Carbide molded body and production method therefor

Abstract

Provided is a carbide molded body obtained by molding a carbide powder / particular material and a binder. The carbide in the carbide molded body is configured such that: (a) the maximum particle diameter is less than 1 / 2 of the representative length d of the carbide molded body; and (b) when the carbide molded body comprises large particles that account for 70 vol% and small particles that account for 30 vol%, the ratio of the median particle diameter D50 of the large carbide particles to the median particle diameter D50 of the small carbide particles is 1.5 to 2.3. Also provided is a method for producing the carbide molded body in which the carbide raw material is subjected to sieving and crushing such that the carbide powder / particles have a particle size distribution similar to (a) and (b).

Description

Carbide compact and manufacturing method thereof

[0001] The present invention relates to a carbide molded body and a manufacturing method thereof, and more particularly to a carbide molded body and a manufacturing method thereof in which the packing ratio of carbide, which is the raw material of the carbide molded body, is increased to increase strength while optimizing the amount of binder added.

[0002] The converter in the steelmaking process is a key process for producing molten steel by decarburizing the molten iron tapped from the blast furnace by blowing high-purity oxygen at high speed onto it. At the same time, auxiliary materials, mainly quicklime, are added to remove impurities (such as phosphorus) from the molten iron.

[0003] On the other hand, a hot metal pretreatment process may be provided as a pretreatment process for the converter to remove sulfur, phosphorus, and other elements from the hot metal in order to meet the requirements for the material properties of steel products. In this case, there is a problem that the hot metal temperature decreases due to the hot metal pretreatment process.

[0004] In recent years, from the perspective of environmental protection, CO ２ Reducing CO emissions has become an important issue. To address this issue, attempts have been made to increase the blending ratio of cold iron sources such as iron scrap as iron sources used in the steelmaking process and to reduce the blending ratio of molten pig iron. This is because, when producing molten pig iron in a blast furnace in the manufacture of steel products, a great deal of energy is required to reduce and melt the iron ore, and at the same time, a large amount of CO is produced. ２ In addition, when a part of the molten iron is replaced with a cold iron source in the steelmaking process, the cold iron source only requires the heat of dissolution and does not require the energy for the heat of reduction of iron ore, so CO ２ However, in a converter, the heat sources for melting the cold iron source are the sensible heat of the molten iron and the heat of oxidation of carbon and silicon in the molten iron, and there is a natural limit to the amount of cold iron source that can be melted within the range of such sensible heat and heat of oxidation.

[0005] Therefore, in the dephosphorization or decarburization refining of molten pig iron, carbide molded bodies are used to supply an additional carbon source to the molten pig iron in order to increase the thermal margin of the molten pig iron and increase the blending ratio of the cold iron source. For example, various carbide molded bodies have been proposed, which are formed by molding coal, coke powder, graphite, electrode powder, SiC, etc. into a lump. Note that amorphous graphite, a natural mineral product that is not molded, is also sometimes used as a carbon source because it is relatively inexpensive.

[0006] As a method for producing such a carbide molded body, for example, Patent Document 1 discloses an invention using carbon powder (such as coal, coke powder, or graphite) as a raw material, with 30% to 70% of particles having a particle size of 1 mm or less, and the rest having 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 of 0.5 to 1.0 wt % of the carbon powder, and the mixture is refined to a moisture content of 10% or less, kneaded, molded under high pressure, and then dried. In this way, the invention described in this document makes it possible to mold the carbon powder into a solid body with sufficient strength.

[0007] Patent Document 2 discloses an invention of a converter heating material formed by molding biochar obtained by carbonizing carbon-neutral plant biomass and a binder such as starch, carboxymethyl cellulose, or corn starch in an amount of 1% by mass to 15% by mass relative to the mass of the biochar. Conventional converter heating materials are made from fossil resources such as coal, coke, or graphite, but the invention described in this document provides an alternative converter heating material, thereby reducing the consumption of fossil resources and reducing CO2, a greenhouse gas. ２ At the same time, by making the particle size of the carbide 3 mm or less, the solid material has a crushing strength of 490 N / piece or more, which is sufficient for use as a converter heating material.

[0008] JP-A-2-270922 Patent No. 5846289

[0009] However, the carbon powder used as the raw material for the recarburizer for steelmaking described in Patent Document 1 has a high proportion of particles having a particle size of 1 mm or less, at 30% to 70%, which poses a problem of a high processing load for particle size adjustment, such as a pulverization process of the carbon raw material.

