A high-grade salt purification and preparation system

Abstract

This invention relates to the field of sodium chloride preparation system technology, and in particular to a high-grade salt purification and preparation system, including a brine purification module for removing suspended solids and calcium, magnesium, and iron ions from the brine to obtain refined brine. A dual-membrane method is then used to remove suspended solids and sodium sulfate from the refined brine. A first heat exchanger, whose heat exchange input end is connected to the output end of the brine purification module, is used to control the temperature of the purified brine. In use, the brine purification module removes suspended solids and calcium, magnesium, and iron ions from the brine to obtain refined brine. Membrane filtration is used to remove suspended solids from the refined brine, and nanofiltration is used to separate and control the sodium sulfate content. A near-spherical salt crystallization module controls factors such as brine temperature, salt concentration, impurity content, stirring speed, evaporation intensity, supersaturation, residence time, and solution pH to reduce and dissolve powdered salt, separating large and small crystal particles, with large particles settling and small particles suspended.

Description

Technical Field

[0001] This invention relates to the field of sodium chloride preparation system technology, and in particular to a high-grade salt purification and preparation system. Background Technology

[0002] The purity, particle size, morphology, strength, and flowability of salt are important indicators for evaluating salt quality. Sodium chloride prepared by sun-drying crystallization or slow evaporation crystallization is mostly cubic. Currently, salt-producing enterprises using MVR evaporation, single-effect evaporation, and multi-effect evaporation systems to crystallize sodium chloride have a wide particle size distribution and a high proportion of irregular morphological crystals. Different crystal morphologies affect many properties of the crystal, including the efficiency of crystal nucleation or crystal regrowth, centrifugal filtration, and drying during evaporation crystallization, as well as application performance such as dissolution rate, bulk density, and flowability. A wider particle size distribution and a higher proportion of fine powder particles increase the contact area between particles, thus increasing the tendency to agglomerate, which affects the storage, transportation, and use of salt. This directly impacts product quality, performance, salt variety adjustment, and sales price.

[0003] Many factors influence the crystallization particle size and crystal form in existing salt production processes. Sodium chloride crystal growth is affected by various factors, including temperature, supersaturation, stirring, impurities, solution pH, and crystal particle size. Since different salt mines have different impurities and content in their raw brine, the brine purification processes and technologies used to prepare refined brine vary. CN111302362A describes a method of obtaining large-particle spherical salt by adding gum arabic to a saturated sodium chloride solution and evaporating it. The particle size of the large-particle spherical salt is 400-950 μm, and the sphericity is 0.5-1.0. This technical solution requires pretreatment of the sodium chloride slurry solution, and the particle size and sphericity of the obtained salt still need improvement. "Research on the Regulation of Spherical and Flaky Sodium Chloride Crystal Habits". Chemical Industry and Engineering [J]. 2018, 35(3). The influence of small-scale liquid evaporation on the growth of sodium chloride crystals was studied. In the vacuum evaporation crystallization experiment, when a stirring rate of 650 r / min, a liquid evaporation volume of 122 mL and an evaporation rate of 5.2 min / mL were used, the best spherical sodium chloride crystals could be obtained. However, this technology and parameters cannot realize the production application of high-yield spherical sodium chloride. "The Influence of Brine Performance Indicators on Spherical Sodium Chloride Crystallization". Salt Science and Chemical Engineering [J]. 2022, 59(5). Under the condition of pH 2, brine can produce spherical sodium chloride products with large particle size, good sphericity and easy agglomeration. When the pH value is increased to 11, spherical sodium chloride products with small particle size, poor sphericity and easy agglomeration are obtained. Introducing CaSO4 can increase the boiling point of sodium chloride solution. Since brine salt production is a neutral or alkaline system and increases energy consumption, this technology and parameters cannot achieve high-yield production application.

