Process for the preparation of a precipitated silica with low hydroxyl content

Abstract

The application relates to the field of silica preparation, and particularly discloses a preparation method of precipitated silica with low hydroxyl content, wherein solid sodium silicate with a modulus of 3.0-3.5 is selected, a water glass solution with a concentration of 15-20 wt% is prepared, and sulfuric acid with a concentration of 18-25 wt% is prepared as an acidifying agent; SO2, calcium hydroxide is added into the prepared sodium silicate solution, impurity removal and purification treatment are carried out, and after standing and filtering, the purified sodium silicate solution which can be used for reaction is obtained. Through precise control of the reaction conditions, the parameters such as acid-base flow rate, reaction temperature and pH in different reaction stages are controlled, the surface hydroxyl content of the target product is significantly reduced, and before preparation, the impurity-removed water glass solution is obtained by taking calcium hydroxide as a sodium silicate purifying agent, so that the purity of the reaction product is higher, and when the reaction product is used for rubber mixing, the reaction product is not prone to agglomeration and thickening.

Description

Technical Field

[0001] This invention relates to silica preparation and processing technology, specifically a method for preparing precipitated silica with low hydroxyl content. Background Technology

[0002] As is widely known, silicon dioxide, also known as silica, is commonly used in the industrial fields of reinforcement, thickening, and adsorption in the form of white carbon black. It is an inorganic compound formed by silicon and oxygen atoms through covalent bonds and is widely found in natural minerals and biomass such as quartz, sandstone, diatomaceous earth, and rice husk ash.

[0003] Precipitated silica is a commonly used industrial inorganic material produced through acid-base neutralization reactions. It typically uses solid sodium silicate and inorganic acids as raw materials, and by selecting appropriate reaction conditions, silica with different properties can be obtained. Precipitated silica has a relatively large production volume, low production cost, and rich pore structure, and is mainly used in rubber reinforcement, toothpaste, animal feed, coatings, and other fields. Precipitated silica has multiple advantages: good chemical stability, high specific surface area and porosity, and strong surface modification potential, making it an excellent chemical material.

[0004] For example, the invention patent with publication number CN112408400B, publication date July 26, 2022, entitled "A Water-Saving and Energy-Saving Precipitation Production Method for Silica," uses water glass as raw material and sulfuric acid as a precipitant to prepare silica via a hydrothermal method. It reduces the silanol content on the silica particle surface by treating the silanol groups with metal ions, thereby reducing the strong hydrogen bond adsorption of water by the silica particles and achieving the goal of reducing production water consumption and drying energy consumption. The filter cake produced using this invention's technology has a water content reduced from over 90% before treatment to below 75% after alkaline earth metal ion treatment. Compared to traditional precipitation methods for producing silica powder, this invention features a simple process, low water consumption, low pollution, and low energy consumption. Compared to gas-phase silica production, this invention has the advantages of simple equipment requirements, low cost, low pollution, and low energy consumption.

[0005] The shortcoming of existing technologies lies in the fact that water absorption significantly affects the properties of silica, mainly in terms of dispersibility, rheological behavior, mechanical properties, chemical stability, and application suitability. Its water absorption is related to the hydroxyl content on the silica surface. Excessive hydroxyl content leads to easy absorption of moisture in humid environments, causing powder agglomeration, reduced flowability, and affecting storage and processing stability. This is particularly true when silica is mixed with rubber, leading to easy aggregation and reduced reinforcing or thickening effects. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing precipitated silica with low hydroxyl content, so as to overcome the above-mentioned shortcomings in the prior art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing precipitated silica with low hydroxyl content, specifically comprising the following steps:

[0008] S01. Select solid sodium silicate with a modulus of 3.0-3.5, prepare a water glass solution with a concentration of 15-20wt%, and prepare sulfuric acid with a concentration of 18-25wt% as an acidifying agent for later use.

[0009] S02. Add calcium hydroxide to the prepared sodium silicate solution as a purification agent, let it stand, filter and obtain a purified sodium silicate solution that can be used for the reaction.

