MXPA97003441A - Iron oxide structures monolithic depared thick steel made and methods for manufacturing those structures - Google Patents

Abstract

The present invention relates to a method for forming a monolithic iron oxide structure which comprises providing a metallic structure containing hydroxide and heating the metal structure containing iron in an oxidative atmosphere having an oxygen source consisting essentially of free oxygen at a temperature below the melting point of iron to oxidize the iron-containing structure and directly transform the iron-oxide-iron, where the iron is first oxidized to hematite to transform the iron-containing structure to a monolithic structure of hematite and then heat the monolithic structure of hematite at a temperature of about 1350§a around 1550§C to deoxidize the hematite to magnetite, such that the monolithic structure of magnetite retains substantially the same shape, size and wall thickness of the hemati structure

Description

SINGLE-WAVE MONOLITHIC IRON OXIDE STRUCTURES STEEL MADE OF STEEL AND METHODS TO MANUFACTURE SUCH STRUCTURES FIELD OF THE INVENTION This invention relates to monolithic iron oxide structures made of steel and method for manufacturing said structures by steel heating treatment. BACKGROUND OF THE INVENTION Thin wall monolithic structures, combining a variety of thin wall shapes with the mechanical strength of monoliths, have diverse technological and engineering applications. Typical applications for such materials include gas and liquid flow dividers used in heat exchangers, silencers, catalytic vehicles used in various chemical industries and in emission control for vehicles, etc. In many applications, the operating environment requires a monolithic thin-walled structure that is effective at elevated temperatures and / or in corrosive environments. In said demanding conditions, two types of refractory materials, metals and ceramics have been used in the art. Each one suffers from disadvantages. Although metals can be mechanically strong and relatively easy to configure in various structures of varying wall thicknesses, they typically have poor performance in environments that include elevated temperatures or corrosive media (particularly acidic and oxidizing environments). Although many ceramics can withstand ambient temperatures and environments that are corrosive better than many metals, they are difficult to configure, suffer from decreased strength compared to metals and require thicker walls to compensate for their relative weakness compared to metals. In addition, chemical processes to form ceramics are often environmentally damaging. These processes may include toxic ingredients and waste. In addition, the processes commonly used to make ceramic structures by concreting powders, is a difficult manufacturing process which requires the use of very simple powders with grains of particular size to provide convenient densification of the material at high temperature and pressure. Frequently, the process results in cracks in the structure formed. Metal oxides are useful ceramic materials. In particular, iron oxides in their high oxidation states, such as hematite (a-Fe203) and magnetite (Fe2O) are thermally stable refractory materials. For example, hematite is stable in air except at temperatures in excess of 1400 ° C and the melting point of magnetite is 1594 ° C, and the melting point of magnetite is 1594 ° C. These iron oxides, in volume, are also chemically stable in normal acidic, basic and oxidant environments. Iron oxides such as magnetite and hematite, have similar densities, exhibit similar coefficients of thermal expansion and similar mechanical strength. The mechanical strength of these materials is superior to that of ceramic materials such as cordierite and other aluminosilicates. Hematite and magnetite differ substantially in their magnetic and electrical properties. The hematite is practically not magnetic and is not electrically conductive. On the other hand, magnetite is ferromagnetic at temperatures below approximately 575 ° C and is highly conductive (approximately 106 times greater than hematite). In addition, both hematite and magnetite are environmentally benign, which makes them particularly suitable for applications where environmental or health issues are important. In particular, these materials do not have toxicological or environmental limitations imposed by the OSHA regulations of E.U.A. Metal oxide structures have traditionally been manufactured by providing a mixture of metal oxide powders (as opposed to metal powders) and reinforcing components, forming the dough in a desired configuration and then compacting the powder into a final structure. However, these process have many disadvantages including some associated with the process of other ceramic materials. In particular, they suffer from dimensional changes, they generally require a binder or lubricant to pack the powder that will be formed and suffer from decreased porosity and increased shrinkage at higher temperatures of compression.

The use of metallic powders has been reported for the manufacture of metallic structures. However, the formation of metallic oxides containing metallic powders has not been considered convenient. In addition, the formation of metal oxides during the grinding of metal powders is considered a detrimental effect which is opposed to the desired formation of metal ligatures. "Oxidation and especially the reaction of metals and ceramics of monoxide with oxygen, has generally been considered an undesirable aspect that needs to be avoided." Concise Encyclopedia of Advanced Ceramic Materials, R.J. Brook, de., Max-Planck-Institut fur Metalfoschung, Pergamon Press, pp. 124-125 (1991). In the prior art, it has been unacceptable to use steel starting materials to make uniform monolithic iron oxide structures, at least in part, because the oxidation has been incomplete in prior art processes. In addition, the surface layers of iron oxides according to the processes of the prior art suffer from easily peeling off the thickness of the steel. The heat treatment of steel has often been referred to as annealing. Although the procedures of annealing are diverse and can modify strongly, or even improve, some properties of the steel, annealing occurs only with slight changes in the chemical composition of the steel at elevated temperatures in the presence of oxygen, particularly in air, coal and steel. with low alloy they can be partially oxidized, but this penetrating oxidation has been considered to be universally harmful. Said partially oxidized steel has been considered useless and in the art it has been characterized as "burned", which taught that "burned steel can be recovered little and can normally be discarded". "The Making, Shaping and Testing of Steel" W.S. Steel, 10th, 10th ed., Section 3, p. 730. "Annealing is [] used to remove thin oxide films from dusts that fade during prolonged storage or exposure to moisture", Metals Handbook, Vol. 7, p. 182 Powder Metallurgy, ASM (9th Ed. 1984). An attempt to manufacture a metal oxide by oxidation of a parent metal is described in the U.S. Patent. 4,713,360. The '360 patent discloses a self-supporting ceramic body produced by oxidation of a molten mother metal to form a polycrystalline material consisting essentially of the oxidation reaction product of the parent metal with a vapor phase oxidant, and, optionally, one or more non-oxidized constituents of the mother metal. The '360 patent discloses that the parent metal and the oxidant evidently form a favorable polycrystalline oxidation reaction product having a surface-free energy ratio with the molten mother metal so that within some portion of a region or temperature in the Where the parent metal is melted, at least some of the grain intersections (ie, grain boundaries or three grain intersections) of the polycrystalline oxidation reaction product are replaced by flat or linear channels or molten metal.

