A method for recovering nitric acid / hydrofluoric acid from stainless steel pickling spent liquor

Abstract

The present application belongs to the technical field of waste liquid environmental protection recycling, and particularly relates to a method for recovering nitric acid / hydrofluoric acid from stainless steel pickling waste liquid, which comprises the following steps: (1) diffusion dialysis treatment: the stainless steel pickling waste liquid is introduced into a diffusion dialysis device for diffusion dialysis, to obtain recovered nitric acid and diffusion dialysis residual liquid respectively; (2) potassium hydroxide neutralization and solid-liquid separation of diffusion dialysis residual liquid: the diffusion dialysis residual liquid is neutralized with potassium hydroxide to form potassium nitrate, potassium fluoride and metal hydroxide precipitate, and then solid-liquid separation is performed to remove the metal hydroxide precipitate, to obtain a KNO3 / KF solution; (3) bipolar membrane electrodialysis treatment: the KNO3 / KF solution obtained in step (2) is introduced into a three-compartment bipolar membrane electrodialysis device, to regenerate HNO3 / HF and KOH. The present application can effectively recover dissociated acid (mainly nitric acid), nitrate in the form of metal salt, molecular hydrofluoric acid and complex fluoride.

Description

Technical Field

[0001] This invention belongs to the field of waste liquid environmental recycling technology, specifically a method for recovering nitric acid / hydrofluoric acid from stainless steel pickling waste liquid. Background Technology

[0002] After hot rolling or annealing, steel products require the use of strong inorganic acid solutions to clean the oxide scale and rust generated on the surface. Pickling is a crucial process in steel production and processing. my country is the world's largest steel producer, and in 2021, it generated approximately 200 million tons of pickling wastewater. This wastewater severely restricts the sustainable development of the steel industry, and the post-treatment of pickling wastewater urgently needs to be addressed.

[0003] The composition of steel pickling waste liquid is related to the type of steel being processed. The most common pickling process for 300 series stainless steel often uses a mixture of nitric acid and hydrofluoric acid. After surface treatment of the stainless steel, NO3... - Some of it still exists in the form of nitric acid, and some exists in the form of metal salts, such as Fe(NO3)3, Cr(NO3)3, Ni(NO3)2; F - Mostly Fe 3+ Cr 3+ It exists in the form of a complex, with a small portion existing as hydrofluoric acid. The typical composition of stainless steel pickling waste liquid is: H... + 2-5 mol / L, NO3 - 150-200g / L, F - 50-60g / L, Fe 3+ 30-40g / L, Cr 3+ 10-15g / L, Ni 2+ 5-10g / L.

[0004] Stainless steel pickling wastewater must undergo post-treatment to meet standards before discharge. The traditional post-treatment process—neutralization and precipitation—involves adding alkali (usually calcium hydroxide) for neutralization and precipitation. The precipitate is then dehydrated and treated as solid hazardous waste, while the aqueous phase can be discharged as ordinary wastewater. This method not only consumes large amounts of reagents and incurs high treatment and disposal costs, but also generates large amounts of sludge or high-salinity wastewater.

[0005] To address this problem, researchers have developed various methods for recovering acid from stainless steel pickling waste.

[0006] Evaporation method: Nitric acid and hydrofluoric acid can be evaporated by heating under reduced pressure, with a recovery rate of 93-96%. However, most of the sulfur dioxide in the pickling waste liquid... - It does not exist in the form of hydrofluoric acid; the process requires the addition of sulfuric acid first, using sulfate ions to replace F. -This method requires a large amount of sulfuric acid. Furthermore, it places high demands on the corrosion resistance of the equipment, resulting in significant investment, high maintenance and operating costs. Additionally, the crystallization of the mother liquor during distillation easily clogs the equipment, leaving behind sulfuric acid-containing sulfate sludge, which is difficult to process.

[0007] The roasting method, pioneered in 1959 by the Austrian company Andritz, involves a thermochemical reaction between metal fluorides and metal nitrates under high-temperature conditions. This reaction produces metal oxide powder, hydrofluoric acid vapor, and nitrogen oxide gas. Free acid is converted into acid vapor, which is then absorbed and recovered in an absorption tower. The nitrogen oxide tail gas is discharged after denitrification. Recovery rates for hydrofluoric acid, nitric acid, and metal salts can reach 97-99%, 60-70%, and 90%, respectively. The main problems with this method are strong corrosion of equipment, high maintenance and operating costs, large initial investment, and operating costs far exceeding the recovery benefits.

