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WO1998059079A1 - Technique de reduction de fer a faible teneur en soufre a l'aide d'un four a sole rotatif - Google Patents

Technique de reduction de fer a faible teneur en soufre a l'aide d'un four a sole rotatif Download PDF

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Publication number
WO1998059079A1
WO1998059079A1 PCT/US1998/012347 US9812347W WO9859079A1 WO 1998059079 A1 WO1998059079 A1 WO 1998059079A1 US 9812347 W US9812347 W US 9812347W WO 9859079 A1 WO9859079 A1 WO 9859079A1
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Prior art keywords
iron
reductant
agglomerates
matrix
carbonaceous reductant
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PCT/US1998/012347
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English (en)
Inventor
Larry J. Lehtinen
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Iron Dynamics Inc
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Iron Dynamics Inc
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Priority to AU81433/98A priority Critical patent/AU8143398A/en
Publication of WO1998059079A1 publication Critical patent/WO1998059079A1/fr
Anticipated expiration legal-status Critical
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B13/00—Making spongy iron or liquid steel, by direct processes
    • C21B13/10—Making spongy iron or liquid steel, by direct processes in hearth-type furnaces
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B13/00—Making spongy iron or liquid steel, by direct processes
    • C21B13/10—Making spongy iron or liquid steel, by direct processes in hearth-type furnaces
    • C21B13/105—Rotary hearth-type furnaces

Definitions

  • the present invention relates to the direct reduction of iron oxides to elemental iron. More specifically, it relates to direct iron reduction processes utilizing solid carbon reducing agents and a rotary hearth furnace to achieve continuous direct iron reduction. Discussion of Related Art Skilled artisans in the field of refining iron are increasingly recognizing direct reduction, which involves a chemical reduction reaction at a temperature below the melting temperature of the materials present, as a useful method of converting iron oxides, such as, for example, iron ore, into elemental iron.
  • the two general categories of direct reduction are (1) those that utilize natural gas as the source of reducing gases, and (2) those that utilize solid carbonaceous materials such as coal as the source of reducing gases (i.e. solid-based direct iron reduction processes).
  • sponge iron refers to the product of a direct reduction process and is used interchangeably herein with the term "DRI”.
  • the sponge iron which is still in agglomerate form, is then optionally densified by briquetting, transported, melted and treated to extract the reduced elemental iron from contaminants such as sulfur, ash, silica, or slag which are tightly bound to the elemental iron in the sponge iron product.
  • the present invention overcomes the aforementioned problems of high levels of sulfur and ash contamination in the DRI product and the need for large quantities of binders in the formation of green balls by teaching a process for achieving direct reduction of iron which minimizes the sulfur and ash that are allowed into the agglomerate and also minimizes the binder needed in the agglomerate.
  • the present invention overcomes these hurdles by providing a solid-based direct reduction process which involves the use of relatively small green balls comprising an iron oxide tightly bound to an internal low-sulfur, reductant, that supplies only part of the carbon required for the reduction of the iron oxide.
  • These green balls are placed on the hearth of a rotary hearth furnace in intimate contact with a matrix comprising an external solid carbon reductant typically of lower cost and quality than the internal reductant, and preferably also one or more de-sulfurizing agents.
  • the external reductant preferably supplies more than 10% of the carbon required for iron oxide reduction, more preferably in excess of 25% of the required carbon, more preferably in excess of 50% and most preferably more than 67% of the required carbon.
  • the optimal amount of carbon for reduction that is supplied via external carbon would depend on the relative economics of the two sources of carbon as well as the productivity that is sacrificed as more carbon is supplied externally, and the cost penalties associated with de-sulfurizing and melting the gangue in the DRI by the end user.
  • the present invention provides methods for achieving direct reduction of iron by providing a relatively small agglomerate comprising an iron oxide composition and an internal carbonaceous reductant; placing the agglomerate in intimate contact with a matrix comprising an external carbonaceous reductant and, optionally, one or more de-sulfurizing agents to make a furnace charge; and introducing this furnace charge into a rotary hearth furnace for direct reduction.
