Method for producing reduced iron

Abstract

Provided is a method for producing reduced iron capable of suppressing agglomeration of iron ore pellets. A method for producing reduced iron according to the present invention is characterized by comprising a pellet production step in which an iron ore powder, a binder, and auxiliary raw materials are mixed and granulated to obtain green pellets, and then the green pellets are fired to obtain iron ore pellets, and a reduction step in which the iron ore pellets are charged into a vertical shaft furnace, and the iron ore pellets are reduced to obtain reduced iron ore pellets. The present invention is characterized in that iron ore having a composition in which the total amount of Fe is 63% by mass or less and the sum of SiO2 and Al2O3 is 4% by mass or more is contained as a raw material for the iron ore powder to obtain reduced iron pellets by direct reduction with a reducing gas, and in that the reduced iron pellets satisfy the following formula. (SiO2 + Al2O3 + CaO + MgO + FeO) / M.Fe is greater than or equal to 0.15.

Description

Technical Field

[0001] The present invention relates to a method for producing reduced iron, which comprises the step of directly reducing iron ore pellets in a vertical shaft furnace. Background Art

[0002] In the current mainstream reduction method of direct reduction ironmaking, iron ore pellets or lump ore are used as raw materials in the vertical shaft furnace. In the high-temperature reducing atmosphere in the vertical shaft furnace, the raw materials in the furnace are caused to stick together, which is called agglomeration. This makes it difficult for the raw materials in the furnace to fall, which reduces productivity. Therefore, it is necessary to control the maximum temperature in the furnace to suppress agglomeration, so it is impossible to increase the temperature in the furnace to increase the reduction rate.

[0003] As a technology to suppress agglomeration, research on auxiliary raw materials for iron ore pellets and evaluation of the iron grade of iron ore have been conducted. Patent Document 1 describes pellets in which "the surface of the raw iron ore is coated with cement." In addition, Non-Patent Document 1 states that "the higher the iron grade of the pellets, the higher the shrinkage rate and the easier it is to form agglomerates."

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 62-7806

[0007] Non-patent literature

[0008] Non-patent document 1: Dentaro Kaneko and two others, "Study on the agglomeration phenomenon during the reduction process of iron oxide", Iron and Steel, 64th year (1978), No. 6, pp. 681-690 Summary of the invention

[0009] However, if impurities not needed in actual reduction are to be added as in Patent Document 1, large-scale equipment modification is required, which is not ideal because the increase in slag ratio leads to an increase in power consumption, etc., and there is still room for improvement in suppressing agglomeration. In addition, as shown in the embodiments of the present invention described later, the metallization rate in the reduction process and the basicity of the iron ore pellets will affect agglomeration. Therefore, if only the grade of the iron ore is concerned as in Non-Patent Document 1, it does not mean that the operation of reducing iron with suppressed agglomeration can be achieved. Therefore, a method for manufacturing reduced iron that can suppress the agglomeration of iron ore pellets based on a comprehensive evaluation criteria including the type of iron ore raw material, the metallization rate in the reduction process, the basicity of the iron ore pellets, etc. is needed.

[0010] In view of the above problems, an object of the present invention is to provide a method for producing reduced iron capable of suppressing agglomeration of iron ore pellets.

[0011] The present inventors have conducted intensive research to solve the above problems and found that SiO contained in reduced iron pellets 2 、Al 2 O 3 When the contents of CaO and MgO, the content of iron oxide FeO remaining in the reduced iron pellets, and the content of metallic iron M.Fe contained in the reduced iron pellets satisfy the following formula (1), agglomeration of iron ore pellets can be suppressed.

[0012] (SiO 2 +Al 2 O 3 +CaO+MgO+FeO) / M.Fe≥0.15···(1)

[0013] That is, the gist of the present invention is as follows.

[0014] [1] A method for producing reduced iron, characterized by comprising:

[0015] a pellet manufacturing step of mixing iron ore powder, a binder and auxiliary raw materials and granulating them to obtain green pellets, and then calcining the green pellets to obtain iron ore pellets; and

[0016] In the reduction step, the iron ore pellets are loaded into a vertical shaft furnace and directly reduced by reducing gas to obtain reduced iron pellets.

[0017] The total amount of Fe is 63 mass % or less and SiO 2 With Al 2 O 3 As a raw material of the iron ore powder,

[0018] The reduced iron pellets satisfy the following formula (1).