[0008] Resin adsorption method: The resin exchange method for treating pickling waste liquid utilizes ion exchange resins to exchange acid radicals or metal ions from the pickling waste liquid, thereby achieving the separation between different acids and metal salts. SCANACON is a representative company using this method. The problems with this method are: ① It can only recover dissociated acids (i.e., acids that can dissociate into hydrogen ions in solution); ② The metal removal effect is unstable, and the recovered acid solution still contains some metal ions, with a concentration of approximately 30% of the original acid concentration; ③ It produces an equal volume of waste acid residue, requiring chemical treatment to generate waste salt sludge.

[0009] Diffusion dialysis is a membrane separation method that uses the concentration difference across a membrane as the driving force and the selective permeability of the ion exchange membrane to separate acids and metal salts. As a simple, low-energy, and environmentally friendly membrane separation technology, diffusion dialysis can effectively separate acids from impurities in waste liquids. The recovered acid can be reused, generating good economic and environmental benefits, and has received widespread attention in recent years.

[0010] The invention patent with application number 201911301574.1 discloses a resource recovery process for iron-containing waste sulfuric acid, which mainly includes the following steps: (1) Pretreatment: The acidic wastewater is subjected to multi-stage filtration to remove impurities and make it meet the requirements of a diffusion dialysis unit; (2) Dialysis treatment: The pretreated acidic wastewater is passed into a diffusion dialysis unit and subjected to diffusion dialysis treatment to obtain concentrated acid and waste liquid respectively; (3) Posttreatment: The obtained concentrated acid is evaporated to obtain regenerated sulfuric acid, and the condensate generated during the evaporation process is treated in a biochemical system; the obtained waste liquid is neutralized with alkali and then discharged. Using this method to treat iron-containing sulfuric acid waste liquid, the sulfuric acid recovery rate is 80%.

[0011] The problem with using the above-mentioned diffusion dialysis method to recover acid from stainless steel pickling waste liquid is that it can only recover dissociated acid (mainly nitric acid), and cannot recover non-dissociated acid (i.e., acid that cannot dissociate into hydrogen ions in solution, such as molecular hydrofluoric acid, because molecular hydrofluoric acid can hardly cross the diffusion dialysis anion membrane). At the same time, it cannot effectively recover acid radicals existing in salt form (such as nitrate ions in Fe(NO3)3, Cr(NO3)3, Ni(NO3)2, etc.) and complexed acid radicals (i.e., acid radicals that form complexes with metal ions, such as those with Fe...). 3+ Cr 3+ Fluoride ions that form complexes (e.g., fluoride ions).

[0012] Patent application number 200810118723.6 discloses a method for recovering alcoholic acid and regenerating alkali from waste acid containing metal ions. The main steps are: (1) subjecting the waste acid solution to a first-stage diffusion dialysis; (2) subjecting the primary residue after step 1 to a second-stage diffusion dialysis; (3) subjecting the secondary residue after step 2 to neutralization and precipitation; and (4) allowing the neutralized filtrate after solid-liquid separation and filtration to enter the intermediate chamber of a three-compartment electrodialysis tank to obtain regenerated acid and regenerated alkali. This method is not suitable for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid, and is especially unsuitable for recovering hydrofluoric acid. The acid-base neutralization step in the above-mentioned patented method uses sodium hydroxide as the alkali. If sodium hydroxide is used for acid-base neutralization, the solubility of the resulting NaF is only 4.17 (g / 100g water) (20℃). A large amount of NaF precipitates out along with the metal precipitate and cannot enter the bipolar membrane electrodialysis step for HNO3 / HF regeneration along with the neutralized salt solution (i.e., NaNO3 / NaF solution). Therefore, F cannot be effectively recovered. - To dissolve the NaF formed by acid-base neutralization, a large amount of water needs to be added and the precipitate repeatedly washed to dissolve the NaF, resulting in a very dilute NaNO3 / NaF solution. This leads to low acid-base concentrations during bipolar membrane electrodialysis regeneration, and the low-concentration recovered acid cannot be directly reused in the upstream process.

[0013] In the aforementioned patented method, most of the acid (approximately 80%) is recovered via diffusion dialysis, and the remaining acid in metal salt form is recovered through acid-base neutralization and electrodialysis. However, in the nitric acid / hydrofluoric acid system, hydrofluoric acid is a weak acid and hardly dissociates; simultaneously, some fluoride ions form complexes with metal ions. Therefore, fluoride ions can hardly be recovered via diffusion dialysis. - The main recovery process involves acid-base neutralization and electrodialysis. The salt solution obtained from the acid-base neutralization step contains F... - The recovery rate determines F - Can it be effectively recovered in the bipolar membrane electrodialysis step, that is, in the acid-base neutralization step F? - The recovery rate determines the overall impact of the process on F. - Recovery rate. Summary of the Invention

[0014] To address the aforementioned problem of recovering nitric acid / hydrofluoric acid from stainless steel pickling waste containing metal ions, this invention provides a method for recovering nitric acid / hydrofluoric acid from stainless steel pickling waste. Specifically, it involves the effective recovery of HNO3 and HF from stainless steel pickling waste containing metal ions and nitric acid / hydrofluoric acid, including the recovery of dissociated acid (mainly nitric acid), nitrate ions in metal salt form, molecular hydrofluoric acid, and complexed fluoride ions.