  • metallic iron is present in the agglomerated sponge iron and a significant amount of ash and sulfur contaminants are present as a byproduct exterior to the sponge iron agglomerates.
  • the reduced balls are then readily separated from contaminating byproducts such as ash and sulfur in post-reduction beneficiation steps.
  • the beneficiated sponge iron may then be conveyed by insulated bottles, or other means such as an inert pneumatic conveyor or heat resistant metal conveyor, to downstream users such as a smelter, an electric arc furnace, or a hot briquetting machine where the product is densified to facilitate storage and transportation.
  • the product may be sold as merchant sponge iron, or advantageously used as hot or cold feedstock for iron making or steel-making operations.
  • Figure 1 provides a top plan view of a portion of a rotary hearth furnace along with apparatus for introducing furnace charge material onto the hearth according to a preferred aspect of the invention, the apparatus including a green ball drier 135, a matrix feed bin 150 and weigh feeder 170, a balling machine 100, a wet green ball conveyor 1 10, a green ball drier loading zone 140, a green ball drier discharge zone
  • the present invention provides improved methods for the direct reduction of iron oxides.
  • the present invention involves iron reduction methods wherein the iron oxide composition charged to a rotary hearth furnace is bound together in a relatively small green ball with an internal reductant, the internal reductant preferably having a low sulfur content, and being dosed in a quantity less than the total amount of reductant needed to fully reduce the iron oxide in the small green balls.
  • the green balls are placed on the hearth of a rotary hearth furnace 1 in intimate contact with a matrix comprising an external carbonaceous reductant preferably of lower cost than the internal reductant, typically due to, for example, higher levels of sulfur, ash and/or volatiles.
  • An example of a material useful as the external reductant is a waste byproduct such as petroleum coke.
  • the carbon in the reductant matrix provides a portion of the carbon needed for reduction and, therefore, the amount of reductant in the green ball need only be a fraction of the total reductant needed to achieve substantially complete reduction.
  • the use of smaller green balls, the presence of a lesser amount of carbonaceous reductant in the agglomerate, and the padding aspect of the external reductant in the matrix during handling of the small green balls all serve to decrease the amount of binder which must be used to achieve satisfactory green ball strength and durability. Since these binders are very expensive, accounting for as much as 15%> of the total costs in prior art solid-based reduction processes, this feature of the invention increases the economic value of carbonaceous direct reduction processes.
  • the internal reductant bound to the oxide in the green ball oxide is only a fraction of the total reductant present and the internal reductant preferably has a relatively low sulfur and ash content, the resulting sponge iron agglomerates may be readily separated from the sulfur and ash byproduct contaminants present in the matrix yielding a DRI product with purity, quality and value substantially exceeding that of DRI produced by prior art solids-based rotary hearth reduction processes.
  • a furnace charge is prepared using appropriate proportions of particulate starting materials, including one or more particulate iron oxides, one or more particulate carbonaceous reductants, one or more binder materials and, optionally, one or more de-sulfurizing agents, one or more beneficial catalyzing additives and water.
  • the furnace charge is prepared using two separate compositions: (1) agglomerates of one or more iron oxides one or more binder materials, and one or more particulate carbonaceous reductants, referred to herein as "internal reductant," and also, in certain preferred embodiments, one or more beneficial catalyzing additions; and (2) a particulate matrix of an additional quantity of one or more carbonaceous reductants (may preferably be a low-cost, high- ash/sulfur reductant), referred to herein as the "external reductant.”
  • the matrix is first fed onto a conveyor and, subsequently, the agglomerates are fed onto the matrix.
  • the matrix acts as a padding material to cushion the impact of the agglomerates falling down onto the conveyor belt, and the subsequent agitation, handling and conveyor transfers and discharge impacts cause less degradation of the agglomerates.
  • the agglomerates become intimately coated with and embedded into the matrix, and this combined composition constitutes an inventive furnace charge.
