Patent Application
20140327191
The present invention relates to a technique for producing a fired body, and more specifically relates to a technique for producing a fired body from a green body containing an organic component.
Conventionally, a honeycomb filter structure having a large number of through-holes has been widely known for diesel particulate filters and the like. Such a honeycomb filter structure is produced by plugging, with a plugging material, one end of each of a part of through-holes of a green body produced by an extruder, while also plugging, with a plugging material, one end on the opposite side of each of the remaining through-holes, and then firing this. In Patent Literatures 1 and 2, a tunnel kiln for firing a ceramic honeycomb structure is disclosed.
Patent Literature 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-525531
Patent Literature 2: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-527202
Meanwhile, the honeycomb filter structure for diesel particulate filters has a cell structure made of a porous partition wall. Exhaust gas of a diesel engine passes through the partition wall to thereby allow a particulate substance included in the exhaust gas to be removed. The green body contains an organic component such as an organic binder and a pore-foaming agent. The pore-forming agent disappears by firing to thereby form the porous partition wall.
When the green body is fired, a high oxygen concentration in a furnace may cause the organic component contained in the green body to rapidly combust. Such rapid combustion of the organic component results in a rapid increase in the temperature of a body to be fired. As a result, cracks tend to occur on the resulting fired body. As a measure for solving such a drawback, it is considered to reduce the oxygen concentration in the furnace to about 1 to 5% by volume in a debinding step (step of heating the body to be fired to remove the organic component).
While the reduction in the oxygen concentration in the furnace overcomes the above drawback, however, the organic component contained in the green body does not completely combust, and hydrocarbon (HC) and carbon monoxide (CO) which cause tar are generated in the furnace. In the case of a batch type firing furnace, even if tar is generated in the furnace, most thereof combusts and disappears due to the subsequent increase in temperature. On the contrary, the problem with the tunnel kiln is that tar tends to accumulate in a region of a temperature band in which the debinding step is performed.
In the tunnel kiln, the temperature distribution in a furnace is set in advance and a wheeled platform that loads a body to be fired sequentially passes through the respective temperature bands in the furnace to thereby allow processes such as debinding and sintering to proceed. The region in which the debinding step is performed is maintained at a temperature proper for debinding (for example, 100 to 600° C.), and the temperature thereof is not raised at a normal operation to such a degree that tar combusts and disappears. Therefore, tar is attached to a furnace material (heat-insulating material such as a firebrick and ceramic fibers) constituting the inner wall of the region to contaminate the furnace material. Tar may cause the furnace material to be degraded and also be brought into contact with a gas having a high oxygen concentration (for example, air) under a high temperature condition to ignite. Tar attached or soaked into the furnace material is difficult to remove, and it is thus required to periodically replace the furnace material.
The present invention has been made in view of the above problems, and an object thereof is to provide a tunnel kiln whose maintenance is easy and where the lifetime of a furnace material can be longer even if tar is generated in a furnace in a debinding step of an organic component contained in a body to be fired, and a method for producing a fired body using the same.
The tunnel kiln according to the present invention comprises a tunnel kiln main body having an inner wall constituted by a furnace material, and having a debinding zone and a sintering zone wherein respectively debinding and sintering of a body to be fired containing an organic component are performed; a conveying unit for conveying the body to be fired from an inlet side to an outlet side of the tunnel kiln main body; and a lining provided so that at least an inner wall of the debinding zone, of the inner wall of the tunnel kiln main body, is covered therewith.
According to the tunnel kiln, tar can be sufficiently prevented from attaching or soaking into the furnace material by the lining with which the inner wall of the debinding zone is covered. The lining can be constituted by a material that is dense and that has shielding property (for example, stainless). Even if being attached to the lining, tar can be easily cleaned off as compared with the case where tar is attached to the furnace material. In the case where the lining is degraded by tar or the like, it is sufficient only to replace the lining with new one.
