Method for producing carbon briquette

Abstract

A method of producing carbon agglomerate is provided, capable of producing carbon agglomerate having high strength capable of withstanding use in a blast furnace even when a usage amount of carbonaceous raw material that has poor softening and melting property is increased. The method includes: a powder preparation process of preparing a carbonaceous powder having a volatile content of 6 wt% D.B. or more and less than 20 wt% D.B., an O / C ratio representing an atomic ratio of O atoms to C atoms of 0.040 or more, and a maximum particle size of 300 µm or less; and a hot pressing process of pressure molding the carbonaceous powder under a set of conditions including a maximum arrival temperature of 600 °C to 1250 °C in an oxygen-excluded environment to obtain the carbon agglomerate.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method of producing carbon agglomerate such as coke.BACKGROUND

[0002] A typical conventional method of producing coke involves grinding coal having caking properties (caking coal) to produce powder in which 70 wt% to 100 wt% of particles have a particle size of 3 mm or less, and then dry distilling the powder, causing the caking coal particles to soften and melt in the softening and melting temperature range of about 400 °C to 500 °C and adhere to each other, thereby obtaining coke as carbon agglomerate.

[0003] Patent Literature (PTL) 1 describes a method of producing a high-density carbon material. In this production method, a carbonaceous raw material consisting of a self-sintering carbonaceous powder is heated to 400 °C to 600 °C under atmospheric pressure, and then, while maintaining the heated carbonaceous raw material in the temperature range, a pressure of 50 kg / cm 2< to 400 kg / cm 2< is applied to form the carbonaceous raw material into a desired shape, and further, the obtained formed body is sintered and graphitized. Examples of self-sintering carbonaceous powder include bulk mesophase, mesocarbon microbeads, and petroleum-based or coal-based raw coke.

[0004] Thus, conventionally, carbon agglomerate is produced by using coal having caking properties (caking coal) as the main carbonaceous raw material, adding a binder such as pitch as required, and agglomerating the particles of the carbonaceous powder by bonding or fusing together using liquid-phase components such as the caking coal and the binder. In conventional production processes, when caking coal, pitch, or the like is not used, carbon agglomerate is not obtained, and even when carbon agglomerate is obtained, there is a problem of sufficient strength not being obtained. In particular, among carbon agglomerates, coke used in a blast furnace process is required to have high strength. For this reason, conventionally, coal having caking properties (caking coal) has been used as a raw material for producing coke used in blast furnaces.

[0005] Non-Patent Literature (NPL) 1 describes results of a study on coke strength (strength of blast furnace coke) using an indirect tensile strength test method.CITATION LISTPatent Literature

[0006] PTL 1: JP S61-122110 ANon-Patent Literature

[0007] NPL 1: Tsugio Miyagawa et al., A Study on The Tensile Strength of Coke (I), Journal of the Fuel Society of Japan, Vol. 54, No. 584, 1975, pp. 983-993SUMMARY(Technical Problem)

[0008] In recent years, depletion of caking coal suitable for producing coke is occurring, and there is a demand for expanded use of carbonaceous materials that do not exhibit softening and melting property, such as coal with poor softening and melting property and materials with high carbon content derived from coal or biomass, that have not been used in coke production until now. However, in order to increase a usage amount of carbonaceous raw material that has poor softening and melting property, producing carbon agglomerate by an agglomeration technique only using a liquid-phase component to bond or fuse particles together is difficult. For this reason, there is a need for a method of producing carbon agglomerate using a mechanism different from that of conventional methods.

[0009] In view of the above, it would be helpful to provide a method of producing carbon agglomerate that can produce high-strength carbon agglomerate that can withstand use in a blast furnace, even when a usage amount of carbonaceous raw material that has poor softening and melting property is increased.(Solution to Problem)

[0010] The inventors have conducted intensive studies to solve the above problem and have discovered the following.

[0011] First, the inventors discovered a technique for agglomerating carbonaceous powder based on a new mechanism that does not rely on a liquid-phase sintering phenomenon in which a liquid phase component that can become volatile is used to bond or fuse carbonaceous powder particles together. That is, the inventors have discovered that by pressure molding a defined carbonaceous powder while heating at a temperature range of 600 °C or more, the particles of the carbonaceous powder are bonded to each other by a solid-phase sintering phenomenon, as opposed to conventional agglomeration by a liquid-phase sintering phenomenon, and carbon agglomerate can be produced. Specifically, the defined carbonaceous powder is a fine carbonaceous powder in which the amount of volatile matter is appropriately controlled and the maximum particle size is 300 µm or less. Conventional agglomeration using a liquid-phase sintering phenomenon occurs when caking coal powder is heated to 400 °C to 500 °C to soften and melt the powder. In contrast, according to the present disclosure, a defined carbonaceous powder is pressure molded at a temperature higher than 500 °C, so that adhesion between particles of the carbonaceous powder progresses in a solid-phase sintering-like process. Further, in conventional agglomeration using a liquid-phase sintering phenomenon, excessive particle size reduction of caking coal leads to a decrease in plasticity and was therefore considered inappropriate. On the other hand, carbonaceous powder that has a small particle size is suitable for agglomeration by a solid-phase sintering-like phenomenon.

