Method and system for concentrating waste acid

Abstract

The invention provides a waste acid concentration method and system, and belongs to the field of acid pickling waste liquid treatment. The method comprises the following steps: filtering waste acid to obtain a first mixed solution; carrying out thermal refining on the first mixed solution to obtain a second mixed solution; freezing and crystallizing the second mixed solution to separate out FeSO4 crystals in the mixed solution, so as to obtain a solid-liquid mixture; and carrying out solid-liquid separation on the solid-liquid mixture to respectively obtain regenerated acid and FeSO4. 7H2O crystals. The method comprises the following steps: firstly, filtering the waste acid to obtain a relatively pure first mixed solution; secondly, the first mixed solution is subjected to thermal refining, so that FeSO4 can be separated out more easily in the subsequent freezing crystallization process; then, effective separation of Fe < 2 + > and sulfuric acid is realized through freezing crystallization; and finally, respectively obtaining the regenerated acid and the FeSO4. 7H2O crystal. Therefore, the tempering-freezing crystallization method is utilized, and the resource utilization rate of waste acid generated by steel pickling is increased.

Description

Technical Field

[0001] The present application relates to the technical field of pickling waste liquid treatment, and in particular to a method and system for concentrating waste acid. Background Art

[0002] Pickling is an important part of steel processing. Its main purpose is to remove oxides, rust and pollutants on the surface of steel to improve the surface quality and subsequent processing performance of steel. During the pickling process, the acid solution reacts chemically with the oxides and rust on the surface of the steel to generate corresponding salts and water. At the same time, the acid will also react with the iron in the steel matrix. As the pickling process proceeds, the concentration of ferrous ions and ferric ions in the acid solution gradually increases. When the concentration of these ions reaches a certain level, the pickling efficiency will decrease and the pickling solution will lose its value for continued use. At this time, the waste liquid containing high concentrations of ferrous ions, ferric ions and unreacted acid solution is steel pickling waste liquid. Since steel pickling waste liquid contains high concentrations of acid and metal ions, direct discharge will cause pollution to the environment.

[0003] In the prior art, the waste acid is collected by neutralization filter pressing, and then mixed with quicklime for acid-base neutralization. The neutralization process produces a large amount of sludge containing precipitates such as iron hydroxide, calcium sulfate, and lime slag. The slurry is then filtered and dehydrated, the neutralized water is recycled and reused, and the mud cake is stored in a warehouse and uniformly handled by an institution. The discharge standard of the waste acid liquid after neutralization is a pH value of 7 to 8.5 and suspended solids <160 mg / L. However, the neutralization filter pressing method used consumes a large amount of quicklime and produces a large amount of sludge containing precipitates such as iron hydroxide, calcium sulfate, and lime slag. The operating cost is high and it will cause environmental pollution. Therefore, how to improve the resource utilization rate of waste acid generated by steel pickling is a technical problem that urgently needs to be solved. Summary of the invention

[0004] The present application provides a method and system for concentrating waste acid to solve the following technical problem: how to improve the resource utilization rate of waste acid generated by steel pickling.

[0005] In a first aspect, the present application provides a method for concentrating waste acid, wherein the waste acid contains H2SO4 and FeSO4, and the method comprises:

[0006] filtering the waste acid to obtain a first mixed solution;

[0007] Performing a conditioning treatment on the first mixed solution to obtain a second mixed solution;

[0008] Freezing and crystallizing the second mixed solution to crystallize FeSO4 in the mixed solution to obtain a solid-liquid mixture; and

[0009] The solid-liquid mixture is subjected to solid-liquid separation to obtain regenerated acid and FeSO4·7H2O crystals respectively.

[0010] Optionally, the final temperature of the frozen crystallization is -2°C to -5°C, and the effective freezing time of the frozen crystallization is 110 min to 130 min.

[0011] Optionally, the waste acid is steel pickling waste liquid.

[0012] Optionally, the concentration of free sulfuric acid in the steel pickling waste liquid is 3% to 4%.

