Method and device for removing micro-plastics in crushed washing type sea salt

Abstract

The invention relates to the field of sea salt processing, in particular to a method and a device for removing micro-plastics in crushed and washed sea salt. The method comprises the following steps: washing crushed crude salt with saturated brine to obtain mixed salt slurry; heating the mixed salt slurry to a temperature higher than room temperature, carrying out centrifugal separation treatment, and drying the cleaned salt to obtain refined salt; reducing the temperature of the saturated brine washing liquid containing the micro-plastics to room temperature under air floatation treatment; and finally, halogen salt crystals containing the micro-plastics are separated from the saturated brine, and the obtained saturated brine is reused in crude salt washing, so that removal of the micro-plastics and circulation of the brine are completed. According to the method, salt leaching brine recycling systems owned by many salt production units at present are combined, on the premise of not additionally adding chemical flocculants, effective removal of micro-plastics is achieved, the technological process is simplified, the production cost is saved, and the method is suitable for being popularized in production practice.

Description

Technical Field

[0001] The invention relates to the field of sea salt processing, and in particular to a method and a device for removing microplastics in crushed and washed sea salt. Background Art

[0002] Raw salt, also known as coarse salt or unprocessed salt, refers to salt that is directly collected from salt fields or salt mines without further processing. It usually contains a certain amount of impurities, such as silt, minerals, brine and other soluble substances. Raw salt cannot meet the requirements of daily salt use in terms of physical and chemical properties or appearance without further processing. Using raw salt resources such as sea salt as raw materials and processing them using crushing and washing technology is a common processing method for edible salt products. Microplastics (MPs) are plastic fragments or particles with a diameter of less than 5 mm. They are widely distributed in natural environments such as the ocean due to the decomposition of plastic products in daily life and the use of products containing tiny plastic particles such as cosmetics and detergents. In the process of sea salt processing, sea salt is prone to absorb microplastics in seawater during the sun-drying process, resulting in salt contamination. After these microplastics enter the human body, they may accumulate in the body, interfere with the functions of the digestive system, endocrine system, and even cause more serious health problems.

[0003] At present, there is little research and development on the removal technology of microplastics in sea salt or brine. In the traditional preparation process of crushed and washed sea salt, the removal rate of microplastics through the salt washing process is between 50% and 90%. The removal of microplastics in sea salt is mostly done by chemically induced coagulation-flocculation-precipitation. The chemically induced coagulation-flocculation-precipitation method is to make colloids and fine suspended matter in the water environment coagulate into floccules under the action of coagulation / flocculation / coagulant, and then separate and remove them after precipitation. For example, in the article "Quantitative Analysis of Coagulation Removal Process of Microplastics in Clean Sea Salt Production" published in the 19th (6) issue of the International Journal of Environmental Science and Technology in 2022, aluminum chloride, aluminum sulfate, ferric chloride and γ-chloropropyltrichlorosilane are used as coagulants for sea salt treatment, and the microplastic removal rate can reach more than 90%. However, the biggest problem with the chemical induction method is that it will introduce new chemical substances into the brine, which increases the difficulty of the subsequent preparation of refined salt.

[0004] Flotation Process is a water treatment technology that is commonly used to separate and remove suspended solids, oils and other pollutants from water. As stated in the article "Microplastic Remediation Technology in Water and Wastewater Treatment: Current Status and Future Prospects" published in the journal "Science of the Holistic Environment" in 2023, the microplastic removal rate of flotation is between 32.7-95%. In addition, since the particles removed by the flotation method are different, the probability of collision with bubbles is also different, so the flotation method often requires the addition of flocculants to increase the particle size of the particles and improve the removal rate. For example, a microplastic separation method based on flotation disclosed in Patent 201910333601.7 improves the removal rate of microplastics by flotation by adding triheptyl dodecyl ammonium iodide as a flocculant. Summary of the invention

[0005] The present invention aims to overcome the defects in the prior art that the removal rate of microplastics in sea salt is not high, and the removal of microplastics in sea salt by traditional flotation method will introduce other chemical substances, making the sea salt refining process more complicated. A method and device for removing microplastics in crushed and washed sea salt are proposed.