[0004] Brine purification is a crucial process in the salt-making industry for reducing impurities and improving product quality. The content of sodium sulfate, calcium, magnesium, and iron ions in the raw brine is unstable. Instability in raw material parameters, reaction parameters, process parameters, operating parameters, and operating procedures during purification prevents the full realization of the process's advantages. The sodium sulfate and suspended solids in the purified brine have an impact; as the salt-making system concentrates during evaporation, the content of sodium sulfate and suspended solids gradually increases, affecting the crystal form and particle size of the crystalline salt. This results in incomplete crystal formation, which is easily broken during centrifugation, drying, and transportation. Increased powder content further impacts salt quality, leading to smaller crystal size and a higher proportion of irregular crystals. This results in a high proportion of fine powder particles in the salt production, which are easily dispersed during centrifugation, drying, packaging, and transportation, potentially forming salt mist and affecting the surrounding environment, equipment, and production. The crystallization and growth of vacuum-evaporated sodium chloride crystals are influenced by various factors, including temperature, supersaturation, stirring, impurities, solution pH, and crystal morphology and size. Crystal particle size has a significant impact on agglomeration. The larger the proportion of small particles, the greater the contact area between particles, and the greater the tendency to agglomerate, thus affecting the storage, transportation, and use of salt.

[0005] This patent provides a high-quality salt purification and preparation system. By optimizing and improving the purification process, it controls the content of sodium sulfate, calcium, magnesium, and iron ions, and suspended solids in the brine, thereby improving the quality of the purified brine. During the evaporation and salt production process, by controlling temperature, solution concentration, impurity content, stirring speed, evaporation intensity, supersaturation, and residence time, the nucleation rate can be adjusted to achieve isotropic crystal growth or agglomerated crystal forms. Simultaneously, the shear force generated by fluid flow can smooth the edges of crystals or aggregates. The synergistic effect of these factors allows sodium chloride crystals to grow into regular, near-spherical particles. Larger and more uniform near-spherical or spherical sodium chloride crystals exhibit better anti-caking and flowability, which is beneficial for production, transportation, packaging, and storage. Near-spherical sodium chloride has the widest application prospects due to its outstanding anti-caking properties and good flowability, offering unique advantages in industrial production and daily life applications. Summary of the Invention

[0006] The technical problem this invention aims to solve is: In existing salt production, the presence of sodium sulfate, suspended solids, and ionic impurities in the purified brine leads to a gradual increase in the concentration of sodium sulfate, suspended solids, and impurity ions during the evaporation and salt-making process. This affects the crystal form and particle size of the crystalline salt, resulting in incomplete crystal forms. Consequently, the proportion of fine powder particles and irregular crystal forms in the salt production is high. During subsequent centrifugation, drying, packaging, and transportation, the incomplete salt crystals are easily broken, increasing the powder salt content. The fine powder particles are also easily dispersed, even forming salt mist, which affects the surrounding environment, equipment, and production. This invention provides a high-quality salt purification and preparation system.

[0007] The technical solution adopted by this invention to solve its technical problem is: a high-grade salt purification and preparation system, comprising: The brine purification module is used to remove light suspended solids from the brine and to obtain refined brine after calcium, magnesium and iron ions are removed and precipitated. The first heat exchanger has its heat exchange input end connected to the output end of the brine purification module, and is used to control the temperature of the purified brine. A near-spherical salt crystallization module includes an upper tank, a middle tank, and a lower tank, which are arranged sequentially from top to bottom and interconnected. The capacity of the upper, middle, and lower tanks gradually increases from top to bottom. The upper tank is provided with an input port and a vacuum port. The input port is connected to the heat exchange output end of a first heat exchanger, and the vacuum port is connected to an external vacuum system. A first stirring mechanism is provided in each of the upper, middle, and lower tanks, with the first stirring mechanism located in the upper tank. The first stirring mechanism is located in the middle tank and is used to separate large and small crystal particles, allowing large crystal particles to descend, small crystal particles to rise, and large crystal particles to be rounded and solidified. The first stirring mechanism is located in the lower tank and is used to round and solidify large crystal particles again to form near-spherical or spherical salt. The lower tank is provided with an outlet for outputting near-spherical or spherical salt. The first heat exchanger is used to control the temperature of the upper tank, middle tank and lower tank and to gradually decrease the temperature from top to bottom. Compared to existing technologies, this solution removes light suspended solids from the brine and removes calcium, magnesium, and iron ions to obtain refined brine through a brine purification module. Then, a near-spherical salt crystallization module controls factors such as brine temperature, salt concentration, impurity content, stirring speed, evaporation intensity, supersaturation, residence time, and solution pH to reduce and dissolve powdered salt, separate large and small crystal particles, allow large particles to descend and be rounded and solidified, and allow small particles to rise. Finally, the large crystal particles are rounded and solidified again to form near-spherical or spherical salt, thereby improving the particle size of the salt product, reducing the proportion of powdered salt, and improving the quality of the salt product.

