A preparation method of tremella-like precipitated calcium carbonate

Abstract

The present invention discloses a preparation method of tremella-like precipitated calcium carbonate, comprising the following steps: S1: Take refined lime milk and prepare it with a solid content for standby; S2: Inject water into the carbonization tower and introduce kiln gas, controlling the temperature and stirring frequency; S3: Pump the lime milk and dilute EDTA-2Na solution prepared in step S1 into the carbonization tower simultaneously; S4: Immediately pour the phosphate solution when the lime milk pumping is completed, and stir well to obtain a suspension; S5: Filter press, dry, crush, and screen the suspension obtained in step S4 to obtain tremella-like precipitated calcium carbonate. The present invention can prepare tremella-like precipitated calcium carbonate. This product has strong surface adsorption force and good morphological dispersion, and has potential application value in sewage adsorption and drug loading and slow release. The product has high whiteness and large tapped density, and has good reinforcing effect and whitening effect in PVC plastics.

Description

Technical Field

[0001] The present invention relates to the technical field of calcium carbonate preparation, and particularly relates to a method for preparing tremella-like precipitated calcium carbonate. Background Art

[0002] Calcium carbonate is an inorganic mineral commonly found in nature. To expand its application fields and values, calcium carbonate has been prepared by physical and chemical methods in industries, biology, medicine, etc. Among them, calcium carbonate prepared by chemical synthesis methods has three relatively stable crystal forms, namely calcite type, aragonite type, and vaterite type. Of course, the precursor forms of these three crystal forms are considered to be amorphous calcium carbonate.

[0003] The morphology control of calcium carbonate has been the focus of research in recent years. Calcium carbonates with different micro morphologies also have different application fields. Spherical calcium carbonate can be used in mechanical lubrication, drug loading, etc., and cubic calcium carbonate can be applied to coatings and sealants with good effects. Micron-sized cubic precipitated calcium carbonate can be prepared by the bubbling carbonation method. In its preparation process, carbon dioxide is continuously introduced into water to form carbonated water first, and then calcium hydroxide lime slurry is pumped into the water. By controlling the slurry feeding rate and carbonation parameters, micron-sized cubic precipitated calcium carbonate can be formed, and the particle size can reach 2 - 15 μm. Some researchers have studied its mechanism. Among them The research results of Y.M.H. Seepma et al. (Seepma Sergěj YM H, et al. "Controlling CaCO3 ParticleSize with{Ca 2+}:{CO3 2-}Ratios in Aqueous Environments.."Crystal growth&design21.3(2021).doi:10.1021 / ACS.CGD.0C01403.) show that when{Ca 2+}={CO3 2-}, the reaction rate is the fastest. In an excessive Ca 2+ solution, the reaction rate is slower, especially at a lower{Ca 2+}·{CO3 2-} / Ksp value; at a high{Ca 2+}·{CO3 2-} / K sp value, it occurs through the aggregation of prenucleation clusters. At a high{Ca 2+}·{CO3 2-} / K sp value, the reaction time is less affected by the ion activation degree. Therefore, when the initial carbonate ions in the solution are relatively stable, the{Ca 2+} and{CO32- The ratio of {} is used to control the reaction rate.

[0004] For micron-sized precipitated calcium carbonate, it is mainly prepared by process control and the addition of crystal form control agents. The purpose is also to well control the ratio of carbonate ions and calcium ions in the solution, achieve the purpose of controlling the reaction rate, and prepare calcium carbonate with the target morphology.

[0005] Chinese Patent CN108821322A discloses a method for preparing layered structure microsphere precipitated calcium carbonate. Limestone is calcined to obtain quicklime, and the obtained quicklime reacts with water for digestion to obtain lime milk. After sieving through a 100-mesh sieve to remove impurities and refining, the concentration of the lime milk is adjusted to 10 - 100 g / L to obtain a lime milk slurry. After heating the lime milk slurry to 40 - 90 °C, it is pumped into a carbonization tower. A soluble ionic compound additive is added to the lime milk, and after stirring evenly, a mixed gas containing carbon dioxide is introduced. During the reaction process, no stirring or low-speed stirring is carried out. When the pH value of the slurry drops to 7.5, the reaction is stopped to obtain a calcium carbonate slurry. Then, it is washed with absolute ethanol, filtered by suction, dried at 60 - 70 °C, and pulverized to obtain a layered structure microsphere precipitated calcium carbonate product.

