U.S. patent application number 09/770402 was filed with the patent office on 2001-08-16 for method for decomposing nitrogen fluoride or sulfur fluoride and decomposing reagent used therefor.
Invention is credited to Atobe, Hitoshi, Ito, Kazuto, Izumikawa, Chiaki, Kaneko, Toraichi, Tezuka, Kazumasa.
Application Number | 20010013590 09/770402 |
Document ID | / |
Family ID | 26526797 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010013590 |
Kind Code |
A1 |
Izumikawa, Chiaki ; et
al. |
August 16, 2001 |
Method for decomposing nitrogen fluoride or sulfur fluoride and
decomposing reagent used therefor
Abstract
A method for decomposing nitrogen fluoride or sulfur fluoride,
comprising contacting gaseous nitrogen fluoride or sulfur fluoride
with a solid reagent comprising elemental carbon, one or more of
the alkaline earth metal elements and optionally one or more of the
alkali metal elements, to fix the fluorine component in the
nitrogen fluoride or sulfur fluoride in said reagent.
Inventors: |
Izumikawa, Chiaki; (Tokyo,
JP) ; Tezuka, Kazumasa; (Okayama-shi, JP) ;
Ito, Kazuto; (Okayama-shi, JP) ; Atobe, Hitoshi;
(Kawasaki-shi, JP) ; Kaneko, Toraichi;
(Kawasaki-shi, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
WASHINGTON
DC
20037-3213
US
|
Family ID: |
26526797 |
Appl. No.: |
09/770402 |
Filed: |
January 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09770402 |
Jan 29, 2001 |
|
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|
09129841 |
Aug 6, 1998 |
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Current U.S.
Class: |
252/181.1 |
Current CPC
Class: |
B01D 53/685 20130101;
Y02C 20/30 20130101 |
Class at
Publication: |
252/181.1 |
International
Class: |
H01K 001/56 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 7, 1997 |
JP |
9-225723 |
Aug 7, 1997 |
JP |
9-225851 |
Claims
1. A method for decomposing nitrogen fluoride and/or sulfur
fluoride, comprising contacting at least one of gaseous nitrogen
fluoride and gaseous sulfur fluoride with a solid reagent
containing elemental carbon and one or more of the alkaline earth
metal elements.
2. The method according to claim 1, wherein said reagent further
comprises one or more of the alkali metal elements.
3. The method according to claim 1, wherein gaseous nitrogen
fluoride is contacted with said reagent at a temperature of not
lower than 200.degree. C. to decompose the nitrogen fluoride.
4. The method according to claim 3, wherein the temperature of said
contact between said nitrogen and said reactant is selected to be
up to 450.degree. C. to prevent by-production of carbon
fluoride.
5. The method according to claim 3, wherein the temperature of said
contact between said nitrogen fluoride and said reagent is selected
to be not lower than 700.degree. C. to prevent by-production of
carbon fluoride.
6. The method according to claim 3, wherein said reagent contains
an alkali metal element, and the temperature of said contact
between the nitrogen fluoride and said reagent is selected to be
not less than 350.degree. C. to prevent by-production of nitrogen
oxide.
7. The method according to claim 1, wherein gaseous sulfur fluoride
is contacted with said reagent at a temperature of not lower than
300.degree. C. to decompose the sulfur fluoride.
8. The method according to claim 7, wherein the temperature of said
contact between the sulfur fluoride and said reagent is selected to
be not less than 450.degree. C. to prevent by-production of sulfur
oxide.
9. The method according to claim 7, wherein said reagent further
comprises an alkali metal element, and the temperature of said
contact between the sulfur fluoride and said reagent is selected to
be not lower than 350.degree. C. to prevent by-production of sulfur
oxide.
10. The method according to claim 2, wherein the content proportion
in terms of the atomic ratio of said alkaline earth metal element
to said elemental carbon to said alkali metal element is in a range
of 1:0.25-4.0:0-0.3.
11. The method according to claim 1, wherein said alkaline earth
metal element comprises calcium or magnesium and is incorporated in
said reagent in the form of an oxide, hydroxide and/or carbonate
thereof.
12. The method according to claim 2, wherein said alkali metal
element comprises potassium and is incorporated in said reagent in
the form of a hydroxide, carbonate, phosphate, aluminate, nitrate
and/or sulfate thereof.
13. The method according to claim 1, wherein said elemental carbon
is incorporated in said reagent in the form of charcoal, activated
carbon, carbon black or coke powder.
14. A reagent for decomposing nitrogen fluoride, comprising a solid
material containing an alkaline earth metal element, elemental
carbon and optionally an alkali metal element, wherein the content
proportion in terms of the atomic ratio of said alkaline earth
metal element to said elemental carbon to said alkali metal element
is 1.0:0.25-4.0:0-0.3.
15. The reagent for decomposing nitrogen fluoride according to
claim 14, wherein said alkaline earth metal element comprises
calcium or magnesium and is incorporated in said solid material in
the form of an oxide, hydroxide or carbonate thereof.
16. The reagent for decomposing nitrogen fluoride according to
claim 14, wherein said alkali metal element comprises potassium and
is incorporated in said solid material in the form of a hydroxide,
carbonate, phosphate, aluminate, nitrate or sulfate thereof.
17. The reagent for decomposing nitrogen fluoride according to
claim 14, wherein said elemental carbon is incorporated in said
solid material in the form of charcoal, activated carbon, carbon
black or coke powder.
18. The reagent for decomposing sulfur fluoride, comprising a solid
material containing elemental carbon, an alkaline earth metal
element and optionally an alkali metal element, wherein the content
proportion in terms of the atomic ratio of said alkaline earth
metal element to said elemental carbon to said alkali metal element
is 1.0:0.25-4.0:0-0.3.
19. The reagent for decomposing sulfur fluoride according to claim
18, wherein said alkaline earth metal element comprises calcium or
magnesium and is incorporated in said solid material in the form of
an oxide, hydroxide or carbonate thereof.
20. The reagent for decomposing sulfur fluoride according to claim
18, wherein said alkali metal element comprises potassium and is
incorporated in said solid material in the form of a hydroxide,
carbonate, phosphate, aluminate, nitrate or sulfate thereof.
21. The reagent for decomposing sulfur fluoride according to any
one of claims 18, wherein said elemental carbon is incorporated in
said solid material in the form of charcoal, activated carbon,
carbon black or coke powder.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for decomposing
nitrogen fluoride or sulfur fluoride and a decomposing reagent used
therefor.
