A method for preparing high specific surface area and high dispersion white carbon black by using fluorosilicic acid to produce hydrogen fluoride and by-product silicon residue

Abstract

The present application belongs to the field of inorganic chemical solid waste resource utilization and high value-added white carbon black preparation technology, and relates to a method for preparing high specific surface area and high dispersion white carbon black by using by-product silicon slag in hydrogen fluoride production from fluosilicic acid. The method comprises: after drying, the by-product silicon slag is subjected to graded alkali dissolution activation, step-by-step aging, solid-liquid separation to obtain sodium silicate filtrate; a composite dispersing agent is added to the filtrate, three-stage pH grading precipitation reaction is adopted, and white carbon black nucleation, mesoporous oriented growth and complete precipitation are precisely controlled; the precipitation slurry is subjected to aging, acid washing, spray drying and surface treatment, and crushing to obtain the product. Through the system synergy of graded alkali dissolution activation, step-by-step aging, three-stage pH grading precipitation and composite dispersing agent, the specific surface area and dispersibility of the white carbon black are significantly improved, the product BET is 330-360 m 2 / g, the DBP absorption value is greater than or equal to 2.85 cm 3 / g, the purity is greater than or equal to 98.7%, the silicon recovery rate is greater than 92%, and the wastewater discharge amount is reduced by more than 50%. The method has been verified by industrialized continuous production, and has good industrialization prospect.

Description

Technical Field

[0001] This invention belongs to the field of inorganic chemical solid waste resource utilization and high value-added silica preparation technology, specifically involving a method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid. Background Technology

[0002] Silica (hydrated silica, SiO2•nH2O), as an important inorganic fine chemical product, possesses characteristics such as high specific surface area, excellent reinforcing properties, and good dispersibility, and is widely used in rubber, plastics, coatings, pharmaceuticals, and daily chemicals. Currently, the main methods for preparing silica include precipitation, gas-phase, and gelation methods. Among these, precipitation is widely used due to its simple process, moderate cost, and ease of large-scale production.

[0003] The fluorosilicic acid process for producing anhydrous hydrogen fluoride is an important way to utilize the fluorine resources byproduct of the phosphate fertilizer industry. However, in this process, after fluorosilicic acid reacts with concentrated sulfuric acid, silicon precipitates as silicon dioxide and encapsulates fluorosilicic acid, mechanical residues, and metal ions such as calcium and magnesium, forming a large amount of fluorine-containing silicon slag. According to industry statistics, approximately 0.8 to 1.2 tons of fluorine-containing silicon slag are produced as a byproduct for every ton of anhydrous hydrogen fluoride produced. Currently, only the fluorine in the fluorosilicic acid is effectively utilized in China, while the silicon is discharged with the silicon slag, mostly stored in slag dumps as slag paste. This not only causes a huge waste of silicon resources but also brings a serious environmental burden.

[0004] The existing technology for preparing silica using fluorinated silica slag has obvious defects: (1) The specific surface area of ​​the product obtained by the physical dispersion sedimentation method is usually less than 200 m². 2 / g; (2) The particle size of the product of the alkali dissolution-carbonation method is too large and the specific surface area is low; (3) The energy consumption of the ammonia-grinding pore-forming method is high and it is difficult to produce continuously; (4) Although there are reports of multi-stage precipitation or composite dispersants in the field of precipitation method, they all use commercial water glass as raw material, and their multi-stage precipitation adopts the intermittent operation mode of "pausing feeding and stirring", which does not achieve continuous production; composite dispersants are only used as auxiliary means and do not form a system synergy with the staged precipitation.

[0005] Therefore, developing a process that can simultaneously achieve high-value utilization of fluorinated silicon slag, high specific surface area and high dispersibility of the product, and is suitable for continuous production remains a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0006] Based on this, the purpose of this invention is to provide a method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid. By integrating graded alkali dissolution activation, stepwise aging, graded precipitation with synergistic regulation of composite dispersants and drying modification, this method solves the problems of difficulty in achieving both specific surface area and dispersibility, complex process flow, and large wastewater discharge in the prior art.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, comprising the following steps: S1, the silicon slag, a byproduct of the production of hydrogen fluoride from fluorosilicic acid, is mixed with an alkaline solution and subjected to an alkaline dissolution reaction to obtain a reaction slurry; the reaction slurry is then diluted, aged, and subjected to solid-liquid separation to obtain a filter cake and sodium silicate filtrate; S2, a composite dispersant is first added to the sodium silicate filtrate described in S1 to form a mixed solution, and then an acid solution is added to adjust the pH range of the mixed solution to carry out a precipitation reaction to obtain a precipitated slurry; after aging and solid-liquid separation, the precipitated slurry is used to obtain a crude silica filter cake. S3, after washing, drying and pulverizing the crude silica filter cake described in S2, the high specific surface area and high dispersion silica is obtained.

