Method and device for preparing high-purity calcium fluoride and white carbon black from silicon tetrafluoride waste gas

Abstract

The invention relates to a waste gas resourceful treatment technology, and discloses a method and device for preparing high-purity calcium fluoride and white carbon black from silicon tetrafluoride waste gas, and the method comprises the following steps: introducing silicon tetrafluoride gas into an absorption tower for absorption reaction, and introducing recycled ammonia gas and high-content ammonia water at the same time to generate a silicic acid-containing solution; conveying the obtained solution into an aging tank for aging, and separating by a filter press to obtain a white carbon black product and filtrate; introducing the filtrate into an ammoniation defluorination crystallization reactor, and separating out and crystallizing the fluorine element through ammoniation reaction; the crystals are collected into a crystal collecting tank and conveyed to a dehydrator for solid-liquid separation, and high-purity calcium fluoride crystals are obtained; by means of the integrated design, silicon tetrafluoride resource utilization and fluorine element high-value byproduct production are achieved, the technological process is simple, the product purity is high, and good industrial application value is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to waste gas resource treatment technology, in particular to a process method and device for preparing high-purity calcium fluoride and white carbon black from phosphorus chemical silicon tetrafluoride waste gas. BACKGROUND

[0002] The main component of phosphate rock is calcium fluorophosphate, and the fluorine content is about 3%. The fluorine resource reserves of phosphate rock account for 94% of the global fluorine resource reserves. China's phosphate rock reserves are 3.1 billion tons, ranking second in the world. China's phosphate fertilizer and phosphoric acid production ranks first in the world. In 2019, the production of wet-process phosphoric acid was 15.5 million tons. In the production process of wet-process phosphoric acid, sulfuric acid is used to decompose phosphate rock to produce phosphoric acid, and hydrogen fluoride gas is generated at the same time. The generated HF reacts with the silicon dioxide (SiO2) impurities in the phosphate rock to generate gaseous silicon tetrafluoride and water. About 50% of the fluorine in the phosphate rock is discharged as silicon tetrafluoride gas. The chemical equation is as follows: Ca5F(PO4)3 + 5H2SO4+ 10H2O → 3H3PO4 + 5CaSO4·2H2O + HF ; 4HF + SiO2 → SiF4 + 2H2O.

[0003] If these SiF4-containing waste gases are not effectively treated and directly discharged, it will cause serious harm to the environment and human health: such as forming smog air pollution and corrosive, toxic acid rain containing fluorine pollution stronger than ordinary sulfuric acid type acid rain; causing fluorine poisoning of animals and plants; damaging the respiratory system, bones and teeth of the human body; causing corrosion to buildings and materials.

[0004] In order to reduce silicon tetrafluoride gas pollution and realize resource utilization, water absorption method is usually used to process into fluorosilicic acid, and then through fluorosilicic acid to process into various products. At present, fluorosilicic acid has developed processes such as fluorosilicate, cryolite, anhydrous hydrofluoric acid, etc. Among them, the technology of preparing fluorosilicate from fluorosilicic acid is mature, but the product has low added value, and a large amount of hydrochloric acid or sulfuric acid waste liquid is generated in the production process, causing secondary pollution and increasing the comprehensive treatment cost. The cryolite technology route is mature and the demand is stable, but the process cost is high and the economy is poor. The preparation of anhydrous hydrofluoric acid product has high added value, but the investment is huge, the equipment is severely corroded, and the requirements for equipment material and process control are extremely high, at the same time, the energy consumption is large, and the conversion rate of fluorine is low.

[0005] In recent years, the fluorine chemical industry has focused on producing hydrofluoric acid from fluorite, and the consumption of fluorite for fluorine chemical industry accounts for about 50% of the total annual consumption. The fluorite resources in nature are gradually depleted, and the purity and particle size specifications of fluorite raw materials for hydrofluoric acid preparation are very high, so the preparation of high-purity artificial fluorite that can be used for hydrofluoric acid production has become a key way to solve the problem of silicon tetrafluoride gas. Among them, the ammonolysis of fluorosilicic acid is an important means of separating fluorine and silicon.

[0006] Patent CN 108025923 B discloses a method for preparing calcium fluoride from fluosilicic acid, comprising the following steps: (1) first reacting fluosilicic acid with ammonium hydroxide or ammonia to obtain a first concentrated suspension; filtering the first suspension to obtain a filtrate containing an ammonium fluoride solution; (2) then precipitating the ammonium fluoride solution obtained in step (1) as a filtrate with calcium carbonate to produce a second suspension containing calcium fluoride and ammonium carbonate, wherein the calcium carbonate is in dry form or is in the form of a suspension with a concentration of between 10 and 80% by weight; filtering the second suspension to obtain a filter cake containing calcium fluoride and a filtrate containing an ammonium carbonate solution; washing and drying the filter cake to obtain calcium fluoride and a filter cake wash liquid containing an ammonium hydroxide solution; wherein 10 to 70% of the second suspension is recycled to enhance the crystallization of calcium fluoride; due to the partial decomposition of ammonium carbonate under reactor conditions, a portion of the ammonia from the second reactor in step (2) is converted, which is then purified and returned to the first reactor, and the ammonium carbonate solution obtained as a filtrate and filter cake wash liquid in step (2) is collected and processed by distillation and condensation to recover the liquid ammonia recycled to the first reactor.

