Patent Publication Number: US-2018051397-A1

Title: Method for producing carbon fibers

Description:
TECHNICAL FIELD 
     The present invention relates to a method for producing carbon fibers. 
     BACKGROUND ART 
     Carbon fibers are in extensive use as a reinforcing material for structural materials such as, for example, resins, concrete and ceramic. Carbon fibers are used also as, for example, a heat insulator, raw material for activated carbon, electrically conductive material, and heat transfer material. 
     Carbon fibers are produced by forming a synthetic resin, e.g., polyacrylonitrile, or a pitch obtained from petroleum or coal into fibers by spinning and then subjecting the fibers to infusibilization (air oxidation) and carbonization. Coal derived pitch is a viscous black substance and is a residue that remains after volatile components including naphthalene are taken out by distillation from coal tar, which is a liquid substance yielded as a by-product when producing a coke by dry distillation of coal. Such coal derived pitch is a mixture of a large number of compounds including a large proportion of aromatic compounds each having many benzenes in the skeleton. 
     Upon heating about from 100° C. to 200° C., coal derived pitch melts to become a viscous liquid. By extruding this from a nozzle, the coal derived pitch can hence be spun. However, since coal derived pitch is a by-product of coke production and is recovered as a residue as stated above, it contains various components, e.g., metallic impurities and solid carbon matter, which inhibit the spinning and subsequent infusibilization and carbonization. It is therefore difficult to stably and efficiently produce carbon fibers. 
     Furthermore, these impurities can be causative of defects of the carbon fibers to be produced. 
     Namely, it is preferable that the pitch to be used for producing carbon fibers should have a high carbon content and contain neither metallic impurities nor solid carbon matter. 
     It is also preferable that the pitch to be used for producing carbon fibers should evenly melt at a certain temperature when subjected to spinning. The softening point of the pitch is preferably 150° C. or higher in order that the infusibilization treatment for fixing the shape of fibers obtained by spinning the pitch can be performed at an elevated temperature to heighten the efficiency, and is preferably 300° C. or lower in order that the spinning can be conducted at temperatures which do not result in a pyrolysis reaction during the spinning. 
     In order to meet such requirements, it has been proposed to modify coal derived pitch by subjecting the coal derived pitch to, for example, treatments such as regulation of the viscosity or components and removal of impurities (see, for example, JP-B2-7-15099). 
     However, such coal derived pitch modification treatments result in an increase in the cost of carbon fiber production. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: JP-B2-7-15099 
       