  • a mixture provided to form agglomerates is termed "balling feed mixture” herein, and a mixture prepared to be the matrix is simply referred to as the "matrix mixture.”
  • the charge is placed on the hearth of a rotary furnace where it is exposed to appropriate reaction conditions to achieve direct reduction of the iron oxide into sponge iron.
  • the term "sponge iron” is intended to refer to the product of a direct reduction process which includes elemental iron therein, and is used interchangeably with the term "DRI".
  • the agglomerates preferably remain substantially intact during the reduction reaction and, thereby, the sponge iron exists in the reduced agglomerates.
  • the sponge iron is then discharged from the rotary hearth furnace for subsequent processing or merchant sale.
  • One starting material required to practice the present invention therefore, is a particulate iron oxide composition, used in the formation of agglomerates.
  • the particulate iron oxide composition comprises a sufficient amount of iron oxide to make the direct reduction into metallic iron economically feasible.
  • a preferred level of iron oxide in such a composition may be determined by skilled artisan on a case- by-case basis for a wide variety of economic conditions and situations. It is contemplated according to the present invention that a wide variety of iron ores, such as virgin ores, or concentrates thereof, may be used in inventive processes.
  • iron oxide compositions suitable for use according to the invention include virgin iron ore, such as hematite iron ore fines, lump ores, iron oxide pellet fines, hematite iron ore, specular hematite concentrate, earthy hematite, magnetite iron ore, magnetite concentrate, limonite, limonite concentrate, taconite concentrate, semi-taconite concentrate, pyrolusite and pyrolusite concentrate; and steel mill waste oxides such as mill scale, EAF dust and drop out dust. It is not intended, however, that this list be limiting and is readily understood by a skilled artisan that additional compositions or combinations of reducible oxides therein may find advantageous use according to the present invention.
  • Suitable iron oxide compositions for use according to the present invention include but are not limited to magnetite concentrates from Minnesota and Michigan, semi-taconites from Minnesota, specular hematite concentrates from Eastern Canada, or Michigan, hematite lump or fines from Brazil, hematite lump or fines from Australia, hematite lump or fines from India, iron ores from Sweden, magnetite concentrates or fines from Peru or Chile, and limonite or hematite ores from Africa.
  • Suitable iron oxide compositions may be obtained from companies which are in the business of iron ore mining, such as, for example, Cleveland Cliffs, Inc.. Quebec Cartier Mining company, Iron Ore Company of Canada, CVRD, Hammersley Iron, BHP or MBR.
  • carbonaceous reductant is used to prepare agglomerates and is also used to prepare a matrix according to the invention.
  • the former is termed “internal reductant,” and the latter is termed “external reductant.”
  • the internal reductant and the external reductant may be compositionally similar or identical, in preferred aspects of the invention, the internal reductant is a low-sulfur, low-ash, carbonaceous reductant and the external carbonaceous reductant may be a cheaper high-sulfur, high-ash reductant.
  • the external reductant may preferably be a low cost carbon source since a wide variety of combinations of ash, sulfur, carbon and volatile content may be advantageously used including low cost carbons such as petroleum coke or coke braize.
  • the ash content is less than about 10% dry weight basis and most preferably less than about 6% on a dry weight basis.
  • particulate carbonaceous reductants which are advantageously used as the internal carbonaceous reductant in accordance with the invention include low to medium volatile subbituminous to bituminous coals, anthracite, lignite, coke, coke braize, graphite and char.
  • the external reductant can be one or more of a wide variety of coals and cokes including those described above as suitable internal reductants, and also including cheaper materials such as, for example. Powder River basin coals, petroleum cokes, lignites, Indiana and Illinois coals, and a wide variety of coals produced around the world.
  • the most economic choice of an external reductant under a particular set of circumstances can be easily made by a skilled artisan based upon delivered economics per ton of contained carbon.
  • a very important and significant advantage of the invention is that reductants high in sulfur and ash may be used in the matrix as the external carbonaceous reductant.
