Green coke for graphite electrodes and its manufacturing method, needle coke for graphite electrodes and its manufacturing method, manufacturing method of graphite electrodes

Abstract

To provide green coke for graphite electrodes and a method for producing the same, needle coke for graphite electrodes and a method for producing the same, and a method for producing graphite electrode which allow for the suppression of puffing in needle coke, enhancing the manufacturing yield and properties of graphite electrodes, without the imposition of significant costs during the production of needle coke.SOLUTION: Green coke for graphite electrodes is used in the production of needle coke for graphite electrodes, wherein an inhibitor for the production of graphite electrodes is contained. The green coke for graphite electrodes is calcined at temperatures of 1000°C or more and 1700°C or less to make needle coke for graphite electrodes. The needle coke for graphite electrodes is used as aggregate, which is subjected to graphitization to make a graphite electrode.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to green coke for graphite electrodes and a method for producing the same, needle coke for graphite electrodes and a method for producing the same, and a method for producing graphite electrodes. [Background technology]

[0002] Coal tar, a by-product of coal carbonization, is mostly composed of condensed polycyclic aromatic compounds and has long been used as a raw material for various carbon products. The coal tar product group consists of approximately 30% products such as creosote oil and naphthalene obtained from distillate components, and the remaining 70% products obtained from coal tar pitch, a heavy component that is not a distillate. Among these, needle coke, produced from coal tar pitch, occupies a particularly important position as a high-value-added product and is mainly used as aggregate for graphite electrodes in electric steelmaking. In the production of needle coke, green coke is obtained by coking pitch, from which impurities have been removed in the refining process, at a temperature of 400°C or higher using a delayed coker or the like. Next, needle coke is obtained by heat-treating this green coke in a calcination process, which removes the water and volatile components contained in the green coke. In the manufacturing process of graphite electrodes, needle coke granules and binder pitch, which is used as a binder in the production of molded bodies, are first mixed in a predetermined ratio, heated and kneaded, and then extruded to produce a green electrode. This green electrode is then calcined to graphitize it, and after further processing, a graphite electrode product is obtained.

[0003] These graphite electrodes are used under harsh high-temperature conditions, requiring extremely high thermal shock resistance. To manufacture graphite electrodes with high thermal shock resistance, needle coke with a low coefficient of thermal expansion is necessary. Needle coke made from coal tar pitch (hereinafter sometimes referred to as pitch-based needle coke) has the lowest coefficient of thermal expansion among all cokes, making it the most preferable raw material for graphite electrodes. However, while pitch-based needle coke yields high-quality graphite electrodes, it is prone to an irreversible expansion phenomenon called puffing during the graphitization process to manufacture the electrodes. If graphitization is rapid, cracks can form in the product, significantly reducing the yield.

[0004] Therefore, the production of graphite electrodes required a long heating process for graphitization, resulting in extremely low productivity. This puffing phenomenon is thought to be due to abnormal expansion caused by the rapid desorption and volatilization of nitrogen and sulfur contained in pitch-based needle coke, mainly in the 1500-2100°C and 2500-2800°C regions during the graphitization process.

[0005] For example, Patent Documents 1 and 2 propose a method to reduce buffing by heat-treating pitch coke at 1500°C or higher to denitrify it. Patent Document 3 also describes a method in which green coke is pre-treated, such as through oxidation, and then heat-treated at a normal calcination temperature. These methods have drawbacks: the former involves high energy consumption due to high-temperature heating, and the latter involves a more complex process compared to conventional methods. Furthermore, it has been proposed to increase the puffing suppression effect while reducing the amount of inhibitor added by adding a metal compound used as a puffing inhibitor in solution only to the surface of the lump and granular coke before mixing it with binder pitch, etc., and then heat-treating it (Patent Document 4). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 60-33208 [Patent Document 2] Japanese Patent Application Publication No. 60-208392 [Patent Document 3] Japanese Patent Publication No. 135486 / 1986 [Patent Document 4] Japanese Patent Publication No. 2001-329271 [Overview of the initiative] [Problems that the invention aims to solve]

[0007] However, these methods for producing low-puffing needle coke all have economic drawbacks, preventing them from being put into practical use, or failing to achieve a sufficient reduction in puffing.

[0008] The present invention has been made in view of the above circumstances, and aims to provide green coke for graphite electrodes and a method for producing the same, needle coke for graphite electrodes and a method for producing the same, and a method for producing graphite electrodes, which can suppress puffing of needle coke and improve the manufacturing yield and characteristics of graphite electrodes without incurring significant costs during the production of needle coke. [Means for solving the problem]

[0009] One embodiment of the present invention includes the following embodiments: [1] Green coke used in the production of needle coke for graphite electrodes, Green coke for graphite electrodes, containing an inhibitor for graphite electrode manufacturing. [2] The graphite electrode raw coke according to [1], wherein the inhibitor for manufacturing graphite electrodes comprises at least one of a metal consisting of the following elements (Mβ) and an oxide having the following elements (Mβ). Element (Mβ): At least one element selected from the group consisting of Group 4, Group 8, Group 9, Group 10, Group 13, Group 14, and Group 15 of the long-period periodic table. [3] The green coke for graphite electrode according to [2], wherein the element (Mβ) is at least one element selected from the group consisting of Si, Ge, Al, B, Ti, Fe, and P. [4] The green coke for graphite electrode according to [2] or [3], wherein the oxide having the element (Mβ) is a composite oxide having the following element (Mα) and the element (Mβ). Element (Mα): At least one metal element (excluding the element (Mβ)). [5] The green coke for graphite electrode according to [4], wherein the element (Mα) is at least one metal element selected from the group consisting of K, Sc, alkaline earth metal elements, and rare earth metal elements. [6] The green coke for graphite electrode according to [4] or [5], wherein the composition formula of the composite oxide is the following formula (1). Mα , , 5-z , , , 1-y , , , Mβ 1-y O 5-z ···(1) (In the formula, 0 ≦ x < 3, 0 ≦ y < 1, and 0 ≦ z < 5.) [7] The green coke for graphite electrode according to any one of [1] to [6], wherein the green coke is a carbide of a raw material pitch containing one or more of pitches derived from coal tar, FCC decant oil, ethylene heavy end, petroleum residue, petroleum waste, biomass oil, or biomass tar. [8] The green coke for graphite electrode according to any one of [1] to [7], wherein when the ratio (wt%) of the inhibitor for graphite electrode production to the total weight of the green coke is X, 0.1 < X < 9.5. [9] A method for producing the green coke for graphite electrode according to any one of [1] to [8], A method for producing green coke for graphite electrode, comprising applying an inhibitor for graphite electrode production to the surface of the green coke and heat-treating it to incorporate the inhibitor for graphite electrode production into the green coke.

