JP2010222723A - Flameproof fiber bundle and method for producing carbon fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a flameproof fiber bundle capable of providing a carbon fiber bundle having few structural defects in a single fiber in a method for producing the carbon fiber bundle; and to provide the method for producing the carbon fiber bundle capable of achieving the stabilization of the strength at the production of the carbon fiber bundle, especially the stabilization of the strength at the start of the production. <P>SOLUTION: The method for producing the flameproof fiber bundle by heating a polyacrylonitrile-based fiber bundle having a fiber fineness of 100-30,000 dtex and a number of filaments of 500-70,000 in a heat treating furnace in which an oxidizing gas is circulated in an oxidizing atmosphere at 200-300°C. The method includes: circulating the oxidizing gas through a filtering device which has a filter medium unit having an aggregate in which 97-100% by volume of ceramic particles or metal particles have a particle diameter of 2-80 mm, and packed in the interior thereof, and which is installed in the interior of the heat-treating furnace and/or the circulating duct part of the heat-treating furnace. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention is a method for producing a flame-resistant fiber capable of producing a flame-resistant fiber that gives a carbon fiber having a stable strength in the carbon fiber production method.

  It is known that a composite having excellent mechanical properties can be obtained by using, as a reinforcing material, a carbon fiber having few structural defects of a single fiber constituting a carbon fiber bundle. In order to provide such a carbon fiber bundle with few structural defects of a single fiber, various technical developments are performed.

  As one of such technologies, a carbon fiber bundle having few structural defects of single fibers is finally obtained by removing dust contained in the oxidizing gas in a flameproofing furnace used in the process of carbon fiber production. Proposed to provide. For example, a method has been proposed in which air blown to a flameproofing furnace is filtered and cleaned using a sintered metal filter of 1 μm or less (for example, Patent Document 1). However, in this method, the pressure loss due to dust collection increases, and fluctuations in the circulating wind speed in the flameproofing furnace can occur, so it is not easy to control the conditions. In addition, the sintered metal filter is highly likely to be deformed by heat, and the glazing changes due to the deformation, so that maintenance becomes complicated.

  Further, as another technique for the same purpose, in order to prevent foreign matter such as strands and powder derived from strands from accumulating and contaminating the flameproof fiber, foreign matter removal means such as a wire mesh is provided in the hot air circulation path 42. It is conceivable to provide it. In the flameproofing furnace used in the carbon fiber manufacturing process, the air volume inside the hot air circulating furnace is detected by the hot air speed sensor, and the control unit converts the detected air signal into an output control signal of the hot air circulating means based on the value of the detection signal, and output control. A method has been proposed in which the output of the hot air circulating means is controlled by a signal to keep the air blowing amount of the hot air circulating means constant, and foreign matter in the furnace is removed using a porous plate such as a wire mesh or a punching plate (for example, Patent Document 2). However, the method of controlling the output of the fan, which is the hot air circulation means, to keep the hot air circulation air flow constant is to control the rotation speed of the fan when extreme pressure loss occurs due to clogging of the metal mesh part or punching plate. Therefore, it is practically difficult to keep the wind speed constant.

Japanese Patent Laid-Open No. 58-220821 JP 2006-57222 A

  Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art, and in the carbon fiber bundle manufacturing method, even if foreign matter is generated or mixed in the flameproofing furnace, it can be efficiently removed, and the above background The object is to obtain a production method that does not cause the problems described in the technology. According to this manufacturing method, it can be expected that a carbon fiber bundle with few structural defects of single fibers can be provided even if foreign matter is generated or mixed in the flameproofing furnace.

The present invention employs the following means in order to solve such problems. That is,
(1) A flame-resistant fiber bundle in which a polyacrylonitrile fiber bundle having a yarn fineness of 1000 to 30000 dtex and a filament number of 500 to 70000 is heated at 200 to 300 ° C. in an oxidizing atmosphere in a heat treatment furnace in which an oxidizing gas is circulated. It is a manufacturing method, Comprising: In the said heat processing furnace (The heat processing furnace which obtains a flame-resistant fiber bundle by heating at 200-300 degreeC by said oxidizing atmosphere is also described as a flame-resistant furnace. Alternatively, a filtration device having a filter medium unit in which an aggregate of ceramic particles or metal particles having a particle size of 2 to 80 mm on a volume basis is installed in the circulation duct portion of the heat treatment furnace is installed. A method for producing a flame-resistant fiber bundle in which an oxidizing gas is circulated through an apparatus.

