GB2138114A - Method and apparatus for continuous production of carbon fibers - Google Patents

Abstract

The furnace includes a heating chamber 2 for carbonizing fibers, a fiber inlet 3 at the upper end of the chamber 2, an air tight sealed fiber outlet 7 at the lower end of the furnace, an inert gas inlet 6 provided on the wall of the chamber and above the fiber outlet 7, at least one inert gas injection portion 8a, 8b, formed on the wall of the chamber, each capable of forming a curtain of inert gas across the heating chamber, each injection portion being provided between the gas inlet 6 and the fiber inlet 3, at least one outlet 10 each being provided below each inert gas injection portion, and a heating member capable of controlling the temperature in the heating chamber so that the temperature gradually increases from the upper end towards a lower end of the heating chamber. <IMAGE>

Description

SPECIFICATION Method and apparatus for continuous production of carbon fibers The present invention relates to a method for continuous production of carbon fibers and a vertical carbonizing apparatusforconducting the method.

More particularly, the invention relates to a method using a vertical carbonizing furnace through which a fiber stock is guided downwardly and which is provided in the carbonizing chamber with at least one inert gas injection holeforforming a curtain of inert gas, as well as another hole made in the vicinity of said injection hole through which to draw a gas out ofthe carbonizing chamber, and to an apparatus for producing carbon fibers in such a manner.

The production of carbon fibers generally consists of preoxidizing organicfibers (e.g. polyacrylonitrile fibers orcellulosefibers) in an oxidizing atmosphere to renderthem flame-retardant, and feeding the preoxidized fibers into a carbonizing furnace where they are carbonized in an inert gas atmosphere or a non-oxidizing atmosphere at a temperature of 300"C orhigher. In this carbonizing step,the preoxidized organic fibers are thermally decomposed into carbon fibers. The carbonization is usually effected at a temperature between 300 and 1,500 C, sometimes higherthan 1,500"C, and if necessary, atthegraphi- tization temperature of 2,000"C or more (see U.S.

Patents No. 4,073,870 and No. 4,321,446).

The carbon fibers produced by the above described conventional method has very low strength and ductility due not only to internal defectsfrom microvoids but also to surface defects such as cracks.

Therefore, to produce carbon fibers of high performance, the presence of surface defects must be minimized. In the carbonizing step, the preoxidized fibers release various decomposition products as they are carbonized at increasing temperatures, and the release of most decomposition products is known to occur in a temperature range of 300 to 9000C. The decomposition productsformed inthistemperature range, for example, HCN, NH3, CO, H2, H2O, CH4, CO2 and higher molecularweight saturated and unsatureated hydrocarbons having 3to 7 carbon atoms are gaseous underthe temperature conditions where they are produced.However, in a vertical carbonizing furnace where preoxidized fibers are guided down through a heating chamber in which the temperature increases from thetop to bottom, the gaseous decomposition products (hereunder decomposition gases) are carried by the ascending gas stream into the low-temperaturn zone of the furnace where the higher molecular lar weight hydrocarbons are cooled to form a tar mist. Part of the decomposition products now in the form of a tar mist is deposited on the inner surface of the furnace wall orthe fiber surfaces. The sticky tar mist on the waIl surfaces catches fiber fuzz adrift in thefurnace and grows during continuous furnace operation.Ultimately it contacts and damages the surface of the fiber passing through the furnace or partiallyobstructsthe passageofthefibersto upset the uniform flow of the gas stream. if the contact between the fibers and the tar mist is extreme, the individual filaments stickto each other, and the buildup of tar mist at elevated temperatures causes surface defects that greatly reduce the strength and ductility of the carbon fiber product. Furthermore, decomposition gases such as H20, CO2 and CO lower the fiber strength appreciably when they contact the fibers in the high-temperaturezone ofthefurnace.

An object ofthe present invention is to provide a method for continuous production of carbon fibers having high performance.

Another object of the present invention is to provide an apparatus capable of continuous production of carbon fibers having high performance.

The present invention has been accomplished as a result of studies to develop an effective method and apparatus of removing decomposition gases (that have been produced at between about 300 and 900"C) from a vertical carbonizing furnace ofthetype described above wherein preoxidized filaments are fed from above and are carbonized as they are guided substantially vertically through the furnace.

