Electrochemical reduction method for liquid indigo with controllable alkali concentration

Abstract

The invention discloses an electrochemical reduction method of liquid indigo with controllable alkali concentration. According to the method, a plate and frame type electrolytic cell is adopted, a mixed alkaline aqueous solution of indigo and a chemical reducing agent is used as a catholyte, an acidic aqueous solution is used as an anolyte, a three-dimensional porous carbon-based conductive material is used as a cathode, and a noble metal electrode is used as an anode; a direct current or pulse current sequentially passes through an anolyte, a diaphragm and a catholyte from an anode to a cathode for an electrochemical reduction reaction, and indigo in the catholyte is electrochemically reduced into indigo, so that the concentration of alkali in the catholyte is controllable by using a sulfuric acid solution in the anolyte, and the ratio of the concentration of alkali in the electrolyte to the concentration of indigo is less than 3.4; in addition, under the condition that the alkali concentration is controllable, it can be guaranteed that the current efficiency of indigo electroreduction reaches 58.7%, and the indigo conversion rate reaches 93.1%.

Description

(I) Technical field

[0001] The invention relates to a liquid indigo electrochemical reduction method with controllable alkali concentration. (II) Background technology

[0002] Indigo (I) is an important non-azo colorant with a unique hue. It is one of the oldest pigments known to mankind and is widely used in food, medicine, and printing and dyeing industries, especially in the textile industry. It is often used to make denim and blue jeans, etc. Its production volume is huge. Therefore, it is of great significance to study environmentally friendly and efficient indigo production processes. Indigo is difficult to dissolve in water, but the indigo white (II) obtained after reduction has good solubility in alkaline aqueous solutions. Therefore, before dyeing, indigo needs to be reduced to indigo white in an alkaline aqueous solution. After the indigo white (golden yellow) dissolved in the alkaline aqueous solution is evenly adsorbed on the cotton yarn and exposed to air, the indigo white is oxidized to indigo, thereby dyeing the cotton yarn evenly and firmly.

[0003]

[0004] At present, the main method of indigo dyeing in industry is the hydrosulfite reduction dyeing method. Figure 1 The process flow and general material flow are as follows. In this process flow, there is a high concentration of indigo in the dye vat, which is reduced with insurance powder under alkaline conditions. The indigo white solution obtained by reduction is diluted in the dye bath. The diluted indigo white solution can be dyed. The optimal concentration ratio of sodium hydroxide and indigo white in the dye vat is 3.4. Dyeing according to this ratio will produce a good cloth. It is mainly reflected in: the color of the fabric is relatively pure and bright, and the saturation and brightness of the color are relatively ideal; the fabric has a uniform dyeing effect without obvious color difference problems. However, this process also has a large environmental pollution problem. The discharged wastewater contains high concentrations of sulfates and sulfites, which are not only highly corrosive but also very difficult to treat, seriously polluting the environment.

[0005] Electrochemical reduction of indigo is a safe and environmentally friendly indigo reduction technology that conforms to the concept of modern environmental protection economy. At present, the electrochemical reduction of indigo technology uses alkaline solution as the anode, but this technology has the following problems: the ratio of sodium hydroxide to indigo white in the final electrolyte is greatly increased, exceeding the ratio in the industrial dyeing tank.

[0006] Excessive alkali concentration in the dye vat will have a variety of adverse effects: (1) For fabrics, it may damage the fibers, reduce the polymerization degree of cotton fibers, and make the fabric fragile and vulnerable; the high alkali environment may cause the indigo to change color or over-react, and it may not be able to color normally. (2) For the dyeing process, high alkali concentration may cause dye precipitation, affecting the dyeing effect, and may also block the circulation system of the dye vat. (3) High alkali concentration will also increase the difficulty and cost of wastewater treatment.

[0007] Therefore, finding a liquid indigo electrochemical reduction method with controllable alkali concentration is crucial in the textile industry. (III) Summary of the invention

[0008] The purpose of the present invention is to provide a liquid indigo electrochemical reduction method with controllable alkali concentration, which ensures that the alkali concentration in the cathode chamber is controllable and the ratio of the alkali solution to indigo white is close to the industrial optimal ratio of 3.4:1, effectively improving the indigo white yield and current efficiency. It solves the problems of poor dyeing effect, low current efficiency, and difficult wastewater treatment.