[0012] However, even when carbonaceous powders having the same volatile content and particle size are subjected to pressure molding, there are cases where high strength sufficient to withstand use in a blast furnace is obtained and cases where it is not. Therefore, the inventors further investigated carbonaceous powder that can be used to obtain high-strength carbon agglomerate by a solid-phase sintering-like phenomenon. As a result, the inventors discovered that when the carbonaceous powder used in the pressure molding has many oxygen-containing functional groups, it is possible to obtain carbon agglomerate having high strength capable of withstanding use in a blast furnace.

[0013] The present disclosure is made based on these discoveries, and primary features are as described below. [1] A method of producing carbon agglomerate, the method comprising: a powder preparation process of preparing a carbonaceous powder having a volatile content of 6 wt% D.B. or more and less than 20 wt% D.B., an O / C ratio representing an atomic ratio of O atoms to C atoms of 0.040 or more, and a maximum particle size of 300 µm or less; and a hot pressing process of pressure molding the carbonaceous powder under a set of conditions including a maximum arrival temperature of 600 °C or more and 1250 °C or less in an oxygen-excluded environment to obtain the carbon agglomerate. [2] The method of producing carbon agglomerate according to [1], above, wherein molding pressure in the pressure molding is 11 MPa or more. [3] The method of producing carbon agglomerate according to [1] or [2], above, wherein the powder preparation process further comprises a process of subjecting a carbonaceous raw material to a heat treatment and an optional pulverization treatment to obtain the carbonaceous powder. (Advantageous Effect)

[0014] According to the method of producing carbon agglomerate of the present disclosure, it is possible to produce high-strength carbon agglomerate that can withstand use in a blast furnace, even when a usage amount of carbonaceous raw material that has poor softening and melting property is increased.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the accompanying drawings: FIG. 1 is a graph illustrating a relationship between volatile content of carbonaceous powder and indirect tensile strength of carbon agglomerate according to tested examples; and FIG. 2 is a graph illustrating a relationship between an O / C ratio of carbonaceous powder and indirect tensile strength of carbon agglomerate according to tested examples. DETAILED DESCRIPTION

[0016] A method of producing carbon agglomerate according to an embodiment of the present disclosure includes: a powder preparation process of preparing a carbonaceous powder having a volatile content of 6 wt% D.B. or more and less than 20 wt% D.B., an O / C ratio representing an atomic ratio of O atoms to C atoms of 0.040 or more, and a maximum particle size of 300 µm or less; and a hot pressing process of pressure molding the carbonaceous powder under a set of conditions including a maximum arrival temperature of 600 °C or more and 1250 °C or less in an oxygen-excluded environment to obtain the carbon agglomerate. In the production method according to the present embodiment, particles of carbonaceous powder are bonded to each other by a solid-phase sintering-like phenomenon rather than a liquid-phase sintering phenomenon in which liquid-phase components that may become volatile are used to bond or fuse the particles of carbonaceous powder to each other, thereby forming carbon agglomerate.[Powder preparation process]

[0017] The powder preparation process, as an example, includes a process of subjecting carbonaceous raw material to a heat treatment to obtain a heat-treated carbonaceous material, and a process of pulverizing the heat-treated carbonaceous material to obtain the carbonaceous powder.

[0018] The carbonaceous raw material to be subjected to the heat treatment and the carbonaceous powder obtained by subjecting the carbonaceous raw material to the heat treatment and optional pulverization may be, for example, one or more types selected from coal and biomass.

[0019] Biomass is a general term for a certain amount of accumulated plant and animal resources and waste derived from these resources (excluding fossil resources). Biomass according to the present embodiment includes any biomass that produces charcoal when pyrolyzed, such as agricultural, forestry, livestock, fisheries, and waste biomass.

[0020] According to the present embodiment, the biomass used as the carbonaceous raw material preferably includes biomass that effectively generates a high amount of heat, for example, woody biomass.