[0015] To achieve the above objectives, the present invention adopts the following technical solution:

[0016] This invention provides a method for recovering nitric acid / hydrofluoric acid from stainless steel pickling waste liquid, which can be called a "diffusion dialysis-potassium hydroxide neutralization-bipolar membrane electrodialysis combined process," and specifically includes the following steps:

[0017] (1) Diffusion dialysis treatment:

[0018] Stainless steel pickling waste liquid containing metal ions from a nitric acid / hydrofluoric acid mixed acid system is passed into a diffusion dialysis apparatus for diffusion dialysis, yielding recovered acid and diffusion dialysis residue. The recovered acid mainly consists of nitric acid and can be referred to as recovered nitric acid. The diffusion dialysis residue mainly contains nitrate ions in salt form, molecular hydrofluoric acid, and complexed fluoride ions.

[0019] (2) Neutralization of diffusion dialysis residue with potassium hydroxide and solid-liquid separation:

[0020] The diffusion dialysis residue is neutralized with potassium hydroxide to form potassium nitrate, potassium fluoride, and metal hydroxide precipitates. Solid-liquid separation is then performed to remove the metal hydroxide precipitates, yielding a KNO3 / KF solution.

[0021] (3) Bipolar membrane electrodialysis treatment:

[0022] The KNO3 / KF solution obtained in step (2) is passed into a three-compartment bipolar membrane electrodialysis unit to regenerate HNO3 / HF and KOH.

[0023] This achieves the removal of NO3 from pickling waste liquid. - and F - Effective recovery in the form of acid, especially F - Effective recycling of NO3. - and F - The recovery rates reached over 95% and over 85%, respectively; the concentrations of recovered HNO3 and HF reached 139.3 g / L and 10.29 g / L, respectively. The recovered acid can be mixed with commercial nitric acid (65% concentration) and commercial hydrofluoric acid (40% concentration) in a ratio of 7:2:1 and reused in the stainless steel pickling section.

[0024] The process route diagram of this invention is as follows: Figure 1 As shown.

[0025] The specific method for the diffusion dialysis treatment in step (1) is as follows:

[0026] A diffusion dialysis unit is a diffusion dialysis membrane stack consisting of anion exchange membranes, dialysis dialysis plates, and clamping plates. The compartment through which stainless steel pickling waste containing metal ions is introduced is the dialysis chamber, where diffusion dialysis residue is obtained after treatment. The compartment through which the initial recovery solution (usually water or a dilute solution) is introduced is the diffusion chamber, where recovered acid is obtained after treatment. In this process, the acid in the dialysis chamber permeates through the anion exchange membrane into the diffusion chamber, where metal ions are retained. This results in a relatively pure acid solution in the diffusion chamber. The liquid retained in the dialysis chamber at the end of the diffusion dialysis process is the diffusion dialysis residue.

[0027] The specific method for neutralizing the diffusion dialysis residue with potassium hydroxide and separating the solid and liquid in step (2) is as follows:

[0028] Potassium hydroxide is added to the diffusion dialysis residue, causing a metal hydroxide precipitate. The precipitate is removed by solid-liquid separation, yielding a salt solution of potassium nitrate and potassium fluoride. Potassium fluoride obtained by acid-base neutralization with potassium hydroxide has high solubility, which can increase the solubility of potassium nitrate. - The recovery rate is improved, resulting in a high concentration of nitrate and fluoride solution, which increases the concentration of HNO3 / HF regenerated by bipolar membrane electrodialysis.

[0029] The specific method for the bipolar membrane electrodialysis treatment in step (3) is as follows:

[0030] The KNO3 / KF solution obtained in step (2) is passed into the salt chamber of a three-compartment bipolar membrane electrodialysis device as the initial solution for the salt chamber. The anions and cations in the solution migrate across the anion exchange membrane and cation exchange membrane respectively under the drive of the electric field and enter the acid chamber and the alkali chamber. With the help of the bipolar membrane dissociating water, HNO3 / HF and KOH are obtained in the acid chamber and the alkali chamber respectively.