[0008] To better control the temperature of the brine in the upper, middle, and lower tanks, in some preferred embodiments, a first outer shell is provided on the outer circumferential surface of the upper tank, forming a first insulation cavity between the first outer shell and the upper tank; a second outer shell is provided on the outer circumferential surface of the middle tank, forming a second insulation cavity between the second outer shell and the middle tank; a third outer shell is provided on the outer circumferential surface of the lower tank, forming a third insulation cavity between the third outer shell and the middle tank; and the condenser end of the first heat exchanger is connected to the first, second, and third insulation cavities respectively.

[0009] To implement the brine purification module, some preferred embodiments include a suspended solids removal tank, a purification reaction tank, a refined brine storage tank, a flat sheet membrane, a nanofiltration membrane, and a low-nitrate refined brine tank. The suspended solids removal tank is equipped with a brine inlet communicating with external brine. An overflow port for discharging suspended solids is provided above the suspended solids removal tank. A removal mechanism for removing suspended solids is provided inside the suspended solids removal tank. The suspended solids removal tank is connected to the feed and sludge discharge port of the purification reaction tank via a first conveying pump. The purification reaction tank is connected to the input end of the refined brine storage tank via a second conveying pump. The refined brine storage tank is connected to the input end of the flat sheet membrane via a third conveying pump. The output end of the flat sheet membrane is connected to the input end of the nanofiltration membrane via a fourth conveying pump. The output end of the nanofiltration membrane is connected to the input end of the low-nitrate refined brine tank. The output end of the low-nitrate refined brine tank is connected to the heat exchange input end of the first heat exchanger via a fifth conveying pump.

[0010] In some preferred embodiments, the bottom of the lower tank is connected to the input port on the upper tank via a sixth delivery pump.

[0011] In some preferred embodiments, the bottom of the intermediate tank is connected to the outlet of the refined brine storage tank via a seventh delivery pump.

[0012] To achieve the removal mechanism, in some preferred embodiments, the removal mechanism includes an air compressor, an air tank, a first aeration pipe and a second aeration pipe. The first aeration pipe and the second aeration pipe are arranged side by side at the bottom of the suspended solids removal tank. The output end of the air compressor is connected to the input end of the air tank, and the output end of the air tank is connected to the first aeration pipe and the second aeration pipe respectively.

[0013] In some preferred embodiments, the purification reaction tank is provided with a second stirring mechanism.

[0014] In some preferred embodiments, a second heat exchanger is also included, wherein the output end of the third delivery pump is connected to the heat exchange input end of the second heat exchanger, the heat exchange output end of the second heat exchanger is connected to the input end of the flat plate membrane, and the output end of the condenser end of the first heat exchanger is connected to the input end of the condenser end of the second heat exchanger.

[0015] In some preferred embodiments, the first stirring mechanism includes a first stirring shaft, one end of which is rotatably mounted on the upper tank, and the other end of which extends from the upper tank to the lower tank. A first stirring paddle is provided in the upper tank, a second stirring paddle is provided in the middle tank, and a third stirring paddle is provided in the lower tank.

[0016] In some preferred embodiments, a middle guide tube is provided inside the middle tank, the second stirring paddle is located inside the middle guide tube, a lower guide tube is provided inside the lower tank, and the third stirring paddle is located inside the lower guide tube.

