A method for efficiently removing zinc from the surface of galvanized steel

Abstract

The application provides a method for efficiently removing zinc on the surface of galvanized steel, and belongs to the technical field of metal recycling. The method for removing zinc on the surface of galvanized steel comprises step-by-step zinc removal and use of a special compound sulfuric acid solution. The application solves the problem that the reaction of dilute sulfuric acid and zinc cannot prevent the reaction of dilute sulfuric acid and iron at the same time in the zinc removal process by controlling the concentration composition of the sulfuric acid solution, adjusting the concentration and ratio of each additive, performing the reaction in steps, and accurately controlling the reaction time and reaction speed, and has a wide application prospect in the field of zinc removal of galvanized steel.

Description

Technical Field

[0001] This invention belongs to the field of metal recycling technology, specifically relating to a method for efficiently removing zinc from the surface of galvanized steel. Background Technology

[0002] Galvanized steel sheet refers to steel sheet with a zinc coating on its surface. The zinc coating effectively improves the corrosion resistance of the steel sheet, and galvanized steel sheet is widely used in construction, automobiles, home appliances, shipbuilding, heating facilities, and other fields. Due to the large-scale use of galvanized steel sheet, the production of galvanized steel sheet scrap and defective products has increased. Currently, my country's annual production of galvanized steel sheet scrap is typically over 50 million tons. If all of this scrap could be dezincified and recycled, it would reduce approximately 1 million tons of zinc-containing hazardous waste dust annually, convert 50 million tons of low-grade steel into high-grade steel, and recover over 150,000 tons of zinc. Therefore, finding a method to quickly remove zinc from the surface of galvanized steel is a pressing technical problem that the galvanized steel sheet scrap recycling industry needs to solve.

[0003] Traditional methods for removing zinc from the surface of galvanized steel sheets generally fall into three categories: vacuum baking, ammonia-ammonium chloride solution dissolution, and strong alkali solution zinc removal. Vacuum baking requires harsh conditions under vacuum and high temperatures, and the dezincified steel is prone to rusting. Ammonia-ammonium chloride solution dissolution is not used due to environmental pollution concerns. Strong alkali solution zinc removal is more commonly used, but it requires high temperatures, and each ton of zinc powder requires approximately 4000 kWh of electricity, resulting in high energy consumption, high cost, and low production efficiency. Alkali dezincification via electrolysis can only produce a single product, zinc powder, limiting its market potential. During the recycling and smelting of galvanized steel sheet scrap, every 100 tons of steel generates 2 kg of zinc-containing hazardous waste dust. Although steel mills have dust collection measures, the environmental pollution remains severe. Therefore, complete remediation during the smelting process is impossible; only by thoroughly removing zinc from the surface of the galvanized steel sheet scrap before smelting can complete remediation be achieved. Alkaline dezincification suffers from drawbacks such as slow speed, low efficiency, and high cost, making it difficult to break even and hindering its rapid adoption. This method fails to meet the urgent national and industry demand for dezincification of galvanized steel scrap. While the ordinary sulfuric acid solution method can remove the zinc layer from the surface of galvanized steel sheets, it cannot effectively prevent the reaction between sulfuric acid and iron. The resulting zinc sulfate solution has a high iron content, making it impossible to effectively separate the iron from the zinc sulfate solution at a reasonable cost, thus hindering the full utilization of the zinc sulfate solution.

[0004] Therefore, the present invention aims to develop an environmentally friendly, efficient, low-cost, diversified by-product, high-efficiency, and easily promoted method for dezincification of galvanized steel. Summary of the Invention

[0005] In view of this, the present invention provides a method for efficiently removing zinc from the surface of galvanized steel. This invention utilizes a composite sulfuric acid solution of specific concentration and ratio, combined with step-by-step zinc removal, precise control of the zinc removal time, and appropriate control of the zinc removal reaction rate in the third step. This solves the problem that the reaction between dilute sulfuric acid and zinc cannot be prevented from simultaneously reacting with iron during the zinc removal process. This achieves the effect of efficiently removing zinc from the surface of galvanized steel sheets without damaging the steel substrate, thus solving the environmental pollution problem during the smelting and recycling of galvanized scrap steel, and also enabling the effective recovery and utilization of valuable zinc resources.

