JP2000290787A - Method for suppressing gaseous nox by hydrogen peroxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to automatically control the generation amount of gas corresponding to a current value by electrolyzing a nitric acid-containing liquid, such as a stainless steel pickling liquid containing hydrofluoric acid, at a specified potential and regulating the addition amount of the hydrogen peroxide to oxidize NOx to nitric acid in accordance with this electrolytic current value. SOLUTION: The nitric acid-containing liquid, for example, nitric acid-containing liquid 2 of a stainless steel pickling bath 1 consisting of mixed acids containing, for example, nitric acid and hydrofluoric acid is electrolyzed with a specified voltage via a three electrode type voltage controller 6 having a platinum electrode 4 and a reference electrode 5. The electrolytic current value having a proportional relation with the generation amount of the gaseous NOx of the nitric acid-containing liquid 2 is then detected. A supply pump drive control signal 8 is transmitted from the voltage controller 6 to a hydrogen peroxide supply pump 3 to add a prescribed amount of hydrogen peroxide to the pickling bath 1. As a result, the NOx formed in the nitric acid-containing liquid 2 is oxidized as the nitric acid and the generation amount thereof is automatically maintained at a prescribed level and the excessive consumption of hydrogen peroxide can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for suppressing the generation of NOx gas when a metal is treated in a nitric acid-containing solution by adding hydrogen peroxide.

[0002]

2. Description of the Related Art Nitric acid is widely used in the industrial field, and when a metal is treated with a nitric acid-containing liquid, NOx gas which often has an adverse effect on the environment and the human body is generated. For example, in the case of pickling treatment of stainless steel with a mixed acid of nitric acid and hydrofluoric acid, nitrous acid is generated in the solution together with dissolution of the stainless steel in the treatment solution. Nitric acid undergoes various reactions in a solution to become NO or NO ₂ ,
It is released outside the system as Ox gas. Here, when a scrubber or the like is used for the treatment of NOx gas, the equipment cost of the exhaust gas treatment device is increased, and periodic maintenance is also required.

On the other hand, there is a method of suppressing NOx gas by adding hydrogen peroxide to a nitric acid-containing liquid (US Pat. No. 3,945,865). Preparation is problematic. This is because excessive addition only decomposes itself in a nitric acid or hydrofluoric acid solution in which metal ions are present, and wastefully consumes hydrogen peroxide. Japanese Patent Application Laid-Open No. 55-134694 discloses a method of adjusting the addition of hydrogen peroxide based on the oxidation-reduction potential of a nitrite-containing solution. Without this, it is difficult to adjust the amount of added hydrogen peroxide.

[0004]

An object of the present invention is to solve the above-mentioned problems in the prior art and to provide a method for effectively suppressing the generation of NOx gas in a nitric acid-containing liquid.

[0005]

The present inventors have conducted intensive studies on the adjustment of the amount of added hydrogen peroxide. As a result, when a nitric acid-containing solution was subjected to constant potential electrolysis, the electrolytic current value and the amount of nitrous acid in the solution were measured. The inventors have found that there is a certain relationship between the ion concentration and the amount of generated NOx gas, and have reached the present invention.

That is, the present invention provides a method for suppressing the release of NOx gas from a nitric acid-containing liquid by adding hydrogen peroxide. In the method, the nitric acid-containing liquid is electrolyzed at a constant potential, and hydrogen peroxide is added based on the electrolytic current value. The present invention relates to a method for suppressing NOx gas, characterized by adjusting the amount of NOx added.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is suitably used for a mixed acid of nitric acid and hydrofluoric acid used for pickling stainless steel and a nitric acid-containing liquid used for surface treatment of copper, brass and the like. For example, pickling of stainless steel is performed in a batch or continuous manner, and the temperature of the nitric acid-containing solution and the amount of dissolved metal also change, so that the amount of generated NOx also changes. Therefore, the required amount of added hydrogen peroxide also changes with time.

The present invention can also be applied to a case in which nitric acid is oxidized to nitric acid by adding hydrogen peroxide to a nitric acid-containing solution used as a NOx absorbing solution. For example, NO from combustion of coal, oil or other fuels
In this case, NOx gas generated from equipment for nitrification and oxidation of organic compounds using x gas and nitric acid is absorbed and oxidized.

The constant potential electrolysis method of the present invention is a method of performing electrolysis while keeping the potential of a cathode constant. Specifically, a so-called three-electrode potential control device having a working electrode, a counter electrode, and a reference electrode is used. In this method, a nitric acid or hydrofluoric acid solution is electrolyzed and a current value is detected. It is necessary that the material of the working electrode and the counter electrode is stable and does not dissolve, and platinum is preferable because the electrolytic solution is nitric acid or hydrofluoric acid. The reference electrode is not particularly limited, but is preferably a silver / silver chloride electrode made of resin and a double junction type which does not cause contamination of the electrolytic solution because the glass is dissolved in a hydrofluoric acid solution because it is easy to handle.

In addition, a three-electrode potential control device is used even if a two-electrode potential control device is used, platinum is used for the working electrode and the counter electrode, and the current is detected while the potential applied between the two electrodes is kept constant. The same correlation is obtained as in the case where FIG. 1 is a schematic diagram of an apparatus using a three-electrode potential controller, and FIG.
It is the schematic of an apparatus at the time of using a bipolar potential control apparatus.