[0006] Chinese Patent CN108793217A discloses a method for preparing spherical cluster-shaped light calcium carbonate. Quicklime is calcined to become quicklime, and quicklime and water undergo a digestion reaction to obtain lime slurry. After aging, sieving, impurity removal, and dilution to a solid content of 10 - 13%, a Ca(OH)₂ suspension is obtained. Then, a first crystal form regulator and a second crystal form regulator are added and stirred and mixed evenly to obtain a mixed suspension. The first crystal form regulator is one or more of triethanolamine dodecyl phosphate, dodecyl phosphate salt, and hexadecyl glucoside; the second crystal form regulator is one or more of sodium N-lauroyl amide, sodium N-lauroyl glutamate, and polysorbate. Carbon dioxide gas is introduced into the mixed suspension for carbonation reaction. When the pH value of the slurry drops to 7.0 - 7.5, the reaction is stopped. Then, through dehydration, drying, pulverization, and screening, a calcium carbonate product is obtained. This method uses crystal form control agents to control the morphology of the reaction, with high cost and a relatively cumbersome process.

[0007] Chinese Patent CN111592025A discloses a preparation method of chrysanthemum-shaped micron calcium carbonate, which includes the following steps: 1) Preparation of reverse microemulsion: Water, oil, co-surfactant and surfactant are fully mixed according to the volume ratio to prepare a clear reverse microemulsion; 2) Preparation of chrysanthemum-shaped micron calcium carbonate: Under stirring, carbon dioxide is introduced into the reverse microemulsion in step 1) for a certain reaction time until no more precipitation occurs, then the introduction of carbon dioxide gas is stopped, the stirring is ended, and after standing and aging at room temperature, centrifugal separation is carried out, and the obtained precipitate is dried to obtain the chrysanthemum-shaped micron calcium carbonate. This method uses the two-phase difference to control the distribution of carbonate and calcium ions, and with the assistance of surfactants, calcium carbonate with a special morphology is prepared.

[0008] Chinese Patent CN104250020A discloses a vaterite-type calcium carbonate micro-nano hierarchical structure and its additive-free preparation method. 1) Preparation of precursor solution: Calcium acetate is dissolved in a mixed solvent of N,N-dimethylformamide and ultrapure water, where the volume ratio of N,N-dimethylformamide to ultrapure water is 7:1 to 12:1, to obtain a precursor solution; 2) Preparation of vaterite-type calcium carbonate micro-nano hierarchical structure: The precursor obtained in step 1) is transferred to a high-pressure reaction kettle and placed in a muffle furnace for solvothermal reaction at 100-160°C for 2-12 h. After the reaction is completed, solid-liquid separation of the reaction solution is carried out by centrifugation, and the obtained solid product is post-treated to obtain the vaterite-type calcium carbonate micro-nano hierarchical structure. This method first prepares a precursor solution containing additives, and after high-temperature reaction in a high-pressure reaction kettle, centrifugal separation is used to prepare the vaterite-type calcium carbonate micro-nano hierarchical structure.

[0009] Chinese Patent CN103466580A discloses a preparation method of hydroxyapatite microspheres, which includes the following steps: Step (1): Adjust the phosphate aqueous solution with a concentration of 2.17-27.6 g / L with an alkali solution until the pH value is 10; Step (2): Add calcium carbonate powder to the phosphate aqueous solution after adjusting the pH value and stir at room temperature to form a suspension; the molar ratio of the added calcium carbonate to phosphate is 1:0.6-7.5; Step (3): Place the obtained suspension in a microwave oven for heating reaction, the microwave power is 60-100 W, and the reaction time is 5-15 min; Step (4): Filter the solution heated in step (3), and alternately rinse the filter cake with deionized water and absolute ethanol for more than 3 times each; Step (5): Put the filter cake rinsed clean in step (4) into a constant temperature drying oven and dry at 40-60°C for 24-48 h, and then grind the solid substance into dry powder to obtain hydroxyapatite microspheres. The hydrothermal method used in this method is difficult to scale up production, and microwave radiation is also a device that increases costs.