[0017] The beneficial effects of this invention are as follows: When using the high-grade salt purification and preparation system of this invention, the brine purification module removes suspended solids and calcium, magnesium and iron ions from the brine to obtain refined brine. Then, the double-membrane method is used to remove suspended solids and reduce sodium sulfate to refine the refined brine. Then, the near-spherical salt crystallization module controls the temperature, solution concentration, impurity content, stirring speed, evaporation intensity, supersaturation, residence time and other factors during the evaporation and salt production process. This allows for the adjustment of the nucleation rate, isotropic crystal growth or agglomeration of crystals. At the same time, the shear force generated by the fluid flow can smooth the edges of the crystals or aggregates. The synergistic effect of these factors enables sodium chloride crystals to grow into regular near-spherical or spherical particles. This process reduces and dissolves powdered salt, separating crystal particles of varying sizes. Larger particles descend and are rounded and solidified into spherical shapes, while smaller particles rise, dissolve, or enlarge further. Finally, the larger crystal particles are solidified again to form near-spherical or spherical salt, thereby improving the particle size of the salt product, reducing the proportion of powdered salt, and enhancing the quality of the salt product. The resulting salt has better quality, with higher sphericity or a higher proportion of spherical particles, resulting in high particle size uniformity. This reduces the contact area between crystals, significantly decreasing the tendency to agglomerate. This crystal form exhibits good anti-caking properties, extending the storage time of the salt product without the need for anti-caking agents. By optimizing the purification process and adding parameters such as raw materials, reactions, and processes, the content of calcium, magnesium, and iron ions and suspended solids is reduced, and the sodium sulfate content in the raw brine is controlled. This process leverages its advantages and solves the problems caused by the gradual increase in sodium sulfate and suspended solids content during the evaporation and salt production system, which affects the crystal form and particle size of the crystalline salt. This results in incomplete crystal forms, which are easily broken during centrifugation, drying, packaging, and transportation. The increased content of powdery salt also affects salt quality, leading to smaller crystal particle size, a high proportion of irregular crystal forms, and a high proportion of fine powder particles in the salt production. These fine powder particles are easily dispersed and may even form salt mist, affecting the surrounding environment, equipment, and production. Attached Figure Description

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] Figure 1 This is a schematic diagram of the structure of the present invention; Figure 2 This is a schematic diagram of the brine purification module in this invention; Figure 3 This is a schematic diagram of the structure of the first heat exchanger in this invention; Figure 4 This is a schematic diagram of the structure of the near-spherical salt crystallization module in this invention; Figure 5 This is a diagram of sodium chloride crystals prepared according to the present invention.

[0020] In the diagram: 1. Brine purification module; 101. Suspended solids removal tank; 1011. Brine inlet; 1012. Overflow outlet; 102. Purification reaction tank; 1021. Feed and sludge discharge outlet; 103. Refined brine storage tank; 104. Flat sheet membrane; 105. Nanofiltration membrane; 106. Low-nitrate refined brine tank; 107. First transfer pump; 108. Second transfer pump; 109. Third transfer pump; 110. Fourth transfer pump; 111. Fifth transfer pump; 112. Removal mechanism; 1121. Air compression device; 1122. Air storage tank; 1123. First aeration pipe; 1124. Second aeration pipe; 113. Second stirring mechanism; 114. Second heat exchanger. 2. First heat exchanger; 3. Near-spherical salt crystallization module, 301, upper tank, 3011, input port, 3012, vacuum port, 3013, first outer shell, 302, middle tank, 3021, second outer shell, 303, lower tank, 3031, discharge port, 3032, third outer shell, 304, first stirring mechanism, 305, sixth conveying pump, 306, seventh conveying pump. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the embodiments: This invention is not limited to the specific embodiments listed below. Those skilled in the art can implement this invention using various other specific embodiments based on the content disclosed herein. Any modifications or alterations made to the design structure and concept of this invention fall within the protection scope of this invention. It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0022] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0023] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0024] like Figure 1-5 As shown, a high-grade salt purification and preparation system includes: Brine purification module 1 is used to remove light suspended solids from the brine and to obtain refined brine after calcium, magnesium and iron ions are removed and settled. The first heat exchanger 2 has its heat exchange input end connected to the output end of the brine purification module 1, and is used to control the temperature of the purified brine. The near-spherical salt crystallization module 3 includes an upper tank 301, a middle tank 302, and a lower tank 303, which are arranged sequentially from top to bottom and interconnected. The capacity of the upper tank 301, middle tank 302, and lower tank 303 gradually increases from top to bottom. The upper tank 301 is provided with an input port 3011 and a vacuum port 3012. The input port 3011 is connected to the heat exchange output end of the first heat exchanger 2, and the vacuum port 3012 is connected to an external vacuum system. A first stirring mechanism 304 is provided in each of the upper tank 301, middle tank 302, and lower tank 303. Mechanism 304 is located inside the upper tank 301 and is used to reduce and dissolve the powdered salt in the upper tank 301. The first stirring mechanism 304 is located inside the middle tank 302 and is used to separate large and small crystal particles and realize the downward movement of large crystal particles, the upward movement of small crystal particles, and the rounding and solidification of large crystal particles. The first stirring mechanism 304 is located inside the lower tank 303 and is used to round and solidify large crystal particles again to form near-spherical salt. The lower tank 303 is provided with an outlet 3031 for the output of near-spherical or spherical salt. The first heat exchanger 2 is used to control the temperature of the brine in the upper tank 301, the middle tank 302 and the lower tank 303 and make the temperature gradually decrease from top to bottom.

