JP2017119910A - Reduction raw material, and method for producing reduction raw material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the reduction powdering and ensure the strength of a reduction raw material by suppressing hydrogen-based reduction of iron oxide at a temperature range of 550-600°C, and proceeding reduction of iron oxide at 700°C or higher.SOLUTION: A reduction raw material having a porous body comprising FeO, and a coating layer which covers 70% or more of the entire surface of the porous body, is in an amount of 5.0 parts by mass or more and 50.0 parts by mass or less of the mass of the porous body, and comprises a calcium iron compound composed of CaFeO(1<y/x≤2; 1≤z), and a method for producing the reduction raw material are provided.SELECTED DRAWING: Figure 10

Description

  The present invention relates to a reducing material used in a solid reduction furnace and a method for producing the reducing material.

  A solid reduction furnace, which is a hydrogen-based reduction process, uses calcined pellets manufactured using iron ore fine powder as a main raw material and reduces it to Fe with a reduction rate of about 90% or more without melting it. Inside the solid reduction furnace, the hematite phase in the calcined pellets is reduced to change into a magnetite phase while passing through a temperature range of 500 to 600 ° C. This phase change is accompanied by volume expansion, Since the reduction of hematite is an endothermic reaction, the temperature rise tends to stagnate in the temperature range. For this reason, the fired pellets are liable to cause a pulverization phenomenon in a solid reduction furnace. In addition, the generated powder is reduced to act as a paste that hardens the fired pellets, and the pellets are welded to each other, causing clustering, and a shelf hanging that prevents the pellets in the furnace from moving and cannot be removed from the bottom of the furnace. Similar reduction pulverization problems may also occur in a blast furnace that performs CO-based reduction, but in a solid reduction furnace that performs hydrogen-based reduction, the problems of reduced pulverization and clustering become even greater.

Conventionally, in order to suppress reductive pulverization in a blast furnace or a solid reduction furnace, an aqueous solution of chloride such as CaCl ₂ is sprayed on the fired pellets and dried to provide chloride such as anhydrous CaCl ₂ on the surface. Has been done. Since anhydrous CaCl _{2 has} a melting point of 772 ° C., the chloride inhibits the progress of reduction by preventing the reduction gas from diffusing into the sintered ore and sintered pellets in the temperature range where reduction powdering is a problem. It functions to prevent reducing powdering and to melt at higher temperatures and be removed from the pellet surface without interfering with pellet reduction. However, it takes time and effort to spray and dry chloride on the sintered ore and fired pellets, and the added chloride adheres to reduction equipment such as blast furnaces and solid reduction furnaces, corroding the equipment. There was a problem such as doing.

In order to expand the use of inexpensive inferior raw materials, without using chlorides such as CaCl ₂ and to suppress the above-mentioned reduction powdering, it is difficult for the clustering of pellets to occur, and to perform stable furnace operation and reduction treatment, Various sintered ores and methods for producing the sintered ores have been proposed.

  For example, Patent Document 1 uses a coarse-grained sintering raw material that remains solid during sintering or does not contribute to the formation of calcium ferrite, and a fine-sintered raw material that is melted during sintering to generate calcium ferrite. There is disclosed a sintered ore that performs granulation by adhering a fine powder sintered raw material around a grain sintered raw material, and sintering the resulting product from the coarse grain sintered raw material with calcium ferrite. Yes. Note that a two-layered pellet obtained by granulating a core made of a raw material of coarse particles or fine powder and fine powder around the core is sometimes referred to as a “two-layer pellet”.

Non-Patent Document 1 discloses a two-layer pellet obtained by granulating a core made of raw material of coarse particles or fine powder and CaO-Fe ₂ O ₃ -based fine powder around the core. The ratio of about 50:50 is reported, and the experimental results are reported in which the ratio of hematite ore and limestone as the raw material for the adhesion layer is changed.

  In Patent Document 1 and Non-Patent Document 1, firing is performed at a temperature at which the coating layer is melted to form a connecting portion made of calcium ferrite or a mixed phase of calcium ferrite and hematite, and this connecting portion is formed by a plurality of two-layer pellets. Are bonded to each other, and a two-layer pellet connected body having excellent mechanical strength, that is, a sintered ore is produced. However, since the connecting part or the layer containing calcium ferrite melts to form a liquid phase and flows here, the shape is not controlled, and the melt is dripped to cause uneven distribution of calcium ferrite. This causes a break in the layer containing calcium ferrite, resulting in a problem that the calcium ferrite permeates into the iron oxide granule portion inside and impairs the reducibility of the pellet.

  In Patent Document 2, 3-12φ raw pellets coated with a particulate solid fuel are fired on the surface, and the obtained calcined agglomerated ore is composed of at least one of a calcium ferrite phase and a slag phase. A combined, irregularly shaped assembly of a plurality of sintered pellets is disclosed. However, in Patent Document 2, the calcium ferrite phase is only used for bonding pellets, and there is no disclosure or suggestion about the control of the composition and distribution shape of the calcium ferrite phase.

Patent Document 3, CaO / a 2-layer structure pellets for blast furnace insertion of basicity 1.8-2.4 represented by SiO _2, core basicity from 1.8 to 2.0 and basicity 1. A two-layer pellet for blast furnace insertion having a two-layer structure consisting of 2 to 1.5 outer peripheral portions is disclosed. In this way, by forming a two-layer pellet with different basicity, 2CaO / SiO ₂ is generated, and the melt consisting of the melt and FeO oozes from the inside of the pellet to the surface up to 1290 ° C. even in the blast furnace. Sintering of the part of metallic iron is suppressed.

  In the invention disclosed in Patent Document 3, when a dense calcium ferrite structure is developed by firing at 1270 ° C. or higher, reducing powdering in the low temperature region of the blast furnace becomes intense. It is configured to generate a developed non-dense calcium ferrite structure to prevent reduced powdering. The prior art two-layer pellet is characterized in that the CaO ratio in the outer peripheral portion is lower than that in the core portion.

Patent Document 4, to form a multilayer structure pseudo particles the surface of the granules of iron oxide powder was coated with a mixture molar ratio was blended so that 0.5 to 2 of CaO and Fe ₂ O _3, The sintered ore in which the outer periphery generates a melt to form a connecting portion that joins the pseudo particles to each other and the strength of the sintered ore is maintained is disclosed.

In Patent Document 4, since the connecting portion is formed by firing at a temperature at which the coating layer is melted, iron oxide is supplied from the iron oxide granule to the calcium ferrite phase generated in the connecting portion, and CaO and Fe ₂ O _3. The ratio cannot be controlled to the targeted composition by changing from the composition in the initial coating layer so that Fe ₂ O ₃ increases. As a result, in Patent Document 4, the reduction behavior of the calcium ferrite phase of the coating layer cannot be controlled. Further, in Patent Document 4, since the coating layer is melted, the shape of the layer containing calcium ferrite cannot be controlled by dripping the melt. Therefore, the distribution of calcium ferrite is uneven, the cuts of the layers are not formed continuously, and problems such as impairing the reducibility of the pellets by penetrating into the inner iron oxide granule portion occur. .

  Patent Document 5 discloses a sintered ore by a two-layer pellet in which an outer layer containing a CaO source is melted and FeO is supplied from the inner layer surface to form a binder phase. However, the invention disclosed in Patent Document 5 also uses a calcium ferrite phase formed by supplying an iron oxide component from the inner layer as a binder phase while melting the coating layer. There is a problem that the composition, distribution and density of the calcium ferrite phase to be generated cannot be controlled.

  Patent Document 6 discloses a reducing raw material including a porous coating layer containing CaO and / or MgO and having a porosity of 20% by volume or more. The raw material for reduction prevents clustering due to the presence of CaO and / or MgO on the surface, and since the coating layer is porous and air permeable, the reduction of pellets is not hindered by the presence of the coating layer and becomes uniform. It is characterized by being difficult to reduce powder. Thus, the coating layer of this prior art is characterized by high air permeability, does not hinder the reduction of iron oxide, and does not exhibit gas barrier properties.

