MXPA99002982A - Process for the production of iron carbide from iron oxide using external sources of carbon monoxide - Google Patents

Abstract

The present invention is a process for the rapid conversion of iron oxide-containing material into iron carbide. The process includes a first step in which the material containing iron oxide is contacted with a reducing gas containing a high concentration of hydrogen gas, to form an intermediate product containing metallic iron, and a second step in which the Product containing metallic iron is contacted with a carburizing gas having high concentrations of hydrogen gas and carbon monoxide, to produce iron carbide. Unused carbon monoxide in the second stage outlet gas is not recycled to the second step, but is used as a fuel source.

Description

PROCESS FOR THE PRODUCTION OF IRON CARBON FROM HI OXIDE USING EXTERNAL SOURCES OF MONOXIDE OF CARBON FIELD OF THE INVENTION The present invention relates to a method for producing iron carbide from a feedstock containing iron. More specifically, the present invention utilizes a two-step process for converting iron oxide to metallic iron in the first step, and metallic iron to iron carbide in the second step, for use in the manufacture of steel.

BACKGROUND OF THE INVENTION The steel industry has depended on a process, which has been in use for many years, for the conversion of iron ore into steel. The process converts iron ore into iron ingots in a smelting furnace using coke produced in a coking oven.

The process then converts the ingot of iron or hot metal into steel in a basic oxygen furnace or Siemens-Martin. In recent years, local and federal environmental regulations have caused numerous problems for steel producers using this steelmaking process. The smelting furnace and the coking ovens used in the process not only waste a lot of energy, but also are responsible for most of the environmentally damaging emissions of the steel producers. Redesigning or modifying smelting furnaces and coking ovens to meet pollution standards is costly. Expenses would cause the cost of steel produced by the conventional steelmaking process not to be competitive with steel produced by foreign competitors. To address these problems, a process was developed for the production of steel that eliminates the smelting furnace and the coking oven in the steelmaking process. In the process, an iron oxide bed is fluidized by a simple multi-component gas stream and converted directly to a product containing iron carbide, consisting mainly of Fe3C. The iron carbide is then added to an electric arc furnace or basic oxygen to produce steel. In the process, the reduction and carburization reactions occur together in the same fluidized bed. Another process has been applied to produce acicular iron carbides having magnetic characteristics desired for use in the magnetic register and as catalysts for converting CO and H2 to lower aliphatic hydrocarbons. In the process, a bed of acicular iron oxide is reduced by a gas and a bed of the reduced product is then carburized by another gas, to produce acicular iron carbides of the Fe 5 C 2 form. The process suffers from slow reaction kinetics, large amounts of impurities (including iron oxide, free carbon, and metallic iron) in the acicular iron carbide product, and poor gas efficiency (ie, poor use of reactants in the gas ). The product, Fe5C2, is quite unstable and requires more carbon reactant to form Fe3C (and therefore, more expensive to produce). Other techniques for converting an iron-containing feedstock into a product containing iron carbide require expensive components, suffer from poor gas efficiency, and / or give rise to other operational complications. It would be advantageous to provide a process for converting iron-containing materials into iron carbide having a high gas utilization. Furthermore, it would be advantageous to produce an iron carbide product with environmentally friendly and / or non-hazardous byproducts. It would also be an advantage to optimize the reaction kinetics of the chemical reactions to convert iron-containing materials to iron carbide and produce an iron carbide product having high purity and low residual iron oxide. Additionally, it would be advantageous to develop an environmentally friendly, energy efficient and inexpensive process to produce steel. In addition, it would be advantageous to convert, in a non-costly and efficient manner, iron-containing materials into iron carbide for use in the production of steel. In addition, it would be an advantage to eliminate the melting furnace and the coking oven from the steelmaking process.

