JP3263264B2 - Exhaust gas treating agent and exhaust gas treating method using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas treating agent for detoxifying a gas discharged in a semiconductor manufacturing process and an exhaust gas treating method using the same.

[0002]

2. Description of the Related Art Nitrogen trifluoride (hereinafter referred to as NF ₃ ) is a strong fluorinating agent at high temperatures and in a plasma state, but is stable at room temperature and is easy to store and handle. Another characteristic is that the residual component derived from NF ₃ after fluorinating the fluorinated substance is gaseous (mainly nitrogen). For this reason,
In a semiconductor manufacturing process, it is used as a dry etching agent or a cleaning gas for semiconductor manufacturing equipment and the like, and in recent years, its demand has been growing.

While useful as described above, NF₃Is poison
The allowable concentration is set to 10 ppm
ing. Therefore, after being used in the semiconductor manufacturing process
NF ₃Exhaust gas containing
It is desired to perform harmful treatment (treatment). However, N
F₃Is stable at room temperature and hardly absorbed by water
Therefore, processing is not easy.

Heretofore, several methods have been proposed for treating such exhaust gas, and a method of treating the exhaust gas by contact with a solid chemical and a method of treating the exhaust gas in a flame using hydrogen or the like as a fuel. Are roughly divided into Among them, the former is a simple and easy technique because the configuration of the device can be simplified in principle in comparison with the latter. NF ₃ removal means the fluorine component (F)
Is converted to a readily decomposable or less toxic fluoride form. Since NF ₃ has a high thermodynamic potential among fluorides, the reaction of decomposing itself and converting other chemical species to fluoride is thermodynamically spontaneous in most cases. Therefore, a wide variety of chemical species can be used in principle as solid drugs.

[0005] NF ₃ and I ₂ , B, P, Si, As, S
b, alkali metal, alkaline earth metal, Zn, Cd, A
There have been cases where attempts have been made to react with chemical species such as 1, Fe, Cu, SiO ₂ (GMELIN HANDUBUCH, Vol.
umeF: SVol. 1, 1959, p. 244-24
5), these are generally listed as solid drug candidates.
First, as a specific example of a method by contact with a solid drug,
A method of treating by contacting with activated carbon or the like at 300 to 600 ° C. (Japanese Patent Laid-Open No. 62-237929), or a method of treating by contacting with silicon or the like at 200 to 800 ° C.
1-204025).

[0006] In contact with activated carbon, the treatment results in the production of mainly non-toxic carbon tetrafluoride (CF ₄ ).
Although CF ₄ is not subject to regulation, it can be released to the atmosphere, but from the viewpoint of preventing global warming, releasing it to the atmosphere is an undesirable substance. In contact with silicon etc., as a result of processing,
Silicon tetrafluoride (SiF ₄ ) which is toxic but easy to decompose
Etc. are generated. In this method, a device for decomposing and processing SiF ₄ or the like needs to be provided, and the processing process as a whole is not always simple.

Moreover, they are easy to handle drugs commonly solid, also the capacity of the NF ₃ that can be treated with an agent per unit volume (hereinafter, processing power) is large. However, on the other hand, it is practically necessary to bring the exhaust gas containing NF ₃ into contact with the solid chemical at a high temperature. For this reason, when the amount of drug used is large and the filling tube becomes large, there is a disadvantage that the configuration of the device becomes complicated because it is necessary to maintain the entire drug at a high temperature.

[0008] An agent obtained by refining a catalyst mainly composed of nickel and activating it with hydrogen at 180 to 200 ° C;
Method of contacting exhaust gas containing NF ₃ (Japanese Unexamined Patent Application Publication No.
No. 261247) is an improvement of the above method. In this method, since substantially only nitrogen is generated as a result of the treatment, no additional treatment device is required, and the treatment can be performed at a relatively low temperature. However, the above-mentioned method has a lower processing capacity than the method using activated carbon or silicon as a chemical, and upon contact with air,
There is a disadvantage that handling is difficult because the reactivity (activity) with NF ₃ is reduced.