  (2) The flameproof fiber according to (1), wherein the aggregate filled in the filter medium unit is formed of at least one material selected from the group consisting of alumina, iron, aluminum, and stainless steel. A method of manufacturing a bundle.

  (3) The above (1) or (2), wherein a total surface area of the aggregate filled in the filter medium unit is 7 to 600 times a cross-sectional area perpendicular to the direction of the oxidizing gas flow in the heat treatment furnace. ) For producing a flame-resistant fiber bundle.

(4) The time required for the oxidizing gas to pass through the filter medium unit is 0.15 to 0.15.
The method for producing a flame-resistant fiber bundle according to any one of (1) to (3), which is 1.0 seconds.

  (5) Said (1)-(4) whose dispersion | variation in the wind speed of the said oxidizing gas in each position of the cross section orthogonal to the direction of the flow of the oxidizing gas of the said heat treatment furnace is less than +/- 20% with respect to the average The manufacturing method of the flame-resistant fiber bundle in any one of.

  (6) The method for producing a flameproof fiber bundle according to any one of (1) to (5), wherein a heater is installed in the circulation duct at a position upstream of the filtration device.

  (7) The flame resistance according to any one of (1) to (6), wherein a bypass line is provided in the circulation duct, and a filtration device and a circulation fan are provided in the bypass line. A method of manufacturing a fiber bundle.

  (8) A method for producing carbon fibers, wherein the flame-resistant fiber bundle obtained by the production method according to any one of (1) to (7) is fired at 300 to 3000 ° C. in an inert atmosphere.

  According to the present invention, the structural defect of the single fiber is removed by continuously removing the dust in the flameproofing furnace using a filtration device filled with a ceramic or metal aggregate having a diameter of 2 to 50 mm. A flame-resistant fiber bundle capable of providing a small number of carbon fiber bundles can be obtained, and even if foreign matter is generated or mixed in the flame-proofing furnace, the strength of the carbon fiber bundle is suppressed and maintained stably. can do. Moreover, by applying this manufacturing method, the wind speed in a flame-resistant furnace can be kept constant, and it can produce without causing a problem also in a disaster prevention aspect.

  Moreover, strength stabilization can be achieved in a short time by applying the present invention at the start of production of carbon fiber bundles where the possibility of mixing such foreign substances is relatively high.

It is a schematic model diagram which shows an example of the flameproofing furnace used by this invention. It is a schematic model diagram which shows an example of the filter medium unit used by this invention. It is a schematic model diagram which shows another example of the flameproofing furnace used by this invention. It is a schematic model diagram which shows another example of the flameproofing furnace used by this invention. It is a schematic model diagram which shows an example of the flameproofing furnace used by a comparative example. It is a schematic model diagram which shows an example of the flameproofing furnace used by a comparative example. It is a schematic diagram which shows an example of the filter used by a comparative example.

  In the present invention, a plurality of polyacrylonitrile fiber bundles are allowed to run in parallel at a predetermined interval in an oxidizing atmosphere in a heat treatment furnace in which an oxidizing gas is circulated, and heat treatment is performed to produce a flameproof fiber bundle. The fineness of a single polyacrylonitrile fiber bundle targeted by the present invention is 1000 to 30000 dtex. If it is less than 1000 dtex, it is difficult to improve the strength even if the present invention is applied. If it exceeds 30000 dtex, the amount of dust passing through the heat treatment furnace increases due to the large amount of yarn passing through. Therefore, it is necessary to frequently perform maintenance such as dust removal due to clogging. Preferably it is 1500-20000 dtex. In addition, the fineness of the yarn of the polyacrylonitrile fiber bundle indicates an average value per 1 m of the yarn weight obtained by sampling the yarn length of 10 m with N number of 5 while preventing the yarn from loosening.

  Moreover, the polyacrylonitrile fiber bundle used for this invention is the filament number 500-70000 of one thread | yarn. If the number of filaments is less than 500, the number of filaments is small and the proportion of defective single yarns may increase. Therefore, it is difficult to improve the strength even if the present invention is applied. This is because during the production, dust is clogged in the aggregate of ceramic grains or metal grains, and thus maintenance such as dust removal needs to be frequently performed. The number of filaments is more preferably 1000 to 60000.

