Production method for reduced product of metal oxide

Abstract

To provide a production method for a reduced product of a metal oxide in which carbon dioxide can be inhibited from being produced.SOLUTION: A production method for a reduced product of a metal oxide includes irradiating the metal oxide and a high-melting-point material not in contact with the metal oxide with an electromagnetic wave which is at least one of a microwave and a millimeter wave to reduce at least a part of the metal oxide. A partition material is disposed between the metal oxide and the high-melting-point material. The melting point of the high-melting-point material is higher than that of the metal oxide. The high-melting-point material includes: an absorbing material which absorbs the electromagnetic wave in a temperature zone which is at least partially lower than that in which the metal oxide absorbs the electromagnetic wave; and an insulation material which has a lower level of absorption of the electromagnetic wave than the metal oxide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] Some aspects of the present invention relate to methods for producing reduced metal oxides. [Background technology]

[0002] Since iron ore is primarily composed of iron oxide, the iron oxide is generally reduced by the carbon contained in coke when producing iron. The reduction reaction of iron oxide is expressed by the following formula: 3Fe2O3+CO → 2Fe3O4+CO2 Fe3O4+CO→3FeO+CO2 FeO+CO→Fe+CO2 As shown in the above formula, conventional methods for reducing iron oxide generate carbon dioxide (CO2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-138023 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-184718 [Patent Document 3] International Publication No. 2022 / 195989 [Patent Document 4] International Publication No. 2022 / 196681 Summary of the Invention [Problem to be solved by the invention]

[0004] It is desirable to suppress the generation of carbon dioxide when reducing metal oxides, including but not limited to iron oxide. Therefore, one object of some aspects of the present invention is to provide a method for producing a reduced metal oxide that can suppress the generation of carbon dioxide. [Means for solving the problem]

[0005] A method for producing a reduced metal oxide according to an embodiment of the present invention includes irradiating a metal oxide and a high-melting-point material that is not in contact with the metal oxide with electromagnetic waves, which are at least one of microwaves and millimeter waves, to reduce at least a portion of the metal oxide, wherein a partition material is disposed between the metal oxide and the high-melting-point material, the melting point of the high-melting-point material being higher than the melting point of the metal oxide, and the high-melting-point material includes an absorbing material that absorbs electromagnetic waves in a temperature range that is at least partially lower than the temperature range in which the metal oxide absorbs electromagnetic waves.

[0006] In the method for producing a reduced metal oxide, the metal oxide may not be brought into contact with a carbon-containing reducing agent when at least a portion of the metal oxide is reduced. The carbon-containing reducing agent may be a gas, liquid, or solid.

[0007] In the method for producing a reduced metal oxide, carbon monoxide (CO) does not need to be supplied to the metal oxide when at least a portion of the metal oxide is reduced.

[0008] In the above-mentioned method for producing a reduced metal oxide, the metal oxide may be in the form of a powder.

[0009] In the method for producing a reduced metal oxide, in which at least a portion of the metal oxide is reduced, the partition material may be a container, the metal oxide may be placed inside the container, and the high-melting point material may be placed outside the container.

[0010] In the method for producing a reduced metal oxide, in which at least a portion of the metal oxide is reduced, the partition material may be a container, the high-melting point material may be placed inside the container, and the metal oxide may be placed outside the container.

[0011] In the above-described method for producing a reduced metal oxide, the container may be a crucible.

[0012] In the above-mentioned method for producing a reduced metal oxide, the metal oxide may be iron oxide.

[0013] In the above-described method for producing a reduced metal oxide, the iron oxide may be at least one selected from iron oxide (III) (Fe2O3), triiron tetroxide (Fe3O4), and iron oxide (II) (FeO).

[0014] In the above-mentioned method for producing a reduced metal oxide, the iron oxide may be iron (III) oxide (Fe2O3).

[0015] In the above-described method for producing a reduced metal oxide, the absorbing material may contain a carbon material.

[0016] In the above-described method for producing a reduced product of a metal oxide, the high-melting-point material may further contain a heat insulating material that absorbs electromagnetic waves to a lesser extent than the metal oxide.

[0017] In the above-described method for producing a reduced metal oxide, the heat insulating material may contain an oxide.

[0018] In the above-described method for producing a reduced metal oxide, a high-melting-point material may be contained in the mold.

[0019] In the above-described method for producing a reduced metal oxide, the high-melting-point material may be in the form of a powder.

[0020] A method for reducing metal oxide according to an embodiment of the present invention includes irradiating a metal oxide and a high-melting-point material that is not in contact with the metal oxide with electromagnetic waves, which are at least one of microwaves and millimeter waves, to reduce at least a portion of the metal oxide, wherein a partition material is disposed between the metal oxide and the high-melting-point material, the melting point of the high-melting-point material being higher than the melting point of the metal oxide, and the high-melting-point material includes a mixture of an absorbing material that absorbs electromagnetic waves in a temperature range that is at least partially lower than the temperature range in which the metal oxide absorbs electromagnetic waves.

