JP2009084763A - Method for preparing carbon fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preparing a carbon fiber at high yield by stabilizing infusibilization reaction by maintaining a certain state of the penetration of an oxidizing gas regardless of the fluctuation in basis weight of a precursor mat of a carbon fiber and regardless of the clogging of its protection filter for protecting a heating device without using a shape-retention mechanism. <P>SOLUTION: The method for preparing a carbon fiber employs a continuous-charge type infusibilization furnace having a plurality of chambers in series in a direction of transferring a carbon-fiber precursor mat 16, having the function of circulating an oxidizing gas in the furnace, and having an air feeding/discharging function, in an infusibilization step by measuring a face wind velocity V of the oxidizing gas penetrating the carbon-fiber precursor mat 16 in each of the chambers to control the velocity V to be the target value ±10% by adjusting the pressure of circulating air according to the rate of change in the velocity V. The method for preparing a carbon fiber is further measures the amount of air supplied (Q1) and the amount of air exhausted (Q2) at the individual chambers of the infusibilization furnace to control the ratio of Q1 to Q2, Q1/Q2 to be 1.0±0.3 by varying the amount of air supplied (Q1) and the amount of air exhausted (Q2) according to the variation in pressure of air circulating therein. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a carbon fiber production method suitable for use as a heat dissipation material and a fiber-reinforced resin molding material. Furthermore, the present invention relates to a carbon fiber production method that can maintain the yield of the infusibilization process high for a long period of time.

  Pitch-based carbon fibers are attracting attention as a material with excellent thermal conductivity because of their high graphitization properties. Examples of materials having excellent thermal conductivity include metals such as aluminum oxide, boron nitride, aluminum nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, and aluminum hydroxide, metal oxides, metal nitrides, metal carbides, and metal water. Oxides are known. However, metallic material-based fillers have high specific gravity and become heavy when used as a composite material. Further, when a spherical material such as carbon black, which is a carbon-based material, is added in a high amount, so-called powder falling occurs, and particularly in an electronic device, its conductivity adversely affects the device. In contrast, pitch-based carbon fibers have the advantage that the specific gravity is small and the weight of the composite material can be reduced when added in the same volume as the metal-based filler, and the shape is fibrous. In addition, there is also an advantage that powder falling hardly occurs.

  The production process of pitch-based carbon fibers mainly includes a spinning process, an infusibilization process, and a firing process. In the process of producing pitch-based carbon fibers, when carbon fiber precursor mat obtained by spinning mesophase pitch by melt blow method is heated in a high temperature inert atmosphere of 500-2000 ° C, most of the atoms other than carbon are removed. Carbon fiber. However, the carbon fiber precursor obtained in the spinning process has a low melting point, and melts and loses its fiber shape when subjected to rapid high-temperature heating. Therefore, in order to produce carbon fibers, an infusibilization treatment is required before the heat treatment.

  Infusibilization is usually achieved by heat treatment with a gentle gradient in the range of 150 to 350 ° C. in an oxidizing atmosphere. The infusibilization treatment includes a method in which the carbon fiber precursor is placed on a tray or the like and intermittently charged into the infusibilization furnace, and a method in which the carbon fiber precursor is matted and placed on the transport belt and continuously fed into the infusibilization furnace. In general, the latter with high production efficiency is selected. The continuous charging type infusible furnace has a plurality of chambers in series with the flow direction of the carbon fiber precursor mat, and each chamber is provided with a heating device such as a heater and an oxidizing gas circulation device such as a fan.

  On the other hand, in order to perform a uniform infusibilization treatment to the inside of the carbon fiber precursor mat, it is not sufficient to simply introduce the carbon fiber precursor mat into an oxidizing gas atmosphere. For example, as disclosed in Patent Document 1, an oxidizing gas is used. Must be forced to penetrate in the thickness direction of the carbon fiber precursor mat. However, in this method, when the carbon fiber precursor mat has a bulky part than the surrounding part, the penetrability of the oxidizing gas is deteriorated in the part, and the heat of infusible reaction is stored in the mat. From this point, the carbon fiber precursor melts or burns, which significantly deteriorates the process yield.