  The number of yarns of the polyacrylonitrile fiber bundle that is simultaneously processed in the method for producing a flame resistant fiber bundle of the present invention is not particularly limited, but is preferably 10 to 2000. If the number is less than 10, the amount of dust generated due to the raw yarn is small, so it is not necessary to apply the present application. On the other hand, if the number exceeds 2000, the amount of dust increases. The process may not be stable. From this point, more preferably 20 to 1500.

  In the method for producing a flameproof fiber bundle of the present invention, the running speed of the yarn is not particularly limited, but is preferably 0.5 to 40 m / min. If it is slower than 0.5 m / min, the time required to pass through the flameproofing furnace becomes longer, so that even if the present invention is applied, a small amount of dust floating in the furnace adheres to the running yarn, and pressure bonding such as a roller. May cause a defect in the running yarn and reduce the strength, and if it is faster than 40 m / min, it may be difficult to cope with the yarn breakage treatment, and the difficulty of production management becomes high, which is not preferable. From this viewpoint, 1.0 to 30 m / min is more preferable.

  In the method for producing a flameproof fiber bundle of the present invention, the treatment temperature of the heat treatment furnace is 200 to 300 ° C. If the temperature is lower than 200 ° C., the heat treatment is insufficient, so that yarn breakage occurs in a high-temperature furnace. If the temperature exceeds 300 ° C., the running yarn accumulates heat and causes a runaway reaction, resulting in a fire. Preferably it is 220-290 degreeC.

  The material of the heat treatment furnace and the circulation duct part of the heat treatment furnace applied to the method for producing a flame resistant fiber bundle of the present invention is not particularly limited as long as it can withstand the above treatment temperature, but generates metal dust. Stainless steel (SUS) is desirable because it is difficult.

  In the flameproof fiber bundle manufacturing method of the present invention, a filtration device is installed in the heat treatment furnace and / or in a circulation duct portion of the heat treatment furnace, and an oxidizing gas is circulated through the filtration device. The filter medium unit in the filtration device is filled with an aggregate of ceramic particles or metal particles having a particle size range of 2 mm to 80 mm in a proportion of 97 to 100% based on the volume of all particles. ing. The shape of the ceramic grains or metal grains is preferably a sphere from the viewpoint of securing a void. If particles with a particle size of less than 2 mm exceed 3% on the basis of the volume of all particles, when the dust in the heat treatment furnace accumulates on the aggregate, it tends to be subject to pressure loss and the wind speed is lowered, which is dangerous for disaster prevention. Moreover, when the particle size of particles larger than 80 mm exceeds 3% on a volume basis in all particles, it is difficult to collect the dust in the furnace, and the filtration effect cannot be exhibited. A more preferable range of the particle diameter is 3 mm to 30 mm. Moreover, as long as it is in this range, aggregates having different particle sizes may be mixed. In addition, the particle diameter here means (i) the diameter of a volume equivalent sphere as a representative diameter, and (ii) the diameter of individual particles.

  As the aggregate of ceramic particles or metal particles filled in the filter medium unit, a material having excellent heat resistance such as alumina, iron, aluminum, and stainless steel can be preferably used, but stainless steel is preferable from the viewpoint of being resistant to corrosion, and dust. From the viewpoint of not only collecting silica, which is the main component, but also adsorbing silicon as a silica generation source to suppress dust generation, it is preferable to use alumina that is excellent in silicon adsorption. In addition to the filter medium unit, it is also preferable to install honeycomb-like, foam-like, powder-like, or filter-like alumina and / or ceramic in a heat treatment furnace for the purpose of silicon adsorption.

  The surface area of the aggregate of ceramic particles or metal particles filled in the filter medium unit is preferably 7 to 600 times the cross-sectional area perpendicular to the direction of the oxidizing gas flow in the heat treatment furnace. If it is less than 7 times, the dust may not be sufficiently removed and the effect may not be exhibited. On the other hand, if it exceeds 600 times, it tends to be subject to pressure loss, and the wind speed in the heat treatment furnace may decrease, which may be dangerous for disaster prevention. More preferably, it is 10 to 350 times. Here, the method of measuring the surface area of the aggregate of ceramic particles or metal particles is to calculate the surface area of the sphere by measuring the diameter of 100 particles randomly sampled from the spherical particle aggregate one by one. The average value was used as a representative surface area for the entire particle size. As for the shape and attachment method of the filter medium unit, if the structure can collect dust efficiently by installing a large surface area of the aggregate of ceramic particles or metal particles filled in the wind direction, it is cylindrical. Although not particularly limited, such as a rectangular parallelepiped or an aggregate thereof, for example, a rectangular parallelepiped filter medium unit having a length of 1000 mm and a width of 200 mm as shown in FIG. 2 is inclined at an angle of 45 ° with respect to the wind speed in the heat treatment furnace. A preferred method is to install and collect the dust in the heat treatment furnace.

