EP2612894A1

EP2612894A1 - Metallurgical coke production method

Info

Publication number: EP2612894A1
Application number: EP11821997.1A
Authority: EP
Inventors: Yusuke Dohi; Izumi Shimoyama; Kiyoshi Fukada; Tetsuya Yamamoto; Hiroyuki Sumi
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2010-09-01
Filing date: 2011-08-31
Publication date: 2013-07-10
Anticipated expiration: 2031-08-31
Also published as: JP2012072388A; KR20130081702A; WO2012029987A1; EP2612894A4; KR101461838B1; JP5152378B2; CN103180414A; EP2612894B1; CN103180414B

Abstract

There is provided a method for producing a metallurgical coke having higher quality such as strength than before, using a coal blend including coals whose thermal plasticity is accurately evaluated by measuring the thermal plasticity of coal in a state in which an environment surrounding plastic coal in a coke oven is simulated. In the method for producing a metallurgical coke by carbonizing a coal blend obtained by blending at least two coals and a caking additive, a sample including the coals and the coking additive that constitute the coal blend is packed in a vessel in a predetermined amount, a material having through-holes that connect upper and lower surfaces is disposed on the sample, the sample is heated at a predetermined heating rate while a constant load is imposed on the material having through-holes that connect upper and lower surfaces, and a permeation distance of the sample that has permeated into the through-holes is measured in advance. Coals and a caking additive whose permeation distance is larger than a predetermined control value are pulverized into particles having a particle size smaller than a predetermined particle size and then blended.

Description

Technical Field

The present invention relates to a method for producing a metallurgical coke that uses a test method for evaluating thermal plasticity during carbonization of coal. In particular, the present invention relates to a method for producing a metallurgical coke that can reduce the amount of high grade coals used while maintaining the coke strength or a method for producing a metallurgical coke in which a high-strength coke can be obtained from the same coal blend.

Background Art

Coke used in a blast furnace process that is most commonly used as an iron-making process variously serves as a reducing agent for iron ore, a heat source, a spacer, and the like. In order to stably and efficiently operate a blast furnace, it is important to maintain the gas permeability in the blast furnace and thus the production of a coke having high strength has been required. Coke is produced by carbonizing, in a coke oven, a coal blend produced by blending various coals for coke making which are prepared by being pulverized so as to have an appropriate particle size. The coals for coke making are softened and melted in a temperature range of about 300°C to 550°C during carbonization. At the same time, the coals foam and swell due to the generation of volatile matter. Consequently, particles adhere to each other and form into lump semi-coke. Semi-coke contracts in a process of increasing the temperature to approximately 1000°C and thus forms into hard coke. Therefore, it is said that the adhesive properties in a plastic phase of coal considerably affect properties such as coke strength and particle size after carbonization.
In order to reinforce the adhesion of coals for coke making (coal blend), a method for producing a coke by adding a caking additive that exhibits high fluidity in a plastic temperature range of coal to the coal blend has been commonly employed. Specific examples of the caking additive include tar pitch, petroleum pitch, solvent-refined coal, and solvent-extracted coal. Similarly to the coal, it is also said that the adhesive properties of the caking additive in a plastic phase considerably affect the properties of coke after carbonization.
As described above, the thermal plasticity of coal is extremely important because the thermal plasticity considerably affects the properties of coke and coke cake structures after carbonization. Thus, the measurement method of thermal plasticity has been actively studied for a long time. In particular, coke strength, which is an important quality of coke, is considerably affected by the properties of coal serving as a raw material of coke, namely, coal rank and thermal plasticity. The thermal plasticity is a property of coal that is softened and melted by heating.
In general, the thermal plasticity is measured and evaluated using, for example, the fluidity, viscosity, adhesive properties, and swelling properties of a plastic product.
Regarding the thermal plasticity of coal, a typical method for measuring the fluidity in a plastic phase may be a coal fluidity test method that uses a Gieseler plastometer method specified in JIS M 8801. The Gieseler plastometer method is a method in which a coal pulverized so as to have a particle size of 425 µm or less is placed into a particular crucible and heated at a predetermined heating rate, and the rotation speed of a stirring rod on which a predetermined torque is exerted is read from a dial plate and given in units of ddpm (dial division per minute).
The Gieseler plastometer method is a method in which the rotation speed of a stirring rod at a constant torque is measured, and furthermore a method in which a torque at a constant rotation speed is measured has been developed. For example, Patent Literature 1 discloses a method in which a torque is measured while a rotor is rotated at a constant rotation speed.
There is also a method for measuring a viscosity with a dynamic viscoelastometer for the purpose of measuring a viscosity that has a physical significance as the thermal plasticity (e.g., refer to Patent Literature 2). The measurement of dynamic viscoelasticity is a measurement of viscoelastic behavior observed when a force is periodically applied to a viscoelastic body. In the method disclosed in Patent Literature 2, the viscosity of plastic coal is evaluated using a complex viscosity among parameters obtained in the measurement and thus the viscosity of plastic coal can be measured at a desired shear rate.
It has been reported that the adhesive properties of plastic coal that adheres to activated carbon or glass beads are measured as the thermal plasticity of coal. This is a method in which a small amount of coal sample is heated while being sandwiched by activated carbon or glass beads in a vertical direction and cooled after the softening and melting, and the appearance of the adhesion state between the coal sample and the activated carbon or glass beads is observed.
A typical method for measuring the swelling properties of coal in a plastic phase may be a dilatometer method specified in JIS M 8801. The dilatometer method is a method in which a coal pulverized so as to have a particle size of 250 µm or less is molded by a prescribed method, inserted into a designated crucible, heated at a predetermined heating rate, and measuring the displacement of the coal over time with a detection rod disposed above the coal.
There has been also known a coal dilatation test method in which the permeating behavior of gas generated in a plastic phase of coal is improved for the purpose of simulating the thermoplastic behavior of coal in a coke oven (e.g., refer to Patent Literature 3). This is a method in which a permeable material is disposed between a coal layer and a piston or between a coal layer and a piston and under the coal layer to increase the number of paths through which the volatile matter and liquid substances generated from the coal pass, whereby the measurement environment is brought closer to the swelling behavior in a coke oven. Similarly, there has been also known a method in which a material having permeation paths is disposed on a coal layer and a coal is heated with a microwave while imposing a load to measure the swelling properties of coal (refer to Patent Literature 4).

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 6-347392
PTL 2: Japanese Unexamined Patent Application Publication No. 2000-304674
PTL 3: Japanese Patent No. 2855728
PTL 4: Japanese Unexamined Patent Application Publication No. 2009-204609

Non Patent Literature

NPL 1: MOROTOMI et al., "Journal of the Fuel Society of Japan", Vol. 53, 1974, pp 779 to 790
NPL 2: MIYAZU et al., "Nippon Kokan Technical Report", Vol. 67, 1975, pp 125 to 137

