A method for preparing low turbidity refined brine

Abstract

This invention belongs to the field of ammonia-soda process soda ash production technology, specifically relating to a method for preparing low-turbidity refined brine. The method involves sending calcium-removed brine into a clarification tank for clarification and sedimentation to obtain a supernatant, which is then pressurized and filtered through a microfiltration membrane device to obtain low-turbidity refined brine. The microfiltration membrane device includes a first chamber and a second chamber. Several high-density polyethylene microfiltration membrane filter elements are vertically arranged in the second chamber to connect the first and second chambers. The microfiltration membrane filter element has a double-layer structure including a filter layer and a support layer. This invention utilizes an asymmetric double-layer rigid membrane material sintered from high-density polyethylene of different particle sizes to form a precise pore size filtration channel. Combined with an efficient backwashing mechanism of online gas washing and acid washing, this achieves long-cycle, low-cost operation of the device.

Description

Technical Field

[0001] This invention belongs to the field of ammonia-soda process soda ash production technology, specifically relating to a method for preparing low-turbidity refined brine. Background Technology

[0002] In the traditional lime-ammonia process for alkali production, the brine refining process first uses lime slurry to remove magnesium ions from the crude brine to obtain primary brine, and then uses carbonized tail gas containing carbon dioxide and ammonia to remove calcium ions from the primary brine. The principle of calcium removal is as follows:

[0003] Specific method: Carbonation tail gas is passed into a calcium removal tower and reacts with primary brine to remove calcium, producing decalcified brine. After clarification and sedimentation, a clear supernatant with lower turbidity is obtained, which is the refined brine. Turbidity of the refined brine is one of the important indicators for ensuring brine quality, mainly reflecting the degree of calcium ion removal. The turbidity control index for refined brine is below 10 NTU. When the turbidity of the refined brine exceeds the control index, the amount of residual calcium ion impurities increases. These calcium ions enter subsequent processes, not only accelerating scaling inside the equipment and affecting production stability, but also impacting the quality of the soda ash product.

[0004] Because the production load changes frequently during the production process, the volume of carbonization tail gas fluctuates greatly, resulting in large fluctuations in the turbidity of refined brine, ranging from 10 to 50 NTU. In order to stabilize the turbidity of refined brine, Shandong Haihua Soda Ash Plant currently uses tubular membrane devices to filter the refined brine to further reduce the turbidity. The turbidity of refined brine after tubular membrane filtration can be reduced to 0.3 to 1 NTU.

[0005] However, the production method of using tubular membrane filtration to reduce the turbidity of purified brine has the following problems: First, the turbidity of refined brine needs to be controlled below 10 NTU to avoid affecting subsequent soda ash production. While tubular membrane filtration can reduce the turbidity of refined brine to 0.3–1 NTU, although the turbidity of the refined brine filtered by tubular membranes is far below the control target of 10 NTU, the tubular membranes are prone to clogging during operation, have a rapid flux decline rate, and short operating cycles, requiring frequent switching of multiple tubular membrane units. Second, tubular membranes have a relatively low flux, slow filtration speed, high operating pressure, and high energy consumption. Third, maintenance costs are high, cleaning is frequent, and the cleaning effect is poor. Fourth, using tubular membrane filtration requires the return of some concentrated water to the clarification tank, increasing the load on the clarification tank and reducing the clarification effect. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing low-turbidity refined brine. This method uses a microfiltration membrane device to replace a tubular membrane to reduce the turbidity of the refined brine. While ensuring that the turbidity of the refined brine meets the standard, it can extend the equipment operating cycle, increase the filtration speed, reduce the operating pressure, reduce the cleaning frequency, reduce the load on the clarification tank, reduce operating costs, and increase production efficiency.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing low-turbidity purified brine includes the following steps: The calcium-deionized brine is sent to a clarification tank for clarification and sedimentation to obtain a supernatant. The supernatant is then pressurized and sent to a microfiltration membrane device for filtration to obtain a low-turbidity refined brine. The microfiltration membrane device includes a first chamber and a second chamber, with a partition between the first and second chambers. The partition has several through holes. Several microfiltration membrane filter elements are vertically arranged in the second chamber. The top of each microfiltration membrane filter element passes through the through holes and is fixedly connected to the partition, forming a filtration channel connecting the first and second chambers. The first chamber has a liquid outlet on its side wall and a backflush port on its top. The second chamber has a liquid inlet on its side wall and a sludge discharge port on its bottom. The supernatant enters the second chamber through the liquid inlet, passes through the microfiltration membrane filter elements into the first chamber, and is finally discharged through the liquid outlet.