[0025] A first outer shell 3013 is provided on the outer peripheral surface of the upper tank 301, and a first heat-insulating cavity is formed between the first outer shell 3013 and the upper tank 301. A second outer shell 3021 is provided on the outer peripheral surface of the middle tank 302, and a second heat-insulating cavity is formed between the second outer shell 3021 and the middle tank 302. A third outer shell 3032 is provided on the outer peripheral surface of the lower tank 303, and a third heat-insulating cavity is formed between the third outer shell 3032 and the middle tank 302. The condensing end of the first heat exchanger 2 is connected to the first heat-insulating cavity, the second heat-insulating cavity and the third heat-insulating cavity respectively. A condensate drain outlet is provided on the first heat-insulating cavity, the second heat-insulating cavity and the third heat-insulating cavity.

[0026] The brine purification module 1 includes a suspended solids removal tank 101, a purification reaction tank 102, a refined brine storage tank 103, a flat sheet membrane 104, a nanofiltration membrane 105, and a low-nitrate refined brine tank 106. The suspended solids removal tank 101 is equipped with a brine inlet 1011 that communicates with external brine. An overflow port 1012 for discharging light suspended solids is located above the tank 101. A removal mechanism 112 for removing light suspended solids is installed inside the tank 101. The suspended solids removal tank 101 is connected to the purification reaction tank 102 via a first transfer pump 107. The feed and sludge discharge port 1021 is connected. The purification reaction tank 102 is connected to the input end of the refined brine storage tank 103 through the second conveying pump 108. The refined brine storage tank 103 is connected to the input end of the flat plate membrane 104 through the third conveying pump 109. The output end of the flat plate membrane 104 is connected to the input end of the nanofiltration membrane 105 through the fourth conveying pump 110. The output end of the nanofiltration membrane 105 is connected to the input end of the low-nitrate refined brine tank 106. The output end of the low-nitrate refined brine tank 106 is connected to the heat exchange input end of the first heat exchanger 2 through the fifth conveying pump 111.

[0027] The bottom of the lower tank 303 is connected to the input port 3011 on the upper tank 301 via the sixth transfer pump 305, and the bottom of the middle tank 302 is connected to the outlet of the refined brine storage tank 103 via the seventh transfer pump 306.

[0028] The removal mechanism 112 includes an air compressor 1121, an air storage tank 1122, a first aeration pipe 1123, and a second aeration pipe 1124. The first aeration pipe 1123 and the second aeration pipe 1124 are arranged side by side at the bottom of the suspended solids removal tank 101. The output end of the air compressor 1121 is connected to the input end of the air storage tank 1122. The output end of the air storage tank 1122 is connected to the first aeration pipe 1123 and the second aeration pipe 1124 respectively. The air storage tank 1122 is used to store air and also acts as a buffer. When conveying gas, in order to prevent insufficient air output or blockage of the outlets of the first aeration pipe 1123 and the second aeration pipe 1124, an automatic control valve of the air storage tank 1122 is set during gas conveying. The valve is closed for 30 to 60 seconds after 4 to 8 hours of operation and then reopened to reduce blockage of the outlets of the first aeration pipe 1123 and the second aeration pipe 1124.

[0029] A second stirring mechanism 113 is provided on the purification reaction tank 102.