[0021] Examples of woody biomass include forestry-derived biomass such as papermaking by-products such as pulp black liquor and chip dust, sawmill by-products such as bark and sawdust, forest residual material such as branches, leaves, tops, and cut off lumber, thinned timber such as cedar, cypress, and pine, material from specialty forest products such as waste logs from edible fungi cultivation, firewood and charcoal trees such as castanopsis, oak, and pine, and short-rotation forestry trees such as willow, poplar, eucalyptus, and pine. Further examples of woody biomass include general waste such as pruned branches from city or town roadside trees and private garden trees, pruned branches from national or prefectural roadside trees and corporate garden trees, and industrial waste such as construction and building waste.

[0022] Some agricultural biomass such as rice husks, wheat straw, rice straw, sugarcane waste, and palm oil, which are classified as agricultural biomass and are generated from waste or by-products, and rice bran, rapeseed, soybeans, and the like that are generated from energy crops, can also be suitably used as woody biomass.

[0023] According to the present embodiment, it is important that the volatile content of the carbonaceous powder to be subjected to pressure molding is 6 wt% D.B. or more and less than 20 wt% D.B.

[0024] When the volatile content of the carbonaceous powder is less than 6 wt% D.B., it is not possible to obtain carbon agglomerate having a high strength sufficient for use in a blast furnace. The solid-phase sintering-like phenomenon in the method of producing carbon agglomerate according to the present embodiment is believed to be driven by the aromatization or polycyclization reaction of the carbonaceous powder. This reaction involves the release of hydrogen and other functional groups, and is accompanied by gas generation. That is, volatile content of carbonaceous powder corresponds to the amount of hydrogen and other functional groups released when aromatization or polycyclization reactions occur, which drive the solid-phase sintering-like phenomenon, and indicates the potential for the solid-phase sintering-like phenomenon. Accordingly, when the volatile content is less than 6 wt% D.B., it is not possible to obtain carbon agglomerate having sufficient strength. Therefore, the volatile content of the carbonaceous powder is 6 wt% D.B. or more. The volatile content of the carbonaceous power is preferably 8 wt% D.B. or more.

[0025] On the other hand, when the volatile content of the carbonaceous powder is 20 wt% D.B. or more, the gas generated during heating (gas generated by volatilization of the volatile content or gas generated by decomposition of the volatile content) expands, and the carbonaceous powder foams. Therefore, compression of the carbonaceous powder during pressure molding is hindered, the carbonaceous powder cannot be sufficiently pressurized, and formation of bonds between particles of the carbonaceous powder due to the solid-phase sintering-like phenomenon is hindered. Further, there is a risk that the internal pressure of a space surrounded by walls of a mortar, press mold, or the like during pressure molding may exceed the pressure to be applied to the carbonaceous powder due to gas generated during heating. In such a case, there is a possibility that the pressure molding apparatus or wall may be damaged. Further, gas generated during heating can more easily cause walls to become fouled. Accordingly, the volatile content of the carbonaceous powder is less than 20 wt% D.B. The volatile content of the carbonaceous power is preferably 18 wt% D.B. or less.

[0026] Further, when the volatile content of the carbonaceous powder is in the range from 6 wt% D.B. to less than 20 wt% D.B., the greater the volatile content, the stronger the carbon agglomerate that can be obtained.

[0027] According to the present embodiment, the volatile content of the carbonaceous powder is a value measured in accordance with "Coal and coke - Methods for proximate analysis" (JIS M 8812:2004) specified according to the Japanese Industrial Standard (JIS).

[0028] The carbonaceous powder according to the present embodiment is obtainable through a process of subjecting a carbonaceous raw material to a heat treatment to obtain a heat-treated carbonaceous material having a volatile content of 6 wt% D.B. or more and less than 20 wt% D.B., and a process of pulverizing the heat-treated carbonaceous material. However, the order of pulverization and heat treatment is not limited to this, and carbonaceous raw material may be pulverized and then heat-treated to obtain the carbonaceous powder. Further, when carbonaceous raw material is originally in powder form, it may be possible to heat-treat the carbonaceous raw material without pulverization to produce the carbonaceous powder having a volatile content of 6 wt% D.B. or more and less than 20 wt% D.B. Alternatively, carbonaceous raw material may be roughly ground, heat-treated to reduce the volatile content to 6 wt% D.B. or more and less than 20 wt% D.B., and then pulverized to obtain the carbonaceous powder.

