JPH10141648A - Method and apparatus for treating waste gas from gas nitriding process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectually treat waste gas produced by a nitriding treatment and reduce NOx concentration in waste gas by bringing into a neutralization reaction nitric acid or nitrous acid and ammonium hydroxide produced in predetermined processes. SOLUTION: Waste gas of gas nitriding fed into a neutralization water tank 14 from a nitriding case 10 is directly discharged into water collected in the neutralization water tank 14 from a waste gas line 50 with the action of an air pump 54, and ammonia contained in the exhaust gas is brought into a reaction with water to be changed to ammonium hydroxide and be dissolved into water. Further, there is a reaction in the neutralization water tank 14 where the nitric acid and nitrous acid are neutralized with the ammonium hydroxide, and finally the waste gas of gas nitriding is converted into harmless ammonium nitride or ammonium nitride and is treated. In the neutralization reaction, combustion gas is energetically discharged from fine holes in a nozzle 35 so that the reaction is promoted, and hence pH of the water in the water tank is lowered to a value close to neutrality.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ammonia (NH)
₃₎ relates to a gas nitriding exhaust gas treatment technique to be discharged along with the nitriding treatment of the steel by gas, in particular, to detoxified gas exhaust nitride, eventually nitrogen oxides released into the atmosphere (NO _x) The present invention relates to a gas nitriding exhaust gas treatment method and apparatus for reducing environmental pollution at a minimum.

[0002]

2. Description of the Related Art In general gas nitriding as a nitriding treatment of steel, steel is heated to 500 to 550 ° C. in an ammonia gas flow.
For 20 to 100 hours. In this gas nitriding, ammonia (NH ₃ ) is decomposed as follows, and the generated nascent nitrogen (N) diffuses into the surface of the steel to form a hard nitrided layer. In order to efficiently perform gas nitriding of NH ₃ → N + 3H steel, at the initial stage of nitriding, the steel is treated with an ammonia decomposition degree of 15 to 30% (hereinafter, simply referred to as decomposition degree) for about 20 to 25 hours, and thereafter. Since it is necessary to carry out with a predetermined degree of decomposition according to the purpose of nitriding, the amount of treatment, etc., it is common that the gas nitriding exhaust gas contains about 40 to 70% of undecomposed ammonia gas. is there.

Therefore, if such a gas containing a large amount of ammonia gas is released into the atmosphere without any treatment, a strong ammonia smell causes environmental pollution. To be discarded, or decomposed by burning undecomposed ammonia and then released to the atmosphere.

[0004]

However, if the gas nitriding exhaust gas is dissolved in water to be discarded, the hydrogen ion concentration (hereinafter referred to as pH) becomes strongly alkaline at about 12 pH and cannot be discharged as it is. It must be diluted below the concentration.

[0005] When ammonia is burned, most of it is decomposed into nitrogen and water vapor, but a small amount of nitric oxide (NO _x ) is produced as a product which is environmentally problematic.

[0006] The NO _x concentration is, for example, a hydrolysis degree of 45
% When the gaseous nitriding exhaust gas was burned, a result of 300 ppm was obtained.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art, to effectively treat gas nitriding exhaust gas generated during nitriding treatment, and to reduce NO _{x in} exhaust gas, which poses a problem of environmental pollution. An object of the present invention is to provide a method and an apparatus for treating a gas-nitrided exhaust gas whose concentration can be reduced.

[0008]

In order to achieve the above object, according to the present invention, a metal material is heated in a stream of ammonia for a predetermined time to perform a gas nitriding treatment for forming a nitride layer on the surface of the metal material. A method for treating a gas nitriding exhaust gas discharged from a nitriding treatment vessel, wherein at least a part of the gas nitriding exhaust gas is guided to a combustion device, and ammonia (NH ₃ ) contained in the gas nitriding exhaust gas is burned, and nitrogen ( A first step of decomposing into N ₂ ) and water (H ₂ O);
The combustion gas generated in the first step is led to a water tank and reacted with water, whereby nitrogen oxides (NO
_x ) to nitric acid (HNO ₃ ) or nitrous acid (HNO ₂ ), and the remaining gas nitriding exhaust gas and / or undecomposed NH ₃ contained in the combustion gas are introduced into a water tank and reacted with water. Ammonium hydroxide (N
H ₄ OH), a _fourth step of neutralizing the nitric acid or nitrous acid generated in the second step with the ammonium hydroxide generated in the third step,
It is characterized by including.

