A method for producing large size micelles in a sodium aluminate solution

Abstract

The application provides a method for generating large-size micelles in sodium aluminate solution and relates to the technical field of alumina production. The method mainly adds organic additives such as surfactants to cooperatively promote the self-assembly of organic matters into large-size micelles, and then facilitates the economic removal of the organic matters. Specifically, after adding a cationic, amphoteric or non-ionic surfactant aqueous solution into an organic matter-containing sodium aluminate solution, the temperature is reduced, and slow stirring is performed, and then by adjusting the repulsion of the polar head, the large-size micelles are self-assembled in the high ionic strength sodium aluminate solution; the mixed slurry is filtered to separate the large-size micelles from the sodium aluminate solution, and the efficient removal of the organic matters is realized. The method for generating large-size micelles in sodium aluminate solution can realize the removal rate of the organic matters in the sodium aluminate solution of more than 30%, significantly reduces the total organic carbon concentration in the Bayer system, eliminates the negative influence of the organic matters on the production process, and has the characteristics of simplicity, high efficiency, wide application range and the like.

Description

Technical Field

[0001] This invention relates to the field of alumina production technology, and in particular to a method for generating large-sized micelles in sodium aluminate solution. Background Technology

[0002] The Bayer process is the primary method for alumina production. During the alkaline circulation process in the Bayer process, organic matter rapidly accumulates in the circulating mother liquor, leading to a rapid increase in the total organic carbon (TOC) concentration in the sodium aluminate solution. Imported ore has a high TOC content, resulting in more organic matter being degraded during leaching, primarily entering the sodium aluminate solution in anionic form, thus causing a high concentration of organic matter in the sodium aluminate solution using imported ore. This organic matter reduces decomposition rates, refines the product, promotes scaling, colors the product, generates foam and harmful gases, severely impacting alumina production. Therefore, developing an efficient and economical method for removing organic matter from sodium aluminate solutions is urgently needed.

[0003] Organic matter in sodium aluminate solutions is generally classified into three categories: high, medium, and low molecular weight organic matter. High and medium molecular weight organic matter are the main substances causing changes in solution properties, altering the gas-solid-liquid interface, generating abundant foam, and coloring products. Low molecular weight organic matter, such as sodium oxalate, mainly causes equipment scaling and product refinement. Currently, there are many methods for removing organic matter both domestically and internationally. Oxidation, adsorption, flotation, and seawater methods all target high and medium molecular weight organic matter; crystallization, precipitation, and causticization primarily target low molecular weight organic oxalates; ion exchange and combustion methods are effective for removing various organic sodium salts. However, due to the high ionic strength, high temperature, complex ionic structure, and unclear interactions between organic matter in sodium aluminate solutions, the interfacial behavior of organic matter is extremely complex, which is the main reason for the low efficiency, poor applicability, complex processes, and high costs of organic matter removal to date.

[0004] Therefore, regulating the interfacial behavior of organic matter in sodium aluminate solution is key to improving removal efficiency and simplifying the process. However, there are few reports on the interfacial behavior of organic matter in sodium aluminate solution, and even fewer reports on the preparation of large-size micelles of organic matter. In aqueous solution, surfactant micelles are generally smaller than 0.5 μm, with most ranging from 1 to 100 nm. To prepare large-size micelles, most studies focus on fields such as oilfield recovery and wastewater treatment. Cao Xulong (Acta Physico-Chimica Sinica, 2014, 30(7), 1297-1302) et al. compounded diethylenetriamine with oleic acid to construct mixed micelle and anti-miclle structures, achieving an 80-fold thickening of water-in-oil emulsions. When water-in-oil emulsions are formed, crude oil molecules can also be smoothly solubilized and enter the micelles; these research results greatly improve the recovery efficiency of crude oil in oilfields. Rao Xiaoping (Patent Announcement No. CN)

[0005] 107519814B et al. added a maleic anionic surfactant to a fixed concentration of cationic surfactant, and through the association of the two, prepared a viscoelastic solution with gel characteristics, which has significant application value in oilfield flooding, daily cleaning agents, and industrial detergents. Liu Guihua (patent application numbers 202010112367.8 and 202010112032.6) added surfactants to remove organic matter by adsorption and flotation methods to control the zeta potential of hydrated calcium aluminate, control the critical micelle concentration, and promote the formation of micelles, thereby achieving the goal of efficient removal of organic matter, but did not address the micelle size in sodium aluminate solution.