[0013] Optionally, the mass concentration of ferrous sulfate in the steel pickling waste liquid is 150g / L to 250g / L.

[0014] Optionally, the regenerated acid is circulated for steel pickling, and the sulfuric acid concentration of the regenerated acid is ≥15%.

[0015] Optionally, the recovery rate of H2SO4 in the waste acid is ≥80%.

[0016] Optionally, the purity of the FeSO4·7H2O crystals is ≥80%.

[0017] In a second aspect, the present application provides a system for concentrating spent acid, the system being adapted to the method described in any one of the embodiments of the first aspect, the system comprising:

[0018] an acid-resistant filter, the acid-resistant filter being connected to the waste acid tank and used for filtering the waste acid to obtain a first mixed solution;

[0019] A conditioning device, the conditioning device is connected to the liquid phase outlet of the acid-resistant filter, and is used to sequentially condition the first mixed liquid to obtain a second mixed liquid;

[0020] A freezer, wherein the crystallization kettle of the freezer is connected to the outlet of the quenching and tempering device, and is used to freeze and crystallize the second mixed liquid to crystallize FeSO4 in the mixed liquid to obtain a solid-liquid mixture;

[0021] A solid-liquid separation device is connected to the outlet of the crystallization kettle of the refrigerator and is used to perform solid-liquid separation on the solid-liquid mixture to obtain regenerated acid and FeSO4·7H2O crystals respectively.

[0022] Optionally, the system further includes:

[0023] An acid recovery tank, wherein the inlet of the acid recovery tank is communicated with the liquid phase outlet of the solid-liquid separation device, and the outlet of the acid recovery tank is communicated with the inlet of the pickling tank.

[0024] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0025] The present application provides a method for concentrating waste acid, wherein the waste acid contains H2SO4 and FeSO4, and the method comprises: filtering the waste acid to obtain a first mixed solution; subjecting the first mixed solution to a tempering treatment to obtain a second mixed solution; subjecting the second mixed solution to freeze crystallization, so that the FeSO4 in the mixed solution is crystallized to obtain a solid-liquid mixture; and subjecting the solid-liquid mixture to solid-liquid separation to obtain regenerated acid and FeSO4·7H2O crystals, respectively. First, the waste acid is filtered to remove suspended matter, impurities, etc. therein, so as to obtain a relatively pure first mixed solution; secondly, the first mixed solution is tempered to adjust the acidity of the solution to change the solubility of ferrous sulfate, which is conducive to easier precipitation of FeSO4 in the subsequent freezing and crystallization process; thirdly, the temperature is lowered to below the solubility critical point of FeSO4 through freezing and crystallization, so that FeSO4 is precipitated in the form of crystals, thereby achieving FeSO4. 2+ Effective separation from sulfuric acid; finally, through solid-liquid separation, regenerated acid and FeSO4·7H2O crystals are obtained respectively, realizing effective separation and recovery of resources. Therefore, the present application utilizes the quenching and tempering-freezing crystallization method to improve the resource utilization rate of waste acid generated by steel pickling. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0027] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0028] Figure 1 A flow chart of a method for concentrating waste acid provided in an embodiment of the present application;

[0029] Figure 2 A schematic diagram of a method for concentrating spent acid provided in an embodiment of the present application;

[0030] Figure 3 A schematic diagram of a system for concentrating spent acid provided in an embodiment of the present application;

[0031] Reference numerals:

[0032] 1-waste acid tank, 2-acid-resistant filter, 3-conditioning device, 4-refrigerator, 5-solid-liquid separation device, 6-recovery acid tank, 7-pickling tank. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0034] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0035] In addition, in the description of the specification of the present application, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second", etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" means one or more, and "plurality" means two or more. "At least one", "the following at least one item (items)" or similar expressions refer to any combination of these items, including any combination of single items (items) or plural items (items). For example, "at least one of a, b, or c", or "at least one of a, b, and c", can all represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, the mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one-to-one with the proportional numbers in the proportional formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0036] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0037] Figure 1 A flow chart of a method for concentrating waste acid provided in an embodiment of the present application; Figure 2 A schematic diagram of a method for concentrating spent acid provided in an embodiment of the present application.