[0030] It also includes a second heat exchanger 114, the output end of the third transfer pump 109 is connected to the heat exchange input end of the second heat exchanger 114, the heat exchange output end of the second heat exchanger 114 is connected to the input end of the flat plate membrane 104, and the output end of the condenser end of the first heat exchanger 2 is connected to the input end of the condenser end of the second heat exchanger 114.

[0031] The first stirring mechanism 304 includes a first stirring shaft. One end of the first stirring shaft is rotatably mounted on the upper tank 301, and the other end of the first stirring shaft extends from the upper tank 301 to the lower tank 303. A first stirring paddle is provided inside the upper tank 301. A second stirring paddle is provided inside the middle tank 302. A third stirring paddle is provided inside the lower tank 303. A middle guide tube is provided inside the middle tank 302, and the second stirring paddle is located inside the middle guide tube. A lower guide tube is provided inside the lower tank 303, and the third stirring paddle is located inside the lower guide tube. In this embodiment, both the middle and lower guide tubes are used to crystallize near-spherical or spherical salt particles in the brine, separating large and small crystal particles, causing large crystal particles to descend and small crystal particles to rise, and the crystal particles to be rounded and solidified to form near-spherical or spherical salt.

[0032] Near-spherical salt crystals are measured by their roundness. The formula for roundness is: projected area multiplied by 4π divided by the square of the perimeter. ; ; ;

[0033] Where e represents the circularity, and S represents the area of ​​the circle, with S in μm. 2 C is the circumference of the circle, and the unit of C is μm or d. x and d y d represents the maximum and minimum diameters of the projected plane of sodium chloride crystals. x and d y Units: μm, r 平 For d x and d y The sum divided by 4, r 平 Unit: μm. When e=1, sodium chloride crystals are spherical. When 0.9≤e<1, sodium chloride crystals are nearly spherical. The closer the e value is to 1, the higher the sphericity. The smaller the e value, the lower the sphericity.

