EP4779040A1

EP4779040A1 - Carbide molded body and production method therefor

Info

Publication number: EP4779040A1
Application number: EP23952208.9A
Authority: EP
Inventors: Atsushi Yumoto; Toshikazu Aoki; Masahiro Sekiya; Tetsuya Kato; Muneyuki Kamata
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-09-12
Filing date: 2023-09-12
Publication date: 2026-07-22
Also published as: WO2025057314A1; CN121511316A; KR20260022463A

Abstract

There is provided a carbide molded body formed by molding carbide granules together with a binder, in which carbides in the carbide molded body satisfy (a) and (b): (a) a maximum particle size is less than one-half of a representative length d of the carbide molded body; and (b) when the carbides are classified into 70 vol% on a large-particle side and 30 vol% on a small-particle side, a ratio of a median particle size D50 of the carbides on the large-particle side to the median particle size D50 of the carbides on the small-particle side is from1.5 to 2.3. There is provided a method for producing the carbide molded body, the method including subjecting a carbide raw material to sieving treatment and crushing treatment such that the carbide granules have a particle size distribution the same as described in the above (a) and (b).

Description

TECHNICAL FIELD

The present invention relates to a carbide molded body and a method for producing the same, and more particularly, to a carbide molded body and a method for producing the same that can increase packing density of carbides serving as a raw material of the carbide molded body to enhance strength while optimizing an amount of binder added.

BACKGROUND ART

A converter process in steelmaking is a main process for producing molten steel, in which decarburization is carried out by blowing high-purity oxygen at high speed onto hot metal tapped from a blast furnace. At the same time, auxiliary raw materials mainly composed of quicklime are added to remove impurities such as phosphorus contained in the hot metal.
In addition, a hot metal pretreatment process may be introduced prior to the converter process to remove elements such as sulfur and phosphorus from hot metal, in order to meet material property requirements for steel products. In this case, there arises a problem in that a temperature of the hot metal decreases due to the hot metal pretreatment process.
In recent years, from the viewpoint of environmental protection, reduction of CO₂ emissions has become an important issue in ironmaking. In order to solve this issue, attempts have been made to increase a blending ratio of a cold iron source, such as iron scrap, used as an iron source in steelmaking, and thereby decrease a blending ratio of hot metal. This is because, when producing hot metal in a blast furnace in the manufacture of steel products, a large amount of energy is required for reducing and melting iron ore, and at the same time, a large amount of CO₂ is emitted. Further, when a part of hot metal in the steelmaking is replaced with a cold iron source, the cold iron source requires only heat for melting, and does not require the energy corresponding to the heat of reduction of iron ore, thereby enabling a substantial decrease in CO₂ emissions. However, in the converter, the heat source for melting the cold iron source is sensible heat possessed by the hot metal, as well as heat of oxidation of carbon and silicon contained in the hot metal, and therefore there is naturally a limit to the amount of cold iron source that can be melted within the range of such sensible heat and oxidation heat.
Accordingly, in dephosphorization treatment or decarburization refining of hot metal, a carbide molded body is used which supplies an additional carbon source to the hot metal in order to increase the thermal margin of the hot metal and thereby increase the blending ratio of the cold iron source. For example, various carbide molded bodies have been proposed in which coal, coke powder, graphite, electrode powder, or SiC is molded into a lump form. It should be noted that natural earthy graphite, which is a mineral product used without molding, may also be used as a carbon source because it is relatively inexpensive.
As a method for producing such a carbide molded body, for example, Patent Literature 1 discloses an invention in which carbon powder (such as coal, coke powder, or graphite) is used as a raw material, in which 30% to 70% of the carbon powder has a particle size of 1 mm or less, and the remainder has a particle size of 1 mm to 8 mm. In this invention, a binder such as polyvinyl alcohol, carboxymethyl cellulose, or α-starch is added to the carbon powder (raw material) in an amount from 0.5 wt% to 1.0 wt% relative to the weight of the carbon powder, the moisture content is adjusted to 10% or less and kneaded into a mixture, and subsequently the mixture is subjected to high-pressure molding and further dried. In this manner, the invention described in the above literature makes it possible to mold the carbon powder into a solid having sufficient strength.
Patent Literature 2 discloses an invention of a converter heating material formed by molding biochar, which is obtained by carbonizing carbon-neutral plant-based biomass, together with a binder such as starch, carboxymethyl cellulose, or cornstarch in an amount from 1 mass% to 15 mass% relative to the mass of the biochar. The invention described in the above literature, in contrast to conventional converter heating materials which use fossil resources such as coal, coke, or graphite as raw materials, provides an alternative converter heating material, thereby making it possible to reduce consumption of fossil resources and to reduce generation of CO₂, which is a greenhouse gas. At the same time, by setting the particle size of the carbides to 3 mm or less, the material is formed into a solid having crushing strength of 490 N/piece or more, which is sufficient for use as a converter heating material.