[0029] Heat treatment of carbonaceous raw material is preferably carried out by heating the carbonaceous raw material to a heat treatment temperature of 500 °C or more and less than 900 °C in an oxygen-excluded environment. When the heat treatment temperature is less than 500 °C, volatile matter remaining in the carbonaceous powder becomes 20 wt% D.B. or more. The heat treatment temperature is therefore preferably 500 °C or more. The heat treatment temperature is more preferably 600 °C or more. On the other hand, when the heat treatment temperature is 900 °C or more, the volatile matter remaining in the carbonaceous powder becomes less than 6 wt% D.B., and the solid-phase sintering-like phenomenon becomes less likely to occur. The heat treatment temperature is therefore preferably less than 900 °C. The heat treatment temperature is more preferably 800 °C or less. Further, in the range of the heat treatment temperature from 500 °C to less than 900 °C, the lower the heat treatment temperature, the more volatile matter in the carbonaceous powder, and therefore the solid-phase sintering-like phenomenon is strongly expressed in the hot pressing process, and high-strength carbon agglomerate is obtainable.

[0030] The heat treatment of the carbonaceous raw material is preferably carried out in an atmosphere in which the supply of oxygen is blocked. The heat treatment of the carbonaceous raw material may be carried out, for example, in a state where the carbonaceous raw material is accommodated in a vessel that forms a space where the inflow of air is prevented and through which an inert gas is flowed. The heat treatment of the carbonaceous raw material may be carried out by heating a vessel containing the carbonaceous raw material and heat transfer from the vessel.

[0031] Normally, the reaction rate of the pyrolysis reaction of carbonaceous raw material in the heat treatment is fast, and therefore the time required for completion of the pyrolysis reaction is short. The heat treatment time is therefore preferably 1 minute or longer. The heat treatment time is more preferably 10 minutes or more. This eliminates a temperature difference between carbonaceous raw material and the vessel, allowing the carbonaceous raw material to be uniformly heat-treated in its entirety. Further, it is possible to reliably raise the temperature of the entire carbonaceous raw material to the heat treatment temperature (that is, heat evenly) to carry out the heat treatment, and thus it is possible to suppress variation in the quality of the heat-treated carbonaceous material and the carbonaceous powder. There is no particular upper limit for the heat treatment time, but when the heat treatment time is too long, the energy required for the heat treatment increases, which undesirably increases costs. The heat treatment time is normally sufficient when 60 minutes or shorter.

[0032] The heat treatment time refers to the time during which the temperature of carbonaceous raw material is maintained at a defined heat treatment temperature of 500 °C or more and less than 900 °C from the time the temperature of the carbonaceous raw material reaches the heat treatment temperature.

[0033] The heat treatment may be carried out using a heating apparatus such as an electric furnace, a rotary kiln, a fluidized bed heating furnace, a screw-type heating furnace, a shaft furnace, a dry distilling furnace, or the like.

[0034] Further, when using carbonaceous raw material having a volatile content of 6 wt% D.B. or more and less than 20 wt% D.B. before the heat treatment, it is not necessarily required to carry out the heat treatment in advance, and it may be sufficient to pulverize the carbonaceous raw material into carbonaceous powder. However, in order to suppress the generation of water vapor in the hot pressing process described below, it is desirable to carry out a heat treatment to decrease moisture content.

[0035] According to the present embodiment, it is important that the maximum particle size of the carbonaceous powder to be subjected to pressure molding is 300 µm or less. When the carbonaceous powder contains a large amount of coarse particles having a particle size exceeding 300 µm, the coarse particles may remain in carbon agglomerate and cause a decrease in strength. In the subsequent hot pressing process, the smaller the particle size of the carbonaceous powder, the more the solid-phase sintering-like phenomenon that causes bonding between particles of the carbonaceous powder is promoted. Accordingly, remaining coarse particles inhibit bonding between particles, causing a decrease in strength. Further, defects are likely to form around coarse particles in carbon agglomerate, which can cause stress concentration and become initiation points of fracture when an external force is applied, resulting in a decrease in strength.

[0036] The maximum particle size of the carbonaceous powder is preferably 100 µm or less. When the particle size of the carbonaceous powder is appropriately small, the physical structure in the carbon agglomerate becomes dense and uniform, which contributes to increasing the strength of the carbon agglomerate.

[0037] The smaller the particle size of the carbonaceous powder, the more the strength of the carbon agglomerate is improved, and therefore smaller particle size is preferred. Accordingly, there is no particular lower limit for the maximum particle size of the carbonaceous powder. However, taking productivity into consideration, setting the maximum particle size of the carbonaceous powder to less than 20 µm increases the cost of fine pulverization, while the improvement in performance of the carbon agglomerate is limited. Accordingly, when the maximum particle size of the carbonaceous powder is 20 µm or more, carbon agglomerate having sufficient strength can be produced.

[0038] According to the present embodiment, the particle size and particle size distribution (volume basis) of the carbonaceous powder can be measured using a commercially available particle size distribution measurement device. For example, a laser diffraction / scattering type particle size distribution measuring instrument "Laser Mastersizer LMS-3000", produced by Malvern Panalytical Ltd., may be used. In the particle size distribution of the carbonaceous powder, the particle size (circle-equivalent particle diameter) that accounts for 95 % of the particles in the carbonaceous powder, calculated from the smallest particles, is defined as the maximum particle size.

