This invention relates to reduction of iron bearing materials such as iron ore to metallic iron nodules (known as “NRI”).
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. Such fusion reduction processes generally involve the following processing steps: feed preparation, drying, preheating, reduction, fusion/melting, cooling, product discharge, and metallic iron/slag product separation. These process steps result in direct reduction of iron bearing material to metallic iron nodules (NRI) and slag. Metallic iron nodules produced by these direct reduction processes are characterized by near total reduction, approaching 100% metal (e.g., about 96% or more metallic Fe). Percents (%) herein are percents by weight unless otherwise stated.
Unlike conventional direct reduced iron (DRI) product, the metallic iron nodule (NRI) product has little or no gangue and little or no porosity. NRI is substantially metallic iron product desirable for many applications, such as use in place of scrap in steelmaking by electric arc furnaces. Such metallic iron nodules may be made by processing beneficiated taconite iron ore, which may contain 30% oxygen and 5% gangue. In addition to advantages of the NRI product, there is less bulk to transport than with beneficiated taconite pellets or DRI, as well as a lower rate of oxidation and a lower porosity than DRI. In addition, generally, such metallic iron nodules are just as easy to handle as taconite pellets and DRI. Moreover, NRI is a more efficient and effective substitute for scrap in steel making by electric arc furnace (EAF) without extending heat times and increasing energy cost in making steel.
Various types of hearth furnaces have been described and used for direct reduction of metallic iron nodules (NRI). One type of hearth furnace used to make NRI is a rotary hearth furnace (RHF). The rotary hearth furnace is partitioned annularly into a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the supply location and the discharge location of the furnace. An annular hearth is supported rotationally in the furnace to move from zone to zone carrying reducible material the successive zones. In operation, the reducible material comprises a mixture of iron ore or other iron oxide source and reducing material such as carbonaceous material, which is charged onto the annular hearth and initially subject to the drying/preheat zone. After drying and preheating, the reducible material is moved by the rotating annular hearth to the reduction zone where the iron ore is reduced in the presence of the reducing material, and then to the fusion zone where the reduced reducible material is fused into metallic iron nodules, using one or more heating sources (e.g., natural gas burners). The reduced and fused NRI product, after completion of the reduction process, is cooled on the moving annular hearth in the cooling zone to prevent reoxidation and facilitate discharge from the furnace. Another type of furnace used for making NRI is the linear hearth furnace such as described in U.S. Pat. No. 7,413,592, where similarly prepared mixtures of reducible material are moved on moving hearth sections or cars through a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the charging end and discharging end of a linear furnace while being heated above the melting point of iron.
A limitation of these methods and systems of making metallic iron nodules has been their energy efficiency. The iron oxide bearing material and associated carbonaceous material generally had to be heated in a reduction furnace to about 2500° F. (about 1370° C.), or higher, to reduce the iron oxide and produce metallic iron nodules. The furnace generally required natural gas methane or propane to be burned to produce the heat necessary to heat the iron oxide bearing material and associated carbonaceous material to the high temperatures to reduce the iron oxide and produce a metallic iron material. Furthermore, the reduction process involved production of volatiles in the furnace that had to be removed from the furnace and secondarily combusted to avoid an environmental hazard, which added to the energy needs to perform the iron reduction. See, e.g., U.S. Pat. No. 6,390,810.
In the past, furnace systems for production of iron nodules heated by oxy-fuel burners had reduced efficiency due to loss of heat through the exhaust stack. Recovery of heat through preheating of oxygen and fuel entering the oxy-fuel burners has not been possible as oxygen gas and fuel sources contain too little mass to efficiently transfer heat from one location in the furnace to another, and tend to be more volatile when heated. Additionally, the oxy-fuel burners have produced flame temperatures resulting in internal furnace temperatures causing damage to the burner and the furnace refractory. We have found a method and reduction furnace system for making metallic iron nodules that reduces the energy consumption needed to reduce the iron oxide bearing material to produce metallic iron nodules more efficiently.
A method of production of metallic iron nodules is disclosed comprising the steps of assembling a hearth furnace having a moveable hearth comprising refractory material and having at least a conversion zone and a fusion zone, providing a hearth material layer comprising at least carbonaceous material on the refractory material, providing a layer of reducible material comprising a mixture of at least reducing material and reducible iron bearing material arranged in a plurality of discrete portions over at least a portion of the hearth material layer, delivering oxygen gas into the hearth furnace in the conversion and fusion zones to at a ratio of at least 0.7:1 pounds of oxygen to pounds of iron in the reducible iron bearing material to heat the conversion zone to a temperature sufficient to at least partially reduce the reducible material and to heat the fusion zone to a temperature sufficient to at least partially reduce the reducible material, and heating the layer of reducible material in the fusion zone to form from the discrete portions one or more metallic iron nodules and slag. As used herein, the ratio of pounds of oxygen gas to pounds of iron in the reducible iron bearing material is based on the overall amount of oxygen gas delivered to the conversion and fusion zones of the furnace, and the ratio of pounds of oxygen gas to pounds of iron in the reducible material may be more or less than the overall ratio in any particular location along the length of the conversion and fusion zones of the furnace as described below.
Alternatively, the ratio of pounds of oxygen to pounds of iron in the reducible material may be at least 0.8:1, 0.9:1, 1:1, 1.2:1, 1.5:1, or 1.7:1 based on oxygen delivered to the conversion and fusion zones of the furnace. The oxygen gas may be delivered to the conversion zone and the fusion zone through one or more oxygen lances or oxy-burners. The oxygen gas may be delivered through oxygen lances from a position less than 18 inches from the top of the interior of the hearth furnace and alternately or in addition through the oxy-burners positioned in the walls of the furnace housing in the conversion zone and the fusion zone.