The tunnel kiln may further comprise an oxygen concentration adjustment unit for reducing an oxygen concentration in the debinding zone. As the oxygen concentration in the debinding zone is reduced, the organic component is hard to completely combust, and therefore the amount of tar generated is increased. Accordingly, in the case where operation is carried out under a low oxygen concentration in a furnace, disposing of the lining is particularly useful. In addition, in the case where the content of the organic component in the body to be fired is relatively high, there is such an advantage that cracks can be highly inhibited from occurring on a fired body by performing debinding and firing under a low oxygen concentration.
The debinding zone of the tunnel kiln main body may have a region where the organic component contained in the body to be fired is to be gasified and a region where the gasified organic component is to be condensed. For example, the lining may have a cold spot where a temperature of a surface of the lining is partially lower. The cold spot is provided on the lining to thereby allow the gasified organic component to be easily cooled and condensed on the spot. Thus, it is possible to inhibit tar from attaching to a wide range of the lining to thereby make maintenance further effective.
The tunnel kiln main body may further have a cooling zone wherein the fired body is cooled, at an outlet side over the sintering zone. When the tunnel kiln main body is provided with the cooling zone to allow the fired body to move through the zone to be cooled, it is possible to take out the fired body without decreasing the temperature of the entire of the tunnel kiln main body after firing. Thus, it is possible to use the tunnel kiln as a continuous type firing furnace.
The present invention provides a method for producing a fired body using a tunnel kiln having a lining with which a part of its inner wall is covered. That is, this method comprises the steps of: (A) conveying a body to be fired containing an organic component from an inlet side to an outlet side of a tunnel kiln whose inner wall is constituted by a furnace material; (B) debinding the body to be fired in the tunnel kiln; (C) condensing the organic component gasified in step (B) on a surface of a lining provided so that a part of the inner wall of the tunnel kiln is covered therewith; and (D) sintering the body to be fired after debinding.
According to the method for producing the fired body, tar can be sufficiently prevented from attaching or soaking into the furnace material by the lining with which the inner wall of a debinding zone is covered. Even if being attached to the lining, tar can be easily cleaned off as compared with the case where tar is attached to the furnace material. In the case where the lining is degraded by tar or the like, it is sufficient only to replace the lining with new one.
According to the present invention, a tunnel kiln whose maintenance is easy and which allows its furnace material to have longer lifetime, even if tar is generated in a furnace in a debinding step of an organic component contained in a body to be fired, and a method for producing a fired body using the same are provided.
a) is a perspective view illustrating one example of a green molded body for honeycomb structures, and
Hereinafter, a suitable embodiment of the present invention is described in detail with reference to the drawings. Herein, a green molded body for producing a honeycomb filter structure is described with an example. This green molded body is a body to be fired containing an organic component in a relatively large amount.
<Green Molded Body>
A green molded body 70 illustrated in
The length of the direction in which the through-holes 70a of the green molded body 70 extend is not particularly limited, but can be, for example, 40 to 350 mm. In addition, the outer diameter of the green molded body 70 is also not particularly limited, but can be, for example, 100 to 320 mm.
The raw material composition that forms the green molded body 70 is not particularly limited, but in the case of producing a honeycomb structure for diesel particulate filters, includes an inorganic compound source powder serving as a ceramics raw material, and an organic binder such as methylcellulose, as well as an additive added if necessary. From the viewpoint of high-temperature resistance of the honeycomb structure, a suitable ceramics material includes oxides such as alumina, silica, mullite, cordierite, glass and aluminum titanate, silicon carbide, and silicon nitride. Herein, aluminum titanate can further include magnesium and/or silicon.
For example, in the case where a green molded body of aluminum titanate is produced, the inorganic compound source powder can include an aluminum source powder such as an α-alumina powder, and a titanium source powder such as an anatase type or rutile type titania powder, and if necessary, further a magnesium source powder such as a magnesia powder or a magnesia spinel powder and/or a silicon source powder such as a silicon oxide powder or a glass frit.
Examples of the organic binder include celluloses such as methylcellulose, carboxylmethylcellulose, hydroxyalkylmethylcellulose and sodium carboxylmethylcellulose; alcohols such as polyvinyl alcohol; and lignosulfonate.
Examples of the additive include a pore-forming agent, a lubricant and a plasticizer, a dispersant, and a solvent.