[0039] A fine pulverizing method and a fine pulverizing apparatus for pulverizing the carbonaceous raw material or the heat-treated carbonaceous material are not particularly limited. As the fine pulverizing apparatus, a media mill such as a cutter mill, a hammer mill, a pin mill, a jet mill, or a ball mill may be used. The fine pulverizing apparatus is not limited to an apparatus that carries out only fine pulverization, and for example, a pulverizer with a built-in classifier may be used.

[0040] According to the present embodiment, it is important that the O / C ratio, which represents the atomic ratio of O atoms to C atoms in the carbonaceous powder to be subjected to pressure molding, is 0.040 or more. In other words, the carbonaceous powder that is the raw material for the carbon agglomerate has many oxygen-containing functional groups. In the method of producing carbon agglomerate by a solid-phase sintering-like phenomenon according to the present embodiment, the carbonaceous powder subjected to pressure molding having a large number of oxygen-containing functional groups is an important factor in strength development.

[0041] The solid-phase sintering-like phenomenon in the method of producing carbon agglomerate according to the present embodiment is believed to be driven by an aromatization or polycyclization reaction of the carbonaceous powder, as described above. Accordingly, in order to form bonds between particles of the carbonaceous powder, the reaction needs to proceed across particles. A large O / C ratio means that the carbonaceous powder has many oxygen-containing functional groups that can be released when heated. The release of oxygen-containing functional groups contributes to a polycyclization reaction of aromatic rings, and therefore it is believed that the more oxygen-containing functional groups in a carbonaceous powder, the more the polycyclization reaction is promoted and the more likely the solid-phase sintering-like phenomenon is to occur.

[0042] Normally, the aromatization or polycyclization reaction of carbonaceous powder proceeds at the edges of crystallites that are composed of relatively planar aromatic ring precursors or layers of monocyclic or polycyclic aromatic carbons. For this reason, in order to form bonds between particles, it is necessary to increase the number of states in which the edges of the crystallites between adjacent particles are close to each other and the stacking direction is aligned. On the other hand, the particles of the carbonaceous powder form a packed bed in the stage prior to agglomeration, but the orientation of each particle cannot be arbitrarily controlled, and so the particles are arranged randomly. Further, it is believed that the solid-phase sintering-like phenomenon occurs only at limited contact surfaces between particles in the packed bed. That is, it is difficult to intentionally align crystallites present on the surfaces of the particles facing each other at contact surfaces. Oxygen-containing functional groups are thought to be present at the edges of crystallites, and therefore it is believed that using a carbonaceous powder containing a large number of oxygen-containing functional groups increases the likelihood that the edges of crystallites at contact surfaces between adjacent particles will come into close proximity with each other. Further, it is believed that the release of oxygen-containing functional groups present at the edges of crystallites promotes the polycyclization reaction of aromatic rings, and therefore it is preferable to use a carbonaceous powder containing a large number of oxygen-containing functional groups in the method of producing carbon agglomerate according to the present disclosure using the solid-phase sintering-like phenomenon.

[0043] When the O / C ratio of the carbonaceous powder is 0.040 or more, oxygen-containing functional groups necessary for promoting the reaction are sufficiently present, and carbon agglomerate having high strength is obtainable by the solid-phase sintering-like phenomenon. When the O / C ratio of the carbonaceous powder is less than 0.040, the solid-phase sintering-like phenomenon does not proceed sufficiently, making it difficult to obtain carbon agglomerate having high strength. Accordingly, it is important that the O / C of the carbonaceous material powder is 0.040 or more. The O / C of the carbonaceous powder is more preferably 0.045 or more. The higher the O / C of the carbonaceous powder, the better, and therefore an upper limit is not particularly limited. The O / C of the carbonaceous powder may be 0.12 or less.

[0044] Carbonaceous powder having an O / C ratio of 0.040 or more is obtainable, for example, by pulverizing heat-treated carbonaceous material obtained by heat-treating carbonaceous raw material that has not been significantly subjected to carbonization, such as coal that has a low degree of coalification or biomass. The caking coal used in normal coke production has a decreased O / C ratio due to release of carbon dioxide as gas through heating and pressurization during the carbonization process. Therefore, both the heat-treated carbonaceous material obtained by heat-treating this coal, and the carbonaceous powder obtained by further pulverization, have a low O / C ratio and therefore the solid-phase sintering-like phenomenon is less likely to occur. On the other hand, coal that has a low degree of coalification and biomass have not been significantly subjected to carbonization, and therefore release of carbon dioxide as gas is not promoted. Therefore, the heat-treated carbonaceous material obtained by heat-treating such carbonaceous raw material, as well as the carbonaceous powder obtained by pulverizing the material, have a large O / C ratio, and it is thought that the solid-phase sintering-like phenomenon is advantageously promoted.