[0031] The diffusion dialyzer in step (1) includes, but is not limited to, the traditional plate-and-frame diffusion dialyzer, which is well-known and common to those skilled in the art. It can be purchased from the market or assembled independently. Examples include, traditional plate-and-frame diffusion dialyzers, static diffusion dialyzers, spiral wound diffusion dialyzers (for specific structures, please refer to the patent with publication number CN101983756B), and baffled plate-and-frame rapid diffusion dialyzers (whose structure and feed flow direction are as follows). Figure 4a and Figure 4b (as shown in the image) etc.

[0032] The structure of the baffle-frame type rapid diffusion dialysis unit is as follows: Figure 4aAs shown: Several redirecting partitions 2 and ion exchange membranes 3 are staggered between two clamping plates 1. The redirecting partitions 2 are located on both sides of the ion exchange membrane 3. The redirecting partitions 2 and the ion exchange membrane respectively form dialysis chamber compartments and diffusion chamber compartments. Selective ion channels are formed between the dialysis chamber compartments and the diffusion chamber compartments through the ion exchange membrane. The compartments of the dialysis chamber and the compartments of the diffusion chamber are connected in series. The partition includes a partition frame 4 and a partition mesh 5. The partition mesh 5 is fixedly connected to the inner wall of the partition frame 4, and the partition mesh 5 forms the compartment. The partition frame is made of polypropylene, and the partition mesh is made of PVC. The partition frame has two liquid distribution holes 6 and one liquid passage hole 8. One liquid distribution hole 6 and one liquid passage hole 8 are located at the top of the partition frame, and one liquid distribution hole 6 is located at the bottom of the partition frame. The liquid distribution hole 6 at the top of the partition frame is connected to the compartment through a liquid distribution groove 7, while the other liquid passage hole 8 is not connected to the compartment. The liquid distribution hole at the bottom of the partition frame is connected to the compartment through a liquid distribution groove. The liquid distribution hole at the top of the partition frame connected to the compartment and the liquid distribution hole at the bottom of the partition frame connected to the compartment are arranged diagonally and centrally symmetrically. Sealing gaskets 9 are located on both sides of the liquid distribution hole 6 connected to the compartment on the partition to prevent leakage of the liquid.

[0033] The feed flow direction of the baffle-frame type rapid diffusion dialysis unit is as follows: Figure 4b As shown: The feed flow in the dialysis chamber and diffusion chamber compartments is counter-current. The feed in the dialysis chamber flows from left to right of the membrane stack. The sixth, fourth, and second partitions are dialysis chamber partitions (i.e., dialysis chamber compartments). The feed enters through the feed inlet on the clamping plate into the distribution hole in the lower right corner of the sixth partition, which connects to the compartment, and flows from bottom to top. Since the fifth partition does not have a flow hole in its lower right corner, it cannot flow into the fourth partition through the flow hole in the lower right corner of the fifth partition. Instead, it enters the fourth partition through the flow hole in the upper left corner of the fifth partition, and then flows from top to bottom through the distribution channel of the fourth partition. This achieves one change in feed direction, and so on. The feed in the diffusion chamber flows from right to left of the membrane stack. The first, third, and fifth partitions are diffusion chamber partitions (i.e., diffusion chamber compartments). Water enters the first partition through the water inlet on the clamping plate and flows down through the distribution hole in the upper right corner of the first partition, which is connected to the compartment. Since the second partition does not have a liquid passage hole in the upper right corner, it cannot flow into the third partition through the liquid passage hole in the upper right corner of the second partition. Instead, it can only enter the third partition through the liquid passage hole in the lower left corner of the second partition and then flow up through the distribution groove of the third partition. In this way, the direction of the liquid is changed once, and so on.

[0034] The solid-liquid separation method in step (2) includes, but is not limited to, one or more of the solid-liquid separation methods well-known and commonly used by those skilled in the art, such as sedimentation, filtration, and centrifugation, used in combination. Preferably, one or more of the solid-liquid separation methods such as filtration and centrifugation are used in combination.

[0035] The bipolar membrane electrodialysis unit in step (3) includes, but is not limited to, the traditional plate-and-frame type "acid-salt-alkali" three-compartment bipolar membrane electrodialysis unit, which is well-known and common to those skilled in the art. It can be purchased from the market or assembled by oneself. Figure 2 This is a schematic diagram illustrating the principle of generating HNO3 / HF and KOH in a common plate-and-frame type "acid-salt-alkali" three-compartment bipolar membrane electrodialysis system. When using the "acid-salt-alkali" three-compartment bipolar membrane electrodialysis treatment, the KNO3 / KF solution obtained in step (2) is passed into the salt compartment. Driven by an electric field, the anions and cations migrate across the anion exchange membrane and cation exchange membrane, respectively, into the acid and alkali compartments. Combined with the dissociation of water by the bipolar membrane, corresponding regenerated HNO3 / HF and KOH are obtained in the acid and alkali compartments, respectively. The salt compartment yields a desalination residue (a low-concentration KNO3 / KF solution).

