Crushing and washing salt processing method for removing microplastics in raw salt

Abstract

This invention belongs to the field of salt production technology, specifically relating to a method for processing raw salt by crushing and washing to remove microplastics. The method, in the order of operation, includes at least a raw salt feeding and crushing step, a saturated brine washing step, and a dehydration and drying step. The key feature is that in the saturated brine washing step, the saturated brine and the crushed raw salt are mixed at a ratio of 7.5–15 ml. 3 A brine slurry is prepared by mixing a certain amount of brine consumed per ton of salt and stirring, and is used to remove microplastics from the raw salt. The preferred range for the brine consumption per ton of salt is 10–12.5 m³. 3 Within this preferred range, the microplastic removal rate can reach over 89.8%. This invention improves the microplastic removal effect of the saturated brine washing process by optimizing the brine consumption per ton of salt, and it shows good removal effects on common microplastics of different types and densities.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of salt production, and particularly relates to a crushing and washing salt processing method for removing microplastics in raw salt. BACKGROUND

[0002] According to the definition of the national standard, the crushed and washed salt refers to the edible salt prepared by using sea salt, lake salt or salt ore as raw materials through the crushing and washing process. The crushing and washing method is the mainstream process route for the production of edible salt at present, and mainly includes six processes: raw salt feeding and crushing, saturated brine cleaning, centrifugal dewatering, fluidized bed drying, particle grading, and laser color selection. The saturated brine cleaning process is a key step for removing impurities in the raw salt, and the principle is to dissolve the soluble impurity particles in the raw salt through the saturated brine, and at the same time, to make the insoluble impurity particles suspended upward by using the high buoyancy of the saturated brine, so that the two types of impurity particles are finally separated out with the flow of the saturated brine.

[0003] The microplastics, which are generally defined as plastic fragments with a diameter less than 5 mm, are a kind of pollutants that have gradually attracted public attention in recent years. In 2022, the microplastics were listed as a key new pollutant in the Action Plan for New Pollutant Governance. After entering the human body, the microplastics will not be metabolized and excreted, but will remain in organs such as the lungs, liver, kidneys and cardiovascular system for a long time, causing chronic inflammatory reactions and affecting human health.

[0004] The microplastic pollution widely exists in the human activity environment and the natural environment. As a basic food that is stably and long-term ingested by residents, the microplastic pollution in the edible salt will directly cause food safety hazards. For example, according to a number of investigations on commercially available sea salt, the content of microplastics in the sea salt is 550-681 particles / kg; according to the dietary habits in China, the average daily salt intake of an adult is more than 10 grams, which is equivalent to the intake of more than 2000 particles of microplastics per person per year. This shows that the cleaning effect of the saturated brine cleaning process in the existing crushing and washing process on the microplastics in the raw salt is not ideal. SUMMARY

[0005] In order to propose a cleaning scheme for the microplastics, the inventors have extensively studied the difference characteristics of the microplastic impurities and other insoluble impurities in the raw salt, and other inherent characteristics of the microplastic pollutants.

[0006] The reason why the current saturated brine cleaning process cannot effectively remove the microplastics in the raw salt mainly lies in two aspects. Part of the reason is that the density difference between the saturated brine and the microplastic pollutants is not significant, which affects the buoyancy screening effect. The density of the microplastic pollutants in the raw salt ranges from 0.9 to 2.5 g / cm 3 , and the plastic type with a density of 0.9-1.6 g / cm 3 accounts for the majority, while the density of the saturated brine is about 1.2 g / cm3 More importantly, most of the microplastic particles in the raw salt have a particle size of less than 200 μm, have strong adsorption, and are tightly combined on the surface of the raw salt crystal and cannot be easily separated out with the flushing flow of the saturated brine. In the field of water pollution treatment, researchers use the adsorption characteristics of microplastics to achieve the purpose of removing microplastics in water bodies: researchers in this field have found that stirring and neutral pH environment can promote the adsorption between microplastics in water bodies and inorganic flocculants, so as to jointly settle.

[0007] The inventors believe from the above analysis that the key to removing microplastics in crushed washing salt is to overcome the adsorption between microplastics and raw salt crystals.