Japanese Unexamined Patent Publication No. 57-200529 JP 62-37325 A JP-A-2-82522 JP 2004-225147 A JP 2002-129247 A International Publication No. 2015/101614

Iron and Steel 70th Year (1984) No. 6, pp. 520-526, Eiki Kasai et al.

As described above, in the solid reduction furnace, since reduction of hydrogen-based iron oxide proceeds by an endothermic reaction, the furnace temperature is in a temperature range of 550 to 600 ° C., which is a reaction zone from Fe ₂ O ₃ to Fe ₃ O ₄ . When the rise in the furnace temperature is stagnant in the temperature range of 550 to 600 ° C., there is a reduction pulverization phenomenon that cracks are generated in the fired pellets due to the volume expansion accompanying the reaction. Easy to get up. Reduction powdering is more likely to occur with hydrogen reduction than Fe ₂ O ₃ reduction with CO, which is an exothermic reaction. When reduced pulverization occurs, the air permeability in the furnace is reduced, the generated powder is reduced to work like a paste that hardens the fired pellets, and clustering occurs that creates a lump of pellets, or pellets in the furnace This causes problems such as a suspended shelf that cannot be removed from the bottom of the furnace, and the solid reduction furnace cannot be operated continuously. However, none of Patent Documents 1 to 6 and Non-Patent Document 1 disclose or suggest a solution to these problems.

  Therefore, the problem to be solved by the present invention is to cover a mass of iron oxide with a coating layer having a desired thickness and having a continuous and dense gas barrier property, so that a temperature range of 550 to 600 ° C. This suppresses the reduction of hydrogen-based iron oxide, and the coating layer loses the gas barrier property at 700 ° C. or higher, and the reduction of reduced iron powder is suppressed by promptly reducing the iron oxide, thereby reducing the strength of the calcined pellets. It is to solve the problem that cannot be achieved by the prior art.

A large number of cracks are formed in the sintered body made of Fe ₂ O ₃ powder during the reduction process. Hydrogen enters the sintered body from such a crack, and Fe ₃ O from the surface of the sintered body to the inside along the crack formed in the sintered body at a temperature range of 550 ° C. to 600 ° C. Reaction to ₄ proceeds.

FIG. 1 is a schematic diagram of the reduction reaction of the fired pellet when the reduction reaction is advanced in a solid reduction furnace using a porous body made of Fe ₂ O ₃ as a fired pellet. As shown in the following chemical formula, initially, the surface of Fe ₂ O ₃ of the fired pellet is changed to Fe ₃ O ₄ by contacting with a hydrogen atmosphere, and the Fe ₃ O ₄ is the outermost surface (interface II in FIG. 1). ) And then reduced to Fe ²⁺ by contact with a hydrogen atmosphere.
Fe ₃ O ₄ + 4H ₂ → 3Fe ²⁺ + 6e ⁻ + 4H ₂ O

On the other hand, _Fe 3 _O slow movement of oxide ions in ₄ in the crystal grains, as shown in the following chemical formula, _Fe 3 _{O 4} portion of reduced ^{Fe 2+} ions from the _Fe 3 _{O 4} in the crystal grains the diffused by being supplied to the interface between the _{Fe 2 O 3, Fe 2 O} 3 is changed to _Fe 3 _{O 4} (the interface I of Figure 1).
4Fe ₂ O ₃ + Fe ²⁺ + 2e ⁻ → 3Fe ₃ O ₄

For specific volume of the Fe ₃ O ₄ is 1.105 times the _Fe 2 _{O 3,} at the interface between the crystal grains and the _Fe 3 _{O 4} crystal grains of the _Fe 2 _{O 3,} resulting volume expansion, a crack is generated .

When Fe ₃ O ₄ reacts and is reduced to Fe ²⁺ in the solid reduction furnace as described above, the crystal lattice of Fe ₃ O _{4 on} the surface of the fired pellet disappears, so that the outermost surface of the fired pellet (see FIG. 1 interface II) becomes porous and the strength decreases. FIG. 2A shows the relationship between the reduction rate of Fe ₂ O ₃ to Fe and the porosity of the fired pellets (change in open porosity due to reduction). Also, it is shown in FIG. 2 (b) the change in the bulk density of the fired pellets due to reduction of the fired pellets of Fe ₂ O _3. The data shown in FIGS. 2 (a) and 2 (b) are obtained by performing a reduction reaction in an argon-hydrogen atmosphere containing 3.6% of H ₂ gas at 550 ° C. while changing the reduction time up to 10 hours. It was.
2 (a) and 2 (b), it can be seen that the open pores increase and the bulk density decreases with the reduction of the Fe ₂ O ₃ fired pellets.

3 (a) and 3 (b) show a case where a porous body made of Fe ₂ O ₃ is used as a fired pellet, and the reduction reaction proceeds by heat treatment at 550 ° C. in an atmosphere firing furnace while flowing a hydrogen-containing gas. It is the figure which showed an example of the structural phase and intensity | strength change of the said baking pellet. 3A and 3B, the graph indicated by “H” corresponds to the Fe ₂ O ₃ phase, and the graph indicated by “M” corresponds to the Fe ₃ O ₄ phase. The graph indicated by “Fe” corresponds to the Fe phase, and the graph indicated by “FeO” corresponds to the FeO phase. As shown in FIG. 3 (a), when the degree of reduction powdering is small, the reduction to Fe begins before the change from Fe ₂ O ₃ to Fe ₃ O ₄ is completed at 550 to 600 ° C. The strength of the fired pellet is ensured to some extent.

On the other hand, as shown in FIG. 3B, when the degree of reduction powdering is large, the reduction to Fe starts after the change from Fe ₂ O ₃ to Fe ₃ O ₄ is completed at 550 to 600 ° C. The strength of the fired pellets is weakened, and crack formation and destruction associated with reduction, that is, reduction powdering occurs during a low strength state. On the other hand, even in the case of pellets having a large degree of reduction powdering at 550 to 600 ° C., the change from Fe ₂ O ₃ to Fe ₃ O ₄ is suppressed at 550 to 600 ° C., and at a high temperature of about 700 ° C. If it is reduced, reduction to Fe is not likely to occur, and reduction to Fe proceeds rapidly, and the reduction in pellet strength becomes serious.

Thus, in order to secure the strength of the fired pellets, suppressing the change from Fe ₂ O ₃ to Fe ₃ O _{4 at} 550 to 600 ° C. is effective in suppressing reduction powdering.

Incidentally, cement or Mg (OH) ₂ has been put into practical use for firing pellet coating in a solid reduction furnace for the purpose of preventing sticking or clustering. Therefore, the present inventors have intensively studied to control the reducibility and reduced powdering characteristics of the fired pellets by coating the fired pellets with a calcium ferrite-based oxide.

FIG. 4 shows the case where CaFe ₂ O ₄ (hereinafter sometimes referred to as “CF2”) is subjected to a reduction reaction in an argon-hydrogen atmosphere containing 3.6% of H ₂ gas at different reduction temperatures. A line diffraction pattern is shown. The x-ray diffraction pattern of the sample obtained by reducing the CF2 at 550 ° C. for 5 hours has almost no change from the x-ray diffraction pattern of CF2 before the reduction. However, after the reduction reaction of CF2 at 700 ° C. for 5 hours, the peak intensity of the x-ray diffraction pattern decreases, and a diffraction peak of CaFeO _2.5 (hereinafter sometimes referred to as “CF1”) can be observed. From this, it is considered that CF2 undergoes a reduction reaction at 700 ° C. as follows.
2CaFe ₂ O ₄ → CaFeO _2.5 + CaFe ₃ O ₅ + 1 / 4O ₂
CaFe ₃ O ₅ → CaFeO _2.5 + 2Fe + 5 / 4O ₂

  Thus, it was revealed that the calcium ferrite phase such as CF2 does not change at 550 ° C., but the reduction proceeds at 700 ° C. That is, the idea that this material has a reducible property suitable as a coating layer of a fired pellet was obtained. That is, one of the points of the present invention is that the calcium ferrite phase is not used as a bonding layer between pellets such as the conventional two-layer pellet sintered ore, but pays attention to the reducibility in a hydrogen atmosphere. And is used as a barrier layer for reducing powder reduction.