[0008] Further, in S1, the particle size of the silicon slag by-product from the production of hydrogen fluoride from fluorosilicic acid is ≤150 mesh, and its moisture content is ≤5%; the mass ratio of the silicon slag by-product from the production of hydrogen fluoride from fluorosilicic acid to the alkaline solution is 1:(1.5~2.5); the alkaline solution includes a sodium hydroxide solution with a mass concentration of 15~25 wt%; the reaction slurry is diluted with water as a diluent, and the solid content of the reaction slurry is adjusted by adding the diluent, and the volume ratio of the reaction slurry to water is 1:(0.5~1).

[0009] Furthermore, the alkali dissolution reaction described in S1 employs a two-stage temperature gradient control, comprising a first-stage control and a second-stage control. The first-stage control involves a reaction temperature of 60–80°C and a reaction time of 20–40 min, initially activating the silica in the silicon slag. The second-stage control involves a reaction temperature of 95–105°C and a reaction time of 10–20 min, ensuring the complete conversion of silica into sodium silicate. The advantage of this staged activation is that the low-temperature stage avoids rapid decomposition of fluorosilicic acid leading to localized overconcentration, while the high-temperature stage fully dissolves silicon elements and reduces excessive hydrolysis of sodium silicate.

[0010] Furthermore, the aging process described in S1 employs a stepped aging method, comprising a first-stage aging and a second-stage aging. The first-stage aging is conducted at a temperature of 70-90°C for 30-60 minutes to promote further dissolution of unreacted particles. The second-stage aging is conducted at a temperature of 40-60°C for 20-40 minutes to allow impurities such as calcium, magnesium, and iron to precipitate as hydroxides, while simultaneously stabilizing and clarifying the sodium silicate solution. Stepped aging, compared to single-stage aging, can improve the purity of the sodium silicate filtrate and reduce the content of metallic impurities in the subsequently precipitated silica.

[0011] Furthermore, the solid-liquid separation in S1 is performed by pressure filtration, the pressure of which is 0.4~0.6MPa, the moisture content of the filter cake is ≤35%, and the filter cake mainly consists of unreacted silica residue and heavy metal precipitates, which can be returned to step S1 for further processing or utilized as a resource; the concentration of SiO2 in the sodium silicate filtrate is 6~9wt%, and the molar ratio of SiO2 to Na2O in the sodium silicate is (2.4~2.8):1, that is, the modulus (SiO2 / Na2O molar ratio) is controlled to be 2.4-2.8.

[0012] Further, the amount of the composite dispersant in S2 is 0.5~2% of the volume of the sodium silicate filtrate; the composite dispersant is compounded from an aqueous polymeric dispersant, a nonionic surfactant, and an organosilicon dispersant stabilizer in a mass ratio of (6~9):(1~2):(0.8~1.2); preferably, the mass ratio of the aqueous polymeric dispersant, the nonionic surfactant, and the organosilicon dispersant stabilizer is 8:1.5:1, at which point the synergistic effect is optimal.

[0013] Furthermore, the critical micelle concentration (CMC) of the composite dispersant is 0.05~0.2 wt%, forming a stable micelle structure in the reaction system, which can be effectively adsorbed on the surface of silica particles and inhibit agglomeration through a dual mechanism of steric hindrance and electrostatic repulsion.

[0014] Furthermore, the water-based polymeric dispersant has a weight-average molecular weight of 2000-10000, including any one or more of sodium polyacrylate, ammonium polycarboxylate, and sodium maleic acid-acrylic acid copolymer.

[0015] Furthermore, the nonionic surfactant has an HLB value of 10 to 15, including any one or more of Tween-80, Tween-60, Span-80, and alkylphenol polyoxyethylene ether.

[0016] Furthermore, the organosilicon dispersant stabilizer includes any one or more of polyether-modified polysiloxane, polydimethylsiloxane emulsion, and silane coupling agent.