[0009]

As described above, conventional treatment methods using solid chemicals have advantages, but require high temperatures for the treatment reaction and are harmful to the chemical reaction. Therefore, there are drawbacks such as the necessity of an additional processing device due to the generation of the gas, the low processing capacity, and the difficulty in handling solid chemicals. Therefore, there is a need for a solution in practical use. The present invention is directed to the development of a novel solid processing drug that does not have these disadvantages.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an exhaust gas treating agent which solves the above-mentioned conventional problems and an exhaust gas treating method using the same. An object of the present invention is a treatment which is relatively easy to handle and does not require a high temperature for the reaction, and thus does not contain chemical species which require a secondary treatment in the exhaust gas after the treatment, which is comparable to the conventional treatment. NF ₃ with ability
The aim is to develop a method for treating contained exhaust gas. The inventors have conducted intensive studies on solving the above-mentioned problems, and as a result, have an activity at a relatively low temperature, have a practical processing capacity, and the gas generated as a result of the processing reaction is harmless. In addition, they have found a novel solid drug that is stable even in the air, and have completed the present invention.

That is, in the present invention, an exhaust gas treating agent containing a metal whose surface has been preliminarily fluorinated is mainly filled in a treatment tube, and then an exhaust gas containing at least nitrogen trifluoride as a substance to be treated is supplied to the treatment tube. The present invention relates to a method for treating exhaust gas, which comprises ventilating and contacting with a treating agent, and an exhaust gas treating agent for treating exhaust gas containing nitrogen trifluoride whose surface is mainly composed of fluorinated metal.

The present invention will be described more specifically and in detail. In the present invention, a metal having a surface fluorinated is used. This metal is a chemically base metal having substantially no vapor pressure at a use temperature when it is made of fluoride. The metals in question are extensive, but generally, the periodic tables IB, IIB, III
Group B, VIIA, and VIII metals are suitable, and specifically, Mn, Fe, Cu, Zn, Al and the like can be advantageously used. It is known that NF ₃ reacts with these, as described above. However, when used without using the present invention, the superiority over the prior art is lost due to any of the various problems described above.

As far as the above-mentioned metals are contained as an active ingredient, other metals or components which are inactive against the treatment reaction may be contained in the practice of the present invention. Alkali metals are active chemical species, but are excluded because they are not suitable in chemical and mechanical properties for the physical form as in the present invention. It should be noted that the processing capability of Fe can be improved by coexisting Cr. The mechanism is not clear, but C
It is presumed that the coexistence of r has an effect of suppressing a decrease in the functional surface (effective surface involved in the exhaust gas treatment reaction) of the exhaust gas treating agent in the process of exhaust gas treatment.

As a method of adding Cr, for example, Fe-
A method in which a Cr alloy is pulverized or a precipitate obtained by a coprecipitation method using a water-soluble salt as a raw material is used as a precursor is also applicable.
From this, it is considered that the state of existence of Cr is preferably a state substantially mixed with Fe. It is appropriate that the amount of Cr added is such that the ratio of Cr to Fe is 1 to 30% by weight. If the amount is extremely small, no effect can be obtained, and if it is too large, an effect commensurate with the added amount cannot be expected.

A method for obtaining a metal whose surface is fluorinated is not specified, but a method of treating metal particles with a liquid or gaseous fluorinating agent is generally applicable. At this time, it is advantageous that the surface is not covered with a film such as an oxide in order to smoothly perform the fluorination treatment. In consideration of the post-process of the fluorination treatment, it is more convenient to use a gaseous fluorinating agent.

A typical method for obtaining metal particles is a grinding method. Milling is one method of metal crushing, and a crusher that uses mechanical impact caused by the movement of a crushing rod is used. 3 with a dedicated crusher that handles coarse, medium and fine grinding
This is a method of pulverizing metal in a step process. It is preferable to use an inert gas atmosphere so that the metal particles do not come into contact with air during the pulverization process for the above-described reason.

Further, a ball mill method is also applicable. The basic principle is to use the same mechanical impact as the grinding method. To increase the crushing efficiency, an improved mill with obstacles attached to the inner surface of the drum is used to increase the ball drop height. An advantage of the ball mill method is that it is easy to use an inert gas atmosphere in order to avoid contact between metal particles and air, and in this respect, the degree of freedom in equipment design is higher than in the grinding method.

As a method not relying on mechanical impact, there is a spraying method. This is a method in which molten metal is sprayed, scattered, and solidified by an air current to obtain metal particles. This method can be advantageously applied to a metal having a low melting point, but Fe, Cu, Zn,
In addition to Al, it is applicable to alloys. In the present method, it is desirable that both the gas stream for spraying and the atmosphere for solidification be an inert gas.