The object ofthe present invention can be attained by a method which comprises using a vertical carbonizing furnace having a heating chamber, heating the chamber in such a mannerthatthetempera ture gradually increases from the upper end toward a lower end of the heating chamber, introducing a fiber to be carbonized from a fiber inlet provided at the upper end ofthe chamber, introducing an inert gas from the gas inlet provided at lower end of the chamberto maintain the atmosphere in the chamber non-oxidizing atmosphere, injecting an inert gas from at least one portion between the fiber inlet and the gas inletto form a curtain ofthe inert gas across the heating chamberto prevent decomposition gases formed in the heating chamberto ascend, discharging the decomposition gases with the inert gas from at least one gas outlet each being provided at a lower portion of each inert gas injection portion, and recovering carbonized fiberfrom a fiber outlet pro- vided atthe lower portion of the heating chamber.

The method ofthe the present invention can be carried out by using an apparatus which comprises: a vertical carbonizing furnace having a heating chambertherein for carbonizing fibers, the furnace including, (i) a fiber inlet atthe upper end of the chamber, (li) an airtight sealed fiber outlet at the lower end of the furnace, (iii) an inert gas inlet provided on the wall ofthe chamber and above the fiber outlet, (iv) at least one inert gas injection portion, formed on the wall of the chamber, each capable offorming a curtain of inert gas across the heating chamber, each injection portion being provided between the gas inlet and the fiber inlet, (v) at least one gas outlet each being provided at a lower portion of each inert gas injection portion, and (vi) a heating member capable of controlling the temperature in the heating chamber in such a manner thatthetemperature gradually increases from the upper end toward a lower end of the heating chamber.

Fig. lisa schematic cross section of one embodi ment ofthe apparatus ofthe present invention; Fig. 2 is an enlarged schematic view showing the inert gas injection portions, gas outlets and the nearby area of the apparatus according to another embodi rnentofthepresentinvention; and Fig. 3 is a schematic cross section of an apparatus according to another embodiment ofthe present invention.

When preoxidized fibers are carbonized by the method of the present invention or carbonized in the apparatus ofthe present invention, the flowing of the decomposition gases produced in the higher-temper- ature zone into the lower-temperature zone can be prevented or reduced, thereby tar mist deposition on the innerwall surface orfiber surfaces can also be prevented or reduced Furthermore, it is also possible to prevent or reduce the decomposition gases from contacting the surface ofthe fibers being carbonized.

Thus, carbon fibers of consistently good quality can be produced over an extended period. The apparatus of the present invention is effectively used for carbonizing preoxidizedfibers in a temperature range of about 300 to 900"C wh ere the formation ofthermal decomposition gases is particularly noticeable.

Illustrative fibers that can be effectively treated by the method or by the apparatus of the present invention include preoxidizedfibers obtained from acrylic or cellulose fibers that generate thermal decomposition gaseswhen they are subjected tothe ordinary carbonization step. These fibers are fed to the heating chamber usually in the form of a strand or tow made up of about 100 to 500,000 filaments, or in a fabric or nonwoven cloth form. Any number of strands ortows may be guided through a single heating furnace atthesame time.When fibers are supplied as strands, the apparatus of the present invention is able to increase the strand spacing to abouttwice as large asthatpermissiblewith an apparatus having neither inert gas injecting portion nor gas outlet provided below the gas injection portion.

The method and the apparatus ofthe present invention is hereunder described in greater detail by reference to the accompanying drawings. Fig. lisa schematic cross section of one embodiment of the apparatus. In this figure, fibers 1 to be treated are introduced into a heating chamber 2 for carbonizing the fibers. The inner space of the heating chamber 2 serves both as a carbonizing chamber and as the passage wayforthe fibers. The upper end ofthe heating chamber is provided with a fiber inlet 3 and is open to air. The lower end ofthe heating chamber is provided with a fiber outlet 7 which communicates with a sealing mechanism (not shown). The heating chamber 2 is surrounded by heating elements 4a, 4b and4c.