[0006] A method for efficiently removing zinc from the surface of galvanized steel includes the following steps:

[0007] (1) Immerse the galvanized steel sheet in a mixed solution composed of sulfuric acid aqueous solution and surfactant to carry out the dezincification process 1. Detect the zinc online. Stop the reaction when the remaining zinc content reaches 18-25% of the initial zinc content.

[0008] (2) Immerse the galvanized steel sheet obtained in step (1) in a mixed solution composed of sulfuric acid aqueous solution, composite corrosion inhibitor and surfactant to carry out dezincification process 2. Detect the surface condition of the galvanized steel sheet online. Stop the reaction when exposed iron spots appear.

[0009] (3) Immerse the galvanized steel sheet obtained in step (2) in a mixed solution composed of sulfuric acid aqueous solution, composite grease, composite corrosion inhibitor and surfactant to carry out the dezincification process 3. Observe the reaction on the surface of the galvanized steel online. Stop the reaction when no bubbles are generated.

[0010] (4) Clean the galvanized steel sheet obtained in step (3) to obtain a clean steel sheet.

[0011] Preferably, the concentration of the sulfuric acid aqueous solution in step (1) is 10 g / L to 450 g / L.

[0012] Preferably, the surfactant in step (1) includes any one of sodium dodecylbenzenesulfonate, sodium lauryl polyoxyethylene ether sulfate, and sorbitan monostearate.

[0013] Preferably, the mass ratio of sulfuric acid aqueous solution to surfactant in the mixed solution in step (1) is 100: (0.1~1.5).

[0014] Preferably, the composite corrosion inhibitor in step (2) is prepared by compounding thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide and fatty alcohol polyoxyethylene ether in a mass ratio of (0.5~1.5):(2~3):(2.5~3.1):(1.5~2):(1~2.2).

[0015] Preferably, the mass ratio of sulfuric acid aqueous solution, composite corrosion inhibitor and surfactant in the mixed solution in step (2) is 100:(0.1~1.0):(0.1~1.0).

[0016] Preferably, the compound oil in step (3) is composed of glyceryl castor oil, ethyl olive oil, and palm oil in a mass ratio of (1~2.5):(0.5~1):(2~3).

[0017] Preferably, the mass ratio of sulfuric acid aqueous solution, composite oil, composite corrosion inhibitor and surfactant in the mixed solution in step (3) is 100: (0.1~1.5): (0.1~1.5): (0.1~0.5).

[0018] Preferably, the reaction temperature in step (3) is less than 10°C.

[0019] Preferably, the iron content in the zinc sulfate solution obtained in steps (1) and (2) is ≤0.001 g / L, and the iron content in the zinc sulfate solution obtained in step (3) is ≤2 g / L.

[0020] Preferably, step (4) involves washing with water, and the resulting washing water is used to prepare a sulfuric acid aqueous solution.

[0021] The beneficial effects of this invention are as follows:

[0022] (1) The zinc removal method provided by the present invention can obtain a zinc sulfate solution with low iron content. Using this solution as the mother liquor, a variety of zinc-containing products can be produced, such as feed-grade monohydrate (heptahydrate) zinc sulfate, nano zinc oxide, lithopone, electrolytic zinc, etc. The products are diverse and have broad market prospects.

[0023] (2) The products of traditional alkaline dezincification are usually electrolytic zinc powder, but the quality of these electrolytic zinc powders is poor and they cannot be sold directly as qualified commercial zinc powder. They need to be reprocessed, resulting in a single product and a small market.

[0024] (3) The zinc removal method provided by the present invention can not only improve the zinc removal speed, but also completely solve the problem that dilute sulfuric acid cannot prevent the reaction of iron while reacting with zinc during the zinc removal process. This enables the sulfuric acid method to be quickly applied to the recycling industry of waste galvanized steel, solving the problem that the entire industry cannot quickly recycle galvanized waste steel on a large scale.