[0003] In the present specification, the nitrogen fluoride
designates a compound which comprises fluorine and nitrogen as
essential constituent elements and which easily volatilizes. A
typical example of nitrogen fluoride is nitrogen trifluoride
(NF.sub.3). Sulfur fluoride designates a compound which comprises
fluorine and sulfur as essential constituent elements and which
easily volatilizes. A typical example of sulfur fluoride is sulfur
hexafluoride (SF.sub.6). "A compound which easily volatilizes"
herein denotes a compound which is a gas at room temperature and
normal pressure, or a compound which is a liquid at room
temperature but which forms a gas mixture containing at least 0.01%
by volume of the compound as a vapor it an inert gas is
co-present.
[0004] 2. Description of the Related Art
[0005] Since nitrogen fluoride and sulfur fluoride defined above
are thermally stable, they are used, for example, as gases for
etching or cleaning in processes for manufacturing semiconductor
devices. However, nitrogen fluoride and sulfur fluoride are
substances which are suspected to have an influence on global
warming because they have a large global warming potential (GWP)
value and remain in the atmosphere without decomposition when
released in the air, and it is said that they are preferably
decomposed after use. Accordingly, decomposition of used nitrogen
fluoride and sulfur fluoride into nontoxic substances is
required.
[0006] A combustion decomposition method, a reagent decomposition
method, a catalytic decomposition method, and the like have
heretofore been proposed as technologies for decomposing nitrogen
fluoride.
[0007] Decomposition of nitrogen fluoride by the combustion method
unavoidably forms by-product NO.sub.x. The combustion method,
therefore, requires removal of the by-product and an additional
treatment of the fluorine component. Accordingly, the combustion
method is not efficient from the standpoint of recovering the
fluorine component. The proposed reagent decomposition methods and
catalytic decomposition methods require special treating conditions
to increase the decomposition efficiency, and special treatment is
required for recovery of the fluorine component formed by the
decomposition. Accordingly, the decomposition operation in situ
where nitrogen fluoride is used (generation source of a nitrogen
fluoride gas) cannot be simply conducted.
[0008] Furthermore, no method for efficiently and completely
decomposing sulfur fluoride such as sulfur hexafluoride (SF.sub.6)
into nontoxic substances has ever been known.
[0009] An object of the present invention is, therefore, to provide
a method for decomposing nitrogen fluoride or sulfur fluoride,
which allows decomposition of nitrogen fluoride or sulfur fluoride
at a high efficiency by a simple operation and efficient recovery
of decomposed fluorine, and to provide a decomposing reagent
therefor.
SUMMARY OF THE INVENTION
[0010] The above object is solved in accordance with the present
invention by providing a method for decomposing nitrogen fluoride
or sulfur fluoride comprising contacting at least one of nitrogen
fluoride and sulfur fluoride gases with a solid reagent containing
elemental carbon and one or more of the alkaline earth metal
elements.
[0011] More specifically, nitrogen fluoride can be decomposed by
contacting a nitrogen fluoride gas with the reagent as mentioned
above at a temperature of 200.degree. C. or more. In the
decomposition, formation of by-product carbon fluoride can be
inhibited by contacting the nitrogen fluoride gas with the reagent
at a temperature of 200 to 450.degree. C. or at a temperature of
not less than 700.degree. C. Moreover, formation of by-product
nitrogen oxide can be inhibited by contacting the nitrogen fluoride
gas with the reagent at a temperature of 350.degree. C. or
more.
[0012] Furthermore, when the reagent mentioned above further
contains an alkali metal element, a nitrogen fluoride gas can be
similarly decomposed at a contact temperature of 200.degree. C. or
more. Moreover, formation of by-product carbon fluoride can be
inhibited by contacting the nitrogen fluoride gas with the reagent
at a temperature of 200 to 450.degree. C. or at a temperature of
not less than 700.degree. C. Furthermore, formation of by-product
nitrogen oxide can be inhibited at the contact temperature of
250.degree. C. or more, lower than the lower limit of the contact
temperature in the above case where the reagent does not contain an
alkali metal element.
[0013] Similarly, sulfur fluoride can be decomposed by contacting a
sulfur fluoride gas with a solid reagent containing elemental
carbon and an alkaline earth metal element at a contact temperature
of 300.degree. C. or more. In the decomposition, formation of
by-product sulfur oxide such as SO.sub.2 can be inhibited by
contacting the sulfur fluoride gas with the reagent at a
temperature of 450.degree. C. or more.
[0014] Furthermore, a sulfur fluoride gas can be similarly
decomposed at a contact temperature of 300.degree. C. or more when
the reagent further contains an alkali metal element. In addition,
in this case, formation of by-product sulfur oxide can be inhibited
at a temperature lower than in the case where the reagent does not
contain an alkali metal element. More concretely, formation of
by-product SO.sub.2 can be inhibited from a temperature of
350.degree. C. or more.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic diagram of an apparatus arrangement
showing an embodiment of a system for carrying out the method of
the present invention.
[0016] FIG. 2 is a schematic diagram of an apparatus arrangement
showing another embodiment of a portion for introducing a gas to be
treated for carrying out the method of the present invention.
[0017] FIG. 3 is a schematic cross-sectional view of a reaction
vessel portion showing an embodiment of heating the reagent from
the interior of the reaction vessel according to the method of the
present invention.
[0018] FIG. 4 is a schematic cross-sectional view of a reaction
vessel portion showing another embodiment of heating the reagent
from the interior of the reaction vessel according to the method of
the present invention.
[0019] FIG. 5 is a diagram showing one embodiment of conducting a
heat exchange between a gas to be treated before entering a
reaction vessel and an exhaust gas discharged from the reaction
vessel, in carrying out the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0020] The method for decomposing nitrogen fluoride or sulfur
fluoride according to the present invention is characterized by
using a solid reagent for decomposition comprising elemental
carbon, one or more of the alkaline earth metal elements, and
optionally one or more of the alkali metal elements. This reagent
for decomposition is a solid material comprising elemental carbon,
an alkaline earth metal element and optionally an alkali metal
element, in which the proportional content in terms of the atomic
ratio of the alkaline earth metal element to elemental carbon to
the alkali metal element preferably be 1.0:0.25-4.0:0-0.3. The
proportion is represented in terms of the atomic ratio because the
proportional content of the metal element components in the reagent
are important when the alkaline earth metal element or the alkali
metal element is contained therein in the form of various compounds
such as oxides and carbonates.