[0010] S03. First, add 1 / 6 of the reactor volume of industrial water as bottom water to the reactor, control the water temperature in the range of 60-65℃, add the prepared water glass solution, set the stirring speed to 70-80r / min, and the folded unit on the stirring head in the reactor is in a fully unfolded state to form a rhombus.

[0011] S04. Simultaneously add sulfuric acid solution and sodium silicate solution to the reactor, and make the flow rate of the alkali solution greater than the flow rate of the acid solution. Keep the reaction temperature and stirring speed constant. Gradually fold the folding unit on the stirring head in the reactor to reduce the rhomboid area. Keep the pH value of the reaction system in the range of 9.0-10.5 until the volume of the reaction system reaches 60% of the reactor. Control the reaction time within 30-50 minutes.

[0012] S05. Continue to add sulfuric acid solution and sodium silicate solution to the reactor simultaneously, keeping the flow rate of the alkali solution higher than that of the acid solution. Control the reaction temperature within the range of 80-95℃, maintain the stirring speed at 70-80 r / min, and fully fold the folding unit on the stirring head in the reactor. Keep the pH value of the reaction system within the range of 9.0-10.5 until the volume of the reaction system reaches 80% of the volume of the reactor. Set the reaction time to 60 min.

[0013] S06. Continue adding sulfuric acid solution, keep the reaction temperature controlled within the range of 80-95℃, keep the stirring speed at 70-80r / min, adjust the pH of the reaction system to the range of 3.5-5.5, then stop adding sulfuric acid solution, age the reaction system for 30min, and the reaction is complete.

[0014] S07. The reaction slurry is filtered by pressure and washed with industrial water. The sodium sulfate content in the filter cake is determined by the conductivity of the washing water, and the sodium sulfate content in the final product is controlled to be less than 2%. The solid content of the filter cake is controlled at 20-25%, and it is directly sent to a pulping machine to be made into a concentrated slurry.

[0015] S08. The concentrated slurry is fed into a spray dryer for drying to obtain the target product.

[0016] As a further description of the above technical solution: the folding unit includes a hinge joint that is slidably connected to the stirring head, and folding plates are symmetrically hinged between the hinge joint and the end of the stirring head. The two sets of folding plates form a rhombus, and the rhombus deforms as the hinge joint moves.

[0017] As a further description of the above technical solution: the folding plate includes a hinged upper folding plate and a lower folding plate, and a woven mesh is provided between the upper folding plate and the lower folding plate, the woven mesh deforming according to the rhombus shape.

[0018] As a further description of the above technical solution: the upper receiving plate is provided with stirring blades.

[0019] As a further description of the above technical solution: an input pipe is rotatably arranged on the reactor, and liquid tanks are opened in a linear array on the input pipe. A matching groove corresponding to the input pipe is opened inside the reactor.

[0020] As a further description of the above technical solution: an inner tube is slidably connected inside the input tube, and a limiting groove corresponding to the liquid-carrying tank is opened on the inner tube. The limiting groove and the liquid-carrying tank are staggered to restrict the passive outflow of liquid.

[0021] As a further description of the above technical solution: an expansion cone is movably provided at the end of the input tube, and the expansion cone has arc-shaped recesses arranged in a circumferential array.

[0022] As a further description of the above technical solution: a limiting rod is provided on the expansion cone head, and a guide groove is provided on the input tube for the limiting rod to slide. The expansion cone head flips along the input tube to fit against the input tube.

[0023] As a further description of the above technical solution: the bottom of the reactor is provided with a limiting protrusion, and the input pipe is flipped to fit against the side wall of the reactor, so that the limiting protrusion restricts the expansion cone from moving away from the input pipe.

[0024] As a further description of the above technical solution: the woven mesh is arranged in an alternating pattern of horizontal and vertical lines, with the intersection points rotating as the upper gathering plate and the lower folding plate rotate.

