Process for producing regenerated acid from spent pickling acid

Abstract

The present application relates to a method for preparing regenerated acid from stainless steel pickling waste acid, comprising the following steps: (1) pretreatment and collection of the stainless steel pickling waste acid; (2) setting an anion membrane in an electrolytic cell, placing the pretreated pickling waste acid in the cathode cavity, and placing a low-concentration nitric acid solution in the anode cavity, and treating the waste acid in the cathode cavity to a pH of 1-3 by electrolysis; meanwhile, performing cyclic pressure filtration during the process of pH 1-3; (3) continuing electrolysis to treat the waste acid to a pH of 3-7, obtaining a nitric acid solution with increased concentration to a preset concentration range in the anode cavity, and obtaining a metal ion solution with reduced concentration to a preset concentration range and a hydrofluoric acid solution with increased concentration to a preset concentration range in the cathode cavity; (4) periodically cleaning the elemental metal attached to the cathode electrode plate during the electrolysis processes of (2) and (3); and (5) mixing the obtained solution in the anode cavity with the solution in the cathode cavity after electrolysis. The above method can realize the recycling of waste acid.

Description

Technical Field

[0001] This invention belongs to the field of waste acid treatment technology, specifically relating to a method for producing regenerated acid from waste acid from stainless steel pickling. Background Technology

[0002] In the stainless steel pickling industry, pickling is typically used to remove the iron oxide scale generated on the surface of annealed strip steel, while simultaneously passivating the strip surface to improve the product's surface quality and corrosion resistance. Traditional stainless steel pickling methods mostly employ sulfuric acid / neutral salt electrolysis + mixed acid (HNO3 + HF) pickling. During the pickling process, the mixed acid reacts with the oxide scale and metal elements such as Fe and Cr in the stainless steel matrix to form metal salts. As the metal ion content in the acid solution increases, the free acid component decreases, and the acid activity continuously declines, directly leading to a decrease in pickling efficiency. Simultaneously, excessively high levels of metal ions can combine with fluoride ions to form insoluble metal fluorides, which easily clog pumps and pipelines. Neutralizing the generated waste acid before discharge requires a large amount of alkali, and the nitrate ions in the waste liquid cannot be removed, requiring biological treatment, which is extremely costly. Currently, the commonly used waste acid recovery technologies are mainly of two types: resin acid bed method and spray roasting method.

[0003] (1) Resin Bed Acid Recovery Technology: The core component of resin bed acid recovery technology is the resin bed, which utilizes the principle of acid retardation. When a mixed solution of acid and its corresponding salt is pumped into a resin bed containing anion exchange resin, the salt solution flows out from the other end of the resin bed, while the acid is adsorbed by the resin and "retarded" in the resin column, thus separating the acid and salt solution. Therefore, this method is also called the acid retardation method. This method can recover free acid and remove some metal salts, but the deacidified metal salt waste liquid needs further treatment, so the subsequent environmental protection pressure remains huge.

[0004] (2) The principle of spray roasting acid regeneration technology is to first use high-temperature flue gas to heat and evaporate part of the free acid and water in the mixed acid waste liquid, thus pre-concentrating the acid liquid. The concentrated acid liquid is then heated and roasted to decompose the metal salts into acid gas and metal oxides. The gaseous products after roasting are absorbed by spraying to form regenerated acid, and the metal oxides can be reused. This method can simultaneously recover free acid and combined acid, and recover metal elements into metal oxide powder. However, the initial investment is large, the tail gas needs to be further denitrified, and the maintenance and operation costs are also high, making it unsuitable for small and medium-sized enterprises. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a novel method for producing regenerated acid from waste acid from stainless steel pickling, which can meet the processing needs of small and medium-sized stainless steel enterprises. This method significantly reduces the metal ions in the waste acid while significantly increasing the acid concentration, thereby achieving the effect of waste acid recycling.

[0006] The technical solution of the present invention is a method for producing regenerated acid from waste acid from stainless steel pickling, comprising the following steps:

[0007] (1): Pretreatment of stainless steel pickling waste acid, collection of pretreated pickling waste acid, the pickling waste acid contains high concentration of metal ions;

[0008] (2): Provide an electrolytic cell and set an anion exchange membrane in the electrolytic cell to divide the electrolytic cell into a cathode cell and an anode cell. Place a cathode electrode in the cathode cell and an anode electrode in the anode cell. Place pretreated pickling waste acid in the cathode cell and add dilute nitric acid solution to the anode cell. Utilize the penetration characteristics of the anion exchange membrane to treat the waste acid in the cathode cell to a pH of 1-3 through electrolysis.

