High-purity quartz sand and method for producing the same

Abstract

The application relates to the field of quartz sand purification, in particular to high-purity quartz sand and a preparation method thereof, which comprises pretreatment and acid leaching. The pretreatment is as follows: quartz sand (SiO2 content >= 99%) is put into a mixed solution composed of a first additive, a second additive and water, then a third additive solution is added under ultrasonic conditions to obtain pretreated quartz sand; the first additive comprises sodium sulfate and sodium chloride; the second additive comprises sodium carbonate and sodium bicarbonate; and the third additive comprises sulfuric acid and hydrochloric acid. The first additive, the second additive and the third additive cooperate with each other to reduce the formation threshold of cavitation bubbles and form more cavitation bubbles; meanwhile, the inorganic salts in the solution increase the surface tension of the solution, so that the cavitation bubbles can reach sufficient cavitation intensity before being broken; the cooperation among the three reagents makes the cavitation bubbles in the solution more and have sufficient strength, so that the impurities in the quartz sand can be removed more deeply, and the purity of the quartz sand is improved.

Description

Technical Field

[0001] This application relates to the field of quartz sand purification, specifically to a high-purity quartz sand and its preparation method. Background Technology

[0002] High-purity silica sand refers to silica sand with a SiO2 content higher than 99.9%. It has extremely low impurity content and possesses high temperature resistance, excellent chemical properties, and superior electrical insulation and light transmittance. It is an indispensable basic material for high-tech industries and is widely used in numerous fields such as solar energy, electric light sources, semiconductors, electronics, communications, and optical instruments. For example, in the semiconductor industry, molten silica can be used to produce wafers with excellent high-temperature thermal properties, which are then used to manufacture high-performance heat pipes with good high-temperature thermal stability. In the communications industry, silica sand-based silica glass is a fundamental material for optical fibers and other optoelectronic devices. Furthermore, it can be used to produce silicone resins and solar cell substrates, and its demand is even greater in industries such as abrasives, fillers, coatings, and silicone rubber.

[0003] During quartz mineralization, many impurity minerals are associated with it, such as feldspar, mica, and clay minerals. The iron and aluminum content in these impurity minerals significantly affects the quality of quartz sand. Therefore, purification is necessary for the large-scale production of high-purity quartz sand. Since most impurity minerals adhere to the quartz sand as submicron-sized inclusions, directional fracturing (hot quenching) is often required to break them down. This purification method consumes a significant amount of energy and is increasingly difficult to adapt to the energy-saving principles of current production systems. Summary of the Invention

[0004] To address the problem that current methods for removing impurities from quartz sand primarily employ directional blasting, which consumes a significant amount of energy, this application provides a method for preparing high-purity quartz sand.

[0005] In a first aspect, this application provides a method for preparing high-purity quartz sand, employing the following technical solution:

[0006] A method for preparing high-purity quartz sand includes the following steps:

[0007] Pretreatment: Quartz sand (SiO2 content ≥99%) is placed in a mixed solution of the first additive, the second additive and water, and then the third additive solution is added under ultrasonic conditions to obtain pretreated quartz sand;

[0008] Acid leaching: The pretreated quartz sand is rinsed and dehydrated, then acid leaching is performed to remove impurities, followed by deacidification, water washing and filtration, and drying to obtain high-purity quartz sand;

[0009] The first auxiliary agent includes at least one of sodium sulfate and sodium chloride;

[0010] The second auxiliary agent includes at least one of sodium carbonate and sodium bicarbonate;

[0011] The third auxiliary agent includes at least one of sulfuric acid and hydrochloric acid.

[0012] By employing the above technical solution, the first auxiliary agent (at least one of sodium sulfate and sodium chloride), as an inorganic salt, increases the surface tension of the mixed solution. The second auxiliary agent (at least one of sodium carbonate and sodium bicarbonate) reacts with the third auxiliary agent (at least one of sulfuric acid and hydrochloric acid) under ultrasonic conditions to generate CO2 bubbles and inorganic salts. The tiny bubble nuclei (cavitation nuclei) present in the liquid are the starting point of ultrasonic cavitation; increasing the number of cavitation nuclei in the liquid can promote the generation of cavitation effect. CO2 bubbles, as cavitation nuclei, significantly enhance cavitation, which helps to remove tiny impurities and adsorbates from the surface of quartz sand.