[0008] Preferably, the microfiltration membrane filter element is made of high-density polyethylene (HDPE).

[0009] Preferably, the microfiltration membrane filter element has a double-layer structure including a filter layer and a support layer. The filter layer is composed of small-particle high-density polyethylene with a pore size of 1.0 to 1.05 micrometers; the support layer is composed of large-particle high-density polyethylene with an open porosity of 40 to 42%.

[0010] Preferably, the pressure of the supernatant after pressurization is 0.3-0.4 MPa, and the turbidity of the supernatant is 10-50 NTU.

[0011] Preferably, when the pressure difference between the product water inside and outside the microfiltration membrane is greater than 0.10 MPa, the system performs an air washing operation for 10 to 15 minutes; when the pressure difference between the product water and the previous air washing is greater than 0.10 MPa and the time interval between the previous air washing and the previous air washing is less than 6 hours, the system enters an acid washing operation for 1.5 to 2 hours.

[0012] Compared with the prior art, the technical solution of the present invention has at least the following beneficial effects: (1) The inventors found that the main reason for the short operating cycle of the tubular membrane is that the membrane tube is easily blocked. The main reasons are as follows: (1) The tubular membrane is a hollow fiber membrane with a pore size of 0.01 to 0.1 micrometers. The material is polyetherketone (PEK), which has the characteristics of strong fiber elasticity and fatigue resistance. However, it is precisely because of the elasticity of the filtration channel of PEK fiber that large particles larger than the pore size are easily squeezed into the filtration channel and stuck in the fiber filtration channel. After being stuck, the channel becomes narrower, which causes small particles to block the channel continuously, eventually blocking the entire filtration channel; (2) It was found by testing that the pH value of the clear liquid in the upper layer of the clarification tank is 10.5. At a temperature of ~11.5, the supernatant contains no organic matter or elastic, stretchable solid particles such as magnesium hydroxide colloid. It contains only hard inorganic solid particles such as calcium carbonate. The crystal form of calcium carbonate is mainly rhombohedral or polyhedral with relative hardness. The non-smooth shape of the crystals makes the particles easy to get stuck in the elastic fiber filter channels and push forward under pressure. However, the further forward they push, the tighter they get stuck. This blockage caused by the elasticity of the fibers is unrelated to the pore size. If the pore size increases, the stuck particles will also become larger. Therefore, even if the pore size of the tubular membrane is enlarged, the problem of easy blockage cannot be solved.

[0013] The microfiltration membrane device of this invention employs a double-layer structure comprising a filter layer and a support layer. The filter layer is composed of small-particle high-density polyethylene with a pore size of 1.0–1.05 micrometers; the support layer is composed of large-particle high-density polyethylene with a porosity of 40–42%. The filter layer and support layer are sintered at high temperature to form an asymmetric double-layer structure with inelastic filtration channels. The pore size of these filtration channels has absolute precision (1 micrometer). On the one hand, because the filtration channels of the microfiltration membrane are inelastic and rigid, large calcium carbonate particles larger than the pore size of the filtration channels cannot be forced into the channels, preventing them from becoming stuck. On the other hand, the absolute precision (1 micrometer) of the filtration channel pore size ensures uniform pore size within the channels, preventing small calcium carbonate particles from clogging the channels due to uneven pore size. Thus, this invention, through the inelastic filtration channels formed by the asymmetric double-layer structure of the microfiltration membrane filter element, prevents calcium carbonate particles from becoming stuck in the filtration channels, preventing clogging and extending the operating cycle of the microfiltration membrane device.