BRIEF DESCRIPTION OF THE INVENTION According to one embodiment of the present invention, a two-step process for producing iron carbide from a feedstock containing iron oxide is provided. As used herein, "iron carbide" preferably includes Fe2C and Fe3C, and "iron oxide" preferably includes FeO, Fe2O3 and Fe3O. In the first step (reduction), a feed material containing iron oxide is converted to an intermediate product by contacting the feed material with a reducing gas, to reduce the iron oxide to metallic iron, and in a second step (carburization), the metallic iron becomes a product of iron carbide. The reducing gas preferably contains sufficient hydrogen gas, the primary reducing agent, to substantially carry out the complete reduction of the iron oxides in the metallic iron feed material. Normally, the step reducer is a process of closed circuit, so that virtually all the reducing reagent is used by the process to remove oxygen from the feed material. Preferably, the predominant component of the reducing gas is hydrogen gas, and more preferably the reducing gas contains at least about 80 mol% hydrogen gas. Water, the byproduct of the reduction reaction, is easily removed from the first stage outlet gas by suitable techniques. At least the majority of the iron in the intermediate product is in the form of metallic iron. Preferably, at least about 70 and more preferably at least about 90 mol% of the iron in the intermediate product is in the form of metallic iron. The intermediate product typically contains no more than about 35 percent mole of iron carbide, more usually no more than about 25 percent mole, and more usually no more than about 10 percent mole of iron carbide. It is preferred that the iron oxide is at least about 90 mole percent of the feed material in the first step in a water-free base. Preferably, a substantial portion, and more preferably at least the majority, of the iron oxide in the feedstock is converted to metallic iron in the first step (reduction). The presence of iron oxides in the intermediate product is not desired since the iron oxide can decrease the reaction kinetics in the carburization step and prolong the necessary residence time of the material in the carburization step for a desired degree of carburization . In the carburization step, the intermediate product is contacted with a carburizing gas to produce an iron carbide product. The carburizing gas includes carbon monoxide and hydrogen gas. Preferably, the carburizing gas contains at least about 5 and more preferably at least about 1.5 mole% carbon monoxide and at least about 80 mole% hydrogen gas. The carburizing gas may also include other components, such as carbon dioxide, methane, water vapor and a diluent, such as nitrogen or other inert gas. Preferably, the carburizing gas does not include more than about 5, more preferably not more than about 3, and more preferably not more than about 1 mole% of carbon dioxide; preferably not more than about 1.5, more preferably not more than about 10, and more preferably not more than about 5 mol% methane; preferably not more than about 10, more preferably not more than about 1, and more preferably not more than about 0.5 mole% of water vapor; and no more than about 10 mol% of inert gases. As will be appreciated, the temperatures of the carburizing gas and the bed of the intermediate product during carburization are important for the reaction kinetics. Preferably, the carburizing gas has a gas temperature of at least about 550 ° C and the intermediate product a bed temperature of at least about 500 ° C. Due to the high concentration of carbon monoxide in the carburizing gas and the fact that carbon monoxide directly converts metallic iron into iron carbide, less gas is required for complete carburization than in two-step processes and speed of the carburization reaction is relatively high. Compared with other two-step processes, the process of the present invention requires components of lower capacity for a given performance of feedstock. Network capacity components significantly reduce capital and operating costs and water consumption. At least the majority of the carbon monoxide in the carburizing gas is passed through the intermediate product only once (ie, the carburization step is preferably an open circuit, while the reduction step is preference, a closed circuit). Stated another way, at least most of the unreacted carbon monoxide in the carburizing gas is not recycled to the second step. Preferably, at least about 30%, more preferably at least about 50%, and most preferably at least about 65% of the carbon monoxide in the carburizing gas is reacted with the intermediate product in the simple contact of the carburization gas with the intermediate product. Not more than about 30, more preferably not more than about 20, and more preferably no more than about 10% by volume of the carburizing exit gas is recycled to the carburization step, because if too much of the exhaust gas is recycled, Methane will form in the carburizing gas and dilute the concentration of carbon monoxide to relatively low levels. The use of the carbon monoxide in the carburizing gas only for a simple passage through the intermediate product is made at least in part economical, by using the second stage outlet gas as a fuel source in other steps of the process, for example, the heating of the feed material and / or the reduction and / or carburization gases before contacting the gases with the feed material and the intermediate product, respectively. Preferably, at least about 80%, more preferably at least about 90%, and more preferably all of the carbon monoxide in the carburization outlet gage is used as a fuel source in one or more steps of the process. Normally, these preferred percentages of carbon monoxide do not represent more than about 50, more usually not more than about 40, and more usually not more than about 30% of the carbon monoxide in the carburizing gas. Lower fuel costs offset the higher expense associated with forming more carburizing gas. Additionally, the hydrogen gas can be separated from the exhaust gas from the carburization step to reconstitute the reducing gas in the reduction step. At least the majority of the iron carbide product is preferably iron carbide. It is desired that at least about 90 mole percent, and more preferably at least about 95 mole percent of the iron carbide be in the Fe3C form. Fe2C is not desired since, unlike Fe3C, it is highly reactive and will oxidize on air exposure. Preferably, the iron carbide product contains no more than about 25 and more preferably no more than about 5 percent mol of im purities, including metallic iron, free carbon and iron oxide. Impurities, such as metallic iron, free carbon, and iron oxide, can cause problems if the iron carbide product is converted to steel and the steel is processed into useful items. As an example, the metallic iron in the iron carbide product can be oxidized to form iron oxides, which create difficulties in converting the iron carbide product into steel. The high level of product purity is made possible by monitoring the composition of the carburizing gas. Since methane is normally absent from the carburizing gas, it is possible to have iron carbide react again to form iron and methane. When or at some time before sufficient iron in the intermediate product has formed iron carbide, to allow the iron carbide to react again to form methane and metallic iron, the iron carbide product is removed from the reactor. In one embodiment, the iron carbide product is fed directly to an appropriate reactor for conversion into steel. In one embodiment of the present invention, the process is a continuous process. Preferably, the two steps of the process are conducted in separate reaction zones to facilitate the continuity of the process. Preferably, in one or both steps of the process, the reaction zone is a fluidized bed. The present invention can have numerous advantages over existing methods in addition to those advantages discussed above. One embodiment of the present invention advantageously provides a continuous process for converting iron-containing materials into iron carbide. The present invention thus avoids the increase in operating expenses associated with batch processes. Another embodiment of the present invention advantageously provides a process with rapid reaction kinetics. The composition of each gas can be selected to optimize the kinetics of the reaction at each step of the process. The reaction conditions, such as pressure, temperature and time, can also be selected to optimize the kinetics of each reaction.

