Method for circularly absorbing NO in flue gas of rotary kiln by using acidic KMnO4 solution

Abstract

The invention discloses a method for circularly absorbing nitric oxide (NO) in flue gas of a rotary kiln by using an acidic potassium permanganate (KMnO4) solution, and belongs to the technical field of flue gas purification. The method comprises the following steps: introducing flue gas containing NO into an acidic KMnO4 solution added with EDTA (Ethylene Diamine Tetraacetic Acid) for oxidation absorption; the high-temperature flue gas in the settling chamber is led to concentrate the absorption liquid and led into the rotary kiln, and the carbonaceous fuel is used for reducing HNO3 into N2; and regenerating KMnO4 by electrolyzing residual mother liquor, and recycling the regenerated solution. The method is simple in process, low in operation cost and high in NO removal rate.

Description

Technical Field

[0001] The invention belongs to the technical field of industrial flue gas purification, and particularly relates to a method for treating nitrogen monoxide in rotary kiln flue gas. Background Art

[0002] With the gradual improvement of pollution control requirements, nitrogen oxides (NO x ) The emission limit has been increased from the current 100mg / Nm 3 Tightened to 50mg / Nm 3 Although existing technology can make NO x The concentration dropped to 80-100 mg / Nm 3 , but further reduced to 50mg / Nm 3 The following technical and economic bottlenecks are faced: Although the SCR catalyst can achieve a 90% denitrification rate in the low temperature range of 300-400℃, if the inlet concentration exceeds 300mg / Nm 3 , the outlet concentration is still 30-35mg / Nm 3 , combined with secondary pollution derived from ammonia escape, it is difficult to stably meet the standards; SNCR technology is limited by the reaction temperature window, and the actual average emission value is still 100-150mg / Nm 3 Because NO is poorly soluble in water, it is difficult to reduce NO emissions to 50 mg / Nm3 using oxidative absorption technologies such as ozone, hypochlorite, and perchlorate. 3 Potassium permanganate solution also faces the problem of NO insolubility in the aqueous phase. Furthermore, manganese salt (MnSO4) concentrations above 5% in the wastewater are listed on the National Hazardous Waste List (HW17) and require specialized disposal. Regeneration technologies, such as the electrolysis method proposed in patent CN113636589A, require additional KOH and consume energy of 4.5 kWh / kg KMnO4. This overall cost still represents 65% of fresh reagents, making it unsustainable for long-term operation.

[0003] Currently, there is a lack of economical and applicable ultra-low NO emission technology for rotary kiln flue gas. In view of the operating characteristics of rotary kilns, it is urgent to develop a NO deep treatment technology that comprehensively utilizes flue gas energy and other convenient conditions. Summary of the Invention

[0004] This integrated method combines KMnO₄ solution absorption with flue gas thermal energy concentration followed by electrolytic regeneration of the absorbent. This method achieves efficient NO removal and absorbent recycling, reducing operating costs and avoiding solid waste generation. A small amount of sulfuric acid is added to the absorbent to adjust the acidity, allowing the KMnO₄ to exert its strong oxidizing properties. Manganese is reduced to Mn(II), fully utilizing manganese's electron resources. A small amount of EDTA is added as a ligand for Mn(II), forming a complex capable of complexing NO, thereby increasing its solubility in the solution.

[0005] The technical solution of the present invention is:

[0006] It includes two core steps: absorption and regeneration.

[0007] (1) NO absorption stage

[0008] The acidic KMnO4 solution undergoes an oxidation reaction with NO in the flue gas to generate nitric acid (HNO3) and a mixture of manganese salts and potassium salts:

[0009] MnEDTA + NO = Mn(NO)EDTA

[0010] 10Mn(NO)EDTA+6KMnO4+9H2SO4=10HNO3+6MnSO4+3K2SO4+10MnEDTA+4H2

[0011] O

[0012] (2) KMnO4 regeneration stage: closed-loop regeneration is achieved in two steps.

[0013] (1) Absorption liquid concentration and HNO3 high temperature carbon reduction

[0014] The high-temperature flue gas in the settling chamber is concentrated into the absorption liquid, which vaporizes HNO3 and H2O and introduces them into the rotary kiln. The high temperature and carbon environment in the kiln are used to reduce HNO3 to N2:

[0015] 4HNO3+5C→2N2+5CO2+2H2O

[0016] (2) Electrolytic regeneration of KMnO4

[0017] The concentrated solution of MnSO4+K2SO4 is electrolyzed:

[0018] Anode chamber:

[0019] Mn 2+ +4H2O→MnO4-+8H++5e -

[0020] Cathode chamber:

[0021] 2H + +2e - →H2↑

[0022] Overall reaction (including potassium salt conversion):

[0023]

[0024] The KMnO4 mixed solution after electrolysis is supplemented with sulfuric acid and EDTA and then returned to the absorption process for recycling.