[0014] In the field of ammonia-soda process soda ash production technology, as long as the turbidity of the refined brine is reduced to below 10 NTU, the subsequent production requirements can be met. The filter element pore size of the microfiltration membrane device used in this invention is 1.0 to 1.05 micrometers, which is much larger than the pore size of tubular membranes (0.01 to 0.1 micrometers). From the actual production operation results, although the turbidity of the refined brine increases (from 1 NTU to 1.5 to 1.9 NTU) when using the microfiltration membrane device to filter the refined brine compared with the tubular membrane, the turbidity of the refined brine is still far below the control standard and will not affect subsequent production.

[0015] Traditional tubular membranes are made of polyetherketone (PEK) and operate at pressures of 0.7–0.9 MPa. To improve the pressure resistance of tubular membranes, the pore size of the filter channels must be controlled within 0.01–0.1 micrometers. In contrast, the filter element of this invention is made of high-density polyethylene (HDPE), which, after high-temperature sintering, forms a double-layer asymmetric structure comprising a filter layer and a support layer. The filter layer acts as a sieving layer. Compared to tubular membranes, although the pore size of the filter layer increases from 0.01–0.1 micrometers to 1.0–1.05 micrometers, resulting in a decrease in its pressure resistance, the support layer provides support, thus improving the pressure resistance of the filter layer. This allows the double-layer structure of the filter element to withstand operating pressures not exceeding 0.6 MPa in actual production, fully meeting production requirements. Furthermore, the larger pore size of the filter layer also offers advantages such as increased throughput, reduced operating pressure, and easier cleaning.

[0016] Large calcium carbonate particles with a diameter exceeding the pore size of the filtration channel adhere to the surface of the microfiltration membrane to form a filter cake layer. The filter cake layer filters the brine and reduces the turbidity of the refined brine.

[0017] (2) Traditional tubular membrane filters the upper layer of clear liquid in the clarifier using an inlet-outlet filtration method. The upper layer of clear liquid permeates through the tubular membrane to form low-turbidity refined brine, which is then sent to the subsequent production process. The high-turbidity concentrate that does not permeate through the tubular membrane is mixed into the upper layer of clear liquid and then sent back to the tubular membrane for circulation filtration. In order to control the turbidity index of the tubular membrane feed water and reduce the clogging of the tubular membrane, some of the high-turbidity concentrate must be discharged as wastewater. To reduce brine loss, this discharged wastewater is sent back into the clarifier for settling and sludge removal. This invention uses a microfiltration membrane device to filter the upper layer of clear liquid. After the upper layer of clear liquid enters the second chamber, it is filtered through the microfiltration membrane filter element and then enters the first chamber. During the filtration process, solid particles are intercepted by the filter layer and gradually form a filter cake layer on the surface of the filter layer. When the pressure difference between the product water inside and outside the microfiltration membrane is greater than 0.10 MPa, the device will automatically backflush once to blow off the filter cake layer attached to the membrane surface and discharge it from the sludge discharge port at the bottom of the second chamber. Compared with the traditional tubular membrane wastewater discharge method, the microfiltration membrane device produces less wastewater with a higher solid content, which greatly improves the water production rate of the device. Since the solid content in the wastewater discharged by the microfiltration membrane device of this invention is 60-70 wt%, there is no need to send it back to the clarification tank for sedimentation treatment. It can be directly sent to the calcium carbonate plant to produce calcium carbonate, which not only increases efficiency, but also reduces the load on the clarification tank and improves the working condition of the clarification tank.

