Method for preparing basic chromium sulfate from sodium-modified vanadium extraction wastewater

Abstract

The invention provides a method for preparing basic chromium sulfate from sodium-modified vanadium extraction wastewater, which comprises the following steps: sequentially mixing the sodium-modified vanadium extraction wastewater with a reducing agent and alkali liquor, and carrying out solid-liquid separation to obtain wet-based chromium hydroxide; mixing the first wet-based chromic hydroxide with sulfuric acid, stirring after standing, and adding the second wet-based chromic hydroxide while stirring to obtain an intermediate solution; and sequentially and circularly adding sulfuric acid and wet-based chromium hydroxide into the intermediate solution until the charging is finished, thereby obtaining the liquid basic chromium sulfate. According to the method provided by the invention, the sodium-modified vanadium extraction wastewater is converted into the wet-based chromium hydroxide with a certain water content, and the basic chromium sulfate is prepared through the process design of the reaction of the wet-based chromium hydroxide and the sulfuric acid, so that the product purity and the raw material utilization rate are improved, the energy consumption is remarkably reduced, and the method has a great industrial value.

Description

Technical Field

[0001] The invention belongs to the technical field of chromium salt preparation, and particularly relates to a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater. Background Art

[0002] Basic chromium sulfate (Cr(OH)SO4) is a core product of the chromium salt industry. Its molecular structure contains both hydroxyl and sulfate groups, giving it excellent complexing and tanning properties. As the most widely used tanning agent in the leather industry, it significantly improves leather's heat resistance and mechanical strength by cross-linking collagen fibers. It also serves as a mordant in textile printing and dyeing, enhancing dye fixation. It is used in chromium plating pretreatment and passivation processes in metal surface treatment. Furthermore, it can serve as a catalyst support in polymer polymerization reactions.

[0003] The traditional production process for basic chromium sulfate relies primarily on the dichromate reduction method, where sodium dichromate and sulfur dioxide are reduced to form basic chromium sulfate, with sodium sulfate as a byproduct. However, this process has certain technical drawbacks: the raw materials are highly toxic; the production process generates large amounts of chromium-containing wastewater, making hexavalent chromium residue difficult to remove; it consumes a lot of energy, requiring continuous heating during the reduction phase; and the product contains a high sodium ion content. These issues hinder the sustainability of this traditional process.

[0004] Meanwhile, in the vanadium metallurgy sector, the sodium roasting process for vanadium extraction produces large amounts of wastewater containing high-valent chromium. Approximately 5–15% of the chromium in vanadium-chromium coexisting ores is converted to soluble hexavalent chromium during the vanadium extraction process, making use of this chromium resource to produce basic chromium sulfate is crucial for the sustainability and economic viability of the chromium salt process.

[0005] CN107954474A discloses a method for producing vanadium products and basic chromium sulfate using a vanadium-chromium solution. The method involves precipitating vanadium from the vanadium-chromium solution to obtain a chromium-containing material. The vanadium-chromium solution is then acid-dissolved with sulfuric acid, an oxidant is added, and the pH is adjusted to obtain a chromium sulfate solution. The basicity is then adjusted to obtain basic chromium sulfate. CN109399714A discloses a method for producing basic chromium sulfate using chromium-containing wastewater. The chromium-containing wastewater is treated with an oxidant, adjusted to alkalinity, impurities are removed, and acidified for reduction to obtain a chromium sulfate solution. The chromium sulfate solution is then added with alkali and stirred to obtain liquid basic chromium sulfate. The solution is then concentrated, washed, dried, and pulverized to obtain the basic chromium sulfate product. Existing methods for producing basic chromium sulfate using chromium wastewater generally involve first separating solid chromium oxide or chromium hydroxide, then acid-hydrolyzing the solution to obtain a chromium sulfate solution. The basicity is then adjusted with alkali to obtain basic chromium sulfate. This process route has the following main problems: it requires multiple steps such as drying, acid hydrolysis, and alkaline adjustment, which makes the process lengthy and inefficient; the drying and acid hydrolysis processes take a very long time, and the energy consumption and cost are extremely high; residual sodium ions are easily retained in the product, affecting product performance.