[0009] The technical solution adopted by the present invention is:

[0010] The present invention provides a method for electrochemical reduction of liquid indigo with controllable alkali concentration. The method adopts a plate-and-frame electrolytic cell, uses a mixed alkaline aqueous solution of indigo (I) and a chemical reducing agent as a cathode liquid, an acidic aqueous solution as an anode liquid, a three-dimensional porous carbon-based conductive material as a cathode, and a noble metal electrode as an anode; direct current or pulse current is passed from the anode to the cathode in sequence through the anode liquid, a diaphragm and the cathode liquid to perform an electrochemical reduction reaction, and the indigo in the cathode liquid is electrochemically reduced to indigo white (II).

[0011]

[0012] Preferably, the alkaline aqueous solution in the cathode liquid is an aqueous solution containing a supporting electrolyte, and the supporting electrolyte includes alkali metal hydroxides (such as NaOH, KOH, etc.), carbonates (such as Na2CO3, K2CO3, etc.), quaternary ammonium bases (such as tetramethylammonium hydroxide, tetraethylammonium hydroxide), and ammonia monohydrate.

[0013] Preferably, the chemical reducing agent includes any chemical reagent that can theoretically reduce indigo in an alkaline aqueous solution, more preferably a mixture of one or more of thiourea dioxide, sodium dithionite, sodium formaldehyde sulfoxylate (Rongalit C, BASF), glucose, and α-hydroxyaldehyde.

[0014] Preferably, the indigo concentration in the cathode liquid is 0.2-2 mol / L (preferably 0.34 mol / L), the chemical reducing agent concentration is 0.01-0.1 mol / L (preferably 0.05 mol / L); and the supporting electrolyte concentration is 0.1-10 mol / L (preferably 0.2-2 mol / L, more preferably 1.5-2 mol / L).

[0015] Preferably, the indigo concentration in the cathode liquid is 0.34 mol / L, the chemical reducing agent concentration is 0.05 mol / L, and the supporting electrolyte concentration is 1.5-2 mol / L.

[0016] Preferably, after the cathode liquid is prepared, nitrogen is circulated for 40 minutes.

[0017] Preferably, the anolyte is an aqueous sulfuric acid solution with a concentration of 0.1 to 10 mol / L (preferably 0.5 to 6 mol / L).

[0018] Preferably, the anode is a noble metal electrode such as a platinum electrode, a ruthenium titanium electrode, a titanium iridium electrode, etc., preferably a titanium iridium electrode. The anode electrode can be in different shapes, such as a flat plate, a wire mesh, or a mesh shape, preferably a diamond mesh shape that is conducive to gas discharge; the oxidation reaction occurs on the anode.

[0019] Preferably, the cathode is an electrode made of carbon-containing materials such as graphite felt, carbon felt or foamed glassy carbon, preferably graphite felt; the cathode is used after heat treatment at 300-1000° C. for 1-10 h in a muffle furnace containing an oxygen atmosphere.

[0020] Preferably, the temperature range of the electrochemical reduction is 5 to 90°C, preferably 60 to 80°C; the current density range is 1 to 10 A / dm 2 , preferably 3 to 7A / dm 2 . Catholyte flow rate 400mL / min.

[0021] Preferably, the diaphragm of the plate-and-frame electrolyzer is a cationic membrane (eg, Nafion 324) or a microporous membrane, preferably a cationic membrane.

[0022] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in:

[0023] The present invention ensures that the alkali concentration in the cathode liquid is controllable by using an acidic aqueous solution (especially a sulfuric acid aqueous solution) as the anolyte, so that the ratio of the alkali concentration to the indigo concentration in the electrolyte is below 3.4 and does not exceed the industrial indicators. While ensuring that the alkali concentration is controllable, the current efficiency of the indigo electroreduction can reach 58.7% and the indigo conversion rate can reach 93.1%. (IV) Description of the drawings

[0024] Figure 1 , schematic diagram of the indigo dyeing process and the general material flow.

[0025] Figure 2 , schematic diagram of the structure of the plate and frame electrolyzer. (V) Specific implementation methods

[0026] The present invention is further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto:

[0027] Indigo yield = (the amount of indigo in the cathode liquid at the end of the reaction (mol) / the amount of indigo added to the cathode liquid at the beginning of the reaction (mol)) × 100%

[0028] Current efficiency = (effective power (Ah) / input power (Ah)) × 100%

[0029] Effective electricity (Ah) = the amount of indigo in the cathode liquid at the end of the reaction (mol) × 2 (e-) × 26.8 (Ah / (mol×e-)) - the amount of reducing agent added to the cathode liquid (mol) × 2 (e-) × 26.8 (Ah / (mol×e-))

[0030] Electricity input (Ah) = electrolysis current (A) × electrolysis time (h)

[0031] The optimal concentration ratio of sodium hydroxide to indigo in industrial dyeing vats is 3.4:1.