In the method for producing a flame-resistant fiber bundle of the present invention, it is preferable that the time during which the oxidizing gas in the heat treatment furnace contacts the aggregate of ceramic particles or metal particles in the filter medium unit is 0.15 seconds or more. This is because if it is less than 0.15 seconds, the dust collection efficiency is lowered, and dust in the oxidizing gas circulating in the heat treatment furnace may not be collected sufficiently. In consideration of the upper limit of efficiency, the contact time between the oxidizing gas and the aggregate of ceramic particles or metal particles in the filter medium unit is preferably 1.0 second or less. Here, the contact time between the ceramic particles or metal particles aggregate in the filter medium unit and the oxidizing gas is the filling volume of the ceramic particles or metal particle aggregate in the filter medium unit, that is, the apparent volume including the internal voids. It is a value obtained by dividing [m ³ ] by the flow rate of oxidizing gas [Nm ³ / sec].

  In the heat treatment furnace used in the method for producing a flame-resistant fiber bundle of the present invention, the variation in the wind speed of the oxidizing gas at each position of the cross section perpendicular to the direction of the oxidizing gas flow in the heat treatment furnace is relative to the average. It is preferable to control the wind speed to be within ± 20%. When the wind speed spot exceeds ± 20%, the wind speed spot causes dust to be collected at a local portion of the filtration device, which may easily cause pressure loss. In order to improve wind speed spots, a method of installing a baffle plate in the heat treatment furnace can be considered, but the shape of the baffle plate may be set according to the position where the filtration device is installed. The shape and the like are not particularly limited as long as the rectifying plate is designed to be inserted at a point where the wind speed is high so that the point wind speed is uniform. For example, as shown in FIG. 1, it is preferable to install a rectifying plate on the upstream side of the shaded part filtration device so that the wind speed spots in the machine width direction are constant.

  When installing the filtration device in the circulation duct, it is preferable to install the filtration device on the downstream side of the heater, which is a dust generation source, and the dust generated in the heater section can be collected by the filtration device.

  In the heat treatment furnace used in the method for producing a flameproof fiber bundle of the present invention, in order to collect dust in the heat treatment furnace over a long period of time, a bypass line is provided in the circulation duct, and a filtration device is provided in the bypass line. It is also preferable to provide a circulation fan. In such a case, in order to make it difficult to receive a change in the wind speed due to the pressure loss, it is preferable to provide a damper capable of adjusting the pressure at the branch portion of the bypass line so as to manage the pressure constant.

  The flame-resistant fiber bundle obtained by the method for producing a flame-resistant fiber bundle of the present invention can be baked in the range of 300 to 3000 ° C. under an inert atmosphere to produce carbon fibers. At this time, it is preferable that the baking is performed in two steps or three steps at the above temperature. For example, when firing in three steps, a step of firing in an inert atmosphere of 300 to 1000 ° C, a step of firing in an inert atmosphere of 1000 to 2000 ° C, and an inertness of 2000 to 3000 ° C Firing is divided into three steps of firing in an atmosphere. When firing in two steps, the step of firing in an inert atmosphere of 300 to 1000 ° C. and the step of firing in an inert atmosphere of 1000 to 2000 ° C. are performed. Which of the two steps or the three steps is applied is selected depending on the required characteristics of the carbon fiber. An apparatus responsible for these two to three steps is called a carbonization furnace. The carbonization furnace includes a heat treatment chamber for heat treatment in an inert atmosphere, and an inlet portion and an outlet portion before and after the heat treatment chamber. A seal chamber for maintaining an inert atmosphere is provided. The carbonization treatment of the flame-resistant fiber bundle is continuously performed by the carbonization furnace provided with the seal chamber and the heat treatment chamber, thereby enabling continuous production of the carbon fiber bundle. A carbon fiber bundle can be obtained by performing energization treatment in a chemical solution after firing in a carbonization furnace and / or sizing treatment with a resin.