FIG. 5A and FIG. 5B show the results of comparing the reduction rate, open porosity, and bulk density of Fe ₂ O ₃ with the fired pellets made of CF1 and CF2. The graph of “model pellet 1200 ° C. firing” in FIGS. 5A and 5B corresponds to the graph of the Fe ₂ O ₃ fired pellet in FIGS. 2A and 2B.

CF1 and CF2 undergo little reduction when reduced at 550 ° C. Therefore, each of the fired pellets composed of fired pellets and CF2 consisting CF1, argon containing H ₂ gas 3.6% - hydrogen atmosphere, the measurement results of performing the reduction reaction for 5 hours at 550 ° C. FIG. 5 (a) and FIG. 5 (b) together show the measurement results when each of these pellets was subjected to a reduction reaction at 700 ° C. for 5 hours in the above atmosphere.

FIG. 5A shows a calcined pellet made of CF1 (“CF1” in the figure), a calcined pellet made of CF2 (“CF2” in the figure), and the Fe ₂ O of FIGS. 2A and 2B. ₃ is a graph comparing changes in the reduction rate and open porosity of fired pellets of No. ₃ . Moreover, FIG.5 (b) is a graph which compares the reduction rate of each said baking pellet, and the change of a bulk density.

  According to FIGS. 5A and 5B, it can be seen that both CF1 and CF2 generate less open pores due to reduction, and the decrease in bulk density is also small. Further, CF2 is difficult to reduce at 550 ° C. as will be described later, but at 700 ° C., it is reduced by itself through the above-described reduction reaction. Even though it is initially dense and has gas barrier properties, Loss gas barrier properties during the reduction process. In contrast, CF1 does not easily proceed under any reduction condition, and there is no change in gas barrier properties.

  Based on these findings, the inventors of the present invention made a sintered pellet having a structure having a coating layer of a calcium ferrite phase such as CF2 on the surface of a spherically shaped iron oxide pellet. It has been found that the reduction behavior can be controlled and the strength can hardly be lowered after the reduction reaction.

  In addition, the present inventors set the composition of the calcium ferrite phase such as CF2 and the like, the amount or thickness of the coating layer, the method of applying the coating layer, the iron oxide pellets of the base and the firing conditions of the coating layer as appropriate ranges. The present inventors have found that the reaction between the iron oxide pellets as a base and the coating layer can be suppressed, and that the film can be formed so as to have a denseness excellent in gas barrier properties.

In addition, as a result of further studies on the improvement of the density of the coating layer, the present inventors obtained CaFe _3.23 O _z that becomes a liquid phase near 1200 ° C. and CaFeO _z that becomes a liquid phase above 1440 ° C., respectively. When mixed powder mixed at a weight ratio of 23:77 is used as the raw material powder for the coating layer, CaFe _1.25 O _z is generated, and the coating layer does not flow down and a healthy coating layer is formed. I found. Further, the coating layer formed by reacting the mixed powder, said CaFe _1.25 O _z is a product after the reaction is formed by firing the coated iron oxide pellets of the underlying Fe ₂ O ₃ coating The present inventors have found that the sinterability is promoted more than the layer, and is denser and has a higher gas barrier property.

  This is considered as follows. That is, in the case of a liquid phase due to macro melting of the constituent particles (hereinafter referred to as “macro liquid phase”), the coating layer flows and melts away, so the coating layer is formed densely and continuously. Can not. On the other hand, if the melting level of the coating layer is microscopic as described above, it reacts with other unmelted constituent particles existing in the vicinity of the molten constituent particles to form a solid calcium ferrite phase. I think that.

As described above, the present inventors baked a mixture prepared by adjusting the Fe / Ca ratio so that the calcium ferrite phase that easily generates the liquid phase and the calcium ferrite phase that is difficult to generate the liquid phase are appropriate. It has been found that the formation of the target coating layer is promoted. Further, using at least one of a calcium ferrite phase, CaO and CaCO ₃ that hardly generates a liquid phase, and mixing with a calcium ferrite phase that easily generates a liquid phase so that the Fe / Ca ratio is appropriate, The present inventors have found that the formation of the target coating layer is promoted by firing.

  Therefore, a further point of the present invention is that the formation process of the coating layer is controlled in the formation process of the calcium ferrite phase that generates a melt like the above-described two-layer pellet of the prior art, and the effect of suppressing the reduction powdering can be obtained. While it is impossible to provide a suitable coating layer, it is possible to coat the calcium ferrite phase while controlling the composition, basis weight, denseness, continuity, etc. by solid phase sintering or local melting of the constituent particles. It is possible to form a layer.

  The solid-phase sintering in the present invention is not the above-described movement / diffusion in the macroscopic liquid phase, but does not generate the macroscopic liquid phase, and the mass transfer in the solid phase through the contact points of the constituent particles. -Refers to the process of improving the density by diffusion.

  In the present invention, “local melting of constituent particles” means melting a part of constituent particles at the constituent particle level to generate a local liquid phase (hereinafter referred to as “micro level liquid phase”). A process.

  The present invention has been made on the basis of the above findings, and the gist thereof is as follows.