CITATION LIST

PATENT LITERATURE(S)

Patent Literature 1: JP H 2-270922 A
Patent Literature 2: JP Patent No. 5846289

SUMMARY OF THE INVENTION

PROBLEM(S) TO BE SOLVED BY THE INVENTION

However, since the carbon powder used as the raw material for the carburizing material for steelmaking described in Patent Literature 1 has a large percentage, namely from 30% to 70%, of particles having a size of 1 mm or less, there is a problem in that the processing load for particle size adjustment, such as pulverization treatment of the carbon raw material, is high.
In addition, the carbon powder used as the raw material for the converter heating material described in Patent Literature 2 is required to be classified into a relatively narrow particle size range of 3 mm or less, and, similar to the invention described in Patent Literature 1, there is a problem in that the processing load for particle size adjustment, such as pulverization treatment of the carbon raw material, is high.
In the carburizing material for steelmaking described in Patent Literature 1, a binder is used in an amount from 0.5 wt% to 1.0 wt% relative to the weight of the carbon powder. In the converter heating material described in Patent Literature 2, a binder is used in an amount from 1 mass% to 15 mass% relative to the mass of the carbide. However, in recent years, fluctuations in the price of plant-derived binders, specifically in the price of cornstarch derived from corn, have increased. This is due to price fluctuations linked to poor corn harvests caused by abnormal weather conditions, and to upward pressure on prices resulting from increased demand for corn-derived bioethanol for reducing environmental impact. Accordingly, there is a growing demand for technologies that reduce the amount of binder used in molding carbide molded bodies.
On the other hand, if the amount of binder added is less than the required amount, abnormalities such as insufficient strength of the molded carbon material occur, which raises problems such as operational troubles during transportation or a decrease in yield. If such problems are to be solved by ensuring the strength of the carbide molded body through complete fine pulverization of the carbides serving as the raw material, there arises a problem in that the cost for fine pulverization of the carbide increases, thereby impairing overall economic efficiency.
The invention has been made in view of the above circumstances, and an object of the invention is to provide a carbide molded body and a method for producing the same that can optimize a particle size distribution of carbides serving as a raw material to increase packing density of the carbides and thereby enhance the strength thereof, while reducing the amount of binder added.

MEANS FOR SOLVING THE PROBLEM(S)

[1] A carbide molded body formed by molding carbide granules together with a binder, in which carbides in the carbide molded body satisfy (a) and (b): (a) a maximum particle size is less than one-half of a representative length d of the carbide molded body; and (b) when the carbides are classified into 70 vol% on a large-particle side and 30 vol% on a small-particle side, a ratio of a median particle size D50 of the carbides on the large-particle side to the median particle size D50 of the carbides on the small-particle side is from1.5 to 2.3.
[2] The carbide molded body according to [1], in which the carbide molded body has crushing strength of 490 N/piece or more.
[3] A method for producing a carbide molded body, including molding carbide granules together with a binder, the method further including: carbonizing at least one material selected from the group consisting of coal, plant-based biomass, and waste plastic to produce a carbide raw material; and subjecting the carbide raw material to sieving treatment and crushing treatment such that (a') and (b') are satisfied, to provide the carbide granules prior to the molding,
- (a'): forming the carbide granules from the carbide raw material such that a maximum particle size is less than one-half of a representative length d of the carbide molded body, and classifying the carbide granules into a plurality of groups having different particle sizes; and
- (b'): formulating the carbide granules such that, when the carbide granules are classified into 70 vol% on a large-particle side and 30 vol% on a small-particle side, a ratio of a median particle size D50 of carbides on the large-particle side to the median particle size D50 of carbides on the small-particle side is from 1.5 to 2.3.
[4] The method for producing a carbide molded body according to [3], in which the carbide molded body has crushing strength of 490 N/piece or more.

According to the above aspects of the invention, by setting the particle size distribution of the carbides in the carbide molded body to a predetermined distribution, it is possible to provide a carbide molded body and a method for producing the same, in which the packing density of the carbides is increased to improve the strength thereof, while the amount of binder to be added can be reduced within an economically reasonable range.

BRIEF DESCRIPTION OF DRAWING(S)