[0017] Further, in step S2, the mixture is first heated to 75-95°C, and then an acid solution is added at a rate of 50-150 mL / min under stirring. The acid solution is a 10-25 wt% dilute sulfuric acid solution. The pH range includes three segments: the first segment has a pH of 9.0-10.0 and a residence time of 10-20 min; the second segment has a pH of 7.0-8.0 and a residence time of 15-25 min; and the third segment has a pH of 5.5-6.5 and a residence time of 5-15 min. Preferably, the optimal control points for fractional precipitation in the three pH segments are: the first segment pH 9.8±0.1, the second segment pH 7.2±0.1, and the third segment pH 5.8±0.1. The three-stage pH continuous fractional precipitation is specifically as follows: The first stage is the nucleation stage: the pH of the system is slowly reduced from the initial alkalinity to 9.0-10.0, and the reaction time is 10-20 minutes. Within this pH range, the polymerization rate of silicate ions is moderate. The polymeric dispersant and nonionic surfactant in the composite dispersant are synergistically adsorbed on the surface of the newly formed crystal nuclei, inhibiting excessive growth of crystal nuclei and forming a large number of crystal nuclei with uniform particle size (5-10 nm). The second stage is the growth stage: continue adding dilute sulfuric acid, controlling the pH to drop to 7.0-8.0, with a reaction time of 15-25 minutes. At this pH, silicic acid further condenses, and crystal nuclei grow into silica precursor particles (particle size 20-50 nm) with a loose, porous structure. The organosilicon dispersant stabilizer in the composite dispersant combines with the hydroxyl groups on the particle surface through silicon-oxygen bonds, forming hydrophobic microdomains and guiding the formation of mesoporous structures inside the particles. The third stage is the precipitation and defluorination phase: continue adding dilute sulfuric acid until the pH of the system drops to 5.5-6.5, with a reaction time of 5-15 minutes. Under these acidic conditions, the silica precipitates completely, while fluoride ions react with hydrogen ions to form HF, which escapes or is converted into soluble fluorides, thus achieving effective removal of fluoride. Throughout the fractional precipitation process, the composite dispersant plays a continuous role, exhibiting different dispersion and stabilization mechanisms at different pH ranges, thereby achieving precise control over the entire process of silica nucleation, growth, and precipitation. Compared to single-stage or two-stage precipitation (such as maintaining alkalinity until the final acidification), the three-stage pH continuous fractional precipitation of this invention does not require interrupting stirring, making it suitable for continuous production. Furthermore, through the synergy of pH and the dispersant, a higher specific surface area and a more uniform mesoporous structure can be obtained.

[0018] Furthermore, the aging temperature of the precipitated slurry in S2 is 50~70℃ and the time is 1~3h, so as to further stabilize the structure of the silica particles; after aging, the silica is separated by pressure filtration to obtain a crude silica filter cake.

[0019] Furthermore, the washing described in S3 employs a two-stage acidification washing process. The first stage of the two-stage acidification washing uses a 0.5-2wt% dilute sulfuric acid solution to wash at 40-60°C for 15-30 minutes to remove residual fluoride ions and metal ions. The second stage uses deionized water to wash until the pH of the washing solution is ≥6.0 to remove residual acid and soluble salts.

[0020] Further, the crude silica filter cake described in S3 is washed to obtain a silica filter cake. After the silica filter cake is broken up, 1-3% (by weight of dry silica) of n-butanol is added for treatment, followed by spray drying. Simultaneously, surface hydrophobic modification is completed during drying. The preferred amount of n-butanol added is 2% (by weight of dry silica). The inlet air temperature of the spray dryer is 220-280℃, the outlet air temperature is 90-110℃, and the spray pressure is 1.5-2.5 MPa. The preferred inlet air temperature is 250-270℃, and the outlet air temperature is 95-105℃. During the spray drying process, n-butanol promotes rapid evaporation of moisture (n-butanol azeotropically reacts with water, lowering the evaporation temperature). Furthermore, its hydroxyl groups undergo esterification or hydrogen bonding with the silanol hydroxyl groups on the silica surface, forming a hydrophobic coating layer, significantly improving the product's dispersibility and flowability. This integrated treatment shortens the process, reduces energy consumption, and achieves more uniform modification compared to the traditional "drying + separate surface modification" process.

[0021] Furthermore, the pulverization described in S3 is carried out using an air jet mill, and the pulverization is performed until the D50 of the silica is 5-15μm.

[0022] The high specific surface area and highly dispersed silica prepared by the above-described preparation method of the present invention has a BET specific surface area of ​​330~360 m². 2 / g, DBP absorption value ≥2.85 cm 3 / g, purity ≥98.7%, silicon recovery rate >92%. This invention achieves a high specific surface area (330~360 m²). 2 / g) and high dispersibility (DBP≥2.85 cm) 3 The core technology path of / g) is not a simple superposition of the steps, but rather a dynamic synergy and precise control of three factors: "silicate ion supersaturation - dispersant molecular conformation - pH environment". 1. The pre-conditioning effect of graded alkali dissolution activation and stepped aging: The first step, low-temperature alkali dissolution, controls the slow release of silicate ions, avoiding the formation of dense silica gel; the second step, high-temperature complete dissolution. Subsequent stepped aging yields a high-purity (metal ion ≤0.05%) silicate ion cluster with a narrow activity distribution, laying a molecular-level foundation for precise control of nucleation and growth.

[0023] 2. Synergistic effect of three-stage pH fractionation precipitation and composite dispersant: The first stage (pH 9.0~10.0): The polymeric dispersant and the nonionic surfactant form mixed micelles that adsorb onto the newly generated negatively charged crystal nuclei, creating a dual barrier of "steric hindrance and electrostatic repulsion" that restricts the growth of the crystal nuclei to 5~10nm.

