METHOD FOR PRODUCING PIG IRON

Abstract

A method for producing pig iron may use a blast furnace with a tuyere. Such a method may include: charging a first layer containing an iron ore material and a second layer containing coke alternately in the blast furnace; and reducing and melting the iron ore material in the charged first layer while injecting an auxiliary reductant into the blast furnace by hot air blown from the tuyere. An aggregate containing a reduced iron molded product obtained through compression molding of reduced iron may be blended into the first layer. The iron ore material may contain iron ore pellets as a principal material. An average basicity of the reduced iron molded product may be less than or equal to 0.5, and an average basicity of the iron ore pellets may be greater than or equal to 0.9.

Description

TECHNICAL FIELD

The present invention relates to a method for producing pig iron.

BACKGROUND ART

A method of producing pig iron through charging a first layer containing an iron ore material and a second layer containing coke alternately in a blast furnace, and reducing and melting the iron ore material while injecting an auxiliary reductant into the blast furnace by hot air blown from a tuyere is known. During this, the coke serves as a heat source for melting the iron ore material, a reducing agent for the iron ore material, a recarburizing agent for carburizing the molten iron to lower the melting point, and a spacer for ensuring gas permeability in the blast furnace. Due to the coke maintaining gas permeability, descent of the burden is stabilized, and in turn, stable operation of the blast furnace is enabled.

In operation of the blast furnace, it is desirable that the proportion of the coke is low in light of cost reduction. However, a decrease in the proportion of the coke leads to attenuation of the above-described roles played by the coke. For example, as a method of decreasing the proportion of the coke, in other words increasing the proportion of the iron ore material, a blast furnace operation method of limitedly charging reduced iron of a small grain size to a peripheral portion of the blast furnace has been proposed (see Japanese Unexamined Patent Application, Publication No. H11-315308). In the blast furnace operation method, it is reportedly possible to increase the filling rate of the raw material while maintaining the roles of the coke as the heat source, the reducing agent, the recarburizing agent, and the spacer in the central portion of the furnace, by charging the reduced iron requiring no reduction only to the peripheral portion of the furnace.

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: Japanese Unexamined Patent Application, Publication No. H11-315308

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In light of recent requirement of a decrease in CO₂ emission, a further decrease in the amount of coke used in the blast furnace operation is demanded. In the conventional blast furnace operation method, of the roles played by the coke, the roles as the heat source, the reducing agent, and the recarburizing agent may be substituted by an auxiliary reductant injected from a tuyere. On the other hand, the role as the spacer is played only by the coke. In the conventional blast furnace operation method, the charging position of the reduced iron is limited to the peripheral portion of the furnace. In addition, the amount of the coke used is only relatively reduced by the charging of the reduced iron. Therefore, in the conventional blast furnace operation method, only a limited decrease in the amount of the coke used is possible, and the recent demand for a decrease in the CO₂ emission may not be sufficiently met.

The present invention was made in view of the foregoing circumstances, and an objective thereof is to provide a method for producing pig iron enabling a decrease in the amount of the coke used while maintaining stable operation of the blast furnace.

Means for Solving the Problems

A production method of pig iron using a blast furnace with a tuyere according to one aspect of the present invention includes: charging a first layer containing an iron ore material and a second layer containing coke alternately in the blast furnace; and reducing and melting the iron ore material in the charged first layer while injecting an auxiliary reductant into the blast furnace by hot air blown from the tuyere, wherein: an aggregate containing a reduced iron molded product obtained through compression molding of reduced iron is blended into the first layer, the iron ore material contains iron ore pellets as a principal material, an average basicity of the reduced iron molded product is less than or equal to 0.5, and an average basicity of the iron ore pellets is greater than or equal to 0.9.

In the method for producing pig iron, the first layer containing the iron ore material contains, as an aggregate, a reduced iron molded product obtained through compression molding of reduced iron. Since the reduced iron molded product facilitates permeation of hot air during softening and fusing of the first layer in the melting step, the method for producing pig iron can decrease the amount of the coke for ensuring gas permeability. Furthermore, since the method for producing pig iron uses the reduced iron molded product in which the average basicity is less than or equal to 0.5, the reduced iron molded product can be obtained at relatively low cost. Moreover, since the method for producing pig iron uses, as the principal material, the iron ore pellets in which the average basicity is greater than or equal to 0.9, an increase in viscosity can be inhibited when the reduced iron molded product, having the low basicity, has melted, thereby promoting melting down. Thus, gas permeability in mainly a cohesive zone can be improved, and furthermore, an amount of the coke used can be decreased. Consequently, using the method for producing pig iron enables the amount of the coke used to be decreased while maintaining stable operation of the blast furnace.

A content of the iron ore pellets in the iron ore material is preferably greater than or equal to 50% by mass. When the content of the iron ore pellets is thus greater than or equal to the lower limit, the gas permeability can be further improved. The iron ore pellets are preferably self-fluxing. When the iron ore pellets are thus self-fluxing, melting down of the reduced iron molded product is promoted, whereby the gas permeability can be further improved.

A ratio R of a consumption of the iron ore pellets to a consumption of the reduced iron molded product preferably satisfies the following inequality 1. When the ratio R of the consumption of the iron ore pellets to the consumption of the reduced iron molded product thus satisfies the following inequality 1, the effect of improving the gas permeability due to the melting down of the reduced iron molded product can be more certainly expressed.