A method that does not rely on mechanical shock includes a reduction method. The reduction method is a method in which an easily reducible metal compound such as an oxide, a hydroxide or a halide of a target metal is reduced with a reducing agent such as hydrogen, carbon monoxide, a lower hydrocarbon or a lower alcohol. The reduction is performed by contacting with a reducing agent in an atmosphere furnace or a column at about 200 to 1000 ° C. for reduction. It has a feature that metal particles having a relatively large specific surface area can be easily obtained.

The present invention is not particularly limited as long as predetermined metal particles can be obtained, including these various metal particle production methods and other metal particle production methods. Next, a method of fluorinating the obtained metal particles will be described. At this time, the fluorinating agent is not particularly limited,
As mentioned above, gaseous fluorinating agents have some advantages.
As the gaseous fluorinating agent, chemical species having a high thermodynamic potential such as fluorine gas (F ₂ ), nitrogen trifluoride gas, and hydrogen fluoride gas (HF) are recommended. Fluorination is carried out using these, and is usually performed with an inert gas such as nitrogen.
The predetermined fluorination is achieved by bringing the gas diluted to about% into contact with the metal particles at around room temperature. When performed at a high temperature, not only is the fluorination excessive, but also the fluorinated film tends to lack stability, possibly due to lack of stability.

The treatment can be carried out using a general reaction apparatus such as a fixed bed or a fluidized bed, and is not particularly limited as long as the apparatus can uniformly contact the fluorinating agent with the metal particles. As an example, as a fixed bed type, a fixed bed type column is filled with metal particles through a packing hopper or the like having a structure in which contact with air does not substantially occur, and then a diluted fluorinating agent is passed through the column. I do. Although the gas flow rate can be selected as appropriate, the space velocity is several thousand to several tens [h].
r-1] is a standard.

The fluorinated metal particles obtained as described above can be used as they are, but it is also practically preferable to form them into pellets in accordance with the operating conditions during processing. The molding is carried out using a general tableting device, and at this time, a molding aid and a lubricant can be appropriately added. It is also possible to carry out pellet molding before the fluorination treatment, and in this case, the addition of a molding aid and a lubricant is the same as described above.

In the treatment of an exhaust gas containing NF ₃ , the exhaust gas is brought into contact with the obtained metal particles or a molded product thereof (hereinafter, a treating agent). At this time, the metal particles or the treating agent are generally used after being filled in a column having gas inlets and outlets at both ends. The exhaust gas enters through a gas inlet, comes into contact with the treating agent, and is discharged through a gas outlet. The shape and structure of the column to be packed are not limited as long as the column has substantially the above-mentioned functions. A more preferred embodiment is provided if a column is provided with a function of measuring the temperature of the treatment agent packed layer, a pressure measuring device, and the end point of the life of the treatment agent. As a function of detecting the end point of the life of the treatment agent, a method of detecting a change in the weight of the column as a monitor can be used.

After the column is filled with the treating agent, NF ₃
Vent the contained exhaust gas. At this time, N contained in the exhaust gas
There is no particular limitation on the concentration of F ₃ , the superficial velocity and space velocity of the exhaust gas in the column, gas components other than NF ₃ , and the like. However, NF is surely
If it is necessary to remove the _three concentrations substantially completely, it is desirable to consider the conditions for ventilation in advance.

For example, the concentration of NF ₃ gas is usually several tens ppm to several tens%, preferably several hundred ppm to several%. The superficial velocity of the exhaust gas depends on the diameter of the column, the shape of the treating agent, etc., but is usually 50 cm / s.
ec or less, preferably 20 cm / sec or less. The space velocity is generally 10,000 [hr-1] or less. As long as the pressure is not extremely low or high, there is practically no problem, and about 100 kPa to 200 kPa can be usually applied.

The temperature of the treating agent is preferably from 40 to 700 ° C., more preferably from 50 to 300 ° C. If the temperature is lower than 40 ° C., the substantial processing capacity is reduced. 70
Even if the temperature exceeds 0 ° C., although the processing capacity is present, it hardly changes substantially, so that it is not particularly necessary to select a high temperature. NF
For the gas components other than ₃ , for example, oxygen, nitrogen oxides, hydrogen halides, halogenated silanes, gaseous silicon compounds, and the like may be mixed, but these may be accompanied.