[0006] Therefore, although organic matter in sodium aluminate solution mainly exists in the form of anionic surfactants, and micelles can be formed when the concentration exceeds the critical micelle concentration, the control of micelle size remains unclear due to the strong repulsive effect of polar groups. This is the key reason why the process for removing organic matter from sodium aluminate solution is relatively complex and costly. Summary of the Invention

[0007] This invention provides a method for generating large micelles in sodium aluminate solution, the purpose of which is to solve the aforementioned problems existing in the prior art.

[0008] To achieve the above objectives, embodiments of the present invention provide a method for generating large-sized micelles in a sodium aluminate solution. This method primarily involves adding organic additives such as surfactants, or further adding high-valence inorganic cations, to synergistically promote the self-assembly of organic matter into large-sized micelles, thereby efficiently and economically removing organic matter. Specifically, an aqueous solution of a cationic, amphoteric, or nonionic surfactant is added to a sodium aluminate solution containing organic matter. The temperature is lowered, and the mixture is slowly stirred. By controlling the repulsive force of the polar heads, large-sized micelles are self-assembled in the high-ionic-strength sodium aluminate solution. This mixed slurry is then filtered to separate the large-sized micelles from the sodium aluminate solution, achieving efficient removal of organic matter. The method of generating large-sized micelles in a sodium aluminate solution of the present invention can achieve an organic matter removal rate of over 30% in sodium aluminate solution, significantly reducing the total organic carbon concentration in the Bayer system, eliminating the negative impact of organic matter on the production process, and is characterized by its simplicity, high efficiency, and wide applicability.

[0009] Embodiments of the present invention provide a method for generating large-sized micelles in a sodium aluminate solution, comprising the following steps:

[0010] S1. Dissolve the surfactant in deionized water to prepare an aqueous surfactant solution;

[0011] S2. Add the surfactant aqueous solution to the sodium aluminate solution containing organic matter, control the temperature, stir slowly, and carry out the reaction; then add the surfactant aqueous solution and / or high-valence inorganic salt solution, continue to stir slowly, and obtain a slurry;

[0012] S3. Filter the slurry to obtain sodium aluminate solution and removed organic matter.

[0013] Preferably, the surfactant in step S1 is at least one of cationic, amphoteric, or nonionic surfactants.

[0014] More preferably, the cationic surfactant is at least one of polyacrylamide, pyridine salt, imidazole salt, quaternary ammonium salt, and morpholine salt; the amphoteric surfactant is an amino acid or betaine; and the nonionic surfactant is dodecyl dimethylamine oxide, triethanolamine, chitosan, or polycondensation amine.

[0015] Preferably, the concentration of the surfactant aqueous solution in step S1 is 1-150 g / L, and the amount added is based on 2-20 ppm of surfactant in sodium aluminate solution.

[0016] Preferably, in step S2, the amount of surfactant aqueous solution added for the first time is 70-100% of the total amount, and the amount added for the second time is 0-30% of the total amount.

[0017] Preferably, the Na2O in the sodium aluminate solution containing organic matter in step S2 is... k 90~190g / L, Al2O380~195g / L, α k 1.35~3.2, Na2O c 5~15g / L, TOC 5~30g / L.

[0018] Preferably, in step S2, the initial stirring time is 5-60 min and the stirring intensity is 20-200 r / min; the subsequent stirring time is 30-120 min and the stirring intensity is 0-200 r / min.

[0019] Preferably, the temperature control in step S2 is as follows: initial temperature 70-100℃, final temperature 30-80℃.

[0020] Preferably, the high-valent inorganic salt in step S2 is one or two of aluminum salt, calcium salt, iron salt and barium salt, and the amount added is less than 300 ppm.

[0021] Preferably, the diameter of the organic micelles in the slurry in step S3 is greater than 100 nm.

