JPS60121283A - Method for pickling metal with nitric acid - Google Patents

Abstract

PURPOSE:To reduce the amt. of harmful gaseous NOx generated from a pickling liquid during the pickling treatment by adding continuously a specified amt. of a compd. contg. a sulfamic acid group in pickling the surface of a metal with an aq. soln. contg. nitric acid. CONSTITUTION:The surface of stainless steel, Ti, and Cu is treated with a pickling liquid consisting of an aq. nitric soln. added with fluoric acid to dissolve and remove the oxides on the surface and to clean the surface, and the surface of the metal is cleaned. In this case, 0.1-30kg as a sulfamic acid group per 100m<2> surface area of the metal to be treated of sulfamic acid, sodium sulfamate, ammonium sulfamate, or urea sulfamate is added into the pickling liquid. The amt. of gaseous NOx generated from the pickling liquid during the pickling treatment can be reduced in this way, and the contamination of the pickling environment with NOx can be completely eliminated by denitrating the gas contg. NOx with an aq. NaOH soln.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of pickling a metal surface with a pickling solution containing nitric acid.

[Prior art]

D-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.

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, resulting in so-called smut. Things tend to happen. Therefore, a mixture of nitric acid and hydrofluoric acid is used especially for pickling stainless steel fields. Nitric acid is also used in the gloss finishing of nonferrous metals such as copper, brass, and 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 metal oxides contain a large amount of NOx in the air above the pickling bath, and if they are released into the atmosphere, they immediately cause environmental pollution. This is aimed at reducing NOx in exhaust gas.

A common wet denitrification method is a method of absorbing in an alkaline aqueous solution, but the tank capacity, absorption equipment, blower, etc. are larger, and the dry denitrification method significantly consumes chemicals such as oxidizing agents and reducing agents. Moreover, when using these methods, the practical limit of NOx@IIH in the air above the pickling bath is 1500 pI)m.

Since this problem cannot be solved with a denitrification device alone, methods have been attempted to reduce the NOx itself generated on the 1V washing bath. For example, there is a method of adding sulfamino acid to the bath as described in % 102232/1983. In this method, a large amount of sulfamino acid is added to the bath at once, so pickling is carried out in the presence of a large amount of sulfamino acid groups, and smut adheres to the concave surfaces of the gold maple after pickling. In addition, the sulfuric acid generated by the reaction between excess sulfamic acid and nitrous acid causes F to enter the bath.
Fe exists, and when discarding the pickling solution after use, Fe @
There are problems in that an aeration step (oxidation treatment through air passage) is required for Fe to K conversion, and bumping occurs when concentrated nitric acid is completely replenished into the pickling bath during pickling.

[Purpose of the invention]

An object of the present invention is to provide a method for cleaning metal with nitric acid, which can solve the above-mentioned problems and suppress the generation of NOx in the pickling solution.

[Structure of the invention]

The present inventors discovered that HNO present in a pickling solution containing nitric acid
We discovered that it is possible to reduce NOx on the pickling bath by adjusting the amount of sulfamic acid radicals supplied without overdoing the amount of sulfamic acid radicals used to eliminate A compound having a sulfamino acid group of 0.1 to 30 kg per 10 o- of sulfamino acid groups is continuously or intermittently supplied in small amounts to the nitric acid pickling solution, so that no sulfamino acid groups remain in the nucleic acid washing solution. These are all features.

Sulfamic acid group per 1ooy of pickled metal surface is 0.
Below I Kf, nitrous acid accumulates in the pickling solution, and the ability to directly convert nitrous acid into harmless N2 as shown by the following reaction formula: % formula % is insufficient. In addition, if the sulfamic acid group per 100 m' of pickling metal exceeds 30 kg, the consumption cost of the compound having 7 amino acid groups will increase, and ferrous ions will be generated in the pickling solution and air will be absorbed. This causes an increase in costs, such as the need for oxidation treatment. Although unconfirmed, it is also thought to be a cause of smut that forms on curved surfaces of stainless steel and other materials.

On the other hand, occasionally add concentrated nitric acid to add the consumed nitric acid.
It is necessary to replenish the liquid, and if a large amount of sulfamino acid groups remain in the liquid, it will cause bumping gas.

In this way, the amount of sulfamic acid groups added to the pickling solution is 0.1 to 30, preferably 1 to 10, per 100 tons of the metal surface to be pickled. It is.

Compounds having a sulfamino acid group include sulfamic acid and its salts, such as sodium sulfamate, -
ammonium, -potassium, -IJ thium, -calcium, -gnesium, -aluminum, -nickel, -
Water-soluble salts such as alkanolamines and sulfamino acid ureas are mentioned.