[0009] According to the gas nitriding exhaust gas treatment method of the present invention, the combustion of part of the ammonia gas contained in the exhaust gas nitriding gas, reacting the NO _x contained in the combustion gas and the rest of the ammonia gas in water, Because it is neutralized to harmless ammonium nitrate or ammonium nitrite, the NO _x concentration of the gas finally released to the atmosphere can be considerably reduced, and the pH of the water in the aquarium can be reduced to a value close to neutrality. Can be lowered.

According to a preferred embodiment of the present invention, the combustion gas and the remaining gas nitriding exhaust gas are introduced into a neutralized water tank filled with water, and the second to fourth steps are performed in the same water tank. In this case, it is preferable that a part of the gas nitriding exhaust gas is introduced into the neutralization water tank through a combustion device, and the remaining gas nitriding exhaust gas is directly introduced into the neutralization tank without passing through the combustion device. At that time, by smoothly performing the reaction, the concentration of NO _x lower, and in order to approximate the pH by neutral appropriately control the degradation rate by combustion, furthermore, leads to neutralization aquarium through combustion apparatus It is preferable to adjust the ratio of the combustion gas and the gas nitriding exhaust gas directly guided to the neutralization water tank without passing through the combustion device.

When the decomposition rate in the combustion apparatus is low, the entire gas nitriding exhaust gas is guided to a water tank through the combustion apparatus, and a part of NH3 contained in the gas nitriding exhaust gas is decomposed in the combustion apparatus. You may make it.

The gas-nitriding exhaust gas treatment apparatus according to the present invention comprises:
A method for treating a gas nitriding exhaust gas discharged from a nitriding treatment vessel that heats a metal material in an ammonia stream for a predetermined time and performs a gas nitriding treatment for forming a nitride layer on the surface of the metal material, comprising: A combustion device connected to an exhaust line, a neutralization water tank for storing water therein, and a combustion gas line for guiding combustion gas exhausted from the combustion device to the neutralization water tank are provided. Things.

According to a preferred embodiment of the gas nitriding exhaust gas treatment apparatus, an exhaust gas line connected to the exhaust line and directing the gas nitriding exhaust gas directly to the neutralization water tank, and the neutralization water tank via the combustion gas line Flow rate control means for controlling the ratio between the flow rate of the combustion gas reaching the exhaust gas and the flow rate of the gas nitrided exhaust gas flowing directly from the exhaust gas to the neutralization water tank at a predetermined ratio.

In order to promote the neutralization reaction, the discharge section where the exhaust gas line is connected to the neutralization water tank is provided with a nozzle for discharging combustion gas into the water in the neutralization water tank. Preferably has a large number of pores.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method and an apparatus for treating a gas nitrided exhaust gas according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is an explanatory view of the structure of an apparatus for carrying out a gas nitriding exhaust gas treatment method according to the present invention.
A nitriding case 12 is used as a nitriding container for performing a nitriding treatment on the surface of the surface. Numeral 12 denotes a combustion device for burning and decomposing ammonia contained in the gas nitriding exhaust gas discharged from the nitriding case 10. Numeral 14 is for performing a process of reacting the gas nitriding exhaust gas and the combustion gas led from the gas nitriding case 10 and the combustion device 12 with water and a process of neutralizing nitric acid or nitrous acid and ammonium hydroxide generated as a result. It is a Japanese aquarium.

Ammonia gas is supplied to the nitriding case 10 from an ammonia gas line 15 through a normally open solenoid valve 16, and the steel material 11 is heated to a predetermined temperature (5 ° C.) by a built-in heater (not shown) in an ammonia gas flow.
(00 to 550 ° C.), thereby performing a nitriding treatment.

A nitrogen gas line 17 is connected to the ammonia gas line 15 so as to merge with the ammonia gas line 15.
Normally closed solenoid valve 1 instead of closing 6
When the nozzle 8 is opened, the nitrogen gas is pumped to the nitriding case 10 through the ammonia gas line 15 so that the ammonia gas can be purged.

Reference numeral 20 denotes a gas nitriding exhaust gas (including ammonia, nitrogen, and hydrogen) generated as a result of the nitriding treatment.
This is an exhaust line for exhausting from zero. In the exhaust line 20, the gas nitriding exhaust gas is cooled while passing through the cooling jacket 21, and branches off into a line for supplying the gas nitriding exhaust gas to the combustion device 12 and the neutralization water tank 14 downstream of the cooling jacket 21. ing.