    
     SUMMARY OF THE INVENTION 
     Problem that the Invention is to Solve 
     In view of the drawback described above, an object of the present invention is to provide a method capable of producing carbon fibers at low cost. 
     Means for Solving the Problem 
     The invention, which has been achieved in order to solve the problem described above, is a method for producing a carbon fiber, including a step of separating an ash-free coal obtained from a bituminous coal or a sub-bituminous coal, into a soluble component(s) and an insoluble component(s) by a solvent extraction treatment, a step of heat-treating the soluble component(s), a step of melt-spinning the soluble component(s) subjected to the heat treatment, a step of infusibilizing a filament body obtained by the melt spinning, and a step of carbonizing the filament body subjected to the infusibilization. 
     According to this method for producing carbon fibers, an ash-free coal having a low content of impurities, e.g., ash matter, which inhibit spinning, is subjected to a solvent extraction treatment to thereby extract therefrom soluble components including organic substances having relatively low molecular weights as main components, and from the soluble components are further removed volatile components and low-temperature pyrolizable components, which inhibit spinning, by a heat treatment, thereby obtaining a pitch. This pitch is low in the content of impurities and components having relatively high molecular weights and has a softening temperature which is suitable for melt spinning and capable of performing infusibilization at a relatively high temperature. Because of this, carbon fibers can be efficiently produced by this method for carbon fiber production. Furthermore, since the pitch can be obtained by merely performing a solvent extraction treatment and a heat treatment, carbon fibers of high quality can be produced at relatively low cost. 
     It is preferable that in the separation step, a solvent extraction temperature should be lower than 300° C. In the cases when the solvent extraction temperature in the separation step is thus regulated to be below the upper-limit temperature, the pitch obtained does not contain components having relatively high molecular weights and has a softening temperature at which the melt-spinning can be performed relatively easily. The efficiency of spinning is thus heightened, and the cost of carbon fiber production can be further reduced thereby. 
     It is preferable that in the heat treatment step, a heat treatment temperature should be 150° C. or higher. By thus regulating the heat treatment temperature to be equal to or higher than the lower limit, volatile components which inhibit spinning can be more reliably removed from the pitch, and the efficiency of melt spinning can hence be improved. In addition, components which soften at low temperatures are removed from the pitch to narrow the range of softening temperature, thereby enabling the infusibilization treatment at a higher temperature. The efficiency of melt spinning and infusibilization can be thus heightened, thereby further reducing the cost of carbon fiber production. 
     It is preferable that the heat treatment temperature in the heat treatment step should be higher than the solvent extraction temperature in the separation step. By thus regulating the heat treatment temperature so as to be higher than the solvent extraction temperature, low-molecular-weight volatile components can be more reliably removed from the pitch and the efficiency of spinning can hence be further heightened, thereby as further reducing the cost of carbon fiber production. 
     It is preferable that the heat treatment temperature in the heat treatment step should be higher than a spinning temperature in the melt spinning step. By thus regulating the heat treatment temperature to be higher than the spinning temperature, components which are pyrolizable during spinning can be removed from the pitch and the efficiency of spinning can hence be further heightened, thereby further reducing the cost of carbon fiber production. 
     The terms “bituminous coal” and “sub-bituminous coal” herein mean coals each having the coal properties as provided for in JIS-M1002 (1978). The term “ash-free coal” means a modified coal which was obtained by modifying a coal and has an ash matter content of 5% or less, preferably 3% or less, more preferably 1% or less. The term “ash matter” means a value determined in accordance with JIS-M8812 (2004). 
     Effect of the Invention 
     As explained above, since carbon fibers are obtained, by the method of the present invention for carbon fiber production, by subjecting the pitch to melt spinning, infusibilization and carbonization, the pitch being obtained by heat-treating soluble components obtained by a solvent extraction treatment of an ash-free coal, the pitch used is low in the content of components which inhibit the spinning and the efficiency of spinning is hence high. Consequently, carbon fibers of high quality can be provided at relatively low cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a flowchart which illustrates the procedure of a method for carbon fiber production as an embodiment of the present invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention are explained below in detail while appropriately referring to the drawing. 