Another embodiment of the present invention advantageously provides a process that produces a high purity iron carbide product. The iron carbide product is substantially free of impurities, including free carbon, iron oxide and metallic iron. Another embodiment of the present invention conveniently produces byproducts that are environmentally friendly and non-hazardous. The main by-product is water vapor. Another embodiment of the present invention advantageously provides an environmentally friendly, energy efficient, and inexpensive process for making steel. The process eliminates the melting furnace and the coking oven by direct conversion of iron-containing materials to iron carbide, followed by steel production.

BRIEF DESCRIPTION OF THE DIAMETERS Fig. 1 is a flow diagram of one embodiment of a process according to the present invention, illustrating the approach of two steps for converting an iron-containing feedstock into a carbide containing product. of iron.

DETAILED DESCRIPTION DETAILED Referring to Fig. 1, in one embodiment of the present invention, a feed material containing iron oxide 1 0 is transported from a storage container 12 first by a conveyor 1 4 and then by entrainment in an oxidation gas through first , second and third of particle separators (for example, cyclones) 1 8a-ca a hopper secured 1 9 and finally to a reduction reactor 22. The feed material containing 1 0 iron oxide can be selected from a wide range of iron-containing materials, including iron ores and iron ore concentrates. Preferably, the feedstock is one of or a mixture of several iron oxides, including magnetite (Fe3O4), hematite (Fe2O3), limonite (Fe2O3 • H2O), FeO and iron hydroxides, including Fe (OH) 2 and Fe (OH) 3 More preferably, the feedstock is a mixture of iron oxides and more preferably the feedstock contains at least about 90 mole percent of iron oxides in a water-free base. In some applications, such as iron ores and iron ore concentrates, the 1 0 feedstock may include small amounts of water. To oxidize the iron oxides and remove free water and other contaminants from the feed material 1 0, a slit 26 of air or other oxidizing gas is passed through a heat exchanger 30 and a burner 34 where it is heated to a temperature preferred ranging from about 700 to about 1,000 ° C. The heated air 38 is contacted with the feed material 10 in the particle separators 1 8a-c to preheat the material to a preferred temperature ranging from about 150 to about 800 ° C. After heating the feed material 10, the air is passed through a baghouse or other filtration unit 42 from which it is vented to the atmosphere.

