Pretreatment method and application of red mud

Abstract

The invention belongs to the technical field of electro-catalysis, and discloses a red mud pretreatment method and application thereof.The red mud pretreatment method comprises the steps that industrial waste red mud generated in the aluminum smelting industry is recycled, subjected to high-speed centrifugation, washed to be neutral in a graded mode, filtered, dried and ground into powder, and a working electrode is prepared; through a pre-reduction step, a certain potential is applied to carry out electrochemical in-situ reconstruction on a working electrode. The surface part Fe < 3 + > is reconstructed into high-activity Fe < 0 > through a pre-reduction step. The red mud is used as the electrocatalyst to be applied to the nitrate reduction reaction, the catalytic performance is good, the ammonia production performance is excellent, and meanwhile the method can be applied to treatment of industrial flue gas and industrial wastewater.

Description

Technical Field

[0001] The invention belongs to the technical field of electrochemistry and relates to red mud, and in particular to a red mud pretreatment method and application thereof. Background Art

[0002] Ammonia is an important chemical substance widely used in the production of fertilizers and various industrial products. The traditional Haber-Bosch process for ammonia synthesis has harsh reaction conditions and uses methane as a hydrogen source, which leads to the emission of a large amount of carbon dioxide, causing serious harm to the environment. Finding a cleaner and more sustainable alternative process for ammonia production has important research value and application prospects. At the same time, nitrate is a common nitrogen source pollutant. The burning of fossil fuels, excessive use of nitrogen fertilizers and the discharge of nitrate wastewater have led to a continuous increase in nitrate in the environment, which has a serious impact on aquatic ecosystems and leads to eutrophication of water bodies. In addition, gastrointestinal microorganisms in the human body can convert nitrate into nitrite. Long-term exposure to nitrite can cause damage to the human body and may even cause cancer in severe cases.

[0003] Electrocatalytic nitrate reduction to produce ammonia provides an effective solution to reuse waste nitrogen to produce high value-added ammonia. Currently, due to the transfer process involving 8 electrons and the complex reaction pathway, multiple by-products are produced, the selectivity is poor, and the competitive hydrogen evolution reaction seriously affects the Faraday efficiency. Therefore, it is particularly important to find electrocatalysts with high activity and high selectivity.

[0004] In addition, red mud is a common industrial waste, and the environmental problems it generates during its treatment process are becoming increasingly prominent. However, these wastes contain a small amount of iron and other metal substances, which provides a possibility for finding a sustainable way to utilize these wastes. At the same time, the industrial application of electrocatalysts is in great demand and the cost issue cannot be ignored. It is necessary to find cheaper and more efficient catalysts to make them a viable option for practical applications. Summary of the invention

[0005] Based on the above, one of the purposes of the present invention is to provide a method for pretreating red mud, in which the red mud is pretreated in order to be used as an electrocatalyst, and to exhibit excellent electrocatalytic reduction of nitrate to produce ammonia performance, so that it has certain application value in the treatment of industrial flue gas and industrial wastewater.

[0006] The present invention provides a method for pretreating red mud, which specifically comprises the following steps: Step 1, high-speed centrifugation of red mud, wherein the red mud is waste residue produced by the aluminum smelting industry; Step 2, washing the red mud after centrifugation with water for 3 times until it is neutral, wherein the first washing is performed with water having an EC of 10 to 500 us / cm for 5 times, followed by washing with water having an EC of 1 to 10 us / cm for 3 times, and finally washing with water having an EC of ≤ 5 us / cm for 2 times, filtering after each washing, and finally drying at 80°C; Step 3, mixing the red mud obtained in step 2 with SiO2, Al2O3, and CaO, grinding the mixed sample into a powder, wherein the elements in the powder are as follows by mass percentage: W(Fe): 16.31, W(Al): 11.53%, W(Si): 20.30%, and W(Ca): 5.81%, mixing the mixed powder with isopropanol and 5wt% Nafion solution, ultrasonicating at room temperature for 25-35min to obtain a uniformly mixed dispersion, and then dropping it onto a glassy carbon sheet to prepare a working electrode, and controlling the red mud powder loading per unit area of ​​the glassy carbon sheet to be 0.18μg; Step 4, through a pre-reduction process, a certain potential is applied to electrochemically reconstruct the working electrode obtained in step 3 in situ.

[0007] Furthermore, in step 3, the mixed powder is 3 mg, the isopropanol is 1 mL, and the 5 wt % Nafion solution is 20 μL, and the three are mixed in this proportion.