The following examples further illustrate the present invention. In this example, 20 polyacrylonitrile fiber bundles (single fiber fineness: 1.1 dtex, number of single fibers: 12,000) which are carbon fiber precursor fibers. Machine width 0.5 m, length A flame-resistant fiber bundle was produced by performing an oxidation treatment at an in-furnace temperature of 250 ° C. using an outside-roller flame-proofing furnace similar to that shown in FIG. The average wind speed in the flameproofing furnace was set to 1.0 m / sec, and the oxidation treatment time was set to 60 minutes in total.In addition, Tosoh was forcibly scattered in the flameproofing furnace. Silica type VN3 manufactured by Co., Ltd. was placed in a 100 g flameproofing furnace and circulated, and each time the examples were completed, the inside of the flameproofing furnace was washed with ion-exchanged water to discharge the silica. The flame-resistant fiber is then heated up to 700 ° C in the previous carbonization furnace. After firing, it was fired in a carbonization furnace at a maximum of 1400 ° C. Then, it was energized with dilute sulfuric acid, dried after sizing treatment and rolled up into a carbon fiber package, which started to circulate silica in a flameproofing furnace. 5 hours later, the strand tensile strength was measured using the running yarn that passed through the flameproofing furnace, and the strand tensile strength was measured as follows: ERL 4221 (Dow Chemical Japan Co., Ltd.) Manufactured) / boron trifluoride monoethylamine (BF ₃ · MEA) / acetone = 100/3/4 parts of resin is impregnated into a carbon fiber bundle, and the resulting resin-impregnated strand is heated at 130 ° C. for 30 minutes. After curing, it was measured according to the resin impregnated strand test method specified in JIS R 7608 (2007), and circulation of the silica was started in a flameproofing furnace. Using an electrostatic capacity type particle counter (Kansai Automation Co., Ltd. Model DT270) to monitor the number of dust in the oxidization oven after 5 hours.
Example 1
As shown in FIG. 1, a filter medium unit having a combination of a V shape having a thickness of 200 mm, a length of 1000 mm, and an angle of 45 ° as shown in FIG. 2 is installed on the downstream side of the heater 3 of the circulation duct 2 of the flameproofing furnace 1. Then, an aggregate of alumina having a particle size of 5 mm was placed in the filter medium unit. The surface area of the aggregate is 100 times the cross-sectional area perpendicular to the flow direction of the oxidizing gas, and the time required for the oxidizing gas to pass through the filter medium unit is 0.2 seconds. The polyacrylonitrile fiber was oxidized while operating the circulation duct. Thereafter, pre-carbonization and carbonization were performed as described above. Table 1 shows the strand physical property evaluation results of the carbon fiber and the amount of dust in the flameproofing furnace.
(Example 2)
As shown in FIG. 3, a filter medium unit having a combination of the V shapes shown in FIG. 2 was installed at the bottom of the flameproofing furnace 1, and an aggregate of alumina having a diameter of 5 mm was placed in the filter medium unit. The surface area of the aggregate is 100 times the cross-sectional area perpendicular to the flow direction of the oxidizing gas, and the time required for the oxidizing gas to pass through the filter medium unit is 1.0 second. The polyacrylonitrile fiber was oxidized while operating the circulation duct. Table 1 shows the strand physical property evaluation results of the carbon fiber and the amount of dust in the flameproofing furnace.
Example 3
As shown in FIG. 4, a V-shape having a thickness of 200 mm, a length of 1000 mm, and an angle of 45 ° as shown in FIG. 2 is combined on the downstream side of the heater 3 in the circulation duct 2 of the flameproofing furnace 1 and the lower part of the flameproofing furnace 1. A filter medium unit having a shape of the shape was installed, and an aggregate of alumina having a diameter of 5 mm was placed in the filter medium unit. The variation in the wind speed of the oxidizing gas at each position of the cross section perpendicular to the direction of the oxidizing gas flow in the flameproofing furnace 1 is ± 5% at maximum with respect to the average for the duct portion, and in the flameproofing furnace. Was ± 20% at maximum with respect to the average. Further, the surface area of the aggregate is 100 times the cross-sectional area orthogonal to the direction of the flow of the oxidizing gas, and the time required for the oxidizing gas of the filter medium unit to pass is 0. 0 for the filtration device installed in the duct portion. 2 seconds and 1.0 second in the filtration apparatus installed in the flameproofing furnace. The polyacrylonitrile fiber was oxidized while operating the circulation duct. Table 1 shows the strand physical property evaluation results of the carbon fiber and the amount of dust in the flameproofing furnace.
(Comparative Example 1)
As shown in FIG. 5, the oxidation treatment of the polyacrylonitrile fiber was carried out while operating the circulation duct without installing the filtration device having the filter medium unit as shown in FIG. Thereafter, pre-carbonization and carbonization were performed as described above. Table 1 shows the strand physical property evaluation results of the carbon fiber and the amount of dust in the flameproofing furnace.
(Comparative Example 2)
As shown in FIG. 6, a metal filter having an opening of 1 μm as shown in FIG. 7 was installed in the flameproofing furnace 1 in the flameproofing furnace duct, and the polyacrylonitrile fiber was oxidized while operating the circulation duct. . Thereafter, pre-carbonization and carbonization were performed as described above. However, since the average wind speed in the flameproofing furnace fell below 0.5 m / sec due to clogging in the filter part, it was dangerous for disaster prevention and the test was stopped.
(Reference example)
A pre-carbonization treatment and a carbonization treatment were performed under the same conditions as in Comparative Example 1 except that silica was not forcedly introduced into the flameproofing furnace. Five hours after the start of carbonization, the carbon fiber was sampled, and the strand physical property evaluation results and the amount of dust in the flameproofing furnace are shown in Table 1. In addition, five hours after the start of the carbonization process at the sampling timing is a time when the operation is stabilized.