Fig. 1 is a schematic cross-sectional view of an example of a converter facility in which a carbide molded body according to an exemplary embodiment of the invention is used.
Fig. 2 is a schematic cross-sectional view illustrating a state in which biochar is pulverized by a roll crusher used in preliminary experiments and Examples leading to the invention.
Fig. 3 is a schematic cross-sectional view illustrating a state in which pulverized biochar is compression-molded by a twin-roll briquetting machine used in the preliminary experiments and Examples leading to the invention.
Fig. 4 is a schematic cross-sectional view illustrating a state in which pulverized biochar is extrusion-molded by a pelletizer used in the preliminary experiments and Examples leading to the invention.
Fig. 5 is a diagram obtained in the preliminary experiments leading to the invention, illustrating a relationship between fractions of large particle size carbides and small particle size carbides in a carbide molded body and porosity of the carbide molded body, stratified according to the particle size ratio of the large particles to the small particles.
Fig. 6 is a diagram obtained in the preliminary experiments leading to the invention, illustrating a relationship between the ratio of the large particle size to the small particle size of carbides in a carbide molded body and the minimum porosity of the carbide molded body.
Fig. 7 is a diagram obtained in the preliminary experiments leading to the invention, illustrating a relationship between a ratio of a median particle size D50 of carbides on the large particle size side to a representative length d of a carbide molded body, and a binder concentration required for obtaining the carbide molded body.
Fig. 8A is a diagram illustrating measurement results of a particle size of pulverized biochar of Comparative, which was used in Examples of a pellet-shaped carbide molded body according to an exemplary embodiment of the invention.
Fig. 8B is a diagram illustrating measurement results of a particle size of pulverized biochar of Inventive Example, which was used in Examples of a pellet-shaped carbide molded body according to the exemplary embodiment of the invention.
Fig. 8C is a diagram illustrating measurement results of a particle size of pulverized biochar of another Inventive Example, which was used in Examples of a pellet-shaped carbide molded body according to the exemplary embodiment of the invention.
Fig. 9A is a diagram illustrating measurement results of a particle size of pulverized biochar of Inventive Example, which was used in Examples of a briquette-shaped carbide molded body according to the exemplary embodiment of the invention.
Fig. 9B is a diagram illustrating measurement results of a particle size of pulverized biochar of Comparative, which was used in Examples of a briquette-shaped carbide molded body according to the exemplary embodiment of the invention.

DESCRIPTION OF EMBODIMENT(S)