$\begin{array}{cc}R\ge \frac{\left[{\left(C/S\right)}_{\mathrm{Critical}}-{\left(C/S\right)}_{\mathrm{HBI}}\right]}{\left[{\left(C/S\right)}_P-{\left(C/S\right)}_{\mathrm{Critical}}\right]}\times \frac{{\left(\%{\mathrm{SiO}}_2\right)}_{\mathrm{HBI}}}{{\left(\%{\mathrm{SiO}}_2\right)}_P}& 1\end{array}$

In the above inequality 1: (C/S) represents an average basicity; (% SiO₂) represents a content of SiO₂ (% by mass); HBI, being in subscript, represents the reduced iron molded product; and P represents the iron ore pellets, wherein (C/S)_Critical represents a critical basicity of the HBI.

Herein, the “principal material” as referred to means a material having the greatest content in terms of mass. The “basicity” as referred to means a ratio of the mass of CaO to the mass of SiO₂. It is to be noted that the “average basicity” as referred to means, in a case in which the target mass is constituted by a plurality of granular bodies, a ratio of a total mass of CaO to a total mass of SiO₂ in the plurality of granular bodies.

With regard to the “critical basicity”, as shown in FIG. 3, when the average basicity of the HBI is adopted as a parameter, pressure loss of a sample-packed bed is continuously measured, and a maximum value thereof (maximum pressure loss) is plotted, the “critical basicity” means the average basicity at which the maximum pressure loss begins to decrease. It is to be noted that as shown in FIG. 5, for example, the sample-packed bed can be constituted of, from the top: an upper coke layer 72a (20 mm in height); an iron ore layer 72b (110 mm in height); and a lower coke layer 72c (40 mm in height), using a furnace for a large-scale reduction under load test 7 in which a graphite crucible 71 to be filled with a sample has an inner diameter of 75 mm.

Effects of the Invention

As explained in the foregoing, the method for producing pig iron according to the present invention enables a decrease in the amount of the coke used while maintaining stable operation of the blast furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating the method for producing pig iron according to an embodiment of the present invention.

FIG. 2 is a schematic view illustrating the inside of the blast furnace used in the method for producing pig iron in FIG. 1.

FIG. 3 is a graph showing a relationship between the average basicity of the reduced iron molded product and the maximum pressure loss.

FIG. 4 is a schematic partial enlarged view of the vicinity of an area from a cohesive zone to a dripping zone in FIG. 2.

FIG. 5 is a schematic cross-sectional view illustrating a configuration of a furnace for a large-scale reduction under load test used in Examples.

FIG. 6 is a graph showing a temperature profile of heating a sample-packed bed in the Examples.

FIG. 7 is a graph showing a relationship between the temperature of the sample-packed bed and a flow rate of gas supplied.

FIG. 8 is a graph showing results of the Examples.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the method for producing pig iron according to each embodiment of the present invention will be described.

The method for producing illustrated in FIG. 1 uses a blast furnace 1 illustrated in FIG. 2, and includes a charging step S1 and a reducing/melting step S2.

Blast Furnace

The blast furnace 1 includes a tuyere 1a and a taphole 1b provided in a furnace lower portion as illustrated in FIG. 2. Typically, a plurality of tuyeres 1a are provided. The blast furnace 1 is a solid-gas countercurrent type shaft furnace that enables: hot air, which is high-temperature air with high-temperature or normal-temperature oxygen being added as needed, to be blown from the tuyere la into the furnace; a series of reactions such as reduction and melting of an iron ore material 11 described later to take place; and pig iron to be tapped from the taphole 1b. In addition, the blast furnace 1 is equipped with a bell-armor type raw material charging device 2. The raw material charging device 2 will be described later.

Charging Step

In the charging step S1, a first layer 10 and a second layer 20 are alternately charged in the blast furnace 1 as illustrated in FIG. 2. In other words, the numbers of the first layers 10 and the second layers 20 are at least two, respectively.

(First Layer)

The first layer 10 contains the iron ore material 11. Further, an aggregate 12 is blended into the first layer 10. In addition to the iron ore material 11 and the aggregate 12, auxiliary materials such as limestone, dolomite, and silica may also be charged into the first layer 10.

The iron ore material 11 refers to mineral ore serving as an iron raw material. In the reducing/melting step S2, the iron ore material 11 is heated and reduced into molten iron by the hot air blown from the tuyere la. In the method for producing pig iron, the iron ore pellets are the principal material. The “iron ore pellets” are referred to herein are made by using iron ore fine powder in an order of several tens of um, and by improving quality to have characteristics (for example, size, strength, reducibility, and the like) suitable for a blast furnace. It is to be noted that in the method for producing pig iron, the iron ore pellets preferably do not contain sintered iron ore powder.

The lower limit of the average basicity of the iron ore pellets is 0.9, and is more preferably 1.0, being basic, and still more preferably 1.4. When the basicity of the iron ore pellets is less than the lower limit, the melting down of the reduced iron molded product may be less likely to be promoted, and the gas permeability may deteriorate. The upper limit of the average basicity of the iron ore pellets is not particularly limited, and the average basicity of the iron ore pellets is typically less than or equal to 2.0.