[0010] Furthermore, the carbon powder used as the raw material for the converter heating material described in Patent Document 2 needs to be sized to a relatively narrow particle size range of 3 mm or less, and similarly to the invention described in Patent Document 1, there is a problem in that the processing load for particle size adjustment, such as pulverization of the carbon raw material, is high.

[0011] Furthermore, the steelmaking recarburizer described in Patent Document 1 uses a binder in a range of 0.5% by weight to 1.0% by weight of the carbon powder, while the converter heating material described in Patent Document 2 uses a binder in a range of 1% by weight to 15% by weight of the carbide. However, in recent years, price fluctuations of plant-derived binders, specifically, the price fluctuations of cornstarch derived from corn, have become a problem. This is due to price fluctuations linked to poor corn harvests caused by abnormal weather and price pressures associated with increased demand for corn-derived bioethanol to reduce environmental impact. Therefore, there is an increasing demand for technology that reduces the amount of binder used when molding carbide molded bodies.

[0012] On the other hand, if the amount of binder added is less than the required amount, abnormalities such as insufficient strength of the molded carbonaceous material occur, resulting in problems such as operational troubles during transportation or reduced yields. If such problems are solved by finely crushing all of the raw carbide material to ensure the strength of the carbide molded body, this will result in an increase in costs for finely crushing the carbide, which will impair overall economic efficiency.

[0013] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a carbide molded body and a method for manufacturing the same that enable the amount of binder added to be reduced while optimizing the particle size distribution of the raw material carbide to increase the carbide filling rate and strength.

[0014] [1] A carbide molded body obtained by molding carbide powder particles and a binder, wherein the carbides in the carbide molded body (a) have a maximum particle size less than 1 / 2 of a characteristic length d of the carbide molded body, and (b) when divided so that the large particle size side accounts for 70 volume % and the small particle size side accounts for 30 volume %, the ratio of the median particle size D50 of the carbides on the large particle size side to the median particle size D50 of the carbides on the small particle size side is 1.5 or more and 2.3 or less. [2] The carbide molded body according to [1], wherein the carbide molded body has a crushing strength of 490 N / piece or more. [3] A method for producing a carbide molded body produced by molding carbide powder and a binder, the method comprising: carbonizing at least one selected from the group consisting of coal, plant biomass, and waste plastic to produce a carbide raw material; sieving and crushing the charcoal raw material to (a') produce the charcoal powder and granules from the charcoal raw material, the charcoal powder and granules being divided into a plurality of groups with different particle sizes within a range in which the maximum particle size is less than 1 / 2 of the characteristic length d of the charcoal molded body; and (b') dividing the charcoal powder and granules so that the large particle size side accounts for 70 volume % and the small particle size side accounts for 30 volume %, blending the charcoal powder and granules so that the ratio of the median particle size D50 of the charcoal on the large particle size side to the median particle size D50 of the charcoal on the small particle size side is in the range of 1.5 to 2.3, thereby producing the charcoal powder and granules before molding. [4] The method for producing a carbide molded body according to [3], wherein the carbide molded body has a crushing strength of 490 N / piece or more.

[0015] According to the present invention, it is possible to provide a carbide molded body and a method for manufacturing the same, which can increase the carbide filling rate and strength by adjusting the particle size distribution of the carbide in the carbide molded body to a predetermined distribution, while reducing the amount of binder added within an economically reasonable range.

[0016] FIG. 1 is a schematic cross-sectional view of an example of converter equipment in which a carbide molded body according to an embodiment of the present invention is used. FIG. 2 is a schematic cross-sectional view showing how biochar is crushed by a roll crusher used in the preliminary experiments and examples leading to the present invention. FIG. 3 is a schematic cross-sectional view showing how pulverized biochar is compressed and molded by a twin-roll briquetting machine used in the preliminary experiments and examples leading to the present invention. FIG. 4 is a schematic cross-sectional view showing how pulverized biochar is extruded and molded by a pellet manufacturing machine used in the preliminary experiments and examples leading to the present invention. FIG. 5 is a diagram obtained in a preliminary experiment leading to the present invention, which summarizes the relationship between the ratio of large to small particle sizes of carbide particles in a carbide molded body and the porosity of the carbide molded body, stratified by the particle size ratio of large to small particle sizes. FIG. 6 is a diagram obtained in a preliminary experiment leading to the present invention, which summarizes the relationship between the particle size ratio of large to small particle sizes of carbide particles in a carbide molded body and the minimum porosity of the carbide molded body.