[0034] In use, the aforementioned high-grade salt purification and preparation system first introduces external brine into the suspended solids removal tank 101. The air compressor 1121 is then activated to supply compressed air to the air storage tank 1122 until the required pressure is reached. The output of the air storage tank 1122 is then opened and supplied to the first aeration pipe 1123 and the second aeration pipe 1124 within the brine. The first and second aeration pipes 1123 and 1124 aerate the brine, and the light suspended solids in the brine are discharged through the overflow port 1012. The first delivery pump 107 then delivers the brine, after the suspended solids have been removed, to the purification reaction tank 102. Sodium carbonate and hydroxide are added to the purification reaction tank 102. After sodium is used to remove calcium, magnesium, and iron ions and the precipitates, a refined brine is obtained. The total calcium and magnesium ion content is ≤4.5 mg / L, the iron ion content is ≤0.8 mg / L, the sodium chloride content is 290~305 g / L, the sodium sulfate content is 9~18 g / L, the pH value is 11.0~11.8, and the suspended solids in the refined brine are ≤5 mg / L. Simultaneously, the second stirring mechanism 113 on the purification reaction tank 102 rotates and stirs the brine, and the brine is transported to the refined brine storage tank 103 by the second transfer pump 108. Hydrochloric acid is added to the refined brine storage tank 103 to adjust the pH value of the refined brine to 8.5~9.5. Then, after heat exchange through a heat exchanger, the brine is transported to the flat plate membrane 104, where the flat plate membrane 104 removes the impurities from the refined brine. Trace suspended solids, with a concentration ≤0.8 mg / L, then enter the nanofiltration membrane 105. The nanofiltration membrane 105 uses alkali-resistant saturated brine to prepare low-nitrate refined brine. Its main components are sodium sulfate ≤3.0 g / L, total calcium and magnesium ion content ≤4.5 mg / L, iron ion content ≤0.8 mg / L, silicon (as SiO2) content ≤0.5 mg / L, pH value: 8.5~9.5, and suspended solids ≤0.8 mg / L. The brine then enters the low-nitrate refined brine tank 106 for storage. The brine in the low-nitrate refined brine tank 106 undergoes heat exchange through the first heat exchanger 2 before entering the near-spherical salt crystallization module 3. The brine is then processed in the upper tank 301, middle tank 302, and lower tank. The temperature of tank 303 is controlled by the first heat exchanger 2. The brine temperature in the upper tank 301 is higher than that in the middle tank 302. The condensate inlet temperature of the upper tank 301 is 81-85℃, and that of the middle tank 302 is 78-82℃. The brine temperature in the middle tank 302 is slightly higher than that in the lower tank 303, which also has a condensate inlet temperature of 78-82℃. The capacity of the upper tank 301 is smaller than that of the middle tank 302, and the capacity of the middle tank 302 is smaller than that of the lower tank 303. This design allows for better and more stable control of the brine in the upper, middle, and lower tanks 301 and 303, maintaining a vacuum level of 35-45%. kPa, automatic variable speed stirring rate is 550~680 r / min, automatic variable speed stirring setting operating parameters: counterclockwise rotation (top view), speed is 680 r / min, run for 5 hours, automatically change speed to 550 r / min, run for 0 hours.After 5 hours, the automatic speed is adjusted to 680 r / min for continuous operation, achieving rapid separation of large and small crystal particles. The evaporation rate of refined brine is 38~45 ml / h. The prepared salt crystal particle size distribution is as follows: 450~600 μm accounts for 88~95%, less than 300 μm accounts for less than 3%, and fine powder crystals less than 150 μm account for less than 0.3%. The projected roundness of salt crystals is ≥95% (0.92~1.0), with high sphericity and strength of 2.50-8.50 N. This results in high particle size uniformity of the salt product, reduces the proportion of powdered salt, has better anti-caking properties, and improves the quality of the salt product. Simultaneously, the upper tank 301 better reduces the amount of powdered salt and dissolves it more quickly. The middle tank 302 rapidly separates large and small crystal particles; small particles rise and dissolve, while large particles descend, are rounded, and fixed. The lower tank 303 further rounds and solidifies the large crystal particles, forming near-spherical or spherical salt. If ultrafine powdered salt particles are encountered when forming near-spherical or spherical salt in the lower tank 303, the brine containing ultrafine powdered salt particles is transported back to the upper tank 301 via the sixth transfer pump 305. The upper tank 301 dissolves the ultrafine powdered salt particles. As the evaporation process of salt production gradually increases with the concentration of sodium sulfate and suspended matter in the salt production system, it affects the crystal form and particle size of the crystalline salt, thus affecting the crystal form of the crystalline salt. Incomplete crystals in the near-spherical salt crystallization module lead to breakage during centrifugation, drying, and transportation, increasing the powdery salt content and affecting salt quality. To prevent high sodium sulfate content in the near-spherical salt crystallization module, the brine in the intermediate tank 302 is transferred to the refined brine storage tank 103 via the seventh transfer pump 306. Steps 104 and 105 reduce the suspended solids and sodium sulfate content in the brine. This solves the problem of the increasing sodium sulfate and suspended solids content during the evaporation salt production system affecting the crystal form and particle size of the crystallized salt, resulting in more powdery salt particles and a higher proportion of irregular crystals. This leads to a high proportion of fine powder particles in the salt production, which easily become airborne and even form salt mist during centrifugation, drying, packaging, and transportation, affecting the surrounding environment, equipment, and production. Furthermore, crystal particle size has a significant impact on agglomeration; a higher proportion of small particles increases the contact area between particles, greatly increasing the tendency to agglomerate, thus affecting the storage, transportation, and use of the salt.

[0035] The above description, based on the preferred embodiments of the present invention, provides inspiration. Those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification but must be determined according to the claims.

Claims

1. A high-grade salt purification and preparation system, characterized in that, include: The brine purification module is used to remove light suspended solids from the brine and to remove calcium, magnesium and iron ions after sedimentation to obtain refined brine. Membrane filtration is used to remove suspended solids from the refined brine, and nanofiltration membrane is used to separate and control the sodium sulfate content. The first heat exchanger has its heat exchange input end connected to the output end of the brine purification module, and is used to control the temperature of the purified brine. A near-spherical salt crystallization module includes an upper tank, a middle tank, and a lower tank, which are arranged sequentially from top to bottom and interconnected. The capacity of the upper, middle, and lower tanks gradually increases from top to bottom. The upper tank is provided with an input port and a vacuum port. The input port is connected to the heat exchange output end of a first heat exchanger, and the vacuum port is connected to an external vacuum system. A first stirring mechanism is provided in each of the upper, middle, and lower tanks, with the first stirring mechanism located in the upper tank. The first stirring mechanism is located in the middle tank and is used to reduce and dissolve the powdered salt in the upper tank. It is used to separate large and small crystal particles and realize the functions of lowering large crystal particles, raising small crystal particles, and rounding and solidifying large crystal particles. The first stirring mechanism is located in the lower tank and is used to further solidify large crystal particles and form near-spherical salt or spherical salt. The lower tank is provided with a discharge port for outputting near-spherical or spherical salt. The first heat exchanger is used to control the temperature of the upper tank, middle tank and lower tank and make the temperature gradually decrease from top to bottom.