The lower limit of the content of the iron ore pellets in the iron ore material 11 is preferably 50% by mass, more preferably 90% by mass, and still more preferably 100% by mass, i.e., the iron ore material 11 is still more preferably completely constituted by the iron ore pellets. When the content of the iron ore pellets is thus greater than or equal to the lower limit, the gas permeability can be further improved.

The iron ore pellets are preferably self-fluxing. When the iron ore pellets are thus self-fluxing, the melting down of the reduced iron molded product may be promoted, whereby the gas permeability can be further improved.

It is preferred that the iron ore pellets have a porosity resulting from large open pores having a pore size of greater than or equal to 4 μm which is greater than or equal to 21%. When the iron ore material contain the iron ore pellets, of which the porosity resulting from the large open pores having the pore size of greater than or equal to 4 μm is greater than or equal to 21%, a reduction percentage of the iron ore material can be increased, whereby the amount of the coke used can be further decreased. As used herein, the “porosity resulting from large open pores having a pore size of greater than or equal to 4 μm” refers to a percentage of a volume of the large open pores having the pore size of greater than or equal to 4 μm with respect to an apparent volume of the iron ore pellets, the percentage being calculated by ε₀×A₊₄/A [%], wherein: ε₀ [%] is an open porosity of the iron ore pellets; A [cm³/g] is a total capacity of pores per unit weight of the iron ore pellets; and A₊₄ [cm³/g] is a total capacity of pores having a pore size of greater than or equal to 4 μm per unit weight of the iron ore pellets, each of these being measured by using a mercury intrusion porosimeter (for example, AutoPorc III 9400, manufactured by Shimadzu Corporation). Note that an open pore refers to a pore connected to the outside of the iron ore pellets, while a closed pore refers to a pore closed inside the iron ore pellets.

The iron ore pellets preferably contain MgO. MgO has the effects of enhancing a slag desulfurization ability at a hearth level, and enhancing reducibility at high temperatures. Thus, it is considered that by making the behavior of the melting down of the iron ore material 11 become closer to that of the reduced iron molded product, there is an effect of promoting the melting down of the reduced iron molded product. The lower limit of a content of the MgO in the iron ore material 11 is preferably 1% by mass, and more preferably 1.5% by mass. On the other hand, the upper limit of the content of the MgO is preferably 4% by mass, and more preferably 3% by mass. When the content of the MgO is less than the lower limit, the effect of promoting the melting down of the reduced iron molded product may not be sufficiently obtained. Conversely, when the content of the MgO is greater than the upper limit, strength of the iron ore pellets may deteriorate.

The iron ore material 11 may include, in addition to the iron ore pellets: sintered iron ore, lump iron ore, carbon composite agglomerated iron ore, metal, and/or the like. It is to be noted that in view of improving the gas permeability, a content of the sintered iron ore in the iron ore material 11 is preferably less than or equal to 10% by mass, and more preferably 0% by mass, i.e., the sintered iron ore is more preferably not contained in the iron ore material 11.

It is to be noted that the reduced iron molded product contained in the aggregate 12, described later, may also serve as the iron raw material, but in the present specification, the reduced iron molded product is not contained in the iron ore material 11.

The aggregate 12 is for improving gas permeability in a cohesive zone D described later, whereby the hot air is permeated to the central portion of the blast furnace 1. The aggregate 12 contains a reduced iron molded product (hot briquette iron: HBI) obtained through compression molding of reduced iron.

The HBI is obtained by molding direct reduced iron (DRI) in a hot state. The DRI is high in porosity and has a drawback of oxidization and heat generation during marine transportation and outdoor storage, while the HBI is low in porosity and not likely to be re-oxidized. After serving to ensure gas permeability in the first layer 10, the aggregate 12 functions as a metal and becomes molten iron. Since the aggregate 12 has a high metallization degree and requires no reduction, the reduction agent is not much required for becoming the molten iron. CO₂ emission can thus be reduced. Note that the “metallization degree” refers to a proportion [% by mass] of metallic iron with respect to the total iron content.

The upper limit of the average basicity of the reduced iron molded product is 0.5, and more preferably 0.4. The reduced iron molded product contains, as slag components derived from iron ore, SiO₂ and/or Al₂O₃, and the average basicity typically tends to be low. Since in the method for producing pig iron, the reduced iron molded product having a basicity of less than or equal to the upper limit is used, it is not necessary to prepare a high-grade reduced iron molded product which has an increased basicity by SiO₂ and/or Al₂O₃ being eliminated, CaO being added, and/or the like. Therefore, the pig iron can be produced at low cost. On the other hand, the lower limit of the average basicity of the reduced iron molded product is not particularly limited, and may be 0.

A ratio R of a consumption of the iron ore pellets to a consumption of the reduced iron molded product preferably satisfies the following inequality 1. When the ratio R of the consumption of the iron ore pellets to the consumption of the reduced iron molded product thus satisfies the following inequality 1, the effect of improving the gas permeability due to the melting down of the reduced iron molded product can be more certainly expressed.

