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Claims  |
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What is claimed is:
1. An air filter comprising: a supporter and an inorganic material layer that is formed by fixing powder of fraipontite mineral to the surface of the supporter, using powder
of an inorganic substance as a binder.
2. An air filter comprising: a supporter, a first inorganic material layer formed of powder of fraipontite mineral and powder of an inorganic substance as a binder, and a second inorganic material layer formed of powder of an inorganic
substance, wherein one of the first inorganic material layer and the second inorganic material layer is fixed to the surface of the supporter, and the other of the first inorganic material layer and the second inorganic material layer is fixed to the
surface of the one inorganic material layer.
3. An air filter comprising: a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is formed by pelletizing a mixture of powder of fraipontite mineral and powder of an inorganic
substance as a binder.
4. An air filter comprising: a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is formed by pelletizing a mixture of powder of fraipontite mineral and powder of an inorganic
substance as a binder and is then coated with powder of an inorganic substance, or the pellet is formed by pelletizing powder of an inorganic substance and is then coated with powder of fraipontite mineral, using powder of an inorganic substance as a
binder.
5. An air filter comprising: a casing filled with a plurality of pellets, wherein the pellet is formed by pelletizing powder of fraipontite mineral, using powder of an inorganic substance as a binder, or the pellet is formed by pelletizing
powder of fraipontite mineral, using powder of an inorganic substance as a binder and is then coated with powder of an inorganic substance, or the pellet is formed by pelletizing powder of an inorganic substance and is then coated with powder of
fraipontite mineral, using powder of an inorganic substance as a binder.
6. An air filter comprising: a supporter and an inorganic material layer that is formed by fixing a mixture to the surface of the supporter, using an inorganic substance as a binder, the mixture comprising powder of fraipontite mineral and one
or both of powder of manganese oxide and permanganate.
7. An air filter comprising: a supporter, a first inorganic material layer formed of powder of fraipontite mineral and an inorganic substance as a binder, and a second inorganic material layer formed of one or both of powder of manganese oxide
and permanganate, and powder of an inorganic substance as a binder, wherein one of the first inorganic material layer or the second inorganic material layer is fixed to the surface of the supporter, and the other inorganic material layer is fixed to the
surface of the one inorganic material layer.
8. An air filter comprising: a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is formed by pelletizing a mixture of powder of fraipontite mineral and one or both of powder of
manganese oxide and permanganate, using powder of an inorganic substance as a binder.
9. An air filter comprising: a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is formed by pelletizing powder of fraipontite mineral, using powder of an inorganic substance as a
binder and is then coated with one or both of powder of manganese oxide and permanganate, using powder of an inorganic substance as a binder, or the pellet is formed by pelletizing one or both of powder of manganese oxide and permanganate, using powder
of an inorganic substance as a binder and is then coated with powder of fraipontite mineral, using powder of an inorganic substance as a binder.
10. An air filter comprising: a casing filled with a plurality of pellets, wherein the pellet is formed by pelletizing a mixture of powder of fraipontite mineral and one or both of powder of manganese oxide and permanganate, using powder of an
inorganic substance as a binder, or formed by pelletizing powder of fraipontite mineral, using the powder of an inorganic substance as a binder and is then coated with one or both of powder of manganese oxide and permanganate, using an inorganic
substance as a binder, or the pellet is formed by pelletizing one or both of powder of manganese oxide and permanganate, using an inorganic substance as a binder and is then coated with powder of fraipontite mineral, using powder of an inorganic
substance as a binder.
11. An air filter as claimed in claim 1, 2, 3, 4, 6, 7, 8, or 9, wherein said supporter is a honeycomb structure.
12. An air filter as claimed in claim 6, 7, 8, 9, or 10 wherein said manganese oxide is selected from the group consisting of tri-manganese tetraoxide (Mn.sub.3 O.sub.4), di-manganese trioxide (Mn.sub.2 O.sub.3), and manganese dioxide
(MnO.sub.2) and combinations thereof.
13. An air filter as claimed in claim 6, 7, 8, 9, or 10 wherein said permanganate is selected from the group consisting of M.sup.I MnO.sub.4 (M.sup.I : alkali metal) and M.sup.II (MnO.sub.4).sub.2 (M.sup.II : alkaline earth metal).
14. An air filter as claimed in claim 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, wherein said inorganic substance is selected from the group consisting of talc, kaolin mineral, bentonite, diatom earth, silica, alumina, mixture of silica and alumina,
aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, and activated bentonite and combinations thereof.
15. An air filter as claimed in claim 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 wherein either or both of said inorganic material layer and said pellet may include an inorganic adhesion assisting agent.
16. A high efficiency air cleaning apparatus for keeping a space or room at a level of required cleanliness, comprising: an air circulation path, an air filter recited in claim 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, and a particulate filter, said air
filter being installed within or adjacent to the space or room, and said particulate filter being positioned on the upstream side of said space or room and on the down stream side of said air filter, whereby said space or room is cleaned by circulating
the air therein through said air circulation path.
17. A high efficiency air cleaning apparatus as claimed in claim 16, wherein said air filter and said particulate filter are installed on the ceiling portion of said space or room.
18. A method for manufacturing an air filter comprising the steps of:
immersing a supporter in a suspension in which powder of fraipontite mineral and powder of an inorganic substance as a binder are dispersed as the suspensoid, and
drying the supporter impregnated with the suspensoid, thereby forming an inorganic material layer fixed to the surface of the supporter.
19. A method for manufacturing an air filter comprising the steps of:
immersing a supporter in a suspension in which powder of fraipontite mineral and powder of an inorganic substance as a binder are dispersed as the suspensoid,
drying the supporter impregnated with the suspensoid, thereby forming a first inorganic material layer fixed to the surface of the supporter,
immersing the supporter with the first inorganic material layer in a suspension in which powder of an inorganic substance is dispersed as the suspensoid,
drying the supporter impregnated with the suspensoid, thereby forming a second inorganic material layer fixed to the surface of the first inorganic material layer; or
forming said second inorganic material layer so as to be fixed to the surface of the supporter, and
forming said first inorganic material layer so as to be fixed to the surface of said second inorganic material layer.
20. A method for manufacturing an air filter comprising the steps of:
immersing a supporter in a suspension in which powder of fraipontite mineral, one or both of powder of manganese oxide and permanganate, and powder of an inorganic substance as a binder are dispersed as the suspensoid, and
drying the supporter impregnated with the suspensoid, thereby forming an inorganic material layer fixed to the surface of the supporter.
21. A method for manufacturing an air filter comprising the steps of:
immersing a supporter in a suspension in which powder of fraipontite mineral and powder of an inorganic substance as a binder are dispersed as the suspensoid,
drying the supporter impregnated with the suspensoid, thereby forming a first inorganic material layer fixed to the surface of the supporter,
immersing the supporter with the first inorganic material layer in a suspension in which one or both of powder of manganese oxide and permanganate, and powder of an inorganic substance as a binder are dispersed as the suspensoid,
drying the supporter impregnated with the suspensoid, thereby forming a second inorganic material layer fixed to the surface of the first inorganic material layer; or
forming said second inorganic material layer so as to be fixed to the surface of the supporter, and
forming said first inorganic material layer so as to be fixed to the surface of said second inorganic material layer. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to an air filter capable of being installed in a high efficiency air cleaning apparatus, an air cleaning device or the like such as a clean room, a clean bench, a storage means (stocker) for half-finished products,
etc., for removing gaseous inorganic and/or organic impurities contained in the limited space that is defined by the high efficiency air cleaning apparatus, the device or the like. The invention also relates to a method for manufacturing the air filter
and also relates to the high efficiency air cleaning apparatus, the device or the like provided with the filter manufactured according to the method.
In the following description, the expression "air cleaning apparatus," and "device" seem to be too general, so that it would be described in terms of "clean room," "clean bench," "storage means (stocker)" or others having the function similar
thereto, without altering the gist and scope of the invention, for more practical and better understanding of the invention.
BACKGROUND OF THE INVENTION
Today, the clean room, the clean bench, and the like are widely used for manufacturing a large scale integrated (LSI) circuit, a liquid crystal display (LCD) panel and so forth, all of which have to be carefully protected from specific impurities
or contaminants contained in the ordinary air. In case of manufacturing products like these items, however, it is not rare that they are forced to be kept inside a carrier or a stocker in the form of a half-finished product after having passed through
predetermined manufacturing steps. Consequently, they come to exposed to the inside atmosphere of the carrier or stocker until they are transferred to the next manufacturing step. Furthermore, it is also not rare that this intermediate storage or
waiting state is continued over a period of several to several tens hours. Consequently, should the atmosphere of the clean room or the like be insufficiently cleaned, gaseous impurities contained therein would be deposited on the semiconductor
substrate surface of LSI or glass substrate surface of LCD. Acid substances, alkaline substances, organic substances, and various dopants are considered as impurities that would give an ill influence to the performance of LSI or LCD when they are
deposited on the substrate surface thereof. Especially, sulfur oxides SOx among acid substances reacts with ammonia gas contained, if any, in the clean room atmosphere to unfavorably form a thin cloudy matter consisting of ammonium sulfate over the
surface of a silicon wafer, a glass substrate, a lens, a mirror, a hard disk, a magnetic disk, and so forth. Thus, this would give the ill influence to the manufacture of semiconductor elements, LCD substrates, and magnetic memory parts. The most
popular sulfurous acid gas representing sulfur oxides may be generated from various sources. For instance, it may be generated from sulfuric acid that is used in the rinsing process carried out inside the clean room, and also it might be contained as an
atmospheric pollutant or volcanic gas in the outdoor air introduced to the clean room. Boron B also may act as a contaminant against thin film transistors (TFT), and it contaminates the channel region of TFT to make the characteristic thereof worse.
According to one example of the actual measurement of various contaminants contained in the atmosphere of the ordinary clean room that is not provided with any chemical protective measure against gaseous contaminants, it is reported that there
are contained in such atmosphere acid substances of about 100 ppt-1,000 ppt, basic substances of about 1,000 ppt-10,000 ppt, organic substances of about 1,000 ppt-10,000 ppt, and dopants of about 10 ppt-100 ppt, respectively.
According to the allowable concentration (ppt) of the chemical contaminant required for 0.25.mu. process (after '98), which is disclosed in the article entitled "Forecast Of Airborne Molecular Contamination Limits For The 0.25 Micron High
Performance Logic Process" of Technology Transfer #95052812A-TR published by SEMATECH (U.S.A.) on May 31, 1995, severe limitations are imposed on the concentration of various chemical contaminants. That is, with regard to acid substances, they should be
limited to less than 180 ppt in the salicidation process, and less than 5 ppt in the contact formation process. As for basic substances, they should be limited to less than 1 ppb in the photolithographic process. With respect to the dopant, it should
be limited to less than 0.1 ppt for the pre-gate oxidation process. As to organic substances, they should be limited to less than 1 ppb in the pre-gate oxidation process, and less than 2 ppb in the contact formation process.
Among these four chemical contaminants, almost all of acid substances, basic substances and dopants are water-soluble, so that they can be removed by means of the wet type rinsing (scrubber) or by means of chemical adsorption using a chemical
filter that is formed of ion-exchange fiber or activated charcoal, to the surface of which a selected chemical is impregnated (referred to as "chemical impregnated activated charcoal" hereinafter). A cylindrical case filled up with chemical impregnated
activated charcoal is well known as the most simple configuration of a chemical filter using chemical impregnated activated charcoal. Activated charcoal impregnated with potassium permanganate solution has been used as the filter media of a chemical
filter for removing sulfur oxides SOx. Chemical filters of other types are available. For instance, some chemical filters are made in the form of a felt or a nonwoven fabric that is formed by interweaving chemical impregnated fibrous activated charcoal
with the polyester fiber, using an organic binder, and the others are formed in the shape of a block or a sheet by binding chemical impregnated activated charcoal powder to an air permeable urethane foam or a nonwoven fabric with a proper adhesive. The
chemical filter using chemical impregnated activated charcoal is disclosed in the Japanese unexamined patent publication No. 61-103518, which is incorporated by reference herein. This filter uses the acid cation exchange fiber and the basic anion
exchange fiber as its basic filter media, and is made in the form of a nonwoven fabric, a sheet, or a felt. Various acid and basic ions can be ion-exchanged and removed with this filter.
On one hand, organic substances are generally insoluble in water, so that they may be basically removed by means of the physical adsorption function of activated charcoal.
In case of the wet type air rinsing system, an atomizer will require a rather larger initial cost for installation thereof and also require a considerable running cost for generating a high pressure spray, which can not be ignored. Also, it
should be noted that this system relates to the humidity and temperature control of the clean room air. Consequently, in case of circulating the air through the clean room, if it is rinsed by this system, it is additionally required to control the
humidity and temperature of the circulating clean room air and to remove tiny water droplets contained therein immediately after being rinsed thereby. Furthermore, the rinsing water has to be always kept clean and free from any of bacteria, water
soluble contaminants, and so forth, so that it is required, for instance, to additionally arrange other facilities for preventing bacteria from being generated and/or for condensing and separating water soluble contaminants.
In view of disaster prevention standpoint, however, it is not preferable, should rather be avoided, to install the chemical filter that uses chemical impregnated activated charcoal or ion exchange fiber, on the ceiling portion of the clean room,
because they are inflammable. The prior art chemical filter generally includes, as its components, a nonwoven fabric, an organic binder for fixing activated charcoal to a sheet of filter media, a sealing member, and so forth. However, these components
are apt to desorb gaseous organic impurities, which are readily mixed with the air once cleaned by passing through the chemical filter, and would never fail to give a wrong influence to the manufacture of semiconductor devices. Dusts flying away from
the filter media are also one of wrong factors causing the product contamination. If a plurality of processes which might cause various gaseous inorganic impurities, are executed in one clean room, the chemical filter is naturally required to have an
excellent filtering performance against such gaseous inorganic impurities. At the same time, it is strictly required that the chemical filter never desorbs any of gaseous organic impurities by itself. In general, in order to block dusts which might fly
away from the chemical filter, a particulate air filter is installed on the downstream side of the chemical filter. However, if such particulate air filter desorbs some gaseous organic impurities by itself, the function of the chemical filter would be
in vain even though it is so excellent. Furthermore, the prior art chemical filter including basic chemicals and metals fails to remove sulfurous acid gas sufficiently.
In case of the chemical filter using the ion exchange fiber, gaseous organic substances might be desorbed from various additive agents included in the polymer fiber forming the ion exchange fiber. In this case, it sometimes takes place that a
part of the ion exchange group is desorbed as sulfonic acid gas, carboxylic acid gas, phosphoric acid gas, ammonia gas, or amine gas.
Furthermore, in case of filtering the clean room air including both of acid and basic inorganic impurities by means of the prior art chemical filter that uses chemical impregnated activated charcoal or ion exchange fiber, it is required to
separately prepare both of an adsorptive media for removing acid inorganic impurities and an adsorptive media for removing basic inorganic impurities.
Accordingly, an object of the invention is to provide an air filter that is excellent from the standpoint of disaster prevention, and is capable of removing both of organic and inorganic impurities contained in the object air, and desorbs neither
gaseous inorganic impurities nor gaseous organic impurities by itself. Furthermore, the other object of the invention is to provide a method for manufacturing such filters and a high efficiency clean room or the like installed with such filters as
manufactured according to the above method.
Another object of the invention is to provide an air filter that is able to work for a long time and efficiently remove sulfurous acid gas which could not be sufficiently removed by the prior art chemical filter.
SUMMARY OF THE INVENTION
According to the invention, there is provided an air filter including a supporter and an inorganic material layer that is formed by fixing the powder of the fraipontite mineral to the surface of the supporter, using the powder of an inorganic
substance as a binder.
Furthermore, according to the invention, there is provided an air filter including a supporter, a first inorganic material layer formed of the powder of the fraipontite mineral and the powder of an inorganic substance as a binder, and a second
inorganic material layer formed of the powder of an inorganic substance, wherein either the first inorganic material layer or the second inorganic material layer is directly fixed to the surface of the supporter, and the remaining inorganic material
layer is additionally fixed to the surface of the first fixed inorganic material layer.
Further, according to the invention, there is provided an air filter including a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is formed by pelletizing a mixture of the powder of
the fraipontite mineral and the powder of an inorganic substance as a binder.
Still further, according to the invention, there is provided an air filter including a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is first formed by pelletizing a mixture of
the powder of the fraipontite mineral and the powder of an inorganic substance as a binder and is then coated with the powder of an inorganic substance, or the pellet is first formed by pelletizing the powder of an inorganic substance and is then coated
with the powder of the fraipontite mineral, using the powder of an inorganic substance as a binder.