[0008] Researchers in other research fields have conducted extensive research on factors affecting the adsorption of microplastics. Many studies in the field of water pollution treatment have found that water salinity is the main factor affecting the adsorption strength between microplastics and metal ions, other types of microplastics or other organic pollutants. Salinity refers to the number of grams of dissolved substances per unit weight of water, and the higher the salinity, the more anions and cations contained in the water body; these ions will compete with each other for the binding sites of the functional groups of microplastics, change the double-layer state of the surface of microplastics, or form complexes on the surface of microplastics, thereby affecting the adsorption of microplastics. The inventors found by comparison that the research object of the field of water pollution treatment is water bodies, and the number of ions that can be contacted by microplastics in water bodies depends on the salinity; while the object to be treated in the field is the salt slurry suspension formed by saturated brine and crushed raw salt, and the number of ions that can be contacted by microplastics in the salt slurry depends on the volume of saturated brine.

[0009] The inventors believe that increasing the volume of saturated brine can increase the number of ions that can be contacted by microplastic particles, and may be able to reduce the adsorption of microplastics. But as mentioned earlier, in order for microplastic particles to fully contact the ions in the system, continuous stirring must be carried out; for some inorganic substances, stirring will promote the adsorption between microplastics and them.

[0010] The inventors found through experiments that the effect of the volume of saturated brine on the number of microplastic particles remaining in the raw salt is not a linear relationship: only when the volume of saturated brine is within the preferred range, can the microplastic particles in the raw salt be effectively removed. Further, the inventors also conducted a control test of first treating the saturated brine to remove microplastics before the saturated brine cleaning process. From the results of the control test, on the one hand, the overall removal effect of microplastics is improved, and on the other hand, the relationship between the volume of saturated brine and the number of microplastic residues in the raw salt remains the same as before.

[0011] The present application is realized by the following technical solutions:

[0012] A crushing and washing salt processing method for removing microplastics in raw salt, according to the operation sequence, at least including a raw salt feeding and crushing process, a saturated brine washing process, a dehydration and drying process, characterized in that:

[0013] In the saturated brine washing process, the saturated brine is mixed with the crushed raw salt at a salt consumption brine amount of 7.5-15 m 3 / t of salt to form a salt slurry and stirred to remove microplastics in the raw salt.

[0014] The salt consumption brine amount refers to the volume of saturated brine consumed to wash one ton of raw salt in the raw salt production process. The present application realizes the effect of removing microplastics by optimizing the salt consumption brine amount parameter in the saturated brine washing process.

[0015] As a preferred, the salt consumption brine amount is 10-12.5 m 3 / t. Within this preferred range, the microplastic removal rate can reach more than 89.8%.

[0016] As a preferred, the raw salt is sea salt.

[0017] As a preferred, the saturated brine contains at least one or more of calcium sulfate, magnesium sulfate, magnesium chloride, and sodium chloride.

[0018] As a preferred, the types of microplastics include at least one or more of polyamide PA, ACR resin, ethylene-vinyl acetate copolymer EVA, polyvinyl chloride PVC, polyurethane PU, polyethylene PE, polymethyl methacrylate PMMA, polybutylene PB, polycarbonate PC, polylactic acid PLA, polycaprolactone PCL, polyimide PI, thermoplastic polyimide CTPI, polypropylene PP, polystyrene PS, polytetrafluoroethylene PTFE, polyethylene terephthalate PET, and polysulfone PSU.

[0019] As a preferred, the particle size range of the microplastics is 20-500 μm.

[0020] As a preferred, the average particle size of the crushed raw salt is below 3 mm.

[0021] As a preferred, the salt slurry is stirred in the saturated brine washing process using a stirring salt washing machine, and the paddle speed is 46-56 revolutions per minute.

[0022] As a preferred, the crushing and washing salt processing method further includes a saturated brine pretreatment process; the saturated brine pretreatment process is preferentially operated in the saturated brine washing process to remove microplastics in the saturated brine.

[0023] Further preferably, the saturated brine pretreatment process comprises a gas floatation treatment, the gas floatation treatment is that bubbles are distributed in the saturated brine in a large amount by a bubble generating device, the bubbles are combined with the microplastics dispersed in the saturated brine and then enriched in a surface layer of the saturated brine, and the microplastics are removed by removing the surface layer of the saturated brine.