[0024] The second stage (pH 7.0~8.0): Organosilicon dispersants and stabilizers preferentially anchor to "active defect sites" on the particle surface through hydrogen bonds or Si-O-Si bonds, forming nanoscale "hydrophobic templates". During the polymerization of silicate ions, they are forced to bypass these templates, inducing the generation of a large number of interconnected mesopores (pore size 2~10nm) in situ.

[0025] The third stage (pH 5.5~6.5): The low pH environment causes the residual component A to desorb from the particle surface, while promoting the entry of soluble metal ions and fluoride ions into the liquid phase, thus achieving high product purity.

[0026] 3. Spray drying-surface modification integration for "shaping and enhancement" of dispersibility: n-Butanol acts as both an azeotropic agent and a modifier in spray drying. It undergoes a gas-solid phase esterification reaction with the silanol on the surface of silica at high temperature, forming a uniform and dense hydrophobic layer, which effectively prevents hard agglomeration during drying and subsequent pulverization.

[0027] The beneficial effects of this invention are: 1. High specific surface area and high dispersibility are combined: High-purity and highly active sodium silicate filtrate is obtained through staged alkali dissolution activation and stepwise aging; through three-stage pH staged precipitation and synergistic regulation by composite dispersants, the BET specific surface area of ​​the product reaches 330~360 m². 2 / g, DBP absorption value ≥2.85 cm 3 / g, significantly superior to ordinary precipitated silica (150-200 μg). 2 / g, 2.0-2.5 cm 3 / g), reaching a medium level of fumed silica but with significantly reduced costs.

[0028] 2. High silicon recovery rate and thorough impurity removal: Total silicon recovery rate > 92%, fluorine removal rate > 98%, and product purity ≥ 98.7%.

[0029] 3. The process is continuous and environmentally friendly: the mother liquor can be recycled throughout the process, and the wastewater discharge is reduced by more than 50% compared with the traditional process; n-butanol can be partially recovered and reused in spray drying.

[0030] 4. Industrialization verified: After 30 days of continuous operation, the product quality is stable (BET RSD≤2.4%, DBP RSD≤2.8%), and it is ready for large-scale promotion and application. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. All mentioned embodiments are implemented based on the technical solutions of the present invention, and detailed implementation processes are given. However, it should be stated that the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available reagents and materials; and unless otherwise specified, the room temperature or room temperature refers to 25±5℃.

[0032] Example 1

[0033] The steps for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, are as follows: Raw material: Silicon slag (wet basis) by-product of fluorosilicic acid to hydrogen fluoride production in a certain phosphate chemical enterprise. Composition: SiO2 31.2%, total fluorine 7.3%, moisture 59.8%, calcium 0.25%, magnesium 0.10%.

[0034] Step S1: Dry 500g of silica slag at 105℃ until the moisture content is ≤5%, and pulverize it through a 150-mesh sieve. Mix it with a 20% NaOH solution at a mass ratio of 1:2, and react in a 1L four-necked flask at 75℃ for 30 min for the first stage and at 100℃ for 15 min for the second stage.

[0035] Step S2: Dilute with 400mL of deionized water, age at 85℃ for 45min for the first stage, and then age at 50℃ for 30min for the second stage.

[0036] Step S3: Pressure filtration (0.5MPa) yields 950mL of sodium silicate filtrate with a SiO2 concentration of 6.8% and a modulus of 2.6.

[0037] Step S4: Composite dispersant formulation: 8g sodium polyacrylate, 1.5g Tween-80, 1.0g polyether-modified polysiloxane, diluted with water to 60mL. Add 12mL of this dispersant to the filtrate.

[0038] Step S5: Heat to 85℃ and add 15% dilute sulfuric acid dropwise at 100 mL / min. Control the pH to 9.8±0.1 in the first stage (reaction 15 min), the pH to 7.2±0.1 in the second stage (20 min), and the pH to 5.8±0.1 in the third stage (12 min).

[0039] Step S6: Aging at 60℃ for 2 hours, pressure filtration, washing the filter cake with 1% dilute sulfuric acid at 50℃ for 20 minutes at a flow rate of 80 mL / min, and washing with deionized water until pH ≥ 6.0.

[0040] Step S7: Break up the filter cake, add 1.5g n-butanol (1.5% on a dry basis), and spray dry (inlet air 260℃, outlet air 100℃, pressure 2.0MPa).

[0041] Step S8: Airflow pulverization and sieving.

[0042] Product: BET 358.4 m 2 / g, DBP 3.02 cm 3 / g, purity 99.1%, whiteness 95.2, D50=7.5μm, silicon recovery rate 94.1%.

[0043] Example 2

[0044] This embodiment is a pilot-scale amplification experiment based on Example 1: Three batches of silicon slag (50 kg / batch) were processed. Equipment included a 100L reactor, a plate and frame filter press, a 200L sedimentation reactor, and a small spray drying tower. Process parameters were as described in Example 1.