[0007] The present invention is a process for preparing calcium fluoride from fluosilicic acid, ammonia water and calcium carbonate. Fluosilicic acid is ammoniated to produce ammonium fluoride and silicon dioxide, and ammonium fluoride is reacted with calcium carbonate to produce calcium fluoride. However, calcium carbonate is a solid powder, which is difficult to dissolve, has a slow reaction rate, and the reaction is not complete. The purity of calcium fluoride is insufficient due to the residual raw materials in the reaction product. In addition, appropriate heating is required during the crystallization process, which increases the preparation cost. In the production process of the preparation of calcium fluoride, ammonia gas and carbon dioxide gas are simultaneously produced, which is difficult to separate, affecting the reuse of ammonia.

[0008] Patent CN102795601A discloses a method for producing anhydrous hydrogen fluoride and co-producing white carbon black from fluosilicic acid. The invention ammoniates the fluosilicic acid solution byproduct of phosphate fertilizer to obtain an ammonium fluoride solution and silicon dioxide. The filter cake is washed and dried to obtain white carbon black product. The filtrate ammonium fluoride solution is used for the next step of production. The ammonium fluoride solution is concentrated and decomposed at high temperature to obtain ammonium hydrogen fluoride solution and ammonia gas. The ammonium hydrogen fluoride solution is used for the next step of production, and the ammonia gas is used for the ammonolysis of the fluosilicic acid solution. This technical route can realize the reuse of ammonia gas, but the preparation of ammonia gas by recovery requires heating and high-temperature decomposition, which has high equipment investment and high operating cost, and has certain safety risks.

[0009] In summary, the existing technology for preparing calcium fluoride from fluosilicic acid involves adding ammonium hydroxide or ammonia and calcium carbonate to prepare calcium fluoride crystals, but there are still problems of difficult dissolution of reagents, incomplete reaction, high cost, and secondary pollution. In addition, in the existing technology, the preparation of ammonia gas by recovery requires heating and high-temperature decomposition, which has high equipment investment and high operating cost, and has certain safety risks.

[0010] The purpose of this application is to provide a novel process for the resource-based preparation of high-purity calcium fluoride and silica from silicon tetrafluoride gas produced in the phosphate chemical industry. This process effectively recovers silicon tetrafluoride gas while producing high-purity usable silica and calcium fluoride products. Furthermore, it enables the recycling of ammonia generated during the reaction process. Summary of the Invention

[0011] The purpose of this invention is to provide a novel process for the resource-based preparation of high-purity calcium fluoride and silica from silicon tetrafluoride gas produced in the phosphorus chemical industry. This process effectively recovers silicon tetrafluoride gas while producing high-purity, usable silica and calcium fluoride products. Furthermore, it enables the recycling of ammonia generated during the reaction process.

[0012] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing high-purity calcium fluoride and silica from silicon tetrafluoride waste gas in a phosphorus chemical industry, characterized by comprising the following steps: (1) Silicon tetrafluoride gas generated during the phosphorus chemical production process is introduced into the absorption tower, and recycled ammonia gas and high ammonia water are introduced at the same time. The absorption tower is filled with Pall rings or Raschig rings. The pH of the water collection pool at the bottom of the absorption tower is controlled between 6.5 and 7.5. The absorption is assisted by a circulating pump. Under normal temperature conditions, the silicon tetrafluoride gas is absorbed and converted into fluorosilicic acid solution and the ammonia neutralization reaction is completed to generate silicon dioxide precipitate. (2) Send the water obtained in step (1) into the aging tank and stir for 5 to 15 minutes to allow the generated silica to fully precipitate and ensure the quality of the silica. (3) The solid-liquid mixture obtained in step (2) is subjected to pressure filtration. The obtained filter cake is washed and dried to obtain white carbon black product. The filtrate enters the filtrate tank, where the silicon removal rate is ≥99% and the main component of the filtrate is ammonium fluoride. (4) The filtrate obtained in step (3) is sent into the ammoniation defluorination crystallization reactor. While stirring rapidly, 2% to 5% lime solution is added. The calcium-fluoride molar ratio is controlled at 0.46 to 0.52, the hydraulic retention time is 60 to 150 min, and 1% to 10% calcium fluoride seed crystals are added. The induced crystallization reaction is carried out under a negative pressure environment of -300 to -500 Pa, so that fluoride ions are converted into calcium fluoride precipitate and ammonia gas is released. The effluent from the ammoniation defluorination crystallization reactor enters the intermediate tank. (5) Collect the solution containing large-particle calcium fluoride crystals at the bottom of the reactor in step (4) into a crystal collection tank. After dehydration, washing and drying, high-purity calcium fluoride crystals with a purity ≥98%, an average particle size ≥20 μm and a silicon content ≤0.1% are obtained. The dehydrated filtrate enters the intermediate tank. (6) The ammonia gas generated in steps (4) and (5) is collected by the induced draft fan and returned to the absorption tower in step (1). The ammonia water formed in the intermediate pool is returned to the absorption tower to realize the recycling of ammonia.

[0013] Preferably, the pH value in the bottom water collection tank of the absorption tower in step (1) is linked with the intermediate tank reflux pump to achieve automatic adjustment.

[0014] Preferably, the power of the stirring device in the aging tank in step (2) is 100-200W / m³.

[0015] Preferably, the purity of the silica product in step (3) is ≥99.8%.

[0016] Preferably, the fluorine recovery rate in the ammoniation defluorination crystallization reactor described in step (4) is ≥99%.

[0017] Preferably, the dehydration in step (5) can be performed using a centrifuge or a filter press.