At the upper end of the heating chamber, an ascending gas stream establishes a seal to preventthe entrance of the atmosphere into the chamber. It is preferred to provide a gas outlet 5 below the fiber inlet 3 atthe upper portion of the chamber. The function of this gas outlet 5 isto maintain an inert gas atmosphere in the interiorofthe heating chamber2bydisplacing eternal gases (e.g. air and watervaporthat have entered the chamberthrough the fiber inlet together with the fibers) with the ascending flow ofthe gas that has been introduced into the chamberfrom below.

When the ascending flow of gas introduced into the furnace from below is drawn out of the system through the fiber inlet 3, the gas in the furnace is quenched atthe inlet3 and its nearby area, whereupon the decomposition gases in thefurnace gas form a tar mist which builds up on the surface ofthe the fibers or the fiber inlet to cause various defects such as the breakage ofthe fibers orthe adhesion between filaments. This can be effectively prevented by disposing the gas outlet 5 between the fiber inlet3 and the first heating element4a positioned below it.The gas outlet 5 is provided at such a position (i.e. distance from the fiber inlet 3) thatthe above-stated two objects are achieved: 1 the greatest portion of the decomposition gases in the heating chamber is drawn out of the system through the outlet 5, and 2) the air in the bundle offibers introduced into the heating chamber is substantially completely replaced by an inert gas by the time the fibers have travelled from the fiber inlet3 and the gas outlet 5. If necessary, the fiber inlet3 may be heaped to preventthebuild-up oftarmistinthat area.

The lower end ofthe heating chamber is provided with a fiber outlet7 which communicates with a sealing mechanism (not shown). Abovethe fiber outlet7 is positioned an inert gas inlet 6. An inert gas is usually supplied in the ratefrom 0.02 - 0.50 Nmlsec (calculated to the rate at the normal state). Preoxidized fiber is supplied tothe heating chamberhavingthe construction described above, where it is carbonized in the inner space (carbonizing chamber) and subsequently recovered through the sealing mechanism at the lower end. The sealing mechanism may be in any suitable form such as a liquid seal, rolierseal or an inert gas curtain seal.Thefibers corking outofthe carbonizing chamber are eitherwound on a take-up roll or continuously supplied to anotherfurnace held at higher temperatures. The heating elements 4a, 4b and 4c are so designed thatthetemperature within the heating chamber increases gradually in thetravelling direction ofthe fibers. The stream of inert gas (which was not drawn out ofthe chamber) flows in the heating chamber in the direction opposite the travelling direction of the fibers.

In this embodiment of the apparatus of the present invention, inert gas injecting portions 8a and 8b are provided between the inert gas inletS at the bottom of the heating chamber and the gas outlet5 atthe upper portion. Each ofthe inert gas injecting portions may be composed of a single hole (usually in the form of a horizontally elongated slit) or it may comprise a plurality of slit-iike openings arranged side by side horizontally as shown in Fig. 2.The inert gas injecting portion maybeformed on only one ofthetwo opposing faces of the heating chamberwall, or it may be formed on both walls as shown in Figs. 1 and 2.

More effective removal of decomposition gases and the displacement ofthefurnace gas with an inert gas may be accomplished by disposing another injecting portion 8c aboveand in close proximitywith the gas outlet 5 as shown in Fig. 1 Fig. 2 is an enlarged schematioview of inert gas injecting portions 8 and 8', gas outlets 10 and 10', and the nearby area.

Suitable inert gases are, for example, nitrogen, argon, helium and mixtures thereof.