[0021] When such a reagent is contacted with a nitrogen fluoride
gas at an appropriate temperature, the nitrogen fluoride is
decomposed. Fluorine formed by decomposition is fixed to the
reagent, and nitrogen formed by the decomposition can be inhibited
from forming NO.sub.x (nitrogen oxide such as N.sub.2O, NO and
NO.sub.2). That is, in accordance with the present invention,
nitrogen fluoride can be efficiently decomposed without generating
toxic by-product gases such as fluorine gas, carbon fluoride gas
and NO.sub.x. Similarly, when a sulfur fluoride gas is contacted
with such a reagent as mentioned above at an appropriate
temperature, the sulfur fluoride is decomposed. Fluorine formed by
decomposition is fixed to the reagent, and generation of sulfur
oxide by the decomposition can be inhibited. That is, in accordance
with the present invention, sulfur fluoride can be efficiently
decomposed without generating toxic by-products such as fluorine
gas, carbon fluoride gas and SO.sub.x. In order not to generate
such by-products in the decomposed gases, the reaction conditions
such as the reaction temperature, the concentration of sulfur
fluoride and presence or absence of other components such as oxygen
in the starting gases to be decomposed, the form and the component
proportion of the reagent, and the gas feed rate should be
appropriately adjusted. The most important condition is the
reaction temperature, as will be shown in the following
Examples.
[0022] That is, it has been found that when nitrogen fluoride is
contacted with the reagent at a temperature of 200 to 450.degree.
C. or of 700.degree. C. or more (actually the temperature of the
reagent), the fluorine component in nitrogen fluoride can be
completely fixed to the reagent while generation of fluorine gas is
inhibited, and formation of carbon fluoride caused by the reaction
of the fluorine component with carbon in the reagent is inhibited.
When the contact temperature is higher than 450.degree. C. and less
than 700.degree. C., carbon fluoride such an CF.sub.4 may be formed
in a slight amount. The formed carbon fluoride gas is not
decomposed by the reagent in this temperature range. However, it is
considered that in the temperature range of 700.degree. C. or more,
carbon fluoride, even if it is temporarily formed, is decomposed by
the reagent, and fluorine formed by the decomposition is fixed to
the reagent. At a temperature of 450.degree. C. or less, it is
considered that carbon fluoride is not formed at all.
[0023] The discharged gas sometimes contains CO. In such a case, it
is satisfactory that the discharged gas be released outside the
system after oxidation treatment thereof. Moreover, in a low
reaction temperature range, NO.sub.x may be formed. For example,
nitrogen oxide is sometimes formed in a slight amount at a
temperature lower than 350.degree. C. by the reagent comprising
elemental carbon and an alkaline earth metal element, at a
temperature less than 250.degree. C. by the reagent comprising
elemental carbon, an alkaline earth metal element and an alkali
metal element. Accordingly, formation of nitrogen oxide can be
inhibited by employing the reaction temperature of 350.degree. C.
or more for the former reagent, and 250.degree. C. or more for the
latter reagent.
[0024] Furthermore, it has been found that fluorine in sulfur
fluoride is completely fixed to the reagent and the fluorine
component is not involved in the discharged gas if the reaction
temperature is held at 300.degree. C. or more. It has also been
found that a reaction of carbon in the reagent with the fluorine to
form carbon fluoride is also inhibited. Furthermore, it has also
been found that when the reaction temperature is held at
450.degree. C. or more, a reaction of sulfur with oxygen in the gas
to be treated or in the reagent to form sulfur oxide is also
inhibited. However, as will be shown in the following Examples,
sulfur oxide is sometimes formed to some extent in a low reaction
temperature range, depending on the reaction conditions.
[0025] The discharged gas contains CO sometimes. When the
discharged gas contains CO, the discharged gas may be released
outside the system after oxidation treatment. In the cases where
the reaction temperature is low as described above, SO.sub.2 may be
formed sometimes, for example, at a temperature of less than
450.degree. C. when the reagent does not contain an alkali metal
element, and at a temperature of less than 350.degree. C. when the
reagent contains an alkali metal element, but in such a case, the
discharged gas may be released outside the system after
desulfurization treatment. Moreover, it was considered that
elemental carbon in the reagent may react with fluorine to form
carbon fluoride, but it has been found that carbon fluoride is not
substantially formed under the conditions shown in the following
Examples.
[0026] The elemental carbon in the reagent is considered to
contribute to the decomposition of nitrogen fluoride and sulfur
fluoride in the presence of an alkaline earth metal element.
Although the content of the elemental carbon in the reagent may be
varied as the decomposition reaction proceeds, it is preferred that
the reagent contains the same in an atomic ratio of elemental
carbon to the alkaline earth metal element of at least 0.25:1.0 at
least in the initial stage of the decomposition reaction. If the
ratio is less than 0.25, decomposition of nitrogen fluoride or
sulfur fluoride sometimes does not proceed sufficiently. However,
if the reagent contains the elemental carbon in such an amount that
the ratio exceeds 4.0, the alkaline earth metal element content is
reduced in accordance with the ratio, and the decomposition
reaction may not be effected sufficiently. Accordingly, the atomic
ratio of the elemental carbon to alkaline earth metal element in
the reagent may be 0.25-4.0:1.0, preferably 0.5-3.0:1.0, more
preferably 1.0-2.0:1.0. Moreover, the amount of the elemental
carbon in the reagent is desirably maintained from the initial
stage to the midpoint of the decomposition reaction. The elemental
carbon herein denotes solid carbon present in uncompounded form,
and carbon atoms constituting a specific compound are not included
in the elemental carbon. An example of the carbon in a specific
compound is carbon in a carbonate. The carbon in the reagent
denoted in the present specification means elemental carbon, unless
otherwise noted.
[0027] The elemental carbon can be incorporated in the reagent in
the form of charcoal, activated carbon, carbon black or coke
powder. The elemental carbon may also be in the form of carbon
fibers, graphite or materials containing inorganic carbonaceous
material as the principal component. When the reagent is a pellet,
such a carbonaceous material in powder in combination with other
raw materials (an alkaline earth metal compound and optionally an
alkali metal compound) may be pelletized. Moreover, in order to
obtain a reagent of a fired product, a powder material of such a
carbonaceous material mixed with other raw materials (an alkaline
earth metal compound, or an alkaline earth metal compound and an
alkali metal compound) may be fired.
[0028] The particle size of the elemental carbon is not
particularly limited but is preferably in a range of 1 to 5 mm,
particularly in a range of 2 to 3 mm. If the particle size is too
large, the efficiency of contact with a gas is low, which is not
preferred.
[0029] The alkaline earth metal element in the reagent acts to
decompose nitrogen fluoride and sulfur fluoride, in the presence of
the elemental carbon. For example, when the alkaline earth metal
element, nitrogen fluoride and sulfur fluoride to be used for
decomposition are Ca, NF.sub.3 and SF.sub.6, respectively, examples
of the reactions are as follows:
[0030] 6CaO+4NF.sub.3+3C.fwdarw.6CaF.sub.2+2N.sub.2+3CO.sub.2
[0031] 6CaO+4NF.sub.3.fwdarw.6CaF.sub.2+2N.sub.2+3O.sub.2
[0032] 4CaO+SF.sub.6.fwdarw.3CaF.sub.2+CaSO.sub.4
[0033] As a result, Ca plays the role of fixing fluorine in
nitrogen fluoride or sulfur fluoride in the form of CaF.sub.2.