[0021] In the method for reducing a metal oxide, the metal oxide may not be brought into contact with a carbon-containing reducing agent when at least a portion of the metal oxide is reduced. The carbon-containing reducing agent may be a gas, liquid, or solid.

[0022] In the above-described method for reducing a metal oxide, when at least a portion of the metal oxide is reduced, carbon monoxide (CO) does not need to be supplied to the metal oxide.

[0023] In the above-mentioned method for reducing a metal oxide, the metal oxide may be in the form of a powder.

[0024] In the above-described method for reducing a metal oxide, in which at least a portion of the metal oxide is reduced, the partition material may be at least a part of the container, the metal oxide may be disposed inside the container, and the high-melting-point material may be disposed outside the container.

[0025] In the above-described method for reducing a metal oxide, in which at least a portion of the metal oxide is reduced, the partition material may be at least a part of the container, the high-melting point material may be disposed inside the container, and the metal oxide may be disposed outside the container.

[0026] In the above method for reducing a metal oxide, the container may be a crucible.

[0027] In the above method for reducing a metal oxide, the metal oxide may be iron oxide.

[0028] In the above-described method for reducing a metal oxide, the iron oxide may be at least one selected from iron oxide (III) (Fe2O3), triiron tetroxide (Fe3O4), and iron oxide (II) (FeO).

[0029] In the above method for reducing a metal oxide, the iron oxide may be iron(III) oxide (Fe2O3).

[0030] In the above method for reducing metal oxide, the absorbing material may include a carbon material.

[0031] In the above-described method for reducing metal oxide, the high-melting-point material may further contain a heat insulating material that absorbs electromagnetic waves to a lesser extent than the metal oxide.

[0032] In the above method for reducing metal oxide, the heat insulating material may contain an oxide.

[0033] In the above method for reducing metal oxide, a high melting point material may be contained in the mold.

[0034] In the above-described method for reducing metal oxide, the high-melting-point material may be in the form of a powder.

[0035] A method for producing a reduced metal oxide according to an embodiment of the present invention includes placing a partition material between the metal oxide to be reduced and a high-melting point material including an absorbing material and a heat insulating material that absorbs electromagnetic waves in a temperature range lower than the temperature range in which the metal oxide changes its absorptivity for electromagnetic waves, and irradiating the metal oxide, the high-melting point material, and the partition material with electromagnetic waves to heat the high-melting point material and the metal oxide and cause a reduction reaction in the metal oxide.

[0036] In the above-mentioned method for producing a reduced metal oxide, the partition material may be at least a part of the first container, and one of the metal oxide to be reduced or the high-melting-point material may be placed inside the first container and the other placed outside the first container, the first container, the metal oxide, and the high-melting-point material may be placed in a second container, and electromagnetic waves may be irradiated from outside the second container to heat the high-melting-point material and the metal oxide, thereby causing a reduction reaction in the metal oxide.

[0037] In the above-described method for producing a reduced metal oxide, the metal oxide may be placed in the first container, and the high-melting-point material may be placed outside the first container.

[0038] In the above-mentioned method for producing a reduced metal oxide, at least a portion of the metal oxide may be covered with a powder.

[0039] In the above-mentioned method for producing a reduced metal oxide, the powder may contain aluminum oxide.

[0040] In the above-described method for producing a reduced metal oxide, the absorbing material may contain a carbon material.

[0041] In the above-mentioned method for producing a reduced metal oxide, the metal oxide may be hematite and the reduced metal may be magnetite.

[0042] In the above-mentioned method for producing a reduced form of a metal oxide, the temperature of the metal oxide may be measured, and the irradiation of the electromagnetic waves may be stopped when the temperature no longer increases. [Effects of the Invention]

[0043] According to some aspects of the present invention, it is possible to provide a method for producing a reduced metal oxide that can suppress the generation of carbon dioxide. [Brief explanation of the drawings]

[0044] [Figure 1] 1A and 1B are schematic diagrams showing examples of arrangements of metal oxides and high-melting-point materials according to an embodiment. [Figure 2] 1 is a schematic diagram of an electromagnetic wave irradiation device according to an embodiment. [Figure 3] 4 is a graph showing temperature changes of a sample according to Example 1. [Figure 4] 1 is a photograph showing a reduced sample according to Example 1. [Figure 5] 1 is a graph showing the X-ray diffraction peaks of the reduced sample according to Example 1. [Figure 6] FIG. 10 is a schematic diagram of an electromagnetic wave irradiation device according to a second embodiment. [Figure 7] 10 is a graph showing the temperature change of a high-melting-point material and a sample according to Example 2. [Figure 8] FIG. 2 is a schematic diagram showing an example of the arrangement of iron oxide and aluminum oxide according to Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0045] Hereinafter, embodiments and examples of the present invention will be described with reference to the drawings. However, the drawings are schematic. Therefore, specific dimensions and the like should be determined in light of the following description. Furthermore, it goes without saying that the dimensional relationships and ratios between the drawings may differ.