  • the particle size may advantageously be coarser than the preferred ranges for the internal reductant so that particles in the matrix, once dried, are not lost to the process gas stream flowing above the charge layer on the hearth.
  • the external reductant is crushed, ground and sized to at least about 90% smaller than one (1) inch in particle diameter, more preferably to at least about 90% passing one-half (1/2) inch particle size, more preferably smaller than about 90% passing one-quarter inch, more preferably smaller than about 90% passing 10 mesh and most preferably smaller than about 90% passing 50 mesh.
  • de-sulfurizing agent is intended to refer to a composition that, when present in a particulate external reductant matrix selected according to the invention, "captures” sulfur present in the furnace charge, and thereby prevents significant accumulation of sulfur in the agglomerate during and/or after the reduction reaction.
  • Production of agglomerated sponge iron with a relatively low sulfur content significantly reduces the cost of purifying liquid iron and/or steel in downstream refining process.
  • Suitable de-sulfurizing agents for use in the practice of this invention include, for example, high calcium hydrated or slaked lime, dolomitic slaked lime, magnesian slaked lime, high calcium caustic lime, dolomitic caustic lime, magnesian caustic lime, high calcium limestone, dolomitic limestone, magnesian limestone, calcite and calcium carbide.
  • a plurality of de- sulfurizing agents may be blended prior to mixing with the other starting materials.
  • de-sulfurizing agents may be obtained in the preferred powdered form from a wide variety of commercial outlets well known to a person skilled in the art.
  • the three above-listed varieties of limestone may advantageously be ground in a batch using the same grinding system used to grind the reductants, thereby reducing the capital cost of the DRI production facility and reducing the cost of the de-sulfurizing agents.
  • Some de-sulfurizing agents are detrimental to carbonaceous direct iron reduction processes taught in the prior art which utilize binders to form agglomerates or briquettes, because the de-sulfurizing agents are mixed with other starting materials prior to agglomeration thereof, and interfere with the binders' ability to give the agglomerate adequate dry strength and durability. Therefore, if a de-sulfurizing agent is used in these processes, a much greater amount of binder is typically required to achieve satisfactory agglomeration, especially in the case of the FASTMET process, which handles dry green balls between the drier and the hot hearth.
  • the de-sulfurizing agents that interfere with binders preferably are not admixed with the starting materials to be formed into the agglomerate, and only those additives that enhance the reduction reaction without substantially interfering with the binder or binders (i.e.
  • beneficial non-interfering catalysts are added with the starting materials to be agglomerated (i.e., the iron oxide composition and the internal carbonaceous reductant).
  • one or more de-sulfurizing agents are preferably admixed with the external carbonaceous reductant, and thereby substantially prevent sulfur in the external carbonaceous reductant from contaminating the DRI in the agglomerate.
  • the absence of or scarcity of interfering de-sulfurizing agents in the green ball eliminates the need for excessive amounts of binder material, and therefore helps minimize the cost of binder material per ton of DRI produced.
  • Some non-interfering de-sulfurizing agents in small dosages also function to increase the rate of an iron reduction reaction and/or serve as a flux of silica and alumina for downstream iron and steel-making and, therefore, are beneficial additives (termed "beneficial non-interfering catalysts" herein) to the balling feed mixture.
  • limestone derivations including, for example, powdered forms of hydrated or slaked lime, caustic lime, finely ground limestone, dolomite or any combination thereof, used in accordance with the invention achieve a particularly advantageous rate of the direct reduction reaction, increasing metallization of iron oxides by about 1 to about 10 percentage points for a given reaction time and reaction condition.
  • the limestone derivations are especially advantageous in certain embodiments of the invention, i.e.
  • binders Since the iron oxide composition and internal carbonaceous reductants are agglomerated according to the invention, it is critical that binders also be mixed therewith.
  • a number of binders suitable for use in this manner are well known in the art and may be advantageously used in accordance with the invention.
