Crushing and washing salt processing method for removing micro-plastics in crude salt

Abstract

The invention belongs to the technical field of salt production, and particularly relates to a crushing and washing salt processing method for removing micro-plastics in crude salt, which at least comprises a crude salt feeding and crushing process, a saturated brine cleaning process and a dewatering and drying process according to an operation sequence, the method is characterized in that in the saturated brine cleaning procedure, saturated brine and crushed crude salt are mixed according to the brine consumption per ton of 7.5-15m < 3 > / t to prepare salt slurry, and the salt slurry is stirred and used for removing micro-plastics in the crude salt. The preferable range of the brine consumption per ton of salt is 10-12.5 m < 3 > / t, and within the preferable range, the micro-plastic removal rate can reach 89.8% or above. According to the method, the micro-plastic removal effect of the saturated brine cleaning procedure is improved by optimizing the brine consumption per ton, and the method has a good removal effect on common micro-plastics of different types and different densities.

Description

Technical Field

[0001] The invention belongs to the technical field of salt production, and specifically relates to a method for processing crushed washed salt for removing microplastics in raw salt. Background Art

[0002] According to the national standard definition, crushed and washed salt refers to edible salt made from sea salt, lake salt or rock salt ore by crushing and washing. The crushing and washing method is the mainstream process route for salt production at present, which mainly includes six processes: raw salt crushing, saturated brine washing, centrifugal dehydration, fluidized bed drying, particle grading, laser color sorting, etc. The saturated brine washing process is a key step in removing impurities from raw salt. Its principle is to dissolve the soluble impurity particles in the raw salt through saturated brine, and at the same time use the high buoyancy of saturated brine to suspend the insoluble impurity particles upward, so that both types of impurity particles are finally separated with the flow of saturated brine.

[0003] Microplastics are broadly defined as plastic fragments with a diameter of less than 5mm. They are a type of pollutant that has gradually attracted public attention in recent years. In 2022, the Action Plan for the Control of New Pollutants listed microplastics as key new pollutants. After entering the human body, microplastics are not metabolized or 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] Microplastic pollution is widely present in human activity environments and natural environments. As a basic food that residents consume in a long-term and stable manner, the microplastic pollution in salt will directly pose a food safety hazard. Taking sea salt as an example, multiple surveys on commercially available sea salt show that the content of microplastics in sea salt is 550-681 grains / kg; according to Chinese dietary habits, the average daily salt intake of adults is more than 10 grams, which is equivalent to more than 2,000 microplastics per person per year. This shows that in the existing crushing and washing process, the saturated brine washing process is not ideal for removing microplastics in raw salt. Summary of the invention

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

[0006] There are two main reasons why the current saturated brine cleaning process cannot effectively remove microplastics from raw salt. Part of the reason is that the density difference between saturated brine and microplastic pollutants is not significant, which affects the buoyancy screening effect. The density of microplastic pollutants in raw salt ranges from 0.9 to 2.5 g / cm 3 , where the density is 0.9~1.6g / cm 3 The plastic type accounts for the majority, and the density of saturated brine is about 1.2g / cm3 The more important reason is that most of the microplastic particles in the raw salt have a particle size of less than 200μm, have strong adsorption, and will be tightly bound to the surface of the raw salt crystals, and will not be easily separated with the flushing and flow of saturated brine. In the field of water pollution control, researchers are using the adsorption characteristics of microplastics to achieve the purpose of removing microplastics from water bodies: researchers in this field have found that stirring and a neutral pH environment can enhance the adsorption between microplastics in water bodies and inorganic flocculants, thereby settling together.

[0007] Based on the above analysis, the inventor believes that the key to removing microplastics from 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 that affect the adsorption of microplastics. Many studies in the field of water pollution control have found that water salinity is the main factor affecting the intensity of adsorption between microplastics and metal ions, other types of microplastics or other organic pollutants. Salinity refers to the number of grams of dissolved substances in a unit weight of water. The higher the salinity, the more anions and cations are contained in the water body; these ions will compete with each other for binding sites with the functional groups of microplastics, change the double layer state of the microplastic surface, or form complexes on the microplastic surface, thereby affecting the adsorption of microplastics. The inventors compared and found that the research object in the field of water pollution control is water bodies, and the number of ions that microplastics can contact in water bodies depends on the salinity; while the object to be treated in this field is a salt slurry suspension formed by saturated brine and crushed raw salt, and the number of ions that microplastics can contact 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 microplastic particles can contact, which may help reduce the adsorption of microplastics. However, as mentioned above, continuous stirring is required to allow microplastic particles to fully contact the ions in the system; for some inorganic substances, the stirring process will promote the adsorption of microplastics with them.