[0046] A method for producing a reduced metal oxide according to an embodiment includes irradiating a metal oxide and a high-melting-point material not in contact with the metal oxide with electromagnetic waves, which are at least one of microwaves and millimeter waves, to reduce at least a portion of the metal oxide. A partition material is disposed between the metal oxide and the high-melting-point material. The microwaves are, for example, electromagnetic waves with a frequency of 300 MHz or more and 30 GHz or less. The millimeter waves are, for example, electromagnetic waves with a frequency of 30 GHz or more and 300 GHz or less. Hereinafter, the term "electromagnetic waves" refers to at least one of microwaves and millimeter waves. In the method for producing a reduced metal oxide according to an embodiment, the melting point of the high-melting-point material is higher than the melting point of the metal oxide. Furthermore, the high-melting-point material includes an absorbing material that absorbs electromagnetic waves in at least a temperature range that is partially lower than the temperature range in which the metal oxide absorbs electromagnetic waves.

[0047] Examples of metal oxides to be reduced include iron oxide, nickel oxide, copper oxide, gold oxide, silver oxide, aluminum oxide, cobalt oxide, tungsten oxide, titanium oxide, chromium oxide, molybdenum oxide, beryllium oxide, magnesium oxide, tin oxide, cerium oxide, lead oxide, mercury oxide, sodium oxide, bismuth oxide, and gallium oxide.

[0048] Examples of iron oxides include iron(III) oxide (Fe2O3), triiron tetroxide (Fe3O4), and iron(II) oxide (FeO). Reduction of iron(III) oxide (Fe2O3) yields triiron tetroxide (Fe3O4). Reduction of triiron tetroxide (Fe3O4) yields iron(II) oxide (FeO). Reduction of iron(II) oxide (FeO) yields iron (Fe). Iron(III) oxide (Fe2O3) is also called hematite and ferric oxide. Triiron tetroxide (Fe3O4) is also called magnetite and triiron tetroxide. Iron(II) oxide (FeO) is also called wustite and ferrous oxide.

[0049] Metal oxides do not necessarily have to be reduced to completely unoxidized metals. For example, iron (III) oxide (FeO) does not necessarily have to be reduced to iron (Fe). For example, iron (III) oxide (FeO) may be reduced to triiron tetroxide (FeO). Because triiron tetroxide (FeO) is magnetic, if iron (III) oxide (FeO) contains impurities, it is possible to reduce iron (III) oxide (FeO) to triiron tetroxide (FeO), and then separate the triiron tetroxide (FeO) from the impurities using magnetic force. As described above, three reaction steps are required to reduce iron (III) oxide (FeO) to iron (Fe). However, even if one or two of the three reaction steps are performed using the reduction method according to this embodiment, carbon dioxide generation can still be suppressed.

[0050] The metal oxide to be reduced may contain one type of metal or may contain multiple types of metals. The metal oxide to be reduced may be an alloy. The metal oxide to be reduced may contain, as alloy components, for example, silicon, manganese, chromium, nickel, boron, copper, aluminum, titanium, niobium, vanadium, zinc, antimony, palladium, lanthanum, gold, potassium, cadmium, indium, molybdenum, sulfur, etc.

[0051] The shape of the metal oxide to be reduced is not particularly limited. The metal oxide may be in the form of flakes. The metal oxide flakes may be, for example, metal oxide slices, metal oxide fragments, metal oxide chips, metal oxide cuttings, or metal oxide powder. The metal oxide may also be an aggregate of metal oxide flakes. The metal oxide to be reduced may be a molded body of multiple metal oxide flakes. For example, a molded metal oxide body may be produced by filling multiple metal oxide flakes into a mold and applying pressure to the multiple metal oxide flakes. The molded body may be a briquette. The molded body may be disk-shaped, but is not limited to this. The pressure applied to the multiple metal oxide flakes is not limited, and is, for example, 1 MPa or more, 100 MPa or more, or 200 MPa or more, and 2000 MPa or less, 1900 MPa or less, or 1800 MPa or less. Pressurizing methods include uniaxial molding, cold isostatic pressing (CIP) molding, hot isostatic pressing (HIP) molding, and roller pressing.

