AU2007307596A1

AU2007307596A1 - Method for production of ashless coal

Info

Publication number: AU2007307596A1
Application number: AU2007307596A
Authority: AU
Inventors: Atsushi Furuya; Nobuyuki Komatsu; Noriyuki Okuyama; Naoji Tada
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2006-10-12
Filing date: 2007-10-11
Publication date: 2008-04-17
Anticipated expiration: 2027-10-11
Also published as: CN101511977A; JP2008115369A; JP4061351B1; US20100006477A1; CN101511977B; KR101151556B1; WO2008044728A1; KR20090060339A; AU2007307596B2

Description

DESCRIPTION METHOD OF PRODUCING ASHLESS COAL Technical Field The present invention relates to a method of producing ashless coal in which ashless coal used in caking coal for coke for iron making is produced from coal. Background Art Hitherto, as caking coal for coke for iron making, such as blast furnace coke, blended coal containing high-rank caking coal as a main component, weakly caking coal, and non- or slightly caking coal has been used. Recently, an attempt has been made to extract a component soluble in a solvent from coal to obtain extracted coal having a quality higher than that of raw coal. For example, the following method has been disclosed (see, for example, Patent Document 1): Bituminous coal, sub-bituminous coal, brown coal, lignite, or the like is used as raw coal and mixed with liquefaction oil used as a solvent to prepare a slurry, the slurry is hydrogenated at a high temperature and a high pressure in the presence of a catalyst to liquefy the resulting product, and solvent refined coal (SRC) that is finally produced is separated and extracted. The SRC is used in caking coal for coke. In addition, the amount of caking coal resource has - 2 been insufficient, and caking coal is expensive. Accordingly, coal such as non- or slightly caking coal, or low-rank brown coal or sub-bituminous coal, that is, low quality coal has been attracting attention. Accordingly, a development and a proposal has also been made in which such low-quality coal is used as raw coal to produce extracted coal having characteristics equivalent to caking coal and the extracted coal is used as caking coal for coke. For example, a method has been disclosed in which low rank coal such as brown coal or sub-bituminous coal is heat treated in a solvent (medium liquid) at a pressure in the range of 1 to 20 MPa and a temperature of 400*C or lower, the solvent and the resulting heat-treated coal are then separated to obtain heat-treated coal, and the heat-treated coal is used as part of caking coal for coke (see, for example, Patent Document 2). Furthermore, the following method of producing ashless coal in which ash in coal is efficiently removed has been disclosed (see, for example, Patent Document 3). In the method, ashless coal is extracted from raw coal by bringing the raw coal into contact with a solvent of N-methyl-2 pyrrolidinone (NMP) or a mixed solvent of carbon disulfide and N-methyl-2-pyrrolidinone in the presence of chlorine or a fluorine compound. Patent Document 1: Japanese Unexamined Patent Application - 3 Publication No. 8-269459 (paragraphs 0010 to 0032) Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-55668 (paragraphs 0017 to 0030) Patent Document 3: Japanese Unexamined Patent Application Publication No. 2001-26791 (paragraphs 0009 to 0022) Disclosure of Invention Problems to be Solved by the Invention However, in the above-described methods of producing extracted coal, problems described below occur. In the production method described in Patent Document 1, ash and the used catalyst are concentrated in the resulting SRC, and thus the quality of the SRC is not satisfactory when the SRC is used in caking coal for coke for iron making. Although the SRC is provided with thermal plasticity (i.e., softening fluidity), which is an important quality as a binder (caking additive) for a raw material of coke, the volatility of the SRC is too high and thus a solidifying property at a temperature in the range of 400 0 C to 5000C is not sufficient. Accordingly, even when the SRC is used as a binder, it is difficult to produce coke having a sufficiently high strength. Furthermore, from the standpoint of a production method, this SRC is disadvantageous in that the method requires expensive hydrogen and a catalyst and must be performed under the condition of a high temperature and a high pressure, and - 4 thus production cost and equipment cost are enormous and this method is not economical. Regarding the production method described in Patent Document 2, although the cost is lower than that of the above method including liquefaction, the resulting heat treated coal is a mixture of a product extracted by the solvent and a non-extracted product. Therefore, the quality such as thermal plasticity, which is important as caking coal for coke, is not satisfactory. In the production method described in Patent Document 3, a component soluble in a solvent is extracted from coal using a strong polar solvent such as NMP without adding hydrogen. However, when a polar solvent is used as the solvent, the solvent forms a strong bond with coal. Accordingly, the solvent is not readily recovered, resulting in a problem of increasing production cost of ashless coal. The present invention has been made in view of the above problems. It is an object of the present invention to provide a method of producing ashless coal having an excellent quality as caking coal used as coke for iron making with a high efficiency and at a low cost. Means for Solving the Problems As a result of intensive studies on a method of producing ashless coal used in caking coal for coke for iron making, the inventors of the present invention found the - 5 relationship between the temperature and the time in an extraction step wherein ashless coal can be efficiently produced and the ashless coal can have a quality that does not obstruct thermal plasticity (softening fluidity) when used in blended coal. This finding led to the invention of a method of producing ashless coal that can be used in caking coal for coke for iron making with a high efficiency and at a low cost. Specifically, according to a method of producing ashless coal in the present invention, a method of producing ashless coal used in caking coal for coke for iron making includes a slurry preparation step of mixing a solvent with coal to prepare a slurry; an extraction step of extracting the slurry obtained in the slurry preparation step at a temperature in the range of 400*C to 420*C for 20 minutes or less, and then cooling the slurry to 370'C or lower; a separation step of separating the slurry obtained in the extraction step into a liquid portion and a non-liquid portion; and an improved-coal-obtaining step of separating the solvent from the liquid portion separated in the separation step to obtain ashless coal which is improved coal. According to this production method, in the slurry preparation step, a solvent is mixed with coal which is a raw material for ashless coal to prepare a slurry. In the -6 extraction step, by treating the slurry obtained in the slurry preparation step under the condition of a predetermined temperature and time, the proportion of a coal component extracted in the solvent is increased, the coal component is extracted in the solvent with a high efficiency, and a resolidification temperature of the resulting ashless coal is increased. Furthermore, in the separation step, the slurry obtained in the extraction step is separated into a liquid portion, which is a solution containing the coal component extracted in the solvent, and a non-liquid portion, which is a slurry containing a coal component insoluble in the solvent. In the improved-coal-obtaining step, the solvent is separated from the liquid portion separated in the separation step, thus producing ashless coal. In the method of producing ashless coal according to the present invention, in the improved-coal-obtaining step, in addition to the production of the ashless coal, the solvent is separated from the non-liquid portion separated in the separation step to obtain residue coal which is improved coal. According to this production method, in the improved coal-obtaining step, the ashless coal is produced, and in addition, the solvent is separated from the non-liquid portion separated in the separation step, thus producing residue coal.