[0025] (4) The zinc removal method provided by the present invention can be carried out at room temperature, while alkaline zinc removal is usually carried out at 80°C. The present invention has the characteristics of low energy consumption and fast zinc removal speed. Under the same production capacity, the total energy consumption of the zinc removal method of the present invention is only 60% of that of alkaline zinc removal, and the zinc removal speed is more than 3 times that of alkaline zinc removal.

[0026] (5) The dezincification method provided by the present invention produces steel plates with high cleanliness, and the surface cleanliness is 5% higher than that of alkaline dezincification, which can increase the steel yield during scrap steel smelting and improve the economic benefits of steel plants. Detailed Implementation

[0027] In some specific embodiments of the present invention, the present invention provides a method for efficiently removing zinc from the surface of galvanized steel, comprising the following steps:

[0028] (1) Immerse the galvanized steel sheet in a mixed solution composed of sulfuric acid aqueous solution and surfactant to carry out the dezincification process 1. Detect the zinc online. Stop the reaction when the remaining zinc content reaches 18-25% of the initial zinc content.

[0029] (2) Immerse the galvanized steel sheet obtained in step (1) in a mixed solution composed of sulfuric acid aqueous solution, composite corrosion inhibitor and surfactant to carry out dezincification process 2. Detect the surface condition of the galvanized steel sheet online. Stop the reaction when exposed iron spots appear.

[0030] (3) Immerse the galvanized steel sheet obtained in step (2) in a mixed solution composed of sulfuric acid aqueous solution, composite grease, composite corrosion inhibitor and surfactant to carry out the dezincification process 3. Observe the reaction on the surface of the galvanized steel online. Stop the reaction when no bubbles are generated.

[0031] (4) Clean the galvanized steel sheet obtained in step (3) to obtain a clean steel sheet.

[0032] In some specific embodiments of the present invention, the concentration of the sulfuric acid aqueous solution in step (1) is 10 g / L to 450 g / L. Preferably, the concentration of the sulfuric acid aqueous solution is 150 g / L.

[0033] In some specific embodiments of the present invention, the surfactant in step (1) includes any one of sodium dodecylbenzenesulfonate, sodium lauryl polyoxyethylene ether sulfate, and sorbitan monostearate.

[0034] In some specific embodiments of the present invention, the mass ratio of sulfuric acid aqueous solution to surfactant in the mixed solution of step (1) is 100:(0.1~1.5). Preferably, the mass ratio of sulfuric acid aqueous solution to surfactant in the mixed solution of step (1) is 100:1.

[0035] In some specific embodiments of the present invention, the composite corrosion inhibitor in step (2) is prepared by compounding thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide, and fatty alcohol polyoxyethylene ether in a mass ratio of (0.5~1.5):(2~3):(2.5~3.1):(1.5~2):(1~2.2). Preferably, the composite corrosion inhibitor in step (2) is prepared by compounding thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide, and fatty alcohol polyoxyethylene ether in a mass ratio of 1:3:2.8:1.7:1.5. The composite corrosion inhibitor obtained by the special composition and specific raw material ratio provided by the present invention has better anti-pitting corrosion effect, protects the uniformity and brightness of the steel surface, and improves the cleanliness of molten steel.

[0036] In some specific embodiments of the present invention, the mass ratio of sulfuric acid aqueous solution, composite corrosion inhibitor, and surfactant in the mixed solution of step (2) is 100:(0.1~1.0):(0.1~1.0). Preferably, the mass ratio of sulfuric acid aqueous solution, composite corrosion inhibitor, and surfactant in the mixed solution of step (2) is 100:0.5:0.8.

[0037] In some specific embodiments of the present invention, the composite oil in step (3) is composed of glyceryl ricinoleate, ethyl oleate, and palm oil in a mass ratio of (1~2.5):(0.5~1):(2~3). Preferably, the composite oil in step (3) is composed of glyceryl ricinoleate, ethyl oleate, and palm oil in a mass ratio of 2:0.6:2.5. The specific raw material composition and specific ratio of the composite oil used in the present invention have better dispersibility and permeability, ensuring the uniformity of the reaction.

