JPS6314065B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for pickling metal surfaces with a pickling solution containing nitric acid. [Prior art] Oxide scale adheres to the surface of metal materials that have been subjected to high-temperature processing or annealing, and the method for removing this scale is generally pickling with an inorganic acid such as sulfuric acid, hydrochloric acid, phosphoric acid, or nitric acid. It is. Warm sulfuric acid or cold hydrochloric acid aqueous solution is usually used to pickle steel, but with steel and stainless steel that have a high carbon content, carbon remains on the metal surface after pickling, causing what is called smut. easy. Therefore, a mixture of nitric acid and hydrofluoric acid is used especially for pickling stainless steel. Nitric acid is also used in the bright finishing of non-ferrous metals such as copper and brass or magnesium alloys, and in the pre-plating process of aluminum and its alloys. However, a problem with metal descaling or chemical polishing using an acid solution containing nitric acid is the intense generation of NOx gas. This is because when HNO ₂ , which is produced when metal oxides melt, reaches supersaturation, NO
This is thought to be due to the generation of a mixed gas of NO 2 and NO ₂ .
If a large amount of such NOx is contained in the air above the pickling bath and is released into the atmosphere, it will immediately cause environmental pollution, so normally a wet or dry denitrification device is introduced to remove NOx from the exhaust gas. It is on the decline. The common wet denitrification method is to absorb in an alkaline aqueous solution, but tank capacity, absorption equipment, etc.
As blowers and the like become larger, the dry denitrification method significantly consumes chemicals such as oxidizing agents and reducing agents. Moreover, when using these methods, the practical limit for the NOx concentration in the air above the pickling bath is 1500 ppm. Since this problem cannot be solved with a denitrification device alone, methods have been tried to reduce the NOx itself generated on the pickling bath. In this method, a large amount of sulfamic acid is added to the bath at once, so pickling is performed in the presence of a large amount of sulfamic acid groups, and smut adheres to the metal surface after pickling. In addition, sulfuric acid generated by the reaction between excess sulfamic acid and nitrous acid causes Fe ²⁺ in the bath.
exists, and when disposing of the pickling solution after use, Fe ²⁺
There are problems in that an aeration process (oxidation treatment through air passage) is required to convert F ³⁺ into F 3+ , and bumping occurs when concentrated nitric acid is supplied to the pickling bath during pickling. [Object of the invention] The present invention solves the above problems and improves the
The purpose of this invention is to provide a method for cleaning metal with nitric acid that can suppress the generation of NOx. [Structure of the Invention] The present inventors developed a pickling bath by adjusting the supply amount of a sulfamic acid group compound so that the sulfamic acid group for eliminating HNO ₂ present in a nitric acid solution containing nitric acid did not become excessive. They discovered that the above NOx can be reduced, and after further research, they completed the present invention. In other words, the method of pickling metal with nitric acid of the present invention uses sulfamic acid radicals per 100 ^m2 of surface area of the metal to be pickled.
The method is characterized in that 0.1 to 30 kg of a compound having a sulfamic acid group is continuously or intermittently supplied into the nitric acid pickling solution in small amounts, so that no sulfamic acid residue remains in the pickling solution. be. Sulfamine acid radical per ^100m2 of pickled metal surface
Below 0.1Kg, nitrous acid accumulates in the pickling solution, and the reaction formula is as follows: HNO ₂ +NH ₂ SO ₃ ^- →N ₂ +H ₂ SO ₄ +OH ^- The nitrite is directly converted into harmless N ₂ . Lack of ability to convert. Also pickled metal surface 100
If the amount of sulfamic acid groups exceeds 30 kg per ^m2 , not only will the consumption cost of the compound with sulfamic acid groups increase, but also ferrous ions will be generated in the pickling solution, which will require oxidation treatment by air suction, which will increase costs. It causes Although unconfirmed, it is also thought to be the cause of smuts forming on the surfaces of stainless steel, etc. On the other hand, it is necessary to occasionally replenish concentrated nitric acid to replace the consumed nitric acid content, and if a large amount of sulfamic acid groups remain in the solution, this may cause bumping gas. The amount of sulfamine acid radical added to the pickling solution is 0.1 to 30 kg, preferably 1 to 10 kg, per 100 m ² of the metal surface to be pickled. Examples of compounds having a sulfamic acid group include sulfamic acid and its salts, such as water-soluble salts of sodium sulfamate, -ammonium, -potassium, -lithium, -calcium, -magnesium, -aluminum, -nickel, and -alkanolamine, and sulfamic acid. Examples include urea. Metals to be pickled include stainless steel, titanium,