[0052] The absorbing material contained in the high-melting-point material has a melting point higher than the melting point of the metal oxide to be reduced. At least a portion of the temperature range in which the absorbing material absorbs electromagnetic waves is lower than the temperature range in which the metal oxide to be reduced absorbs electromagnetic waves. It is desirable that the absorbing material contained in the high-melting-point material has a peak absorption rate in a temperature range lower than the absorptance change temperature range of the metal oxide to be reduced, and does not absorb electromagnetic waves much in the temperature range in which the metal oxide to be reduced absorbs electromagnetic waves. The absorptance change temperature range is the temperature range in which the absorption rate of the metal oxide to be reduced changes. Generally, metal oxides have low electromagnetic wave absorption rates below the absorptance change temperature range and high electromagnetic wave absorption rates above the absorptance change temperature range. The temperature range in which the metal oxide to be reduced absorbs electromagnetic waves is, for example, 300°C to 3000°C, 300°C to 2500°C, 300°C to 1200°C, 450°C to 1100°C, or 600°C to 800°C. The temperature range in which the absorbing material absorbs electromagnetic waves is, for example, 50°C to 1000°C, 100°C to 1000°C, 250°C to 900°C, or 300°C to 600°C. The absorbing material can be suitably used for many types of metal oxides as long as it exhibits such a temperature range in which it absorbs electromagnetic waves. In particular, in one example of this embodiment, hematite, an iron oxide, is used as the metal oxide. The temperature range in which the absorptivity of hematite changes is about 400°C, and as will be described later, hematite and its reduced form are heated by electromagnetic waves to above 1200°C. Therefore, a suitable absorbing material is one that has a temperature range in which it absorbs electromagnetic waves of 300°C to 600°C.

[0053] Preferably, at least a portion of the temperature range in which the absorbing material absorbs electromagnetic waves overlaps with the temperature range in which the metal oxide to be reduced absorbs electromagnetic waves. The absorbing material absorbs electromagnetic waves in a temperature range at least partially lower than the temperature range in which the metal oxide to be reduced absorbs electromagnetic waves, and thus generates heat faster than the metal oxide to be reduced. Therefore, the absorbing material can accelerate the heating of the metal oxide to be reduced before it reaches the temperature range in which the metal oxide to be reduced absorbs electromagnetic waves. Therefore, when the high-melting-point material contains an absorbing material, the temperature of the metal oxide to be reduced reaches the temperature range in which it absorbs electromagnetic waves more quickly, thereby shortening the heating time of the metal oxide to be reduced. Furthermore, since the absorbing material absorbs electromagnetic waves in at least a portion of the temperature range lower than the temperature range in which the metal oxide to be reduced absorbs electromagnetic waves, it is possible to prevent the high-melting-point material from being heated more than necessary. Therefore, when the high-melting-point material has a fixed shape, the shape of the high-melting-point material can be stable even while the metal to be heated is being irradiated with electromagnetic waves.

[0054] As described above, the absorbing material preferably has the property of absorbing electromagnetic waves in a temperature range lower than the absorptivity change temperature range of the metal oxide, and has a melting point higher than that of the metal oxide. Examples of materials with such properties include carbon materials. Suitable examples of carbon materials include, but are not limited to, carbon black, amorphous carbon, graphite, silicon carbide, carbon resin, and metal carbide. The absorbing material may include metals, metal nitrides, metal oxides, metal borides, etc. that absorb electromagnetic waves in a temperature range at least partially lower than the temperature range in which the metal oxide to be reduced absorbs electromagnetic waves. The absorbing material may also be a compound of these. Note that the carbon material contained in the high-melting point material does not come into contact with the metal oxide to be reduced, and therefore does not function as a reducing agent for the metal oxide to be reduced.

[0055] The high-melting-point material may further include an insulating material that absorbs electromagnetic waves to a lesser extent than the metal oxide. The insulating material, for example, has higher electromagnetic wave transparency than the metal oxide to be reduced and absorbs electromagnetic waves to a lesser extent than the metal oxide to be reduced. The insulating material has a melting point higher than that of the metal oxide to be reduced. Because the insulating material has a low absorption of electromagnetic waves, it generates less heat even when irradiated with electromagnetic waves, thereby exhibiting an insulating effect. Furthermore, because the insulating material has a higher melting point than the metal oxide to be reduced, its shape remains stable even when irradiated with electromagnetic waves. Therefore, when the high-melting-point material has a fixed shape, the high-melting-point material including the insulating material can maintain its shape stable even while the metal oxide to be reduced that has been irradiated with electromagnetic waves is being heated.

[0056] The insulating material may include an oxide. The insulating material may include a metal oxide or a metalloid oxide. Examples of metal and metalloid oxides include, but are not limited to, aluminum oxide, silicon oxide, magnesium oxide, zirconium oxide, and titanium oxide. For example, the melting point of aluminum oxide is 2072°C. The melting point of silicon oxide is 1710°C. The melting point of magnesium oxide is 2852°C. The insulating material may be a compound of these.

[0057] The high melting point material may be a mixture of an absorbent material and a heat insulating material. It is preferable that the absorbent material and the heat insulating material are uniformly distributed in the high melting point material. The high melting point material may be a powder or a molded body. The high melting point material may be contained in a mold. A molded high melting point material may be obtained by solidifying a powder absorbent material and a heat insulating material with a binder. A molded high melting point material may be obtained by solidifying a mixture of a powder absorbent material and a heat insulating material with a binder. A molded high melting point material may be obtained by solidifying a mixture of a powder absorbent material and a heat insulating material with a binder.

