Method for removing nitrate in silver powder wastewater by liquid phase reduction

Abstract

The application discloses a method for removing nitrate in liquid-phase reduction silver powder wastewater. The method comprises the following steps: (1) filtering the liquid-phase reduction silver powder wastewater containing nitrate; (2) removing PVP in the liquid-phase reduction silver powder wastewater filtered in step (1) by adopting an electrochemical flotation method to obtain foam liquid and floating residual liquid; (3) defoaming the foam liquid to obtain PVP enrichment liquid, drying the PVP enrichment liquid to obtain PVP powder; (4) treating the floating residual liquid by using alkali liquid; (5) reducing the nitrate in the floating residual liquid by adopting an electrochemical method; and (6) treating the liquid after electrolysis by using acid liquid to obtain silver powder wastewater containing sulfate. The method provided by the application converts hydroxide into sulfate by using sulfuric acid, the final wastewater becomes sulfate-containing wastewater, the silver powder wastewater containing sulfate is weakly alkaline, does not contain nitrate, has no strong oxidizing property and is suitable for evaporation treatment.

Description

Technical fields:

[0001] This invention relates to the field of silver powder wastewater treatment technology, specifically to a method for removing nitrate ions from liquid-phase reduction silver powder wastewater. Background technology:

[0002] Currently, silver powder is an important functional metallic material, mainly used in microelectronics fields such as conductive pastes, conductive adhesives, and printed circuits. Ultrafine spherical silver powder is generally prepared using a liquid-phase reduction method. A typical liquid-phase reduction method uses silver nitrate as the silver source, ascorbic acid as the reducing agent, and polyvinylpyrrolidone (PVP) or polyvinyl alcohol (PVA) as the dispersant. During the silver powder preparation process, a large amount of highly concentrated acidic wastewater is discharged. This wastewater mainly includes the silver powder reduction mother liquor and the washing water from the first two stages of silver powder production. Typically, high-concentration silver powder wastewater contains silver powder (below 20 mg / L), nitric acid (20–30 g / L), ascorbic acid, dehydroascorbic acid (20–40 g / L), and polyvinylpyrrolidone or polyvinyl alcohol (15–20 g / L). Therefore, this silver powder production wastewater is classified as difficult-to-treat industrial wastewater due to its high acidity, high COD, and high nitrogen content. In recent years, with the rapid development of the solar cell industry, the demand for spherical silver powder has increased accordingly, leading to a significant increase in the amount of wastewater discharged during silver powder production.

[0003] Currently, high-concentration silver powder production wastewater is generally treated using an alkali neutralization-evaporation method. While this method offers advantages such as simple operation and high treatment efficiency, it suffers from safety concerns. This is because the precipitate from this wastewater evaporation process is a mixture of nitrates and organic matter. Due to the oxidizing properties of nitrates and the reducing properties of organic matter, the mixture poses significant risks of spontaneous combustion and explosion during evaporation, transportation, and storage. In the context of safe production, a safe and environmentally friendly clean treatment method for silver powder production wastewater is urgently needed to comply with current safety regulations and environmental policies. Therefore, the pre-removal of nitrates from silver powder production wastewater is crucial for safe evaporation treatment and is also of great significance for cost reduction and efficiency improvement in the preparation of spherical silver powder. Summary of the Invention:

[0004] This invention addresses the problems of spontaneous combustion and explosion of precipitates in existing evaporation treatment methods for silver powder production wastewater. It provides a liquid-phase reduction method for removing nitrates from silver powder wastewater. This method is based on the principle of acid-base neutralization, using sulfuric acid to convert hydroxides into sulfates, ultimately turning the wastewater into sulfate-containing wastewater. The sulfate-containing silver powder wastewater is weakly alkaline, contains no nitrates, and has no strong oxidizing properties, thus reducing the risk of combustion and explosion of organic matter and making it suitable for evaporation treatment.

[0005] The purpose of this invention is to provide a method for removing nitrate ions from liquid-phase reduced silver powder wastewater, comprising the following steps:

[0006] (1) Filter the liquid-phase silver powder reduction wastewater containing nitrate ions;

[0007] (2) PVP in the liquid phase reduction silver powder wastewater after filtration in step (1) was removed by electrochemical flotation to obtain foam liquid and float liquid.