[0006] Therefore, based on the problems of the existing process, it is necessary to further develop a short-process, low-cost basic chromium sulfate preparation process. Summary of the Invention

[0007] The object of the present invention is to provide a method for preparing basic chromium sulfate from sodium vanadium extraction wastewater, which converts chromium in sodium vanadium extraction wastewater into basic chromium sulfate through low cost and short process.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater, the method comprising the following steps:

[0010] (1) mixing sodium vanadium extraction wastewater with a reducing agent and an alkali solution in sequence, and performing solid-liquid separation to obtain wet-based chromium hydroxide;

[0011] (2) mixing the first wet-based chromium hydroxide and sulfuric acid, allowing to stand and then stirring, and adding the second wet-based chromium hydroxide while maintaining stirring to obtain an intermediate solution;

[0012] (3) adding sulfuric acid and wet-base chromium hydroxide to the intermediate solution in a cyclic manner until the addition is completed to obtain liquid basic chromium sulfate.

[0013] The method provided by the present invention reduces and reacts the high-valent chromium ions in the sodium vanadium extraction wastewater to convert them into wet-based chromium hydroxide with a certain water content. First, a portion of the wet-based chromium hydroxide is reacted with sulfuric acid. By utilizing the characteristic that the wet-based chromium hydroxide has a certain water content, the sulfuric acid disperses and dissolves the chromium hydroxide, and the chromium hydroxide is evenly dispersed in the sulfuric acid, avoiding excessive acidification that causes passivation or precipitation of trivalent chromium, facilitating further reaction. At the same time, during the mixed reaction process, dissolution and reaction continue to release heat to form a continuous reaction; there is no need to dehydrate and dry the chromium hydroxide, greatly reducing energy consumption and cost. In the process of cyclically adding raw materials, trivalent chromium ions and sulfate ions are maintained under stable pH and temperature conditions to form the target product, basic chromium sulfate. The reaction is continuous and stable, and is not prone to side reactions and impurities. The product purity and yield are high, and the raw material utilization rate is high.

[0014] In the present invention, the sulfuric acid is concentrated sulfuric acid, and the mass fraction of the concentrated sulfuric acid is 98%.

[0015] Preferably, the sodium-treated vanadium extraction wastewater is obtained by subjecting vanadium-containing raw materials to sodium-treatment roasting, leaching and ammonium salt precipitation of vanadium.

[0016] Preferably, the hexavalent chromium content of the sodium vanadium extraction wastewater is 1.2-1.4 g / L, for example, it can be 1.2 g / L, 1.25 g / L, 1.3 g / L, 1.35 g / L or 1.4 g / L, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0017] Preferably, the pH of the sodium vanadium extraction wastewater is 1-2, for example, 1, 1.2, 1.5, 1.8 or 2, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0018] Preferably, the reducing agent comprises sodium metabisulfite.