[0022] Mechanism Explanation: In studying the self-assembly of organic matter in sodium aluminate solution to form large-sized micelles, this invention specifically discovered:

[0023] (1) Sodium aluminate solution has high ionic strength, and the critical micelle concentration of surfactants in sodium aluminate solution is extremely low, almost always around 10. -5 -10 -4Within the concentration range of mol / L.

[0024] (2) In sodium aluminate solution, due to different charges, certain cationic, amphoteric and nonionic surfactants can easily interact with the polar groups of anionic organic compounds in sodium aluminate solution, promoting self-assembly to form micelles.

[0025] (3) When specific cationic, amphoteric or nonionic surfactants are combined with anionic surfactants, the ion pairs of the surfactants can be easily generated through association, which can significantly increase the cross-sectional area of ​​the nonpolar groups and is conducive to the self-formation of large-sized layered micelles.

[0026] (4) A large number of micelles can re-aggregate, resulting in micelles of even larger size.

[0027] (5) A suitable cooling regime is conducive to the self-assembly of surfactant molecules and the generation of large micelles.

[0028] (6) In sodium aluminate solution, the formation of large micelles will lead to an increase in the absorbance of the solution; while even larger micelles (<5μm) will increase the turbidity of the solution.

[0029] The above-described solution of the present invention has the following beneficial effects:

[0030] (1) Large micelles can be prepared by a simple method. By adding a specific surfactant solution, the repulsive force of polar groups of medium and high molecular weight organic compounds and the hydrophobic effect of nonpolar groups can be controlled to generate large micelles.

[0031] (2) Coexistence of micelles of different sizes. Pre-micelles, micelles and condensed micelles coexist in the solution. The micelle diameter is generally greater than 100 nm, and the diameter of large micelles can reach 5 μm.

[0032] (3) High versatility. Organic matter in sodium aluminate solution mainly exists in anionic form, with high concentration, high molecular weight, high ionic strength, and low critical micelle concentration; after the addition of surfactant, large micelles can be generated. It is especially suitable for sodium aluminate solutions containing medium to high molecular weight organic matter.

[0033] (4) It is easy to graft into the Bayer process. It can be carried out in the crude liquor, concentrate or seed mother liquor in the Bayer process, or large micelles can be prepared in the crude liquor tank, concentrate tank, seed tank or washing tank.

[0034] (5) Low cost of organic matter removal. After generating large micelles, they can be removed by filtration, adsorbed on red mud, or adsorbed on hydrated calcium aluminate or fly ash.

[0035] (6) High organic matter removal rate. For the large amount of high molecular weight organic matter in sodium aluminate solution, the organic matter removal rate is greater than 30% based on TOC. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a cryo-electron microscopy image of a large-size micelle generated according to Embodiment 1 of the present invention;

[0038] Figure 2 This is a micelle size distribution diagram generated in Example 4 of the present invention. Detailed Implementation

[0039] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0040] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0041] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0042] Unless otherwise specified, the percentage of solution involved in this invention refers to the number of grams of solute contained in 100 mL of solution; caustic ratio (α) k () refers to the mass concentration ratio of alkali (calculated as Na2O) to aluminum oxide molecules in the solution.

[0043] The organic concentration of the solution involved in this invention is expressed as total organic carbon (TOC) concentration. The stirring speed is expressed in revolutions per minute (r / min).

[0044] The weight parts mentioned in this invention can be weight units known in the art such as μg, mg, g, kg, or multiples thereof, such as 1 / 10, 1 / 100, 10 times, 100 times, etc.

[0045] Studies on the interfacial behavior of organic matter in sodium aluminate solution are scarce, and research on the preparation of large-sized organic micelles is even rarer. For example, surfactant micelles in aqueous solutions are generally smaller than 0.5 μm, with most ranging from 1 to 100 nm. Most research on preparing large-sized micelles focuses on fields such as oilfield recovery and wastewater treatment. While existing techniques employ adsorption and flotation to remove organic matter by adding surfactants to control the zeta potential of hydrated calcium aluminate, regulate the critical micelle concentration, and promote micelle formation to achieve efficient organic matter removal, they do not address the size of micelles in solution. The control of micelle size in sodium aluminate solution remains unclear, which is a key reason for the complexity and high cost of organic matter removal processes in sodium aluminate solution. Therefore, this invention provides a method for generating large-sized micelles in sodium aluminate solution.