The branch line on the combustion device 12 side is connected via a flow control valve 22 and a solenoid valve 23. The combustion device 12 includes a combustion chamber 25 in an upper section inside a combustion cylinder 24.
The combustion chamber 25 has an exhaust line 2
An ammonia burner 26 for taking in and burning the gas nitriding exhaust gas from zero, and a pilot burner 27 for igniting and piloting the ammonia burner 26 are arranged. In this case, an air self-supply type burner is used as the ammonia burner 26, and a cut valve 2 for preventing backflow is provided between the ammonia burner 26 and the electromagnetic valve 23.
9 are provided. Fuel gas such as city gas is supplied to the pilot burner 27 via an electromagnetic valve 28. It is preferable that a catalyst made of steel or the like be provided above the ammonia burner 26 in order to increase the combustion efficiency.

An exhaust port 30 for discharging a combustion gas generated by the combustion of the gas nitrided exhaust gas is provided at an upper portion of the combustion tube 24.
Is provided, and the combustion gas is introduced from the exhaust port 30 into the neutralization water tank 14 through the combustion gas line 32. In the combustion gas line 32, the combustion gas is forcibly neutralized.
A water ring vacuum pump 33 is incorporated as a means for leading to 4, and the discharge part of the discharge pipe 34 drawn from the vacuum pump 33 and immersed in the water in the neutralization water tank 14 includes: A nozzle 35 is attached, and the combustion gas is discharged from the nozzle 35 into water. In the case of this embodiment, the surface is processed so that many pores are opened so that the gas is easily dispersed in water.

Water is supplied to the neutralizing water tank 14 through a water supply line 36. The water supply section of the water supply line 36 includes a first water supply port opened and closed by a manual valve 37, and a second water supply port opened and closed by a manual valve 38 and a solenoid valve 39. When water is first charged into the neutralization water tank 14, water is supplied by opening both the manual valves 37 and 38 and the solenoid valve 39, and the temperature of the nitriding case 10 is raised to a predetermined temperature to prepare for the nitriding treatment. After the adjustment, the manual valve 37 is closed so that a required amount of water is supplied from the second water supply port.

The neutralization water tank 14 is provided with a stirrer 40 for stirring water in the water tank by a rotating propeller, and a pH sensor 41 for managing a neutralization reaction in the water tank. In the neutralization water tank 14, the finally treated gas is released into the atmosphere through an exhaust port 42 provided at the upper part of the water tank.
The drain is led to an external wastewater treatment unit (not shown) through a drain outlet 43 provided at the lower part of the water tank.

Next, the neutralized water tank 1 constructed as described above
4 shows a line for directly leading the gas nitriding exhaust gas from the nitriding case 10. In this embodiment, an exhaust gas line 50 for branching from the exhaust line 20 of the nitriding case 10 to introduce the gas nitriding exhaust gas into the neutralization water tank 14, a power outage, overheating in the furnace in the nitriding case 10, or Abnormality line 5 for introducing gas nitriding exhaust gas to cope with unexpected situations such as misfire of pilot burner 27
1, and an ammonia decomposition degree measurement line 52 for measuring the degree of decomposition of ammonia in the gas nitriding exhaust gas in the course of the nitriding treatment is provided in parallel.

A flow control valve 53, an air pump 54, and a solenoid valve 55 are incorporated in the exhaust gas line 50 in order from the upstream, and the gas nitriding exhaust gas is forcibly immersed in the water of the neutralization water tank 14 by the air pump 54. From the discharge section at the end of the line. In addition, the flow control valve 53, together with the flow control valve 22 installed on the intake side of the combustion device 12, sets a flow rate that sets the ratio of the flow rate of the combustion gas flowing through the combustion gas line 32 to the flow rate of the gas nitrided exhaust gas flowing through the exhaust gas line 50. It constitutes control means.

A manual valve 56,
A solenoid valve 57 and a check valve 58 are provided. By opening the manual valve 56 during operation, gas can be automatically discharged to the neutralizing water tank 14 in the event of an abnormal situation. A decomposition degree measuring device 60 is connected to the ammonia decomposition degree measurement line 52 downstream of the manual valve 59.

Next, a gas nitriding exhaust gas treatment method carried out by the gas nitriding exhaust gas treatment apparatus configured as described above will be described with reference to FIG. 1 and FIG. 2 which is a flowchart of the process. First, from the nitriding case 10, an undecomposed ammonia gas, and a gas nitriding exhaust gas containing a nitrogen gas and a hydrogen gas generated by the decomposition of the ammonia are supplied to the exhaust line 2.
Exhausted through 0. The gas nitriding exhaust gas is distributed to a predetermined ratio by the flow control valves 22 and 53 and sent to the combustion device 12 and the neutralization water tank 14.