The structures formed according to the methods described in the '360 patent, require the formation of molten metal prior to the oxidation of the metal. In addition, the materials formed in accordance with said processes do not greatly improve the strength compared to the compression processes known in the art., The metallic structure originally present, can not be maintained since the metal must be melted in order to form the metal oxide. Therefore, after the ceramic structure was formed, whose thickness is not specified, it is configured to the final product. Another attempt to make a metal oxide by oxidation of a parent metal is described in the U.S. Patent. 5,093,178. The '178 patent discloses a flow divider that states that it can be produced by configuring the flow divider of metallic aluminum by extrusion or winding, then converting it to hydrated aluminum oxide by anodic oxidation while moving slowly downward in an electrolyte bath and finally converting it to α-alumina by heat treatment. The '178 patent relates to an unmanageable electrochemical process that is expensive and requires strong acids that are corrosive and environmentally damaging. The process requires slow movement of the structure in the electrolyte, apparently to provide a fresh surface for oxidation and only allows partial oxidation. In addition, the oxidation step of the process of this' 178 patent produces a hydrous oxide which must then be further treated to produce a useful working body. In addition, the description of the '178 patent is limited to processing aluminum and does not suggest that the process may be applicable to iron. Also refer to "Directed Metal Oxidation" in The Encyclopedia of Advanced Materials, vol. 1, p. 641 (Bloor et al., Eds., 1994). Accordingly, there is a need for monolithic iron oxide structures that are highly resistant, manufactured efficiently and economically, in environmentally benign processes, and capable of providing characteristics such as those required in demanding temperature and chemical environments. Also needed are monolithic iron oxide structures that are capable of operating in demanding environments, and that have a variety of configurations and wall thicknesses. OBJECTIVES AND COMPENDIUM OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide a monolithic structure of iron oxide having high strength, which is manufactured efficiently and which is capable of providing refractory characteristics such as those required in demanding temperature and chemical environments. It is a further object of the invention to provide monolithic structures of iron oxide directly from simple steel structures and to substantially retain the physical configuration of the steel structure. These and other objects of the invention are achieved by a thin-walled iron oxide structure manufactured by providing a monolithic metal structure containing iron (such as a steel structure), and heating the metal structure containing iron at a temperature below from the iron melting point to oxidize the iron-containing structure and directly convert iron to iron oxide, so that the iron oxide structure retains substantially the same physical configuration as the iron-containing metal structure. In one embodiment of the invention, a thin-walled iron oxide structure is manufactured by providing a monolithic metal structure containing iron (such as a steel structure) and heating the metal structure containing iron at a temperature below the point of fusion of iron to oxidize the iron-containing structure and directly transform the iron into hematite and then deoxidize the hematite structure into a magnetite structure. The iron oxide structures of the invention can be made directly from ordinary steel structures and will substantially retain the shape of the ordinary steel structures from which they are made. The thin walled iron oxide structures of the invention can be used in a wide variety of applications, including flow splitters, corrosion resistant components of automobile exhaust systems, catalytic supports, filters, thermal insulation materials and sound insulating materials. An iron oxide structure of the invention predominantly containing magnetite, which is magnetic and electrically conductive, can be heated and, therefore, can be applicable in applications such as electrically heated thermal insulation, electrical heating of liquids and gases which they pass through channels and incandescent devices that are stable in the air. Additionally, the combination structures can be manufactured using both magnetite and hematite.

For example, the materials of the invention can be combined in a magnetite heating element surrounded by hematite insulation. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plan view of an illustrative steel structure, configured as a cylindrical flow divider and useful as a starting material for manufacturing iron oxide structures of the invention. Figure 2 is a cross-sectional view of an iron oxide structure of the invention configured as a cylindrical flow divider. Figure 3 is a schematic cross-sectional view of a cubic sample of an iron oxide structure of the invention configured as a cylindrical flow divider, showing the axes of coordinates and direction of the forces. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The present invention relates to the direct transformation of structures formed of iron-containing materials, such as thin single steel sheets, tapes, gauzes, wires, etc., into structures made of iron oxide, such as as hematite, magnetite and combinations thereof. The thickness of the wall of the iron-containing starting structure is important, preferably less than about 0.6 mm, more preferably less than about 0.3 μm, and even more preferably less than about 0.1 mm. The process for carrying out said transformation comprises forming an iron-containing material in a desired structural configuration and then heating the iron-containing structure to a temperature below the melting point of the iron to form an iron oxide structure having substantially the same way as the starting structure that contains iron. Oxidation preferably occurs below the iron melting point, which is about 1,536 ° C. The formation of hematite structures occurs preferably in air of about 725 to about 1350 ° C and more preferably of about 800 to about 1200 ° C. Although the magnetite structures can be made by direct transformation of iron-containing structures to magnetite structures, the magnetite structures are preferably obtained by deoxidating hematite structures by heating them in air at a temperature of about 1420 to about 1550 ° C. The processes of the invention are simple, efficient and environmentally benign since they contain substituents or toxic and do not create toxic waste.