[0036] Preferably, the potassium hydroxide obtained in the bipolar membrane electrodialysis treatment in step (3) is used to neutralize the diffusion dialysis residue in step (2).

[0037] Preferably, the acid concentration of the initial solution in the acid chamber during the bipolar membrane electrodialysis treatment in step (3) is 0-5 mol / L.

[0038] Preferably, the initial alkali concentration of the alkali chamber solution in the bipolar membrane electrodialysis treatment in step (3) is 0-5 mol / L.

[0039] Preferably, in step (3), the volume ratio of the initial solution in the acid or alkali chamber to the initial solution in the salt chamber during bipolar membrane electrodialysis is 0.1-1:1.

[0040] The technical solution provided by this invention has the following advantages compared with the prior art:

[0041] (1) The method for recovering nitric acid / hydrofluoric acid from stainless steel pickling waste liquid of the present invention, namely the "diffusion dialysis-potassium hydroxide neutralization-bipolar membrane electrodialysis combined process", can not only recover the dissociated acid (mainly nitric acid), but also recover nitrate ions in the form of salts, molecular hydrofluoric acid, and complexed fluoride ions, thus realizing NO3 - and F - The effective recovery of fluoride ions, in particular, has economic value.

[0042] (2) It overcomes the problem of difficult treatment of residual liquid generated by general diffusion dialysis method, and solves the problem of large amount of waste brine generated after simple acid-base neutralization treatment, which is environmentally friendly and practical.

[0043] (3) The method of the present invention is used to recover nitric acid / hydrofluoric acid and NO3 from stainless steel pickling waste liquid. - and F -The recovery rates reached over 95% and over 85%, respectively; the concentrations of recovered HNO3 and HF reached 139.3 g / L and 10.29 g / L, respectively. The recovered acid can be mixed with commercial nitric acid (65% concentration) and commercial hydrofluoric acid (40% concentration) in a ratio of 7:2:1 and reused in the stainless steel pickling section. Attached Figure Description

[0044] Figure 1 This is a process route diagram for recovering nitric acid / hydrofluoric acid from stainless steel pickling waste liquid according to the present invention;

[0045] Figure 2 The schematic diagram of the "acid-salt-base" three-compartment bipolar membrane electrodialysis device of the present invention for generating HNO3 / HF and KOH is shown below (BM: bipolar membrane; AM: anion exchange membrane; CM: cation exchange membrane).

[0046] Figure 3 This is a flowchart illustrating the "diffusion dialysis-potassium hydroxide neutralization-'acid-salt-alkali' three-compartment bipolar membrane electrodialysis combined process" of the present invention for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid;

[0047] Figure 4a The schematic diagram of the baffle diffusion dialysis device of the present invention is as follows: (1. clamping plate, 2. diversion baffle, 3. ion exchange membrane, 4. baffle frame, 5. baffle mesh, 6. liquid distribution hole, 7. liquid distribution tank, 8. liquid passage hole, 9. sealing gasket).

[0048] Figure 4b This is a schematic diagram of the feed liquid flow direction in the baffle diffusion dialysis device of the present invention;

[0049] Figure 5 This is a flowchart illustrating the process of recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid using the "diffusion dialysis-regenerated potassium hydroxide reuse neutralization-'acid-salt-alkali' three-compartment bipolar membrane electrodialysis combined process" of the present invention.

[0050] Figure 6 This is a schematic diagram of a traditional plate-and-frame diffusion dialysis unit.

[0051] Figure 7 A roadmap for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid using a "diffusion dialysis-sodium hydroxide neutralization-'acid-salt-alkali' three-compartment bipolar membrane electrodialysis combined process";

[0052] Figure 8 The route diagram for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid using the "diffusion dialysis-sodium hydroxide neutralization-'acid-salt-alkali' three-compartment bipolar membrane electrodialysis combined process" (the sodium hydroxide neutralization step adds water at twice the volume of the diffusion dialysis residue). Detailed Implementation

[0053] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0054] Example 1

[0055] The "diffusion dialysis-potassium hydroxide neutralization-bipolar membrane electrodialysis combined process" of this invention is used to recover nitric acid and hydrofluoric acid from stainless steel pickling waste liquid. The process route is as follows: Figure 3 As shown. The liquid to be treated is stainless steel pickling waste liquid, and its composition is NO3. - 5.38 mol / L, F - 1.91 mol / L, Fe 3+ 0.54 mol / L, Cr 3+ 0.21 mol / L, Ni 2+ 0.11 mol / L. Total feed volume: 1 L.

