RU2800053C1 - Needle coke with low coefficient of thermal expansion and low cracking - Google Patents

Abstract

FIELD: coking.

SUBSTANCE: group of inventions is related to variants of needle coke with a low coefficient of thermal expansion (CTE) and low cracking. One variant is needle coke, obtained by mixing and coking 100 weight fractions of a basic raw material for needle coke, consisting of a heavy oil based on coal tar or based on petroleum, having a weak hydrogen releasing property with a PDQI value calculated using equation (1), less than 5.0, with 10-80 weight fractions of secondary raw materials having a strong hydrogen release property with a PDQI value calculated using equation (1), 5.0 or more, and calcining the obtained raw coke: where [Equation (1)] has PDQI=H%× 10× (HNβ /H), where H% is the amount of hydrogen (wt.%) obtained using elemental analysis, and HNβ/H is the ratio of β-naphthenic hydrogen to total hydrogen, measured with ¹H-NMR. Moreover, the specified coke has a crystallite size Lc 4.0-10.0 nm, interplanar interval d002 0.340-0.350 nm, proportion of open pores 0.15-0.40 and the standard deviationσ (θ) of the average orientation angle 0.01-0.85.

EFFECT: production of needle coke with low CTE and low cracking by limiting the characteristics of the main raw materials and secondary raw materials, and by controlling the structure of needle coke obtained by coking and calcining to ensure a specific structure.

5 cl, 3 dwg, 2 tbl, 3 ex

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

[0001] The present invention relates to needle coke with a low coefficient of thermal expansion (CTE) and low cracking, as well as an artificial graphite electrode for electric steelmaking.

BACKGROUND OF THE INVENTION

[0002] Needle coke is generally produced using petroleum-based or coal-tar-based heavy oil as a raw material, and is used as an artificial graphite electrode assembly for electric steelmaking. This graphite electrode is made by grinding needle coke to specified particles, kneading it with a binder pitch, extrusion molding, and then subjecting it to primary firing, impregnation, secondary firing, and graphitization.

[0003] It is desirable that the graphite electrode has a low coefficient of thermal expansion (CTE) to withstand harsh operating conditions in a high temperature atmosphere. Lower CTE reduces electrode consumption during electric steelmaking, thus contributing to cost savings.

[0004] The graphitization processing in the production process of graphite electrodes is to carry out heat treatment at a high temperature, about 3000°C, and a method using an LWG (flow-through type) furnace is widely used. The graphitization carried out in this LWG furnace achieves a faster rate of temperature rise, thereby increasing the rate of gas generation from the graphite electrode material, and therefore an abnormal expansion phenomenon called swelling or cracking is likely to occur. This cracking causes the electrode to have a low density and sometimes the electrode is damaged. Thus, processes for the production of needle coke to reduce cracking or crack inhibitors added during the manufacture of electrodes have been studied.

[0005] It is assumed that the CTE value generally decreases as the uniformity of the needle coke texture orientation increases or as the number of hairline cracks increases. The direction of the carbon hexagonal network in the plane of the graphite structure, which is a crystalline structure forming needle coke, has less thermal expansion than the delamination direction, and therefore it is considered that the alignment of the plane direction in the longitudinal direction of the electrode provides a low CTE of the electrode. It is also believed that the presence of hairline cracks reduces thermal expansion, resulting in low CTE.

It is generally believed that cracking is created by gas pressure caused by the evaporation of nitrogen or sulfur compounds in needle coke under high temperature conditions.

[0006] What is required of needle coke for artificial graphite electrode is low CTE during electrode use and low cracking during electrode production; and traditionally, the following techniques have been used as a production method for needle coke with low CTE and low cracking.

[0007] Patent Document 1 discloses a method for: reducing cracking by removing a substance causing cracking by hydrogenating a feedstock and removing nitrogen or sulfur contained in the feedstock; and reducing CTE by exhibiting preferential carbonization behavior by reducing oxygen and sodium and reducing high temperature viscosity by increasing the naphthenic ring. Patent Document 2 discloses a co-carbonization method for reducing CTE by mixing a heavy petroleum-based oil having a specific property adjusted with coal tar pitch insoluble in quinoline and coking the mixture, thereby diluting the nitrogen or sulfur content of a cracking agent and reducing cracking, and by achieving a balance between gas production and carbonization rate, which creates an anisotropic texture, showing low CTE. In addition, Patent Document 3 discloses a method for producing needle coke with low CTE and low cracking by forming a good bulk mesophase by mixing two or more raw materials and by generating a gas to orient crystals during solidification. Patent Document 4 and Non-Patent Document 1 disclose that coke structure change caused during two-stage calcination results in low CTE and low cracking. Patent Document 5 discloses that once-calcined coke is calcined again in an oxidizing atmosphere, thereby increasing fine pores, and thus low CTE and low cracking coke can be produced. Patent Document 6 discloses a method for producing needle coke with low CTE and low cracking by mixing a quinoline-insoluble coal tar-based heavy oil and a petroleum-based heavy oil, and coking the mixture, and performing two calcination steps.

[0008] As described above, it is known that needle coke with low CTE and low cracking can be obtained by converting raw materials by hydrogenation, co-carbonization by mixing and coking two or more types of raw materials, two stages of calcination, re-calcination under an oxidizing atmosphere, as well as combinations thereof.

Although it is not a way to reduce CTE or cracking, Patent Document 7 discloses that when a hydrogen donor solvent is added to low-temperature pitch and a heat treatment is carried out to thermally convert it, its quality becomes suitable for use as a raw material for needle coke.