An exemplary embodiment of the invention will be described in detail below. First, among the refining facilities in which a carbide molded body according to the exemplary embodiment of the invention is used, a converter facility, which is a typical refining facility, will be described. It should be noted that the carbide molded body of the invention may be referred to as a converter heating material when used in the converter facility. Moreover, the invention is applicable to not only the converter facility but also refining facilities in general, including hot metal pretreatment facilities, and is not limited to the exemplary embodiment described below.
Fig. 1 is a schematic cross-sectional view of an example of a converter facility in which a carbide molded body according to the exemplary embodiment of the invention is used. In Fig. 1, a top-blowing lance 2 is inserted from above into an interior of a converter body 1 containing hot metal 8, and oxygen gas is blown from the top-blowing lance 2 onto the hot metal 8. At the same time, bottom-blowing gas for stirring is blown in from a plurality of bottom-blowing tuyeres 3 arranged at the bottom of the converter body 1. By blowing gases from both the upper and lower directions in this manner, the hot metal 8 and slag 9 are stirred, while decarburization refining of the hot metal 8 is carried out. Through the decarburization refining of the hot metal 8, converter off-gas 10 mainly composed of CO gas is generated from inside the furnace.
A flue 4 is installed above the converter body 1, and downstream of the flue 4, a primary dust collector (not illustrated), a secondary dust collector (not illustrated), and an induced draft fan (not illustrated) are installed in this order. By means of such treatment equipment of the converter off-gas 10, the converter off-gas 10 generated inside the converter body 1 during decarburization refining is cooled and dedusted, and, in an unburned state, is recovered in a gas holder (not illustrated) located downstream of the induced draft fan (not illustrated).
A side of the flue 4 connected to a throat of the converter body 1 is referred to as a skirt 5, which has a structure capable of moving up and down. When the off-gas is recovered, the skirt 5 and the throat of the converter body 1 are, in principle, kept in close contact with each other. In addition, an auxiliary material charging device is installed in the flue 4 in order to charge auxiliary materials such as quicklime, calcined dolomite, iron ore, mill scale, manganese ore, carbide molded bodies (carbon materials such as coke and earthy graphite), and ferroalloys (such as Fe-Mn and Fe-Si) into the converter body 1. The auxiliary material charging device includes a hopper 6, a charging chute 7, and the like. The slag 9 is formed from auxiliary materials such as quicklime, calcined dolomite, iron ore, mill scale, or manganese ore charged into the furnace from the auxiliary material charging device.
The carbide molded body according to the exemplary embodiment of the invention is a carbide molded body formed by molding carbide granules together with a binder. Carbides in the carbide molded body satisfy (a): the maximum particle size is less than one-half of a representative length d of the carbide molded body. This is because, if the maximum particle size is one-half or more of the representative length d of the carbide molded body, the spacing between large particles becomes excessively wide, requiring a large liquid bridge adhesion force, and thus making it necessary to increase the binder concentration, with the result that the effect of reducing the amount of binder cannot be achieved. In addition, the carbide molded body according to the exemplary embodiment of the invention satisfies (b): when the carbides are classified into 70 vol% on a large-particle side and 30 vol% on a small-particle side, a ratio of a median particle size D50 of the carbides on the large-particle side to a median particle size D50 of the carbides on the small-particle side is from1.5 to 2.3. The reason for setting the lower limit of this ratio to 1.5 or more is that, if the ratio is less than 1.5, the minimum porosity cannot be significantly reduced as compared with the porosity when large and small particles are not blended, and thus the effect of reducing the amount of binder cannot be achieved. The lower limit of this ratio is preferably 1.6 or more in order to reliably reduce the porosity, and more preferably 1.7 or more in order to reliably obtain the effect. The reason for setting the upper limit of this ratio to 2.3 or less is that, if the ratio exceeds 2.3, the rate of change in porosity with respect to the blending ratio in the vicinity of the minimum porosity becomes large, raising concerns that abnormalities may occur in the carbide molded body due to production variations, thereby reducing the product yield. The upper limit of this ratio is preferably 2.2 or less in order to stably keep low the rate of change in porosity with respect to the blending ratio in the vicinity of the minimum porosity and thereby stably ensure the product yield, and more preferably 2.1 or less in order to more reliably obtain the effect.
The composition of the carbide molded body according to the exemplary embodiment of the invention, in a dry state excluding moisture, has 70 mass% or more of fixed carbon, 5 mass% to 20 mass% of volatile matter, 0.1 mass% or less of sulfur content, and the balance being ash. It should be noted that the moisture is adjusted to 5 mass% or less relative to the mass in the dry state.
In the method for producing the carbide molded body according to the exemplary embodiment of the invention, the carbide molded body is formed by molding carbide granules together with a binder. The carbide granules are produced by carbonizing at least one material selected from the group consisting of coal, plant-based biomass, and waste plastic to produce a carbide raw material, and subjecting the carbide raw material to sieving treatment and crushing treatment such that the following (a') and (b') are satisfied. That is, as (a'), carbide granules are formed from the carbide raw material such that the maximum particle size is less than one-half of the representative length d of the carbide molded body, and the carbide granules are classified into a plurality of groups having different particle sizes. Further, as (b'), the carbide granules are formulated such that, when the carbide granules are classified into 70 vol% on a large-particle side and 30 vol% on a small-particle side, a ratio of a median particle size D50 of the carbides on the large-particle side to a median particle size D50 of the carbides on the small-particle side is from 1.5 to 2.3. As described above, the particle size distribution of the carbide granules herein conforms to the above-described (a) and (b). Accordingly, since the reasons for limiting to (a') and (b') are the same, description thereof is omitted here.
It should be noted that the sieving treatment and the crushing treatment may be performed once or multiple times depending on the particle size, hardness, and other properties of the carbide raw material.
The more specific production steps used in the method for producing the carbide molded body according to the exemplary embodiment of the invention are as follows. That is, in the exemplary embodiment, the carbide molded body is obtained by adding a binder and water to the carbide granules serving as a main raw material, mixing and stirring them with a mixer (not illustrated) to form a kneaded mixture, molding the kneaded mixture with a molding device (see Figs. 3 and 4), and then drying the molded product to a predetermined moisture content. In producing the carbide granules as the main raw material, first, raw materials such as coal, plant-based biomass, or waste plastic are carbonized into carbides by using a carbonization furnace (not illustrated), such as a rotary kiln, a batch furnace, or a shaft furnace. Next, the carbonized carbides are, if necessary, passed through a cutter mill (see Fig. 2) to be crushed into various particle sizes, and further, the crushed material of various particle sizes is subjected to particle size classification by using a sieving machine (not illustrated) or the like, thereby producing carbide granules satisfying the above-described (a') and (b'). As the binder used for kneading the carbide granules, inorganic bentonite, organic carboxymethyl cellulose, cornstarch, or the like can be used.