[0056] Diffusion dialysis treatment: A baffled diffusion dialysis unit is used. A schematic diagram of the diffusion dialysis unit is shown below. Figure 4a and Figure 4b As shown. Eleven anion exchange membranes (Asahi Glass DSVN anion exchange membranes from Japan) and twelve separators were used, with the anion exchange membranes and separators arranged alternately. The effective membrane area of ​​a single anion exchange membrane was 0.0088 m². 2 The effective membrane area of ​​the entire membrane module is 0.097m². 2 The initial liquid in the diffusion chamber is water. The feed liquid in the diffusion chamber and the feed liquid in the dialysis chamber operate in countercurrent flow. The linear velocity of the feed liquid flow in the membrane stack is 8.78 cm / min.

[0057] Potassium hydroxide neutralization: Acid-base neutralization was performed using potassium hydroxide tablets, and solid-liquid separation was performed by centrifugation. Centrifugation conditions: 10000 r / min, 10 min, to obtain KNO3 / KF solution.

[0058] Bipolar membrane electrodialysis treatment: A three-compartment bipolar membrane electrodialysis unit with an acid-salt-alkali structure is used. The principle of generating HNO3 / HF and KOH in the bipolar membrane electrodialysis unit is as follows: Figure 2 As shown. Five bipolar membranes (Beijing Tingrun), four anion exchange membranes (Asahi Glass AMVN, hereinafter referred to as anion membranes), four cation exchange membranes (Asahi Glass CMVN, hereinafter referred to as cation membranes), and twelve separators were used, arranged in a "bipolar membrane-separator-cation membrane-separator-anion membrane-separator" configuration. The effective membrane area of ​​each membrane was 0.0088 m². 2 The membrane uses titanium-coated ruthenium anode plates and stainless steel cathode plates. The initial solution for the acid chamber is water, the initial solution for the alkali chamber is water, and the initial solution for the salt chamber is a KNO3 / KF solution obtained from the potassium hydroxide neutralization step. The volume ratio of the initial acid, alkali and salt chamber solutions is 1:1:1, and the linear velocity of the solution flow in the membrane stack is 3 cm / s.

[0059] The results are shown in Table 1. NO3- and F - The recovery rates reached 97.35% and 89.7%, respectively; the recovered concentrations of HNO3 and HF reached 139.3 g / L and 10.29 g / L, respectively.

[0060] Table 1. Results of the recovery of nitric acid and hydrofluoric acid from stainless steel pickling waste liquid using the "diffusion dialysis-potassium hydroxide neutralization-'acid-salt-alkali' three-compartment bipolar membrane electrodialysis combined process".

[0061]

[0062]

[0063] Example 2

[0064] The "diffusion dialysis-regenerated potassium hydroxide reuse neutralization-bipolar membrane electrodialysis combined process" of this invention is used to recover nitric acid and hydrofluoric acid from stainless steel pickling waste liquid. The process flow is as follows: Figure 5 As shown. The liquid to be treated is stainless steel pickling waste liquid, and its composition is NO3. - 5.38 mol / L, F - 1.91 mol / L, Fe 3+ 0.54 mol / L, Cr 3+ 0.21 mol / L, Ni 2+ 0.11 mol / L. Total feed volume: 1 L.

[0065] Diffusion dialysis treatment: A traditional plate and frame diffusion dialysis unit is used. The structure of the diffusion dialysis unit is as follows: Figure 6 As shown. Eleven anion exchange membranes (Asahi Glass DSVN anion exchange membranes from Japan) and twelve separators were used, with the anion exchange membranes and separators arranged alternately. The effective membrane area of ​​a single anion exchange membrane was 0.0088 m². 2 The effective membrane area of ​​the entire membrane module is 0.097m². 2 The initial liquid in the diffusion chamber is water. The feed liquid in the diffusion chamber and the feed liquid in the dialysis chamber operate in countercurrent flow. The linear velocity of the feed liquid flow in the membrane stack is 0.73 cm / min.

[0066] Regenerated potassium hydroxide for neutralization: The potassium hydroxide solution obtained from the bipolar membrane electrodialysis treatment step in Example 1 was used for acid-base neutralization. The insufficient part was supplemented with potassium hydroxide tablets. Solid-liquid separation was performed by multi-stage vacuum filtration: the first stage filter had a pore size of 70-80 μm, the second stage filter had a pore size of 15-20 μm, and the third stage filter had a pore size of 0.45 μm, to obtain a KNO3 / KF solution.