A significant advantage of the present invention is that relatively inexpensive and abundant starting materials such as simple steel can be used for the formation of iron oxide structures. As used in this application, simple steel refers to alloys comprising iron and less than about 2 percent carbon, with or without other substituents that can be found in the steel. In general, any steel and other iron-containing material, which can be oxidized to iron oxide by heat treatment below the melting point of the iron metal, is within the scope of the present invention. It has been found that the process of the invention is applicable for steels having a broad scale of carbon content, for example, from about 0.04 to about 2 weight percent. In particular, high carbon steel, such as Russian Steel 3 and low carbon steel such as AISI-SAE 1010, are suitable for use in the invention. Russian steel 3 contains more than about 97 weight percent iron, less than about 2 weight percent carbon, and less than about 1 weight percent other elements (including from about 0.3 to about 0.7 weight). weight percent manganese, from about 0.2 to about 0.4 weight percent silicone, from about 0.01 to about 0.05 percent by weight of phosphorus and from about 0.01 to about 0I04 percent by weight of sulfur). ISI-SAE 1010 contains more than about 99 weight percent iron, from about 0.08 to about 0.13 weight percent carbon, from about 0.3 to about 0.6 weight percent manganese, of about 0.4. percent by weight of phosphorus, and approximately 0.05 percent by weight of sulfur. To improve the efficiency and integrity of the transformation of the starting material to iron oxide, it is important that the initial structure be sufficiently thin wall. It is preferred that the starting structure be less than about 0.6 mm in thickness, more preferably less than about 0.3 mm in thickness, and even more preferably less than about 0.1 mm in thickness. The starting material can take virtually any desired suitable shape in the final product such as thin sheets, tapes, gauzes, meshes, wires, etc. Significantly, it is not necessary that any binder or organic or inorganic matrices be present to maintain the oxide structures formed during the process of the invention. Therefore, the thermal stability, mechanical strength and uniformity of shape and thickness of the final product can be greatly improved over the products incorporating said binders. Simple steel has a density of about 7.9 gm / cm3 while the density of hematite and magnetite is about 5.2 gm / cm3 and about 5.1 gm / cm3, respectively. Since the density of the steel starting material is higher than that of the iron oxide product, the walls of the iron oxide structure will normally be thicker than the walls of the starting material structure, as illustrated by the data provided in Table I of Example 1 below. The wall of the oxide structure normally also contains an internal space whose width correlates with the thickness of the wall of the starting structure. It has been found that the thinner wall starting structures will have a smaller internal space after oxidation compared to the thicker wall starting structures. For example, as seen in Table I in Example 1, the width of the space was 0.04 and 0.015 mm, respectively, for the iron oxide structures made of 0.1 and 0.025 mm thick sheets. In particular it is preferred that a maximum amount of the surface area of the structure be exposed to the oxidative atmosphere during the heating process for formation of hematite. In a preferred embodiment of the invention, the starting structure is a cylindrical steel disk configured as a flow divider as described in Figure 1. Said flow divider can be useful, for example, as an automotive catalytic converter. Typically, the disc comprises a first flat sheet of steel adjacent to a second corrugated sheet of steel, forming a triangular cell (mesh) that are wound together to form a disc of suitable diameter. Preferably the roll is sufficiently tight to provide physical contact between the adjacent sheets. Alternatively, the disk could comprise three adjacent sheets, such as an adjacent flat sheet, a first corrugated sheet that is adjacent to a second corrugated sheet, with the corrugated sheets having different triangular cell sizes. The size of the structures that can be formed in most conventional ceramic processes is limited. However, there are no significant size limitations for structures formed with the present invention. For example, the steel flow dividers of said construction that are useful in the invention may vary based on the size of the furnace, the requirements of the finished product, and other factors. Steel flow dividers may vary, for example, from about 50 to about 100 mm in diameter, and from about 35 to about 75 mm in height. The thickness of the flat sheets is from about 0.025 to about 0.1 mm and the thickness of the corrugated sheets is from about 0.025 to about 0.3 mm. The triangular cell formed by the flat and corrugated sheets in said illustrative flow dividers, can be adjusted to suit the particular characteristics desired for the iron oxide structure to be formed, depending on the thickness of the sheet and the design of the equipment ( such as a serrated roller) used to form the corrugated sheets. For example, for sheets from 0.1 mm to 0.3 mm, the base of the cell may be approximately e4.0 mm and the cell height approximately 1.3 mm. For sheets of 0.025 to 0.1 mm in thickness, a smaller cell structure could have a base of about 1.9 to about 2.2 mm, and a cell height of about 1.0 to about 1.1 mm. Alternatively, for sheets of 0.025 to 0.1 mm in thickness, an even smaller cell structure could have a base of about 1.4 to about .15 mm, and a cell height of about 0.7 to about 0.8 mm. For different applications, or different furnace sizes, the dimensions may vary the above dimensions. The oxidative atmosphere could provide a sufficient supply of oxygen to allow the transformation of iron to iron oxide. The particular amounts of oxygen, source, concentration and delivery rate can be adjusted according to the characteristics of the starting material, the requirements for the final product, the equipment used and processing details. A simple oxidative atmosphere is air. The exposure of both sides of a sheet of the structure, allows oxidation to be present from both sides, thus increasing the efficiency and uniformity of the oxidation process. Without wishing to be bound by theory, it is thought that the oxidation of the iron in the starting structure occurs via a diffusion mechanism, more preferably by diffusion of iron atoms from the metal network to a