After venting the exhaust gas, the column outlet gas is
Chromatographic analysis shows NF ₃Is not detected.
The by-product gas is nitrogen. This is NF₃And treatment agent
Contact with NF₃The F component inside is fixed to the treating agent and N
This indicates that only minutes have been discharged. Also processing capacity
Although it depends on the conditions at the time of ventilation, generally the unit volume of the treatment agent is
NF of 20 times or more capacity per₃(Concentration 100% conversion)
Processing is possible and processing capacity is sufficiently ensured.

The specific surface area of the fluorinated metal particles is usually preferably about 0.1 to 120 m ² / g. This is because if the specific surface area is extremely small, a sufficient processing capacity cannot be obtained, or a restriction such as a high temperature is required for the treatment. The amount of the fluorinating agent required for the formation also increases, which is economically undesirable.

Incidentally, the specific surface area of the target metal in the present invention can be generally determined by the BET method.

The fluorination rate for a metal is related to the specific surface area of the metal used. When the fluorination treatment is performed under the same conditions, a correlation is generally recognized between the specific surface area and the fluorination rate. Therefore, it is considered that fluorination occurs in the surface layer of the metal.

The fluorination rate is preferably from 0.1 to 5%. When the fluorination rate is extremely low, when the metal particles are exposed to air, heat is generated, and deterioration in the storage state may be a problem. In the case of an extremely high fluorination rate, the activity in a relatively low temperature range is reduced, and the processing capacity is substantially reduced by the fluorination treatment. Note that the determination of the fluorination rate only involves quantifying the fluorine content in a predetermined amount of the fluorinated metal, and a method of dissolving it in an acid or the like and then quantifying this with a fluorine ion electrode can be applied.

The metal particles obtained as described above are relatively stable in the air, and can be handled in accordance with the conventional solid chemicals for processing on the assumption that they are used at a high temperature. On the other hand, in the case of a catalyst in which a catalyst mainly composed of nickel is finely divided and activated with hydrogen at 180 to 200 ° C., considerable heat is generated when handled in the air. Even so, it can be said that handling in air is impossible because the processing capacity is almost lost.

[0033]

The present invention will be described in more detail by way of examples with reference to examples. Example 1 Ball mill method In order to increase the crushing efficiency, 1 kg of coarsely crushed zinc pieces was put into a mill having an obstacle for increasing the drop height of a ball mounted on the inner surface of a drum, and the inside of the mill was completely filled with dry nitrogen gas. Replaced. Subsequently, the mill was operated for 24 hours to grind the zinc. A stainless steel column having an inner diameter of about 16 mm having a gas inlet / outlet at the front and rear through a hopper was attached to the mill outlet, and the inside of the column and the hopper was purged with nitrogen by flowing dry nitrogen. Was transferred to the column. Separately, 10 g of milled zinc was taken out to a stainless steel dish and exposed to an air atmosphere. As a result, zinc generated a strong heat. Next, NF ₃ was brought into contact with the zinc transferred to the column to fluorinate the surface. NF ₃ gas diluted with nitrogen to 0.5% by volume was aerated at a flow rate of 1 cm / sec for 5 hours or more. At this time, the column was immersed in a thermostat at 25 ° C. to control the internal temperature. 10 g of the obtained zinc was taken out to a stainless steel dish and exposed to an air atmosphere, but no heat generation was observed. The specific surface area of the obtained zinc was 0.14 m ² / g. The fluorination rate was 0.82% by weight.

Example 2 The same operation as in Example 1 was performed, except that zinc was changed to copper. The specific surface area of the obtained copper was 0.11 m ² / g. The fluorination rate was 0.4% by weight.