The feed material 10 is preferably heated in an oxidizing atmosphere (for example, air) at a temperature sufficient to oxidize the iron oxide and other constituents of the feed material 1 0. This step increases the production of iron carbide by converting magnetite to hematite and sulfur sulfur to sulfur dioxide gas or more stable sulfur compounds (eg, thermally stable sulphates formed by the reaction of oxidized sulfur sulfur with alkali and alkaline earth oxides) and by removing or removing water as steam from Water. The conversion of magnetite to hematite increases the production of iron carbide, since hematite is more readily reduced to metallic iron in reactor 22 than magnetite. The unheated feed material 1 0 preferably contains at least about 70, more preferably at least about 85, and more preferably at least about 98 percent mole of iron oxides in a water-free base. After heating, the iron oxide constituent in the feedstock should preferably contain more than about 50, more preferably more than about 75, and more preferably more than about 95 mole percent of hematite and preferably less than about 50, more preferably less than about 25, and more preferably less than about 5 percent mole of magnetite. Preferably, at least about 50, more preferably at least about 75, and more preferably at least about 95 percent mole of the magnetite in the iron material is converted to hematite during heating. The production of iron carbide is further decreased by the presence of sulfur sulfur in the preheated feed material, because sulfur sulfur retards the conversion of metallic iron to iron carbide. The preheated feedstock should preferably contain less than about 2, more preferably less than about 1, and more preferably less than about 0.1 mole percent of sulfur sulfur. The production of iron carbide decreases due to the presence of water or water vapor, since water vapor, being a by-product of the conversion of iron oxides into metallic iron, and metallic iron into iron carbide, can impose restrictions of balance in the production of metallic iron and iron carbide. After preheating, the feedstock should preferably have less than about 2, more preferably less than about 1, and more preferably less than about 0.5 percent mole of water. The heating of the feedstock is discussed in detail in the serial codependent US patent application no. 08/1 81, 997, submitted on January 14, 1994, entitled "TWO STEP PROCESS FOR THE PRODUCTION OF I RON CARBI DE FROM I RON OXI DE" (Two-step process for the production of iron carbide from iron oxide); US serial patent application no. 08/596, 954, filed on March 31, 1998 and entitled PROCESS FOR CONVERTI NG I RON OXI DES TO I RON CARBI OF EMPLOYI NG I NTERNALLY GEN ERATED OXY CARBON OF AS THE CARBI D1 NG AGENT "(Process to convert oxides from iron to iron carbide using internally generated carbon oxide as the carburizing agent) (now issued as U.S. patent 5,733,357); and U.S. patent application Serial No. 08 / 703,981, filed August 28, 1996 and entitled "METHOD FOR PREHEATI NG FEED MATERIALS FOR THE PRODUCTION OF IRON CARBI DE "(Method for preheating feed materials for the production of iron carbide), each of which is incorporated herein by reference. contact with a reducing gas 48 in the reduction reactor 22 to produce a reducing outlet gas 50 and an intermediate 54. The reducing gas 48 contains hydrogen gas as the reducing agent to reduce the iron oxides in the metal iron feed material. Although not wishing to be bound by any theory, it is believed that hydrogen gas reduces oxides and hydroxides from iron to metallic iron according to one or more of the following equations: (1) Fe203 + 3 H2? 2 Fe + 3 H2O (2) Fe304 + 4 H2? 3 Fe + 4 H2O (3) Fe (OH) 2 + H2? Fe + 2 H2O (4) 2 Fe (OH) 3 + 3 H2? 2 Fe + 6 H20 Although carbon monoxide can also be used to reduce oxides and hydroxides from iron to metallic iron, it is preferred that the reducing gas be substantially free of carbon monoxide. It is difficult to remove carbon dioxide, the by-product of the reduction reaction, from the reducing outlet gas 50 and free carbon can be deposited in the intermediate product 54 by the reaction. Commonly, a small amount of carbon monoxide will be present in the reducing gas because, when the hydrogen gas is separated from the carburizing exhaust gas as discussed below, a small amount of carbon monoxide normally contaminates the hydrogen gas removed from the device. from separation. The reducing gas has a relatively high concentration of hydrogen gas to rapidly convert the iron oxides in the metal iron feed material. Normally, the reducing gas 48 will contain at least about 1 5 times and more usually at least about 4 times the stoichiometric amount required to convert all of the iron oxides in the metal iron feed material. Preferably, the reducing gas contains at least about 80, more preferably at least about 85, and more preferably at least about 95 mole% hydrogen gas. The reducing gas preferably does not contain more than about 5, more preferably