[0018] An apparatus for preparing high-purity calcium fluoride and silica from silicon tetrafluoride waste gas in the phosphorus chemical industry, characterized in that it comprises: An absorption tower is used to contact silicon tetrafluoride waste gas with recycled ammonia gas and recycled ammonia-containing water to generate a fluorosilicic acid solution and generate silicon dioxide precipitate. An aging tank, connected to the downstream of the absorption tower, is used to stir and age the water effluent from the bottom of the absorption tower to promote the full precipitation of silica. A filter press, connected to the aging tank, is used to perform solid-liquid separation of the aging products to obtain silica filter cake and filtrate; The filtrate tank is used to collect the filtrate from the filter press and quantitatively feed the filtrate into the ammoniation defluorination crystallization reactor. The ammoniation defluorination crystallization reactor is connected to the filtrate tank and the lime solution tank. It is used to convert fluoride ions in the filtrate into calcium fluoride crystals and release ammonia gas under stirring and seed induction. The effluent from the ammoniation defluorination crystallization reactor enters the intermediate tank. A crystal collection tank, connected to the ammoniation defluorination crystallization reactor, is used to collect the calcium fluoride slurry obtained from crystallization; A dehydrator, connected to the crystal collection tank, is used to dehydrate, wash and dry the crystal slurry to produce high-purity calcium fluoride crystals, and the dehydrated filtrate enters the intermediate tank. An intermediate tank is located between the ammoniation defluorination crystallization reactor and the absorption tower. It is used to collect free ammonia from the effluent of the ammoniation defluorination crystallization reactor and the dewatering filtrate of the dewatering machine, and to return the ammonia water to the absorption tower. And a negative pressure suction and conveying pipeline for gas recovery, used to collect ammonia and return the recovered ammonia and ammonia water to the absorption tower.

[0019] Preferably, the absorption tower is a packed tower structure, and the tower is equipped with Pall rings or Raschig rings, a circulating pump, and an online pH sensor.

[0020] Preferably, the pH sensor is linked to the intermediate tank circulation pump to maintain the pH at the bottom of the absorption tower at 6.5 to 7.5.

[0021] Preferably, the ammoniation defluorination crystallization reactor is equipped with a rapid stirring device, a seed crystal addition port, a lime solution inlet, and a negative pressure exhaust port; the lime solution tank provides the crystallization reactor with a lime solution of 2% to 5% concentration to control the calcium-fluoride molar ratio at 0.46 to 0.52; after dehydration, washing, and drying, high-purity calcium fluoride crystals with a purity ≥98%, an average particle size ≥20 μm, and a silicon content ≤0.1% are obtained.

[0022] Compared with the prior art, the present invention has the following beneficial effects: The waste gas from silicon tetrafluoride in the phosphorus chemical industry is treated through processes such as absorption, silicon removal from ammonia, and fluorine removal by calcium addition. The concentrations of fluorine and silicon are significantly reduced. The recovery rate of silicon in the fluorosilicic acid gas is over 99%, and the recovery rate of fluorine is over 99%, thus solving the pollution problem of silicon tetrafluoride waste gas. While solving the problems of fluorine and silicon pollution, it can obtain high-purity calcium fluoride and co-produce precipitated silica.

[0023] By using lime to remove fluoride combined with induced crystallization, fluoride in wastewater is recovered in the form of calcium fluoride crystals, forming large-particle high-purity calcium fluoride with an average particle size of more than 20μm, which improves product quality and meets the requirements for raw materials for hydrofluoric acid preparation. The ammonia gas and ammonia water gas-liquid dual circulation process is used to react with silicon tetrafluoride gas in the absorption tower. The one-step process is simple, requires less investment, and has low operating costs. It breaks through the bottleneck of high temperature and high energy consumption in the existing ammonia recovery process and reduces the risk of environmental pollution. Attached Figure Description

[0024] Figure 1 This is a frame diagram of an apparatus for preparing high-purity calcium fluoride and silica from silicon tetrafluoride waste gas in the phosphorus chemical industry. Figure 2 This is a process flow for preparing high-purity calcium fluoride and silica from silicon tetrafluoride waste gas in the phosphorus chemical industry. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0026] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0027] The purpose of this invention is to provide a novel process for the resource-based preparation of high-purity calcium fluoride and silica from silicon tetrafluoride gas produced in the phosphorus chemical industry. This process effectively recovers silicon tetrafluoride gas while producing high-purity, usable silica and calcium fluoride products. Furthermore, it enables the recycling of ammonia generated during the reaction process.

[0028] Specifically: Silicon tetrafluoride gas is introduced into an absorption tower for absorption and reaction, while recycled ammonia and high-ammonia water are introduced simultaneously to generate a silicic acid solution; the resulting solution is transported to an aging tank for aging, and then separated by a filter press to obtain silica product and filtrate; the filtrate is introduced into an ammoniation defluorination crystallization reactor, where fluorine crystals are precipitated through an ammoniation reaction; the crystals are collected in a crystal collection tank and transported to a dehydrator for solid-liquid separation to obtain high-purity calcium fluoride crystals; the by-product liquid flows into an intermediate tank, and after adjustment, it is recycled or discharged. This invention, through integrated design, realizes the resource utilization of silicon tetrafluoride and the high-value by-product of fluorine, with a simple process flow, high product purity, and good industrial application value.

[0029] The reaction equations for the entire process are as follows: SiF4+ H2O → SiO2·H2O + H2SiF6; H2SiF6.2H2O + 6NH4OH → 6NH4F + SiO2↓ + 6H2O; 2NH4F + Ca(OH)2 → CaF2↓ + 2NH3↑ + 2H2O.