- 7 In the method of producing ashless coal according to the present invention, in the extraction step, the slurry obtained in the slurry preparation step is heated to and extracted at a temperature in the range of 400 0 C to 420 0 C and then immediately cooled to 370 0 C or lower. According to this production method, in the extraction step, the slurry obtained in the slurry preparation step is heated to and extracted at a predetermined temperature and then immediately cooled to 370'C or lower without maintaining the temperature. Consequently, the proportion of the coal component extracted in the solvent is further increased, and the coal component is extracted in the solvent with a higher efficiency. In the method of producing ashless coal according to the present invention, the coal is low-quality coal. According to this production method, by using inexpensive low-quality coal as coal which is a raw material for ashless coal, the ashless coal can be produced at a lower cost. Advantages According to a method of producing ashless coal of the present invention, ashless coal used in caking coal for coke for iron making can be produced with a high efficiency and at a low cost. When this ashless coal is blended with raw coal, thermal plasticity of the resulting blended coal can - 8 be increased, and the amount of expensive strongly caking coal blended can be reduced. Consequently, the cost of the raw coal for coke for iron making can be reduced. Furthermore, by improving the caking property of the blended coal, the strength of coke for iron making can also be improved. Furthermore, in addition to the ashless coal, residue coal can also be produced with a high efficiently and at a low cost. Brief Description of Drawings [Fig. 1] Fig. 1 is a flowchart illustrating steps of a method of producing ashless coal. [Fig. 2] Fig. 2 is a schematic view showing a sold-liquid separator for performing a gravitational settling method. [Fig. 3] Fig. 3 is a graph showing Gieseler curves obtained by a Gieseler plastometer test in Example 1. [Fig. 4] Fig. 4 is a graph showing the relationship between the extraction temperature and the resolidification temperature of prepared ashless coal c in the case where an extraction treatment was performed with an extraction time of one hour using sub-bituminous coal C was used as raw coal in Example 2. [Fig. 5] Fig. 5 is a graph showing the relationship between the extraction time and the extraction yield in the case where sub-bituminous coal C was heated to an extraction temperature of 370"C, 400'C, or 420*C with a preheater, and - 9 kept in an extractor for a predetermined time, and then rapidly cooled to 360 0 C to perform an extraction treatment in Example 3. Reference Numerals Sl slurry preparation step S2 extraction step S3 separation step S4 improved-coal-obtaining step 1 coal slurry preparation tank 2 pump 3 preheater 4 extraction chamber 5 gravity settling chamber 6 solid-concentrated liquid receiver 7 cooler 8 filter unit 9 supernatant receiver 10 stirrer 100 sold-liquid separator Best Mode for Carrying Out the Invention Next, a method of producing ashless coal according to the present invention will be described in detail with reference to the drawings. In the drawings to be referred, Fig. 1 is a flowchart illustrating steps of the method of producing ashless coal, and Fig. 2 is a schematic view - 10 showing a sold-liquid separator for performing a gravitational settling method. <<Method of producing ashless coal>> As shown in Fig. 1, the method of producing ashless coal includes a slurry preparation step (Sl), an extraction step (S2), a separation step (S3), and an improved-coal obtaining step (S4). The steps will be described below. <Slurry preparation step (Si)> The slurry preparation step (Sl) is a step of mixing coal with a solvent to prepare a slurry. As a solvent for dissolving coal, a monocyclic aromatic compound such as benzene, toluene, or xylene; or a polar solvent such as N-methyl-pyrrolidone (NMP) or pyridine is generally used. However, in the present invention, a non hydrogen-donor solvent, examples of which mainly include bicyclic aromatic compounds, is used. Non-hydrogen-donor solvents are coal derivatives which are mainly bicyclic aromatic solvents which are mainly obtained by purifying a coking product of coal. Such non hydrogen-donor solvents are stable even in a heated state and have good affinity with coal. Therefore, a proportion of a coal component extracted in such a solvent (hereinafter, also referred to as "extraction yield") is high, and such a solvent can be easily recovered by a distillation method or - 11 the like. In addition, the recovered solvent can be recycled in order to improve the economical efficiency. Examples of the non-hydrogen-donor solvent mainly include bicyclic aromatic compounds such as naphthalene, methylnaphthalene, dimethylnaphthalene, and trimethylnaphthalene. Examples thereof also include naphthalenes having an aliphatic side chain, biphenyl, and alkylbenzenes having a long aliphatic side chain. The non-hydrogen-donor