[0010] The inventors found through experiments that the effect of the volume of saturated brine on the number of residual microplastic particles 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. Furthermore, the inventors also conducted a control test in which the saturated brine was treated 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 was improved, and on the other hand, the relationship between the volume of saturated brine and the number of residual microplastics in the raw salt remained the same.

[0011] The present invention is achieved through the following technical solutions:

[0012] A method for processing crushed washed salt for removing microplastics from raw salt, which comprises at least a raw salt feeding and crushing step, a saturated brine washing step, and a dehydration and drying step in accordance with the operation sequence, and is characterized in that:

[0013] In the saturated brine cleaning process, the saturated brine and the crushed raw salt are mixed at a temperature of 7.5 to 15 m 3 / t of brine consumption is mixed into salt slurry and stirred to remove microplastics in raw salt.

[0014] The brine consumption per ton of salt refers to the volume of saturated brine required to wash one ton of raw salt during the raw salt production process. The present invention achieves the effect of removing microplastics by optimizing the brine consumption per ton of salt in the saturated brine washing process.

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

[0016] Preferably, the raw salt is sea salt.

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

[0018] Preferably, 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] Preferably, the particle size range of the microplastics is 20 to 500 μm.

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

[0021] Preferably, in the saturated brine cleaning process, a stirring salt washer is used to stir the salt slurry, and the blade speed is 46 to 56 rpm.

[0022] Preferably, the crushing and washing salt processing method also includes a saturated brine pretreatment process; the saturated brine pretreatment process is performed preferentially over the saturated brine cleaning process to remove microplastics in the saturated brine.

[0023] Further preferably, the saturated brine pretreatment process includes flotation treatment, which is to distribute bubbles in large quantities in the saturated brine through a bubble generating device, and the bubbles are combined with microplastics dispersed in the saturated brine and enriched in the surface layer of the saturated brine, and the microplastics are removed by removing the surface layer of the saturated brine.

[0024] In the existing technology, the brine consumption per ton of salt is usually controlled at 5.5m 3 / t, the removal rate of microplastic particles is about 50% or lower. Compared with the prior art, the present invention improves the removal effect of microplastics in raw salt by optimizing the brine consumption per ton of salt. 3 / t, the removal rate of microplastics can reach 89.8% to 91.9%. This method has a good removal effect on common microplastics of different types and densities, including 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, polysulfone PSU. Under the premise of not changing the solute components in the saturated brine, the removal rate of microplastics can be increased from about 90% to more than 98% by pre-treating the saturated brine for microplastic removal.

[0025] The optimization method proposed in the present invention improves the original process parameters, is easy to implement and has applicability; the technical solution of the present invention does not change the composition of the saturated brine itself, and does not require the addition of biological or chemical reagents for assistance, meets the safety requirements of salt production, and has good practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is part of the main process of the crushing and washing process;

[0027] Figure 2 The influence of brine consumption per ton of salt on the number of microplastic particles in washed salt;

[0028] Figure 3 The influence of brine consumption parameters per ton of salt on the types of microplastic particles in washed salt. DETAILED DESCRIPTION

[0029] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments of the specification. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention involved in the following description are generally only embodiments of a part of the present invention, rather than all embodiments. Therefore, based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work should fall within the scope of protection of the present invention.

[0030] Example

[0031] 1. Production method of crushed washing salt

[0032] Figure 1 The main steps in the crushing and washing process route related to the present invention are shown, and are specifically described as follows:

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

[0034] S2, the crushed raw salt and saturated brine are mixed in a stirring salt washer to form a salt slurry; the paddles of the stirring salt washer stir the salt slurry at a speed of 46 to 56 rpm;

[0035] S3, dehydrating and drying the salt slurry to obtain washed salt.

[0036] The technical solution of the present invention is mainly optimized based on the parameter "brine consumption per ton of salt" in the S2 saturated brine cleaning process. The brine consumption per ton of salt refers to the volume of saturated brine required to wash one ton of raw salt in the raw salt production process.