[0025] In the above technical solution, the present invention provides a method for preparing precipitated silica with low hydroxyl content, which has the following beneficial effects: by precisely controlling the reaction conditions, parameters including acid-base flow rate, reaction temperature and pH are controlled at different reaction stages, significantly reducing the hydroxyl content on the surface of the target product. In addition, calcium hydroxide is used as a sodium silicate purifying agent before preparation to obtain a purified water glass solution, resulting in higher purity of the reaction product. When used in rubber compounding, it is less likely to cause agglomeration, thickening and other problems. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0027] Figure 1 This is a schematic diagram of the overall structure provided for an embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram of the end cap, stirring mechanism, and input pipe structure provided in an embodiment of the present invention;

[0029] Figure 3 This is a schematic diagram of the stirring rod structure provided in an embodiment of the present invention;

[0030] Figure 4 This is a schematic cross-sectional view of the overall structure provided in an embodiment of the present invention;

[0031] Figure 5 for Figure 4 Enlarged view of point A in the middle;

[0032] Figure 6 for Figure 4 Enlarged view of point B in the middle;

[0033] Figure 7 This is a schematic cross-sectional view of the entire structure from another angle, provided for an embodiment of the present invention.

[0034] Figure 8 This is an exploded view of the stirring rod structure provided in an embodiment of the present invention;

[0035] Figure 9 for Figure 8 Enlarged view of point C in the middle;

[0036] Figure 10 This is a schematic diagram of the input tube structure provided in an embodiment of the present invention;

[0037] Figure 11 This is an exploded view of the input tube structure provided in an embodiment of the present invention;

[0038] Figure 12 for Figure 11 Enlarged view of point D;

[0039] Figure 13 for Figure 11 Enlarged view of point E in the middle;

[0040] Figure 14 for Figure 4 Enlarged diagram at point F;

[0041] Figure 15 This is a schematic diagram of the structure of the woven mesh provided in an embodiment of the present invention.

[0042] Explanation of reference numerals in the attached figures:

[0043] 1. Reactor; 10. End cap; 11. Outer shell layer; 12. Inner wall layer; 121. Restricting protrusion; 122. Matching groove; 13. Temperature control jacket; 2. Stirring mechanism; 21. Stirring motor; 22. Shaft seal; 221. Telescopic motor; 23. Stirring head; 230. Restricting protrusion ring; 231. Extension rod; 2311. Hinge joint; 2312. Rotating groove; 24. Folding unit; 241. Mesh; 242. Upper retracting plate; 2421. Stirring blade; 243. Lower folding plate; 31. Input pipe; 311. Liquid tank; 312. Expansion cone; 3121. Arc recess; 3122. Restricting rod; 313. Guide groove; 32. Tilting seat; 321. Liquid guide head; 322. Cover; 33. Inner tube; 331. Restricting groove; 332. Stud head. Detailed Implementation

[0044] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0045] Please see Figure 1-15 This invention provides a technical solution: a method for preparing precipitated silica with low hydroxyl content, specifically including the following steps:

[0046] S01. Select solid sodium silicate with a modulus of 3.0-3.5, prepare a water glass solution with a concentration of 15-20wt%, and prepare sulfuric acid with a concentration of 18-25wt% as an acidifying agent for later use.

[0047] S02. Add calcium hydroxide (Ca(OH)2) to the prepared sodium silicate solution to remove impurities from the sodium silicate solution. After standing and filtration, a purified sodium silicate solution that can be used for the reaction is obtained, thus achieving the purification of the water glass solution.

[0048] S03. First, add 1 / 6 of the volume of industrial water into the reactor 1 as bottom water. The water temperature is controlled within the range of 60-65℃ by steam heating. Add the prepared water glass solution and set the stirring speed to 70-80 r / min. The folded unit 24 on the stirring head 23 in the reactor 1 is fully unfolded and forms a rhombus. At this time, the rhombus area of ​​the folded unit 24 on the stirring head 23 is the largest, corresponding to the bottom liquid without bubbles.

[0049] S04. Simultaneously add sulfuric acid solution and sodium silicate solution to reactor 1, and make the flow rate of the alkali solution greater than the flow rate of the acid solution. Keep the reaction temperature and stirring speed constant. The folding unit 24 on the stirring head 23 in reactor 1 is gradually folded to reduce the area of ​​the rhombus. The pH value of the reaction system is kept in the range of 9.0-10.5 until the volume of the reaction system reaches 60% of the reactor 1. The reaction time is controlled within 30-50 minutes. The area of ​​the rhombus that is gradually reduced by the folding unit 24 corresponds to the bottom liquid that is gradually expanding in reactor 1.