[0019] (SiO 2 +Al 2 O 3 +CaO+MgO+FeO) / M.Fe≥0.15···(1)

[0020] Among them, in formula (1), SiO 2 、Al 2 O 3 , CaO and MgO are respectively SiO contained in the above-mentioned reduced iron pellets. 2 、Al 2 O 3 , CaO and MgO contents (mass %), FeO is the content (mass %) of iron oxide remaining in the above-mentioned reduced iron pellets, and M.Fe is the content (mass %) of metallic iron contained in the above-mentioned reduced iron pellets.

[0021] It should be noted that in this specification, the left side of formula (1) (SiO 2 +Al 2 O 3 +CaO+MgO+FeO) / M.Fe is called the "agglomeration index".

[0022] [2] The method for producing reduced iron according to [1], wherein (A) the SiO contained in the iron ore pellets is intentionally set to 2 、Al 2 O 3 , the contents of CaO and MgO, and (B) one or both of the metallization rate in the above-mentioned reduction step so that the above-mentioned reduced iron pellets satisfy the above-mentioned formula (1).

[0023] [3] The method for producing reduced iron according to [2], wherein the above (B) is predetermined, and the above (A) is intentionally set so that the above reduced iron pellets satisfy the above formula (1).

[0024] [4] A method for producing reduced iron according to [2] or [3], wherein the setting of the above-mentioned (A) is carried out by setting (A-1) the composition of the above-mentioned iron ore powder in the above-mentioned pelletizing step, and (A-2) one or both of the types and contents of the above-mentioned binder and the above-mentioned auxiliary raw materials.

[0025] [5] The method for producing reduced iron according to [4], wherein the above (A-2) is predetermined, and the above (A-1) is intentionally set so that the above reduced iron pellets satisfy the above formula (1).

[0026] [6] The method for producing reduced iron according to any one of [1] to [5], wherein the metallization rate in the reduction step is set within a range of 88 to 96%.

[0027] [7] The method for producing reduced iron according to any one of [1] to [6], wherein the temperature of the reducing gas in the reducing step is determined based on the value of the left side of the formula (1).

[0028] According to the method for producing reduced iron of the present invention, agglomeration of iron ore pellets can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagrams showing (a) a load softening reduction test apparatus and (b) an I-type rotation test machine used in each experimental example.

[0030] Figure 2 The relationship between T.Fe of the reduced iron pellets in Experimental Example 1 and the agglomeration strength of the reduced iron pellets is shown.

[0031] Figure 3 The relationship between the agglomeration index of the reduced iron pellets in Experimental Example 1 and the agglomeration strength of the reduced iron pellets is shown.

[0032] Figure 4 The relationship between T.Fe of the reduced iron pellets obtained in Experimental Example 2 and the agglomeration strength of the reduced iron pellets is shown.

[0033] Figure 5 The relationship between the agglomeration index of the reduced iron pellets in Experimental Example 2 and the agglomeration strength of the reduced iron pellets is shown.

[0034] Figure 6 The relationship between the metallization rate and the agglomeration index of the reduced iron pellets in Experimental Example 3 is shown.

[0035] Figure 7 The relationship between the blending ratio of low- and medium-grade iron ore in the iron ore pellets in Experimental Example 4 and the agglomeration index of the reduced iron pellets is shown.

[0036] Figure 8 The relationship between the agglomeration index of the reduced iron pellets in Experimental Example 4 and the agglomeration strength of the reduced iron pellets is shown.

[0037] Fig. 9 The relationship between the agglomeration index of the reduced iron pellets in Experimental Example 5 and the upper limit temperature in the furnace is shown. DETAILED DESCRIPTION

[0038] The following is a description of the method for producing reduced iron according to the present invention. It should be noted that the embodiment described below is only an example of the present invention, and the specific example does not limit the configuration of the present invention.

[0039] A method for producing reduced iron according to an embodiment of the present invention is characterized in that it comprises: a pellet production step of mixing iron ore powder, a binder and an auxiliary raw material and granulating to obtain green pellets, then calcining the green pellets to obtain iron ore pellets, and a reduction step of charging the iron ore pellets into a vertical shaft furnace and directly reducing them with a reducing gas to obtain reduced iron pellets. 2 With Al 2 O 3 The iron ore powder is made of an iron ore having a composition in which the sum of the above-mentioned iron ore and the above-mentioned reduced iron pellets is 4% by mass or more, and the above-mentioned reduced iron pellets satisfy the following formula (1).