solvent preferably has a boiling point in the range of 1800C to 330*C. When the boiling point is lower than 1800C, pressures necessary in the extraction step (S2) and the separation step (S3) are high, and a loss due to volatilization is increased in the step of recovering the solvent, thereby decreasing the rate of recovery of the solvent. Furthermore, the extraction yield in the extraction step (S2) is decreased. On the other hand, when the boiling point is higher than 3300C, it is difficult to separate the solvent from a liquid portion and a non liquid portion described below, thereby decreasing the rate of recovery of the solvent. As described above, by performing thermal extraction using the non-hydrogen-donor solvent, the extraction yield of coal can be increased. Furthermore, unlike polar solvents, the solvent can be easily recovered, and thus the solvent can be readily recycled. In addition, expensive - 12 hydrogen or a catalyst need not be used, and thus coal can be solubilized at a low cost to obtain ashless coal, thus improving the economical efficiency. As coal (hereinafter, also referred to as "raw coal") used as a raw material for ashless coal, low-quality coal, which hardly has thermal plasticity, such as non- or slightly caking coal, steaming coal, or low-rank coal, e.g., brown coal or sub-bituminous coal, is preferably used. By using such inexpensive coal, the ashless coal can be produced at a lower cost, and thus the economical efficiency can be further improved. However, the coal used is not limited to such low-quality coal, and caking coal may be used according to need. Note that, the term "low-quality coal" used here refers to coal such as non- or slightly caking coal, steaming coal, low-rank coal (such as brown coal and sub-bituminous coal). The term "low-rank coal" refers to coal that contains moisture in an amount of 20% or more and that is desirably dehydrated. Examples of the low-rank coal include brown coal, lignite, and sub-bituminous coal. Specific examples of the brown coal include Victorian coal, North Dakota coal, and Berga coal. Examples of the sub-bituminous coal include West Banco coal, Binungan coal, and Samarangau coal. The low-rank coal is not limited to the above examples. Any coal that contains a large amount of moisture and that is - 13 desirably dehydrated is included in the low-rank coal in the present invention. The concentration of the coal relative to the solvent is preferably in the range of 10 to 50 mass percent, and more preferably in the range of 20 to 35 mass percent on the basis of dry coal, though it depends on the type of raw coal. If the concentration of the coal relative to the solvent is less than 10 mass percent, the ratio of a coal component extracted in the solvent to the amount of solvent is low, which is not economical. The coal concentration is preferably as high as possible. However, if the coal concentration exceeds 50 mass percent, the prepared slurry has a high viscosity, and thus transfer of the slurry and separation of a liquid portion and a non-liquid portion in the separation step (S3) tend to be difficult. <Extraction step (S2)> The extraction step (S2) is a step of extracting (hereinafter, also referred to as "heating") the slurry obtained in the slurry preparation step at a temperature in the range of 400 0 C to 420 0 C for 20 minutes or less, and then cooling the slurry to 370 0 C or lower. The heating temperature of the slurry in the extraction step (S2) is in the range of 400 0 C to 420 0 C. If the heating temperature is lower than 400 0 C, a bond between molecules constituting coal is not sufficiently weakened. Accordingly, - 14 in the case where low-quality coal is used as the raw coal, a resolidification temperature of the resulting ashless coal cannot be increased to a temperature equivalent to the resolidification temperature of strongly caking coal. On the other hand, if the heating temperature exceeds 4200C, a thermal decomposition reaction of the coal becomes very active and generated thermal decomposition radicals are recombined, thereby decreasing the extraction yield. In the heating temperature range of 4000C to 4200C, as the extraction time increases, the thermal decomposition reaction is excessively carried out and radical polymerization proceeds, thus decreasing the extraction yield. However, a relatively high extraction yield can be maintained in an extraction time of 20 minutes or less. In addition, at a temperature of 3700C, the extraction yield becomes the maximum in an extraction time of 30 minutes or more. Even if the extraction time is then prolonged to several hours, the resolidification temperature of the resulting ashless coal is not increased, though the extraction yield is not significantly changed. Accordingly, in order to increase the resolidification temperature of the resulting ashless coal and to improve the extraction yield, the most preferable condition is heating at a temperature in the range of 4000C to 4200C for 20 minutes or less, and then cooling to 3700C or lower.