[0030] See appendix Figures 1-2 The specific implementation steps of the process flow of the present invention are as follows: A method for preparing high-purity calcium fluoride and silica from silicon tetrafluoride waste gas in a phosphorus chemical industry, characterized by comprising the following steps: (1) Silicon tetrafluoride gas generated during the phosphorus chemical production process is introduced into the absorption tower, and recycled ammonia gas and high ammonia water are introduced at the same time. The absorption tower is filled with Pall rings or Raschig rings. The pH of the water collection pool at the bottom of the absorption tower is controlled between 6.5 and 7.5. The absorption is assisted by a circulating pump. Under normal temperature conditions, the silicon tetrafluoride gas is absorbed and converted into fluorosilicic acid solution and the ammonia neutralization reaction is completed to generate silicon dioxide precipitate.

[0031] Specifically, the waste gas containing silicon tetrafluoride (SiF4) generated from phosphorus chemical industry is absorbed and neutralized simultaneously by gas-liquid contact with recovered ammonia / high ammonia water under normal temperature conditions; thus, the silicon tetrafluoride is efficiently absorbed and hydrolyzed to generate fluorosilicic acid and primary silica precipitates.

[0032] Specifically, the silicon tetrafluoride waste gas is introduced into the lower part or middle section of the absorption tower according to the designed air volume (depending on the tower type); simultaneously, recycled ammonia gas and high-ammonia water are introduced into the top or middle section of the tower. The waste gas and absorbent liquid form a counter-current flow (gas rises, liquid falls), or a parallel flow / lateral replenishment of liquid, ensuring sufficient gas-liquid contact. The circulating pump in the water collection tank at the bottom of the absorption tower draws the absorbent liquid back to the top of the tower and distributes it evenly in the form of sprays / distributors, so as to fully wet the surface of the packing and maintain the gas-liquid film mass transfer with a high specific surface area. The circulating pump is often equipped with a frequency converter to realize the adjustment of the gas-liquid ratio according to the operating conditions (controlling the gas-liquid ratio is the key to ensuring high absorption efficiency).

[0033] Specifically, an online pH probe is installed in the bottom collection tank of the tower to maintain the pH at 6.5–7.5 (the specified value for this process). The pH signal is linked to the intermediate tank / reflux pump and the addition of ammonia water (or ammonia gas): when the pH is close to the lower limit (<6.5), it indicates that the system is too acidic, and it is necessary to increase the ammonia reflux flow rate or start the addition of ammonia water to increase alkalinity and promote precipitation; when the pH exceeds the upper limit (>7.5), it is necessary to reduce the reflux ammonia water and / or adjust the ammonia gas return to avoid the redissolution of silicon and excessive ammonia volatilization. The gas-liquid ratio and absorption intensity in the tower can be changed by adjusting the reflux pump flow rate (increase / decrease), thereby quickly adjusting the system pH.

[0034] Optionally, the process operates at room temperature. The SiF4 hydrolysis provides heat release, but the amount is generally small, making the temperature rise controllable. A temperature monitoring device and a bypass cooling device should be installed inside the tower. If the temperature is too high, the bypass cooling device should be turned on to cool the absorption tower in order to prevent it from affecting the packing or sedimentation behavior.

[0035] Specifically, SiF4 hydrolysis and subsequent ammonia neutralization occur inside the tower (or in the bottom circulation tank), and SiO2 precipitates in colloidal / aggregate form and flows back to the bottom collection tank with the liquid phase. The turbulence and packing film inside the tower promote rapid contact and reaction between SiF4 and water, improving the absorption and conversion rate of a single tower.

[0036] This step has the following technical effects: through the combination of absorption tower circulation and ammonia recovery, SiF4 is efficiently captured and converted, so that SiF4 is efficiently absorbed and converted into fluorosilicic acid at room temperature, reducing volatile fluorine emissions; the absorption tower and the downstream ammonia / ammonia water circulation (achieved by induced draft fan return and intermediate pool return) maximize the utilization of ammonia resources, reduce operating costs, reduce the amount of external ammonia added, and reduce the risk of ammonia discharge.

[0037] (2) Send the water obtained in step (1) into the aging tank and stir for 5 to 15 minutes to allow the generated silica to fully precipitate and ensure the quality of the silica.

[0038] Specifically, the effluent from the bottom of the absorption tower continuously or quantitatively enters the aging tank. The optimal hydraulic retention time is 5–15 minutes (which can be finely adjusted within this range when scaling up operations), and this is precisely achieved through intermediate tank / valve control and tank liquid level control.

[0039] Optionally, the aging tank adopts a two-stage stirring strategy: the initial stage (mixing stage, about 1–2 min): using a power density close to the upper limit (close to 200 W / m³) to quickly disperse the feed liquid, uniform pH and ionic strength, and prevent local over-acidity / over-alkalinity; the maturation stage (aging / flocculation stage, remaining time, about 3–14 min): reducing the stirring power (close to 100 W / m³) to create a mild shear field, which is conducive to colloidal polymerization, floc growth and formation of a structure strong enough to maintain its structure during the subsequent pressure filtration.

[0040] Specifically, the settled / flocculated material after aging is extracted by a bottom pump or static separation device and sent directly to the filter press. To avoid excessive aging or redispersibility of the flocs, the transport time from the aging tank to the filter press is ≤ 2 hours.