Still further, according to the invention, there is provided an air filter including a casing filled up with a plurality of pellets, wherein the pellet is formed by pelletizing the powder of the fraipontite mineral, using the powder of an
inorganic substance as a binder, or the pellet is first formed by pelletizing the powder of the fraipontite mineral, using the powder of an inorganic substance as a binder and is then coated with the powder of an inorganic substance, or the pellet is
first formed by pelletizing the powder of an inorganic substance and is then coated with the powder of the fraipontite mineral, using the powder of an inorganic substance as a binder.
Still further, according to the invention, there is provided an air filter including a supporter and an inorganic material layer that is formed by fixing a mixture to the surface of the supporter, using an inorganic substance as a binder, the
mixture consisting of the powder of the fraipontite mineral and the powder of manganese oxide and/or permanganate.
Still further, according to the invention, there is provided an air filter including a supporter, a first inorganic material layer formed of the powder of the fraipontite mineral and an inorganic substance as a binder, and a second inorganic
material layer formed of the powder of manganese oxide and/or permanganate and the powder of an inorganic substance as a binder, wherein either the first inorganic material layer or the second inorganic material layer is directly fixed to the surface of
the supporter, and the remaining inorganic material layer is additionally fixed to the surface of the first fixed inorganic material layer.
Still further, according to the invention, there is provided an air filter including a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is formed by pelletizing a mixture of the
powder of the fraipontite mineral and the powder of manganese oxide and/or permanganate, using the powder of an inorganic substance as a binder.
Still further, according to the invention, there is provided an air filter including a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is first formed by pelletizing the powder of
the fraipontite mineral, using the powder of an inorganic substance as a binder and is then coated with the powder of manganese oxide and/or permanganate, using the powder of an inorganic substance as a binder, or the pellet is first formed by
pelletizing the powder of manganese oxide and/or permanganate, using the powder of an inorganic substance as a binder, and is then coated with the powder of the fraipontite mineral, using the powder of an inorganic substance as a binder.
Still further, according to the invention, there is provided an air filter including a casing filled up with a plurality of pellets, wherein the pellet is first formed by pelletizing a mixture of the powder of the fraipontite mineral and the
powder of manganese oxide and/or permanganate, using the powder of an inorganic substance as a binder, or the pellet is first formed by pelletizing the powder of the fraipontite mineral, using the powder of an inorganic substance as a binder and is then
coated with the powder of manganese oxide/permanganate, using an inorganic substance as a binder, or the pellet is first formed by pelletizing the powder of manganese oxide and/or permanganate, using an inorganic substance as a binder and is then coated
with the powder of the fraipontite mineral, using the powder of an inorganic substance as a binder.
The fraipontite mineral is aluminosilicate belonging to a 1:1 type layer clay mineral of serpentine subgroup and its composition is expressed by the following formulas (I) and (2). As shown in FIG. 1, the fraipontite mineral is a crystal having
a double structure, of which one side exhibits solid basic properties while the other side has solid acid properties. These solid basic and acid sides respectively act as the adsorption site for acid and basic substances. According to the report by N.
Takahashi, M. Tanaka and T. Sato, entitled "Structure of Synthetic Fraipontite" (The Chemical Society of Japan, 1991, No. 7, pp. 962-967), a single layer of the fraipontite mineral has a thickness of 7.1 angstroms.
In case of using solely or in combination the powder of the fraipontite mineral, the powder of an inorganic substance as a binder, and the powder of manganese oxide and/or permanganate in order to form the above-mentioned inorganic material layer
or pellet layer for use in the filter according to the invention, adjacent powder particles make gaps or air permeable pores therebetween. When the object gas passes through the filter, it comes in and comes out such gaps or air permeable pores. While
the object gas passes through the filter, the fraipontite mineral contained in the inorganic material layer acts on the object gas, thereby adsorbing and removing gaseous acid and/or basic impurities contained in the object gas. When the powder of
manganese oxide and/or permanganate is contained in the inorganic material layer, it will act on and remove sulfur oxides SOx which is harmful for manufacture of semiconductor devices, magnetic disks, and so forth, even though contained in the object
gas. For instance, sulfuric acid used in the rinsing process inside the clean room, atmospheric pollutants and volcanic gas contained in the outdoor air introduced into the clean room and so on will serve as a potential source of sulfurous acid gas,
which reacts with ammonia gas in the atmosphere as mentioned above and gives the wrong influence to the manufacture of semiconductor devices, LCD substrates, magnetic recording parts, and so forth.