  Further, in Patent Document 2, by providing a three-dimensional holding mechanism above the conveyor belt, the poor penetration due to compression of the bulky portion that occurs when the oxidizing gas penetrates from below to above the carbon fiber precursor mat is reduced. There is a description of a method for uniformly penetrating a property gas. However, the installation of the three-dimensional holding mechanism in the furnace increases the complexity of the equipment, and the three-dimensionally held mat does not peel off from the mechanism at the process outlet, so that the transfer is not smoothly performed and the yield is lowered. is there. In addition, in this document, each infusibilization furnace has a structure for supplying and exhausting air and the structure for controlling the amount of air supplied and exhausted for each room is not disclosed.

Japanese Patent Application Laid-Open No. 60-167928 JP-A-2-169727

  The carbon fiber precursor obtained by spinning the mesophase pitch by the melt blow method is collected on the wire mesh belt and becomes a mat shape. Furthermore, the collected mats are cross-wrapped to reduce the equipment size in the subsequent process, and a plurality of mats are stacked. However, it is difficult in the above operation to keep the basis weight of the carbon fiber precursor mat thus obtained, and the disturbance makes the penetration of the oxidizing gas in the infusible treatment nonuniform. Further, in the case of an infusibilizing furnace having a structure in which an oxidizing gas is circulated, the heating device protection filter provided in the circulation system is clogged, and the penetration of the oxidizing gas becomes unstable. When the penetration of the oxidizing gas becomes unstable, the heat of infusible reaction is stored inside the carbon fiber precursor mat, causing melting or combustion phenomenon, and the process yield is remarkably deteriorated. Furthermore, if a mat shape holding mechanism is installed in the furnace as a measure for preventing compression of the bulky portion, the equipment becomes complicated and the transportability is lowered.

  Accordingly, the object of the present invention is to achieve an infusible reaction by keeping the oxidizing gas penetration state constant against changes in the weight per unit area of the carbon fiber precursor mat and clogging of the filter for heating device protection without using a shape holding mechanism. Is to provide a production method that can achieve a high process yield.

  The present inventors examined a method for infusibilizing the carbon fiber precursor mat with a high yield over a long period of time, even when the carbon fiber precursor mat has a bulky part than the periphery, The present inventors have found a method of controlling the supply / exhaust amount control for each chamber by the method of the present invention to keep the amount of oxidizing gas penetrating the precursor mat uniform, and have reached the present invention. That is, in the present invention, the change in the basis weight of the carbon fiber precursor mat and the clogging of the filter for protecting the heating device are detected by the surface wind speed of the oxidizing gas in each chamber of the infusibilizing furnace, and the surface wind speed is changed by changing the circulating wind pressure. This is a carbon fiber manufacturing method characterized by keeping the amount of oxidizing gas penetrating the precursor mat uniform by following the target value.

  Furthermore, the present inventors have found that the change in the supply / exhaust balance of each chamber due to the change in the circulating wind pressure due to the above operation affects the adjacent chamber, and as a result, the oxidizing gas penetration state and the indoor temperature distribution in the adjacent chamber are disturbed. The present inventors have found a method capable of suppressing the influence on the adjacent room by adjusting the supply / exhaust balance with respect to the change in the circulating wind pressure, and have reached the present invention.

  That is, the present invention relates to (1) a method for producing a carbon fiber in which a mesophase pitch is spun by a melt blow method, (2) infusible in an oxidizing gas atmosphere, and (3) fired in an inert gas atmosphere. The infusibilization furnace in the infusibilization step has a plurality of chambers in series with the conveying direction of the carbon fiber precursor mat, and has a function of circulating an oxidizing gas in the furnace, and for each chamber. Is a continuous charging type infusible furnace with an air supply / exhaust function, which measures the surface wind speed V of the oxidizing gas passing through the carbon fiber precursor mat in each furnace chamber and changes the circulating wind pressure with respect to the amount of change in V Thus, the carbon fiber manufacturing method is characterized in that V is controlled to a target value ± 10%.

  In the carbon fiber manufacturing method of the present invention, the air supply amount Q1 and the exhaust amount Q2 of each chamber of the infusibilizing furnace are further measured, and the supply air amount and the exhaust amount are changed with respect to the change of the circulating wind pressure. It is preferable to control the ratio Q1 / Q2 to 1.0 ± 0.3.

Furthermore, the surface wind velocity V of the circulating gas in each infusibilizing furnace chamber is 0.5 m / s or more and less than 5.0 m / s, and the total air supply amount Qt [Nm ³ / h] in each infusibilizing furnace chamber And the ratio Qt / W of the carbon fiber mat amount W [kg / h] charged into the infusibilizing furnace is preferably 5 or more and less than 50.

  Even if the carbon fiber precursor mat has a bulky part than the surrounding area, the method of the present invention controls the supply / exhaust amount control for each chamber and keeps the amount of oxidizing gas to be penetrated uniformly, so that it is stable for a long time. Infusibilization treatment can be performed, and the infusibilization process yield can be maintained high over a long period of time.

Next, embodiments of the present invention will be sequentially described.
Examples of the raw material for the carbon fiber produced in the present invention include condensed polycyclic hydrocarbon compounds such as naphthalene and phenanthrene, and condensed heterocyclic compounds such as petroleum pitch and coal pitch. Among them, condensed polycyclic hydrocarbon compounds such as naphthalene and phenanthrene are preferable, and optically anisotropic pitch, that is, mesophase pitch is particularly preferable. These may be used singly or in combination of two or more, but it is particularly desirable to use mesophase pitch alone in order to improve the thermal conductivity of the carbon fiber.

The softening point of the raw material pitch can be determined by the Mettler method, and is preferably 250 ° C. or higher and 350 ° C. or lower. When the softening point is lower than 250 ° C., fusion between fibers and large heat shrinkage occur during infusibilization. On the other hand, when the temperature is higher than 350 ° C., thermal decomposition of the pitch occurs and it becomes difficult to form a yarn.
The raw material pitch is spun by a melt blow method, and then made into a three-dimensional random mat-like carbon fiber by infusibilization and firing. Each step will be described below.

(1) Process for producing carbon fiber precursor from mesophase pitch The raw material pitch can be spun by a known method. A continuous fiber or a short fiber by a melt blow method is generally used. The main object of the present invention is to perform spinning by the melt blow method from the viewpoint of high productivity.
In the present invention, the shape of the spinning nozzle is not particularly limited, but those having a ratio L / D of the nozzle hole diameter D to the nozzle hole length L of preferably 5 or more and 20 or less, more preferably 8 or more and 15 or less. It is. The temperature of the nozzle at the time of spinning is not particularly limited, and may be a temperature at which a stable spinning state can be maintained, that is, a temperature at which the viscosity of the raw material pitch is 2 to 25 Pa · s, preferably 8 to 17 Pa · s.

  The pitch fibers drawn out from the nozzle holes are shortened by blowing a gas having a linear velocity of 100 to 10000 m / minute heated to 100 to 350 ° C. in the vicinity of the thinning point to become a carbon fiber precursor. . As the gas to be blown, air, nitrogen, or argon can be used, but air is preferable from the viewpoint of cost performance.

The carbon fiber precursor is collected on a wire mesh belt to form a continuous mat, and further cross-wrapped to form a three-dimensional random mat.
The three-dimensional random mat refers to a mat in which pitch fibers are entangled three-dimensionally in addition to being cross-wrapped. This entanglement is achieved in a cylinder called chimney while reaching the wire mesh belt from the nozzle. Since the linear fibers are entangled three-dimensionally, the characteristics of the fibers that normally exhibit only one-dimensional behavior are reflected in the three-dimensional.

(2) Process for producing infusible carbon fiber from carbon fiber precursor Hereinafter, the infusible process in the present invention will be described with reference to the drawings.
FIG. 1 is a schematic sectional side view of an infusibilizing furnace used in the present invention. The carbon fiber precursor mat is conveyed while being sandwiched between an upper conveyance belt 1 and a lower conveyance belt 2. The transport belt is not particularly limited as long as it is air permeable so that the oxidizing gas can penetrate and can withstand the infusible reaction temperature. The upper and lower belt intervals may be fixed, but it is preferable to provide a mechanism that can be arbitrarily adjusted according to the basis weight of the mat. Furthermore, it is preferable that the conveyor belt is variable so that the infusibility reaction time can be set arbitrarily.