[0006] In order to achieve the above-mentioned invention object, the present invention adopts the following technical solutions: In a first aspect, the present invention discloses a method for removing microplastics from crushed and washed sea salt, comprising the following steps: S1, washing the crushed raw salt with saturated brine in a stirring salt washing tank to obtain a mixed salt slurry; S2. The mixed salt slurry is heated to a temperature higher than room temperature and centrifuged to obtain washed salt and a saturated brine washing solution containing microplastics. The washed salt is dried to obtain refined salt; S3, lowering the temperature of the saturated brine washing liquid containing microplastics to room temperature under air flotation treatment, so that the saturated brine washing liquid in step S2 is converted into a supersaturated brine washing liquid, and the halide salt in the supersaturated brine washing liquid is crystallized with the microplastics as crystal nuclei, thereby obtaining halide salt crystals containing microplastics and saturated brine; S4. Separate the halide salt crystals containing microplastics from the saturated brine, and reuse the obtained saturated brine in step S1, thereby completing the removal of microplastics.

[0007] Research on the removal of microplastics in water bodies has a long history. Chemical methods mainly use chemically induced coagulation-flocculation-precipitation to achieve good microplastic removal effects. Physical methods mainly remove microplastics through sedimentation, dissolved air flotation and filtration. However, unlike the removal of microplastics in traditional sewage treatment plants, the removal of microplastics in the sea salt refining process is limited in many aspects. On the one hand, the particulate matter in the salt slurry dissolved in sea salt is less than that in sewage, and it is difficult for microplastics to attach to other large particle pollutants and then precipitate or float. On the other hand, refined salt is closely related to food safety, and the added flocculants need to be removed in subsequent processes. Therefore, whether based on safety or cost considerations, without the addition of external chemicals, the removal of microplastics in sea salt refining has always been a problem to be solved.

[0008] Crushed and washed sea salt is a process in which the crushed sea salt is washed with saturated brine to remove soluble impurities and insoluble sediment. After the sea salt is crushed, the original plastic impurities will be further crushed, so that the abundance of microplastics is further increased, and the untreated saturated brine itself contains microplastics, making it difficult to further reduce the content of microplastics in the traditional crushed and washed sea salt. On the basis of the traditional crushed and washed sea salt process, the present invention obtains saturated brine with less microplastic content by temperature regulation without using chemical flocculants, and then uses it to wash the crushed raw salt, and removes the microplastics in the crushed raw salt in the circulation of saturated brine.

[0009] During the washing process, the crushed raw salt is heated to increase the solubility of the bittern. The temperature solubility of sodium chloride in the sea salt system changes little. For every 100 mL of water from 10°C to 90°C, only 4 g of sodium chloride can be dissolved. The solubility of calcium chloride and magnesium chloride is more sensitive to temperature. For every 100 mL of water from 10°C to 90°C, nearly 100 g of calcium chloride can be dissolved. Therefore, during the heating and washing process, not too much salt will enter the water to cause waste, and more other inorganic salts will melt into the brine. The precipitate after centrifugation is the washed salt, and the supernatant is the saturated brine containing a large amount of microplastics. The precipitated salt is the refined salt with microplastics removed after drying, and the supernatant enters the flotation tank for the next step of treatment.

[0010] In the flotation tank, as the brine returns to room temperature, a supersaturated solution of inorganic salts is formed, in which calcium chloride, magnesium chloride and sodium chloride are crystallized with microplastics suspended in the water as condensation nuclei. These crystals wrapped in microplastics are more likely to collide with and adhere to bubbles, thereby floating to the water surface and being removed by the foam scraper. During this process of crystallization, the crystals will also capture the rising tiny bubbles, thereby further reducing the density of the crystals and making them easier to float. For micron-sized particles, the larger the size, the higher the removal rate due to the increased probability of collision with bubbles; and for millimeter-sized particles, the larger the size, the lower the removal rate due to the influence of their own gravity. Therefore, by supplementing the crystallization process with flotation treatment, the size of the crystallized particles can be controlled within a reasonable range, making it easy for them to adhere to bubbles and not easy to precipitate due to their own large size, and the removal rate of microplastics treated by flotation can be further improved.