Examples of the pore-forming agent include a carbon material such as graphite; resins such as polyethylene, polypropylene and polymethylmethacrylate; a vegetable material such as starch, a nuts shell, a walnut shell and corn; ice; and dry ice.
Examples of the lubricant and the plasticizer include alcohols such as glycerin; higher fatty acids such as caprylic acid, lauric acid, palmitic acid, arachidic acid, oleic acid and stearic acid; a stearic acid metal salt such as stearic acid Al, and polyoxyalkylene ether.
Examples of the dispersant include inorganic acids such as nitric acid, hydrochloric acid and sulfuric acid; organic acids such as oxalic acid, citric acid, acetic acid, malic acid and lactic acid; alcohols such as methanol, ethanol and propanol; and a surfactant such as ammonium polycarboxylate.
As the solvent, it is possible to use alcohols such as methanol, ethanol, butanol and propanol; glycols such as propylene glycol, polypropylene glycol and ethylene glycol; and water.
The total amount of the organic component contained in the green molded body 70 is preferably 10 to 25 parts by mass and more preferably 15 to 20 parts by mass relative to 100 parts by mass of the green molded body 70. If the amount of the organic component is less than 10 parts by mass, the moldability of the green molded body 70 tends to be insufficient and the porosity after firing tend to be small. On the other hand, if the amount of the organic component is more than 25 parts by mass, the amount of tar generated in the debinding step tends to be increased and cracks tend to occur on a fired body.
<Tunnel Kiln>
With reference to
The replacement chamber R is provided so as not to increase the oxygen concentration in the furnace along with carrying of the wheeled platform 5 into the tunnel kiln main body 1. The replacement chamber R has a door D1 at the inlet side, D2 at the outlet side, a nitrogen source N2 and a pipe L1 for feeding nitrogen gas into the replacement chamber R, and a sensor 3 for measuring the oxygen concentration in the replacement chamber R. When the wheeled platform 5 is put in the replacement chamber R, the door D1 at the inlet side is opened in the state where the door D2 at the outlet side (the door of the inlet of the tunnel kiln main body) is closed. After the wheeled platform 5 is put in the replacement chamber R, the door D1 is closed. Thereafter, nitrogen gas is fed into the replacement chamber R to reduce the oxygen concentration in the replacement chamber R. After the oxygen concentration in the replacement chamber R is sufficiently reduced, the door D2 is opened to put the wheeled platform 5 in the tunnel kiln main body 1. Herein, nitrogen gas is preferable as a gas for reducing the oxygen concentration in terms of cost, but helium gas, argon gas, or a mixed gas thereof may also be used instead of nitrogen gas.
When the wheeled platform 5 is put in the tunnel kiln main body 1 from the replacement chamber R, the state where the oxygen concentration in the replacement chamber R is lower than the oxygen concentration in the tunnel kiln main body 1 is preferable. For example, in the case where a debinding treatment is performed in the tunnel kiln main body 1 (debinding zone Z1 described later) in an oxygen concentration of 2% by volume or less, it is preferable to reduce the oxygen concentration in the replacement chamber R to less than 2% by volume (more preferably 1% by volume or less). In the case where the debinding treatment is performed in an oxygen concentration of 1% by volume or less, it is preferable to reduce the oxygen concentration in the replacement chamber R to less than 1% by volume (more preferably 0.5% by volume or less).
The tunnel kiln main body 1 is provided so that the green molded body 70 loaded on the wheeled platform 5 is moved from the inlet side to the outlet side to thereby provide a fired body of the green molded body 70. The inner wall 1a of the tunnel kiln main body 1 is made of a heat-insulating material (furnace material) such as firebrick, a ceramic fiber and a glass fiber. The interior of the tunnel kiln main body 1 is divided into a debinding zone Z1, a sintering zone Z2 and a cooling zone Z3 depending on the temperature conditions. The total length of the tunnel kiln main body 1 may also extend over 100 m. The total length depends on the temperature setting of the respective zones, the composition of the green molded body 70, and the like.
