Tail gas treatment method for sodium fluoride production process

Abstract

The invention relates to a sodium fluoride production process tail gas treatment method, which comprises: 1) carrying out alkali washing on tail gas by using a NaCOO solution, and carrying out a reaction on NaCOO and hydrogen fluoride to generate sodium fluoride, carbon dioxide and water; sodium fluoride is recycled, and the tail gas contains a large amount of carbon dioxide, water vapor and hydrogen fluoride which does not participate in reaction; (2) firstly condensing at low temperature, and then adsorbing and drying to remove not less than 99% of water vapor in the tail gas; 3) thoroughly removing hydrogen fluoride by using a protection bed filled with an HF-resistant adsorbent, and removing residual water vapor and other impurities in the tail gas through a molecular sieve adsorption tower to obtain pure carbon dioxide; according to the method, hydrogen fluoride in tail gas containing trace hydrogen fluoride is deeply removed, and besides a small amount of recycled sodium fluoride, pure carbon dioxide formed through treatment is ingeniously prepared into the dry ice.

Description

Technical Field

[0001] This invention relates to the field of chemical waste gas treatment technology, specifically to the treatment technology of hydrogen fluoride-containing waste gas from sodium fluoride production processes. Background Technology

[0002] The main component of NaF production exhaust gas is carbon dioxide, along with water vapor and trace amounts of hydrogen fluoride. When untreated, the volume fraction of CO2 is approximately 89%–93%, the water vapor content is approximately 5.5%–8%, the air content is approximately 1%–2%, and the HF content is 0.5%–1%. Currently, industrial treatment of this exhaust gas mainly involves simple defluorination and dust removal before direct emission, which poses a risk of environmental pollution from harmful impurities such as HF.

[0003] To completely eliminate environmental risks, it is necessary to perform advanced treatment on exhaust gases containing trace amounts of hydrogen fluoride, which significantly increases costs. Summary of the Invention

[0004] The purpose of this patent is to provide a feasible tail gas treatment method that can deeply remove hydrogen fluoride gas from the tail gas of sodium fluoride production process, eliminate environmental risks, and is specifically achieved through the following technical solution: A method for treating tail gas from a sodium fluoride production process, wherein the main component of the tail gas is carbon dioxide, and it also contains water vapor and trace amounts of hydrogen fluoride, the method comprising the following steps: 1) Alkaline washing of the tail gas with Na2CO3 solution, where Na2CO3 reacts with hydrogen fluoride to produce sodium fluoride, carbon dioxide, and water; the sodium fluoride is recycled, and at this time the tail gas contains a large amount of carbon dioxide, water vapor, and unreacted hydrogen fluoride; 2) Low-temperature condensation followed by adsorption drying to remove at least 99% of the water vapor from the tail gas; 3) Thorough removal of hydrogen fluoride using a protective bed filled with HF-resistant adsorbent, followed by removal of the remaining water vapor and other impurities from the tail gas via a molecular sieve adsorption tower to obtain pure carbon dioxide; 4) Liquefaction of the carbon dioxide gas; 5) Injection of the liquid carbon dioxide into a dry ice granulator to produce dry ice.

[0005] The method for treating the tail gas of the sodium fluoride production process is further designed in that, in step 1), the tail gas is subjected to alkaline washing in an alkaline absorption tower using a 10% Na2CO3 solution.

[0006] The method for treating the tail gas of the sodium fluoride production process is further designed in that, in step 2), the tail gas is first cooled to 2~5℃ to remove a large amount of free water, and then further cooled by cascade deep refrigeration. The cooled tail gas is then injected into the adsorption drying unit to stabilize the atmospheric dew point of the tail gas below -40℃, ensuring that the moisture removal rate is ≥99%.

[0007] The method for treating the tail gas of the sodium fluoride production process is further designed in that the HF-resistant adsorbent in step 3) is modified activated carbon or alumina.

[0008] The method for treating the tail gas of the sodium fluoride production process is further designed in that, in step 4), a screw compressor is used to compress the refined CO2 to 5-7 MPa, which is then cooled to 15-20°C by a shell-and-tube condenser, and then depressurized to 1.5-2.0 MPa by a throttling expansion valve and cooled to below -56°C to achieve CO2 liquefaction with a liquefaction efficiency of ≥95%.