Summary of Invention

Technical Problem

In the production of a metallurgical coke, a coal blend produced by blending a plurality of brands of coals at a particular ratio is commonly used. However, if the thermal plasticity cannot be accurately evaluated, there is a problem in that required coke strength cannot be satisfied. When a low-strength coke that does not satisfy the predetermined strength is used in a shaft furnace such as a blast furnace, the amount of dust generated in the shaft furnace increases and the pressure loss increases, which may cause the instability of the operation of the shaft furnace and may cause a trouble called channeling in which the flow of gas is locally concentrated.
With the existing index for thermal plasticity, strength often cannot be accurately estimated. Therefore, the coke strength is empirically controlled to be higher than or equal to a particular coke strength by setting the target coke strength on the high side in advance in consideration of variations in coke strength resulting from inaccuracies in evaluation of thermal plasticity. However, in this method, the average grade of a coal blend needs to be set on the high side by using relatively expensive coals having so-called excellent thermal plasticity, which increases the cost.
In a coke oven, coal is softened and melted while being restricted by adjacent layers. Since the thermal conductivity of coal is low, coal is not uniformly heated in the coke oven and a coke layer, a plastic layer, and a coal layer are formed in different states in that order from an oven wall serving as a heating surface. The coke oven itself swells to a degree during carbonization, but substantially does not deform. Thus, the plastic coal is restricted by the adjacent coke layer and coal layer.
There are many defect structures around the plastic coal, such as gaps between coal particles in the coal layer, gaps between particles of the plastic coal, large pores generated by volatilization of pyrolytic gas, and cracks formed in the adjacent coke layer. In particular, the cracks formed in the coke layer are believed to have a width of about several hundred micrometers to several millimeters, which are larger than the gaps between coal particles and the large pores each having a size of about several tens of micrometers to several hundred micrometers. Therefore, it is believed that not only the pyrolytic gas and liquid substances, which are by-products generated from the coal, but also the plastic coal itself permeates into the large defects formed in the coke layer. The rate of shear exerted on the plastic coal during the permeation is expected to be different depending on brands.
The inventors have considered that the thermal plasticity of coal measured under the conditions that an environment surrounding the above-described coal in a coke oven is simulated needs to be used as an index to more precisely control the coke strength. In particular, the inventors have considered that it is important to perform the measurement under the conditions that the plastic coal is restricted and under the conditions that the movement and permeation of plastic products into defect structures around the plastic products are simulated. However, the existing measurement method has the following problems.
The Gieseler plastometer method in which the measurement is performed while coal is packed in a vessel poses a problem because the restriction and permeation conditions are not taken into account at all. This method is also not suitable for the measurement of a coal that exhibits high fluidity. This is because, when a coal that exhibits high fluidity is measured, a phenomenon (Weissenberg effect) occurs in which a hollow space is formed in a portion close to the sidewall of the vessel and a stirring rod rotates without making contact, and consequently the fluidity sometimes cannot be accurately evaluated (e.g., refer to Non Patent Literature 1).
The method in which a torque is measured at a constant rotation speed also poses a problem because the restriction and permeation conditions are not taken into account. In addition, since the measurement is performed at a constant shear rate, the thermal plasticity of coal cannot be accurately evaluated as described above.
The dynamic viscoelastometer is a device in which viscosity is targeted as the thermal plasticity and the viscosity can be measured at a desired shear rate. By setting the shear rate in the measurement to a rate of shear exerted on the coal in a coke oven, the viscosity of plastic coal in the coke oven can be measured. However, it is generally difficult to measure or estimate the shear rate of each brand in a coke oven in advance.
The method in which the adhesive properties of plastic coal that adheres to activated carbon or glass beads are measured as the thermal plasticity of coal attempts to reproduce the permeation conditions in consideration of the presence of the coal layer, but poses a problem in that the coke layer and large defects are not simulated. Furthermore, the reproduction is not sufficient because the measurement is not performed under the restriction conditions.
The coal dilatation test method disclosed in Patent Literature 3 in which a permeable material is used poses a problem in that the movement of gas and liquid substances generated from coal is taken into account, but the movement of the plastic coal itself is not taken into account. This is because the permeability of the permeable material used in Patent Literature 3 is not so high to the degree that the plastic coal moves through the material. When the inventors of the present invention conducted the test disclosed in Patent Literature 3, the permeation of plastic coal into a permeable material did not occur. Therefore, different conditions need to be employed to cause the permeation of plastic coal into a permeable material.
Patent Literature 4 also discloses a coal dilatation measurement method in which the movement of gas and liquid substances generated from coal is taken into account by disposing a material having permeation paths on a coal layer. However, the method poses problems in that the heating method is restricted and the conditions for evaluating a permeation phenomenon in a coke oven are unclear. Moreover, in Patent Literature 4, the relationship between the permeation phenomenon of plastic coal and the thermoplastic behavior is unclear, the relationship between the permeation phenomenon of plastic coal and the quality of coke produced is not mentioned, and the production of high quality coke is not mentioned.
As described above, in the related art, the thermal plasticity such as fluidity, viscosity, adhesive properties, permeation properties, dilatation during permeation, or pressure during permeation of coals and caking additives cannot be measured in a state in which an environment surrounding plastic coals and caking additives in a coke oven is sufficiently simulated.
Accordingly, it is an object of the present invention to provide a method for producing a metallurgical coke having better quality such as strength than a metallurgical coke produced by an existing method. In the method, the thermal plasticity of coals used for a coal blend is accurately evaluated by measuring the thermal plasticity of coal in a state in which an environment surrounding plastic coal in a coke oven is simulated, to clarify the effects of the coals on coke strength; and the adverse effects on coke strength are reduced by adjusting the pretreatment conditions of coals that adversely affect coke strength. Solution to Problem
To solve the problems above, the present invention is characterized as follows.

[1] A method for producing a metallurgical coke by carbonizing a coal blend obtained by blending at least two coals or a coal blend obtained by blending at least two coals and a caking additive includes:
- packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing a material having through-holes that connect upper and lower surfaces on the sample, heating the sample, and measuring a permeation distance of the sample that has permeated into the through-holes ; and
- performing blending after at least part of coals and a caking additive whose permeation distance is larger than a control value is pulverized into particles having a particle size smaller than a predetermined particle size.
[2] A method for producing a metallurgical coke by carbonizing a coal blend obtained by blending at least two coals or a coal blend obtained by blending at least two coals and a caking additive includes:
- packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing a material having through-holes that connect upper and lower surfaces on the sample, heating the sample, and measuring a permeation distance of the sample that has permeated into the through-holes; and
- performing blending after coals and a caking additive whose permeation distance is larger than a control value are pulverized so that an average particle size of the coals and caking additive is smaller than an average particle size of coals and a caking additive whose permeation distance is smaller than the control value.
[3] A method for producing a metallurgical coke by carbonizing a coal blend obtained by blending at least two coals or a coal blend obtained by blending at least two coals and a caking additive includes:
- packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing a material having through-holes that connect upper and lower surfaces on the sample, heating the sample, and measuring a permeation distance of the sample that has permeated into the through-holes in advance; and
- when coals and a caking additive whose permeation distance is larger than a control value are blended into the coal blend, performing blending after all the coals and caking additive that constitute the coal blend are pulverized into particles having a particle size smaller than a predetermined particle size.
[4] In the method for producing a metallurgical coke according to (1) or (3), the predetermined particle size is a particle size having a particle size distribution in which a ratio of particles having a particle size of 6 mm or more to all particles is 5 mass% or less.
[5] In the method for producing a metallurgical coke according to any one of [1] to [4], the control value of the permeation distance of each of the coals and the caking additive that constitute the coal blend is specified by formula (1) below $Permeation distance = 1.3 \times a \times log MF$

where a is a constant that is 0.7 to 1.0 times a coefficient of log MF obtained by measuring the permeation distance and log MF of at least one of coals and a caking additive that satisfy log MF < 2.5 among the coals and the caking additive that constitute the coal blend and making a regression line that passes through the origin using the measured values, and
the log MF is a common logarithm of Gieseler maximum fluidity MF of each of the coals and the caking additive that constitute the coal blend.
[6] In the method for producing a metallurgical coke according to any one of [1] to [4], the control value of the permeation distance of each of the coals and the caking additive that constitute the coal blend is specified by formula (2) below $Permeation distance = aʹ \times log MF + b$