[0032] The heat-treated graphite felt refers to the graphite felt that is heat-treated at 500° C. for 2 hours in a muffle furnace containing an oxygen atmosphere before use.

[0033] Example 1: Electrochemical reduction of indigo (0.34 mol / L)

[0034] by Figure 2 The plate-and-frame electrolyzer shown in the figure is a reactor, with Nafion-324 cationic membrane as the diaphragm and heat-treated graphite felt (3×10×2 cm 3 ) is the cathode, and the diamond mesh titanium plated with iridium is the anode. Add the NaOH aqueous solution into the liquid storage tank and circulate it with nitrogen for 40 minutes, then add 35.7g indigo and 2g thiourea dioxide (reducing agent), stir evenly, and prepare 400mL of 2mol / L NaOH + 0.34mol / L indigo + 0.05mol / L thiourea dioxide aqueous solution, which is the cathode liquid; 400mL 0.5mol / L sulfuric acid aqueous solution is the anode liquid. During the electrolysis process, the temperature is controlled at 60°C and the current density is controlled at 6A / dm 2 (current is 3.6A), the cathode liquid flow rate is 400mL / min. Samples were taken at 0, 1, and 2 hours of electrolysis, and the indigo content in the cathode liquid was analyzed by potentiometric titrator. The electrolysis was stopped after 2 hours of electrolysis, and the concentration ratio of sodium hydroxide to indigo in the cathode liquid was 3.3:1. According to the titration results, the yield of indigo was 93.1% and the current efficiency was 58.7%.

[0035] Examples 2-3: Effect of alkali concentration in cathode liquid on electrochemical reduction of indigo

[0036] The concentration of sodium hydroxide in the cathode liquid of Example 1 was changed to that shown in Table 1, and other operations were the same as those of Example 1. The results are shown in Table 1. It can be seen that when the ratio of sodium hydroxide to indigo white is ensured to be at or below the optimal industrial ratio (3.4:1), when the NaOH concentration in the cathode liquid is 2M, the yield of indigo white and the current efficiency are the highest. Combined with Example 1, the optimal concentration is 2M.

[0037] Table 1 Different concentrations of sodium hydroxide solution added to the cathode

[0038]

[0039] Examples 4-6 Effect of sulfuric acid concentration in anolyte on electrochemical reduction of indigo

[0040] The concentration of sulfuric acid in the anolyte of Example 1 was changed to that shown in Table 2. Other operations were the same as those of Example 1. The results are shown in Table 2. It can be seen that the concentration of sulfuric acid in the anolyte has no significant effect on the yield of indigo white and the current efficiency.

[0041] Table 2 Sulfuric acid solution with different concentrations added to the anode

[0042]

[0043] Examples 7-18: Effects of current density, temperature, anode electrode, etc. on electrochemical reduction of indigo

[0044] 1. Temperature

[0045] The 60°C in Example 1 was changed to 5, 30, 50, 80, and 90°C respectively. Other operations were the same as in Example 1. The results are shown in Table 3. It can be seen from Example 1 that with the increase of temperature, the indigo yield and the current efficiency first increased and then decreased. The indigo yield and the current efficiency effects were within an acceptable range when the temperature was between 50 and 90°C. In Example 1, the optimal temperature was 60°C.

[0046] Table 3 Effect of temperature on electrochemical reduction of indigo

[0047]

[0048] 2. Current density

[0049] The current density in Example 1 was changed to 1, 3, 7, 10 A / dm 2 , other operations are the same, the results are shown in Table 4, it can be seen that the current density is 3-10A / dm 2 The indigo yield and current efficiency are within an acceptable range. Combined with Example 1, the optimal current density is 6A / dm 2 .

[0050] Table 4 Effect of current density on electrochemical reduction of indigo

[0051]

[0052] 3. Anode

[0053] The anode electrode in Example 1 was changed to a flat titanium-iridium, platinum sheet or ruthenium-titanium, and other operations were the same. The results are shown in Table 5. It can be seen that the anode material has no obvious effect on the indigo yield and current efficiency.