[0008] Furthermore, in step 4, the pre-reduction process is to place the working electrode and the reference electrode (Hg / HgO electrode) in a cathode electrolyte, wherein the cathode electrolyte is a mixed solution of 1M KOH and 0.1M KNO3, and the counter electrode (platinum electrode) is placed in an anode electrolyte, wherein the anode electrolyte is 1M KOH; the applied potential is -0.8V (vs. RHE) until the current density reaches a stable state.

[0009] Another object of the present invention is to provide an application of red mud material in electrocatalytic nitrate reduction to produce ammonia, specifically: Step 1, conducting electrochemical performance testing in a three-electrode system with good airtightness; Step 2, applying different cathode potentials, maintaining them for a certain period of time, and measuring the ammonia yield and Faraday efficiency at a certain potential.

[0010] Furthermore, the electrochemical performance test described in step 1 is to place the pre-reduced working electrode and the reference electrode (Hg / HgO electrode) in the cathode electrolyte, the cathode electrolyte is a mixed solution of 1M KOH and 0.1M KNO3, and the counter electrode (Pt sheet electrode) is placed in the anode electrolyte, and the anode electrolyte is 1M KOH.

[0011] Furthermore, in step 2, different cathode potentials of 0.4 V, 0.5 V, 0.6 V, 0.7, and 0.8 V (vs. RHE) were applied, the test time was 600 s, ammonium chloride was used as a standard sample, and the indophenol blue colorimetric method was used to quantitatively measure the concentration of ammonia.

[0012] The present invention pre-treats red mud and uses it as a catalyst in the electrocatalytic nitrate reduction reaction. The experimental results show that the electrochemical in-situ reduction step reduces the Fe 3+ Reconstruction to Fe 0 This structural optimization significantly improves the electrocatalytic efficiency of the material; when it is used for electrocatalytic nitrate reduction to synthesize ammonia, it can achieve high ammonia yield and high ammonia FE. The ammonia yield is 3.34 mol h at -0.8 V (vs. RHE). -1 g cat -1 , the Faraday efficiency reaches 90.91%. In addition, this excellent catalytic performance can also be used in industrial flue gas treatment or industrial wastewater treatment.

[0013] Beneficial effects of the present invention: 1. Wide sources of raw materials and low cost: The red mud provided by the present invention is a global industrial waste and is widely distributed. Using it for electrocatalytic nitrate reduction to synthesize ammonia not only has certain advantages in reducing the industrial production cost of traditional catalysts, but also provides another solution for the recycling of red mud.

[0014] 2. Good catalytic performance of the product: The red mud provided by the present invention can maintain a Faradaic efficiency of more than 90% within the applied potential range of electrocatalytic nitrate reduction to synthesize ammonia, and at the same time achieves a 3.34 mol h -1 g cat -1 ammonia yield.

[0015] 3. The product has a wide range of applications: The red mud provided by the present invention exhibits excellent electrocatalytic reduction of nitrates to produce ammonia, which also makes it have certain application value in the treatment of industrial flue gas and industrial wastewater. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 . is the X-ray diffraction pattern (XRD) of the red mud material in Example 1; Figure 2 .a and b are X-ray photoelectron spectra (XPS) of the red mud material of Example 1 before and after pre-reduction, respectively; Figure 3 .It is a schematic diagram of a gas diffusion electrolytic cell device using red mud materials for industrial flue gas treatment; Figure 4 . is a schematic diagram of a flow electrolysis cell device using red mud materials for industrial wastewater treatment. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0018] A red mud pretreatment method specifically comprises the following steps: washing the red mud after high-speed centrifugation with water in batches until it is neutral, the washing conditions are washing with water 5 times (EC=10-500us / cm), then washing with water 3 times (EC=1-10us / cm), and finally washing with water 2 times (EC≤5us / cm), filtering and drying in an oven at 80°C for 24 hours; mixing the dried red mud with SiO2, Al2O3, and CaO, and the elements in the mixed sample are calculated by mass percentage as follows: W(Fe):16.31, W(Al):11.53%, W(Si):20.30%, and W(Ca):5.81%. Grinding the mixed sample into powder, weighing 3 mg of the mixed powder, adding 1 ml of isopropanol and 20 μL of Nafion (5wt%) solution to mix, and ultrasonicating at room temperature for 25-35 minutes to obtain a mixed solution; then, dripping the mixed solution onto a glassy carbon sheet to prepare a working electrode, ensuring that the catalyst loading per unit area of ​​the glassy carbon sheet is 0.18 μg. The working electrode and the reference electrode (Hg / HgO electrode) were placed in the cathode electrolyte, which was a mixed solution of 1M KOH and 0.1M KNO3, and the counter electrode (platinum electrode) was placed in the anolyte, which was 1M KOH. In a well-sealed three-electrode system, the catalyst was electrochemically reconstructed in situ by applying a potential of -0.8V (vs. RHE) until the current density reached a stable level. Figure 1 As shown, the structure of red mud was characterized and analyzed by X-ray diffraction (XRD), showing relevant diffraction peaks belonging to Fe2O3. Figure 2 As shown, a and b are the X-ray photoelectron spectra (XPS) of the red mud material before and after pre-reduction, respectively. The results show that the valence state of Fe in the red mud material before pre-reduction was trivalent and divalent, and zero-valent iron appeared after applying potential for pre-reduction. The results confirm that the pre-reduction step reconstructs part of the trivalent iron on the surface into zero-valent iron.