where a' is a constant that is 0.7 to 1.0 times a coefficient of log MF obtained by measuring the permeation distance and log MF of at least one of coals and a caking additive that satisfy log MF < 2.5 among the coals and the caking additive that constitute the coal blend and making a regression line that passes through the origin using the measured values; b is a constant that is higher than or equal to a mean value of standard deviations and lower than or equal to five times the mean value, the standard deviations being obtained when the sample used in the making of the regression line is measured multiple times; and
the log MF is a common logarithm of Gieseler maximum fluidity MF of each of the coals and the caking additive that constitute the coal blend.
[7] In the method for producing a metallurgical coke according to [5], the constant a of the formula (1) is determined using measured values of the permeation distance and log MF of at least one of coals and a caking additive that satisfy 1.75 < log MF < 2.50,
where the log MF is a common logarithm of Gieseler maximum fluidity MF.
[8] In the method for producing a metallurgical coke according to [6], the constant a' of the formula (2) is determined using measured values of the permeation distance and log MF of at least one of coals and a caking additive that satisfy 1.75 < log MF < 2.50,
where the log MF is a common logarithm of Gieseler maximum fluidity MF.
[9] In the method for producing a metallurgical coke according to any one of [1] to [4],
brands of the coals or the caking additive included in the coal blend used in the production of coke and a blending ratio of the brands of the coals or caking additive are determined in advance; and
the permeation distances and log MF of the brands of the coals or caking additive are measured, and a value two times or more a weighted average permeation distance calculated from the permeation distances and blending ratio of brands of coals or a caking additive that are included in the coal blend and satisfy log MF < 3.0 is defined as the control value of the permeation distance,
where the log MF is a common logarithm of Gieseler maximum fluidity MF.
[10] In the method for producing a metallurgical coke according to any one of [1] to [4], when a coal sample or a caking additive sample pulverized so that a ratio of particles having a particle size of 2 mm or less is 100 mass% is packed in a vessel at a packing density of 0.8 g/cm³ with a thickness of 10 mm to prepare a sample, glass beads having a diameter of 2 mm are disposed on the sample so as to form a layer having a thickness larger than or equal to the permeation distance, and heating is performed in an inert gas atmosphere from room temperature to 550°C at a heating rate of 3 °C/min while a load of 50 kPa is imposed from the above of the glass beads, the control value of the permeation distance is 15 mm or more.
[11] In the method for producing a metallurgical coke according to any one of [1] to [9], the permeation distance is measured by packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing a material having through-holes that connect upper and lower surfaces on the sample, heating the sample while a constant load is imposed on the material having through-holes that connect upper and lower surfaces, and measuring a permeation distance of the sample that has permeated into the through-holes .
[12] In the method for producing a metallurgical coke according to any one of [1] to [9], the permeation distance is measured by packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing the sample and a material having through-holes that connect upper and lower surfaces on the sample, heating the sample while the material having through-holes that connect upper and lower surfaces is kept at a constant volume, and measuring a permeation distance of the sample that has permeated into the through-holes.

Advantageous Effects of Invention

According to the present invention, the thermal plasticity of coals or caking additives can be evaluated in a state in which the effects of defect structures that are present around a plastic layer of coal in a coke oven, which is believed to considerably affect the thermal plasticity of coal in a coke oven, in particular, the effects of cracks that are present in a coke layer adjacent to the plastic layer are simulated and the restriction conditions around a plastic product in a coke oven are properly reproduced. Thus, the formation of defects derived from coals or caking additives that exhibit excessively high fluidity, which cannot be detected by an existing method for evaluating thermal plasticity, can be estimated and coals or caking additives that adversely affect the coke quality can be specified. By performing blending after such coals or caking additives are pulverized into fine particles, the adverse effects on coke quality can be reduced and a high-strength metallurgical coke can be produced.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a schematic view showing an example of an apparatus for measuring thermal plasticity while imposing a constant load on a coal or caking additive sample and a material having through-holes that connect upper and lower surfaces, the apparatus being used in the present invention.
[Fig. 2] Fig. 2 is a schematic view showing an example of a material having circular through-holes among materials having through-holes that connect upper and lower surfaces used in the present invention.
[Fig. 3] Fig. 3 is a schematic view showing an example of a spherical particle-packed layer among materials having through-holes that connect upper and lower surfaces used in the present invention.
[Fig. 4] Fig. 4 is a schematic view showing an example of a cylinder-packed layer among materials having through-holes that connect upper and lower surfaces used in the present invention.
[Fig. 5] Fig. 5 is a schematic view showing the state of formation of defect structures when a coke is produced from a coal blend obtained by blending the coals or caking additives that satisfy the ranges (A) to (D). Fig. 5(a) shows the state in which coals are packed before the production of coke. Fig. 5(b) shows the state of formation of defects after the production of coke.
[Fig. 6] Fig. 6 is a schematic view showing the state of formation of defect structures when a coke is produced from a coal blend obtained by blending the coals or caking additives that do not satisfy the ranges (A) to (D). Fig. 6(a) shows the state in which coals are packed before the production of coke. Fig. 6(b) shows the state of formation of defects after the production of coke.
[Fig. 7] Fig. 7 is a schematic view showing the state of formation of defect structures when a coke is produced from a coal blend obtained by performing blending after coals or caking additives that satisfy the ranges (A) to (D) are pulverized into fine particles. Fig. 7(a) shows the state in which coals are packed before the production of coke. Fig. 7(b) shows the state of formation of defects after the production of coke.
[Fig. 8] Fig. 8 is a schematic view showing the state of formation of defect structures when a coke is produced from a coal blend obtained by performing blending after coals or caking additives other than the coals or caking additives that satisfy the ranges (A) to (D) are pulverized into fine particles. Fig. 8(a) shows the state in which coals are packed before the production of coke. Fig. 8(b) shows the state of formation of defects after the production of coke.
[Fig. 9] Fig. 9 is a graph showing the measurement results of the permeation distance of plastic coals in the present invention.
[Fig. 10] Fig. 10 is a graph showing the positional relationship between the permeation distance and maximum fluidity of an A coal and an F coal used in Example 1 and the range (A) of the permeation distance and maximum fluidity.
[Fig. 11] Fig. 11 is a graph showing the positional relationship between the permeation distance and maximum fluidity of an A coal and an F coal used in Example 1 and the range (B) of the permeation distance and maximum fluidity.
[Fig. 12] Fig. 12 is a graph showing the measurement results of the drum strength of coke measured in Example 1.
[Fig. 13] Fig. 13 is a graph showing the measurement results of the drum strength of coke measured in Example 2.
[Fig. 14] Fig. 14 is a schematic view showing an example of an apparatus for measuring thermal plasticity while keeping a coal sample and a material having through-holes that connect upper and lower surfaces at a constant volume, the apparatus being used in the present invention. Description of Embodiments