[0058] A partition material is disposed between the metal oxide and the high-melting-point material to prevent contact between them. The partition material preferably does not contain any carbon-containing substances. The partition material also inhibits the permeation of carbon-containing substances. The partition material preferably inhibits the permeation of gases. The partition material preferably has high thermal conductivity so that heat from the high-melting-point material can be transferred to the metal oxide. The efficiency of electromagnetic wave absorption by the partition material is preferably lower than the efficiency of electromagnetic wave absorption by the metal oxide so that the metal oxide can efficiently absorb electromagnetic waves. The partition material preferably has low dielectric loss so that the metal oxide can efficiently absorb electromagnetic waves. The material of the partition material is preferably a material that does not generate carbon dioxide when reduced by the high-melting-point material at the temperature at which the metal oxide to be reduced is reduced. The material of the partition material is, for example, an oxide of a metal different from the metal oxide to be reduced. When the metal oxide to be reduced is iron oxide, an example of the material of the partition material is aluminum oxide (Al2O3). The temperature at which iron oxide is reduced is, for example, 400 to 500°C, but the temperature at which aluminum oxide is reduced is 2000°C or higher, so if the electromagnetic wave conditions are set to a temperature at which aluminum oxide is not reduced but iron oxide is reduced, the aluminum oxide in contact with the high-melting point material will not be reduced while the iron oxide is reduced, and carbon dioxide will not be generated. Other examples of materials for the partition include silicon nitride (Si3N4), aluminum nitride (AlN), and zirconia (ZrO2).

[0059] The partition may be a container, and the metal oxide may be placed inside the container, while the high-melting-point material may be placed outside the container. The container may be a crucible. Irradiating the metal oxide and the high-melting-point material with electromagnetic waves may be performed in an inert gas atmosphere. Examples of inert gases include argon (Ar) and helium (He). Irradiating the metal oxide and the high-melting-point material with electromagnetic waves may also be performed in a neutral gas atmosphere. Examples of neutral gases include nitrogen (N). Irradiating the metal oxide and the high-melting-point material with electromagnetic waves may also be performed in a carbon-free reducing gas atmosphere. Examples of carbon-free reducing gases include hydrogen (H) and ammonia (NH). Because hydrogen does not contain carbon, carbon dioxide is not generated even when hydrogen functions as a reducing agent. Furthermore, the presence of hydrogen promotes the reduction of the metal oxide due to a synergistic effect with the high-melting-point material. Furthermore, ammonia is decomposed into nitrogen and hydrogen by heat. Therefore, the presence of ammonia generates hydrogen, which can function as a reducing agent. The irradiation of the metal oxide and the high melting point material with electromagnetic waves may be carried out in a vacuum.

[0060] When carrying out the method for producing a reduced metal oxide according to the embodiment, for example, as shown in FIG. 1 , a metal oxide 2 to be reduced is placed in a first container 1, such as a crucible. Powders 7 may be placed above and below the metal oxide 2. Alternatively, a high-melting-point material 4 and an insulating material 6 are placed in a second container 3, such as a crucible, that is larger than the first container 1. Next, the first container 1, containing the metal oxide 2, is placed in the second container 3 containing the high-melting-point material 4 and the insulating material 6. The first container 1 functions as a partition between the metal oxide 2 and the high-melting-point material 4. To prevent the high-melting-point material 4 from scattering, the exposed portion of the opening of the second container 3, where the high-melting-point material 4 is located, may be covered with a lid 5, such as a plate. It is preferable that the lid 5 be made of a highly gas-permeable material to prevent desorbed oxygen plasma or oxygen molecules from returning to the metal oxide and recombining. As long as the metal oxide 2 can be separated from the high-melting-point material 4, the partition need not come into contact with the metal oxide 2. The arrangement of the metal oxide 2 and the high-melting-point material 4 may be reversed. Specifically, the high-melting-point material 4 to be reduced may be placed in the first container 1, and the metal oxide 2 may be placed around the first container 1.

[0061] The powder 7 below the metal oxide 2 to be reduced in the first container 1 may be placed in the first container 1 to prevent the metal oxide 2 from coming into contact with the high-melting point material 4 in the unlikely event that the first container 1 is broken. The powder 7 above the metal oxide 2 in the first container 1 may be placed to reduce the chance of contact between the reduced form of the metal oxide 2 and oxygen in the gas. Placing the powder 7 is optional. The powder 7 is, for example, a metal or metal oxide having a higher melting point than the metal oxide 2 to be reduced. The powder 7 is, for example, aluminum oxide powder. When the metal oxide to be reduced is iron oxide, aluminum oxide bonds more strongly with oxygen than the iron oxide to be reduced, and therefore almost never releases oxygen plasma or oxygen molecules before the iron oxide to be reduced.

