Abatement of F2 using small particle fluidized bed

Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Halogenous component

Reexamination Certificate

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Details

C423S241000, C423S24000R, C423S493000

Reexamination Certificate

active

06352676

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
In the process of making semiconductor chips, the exhaust streams of some tools may contain F
2
. For example, a new tool from Applied Materials uses microwave to dissociate NF
3
to clean a reactor chamber. Although the exhaust stream contains only traces of NF
3
, significant quantities of F
2
exist. Streams, such as the exhaust from the Applied Materials tool, have to be treated to remove F
2
before venting to the atmosphere. Fluorine is a corrosive and toxic chemical with a TLV of 1 ppm.
Conventionally, F
2
is removed using wet caustic scrubbers where F
2
reacts with the caustic solution to form fluoride salt. The wet scrubber, although efficient, produces large quantities of liquid wastes that need to be treated later. In addition, the caustic scrubbers can generates toxic byproducts, OF
2
and NO
3
F, that causes additional safety concerns.
Alternatively, F
2
has been removed using solids. A packed bed of charcoal has been used to remove F
2
, but it generates global warming carbon-fluorine gases. Safety is another concern in using carbon, as the possibility exists of forming explosive CF
x
compounds in the packed bed. Packed beds of soda lime (NaOH/Ca(OH)
2
), limestone (CaCO
3
), and alumina (Al
2
O
3
) have been employed to remove F
2
. These dry scrubbers frequently plug and require considerable manpower to empty and refill the reactors. In addition, the heat generation inside a packed bed could present a problem when F
2
concentration in the feed becomes too high, as the heat removal inside a packed bed is a challenge. Other methods of F
2
disposal can be found in the report by Netzer, W. D., Fluorine Disposal Processes for Nuclear Applications, Goodyear Atomic Corp., Apr. 8, 1977.
Holmes, et. al., Fluidized Bed Disposal of Fluorine, I & EC Process Design and Development, Vol. 6, No. 4, October 1967, pp. 408-413, describes the abatement of fluorine using a fluidized bed of activated alumina. High F
2
removal efficiency (>99%) was easily achieved at a reaction temperature between 300 to 400° C. The flow rate was limited to 1.25 to 1.65 minimum fluidization velocity. The definition of minimum fluidization velocity can be found in many textbooks, e.g. “Fluidization Engineering” by Kunii and Levenspiel, John Wiley & Sons, 1969. In this velocity range, the fluidized bed is considered at a “bubbling” fluidization mode. Soda ash was also found to be effective.
U.S. Pat. No. 5,417,948 discloses the use of zirconium alloys to abate NF
3
. It lists fluidized beds as a possible means of contacting the alloys with NF
3
. A control example used iron wire cut into 5 to 10 mm pieces as a bed material.
The prior art has attempted to provide various methods and means of abating fluorine. However, the prior art has not achieved a commercially viable process for fluorine abatement which generates no pollutants, allows for high throughput, avoidance of clogging, efficient destruction of fluorine and a method for recharging of the abatement system for continuous processing. These advantages are achieved by the present invention, as will be set forth in greater detail below.
BRIEF SUMMARY OF THE INVENTION
The present invention is a process of destroying a fluorine specie selected from the group consisting of fluorine, chlorine trifluoride and mixtures thereof from a gas mixture containing a fluorine specie by contacting the gas with a fluidized bed of metal particles capable of reacting with a fluorine specie wherein the particles have a particle size essentially no greater than approximately 300 microns.
Preferably, up to 10 wt. % of the bed of metal particles are large particles having a particle size sufficiently larger than 300 microns to assist in mixing of the bed of metal particles.
Preferably, the large particles have a particle size of approximately 500 to 2000 microns.
Preferably, the metal particles are selected from the group consisting of iron, nickel, copper, calcium, aluminum, magnesium, manganese, cobalt, zinc, tin and mixtures thereof.
Preferably, the contacting is performed at a temperature in the range of 150 to 550° C.
Preferably, the gas is contacted with the fluidized bed until the height of the fluidized bed increases due to reaction with the fluorine to a predetermined increased bed height.
Preferably, the gas alternately contacts one of at least two parallel switching fluidized beds where one bed is contacting the gas while one or more other fluidized beds are being recharged.
Preferably, the gas contact is switched from a first of at least two parallel switching fluidized beds to an other fluidized bed that has been recharged when the first bed has expanded to at least approximately 90 percent of the original bed height.
Preferably, after the gas contacts the fluidized bed the gas contains no greater than 1 part per million by volume of fluorine specie.
Preferably, the flow of the gas is sufficient to obtain at least a minimum fluidization velocity of the fluidized bed for the metal particles contained in the fluidized bed.
Preferably, the gas has a residence time, defined by the ratio of packed bed volume to the volumetric feed flowrate at normal conditions, in the fluidized bed of greater than approximately 3 seconds.
Preferably, the flow of the gas is approximately at least two times the minimum fluidization velocity.
Preferably, the gas contains a gas component selected from the group consisting essentially of N
2
, O
2
, Ar, He, SiF
4
, NF
3
, CF
4
, C
2
F
6
, CHF
3
, SF
6
and mixtures thereof.
Most preferably, the present invention is a process of destroying fluorine in a gas containing fluorine and one or more gas components of N
2
, O
2
, Ar, He, SiF
4
, NF
3
, CF
4
, C
2
F
6
, CHF
3
, SF
6
, and mixtures thereof by contacting said gas alternately with one of a pair of switching parallel connected fluidized beds of iron metal particles capable of reacting with fluorine wherein the particles have a particle size essentially no greater than approximately 300 microns, and switching beds when the height of the bed being contacted with said gas increases to a predetermined bed height corresponding to substantially stoichiometric reaction of fluorine with the iron metal particles.
Preferably, the iron metal particles are at least approximately 99% iron by weight.
Preferably, the iron metal particles have an average particle size of approximately 100 microns.
Preferably, the iron metal particles react with the fluorine to generate a mixture of FeF
2
and FeF
3
.
In one preferred embodiment, the gas containing fluorine contains predominantly nitrogen and such fluorine.
In an alternate embodiment, the metal particles are continuously being added to the fluidized bed and metal particles that have been reacted with the fluorine are continuously being removed. In this alternate embodiment, preferably a single fluidized bed is utilized.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Not Applicable
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a method of destroying a fluorine specie of fluorine and/or chlorine trifluoride using fine metal powder in a fluidized bed reactor. The preferred metal powder is iron and should have a purity of greater than 90%, preferably 99% by weight and a particle size essentially no greater than approximately 300 microns, preferably an average particle size of approximately 100 microns. Preferably, up to 10 wt. % of the particles in the bed can be larger than 300 microns with sufficient size to assist in mixing of the particles of essentially no greater than approximately 300 microns during operation and fluidization to prevent caking of the latter particles. These larger particles could preferably have a size of 500 to 2000 microns.
The preferred operating conditions are:
temperature; 150° C. to 550° C.;
feed flow; greater than the minimum fluidization velocity of the powder;
residence time; greater than 3 seconds.
Other metals such as Ni

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