[0040] (SiO 2 +Al 2 O 3 +CaO+MgO+FeO) / M.Fe≥0.15···(1)

[0041] Among them, in formula (1), SiO2 、Al 2 O 3 , CaO and MgO are respectively SiO contained in the above-mentioned reduced iron pellets. 2 、Al 2 O 3 , CaO and MgO contents (mass %), FeO is the content (mass %) of iron oxide remaining in the above-mentioned reduced iron pellets, and M.Fe is the content (mass %) of metallic iron contained in the above-mentioned reduced iron pellets.

[0042] The raw materials of iron ore pellets are generally composed of iron ore powder, binder and auxiliary materials. 2 O 3 Content and SiO 2 Content, high grade iron ore, medium grade iron ore and low grade iron ore are defined as follows.

[0043] High-grade iron ore: T.Fe>63%, or Al 2 O 3 +SiO 2 <4%

[0044] Medium-grade iron ore: T.Fe≤63%, and 4%≤Al 2 O 3 +SiO 2 <6%

[0045] Low-grade iron ore: T.Fe≤63%, and Al 2 O 3 +SiO 2 ≥ 6%

[0046] It should be noted that, when analyzing the composition of iron ore, part of the components volatilize due to heating, and the volatilization amount (ignition loss) is measured. T.Fe is the content of the iron ore composition including the volatilization amount (the content in the iron ore before volatilization), and SiO 2 and Al 2 O 3 It is the content in the composition of iron ore excluding the volatile content (the content in the iron ore after volatilization).

[0047] In the present invention, as a raw material of iron ore powder, the total amount of Fe is 63% by mass or less and SiO 2 With Al 2 O 3The iron ore having a composition of 4 mass % or more is medium-grade iron ore or low-grade iron ore. By using medium-grade iron ore or low-grade iron ore, the agglomeration index can be appropriately adjusted. In addition, there is no upper limit to the usage ratio of medium-grade iron ore or low-grade iron ore in the raw material of iron ore powder, and the raw material of iron ore powder can be all medium-grade iron ore and low-grade iron ore or both.

[0048] The iron ore pellets preferably contain one or more binders. Bentonite is preferred as the binder for the iron ore pellets, but any known or arbitrary binder such as an organic or inorganic binder that can achieve the same effect may be used.

[0049] Iron ore pellets can be mixed with quicklime, limestone (CaCO 3 )、Dolomite(CaMg(CO 3 ) 2 ) etc. as auxiliary raw materials. The auxiliary raw materials are used to adjust the basicity of the iron ore pellets (adjust the CaO content). The basicity of the iron ore pellets is determined by the CaO / SiO ratio contained in the iron ore pellets. 2 The weight ratio of CaO / SiO 2 Adjusting to a low value can increase the iron content in the iron ore pellets, that is, the hematite (Fe 2 O 3 ) content. Iron ore pellets generally exhibit strength through the solid-state bonding of hematite, so by 2 By adjusting to a low value, the ratio of solid-state bonding of hematite increases, and a higher strength iron ore pellet can be obtained. Therefore, from the viewpoint of appropriately obtaining its strength, the basicity of the iron ore pellet is preferably 0.6 or less. In addition, if the basicity is 0.6 or less, the slag ratio can be appropriately suppressed. It should be noted that the lower limit of the basicity of the iron ore pellet is not particularly limited, and is generally 0.1 or more.

[0050] The above raw materials are used to produce iron ore pellets through conventional crushing, mixing, granulation, and sintering processes. The raw materials crushed by a pulverizer such as a ball mill are mixed using a high-speed stirring mixer, processed into a pellet shape using a granulator or a drum mixer, and sintered using an electric furnace or a rotary kiln. The crushing strength of the iron ore pellets is preferably greater than 250 kg. In order to obtain sufficient crushing strength, the sintering conditions are preferably: the sintering temperature (furnace temperature) is 1200-1350°C, and the holding time at the sintering temperature is 10-25 minutes. There is no particular restriction on the particle size of the iron ore pellets, and preferably a particle size of about 10-12.5 mm.