[0018] (3) Due to the non-uniformity and elasticity of the pore size of traditional tubular membrane filters, large particles are tightly stuck inside, making it impossible to clean the membrane by gas backflushing. Only acid washing can be used for cleaning. However, the microfiltration membrane device used in this invention can use a gas washing + acid washing cleaning method. Compared with the tubular membrane cleaning method, the microfiltration membrane device reduces the number of acid washings. In actual operation, tubular membranes are generally automatically acid washed once every 10 hours, with each acid washing lasting 6 to 7 hours. In contrast, the microfiltration membrane device is automatically gas washed online once every 6 to 6.5 hours, lasting 10 to 15 minutes. It is also automatically acid washed online once every 180 to 190 hours, lasting 1.5 to 2 hours. This greatly reduces the frequency and time of acid washing, lowers maintenance costs, and extends the equipment operating cycle.

[0019] (4) The present invention uses a microfiltration membrane device to filter the supernatant, eliminating the backflow of concentrated water. Compared with the original tubular membrane process, this reduces the load on the clarification tank and produces the following technical effects: The amount of water entering the clarifier was reduced, thus lowering the operating load on the clarifier. This increased the residence time of brine in the clarification tank, improved the clarification effect, further reduced the turbidity of the supernatant, and thus reduced the microfiltration membrane load. The amount of salt sludge produced in the clarification tank is reduced because the solid content of the salt sludge produced in the clarification tank is about 20-30 wt%, and the remaining components are refined brine. The solid content of the salt sludge in the clarification tank is much lower than the solid content of the filter residue discharged from the microfiltration membrane (about 60-70 wt%). Therefore, there is no concentrated water recirculation, which reduces the amount of salt sludge discharged from the clarification tank and also reduces the amount of refined brine discharged, thus reducing the loss of refined brine. Reducing the load on the clarifier can lower the speed of the scraper, thereby reducing the power consumption of the equipment and the operating cost of the clarifier. At the same time, it can reduce the stirring phenomenon inside the clarifier caused by the rotation of the scraper, thereby reducing the particle suspension phenomenon caused by stirring and increasing the settling effect.

[0020] (5) This invention uses a microfiltration membrane device instead of a tubular membrane to filter the clear liquid in the clarifier tank to prepare low-turbidity refined brine. This not only improves the operating conditions of the device, shortens the pickling time, and stabilizes production, but also eliminates the need for concentrated water circulation, reduces the load on the clarifier tank, and increases the water production rate. As shown in Table 2, compared with the tubular membrane filtration process, the microfiltration membrane filtration process of this invention reduces the inlet water pressure from 0.7-0.9 MPa to 0.3-0.4 MPa, increases the water production rate from 66-75% to 99.0-99.2%, reduces the power consumption of the clarifier tank from 6.5 kWh / h to 4.2-4.3 kWh / h, and reduces the amount of salt mud in the clarifier tank from 9-10 m³. 3 / h dropped to 7-8m 3 / h, effectively reducing production operating costs and equipment maintenance costs. Attached Figure Description

[0021] Figure 1 This is a diagram of the asymmetric double-layer structure of the microfiltration membrane filter element of the present invention; Figure 2 This is a schematic diagram of the microfiltration membrane device of the present invention; Figure 3 This is a process flow diagram of a specific embodiment of the method for preparing low-turbidity refined brine of the present invention; In the diagram: 1. Filter housing; 2. Liquid inlet; 3. Baffle; 4. Backflush port; 5. Liquid outlet; 6. First chamber; 7. Microfiltration membrane filter element; 8. Second chamber; 9. Sludge discharge port. Detailed Implementation

[0022] Explanation of relevant terms in this invention: Calcium-removed brine: refers to the crude brine after the raw salt has been dissolved, precipitated and filtered. The crude brine still contains a small amount of impurities such as Ca²⁺ and Mg²⁺. Lime milk and polyacrylamide flocculant are added to the crude brine to clarify and remove magnesium ion impurities by sedimentation, thus obtaining primary brine. The primary brine is then sent to a calcium removal tower to react with carbonization tail gas to obtain calcium-removed brine.