[0045] According to the present embodiment, the atomic ratio O / C of O atoms to C atoms of the carbonaceous powder is calculated from the results of quantifying O and C according to "Coal and coke - Determination of constituents" (JIS M 8813:2004) specified in JIS.[Hot pressing process]

[0046] In the hot pressing process, the carbonaceous powder obtained in the powder preparation process is pressure molded under a set of conditions including a maximum arrival temperature of 600 °C or more and 1250 °C or less in an oxygen-excluded environment to obtain carbon agglomerate.

[0047] In the hot pressing process, the carbonaceous powder is mechanically pressed and molded, that is, pressure molded. Mechanical pressure refers to compressing the carbonaceous powder with a physical wall such as a pestle and mortar, a press mold, or a compression roller.

[0048] In the hot pressing process, the carbonaceous powder is pressed while being heated (that is, hot pressed). Pressurizing the carbonaceous powder during the heating process includes a case where pressurization is carried out only during a part of the entire process of heating the carbonaceous powder, and a case where pressurization is carried out during the entire process. Heating the carbonaceous powder means, in other words, a state in which the temperature of the carbonaceous powder is increased.

[0049] The hot press apparatus for pressure molding while heating the carbonaceous powder is not particularly limited. The carbonaceous powder may be pressed by storing the carbonaceous powder in a space surrounded by a wall such as described above (for example, in a mold for hot pressing) and compressing the powder through the wall. A heat source for heating the carbonaceous powder may be, for example, electric resistance heating, microwave heating, or high-frequency induction heating.

[0050] When the carbonaceous powder is heated, the carbonaceous powder undergoes thermal expansion. Accordingly, the bulk density of the packed bed of the carbonaceous powder decreases. In contrast, by heating the carbonaceous powder while applying pressure, the packed bed of the carbonaceous powder can be compressed against thermal expansion, increasing the number of contact points between the particles of the carbonaceous powder, and promoting the solid-phase sintering-like phenomenon.

[0051] In the hot pressing process, the carbonaceous powder is heated in an atmosphere in which the supply of oxygen is blocked, so that the particles of the carbonaceous powder are bonded to each other by the solid-phase sintering-like phenomenon. An atmosphere in which the supply of oxygen is blocked is, for example, an atmosphere in a space where the inflow of air (oxygen) is prevented and an inert gas is circulated. In an environment where oxygen is supplied, the carbonaceous powder burns and disappears.

[0052] The carbonaceous powder may be heated through the wall in the hot pressing process. In the hot pressing process, the temperature of the carbonaceous powder at the start of pressing the carbonaceous powder is referred to as the "molding start temperature", and the maximum temperature of the carbonaceous powder during the pressing period is referred to as the "maximum arrival temperature".

[0053] The maximum arrival temperature in the hot pressing process needs to be 600 °C or more and 1250 °C or less. This is because in this temperature range, bonding between particles occurs significantly due to the solid-phase sintering-like phenomenon.

[0054] When the maximum arrival temperature in the hot pressing process is less than 600 °C, bonding between particles due to the solid-phase sintering-like phenomenon does not proceed sufficiently. From the viewpoint of sufficiently promoting the bonding between particles by the solid-phase sintering-like phenomenon, the maximum arrival temperature is 600 °C or more. The maximum arrival temperature is preferably 700 °C or more. The maximum arrival temperature is more preferably 900 °C or more.

[0055] When the maximum arrival temperature in the hot pressing process exceeds 1250 °C, hetero elements are removed, the bonding between the particles does not progress sufficiently, and the formation of bonds between the particles due to the solid-phase sintering-like phenomenon is inhibited. The maximum arrival temperature is therefore 1250 °C or less. The maximum arrival temperature is preferably 1100 °C or less.

[0056] After the hot pressing process, a carbonization treatment may be further carried out. That is, after the pressure molding, the load may be removed, and the carbon agglomerate may be heated without being pressurized to carry out the carbonization treatment. In such a case, from the viewpoint of increasing the strength of the carbon agglomerate, carbonization temperature (the maximum temperature of the carbon agglomerate during the carbonization treatment) is preferably higher than the maximum arrival temperature in the hot pressing process. However, for the same reasons as those for the maximum arrival temperature in the hot pressing process, the carbonization temperature is 1250 °C or less. The carbonization temperature is preferably 1100 °C or less. According to the present embodiment, the heating in the hot pressing process can also serve as a carbonization treatment, and therefore carbonization treatment after the hot pressing is optional.