[0051] In the reduction step of the iron ore pellets, a vertical shaft furnace is used to obtain reduced iron pellets. The reducing gas is not particularly limited, and for example, a gas containing H 2: 55% and CO: 35% and the remainder is CO 2 and CH 4 The mixed gas, including H 2 : 75% and CO: 20% and the remainder is CO 2 and N 2 The mixed gas formed.

[0052] The temperature of the reducing gas is preferably determined based on the agglomeration index in a manner that agglomeration does not occur. The temperature of the reducing gas is preferably 900°C or higher, more preferably 950°C or higher, and further preferably 1000°C or higher. If the reducing gas temperature is 900°C or higher, the reduction efficiency is appropriate. It should be noted that in the present invention, the reduction efficiency can be increased as described in Experimental Example 5 (refer to Fig. 9 ) is performed, the upper limit of the reducing gas temperature at which agglomeration is less likely to occur is determined based on the agglomeration index, and therefore the reducing gas temperature is preferably below the determined upper limit value.

[0053] In the reduction process of iron ore pellets, the ratio of T.Fe (total iron) in the reduced pellets to M.Fe (metallic iron) is called the metallization rate. It should be noted that the unreduced iron is only FeO. When the metallization rate is low, the strength of the iron ore pellets during reduction decreases and they are pulverized in the furnace, so the productivity decreases. Therefore, the metallization rate is preferably 88% or more. On the other hand, when the metallization rate is too high, agglomeration occurs and the pellets get stuck in the furnace, so the productivity decreases. Therefore, the metallization rate is preferably 96% or less. In summary, if the metallization rate is 88% to 96%, the slag ratio can be properly controlled and agglomeration can be suppressed. The metallization rate is generally determined based on the operating conditions of the reduction process (reducing gas amount, temperature, holding time, etc.) and the equipment used.

[0054] In the present embodiment, it is important that the reduced iron pellets satisfy the following formula (1).

[0055] (SiO 2 +Al 2 O 3 +CaO+MgO+FeO) / M.Fe≥0.15···(1)

[0056] Among them, in formula (1), SiO 2 、Al 2 O 3 , CaO and MgO are respectively SiO contained in the above-mentioned reduced iron pellets. 2 、Al 2 O 3, CaO and MgO contents (mass %), FeO is the content of iron oxide remaining in the above-mentioned reduced iron pellets (mass %), and M.Fe is the content of metallic iron contained in the above-mentioned reduced iron pellets (mass %). It should be noted that the contents of various oxides and M.Fe are the contents when the volatilization amount of iron ore is not included.

[0057] As shown in formula (1), the agglomeration index is 0.15 or more. The agglomeration index is preferably 0.2 or more. When the agglomeration index is lower than 0.15, agglomeration cannot be suppressed. In addition, the agglomeration index is preferably 0.4 or less, and more preferably 0.3 or less. If the agglomeration index is 0.4 or less, the amount of slag is appropriate and can be effectively reduced.

[0058] It is preferred to intentionally set the SiO contained in the (A) iron ore pellets to 2 、Al 2 O 3 The content of SiO in the iron ore pellets is preferably set to (A) 1.0, (B) 2.0, (C) 3.0 and (D) 4.0, (E) 5.0, (F) 6.0 and (F) 7.0, (G) 8.0 and (G) 9.0, (G) 10.0 and (G) 11.0, (G) 12.0 and (G) 13.0. 2 、Al 2 O 3 , CaO and MgO contents so that the reduced iron pellets satisfy formula (1).

[0059] (A) SiO contained in iron ore pellets 2 、Al 2 O 3 The setting of the contents of CaO and MgO is preferably carried out by setting the composition of the (A-1) iron ore powder in the iron ore pellet manufacturing process, and one or both of the types and contents of the (A-2) binder and auxiliary raw materials. The types and contents of the (A-2) binder and auxiliary raw materials are usually set according to the properties (strength, basicity, etc.) required for the iron ore pellets without considering the agglomeration index. In this way, when the types and contents of the (A-2) binder and auxiliary raw materials are fixed, it is more preferable to intentionally set the composition of the (A-1) iron ore powder so that the reduced iron pellets satisfy formula (1). At this time, in order to make the agglomeration index greater than 0.15 without changing the basicity of the iron ore pellets, it is preferably set by mixing iron ore containing a large amount of gangue components.