The debinding zone Z1 is a zone for performing a removing step (debinding step) of heating the green molded body 70 to remove the organic component contained in the body 70. The range of the debinding zone Z1 can be a range of temperature conditions where the organic component contained in the green molded body 70 is gasified or combusts. While depending on the organic component contained in the green molded body 70, the inlet side temperature (lower limit temperature) of the debinding zone Z1 is preferably 120° C. or lower, more preferably 100° C. or lower, and further preferably 80° C. or lower. On the other hand, the outlet side temperature (upper limit temperature) of the debinding zone Z1 is preferably 600° C. or higher, more preferably 700° C. or higher, and further preferably 800° C. or higher. If the inlet side temperature of the debinding zone Z1 is more than 120° C., debinding may be started before the green molded body 70 reaches the debinding zone Z1. If the outlet side temperature of the debinding zone Z1 is less than 600° C., the debinding treatment of the green molded body 70 may not be sufficiently completed in the debinding zone Z1.
As illustrated in
In order to adjust the temperature and the oxygen concentration in the sintering zone Z1, for example, a combustion gas may be fed from a combustion chamber 7 (see
While the inner wall 1a of the tunnel kiln main body 1 is made of the heat-insulating material described above, it is provided with a metal case (lining) 1b with which at least the inner wall 1a of the debinding zone Z1 of the entire of the inner wall 1a is covered. The metal case 1b prevents tar generated in the furnace by the debinding treatment from being attached or soaked into the inner wall 1a. While the metal case 1b may be provided on at least the debinding zone Z1, the metal case 1b may also be provided so as to extend over the boundary at the inlet side of the debinding zone Z1 since tar is condensed at a place where the temperature is low. For example, the inner wall 1a may also be covered with the metal case 1b from a position immediately following the door D2 of the replacement chamber R. In this case, the debinding zone Z1 has a region in which the organic component is to be gasified and a region in which the gasified organic component is to be condensed.
The material of the metal case 1b is not particularly limited as long as it can be withstand the temperature and the atmosphere of the debinding treatment, and examples thereof include stainless steel. Herein, the lining with which the inner wall 1a is covered is not limited to one made of metal, and may be, for example, one made of a ceramic material or the like. From the viewpoint of easiness of maintenance, however, a metal case having dense characteristics and having a flat inner surface is preferable. Herein, there is also an advantage of covering a furnace material containing air with the metal case 1b to thereby easily reduce the oxygen concentration in the debinding zone Z1.
The metal case 1b may also have a cold spot 1c where the temperature of the surface is partially low (see
The sintering zone Z2 is a zone for performing firing of the green molded body 70 after debinding. The range of the sintering zone Z2 can be from a temperature at which the debinding treatment of the green molded body 70 is completed to a retention temperature for firing. From the viewpoint of preventing defects such as cracks from occurring at the time of raising the temperature of the body to be fired, the rate of temperature rise is preferably 100 to 20° C./hr and more preferably 80 to 40° C./hr. The rate of temperature rise can be set by adjusting the temperature distribution of the sintering zone Z2 or the speed of the wheeled platform 5. Depending on the composition of the green molded body 70 or the like, the firing temperature (retention temperature for firing) is preferably 1400° C. or higher, more preferably 1450° C. or higher, and further preferably 1500° C. or higher.
The oxygen concentration in the sintering zone Z2 is not required to be particularly controlled, and is determined by the ratio of the amount of a fuel combusted to the amount of air used. While differing depending on the temperature, the oxygen concentration is usually about 5 to 15% by volume.
The cooling zone Z3 is a zone for gradually cooling the fired body. The range of the cooling zone Z3 can be from the retention temperature for firing to a temperature at which the fired body can be removed from a door D3 to the outside. From the viewpoint of preventing defects such as cracks from occurring at the time of cooling the fired body, the rate of temperature drop is preferably 50 to 150° C./hr and more preferably 80 to 100° C./hr. The rate of temperature drop can be set by adjusting the temperature distribution of the cooling zone Z3 or the speed of the wheeled platform 5.
