JP7469166B2 - Iron structure, its manufacturing method, and method for manufacturing iron powder for heat generating composition - Google Patents

Description

The present invention relates to an iron structure and a method for manufacturing the same, as well as a method for manufacturing iron powder for use in heat-generating compositions.

Heating devices in which a heat generating composition containing an oxidizable metal such as iron powder and activated carbon is enclosed in a breathable packaging material have been widely used to provide heat to the human body, such as disposable hand warmers. Such heat generating compositions generate heat by utilizing the heat of reaction generated by the oxidation reaction of the iron powder contained therein.

The present applicant has proposed a method for producing iron powder for use in a heat-generating composition having a bulk density of 0.3 g/ ^cm3 or more and 1.5 g/ ^cm3 or less (Patent Document 1). The iron powder obtained by this method has excellent heat-generating properties and handleability.

Patent Document 2 discloses reduced iron powder for use as an oxygen scavenger having a specific surface area of 1.0 m ² /g or more, an apparent density of 1.0 g/cm ³ or less, and an average particle size of 9.0 μm or less, and a method for producing the same.

Patent Document 3 discloses a method for producing pellets, which includes a mixing process in which an oxide ore containing nickel and iron oxide is mixed with a carbonaceous reducing agent to form a mixture, and a mixture molding process in which the resulting mixture is press-molded into pellets, with the aim of efficiently recovering ferronickel.

JP 2019-112710 A JP 2002-292276 A JP 2019-99832 A

The method described in Patent Document 1 makes it possible to obtain iron powder with high heat generating properties, but when obtaining reduced iron, the reduced iron particles may bond together or may adhere to the inside of the reduction furnace, leaving room for improvement in terms of improving the yield and purity of reduced iron, and ultimately productivity.

The metal powder obtained by the techniques described in Patent Documents 2 and 3 has not been studied in any way with respect to improving heat generation characteristics, nor has it been studied in any way with respect to the efficient production of metal powder with excellent heat generation characteristics.

Therefore, the present invention relates to an iron structure that is the raw material for iron powder with excellent heat generation properties, and a method for producing the same.

The present invention relates to a lump having at least a surface composed of an aggregate of fibrous materials containing metallic iron,
The bulk density is 0.3 g/cm ³ or more and 1.5 g/cm ³ or less,
The present invention relates to an iron structure having an average grain size of 1 mm or more and 100 mm or less.

The present invention also relates to a method for producing an iron structure, in which granular material containing iron oxide, a carbonaceous reducing agent, and an inorganic binder and having an average particle size of 1 mm to 100 mm is heated in a rotary furnace at 850°C to 1000°C.

The present invention further relates to a method for producing iron powder for use in heat generating compositions, in which the iron structure obtained by the method for producing the iron structure is pulverized to an average particle size of 5 μm or more and 500 μm or less.

According to the present invention, there is provided an iron structure that serves as a raw material for iron powder having excellent heat generation characteristics.
Furthermore, according to the present invention, there is provided a production method that, when reducing iron oxide, makes it difficult for reduced iron to adhere to the wall surface of a reduction furnace, and also makes it possible to obtain iron structures with uniform particle size.

FIG. 1 is a scanning electron microscope image of the surface of an iron structure. FIG. 2(a) is a cross-sectional view showing a schematic representation of one embodiment of a heating element containing a heat generating composition, FIG. 2(b) is a plan view showing a schematic representation of one embodiment of a heating tool equipped with a heating element, and FIG. 2(c) is a schematic cross-sectional view of the heating tool along line II in FIG. 2(b). FIG. 3 is a graph comparing the heating characteristics of a heating tool made using the iron powder obtained in Example 1 with the heating characteristics of a heating tool made using the iron powder in Comparative Example 3.

The present invention will be described below based on its preferred embodiments. The iron structure of the present invention is a lump. The lump has an irregular shape and a variety of sizes. Typical shapes of the lump are approximately spherical, approximately ellipsoidal, approximately pellet-shaped, crushed stone having one or more ridges, coal-shaped, etc. At least a part or the entire surface of the iron structure is composed of an aggregate of fibrous material containing metallic iron. By subjecting the iron structure to a process such as crushing, iron powder containing metallic iron can be obtained. In other words, the iron structure is suitable as a raw material for manufacturing iron powder.

At least the surface of the iron structure is porous, with a mesh-like structure in which numerous fibrous materials are arranged randomly in three dimensions. The iron structure has a plurality of pores formed by the assembly of a plurality of fibrous materials. The pores present in the iron structure may have an open cell structure or a closed cell structure, and both of these structures may be present in one mass.

The fibrous materials constituting the iron structure of the present invention have various forms. In particular, when focusing on any one fibrous material, the fibrous material may be elongated and unidirectional, or may be branched. The length of each fibrous material may be uniform or non-uniform. The thickness of each fibrous material may be the same along the extension direction of the fibrous material, or may vary regularly or irregularly along the extension direction. The above-mentioned aggregate is formed by entangling multiple fibrous materials having at least one of these forms, or by bonding the intersections of the fibrous materials with each other.

The thickness of the fibrous material in the iron structure is about 10 μm or less on average. The thickness of the fibrous material can be obtained by performing image analysis of an electron microscope photograph and measuring the minimum diameter of the fibrous material in a direction perpendicular to the extension direction.
Specifically, the thickness of the fibrous object is determined as the arithmetic average of the measurements of 20 or more fibrous objects randomly selected from the two-dimensional screen observed under an electron microscope at a magnification of 1000 times.

An electron microscope image of one embodiment of the iron structure is shown in Fig. 1. As shown in the figure, the iron structure of this embodiment is composed of a continuum in which fibrous materials having various forms are randomly arranged, and a plurality of pores are formed. In this embodiment, the fibrous materials are formed not only on the surface of the iron structure but also inside it.
In addition, since the iron structure of the present invention is a continuous body, those that maintain their shape only by the entanglement of fibrous metal fibers, such as steel wool, are excluded from the present invention.

The iron structure of the present invention preferably contains metallic iron as a whole. Specifically, the iron structure may be a mass whose surface is composed of an aggregate of fibrous materials containing metallic iron, and whose interior is a structure containing metallic iron but not fibrous materials. Alternatively, the iron structure may be a mass whose surface and interior are both composed of an aggregate of a plurality of fibrous materials containing metallic iron. When the iron structure has the latter structure, both the surface and interior of the iron structure have a mesh-like structure. The iron structure of the present invention encompasses these embodiments.
Whether or not the surface of the iron structure is composed of an aggregate of fibrous materials can be determined by observing the surface of the iron structure with a scanning electron microscope (SEM). Whether or not the fibrous materials on the surface of the iron structure contain metallic iron can be determined by using energy dispersive X-ray analysis with an electron microscope (SEM-EDX).