[0013] Due to the presence of the first additive, and the fact that the products of the reaction of the second and third additives are still inorganic salts, the solution maintains a high surface tension. This high surface tension allows cavitation bubbles to break only when they are large, further increasing the cavitation intensity and making the cavitation process more intense and effective, thus removing impurities from the quartz sand more thoroughly.

[0014] This application utilizes the synergistic effect of the first, second, and third additives to lower the cavitation bubble formation threshold and generate more cavitation bubbles. Simultaneously, the inorganic salts in the solution increase the surface tension of the solution, ensuring that the cavitation bubbles reach sufficient cavitation intensity before breaking. The synergistic effect of the three reagents results in a large number of cavitation bubbles with sufficient intensity in the solution, thereby enabling more thorough removal of impurities from the quartz sand and improving the purity of the quartz sand.

[0015] Preferably, the first auxiliary agent is sodium sulfate.

[0016] By adopting the above technical solution, the cations generated after the ionization of inorganic salts (such as Na) can be reduced. + K + ) and anions (such as Cl) - SO4 2- High charge density ions (such as SO42-) form a strong electrostatic interaction with water molecules, "dragging" surface water molecules into the solution interior, resulting in a more compact molecular arrangement on the surface layer and thus increasing surface tension. 2- It can enhance the hydrogen bond network between water molecules, further increasing the tension.

[0017] Therefore, sodium sulfate can increase the surface tension of the solution more effectively than sodium chloride, resulting in higher cavitation intensity of cavitation bubbles and better removal of impurities from quartz sand.

[0018] Preferably, the sodium sulfate accounts for 13%-18% of the mass of the mixed solution.

[0019] By adopting the above technical solution, when the mass ratio of sodium sulfate is too low, the surface tension of the solution is only increased to a limited extent, the cavitation intensity of cavitation bubbles is not increased sufficiently, and the removal effect does not reach the ideal state; when the mass ratio of sodium sulfate is too high, the surface tension of the solution is difficult to continue to increase, and the removal effect is difficult to further improve. Therefore, after a lot of research and experimental verification, the applicant finally determined that the mass ratio of sodium sulfate in the mixed solution of this application is preferably as described above.

[0020] Preferably, the second additive is sodium carbonate.

[0021] By employing the above technical solution, when the third auxiliary agent (sulfuric acid, hydrochloric acid) is added to the mixed solution and reacts with the second auxiliary agent, it reacts with sodium carbonate to first form sodium bicarbonate, and then generates CO2 bubbles. However, its reaction with sodium bicarbonate directly generates CO2 bubbles. If a large number of CO2 bubbles are generated in a short period, the excessive CO2 bubbles will increase the scattering of ultrasound in the liquid, causing the acoustic energy to be dispersed by the bubble cluster rather than concentrated on the target cavitation bubble, thus reducing the effective cavitation intensity. Therefore, compared to sodium bicarbonate, sodium carbonate can continuously generate CO2 bubbles for a period of time, better ensuring the impurity removal effect on quartz sand.

[0022] Preferably, the sodium carbonate accounts for 3%-6% of the mass of the mixed solution.

[0023] By adopting the above technical solution, when the mass ratio of sodium carbonate is too low, there are fewer cavitation nuclei in the solution, making it difficult to achieve a better cavitation effect; when the mass ratio of sodium carbonate is too high, a large number of CO2 bubbles will be generated in the solution. Excessive CO2 bubbles will increase the scattering of ultrasound in the liquid, causing the acoustic energy to be dispersed by the bubble group rather than concentrated on the target cavitation bubble, thereby reducing the effective cavitation intensity. Therefore, after extensive research and experimental verification, the applicant finally determined that the mass ratio of sodium carbonate in the mixed solution of this application is preferably as described above.

[0024] Preferably, the third auxiliary agent is sulfuric acid.

[0025] By adopting the above technical solution, the third auxiliary agent reacts with the second auxiliary agent. When the third auxiliary agent is sulfuric acid, the reaction product is sodium sulfate; when the third auxiliary agent is hydrochloric acid, the reaction product is sodium chloride. Sodium sulfate, compared to sodium chloride, can better increase the surface tension of the solution, resulting in higher cavitation intensity of the cavitation bubbles and a better removal effect on impurities in quartz sand. Therefore, sulfuric acid, compared to hydrochloric acid, has a higher cavitation intensity and a better removal effect on impurities in quartz sand.