[0033] Figures obtained in preliminary experiments leading to the present invention summarize the relationship between the ratio of the median particle size D50 of carbide particles on the larger particle size side to the characteristic length d of the carbide molded body, and the binder concentration required for molding the carbide molded body. Figures also show the particle size measurement results of pulverized biochar from a comparative example used in an example of a pellet-shaped carbide molded body according to an embodiment of the present invention. Figures also show the particle size measurement results of pulverized biochar from an example of the present invention used in an example of a pellet-shaped carbide molded body according to an embodiment of the present invention. Figures also show the particle size measurement results of pulverized biochar from another example of the present invention used in an example of a pellet-shaped carbide molded body according to an embodiment of the present invention. Figures also show the particle size measurement results of pulverized biochar from an example of the present invention used in an example of a briquette-shaped carbide molded body according to an embodiment of the present invention.

[0017] Hereinafter, embodiments of the present invention will be described in detail. First, a description will be given of a converter furnace, which is a typical example of refining equipment in which carbide molded bodies according to embodiments of the present invention can be used. When the carbide molded body of the present invention is used in a converter furnace, it may be referred to as a converter heating material. Furthermore, the present invention can be used not only in converter furnaces but also in refining equipment in general, including hot metal pretreatment equipment, and is not limited to the following embodiments.

[0018] Fig. 1 is a schematic cross-sectional view of an example of converter equipment in which carbide molded bodies according to an embodiment of the present invention are used. In Fig. 1, a top-blowing lance 2 is inserted from above into a converter body 1 containing molten pig iron 8, and oxygen gas is blown onto the molten pig iron 8 from this top-blowing lance 2. 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. This gas injection from above and below stirs the molten pig iron 8 and slag 9, and decarburization and refining of the molten pig iron 8 is carried out. Converter exhaust gas 10, mainly composed of CO gas, is generated from the furnace during decarburization and refining of the molten pig iron 8.

[0019] A flue 4 is installed above the converter body 1, and a primary dust collector (not shown), a secondary dust collector (not shown), and an induced draft blower (not shown) are installed downstream of the flue 4 in this order. By using this treatment equipment for converter exhaust gas 10, the converter exhaust gas 10 generated inside the converter body 1 during decarburization refining is cooled and dust-removed, and is collected unburned in a gas holder (not shown) downstream of the induced draft blower (not shown).

[0020] The side of the flue 4 that connects to the throat of the converter body 1 is called the skirt 5, and is designed to be movable up and down. When exhaust gas is recovered, the skirt 5 and the throat of the converter body 1 are generally in close contact with each other. The flue 4 is also equipped with an auxiliary material charging device for charging and adding auxiliary materials such as quicklime, burned dolomite, iron ore, mill scale, manganese ore, carbide molded bodies (carbon materials such as coke and soil graphite), and ferroalloys (Fe-Mn, Fe-Si, etc.) to the converter body 1. This auxiliary material charging device is composed of a hopper 6 and a charging chute 7. The quicklime, burned dolomite, iron ore, mill scale, manganese ore, etc., charged into the converter body 1 form slag 9.

[0021] A carbide molded body according to an embodiment of the present invention is a carbide molded body obtained by molding carbide powder particles and a binder. The carbide in this carbide molded body (a) has a maximum particle size less than half the characteristic length d of the carbide molded body. If the maximum particle size is equal to or greater than half the characteristic length d of the carbide molded body, the spacing between large particles becomes too wide, requiring a large liquid bridging adhesive force, which necessitates an increased binder concentration, resulting in an inability to achieve the effect of reducing the binder amount. Furthermore, the carbide molded body according to an embodiment of the present invention (b) has, when divided so that the large particle side accounts for 70 volume % and the small particle side accounts for 30 volume %, a ratio of the median particle size D50 of the carbide on the large particle side to the median particle size D50 of the carbide on the small particle side is 1.5 or more and 2.3 or less. The reason why the lower limit of this ratio is set to 1.5 or more is that if this ratio is less than 1.5, the minimum porosity cannot be significantly reduced compared to the porosity when large and small particles are not blended, and the effect of reducing the binder amount cannot be obtained. The lower limit of this ratio is preferably 1.6 or more to reliably reduce the porosity, and more preferably 1.7 or more to reliably obtain this effect. The reason why the upper limit of this ratio is set to 2.3 or less is that if this ratio is exceeded, the rate of change in porosity due to blending ratios near the minimum porosity becomes large, which may cause abnormalities in the carbide molded body due to manufacturing variations and reduce product yield. The upper limit of this ratio is preferably 2.2 or less to stably reduce the rate of change in porosity due to blending ratios near the minimum porosity and ensure stable product yield, and more preferably 2.1 or less to reliably obtain this effect.