[0045] Average product: BET 341.7 m 2 / g, DBP 2.91 cm 3 / g, purity 98.9%, whiteness 94.8%, D50=8.5μm, RSD less than 2%. Silicon recovery rate 93.5%, fluoride removal rate 98.5%. Wastewater discharge is 2.2L / kg of product, a 45% reduction compared to traditional processes.

[0046] Example 3

[0047] This embodiment is a continuous production verification test based on Embodiment 1: 1. Raw materials and equipment Raw materials: Silicon slag (wet basis) is a stable by-product of the anhydrous hydrogen fluoride production line of a large phosphate chemical enterprise producing fluorosilicic acid. The daily output is about 12 tons, and the composition is stable: SiO2 30-32 wt%, total fluorine 7.0-7.5 wt%, moisture 59-61 wt%, calcium ≤0.3 wt%, magnesium ≤0.1 wt%.

[0048] Production equipment configuration: Table 1 Summary Table of Production Unit Configuration

[0049] 2. Method and Flow (1) Raw material pretreatment Wet silica slag is fed into a rotary dryer via a bucket elevator and dried to a moisture content of ≤5% using waste heat flue gas, with an outlet temperature of approximately 90°C. After drying, the silica slag is pulverized to below 150 mesh (≤106 μm) by a ball mill and then stored in a silica slag silo.

[0050] (2) Graded alkaline dissolution activation Pretreated silica slag and 20 wt% sodium hydroxide solution were added to reactor #1 at a mass ratio of 1:2. The mixture was heated to 75±2℃ and stirred for 30 min to complete the first-stage activation. The slurry was then pumped into reactor #2, heated to 100±2℃, and stirred for another 15 min to complete the second-stage activation.

[0051] (3) tiered aging Add dilution water (volume ratio 1:0.8) to reactor #2, keep it at 90±2℃ for 40 min, then cool it down to 50±2℃ through a heat exchanger and continue aging for 30 min.

[0052] (4) Pressure filtration separation The aged slurry is pumped into a plate and frame filter press, and the filtration pressure is controlled at 0.45~0.55 MPa. The filtrate (sodium silicate solution) is pumped into a filtrate storage tank for later use. The filter residue (unreacted residue, accounting for 8-10% of the raw material dry basis) is cleaned regularly and returned to the front end to be mixed with fresh silica slag for treatment.

[0053] (5) Formulation and addition of composite dispersants A composite dispersant was prepared in a dispersant preparation tank at a mass ratio of BYK190 (aqueous polymeric dispersant): Tween-80 (nonionic surfactant): polyether-modified polysiloxane (organosilicon dispersant and stabilizer) = 8:1.5:1, and diluted with water to a mass concentration of 15 wt%. This dispersant was then continuously added to the sodium silicate filtrate using a metering pump at a rate of 1.2% of the filtrate volume.

[0054] (6) Three-stage pH-fractionated precipitation reaction The sodium silicate filtrate, after being preheated to 85±2℃, is sequentially fed into three series-connected precipitation reactors. The DCS system monitors the pH value in real time and automatically controls the dropping rate of 15 wt% dilute sulfuric acid. Reactor #1 (nucleation stage): pH controlled at 9.8±0.1, residence time 15 min, forming a large number of uniform fine crystal nuclei (5~10 nm).

[0055] Reactor #2 (Growth Stage): pH controlled at 7.2±0.1, residence time 20 min, crystal nuclei grow into loose and porous silica precursor (20-50 nm).

[0056] Reactor #3 (complete precipitation and defluorination stage): pH controlled at 5.8±0.1, residence time 15 min, complete precipitation, and simultaneous removal of fluoride ions. The outlet slurry pH is stable at 5.5~6.0.

[0057] (7) Aging The precipitated slurry was pumped into an aging tank and aged at 60±2℃ for 2 hours with intermittent stirring.

[0058] (8) Washing and filtration After aging, the slurry is fed into a belt vacuum filter: First-stage washing: Use 0.8 wt% dilute sulfuric acid for spray washing. After collection, part of the washing solution is returned to the dilute sulfuric acid preparation tank for reuse.

[0059] Secondary washing: The filter cake is washed with deionized water until its pH reaches ≥ 6.0. After neutralization, part of the washing liquid is reused as process makeup water, and the rest is discharged into the wastewater treatment system after meeting discharge standards.

[0060] (9) Integration of drying and surface modification After washing, the filter cake (moisture content 65-70%) is conveyed to a dispersant via a screw conveyor, while n-butanol is atomized and sprayed at a ratio of 2% by dry weight. The dispersed material then enters a spray drying tower: inlet air temperature 260±10℃, outlet air temperature 100±5℃, spray pressure 2.2 MPa. The dried product is collected in two stages: a cyclone separator and a bag filter.

[0061] (10) Crushing and Packaging The collected coarse silica powder was pulverized to D50=7~10 μm by an air jet mill and then packaged into 20kg / bags by an automatic packaging machine.