The surface area of the electrode is not particularly limited, but affects the detected current value, and is determined according to the magnitude of the current to be obtained. When it is desired to control the generation amount of NOx gas to a constant level, it is preferable to automatically supply hydrogen peroxide for reacting with nitrite ions in accordance with the obtained current value. An electrode surface having a size capable of extracting an electrolytic current value necessary for controlling the supply device is required. The distance between the electrodes and the electrolysis temperature are preferably constant in order to detect a stable electrolysis current value.

FIG. 3 is a graph showing the relationship between the potential of the pickling liquid and the current value when a bipolar potential control device is used. Nitrite ion was measured by ion chromatography.
The nitric acid and hydrofluoric acid solutions are generally used when pickling stainless steel, and the measurement conditions are as follows. Pickling solution to be measured: nitric acid 10% by weight, hydrofluoric acid 4% by weight, electrolysis temperature: 40 ° C. (stirred with stirring), working electrode and counter electrode: platinum wire (surface area 4.7 cm ² ), distance between electrodes: 4 cm, liquid Volume: 400 ml. Potential-current curve immediately after bathing and stainless steel (SUS4
Comparing the potential-current curve after immersion of 30),
The current value after immersion is always larger. When the voltage is 0.
Between 20V and 1.25V, the current after bathing is about 1
It is constant at 0 mA.

FIG. 4 shows the relationship between the electrolysis current value at an electrolysis potential of 0.5 V, the measured value of the nitrite ion concentration by ion chromatography, and the measured value by a NOx gas detector tube above the nitric acid and hydrofluoric acid liquid levels. It is a graph which shows a relationship. From this, it is clear that there is a proportional relationship between the NOx gas generation amount and the electrolytic current value. By utilizing this proportional relationship, the amount of NOx gas generated can be suppressed by adding hydrogen peroxide so as to be equal to or less than a certain electrolytic current value.

In the present invention, the set value of the maximum value of the electrolytic current is appropriately selected according to the upper limit of the NOx gas concentration. For example, when the NOx gas concentration is 80 ppm or less, the current value is 2
Hydrogen peroxide may be added until the current reaches 0 mA. According to the setting of the electrolytic current value, the NOx gas generation amount can be kept constant at a certain reference value. Further, as a method for adding hydrogen peroxide, simple on / off control can be used.

[0015]

EXAMPLES The present invention will be described below in detail with reference to examples, but the present invention is not limited to the examples.

Example 1 Using a two-electrode voltage controller shown in FIG.
0% by weight) and a pickling solution (1 L,
SUS430 (3 × 5 cm square plate) was immersed at 40 ° C.) and dissolved. When the electrolytic potential is 0.5 V and the electrolytic current value is 2
When it exceeded 0 mA, it was controlled so that hydrogen peroxide was added. The behavior of the electrolytic current value due to the addition of hydrogen peroxide is the fifth.
The change was as shown in the figure. In this case, the amount of NOx gas above the liquid level of the pickling solution was always about 80 ppm or less.

Example 2 The same processing as in Example 1 was performed except that the hydrogen peroxide was added when the electrolytic current value exceeded 5 mA.
In this case, the amount of NOx gas above the liquid level of the pickling solution is always about 1
It was 0 ppm or less.

Example 3 Using a three-electrode voltage controller (a silver / silver chloride double junction electrode as a reference electrode) shown in FIG.
%) And hydrofluoric acid (4% by weight).
At 0 ° C.), SUS430 (3 × 5 cm square plate) was immersed and dissolved. The electrolytic potential was set to 1.1 V (vsAg / AgCl), and control was performed so that hydrogen peroxide was added when the electrolytic current value exceeded 20 mA. In this case, the amount of NOx gas above the liquid level of the pickling solution was always about 70 ppm or less.

Comparative Example 1 SUS430 (3 × 5 cm square plate) was immersed and dissolved in an acid washing solution (1 L, 40 ° C.) composed of nitric acid (10% by weight) and hydrofluoric acid (4% by weight). In this case, the N
Ox gas volume increases with each treatment of stainless steel, up to 10
00 ppm was reached.

[0020]

According to the present invention, it is possible to control the required minimum amount of hydrogen peroxide added according to the amount of NOx gas.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an apparatus using three-electrode voltage control.

FIG. 2 is a schematic diagram of an apparatus using two-electrode voltage control.

FIG. 3 shows the relationship between the potential of the pickling solution and the current value when a two-electrode potential controller is used.

FIG. 4 shows a relationship between an electrolytic current value when an electrolytic potential of 0.5 V is applied, a nitrite ion concentration, and a NOx gas generation concentration.

FIG. 5 shows the behavior of the electrolytic current value in Example 1.

[Explanation of symbols]

1: Pickling bath 2: Nitric acid-containing liquid 3: Hydrogen peroxide supply pump 4: Platinum electrode 5: Reference electrode 6: 3-electrode voltage controller (potentiostat) 7: 2-electrode voltage controller 8: Supply pump Drive control signal 9: Potential-current curve when nitrite ion does not exist 10: Potential-current curve when nitrite ion is 0.55 g / L 11: Relationship between current value and NOx gas generation concentration 12: Relationship between current value and nitrite ion concentration

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Claims (2)

[Claims] In a method for suppressing the release of NOx gas from a nitric acid-containing liquid by adding hydrogen peroxide, the nitric acid-containing liquid is electrolyzed at a constant potential, and the amount of hydrogen peroxide added is determined based on the electrolytic current value. A method for suppressing NOx gas, comprising adjusting. 2. The method according to claim 1, wherein the nitric acid-containing liquid is a fluorine-containing pickling bath for stainless steel.