[0062] Next, the metal oxide 2, the first container 1, and the second container 3 containing the high-melting-point material 4 are placed in the chamber 21 of the electromagnetic wave irradiation device 20 shown in FIG. 2 . The chamber 21 is closed, and electromagnetic waves are irradiated into the chamber 21 from an oscillator 23 via a waveguide 22. The irradiated high-melting-point material 4 generates heat, accelerating the heating of the metal oxide 2. After the metal oxide 2 begins to self-heat, the partition material and the high-melting-point material 4 may be removed. The heated metal oxide 2 is reduced by heating even without a reducing agent. Before and during the irradiation of the electromagnetic waves into the chamber 21, an inert gas, a neutral gas, or a carbon-free reducing gas may be introduced into the chamber 21 via an inlet pipe 24. Furthermore, before and during the irradiation of the electromagnetic waves into the chamber 21, the gas in the chamber 21 may be discharged to the outside of the chamber 21 via an outlet pipe 25.

[0063] In this embodiment, the temperature rise from room temperature to the absorptivity change temperature range is primarily due to heat conduction from the high-melting-point material to the metal oxide, so it is best to place the high-melting-point material and the metal oxide as close together as possible. Furthermore, the aforementioned carbon material is suitable as a material that satisfies the requirements of having an electromagnetic wave absorptivity peak below the absorptivity change temperature range, having a melting point higher than that of the metal oxide, and being industrially readily available and inexpensive. However, when a carbon material is used as an absorbing material and placed in contact with a metal oxide, the carbon acts as a strong reducing agent, bonding with oxygen atoms bonded to the metal oxide, thereby accelerating reduction. However, the carbon also bonds with oxygen dissociated from the metal oxide, generating large amounts of carbon dioxide. In contrast, according to the method for producing a reduced metal oxide of this embodiment, even if the high-melting-point material contains a carbon material, the metal oxide and the high-melting-point material are separated by a partition material and do not come into contact with each other during electromagnetic wave irradiation. Therefore, the oxygen contained in the metal oxide and the carbon contained in the high-melting-point material do not react to generate carbon dioxide. Therefore, the method for producing a reduced metal oxide of this embodiment can suppress the generation of carbon dioxide. Furthermore, as will be shown in the examples described later, according to the method for producing a reduced metal oxide of the embodiment, it is possible to reduce the metal oxide without contacting the metal oxide to be reduced with a reducing agent containing carbon. Furthermore, according to the method for producing a reduced metal oxide of the embodiment, it is possible to reduce the metal oxide without contacting the metal oxide to be reduced with a reducing agent containing carbon. 10 It is possible to reduce metal oxides without supplying a reducing gas such as a fluorine-containing gas (e.g., HCl).

[0064] Example 1 Ten grams of iron oxide powder (hematite, Fe2O3, High Purity Chemical Laboratory, purity 99.9%, particle size <1 μm) was prepared as a sample. It was placed in a small aluminum oxide (Al2O3) crucible in the following order, avoiding mixing: aluminum oxide (Al2O3) powder, iron oxide powder (hematite, Fe2O3), and aluminum oxide (Al2O3) powder. High-melting-point materials, including silicon carbide (SiC) as an absorber and aluminum oxide (Al2O3) as an insulator, were also prepared. Furthermore, a large aluminum oxide (Al2O3) crucible was placed with insulation, the small crucible, and the high-melting-point material, with the bottom of the small crucible touching the high-melting-point material, and the surrounding area was covered with insulation. The small crucible separated the sample from the high-melting-point material. The top of the large crucible was covered with insulation with a hole for measuring the radiation thermometer. The aluminum oxide powder below the iron oxide powder in the small crucible was placed in the small crucible to prevent the iron oxide from coming into contact with the high-melting-point material in the event that the crucible were to break, and the aluminum oxide powder above the iron oxide powder in the small crucible was placed to reduce the chance of contact between the reduced iron oxide and oxygen in the gas.

[0065] Next, the large crucible was placed in the chamber of a microwave irradiation device, and microwaves with a frequency of 2.45 GHz and an output of 1.7 kW were irradiated onto the large crucible. The temperature of the sample inside the large crucible was measured with a radiation thermometer. As shown in Figure 3, the temperature of the sample rose rapidly from approximately 600°C to approximately 1400°C in about four minutes after the start of microwave irradiation, and finally reached 1600°C. Figure 4 shows a photograph of the sample that was then recovered. The sample was sintered and black.

[0066] The collected sample was analyzed using an X-ray diffractometer (Ultima IV, Rigaku). As shown in Figure 5, peaks consistent with the X-ray diffraction peaks of magnetite (Fe3O4) were confirmed in the sample, and the X-ray diffraction intensity identified the sample as 100% magnetite (Fe3O4). Therefore, it was confirmed that hematite (Fe2O3) was reduced to magnetite (Fe3O4) by microwave irradiation without contact with carbon monoxide (CO) or a carbon-containing reducing agent. In this example, because hematite was not in contact with a carbon-containing reducing agent, theoretically, hematite could be reduced to magnetite without generating carbon dioxide.