[0034] The alkaline earth metal element is thus a fundamental
component of the reagent in the present invention. Therefore, in
the present specification, the relative proportions of the
elemental carbon and the alkali metal element are defined based on
the content of the alkaline earth metal element. Accordingly, the
actual amount of the alkaline earth metal element in the reagent is
determined in relation to the content of the elemental carbon and
further that of the alkali metal element. If the relative amount of
the alkaline earth element is too small, the relative ratio of the
amount of the alkaline earth metal element, contributing to the
decomposition reaction, to the amount of carbon lowers as the
reaction proceeds, and it becomes impossible to obtain a high
decomposition ratio. Conversely, if the relative amount of the
alkaline earth metal element is excessive, the relative ratio of
the amount thereof to the amount of carbon becomes too high, and it
also becomes impossible to obtain a high decomposition
efficiency.
[0035] The alkaline earth metal element can be Be, Mg, Ca, Sr, Ba
and Ra and may be contained in the reagent in the form of an oxide,
hydroxide or salt such as carbonate of these elements. Of these
alkaline earth metal elements, Ca and Mg are preferred and can be
easily treated since the starting materials and the decomposition
reaction products thereof are nontoxic. A raw material in an
oxygen-containing form, such as an oxide, a hydroxide or a
carbonate of Ca or Mg is stable, and can be treated easily.
Concrete examples of such starting materials include quick lime,
slaked lime, marble, magnesium carbonate and dolomite. Of these
compounds, compounds in the form of oxides are particularly
preferred to be contained in the reagent.
[0036] When Ca or Mg are contained in the reagent in the form of an
oxygen-containing compound as explained above, nitrogen fluoride or
sulfur fluoride can be decomposed more advantageously, because it
is supposed that the reagent becomes more active in the
decomposition reaction of nitrogen fluoride or sulfur fluoride when
the oxygen in the oxygen-containing compound reacts with carbon
fluoride or substitutes for fluorine in nitrogen fluoride or sulfur
fluoride, or the oxygen in a gaseous phase reacts with carbon in
the reagent.
[0037] When an alkali metal element is present in the reactants in
addition to the elemental carbon and the alkaline earth metal
element, nitrogen fluoride and sulfur fluoride can be decomposed at
a high decomposition ratio even at a lower decomposition
temperature compared with the decomposition using the similar
reagent containing no alkali metal element. Moreover, the reaction
temperature range where nitrogen oxide and sulfur oxide are not
formed can be shifted to the lower temperature side, and the
temperature range where carbon fluorides are not formed can also be
shifted to the lower temperature side. Although the alkali metal
element includes Li, Na, K, Rb, Cs, etc., K has been found to show
such effects to a significant extent. The content ratio in terms of
atomic ratio of the alkali metal element to the alkaline earth
metal element in the reagent may be 0-0.3:1.0. When the reagent
contains the alkali metal element in such an amount that the ratio
exceeds 0.3, the effects of the alkali metal element are saturated,
and the contents of the elemental carbon and the alkaline earth
metal element are relatively lower, which is not preferred.
[0038] The alkali metal element, for example, K, may be contained
in the reagent by incorporating the same in the form of a compound
such as a hydroxide, a carbonate, a phosphate, an aluminate, a
nitrate or a sulfate. These compounds may be incorporated singly or
in combination, and as powder if the compound is in powder form, or
after grinding to a particle size of up to 100 .mu.m if it is in
bulk form. These compounds may also be added in the form of an
aqueous solution.
[0039] It is desirable that the total amount of the elemental
carbon, the alkaline earth metal element and optionally the alkali
metal element be 50% by weight or more of the entire reagent.
Although nitrogen fluoride or sulfur fluoride can be decomposed
even if the total amount of these substances is less than 50% by
weight, a total amount of 50% by weight or more is preferred to
decompose the fluorides efficiently. The remaining components may
be the components of compounds containing the alkaline earth
elements and or other compounds and it is desired that the majority
of the remaining components be oxygen, but the remaining components
may include impurities such as moisture and CO.sub.2 accompanying
the raw materials.
[0040] As described above, the reagent for nitrogen fluoride or
sulfur fluoride of the present invention is a solid material which
contains elemental carbon and an alkaline earth metal element as
essential components, and which may also contain an alkali metal
element as an optional component. The solid reagent is preferably
pelletized because the pellets can have increased contact with
nitrogen fluoride or sulfur fluoride gases to be decomposed. In
order to pelletize the solid reagent, the aforementioned starting
materials in powder form re mixed, pelletized with a suitable
amount of water and if necessary an appropriate binder, and dried
to evaporate the water and obtain the the pellets.
[0041] Fired pellets are the most preferred pelletized material.
That is, the most preferred pelletized material is a fired material
obtained by mixing a carbonaceous material powder, an alkaline
earth metal compound powder and optionally an alkali metal compound
powder, and firing the powder mixture, or by mixing a carbonaceous
material powder and an alkaline earth metal compound powder, firing
the powder mixture, coating or impregnating the fired material with
a solution of an alkali metal compound, and drying the coated or
impregnated material to remove the volatile component (solvent).
The materials for the starting materials for obtaining the fired
material can comprise the carbonaceous material, the alkaline earth
metal compound and the alkali metal compound which are described
above.
[0042] Typical examples of the fired material include a material
obtained by firing a kneaded material containing a carbonaceous
material powder and slaked lime (and optionally a potassium
compound) under conditions sufficient to induce a reverse slaking
reaction of slaked lime while carbon remains, or a material
obtained by firing a kneaded material containing a carbonaceous
material powder and calcium carbonate (and optionally a potassium
compound) under conditions sufficient to induce a decomposition
reaction of calcium carbonate while carbon remains. In the
incorporation of the potassium compound into the kneaded material,
the compound can be in a powder state or in an aqueous
solution.
[0043] The kneaded material preferably comprises a pelletized
material having a particle size of up to 100 .mu.m, in which the
raw materials are homogeneously mixed. The production of the
pelletized material comprises weighing the mixed raw materials, and
adding a suitable amount of water for the kneading. The addition of
water can be replaced with an aqueous solution containing an alkali
metal element. A kneader which can conduct mixing and pelletization
simultaneously is suitable, but mixing and pelletization may be
separately conducted. For example, a Henschel mixer or vertical
mixer can conduct mixing and pelletization simultaneously. However,
only mixing of the raw materials may be conducted by a Henschel
mixer or V-type mixer, followed by conducting pelletization with a
dish type granulator or drum pelletizer.