[0050] S05. Continue to add sulfuric acid solution and sodium silicate solution to reactor 1 simultaneously, keeping the flow rate of the alkali solution greater than that of the acid solution. Control the reaction temperature within the range of 80-95℃ and maintain the stirring speed at 70-80 r / min. The rhomboid shape of the folding unit 24 on the stirring head 23 in reactor 1 is completely folded to minimize the area, so that the pH value of the reaction system is maintained within the range of 9.0-10.5 until the volume of the reaction system reaches 80% of the volume of reactor 1. Set the reaction time to 60 min.

[0051] S06. Continue adding sulfuric acid solution, keeping the reaction temperature and stirring speed constant. After adjusting the pH of the reaction system to the range of 3.5-5.5, stop adding acid. Let the reaction system age for 30 minutes to complete the reaction.

[0052] S07. The reaction slurry is filtered by pressure and washed with industrial water. The sodium sulfate content in the filter cake is determined by the conductivity of the washing water, and the sodium sulfate content in the final product is controlled to be less than 2%. The solid content of the filter cake is controlled at 20-25%, and it is directly sent to a pulping machine to be made into a concentrated slurry.

[0053] S08. The concentrated slurry is fed into a spray dryer for drying, using hot air from natural gas combustion as the heat source to obtain the target product.

[0054] The target product should meet the following criteria: 1. BET: 175-250 m² / g; 2. Particle size (D50): 200-250 μm; 3. DBP oil absorption value: 2.0-3.0 cm. 3 / g; 4. Loss on heating: 5.0-7.0%; 5. Silica content: ≥95-98%; 6. Loss on ignition: ≤7.0%; 7. pH: 6.0-7.5; 8. Soluble dissociated salts (calculated as sodium sulfate): ≤2.0%; 9. Dispersion grade: ≥9.5; 10. Iron content: ≤500mg / kg; 11. Copper content: ≤10mg / kg; 12. Manganese content: ≤40mg / kg; 13. Surface hydroxyl content: 1.0-2.5 hydroxyl groups / nm2; 14. Water absorption rate ≤5.50% after 8 hours.

[0055] By precisely controlling the reaction conditions, parameters including acid-base flow rate, reaction temperature, and pH are controlled at different reaction stages, significantly reducing the hydroxyl content on the surface of the target product. Before preparation, calcium hydroxide is used as a sodium silicate purifier to obtain a purified water glass solution, resulting in higher purity of the reaction product. The surface hydroxyl density of the target product is 1.0-2.5 hydroxyl groups / nm2, the dispersion grade is higher than 9.5, the BET specific surface area is 175-250m2 / g, and the water absorption rate is less than 5.50% after 8 hours. When used in rubber compounding, it is less prone to agglomeration and thickening.

[0056] In another embodiment of the present invention, the reactor 1 includes a support portion and an end cap 10. The support portion is formed by combining an outer shell layer 11 and an inner wall layer 12. A temperature-controlled interlayer 13 is formed between the outer shell layer 11 and the inner wall layer 12. Liquid or gas is introduced into the temperature-controlled interlayer 13 to control the temperature. A stirring mechanism 2 is provided on the reactor 1. The stirring mechanism 2 includes a stirring head 23 rotatably connected to a shaft seal 22 on the end cap 10. One end of the stirring head 23 is connected to the output end of a stirring motor 21 on the shaft seal 22 via a coupling. The other end of the stirring head 23 is a stirring part. The stirring part is provided with a folding unit 24. The folding unit 24 is slidably connected to a hinge joint 2311 on the stirring head 23. Figure 3 As shown, folding plates are symmetrically hinged between the hinge joint 2311 and the end of the stirring head 23. The two sets of folding plates are arranged in a rhombus shape. During step S04, the liquid in the reactor 1 will expand. This expansion is caused by bubbles, which will lead to a decrease in the fluidity of the liquid. At this time, the rhombus shape of the folding unit 24 is in an expanded state, which puts a heavy burden on the stirring and the mixing efficiency is not as high as that of the fine rod stirring. At this time, the folding unit 24 is folded to reduce the area of ​​the rhombus formed by the two sets of folding plates. This adjusts the bubble expansion, reduces the burden on the stirring, and maintains the mixing efficiency.