[0010] Chinese Patent CN101920947A provides a method for preparing sheet-like B-type carbonate apatite, which comprises the following steps: using calcium carbonate or biological minerals as raw materials, placing them in an excessive amount of phosphate buffer solution, with the molar ratio of Ca to P in the reactants not exceeding 1.67, heating by microwave, reacting at 50-100 °C for 1-30 min, and taking the precipitate; the pH of the phosphate buffer solution is 4.0-12, containing phosphate ions, dihydrogen phosphate ions and hydrogen phosphate ions, and the total concentration is 0.05-0.50 mol / L. This method uses calcium carbonate or calcium carbonate-containing shellfish bones, places them in a phosphate buffer solution, and obtains an apatite material by microwave heating. The material has good bioactivity and biocompatibility.

[0011] The above patent prepared calcium carbonate with different morphologies through process regulation and crystal form control agents, etc., providing a basis for the production technology of calcium carbonate and a theory for the crystallization control of inorganic materials. However, there is no relevant report on the preparation of tremella-like precipitated calcium carbonate. Summary of the Invention

[0012] In view of the above problems, the present invention provides a method for preparing tremella-like precipitated calcium carbonate. Through slurry feeding and carbonization, while feeding the slurry, EDTA-2Na is added dropwise to promote the dissolution of Ca in the lime milk, accelerate the reaction, form micron-sized precipitated calcium carbonate, and ensure the dispersibility; at the end of the reaction, a phosphate solution is added dropwise to prepare tremella-like precipitated calcium carbonate. This product has strong surface adsorption force and good morphological dispersion, and has potential application value in sewage adsorption and drug loading and slow release. The product has high whiteness and large tapped density, and has good reinforcing effect and whitening effect in PVC plastics. 2+ To achieve the above object, the present invention adopts the following technical solutions:

[0013] A method for preparing tremella-like precipitated calcium carbonate, comprising the following steps:

[0014] S1: Take refined lime milk, prepare the solid content for use;

[0015] S2: Inject water into the carbonization tower, and introduce kiln gas, controlling the temperature and stirring frequency;

[0016] S3: Pump the lime milk prepared in S1 and the dilute EDTA-2Na solution into the carbonization tower at the same time;

[0017] S4: Immediately pour the phosphate solution when the lime milk pumping is completed, stir well to obtain a suspension;

[0018] S5: Filter the suspension obtained in S4, dry, crush, and sieve to obtain tremella-like precipitated calcium carbonate.

[0019]

[0020] ​Preferably, in step S1, the solid content of the lime milk is 1.020 - 1.050.

[0021] Preferably, in step S2, the water injection volume is 10 - 15 kg.

[0022] Preferably, in step S2, the carbon dioxide concentration in the kiln gas is 40 - 50%.

[0023] Preferably, in step S2, the temperature is controlled at 50 - 70 °C.

[0024] Preferably, in step S2, the stirring frequency is 15 - 25 HZ.

[0025] Preferably, in step S3, the dosage of EDTA-2Na is 0.15 - 0.30% of the dry basis calcium carbonate content, and the concentration is 1 - 3%.

[0026] Preferably, in step S4, the phosphate is a compound of one or more of orthophosphate, hydrogen phosphate, dihydrogen phosphate, and phosphoric acid.

[0027] Preferably, in step S4, the dosage of the phosphate is 0.5% - 2.0% of the dry basis calcium carbonate content, and the concentration is 1 - 3%.

[0028] Preferably, in step S4, the stirring frequency is 25 - 30 HZ, and the stirring time is 10 - 30 min.

[0029] The principle of the preparation method of the present invention is as follows:

[0030] Carbon dioxide kiln gas is introduced into water. Carbon dioxide reacts with water to form bicarbonate and carbonate ions. The water is weakly acidic, and the carbonate ions in the water are in excess relative to calcium ions. When calcium hydroxide is added to the water, calcium ions react fully with carbonate ions to form cubic precipitated calcium carbonate. EDTA-2Na is added dropwise simultaneously with the lime milk instead of being mixed first and then added, in order to make calcium ions dissolve more fully in water under the synergistic action of dilution and complexation, promoting the reaction to accelerate. This makes the cubic precipitated calcium carbonate formed in the reaction have more calcium hydroxide on its surface, and these sites are more likely to serve as nucleation sites. After adding phosphate at the end of the reaction, a tremella-like coating is formed on the cubic precipitated calcium carbonate.