$\begin{array}{cc}R\ge \frac{\left[{\left(C/S\right)}_{\mathrm{Critical}}-{\left(C/S\right)}_{\mathrm{HBI}}\right]}{\left[{\left(C/S\right)}_P-{\left(C/S\right)}_{\mathrm{Critical}}\right]}\times \frac{{\left(\%{\mathrm{SiO}}_2\right)}_{\mathrm{HBI}}}{{\left(\%{\mathrm{SiO}}_2\right)}_P}& 1\end{array}$

The above inequality 1 is explained in detail below. FIG. 3 is a graph showing a relationship between the average basicity of the HBI and the maximum pressure loss of the packed bed in which the first layer 10 and the second layer 20 are alternately charged. It can be understood that this maximum pressure loss being lower indicates that the gas permeability is higher. From FIG. 3, it is revealed that when the average basicity of the HBI is greater than a certain value, an improvement in the gas permeability is recognized. This certain value is the critical basicity. It is considered that in a case in which CaO being greater than or equal to this critical basicity is present, the SiO₂ in the HBI changes into a calcium silicate melt, and the viscosity of the molten iron generated from the HBI decreases, whereby melting down is promoted. In other words, it can be deemed that in order to obtain the melting down-promoting effect of the HBI, CaO being greater than or equal to the critical basicity is needed.

In FIG. 3, the CaO is provided from the HBI, but the CaO can also be provided from the iron ore pellets. In this case, it is considered that when the CaO amount, with respect to the SiO₂ amount in the HBI and the iron ore pellets combined, is greater than the critical basicity, melting down of the HBI is promoted, whereby the gas permeability of the packed bed can be enhanced.

The SiO₂ amount and the CaO amount in the HBI and the iron ore pellets combined are represented in the following formulae 2, with the consumption of the reduced iron molded product being represented by M_HBI [kg], and the consumption of the iron ore pellets being represented by M_P [kg].

[SiO₂ amount]=(% SiO₂)_HBI×M_HBI+(% SiO₂)_p×M_P

[CaO amount]=(C/S)_HBI×(% SiO₂)_HBI×M_HBI+(C/S)_HBI×(% SiO₂)_p×M_P   (2)

Here, since it is considered that when, as described above, the melting down of the HBI is promoted when CaO amount/SiO₂ amount≥(C/S)_Critical is satisfied, the above formulae 2 are substituted into this inequality and R=M_P/M_HBI is solved for to obtain the above inequality 1.

The lower limit of a charged rate of the reduced iron molded product is preferably 100 kg and more preferably 150 kg per 1 ton of the pig iron. When the charged rate of the reduced iron molded product is less than the lower limit, the function of the aggregate 12 ensuring gas permeability in the cohesive zone D in the reducing/melting step S2 may not be sufficiently exerted. On the other hand, the charged rate of the reduced iron molded product is defined as appropriate in such a range that the aggregate is not excessive and does not diminish the effect of the aggregate, and the upper limit of the charged rate of the reduced iron molded product is, for example, 700 kg per 1 ton of the pig iron.

The lower limit of a ratio of an average grain size of the reduced iron molded product to an average grain size of the iron ore material 11 is preferably 1.3, and more preferably 1.4. As illustrated in FIG. 4, even when a part of the iron ore material 11 in the first layer 10 is melted and moves to the lower side of the blast furnace 1 as a drip slag 13 and the iron ore material 11 is softened and shrunk, the reduced iron molded product having a high melting point is not softened. Blending the reduced iron molded product, which is larger than the iron ore material 11 to at least a certain degree, as the aggregate 12 facilitates the aggregate effect of the reduced iron molded product to be exerted and enables suppression of layer shrinkage of the entire first layer 10. Consequently, due to the ratio of the average grain sizes being greater than or equal to the lower limit, a channel of the hot air shown by an arrow in FIG. 4 can be secured, whereby gas permeability in the reducing/melting step S2 can be improved. Meanwhile, the upper limit of the ratio of the average grain sizes is preferably 10 and more preferably 5. When the ratio of the average grain sizes is greater than the upper limit, it may be difficult to blend the reduced iron molded product uniformly into the first layer 10, leading to an increase in segregation. It is to be noted that the “average grain size” as referred to means a grain size in which a total mass accounts for 50% in a grain size distribution.

The upper limit of a gas permeability resistance index of the reduced iron molded product after a tumbler rotation test is preferably 0.1, and more preferably 0.08. The reduced iron molded product is typically produced and used in different plants, and subjected to transportation. Since volume can be broken and grain size distribution can be altered during the transportation, by using the reduced iron molded product, which ensures that the gas permeability resistance index is less than or equal to a certain value even after the tumbler rotation test, gas permeability in the lumpy zone E, described later, can be improved in actual blast furnace operations. On the other hand, the lower limit of the gas permeability resistance index is not particularly limited and may be a value close to zero, which is a theoretical limit value, but is typically about 0.03. Note that it is only required to use the reduced iron molded product having the gas permeability resistance index less than or equal to a predetermined value as a characteristic, and this does not mean that the tumbler rotation test is required in the method for producing pig iron.