[0062] 3. Production operation data After 30 days of continuous and stable operation, it processes an average of about 12 tons of wet-based silica slag per day and produces about 2.6-2.8 tons of silica products per day.

[0063] 4. Product quality indicators (30-day average) Table 2 Summary of Product Quality Indicators

[0064] 5. Process control and stability The DCS system automatically controls key processes such as fractional alkali dissolution (temperature, residence time) and fractional precipitation (pH, temperature, residence time), keeping process parameter fluctuations within ±2% of the set values. During 30 days of continuous production, product quality indicators showed minimal fluctuations: BET specific surface area RSD = 2.4%, DBP absorbance RSD = 2.8%, and good batch-to-batch consistency, meeting the quality stability requirements for industrial-grade silica.

[0065] 6. Material and energy consumption (per ton of product) Table 3 Summary of Material and Energy Consumption

[0066] 7. Resource utilization and environmental benefits The comprehensive recovery rate of silicon resources is 92-94%, and approximately 800-850 tons of precipitated silica can be recovered annually (based on 12 tons / day of silicon slag).

[0067] Fluorine resources: About 98% of the fluorine in silicon slag enters the precipitation mother liquor. After neutralization, evaporation and concentration, sodium fluoride / ammonium fluoride products can be recovered from the mother liquor, realizing the secondary utilization of fluorine resources.

[0068] Wastewater discharge: approximately 1.5~2.0 tons / ton of product, which is more than 50% less than that of traditional processes. The wastewater mainly comes from the washing process and is discharged in compliance with standards after neutralization and sedimentation treatment.

[0069] Mother liquor recycling: Sodium silicate filter press mother liquor is completely returned to the system for reuse, washing mother liquor is partially reused, and dispersant recovery rate is about 80%.

[0070] 8. Economic Benefit Estimation (Based on an annual production capacity of 250 tons of silica) Table 4 Summary of Economic Benefit Estimates

[0071] 9. Prospects for Industrial Application This method has been validated in 30 days of continuous industrial production at a large-scale phosphate chemical enterprise, demonstrating stable product quality, reliable process, and economic feasibility, thus meeting the conditions for large-scale promotion and application. This complete set of technologies can be extended to phosphate and fluorochemical enterprises nationwide, and is of great significance for solving the environmental problems associated with the storage of fluorinated silicon slag and achieving dual recovery of silicon and fluorine resources.

[0072] Comparative Example 1 Compared with Example 1, the comparative example uses the same raw materials as Example 1, but the two-stage temperature gradient is eliminated, and the mixture is directly dissolved in alkali at 100°C for 45 minutes (the total time is the same as in Example 1). The remaining steps are the same as in Example 1.

[0073] Results: The SiO2 concentration in the sodium silicate filtrate was 5.4%, the modulus was 2.3, and the silicon recovery rate was 78.6%. Product BET 215.3 m 2 / g, DBP 2.12 cm 3 / g, purity 96.2%, whiteness 91.5. This indicates that graded alkali dissolution activation significantly improved silicon dissolution rate and product quality.

[0074] Comparative Example 2 Compared with Example 1, this comparative example is the same as Example 1 except that the tiered aging process is cancelled and replaced with aging at 85°C for 75 minutes (the total time is the same).

[0075] Results: The calcium and magnesium ion content in the sodium silicate filtrate was approximately 40% higher than in Example 1, and the content of metal impurities in the product was increased, BET 286.4 m 2 / g, DBP 2.43 cm 3 / g, purity 97.1%, whiteness 92.8. This indicates that the step-age process effectively removed metallic impurities, which is beneficial for improving the purity and specific surface area of ​​the silica.

[0076] Comparative Example 3 Compared with Example 1, this comparative example is the same as Example 1 except that step S4 (without adding the composite dispersant) is omitted.

[0077] Results: Significant aggregation occurred during the precipitation process, with a product BET of 208.7 m. 2 / g, DBP 1.95 cm 3 The particle size was uneven (D50=15.6μm), indicating poor dispersibility. This demonstrates that composite dispersants are crucial for inhibiting agglomeration, improving specific surface area, and enhancing dispersibility.

[0078] Comparative Example 4 Compared with Example 1, this comparative example is the same as Example 1 except that the three-stage pH precipitation was changed to the direct addition of dilute sulfuric acid to the endpoint pH=6.0 (one-time precipitation).

[0079] Result: Product BET 245.3 m 2 / g, DBP 2.18 cm 3 / g, D50=12.4μm, with a specific surface area and dispersibility significantly lower than that of the three-stage pH precipitation (Example 1). This indicates that three-stage pH fractional precipitation is key to obtaining a high specific surface area.

[0080] Comparative Example 5 This comparative example simulates existing technology, using the same silicon slag as in Example 1, and the following steps are taken: (1) Alkali dissolution: The most common single-stage alkali dissolution method in the existing technology is adopted (100℃, 45min).

