Method and system for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution

Abstract

This invention relates to the field of industrial waste gas treatment and resource utilization technology, and particularly to a method and system for treating hydrofluoric acid tail gas using a circulating alkaline solution to purify calcium fluoride. The invention includes the following steps: S1. Tail gas absorption and alkaline regeneration: HF-containing tail gas is reacted with KOH absorbent to generate a KF solution, and then Ca(OH)2 is added to the KF solution to generate a slurry; S2. Solid-liquid separation: The slurry generated in S1 is separated to obtain a regenerated KOH solution and a wet CaF2 filter cake; S3. Multi-stage washing and purification: The CaF2 filter cake is sequentially washed with a saturated KF solution, a dilute hydrofluoric acid solution, and clean water; S4. Drying and calcination: The washed CaF2 filter cake is dried and calcined to obtain a high-purity calcium fluoride product. This invention, through an innovative three-stage washing process using saturated KF solution washing + dilute HF acid washing + water washing, effectively removes different types of impurities, resulting in a final calcium fluoride product with a stable purity of over 96%.

Description

Technical Field

[0001] This invention relates to the field of chemical waste treatment and resource utilization technology, and in particular to a method and system for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution. Background Technology

[0002] In industries such as hydrofluoric acid production and fluorine-containing chemical processing, large quantities of hydrogen fluoride (HF) tail gas are generated during the production process. Currently, the industrial practice commonly uses alkaline absorption to treat this tail gas, with potassium hydroxide (KOH) or sodium hydroxide (NaOH) solutions being the commonly used absorbents. After absorption, potassium fluoride (KF) or sodium fluoride (NaF) solutions are generated.

[0003] In traditional processes, an excess of calcium salt (such as calcium chloride CaCl2) or lime (calcium hydroxide Ca(OH)2) is usually added to the fluoride solution to form calcium fluoride (CaF2) precipitate, thereby achieving fluoride immobilization. The reaction equation is as follows: 2KF + Ca(OH)2→ CaF2↓ + 2KOH While this method achieves the harmless treatment of exhaust gases, it has the following significant drawbacks: High alkali consumption and high operating costs: The generated potassium hydroxide cannot be effectively recovered for tail gas absorption, requiring a continuous replenishment of fresh alkali solution, resulting in high chemical consumption and high operating costs.

[0004] Calcium fluoride products are low in purity and value: The resulting calcium fluoride sludge has a complex composition, containing unreacted calcium salts, entrained alkali metal salts, heavy metal impurities, and silicates, with a purity typically between 70% and 85%. This low-grade calcium fluoride can usually only be disposed of as hazardous waste in landfills, resulting in high disposal costs and a serious waste of fluorine resources.

[0005] Environmental risks: Calcium fluoride sludge with high impurity content poses an environmental risk of pollutant leaching during stockpiling and landfilling. Summary of the Invention

[0006] The purpose of this invention is to provide a method and system for treating hydrofluoric acid tail gas that can realize alkali recycling and simultaneously purify the by-product calcium fluoride into a high-value-added product by using recycled alkali to purify calcium fluoride, thereby solving the problems of low purity and low resource value of calcium fluoride sludge generated by the existing hydrofluoric acid tail gas alkali absorption method.

[0007] The technical solution of this invention: A method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution includes the following steps: S1. Tail gas absorption and alkaline regeneration: The tail gas containing HF is reacted with KOH absorption liquid to generate KF solution, and then Ca(OH)2 is added to the KF solution to generate slurry. S2. Solid-liquid separation: Separate the slurry produced in S1 to obtain a regenerated KOH solution and a wet CaF2 filter cake; S3. Multi-stage washing and purification: The CaF2 filter cake is washed sequentially with saturated KF solution, dilute hydrofluoric acid solution, and water. S4. Drying and calcination: The washed CaF2 filter cake is dried and calcined to obtain high-purity calcium fluoride product.

[0008] Preferably, in step S1, the HF-containing tail gas is contacted countercurrently with the KOH absorption liquid, and the reaction time of the KF solution and Ca(OH)2 to generate slurry is 30-60 minutes.

[0009] Preferably, in step S1, the mass concentration of the KOH absorption solution is 20%-25%; the mass concentration of the KF solution is 19.3%-23.8%; and the molar ratio of Ca(OH)2 to KF is 1.05:1.

[0010] Preferably, the slurry separation method in step S2 is centrifugation or pressure filtration.