The inert gas is injected through 8a and 8b after having treated by preheating elements 9a and 9b (and 9c if injecting portion 8c is also provided) to the temperature inthefurnace ora highertemperature but not higherthan the temperature in the furnace by morethan 200 C.The inertgas inJected intothe heating chamberthrough the inert gas injecting portionstraversestheheating chambertoform a curtain of inert gas around each fiberthus providing a shield from the gas stream coming upfromthe lower part ofthe heating chamber.The ascending internal gas obstructed bythecurtain of inert gas is drawn from the system through gas outlets 10a and 1 0b (and Swhen 8c is provided).The interiorofthe heating chamber is usually held art a pressureofapproximate- ly 2to 100 mmH2O, so by connecting the gas outlets 1 0a, 1 0b and 5 to pressure regulating valves 11 a, 11 b and 1 1c,the pressure within the heating chamber can be held constant asthe gas is ejected from these outlets. Accordingly, no airwill be drawn into the chamberthrough the fiber inlet 3.Like the inert gas injecting portion(s), the gas outlet(s) may be provided in one of the opposing faces of the chamberwall (as in Fig. 1) or in both walls (as in Fig. 2). In the former case, the outlet(s) maybeformed belowand in close proximity with the inert gas injecting portion orthey may be formed in an area of the chamberwall which is the opposite side to the wall where the injection holes are formed and which is below and in close proximity with the injection holes. The gas outlets are preferably provided at a position as close as possible to the injection holes. If the fibers to be carbonized are intheform having a very great density (strand spacing in the case of strand) in the heating chamber, the hole arrangement shown in Fig. 2 is suitable, and if the density is small, any arrangement may be used.

Referring to Fig. 2, the inert gas injected through the injecting portions 8, 8' toward the fibers 1 forms a gaseous curtain around each fiberto obstruct the flowofthe ascending gas, which is drawn outofthe furnacethrough outlets 10 and 10'. At least one layer (usually more than one layer) of inert gas injecting portion is formed within theheating chamber, and a numberofgaseous curtains equal to numberof layer ofthe injecting portions are formed. One layer of injecting portion is usually formed between each of heating elements 4a,4b and 4c inthefurnace, and at least two layers of injecting portions preferably formed. The purpose of the present invention is satisfactorily achieved by not more than five layers of injecting portions.

Usually, fibers arranged into one vertical plane are supplied to the chamber. When fibers are supplied to the chamber as strands the strand spacing (number of strands per meter of width ofthe fiber plane) is usually from 50 two 400 strands/m (provided strands of 1,000 - 50,000 filaments/strand are used) and when fibers are supplied as tows they are usually spread to 2,000,000 to 10,000,000 denier/m. When fibers are supplied as fabric of non-woven cloth of not more than 500 glum2 can be effectively treated in the apparatus of the present invention. The fibers travel through the heating chamber under a tension which is at least sufficient to prevent them from contacting the wall of the chamber. The tension generally ranges from 1 to 600 mg/d, preferably from 50 two 300 mg/d.

Thetravelling speed ofthefibers depends on the iength ofthe heating chamber and the temperature withinthatchamber. The speed usually ranges from 0.02 to 0.20 m/sec. The inert gas is injected at a flow rate sufficientto perm it the ascending gas to be drawn out of the furnace th rough the gas outlets so that the concentration ofthe decomposition products in the ascending gas is preferably reduced to less than about 50%. For this purpose, when the inert gas is injected from the both sides ofwalls ofthe chamber wherein strands are arranged side by side, the flow rate of the inert gas in the direction vertical to the fiber surface generally ranges from 0.3 to 3 Nm/sec, preferably from 0.5 to 1.5 Nm/sec.The inert gas is preferably injected in such a direction that a horizontal gaseous curtain is formed within the heating chamber; therefore, it is directed into the heating chamber either horizontally or slightly downwardly.

Part ofthe inert gas introduced is drawn out ofthe furnace together with the decomposition gases and the remainder ascends the furnace. In the apparatus of the present invention, the fibers are carbonized by being heated in a temperature which is gradually raised from about 300"C to a temperature of not more than about 950"C, usually, about 9000C.

When the apparatus of the present invention is used to produce carbon fibers, the decomposition gases formed within the heating chamber can be discharged from the furnace with reduced chance of contacting the fibers being carbonized orthe gas in the upper part of the furnace which is in the lower temperature zone. As a result,theamountofthe decomposition gasesthat build up on the surface of thefibersorthewall ofthefurnace asatar rnist is reduced to such an extent that carbon fibers of good quality can be consistently produced over an extended period.

One embodiment ofthe present invention where carbon fibers are produced from acrylonitrilefibers with the apparatus of Fig. 1 is hereunder described. A strand or tow of preoxidized acrylonitrilefibers having a bonded oxygen content of 6-15wt%, preferably 8 to 12 wt% is fed to thefurnacethrough inlet 3, which is preferably preheated to 250 - 350"C to preventtardeposition.Thefibers pass through the upperpartofthe heating chamberthatis being heated usually at approximately a temperature having an incline of from 300 to 500"C by heating element 4a, and by the time when they reach the gas outlet 5, the gas, particularly air, contained in the bundle of fibers is replaced by the internal gas that has been present in the heating chamber, and is then discharged from the system through outlet 5. The replacement of the confined air by the internal gas must be thorough for the fibers which are usually supplied in the form ofthe bundle comprising 1 00 to 500,000 filaments. The fibers then pass through a zone where a curtain of an inert gas such as nitrogen, argon or helium isformed.Thereafter,theyentera second hot zone which is usually heated to have an incline ofatemperature from about 500 to 700"C by heating element4b. The inert gas is preheated to the temperature ofthe zone belowthe gas inlet or a highertemperatu re that does not exceed that temperature by more than 200"C. The purpose of this preheating is to prevent the decomposition gases from being quenched by the introduced inert gas to form a mist and for minimizing the fiuctuation ofthe temperature in the furnace. The inert gas should be blown against the fibers gently to prevent the formation of fiber fuzz orfiuffs.

In the second hot zone, the fibers are subjectedto a heat treatment at about 500 - 700 C for a period of about 1 Oto 60 seconds. Thereafter, they are passed through another curtain of inert gas, then to a third hot zone which is usually heated to a temperature having an incline of from about 750 to a temperature of 900"C ormore than 900 C but not more than 950"C by heating element4c. The fibers are retained in this zone for about 5 to 40 seconds. The temperatures provided by heating elements 4a, 4b and 4c vary stepwise but the temperature within the heating tube gradually increases from top to bottom. Finally, the fibers are recovered from the system through fiber outlet7 and a sealing mechanism.A preferred sealing mechanism is the combination of a curtain of nitrogen gas anda rollerseal.The recoveredfibers that have been carbonized to a small extent (so called pre-carbonized) are then fed to a furnace which is held at a highertemperature of about 900 to 1 ,5000C in an inert gas atmosphere, and by holding them in that furnace for a period of about 35 to 200 seconds, carbon fibers having the following properties are obtained.

Fineness: 790 - 810 tex Tensile modulus of elasticity 23,900 - 25,000 kg/mm2 Ultimate tensile strength 415-450 kg/mm2, co- efficient of variation (CV) = 4% or less Elongation at failure 1.72 - 1.86% The apparatus ofthe present invention can be operated continuously, for example, for480 hours, with 300 bundles of 12,000 preoxidized filaments being fed simultaneously. The resulting carbon fibers have high quality in that they have few fluffs and cohering filaments and have uniform strength properties. As another advantage, decomposition gases formed in the apparatus can be recovered in high concentration, so the emission gas from the appar- atus can be easily disposed in an incinerator.

When the same apparatus was operated continuouslyforabout320 hours without injecting an inert gas into the heating chamber and without drawing the internal gas from the furnace th rough several outlets, thefurnace was partly obstructed by the fiber fluffs and tar mist deposited on the wall of the zone heated at temperatures between 300 and 700"C. The resulting product was fluffy, had a tensile strength of lessthan 350 kg/mm2 (CV = 9% or more) and was not uniform in its strength.

Fig.3 shows an apparatus of another embodiment ofthe present invention. This apparatus isthe same as that shown in Fig. 1 exceptthatthe apparatus of Fig. 3 has an additional heating chamber 12 which is provided downwardly in contact with the heating chamber2. In the heating chamber 12further carbonization of the fiber is conducted. In the heating chamber 12 thetemperature is kept at a higher temperature fan that ofthe heating chamber 2. The fibers which have been heated in the heating chamber 2 to a temperature up to 900 - 950"C are continuously path through the heating chamber 12.In the heating chamber 12 the fibers are heated in an inert gas atmosphere and at a temperature having a incline of from a temperature higherthan the temperature ofthe heating chamber 2 to a temperature of not more than 1 500"C. The thus carbonized fibers are recovered from the outlet7.