As an evaluation of hydrogen donating ability, Non-Patent Document 2 proposes PDQI (Proton Donor Quality Index) which is for evaluation of recycle solvent in coal hydrogenation liquefaction reaction, without saying that it is useful as an indicator for improving the quality of needle coke.

BIBLIOGRAPHY

PATENT LITERATURE

[0009] Patent Document 1:

Japanese Patent Pending Application No. 60-149690

Patent Document 2:

Japanese Patent Pending Application No. 4-145193

Patent Document 3:

WO2009/1610

Patent Document 4:

Japanese Patent Pending Application No. 52-29801

Patent Document 5:

Japanese Patent Pending Application No. 61-218686

Patent Document 6:

Japanese Patent Pending Application No. 5-163491

Patent Document 7:

WO2011/48920

NON-PATENT LITERATURE

[0010] Non-Patent Document 1:

Carbon, Vol. 19, no. 5, pages 347 to 352

Non-Patent Document 2:

Journal of the Fuel Society of Japan, Vol. 65, no. 12, pages 1012 to 1019, 1986

SUMMARY OF THE INVENTION

TECHNICAL PROBLEM

[0011] Needle coke with low CTE and low cracking is known to be obtained by using a converted feedstock, co-carbonization using two or more feedstocks, changing calcination conditions; however, there is a need for a more stable production of needle coke with low CTE and low cracking as feedstock properties change.

The use of needle coke for artificial graphite electrodes gives low CTE and low cracking, and it is considered possible to achieve more stable low CTE and low cracking by combining feedstock selection, coking conditions, and calcination conditions while clarifying which structure of the needle coke promotes low CTE and low cracking.

In a method for producing low CTE and low cracking needle coke by mixing and coking two or more raw materials, the present invention is to obtain low CTE and low cracking needle coke by limiting the characteristics of the main raw material and the secondary raw material.

In addition, the present invention is directed to obtaining needle coke with low CTE and low cracking by controlling the structure of needle coke obtained by coking and calcining to provide a specific structure.

SOLUTION

[0012] As a result of intensive research to solve the above problems, the inventor of the present invention has found that: needle coke with low CTE and low cracking is obtained by coking and calcining an oil mixture obtained by mixing a primary feedstock for needle coke having a weak hydrogen releasing property with a secondary feedstock having a strong hydrogen releasing property to reduce CTE and cracking; and needle coke with low CTE and low cracking is obtained by adjusting the resulting needle coke to have a specific structure, by selecting raw materials, coking conditions and calcination conditions, and thereby complete the present invention.

[0013] Thus, the present invention is a needle coke with low CTE and low cracking, obtained by mixing and coking 100 ppm. a basic raw material for needle coke, consisting of a heavy oil based on coal tar or based on petroleum, having a weak hydrogen releasing property with a PDQI value calculated by equation (1) of less than 5.0, with 10-80 ppm. a secondary raw material having a strong hydrogen release property with a PDQI value calculated from Equation (1) of 5.0 or more, and calcining the obtained raw coke.

[Equation (1)]

PDQI=H% × 10 × (HNβ/H)

where H% is the amount of hydrogen (wt.%) obtained using elemental analysis, and HNβ/H is the ratio of β-naphthenic hydrogen to the total amount of hydrogen, measured using ¹ H-NMR.

[0014] The low CTE and low cracking needle coke of the present invention preferably has a CTE related structure index (NCSIC: CTE needle coke structure index) expressed by equation (2) of more than 25.0 and a cracking related structure index (NCSIP: cracking needle coke structure index) of more than 5.0.

[Equation (2)]

NCSIC=((Lc/d002) + (percentage of open pores))/(σ (θ))

where Lc is the crystallite size, d002 is the interplanar spacing, and σ(θ) is the standard deviation of the average orientation angle;

[Equation (3)]

NCSIP=(number of open pores/number of closed pores) + (HIT/1000)/EIT

where HIT is the indentation hardness and EIT is the indentation modulus.

[0015] For the low CTE and low cracking needle coke of the present invention, the main raw material is preferably QI-removed pitch having a weak hydrogen donating property obtained by distilling and removing QI from coal tar. In particular, the main raw material is preferably a QI-removed pitch having a weak hydrogen donating capability obtained by distilling and removing QI from coal tar having a PDQI value calculated from Equation (1) of less than 2.

[0016] The low CTE and low cracking needle coke of the present invention preferably has a crystallite size Lc of 4.0-10.0 nm, interplanar spacing d002 of 0.340-0.350 nm, an open pore fraction of 0.15-0.40, and a mean orientation angle standard deviation of 0.01-0.85.

The needle coke of the present invention preferably has an open cell number of 0.040-0.070 ^cm3 /g, a closed cell number of 0.001-0.015 ^cm3 /g, an indentation hardness HIT of 800-1500 MPa, and an indentation modulus EIT of 5.0-15.0 GPa.

[0017] In the present invention, the crystallite size Lc and the interplanar spacing d002 in equation (2) are obtained by XRD measurement and Gakushin analysis. The proportion of open pores is calculated as the ratio of the volume of pores with a diameter of 1-10 microns, measured using mercury porosimetry, to the volume of pores with a diameter of 120 microns or less. The standard deviation σ(θ) of the average orientation angle is calculated as the standard deviation, which is the variation index of the orientation angles measured with a birefringence meter.

BENEFICIAL EFFECTS OF THE INVENTION

[0018] According to the present invention, needle coke with low CTE and low cracking can be stably produced by: coking a primary feedstock of needle coke having a weak hydrogen releasing property with a secondary feedstock having a strong hydrogen releasing property, and calcining the resulting raw coke; as well as controlling the pore structure and texture structure to provide a specific coke structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] [Fig. 1] FIG. 1 shows the displacement-load curve measured with a microhardness tester.