[0008] (3) Defoam the foam liquid to obtain PVP enriched liquid, and dry the PVP enriched liquid to obtain PVP powder;

[0009] (4) Treat the floating liquid with alkaline solution;

[0010] (5) Use an electrochemical method to reduce nitrate in the residual liquid;

[0011] (6) Treat the electrolyte with acid to obtain silver powder wastewater containing sulfate.

[0012] The liquid-phase reduction silver powder wastewater proposed in this invention comprises silver powder, ascorbic acid, dehydroascorbic acid and polyvinylpyrrolidone, and is a difficult-to-treat organic wastewater with high nitrogen and high COD.

[0013] Step (1) uses filtration to remove suspended solids and other substances from silver powder wastewater, which is beneficial for the flotation of polymer PVP in the wastewater.

[0014] Preferably, in the electrochemical flotation method described in step (2), the anode is an iridium oxide-based electrode or a platinum electrode, and the cathode is a titanium electrode or a graphite electrode, with the current density controlled at 100–200 A / m. 2 The flotation time is 20 to 60 minutes.

[0015] Electrochemical flotation utilizes tiny bubbles that precipitate in situ from the solution to adsorb PVP and float it to the surface, thus separating and enriching PVP. The bubbles in electroflotation are oxygen bubbles precipitated at the anode and hydrogen bubbles precipitated at the cathode under specific electrode potentials. These bubbles are small in diameter and numerous, enabling efficient removal of PVP from wastewater.

[0016] Further preferably, the iridium oxide-based electrode includes an iridium-tantalum oxide electrode, an iridium-ruthenium oxide electrode, an iridium-tin oxide electrode, and an iridium-manganese oxide electrode. Even more preferably, the iridium oxide-based electrode is an iridium-tantalum oxide electrode or an iridium-tin oxide electrode.

[0017] Preferably, the defoaming method in step (3) is ultrasonic defoaming. During ultrasonic defoaming, the ultrasonic power is 25-40 kW, and the time is 10-30 min. Ultrasonic defoaming is based on the cavitation and sound pressure effects of ultrasound. In addition, thermal and resonance mechanisms are also ultrasonic defoaming mechanisms. During the defoaming process, ultrasound can effectively promote foam collapse and accelerate foam drainage. After defoaming, a PVP-enriched solution is obtained.

[0018] Preferably, the drying method described in step (3) is spray drying, with a controlled pressure of -700 to -1000 Pa, a temperature of 140 to 160 °C, and a time of 10 to 30 seconds. Spray drying is a process in which atomizers disperse liquid into fine droplets, and in a hot drying medium, the heat causes the solvent in the droplets to evaporate rapidly, forming a dried powder.

[0019] Preferably, the alkali in step (4) is sodium hydroxide or potassium hydroxide. The alkali treatment utilizes hydroxide ions to attack the lactam groups in the residual PVP in the residual liquid, thereby reducing its surface activity and weakening its adsorption on the electrode surface, which is beneficial for subsequent electrochemical treatment.

[0020] Further preferred, the mass concentration of the alkaline solution is 25% to 40%, and the endpoint of the alkaline treatment is that the pH of the residual liquid is 6.5 to 7.5.

[0021] Preferably, in step (5) of the electrochemical method, the anode is an iron electrode or a stainless steel electrode; the cathode is a copper-based electrode, and the current density is 200–500 A / m. 2 The electrochemical treatment time is 2 to 5 hours.

[0022] Electroreduction treatment (electrochemical method) is based on the gradual reduction of nitrate ions to nitrogen gas at the cathode surface under a certain electrode potential, while water is oxidized to oxygen gas at the anode surface. The final products of the electrochemical conversion reaction are oxygen, nitrogen gas, and hydroxide ions. In addition, ascorbic acid and dehydroascorbic acid in the wastewater are also degraded into organic acids such as gulonic acid at the anode surface.

[0023] Further preferably, the copper-based electrode is selected from one of a copper plate electrode, a porous copper electrode, a copper-zinc alloy electrode, and a copper-palladium alloy electrode. Even more preferably, the copper-based electrode is a copper-zinc alloy electrode or a copper-palladium alloy electrode.