[0011] Preferably, the mass concentration of the regenerated KOH solution in step S2 is 19.7%-24.7%; Return to step S1 to use KOH absorbent solution to absorb HF-containing tail gas.

[0012] Preferably, the multi-stage washing and purification is a multi-stage countercurrent washing and purification process, which continues until the effluent pH is 6.5-7.5 and the conductivity is <50μS / cm.

[0013] Preferably, the mass concentration of the saturated KF solution in step S3 is 19.3%-23.8%; and the mass concentration of the dilute hydrofluoric acid solution in step S3 is 3%-8%.

[0014] Preferably, the drying process in S4 is carried out at a temperature of 100℃-105℃ for 1-4 hours.

[0015] Preferably, in the calcination process of S4, the calcination temperature is 600℃-800℃ and the calcination time is 1-3 hours.

[0016] A hydrofluoric acid tail gas treatment system utilizing circulating alkaline solution to purify calcium fluoride includes an absorption tower, a reaction vessel, a solid-liquid separation device, a multi-stage series-connected scrubbing device, a dryer, and a calcining furnace. The absorption tower is equipped with an absorption tower inlet, an absorption liquid circulation inlet, and an absorption tower outlet. The reaction vessel is equipped with a reaction vessel inlet and a reaction vessel outlet. The solid-liquid separation device is equipped with a solid-liquid separation device inlet, a solid-liquid separation device liquid outlet, and a solid separation device solid outlet. The multi-stage series-connected scrubbing device includes a multi-stage series-connected scrubbing device inlet and a multi-stage series-connected scrubbing device outlet. The outlet of the absorption tower is connected to the inlet of the reactor, the outlet of the reactor is connected to the inlet of the solid-liquid separation device, the liquid outlet of the solid-liquid separation device is connected to the absorption liquid circulation inlet of the absorption tower, the solid outlet of the solid-liquid separation device is connected to the inlet of a multi-stage series washing device, and the outlet of the multi-stage series washing device is sequentially connected to a dryer and a calcining furnace.

[0017] A method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution includes the following steps: S1. Tail gas absorption and alkali regeneration: The HF-containing tail gas is passed into an absorption tower and comes into countercurrent contact with the circulating KOH absorbent to generate potassium fluoride (KF) solution. The main reaction is as follows: HF + KOH → KF + H2O Subsequently, a measured amount of calcium hydroxide (Ca(OH)2) slurry was added to the generated KF solution, reacting to form calcium fluoride (CaF2) precipitate and potassium hydroxide (KOH). The main reaction is as follows: 2KF + Ca(OH)₂ → CaF₂↓ + 2KOH This step regenerates KOH, and the regenerated KOH solution is returned to the absorption tower for tail gas absorption, forming a closed alkali cycle system that significantly reduces the consumption of fresh KOH.

[0018] S2. Solid-liquid separation and primary calcium fluoride collection: The slurry that has completed the reaction in S1 is subjected to solid-liquid separation (e.g., through a filter press or centrifuge) to obtain a regenerated KOH solution (returned to S1) and a wet calcium fluoride filter cake.

[0019] S3. Multi-stage countercurrent washing and purification: The calcium fluoride filter cake obtained from S2 is subjected to multi-stage deep washing and purification, specifically including: S3a. Washing with saturated potassium fluoride solution: A near-saturated KF solution from the tail gas absorption system is used as the primary washing solution. This step utilizes the common ion effect (CaF2-KF-H2O system) to significantly reduce the dissolution loss of calcium fluoride during washing, while simultaneously washing away some soluble impurities (such as excess Ca) carried in the filter cake.2+ ).

[0020] S3b. Dilute hydrofluoric acid (HF) washing: A two-stage washing process is performed using a low-concentration hydrofluoric acid solution (e.g., 3%-8%). This step effectively dissolves and removes metal oxide impurities (such as SiO2, Fe2O3, and Al2O3) from calcium fluoride, reacting to form soluble fluorosilicic acid and fluoroferric acid, which are then washed away. The general reaction formula is: MOx + 2xHF → MF2x + xH2O (where M represents a metallic impurity) S3c. Rinse with water: Finally, rinse with deionized water to thoroughly remove residual acid and soluble fluoride salts until the washing solution is neutral.

[0021] The crude calcium fluoride is first washed with a potassium fluoride solution saturated with exhaust gas to dissolve the co-precipitated hydroxides; then it is washed a second time with hydrofluoric acid of a specific concentration to dissolve acid-soluble impurities such as silica; then it is washed with water to remove soluble salts; finally, the bound water is removed by controlling the calcination temperature to obtain a calcium fluoride product with a specific crystal form.

