Composite catalyst and nitrate waste acid treatment method

Abstract

The invention discloses a composite catalyst and a nitrate waste acid treatment method, and belongs to the technical field of industrial waste acid treatment. The composite catalyst provided by the invention is used for treating nitrate waste acid and catalyzing formic acid / methanol nitrate reduction reaction, nitrate in a complex waste acid system is reduced into N2, generation of by-products is reduced, meanwhile, the composite catalyst can be recycled for more than 8 times, and the composite catalyst has remarkable industrial application value and environmental protection significance.

Description

Technical Field

[0001] This application belongs to the field of industrial waste acid treatment technology, and in particular relates to a composite catalyst and a method for treating nitrate waste acid. Background Technology

[0002] High-nitrate waste acid is a typical hazardous waste generated during the production processes of industries such as metallurgy, chemical engineering, electroplating, and electronic etching. Its nitrate (NO3) content is high. - The concentration of nitrates is typically as high as 5% to 20%, and they contain heavy metals such as copper, nickel, chromium, and lead, as well as small amounts of organic pollutants. If this type of waste acid is discharged directly without proper treatment, it will cause serious environmental problems: nitrates entering water bodies will cause eutrophication and disrupt the ecological balance; seeping into the soil will affect crop growth and harm human health through the food chain; the heavy metals in them have bioaccumulative toxicity, and the high acidity environment will also corrode facilities and, under anaerobic conditions, promote the conversion of nitrates into carcinogenic nitrosamines.

[0003] Currently, commonly used industrial treatment methods include chemical precipitation, evaporation concentration, biological treatment, and electrochemical reduction. However, these technologies all have significant drawbacks. Chemical precipitation consumes large amounts of alkali and produces nitrate-containing sludge, causing secondary pollution. Evaporation concentration is extremely energy-intensive and cannot effectively separate nitrates from heavy metals. Biological treatment is inefficient under high acidity conditions and easily generates ammonia nitrogen byproducts. Membrane separation technology faces problems such as easy corrosion and short lifespan of membrane materials. While electrochemical reduction has some effectiveness, the electrodes are prone to passivation and may produce harmful intermediate products. These limitations of existing technologies keep the treatment cost of high-nitrate waste acid high and make it difficult to achieve true harmlessness and resource recovery.

[0004] Existing technology discloses a metal catalyst formed by CeO2 supported on palladium (Pd) and indium (In) for the reduction of nitrate to nitrogen by formic acid at room temperature and pressure. When the amount of formic acid is 8 mmol / L, i.e., the molar ratio with nitrate is 0.5:1, the N2 conversion rate is 32%, with poor selectivity for N2. When the amount of formic acid is increased to 32 mmol / L, i.e., the molar ratio with nitrate is 2:1, the N2 conversion rate can reach 99%. However, the existing technology cannot efficiently achieve the reduction of nitrate to nitrogen by formic acid with a small amount of formic acid added, and it is not applicable to the removal of nitrate in complex waste acid systems.

[0005] Therefore, developing a composite catalyst composed of a mixture of ruthenium-supported activated carbon and cerium oxide for the treatment of nitrate waste acid, catalyzing the reduction of nitrate by formic acid / methanol, reducing nitrate in complex waste acid systems to N2, reducing byproduct formation, and the composite catalyst can be recycled more than 8 times, has important research significance and application value. Summary of the Invention

[0006] To address the technical problems existing in the prior art, the primary objective of this invention is to provide a composite catalyst.

[0007] Another object of the present invention is to provide the application of the above-mentioned composite catalyst in the treatment of nitrate-containing waste acid.

[0008] Another object of the present invention is to provide a method for treating nitrate waste acid using the above-mentioned composite catalyst.

[0009] Another object of the present invention is to provide a system for implementing the above-mentioned nitrate waste acid treatment method.

[0010] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: This invention protects a composite catalyst obtained by ball milling and mixing ruthenium-supported activated carbon and cerium oxide; The mass ratio of the ruthenium-supported activated carbon to cerium oxide is 1:(1~3).

[0011] The ruthenium-loaded activated carbon is prepared by the following method: after impregnating activated carbon with a ruthenium salt solution, it is thermally reduced in a hydrogen atmosphere to obtain activated carbon loaded with ruthenium atoms, i.e., ruthenium-loaded activated carbon. The specific surface area of ​​the activated carbon is 800~1500 m². 2 / g.