As used herein, the “gas permeability resistance index after a tumbler rotation test” of the reduced iron molded product is calculated as follows. First, the tumbler rotation test is carried out pursuant to Iron Ores—Determination Of Tumble Strength (JIS-M8712:2000) to obtain a grain size distribution of the reduced iron molded product through screening. The grain size distribution is indicated with d_i [cm] being a typical grain size (median) of mesh opening used for the screening, and w_i being a weight fraction of the reduced iron molded product belonging to the typical grain size d_i. By using this grain size distribution, a harmonic mean diameter D_p [cm] and a granularity composition index I_sp are calculated by the following formula 3. Furthermore, by using a gravitational conversion factor g_c [9.807 (g·cm)/(G·sec²)], a gas permeability resistance index K is obtained by the following formula 3. Note that rotational conditions of a tumbler in the tumbler rotation test are 24±1 rpm and 600 times.

D_p=1/(Σ^Wi/_di)

I_sp=100×√{square root over (I_s)}I_p

where I_s=D_p²×Σw_i×(¹/_di−¹/_Dp)²

I_p=1/_Dp²×Σw_i×(d_i−D_p)²   (3)

K=C×^(1.06Ispn)/_(gc×_Dp1.5)

where n=0.47, C=0.55

In addition, in a case in which the reduced iron molded product contains aluminum oxide, the upper limit of the content of the aluminum oxide in the reduced iron molded product is preferably 1.5% by mass and more preferably 1.3% by mass. When the content of the aluminum oxide is greater than the upper limit, it may be difficult to ensure gas permeability in the furnace lower portion due to increases in the melting point and the viscosity of the slag. Consequently, by configuring the content of aluminum oxide in the reduced iron molded product to be less than or equal to the upper limit, an increase in the amount of the coke used can be inhibited. Note that the content of the aluminum oxide may be 0% by mass, i.e., the reduced iron molded product may not contain aluminum oxide, but the lower limit of the content of the aluminum oxide is preferably 0.5% by mass. When the content of the aluminum oxide is less than the lower limit, the reduced iron molded product becomes expensive, and the production cost of the pig iron may be increased.

(Second Layer)

The second layer 20 contains coke 21.

The coke 21 serves: as a heat source for melting the iron ore material 11; to generate CO gas as a reducing agent necessary for reduction of the DIR iron ore material 11; as a recarburizing agent for carburizing the molten iron to lower the melting point; and as a spacer for ensuring gas permeability in the blast furnace 1.

(Charging Method)

Various methods can be used as a method for alternately charging the first layer 10 and the second layer 20. The method is described herein with reference to, as an example, the blast furnace 1 equipped with a bell-armor type raw material charging device 2 (hereinafter, may be also merely referred to as “raw material charging device 2”) illustrated in FIG. 2.

The raw material charging device 2 is provided in a furnace top portion. In other words, the first layer 10 and the second layer 20 are charged from the furnace top. The raw material charging device 2 includes, as illustrated in FIG. 2, a bell cup 2a, a lower bell 2b, and an armor 2c.

The bell cup 2a is where the raw material to be charged is loaded. When the first layer 10 is charged, the raw material constituting the first layer 10 is loaded into the bell cup 2a, and when the second layer 20 is charged, the raw material constituting the second layer 20 is loaded into the bell cup 2a.

The lower bell 2b is in a cone shape expanding downward, and is provided inside the bell cup 2a. The lower bell 2b is vertically movable (FIG. 2 shows an upward moved state with a solid line, and a downward moved state with a dotted line). The lower bell 2b is configured to seal a lower portion of the bell cup 2a when moved upward, and to form a gap on an extended line of a lateral wall of the bell cup 2a when moved downward.

The armor 2c is provided on a lower side with respect to the lower bell 2b, in a furnace wall portion of the blast furnace 1. When the lower bell 2b is moved downward, the raw material falls from the gap, while the armor 2c serves as a rebound plate for rebounding the fallen raw material. In addition, the armor 2c is configured to be protrudable and retractable with respect to a center (central portion) of the blast furnace 1.

By using the raw material charging device 2, the first layer 10 can be charged as follows. Note that the same applies to the second layer 20. In addition, the first layer 10 and the second layer 20 are alternately charged.

First, the lower bell 2b is positioned on the upper side and the raw material of the first layer 10 is charged into the bell cup 2a. When the lower bell 2b is positioned on the upper side, the lower portion of the bell cup 2a is sealed, whereby the raw material is loaded in the bell cup 2a. Note that the loaded amount is an amount of each layer to be charged.

Next, the lower bell 2b is moved downward. As a result, a gap is generated from the bell cup 2a, and the raw material falls through the gap in the furnace wall direction to hit the armor 2c. The raw material that has hit and been rebounded by the armor 2c is charged into the furnace. The raw material falls while moving toward the furnace interior due to the rebound at the armor 2c, and is accumulated while flowing from the fallen position toward the central side of the furnace interior. Since the armor 2c is configured to be protrudable and retractable with respect to the central portion, the fallen position of the raw material can be adjusted by protruding and retracting the armor 2c. This adjustment enables the first layer 10 to be accumulated in a desired shape.

Reducing/Melting Step

In the reducing/melting step S2, the iron ore material 11 in the charged first layer 10 is reduced and melted while an auxiliary reductant is injected into the blast furnace by hot air blown from the tuyere 1a. Note that the operation of the blast furnace is continuous, and thus the reducing/melting step S2 is carried out continuously. On the other hand, the charging step S1 is carried out intermittently, and the first layer 10 and the second layer 20 to be processed in the reducing/melting step S2 are added according to the circumstances of the reduction and melting process of the first layer 10 and the second layer 20 in the reducing/melting step S2.