no more than about 2, and more preferably no more than about 1 mol% of carbon oxides and not more than about 1.5, more preferably not more than about 10. , and more preferably no more than about 5 mol% methane. The reaction conditions are maintained at levels that promote a relatively high rate of conversion of the iron oxides and hydroxides into metallic iron. Preferred reactions are: (i) a temperature of the feed material in the reducing reactor ranging from about 500 to about 800 ° C and more preferably from about 550 to about 650 ° C to produce pyrophoric metallic iron; (ii) a pressure in the reducing reactor ranging from about 1 to about 6 atm; and (iii) a gas velocity of at least about 0.1 524 and more preferably at least about 1.524 m / s. The number of moles of hydrogen gas that must be contacted with each mole of iron in the feedstock to produce the desired degree of reduction of oxides and hydroxides from iron to metallic iron is, preferably, at least about 20 and more preferably at least about 10, and more preferably ranges from about 8 to about 4 moles of hydrogen gas / mol of iron. It is preferred that at least about 65, more preferably at least about 80 and more preferably at least about 95 percent mole of the iron oxides and hydroxides in the feed material be converted to metallic iron in the reduction reactor 22. The com The iron component in the intermediate product 48 is preferably at least about 65, more preferably at least about 75, and more preferably at least about 90 percent mol of metallic iron. The intermediate product 48 preferably does not contain more than about 35, more preferably not more than about 25, and more preferably not more than about 10 percent mole of iron carbide. Preferably, the intermediate product 48 contains no more than about 20 mole percent iron oxides and more preferably no more than about 10 mole percent iron oxides. The presence of iron oxides in the intermediate product 48 decreases the kinetics of the reaction when the intermediate product is subsequently converted into a carburization reactor to iron carbide. The reduction reactor 22 may be selected from a variety of reactors, including a fluidized bed reactor, rotary kiln, or a multi-furnace or shaft furnace. For the feed material having a D8o size of no more than about 1 mm, the preferred reactor is a fluidized bed reactor. As used herein, size D80 refers to the particle size in which 80% of the material is less than or equal to. For the feed material having a size D80 of at least about 10 mm, the preferred reactor is a shaft furnace. The reduction reactor 22 could be a circulating fluidized bed reactor. The term "fluidized bed reactor" as used herein refers to a fluidized bed-bubbling bed reactor. For a fluidized bed reactor, the depth of the bed of the feed material is preferably from about 0.3048 to about 3.657 meters, more preferably from about 0.6096 to about 2.4384 meters, and more preferably about 0.9144 meters. The size D80 of the feed material in the bed is preferably no more than about 2.5 mm, more preferably no more than about 2 mm, and more preferably varying from about 0.1 to about 1 mm. In order to efficiently convert the feedstock to metallic iron in a fluidized bed reactor, it is preferred that the feedstock remain in contact with the reducing gas 48 for a sufficient length of time to allow controlled formation by diffusion of metallic iron to proceed with the termination. Preferably, the residence time is from about 30 to about 1,200 minutes, more preferably from about 60 to about 300 minutes, and more preferably from about 90 to about 150 minutes. The reducing outlet gas 50 is passed through a particulate separation device, such as a cyclone, to form a treated exit gas 58 and removed particles 62. The treated exit gas 58 is passed through a heat exchanger. heat 66 and a gas purification system 60 discussed below, to produce a purified waste gas 70. The purified waste gas 70 is reconstituted with hydrogen gas 74 from a hydrogen reformer 78 to form the reducing gas 48. As will be appreciated, the hydrogen reformer 78 converts methane (or other hydrocarbon) and water into carbon monoxide and hydrogen gas. The reducing gas 48 is passed sequentially through the heat exchanger 66 and the process gas heater 82, to heat the reducing gas 48 to a preferred temperature ranging from about 500 to about 900 ° C and back to the reduction reactor 22. The process gas heater 82 uses a preheated combustion air, which is heated by a 90 heat exchanger to burn the fuel to the heater. The removed particles are recombined with the intermediate 54. The intermediate 54 is subsequently transferred from the reduction reactor 22 to a carburization reactor 1 00 and contacted with a carburizing gas 1 04 to produce a carburizing exit gas. 1 08 and an iron carbide product 1 12. The carburizing gas 1 04 is a multi-component gas stream that converts metallic iron and to a lesser degree the iron oxides into the intermediate product 54 into iron carbide.