[0022] The composition of the carbide molded body according to the embodiment of the present invention, in a dry state excluding moisture, is 70% by mass or more of fixed carbon, 5% by mass to 20% by mass or less of volatile components, 0.1% by mass or less of sulfur, and the remainder is ash. The moisture content is adjusted to 5% by mass or less of the mass in the dry state.

[0023] In a method for producing a carbide molded body according to an embodiment of the present invention, a carbide molded body is produced by molding carbide powder and a binder. The carbide powder is produced by carbonizing at least one material selected from the group consisting of coal, plant biomass, and waste plastic to produce a carbide raw material, which is then sieved and crushed to satisfy the following requirements (a') and (b'). Specifically, in step (a'), the carbide raw material is divided into multiple groups of carbide powder with different particle sizes, each group having a maximum particle size less than half the characteristic length d of the carbide molded body. Furthermore, in step (b'), the carbide powder is divided into groups with 70% by volume of larger particles and 30% by volume of smaller particles, and the ratio of the median particle size D50 of the larger particles to the median particle size D50 of the smaller particles is blended to be 1.5 to 2.3. As described above, the particle size distribution of the carbide powder particles here conforms to (a) and (b) above. Therefore, the reasons for limiting (a') and (b') are also the same, and therefore explanations thereof will be omitted here. Note that the sieving treatment and crushing treatment may be carried out once or multiple times depending on the particle size, hardness, and other properties of the carbide raw material.

[0024] More specific manufacturing steps used in the manufacturing method of a carbide molded body according to an embodiment of the present invention are as follows. That is, in this embodiment, the carbide molded body is obtained by adding a binder and water to a carbide powder as a main raw material, mixing and stirring the mixture in a mixer (not shown), molding the mixture in a molding device (see FIGS. 3 and 4), and then drying the mixture to a predetermined moisture content. To manufacture the carbide powder as a main raw material, first, the raw material, such as coal, plant biomass, or waste plastic, is carbonized into a carbide using a carbonization device (not shown), such as a rotary kiln furnace, a batch furnace, or a shaft furnace. Next, the carbonized carbide is crushed into various particle sizes by passing it through a cutter mill (see FIG. 2) as needed, and the crushed particles of various particle sizes are further subjected to particle size adjustment using a sieving machine (not shown) or the like, to obtain a carbide powder satisfying the above (a') and (b'). As the binder used for kneading the carbide powder particles, inorganic bentonite, organic carboxymethyl cellulose or corn starch, etc. can be used.

[0025] Figure 2 schematically shows how dry-distilled carbide 21 is supplied from a hopper 27 to a roll crusher 26, a type of cutter mill, crushed by crusher rolls 28 to form carbide powder granules 22, and then transported by a belt conveyor 29. A twin-roll briquetting machine, a pellet manufacturing machine, or the like can be used as a molding device for forming carbide molded bodies. Figure 3 also schematically shows how a kneaded mixture 23 of carbide powder granules kneaded with a binder and water is supplied from a hopper 32 of a twin-roll briquetting machine 31 to a molding roll 33, where it is compressed and molded to form carbide molded bodies 24 (briquettes 24a). The figure also shows how the carbide molded bodies 24 (briquettes 24a) immediately after molding are transported by a belt conveyor 34. FIG. 4 shows a schematic diagram in which a similar kneaded material 23 is supplied from a hopper 42 of a pellet manufacturing machine 41 to a screw feeder 43, extrusion-molded, and cut to a predetermined length by a cutter 44 to become a carbide molded body 24 (pellets 24b), which is then transported out on a belt conveyor 45.