[0024] In the prior art, the ton salt consumption brine amount is generally controlled at about 5.5 m 3 / t, and the removal rate of the microplastic particles is about 50% or lower. Compared with the prior art, the present application improves the removal effect of the microplastics in the crude salt by optimizing the ton salt consumption brine amount. When the ton salt consumption brine amount is 10-12.5 m 3 / t, the removal rate of the microplastics can reach 89.8%-91.9%. The present method has good removal effect on common microplastics of different types and different densities, and the microplastics include at least one or more of polyamide PA, ACR resin, ethylene-vinyl acetate copolymer EVA, polyvinyl chloride PVC, polyurethane PU, polyethylene PE, polymethyl methacrylate PMMA, polybutylene PB, polycarbonate PC, polylactic acid PLA, polycaprolactone PCL, polyimide PI, thermoplastic polyimide CTPI, polypropylene PP, polystyrene PS, polytetrafluoroethylene PTFE, polyethylene terephthalate PET, and polysulfone PSU. Without changing the solute components in the saturated brine, the removal rate of the microplastics can be improved from about 90% to more than 98% by the microplastic removal pretreatment of the saturated brine.

[0025] The optimization method of the present application is improved on the basis of the original process parameters, is easy to implement, and has good popularization. The technical scheme of the present application does not change the composition of the saturated brine itself, does not need to add biological reagents or chemical reagents for assistance, meets the safety requirements of salt production, and has good practicability. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 Some main processes of the crushing and washing method process route;

[0027] Figure 2 Effect of ton salt consumption brine amount parameter on the number of microplastic particles in the washed salt;

[0028] Figure 3 Effect of ton salt consumption brine amount parameter on the type of microplastic particles in the washed salt. DETAILED DESCRIPTION

[0029] The present application will be further described in conjunction with the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present application based on these descriptions. In addition, the embodiments of the present application described in the following description are generally only embodiments of a part of the present application, not all embodiments. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments in the present application without creative labor should be within the scope of protection of the present application.

[0030] Embodiments

[0031] 1. A method for producing crushed washing salt

[0032] Figure 1 The main processes related to the present application in the process route of the crushed washing method are shown, and the specific description is as follows:

[0033] S1, the raw salt is crushed to obtain crushed raw salt with an average particle size of less than 3 mm;

[0034] S2, the crushed raw salt is mixed with saturated brine in a stirring salt washing machine to form a salt slurry; the paddle of the stirring salt washing machine stirs the salt slurry at a speed of 46-56 revolutions per minute;

[0035] S3, the salt slurry is dewatered and dried to obtain washed salt.

[0036] The technical solution of the present application is mainly optimized based on the parameter "ton salt halogen consumption" in the saturated brine washing process of S2. The ton salt halogen consumption refers to the volume of saturated brine consumed for washing one ton of raw salt in the production process of raw salt.

[0037] As a preferred solution, the present application adds a saturated brine pretreatment process before the saturated brine washing process of S2. Microplastics carried in the saturated brine are removed by methods such as membrane filtration and air flotation without adding biological reagents or chemical reagents.

[0038] 2. Laboratory testing and verification

[0039] 2.1 Method for extracting, purifying and identifying microplastic particles in raw salt

[0040] Based on the previous research of the inventors, the density of microplastics is 0.9-2.5 g / cm 3 , but most of them are 0.9-1.6 g / cm 3 , so zinc chloride with a density of 1.7-1.8 g / cm 3 is selected as the floating agent. By extracting microplastics in the sample in this way, high-density microplastics (about 1.40 g / cm 3 ) such as PVC, PS, additives and plasticizers in the sample can be better separated and extracted.

[0041] In this experiment, 10g of the salt sample containing microplastics was dissolved in 100mL of ultrapure water. The solution was then filtered through a 1000-mesh (13μm pore size) stainless steel membrane. The stainless steel membrane was then placed in a saturated zinc chloride flotation solution (1.7–1.8 g / cm³). 3 The solution was ultrasonically separated for 20 min, and the filter membrane was rinsed again with clean zinc chloride solution. All zinc chloride solutions containing microplastic particles were centrifuged at 3000 rpm for 5 min, and the supernatant was collected and filtered onto a new membrane. The zinc chloride on the filter membrane was cleaned by filtration with ultrapure water, and the filter membrane was removed and immersed in anhydrous ethanol, followed by ultrasonic separation for 20 min. The filter membrane was then removed and rinsed again with clean anhydrous ethanol. The ethanol solution containing microplastic fragments was concentrated to 100 μL by blowing with high-purity nitrogen for subsequent analysis.