[0041] This step has the following technical effects: by using a two-stage stirring strategy, precipitated silicon with good particle size and high floc strength is generated, ensuring the purity and physical property stability of the final silica (target purity ≥99.8%); and most of the silicon is solidified in the aging tank (≥95%), making the filter press and washing system more efficient and energy-saving.

[0042] (3) The solid-liquid mixture obtained in step (2) is subjected to pressure filtration. The obtained filter cake is washed and dried to obtain white carbon black product. The filtrate enters the filtrate tank, where the silicon removal rate is ≥99% and the main component of the filtrate is ammonium fluoride.

[0043] Specifically, the slurry is transported from the aging tank to the filter press inlet via a metering pump or gravity conveying. The solid content can be confirmed using an online concentration meter before transport. The filter chambers are filled evenly according to equipment specifications, controlling the thickness of the filter cake per pass (typically 10–50 mm, adjusted based on filter cloth permeability and floc characteristics). Hydraulic pressurization is used, with an operating pressure range of 0.4–1.6 MPa (selected according to the filter press's rated pressure and floc strength). The pressure curve can be applied in stages (slow pressure increase → pressure holding → increased dewatering) to prevent filter cake cracking / rupture. The endpoint is determined by the discharge rate decreasing to the set value, the turbidity of the permeate decreasing, or the pressure / flow rate curve. Washing methods can include: co-current initial washing combined with stepped backwashing to achieve maximum replacement efficiency with minimal water consumption; pressurized washing or gravity washing can also be used, with the washing liquid pressure approaching 30–80% of the filtration pressure. After washing, the filter cake is dehydrated again by pressure filtration to the target moisture content (the moisture content of the filter cake is typically 35–60%, depending on the flocs and equipment). Then, the pressure plate is opened to unload the filter cake. After unloading, the filter cake is transported by scraper / conveyor belt to a dryer or directly into the drying equipment. Finally, belt hot air drying, box drying, vacuum drying, etc., can be used to reduce the moisture content to the final moisture required by the process (e.g., <1–5%) to ensure the storage and transportation stability and physical properties of the silica.

[0044] (4) The filtrate obtained in step (3) is sent into the ammoniation defluorination crystallization reactor. While stirring rapidly, 2% to 5% lime solution is added to control the calcium-fluoride molar ratio at 0.46 to 0.52 and the hydraulic retention time at 60 to 150 min. At the same time, 1% to 10% calcium fluoride seed crystals are added. The induced crystallization reaction is carried out under a negative pressure environment of -300 to -500 Pa, so that fluoride ions are converted into calcium fluoride precipitate and ammonia gas is released. The effluent from the ammoniation defluorination crystallization reactor enters the intermediate tank.

[0045] Specifically, a certain amount of filtrate (NH4F solution) from the filtrate tank is injected into the reactor to the working level. Circulation stirring (at low speed) is started, and gas-phase suction is activated to establish the target negative pressure. The system temperature is controlled (20–35 °C). Under stable system conditions and mild shear, calcium fluoride seed slurry (1%–10% of the theoretical product mass) is added first. The seed crystals provide a preferential growth surface, which helps to inhibit a large number of spontaneous nucleations and the formation of large crystal particles. During addition, ensure uniform dispersion of the seed crystals to avoid local supersaturation leading to new nucleation. Calculate and slowly add 2%–5% lime solution (Ca(OH)2) in stages according to the designed Ca / F molar ratio of 0.46–0.52. Use multi-point dispersed addition or flow-proportional addition to avoid instantaneous supersaturation. The dosing rate is controlled in a closed loop using an online fluoride ion electrode: if the residual fluoride ion concentration in the effluent exceeds the standard, slightly increase the lime solution (Ca(OH)2) rate or extend the residence time; if excessive spontaneous nucleation occurs (increased fine powder), reduce the dosing rate and increase the amount of seed crystals. The stirring strategy is as follows: In the initial stage (dispersion / mixing stage, 1–5 min), rapid stirring is used to ensure rapid and uniform contact between lime, NH4F, and seed crystals (power density 200–800 W / m³) to avoid localized oversaturation. During the growth / maturation stage (residual hydraulic residence time): stirring is reduced to a mild level (50–300 W / m³) to create gentle turbulence that promotes crystal growth and reduces breakage; the total hydraulic residence time is 60–150 min.

[0046] Optionally, the ammoniation defluorination crystallization reactor is equipped with a cooling or jacketed temperature control device, as excessive temperature rise will alter the solubility and dissolution behavior.

[0047] Optionally, an online pH monitoring device can be installed, and the pH control range during the reaction can be 10.0–12.0. Since the pH in the alkalization reaction system will increase, excessively high pH may cause impurities to dissolve or equipment to scale. Setting a high pH is beneficial for the conversion of NH4⁺ to NH3 and facilitates ammonia recovery.

[0048] Specifically, larger crystals accumulated at the bottom / sidewall of the ammoniation defluorination crystallization reactor are sent to a crystal collection tank via periodic bottom discharge or continuous bottom discharge (depending on reactor type). Overflow / effluent enters an intermediate tank for further treatment or recycling. The residual fluoride ion concentration in the effluent is used to evaluate the recovery rate (target ≥99% recovery).

[0049] Optionally, the ammonia (NH3) released during the reaction is extracted by an induced draft fan, first passing through a gas-liquid separator and a demister to remove droplets, and then returned to the absorption tower for reabsorption (or first condensed and dried), completing the closed-loop reuse of ammonia. Negative pressure helps suppress the escape of volatile fluorides and facilitates ammonia recovery; removing droplets effectively prevents ammonia from carrying droplets into the recovery system, improving the purity of ammonia recovery and reducing equipment corrosion.

