SYSTEM AND METHOD FOR PRODUCING METALLIC IRON

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] This invention relates generally to a system and method for producing metallic iron by thermally reducing a metal oxide in a moving hearth furnace.

[0002] Metallic iron has been produced by reducing iron oxide such as iron ores, iron pellets and other iron sources. Various such methods have been proposed so far for directly producing metallic iron from iron ores or iron oxide pellets by using reducing agents such as coal or other carbonaceous material.

[0003] These processes have been carried out in rotary hearth and linear hearth furnaces. An example of such a rotary hearth furnace is described in U.S. Pat. No. 3,443,931. An example of such a linear hearth furnace is described in U.S. 2005/0229748. Both the rotary hearth furnace and the linear hearth furnace involve making mixtures of carbonaceous material with iron ore or other iron oxide fines and heating them on a moving hearth to reduce the iron oxide to metallic iron nuggets and slag.

[0004] Hearth furnaces are largely operated with combustion gases from the heating burners flowing counter to the movement of the hearth and the charge materials. Thermal energy is transferred to the charge materials by direct radiation from the burner flame and the furnace walls as well as by direct contact of the combustion gases with the charge materials. The open nature of these systems, even if divided into zones by baffle walls, does not allow much control the furnace atmosphere, which is predominantly burner combustion products with lesser amounts of reaction products from the charge materials.

[0005] Hearth furnaces are generally heated by natural gas burners that provide thermal energy to the system to raise the temperature of the charge materials and initiate the reduction process, that is, the reaction of the carbon in the charge materials with the iron oxides in the charge materials. The carbon dioxide in the combustion gases also reacts with the carbon in the charge materials to produce carbon monoxide through the Boudouard reaction at about 1830 ⁰ F (1000 ⁰ C). This reaction removes carbon from the charge materials at relatively low temperatures. At these temperatures the reaction rate between the carbon monoxide formed and the iron oxide is relatively slow. Therefore, carbon is leached from the system before the reduction process can be completed and has a negative effect on the process.

[0006] The effect is that final reduction, in the case of forming iron nuggets, relies on production of carbon monoxide through interaction of the combustion gases with the carbon in the charge materials, which requires high temperatures approaching or exceeding 2550 ⁰ F (1400 ⁰ C). This high temperature requires both extra burner energy and time to allow completion of the reduction process. These higher temperature also increase construction and maintenance costs because more costly refractory is required. A further impediment is the removal of carbon from the charge materials before metallization is complete so that the carbon is not available to be absorbed by the metallic iron formed reducing its melting temperature.

SUMMARY OF THE INVENTION

[0007] A hearth furnace for producing metallic iron material is disclosed that comprises:

(a) a furnace housing having a drying/preheat zone capable of providing a drying/preheat atmosphere for reducible material, a conversion zone capable of providing a reducing atmosphere for reducible material, a fusion zone capable of providing an atmosphere to at least partially reduced metallic iron material, and optionally a cooling zone capable of providing a cooling atmosphere for reduced material containing metallic iron material, the conversion zone being positioned between the drying/preheat zone and the fusion zone,

(b) a hearth capable of being movable within the furnace housing in a direction through the drying/preheat zone, then the conversion zone, then the fusion zone, and then, if present, the cooling zone,

(c) a hood within at least a portion of the conversion zone, the hood separating the conversion zone into an upper region and a lower region with the lower region adjacent the hearth and the upper region adjacent the lower region and spaced from the hearth, and

(d) at least one reductant injector capable of introducing a gaseous reductant into the lower region adjacent the hearth.

[0008] The moveable hearth of the hearth furnace may have a linear hearth or a rotary hearth.