Fig. 2 schematically illustrates a state in which carbonized carbides 21 are supplied from a hopper 27 in a roll crusher 26, which is a type of cutter mill, crushed by crusher rolls 28 to become carbide granules 22, and conveyed by a belt conveyor 29. As the molding device for the carbide molded body, a twin-roll briquetting machine, a pelletizer, or the like can be used. Fig. 3 schematically illustrates a state in which a kneaded mixture 23 of carbide granules kneaded with a binder and water is supplied from a hopper 32 of a twin-roll briquetting machine 31 to molding rolls 33, compressed and molded, and formed into a carbide molded body 24 (briquette 24a). Fig. 3 also illustrates a state in which the carbide molded body 24 (briquette 24a) immediately after molding is conveyed by a belt conveyor 34. Fig. 4 schematically illustrates a state in which the same kneaded mixture 23 is supplied from a hopper 42 of a pelletizer 41 to a screw feeder 43 to be extrusion-molded, cut to a predetermined length by a cutter 44 to become the carbide molded body 24 (pellet 24b), and conveyed by a belt conveyor 45.
The reason why the carbides in the carbide molded body according to the exemplary embodiment of the invention are required to have the maximum particle size less than one-half of the representative length d of the carbide molded body is to prevent the binder concentration during production of the carbide molded body from being unnecessarily increased. That is, when granulating the carbide granules, contrary to the expectation that the small particles will enter between the large particles to achieve dense packing, there may occur portions where the small particles do not enter between the large particles, depending on the location. As explained in results of preliminary experiments described later, such a phenomenon becomes particularly significant when the maximum particle size is one-half or more of the representative length d of the carbide molded body. Even between such large particles, the particles are bonded together by the liquid bridge adhesion force of water containing the binder; however, when the maximum particle size is one-half or more of the representative length d of the carbide molded body and the spacing between the large particles becomes wide, a large liquid bridge adhesion force is required, and it becomes necessary to increase the binder concentration.
Here, the representative length d of the carbide molded body according to the exemplary embodiment of the invention will be described. Specifically, the representative length d is a length representing the outer shape of the carbide molded body when dealing with the balance between the gravity acting on the carbide molded body and the drag force exerted from the converter off-gas according to the outer shape. More specifically, the representative length d is a length representing the cross-sectional area of the carbide molded body perpendicular to the drag force received from the converter off-gas. The representative length d can be exemplified by the maximum diagonal length in the case of an irregular cross-section, one side in the case of a quadrilateral cross-section, and the diameter in the case of a circular cross-section. By using this representative length, it is possible, for example, to preliminarily examine that, in the case of a carbide molded body having a representative length at which the gravity is smaller than the drag force received from the converter off-gas, the carbide molded body will float together with the converter off-gas and fail to reach a molten bath. It should be noted that a representative length d of a pellet-shaped carbide molded body can be represented by a pellet diameter and a representative length d of a briquette-shaped carbide molded body can be represented by the maximum diagonal length or one side length.
The carbides in the carbide molded body according to the exemplary embodiment of the invention are to satisfy that, when the carbides are classified into 70 vol% on the large-particle side and 30 vol% on the small-particle side, the ratio of the median particle size D50 of the carbides on the large-particle side to the median particle size D50 of the carbides on the small-particle side is from 1.5 to 2.3. This has been derived based on the findings obtained from the preliminary experiments and Examples described later. Specifically, with this, the porosity is reduced as compared with the case where large and small particles are not blended, the rate of change in porosity is also smaller, and the tolerance range of porosity has a wider margin, so that even when production variations are taken into account, no abnormalities occur in the carbide molded body, and a high product yield can be maintained. It should be noted that the term "abnormality" of the carbide molded body herein refers to a state in which, for example, a briquette molded by a twin-roll briquetting machine or the like cracks and a lamination defect or the like occurs, so that a molded body of normal shape cannot be obtained.
Here, a method for measuring the ratio between the median particle sizes of the carbide particles in the carbide molded body according to the exemplary embodiment of the invention will be described. First, after immersing the carbide molded body in water to dissolve the binder, filtration is performed to collect the carbide, which is then washed with pure water or methanol, and the particle size distribution of the carbide particles is determined by laser diffraction particle size distribution measurement (JIS Z 8825:2013, Particle size analysis - Laser diffraction methods). Next, by performing computer analysis on the particle size distribution, the ratio of the median particle size D50 of the carbides on the large-particle side to the median particle size D50 of the carbides on the small-particle side can be obtained when the carbides are classified into 70 vol% on the large-particle side and 30 vol% on the small-particle side. It should be noted that the median particle size D50 herein is defined as the particle size read from a cumulative distribution curve on a volume basis, in which the particle size is plotted on the horizontal axis and the cumulative particle amount is expressed as a volume fraction (%) on the vertical axis, at which the cumulative value corresponds to 50%.
In the method for producing the carbide molded body according to the exemplary embodiment of the invention, the ratio between the median particle sizes of the carbide granules, which are the main raw material of the carbide molded body, can also be obtained by the same method as described above for obtaining the ratio between the median particle sizes of the carbide particles in the carbide molded body. That is, also in this case, the carbide granules are sieved through sieve openings corresponding to the respective predetermined particle sizes to obtain the particle size distribution of the carbide granules, and the ratio between the median particle sizes can be obtained from the particle size distribution by the same method as described above for obtaining the ratio between the median particle sizes of the carbide particles in the carbide molded body. It should be noted that the measurement results of the ratio between the median particle sizes of the carbide granules, which are the raw material of the carbide molded body, and the measurement results of the ratio between the median particle sizes of the carbide particles in the carbide molded body produced from the carbide granules can be regarded as being equivalent both in principle and statistically.
Next, the results of the preliminary experiments on the range of such carbide particle sizes will be described.
The inventors first examined a relationship between crushing strength of a carbide molded body obtained by molding carbide granules and packing density of the carbide particles. In general, it is known that the crushing strength is proportional to the packing density (= 1 - porosity), because when the packing density is high, the capillary suction force in the interstices between the particles increases and the particles mechanically bond together by fitting into each other's irregularities.
With respect to the packing density, a packing model by Horsfield is known, in which, when monosized spherical particles having a particle size φ are arranged in hexagonal closest packing, the porosity becomes 25.9%. Further, by filling spherical particles having a particle size φ of 0.414 into the remaining spaces among the monosized spherical particles with the above particle size φ, the porosity can be reduced to 20.7%. However, in the case of molding carbides, the packing structure is not in hexagonal closest packing, the particle shape is not spherical, and furthermore, the particle size is not uniform but has a distribution, and therefore the application of the packing model by Horsfield is difficult.

