JP2001009477A - NOx GAS SUPPRESSING METHOD USING HYDROGEN PEROXIDE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide method for efficiently suppressing NOx gas generated at the time of pickling or surface-treating a metal with a nitric acid-containing liquid by adding hydrogen peroxide. SOLUTION: The method, is to control the quantity of hydrogen peroxide to be added based on the oxidation-reduction potential of the nitric acid- containing liquid and the constant potential electrolytic current value. When each of the oxidation-reduction potential and the constant potential electrolytic current value exceed the respective max. acceptable value, the generating of NOx gas is suppressed to a fixed value or below by adding hydrogen peroxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for suppressing NOx gas generated when a metal is treated in a nitric acid-containing liquid 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 solution, 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, equipment costs for an exhaust gas treatment device are required, 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). However, the amount of hydrogen peroxide added is appropriately controlled. Control is a problem. 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 for controlling the addition of hydrogen peroxide based on the oxidation-reduction potential of a nitrite-containing solution, but the control method is complicated.

[0004]

SUMMARY OF THE INVENTION 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. There is a certain relationship between the ion concentration and the amount of NOx gas generated. By using this method together with the method based on the oxidation-reduction potential, the generation of NOx gas can be effectively suppressed without excessive addition of hydrogen peroxide. They found what they could do and arrived at the present invention.

That is, the present invention relates to a method for suppressing the release of NOx gas from a nitric acid-containing liquid by adding hydrogen peroxide, wherein the amount of hydrogen peroxide added is determined based on the oxidation-reduction potential of the nitric acid-containing liquid and the electrolytic current. And controlling a NOx gas.

[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. Further, it is also used as a pickling treatment liquid for carbon steel, low alloy steel, aluminum and the like. For example, pickling of stainless steel is performed batchwise or continuously, 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 measuring electrode for measuring the oxidation-reduction potential of the present invention is not particularly limited as long as the electrode is made of a material inert to the nitric acid-containing solution. For example, if the nitric acid-containing liquid contains hydrofluoric acid, a platinum electrode is preferably used, and a double-junction silver / silver chloride electrode made of resin is preferably used as the comparative electrode.

FIG. 1 shows a solution of nitric acid and hydrofluoric acid in stainless steel (S
FIG. 10 is a graph showing changes in oxidation-reduction potential when hydrogen peroxide is intermittently added while dissolving US430). Nitrite ions are present in the high potential state (region),
Hydrogen peroxide is present in (the region of) a low potential. The absolute value of the oxidation-reduction potential varies depending on the type of electrode, the temperature of the solution, the acid and metal concentrations in the solution, etc.
A potential difference of about 200 mV is observed between the state where nitrite ions are present and the state where excess hydrogen peroxide is present. Therefore, it is possible to detect whether or not excess hydrogen peroxide exists. In the present invention, the set value of the oxidation-reduction potential is preferably about 700 mV where there is no excess hydrogen peroxide.

[0010] The constant potential electrolysis of the present invention is a method of performing electrolysis while keeping the potential of a cathode constant.
By a three-electrode potential control device (so-called potentiostat) having a counter electrode and a reference electrode. 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 resin-made silver / silver chloride electrode 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 of ease of handling.
Also, the same correlation as that of the three-electrode potential controller can be obtained by using the two-pole potential controller.

The surface area of the electrode is not particularly limited, but is determined according to the magnitude of the required current. In order to automatically supply hydrogen peroxide for reacting with nitrite ions according to the obtained current value, an electrode surface large enough to take out an electrolytic current value necessary for control is required. The distance between the electrodes and the electrolysis temperature are preferably constant in order to detect a stable electrolysis current value.

FIG. 2 is a graph showing the relationship between the electrolytic current value, the measured value of the nitrite ion concentration, and the NOx gas concentration on the upper surface of the nitric acid and hydrofluoric acid solutions when a three-electrode potential controller is used. is there. Nitrite ion was measured with an ion chromatograph, and NOx gas concentration was measured with a gas detector tube. 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), electrolysis potential: 1.1 V, working electrode and counter electrode: platinum wire (surface area: 4.7 cm ² ), see Electrode: copper / silver chloride (double junction type), distance between electrodes: 4 cm, liquid volume: 500 ml.

FIG. 2 shows that nitrite ion concentration 1 and N
It is shown that there is a proportional relationship between the Ox gas concentration 2 and the electrolytic current value. Using this proportional relationship, the amount of hydrogen peroxide added to suppress NOx gas generation can be controlled based on the electrolytic current value. In the present invention, the set 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 set to 20 ppm or less, hydrogen peroxide may be added when the current value is 20 mA or more according to FIG. According to the setting of the electrolytic current value, the NOx gas generation amount can be kept below a certain reference.

However, as shown in FIG. 3, when the addition of hydrogen peroxide is continued, the nitrite ion concentration 3 disappears, and when excess hydrogen peroxide is present, the hydrogen peroxide The electrolytic current value 4 increases in proportion to the amount. Therefore,
It is necessary to determine whether the detected electrolytic current value is due to nitrite ions or hydrogen peroxide.