[0044] In the mixing and pelletization, a suitable amount of a
binder may be added in addition to water or an aqueous solution. An
organic binder such as polyvinyl alcohol (PVA) can be used as the
binder. Moreover, an inorganic binder such as aluminum oxide-based
or silicon oxide-based ones can also be used. However, the amount
thereof must be restricted such that the performance of the reagent
for decomposition thus obtained is not influenced.
[0045] The kneaded and pelletized particles are preferably fired
under such conditions that the moisture and volatile components are
removed and the fired material has a suitable strength. It is
preferred that heat treatment for firing be conducted in an inert
atmosphere for the purpose of preventing carbon in the mixed raw
materials from being oxidized and consumed. If the firing is
conducted in an inert atmosphere, a heat treatment at a high
temperature is possible, and a fired material can have a high
strength. Although a continuous system such as a rotary kiln can be
used an apparatus for firing, a fixed furnace can also be used.
[0046] The reagent for decomposition according to the present
invention preferably has a low water content and generates no water
during the decomposition reaction. A reagent which releases water
in an amount of not more than 15% by weight when heated at
800.degree. C. in an inert atmosphere is preferred.
[0047] Next, a method and a system for decomposing nitrogen
fluoride or sulfur fluoride using the reagent as described above
will be described.
[0048] The decomposition treatment of nitrogen fluoride or sulfur
fluoride according to the present invention can be carried out by
feeding a nitrogen fluoride-containing gas or a sulfur
fluoride-containing gas to be treated to a reaction vessel charged
with the reagent. In the decomposition treatment, the temperature
of the reactants, namely the reaction temperature, is held at
200.degree. C. or more for nitrogen fluoride, and at 300.degree. C.
or more for sulfur fluoride and the reaction atmosphere may be a
nonoxidizing atmosphere or weakly oxidizing atmosphere. Since the
reagent containing an alkali metal element has a higher activity
compared with those containing no alkali metal element, the
decomposition may sometimes be adequately achieved even in a
nonoxidizing atmosphere but can also be carried out in a weakly
oxidizing atmosphere, for example, in an atmosphere of the gas to
be treated containing from 0.01 to 25% by volume of oxygen. There
is no specific limitation on the concentration of nitrogen fluoride
or sulfur fluoride in the gas to be treated, and even a gas
containing 100% of nitrogen fluoride or sulfur fluoride can be
decomposed. However, the gas to be treated may also be diluted with
an inert gas, and further with an oxygen-containing gas. Preferred
decomposition temperatures are determined in accordance with the
concentration of nitrogen fluoride or sulfur fluoride in a gas to
be treated for decomposition, the concentration of an
oxygen-containing gas in the gas to be treated, the SV (superficial
velocity), the LV (linear velocity), the state of mixing of the gas
with other gases, the component ratio and the form of the reagent
of the present invention, and moreover the degree of inhibition of
formation of by-products such as nitrogen oxide, sulfur oxide and
carbon fluorides.
[0049] The decomposition treatment can be carried out with a
decomposition system for nitrogen fluoride or sulfur fluorides,
comprising a reaction vessel charged with the reagent, an inlet for
a gas to be treated communicating to the reaction vessel, a gas
discharge outlet provided so that the gas after the reaction is
discharged from the reaction vessel, a furnace for accommodating
the reaction vessel, a heating source for elevating the temperature
of the atmosphere within the furnace to 200.degree. C. or more, a
pipe connecting the inlet for a gas to be treated and a nitrogen
fluoride-containing gas source or sulfur fluoride-containing gas
source, and optionally an exhaust gas oxidizer communicating
through a pipe to the gas discharge outlet.
[0050] FIG. 1 shows one embodiment of a system for carrying out the
method of the present invention. In the figure, the reference
numeral 1 designates a reaction vessel (tube) made of a metal which
is charged with a reagent 2 composed of the reagent as described
above. In the figure, a tubular reaction vessel 1 is vertically
arranged and the reagent 2 is placed on a through-flow bed 3 fixed
within the vessel. The metal pipe of the reaction vessel 1 can be a
pipe of a stainless steel or a nickel-based alloy.
[0051] The reaction vessel 1 is installed within a heating furnace
4. The heating furnace 4 shown in the figure has an electrical
heater 5 having a heating element which generates heat when a
current is applied to it, as a heating source. The in-furnace
atmosphere 6 is heated with the electrical heater 5 to a
predetermined temperature, and the heat within the furnace is
transferred to the reagent 2 through the wall of the metal-made
reaction vessel. The heating source is not limited to an electrical
heater so long as the temperature of the in-furnace atmosphere 6
can be raised to the predetermined temperature. For example, a high
temperature gas such as a combustion exhaust gas can also be used
as a heating source.
[0052] An inlet 7 for a gas to be treated is provided for the
reaction vessel 1 installed within the heating furnace 4. The inlet
7 for a gas to be treated is connected to a container 8 containing
nitrogen fluoride or sulfur fluoride through a pipe. The container
8 can be indirectly heated by heating means 9 if necessary, and the
gas pressure of nitrogen fluoride or sulfur fluoride within the
container 8 is increased by the heating. A gas discharge pipe 10
from the container 8 is provided with a flow rate control valve 11.
In the embodiment in FIG. 1, an oxygen gas bomb 12 and a nitrogen
gas bomb 13 are separately placed in addition to the container 8.
An oxygen gas and a nitrogen gas are once introduced into a gas
header 18 from the respective bombs through gas discharge pipes 16,
17 provided with flow rate control valves 14, 15, respectively, and
nitrogen fluoride or sulfur fluoride is introduced into the header
18, so that the nitrogen gas is mixed as a carrier with gaseous
nitrogen fluoride or gaseous sulfur fluoride and the oxygen gas can
be optionally added. Consequently, the gas to be treated, mixed in
the header 18, is fed to the inlet 7 for a gas to be treated of the
reaction vessel 1 through a gas feed pipe 19.
[0053] In addition, the following procedures may also be conducted
in place of the above embodiment. A gas mixture prepared in advance
by mixing nitrogen fluoride or sulfur fluoride, nitrogen and oxygen
may be provided in one container, and the gas mixture directly fed
to the inlet 7 for a gas to be treated. Alternatively, a nitrogen
gas is fed to the container 8 for nitrogen fluoride or sulfur
fluoride so that nitrogen fluoride or sulfur fluoride is forcibly
expelled from the container, and an oxygen gas may be added to the
discharge pipe path. In any case, an oxygen gas feed pipe is
preferably connected to the container 8 itself or a pipe from the
container 8 to the inlet 7 for a gas to be treated.
[0054] On the other hand, an exhaust gas pipe 21 is connected to a
gas discharge outlet 20 of the reaction vessel 1, and the exhaust
gas pipe 21 is connected to a halogen-absorbing bottle 22 to which
a gas discharge pipe 23 is attached. Moreover, a sampling pipe 24
is attached to the exhaust gas pipe 21, and an exhaust gas sampled
with the sampling pipe 24 is fed to a gas analyzer 25.
