Method for separating anion exchange resin and cation exchange resin in mixed ion exchange resin

Abstract

To provide a method for separating a mixed ion exchange resin in which an anion exchange resin and a cation exchange resin can be separated with extremely high accuracy by using a Seprex method.SOLUTION: A method for separating an anion exchange resin 2 and a cation exchange resin 3 is carried out by a device including a separation tower 1, a Seprex tower 8, and a mixing tank 5 for mixing the anion exchange resin 2 and an aqueous NaOH solution 6. The anion exchange resin 2 taken out during a separation step contains a small amount of the cation exchange resin 3. The anion exchange resin 2 is put into the mixing tank 5 to remove water therefrom. Thereafter, the aqueous NaOH solution 6 having an intermediate specific gravity between specific gravities of the anion exchange resin 2 and the cation exchange resin 3 is injected for immersion. The anion exchange resin 2 is then transferred together with a liquid mixture 2A with the aqueous NaOH solution 6 to the Seprex tower 8 into which water W is injected in advance.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for separating anion exchange resins and cation exchange resins from mixed ion exchange resins used in non-regenerative ion exchange devices or mixed bed ion exchange devices used in pure water production systems. [Background technology]

[0002] Pure water production systems remove impurities from raw water to improve its purity. To remove ionic impurities, i.e., anionic and cationic impurities, a mixed-bed ion exchange system packed with a mixture of anion and cation exchange resins is commonly used. In this mixed-bed ion exchange system, once the ion exchange resin has exchanged ions equivalent to its ion exchange capacity, any further ionic impurities cannot be removed and break through. Therefore, after treating a certain amount of treated water, the ion exchange resins are recovered from the mixed-bed ion exchange system and regenerated in a cation exchange resin regeneration tower and an anion exchange resin regeneration tower using hydrochloric acid or caustic soda, respectively, for reuse. In this process, the anion and cation exchange resins are typically separated by passing water through the system in an upward flow, utilizing the difference in sedimentation velocity due to the difference in specific gravity between the anion and cation exchange resins.

[0003] An example of this mixed ion exchange resin separation tower is shown in Figure 3. In Figure 3, the mixed ion exchange resin separation tower 21 has a cylindrical separation tower body 21A with an inlet / outlet port 22 at the bottom, a water supply pipe 23 with multiple discharge nozzles 23A as a water outlet, and a drain port 24 at the top. A water collecting plate 25 is disposed below the discharge nozzles 23A of this separation tower 21. An anion exchange resin withdrawal pipe 26 as an anion exchange resin withdrawal section is disposed near the middle of the separation tower 21 in the vertical direction, and a cation exchange resin withdrawal pipe 27 is disposed below the anion exchange resin withdrawal pipe 26 and slightly above the water supply pipe 23. A peephole 28 is formed in the side of the separation tower 21. Reference numeral 29 denotes an inlet for used mixed ion exchange resin, disposed on the upper side of the side of the separation tower 21.

[0004] In such a mixed ion exchange resin separation column 21, used mixed ion exchange resin is placed in the separation column 21, followed by passing an approximately 4 wt % NaOH aqueous solution (hereinafter simply referred to as NaOH aqueous solution). After leaving the column for a predetermined time, pure water is poured in through the inlet / outlet 22 and the NaOH aqueous solution in the separation column is pushed out through the outlet 24 for washing. Then, the separation column 21 is filled with a predetermined amount of separation water (pure water). At this time, the level of the separation water is set to be higher than the top surface of the mixed ion exchange resin, specifically, approximately 500 mm or less above.

[0005] Next, air is injected into the separation tower through inlet / outlet 22 to bubble the mixed ion exchange resin, loosening the colloidal entangled resin particles. Then, the bubbling is stopped and the mixed ion exchange resin is allowed to settle on water collection plate 25. At this time, the cation exchange resin with a high specific gravity settles first, followed by the anion exchange resin with a low specific gravity. Next, in preparation for backwashing, pure water (water for separation) is introduced through inlet / outlet 22 so that separation tower 21 is filled with water.