     [Method for Producing Carbon Fibers] 
     As illustrated in  FIG. 1 , the method for carbon fiber production according to one embodiment of the present invention includes a step of subjecting bituminous coal or sub-bituminous coal to pyrolysis and a solvent extraction treatment to thereby form an ash-free coal (ash-free coal formation step: step S 1 ), a step of separating the ash-free coal obtained from bituminous coal or sub-bituminous coal into soluble components and insoluble components by a low-temperature solvent extraction treatment (separation step: step S 2 ), a step of heat-treating the soluble components obtained (heat treatment step: step S 3 ), a step of melt-spinning the soluble components subjected to the heat treatment (melt spinning step: step S 4 ), a step of infusibilizing a filament body obtained by the melt spinning (infusibilization step: step S 5 ), and a step of carbonizing the shaped mixture (carbonization step: step S 6 ). 
     &lt;Ash-Free Coal Formation Step&gt; 
     In the ash-free coal formation step as step S 1 , a slurry obtained by mixing bituminous coal or sub-bituminous coal with a solvent is heated to equal to or higher than the pyrolysis temperature of the bituminous coal or sub-bituminous coal, and soluble components of the pyrolyzed bituminous coal or sub-bituminous coal are extracted with the solvent, thereby obtaining an ash-free coal. Bituminous coal and sub-bituminous coal are superior in yield and pitch property compared to other kinds of coals. For example, brown coal and lignite have a high oxygen content and a low carbon content, and there are 1.5 cases where these properties are problematic as a raw material for carbon fibers. Ones having a high degree of coalification, such as anthracite, are unfavorable because the yield of ash-free coal is low. 
     The solvent is not particularly limited so long as it has a property of dissolving bituminous coal or sub-bituminous coal. For example, use can be made of monocyclic aromatic compounds such as benzene, toluene and xylene and bicyclic aromatic compounds such as naphthalene, methylnaphthalene, dimethylnaphthalene, and trimethylnaphthalene. The bicyclic aromatic compounds include naphthalenes having an aliphatic chain and biphenyls having a long aliphatic chain. 
     Preferred of those solvents are bicyclic aromatic compounds which are coal derivatives obtained by purifying products of dry coal distillation. The bicyclic aromatic compounds which are coal derivatives are stable even in a heated state and have an excellent affinity for coals. Because of this, by using such bicyclic aromatic compounds as the solvent, not only the proportion of coal components to be extracted with the solvent can be heightened but also the solvent can be easily recovered by a method such as distillation and circulated and used. 
     A lower limit of the slurry heating temperature (pyrolysis and extraction temperature) is preferably 300° C., more preferably 350° C., even more preferably 380° C. Meanwhile, an upper limit of the slurry heating temperature is preferably 470° C., more preferably 450° C. In the case where the slurry heating temperature is below the lower limit, the bonds between the molecules constituting the coal cannot be sufficiently weakened and, hence, in the case of using, for example, a low-rank coal as the raw material coal, there is a possibility that the ash-free coal obtained by the extraction cannot have a heightened re-solidification temperature or there is a possibility that the yield might be low, making it uneconomical. Conversely, in the case where the slurry heating temperature exceeds the upper limit, there is a possibility that pyrolysis reactions of the coal might occur very vigorously and the yielded pyrolysis radicals undergo recombination, resulting in a decrease in the extraction rate. 
     The extraction rate from the bituminous coal or sub-bituminous coal (yield of ash-free coal) in the ash-free coal formation step is, for example, 40% by mass or more and 60% by mass or less, although it depends on the quality of the bituminous coal or sub-bituminous coal used as a raw material coal. 
     &lt;Separation Step&gt; 
     In the separation step as step S 2 , the ash-free coal obtained in the ash-free coal formation step as step S 1  is subjected to a low-temperature solvent extraction treatment, thereby separating into relatively low-molecular-weight soluble components, which are solvent-extractable at low temperatures, and relatively high-molecular-weight insoluble components, which are not solvent-extractable. Thus, soluble components capable of being melt-spun are obtained. 
     More specifically, the ash-free coal which has been pulverized is dispersed in a solvent to prepare a slurry. This slurry is held for a certain time period at a given temperature range, followed by separating a solid matter, i.e., insoluble components of the slurry, and liquid matter, i.e., the solvent in which soluble components have dissolved. 