[0031] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0032] 1. The raw materials of the present invention are easily available, with low cost, economic and environmentally friendly, few by-products, few parameters to be controlled in the key carbonation step, easy to control, simplified process steps, short carbonation time, and high efficiency.

[0033] 2. The tremella-like precipitated calcium carbonate prepared by the present invention has strong surface adsorption force and good morphological dispersion, and has potential application value in sewage adsorption and drug loading and slow release. The product has high whiteness and large tapped density, and has good reinforcing effect and whitening effect in PVC plastics. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is the SEM image of the tremella-like precipitated calcium carbonate prepared in Example 1;

[0035] Figure 2 It is the SEM image of the tremella-like precipitated calcium carbonate prepared in Example 2;

[0036] Figure 3 It is the SEM image of the tremella-like precipitated calcium carbonate prepared in Example 3;

[0037] Figure 4 It is the SEM image of the cubic precipitated calcium carbonate prepared in Comparative Example 1;

[0038] Figure 5 It is the SEM image of the spindle-shaped precipitated calcium carbonate prepared in Comparative Example 2;

[0039] Figure 6 It is the SEM image of the tremella-like precipitated calcium carbonate prepared in Comparative Example 3;

[0040] Figure 7 It is the SEM image of the tremella-like precipitated calcium carbonate prepared in Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0041] In order to better present the present invention, it is illustrated by specific implementation cases. These implementation cases belong to the protection scope of the present invention but do not limit the protection scope of the present invention.

[0042] Example 1

[0043] S1: Take the refined lime milk, adjust the solid content to 1.020, making it lower than the solid content of the lime milk used for ordinary nano calcium carbonate;

[0044] S2: Introduce 10 kg of water into the carbonization tower, introduce the kiln gas with a carbon dioxide concentration of 45%, control the temperature at 60 °C, and the stirring frequency at 20 HZ;

[0045] S3: Pump the lime milk prepared in step S1 and the EDTA-2Na dilute solution with a dry basis calcium carbonate content of 0.15% and a concentration of 1.0% into the carbonization tower at the same time;

[0046] S4: Immediately pour the sodium orthophosphate solution with a dry basis calcium carbonate content of 2.0% and a concentration of 1.0% when the lime milk pumping is completed, and start stirring at a speed of 30 HZ for 10 min to obtain a suspension;

[0047] S5: Dry, crush, and sieve the suspended hydraulic filter obtained in step S4 to obtain tremella-like precipitated calcium carbonate.

[0048] Example 2

[0049] S1: Take refined lime milk, adjust the solid content to 1.050, making it lower than the solid content of the lime milk used for ordinary nano calcium carbonate.

[0050] S2: Introduce 10 kg of water into the carbonization tower, introduce kiln gas with a carbon dioxide concentration of 50%, control the temperature at 70°C, and the stirring frequency at 25 HZ.

[0051] S3: Pump the lime milk prepared in step S1 and a dilute EDTA-2Na solution with a dry basis calcium carbonate content of 0.30% and a concentration of 3.0% into the carbonization tower simultaneously.

[0052] S4: Immediately pour a mixed solution of sodium hydrogen phosphate and phosphoric acid with a dry basis calcium carbonate content of 2.0% and a concentration of 2% when the lime milk pumping is completed, and start stirring at a speed of 25 HZ for 30 min to obtain a suspension.

[0053] S5: Filter press, dry, crush, and sieve the suspension obtained in step S4 to obtain tremella-like precipitated calcium carbonate.

[0054] Example 3

[0055] S1: Take refined lime milk, adjust the solid content to 1.045, making it lower than the solid content of the lime milk used for ordinary nano calcium carbonate.

[0056] S2: Introduce 15 kg of water into the carbonization tower, introduce kiln gas with a carbon dioxide concentration of 40%, control the temperature at 50°C, and the stirring frequency at 15 HZ.

[0057] S3: Pump the lime milk prepared in step S1 and a dilute EDTA-2Na solution with a dry basis calcium carbonate content of 0.15% and a concentration of 1.0% into the carbonization tower simultaneously.