[0067] Bipolar membrane electrodialysis treatment: The initial solution in the acid chamber is water, the initial solution in the alkali chamber is water, and the initial solution in the salt chamber is the KNO3 / KF solution obtained from the regeneration potassium hydroxide recycling neutralization step. The volume ratio of the initial acid chamber, alkali chamber and salt chamber solutions is 0.5:0.5:1, and other aspects are the same as in Example 1.

[0068] The results are shown in Table 2, NO3 - and F - The recovery rates reached 97.01% and 85.6%, respectively; the recovered concentrations of HNO3 and HF reached 118.44 g / L and 9.29 g / L, respectively.

[0069] Table 2. Results of the recovery of nitric acid and hydrofluoric acid from stainless steel pickling waste liquid using the "diffusion dialysis-regenerated potassium hydroxide reuse neutralization-'acid-salt-alkali' three-compartment bipolar membrane electrodialysis combined process".

[0070]

[0071] Comparative Example 1

[0072] The combined process of diffusion dialysis-sodium hydroxide neutralization-bipolar membrane electrodialysis is used to recover HNO3 and HF from stainless steel pickling waste liquid. The process flow is as follows: Figure 7 As shown. The liquid to be treated is stainless steel pickling waste liquid, and its composition is NO3. - 5.38 mol / L, F - 1.91 mol / L, Fe 3+ 0.54 mol / L, Cr 3+ 0.21 mol / L, Ni 2+ 0.11 mol / L. Total feed volume: 1 L.

[0073] Diffusion dialysis treatment: Same as in Example 2

[0074] Sodium hydroxide neutralization: Sodium hydroxide tablets were used to neutralize the acid and base of the diffusion dialysis residue. Solid-liquid separation was carried out by multi-stage vacuum filtration: the first stage filter had a pore size of 70-80 μm, the second stage filter had a pore size of 15-20 μm, and the third stage filter had a pore size of 0.45 μm, to obtain a NaNO3 / NaF solution.

[0075] Bipolar membrane electrodialysis treatment: the initial solution for the acid chamber is water, the initial solution for the alkali chamber is water, and the initial solution for the salt chamber is the NaNO3 / NaF solution obtained from the sodium hydroxide neutralization step. Other steps are the same as in Example 1.

[0076] The results are shown in Table 3. NO3 - and F - The recoveries were 94.26% and 47.21%, respectively; the recovered concentrations of HNO3 and HF were 114.32 g / L and 5.13 g / L, respectively.

[0077] Table 3. Results of the recovery of nitric acid and hydrofluoric acid from stainless steel pickling waste liquid using the "diffusion dialysis-sodium hydroxide neutralization-'acid-salt-alkali' three-compartment bipolar membrane electrodialysis combined process".

[0078]

[0079]

[0080] Comparative Example 2

[0081] The combined process of diffusion dialysis-sodium hydroxide neutralization-bipolar membrane electrodialysis is used to recover HNO3 and HF from stainless steel pickling wastewater. The process route is as follows: Figure 8 As shown. The feed solution to be treated is stainless steel pickling waste liquid, which consists of 2.8 mol / L HNO3, 2 mol / L HF, 0.54 mol / L Fe(NO3)3, 0.19 mol / L Cr(NO3)3 and 0.085 mol / L Ni(NO3)2. The total volume of the feed solution is 1 L.

[0082] Diffusion dialysis treatment: Same as in Example 1

[0083] Neutralization with sodium hydroxide: Sodium hydroxide tablets were used to neutralize the diffusion dialysis residue. To dissolve more NaF, water with a volume twice that of the diffusion dialysis residue was added. Solid-liquid separation was performed by centrifugation at 10,000 r / min for 10 min to obtain a NaNO3 / NaF solution.

[0084] Bipolar membrane electrodialysis treatment: the initial solution for the acid chamber is water, the initial solution for the alkali chamber is water, and the initial solution for the salt chamber is the NaNO3 / NaF solution obtained from the sodium hydroxide neutralization step. Other steps are the same as in Example 1.

[0085] The results are shown in Table 4. NO3 - and F - The recoveries were 97.17% and 73.14%, respectively; the recovered concentrations of HNO3 and HF were 83.21 g / L and 4.73 g / L, respectively.

[0086] Table 4. Results of the recovery of nitric acid and hydrofluoric acid from stainless steel pickling waste liquid using the "diffusion dialysis-sodium hydroxide neutralization-'acid-salt-alkali' three-compartment bipolar membrane electrodialysis combined process".