[0009] (3): In (2), the waste acid electrolyzed into the cathode cell has a pH of 1-3 and is subjected to circulating pressure filtration.

[0010] (4): After (2), electrolysis continues to treat the waste acid in the cathode cell to pH 3-7. At this time, the anolyte is high-concentration nitric acid, and the concentration of metal ions in the cathode is greatly reduced. With the removal of metal ions, a large amount of nitrate and fluoride ions are released. Nitrate ions penetrate to the anode through the selective permeability of the anion exchange membrane, while fluoride ions are blocked at the cathode. The cathode solution is formed as a hydrofluoric acid solution containing a small amount of metal ions.

[0011] (5): In (2) and (4), the metal ions in the pickling waste acid are reduced and attached to the cathode electrode plate. The metal elements attached to the cathode electrode plate are periodically cleaned off, or the cathode electrode plate is periodically replaced.

[0012] (6): After the electrolysis in (4) is completed, the solution obtained in the anode cell is mixed with the solution in the cathode cell and used again as a mixed acid solution.

[0013] Furthermore, in (3), the filter press is an external filter press, and both the chamber plate and the filter cloth are made of acid-resistant material.

[0014] Further, in (1), the pretreatment is filtration, which is used to filter out particulate matter in the pickling waste acid and store it in an acid storage tank after preliminary filtration.

[0015] Furthermore, the cathode electrode is made of stainless steel, copper or lead, and the anode electrode is made of lead dioxide or platinum.

[0016] Furthermore, in (2), the stainless steel pickling waste liquid added to the cathode tank has a nitric acid concentration of 20-200 g / L, a hydrofluoric acid concentration of 0-50 g / L, and a metal ion concentration of 20-200 g / L. The same level of dilute nitric acid is added to the anode tank as an electrolyte, and the initial dilute nitric acid concentration of the anode liquid is 0-100 g / L.

[0017] Furthermore, in (2), when the waste acid in the cathode cell is treated to pH 1 by electrolysis, the filter press is started to filter the cathode cell solution until the pH exceeds 3, at which point the filter press stops.

[0018] Compared to existing technologies, this invention provides a method for producing regenerated acid from stainless steel pickling waste acid, relating to the resource utilization of waste acid. It employs membrane electrolysis, using mixed acid waste liquid as the cathode and nitric acid of a certain initial concentration as the anode. Utilizing the reducing properties of the cathode, metal ions in the cathode solution are largely reduced and enriched on the electrode plates. By periodically replacing and cleaning the cathode electrode plates, metal ions are removed from the waste acid. During this process, due to the selective permeability of the anion exchange membrane, a large amount of nitrate ions pass through the membrane into the anode, while most of the hydrofluoric acid remains at the cathode. Finally, mixing the anode and cathode electrolytes yields regenerated acid with high acid concentration and low metal ion content. Although this method cannot separately obtain pure nitric acid and hydrofluoric acid, it can reduce the metal ion content in the waste acid to a very low level according to the actual needs of the enterprise, while significantly increasing the acid concentration, thereby achieving the effect of waste acid regeneration. This method involves simple equipment, low initial investment, and high efficiency, making it suitable for small and medium-sized stainless steel enterprises. Attached Figure Description

[0019] Figure 1 This is a flowchart of the method for producing regenerated acid from waste acid from stainless steel pickling according to the present invention;

[0020] Figure 2 This is a schematic diagram illustrating the electrolysis principle of the method for producing regenerated acid from waste acid from stainless steel pickling according to the present invention.

[0021] The annotations in the attached figures are explained as follows:

[0022] 10-Electrolytic cell; 11-Cathode cell; 12-Anode cell; 21-Cathode electrode; 22-Anode electrode; 31-Ion membrane; 41-Metallic element. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] like Figure 1As shown, a method for producing regenerated acid from stainless steel pickling waste acid includes the following steps:

[0025] (1): Pretreatment and collection of waste acid from stainless steel pickling; the cold acid unit discharges the waste acid from the production line to the pretreatment filtration system to filter out particulate matter in the pickling waste acid; then the treated waste acid is discharged to the waste acid storage tank. This treatment process has no specific restrictions on the acid concentration and metal ion concentration of the waste acid. After preliminary filtration, it is stored in the acid storage tank.