The primary components of the carburizing gas 1 04 are hydrogen gas and carbon monoxide. While not wishing to be bound by any theory, it is believed that carburization gas 1 04 converts metallic iron to iron carbide according to one or more of the following equations: (5) 3 Fe + CO + H2? Fe3C + H2O (6) 2 Fe + CO + H2? Fe2C + H20 Carbon monoxide provides carbon to form iron carbide, while hydrogen gas converts the oxygen released into carbon monoxide into water, which is easily removed from the carburizing exhaust gas by suitable techniques.

The carburization gas contains relatively high concentrations of carbon monoxide and hydrogen gas to provide a fairly rapid rate of conversion of metallic iron into iron carbide. Preferably, the carburizing gas includes at least about 300%), more preferably at least about 50%), and more preferably at least about 20% of the stoichiometric amount of carbon monoxide required to convert all of the metallic iron into the intermediate product in iron carbide. The carburizing gas preferably contains at least about 5, more preferably at least about 10, and more usually at least about 15% mol carbon monoxide, but usually not more than about 30 mol%, more usually not more than about 25 mol%, and more usually no more than about 20 mol% carbon monoxide. The carburizing gas preferably contains at least about 50, more preferably at least about 65, and more preferably at least about 80 mol% hydrogen gas. The carburizing gas may contain a diluent, such as an inert gas. Preferably, the concentration of the diluent in the carburizing gas is not more than about 10 and more preferably not more than about 5 mol%. The conditions in the carburization reactor 1 00 are selected to provide a fairly rapid rate of conversion of metallic iron to iron carbide. The reaction conditions are: (i) a temperature of the intermediate product in the reduction reactor ranging from 450 to about 750 ° C, more preferably from about 500 to about 700 ° C, and more preferably from about 550 to about 650 ° C C, to inhibit the metallic iron in the intermediate feed material from melting or sticking; (ii) a pressure in the reduction reactor ranging from about 1 to about 6 atm; and (iii) a gas velocity of at least about 0.1524 and more preferably no more than about 1.524 m / s. The number of moles of carbon monoxide to be brought into contact with the intermediate product to produce the desired degree of conversion of metallic iron to iron carbide, preferably is not greater than about 1 and more preferably not greater than about 0.7, and more preferably ranges from about 0.35 to about 0.5 moles of carbon monoxide / mole of iron. To efficiently convert the intermediate product 54 to iron carbide, it is preferred that the intermediate 54 remain in contact with the carburizing gas 1 04 for a sufficient length of time to allow controlled diffusion formation of iron carbide to proceed to the termination. Although longer residence times may be used, the residence time in the carburization reactor 1 00 is preferably from about 60 to about 300 minutes, more preferably from about 45 to about 90 minutes, and more preferably from about 30 to about 60 minutes .

A relatively high rate of conversion of metallic iron to iron carbide is carried out in the carburization step. Preferably, at least about 80, more preferably at least about 90, and more preferably at least about 95 percent by weight of the metallic iron in intermediate 54 is converted to iron carbide in the carburization reactor 1 00. The iron carbide product 1 12 is not pyrophoric, contains a high percentage of iron, and has a high purity. The iron-containing materials in the 1 1 2 iron carbide product are preferably at least about 70, more preferably at least about 80, and more preferably at least about 90 percent mole of iron carbide. The iron carbide product 12 preferably contains less than about 10 percent mole of iron oxides, less than about 1 percent mole of free carbon, and less than about 10, and more preferably less than about 2 percent mole of carbon. metallic iron. The iron carbide product 12 may have a layer of hydrogen on its surface on the outlet of the carburization reactor 1 00. Since the catalytic combustion of hydrogen can cause the material to become hot enough to oxidize, it is desirable to treat the product of iron carbide to remove the hydrogen layer. The iron carbide product can, for example, undergo a flow of inert gas or be placed in a vacuum to remove the hydrogen. The carburization reactor 1 00 is commonly the capacity and type of reactor as the reduction reactor 22. Similarly, for a fluidized bed reactor such as the carburization reactor 100, the depth of the bed of the intermediate product 54 is substantially the same that the depth of the bed of the feedstock in the reduction reactor 22. The iron carbide product is removed from the reactor and sent to product storage facilities or sent directly to a facility for manufacturing steel. The carburizing exhaust gas has a composition that includes a number of components not necessarily present in the carburetion gas. Commonly, the carburizing exit gas includes from about 70 to about 95 mole% hydrogen gas, from about 2 to about 10 mole percent methane, from about 1 to about 5 mole percent carbon monoxide, from about 1 to about about 2% mole of carbon dioxide, and water vapor. The carburization exit gas 1 08 is passed through a cyclone 1 1 6 or other separation device of suitable particles, to produce recovered particles 120 (which can recombine with the iron carbide product 1 1 2 or return to the carburization reactor) and a treated carburization outlet gas 1 24. The treated carburization exit gas 1 24 is passed through a heat exchanger 128 and treated by the gas purification system 60 to produce an exit gas of purified carburization 1 32. Purified carburization salt gas 1 32 is preferably passed through a hydrogen gas separator 1 36 to produce a stream of hydrogen gas 140 which can be used to reconstitute the reducing gas and a stream of suppressed hydrogen-gas gas 144. Preferably, at least about 50% and more preferably at least about 75% of the hydrogen gas in the purified exhaust gas 132 it is recovered in the 1 1 0 hydrogen gas stream to be reused in the reduction step. The hydrogen gas separator is preferably a pressure swing absorber, which uses a pressurized porous substrate, such as a zeolite, to trap gases heavier than hydrogen gas in the pores. When the substrate is substantially filled with non-hydrogen gas, the exit gas deviates from the unit, while the non-hydrogen gas is released from the substrate and thereby regenerates the substrate. The carbon monoxide in the carburizing gas 1 04 only makes one passage through the intermediary product 54. The unreacted carbon monoxide n the exit gas 1 14 is subsequently used as a fuel source in other parts of the process, specifically to provide fuel for one or more of the reduction reactor 22, the gas heaters 82 and 148 and the hydrogen reformer 78. The carburization gas 1 04 is made of hydrogen gas and carbon monoxide from the hydrogen reformer 78. Preferably, at least about 50%, more preferably at least about 65% >; and more preferably at least about 75% of the carbon monoxide in the carburizing gas is produced directly by the reformer, before the carburizing gas comes into contact with the intermediate product.