[0042] Qualitative and quantitative analysis of microplastic samples was performed using a laser infrared imaging spectrometer (LDIR). The purified sample was dropped onto the highly reflective glass surface of the Agilent 8700 LDIR imaging system's stage. After the ethanol evaporated, the Agilent Clarity software was used to control the process. First, the morphology of microplastic particles with a diameter range of 20–500 μm was located and described using a scanning mode. Then, the spectrum of each particle was acquired using a scanning mode, and real-time matching and reporting were performed. Microplastics with a match degree of 0.65 or higher with the standard spectrum were identified.

[0043] To prevent contamination by microplastics in the environment during the experiment, the following precautions were taken: cotton lab coats and latex gloves were worn throughout the experiment; all instruments were rinsed with ultrapure water; samples were wrapped in aluminum foil to prevent them from being exposed to air; and each batch of samples underwent three blank tests and blank method tests before infrared imaging spectroscopy identification.

[0044] The effect of 2.2 tons of brine consumption on the number of microplastics removed.

[0045] Saturated brine and crushed raw salt were collected from the sea salt production workshop directly under Zhejiang Salt Industry Group Co., Ltd. In a laboratory setting, simulating the salt washing process, 60g, 90g, 120g, 150g, and 180g of crushed raw salt were placed in 2L beakers, and 1L of saturated brine was added to prepare a solution equivalent to 5.0m³ of brine. 3 / t, 7.5m 3 / t, 10.0m 3 / t, 12.5m 3 / t, 15.0m 3A salt washing system with a brine consumption of / t of salt was used, and the salt was thoroughly washed by stirring at 600 rpm for 30 min with a magnetic stirrer. The supernatant was poured off and reserved for Experiment 2.3. The wet salt was dried to obtain the washed salt. Salt samples obtained from different brine consumption rates were collected for microplastic particle extraction, purification, and identification. The results are shown in Table 1 and 2. Figure 2 As shown.

[0046] Table 1. Residual microplastic particles in salt samples after washing with different amounts of brine per ton of salt.

[0047]

[0048] From Table 1 and Figure 2 It can be seen that when the brine consumption per ton of salt used for salt washing increases from 5.5 to 7.5 m³ in existing technologies... 3 The range was increased from 10.0 to 12.5m / t. 3 When the concentration of brine per ton of salt is between 1 / t, the abundance of residual microplastics in the washed salt sample is the lowest, and the microplastic removal rate can reach 89.8% to 91.9%. Further increasing the brine consumption per ton of salt actually decreases the microplastic removal rate.

[0049] The effect of 2.3 tons of brine consumption on the types of microplastics removed.

[0050] The samples from section 2.2 were subjected to qualitative and quantitative analysis of microplastics using LDIR spectroscopy, and the results are as follows: Figure 3 As shown. Figure 3 The abundance of microplastics was distinguished by color gradients; the lighter the blue, the lower the abundance of that type of microplastic, and the darker the blue, the higher the abundance of that type of microplastic. The abundance of polyamide (PA) in the original salt was an extreme value, marked in green.

[0051] Brine consumption per ton of salt: 10.0m³ 3 At / t, it has a good removal effect on various types of microplastics within the measured range, including polyamide PA, ACR resin, ethylene-vinyl acetate copolymer EVA, polyvinyl chloride PVC, polyurethane PU, polyethylene PE, polymethyl methacrylate PMMA, polybutene PB, polycarbonate PC, polylactic acid PLA, polycaprolactone PCL, polyimide PI, thermoplastic polyimide CTPI, polypropylene PP, polystyrene PS, polytetrafluoroethylene PTFE, polyethylene terephthalate PET, polysulfone PSU, etc.

[0052] 2.4 The impact of saturated brine pretreatment process on microplastic removal efficiency

[0053] The saturated brine pretreatment process is prioritized over the saturated brine washing process. To avoid altering the salinity and composition of the saturated brine, the saturated brine pretreatment process does not employ methods requiring additional auxiliary substances, such as adsorption, biodegradation, or advanced oxidation. Preferably, the pretreatment process uses air flotation; the principle of air flotation is to disperse air into the water under high pressure to form dense microbubbles. Insoluble impurity particles are encapsulated by these microbubbles, gaining greater buoyancy and floating to the surface to form foam. Finally, the contaminants are separated by collecting the foam.