[0067] According to this example, metal oxides were successfully reduced without the use of a reducing agent by irradiating them with microwaves for just a few minutes. Reducing agents are primarily carbon-containing substances that extract oxygen atoms bonded to the metal oxide in the form of carbon dioxide. However, since reduction was performed without the use of a reducing agent in this example, no carbon dioxide was emitted from the metal oxide. In this example, reduction proceeded to magnetite, and wüstite or pure iron was not detected. Microwave heating microscopically exceeded the oxygen bond energy of hematite, causing oxygen atoms to dissociate and be released outside the metal oxide in the form of plasma or oxygen molecules, resulting in magnetite. Since the oxygen bond energy of magnetite was not exceeded, it is believed that reduction to wüstite or pure iron was not achieved. However, it is believed that increasing the electromagnetic wave energy could reduce metal oxides to wüstite or pure iron. Many metals contain a mixture of metal and oxygen elements in various mixture ratios depending on the degree of oxidation. In this example, metal oxides can be reduced to any desired degree by irradiating them with optimal electromagnetic energy. In particular, in this example, the non-magnetic hematite is reduced to magnetite, which is magnetic and has high industrial utility value, and the reduction stops there, making it particularly valuable. Also, in this example, the reduction stops at magnetite, but the energy of the irradiated electromagnetic waves may be increased to further promote the reduction, or at this stage, the container separating the metal oxide to be reduced from the absorbent material may be removed, and the carbon material contained in the absorbent material may be brought into contact with the metal oxide, thereby promoting the reduction through a stronger reduction action.

[0068] Example 2 As shown in Figure 6, hematite (Fe2O3), a metal oxide 2 to be reduced, was placed in a first container 1, which was a crucible. Furthermore, the first container 1, containing the metal oxide 2, was placed in a second container 3, which was a larger crucible, so that the first container 1 was covered with a high-melting-point material 4. To prevent the high-melting-point material 4 from scattering, the exposed portion of the opening of the second container 3, where the high-melting-point material 4 was located, was covered with a lid 5, which was an aluminum oxide plate. The lid 5 had a hole for temperature measurement. While the sample was irradiated with microwaves, the temperature of the metal oxide 2 to be reduced was measured with a radiation thermometer. Except for the above, the sample was irradiated with microwaves using the same method as in Example 1. As shown in Figure 7, for approximately 4 minutes after the start of microwave irradiation, the temperature of the high-melting-point material rose faster than that of the sample. This indicates that the absorbing material used in this example had a higher electromagnetic wave absorption rate than the metal oxide at temperatures below 450 °C. After about 4 minutes, when the temperature exceeded 450°C, the sample temperature rose rapidly, as described in Example 1, and became higher than that of the high-melting-point material. This is because the metal oxide absorbs more electromagnetic waves around 500°C, the temperature range where the absorptivity of metal oxides changes, and heats up faster than the absorbing material. From this point, the sample's temperature rise curve became steeper, reaching approximately 1150°C within about 7 minutes. This suggests that at least a portion of the sample converted to magnetic magnetite, and that the magnetite began to self-heat due to the magnetic field. After that, the high-melting-point material continued to heat up gradually, stopping at around 1350°C. This is presumably because most of the hematite in the sample was reduced to magnetite, the electromagnetic absorptivity of magnetite decreased at 1350°C, and the heating due to electromagnetic waves and the radiation of heat to the surroundings were balanced. X-ray diffraction also confirmed that the sample collected in Example 2 was reduced to magnetite (Fe3O4). It is believed that the reduction of the sample is in progress even before the temperature of the sample exceeds the temperature of the high-melting-point material.In this example, the temperature rise of the absorbing material occurs first, and the temperature of the metal oxide rises above that of the absorbing material midway through the process. This suggests that the absorbing material first absorbs the electromagnetic waves and its temperature rises first. The temperature of the metal oxide rises due to the radiant heat or heat transfer, and after the absorptivity change temperature range is exceeded, the metal oxide primarily absorbs the electromagnetic waves, and the temperature of the absorbing material rises due to the radiation or heat transfer from the metal oxide. The absorbing material used in this example should be selected so that its temperature rises prior to the metal oxide upon irradiation with the electromagnetic waves, and its temperature rise is lower than that of the metal oxide after the absorptivity change temperature range is exceeded. Furthermore, if the electromagnetic wave radiation is continued until the temperature rise of the metal oxide to be reduced reaches its peak—in this example, 12 minutes have elapsed and the measured temperature reaches 1350°C—it can be assumed that the hematite has been sufficiently reduced to magnetite.

[0069] The results of Example 2 show that for about four minutes after the start of microwave irradiation, the high-melting-point material absorbed more microwaves than the sample, causing the high-melting-point material to heat up faster than the microwaves, and the temperature of the high-melting-point material was transmitted to the sample via the crucible. Also, about four minutes after the start of microwave irradiation, the sample changed from hematite to magnetite, which has magnetic properties, causing a sudden increase in the microwave absorption efficiency of the sample and self-heating of the sample.