Example I A strand (comprising 12,000 filaments) of fibers prepared from a copolymerconsisting of 98% by weight of acryolnitrile and 2% byweight ofmethy- lacrylate, and having an individual fineness of 0.9 denier ways preoxidized in the air at 265"C for 0.38 hour, at 2750for 0.20 hour and at 2830C for 0.15 hour under a tension so that shrinkage ofthe fiber reached 50% ofthefree shrinkage atthattemperature. The thus obtained preoxidized fibers had bonded-oxygen of 9.8% by weighs The tow of preoxidized fibers was carbonized using the apparatus shown in Fig. 1.The strand was fed to the furnace through inlet 3, which was preheated to 350"C. The strand spacing was 140 strands/m. The temperature ofthe upperzone was heated to have an incline of a temperature of from 300 to 5000C by the heating element4a, in the same mannerthe middle zone was heated to 500 - 700"C by the heating element 4b and the lowerzonewas heated to 700-9000Cby the heating element 4c. Nitrogen gas was used as the inert gas. The gas which introduced from the gas inlet 6 was heated to 600"C, the gases which were injected from 8c, 8a and 8were heated to 400"C, 600"C and 750"C, respectively.The flow rate of the gas in the chamber 2 was 0.15 Nm/sec. Flow rates at fiber surfaces at8c, 8a and 8b were 1.00 Nm/sec, 0.75 Nm/sec and 0.50 Nm/sec, respectively. The carbonization ofthe fiber was conducted under a tension of 80 mg/d. The speed ofthe fiber was 0.11 m/sec and the residence time was 66 sec.

The interior pressure ofthe heating chamberwas maintained at3-7mmH2O and decomposition gases were discharged from gas outlets 10a, 1 Ob and 5. The recovered fibers that have been carbonized (precarbonized) were then fed to a furnace which was heated to a temperature having an incline offrom 900 to 1420"C and which was kept under N2 gas atmosphere, and the fibers were held in thatfurnacefor 60 seconds.

For comparison the same experimentwas conducted except that the inert gas was not injected from 8a,8b and 8candthedecomposition gas was not discharged from 10a and lOb.

The thus obtained carbon fibers had thefollowing properties as shown in the following table.

The Present Invention Comparison Tensile Strength 450 kg/mm2 350 kg/mm2 (kg/mm2) Tensile Modulus of 24.0 x 103 kg/mm2 24.0 x 103 kg/mm2 Elasticity (kg/mm2) Elongation at Failure 1.88 1.46 Continuous Stable Manufacturing Period (Period during which more than about 200 hours continuous manufacturing 480 hours carbon fibers can be conducted without causing fuzzy strands or breakage of fibers) While the invention has been described in detail and with reference to specific embodiments thereof, itwill be apparent to one skiiled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims (26)

1. An apparatus for producing carbon fibers, comprising: a vertical carbonizing furnace having a heating chambertherein forcarbonizing fibers, the furnace including, (i) a fiber inlet at the upper end of the chamber, (ii) an airtight sealed fiber outlet at the lower end of the furnace, (iii) an inert gas inlet provided on the wall ofthe chamber and above the fiber outlet, (iv) at least one inert gas injection portion, formed on the wall of the chamber, each capable offorming a curtain of inert gas across the heating chamber, each injection portion being provided between the gas inletandthefiberinlet, (v) at least one gas outlet each being provided at a lower portion of each inert gas injection portion, and (vi) a heating member capable of controlling the temperature in the heating chamber in such a manner that the temperature gradually increases from the upper end toward a lower end ofthe heating chamber.

2. An apparatus for producing carbon fibers as claimed in Claim 1, wherein an additional gas outlet is provided on the wall of an upper portion of the heating chamber ata point between the fiber inlet and the heating member.

3. An apparatus for producing carbon fibers as claimed in Claim 1,wherein the inert gas injecting portion comprises of a layer of at least one slit like opening arranged horizontally on the wall ofthe chamber.

4. An apparatusfor producing carbon fibers as claimed in Claim 3, wherein the furnace comprises of a plurality of layers of said inert gas injection portion comprising at least one slit like opening, forming a plurality of injecting portions capable of forming curtains of inert gas.