[0009] The beneficial effects of this invention are as follows: This patent targets exhaust gas containing trace amounts of hydrogen fluoride. It combines alkaline washing with Na2CO3 solution and molecular sieve adsorption technology to deeply remove hydrogen fluoride from the exhaust gas. In addition to the small amount of sodium fluoride that is reused, the pure carbon dioxide generated during the treatment is cleverly used to produce dry ice, thereby completely covering the cost required for the initial deep treatment. This makes the results of the initial deep treatment of trace hydrogen fluoride a high-quality raw material for the preparation of dry ice, and the cost of the initial deep treatment is completely covered by the by-product dry ice. Attached Figure Description

[0010] Figure 1 This is the process flow diagram of this patent. Detailed Implementation

[0011] The present invention will be further described below with reference to the accompanying drawings and embodiments: A method for treating tail gas from a sodium fluoride production process, wherein the main component of the tail gas is carbon dioxide, and it also contains water vapor and trace amounts of hydrogen fluoride, the method comprising the following steps: 1) Alkaline washing of the tail gas with Na2CO3 solution, where Na2CO3 reacts with hydrogen fluoride to produce sodium fluoride, carbon dioxide, and water; the sodium fluoride is recycled, and at this time the tail gas contains a large amount of carbon dioxide, water vapor, and unreacted hydrogen fluoride; 2) Low-temperature condensation followed by adsorption drying to remove at least 99% of the water vapor from the tail gas; 3) Thorough removal of hydrogen fluoride using a protective bed filled with HF-resistant adsorbent, followed by removal of the remaining water vapor and other impurities from the tail gas via a molecular sieve adsorption tower to obtain pure carbon dioxide; 4) Liquefaction of the carbon dioxide gas; 5) Injection of the liquid carbon dioxide into a dry ice granulator to produce dry ice.

[0012] Step 1) The tail gas is subjected to alkaline washing using a 10% Na₂CO₃ solution in an alkaline absorption tower. The reaction equation is: HF + Na₂CO₃ = NaF + CO₂↑ + H₂O. The Na₂CO₃ solution after the first-stage HF removal can be used for the synthesis of NaF.

[0013] Step 2) First, cool the exhaust gas to 2~5℃ to remove a large amount of free water, then further cool it down through cascade deep cooling, and then inject the cooled exhaust gas into the adsorption drying unit to stably reduce the atmospheric pressure dew point of the exhaust gas to below -40℃, ensuring that the moisture removal rate is ≥99%.

[0014] In step 3), the adsorbent is 13X molecular sieve, the adsorption temperature is 20-30℃, the pressure is 0.6-0.8MPa, and the adsorption cycle is 8-10h. After passing through the adsorption tower, the CO2 purity is increased to ≥99.99%. The molecular sieve regeneration temperature is 220-250℃, and the regeneration time is 2-3h. To completely remove hydrogen fluoride and protect the molecular sieve adsorption tower, a protective bed is set up before the molecular sieve adsorption tower, filled with an HF-resistant adsorbent, specifically modified activated carbon or alumina.

[0015] Step 4) The refined CO2 is compressed to 5-7 MPa using a screw compressor, cooled to 15-20°C by a shell-and-tube condenser, and then depressurized to 1.5-2.0 MPa by a throttling expansion valve and cooled to below -56°C to achieve CO2 liquefaction with a liquefaction efficiency of ≥95%.

[0016] Finally, the liquefied CO2 enters a dry ice granulator, where it is extruded and molded at a pressure of 10-15 MPa and a molding temperature of -78.5℃ to produce dry ice granules with a diameter of 3-10 mm. Alternatively, it enters a dry ice block molding machine and is pressed into dry ice blocks of 200×100×50 mm with a density ≥1.5 g / cm³.

[0017] In practical application, the NaF production exhaust gas is first transported through a sealed, corrosion-resistant pipeline using an induced draft fan. The pipeline velocity is controlled at 15-20 m / s to ensure a completely leak-free and stable delivery of the exhaust gas to the pretreatment system, preventing gas escape and environmental pollution. The exhaust gas is then treated according to the method provided in this invention until dry ice is obtained. The finished dry ice is promptly transferred to a low-temperature insulated warehouse for storage, sealed with specialized insulation materials to minimize sublimation loss. Throughout the production process, pH meters monitor the alkali concentration, CO2 purity analyzers monitor gas purity, and temperature and pressure transmitters monitor process parameters in real time. Safety valves, rupture discs, leak detectors, and emergency ventilation systems ensure safe, stable, and continuous operation of the entire process.

[0018] The present invention provides two sets of embodiments based on different production conditions (i.e., different NaF yields): Example 1 (1 ton NaF / h operating condition, exhaust gas 300 Nm³) 3 / h) This embodiment corresponds to the operating condition of the NaF production unit with an hourly output of 1 ton. The reaction tail gas is collected and then enters this system.

[0019] 1. Raw material exhaust gas parameters Exhaust gas treatment capacity: 300 Nm 3 / h CO2 volume fraction: 90.2% (corresponding to 270.6 Nm³ of pure CO2) 3 / h) Initial HF concentration: 1850 mg / m³ Water vapor volume fraction: 6.8% Operating pressure: atmospheric pressure 2. Specific Implementation Steps Step 1: Exhaust gas collection The system uses a closed PTFE-lined pipe and an induced draft fan to transport 300 Nm³ of fluid, controlling the pipe velocity at 15 m / s. 3 The exhaust gas is stably fed into the subsequent pretreatment system at a rate of / h, ensuring no leakage throughout the process.