[10] The method for producing green coke for graphite electrode according to [9], wherein the temperature of the heat treatment is 300°C or higher and lower than 600°C.

[11] A method for producing green coke for graphite electrodes according to [9] or

[10] , wherein the heat treatment is performed in a rotary kiln furnace. Needle coke for graphite electrodes, which is a carbonized form of green coke for graphite electrodes as described in any of

[12] [1] to [8].

[13] The puffing value P calculated using the following formula (I) for the test piece created in the evaluation test (i) below. 2800 Needle coke for graphite electrodes as described in

[12] , wherein the content is 2.040% or less. P 2800 =(L2-L1) / L1×100 ···(I) However, L1 and L2 in equation (I) have the following meanings. L1: Thickness of the test piece before firing (mm) L2: Thickness of the test piece after firing at 2800℃ (mm) <Evaluation Test (i)> The graphite electrode needle coke and 30% by weight of binder pitch relative to the graphite electrode needle coke are mixed and kneaded for 5 minutes while heating at 165°C. This is molded into a disc shape of 20mmΦ × 3mm to 15mm and fired in a firing furnace at 1000°C for 3 hours to burn off the binder pitch and create a test piece. The test piece is fired at a heating rate of 20°C / min to 2800°C, and L1 and L2 of the test piece are measured before and after firing.

[14] The puffing value P calculated using the following formula (II) for the test piece created in the evaluation test (ii) below. 1700-2100 Needle coke for graphite electrodes as described in

[12] or

[13] , wherein the content is 1.10% or less. P 1700-2100 =(L3-L4) / L5×100 ···(II) However, L3, L4, and L5 in equation (II) have the following meanings. L3: Thickness of the test piece at 2100℃ firing (mm) L4: Thickness of the test piece at 1700℃ firing (mm) L5: Thickness of the test piece at the time of firing at 1000℃ (mm) <Evaluation Test (ii)> Mix the needle coke for the graphite electrode with a binder pitch of 30% by weight based on the outside ratio with respect to the needle coke for the graphite electrode, and knead for 5 minutes while heating at 165°C. Mold this into a disk shape with a diameter of 20 mm Φ × 3 mm to 15 mm, and bake it at 1000°C for 3 hours using a firing furnace to burn off the binder pitch and obtain a test piece. Heat the test piece to 2800°C at a heating rate of 20°C / min using a thermal expansion measuring device, and measure L3, L4, and L5 of the test piece during firing.

[15] In the evaluation test (i), a test piece is prepared in the same manner except that green coke not containing the inhibitor for manufacturing the graphite electrode is used, and the puffing value (blank) calculated by the formula (I) of the test piece is P 2800b When it is 2800 / P 2800b The needle coke for the graphite electrode according to

[13] , wherein the ratio of the puffing value calculated by is less than 1.

[16] In the evaluation test (ii), a test piece is prepared in the same manner except that green coke not containing the inhibitor for manufacturing the graphite electrode is used, and the puffing value (blank) calculated by the formula (II) of the test piece is P 1700-2100b When it is 1700-2100 / P 1700-2100b The needle coke for the graphite electrode according to

[14] , wherein the ratio of the puffing value calculated by is less than 1.

[17] A method for manufacturing needle coke for a graphite electrode, wherein the green coke for the graphite electrode according to any one of [1] to [8] is calcined at 1000°C or higher and 1700°C or lower to obtain needle coke.

[18] The method for manufacturing needle coke for a graphite electrode according to

[17] , wherein the calcination is performed in a rotary kiln furnace.

[19] When the ratio (weight%) of the inhibitor for manufacturing the graphite electrode to the total weight of the needle coke is Y, 0.02 < Y < 15, the method for manufacturing needle coke for a graphite electrode according to

[17] .

[20] A method for manufacturing a graphite electrode, wherein the needle coke for the graphite electrode according to any one of

[12] to

[16] is used as an aggregate and graphitized to obtain a graphite electrode.

[21] The method for manufacturing a graphite electrode according to

[20] , wherein the temperature of the graphitization treatment is 2500°C or more and 3000°C or less.

[22] The method for manufacturing a graphite electrode according to

[20] or

[21] , wherein the proportion of the inhibitor for manufacturing the graphite electrode remaining in the graphite electrode is 0.01% by weight or less. [Effects of the Invention]

[0010] According to the present invention, by using green coke for graphite electrodes in which a graphite electrode manufacturing inhibitor is contained in the green coke used in the production of needle coke for graphite electrodes, it is possible to suppress puffing of needle coke and improve the production yield and characteristics of graphite electrodes without incurring significant costs during the production of needle coke. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows the hot puffing values ​​measured by a thermal expander for Example 1 and Comparative Example 1. [Figure 2] This figure shows the data obtained from the hot buffing measurements of Example 1 and Comparative Example 1, calculated using the difference method. [Modes for carrying out the invention]

[0012] The present invention will be described in detail below, but the present invention is not limited to the following description and can be modified and implemented as such without departing from the spirit of the invention. In the following explanation, "weight%" is synonymous with "mass%", and "parts by weight" is synonymous with "parts by mass". In this invention, "pitch-based" shall be treated as synonymous with "coal-based." In this invention, "sulfur content of coke" refers to the value measured in accordance with JIS M8813. Furthermore, in this invention, "nitrogen content of coke" refers to the value measured in accordance with JIS M8819.

[0013] 1. Green coke for graphite electrodes One embodiment of the present invention relates to green coke for graphite electrodes. The green coke for graphite electrodes in this embodiment is green coke used in the manufacture of needle coke for graphite electrodes, and contains a graphite electrode manufacturing inhibitor (hereinafter sometimes simply referred to as "inhibitor").

[0014] 1.1. Inhibitors for Graphite Electrode Manufacturing The inhibitor used in the binder composition for manufacturing graphite electrodes in this embodiment is intended to suppress the puffing of needle coke when obtaining graphite electrodes by firing them simultaneously with needle coke.

[0015] One inhibitor in the embodiment includes at least one of a metal composed of element (Mβ) and an oxide having element (Mβ). Element (Mβ): At least one element selected from the group consisting of Group 4 elements (Ti, Zr, Hf), Group 8 elements (Fe, Ru, Os), Group 9 elements (Co, Rh, Ir), Group 10 elements (Ni, Pr, Pt), Group 13 elements (B, Al, Ga, In), Group 14 elements (Si, Ge, Sn), and Group 15 elements (P, Sb, Bi) of the long-period periodic table.