- 15 The lower limit of the temperature in the cooling is preferably 350 0 C. If the temperature is decreased to lower than 350 0 C, a dissolving power of the solvent decreases, and reprecipitation of an extracted coal component occurs, thereby decreasing the yield of the ashless coal. As described below, in the extraction step (S2), for example, the temperature of an extraction chamber may be increased to 400*C to 420 0 C, and the extraction chamber may then be cooled immediately. The lower limit of the extraction time is not determined uniquely, but is preferably determined to be one minute from the standpoint of the operation of the extraction chamber. That is, in this case, the extraction time is preferably in the range of 1 to 20 minutes. After heating is performed at a temperature in the range of 400 0 C to 420*C for 20 minutes or less, cooling is immediately performed to 370 0 C or lower. This is because if it takes a long time to cool to 370 0 C or lower, the extraction yield is decreased accordingly. Here, the phrase "cooling is performed immediately" means that cooling is performed by performing a cooling treatment as soon as possible. For example, it means that cooling is performed as soon as possible by a cooling treatment until the slurry is transferred to a gravity settling chamber described below.

- 16 The shorter the heating time (extraction time) at a temperature in the range of 400 0 C to 420 0 C, the higher the extraction yield. Accordingly, in order to further improve the extraction yield, the heating time (extraction time) is preferably 15 minutes or less, more preferably 10 minutes or less, and further preferably 5 minutes or less. Furthermore, zero minutes are preferable, more specifically, it is preferable that the temperature be increased to 400 0 C to 4200C to perform extraction, and immediately decreased to 370*C or lower. Furthermore, in the temperature range of 4000C to 420C, a temperature close to 400*C is preferable, and 4000C is preferable. The reason for this is that, as the temperature is close to 4000C, the extraction yield increases. In the extraction in this extraction step (S2), components that are rich in aromatic components having an average boiling point (Tb5O: 50% distillation temperature) mainly in the range of 2000C to 3000C are produced by the thermal decomposition of the coal, and the components can be suitably used as a part of the solvent. The extraction step (S2) is preferably performed in the presence of an inert gas. The reason for this is that contact with oxygen in the extraction step (S2) is hazardous because of a fear of ignition, and that use of hydrogen increases the cost.