[0009] In addition, a method of reducing iron ore and other iron oxide sources is disclosed comprised of:

(a) forming a furnace housing having a drying/preheat zone capable of providing a drying/preheat atmosphere for reducible material, a conversion zone capable of providing a reducing atmosphere for reducible material, a fusion zone capable of providing an atmosphere to

at least partially reduced metallic iron material, and a cooling zone capable of providing a cooling atmosphere for reduced material containing metallic iron material, the conversion zone being positioned between the drying/preheat zone and the fusion zone,

(b) providing a hearth capable of being movable within the furnace housing in a direction through the drying/preheat zone, then the conversion zone, then the fusion zone, and then the cooling zone,

(c) providing a hood within at least a portion of the conversion zone separating the atmosphere of the conversion zone into an upper region and a lower region, with the lower region adjacent the hearth and the upper region adjacent the lower region and spaced from the hearth,

(d) injecting a gaseous reductant into the lower region adjacent the hearth, and

(e) moving the hearth containing iron oxide bearing material and carbonaceous material in the furnace housing through the drying/preheat zone to dry and preheat the iron oxide bearing material and carbonaceous material, then through the lower region of the conversion zone to heat the iron oxide bearing material and carbonaceous material in the presence of the injected gaseous reductant to at least partially reduce the iron oxide bearing material, then through the fusion zone to fuse the reduced iron oxide bearing material to metallic iron material, and then through the cooling zone to cool the metallic iron material.

[0010] The carbonaceous material may be selected from the group consisting of coke, char and other carbon containing materials.

[0011] The gaseous reductant may be selected from the group consisting of carbon monoxide, hydrogen, combustion gases, natural gas, or mixtures thereof.

[0012] The hood may be within 8" (200 mm) or 6" (150 mm) of the reducible mixture. The hood maybe comprised of a plurality of spaced apart pipes.

[0013] The reductant gas injectors may be spaced close to the upper surface of the material, but not so close the injected gas directly impinges on the surface of the reducible material. If the gas injectors are further spaced from the reducible material, more injected gas is indicated to be used to achieve the same effect in reduction. The injectors should be spaced to provide efficient reduction of the reducible material. The injectors may be apertures in pipes used to form the hood.

[0014] The method of reducing iron ore and other iron oxide sources may be further comprised of forming a hearth layer of carbonaceous material on the movable hearth, on which mixtures of the iron oxide bearing material and the carbonaceous material is positioned in preformed or in situ formed discrete portions.

[0015] Alternatively, the disclosed method of reducing iron ore and other iron oxide sources may be comprised of:

(a) forming a furnace housing having a drying/preheat zone with a drying/preheat atmosphere, and a conversion zone with a reducing atmosphere for reducing reducible material,

(b) providing a hearth capable of being movable within the furnace housing in a direction through the drying/preheat zone, and then the conversion zone,

(c) providing a hood within at least a portion of the conversion zone separating the conversion zone into an upper region and a lower region with the lower region adjacent the hearth and the upper region adjacent the lower region and spaced from the hearth,

(d) injecting a gaseous reductant into the lower region adjacent the hearth, and

(e) moving the hearth containing iron oxide bearing material and carbonaceous material in the furnace housing through the drying/preheat zone to dry and preheat the iron oxide bearing and carbonaceous material, and then through the lower region of the conversion zone to heat the iron oxide bearing material and carbonaceous material in the presence of the injected gaseous reductant, and at least partially reduce the iron oxide bearing material.

[0016] The method of producing metallic iron material may involve movement of the hearth linearly at least through the reducing region of the conversion zone. Alternatively, the method of producing metallic iron material may involve the movement of the hearth in a rotary furnace.

[0017] Also disclosed in a method of reducing iron oxide to produce metallic iron material, the steps comprising:

(a) providing a furnace having a moving hearth and a hood dividing a part of the atmosphere of the furnace into a lower region adjacent the moving hearth and an upper region adjacent the lower region and spaced from the moving hearth,

(b) placing at least a reducible layer of mixed iron oxide and carbonaceous material on the moving hearth,

(c) moving the reducible layer of mixed iron oxide and carbonaceous material on the moving hearth into the lower region,

(d) injecting a gaseous reductant into the lower region adjacent the hearth, and

(e) heating the reducible layer of mixed iron oxide and carbonaceous material in the presence of the injected gaseous reductant.

[0018] The gaseous reductant may be injected at a point where the temperature near the hearth is about 2280 ⁰ F (1250 ⁰ C), or may be injected at a point where the temperature near the hearth is about 2100 ⁰ F (1150 ⁰ C).