[0054] Table 5 Effect of anode electrode on electrochemical reduction of indigo

[0055]

[0056]

[0057] Comparative Example 1: Electrochemical Reduction of Indigo (0.34 mol / L) by Changing the Anode and Anolyte

[0058] The anode in Example 1 was changed to 316 stainless steel, the anolyte was changed to 400 mL 2 mol / L sodium hydroxide aqueous solution, and the other operations were the same as in Example 1. The electrolysis was stopped after 2 hours of electrolysis. According to the titration results, the yield of indigo white was 94.4%, the current efficiency was 61%, and the concentration ratio of sodium hydroxide to indigo white was 5.3:1. Although the current efficiency and indigo white conversion rate in Comparative Example 1 were higher than those in Example 1, the ratio of sodium hydroxide to indigo white in the final electrolyte was much higher than the industrial ratio.

[0059] Comparative Examples 2-4: Effect of Changing the Anode and Anolyte Catholyte Concentration on the Electrochemical Reduction of Indigo

[0060] The anode in Example 1 was changed to 316 stainless steel, the anolyte was changed to 400 mL 2 mol / L sodium hydroxide aqueous solution, the sodium hydroxide concentration in the cathode liquid was changed to 1.5, 1, and 0.8, and other operations were the same. The results are shown in Table 6. It can be seen that the concentration ratio of sodium hydroxide to indigo in Comparative Examples 3 and 4 is lower than the optimal concentration ratio in the industry, but the indigo yield and current efficiency are significantly lower than those in Example 1. Combined with Comparative Example 1, it is shown that when the ratio of sodium hydroxide to indigo in the industry is reached, the anode is not suitable for electrochemical reduction of indigo with an alkaline solution.

[0061] Table 6 Different concentrations of sodium hydroxide solution added to the cathode

[0062]

Claims

1. A liquid indigo electrochemical reduction method with controllable alkali concentration, characterized in that: The method adopts a plate-and-frame electrolytic cell, with a mixed alkaline aqueous solution of indigo and a chemical reducing agent as the cathode liquid, an acidic aqueous solution as the anode liquid, a three-dimensional porous carbon-based conductive material as the cathode, and a noble metal electrode as the anode; a direct current or a pulse current is passed from the anode to the cathode in sequence through the anode liquid, a diaphragm and the cathode liquid to perform an electrochemical reduction reaction, and the indigo in the cathode liquid is electrochemically reduced to indigo white.

2. The method according to claim 1, characterized in that The alkaline aqueous solution in the cathode liquid is an aqueous solution containing a supporting electrolyte, and the supporting electrolyte includes NaOH, KOH, Na2CO3, K2CO3, tetramethylammonium hydroxide, tetraethylammonium hydroxide, and ammonia monohydrate.

3. The method according to claim 1, characterized in that The chemical reducing agent includes a mixture of one or more of thiourea dioxide, sodium dithionite, sodium formaldehyde sulfoxylate, glucose, and α-hydroxyaldehyde.

4. The method according to claim 2, characterized in that The indigo concentration in the cathode liquid is 0.2-2 mol / L, the chemical reducing agent concentration is 0.01-0.1 mol / L, and the supporting electrolyte concentration is 0.1-10 mol / L.

5. The method according to claim 4, characterized in that The indigo concentration in the cathode liquid is 0.34 mol / L, the chemical reducing agent concentration is 0.05 mol / L, and the supporting electrolyte concentration is 1.5-2 mol / L.

6. The method according to claim 1, characterized in that After the cathode liquid is prepared, nitrogen is circulated for 40 minutes.

7. The method according to claim 1, characterized in that The anolyte is a sulfuric acid aqueous solution with a concentration of 0.1 to 10 mol / L.

8. The method according to claim 1, characterized in that The cathode is graphite felt, carbon felt or foamed glassy carbon; the cathode is used after heat treatment at 300-1000° C. for 1-10 hours in a muffle furnace containing an oxygen atmosphere.

9. The method according to claim 1, characterized in that The temperature range of the electrochemical reduction is 5 to 90°C; the current density range is 1 to 10A / dm 2 , cathode liquid flow rate 400mL / min.

10. The method according to claim 1, characterized in that The diaphragm of the plate-and-frame electrolyzer is a Nafion 324 cationic membrane.

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