L of extract was obtained, with soluble solids of 6.2°Brix.

[0026] Container and equipment: Square container with a 2 mm thick 304 stainless steel bottom wall (thermally conductive) and a 50 mm thick polyurethane foam side wall (thermally insulated). The internal dimensions are 300 mm × 300 mm × 150 mm (height). The freezing platform is a flat-plate freezer with a temperature control accuracy of ±0.5℃.

[0027] First-level concentration: Inject the extract into the container to a height of 100 mm (volume 4 L).

[0028] The container was placed on a freezing platform with the cold source temperature set to -18℃ (15.5℃ difference from the freezing point).

[0029] Freeze for 6 hours. During this time, ice crystals were observed to grow uniformly upwards from the bottom, while the liquid phase at the top gradually concentrated.

[0030] Stop freezing when the volume of the top concentrate has decreased to about 35% of its initial volume (approximately 1.4 L). At this point, a lower layer of ice crystals (approximately 65 mm thick) and an upper layer of concentrate (approximately 35 mm high) will form inside the container.

[0031] The upper concentrate was separated by pouring to obtain approximately 1.4 L of primary concentrate with 16.8°Brix soluble solids (concentration factor 2.71 times).

[0032] After the lower layer of ice crystals was removed and melted, the solid content was measured to be 0.11°Brix, and the entrainment rate was 0.7%.

[0033] The entrainment rate in this application is defined as: Ice crystal entrainment rate = (Soluble solids content of the liquid after the ice crystals have completely melted ÷ Soluble solids content of the original solution) × 100%.

[0034] Secondary concentration: The first-stage concentrate was injected into the same container (liquid layer height 70 mm), and the freezing point was measured to be -3.8℃.

[0035] Set the cold source temperature to -20℃ (16.2℃ difference from the freezing point) and freeze for 5 hours.

[0036] Collect approximately 0.5 L of the concentrate, with soluble solids of 39.2°Brix (total concentration factor of 6.32 times).

[0037] The entrainment rate of ice crystals in the lower layer is 0.9%.

[0038] Level 3 Concentration (Optional): The secondary concentrate was further concentrated to a liquid level of 50 mm, a freezing point of -5.2℃, and a cold source of -22℃ for 4 hours.

[0039] 0.2 L of concentrate was obtained, with a solids content of 53.8°Brix (total concentration factor of 8.68 times) and an entrainment rate of 1.1%.

[0040] Quality Inspection: The secondary concentrate retained 91.8% of the total volatile components and 96.1% of the total flavonoids. Sensory evaluation: It has a rich and pure mint aroma with no cooked taste.

[0041] Example 2: Gradient-directed freeze concentration of Hangzhou white chrysanthemum extract Extraction solution: 100 g of dried Hangzhou white chrysanthemum was extracted twice with 2 L of pure water at 80℃. The extracts were combined, centrifuged and clarified to obtain 1.8 L of soluble solids with a concentration of 5.8 Brix.

[0042] operate: Container: Same as in Example 1, with an inner cavity size of 150 mm × 150 mm × 100 mm (height) and a liquid layer thickness of 90 mm (volume 2.0 L).

[0043] First-stage concentration: -16℃ cold source (difference 13.7℃), frozen for 7 hours, collect 0.7 L of concentrate (35% by volume), solids 15.2°Brix (fold 2.62), ice crystal entrainment 0.8%.

[0044] Secondary concentration: -18℃ cold source, frozen for 5.5 hours, 0.25 L of concentrate was collected, solids 36.8°Brix (total multiple 6.34), entrainment rate 1.0%.

[0045] Three-stage concentration: -20℃ cold source, freeze for 4 hours, collect 0.1 L of concentrate, solids 52.4°Brix (total multiple 9.03), entrainment rate 1.2%.

[0046] Test results: Chlorogenic acid retention rate 94.8%, volatile oil retention rate 91.5%.

[0047] Example 3: Gradient-directed freeze concentration of rose extract Preparation of extract: 300 g of fresh rose petals were added to 3 L of pure water at 40℃ and extracted for 2 hours. After filtration, 2.5 L of soluble solids were obtained, with a soluble solids content of 4.2°Brix and a freezing point of -1.8℃.

[0048] Container: Internal dimensions 250 mm × 250 mm × 100 mm (height), liquid layer thickness 80 mm (volume 5 L).

[0049] First-stage concentration: -15℃ cold source (difference 13.2℃), frozen for 8 hours, collect 1.8 L of concentrate (volume ratio 36%), solids 10.8°Brix (fold 2.57), entrainment rate 0.6%.