[0038] See also Figure 1 The present application provides a method for concentrating waste acid, wherein the waste acid contains H2SO4 and FeSO4, and the method comprises:

[0039] S1, filtering the waste acid to obtain a first mixed solution;

[0040] The waste acid is filtered to remove suspended matter, large particle impurities, etc., to obtain a relatively pure first mixed solution.

[0041] In some embodiments, the waste acid is steel pickling waste liquid.

[0042] In some embodiments, the concentration of free sulfuric acid in the steel pickling wastewater is 3% to 4%.

[0043] In some embodiments, the mass concentration of ferrous sulfate in the steel pickling wastewater is 150 g / L to 250 g / L.

[0044] The concentration of free sulfuric acid in the steel pickling waste liquid is limited to 3% to 4%, and the mass concentration of ferrous sulfate in the steel pickling waste liquid is 150g / L to 250g / L, so that the waste acid is more suitable for subsequent freezing and crystallization treatment, and ferrous sulfate and regenerated acid are recovered more effectively. Exemplarily, the free sulfuric acid content of the steel pickling waste liquid can be 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, etc., and the mass concentration of ferrous sulfate in the steel pickling waste liquid can be 150g / L, 160g / L, 180g / L, 200g / L, 220g / L, 240g / L, 250g / L, etc.

[0045] S2, subjecting the first mixed solution to a tempering treatment to obtain a second mixed solution;

[0046] The conditioning treatment is mainly based on the principle of acid-base neutralization. In this process, an appropriate amount of alkaline substances (such as lime milk, sodium hydroxide, etc.) are added to the waste acid, which will react with the sulfuric acid in the waste acid to generate sulfate and water. As the neutralization reaction proceeds, the concentration of sulfuric acid in the waste acid gradually decreases until it reaches the predetermined acidity range. The core purpose of the conditioning treatment is to adjust the acidity of the waste acid, that is, to reduce the concentration of sulfuric acid therein. Through this process, the waste acid can be made more suitable for subsequent freeze crystallization treatment, thereby more effectively recovering ferrous sulfate and regenerating acid.

[0047] S3, freezing and crystallizing the second mixed solution to crystallize FeSO4 in the mixed solution to obtain a solid-liquid mixture; and

[0048] The second mixed solution after quenching and tempering is frozen to reduce its temperature. During the freezing crystallization process, as the temperature gradually decreases, FeSO4 molecules gradually aggregate and form a stable crystal structure. These crystals continue to grow in the solution and eventually form visible FeSO4·7H2O crystals.

[0049] In some embodiments, the final temperature of the frozen crystallization is -2°C to -5°C, and the effective freezing time of the frozen crystallization is 110 min to 130 min.

[0050] During the freezing crystallization process, as the temperature gradually decreases, the solute in the ferrous sulfate solution begins to precipitate and form crystals. These crystals continue to grow in the solution and eventually form visible ferrous sulfate crystal particles. At the same time, since the solubility of ferrous sulfate decreases with decreasing temperature, more ferrous sulfate will precipitate from the solution during the freezing process, thus achieving Fe 2+ Effective separation from sulfuric acid. The final temperature of frozen crystallization is limited to -2°C to -5°C, and the solubility of ferrous sulfate will be significantly reduced, thereby prompting it to precipitate from the solution in the form of crystals. The effective freezing time of frozen crystallization is limited to 110min to 130min, and the ferrous sulfate molecules in the solution will gradually aggregate and form a stable crystal structure. Through frozen crystallization, ferrous sulfate can be effectively separated from waste acid, thereby improving its recovery efficiency. At the same time, compared with the traditional evaporation and concentration process, the frozen crystallization process has lower energy consumption and less equipment investment, so the processing cost can be reduced. In addition, the frozen crystallization process can reduce the generation of waste residues and wastewater, thereby reducing pollution to the environment. Exemplarily, the final temperature of frozen crystallization can be -2°C, -3°C, -4°C, -5°C, etc., and the effective freezing time of the frozen crystallization can be 110min, 115min, 116min, 117min, 118min, 119min, 120min, 125min, 130min, etc.