  • acceptable binders include bentonite clay and organic compositions such as water solutions of molasses, water solutions of lignin sulfate, Peridur, Alcotac and other forms of carboxymethychloride together with soda ash.
  • water Another material which imparts advantageous features to inventive methods is water.
  • water is mixed, optionally together with a surfactant or other wetting agent, into the external carbonaceous reductant in a sufficient amount, it allows the material to be conventionally conveyed by belt conveyors such that the material can be introduced onto a furnace charge conveyor ahead of the dry green balls and thereby act as a cushion for the arriving fragile green balls.
  • the moist nature of the matrix also prevents or minimizes the loss of reductant matrix particles in fugitive fashion to the process gases flowing above the furnace charge layer on the heath.
  • the moisture content of the external carbonaceous reductant also imparts the following advantageous characteristics to inventive mixtures; (1) elimination of dust problems, (2) prevention of the ignition of the furnace charge upon contact with a hot hearth, and (3) delay of coal volatization so that, if the rotary hearth furnace flue is located at the beginning of the furnace loading zone, as is common in a variety of prior art rotary hearth furnace designs, then the hydrocarbon volatiles of the coal have increased time to combust usefully in the hood to the benefit of the oxidic mixture.
  • water is preferably mixed with the iron oxide composition, internal carbonaceous reductant and binder material in the balling feed mixture prior to agglomeration thereof in order to achieve sufficiently strong and properly sized green balls.
  • the water content of the balling feed mixture is less than about 15% by weight. More preferably the water content is less than about 12%. Most preferably the water content is less than about 10 all by weight.
  • the proportions of iron oxide composition to carbonaceous reductant in the furnace charge material are selected according to the invention based upon the amount of iron in the oxide composition and the amount of fixed carbon in the carbonaceous reductant. According to a preferred aspect of the invention, the ratio of carbon to iron in the furnace charge is selected to optimize reduction of the iron oxide without wasting reductant.
  • the ratio of fixed carbon in the total reductant present in the furnace charge to iron in the iron oxide composition is between about 4.0:10.0 and about 2.4:10.0 by weight. More preferably, the ratio is between about 3.4: 10 and about 3.0: 10 by weight. Most preferably the ratio of carbon to iron in the mixture is about 3.2:10.0 by weight.
  • the optional ratio will be dependent upon such factors as the delivered cost and fixed carbon of reductants, the reactivity of each reductant, the rate of metalization achieved with various ratios of reductant and the sensitivity of the end user of the DRI to gangue and sulfur content.
  • the relative amount of de-sulfurizing agent which is included in the matrix material according to the present invention will be dependent upon the specific agent selected for use, the amount of sulfur in the external reductant, and the amount of iron and fixed carbon in the iron oxide composition and reductant in the charge, respectively. Additionally, the amount of water present may be optimized by taking into account the factors set forth herein. An optimum amount of each of these components may readily be determined by one skilled in the art with minimal experimentation.
  • the inventive matrix mixture comprises less than about 0.1% de-sulfurizing agent and from about 0.5% to about 14% water, both by weight. In another preferred embodiment, the matrix mixture according the invention comprises from about 0.1% to about 30% de-sulfurizing agent by weight; and from about 0.5% to about 14% water by weight. To increase the rate of reaction, it may also be desirable to place a beneficial non-interfering catalyst into the green ball by including it in the balling feed mixture prior to agglomerate thereof.
  • the balling feed mixture fed to the agglomerating circuit comprises a particulate iron oxide composition, which is a hematitic or magnetitic virgin iron ore or concentrate thereof; an internal carbonaceous reductant, which is one or a blend of more than one low or medium volatile bituminous or subbituminous coals; and a binder material, which is one or a blend of more than one of the following compositions: bentonite, molasses, lignin sulfate, Peridur, Perispray or another similar carboxyme hylchloride and soda ash composition.