[0011] Preferably, in step S1, the saturated brine is washed with a brine ratio of 120-150 g / L. Experiments have shown that within this range, the removal rate of microplastics in the salt sample after washing can reach about 90%. In traditional salt-making enterprises where brine is not recycled, the old brine after washing is directly discarded, and the brine ratio used for washing is generally 5.5-7.5 m / L. 3 / t (133.3g / L-181.9g / L).

[0012] Preferably, the water content of the washed salt after centrifugal separation in step S2 is controlled to be below 2.0%. In order to prevent the salt from agglomerating before the subsequent drying process and to improve the efficiency of the drying process, the salt slurry should be fully centrifuged to control its water content.

[0013] Preferably, the flotation treatment time in step S3 is between 30 min and 45 min.

[0014] Preferably, in step S2, the mixed salt slurry is heated to 30-40° C. above room temperature.

[0015] Preferably, the flotation treatment adopts a reflux pressurized dissolved air method, wherein a portion of the effluent from which microplastics are removed is sucked back for pressurization and dissolved air, and then enters the flotation tank after decompression, thereby generating a large number of tiny bubbles.

[0016] Preferably, the diameter of the bubbles generated during the flotation treatment is between 70 and 90 μm. The present invention also discloses a device for removing microplastics in crushed and washed sea salt, comprising: a crusher for crushing raw salt, and a stirring salt-washing tank for stirring the crushed raw salt with saturated brine, the stirring salt-washing tank is provided with a heating device, the stirring salt-washing tank is connected to a dehydration device capable of separating salt and brine, the dehydration device comprises a precipitation output end and a supernatant output end; the precipitation output end of the dehydration device is connected to a drying device, the supernatant output end of the dehydration device is connected to an air flotation unit, and the air flotation unit is connected to the stirring salt-washing tank.

[0017] As a further preferred embodiment, the air flotation unit includes a reflux device, which generates a large number of tiny bubbles by sucking saturated brine after microplastics are removed, pressurizing it with gas, and then passing it through a decompression device.

[0018] As a further preferred embodiment, the flotation unit further comprises a sedimentation tank. The flocs generated during the flotation process may contain some heavier particles that fail to float completely in the flotation tank. Therefore, the clear liquid output end of the flotation tank is connected to one or two sedimentation tanks for supplementary treatment and water quality monitoring.

[0019] Therefore, the present invention has the following beneficial effects: (1) The present invention integrates the microplastic removal process into the traditional crushing and washing sea salt preparation process through the improved flotation method without introducing other chemical impurities, thereby achieving a considerable microplastic removal rate.

[0020] (2) The present invention combines the optimization of the salt-bitter ratio in the preferred embodiment, so that the removal rate of microplastics in salt particles can reach more than 95% without adding other chemical substances.

[0021] (3) The present invention is based on the traditional crushing and washing sea salt process. Through temperature control, it not only achieves the removal of microplastics, but also avoids the process of removing flocculants, making the overall process simpler and reducing costs, which is suitable for promotion in production practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a flow chart of a method for removing microplastics from crushed and washed sea salt according to the present invention.

[0023] Figure 2 Schematic diagram of the structure of the air flotation unit in an embodiment of the present invention.

[0024] Figure 3 This is a comparison chart of the residual amount of MPs particles in salt samples after washing with different brine ratios. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is further clearly and completely described below through specific examples and in combination with the accompanying drawings. It should be noted that the embodiments described in the present invention are implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating processes are given, but they are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. The experimental methods used in the following embodiments are conventional methods unless otherwise specified, and the materials, reagents, etc. used in the embodiments, unless otherwise specified, can be obtained from commercial channels.

[0026] In the present invention, unless otherwise specified, the required structural components are structural components well known to those skilled in the art or disclosed in the prior art.