(1) a porous body containing Fe ₂ O ₃ ;
70% or more of the entire surface of the porous body is covered, and is 5.0 parts by mass or more and 50.0 parts by mass or less of the mass of the porous body, and Ca _x Fe _y O _z (1 <y / x ≦ And a coating layer containing a calcium iron compound consisting of 1 ≦ z).
(2) The porous body contains bentonite in an amount of 0.1 parts by mass or more and 10.0 parts by mass or less with respect to the content of Fe ₂ O ₃ of the porous body. The raw material for reduction as described.
(3) The coating layer contains bentonite in an amount of 0.1 parts by mass or more and 10.0 parts by mass or less based on the total of the Ca compound and the Fe compound and the Ca compound and the Fe compound (1 ) Or the raw material for reduction according to (2).
(4) Granulating a powder containing Fe ₂ O ₃ to produce a granulated body,
Using a calcium iron compound composed of Ca _x Fe _y O _z (1 <y / x ≦ 2; 1 ≦ z), 70% or more of the entire surface of the granulated body is accounted for by 5% of the mass of the granulated body. A coating layer is formed by coating with 0 to 50.0 parts by mass,
A method for producing a reducing raw material, characterized in that the granulated body on which the coating layer is formed is fired in the air at 1150 ° C. or higher and 1200 ° C. or lower.
(5) Fe ₂ O ₃ is mixed with at least one of CaO and CaCO ₃ so that the Fe content with respect to the Ca content is more than 1.0 and less than 2.0 in terms of molar ratio,
Subsequently, the said calcium iron compound is obtained by baking at 800 degreeC-1000 degreeC in air | atmosphere, The manufacturing method of the raw material for reduction | restoration as described in (4) characterized by the above-mentioned.
(6) Granulate a powder containing Fe ₂ O ₃ to produce a granulated body,
Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2), at least one of calcium iron compound, CaO and CaCO ₃ , and Ca _x1 Fe _y1 O _z1 (2 <y1 / a mixture satisfying a molar ratio of Fe and Ca of 1 <Fe / Ca ≦ 1.5 using a calcium iron compound consisting of x1 ≦ 4; 1 ≦ z1)
Using the mixture, 70% or more of the entire surface of the granulated body is coated with 5.0 to 50.0 parts by mass of the mass of the granulated body to form a coating layer,
A method for producing a reducing raw material, characterized in that the granulated body on which the coating layer is formed is fired in the air at 1150 ° C. or higher and 1200 ° C. or lower.
(7) Calcium iron compound composed of Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2) and Ca _x1 Fe _y1 O _z1 (2 <y1 / x1 ≦ 3.23; 1 ≦ Preparation of a reducing raw material according to (6), wherein a mixture satisfying a molar ratio of Fe and Ca of 1 <Fe / Ca ≦ 1.5 is prepared using a calcium iron compound comprising z1) Method.
(8) At least one of CaO and CaCO ₃ and Fe ₂ O ₃ are mixed so that the Fe content with respect to the Ca content is 1.0 to 1.5 in terms of molar ratio, Calcining in the atmosphere at 800 ° C. to 1000 ° C. to obtain a calcium iron compound composed of Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2) (6) or The manufacturing method of the raw material for reduction | restoration as described in (7).
(9) At least one of CaO and CaCO ₃ and Fe ₂ O ₃ are mixed so that the Fe content with respect to the Ca content is more than 2.0 and 4.0 or less in terms of molar ratio, (6) or (7) characterized by obtaining a calcium iron compound comprising Ca _x1 Fe _y1 O _z1 (2 <y1 / x1 ≦ 4; 1 ≦ z1) by firing in the air at 800 ° C. to 1000 ° C. The manufacturing method of the raw material for reduction | restoration as described in).
(10) Mixing the powder containing Fe ₂ O ₃ with bentonite at 0.1 parts by mass or more and 10.0 parts by mass or less with respect to the mass of the powder,
The mixture containing the powder containing Fe ₂ O ₃ and the bentonite is granulated to an average particle diameter of 5 mm to 20 mm in diameter to form a granulated body,
Next, the granulated body is dried in the air at 100 ° C. or higher and 600 ° C. or lower to produce a granulated body containing the Fe ₂ O _3. (4) to (9), The manufacturing method of the raw material for a reduction | restoration as described in 2.
(11) The coating layer contains bentonite in an amount of 0.1 parts by mass or more and 10.0 parts by mass or less with respect to the Ca compound and the Fe compound contained in the coating layer. The method for producing a reducing raw material according to any one of 10).

  In the temperature range near 550 ° C., where reduction powdering is a problem, the coating layer of the raw material for reduction according to the present invention suppresses reduction of the iron oxide portion of the fired pellet, and reduction powdering hardly occurs. In the temperature range of 700 ° C. or higher at which reduced powdering does not easily occur, the coating layer itself is reduced and loses its gas barrier properties, and does not hinder the reduction of iron oxide, allowing rapid reduction progress. Also, during the reduction process, the coating layer does not generate coarse pores inside and is reduced while maintaining a structure that maintains high strength, so there is no problem of peeling or chipping of the coating layer. CaO remains evenly. For this reason, the coating layer of the raw material for reduction of the present invention suppresses the fusion of pellets in the solid reduction furnace and the occurrence of clustering due to the reduction even after the reduction.

  Therefore, according to the present invention, it is possible to suppress the reduction pulverization that is likely to occur in the solid-state reduction furnace, which is a hydrogen-based reduction process, and the clustering of pellets hardly occurs, and stable furnace operation / reduction treatment becomes possible. .

In addition, the method for producing a reducing raw material provided by the present invention is the same as the conventional method of applying chloride such as CaCl ₂ to the pellet surface, and spraying and drying chloride on the fired pellet newly to the surface. There is no need for the step of applying, and problems such as corrosion of equipment such as a solid reduction furnace do not occur.

It is a schematic diagram of the reduction reaction of the baked pellets when the reduction reaction of the baked pellets made of Fe ₂ O ₃ is advanced by a solid reduction furnace. (A) is a graph showing the relationship between the change in the reduction rate and the open porosity by reduction of fired pellets to Fe from Fe ₂ O _3, (b) is the accompanying reduction of fired pellets of Fe ₂ O ₃ It is a graph which shows the change of the bulk density of a baking pellet. (A) and (b), as fired pellets a porous body consisting of Fe ₂ O _3, in the case of advanced reduction by hydrogen-containing gas, be a graph illustrating the construction phase and intensity change of the fired pellets (A) is a result when reduction powdering is small, (b) is a result when reduction powdering is large. It is an x-ray diffraction pattern when CF2 is subjected to a reduction reaction in an argon-hydrogen atmosphere under different reduction temperatures. (A) is a graph comparing changes in reduction rate and open porosity when fired pellets manufactured with different compositions are subjected to a reduction reaction in an argon-hydrogen atmosphere, and (b) is a change in composition. It is a graph which compares the change of a reduction rate and a bulk density when carrying out the reduction reaction in the argon-hydrogen atmosphere. (A) is a graph which shows the relationship between the calcination temperature and relative density of the baked pellet manufactured by changing a composition, (b) is the relationship between the calcination temperature and crush strength of the baked pellet manufactured by changing a composition. It is a graph which shows. It is a graph which shows the relationship between the reduction temperature and crushing strength of the firing pellet of CF1 and CF2. It is an enlarged photograph which shows the state of the cross-sectional structure | tissue of the surface vicinity before the reduction | restoration of the baking pellet of CF1 and CF2, and after reduction | restoration. A Fe ₂ O ₃ -CaO phase diagram. (A) is an electron micrograph of a section of CF2 phase coating layer obtained by the production method of the present invention, (b) the pellets produced by firing at 1250 ° C. The Fe ₂ O ₃ powder after granulation It is the electron micrograph of the cross section of the said coating layer when baking at 1200 degreeC on the surface and forming the coating layer of CF2 phase. (A) is a graph which shows the relationship between the bulk density by Fe / Ca ratio, and a calcination temperature (degreeC) at the time of making a calcium pellet phase with the Fe / Ca ratio of 1 to 2 into a baking pellet, (b ) Is a graph showing the relationship between the bulk density / apparent density according to the Fe / Ca ratio and the firing temperature. The bulk density of a coating layer formed from a raw material powder of a calcium ferrite phase having a Fe / Ca ratio of 1.25, a calcium iron compound having a Fe / Ca ratio of 3.23, and a Fe / Ca ratio of 1.0 It is a graph which compares the relative bulk density of the coating layer formed using the mixed powder which prepared the calcium iron compound so that Fe / Ca ratio might be 1.25.

  Hereinafter, the present invention will be described in detail.

[Structure and composition of raw material for reduction]
The raw material for reduction of the present invention has a gas barrier property in a temperature range near 550 ° C. with respect to hydrogen on the surface of a porous body made of Fe ₂ O ₃ raw material granulated in a bulk shape such as a sphere or a spheroid. And a coating layer that loses gas barrier properties in a temperature range of 700 ° C. or higher is covered.

The porous body made of the Fe ₂ O ₃ raw material described above mainly contains Fe ₂ O ₃ powder, and may contain a molding aid for imparting formability to the Fe ₂ O ₃ raw material. As the molding aid, bentonite is preferable, and ion-exchanged bentonite may be used. From the viewpoint of increasing the purity of the reduced iron obtained from the solid reduction furnace, the content of the molding aid is preferably 0.1 parts by mass or more and 10.0 parts by mass or less with respect to the Fe ₂ O ₃ powder. . In addition, it is preferable that the said porous body shall be 1.0 g-4.0 g, and an average particle diameter is a magnitude | size of diameter 5 mm or more and 20 mm or less.