Example 3 A mill was operated for 48 hours in the same manner as in Example 1 to pulverize aluminum. Further, as in Example 1, 30 g of aluminum was transferred to the column. Separately, 5 g of aluminum pulverized by a mill was taken out to a stainless steel dish and exposed to an air atmosphere, whereby the aluminum generated heat. Next, fluorine gas was brought into contact with the aluminum transferred to the column to fluorinate the surface. Fluorine gas diluted to 0.1% by volume with dry nitrogen was passed at a flow rate of 1 cm / sec for 5 hours or more. At this time, the column was immersed in a constant temperature water bath at 30 ° C. to control the temperature inside the column. 5 g of the obtained aluminum was taken out to a stainless steel dish and exposed to an air atmosphere, but no heat generation was observed. The obtained aluminum had a specific surface area of 0.22 m ² / g and a fluorination ratio of 0.65% by weight.

Example 4 A wet cake obtained by oxidizing a precipitate obtained by a reaction between ferric sulfate and sodium hydroxide with air was dried. After forming into / 8 inch × 1/8 φ inch pellets, 80 g of the pellets were packed in a 100 cc stainless steel column, and while heating to 500 ° C., hydrogen gas diluted to 50% by volume with nitrogen was applied to the column. Circulation,
Reduced over 15 hours. 10g iron pellet from column
Was taken out to a stainless steel dish and exposed to an air atmosphere, and significant heat generation was observed. Next, NF ₃ was brought into contact with the iron pellets transferred to the column to fluorinate the surface. NF ₃ diluted to 1% by volume with nitrogen was aerated at 2 cm / sec for 5 hours or more. At this time, the temperature inside the column was controlled by immersing the column in a thermostat at 20 ° C. 10 g of the obtained iron pellet was taken out to a stainless steel dish and exposed to an air atmosphere, but no heat generation was observed. The specific surface area of the obtained iron pellet was about 30 m ² / g, and the fluorination rate was 1.2% by weight.

Example 5 Each of the metal particles obtained in Examples 1 to 3 was separately mixed with calcium fluoride powder at an apparent volume ratio of 1: 1 and then molded by an extruder (generally). Size 1/8
Inch × １ ／ φ inch) to obtain a pellet mainly containing zinc, a pellet mainly containing copper, and a pellet mainly containing aluminum. Calcium fluoride is stable and inert in the treatment reaction. Each of these pellets and the iron pellets obtained in Example 4 was packed in a stainless column having an inner diameter of about 23 mm to a height of about 200 m.
After filling with m, NF ₃ gas diluted to 0.2% by volume with nitrogen was passed, and the components of the gas at the outlet of the column were analyzed over time (Table 1). As a result, in each case, NF ₃
(The state in which NF ₃ is not detected at the column outlet is maintained with time) and that the column outlet gas does not contain NF ₃ . No generation of components requiring secondary treatment was observed. The pressure during ventilation was approximately atmospheric pressure.

[0038]

[Table 1]

Example 6 After a commercially available reduced iron fine powder was further reduced, it was formed into a pellet of approximately 1/8 inch × 1/8 φ inch by an extruder. The pellet is then transferred to a column and
It was fluorinated surface by contacting the F _3. NF ₃ diluted to 0.3% by volume with nitrogen was aerated at 10 cm / sec for 5 hours or more. At this time, the temperature inside the column was controlled by immersing the column in a thermostat at 20 ° C. The specific surface area of the obtained iron pellet was about 140 m ² / g, and the fluorination rate was 4.7% by weight.

Example 7 10% iron (III) nitrate and 10% chromium (III) nitrate were mixed at a mixing ratio of 100: 15, respectively.
Various kinds of aqueous solutions were prepared. Add a sodium hydroxide solution to each aqueous solution created until it becomes weakly alkaline,
Two types of precipitates were obtained. This was washed with water, filtered, dried, and tableted to obtain a tablet of 3 mmφ × 3 mm. next,
After the reduction treatment, the tablets having a chromium ratio of 15% were subjected to a fluorination treatment (fluorination ratio: about 1%) with a dilute NF ₃ gas (concentration: 0.05% by volume), which was used as an exhaust gas treating agent. Next, 30 g of an exhaust gas treating agent was packed in a column having an inner diameter of 16 mm, the column temperature was set to 200 ° C., and NF ₃ gas (balance gas was nitrogen) diluted to 1% by volume was 1200 cc / min (empty column). Speed about 10cm / s
ec). As a result, NF ₃ was detected 240 minutes later.