[0087]

[0088]

[0089] As can be seen from Examples 1-2 and Comparative Examples 1-2, the diffusion dialysis step mainly recovers dissociated HNO3. Hydrofluoric acid is a weak acid and hardly dissociates; simultaneously, some fluoride ions form complexes with metal ions, therefore fluoride ions can hardly be recovered by diffusion dialysis. Furthermore, nitrate ions in metal salt form also cannot be recovered by diffusion dialysis. Molecular hydrofluoric acid, complexed fluoride ions, and nitrate ions in metal salt form remain in the diffusion dialysis residue. The diffusion dialysis residue is neutralized with acid and base to remove NO3 from the residue. - and F - Both can be effectively recovered. The alkali used for acid-base neutralization must be potassium hydroxide because potassium fluoride and potassium nitrate have high solubility (potassium fluoride solubility is 94.9 g / 100g water (20℃), and potassium nitrate solubility is 33 g / 100g water (20℃)). This ensures the safe recovery of NO3. - and F - In the acid-base neutralization step, the recovery rate is as follows: nitrate in metal salt form, hydrofluoric acid in molecular form, and fluoride ions in complex form are converted into potassium salt, forming a KNO3 / KF solution, Fe... 3+ Cr 3+ Ni 2+ After the metal ions are converted into hydroxide precipitates and removed, KNO3 / KF is regenerated into HNO3 / HF and KOH using bipolar membrane electrodialysis. Thus, NO3- precipitates are simultaneously removed. - and F - It can be efficiently recovered in the form of acid.

[0090] The upper and lower limits of the process parameters (such as temperature, time, etc.) and the range values ​​of the present invention can all achieve the method, and examples are not listed here.

[0091] All aspects not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0092] 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 it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid, the method comprising the following steps: (1) Diffusion dialysis treatment: Stainless steel pickling waste liquid is passed into a diffusion dialysis unit for diffusion dialysis to obtain recovered nitric acid and diffusion dialysis residue. The diffusion dialysis unit is a diffusion dialysis membrane stack composed of anion exchange membranes, redirecting baffles, and clamping plates. Several redirecting baffles and ion exchange membranes are staggered between two clamping plates. The redirecting baffles are located on both sides of the ion exchange membranes, forming dialysis chambers and diffusion chambers respectively. Selective ion channels are formed between the dialysis chambers and diffusion chambers through the ion exchange membranes. The dialysis chambers and diffusion chambers are connected in series. The feed flow in the dialysis chambers and diffusion chambers is countercurrent. (2) Neutralization of diffusion dialysis residue with potassium hydroxide and solid-liquid separation: The diffusion dialysis residue is neutralized with potassium hydroxide to form potassium nitrate, potassium fluoride and metal hydroxide precipitates. Solid-liquid separation is then performed to remove the metal hydroxide precipitates, yielding KNO3 and KF solutions. (3) Bipolar membrane electrodialysis treatment: The KNO3 and KF solutions obtained in step (2) are passed into a three-compartment bipolar membrane electrodialysis unit to regenerate HNO3, HF and KOH.

2. The method for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid according to claim 1, characterized in that, The solid-liquid separation method in step (2) includes one or more of sedimentation, filtration and centrifugation.

3. The method for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid according to claim 1, characterized in that, The specific method for the bipolar membrane electrodialysis treatment in step (3) is as follows: The KNO3 and KF solutions obtained in step (2) are passed into the salt chamber of a three-compartment bipolar membrane electrodialysis apparatus as the initial solution for the salt chamber. The anions and cations in the solution migrate across the anion exchange membrane and cation exchange membrane respectively under the drive of the electric field and enter the acid chamber and the alkali chamber. With the help of the bipolar membrane dissociating water, HNO3, HF and KOH are obtained in the acid chamber and the alkali chamber respectively.

4. The method for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid according to claim 1, characterized in that, The three-compartment bipolar membrane electrodialysis unit in step (3) includes the traditional plate and frame type "acid-salt-alkali" three-compartment bipolar membrane electrodialysis unit.

5. The method for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid according to claim 1, characterized in that, The potassium hydroxide obtained in step (3) during bipolar membrane electrodialysis is used to neutralize the diffusion dialysis residue in step (2).

6. The method for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid according to claim 1, characterized in that, In step (3), the initial acid concentration of the acid chamber solution in the bipolar membrane electrodialysis treatment is 0-5 mol / L, and the initial alkali concentration of the alkali chamber solution is 0-5 mol / L.

7. The method for recovering nitric acid and hydrofluoric acid from stainless steel pickling waste liquid according to claim 1, characterized in that, In step (3), the volume ratio of the initial solution in the acid or alkali chamber to the initial solution in the salt chamber during bipolar membrane electrodialysis is 0.1-1:1.

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