[0057] When the carbonaceous powder is heated while being pressure molded, the molding start temperature in the hot pressing process is preferably low, and is typically room temperature (for example, 10 °C or more and 35 °C or less). This allows a wider temperature range for the carbonaceous powder in the hot pressing process and a longer reaction time. That is, it is preferable to start pressing when or immediately after heating of the carbonaceous powder is started.

[0058] The heating rate from the molding start temperature to the maximum arrival temperature is preferably 1 °C / min or more. The heating rate is preferably 30 °C / min or less. By setting the heating rate to be 1 °C / min or more and 30 °C / min or less, a strength decrease can be avoided.

[0059] Holding time at the maximum arrival temperature is preferably 1 minute or more from the viewpoint of suppressing variation in strength due to temperature unevenness in the carbon agglomerate. Further, even when the maximum arrival temperature is held for a long period of time, almost no further improvement in performance is observed, while there is a problem of reduced productivity, and therefore the holding time is preferably 60 minutes or less.

[0060] In the hot pressing process, the pressure that is mechanically applied to the carbonaceous powder is referred to as molding pressure. The higher the molding pressure, the more the number of contact points between particles of the carbonaceous powder increases, promoting the solid-phase sintering-like phenomenon. Accordingly, the higher the molding pressure, the stronger the carbon agglomerate. When the molding pressure is 11 MPa or more, the strength of the carbon agglomerate becomes stable. When the molding pressure is less than 11 MPa, obtaining carbon agglomerate with high strength may not be possible. The molding pressure is therefore preferably 11 MPa or more. The molding pressure is more preferably 20 MPa or more. However, when the molding pressure is too high, production costs may increase. A molding pressure of 300 MPa or less is sufficient.[Carbon agglomerate]

[0061] According to the method of producing carbon agglomerate of the present embodiment, it is possible to produce carbon agglomerate having high strength capable of withstanding use in a blast furnace without use of a liquid-phase component. According to the present embodiment, when strength is 4 MPa or more, the carbon agglomerate is evaluated as having a strength sufficient for use in a conventional blast furnace process (that is, high-strength coke). According to the present embodiment, the strength of the carbon agglomerate refers to cold indirect tensile strength measured by the method described in NPL 1.

[0062] According to the present embodiment, "carbon agglomerate" refers to an agglomerate that is mainly carbon, is produced by the production method according to the present embodiment, and has a carbon content of 70 wt% D.B. or more and 100 wt% D.B. or less.EXAMPLES

[0063] The carbonaceous raw materials listed in Table 1 were subjected to heat treatment to obtain heat-treated carbonaceous materials. The heat treatment was carried out in an electric furnace through which nitrogen gas was circulated, under conditions in which the carbonaceous raw materials were heated to the heat treatment temperatures listed in Table 1 and then held for 30 minutes.

[0064] The heat-treated carbonaceous materials were then pulverized to obtain carbonaceous powders having maximum particle sizes listed in Table 1. The pulverization treatment was carried out using an ultra centrifugal mill (Model: ZM 200, produced by Verder Scientific GmbH). Table 1 also lists the volatile content and O / C of the carbonaceous powders.

[0065] In each case, 1.32 g of the carbonaceous powder was filled into a mold (a press mold having a diameter of 12 mm), and the carbonaceous powder was subjected to pressure molding under the pressure molding conditions listed in Table 1. For a case where the molding start temperature was room temperature, the carbonaceous powder was compressed at room temperature with the molding pressure listed in Table 1, and the temperature of the carbonaceous powder was increased at a heating rate of 20 °C / min until the maximum arrival temperature listed in Table 1 was reached while applying the molding pressure listed in Table 1 under a nitrogen gas flow. Further, for a case where the molding start temperature was other than room temperature, the carbonaceous powder was heated at a heating rate of 20 °C / min under nitrogen gas flow until the molding start temperature listed in Table 1 was reached without applying molding pressure, and then the carbonaceous powder was heated at a heating rate of 20 °C / min until the maximum arrival temperature listed in Table 1 was reached while applying the molding pressure listed in Table 1. Further, the carbonaceous powder was held at the maximum arrival temperature listed in Table 1 for 5 minutes. The resulting carbon agglomerate was then allowed to cool down and recovered.

[0066] In a case where the maximum arrival temperature under the pressure molding conditions listed in Table 1 was less than 900 °C, a separate carbonization treatment was carried out after the pressure molding. That is, after the molding pressure was released at the maximum arrival temperature, the carbonaceous powder was heated at a heating rate of 20 °C / min until the carbonization temperature listed in Table 1 was reached, and was held at that carbonization temperature for 5 minutes. The resulting carbon agglomerate was then allowed to cool down and recovered.