     A lower limit of the average particle diameter of the ash-free coal to be dispersed in the solvent is preferably 50 μm, more preferably 100 μm. Meanwhile, an upper limit of the average particle diameter of the ash-free coal to be dispersed in the solvent is preferably 3 mm, more preferably 1 mm. In the case where the average particle diameter of the ash-free coal to be dispersed in the solvent is less than the lower limit, there is a possibility that it might be difficult to separate the liquid containing extracted soluble components and the solid matter that is insoluble components. Conversely, in the case where the average particle diameter of the ash-free coal to be dispersed in the solvent exceeds the upper limit, there is a possibility that the efficiency of extraction of soluble components might decrease. The term “average particle diameter” means the volume-based 50% cumulative particle diameter in a particle size distribution measured by the laser diffraction scattering method. 
     A lower limit of the mixing proportion of the ash-free coal to the solvent in the slurry is preferably 3% by mass, more preferably 5% by mass. Meanwhile, an upper limit of the mixing proportion of the ash-free coal to the solvent is preferably 40% by mass, more preferably 30% by mass. In the case where the mixing proportion of the ash-free coal to the solvent is less than the lower limit, there is a possibility that the production efficiency might be low, making it uneconomical. Conversely, in the case where the mixing proportion of the ash-free coal to the solvent exceeds the upper limit, there is a possibility that it might be difficult to handle the slurry or separate the insoluble components. 
     Methods for separating the solvent in which soluble components are dissolved and the insoluble components are not particularly limited. Use can be made of any of known separation methods including a filtration method, centrifugal method and gravitational settling method, or a combination of two methods of these. Preferred of these is a combination of a centrifugal separation method and a filtration method, in which a continuous processing of fluids is possible, which is low in cost and suitable for large-quantity treatments, and which is capable of reliably removing the insoluble components. 
     The solvent is then removed from the liquid (supernatant) obtained by separating the insoluble components, thereby separating and recovering the soluble components of the ash-free coal. Meanwhile, the solvent is removed from the high-solid-content liquid, thereby separating and recovering the insoluble components of the ash-free coal. Methods for removing the solvent from the supernatant and from the high-solid-content liquid are not particularly limited, and use can be made of a general distillation method, a vaporization method or the like. Especially for removing the solvent from the insoluble components, use of distillation is preferred for recovering and reutilizing the solvent. 
     The solvent to be used in the separation step may be any one as long as it can dissolve low-molecular-weight components of the ash-free coal. Use can be made of the one similar to the solvent used in the ash-free coal formation step. In particular, as the solvent for use in the separation step, preferred is a solvent with which a sufficiently high extraction rate is obtained at low temperatures, preferably ordinary temperature. Examples of such preferred solvent include pyridine, methylnaphthalene and tetrahydrofuran. 
     The solvent extraction treatment temperature in the separation step varies in optimal temperature depending on the kind of the solvent. In general, however, the solvent extraction treatment temperature is preferably lower than 300° C., more preferably 200° C. or lower, even more preferably 150° C. or lower. Meanwhile, there is no particular lower limit on the solvent extraction treatment temperature, but it is preferably ordinary temperature, e.g., 20° C. In the case where the solvent extraction treatment temperature exceeds the upper limit, there is a possibility that the molecular weights of the soluble components to be extracted might increase, resulting in too high a softening temperature and a decrease in spinning efficiency in step S 4 . Conversely, in the case where the solvent extraction treatment temperature is below the lower limit, there is a possibility that cooling might be necessary, resulting in an unnecessary increase in cost. 
     A lower limit of the extraction period in the separation step, that is, the time period for holding at the solvent extraction treatment temperature, is preferably 10 minutes, more preferably 15 minutes. Meanwhile, an upper limit of the extraction period is preferably 120 minutes, more preferably 90 minutes. In the case where the extraction period is shorter than the lower limit, there is a possibility that low-molecular-weight components of the ash-free coal cannot be sufficiently dissolved away. Conversely, in the case where the extraction period exceeds the upper limit, there is a possibility that the production cost might increase unnecessarily. 