[0016] (First embodiment) The inhibitor of the first embodiment includes a composite oxide having elements (Mα) and (Mβ). Element (Mα): At least one metallic element (excluding element (Mβ)). Element (Mβ): At least one element selected from the group consisting of Group 4 elements (Ti, Zr, Hf), Group 8 elements (Fe, Ru, Os), Group 9 elements (Co, Rh, Ir), Group 10 elements (Ni, Pr, Pt), Group 13 elements (B, Al, Ga, In), Group 14 elements (Si, Ge, Sn), and Group 15 elements (P, Sb, Bi) of the long-period periodic table.

[0017] As for the element (Mα), it is preferable that it is at least one metallic element selected from the group consisting of K, Sc, alkaline earth metal elements (Mg, Ca, Sr, Ba), and rare earth metal elements (Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), as it is easier to reduce puffing; more preferably that it is at least one metallic element selected from the group consisting of alkaline earth metals and rare earth metal elements; particularly preferable that it is at least one metallic element selected from the group consisting of alkaline earth metals and rare earth metal elements; and most preferably an alkaline earth metal.

[0018] The element (Mβ) is preferably at least one element selected from the group consisting of Si, Ge, Al, B, Ti, Fe, and P, more preferably at least one element of Si and Ge, and most preferably Si, as it is easier to reduce puffing.

[0019] The compositional formula of the inhibitor in the first embodiment is not particularly limited, but it is preferably the following formula (1) because it is easier to reduce puffing. Mα 3-x Mβ 1-y O 5-z ...(1) (In the equation, 0 ≤ x < 3, 0 ≤ y < 1, and 0 ≤ z < 5.)

[0020] In the formula, x is preferably 0 ≤ x < 3, more preferably 0.05 ≤ x ≤ 2.5, and particularly preferably 0.1 ≤ x ≤ 2, as this makes it easier to reduce puffing. In the formula, y is preferably 0 ≤ y < 1, more preferably 0.01 ≤ y ≤ 0.8, and particularly preferably 0.1 ≤ y ≤ 0.5, as this makes it easier to reduce puffing. In the formula, z is preferably 0 ≤ z < 5, more preferably 0.05 ≤ z ≤ 4, and particularly preferably 0.1 ≤ z ≤ 3, as this makes it easier to reduce puffing.

[0021] Specific examples of composite oxides containing elements (Mα) and (Mβ) include, for example, MgSi3O7, Mg3SiO5, and Mg 14 Si5O 24 , MgO, Ca3SiO5, Ca2SiO4, CaSiO3, Ca8Si5O 18 , CaSi2O5, CaO, CaCO3, Ca(OH)2, SrSiO3, Sr3SiO5, SrSi2O5, Sr2SiO4, Ba3SiO5, Ba2SiO4, BaSiO3, BaSi2O5, Ba2Si3O8, Ba5Si8O 21 Ba3Si5O 13 , Ba2SiO4, BaO, BaCO3, Ce2SiO5, Ce2Si2O7, Ce2Si4O 11 , CeSi3O8, Ce3SiO8, Ce6Si6O 21 La5Si3O 13.5 La6Si6O 21 , La2SiO5, La2Si4O 11 La2Si6O 16 , Pr 4.67 Si3O 13 ,Pr2Si2O7,Pr6Si2,Pr6Si 12 O 25 ,Eu5Si3O 13 Eu2Si2O7, Eu2SiO4, Eu6Si 12 O 33 , CaGe2O5, Ca3GeO5, Ca2GeO4, Ca5Ge3O 11 CaGeO3, Ca2Ge7O 16 CaAl 12 O 19 CaAl4O7, Ca4Al6O 13 Ca5Al6O 14 Ca6Al7O 16 , Ca2Al2O5, Ca3Al2O6, CaFeO2, Ca2Fe2O5, CaFeO3, CaFe2O4, CaFe3O5, CaFe4O6, CaFe5O7, CaFe6O8, CaTi2O4, CaTi2O5, Ca3Ti2O7, Ca4Ti3O 10 CaTiO3, CaTi5O 11、Mg2Al4O8、MgAl2O4、Mg2Al2O5、MgGe2O4、Mg2TiO4、MgTiO3、MgTi2O4、MgTi2O5、MgFe2O4、SrAl 12 O 19 SrAl4O7 Sr4Al 14 O 25 Sr 12 Al 14 O 33 Sr3Al2O6 Sr 10 Al6O 19 、Sr3GeO、SrGe4O9、SrGeO3、Sr3GeO5、SrTi 11 O 20 Sr2Ti6O 13 SrTiO3, Sr4Ti3O 10 Sr3Ti2O7, Sr2TiO4, SrFe 12 O9, SrFe2O4, Sr2Fe3O6, SrFeO2, Sr4Fe6O 13 Sr2Fe2O5, Sr3Fe2O5, Sr4Fe4O 11 、SrFeO3、Sr3Fe2O6、Sr2FeO3、Sr3Fe2O7、SrFeO4、Sr2FeO4、BaAl 12 O 19 ,BaAl4O7、BaAl2O4、Ba3Al2O6、Ba4Al2O7、Ba7A l2 O 10 Ba 17 Al3O7、Ba 21 Ge2O5, Ba3GeO, Ba3GeO5, Ba 10 Ge7O3, BaGeO3, BaGe2O5, Ba2Ge5O 12 BaGe4O9, Ba3TiO5, Ba2TiO4, Ba4Ti5O 10 Ba4Ti4O 11 BaTiO3, BaTi2O5, BaTi4O9, Ba2Ti9O 20 BaTi5O 11 Ba2Ti 13 O 22 Ba3FeO5, Ba2FeO4, BaFeO2, Ba2Fe2O5, Ba8Fe8O 21 Ba5Fe5O 14 BaFeO3, BaFe2O4, Ba2Fe6O 11BaFe4O7, BaFe7O 11 BaFe 12 O 19 BaFe 15 O 23 Examples can be given. The complex oxide contained in the inhibitor of the first embodiment may be one type or two or more types.