FIG. 2 illustrates a state in the reducing/melting step S2. As illustrated in FIG. 2, a raceway A, which is a hollow portion in which the coke 21 whirls and is present in an extremely sparse state, is formed in the vicinity of the tuyere la due to the hot air from the tuyere la. In the blast furnace 1, the temperature in the raceway A is the highest, being about 2,000° C. A deadman B, which is a pseudo-stagnation zone of the coke inside the blast furnace 1, is present adjacent to the raceway A. In addition, the dripping zone C, the cohesive zone D, and the lumpy zone E are present in an upward direction in this order from the deadman B.

The temperature in the blast furnace I increases from a top portion toward the raceway A. In other words, the temperature increases in the order of the lumpy zone E, the cohesive zone D, and the dripping zone C. For example, the temperature of the lumpy zone E is about greater than or equal to 20° C. and less than or equal to 1,200° C., while the temperature of the deadman B is about greater than or equal to 1,200° C. and less than or equal to 1,600° C. Note that the temperature of the deadman B varies in the radial direction, and the temperature of a central portion of the deadman B may be lower than the temperature of the dripping zone C. In addition, by stably circulating the hot air in the central portion in the furnace, the cohesive zone D having an inverted V-shaped cross section is formed, whereby gas permeability and reducibility are ensured in the furnace.

In the blast furnace 1, the iron ore material 11 is first heated and reduced in the lumpy zone F. In the cohesive zone D, the iron ore reduced in the lumpy zone E is softened and shrunk. The softened and shrunk iron ore falls as the drip slag, and moves to the dripping zone C. In the reducing/melting step S2, reduction of the iron ore material 11 proceeds principally in the lumpy zone E, while melting of the iron ore material 11 proceeds principally in the dripping zone C. Note that in the dripping zone C and the deadman B, direct reduction proceeds, which is a direct reaction between the fallen liquid iron oxide FeO and carbon in the coke 21.

The aggregate 12 containing the reduced iron molded product exerts the aggregate effect in the cohesive zone D. In other words, even in a state in which the iron ore has been softened and shrunk, the reduced iron molded product having a high melting point is not softened, and secures a gas permeation channel ensuring permeation of the hot air to the central portion of the blast furnace 1.

The reduced iron molded product has a high melting point, but by a carburization reaction from carbon monoxide in the reducing gas and/or carbon in the coke, the melting point becomes lower, whereby the reduced iron molded product becomes molten iron in a temperature range of about 1,500° C. in a lower portion of the cohesive zone D. Even at this point in time, the SiO₂ of the slag components contained in the reduced iron molded product are present in a solid state, resulting in a state of high viscosity due to a state of solid/liquid co-presence with the molten iron from the reduced iron molded product which had melted earlier, whereby the melting down stagnates. Here, in a case of the reduced iron molded product having high basicity, the CaO reacts with the SiO₂ to form a calcium silicate melt, thereby resolving the solid/liquid co-presence and thus promoting the melting down. Also in the case of the reduced iron molded product having low basicity, i.e., containing SiO₂ in a high amount, the SiO₂ supplied from the reduced iron molded product reacts with the CaO supplied from the iron ore pellets having a high basicity, i.e., containing CaO in a high amount, to generate a calcium silicate melt, thereby resolving the solid/liquid co-presence and promoting the melting down of the reduced iron molded product.

The molten iron F obtained by melting the reduced iron is accumulated on a hearth portion, and a molten slag G is accumulated on the molten iron F. The molten iron F and the molten slag G can be tapped from the taphole 1b.

The auxiliary reductant to be injected from the tuyere 1a is exemplified by: finely pulverized coal obtained by finely pulverizing coal to have a grain size of about 50 μm; heavy oil; natural gas; and the like. The auxiliary reductant serves as a heat source, a reduction agent, and a recarburizing agent. In other words, of the roles played by the coke 21, the roles other than that of the spacer are substituted by the auxiliary reductant.

Advantages

In the method for producing pig iron, the first layer 10 containing the iron ore material 11 contains, as an aggregate 12, the reduced iron molded product obtained through compression molding of reduced iron. Since the reduced iron molded product facilitates permeation of hot air during softening and fusing of the first layer 10 in the reducing/melting step S2, the method for producing pig iron can decrease the amount of the coke for ensuring gas permeability. Furthermore, since the method for producing pig iron uses the reduced iron molded product in which the average basicity is less than or equal to 0.5, the reduced iron molded product can be obtained at relatively low cost. Moreover, since the method for producing pig iron uses, as the principal material, the iron ore pellets in which the average basicity is greater than or equal to 0.9, an increase in viscosity can be inhibited when the reduced iron molded product, having the low basicity, has melted, thereby promoting melting down. Thus, gas permeability in mainly the cohesive zone D can be improved, and furthermore, the amount of the coke used can be decreased. Consequently, using the method for producing pig iron enables the amount of the coke used to be decreased while maintaining stable operation of the blast furnace 1.

Other Embodiments

The present invention is not in any way limited to the above-described embodiments.