[Fig. 2] FIG. 2 shows the displacement-load curve of the needle coke of Example 1.

[Fig. 3] FIG. 3 shows the displacement-load curve of the needle coke of Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

[0020] It is believed that important factors in the structure of needle coke for CTE needle coke are crystallinity, number of pores, texture orientation, etc. Many of these are formed during the coking process. It is known that in the process of coking raw materials by heating: organic molecules repeat the dehydrogenation-polycondensation reactions; in the process of their separation, mesophase spherulites with optical anisotropy are formed; and due to their growth and fusion, an optical anisotropic texture is provided. It is known that as the mesophase grows and coalesces during coking, the viscosity of the system increases. However, when the balance between the increase in the viscosity of the system and the growth and coalescence of the mesophase is disturbed, causing an increase in viscosity first, the mesophase cannot grow sufficiently to provide low crystallizability; while maintaining a low viscosity for sufficient mesophase growth and coalescence to occur increases crystallizability. In addition, just before the end of coking, the texture is oriented uniaxially by the shear force of the generated or introduced gas, thereby improving the orientation of the optically anisotropic texture. An efficient feedstock for these factors influencing CTE is hydrogenated feedstock.

As a result of examining the difference between the hydrogenated feedstock and the non-hydrogenated feedstock, it was found that the hydrogenated feedstock has a larger PDQI value expressed by equation (1) than the non-hydrogenated feedstock, and it was found that the hydrogenated feedstock has a strong hydrogen donor property. It has been found that among hydrogenated feedstocks, in particular the preparation of feedstocks having a strong hydrogen donor property makes it possible to obtain needle coke with a lower CTE. The reason why the use of a feedstock with a strong hydrogen donor property allows the production of needle coke with a lower CTE is not clear; however, the use of a feedstock having a strong hydrogen donor property causes hydrogen transfer of the naphthene during coking, thereby maintaining the low viscosity region during coking, causing sufficient growth and coalescence of the mesophase, and generating an increased amount of gas so that just before the end of coking, the orientation of the optically anisotropic texture is improved by shear away from the gas.

However, when a raw material having a strong hydrogen donor property is used alone as a raw material, its boiling point is lower than that of a main raw material having a weak hydrogen donor property, and the amount of gas generated is too large; and therefore it is assumed that crude coke cannot be obtained, or even if it is obtained, its yield will be low. At the same time, in the case of using a mixed feedstock consisting of a commonly used feedstock having a weak hydrogen donor property as a main feedstock and a feedstock having a strong hydrogen donor property as a secondary feedstock, it is considered that: the naphthenic hydrogen contained in the recycled feedstock is transferred to the aromatic ring of the main feedstock; this makes it possible to preserve the region of low viscosity for sufficient development of growth and coalescence of the mesophase, which cannot be done using a single feedstock; and the orientation is also improved by the gas generated from the recycled material having a strong property of giving off hydrogen. In addition, the secondary raw material having a strong hydrogen donating property serves as a reaction regulator, and increasing or decreasing its amount can control the property improvement. Thus, the amount of recycled material having a strong hydrogen donating property to be mixed with 100 ppm the main raw material having a weak hydrogen donating property is preferably 10 m.h. or more and 80 m.h. or less; more preferably 20 m.h. or more and 50 m.h. or less; and even more preferably 25 m.h. or more and 45 m.h. or less.

Simultaneously with the selection of feedstock for needle coke, coking conditions and calcination conditions are appropriately changed to obtain a needle coke structure suitable for low CTE and low cracking, so that further improvement in properties is expected in addition to the improvement in properties due to the feedstock.

[0021] It is believed that the structure of needle coke obtained by raw material selection, coking and calcination is most important for CTE and cracking characteristics. It is believed that evaluation of coke structure is done by the degree of development of crystallizability by XRD, by the orientation of the optically anisotropic texture by birefringence, and by the number of pores by mercury porosimetry; and the needle coke structure index (equation (2)) for the CTE obtained from these evaluation results is thereby used to enable the structure of the low CTE needle coke to be converted into a numerical form. Since the development of crystallizability increases the crystallite size Lc and reduces the interplanar spacing d002, it is believed that when the ratio Lc/d002 is large, crystallizability develops. The Lc crystallite size is preferably 5.5 nm or more, and more preferably 6.0 nm or more.

The standard deviation σ(θ) of the average orientation angle θav measured by birefringence becomes smaller as the orientation is high, showing a uniform orientation; and therefore it is considered that if the coke has a smaller value of σ(θ), it has a uniform orientation. The standard deviation σ(θ) is preferably less than 0.80, and more preferably less than 0.70.

With regard to the number of pores, it is believed that when the ratio of the number of pores with a size of 1 to 10 µm to the number of pores with a size of 120 µm or less is large, the pores function as places of weakness during expansion, contributing to low CTE. The ratio of the number of pores with a size of 1 to 10 µm to the number of pores with a size of 120 µm or less is called the proportion of open pores. The open pore ratio is preferably in the range of 0.15 to 0.40, more preferably 0.30 or more, and even more preferably 0.33 or more.

[0022] The NCSIC value is obtained by converting the coke structure suitable for low CTE into a numerical form using the coke structure, including crystallizability, orientation, and number of pores. It is believed that when the NCSIC value is higher (developed crystallizability, higher orientation and more pores), needle coke has a low CTE.