One of the features of the iron structure of the present invention is that its bulk density is lower than that of conventional reduced iron powders. In detail, the bulk density of the iron structure is preferably 0.3 g/ ^cm3 or more, more preferably 0.4 g/ ^cm3 or more, and even more preferably 0.5 g/ ^cm3 or more.
The bulk density of the iron structure is preferably 1.5 g/ ^cm3 or less, more preferably 1.4 g/ ^cm3 or less, and even more preferably 1.3 g/ ^cm3 or less.
By making the iron structure have such a bulk density, it is possible to reduce the external force required for pulverization when pulverizing the iron structure to produce iron powder, and it is possible to produce iron powder having excellent heat generation properties with high productivity. Such an iron structure can be produced, for example, by the production method described below.
The bulk density of the iron structure of the present invention can be measured in accordance with JIS Z2504 using a bulk density measuring device (JIS bulk specific gravity measuring device manufactured by Tsutsui Rikagaku Kikai Co., Ltd.).

The iron structure of the present invention preferably has an average grain size of 1 mm or more, more preferably 1.5 mm or more.
The iron structure of the present invention preferably has an average grain size of 100 mm or less, more preferably 50 mm or less.
By making the iron structure have such an average particle size, the iron structure can be easily handled when used as a raw material for iron powder, and as a result, iron powder having excellent heat generation properties can be obtained with high productivity. Such an iron structure can be manufactured, for example, by the manufacturing method described below.
The average particle size of the iron structure is determined by directly measuring the longest Feret diameter and the shortest Feret diameter as specified in JIS Z8827-1 as the dimensions per iron structure, calculating the average Feret diameter by arithmetically averaging the measured values, and the arithmetic mean value of the average Feret diameters when 20 or more particles are measured is defined as the average particle size in the present invention.

The iron structure of the present invention has a metallic iron content of preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more.
The content of metallic iron in the iron structure is preferably 99.5% by mass or less, more preferably 97% by mass or less, and even more preferably 95% by mass or less.
By setting the metallic iron content within the above-mentioned range, the oxidation reaction of the iron powder obtained using the iron structure proceeds efficiently, resulting in even better heat generation properties, and further improving the production efficiency of such iron powder.
The metallic iron content is measured by the bromine-methanol dissolution method specified in ISO 5416. The metallic iron content in the iron structure can be adjusted by appropriately changing the iron element content of the raw material, reduction conditions, etc., in the manufacturing method described below, for example.

The iron structure of the present invention has a total iron element content of preferably 50 mass % or more, more preferably 60 mass % or more, and preferably 99.5 mass % or less, more preferably 99 mass % or less.
The iron element content can be measured in accordance with JIS M8212:2005, or by subjecting the iron structure to be measured to SEM-EDX or X-ray fluorescence analysis and performing elemental quantitative analysis within a measurement range surface in which the surface of the iron structure is designated as the measurement range, or by dissolving the iron structure in an acid such as nitric acid to prepare a solution, and then subjecting the solution to ICP atomic emission spectrometry.

The iron structure preferably has a crushing strength of 40N or more, more preferably 45N or more, and further preferably 50N or more.
The crushing strength of the iron structure is preferably 400N or less, more preferably 350N or less, and further preferably 300N or less.
By making the crushing strength of the iron structure fall within the above-mentioned range, the strength of the iron structure itself can be appropriately expressed, and unintended collapse of the iron structure can be prevented, thereby improving the handleability of the iron structure. At the same time, when producing iron powder by crushing the iron structure, it is possible to produce iron powder that maintains the above-mentioned bulk density with high productivity without applying excessive external force, and the obtained iron powder has excellent heat generation properties.
The crushing strength of the iron structure can be measured using a hardness tester (digital hardness tester KHT-40N manufactured by Fujiwara Seisakusho Co., Ltd.) in accordance with JIS Z8841.

At least the surface of the iron structure is formed porously by an aggregate of fibrous material, and the pore volume of the surface portion of the iron structure is preferably 3 mL/g or less, more preferably 2 mL/g or less, and even more preferably 1.5 mL/g or less, in the range of pores of 0.01 μm or more and 100 μm or less.
The pore volume of the surface portion of the iron structure is preferably 0.25 mL/g or more, more preferably 0.3 mL/g or more, and further preferably 0.35 mL/g or more, for pores in the range of 0.01 μm or more and 100 μm or less.
By having the pore volume of the surface portion in such a range, the contact area between oxygen and iron element can be increased, and the reaction rate of the oxidation reaction of the iron powder produced using the iron structure can be further increased, thereby enabling excellent heat generation characteristics to be exhibited. The pore volume of the surface portion of the iron structure can be measured in accordance with the mercury intrusion method described in JIS R1655:2003. The pore volume is measured using a sample scraped off from the surface portion of the iron structure.

The iron structure may be composed only of unoxidized iron (hereinafter also referred to as "metallic iron" or "reduced iron"), or may be composed of iron compounds such as iron oxide (II) or iron oxide (III) in addition to metallic iron.
From the viewpoint of stably obtaining iron powder that simultaneously increases the reaction rate of the oxidation reaction to enhance heat generation performance and suppresses the occurrence of excessive oxidation reactions to enhance handleability, it is preferable that the iron structure is configured to have iron oxide (II) or iron oxide (III) on the surface of the iron structure and metallic iron inside the iron structure.
Whether an iron structure has iron oxide on its surface and metallic iron inside can be determined by embedding the iron structure in epoxy resin or the like, preparing an observation cross-section of the embedded iron structure using a mechanical polishing device or a cross-section polisher device or the like, and observing the cross-section using SEM-EDX.
Furthermore, an iron structure having such a configuration can be easily obtained by appropriately adjusting the heating temperature and heating time conditions in the reduction step described below, or by performing a mild oxidation treatment that is commonly performed in this technical field after the reduction step.

The iron structure may further contain a non-ferrous element in addition to the iron element. From the viewpoint of improving the crushing strength of the iron structure, it is preferable that the iron structure further contains at least one non-ferrous element selected from the group consisting of alkali metals, alkaline earth metals, silicon, titanium, aluminum, and zirconium.
The non-ferrous elements are preferably at least one of non-ferrous metal elements and semi-metal elements. The alkali metals include sodium and potassium. The alkaline earth metals include magnesium and calcium.

_The non-ferrous elements in the iron structure may be present in one or more of the following forms: a simple non-ferrous element, an oxide of a non-ferrous element such as _SiO2 , _Al2O3 , or _ZrO2 , a complex compound of a non-ferrous element and an iron element, or a complex compound of two or more non-ferrous elements. Examples of such compounds of non-ferrous elements include clay minerals such as montmorillonite, nontronite, beidellite, saponite, hectorite, sauconite, stevensite, and bentonite.

When the iron structure contains the non-ferrous elements, the content of the non-ferrous elements is preferably 0.5% by mass or more, more preferably 0.8% by mass or more. The content of the non-ferrous elements is preferably 5% by mass or less, more preferably 4.5% by mass or less. By being in such a range, the crush strength of the iron structure can be further improved, and the fibrous material can be easily formed on the surface during the production of the iron structure.
The content of non-ferrous elements can be measured by performing elemental quantitative analysis within a measurement range surface designated as the measurement range on the surface of the iron structure using SEM-EDX or X-ray fluorescence analysis, or by dissolving the iron structure in an acid such as nitric acid to prepare a solution, and then subjecting the solution to ICP optical emission spectrometry.