[0046] In studying the self-assembly of organic matter in sodium aluminate solution to form large-sized micelles, this invention specifically discovered:

[0047] (1) Sodium aluminate solution has high ionic strength, and the critical micelle concentration of surfactants in sodium aluminate solution is extremely low, almost always around 10. -5 -10 -4 Within the concentration range of mol / L.

[0048] (2) In sodium aluminate solution, due to different charges, certain cationic, amphoteric and nonionic surfactants can easily interact with the hydrophobic polar groups of anionic organic compounds in sodium aluminate solution, promoting self-assembly to form micelles.

[0049] (3) When specific cationic, amphoteric or nonionic surfactants are combined with anionic surfactants, the ion pairs of the surfactants can be easily generated through association, which can significantly increase the cross-sectional area of ​​the nonpolar groups and is conducive to the self-formation of large-sized layered micelles.

[0050] (4) A large number of micelles can re-aggregate, resulting in micelles of even larger size.

[0051] (5) A suitable cooling regime is conducive to the self-assembly of surfactant molecules and the generation of large micelles.

[0052] (6) In sodium aluminate solution, the formation of large micelles will lead to an increase in the absorbance of the solution; while even larger micelles (<5μm) will increase the turbidity of the solution.

[0053] Compared with the prior art, the present invention has the following advantages:

[0054] (1) Large micelles can be prepared by a simple method. By adding a specific surfactant solution, the repulsive force of polar groups of medium and high molecular weight organic compounds and the effect of non-polar groups can be controlled to generate large micelles.

[0055] (2) Coexistence of micelles of different sizes. Pre-micelles, micelles and condensed micelles coexist in the solution. The micelle diameter is generally greater than 100 nm, and the diameter of large micelles can reach 5 μm.

[0056] (3) High versatility. Organic matter in sodium aluminate solution mainly exists in anionic form, with high concentration, high molecular weight, high ionic strength, and low critical micelle concentration; after the addition of surfactant, large micelles can be generated. It is especially suitable for sodium aluminate solutions containing medium to high molecular weight organic matter.

[0057] (4) It is easy to graft into the Bayer process. It can be carried out in the crude liquor, concentrate or seed mother liquor in the Bayer process, or large micelles can be prepared in the crude liquor tank, concentrate tank, seed tank or washing tank.

[0058] (5) Low cost of organic matter removal. After generating large micelles, they can be removed by filtration, adsorbed onto red mud, or adsorbed onto hydrated calcium aluminate.

[0059] (6) High organic matter removal rate. For the large amount of high molecular weight organic matter in sodium aluminate solution, the organic matter removal rate is greater than 30% based on TOC.

[0060] The following will be explained through specific embodiments.

[0061] Example 1

[0062] S1. Weigh 0.3g of 1-butyl-2,3-dimethylimidazolium chloride and 0.05g of chitosan, dissolve them in deionized water and bring the volume to 100mL to prepare a surfactant mixed solution with a concentration of 3.5g / L.

[0063] S2, at 95℃, in 100mL of sodium aluminate solution containing organic matter (Na2O=161.55g / L, Al2O3=181.12g / L, α k =1.47, sodium dodecyl sulfate 1.76 g / L, TOC 880.4 ppm) Add 4 mL of surfactant mixed solution (of which 1-butyl-2,3-dimethylimidazolium chloride solution accounts for 75% of the total) and stir at 50 r / min for 30 min until homogeneous; slowly cool to 80℃, then add another 1 mL of surfactant mixed solution and stir slowly for 60 min. The solution is slightly turbid, and the absorbance increases from 0.08 to 0.17. Take a sample and freeze it for sample preparation. The micelle morphology is as follows. Figure 1 As shown, the maximum diameter reaches 3μm.

[0064] S3. The solution after standing in S2 is vacuum filtered using slow quantitative filter paper as the filter medium. The resulting filtrate is the sodium aluminate solution with organic matter removed. The total organic carbon concentration in the filtrate decreases to 320.8 ppm, and the organic matter removal rate can reach 63.6%.