- 17 As the inert gas used in the extraction step (S2), inexpensive nitrogen is preferably used, but the inert gas is not particularly limited thereto. The pressure in the extraction step (S2) depends on the temperature during extraction and the vapor pressure of the solvent used, but is preferably in the range of 1.0 to 2.0 MPa. If the pressure is lower than the vapor pressure of the solvent, the solvent vaporizes and is not confined in a liquid phase, and thus extraction cannot be performed. In order to confine the solvent in the liquid phase, a pressure higher than the vapor pressure of the solvent is necessary. On the other hand, if the pressure is excessively high, the cost of the apparatus and the operation cost increase, which is not economical. <Separation step (S3)> The separation step (S3) is a step of separating the slurry obtained in the extraction step (S2) into a liquid portion and a non-liquid portion. Here, the term "liquid portion" refers to a solution containing a coal component extracted in a solvent, and the term "non-liquid portion" refers to a slurry containing a coal component that is insoluble in the solvent (coal containing ash, i.e., ash coal). A method of separating the slurry into the liquid portion and the non-liquid portion in the separation step - 18 (S3) is not particularly limited, but a gravitational settling method is preferably used. Various types of filtering methods and methods of centrifugal separation are generally known as a method of separating a slurry into a liquid portion and a non-liquid portion. However, in the methods of filtration, it is necessary to change a filter aid frequently. In addition, in the methods of centrifugal separation, clogging due to an undissolved coal component readily occurs. Therefore, it is difficult to carry out these methods on an industrial scale. Accordingly, the gravitational settling method, in which a continuous operation of a fluid can be performed, which can be performed at a low cost, and thus which is suitable for treatment of a large amount, is preferably used. Thereby, a liquid portion (hereinafter, also referred to as "supernatant"), which is a solution containing a coal component extracted in the solvent, can be obtained from an upper portion of a gravity settling chamber, and a non liquid portion (hereinafter, also referred to as "solid concentrated liquid"), which is a slurry containing a coal component insoluble in the solvent, can be obtained from an lower portion of the gravity settling chamber. An example of the gravitational settling method will now be described with reference to Figs. 1 and 2. As shown in Fig. 2, in the gravitational settling - 19 method, in a sold-liquid separator 100, first, powdery coal which is a raw material for ashless coal is mixed with a solvent in a coal slurry preparation tank 1 to prepare a slurry (slurry preparation step (Sl).). Next, a predetermined amount of the slurry is supplied from the coal slurry preparation tank 1 to a preheater 3 using a pump 2, and the slurry is heated to a temperature in the range of 4000C to 4200C. The heated slurry is then supplied to an extraction chamber (extractor) 4. The slurry is heated at a temperature in the range of 4000C to 4200C for 20 minutes or less under stirring with a stirrer 10, and then immediately cooled to 3700C or lower with a cooler 7 (extraction step (S2)). Note that, in order to immediately perform cooling, a cooling mechanism is preferably provided in the extraction chamber 4. The term "20 minutes or less" used here means the total of the heating time in the preheater 3 and the heating time in the extraction chamber 4, that is, the time from the start of heating in the preheater 3 at a temperature in the range of 4000C to 4200C to the immediate cooling to 3700C or lower. The slurry obtained after this extraction treatment is supplied to a gravity settling chamber 5, and the slurry is separated into a supernatant and a solid-concentrated liquid (separation step (S3)). The solid-concentrated liquid settled in a lower portion of the gravity settling chamber 5 is discharged to a solid- - 20 concentrated liquid receiver 6, and a predetermined amount of supernatant in an upper portion thereof is discharged to a filter unit 8. Here, in order to prevent reprecipitation of a solute eluted from the coal of the raw material, the gravity settling chamber 5 is preferably maintained at a temperature in the range of 350*C to 370'C, i.e., a temperature cooled after the heating of the slurry, and the pressure therein is preferably controlled to be in the pressured range of 1.0 to 2.0 MPa. The time during which the slurry is maintained at the cooled temperature in the gravity settling chamber 5 is a time required for separating the slurry into the supernatant and the solid-concentrated liquid. This time is generally in the range of 60 to 120 minutes, but is not particularly limited. By increasing the number of gravity settling chambers 5, a component soluble in the solvent contained in the solid concentrated liquid can be recovered. In order to efficiently recover the component, the gravity settling chambers 5 are preferably arranged in two stages. The supernatant discharged from the gravity settling chamber 5 is filtered through the filter unit 8 according to need, and is recovered to a