     A lower limit of the extraction rate of soluble components from the ash-free coal in the separation step is preferably 10% by mass, more preferably 20% by mass, even more preferably 30% by mass. Meanwhile, an upper limit of the extraction rate of soluble components from the ash-free coal is preferably 90% by mass, more preferably 70% by mass, even more preferably 50% by mass. In the case where the extraction rate of soluble components from the ash-free coal in the separation step is less than the lower limit, there is a possibility that the yield might be low, resulting in an increase in production cost. Conversely, in the case where the extraction rate of soluble components from the ash-free coal in the separation step exceeds the upper limit, there is a possibility that the soluble components might have a higher softening temperature, resulting in a decrease in spinning efficiency. 
     &lt;Heat Treatment Step&gt; 
     In the heat treatment step as step S 3 , the soluble components obtained in the separation step as step S 2  are heated to volatilize low-molecular-weight components and to decompose and remove, in advance, components which are pyrolizable at low temperatures, thereby obtaining a pitch to be used in the melt spinning step as step S 4 . 
     It is preferable that the beat treatment should be conducted by heating in a non-oxidizing gas atmosphere. By thus conducting the heating in a non-oxidizing gas atmosphere to prevent oxidative crosslinking, troubles such as an increase in softening temperature can be avoided. The non-oxidizing gas is not particularly limited so long as the pitch can be inhibited from oxidizing therein. However, nitrogen gas is more preferred from the standpoint of profitability. 
     It is preferable that the heat treatment should be conducted under reduced pressure. By thus conducting the heat treatment under reduced pressure, the vapor of volatile components and the gas of pyrolysis products can be efficiently removed from the pitch. 
     A lower limit of the heat treatment temperature in the heat treatment step is preferably 150° C., more preferably 170° C. Meanwhile, an upper limit of the heat treatment temperature is preferably 320° C., more preferably 280° C. In the case where the beat treatment temperature is below the lower limit, there is a possibility that volatile components in the insoluble components cannot be sufficiently removed, resulting in a pitch which has insufficient stringiness to reduce the spinning efficiency. Conversely, in the case where the heat treatment temperature exceeds the upper limit, there are possibilities that the energy cost might increase unnecessarily, that useful components might be pyrolyzed to reduce the production efficiency, and that carbonization might proceed further to reduce the spinnability. 
     It is preferable that the heat treatment temperature in the heat treatment step should be higher than the solvent extraction treatment temperature in the separation step as step S 2 . By thus regulating the heat treatment temperature so as to be higher than the solvent extraction treatment temperature, volatile components each having a boiling point higher than the solvent extraction treatment temperature can be removed from the pitch. Thus, the trouble in which volatile components come off from the pitch formed into filament shape in the spinning step as step S 4  to form voids or to break the filament body can be avoided. 
     It is more preferable that the heat treatment temperature in the heat treatment step should be higher than the melt spinning temperature in the melt spinning step as step S 4 , which will be described later. By thus regulating the heat treatment temperature so as to be higher than the melt spinning temperature, components which are pyrolizable during the melt spinning can be pyrolyzed and removed beforehand in this heat treatment step. Thus, it is possible to avoid the trouble in which pyrolysis products yielded during spinning break the filament body obtained by the spinning of the pitch and the trouble in which these pyrolysis products form defects in the carbon fibers finally obtained. 
     A lower limit of the heat treatment period (period for holding at the heat treatment temperature) in the heat treatment step is preferably 10 minutes, more preferably 15 minutes. Meanwhile, an upper limit of the heat treatment period in the heat treatment step is preferably 120 minutes, more preferably 90 minutes. In the case where the heat treatment period in the heat treatment step is shorter than the lower limit, there is a possibility that low-molecular-weight components cannot be sufficiently removed. Conversely, in the case where the heat treatment period in the heat treatment step exceeds the upper limit, there is a possibility that the treatment cost might increase unnecessarily. 
     A lower limit of the softening temperature of the pitch obtained by heat-treating the soluble components is preferably 150° C., more preferably 170° C. Meanwhile, an upper limit of the softening temperature of the pitch is preferably 280° C., more preferably 250° C. In the case where the softening temperature of the pitch is below the lower limit, there is a possibility that the infusibilization treatment temperature in the infusibilization step as step S 5  cannot be heightened, making the infusibilization treatment inefficient. Conversely, in the case where the softening temperature of the pitch exceeds the upper limit, it is necessary to heighten the spinning temperature in the melt spinning step as step S 4  and there are possibilities that the spinning becomes unstable and that the cost increases. The “softening temperature” is a value measured by a ring-and-ball method according to ASTM-D36. 