The carburizing gas 1 04 is passed through the heat exchanger 128 and the process gas heater 148, and is heated to a temperature preferably varying from about 500 to about 750 ° C and more preferably from about 600 to about 650 ° C, before being introduced into the reduction reactor. The gas treatment system 60 includes packed towers 1 52a and 1 52b, washers 1 56a and 1 56b for contacting the reduction and carburization exit gases 58 and 1 24, respectively. A thickener 1 60 and a cooling tower 164 treat the water and / or steam after use in a scrubber 1 56 or a packed tower 1 52 for purification of the gas. Although various embodiments of the present invention have been described in detail, it is evident that modifications and adaptations of those modalities will occur to those skilled in the art. However, it will be expressly understood that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims.

Claims (35)

1. REVIVALATION IS 1 . A process for producing iron carbide, comprising: (a) contacting a feedstock comprising an iron oxide with a reducing gas predominantly comprising hydrogen gas, to produce a first product comprising predominantly metallic iron, wherein said first product does not contains more than about 35 percent mol of iron carbide; and (b) contacting said first product with a carburizing gas comprising hydrogen gas and carbon monoxide to form a carburization exit gas and an iron carbide product, wherein at least the majority of the carbon monoxide in the Carburizing gas is free from recycling. 2. The process, as claimed in claim 1, wherein said reducing gas comprises at least about 80 mol% of hydrogen gas. 3. The process, as claimed in claim 1, wherein said reductive gas comprises at least about 90 mol% hydrogen gas. 4. The process, as claimed in claim 1, wherein steps (a) and (b) are conducted in separate reaction zones. The process, as claimed in claim 1, wherein said contact in step (a) comprises fluidizing a bed of said first feed material with said reducing gas, and said contacting in step (b) comprises fluidizing a bed of said first product with said carburizing gas. 6. The process, as claimed in claim 1, wherein said feedstock comprises magnetite and hematite. 7. The process, as claimed in claim 1, wherein the feedstock has a temperature ranging from about 150 to about 800 ° C. 8. The process, as claimed in claim 1, wherein said feed material has a size D80 which varies from about 10 mm to about 0.1 mm. 9. The process, as claimed in claim 1, wherein the exhaust gas comprises carbon monoxide and at least the majority of the carbon monoxide is used as a source of fuel in the process. The process, as claimed in claim 1, wherein at least about 4 moles of hydrogen gas / mol of iron in the feedstock comes into contact with the feedstock. eleven . The process, as claimed in claim 1, wherein at least the majority of the carbon monoxide in the carburizing gas is produced in a reformer from water and methane.
2. 2. The process, as claimed in claim 1, wherein said first product comprises pyrophoric metallic iron.
3. 3. The process, as claimed in claim 1, wherein the carburizing gas comprises at least about 1.5 mole percent of carbon monoxide. The process, as claimed in claim 1, wherein the carburizing gas comprises an amount of hydrogen gas ranging from about 70 to about 90 mol%. 15. The process, as claimed in claim 14, wherein the carburizing gas does not comprise more than about 15 mol% methane. The process, as claimed in claim 14, wherein the carburizing gas does not comprise more than about 5% > mole of carbon dioxide. 17. The process, as claimed in claim 1, wherein the temperature of said first product in step (b) ranges from about 500 to about 800 ° C. The process, as claimed in claim 1, wherein at least about 90% of the metallic iron in the first product is converted to iron carbide in step (b). 19. The process, as claimed in claim 1, wherein no more than about 30% of the carburizing exhaust gas is recycled to the contact passage (a). The process, as claimed in claim 1, wherein at least about 2.7 moles of hydrogen gas / mol of iron in the first product is contacted with the first product. twenty-one . The process, as claimed in claim 1, wherein at least about 0.5 moles of carbon monoxide / mol of iron in the first product is contacted with the first product. 22. The process, as claimed in claim 1, wherein at least about 50% of the carbon monoxide in the carburizing gas reacts with the metallic iron to form iron carbide. 