In the combustion device 12, the ammonia contained in the gas nitriding exhaust gas is decomposed into nitrogen oxides such as nitrogen dioxide (NO ₂ ) and nitrogen monoxide (NO) and water by combustion.

The combustion gas containing such nitrogen oxides is as follows:
The gas is discharged from the nozzle 35 into the water in the neutralization water tank 14 via the vacuum pump 33 through the combustion gas line 32, and the nitrogen dioxide and the nitrogen monoxide react with the water to change into nitric acid and nitrous acid.

On the other hand, the gas nitriding exhaust gas sent from the nitriding case 10 to the neutralizing water tank 14 is discharged from the exhaust gas line 50 directly into the water stored in the neutralizing water tank 14 by the action of the air pump 54, and is discharged into the exhaust gas. The contained ammonia reacts with water and changes into ammonium hydroxide (NH ₄ OH) to be dissolved.

Further, in the neutralization water tank 14, a reaction is performed in which nitric acid and nitrous acid are neutralized by ammonium hydroxide. .
In this neutralization reaction, the combustion gas is vigorously released from the fine pores of the nozzle 35, so that the reaction is promoted.

The above is an example in which most of the ammonia contained in the gas nitrided exhaust gas sent to the combustion device 12 is considered to be decomposed by combustion in accordance with the flow chart of FIG. Most preferably, the proportions of the combustion gas guided to the neutralization water tank 14 through the combustion tank 12 and the gas nitrided exhaust gas directly guided to the neutralization water tank without passing through the combustion device 12 are adjusted to 50% by the flow control valves 22 and 53, respectively. However, in practice, decomposition by ammonia combustion and dissolution in water do not always progress 100%.

Therefore, when the combustion in the combustion device 12 is incomplete, it is necessary to increase the ratio of the flow rate sent from the nitriding case 10 to the combustion device 12 in order to increase the amount of nitrogen oxide generated by decomposition. is there.

FIG. 3 is a flow chart of the reaction when the decomposition ratio of ammonia in the combustion device 12 is 50%. In this case, the undecomposed ammonia is sent to the neutralization water tank 14 through the combustion gas line 32 and is dissolved in the water.
As a result, the same reaction can be obtained, and the exhaust gas line 50 that directly guides the gas to the neutralization water tank 14 without passing through the combustion device 12 may be omitted.

[0035]

Next, as an example, a diameter of 500 mm and a capacity of 3
The processing results of the gas nitriding exhaust gas discharged when the nitriding treatment is performed by flowing ammonia gas at a flow rate of 17 l / min using the 800 liter nitriding case 10 will be described. In this case, the decomposition degree of the ammonia gas in the nitriding case 10 was set to 50%.

The concentration of nitrogen oxide (NO _x ) contained in the combustion gas discharged from the exhaust port 30 of the combustion tube 24 of the combustion device 12 and the concentration of the processed gas discharged from the exhaust port 42 of the neutralization water tank 14 are determined. When measured, the former is 135 pp
m, the latter was 100 ppm. In addition, these N
When the NO _x concentration per unit time was calculated from the O _x concentration and the gas flow rate passing through both the exhaust ports 30 and 42, the former was 1.8 × 10 ⁻³ m ³ / h, and the latter was 1.8 × 10 ⁻³ m ³ / h. Was 0.13 × 10 ⁻³ m ³ / h. Since the gas temperature of the former was 300 ° C. and the latter was 17 ° C., the flow rate of the former combustion gas was converted to the flow rate at 17 ° C.

According to these NO _x concentration values, the amount of the treated NO _x released to the atmosphere via the neutralizing water tank 14 is reduced to about 1/14 of that of the combustion gas discharged from the combustion gas. I have.

Further, the amount of water in the neutralizing water tank 14 was set to 1000 liters, kept without flowing, and subjected to continuous treatment for 56 hours under the above conditions. As a result, the pH of the water in the neutralizing water tank 14 became It was about 8 near neutrality.