[0023] Important factors in the cracking of needle coke are nitrogen and sulfur. It is known that reducing the content of nitrogen and sulfur as a cause of gassing can reduce cracking. As to which needle coke structure is effective in reducing cracking, more pores are known to be beneficial, but other structures have not been investigated. The inventor of the present invention has found that the number of pores as the exit path of the generated gas and the strength of the coke to withstand the pressure of the gas are important; and as for the structure of the needle coke for low cracking, both the pore structure to allow the gas generated during cracking to escape, and the texture structure related to strength to withstand the pressure of the gas should be optimal.

[0024] Since nitrogen and sulfur are obtained from the feedstock, the feedstock has a low content of nitrogen and sulfur. However, in recent years, the content of nitrogen and sulfur in raw materials is high and therefore there are difficulties with the choice of raw materials. The addition of a secondary feedstock with a high hydrogen donating capacity during coking results in expected moderate hydrodesulfurization/denitrification, so the hydrogen yielding capacity of the secondary feedstock is an important factor in the selection of secondary feedstock.

[0025] The number of pores and strength can be changed accordingly due to the conditions of coking or calcination. As for the structure index of coke (Equation (3)), suitable for low cracking, it is considered that the use of coke strength calculated from: two kinds of pore number, the number of open pores obtained by mercury porosimetry, and the number of closed pores calculated from true density and apparent density; indentation hardness (HIT), calculated from the applied load and the indentation depth of the indenter, obtained using a microhardness tester; and the indentation modulus (EIT), which is the degree of recovery after unloading, allows the coke structure suitable for cracking to be converted into a numerical form.

In the present invention, the indentation hardness (HIT) is preferably 800-1500 MPa, and more preferably 900-1400 MPa. The indentation modulus (EIT) is preferably 5.0-15.0 GPa, and more preferably 8.0-13.0 GPa.

[0026] Optimization of the texture structure, related to pore structure and strength, produces needle coke with low cracking, which is considered to have a coke structure, i.e.

a porous structure in which the number of open pores for connection with the outside of the coke is large, and the number of closed pores without connection with the outside is small; and a texture structure related to strength in order to withstand gas pressure, wherein the structure is highly resistant to deformation by an external force (rigid texture) and is flexible to easily recover from deformation by an external force (low elastic modulus), in particular, a coke structure in which the orientation of the optically anisotropic texture is disordered at the micron level, while it is uniform at the submicron and nanoscale levels. With regard to the pore structure, it is believed that more pores for the gas exit paths at the time when nitrogen and sulfur are volatilized as gas during graphitization can reduce cracking. It is believed that fewer closed pores can reduce cracking as well as the amount of gas remaining in the coke. At the same time, it is believed that the texture structure of coke with high strength can withstand gas pressure at the time of graphitization or easily recover even after being deformed by gas pressure to reduce cracking. In coking, an exemplary method includes: in the initial stage of coking to develop optical anisotropy from submicron to nanoscale, developing a mesophase when conducting coking at a low temperature or maintaining a low viscosity state using hydrogenated feedstock; and in the last stage, to disrupt the orientation of the micron optically anisotropic texture, increase the temperature, pressure and amount of steam, or to disrupt the interior of the system with a combination of these, change the coking conditions in the middle of coking. Optical anisotropy from submicron to nanoscale and optically anisotropic texture from micron to millimeter are changed by changing coking conditions such as temperature, pressure, amount of steam, and the like. in the middle of the process, which leads to an increase or decrease in the number of closed coke pores, which are obtained due to the orientation of the texture structure. With regard to calcination, examples of methods for increasing the number of open pores include two or more stages of calcination, high temperature calcination, and oxidative calcination. In two or more stages of calcination, first, calcination is carried out at a low temperature, followed by a single cooling, and then a second or subsequent calcination is carried out; and cooling and heating create hairline cracks, so that an increase in open pores and a decrease in closed pores can be achieved. In addition, with regard to high-temperature calcination, calcination at a higher temperature than usual causes more shrinkage than ordinary calcination, and stress causes cracking, so that the number of open pores increases. As for the oxidative calcination, it is carried out with the introduction of an oxidizing gas, and thus the surfaces of the needle coke are oxidized to form pores, resulting in an increase in the number of open pores.

The number of open pores is in the range of 0.040-0.070 cm ³ /g, and preferably 0.05-0.065 cm ³ /g. On the other hand, the amount of closed pores is 0.001-0.015 cm ³ /g, and preferably 0.005-0.009 cm ³ /g.

[0027] The needle coke of the present invention is produced by coking a mixed feedstock consisting of a main feedstock having a weak hydrogen releasing property and a secondary feedstock having a strong hydrogen releasing property, and calcining the resulting raw coke.

[0028] Examples of the main raw material for needle coke include coal tar heavy oil, petroleum heavy oil, etc.

[0029] Examples of the heavy oil based on coal tar include coal tar obtained as a by-product of coke production, coal tar pitch obtained by distillation of coal tar, and oil obtained by coal liquefaction. It is preferable to use coal tar pitch obtained by removing a quinoline insoluble substance, the content of which is usually 0.1% or less. Pitch obtained by distillation and thermal reforming of coal tar pitch, from which substances insoluble in quinoline have been removed, can be used as a feedstock.

Examples of the petroleum-based heavy oil include catalytic cracking oil, pyrolysis oil, semi-tar, vacuum distillation oil residue and ethylene residue oil, and in particular, decanted oil (FCC-DO), which is a heavy component of catalytic cracking oil, is preferred.

The feedstock can be a mixture of a heavy oil based on coal tar and a heavy oil based on petroleum, or a mixture of a coking by-product and a thermally reformed blended oil.