[0019] Application Example 1 In a three-electrode system with good airtightness, the pre-reduced working electrode and the reference electrode (Hg / HgO electrode) in Example 1 were placed in a cathode electrolyte, which was a mixed solution of 1M KOH and 0.1M KNO3; the counter electrode (Pt sheet electrode) was placed in an anolyte, which was 1M KOH; it tests were performed at potentials of 0.4V, 0.5V, 0.6V, 0.7, and 0.8V (vs. RHE), respectively, for 600s, and each group of experiments was repeated at least three times. Using ammonium chloride as a standard sample, the concentration of ammonia was quantitatively measured using the indophenol blue colorimetric method.

[0020] Table 1 Ammonia yield and FE of each product at different potentials during nitrate reduction of red mud materials for ammonia synthesis Table 1 Ammonia yield and FE, by-products (NO2 - In Table 1, the red mud material exhibited excellent Faradaic efficiency and ammonia production performance in the potential range of 0.4 to 0.8 V (vs. RHE). The performance was optimal at 0.8 V (vs. RHE), with an ammonia yield of 56749.75 μg h -1 mg cat -1 , the Faradaic efficiency reached 90.91%. The possible by-products (NO2 - , H2) and the Faraday efficiency of red mud materials for NH4 + The Faradaic efficiency can be maintained above 80%, showing a good + Higher selectivity.

[0021] Application Example 2 Application of the pre-reduced working electrode obtained in Example 1 in industrial flue gas treatment, Figure 3 Schematic diagram of a gas diffusion electrolytic cell for industrial flue gas treatment, the main components include: cathode gas chamber plate 1, diffusion medium 2, sealing gasket 3, cathode 4, cathode liquid chamber plate 5, nafion membrane 6, anode liquid chamber plate 7, anode 8, anode end plate 9; Among them, the diffusion medium 2 is foamed titanium, the cathode 3 is the pre-reduced working electrode in Example 1, the anode 8 is a platinum sheet electrode, the cathode gas chamber plate 1 has an air outlet and an air inlet at the upper and lower ends, and the cathode liquid chamber plate 5 and the anode liquid chamber plate 7 have a liquid outlet and a liquid inlet at the upper and lower ends.

[0022] When used, the industrial flue gas tail gas bypass pipe is connected to the electrolytic cell, and the industrial flue gas is introduced into the electrolytic cell through the air inlet below the cathode gas chamber plate 1, with a gas flow rate of 5 to 6 m 3 / h, and then inject electrolyte (1M KOH solution) into the electrolytic cell through the liquid inlet below the cathode liquid chamber plate 5 and the anode liquid chamber plate 7. The industrial flue gas is dispersed by the diffusion medium and forms a gas-liquid-solid three-phase interface with the electrolyte on the surface of the working electrode. A DC power supply is used to apply current, and NO in the industrial flue gas is X Get electrons and protons to generate NH4 + The final product emission is flue gas particles ≤ 10mg / Nm 3 ,SO2≤30mg / Nm 3 , NO x ≤100mg / Nm 3 .