Examples of metals to be pickled include metals such as stainless steel, titanium, aluminum, and copper, and alloys thereof.

The compound having a sulfamine radical can be supplied to the pickling solution in the form of a powder or an aqueous solution. However, a powder feeder is required to charge all 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 a 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 compound having a sulfamino acid group in water at 25°C is as follows.

fr/100fr water Sulfamic acid 23 Sodium sulfamate 166 Ammonium sulfamino acid 193 Nickel sulfamino acid 70 Barium sulfamino acid 34.2 An aqueous solution of 50 to 50 qb can be obtained as described above, but the water balance between 10 and 50 qb is obtained depending on the water balance with the pickling solution. A water bath solution is preferred, and it is impractical to add the solution when producing less than 1 inch of water.

A compound having a sulfamino acid group is supplied to the pickling solution in the form of an aqueous solution or powder to pickle the metal surface. It varies depending on the amount of metal used, the speed of pickling, etc. Therefore, it is preferable to continuously supply a small amount of sulfamine in the pickling solution so that the value can be maintained at 0 or close to 0, but in cases where the amount of desired oxides generated is small, it is preferably supplied intermittently. You can also.

As mentioned above, if a compound containing sulfamic acid groups is added in small quantities to the pickling solution at a rate of 0.1 to 30% per 100 m' of surface area of the metal to be pickled, almost no sulfamic acid groups will be present in the pickling solution, and NOx gas will be generated. 1500 in the air above the bath
Although it can be suppressed to below ppm, the air above this bath remains as it is.
It contains too much NOx to be released into the atmosphere. The present invention significantly reduces NOx f in the exhaust gas by carrying out pickling as described above and by passing the air above the bath containing the generated NO^ gas to a denitrification device.

As a denitrification device, a conventionally used type is sufficient, and a large one is not required. Therefore, not only can equipment costs be reduced, but even if the pickling speed is increased,
Since the NOx concentration in the air above the bath is low, it can be efficiently denitrified 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 of 1.2 m in thickness to 5 strips in length 1 was filled with a nitric acid pickling solution at a temperature of 40 to 60°C.
10 to 30m/Ill in a vertical pickling tank of Qm
The scales deposited on the stainless steel copper were removed by feeding at a speed of In (residence time: 20 to 60 seconds). One pickling solution contains 7 to 18 units of NOS and 2 to 4 units of HF, and furthermore, 770.5 to 10K per 100m' of surface area of the metal to be pickled is added from a predetermined location in the pickling tank.
The sulfamic acid compound of f was continuously fed.

The upper part of the pickling tank is a draft, and 1
The air on the surface of the pickling tank was continuously sucked out using a blower with a capacity of 5000 Ntl/hr, and 3 to 6 inches of Na was removed.
NOx in the exhaust gas was absorbed and removed by a denitrification device using the aqueous solution of 0)1 as a total absorption liquid. Tables 1 and 2 below show the pickling conditions for each example and the NO at the inlet and outlet of the denitration equipment.
x@ indicates degrees.

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 sulfamino acid radicals supplied was entirely outside the range of the present invention. The results are also shown in Tables 1 and 2 below.

Table 2 Pickling test for SU8 410
In the examples, the NOx in the inlet gas was 950 to 11,000 ppI), but the outlet gas was denitrated to 80 to 1,100 pp. On the other hand, in Comparative Examples 4.5 and 10, the NOx in the inlet gas was 1,600 to 3,000 ppm, and the outlet gas still contained 150 to 3,001) m) of NOx 2. Note that when the amount of sulfamic acid radicals supplied is too large as in Comparative Example 11, HNOa almost disappears and NOx in the inlet gas becomes extremely small, but smut adheres to the metal surface after pickling.

〔Effect of the invention〕

As described above, the production of NOx gas can be suppressed by adjusting the supply amount of the sulfamic acid group compound added to the pickling solution according to the method of the present invention, and as a denitrification device, it is possible to reduce manufacturing costs even though it requires an increase in capacity. effective. Therefore, the present invention is applicable not only to stainless steel but also to pickling of aluminum, copper, titanium, or alloys thereof.

Patent applicant: Nissan Chemical Industries, Ltd. Nippon Stainless Co., Ltd.

Claims (1)

[Claims] A chemical compound having 0.1 to aoKy of sulfamic acid groups per imHR1ooi of the metal to be pickled (Ij'j (4) continuously in small amounts in the pickling solution or I8) Indispensably, the sulfamine 111ν group is not allowed to remain in the pickling solution, and the air above the pickling solution containing the generated nitrogen oxide gas is conducted to a total denitrification device. A method for pickling metal with nitric acid, which is characterized by: