FLUORINE-CONTAINING GAS DECOMPOSING/REMOVING AGENT, METHOD FOR PRODUCING SAME, AND FLUORINE-CONTAINING GAS REMOVING METHOD AND FLUORINE RESOURCE RECOVERY METHOD EACH USING SAME

Abstract

The present invention relates to a fluorine-containing gas removing agent comprising an alumina and an alkali earth metal compound, wherein an ammonia desorption curve obtained by an ammonia TPD-MS method having a mass-to-charge ratio of 15 has a peak in a range lower than 200 C. and has a shoulder in a range of 200 C. or higher.

Claims

1. A fluorine-containing gas removing agent comprising alumina and an alkaline earth metal compound, wherein an ammonia desorption curve by an ammonia TPD-MS method of mass-to-charge ratio 15 has a peak in a range of less than 200 C. and has a shoulder in a range of 200 C. or more.

2. The removing agent according to claim 1, wherein the shoulder is present in the range of 230 to 350 C.

3. The removing agent according to claim 1, wherein the amount of ammonia desorbed in a temperature range of 100 to 450 C. is 10.0 to 100.0 mmol/kg per weight of the removing agent in the ammonia TPD-MS spectrum measurement of the mass-to-charge ratio 15.

4. The removing agent according to claim 1, wherein the amount of ammonia desorbed in a temperature range of 230 to 450 C. is 35.0 to 55.0 when the amount of ammonia desorbed in a temperature range of 100 to 450 C. is 100 in the ammonia TPD-MS spectrum measurement of the mass-to-charge ratio 15.

5. The removing agent according to claim 1, wherein the alumina comprises a crystalline alumina.

6. The removing agent according to claim 5, wherein the crystalline alumina has a single peak in 2=45 to 47 in the X-ray diffraction pattern.

7. The removing agent according to claim 5, wherein the crystalline alumina comprises a alumina.

8. The removing agent according to claim 5, wherein the crystalline alumina has a peak at 2=42.60.5 in the X-ray diffraction pattern.

9. The removing agent according to claim 5, wherein the crystalline alumina comprises a alumina.

10. The removing agent according to claim 1, wherein the alkaline earth metal compound is at least one compound selected from the group consisting of a magnesium oxide, a calcium oxide, a magnesium carbonate, a calcium carbonate, a calcium hydroxide and a magnesium hydroxide.

11. The removing agent according to claim 1, wherein the molar ratio of aluminum atoms:alkaline earth metal atoms is 1:9 to 5:5.

12. The removing agent according to claim 1, wherein the sum of the weight of the aluminum oxide and the alkaline earth metal oxide is 70 weight % or more based on the total weight of the removing agent as aluminum atom is converted to an aluminum oxide (Al.sub.2O.sub.3) and the alkaline earth metal atom is converted to an alkaline earth metal oxide.

13. The removing agent according to claim 1, wherein the removing agent comprises no metal element other than aluminum and the alkaline earth metal.

14. The removing agent according to claim 1, wherein the fluorine-containing gas is selected from the group consisting of a fluorinated hydrocarbon and a perfluoro compound.

15. The removing agent according to claim 14, wherein the fluorinated hydrocarbon is selected from the group consisting of CHF.sub.3, CH.sub.2F.sub.2, CH.sub.3F, C.sub.2HF.sub.5, C.sub.2H.sub.2F.sub.4, C.sub.2H.sub.3F.sub.3, C.sub.2H.sub.4F.sub.2 and C.sub.2H.sub.5F.

16. The removing agent according to claim 14, wherein the perfluoro compound is selected from the group consisting of CF.sub.4, C.sub.2F.sub.6, C.sub.3F.sub.8, C.sub.4F.sub.10, C.sub.4F.sub.8, C.sub.4F.sub.6, C.sub.5F.sub.12, C.sub.5F.sub.10, C.sub.5F.sub.8, SF.sub.6 and NF.sub.3.

17. A method of manufacturing the removing agent according to claim 1, the method comprises steps of: mixing and/or kneading alumina and/or a alumina with an alkaline earth metal compound and optionally a dispersion medium; forming the resulting mixture; and optionally drying and/or calcining the formed mixture.

18. The method according to claim 17, wherein the drying is performed at 100 to 150 C.

19. The method according to claim 17, wherein the calcining is performed at 550 to 800 C.

20. A method of manufacturing the removing agent according to claim 1, the method comprises steps of: mixing and/or kneading a bayalite and/or a gibbsite with an alkaline earth metal compound and optionally a dispersion medium; forming the resulting mixture and optionally drying; and calcining the formed mixture.

21. The method according to claim 20, wherein the drying is performed at 100 to 150 C.

22. The method according to claim 20, wherein the calcining is performed at 550 to 800 C.

23. A method of decomposing a fluorine-containing gas and immobilizing fluorine generated by the decomposition in a removing agent, the method comprises steps of: heating the removing agent according to claim 1 to a temperature of 350 to 800 C.; and inletting the fluorine-containing gas in the said removing agent at a space velocity of 100 to 1000 h.sup.1, while maintaining the above temperature.

24. The method according to claim 23, wherein the temperature is 350 to 600 C.

25. The method of decomposing and removing a fluorine-containing gas, the method comprises a step of: contacting the fluorine-containing gas with the removing agent according to claim 1 without supplying water or oxygen from the outside.

26. A method of recovering fluorine from a fluorine-containing gas, the method comprises steps of: decomposing the fluorine-containing gas by contacting the fluorine-containing gas and the removing agent according to claim 13, and immobilizing fluorine generated from the decomposition in the form of an alkaline earth metal fluoride in the removing agent; optionally grinding the fluorine-immobilized removing agent to separate the alkaline earth metal fluoride and the alumina; and separating fluorine from the removing agent by treating the fluorine-immobilized removing agent or the alkaline earth metal fluorides obtained from grinding/separating the removing agent with a solution to dissolve the alkaline earth metal fluoride.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 Particle size distribution curve of the aluminum raw material used for the preparation of removing agent of each examples. Example 1 (Al raw material: bayalite), Example 5 (Al raw material: gibbsite), Comparative Example 2 (Al raw material: boehmite).

[0030] FIG. 2 X-ray diffraction pattern of Example 2, Example 6, Comparative Example 2 and Comparative Example 1

[0031] FIG. 3 Enlarged view of FIG. 2 focusing on the low diffraction intensity region of Example 2, Example 6, Comparative Example 2, Comparative Example 1

[0032] FIG. 4 Difference spectrum between Example 2 (crystalline alumina+calcium oxide) and Comparative Example 1 (calcium oxide)

[0033] FIG. 5 Shape comparison of the diffraction peaks (2=44.0 to 48.0) for Example 2 and Comparative Example 2

[0034] FIG. 6 Difference spectrum between Example 6 (crystallic aluminum+calcium oxide) and Comparative Example 1 (calcium oxide)

[0035] FIG. 7 X-ray diffractive patterns of analysis example 7 (in-progress product of Example 2 for evaluating CF.sub.4 removing capability)

[0036] FIG. 8 Ammonia TPD-MS spectra of Example 2, Example 6, and Comparative Example 2

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention is a removing agent which decomposes a fluorine-containing gas and fixes to removes fluorine produced by the decomposition as an alkaline earth metal fluoride containing alumina having specific acid properties, preferably crystalline alumina and an alkaline earth metal compound. In this specification, from the viewpoint of decomposing and removing the fluorine-containing gas by treating the gas with the above agent, removing agent is also called decomposition removing agent and treatment agent.

[0038] As described above, the removing agent comprises an alkaline earth metal compound. The alkaline earth metal is selected from the group consisting of beryllium, magnesium, calcium, strontium, barium and radium, preferably magnesium or calcium. Alkaline earth metal compounds which can be used in the present invention include, for example, oxides, carbonates and hydroxides of alkaline earth metals, preferably oxides, among which, for example, calcium oxide, calcium carbonate, calcium hydroxide, magnesium oxide, magnesium carbonate and/or magnesium hydroxide, particularly preferably calcium oxide and/or magnesium oxide, are used.

[0039] In the following, for the sake of simplicity, embodiments using calcium oxide as the alkaline earth metal compound are described in detail. The descriptions of these embodiments are also applicable to other embodiments employing alkaline earth metal compounds other than calcium oxide, and those skilled in the art will be able to properly understand other embodiments by reference to these descriptions.

[0040] As described above, the fluorine-containing gas removing agent of the present invention comprises alumina having specific acid properties. Whether or not the alumina (preferably crystalline alumina) comprised in the fluorine-containing gas removing agent has a particular acid property is determined by shape of the ammonia desorption curve (curve plotting the ion current value against temperature) obtained by ammonia TPD-MS method (using a signal having a mass-to-charge ratio of 15, which is a fragment of ammonia), as explained in the examples. The shape is characterized by having a peak in the range of less than 200 C. and a shoulder in the range of 200 C. or more, as shown in FIG. 8 (Example 2 or 6). Preferably, the ammonia desorption curve has a peak in a range from 160 C. to less than 200 C., more preferably in a range centered at 180 C., e.g., 170 to 190 C., and a shoulder in a range from 230 to 450 C., more preferably in a range from 240 to 400 C., and particularly preferably in a range from 250 to 350 C., e.g., 250 to 340 C., 250 to 330 C., or 250 to 320 C.). Here, preferably the peak is the main peak, that is, the peak with the largest ion current value in the curve. Preferably, the peak can be a single peak in the curve, except for a shoulder. In addition, the above-mentioned ammonia desorption curve may have one or more shoulders, but preferably has one shoulder.

[0041] In the present specification, the peak represents a convex apex portion of the ammonia desorption curve.

[0042] In the present specification, the term shoulder denotes a shoulder or stepped portion in the ammonia desorption curve, i.e., a relatively short portion projecting transversely to a smooth curve slope in the curve.

[0043] In other words, the ammonium TPD-MS spectrum has the above peaks, and further has an inflection point in the range of 200 C. or more, preferably in the range of 240 to 300 C., more preferably 250 to 290 C., in particular 260 to 280 C.

[0044] In the present specification, inflection point represents a point at which the slope of the tangent line at a point of the ammonia desorption curve changes from an increase to a decrease, or a point at which the slope of the tangent line changes from a decrease to an increase. In one preferred embodiment of the present invention, the ammonia desorption curve has the former, i.e. an inflection point at which the slope of the tangent changes from increasing to decreasing, in the above temperature range.

[0045] Therefore, in one embodiment of the present invention, the ammonia desorption curve has a main peak in a range of 160 C. or more and less than 200 C., preferably in a range centered around 180 C., and the ion current value decreases toward a higher temperature side than the peak, in order to have the above-mentioned inflection point (the point at which the slope of the tangent at a certain point of the ammonia desorption curve starts to increase to decrease) in the range of 200 C. or more, in particular, in the range of 260 to 280 C., the above-mentioned shoulder is formed within a specific temperature range. Typically, as shown in FIG. 8 (Examples 2 and 6), the ion current value increases from around 100 C. to reach the main peak value, but the ion current value decreases from this main peak to the high temperature side and decreases to 0 (or around 0) in the temperature range of over 400 C. Then, one shoulder as described above is formed until the ion current value decreases from the main peak value to 0 (or near 0).

[0046] The shoulder may have a sharp shape or may be gentle as long as there is any portion to be recognized as a shoulder compare to a curve having only the main peak (e.g., Comparative Example 2 of FIG. 8).

[0047] The above ammonia desorption curve can have other inflection point other than the above temperature range as long as the above-mentioned shoulder is formed.

[0048] In one embodiment of the present invention, the ion current value at the inflection point (the point at which the slope of the tangent line at a point of the ammonia desorption curve changes from an increase to a decrease) can be 10 to 80%, preferably 30 to 75%, for example 50 to 70% of the ion current value of the peak.

[0049] As described above, the fluorine-containing gas removing agent of the present invention comprises an alumina having specific acid properties, that is, an alumina having the characteristics described above in the ammonia desorption curve of the removing agent containing the alumina. Preferably, the alumina is a crystalline alumina.

[0050] The fluorine-containing gas removing agent of the present invention is required compatibility of decomposition activity at low temperature and high removal capability. In the present invention found that the removing agent could have both decomposition activity at low temperature and high removal capability when the amount of desorbed ammonia per weight of the removing agent (the amount of desorbed ammonia per 1 kg of removing agent) is 5.0 to 150 0 mmol/kg, preferably 7.0 to 120 0 mmol/kg, particularly preferably 10.0 to 100.0 mmol/kg when the amount of ammonia desorbed in the range of 100 to 450 C. is determined by ammonia TPD-MS spectrum. As will be described later, the above-mentioned alumina, preferably crystalline alumina, functions as a catalyst, and calcium oxide functions to fix fluorine as calcium fluoride. The small amount of desorbed ammonia per weight of the removing agent indicates that the number of acid points which can be the active center of the catalyst is small, which leads to a decrease in the fluorine-containing gas decomposition rate. On the other hand, it is also important for the present removing agent that the fixing (removing) ability of fluorine generated by decomposition is high. For this purpose, it is necessary to increase the calcium oxide content as much as possible. Excessively high amounts of desorbed ammonia per weight of removing agent lead to a sacrifice in calcium oxide content. Therefore, the amount of desorbed ammonia per weight of the removing agent is preferably controlled within a range suitable for maximizing the decomposition and removal ability of the removing agent.

[0051] In the case of the present removing agent in which alumina (preferably crystalline alumina) has a specific acid property, which is compatible with the fluorine-containing gas decomposition activity at a low temperature and the removing ability, in addition to a peak (preferably a main peak) centered at less than 200 C., preferably 180 C., it has a shoulder in the range of 230 to 450 C. Since the removing agent containing alumina having such shoulder, preferably crystalline alumina, exhibits excellent performance, it is presumed that the acid point which forms the shoulder is the active center of the fluorine-containing gas decomposition reaction. In the case of such removing agent, it is found in the present invention that in the ammonia TPD-MS spectrum with a mass-to-charge ratio of 15, the amount of ammonia desorbed from the shoulder from 230 to 450 C. is 35 to 55, for example 40 to 50, when the amount of ammonia desorbed from 100 to 450 C. is set to 100.

[0052] The alumina contained in the fluorine-containing gas removing agent having the above-mentioned characteristics in the ammonia desorption curve is preferably crystalline alumina as described above. Preferably, the type of crystalline alumina included in the fluorine-containing gas removing agent having the above characteristics in the ammonia desorption curve is alumina and/or alumina. Accordingly, in one embodiment of the present invention, the alumina may comprise alumina and/or alumina. In a further embodiment of the invention, the alumina consists essentially only of alumina and/or alumina.

[0053] The fluorine-containing gas to be decomposed (to be removed) by the removing agent of the present invention is not particularly limited as long as to be a gas comprising alone or in combination of fluorinated hydrocarbons such as CHF.sub.3, CH.sub.2F.sub.2 and CH.sub.3F, and PFCs such as CF.sub.4, C.sub.2F.sub.6, C.sub.4F.sub.8, NF.sub.3 and SF.sub.6. In one embodiment, the fluorine-containing gas comprises CF.sub.4 and/or C.sub.2F.sub.6, or is a gas consisting of CF.sub.4, C.sub.2F.sub.6, or CF.sub.4 and C.sub.2F.sub.6. In the following, for the sake of brevity, the embodiments in which the removing agent using crystalline alumina and calcium oxide and the CF.sub.4 is subject to decomposition is mainly described in detail.

[0054] The reaction formula for removing fluorine-containing gas by the removing agent of the present invention is represented by (1) to (3) as an example when the alkaline earth metal is calcium oxide and the fluorine-containing gas is CF.sub.4. In formula (1) to (3), ads at the lower right of the molecular formula represents a state adsorbed on the surface of crystalline alumina. Among them, it is presumed that the reaction (1) is such that a specific acid point on the surface of the crystalline alumina particle becomes an active center, and the reaction (2) and (3) proceeds at the interface between the crystalline alumina and calcium oxide.

CF.sub.4.fwdarw.C.sub.ads+4F.sub.ads(1)

CaO+2F.sub.ads.fwdarw.CaF.sub.2+O.sub.ads(2)

C.sub.ads+2O.sub.ads.fwdarw.CO.sub.2(3)

[0055] It can be seen that crystalline alumina is indispensable as a decomposition catalyst of the formula (1) and affect removing ability since the calcium oxide fixes fluorine as calcium fluoride. That is, it can be said that the crystalline alumina and calcium oxide play different roles. As a whole, it proceeds as formula (4).

CF.sub.4+2CaO.fwdarw.2CaF.sub.2+CO.sub.2(4)

[0056] As mentioned above, the removing agent of the present invention comprises the above-mentioned alumina and alkaline earth metal compound, wherein the ratio of the number of aluminum atoms to the number of alkaline earth metal atoms in the removing agent is preferably 0.1:99.9 to 8:2, more preferably 0.5:99.5 to 6:4, particularly preferably 1:9 to 5:5, for example 1.5:8.5 to 5:5 or 2:8 to 5:5. When the number of aluminum atoms and the number of alkaline earth metal atoms are in this range, particularly good fluorine-containing gas decomposition activity at low temperature and removal ability can be compatible. In addition, when the raw material is input to the kneader as described below, the amount of raw material used is measured based on this ratio.

[0057] The total weight of alumina and alkaline earth oxide in the present removing agent is from 50 to 100% by weight, more preferably from 60 to 100% by weight, particularly preferably from 70 to 100% by weight, for example from 80 to 100% by weight or from 90 to 100% by weight, based on the total weight of the removing agent, when calculated assuming that the all aluminum atoms in the removing agent are present as aluminum oxides (Al.sub.2O.sub.3) and all alkaline earth metal atoms are present as their oxides (CaO in the case of Ca, MgO in the case of Mg).

Particularly good removal capabilities can be obtained when the sum of the weights of aluminum oxide and alkaline earth metal oxide based on the total weight of the removal agent, is in this range. In addition, when the raw material is input to the kneading machine as described below, the amount of raw material used is measured based on this ratio.

[0058] In one embodiment of the present invention, the removing agent of the present invention does not comprise metal elements other than aluminum and alkaline earth metals. In this case, the removing agent has the advantage that a complicated operation is not required to recover the calcium fluoride. The removing agent may contain other components, for example, a dispersion medium, a molding aid, and the like, as long as the effects of the present invention are not impaired. In another embodiment of the present invention, the removing agent of the present invention may consist substantially only of alumina and alkaline earth metal oxides.

[0059] The removing agent may also have tap densities of 0.5 to 1 g/ml, preferably 0.6 to 0.9 g/ml, for example 0.7 to 0.85 g/ml. The removing agent having such tap densities are capable of achieving sufficient throughput of fluorine-containing gases.

[0060] Hereinafter, a method for producing the fluorine-containing gas removing agent of the present invention is explained.

[0061] For example, the removing agent can be produced by mixing the alumina with the alkaline earth metal compound (or a raw material of the alkaline earth metal compound), forming the resulting mixture, and optionally drying and/or calcining. Alternatively, the removing agent can be produced by mixing the raw material of the alumina with the raw material of the alkaline earth metal compound, forming the resulting mixture, optionally drying, and calcining.

[0062] The above-mentioned alumina, preferably crystalline alumina, e.g. alumina or alumina, can be prepared, e.g. by calcination, from the raw materials to generate the above-mentioned alumina. For example, in the case of alumina or alumina, bayerite or gibbsite, respectively, can be used as the raw material. Bayerite as a raw material of alumina can be, for example, PURAL BT from SASOL Corporation, and gibbsite as a raw material of alumina can be, for example, CW-350 from Sumitomo Chemical Corporation. The raw material of the alumina may preferably have a median diameter of 45 m or less, for example 20 to 45 m.

[0063] The alkaline earth metal compounds described above, such as calcium oxide or magnesium oxide, are commercially available or can be prepared from raw materials that yield the alkaline earth metal compounds described above, for example, by calcination. For example, in the case where the alkaline earth metal compound is calcium oxide or magnesium oxide, calcium hydroxide or magnesium hydroxide can be used as a raw material. The calcium hydroxide as a raw material of the calcium oxide of the present invention can be, for example, JIS special number slaked lime from Ube Materials Co., Ltd. Since the removing agent of the present invention is typically produced through a paste state in a manufacturing process as shown in the examples below, the raw material in powder form that is easy to handle is preferable.

[0064] In one embodiment, the present invention relates to a method of the above mentioned fluorine-containing gas removing agent, the method comprises steps of:

[0065] mixing and/or kneading the said alumina (preferably alumina and/or alumina) with an alkaline earth metal compound (preferably calcium oxide and/or magnesium oxide) and optionally a dispersion medium;

[0066] forming the resulting mixture; and

[0067] optionally drying and/or calcining the formed mixture.

In another embodiment of the present invention, the present invention relates to a method of manufacturing the fluorine-containing gas removing agent, the method comprises steps of:

[0068] preparing the said alumina (preferably alumina and/or alumina) by calcining the said alumina raw material (preferably bayalite and/or gibbsite),

[0069] mixing and/or kneading the obtained alumina ( alumina and/or alumina) with an alkaline earth metal compound and optionally a dispersion medium;

[0070] forming the resulting mixture; and

[0071] optionally drying and/or calcining the formed mixture.

[0072] In a further embodiment of the present invention, the present invention relates to the method of manufacturing the above said removing agent, the method comprises the steps of:

[0073] mixing and/or kneading the above said alumina raw material (preferably bayalite and/or gibbsite) with an alkaline earth metal compound (preferably calcium hydroxide and/or magnesium hydroxide) and optionally dispersing with a medium;

[0074] forming the resulting mixture and optionally drying; and

[0075] calcining the formed mixture.

The present invention also relates to a fluorine-containing gas removing agent produced by the method.

[0076] In the mixing and/or kneading step described above (hereinafter also referred to simply as kneading collectively), a dispersion medium may be used. Water is suitably used as the dispersion medium, and an organic solvent such as an alcohol or other additives may be used too if necessary. The mixing and/or kneading can be carried out by the methods commonly used in the art (in particular those used for mixing powders), for example using a kneader. The kneader is not particularly limited as long as it can uniformly mix the powder, such as a ribbon blender, a kneader, a mix muller, or a mortar machine.

[0077] Mixed and/or kneaded raw materials (i.e., the resulting mixture/kneaded product, hereinafter, also simply referred to as the mixture obtained) can then be formed.

[0078] When the removing agent is used in powder form the fluorine-containing gas removing ability of the removing agent may be significantly impaired as the contact between the alumina particles (preferably crystalline alumina particles) responsible for the fluorine-containing gas decomposition reaction and the calcium oxide particles responsible for fluorine immobilization is weak, mass transfer at the interface between the two particles is hindered. In order to facilitate mass transfer at the interface between the alumina particles (preferably crystalline alumina particles) and the calcium oxide particles in the fluorine-containing gas removal reaction, it is preferable to consolidate the kneading raw material with an appropriate mechanical load strength and to impart a form to the removing agent. Thus, in one embodiment of the present invention, the removing agent is in the form of a shaped body.

[0079] The shape and size of the removing agent of the present invention can be appropriately selected depending on its use mode, but generally, granules or cylindrical pellets having a diameter of 1 to 5 mm and a length of about 3 to 20 mm are suitably used. However, the present invention is not limited thereto, and various different shapes of pellets, tablet shapes, granules, crushed granules, and the like can be used. For this purpose, any molding equipment capable of consolidating the obtained mixture can be used without any particular limitation, for example, a tablet press, a briquette machine, a pelletizer, a disk pelleter, a plunger extruder, or the like can be used as the molding machine.

[0080] It is also possible to use a kneading molding machine to complete the mixing and molding of the kneading raw materials in one equipment.

[0081] The shaped removing agent can then be dried. -alumina and -alumina which can be used as components of the removing agent of the present invention, and bayerite and gibbsite which can be used as aluminum raw materials for the alumina could form boehmite under a hydrothermal condition. In addition, bayerite and gibbsite could form a complex hydroxide under a hydrothermal condition in admixture with calcium hydroxide.

[0082] Since -alumina produced at the boehmite calcination and the complex oxides produced at the complex hydroxides have a low CF.sub.4 removing capability, the materials after kneading need to be dried under conditions in which they are not produced. Drying temperature for that purpose is preferably from 50 to 200 C., more preferably from 60 to 170 C., still more preferably from 80 to 160 C., particularly preferably from 100 to 150 C., for example from 10 to 130 C. The drying time is preferably from 1 minute to 30 minutes, more preferably from 2 minutes to 15 minutes, particularly preferably from 3 minutes to 10 minutes, for example from 3 minutes to 5 minutes. If the time is too short, residual moisture may adversely effect at the firing process forming -alumina and complex oxides. If the drying time is too long, water vapor in the dryer may be contacted for a long time to form boehmite and complex hydroxides.

[0083] The dryer can be selected from a mesh belt oven, a rotary dryer, an infrared heat dryer, a hot air circulation type dryer or the like without any particular limitation. However, for the reasons described above, it is preferable to select an apparatus capable of reducing the concentration of retained water vapor in the furnace, and to operate the apparatus under conditions suitable for the apparatus.

[0084] The formed removing agent, preferably a removing agent dried after forming, can then be calcined. The precautions to be taken during drying also apply to calcination. The calcination can be carried out, for example, in an air atmosphere. The alumina and alumina that can be used as components of the removing agent of the present invention, as well as the bayalite and gibbsite that can be used as aluminum raw materials for the alumina, can all form boehmite under hydrothermal conditions. In addition, bayerite and gibbsite form complex hydroxides when mixed with calcium hydroxide and subjected to hydrothermal conditions. -alumina oxide formed in the firing of boehmite and the complex oxide produced by complex hydroxides calcination have a low CF.sub.4 removing capability, a firing condition not to produce them is required.

[0085] If the firing temperature is too low, it becomes a removing agent containing a large amount of calcium hydroxide, a water-rich exhaust gas (off-gas) is discharged when being filled into a reaction vessel actually used and heated, which becomes a source of trouble in the post-stage equipment. If the firing temperature is excessively high, crystal components such as alumina, alumina, and alumina, which are not preferable for the present invention, are increased. Therefore, the firing temperature is preferably 450 to 900 C., more preferably 500 to 850 C., particularly preferably 550 to 800 C., for example 650 to 750 C.

[0086] The firing time is preferably 10 minutes to 90 minutes, more preferably 15 minutes to 50 minutes, particularly preferably 20 minutes to 40 minutes. If the time is too short, the calcium hydroxide content becomes high to cause the same trouble as when the calcination temperature is low. If the time is too long, the aluminum raw material may form a composite oxide via alumina or a composite hydroxide via boehmite as it comes into contact with the water vapor in the furnace for a long time.

[0087] The furnace can be selected from a mesh belt furnace, a rotary kiln, an infrared heating furnaces and the like without any particular limitation. However, for the reasons described above, it is favorable to select a device capable of lowering the concentration of retained water vapor in the furnace, and to operate it under conditions suitable for that.

[0088] As described above, the fluorine-containing gas removing agent of the present invention can be produced.

[0089] By using a removing agent produced in this manner, specifically, the removing agent can be in contact with a fluorine-containing gas, typically by maintaining its contact state, to decompose the fluorine-containing gas to fix the generated fluorine in the removing agent, and consequently to remove the fluorine-containing gas.

[0090] Devices in which the removing agent of the present invention is used comprises, for example, moving beds and fluidized beds, and usually used in fixed beds. Further, the details of the structure of these devices are not particularly limited. As a specific example, the removing agent of the present invention is filled in, for example, a cylindrical reaction vessel, and the exhaust gas containing fluorine-containing gas is flown therethrough, whereby the fluorine-containing gas in the exhaust gas can be removed safely and efficiently.

[0091] The removal of the fluorine-containing gas to be treated and removed by the removing agent of the present invention can be carried out on an exhaust gas comprising a fluorine-containing gas of, for example, 0.01 ppmv (parts per million by volume) to 100 vol. %, preferably from 1 ppmv to 10 vol. %, more preferably from 1 ppmv to 5 vol. %; and/or at reaction temperature of 800 C. or less, preferably 350 to 800 C., more preferably 350 to 720 C., even more preferably 350 to 600 C., for example 400 to 580 C. or 460 to 580 C.; and/or at a removing agent layer thickness of 1 to 1000 cm, for example 50 cm to 300 cm; and/or at a space velocity of fluorine-containing gas of 1 to 2000 h.sup.1, for example 100 to 1000 h.sup.1. With respect to the above-mentioned reaction temperature, for example, when the object to be removed is CF.sub.4, when a removing agent containing alumina is used as the alumina, it is possible to perform removal at preferably 350 to 800 C., more preferably 350 to 720 C., still more preferably 350 to 600 C., for example 400 to 520 C., and when a removing agent containing alumina is used as the alumina, it is possible to perform removal at preferably 400 to 800 C., more preferably 450 to 720 C., still more preferably 480 to 600 C., for example 500 to 580 C. On the other hand, for example, when the object to be removed is a C.sub.2F.sub.6, when using a removing agent containing alumina as the above alumina, it is possible to perform removal at preferably 350-800 C., more preferably 350 to 720 C., for example 500 to 620 C., and when using a removing agent containing alumina as the above alumina, it is possible to carry out the removal at preferably 350 to 800 C., for example 600 to 720 C.

[0092] When the removing agent of the present invention is used, the fluorine-containing gas can be removed without supplying water or oxygen from the outside of the reaction system.

[0093] Thus, in one embodiment of the present invention, the present invention relates to a method of treating a fluorine-containing gas, preferably a method of decomposing a fluorine-containing gas, more preferably a method of decomposing a fluorine-containing gas and immobilizing fluorine produced by decomposition in a removing agent (preferably in the form of an alkaline earth metal fluoride), the method comprises steps of:

[0094] heating the above said removing agent at a temperature of 350 to 800 C.; and

[0095] inletting a fluorine-containing gas in the removing agent at a space velocity of 100 to 1000 h.sup.1 while maintaining the above-mentioned temperature.

Preferably, in the method, neither water nor oxygen is supplied from outside. By this method, for example, the fluorine-containing gas in the exhaust gas is decomposed and the fluorine-containing gas is removed from the exhaust gas.

[0096] In another embodiment of the present invention, the invention relates to the use of said removing agent for treating a fluorine-containing gas, preferably for decomposing a fluorine-containing gas, more preferably for decomposing a fluorine-containing gas and immobilizing fluorine produced by decomposition in a removing agent (preferably in the form of an alkaline earth metal fluoride).

[0097] In another embodiment of the present invention, the invention relates to a method recovering fluorine from a fluorine-containing gas, the method comprises steps of:

[0098] heating the removing agent at temperature of 350 to 800 C.,

[0099] decomposing a fluorine-containing gas by inletting a fluorine-containing gas in the removing agent at a space velocity of 100 to 1000 h.sup.1 while maintaining the above-mentioned temperature, and immobilizing fluorine generated by decomposition in the removing agent preferably in the form of an alkaline earth metal fluoride;

[0100] optionally grinding the fluorine immobilized removing agent to separate the alkaline earth metal fluoride and alumina; and

[0101] separating fluorine from the removing agent as hydrogen fluoride by treating the fluorine-immobilized removing agent or the alkaline earth metal fluorides obtained from grinding/separating the removing agent with a sulfuric acid solution to dissolve the alkaline earth metal fluoride. Preferably, in the method neither water nor oxygen is supplied from the outside. By the method the useful resource fluorine can be recovered from the fluorine-containing gas. In particular, this method is advantageous when the removing agent does not contain metal elements other than aluminum and alkaline earth metals.

[0102] In a further embodiment of the present invention, the present invention relates to the use of said removing agent for recovering fluorine from a fluorine-containing gas.

[0103] The end point of use of the removing agent of the present invention can be achieved by monitoring the concentration of the fluorine-containing gas in the gas discharged from the removing agent. The removing agent determined to have reached the throughput limit is removed from the apparatus and disposed. As a disposal method, a treatment solution capable of selectively dissolving only alkaline earth metal fluorides, for example, immersed in a sulfuric acid to recover as hydrogen fluoride. In addition to harvesting, only alkaline earth metal fluoride can be taken out from a mixture of alkali metal fluoride and alumina having different specific gravities and particle diameters, which is obtained by crushing a removing agent judged to have reached the limit of the throughput, by using a gravity concentrating method or a elutriation method, and hydrogen fluoride can be recovered by immersing in sulfuric acid. Also, it can be simply discarded.

[0104] The following examples are provided to illustrate the present invention in detail, but not limited in any way.

EXAMPLES

[0105] Physical property evaluation and performance evaluation of the removing agent used in the following Examples and Comparative Examples were carried out by the following methods.

(1) Particle Size Distribution Measurement: A laser diffraction scattering particle size distribution measuring device, Model Micro79 Track MT3300EX from Microtrack-Bell Co., Ltd. was used. Solvent was water (refractive index: 1.333), and the properties to be measured were aspherical, transmission, refractive index: 1.81.
(2) Tap Density Measurement: 70 g of removing agent was filled in a 100 ml cylinder and read the removing agent fill volume after 100 taps, tap density (g/ml) was determined. The instrument used was according to a model Autotap manufactured by Qantachrome Instruments Japan.
(3) X-ray diffraction measurement: X-ray diffraction method with CuK radiation (45 kV, 40 mA) by using model X'Pert PRO MPD from Spectris Co., Ltd. The detector was X'Celerator (one-dimensional silicon strip detector) also from the same company. In addition, a 1 divergent slit and a 0.04 rad solar slit were attached. In the scan of 2=20 to 70, the step size was 0.017, the scan speed was 0.060/sec. In the scan of 2=44 to 48, the step size was 0.00 2, and the scan speed was 0.004/sec.
(4) CF.sub.4 removal capacity evaluation: The removing agent of 31.4 ml (layer thickness: 10.0 cm) was filled in a 2.0-cm ID reactor made of high-corrosion-resistant nickel-alloy (Hastelloy) installed in a ceramics electrotubular oven and used for the evaluation. Dry N.sub.2 gas was passed through the reactor at a space velocity (GHSV) of 502 h.sup.1, and the temperature was raised to the test reaction temperature over a period of 3 hours, and then the temperature was maintained. The overshoot at the time of temperature rise was controlled within 20 C. After 30 minutes when the test temperature stabilized, the gas flowing into the reactor was switched to a dry N.sub.2 gas (GHSV: 502 h.sup.1) containing 1.00 vol. % of CF.sub.4 gas. The time from the start of the flow of the CF.sub.4 gas into the reactor to the detection of the CF.sub.4 gas of 500 ppmv in the processed gas (decomposition rate: 95%) was defined as the CF.sub.4 gas treatment time, and the CF.sub.4 removing capacity (L/kg) was estimated using the equation (5). A gas chromatograph with a thermal conductivity detector (TCD) (GC-2014, Shimadzu Co., Ltd., packed column filler: Porapack Q, carrier gas: He) was used to measure CF.sub.4 concentration and other reaction products in the decomposition processed gas. With respect to all the test specimens, when a CF.sub.4 gas of 500 ppmv was detected in the processed gas, it was confirmed that F2 and HF gases were not present in the processed gas by using a detection tube (No. 17) from Gastec Corporation. For the decomposition gas detected in the processed gas was only CO.sub.2 except for N.sub.2.

[Chemical Formula 1]

CF.sub.4 removal capacity (L/kg)=space velocity (502 h.sup.1)CF.sub.4 concentration (1.00 vol. %)CF.sub.4 gas treatment time (h)/tap density (kg/L)(5)

(5) Evaluation of C.sub.2F.sub.6 removal capacity evaluation: The evaluation was carried out by the same facilities and procedures as the evaluation of CF.sub.4 removal capacity, except that dry N.sub.2 containing 0.67 vol. % of C.sub.2F.sub.6 gas was used for the test. The time till detect 333 ppmv of C.sub.2F.sub.6 in the processed gas (decomposition rate: 95%) was defined as the C.sub.2F.sub.6 gas treatment time, and the C.sub.2F.sub.6 removing capacity was estimated using equation (6). The reason why the concentration of the decomposition rate C.sub.2F.sub.6 gas was set to 0.67 vol. % was that the concentration of fluorine atoms per unit gas volume in the CF.sub.4 test and the C.sub.2F.sub.6 test were the same. With respect to all the test specimens, when a C.sub.2F.sub.6 gas of 333 ppmv was detected in the processed gas, it was confirmed that F.sub.2 and HF gases were not present in the processed gas by using a detection tube (No. 17) from Gastec Corporation. For the decomposition gas detected in the processed gas was only CO.sub.2 except for N.sub.2. The decomposition gases detected in the natural gas were CO.sub.2 and CO, except for N.sub.2.

[Chemical Formula 1]

C.sub.2F.sub.6 removal capacity (L/kg)=space velocity (502 h.sup.1)C.sub.2F.sub.6 concentration (0.67 vol. %)C.sub.2F.sub.6 gas treatment time (h)/tap density (kg/L)(6)

(6) Effect of Reaction Temperature on CF.sub.4 Decomposition Rate: The removing agent of 31.4 ml (layer thickness: 10.0 cm) to be tested was filled in a Hastelloy reactor with an inner diameter of 2.0 cm installed in a ceramic electrotubular oven and used for evaluation. In the test, 15 steps of temperature raising operation were performed at 30 C. intervals from 300 C. to 720 C. In the respective temperature step, when the temperature became constant, dry N.sub.2 (GHSV: 502 h-1) containing 1,000 ppmv of CF.sub.4 gas was flown in, and after 15 minutes the CF.sub.4 concentration on the inlet side of the reactor gas and after 30 minutes the CF.sub.4 concentration on the outlet side of the reactor gas were examined. Equation (7) was used to calculate the CF.sub.4 decomposition rate. The gas chromatograph with a TCD detector (GC-2014, Shimadzu Co., Ltd., packed column filler: Porapack Q, carrier gas: He) was used for CF.sub.4 concentration. The dry N.sub.2 gas (GHSV: 502 h.sup.1) was circulated during the period the 1,000 ppmv CF.sub.4 gas was not circulated when the temperature was not stable during the period of the temperature rise and the temperature hold thereafter.

CF.sub.4 decomposition rate (%)=[inlet gas CF.sub.4 concentration (ppmv)processed gas CF.sub.4 concentration (ppmv)]/inlet gas CF.sub.4 concentration(ppmv)100(7)

(7) Ammonia TPD-MS measurements: Measured with a catalytic analyzer model BELCAT-B from Microtrack Bell Co., Ltd. A mass-spectrometer was OmniStar GSD301 O.sub.2 from Pfeiffer Vacuum Inc. In order to avoid overlapping of the signals caused by water and CO.sub.2 fragments (mass-to-charge ratio 16 and 17), mass-to-charge ratio 15 of the ammonium fragment was used for data analysis. The sample to use in the test was quickly crushed with agate mortar and pestle to prevent exposure to the outside air. Approximately 100 mg of sample was placed in the sample cell and used for the measurement. As a pretreatment for the measurement, the temperature was maintained at 500 C. for 60 minutes under a He gas flow at a flow rate of 50 ml/min, and then, 5% ammonia-He at temperature of 100 C. and flow rate of 50 ml/min was circulated for 30 minutes for ammonia adsorption. After switching to a He gas of the flow rate 30 ml/min and holding for 30 minutes, the ammonia desorption curve was obtained of between 100 to 450 C. while rising the temperature at rate of 10 C./min from 100 C. up to 610 C. When the temperature reached 610 C., the temperature was held for 10 minutes to terminate the measurement.

[0106] In determining the amount of desorbed ammonia (unit: mmol/kg) per weight of the removing agent, the weight of the removing agent after the ammonia TPD-MS measurement above was used for the calculations. The removing agent weight was determined by subtracting the previously determined empty sample cell weight from the sample cell weight after the ammonia TPD-MS measurement.

Example 1

[0107] Preparation of a removing agent sample comprising alumina and a calcium oxide is as follows. Bayerite (Al(OH).sub.3) powder and a calcium hydroxide powder were weighed so that the molar ratio Al(OH).sub.3:Ca(OH).sub.2 was 5:5, made a kneaded cake (mixture) by mixing in a mix muller (Shinto Kogyo Co., Ltd., Model MSG-0LS) while adding water. Disk Peletor (Non-2 Paudal Co., Ltd., Model F-5) was used to make the kneaded cake in a grain shape of a diameter approximately 2 mm and a length about 6 mm. The obtained grain shape was dried for 5 minutes in a hot air circulation type electric dryer maintained at 120 C. The dried grain shape was calcined in a rotary kiln (an external heat batch type rotary kiln from Takasago Kogyo Co., Ltd.) under a flow of dry air (dew point: 50 C.) of 50 L/min while rotating the retort at 1.5 rpm. The calcining was carried out by holding 700 C. for 30 minutes. The removing agent sample was obtained of Example 1 after cooling to room temperature by operating a cooling blower attached to the rotary kiln.

[0108] The resulting sample was stored in a desiccator containing silica gel and taken out at various tests. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2. The results of analysis on the amount of desorbed ammonia per weight of the removing agent obtained by the ammonia TPD-MS measurement are shown in Table 3.

Example 2

[0109] A removing agent sample of Example 2 of the molar ratio Al(OH).sub.3:Ca(OH).sub.2 of 3:7 was prepared and stored in the same method and conditions of Example (Ex.) 1. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2. The results of analysis on the amount of desorbed ammonia per weight of the removing agent obtained by the ammonia TPD-MS measurement are shown in Table 3.

Example 3

[0110] A removing agent sample of Example 3 of the molar ratio Al(OH).sub.3:Ca(OH).sub.2 of 2:8 was prepared and stored in the same method and conditions of Example 1. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2. The results of analysis on the amount of desorbed ammonia per weight of the removing agent obtained by the ammonia TPD-MS measurement are shown in Table 3.

Example 4

[0111] A removing agent sample of Example 4 of the molar ratio Al(OH).sub.3:Ca(OH).sub.2 of 1:9 was prepared and stored in the same method and conditions of Example 1. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2. The results of analysis on the amount of desorbed ammonia per weight of the removing agent obtained by the ammonia TPD-MS measurement are shown in Table 3.

Example 5

[0112] A removing agent sample of Example 5 comprising alumina and a calcium oxide was prepared and stored in the same method and conditions of Example 1 except that a gibbsite powder (Al(OH).sub.3) and a calcium hydroxide powder were used as raw materials and the molar ratio Al(OH).sub.3:Ca(OH).sub.2 was 5:5. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2. The results of analysis on the amount of desorbed ammonia per weight of the removing agent obtained by the ammonia TPD-MS measurement are shown in Table 3.

Example 6

[0113] A removing agent sample of Example 6 of the molar ratio Al(OH)3:Ca(OH)2 of 3:7 was prepared and stored in the same method and conditions of Example 5. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2. The results of analysis on the amount of desorbed ammonia per weight of the removing agent obtained by the ammonia TPD-MS measurement are shown in Table 3.

Example 7

[0114] A removing agent sample of Example 7 of the molar ratio Al(OH)3:Ca(OH)2 of 2:8 was prepared and stored in the same method and conditions of Example 5. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2. The results of analysis on the amount of desorbed ammonia per weight of the removing agent obtained by the ammonia TPD-MS measurement are shown in Table 3.

Example 8

[0115] The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 500 C. of the sample prepared and stored in exactly the same method and conditions as in Example 3 are shown in Table 2.

Example 9

[0116] The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 570 C. of the sample prepared and stored in exactly the same method and conditions as in Example 6 are shown in Table 2.

Example 10

[0117] A removing agent sample comprising alumina and a magnesium oxide was prepared and stored in the same method and conditions of Example 1 except that a bayerite powder and a magnesium hydroxide powder were used as raw materials and the molar ratio Al(OH).sub.3:Ca(OH).sub.2 was 3:7. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2.

Example 11

[0118] A removing agent sample of Example 11 comprising alumina and a magnesium oxide was prepared and stored in the same method and conditions of Example 1 except that a gibbsite powder and a magnesium hydroxide powder were used as raw materials and the molar ratio Al(OH).sub.3:Ca(OH).sub.2 was 3:7. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2.

Comparative Example 1

[0119] The removing agent sample of Comparative Example (Com. Ex.) 1 consisting only of a calcium oxide was prepared and stored in the same manner and conditions as in Example 1 except that only calcium hydroxide powder was used as a raw material. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 3.

Comparative Example 2

[0120] The removing agent sample of Comparative Example 1 comprising alumina and a calcium oxide was prepared and stored in the same manner and conditions as in Example 1 except for using boehmite (AlOOH) powder and calcium hydroxide powder as raw materials and the molar ratio AlOOH:Ca(OH).sub.2 was 3:7. The tap density and the CF.sub.4 removing agent capacity at the test temperatures of 600 C. are shown in Table 2. The results of analysis on the amount of desorbed ammonia per weight of the removing agent obtained by the ammonia TPD-MS measurement are shown in Table 3.

TABLE-US-00002 TABLE 2 CF.sub.4 removing agent capacity of the fluorine-containing gas removing agent comprising a crystalline alumina and an alkaline earth metal compound Molar ratio CF.sub.4 Al atoms:Alkaline removing Crystalline Alkaline earth earth Tap CF.sub.4 gas agent Alumina metal compound metal atoms density test temp. capacity Ex. 1 alumina Calcium oxide 5:5 0.76 g/ml 600 C. 99 L/kg Ex. 2 alumina Calcium oxide 3:7 0.74 g/ml 600 C. 120 L/kg Ex. 3 alumina Calcium oxide 2:8 0.74 g/ml 600 C. 129 L/kg Ex. 4 alumina Calcium oxide 1:9 0.8 g/ml 600 C. 81 L/kg Ex. 5 alumina Calcium oxide 5:5 0.75 g/ml 600 C. 96 L/kg Ex. 6 alumina Calcium oxide 3:7 0.74 g/ml 600 C. 99 L/kg Ex. 7 alumina Calcium oxide 2:8 0.8 g/ml 600 C. 98 L/kg Ex. 8 alumina Calcium oxide 2:8 0.74 g/ml 500 C. 84 L/kg Ex. 9 alumina Calcium oxide 3:7 0.74 g/ml 570 C. 82 L/kg Ex. 10 alumina Magnesium oxide 3:7 0.71 g/ml 600 C. 137 L/kg Ex. 11 alumina Magnesium oxide 3:7 0.71 g/ml 600 C. 108 L/kg Com. Ex. 1 None Calcium oxide 0:10 0.79 g/ml 600 C. 0 L/kg Com. Ex. 2 alumina Calcium oxide 3:7 0.77 g/ml 600 C. 0 L/kg

TABLE-US-00003 TABLE 3 Ratio of the desorbed ammonia amount (100 to 450 C.) <A> of the fluorine-containing gas removing agents comprising a crystalline alumina and an alkaline earth metal compound and as the value is 100, the desorbed ammonia amount (230 to 450 C.) <B>. Desorbed ammonia Desorbed ammonia <B> ratio (100 to 450 C.) (230 to 450 C.) when <A> is <A> <B> 100 Ex. 1 73.0 mmol/kg 33.0 mmol/kg 46 Ex. 2 47.6 mmol/kg 22.7 mmol/kg 48 Ex. 3 34.9 mmol/kg 15.4 mmol/kg 44 Ex. 4 21.7 mmol/kg 7.9 mmol/kg 44 Ex. 5 25.5 mmol/kg 10.2 mmol/kg 40 Ex. 6 14.9 mmol/kg 6.4 mmol/kg 43 Ex. 7 10.7 mmol/kg 4.4 mmol/kg 41 Com. Ex. 2 38.0 mmol/kg 11.4 mmol/kg 30

Example 12

[0121] The tap density and the C.sub.2F.sub.6 removing agent capacity at the test temperatures of 600 C. of the sample prepared and stored in exactly the same method and conditions as in Example 3 are shown in Table 4.

Example 13

[0122] The tap density and the C.sub.2F.sub.6 removing agent capacity at the test temperatures of 700 C. of the sample prepared and stored in exactly the same method and conditions as in Example 6 are shown in Table 4.

Comparative Example 3

[0123] The tap density and the C.sub.2F.sub.6 removing agent capacity at the test temperatures of 600 C. of the sample prepared and stored in exactly the same method and conditions as in Comparative Example 2 are shown in Table 4.

TABLE-US-00004 TABLE 4 C.sub.2F.sub.6 removing agent capacity of the fluorine-containing gas removing agents comprising a crystalline alumina and an alkaline earth metal compound. Molar ratio Al C.sub.2F.sub.6 atoms:Alkaline C.sub.2F.sub.6 removing Crystalline Alkaline earth earth Tap gas test agent Alumina metal compound metal atoms density temp. capacity Ex. 12 alumina Calcium oxide 2:8 0.74 g/ml 600 C. 78 L/kg Ex. 13 alumina Calcium oxide 3:7 0.74 g/ml 700 C. 56 L/kg Com. Ex. 3 alumina Calcium oxide 3:7 0.77 g/ml 600 C. 0 L/kg

Example 14

[0124] The effect of the reaction temperature on the CF.sub.4 decomposition rate of the sample prepared and stored in exactly the same method and conditions as in Example 3 is shown in Table 5.

Example 15

[0125] The effect of the reaction temperature on the CF.sub.4 decomposition rate of the sample prepared and stored in exactly the same method and conditions as in Example 6 is shown in Table 5.

TABLE-US-00005 TABLE 5 Effect of reaction temperature to CF.sub.4 decomposition rate of the fluorine- containing gas removing agents comprising a crystalline alumina and an alkaline earth metal compound. Example 14 Example 15 CF.sub.4 CF.sub.4 decomposition decomposition Temperature rate Temperature rate 299 C. 0% 301 C. 0% 329 C. 7% 331 C. 0% 359 C. 25% 361 C. 1% 389 C. 64% 392 C. 5% 420 C. 99% 422 C. 13% 450 C. or more 100% 453 C. 30% 480 C. 60% 510 C. 93% 543 C. or more 100%

Analysis Example 1

[0126] Particle size distribution measurements were carried out on the aluminum raw materials used for the preparation of the samples of Example 1 (aluminum raw material: bayalite), Example 5 (aluminum raw material: gibbsite), and Comparative Example 2 (aluminum raw material: boehmite), and the results are shown in FIG. 1.

[0127] Median diameter (Cumulative volume: Particle size at 50%) of the aluminum raw materials were bayerite: 22 m, gibbsite: 4 2 m and boehmite: 46 m.

Analysis Example 2

[0128] X-ray diffraction measurements were carried out on the samples of Example 2, Example 6, Comparative Example 1, and Comparative Example 2. These results are shown in FIG. 2. In drawing FIG. 2, the interval of each spectrum was shifted by 20 cps as the diffraction intensity in order to make each spectrum easier to compare.

[0129] FIG. 2 shows high intensity diffraction peaks (2=32.2, 37.3, 53.9, 64.2, 67.4) derived from calcium oxide (PDF: 37-1497) for all samples. From this, it is understood that the calcium hydroxide used as a raw material was calcined and changed to calcium oxide. The calcination was carried out in an air atmosphere, but calcium carbonate was not confirmed. The PDF used as the basis for the identification of the diffractive patterns refers to the Powder Diffraction File (PDF) of the International Center for Diffraction Data (ICDD) (hereinafter, the same applies).

Analysis Example 3

[0130] An enlarged view of focusing on the low-diffraction-intensity range (Intensity: 0-100 cps) in FIG. 2. is shown in FIG. 3. Aluminum hydroxide represented by Al(OH).sub.3 in the chemical formula includes bayerite, gibbsite, and nordstrandite. When they are fired at 700 C. under atmospheric pressure, it is possible to produce alumina, alumina, and alumina via boehmite. As features of these activated alumina, it is known that they have a peak derived from of the oxygen interatomic distance closely packed at a diffraction angle of 2=67, and that the diffraction peak is broad at low intensity. Looking at the diffraction angle of 2=67 in FIG. 3, it can be seen that the other samples have a wider peak tail than the sample of the calcium oxide itself of Comparative Example 1. This shows that in Comparative Example 1 containing no aluminum raw material, only a high intensity and sharp peak of 2=67.4 caused by calcium oxide appears, whereas in the other samples, a low intensity and broad peak caused by activated alumina overlaps with a peak of 2=67.4 caused by calcium oxide. Activated alumina is known to have spots in electron diffraction images because it is crystalline. From these results and findings, it is concluded that the aluminum compounds contained in the samples of Example 2, Example 6, and Comparative Example 2 are all crystalline alumina.

Analysis Example 4

[0131] Difference spectra between Example 2 (crystalline alumina+calcium oxide) and Comparative Example 1 (calcium oxide) were drawn and shown in FIG. 4 for the purpose of identifying the crystalline alumina comprised in Example 2.

[0132] In the difference spectrum of FIG. 4, the spectrum was disturbed at the diffraction angle of calcium oxide. Focusing on the part where the spectrum was not disturbed, diffraction peaks that could be obtained from either alumina (PDF: 4-0875) or alumina (PDF: 10-0425) were confirmed. From the thermal transition sequence of aluminum hydroxide, it is reasonable to generate alumina from bayerite via alumina or boehmite (if under hydrothermal conditions).

Analysis Example 5

[0133] For the purpose of specifying the crystalline alumina ( alumina and/or alumina) comprised in Example 2, the diffraction patterns of Example 2 and Comparative Example 2 in the vicinity of 2=46 were compared in FIG. 5. Comparative Example 2 clearly contained -alumina from the thermal transition sequence of aluminum hydroxide.

[0134] It is known that the diffraction patterns of alumina and alumina are very similar but there are differences in diffraction patterns around 2=46 and 2=67. At both diffraction angle, -alumina has one diffraction peak, whereas -alumina has two diffraction peaks, which may appear to be the main peak and shoulder. Referring to FIG. 5, it can be seen that Example 2 is alumina having one diffraction peak, whereas Comparative Example 2 is alumina having two diffraction peaks.

Analysis Example 6

[0135] For the purpose of specifying the crystalline alumina comprised in Example 6, the difference spectra of Example 6 (crystalline alumina+calcium oxide) and Comparative Example 1 (calcium oxide) were drawn and shown in FIG. 6. In the difference spectrum of FIG. 6, the spectrum is disturbed in the diffraction angle of calcium oxide. Focusing on the portion where the spectrum was not disturbed, a diffraction peak derived from alumina (PDF: 13-0373) was confirmed. As produced by firing gibbsite at atmospheric pressure, it is alumina, or -alumina via boehmite (if under hydrothermal conditions). It's known only alumina has a diffraction peak at 2=42.6.

Analysis Example 7

[0136] The evaluation test for the CF.sub.4 removing capability at the test temperatures of 600 C. about the sample prepared in exactly the same method and conditions as in Example 2 for the purpose of examining the reaction product. When the CF.sub.4 flow reached 15 hours, the flow was switched to N.sub.2 air flow, and X-ray diffractometry was performed on samples cooled to room temperature. These results are shown in FIG. 7. In FIG. 7, high-intensity diffractive peaks derived from calcium oxide (37-1497) and calcium fluoride (PD F: 87-0971) were confirmed. Diffraction peaks derived from aluminum fluoride were not observed.

Analysis Example 8

[0137] Ammonia desorption curves with mass-to-charge ratios of 15, which were investigated using the ammonia TPD-MS method, were compared for the removing agents of different types of crystalline aluminas. The result is shown in FIG. 8. The peak centrally 180 C. for all removing agent was confirmed. In addition, it was found that the removing agent comprising Example 2 ( alumina) and Example 6 ( alumina), which had a high CF.sub.4 removal capacity, had a common feature with a shoulder in the region of was 230-450 C. On the other hand, no shoulders or peaks were observed in the same regions in Comparative Example 2 (-alumina) in which the CF.sub.4 removing capability was low. Based on these characteristics, it can be seen that the removing agent having a high CF.sub.4 removing capability has an inflection point in the range of 260 to 280 C. in the ammonia desorption curve.

[0138] The results are summarized as follows.

(1) Alkaline earth metal oxides alone cannot decompose fluorine-containing gases (Comparative Example 1).
(2) The removing agent in which the alkaline earth metal oxide and the crystalline alumina alumina or alumina are present in the form of a mixture exhibits a high fluorine-containing gas removal capacity ( alumina: Examples 1 to 4, Example 10, Example 12) ( alumina: Examples 5 to 7, Example 11, Example 13). On the other hand, some removing agents even comprising a crystalline alumina have a low fluorine-containing gas removing ability, such as removing agents consisting of -alumina and alkaline earth metal oxides (-alumina: Comparative Example 2, Comparative Example 3).
(3) The removing agent containing -alumina or -alumina having a high fluorine-containing gas-removing capability has a common characteristic in the ammonia TPD-MS spectrum. These removing agents have shoulders in the region of 230 to 450 C. in addition to peaks centered at 180 C. In view of these characteristics, a removing agent containing alumina or alumina having a high fluorine-containing gas removing ability has an inflection point in a region of 260 to 280 C. On the other hand, since no shoulders or peaks are observed in the region of 230 to 450 C. in the removing agent containing -alumina having a low CF.sub.4 removal capability, the removing agent does not have an inflection point in the region of 260 to 280 C. (Analysis Example 8).
(4) In the removing agent during the decomposition treatment of the fluorine-containing gas, the formation of aluminum fluoride is not observed while the alkaline earth metal oxide is changed to alkaline earth metal fluoride. Crystalline alumina does not change before and after the reaction, but serves as a catalyst to accelerate the reaction. Also, the maximal CF.sub.4 decomposition throughput is determined by the content of alkaline-earth-metal compounds.
(5) From the above results, it is inferred that the fluorine-containing gas removal proceeds in parallel with the following three kinds of reactions.
(i) decomposition reaction of fluorine-containing gases on crystalline alumina catalyst surfaces with specific acid properties,
(ii) reaction where fluorine, the product of decomposition reaction, reacts with alkaline earth metal oxides to form alkaline earth metal fluorides, and
(iii) reaction where oxygen derived from alkaline earth metal oxide subjected to fluorination reaction combines with carbon of the product from reaction (i) to form CO.sub.2.
(6) Both the removing agent containing -alumina and the removing agent containing -alumina can decompose not only CF.sub.4 but also C.sub.2F.sub.6, but a higher temperature is required to achieve the same level of fluorine-containing-gas removing capacity in the C.sub.2F.sub.6 process as during the CF.sub.4 process ( alumina: Example 8, Example 12) ( alumina: Example 9, Example 13).
(7) The alumina-containing removing agent exhibits the same level of fluorine-containing gas removal capability at a lower reaction temperature than the alumina-containing removing agent (Example 8, Example 9). For example the reaction temperature required to remove 1000 ppmv of CF.sub.4 was 450 C. or higher for the removing agent containing alumina, whereas the reaction temperature required 543 C. or higher for the removing agent containing alumina (Examples 14 and 15).

[0139] From the above results, the removing agent comprising an alumina and an alkaline earth metal oxide and having the above characteristics of ammonia desorption curves by ammonia TPD-MS method could decompose and remove the fluorine-containing gas with high efficiency at a lower temperature compare to the conventional removing agent. In the decomposition treatment, there is no need to supply water vapor or oxygen gas from the outside, and in the removing agent the weight composition of the alkaline earth metal element which plays a fluorine immobilization function is large, and as a result, the treatment capacity as a removing agent is enhanced. It has been found that many specifications required as a fluorine-containing gas removing agent can be satisfied where high performance such as recover of fluorine by easily separating the calcium fluoride produced by immobilization, since the constituent metal of the removing agent comprises aluminum and alkaline earth metal elements and does not need to contain any third metal element.