surface where they are oxidized. This mechanism is consistent with the formation of an internal space in the structure during the oxidation process. Where the oxidation of both sides of a sheet 10 occurs, the internal space 20 can be seen in a cross-sectional view of the structure, as shown in Figure 2. Where an iron structure contains regions that vary in their openings for air flow, internal spaces have been found in most open regions of a structure, which suggests that oxidation can occur more evenly on both sides of the iron-containing structure than in other regions of the structure. In less open regions of the iron structure, particularly at points of contact between sheets of the iron-containing structure, it has been found that the spaces are narrower or even or are visible. Similarly, the iron-containing wires can form hollow iron oxide tubes having a central cylindrical void space analogous to the internal space that can be found in sheets of iron oxide. When iron is oxidized (atomic weight 55.85) it is oxidized to Fe2O3 (molecular weight 159.69) or Fe2O4 (molecular weight 231.54), the oxygen content comprising the theoretical weight gain is 30.05 percent or 27.64 percent, respectively , of the final product. The oxidation takes place in a significantly decreasing way over time. That is, at the beginning during the heating process, the oxidation rate is relatively high, but decreases significantly as the process continues. This agrees with the mechanism of diffusion oxidation that is thought to occur, since the length of the diffusion path of iron atoms could increase with time. The quantitative rate of hematite formation varies with factors such as the heating regime, and details of the design of the structure containing iron, such as sheet thickness and cell size. For example, when an iron-containing structure made of simple sheets of 0.1 mm thick flat and corrugated steel and having large cells as described above, is heated to about 850 ° C more than forty percent of the iron can be oxidized in one hour. For such a structure, more than sixty percent of the iron can be oxidized in about four hours, while it can take about 100 hours for total oxidation (substantially 100 percent) of iron to hematite. Impurities in the steel starting structures, such as P, Si and Mn, can form solid oxides that lightly contaminate the final iron oxide structure. In addition, the use of an asbestos insulation layer in the process of the invention can also introduce impurities into the structure of the iron oxide. Factors such as these can lead to weight gain slightly greater than the theoretical weight gain of 30.05 percent or 27.64 percent, respectively, for the formation of hematite and magnetite. Incomplete oxidation can lead to a weight gain less than the theoretical weight gain of 20.05 percent or 27.64 percent, respectively, for the formation of hematite and magnetite. Also when a magnetite is formed by deoxidizing hematite, incomplete deoxidation of hematite can lead to a weight gain of more than 27.64 percent for netite mag formation. Therefore, for practical reasons. The terms "iron oxide structure", "hematite structure" and "magnetite structure", as used herein, refer to structures consisting substantially of iron oxide, hematite and magnetite, respectively. The oxygen content and X-ray diffraction spectrum can provide useful iron oxide structures forming indicators of the invention of iron-containing structures. In accordance with this invention, the term "hematite structure" encompasses structures that are substantially non-magnetic at ambient temperature and substantially not electrically conductive and contain more than about 29 weight percent oxygen. The normal X-ray diffraction data for hematite powder is shown in Table IV in Example 1 below. The magnetite structure refers to structures that are magnetic and electrically conductive at room temperature and contain from about 27 to about 29 weight percent oxygen. If the magnetite was formed by a deoxidation of hematite, the hematite may also be present in the final structure as seen, for example in the x-ray data illustrated in Table V in Example 2 below. Depending on the characteristics and desired uses of the final product, the deoxidation process proceeds until enough magnetite is formed. It may be convenient to obtain the stoichiometric oxygen content in the iron oxide present in the final structure. This can be achieved by controlling such factors as heating regime, heating temperature, heating time, air flow and shape of the iron-containing structure, as well as the choice and handling of an insulating layer. The formation of hematite is preferably carried out by heating a simple steel material at a temperature lower than the melting point of the iron (about 1536 ° C), more preferably at a temperature of less than about 1350 ° C, even more preferably at a temperature of about 725 to about 1200 ° C and more preferably from about 750 to about 850 ° C. Oxidation at temperatures below about 700 ° C may be very slow to be practical in some cases, while oxidation of iron to hematite at temperatures above about 1400 ° C may require careful control to avoid localized overheating and melting due to the strong exothermicity of the oxidation reaction. The temperature at which iron oxidizes to hematite, is inversely related to the surface area of the product obtained. For example, oxidation at about 750 to about 850 ° C can produce a hematite structure having an area of approximately four times greater than that obtained at 1200 ° C. A suitable and simple oven to carry out the heating is a conventional driving oven. The air access in a conventional driving oven mainly comes from the bottom of the oven. The electrically heated metallic elements can be used around the structure to be heated to provide relatively uniform heating to the structure, preferably within about 1 ° C. In order to provide a relatively uniform heating regime, a normal electronic control panel can be provided, which can also help provide uniform heating to the tube. It is not thought that any particular oven design is critical while providing an oxidative environment and heating to the desired temperature for the starting material. The starting structure can be placed inside a shirt that can serve to fix the external dimensions of the structure. For example, an indian cylinder can be placed inside a cylindrical quartz tube which serves as a shirt. If a jacket is used for the starting structure, an insulating layer is preferably provided between the outer surface of the starting structure and the internal surface of the bed. The insulating material