[0067] The indirect tensile strength of the resulting carbon agglomerate was calculated. The indirect tensile strength was measured according to the method described in NPL 1. Table 1 lists the indirect tensile strength of the carbon agglomerate for each case. FIG. 1 is a graph illustrating a relationship between the volatile content of the carbonaceous powder and the indirect tensile strength of the carbon agglomerate according to tested examples (Examples No. 1 to 9 and Comparative Examples No. 10 to 18). Further, FIG. 2 is a graph illustrating a relationship between the O / C ratio of the carbonaceous powder and the indirect tensile strength of the carbon agglomerate according to tested examples (Examples No. 1 to 9 and Comparative Examples No. 10 to 18). For the Examples No. 1 to 9, 19, and 21 to 24, carbon agglomerate having a strength of 4 MPa or more that can withstand use in a blast furnace was obtained. In contrast, for Comparative Examples No. 10 and 11, in which the volatile content of the carbonaceous powder was less than 6 %, and for Comparative Examples No. 12 to 18, in which the O / C ratio of the carbonaceous powder was less than 0.040, the strength of the carbon agglomerate was less than 4 MPa. Further, for Comparative Example No. 20, in which the maximum arrival temperature during pressure molding was changed to 500 °C from Example No. 2, no agglomeration occurred.[Table 1]

[0068] Table 1No.Carbonaceous raw materialHeat treatment temp.Carbonaceous powderPressure molding conditionsCarbonization treatmentCarbon agglomerateClassificationVolatile contentO / CMaximum particle sizeMolding start temperatureMaximum arrival temp.Molding pressureCarbonization temperatureIndirect tensile strength(°C)(wt% D.B.)-(µm)-(°C)(MPa)(°C)(MPa)1bituminous coal60017.330.07759room temp.100050-11.63Example2lignite coal60016.980.08676room temp.100050-11.75Example3bituminous coal / biomass600 / 60015.840.07567room temp.100050-9.76Example4biomass60014.350.07276room temp.100050-11.26Example5bituminous coal70013.200.05152room temp.100050-7.12Example6lignite coal70010.320.06276room temp.100050-6.85Example7biomass6009.040.05286room temp.100050-6.66Example8biomass7008.860.04976room temp.100050-5.34Example9biomass8006.430.04276room temp.100050-4.13Example10biomass9004.750.02867room temp.100050-1.54Comparative Example11bituminous coal9004.300.01876room temp.100050-0.98Comparative Example12bituminous coal60014.720.03967room temp.100050-3.00Comparative Example13bituminous coal60013.180.03876room temp.100050-2.18Comparative Example14bituminous coal60012.020.03467room temp.100050-3.88Comparative Example15bituminous coal60011.620.02067room temp.100050-2.42Comparative Example16bituminous coal60011.000.01859room temp.100050-2.57Comparative Example17bituminous coal60010.400.02059room temp.100050-3.33Comparative Example18bituminous coal7007.230.01767room temp.100050-2.26Comparative Example19lignite coal60016.980.08676600100050-8.87Example20lignite coal60016.980.08676room temp.500501000no agglomerationComparative Example21lignite coal60016.980.08676room temp.8005010006.40Example22lignite coal60016.980.08676room temp.100020-4.43Example23lignite coal60016.980.08676room temp.100035-8.31Example24lignite coal60016.980.086211room temp.100050-6.48ExampleNote: underlining indicates a value outside the scope of the present disclosure. INDUSTRIAL APPLICABILITY

[0069] According to the method of producing carbon agglomerate of the present disclosure, it is possible to produce high-strength carbon agglomerate that can withstand use in a blast furnace, even when a usage amount of carbonaceous raw material that has poor softening and melting property is increased.

Claims

1. A method of producing carbon agglomerate, the method comprising: a powder preparation process of preparing a carbonaceous powder having a volatile content of 6 wt% D.B. or more and less than 20 wt% D.B., an O / C ratio representing an atomic ratio of O atoms to C atoms of 0.040 or more, and a maximum particle size of 300 µm or less; and a hot pressing process of pressure molding the carbonaceous powder under a set of conditions including a maximum arrival temperature of 600 °C or more and 1250 °C or less in an oxygen-excluded environment to obtain the carbon agglomerate.

2. The method of producing carbon agglomerate according to claim 1, wherein molding pressure in the pressure molding is 11 MPa or more.

3. The method of producing carbon agglomerate according to claim 1 or 2, wherein the powder preparation process further comprises a process of subjecting a carbonaceous raw material to a heat treatment and an optional pulverization treatment to obtain the carbonaceous powder.

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