[0057] Preferred, such as Figure 8As shown, the folding plate includes a hinged upper folding plate 242 and a lower folding plate 243. The upper folding plate 242 is hinged to the hinge joint 2311, and the lower folding plate 243 is hinged to the end (the lower end) of the stirring head 23. A mesh 241 is provided between the disjointed upper folding plate 242 and lower folding plate 243. The mesh 241 forms a mesh structure to increase the stirring capacity of the rhombus area. When the upper folding plate 242 and lower folding plate 243 deform with the movement of the hinge joint 2311, the mesh 241 deforms with the rhombus, thereby changing the stirring capacity when the rhombus changes. The lower folding plate 243 can be retracted into the upper folding plate 242 to store the folded mesh 241.

[0058] Furthermore, the hinge joint 2311 is provided with a rotating groove 2312, and an extension rod 231 is rotatably connected to the rotating groove 2312. The extension rod 231 extends out of the end cap 10, and a limiting protrusion ring 230 is provided on the part of the extension rod 231 extending out of the end cap 10. A telescopic motor 221 is provided on the shaft seal 22. The output end of the telescopic motor 221 corresponds to the limiting protrusion ring 230. When the telescopic motor 221 extends, it locks the limiting protrusion ring 230, restricting it from rotating together with the stirring head 23. The stirring head 23 and the extension rod 231 are provided with mutually cooperating transmission threads, and the extension rod 231 can move along the axis of the stirring head 23 through the transmission threads.

[0059] Furthermore, such as Figure 15 As shown, the net 241 is arranged in a staggered pattern of horizontal and vertical lines, with the staggered points rotating as the upper closing plate 242 and the lower folding plate 243 rotate. The net 241 forms multiple small quadrilaterals, which increase in size as the area of ​​the rhombus increases and decrease in size as the area of ​​the rhombus decreases.

[0060] In another embodiment of the present invention, an stirring blade 2421 is provided on the upper receiving plate 242. The stirring blade 2421 is inclined. When the upper receiving plate 242 moves as the area of ​​the rhomboid part decreases, the stirring blade 2421 will flip and move closer to the vertical direction.

[0061] In another embodiment provided by the present invention, such as Figure 2As shown, a tilting seat 32 is provided on the reactor 1, and an input pipe 31 is rotatably mounted on the tilting seat 32. A liquid guide head 321 is provided at the upper end of the input pipe 31, and a cover 322 is provided on the liquid guide head 321. The cover 322 is made of flexible material to isolate the reactor 1 from the outside. The liquid guide head 321 is connected to a material pump to pump material into the input pipe 31. Liquid grooves 311 are linearly arrayed on the input pipe 31, and matching grooves 122 corresponding to the input pipe 31 are provided inside the reactor 1. When a short-term reaction is carried out... When administering a large volume of fluid, the input tube 31 flips and embeds itself into the anastomosis groove 122, at which point the anastomosis groove 122 is closed. However, when a long-term, gradual infusion is required, as in steps S03 and S04, the input tube 31 flips and moves closer to the stirring head 23. At this point, the liquid-carrying groove 311 is exposed and connected to the outside. As the stirring head 23 rotates continuously, the liquid trajectory passes through the liquid-carrying groove 311, carrying out the liquid in the input tube 31. This natural outflow along the trajectory allows for more thorough mixing during the expansion process, where the liquid's fluidity is not very good.

[0062] Preferred, such as Figure 8 As shown, an inner tube 33 is slidably connected inside the input tube 31. A limiting groove 331 corresponding to the liquid-carrying tank 311 is opened on the inner tube 33. A stud head 332 is rotatably connected to the upper end of the inner tube 33. A liquid guide head 321 extends from the stud head 332. By rotating the stud head 332, the inner tube 33 is driven to slide along the input tube 31, so that the limiting groove 331 and the liquid-carrying tank 311 are staggered to restrict the passive flow of liquid. The flow rate of liquid carried out can be controlled by moving the staggered range of the limiting groove 331 and the liquid-carrying tank 311, which is suitable for long-term gradual infusion.