The iron structure reacts with oxygen in the air at room temperature and pressure, and some of the constituent elements are inevitably oxidized. Therefore, the iron structure may contain about 10 mass % or less of oxygen (O).
The iron structure may also contain about 10 mass % or less of carbon (C) as an unavoidable component derived from the raw materials used for its production.
When the iron structure contains oxygen and carbon, oxygen and carbon are excluded from the "non-ferrous elements" in this specification.

The above-mentioned iron structure can be manufactured, for example, as follows.
The method includes a step of heating granular material having an average particle size within a predetermined range, the granular material including iron oxide, a carbonaceous reducing agent, and an inorganic binder, in a rotary furnace, thereby reducing the iron oxide to produce metallic iron (reduced iron).

In manufacturing the iron structure, it is preferable to first prepare granular materials containing iron oxide, a carbonaceous reducing agent, and an inorganic binder. By preparing the granular materials, it is possible to effectively prevent each raw material from flowing out of the system, while reducing the area of contact of the iron oxide with the inner wall of the rotary furnace, thereby preventing the reduced iron from adhering to the inner wall of the furnace and forming a solidified state. In addition, since the granular materials are easily stirred together in the rotary furnace, it is possible to prevent the granular materials from bonding together during reduction. Furthermore, it is possible to efficiently proceed with the reduction reaction of iron, and to highly productively manufacture iron structures having a specific surface structure and a predetermined bulk density and average particle size.

As the iron oxide, from the viewpoint of efficiently producing metallic iron, it is preferable to use a raw material containing iron oxide (III). Examples of the raw material containing iron oxide (III) include one or more of iron ore, iron oxide (so-called mill scale) generated on the surface of steel materials during hot working of steel materials such as steel plates, steel pipes, and shape steel, and iron oxide powder (so-called spray roasted powder) obtained by spray drying pickling wastewater from a pickling line in a steel manufacturing process.
The raw material containing iron (III) oxide may be in the form of a lump or powder.
Among these, from the viewpoint of efficiently producing reduced iron with fewer impurities, it is preferable to use iron oxide powder, and from the viewpoint of achieving both efficient production of reduced iron and reduced environmental load, it is even more preferable to use sprayed roasted powder.

The carbonaceous reducing agent is a reducing agent whose main component is carbon alone, and functions as an auxiliary material for reducing iron oxide. Examples of carbon alone include amorphous carbon and graphite. Examples of such carbonaceous reducing agents include coal-based reducing agents such as coal, coal char, and coke; and biomass charcoal such as sawdust charcoal, coconut shell charcoal, and wood charcoal. These can be used alone or in combination of two or more.
The carbonaceous reductant may be in the form of a lump or a powder.
From the viewpoint of more efficiently progressing the reduction reaction of iron oxide and highly productively producing an iron structure having a specific surface structure and a predetermined bulk density, it is preferable to use a powder of a carbonaceous reducing agent, and it is also preferable to use biomass charcoal as the carbonaceous reducing agent, and it is even more preferable to use powder of biomass charcoal.

The inorganic binder is used to physically bind the iron oxide and the carbonaceous reducing agent to enable the desired shape to be retained. As the inorganic binder, one that has high formation efficiency of granular material and shape retention of the granular material, while not inhibiting the reduction reaction of the iron oxide, can be used. In particular, the inorganic binder is preferably at least one of water glasses such as sodium silicate, lithium silicate, and potassium silicate; inorganic particles such as silica sol, titania sol, alumina sol, and zirconia sol; alkoxides such as alkoxysilane, ethoxysilane, titanium alkoxide, titanium ethoxide, zirconium alkoxide, zirconium ethoxide, titanasiloxane, and zirconasiloxane; and clay minerals including smectites such as montmorillonite, nontronite, beidellite, saponite, hectorite, sauconite, stevensite, and bentonite, and more preferably uses at least water glasses such as sodium silicate from the viewpoint of further increasing the shape retention of the granular material.

Whether the granular material contains the inorganic binder described above can be identified by using known analytical devices such as the SEM-EDS and X-ray fluorescence analysis described above to perform elemental mapping of the surface of the granular material to analyze the type and amount of contained elements, identify the type of compound based on the position of the detection peak measured by X-ray diffraction (XRD) analysis, and analyze the content of each compound based on the intensity of the detected peak measured, either alone or in combination.

As a method for producing granules, for example, iron oxide, a carbonaceous reducing agent and an inorganic binder can be charged into a mixer in any order or simultaneously and mixed to obtain a mixture, and then the mixture can be charged into a granulator and granulated into granules having a predetermined particle size.
From the viewpoint of uniformly mixing the iron oxide and the carbonaceous reducing agent, it is preferable to use powders of both the iron oxide and the carbonaceous reducing agent.
From the viewpoints of uniformly mixing the raw materials and improving the moldability of the particles during granulation, it is preferable to add water, an organic solvent, or the like to the above-mentioned mixture to increase the viscosity.

The components are preferably mixed using a mixer that is used for kneading high-viscosity materials, such as a pressure kneader, an open kneader, or a roll kneader.
Furthermore, when granulating the mixture obtained using the mixing device, a granulating device capable of granulating by compression can be used, for example, a briquette manufactured by Shinto Kogyo Co., Ltd.

The granulated material may be directly used in the subsequent steps, or may be dried to remove the liquid and then used in the subsequent steps. When drying is performed, it is preferably performed at a temperature of 80°C or higher and 150°C or lower, and for a period of 1 hour or higher and 24 hours or lower.

The granules thus obtained have geometric shapes such as spheres, ellipsoids, cylinders, etc., or are like crushed stone, coal, etc.

The granular material preferably has an average particle size of 1 mm or more, more preferably 1.5 mm or more.
The granular material preferably has an average particle size of 100 mm or less, more preferably 50 mm or less.
By making the granules have such an average particle size, it is possible to improve the granulation yield while preventing the reduced iron from adhering to the inner wall of the rotary furnace when heated in the rotary furnace as described below. It is also possible to prevent the granules from bonding with each other, thereby allowing the reduction reaction of the iron oxide to proceed efficiently, and to produce iron structures having a specific surface structure and a predetermined bulk density with high productivity.
The average particle size of the granular material can be measured by the same method as that for the iron structure described above.
The average particle size of the granules can be adjusted, for example, by appropriately changing the shape of the pockets of the granulation device and the dimensions thereof such as the inner diameter during granulation.