  • a particulate iron oxide composition which is a hematitic or magnetitic virgin iron ore or concentrate thereof
  • an internal carbonaceous reductant which is one or a blend of more than one low or medium volatile bituminous or subbituminous coals
  • a binder material which is one or a blend of more than one of the following compositions: bentonite, molasses, lignin
  • the matrix mixture preferably includes an external reductant and a de- sulfurzing agent that is a blend of one or more of the following: high calcium hydrated or slaked lime, dolomitic slaked lime, magnesian slaked lime, high calcium caustic lime, dolomitic caustic lime, magnesian caustic lime, high calcium limestone, dolomitic limestone, magnesian limestone, calcite and calcium carbide.
  • the external reductant is a blend of one or more coals, cokes, petcokes, chars, lignites, anthracites, graphites or other economically attractive sources of carbon.
  • the overall furnace charge comprises from about 65% to about 85% virgin ore; from about 10% to about 40% total reductants (internal and external); from about 0.5% to about 3% beneficial additive; and from about 1% to about 15% water, from about .03% to about 3% binding agents, and from about 1% to about 25% desulfurizing agents, all by weight.
  • the dry-weight- basis iron content of the iron ore is from about 64% to about 70% and the dry- weight- basis fixed carbon of the internal carbonaceous reductant is from about 60% to about 80%.
  • starting materials used to form green balls i.e. an iron oxide composition, an internal carbonaceous reductant, a binder material, water and, optionally, additional additives selected by a skilled artisan
  • green balls produced be no larger than about 15mm in diameter, more preferably no larger than about 10mm in diameter more preferably no larger than about 6mm in diameter, and most preferably no larger than about 4mm in diameter.
  • the green balls are then placed in intimate contact with a matrix as discussed more fully below with respect to particular preferred embodiments of the invention.
  • the green balls may preferably be dried before they are placed in intimate contact with the matrix.
  • starting materials used to form the matrix i.e. an external carbonaceous reductant, which may be compositionally the same as or different than the internal carbonaceous reductant, a de-sulfurizing agent and, optionally, water and additional additives selected by a skilled artisan
  • an intensive mixer or plug mill to thoroughly blend the mixture.
  • the matrix mixture may then preferably be conveyed to a surge bin, from which it is advantageously delivered to an apparatus for contacting the mixture with agglomerates in accordance with the invention.
  • the matrix mixture may advantageously be delivered at a predetermined rate to 11
  • a matrix mixture prepared in accordance with the invention is preferably fed onto a conveyor at a rate determined by a desired weight or volumetric ratio with respect to the weight or volume component of the dry or wet green balls.
  • the matrix mixture may optionally be first placed in a feed bin, or surge bin, before being weigh fed onto a conveyor that subsequently also receives the wet or dry green balls.
  • the green balls and the matrix material having been placed in intimate contact with one another in carefully-monitored ratios to provide a furnace charge material, are then placed onto a rotary hearth furnace 1 for reduction of the iron oxide composition.
  • the movement of the charge across conveyor idlers and transfer points and the subsequent introduction of the charge onto the hearth loading zone 10 advantageously mixes the green balls and the matrix together, causing the matrix to fill the interstitial space between the green balls, bringing the matrix material and the green balls into intimate contact with one another.
  • the hearth loading zone 10 is optionally shrouded, sealed, and fed with nitrogen gas to purge air and eliminate the hazard of combustion of the carbon in the furnace charge material upon contact with the hot hearth and to avoid air leakage into the discharge area whereby sponge iron reoxidation could occur.
  • An example of a preferred feed system used in accordance with the invention is an oscillating conveyor system, as mentioned above.
  • the oscillating conveyor from which the furnace charge drops onto the hearth may advantageously be moved back and forth repeatedly across the width of the hearth, thus introducing furnace charge material onto the hearth as the hearth steadily moves thereunder.
  • the oscillating conveyor 40 in a preferred embodiment is oriented as shown in Figure 1, wherein the oscillating conveyor 40 oscillates under the furnace charge conveyor 50, and a relatively even layer of furnace charge material on the hearth is achieved by a movement of the oscillating conveyor 40 back and forth with greater speed at the "inside" end of the stroke.