[0022] The method for producing the inhibitor of the first embodiment is not particularly limited, and examples of known methods include weighing and mixing raw material compounds to achieve the composition ratio of the desired composite oxide, and calcining them in an air atmosphere and a reducing atmosphere at a temperature range of 1000°C to 1500°C. Furthermore, while there is no particular upper limit to the particle size of the obtained inhibitor, it is more preferable to be 10,000 μm or less, particularly preferable to be 1,000 μm or less, and most preferable to be 100 μm or less. Similarly, while there is no particular lower limit to the particle size, it is more preferable to be 1 nm or more, particularly preferable to be 10 nm, and most preferable to be 100 nm or more. By controlling the particle size within this range, uniform dispersion becomes easier, and the crystallinity of the obtained inhibitor is maintained, which is expected to enhance the puffing effect. In this specification, "inhibitor particle size" refers to the mode diameter measured using a laser diffraction particle size analyzer MT3300EX (manufactured by Microtrac-Bel) with ethanol as the dispersion medium.

[0023] By using the inhibitor of the first embodiment, which includes a composite oxide having elements (Mα) and (Mβ), the puffing suppression effect is increased, improving the manufacturing yield and characteristics of graphite electrodes. The reason why the inhibitor of the first embodiment exhibits such effects is not yet clear, but it is presumed to be as follows. It is presumed that the presence of the inhibitor of the first embodiment during the calcination of needle coke for graphite electrodes causes the formation of a nitrogen-containing complex compound or a sulfur-containing complex compound at a temperature lower than the temperature at which nitrogen is desorbed from the graphite electrode, resulting in a difference in the timing of nitrogen desorption and sulfur desorption compared to the system without the inhibitor, thus suppressing the puffing phenomenon. In other words, it is presumed that the inhibitor of the first embodiment contains a complex oxide having elements (Mα) and (Mβ), and therefore reacts with nitrogen or sulfur in the coke during the heating process during calcination to form a complex compound (nitride, oxynitride, sulfide, oxysulfide), thus producing the effect of the first embodiment.

[0024] (Second Embodiment) The inhibitor of the second embodiment includes a metal composed of element (Mα) or an oxide having element (Mα), and a metal composed of element (Mβ) or an oxide having element (Mβ). The inhibitor of the second embodiment is preferably composed of an oxide having element (Mα) and an oxide having element (Mβ) because it is easier to reduce puffing. Element (Mα): At least one metallic element (excluding element (Mβ)). Element (Mβ): At least one element selected from the group consisting of Group 4, Group 8, Group 9, Group 10, Group 13, Group 14, and Group 15 of the long-period periodic table.

[0025] <Metals composed of element (Mα) or oxides containing element (Mα)> As for the element (Mα), it is preferable that it is at least one metallic element selected from the group consisting of K, Sc, alkaline earth metal elements and rare earth metal elements, more preferably at least one metallic element selected from the group consisting of Sc, alkaline earth metals and rare earth metal elements, particularly preferably at least one metallic element selected from the group consisting of alkaline earth metals and rare earth metal elements, and most preferably alkaline earth metals.

[0026] The compositional formula of the oxide containing the element (Mα) is not particularly limited, but it is preferably the following formula (2) because it is easier to reduce puffing. Mα 3-x1 O 3-z1 ...(2) (In the formula, Mα is the element (Mα), and 0 ≤ x1 < 3 and 0 ≤ z1 < 3.)

[0027] In the formula, x1 is preferably 0 ≤ x1 < 3, more preferably 0.05 ≤ x1 ≤ 2.5, and particularly preferably 0.1 ≤ x1 ≤ 2, as this makes it easier to reduce puffing. In the formula, z1 is preferably 0 ≤ z1 < 3, more preferably 0.05 ≤ z1 ≤ 2.5, and particularly preferably 0.1 ≤ z1 ≤ 2, as this makes it easier to reduce puffing.

[0028] Specific examples of metals composed of element (Mα) or oxides containing element (Mα) include, for example, CaO, Ca(OH)2, CaCO3, Ca, MgO, MgCO3, Mg(OH)2, Mg, SrO, SrCO3, Sr(OH)2, Sr, BaO, BaCO3, Ba(OH)2, Ba, CeO2, Ce, and Pr6O. 11 Examples include Pr, Eu2O3, and Eu. The oxide containing the element (Mα) in the inhibitor of the second embodiment may be one or two or more.

[0029] <Metals composed of element (Mβ) or oxides containing element (Mβ)> The element (Mβ) is preferably at least one element selected from the group consisting of Si, Ge, Al, B, Ti, Fe, and P, more preferably at least one element of Si and Ge, and most preferably Si, as it is easier to reduce puffing.

[0030] The compositional formula of the oxide containing the element (Mβ) is not particularly limited, but it is preferable to have the following formula (3) because it is easier to reduce puffing. Mβ 1-y1 O 2-z2 ...(3) (where Mβ is an element (Mβ), 0 ≦ y1 < 1, and 0 ≦ z2 < 2.)

[0031] In the formula, since y1 makes it easy to reduce puffing, 0 ≦ y1 < 1 is preferable, 0.01 ≦ y1 ≦ 0.8 is more preferable, and 0.1 ≦ y1 ≦ 0.5 is particularly preferable. In the formula, since z2 makes it easy to reduce puffing, 0 < z2 ≦ 2 is preferable, 0.05 ≦ z2 ≦ 0.15 is more preferable, and 0.1 ≦ z2 ≦ 0.2 is particularly preferable.

[0032] Specific examples of the metal composed of the element (Mβ) or the oxide having the element (Mβ) include, for example, SiO2, Si, SiO x , GeO2, Ge, Al2O3, Al, B2O3, P2O5. The oxide having the element (Mβ) contained in the inhibitor of the second embodiment may be one kind or two or more kinds.

[0033] The method for producing the inhibitor of the second embodiment is not particularly limited. For example, the element (Mα) and the element (Mβ) are weighed and mixed so as to be in the target ratio, and fired in an air atmosphere and a reducing atmosphere in a temperature range of 1000 °C to 1500 °C, and an example of a method for obtaining an inhibitor in which the oxide having the element (Mα) and the oxide having the element (Mβ) are combined can be given. Alternatively, an oxide having the element (Mα) and an oxide having the element (Mβ) may be mixed and prepared. The particle size of the combined inhibitor is not particularly limited, but 10000 μm or less is more preferable, 1000 μm or less is particularly preferable, and 100 μm or less is most preferable. The lower limit of the particle size is not particularly limited, but 1 nm or more is more preferable, 10 nm is particularly preferable, and 100 nm or more is most preferable. By controlling the particle size within this range, it is expected to be easily dispersed uniformly and enhance the puffing effect.

[0034] The mixing ratio of the oxide containing element (Mα) and the oxide containing element (Mβ) is not particularly limited, but in order to achieve low puffing, the lower limit of the weight ratio of the oxide containing element (Mβ) to the oxide containing element (Mα) is usually 0.01 or more, preferably 0.05 or more, and more preferably 0.1 or more, while the upper limit is usually 1 or less, preferably 0.7 or less, and more preferably 0.5 or less.