A typical method for producing metallic iron by reduction reaction is to charge an iron-containing raw material such as iron ore containing iron oxide (III) and a solid reducing agent containing a carbon source such as coal into a rotary kiln or other rotary furnace, reduce the iron oxide, and produce metallic iron (reduced iron) as the main product. However, when iron ore or mill scale is used as the iron-containing raw material, these raw materials contain a large amount of elements other than iron in addition to the target iron, so it is difficult to increase the purity of metallic iron in the reduced product, which is the product, and there is room for improvement in terms of stably producing a sufficient amount of reduced iron. In addition, it has not been possible to stably produce an iron structure having the above-mentioned surface structure and bulk density. Furthermore, the iron powder obtained by pulverizing this reduced product does not fully exhibit the desired heat generating properties, and there is room for improvement.
In addition, in order to stably produce a sufficient amount of reduced iron, an iron-containing raw material having a high content of iron oxide (III) may be used instead of iron ore. For example, a mixed powder obtained by mixing iron oxide (III) powder and a powder of a solid reducing agent containing carbon may be heated in a rotary furnace, or a mixture obtained by mixing a lump of iron oxide (III) and a powder or lump of a solid reducing agent containing carbon may be heated in a rotary furnace. However, when an iron structure having the above-mentioned surface structure and bulk density is produced by the former method, the constituent particles of the mixed powder may bond with each other, or the mixed powder may have a tendency to easily adhere to the inner wall of the rotary furnace, so that a large amount of the mixed powder or the reduced iron powder may adhere firmly to the inner wall of the rotary furnace. As a result, a separate operation such as peeling off the reduced iron powder from the inner wall must be performed, which may cause inconvenience in terms of increasing productivity. In the latter case, the reduction reaction of the iron oxide may be uneven, and the reduced iron may not be produced uniformly and sufficiently, or the mixture may be strongly bonded to each other, resulting in an excessively large particle size of the product after reduction.

The inventors have conducted extensive research into ways to improve these points, and have found that by heating granular material containing iron oxide, a carbonaceous reducing agent, and an inorganic binder and having a specified particle size in a rotary furnace, the reduction reaction of the iron oxide can proceed uniformly and efficiently without the granular material bonding to itself, and that the reduced iron produced can be prevented from adhering to the inner wall of the rotary furnace or bonding to itself. They have also found that the effects of the present invention can be significantly achieved by using a granular material prepared by mixing an inorganic binder with iron oxide powder and a carbonaceous reducing agent powder, and granulating the granular material to a specified particle size.

The content of iron (III) oxide in the granules is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more.
The content of iron (III) oxide in the granules is preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less.
By setting the content of iron (III) oxide in the granules within the above-mentioned range, reduced iron with high purity can be efficiently obtained.
The content of iron (III) oxide can be calculated by subtracting the content of metallic iron measured in accordance with ISO 5416 and the content of iron (II) oxide measured in accordance with JIS M8213 from the content of total iron measured in accordance with JIS M8212:2005. When multiple raw materials containing iron (III) oxide are used, the content of iron (III) oxide is based on the total amount thereof.

The content of the carbonaceous reducing agent in the granular material, expressed in terms of carbon content, is preferably 5 mass% or more, more preferably 10 mass% or more, even more preferably 20 mass% or more, and is preferably 50 mass% or less, more preferably 45 mass% or less, even more preferably 40 mass% or less.
By setting the content of the carbonaceous reducing agent in the granular material within the above-mentioned range, the reduction reaction of iron oxide can be promoted, and reduced iron can be efficiently obtained.
The content of the carbonaceous reducing agent can be measured in accordance with the method specified in JIS M 8812. When a plurality of carbonaceous reducing agents are used, the content of the carbonaceous reducing agents is based on the total amount thereof.

The content of the inorganic binder in the granular material is preferably 0.5% by mass or more, more preferably 0.8% by mass or more, and further preferably 1% by mass or more.
The content of the inorganic binder in the granular material is preferably 5% by mass or less, more preferably 4.5% by mass or less, and further preferably 4% by mass or less.
By setting the content of the inorganic binder in the granular material within the above-mentioned range, the iron oxide and the carbonaceous reducing agent can be mixed uniformly, and the granular material can be granulated with high productivity.

The crushing strength of the granules is preferably 30N or more, more preferably 40N or more, and further preferably 50N or more.
The crushing strength of the granules is preferably 400N or less, more preferably 350N or less, and further preferably 300N or less.
When the granules have such a crushing strength, unintended collapse during heating in the rotary furnace can be prevented, and reduced iron can be efficiently produced.
The crushing strength of the granular material can be measured in the same manner as in the above-mentioned method for measuring the crushing strength of an iron structure.
The crushing strength of the granules can be adjusted, for example, by adjusting the contents of iron oxide, carbonaceous reducing agent and inorganic binder so that they fall within the above-mentioned ranges, by using a suitable type of inorganic binder, or by changing the granulation conditions.

When iron oxide is used as a powder, from the viewpoint of uniformly mixing the raw materials and easily obtaining granular material with a uniform distribution of iron oxide, the average particle size of the particles constituting the iron oxide powder is preferably 0.1 μm or more, more preferably 0.5 μm or more, and is preferably 20 μm or less, more preferably 10 μm or less.
Furthermore, when the carbonaceous reducing agent is used in powder form, from the viewpoint of uniformly mixing the raw materials, uniformly distributing the carbonaceous reducing agent, and easily obtaining a granular material capable of efficiently progressing the reduction reaction, the average particle size of the particles constituting the carbonaceous reducing agent is preferably 10 μm or more, more preferably 20 μm or more, and is preferably 5000 μm or less, more preferably 3000 μm or less.

The average particle size of the iron oxide powder and the carbonaceous reducing agent powder can be the volume-based median size measured independently by a laser diffraction particle size distribution measuring device. This measurement can be performed in the usual manner using a HORIBA, Ltd. LA-950V2, a standard wet circulation cell, a refractive index of 3.5 for the real part and 3.8i for the imaginary part, a refractive index of 1.33 using water as the dispersion medium, a circulation speed of 15, and an agitation speed of 5.

The granules are then heated in a rotary furnace, such as a rotary kiln, to reduce the iron oxide in the granules to metallic iron.
The heating method of the rotary kiln may be an internal combustion type that directly heats the granular material, or an external combustion type that heats the outer wall of the rotary kiln and transfers the heat to the granular material inside the rotary kiln. From the viewpoint of preventing the intrusion of dust and the like from the outside and obtaining a high-quality iron structure, it is preferable to use an external combustion type rotary kiln.
The granular material may be fed into the rotary furnace once, or may be fed intermittently multiple times or continuously during heating of the rotary furnace. The granular material may be fed before heating or may be fed during heating.

The heating temperature of the granular material is preferably 850° C. or higher, more preferably 880° C. or higher, and even more preferably 900° C. or higher. The heating temperature of the granular material is preferably 1000° C. or lower, more preferably 980° C. or lower, and even more preferably 950° C. or lower.
The heating time for the granules is preferably 0.5 hours or more, more preferably 1 hour or more, and is preferably 10 hours or less, more preferably 8 hours or less, provided that the heating temperature is within the above-mentioned range.
By heating the granules under such conditions, the iron oxide present inside the granules can be efficiently reduced, and iron structures having a specific surface structure and a predetermined bulk density can be produced with high productivity.