An inorganic substance used as a binder for forming the first or second inorganic material layers may be the same as or different from the inorganic substance for forming the second or first inorganic material layer. For instance, if the
inorganic substance used as a binder for forming the first inorganic material is the clay mineral such as talc, kaolin mineral, bentonite, etc., the inorganic material layer for forming the second inorganic material may be selected from such clay
mineral. On one hand, if the inorganic substance such as silica or the like having the strong affinity for the boric compound is included in both of the first and second inorganic material layers, the total quantity of silica or the like is naturally
increased, so that the change with the passage of time with regard to the protection efficiency against the boric compound is naturally made smaller, thus making it possible to effectively remove the boric compound over a long period of time. In order
to prevent the fraipontite mineral from its exfoliating and acting as a dust source, if the inorganic material layer that is directly fixed to the supporter surface of the filter i.e. the first inorganic material layer is formed including the powder of
the fraipontite mineral, the outermost surface of it is preferably covered by another inorganic material layer i.e. the second inorganic material layer that is formed of, for instance, the powder of an inorganic substance or a mixture of the powder of an
inorganic substance and the powder of manganese oxide and/or permanganate. In case of the pellet, if it is formed including the powder of the fraipontite mineral, it is preferable to coat its outermost surface in the same way as taken in respect of the
inorganic material layer. Especially, in case of forming the outermost layer or coat layer including silica gel, the most effective protection may be achieved against the dust that is caused by exfoliation of the fraipontite mineral.
Gaseous acid and/or basic impurities in the clean room air can be adsorbed and removed by means of the fraipontite mineral. On the other hand, gaseous organic impurities such as DOP, DBP, siloxane and so forth, which contaminate the surface of
the substrate, can be adsorbed and removed by properly selecting an inorganic substance as a binder, which includes micropores and/or mesopores. Accordingly, the filter according to the invention is applicable to even the atmosphere including various
chemical contaminants which cause the surface contamination of the substrate, and may capture almost all of such.
In case of making use of the fraipontite mineral for removing gaseous acid and/or basic impurities contained in the atmosphere, as shown in FIG. 2, a single layer or stacked several layers of the fraipontite mineral are preferably arranged in
random around an inorganic binder particulate without overlapping with each other, such that the object air to be cleaned may come in contact with the adsorption site that exists on the surface of the single layer or stacked several layers of the
fraipontite mineral. The object air may come in contact with the fraipontite mineral directly or through the inorganic material layer, which is formed of the inorganic substance such as manganese oxide, permanganate, or the like, and includes such air
permeable gaps or pores that the object air can pass therethrough. Accordingly, the single layer or stacked several layers of the fraipontite mineral have to be evenly distributed inside a porous structure (i.e. inorganic material layer of the
invention) such that the object air entering and coming out of this porous structure may come in contact with the adsorption site existing on the surface of the microcrystal (shape: circular, thickness: several tens angstroms, diameter: 100 angstroms to
1 micrometer) of the fraipontite mineral, thereby removing gaseous acid and/or basic impurities.
The microcrystal of the fraipontite mineral includes neither micropore nor mesopore capable of physically adsorbing gaseous organic impurities, thus it having no capability to adsorb and remove gaseous organic impurities.
It is preferable that the supporter for use in the filter according to the invention is made in the form of a honeycomb structure. It is also preferable that the honeycomb structure is formed of inorganic fiber as the indispensable requirement
therefor. Furthermore, if the supporter is made of ceramics, it will be provided with a strong and hard surface, which is preferable from the standpoint of installation and maintenance of the filter. The term "honeycomb structure" as used herein stands
for not only a structure with the shape of a honeycomb but also any structure including a plurality of cells the air can flow therethrough, for instance a structure having a grid shape in section, a structure formed of a plurality of corrugated sheets
and flat sheets which are stacked alternately, and so on. Furthermore, the term "supporter" is not limited to the honeycomb structure but may be a three dimensional mesh structure like rock wool. In this case, the powder of the fraipontite mineral is
preferably fixed not only to the surface of the mesh structure but also to the inside thereof.