[0006] In this full-water state, pure water is discharged from the discharge nozzle 23A and passed through in an upward flow, and adjustment is made while visually checking through the observation window 28 so that the separation interface is at the lower end of the suction port of the anion exchange resin withdrawal piping 26. Then, the anion exchange resin is sucked through the anion exchange resin withdrawal piping 26 and discharged as an anion exchange resin / water mixed phase flow, which is then removed. After draining, this anion exchange resin / water mixed phase flow is transferred to an anion exchange resin regeneration tower for anion exchange resin regeneration treatment.

[0007] After the anion exchange resin has been extracted in this way, pure water is continuously discharged from the discharge nozzle 23A while being sucked through the cation exchange resin extraction pipe 27, and is discharged as a cation exchange resin-water mixed-phase flow and extracted. After draining the water from this cation exchange resin-water mixed-phase flow, it is transferred to a cation exchange resin regeneration tower for cation exchange resin regeneration treatment. At this time, not all of the cation exchange resin is extracted, but some is left behind to prevent contamination with anion exchange resin.

[0008] However, the above-mentioned method for separating anion exchange resin and cation exchange resin has a problem in that the separation between the two is insufficient. In particular, although the cation exchange resin can be separated well by leaving the interface portion in the separation column 21, there is a problem in that the cation exchange resin is likely to be mixed with the anion exchange resin that is extracted first.

[0009] Therefore, a method called the Seplex method is used to separate the anion exchange resin and the cation exchange resin using a highly concentrated NaOH aqueous solution. This Seplex method uses the process shown in Figure 4.

[0010] Specifically, using the separation tower 21 shown in Figure 3, after the anion exchange resin is extracted in the separation process shown in Figure 4, the anion exchange resin and trace amounts of thione exchange resin mixed in with the ultrapure water used for separation are temporarily stored in a mixing tank and drained. An aqueous NaOH solution with a specific gravity intermediate between the specific gravities of the wet anion exchange resin and the wet cation exchange resin is then injected into the mixing tank. In this case, if the wet specific gravity of the porous anion exchange resin is 1.05 g / mL and the wet specific gravity of the porous cation exchange resin is 1.28 g / mL, as shown in Figure 5, an aqueous NaOH solution with a specific gravity intermediate between the two (e.g., 16 wt% NaOH solution (specific gravity 1.18 g / mL)) is injected. The resulting mixture is then transferred to a high-speed separation tower (Seplex tower) dedicated to separation. The anion exchange resin floats and the cation exchange resin settles, and the cation exchange resin is then extracted and removed from the bottom of the Sepplex tower. Then, pure water is supplied into the Seplex column to extrude and wash the NaOH aqueous solution, and the remaining anion exchange resin is then extracted (advanced separation process). The extracted anion exchange resin is transferred to an anion exchange resin regeneration column where it is regenerated and washed by a standard method. Meanwhile, the cation exchange resin separated in the separation column is transferred to a cation exchange resin regeneration column where it is regenerated and washed by a standard method.

[0011] Specifically, as shown in Figure 6(a), anion exchange resin 32 and cation exchange resin 33 are separated by backwash separation in separation tower 31, and then only the anion exchange resin 32 is transferred to mixing tank 34 together with ultrapure water for separation and drained. Then, an NaOH aqueous solution 37 with a specific gravity intermediate between the anion exchange resin 32 and the cation exchange resin 33 is injected into mixing tank 34 and mixed. A mixture 32A of the NaOH aqueous solution 37 and anion exchange resin 32 is introduced into Sepplex tower 36 through transfer pipe 35 equipped with pump 35A. As a result, anion exchange resin 32 floats and cation exchange resin 33 settles, as shown in Figure 6(b). The cation exchange resin 33 is then extracted and removed from the bottom of Sepplex tower 36. Reference numeral 38 denotes an anion exchange resin extraction pipe. Summary of the Invention [Problem to be solved by the invention]

[0012] The Seplex method described above can separate a trace amount of cation exchange resin 33 mixed in anion exchange resin 32. However, the anion exchange resin 32 introduced into the Seplex column 36 quickly forms a dense layer after introduction, and if a trace amount of cation exchange resin 33 mixed in the anion exchange resin layer 32 is surrounded by the anion exchange resin layer 32, the cation exchange resin 33 does not settle. As a result, there is a problem in that it is difficult to separate the cation exchange resin 33 from the anion exchange resin 32 with high accuracy.