[0019] In the method of producing metallic iron material, the movement of the hearth may be linear at least through the reducing region of the conversion zone. Alternatively, the movement of the hearth is rotary through the furnace.

[0020] In the method for reducing iron oxide to produce metallic iron material may further comprise the step of placing a layer of carbonaceous material on the moving hearth below the reducible layer containing a mixture of iron oxide and carbonaceous material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is an elevation view illustrating a hearth furnace for producing metallic iron material and a method for producing same; and

[0022] FIG. 2 is an elevation view illustrating an alternative embodiment of a hearth furnace for producing metallic iron material, and method for operation for the same.

DETAILED DESCRIPTION OF THE DRAWINGS

[0023] Referring to FIG. 1, a hearth furnace 10 for producing metallic iron material directly from iron ore and other iron oxide sources is shown. The furnace 10 has a furnace housing 11 internally lined with a refractory material suitable to withstand the temperatures involved in the metallic reduction process carried out in the furnace. The hearth furnace 10 is divided into a drying/preheat zone 12 capable of providing a drying/preheat atmosphere for reducible material, a conversion zone 13 capable of providing a reducing atmosphere for reducible material, a fusion zone 14 capable of providing an atmosphere to at least partially form metallic iron material, and optionally a cooling zone 15 capable of providing a cooling atmosphere for reduced material containing metallic iron material. The conversion zone 13 is positioned

between the drying/preheat zone 12 and the fusion zone 14. The conversion zone 13 is the zone in which at least the initial reduction of metallic iron material occurs. The entry end of the hearth furnace 10, at the drying/preheat zone 12, is closed by a restricting baffle 19 that inhibits fluid flow between the outside ambient atmosphere and the atmosphere of the drying/preheat zone 12, yet provides clearance so as not to inhibit the movement of reducible material into the furnace housing 11. The baffle 19 may be made of suitable refractory material or a metal material if the temperatures are sufficiently low. The exit end of the hearth furnace 10, at the cooling zone 15, is closed by a restricting baffle 65 that inhibits fluid flow between the outside ambient atmosphere and the atmosphere of the cooling zone 15, yet provides clearance so as not to inhibit the movement of reducible material out of the furnace housing 11. The baffle 65 may be made of a suitable refractory material or a metal material if the temperatures are sufficiently low.

[0024] Hearth 20, provided within the furnace housing 11 , is comprised of a series of movable hearth cars 21, which are positioned contiguously end to end as they move through the furnace housing 11. Hearth cars 21 are moved on wheels 22 which typically engage railroad rails 23. The upper portion of the hearth cars 21 are lined with a refractory material suitable to withstand the temperatures for reduction of the iron oxide bearing material into metallic iron as explained herein. The hearth cars are positioned contiguously end to end to move through the furnace housing 11 , so that the lower portions of the hearth cars are not damaged by the heat generated in the furnace as the process of reducing iron oxide-bearing material into metallic iron proceeds.

[0025] The reducible material is positioned on the hearth cars 21 outside the furnace generally in the form of a mixture of finely divided iron ore, or other iron oxide bearing material, and a carbonaceous material, such as coke, char, anthracite coal or non-caking bituminous and sub- bituminous coal. The reducible material is in mixtures of finely divided iron oxide-bearing material that are formed into compacts. The compacts may be briquettes or mounds preformed or formed in situ on the hearth cars 21 so that the mixtures of reducible material are presented to the furnace 10 in discrete portions. Also, a hearth layer of finely divided carbonaceous material, such as coke, char or coal, may be provided on the hearth cars with the reducible material positioned on the hearth layer, to avoid damage to the refractory material used in the upper portion of the hearth cars 21 from the related slag generated on reducing the metallic iron in the furnace.

[0026] The hearth furnace may be linear as generally illustrated in FIG. 1. In this connection, the building in which the furnace is housed, or other considerations, may require that certain

parts of the furnace be arcuate or at angles, to accommodate these needs. For these purposes, the hearth furnace is classified as linear if a part of its length, usually the conversion zone 13, is substantially linear in the direction of travel of the hearth 20. The hearth furnace may also be rotary, in which case the hearth cars are pie-shaped or in the form of replaceable sections of a contiguous hearth.