The amount of granular material added to the rotary furnace is preferably 5% by volume or more, and more preferably 10% by volume, in order to heat the granular material uniformly to promote the reduction reaction and to produce iron structures with a specific surface structure and a specified bulk density with high productivity.

The granular material is preferably heated while supplying a gas into the rotary furnace into which the granular material is introduced. Examples of the gas to be supplied include an oxidizing gas such as air, and an inert gas such as nitrogen gas or a rare gas. In other words, the granular material is preferably heated in an oxidizing gas atmosphere or an inert gas atmosphere.
In addition, the flow rate of the gas supplied can be appropriately changed depending on the volume of the rotary furnace. For example, when the internal volume of the rotary furnace is 19.6 L, the flow rate can be preferably 0.1 L/min or more and preferably 0.2 L/min or less under standard gas conditions.

When the granular material starts to be heated, the inside of the rotary furnace is an air atmosphere or an inert gas atmosphere such as nitrogen, but as the heating temperature rises, the carbon-based reducing agent decomposes to generate carbon monoxide, and the rotary furnace becomes an atmosphere containing a reducing gas. In this atmosphere, the carbon monoxide reduces the iron oxide to generate metallic iron (Fe). In addition, as metallic iron is generated, carbon dioxide is generated from the carbon monoxide, which reacts with the carbon-based reducing agent to become carbon monoxide, which diffuses into the heating furnace and reduces the iron oxide. These chemical reactions are repeated in the rotary furnace, and multiple fibrous materials containing metallic iron are formed in a mesh pattern at least on the surface, generating an iron structure with a predetermined bulk density.

The iron structure is preferably manufactured using granular materials having a predetermined average particle size, and in the rotary furnace during the reduction treatment, the granular materials may come into contact with the inner wall of the rotary furnace or with each other. Therefore, depending on the manufacturing conditions, the relationship between the shape of the granular materials and the iron structure before and after the reduction treatment may be such that (i) the shape is substantially the same before and after the reduction treatment, (ii) the shape is irregular, such as when corners or protrusions are formed or the corners or protrusions are chamfered after the reduction treatment, or (iii) the shape of a mass in which the granular materials are bonded during the reduction treatment to form a particle size larger than the average particle size of the granular materials used may be produced.
In the assembly of iron structures that is the product, only one of these forms may be present, or a mixture of a plurality of forms may be present.
In the cases of forms (i) and (ii) above, the average particle size of the raw material granules and the average particle size of the finished iron structure are substantially the same, or the average particle size of the iron structure is smaller than the average particle size of the granules.
In a preferred embodiment of the present production method, an iron structure having the above forms (i) and (ii) is produced as a main product.

Through the above steps, the desired iron structure can be obtained. This iron structure preferably has a bulk density of 0.3 g/ ^cm3 or more and 1.5 g/ ^cm3 or less. In addition, this iron structure preferably has a mesh-like structure in which both the surface and the interior are composed of a plurality of fibrous materials containing metallic iron.
If necessary, the iron structure can be subjected to a dressing process to select an iron structure with high purity, and then subjected to subsequent processes such as pulverization to produce iron powder. This iron powder is used as a material for a heat-generating composition that generates heat by utilizing heat generated by an oxidation reaction between the iron powder (an oxidizable metal) and oxygen in the air. In other words, the iron powder obtained by pulverizing the iron structure is suitable for use in a heat-generating composition.

In addition, since this iron powder is obtained by pulverizing an iron structure, it has substantially the same structure and characteristics as the iron structure, except for the average particle size. Specifically, it is preferable that at least a part of the surface of the iron powder particle is an aggregate of fibrous materials, in which a plurality of fibrous materials containing metallic iron are randomly present to form a mesh-like structure. It is also preferable that the bulk density of the iron powder is 0.3 g/ ^cm3 or more and 1.5 g/ ^cm3 or less. By maintaining such characteristics, the iron powder has excellent heat generation characteristics.

When performing ore-dressing, any known method can be used without particular restrictions, for example, a method of separating magnetic metallic iron from non-magnetic components (gangue) using magnetic force (magnetic ore-dressing method) can be exemplified.

The method for crushing the iron structure is not particularly limited, and any known method can be used, for example, using a known crusher such as a rod mill, roll crusher, ball mill, disk mill, cutter mill, or hammer mill.

When producing iron powder from the obtained iron structure, it is preferable to pulverize the iron structure to a predetermined particle size from the viewpoint of fully expressing the heat generating properties of the iron powder. Specifically, the iron structure is pulverized to an average particle size of preferably 5 μm or more, more preferably 20 μm or more, and preferably 500 μm or less, more preferably 100 μm or less. The average particle size of the iron powder can be measured, for example, by a method similar to the method for measuring the average particle size of iron oxide powder and carbonaceous reducing agent powder.

In addition, from the viewpoint of obtaining iron powder with excellent heat generation properties, it is preferable that the specific surface structure and the specified bulk density of the iron structure are maintained as much as possible during powderization, and for this purpose, it is preferable to crush the iron structure to a lesser degree than in the past.

In order to obtain iron powder that satisfies the above-mentioned average particle size and maintains a specific surface structure and a predetermined bulk density in the iron structure, for example, the following crushing method can be adopted.
That is, a 0.05 kg iron structure can be pulverized using a vibrating disc mill (e.g., Verder Scientific, product name "RS200") at a rotation speed of 700 to 1000 rpm for about 5 to 300 seconds. By carrying out such pulverization, it is possible to easily obtain iron powder consisting of an aggregate of metallic iron particles that satisfy the above-mentioned average particle size, specific surface structure, and predetermined bulk density.
The obtained iron powder may be further sieved as necessary to adjust the particle size and particle diameter.

The iron powder obtained by pulverizing the iron structure preferably has a bulk density of 0.3 g/ ^cm3 or more and 1.5 g/ ^cm3 or less. Due to this, the iron powder has high handleability and excellent heat generation properties, and is therefore suitable for use in a heat-generating composition.
The heat generating composition typically contains an oxidizable metal, a carbon material such as activated carbon, an electrolyte such as salt or a metal hydroxide, and water. As the oxidizable metal, iron powder obtained by pulverizing an iron structure can be used.

The heat generating composition can be laminated, for example, on a sheet-like substrate to form a heat generating element, and a heat generating tool equipped with the heat generating element can be made. FIG. 2(a) shows one embodiment of a heat generating element including the heat generating composition, and FIGS. 2(b) and (c) show one embodiment of a heat generating tool equipped with a heat generating element.