[0019] Preferably, the water content of the wet-based chromium hydroxide is 35%-40%, for example, 35%, 36%, 37%, 38%, 39% or 40%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0020] Preferably, in step (2), the mass ratio of the first wet-basis chromium hydroxide to sulfuric acid is 1:(1.5-1.7) based on Cr(OH)3 and H2SO4, for example, 1:1.5, 1:1.55, 1:1.6, 1:1.65 or 1:1.7, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0021] Preferably, in step (2), the mass ratio of the first wet-basis chromium hydroxide to the second wet-basis chromium hydroxide is 1:(0.9-1.1), for example, 1:0.9, 1:0.95, 1:1, 1:1.05 or 1:1.1, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0022] The total mass of the wet-basis chromium hydroxide added in step (2) accounts for 20%-30% of the total mass of the wet-basis chromium hydroxide added in the method, for example, it can be 20%, 22%, 24%, 25%, 26%, 28% or 30%, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0023] Preferably, the standing time in step (2) is 1-3 hours, for example, 1 hour, 1.5 hours, 2 hours, 2.5 hours or 3 hours, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0024] Preferably, in step (3), the mass ratio of sulfuric acid and wet-basis chromium hydroxide added in each cycle is 1:(0.9-1.1) according to which H2SO4 and Cr(OH)3 are calculated, for example, it can be 1:0.9, 1:0.95, 1:1, 1:1.05 or 1:1.1, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0025] Preferably, in step (3), the mass of the wet-basis chromium hydroxide added in each cycle accounts for 20%-50% of the total mass of the wet-basis chromium hydroxide added in step (3), for example, it can be 20%, 25%, 30%, 35%, 40%, 45% or 50%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0026] Preferably, the number of cyclic additions in step (3) is 3-5 times, for example, 3 times, 4 times or 5 times.

[0027] Preferably, the endpoint pH at the end of the addition in step (3) is 2.4-2.6, for example, it can be 2.4, 2.45, 2.5, 2.55 or 2.6, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0028] Preferably, after the addition in step (3) is completed, stirring and aging are carried out.

[0029] Preferably, the stirring and aging time is 2-4 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The method provided by the present invention utilizes sodium-treated vanadium extraction wastewater to convert it into wet-based chromium hydroxide with a certain water content, and prepares basic chromium sulfate through a process design in which the wet-based chromium hydroxide reacts with sulfuric acid, thereby improving product purity and raw material utilization rate, and significantly reducing energy consumption, thereby having great industrial value. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0033] In order to clearly illustrate the technical solution of the present invention, in a specific embodiment, the hexavalent chromium content in the sodium vanadium extraction wastewater is 1.2-1.4 g / L, the pH is 1-2; and the mass fraction of the concentrated sulfuric acid used is 98%.

[0034] Example 1

[0035] This embodiment provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater, the method comprising the following steps:

[0036] (1) adding sodium metabisulfite and sodium hydroxide solution sequentially to sodium vanadium extraction wastewater, and performing filter pressing to obtain wet-basis chromium hydroxide with a water content of 40%;

[0037] (2) turning on the screw conveyor above the acid leaching tank, turning on the electric hoist for feeding, adding the first wet-based chromium hydroxide to the acid leaching tank, opening the acid inlet valve and the sulfuric acid storage tank valve of the acid leaching tank, and adding concentrated sulfuric acid to the acid leaching tank by flow meter, wherein the mass ratio of the first wet-based chromium hydroxide to the concentrated sulfuric acid is 1:1.6 according to the ratio of Cr(OH)3 to H2SO4, and starting stirring after standing for 1 hour, and then adding the second wet-based chromium hydroxide, wherein the mass ratio of the first wet-based chromium hydroxide to the second wet-based chromium hydroxide is 1:1, and stirring is carried out uniformly. The total mass of the wet-based chromium hydroxide added in this step accounts for 25% of the total mass of the wet-based chromium hydroxide added in the method;

[0038] (3) Concentrated sulfuric acid and wet-based chromium hydroxide were added to the acid leaching tank in a cyclic manner, wherein the mass ratio of concentrated sulfuric acid and wet-based chromium hydroxide added in each cycle was 1:1 according to H2SO4 and Cr(OH)3. The addition was stopped when the liquid level in the acid leaching tank reached 3.5 meters and the liquid pH was measured to be 2.5. Concentrated sulfuric acid and wet-based chromium hydroxide were added four times respectively, and the stirring and aging were continued for 3 hours before stopping to obtain liquid basic chromium sulfate.