[0065] Example 2

[0066] S1. Weigh 10g of dodecyltrimethylammonium bromide and 0.1g of dodecyldimethylamine oxide, dissolve them in deionized water and bring the volume to 100mL to prepare a surfactant mixed solution with a concentration of 100g / L.

[0067] S2, at 95℃, in 100mL of sodium aluminate solution containing organic matter (Na2O=161.55g / L, Al2O3=172.12g / L, α k =1.54, sodium lauryl 1.03 g / L, TOC 673.5 ppm) 0.5 mL surfactant mixture was added and stirred at 50 r / min for 90 min; slowly cooled to 70 °C, the absorbance of the solution increased from 0.08 to 0.19.

[0068] S3. The solution after standing in S2 is vacuum filtered using slow quantitative filter paper as the filter medium. The resulting filtrate is the sodium aluminate solution with organic matter removed. The total organic carbon concentration in the filtrate decreases to 110.5 ppm, and the organic matter removal rate can reach 43.1%.

[0069] Example 3

[0070] S1. Weigh 1.75g ​​of hexadecylpyridine bromide, dissolve it in deionized water and bring the volume to 100mL to prepare a surfactant mixed solution with a concentration of 17.5g / L.

[0071] S2, at 70℃, in 200mL of sodium aluminate solution containing organic matter (Na2O=141.25g / L, Al2O3=78.28g / L, α... k =2.98, sodium lauryl 1.03 g / L, TOC 673.5 ppm) First, add 1 mL of surfactant mixture to the solution and stir at 50 rpm for 30 min until homogeneous. Slowly cool to 30℃, then add 0.1 mL of 3 g / L FeCl3 solution and stir at 50 rpm for 30 min until homogeneous. The absorbance of the solution increased from 0.05 to 0.26. Samples were frozen for preparation, and the maximum micelle diameter reached 5 μm. Dynamic light scattering measurements were performed on the solution samples, and the micelle size distribution is shown in the figure. Figure 2 As shown.

[0072] S4. The solution after standing in S2 is vacuum filtered using slow quantitative filter paper as the filter medium. The resulting filtrate is the sodium aluminate solution with organic matter removed. The total organic carbon concentration in the filtrate decreases to 87.5 ppm, and the organic matter removal rate can reach 87%.

[0073] Example 4

[0074] S1. Weigh 0.1g of cationic polyacrylamide and 1.75g ​​of hexadecylpyridine bromide, dissolve them in deionized water and bring the volume to 100mL to prepare a surfactant mixed solution with a concentration of 18.5g / L.

[0075] S2, at 95℃, in 100mL of sodium aluminate solution containing organic matter (Na2O=141.25g / L, Al2O3=167.28g / L, α... k =1.39, sodium humate 1.52 g / L, TOC 1273.5 ppm) 0.5 mL of surfactant mixture was added, and the mixture was stirred at 50 r / min for 90 min. At this time, the absorbance of the solution increased from 0.1 to 0.37.

[0076] S4. The solution after standing in S3 is vacuum filtered using slow quantitative filter paper as the filter medium. The resulting filtrate is the sodium aluminate solution with organic matter removed. The total organic carbon concentration in the filtrate decreases to 235.8 ppm, and the organic matter removal rate can reach 81.5%.

[0077] Example 5

[0078] S1. Weigh 5g of 4-ethyl-4-methylmorpholine bromide, 0.05g of polyamine condensation and 3g of betaine, dissolve them in deionized water and bring the volume to 100mL to prepare a surfactant mixed solution with a concentration of 80g / L.

[0079] S2, at 65℃, in a sodium aluminate solution containing organic matter (Na2O=155.25g / L, Al2O3=93.28g / L, α k =2.74, sodium humate 1.52 g / L, TOC 1273.5 ppm) Add 0.5 mL of surfactant mixed solution to 100 mL of the solution, stir at 50 r / min for 30 min until homogeneous; then cool to 40 °C, add 0.5 mL of surfactant solution and 1 mL of lime water (Ca 2+ Add approximately 0.3 g / L of the solution, stir at 50 rpm for 30 minutes, and let stand for 60 minutes. At this point, the absorbance of the solution increases from 0.1 to 0.39.