The coating layer covering the porous body contains a calcium iron compound mainly composed of Ca _x Fe _y O _z (1 <y / x ≦ 2; 1 ≦ z). In addition, since the calcium iron compound remains as CaO on the surface of the reduced iron after reduction, fusion between pellets in the solid reduction furnace and clustering due thereto are less likely to occur. Incidentally, the calcium ferrite phase such as calcium iron compound is originally a substance often contained in sintered ore in the field of ironmaking. Even if the substance is applied to the surface of the porous body made of the Fe ₂ O ₃ raw material while controlling the composition, and the reduced product is introduced into the blast furnace as a reduced iron content, It has been introduced from this sinter, and does not become a new foreign material, and it is difficult to cause problems that hinder the operation of the conventional blast furnace or damage the equipment of the blast furnace.

The molar ratio of Fe to Ca in the calcium iron compound is more than 1, as shown in FIGS. 6 (a) and 6 (b), when CF1 (CaFeO _2.5 ) with the ratio of 1 is 1300 ° C. If the above baking is performed, it densifies and shows the same strength as hematite. However, even if CF1 is baked at 1200 ° C., the coating layer does not become sufficiently dense. Further, CF1 has a reduction rate of only 2.47% even at 700 ° C. as shown in FIG. 7 and gas barrier properties remain, and the reduction reaction hardly proceeds as shown in FIG. Therefore, the molar ratio of Fe to Ca of the calcium iron compound constituting the coating layer needs to be adjusted to more than 1.

On the other hand, the coating layer containing CF2 (CaFe ₂ O ₄ ) has a relative density of more than 80% to 90% by firing at 1200 ° C. as shown in FIGS. 6 (a) and (b). It is dense to the extent and has a crushing strength of 2000N. Further, as shown in FIG. 7, at 700 ° C., the coating layer containing CF 2 is reduced to a reduction rate of 12.9%, and the reduction reaction proceeds as shown in FIG. However, as shown in FIG. 7, even in this state, the crushing strength of CF2 is maintained at 1000 N or more, and problems such as peeling during the reduction process hardly occur. The reduced tissue having a CF2 coating layer does not generate coarse pores, and the reduced tissue maintains high strength even after reduction.

However, when the molar ratio y / x value exceeds 2, as shown in the Fe ₂ O ₃ —CaO phase diagram of FIG. 9, the liquid phase generation temperature is lowered by 10 ° C. or more, and therefore, about 1200 ° C. Also in firing, a liquid phase is generated and melted in the firing process, making it difficult to form a healthy coating layer. In addition, the proportion of CaO produced after the coating layer is reduced is reduced, so that the pellets are fused in the solid reduction furnace, and the pellets are fused in the solid reduction furnace, thereby clustering. Is likely to occur.

On the other hand, from FIG. 9, if the y / x value is controlled to 2 or less, in the process of applying the coating layer, the molten state to the extent that the layer flows, that is, the liquid phase due to the macro melting described above is obtained. It is suggested that it is possible to prevent the formation and form a dense layer by solid phase sintering. FIG. 10 shows a structure in which Fe ₂ O ₃ powder is granulated and then fired at 1,250 ° C. on the surface of pellets produced by firing at 1,250 ° C. to form a CF2 phase coating layer (FIG. 10B). It is an electron micrograph which compares the coating layer (FIG. 10 (a)) of CF2 phase obtained by the manufacturing method of this invention. Incidentally, invention example of FIG. 10 (a) was obtained by baking at coating the obtained pellet CF2 phase by granulating the _Fe 2 _{O 3} powder 1200 ° C.. As shown in FIG. 10, it was confirmed that a dense layer could be formed without melting by solid phase sintering.

  The coating layer may contain a molding aid for imparting formability to the coating layer and for imparting adhesiveness to the porous body. As the molding aid, bentonite is preferable, and ion-exchanged bentonite may be used. From the viewpoint of increasing the purity of the reduced iron obtained from the solid reduction furnace, the content of the molding aid is preferably 0.1 parts by mass or more and 10.0 parts by mass or less with respect to the calcium iron compound. In order to precisely control the formation temperature of the coating layer, it is preferable not to include a combustible material such as coke powder in the components of the coating layer or the starting material for shaping the coating layer.

  The coating layer covers 70% or more of the entire surface of the porous body. When the coverage of the coating layer is less than 70%, the gas barrier property due to the coating layer is lowered. For this reason, in the reduction in a hydrogen atmosphere at 500 to 600 ° C., the reduction of the pellets proceeds, exhibits the same reducibility as in the case where there is no coating layer, and the problem of reduced powdering occurs. The ratio of the coating layer covering the entire surface of the porous body is preferably 85% or more. If over 90% of the entire surface of the porous body is coated, there is no problem in reducing characteristics, but the yield and productivity are impaired. Therefore, it is sufficient to cover up to about 90%. Note that the coverage or porosity of the coating layer can be measured by observing the cross-sectional structure of the reducing raw material with an optical microscope and performing image processing on the structure photograph. In particular, in the cross-sectional structure observation, the coverage can be obtained as a ratio of the length of the portion where the coating layer is directly bonded to the solid phase without interposing pores or cracks on the surface of the porous body.

  Moreover, the said coating layer is 5.0 mass parts or more and 50.0 mass parts or less of the mass of the said porous body. When the coating layer is less than 5.0 parts by mass of the porous body, the effect of reducing powder reduction is insufficient, and when it exceeds 50.0 parts by mass of the porous body, the iron oxide raw material The ratio is reduced and productivity is impaired. For example, when the mass of the porous body is 2.0 g, the mass of the coating layer is 0.1 g or more and 1 g or less with respect to one porous body. Moreover, it is preferable that the said coating layer shall be 5 to 20 mass parts of the said porous body.

In the present invention, by forming the coating layer into a porous body made of Fe ₂ O ₃ raw material, the reaction between the coating layer and the porous body is suppressed, and gas barrier properties against hydrogen gas can be exhibited. Denseness can be ensured.

[Production method of raw material for reduction]
The porous body serving as the core of the raw material for reduction is manufactured by granulating Fe ₂ O ₃ powder in bulk. In order to adjust the particle size of the reducing raw material Fe ₂ O ₃ , the raw material powder may be pulverized in advance, or calcined at a temperature of about 1000 ° C., and then granulated in bulk. If necessary, a solvent such as water and a molding aid such as bentonite are added to the Fe ₂ O ₃ powder and granulated in bulk. The granulation method is not particularly limited, but it is desirable that the granulation is performed to 1.0 g to 4.0 g and the average particle size is 5 mm or more and 20 mm or less. The granulated body is dried within a range of 100 ° C to 600 ° C. For granulation, a method in which raw powder is packed into a mold and press-molded may be used, but it is suitable for mass production at low cost, and most suitable is that raw material is put into a bread granulator and rolled while rolling It is a method of growing into a spherical shape of a large size.

The calcium iron compound constituting the coating layer includes Fe ₂ O and at least one of CaO and CaCO ₃ so that the Fe content with respect to the Ca content is more than 1.0 and less than 2.0 in terms of molar ratio. ₃ and then calcining in the air at 800 ° C. to 1000 ° C. for production. Although it is possible to use the mixed powder for coating as it is, in this way, by forming a calcium iron compound having a desired Ca / Fe molar ratio in advance and coating the granulated body with the calcium iron compound. It is more desirable because the uniformity of the coating layer can be achieved, the solid phase sinterability of the coating layer can be easily controlled, and the gas barrier property of the coating layer can be reliably realized.

  The coating layer preferably contains bentonite. The material constituting the coating layer can be produced by mixing bentonite at 0.1 parts by mass or more and 10.0 parts by mass or less with respect to the calcium iron compound. Using the coating layer material thus obtained, the surface of the granule is coated with 5.0 to 50.0 parts by mass of the granule to form a coating layer.

  The coating layer is prepared by putting iron oxide pellets granulated in advance into a spherical shape and a material or starting material (hereinafter referred to as “coating layer material”) constituting the coating layer into a bread granulator. It is preferably formed by rolling the bread granulator and adhering it to a granulated body such as the iron oxide pellets. More preferably, the granulated body such as the iron oxide pellets is immersed in a container storing the material of the coating layer and the like, and after the material of the coating layer is adhered to the surface of the granulated body, the The granular body is taken out from the container, and the granulated body is rolled in a dish-shaped container to adjust the material or the like of the coating layer to a desired amount. According to a more preferable method described later for forming the coating layer, it is possible to easily form a coating layer that is difficult to peel off from the surface of the iron oxide granule without using a solvent such as water.