Accordingly, preliminary experiments were conducted for the case of pelletizing biochar in order to increase the packing density of the carbides and thereby enhance the crushing strength of the carbide molded body. First, the carbides raw material were crushed and sieved, and thereby classified into groups having average particle sizes corresponding to integer divisions of the pellet diameter A (A = 20 mm), namely 1/2A, 1/3A, 1/4A, 1/5A, and 1/6A. Among these groups of carbides, any of 1/2A, 1/3A, 1/4A, and 1/5A was used as the "large particle size" carbides. Any of 1/3A, 1/4A, 1/5A, and 1/6A was used as the "small particle size" carbides. It should be noted that the "small particle size" carbides are selected such that their average particle size is smaller than that of the "large particle size" carbides. As shown in Table 1, ten cases having different combinations of the average particle sizes of the "large particle size" carbides and the "small particle size" carbides were studied. Fig. 5 illustrates results of measurements of the porosity (= 1 - packing density) for each sample, in four cases in which the ratio of the average particle size of the large particle size carbides to that of the small particle size carbides was 1.3, 2.0, 2.5, and 3.0, respectively, with the volume fraction (blending ratio) of the "small particle size" carbides being changed. As for each sample, first, for each mixture of carbides in which the volume fraction (blending ratio) of the small particles was changed within each case, 8 mass% cornstarch as a binder based on the carbides and an appropriate amount of water were added and kneaded to prepare a kneaded mixture. Further, the kneaded mixture in each case was molded into pellets by a pelletizer, and thereafter dried so that the moisture content was within 3 mass%, thereby preparing a test material. In addition, Table 1 also lists the minimum porosity obtained when the ratio of the average particle sizes (also simply referred to as the particle size ratio) was varied, for cases other than those illustrated in Fig. 5.