[0027] The zones of the furnace 10 are generally characterized by the temperature reached in each zone. In the drying/preheat zone 12, moisture is generally driven off from the reducible material and the reducible material is heated to a temperature short of fiuidizing volatiles in and associated with the reducible material positioned on the hearth cars 21. The design is to reach in the drying/preheat zone a cut-off temperature in the reducible material just short of significant volatilization of carbonaceous material in and associated with the reducible material. This temperature is generally somewhere in the range of about 300-600 ⁰ F (150 - 315 ⁰ C), depending in part on the particular composition of the reducible material.

[0028] The conversion zone 13 is characterized by heating the reducible material to initiate the reduction process in forming the reducible material into metallic iron material and slag. The conversion zone 13 is generally characterized by heating the reducible material to about 1500 to 2100 ⁰ F (815 to 1150 ⁰ C), depending on the particular composition and form of reducible material.

[0029] The fusion zone 14 involves further heating the reducible material, now absent of most volatile materials and commencing to form metallic iron, to fuse the metallic iron material and separate slag. The fusion zone generally involves heating the reducible material to about 2400 to 2550 ⁰ F (1315 - 1370 ⁰ C), or higher, so that metallic iron nuggets are formed with only a low percentage of iron oxide in the metallic iron. If the process is carried out efficiently, there will also be a low percentage of iron oxide in the slag, since the process is designed to reduce very high percentage of the iron oxide in the reducible material to metallic iron.

[0030] The heating of the reducible material in the conversion zone 13 and fusion zone 14 may be done by oxy-fuel burners 16 in the side wall of the furnace housing 11 as shown in FIG. 1. The oxy-fuel burners 16 are positioned to efficiently reduce the reducible material to metallic iron material in fusion zone 14. The oxy-fuel burners 16 should be positioned to provide for efficient heat transfer and efficient reduction of the iron oxide in the reducible material with the least energy consumption. The oxy-fuel burners 16 may be positioned on about 10 foot centers (about 3m), staggered along opposite side walls, about a foot down from the roof 17 of the furnace housing 11. Alternatively, or in addition, the oxy-fuel burners 16 may be positioned

opposite each other in the side walls and/or in the roof 17 of the furnace housing 11. In addition, oxygen lances 29 may be positioned in the roof 17 of the furnace housing 11 of the conversion zone 13 and the fusion zone 14 to provide additional energy for generation of heat and efficient conversion of the reducible material in the furnace. Combustion gases are exhausted via an exhaust conduit 41. FIG. 1 shows an exemplary placement of exhaust conduit 41. Depending on desired operating conditions, exhaust conduit 41 may be placed elsewhere in conversion zone 13, may be placed in fusion zone 14, or may be placed in drying/preheat zone 12. There may be a single exhaust conduit 41, or there may be multiple exhaust conduits placed in diverse locations within furnace 10.

[0031] Cooling zone 15 cools the metallic iron material from its formation temperature in the conversion zone 13 and fusion zone 14 to a temperature at which the metallic iron material can be reasonably handled and further processed. This temperature is generally about 500 ⁰ F (260 ⁰ C) or below. The cooling can be achieved by injection of nitrogen through nozzles 27 in the roofs and/or side walls of the furnace housing 11 and/or indirect water cooling. Also, water spray may be used for the cooling in the cooling zone 15, if desired and provision made for water handling within the system.

[0032] Shown in FIG. 1, a hood or barrier 30 is positioned in the conversion zone 13. In one embodiment, the hood 30 may be comprised of spaced pipes 33, e.g., 2 foot on center (about .6m), positioned transverse between the furnace side walls, and supporting a plate or grate 34 as shown in FIG. 1. The plate or grate 34 may be a ceramic, silicon carbide, or refractory material. In hood 30, there may also be provided intermediately along its length and at its end gaps 35. The hood 30 may be of a heat conductive material capable of conducting the heat generated in the region above the hood to the region below the hood to reduce the reducible material positioned on the hearth 20, or heat radiating material capable of absorbing heat from combustion in the region above the hood and radiating heat into the region below the hood to reduce the reducible material, or both. The hood may be made of silicon carbide or other such higher heat conductive refractory material.