In the above-described embodiment, the case was described in which it was assumed that the iron ore material of all of the first layers charged contains the iron ore pellets as the principal material, the average basicity of the reduced iron molded product is less than or equal to 0.5, and the average basicity of the iron ore pellets is greater than or equal to 0.9; however, the present invention also includes a configuration in which the iron ore material of at least one of the first layers contains the iron ore pellets as a principal material, the average basicity of the reduced iron molded product is less than or equal to 0.5, and the average basicity of the iron ore pellets is greater than or equal to 0.9. However, of total first layers, the first layer having the above-described configuration preferably account for greater than or equal to 90%, more preferably account for greater than or equal to 95%, and still more preferably account for 100%, i.e., it is still more preferable that the all layers of the first layers have the above-described configuration.

In the above-described embodiment, the case was described in which the method for producing pig iron of the present invention includes only the charging step and the reducing/melting step; however, the method for producing pig iron may include other step(s).

For example, the method for producing pig iron may include a step of charging, into the central portion of the blast furnace, a mixture of coke and a reduced iron molded product. In this case, in the reduced iron molded product in the mixture, it is preferred that a proportion of the reduced iron molded product having a grain size of greater than or equal to 5 mm is greater than or equal to 90% by mass, and a content of the reduced iron molded product in the mixture is less than or equal to 75% by mass. When hot air reaches the central portion of the blast furnace, the hot air goes up in the central portion. By thus including, in the central portion, the reduced iron molded product of a large grain size at a content being less than or equal to the upper limit, the sensible heat can be effectively used without disturbing the flow of the hot air. Consequently, a further decrease in the amount of the coke used is enabled. Here, the “central portion” of the blast furnace refers to a region at a distance of less than or equal to 0.2 Z from the center, Z being a radius of a furnace throat portion.

Furthermore, the method for producing pig iron may include a step of finely pulverizing powder derived from the reduced iron molded product and coal. In this case, it is preferred that the fine powder obtained by the fine pulverizing step is included as the auxiliary reductant. A part of the reduced iron molded product is pulverized into powder due to a conveying process and the like. Such powder lowers gas permeability in the blast furnace, and is not appropriate for use in the first layer. In addition, the powder has a large specific surface area, and is thus re-oxidized into iron oxide. Injecting the auxiliary reductant containing the iron oxide from the tuyere enables improvement of gas permeability. Consequently, by finely pulverizing powder derived from the reduced iron molded product together with coal and using fine powder obtained by finely pulverizing the powder and the coal as the auxiliary reductant to be injected from the tuyere, the reduced iron molded product can be effectively used and gas permeability in the blast furnace can be improved.

Although the case of employing the bell-armor type as the charging step according to the above-described embodiment has been described, other types may also be employed. The other types include a bell-less type. With the bell-less type, charging can be carried out by using a swivel chute and adjusting the angle thereof.

EXAMPLES

Hereinafter, the embodiments of the present invention will be explained in detail by way of Examples; however, the present invention is not limited to these Examples. An effect of the basicity of the iron ore pellets on gas permeability was studied by conducting a large-scale reduction under load test simulating the peripheral portion of the blast furnace.

FIG. 5 illustrates a furnace for a large-scale reduction under load test 7 used in this experiment. A graphite crucible 71 to be filled with a sample was configured to have an inner diameter of 75 mm. A sample-packed bed 72 was constituted of, from the top, an upper coke layer 72a (20 mm in height), an iron ore layer 72b (110 mm in height), and a lower coke layer 72c (40 mm in height). The iron ore layer 72b corresponds to the first layer 10 of the present invention, and the upper coke layer 72a and the lower coke layer 72c correspond to the second layer 20.

The iron ore layer 72b was configured with a mixture of the reduced iron molded product (HBI) and the iron ore material. It is to be noted that in the iron ore layer 72b, the total iron content (T. Fe) was configured to be constant.

Chemical characteristics of the HBI used are shown in Table 1. The average basicity of the HBI was 0.46. A charged rate of the HBI was 250 kg per 1 ton of pig iron.

TABLE 1
Contents
T•Fe	FeO	M•Fe	SiO2	CaO	Al2O3	MgO	Metallization	Basicity
[% by	[% by	[% by	[% by	[% by	[% by	[% by	Degree	C/S
mass]	mass]	mass]	mass]	mass]	mass]	mass]	[% by mass]	(—)
92.02	4.66	85.50	1.97	0.91	0.80	0.05	92.9	0.46

As the iron ore material, the following three types were prepared: (1) iron ore pellets having an average basicity of 0.04 (SiO₂ content: 5.44% by mass; MgO content: 0.54% by mass); (2) iron ore pellets having an average basicity of 1.20 (SiO₂ content: 4.23% by mass; MgO content: 2.11% by mass; and (3) self-fluxing sintered iron ore having an average basicity of 2.10 (SiO₂ content: 5.40% by mass; MgO content: 1.00% by mass). While heating each sample-packed bed 72 using the iron ore materials of the above-described (1) to (3) with a temperature profile shown in FIG. 6 by using an electric furnace 73, gas (reducing gas) of a composition shown in FIG. 7 was supplied thereto. The gas was supplied from a gas supply pipe 74 provided in a lower portion of the furnace for a large-scale reduction under load test 7, and discharged from a gas discharge pipe 75 provided in an upper portion. A total feed rate of the gas was 40 NL/min, and temperature control was carried out by two thermocouples 76. In addition, a load applied to the sample-packed bed 72 was 1 kgf/cm². The load was applied by applying a weight of a weight 78 via a loading rod 77.