[0037] As a preferred embodiment, the present invention adds a saturated brine pretreatment process before the S2 saturated brine cleaning process, and removes microplastics carried in the saturated brine through membrane filtration, flotation and other methods that do not require the addition of biological or chemical reagents.

[0038] 2. Laboratory testing and verification

[0039] 2.1 Extraction, purification and identification of microplastic particles in raw salt

[0040] Based on the inventor’s previous research, the density of microplastics is between 0.9 and 2.5 g / cm 3 , but most of the density is between 0.9 and 1.6 g / cm 3 , so the density is selected as 1.7~1.8g / cm 3 Zinc chloride is used as a flotation agent. This can extract microplastics from samples, and high-density microplastics (about 1.40 g / cm 3 ) can be better separated and extracted.

[0041] In this experiment, 100 mL of ultrapure water was added to 10 g of the sample of salt containing microplastics to be tested for dissolution. The sample was filtered through a 1000 mesh (pore size 13 μm) stainless steel filter membrane. The stainless steel filter membrane was placed in a saturated zinc chloride flotation solution (1.7-1.8 g / cm 3 ) for 20 minutes, and rinse the filter membrane again with a clean zinc chloride solution. Centrifuge all the zinc chloride solutions containing microplastic particles at 3000rpm for 5 minutes, collect the supernatant, and filter it onto a new membrane again. Clean the zinc chloride on the filter membrane with ultrapure water, remove the filter membrane and immerse it in anhydrous ethanol for 20 minutes of ultrasonic separation. Take out the filter membrane and rinse it again with clean anhydrous ethanol. Concentrate the ethanol solution containing microplastic fragments to 100 μL with high-purity nitrogen gas blowing for subsequent verification.

[0042] A laser infrared imaging spectrometer (LDIR) was used to qualitatively and quantitatively analyze microplastic samples. The purified sample was dropped onto the high-reflective glass on the surface of the Agilent 8700 LDIR imaging system sample table. After the ethanol evaporated, the Agilent Clarity software was used to control the morphology of microplastic particles with a particle size range of 20 to 500 μm. The scanning mode was used to locate and describe the morphology of microplastic particles, and then the scanning mode was used to obtain the spectrum from each particle for real-time matching and reporting. The microplastics with a matching degree of more than 0.65 with the standard spectrum were recorded as corresponding microplastics.

[0043] In order to prevent contamination by microplastics in the environment during the experiment, the following precautions were taken during the experimental operation: wearing cotton laboratory coats and latex gloves throughout the experiment; rinsing all instruments with ultrapure water; wrapping the samples with aluminum foil to prevent the samples from being exposed to the air; and completing three blank tests and blank method tests for each batch of samples before infrared imaging spectroscopy identification.

[0044] 2.2 The impact of brine consumption on the amount of microplastics removed

[0045] Saturated brine and crushed raw salt were collected in the sea salt production workshop directly under Zhejiang Salt Industry Group Corporation. The salt washing process was simulated in the laboratory. 60g, 90g, 120g, 150g, and 180g of crushed raw salt were taken in a 2L beaker, and 1L of saturated brine was added to prepare the equivalent of 5.0m 3 / t、7.5m 3 / t, 10.0m 3 / t、12.5m 3 / t、15.0m 3 / t ton salt consumption brine washing system, use a magnetic stirrer at 600rpm for 30min to fully wash the salt. Pour out the supernatant and save it for experiment 2.3. Dry the wet salt to obtain washed salt. Collect the salt samples obtained from the above different ton salt consumption treatments for microplastic particle extraction, purification and identification. The results are shown in Table 1 and Figure 2 shown.

[0046] Table 1 Residual microplastic particles in salt samples after washing with different tons of brine consumption

[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 is increased from 5.5 to 7.5 m3 in the prior art, 3 / t increased to 10.0~12.5m 3 When the salt consumption per ton was between 100 and 200 kg, the abundance of residual microplastics in the salt samples after washing was the lowest, and the microplastic removal rate could reach 89.8% to 91.9%. When the brine consumption per ton of salt continued to increase, the microplastic removal rate actually decreased.