[0060] Example

[0061] As raw materials for iron ore pellets, iron ore, a binder, and auxiliary materials were prepared. Table 1 shows the composition of the iron ore used in Experimental Examples 1 to 5. As a binder, a mixture containing 3% CaO, 60% SiO 2 , 15% Al2 O 3 As auxiliary materials, a material containing 53% CaO, less than 1% SiO 2 , less than 1% Al 2 O 3 and 1% MgO of limestone.

[0062]

[0063] (Experimental Example 1)

[0064] Iron ore pellets were produced by changing the blending ratio of the five kinds of iron ores shown in Table 1 and the contents of limestone and bentonite. First, the iron ore was crushed with a ball mill to obtain iron ore powder. In each case, in order to make the T.Fe of the reduced iron pellets have various values, the blending ratio of the five kinds of iron ores shown in Table 1 was changed, and a total of 2000 g of mixed iron ore powder was prepared. In addition, limestone was added to the mixed iron ore powder in an amount set so that the basicity of the iron ore pellets would be 0.2 or 1.1. Generally, a basicity of 0.2 is called acidic, and a basicity of 1.1 is called alkaline. In the following description, Figure 2 , Figure 3 This is also stated in. Further add a specified amount of bentonite, and use a high-speed stirring mixer to mix at 20rpm for 3 minutes. Next, load the mixed raw materials into a 1.2mφ granulator, and granulate while adding water. Collect pellet particles with a particle size of 10 to 12.5mm, and further rotate them in a granulator for 10 minutes to obtain green pellets. The green pellets are fired in an electric furnace with a firing temperature (temperature in the furnace) controlled at 1200 to 1350°C for 25 minutes to obtain iron ore pellets (fired pellets). The crushing strength of the iron ore pellets was measured using a universal testing machine (Autograph) at a speed of 1mm / min. The results showed that the crushing strength (the point where the load shows a maximum) exceeded 250kg.

[0065] [Agglomeration strength evaluation]

[0066] In each case, an evaluation test of agglomeration strength was conducted. This evaluation test was conducted by reducing iron ore pellets under conditions where agglomeration is more likely to occur than in the usual production of reduced iron, and analyzing the degree of agglomeration of the obtained reduced iron pellets. This evaluation test was conducted using a load softening reduction test apparatus and an I-type rotation test machine. Figure 1 (a) shows a schematic diagram of a load softening reduction test apparatus. Alumina balls 12, 500 g of iron ore pellets (14), and alumina balls 12 are sequentially filled in a φ100 mm carbon crucible 10. A 1 kg / cm 2 The load is set to H. 2 :N 2=20:80 reducing gas 16 is circulated at 24L / min, the temperature inside the test device is set to 900°C (heated by heater 18), and the holding time is set to 360 minutes in the example of metallization rate 94% and 400 minutes in the example of metallization rate 98% to produce an aggregate of agglomerated reduced iron pellets.

[0067] Figure 1 (b) shows a schematic diagram of a type I rotary test machine. The agglomerated reduced iron pellets 20 are placed in a type I rotary test machine 22 and are rotated at 30 rpm for 5 minutes to be crushed. The crushed sample is taken out and sorted using a 15 mm sieve. The weight of the sample on the sieve is measured, and the agglomeration strength CR (%) is calculated by the following formula. The higher the agglomeration strength, the easier it is for agglomeration to occur in a vertical furnace during the actual manufacture of reduced iron.

[0068] CR=Ws / W×100

[0069] W: Total mass of the sample before crushing

[0070] Ws: The mass on the 15mm sieve after crushing

[0071] Figure 2 The relationship between the T.Fe of the reduced iron pellets and the agglomeration strength of the obtained reduced iron pellets is shown in FIG. It should be noted that in the example where the T.Fe is lower than the T.Fe of the iron ore raw material, the T.Fe is reduced by adding limestone. It is confirmed that if the T.Fe of the reduced iron pellets increases, there is a trend that the agglomeration strength increases, but the influence of the metallization rate and the basicity of the iron ore pellets is also significantly manifested. For example, in a sample with a T.Fe of about 65% of the reduced iron pellets, the agglomeration strength varies in the range of 5 to 13% depending on the basicity. It can be seen from this that in the existing method of suppressing agglomeration using T.Fe (iron grade) as an indicator described in Non-Patent Document 1, it is sometimes difficult to suppress agglomeration depending on the type of iron ore or the metallization rate.