[0024] Preferably, the acid solution in step (6) is a sulfuric acid solution with a mass concentration of 20% to 30%, and the endpoint of the acid treatment is that the pH of the electrolyte after electrolysis is 7 to 9.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] 1. Regarding evaporation treatment, the method proposed in this invention can significantly improve the safety of the evaporation process by pre-removing nitrate from silver powder wastewater;

[0027] 2. This invention converts nitrates in wastewater into sulfates, which can reduce the energy consumption of wastewater evaporation and improve the economy and safety of evaporation.

[0028] 3. The method proposed in this invention can effectively recover PVP powder and improve the economic efficiency of wastewater treatment. Attached image description:

[0029] Figure 1 This is a process flow diagram of a method for removing nitrate from wastewater containing liquid-phase silver powder reduction. Detailed implementation method:

[0030] The following embodiments are further illustrations of the present invention, but not limitations thereof.

[0031] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention. Unless otherwise specified, the experimental materials and reagents used herein are commercially available products conventionally available in this technical field.

[0032] In the examples below, the polyvinylpyrrolidone (PVP) content of the silver powder wastewater was 12–18 g / L, and the pH value was 0.8–1.0.

[0033] Example 1

[0034] 1000 mL of silver powder wastewater (polyvinylpyrrolidone, PVP, 15 g / L) was taken and filtered to remove suspended solids with a particle size greater than 0.1 μm, resulting in clarified wastewater. Using an iridium-tantalum oxide electrode as the anode and a titanium sheet as the cathode, a current density of 150 A / m was applied. 2 The wastewater was electrofloted for 20 minutes to obtain foam liquid and 900 mL of residual liquid.

[0035] The foaming liquid was ultrasonically defoamed for 15 minutes at a power of 25 kW to obtain 100 mL of enriched liquid. The enriched liquid was spray-dried for 10 seconds at a pressure of -800 Pa to obtain 12.2 g of white PVP powder. Liquid chromatography analysis showed that the purity of the PVP powder was 98%.

[0036] A 30% sodium hydroxide solution was slowly added to the residual liquid until the pH of the solution reached 7.0. A stainless steel electrode was used as the anode, a copper sheet as the cathode, and the current density was 500 A / m. 2 The residual liquid wastewater after alkaline treatment was electrolyzed for 4 hours. After electrolysis was terminated, the pH of the wastewater was 12.5. The post-electrolysis liquid was treated with a 30% sulfuric acid solution until the pH of the wastewater reached 7.5. At this point, the oxidation-reduction potential of the wastewater was -400mV. Chemical analysis showed that the removal rate of nitrate in the wastewater was 98%.

[0037] Comparative Example 1

[0038] Similar to Example 1, except that sodium hydroxide solution was not used to treat the flocculated residue; instead, the residue was directly electrolyzed. A stainless steel electrode was used as the anode, a copper sheet as the cathode, and the current density was 500 A / m. 2The residual liquid wastewater after alkaline treatment was electrolyzed for 4 hours. After electrolysis was terminated, the pH of the wastewater was 12.5. The post-electrolysis liquid was treated with a 30% sulfuric acid solution until the pH of the wastewater reached 7.5. At this point, the oxidation-reduction potential of the wastewater was 363 mV. Chemical analysis showed that the removal rate of nitrate in the wastewater was 5%.

[0039] Compared with Example 1, in Comparative Example 1, the direct electrolysis of the residual liquid resulted in a lower reduction efficiency of nitrate on the cathode surface and a shorter lifespan of the stainless steel electrode.

[0040] Comparative Example 2

[0041] Same as Example 1, except that: graphite is used as the anode and titanium sheet as the cathode, at a current density of 150 A / m 2 The wastewater was electrofloted for 20 minutes to obtain foam liquid and 900 mL of residual liquid.

[0042] Compared with the graphite anode in Comparative Example 2, the iridium tantalum oxide electrode in Example 1 produces smaller and more numerous bubbles, resulting in higher efficiency of electroflotation.

[0043] Example 2

[0044] Same as Example 1, except that: a stainless steel electrode is used as the anode, a copper-zinc alloy electrode is used as the cathode, and the current density is 500 A / m. 2 The residual liquid wastewater after alkaline treatment was electrolyzed for 3 hours. After electrolysis was terminated, the pH of the wastewater was 12.5. The post-electrolysis liquid was treated with a 30% sulfuric acid solution until the pH of the wastewater reached 7.5. At this point, the oxidation-reduction potential of the wastewater was -400mV. Chemical analysis showed that the removal rate of nitrate in the wastewater was 98%.