supernatant receiver 9. Next, as described below, the solvent is separated and - 21 recovered from the liquid portion and the non-liquid portion by a distillation method or the like, and ashless coal which is improved coal not containing ash is obtained from the liquid portion (improved-coal-obtaining step (S4)). Furthermore, residue coal which is improved coal containing concentrated ash can be obtained from the non-liquid portion, as required. <Improved-coal-obtaining step (S4)> The improved-coal-obtaining step (S4) is a step of separating the solvent from the liquid portion separated in the separation step (S3) to obtain ashless coal which is improved coal (ashless coal-obtaining step). As a method of separating the solvent from the supernatant (liquid portion), a general distillation method, evaporation method (such as a spray drying method), or the like can be employed. The solvent recovered by the separation can be circulated in the coal slurry preparation tank 1 (see Fig. 2) and used repeatedly. By separating and recovering the solvent, ashless coal that does not substantially contain ash can be obtained from the supernatant. This ashless coal hardly contains ash, is free of moisture, and has a heat value higher than that of the raw coal. Furthermore, thermal plasticity, which is a particularly important quality as a raw material for coke - 22 for iron making, is markedly improved, and the ashless coal exhibits a performance (fluidity) superior to that of the raw coal. Accordingly, this ashless coal can be used as blended coal for a raw material for coke. In addition, the ashless coal can also be used as blended coal by mixing with the residue coal. According to need, in the improved-coal-obtaining step (S4), in addition to the production of the ashless coal, residue coal which is improved coal may be produced by separating the solvent from the non-liquid portion separated in the separation step (S3) (residue-coal-obtaining step). As a method of separating the solvent from the solid concentrated liquid (non-liquid portion), a general distillation method or an evaporation method can be employed as in the ashless coal-obtaining step described above. The solvent recovered by the separation can be circulated in the coal slurry preparation tank 1 (see Fig. 2) and used repeatedly. By separating and recovering the solvent, residue coal in which ash is concentrated can be obtained from the solid-concentrated liquid. This residue coal contains ash, but is free of moisture and has a sufficient heat value. The residue coal does not exhibit thermal plasticity, but an oxygen-containing functional group is eliminated from the residue coal. Therefore, in the case where the residue coal is used as - 23 blended coal, the residue coal does not obstruct thermal plasticity of other coal contained in the blended coal. Accordingly, this residue coal can be used as part of blended coal for a raw material for coke as in general non or slightly caking coal. Alternatively, the residue coal can be used as various types of fuels instead of being used in caking coal for coke. Only the ashless coal not containing ash may be produced from the liquid portion as caking coal for coke. Only the solvent may be recovered from the non-liquid portion, and the residue coal in which the ash is concentrated may be disposed of without being recovered. The present invention has been described above. However, when the present invention is carried out, as long as the above-described steps are not adversely affected, other steps such as a coal-crushing step of crushing raw coal, a removing step of removing undesired substances such as contaminations, and a drying step of drying the prepared ashless coal may be performed between the steps or before or after the steps. EXAMPLES Next, a method of producing ashless coal according to the present invention will be specifically described using Examples. [EXAMPLE 1] - 24 In Example 1, in the case where an extraction temperature in an extraction step was 370'C, changes in thermal plasticity (softening fluidity), a resolidification temperature, and the like of raw coal samples and ashless coal samples obtained from the raw coal samples were examined (Experimental Example 1). Strongly caking coal A, strongly caking coal B, and sub-bituminous coal C having industrial analysis values and elemental analysis values shown in Table 1 were used as raw coal samples. Five kilograms of each of the raw coal samples was mixed with 20 kg (fourfold) of a solvent (1 methylnaphthalene (produced by Nippon Steel Chemical Co., Ltd.) to prepare slurries. Extraction was performed by pressing each of the slurries with nitrogen at a pressure of 1.2 MPa in an autoclave having an internal volume of 30 L at 370 0 C for one hour. Each of the slurries was separated into a supernatant and a solid-concentrated liquid in a gravity settling chamber in which the same temperature and the same pressure were maintained. The solvent was separated and recovered from the supernatant by a distillation method. Ashless coal a was prepared from the strongly caking coal A, ashless coal b was prepared from the strongly caking coal B, and ashless coal c was prepared from the sub-bituminous coal C. Industrial analysis values and elemental analysis values of these samples are shown in Table 1.