Comparative Example 1 A column equivalent to that in Example 5 was packed with granular activated carbon,
Heated to ° C. NF ₃ diluted to 0.2% by volume was passed through the column at a gas flow rate of 10 cm / sec. Analysis of the outlet gas indicated that NF ₃ was hardly removed. Therefore, the column was heated to 500 ° C., and NF ₃ diluted to 0.2% by volume was passed through the column at a gas flow rate of 10 cm / sec. When the outlet gas was analyzed, NF ₃ was not detected, but CF 4 and a trace amount of dinitrogen difluoride were detected. CF ₄ has low toxicity,
Although there is no regulation on emission due to air pollution, it is a substance that is attracting attention as a chemical species that affects global warming.

Comparative Example 2 A column equivalent to that of Example 5 was filled with granular metallic silicon,
Heated to 100 ° C. NF3 diluted to 0.2% by volume was passed through the column at a gas flow rate of 10 cm / sec. Analysis of the outlet gas indicated that NF ₃ was hardly removed. Therefore, the column was heated to 500 ° C., and NF ₃ diluted to 0.2% by volume was passed through the column at a gas flow rate of 10 cm / sec. When the outlet gas was analyzed, NF ₃ was not detected, but silicon tetrafluoride having a strong pungent odor was detected. Since silicon tetrafluoride is toxic, it is inconvenient to release it directly to the atmosphere. Secondary treatment such as washing with water utilizing hydrolysis is necessary.

Comparative Example 3 A catalyst mainly containing nickel was crushed, and particles of about 1 mm were obtained by sieving. This was packed in a column equivalent to that of Example 5, and heated to 200 ° C. After bubbling hydrogen through the column for 20 hours, the column was cooled to room temperature while bubbling nitrogen through. NF ₃ diluted to 0.2% by volume was passed through the column at a gas flow rate of 15 cm / sec.
When the outlet gas was analyzed, NF ₃ was not detected for about 10 minutes, but about 100 ppm of NF ₃ was detected thereafter. Next, the granular nickel-based catalyst treated with hydrogen in the same manner as described above and subsequently treated with nitrogen was taken out to a stainless steel dish and exposed to air.

[0044]

As described above, the implementation procedure of the present invention will be described in detail.
As described above, the effect of this will be described below. Eye of the invention
The target is relatively easy to handle and requires high temperatures for the reaction.
And additional processing is required in the exhaust gas after processing.
Species that do not include
NF₃The aim was to develop a treatment method for contained exhaust gas.
is there. According to the present invention, as described above, the air
There is no practical problem in handling, even at relatively low temperatures
F₃It is possible to treat exhaust gas containing₃ place
Exhaust gas after processing is only nitrogen, including by-products that require processing
It has practical processing ability. As a result,
NF with conventional solid drug₃Compared with the treatment of contained exhaust gas
Therefore, the equipment can be simplified and downsized easily,
Updating can be performed in the same manner as the conventional method. N
F₃It has epoch-making advantages in treating waste gas
It can be said that.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI B01J 27/125 ZAB B01D 53/34 ZAB 27/128 ZAB 128 27/138 ZAB 53/36 ZABZ (58) Investigated field (Int. Cl. ⁷ , DB name) B01D 53/14 B01D 53/34 B01J 20/00 B01J 23/00

Claims (7)

(57) [Claims] An exhaust gas treating agent containing a metal whose surface has been preliminarily fluorinated as a main component is filled in a treatment cylinder, and subsequently, an exhaust gas containing at least nitrogen trifluoride as a substance to be treated is passed through the treatment cylinder, A method for treating exhaust gas, which comprises contacting with a treating agent. 2. The method according to claim 1, wherein the temperature at which the exhaust gas is brought into contact with the treating agent is 40 to 700 ° C. 3. The fluorinated metal has a specific surface area of 0.1.
The method of claim 1, wherein at m ² / g or more. 4. A fluorine content of 0.1 to metal.
4. The method according to claim 1, wherein the amount is not less than% by weight. 5. The method according to claim 1, wherein the metal is iron, copper, zinc, aluminum and manganese, or an alloy or a mixture containing these as an active ingredient. 6. The metal is iron, and 1 to 1 for iron.
6. The method according to claim 5, wherein 30% by weight of chromium is substantially homogeneously mixed with said iron. 7. An exhaust gas treating agent for treating an exhaust gas containing nitrogen trifluoride as a substance to be treated whose main component is a metal whose surface has been fluorinated in advance.