If the method for producing a carbon fiber bundle of the present invention is applied, a dust collector is installed in a flameproofing furnace or / and a duct at the start of production, and dust in the flameproofing furnace is continuously removed, so that Dust adhesion can be suppressed, and defects caused by dust cannot be generated in a high-temperature furnace, and stable strength can be maintained. Further, by removing the SiO ₂ occupying most of the dust components at the same time, it is possible to significantly reduce the SiO ₂ to be discharged to the atmosphere, is very effective in terms of environmental measures.

1: Flame-proofing furnace 2: Circulating duct 3: Heater 4: Filtration device 5: Circulating fan 6: Exhaust line 7: Fresh air supply line 8: Traveling yarn 9: Granular alumina 10: Metal filter having an opening of 1 μm

Claims (8)

  In a method for producing a flame-resistant fiber bundle, a polyacrylonitrile fiber bundle having a yarn fineness of 1000 to 30000 dtex and a filament number of 500 to 70000 is heated at 200 to 300 ° C. in an oxidizing atmosphere in a heat treatment furnace in which an oxidizing gas is circulated. And a filter medium unit in which an aggregate of 97-100% of ceramic particles or metal particles having a particle size of 2 mm to 80 mm on a volume basis is filled in the heat treatment furnace and / or a circulation duct portion of the heat treatment furnace. The manufacturing method of the flame-resistant fiber bundle which installs the filtration apparatus which has and circulates oxidizing gas through this filtration apparatus.   The method for producing a flameproof fiber bundle according to claim 1, wherein the aggregate filled in the filter medium unit is formed of at least one material selected from the group consisting of alumina, iron, aluminum, and stainless steel. .   The total surface area of the aggregate filled in the filter medium unit of the filtration device is 7 to 600 times the cross-sectional area perpendicular to the direction of the oxidizing gas flow in the heat treatment furnace. A method for producing a flameproof fiber bundle.   The method for producing a flame resistant fiber bundle according to any one of claims 1 to 3, wherein a time required for the oxidizing gas to pass through the filter medium unit is 0.15 to 1.0 seconds.   The variation in the wind speed of the oxidizing gas at each position of the cross section orthogonal to the direction of the oxidizing gas flow in the heat treatment furnace is within ± 20% of the average thereof. A method for producing a flameproof fiber bundle.   The manufacturing method of the flame-resistant fiber bundle in any one of Claims 1-5 in which the heater is installed in the position which becomes the upstream of the said filtration apparatus in the said circulation duct.   The method for producing a flameproof fiber bundle according to any one of claims 1 to 6, wherein a bypass line is provided in the circulation duct, and a filtration device and a circulation fan are provided in the bypass line.   The manufacturing method of the carbon fiber which bakes the flame-resistant fiber bundle obtained by the manufacturing method in any one of Claims 1-7 at 300-3000 degreeC by inert atmosphere.