can be any material that serves to prevent the outer surface of the structure of the iron oxide formed during the oxidation process from welding the internal surface of the jacket. Asbestos is an adequate insulating material. For ease of handling, the starting structure can be placed inside the oven or heating area, while the oven is still cold. The oven can then be heated to the working temperature and maintained during the heating period. Alternatively, the furnace or heating area can be heated to the working temperature and then the metal starting structure can be placed in the heating area during the heating period. The rate at which the heating area is brought to the working temperature is not critical and will ordinarily vary merely with the design of the furnace. For formation of hematite using a driving furnace at a working temperature of about 790 ° C, it is preferred that the furnace be heated to the working temperature for a period of about 24 hours, a heating rate of about 35 ° C. per hour. The heating time of the structure (the heating period) varies with such factors as furnace design, air flow rate (oxygen), and weight, wall thickness, shape, size, and open cross section of the material. departure. For example, for formation of simple steel sheet hematite of approximately 0.1 mm in thickness, in a driving furnace, a heating time of less than about one day, and more preferably from about 3 to about 5 hours, is preferred for cylindrical disc structures approximately 20 mm in diameter, about 15 mm high, and pressing about 5 grams. For larger samples, the heating time should be longer. For example, for the formation of hematite of said single steel sheets in a driving furnace, a heating time of less than about ten days and more preferably of about 2 to about 5 days, it is preferred for the surrounding disk structures. 95 mm in diameter, about 70 mm in height and weighing up to about 1000 grams. After heating, the structure was cooled. Preferably the heat was turned off in the oven and the structure was simply allowed to cool inside the oven under ambient conditions for about 12 to 15 hours. Warming must not be rapid, in order to minimize any adverse effects on the integrity and mechanical strength of the iron oxide structure. The cooling of the iron oxide structure should be avoided commonly. The monolithic hematite structures of the invention have shown remarkable mechanical strength, as can be seen in Tables 11 and VI in the following Examples. For hematite structures configured as flow dividers, structures that have smaller cell size and larger wall thicknesses exhibit the highest strength. Of these two characteristics, as can be seen in Tables 11 and 14, the increase in primary resistance seems to come from the size of the cell, not the thickness of the wall. Therefore, the hematite structures of the invention are particularly suitable for use as light flow dividers having a large open cross section. One particularly promising application of monoliths of the invention is a ceramic support in catalytic converters, a current industrial standard is a cordierite flow divider having, without thin coating, a wall thickness of about 0.17 mm, an open cross section of 65 percent and a limiting resistance of approximately 0.3 Mpa. P.S. Strom et al., SAE PAPER 900500, pages 40-41, "Recent Trends in Automotive Emission Control", SAE (Feb. 1990). As can be seen in the following Tables I and III, the present invention can be used to make a hematite flow divider having thinner walls (approximately 0.07 mm), upper open cross section (approximately 80 percent), and twice the limiting resistance (approximately 0.5 to around 0.7 Mpa) compared to the cordierite product. With the present invention, hematite flow dividers having thin walls, such as, for example, from 0.07 to about 0.3 mm can be obtained. The preferred method for forming magnetite structures of the invention comprises the transformation first of an iron-containing structure to hematite, as described above, and then the hematite is deoxidized to magnetite. Following the oxidation of a starting structure to hematite, the hematite can be deoxidated to magnetite by heating at about 1350 ° to about 1 550 ° C. Optionally, after heating to form a hematite structure, the structure can be cooled, such as at a temperature at or above room temperature, before the deoxidation of hematite to magnetite. Alternatively, the hematite structure does not need to be cooled before deoxidation to magnetite. The sufficient heating time for the deoxidation of hematite to magnetite is generally much shorter than the period sufficient to oxidize the material to hematite initially. Preferably, to use hematite structures as described above, the heating time for deoxidation to magnetite structures is less than about twenty-four hours, and in most cases, it is preferably less than about six hours, in order to form structures that contain adequate magnetite. A heating time of less than about one hour for deoxidation may be sufficient in many cases. A simple deoxidative atmosphere is ai re. The alternate useful deoxidizing atmospheres are air enriched with nitrogen, pure nitrogen (or any suitable inert gas) or a vacuum. The presence of a reducing agent, such as carbon monoxide, can help efficiency in the deoxidation reaction.Magnetite structures can also be formed directly from iron-containing structures by heating iron-containing structures in an oxidative atmosphere. To avoid a substantial presence of hematite in the final product, the preferred working temperatures for a direct transformation of iron-to-magnetite-containing structures are from about 1350 to about 1500 ° C. Since the oxidation reaction is strongly exothermic, it is a significant risk that the temperature in localized areas may rise above the melting point of about 1536 ° C, resulting in local mergers of the structure. Since the deoxidation of hematite to magnetite is endothermic, unlike the exothermic oxidation of steel to magnetite, the risk of localized fusions is reduced to a minimum if the iron is first oxidized to hematite and then deoxidized to magnetite. Therefore, the formation of a magnetite structure by oxidation of a structure containing iron to a hematite structure at a temperature below about 1200 ° C, followed by the deoxidation of hematite to magnetite, is the preferred method. The thin-walled iron oxide structures of the invention can be used in a wide variety of applications. The relatively open top area, which can be obtained, can form the useful products as catalytic supports, filters, thermal insulating materials and sound insulating materials.