[0070] (Comparative Example 1) As in Example 1, 1 g of iron oxide powder (hematite, Fe2O3, Kojundo Chemical Laboratory, purity 99.9%, particle size <1 μm) was prepared as a sample. As shown in FIG. 8, a sample 33 surrounded by aluminum oxide (Al2O3) powder 32 was placed in a large aluminum oxide crucible 31. Microwaves were irradiated onto sample 33 under the same conditions as in Example 1, but the temperature of sample 33 did not exceed 140°C, the lower limit of measurement of the radiation thermometer, and sample 33 remained as hematite and was not reduced. [Explanation of symbols]

[0071] REFERENCE SIGNS LIST 1 First container, 2 Metal oxide, 3 Second container, 4 High melting point material, 5 Lid, 6 Heat insulating material, 20 Electromagnetic wave irradiation device, 21 Chamber, 22 Waveguide, 23 Oscillator, 24 Inlet tube, 25 Outlet tube, 32 Aluminum oxide powder

Claims

1. a method for reducing at least a portion of the metal oxide by irradiating a metal oxide and a high-melting-point material that is not in contact with the metal oxide with electromagnetic waves that are at least one of microwaves and millimeter waves; a partition material is disposed between the metal oxide and the high-melting-point material; the melting point of the high-melting-point material is higher than the melting point of the metal oxide; The high melting point material is an absorbing material that absorbs the electromagnetic waves in a temperature range at least part of which is lower than the temperature range in which the metal oxide absorbs the electromagnetic waves; a heat insulating material that absorbs the electromagnetic waves to a lesser extent than the metal oxide; Including, A method for producing reduced metal oxides.

2. The method for producing a reduced metal oxide according to claim 1 , wherein the metal oxide is not brought into contact with a carbon-containing reducing agent in reducing at least a portion of the metal oxide.

3. The method for producing a reduced metal oxide according to claim 1 , wherein carbon monoxide (CO) is not supplied to the metal oxide in reducing at least a portion of the metal oxide.

4. The method for producing a reduced metal oxide according to claim 1 , wherein the metal oxide is in the form of a powder.

5. 2. The method for producing a reduced metal oxide according to claim 1, wherein the partition material is at least a part of a container, and in reducing at least a part of the metal oxide, the metal oxide is placed inside the container and the high-melting point material is placed outside the container.

6. 2. The method for producing a reduced metal oxide according to claim 1, wherein the partition material is at least a part of a container, and in reducing at least a part of the metal oxide, the high-melting point material is placed inside the container and the metal oxide is placed outside the container.

7. The method for producing a reduced metal oxide according to claim 1 , wherein the metal oxide is iron oxide.

8. The iron oxide is iron (III) oxide (Fe 2 O 3 ), triiron tetroxide (Fe 3 O 4 8. The method for producing a reduced metal oxide according to claim 7, wherein the reduced metal oxide is at least one selected from the group consisting of iron(II) oxide (FeO), ... and iron(II) oxide (FeO).

9. The method for producing a reduced metal oxide according to claim 1 , wherein the absorbent material comprises a carbon material.

10. The method for producing a reduced metal oxide according to claim 1 , wherein the insulating material comprises an oxide.

11. a partition material is disposed between a metal oxide to be reduced and a high-melting-point material including an absorbing material that absorbs electromagnetic waves in a temperature range lower than the temperature range in which the absorptivity of the metal oxide changes for electromagnetic waves, and a heat insulating material; and irradiating the metal oxide, the high-melting point material, and the partition material with electromagnetic waves to heat the high-melting point material and the metal oxide, thereby causing a reduction reaction in the metal oxide. A method for producing reduced metal oxides.

12. the partition material is at least a part of a first container, and one of the metal oxide to be reduced and the high-melting-point material is placed inside the first container, and the other is placed outside the first container; placing the first container, the metal oxide, and the high melting point material in a second container; the electromagnetic waves are applied from the outside of the second container to heat the high-melting point material and the metal oxide, thereby causing a reduction reaction in the metal oxide; The method for producing the reduced metal oxide according to claim 11.

13. The method for producing a reduced metal oxide according to claim 11, wherein at least a portion of the metal oxide is covered with a powder.

14. The method for producing a reduced metal oxide according to claim 13, wherein the powder contains aluminum oxide.

15. The method for producing a reduced metal oxide according to claim 11 , wherein the absorbent material comprises a carbon material.

16. The method for producing a reduced metal oxide according to claim 11, wherein the metal oxide is hematite and the reduced material is magnetite.

17. The method for producing a reduced metal oxide according to claim 11, wherein the temperature of the metal oxide is measured, and when the temperature no longer increases, the irradiation of the electromagnetic waves is stopped.

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