A pressure loss of the sample-packed bed 72 was continuously measured under the above-described conditions, and a time-integrated value (S value) of the pressure loss was calculated. The S value can be used as an indicator for evaluating softening and melting behavior of the iron ore layer 72b, and it is considered that the S value being lower indicates higher gas permeability. The results are shown in FIG. 8.

From the results of FIG. 8, the S values are in the order of: the iron ore pellets having the average basicity of 1.20, the iron ore pellets having the average basicity of 0.04, and the self-fluxing sintered iron ore having the average basicity of 2.10, whereby it is revealed that by using the iron ore pellets having the average basicity of greater than or equal to 0.9 as the iron ore material, the gas permeability is improved.

The average basicity (being CaO amount/SiO₂ amount) determined from the CaO amount and the SiO₂ amount calculated based on the above-described formulae 2 was: (1) 0.10 in the case of using the iron ore pellets having the average basicity of 0.04; and (2) 1.13 in the case of using the iron ore pellets having the average basicity of 1.20. The critical basicity of the HBI used was 0.88, and it can be deemed that by setting the basicity determined from the CaO amount and the SiO₂ amount calculated based on the above-described formulae 2 to greater than or equal to the critical basicity of the HBI, i.e., by satisfying the above-described inequality 1, the gas permeability is improved.

INDUSTRIAL APPLICABILITY

Using the method for producing pig iron according to the present invention enables a decrease in the amount of the coke used while maintaining stable operation of the blast furnace.

EXPLANATION OF THE REFERENCE SYMBOLS

• 1 Blast furnace
• 1a Tuyere
• 1b Taphole
• 2 Raw material charging device
• 2a Bell cup
• 2b Lower bell
• 2c Armor
• 10 First layer
• 11 Iron ore material
• 12 Aggregate
• 13 Drip slag
• 20 Second layer
• 21 Coke
• 7 Furnace for large-scale reduction under load test
• 71 Graphite crucible
• 72 Sample-packed bed
• 72a Upper coke layer
• 72b Iron ore layer
• 72c Lower coke layer
• 73 Electric furnace
• 74 Gas supply pipe
• 75 Gas discharge pipe
• 76 Thermocouple
• 77 Loading rod
• 78 Weight
• A Raceway
• B Deadman
• C Dripping zone
• D Cohesive zone
• E Lumpy zone
• F Molten iron
• G Molten slag

Claims

1. A method for producing pig iron using a blast furnace with a tuyere, the method comprising:

charging a first layer comprising an iron ore material and a second layer comprising coke alternately in the blast furnace; and

reducing and melting the iron ore material in the first layer that is charged while injecting an auxiliary reductant into the blast furnace by hot air blown from the tuyere,

wherein an aggregate comprising a reduced iron molded product obtained through compression molding of reduced iron is blended into the first layer,

wherein the iron ore material comprises iron ore pellets as a principal material,

wherein an average basicity of the reduced iron molded product is less than or equal to 0.5, and

wherein an average basicity of the iron ore pellets is greater than or equal to 0.9.

2. The method of claim 1, wherein a content of the iron ore pellets in the iron ore material is greater than or equal to 50% by mass.

3. The method of claim 1, wherein the iron ore pellets are self-fluxing.

4. The method of claim 1, wherein an R ratio of a consumption of the iron ore pellets to a consumption of the reduced iron molded product satisfies inequality (1): R ≥ [ ( C / S ) Critical - ( C / S ) HBI ] [ ( C / S ) P - ( C / S ) Critical ] × ( % ⁢ SiO 2 ) HBI ( % ⁢ SiO 2 ) P, ( 1 )

wherein (C/S) is an average basicity, (% SiO2) is a content of SiO2 in % by mass, HBI, in subscript, is the reduced iron molded product, P is the iron ore pellets, and (C/S)Critical is a critical basicity of the HBI.

5. The method of claim 1, wherein the average basicity of the iron ore pellets is greater than or equal to 1.0.

6. The method of claim 1, wherein the average basicity of the iron ore pellets is greater than or equal to 1.4.

7. The method of claim 1, wherein the average basicity of the iron ore pellets is less than or equal to 2.0.

8. The method of claim 1, wherein the content of the iron ore pellets in the iron ore material is greater than or equal to 90% by mass.

9. The method of claim 1, wherein the content of the iron ore pellets in the iron ore material is greater than or equal to 100% by mass.

10. The method of claim 1, wherein the iron ore pellets have a porosity resulting from open pores of a pore size of greater than or equal to 4 μm which is greater than or equal to 21%.

11. The method of claim 1, wherein the iron ore pellets comprise MgO.

12. The method of claim 1, wherein the iron ore pellets comprise MgO in at least 1% by mass.

13. The method of claim 1, wherein the iron ore pellets comprise MgO in at least 1.5% by mass.

14. The method of claim 1, wherein the iron ore pellets comprise MgO in no more than 4% by mass.

15. The method of claim 1, wherein the iron ore pellets comprise MgO in no more than 3% by mass.