JP2003268580A - Method of pickling stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of pickling stainless steel giving deodorization effect and a safe working environment by effectively decomposing nitrous acid gas produced by the reaction between stainless steel and nitric acid, and effectively shielding gas and mist caused from the surface of a pickling liquid, and to provide an environment improving effect by reusing and reproducing an aged and deteriorated pickling liquid. <P>SOLUTION: Solid urea is bound to a suspended body, or urea impregnated into a porous substance is bound to a suspended body, and the bound material is floated near the boundary of a liquid, and pickling is performed while the urea is slowly eluted, so that harmful nitrous acid gas caused on the pickling is decomposed, and simultaneously, the occurrence of acid mist and gas produced from the boundary of the liquid is suppressed to provide deodorization and safe working environment. <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to burns generated in various steps such as cutting, welding, assembling and heat treatment of stainless steel.
In the pickling operation method for removing rust and dirt, an appropriate amount of urea is bound and floated near the liquid surface of a mixed acid mainly composed of nitric acid and hydrofluoric acid or nitric acid and ammonium acid fluoride. By gradually dissolving and lowering the urea, the urea is decomposed into safe and harmless nitrogen and carbon dioxide while facilitating the reaction with the harmful nitrous acid generated and raised from the bottom of the mixed acid during the pickling reaction. The shielding effect of the mixed acid and gas phase by the buoyant substance bound with urea effectively shields the gas and mist scattered from the mixed acid surface to provide odorless realization and a safe working environment, and furthermore The present invention relates to a method for pickling stainless steel, which aims to drastically restore its pickling ability by applying to this kind of mixed acid whose performance has deteriorated and aged. 2. Description of the Related Art Conventionally, this type of pickling operation has been a mixed acid of nitric acid and hydrofluoric acid or acidic ammonium fluoride (hereinafter referred to as "nitrogen") since the history of stainless steel technology introduction from developed countries. Hydrofluoric acid) has been exclusively employed exclusively. On the other hand, in the manufacturing process of drawing stainless steel pipes, a method of adding urea to nitric acid has been adopted for the purpose of extending the life of nitric acid, but the amount of urea added and the pickling temperature If it is not appropriate, (a) nitric hydrofluoric acid will be significantly consumed (b) The surface of the stainless steel tube base material will be roughened (c) Inefficient reaction of urea (2) There will be various problems such as urea hydrolysis In part, as a countermeasure, it is stated that Japanese Patent Application Laid-Open No. 54-123529 "Method for pickling stainless steel" enables efficient use of urea and extension of the life of fluorine. As a specific example, the liquid temperature of nitric hydrofluoric acid is suitably from 20 ° C to 50 ° C,
The method of adding urea is preferably such that the molar ratio of urea to nitrite in nitric hydrofluoric acid is less than 1, and it is preferably added in several portions. The data in Table 1 shows that the addition of the total amount while splitting can greatly increase the surface area of the stainless steel that can be pickled. In the pickling method in which such urea is added, the nitrite gas generated by the reaction of stainless steel with nitric acid can be decomposed into nitrogen and carbon dioxide gas. It is difficult to dissolve because it accumulates on the bottom, does not react effectively with nitrous acid generated by pickling, and there is no specific measure to prevent the generation of vapor pressure of nitric hydrofluoric acid evaporating from the liquid surface. In general, at the seed pickling site, an unbearable odor caused by nitrous acid drifts, and measures for improving the working environment have been strongly desired from the viewpoint of occupational safety and health. DISCLOSURE OF THE INVENTION The disadvantages of the known pickling method of adding urea to nitric hydrofluoric acid are that the present invention has a disadvantage that a fixed amount of urea must be added in a divided amount, Furthermore, since the added urea is difficult to dissolve, the reaction with nitrous acid which is scattered at the bottom of the nitric hydrofluoric acid solution and generated during the pickling operation is hindered and is not effectively performed, and it is generated from nitric hydrofluoric acid itself. The production of nitrous acid with a strong odor and a dangerous odor that overlaps with anything makes it impossible to work in a normal room without ventilation equipment at all. The purpose is to improve the pickling work, which is so poor that it cannot be done without protective equipment, and to prolong the life of the nitric hydrofluoric acid solution in use and to regenerate and reuse the aged and deteriorated nitric hydrofluoric acid solution. And [0004] In view of the above problems, the following is proposed as means for solving the problems. That is, the gist is that solid urea having a molar ratio to nitrite of about 1 is directly bound to the floating body or is impregnated with a nonwoven fabric or an acid-resistant porous material and then bound to the floating body. A pickling method characterized by floating acid near the liquid surface to thereby suppress the generation of nitric hydrofluoric acid vapor from the liquid surface and performing pickling while gradually