[0081] (2) Aging: Single-stage aging (85℃, 75min).

[0082] (3) After solid-liquid separation, the remaining steps are completely in accordance with the scheme of Example 1 of this invention (i.e., using the best ratio of composite dispersant and performing three-stage pH-graded precipitation, integrated drying, etc.).

[0083] Result: The product's BET specific surface area is only 261.5 m². 2 / g, DBP absorbance value is 2.31 cm⁻¹ 3 / g, product purity 97.2%, silicon recovery rate 80.5%.

[0084] Conclusion and Analysis: Although the core "composite dispersant + three-stage pH fractional precipitation" scheme of this invention was subsequently adopted, insufficient pre-treatment alkali dissolution activation (single-stage) and ineffective aging for impurity removal (single-stage) resulted in low silicate ion activity, a wide activity distribution, and a high content of Ca in the sodium silicate filtrate. 2+ Mg 2+ Al 3+ Impurities such as metal ions can disrupt the synergistic environment of the dispersant during subsequent fractional precipitation (e.g., metal ions can complex the dispersant or cause premature local aggregation), preventing the efficient formation of mesoporous guiding templates in the second stage. Consequently, the final product performance is far lower than the 330-360 of this invention. This strongly demonstrates a strong synergistic dependency between the "graded activation-step aging" and "fragmented precipitation-composite dispersant" processes of this invention, rather than a simple additive relationship.

[0085] Comparative Example 6 Compared with Example 1, this comparative example changed the spray drying + n-butanol integration method to first drying in an oven (105°C, 4h), then soaking and modifying in n-butanol for 2h, followed by filtration and low-temperature drying.

[0086] Product: BET 320.8 m 2 / g, DBP 2.79 cm 3 / g, but the modification was uneven; some areas of the silica were hydrophobic while others were hydrophilic, resulting in poor flowability. This indicates that integrated spray drying modification is more uniform and simplifies the process.

[0087] Summary of technical effects By comparing the data from the above embodiments with those from the comparative examples, the following conclusions can be drawn: The staged alkali dissolution activation + stepped aging combination of the present invention, compared with single-stage alkali dissolution + single-stage aging (Comparative Example 1+2), increases silicon recovery by approximately 14 percentage points and product BET by approximately 50 m. 2 / g.

[0088] The three-stage pH fractionation precipitation and the composite dispersant exhibit a significant synergistic effect: when using the three-stage pH fractionation precipitation alone (without dispersant), the BET is only 208.7 m. 2 / g; When using the dispersant alone (single-stage precipitation), BET is only 245.3 m. 2 / g; the combined BET reaches 358.4 m 2 / g is greater than the sum of the individual effects of the two, proving that there is synergy.

[0089] The pretreatment (graded activation + stepped aging) and the post-processing core process (segmented precipitation + composite dispersant) have a strong synergistic dependency relationship and neither can be omitted (Comparative Example 5 vs Example 1).

[0090] Compared to existing technologies for preparing silica from fluorinated silica slag, this application achieves BET ≥ 330 m for the first time. 2 / g and DBP≥2.85 cm 3 The high performance index of / g has been verified in industrial applications. This invention achieves high specific surface area and high dispersibility of silica through the dynamic synergy and precise control of three factors: silicate ion supersaturation, dispersant molecular conformation, and pH environment. Specifically: 1. The pre-conditioning effect of graded alkali dissolution activation and stepped aging: The first step, low-temperature alkali dissolution, controls the slow release of silicate ions, avoiding the formation of dense silica gel; the second step, high-temperature complete dissolution. Subsequent stepped aging yields a high-purity (metal ion ≤0.05%) silicate ion cluster with a narrow activity distribution, laying a molecular-level foundation for precise control of nucleation and growth.

[0091] 2. Synergistic effect of three-stage pH fractionation precipitation and composite dispersant: The first stage (pH 9.0~10.0): The polymeric dispersant and the nonionic surfactant form mixed micelles that adsorb onto the newly generated negatively charged crystal nuclei, creating a dual barrier of "steric hindrance and electrostatic repulsion" that restricts the growth of the crystal nuclei to 5~10nm.

[0092] The second stage (pH 7.0~8.0): Organosilicon dispersants and stabilizers preferentially anchor to "active defect sites" on the particle surface through hydrogen bonds or Si-O-Si bonds, forming nanoscale "hydrophobic templates". During the polymerization of silicate ions, they are forced to bypass these templates, inducing the generation of a large number of interconnected mesopores (pore size 2~10nm) in situ.

[0093] The third stage (pH 5.5~6.5): The low pH environment causes the residual component A to desorb from the particle surface, while promoting the entry of soluble metal ions and fluoride ions into the liquid phase, thus achieving high product purity.