[0050] This step has the following technical effects: by controlling the Ca / F molar ratio and inducing crystallization, fluorine in the aqueous phase can be recovered in the form of CaF2 crystals, with a fluorine recovery rate of ≥99%, significantly reducing fluorine emissions and resource losses; the large crystal particles formed are easy to separate from solids and liquids and wash, reducing fine powder and difficult-to-treat slurry, reducing downstream dewatering energy consumption and treatment difficulty, improving ammonia recovery purity and reducing equipment corrosion.

[0051] (5) Collect the solution containing large-particle calcium fluoride crystals at the bottom of the reactor in step (4) into a crystal collection tank. After dehydration, washing and drying, high-purity calcium fluoride crystals with a purity ≥98%, an average particle size ≥20μm and a silicon content ≤0.1% are obtained. The dehydrated filtrate enters the intermediate tank.

[0052] Specifically, after continuous operation of the ammoniation crystallization reactor, crystals gradually grow and settle at the bottom of the reactor or in a dedicated crystal collection area. The slurry containing large-particle CaF2 crystals accumulated at the bottom is periodically or continuously discharged into a crystal collection tank. A positive displacement pump or gravity conveying is used, with the conveying pipe diameter as large as possible, gentle bends, and minimal abrupt changes in cross-section. The conveying speed is controlled to keep the flow rate below the shear threshold (<1–2 m / s) to reduce crystal breakage. A hydrocyclone or screening step is added to the crystal collection process to return fine powder <10–15 μm to the crystallizer or intermediate tank for further growth, while coarse crystals enter the dehydration process to improve dehydration efficiency and reduce the loss of fine powder with the solid phase. The collection tank can serve as a short-term maturation tank for the crystals (gentle stirring, short-term maturation 30–120 min), which helps to smooth the crystal surface, eliminate stress, and improve particle strength, thereby enhancing subsequent dehydration and washing performance. During crystal discharge and unloading, avoid direct impact of crystals on metal surfaces or high-speed impellers; the unloading channel should be soft-lined or use a buffer platform.

[0053] Specifically, a large amount of mother liquor and dissolved components (NH4F, etc.) in the crystal slurry can be partially recycled back to the intermediate tank or directly returned to the crystallizer / absorption section after sedimentation / setting. The crystal collection tank should be equipped with a supernatant return port and a discharge port. When using a hydrocyclone / screen, the fine powder is returned to the crystallizer or recycled back to the intermediate tank for regrowth, reducing product loss and maintaining a closed-loop system. Low-intensity washing or displacement (using low-Si reflux water) can be performed in the collection tank to reduce the entrainment of residual dissolved impurities from the mother liquor and reduce the amount of water used for subsequent washing.

[0054] This step has the following technical effects: regular or continuous crystal removal prevents excessive accumulation of the crystal bed in the crystallizer, avoids blockage and nodules, and maintains good mass transfer and controllable supersaturation in the reactor, thereby maintaining high recovery efficiency (≥99%). The fine powder reflux and mother liquor diversion strategy reduces product loss and returns soluble fluorine. Combined with front-end ammonia recovery, the entire system achieves efficient closed-loop utilization of ammonia and fluorine.

[0055] (6) The ammonia gas generated in steps (4) and (5) is collected by the induced draft fan and returned to the absorption tower in step (1). The ammonia-containing water formed in the intermediate pool is returned to the absorption tower to realize the recycling of ammonia.

[0056] Specifically, the ammonia (NH3) released during the crystallization reaction is extracted by an induced draft fan, first passing through a gas-liquid separator and a demister to remove droplets, and then being returned to the absorption tower for reabsorption (or first condensed and dried), completing the closed-loop reuse of ammonia. Negative pressure helps suppress the escape of volatile fluorides and facilitates ammonia recovery; removing droplets effectively prevents ammonia from carrying droplets into the recovery system, improving the purity of ammonia recovery and reducing equipment corrosion.

[0057] Specifically, the effluent from the crystallization reactor, condensate, and reflux liquid from the demisting reactor are collected in an intermediate tank. Due to its high pH (resulting from alkalization in the crystallization reactor), the intermediate tank contains a large amount of free NH3 and NH4⁺ in equilibrium. Stirring ensures the system is homogenized. The intermediate tank liquid is then pumped back to the top of the absorber or the packing section of the absorber as absorbent via a metered-flow rate (or based on pH / NH4⁺ / NH3 concentration feedback) as liquid, completing the liquid-phase reuse of ammonia. The reflux flow rate is linked to the absorber pH for control, ensuring the pH in the bottom collection tank of the absorber is maintained at the process target of 6.5–7.5.

[0058] Optionally, a gas phase NH3 alarm device and absorption bypass are installed. If the gas phase NH3 concentration exceeds the safety threshold or the absorption tower is close to the alkalinity limit, an audible and visual alarm is issued. Part of the gas phase is guided to the bypass absorption (such as a dilute acid scrubbing tower) for treatment and is recovered as usable ammonium sulfate or neutralized before being discharged.

[0059] This step has the following technical benefits: it ensures a closed reflux between the crystallization reactor, gas path, and absorption tower, preventing the escape of ammonia and volatile fluorides. The negative pressure is maintained by a corrosion-resistant induced draft fan, which must have a corrosion-resistant lining and be explosion-proof / fireproof. The gas phase NH3 monitoring and alarm system is equipped with safety redundancy.