[0033] Alternatively, or in addition to gaps 35, the hood 30 may be perforated, as with a grate for example, or otherwise discontinuous to allow for controlled flow of fluidized material from the region below the hood into the region above the hood.

[0034] The hood 30 impedes direct impingement of combustion gases with the reducible material on the hearth 20 and impedes reaction of furnace combustion gases with the reducible material.

[0035] Reductant injectors are provided to inject gaseous reductants under the hood 30 to react with the reducible materials to accelerate the reduction of iron oxide and supplement the reduction potential provided by solid reductants such as coal, coke, coke breeze, or coal char that have been mixed with the iron oxide materials.

[0036] The gaseous reductant may be selected from the group consisting of carbon monoxide, hydrogen, combustion gases, natural gas, or mixtures thereof.

[0037] The reductant injectors may be positioned close to the upper surface of the reducible materials on the moving hearth 20 to provide for efficient reduction of the reducible material. For example the reductant injectors may be placed within 8" (about 200 mm) or 6" (about 150 mm) of the materials on the hearth 20, or may be placed within 2" or 3" (about 50 mm or about 75 mm) of the materials on the hearth 20. In the embodiment shown in FIG. 1, the reductant injectors may be apertures in pipes 33 spaced along the length of the pipe as the pipe extends from adjacent one side wall of the furnace to adjacent the opposite side wall of the furnace. The apertures may be directed upstream, directed downstream, or directed up or down. The reductant injector may inject the gaseous reductant at any angle. The injection points and direction may be selected to reduce direct impingement on the injected gaseous reductant onto the materials on the hearth 20 and/or to promote mixing of the injected gaseous reductant with the atmosphere below the hood 30.

[0038] The injected reductant gases can include carbon monoxide, natural gas, hydrogen, effluent gases or mixtures of any of these gases. Where the furnace temperatures are lower, i.e., below about 2000 ⁰ F (1100 ⁰ C), carbon monoxide and hydrogen gas are better choices than natural gas.

[0039] Where the hood 30 is comprised of a plurality of close spaced pipes 33, the pipes may have apertures such that any reductant gases introduced into the pipes 33 will flow out of the pipes to the area below the hood 30. Depending upon the spacing of the pipes 33, every pipe may include apertures or only certain pipes may include apertures. Conduits, injectors, or other devices may also be provided to effect the injection of reductant gases below the hood 30.

[0040] In one particular instance, the hood 30 can be installed towards the feed end of the furnace where the temperatures are relatively low, i.e., 2280 ⁰ F (1250 ⁰ C). The injection of a gaseous reductant at this point in the process will allow addition of less than stoichiometric amounts of solid reductant in the reducible materials, if desired, and also allows the carryover of some solid carbonaceous material into the fusion zone of the furnace where reduction of the iron

oxides is typically completed, and provides carbon to be absorbed by the metallic iron formed, which will reduce the melting point of the metallic iron and facilitate separation of the metallic iron and slag on the hearth.

[0041] To provide for control of the flow of fluids in the furnace 10, a first baffle 40 is provided between drying/preheat zone 12 and conversion zone 13. This first baffle 40 is capable of inhibiting direct fluid communication between the atmosphere of the conversion zone 13 and the atmosphere of the drying/preheat zone 12. First baffle 40 may be made of a suitable refractory material, such as silicon carbide, and may extend downwardly to within a few inches of the reducible material on the hearth 20. The design is to provide for efficient inhibiting of direct fluid communication between the conversion zone 13 and the drying/preheat zone 12 in the furnace 10, without interfering with movement of reducible material on hearth 20 through furnace housing 11. Also, horizontal baffle 49 of refractory material may be extended from first baffle 40 into the drying/preheat zone 12 to further facilitate flow of fluid through the drying/preheat zone 12.