[0094] 3. Spray drying-surface modification integration for "shaping and enhancement" of dispersibility: n-Butanol acts as both an azeotropic agent and a modifier in spray drying. It undergoes a gas-solid phase esterification reaction with the silanol on the surface of silica at high temperature, forming a uniform and dense hydrophobic layer, which effectively prevents hard agglomeration during drying and subsequent pulverization.

[0095] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, characterized in that, Includes the following steps: S1, the silicon slag, a byproduct of the production of hydrogen fluoride from fluorosilicic acid, is mixed with an alkaline solution and subjected to an alkaline dissolution reaction to obtain a reaction slurry; the reaction slurry is then diluted, aged, and subjected to solid-liquid separation to obtain a filter cake and sodium silicate filtrate; S2, a composite dispersant is first added to the sodium silicate filtrate described in S1 to form a mixed solution, and then an acid solution is added to adjust the pH range of the mixed solution to carry out a precipitation reaction to obtain a precipitated slurry; after aging and solid-liquid separation, the precipitated slurry is used to obtain a crude silica filter cake. S3, after washing, drying and pulverizing the crude silica filter cake described in S2, the high specific surface area and high dispersion silica is obtained.

2. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... The silicon slag produced as a byproduct of the fluorosilicic acid-to-hydrogen fluoride production process in S1 has a particle size ≤150 mesh and a moisture content ≤5%; the mass ratio of the silicon slag produced as a byproduct of the fluorosilicic acid-to-hydrogen fluoride production process to the alkaline solution is 1:(1.5~2.5), and the alkaline solution includes a sodium hydroxide solution with a mass concentration of 15~25 wt%; the reaction slurry is diluted with water as the diluent, and the volume ratio of the reaction slurry to water is 1:(0.5~1).

3. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... The alkali dissolution reaction described in S1 is controlled by a two-stage temperature gradient, which includes a first-stage control and a second-stage control. The reaction temperature of the first-stage control is 60~80℃ and the reaction time is 20~40min. The reaction temperature of the second-stage control is 95~105℃ and the reaction time is 10~20min.

4. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... The aging process described in S1 employs a stepped aging process, which includes a first-stage aging process and a second-stage aging process. The temperature of the first-stage aging process is 70~90℃, and the holding time is 30~60min. The temperature of the second-stage aging process is 40~60℃, and the holding time is 20~40min.

5. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... The solid-liquid separation described in S1 is performed by pressure filtration, the pressure of which is 0.4~0.6MPa, and the moisture content of the filter cake is ≤35%; the concentration of SiO2 in the sodium silicate filtrate is 6~9wt%, and the molar ratio of SiO2 to Na2O in the sodium silicate is (2.4~2.8):

1.

6. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... The amount of the composite dispersant in S2 is 0.5-2% of the volume of the sodium silicate filtrate; the composite dispersant is compounded from an aqueous polymeric dispersant, a nonionic surfactant, and an organosilicon dispersant stabilizer in a mass ratio of (6-9):(1-2):(0.8-1.2); the aqueous polymeric dispersant has a weight-average molecular weight of 2000-10000, including any one or more of sodium polyacrylate, ammonium polycarboxylate, and sodium maleic acid-acrylic acid copolymer; the nonionic surfactant has an HLB value of 10-15, including any one or more of Tween-80, Tween-60, Span-80, and alkylphenol polyoxyethylene ether; the organosilicon dispersant stabilizer includes any one or more of polyether-modified polysiloxane, polydimethylsiloxane emulsion, and silane coupling agent.

7. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... In step S2, the mixture is first heated to 75-95°C, and then acid solution is added at a rate of 50-150 mL / min. The acid solution is a dilute sulfuric acid solution with a concentration of 10-25 wt%. The pH range includes three segments: the first segment has a pH of 9.0-10.0 and a residence time of 10-20 min; the second segment has a pH of 7.0-8.0 and a residence time of 15-25 min; and the third segment has a pH of 5.5-6.5 and a residence time of 5-15 min.

8. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... The aging temperature of the precipitated slurry described in S2 is 50~70℃ and the time is 1~3h.

9. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... The washing described in S3 adopts a two-stage acidification washing method. The first stage of the two-stage acidification washing uses a 0.5~2wt% dilute sulfuric acid solution to wash at 40~60℃ for 15~30min. The second stage involves washing with deionized water until the pH of the washing solution is ≥6.

0.

10. The method for preparing high specific surface area and highly dispersed silica using silicon slag, a byproduct of hydrogen fluoride production from fluorosilicic acid, according to claim 1, is characterized in that... The crude silica filter cake described in S3 is washed to obtain silica filter cake. After the silica filter cake is broken up, 1-3% of n-butanol (by dry basis silica mass) is added for treatment, and then spray drying is carried out. The inlet air temperature of the spray drying is 220-280℃, the outlet air temperature is 90-110℃, and the spray pressure is 1.5-2.5MPa.

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