[0013] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for separating anion exchange resins and cation exchange resins of a mixed ion exchange resin mixture using the Seplex method, which enables separation of anion exchange resins and cation exchange resins with extremely high accuracy. [Means for solving the problem]

[0014] In view of the above object, the present invention provides a method for separating anion exchange resins and cation exchange resins from a mixed ion exchange resin of anion exchange resins and cation exchange resins, the method comprising: a separation step in which the mixed ion exchange resin is charged into a substantially cylindrical mixed ion exchange resin separation tower, and a separation liquid is passed through an inlet for air and separation water in an upward flow into the separation tower to separate the mixed ion exchange resin by utilizing the difference in specific gravity; and an anion exchange resin transfer step in which the anion exchange resin above the separation interface between the anion exchange resin and the cation exchange resin is extracted from the separation tower and transferred to a high-speed separation tower, wherein the anion exchange resin is transferred in the anion exchange resin transfer step while immersed in a solution having a specific gravity intermediate between the anion exchange resin and the cation exchange resin, and a liquid having a specific gravity smaller than that of the solution having a specific gravity intermediate between the anion exchange resin and the cation exchange resin is previously placed in the high-speed separation tower, and the cation exchange resin immersed in the solution having a specific gravity intermediate between the anion exchange resin and the cation exchange resin is transferred to the high-speed separation tower (Invention 1).

[0015] According to this invention (Invention 1), during the transfer of anion exchange resin, the anion exchange resin containing a small amount of extracted cation exchange resin is immersed in a solution with a specific gravity intermediate between the anion exchange resin and the cation exchange resin. This allows the anion exchange resin to float and the cation exchange resin to settle during transfer. However, if a dense layer is formed due to entanglement, the cation exchange resin contained in the anion exchange resin becomes difficult to remove. Therefore, the separated anion exchange resin is mixed with a solution with a specific gravity intermediate, such as an aqueous NaOH solution, and then introduced into a Seplex column containing a solution with a specific gravity lower than that of the intermediate solution, such as water. The mixing of the solutions with different specific gravities agitates and loosens the anion exchange resin, allowing the cation exchange resin mixed in the anion exchange resin to settle. This reduces the contamination rate of the cation exchange resin when the anion exchange resin is separated in the high-speed separation column.

[0016] In the above invention (invention 1), the solution having an intermediate specific gravity between the anion exchange resin and the cation exchange resin is preferably an aqueous NaOH solution (invention 2). In particular, in the above invention (invention 2), it is preferable that the concentration of the aqueous NaOH solution is 8% by weight or more (invention 3).

[0017] According to such inventions (Inventions 2 and 3), the specific gravity of the NaOH aqueous solution can be easily adjusted to a value between the specific gravity of the anion exchange resin when wet and the specific gravity of the cation exchange resin when wet, so that the anion exchange resin can be suspended and the cation exchange resin mixed in the anion exchange resin can be suitably separated.

[0018] In the above inventions (Inventions 2 and 3), when the anion exchange resin is transported while immersed in a solution having an intermediate specific gravity between the anion exchange resin and the cation exchange resin, it is preferable that the anion exchange resin extracted from the separation tower is temporarily placed in a mixing tank, mixed with an aqueous NaOH solution in the mixing tank, and then transported to the high-speed separation tower (Invention 4).

[0019] According to this invention (Invention 4), the anion exchange resin extracted from the separation tower is first placed in a mixing tank, where it is drained and then mixed with an aqueous NaOH solution. This makes it easier to control the concentration of the aqueous NaOH solution in the mixing tank, and enables the anion exchange resin to be separated with higher precision. [Effects of the Invention]