[0072] Figure 3 The relationship between the agglomeration index of the reduced iron pellets concerned by the present invention and the agglomeration strength of the reduced iron pellets is shown in FIG. Regardless of the metallization rate and basicity, when the agglomeration index is above 0.15, the agglomeration strength is below 9.5%, which is a level that can fully suppress agglomeration. It can be seen from this that when the agglomeration index is used to evaluate the reduced iron pellets as in the present invention, the agglomeration strength can be evaluated regardless of the metallization rate and basicity.

[0073] (Experimental Example 2)

[0074] The iron ore pellets were manufactured in the same manner as in Experimental Example 1 by changing the blending ratio of the five iron ores shown in Table 1 and the basicity of the iron ore pellets to increase the number of test cases. The obtained iron ore pellets were subjected to an agglomeration strength evaluation test, and the results are shown in FIG. Figure 4 , 5 At this time, the metallization rate of the reduced iron pellets was changed by changing the holding time in the agglomeration strength evaluation test. Figure 4 It can be seen that when the T.Fe (iron grade) of the reduced iron pellets is used as an indicator, the difference in agglomeration strength is greater. Figure 5 It is understood that even when the number of test cases is increased, when the agglomeration index is 0.15 or more, the agglomeration strength is 9.5% or less, which is a level at which agglomeration can be sufficiently suppressed.

[0075] (Experimental Example 3)

[0076] Low-grade iron ore OreA, medium-grade iron ore OreC, and high-grade iron ore OreD are used as raw materials to manufacture iron ore pellets. First, each iron ore is crushed with a ball mill to obtain iron ore powder. Prepare 2000g of each iron ore powder, and add limestone to the iron ore powder according to the addition amount set in such a way that the basicity of the iron ore pellet becomes 0.2. Further, 1.0wt% of bentonite is added relative to the amount of iron ore powder, and iron ore pellets are manufactured in the same manner as in Experimental Example 1. The obtained iron ore pellets are subjected to an agglomeration strength evaluation test, and the metallization rate of the reduced iron pellets is changed by changing the holding time. The holding time is 360 minutes when the metallization rate is 92%. For every 2% increase in the metallization rate, the holding time is extended by 20 minutes until the metallization rate reaches 100%, and the test is carried out in this way.

[0077] Figure 6 The relationship between the metallization rate and the agglomeration index obtained in this experimental example is shown in FIG. It can be seen that even if the iron ore pellets are produced using the same raw materials, the agglomeration index decreases if the metallization rate is increased. This is believed to be because if the metallization rate is high, the amount of unreduced FeO in the reduced iron pellets decreases, so the slag ratio decreases and agglomeration is more likely to occur. According to Figure 6 It can be seen that when using low-grade iron ore or medium-grade iron ore, the agglomeration index can be made above 0.15 by making the metallization rate below 96%. Therefore, the metallization rate is preferably below 96%. On the other hand, if the metallization rate is reduced, the reducibility is impaired, which is not preferred. From this point of view, the metallization rate is preferably above 88%. Furthermore, according to Figure 6 It is understood that it is difficult to make the agglomeration index 0.15 or more when only high-grade iron ore is used as the raw material.

[0078] (Experimental Example 4)

[0079] Iron ore pellets were produced by blending low-grade iron ore OreA or medium-grade iron ore OreC with high-grade iron ore OreD used in the experimental example. First, each iron ore was crushed with a ball mill to obtain iron ore powder. Mixed iron ore powders were prepared by blending 0, 10, 20, and 40% by mass of low-grade iron ore or medium-grade iron ore powder with the high-grade iron ore powder, and a total of 2000 g of each mixed iron ore powder was prepared. Limestone was added to the mixed iron ore powder in an amount set so that the basicity of the iron ore pellets became 0.2. Further, 1.0 wt% of bentonite was added relative to the amount of the mixed iron ore powder, and iron ore pellets were produced in the same manner as in Experimental Example 1. The obtained iron ore pellets were subjected to an agglomeration strength evaluation test, and the results are shown in Figure 7 , 8 At this time, the metallization rate of the reduced iron pellets was changed by changing the holding time in the agglomeration strength evaluation test. When the medium-grade iron ore was mixed, the holding time was 380 minutes, and the metallization rate was 94%. When the low-grade iron ore was mixed, the holding time was 400 minutes, and the metallization rate was 96%.