[0027] like Figure 1 , Figure 2 As shown, the present invention provides a device for removing microplastics from crushed and washed sea salt, comprising a crusher 1 for crushing raw salt, preferably a double-roll crusher, the belt output end of the double-roll crusher is connected to the stirring and washing salt tank 2, a heating device 3 is provided below the stirring and washing salt tank 2 so that it can be heated synchronously during the stirring process, the salt slurry of the stirring and washing salt tank 2 is connected to the dehydration device 4 through the bottom pump port, the dehydration device used is a centrifuge, and its precipitation output end is connected to the drying device 5, the drying device used is a fluidized drying bed, and the supernatant output end of the dehydration device 4 is connected to the flotation unit 10. The primary sedimentation tank in the sedimentation tank processing link of the original sea salt processing plant is used as a flotation tank, which is divided into three circulated spaces, a flotation chamber, a flotation chamber and a primary sedimentation chamber, a froth scraping device is provided above the flotation chamber and the flotation chamber, the primary sedimentation chamber is separated from the air and the flotation chamber by a surrounding structure, and the primary sedimentation chamber is also provided with a brine outlet, and the outlet is connected to the secondary sedimentation tank and the tertiary sedimentation tank in sequence.

[0028] The implementation method of the above device is as follows: like Figure 1As shown, a large piece of raw salt is added to a double-roll crusher with an output specification of about 5 mm. The crushed raw salt and saturated brine are passed into the stirring salt washing tank 2 together with stirring, and the heating device 3 is turned on at the same time. By monitoring the temperature of the salt slurry output from the stirring salt washing tank 2, the heating device 3 is adjusted so that the temperature of the output salt slurry is higher than the room temperature. The output salt slurry enters the dehydration device 4 and starts centrifugation, and the salt slurry is divided into washed salt and saturated brine washing liquid containing microplastics. The washed salt enters the fluidized drying bed and is dried to become refined salt; the saturated brine washing liquid containing microplastics enters the flotation unit 10 for flotation treatment. The water pump draws the reflux water at the outlet of the primary sedimentation chamber, and injects the air and water into the pump at the inlet of the dissolved air pump. At this time, the high-speed rotating impeller cuts the air into bubbles. Under the high pressure in the pump, the bubbles can quickly dissolve in the water to achieve gas-liquid mixing, and then pump microbubbles into the flotation chamber through the pressure reducing valve. At this time, as the temperature drops, the supersaturated brine begins to crystallize. The microbubbles, as the carriers of MPs and the precipitated salt, form small floating bodies, float up to form scum, and are regularly scraped off with a scraper. The brine after scraping off the foam enters the second sedimentation tank and the third sedimentation tank in turn to stand still so that the high-density insoluble matter can settle. Finally, the upper layer of clear brine is taken for reuse, and the saturated brine is passed into the stirring and salt washing tank 2 again to realize the circulation of brine.

[0029] The extraction, purification and identification of microplastic (MPs) particles in the following experiments and examples are as follows: Since the sample in this experiment contains less organic matter and other impurities, the density of MPs is 0.9-2.5 g / cm³, but most of them are 0.9-1.6 g / cm³, so ZnCl2 with a density of 1.7-1.8 g / cm³ is selected as the flotation agent. This can extract MPs from the sample, and high-density MPs (about 1.40 g / cm³) such as PVC, PS, additives and plasticizers in the sample can be removed. 3 ) can be better separated and extracted.

[0030] Dissolve or dilute 10 g of salt solid sample in 100 mL of ultrapure water. Filter through a 1000 mesh (pore size 13 μm) stainless steel filter. Place the filter membrane in a saturated zinc chloride flotation solution (1.7-1.8 g / cm3) for ultrasonic separation for 20 minutes, and rinse the filter membrane again with a clean ZnCl2 solution. Centrifuge all the above ZnCl2 solutions containing MPs particles at 3000 r / min, collect the supernatant, and filter it again onto a new membrane. Clean the ZnCl2 on the filter membrane by filtering with ultrapure water, remove the filter membrane and immerse it in anhydrous ethanol, and ultrasonically separate it for 20 minutes. Remove the filter membrane and rinse the filter membrane again with clean anhydrous ethanol. Concentrate the above ethanol solution containing MPs fragments to 100 μL with high-purity nitrogen gas blowing for subsequent identification.

[0031] The purified sample was dropped onto the high-reflective glass on the surface of the sample stage of the Agilent 8700 LDIR imaging system. After the ethanol evaporated, the Agilent Clarity software was used to control the particle analysis mode, select the MPs library establishment method, and set the automatic test method (matching degree > 0.65, particle size range 20-500 μm) for testing.