In the Fe ₂ O ₃ granules of the granulated material was fired in advance than about bulk density of 70%, it is impossible to form a dense coating layer on its surface. As can be seen from FIG. 10 (b), when the coating layer is formed after the granulated body is fired, the dense coating layer of the present invention cannot be formed.

  The granulated body on which the coating layer is formed is fired in the air at 1150 ° C. or more and 1200 ° C. or less. When the firing temperature exceeds 1200 ° C., the reaction between the coating layer and iron oxide starts as shown in FIG. 9, and the above-described macroscopic liquid phase is generated. If a liquid phase is generated instead of solid phase sintering at the firing stage, the components of the coating layer flow down during firing or permeate and react inside the iron oxide granule part. The desired uniform gas barrier property cannot be exhibited. Due to the composition change due to the reaction with the iron oxide granule part, the reducibility of the coating layer is changed, and the coating layer is reduced due to the occurrence of flow-down, resulting in problems such as discontinuity of the coating layer. Arise. On the other hand, when the temperature is lower than 1150 ° C., the strength of the reducing raw material obtained by firing is insufficient.

11A and 11B show the molar ratio of Ca to Fe (hereinafter also referred to as “Fe / Ca ratio”) of the calcium iron compound that is the main component of the coating layer, and the bulk density (g / cm ³ ). ), Bulk density / apparent density, and firing temperature (° C.). As shown in FIG. 11 (b), the calcium iron compound having an Fe / Ca ratio of 1.25 to 2 has a value of “bulk density / apparent density” of 90% or more after firing at 1200 ° C. . Further, as shown in FIG. 11A, the bulk density of the calcium iron compound having an Fe / Ca ratio of 1.25 to 2 is the bulk density of the calcium iron compound having an Fe / Ca ratio of 1 after firing at 1200 ° C. About 20% higher than the density. Thus, the coating layer made of a calcium iron compound having an Fe / Ca ratio of more than 1 has good denseness, and the granulated body on which the coating layer is formed is desirably fired at 1200 ° C.

  In this way, the iron oxide material and the coating layer are integrally fired, so that defects such as cracks do not occur in the coating layer, and there is no peeling of the coating layer, so that it can be densely and soundly formed on the pellet surface. . In addition, when the fired pellets are reduced in a hydrogen atmosphere by the application of the coating layer controlled as described above, the portion of iron oxide under the coating layer is in a temperature region near 550 ° C. where reduction powdering becomes a problem. Is reduced, and reduced powdering is less likely to occur. On the other hand, in a temperature range of 700 ° C. or higher at which reduced powdering does not easily occur, the coating layer itself is reduced and loses its gas barrier property, and the reduction proceeds promptly without impeding the reduction of iron oxide.

  In addition, a mixed powder prepared by adjusting a calcium ferrite phase that easily generates a liquid phase and a calcium ferrite phase that hardly generates a liquid phase so that the Fe / Ca molar ratio satisfies 1 <Fe / Ca ≦ 1.5 is used. A coating layer may be formed.

As the calcium ferrite phase that easily forms a liquid phase, a calcium iron compound composed of Ca _x1 Fe _y1 O _z1 (2 <y1 / x1 ≦ 4; 1 ≦ z1) is preferable, and in particular, Ca _x1 Fe _y1 O _z1 (y1 / x1) = 3.23; 1 ≦ z1) (hereinafter referred to as “CaFe _3.23 O _z ”) is preferably used. This is because in the phase diagram of Fe ₂ O ₃ —CaO, the composition position where the melting point of the liquid phase is the lowest is near F ₂ eO ₃ : 80 wt%, and the composition corresponding to this position is “CaFe _3.23 O _z ”. This is because the effect of promoting the densification of the coating layer can be obtained most.

As the calcium ferrite phase that hardly forms a liquid phase, a calcium iron compound composed of Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2) is preferable. Further, the product is hard calcium ferrite phase liquid phase, or as a substitute for generating hard calcium ferrite phase of the liquid phase may be used at least one of CaO and CaCO _3. However, when a calcium ferrite phase having the same Fe / Ca molar ratio (1 <Fe / Ca ≦ 1.5) is generated, Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1) rather than CaO and CaCO _3. 5; 1 ≦ z 2) is preferably used. This is because when the calcium ferrite phase powder having a low Fe / Ca molar ratio is used, the amount of use can be increased as compared with the CaO and CaCO ₃ powders. This is because the liquid phase distribution can be made uniform.

When CaFe _3.23 O _z is used as the calcium ferrite phase that easily generates the liquid phase, Ca _x2 Fe _y2 O _z2 (y 2 / x 2 = 1.0; 1 ≦ _z 2) (hereinafter referred to as “calcium ferrite phase that is difficult to generate the liquid phase”) "CaFeO _{z '"} is described as) using, CaFe _3.23 O _z in a weight ratio _{of:. CaFeO z' = 1:} 99~40: 60 range, more preferably 9: 91-40: in the range of 60 It is preferable to mix as described above. Within this range, the proportion of CaFeO _{z 'in} the coating layer raw material powder is larger than that of Ca _x2 Fe _y2 O _z2 , and a sufficiently large amount of reaction partner CaFeO _z' exists around the microscopic liquid phase. In addition, the coating layer can be formed stably. However, if the ratio of CaFe _3.23 O _z is lower than the above range, the formation of a microscopic liquid phase becomes insufficient, and it becomes difficult to obtain the effect of promoting the densification of the coating layer. On the other hand, when the proportion of CaFe _3.23 O _z is higher than the above range, the reaction partner CaFeO _{z ′} around the microscopic liquid phase is insufficient, so that a coating layer using a calcium ferrite phase powder of a single composition from the beginning. Compared with the case of forming, it becomes difficult to obtain the effect of promoting the densification of the coating layer.

  In addition, it is preferable that the powder of the calcium ferrite phase which is easy to produce | generate a liquid phase, and the powder of the calcium ferrite phase which is hard to produce | generate a liquid phase are within the range of 0.5 micrometer-500 micrometers in terms of a projected area circle. . If the average particle size of the calcium ferrite phase powder that is liable to form a liquid phase is less than 0.5 μm, the formation of a microscopic liquid phase becomes insufficient, and the effect of promoting the densification of the coating layer is difficult to obtain. . In addition, when the average particle diameter of the calcium ferrite phase powder that easily generates the liquid phase exceeds 500 μm, the reaction partner of the calcium ferrite phase that easily generates the liquid phase is locally insufficient, and the effect of promoting the densification of the coating layer Is difficult to obtain.

  By using the mixed powder having the above-described composition, it is possible to form a healthy coating layer without causing the problems caused by the above-described macroscopic liquid phase generation. FIG. 12 shows the relative bulk density (“CF1.25” in the figure) of the coating layer formed from a raw material powder of a calcium ferrite phase having a Fe / Ca ratio of 1.25 and an Fe / Ca ratio of 3. The bulk density of the coating layer formed by using a mixed powder prepared by adjusting the Fe / Ca ratio to 1.25 with a calcium iron compound having a Fe / Ca ratio of 1.0 and a calcium iron compound having a Fe / Ca ratio of 1.0 (in the figure) 12 shows that “CF1.25 by (CF3.23 + CF).” From FIG. 12, the coating layer formed by using the mixed powder having the composition described above uses a powder composed of a calcium ferrite phase having a single composition. It can be seen that the resulting coating layer is denser and has better gas barrier properties, and the effect of this effect is higher as the firing temperature at the time of forming the coating layer is higher and approaches 1200 ° C. I understand.