In the embodiment shown in FIG. 2(a), the heat generating element 1 has a structure in which the heat generating composition 2 is laminated between a first sheet-like substrate 11 and a second sheet-like substrate 12.
The constituent materials of each sheet-like substrate 11, 12 may be the same as those conventionally used in the relevant technical field, and examples thereof include impermeable materials such as synthetic resin films, permeable materials made of fiber sheets such as nonwoven fabrics or paper, or laminates of the impermeable materials and the fiber sheets.
In addition, each sheet-like substrate 11, 12 may have water absorption properties, such as a fiber sheet containing hydrophilic fibers, a highly absorbent sheet containing particles of water-absorbing polymer and hydrophilic fibers, etc. Each sheet-like substrate 11, 12 may be made of the same material or different materials. From the viewpoint of the heat generation sustainability of the heating element caused by the oxidation reaction, it is preferable that at least one sheet of each sheet-like substrate 11, 12 has air permeability.
Alternatively, the heating element may have a two-layer structure in which the heating composition is laminated on one side of a sheet-like substrate. The heating element 1 can be formed, for example, by applying a slurry-like heating composition to one side of the sheet-like substrate, or by scattering a powder or lump-like heating composition on one side of the sheet-like substrate.

The heating tool 10 shown in Figs. 2(b) and (c) is configured to include a heating element 1 and a flat bag body 20 that contains the heating element 1. The heating element 1 is a member that generates heat in the heating tool 10, and is configured to include a heating composition 2 having the above-mentioned composition. The bag body 20 is a member that surrounds the entire heating element 1 and forms the outer surface of the heating tool 10, and is breathable in part or in its entirety. In the heating tool 10, the heating element 1 may be fixed to the bag body 20, or the heating element 1 and the bag body 20 may not be fixed and may be movable separately.

As shown in FIG. 2(c), the bag body 20 comprises a first sheet material 21 and a second sheet material 22. The first sheet material 21 and the second sheet material 22 each have an extension portion that extends outward from the periphery of the heating element 1, and the extension portions are joined together to surround the heating element 1, forming a bag shape with a space inside. Each sheet material can be the above-mentioned gas permeable material, gas impermeable material, a porous sheet with gas permeability, a water-absorbent sheet with a water-absorbent polymer held on one side of the sheet, or the like.

When measuring the physical properties of the iron powder contained in the heat generating composition, heat generating element, and heat generating tool, methods for recovering the iron powder include a method in which the contents of the heat generating composition or heat generating element are dried in an inert atmosphere, and then magnetically separated to recover only the iron powder that adheres to a magnet, or a method in which the contents of the heat generating composition or heat generating element are dispersed in a large amount of water, centrifuged, and the precipitated iron powder is recovered by drying in an inert atmosphere. The oxidation reaction of iron requires metallic iron, water, and oxygen, but all of the above-mentioned recovery methods are carried out in the absence of water or oxygen, so the iron powder can be recovered without causing an oxidation reaction of the iron powder.

The heating tool having the above-mentioned configuration can be used as a heating article such as a disposable hand warmer. The heating tool to which the present invention is applicable is applied directly to the human body or to clothing and is preferably used to warm the human body. Examples of application areas on the human body include the shoulders, neck, face, eyes, lower back, elbows, knees, thighs, lower legs, abdomen, lower abdomen, hands, and soles of the feet. In addition to the human body, the heating tool to which the present invention is applicable can also be applied to various items and is preferably used to warm or keep them warm. In addition, a known adhesive may be applied to the outer surface of the packaging material in order to fix the heating tool to the application area.

The present invention has been described above based on its preferred embodiments, but the present invention is not limited to the above embodiments.

The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples.

Example 1
(1. Mixing step)
First, 1.3 kg of iron oxide, 0.3 kg of carbonaceous reducing agent, and 0.06 kg of inorganic binder were charged into a mixer (open kneader, manufactured by Moriyama Seisakusho Co., Ltd., model number: SV5-3) and mixed for 30 minutes to obtain a viscous mixture. The iron oxide, carbonaceous reducing agent, and inorganic binder were the raw materials shown below.
<Ingredients>
Iron oxide: iron (III) oxide powder (spray roasted powder, manufactured by JFE Chemical Corporation, product name: JC-DS, iron (III) oxide content: 99% by mass or more, average particle size: 6 μm)
Carbonaceous reducing agent: Biomass charcoal powder (manufactured by Osaka Gas Chemicals Co., Ltd., product name: coconut shell carbonized product, carbon content: 78% by mass, average particle size: 0.2 to 2.8 mm, moisture content: 12% by mass)
Inorganic binder: sodium calcium silicate aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., sodium silicate concentration: 54.5% by mass)

(2. Granular material preparation process)
The mixture was then charged into a briquette BGS-0N manufactured by Shinto Kogyo Co., Ltd. and granulated under the following granulation conditions, and the granulated material was dried at 105° C. for 24 hours to obtain a granular material. The contents of iron oxide (III), carbonaceous reducing agent, and sodium silicate in the granular material, as well as the average particle size of the granular material, are shown in Table 1 below.
<Granulation conditions>
Roll pocket size: 18.0 mm x 14.1 mm x 3.3 mm, roll diameter: 114 mm, roll thickness: 3.5 mm, pressure between rolls: 35 kN, roll rotation speed: 3.4 rpm, mixture supply rate: 22 kg/h.

(3. Reduction step)
Finally, an external combustion rotary kiln (a batch type rotary kiln manufactured by Takasago Industrial Co., Ltd. (inner diameter: 250 mm, heating zone length: 400 mm, heating zone composition: INC601) was used as the heating furnace, and 2.1 kg of granular material was put into the rotary kiln. The kiln rotation speed was set to 1 rpm, nitrogen was used as the gas supplied into the rotary kiln, and the nitrogen flow rate in the rotary kiln was set to 1 L/min. The iron oxide was reduced to metallic iron by heating under the temperature and time conditions shown in Table 1 in a nitrogen atmosphere, and an iron structure was obtained.
The iron structure obtained was a lump, and it was confirmed by an electron microscope that a plurality of fibrous materials present at least on the surface formed a mesh structure. The inside of the iron structure had a mesh structure composed of a plurality of fibrous materials containing metallic iron. The physical properties of the iron structure of this example are shown in Table 1 below.

Example 2
The mixture was produced using the same raw materials as in Example 1. The mixture was produced by first putting 5.4 kg of iron oxide and 1.35 kg of carbonaceous reducing agent into a mixer (Mixmuller MSG-05, manufactured by Shinto Kogyo Co., Ltd.) and mixing for 10 minutes, and then putting 1.07 kg of inorganic binder into the mixer and mixing for an additional 20 minutes to obtain a viscous mixture. The mixing conditions were set to a compression pressure of 28 kg and a clearance of 1 mm from the start to the end of the production of the mixture.

(2. Granular material preparation process)
Next, the obtained mixture was put into a briquette BGS-3N manufactured by Shinto Kogyo Co., Ltd. and granulated under the following granulation conditions. The granulated product was dried at 105°C for 24 hours to obtain granules. The contents of iron oxide (III), carbonaceous reducing agent and sodium silicate in the granules and the average particle size of the granules are shown in Table 1 below.
<Granulation conditions>
Roll pocket size: 18.6 mm x 14.1 mm x 3.3 mm, roll diameter: 228 mm, roll thickness: 4.8 mm, pressure between rolls: 75 kN, roll rotation speed: 10 rpm, mixture supply rate: 170 kg/h.