[0032] Laser infrared imaging spectrometer (LDIR), LDIR is an emerging simple and highly automated MPs analysis instrument. The instrument uses a semiconductor-based quantum cascade laser (QCL) by combining the QCL with a fast scanning optical system and a highly sensitive mercury cadmium telluride detector. The QCL can emit a single wavelength of infrared light to scan a large area ultra-fast, or it can obtain a full spectrum from a single point in less than a second. When identifying MPs in a sample, the scanning mode is first used to locate and describe the morphology of the MPs particles, and then the scanning mode is used to obtain a spectrum from each particle for real-time library matching and reporting.

[0033] The LDIR spectrum scan results match the microscopic infrared library. In actual measurements, some characteristic peaks of plastic polymers in the LDIR spectrum do not completely match the library. The possible reason is that during the aging process, plastic fragments can be weathered through physical and chemical processes and photodegradation. Natural weathering processes can modify MPs by adding and / or removing different functional groups, for example, adding hydroxyl groups through a slow hydrolysis process, which may mask some characteristic peaks of secondary MPs. Therefore, the standard spectrum matching degree above 0.65 is recorded as the corresponding MPs.

[0034] Determine the optimal brine ratio through experiments: Clear brine and raw salt were collected in the sea salt production workshop, and the salt washing process was simulated in the laboratory. 60 g, 90 g, 120 g, 150 g, and 180 g of raw salt were taken in a 2 L beaker, and 1 L of clear brine was added to prepare a salt washing system equivalent to 60 g / L, 90 g / L, 120 g / L, 150 g / L, and 180 g / L brine ratios. The salt was fully washed with a magnetic stirrer for 30 min, the supernatant was poured out, and the wet salt was dried to obtain washed salt. The salt samples washed with the above different brine ratios were collected for MPs particle extraction, purification, and identification. Table 1 shows the MPs particle residues in the salt samples after washing with different brine ratios.

[0035] Table 1 From Table 1 and Figure 3 It can be seen that when the brine used for washing salt is increased from the traditional 5.5-7.5 m 3 / t (133.3g / L-181.9g / L) is accurate to 120-150 g / L, and the residual MPs particles in the salt sample after salt washing are the least.

[0036] Example 1 Take 120 g of raw salt in a 2 L beaker, add 1 L of saturated brine after flotation to form a salt washing system with a salt-bite ratio of 120 g / L. Stir under a magnetic stirrer for 30 min, heat the mixed salt slurry to 30°C above room temperature, centrifuge it, and pour the saturated brine washing liquid containing microplastics into a 1 L beaker.

[0037] At room temperature, the brine after salt washing was subjected to flotation treatment using an air pump for 30 min, and the foam was skimmed off. After settling for 1 h, saturated brine was obtained.

[0038] The obtained saturated brine was mixed with an appropriate amount of raw salt to form a salt washing system with a brine ratio of 120 g / L, and the above operation was repeated for a total of 3 cycles. All washed salts were dried to obtain refined salt, which was then subjected to MPs particle extraction, purification and identification. Example 2 Take 150 g of raw salt in a 2 L beaker, add 1 L of saturated brine after flotation to form a salt washing system with a salt-bite ratio of 150 g / L. Stir under a magnetic stirrer for 30 min, heat the mixed salt slurry to 30°C above room temperature, centrifuge it, and pour the saturated brine washing liquid containing microplastics into a 1 L beaker.

[0039] At room temperature, the brine after salt washing was subjected to flotation treatment with an air pump for 30 min, and the foam was skimmed off. After settling for 1 h, saturated brine was obtained.

[0040] The obtained saturated brine was mixed with an appropriate amount of raw salt to form a salt washing system with a brine ratio of 150 g / L, and the above operation was repeated for a total of 3 cycles. All washed salts were dried to obtain refined salt, which was then subjected to MPs particle extraction, purification and identification.

[0041] Example 3 Take 140 g of raw salt in a 2 L beaker, add 1 L of saturated brine after flotation to form a salt washing system with a salt-bite ratio of 140 g / L, stir under a magnetic stirrer for 30 min, heat the mixed salt slurry to 40°C above room temperature, centrifuge it, and pour the saturated brine washing liquid containing microplastics into a 1 L beaker.