Calcium iron compound composed of Ca _x1 Fe _y1 O _z1 (2 <y1 / x1 ≦ 4; 1 ≦ z1) and calcium iron composed of Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2) The compound is prepared by mixing at least one of CaO and CaCO ₃ with Fe ₂ O ₃ so that the Fe content is in a desired molar ratio with respect to the Ca content, and then at 800 ° C. to 1000 ° C. It can manufacture by baking in air | atmosphere.

Further, from a calcium iron compound made of Ca _x1 Fe _y1 O _z1 (2 <y1 / x1 ≦ 4; 1 ≦ z1) and Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2) While adjusting so that it may satisfy | fill 1 <Fe / Ca <= 1.5, you may form a coating layer using the powder which mixed the bentonite. The material which comprises a coating layer can mix and manufacture a bentonite by 0.1 to 10.0 mass parts with respect to said 2 types of calcium iron compound whole quantity. If the bentonite is added in a large amount, the reaction between the calcium ferrite phase powder that easily forms the liquid phase and the calcium ferrite phase powder that hardly forms the liquid phase may be inhibited. For this reason, in the method of coating the mixed powder, the amount of bentonite added is more preferably 2 parts by mass or less. Using the coating layer material thus obtained, the surface of the granule is coated with 5.0 to 50.0 parts by mass of the granule to form a coating layer.

(Coating layer formation test 1)
Fe ₂ O ₃ and the mass ratio of the bentonite 20: to be 1, were weighed and mixed with _Fe 2 _{O 3} powder and bentonite, then, as the mass per becomes 2 g, _Fe 2 O ₃ and iron oxide pellets consisting of bentonite were formed with a bread granulator.

Meanwhile, CaCO ₃ and Fe ₂ O ₃ are weighed and mixed with CaCO ₃ powder and Fe ₂ O ₃ powder so that the molar ratio of Ca to Fe is 1: 1.5, and calcined at 900 ° C. Thus, a sintered body of CaFe _1.5 O was manufactured. Next, the fired body of CaFe _1.5 O is crushed, the bentonite is weighed so that the mass ratio of CaFe _1.5 O and bentonite is 20: 1, and CaFe _1.5 O powder and bentonite are mixed. The material of the coating layer was prepared by mixing uniformly.

The iron oxide pellet is immersed in a container in which a mixed powder of the CaFe _1.5 O powder and bentonite is stored, and the coating layer material is attached to the surface of the iron oxide pellet. The iron oxide pellets were removed from the container and rolled in a dish-shaped container to adjust the amount of the coating layer material to a desired amount.

  The granulated body to which the material of the coating layer was adhered was fired at 1200 ° C. in the atmosphere to obtain the reducing raw materials of Comparative Examples 1 to 6 and Examples 1 to 6. The molar ratio (y / x) of the Fe element to the Ca element of the calcium iron compound contained in the coating layers of Comparative Examples 1 to 6 and Examples 1 to 6, and the mass% (parts by mass) of the coating layer with respect to the iron oxide pellets ) Are shown in Table 1.

When Examples 1-6 and Comparative Examples 1-6 were subjected to a reduction reaction in an atmospheric heat treatment furnace, the reduction rate and crushing strength of the reducing raw material were measured. In order to prevent damage due to contact between the pellets in the solid reduction furnace, and to prevent reducing powdering and clustering, the raw material for reduction shall pass that satisfies both the following requirements (i) and (ii), Those that did not satisfy at least one of the requirements (i) and (ii) were judged as rejected. The measurement results are shown in Table 1.
(i) Reduction reaction at 550 ° C .: The reduction rate after 5 hours is 5% or less and the crushing strength is 1000 N or more.
(ii) Reduction reaction at 700 ° C .: Reduction rate after 5 hours is 15% or more and crushing strength is 500 N or more.

The reduction reaction was performed by heating each sample for 5 hours at 550 ° C. and 700 ° C. in an Ar air flow containing 3.6% H ₂ in an atmosphere heat treatment furnace. In any case, the weight of the reducing material before coating is 2 g per piece.

  As can be seen from Examples 1 to 4 having coating layers having the same molar ratio of Ca and Fe, the reduction rate in the reduction test at 550 ° C. decreases as the mass of the coating layer per reducing material increases. However, at 700 ° C., the reduction rate is hardly affected by the mass of the coating layer. From this, the progress of the reduction reaction at 550 ° C. is suppressed by the coating layer, and in the temperature range where reduction powdering becomes a problem, the fired pellets maintain high crushing strength, and therefore reduction powdering is difficult to occur. I understand that. On the other hand, it can be seen that the coating layer does not affect the progress of the reduction reaction at 700 ° C. in the high temperature range where reduction powdering is not a problem. That is, according to the present invention, the reduction of hydrogen-based iron oxide is suppressed in the temperature range of 550 to 600 ° C., and the reduction of iron oxide is advanced at 700 ° C. or higher, thereby suppressing reduction powdering and increasing the strength of the calcined pellets. It became clear that it was possible to secure.

  Further, regarding the reduction rate in the reduction reaction test at 550 ° C., when the results of Example 1 and Comparative Example 3 are compared, if the coverage of the coating layer is 70%, the gas barrier effect is ensured in the reduction at 550 ° C. However, it can be seen that when the coverage of the coating layer is less than 70%, the gas barrier properties are insufficient and the effects of the present invention cannot be obtained. If the gas barrier property is insufficient, the reduced behavior of the fired pellets is almost the same as the pellets without coating. On the other hand, the basis weight of Examples 2 to 6 is 10 parts by mass or more, and the coverage is a preferable coverage of 85% or more. With this coverage, the gas barrier properties at 550 ° C. are stably exhibited with less variation. In the fired pellet of Comparative Example 4, the coating layer is 60 parts by mass of the porous body, and of course the gas barrier property at 550 ° C. is exhibited, but the ratio of iron oxide in the porous body part in the fired pellet is 2 / 3, it becomes necessary to prepare a large amount of coating layer for the production of fired pellets, and the amount of reduced iron produced by reduction is also reduced, both the production of pellets and the production of reduced iron This causes problems in productivity in the process.

  Moreover, since the molar ratio (y / x) of Ca element and Fe element of the calcium iron compound contained in the coating layer of Comparative Example 5 is 1 or less, Comparative Example 5 is 700 compared to Example 5. Low reduction rate in reduction reaction test at ° C. In Comparative Example 6, the molar ratio (y / x) exceeded 2 and a healthy coating layer could not be formed.

(Coating layer formation test 2)
Formed by using a mixed powder in which a calcium ferrite phase that easily generates a liquid phase and a calcium ferrite phase that does not easily generate a liquid phase are adjusted so that the molar ratio of Fe / Ca satisfies 1 <Fe / Ca ≦ 1.5. The reduction rate and crushing strength of the reducing raw material having the coating layer were measured.

  In the coating layer formation test 2, except that the material of the coating layer was prepared based on the conditions in Table 2, the firing conditions of the granulated body after formation of the coating layer, the reduction reaction conditions, the judgment conditions of the crushing strength, etc. All other conditions were the same as in the coating layer formation test 1 described above.

The materials of the coating layers of Examples 7 to 11 were prepared as follows.
First, the CaCO ₃ powder and the Fe ₂ O ₃ powder are weighed and mixed so that the molar ratio of Fe to Ca becomes the value in “y1 / x1” of the column “CaFe Compound 1” in Table 2, A calcined body of calcium iron compound 1 was prepared by calcining. Further, as the calcium iron compound 2 to be reacted with the calcium iron compound 1, the CaO powder or the CaCO _{3 is} adjusted so that the molar ratio of Fe to Ca becomes a numerical value in “y2 / x2” of the column “CaFe compound 2” in Table 2. The powder and Fe ₂ O ₃ powder were weighed and mixed and prepared by firing at 900 ° C. However, CaO was used as the CaFe compound 2 of Example 7. Further, CaCO ₃ was used as the CaFe compound 2 of Example 8.