Manganese oxide as used in the filter according the invention, may preferably be any one selected from tri-manganese tetraoxide (Mn.sub.3 O.sub.4), di-manganese trioxide (Mn.sub.2 O.sub.3), and manganese dioxide (MnO.sub.2). For instance, the
most popular sulfurous acid gas representing the sulfur oxides is hardly neutralized with zinc or magnesium in the fraipontite mineral, if it is left as it is. If, however, sulfurous acid gas is oxidized by manganese oxides, sulfurous acid gas is
transformed into sulfuric acid, which in turn, easily reacts with zinc or magnesium contained in the fraipontite mineral, thus being neutralized and removed. The oxidizing strength of manganese monoxide is not so large, so that it would not be expected
that sulfurous acid gas is sufficiently removed with use of it.
In the next, how manganese dioxide MnO.sub.2 removes sulfurous acid gas SO.sub.2 will be explained in the following. At first, sulfurous acid gas in the air dissolves in moisture adsorbed by pores of the honeycomb structure according to the
following formula.
Then, sulfurous acid is immediately oxidized based on the synergistic effect between the reduction process by sulfurous acid and the oxidation process by manganese dioxide, thereby being transformed into sulfuric acid according to the next
formula.
Sulfuric acid can be also produced by other manganese oxides than manganese dioxide MnO.sub.2 according to the following reaction.
Manganese oxides may transform sulfurous acid into sulfuric acid not only by its oxidizing function but also by its catalytic action. When manganese oxides act as a catalyst, its oxidation number is kept unchanged.
In the next, sulfuric acid as produced above, further reacts with zinc contained in the fraipontite mineral to produce zinc sulfate according to the following formula.
The above chemical reaction formulas regarding manganese dioxide can be arranged as follows.
As mentioned above, sulfurous acid gas reacts with zinc and is removed producing a salt, thereby an environment suitable for manufacturing high precision electronic parts or the like being prepared. Since manganese dioxide is substantially
insoluble in water, it is preferably prepared in the form of the bulk particulate so as to have a diameter of several microns.
During the above chemical reaction, the transformation of MnO.sub.2 .fwdarw.Mn.sub.2 O.sub.3 takes place. Thus, there is fear that Mn.sub.2 O.sub.3 might cover the surface of the bulk particulate of MnO.sub.2 and might prevent manganese dioxide
from carrying out its oxidizing process, thereby its function of adsorbing sulfur oxides being weakened. However, di-manganese trioxide (Mn.sub.2 O.sub.3) is soluble in the acid (sulfuric acid in this case), so that the fresh surface of manganese
dioxide can be always provided by the above-mentioned action of sulfuric acid. This process can take place when tri-manganese tetraoxide (Mn.sub.3 O.sub.4) or di-manganese trioxide (Mn.sub.2 O.sub.3) is used in place of manganese dioxide (MnO.sub.2), so
that the above fear is eliminated.
Permanganate used in the filter according to the invention, may preferably be either M.sup.I MnO.sub.4 (M.sup.I : alkali metal) or M.sup.II (MnO.sub.4).sub.2 (M.sup.II : alkaline earth metal). Sulfurous acid is immediately oxidized with the
synergistic effect between the oxidation process by permanganate and the reduction process by sulfurous acid and is transformed into sulfuric acid, which, in turn, causes the neutralizing reaction with zinc or magnesium contained in the fraipontite
mineral, thereby removing sulfurous acid gas in the same as described above in connection with manganese oxides.
The inorganic substance used in the filter according to the invention, may preferably be the one selected from talc, kaolin mineral, bentonite, diatom earth, silica, alumina, mixture of silica and alumina, aluminum silicate, activated alumina,
porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, and activated bentonite. These inorganic substances can be advantageously used for adsorbing and removing gaseous organic impurities.
The powder of the fraipontite mineral has no self-bonding property, so that a certain binder has to be additionally used, if it is pelletized or fixed to the surface of the supporter to form a layer thereon. The above-mentioned silica may be replaced by
silica gel, while alumina may be replaced by alumina gel. Similarly, the mixture of silica and alumina may be replaced by the mixed gel consisting of silica gel and alumina gel.