[0035] By using the inhibitor of the second embodiment, which includes a metal composed of element (Mβ) or an oxide having element (Mα), and a metal composed of element (Mβ) or an oxide having element (Mβ), the puffing suppression effect is increased, and the manufacturing yield and characteristics of graphite electrodes are improved. The reason why the inhibitor of the second embodiment exhibits such an effect is not yet clear, but it is presumed to be as follows. It is presumed that the presence of the inhibitor of the second embodiment during the calcination of needle coke for graphite electrodes causes the formation of a nitrogen-containing complex compound or a sulfur-containing complex compound at a temperature lower than the temperature at which nitrogen is desorbed from the graphite electrode, resulting in a difference in the timing of nitrogen and sulfur desorption compared to the system without the inhibitor, thus suppressing the puffing phenomenon. In other words, it is presumed that the inhibitor of the second embodiment contains oxides having element (Mα) and oxides having element (Mβ), and therefore reacts with nitrogen or sulfur in the coke during the heating process during calcination to form a complex compound (nitride, oxynitride, sulfide, oxysulfide), thus producing the effect of the second embodiment.

[0036] 1.2. Raw Coke Green coke can be made from carbides obtained, for example, by coking raw pitch. As the raw material pitch, pitch commonly used as a raw material for needle coke for graphite electrodes can be used. For example, pitch derived from coal tar, FCC decanted oil, ethylene heavy end, petroleum residues, petroleum waste, biomass oil, biomaster, etc. The raw material pitch may be used alone or in combination of two or more types.

[0037] Coal tar pitch is preferred as a raw material pitch because it is easier to obtain needle coke with a low coefficient of thermal expansion. Compared to petroleum-based pitches, coal tar pitch has a higher sulfur and nitrogen content, and tends to have a higher buffing rate if used as is, but in this invention, buffing can be sufficiently suppressed even with coal tar pitch. Pitch-based needle coke can be obtained by hydrogenating the raw material coal tar pitch and then coking the resulting hydrogenated coal tar pitch.

[0038] Hydrogenated coal tar pitch is obtained by hydrogenating raw coal tar pitch (hydrogenation step) and separating light oil from the resulting hydrogenated coal tar pitch (separation step). The light oil separated in the separation step can be supplied to the hydrogenation step for recycling. The method for hydrogenating the raw coal tar pitch and the method for separating light oil from the hydrogenated coal tar pitch are not particularly limited, and known methods can be applied.

[0039] The raw material coal tar pitch is not particularly limited. The method for producing the raw material coal tar pitch (pre-preparation method) is not particularly limited, and an example is a method of substantially removing quinoline insoluble matter from coal tar heavy oil. Known methods can be applied as means for removing quinoline insoluble matter, but a method of treatment with an aromatic oil or an aliphatic oil solvent, or a method of treatment with a mixed solvent of aromatic oil and aliphatic oil is preferred. Specifically, the solvent is mixed with coal tar heavy oil under appropriate conditions, heated, and then allowed to stand as necessary, and the mixture is distilled to remove low-boiling point components, thereby obtaining a raw material coal tar pitch that contains almost no quinoline insoluble matter. As aliphatic oils, alicyclic compounds such as cyclohexane and cyclopentane, compounds having carbonyl groups such as acetone and ether, and light oil can be used. As aromatic oils, tar-based cleaning oil and anthracene oil can be used.

[0040] In the production of hydrogenated coal tar pitch, petroleum-based heavy oil may be mixed with the raw material coal tar pitch. When petroleum-based heavy oil is mixed, the quinoline-insoluble components may be removed after mixing the coal tar-based heavy oil and the petroleum-based heavy oil to obtain a mixture of raw material coal tar pitch and petroleum-based heavy oil. Alternatively, the raw material coal tar pitch and petroleum-based heavy oil may be mixed, and the light oil may be separated from this mixture before being used in the hydrogenation process. Furthermore, the light oil-separated coal tar pitch obtained by separating the light oil may be mixed with petroleum-based heavy oil and used in the hydrogenation process.

[0041] The petroleum-based heavy oil is not particularly limited and examples include fluid catalytic cracking oil, atmospheric distillation residue, vacuum distillation residue, shale oil, tar sand bitumen, Orinoco tar, liquefied coal oil, ethylene bottom oil, and heavy oils obtained by hydrorefining these. In addition, the mixture may further contain relatively light oils such as straight-run diesel, vacuum diesel, desulfurized diesel, and desulfurized vacuum diesel.

[0042] The coking method is not particularly limited, and examples thereof include the delayed coking method, the visbreaking method, the flexible coking method, and the Eureka process. Among these, the delayed coking method is preferred from the viewpoints of the productivity and quality stability of needle coke. In the delayed coking method, the raw material pitch rapidly passes through the heating tube while being heated and is introduced into the coke drum to cause coking. For coking, a rotary kiln furnace, a shaft furnace, etc. can be used.

[0043] The coking temperature is preferably 300°C or higher and 600°C or lower, more preferably 450°C or higher and 550°C or lower. The pressure during coking is preferably 0.01 MPa or higher and 0.99 MPa or lower in gauge pressure, more preferably 0.25 MPa or higher and 0.60 MPa or lower. The coking time is preferably 8 hours or longer and 72 hours or shorter, more preferably 10 hours or longer and 36 hours or shorter.

[0044] When the ratio (weight %) of the inhibitor to the total weight of the green coke is X, it is preferably 0.1 < X < 9.5. Thereby, the puffing reduction effect can be easily obtained sufficiently, and it is also easy to reduce the adverse effect on the electrode product. The ratio X of the inhibitor is more preferably 1 < X < 9, and even more preferably 1.5 < X < 8.

[0045] 1.3. Method for producing green coke for graphite electrode The method for producing green coke for graphite electrode according to the embodiment is not particularly limited, and examples thereof include a method of incorporating an inhibitor into green coke by applying the inhibitor to the surface of the green coke and performing heat treatment. More specifically, for example, a method of dissolving or dispersing an inhibitor in an organic solvent or heavy oil, applying it to the surface of the green coke, and performing heat treatment at 300°C or higher and less than 600°C can be exemplified. For example, the inhibitor may be dissolved in a volatile solvent such as alcohol or benzene and applied to the surface of the green coke.