[0051] S4. The solid-liquid mixture is subjected to solid-liquid separation to obtain regenerated acid and FeSO4·7H2O crystals respectively.

[0052] The solid-liquid mixture obtained after freezing and crystallization is subjected to solid-liquid separation, and the ferrous sulfate crystals are separated from the regenerated acid by filtration, centrifugation and other methods. The separated regenerated acid has a higher sulfuric acid concentration and can be directly reused in the pickling process of steel or other industrial production that requires sulfuric acid. This not only reduces the demand for new sulfuric acid, but also reduces the cost of waste acid treatment. FeSO4·7H2O crystals can be sold or reused as valuable by-products, such as for the preparation of other chemicals, as fertilizer components, etc.

[0053] In some embodiments, the regenerated acid is circulated for steel pickling, and the sulfuric acid concentration of the regenerated acid is ≥15%.

[0054] In some embodiments, the recovery rate of H2SO4 in the spent acid is ≥80%.

[0055] In some embodiments, the purity of the FeSO4·7H2O crystals is ≥80%.

[0056] This process uses the quenching and tempering-freezing crystallization method, that is, changing the sulfuric acid concentration in the waste acid solution, and then freezing to lower the temperature to promote the crystallization of ferrous sulfate, so that Fe 2+ Separation from sulfuric acid. The principle is that the solubility of ferrous sulfate changes with the acidity and temperature of the liquid. When the temperature remains unchanged, the solubility of FeSO4 decreases with the increase of acidity. When the acidity remains unchanged, the solubility increases with the increase of temperature until the "critical point". If the temperature continues to rise, the solubility of FeSO4 will decrease instead. This process has the following advantages for the regeneration of waste sulfuric acid:

[0057] 1. Efficient resource recovery and reuse. (1) High recovery rate: This process can achieve more than 80% recovery and reuse of waste acid, greatly improving resource utilization efficiency. This means that waste acid, which was originally regarded as waste, can be converted into valuable resources for production or other purposes. (2) By-product value: Ferrous sulfate crystals, as a valuable by-product, can be sold or used in other industrial production processes, thereby bringing additional economic benefits to the enterprise.

[0058] 2. Energy conservation, emission reduction and environmental protection. (1) Reduce energy consumption: Compared with the traditional evaporation concentration process, this process does not require a lot of energy consumption and has lower operating costs. This helps to reduce the operating costs of enterprises and reduce the impact on the environment. (2) Reduce pollutant emissions: By reducing the amount of water treatment sludge by 70% and significantly reducing the amount of neutralization sludge and neutralization water produced, this process significantly reduces the pressure of environmental pollution. This helps to improve environmental quality and protect ecological balance. (3) Achieve clean production: The process is simple to operate and easy to control, and the treatment facilities do not occupy a large area. At the same time, it makes full use of existing resources, forms a closed-loop circulation of raw materials, ensures that the waste acid treatment and recovery meet environmental protection requirements, and achieves clean production.

[0059] 3. Economic and social benefits. (1) Reduce production costs: By recycling and reusing waste acid and ferrous sulfate, enterprises can reduce the cost of purchasing raw materials, thereby reducing production costs. (2) Improve corporate image: The use of environmentally friendly and energy-saving waste acid treatment solutions helps to improve the social image and reputation of enterprises and enhance market competitiveness. (3) Promote the development of circular economy: The solution is in line with the concept of circular economy, namely "reduction, reuse, and recycling". It helps to promote industrial upgrading and transformation and promote sustainable development.

[0060] Figure 3 A schematic diagram of a system for concentrating spent acid provided in an embodiment of the present application.