[0020] According to the method for separating anion exchange resin and cation exchange resin from a mixed ion exchange resin of the present invention, the separated anion exchange resin is mixed with a solution having an intermediate specific gravity between the anion exchange resin and the cation exchange resin, such as an aqueous NaOH solution, and the mixture is introduced into a high-speed separation tower, such as a Seplex tower, which has previously been charged with a solution having a lower specific gravity than the intermediate specific gravity solution, such as water. This makes it easier for the cation exchange resin mixed in the anion exchange resin to settle, thereby reducing the contamination rate of the cation exchange resin when the anion exchange resin is separated in the high-speed separation tower. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a process diagram showing a method for separating anion exchange resin and cation exchange resin in a mixed ion exchange resin according to one embodiment of the present invention. [Figure 2] FIG. 3 is a schematic diagram showing a transfer step of an anion exchange resin in the embodiment. [Figure 3] FIG. 1 is a schematic diagram showing an example of a separation column for an anion exchange resin and a cation exchange resin of a mixed ion exchange resin. [Figure 4] FIG. 1 is a process diagram showing a conventional method for separating anion exchange resin and cation exchange resin from a mixed ion exchange resin by the Seplex method. [Figure 5] FIG. 1 is a schematic diagram illustrating the principle of the Sepplex method. [Figure 6] FIG. 1 is a schematic diagram showing a conventional transfer process using the Seplex method. DETAILED DESCRIPTION OF THE INVENTION

[0022] The method for separating the anion exchange resin and the cation exchange resin in the mixed ion exchange resin of the present invention will be described in detail below with reference to the accompanying drawings.

[0023] [Configuration of Mixed Ion Exchange Resin Separation System] The method for separating the anion exchange resin and the cation exchange resin in the mixed ion exchange resin of this embodiment can be carried out, for example, by using a configuration including a separation tower as shown in FIG. 3 described above, a cation exchange resin regeneration tower, a Seplex tower (anion exchange resin high-speed separation tower), and a mixing tank for mixing the anion exchange resin and an NaOH aqueous solution (a solution with an intermediate specific gravity between the anion exchange and cation exchange resins).

[0024] (Mixed ion exchange resins) In this embodiment, the mixed ion exchange resin is a mixed resin of anion exchange resin and cation exchange resin. The ratio (volume ratio) of anion exchange resin to cation exchange resin in this mixed ion exchange resin is not particularly limited, but is about 30:70 to 70:30.

[0025] Both the anion exchange resin and the cation exchange resin are preferably porous ion exchange resins. The porous anion exchange resin has a specific gravity (wet) of, for example, about 1.03 to 1.09 g / mL, and the porous anion exchange resin has a specific gravity (wet) of, for example, about 1.22 to 1.30 g / mL.

[0026] [Method for separating anion exchange resin and cation exchange resin from mixed ion exchange resin] Next, the separation method of the mixed ion exchange resin of this embodiment using the above-mentioned apparatus configuration will be described with reference to the flow chart of FIG.

[0027] (separation process) The separation step is the same as that of the Sepplex method shown in FIG. 4, and therefore a detailed description thereof will be omitted.

[0028] (Anion exchange resin advanced separation process) The anion exchange resin removed in the separation step contains a small amount of cation exchange resin. Therefore, in this embodiment, the anion exchange resin is placed in a mixing tank, drained, and then immersed in an aqueous NaOH solution. The NaOH solution is set to a concentration of 5 to 30 wt % so that its specific gravity is intermediate between that of the wet anion exchange resin and that of the wet cation exchange resin. For example, when the anion exchange resin and the cation exchange resin are porous, the NaOH solution is set to a concentration of 9 to 25 wt %. Such an aqueous NaOH solution has a specific gravity of approximately 1.10 to 1.27 g / mL. Specifically, if the wet specific gravity of a porous anion exchange resin is 1.05 g / mL and the wet specific gravity of a porous cation exchange resin is 1.28 g / mL, an aqueous NaOH solution with a specific gravity intermediate between the two should be injected. However, the NaOH solution should be set so that its wet specific gravity is greater than that of the porous cation exchange resin, even when mixed with water previously injected into the Seplex column (described later).

[0029] After the anion exchange resin and the NaOH aqueous solution are mixed in the mixing tank, the anion exchange resin is transferred to a Sepplex column (advanced separation column) together with the NaOH aqueous solution. In this embodiment, this process is carried out according to the procedure shown in FIG. 2. Specifically, as shown in FIG. 2(a), anion exchange resin 2 and cation exchange resin 3 are separated by backwash separation in separation column 1. Then, only anion exchange resin 2 is transferred to mixing tank 5 through anion exchange resin withdrawal pipe 4 together with ultrapure water (separation water) and drained. Subsequently, NaOH aqueous solution 6 with a specific gravity intermediate between that of anion exchange resin 2 and cation exchange resin 3 is injected into mixing tank 5 and mixed to prepare a mixture 2A of NaOH aqueous solution 6 and anion exchange resin 2. This mixture is then introduced into Sepplex column 8 through transfer pipe 7 equipped with pump 7A. A predetermined amount of water W is previously introduced into Sepplex column 8. The amount of water W added to the Seplex column 8 is preferably such that the specific gravity of the NaOH aqueous solution 6A, which is a mixture of the NaOH aqueous solution 6 and the water W, is greater than the specific gravity of the anion exchange resin 2 and slightly smaller than the specific gravity of the cation exchange resin 3.