[0025] Furthermore, in step (1), the KMnO4 solution is controlled to be 10%-15%, and the EDTA is controlled to be 0.3%-0.5%.

[0026] Furthermore, in step (1), the temperature of the high-temperature flue gas is controlled to be 600-1000°C (preferably 700-800°C).

[0027] Furthermore, in step (2), the electrolysis current density is controlled at 80-120 A / dm 2 (Preferably 90-110A / dm 2 ), electrolysis time is controlled at 90-120min.

[0028] The beneficial effects of the present invention are:

[0029] (1) The present invention effectively complexes insoluble NO by adding EDTA, greatly improving its solubility and reaction activity in acidic KMnO4 solution, and achieving a NO removal rate of more than 98%. At the same time, the electrolysis technology is innovatively used to efficiently regenerate the denitrification by-product manganese salt into KMnO4, forming a closed-loop recycling of the denitrification agent, which greatly reduces raw material consumption and operating costs.

[0030] (2) The present invention makes full use of the energy of the high-temperature flue gas at the outlet of the settling chamber to perform gas stripping and concentration on the absorption waste liquid, evaporate water and enrich HNO3 without additional energy consumption; then the concentrated HNO3 vapor is introduced into the rotary kiln, and the high temperature environment and abundant carbonaceous fuel in the kiln are used to reduce it in situ to harmless N2, which completely solves the problem of nitric acid as a secondary pollutant or requiring additional treatment.

[0031] (3) The electrolytic regeneration process of the present invention does not require the addition of alkaline solutions such as KOH, and the energy consumption is significantly reduced to 3.6-4.05 kWh / kgKMnO4, making the regeneration cost much lower than that of traditional methods; more importantly, the entire process completely avoids the generation of high-concentration manganese salt waste liquid, eliminating the environmental risk of being classified as hazardous waste due to excessive manganese salt concentration in the waste liquid, and realizing clean production.

[0032] (4) The present invention demonstrates strong adaptability to complex flue gases such as high calcium, high sulfur, and heavy metal dust. By adding targeted additives such as trace phosphates, oxalic acid, and citric acid, it effectively controls metal ion interference, prevents system scaling and electrolytic pollution, and ensures that the NO emission concentration can be stably reduced to 22 mg / Nm under harsh conditions. 3 Below, meet ≤50mg / Nm 3 Ultra-low emission requirements.

[0033] In summary, the present invention not only achieves efficient and deep removal of NO and economical recycling of absorbents, but also fully utilizes the waste heat of the system and eliminates secondary pollution and hazardous waste, providing an economical, efficient, environmentally friendly and stable technical path for ultra-low NO emissions from flue gas of industrial kilns such as rotary kilns. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0035] The present invention provides a method for removing NO from rotary kiln flue gas by circulating an acidic KMnO4 solution. The core steps include: ① Using an acidic KMnO4 solution to absorb and oxidize NO to produce HNO3 and manganese salts, while EDTA complexation increases NO solubility; ② Using high-temperature flue gas in a settling chamber to concentrate the waste liquid, stripping the HNO3 to the rotary kiln where it is reduced to N2 by carbonaceous fuel; and ③ Electrolyzing the remaining mother liquor to regenerate KMnO4 and recycle it. This method, through a closed-loop process of enhanced EDTA absorption, high-temperature carbon reduction, and electrolytic regeneration, achieves a NO removal rate exceeding 98% and a KMnO4 regeneration rate of 94.5%, significantly reducing operating costs while avoiding the generation of hazardous manganese salt waste.

[0036] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited thereto.

[0037] Example 1: Simulating zinc volatilization flue gas NO control

[0038] Prepare 2L of absorption solution containing 10% KMnO4, 1.5% H2SO4, and 0.5% EDTA and inject it into a glass spray tower (1.2m high, 0.15m in diameter, with 3 spiral nozzles built in). 3 / h, NO 450±20mg / Nm 3 , ZnO dust 200mg / Nm 3 , N2 balance) enters from the bottom of the tower, and the liquid-gas ratio is controlled at 8L / m 3 , absorption temperature 40±2℃. After the reaction, the waste liquid was transferred to the sedimentation tank, 0.1M Na3PO4 was added and stirred for 30min to generate Zn3(PO4)2 precipitation, and the solid phase was separated by 0.45μm filter membrane. The filtrate was pumped into the quartz tube stripping reactor (Φ50×600mm), and 600℃ high temperature flue gas (flow rate 1.2Nm 3 / h) countercurrent contact concentration; when the pH value rises to 7.2 (concentration end point), stop the gas stripping and introduce the steam containing HNO3 into the rotary kiln reduction zone. The remaining concentrated mother liquor (Mn 2+ The concentration was 1.8 mol / L) and transferred into a single-chamber electrolytic cell (graphite anode / stainless steel cathode, plate area 10 cm 2, pole distance 15mm), control current density 100A / dm 2 After 120 min of electrolysis, the regenerated KMnO4 solution was reused after adding fresh EDTA and sulfuric acid. The NO removal rate was 98.2% (the average inlet value was 468 mg / Nm 3 , export 8.4mg / Nm 3 ), the KMnO4 regeneration rate was 94.5% (the KMnO4 concentration of the solution after electrolysis was 9.45%), the electrolytic cell voltage was 3.95V, and the zinc precipitation rate was 92.7%.