The iron oxides of the invention, such as hematite and magnetite, may be useful in applications such as gaseous and liquid flow splitters; Corrosion-resistant components of automotive exhaust systems, such as silencers, catalytic converters, etc .; construction materials (such as pipes, walls, ceilings, etc.); filters such as for water purification, food products, medical products, and for particles that can be regenerated by heating them; thermal insulation in high temperature environments (such as furnaces and / or in chemically corrosive environments) and sound insulations.The iron oxides of the invention which are electrically conductive such as magnetite, can be electrically heated and, therefore, can be It can be applied in applications such as electrically heated thermal insulation, electrical heating of liquids and gases that pass through channels and incandescent devices, etc. In addition, a combination of structures using both magnetite and hematite can be manufactured. The invention will be combined into a magnetite heating element surrounded by hematite insulation The following examples are illustrative of the invention: EXAMPLE 1 The monolithic structures of hematite in the form of a cylindrical flow splitter were manufactured by heating a structure made of steel. simple in air, as described e later, five different samples of steel structure were formed and then transformed into hematite structures. The properties of the structures and processing conditions for the five series are set forth in Table I. TABLE 1 PROPERTIES OF THE FLOW DIVIDER AND CONDITIONS OF PROSECUTION * Calculated from steel weight or hematite using a density of 7.86 g / cm3 for steel and 5.24 g / cm3 for hematite. ** Calculated as the product of real thickness of hematite of the size of the geometric area of steel (one side) (with space) The details of the processes carried out for Sample 1 are given below. Samples 2 to 5 were formed and they tested in a similar way. For Sample 1, a cylindrical flow divider similar to that described in Figure 1 was constructed, measuring approximately 92 mm in diameter and 76 mm in height, it was constructed of two sheets of steel, each of 0.025 mm thick AISI-SAE 1010, a flat and a corrugated. The corrugated sheet of steel had a triangular cell, with a base of 2.15 mm and a height of 1.07 mm. The sheets were wound tight enough so that physical contact was formed between the flat and corrugated sheets. After the winding, an additional flat sheet of steel was placed around the outer layer of the structure to provide ease of handling and additional stiffness. The final weight of the structure was approximately 273.4 grams. The steel structure was wrapped in an insulating sheet of asbestos approximately 1 mm thick and was hermetically placed in a cylindrical quartz tube that served as a jacket to fix the external dimensions of the structure. The tube containing the steel structure was then placed at room temperature on a ceramic support in a driving oven. The ceramic support retained the sidewalk sample at a height in the furnace that subjected the sample to a uniform working temperature varied by or more than about 1 ° C at any point in the sample. Thermocouples were used to uniformly monitor sample temperature. After placing the sample in the oven, the oven was heated electrically for approximately 22 hours at a heating rate of about 35 ° C per hour at a working temperature of about 790 ° C. The sample was then maintained at about 790 ° C for about 96 hours in an ambient atmosphere of air. No special arrangements were made to affect the air flow inside the oven. After about 96 hours, the heat was turned off in the oven and the oven allowed to cool to room temperature over a period of about 20 hours. Then, the quartz tube was removed from the oven. The iron oxide structure was easily separated from the quartz tube and mechanically removed traces of the asbestos insulation from the iron oxide structure by abrasive means. The weight of the structure was approximately 391.3 grams, corresponding to a weight gain (oxygen content) of approximately 30.1 weight percent. It was thought that the very slight increase in weight above the theoretical limit of 30.05 percent was due to impurities that could result from the isolation of asbestos. The X-ray diffraction spectrum for a powder made from the structure showed excellent arrangement with a normal spectrum of hematite, as shown in Table IV. The structure generally retained the shape of the starting structure of the steel, with the exception of some deformations of triangular cells due to the increased thickness of the wall. In the hematite structure, all physical contacts between adjacent steel sheets were "soldered" internally, producing a monolithic structure that has no visible cracks or other defects. The thickness of the wall of the hematite structure was from about 0.07 to about 0.08 mm, resulting in an open cross section of about 80 percent, as shown in Table I. In several cross sections of the structure, which was seen under a microscope, each containing several dozen cells, you could almost always see an internal space of about 0.01 to about 0.02 mm. The surface area of BET was approximately 0.1 m2 / gram. The hematite structure was not magnetic, as it was checked against a common magnet. In addition, the structure was not electrically conductive under the following test. A small rod having a diameter of about 5 mm and a length of about 10 mm is cut from the structure. The rod was contacted with the platen plates that served as electrical contacts. The electrical power capable of supplying from about 10 to about 60 watts was applied to the structure without any noticeable effect on the structure. The structure monolithic hematite was tested for suffers resistance by placing four samples of the structure in sulfuric acid (solutions of five and ten percent water) as shown in Table II below. Samples 1 and 2 included portions of the external surface sheets. It is possible that these samples contained slight traces of insulation and / or were incompletely oxidized when the heating process was stopped. Samples 3 and 4 included internal sections of the structure alone. With the four samples, no visible surface corrosion of the samples was observed, even after 36 days in the sulfuric acid and the amount of iron dissolved in the acid, measured by atomic absorption spectroscopy, was negligible. The samples were also compared to the powder samples made from the same monolithic hematite structure, ground to a quality similar to that used for X-ray diffraction analysis and soaked with H2SO4 for approximately twelve days. After another week of exposure (for a total of 43 days for the monolith samples and 19 days for the powder samples), the amount of dissolved iron remained virtually unchanged, suggesting that the saturation concentrations were reached. The relative dissolution for the powder was higher due to the surface area of the powder samples which is higher than that of the samples of the monolithic structure. However, the amount and percentage solution was negligible both for the monolithic structure and for the powder formed from the structure. TABLE II RESISTANCE TO CORROSION OF SULFURIC ACID Based on the data given in Tables I and II for the monolithic structure, the average corrosion resistance for the samples was less than 0.2 mg / cm2 per year, which was considered non-corrosive by ASM. ASM Engineered Materials Reference Book, ASM International, Metals Park, Ohio 1989. The hematite structure of the example was also subjected to mechanical trituration test, as follows. Seven normal cubic samples, each approximately 2.54 cm x 2.54 cm x 2.54 cm, were cut with a diamond saw from the structure. Figure 3 describes a schematic cross-sectional view of the samples tested and the axes of coordinates and direction of forces. The A axis is parallel to the channel axis, the B axis is normal to the channel axis and almost parallel to the flat sheet and the C axis is normal to the channel axis and almost normal to the flat sheet. The grinding pressures are given in Table III.