[0061] In a second aspect, the present application provides a system for concentrating spent acid, the system being adapted to the method described in any one of the embodiments of the first aspect, the system comprising:

[0062] an acid-resistant filter 2, the acid-resistant filter 2 being connected to the waste acid tank 1 and being used for filtering the waste acid to obtain a first mixed solution;

[0063] A conditioning device 3, the conditioning device 3 is connected to the liquid phase outlet of the acid-resistant filter 2, and is used to sequentially condition the first mixed liquid to obtain a second mixed liquid;

[0064] A freezer 4, wherein the crystallization kettle of the freezer 4 is connected to the outlet of the quenching and tempering device 3, and is used to freeze and crystallize the second mixed liquid so as to crystallize FeSO4 in the mixed liquid to obtain a solid-liquid mixture;

[0065] The solid-liquid separation device 5 is connected to the outlet of the crystallization kettle of the refrigerator 4 and is used for performing solid-liquid separation on the solid-liquid mixture to obtain regenerated acid and FeSO4·7H2O crystals respectively.

[0066] In some embodiments, the system further comprises:

[0067] The inlet of the acid recovery tank 6 is communicated with the liquid phase outlet of the solid-liquid separation device 5 , and the outlet of the acid recovery tank 6 is communicated with the inlet of the pickling tank 7 .

[0068] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to industry standards. If there is no corresponding industry standard, then the conditions recommended by the manufacturer are followed.

[0069] Example 1

[0070] The mass of the treated waste sulfuric acid is 20 tons, the content of ferrous sulfate in the waste sulfuric acid is 200g / L, and the concentration of free sulfuric acid is 3%. The generated waste sulfuric acid is discharged into the waste acid storage tank through a pipeline for collection, and then filtered by an acid-resistant filter and lifted to a freezing crystallization kettle, and then enters a solid-liquid separation device after conditioning, stirring for uniform reaction, freezing crystallization (the temperature of freezing crystallization is -2°C and the time is 110min). The regenerated acid obtained after solid-liquid separation enters a reuse acid pool for reuse in production, the sulfuric acid concentration of the regenerated acid is 20%, and the mass of the generated regenerated acid is 18.2 tons. The crystallized ferrous sulfate (FeSO4·7H2O) is exported for disposal, and the purity of the ferrous sulfate (FeSO4·7H2O) is 98%, and the mass is 4 tons.

[0071] Example 2

[0072] The mass of the treated waste sulfuric acid is 20 tons, the content of ferrous sulfate in the waste sulfuric acid is 200g / L, and the concentration of free sulfuric acid is 3.5%. The generated waste sulfuric acid is discharged into the waste acid storage tank through a pipeline for collection, and then filtered by an acid-resistant filter and lifted to a freezing crystallization kettle, and then enters a solid-liquid separation device after conditioning, stirring for uniform reaction, freezing crystallization (the temperature of freezing crystallization is -5°C and the time is 120min). The regenerated acid obtained after solid-liquid separation enters a reuse acid pool for reuse in production, the sulfuric acid concentration of the regenerated acid is 17%, and the mass of the generated regenerated acid is 18 tons. The crystallized ferrous sulfate (FeSO4·7H2O) is exported for disposal, and the purity of the ferrous sulfate (FeSO4·7H2O) is 90%, and the mass is 3.7 tons.

[0073] Example 3

[0074] The mass of the treated waste sulfuric acid is 20 tons, the content of ferrous sulfate in the waste sulfuric acid is 200g / L, and the concentration of free sulfuric acid is 4%. The generated waste sulfuric acid is discharged into the waste acid storage tank through a pipeline for collection, and then filtered by an acid-resistant filter and lifted to a freezing crystallization kettle, and then tempered, stirred for uniform reaction, frozen crystallized (the temperature of the freezing crystallization is -3°C and the time is 130min), and then enters the solid-liquid separation equipment. The regenerated acid obtained after solid-liquid separation enters the reuse acid pool for reuse in production, the sulfuric acid concentration of the regenerated acid is 15%, and the mass of the generated regenerated acid is 16 tons. The crystallized ferrous sulfate (FeSO4·7H2O) is exported for disposal, and the purity of ferrous sulfate (FeSO4·7H2O) is 80%, and the mass is 3.5 tons.