[0080] S4. The solution after standing in S2 is vacuum filtered using slow quantitative filter paper as the filter medium. The resulting filtrate is the sodium aluminate solution with organic matter removed. The total organic carbon concentration in the filtrate decreases to 214.5 ppm, and the organic matter removal rate can reach 83.2%.

[0081] Example 6

[0082] S1. Weigh 0.5g of cetyltrimethylammonium bromide and 0.01g of cationic polyacrylamide (molecular weight 20000), dissolve them in deionized water and bring the volume to 100mL to prepare a surfactant mixed solution with a concentration of 5.1g / L.

[0083] S2. At 95℃, in a sodium aluminate solution containing organic matter (Na2O = 141.25 g / L, Al2O3 = 167.28 g / L, α... k =1.39, sodium humate 1.52 g / L, TOC 1273.5 ppm) Add 0.5 mL of surfactant mixed solution to 100 mL of the solution, stir at 50 rpm for 60 min; then cool to 80 °C, add 0.1 mL of 3 g / L Ba(OH)₂ solution, stir at 50 rpm for 30 min, and let stand for 60 min. At this time, the absorbance of the solution increases from 0.1 to 0.32.

[0084] S4. The solution after standing in S3 is vacuum filtered using slow quantitative filter paper as the filter medium. The resulting filtrate is the sodium aluminate solution with organic matter removed. The total organic carbon concentration in the filtrate decreases to 278.6 ppm, and the organic matter removal rate can reach 78.1%.

[0085] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for generating large-sized micelles in a sodium aluminate solution, characterized in that, Includes the following steps: S1. Dissolve the surfactant in deionized water to prepare an aqueous surfactant solution; the surfactant is at least one of cationic, amphoteric, or nonionic surfactants. The cationic surfactant is at least one of polyacrylamide, pyridine salt, imidazole salt, quaternary ammonium salt, and morpholine salt; the amphoteric surfactant is an amino acid or betaine; and the nonionic surfactant is dodecyl dimethylamine oxide, triethanolamine, chitosan, or polycondensation amine. S2. Add the surfactant aqueous solution to the sodium aluminate solution containing organic matter, control the temperature, and stir slowly to carry out the reaction; then add the surfactant aqueous solution and the high-valent inorganic salt solution, and continue stirring slowly to obtain a slurry; the Na2O in the sodium aluminate solution containing organic matter... k 90~190g / L, Al2O380~195g / L, α k 1.35~3.2, Na2O c 5-15 g / L, TOC 5-30 g / L; the organic matter in the sodium aluminate solution is one of sodium dodecyl sulfate, sodium laurate, or sodium humate; S3. Filter the slurry to obtain sodium aluminate solution and removed organic matter. The diameter of the organic matter micelles in the slurry in step S3 is greater than 100 nm.

2. The method for generating large-sized micelles in sodium aluminate solution according to claim 1, characterized in that, In step S1, the concentration of the surfactant aqueous solution is 1-150 g / L, and the amount added is calculated as 2-20 ppm of surfactant in a solution containing organic sodium aluminate.

3. The method for generating large-sized micelles in sodium aluminate solution according to claim 1, characterized in that, In step S2, the amount of surfactant aqueous solution added for the first time is 70-100% of the total amount, and the amount added for the second time is 0-30% of the total amount, wherein the amount added for the second time is not 0.

4. The method for generating large-sized micelles in sodium aluminate solution according to claim 1, characterized in that, In step S2, the initial stirring time is 5-60 min, and the stirring intensity is 20-200 r / min; the subsequent stirring time is 30-120 min, and the stirring intensity is 0-200 r / min, wherein the stirring intensity is not 0.

5. The method for generating large-sized micelles in sodium aluminate solution according to claim 1, characterized in that, In step S2, the temperature is controlled as follows: initial temperature 70-100℃, final temperature 30-80℃.

6. The method for generating large-sized micelles in sodium aluminate solution according to claim 1, characterized in that, In step S2, the high-valent inorganic salt is one or two of aluminum, calcium, iron, and barium salts, and the amount added is less than 300 ppm.

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