[0038] In some specific embodiments of the present invention, the mass ratio of sulfuric acid aqueous solution, composite oil, composite corrosion inhibitor and surfactant in the mixed solution of step (3) is 100:(0.1~1.5):(0.1~1.5):(0.1~0.5). Preferably, the mass ratio of sulfuric acid aqueous solution, composite oil, composite corrosion inhibitor and surfactant in the mixed solution of step (3) is 100:1.2:0.7:0.3.

[0039] In some specific embodiments of the present invention, the reaction temperature of step (3) is less than 10°C. Preferably, the reaction temperature of step (3) is 0-10°C. More preferably, the reaction temperature of step (3) is 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, 7°C, 8°C, or 9°C. Most preferably, the reaction temperature of step (3) is 5°C.

[0040] In some specific embodiments of the present invention, the zinc sulfate solution obtained in steps (1) and (2) has an iron content of ≤0.001 g / L, which is a superior grade zinc sulfate solution, and is stored for subsequent production use.

[0041] In some specific embodiments of the present invention, the iron content in the zinc sulfate solution obtained in step (3) is ≤2 g / L.

[0042] In some specific embodiments of the present invention, step (4) involves washing with water, and the resulting washing water is used to prepare an aqueous solution of sulfuric acid.

[0043] The zinc sulfate solution obtained after zinc removal in this invention can be used to produce a variety of products, including feed-grade monohydrate (heptahydrate) zinc sulfate, lithopone, nano zinc oxide, and superior grade electrolytic zinc.

[0044] This invention utilizes the property of sulfuric acid to rapidly dissolve zinc on the surface of galvanized steel, enabling rapid separation of zinc from the steel substrate. The addition of a surfactant reduces surface tension, enhances dezincification ability, and increases the dezincification speed. The addition of a composite corrosion inhibitor forms a dense, hydrophobic protective film on the steel substrate surface, inhibiting the reaction between iron and sulfuric acid, thus solving the problem of simultaneously preventing the reaction between dilute sulfuric acid and iron during the dezincification process. The addition of a composite grease improves dispersibility and penetration, ensuring uniform reaction. The stepwise dezincification process ensures a low iron content in the final zinc sulfate solution, allowing for efficient utilization of the zinc sulfate solution.

[0045] This invention, by controlling the concentration and ratio of the sulfuric acid mixed solution, conducting the reaction in steps, controlling the reaction time, and adjusting the composition and ratio of each additive, successfully achieves low iron content in the zinc sulfate solution after dezincification, and effectively recovers and utilizes the zinc sulfate solution. This transforms galvanized scrap steel into clean steel, increases the steel yield during scrap steel smelting, enhances the efficiency of scrap steel recycling in steel mills, reduces zinc fume pollution to the atmosphere, and effectively protects the environment.

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0047] Unless otherwise specified, the test methods or experimental methods described in the following examples are all conventional methods; unless otherwise specified, the raw materials and additives are obtained from conventional commercial sources or prepared by conventional methods.

[0048] The following examples or comparative examples use galvanized steel sheets with a surface zinc content of 1.5%, and surface impurities and oil stains are removed before dezincification treatment. The room temperature mentioned in this invention refers to 25°C.

[0049] Example 1

[0050] A method for efficiently removing zinc from the surface of galvanized steel, the specific steps of which are as follows:

[0051] (1) The galvanized steel sheet is immersed in a mixed solution composed of sulfuric acid aqueous solution and surfactant sodium dodecylbenzenesulfonate. The concentration of the sulfuric acid aqueous solution is 150 g / L, and the mass ratio of sulfuric acid aqueous solution to surfactant in the mixed solution is 100:1. The zinc removal process 1 is carried out under normal temperature conditions. Online detection is performed. When the amount of remaining zinc reaches 20% of the initial zinc content, the reaction is stopped.

[0052] (2) The galvanized steel sheet obtained in step (1) is immersed in a mixed solution composed of sulfuric acid aqueous solution, composite corrosion inhibitor and surfactant sodium dodecylbenzenesulfonate. The concentration of the sulfuric acid aqueous solution is 150 g / L. The composite corrosion inhibitor is composed of thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide and fatty alcohol polyoxyethylene ether in a mass ratio of 1:3:2.8:1.7:1.5. The mass ratio of sulfuric acid aqueous solution, composite corrosion inhibitor and surfactant in the mixed solution is 100:0.5:0.8. The zinc removal process 2 is carried out at room temperature. The surface condition of the galvanized steel sheet is detected online. The reaction is stopped when exposed iron spots appear.