The inventors of the present invention have conducted thorough studies on the relationship between coke strength and "permeation distance" which is the measured thermal plasticity, by enabling the thermal plasticity to be measured in a state in which an environment surrounding plastic coal in a coke oven is simulated. As a result, the inventors have found that even coals that have been reported that they have almost no difference in terms of thermal plasticity have a difference in terms of thermal plasticity measured by a method of the present invention, that is, thermal plasticity measured in a state in which an environment surrounding plastic coal is simulated. The inventors have also found that, when the coals having a difference in terms of thermal plasticity measured by the method of the present invention are blended to produce coke, the coke strengths of the coke are different from each other. Thus, the present invention has been completed by finding that coals that adversely affect coke strength are used as coals for coke making after the particle size is decreased and thus the adverse effects can be reduced.
Fig. 1 shows an example of an apparatus for measuring thermal plasticity (permeation distance) used in the present invention. The apparatus in Fig. 1 is an apparatus used when a coal sample is heated while a constant load is imposed on the coal sample and a material having through-holes that connect upper and lower surfaces. A sample 1 is prepared by packing a coal in a lower portion of a vessel 3, and a material 2 having through-holes that connect upper and lower surfaces is disposed on the sample 1. The sample 1 is heated to a temperature higher than or equal to the initial softening temperature of the sample 1, and the sample is caused to permeate into the material 2 having through-holes that connect upper and lower surfaces to measure the permeation distance. The heating is performed in an inert gas atmosphere. The permeation distance may be measured by performing heating while the coal and the material having through-holes are kept at a constant volume. Fig. 14 shows an example of an apparatus for measuring thermal plasticity (permeation distance) used in that case.
When the sample 1 is heated while a constant load is imposed on the sample 1 and the material 2 having through-holes that connect upper and lower surfaces as shown in Fig. 1, the sample 1 swells or shrinks and the material 2 having through-holes that connect upper and lower surfaces moves in a vertical direction. Therefore, the dilatation during the permeation of the sample can be measured through the material 2 having through-holes that connect upper and lower surfaces. As shown in Fig. 1, a dilatation detection rod 13 is disposed on the upper surface of the material 2 having through-holes that connect upper and lower surfaces, a loading weight 14 is placed on the upper end of the dilatation detection rod 13, and a displacement meter 15 is disposed above the loading weight 14 to measure the dilatation. A displacement meter that can measure the range (-100% to 300%) of the dilatation of the sample may be used as the displacement meter 15. Since an inert gas atmosphere needs to be kept in the heating system, a non-contact displacement meter is suitable and an optical displacement meter is desirably used. The inert gas is a gas that does not react with coal in the temperature range of the measurement. Typical examples of the gas include argon gas, helium gas, and nitrogen gas, and the nitrogen gas is preferably used. In the case where the material 2 having through-holes that connect upper and lower surfaces is a particle-packed layer, the dilatation detection rod 13 may be buried in the particle-packed layer and thus a plate is desirably disposed between the dilatation detection rod 13 and the material 2 having through-holes that connect upper and lower surfaces. The load is preferably uniformly imposed on the upper surface of the material having through-holes that connect upper and lower surfaces, the material being disposed on the upper surface of the sample. The applied pressure is 5 to 80 kPa, preferably 15 to 55 kPa, and most preferably 25 to 50 kPa relative to the area of the upper surface of the material having through-holes that connect upper and lower surfaces. The pressure is preferably set in accordance with the swelling pressure of a plastic layer in a coke oven. As a result of studies on the reproducibility of measurement results and the power of detecting a difference among various coal brands, it has been found that about 25 to 50 kPa, which is slightly higher than the swelling pressure in the oven, is the most preferable measurement condition.
The heating means is desirably a device that can perform heating at a predetermined temperature-increasing rate while monitoring the temperature of a sample. Specific examples of the heating means include an electric furnace, external heating means that uses a conductive vessel and high-frequency induction in a combined manner, and internal heating means such as a microwave. In the case where the internal heating means is employed, the inside temperature of the sample needs to be made uniform and, for example, a measure of improving the heat-insulating properties of the vessel is preferably taken.
The heating rate is set so as to correspond to the heating rate of coals in a coke oven in order to simulate the thermoplastic behavior of coals and caking additives in a coke oven. The heating rate of coals in a plastic temperature range in the coke oven is dependent on the position in the oven and the operation conditions, but is about 2 to 10 °C/min. The heating rate on average is desirably 2 to 4 °C/min and more desirably about 3 °C/min. However, in the case of coals having low fluidity, such as non- or slightly-caking coal, the permeation distance and dilatation are small at a heating rate of 3 °C/min, which may cause difficulty in detection. It is generally known that the fluidity of coal measured with a Gieseler plastometer is improved by rapidly heating the coal. Therefore, in the case of coals whose permeation distance is 1 mm or less, the heating rate may be increased to 10 to 1000 °C/min to improve the detection sensitivity.
Regarding the heating temperature range, coals and caking additives may be heated to their plastic temperature ranges because the purpose is to evaluate the thermal plasticity of the coals and caking additives. In consideration of the plastic temperature ranges of coals and caking additives for producing coke, the heating may be performed at a predetermined heating rate in a range of 0°C (room temperature) to 550°C and preferably 300°C to 550°C, which is the plastic temperature of coal.
The material having through-holes that connect upper and lower surfaces is desirably a material whose permeability coefficient can be measured or calculated in advance. The material is, for example, a unified material having through-holes or a particle-packed layer. Examples of the unified material having through-holes include a material having circular through-holes 16 shown in Fig. 2, a material having rectangular through-holes , and a material having irregularly shaped through-holes. The particle-packed layer is generally classified into a spherical particle-packed layer and a non-spherical particle-packed layer. An example of the spherical particle-packed layer is a layer composed of packing particles 17 such as beads as shown in Fig. 3. Examples of the non-spherical particle-packed layer include a layer composed of irregularly shaped particles and a layer composed of packing cylinders 18 as shown in Fig. 4. It is desirable that the permeability coefficient in the material be as constant as possible to maintain the reproducibility of the measurement and the permeability coefficient be easily calculated to simplify the measurement. Therefore, it is particularly desirable to use a spherical particle-packed layer for the material having through-holes that connect upper and lower surfaces in the present invention. Any material having through-holes that connect upper and lower surfaces may be used as long as the shape of the material substantially does not change at a temperature higher than or equal to the plastic temperature range of coal, specifically, up to 600°C and the material does not react with coal. The material may have a height sufficiently larger than the height of the permeation of a plastic coal. When a coal layer having a thickness of 5 to 20 mm is heated, the height may be about 20 to 100 mm.
The permeability coefficient of the material having through-holes that connect upper and lower surfaces needs to be set in consideration of the permeability coefficient of large defects in a coke layer. As a result of studies conducted by the inventors of the present invention on the consideration of the cause of large defects and the estimation of the size of large defects, the inventors have found that the permeability coefficient particularly desirable in the present invention is 1 x 10⁸ to 2 × 10⁹ m^-2. The permeability coefficient is derived on the basis of Darcy's law represented by formula (3) below: $ΔP / L = K \cdot μ \cdot u$

where ΔP represents the pressure loss [Pa] in the material having through-holes that connect upper and lower surfaces, L represents the height [m] of the material having through-holes , K represents the permeability coefficient [m^-2], µ represents the viscosity [Pa·s] of a fluid, and u represents the velocity [m/s] of a fluid. For example, when a glass bead layer including glass beads with a uniform particle size is used as the material having through-holes that connect upper and lower surfaces, the diameter of the glass beads selected to provide the above-described suitable permeability coefficient is desirably about 0.2 to 3.5 mm and most desirably 2 mm.
A coal or a caking additive to be used as a measurement sample is pulverized in advance and packed at a predetermined packing density with a predetermined layer thickness. The particle size after the pulverization may be a particle size of coals charged into a coke oven (the ratio of particles having a particle size of 3 mm or less to all particles is about 70% to 80% by mass). The coal or caking additive is preferably pulverized so that the ratio of particles having a particle size of 3 mm or less is 70% by mass or more. However, in consideration of the measurement using a small apparatus, all particles are particularly preferably pulverized so as to have a particle size of 2 mm or less. The packing density of the pulverized product may be 0.7 to 0.9 g/cm³, which corresponds to the packing density in a coke oven. As a result of studies on the reproducibility and detection power, it has been found that the packing density is preferably 0.8 g/cm³. The packed layer may have a thickness of 5 to 20 mm on the basis of the thickness of a plastic layer in a coke oven. As a result of studies on the reproducibility and detection power, it has been found that the packed layer preferably has a thickness of 10 mm.
The main measurement conditions in the measurement of the permeation distance are described below.

(1) A coal or a caking additive is pulverized so that the ratio of particles having a particle size of 2 mm or less is 100% by mass, and the pulverized coal or caking additive is packed in a vessel at a packing density of 0.8 g/cm³ with a layer thickness of 10 mm to prepare a sample;
(2) glass beads having a diameter of 2 mm are disposed on the sample so that the layer thickness of the glass beads is larger than or equal to the permeation distance;
(3) heating is performed from room temperature to 550°C at a heating rate of 3 °C/min in an inert gas atmosphere while a load of 50 kPa is imposed from the above of the glass beads; and
(4) the permeation distance of the plastic sample that has permeated into the glass bead layer is measured.