5. An apparatusforproducing carbon fibers as claimed in Claim 4, wherein the plurality of layers includes 2 to 5 layers of slit like openings capable of forming gas curtains.

6. An apparatusfor producing carbon fibers as claimed in Claim 1, wherein the heating member is capable of heating the heating chamberto a temperature having an incline offrom 300into not more than 950"C.

7. An apparatusfor producing carbonfibersas claimed in Claim 6, wherein said heating chamber has an additional heating chamber provided above the inert gas inlet and downwardlyconnectedwith said heating chamber, said additional heating chamber has a heating member being capable of heating the additional heating chamber to a temperature having an incline of from more than 900to not more than 1500"C.

8. A method for producing carbon fibers using a vertical carbonizing furnace having a heating chambertherein, which comprises heating the chamber in such a mannerthatthetemperature gradually increases from the upper end toward a lower end of the heating chamber, introducing a fiberto be carbonized from a fiber inlet provided atthe upper end ofthe chamber, introducing an inert gas from an gas inlet provided atthe lower end ofthechamberto renderthe atmosphere in the chamber non-oxidizing atmosphere, injecting an inert gas from at least one portion betweentthefiber inlet and the gas inletto form a curtain ofthe inert gas across the heating chamberto prevent decomposition gases formed in the heating chamberto ascend, discharging the decomposition gases with the inert gas from at least one outlet each being provided at a lower portion of each inert gas injection portion, and recovering carbonizedfiberfrom a fiber outlet provided atthe lower portion ofthe heating chamber.

9. Amethodforproducing carbon fibers as claimed in Claim 8, wherein the fibers travel through the heating chamber under a tension which is at least sufficient to prevent them from contacting the wall of the chamber.

10. A method for producing carbon fibers as claimed in Claim 9, wherein the tension ranges from 1 to 600 mg/d.

11. A method for producing carbon fibers as claimed in Claim 8, wherein thefiberstravel through the heating chamberataspeed rangesfrom0.02to 0.20 m/sec.

12. A method for producing carbon fibers as claimed in Claim 8, wherein the fibers are introduced into the heating chamber in the form of a strand, tow, fabric or non-woven cloth.

13. A method for producing carbon fibers as claimed in Claim 12, wherein the strand or two is made up of 100 to 500,000 filaments.

14. Amethodforproducing carbon fibers as claimed in Claim 12, wherein plurality of strands or tows are introduced to the heating chamber.

15. A method for producing carbon fibers as claimed in Claim 13, wherein strands or tows are arranged into one vertical plane and an inert gas is injected from the both sides of walls of the heating chamber.

16. A method for producing carbon fibers as claimed in Claim 12, wherein the strands comprise 1,000to to 50,000 5û,000filamentsand are arranged in strand spacing of from 50 - 400 strands/m.

17. Amethodforproducing carbon fibers as claimed in Claim 8, wherein the flow rate ofthe inert gas in the direction vertical to the fiber is 0.3 to 3 Nm/sec.

18. Amethodforproducing carbonfibersas claimed in Claim 12, wherein the tows are spread to an extent offrom 2,000,000 to 10,000,000 denier/m.

19. A method for producing carbon fibers as claimed in Claim 12 wherein the fibers are fed as fabric or non-woven cloth of up to 500 g/m2.

20. A method for producing carbon fibers as claimed in Claim 8, wherein the fibers are preoxidized fibers obtained from fibers selected from the group consisting acrylic fibers and cellulose fibers.

21. A method for producing carbon fibers as claimed in Claim 8, wherein the heating chamber is heated to a temperature having an incline offrom more than 300into not more than 950"C.

22. A method for producing carbon fibers as claimed in Claim 8,whereinthe inertgas is a gas selected from the group consisting nitrogen, argon, herium and mixtures thereof.

23. A method for producing carbon fibers as claimed in Claim 21,whereinthefibersarefurther treated in atemperature upto 1500"C under an inert gas atmosphere.

24. A method of producing carbon fibers substantially as hereinbefore described with reference to the accompanying drawings.

25. Apparatus for producing carbon fibers substantially as hereinbefore described with reference to the accompanying drawings.

26. Carbon fibers produced bythe method or apparatus claimed in any one ofthe preceding claims.