[0023] Tubular membranes are a type of pressure-driven, high-efficiency separation membrane technology. Their structure consists of hollow fiber tubular structures made of PET polymer material, with pore sizes ranging from 0.01 to 0.1 micrometers. When a liquid flows through the tubular membrane under pressure, water, small molecules, and dissolved ions permeate through the pores (becoming the permeate) and flow out of the membrane, while larger molecules (such as proteins, colloids, bacteria, and suspended particles) are retained inside the tubular membrane because their size exceeds the pore size (becoming the concentrate).

[0024] Microfiltration membranes are pressure-driven, high-efficiency separation membranes with an asymmetric bilayer structure consisting of a support layer and a filter layer. They are made of high-density polyethylene (HDPE), with the support layer being large-particle HDPE and the filter layer being small-particle HDPE. High-temperature sintering gives them relative hardness and an absolute pore size accuracy of 1–1.05 micrometers. The microstructure of the asymmetric bilayer structure of the microfiltration membrane is shown in [link to microfiltration membrane microstructure diagram]. Figure 1 In the second chamber, the supernatant is filtered through the microfiltration membrane under pressure and flows to the first chamber. Water, small molecules, and dissolved ions pass through the membrane pores into the first chamber, while large molecules, due to their size being larger than the pore size, are trapped on the surface of the microfiltration membrane.

[0025] like Figure 2 As shown, the microfiltration membrane device is horizontally arranged with a partition 3, dividing the microfiltration membrane device into a first chamber 6 and a second chamber 8 distributed vertically. Several microfiltration membrane filter elements 7 are vertically arranged in the second chamber 8. The top of the microfiltration membrane filter element 7 is fixedly connected to the partition 3, and a through hole is opened at the connection between the microfiltration membrane filter element 7 and the partition 3 to connect the first chamber 6 and the second chamber 8. The first chamber 6 has a liquid outlet 5 on its side wall and a backflush port 4 on its top. The second chamber 8 has a liquid inlet 2 on its side wall and a slag discharge port 9 at its bottom. The upper clear liquid enters the second chamber 8 through the liquid inlet 2, passes through the microfiltration membrane filter element 7 and enters the first chamber 6 to obtain low-turbidity refined brine, and is finally discharged through the liquid outlet 5.

[0026] like Figure 3 As shown, the process flow of a specific embodiment of the method for preparing low-turbidity purified brine of the present invention is as follows: Calcium-free brine is introduced into a clarification tank for sedimentation and clarification, yielding a supernatant with a turbidity of 10–50 NTU and salt sludge. The salt sludge is discharged, and the supernatant is pressurized to 0.3–0.4 MPa by a pressure pump and sent to the second chamber of the microfiltration membrane device. After passing through the microfiltration membrane filter element, the supernatant enters the first chamber to obtain low-turbidity refined brine, which is finally discharged from the outlet to proceed to subsequent processes. When the pressure difference between the product water inside and outside the microfiltration membrane is greater than 0.10 MPa, the system automatically performs an air washing operation for 10–15 minutes. The filter cake layer adhering to the surface of the filter element is blown off by the air washing backflushing and discharged from the slag discharge port at the bottom of the second chamber. After the air washing is completed, the system automatically switches to the operating mode. When the product water pressure difference is greater than 0.10 MPa and the time interval between the last air washing and the last air washing is less than 6 hours, the system automatically enters an acid washing operation for 1.5–2 hours. After the acid washing, the system automatically switches to the operating mode. The technical features of air washing, acid washing, and automatic control can all be implemented using existing technologies and solutions.

[0027] Based on actual long-term operation tests, the operating time of the microfiltration membrane device when the product water pressure difference is greater than 0.10 MPa is between 6 and 6.5 hours; when the product water pressure difference is greater than 0.10 MPa and the time interval between the last air wash is less than 6 hours, the operating time is between 180 and 190 hours.