[0026] Preferably, the mass ratio of sodium carbonate to sulfuric acid is 1:0.8-1.2.

[0027] By adopting the above technical solution, when the proportion of sulfuric acid is too low, fewer CO2 bubbles are generated by the reaction of sulfuric acid and sodium carbonate, resulting in insufficient cavitation and poor removal of impurities from quartz sand; when the proportion of sulfuric acid is too high, too many CO2 bubbles are generated in a short time, affecting the cavitation effect and reducing the removal effect on impurities from quartz sand. Therefore, after extensive research and experimental verification, the applicant finally determined that the above-mentioned mass ratio of sodium carbonate to sulfuric acid is appropriate.

[0028] Preferably, when the third auxiliary agent solution is added in at least two portions at intervals, the second auxiliary agent is sodium bicarbonate.

[0029] By adopting the above technical solution, when the second auxiliary agent, whether sodium carbonate or sodium bicarbonate, is added all at once, CO2 bubbles will only be generated in the early stage of ultrasound, while in the later stage, cavitation bubbles will be generated more directly by ultrasound waves. However, when the third auxiliary agent solution is added in multiple portions at intervals, CO2 bubbles can be generated in the early, middle, and late stages of ultrasound. In this case, because the third auxiliary agent (sulfuric acid, hydrochloric acid) reacts with sodium carbonate, sodium bicarbonate will be generated first, followed by CO2 bubbles. Especially when the concentration of the third auxiliary agent is low, the amount of CO2 bubbles generated is small or even non-existent. The third auxiliary agent (sulfuric acid, hydrochloric acid) reacts with sodium bicarbonate to directly generate CO2 bubbles. In this case, because the third auxiliary agent is added in multiple portions, a certain amount of CO2 bubbles can be generated each time it is added. Therefore, when the third auxiliary agent solution is added in multiple portions, sodium bicarbonate is more effective as the second auxiliary agent.

[0030] Secondly, this application provides a high-purity quartz sand, employing the following technical solution:

[0031] A high-purity quartz sand is prepared by the above-mentioned method for preparing high-purity quartz sand.

[0032] In summary, this application has the following beneficial effects:

[0033] This application utilizes the synergistic effect of the first, second, and third additives to lower the cavitation bubble formation threshold and generate more cavitation bubbles. Simultaneously, the inorganic salts in the solution increase the surface tension of the solution, ensuring that the cavitation bubbles reach sufficient cavitation intensity before breaking. The synergistic effect of the three reagents results in a large number of cavitation bubbles with sufficient intensity in the solution, thereby enabling more thorough removal of impurities from the quartz sand and improving the purity of the quartz sand. Detailed Implementation

[0034] The present application will be further described in detail below with reference to embodiments and comparative examples.

[0035] Example 1

[0036] A method for preparing high-purity quartz sand includes the following steps:

[0037] Pretreatment: 200g of quartz sand (SiO2 content determined to be 99.0274%) was placed in a 1000g mixed solution consisting of the first auxiliary agent (sodium sulfate), the second auxiliary agent (sodium carbonate), and water. The 1000g mixed solution contained 180g of sodium sulfate and 50g of sodium carbonate. Then, under ultrasonic conditions, the third auxiliary agent solution (sulfuric acid solution) was added to obtain pretreated quartz sand.

[0038] The ultrasonic frequency is 30 kHz, the ultrasonic time is 30 min, the sulfuric acid content in the sulfuric acid solution is 50 g, and the concentration of the sulfuric acid solution can be within the range of 10 wt%-20 wt%. Within the above concentration, the sulfuric acid solution can react fully and is unlikely to cause acid etching to the quartz sand; the concentration of the sulfuric acid solution in this application is 15 wt%.

[0039] Acid leaching: The pretreated quartz sand is rinsed, dehydrated, and then transferred to an acid leaching reactor. A mixed acid (20wt% HCl, 20wt% H2SO4, and 10wt% HF) is added, and the mixture is leached at 85℃ for 5 hours to remove impurities. After acid removal and washing, the sand is filtered and dried to obtain high-purity quartz sand.