2. The high-grade salt purification and preparation system according to claim 1, characterized in that: A first outer shell is provided on the outer peripheral surface of the upper tank, and a first insulation cavity is formed between the first outer shell and the upper tank. A second outer shell is provided on the outer peripheral surface of the middle tank, and a second insulation cavity is formed between the second outer shell and the middle tank. A third outer shell is provided on the outer peripheral surface of the lower tank, and a third insulation cavity is formed between the third outer shell and the lower tank. The condensing end of the first heat exchanger is connected to the first insulation cavity, the second insulation cavity, and the third insulation cavity, respectively.

3. The high-grade salt purification and preparation system according to claim 1, characterized in that: The brine purification module includes a suspended solids removal tank, a purification reaction tank, a refined brine storage tank, a flat sheet membrane, a nanofiltration membrane, and a low-nitrate refined brine tank. The suspended solids removal tank is equipped with a brine inlet communicating with external brine. An overflow port for discharging suspended solids is provided above the suspended solids removal tank. A removal mechanism for removing light suspended solids is provided inside the suspended solids removal tank. The suspended solids removal tank is connected to the feed and sludge discharge port of the purification reaction tank via a first conveying pump. The purification reaction tank is connected to the input end of the refined brine storage tank via a second conveying pump. The refined brine storage tank is connected to the input end of the flat sheet membrane via a third conveying pump. The output end of the flat sheet membrane is connected to the input end of the nanofiltration membrane via a fourth conveying pump. The output end of the nanofiltration membrane is connected to the input end of the low-nitrate refined brine tank. The output end of the low-nitrate refined brine tank is connected to the heat exchange input end of the first heat exchanger via a fifth conveying pump.

4. The high-grade salt purification and preparation system according to claim 3, characterized in that: The bottom of the lower tank is connected to the input port on the upper tank via a sixth delivery pump.

5. The high-grade salt purification and preparation system according to claim 3, characterized in that: The bottom of the intermediate tank is connected to the outlet of the refined brine storage tank via a seventh delivery pump.

6. A high-grade salt purification and preparation system according to any one of claims 3-5, characterized in that: The removal mechanism includes an air compressor, an air tank, a first aeration pipe, and a second aeration pipe. The first and second aeration pipes are arranged side by side at the bottom of the suspended solids removal tank. The output end of the air compressor is connected to the input end of the air tank, and the output end of the air tank is connected to the first and second aeration pipes respectively.

7. The high-grade salt purification and preparation system according to claim 3, characterized in that: The purification reaction tank is equipped with a second stirring mechanism.

8. The high-grade salt purification and preparation system according to claim 3, characterized in that: It also includes a second heat exchanger, the output end of the third delivery pump is connected to the heat exchange input end of the second heat exchanger, the heat exchange output end of the second heat exchanger is connected to the input end of the flat plate membrane, and the output end of the condenser end of the first heat exchanger is connected to the input end of the condenser end of the second heat exchanger.

9. The high-grade salt purification and preparation system according to claim 1, characterized in that: The first stirring mechanism includes a first stirring shaft, one end of which is rotatably mounted on the upper tank, and the other end of which extends from the upper tank to the lower tank. A first stirring paddle is provided in the upper tank, a second stirring paddle is provided in the middle tank, and a third stirring paddle is provided in the lower tank.

10. A high-grade salt purification and preparation system according to claim 9, characterized in that: The middle tank is equipped with a middle guide tube, and the second agitator is located inside the middle guide tube. The lower tank is equipped with a lower guide tube, and the third agitator is located inside the lower guide tube.

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