[0060] Preferably, the pH value in the bottom water collection tank of the absorption tower in step (1) is linked with the intermediate tank reflux pump to achieve automatic adjustment.

[0061] Preferably, the power of the stirring device in the aging tank in step (2) is 100-200W / m³.

[0062] Preferably, the purity of the silica product in step (3) is ≥99.8%.

[0063] Preferably, the fluorine recovery rate in the ammoniation defluorination crystallization reactor described in step (4) is ≥99%.

[0064] Preferably, the dehydration in step (5) can be performed using a centrifuge or a filter press.

[0065] See appendix Figure 1 The above-mentioned process utilizes a schematic diagram of an apparatus for preparing high-purity calcium fluoride and silica from silicon tetrafluoride waste gas in the phosphorus chemical industry. The apparatus is characterized by comprising: An absorption tower is used to contact silicon tetrafluoride waste gas with recycled ammonia gas and recycled ammonia-containing water to generate a fluorosilicic acid solution and generate silicon dioxide precipitate. An aging tank, connected to the downstream of the absorption tower, is used to stir and age the water effluent from the bottom of the absorption tower to promote the full precipitation of silica. A filter press, connected to the aging tank, is used to perform solid-liquid separation of the aging products to obtain silica filter cake and filtrate; The filtrate tank is used to collect the filtrate from the filter press and quantitatively feed the filtrate into the ammoniation defluorination crystallization reactor. The ammoniation defluorination crystallization reactor is connected to the filtrate tank and the lime solution tank. It is used to convert fluoride ions in the filtrate into calcium fluoride crystals and release ammonia gas under stirring and seed induction. The effluent from the ammoniation defluorination crystallization reactor enters the intermediate tank. A crystal collection tank, connected to the ammoniation defluorination crystallization reactor, is used to collect the calcium fluoride slurry obtained from crystallization; A dehydrator, connected to the crystal collection tank, is used to dehydrate, wash and dry the crystal slurry to produce high-purity calcium fluoride crystals, and the dehydrated filtrate enters the intermediate tank. An intermediate tank is located between the ammoniation defluorination crystallization reactor and the absorption tower. It is used to collect free ammonia from the effluent of the ammoniation defluorination crystallization reactor and the dewatering filtrate of the dewatering machine, and to return the ammonia water to the absorption tower. And a negative pressure suction and conveying pipeline for gas recovery, used to collect ammonia and return the recovered ammonia and ammonia water to the absorption tower.

[0066] Preferably, the absorption tower is a packed tower structure, and the tower is equipped with Pall rings or Raschig rings, a circulating pump, and an online pH sensor.

[0067] Preferably, the pH sensor is linked to the intermediate tank circulation pump to maintain the pH at the bottom of the absorption tower at 6.5 to 7.5.

[0068] Preferably, the ammoniation defluorination crystallization reactor is equipped with a rapid stirring device, a seed crystal addition port, a lime solution inlet, and a negative pressure exhaust port; the lime solution tank provides the crystallization reactor with a lime solution of 2% to 5% concentration to control the calcium-fluoride molar ratio at 0.46 to 0.52; after dehydration, washing, and drying, high-purity calcium fluoride crystals with a purity ≥98%, an average particle size ≥20 μm, and a silicon content ≤0.1% are obtained.

[0069] Compared with the prior art, the present invention has the following beneficial effects: The waste gas from silicon tetrafluoride in the phosphorus chemical industry is treated through processes such as absorption, silicon removal from ammonia, and fluorine removal by calcium addition. The concentrations of fluorine and silicon are significantly reduced. The recovery rate of silicon in the fluorosilicic acid gas is over 99%, and the recovery rate of fluorine is over 99%, thus solving the pollution problem of silicon tetrafluoride waste gas. While solving the problems of fluorine and silicon pollution, it can obtain high-purity calcium fluoride and co-produce precipitated silica.

[0070] By using lime to remove fluoride combined with induced crystallization, fluoride in wastewater is recovered in the form of calcium fluoride crystals, forming large-particle high-purity calcium fluoride with an average particle size of more than 20μm, which improves product quality and meets the requirements for raw materials for hydrofluoric acid preparation. The ammonia gas and ammonia water gas-liquid dual circulation process is used to react with silicon tetrafluoride gas in the absorption tower. The one-step process is simple, requires less investment, and has low operating costs. It breaks through the bottleneck of high temperature and high energy consumption in the existing ammonia recovery process and reduces the risk of environmental pollution.

[0071] The silicon tetrafluoride waste gas generated during the phosphorus chemical production process is collected and utilized as a resource, which improves the high-value-added application of silicon tetrafluoride gas. The high-purity silica product obtained by recovering silicon tetrafluoride gas has a purity of over 99.5%, which meets the quality requirements of rubber-grade silica and can be used in rubber, plastics, coatings and other fields. The prepared high-purity calcium fluoride crystals have a purity of over 98% and a silicon content as low as 0.1%, making them suitable for effective utilization as a fluorite resource.

[0072] In summary, this invention provides a novel process for the resource-based preparation of high-purity calcium fluoride and silica from silicon tetrafluoride gas produced in the phosphorus chemical industry. This process effectively recovers silicon tetrafluoride gas while producing high-purity usable silica and calcium fluoride products. Furthermore, it enables the recycling of ammonia generated during the reaction.