Table 1

No.	1	2	3	4	5	6	7	8	9	10
Large particle size	1/2A				1/3A			1/4A		1/5A
Small particle size	1/3A	1/4A	1/5A	1/6A	1/4A	1/5A	1/6A	1/5A	1/6A	1/6A
Particle size ratio	1.5	2.0	2.5	3.0	1.3	1.7	2.0	1.3	1.5	1.2
Minimum porosity	0.37	0.34	0.32	0.25	0.38	0.36	0.34	0.38	0.37	0.39

As seen from Fig. 5, the porosity changes depending on the blending ratio of the "small particle size" carbides and the "large particle size" carbides, and the porosity takes a minimum value when the volume fraction of the small particles is around 0.3. It can also be seen from Fig. 5 that the change in porosity can be stratified and organized according to the ratio of the average particle sizes. Fig. 6 is a diagram in which the relationship is organized in such a stratified manner, with the particle size ratio of the large particle size carbides to the small particle size carbides in the carbide molded body plotted on the horizontal axis, and the minimum porosity of the carbide molded body plotted on the vertical axis. It can be seen from Fig. 6 that increasing the ratio of the average particle size of the large particle size carbides to that of the small particle size carbides makes it possible to reduce the minimum porosity. From this, it was inferred that the amount of added water required to fill voids could be further reduced.
However, in Nos. 3 and 4, since the particle size ratio is as large as 2.5 or more, the minimum porosity can be significantly reduced to 0.32 and 0.25, respectively, but as can be seen from Fig. 5, the change in porosity depending on the blending ratio of the carbide particles is large. Therefore, in Nos. 3 and 4, due to production variations, the amount of added water becomes greater than the proper value, and abnormalities such as lamination defects may occur in the carbide molded body after molding but before drying, raising concerns that the product yield may be reduced. On the other hand, in Nos. 1, 2, and 5 to 10 where the particle size ratio is less than 2.5, although the porosity is inferior to those of Nos. 3 and 4, the rate of change in porosity in the vicinity of the blending ratio at which the porosity becomes minimum is small, and the porosity has a wide tolerance range. Therefore, in Nos. 1, 2, and 5 to 10, even when production variations are taken into account, it is expected that no abnormalities occur in the carbide molded body and a high product yield is obtained. Further, Nos. 1, 2, 6, 7, and 9 have the minimum porosity of 0.37 or less, which is 2% to 5% lower than the porosity of 0.39 in the case without blending large and small particles, and therefore are judged to be suitable, since no abnormalities occur in the carbide molded body and the product yield is not reduced even when production variations are taken into account.
As described above, by increasing the particle size ratio of the carbides, it can be inferred from Fig. 6 that the minimum porosity can be reduced and further the amount of added water required to fill the voids can be reduced. Based on this, it was decided to examine the possibility of reducing the amount of binder required for molding the carbide molded body by increasing the particle size ratio of the carbides. Fig. 7 illustrates the relationship between the ratio of the median particle size D50 of the carbides on the large particle size side to the representative length d of the carbide molded body, and the binder concentration required for molding the carbide molded body. It was found from Fig. 7 that the required binder concentration increased when particles having a large diameter, such as 1/2A, were used as the large particles. The following is presumed to be the cause of this phenomenon. That is, during granulation, it is expected that the small particles enter between the large particles to cause dense packing, but in some locations, portions inevitably occur where the small particles do not enter between the large particles. In such portions, the particles are bonded together by the liquid bridge adhesion force of the water containing the binder. It is presumed that the wider the spacing between the large particles, the greater the liquid bridge adhesion force required, and therefore the binder concentration needs to be increased. In practice, when 1/2A particles were used as the large particles, as in No. 2, a higher binder concentration was required compared to the case where 1/3A particles were used as the large particles, as in No. 7. Therefore, the amount of binder was lower in No. 7 than in No. 2.
From the above preliminary experimental results and Examples described later, the carbides contained in the carbide molded body of the invention are, first, made to satisfy (a): the maximum particle size is less than one-half of the representative length d of the carbide molded body, in order not to unnecessarily increase the binder concentration. At the same time, the carbides contained in the carbide molded body of the invention are made to have the following particle size distribution in order to ensure a preferable control range. That is, the particle size distribution satisfies (b): when the carbides are classified into 70 vol% on the large-particle side and 30 vol% on the small-particle side, the ratio of the median particle size D50 of the carbides on the large-particle side to the median particle size D50 of the carbides on the small-particle side is from1.5 to 2.3. It should be noted that, in the carbides of the invention, when the particle size distribution satisfies (b) as described above, the particle size distribution curve generally has two local maxima.
The crushing strength of the carbide molded body of the invention is preferably 490 N/piece or more. If the crushing strength of the carbide molded body is less than 490 N/piece, the carbide molded body is crushed into dust before being charged into the molten steel, and scatters together with the converter off-gas, so that it is impossible to charge the carbide molded body into the molten bath with a high yield.

Examples

Examples of the invention will be described below.
As the carbide raw material, woody raw materials such as sawmill residues and forest residues were used, and biochar carbonized by heating at 650 degrees C was employed as the material to be molded.
The carbide molded bodies in a form of a pellet having a diameter of 20 mm and in a form of a briquette having a side length of 50 mm were used in Examples. In this case, the target particle size of the crushed biochar was set so as to generally satisfy the requirement that "the maximum particle size be less than one-half of the representative length d of the carbide molded body," namely, 9 mm or less in a form of a pellet having a diameter of 20 mm, and 22 mm or less in a form of a briquette having a side length of 50 mm.
In accordance with such target particle sizes, the above biochar was passed once or multiple times through the roll crusher 26 illustrated in Fig. 2, in the same manner as in the above-described preliminary experiments, and then sieved to prepare crushed biochar having various average particle sizes. These crushed biochars having various average particle sizes were combined, and five mixtures of carbide granules were formulated as shown in Table 2. That is, the five mixtures of carbide granules shown in Table 2 were formulated such that, when each mixture is classified into 70 vol% on the large-particle side and 30 vol% on the small-particle side, the ratio of the median particle size D50 of the carbides on the large-particle side to the median particle size D50 of the carbides on the small-particle side is from1.5 to 2.3. The particle size distributions of the five mixtures of carbide granules thus formulated are illustrated in Figs. 8A, 8B, 8C, 9A, and 9B.
Next, under the mixing conditions shown in Table 2, cornstarch as a binder and water were added to each of the five mixtures of carbide granules and mixed, and the mixtures were compression-molded into briquettes or pellets using the twin-roll briquetting machine depicted in Fig. 3 or the pelletizer depicted in Fig. 4. After molding, the products were left standing at room temperature for five days to dry until the moisture content reached approximately 3%, whereby carbide molded bodies were obtained.
In Comparative 1, as shown in Table 2 and Fig. 8A, since the particle size ratio was as small as 1.3 and there were few small particles to fill voids between the large particles, the minimum porosity was hardly reduced, and the amount of added water required to fill the voids between the large particles increased. Accordingly, the amount of binder added in Comparative 1 became the largest among the five cases. In addition, in Comparative 1, the crushing strength of the dried carbide molded bodies, sampled with n = 10, failed to achieve the crushing strength of 490 N or more required for operation.
In Comparative 2, as shown in Table 2 and Fig. 9B, since the particle size ratio was as large as 2.5, the minimum porosity decreased, and the amount of added water required to fill voids between the large particles was reduced. Accordingly, the amount of binder added was the smallest among the five cases, as in Inventive Example 3. However, in Comparative 2, since the particle size ratio was large, the change in porosity depending on the blending ratio of the carbide particles was large (see Fig. 5), and due to variations in production, the amount of added water tended to be greater than the proper value. Therefore, in Comparative 2, abnormalities such as lamination defects occurred in the carbon material after molding but before drying, the product yield was significantly reduced, which was enough to hamper securing the number of test pieces for crushing strength.
On the other hand, in Inventive Examples 1 to 3, regardless of the difference between the pellet form and the briquette form, it was possible to reduce the amount of binder while maintaining the crushing strength and product yield required for operation.