[0028] Table 1: Specifications and properties of the original tubular membrane process and the microfiltration membrane device used in the embodiments of this invention.

[0029] The technical solution of the present invention will be further described in detail below with reference to the embodiments, but this should not be construed as limiting the present invention. Example 1

[0030] (1) The calcium-removed brine obtained from the calcium removal tower is fed into the clarification tank for sedimentation. The sedimentation tank yields an upper clear liquid of 20 NTU, and the lower layer of salt mud is discharged. (2) After pressurizing the supernatant in step (1) to 0.3 MPa, it is sent to a microfiltration membrane device to filter and obtain refined brine with low turbidity of 1.5 NTU. The refined brine enters the subsequent process and the sludge is discharged for recycling. (3) When the pressure difference between the inside and outside of the microfiltration membrane is greater than 0.10 MPa, the system will automatically perform air washing operation for 10 minutes. The filter cake layer attached to the surface of the filter element will be blown off by air washing backflushing and discharged from the slag discharge port at the bottom of the second chamber. After the air washing is completed, the system will automatically switch to the operation mode. When the pressure difference between the product water and the filter cake is greater than 0.10 MPa and the time interval between the last air washing and the last air washing is less than 6 hours, the system will automatically enter the acid washing operation for 1.5 hours. After the acid washing, the system will automatically switch to the operation mode. Example 2

[0031] (1) The calcium-removed brine obtained from the calcium removal tower is fed into the clarification tank for sedimentation. The sedimentation tank yields an upper clear liquid of 30 NTU, and the salt mud generated in the lower layer is discharged. (2) After pressurizing the upper clear liquid in step (1) to 0.35 MPa, it is sent to a microfiltration membrane device to filter and obtain refined brine with low turbidity of 1.7 NTU. The refined brine is sent to the soda ash production system and the slag is recycled. (3) When the pressure difference between the inside and outside of the microfiltration membrane is greater than 0.10 MPa, the system will automatically perform air washing operation for 10 minutes. The filter cake layer attached to the surface of the filter element will be blown off by air washing backflushing and discharged from the slag discharge port at the bottom of the second chamber. After the air washing is completed, the system will automatically switch to the operation mode. When the pressure difference between the product water and the filter cake is greater than 0.10 MPa and the time interval between the last air washing and the last air washing is less than 6 hours, the system will automatically enter the acid washing operation for 1.5 hours. After the acid washing, the system will automatically switch to the operation mode. Example 3

[0032] (1) The calcium-removed brine obtained from the calcium removal tower is fed into the clarification tank for sedimentation. The sedimentation tank yields an upper clear liquid of 50 NTU, and the lower layer of salt mud is discharged. (2) After pressurizing the upper clear liquid in step (1) to 0.4 MPa, it is sent to the microfiltration membrane device and filtered to obtain refined brine with low turbidity of 1.9 NTU. The refined brine is sent to the soda ash production system and the slag is recycled. (3) When the pressure difference between the inside and outside of the microfiltration membrane is greater than 0.10 MPa, the system will automatically perform air washing operation for 15 minutes. The filter cake layer attached to the surface of the filter element will be blown off by air washing backflushing and discharged from the slag discharge port at the bottom of the second chamber. After the air washing is completed, the system will automatically switch to the operation mode. When the pressure difference between the product water and the pressure difference is greater than 0.10 MPa and the time interval between the last air washing and the last air washing is less than 6 hours, the system will automatically enter the acid washing operation for 2 hours. After the acid washing, the system will automatically switch to the operation mode.

[0033] Comparative Example 1 After pressurizing the supernatant to 0.2 MPa, it was sent into the microfiltration membrane device. Everything else remained the same, and the procedure was carried out according to Example 1.

[0034] Comparative Example 2 After pressurizing the supernatant to 0.5 MPa, it was sent into the microfiltration membrane device. Everything else remained the same, and the procedure was carried out according to Example 1.