The permeation distance of the plastic coal and plastic caking additive can be desirably measured continuously during the heating. However, such a continuous measurement is difficult because of, for example, tar generated from the sample. The swelling and permeation of coal by heating are irreversible phenomena. Once coal is subjected to swelling and permeation, the shape is substantially kept even after cooling. Therefore, after the completion of the permeation of the plastic coal, the entire vessel is cooled and the permeation distance after cooling is measured, whereby the permeation distance during the heating may be measured. For example, the material having through-holes that connect upper and lower surfaces is taken out of the vessel after cooling, and the permeation distance can be directly measured using a vernier caliper or a ruler. In the case where particles are used as the material having through-holes that connect upper and lower surfaces, the plastic product that has permeated into the gaps of the particles fixes the entire particle layer into which the plastic product has permeated. Therefore, the relationship between the mass and height of the particle-packed layer is determined in advance, and then the mass of unfixed particles is measured after the completion of permeation and the mass is subtracted from the initial mass, whereby the mass of fixed particles can be derived and the permeation distance can be calculated.
The superiority of the permeation distance has become obvious not only by the theoretical assumption based on the method for measuring a state similar to the state in a coke oven but also by the result of the study on the effect of the permeation distance on the coke strength. In fact, it has been confirmed by the evaluation method of the present invention that even coals having substantially the same log MF (the common logarithm of the maximum fluidity measured by a Gieseler plastometer method) have a difference in permeation distance depending on the brands thereof and furthermore the effects on the coke strength of coke produced by blending coals having different permeation distances are also different.
In the conventional evaluation of thermal plasticity with a Gieseler plastometer, it has been considered that coals having high fluidity have a higher effect of bonding coal particles to each other. As a result of the study on the relationship between permeation distance and coke strength, it has been found that, if a coal having an extremely large permeation distance is blended, large defects are left and a microstructure having a thin pore wall is formed in the production of coke, whereby the coke strength is decreased compared with the coke strength expected from the average grade of a coal blend. This may be because a coal having an extremely large permeation distance remarkably permeates into portions between coal particles around the coal and thus the regions themselves in which the coal particles have been present become large cavities, resulting in the formation of defects. In particular, it has been found that, in coals exhibiting high fluidity in the evaluation of thermal plasticity with a Gieseler plastometer, the amount of large defects generated in coke is different depending on the magnitude of permeation distance. This relationship has been also seen in caking additives.
As a result of thorough studies conducted by the inventors of the present invention, the inventors have found that the permeation distances of coals or caking additives that reduce the coke strength when blended with a coal for coke making are effectively specified by the four ranges (A) to (D) below.
(A) The range of permeation distance is specified by formula (4). $Permeation distance \geq 1.3 \times a \times log MF$

Herein, a is a constant that is 0.7 to 1.0 times the coefficient of log MF obtained by measuring the permeation distance and log MF of at least one of coals and a caking additive that satisfy log MF < 2.5 among the coals and the caking additive that constitute the coal blend and making a regression line that passes through the origin using the measured values.
(B) The range of permeation distance is specified by formula (5) below $Permeation distance \geq aʹ \times log MF + b$

Herein, a' is a constant that is 0.7 to 1.0 times the coefficient of log MF obtained by measuring the permeation distance and maximum fluidity of at least one of coals and a caking additive that satisfy log MF < 2.5 among the coals and the caking additive that constitute the coal blend and making a regression line that passes through the origin using the measured values; and b is a constant that is higher than or equal to the mean value of standard deviations and lower than or equal to five times the mean value, the standard deviations being obtained when the same sample of at least one type selected from the brands used in making of the regression line is measured multiple times.
(C) When the brands and blending ratio of a coal blend used in the production of coke can be determined in advance, the permeation distance is two times or more the weighted average permeation distance calculated from the permeation distances and blending ratio of brands of coals and caking additives that are included in the coal blend and satisfy log MF < 3.0. The average permeation distance is preferably determined by employing a weighted average in consideration of blending ratio, but may also be determined by employing a simple average.
(D) When a coal sample prepared so that the ratio of particles having a particle size of 2 mm or less is 100 mass% is packed in a vessel at a packing density of 0.8 g/cm³ with a thickness of 10 mm and the coal sample is heated to 550°C at a heating rate of 3 °C/min while a load of 50 kPa is imposed using glass beads having a diameter of 2 mm as the material having through-holes , the permeation distance is 15 mm or more.
The methods for determining the four control values (A) to (D) above are described because the permeation distance varies depending on the set measurement conditions such as a load, a temperature-increasing rate, the type of material having through-holes , and the structure of an apparatus. This is based on the finding that the methods for determining the control values (A) to (C) are effective as a result of studies conducted in consideration of the cases where measurement conditions different from those described above may be employed.
The constants a and a' in the formulae (4) and (5) respectively used when the ranges (A) and (B) are determined are each determined so as to be 0.7 to 1.0 times the coefficient of log MF obtained by measuring the permeation distance and maximum fluidity of at least one of coals that satisfy log MF < 2.5 and making a regression line that passes through the origin using the measured values. This is because, although a substantially positive correlation is seen between the maximum fluidity and permeation distance of coal in the range of log MF < 2.5, the brand that decreases the strength is a brand whose permeation distance considerably deviates from the correlation in a positive direction. As a result of thorough studies, the inventors of the present invention have found that the brand whose permeation distance is 1.3 or more times the permeation distance determined in accordance with the log MF of coal using the above-described regression line decreases the strength, and thus have specified the range of the formula (4). Furthermore, to detect the brand which deviates from the regression equation in a positive direction beyond measurement errors, the inventors have found that the brand whose permeation distance is higher than or equal to a value obtained by adding 5 times the standard deviation obtained when the same sample is measured multiple times to the regression equation decreases the strength, and thus have specified the range of the formula (5). Therefore, the constant b may be 5 times the standard deviation obtained when the same sample is measured multiple times. In the case of the measurement conditions mentioned in the present invention, the constant b is about 3.0 mm. In this case, the formulae (4) and (5) specify, in accordance with the log MF of the coal, the ranges of permeation distance that causes a decrease in the strength. This is because, since the permeation distance generally increases as the MF increases, the degree of the deviation from the correlation is important. The regression line may be made by a linear regression method that uses a publicly known least squares method. The number of coals used for regression is preferably as large as possible because the magnitude of the errors in regression is reduced. In particular, a brand having a low MF has a small permeation distance and thus the magnitude of the errors tends to increase. Therefore, the regression line is particularly preferably determined using at least one coal in the range of 1.75 < log MF < 2.50.
The reason why the constants a, a', and, b are specified in the range is that, by decreasing these values, coals that decrease the strength can be detected with more certainty. The values can be adjusted in accordance with the operational requirements. However, an excessive decrease in these values poses problems in that the amount of coals that may adversely affect the coke strength is excessively increased and coals that, in reality, do not decrease the strength are misunderstood as coals that decrease the strength. Therefore, a and a' are preferably 0.7 to 1.0 times the slope of the regression line and b is preferably 1 to 5 times the standard deviation obtained when the same sample is measured multiple times.
Coals or caking additives used for a coal blend are normally used after various properties are measured for each brand. The permeation distance may also be measured for each lot of brands in advance. The average permeation distance of a coal blend may be obtained by measuring the permeation distances of brands in advance and averaging the permeation distances in accordance with the blending ratio or may be obtained by producing a coal blend and measuring the permeation distance of the coal blend. Thus, a brand having a permeation distance extremely larger than the average permeation distance of a coal blend can be selected. A coal blend used in the production of coke may include oils, coke breeze, petroleum coke, resins, and wastes, in addition to coals and caking additives.
If the coals and caking additives that satisfy the above-described ranges (A) to (D) are used as coals for coke making after subjected to a normal pretreatment, large defects are left and a microstructure having a thin pore wall is formed in the production of coke, resulting in a decrease in the coke strength. Therefore, a measure of restricting the blending ratio of the brands and caking additives is simple and effective means for maintaining the coke strength. In the current production of coke in which many brands from various sources are intended to be blended in terms of stable raw material procurement, even the coals or caking additives that satisfy the above-described ranges (A) to (D) often needs to be used.
The inventors of the present invention have found that, even if a coal blend obtained by blending the coals or caking additives that satisfy the above-described ranges (A) to (D) is used as a coal for coke making, a decrease in the strength can be suppressed by changing the particle size of the coal blend. The process of the consideration is described below with reference to the schematic views.
Fig. 5 schematically shows the state of formation of defect structures when a coke is produced from a coal blend obtained by blending the coals or caking additives that satisfy the ranges (A) to (D). Particles 19 of the coals or caking additives that satisfy the ranges (A) to (D) considerably permeate into gaps between packed particles and large defects in the production of coke. Therefore, thin pore walls are formed and large defects 22 are left in places in which the particles have been originally present, resulting in the decrease in the coke strength (Fig. 5(b)).
Fig. 6 schematically shows the state of formation of defect structures when a coke is produced from a coal blend obtained by blending coals or caking additives 20 that do not satisfy the ranges (A) to (D). Particles 20 of the coals or caking additives that do not satisfy the ranges (A) to (D) do not considerably permeate into gaps between packed particles and large defects in the production of coke. Therefore, thick pore walls are formed and large defects are not left in places in which the particles have been originally present. Consequently, the decrease in the coke strength is not caused (Fig. 6(b)).
Fig. 7 schematically shows the state of formation of defect structures when a coke is produced from a coal blend obtained by performing blending after the coals or caking additives 19 that satisfy the ranges (A) to (D) are pulverized into fine particles. In this case, particles of the coals or caking additives 19 that satisfy the ranges (A) to (D) considerably permeate into gaps between packed particles and large defects in the production of coke. However, the size of defects formed in places in which the particles have been originally present is decreased, and thus the decrease in the coke strength can be suppressed (Fig. 7(b)).
Fig. 8 schematically shows the state of formation of defect structures when a coke is produced from a coal blend obtained by performing blending after the coals or caking additives 20 other than the coals or caking additives that satisfy the ranges (A) to (D) are pulverized into fine particles. In this case, the spaces around the particles of the coals or caking additives 19 that satisfy the ranges (A) to (D) are occupied by the fine particles and defects, which decreases the permeability coefficient. Therefore, the particles of the coals or caking additives 19 cannot considerably permeate into gaps between packed particles and large defects in the production of coke. Consequently, thick pore walls are formed and large defects are not left in places in which the particles have been originally present, which can suppress the decrease in the coke strength (Fig. 8(b)).
As described in the above consideration, in the case where the coals or caking additives that satisfy the ranges (A) to (D) are blended, by decreasing the particle size of the coals or caking additives that satisfy the ranges (A) to (D) or the particle size of coals or caking additives other than the coals or caking additives that satisfy the ranges (A) to (D), the permeation distance of coal can be decreased, the number of large defects can be reduced, and the decrease in the coke strength after carbonization can be suppressed.
When the particle size of a coal blend decreases, the specific surface of coal particles increases and the distance between particles increases. It is generally said that, to maintain the coke strength, the thermal plasticity of the entire coal blend needs to be improved. Therefore, when the coals or caking additives that satisfy the ranges (A) to (D) are blended, it is important to decrease the particle size of the coal blend to the extent that the lack of thermal plasticity of the entire coal blend does not become apparent. Note that the decrease in the particle size of a coal blend to the extent that the lack of thermal plasticity becomes apparent is unusual in the actual operation. Therefore, a high-strength coke can be obtained by employing pulverization conditions severer than normal conditions.