[0022] S4. Drying and calcination: The high-purity calcium fluoride filter cake, after being washed with S3, is dried and then calcined in a high-temperature calcination furnace. The calcination temperature is controlled at 600℃-800℃, and the holding time is 1-3 hours. This step aims to decompose any remaining trace organic impurities, stabilize the calcium fluoride crystal form, further remove volatile impurities, and ultimately obtain a high-purity anhydrous calcium fluoride product.

[0023] The beneficial technical effects of this invention are as follows: Internal circulation of alkali solution is achieved: through the process design of "absorption-precipitation-regeneration", potassium hydroxide can be recycled, which fundamentally reduces the consumption cost of alkali reagents and the amount of waste liquid discharged.

[0024] Producing high-value-added calcium fluoride products: Through the innovative use of a three-stage washing process of "saturated KF solution washing + dilute HF acid washing + water washing", different types of impurities are removed in a highly targeted manner, so that the purity of the final calcium fluoride product can be stably maintained at over 96%.

[0025] Resource recovery and reduction: Hazardous waste (low-grade calcium fluoride sludge) is transformed into commercially valuable high-purity chemical raw materials (which can be used in metallurgy, building materials, fluorochemicals and other fields), realizing the recycling of fluorine resources and completely eliminating the need for landfill disposal of hazardous waste and environmental risks.

[0026] Significant economic benefits: This process not only reduces the cost of exhaust gas treatment, but also creates new economic benefits through the sale of high-purity calcium fluoride products, turning waste into treasure. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the process flow of the present invention.

[0028] Figure 2 This is a schematic diagram of the process flow system of the present invention.

[0029] In the attached diagram, 1-absorption tower, 2-reaction vessel, 3-solid-liquid separation device, 4-washing device, 5-dryer, and 6-calcining furnace. Detailed Implementation

[0030] The following is in conjunction with the appendix Figure 1 and 2 The present invention will be specifically described in conjunction with the embodiments.

[0031] System Implementation Examples like Figure 2 As shown, a hydrofluoric acid tail gas treatment system utilizing circulating alkaline solution to purify calcium fluoride includes an absorption tower 1, a reaction vessel 2, a solid-liquid separation device 3, a multi-stage series-connected scrubbing device 4, a dryer 5, and a calcining furnace 6. The absorption tower 1 is equipped with an absorption tower inlet, an absorption liquid circulation inlet, and an absorption tower outlet. The reaction vessel 2 is equipped with a reaction vessel inlet and a reaction vessel outlet. The solid-liquid separation device 3 is equipped with a solid-liquid separation device inlet, a solid-liquid separation device liquid outlet, and a solid separation device solid outlet. The multi-stage series-connected scrubbing device 4 includes a multi-stage series-connected scrubbing device inlet and a multi-stage series-connected scrubbing device outlet. The solid-liquid separation device 3 is a chamber filter press or a centrifuge. The calcining furnace 6 is a muffle furnace.

[0032] The outlet of the absorption tower is connected to the inlet of the reactor, the outlet of the reactor is connected to the inlet of the solid-liquid separation device, the liquid outlet of the solid-liquid separation device is connected to the absorption liquid circulation inlet of the absorption tower, and the solid outlet of the solid-liquid separation device is connected to the inlet of a multi-stage series washing device. The outlets of the multi-stage series washing device are sequentially connected to the dryer 5 and the calcining furnace 6. All embodiments use the system described in this embodiment for production. Example 1

[0033] like Figure 1 As shown, the treatment involves the tail gas from a specialty gas company's fluoride production process, with an initial HF concentration of 800 mg / m³. 3 .

[0034] S1: The exhaust gas is passed into an absorption tower containing a 20% KOH solution. After purification, the HF concentration in the exhaust gas is <2 mg / m³. 3 The KF solution (concentration 19.3%) produced is introduced into the reactor, and Ca(OH)2 slurry is added according to the Ca(OH)2:KF stoichiometric ratio of 1.05:1. The mixture is stirred and reacted for 30 minutes.

[0035] S2: After the reaction, the slurry is separated into solid and liquid components using a chamber filter press to obtain a regenerated KOH solution (returned to the absorption tower) and a CaF2 filter cake with a water content of 35%.

[0036] S3a: The filter cake is washed in two stages of countercurrent washing with a saturated KF solution with a concentration of approximately 19.3% from the absorption tower.