[0026] (2): An electrolytic cell is provided, and an anion exchange membrane is installed in the electrolytic cell to divide the electrolytic cell into a cathode chamber and an anode chamber. A cathode electrode is placed in the cathode chamber, and an anode electrode is placed in the anode chamber. Pretreated pickling waste acid is placed in the cathode chamber, and dilute nitric acid solution is added to the anode chamber. Utilizing the penetration characteristics of the anion exchange membrane, the waste acid in the cathode chamber is treated to a pH of 1.0-3.0 through electrolysis. The waste acid in the acid storage tank is pumped to the cathode chamber of the electrolytic cell. The concentration of nitric acid in conventional stainless steel pickling waste acid is 20-200 g / L, the concentration of hydrofluoric acid is 0-50 g / L, and the concentration of metal ions is 20-200 g / L. At the same time, dilute nitric acid of the same level is injected into the anode chamber as the electrolyte. The initial concentration of dilute nitric acid in the anolyte is 0-100 g / L.

[0027] Specifically, please refer to Figure 2 An electrolytic cell 10 is provided, which is divided into a cathode chamber 11 and an anode chamber 12 by an ion-exchange membrane 31. A cathode electrode 21 is placed in the cathode chamber 11, and an anode electrode 22 is placed in the anode chamber 12. The ion-exchange membrane 31 is an anion exchange membrane with directional selective permeability, allowing anions to pass through the ion-exchange membrane 31 from the cathode chamber 11 into the anode chamber 12, while preventing cations from passing through the ion-exchange membrane 31 from the cathode chamber 11 into the anode chamber 12. The cathode electrode 21 can be made of stainless steel, copper, lead, etc., and the anode electrode 22 can be made of lead dioxide, platinum, etc. The anion exchange membrane can be purchased commercially. The current density is 1000-4000 A / m². During the process, the catholyte should be kept below 60 degrees Celsius. It can operate stably for a long time in a mixed acid environment of nitric acid and hydrofluoric acid. During electrolysis, metal ions are removed as follows, taking iron and manganese as examples.

[0028] Fe 2+ +2e - →Fe (attached to the cathode plate)

[0029] Fe 3+ +3e - →Fe (attached to the cathode plate)

[0030] Mn 2+ +2e - →Mn (attached to the cathode plate)

[0031] Fe 3+ +3OH - →Fe(OH)3↓ (precipitates in the solution and deposits at the bottom)

[0032] Thus, in the cathode cell solution, metal ions are reduced and adhere to the cathode electrode, resulting in a significant decrease in metal ion concentration and a substantial increase in volume. In the cathode cell, the anion exchange membrane exhibits selectivity, with nitrate ions preferentially passing through, and nitrate anions preferentially permeating through the membrane to fluoride ions. A large number of nitrate ions pass through the anion exchange membrane into the anode, while most of the hydrofluoric acid remains at the cathode. This leads to a significant increase in hydrofluoric acid concentration and a substantial decrease in metal ions. The cathode cell solution can essentially be considered a hydrofluoric acid solution containing a small amount of metal ions. Correspondingly, a high concentration of nitric acid is obtained in the anode cell solution. The solutions from the anode and cathode cells after electrolysis can be mixed and used as a mixed acid solution.

[0033] (3) In (2), when the waste acid in the cathode cell is treated to pH 1.0 by electrolysis, the filter press is started to filter the solution in the cathode cell to remove the hydrolyzed metal ions in the solution. The cathode cell is connected to the filter press to realize the circulating filter press of the waste acid in the cathode cell. The filter press is an external filter press. The chamber plate and filter cloth are made of acid-resistant material. When the pH reaches 1, a lot of precipitates (such as iron hydroxide) will begin to precipitate in the waste acid. When the pH of the waste acid in the cathode cell is between 1 and 3, a lot of precipitates will precipitate in the waste acid. Therefore, in this pH range, the filter press is kept circulating until the pH exceeds 3, and then the filter press stops.

[0034] (4) In (2), during the electrolysis process, the metal ions in the waste acid in the cathode cell are reduced and attached to the cathode electrode plate. The cathode electrode plate is replaced periodically or the metal element 41 attached to the cathode electrode plate is cleaned off. The metal element 41 attached to the cathode electrode plate is cleaned off so that the cathode electrode plate can be reused, thereby achieving the removal of metal ions in the waste acid.

[0035] (5) After step 2, electrolysis continues to treat the waste acid in the cathode chamber to a pH of 3-7. At this point, the anolyte is pure, high-concentration nitric acid, and the metal ion concentration in the cathode solution is significantly reduced. As metal ions are removed, a large amount of nitrate and fluoride ions are released. Nitrate ions penetrate to the anode through the selective permeability of the anion exchange membrane, while fluoride ions are blocked at the cathode and combine with hydrogen ions to form hydrofluoric acid. This process continuously increases the pH of the cathode solution, resulting in high-concentration hydrofluoric acid in the cathode solution and high-concentration nitric acid in the anolyte. The endpoint of electrolysis can be flexibly determined based on the required acid and metal ion concentrations during actual use. The anode and cathode solutions are circulated throughout the entire electrolysis process. The final metal ion concentration of the cathode solution can be set below 20g, and it is perfectly feasible to treat it to near zero based on the treatment time.