[0012] This invention provides a composite catalyst Ru / C-CeO2, where ruthenium (Ru) is an excellent hydrogenation and dehydrogenation catalyst. It can efficiently adsorb and activate hydrogen gas (if the reducing agent is formic acid, formic acid decomposition can produce H2) or directly activate the reducing agent (such as formic acid), and provide active hydrogen species to nitrate molecules, breaking the NO bond. Activated carbon has a molecular weight of 800~1500 μm. 2 With a specific surface area of ​​ / g, ruthenium metal particles, tiny and highly active, can be highly dispersed and firmly fixed on its surface, preventing ruthenium particle agglomeration and deactivation. This greatly increases the contact area between the catalyst and the reactants, improving ruthenium utilization efficiency. Cerium oxide (CeO2) reacts with the intermediate product nitrite (NO2). - It has extremely strong adsorption and activation capabilities. It can rapidly capture NO2 generated during the reaction. - To prevent its accumulation and escape from the reactor (forming "yellow smoke" NO) x (including nitric oxide and nitrogen dioxide), or further excessively reduced to NH4. + .

[0013] Specifically, the specific surface area of ​​the activated carbon is calculated using the BET test.

[0014] Preferably, the ruthenium-supported activated carbon is prepared by immersing the activated carbon in a ruthenium salt solution, soaking it at 100~120°C for more than 3 hours, and then thermally reducing it in a hydrogen atmosphere at 400~500°C.

[0015] Preferably, the ball milling mixing conditions are ball milling at room temperature for 2 to 3 hours.

[0016] Preferably, the activated carbon undergoes pretreatment, including the following steps: crushing and sieving through a 40-60 mesh sieve; soaking the sieved activated carbon in 1-2 mol / L dilute hydrochloric acid for 4-6 hours at a temperature of 60-70°C with stirring; washing with deionized water until the pH is neutral; soaking in a 10-30% hydrogen peroxide solution at room temperature for 6-12 hours; washing the activated carbon after the above treatment with deionized water until neutral, and then drying it at 110-120°C for at least 12 hours to completely remove moisture.

[0017] This invention also protects the application of the above-mentioned composite catalyst in the treatment of nitrate-containing waste acid.

[0018] This invention protects a method for treating nitrate waste acid, comprising the following steps: S1. Add sodium sulfide and / or sodium sulfate to the waste acid to control the pH to below 3, precipitate heavy metal ions, and separate solid and liquid. S2. The filtrate obtained in S1. is introduced into a reactor filled with the above-mentioned composite catalyst, and formic acid and / or methanol are added as reducing agents to catalytically reduce nitrate to N2; The molar ratio of the reducing agent to nitrate is 1~2:1.

[0019] Under pH conditions of 1.0–3.0, precipitants such as sodium sulfide and sodium sulfate can achieve highly selective precipitation of specific heavy metals (such as copper, nickel, chromium, and lead), while amphoteric metal ions such as iron and aluminum in the solution maintain high solubility at this pH and are not easily precipitated. If traditional alkaline agents (such as NaOH) are used to directly adjust the pH to neutral or alkaline to precipitate heavy metal ions, almost all heavy metal ions and most other metal ions will form hydroxide precipitates, resulting in large precipitate volumes, complex sludge composition, and difficult and costly subsequent treatment, as well as waste of aluminum and iron resources and increased reagent costs. Therefore, precipitation under these conditions removes only the target harmful heavy metals, resulting in less precipitate and a more homogeneous composition, which is beneficial for the subsequent resource recovery and utilization of heavy metal sludge and residual liquid. Sodium sulfide is key to selective precipitants, selectively precipitating heavy metals such as copper, nickel, and chromium, while sodium sulfate can precisely precipitate heavy metal lead. Waste acid generated by different industries contains different types of heavy metals, and targeted selective precipitants can be selected according to specific circumstances.

[0020] Preferably, the ratio of the amount of composite catalyst loaded in the reactor to the amount of waste acid treated is 1 g: 1~3 L.

[0021] Preferably, the amount of sodium sulfide and / or sodium sulfate added in step S1 is 1 to 2 times the total molar amount of heavy metal ions.

[0022] Preferably, the reaction time for the catalytic reduction of nitrate in step S2 is 1 to 3 hours.

[0023] Preferably, the reaction pressure for the catalytic reduction of nitrate in step S2 is 0.1~0.5 MPa.

[0024] Preferably, the reaction temperature for the catalytic reduction of nitrate in step S2 is 80~120℃.

[0025] Preferably, the nitrate concentration in the waste acid is 50,000 to 200,000 mg / L.

[0026] Preferably, the process further includes the following step: S2. The tail gas after the reaction is absorbed by alkaline solution and then discharged, and the remaining liquid is recycled.