[0039]

As is apparent from the above description, according to the present invention, at least a part of the gas nitriding exhaust gas is led to a combustion device to burn ammonia (NH ₃ ) contained in the gas nitriding exhaust gas. , Nitrogen (N ₂ ) and water (H ₂ O)
And the combustion gas generated in the first step is introduced into a water tank and reacted with water to convert nitric oxide (NO _x ) in the combustion gas into nitric acid (HNO ₃ ).
Alternatively, a second step of converting to nitrous acid (HNO ₂ ) and ammonium hydroxide (NH ₂ ) by introducing the remaining gas nitriding exhaust gas and / or undecomposed NH ₃ contained in the combustion gas to a water tank and reacting with water. ₄ OH), and a fourth step of neutralizing the nitric acid or nitrous acid generated in the second step with the ammonium hydroxide generated in the third step, thereby producing a gas-nitrided exhaust gas. since the decomposition treatment of the ammonia contained in, ultimately can be reduced to a significant extent the concentration of NO _x gas released to the atmosphere, also, pH even close to neutral value of aquarium water Can be lowered to

[Brief description of the drawings]

FIG. 1 is a configuration explanatory view showing one embodiment of a gas nitriding exhaust gas treatment apparatus according to the present invention.

FIG. 2 is a diagram showing a flow of a process of treating a gas nitriding exhaust gas.

FIG. 3 is a view showing a flow of a process of processing a gas nitriding exhaust gas according to another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Nitriding case 11 Steel material 12 Combustion device 14 Neutralization water tank 20 Exhaust line 22 Flow control valve 24 Combustion cylinder 26 Ammonia burner 27 Pilot burner 30 Exhaust port 32 Combustion gas line 33 Vacuum pump 34 Discharge pipe 35 Nozzle 50 Exhaust gas line 51 Abnormal line 52 Ammonia decomposition measurement line

Claims (7)

[Claims] 1. A method for treating a gas nitriding exhaust gas discharged from a nitriding vessel for heating a metal material in an ammonia stream for a predetermined time to perform a gas nitriding treatment for forming a nitride layer on the surface of the metal material, At least a part of the gas nitriding exhaust gas is led to a combustion device, and ammonia (NH) contained in the gas nitriding exhaust gas is introduced.
₃ ) a first step of burning and decomposing into nitrogen (N ₂ ) and water (H ₂ O); and introducing the combustion gas generated in the first step into a water tank and reacting with water. Nitrogen oxide (NO
_x ) to nitric acid (HNO ₃ ) or nitrous acid (HNO ₂ ), and the remaining gas nitriding exhaust gas and / or undecomposed NH ₃ contained in the combustion gas are introduced into a water tank and reacted with water. ammonium hydroxide than be (NH ₄ OH) a third step of changing the said generated and nitric or nitrous acid in the second step third
And a fourth step of neutralizing the ammonium hydroxide generated in the step. 2. The method according to claim 1, wherein the combustion gas and the remaining gas nitriding exhaust gas are introduced into a neutralized water tank filled with water, and the second to fourth steps are performed in the same water tank. Gas nitriding exhaust gas treatment method. 3. A part of the gas nitriding exhaust gas is introduced into the neutralization water tank through a combustion device, and the remaining gas nitriding exhaust gas is directly introduced into the neutralization tank without passing through the combustion device. The method for treating a gas nitrided exhaust gas according to claim 2. 4. The gas nitriding exhaust gas according to claim 2, wherein the entirety of the gas nitriding exhaust gas is guided to a water tank through the combustion device, and a part of NH ₃ contained in the gas nitriding exhaust gas is decomposed in the combustion device. Nitrogen exhaust gas treatment method. 5. An apparatus for treating a gas-nitrided exhaust gas discharged from a nitriding vessel for heating a metal material in an ammonia gas stream for a predetermined time to perform a gas nitriding treatment for forming a nitride layer on the surface of the metal material, A combustion device connected to an exhaust line of the nitriding treatment vessel; a neutralization water tank for storing water therein; and a combustion gas line for guiding combustion gas exhausted from the combustion device to the neutralization water tank. A gas nitriding exhaust gas treatment apparatus characterized by the above-mentioned. 6. An exhaust gas line connected to the exhaust line for directing gas nitriding exhaust gas to the neutralization water tank; a flow rate of combustion gas reaching the neutralization water tank via the combustion gas line; 6. The gas nitriding exhaust gas treatment apparatus according to claim 5, further comprising: a flow control unit configured to control a ratio of a flow rate of the gas nitriding exhaust gas to the sump tank to a predetermined ratio. 7. A discharge part where the exhaust gas line is connected to the neutralization water tank is provided with a nozzle for discharging combustion gas into water in the neutralization water tank, and the nozzle has a large number of pores. The gas nitriding exhaust gas treatment apparatus according to claim 5 or 6, wherein