[0042] A second baffle 50 is provided either between conversion zone 13 and fusion zone 14 or part way into fusion zone 14. Second baffle 50 is capable of inhibiting direct fluid communication between the atmosphere of the part of the fusion zone 14 downstream of the baffle to the atmosphere of the conversion zone 13. The second baffle 50 may be a refractory material, such as silicon carbide, and extend to within a few inches of the reducible material positioned on the hearth 20 as it moves through the furnace housing 11, to effectively inhibit the direct fluid communication across the second baffle 50. A horizontal baffle 53 of refractory material may extend from second baffle 50 downstream into the fusion zone 14 to facilitate the counter current flow of fluid through the fusion zone and avoid turbulence in the vicinity of the reducible material as it passes under second baffle 50.

[0043] The cooling zone 15 is optional, since it may be desired in certain embodiments to perform the cooling of the metallic iron material outside the furnace housing 11 to reduce furnace costs and other considerations. Alternatively, a third baffle 60 may be provided between the fusion zone 14 and the cooling zone 15. Third baffle 60 is capable of inhibiting direct fluid communication between the atmosphere of at least part of the cooling zone 15 and the atmosphere of the fusion zone 14. The third baffle 60 may be made of a refractory material, such as silicon carbide, and may extend to within a few inches of the reducible material positioned on the hearth 20 as reducible material moves through the furnace housing 11. A

horizontal baffle 63 extends from third baffle 60 to efficiently direct the flow of fluid through the cooling zone 15.

[0044] FlG. 1 shows a first embodiment of hood 30. Hood 30 extends from adjacent first baffle 40 through the conversion zone 13 towards second baffle 50. One or more gaps 35 may be formed in hood 30, as needed, to facilitate the flow of gases through furnace 10. Hood 30 may extend throughout the entire conversion zone 13 or beyond, may extend over only a portion of conversion zone 13, may extend from the beginning of the conversion zone 13 towards second baffle 50 with a gap 35 between hood 30 and second baffle 50, or may cover a latter portion of conversion zone 13 with a gap 35 between first baffle 40 and hood 30. Second baffle 50 may also be placed within the fusion zone 14, such that hood 30 extends from a beginning location in conversion zone 13 to an ending location in fusion zone 14. The hood 30 may be positioned within 8" (about 200 mm) or 6" (about 150 mm) of the materials on the hearth 20, or may be placed within 2" or 3" (about 50 mm or about 75 mm) of the materials on the hearth 20. In any of these configurations, depending on expected furnace conditions, reductant gases may be injected under the entire hood 30 or only under selected portions of hood 30. Provisions may be made in the baffles, or by providing gas conduits, to allow gases to flow between various portions of the furnace as needed for control of pressure, flow of exhaust gases, etc.

[0045] FIG. 2 shows an alternate embodiment of hood 30. Hood 30 extends from a location partway through the conversion zone 13 to second baffle 50. In this alternate embodiment, second baffle 50 can be located at the boundary between conversion zone 13 and the fusion zone 14. Second baffle 50 may also be placed within the fusion zone 14, such that hood 30 extends from a beginning location in conversion zone 13 to an ending location in fusion zone 14. A weir wall 37 may be provided at an upstream end of hood 30. The exhaust conduit 41 may be positioned within the space defined by hood 30, weir wall 37 and second baffle 50. In this alternate embodiment, oxygen lances 29 in the conversion zone 13 might only be provided in the space above hood 30. Hood 30, along with the point of reductant gas injection, may be positioned based upon expected reducible material temperatures to make advantageous use of the injected reductant gases. In this embodiment, hood 30 may, for example, be positioned within 8" (about 200 mm) or 6" (about 150 mm) of the materials on the hearth 20, or may be placed within 2" or 3" (about 50 mm or about 75 mm) of the materials on the hearth 20.

[0046] The position of hood 30 in FIG. 2 illustrates an estimated position where the reducible material has been heated sufficiently in the first part of the conversion zone 13 to bring the reducible material on the hearth 20 up to nearly 2100 ⁰ F (1150 ⁰ C) before it passes under the

hood 30. Injection of the reductant gas under the hood 30 after this point should provide the highly reducing atmosphere needed to produce metallic iron at a temperature low enough to inhibit reaction of the FeO formed with a siliceous gangue material.

[0047] While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described, and that all changes and modifications that come within the spirit of the invention described by the following claims are desired to be protected. Additional features of the invention will become apparent to those skilled in the art upon consideration of the description. Modifications may be made without departing from the spirit and scope of the invention.