[0039] Example 2

[0040] This embodiment provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater, the method comprising the following steps:

[0041] (1) adding sodium metabisulfite and sodium hydroxide solution sequentially to sodium vanadium extraction wastewater, and performing filter pressing to obtain wet-basis chromium hydroxide with a water content of 35%;

[0042] (2) turning on the screw conveyor above the acid leaching tank, turning on the feeding electric hoist, adding the first wet-based chromium hydroxide to the acid leaching tank, opening the acid inlet valve and the sulfuric acid storage tank valve of the acid leaching tank, and adding concentrated sulfuric acid to the acid leaching tank by flow meter, wherein the mass ratio of the first wet-based chromium hydroxide to the concentrated sulfuric acid is 1:1.5 according to the ratio of Cr(OH)3 to H2SO4, and starting stirring after standing for 1 hour, and then adding the second wet-based chromium hydroxide, wherein the mass ratio of the first wet-based chromium hydroxide to the second wet-based chromium hydroxide is 1:0.9, and stirring is carried out uniformly. The total mass of the wet-based chromium hydroxide added in this step accounts for 20% of the total mass of the wet-based chromium hydroxide added in the method;

[0043] (3) Concentrated sulfuric acid and wet-based chromium hydroxide are sequentially added to the acid leaching tank in a cyclic manner, wherein the mass ratio of concentrated sulfuric acid and wet-based chromium hydroxide added in each cycle is 1:0.9 according to the mass ratio of H2SO4 to Cr(OH)3, and the addition is stopped when the pH value of the liquid is measured to be 2.4. Concentrated sulfuric acid and wet-based chromium hydroxide are added 5 times respectively, and the stirring and aging are continued for 4 hours, and then the addition is stopped to obtain liquid basic chromium sulfate.

[0044] Example 3

[0045] This embodiment provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater, the method comprising the following steps:

[0046] (1) adding sodium metabisulfite and sodium hydroxide solution sequentially to sodium vanadium extraction wastewater, and performing filter pressing to obtain wet-basis chromium hydroxide with a water content of 38%;

[0047] (2) turning on the screw conveyor above the acid leaching tank, turning on the electric hoist for feeding, adding the first wet-based chromium hydroxide to the acid leaching tank, opening the acid inlet valve and the sulfuric acid storage tank valve of the acid leaching tank, and adding concentrated sulfuric acid to the acid leaching tank by flow meter, wherein the mass ratio of the first wet-based chromium hydroxide to the concentrated sulfuric acid is 1:1.7 according to the ratio of Cr(OH)3 to H2SO4, and starting stirring after standing for 1 hour, and then adding the second wet-based chromium hydroxide, wherein the mass ratio of the first wet-based chromium hydroxide to the second wet-based chromium hydroxide is 1:0.9, and stirring is carried out uniformly. The total mass of the wet-based chromium hydroxide added in this step accounts for 30% of the total mass of the wet-based chromium hydroxide added in the method;

[0048] (3) Concentrated sulfuric acid and wet-based chromium hydroxide were added to the acid leaching tank in a cyclic manner, wherein the mass ratio of concentrated sulfuric acid and wet-based chromium hydroxide added in each cycle was 1:1.1 according to H2SO4 and Cr(OH)3. The addition was stopped when the pH of the liquid was measured to be 2.6. Concentrated sulfuric acid and wet-based chromium hydroxide were added three times respectively, and the stirring and aging were continued for 4 hours before stopping to obtain liquid basic chromium sulfate.

[0049] Example 4

[0050] This embodiment provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater. Compared with Example 1, concentrated sulfuric acid and wet-based chromium hydroxide are cyclically added in step (3) until the liquid pH is measured to be 2.0. The rest is the same as Example 1.

[0051] Example 5

[0052] This embodiment provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater. Compared with Example 1, concentrated sulfuric acid and wet-based chromium hydroxide are circulated in step (3) until the liquid pH is measured to be 3.0. The rest is the same as Example 1.

[0053] Example 6

[0054] This embodiment provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater. Compared with Example 1, the mass ratio of the first wet-basis chromium hydroxide to concentrated sulfuric acid in step (2) is controlled to be 1:1.2 based on Cr(OH)3 and H2SO4, and the mass ratio of the first wet-basis chromium hydroxide to the second wet-basis chromium hydroxide is 1:0.6. The rest is the same as in Example 1.