This base feed has a PDQI value reflecting the hydrogen releasing property of less than 5.0, preferably less than 1.0, and usually about 0.001, that is, it has a weak hydrogen releasing property.

[0030] The present invention uses a secondary raw material with a strong hydrogen release property together with a main raw material, and the main raw material and the secondary raw material are mixed for use. As this secondary raw material, a raw material having a high PDQI value as described above is used. The PDQI value is preferably 5.0 or more, more preferably 8.0 or more, and even more preferably 10.0 or more.

As secondary raw materials, materials obtained by hydrogenation of heavy oil based on coal tar or petroleum based as the main raw material can be used. The preferred heavy oil is coal tar or a distilled fraction thereof. More preferred is the product obtained by partial hydrogenation of the fraction from 300 to 600°C obtained by distillation of the main raw material. In addition, even if the main feedstock for needle coke is not a starting feedstock, an oil having a hydrogen releasing property of 5 or more, and preferably 10 or more, is suitable as a secondary feedstock.

The hydrogenation condition is preferably to use a hydrogenation reactor using a hydrogenation catalyst at a temperature of 100° C. or higher but lower than 300°, and a hydrogen partial pressure of less than 5 MPa; however, this is not a limitation if the hydrogenation allows a satisfactory PDQI to be obtained.

[0031] The mixing ratio of the main raw material and the secondary raw material is preferably 100 ppm. main raw material 10-80 m.h. secondary raw materials, more preferably 20-50 m.h., and even more preferably 25-45 m.h. If the amount of recycled material is small, the hydrogen donation capability is weak, making it impossible to obtain the low CTE and low cracking needle coke of interest. On the other hand, if the amount of secondary feedstock is too large, the decomposition reaction of the secondary feedstock dominates the reaction between the main feedstock and the secondary feedstock, so that the growth and coalescence of the mesophase of the main feedstock cannot be facilitated by using the hydrogen of the recycled feedstock, which makes it impossible to obtain needle coke of interest with low CTE and low cracking.

[0032] A known delayed coking process can be used to coke the mixed feed. Coking is carried out, for example, at a temperature of 450-550°C and a pressure of 0.2-0.8 MPa for 18-48 hours to obtain raw coke. As a coking method, the following schemes can be used, in which the coking conditions are not maintained constant from the beginning of the raw material loading to the end, or a combination of them: the loading temperature during coking changes in steps; coking pressure changes stepwise; the amount of charge steam during coking changes stepwise; the raw material is separated into two parts, one part of the raw material is fed into the coking oven from the bottom of the coke plant at low temperature, and the other part of the raw material is fed into the coke oven from the side of the coke plant at a high temperature; and the pressure during coking increases and the amount of steam is kept larger than usual.

[0033] As a method for calcining raw coke, a known method can be used. Exemplary methods include a calcination method at a temperature of 800 to 1600° C. using a rotary kiln, a shaft kiln, or a siliconite kiln. The calcination can be carried out in one stage or in two or more stages. The calcination can be carried out at a high temperature, and it can be carried out by blowing an oxidizing gas.

[0034] A method for producing an artificial graphite electrode for electric steelmaking from the above coke with low CTE and low cracking can be a known method, and for example, the electrode can be obtained through the processes of mixing with a binder resin, forming, primary calcination, impregnation, secondary calcination, graphitization, etc.

[0035] Next, the measurement conditions will be explained.

The secondary feedstock for needle coke contains condensed polynuclear aromatic hydrocarbons having a naphthenic ring structure, wherein the hydrogen atoms of the naphthenic ring include hydrogen bonded to carbon at the α position of the aromatic ring (HNα) and hydrogen bonded to carbon at the β or higher position (HNβ). In addition, there are hydrogen atoms derived from an alkyl group and the like bonded as a substituent group to fused polynuclear aromatic compounds, and these hydrogen atoms include hydrogen bonded to carbon at the α position (Hα) and hydrogen bonded to carbon at the β or higher position (Hβ, etc.). In addition to the above, there is a hydrogen bond to the aromatic ring carbon (Ha).

[0036] Identification of these hydrogen atoms, etc. performed with ¹ H-NMR.

For ¹ H-NMR measurement, chloroform is used as a solvent, TMS (tetramethylsilane) is used as a reference substance, and JEOL, Ltd. JNM-LA400; and the proportion of hydrogen is calculated from the value of the integral in the resulting ¹ H-NMR spectrum.

Hα, HNα and HNβ are calculated by integrating chemical shifts from 2.0 to 4.2, from 3.0 to 4.2 and from 1.5 to 2.0, respectively, of the obtained ¹ H-NMR spectrum.

[0037] Analyzes (elemental analysis) of carbon and hydrogen in the main raw materials and secondary raw materials are calculated in accordance with JIS M 8819; nitrogen - in accordance with JIS K 2609; oxygen - in accordance with JIS M 8813; and sulfur - in accordance with JIS K 2541.

[0038] The PDQI value is calculated according to equation (1). H% is the amount of hydrogen, determined using elemental analysis, and HNβ/H is the ratio of β-naphthenic hydrogen, measured using ¹ H-NMR, to total hydrogen.

PDQI is the maximum available hydrogen (mg) of the naphthenic ring contained in 1 g of solvent, and the unit is mg/g.

[0039] The crystallite size Lc and interplanar spacing d002 obtained by XRD are calculated by measuring an angle of 20-30° using an XRD device manufactured by Rigaku Corporation; and crystallite size analysis by the Gakushin method.