[0036] It is understandable that in (5), the metallic elements attached to the cathode electrode plate are also periodically cleaned away.

[0037] (6) After electrolysis, the solution obtained from the anode tank is mixed with the solution from the cathode tank and used again as a mixed acid solution.

[0038] In summary, the method for producing regenerated acid from stainless steel pickling waste acid of this invention employs membrane electrolysis. The mixed acid waste liquid serves as the cathode, and nitric acid of a certain initial concentration serves as the anode. Utilizing the reducing properties of the cathode, a large amount of metal ions in the cathode solution are reduced and enriched on the electrode plates. By periodically replacing and cleaning the cathode electrode plates, metal ions in the waste acid are removed, achieving the effect of waste acid regeneration. During this process, due to the selective permeability of the anion exchange membrane, a large amount of nitrate ions pass through the membrane and enter the anode, while most of the hydrofluoric acid remains at the cathode. Finally, mixing the anode and cathode electrolytes yields regenerated acid with a high acid concentration and low metal ion content. Throughout the treatment process, the pH of the cathode solution gradually increases from low to high, from strong acid to near neutral. The final metal ion concentration of the cathode solution can be set below 20g. Periodically cleaning the metal elements adhering to the cathode electrode plates is carried out throughout the electrolysis process. Furthermore, the pH of the waste acid in the cathode chamber is maintained at 1-3, which plays a crucial role in filtering the hydrolyzed metal precipitates in the cathode chamber solution.

[0039] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made based on the content of the present invention specification and drawings, or direct or indirect applications in other related technical fields, are also included in the same way.

Claims

1. A method for producing regenerated acid from waste acid from stainless steel pickling, characterized in that, Includes the following steps: (1): Pretreatment of stainless steel pickling waste acid, collection of pretreated pickling waste acid, the pickling waste acid contains metal ions; (2): Provide an electrolytic cell and set an anion exchange membrane in the electrolytic cell to divide the electrolytic cell into a cathode cell and an anode cell. Place a cathode electrode in the cathode cell and an anode electrode in the anode cell. Place pretreated pickling waste acid in the cathode cell and add dilute nitric acid solution to the anode cell. Utilize the penetration characteristics of the anion exchange membrane to treat the waste acid in the cathode cell to a pH of 1-3 through electrolysis. The cathode electrode is made of stainless steel, copper or lead, and the anode electrode is made of lead dioxide or platinum. (3): In (2), the waste acid in the cathode cell is electrolyzed to a pH of 1-3 and then circulated and filtered. When the waste acid in the cathode cell is treated to a pH of 1 by electrolysis, the filter press is started to filter the cathode cell solution until the pH exceeds 3 and then the filter press is stopped. (4): After (2), continue electrolysis to treat the waste acid in the cathode cell to pH 3-7. At this time, the anolyte is high-concentration nitric acid and the metal ions in the cathode liquid are greatly reduced. As metal ions are removed, a large amount of nitrate and fluoride ions are released. Nitrate ions pass through the selective permeability of the anion exchange membrane to the anolyte, while fluoride ions are blocked in the catholyte, which forms a hydrofluoric acid solution containing a small amount of metal ions. (5): In (2) and (4), the metal ions in the pickling waste acid are reduced and attached to the cathode electrode plate. The metal elements attached to the cathode electrode plate are periodically cleaned off, or the cathode electrode plate is periodically replaced. (6): After the electrolysis in (4) is completed, the solution obtained in the anode cell is mixed with the solution in the cathode cell and used again as a mixed acid solution.

2. The method for producing regenerated acid from stainless steel pickling waste acid according to claim 1, characterized in that: The filter press described in (3) uses an external filter press, and both the chamber plate and the filter cloth are made of acid-resistant material.

3. The method for producing regenerated acid from stainless steel pickling waste acid according to claim 1, characterized in that: The pretreatment described in (1) is filtration, which is used to filter out particulate matter in the pickling waste acid and store it in an acid storage tank after preliminary filtration.

4. The method for producing regenerated acid from stainless steel pickling waste acid according to claim 1, characterized in that: In (2), the stainless steel pickling waste liquid added to the cathode tank has a nitric acid concentration of 20-200 g / L, a hydrofluoric acid concentration of 0-50 g / L, and a metal ion concentration of 20-200 g / L. The same level of dilute nitric acid is added to the anode tank as an electrolyte, and the initial dilute nitric acid concentration of the anode liquid is 0-100 g / L.

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