[0045] Compared with Example 1, in Example 2, when the copper-zinc alloy electrode is used as the cathode, the nitrate reduction efficiency on the cathode surface is high, that is, the electrolysis time required to achieve the same nitrate removal rate is shorter.

[0046] Example 3

[0047] 2000 mL of silver powder wastewater (polyvinylpyrrolidone, PVP, 12 g / L) was collected and filtered to remove suspended solids with a particle size greater than 0.1 μm, yielding clarified wastewater. Using an iridium tin oxide electrode as the anode and a titanium sheet as the cathode, an iridium tin oxide electrode was used at a current density of 200 A / m². 2 The wastewater was electrofloted for 30 minutes to obtain foam liquid and 1700 mL of residual liquid.

[0048] The foam was ultrasonically defoamed for 10 minutes at a power of 30 kW, yielding 200 mL of enriched solution. The enriched solution was spray-dried for 20 seconds at a pressure of -1000 Pa, yielding 19.6 g of white PVP powder. Liquid chromatography analysis showed that the purity of the PVP powder was 98%.

[0049] A 30% potassium hydroxide solution was slowly added to the residual liquid until the pH of the solution reached 7.5. A stainless steel electrode was used as the anode, a porous copper plate as the cathode, and the current density was 400 A / m². 2 The wastewater was electrolyzed for 5 hours. After electrolysis, the pH of the wastewater was 12.5. The post-electrolysis solution was treated with 30% sulfuric acid until the pH of the wastewater reached 8. At this point, the oxidation-reduction potential of the wastewater was -350 mV. Chemical analysis showed that the removal rate of nitrate in the wastewater was 95%.

[0050] Example 4

[0051] 2500 mL of silver powder wastewater (polyvinylpyrrolidone, PVP, 15 g / L) was collected and filtered to remove suspended solids with a particle size greater than 0.1 μm, yielding clarified wastewater. Using an iridium-tantalum oxide electrode as the anode and a graphite plate as the cathode, a current density of 150 A / m was applied. 2 The wastewater was electrofloted for 30 minutes to obtain foam liquid and 2200 mL of residual liquid.

[0052] The foam was ultrasonically defoamed for 10 minutes at a power of 40 kW, yielding 300 mL of enriched solution. The enriched solution was spray-dried for 10 seconds at a pressure of -800 Pa, yielding 30.6 g of white PVP powder. Liquid chromatography analysis showed that the purity of the PVP powder was 98%.

[0053] Slowly add a 30% sodium hydroxide solution to the residual liquid until the solution pH reaches 7.0. Use an iron sheet as the anode and a copper-zinc alloy sheet as the cathode, with a current density of 400 A / m. 2 The wastewater was electrolyzed for 5 hours. After electrolysis, the pH of the wastewater was 13. The post-electrolysis solution was treated with 30% sulfuric acid until the pH of the wastewater reached 8. At this point, the oxidation-reduction potential of the wastewater was -400mV. Chemical analysis showed that the removal rate of nitrate in the wastewater was 95%.

[0054] Example 5

[0055] 5000 mL of silver powder wastewater (polyvinylpyrrolidone, PVP, 18 g / L) was collected and filtered to remove suspended solids with a particle size greater than 0.1 μm, yielding clarified wastewater. Using an iridium tin oxide electrode as the anode and a titanium sheet as the cathode, an iridium tin oxide electrode was used at a current density of 180 A / m². 2 Under the condition of electroflotation, wastewater was subjected to flotation for 60 minutes to obtain foam liquid and 4500 mL of residual liquid.

[0056] The foam was ultrasonically defoamed for 15 minutes at a power of 35 kW, yielding 500 mL of enriched solution. The enriched solution was spray-dried for 10 seconds at a pressure of -800 Pa, yielding 82.7 g of white PVP powder. Liquid chromatography analysis showed that the purity of the PVP powder was 96%.