[0040] For the standard deviation σ(θ) of the orientation angles measured with a birefringence meter, a particle of needle coke with a size of 1-2 cm is poured into the resin and polished with a polishing composition, and a test piece having a thickness of 8 mm is prepared with needle coke coming to the surface; for the exposed surface of needle coke, a PI-micro birefringence meter manufactured by Photonic Lattice, Inc. is used. with a resolution of 0.9 µm or 2.7 µm per pixel; and the Re value of one field of view, the average orientation angle θav, the standard deviation σ(θ) of the average orientation angles are obtained from the retardation values Re for each pixel and the orientation angle, and a plurality of fields of view are measured. For all measured visual fields, the average numerical values are used as estimates.

[0041] When measuring needle coke using mercury porosimetry, needle coke is reduced to 2-5 mm, measurement is performed at pressures of 1.9-14400 psi (pore diameter conversion: 0.017-120 μm) using Micromeritics AutoPore IV instrument, pore volume relative to pore diameter is calculated from the obtained pressure and the amount of mercury, pore volume of 1-10 is calculated µm and is used as the number of open pores of needle coke. In addition, the open pore ratio is calculated by dividing the volume of pores with a size of 1 to 10 µm by the volume of pores with a size of 120 µm or less. In Tables 1 and 2, the pore volume values mean the total number of pores, and they represent the volume of pores with a size of 120 μm or less to calculate the proportion of open pores.

[0042] The true density of needle coke is measured in accordance with JIS K 2151.

To determine the apparent density, the needle coke is crushed in a jaw crusher, then passed through a sieve with a mesh size of 8 to 16 mesh, and the apparent density is measured using the same measurement procedure as the true density. The number of closed pores is calculated by the following equation (4).

[Equation (4)]

Number of closed pores (cm ³ /g) \u003d (1 / apparent density (g / cm ³ ) - 1 / true density (g / cm ³ ))

[0043] For measurement with a microhardness tester, a particle of needle coke with a size of 1-2 cm is embedded in a resin and polished with a polishing composition, and a test sample having a thickness of 8 mm with needle coke exposed on the surface is used; and the measurement is performed on the exposed surface of the needle coke.

The specific test conditions are as follows: FISHERS COPE, HM2000 manufactured by Fischer Instruments K.K.; the probe is a Vickers indenter; maximum load 2000 mN, loading rate 300 mN/s, creep time 2 s; and unloading is performed at the same speed as loading. For each test sample, 10 points are measured and their average is used as the value for that test sample. Fig. 1 shows a typical displacement-load curve.

To measure the texture structure of needle coke, it is necessary to measure the needle coke in the (elastic deformation) region without plastic deformation (failure), and in the displacement-load curve obtained from the test, the displacement must return to the original point after the load is removed, as shown in Fig. 1.

The indentation depth is the amount of displacement at the moment of maximum load of the indentation test; it varies depending on the test conditions and the substance being measured; the preferred depth is immediately before the destruction of the needle coke. However, when the needle coke of the present invention is measured under the above test conditions, the recess depth is preferably 8-15 µm.

[0044] The indentation hardness (HIT) is calculated from the maximum indentation test load and the indentation depth according to the following equation (5) using the analysis software supplied with the apparatus in accordance with ISO 14577. HIT is considered to indicate a coke structure with high indenter resistance because the coke is not indented during the test when the coke is hard. It is believed that the exemplary structure of hard coke is due to a misorientation of the optically anisotropic texture of the coke.

[Equation (5)]

Indentation hardness, HIT (MPa)=Fmax/Ap

where Fmax is the maximum load and Ap is the projection area where the indenter is in contact with the test piece.

[0045] The indentation modulus (EIT) is calculated based on the initial unload slope from the maximum indentation test load according to the following equation (6) using the analysis software attached to the device in accordance with ISO 14577. With regard to EIT, when the coke has a low elastic modulus, it is easily recovered after the load is removed, and the exemplary low modulus coke structure is considered to be due to the good development of the optical anisotropy coke texture.

[Equation (6)]

Indentation modulus, EIT (GPa)=(1-(Vs) ² )/(1/Er-1-(Vi)2/(Ei))

where Vs is the Poisson's ratio of the sample, Vi is the Poisson's ratio of the indenter, Er is the reduced modulus of elasticity at the point of contact with the indentation, and Ei is the modulus of elasticity of the indenter.

[0046] The structure index associated with CTE (NCSIC) becomes larger as the crystallization ability of graphite in needle coke increases, the proportion of open pores increases, or orientation uniformity increases; and it is an indication that needle coke is more suitable for low CTE.

As described above, the NCSIC value is preferably greater than 25.0, more preferably 27.0 or greater, and even more preferably 29.0 or greater.

[0047] The cracking-related structure index (NCSIP) becomes larger as the number of open pores of the needle coke increases, the number of closed pores decreases, the texture hardness increases, and the elastic modulus decreases; and it is an indication that needle coke is more suitable for low cracking.

As described above, the NCSIP value is preferably greater than 5.0, more preferably 6.0 or greater, and even more preferably 7.0 or greater.

[0048] The nitrogen content of green coke and needle coke is measured in accordance with JIS M 8819.

The sulfur content of raw coke and needle coke is measured according to JIS M 8813.

[0049] Sample preparation for CTE and cracking testing is performed by crushing needle coke with a jaw crusher, passing through an 8-16 mesh screen, then mixing the materials on and under the screen, crushing with a hammer mill, and screening to sizes of 49-200 mesh and 200 mesh or less. After mixing particles of appropriate sizes in an amount of 40 wt.%, 35 wt.% and 25 wt.%, needle coke is kneaded with a binder pitch (BP97 manufactured by C-Chem Co., Ltd.). A kneader is used for kneading, 30 wt.% binder pitch is mixed with 100 wt.% needle coke, and kneading is carried out at 160° C. for 20 minutes to obtain a kneaded product.