In the present invention, by using the properties shown in FIGS. 1 and 2 together, the amount of hydrogen peroxide can be reduced without excessively adding hydrogen peroxide to a level at which nitrite ions cannot exist as much as possible. The addition can be controlled. That is, when the constant potential electrolytic current and the oxidation-reduction potential exceed their respective maximum allowable values, NOx is added by adding hydrogen peroxide.
Gas generation can be suppressed to a certain amount or less, and addition of excessive hydrogen peroxide can be prevented. Note that a simple on / off control can be used as a control method of hydrogen peroxide addition.

Specifically, for example, the electrolytic current is detected by a potentiostat, the oxidation-reduction potential is measured by an oxidation-reduction potential meter, and the detected electrolytic current is compared with the maximum allowable value of the electrolytic current, and the oxidation-reduction potential is measured. The control is performed by determining the start and stop of the addition of hydrogen peroxide by a control signal obtained from a comparison between the measured value of the potential and the maximum allowable value of the oxidation-reduction potential.

Further, according to the present invention, control for maintaining hydrogen peroxide at a constant concentration in the nitric acid-containing liquid can be performed. That is, by adding hydrogen peroxide when the oxidation-reduction potential is equal to or more than a certain value or the constant-potential electrolysis current is equal to or less than a certain value, a state in which a certain amount of excess hydrogen peroxide is present and almost no nitrite ion is present. Can be made. As shown in FIG. 1, when the oxidation-reduction potential is higher than a certain value, nitrite ions are present and hydrogen peroxide is not present, so that hydrogen peroxide is added. When there is no nitrite ion and hydrogen peroxide is present, the oxidation-reduction potential falls below a certain value, but as shown in FIG. 3, the electrolytic current value increases in proportion to the hydrogen peroxide amount. Therefore, the hydrogen peroxide in the nitric acid-containing liquid is maintained at a constant concentration by adding the hydrogen peroxide until a constant potential electrolytic current value corresponding to a desired amount of the hydrogen peroxide is reached.

When the oxidation-reduction potential exceeds the maximum allowable value,
Alternatively, when the electrolysis current is lower than the maximum allowable value, hydrogen peroxide is added, and when the oxidation-reduction potential is equal to or lower than the maximum allowable value and the electrolysis current is equal to or higher than the maximum allowable value, the addition of hydrogen peroxide is stopped. Here, the maximum allowable value of the electrolytic current is determined from the hydrogen peroxide concentration-electrolytic current curve so as to have a constant hydrogen peroxide concentration, and the maximum allowable value of the oxidation-reduction potential is an excess of hydrogen peroxide. Potential.

Specifically, the electrolysis current is detected by a potentiostat, the oxidation-reduction potential is measured by an oxidation-reduction potential measuring device, and the detected electrolysis current is compared with the maximum allowable value of the electrolysis current, and the oxidation is measured. The control is performed by determining the start and stop of the addition of the hydrogen peroxide by a control signal obtained by comparing the reduction potential with the maximum allowable value of the oxidation-reduction potential.

[0020]

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 three-electrode potential control device shown in FIG.
%) And hydrofluoric acid (4% by weight).
At 0 ° C.), SUS430 (3 × 5 cm square plate) was immersed and dissolved. When the oxidation-reduction potential exceeded 700 mV, and the electrolytic current value exceeded 10 mA at an electrolytic potential of 1.1 V, control was performed so that hydrogen peroxide was added. FIG. 6 shows the behavior of the oxidation-reduction potential 14 and the electrolytic current value 15 due to the addition of hydrogen peroxide. In this case, the concentration of NOx gas above the liquid level of the pickling solution was always about 10 ppm or less.

Example 2 Nitric acid (10%) was prepared using a bipolar potential controller shown in FIG.
Wt%) and a pickling solution (500 m
SUS430 (3 × 5 cm square plate) was immersed and dissolved at 1, 50 ° C.). When the oxidation-reduction potential exceeded 750 mV and the electrolysis potential was 0.5 V, and the electrolysis current value exceeded 10 mA, control was performed so that hydrogen peroxide was added. Seventh
The figure shows the behavior of the oxidation-reduction potential 16 and the electrolytic current value 17 due to the addition of hydrogen peroxide. In this case, the concentration of the NOx gas above the liquid level of the pickling liquid was always about 40 ppm or less.

Example 3 Using a bipolar potential control device shown in FIG.
Wt%) liquid (500 ml, 50 ° C) with steel SPCC (4
× 6 cm square plate) was immersed and dissolved. When the oxidation-reduction potential exceeded 720 mV, and the electrolysis potential was 0.7 V, and the electrolysis current value exceeded 10 mA, control was performed so that hydrogen peroxide was added. FIG. 8 shows the behavior of the oxidation-reduction potential 18 and the electrolytic current value 19 due to the addition of hydrogen peroxide. In this case, the concentration of NOx gas above the liquid level of the pickling solution is always 10 pp.
m or less.