EXAMPLES

Example 1

The permeation distance of 18 types of coals (coals A to R) and one type of caking additive (caking additive S) was measured. Table 1 shows the properties of the coals or caking additive used. Herein, Ro represents a mean maximum reflectance of vitrinite in coal according to JIS M 8816, log MF is the common logarithm of the maximum fluidity (MF) measured by a Gieseler plastometer method, and the volatile matter (VM) and the ash content (Ash) are values measured by a proximate analysis method according to JIS M 8812.
The permeation distance was measured with the apparatus shown in Fig. 1. Since a high-frequency induction heating system was employed, a heating element 8 in Fig. 1 was an induction heating coil and a vessel 3 was made of graphite serving as a dielectric. The vessel had a diameter of 18 mm and a height of 37 mm. Glass beads having a diameter of 2 mm were used as a material having through-holes that connect upper and lower surfaces. Into the vessel 3, 2.04 g of a coal sample that was pulverized so as to have a particle size of 2 mm or less and vacuum-dried at room temperature was charged. A weight of 200 g was dropped from 20 mm above the coal sample five times to pack a sample 1 (the thickness of the sample was 10 mm in this state). Subsequently, the glass beads having a diameter of 2 mm were disposed on the packed layer of the sample 1 so as to have a thickness of 25 mm. A sillimanite disc having a diameter of 17 mm and a thickness of 5 mm was disposed on the glass bead-packed layer. A quartz rod serving as a dilatation detection rod 13 was placed on the sillimanite disc. A weight 14 of 1.3 kg was placed on the quartz rod. Consequently, the pressure applied onto the sillimanite disc was 50 kPa. Heating was performed to 550°C at a heating rate of 3 °C/min using a nitrogen gas as an inert gas. After the completion of the heating, cooling was performed in a nitrogen atmosphere. The mass of beads that were not fixed by the plastic coal in the cooled vessel was measured. The above measurement conditions were determined as preferable measurement conditions for permeation distance by the inventors of the present invention through the comparison of the measurement results under various conditions. However, the method for measuring the permeation distance is not limited to the above method.
The glass bead layer may be disposed so as to have a thickness larger than or equal to the permeation distance. In the case where a plastic product permeated to the uppermost portion of the glass bead layer during the measurement, the amount of glass beads was increased and the measurement was performed again. By conducting experiments in which the thickness of glass beads was changed, the inventors of the present invention have confirmed that, as long as the thickness of the glass bead layer is larger than or equal to the permeation distance, the measurement value of the permeation distance of the same sample is the same. When a caking additive having a large permeation distance was measured, a larger vessel was used and the amount of glass beads packed was also increased.

[Table 1]

Coal	- Ro [%]	log MF [log ddpm]	VM [mass%]	Ash [mass%]	Permeation distance [mm]
A coal	0.66	3.55	43.2	5.8	8.0
B coal	0.67	1.00	36.6	9.0	3.3
C coal	0.72	3.61	40.8	9.0	14.9
D coal	0.73	2.29	36.2	8.8	8.1
E coal	0.75	2.32	38.1	9.7	8.0
F coal	0.80	3.17	37.2	7.9	19.5
G coal	0.91	3.59	33.0	7.9	19.0
H coal	1.02	2.48	29.1	8.6	6.3
I coal	1.00	1.71	25.8	9.6	2.5
J coal	1.00	2.20	27.7	10.4	4.8
K coal	1.03	2.97	28.2	9.6	12.1
L coal	1.14	1.77	24.2	9.2	4.9
M coal	1.30	1.34	21.0	9.4	1.3
N coal	1.31	1.26	20.4	7.3	0.9
O coal	1.38	2.49	20.9	10.9	8.7
P coal	1.44	2.03	21.1	9.3	7.8
Q coal	1.54	0.00	16.6	8.3	1.2
R coal	1.62	0.70	18.8	9.6	3.0
Caking additive S	-	4.8 or more	-	less than 1	65.0

The height of the fixed bead-packed layer was defined as the permeation distance. The relationship between the height and mass of the glass bead-packed layer was determined in advance so that the height of the fixed bead-packed layer could be derived from the mass of the beads fixed by the plastic coal. This is represented by formula (6) and the permeation distance was derived from the formula (6). $L = (G - M) \times H$