[0050] For cracking, the mixed product is molded so as to obtain a molded article having a diameter of 20 mm and a length of 10 mm; and the molded article is calcined at 900°C and impregnated with impregnating pitch resin (IP78 manufactured by C-Chem Co., Ltd.), and calcined again at 900°C; and the resulting product is used as a test piece for measuring cracking.

Crack measurement is performed by heating the test piece in a Tamman furnace under argon atmosphere from room temperature to 2550°C at a rate of 10°C/min and measuring the longitudinal expansion of the test piece at 1500°C and 2500°C; and cracking is calculated by the following equation (7).

[Equation (7)]

Cracking (%)=(L2500 - L1500)/L × 100

where L is the initial length of the test piece, L1500 is the length at 1500°C, and L2500 is the length at 2500°C.

[0051] For CTE, the kneaded product is extruded to obtain a molded article having an adjusted size with a diameter of 20 mm and a length of 100 mm; the molded article is calcined at 900°C and graphitized using a Tamman furnace under an argon atmosphere at 2550°C, and the resulting article is used as a test piece.

For CTE measurement, the average coefficient of thermal expansion of the adjusted test piece is measured from room temperature to 500°C.

Examples

[0052] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples; however, the present invention is not limited to these Examples.

[0053] Example 1

Coal tar was distilled; from the obtained coal tar pitch as a heavy component, substances insoluble in quinoline were removed by a solvent separation method; and the resulting pitch was used as the main raw material. Coal tar pitch, corrected by the same method to remove a substance insoluble in quinoline, was distilled at atmospheric pressure; the thus obtained fraction with a temperature of 300 to 600° C. was subjected to periodic hydrogenation treatment in the presence of a hydrogenation catalyst (stabilized by Ni) at 200° C. and 4.5 MPa; and the resulting product was used as a secondary raw material. At 100 m.h. the main raw material was mixed 35 m.h. secondary raw materials; coking of the mixed feedstock was performed at 0.6 MPa with a stepwise increase in the temperature of the coking charge from 470°C to 530°C and with a stepwise increase in the steam ratio (amount of steam (g)/amount of raw material (g)) from 0.15 to 0.30; and thus crude coke was obtained. The crude coke obtained was calcined under nitrogen at 700°C, cooled and calcined again at 1400°C; and thus needle coke was obtained. Test samples for CET and cracking were prepared from this needle coke.

In addition, the characteristic values of the obtained needle coke and test samples are shown in Table 1.

[0054] Example 2

The same coal tar pitch as in Example 1 was used as the main raw material; a fraction of 300-600° C. obtained by atmospheric distillation of coal tar was subjected to periodic hydrogenation treatment in the presence of a hydrogenation catalyst (stabilized by Ni) at 200° C. and 4.5 MPa; and the resulting product was used as a secondary raw material. At 100 m.h. the main raw material was mixed 43 m.h. secondary raw materials; coking of the mixed feedstock was carried out by stepping the feed temperature from 460°C to 550°C, stepping the pressure from 0.5 MPa to 0.65 MPa, and stepping the steam ratio from 0.10 to 0.35; and thus crude coke was obtained. Then the same operations were performed as in Example 1.

[0055] Example 3

The same coal tar pitch as in Example 1 was used as the main raw material; a fraction with a boiling point of 300-600°C, obtained by atmospheric distillation of cracking oil with a fluid catalyst, was subjected to periodic hydrogenation in the presence of a hydrogenation catalyst (stabilized by Ni) at a temperature of 250°C and a pressure of 4.5 MPa; and the resulting product was used as a secondary raw material. 100 m.h. main raw materials and 45 m.h. recycled materials mixed with each other; the coking of the mixture was carried out at a pressure of 0.5 MPa with a stepwise increase in the loading temperature from 470°C to 500°C and with a stepwise increase in the steam ratio from 0.15 to 0.35; and thus crude coke was obtained. Then the same operations were performed as in Example 1.

[0056] Comparative Example 1

The same coal tar pitch as in Example 1 was used as the main raw material; and the 300-600° C. fraction obtained from the atmospheric distillation of coal tar was used as a secondary raw material. 100 m.h. main raw materials and 45 m.h. recycled materials mixed with each other; the coking of the mixture was carried out under constant conditions of 500°C, 0.4 MPa and a steam ratio of 0.12; and thus crude coke was obtained. Then the same operations were performed as in Example 1.

[0057] Comparative Example 2

Used the same basic raw materials as in Example 1; and as a secondary feedstock, a 300-600°C fraction obtained by atmospheric distillation of a mixture of 70 wt.% coal tar pitch and 30 wt.% cracking oil with a fluid catalyst, which was used as the main raw material of Example 1, was used. main raw materials and 45 m.h. recycled materials mixed with each other; coking of the mixture was carried out under constant conditions of 490°C, 0.4 MPa and a steam ratio of 0.12; and thus crude coke was obtained. Then the same operations were performed as in Example 1.

[0058] Comparative Example 3

100 m.h. the main raw material used in Example 1 was mixed with 100 m.h. secondary raw materials used in Example 1, and the coking of the mixed raw materials was carried out under constant conditions of 530°C, 0.5 MPa and a steam ratio of 0.10; and thus crude coke was obtained. Then the same operations were performed as in Example 1.

[0059] Tables 1 and 2 show the types and properties of the feedstock, as well as the characteristics of raw coke and needle coke.