[0040] Example 2

[0041] Example 2 is based on the preparation method of Example 1, except that the first auxiliary agent 180g sodium sulfate is replaced with 180g sodium chloride. The specific adjustments are shown in Table 1.

[0042] Comparative Examples 1-4

[0043] Comparative Example 1 was prepared using the same method as in Example 1, but without the addition of the first auxiliary agent, sodium sulfate.

[0044] Comparative Example 2 was prepared using the same method as in Example 1, but without the addition of the second auxiliary agent, sodium carbonate.

[0045] Comparative Example 3 was prepared by replacing the sulfuric acid solution of the third auxiliary agent solution with water, based on the preparation method of Example 1.

[0046] Comparative Example 4 is based on the preparation method of Comparative Example 3, except that the mixed solution of the first auxiliary agent (sodium sulfate) and the second auxiliary agent (sodium carbonate) is replaced with water.

[0047] The high-purity quartz sands of Examples 1-2 and Comparative Examples 1-4 were subjected to the following performance tests.

[0048] Performance testing

[0049] The SiO2 content of high-purity quartz sand was measured using inductively coupled plasma atomic emission mass spectrometry (ICP-MS).

[0050] Table 1. Content and performance test results of the first, second, and third adjuvants in Examples 1-2 and Comparative Examples 1-4.

[0051]

[0052] Referring to Table 1, comparing Examples 1-2 and Comparative Examples 1-4, it is evident that the synergistic effect between the first additive (sodium sulfate), the second additive (sodium carbonate), and the third additive (sulfuric acid) helps to form more cavitation bubbles with sufficient cavitation intensity under ultrasonic conditions, thereby achieving a better removal effect on impurities in quartz sand and further improving the purity of the quartz sand. This is because the reaction of sodium carbonate and sulfuric acid generates sodium sulfate and CO2 bubbles, with the CO2 bubbles acting as cavitation nuclei, significantly enhancing the cavitation effect. The first additive (sodium sulfate), as an inorganic salt, increases the surface tension of the mixed solution. Since the reaction product of sodium carbonate and sulfuric acid is also sodium sulfate, the solution maintains a high surface tension. This high surface tension allows the cavitation bubbles to break only when they are relatively large, further increasing the cavitation intensity and making the cavitation effect more intense and effective, thus removing impurities from the quartz sand more thoroughly.

[0053] For Comparative Example 1, since no first additive (sodium sulfate) was added, the surface tension of the solution was limited. Although CO2 bubbles were also generated, these CO2 bubbles did not grow to a sufficient cavitation intensity before breaking down, and the removal effect on quartz sand impurities was not as good as in Example 1.

[0054] In Comparative Example 2, CO2 bubbles could not be generated in the solution because the second additive (sodium carbonate) was not added. In addition, Comparative Example 2 also added the first additive (sodium sulfate), which increased the cavitation intensity of cavitation bubbles, but also increased the threshold for cavitation bubble formation. Therefore, the number of cavitation bubbles generated was smaller, which reduced the cavitation effect.

[0055] For Comparative Example 3, the reason is similar to that of Comparative Example 2. Furthermore, since sulfuric acid is a non-surface-active substance, it cannot oriented itself on the solution surface. Instead, it inhibits surface relaxation and increases surface tension by disrupting the hydrogen bond network of water molecules and forming stronger ion-water interactions. Sodium carbonate produces OH- ions upon hydrolysis. - The ions create a weakly alkaline environment, similar to a weak surfactant, which actually reduces the surface tension of the solution. Therefore, the surface tension of the solution in Comparative Example 2 is greater than that in Comparative Example 3, resulting in a better cavitation effect in Comparative Example 3 than in Comparative Example 2.

[0056] Compared to Example 2, Example 1 showed better results. This is because the first additive in Example 1 was sodium sulfate, which has a high charge density of ions (such as SO42-). 2-This can enhance the hydrogen bond network between water molecules, further increasing surface tension. Compared to Example 2, the solution in Example 1 has a higher surface tension, higher cavitation intensity, and better cavitation effect. Therefore, Example 1 is preferred.

[0057] Examples 3-5

[0058] Examples 3-5 are based on the preparation method of Example 1, but the amount of sodium sulfate added in 1000g of mixed solution is adjusted. The specific adjustments are shown in Table 2.