Here, when Figs. 8A, 8B, 8C, 9A, and 9B, which illustrate the respective particle size distributions of the five mixtures of carbide granules formulated, are compared side by side, it can be seen that the particle size distribution of Comparative 1 in Fig. 8A is a single-peaked distribution as a whole. In addition, the particle size distribution of Comparative 2 in Fig. 9B is a gently sloping distribution from the maximum particle size side toward the minimum particle size side. On the other hand, the particle size distributions of Inventive Examples 1 to 3 are each a bimodal particle size distribution having peaks on both the large particle size side and the small particle size side. By having such a particle size distribution with two local maxima, it is possible in the invention to increase the packing density of the carbides and thereby enhance the strength of the carbide molded body, while reducing the amount of binder added within an economically rational range.

Table 2

	Comp. 1	Inventive Ex. 1	Inventive Ex. 2	Inventive Ex. 3	Comp. 2
Shape of carbide molded body	pellet	pellet	pellet	briquette	briquette
Representative length d (mm)	[diameter] 20	[diameter] 20	[diameter] 20	[one side] 50	[one side] 50
Particle size distribution	Fig. 8A	Fig. 8B	Fig. 8C	Fig. 9A	Fig. 9B
Particle size ratio (large particles/small particles)	1.3 (6.7/5.0)	1.5 (5.0/3.3)	2.0 (6.7/3.3)	2.3 (16.7/7.1)	2.5 (25/10)
Minimum porosity	0.38	0.37	0.34	0.33	0.32
Amount of added water (L) per 100 kg of carbides	44	41	33	29	26
Binder concentration (%)	18	17	18	18	20
Amount of binder (kg) per 100 kg of carbides	8	7	6	5	5
Product yield (%)	95	93	96	94	59
Crushing strength (N/piece)	460	500	635	666	-

EXPLANATION OF CODES

1 converter body
2 top-blowing lance
3 bottom-blowing tuyere
4 flue
5 skirt
6 hopper
7 charging chute
8 hot metal
9 slag
10 converter off-gas
21 carbonized carbide
22 carbide granules
23 kneaded mixture of carbide granules
24 carbide molded body immediately after molding
24a briquette
24b pellet
26 roll crusher
27 hopper
28 crusher roll
29 belt conveyor
31 twin-roll briquetting machine
32 hopper
33 molding roll
34 belt conveyor
41 pelletizer
42 hopper
43 screw feeder
44 cutter
45 belt conveyor

Claims

A carbide molded body formed by molding carbide granules together with a binder, wherein
carbides in the carbide molded body satisfy (a) and (b):
(a) a maximum particle size is less than one-half of a representative length d of the carbide molded body; and

(b) when the carbides are classified into 70 vol% on a large-particle side and 30 vol% on a small-particle side, a ratio of a median particle size D50 of the carbides on the large-particle side to the median particle size D50 of the carbides on the small-particle side is from1.5 to 2.3.
The carbide molded body according to claim 1, wherein the carbide molded body has crushing strength of 490 N/piece or more.
A method for producing a carbide molded body, comprising molding carbide granules together with a binder, the method further comprising:
carbonizing at least one material selected from the group consisting of coal, plant-based biomass, and waste plastic to produce a carbide raw material; and

subjecting the carbide raw material to sieving treatment and crushing treatment such that (a') and (b') are satisfied, to provide the carbide granules prior to the molding,
(a'): forming the carbide granules from the carbide raw material such that a maximum particle size is less than one-half of a representative length d of the carbide molded body, and classifying the carbide granules into a plurality of groups having different particle sizes; and

(b'): formulating the carbide granules such that, when the carbide granules are classified into 70 vol% on a large-particle side and 30 vol% on a small-particle side, a ratio of a median particle size D50 of carbides on the large-particle side to the median particle size D50 of carbides on the small-particle side is from 1.5 to 2.3.
The method for producing a carbide molded body according to claim 3, wherein the carbide molded body has crushing strength of 490 N/piece or more.