[0058] S4: Immediately pour a mixed solution of sodium hydrogen phosphate and sodium dihydrogen phosphate with a dry basis calcium carbonate content of 0.5% and a concentration of 1.0% when the lime milk pumping is completed, and start stirring at a speed of 25 HZ for 20 min to obtain a suspension.

[0059] S5: Filter press, dry, crush, and sieve the suspension obtained in step S4 to obtain tremella-like precipitated calcium carbonate.

[0060] Comparative Example 1

[0061] S1: Take refined lime milk, adjust the solid content to 1.020, making it lower than the solid content of the lime milk used for ordinary nano calcium carbonate.

[0062] S2: Inject 10 kg of water into the carbonization tower, introduce kiln gas with a carbon dioxide concentration of 45%, control the temperature at 60 °C, and set the stirring frequency at 20 HZ;

[0063] S3: Pump the lime milk prepared in step S1 into the carbonization tower. When the pumping of the lime milk is completed and the pH drops to 7.0, the reaction ends;

[0064] S4: Filter, dry, crush, and sieve the suspension obtained in step S3 to obtain ordinary cubic precipitated calcium carbonate.

[0065] Comparative Example 2

[0066] S1: Take the refined lime milk and adjust the solid content to 1.020, making it lower than the solid content of the lime milk used for ordinary nano calcium carbonate;

[0067] S2: Inject 10 kg of water into the carbonization tower, introduce kiln gas with a carbon dioxide concentration of 45%, control the temperature at 60 °C, and set the stirring frequency at 20 HZ;

[0068] S3: Pump the lime milk prepared in step S1 and a dilute EDTA-2Na solution with a dry basis calcium carbonate content of 0.15% and a concentration of 1.0% into the carbonization tower simultaneously; when the pH drops to 7.0, the reaction ends;

[0069] S4: Filter, dry, crush, and sieve the suspension obtained in step S3 to obtain ordinary cubic precipitated calcium carbonate.

[0070] Comparative Example 3

[0071] S1: Take the refined lime milk and adjust the solid content to 1.020, making it lower than the solid content of the lime milk used for ordinary nano calcium carbonate;

[0072] S2: Inject 10 kg of water into the carbonization tower, introduce kiln gas with a carbon dioxide concentration of 45%, control the temperature at 60 °C, and set the stirring frequency at 20 HZ;

[0073] S3: Pump the lime milk prepared in step S1 into the carbonization tower;

[0074] S4: Immediately pour a sodium orthophosphate solution with a dry basis calcium carbonate content of 2.0% and a concentration of 1.0% into the carbonization tower when the pumping of the lime milk is completed, and stir at a rotation speed of 30 HZ for 10 min to obtain a suspension;

[0075] S5: Filter, dry, crush, and sieve the suspension obtained in step S4 to obtain surface-rough precipitated calcium carbonate.

[0076] Comparative Example 4

[0077] S1: Take the refined lime milk and adjust the solid content to 1.020, making it lower than the solid content of the lime milk used for ordinary nano calcium carbonate;

[0078] S2: Introduce 10 kg of water into the carbonation tower, introduce kiln gas with a carbon dioxide concentration of 45%, control the temperature at 60 °C, and the stirring frequency at 20 HZ;

[0079] S3: Mix the lime milk prepared in step S1 with a dilute EDTA-2Na solution with a dry basis calcium carbonate content of 0.15% and a concentration of 1.0%, and then pump it into the carbonation tower;

[0080] S4: Immediately pour a sodium orthophosphate solution with a dry basis calcium carbonate content of 2.0% and a concentration of 1.0% into the carbonation tower when the lime milk pumping is completed, start stirring at a speed of 30 HZ for 10 min to obtain a suspension;

[0081] S5: Filter, dry, crush, and screen the suspension obtained in step S4 to obtain surface-rough cubic calcium carbonate precipitate.

[0082] Comparative experiment

[0083] The calcium carbonate powders prepared in Examples 1-3 and Comparative Examples 1-4 were detected by an electron scanning microscope (SEM), and the results are as Figure 1-7 shown.