23. The process, as claimed in claim 1, further comprising: after step (b), burning at least about 30% of the carbon monoxide in the carburizing gas to provide thermal energy in the process. 24. The process, as claimed in claim 1, further comprising: after step (b), separating the hydrogen gas from the other constituents in the carburizing gas to provide the hydrogen gas for the reducing gas. 25. The process, as claimed in claim 1, wherein the product formed in step (b) contains no more than about 20 mole percent of impurities. 26. The process, as claimed in claim 1, further comprising: (c) converting the product formed in step (b) into steel. 27. A process for converting iron oxide into iron carbide, comprising: (a) contacting, in a first reaction zone, a feedstock containing iron oxide with a reducing gas comprising mainly hydrogen gas, for converting said iron oxide-containing feedstock to a product containing metallic iron; (b) transporting the metallic iron-containing product to a second reaction zone different from the first reaction zone; and (c) when the product containing metallic iron is in the second reaction zone, passing a carburizing gas comprising mainly carbon monoxide and hydrogen gas through a bed containing said product containing metallic iron to form a product that it contains iron carbide, wherein at least the majority of the carbon monoxide in the carburizing gas is passed through the bed only once. 28. The process, as claimed in claim 27, wherein said iron-containing material comprises at least 50 percent mole of iron oxide. 29. The process, as claimed in claim 27, wherein said iron oxide is at least about 75 percent mole of hematite. 30. The process, as claimed in claim 27, wherein at least 70 percent mole of said iron oxide is converted to metallic iron in step (a). 31 The process, as claimed in claim 27, wherein the contact time in step (a) between said reducing gas and said iron oxide-containing feedstock varies from about 30 to about 1, 200 m inutes. 32. The process, as claimed in claim 27, wherein said metallic iron-containing product comprises at least about 65 percent mol of metallic iron. 33. The process, as claimed in claim 27, wherein at least 90 mole percent of said iron carbide is Fe3C. 34. A process for converting iron oxide to iron carbide, comprising: (a) contacting, in a first reaction zone, a feedstock containing iron with a reducing gas comprising mainly hydrogen gas, to convert said material of feed containing iron in a product containing metallic iron, wherein the product containing metallic iron comprises at least about 70 mol% of metallic iron; (b) transporting the product containing metallic iron to a second reaction zone different from the first reaction zone; and (c) when the product containing metallic iron is in the second reaction zone, passing a carburizing gas comprising mainly carbon monoxide and hydrogen gas and having a gas temperature through a bed containing said product that it contains metallic iron to form a product containing iron carbide, wherein at least the majority of the carbon monoxide in the carburizing gas is passed through the bed no more than once and the gas temperature is greater than approximately 450 ° C. 35. A process for converting metallic iron to iron carbide, comprising: contacting a feedstock comprising at least about 70 mol% of metallic iron with a carburizing gas, comprising at least about 15 mol% carbon monoxide and at least about about 80 mol% of hydrogen gas to form a product containing iron carbide and a carburization exit gas, wherein at least the majority of the unreacted carbon monoxide in the carburization exhaust gas is consumed as a fuel source in the process. SUMMARY The present invention is a process for the rapid conversion of iron oxide-containing material into iron carbide. The process includes a first step in which the material containing iron oxide is contacted with a reducing gas containing a high concentration of hydrogen gas, to form an intermediate product containing metallic iron, and a second step in which the product containing metallic iron is contacted with a carburizing gas having high concentrations of hydrogen gas and carbon monoxide, to produce iron carbide. Unused carbon monoxide in the second stage outlet gas is not recycled to the second step, but is used as a fuel source.