[0037] S3b: Perform a first-stage wash with a 3% dilute hydrofluoric acid solution.

[0038] S3c: Finally, perform a three-stage countercurrent wash with deionized water until the effluent pH is 6.5-7.5 and the conductivity is <50μS / cm.

[0039] S4: The washed filter cake was dried at 105°C for 4 hours, and then calcined in a muffle furnace at 600°C for 3 hours.

[0040] Product Analysis: X-ray fluorescence spectroscopy (XRF) analysis showed that the purity of the obtained calcium fluoride product was 96.8%, with the main impurities SiO2 content reduced to 0.8% and Fe2O3 content reduced to 0.3%, meeting the high-quality fluorite standard. Example 2

[0041] like Figure 1 As shown, the treatment involves the tail gas from a specialty gas company's fluoride production process, with an initial HF concentration of 800 mg / m³. 3 .

[0042] S1: The exhaust gas is passed into an absorption tower containing a 23% KOH solution. After purification, the HF concentration in the exhaust gas is <2 mg / m³. 3 The KF solution (concentration 22%) produced is introduced into the reactor, and Ca(OH)2 slurry is added according to the Ca(OH)2:KF stoichiometric ratio of 1.05:1. The mixture is stirred and reacted for 40 minutes.

[0043] S2: After the reaction, the slurry is separated into solid and liquid components using a chamber filter press to obtain a regenerated KOH solution (returned to the absorption tower) and a CaF2 filter cake with a water content of about 35%.

[0044] S3a: The filter cake is washed in two stages of countercurrent washing using a 22% saturated KF solution from the absorption tower.

[0045] S3b: Perform a first-stage wash with an 8% dilute hydrofluoric acid solution.

[0046] S3c: Finally, perform a three-stage countercurrent wash with deionized water until the effluent pH is 6.5-7.5 and the conductivity is <50μS / cm.

[0047] S4: The washed filter cake was dried at 105°C for 1 hour, and then calcined in a muffle furnace at 750°C for 3 hours.

[0048] Product Analysis: X-ray fluorescence spectroscopy (XRF) analysis showed that the purity of the obtained calcium fluoride product was 97.6%, with the main impurities SiO2 content reduced to 0.4% and Fe2O3 content reduced to 0.2%, meeting the high-quality fluorite standard. Example 3

[0049] like Figure 1 As shown, the treatment involves the tail gas from a specialty gas company's fluoride production process, with an initial HF concentration of 800 mg / m³. 3 .

[0050] S1: The exhaust gas is passed into an absorption tower containing a 25% KOH solution. After purification, the HF concentration in the exhaust gas is <2 mg / m³. 3 The KF solution (concentration 23.8%) produced is introduced into the reactor, and Ca(OH)2 slurry is added according to the Ca(OH)2:KF stoichiometric ratio of 1.05:1. The mixture is stirred and reacted for 60 minutes.

[0051] S2: After the reaction, the slurry is separated into solid and liquid components by centrifugation to obtain a regenerated KOH solution (returned to the absorption tower) and a CaF2 filter cake with a water content of 35%.

[0052] S3a: The filter cake is washed in two stages of countercurrent washing with a 23.8% saturated KF solution from the absorption tower.

[0053] S3b: Perform a first-stage wash with an 8% dilute hydrofluoric acid solution.

[0054] S3c: Finally, perform a three-stage countercurrent wash with deionized water until the effluent pH is 6.5-7.5 and the conductivity is <50μS / cm.

[0055] S4: The washed filter cake is dried at 100°C for 1 hour, and then calcined in a muffle furnace at 800°C for 1 hour.

[0056] Product Analysis: X-ray fluorescence spectroscopy (XRF) analysis showed that the purity of the obtained calcium fluoride product was 97.3%, with the main impurities SiO2 content reduced to 0.5% and Fe2O3 content reduced to 0.3%, meeting the high-quality fluorite standard.

[0057] Comparative Example 1 The difference between this comparative example and Example 1 is that step S3b is omitted: a first-stage wash with a 3% dilute hydrofluoric acid solution is performed.

[0058] And without step S3c: Finally, perform a three-stage countercurrent wash with deionized water until the effluent pH is 6.5-7.5 and the conductivity is <50μS / cm.

[0059] Product Analysis: X-ray fluorescence spectroscopy (XRF) analysis showed that the purity of the obtained calcium fluoride product was 91.8%, with the main impurities SiO2 content being 2.3% and Fe2O3 content being 0.8%, which did not meet the high-quality fluorite standard.