The oxygen concentration in the cooling zone Z3 is not particularly restricted, and is not required to be highly controlled as in the case of the debinding zone Z1. Herein, the cooling zone Z3 is not necessarily required as long as the temperature in the sintering zone Z2 is decreased at the time of terminating the treatment in the sintering zone Z2 to allow the fired body to be taken out, but the cooling zone Z3 is provided to thereby make such a temperature dropping operation unnecessary, making it possible to use the tunnel kiln 10 as a continuous type firing furnace.
A method for producing a fired body, comprising a step of firing the green molded body 70 using the tunnel kiln 10 to provide a honeycomb filter structure (fired body), is described. A method according to the present embodiment comprises the following step (1) to step (9).
(1) A step of preparing a raw material composition containing an inorganic compound source powder (inorganic compound), an organic binder, an additive and a solvent.
(2) A step of molding the raw material composition to provide the green molded body 70.
(3) A step of loading one or more of the green molded bodies 70 on the wheeled platform 5.
(4) A step of putting the wheeled platform 5 in the replacement chamber R, and then reducing the oxygen concentration in the replacement chamber R.
(5) A step of putting the wheeled platform 5 in the tunnel kiln main body 1 from the replacement chamber R, and then conveying the wheeled platform 5 from the inlet side to the outlet side of the tunnel kiln main body 1.
(6) A step of performing debinding of the green molded body 70 in the debinding zone Z1.
(7) A step of condensing the organic component gasified in step (6) on the surface of the metal case 1b.
(8) A step of performing sintering of a body to be fired, after debinding, in the sintering zone Z2.
(9) A step of cooling a fired body in the cooling zone Z3.
According to the method for producing a fired body, even if tar is generated in the debinding zone Z1, the metal case 1b prevents tar from being attached or soaked into the furnace material. Even if being attached to the inner surface of the metal case 1b, tar can be easily cleaned off as compared with the case where tar is attached to the furnace material. In the case where the metal case 1b is degraded by tar or the like, only the metal case 1b may be replaced with new one. In addition, in the case where the cold spot 1c as illustrated in
Hereinabove, the suitable embodiment of the present invention is described in detail, but the present invention is not limited to the above embodiment. For example, the case of firing the green molded body 70 for producing a honeycomb filter structure is shown in the above embodiment, but the body to be fired that can be fired by the tunnel kiln of the present invention is not limited to this green molded body as long as it contains the organic component.
For example, the case of reducing the oxygen concentration in the tunnel kiln main body 1 is shown in the above embodiment, but in the case of firing a green body made of a material excellent in sintering property, the oxygen concentration in the tunnel kiln main body 1 may not be necessarily reduced. In this case, the replacement chamber R is not necessarily required.
For example, the green molded body 70 that is a cylindrical body is shown in the above embodiment, but the shape or structure of the molded body is not limited thereto. The outer shape of the green molded body 70 may be, for example, a rectangular column such as quadrangular prism, or an elliptic column. In addition, the arrangement of the through-holes 70a may not be square arrangement, and may be, for example, substantially triangle arrangement or substantially hexagonal arrangement. Furthermore, the shape of the through-hole 70a may not be square, and may be, for example, a substantially triangle shape, a substantially hexagonal shape, a substantially octagonal shape, a substantially round shape, and a combination thereof. The combination of a plurality of shapes include a combination of a regular hexagonal shape and an asymmetric hexagonal shape (see
A green molded body 80 illustrated in
<Measurement of Organic Component Gasified in Debinding Step>
A green molded body having a shape illustrated in
According to the present invention, a tunnel kiln whose maintenance is easy and where the lifetime of a furnace material can be longer even if tar is generated in a furnace in a debinding step of an organic component contained in a body to be fired, and a method for producing a fired body using the same are provided.
1 . . . tunnel kiln main body, 1a . . . inner wall, 1b . . . metal case, 1c . . . cold spot, 5 . . . wheeled platform (conveying unit), 10 . . . tunnel kiln, 70, 80 . . . green molded body (body to be fired), L2 . . . pipe (oxygen concentration adjustment unit), N2 . . . nitrogen source (oxygen concentration adjustment unit), Z1 . . . debinding zone, Z2 . . . sintering zone, Z3 . . . cooling zone.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-004299 | Jan 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2013/050206 | 1/9/2013 | WO | 00 | 7/10/2014 |