[0055] A branch pipe 26 is provided to the exhaust gas pipe 21, and
pipes are arranged from the branch pipe 26 to a NO.sub.x decomposer
or desulfurizer 27 and to an oxidizer 28. A returning pipe 29 is
optionally provided so that the gas having passed through said
devices is returned to the exhaust gas pipe 21. That is, if the
exhaust gas is accompanied by NO.sub.x or SO.sub.x, NO.sub.x or
SO.sub.x is decomposed in the NO.sub.x decomposer or desulfurizer
27 charged with a conventional NO.sub.x or SO.sub.x decomposition
catalyst. If the exhaust gas is accompanied by CO, CO is oxidized
to form CO.sub.2 by the oxidizer 28 charged with a noble metal
catalyst such as platinum or palladium, or a hopcalite catalyst.
The gas thus treated is returned to the exhaust gas pipe 21.
[0056] In the system in FIG. 1, the temperature of the atmosphere
within the heating furnace 4 heats the reagent 2 in the reaction
vessel 1 through the vessel wall, and the temperature changes
depending on the heat of reaction and the balance between the heat
capacity of introduced gas and that of the exhaust gas. As shown in
FIG. 1, the temperature of the reaction zone is detected by a
temperature sensor (thermocouple) 31 which is inserted
approximately in the center of the reagent 2 and is connected to a
thermometric device 32. The quantity of heat supplied from the
heating source 5 is controlled so that the detected temperature is
held at a given temperature. Moreover, the temperature of the
in-furnace atmosphere 6 in the heating furnace 4 is also detected
by a temperature sensor 33, and the temperature of the heating
furnace itself is suitably controlled based on the detected
value.
[0057] As explained above, nitrogen fluoride and sulfur fluoride in
the gas to be treated are almost completely decomposed (at a
decomposition ratio close to 100%), and the fluorine thus
decomposed reacts with the alkaline earth metal element in the
reagent to form an alkaline earth metal fluoride. As a result,
nitrogen fluoride or sulfur fluoride and fluorine are substantially
absent from the exhaust gas. Moreover, when the exhaust gas
accompanies NO.sub.x or SO.sub.x and CO, such gases can be treated
to become nontoxic by the NO.sub.x decomposer or desulfurizer 27
and the oxidizer 28.
[0058] Furthermore, if the exhaust gas includes carbon fluorides,
the carbon fluorides can be decomposed and fluorine in carbon
fluorides can be fixed to the reagent by recirculating the carbon
fluoride-containing gas in another system which is substantially
the same as that in FIG. 1 and is placed separately, or in the same
system as in FIG. 1.
[0059] FIG. 2 shows an embodiment in which spent nitrogen fluoride
or sulfur fluoride used in the process of manufacturing
semiconductors is decomposed by the present invention. Spent
nitrogen fluoride or sulfur fluoride 37 discharged from the process
of manufacturing semiconductors is generally fed to a routine
processing step 36 via a pipe 38. In the application of the present
invention, the feed pipe 38 of nitrogen fluoride or sulfur fluoride
is connected to the inlet 7 for a gas to be treated of the reaction
vessel 1. In the embodiment shown in the figure, a branch pipe 40
is attached to the feed pipe 38 through a three way valve 39, and
the branch pipe 40 is connected to the inlet 7 for a gas to be
treated. A nitrogen gas feed pipe 41 is connected to the branch
pipe 40, and a nitrogen gas can be fed to the branch pipe 40 with a
variable flow rate under a pressure from a nitrogen gas source 42.
Consequently, even if the raw material gas is difficult to flow
into the side of the branch pipe 40 through the three way valve,
the raw material gas can be transported to the inlet 7 for a gas to
be treated at a substantially constant flow rate by sending the
necessary amount of nitrogen gas from the nitrogen gas source
42.
[0060] FIG. 3 and FIG. 4 show embodiments of the present invention,
in each of which, a heating source is provided in the interior of
the reaction vessel 1, and heat is transferred to the reagent 2
from the interior of the vessel. In both figures, the reference
numerals 44 denote a heat-resistant furnace material surrounding
the reaction vessel 1, 7 denotes the inlet for a gas to be treated
of the reaction vessel, and 20 denotes the gas discharge outlet
from the reaction vessel.
[0061] In FIG. 3, a heating element 43 which generates heat by
application of current is arranged in the interior of a packed bed
of the reagent 2, and the heating element 43 is covered with a
corrosion-resistant, heat-resistant cover. According to the present
embodiment, since heat is transferred from the interior of the
packed bed of the reagent 2, the rate of heating the reagent to a
desired temperature can increase, and the heat loss can be
reduced.
[0062] In FIG. 4, the interior of the reaction vessel 1 is
separated into a packed bed of the reactants 2 and a heating layer.
A gas to be treated is introduced into the reaction vessel and
passed through the heating layer and allowed to flow into the
packed bed of the reactants. Heating elements 46 which generate
heat in the heating layer by application of current are attached to
a vessel cover 45. Heat is imparted to the gas to be treated when
the gas passes through the heating layer, and heat is also
transferred to the reagent 2 at the same time. Since electrical
heaters are placed within the reaction vessel in the present
embodiment, the system has the following advantages: the
utilization efficiency of heat becomes high and the heating
elements 46 are less deteriorated because they do not contact the
reagent or the gas subsequent to the reaction.
[0063] FIG. 5 shows an embodiment of the present invention wherein
a heat exchanger 48 for exchanging heat between a gas to be treated
prior to introduction to the reaction vessel 1 having a heating
source and an exhaust gas discharged from the reaction vessel 1 is
arranged. By arranging the heat exchanger 48, the sensible heat of
the exhaust gas is imparted to the gas to be treated so that the
heat can be recovered. The heat consumption of the heating source
can, therefore, be lowered.
[0064] In the system of the present invention as described above,
the decomposition reaction finishes when the charged reagent is
entirely consumed. The reaction end point occurs when nitrogen
fluoride or sulfur fluoride are first detected in the exhaust gas
or when a minor amount of NO.sub.x or SO.sub.x is detected. When
the reaction finishes, the operation of the system may be stopped,
and the reagent newly charged, followed by starting the reaction,
that is, the system can be operated in a batch process in which
nitrogen fluorides or sulfur fluoride can be successively
decomposed by the same system. In order to make the batch process
continuous, a double reaction vessel change-over system wherein two
similar systems are installed in parallel can be employed; one
system is operated while the reagent of the other system is renewed
with a fresh reagent; when the operating system is stopped, the gas
flow paths are changed from the stopped system to the other system.
Moreover, the same system can be continuously used over a long
period of time if in the system, a fresh reagent can be
continuously or intermittently supplied to the reaction vessel and
the spent reagent can be continuously or intermittently discharged
therefrom.