Herein, L represents the permeation distance [mm], G represents the mass [g] of the packed glass beads, M represents the mass [g] of the beads not fixed by a plastic product, and H represents the height of the packed layer per gram of glass beads packed in the experimental apparatus [mm/g].
Fig. 9 shows the relationship between the measurement results of the permeation distance and the logarithm (log MF) of the Gieseler maximum fluidity (MF). It is confirmed from Fig. 9 that the permeation distance measured in this Example has a correlation with the maximum fluidity, but there is a difference in permeation distance even at the same MF. For example, as a result of the study on the measurement error of the permeation distance in this apparatus, the standard deviation of three tests under the same conditions was 0.6. In consideration of the standard deviation, a significant difference in permeation distance was recognized between the coal A and coal C having substantially the same maximum fluidity.
Subsequently, the relationship between the coke strength and the particle size of the coals that satisfy the ranges (A) to (D) was investigated. As described below, a coal blend obtained by blending the coal A that does not satisfy the ranges (A) to (D) in a content of 20 mass% and a coal blend obtained by blending the coal F that satisfies the ranges (A) to (D) in a content of 20 mass% were produced, and the coke strength after carbonization was measured by variously changing the particle sizes of only the coal A and coal F.
In the conventional coal blending theory for estimating coke strength, it has been considered that the coke strength is determined by mainly the mean maximum reflectance (Ro) of vitrinite in coal and log MF (e.g., refer to Non Patent Literature 2). Therefore, coal blends (Ro = 0.99, log MF = 2.2) obtained by blending various coals were produced so that all the coal blends had the same weighted average Ro and weighted average log MF. The coal A and coal F were pulverized so that the ratio of particles having a particle size of less than 1 mm was 100 mass%, the ratio of particles having a particle size of less than 3 mm was 100 mass%, and the ratio of particles having a particle size of less than 6 mm was 100 mass%. Coals other than the coal A and coal F were pulverized so that the ratio of particles having a particle size of less than 3 mm was 100 mass%. Six different coal blends shown in Table 2 were produced using these coals.

[Table 2]

	Blending ratio
	Coal blend A1 [%]	Coal blend A2 [%]	Coal blend A3 [%]	Coal blend F1 [%]	Coal blend F2 [%]	Coal blend F3 [%]
A coal	20	20	20	0	0	0
B coal	14	14	14	13	13	13
F coal	0	0	0	20	20	20
H coal	19	19	19	20	20	20
J coal	13	13	13	20	20	20
L coal	11	11	11	11	11	11
N coal	11	11	11	7	7	7
O coal	8	8	8	9	9	9
R coal	4	4	4	0	0	0

Maximum particle size of A coal and F coal [mm]	1	3	6	1	3	6
Welghted average Ro [%]	0.99	0.99	0.99	0.99	0.99	0.99
Weighted average log MF [log ddpm]	2.2	2.2	2.2	2.2	2.2	2.2
Weighted average permeation distance of coals other than A coal and F coal [mm]	4.7	4.7	4.7	5.0	5.0	5.0
Dl 150/15 [-]	80.0	78.8	78.5	79.6	76.9	74.3
CSR (%)	58.0	55.9	55.2	57.6	50.5	47.5
MSI +65 (%)	53.0	51.8	51.5	52.4	49.5	46.7

Table 2 also shows the weighted average permeation distance of the coal blends including coals other than the A coal and F coal, that is, the weighted average permeation distance of coals which are included in the coal blends and whose log MF is less than 3.0. The weighted average permeation distance of coal blends not including the A coal in the coal blends A1 to A3 is 4.7 mm whereas the permeation distance of the A coal is 8.0 mm, which is less than two times the weighted average permeation distance. Therefore, the A coal does not satisfy the ranges (C) and (D). On the other hand, the weighted average permeation distance of coal blends not including the F coal in the coal blends F1 to F3 is 5.0 mm whereas the permeation distance of the F coal is 19.5 mm, which is two times or more the weighted average permeation distance. Therefore, the F coal satisfies the range (C) and also obviously satisfies the range (D).
The constants a and a' in the respective formulae (1) and (2) were determined to be 2.70, which corresponds to the slope of a regression line, the slope being calculated from the permeation distance and maximum fluidity of coals in the range of log MF < 2.5. The constant b in the formula (2) was determined to be 3.0, which is five times the standard deviation of 0.6 obtained under measurement conditions of the invention example. Figs. 10 and 11 show the respective positional relationships between the above ranges (A) and (B) and the permeation distance and maximum fluidity of the caking additive used in this Example, the positional relationships being investigated based on the formulae above. As shown in Figs. 10 and 11, the F coal satisfies both the ranges (A) and (B).
The moisture of all the coal blends shown in Table 2 was adjusted to be 8 mass%. Sixteen kilograms of each of the coal blends was charged into a carbonization can at a bulk density of 750 kg/m³ and a weight of 10 kg was placed thereon. The coal blend was carbonized in an electric oven whose oven wall temperature was 1050°C for six hours. The carbonization can was taken out of the electric oven and cooled using nitrogen to obtain coke. The mass content of coke having a particle size of 15 mm or more after 150 revolutions at 15 rpm was measured in conformity with the drum strength test method of JIS K 2151. The coke strength of the obtained coke was calculated as drum strength DI 150/15, which was the mass ratio between before and after the revolutions. The CSR (coke strength after reaction with CO₂ measured in conformity with ISO 18894) and the micro strength (MSI +65) were also measured.
Table 2 also shows the measurement results of the drum strength. Fig. 12 shows the relationship between the drum strength and the maximum particle size of the coal A and coal F. It has been confirmed that the coal blend obtained by blending the coal F that satisfies the ranges (A) to (D) has lower strength than the coal blend obtained by blending the coal A that does not satisfy the ranges (A) to (D). Therefore, it has been confirmed that the permeation distance measured in the present invention is a factor that affects the strength and cannot be explained using known factors. It has been confirmed that the strength of any coal blend obtained by blending the coal A that does not satisfy the ranges (A) to (D) and the coal F that satisfies the ranges (A) to (D) is improved by decreasing the particle size of the coals. In particular, the strength of the coal blend obtained by blending the coal F that satisfies the ranges (A) to (D) is considerably improved by decreasing the particle size of the coal.
It has been clarified that the decrease in the strength can be suppressed by performing blending after the particle size of the coal F is decreased to a particle size smaller than that of coals that do not satisfy the ranges (A) to (D) (coal blend F1). In the present invention, when the particle size of coals is decreased, the maximum particle size or average particle size of the coals may be decreased. Alternatively, the content of particles whose particle size is larger than a particular sieve opening may be decreased (that is, the content of particles whose particle size is smaller than a particular sieve opening is increased).

Example 2

In the normal operation of an actual coke oven, the particle size of a coal blend is generally controlled using the mass ratio of oversize or undersize relative to the total mass when the coal blend is passed through a sieve with a predetermined opening. Therefore, it is difficult to adjust the particle size of each brand that constitutes the coal blend. When a coal blend obtained by blending the coals or caking additives that satisfy the ranges (A) to (D) is carbonized in an actual coke oven, an operation in which the particle size of all the coals or caking additives that constitute the coal blend is decreased is believed to be a practical and effective operation.
The inventors of the present invention investigated the relationship between the coke strength and the ratio of coals having a particle size of 6 mm or more in the coal blend by carbonizing the coal blends produced by variously changing the blending ratio of the coals or caking additives that satisfy the ranges (A) to (D) and measuring the drum strength DI 150/15 serving as the coke strength after carbonization.
Table 3 shows the average properties of the coal blends used, the carbonization temperature, and the coke temperature at the center of coke oven chamber after carbonization. The ranges of fluctuation in the average properties of a coal blend, the carbonization temperature, and the coke temperature at the center of coke oven chamber after carbonization were reduced so that the effects of these factors on the coke strength were minimized.