TABLE l l l M ECÁN ICA RESISTANCE OF H EMATITA MONOLITHS Sample 4 of Table I was also characterized using a powder diffraction technique of x-rays. Table IV shows the X-ray powder spectrum (Cu Ka radiation) of the sample measured using a HZG-4 x-ray powder diffractometer (Karl Zeiss), as compared to the normal diffraction data for hematite. In the Table, "d" represents interplanetary distances and "J" represents relative intensity.

TABLE IV PATTERNS OF DIFRACTION OF X-RAY DUSTS FOR HEMATITE * Data file 33-0664. The International Center for Diffaction Data, Newton Square, Pa. EXAMPLE 2 A monolithic magnetite structure was manufactured by deoxidizing a monolithic hematite structure. The magnetite structure substantially retained the shape, size and wall thickness of the hematite structure from which it was formed. The hematite structure was made according to a process substantially similar to that established in example 1. The steel sheet from which the hematite flow divider was formed was approximately 0.1 mm thick. The steel structure was heated in an oven at a working temperature of about 790 ° C for about 120 hours. The resulting hematite flow divider had a wall thickness of about 0.27 mm and an oxygen content of about 29.3 percent. A substantially cylindrical section of the hematite structure about 5 mm in diameter, about 12 mm long and weighing about 646.9 milligrams, was separated from the hematite flow divider along the axial direction to form the magnetite structure . This sample was placed in an aluminum oxide crucible and in a TG D7000 differential thermogravimetric analyzer (Sinku Riko, Japan) at room temperature. The sample was heated in air at a rate of about 10 ° C per minute to about 1460 ° C. The sample gained a total of about 1.2 mg by weight (about 0.186%) to a temperature of about 180 ° C, reaching an oxygen content of about 29.4 weight percent. From about 180 ° C to about 1345 ° C, the sample did not gain appreciable weight. At temperatures above about 1345 ° C, the sample began to lose weight. At approximately 1420 ° C, a strong endothermic effect was observed in a differential temperature curve of the spectrum. At 1460 ° C, the total weight measurement compared to the starting structure of hematite was around 9.2 mg. The sample was maintained at about 1460 ° C for about 45 minutes, resulting in an additional weight loss of about 0.6 mg, for a total weight loss of about 9.8 mg. The additional heating at 1460 ° C for about 15 more minutes, did not affect the weight of the sample. The heat was turned off, the sample was allowed to cool slowly (without extinction) at room temperature for several hours and then it was removed from the analyzer. The oxygen content of the final product was about 28.2 weight percent. The product retained substantially the shape and size of the initial sample of hematite, particularly in well thicknesses and internal spaces. In contrast to the sample of hematite, the final product was magnetic, checked by a common magnet, and electrically conductive. The spectrum of X-ray powders, as shown in Table V, showed characteristic peaks of magnetite together characteristic of several peaks of the hematite. The structure was tested for electrical conductivity by cleaning the surface of the sample with a diamond saw, contacting the sample with platinum plates that served as electrical contacts, and applying electrical power of about 10 to about 60 watts (of a current from about 1 to about 5 amps, and a potential of about 10 to about 12 volts) for the structure over a period of about 12 hours. During the test time, the rod was incandescent, from hot red (on the surface) to soft-hot (internally) depending on the powder being applied. Table V shows the spectrum of x-ray powder (Cu Ka radiation) of the sample, measured using a X-ray powder diffractometer HXZG-4 (Karl Zeiss), as compared to normal diffraction data for magnetite. In the Table, "d" represents interplanetary distances and "J" represents relative intensity. TABLE V PATTERNS OF DIFFACTION OF X-RAY DUSTS FOR MAGNETITE * Data file 19.0629. The International Center for Diffaraction Data, Newton Square, Pa.

** Characteristic peaks of hematite. No significant peaks were observed other than those characteristic of hematite or magnetite. EXAMPLE III Two hematite flow dividers were manufactured from simple Russian 3 steel and tested for mechanical strength. Samples were manufactured using the same procedures set forth in Example 1. The steel sheets were approximately 0.1 mm thick. Both steel flow dividers had a diameter of about 95 mm and a height of about 70 mm. The first steel structure had a triangular cell base of approximately 4.0 m and a height of about 1.3 mm. The second steel structure had a triangular cell base of around 2.0 mm and a height of approximately 1.05 mm. Each steel structure was heated to approximately 790 ° C for approximately five days. The weight gain for each structure was approximately 29.8 weight percent. The thickness of the wall for each final hematite structure was approximately 0.27 mm. The hematite structures were subjected to mechanical trituration test as described in Example 1. The cubic samples as shown in Figure 3, each approximately 2.54 cm x 2.54 cm x 2.54 cm, were separated from the structures by a diamond saw. Eight samples were taken from the first structure and the ninth sample was taken from the second structure. The crushed pressures were shown in Table VI. TABLE VI MECHANICAL RESISTANCE OF HEMTITE MONOLITHS

Claims (32)

1. CLAIMS 1. A method for forming a monolithic iron oxide structure comprising providing a monolithic metal structure containing iron and heating the metal structure containing iron in an oxidative atmosphere at a temperature below the iron melting point to oxidize the iron. structure that contains iron and directly transform the iron to iron oxide, so that the iron oxide structure retains substantially the same physical form as the metal structure containing iron.