[0049] 2.3 tons of salt consumption and its impact on the types of microplastics removed

[0050] The samples in 2.2 were subjected to qualitative and quantitative analysis of microplastics by LDIR spectroscopy. The results are as follows Figure 3 shown. Figure 3 The abundance of microplastics is distinguished by color gradient. The lighter the blue, the lower the abundance of this type of microplastics, and the darker the blue, the higher the abundance of this type of microplastics. The abundance of polyamide PA in the raw salt is an extreme value, marked in green.

[0051] Brine consumption per ton of salt: 10.0m 3 / t, it has a good removal effect on all 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 Effect of saturated brine pretreatment on microplastic removal

[0053] The saturated brine pretreatment process is operated prior to the saturated brine cleaning process. In order to avoid changing the salinity and composition of the saturated brine, the saturated brine pretreatment process does not use methods that require the addition of additional auxiliary substances, such as adsorption, biodegradation, and advanced oxidation. Preferably, the pretreatment process uses flotation; the principle of flotation is to disperse air into water through high pressure to form dense micro-bubbles. After being wrapped by micro-bubbles, insoluble impurity particles gain greater buoyancy and float to the water surface to form foam. Finally, the pollutants are separated by collecting the foam.

[0054] Since the salt-making factory adopts a saturated brine circulation system in actual production, in order to restore the actual conditions of the saturated brine during the circulation process, the saturated brine samples to be treated used in this experiment were sampled from the brine supernatant discarded after cleaning the corresponding ton of salt consumption in 2.3.

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

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

[0057]

[0058] SW test and KS test were performed on the data in Table 2. The abundance of microplastics in the non-flotation treatment (p-value = 0.782> 0.05) and the abundance of microplastics in the flotation treatment (p-value = 0.860> 0.05) were both normally distributed. The F test for homogeneity of variance showed p-value = 0.011 < 0.05, and the variance was unequal. The t'-test test (two-sample heteroscedasticity assumption) was used to verify the heterogeneity of the data in the flotation treatment and the non-flotation treatment. The single-tail p-value = 0.034 < 0.05 showed significant differences between the two columns of data, indicating that flotation treatment can effectively reduce the residual microplastics in raw salt and brine.

[0059] The experimental results show that after flotation treatment of saturated brine, the removal effect of microplastics in raw salt was further improved, and the optimal removal rate increased from 91.9% to 98.3%. In addition, flotation treatment did not change the trend of the influence of brine consumption per ton of salt on the removal efficiency of microplastics. Insufficient or excessive volume of saturated brine will reduce the removal effect of microplastics.

Claims

1. A method for processing pulverized washed salt for removing microplastics from raw salt, which comprises at least a raw salt feeding and pulverizing step, a saturated brine washing step, and a dehydration and drying step in accordance with the operating sequence, and is characterized in that: In the saturated brine cleaning process, the saturated brine and the crushed raw salt are mixed at a temperature of 7.5 to 15 m 3 / t of brine consumption is mixed into salt slurry and stirred to remove microplastics in the raw salt.

2. The method for processing crushed washing salt according to claim 1, characterized in that: The brine consumption per ton of salt is 10-12.5 m 3 / t.

3. The method for processing crushed washing salt according to claim 1, characterized in that: The raw salt is sea salt.

4. The method for processing crushed washing 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 crushed washing salt according to claim 1, characterized in that: 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.

6. The method for processing crushed washing salt according to claim 1, characterized in that: The particle size of the microplastics ranges from 20 to 500 μm.

7. The method for processing crushed washing salt according to claim 1, characterized in that: The average particle size of the crushed raw salt is below 3 mm.

8. The method for processing crushed washing salt according to claim 1, characterized in that: In the saturated brine cleaning process, a stirring salt washer is used to stir the salt slurry, and the blade speed is 46 to 56 revolutions per minute.

9. The method for processing crushed washing salt according to claim 1, characterized in that: It also includes a saturated brine pretreatment process; the saturated brine pretreatment process is performed preferentially over the saturated brine cleaning process to remove microplastics in the saturated brine.

10. The method for processing crushed washing salt according to claim 9, characterized in that: The saturated brine pretreatment process includes flotation treatment, which is to distribute bubbles in large quantities in the saturated brine through a bubble generating device. The bubbles combine with microplastics dispersed in the saturated brine and are enriched in the surface layer of the saturated brine. The microplastics are removed by removing the surface layer of the saturated brine.

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