[0026] The reason why the maximum particle size of the carbide particles in the carbide molded body according to the embodiment of the present invention is less than half the characteristic length d of the carbide molded body is to avoid an unnecessary increase in the binder concentration during the production of the carbide molded body. That is, when granulating carbide powder particles, contrary to the expectation that small particles will enter between large particles to form a dense packing, some locations may occur where small particles do not enter between large particles. As will be explained in the preliminary experiment results described below, this phenomenon is particularly pronounced when the maximum particle size is equal to or greater than half the characteristic length d of the carbide molded body. Even between such large particles, the particles are bound by the liquid bridging adhesive force of water containing the binder. However, when the maximum particle size is equal to or greater than half the characteristic length d of the carbide molded body and the large particles are spaced widely apart, a large liquid bridging adhesive force is required, necessitating an increased binder concentration.

[0027] Here, the representative length d of the carbide molded body according to the embodiment of the present invention will be described. That is, the representative length d is a length that represents the outer shape of the carbide molded body when considering the balance between the gravity acting on the carbide molded body and the drag force from the converter exhaust gas depending on the outer shape of the carbide molded body. More simply put, this representative length d is a length that represents the cross-sectional area of ​​the carbide molded body in the direction perpendicular to the drag force from the converter exhaust gas. Examples of the representative length d include the maximum diagonal length when the cross section is irregular, one side when the cross section is rectangular, and the diameter when the cross section is circular. By using this representative length, for example, in the case of a carbide molded body with a representative length such that the gravity force is smaller in the balance between the drag force from the converter exhaust gas and gravity, it is possible to predict in advance whether the carbide molded body will float up with the converter exhaust gas and reach the molten metal. When the carbide molded body is in the form of a pellet, the representative length d can be represented by the pellet diameter, and when the carbide molded body is in the form of a briquette, the representative length d can be represented by the maximum diagonal length or the length of one side.

[0028] Furthermore, when the carbide in the carbide molded body according to the embodiment of the present invention is divided into 70 volume % large particle size carbides and 30 volume % small particle size carbides, the ratio of the median particle size D50 of the carbide on the large particle size side to the median particle size D50 of the carbide on the small particle size side is set to 1.5 or more and 2.3 or less. This was derived based on the findings obtained from the preliminary experiments and examples described below. This results in a lower porosity than when large and small particles are not blended, a smaller rate of change in porosity, and a wider allowable range of porosity. Therefore, even when considering manufacturing variations, abnormalities in the carbide molded body do not occur, and high product yields can be maintained. Note that abnormalities in the carbide molded body here refer to, for example, a state in which briquettes molded using a twin-roll briquetting machine or the like crack, resulting in laminations that prevent the production of molded bodies of a normal shape.

[0029] Here, we will explain a method for measuring the median particle size ratio of carbide particles in a carbide molded body according to an embodiment of the present invention. First, the carbide molded body is immersed in water to dissolve the binder, and then filtered to collect the carbide. The carbide is then washed with pure water or methanol. The particle size distribution of the carbide particles is then determined by laser diffraction particle size distribution measurement (JIS Z 8825:2013 Particle Size Analysis - Laser Diffraction and Scattering Method). Next, the particle size distribution is analyzed by computer to determine the ratio of the median particle size D50 of the carbide particles on the larger particle size side to the median particle size D50 of the carbide particles on the smaller particle size side, when the larger particle size side is divided into 70 volume % and the smaller particle size side is 30 volume %. The median particle size D50 here is determined by reading the particle size at which the cumulative value reaches 50% on a volume-based cumulative distribution curve, where the horizontal axis represents particle size and the vertical axis represents the cumulative particle amount as a volume percentage (%).

[0030] Meanwhile, in the method for producing a carbide compact according to an embodiment of the present invention, the median particle size ratio of the carbide powder particles that are the main raw material of the carbide compact can be measured using a method similar to that for determining the median particle size ratio of the carbide particles in the carbide compact described above. That is, in this case, the carbide powder particles are sieved through a sieve with predetermined particle sizes to determine the particle size distribution of the carbide powder particles, and the particle size distribution can be determined using a method similar to that for determining the median particle size ratio of the carbide particles in the carbide compact described above. The measurement result of the median particle size ratio of the carbide powder particles that are the raw material of the carbide compact and the measurement result of the median particle size ratio of the carbide particles in a carbide compact produced from the carbide powder particles can be treated as being equivalent in principle and statistically.