[0027] Specifically, the remaining liquid is weakly acidic and contains some iron, aluminum and other metal ions, which can be used as raw materials for the production of flocculants, realizing the recovery and utilization of acid, aluminum and iron resources in the remaining liquid.

[0028] A system for implementing the above method includes a sedimentation tank, a catalytic reactor, a gas-liquid separator, and an absorption tower connected in sequence. The catalytic reactor is equipped with a porous plate for fixing the composite catalyst.

[0029] Application of the above composite catalyst in the reduction of nitrate by formic acid and / or methanol; The composite catalyst is a mixture of ruthenium-supported activated carbon and cerium oxide. The mass ratio of ruthenium-supported activated carbon to cerium oxide in the composite catalyst is 1:(1~3).

[0030] Preferably, the preparation method of the composite catalyst includes the following steps: Ruthenium-supported activated carbon is obtained by impregnating it with ruthenium salt and then thermally reducing it. The ruthenium-supported activated carbon is then mixed with cerium oxide powder to obtain a composite catalyst.

[0031] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a composite catalyst and a method for treating nitrate waste acid. The composite catalyst provided by this invention is used for treating nitrate waste acid, catalyzing the reduction of nitrate by formic acid / methanol, reducing the formation of by-products, and the composite catalyst can be recycled more than 8 times, which has significant industrial application value and environmental significance. Detailed Implementation

[0032] The present invention is further illustrated below with reference to specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions in the art or as recommended by the manufacturer; the raw materials and reagents used, unless otherwise specified, are all commercially available from the conventional market. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention are within the scope of protection claimed by the present invention.

[0033] I. Experimental Methods Waste acid to be treated: Nitrate waste acid discharged from a copper factory, containing HNO3. - The mass fraction is 10%, Cu 2+ The concentration is 2 g / L.

[0034] Pre-treated activated carbon: Commercially available activated carbon is crushed and sieved through a 60-mesh sieve; the sieved activated carbon is soaked in 2 mol / L dilute hydrochloric acid for 5 hours, heated to 70°C and stirred during the process, and then repeatedly washed with a large amount of deionized water until the pH is neutral; it is then soaked in a 20% hydrogen peroxide solution at room temperature for 9 hours; the activated carbon after the above treatment is washed with deionized water until neutral, and then dried at 120°C for at least 12 hours to completely remove moisture.

[0035] Composite catalyst 1#: Preparation of S1.Ru / C catalyst: Pretreated activated carbon (with a specific surface area of ​​1459 m² calculated by BET test) was used. 2 / g) is immersed in an aqueous solution of ruthenium trichloride (RuCl3) with a concentration of 0.5 mol / L, stirred at 100℃ for 3 hours, and the water is evaporated to remove the ruthenium salt uniformly loaded on the pores and surface of the activated carbon; heat treatment is carried out at 400℃ in a hydrogen atmosphere to reduce the ruthenium ions to metallic ruthenium nanoparticles; S2. Composite: The Ru / C catalyst prepared in S1 is ball-milled with cerium oxide powder at a mixing mass ratio of 1:1 for 2 hours to obtain composite catalyst 1#.

[0036] Composite catalyst 2#: The preparation method is the same as that of composite catalyst 1#, except that in step S2, the Ru / C catalyst and cerium oxide powder are ball-milled and mixed at a mass ratio of 1:2.

[0037] Composite catalyst 3#: The preparation method is the same as that of composite catalyst 1#, except that in step S2, the Ru / C catalyst and cerium oxide powder are ball-milled and mixed at a mass ratio of 1:3.

[0038] Composite catalyst #4: The preparation method is the same as composite catalyst #1, the difference being that the specific surface area of ​​the pretreated activated carbon, calculated by BET testing, is 805 m². 2 / g; The pretreatment method is as follows: commercial activated carbon is crushed and sieved through a 60-mesh sieve; the sieved activated carbon is soaked in 1 mol / L dilute hydrochloric acid for 4 hours, heated to 70°C and stirred during the process, and then repeatedly washed with a large amount of deionized water until the pH is neutral; it is then soaked in a 20% hydrogen peroxide solution at room temperature for 12 hours; the activated carbon after the above treatment is washed with deionized water until neutral, and then dried at 120°C for at least 12 hours to completely remove moisture.

[0039] Composite catalyst #5: The preparation method is the same as composite catalyst #1, the difference being that the specific surface area of ​​the pretreated activated carbon is calculated to be 700 m² / s². 2 / g; The pretreatment method is as follows: commercial activated carbon is crushed and sieved through a 60-mesh sieve; the sieved activated carbon is soaked in 1 mol / L dilute hydrochloric acid for 2 hours, heated to 70°C and stirred during the process, and then repeatedly washed with a large amount of deionized water until the pH is neutral; it is then soaked in a 20% hydrogen peroxide solution at room temperature for 15 hours; the activated carbon after the above treatment is washed with deionized water until neutral, and then dried at 120°C for at least 12 hours to completely remove moisture.