(3. Reduction step)
Finally, an external combustion rotary kiln (manufactured by Takasago Industrial Co., Ltd., continuous rotary kiln (inner diameter: 300 mm, heating zone length: 2400 mm, heating zone composition: made of SUS310S) was used as the heating furnace, and the granular material was continuously supplied into the rotary kiln at a rate of 9 kg/h. The kiln rotation speed was set to 1.6 rpm, air was used as the gas supplied into the rotary kiln, and the air flow rate in the rotary kiln was set to 1 L/min. The iron oxide was reduced to metallic iron by heating under the conditions of the temperature and time shown in Table 1 in an air atmosphere, and an iron structure was obtained.
The obtained iron structure was a lump, and it was confirmed by an electron microscope that a plurality of fibrous materials present at least on the surface formed a mesh structure. The interior of the iron structure had a mesh structure composed of a plurality of fibrous materials containing metallic iron. It was also confirmed that some of the products containing reduced iron were in a state in which the products were bonded together to form large aggregates whose average particle size was larger than the average particle size of the granular materials. The physical properties of the iron structure of this example are shown in Table 1 below.

Example 3
In producing the mixture, except that the contents of each raw material were 1.3 kg of iron oxide, 0.3 kg of carbonaceous reducing agent, and 0.01 kg of inorganic binder, granules and iron structures were obtained through the steps of mixing, granule preparation, and reduction in the same manner as in Example 1. The contents of iron oxide (III), carbonaceous reducing agent, and sodium silicate in the granules, as well as the average particle size of the granules, are shown in Table 1 below.
The obtained iron structure was a lump, and it was confirmed by an electron microscope that a plurality of fibrous materials present at least on the surface formed a mesh structure. The interior of the iron structure had a mesh structure composed of a plurality of fibrous materials containing metallic iron. A portion of the product containing reduced iron was confirmed as a plate-like material adhering to a portion of the inner wall of the rotary furnace. The physical properties of the iron structure of this example are shown in Table 1 below.

Comparative Example 1
In producing the mixture, the same raw materials as in Example 1 were used, and 1.68 kg of iron oxide and 0.42 kg of a carbonaceous reducing agent were mixed to obtain a powdered mixture (hereinafter, also referred to as a mixed powder). In this comparative example, no inorganic binder was used in the mixing process.
Next, without carrying out the granular material preparation step, the entire amount of the mixed powder was directly subjected to the reduction step in the rotary furnace and under the same heating conditions as in Example 1 to obtain a solid material containing reduced iron. This solid material was a plate-like material adhering to almost the entire surface of the inner wall of the rotary furnace, and the average particle size could not be measured. Therefore, the solid material in this comparative example is not an iron structure according to the present invention.
It was confirmed by an electron microscope that the obtained solid matter had a network structure formed by an aggregate of fibrous matter at least on the surface of the constituent particles. The physical properties of the reduced iron-containing solid matter of this comparative example are shown in Table 1 below.

Comparative Example 2
The same granular material as in Example 2 was used. In the reduction step, a fixed furnace with a non-rotating heating zone (fixed furnace KDF-900GL, manufactured by Denken High Dental Co., Ltd.) was used as the heating furnace. 100 g of the granular material was filled into a stainless steel container (stock pot, 5-372-01, manufactured by AS ONE Corporation: inner diameter 100 mm x height 100 mm), the container was placed in a fixed furnace, and nitrogen was used as the gas to be supplied into the fixed furnace, and the nitrogen flow rate in the fixed furnace was set to 5 L/min. The iron oxide was reduced to metallic iron by heating at 950°C for 3 hours in a nitrogen atmosphere, and a solid material containing reduced iron was obtained. Since all of the products in this solid material were bonded to each other and the average particle size could not be measured, it is not an iron structure of the present invention.
It was confirmed by an electron microscope that the obtained solid matter had a network structure formed by an aggregate of fibrous matter at least on the surface of the constituent particles. The physical properties of the reduced iron-containing solid matter of this comparative example are shown in Table 1 below.

Comparative Example 3
In this comparative example, instead of the iron structure, a general-purpose reduced iron powder (manufactured by DOWAIP CREATION CO., LTD., model number: RK-200) was used as a solid material. This comparative example is an iron powder, not the iron structure of the present invention. In addition, it was confirmed by an electron microscope that this iron powder had no aggregates of fibrous matter on its particle surface.

[Evaluation of physical properties]
The average particle size and crushing strength of the granules in the examples were measured by the methods described above.
In addition, the metallic iron content, carbon element, oxygen element, total content of non-ferrous elements, and thickness of the fibrous material, as well as the average particle size, bulk density, crushing strength, and pore volume of the iron structure in the examples were each measured by the methods described above.
Similarly, the metallic iron content, the total content of carbon, oxygen and non-ferrous elements other than these elements, as well as the bulk density and pore volume of the reduced iron-containing solid materials in the comparative examples except for Comparative Example 3 were measured by the methods described above.
The average particle size of the mixed powder in each of the comparative examples except for Comparative Example 3 was measured as a volume-based median size measured by the above-mentioned laser diffraction type particle size distribution measuring device. The results are shown in Table 1 below.

[Evaluation of Adhesion]
The degree of adhesion of the product to the inner wall of the furnace when the iron structures of the examples and the reduced iron-containing solid materials of the comparative examples except for Comparative Example 3 (hereinafter, these are collectively referred to as products) were produced through the reduction process was evaluated.
The degree of adhesion of the product to the inner wall of the furnace was calculated by measuring the total mass (g) of the product and the mass (g) of the product not adhering to the inner wall of the furnace, and using the following formula (A). The lower the value, the less the degree of adhesion of the product to the inner wall of the furnace, and the better the production efficiency. The results are shown in Table 1.

Product adhesion rate to furnace inner wall (%) = 100 x (1 - ([mass of product not adhering to furnace inner wall (g)] / [total mass of product (g)]) ... (A)

[Assessment of binding]
After each product of the Examples and Comparative Examples was produced, the degree of bonding between the products was visually evaluated according to the following criteria. The results are shown in Table 1.
A: No bonding or aggregation between the products is observed, and the products have excellent handleability.
B: Some bonding and aggregation between products is observed, but the product can be handled without any problems.
C: Much bonding or aggregation between products was observed, making handling difficult.

[Evaluation of Heating Properties of Heating Tools]
A heating tool was produced using the iron structure of Example 1 and the iron powder of Comparative Example 3 by the following method, and its heating characteristics were evaluated.