[0031] The results of preliminary experiments on such a range of carbide particle size are described below. The inventors first investigated the relationship between the crushing strength of a carbide compact obtained by compacting carbide powder and the packing ratio of the carbide particles. It is generally known that a high packing ratio increases the capillary suction force between particles and causes mechanical bonding effects such as particles fitting into each other's uneven surfaces, so that the crushing strength and packing ratio (= 1 - porosity) are proportional to each other.

[0032] Regarding the packing ratio, the Horsfield packing model is known, and when spherical particles of a single particle diameter φ are hexagonally close-packed, the void ratio is 25.9%. Furthermore, by filling the remaining space between the spherical particles with spherical particles of 0.414φ, the void ratio can be increased to 20.7%. However, when molding carbides, the packing structure is not hexagonally close-packed, the particle shape is not spherical, and the particle diameter is not uniform but has a distribution, making it difficult to apply the Horsfield packing model.

[0033] Therefore, we conducted a preliminary experiment to increase the char packing ratio and crush strength of the char pellets produced by molding biochar into pellets. First, we prepared a group of chars with average particle sizes corresponding to integer divisions of the pellet diameter A (A = 20 mm) (1 / 2A, 1 / 3A, 1 / 4A, 1 / 5A, 1 / 6A) by crushing and sieving the raw char. Of these chars, 1 / 2A, 1 / 3A, 1 / 4A, and 1 / 5A were designated as "large particle size" chars. Furthermore, 1 / 3A, 1 / 4A, 1 / 5A, and 1 / 6A were designated as "small particle size" chars. The "small particle size" chars were selected so that their average particle size was smaller than that of the "large particle size" chars. As shown in Table 1, 10 examples with different combinations of average particle sizes for the "large particle size" and "small particle size" chars were examined. Figure 5 shows the results of measuring the porosity (= 1 - packing ratio) for each sample in which the volume fraction (mixing ratio) of the "small particle diameter" carbide was varied for four examples in which the average particle diameter ratio (large particle diameter / small particle diameter) was 1.3, 2.0, 2.5, and 3.0. For each sample, a mixture of carbides with different volume fractions (mixing ratios) of small particles was prepared by adding 8% by mass of cornstarch and an appropriate amount of water as a binder. Each mixture was then molded into pellets using a pelletizer, which were then dried to a moisture content of 3% by mass or less to prepare the test material. Table 1 also lists the minimum porosity for examples other than those shown in Figure 5 when the average particle diameter ratio (also simply referred to as particle diameter ratio) was varied.

[0034]

[0035] Figure 5 shows that the porosity varies depending on the blending ratio of "small particle diameter" to "large particle diameter," reaching a minimum value when the volume fraction of small particles is around 0.3. Figure 5 also shows that the change in porosity can be stratified and organized by the ratio of average particle diameters. Figure 6 is a diagram summarizing the relationship between the particle diameter ratio of large to small particles in a carbide compact, which can be stratified and organized in this way, on the horizontal axis and the minimum porosity of the carbide compact on the vertical axis. Figure 6 shows that the minimum porosity can be reduced by increasing the ratio of average particle diameters of the carbide (large particle diameter / small particle diameter). This suggests that the amount of water added to fill the voids can be further reduced.

[0036] However, in Nos. 3 and 4, the particle size ratio was large (2.5 or greater), significantly reducing the minimum porosity to 0.32 and 0.25, respectively. However, as can be seen from Figure 5, the porosity varied significantly depending on the blending ratio of the carbide particles. Therefore, in Nos. 3 and 4, variations in manufacturing process could result in the amount of added moisture exceeding the appropriate value, raising concerns that irregularities such as laminations could occur in the carbide molded body after molding but before drying, reducing product yield. On the other hand, in Nos. 1, 2, and 5 to 10, which have particle size ratios less than 2.5, although their porosity was inferior to that of Nos. 3 and 4, the rate of change in porosity near the blending ratio at which the porosity was minimum was small, and they had a wide range within the acceptable porosity range. Therefore, even considering variations in manufacturing process, Nos. 1, 2, and 5 to 10 are expected to avoid irregularities in the carbide molded body and achieve high product yield. Furthermore, in Nos. Nos. 1, 2, 6, 7, and 9 had a minimum porosity of 0.37 or less, which was 2 to 5% lower than the porosity of 0.39 when no large or small particles were blended. Even taking into account manufacturing variations, no abnormalities occurred in the carbide molding and no reduction in product yield was observed, and therefore they were deemed appropriate.