[Table 3]

Average properties of coal blend	Ro [%]	0.98 to 1.02
	log MF [log ddpm]	2.6 to 2.8
	Moisture [mass%]	8.5 to 10.5
Carbonization conditions	Carbonization temperature [°C]	1090 to 1105
Carbonization conditions	Coke temperature after carbonization [°C]	990 to 1115

Fig. 13 shows the relationship between the coke strength and the ratio of coals having a particle size of 6 mm or more in the coal blend. As shown in Fig. 13, the following has been confirmed. In the case where the blending ratio of the coals or caking additives that satisfy at least one of the ranges (A) to (D) is relatively high, for example, 8 mass% to 12 mass%, the ratio of particles having a particle size of 6 mm or more increases, which means that the coke strength decreases as the coal particle size increases.
It has been confirmed from this Example that even a coal blend obtained by blending the coals or caking additives that satisfy at least one of the ranges (A) to (D) in a content of 8 mass% or more and less than 12 mass% has the same level of strength as a coal blend substantially not including the coals that satisfy at least one of the ranges (A) to (D), by decreasing the particle size of the entire coal blend until the ratio of particles having a particle size of 6 mm or more reaches 5 mass% or less. The reason for this may be as follows. Since coals having a large permeation distance easily form large defects as shown in Fig. 5, the formation of the large defects is suppressed by decreasing the content of coal particles having a large particle size. In addition to this, the effect of suppressing the permeation shown in Fig. 8 considerably contributes to the increase in the coke strength.
Accordingly, when a coal blend obtained by blending the coals or caking additives that satisfy at least one of the ranges (A) to (D) is carbonized in an actual coke oven, the relationship between particle size and strength is determined for each blending ratio and an operation is conducted in accordance with the control value of particle size which is expected to achieve the control value of strength, whereby the decrease in the strength can be suppressed. In the past, when the coke strength decreased, a large amount of relatively expensive strongly caking coal needed to be blended to increase the strength, which increased the production cost. However, by applying the present invention, the decrease in the strength can be suppressed by the control of pretreatment conditions of coals before charged into a coke oven and thus the increase in the cost due to the blending of strongly caking coal can be prevented.

Reference Signs List

1	sample
2	material having through-holes that connect upper and lower surfaces
3	vessel
5	sleeve
7	thermometer
8	heating element
9	temperature detector
10	thermostat
11	gas inlet
12	gas outlet
13	dilatation detection rod
14	weight
15	displacement meter
16	circular through-hole
17	packing particle
18	packing cylinder
19	coal or caking additive that satisfies the ranges (A) to (D)
20	coal or caking additive that does not satisfy the ranges (A) to (D)
21	pore
22	large defect

Claims

A method for producing a metallurgical coke by carbonizing a coal blend obtained by blending at least two coals or a coal blend obtained by blending at least two coals and a caking additive, the method comprising:
packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing a material having through-holes that connect upper and lower surfaces on the sample, heating the sample, and measuring a permeation distance of the sample that has permeated into the through-holes; and

performing blending after at least part of coals and a caking additive whose permeation distance is larger than a control value is pulverized into particles having a particle size smaller than a predetermined particle size.
A method for producing a metallurgical coke by carbonizing a coal blend obtained by blending at least two coals or a coal blend obtained by blending at least two coals and a caking additive, the method comprising:
packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing a material having through-holes that connect upper and lower surfaces on the sample, heating the sample, and measuring a permeation distance of the sample that has permeated into the through-holes ; and

performing blending after coals and a caking additive whose permeation distance is larger than a control value are pulverized so that an average particle size of the coals and caking additive is smaller than an average particle size of coals and a caking additive whose permeation distance is smaller than the control value.
A method for producing a metallurgical coke by carbonizing a coal blend obtained by blending at least two coals or a coal blend obtained by blending at least two coals and a caking additive, the method comprising:
packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing a material having through-holes that connect upper and lower surfaces on the sample, heating the sample, and measuring a permeation distance of the sample that has permeated into the through-holes in advance; and

when coals and a caking additive whose permeation distance is larger than a control value are blended into the coal blend, performing blending after all the coals and caking additive that constitute the coal blend are pulverized into particles having a particle size smaller than a predetermined particle size.
The method for producing a metallurgical coke according to Claim 1 or 3, wherein the predetermined particle size is a particle size having a particle size distribution in which a ratio of particles having a particle size of 6 mm or more to all particles is 5 mass% or less.
The method for producing a metallurgical coke according to any one of Claims 1 to 4, wherein the control value of the permeation distance of each of the coals and the caking additive that constitute the coal blend is specified by formula (1) below
Permeation distance = 1.3 x a x log MF (1) where a is a constant that is 0.7 to 1.0 times a coefficient of log MF obtained by measuring the permeation distance and log MF of at least one of coals and a caking additive that satisfy log MF < 2.5 among the coals and the caking additive that constitute the coal blend and making a regression line that passes through the origin using the measured values, and
the log MF is a common logarithm of Gieseler maximum fluidity MF of each of the coals and the caking additive that constitute the coal blend.
The method for producing a metallurgical coke according to any one of Claims 1 to 4, wherein the control value of the permeation distance of each of the coals and the caking additive that constitute the coal blend is specified by formula (2) below $Permeation distance = aʹ \times log MF + b$

where a' is a constant that is 0.7 to 1.0 times a coefficient of log MF obtained by measuring the permeation distance and log MF of at least one of coals and a caking additive that satisfy log MF < 2.5 among the coals and the caking additive that constitute the coal blend and making a regression line that passes through the origin using the measured values; b is a constant that is higher than or equal to a mean value of standard deviations and lower than or equal to five times the mean value, the standard deviations being obtained when the sample used in the making of the regression line is measured multiple times; and
the log MF is a common logarithm of Gieseler maximum fluidity MF of each of the coals and the caking additive that constitute the coal blend.
The method for producing a metallurgical coke according to Claim 5, wherein the constant a of the formula (1) is determined using measured values of the permeation distance and log MF of at least one of coals and a caking additive that satisfy 1.75 < log MF < 2.50,
where the log MF is a common logarithm of Gieseler maximum fluidity MF.
The method for producing a metallurgical coke according to Claim 6, wherein the constant a' of the formula (2) is determined using measured values of the permeation distance and log MF of at least one of coals and a caking additive that satisfy 1.75 < log MF < 2.50,
where the log MF is a common logarithm of Gieseler maximum fluidity MF.
The method for producing a metallurgical coke according to any one of Claims 1 to 4,
wherein brands of the coals or the caking additive included in the coal blend used in the production of coke and a blending ratio of the brands of the coals or caking additive are determined in advance; and
the permeation distances and log MF of the brands of the coals or caking additive are measured, and a value two times or more a weighted average permeation distance calculated from the permeation distances and blending ratio of brands of coals or a caking additive that are included in the coal blend and satisfy log MF < 3.0 is defined as the control value of the permeation distance,
where the log MF is a common logarithm of Gieseler maximum fluidity MF.
The method for producing a metallurgical coke according to any one of Claims 1 to 4, wherein, when a coal sample or a caking additive sample pulverized so that a ratio of particles having a particle size of 2 mm or less is 100 mass% is packed in a vessel at a packing density of 0.8 g/cm³ with a thickness of 10 mm to prepare a sample, glass beads having a diameter of 2 mm are disposed on the sample so as to form a layer having a thickness larger than or equal to the permeation distance, and heating is performed in an inert gas atmosphere from room temperature to 550°C at a heating rate of 3 °C/min while a load of 50 kPa is imposed from the above of the glass beads, the control value of the permeation distance is 15 mm or more
The method for producing a metallurgical coke according to any one of Claims 1 to 9, wherein the permeation distance is measured by packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing a material having through-holes that connect upper and lower surfaces on the sample, heating the sample while a constant load is imposed on the material having through-holes that connect upper and lower surfaces, and measuring a permeation distance of the sample that has permeated into the through-holes.
The method for producing a metallurgical coke according to any one of Claims 1 to 9, wherein the permeation distance is measured by packing, as a sample, each of the coals and the caking additive that constitute the coal blend in a vessel, disposing the sample and a material having through-holes that connect upper and lower surfaces on the sample, heating the sample while the material having through-holes that connect upper and lower surfaces is kept at a constant volume, and measuring a permeation distance of the sample that has permeated into the through-holes.