[0023] Application Example 3 Application of the pre-reduced working electrode obtained in Example 1 in industrial wastewater treatment, Figure 4 It is a schematic diagram of an H-type electrolytic cell for industrial wastewater treatment, and its main components include: cathode end plate 1, current collector 2, sealing gasket 3, cathode 4, buffer layer 5, nafion membrane 6, anode 7, and anode end plate 8; The cathode 4 is the pre-reduced working electrode in Example 1, the anode 7 is a platinum sheet electrode, the lower ends of the cathode end plate 1 and the anode end plate 8 are provided with liquid inlets, and a reference electrode hole is reserved on the side of the cathode end plate 1.

[0024] When used, industrial wastewater and 1M KOH solution are injected into the electrolytic cell through the liquid inlets below the cathode end plate 1 and the anode end plate 8, respectively. An electrochemical workstation is used to apply current, and HNO in the industrial wastewater is X Get electrons and protons to generate NH4 + The nitrate content in the treated industrial wastewater meets the industrial discharge standard.

[0025] In summary, the present invention proposes that when red mud is pretreated and used for nitrate reduction reaction to synthesize ammonia, higher ammonia production performance can be achieved. It is applied to the treatment of industrial flue gas and industrial wastewater to effectively remove nitrogen oxides.

[0026] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for pretreating red mud, which specifically comprises the following steps: Step 1, high-speed centrifugation of red mud, wherein the red mud is waste residue produced by the aluminum smelting industry; Step 2, washing the red mud after centrifugation with water for 3 times until it is neutral, wherein the first washing is performed with water having an EC of 10 to 500 us / cm for 5 times, followed by washing with water having an EC of 1 to 10 us / cm for 3 times, and finally washing with water having an EC of ≤ 5 us / cm for 2 times, filtering after each washing, and finally drying at 80°C; Step 3, mixing the red mud obtained in step 2 with SiO2, Al2O3, and CaO, grinding the mixed sample into powder, the elements in the powder are as follows by mass percentage: W(Fe): 16-18%, W(Al): 11-13%, W(Si): 20-22%, W(Ca): 5-7%, the mixed powder is mixed with isopropanol and 5wt% Nafion solution, ultrasonicated at room temperature for 25-35min to obtain a uniformly mixed dispersion, and then dripped onto a glassy carbon sheet to prepare a working electrode, and the red mud powder loading per unit area of ​​the glassy carbon sheet is controlled to be 0.18μg; Step 4, through a pre-reduction process, a certain potential is applied to electrochemically reconstruct the working electrode obtained in step 3 in situ.

2. A method for pretreating red mud according to claim 1, characterized in that: In step 3, 3 mg of mixed powder, 1 mL of isopropanol, and 20 μL of 5 wt % Nafion solution are mixed in this proportion.

3. A method for pretreating red mud according to claim 2, characterized in that: In step 4, the pre-reduction process is to place the working electrode and the reference electrode Hg / HgO electrode in the cathode electrolyte, wherein the cathode electrolyte is a mixed solution of 1M KOH and 0.1 M KNO3, and the counter electrode platinum sheet electrode is placed in the anode electrolyte, and the anode electrolyte is 1M KOH; the applied potential is -0.8V (vs. RHE) until the current density reaches stability.

4. Use of the red mud material obtained according to the method according to any one of claims 1 to 3 in electrocatalytic nitrate reduction to produce ammonia.

5. The method for using the red mud material in electrocatalytic nitrate reduction to produce ammonia according to claim 4, characterized in that: Specifically: Step 1, conducting electrochemical performance testing in a three-electrode system with good airtightness; Step 2, applying different cathode potentials, maintaining them for a certain period of time, and measuring the ammonia yield and Faraday efficiency at a certain potential.

6. The method for using the red mud material in electrocatalytic nitrate reduction to produce ammonia according to claim 5, characterized in that: In step 1, the electrochemical performance test is to place the pre-reduced working electrode and the reference electrode Hg / HgO electrode in a cathode electrolyte, which is a mixed solution of 1M KOH and 0.1M KNO3, and place the counter electrode Pt sheet electrode in an anode electrolyte, which is 1M KOH.

7. The method for using the red mud material in electrocatalytic nitrate reduction to produce ammonia according to claim 5 or 6, characterized in that: In the step 2, different cathode potentials of 0.4 V, 0.5 V, 0.6 V, 0.7, and 0.8 V (vs. RHE) are applied, the test time is 600 s, ammonium chloride is used as a standard sample, and the indophenol blue colorimetric method is used to quantitatively measure the concentration of ammonia.

8. Use of the red mud material obtained according to the method according to any one of claims 1 to 3 in industrial flue gas treatment or industrial wastewater treatment.

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