[0037] As described above, increasing the particle size ratio of the carbide can reduce the minimum porosity, as shown in Figure 6, and it is believed that the amount of water added to fill the voids can also be reduced. Based on this, we decided to investigate the possibility of reducing the amount of binder required for molding a carbide compact by increasing the particle size ratio of the carbide. Figure 7 summarizes the relationship between the ratio of the median particle size D50 of the larger carbide particles to the characteristic length d of the carbide compact, and the binder concentration required for molding the carbide compact. Figure 7 reveals that the required binder concentration increases when large particles, such as particles with a diameter of 1 / 2A, are used as large particles. The following is believed to be the cause of this. During granulation, small particles are expected to fill in between the large particles, resulting in dense packing. However, some areas inevitably occur where the small particles do not fill in between the large particles. In these areas, the large particles are bonded by the liquid bridging adhesive force of the water containing the binder. It is presumed that the wider the spacing between the large particles, the greater the liquid bridging adhesive force required, which in turn requires a higher binder concentration. In fact, when 1 / 2A was used for the large particles, as in No. 2, a higher binder concentration was required compared to when 1 / 3A was used for the large particles, as in No. 7. For this reason, the binder amount was lower in No. 7 than in No. 2.

[0038] Based on the results of the preliminary experiments described above and the examples described below, the carbide contained in the carbide molded body of the present invention (a) has a maximum particle size less than half the characteristic length d of the carbide molded body in order to avoid an unnecessary increase in binder concentration. At the same time, the carbide contained in the carbide molded body of the present invention has the following particle size distribution in order to ensure a preferable control range. That is, the particle size distribution (b) is such that, when the large particle size side is divided into 70 volume % and the small particle size side is divided into 30 volume %, the ratio of the median particle size D50 of the carbide on the large particle size side to the median particle size D50 of the carbide on the small particle size side is 1.5 or more and 2.3 or less. When the particle size distribution of the carbide of the present invention satisfies condition (b), the particle size distribution curve typically has two maxima.

[0039] The crushing strength of the carbide compact of the present invention is preferably 490 N / piece or more. If the crushing strength of the carbide compact is less than 490 N / piece, the carbide compact will be crushed into dust before reaching the molten steel, and will scatter together with the converter exhaust gas, making it impossible to introduce the carbide compact into the molten steel with a high yield.

[0040] Examples of the present invention will be described below.

[0041] Woody raw materials such as sawmill scraps and forest residues were used as the carbonized raw materials, and biochar carbonized at 650°C was used as the material to be molded.

[0042] The shapes of the carbide molded bodies used in the examples were pellets with a diameter of 20 mm and briquettes with a side length of 50 mm. The target particle size of the crushed biochar was set to 9 mm or less for pellets with a diameter of 20 mm and 22 mm or less for briquettes with a side length of 50 mm, in order to roughly satisfy the requirement that "the maximum particle size is less than half the characteristic length d of the carbide molded body."

[0043] Based on these target particle size values, the biochar was passed through the roll crusher 26 shown in Figure 2 one or more times, as in the preliminary experiment described above, and then sieved to prepare pulverized biochar samples with various average particle sizes. These pulverized biochar samples with various average particle sizes were combined to prepare five charcoal powder mixtures, as shown in Table 2. Specifically, the five charcoal powder mixtures in Table 2 were divided into 70 volume % larger particles and 30 volume % smaller particles, with the ratio of the median particle size D50 of the larger particles to the median particle size D50 of the smaller particles ranging from 1.3 to 2.5. The particle size distributions of the five charcoal powder mixtures prepared here are shown in Figures 8A, 8B, 8C, 9A, and 9B.

[0044] Next, cornstarch as a binder and water were added to each of the five sets of carbide powder particles under the mixing conditions shown in Table 2, and the mixture was compression-molded into briquettes or pellets using a twin-roll briquetting machine shown in Figure 3 or a pellet manufacturing machine shown in Figure 4. After molding, the mixture was left to stand at room temperature for five days and dried until the moisture content reached about 3%, to obtain carbide molded bodies.