(Crushing process)
First, the iron structure obtained in step 3 of Example 1 was placed in a vibrating disk mill (Verder Scientific, model: RS200) and pulverized at a rotation speed of 700 rpm for 20 seconds to obtain a crude iron powder. Next, the obtained crude iron powder was sieved for 5 minutes using a test sieve (JTS-200-45-43) with a mesh size of 106 μm and a rotary tap tester (Teraoka, model: S-2) to obtain iron powder (bulk density 0.9 g/m ³ , average particle size 46 μm) as the undersieve fraction. This iron powder had fibrous matter formed on the surface of the iron structure present on its surface.

(Preparation of heat-generating composition)
Using the iron powder obtained from the iron structure of Example 1 and the iron powder of Comparative Example 3, heat generating compositions containing iron powder, activated carbon, and water were prepared. The composition of the heat generating composition was 100 parts by mass of iron powder, 8 parts by mass of activated carbon, 0.3 parts by mass of thickener (guar gum), and 60 parts by mass of deionized water. The heat generating composition was prepared by adding a liquid containing deionized water and a thickener to a mixture of iron powder and activated carbon, and mixing them uniformly.

(Manufacture of heating element)
Using each of the heat generating compositions prepared by the above-mentioned method, a heat generating element having the configuration shown in Fig. 2(a) was manufactured. A water-absorbent sheet having a basis weight of 120 g/ ^m2 was used as the base sheet 11, and a paper made of wood pulp fiber having a basis weight of 32 g/ ^m2 was used as the base sheet 12.
The water-absorbent sheet was produced according to the method described in Example 1 of JP-A-8-246395. This water-absorbent sheet is a single sheet having a structure in which particles of a sodium polyacrylate-based water-absorbent polymer are mainly present in the central region in the thickness direction of the sheet, and the particles are substantially absent on the surface of the sheet. The water-absorbent sheet has particles of a water-absorbent polymer present in the central region in the thickness direction of the sheet, and has a fiber layer containing hydrophilic crosslinked bulky cellulose fibers through the region in which the particles are present, and the fiber layer constitutes the outer surface of the sheet.
The crosslinked bulky cellulose fibers had a fiber coarseness of 0.22 mg/m and an average fiber length of 2.5 mm. The fiber layer further contained softwood bleached kraft pulp and polyvinyl chloride. The water-absorbent polymer particles had an average particle size of 340 μm. The basis weight of the fiber layer was 50 g/ ^m2 , and the basis weight of the water-absorbent polymer particles was 70 g/ ^m2 , resulting in a water-absorbent sheet with a basis weight of 120 g/ ^m2 .

The heating element was manufactured by first uniformly coating the heating composition on the entire surface of one side of the base sheet 12 to form a coating layer. Sodium chloride powder was then uniformly spread over the entire upper surface of the coating layer. The amount of sodium chloride spread was 10 parts by mass per 100 parts by mass of iron powder as the total content of sodium chloride in the heating composition. Then, the base sheet 11 was laminated on the coating layer on which sodium chloride was spread, to manufacture a heating element having the configuration shown in FIG. 2(a). The basis weight of the heating composition in the heating element was 600 g/ ^m2 .
Next, the heating element produced by the above method was cut into 5.0 cm squares and enclosed in a flat bag 20 to produce a heating tool 10. The bag 20 was composed of a 6.3 cm square air permeable sheet with an air permeability of 3500 sec/100 mL as the first sheet material 21 and a 6.3 cm square non-air permeable sheet as the second sheet material 22, and was produced in a bag shape by joining the peripheral parts of each sheet. The heating element was enclosed so that the first sheet material 21 and the base sheet 11 were in contact with each other.

(Evaluation of heat generation characteristics)
The heating characteristics of the heating tool were evaluated according to the method of JIS S4100:2007. In detail, the heating tool was placed in a bag formed by sealing three sides of a needle-punched nonwoven fabric with a basis weight of 100 g/ ^m2 , and placed on a thermostatic chamber at 40°C. A thermometer was placed between the heating tool and the surface of the thermostatic chamber, and the non-permeable sheet of the heating tool was placed so as to face the thermometer. In this state, the temperature change was measured for 60 minutes. In this evaluation, the longer the duration of 60°C or more, the better the heating characteristics. The results are shown in Figure 3.

As shown in Table 1, the manufacturing method of the embodiment using granular material having a predetermined particle size during reduction treatment reduces adhesion of products to the inner walls of the furnace during reduction treatment, compared to the manufacturing method using powder in Comparative Example 1 and the manufacturing method in Comparative Example 2 in which granular material is reduced in a fixed furnace. The iron structures of the embodiment also have a bulk density within a predetermined range. In particular, no bonding between the products is observed in the iron structure of Example 1, and it is also found to be superior in handleability. Therefore, according to the manufacturing method of the embodiment, it is possible to manufacture an iron structure having a predetermined bulk density, good handleability, excellent productivity, and uniform particle size.
In addition, it is found that the iron powder obtained from the iron structure of Example 1 has superior heat generating properties, maintaining a temperature of about 60° C. for a longer period of time, compared to the general-purpose reduced iron powder of Comparative Example 3. Therefore, the iron structure of the Example can be suitably used as a raw material for iron powder with superior heat generating properties.

Claims (10)

The lump comprises a mass having at least a surface composed of an aggregate of fibrous materials containing metallic iron,
The bulk density is 0.3 g/cm ³ or more and 1.5 g/cm ³ or less,
An iron structure having an average grain size of 1 mm or more and 100 mm or less. The iron structure according to claim 1, wherein the metallic iron content is 50% by mass or more and 99.5% by mass or less. The iron structure according to claim 1 or 2, having a crushing strength of 40N or more and 400N or less. Further comprising at least one non-ferrous element selected from the group consisting of alkali metals, alkaline earth metals, silicon, titanium, aluminum, and zirconium;
The iron structure according to any one of claims 1 to 3, wherein the content of the non-ferrous elements is 0.5 mass% or more and 5 mass% or less. A method for manufacturing an iron structure according to any one of claims 1 to 4,
A method for manufacturing an iron structure, comprising heating granular material containing iron oxide, a carbonaceous reducing agent, and an inorganic binder and having an average particle size of 1 mm or more and 100 mm or less at 850°C or more and 1000°C or less in a rotary furnace. The method for manufacturing an iron structure according to claim 5, wherein heating is performed at 850°C or higher and 980°C or lower. The granules contain iron(III) oxide,
The method for producing an iron structure according to claim 5 or 6, wherein the content of iron (III) oxide is 50% by mass or more and 90% by mass or less. The method for manufacturing an iron structure according to any one of claims 5 to 7, wherein the crushing strength of the granular material is 30N or more and 400N or less. The inorganic binder is at least one of water glass, inorganic particles, alkoxides, and clay minerals;
The method for manufacturing an iron structure according to any one of claims 5 to 8, wherein the content of the inorganic binder in the granular material is 0.5 mass% or more and 5 mass% or less. A method for producing iron powder for a heat generating composition, comprising pulverizing an iron structure obtained by the method for producing an iron structure according to any one of claims 5 to 9 so that the average particle size is 5 μm or more and 500 μm or less.

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