[0084] It can be Figure 1-5 seen that in Examples 1-3, due to the addition of phosphates during the calcium carbonate preparation process, it presented a tremella-like shape and had good dispersion; in Comparative Examples 3 and 4, phosphates were also added, and there was a tendency for tremella-like crystallization to grow on the surface of calcium carbonate, but due to the lack of addition of EDTA-2Na or non-sustained release addition of EDTA-2Na, the surface nucleation sites were insufficient and the tremella-like crystals were less. In Comparative Examples 1-2, cubic and spindle-shaped calcium carbonate precipitates were obtained respectively, with a relatively smooth surface and no tremella-like morphology.

[0085] According to GB / T 19281-2014, GB / T 23774-2009, and GB / T 19587-2004, a whiteness meter and a specific surface area tester were used to measure the whiteness and specific surface area of the calcium carbonate prepared in Examples 1-3 and Comparative Examples 1-4, and the measurement results are shown in the following table:

[0086] sample <![CDATA[Specific surface area, m 2 / g]]> Whiteness, % Example 1 11.23 96.3 Example 2 8.32 93.4 Example 3 9.87 94.5 Comparative Example 1 3.45 88.5 Comparative Example 2 5.74 89.1 Comparative Example 3 5.83 90.3 Comparative Example 4 5.76 91.5

[0087] It can be seen from the above table that the specific surface area and whiteness of the tremella-like calcium carbonate precipitate prepared in Examples 1-3 are relatively high.

[0088] On the basis of Example 1, in Comparative Example 1, no EDTA-2Na and sodium orthophosphate were added, and ordinary cubic calcium carbonate precipitate was prepared, and the specific surface area and whiteness were both greatly reduced.

[0089] Comparative Examples 2 and 3 respectively added EDTA-2Na and sodium orthophosphate on the basis of Comparative Example 1, and the specific surface area and whiteness were improved to a certain extent, but were still significantly lower than those of Example 1.

[0090] In Comparative Example 4, on the basis of Example 1, lime milk was pre-mixed with EDTA-2Na and then pumped into the carbonization tower, which had a greater impact on the specific surface area and a smaller impact on the whiteness.

[0091] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing tremella-like precipitated calcium carbonate, characterized in that, The following steps are involved: S1: Take the refined lime milk and adjust the solid content for later use; S2: inject water into the carbonization tower, introduce kiln gas, and control the temperature and stirring frequency; S3: Pumping the lime milk and EDTA-2Na dilute solution prepared in step S1 into the carbonization tower simultaneously; S4: When the lime milk is pumped in, immediately pour in the phosphate solution and / or phosphoric acid and stir thoroughly to obtain a suspension; S5: The suspension obtained in step S4 is filtered, dried, crushed and sieved to obtain tremella-like precipitated calcium carbonate.

2. The method for preparing tremella-like precipitated calcium carbonate according to claim 1, characterized in that: In step S2, water is injected The quantity is 10-15kg.

3. The method for preparing tremella-like precipitated calcium carbonate according to claim 1, characterized in that: In step S2, kiln gas The carbon dioxide concentration is 40-50%.

4. The method for preparing tremella-like precipitated calcium carbonate according to claim 1, characterized in that: In step S2, control The temperature is 50-70℃.

5. The method for preparing tremella-like precipitated calcium carbonate according to claim 1, characterized in that: In step S2, control The stirring frequency is 15-25 Hz.

6. The method for preparing tremella-like precipitated calcium carbonate according to claim 1, characterized in that: In step S3, the amount of EDTA-2Na used is 0.15-0.30% of the dry calcium carbonate content, and the concentration is 1-3%.

7. The method for preparing tremella-like precipitated calcium carbonate according to claim 1, characterized in that: In step S4, phosphoric acid The salt is a combination of one or more of orthophosphate, hydrogen phosphate and dihydrogen phosphate.

8. The method for preparing tremella-like precipitated calcium carbonate according to claim 1, characterized in that: In step S4, phosphoric acid The amount of salt and / or phosphoric acid used is 0.5%-2.0% of the dry calcium carbonate content and the concentration is 1-3%.

9. The method for preparing tremella-like precipitated calcium carbonate according to claim 1, characterized in that: In step S4, stirring The frequency is 25-30Hz and the stirring time is 10-30min.

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