[0020] Step 2: Multi-stage preprocessing (1) Primary alkaline scrubbing to remove HF: The tail gas collected in step 1 is passed into the first stage of a two-stage alkaline scrubbing tower, and a 10% (w / w) Na2CO3 solution is used for countercurrent absorption, with the liquid-to-gas ratio controlled at 15 L / Nm³. 3 The NaF solution generated in the reaction was directly recycled back to the NaF synthesis reactor. Testing showed that the HF content in the tail gas at the tower outlet decreased to 0.7 mg / m³, with an HF removal rate of 99.96%.

[0021] (2) Secondary freeze dehydration: The tail gas after primary alkaline washing and defluorination is introduced into a freeze dryer to reduce the dew point of the treated gas to -41℃ and the moisture removal rate reaches 99.2%.

[0022] Step 3: CO2 Refining The dehydrated gas from step 2 is fed into a dual-tower switching 13X molecular sieve adsorption tower, with a single tower packing capacity of 500 kg. The adsorption temperature is controlled at 25℃, the pressure at 0.7 MPa, and the adsorption cycle at 8 h. During the regeneration stage, the regeneration temperature is controlled at 230℃, the regeneration time at 2.5 h, and nitrogen is used for regeneration with a regeneration air volume of 80 Nm³. 3 / h. Testing showed that the purity of the refined CO2 was increased to 99.992%.

[0023] Step 4: CO2 liquefaction The refined CO2 gas from step 3 is fed into a screw compressor and compressed in three stages to 6.0 MPa, with the exhaust temperature controlled to ≤80℃. The compressed, high-temperature, high-pressure gas is then sent to a shell-and-tube condenser and cooled to 18℃ using circulating water. Finally, the cooled gas is depressurized to 1.8 MPa through a throttling expansion valve, causing the temperature to plummet to -58℃, achieving phase change liquefaction. Testing showed that the CO2 liquefaction efficiency was 96.0%.

[0024] Step 5: Dry ice shaping The liquid CO2 obtained in step 4 was fed into a small dry ice granulator, with the granulation pressure controlled at 12 MPa and the forming temperature at -78.5℃. Testing revealed that the produced dry ice granules had a diameter of 3 mm, a density of 1.55 g / cm³, and a drop strength ≥1.2 m.

[0025] 3. Implementation Results Dry ice production: 507.6 kg / h Total CO2 recovery rate: 95.5% Exhaust emissions: HF was not detected, which complies with the "Integrated Emission Standard for Air Pollutants".

[0026] Example 2 (Scale-up condition, exhaust gas 500 Nm³) 3 / h) This embodiment corresponds to a scale-up operation of approximately 1.67 tons per hour in the NaF production unit, and is used to verify the operational stability of the present invention under different loads.

[0027] 1. Raw material exhaust gas parameters Exhaust gas treatment capacity: 500 Nm 3 / h; CO2 volume fraction: 90.2% (corresponding to 451.0 Nm³ of pure CO2) 3 / h); Initial HF concentration: 1920 mg / m³ (fluctuation under operating conditions); Water vapor volume fraction: 7.0%; Operating pressure: atmospheric pressure.

[0028] 2. Specific Implementation Steps Step 1: Exhaust gas collection By adjusting the frequency converter of the induced draft fan, the duct velocity is controlled at 18 m / s, and 500 Nm³ of air is delivered. 3 The exhaust gas is stably delivered to the subsequent pretreatment system at a rate of / h.

[0029] Steps 2 through 5 are the same as in Example 1.

[0030] 3. Implementation Results Dry ice production: 817 kg / h; Total CO2 recovery rate: 96.0%.

[0031] Operational stability: After 72 hours of continuous operation, the equipment showed no corrosion or blockage, and all process parameters remained stable within the control range.

[0032] 300-500 Nm 3Actual verification under / h operating conditions: The purified CO2 purity is ≥99.99%, the liquefaction efficiency is 96.0%~96.5%, and the total recovery rate is 95.5%~96.0%; the dry ice produced has a density of 1.55~1.58 g / cm³, is dense and non-loose, and has a purity of ≥99.99%. The system operates continuously for 72 hours without corrosion, blockage, or leakage, and the exhaust gas meets emission standards. At the same time, it produces high-value-added dry ice as a byproduct, achieving the dual benefits of environmental protection and resource recovery.

[0033] The calculation is based on an annual production of 5000tNaF.