- 25 Next, for the strongly caking coal A, the strongly caking coal B, the sub-bituminous coal C, the ashless coal a, the ashless coal b, and the ashless coal c, a Gieseler plastometer test defined in JIS M 8801 was performed. The test results are shown in Table 1. Fig. 3 is a graph showing Gieseler curves obtained by the Gieseler plastometer test. [Table 1] 0 cc (o 0o c0 O LO mt -o D0 0 00 co It co S E a) E E~ c oEX 0) Co Co N- Co W cc- 0. r-- Co 0 r- N E~ :3 N c) N Ct) 0= 'a A 0) CD 0) E 04 E CN Nl- LO (V) mV .R0 co qqt LO qq* N 7- cc C N-, q tN V) E C0) NESc - CD LO 0D N4 m a) L.0) mt N* CN 0) Co v) E ) . V CD I >o 0 ) C)N Co N- Co N- Co >~0 C m r- o Co- 0) r- 0 0N CN t Li LA) 0 cc __ - Co Nl- N- 0) 0o > An 6 oi Co 6 6= 6 c cc ClV) 0) 0) Co -0 CC (D CV) D ,o ~ O- - ) Co 0) OD Co Co Wa) 'M Co 0 0)r-:C CD m 0.: c N C14 C) N4 i q C.) cu qc ~- V) ce) LOA N 0 c; C=! C 00 Co 0 Co 0 Co 0n oo 0~ r- ~ 6 0 0 Co W~ Co :30Co O) ir < < cu-c - 27 As shown in Table 1, the ashless coal a, the ashless coal b, and the ashless coal c did not contain moisture, and the ash contents thereof were very small as compared with those of the raw coal samples. In addition, the ashless coal a, the ashless coal b, and the ashless coal c exhibited heat values higher than those of the raw coal samples. The oxygen concentration of the sub-bituminous coal C was high; more than 15%. In addition, although the oxygen concentration of the ashless coal c was decreased to about 10%, a relatively high oxygen concentration was maintained. Here, in the results of the Gieseler plastometer test, the resolidification temperatures of the raw coal samples will be discussed. The resolidification temperature of the strongly caking coal A was 496 0 C, and the resolidification temperature of the strongly caking coal B was 4830C. In contrast, the resolidification temperature of the sub bituminous coal C was low; 4450C. If the sub-bituminous coal C was used as one component of blending coal for coke, the lower resolidification temperature obstructs the melting state of the whole blending coal in the coking process. Accordingly, the sub-bituminous coal C cannot be used as caking coal for coke for iron making. Referring to the values of the maximum fluidity shown in Fig. 3 and Table 1, as for thermal plasticity, the ashless coal a, the ashless coal b, and the ashless coal c - 28 obtained from the above raw coal samples exhibited satisfactory thermal plasticity significantly higher than that of corresponding raw coal samples. However, as for the resolidification temperatures of the prepared ashless coal samples, the ashless coal a obtained from the strongly caking coal A was 508 0 C, and the ashless coal b obtained from the strongly caking coal B was 488*C. That is, the ashless coal a and the ashless coal b were solidified at temperatures higher than those of the strongly caking coal A and the strongly caking coal B, respectively, which were raw coal samples of the ashless coal samples. The resolidification temperature of the ashless coal c obtained from the sub-bituminous coal C was higher than that of the sub-bituminous coal C, which was a raw coal sample, but was relatively low; 463*C. Here, in the case where the ashless coal c is added to caking coal for coke for iron making and coked as blended coal, the ashless coal c is solidified at 463*C, during which strongly caking coal maintains the fluidity, thus obstructing the fluidity of the whole blended coal. As a result, the strength of the resulting coke is decreased. According to the above results, in the case where low quality coal such as sub-bituminous coal is used as raw coal, even when ashless coal is produced under the above conditions, the ashless coal is not particularly excellent - 29 as caking coal for coke for iron making. When strongly caking coal (or caking coal) is used as raw coal, the resulting ashless coal exhibited thermal plasticity superior to that of the raw coal and can be used as caking coal for coke for iron making. However, strongly caking coal is expensive, and thus the raw material cost cannot be reduced. [EXAMPLE 2] In Example 2, the relationship between the extraction temperature when an extraction treatment of the sub bituminous coal C used in Example 1 was performed and the resolidification temperature of the ashless coal c obtained from the sub-bituminous coal C was examined (Experimental Example 2). Fig. 4 shows the relationship between the extraction temperature and the resolidification temperature of the prepared ashless coal c in the case where an extraction treatment was performed with an extraction time of one hour (60 minutes) using the sub-bituminous coal C was used as raw coal. The method of producing ashless coal was performed in accordance with Example 1 except that the extraction temperature was changed. As shown in Fig. 4, at extraction temperatures exceeding about 360 0 C, as the extraction temperature - 30 increased, the resolidification temperature of the ashless coal c increased. At an extraction temperature of 400'C, the resolidification temperature was about 4900C, that is, the resolidification temperature was increased to substantially the same as the resolidification temperature of the above strongly caking coal. The resolidification temperature further increased at temperatures exceeding 4000C. Accordingly, it was found that the resolidification temperature of the resulting ashless coal was increased by increasing the extraction temperature of coal to 4000C or higher. According to the above results, in the case where low quality coal such as sub-bituminous coal is used as raw coal, by controlling the extraction temperature to be 400*C or higher, the resulting ashless coal can be used as caking coal of coke for iron making. [EXAMPLE 3] In Example 3, the relationships between the extraction temperature, the extraction time, and the extraction yield in the case where an extraction treatment of the sub bituminous coal C used in Example 1 was performed were examined (Experimental Example 3). Fig. 5 shows the relationship between the extraction time and the extraction yield in the case where the sub bituminous coal C was heated to an extraction temperature of - 31 370 0 C, 400 0 C, or 420 0 C with a preheater, and kept in an extractor for a predetermined time, and then rapidly cooled to 360 0 C to perform an extraction treatment. In the experiment at 420*C, the time required for increasing the temperature from 400 0 C to 420 0 C in the preheater was eight minutes. Accordingly, in Fig. 5, extraction times which also include the eight minutes, which corresponds to an extraction time from 400 0 C to 420 0 C, in the preheater are shown. The method of producing ashless coal was performed in accordance with Example 1 except that the extraction temperature and the extraction time were changed. The extraction yield of coal was determined from the amount of separated solid residue coal. Specifically, the extraction yield was determined using a formula of (raw coal - residue coal)/raw coal x 100. Note that the amounts of raw coal and residue coal were determined on the basis of the amount of anhydrous ashless coal. Herein, the term "extraction time" means the following temperature-maintaining time: In