[0057] A 30% sodium hydroxide solution was slowly added to the residual liquid until the pH of the solution reached 7.5. A stainless steel electrode was used as the anode, and copper foam as the cathode, with a current density of 500 A / m. 2 The wastewater was electrolyzed for 5 hours. After electrolysis, the pH of the wastewater was 12.5. The post-electrolysis solution was treated with 30% sulfuric acid until the pH of the wastewater reached 7.5, at which point the oxidation-reduction potential of the wastewater was -380mV. Chemical analysis showed that the removal rate of nitrate in the wastewater was 98%.

[0058] Example 6

[0059] Same as Example 5, except that in the electrochemical flotation method, the current density is controlled at 100 A / m. 2 The flotation time was 60 min; during ultrasonic defoaming, the ultrasonic power was 25 kW and the time was 30 min; the spray drying control pressure was -700 Pa, the temperature was 140 ℃, and the time was 10 s; the mass concentration of the alkali solution was 25%, and the endpoint of the alkali treatment was a pH of 6.5 for the residual liquid; in the electrochemical method, the current density was 200 A / m 2 The electrochemical treatment time was 5 hours; the mass concentration of the sulfuric acid solution was 20%; and the endpoint of the acid treatment was when the pH of the electrolyte solution reached 7.

[0060] Example 7

[0061] Same as Example 5, except that in the electrochemical flotation method, the current density is controlled at 200 A / m. 2 The flotation time was 20 min; during ultrasonic defoaming, the ultrasonic power was 40 kW and the time was 10 min; the spray drying control pressure was -1000 Pa, the temperature was 160 ℃, and the time was 30 s; the mass concentration of the alkali solution was 40%, and the endpoint of the alkali treatment was a pH of 7.5 for the residual liquid; in the electrochemical method, the current density was 500 A / m 2 The electrochemical treatment time was 2 hours; the mass concentration of the sulfuric acid solution was 30%; and the endpoint of the acid treatment was a pH of 9 in the electrolyte solution after electrolysis.

[0062] The above description of the embodiments is only for the purpose of helping to understand the technical solution and core idea of ​​the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principle of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for removing nitrate ions from wastewater containing liquid-phase silver powder reduction, characterized in that, Includes the following steps: (1) Filtering liquid-phase silver powder reduction wastewater containing nitrate ions; (2) PVP in the liquid-phase reduction silver powder wastewater after filtration in step (1) is removed by electrochemical flotation to obtain foam liquid and residual liquid. In the electrochemical flotation method, the anode is an iridium oxide-based electrode or a platinum electrode, and the cathode is a titanium electrode or a graphite electrode. The current density is controlled at 100~200 A / m. 2 The flotation time is 20~60 min; (3) Defoam the foam liquid to obtain PVP enriched liquid, and dry the PVP enriched liquid to obtain PVP powder; (4) Treat the floating liquid with an alkaline solution, the alkaline solution being sodium hydroxide or potassium hydroxide, the mass concentration of the alkaline solution being 25%~40%, and the endpoint of the alkaline solution treatment being when the pH of the floating liquid is 6.5~7.5; (5) Nitrate in the residual liquid is reduced by electrochemical method. In the electrochemical method, the anode is an iron electrode or a stainless steel electrode, the cathode is a copper-based electrode, and the current density is 200~500 A / m. 2 The electrochemical treatment time is 2-5 hours; (6) Treat the electrolyte with acid to obtain silver powder wastewater containing sulfate.

2. The method according to claim 1, characterized in that, Iridium oxide-based electrodes include iridium-tantalum oxide electrodes, iridium-ruthenium oxide electrodes, iridium-tin oxide electrodes, and iridium-manganese oxide electrodes.

3. The method according to claim 1, characterized in that, The defoaming method described in step (3) is ultrasonic defoaming. During ultrasonic defoaming, the ultrasonic power is 25~40 kW and the time is 10~30 min.

4. The method according to claim 1, characterized in that, The drying method described in step (3) is spray drying, with a controlled pressure of -700 to -1000 Pa, a temperature of 140 to 160°C, and a time of 10 to 30 seconds.

5. The method according to claim 1, characterized in that, The copper-based electrode is selected from one of the following: copper plate electrode, porous copper electrode, copper-zinc alloy electrode, and copper-palladium alloy electrode.

6. The method according to claim 1, characterized in that, The acid solution mentioned in step (6) is a sulfuric acid solution with a mass concentration of 20% to 30%. The endpoint of the acid treatment is when the pH of the electrolyte solution is 7 to 9.

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