[0030] As a result, as shown in Figure 2(b), due to the mixing action of the mixed liquid 2A and water W, which are liquids with different specific gravities, the anion exchange resin 2 is stirred and loosened together with the mixed liquid 2A, making it difficult for a dense layer of the anion exchange resin to form and making it easier for the cation exchange resin 3 mixed in the anion exchange resin 2 to settle.As a result, the cation exchange resin 3 settles and the rate of its contamination can be reduced.

[0031] Next, the mixture is left to stand in this state to achieve a high degree of separation into anion exchange resin 2 and cation exchange resin 3. Then, the cation exchange resin 3 is drawn out by suction through a cation exchange resin withdrawal pipe (not shown) provided at the bottom of Seplex column 8. After draining the water, this cation exchange resin can be added together with the next separation and regeneration of mixed ion exchange resins.

[0032] (Anion exchange resin regeneration process) Thereafter, pure water is supplied into the Sepplex column 8 to push out and discharge the aqueous NaOH solution from the column, and the anion exchange resin 2 is washed and extracted from the Sepplex column 8. Thereafter, the anion exchange resin 2 can be regenerated by a known regeneration method. Specifically, after the anion exchange resin 2 is charged into the anion exchange resin regeneration column, the anion exchange resin 2 can be regenerated by injecting an aqueous NaOH solution.

[0033] (Cation exchange resin regeneration process) The cation exchange resin 3 separated from the bottom of the separation column 1 may be regenerated by a known method in a cation exchange resin regeneration column. Specifically, after the cation exchange resin 3 is introduced into the cation exchange resin regeneration column, an HCl solution as an inorganic acid is injected to regenerate the cation exchange resin.

[0034] Next, pure water is supplied into the cation exchange resin regeneration tower to push out the HCl solution from the tower and wash the cation exchange resin 3. In this way, the cation exchange resin can be regenerated. The regenerated cation exchange resin can be removed from the cation exchange resin regeneration tower and reused.

[0035] In the present embodiment as described above, the anion exchange resin 2 separated in the separation step is temporarily received in the mixing tank 5 before being introduced into the Sepplex column 8, and mixed with an aqueous NaOH solution having a specific gravity intermediate between that of the anion exchange resin 2 and the cation exchange resin 3. The Sepplex column 8 is then transported with water W already added thereto, thereby enabling the cation exchange resin 3 to be separated with high accuracy.

[0036] Although the present invention has been described above based on the above-described embodiment, various modifications are possible. For example, the solution placed in the mixing tank 5 is not limited to an aqueous NaOH solution, as long as it is a solution with an intermediate ratio of anion exchange resin and cation exchange resin. Furthermore, the liquid preliminarily placed in the Seplex column does not have to be water; it may be any liquid with a lower specific gravity than the aqueous NaOH solution (a solution with an intermediate specific gravity) placed in the mixing tank. Furthermore, in the present invention, the ion exchange resin is not limited to anion exchange resin and cation exchange resin, but also includes catalyst resins in which catalytic metals are supported on these ion exchange resins, boron-selective adsorption resins, and the like. [Example]

[0037] The present invention will be further illustrated by the following specific examples.