eluting urea downward from near the liquid surface. . When stainless steel is pickled while urea is gradually eluted near the liquid surface of a nitric hydrofluoric acid solution or a mixed acid containing nitric acid and ammonium acid fluoride as main components, the stainless steel reacts with nitric acid, Generating harmful nitrous acid based on the formula (1), This nitrous acid dissolves in the liquid, and a part of it becomes nitrous acid gas and rises toward the liquid surface, where urea gradually dissolves and drops from the urea-containing suspension present on the liquid surface. And reacts efficiently, and is decomposed into completely harmless nitrogen and carbon dioxide gas as shown in equation (2). On the other hand, a nitric hydrofluoric acid solution or a mixed acid mainly composed of nitric acid and ammonium acid fluoride has its own vapor pressure, and evaporates from the liquid surface to a gas phase according to the liquid temperature. According to the method of the present invention, the generation of vapor can be effectively prevented from shifting from the liquid phase to the gas phase due to the shielding effect of the floating body existing near the liquid surface. EXAMPLE 1 In order to confirm the nitrite decomposition action and the vapor shielding effect of a urea-containing suspension, the compositions of symbols A, B and C shown in Table 2 were prepared as laboratory tests. Put 2 liters each of nitric hydrofluoric acid solution into a gas phase gas concentration measurement container, and use stainless steel SUS
A 304 test plate (with a scale by welding burn) was inserted into the bottom of the container, and then a urea-containing suspension was added so as to cover about two thirds of the liquid surface.
After standing at 25 ° C. for 30 minutes, the gas phase gas was sampled, and the concentration of nitrous acid gas in the gas was measured using a gas detector tube. In this test, a floating body manufactured by a tempura method described below was used, but there was no significant difference in performance even when a thing manufactured by a tunnel furnace method was used. [Manufacturing method of urea-containing suspension] Tempura method: A porous material such as a nonwoven fabric such as polypropylene or a foamed glass is immersed in a urea solution melted at a temperature lower than the decomposition temperature of urea, impregnated with urea, and pulled up. Then, it is cooled and solidified. Tunnel furnace method: Solid urea (usually granular) is placed in an appropriately sized container, and then covered with a non-woven fabric such as polypropylene or covered with a porous material, and then passed through a heated tunnel furnace, during which the molten urea is converted into a plastic material. A non-woven fabric or a porous material is impregnated and cooled to form a urea-impregnated solid. Since it is not possible to float on the liquid surface of the nitric hydrofluoric acid solution as it is, foamable styrene or foamed cushioning material (commonly known as
The above or the above is directly or indirectly bound to a floating object such as an air cap) or a hollow floating bag. As a method of binding, or together with a floating object, put it in a breathable sheet or a bag made of a plastic sheet such as polyethylene with non-permeable air and add it so that it floats in a nitric hydrofluoric acid solution etc. I do. [Table 2] Table 3 shows the measurement results of the concentration of nitrous acid gas in the gas phase. From these results, it was confirmed that the nitrite decomposition effect by adding urea and the shielding effect by adding floating bodies were observed. [Table 3] Example 2 A commercially available nitric hydrofluoric acid (manufactured by Rasnon Well Co., Ltd.) to which urea was added, and further a urea-containing suspension, in order to confirm the nitrite decomposition action and the vapor shielding effect of the urea-containing suspension A test was conducted on the material to which was added. Table 4
Shows the composition of commercially available nitric hydrofluoric acid used for the test. The method for measuring the concentration of nitrous acid gas in the gas phase was the same as that in Example 1. 2 liters of a commercially available nitric hydrofluoric acid solution and a stainless steel test plate (with a scale by welding burn) were placed in a closed container for measuring the gas phase gas concentration. 25
After standing at 30 ° C. for 30 minutes, the concentration of nitrous acid gas in the gas phase was measured with a gas detector tube. [Table 4] Table 5 shows the measurement results of the concentration of nitrous acid gas in the gas phase. From these results, it was confirmed that the nitrite decomposition effect due to the addition of urea and the shielding effect due to the addition of the suspended solid were extremely effective in preventing the nitrite gas from being scattered. [Table 5] Example 3 In order to confirm the pickling effect by the addition of urea and the addition of a floating body, a stainless steel SUS304 test plate (with a scale by welding burn) was used for 15 minutes using the prepared nitric hydrofluoric acid shown in Table 2 of Example 1 as a test solution. It was immersed and the loss on dissolution was measured. Table 5 shows the test results. The pickling rate of the nitric acid solution to which the urea-containing suspension is added is slightly lower than that of the nitric acid solution in which urea is dissolved in advance, but has a remarkable effect as compared to the case where no urea is added. It is clear. [Table 5] Example 4 In order to confirm the pickling effect of the same commercially available nitric hydrofluoric acid as in Example 2 by the addition of urea and the addition of a suspension, Table 4 of Example 2 was used.