[0034] (1) Capacity Calculation With an annual production capacity of 5,000 tons of NaF, the dry ice output is as follows: Based on the reaction formula: Na2CO3 + 2HF → 2NaF + CO2 + H2O, the theoretical CO2 produced per ton of NaF is approximately 0.5238 tons; the theoretical CO2 produced from 5000 tons of NaF is 2619 tons / year; calculated at a recovery rate of 95.5%, the annual dry ice production is approximately 2500 tons / year.

[0035] (2) Operating cost per ton of dry ice (electricity price calculated at 1 yuan) ① Electricity costs for refrigeration and cryogenics (dehydration + liquefaction + cold preservation) Based on 330 kWh / t, the price is approximately 330 yuan / ton.

[0036] ②Other power and electricity costs (fans, compressors, circulating pumps) Based on 60 kWh / t, the price is approximately 60 yuan / ton.

[0037] ③Na2CO3 consumption Assuming the amount of HF in the exhaust gas is 0.5% of CO2, 13 tons of HF would require 34 tons of Na2CO3 solid. If Na2CO3 solid costs 1,000 yuan per ton, this translates to approximately 14 yuan per ton of dry ice.

[0038] ④ Molecular sieves and adsorbents are amortized at 8 yuan / ton.

[0039] ⑤ Cooling water and circulating water are priced at 6 yuan / ton.

[0040] ⑥ Labor cost is 25 yuan / ton.

[0041] ⑦ Equipment repair and maintenance: 20 yuan / ton.

[0042] ⑧ Packaging cost (insulated bags) is 35 yuan / ton.

[0043] The total operating cost is 330+60+14+8+6+25+20+35=498 yuan / ton of dry ice.

[0044] (3) Equipment investment and depreciation (supporting scale for an annual production of 2,500 tons of dry ice) The total investment for the entire system (collection—pretreatment—refining—liquefaction—forming) is approximately 4.2 million yuan.

[0045] Depreciation is calculated over 10 years, with a residual value of 0.

[0046] Annual depreciation = 4.2 million ÷ 10 = 420,000 yuan / year.

[0047] Depreciation per ton = 420000 ÷ 2500 = 168 yuan / ton.

[0048] (4) Total cost Operating cost: 498 yuan / ton; equipment depreciation: 168 yuan / ton; total cost per ton of dry ice is approximately 498 + 168 = 666 yuan / ton. Assuming granular dry ice costs 2500 yuan / ton, and with a NaF production capacity of 5000t / year, this carbon dioxide recovery unit can generate a profit of 4.58 million yuan annually.

Claims

1. A method for treating tail gas from a sodium fluoride production process, wherein the main component of the tail gas from the sodium fluoride production process is carbon dioxide, and it also contains water vapor and trace amounts of hydrogen fluoride, characterized in that... The method includes the following steps: 1) Alkaline washing of the tail gas with Na2CO3 solution, where Na2CO3 reacts with hydrogen fluoride to produce sodium fluoride, carbon dioxide, and water; sodium fluoride is reused, and at this time the tail gas contains a large amount of carbon dioxide, water vapor, and unreacted hydrogen fluoride; 2) Low-temperature condensation followed by adsorption drying to remove no less than 99% of the water vapor in the tail gas; 3) After thoroughly removing hydrogen fluoride using a protective bed filled with HF-resistant adsorbent, the tail gas passes through a molecular sieve adsorption tower to remove the remaining water vapor and other impurities, obtaining pure carbon dioxide; 4) Liquefying the carbon dioxide gas. 5) Inject liquid carbon dioxide into a dry ice granulator to produce dry ice.

2. The method for treating tail gas from sodium fluoride production process according to claim 1, characterized in that, Step 1) Use a 10% Na2CO3 solution to perform alkaline washing on the tail gas in an alkaline absorption tower.

3. The method for treating tail gas from sodium fluoride production process according to claim 1, characterized in that, Step 2) First, cool the exhaust gas to 2~5℃ to remove a large amount of free water, then further cool it down through cascade deep cooling, and then inject the cooled exhaust gas into the adsorption drying unit to stably reduce the atmospheric pressure dew point of the exhaust gas to below -40℃, ensuring that the moisture removal rate is ≥99%.

4. The method for treating tail gas from sodium fluoride production process according to claim 1, characterized in that, The HF-resistant adsorbent mentioned in step 3) is modified activated carbon or alumina.

5. The method for treating tail gas from sodium fluoride production process according to claim 1, characterized in that, Step 4) The refined CO2 is compressed to 5-7 MPa using a screw compressor, cooled to 15-20°C by a shell-and-tube condenser, and then depressurized to 1.5-2.0 MPa by a throttling expansion valve and cooled to below -56°C to achieve CO2 liquefaction with a liquefaction efficiency of ≥95%.

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