     In this heat treatment step, a lower limit of the yield of the pitch from the soluble components obtained in the separation step is preferably 80% by mass, more preferably 85% by mass. Meanwhile, an upper limit of the yield of the pitch from the soluble components in the heat treatment step is preferably 98% by mass, more preferably 96% by mass. In the case where the yield of the pitch from the soluble components in the heat  8   o  treatment step is below the lower limit, there is a possibility that the yield decreases unnecessarily. Conversely, in the case where the yield of the pitch from the soluble components in the heat treatment step exceeds the upper limit, there is a possibility that volatile components or low-temperature pyrolizable components might remain in the pitch to make the pitch have insufficient stringiness, resulting in a decrease in spinning efficiency. 
     &lt;Melt Spinning Step&gt; 
     In the melt spinning step as step S 4 , the pitch obtained in the heat treatment step as step S 3  is melt-spun by using a known spinning device. Namely, the pitch in a molten state is passed through a nozzle (die) and thereby formed into filament shape, and the shape of the pitch is fixed to the filament shape by cooling. 
     As the nozzle used for this melt spinning, a known one may be used. For example, one having a diameter of 0.1 mm or more and 0.5 mm or less and a length of 0.2 mm or more and 1 mm or less can be used. The filament body formed by melt-spinning the pitch is wound, for example, by a drum having a diameter of about 100 mm or more and 300 mm or less. 
     A lower limit of the melt spinning temperature is preferably 180° C., more preferably 200° C. Meanwhile, an upper limit of the melt spinning temperature is preferably 350° C., more preferably 300° C. In the case where the melt spinning temperature is below the lower limit, there is a possibility that the melting of the pitch might be insufficient, making stable spinning impossible. Conversely, in the case where the melt spinning temperature exceeds the upper limit, there is a possibility that a component of the pitch might be pyrolyzed to break the filament body formed by the spinning. 
     There is no particular lower limit on the linear velocity in the melt spinning. However, it is preferably 100 m/min, more preferably 150 m/min. Meanwhile, an upper limit of the linear velocity in the melt spinning is preferably 500 m/min, more preferably 400 m/min. In the case where the linear velocity in the melt spinning is below the lower limit, there is a possibility that the production efficiency might be low, resulting in an increase in the price of carbon fibers. Conversely, in the case where the linear velocity in the melt spinning exceeds the upper limit, there is a possibility that the spinning might be unstable to rather reduce the production efficiency, resulting also in an increase in the price of carbon fibers. 
     A lower limit of the average diameter of the pitch fibers obtained by spinning in the melt spinning is preferably 7 μm, more preferably 10 μm. Meanwhile, an upper limit of the average diameter of the pitch fibers obtained by spinning in the melt spinning is preferably 20 μm, more preferably 15 μm. In the case where the average diameter of the pitch fibers is less than the lower limit, there is a possibility that stable spinning might be impossible. Conversely, in the case where the average diameter of the pitch fibers exceeds the upper limit, there is a possibility that the pitch fibers might have insufficient flexibility. 
     &lt;Infusibilization Step&gt; 
     In the infusibilization step as step S 5 , the filament body obtained in the melt spinning step as step S 4  is heated in an oxygen-containing atmosphere to thereby perform crosslinking and infusibilization. Air is generally used as the oxygen-containing atmosphere. 
     A lower limit of the infusibilization treatment temperature is preferably 150° C., more preferably 200° C. Meanwhile, an upper limit of the infusibilization treatment temperature is preferably 300° C., more preferably 280° C. In the case where the infusibilization treatment temperature is below the lower limit, there is a possibility that the infusibilization might be insufficient or that a prolonged infusibilization treatment period might be necessary, making the treatment inefficient. Conversely, in the case where the infusibilization treatment temperature exceeds the upper limit, there is a possibility that the filament body might melt without undergoing crosslinking by oxygen. 
     A lower limit of the infusibilization treatment period is preferably 10 minutes, more preferably 20 minutes. Meanwhile, an upper limit of the infusibilization treatment period is preferably 120 minutes, more preferably 90 minutes. In the case where the infusibilization treatment period is shorter than the lower limit, there is a possibility that the infusibilization might be insufficient. Conversely, in the case where the infusibilization treatment period exceeds the upper limit, there is a possibility that the cost of carbon fiber production might increase unnecessarily. 