Examples include metals such as aluminum and copper, and alloys thereof. The compound having a sulfamic acid group may be supplied to the pickling solution in the form of a powder or in the form of an aqueous solution. However, a powder feeder is required to charge the powder, which increases the equipment cost and creates a mist of the pickling solution when the powder is charged. On the other hand, compounds having a sulfamic acid group have high solubility in water and the concentration can be easily adjusted, so it is advantageous to supply them as an aqueous solution. Incidentally, the solubility of the main compounds having a sulfamic acid group in water at 25°C is as follows. gr/100gr water Sulfamic acid 23 Sodium sulfamate 166 Ammonium sulfamate 193 Nickel sulfamate 70 Barium sulfamate 34.2 As mentioned above, a 1 to 50% aqueous solution can be obtained, but a 10 to 50% aqueous solution is preferable from the water balance with the pickling solution. It is impractical to add it at a concentration of less than 1%. The amount of NOx gas generated when a metal surface is pickled by supplying a compound having a sulfamic acid group to the pickling solution in the form of an aqueous solution or powder is determined by the concentration of the pickling solution,
It varies depending on the pickling solution temperature, type of metal material, pickling speed, etc. Therefore, it is preferable to continuously supply small amounts of sulfamine acid groups in the pickling solution to keep it at 0 or a value close to 0, but in cases where the amount of nitrogen oxides generated is small, it is preferably supplied intermittently. You can also do that. As mentioned above, if 0.1 to 30 kg of a sulfamine acid group-containing compound is added to the pickling solution little ^by little as sulfamine acid groups per 100 m2 of surface area of the metal to be pickled, almost no sulfamine acid groups will be present in the pickling solution, and NOx gas will be generated. However, the air above the bath still contains too much NOx to be released directly into the atmosphere.
The present invention not only performs pickling as described above, but also
By passing the air above the bath containing NOx gas through the denitration equipment, NOx in the exhaust gas is significantly reduced. As a denitrification device, a conventionally used type is sufficient, and a large one is not required. Therefore, not only can the equipment cost be reduced, but even if the pickling speed is increased, the NOx concentration in the air above the bath is low, so that the NOx can be efficiently removed using a conventional small-sized denitrification device. The present invention will be explained below with reference to Examples. [Example] Examples 1 to 3 and 6 to 9 A stainless steel strip with a width of 1.2 m and a thickness of 1 to 5 mm was placed in a vertical pickling tank with a length of 10 m in which a nitric acid pickling solution at a temperature of 40 to 60°C was stored. The scale adhering to the stainless steel was removed by feeding the stainless steel at a speed of 10 to 30 m/min (residence time: 20 to 60 seconds). 7 to 18% HNO ₃ and 2 to 4% HF in the pickling solution
Furthermore, from a predetermined location in the pickling tank, 0.5 to 10 kg of a sulfamic acid compound was continuously supplied as a sulfamic acid radical per 100 m ² of surface area of the metal to be pickled. The upper part of the pickling tank is a draft, and a blower with a capacity of 1500Nm ³ /hr continuously sucks air from the surface of the pickling tank through a duct.
NOx in the exhaust gas was absorbed and removed using a denitrification device using a % NaOH aqueous solution as the absorption liquid. 1st below
Table 2 shows the pickling conditions and NOx concentrations at the inlet and outlet of the denitrification equipment for each example. Comparative Examples 4 to 5 and 10 to 11 were then carried out under the same conditions as in the above Example except that the amount of sulfamic acid radicals supplied was outside the range of the present invention. The results are also shown in Tables 1 and 2 below.

【table】

〔Effect of the invention〕

As described above, the generation of NOx gas can be suppressed by adjusting the supply amount of the sulfamine acid group compound added to the pickling solution according to the method of the present invention,
As a denitrification device, it has the effect of reducing manufacturing costs even though it requires an increase in capacity. Therefore, the present invention can be applied to pickling not only stainless steel but also aluminum, copper, titanium, and alloys thereof.

Claims (1)

[Claims] 1. Continuously or intermittently feed a compound having 0.1 to 30 kg of sulfamic acid groups into a nitric acid pickling solution per ¹⁰⁰ m2 of surface area of the metal to be pickled, and add sulfamine to the pickling solution. In addition to preventing acid roots from remaining,
A method for pickling metal with nitric acid, characterized in that air above a pickling solution containing generated nitrogen oxide gas is passed through a denitrification device.