  The infusibilizing furnace is partitioned into a plurality of reaction chambers 3 in series with the conveying direction. In order to maintain the independence of each chamber, the partition plate 4 is preferably narrowed to the extent that it does not contact the conveyor belt. The number of chambers is 4-30, and the infusibilization reaction conditions can be set in each chamber. The number of chambers is preferably 6 to 20 in consideration of rapid reaction temperature changes and equipment costs. Furthermore, it is preferable to install an inlet seal chamber 5 and an outlet seal chamber 6 respectively before and after the front and rear chambers to prevent diffusion of gas in the furnace into the atmosphere.

  FIG. 2 is a schematic cross-sectional view of an infusibilizing furnace used in the present invention. Each chamber 3 partitioned in series with the transport direction has a heating device 7 such as a heater and a burner, an oxidizing gas circulation device 8, an air supply port 9, an exhaust port 10, an air supply amount adjusting device 11, an exhaust gas for each chamber. It has a quantity adjusting device 12 and a rectifying device 13, and it is possible to set infusibilizing conditions independently for each room. It is preferable that the infusibilization reaction temperature is 100 to 200 ° C. in the front chamber and 250 to 350 ° C. in the last chamber, and the chamber is slowly raised to 10 to 30 ° C. in the front chamber. The reaction time is 15 to 120 minutes, preferably 20 to 60 minutes. The gas used is not particularly limited as long as it is oxidizing, but for example, air, oxygen, halogen gas, nitrogen dioxide, ozone and the like can be adopted. Among these, a mixed gas containing air and / or a halogen gas is preferable because it can be infusibilized quickly at low cost and at a low temperature. The direction of the circulation gas may be either upward → downward or downward → upward, but it is preferable to arrange all the chambers downward → upward, or alternately the upward → downward and downward → upward chambers. In order to keep the atmosphere in each room constant, it is preferable to exhaust a part of the circulating gas and supply pure oxidizing gas, and the ratio Q1 / Q2 of the supply amount Q1 and the discharge amount Q2 is 1. A range of 0.0 ± 0.3 is preferable. If it is outside the range of 1.0 ± 0.3, the amount of oxidizing gas that leaks into the adjacent chamber from between the partition plates or leaks from the adjacent chamber increases, and the independence of each chamber cannot be maintained. More preferably, it is 1.0 ± 0.1.

  As described above, the carbon fiber precursor spun by the melt-blowing method often causes spotting spots at the time of mat formation and cross-wrapping, and this causes uneven penetration of the oxidizing gas in a normal method. Further, the clogging of the filter for protecting the heating device 7 makes the penetration of the oxidizing gas unstable. In the present invention, a surface anemometer 14 of an oxidizing gas is installed in each chamber, and the function of detecting spotting spots and filter clogging by sensing changes in the surface wind speed V is provided. The surface wind speed V is defined as the wind speed perpendicular to the mat. When V changes instantaneously, the wind pressure of the oxidizing gas circulation device 8 is changed in a direction in which V approaches the target value. The wind pressure is adjusted by the rotation speed of the circulation device, the damper opening degree, and the like. It is preferable that the control is performed so that V is in the range of the target value ± 10% and automatic control is performed. Further, it is more preferable that the surface anemometer 14 is provided in several places in each room, and the minimum wind speed is controlled so as to fall within the target value ± 10%. More preferably, the target value is ± 5%.

  In the present invention, the exhaust of each chamber is performed by the exhaust fan 15 and the supply of air is performed by the ejector effect of the oxidizing gas circulation device 8. Therefore, when the circulation wind pressure changes for the adjustment of V, the supply amount Q1 varies. End up. When the air supply amount changes, Q1 / Q2 deviates from the appropriate value, and the independence of each room as described above is lost. Therefore, in the present invention, it is preferable that automatic control is performed by adjusting Q1 / Q2 within the range of 1.0 ± 0.3 by the supply air amount adjusting device 11 and the exhaust air amount adjusting device 12 in accordance with the change of the circulating wind pressure. . More preferably, it is 1.0 ± 0.1.

  The infusibilization condition of the present invention is preferably such that the surface wind velocity V of the circulating gas in each infusibilization furnace chamber is 0.5 m / s or more and less than 5.0 m / s. If V is less than 0.5 m / s, the heat inside the mat cannot be sufficiently removed and the mat melts and burns. On the contrary, when V is 5.0 m / s or more, the mat may be compressed too much and the oxidizing gas may not spread over the entire mat. More preferably, it is 1.0 m / s or more and less than 3.0 m / s.