[0046] The heat treatment time is preferably 0.1 hour or more and 10 hours or less, more preferably 0.5 hour or more and 5 hours or less. The heat treatment mode is not particularly limited and can be carried out in a rotary kiln furnace, a shaft furnace, etc., and it is preferably carried out in a rotary kiln furnace.

[0047] 2. Needle coke for graphite electrode Another embodiment of the present invention relates to needle coke for graphite electrodes. The needle coke for graphite electrodes in the embodiment is a carbide of the green coke for graphite electrodes in the embodiment. The needle coke for graphite electrodes in the embodiment is preferably pitch-based needle coke. Since pitch-based needle coke has a small coefficient of thermal expansion, it can be suitably used as an aggregate for graphite electrodes for electric furnace steelmaking. Also, although pitch-based needle coke has a high content of sulfur and nitrogen, in the present invention, even pitch-based needle coke can sufficiently suppress puffing.

[0048] When the ratio (weight %) of the inhibitor to the total weight of the needle coke for graphite electrodes is Y, it is preferably 0.02 < Y < 15. However, the ratio Y of the inhibitor is a value as the ash content (metal component or metal oxide). When the ratio Y of the inhibitor satisfies the above conditions, it is easy to sufficiently obtain the puffing reduction effect, and it is also easy to reduce the adverse effect on the electrode product due to the remaining ash. The ratio Y of the inhibitor is more preferably 0.1 < Y < 10, and even more preferably 0.5 < Y < 8.

[0049] The needle coke for graphite electrodes in the embodiment is easy to suppress the puffing of the needle coke during the production of graphite electrodes. Therefore, the puffing value P calculated by the following formula (I) of the test piece created in the following evaluation test (i) 2800 is preferably not more than 2.040%. P 2800 =(L2 - L1) / L1 × 100 ···(I) However, L1 and L2 in formula (I) have the following meanings. L1: Thickness (mm) of the test piece before firing L2: Thickness (mm) of the test piece after firing up to 2800 °C

[0050] <Evaluation test (i)> Mix needle coke for graphite electrodes and binder pitch at 30% by weight on an external basis with respect to the needle coke for graphite electrodes (that is, 30 parts by weight of binder pitch with respect to 100 parts by weight of needle coke for graphite electrodes), and knead for 5 minutes while heating at 165 °C. Mold this into a disk shape with a diameter of 20 mm Φ × 3 mm to 15 mm, and bake it at 1000 °C for 3 hours using a firing furnace to burn off the binder pitch to obtain a test piece. Heat the test piece to 2800 °C at a heating rate of 20 °C / min for firing, and measure L1 and L2 of the test piece before and after firing.

[0051] Puffing value P 2800 is preferably 2.040% or less, more preferably 2.035% or less, still more preferably 2.030% or less, particularly preferably -1.00% or less, even more preferably -2.00% or less, and most preferably -3.00% or less. Also, the puffing value P 2800 The lower limit of is not particularly limited, but preferably -30% or more, more preferably -20% or more, still more preferably -15% or more, and particularly preferably -10% or more. The puffing value P 2800 The lower limit and the upper limit can be arbitrarily combined. For example, it is preferably -30% or more and 2.040% or less.

[0052] In the evaluation test (i), a test piece is prepared in the same manner except that green coke without an inhibitor applied to its surface is used, and the puffing value (blank) calculated by the formula (I) of the test piece is taken as P 2800b and. At this time, P 2800 / P 2800bThe ratio of the puffing value calculated by 2800 / P 2800b is preferably less than 1, more preferably 0.995 or less, still more preferably 0.990 or less, and particularly preferably 0.987 or less.

[0053] The needle coke for graphite electrodes of the embodiment is easy to suppress the puffing of the needle coke during the production of the graphite electrode. Therefore, the puffing value P calculated by the following formula (II) of the test piece prepared in the following evaluation test (ii) 1700-2100 is preferably 1.10% or less. P 1700-2100 =(L3 - L4) / L5 × 100 ···(II) However, L3, L4, and L5 in formula (II) have the following meanings. L3: Thickness (mm) of the test piece at the time of firing at 2100°C L4: Thickness (mm) of the test piece at the time of firing at 1700°C L5: Thickness (mm) of the test piece at the time of firing at 1000°C <�

[0054] <� <Evaluation test (ii)> Mix the needle coke for graphite electrodes and 30% by weight of binder pitch by external division with respect to the needle coke for graphite electrodes, and knead for 5 minutes while heating at 165°C. This is molded into a disk shape of 20mmΦ × 3mm ~ 15mm, fired at 1000°C for 3 hours using a firing furnace, and the binder pitch is burned off to obtain a test piece. The test piece is fired using a thermal expansion measuring device at a heating rate of 20°C / min to 2800°C, and L3, L4, and L5 of the test piece during firing are measured.

[0055] The puffing value P 1700-2100 is preferably 1.10% or less, more preferably 0.85% or less, particularly preferably 0.80% or less, and most preferably 0.75% or less. Also, the puffing value P 1700-2100The lower limit of P is not particularly limited, but is preferably -5.0% or higher, more preferably -3.0% or higher, particularly preferably -1.0% or higher, and most preferably -0.5% or higher. 1700-2100 The lower and upper limits can be combined in any way, and for example, it is preferable that they be between -5.0% and 1.10%.

[0056] In the evaluation test (ii) described above, test pieces were prepared in the same manner as above, except that green coke without an inhibitor applied to its surface was used, and the puffing value (blank) of the test piece calculated by formula (II) described above was set to P 1700-2100b Let's assume that. At this time, P 1700-2100 / P 1700-2100b The ratio of the puffing value calculated is preferably less than 1, more preferably 0.90 or less, even more preferably 0.80 or less, and particularly preferably 0.75 or less. 1700-2100 / P 1700-2100b A smaller value is preferable, and there is no lower limit, but it may practically be -1.5 or higher.

[0057] The needle coke for graphite electrodes in this embodiment can be produced by calcining the green coke in this embodiment. The needle coke after calcination may be crushed. For the calcination of green coke, a rotary kiln, a shaft furnace, etc., can be used, and the use of a rotary kiln is preferred.

[0058] The baking temperature is preferably between 1000°C and 1700°C, and more preferably between 1000°C and 1500°C. The baking time is preferably between 1 hour and 6 hours, and more preferably between 1.5 hours and 5 hours.