[0053] (3) The galvanized steel sheet obtained in step (2) is immersed in a mixed solution composed of sulfuric acid aqueous solution, composite grease, composite corrosion inhibitor and surfactant. The concentration of the sulfuric acid aqueous solution is 150 g / L. The composite corrosion inhibitor is composed of thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide and fatty alcohol polyoxyethylene ether in a mass ratio of 1:3:2.8:1.7:1.5. The composite grease is composed of glyceryl castor oil, ethyl olive oil and palm oil in a mass ratio of 2:0.6:2.5. The mass ratio of sulfuric acid aqueous solution, composite grease, composite corrosion inhibitor and surfactant in the mixed solution is 100:1.2:0.7:0.3. The dezincification process 3 is carried out, the reaction temperature is controlled at 5℃, and the reaction on the galvanized steel surface is observed online. When no bubbles are generated, the reaction is stopped.

[0054] (4) Clean the galvanized steel sheet obtained in step (3) to obtain a clean steel sheet.

[0055] Comparative Example 1

[0056] The difference from Example 1 is that only a sulfuric acid aqueous solution with a concentration of 150 g / L was used to dezincify the galvanized steel sheet.

[0057] Comparative Example 2

[0058] The difference from Example 1 is that the composite sulfuric acid solution does not contain a composite corrosion inhibitor.

[0059] Comparative Example 3

[0060] The difference from Example 1 is that the compound sulfuric acid solution does not contain compound oils.

[0061] Comparative Example 4

[0062] The difference from Example 1 is that steps (1) and (2) are omitted.

[0063] Comparative Example 5

[0064] The difference from Example 1 is that the reaction temperature in step (3) during zinc removal is room temperature.

[0065] Comparative Example 6

[0066] The difference from Example 1 is that the composite corrosion inhibitor does not contain thiourea.

[0067] Comparative Example 7

[0068] The difference from Example 1 is that the composite corrosion inhibitor does not contain hexamethylenetetramine.

[0069] Comparative Example 8

[0070] The difference from Example 1 is that the composite corrosion inhibitor does not contain imidazoline.

[0071] Comparative Example 9

[0072] The difference from Example 1 is that the composite corrosion inhibitor does not contain hexadecyltrimethylammonium bromide.

[0073] Comparative Example 10

[0074] The difference from Example 1 is that the composite corrosion inhibitor does not contain fatty alcohol polyoxyethylene ether.

[0075] Comparative Example 11

[0076] The difference from Example 1 is that the composite corrosion inhibitor is composed of thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide, and fatty alcohol polyoxyethylene ether in a mass ratio of 0.1:3:2.8:1.7:1.5.

[0077] Comparative Example 12

[0078] The difference from Example 1 is that the composite corrosion inhibitor is composed of thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide, and fatty alcohol polyoxyethylene ether in a mass ratio of 1:4:2.8:1.7:1.5.

[0079] Comparative Example 13

[0080] The difference from Example 1 is that the composite corrosion inhibitor is composed of thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide, and fatty alcohol polyoxyethylene ether in a mass ratio of 1:3:3.5:1.7:1.5.

[0081] Comparative Example 14

[0082] The difference from Example 1 is that the composite corrosion inhibitor is composed of thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide, and fatty alcohol polyoxyethylene ether in a mass ratio of 1:3:2.8:1:1.5.

[0083] Comparative Example 15

[0084] The difference from Example 1 is that the composite corrosion inhibitor is composed of thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide, and fatty alcohol polyoxyethylene ether in a mass ratio of 1:3:2.8:1.7:2.5.

[0085] Performance testing

[0086] The effects of Example 1 and Comparative Examples 1-15 were tested, and the results are shown in the table below.