[0042] At room temperature, the brine after salt washing was subjected to flotation treatment with an air pump for 40 min, and the foam was skimmed off. After settling for 1 h, saturated brine was obtained.

[0043] The obtained saturated brine was mixed with an appropriate amount of raw salt to form a salt washing system with a brine ratio of 140 g / L. The above operation was repeated for a total of 3 cycles. All washed salts were dried to obtain refined salt, which was then subjected to MPs particle extraction, purification and identification. Comparative Example 1 Take 120 g of raw salt in a 2 L beaker, add 1 L of saturated brine after flotation to form a salt washing system with a salt-bite ratio of 120 g / L. Stir under a magnetic stirrer for 30 min, heat the mixed salt slurry to 50°C above room temperature, centrifuge it, and pour the saturated brine washing liquid containing microplastics into a 1 L beaker.

[0044] At room temperature, the brine after salt washing was subjected to flotation treatment with an air pump for 40 min, and the foam was skimmed off. After settling for 1 h, saturated brine was obtained.

[0045] The obtained saturated brine was mixed with an appropriate amount of raw salt to form a salt washing system with a brine ratio of 120 g / L, and the above operation was repeated for a total of 3 cycles. All washed salts were dried to obtain refined salt, which was then subjected to MPs particle extraction, purification and identification.

[0046] Comparative Example 2 Take 150 g of raw salt in a 2 L beaker, add 1 L of saturated brine after flotation to form a salt washing system with a salt-bitter ratio of 150 g / L, stir under a magnetic stirrer for 30 min, perform centrifugal separation, and pour the saturated brine washing liquid containing microplastics into a 1 L beaker.

[0047] At room temperature, the brine after salt washing was subjected to flotation treatment with an air pump for 40 min, and the foam was skimmed off. After settling for 1 h, saturated brine was obtained.

[0048] The obtained saturated brine was mixed with an appropriate amount of raw salt to form a salt washing system with a brine ratio of 150 g / L, and the above operation was repeated for a total of 3 cycles. All washed salts were dried to obtain refined salt, which was then subjected to MPs particle extraction, purification and identification.

[0049] Comparative Example 3 Take 150 g of raw salt in a 2 L beaker, add 1 L of saturated brine after flotation to form a salt washing system with a salt-bite ratio of 150 g / L. Stir under a magnetic stirrer for 30 min, heat the mixed salt slurry to 30°C above room temperature, centrifuge it, and pour the saturated brine washing liquid containing microplastics into a 1 L beaker.

[0050] At room temperature, the brine after salt washing was subjected to flotation treatment with an air pump for 15 min, and the foam was skimmed off. After settling for 1 h, saturated brine was obtained.

[0051] The obtained saturated brine was mixed with an appropriate amount of raw salt to form a salt washing system with a brine ratio of 150 g / L, and the above operation was repeated for a total of 3 cycles. All washed salts were dried to obtain refined salt, which was then subjected to MPs particle extraction, purification and identification.

[0052] Table 2 Table 2 shows the residual MPs particles in the salt samples of different groups after salt washing. Compared with Example 1, the difference between Comparative Example 1 is that the mixed salt slurry is heated to 50°C higher than room temperature. Too high a temperature will cause the inorganic salt to crystallize too quickly during the subsequent flotation crystallization process, resulting in the crystal volume of the microplastics being too large to float up with the bubbles, resulting in a decrease in the removal rate. Compared with Example 2, the difference between Comparative Example 2 is that the salt slurry heating operation is not performed, and its removal rate is greater than the removal rate without flotation at the same brine ratio, but the improvement is not high. Compared with Example 2, the difference between Comparative Example 3 and Example 2 is that the flotation time is shortened, resulting in insufficient removal of microplastics and a decrease in the removal rate.

[0053] In order to prevent contamination by MPs in the environment during the experiment, we took the following precautions, such as wearing cotton lab coats and latex gloves throughout the experiment, rinsing all instruments with ultrapure water, and wrapping samples with aluminum foil to avoid exposure to air. Before LDIR identification, three blank reagent and blank method tests were completed for each batch of samples.