[0050] Secondary concentration: -17℃ cold source, frozen for 6 hours, 0.65 L of concentrate was collected, solids 26.5°Brix (total multiple 6.31), entrainment rate 0.9%.

[0051] Test results: The total retention rate of rose water (based on the main aroma component phenylethanol) was 93.2%, and the total flavonoid retention rate was 95.8%.

[0052] Example 4: Comparison of different container sizes (examining the range of liquid layer thickness) Using peppermint extract as raw material, and with a fixed cold source of -18℃, the effects of different liquid layer thicknesses were compared:

[0053] The results show that the entrainment rate is ≤1.1% in the liquid layer thickness range of 60-150 mm. The entrainment rate increases significantly after exceeding 150 mm, and the optimal range is 60-120 mm.

[0054] Example 5: Comparison of temperatures from different cold sources (examining temperature range) Using peppermint extract as the raw material, with a fixed liquid layer thickness of 100 mm, the effects of different cold source temperatures were compared:

[0055] The results show that within the cold source temperature range of -25℃ to -15℃, the entrainment rate is ≤1.2% and the retention rate is ≥90%. At excessively low temperatures (-30℃), the entrainment rate increases while the retention rate decreases; at excessively high temperatures (-10℃), the entrainment rate is significantly higher. The optimal range is -15℃ to -20℃.

[0056] Comparative Example 1 (conventional isothermal static freezing, without insulating sidewalls) Objective: To verify the necessity of sidewall insulation and vertical gradient freezing.

[0057] Method: A standard stainless steel basin (without heat insulation on the sidewalls, 2 mm thick) was filled with peppermint extract to a liquid layer thickness of 100 mm. The entire basin was placed in a -18°C freezer and frozen evenly (without bottom heating) until completely frozen (approximately 12 hours). After removal, the mixture was allowed to thaw naturally, and the first 40% of the thawed liquid was collected as a concentrate.

[0058] Results: After primary concentration, the soluble solids concentration increased from 6.2°Brix to 11.8°Brix (a factor of 1.90), and the ice crystal entrainment rate (based on the remaining ice core solids) was 5.2%. The volatile component retention rate was 78.6%.

[0059] Compared with Example 1: Example 1 of the present invention achieved a concentration factor of 2.71 (43% increase) at the same freezing temperature, an entrainment rate of 0.7% (87% decrease), and a retention rate of 92.3% (17% increase). This demonstrates that sidewall insulation combined with bottom directional freezing is crucial for improving ice crystal purity.

[0060] Comparative Example 2 (bottom heat conduction + side wall insulation, but using slow freezing temperature -6℃) Method: Using the same insulated container as in Example 1, peppermint extract was added to form a liquid layer 100 mm thick. The cold source temperature was set to -6°C (mild freezing), and the mixture was frozen for 18 hours until completely frozen. Due to the slow freezing rate, although the solute was displaced during ice crystal growth, the vertical temperature gradient was not steep enough, and some solute failed to migrate to the top in time.

[0061] Results: After primary concentration, the concentrate volume ratio was 42%, the solids content was 12.5°Brix (fold increase 2.02), and the ice crystal entrainment rate was 3.6%. The volatile component retention rate was 89.2%.

[0062] Compared with Example 1: Example 1 had a cold source of -18°C and an entrainment rate of 0.7%, which was much lower than the 3.6% of Comparative Example 2. This demonstrates that the steep temperature gradient formed by the faster freezing rate (-25°C to -15°C) is the key to low entrainment, while the traditionally believed "slow freezing reduces entrainment" is less effective under this container structure.

[0063] Comparative Example 3 (bottom heat conduction + side wall insulation, but cold source temperature -30℃) Method: Same container as in Example 1, cold source temperature -30℃, liquid layer thickness 100 mm, frozen for 5 hours.

[0064] Results: After primary concentration, the concentrate volume ratio was 32%, the solids content was 17.5°Brix (fold 2.82), but the ice crystal entrainment rate was 2.4%, and the volatile component retention rate was 84.5%. The ice crystals showed cracks and dendritic structures, and some solute was encapsulated.

[0065] Conclusion: When the cold source temperature is below -25°C, excessive supercooling leads to explosive ice crystal growth, which in turn exacerbates entrainment and damages some heat-sensitive components. This proves that -25°C is the upper limit threshold of this invention.

[0066] Comparative Example 4 (using an insulated bottom wall + a thermally conductive side wall, with a reverse gradient) Method: The container was modified to have a heat-insulated bottom wall (polyurethane) and heat-conducting side walls (stainless steel). Peppermint extract was added, and the container was cooled from the side walls, with no cooling at the bottom. The liquid layer thickness was 100 mm, and the cold source temperature was -18℃.