Further, the infusibilization condition of the present invention is the ratio Qt / Qt of the total air supply amount Qt [Nm ³ / h] of each chamber of the infusibilization furnace and the carbon fiber mat amount W [kg / h] charged into the infusibilization furnace. W is preferably 5 or more and less than 50. When Qt / W is less than 5, the reaction gas generated during the infusibilization reaction is not sufficiently substituted, and the concentration of the oxidizing gas changes, so that a stable infusibilization reaction cannot be performed. On the contrary, when Qt / W is 50 or more, the infusibilization reaction is normally performed, but since the amount of heat for heating the supplied gas is large, the equipment cost and the running cost are increased. More preferably, it is 10 or more and less than 30.

(3) Step of producing carbonized or graphitized carbon fiber from infusible carbon fiber The infusible carbon fiber obtained above can be carbonized or graphitized in an inert gas atmosphere to produce a carbon fiber. Carbonization of the infusible carbon fiber is performed in a vacuum or in an inert gas such as nitrogen, argon, krypton, etc., but it is particularly preferable to carry out the reaction at normal pressure and at low cost. The carbonization temperature is 500 to 2000 ° C, more preferably 800 to 1800 ° C.

  Usually, the firing of carbon fibers exceeding 2000 ° C. is called graphitization, and nitrogen gas and the like cause ionization, and therefore inert gases such as argon and krypton are used. In order to increase the thermal conductivity of the carbon fiber, the treatment is preferably performed at 2300 to 3500 ° C., more preferably 2500 to 3200 ° C.

EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention does not receive any limitation by this. In addition, each value in an Example was calculated | required according to the following method.
(1) Mesophase pitch flow velocity in capillary during spinning It was determined by calculating the pitch speed passing through the capillary from the amount of liquid fed per time fed from the gear pump.
(2) Melt viscosity of mesophase pitch in the capillary It was evaluated using a capillary rheometer from the resin temperature at the time of spinning and the flow velocity in the capillary.
(3) Surface wind speed in each chamber of infusibilization furnace The gas in the furnace is extracted from the top and bottom of the carbon fiber precursor mat, the wind pressure is detected by the Krone wind pressure sensor DP850C, and the wind speed calculation unit V-50 manufactured by Hirano Techseed is used. It was measured.

[Example 1]
A pitch composed of a condensed polycyclic hydrocarbon compound having an optical anisotropy ratio of 100% and a softening point of 283 ° C. was used as a main raw material. This raw material was fed at 330 ° C. using a spinning nozzle consisting of a capillary with an introduction part diameter of 1.2 mm, an introduction angle of 45 °, a diameter of φ0.2 mm, and L / D = 10, and a flow rate in the capillary of 0.15 m / s. In addition, air at 350 ° C. was blown at a rate of 5500 m / min from the slit next to the capillary, and the melt pitch was pulled to create a nonwoven fabric having a basis weight of 300 g / m 2 made of a carbon fiber precursor having an average fiber diameter of 11.0 μm.
The melt pitch density was 1.22 g / cm 2, the melt viscosity in the capillary at 330 ° C. and 0.15 m / s evaluated with a capillary rheometer was 17 Pa · s.

  The nonwoven fabric composed of the carbon fiber precursor has a plurality of chambers in series with the conveying direction of the carbon fiber precursor mat, and has a function of circulating an oxidizing gas in the furnace. The infusibilization treatment was carried out using a continuous charging type infusibilizing furnace with an air supply and exhaust function. The interval between the upper conveyance belt 1 and the lower conveyance belt 2 was 70 mm on the inlet side and 35 mm on the outlet side, and the interval between the partition plates 4 was set so as not to contact the belt. The number of chambers was eight, and the carbon fiber precursor mat was conveyed from the foremost chamber 170 ° C. to the last chamber 300 ° C. over 25 minutes. Air was used as the oxidizing gas, and the flow direction of the circulating gas in the furnace was upward → downward for odd-numbered chambers and downward → upward for even-numbered chambers.