[0039] Example 2: Simulated NO treatment of steel roasting flue gas

[0040] The absorption liquid consists of 15% KMnO4, 2.0% H2SO4, 0.3% EDTA and 0.8% oxalic acid, and is used to treat NO 500mg / Nm 3 and Fe2O3 dust 300mg / Nm 3 The simulated flue gas (air balance) is used. After the absorption is completed in the packed tower, the waste liquid enters a two-stage concentration system: the first stage uses 800℃ high-temperature flue gas stripping (gas-liquid ratio 10:1) to vaporize HNO3 and introduce it into the carbon reduction zone in the kiln; the second stage cools down to 250℃ and stirs at a low speed for 2h to crystallize FeC2O4 (ferrous oxalate). After the solid phase is separated, the mother liquor is transferred to a plate and frame electrolytic cell (titanium-based IrO2 anode / 316L stainless steel cathode, plate spacing 20mm), and the current density is controlled at 110A / dm 2 , 0.1M tartaric acid was continuously added to the cathode chamber to complex the residual iron ions, and KMnO4 was regenerated by electrolysis for 90 minutes. After testing, the outlet NO concentration was 22mg / Nm 3 (removal rate 95.6%), KMnO4 regeneration current efficiency is 90.2% (measured anode current 5.2A, theoretical power consumption 3.1A), iron precipitation amount is 89.4mg / L, and regeneration energy consumption is 3.6kWh / kg KMnO4 (electrolysis voltage 4.1V).

[0041] Example 3: Simulating NOx Control in High-Calcium Flue Gas from a Cement Kiln

[0042] For flue gas (NO 380mg / Nm 3 +Ca(OH)2150mg / Nm 3+SO2 1000ppm), treated with a modified absorption solution (15% KMnO4 + 2.0% H2SO4 + 0.3% EDTA + 0.5% H3PO4) in a bubble reactor (1.5L, gas distributor pore size 20μm), with a gas-liquid contact time of 12s. After absorption, the waste liquid was filtered through a plate and frame filter to remove the mixed CaSO4 / Ca3(PO4)2 scale (the calcium content of the filter residue was >90%). The clear liquid was transferred to a rotary evaporator and concentrated under reduced pressure at 70°C (vacuum degree -0.08MPa) to 40% of the original volume. The concentrated liquid was injected into a double-chamber H-type electrolytic cell, and the MnSO4 solution was electrolyzed in the anode chamber (current density 100A / dm 2 , interelectrode distance 15mm), 0.5M H2SO4 circulated in the cathode chamber to maintain pH <1.0, the electrolysis product was separated by precipitation of potassium manganate crystals and then redissolved and circulated. After testing, the NO removal rate was 97.8% (export NO 8.4mg / Nm 3 ), the SO2 synergistic removal rate was 81.3% (export SO2 187ppm), the calcium precipitation rate was 94.1% (filter residue XRF quantitative), and the electrolytic cell voltage was 4.05V (potential difference between the anode and cathode chambers).

Claims

1. A method for absorbing NO in rotary kiln flue gas by circulating acidic KMnO4 solution, characterized in that The following steps are involved: (a) A small amount of H2SO4 and EDTA is added to a KMnO4 solution to absorb NO in the rotary kiln flue gas in an absorption device, and a liquid phase complexation and oxidation reaction occurs to produce a mixture of HNO3 and potassium manganese salt; (b) a certain amount of waste liquid generated by the flue gas stripping step (a) is introduced into the outlet of the settling chamber to evaporate some water and HNO3, Water and HNO3 vapor are introduced into the rotary kiln inlet, and the carbonaceous fuel in the kiln is used to reduce HNO3 to N2; (c) Preparation of KMnO4 by electrolysis of the remaining mother liquor from the process of (b); (d) The produced KMnO4 solution is circulated to step (a).

2. The method according to claim 1, characterized in that In step (a), the mass concentration of the KMnO4 solution is 5%-30%, the mass concentration of sulfuric acid is 0.5%-5%, and the mass concentration of EDTA is 0.1%-1%.

3. The method according to claim 1, characterized in that In step (b), when the flue gas temperature is 500° C.-800° C. and the pH value of the solution is 7±0.3, the concentration is stopped.

4. The method according to claim 1, wherein: Step (c) electrolysis uses a plate-and-frame electrolytic cell with a controlled current density of 80-120 A / dm 2 , the plate spacing is 10-20mm.

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