[0040] Composite catalyst 6#: The preparation method is the same as that of composite catalyst 1#, except that in step S2, the Ru / C catalyst and cerium oxide powder are ball-milled and mixed at a mass ratio of 1:0.5.

[0041] Composite catalyst 7#: The preparation method is the same as that of composite catalyst 1#, except that in step S2, the Ru / C catalyst and cerium oxide powder are ball-milled and mixed at a mass ratio of 1:4.

[0042] Example 1 S1. Add sodium sulfide to precipitate Cu. 2+ Control the pH to 2.0 and filter. S2. The filtrate is pumped into the catalytic reactor, and composite catalyst #1 is loaded with a loading amount of 100g. The waste acid flow rate is 50L / h. The mass ratio of Ru / C catalyst to cerium oxide powder is 1:1, and the reducing agent is formic acid, calculated as n(formic acid):n(NO3). - Add the ingredients at a ratio of 1:1 and react at 100℃ for 2 hours. S3. The exhaust gas is absorbed by 8% NaOH, and the remaining liquid is reused in the flocculant production process.

[0043] Example 2 The experimental method is the same as in Example 1, except that the pH is controlled to be 1.0 in step S1.

[0044] Example 3 The experimental method is the same as in Example 1, except that the pH is controlled at 3.0 in step S1.

[0045] Example 4 The experimental method is the same as in Example 1, except that composite catalyst 2# is loaded in step S2.

[0046] Example 5 The experimental method is the same as in Example 1, except that composite catalyst 3# is loaded in step S2.

[0047] Example 6 The experimental method is the same as in Example 1, except that in step S2, the reducing agent formic acid is calculated as n(formic acid):n(NO3) - =1.5:1 addition.

[0048] Example 7 The experimental method is the same as in Example 1, except that composite catalyst 4# is loaded in step S2.

[0049] Comparative Example 1 The experimental method is the same as in Example 1, except that step S1, i.e., the step of precipitating heavy metal ions, is not performed, and the waste acid is directly added to the reactor of the composite catalyst.

[0050] Comparative Example 2 The experimental method is the same as in Example 1, except that composite catalyst 5# is loaded in step S2.

[0051] Comparative Example 3 The experimental method is the same as in Example 1, except that composite catalyst 6# is loaded in step S2.

[0052] Comparative Example 4 The experimental method is the same as in Example 1, except that composite catalyst 7# is loaded in step S2.

[0053] Comparative Example 5 The experimental method is the same as in Example 1, except that the pH is controlled at 4.0 in step S1.

[0054] Comparative Example 6 The experimental method is the same as in Example 1, except that no composite catalyst is loaded in step S2.

[0055] Comparative Example 7 The experimental method is the same as in Example 1, except that in step S2, the composite catalyst is replaced with an equal mass of Pd–In / CeO2 bimetallic catalyst.

[0056] Preparation method of Pd–In / CeO2 bimetallic catalyst: CeO2 support was suspended in an aqueous solution containing PdCl2 and stirred for 2 hours (room temperature); after evaporation of water, the catalyst was dried in a sand bath at 80°C, and then further dried in an oven at the same temperature; the prepared single-metal Pd / CeO2 catalyst was first rinsed with nitrogen (N2) for 20 minutes, and then reduced with hydrogen (H2, 250 mL / min) at room temperature for 1 hour; under continuous hydrogen purging, In(NO3)3 (indium nitrate) solution was added to the reduced Pd / CeO2 catalyst, and stirring was continued for 2 hours. After completion, the catalyst was filtered and dried at 80°C.

[0057] II. Test Indicators (1) Nitrate content (NO3) - HJ / T 346-2007 Determination of nitrate nitrogen in water quality by ultraviolet spectrophotometry; (2) Copper content (Cu) 2+ GB 7475-1987 Determination of Copper, Zinc, Lead and Cadmium in Water - Atomic Absorption Spectrophotometry; (3) Number of times the catalyst can be recycled; (4) Gas production phenomenon: By observing the gas produced by the reaction, it is divided into three levels: excellent, good and poor; excellent: colorless, no irritating odor, N2 conversion rate above 95%; good: slightly colored, with irritating odor, N2 conversion rate 70~94%; poor: obvious color, obvious irritating odor, N2 conversion rate 0~69%.