the case where the temperature is increased to a predetermined temperature, the temperature is maintained, and the temperature is then decreased to 3700C or lower, the time during which the predetermined temperature is maintained is referred to as - 32 "extraction time". The term "extraction time zero" means a case where the temperature is increased to a predetermined temperature, and a cooling treatment is then immediately performed without maintaining the temperature. As shown in Fig. 5, at an extraction temperature of 3700C, the maximum extraction yield was obtained at an extraction time of 30 minutes. Even when the extraction time was prolonged for several hours, the extraction yield of coal did not significantly changed. In contrast, at temperatures of 4000C and 420*C, which are in a temperature range in which thermal decomposition of coal rapidly occurs, when the temperature was maintained for a relatively long time, the extraction yield was significantly decreased by, for example, a repolymerization reaction of radicals generated by the thermal decomposition of the coal. Accordingly, it was found that maintaining the temperature for a long time was not economical. It is known that, at temperatures exceeding 4200C, the thermal decomposition significantly occurs, thereby decreasing the extraction yield. Therefore, experiments at such temperatures were omitted here. Specifically, at an extraction temperature of 400*C, the extraction yield was hardly changed in an extraction time in the range of 0 to 20 minutes, and an extraction yield of about 60% or more could be achieved. However, when - 33 the extraction time reached 60 minutes, the extraction yield was decreased to about 50%. In addition, it was found that even when the extraction temperature was increased to 420*C, a relatively high extraction yield (about 52% or more) could be maintained within an extraction time of 20 minutes. Note that, in general, an extraction yield of about 52% or more can be considered to be as a relatively high extraction yield. The above results showed that in the case where the extraction temperature was in the range of 4000C to 4200C and the time before the temperature was cooled to 3700C or lower was 20 minutes or less, ashless coal could be obtained at a high efficiency. Ashless coal obtained under an extraction condition of 400*C for zero minutes had a resolidification temperature of 483*C, and ashless coal obtained under an extraction condition of 4000C for 10 minutes had a resolidification temperature of 4900C. Ashless coal obtained under an extraction condition of 4200C for zero minutes had a resolidification temperature of 4870C, and ashless coal obtained under an extraction condition of 420*C for 22 minutes had a resolidification temperature of 4860C. Accordingly, the resolidification temperatures of the above ashless coal samples obtained by heating low-quality coal such as sub-bituminous coal at a temperature in the range of - 34 400 0 C to 420 0 C for 20 minutes or less and then cooling to 370*C or lower were comparable to the resolidification temperature of the above strongly caking coal. Accordingly, even when such ashless coal is added to caking coal for coke for iron making to prepare blended coal, the fluidity of strongly caking coal contained in the blended coal is not obstructed and the fluidity of the whole blended coal is not obstructed. On the other hand, at an extraction temperature of 370'C, a resolidification temperature of about 460 0 C is merely achieved. Accordingly, even though the extraction yield is high, as described above, the resulting ashless coal is not particularly excellent as caking coal for coke for iron making. According to the results of Examples 1 to 3, it was found that, in the case where low-quality coal such as inexpensive non- or slightly caking coal is used as a raw material, conditions for obtaining ashless coal having a high resolidification temperature at a high efficiency are as follows: As an extraction temperature, the temperature is increased to 400 0 C to 420 0 C, heating is performed at the temperature for 20 minutes or less, and the temperature is then decreased to 370 0 C or lower. In addition, the time (extraction time) during which the temperature is maintained in the range of 400 0 C to 420'C is preferably 15 minutes or - 35 less, and more preferably 10 minutes or less. Furthermore, the extraction time is preferably as short as possible. In addition, even when ashless coal thus obtained is added to caking coal for coke for iron making to prepare blended coal, the strength of the coke is not degraded. In Examples 2 and 3, cases where low-quality coal is used as a raw material have been described. Alternatively, in the case where caking coal (strongly caking coal) is used as a raw material, the resulting ashless coal has a quality higher than the ashless coal produced using low-quality coal as the raw material. Furthermore, the resulting ashless coal has a quality higher than the caking coal used as raw coal. Accordingly, in order to produce ashless coal having a higher quality, caking coal is used as a raw material. However, in the case where a reduction in the cost of raw coal for coke is important, inexpensive low-quality coal is preferably used as the raw material. A method of producing ashless coal according to the present invention has been described in detail using the best mode and Examples. However, the purpose of the present invention is not limited to the features described above, and the scope of right of the present invention should be widely interpreted on the basis of the description of Claims. It is to be understood that the features of the present invention can be widely modified and changed on the basis of - 36 the above description.

Claims

1. A method of producing ashless coal used in caking coal for coke for iron making, comprising: a slurry preparation step of mixing a solvent with coal to prepare a slurry; an extraction step of extracting the slurry obtained in the slurry preparation step at a temperature in the range of 4000C to 4200C for 20 minutes or less, and then cooling the slurry to 3700C or lower; a separation step of separating the slurry obtained in the extraction step into a liquid portion and a non-liquid portion; and an improved-coal-obtaining step of separating the solvent from the liquid portion separated in the separation step to obtain ashless coal which is improved coal.

2. The method of producing the ashless coal according to Claim 1, wherein, in the improved-coal-obtaining step, in addition to the production of the ashless coal, the solvent is separated from the non-liquid portion separated in the separation step to obtain residue coal which is improved coal.

3. The method of producing the ashless coal according to Claim 1 or 2, wherein, in the extraction step, the slurry obtained in the slurry preparation step is heated to and extracted at a temperature in the range of 400 0 C to 420 0 C - 38 and then immediately cooled to 370'C or lower.

4. The method of producing the ashless coal according to Claim 1 or 2, wherein the coal is low-quality coal.

5. The method of producing the ashless coal according to Claim 3, wherein the coal is low-quality coal.