[0063] In another embodiment of the present invention, an expansion cone 312 is movably provided at the end of the input tube 31 (the end being the lower end). The expansion cone 312 has circular arc recesses 3121 arranged in a circular array. During short-term large-volume infusion, the input tube 31 flips and embeds into the anastomosis groove 122. The anastomosis groove 122 is closed, and the liquid is sprayed out along the end of the input tube 31. When sprayed out, it will diffuse along several circular arc recesses 3121 on the expansion cone 312. With the guidance of the expansion cone 312 and the circular arc recesses 3121, the diffusion is more uniform.

[0064] Preferably, the expansion cone 312 is provided with a limiting rod 3122, and the input pipe 31 is provided with a guide groove 313 for the limiting rod 3122 to slide. When the input pipe 31 needs to be gradually infused over a long period of time, it will flip over. When flipping over in the liquid, due to the resistance, the expansion cone 312 will move closer to the end of the input pipe 31 along the guide groove 313 and eventually fit into the input pipe 31. Because of the presence of the arc-shaped depression 3121, the end of the input pipe 31 is not completely blocked and there is a small gap. When the liquid tank 311 continuously carries out liquid, the small gap will also draw liquid from the reactor 1, realizing the exchange of liquid with the liquid in the input pipe 31.

[0065] Preferably, the bottom of the reactor 1 is provided with a limiting protrusion 121. When a large amount of liquid needs to be transferred in a short time, the input pipe 31 is flipped to fit against the side wall of the reactor 1. At this time, the limiting protrusion 121 restricts the expansion cone 312 away from the input pipe 31, ensuring that the liquid can flow out smoothly.

[0066] When producing silica, if a short-term, large-volume infusion is required, the input pipe 31 flips and embeds into the matching groove 122, sealing the groove. Liquid sprays out from the end of the input pipe 31, diffusing along several arc-shaped recesses 3121 on the expanding cone 312. At this time, the liquid in the reactor 1 has not expanded. The hinge joint 2311 moves upward along the stirring head 23, expanding the upper receiving plate 242 and the lower folding plate 243. The rhomboid area is at its maximum, and the mesh 241 expands, stirring the unexpanded liquid. When entering steps S03 and S04, when a long-term, gradual infusion is required, the input pipe 31 flips and moves closer to the stirring head 23. When the liquid tank 311 is exposed and connected to the outside, as the stirring head 23 rotates continuously, the liquid trajectory passes through the liquid tank 311, which will carry out the liquid in the input pipe 31. Due to the existence of the arc-shaped depression 3121, the end of the input pipe 31 is not completely blocked and there is a small gap. When the liquid tank 311 continuously carries out liquid, the small gap will also draw in the liquid in the reactor 1, realizing the exchange with the liquid in the input pipe 31. At this time, the liquid in the reactor 1 expands, and the hinge joint 2311 moves downward along the stirring head 23, folding and shrinking the upper plate 242 and the lower folding plate 243 to reduce the diamond area and cope with the expanding liquid with reduced flow rate.