[0055] Example 7

[0056] This embodiment provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater. Compared with Example 1, the mass ratio of the first wet-basis chromium hydroxide to concentrated sulfuric acid in step (2) is controlled to be 1:2 based on Cr(OH)3 and H2SO4, and the mass ratio of the first wet-basis chromium hydroxide to the second wet-basis chromium hydroxide is 1:1.4. The rest is the same as in Example 1.

[0057] Comparative Example 1

[0058] This comparative example provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater, the method comprising the following steps:

[0059] (1) adding sodium metabisulfite and sodium hydroxide solution sequentially to sodium vanadium extraction wastewater, and performing filter pressing to obtain wet-basis chromium hydroxide with a water content of 40%;

[0060] (2) The screw conveyor above the acid leaching tank is turned on, the loading electric hoist is turned on, wet-based chromium hydroxide is added to the acid leaching tank, the acid inlet valve of the acid leaching tank and the sulfuric acid storage tank valve are opened, concentrated sulfuric acid is added to the acid leaching tank by flow meter, and stirring is started after standing for 1 hour. The stirring is maintained for 4 hours and then stopped. The mass of the wet-based chromium hydroxide and concentrated sulfuric acid added is respectively the same as the total mass of the wet-based chromium hydroxide and concentrated sulfuric acid added in Example 1.

[0061] Comparative Example 2

[0062] This comparative example provides a method for preparing basic chromium sulfate from sodium-treated vanadium extraction wastewater, the method comprising the following steps:

[0063] (1) adding sodium metabisulfite and sodium hydroxide solution sequentially to sodium vanadium extraction wastewater, and performing filter pressing to obtain wet-basis chromium hydroxide with a water content of 40%;

[0064] (2) Turn on the screw conveyor above the acid leaching tank, turn on the feeding electric hoist, add wet-based chromium hydroxide to the acid leaching tank, open the acid inlet valve and the sulfuric acid storage tank valve of the acid leaching tank, add concentrated sulfuric acid to the acid leaching tank by flow meter, let it stand for 1 hour and then start stirring to stir evenly, wherein the mass ratio of wet-based chromium hydroxide to concentrated sulfuric acid is 1:1 according to Cr(OH)3 and H2SO4, and the masses of wet-based chromium hydroxide and concentrated sulfuric acid added in this step are respectively the same as the total mass of wet-based chromium hydroxide and concentrated sulfuric acid added in step (2) of Example 1, and the total mass of wet-based chromium hydroxide added in this step accounts for 25% of the total mass of wet-based chromium hydroxide added in the method;

[0065] (3) Concentrated sulfuric acid and wet-based chromium hydroxide were added to the acid leaching tank in a cyclic manner, wherein the mass ratio of concentrated sulfuric acid and wet-based chromium hydroxide added in each cycle was 1:1 according to H2SO4 and Cr(OH)3. The addition was stopped when the liquid level in the acid leaching tank reached 3.5 meters and the pH value of the liquid was measured to be 2.5. Concentrated sulfuric acid and wet-based chromium hydroxide were added 4 times respectively, and the stirring and aging were continued for 4 hours before stopping.

[0066] The mass fraction and basicity of chromium trioxide (Cr2O3) in the liquid basic chromium sulfate prepared in the examples and comparative examples were determined, and the utilization rate of sulfuric acid raw material and the specific energy consumption of the preparation process were calculated. The results are listed in Table 1.

[0067] Table 1

[0068]

[0069]

[0070] “ / ” in the table means no data.