[0059] Since the needle coke obtained by the above manufacturing method is not particle-size adjusted, it is preferable to mix it with fine coke or the like to obtain needle coke for graphite electrodes with adjusted particle size before binder pitch mixing. In this case, the amount of fine coke to be mixed should preferably be 50% by weight or less, more preferably 30% by weight or less, of the total amount of needle coke for graphite electrodes. Since fine coke does not directly affect the puffing reduction effect, the amount of fine coke to be mixed should be appropriately determined considering electrode quality such as ash content and economic efficiency. Needle coke for graphite electrodes may also be used in the manufacture of graphite electrodes as lump-shaped and granular calcined coke, i.e., coarse-grained coke that has not undergone particle size adjustment.

[0060] 4. Method for manufacturing graphite electrodes Another embodiment of the present invention relates to a method for manufacturing graphite electrodes. The graphite electrode of the embodiment is a graphite electrode obtained by firing needle coke for graphite electrodes of the embodiment. The graphite electrode of the embodiment can be obtained, for example, by using needle coke for graphite electrodes of the embodiment as aggregate, kneading a raw material with an appropriate amount of binder pitch added, molding it to form a green electrode, and then calcining the resulting green electrode to graphitize it. In the manufacture of the graphite electrode, processing may be performed after graphitization as needed.

[0061] A specific example of a method for manufacturing graphite electrodes is to knead binder pitch into needle coke for graphite electrodes according to the embodiment, add iron oxide as needed and knead further, then perform extrusion molding, primary firing, impregnation, secondary firing, graphitization, etc. The use of iron oxide may be omitted, but if iron oxide is used, an even greater reduction in buffing can be obtained, so it should be decided appropriately considering the required quality and economics. For the binder pitch, the same pitch as the raw material pitch can be used as an example.

[0062] The firing temperature is not particularly limited, but it is preferably 500°C to 1200°C, and more preferably 800°C to 1100°C, as this makes it easier to burn off the binder pitch.

[0063] The temperature for the graphitization treatment is preferably between 2500°C and 3000°C, and more preferably between 2600°C and 3000°C. If the temperature for the graphitization treatment is above the lower limit, the inhibitor is more easily burned off. If the temperature for the graphitization treatment is below the upper limit, the graphitization treatment can be performed while suppressing the deterioration and wear of the electrodes due to graphite sublimation and oxidation. The preferred lower and upper limits for the graphitization treatment temperature can be arbitrarily combined.

[0064] The percentage of inhibitor remaining in the graphite electrode is preferably 0.01% by weight or less, more preferably 0.005% by weight or less, and even more preferably 0.001% by weight or less, relative to the total weight of the graphite electrode. If the percentage of residual inhibitor is below the above upper limit, contamination of the iron during electric furnace ironmaking can be reduced.

[0065] As described above, the present invention uses green coke for graphite electrode production that contains an inhibitor. This makes it possible to suppress puffing of needle coke without incurring significant costs during the production of needle coke, thereby improving the production yield and characteristics of graphite electrodes. Furthermore, using an inhibitor containing at least one of a metal composed of element (Mβ) and an oxide containing element (Mβ) increases the puffing suppression effect. In this case, an inhibitor containing a composite oxide containing element (Mα) and element (Mβ) is preferred because it further increases the puffing suppression effect.

[0066] In addition, in the present invention, it is possible to replace the components in the above embodiments with well-known components as long as it does not depart from the spirit of the present invention, and the above-mentioned modifications may be combined as appropriate. [Examples]

[0067] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following description.

[0068] (1) Method for synthesizing inhibitors Ca2SiO4 was prepared by a known method. The obtained Ca2SiO4 was passed through a 100 μm sieve until the particles were less than 100 μm, and this was used as an inhibitor.

[0069] (2) Measurement of cold buffing In each example, the resulting mixture of needle coke and inhibitor (needle coke for graphite electrodes) was classified to a predetermined particle size. 30% by weight of binder pitch was added to the needle coke and inhibitor mixture by external division, and the mixture was kneaded in an oil bath at 165°C for 5 minutes. This was then molded into a disc shape of 20mmΦ × 3mm to 15mm, and fired in a firing furnace at 1000°C for 3 hours to burn off the binder pitch, creating a test piece for puffing measurement. The obtained test pieces were fired by raising the temperature to 2800°C at a heating rate of 20°C / min. L1 and L2 were measured before and after firing, and the puffing value P was calculated using the above formula (I). 2800 The following was calculated. In addition, L3, L4, and L5 of the test piece during firing were measured using a thermal expansion measuring device, and the puffing value P was calculated using the above formula (II). 1700-2100 The result was calculated.

[0070] (3) Measurement of hot buffing In each example, the resulting mixture of needle coke and inhibitor (needle coke for graphite electrodes) was classified to a predetermined particle size. 30% by weight of binder pitch was added to the mixture of needle coke and inhibitor by external division, and the mixture was kneaded in an oil bath at 165°C for 5 minutes. A cylindrical molded body was then produced using an extrusion molding machine. The molded body was fired in a firing furnace at 1000°C for 3 hours to obtain a test piece. The obtained test pieces were heated to 2800°C at a heating rate of 20°C / min. The elongation of the test pieces in the longitudinal direction during this process was measured using a push-rod type thermal expansion meter, and the hot puffing value was calculated using the following formula. Note that the elongation of the test piece (cylindrical body) in the longitudinal direction corresponds to the elongation perpendicular to the extrusion direction in extrusion molding. A lower hot puffing value is desirable. Hot buffing value (%) = (△L / L) × 100 (In the formula, L is the length of the test piece before the test, and △L is the elongation of the test piece in the length direction during heating up to 2800°C.)

[0071] [Example 1] Green coke for graphite electrodes, to which 1.86% by weight of an inhibitor was added, was placed in a heat-resistant container, and then calcined at 1300°C for 2 hours under a nitrogen gas atmosphere and atmospheric pressure to obtain a mixture of needle coke and inhibitor (needle coke for graphite electrodes). Using the resulting mixture of needle coke and inhibitor, P is prepared by method (2). 2800 and P 1700-2100 The values ​​were calculated. The results are shown in Table 1. In addition, the hot puffing value was measured using method (3). The results are shown in Figures 1 and 2.

[0072] [Comparative Example 1] Needle coke for graphite electrodes was manufactured in the same manner as in the example, except that no inhibitor was added to the raw coke, and the blank puffing value P was obtained by method (2). 2800b and P 1700-2100b The values ​​were calculated. The results are shown in Table 1. In addition, the hot puffing value was measured using method (3). The results are shown in Figures 1 and 2.