[0035] Table 2: Comparison of the effects of Examples 1-3 and the original tubular membrane alkali production process

[0036] Note: Water production rate = Water production rate / Water inflow rate As shown in the table above, compared with the prior art, Examples 1-3 reduced the inlet water pressure to the range of 0.3-0.4 MPa, and the water production rate was in the range of 99.0-99.2%, which is much higher than the original 66-75%. Due to the ability to perform air washing, automatic air washing occurs every 6-6.5 hours for 10-15 minutes each time, effectively extending the acid washing interval. Furthermore, due to less clogging, automatic acid washing occurs every 180-190 hours, with each wash controlled between 1-1.5 hours, which is much lower than the original 10-hour automatic acid washing, which lasted 6-7 hours each time. During the filtration process, a sludge with a solid content of 60-70% is directly formed and discharged, while the original tubular membrane's return flow... In contrast to the previous method of settling salt mud with a solid content of 20-30% in the clarification tank, this method is simpler and more efficient. Furthermore, the absence of concentrated water backflow into the clarification tank reduces its load; the absence of concentrated water backflow also reduces the amount of salt mud discharged, and since the salt mud has a high water content, this effectively reduces brine loss; the absence of concentrated water backflow also reduces the amount of salt mud generated, allowing for a lower rotation speed of the scraper, reducing the floating of sediment caused by agitation, and improving the settling effect. Compared to the other two methods, this invention is more cost-effective and efficient. Therefore, this invention achieves several beneficial effects in the brine refining process, including effectively reducing operating and maintenance costs.

[0037] As can be seen from Comparative Example 1 in the table above, when the inlet water pressure is 0.2 MPa, the water production rate decreases rapidly, indicating that a pressure below 0.3 MPa is not conducive to water production. As can be seen from Comparative Example 2, when the inlet water pressure is 0.5 MPa, the water production rate does not change much, indicating that a pressure above 0.4 MPa does not significantly change the effect, but increases operating costs. Therefore, it is not recommended to have too high a pressure.

Claims

1. A method for preparing low-turbidity purified brine, characterized in that, Includes the following steps: The calcium-deionized brine is sent to a clarification tank for clarification and sedimentation to obtain a supernatant. The supernatant is then pressurized and sent to a microfiltration membrane device for filtration to obtain a low-turbidity refined brine. The microfiltration membrane device includes a first chamber and a second chamber, with a partition between the first and second chambers. The partition has several through holes. Several microfiltration membrane filter elements are vertically arranged in the second chamber. The top of each microfiltration membrane filter element passes through the through holes and is fixedly connected to the partition, forming a filtration channel connecting the first and second chambers. The first chamber has a liquid outlet on its side wall and a backflush port on its top. The second chamber has a liquid inlet on its side wall and a sludge discharge port on its bottom. The supernatant enters the second chamber through the liquid inlet, passes through the microfiltration membrane filter elements into the first chamber, and is finally discharged through the liquid outlet.

2. The method for preparing low-turbidity purified brine according to claim 1, characterized in that, The microfiltration membrane filter element is made of high-density polyethylene; the microfiltration membrane filter element has a double-layer structure including a filter layer and a support layer. The filter layer is composed of small-particle high-density polyethylene with a pore size of 1.0 to 1.05 micrometers; the support layer is composed of large-particle high-density polyethylene with an open porosity of 40-42%.

3. The method for preparing low-turbidity purified brine according to claim 1, characterized in that, The pressure of the supernatant after pressurization is 0.3-0.4 MPa, and the turbidity of the supernatant is 10-50 NTU.

4. The method for preparing low-turbidity purified brine according to claim 1, characterized in that, When the pressure difference between the product water inside and outside the microfiltration membrane is greater than 0.10 MPa, the system performs an air washing operation for 10 to 15 minutes. When the pressure difference between the product water and the previous air washing is less than 6 hours, the system enters an acid washing operation for 1.5 to 2 hours.

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