  Next, the calcined bodies of the calcium iron compounds 1 and 2 were crushed, and the powders of the calcium iron compounds 1 and 2 were uniformly mixed at a numerical ratio in the column “weight ratio in raw material powder” in Table 2. Further, the bentonite is weighed so that the mass ratio of the mixed powder and bentonite is 99: 1, and the mixed powder and bentonite are uniformly mixed to prepare the raw material powders for the coating layers of Examples 7 to 11. did. Regarding the Ca element and the Fe element contained in the raw material powders of the coating layers of Examples 7 to 11, the molar ratio of Fe to Ca is shown in the column “Fe / Ca ratio in the whole raw material powder” in Table 2.

Examples 7 and 8 are examples in which the same coating layer as in Example 3 of Table 1 was formed by reacting a calcium ferrite phase (calcium iron compound 1) that easily generates a liquid phase with CaO or CaCO _3. It is. In the raw material for reduction of Example 3, the progress of reduction at 550 ° C. is suppressed, the crushing strength after reduction is high, the reduction proceeds at 700 ° C., and the crushing strength after reduction is also appropriately maintained. . The reducing raw materials of Examples 7 and 8 are also excellent reducing raw materials that exhibit a barrier effect at 550 ° C. reduction equivalent to or higher than that of Example 3 and that the reduction proceeds at 700 ° C.

  In Example 9, a coating layer similar to that in Example 5 in Table 1 is formed by using a calcium ferrite phase (calcium iron compound 1) that easily generates a liquid phase and a calcium ferrite phase (calcium iron compound 2) that is difficult to generate a liquid phase. It is an example formed by reacting. Example 5 in Table 1 is also a reducing raw material provided with the same y / x = 1.2 coating layer. However, in Example 9, the reduction rate in reduction at 550 ° C. was further suppressed as compared with Example 5. A stronger barrier effect is exhibited, suggesting that the coating layer is more dense and sound.

  In Example 10, a coating layer of y / x = 1.1, where y / x is closer to 1.0 than in Example 9, and a calcium ferrite phase (calcium iron compound 1) that easily generates a liquid phase and a liquid phase are generated. This is an example formed by reacting a difficult calcium ferrite phase (calcium iron compound 2). As in Example 9, it can be seen that a reducing raw material that exhibits an excellent barrier effect at 550 ° C. is obtained.

  In Example 11, mixed powder was used as the raw material powder for coating, but y / x = 2.0, and a coating layer outside the above-described preferred range was applied. Example 11 corresponds to Example 6 in Table 1. The raw material for reduction obtained in Example 11 has a reduction rate at 550 ° C. that is almost the same as that of Example 6 and is an excellent raw material for reduction, but unlike Examples 9 and 10, By using the mixed powder of two kinds of calcium ferrite phases, an effect that a more healthy coating layer is easily formed is not shown.

  From the results of Examples 9 to 11, the coating layer formed using the mixed powder having the above-described composition has higher density than the coating layer formed using the powder composed of the calcium ferrite phase having a single composition, It turns out that it is excellent in gas barrier property.

  INDUSTRIAL APPLICABILITY The present invention can be suitably used as a reducing raw material such as iron oxide fired pellets used as a raw material for a solid reduction furnace.

Claims (11)

A porous body containing Fe ₂ O ₃ ;
70% or more of the entire surface of the porous body is covered, and is 5.0 parts by mass or more and 50.0 parts by mass or less of the mass of the porous body, and Ca _x Fe _y O _z (1 <y / x ≦ And a coating layer containing a calcium iron compound consisting of 1 ≦ z). The reduction according to claim 1, wherein the porous body contains bentonite in an amount of 0.1 parts by mass or more and 10.0 parts by mass or less with respect to the content of Fe ₂ O ₃ of the porous body. Raw material.   The said coating layer contains bentonite in 0.1 mass part or more and 10.0 mass parts or less with respect to the sum total of the Ca compound and Fe compound, and the said Ca compound and Fe compound. Raw materials for reduction as described in 1. Granulating a powder containing Fe ₂ O ₃ to produce a granulated body,
Using a calcium iron compound composed of Ca _x Fe _y O _z (1 <y / x ≦ 2; 1 ≦ z), 70% or more of the entire surface of the granulated body is accounted for by 5% of the mass of the granulated body. A coating layer is formed by coating with 0 to 50.0 parts by mass,
A method for producing a reducing raw material, characterized in that the granulated body on which the coating layer is formed is fired in the air at 1150 ° C. or higher and 1200 ° C. or lower. Fe ₂ O ₃ is mixed with at least one of CaO and CaCO ₃ so that the Fe content with respect to the Ca content is more than 1.0 and not more than 2.0 in molar ratio,
Subsequently, the said calcium iron compound is obtained by baking in air | atmosphere at 800 to 1000 degreeC, The manufacturing method of the raw material for a reduction | restoration of Claim 4 characterized by the above-mentioned. Granulating a powder containing Fe ₂ O ₃ to produce a granulated body,
Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2), at least one of calcium iron compound, CaO and CaCO ₃ , and Ca _x1 Fe _y1 O _z1 (2 <y1 / a mixture satisfying a molar ratio of Fe and Ca of 1 <Fe / Ca ≦ 1.5 using a calcium iron compound consisting of x1 ≦ 4; 1 ≦ z1)
Using the mixture, 70% or more of the entire surface of the granulated body is coated with 5.0 to 50.0 parts by mass of the mass of the granulated body to form a coating layer,
A method for producing a reducing raw material, characterized in that the granulated body on which the coating layer is formed is fired in the air at 1150 ° C. or higher and 1200 ° C. or lower. From a calcium iron compound composed of Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2) and Ca _x1 Fe _y1 O _z1 (2 <y1 / x1 ≦ 3.23; 1 ≦ z1) The manufacturing method of the raw material for reduction | restoration of Claim 6 characterized by preparing the mixture with which the molar ratio of Fe and Ca satisfy | fills 1 <Fe / Ca <= 1.5 using the calcium iron compound which becomes. At least one of CaO and CaCO ₃ is mixed with Fe ₂ O ₃ so that the Fe content with respect to the Ca content is 1.0 to 1.5 in terms of molar ratio, and then 800 ° C to 8. A calcium iron compound comprising Ca _x2 Fe _y2 O _z2 (1 ≦ y2 / x2 ≦ 1.5; 1 ≦ z2) is obtained by firing at 1000 ° C. in the air. Of manufacturing raw materials for reduction. At least one of CaO and CaCO ₃ and Fe ₂ O ₃ are mixed so that the Fe content with respect to the Ca content is more than 2.0 and 4.0 or less in terms of molar ratio, and then 800 ° C to The reduction according to claim 6 or 7, characterized in that a calcium iron compound composed of Ca _x1 Fe _y1 O _z1 (2 <y1 / x1≤4; 1≤z1) is obtained by firing in the atmosphere at 1000 ° C. For manufacturing raw materials. Bentonite is mixed in a powder containing Fe ₂ O ₃ and 0.1 to 10.0 parts by mass with respect to the mass of the powder,
A mixture containing the powder containing Fe ₂ O ₃ and the bentonite is granulated to an average particle size of 5 mm or more and 20 mm or less to form a granulated body;
Then, any one of claims 4-9, characterized in that said dried over granular material to the atmosphere at 100 ° C. or higher 600 ° C. or less to produce a granular material containing the Fe ₂ O ₃ The manufacturing method of the raw material for a reduction | restoration as described in 2.   The said coating layer contains bentonite at 0.1 mass part or more and 10.0 mass parts or less with respect to Ca compound and Fe compound which are contained in the said coating layer, Any one of Claims 4-10 characterized by the above-mentioned. A method for producing the reducing raw material according to claim 1.