[0073] [Table 1]

[0074] As shown in Table 1, in Example 1, where an inhibitor was added, the cold puffing value P was higher compared to Comparative Example 1, where no inhibitor was added.2800 It was small. Also, P represents nitrogen puffing. 1700-2100 Similarly, the puffing suppression effect was high. Furthermore, as shown in Figure 1, in Comparative Example 1, puffing occurred rapidly from around 1800°C, whereas in Example 1, puffing was minimal in the same temperature range, confirming that the inhibitor suppressed puffing. Furthermore, as shown in Figure 2, in Comparative Example 1, the value increased sharply from around 1800°C, whereas in Example 1, where the inhibitor was added, the change in the value was small, confirming that the inhibitor suppressed puffing.

Claims

1. Green coke used in the production of needle coke for graphite electrodes, Green coke for graphite electrodes, containing a graphite electrode inhibitor comprising at least a composite oxide having the following elements (Mα) and (Mβ). Element (Mα): At least one metallic element (excluding element (Mβ)). Element (Mβ): At least one element selected from the group consisting of Group 4, Group 8, Group 9, Group 10, Group 13, Group 14, and Group 15 of the long-period periodic table.

2. The green coke for graphite electrodes according to claim 1, wherein the element (Mβ) is at least one element selected from the group consisting of Si, Ge, Al, B, Ti, Fe, and P.

3. The green coke for graphite electrodes according to claim 1, wherein the element (Mα) is at least one metallic element selected from the group consisting of K, Sc, alkaline earth metal elements, and rare earth metal elements.

4. The green coke for graphite electrodes according to claim 1, wherein the compositional formula of the composite oxide is the following formula (1). Ma ３－ｘ Mβ １－ｙ O ５－ｚ ・・・(1) (In the equation, 0 ≤ x < 3, 0 ≤ y < 1, and 0 ≤ z < 5.)

5. The green coke for graphite electrodes according to claim 1, wherein the green coke is a carbide of raw material pitch containing one or more of the following: coal tar, FCC decanted oil, ethylene heavy end, petroleum residue, petroleum waste, biomass oil, or biomasterl.

6. The graphite electrode raw coke according to claim 1, wherein when the ratio (by weight) of the inhibitor for graphite electrode production to the total weight of the raw coke is X, 0.1 < X < 9.

5.

7. A method for producing green coke for graphite electrodes according to any one of claims 1 to 6, A method for producing green coke for graphite electrodes, comprising applying a graphite electrode production inhibitor to the surface of green coke and heat-treating it to incorporate the graphite electrode production inhibitor into the green coke.

8. The method for producing green coke for graphite electrodes according to claim 7, wherein the temperature of the heat treatment is 300°C or more and less than 600°C.

9. The method for producing green coke for graphite electrodes according to claim 7, wherein the heat treatment is performed in a rotary kiln furnace.

10. Needle coke for graphite electrodes, which is a carbonized product of green coke for graphite electrodes according to any one of claims 1 to 6.

11. The puffing value P calculated using the following formula (I) for the test piece created in the following evaluation test (i) ２８００ Needle coke for graphite electrodes according to claim 10, wherein the content is 2.040% or less. P ２８００ =(L2-L1) / L1×100 ・・・(I) However, L1 and L2 in equation (I) have the following meanings. L1: Thickness of the test piece before firing (mm) L2: Thickness of the test piece after firing at 2800°C (mm) <Evaluation Test (i)> The graphite electrode needle coke and 30% by weight of binder pitch relative to the graphite electrode needle coke are mixed and kneaded for 5 minutes while heating at 165°C. This is molded into a disc shape of 20 mmΦ × 3 mm to 15 mm, and fired in a firing furnace at 1000°C for 3 hours to burn off the binder pitch and create a test piece. The test piece is fired at a heating rate of 20°C / min to 2800°C, and L1 and L2 of the test piece are measured before and after firing.

12. The puffing value P calculated using the following formula (II) for the test piece created in the following evaluation test (ii) １７００－２１００ Needle coke for graphite electrodes according to claim 10, wherein the content is 1.10% or less. P １７００－２１００ =(L3-L4) / L5×100 ・・・(II) However, L3, L4, and L5 in equation (II) have the following meanings. L3: Thickness of the test piece at 2100°C firing (mm) L4: Thickness of the test piece at 1700°C firing (mm) L5: Thickness of the test piece at 1000°C firing (mm) <Evaluation Test (ii)> The graphite electrode needle coke and 30% by weight of binder pitch relative to the graphite electrode needle coke are mixed and kneaded for 5 minutes while heating at 165°C. This is molded into a disc shape of 20 mmΦ × 3 mm to 15 mm and fired in a firing furnace at 1000°C for 3 hours to burn off the binder pitch and create a test piece. The test piece is fired at a heating rate of 20°C / min up to 2800°C using a heating thermal expansion measuring device, and L3, L4, and L5 of the test piece are measured during firing.

13. In the evaluation test (i), a test piece is prepared in the same manner except that green coke not containing the inhibitor for producing the graphite electrode is used, and the puffing value (blank) calculated by the formula (I) of the test piece is defined as P. ２８００ｂ When it is ２８００ / P ２８００ｂ The needle coke for a graphite electrode according to claim 11, wherein the ratio of the puffing value calculated by is less than 1.

14. In the evaluation test (ii) described above, a test piece is prepared in the same manner as above, except that green coke that does not contain the inhibitor for manufacturing graphite electrodes is used, and the puffing value (blank) of the test piece calculated by formula (II) described above is P １７００－２１００ｂ In that case, P １７００－２１００ / P １７００－２１００ｂ Needle coke for graphite electrodes according to claim 12, wherein the ratio of the puffing values ​​calculated is less than 1.

15. A method for producing needle coke for graphite electrodes, comprising calcining the green coke for graphite electrodes described in any one of claims 1 to 6 at a temperature of 1000°C to 1700°C to obtain needle coke.

16. The method for producing needle coke for graphite electrodes according to claim 15, wherein the calcination is performed in a rotary kiln.

17. The method for producing needle coke for graphite electrodes according to claim 15, wherein when Y is the ratio (by weight) of the inhibitor for producing graphite electrodes to the total weight of the needle coke, 0.02 < Y < 15.

18. A method for manufacturing a graphite electrode, comprising using needle coke for graphite electrodes as described in claim 10 as aggregate and obtaining a graphite electrode by graphitization treatment.

19. The method for manufacturing a graphite electrode according to claim 18, wherein the temperature of the graphitization treatment is 2500°C or more and 3000°C or less.

20. The method for manufacturing a graphite electrode according to claim 19, wherein the proportion of the inhibitor for manufacturing the graphite electrode remaining in the graphite electrode is 0.01% by weight or less.

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