[0087] Group Iron content (g / L) in zinc sulfate solution Clean steel surface condition Example 1 0.8 The surface is white, bright, uniform, smooth, and free of pitting and denting. Comparative Example 1 23 The surface is unevenly black and white, with severe pitting and perforation. Comparative Example 2 12 The surface is unevenly black and white, with severe pitting and corrosion. Comparative Example 3 10 The surface is unevenly glossy and pitted. Comparative Example 4 8 The surface is white, bright, uniform, smooth, and free of pitting and denting. Comparative Example 5 4 The surface is white, bright, uniform, smooth, and free of pitting and corrosion. Comparative Example 6 5.5 The surface is a uniform, glossy gray, but not smooth and has pitting corrosion. Comparative Example 7 4.5 The surface is a uniform, glossy gray, but not smooth and has pitting corrosion. Comparative Example 8 5.3 The surface is uniformly shiny but not smooth, with pitting and corrosion. Comparative Example 9 6 The surface is glossy, uniform, smooth, and free of pitting or denting. Comparative Example 10 4 The surface is white, bright, uniform, smooth, and free of pitting and denting. Comparative Example 11 6 The surface is white, bright, uniform, smooth, and free of pitting and denting. Comparative Example 12 0.9 The surface is white, bright, uniform, and smooth with slight pitting corrosion. Comparative Example 13 0.85 The surface is white, bright, uniform, and smooth with slight pitting corrosion. Comparative Example 14 4.8 The surface is uneven in color, smooth, and free of pitting corrosion. Comparative Example 15 0.83 The surface is white, bright, uniform, and smooth with slight pitting corrosion.

[0088] As can be seen from the table above, the method provided in Example 1 of the present invention can effectively inhibit the corrosion of steel substrate by sulfuric acid. The zinc sulfate solution obtained after dezincification has a low iron content, and the surface of the steel substrate is bright, uniform, smooth and free of pitting corrosion. High-quality zinc-containing products can be produced using this solution as raw material.

[0089] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for efficiently removing zinc from the surface of galvanized steel, characterized in that, Includes the following steps: (1) Immerse the galvanized steel sheet in a mixed solution composed of sulfuric acid aqueous solution and surfactant to carry out the dezincification process 1. Detect the zinc online. Stop the reaction when the remaining zinc content reaches 18-25% of the initial zinc content. (2) Immerse the galvanized steel sheet obtained in step (1) in a mixed solution composed of sulfuric acid aqueous solution, composite corrosion inhibitor and surfactant to carry out dezincification process 2. Detect the surface condition of the galvanized steel sheet online. Stop the reaction when exposed iron spots appear. (3) Immerse the galvanized steel sheet obtained in step (2) in a mixed solution composed of sulfuric acid aqueous solution, composite grease, composite corrosion inhibitor and surfactant to carry out the dezincification process 3. Observe the reaction on the surface of the galvanized steel online. Stop the reaction when no bubbles are generated. (4) Clean the galvanized steel sheet obtained in step (3) to obtain a clean steel sheet; The concentration of the sulfuric acid aqueous solution in step (1) is 10 g / L to 450 g / L; The surfactant in step (1) includes any one of sodium dodecylbenzenesulfonate, sodium lauryl polyoxyethylene ether sulfate, and sorbitan monostearate. In step (1), the mass ratio of sulfuric acid aqueous solution to surfactant in the mixed solution is 100:(0.1~1.5). The composite corrosion inhibitor in step (2) is composed of thiourea, hexamethylenetetramine, imidazoline, hexadecyltrimethylammonium bromide, and fatty alcohol polyoxyethylene ether in a mass ratio of (0.5~1.5):(2~3):(2.5~3.1):(1.5~2):(1~2.2). In step (2), the mass ratio of sulfuric acid aqueous solution, composite corrosion inhibitor, and surfactant in the mixed solution is 100:(0.1~1.0):(0.1~1.0). The compound oil in step (3) is composed of glyceryl castor oil, ethyl oleate, and palm oil in a mass ratio of (1~2.5):(0.5~1):(2~3). In step (3), the mass ratio of sulfuric acid aqueous solution, composite oil, composite corrosion inhibitor, and surfactant in the mixed solution is 100:(0.1~1.5):(0.1~1.5):(0.1~0.5). The reaction temperature in step (3) is less than 10℃.