[0054] Since the salt factory uses a saturated brine circulation system in actual production, in order to recreate the actual conditions of the saturated brine during the circulation process, the saturated brine sample to be treated in this experiment was taken from the supernatant of the brine discarded after cleaning the brine with the corresponding brine consumption per ton of salt in 2.3.

[0055] In this experiment, saturated brine and gas were treated at a gas-liquid ratio of 1:2 for 30 minutes, skimmed off the foam, and allowed to settle for 1 hour. The saturated brine after air flotation treatment was used to wash the crushed raw salt, following the same washing steps as in 2.2. Raw salt, saturated brine to be treated, treated saturated brine, and washed salt were collected for microplastic abundance determination. The experimental results are shown in Table 2.

[0056] Table 2. Effects of different brine consumption per ton of salt and flotation treatment on the abundance of residual microplastics in the samples.

[0057]

[0058] The data in Table 2 were subjected to the Stern-Warshall (SW) test and the Kirchhoff-Schwarz (KS) test. Both the microplastic abundance data (p-value = 0.782 > 0.05) and the microplastic abundance data (p-value = 0.860 > 0.05) of the unflotation-treated group were normally distributed. The F-test for homogeneity of variance showed p-value = 0.011 < 0.05, indicating heterogeneity. The t'-test (assuming two-sample heteroscedasticity) was used to verify the heterogeneity between the flotation-treated and unflotation-treated groups. The one-tailed p-value = 0.034 < 0.05, indicating a significant difference between the two groups. This suggests that flotation treatment can effectively reduce microplastic residues in raw salt and brine.

[0059] Experimental results showed that flotation treatment of saturated brine further improved the removal efficiency of microplastics in raw salt, with the optimal removal rate increasing from 91.9% to 98.3%. Furthermore, flotation treatment did not alter the trend of brine consumption per ton of salt affecting microplastic removal efficiency; insufficient or excessive saturated brine volume reduced the microplastic removal effect.

Claims

1. A method for processing raw salt by crushing and washing to remove microplastics, comprising, in the order of operation, at least a raw salt feeding and crushing step, a saturated brine washing step, and a dehydration and drying step, characterized in that: In the saturated brine washing process, the saturated brine and crushed raw salt are mixed at a ratio of 7.5–15 ml. 3 The brine consumption per ton of salt is mixed to form a salt slurry, which is then stirred using a stirring salt washing machine with a paddle speed of 46-56 rpm; the average particle size of the crushed raw salt is less than 3 mm. The pulverized washing salt processing method further includes a saturated brine pretreatment step preceding the saturated brine washing step. The saturated brine pretreatment step involves using a bubble generating device to distribute bubbles in large quantities in the saturated brine. After the bubbles combine with microplastics dispersed in the saturated brine, they accumulate on the surface of the saturated brine. The microplastics are removed by removing the surface of the saturated brine.

2. The method for processing crushed and washed salt according to claim 1, characterized in that, The brine consumption per ton of salt is 10–12.5 m³. 3 / t.

3. The method for processing pulverized and washed salt according to claim 1, characterized in that, The raw salt is sea salt.

4. The method for processing pulverized and washed salt according to claim 1, characterized in that, The saturated brine contains at least one or more of calcium sulfate, magnesium sulfate, magnesium chloride, and sodium chloride.

5. The method for processing pulverized and washed salt according to claim 1, characterized in that, The microplastics include at least one or more of the following: polyamide (PA), ACR resin, ethylene-vinyl acetate copolymer (EVA), polyvinyl chloride (PVC), polyurethane (PU), polyethylene (PE), polymethyl methacrylate (PMMA), polybutene (PB), polycarbonate (PC), polylactic acid (PLA), polycaprolactone (PCL), polyimide (PI), thermoplastic polyimide (CTPI), polypropylene (PP), polystyrene (PS), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), and polysulfone (PSU).

6. The method for processing pulverized and washed salt according to claim 1, characterized in that, The microplastic particle size ranges from 20 to 500 μm.

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