[0075] In addition, one or more technical solutions in the embodiments of the present application have at least the following technical effects or advantages:

[0076] In the embodiments of the present application, more than 80% of the waste acid can be recovered and reused.

[0077] In the embodiments of the present application, the process flow is short, no evaporation concentration process is required, power consumption is low, and operating costs are low.

[0078] In the embodiment of the present application, the equipment process is short, the construction investment is low, the labor quota is small, and the operation and maintenance are convenient.

[0079] In the embodiment of the present application, the concentration of sulfuric acid produced by the equipment can reach more than 15%, which can be directly used for pickling; ferrous sulfate crystals can be sold as valuable by-products, bringing certain economic value

[0080] In the embodiments of the present application, the process is simple to operate and easy to control. The treatment facility occupies a small area and has low operating costs. At the same time, the waste acid is recycled and utilized, and existing resources are fully utilized to form a closed-loop circulation of raw materials, thereby ensuring that the waste acid treatment and recycling meets environmental protection requirements and realizes clean production.

[0081] In the embodiment of the present application, waste sulfuric acid freezing crystallization recovery equipment is used to return the sulfuric acid after ferrous sulfate is precipitated and crystallized to production and recycling, reducing the amount of water treatment sludge by 70%, greatly reducing the amount of neutralization mud and neutralization water generated, and alleviating environmental pollution pressure.

[0082] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A method for concentrating waste acid, wherein the waste acid contains H2SO4 and FeSO4, the method comprising: filtering the waste acid to obtain a first mixed solution; Performing a conditioning treatment on the first mixed solution to obtain a second mixed solution; The second mixed solution is subjected to freeze crystallization to crystallize FeSO4 in the mixed solution to obtain a solid-liquid mixture; as well as The solid-liquid mixture is subjected to solid-liquid separation to obtain regenerated acid and FeSO4·7H2O crystals respectively.

2. The method according to claim 1, characterized in that The final temperature of the frozen crystallization is -2°C to -5°C, and the effective freezing time of the frozen crystallization is 110min to 130min.

3. The method according to claim 1, characterized in that The waste acid is steel pickling waste liquid.

4. The method according to claim 3, characterized in that The concentration of free sulfuric acid in the steel pickling waste liquid is 3% to 4%.

5. The method according to claim 3, characterized in that: The mass concentration of ferrous sulfate in the steel pickling waste liquid is 150 g / L to 250 g / L.

6. The method according to claim 1, characterized in that The regenerated acid is circulated for pickling of steel materials, and the sulfuric acid concentration of the regenerated acid is ≥15%.

7. The method according to claim 1, characterized in that The recovery rate of H2SO4 in the waste acid is ≥80%.

8. The method according to claim 1, characterized in that The purity of the FeSO4·7H2O crystals is ≥80%.

9. A system for concentrating spent acid, the system being adapted to the method according to any one of claims 1 to 8, the system comprising: an acid-resistant filter (2), the acid-resistant filter (2) being connected to the waste acid tank (1) and used for filtering the waste acid to obtain a first mixed liquid; A conditioning device (3), the conditioning device (3) being connected to the liquid phase outlet of the acid-resistant filter (2), and being used for conditioning the first mixed liquid in sequence to obtain a second mixed liquid; A freezer (4), wherein the crystallization kettle of the freezer (4) is connected to the outlet of the tempering device (3), and is used to freeze and crystallize the second mixed liquid so as to crystallize FeSO4 in the mixed liquid to obtain a solid-liquid mixture; A solid-liquid separation device (5) is connected to the outlet of the crystallization kettle of the refrigerator (4) and is used to perform solid-liquid separation on the solid-liquid mixture to obtain regenerated acid and FeSO4·7H2O crystals respectively.

10. The system according to claim 9, characterized in that The system further comprises: A recovery acid tank (6), the inlet of the recovery acid tank (6) is connected to the liquid phase outlet of the solid-liquid separation device (5), and the outlet of the recovery acid tank (6) is connected to the inlet of the pickling tank (7).

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