[0073] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing high-purity calcium fluoride and silica from silicon tetrafluoride waste gas, characterized in that, Includes the following steps: (1) Silicon tetrafluoride gas generated during the phosphorus chemical production process is introduced into the absorption tower, and recycled ammonia gas and high ammonia water are introduced at the same time. The absorption tower is filled with Pall rings or Raschig rings. The pH of the water collection pool at the bottom of the absorption tower is controlled between 6.5 and 7.

5. The absorption is assisted by a circulating pump. Under normal temperature conditions, the silicon tetrafluoride gas is absorbed and converted into fluorosilicic acid solution and the ammonia neutralization reaction is completed to generate silicon dioxide precipitate. (2) Send the water obtained in step (1) into the aging tank and stir for 5 to 15 minutes to allow the generated silica to fully precipitate and ensure the quality of the silica. (3) The solid-liquid mixture obtained in step (2) is subjected to pressure filtration. The obtained filter cake is washed and dried to obtain white carbon black product. The filtrate enters the filtrate tank, where the silicon removal rate is ≥99% and the main component of the filtrate is ammonium fluoride. (4) The filtrate obtained in step (3) is sent into the ammoniation defluorination crystallization reactor. While stirring rapidly, 2% to 5% lime solution is added. The calcium-fluoride molar ratio is controlled at 0.46 to 0.52, the hydraulic retention time is 60 to 150 min, and 1% to 10% calcium fluoride seed crystals are added. The induced crystallization reaction is carried out under a negative pressure environment of -300 to -500 Pa, so that fluoride ions are converted into calcium fluoride precipitate and ammonia gas is released. The effluent from the ammoniation defluorination crystallization reactor enters the intermediate tank. (5) Collect the solution containing large-particle calcium fluoride crystals at the bottom of the reactor in step (4) into a crystal collection tank. After dehydration, washing and drying, high-purity calcium fluoride crystals with a purity ≥98%, an average particle size ≥20 μm and a silicon content ≤0.1% are obtained. The dehydrated filtrate enters the intermediate tank. (6) The ammonia gas generated in steps (4) and (5) is collected by the induced draft fan and returned to the absorption tower in step (1). The ammonia water formed in the intermediate pool is returned to the absorption tower to realize the recycling of ammonia.

2. The method according to claim 1, characterized in that, In step (1), the pH value in the water collection tank at the bottom of the absorption tower is linked with the intermediate tank reflux pump to achieve automatic adjustment.

3. The method according to claim 1, characterized in that, The power of the stirring device in the aging tank in step (2) is 100-200W / m³.

4. The method according to claim 1, characterized in that, The purity of the silica product described in step (3) is ≥99.8%.

5. The method according to claim 1, characterized in that, The fluorine recovery rate in the ammoniation defluorination crystallization reactor described in step (4) is ≥99%.

6. The method according to claim 1, characterized in that, The dehydration in step (5) can be performed using a centrifuge or a filter press.

7. An apparatus for preparing high-purity calcium fluoride and silica from silicon tetrafluoride waste gas in the phosphorus chemical industry, characterized in that, include: An absorption tower is used to contact silicon tetrafluoride waste gas with recycled ammonia gas and recycled ammonia-containing water to generate a fluorosilicic acid solution and generate silicon dioxide precipitate. An aging tank, connected to the downstream of the absorption tower, is used to stir and age the water effluent from the bottom of the absorption tower to promote the full precipitation of silica. A filter press, connected to the aging tank, is used to perform solid-liquid separation of the aging products to obtain silica filter cake and filtrate; The filtrate tank is used to collect the filtrate from the filter press and quantitatively feed the filtrate into the ammoniation defluorination crystallization reactor. The ammoniation defluorination crystallization reactor is connected to the filtrate tank and the lime solution tank. It is used to convert fluoride ions in the filtrate into calcium fluoride crystals and release ammonia gas under stirring and seed induction. The effluent from the ammoniation defluorination crystallization reactor enters the intermediate tank. A crystal collection tank, connected to the ammoniation defluorination crystallization reactor, is used to collect the calcium fluoride slurry obtained from crystallization; A dehydrator, connected to the crystal collection tank, is used to dehydrate, wash and dry the crystal slurry to produce high-purity calcium fluoride crystals, and the dehydrated filtrate enters the intermediate tank. An intermediate tank is located between the ammoniation defluorination crystallization reactor and the absorption tower. It is used to collect free ammonia from the effluent of the ammoniation defluorination crystallization reactor and the dewatering filtrate of the dewatering machine, and to return the ammonia water to the absorption tower. And a negative pressure suction and conveying pipeline for gas recovery, used to collect ammonia and return the recovered ammonia and ammonia water to the absorption tower.

8. The apparatus according to claim 7, characterized in that, The absorption tower is a packed tower structure, and the tower is equipped with Pall rings or Raschig rings, a circulating pump, and an online pH sensor.

9. The apparatus according to claim 8, characterized in that, The pH sensor is linked to the intermediate pool circulation pump to maintain the pH at the bottom of the absorption tower at 6.5 to 7.

5.

10. The apparatus according to claim 8 or 9, characterized in that, The ammoniation defluorination crystallization reactor is equipped with a rapid stirring device, a seed crystal addition port, a lime solution inlet, and a negative pressure exhaust port. The lime solution tank provides the crystallization reactor with a lime solution of 2% to 5% concentration to control the calcium-fluoride molar ratio at 0.46 to 0.

52. After dehydration, washing, and drying, high-purity calcium fluoride crystals with a purity ≥98%, an average particle size ≥20 μm, and a silicon content ≤0.1% are obtained.

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