[0045] In Comparative Example 1, as shown in Table 2 and Figure 8A, the particle size ratio was small at 1.3, meaning there were few small particles to fill the gaps between the large particles, so the minimum porosity could hardly be reduced, and the amount of water added to fill the gaps between the large particles increased. Accordingly, the amount of binder added in Comparative Example 1 was the largest among the five groups. Furthermore, in Comparative Example 1, the crushing strength of n = 10 samples taken from the dried carbide molded bodies was not able to achieve the crushing strength of 490 N or more required for operation.

[0046] Furthermore, in Comparative Example 2, as shown in Table 2 and Figure 9B, the particle size ratio was large at 2.5, resulting in a low minimum porosity and a reduced amount of added water to fill the gaps between the large particles. Consequently, the amount of added binder was the smallest among the five groups, along with Inventive Example 3. However, in Comparative Example 2, the large particle size ratio resulted in a large change in porosity depending on the blending ratio of carbide particles (see Figure 5), and variations during production easily led to the amount of added water being higher than the appropriate value. Therefore, in Comparative Example 2, abnormalities such as lamination occurred in the carbonaceous material after molding but before drying, significantly reducing product yield and making it impossible to prepare a sufficient number of test specimens for crushing strength.

[0047] On the other hand, in Examples 1 to 3 of the present invention, regardless of the difference between pellet and briquette shapes, it was possible to reduce the amount of binder while maintaining the crushing strength and yield required for operation.

[0048] Comparing the particle size distributions of the five blended carbide powders in Figures 8A, 8B, 8C, 9A, and 9B, it can be seen that the particle size distribution of Comparative Example 1 in Figure 8A is a single-peak distribution overall. The particle size distribution of Comparative Example 2 in Figure 9B is a distribution that slopes gently from the maximum particle size side to the final particle size side. On the other hand, the particle size distributions of Invention Examples 1 to 3 are all bimodal, with peaks on the large particle size side and the small particle size side. Such particle size distributions with two maxima allow the present invention to increase the packing density of the carbide and thereby increase the strength of the carbide molded body, while reducing the amount of binder added within an economically reasonable range.

[0049]

[0050] REFERENCE SIGNS LIST 1 Converter body 2 Top-blowing lance 3 Bottom-blowing tuyere 4 Flue 5 Skirt 6 Hopper 7 Charge chute 8 Molten iron 9 Slag 10 Converter exhaust gas 21 Carbonized carbide 22 Carbide powder 23 Carbide powder mixture 24 Carbide molded body immediately after molding 24a Briquette 24b Pellets 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 Pellet manufacturing machine 42 Hopper 43 Screw feeder 44 Cutter 45 Belt conveyor

Claims

1. A carbide molded body obtained by molding carbide powder and a binder, wherein the carbide in the carbide molded body has: (a) a maximum grain size that is less than 1 / 2 the characteristic length d of the carbide molded body; and (b) when divided so that the large grain size side accounts for 70 volume % and the small grain size side accounts for 30 volume %, the ratio of the median grain size D50 of the carbide on the large grain size side to the median grain size D50 of the carbide on the small grain size side is 1.5 or more and 2.3 or less.

2. The carbide molded body according to claim 1, wherein the carbide molded body has a crushing strength of 490 N or more per piece.

3. A method for producing a charcoal molded body produced by molding charcoal powder and a binder, comprising: carbonizing at least one selected from the group consisting of coal, plant biomass, and waste plastic to produce a charcoal raw material; sieving and crushing the charcoal raw material; (a') generating the charcoal powder from the charcoal raw material, the charcoal powder being divided into a plurality of groups of different particle sizes within a range in which the maximum particle size is less than 1 / 2 of the representative length d of the charcoal molded body; and (b') dividing the charcoal powder so that the large particle size side accounts for 70 volume % and the small particle size side accounts for 30 volume %, and mixing the charcoal powder so that the ratio of the median particle size D50 of the charcoal on the large particle size side to the median particle size D50 of the charcoal on the small particle size side is in the range of 1.5 to 2.3 to produce the charcoal powder before molding.

4. The method for producing a carbide molded body according to claim 3, wherein the carbide molded body has a crushing strength of 490 N or more per piece.

Images