[0067] Results: Ice crystals grew from the sidewalls inwards, creating a radial temperature gradient. The solute was displaced to the central region, ultimately forming a structure where the concentrated liquid was encased by outer ice crystals, making separation impossible by simple pouring. The container had to be broken to remove the concentrated liquid, making the operation extremely inconvenient, and the ice crystal entrainment rate was as high as 6.8%.

[0068] Conclusion: A unidirectional vertical gradient (from bottom to top) is the preferred direction of this invention; temperature gradients in other directions cannot achieve in-situ separation.

[0069] Comparative Example 5 (no adjustment of cold source temperature during multi-stage concentration) Method: The same peppermint extract as in Example 1 was used for three-stage concentration, but the temperature of the cold source for each stage was fixed at -18°C (not adjusted as the freezing point of the concentrate decreased).

[0070] result: Level 1: Freezing point -2.5℃, cold source -18℃, entrainment rate 0.7%, normal.

[0071] Level 2: The freezing point of the first-stage concentrate is -3.8℃, the cold source is -18℃, the difference is 14.2℃, the entrainment rate is 1.2%, which is normal.

[0072] Level 3: The freezing point of the secondary concentrate is -5.2℃, the cold source is -18℃, the difference is 12.8℃, and the entrainment rate is 1.8%, which is slightly higher but still acceptable.

[0073] Level 4: The freezing point of the Level 3 concentrate is -7.0℃, the cold source is -18℃, the difference is 11.0℃, the freezing time is extended to 7 hours, and the entrainment rate is 2.5%, which is significantly higher.

[0074] Conclusion: When the freezing point of the concentrate decreases, maintaining a fixed cold source temperature leads to reduced subcooling, decreased driving force, prolonged freezing time, and increased entrainment. Therefore, no adjustment is necessary for 2-3 stage concentrations, but for stages beyond 3, it is recommended to reduce the cold source temperature by 1-3°C per stage.

[0075] Comparative Example 6 (liquid layer thickness greater than 150 mm) Method: The same container as in Example 1 was used, but the liquid layer thickness was increased to 200 mm, and the container was frozen at -18°C for 14 hours.

[0076] Results: Due to the excessive thickness of the liquid layer, the temperature gradient between the top and bottom was insufficient to drive the solute to migrate completely to the top, resulting in solute encapsulation in the upper part of the ice crystals. The ice crystal entrainment rate was 3.2%, and the first-stage concentration factor was only 1.9. During separation, the upper concentrate volume accounted for 55% of the initial volume, but it still contained some suspended fine ice crystals, requiring filtration to obtain a clear concentrate.

[0077] Conclusion: The advantages of this invention significantly decrease when the liquid layer thickness exceeds 150 mm. Therefore, limiting the liquid layer thickness to 60-150 mm is more reasonable.

[0078] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for concentrating plant extracts based on gradient freezing and directional salt removal, characterized in that, Includes the following steps: (1) The plant extract stock solution was pretreated to obtain a clear plant extract solution; (2) Dispense the extract into containers, the containers having a heat-conducting bottom wall and an insulated side wall to ensure that heat transfer is mainly carried out in the vertical direction, and the liquid layer thickness is 60-150 mm. (3) Place the container on a freezing platform and cool the bottom of the container. The temperature of the cold source is -25℃ to -15℃ and the freezing time is 4-12 hours. This allows the ice crystals to grow unidirectionally from bottom to top, and the solute is continuously squeezed into the unfrozen liquid phase at the top, forming a layered structure of lower ice crystals and upper concentrated liquid. (4) Stop freezing, separate the upper layer of concentrate from the lower layer of ice crystals, and collect the concentrate as the first-stage concentrate; (5) Repeat steps (2) to (4) to concentrate the first-stage concentrate until the target concentration is reached.

2. The method according to claim 1, characterized in that, The heat-conducting bottom wall is made of stainless steel or aluminum with a thickness of 1-5 mm; the heat-insulating side wall is made of polyurethane foam, polystyrene foam or vacuum insulation board with a thickness of ≥30 mm.

3. The method according to claim 1, characterized in that, In step (5), the number of stages of multi-stage concentration is 2-4, and the temperature of the cold source in each stage decreases by 1-3℃ as the freezing point of the concentrate decreases.

4. The method according to claim 1, characterized in that, The solute entrainment rate of the lower ice crystals obtained by the method is ≤2%, and they can be used as process water after melting.

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