Example 4 Nitric acid (10%) was prepared using the bipolar potential control device shown in FIG.
Wt%) and a pickling solution (500 m
SUS430 (3 × 5 cm square plate) was immersed and dissolved at 1, 40 ° C.). When the oxidation-reduction potential exceeds 700 mV, or when the electrolytic potential is 0.5 V and the electrolytic current value is 5
Control was performed so that hydrogen peroxide was added in the case of mA or less. In this case, the concentration of NOx gas above the liquid level of the pickling solution was zero (not detected by the nitric oxide detector tube). At this time, the concentration of hydrogen peroxide in the pickling solution was maintained at about 0.05% by weight.

Comparative Example 1 Nitric acid (10% by weight) without adding hydrogen peroxide
Pickling solution (1 L, 40 ° C) consisting of hydrofluoric acid (4% by weight)
US430 (3 × 5 cm square plate) was immersed and dissolved.
In this case, the concentration of NOx gas above the liquid level of the pickling solution increased every time the stainless steel was treated, and reached a maximum of 1000 ppm.

[0026]

According to the present invention, it is possible to control the required minimum amount of hydrogen peroxide addition in accordance with the amount of NOx gas.

[Brief description of the drawings]

FIG. 1 shows the behavior of hydrogen peroxide addition and oxidation-reduction potential.

FIG. 2 shows the behavior of the electrolytic current value, the nitrite ion concentration and the NOx gas concentration.

FIG. 3 shows the behavior of the amount of added hydrogen peroxide, the concentration of nitrite ion, and the electrolytic current value.

FIG. 4 is a schematic diagram of an apparatus using three-electrode voltage control.

FIG. 5 is a schematic diagram of an apparatus using two-electrode voltage control.

FIG. 6 shows a behavior of a current electrolysis value and an oxidation-reduction potential in Example 1.

FIG. 7 shows a behavior of a current electrolysis value and an oxidation-reduction potential in Example 2.

FIG. 8 shows a behavior of a current electrolysis value and an oxidation-reduction potential in Example 3.

[Explanation of symbols]

 5: Pickling bath 6: Nitric acid-containing liquid 7: Hydrogen peroxide supply pump 8: Platinum electrode 9: Reference electrode 10: Three-electrode voltage controller (potentiostat) 11: Oxidation-reduction potential measuring instrument 12: Supply pump drive Control signal 13: Two-electrode voltage controller

Claims (8)

[Claims] 1. A method for suppressing the release of NOx gas from a nitric acid-containing liquid by adding hydrogen peroxide, wherein the amount of hydrogen peroxide added is controlled based on an oxidation-reduction potential of the nitric acid-containing liquid and an electrolytic current. A method for suppressing NOx gas. 2. The method according to claim 1, wherein hydrogen peroxide is added when the oxidation-reduction potential and the electrolysis current exceed their respective maximum allowable values. 3. The maximum allowable value of the electrolytic current is determined from a NOx concentration-electrolytic current curve so that the NOx gas generation amount is equal to or less than a predetermined allowable amount, and the maximum allowable value of the oxidation-reduction potential is excessive peroxidation. 3. The method according to claim 2, wherein the potential is a hydrogen-free potential. 4. The electrolysis current is detected by a potentiostat, the oxidation-reduction potential is measured by an oxidation-reduction potential meter, the detected electrolysis current is compared with the maximum allowable value of the electrolysis current, and the oxidation-reduction potential is measured. The method according to claim 2 or 3, wherein the start and stop of the addition of the hydrogen peroxide are determined by a control signal obtained from a comparison between the value and a maximum allowable value of the oxidation-reduction potential. 5. When the oxidation-reduction potential exceeds the maximum allowable value, or when the electrolytic current is lower than the maximum allowable value, hydrogen peroxide is added, and the oxidation-reduction potential is equal to or less than the maximum allowable value and the electrolytic current is maximum. 2. The method according to claim 1, wherein the addition of hydrogen peroxide is stopped when the value exceeds an allowable value. 6. The maximum allowable value of the electrolytic current is determined from a hydrogen peroxide concentration-electrolytic current curve so as to have a constant hydrogen peroxide concentration, and the maximum allowable value of the oxidation-reduction potential is determined by the amount of excess hydrogen peroxide. 6. The method according to claim 5, wherein the potential is present. 7. The electrolysis current is detected by a potentiostat, the oxidation-reduction potential is measured by an oxidation-reduction potential measuring device, and the detected electrolysis current is compared with the maximum allowable value of the electrolysis current, and the measured oxidation-reduction potential is measured. 7. The method according to claim 5, wherein the start and stop of the addition of the hydrogen peroxide are determined by a control signal obtained from a comparison between the hydrogen peroxide and the maximum allowable value of the oxidation-reduction potential. 8. The method according to claim 1, wherein the nitric acid-containing liquid contains hydrofluoric acid.