     &lt;Carbonization Step&gt; 
     In the carbonization step as step S 6 , the filament body infusibilized in the infusibilization step as step S 5  is heated and carbonized, thereby obtaining carbon fibers. 
     Specifically, the filament body is introduced into any desired heating device, e.g., an electric furnace, and the inside thereof is replaced with a non-oxidizing gas, followed by heating while supplying a non-oxidizing gas into this heating device. 
     A lower limit of the heat treatment temperature in the carbonization step is preferably 800° C., more preferably 1,000° C. Meanwhile, an upper limit of the heat treatment temperature is preferably 3,000° C., more preferably 2,800° C. In the case where the heat treatment temperature is below the lower limit, there is a possibility that the carbonization might be insufficient. Conversely, in the case where the heat treatment temperature exceeds the upper limit, there is a possibility that the production cost might increase from the standpoints of improving the heat resistance of the equipment, and the fuel consumption. 
     A heating period in the carbonization step may also be suitably set in accordance with the properties required of the carbon material, without particular limitations. However, the heating period is preferably 15 minutes or more and 10 hours or less. In the case where the heating period is shorter than the lower limit, there is a possibility that the carbonization might be insufficient. Conversely, in the case where the heating period exceeds the upper limit, there is a possibility that the efficiency of carbon material production might decrease. 
     The non-oxidizing gas is not particularly limited so long as it is capable of inhibiting the carbon material from oxidizing. However, nitrogen gas is preferred from the standpoint of profitability. 
     [Carbon Fibers] 
     According to the method for carbon fiber production of  FIG. 1 , carbon fibers obtained through the melt spinning, infusibilization and carbonization of a pitch, the carbon fibers using a pitch obtained by heat-treating soluble components obtained by a solvent extraction treatment of an ash-free coal obtained from bituminous coal or sub-bituminous coal, are produced. 
     Advantages 
     According to this method for producing carbon fibers, an ash-free coal having a low content of impurities, e.g., ash matter, which inhibit spinning, is subjected to a solvent extraction treatment to thereby extract therefrom soluble components including organic substances having relatively low molecular weights as main components, and from the soluble components are further removed volatile components and low-temperature pyrolizable components, which inhibit spinning, by a heat treatment. Thus, a pitch which is low in the content of impurities that inhibit spinning and which has a softening temperature which is suitable for melt spinning and capable of performing infusibilization at a relatively high temperature is obtained. Because of this, this method for producing carbon fibers is high in the efficiency of carbon fiber production and is capable of producing carbon fibers of high quality at relatively low cost. 
     Other Embodiments 
     The embodiment described above should not be construed as limiting the configuration of the present invention. Consequently, in the embodiments, the constituent elements of each part of the embodiments can be omitted, replaced or added, on the basis of the statements in the present description or of common technical knowledge, and all these modifications should be construed as being within the scope of the present invention. 
     The method of the present invention for producing carbon fibers does not require, for itself, producing an ash-free coal from bituminous coal or sub-bituminous coal. Namely, in the method of the present invention for carbon material production, an ash-free coal produced by a third party may be used as a starting material. 
     This method for producing carbon fibers may include, after the carbonization step, a step of further graphitizing the carbon fibers by heating in a non-oxidizing atmosphere to a temperature higher than in the carbonization step. 
     Examples 
     The present invention is explained below in detail by reference to Examples, but the present invention should not be construed as being limited on the basis of the Examples. 
     Carbon fibers of Examples 1 to 4 were produced experimentally through the ash-free coal formation step, separation step, heat treatment step, melt spinning step, infusibilization step, and carbonization step which are explained below. Carbon fibers of Comparative Example 1 were produced experimentally by omitting the heat treatment step from Example 1. Furthermore, carbon fibers of Comparative Example 2 were produced experimentally by melt-spinning the ash-free coal. 
     Differences in production conditions among Examples 1 to 4 and Comparative Examples 1 and 2 and various measured values obtained during the production thereof are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 kind of solvent 
                 pyridine 
                 methyl- 
                 methyl- 
                 tetrahydrofuran 
                 pyridine 
                 — 
               