[0065] In accordance with the present invention, nitrogen fluoride
or sulfur fluoride can be efficiently decomposed at a relatively
low temperature by a simple process and the fluorine obtained by
the decomposition can be fixed to the reagent as a nontoxic
substance. That is, the method for decomposing nitrogen fluoride
and sulfur fluoride of the present invention, can be carried out
with a simple decomposition system, the decomposition operation is
simple, the decomposition efficiency is high, and the decomposition
products are stabilized fluorides such as CaF.sub.2 and can be
easily subjected to after-treatment. Furthermore, low cost of the
reagent provides novel effects. In particular, the method of the
present invention can greatly contribute to the decomposition of
spent nitrogen fluoride generated in the production process of
semiconductors.
[0066] The above descriptions have been principally directed to
separate decomposition of nitrogen fluoride and sulfur fluoride.
However, it is clear that the present invention can also be applied
to the decomposition of a mixture of nitrogen fluoride or sulfur
fluoride with other fluorides, particularly to a mixture of
nitrogen fluoride and sulfur fluoride.
EXAMPLES
Example 1
[0067] The method of the present invention was carried out using a
system in accordance with the same principle as that in FIG. 1
(however the NO.sub.x decomposer and the oxidizer were excluded).
That is, a tubular furnace equipped with a heating element (kanthal
alloy) which generated heat on application of an electric current
(electric capacity of 0.4 kW) was penetrated by a reaction tube
having an inside diameter of 16 mm and a length of 300 mm and
composed of Inconel 600 along the central axis of the furnace. A
reagent for decomposing nitrogen fluoride in an amount of 35 ml was
charged in the central portion of the reaction tube.
[0068] The reagent was in the form of pellets prepared from
charcoal, slaked lime and potassium hydroxide as raw materials, and
was prepared in the following manner.
[0069] Charcoal having a particle size of up to 250 .mu.m, slaked
lime having a particle size of up to 250 .mu.m and potassium
hydroxide (cases where potassium hydroxide was not added are also
included) with varied mixing ratios were mixed in a Henschel mixer,
and water was added to the mixture, followed by pelletizing the
mixture. The resultant pellets were dried at 110.degree. C. for 4
hours, and heat treated in a nitrogen atmosphere at 800.degree. C.
for 8 hours to dehydrate and fire the same. The fired material thus
obtained was screened to give pellets having a particle size of up
to 10 mm and an average particle size of about 3 mm.
[0070] The charcoal used as a raw material contained 78% of fixed
carbon, 9% of a volatile constituent, 3% ash and 10% of a water.
The slaked lime used as a raw material was a material specified by
JIS R9001. Potassium hydroxide which was a first grade reagent was
used. As a result of analyzing the pellets thus produced, the
reagent pellets were found to contain elemental carbon (C) and
calcium oxide (CaO) as principal components. Those reagents
prepared by adding potassium contained potassium to some extent. Of
these, the following two reagents A and B were selected as typical
ones, and used in Example 1. The atomic ratios of C to Ca to K, and
the total content in terms of weight of these components based on
the entire amount in the reagent pellets A and B are as
follows:
1 Total weight Atomic ratio of C/Ca Atomic ratio of K/Ca % of
Ca,C,K Reagent A 1.56/1 0.04/1 79 wt. % Reagent B 1.56/1 0/1 79 wt.
%
[0071] Nitrogen trifluoride (NF.sub.3) was used as the nitrogen
fluoride to be decomposed. As shown in FIG. 1, nitrogen trifluoride
to which an oxygen gas was added or not added was introduced into
the above-mentioned reaction tube with a nitrogen gas being used as
a carrier.
[0072] The following fixed conditions were used. However, CF.sub.4
was added to the gas to be treated in some tests.
2 Flow rate of gas to be treated: 0.17 l/min Concentration of
nitrogen fluoride 5% by volume in a gas to be treated: Superficial
velocity of a gas to be 291 hr.sup.-1 treated: Linear velocity of a
gas to be 0.85 m/min treated: Concentration of oxygen in a gas 0 or
5% by volume to be treated:
[0073] Furthermore, in all of the examples, application of a
current to a heating element was started, and the temperature of
the central portion of the reagent was confirmed to have reached a
predetermined temperature before introducing the gas to be treated.
During the reaction, the current applied to the tubular furnace was
controlled so that the temperature measured by a thermocouple
inserted in the central portion of the reagent (portion which had
the highest temperature in the bulk of the reagent) was held at the
predetermined temperature. The temperature which was held during
the reaction is referred to as the reaction temperature.
[0074] As shown in FIG. 1, part of the exhaust gas discharged from
the reaction tube was sampled, and introduced into a gas analyzer.
The remainder of the exhaust gas was passed through a
fluorine-absorbing bottle containing a solution of sodium
hydroxide, and discharged outside the system. The analysis of the
exhaust gas was carried out, for nitrogen fluoride, other fluorine
compounds, O.sub.2, N.sub.2, NO, N.sub.2O, CO.sub.2 and CO.
[0075] Table 1 shows the results of decomposing nitrogen
trifluoride under the conditions as mentioned above using the
reagent A or B while the maximum temperature of the reagent was
varied. The decomposition ratio of NF.sub.3 shown in Table 1 is one
determined 30 minutes after starting the reaction. Nitrogen
fluoride remaining in the exhaust gas was measured in a sample
obtained from the exhaust gas 30 minutes after starting the
reaction. The decomposition ratio was represented by a percentage
of nitrogen fluoride in the exhaust gas based on nitrogen fluoride
in the gas to be treated.