The inorganic substance preferably has the pore volume of 0.2 cc/g or more, and the specific surface area of 100 m.sup.2 /g or more, respectively, in its pore diameter range of 15 to 300 angstroms. If inorganic substance selected as such is used
as a binder for the inorganic material layer or the pellet formed of the powder of the fraipontite mineral, the inorganic substance serves as a kind of an adhesive for mechanically fixing the powder of the fraipontite mineral to the surface of the
supporter, or as a bonding agent for pelletizing the powder of the fraipontite mineral. Furthermore, such inorganic substance may provide a porous structure, through which the object gas to be cleaned can reach acid and/or basic points that exist on the
surface of the crystal layer of the fraipontite mineral.
The specific surface area and the total volume of the pore of which the diameter is in range of 15 to 300 angstroms, are measured with regard to various inorganic substances capable of being used as a binder, by means of the BET method and the
nitrogen gas adsorption method, respectively. Table 1 shows the measurement result thereof.
TABLE 1 ______________________________________ Pore Volume (cc/g) Specific by N.sub.2 Adsorption Area (m.sup.2 /g) Pore size: 15-300 Powder Sample by BET Method angstrom ______________________________________ diatom earth 200 0.25 Group
(I) silica gel 400 0.61 alumina gel 250 0.60 aluminum silicate 225 0.60 activated alumina 300 0.30 porous glass 400 0.15 sepiolite 295 0.33 activated clay 300 0.37 activated bentonit 87 0.23 talc 28 0.07 Group (II) kaolin mineral 21 0.06
bentonite 23 0.03 ______________________________________
In the above measurement, silica gel is used as the powder sample on behalf of silica, and alumina gel is used as the powder sample in place of alumina. Similarly, the mixture of silica gel and alumina gel is used as the powder sample in place
of the mixture of silica and alumina. The reason why the measurement is carried out with regard to the pore in its pore diameter range of 15 to 300 angstroms, is that the pore in this range functions well as a physical adsorbent against gaseous organic
impurities. A number of gaps or air permeable pores are formed between the adjacent powder of the fraipontite mineral, and also between the adjacent powder of the inorganic substance as a binder, and further between the powder of the fraipontite mineral
and the powder of the inorganic substance as a binder. The object air to be cleaned may flow through these gaps or air permeable pores and reach the adsorption site existing on the surface of the microcrystal of the fraipontite mineral, where gaseous
acid impurities as well as gaseous basic impurities contained therein are adsorbed and removed. During this process, the object air comes to encounter a countless number of pores which exist on the powder surface of the inorganic substance used as a
binder and have a pore diameter in the above-mentioned range, thereby gaseous organic impurities contained in the object air being adsorbed and removed. Accordingly, besides the removal of both gaseous acid and basic impurities, which is one of the main
purposes of the invention, the removal of gaseous organic impurities can be secondarily achieved by properly selecting the pore of the powder of the inorganic substance used as a binder, and employing the inorganic substance belonging to the group (I) as
shown in Table 1.
It will be understood from Table 1 that the specific surface area and the pore volume of the inorganic binder belonging to the group (I) are considerably larger than those of the inorganic binder belonging to the group (II). Therefore, the
inorganic binder of the group (I) is superior to those of the group (II) in respect of the physical adsorption function against gaseous inorganic impurities. The inorganic binders of the group (II), for instance talc, kaolin mineral, and bentonite are
selected and used primarily taking account of air permeability of the porous structure (e.g. inorganic material layer) constructed therewith. Most of air permeable pores that are formed between adjacent binder particulates and also between the binder
particulate and the adsorbent particulate, have a size of about 500 angstroms or more. In other words, such air permeable pore is in the macropore range and less contributes to the physical. Furthermore, the particulate surface of the inorganic binder
such as talc, kaolin mineral, and bentonite, includes few pores capable of performing the physical adsorption of gaseous organic impurities. Consequently, the inorganic binder of the group (II) is primarily used for the purpose of mechanically holding
the powder of the fraipontite mineral on the surface of the supporter. The quantity of the powder of the fraipontite mineral to be held on the supporter surface is preferably increased as much as possible in order to sufficiently remove gaseous
inorganic impurities. On the other hand, the quantity ratio of the inorganic binder to the powder of the fraipontite mineral is preferably made as less as possible. However, if the quantity of the inorganic binder is reduced excessively, the powder of
the fraipontite mineral is incompletely held on the supporter surface, which might | | |