               
                   
                   
                 naphthalene 
                 naphthalene 
               
               
                 Solvent extraction 
                 115 
                 320 
                 100 
                 65 
                 115 
                 — 
               
               
                 temperature (° C.) 
               
               
                 Yield (% by mass) 
                 42 
                 93 
                 38 
                 36 
                 42 
                 — 
               
               
                 Heat treatment temperature 
                 250 
                 250 
                 250 
                 250 
                 — 
                 — 
               
               
                 (° C.) 
               
               
                 Yield (% by mass) 
                 92 
                 96 
                 97 
                 98 
                 — 
                 — 
               
               
                 Softening temperature (° C.) 
                 205 
                 259 
                 194 
                 188 
                 195 
                 245 
               
               
                 Spinnability 
                 A 
                 B 
                 A 
                 A 
                 C 
                 C 
               
               
                 Tensile strength (MPa) 
                 600 
                 750 
                 800 
                 850 
                 unable to be 
                 unable to be 
               
               
                   
                   
                   
                   
                   
                 measured 
                 measured 
               
               
                 Average fiber diameter (μm) 
                 12.4 
                 13.1 
                 11.0 
                 11.7 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     (Ash-Free Coal Formation Step) 
     The ash-free coal to be used as a raw material in Examples 1 to 4 and the Comparative Examples was produced by using bituminous coal for general use as a fuel for boilers. The yield of the ash-free coal based on the raw material coal was 48% by mass. 
     (Separation Step) 
     The ash-free coal was pulverized to an average particle diameter of 0.5 mm or less. Soluble components were extracted from 100 g of this ash-free coal by using 1 L of a solvent. In Example 1 and Comparative Example 1, pyridine was used as the solvent, and the solvent extraction temperature and the extraction period were set at 115° C. and 60 minutes, respectively. In Example 2, methylnaphthalene was used as the solvent, and the solvent extraction temperature and the extraction period were set at 320° C. and 60 minutes, respectively. In Example 3, methylnaphthalene was used as the solvent, and the solvent extraction temperature and the extraction period were set at 100° C. and 60 minutes, respectively. In Example 4, tetrahydrofuran was used as the solvent, and the solvent extraction temperature and the extraction period were set at 65° C. and 60 minutes, respectively. The specific separation method was as follows. The ash-free coal was dispersed in the solvent, followed by holding at the solvent extraction temperature for the extraction period, thereby obtaining a slurry. The insoluble components were separated from this slurry by vacuum filtration, and the solvent was further removed by vacuum distillation to thereby take out soluble components. 
     The extraction rate, i.e., yield (% by mass), of the soluble components, from the ash-free coal, which were obtained by the separation step was measured. 
     As a result of the measurement, the yields of the soluble components were found to be 42% by mass in Example 1 and Comparative Example 1, 93% by mass in Example 2, 38% by mass in Example 3, and 36% by mass in Example 4. 
     (Heat Treatment Step) 
     The soluble components obtained in the separation step were heat-treated in a nitrogen atmosphere to thereby obtain pitches of each of the Examples. The heat treatment conditions included a heat treatment temperature of 250° C. and a heat treatment period (holding period) of 1 hour. 
     The yield of each pitch in this heat treatment step, i.e., the ratio of the mass of each pitch after the heat treatment to the mass of the soluble components before the heat treatment, was measured. 
     As a result of the measurement, the yields through the heat treatment were found to be 92% by mass in Example 1, 96% by mass in Example 2, 97% by mass in Example 3, and 98% by mass in Example 4. 
     The pitches of Examples 1 to 4, which had been obtained through the heat treatment, the pitch of Comparative Example 1 (the soluble components which had undergone no heat treatment), and the ash-free coal of Comparative Example 2 were examined for softening temperature. The softening temperature of each pitch was measured by the ring-and-ball method according to ASTM-D36, and the softening temperature of the ash-free coal was measured by a Gieseler method according to JIS-M8801 (2004). 
     As a result of the measurements, the softening temperatures of the pitches were found to be 205° C. in Example 1, 259° C. in Example 2, 194° C. in Example 3, 188° C. in Example 4, and 195° C. in Comparative Example 1, and the softening temperature of the ash-free coal of Comparative Example 2 was found to be 245° C. 
     It can be seen from these results of the measurements that the higher the extraction rate (yield) of soluble components, the higher the softening temperature. 
     (Melt Spinning Step) 
     The pitches of Examples 1 to 4 and Comparative Example 1 and the ash-free coal of Comparative Example 2 were melt-spun and formed into filament shape. The melt spinning conditions included: use of a nozzle having a diameter of 0.2 mm and a length of 0.4 mm; a spinning temperature of 250° C.; and a spinning speed of 188 m/min. 
     The spinning stability in the melt spinning step was evaluated. The cases where spinning could be performed continuously are indicated by “A”, the case where breakage of the filament rarely occurred is indicated by “B”, and the cases where breakage of the filament frequently occurred are indicated by “C”. 
     As a result of the evaluation of spinnability, Example 1 and Examples 3 and 4 showed exceedingly satisfactory spinnability. Example 2 was slightly poor in stringiness and rarely suffered breakage of the filament (clogging in nozzle). In Comparative Example 1, the formed filament frequently suffered breakage due to the generation of a gas. Comparative Example 2 showed considerably poor stringiness and frequently suffered breakage of the filament (clogging in nozzle). 
     (Infusibilization Step) 
     The filament bodies formed in the melt spinning step were heat-treated in air and infusibilized thereby. The infusibilization treatment conditions included a treatment temperature of 250° C. and a treatment period of 1 hour. 
     (Carbonization Step) 
     The filaments which had been infusibilized in the infusibilization step were carbonized in a nitrogen atmosphere. The carbonization treatment conditions included a carbonization treatment temperature of 800° C. and a holding period of 30 minutes. 
     The carbon fibers of Examples 1 to 4 and Comparative Examples 1 and 2 thus obtained were examined for tensile strength. A tensile strength measurement was made in accordance with JIS-R7606 (2000). 
     As a result of the measurement, the tensile strengths of the carbon fibers were found to be 600 MPa in Example 1, 750 MPa in Example 2, 800 MPa in Example 3, and 850 MPa in Example 4. Meanwhile, in Comparative Example 1 and Comparative Example 2, carbon fibers were unable to be obtained because it was difficult to perform melt-spinning continuously and stably at a constant fiber diameter. The tensile strength measurement was hence not performed in Comparative Example 1 and Comparative Example 2. 
     While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention. 
     The present application is based on a Japanese patent application (Application No. 2015-053477) filed on Mar. 17, 2015, the content thereof being incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The method for carbon fiber production of the present invention is especially suitably applied to a production of carbon fibers required to have dimensional accuracy. 
     DESCRIPTION OF THE SIGNS 
     
         
         S 0  Ash-free coal formation step 
         S 2  Separation step 
         S 3  Heat treatment step 
         S 4  Melt spinning step 
         S 5  Infusibilization step 
         S 6  Carbonization step