3 TABLE 1 Comp. of Decom- gas to be Reac- Reac- position treated
tion tion ratio By-products Test NF.sub.3 O.sub.2 temp. time of
NF.sub.3 N.sub.2O NO CF.sub.4 No. % % .degree. C. hr % ppm ppm % 1*
5 5 250 0.5 .gtoreq.99.9 0.38 .ltoreq.1 .ltoreq.0.002 2* 5 5 300
0.5 .gtoreq.99.9 .ltoreq.0.002 .ltoreq.1 .ltoreq.0.002 3* 5 5 350
0.5 .gtoreq.99.9 .ltoreq.0.002 .ltoreq.1 .ltoreq.0.002 4* 5 5 400
0.5 .gtoreq.99.9 .ltoreq.0.002 .ltoreq.1 .ltoreq.0.002 5* 5 5 450
0.5 .gtoreq.99.9 .ltoreq.0.002 .ltoreq.1 0.2 6* 5 5 750 0.5
.gtoreq.99.9 .ltoreq.0.002 .ltoreq.1 .ltoreq.0.002 7* 5 0 300 0.5
.gtoreq.99.9 .ltoreq.0.002 .ltoreq.1 .ltoreq.0.002 8* 5 0 750 0.5
.gtoreq.99.9 .ltoreq.0.002 .ltoreq.1 .ltoreq.0.002 9* NF.sub.3 2 5
750 0.5 .gtoreq.99.9 .ltoreq.0.002 .ltoreq.1 .ltoreq.0.002 CF.sub.4
3 NF.sub.3 + CF.sub.4 11# 5 5 200 0.5 .gtoreq.99.9 formed .ltoreq.1
.ltoreq.0.002 12# 5 5 350 0.5 .gtoreq.99.9 formed .ltoreq.1
.ltoreq.0.002 13# 5 5 400 0.5 .gtoreq.99.9 .ltoreq.0.002 .ltoreq.1
.ltoreq.0.002 14# 5 5 450 0.5 .gtoreq.99.9 .ltoreq.0.002 .ltoreq.1
0.03 15# 5 5 750 0.5 .gtoreq.99.9 .ltoreq.0.002 .ltoreq.1
.ltoreq.0.002 Note: *Reagent A containing K #Reagent B containing
no K
[0076] The following conclusions can be drawn from the results in
Table 1. Almost 100% of the NF.sub.3 was decomposed at temperatures
of not less than 200.degree. C. in all of the tests. The
relationship between the reaction temperature and the by-products
will be explained. Generation of CF.sub.4 was observed at a
temperature near 450.degree. C. regardless of whether reagent A or
B was used. When the reagent B containing no K was used, NO.sub.x
was formed at temperatures near 350.degree. C. or less, and it was
not formed at temperatures of 400.degree. C. or more. When the
reagent A containing K was used, NO.sub.x was formed at
temperatures of up to 250.degree. C., and it was not formed at
temperatures of 300.degree. C. or more.
[0077] Even if the gas to be treated contained no oxygen, as in
Test Nos. 7 and 8, NF.sub.3 was completely decomposed, and neither
CF.sub.4 nor NO.sub.x was generated at a reaction temperature of
300.degree. C. or 750.degree. C. When a gas to be treated was
accompanied by carbon fluoride CF.sub.4 as in Test No. 9, it is
seen that CF.sub.4 was completely decomposed together with
NF.sub.3.
Example 2
[0078] The same system as in Example 1 (a desulfurizer and an
oxidizer being excluded) was used. Moreover, 35 ml of reagent for
decomposing sulfur fluoride was charged in the reaction tube at the
furnace central portion. The reagents A and B prepared in Example 1
were used.
[0079] Sulfur hexafluoride (SF.sub.6) was used as sulfur fluoride
to be decomposed. As shown in FIG. 1, sulfur hexafluoride to which
an oxygen gas was added was introduced into the reaction tube with
a nitrogen gas being used as a carrier.
[0080] The following fixed conditions were used. However, CF.sub.4
was added to the gas to be treated in some tests.
4 Flow rate of a gas to be treated: 0.17 l/mm Concentration of
sulfur fluoride 5% by volume in a gas to be treated: Superficial
velocity of a gas 146 or 291 hr.sup.-1 to be treated: Concentration
of oxygen in 0% or 5% by volume a gas to be treated:
[0081] Furthermore, in all of the examples, application of a
current to the heating element was started, and the temperature of
the central portion of the reagent was confirmed to have reached a
predetermined temperature before introducing the gas to be treated.
During the reaction, the current applied to the tubular furnace was
controlled so that the temperature measured by a thermocouple
inserted in the central portion of the reagent (portion which had
the highest temperature in the bulk of the reagent) was held at the
predetermined temperature. The temperature having been held during
the reaction is referred to as the reaction temperature.
[0082] Part of the gas exhausted from the reaction tube was
sampled, and introduced into a gas analyzer as shown in FIG. 1. The
remainder of the exhaust gas was passed through a
fluorine-absorbing bottle containing a solution of sodium
hydroxide, and discharged outside the system. Analysis of the
exhaust gas was carried out for sulfur fluoride, other fluorine
compounds, O.sub.2, SO.sub.2, CO.sub.2 and CO.
[0083] Table 2 shows the results of decomposing sulfur hexafluoride
under the conditions mentioned above using the reagent A or B while
the reaction temperature and the superficial velocity were varied.
The decomposition ratio of SF.sub.6 shown in Table 2 is one
determined 30 minutes after starting the reaction. Sulfur fluoride
remaining in the exhaust gas was measured in a sample obtained from
the exhaust gas 30 minutes after starting the reaction. The
decomposition ratio was represented by a percentage of sulfur
fluoride in the exhaust gas based on sulfur fluoride in the gas to
be treated.
5 TABLE 2 Comp. of Decom- gas to be Reac- Reac- Sup.sup.+ position
treated tion tion veloc- ratio By-products Test SF.sub.6 O.sub.2
temp. time ity of SF.sub.6 SO.sub.2 CF.sub.4 No. % % .degree. C. hr
1/hr % ppm % 21* 5 5 350 0.5 146 .gtoreq.99.7 .gtoreq.2000
.ltoreq.0.002 22* 5 5 400 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 23* 5 5 600 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 24* 5 5 600 0.5 291 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 25* 5 5 700 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 26* 5 5 800 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 27* 5 0 450 0.5 291 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 28* 5 0 750 0.5 291 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 29* SF.sub.6 2 5 750 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 CF.sub.4 3 SF.sub.6 + CF.sub.4 31# 5 5 400 0.5 146 30
35 .ltoreq.0.002 32# 5 5 450 0.5 146 .gtoreq.95.3 .gtoreq.2000
.ltoreq.0.002 33# 5 5 500 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 34# 5 5 600 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 35# 5 5 700 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 34# 5 5 700 0.5 291 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 35# 5 5 800 0.5 146 .gtoreq.99.9 .ltoreq.1
.ltoreq.0.002 Note: *Reagent A containing K #Reagent B containing
no K .sup.+Sup = Superficial
[0084] The following conclusions can be drawn from the results of
Table 2. SF.sub.6 was decomposed at reaction temperatures of
300.degree. C. or more in all of the tests. When the reagent A
containing K was used, the percent decomposition become 99.7% or
more at reaction temperatures of 350.degree. C. or more. When the
reagent B containing no K was used, the decomposition ratio becomes
95.3% or more at reaction temperatures of 450.degree. C. or more.
That is, in all of the tests, SF.sub.6 was decomposed at a percent
age of decomposition near 100%.
[0085] It is understood from the relationship between reaction
temperatures and by-products in the table that under the reaction
conditions, formation of SO.sub.2 is observed at temperatures of up
to 350.degree. C. when the reagent A containing K is used, and at
temperatures of up to 450.degree. C. when the reagent B containing
no K is used, and that no test examples show generation of
CF.sub.4.
* * * * *