[0054] For the data analysis of the obtained MPs, Python's Pandas library was used to clean and organize the identification data, Scipy library was used for statistical analysis, and Seaborn library and Microsoft Office Excel 2021 were used for data visualization. The SW test was used to verify whether the single-factor sequence obeyed the normal distribution. If p-value>0.05, it conformed to the normal distribution. The two sequences specified by the Kolmogorov-Smirnov test (KS test) obeyed the same distribution. For data that conformed to the normal distribution and obeyed the same distribution, the F test was used for variance test (p>0.05 variance homogeneity), and the parametric test t-test was used for significance test for homogeneity of variance. If the variance was not homogeneous, the t'-test (two-sample heteroscedasticity assumption) was used for significance test. If p-value<0.05, it was significant; for data that did not conform to the normal distribution, non-parametric tests were used for significance test, and Wilcoxon signed-rank test was used to analyze paired sequences, and Wilcoxon rank-sum test was used to analyze non-paired sequences. If p-value<0.05, it was significant.

[0055] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any technician familiar with the profession, without departing from the scope of the technical solution of the present invention, according to the technical essence of the present invention, any simple modification, equivalent replacement and improvement made to the above embodiment still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for removing microplastics from crushed and washed sea salt, characterized in that: The steps include: S1, washing the crushed raw salt with saturated brine in a stirring salt washing tank to obtain a mixed salt slurry; S2. The mixed salt slurry is heated to a temperature higher than room temperature and centrifuged to obtain washed salt and a saturated brine washing solution containing microplastics. The washed salt is dried to obtain refined salt; S3, lowering the temperature of the saturated brine washing liquid containing microplastics to room temperature under air flotation treatment, so that the saturated brine washing liquid in step S2 is converted into a supersaturated brine washing liquid, and the halide salt in the supersaturated brine washing liquid is crystallized with the microplastics as crystal nuclei, thereby obtaining halide salt crystals containing microplastics and saturated brine; S4. Separate the halide salt crystals containing microplastics from the saturated brine, and reuse the obtained saturated brine in step S1, thereby completing the removal of microplastics.

2. A method for removing microplastics from crushed and washed sea salt according to claim 1, characterized in that: In step S1, saturated brine is washed with a brine ratio of 120-150 g / L.

3. A method for removing microplastics from crushed and washed sea salt according to claim 1, characterized in that: In step S2, the water content of the washed salt is controlled below 2.0%.

4. The method for removing microplastics from crushed and washed sea salt according to claim 1, characterized in that: In the flotation treatment of step S3, the treatment time is between 30 minutes and 45 minutes.

5. The method for removing microplastics from crushed and washed sea salt according to claim 1, characterized in that: In step S2, the mixed salt slurry is heated to 30-40° C. above room temperature.

6. The method for removing microplastics from crushed and washed sea salt according to claim 1, characterized in that: In the flotation treatment of step S3, the saturated brine from which the microplastics have been removed is sucked back and pressurized and air is dissolved to generate bubbles.

7. A method for removing microplastics from crushed and washed sea salt according to claim 6, characterized in that: In the flotation treatment of step S3, the diameter of the bubbles generated is between 70 and 90 μm.

8. A device for removing microplastics from crushed and washed sea salt, which is based on the method for removing microplastics from crushed and washed sea salt according to any one of claims 1 to 7, characterized in that: The invention comprises a pulverizer (1) for pulverizing raw salt, and also comprises a stirring salt-washing tank (2) for stirring the pulverized raw salt and saturated brine, the stirring salt-washing tank (2) being provided with a heating device (3), the stirring salt-washing tank (2) being connected to a dehydration device (4) capable of separating salt from brine, the dehydration device (4) comprising a precipitation output end and a supernatant output end; the precipitation output end of the dehydration device (4) being connected to a drying device (5), the supernatant output end of the dehydration device (4) being connected to an air flotation unit (10), and the air flotation unit (10) being connected to the stirring salt-washing tank (2).

9. The device for removing microplastics from crushed and washed sea salt according to claim 8, characterized in that: The air flotation unit (10) comprises a reflux device.

10. The device for removing microplastics from crushed and washed sea salt according to claim 8, characterized in that: The air flotation unit (10) further comprises a sedimentation tank (11).

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