The stainless steel SUS304 plate (with scale by welding burn) was immersed in the test solution shown in (1) for 15 minutes, and the dissolution loss was measured. Table 6 shows the test results. The pickling rate of the nitric acid solution to which the urea-containing suspension is added is slightly lower than that of the nitric acid solution in which urea is completely dissolved, but has a remarkable effect as compared to the case where no urea is added. It is clear. [Table 6] Example 5 FIG. 1 shows a mother of acid-washed SUS304 material before and after the addition of a urea-containing suspension using a prepared nitric acid hydrofluoric acid solution of 13.5% by weight of nitric acid and 3.6% by weight of hydrofluoric acid at room temperature. It shows the relationship between the amount of material dissolved and the pickling rate. According to this, the pickling speed is improved at the time of the new solution by the addition of the floating body, and the pickling speed is improved by about 20% even in the case of the deteriorated solution which is reduced to about 1/10 of the initial pickling speed, and is recovered. did. Further, the concentration of nitrous acid gas in the gas phase was measured by a detector tube method after the addition of the floating body, and all the measurement results were 0.06 ppm or less. From these results, it was confirmed that the addition of the urea-containing suspension improved the pickling ability of the nitric hydrofluoric acid having deteriorated performance, regenerated the nitric hydrofluoric acid, and suppressed the emission of nitrous acid gas into the gas phase. Example 6 A SUS304 test plate (with a scale by welding burn) was immersed for 15 minutes in a commercially available nitric hydrofluoric acid solution whose performance had deteriorated after being used for about 10 years, and the stainless steel was immersed for 15 minutes. The nitrite gas concentration in the sample was measured. Table 7 shows the test results. This indicates that the addition of the urea-containing suspension enhances the pickling ability of the degraded nitric hydrofluoric acid, thereby enabling reuse and regenerating, and also suppressing the emission of nitrous acid gas into the gas phase. [Table 7] According to the present invention, the following excellent effects can be expected by adding an appropriate amount of urea suspended on a liquid surface. 1. Harmful nitrous acid is decomposed into harmless nitrogen and carbon dioxide gas, and effective safe work can be performed. 2. Gas and mist are effectively prevented from being scattered due to the shielding effect of the floating object, which is beneficial for safety and environmental protection. 3. If an appropriate amount is added regularly, the regenerating effect of the aging liquid is large, which contributes to cost reduction and furthermore, prevention of pollution and environmental destruction. 4. The pickling speed is also effectively increased.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the effect of the addition of a urea-containing suspension, and shows the relationship between the amount of dissolution of stainless steel before and after the addition of the suspension and the pickling rate.

   ────────────────────────────────────────────────── ─── Continuation of front page    F term (reference) 3B201 AA46 AB01 BB02 BB96 CB12                 4K053 PA03 PA13 QA01 QA04 RA16                       RA17 RA25 RA51 TA16 YA27                       YA28 YA30 ZA10

Claims (1)

Claims: 1. A method for pickling stainless steel using a mixed acid containing nitric acid and hydrofluoric acid or nitric acid and ammonium acid fluoride as main components. After binding to a floating body such as a hollow floating bag, the urea is added to the mixed acid tank so as to float on a mixed acid surface mainly containing nitric acid and hydrofluoric acid or nitric acid and ammonium acid fluoride, and the urea is mixed with the mixed acid. The harmful nitrous acid generated during the pickling reaction is decomposed into safe and harmless nitrogen and carbon dioxide by gradually dissolving the liquid near the liquid surface. Gas and mist generated from the mixed acid surface are effectively shielded by the intercepting effect with the phase to provide odorless and safe working environment and improve pickling performance. Pickling method of stainless steel.