2. 2. A method according to claim 1, wherein the iron oxide is hematite.
3. 3. A method according to claim 1, wherein the iron oxide is magnetite.
4. 4. A method according to claim 1, wherein the iron oxide is a combination of hematite and magnetite.
5. 5. A method according to claim 1, wherein the iron-containing structure is a simple steel.
6. 6. A monolithic iron oxide structure according to claim 5, wherein the steel has a carbon content of about 0.04 to about 2.0 weight percent.
7. 7. A method according to claim 5, wherein the steel is AISI-SAE 1010.
8. 8. A method according to claim 5, wherein the steel is Russian 3 steel.
9. 9. A method according to claim 5, wherein the steel structure has a thickness of less than about 0.3 mm.
10. 10. A method according to claim 1, wherein the oxidizing atmosphere is air. 1.
11. A method of claim 1, wherein the iron-containing structure is heated to a temperature of about 725 to about 1200 ° C to oxidize the hematite iron.
12. 12. A method according to claim 1, wherein the iron-containing structure is heated to a temperature of about 750 to about 850 ° C to oxidize the iron to hematite.
13. A method according to claim 1, wherein the iron is first oxidized to hematite to transform the iron-containing structure into a hematite structure and the monolithic structure of hematite is then heated to a temperature of about 1350 to about 1 550 ° C to deoxidize the hematite to magnetite, so that the magnetite structure retains substantially the same shape, size and wall thickness as the hematite structure.
14. 14. A method according to claim 13, wherein the hematite structure is heated at a temperature of about 1420 to about 1460 ° C to deoxidize the hematite to magnetite.
15. 15. A method according to claim 1, wherein the iron-containing structure is heated to a temperature of about 1350 to about 1500 ° C to oxidize the iron to magnetite.
16. 16. A method for forming a monolithic hematite structure comprising providing a structure consisting essentially of simple steel and heating the simple steel structure in an oxidative atmosphere at a temperature between about 725 and about 1200 ° C to oxidize the structure of simple steel by directly transforming iron into steel into hematite, so that the hematite structure retains substantially the same physical form as the simple steel structure.
17. 17. A method according to claim 16, wherein the oxidative atmosphere is air.
18. 18. A method according to claim 16, wherein the simple steel structure is heated to a temperature between about 750 and about 850 ° C.
19. 19. A method for forming a monolithic magnetite structure comprising providing a structure consisting essentially of simple steel, transforming the simple steel structure to a hematite structure by heating the simple steel structure in an oxidative atmosphere at a temperature between about 725 and about 1200 ° C to oxidize the simple steel structure so that the hematite structure retains substantially the same physical shape as the simple steel structure and then deoxidizing the hematite structure to a magnetite structure by heating the hematite structure in a deoxidative atmosphere at a temperature of about 1350 to about 1550 ° C so that the magnetite structure retains substantially the same shape, size and wall thickness than the hematite structure.
20. 20. A method according to claim 19, wherein the deoxidative atmosphere is selected from the group consisting of air, air enriched with nitrogen, substantially pure nitrogen, and a vacuum.
21. 21. A method according to claim 19, wherein the iron is oxidized to hematite by heating the simple steel structure at a temperature between about 750 and about 850 ° C and the hematite is deoxidized to magnetite by heating the hematite structure to a a temperature between about 1420 and about 1460 ° C.
22. 22. A monolithic iron oxide structure comprising a monolithic iron oxide structure obtained from the oxidation of an iron-containing structure at a temperature below the iron melting point, the monolithic iron oxide structure having substantially the same physical form than the iron structure.
23. 23. A monolithic iron oxide structure according to claim 22, wherein the iron oxide is hematite.
24. 24. A monolithic iron oxide structure according to claim 22, wherein the iron oxide is magnetite.
25. 25. A monolithic iron oxide structure according to claim 22, wherein the iron oxide is a combination of hematite and magnetite.
26. 26. A monolithic iron oxide structure according to claim 22, wherein the iron-containing structure is a simple steel.
27. 27. A monolithic iron oxide structure according to claim 25, wherein the steel has a carbon content of about 0.04 to about 2.0 weight percent.
28. 28. A monolithic iron oxide structure according to claim 26, wherein the steel is AISI-SAE 1010.
29. 29. A monolithic iron oxide structure according to claim 26, wherein the steel is Russian 3 steel.
30. 30. A monolithic iron oxide structure according to claim 26, wherein the steel structure has a thickness of less than about 0.3 mm.
31. 31. A monolithic flow divider consisting essentially of an iron oxide selected from the group consisting essentially of hematite, magnetite and a combination thereof and having a wall thickness of less than about one millimeter.
32. 32. A monolithic flow divider according to claim 31, wherein the wall thickness is from about 0.07 to about 0.3 mm.