[0059] The high-purity quartz sands from Examples 3-5 were subjected to the performance tests described above, and the test results are shown in Table 2.

[0060] Table 2. Data on sodium sulfate addition and performance testing for Examples 1 and 3-5.

[0061]

[0062] Referring to Table 2, comparing Examples 1 and 3-5, it can be seen that as the amount of sodium sulfate added to the mixed solution increases, the purity of the quartz sand gradually increases and then tends to stabilize. This is because as the amount of sodium sulfate added to the mixed solution increases, the surface tension of the solution gradually increases, the cavitation intensity of the cavitation bubbles gradually increases, and the removal effect of impurities in the quartz sand becomes better and better.

[0063] Examples 6-11

[0064] Example 6 is based on the preparation method of Example 1, except that the second auxiliary agent 50g sodium carbonate is replaced with 50g sodium bicarbonate.

[0065] Example 7 is based on the preparation method of Example 6, except that the third auxiliary agent 50g sulfuric acid is replaced with 30g sulfuric acid.

[0066] Examples 8-11 are based on the preparation method of Example 1, but the amount of sodium carbonate added in 1000g of mixed solution is adjusted. The specific adjustments are shown in Table 3.

[0067] The high-purity quartz sands of Examples 6-11 were subjected to the above performance tests, and the test results are shown in Table 3.

[0068] Table 3. Data on the addition amounts of sodium carbonate, sodium bicarbonate, and sulfuric acid in Examples 1, 6-11, and performance testing.

[0069]

[0070] Referring to Table 3, a comparison of Examples 1 and 6-11 shows that when the third auxiliary agent solution (sulfuric acid solution) is directly added to the mixed solution, sodium carbonate is superior to sodium bicarbonate as the second auxiliary agent. This is because the reaction of sulfuric acid with sodium carbonate first produces sodium bicarbonate, which then generates CO2 bubbles. However, the reaction with sodium bicarbonate directly generates CO2 bubbles, especially with excess sulfuric acid reacting with sodium bicarbonate, which generates CO2 bubbles at a faster rate. If a large number of CO2 bubbles are generated in a short time, the excess CO2 bubbles will increase the scattering of ultrasound in the liquid, causing the acoustic energy to be dispersed by the bubble group rather than concentrated on the target cavitation bubble, thus reducing the effective cavitation intensity. Therefore, compared to sodium bicarbonate, sodium carbonate can continuously generate CO2 bubbles for a period of time, better ensuring the impurity removal effect on quartz sand.

[0071] As the proportion of sodium carbonate in the mixed solution increases, the purity of the quartz sand initially rises and then falls. This is because a higher proportion of sodium carbonate in the mixed solution generates more CO2 bubbles, which enhances cavitation and results in better impurity removal. However, beyond a certain range, excessive CO2 bubbles are generated in the solution, reducing the effective cavitation intensity and thus decreasing the impurity removal effect on the quartz sand.

[0072] Examples 12-17

[0073] Example 12 is based on the preparation method of Example 1, except that the third auxiliary agent 50g sulfuric acid is replaced with 50g hydrochloric acid (hydrochloric acid solution concentration 15wt%).

[0074] Example 13 is based on the preparation method of Example 12, except that the third auxiliary agent 50g hydrochloric acid is replaced with 30g hydrochloric acid.

[0075] Examples 14-17 are based on the preparation method of Example 1, but the amount of sulfuric acid added is adjusted, as shown in Table 4.

[0076] The high-purity quartz sands of Examples 12-17 were subjected to the above performance tests, and the test results are shown in Table 4.

[0077] Table 4. Data on the addition of sulfuric acid and hydrochloric acid in Examples 1 and 12-17, and related performance test results.

[0078]

[0079] Referring to Table 4, a comparison of Examples 1 and 12-17 shows that when the sodium carbonate and hydrochloric acid content is 50g, the excessive hydrochloric acid content leads to over-reaction, resulting in too many CO2 bubbles generated in a short time. This affects the cavitation effect and reduces the removal efficiency of impurities from the quartz sand. When the sodium carbonate content is 50g and the hydrochloric acid content is 30-50g, the reaction is complete, but the product is sodium chloride. Its effect on increasing solution tension is not as good as that of sodium sulfate produced by sulfuric acid. Therefore, sulfuric acid produces higher cavitation intensity bubbles than hydrochloric acid, resulting in better impurity removal from the quartz sand.