The surface wind speed of each chamber was set to 2.0 m / s, and the wind speed was adjusted using a fan capable of controlling the rotation speed as the oxidizing gas circulation device 8. Further, the ratio Q1 / Q2 between the air supply amount Q1 and the exhaust amount Q2 of each chamber is set to 1.0, and the adjustment of the air supply amount and the exhaust amount is performed as the air supply amount adjusting device 11 and the exhaust amount adjusting device 12. An automatic adjustable damper was used.
The ratio Qt / W between the total amount Qt [Nm ³ / h] of the air supply amount in each infusibilizing furnace and the carbon fiber mat amount W [kg / h] charged into the infusibilizing furnace was set to 100.

Next, the infusibilized carbon fiber precursor mat was fired at 800 ° C. in a nitrogen gas atmosphere, and then graphitized at 2400 ° C. in an argon gas atmosphere to obtain carbon fibers.
Thus, when the infusibilization treatment of the carbon fiber precursor mat was performed continuously for 14 days, there was no abnormality of the infusibilization treatment, and the infusibilization process yield was 100% on a carbon basis.

[Example 2]
Example except that the ratio Qt / W of the total amount Qt [Nm ³ / h] of the air supply amount in each chamber of the infusibilizing furnace and the carbon fiber mat amount W [kg / h] charged into the infusibilizing furnace was set to 20. The carbon fiber was produced under the same conditions as in 1.
Thus, when the infusibilization treatment of the carbon fiber precursor mat was performed continuously for 14 days, there was no abnormality of the infusibilization treatment, and the infusibilization process yield was 100% on a carbon basis. Furthermore, compared with Example 1, the amount of energy used in the infusibilization process was 70%.

[Example 3]
Carbon fibers were produced under the same conditions as in Example 1 except that the ratio Q1 / Q2 between the air supply amount Q1 and the exhaust amount Q2 in each chamber was set to 2.0.
Thus, when the infusibilization treatment of the carbon fiber precursor mat was performed for 14 days in succession, infusibilization abnormality occurred 8 times, and the infusibilization process yield was 95% on a carbon basis.

[Comparative Example 1]
The carbon fiber was produced under the same conditions as in Example 1 except that the surface wind speed of each chamber was set to 2.0 m / s at the initial stage of matting and the wind speed was not adjusted thereafter.
Thus, when the infusibilization treatment of the carbon fiber precursor mat was performed for 14 days in succession, abnormality of the infusibilization treatment occurred 20 times, and the infusibilization process yield was 90% on a carbon basis.

It is a schematic sectional side view of the infusibilization furnace used by this invention. 1 is a schematic cross-sectional view of an infusibilizing furnace used in the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Upper conveyance belt 2 Lower conveyance belt 3 Reaction chamber 4 Partition plate 5 Inlet seal chamber 6 Outlet seal chamber 7 Heating device 8 Oxidative gas circulation device 9 Air supply port 10 Exhaust port 11 Air supply amount adjustment device 12 Exhaust amount adjustment device 13 Rectifier 14 Surface anemometer 15 Exhaust fan 16 Carbon fiber precursor mat

Claims (4)

  (1) Spinning mesophase pitch by melt blow method, (2) Infusible in oxidizing gas atmosphere, (3) In the method for producing carbon fiber fired in inert gas atmosphere, (2) infusible step The infusibilizing furnace has a plurality of chambers in series with the conveying direction of the carbon fiber precursor mat, has a function of circulating an oxidizing gas in the furnace, and has a function of supplying and exhausting each chamber. A continuous charging type infusibilizing furnace having a surface air velocity V of an oxidizing gas penetrating a carbon fiber precursor mat in each furnace chamber, and a target of V by changing a circulating wind pressure with respect to a change amount of V. A method for producing carbon fiber, wherein the value is controlled to ± 10%.   The air supply amount Q1 and the exhaust amount Q2 of each infusibilizing furnace are measured, and the ratio Q1 / Q2 between Q1 and Q2 is changed to 1.0 ± 0. The method for producing carbon fiber according to claim 1, wherein the carbon fiber is controlled to 3.   The method for producing carbon fiber according to any one of claims 1 to 2, wherein the surface wind velocity V of the circulating gas in each chamber of the infusibilizing furnace is 0.5 m / s or more and less than 5.0 m / s. The ratio Qt / W of the total amount Qt [Nm ³ / h] of the air supply amount in each infusibilizing furnace and the amount of carbon fiber mat W [kg / h] charged into the infusibilizing furnace is 5 or more and less than 50 Item 4. The method for producing a carbon fiber according to any one of Items 1 to 3.

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