[0060] Comparative Example 2 The difference between this comparative example and Example 1 is that step S3c is omitted: a three-stage countercurrent wash with deionized water is performed until the effluent pH is 6.5-7.5 and the conductivity is <50μS / cm.

[0061] Product Analysis: X-ray fluorescence spectroscopy (XRF) analysis showed that the purity of the obtained calcium fluoride product was 90.3%, the main impurity potassium fluoride content was 1.5%, the SiO2 content decreased to 0.8%, and the Fe2O3 content decreased to 0.4%, which did not meet the high-quality fluorite standard.

[0062] The technical solutions disclosed and proposed in this invention can be implemented by those skilled in the art by appropriately modifying the conditions and routes, etc. Although the methods and preparation techniques of this invention have been described through preferred embodiments, those skilled in the art can obviously modify or recombine the methods and technical routes described herein without departing from the content, spirit, and scope of this invention to achieve the final preparation technique. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included within the spirit, scope, and content of this invention.

Claims

1. A method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution, characterized in that, Includes the following steps: S1. Tail gas absorption and alkaline regeneration: The tail gas containing HF is reacted with KOH absorption liquid to generate KF solution, and then Ca(OH)2 is added to the KF solution to generate slurry. S2. Solid-liquid separation: Separate the slurry produced in S1 to obtain a regenerated KOH solution and a wet CaF2 filter cake; S3. Multi-stage washing and purification: The CaF2 filter cake is washed sequentially with saturated KF solution, dilute hydrofluoric acid solution, and water. S4. Drying and calcination: The washed CaF2 filter cake is dried and calcined to obtain high-purity calcium fluoride product.

2. The method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution according to claim 1, characterized in that, In step S1, the HF-containing tail gas is contacted countercurrently with the KOH absorption liquid, and the reaction time of KF solution and Ca(OH)2 to generate slurry is 30-60 minutes.

3. The method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution according to claim 1, characterized in that, In step S1, the mass concentration of the KOH absorption solution is 20%-25%; the mass concentration of the KF solution is 19.3%-23.8%; and the molar ratio of Ca(OH)2 to KF is 1.05:

1.

4. The method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution according to claim 1, characterized in that, In step S2, the slurry is separated by centrifugation or pressure filtration.

5. The method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution according to claim 1, characterized in that, The mass concentration of the regenerated KOH solution in step S2 is 19.7%-24.7%; Return to step S1 to use KOH absorbent solution to absorb HF-containing tail gas.

6. The method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution according to claim 1, characterized in that, Multi-stage washing and purification involves multi-stage countercurrent washing and purification until the effluent pH reaches 6.5-7.5 and the conductivity is <50μS / cm, at which point washing is stopped.

7. The method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution according to claim 1, characterized in that, The mass concentration of the saturated KF solution in step S3 is 19.3%-23.8%; the mass concentration of the dilute hydrofluoric acid solution in step S3 is 3%-8%.

8. The method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution according to claim 1, characterized in that, The drying process in S4 has a temperature of 100℃-105℃ and a time of 1-4 hours.

9. The method for treating hydrofluoric acid tail gas by purifying calcium fluoride using circulating alkaline solution according to claim 1, characterized in that, The calcination process in S4 has a calcination temperature of 600℃-800℃ and a calcination time of 1-3 hours.

10. A hydrofluoric acid tail gas treatment system utilizing circulating alkaline solution and purifying calcium fluoride, for implementing the method according to any one of claims 1-9, characterized in that, The system includes an absorption tower, a reaction vessel, a solid-liquid separation device, a multi-stage series washing device, a dryer, and a calcining furnace. The absorption tower is equipped with an absorption tower inlet, an absorption liquid circulation inlet, and an absorption tower outlet. The reaction vessel is equipped with a reaction vessel inlet and a reaction vessel outlet. The solid-liquid separation device is equipped with a solid-liquid separation device inlet, a solid separation device liquid outlet, and a solid separation device solid outlet. The multi-stage series washing device includes a multi-stage series washing device inlet and a multi-stage series washing device outlet. The outlet of the absorption tower is connected to the inlet of the reactor, the outlet of the reactor is connected to the inlet of the solid-liquid separation device, the liquid outlet of the solid-liquid separation device is connected to the absorption liquid circulation inlet of the absorption tower, the solid outlet of the solid-liquid separation device is connected to the inlet of a multi-stage series washing device, and the outlet of the multi-stage series washing device is sequentially connected to a dryer and a calcining furnace.

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