The step of providing a layer of reducible material may include discrete portions being pre-formed briquettes or balls, or compacts made in situ.
The present method permits metallic iron nodules to be produced with little, if any, additional fuel such as natural gas, methane or propane after start-up of the furnace. The carbonaceous material in and surrounding the reducible material may be the only additional fuel source. In the method, the conversion zone may be heated to a temperature of at least 2350° F. (about 1290° C.), and the fusion zone may be heated to the temperature of at least 2550° F. (about 1400° C.). Additionally, a drying zone may be provided within or adjacent the hearth furnace, and the drying zone may be heated to a temperature between about 200-400° F. (about 90-200° C.). The hearth furnace may also include a cooling zone and/or a cooling zone outside the furnace downstream of the hearth furnace.
The method of production of metallic iron nodules may utilize a linear hearth furnace or a rotary hearth furnace.
The present method of making metallic iron nodules may include the additional step of providing an overlayer of coarse carbonaceous material over at least a portion of the layer of reducible material either before introduction into the furnace as described in PCT/US2007/074471, filed Jul. 26, 2007, or adjacent introduction of the heated reducible material to the fusion zone as described in Ser. No. 12/569,176, filed on Sep. 29, 2009, with this application. The coarse carbonaceous material is greater than 6 mesh in size and may have an average particle size greater than an average particle size of the hearth material layer carbonaceous material. The coarse carbonaceous material may be between 6 mesh and ½ inch in size.
If desired, the oxygen gas may be delivered to the conversion zone and the fusion zone through one or more oxygen lances such that the oxygen gas flow avoids impinging upon the coarse carbonaceous layer.
A stoichiometric amount of reducing material is the amount necessary for complete metallization and formation of metallic iron nodules from a predetermined quantity of reducible iron bearing material. At least a portion of the reducible material has a predetermined quantity of reducible iron bearing material and between about 80 percent and about 110 percent of the stoichiometric amount of reducing material necessary for complete iron reduction of the reducible iron bearing material, or metallization, where the iron bearing material includes waste material such as mill scale as described in U.S. Provisional Patent Application 61/146,455 filed Jan. 22, 2009. Alternatively, at least a portion of the reducible material has a predetermined quantity of reducible iron bearing material and between about 70 percent and about 90 percent of the stoichiometric amount of reducing material necessary for complete iron reduction of the reducible iron bearing material where the iron bearing material is magnetite and/or hematite.
The method may include the additional steps of heating the conversion zone to at a temperature sufficient to at least partially reduce the reducible material and the fusion zone to a temperature sufficient to at least partially reduce the reducible material by the combustion of at least one of the fuels selected from the group consisting of natural gas, methane, propane, fuel oil, and coal, commencing the step of delivering oxygen gas in a ratio of at least 0.8:1 0.9:1, 1:1, 1.2:1, 1.5:1, or 1.7:1 pounds of oxygen to pounds of iron in the reducible material, and substantially reducing if not stopping supply of the fuels in the conversion and fusion zones after initiating the step of delivering oxygen gas.
Alternatively, the method of producing metallic iron nodules may comprise the steps of assembling a hearth furnace having a moveable hearth comprising refractory material and having at least a conversion zone and a fusion zone, providing a hearth material layer comprising at least carbonaceous material on the refractory material, providing a layer of reducible material comprising a mixture of at least reducing material and iron bearing material arranged in a plurality of discrete portions over at least a portion of the hearth material layer, delivering oxygen gas into the hearth furnace in the conversion and fusion zones in a quantity sufficient to heat the conversion zone to a temperature sufficient to at least partially reduce the reducible material and to heat the fusion zone to at least 2650° F. (about 1450° C.), and heating the layer of reducible material to produce from the discrete portions one or more metallic iron nodules and slag.
The reducible iron bearing material may contain at least a material selected from the group consisting of mill scale, magnetite, hematite, and combinations thereof in the proportions as described above. The reducing material may contain at least a material or mixture of materials selected from the group consisting of, anthracite coal, coke, char, bituminous coal and sub-bituminous coal.
A method of production of metallic iron nodules is disclosed comprising the steps of assembling a hearth furnace having a moveable hearth comprising refractory material and having at least a conversion zone and a fusion zone, providing a hearth material layer comprising at least carbonaceous material on the refractory material, providing a layer of reducible material comprising at least reducing material and reducible iron bearing material arranged in a plurality of discrete portions over at least a portion of the hearth material layer, delivering oxygen gas in a ratio of at least 0.8:1 pounds of oxygen to pounds of iron in the reducible material and combustible gases other than air into the hearth furnace in the conversion and fusion zones to heat the conversion zone to a temperature sufficient to at least partially reduce the reducible material and to heat the fusion zone to a temperature sufficient to at least partially reduce the reducible material and in addition to produce a stack gas having a composition of at least 20% carbon dioxide, heating the layer of reducible material to form from the discrete portions one or more metallic iron nodules and slag, and processing the stack gas to produce a gas stream having a composition of at least 90% carbon dioxide by oxidizing carbon monoxide and hydrogen in the stack gas, treating the gas stream to remove at least one of sulfur-containing and halogen-containing compounds, and condensing any water vapor present in the gas stream.
The method may in addition comprise the step of capturing the carbon dioxide gas from the gas stream for use in a subsequent process. The subsequent process may be selected from a group consisting of precipitating carbonates, compressing the carbon dioxide gas to form liquid carbon dioxide and transporting the carbon dioxide gas through a pipeline to a second location.