[0038] [Example 1] Using the separation apparatus shown in Figure 2, a mixed resin of porous anion exchange resin 2 (wet specific gravity 1.05 g / mL) and porous cation exchange resin 3 (wet specific gravity 1.28 g / mL) was separated in separation column 1. After that, only the anion exchange resin 2 was transferred to mixing tank 5 together with ultrapure water for separation and draining. Subsequently, 20 wt% NaOH aqueous solution (specific gravity 1.22 g / mL) was injected into mixing tank 5, and the two were mixed. The resulting mixed solution 2A of NaOH aqueous solution 6 and anion exchange resin 2 was transferred through transfer pipe 7 equipped with pump 7A to Separator column 8. Water W for dilution had been previously charged into Separator column 8, and the mixed solution 2A was separated and purified. The amount of water W charged into Separator column 8 was set so that the specific gravity of NaOH aqueous solution 6A, a mixture of NaOH aqueous solution 6 and water W, was greater than that of anion exchange resin 2 but slightly less than that of cation exchange resin 3. The sunk cation exchange resin 3 was extracted, and then the anion exchange resin 2 was extracted. The contamination rate of the cation exchange resin 3 in the extracted anion exchange resin 2 was measured and found to be less than 0.001% by volume.

[0039] [Comparative Example 1] Using the separation apparatus shown in Figure 6, a mixture of porous anion exchange resin 32 (wet specific gravity 1.05 g / mL) and porous cation exchange resin 33 (wet specific gravity 1.28 g / mL) was separated in separation column 1. The anion exchange resin 32 was then transferred to mixing tank 5 along with ultrapure water for separation, and the water was removed. Subsequently, a 16 wt% NaOH aqueous solution (specific gravity 1.18 g / mL) 37 was injected into mixing tank 34, and the two were mixed. The resulting mixture 32A of the NaOH aqueous solution 37 and anion exchange resin 32 was transferred through transfer pipe 35 equipped with pump 35A to Seplex column 36 for separation and purification. The sunk cation exchange resin 33 was then removed, followed by the anion exchange resin 32. The content of the cation exchange resin 33 in the removed anion exchange resin 32 was measured and found to be 0.003% by volume. [Explanation of symbols]

[0040] 1 Separation tower 2. Anion exchange resin 2A Mixture of anion exchange resin and NaOH aqueous solution 3. Cation exchange resin 4 Anion exchange resin extraction piping 5 Mixing tank 6 NaOH aqueous solution 6A NaOH aqueous solution 7 Transfer pipe 7A Pump 8 Seplex Tower (Advanced Separation Tower) 21 Mixed ion exchange resin separation column 21A Separation tower body 22 Inlet / outlet 23 Water supply pipe 23A Discharge Nozzle 24 Drain 25 Water collection board 26 Anion exchange resin extraction piping 27 Cation exchange resin extraction piping 28 Peephole 29 Inlet for used mixed ion exchange resin

Claims

1. A method for separating anion exchange resin and cation exchange resin from a mixed ion exchange resin of anion exchange resin and cation exchange resin, comprising: a separation step in which the mixed ion exchange resin is introduced into a substantially cylindrical mixed ion exchange resin separation tower, and a separation liquid is passed through an inlet for air and separation water in an upward direction into the separation tower to separate the mixed ion exchange resin by utilizing a difference in specific gravity; an anion exchange resin transfer step of extracting the anion exchange resin above the separation interface between the anion exchange resin and the cation exchange resin from the separation tower and transferring it to an advanced separation tower; In the anion exchange resin transferring step, the anion exchange resin is transferred in a state where it is immersed in a solution having an intermediate specific gravity between the anion exchange resin and the cation exchange resin, A method for separating anion exchange resin and cation exchange resin from a mixed ion exchange resin, comprising: pre-filling the high-speed separation tower with a liquid having a lower specific gravity than the intermediate specific gravity solution; and transferring the cation exchange resin immersed in the intermediate specific gravity solution to the high-speed separation tower.

2. 2. The method for separating the anion exchange resin and the cation exchange resin from a mixed ion exchange resin according to claim 1, wherein the solution having an intermediate specific gravity between the anion exchange resin and the cation exchange resin is an aqueous NaOH solution.

3. 3. The method for separating anion exchange resin and cation exchange resin from a mixed ion exchange resin according to claim 2, wherein the concentration of the aqueous NaOH solution is 8% by weight or more.

4. 4. The method for separating anion exchange resins and cation exchange resins from a mixed ion exchange resin mixture according to claim 2, wherein, in the anion exchange resin transfer step, the anion exchange resin is transferred while immersed in a solution having an intermediate specific gravity between the anion exchange resin and the cation exchange resin, and the anion exchange resin extracted from the separation tower is temporarily placed in a mixing tank, mixed with an aqueous NaOH solution in the mixing tank, and then transferred to the high-speed separation tower.

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