[0071] As can be seen from Table 1:

[0072] The preparation method of basic chromium sulfate provided by the present invention uses sodium vanadium extraction wastewater as a raw material to prepare liquid basic chromium sulfate, wherein the mass fraction of chromium trioxide is ≥12%, the alkalinity is ≥10%, and the utilization rate of the raw sulfuric acid is high, the unit energy consumption is extremely low, the recycling of industrial wastewater is realized, the process efficiency is significantly improved, the process cost is reduced, and the method has great industrial value.

[0073] Compared to Example 1, in Examples 4 and 5, when the pH value at the reaction endpoint was changed, the reaction limit could not be reached, resulting in reduced sulfuric acid utilization or reduced product alkalinity. In Examples 6 and 7, when the amount of raw materials added during the process was adjusted, the reaction was incomplete, resulting in reduced raw material utilization and increased energy consumption. In Comparative Examples 1 and 2, when a one-time addition method was used, the product alkalinity was extremely low and difficult to detect, failing to meet the requirements of use.

[0074] In summary, the method provided by the present invention utilizes sodium-treated vanadium extraction wastewater to convert it into wet-based chromium hydroxide with a certain water content, and prepares basic chromium sulfate through a process design of reacting the wet-based chromium hydroxide with sulfuric acid, thereby improving product purity and raw material utilization, and significantly reducing energy consumption, thereby having great industrial value.

[0075] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing basic chromium sulfate from sodium vanadium extraction wastewater, characterized in that: The method comprises the following steps: (1) mixing sodium vanadium extraction wastewater with a reducing agent and an alkali solution in sequence, and performing solid-liquid separation to obtain wet-based chromium hydroxide; (2) mixing the first wet-based chromium hydroxide and sulfuric acid, allowing to stand and then stirring, and adding the second wet-based chromium hydroxide while stirring to obtain an intermediate solution; (3) adding sulfuric acid and wet-base chromium hydroxide to the intermediate solution in a cyclic manner until the addition is completed to obtain liquid basic chromium sulfate.

2. The method according to claim 1, characterized in that The sodium-treated vanadium extraction wastewater is obtained by subjecting vanadium-containing raw materials to sodium-treatment roasting, leaching and ammonium salt precipitation of vanadium; Preferably, the hexavalent chromium content of the sodium vanadium extraction wastewater is 1.2-1.4 g / L; Preferably, the pH of the sodium vanadium extraction wastewater is 1-2; Preferably, the reducing agent comprises sodium metabisulfite.

3. The method according to claim 1 or 2, characterized in that The water content of the wet-based chromium hydroxide is 35%-40%.

4. The method according to any one of claims 1 to 3, characterized in that In step (2), the mass ratio of the first wet-based chromium hydroxide to sulfuric acid is 1:(1.5-1.7) based on Cr(OH)3 and H2SO4.

5. The method according to any one of claims 1 to 4, characterized in that In step (2), the mass ratio of the first wet-basis chromium hydroxide to the second wet-basis chromium hydroxide is 1:(0.9-1.1).

6. The method according to any one of claims 1 to 5, characterized in that The total mass of the wet-basis chromium hydroxide added in step (2) accounts for 20%-30% of the total mass of the wet-basis chromium hydroxide added in the method; Preferably, the standing time in step (2) is 1-3 hours.

7. The method according to any one of claims 1 to 6, characterized in that In step (3), the mass ratio of sulfuric acid and wet-based chromium hydroxide added in each cycle is 1:(0.9-1.1) according to which H2SO4 and Cr(OH)3 are used.

8. The method according to any one of claims 1 to 7, characterized in that In step (3), the mass of the wet-based chromium hydroxide added in each cycle accounts for 20%-50% of the total mass of the wet-based chromium hydroxide added in step (3); Preferably, the number of cyclic additions in step (3) is 3-5 times.

9. The method according to any one of claims 1 to 8, characterized in that The endpoint pH at the end of the addition of the materials in step (3) is 2.4-2.

6.

10. The method according to any one of claims 1 to 9, characterized in that After the addition of the materials in step (3) is completed, stirring and aging are performed; Preferably, the stirring and aging time is 2-4 hours.

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