[0067] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A method for preparing precipitated silica with low hydroxyl content, characterized in that, Specifically, the following steps are included: S01. Select solid sodium silicate with a modulus of 3.0-3.5, prepare a water glass solution with a concentration of 15-20wt%, and prepare sulfuric acid with a concentration of 18-25wt% as an acidifying agent for later use. S02. Add calcium hydroxide to the prepared sodium silicate solution as a purification agent, let it stand, filter and obtain a purified sodium silicate solution that can be used for the reaction. S03. First, add 1 / 6 of the volume of industrial water into the reactor (1) as bottom water, control the water temperature in the range of 60-65℃, add the prepared water glass solution, set the stirring speed to 70-80r / min, and the folding unit (24) on the stirring head (23) in the reactor (1) is fully unfolded into a rhombus shape. S04. Add sulfuric acid solution and sodium silicate solution to the reactor (1) at the same time, and make the flow rate of the alkali solution greater than that of the acid solution. Keep the reaction temperature and stirring speed constant. Gradually fold the folding unit (24) on the stirring head (23) in the reactor (1) to reduce the rhomboid area. Keep the pH value of the reaction system in the range of 9.0-10.5 until the volume of the reaction system reaches 60% of the reactor (1). Control the reaction time within 30-50 minutes. S05. Continue to add sulfuric acid solution and sodium silicate solution to the reactor (1) at the same time, keep the flow rate of the alkali solution greater than that of the acid solution, control the reaction temperature in the range of 80-95℃, keep the stirring speed at 70-80r / min, and completely fold the folding unit (24) on the stirring head (23) in the reactor (1) to keep the pH value of the reaction system in the range of 9.0-10.5 until the volume of the reaction system reaches 80% of the volume of the reactor. The reaction time is set to 60min. S06. Continue adding sulfuric acid solution, keep the reaction temperature controlled within the range of 80-95℃, keep the stirring speed at 70-80r / min, adjust the pH of the reaction system to the range of 3.5-5.5, then stop adding sulfuric acid solution, age the reaction system for 30min, and the reaction is complete. S07. The reaction slurry is filtered by pressure and washed with industrial water. The sodium sulfate content in the filter cake is determined by the conductivity of the washing water, and the sodium sulfate content in the final product is controlled to be less than 2%. The solid content of the filter cake is controlled at 20-25%, and it is directly sent to a pulping machine to be made into a concentrated slurry. S08. The concentrated slurry is fed into a spray dryer for drying to obtain the target product.

2. The method for preparing precipitated silica with low hydroxyl content according to claim 1, characterized in that, The folding unit (24) includes a hinge joint (2311) slidably connected to the stirring head (23). Folding plates are symmetrically hinged between the hinge joint (2311) and the end of the stirring head (23). The two sets of folding plates form a rhombus, which deforms as the hinge joint (2311) moves.

3. The method for preparing low-hydroxyl-content precipitated silica according to claim 2, characterized in that, The folding plate includes a hinged upper folding plate (242) and a lower folding plate (243), and a woven mesh (241) is provided between the upper folding plate (242) and the lower folding plate (243), which are in opposite groups. The woven mesh (241) deforms according to the rhombus shape.

4. The method for preparing low-hydroxyl-content precipitated silica according to claim 3, characterized in that, The upper plate (242) is provided with stirring blades (2421).

5. The method for preparing precipitated silica with low hydroxyl content according to claim 1, characterized in that, An input pipe (31) is rotatably mounted on the reactor (1). A liquid tank (311) is formed in a linear array on the input pipe (31). A matching groove (122) corresponding to the input pipe (31) is formed inside the reactor (1).

6. The method for preparing precipitated silica with low hydroxyl content according to claim 5, characterized in that, An inner tube (33) is slidably connected inside the input tube (31). A limiting groove (331) corresponding to the liquid-carrying tank (311) is opened on the inner tube (33). The limiting groove (331) and the liquid-carrying tank (311) alternately restrict the passive outflow of liquid.

7. The method for preparing low-hydroxyl-content precipitated silica according to claim 5, characterized in that, An expansion cone (312) is movably provided at the end of the input tube (31), and an arc recess (3121) is formed in a circular array on the expansion cone (312).

8. The method for preparing precipitated silica with low hydroxyl content according to claim 5, characterized in that, The expansion cone (312) is provided with a limiting rod (3122), and the input tube (31) is provided with a guide groove (313) for the limiting rod (3122) to slide. The expansion cone (312) flips along the input tube (31) to fit the input tube (31).

9. The method for preparing precipitated silica with low hydroxyl content according to claim 8, characterized in that, The reactor (1) has a limiting protrusion (121) at its inner bottom. The input pipe (31) is flipped to fit against the side wall of the reactor (1), so that the limiting protrusion (121) restricts the expansion cone (312) from moving away from the input pipe (31).

10. The method for preparing low-hydroxyl-content precipitated silica according to claim 3, characterized in that, The woven net (241) is arranged in a staggered pattern, with the staggered points rotating as the upper gathering plate (242) and the lower folding plate (243) rotate.

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