[0058] Table 1 Experimental Results

[0059] Examples 1-7 provide methods for treating nitrate waste acid, reducing the nitrate content to below 2500 mg / L, while reducing the copper content in the wastewater to <1.0 mg / L. The composite catalyst can be recycled more than 8 times, and the gas produced by the reaction is almost entirely nitrogen.

[0060] In Comparative Example 1, the precipitation of heavy metals in step S1 was omitted, leading to catalyst poisoning and deactivation. This resulted in the reduction product no longer being the highly selective N2, causing a surge in side reactions and the generation of numerous other byproducts. Heavy metals may deposit on the catalyst surface, clogging catalyst pores, or form scale on the reactor inner wall, affecting mass transfer and reaction efficiency, thus significantly reducing the nitrate treatment effect. In Comparative Example 2, the activated carbon used had a specific surface area of ​​less than 800 m². 2The reduced ruthenium loading in Comparative Example 3 resulted in a decrease in overall nitrate removal efficiency and made it difficult for the catalyst to achieve more than 8 cycles. Comparative Example 3 used a composite catalyst with too little cerium oxide, leading to insufficient CeO2, weak control over intermediate products, increased byproducts, worsened gaseous conditions, and reduced denitrification efficiency and catalyst lifespan. Comparative Example 4 used a composite catalyst with too much cerium oxide, which may have caused excess CeO2 to cover some Ru active sites, resulting in decreased reaction activity and worsened gaseous conditions. In Comparative Example 5, the pH was controlled at 4.0 in step S1, causing some other metals (such as iron and aluminum) to begin forming hydroxide colloids, which may have encapsulated small amounts of copper ions or contaminated the catalyst surface, leading to incomplete copper removal and a significant decrease in catalyst cycle count. Comparative Example 6 did not use a composite catalyst, and could not catalyze the reduction reaction of nitrate removal with the reducing agent. The removal efficiency was significantly reduced for the same reaction time, gaseous conditions worsened, selectivity to nitrogen decreased, and a large amount of NO was generated. x Gaseous compounds; Comparative Example 7 used a Pd–In / CeO2 bimetallic catalyst, which resulted in unstable catalyst loading, poor recycling rate, insufficient selectivity for N2, and the generation of more NH4. + .

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A composite catalyst characterized in that, It is obtained by ball milling and mixing ruthenium-supported activated carbon and cerium oxide; The mass ratio of the ruthenium-supported activated carbon to cerium oxide is 1:(1~3). The ruthenium-loaded activated carbon is prepared by the following method: after impregnating activated carbon with a ruthenium salt solution, it is thermally reduced in a hydrogen atmosphere to obtain activated carbon loaded with ruthenium atoms, i.e., ruthenium-loaded activated carbon. The activated carbon has a specific surface area of 800 to 1500 m 2 / g.

2. The application of the composite catalyst according to claim 1 in the treatment of nitrate-containing waste acid.

3. A method for treating nitrate salt waste acid, characterized by, Includes the following steps: S1. Add sodium sulfide and / or sodium sulfate to the waste acid to control the pH to below 3, precipitate heavy metal ions, and separate solid and liquid. S2. The filtrate obtained in S1. is introduced into a reactor filled with the composite catalyst of claim 1, and formic acid and / or methanol are added as reducing agents to catalytically reduce nitrate to N2; The molar ratio of the reducing agent to nitrate is 1~2:

1.

4. The nitrate salt waste acid treatment method according to claim 3, characterized by, The ratio of the amount of composite catalyst loaded in the reactor to the amount of waste acid treated is 1 g: 1~3 L.

5. The method of claim 3, wherein the nitrate waste acid is treated by, The amount of sodium sulfide and / or sodium sulfate added in step S1 is 1 to 2 times the total molar amount of heavy metal ions.

6. The method for treating nitrate waste acid according to claim 3, characterized in that, The reaction time for the catalytic reduction of nitrate in step S2 is 1-3 hours.

7. The method for treating nitrate waste acid according to claim 3, characterized in that, The reaction pressure in step S2 is 0.1~0.5 MPa.

8. The method for treating nitrate waste acid according to claim 3, characterized in that, The nitrate concentration in the waste acid is 50,000~200,000 mg / L.

9. The method for treating nitrate waste acid according to claim 3, characterized in that, It also includes the following steps: S2. The tail gas after the reaction is absorbed by alkaline solution and then discharged, while the remaining liquid is recycled.

10. A system for implementing the method according to any one of claims 3 to 9, characterized in that, It includes a sedimentation tank, a catalytic reactor, a gas-liquid separator, and an absorption tower connected in sequence; The catalytic reactor is equipped with a porous plate for fixing the composite catalyst of claim 1.

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