Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
1998-08-12
2001-09-25
Langel, Wayne (Department: 1754)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C423S24000R
Reexamination Certificate
active
06294709
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for efficiently decomposing fluorocarbons, particularly perfluorocarbons and hydrofluorocarbons with about 1-5 carbon atoms, as well as to a simple apparatus for this purpose.
BACKGROUND ART
Japanese Unexamined Patent Publication No. 6-293501 discloses a process for decomposing chlorofluorocarbons, involving heating magnetite by application of microwaves in an applicator, and then contacting the heated magnetite with chlorofluorocarbon gas. Also, Japanese Unexamined Patent Publication No. 7-24255 discloses a process for decomposing chlorofluorocarbons involving microwave irradiation of a mixture comprising a carbonaceous material and an alkaline earth metal oxide or salt for heat generation, and then contacting of the heated mixture with chlorofluorocarbon gas.
Since the chlorofluorocarbon-decomposing processes in the aforementioned publications all employ microwaves, their drawbacks have been the requirement of costly microwave-generating apparatuses and reactors limited to heat-resistant materials with good permeability to microwaves. Ceramics are materials with good permeability to microwaves, but these tend to react with fluorine and thus deteriorate in quality. Consequently, industrially economical and stable decomposition of chlorofluorocarbons is a problem for which a solution has been sought.
One technique for decomposing chlorofluorocarbons without using microwaves has been proposed in Japanese Unexamined Patent Publication No. 8-187302 by the present applicant, which is a chlorofluorocarbon decomposition process wherein chlorofluorocarbon gas is contacted and reacted with a substance containing a carbonaceous material and an alkaline earth metal compound heated in a non-oxidizing atmosphere. The process of this prior application has the advantage of allowing decomposition of chlorofluorocarbons using a normal heating furnace with an electrical heater, and as stated in the related specification, chlorofluorocarbons such as R-113 may be efficiently decomposed.
Nevertheless, subsequent research has revealed that many kinds of fluorocarbons are not decomposed by the process of this prior application, and the decomposition rate is not necessarily satisfactory for fluorocarbons with no chlorine groups, such as perfluorocarbons and hydrofluorocarbons, which are not mentioned in the specification of the prior application.
SUMMARY OF THE INVENTION
Consequently, an object of the present invention is to provide a process for efficiently decomposing fluorocarbons with no chlorine groups, such as perfluorocarbons and hydrofluorocarbons.
This object may be accomplished by contacting perfluorocarbon or hydrofluorocarbon gas with a reagent comprising carbon and at least one alkaline earth metal, at a temperature of 300° C. or higher and in the presence of 20 vol % or less (but not 0%) of gaseous oxygen.
More specifically, it has been found that fluorocarbon gas can be efficiently decomposed by continuously or intermittently feeding a treatment gas containing a perfluorocarbon or hydrofluorocarbon to a reactor filled with a reagent comprising carbon and at least one alkaline earth metal, continuously or intermittently discharging the exhaust gas from the reactor after the reaction, allowing or causing oxygen to be contained in the treatment gas before it is introduced into the reactor so that the oxygen concentration is 20 vol % or less in the treatment gas, and transferring heat necessary for decomposition of the perfluorocarbon or hydrofluorocarbon from outside the reactor to the reaction zone, or from inside the reactor to the reaction zone.
CO gas is sometimes copresent in the exhaust gas according to the process of the invention. In such a case, there may be added an additional step of oxidation of the CO in the exhaust gas to CO
2
. In addition, for the process of the invention, the reactor may be made of a material such as a heat-resistant alloy and or corrosion-resistant alloy, for example a stainless steel or nickel-based alloy, and ceramics may also be used if they are resistant to fluorine and hydrogen fluoride (examples of which are ceramics made of aluminum fluoride). The reactor may be placed in a furnace which can maintain its internal atmosphere at a desired temperature, to transfer the heat in the furnace to the reagents in the reactor through the wall of the reactor.
Another object of the present invention is to provide a process for efficiently decomposing fluorocarbons which may contain, in addition to carbon and fluroine, hydrogen and/or more halogens other than fluorine, such as chlorine.
Thus, the present invention also provides a process for decomposing a fluorocarbon which comprises contacting a gaseous fluorocarbon with a reagent comprising carbon, at least one alkaline earth metal and at least one alkali metal at a temperature of 200° C. or higher A halogen component from the decomposed fluorocarbon may be fixed to said reagent in a manner such that the halogen elements derived from the fluorocarbon as a result of decomposition of the fluorocarbon are fixed into the reagent as halides of the alkaline earth metal and, optionally, of the alkali metal of the components of the reagent.
The present invention further provides a reagent for decomposing a fluorocarbon which comprises at least one alkaline earth metal, carbon and at least one alkali metal which may be in an atomic ratio of alkaline earth metal:carbon:alkali metal of 1.0:0.25-4.0:0.01-0.3. Preferably, not less than 50% by weight of the alkaline earth metal and alkali metal in the reagent is oxides.
In this second feature of the present invention, the term “fluorocarbon” refers to a fluorocarbon which may contain, in addition to carbon and fluorine, hydrogen and/or one or more halogens other than fluorine, such as chlorine.
The present invention further provides, as an apparatus designed to suitably accomplish the decomposition process described above, a fluorocarbon decomposition apparatus comprising a reactor filled with a reagent comprising carbon and at least one alkaline earth metal or carbon, at least one alkaline earth metal and at least one alkali metal, a treatment gas inlet leading into the reactor, a gas outlet for discharge of reacted gas from the reactor, a furnace in which the reactor is housed, a heating source for raising the atmospheric temperature in the furnace, a duct connecting the aforementioned treatment gas inlet and a fluorocarbon-containing gas source, and if necessary also an exhaust gas oxidizer connected by a tube leading to the gas outlet. The reactor may be constructed of a heat-resistant alloy or corrosion-resistant alloy. The fluorocarbon-containing gas source may be one containing a moderate amount of oxygen, such as a fluorocarbon-containing gas generated by a semiconductor manufacturing process.
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Atobe Hitoshi
Ito Kazuto
Izumikawa Chiaki
Kaneko Toraichi
Tezuka Kazumasa
Dowa Mining Co. Ltd.
Langel Wayne
Nave Eileen E.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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