[0080] Figure 7 The figure shows the relationship between the ratio of low- and medium-grade iron ore in iron ore pellets and the agglomeration index of reduced iron pellets. The higher the ratio of low- and medium-grade iron ore, the higher the agglomeration index. Regardless of whether low-grade iron ore or medium-grade iron ore is used, when the ratio is 40% by mass, the agglomeration index can be above 0.15. Figure 8 It can be seen that by controlling the agglomeration index, the agglomeration strength can be controlled.

[0081] (Experimental Example 5)

[0082] Iron ore pellets with different agglomeration indexes were produced by changing the blending ratios of the five iron ores shown in Table 1. The upper limit of the furnace temperature at which the iron ore pellets could be operated without agglomeration in a vertical shaft furnace was measured for each iron ore pellet. 500 g of iron ore pellets were loaded into the vertical shaft furnace and the volume ratio was 1.5 H. 2 :N 2 =20:80 gas atmosphere, with gas temperature of 800℃ and holding time of 360 minutes. Take out the sample, if it is confirmed that no agglomeration occurs, then increase the gas temperature by 50℃, put in a new sample, and repeat the same test. End the test when agglomeration is confirmed, and take the temperature before agglomeration as the upper limit of the furnace temperature.

[0083] Fig. 9 The relationship between the agglomeration index and the upper limit temperature in the furnace is shown in FIG. It can be seen that the upper limit temperature in the furnace increases linearly with the increase of the agglomeration index. Therefore, it can be seen that by using the agglomeration index, agglomeration can be suppressed and the temperature in the vertical furnace can be made higher than before. Therefore, the effect of the present invention is significant.

[0084] Industrial Applicability

[0085] According to the present invention, a method for producing reduced iron capable of suppressing agglomeration of iron ore pellets can be provided.

[0086] Explanation of symbols

[0087] 10 φ100mm carbon crucible

[0088] 12 Alumina Balls

[0089] 14 Iron ore pellets (burned pellets)

[0090] 16 Reducing gas

[0091] 18 Heater

[0092] 20 Agglomerated reduced iron pellets

[0093] 22 I type rotation testing machine

Claims

1. A method for producing reduced iron, characterized in that: have: a pellet manufacturing step of mixing iron ore powder, a binder and auxiliary raw materials and granulating them to obtain green pellets, and then calcining the green pellets to obtain iron ore pellets, and In the reduction step, the iron ore pellets are loaded into a vertical shaft furnace and directly reduced by reducing gas to obtain reduced iron pellets. The raw material of the iron ore powder includes an iron ore having a composition in which the total amount of Fe is 63% by mass or less and the sum of SiO2 and Al2O3 is 4% by mass or more, The reduced iron pellets satisfy the following formula (1): (SiO2+Al2O3+CaO+MgO+FeO) / M.Fe≥0.15···(1)In which, in formula (1), SiO2, Al2O3, CaO and MgO are the contents of SiO2, Al2O3, CaO and MgO contained in the reduced iron pellets, respectively, FeO is the content of residual iron oxide in the reduced iron pellets, and M.Fe is the content of metallic iron contained in the reduced iron pellets, and the contents are measured in mass %.

2. The method for producing reduced iron according to claim 1, wherein: Intentionally set (A) the contents of SiO2, Al2O3, CaO and MgO contained in the iron ore pellets, and (B) Metallization rate in the reduction step One or both of the above can be used to make the reduced iron pellets satisfy the above formula (1).

3. The method for producing reduced iron according to claim 2, wherein: The above (B) is predetermined, and the above (A) is intentionally set so that the reduced iron pellets satisfy the above formula (1).

4. The method for producing reduced iron according to claim 2 or 3, wherein: The setting of (A) is performed by setting the composition of the iron ore powder (A-1) in the pellet production step, and (A-2) Types and contents of the binder and auxiliary materials One or both of them are carried out.

5. The method for producing reduced iron according to claim 4, wherein: The above (A-2) is predetermined, and the above (A-1) is intentionally set so that the reduced iron pellets satisfy the above formula (1).

6. The method for producing reduced iron according to any one of claims 1 to 5, wherein: The metallization ratio in the reduction step is set within a range of 88 to 96%.

7. The method for producing reduced iron according to any one of claims 1 to 6, wherein: The temperature of the reducing gas in the reducing step is determined based on the value of the left side of the equation (1).

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