[0080] As the amount of sulfuric acid added increases, the purity of the quartz sand initially rises and then falls. This is because as the amount of sulfuric acid increases, more CO2 bubbles are generated, which enhances cavitation and results in better impurity removal. However, beyond a certain range, excessive CO2 bubbles are produced in the solution, reducing the effective cavitation intensity and thus decreasing the impurity removal effect on the quartz sand.

[0081] Examples 18-21

[0082] In Example 18, based on the preparation method of Example 1, the sulfuric acid solution (containing 50g of sulfuric acid) was divided into two equal parts. One part was added at the beginning of sonication, and the other part was added after 5 minutes of sonication.

[0083] In Example 19, based on the preparation method of Example 1, the sulfuric acid solution (containing 50g of sulfuric acid) was divided into three equal parts: one part was added at the beginning of sonication, one part was added after 3 minutes of sonication, and one part was added after 6 minutes of sonication.

[0084] In Example 20, based on the preparation method of Example 7, the sulfuric acid solution (containing 30g of sulfuric acid) was divided into two equal parts. One part was added at the beginning of sonication, and the other part was added after 5 minutes of sonication.

[0085] In Example 21, based on the preparation method of Example 7, the sulfuric acid solution (containing 30g of sulfuric acid) was divided into three equal parts: one part was added at the beginning of sonication, one part was added after 3 minutes of sonication, and one part was added after 6 minutes of sonication.

[0086] The high-purity quartz sands of Examples 18-21 were subjected to the above performance tests, and the test results are shown in Table 5.

[0087] Table 5. Data on the addition amounts of sodium carbonate, sodium bicarbonate, and sulfuric acid, the amount of sulfuric acid, and performance testing data for Examples 1, 7, and 18-21.

[0088]

[0089] Referring to Table 5, a comparison of Examples 1, 7, and 18-21 shows that dividing the same amount of sulfuric acid solution into multiple portions and adding them at intervals is more beneficial for improving the purity of high-purity quartz sand. This is because when the sulfuric acid solution is added in multiple portions at intervals, CO2 bubbles are generated throughout the early, middle, and late stages of ultrasound treatment. Since the reaction rate between sulfuric acid solution and sodium bicarbonate is faster, adding it in multiple portions at intervals better avoids generating excessive CO2 bubbles in a short period. Compared to sodium carbonate, adding sulfuric acid solution in multiple portions during the reaction with sodium bicarbonate results in a better improvement in the purity of high-purity quartz sand.

[0090] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for producing high purity quartz sand, characterized by, The method comprises the following steps: Preprocessing: quartz sand with SiO2 content ≥ 99% is put into a mixed solution composed of a first auxiliary agent, a second auxiliary agent and water, then a third auxiliary agent solution is added under ultrasonic condition to obtain pretreated quartz sand; Acid leaching: the pretreated quartz sand is rinsed and dehydrated, then impurities are removed by acid leaching, and then the quartz sand is deacidified, washed and filtered, and dried to obtain high-purity quartz sand; The first auxiliary agent comprises at least one of sodium sulfate and sodium chloride; The second auxiliary agent comprises at least one of sodium carbonate and sodium bicarbonate; The third auxiliary agent comprises at least one of sulfuric acid and hydrochloric acid.

2. The method of producing high purity quartz sand according to claim 1, characterized in that: The first auxiliary agent is sodium sulfate.

3. The method of producing high purity quartz sand according to claim 2, characterized in that: The mass proportion of the sodium sulfate in the mixed solution is 13%-18%.

4. The method of claim 1, wherein: The second auxiliary agent is sodium carbonate.

5. The method of producing high purity quartz sand according to claim 4, characterized in that: The mass proportion of the sodium carbonate in the mixed solution is 3%-6%.

6. The method of claim 1, wherein: The third auxiliary agent is sulfuric acid.

7. The method of producing high purity quartz sand according to claim 6, characterized in that: The mass ratio of the sodium carbonate to the sulfuric acid is 1:0.8-1.

2.

8. The method of claim 1, wherein: When the third auxiliary agent solution is added in at least two portions at intervals, the second auxiliary agent is sodium bicarbonate.

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