[0060]

[Table 1] Example 1 Example 2 Example 3 Main raw material PDQI (-) 0.001 0.001 0.001 Nitrogen content (%) 1.05 1.05 1.05 Sulfur content (%) 0.59 0.59 0.59 Secondary raw materials Amount to be mixed (m.h.) 35 43 45 PDQI (-) 13.5 12.3 8.3 Nitrogen content (%) 0.89 0.85 0.55 Sulfur content (%) 0.04 0.03 0.57 Needle coke Amount of nitrogen (%) 0.41 0.42 0.42 Sulfur Amount (%) 0.31 0.30 0.30 True density (g / cm ³ ) 2.153 2.152 2.148 Apparent density (g/cm ³ ) 2.125 2.119 2.113 Number of open pores (cm ³ /g) 0.060 0.058 0.053 Number of closed pores (cm ³ /g) 0.006 0.007 0.008 HIT (MPa) 1018 948 1394 EIT (GPa) 9.4 10.9 12.9 Crystallite size Lc (nm) 6.6 6.6 5.7 Interplanar spacing d002 (nm) 0.344 0.344 0.344 Pore volume (cm ³ /g) 0.158 0.148 0.158 Pore volume (-) 0.38 0.39 0.34 σ (θ) (rad) 0.68 0.65 0.54 NCSIC(-) 28.9 29.9 31.3 NCSIP(-) 9.9 8.1 7.0 test sample CTE (×10 ^-6 /°C) 1.12 0.99 0.89 Cracking (%) 0.45 0.75 1.14

[0061]

[Table 2]

Comparative Example 1 Comparative Example 2 Comparative Example 3 Main raw material PDQI (-) 0.001 0.001 0.001 Nitrogen content (%) 1.05 1.05 1.05 Sulfur content (%) 0.59 0.59 0.59 Secondary raw materials Amount to be mixed (m.h.) 45 45 100 PDQI (-) 0.9 0.1 13.5 Nitrogen content (%) 0.89 1.09 0.89 Sulfur content (%) 0.92 0.60 0.04 Needle coke Amount of nitrogen (%) 0.49 0.49 0.40 Sulfur Amount (%) 0.34 0.30 0.19 True density (g / cm ³ ) 2.132 2.132 2.147 Apparent density (g/cm ³ ) 2.097 2.069 2,100 Number of open pores (cm ³ /g) 0.037 0.020 0.035 Number of closed pores (cm ³ /g) 0.008 0.014 0.010 HIT (MPa) 1190 1005 1310 EIT (GPa) 15.6 10.8 13.8 Crystallite size Lc (nm) 5.4 5.6 5.7 Interplanar spacing d002 (nm) 0.344 0.344 0.344 Pore volume (cm ³ /g) 0.133 0.142 0.165 Pore volume (-) 0.28 0.14 0.21 σ (θ) (rad) 0.81 0.75 0.70 NCSIC(-) 19.6 21.8 24.1 NCSIP(-) 4.8 1.5 3.6 test sample CTE (×10 ^-6 /°C) 1.46 1.21 1.15 Cracking (%) 1.29 2.14 1.54

Claims (16)

1. Needle coke with low CTE (coefficient of thermal expansion) and low cracking, obtained by mixing and coking 100 m.h. a basic raw material for needle coke, consisting of a heavy oil based on coal tar or based on petroleum, having a weak hydrogen releasing property with a PDQI value calculated by equation (1) of less than 5.0, with 10-80 ppm. recycled material having a strong hydrogen release property with a PDQI value calculated from Equation (1) of 5.0 or more, and calcining the resulting raw coke: [Equation (1)] PDQI=H% × 10 × (HNβ/H), where H% is the amount of hydrogen (wt.%) obtained using elemental analysis, and HNβ/H is the ratio of β-naphthenic hydrogen to total hydrogen, measured using ¹ H-NMR, said coke has a crystallite size Lc of 4.0-10.0 nm, an interplanar spacing d002 of 0.340-0.350 nm, a fraction of open pores of 0.15-0.40, and a standard deviation σ(θ) of the average orientation angle of 0.01-0.85. 2. Low CTE, low cracking needle coke having a CTE structure index (NCSIC) expressed by equation (2) greater than 25.0 and a cracking structure index (NCSIP) expressed by equation (3) greater than 5.0: [Equation (2)] NCSIC=((Lc/d002) + (share of open pores))/(σ (θ)), where Lc is the crystallite size, d002 is the interplanar spacing, and σ(θ) is the standard deviation of the average orientation angle; [Equation (3)] NCSIP=(number of open pores/number of closed pores) + (HIT/1000)/EIT, where HIT is the indentation hardness and EIT is the indentation modulus, said coke has a crystallite size Lc of 4.0-10.0 nm, an interplanar spacing d002 of 0.340-0.350 nm, a fraction of open pores of 0.15-0.40, and a standard deviation σ(θ) of the average orientation angle of 0.01-0.85. 3. The low CTE, low cracking needle coke according to claim 1, wherein the main raw material is QI-removed pitch having a weak hydrogen donation property obtained by distilling and removing QI from coal tar. 4. The low CTE and low cracking needle coke according to claim 1, wherein the main feedstock is a QI-removed pitch having a weak hydrogen donation property obtained by distilling and removing QI from coal tar, and having a PDQI value calculated by Equation (1) of less than 2. 5. The low CTE and low cracking needle coke according to claim 1 or 2, which has an open cell number of 0.040-0.070 ^cm3 /g, a closed cell number of 0.001-0.015 ^cm3 /g, an indentation hardness HIT of 800-1500 MPa, and an indentation modulus EIT of 5.0-15.0 GPa.