  • the eccentric oscillation allows the oscillating conveyor to introduce the charge material across the width of the hearth.
  • the layer be smoothed even more uniformly before entering the reaction zone of the rotary hearth furnace.
  • a charge layer on the hearth may be leveled using, for example, a set of stationary or oscillating plows or a leveling screw that sweeps excess material to the inside or outside of the hearth. Excess material may preferably be swept off the hearth and recycled to the furnace charge conveyor system or optionally spread over the hearth width by the plows or screws. It is also preferred that the furnace charge layer which enters the reaction zone of the rotary hearth furnace has a thickness of less than about 50mm.
  • the thickness is less than about 25mm, most preferably less than about 15mm; however, it is readily understood that the most preferred thickness may be dependent upon the heat transfer conditions of the particular furnace being used.
  • the type and arrangement of burners, the type and arrangement of secondary air used to combust the coal volatiles and carbon monoxide, and the type of combustion fuel used are examples of some of the variables that will impact heat transfer conditions of the particular furnace. Additionally, if the hearth is superheated, a thicker layer may be loaded onto the hearth since heat will be conducted from the hearth into the bottom of the layer, thereby achieving the direct reduction of material not directly exposed to the radiant heat from above the hearth.
  • a thicker layer may also be placed on the hearth in alternative embodiments of the invention where the furnace charge layer is "plowed" or "churned” at one or more points within the reaction zone of the rotary hearth furnace.
  • the charge material is heated to a temperature and for a period of time sufficient to achieve a high degree of reduction of the iron oxide composition to metallic iron.
  • the charge is heated to a temperature of from about 1000°C to about 1500°C and for a period of time of from about 5 minutes to about 50 minutes.
  • Conventional rotary hearth furnaces having multiple reaction zones may be advantageously used in accordance with the present invention.
  • the reaction zone after the charge has passed through the reaction zone, a substantial portion of the starting materials in the charge will have been chemically converted to metallic iron (in the reduced agglomerates, or sponge iron) and sulfur-containing ash byproducts.
  • metallic iron in the reduced agglomerates, or sponge iron
  • sulfur-containing ash byproducts After passing through the reaction zone or zones of the rotary hearth furnace, the reduced agglomerates (sponge iron) and sulfur-containing ash byproduct material are removed from the hearth.
  • the sponge iron and byproduct material are preferably removed using one or more water-cooled discharge plows or screws, and may then be discharged through refractory-lined chutes into, for example, an insulated and nitrogen purged buffer bin and thence into insulated and nitrogen purged bottles, or an inert pneumatic conveyance system for transport to further processing steps, or into an inert atmosphere cooler.
  • at least about 70% of the iron in the sponge iron is in metallic form, more preferably, at least about 80% of the iron is in metallic form, still more preferably at least about 90% and, most preferably, at least about 92%.
  • a portion, preferably a large fraction, of the carbonaceous reductant material is in the matrix, exterior to the green balls (i.e., the external reductant).
  • This feature of the invention advantageously facilitates beneficiation of the agglomerated sponge iron because a large percentage of the ash and the sulfur contaminants are in the loose matrix material. Therefore, the reduced agglomerates may preferably be separated from the byproduct materials by screening, by utilization of an gaseous cyclone, by magnetic separation, or by a combination of these or other known beneficiation techniques.
  • the resulting beneficiated sponge iron is significantly more pure than DRI produced using other solid-based rotary hearth reduction processes known in the prior art.
  • Processes according to the invention may result in sponge iron having as low as about 0.01 to 0.10 percent sulfur and as low as 1.0% to 5.0% gangue present therein.
  • a larger or smaller proportion of the carbonaceous reductant may be placed in the green ball, depending upon whether it is more desirous in a particular application of the invention to achieve extremely pure DRI or to achieve maximum hearth productivity.
  • Production of DRI having such low sulfur and gangue content advantageously reduces costly downstream purifying steps otherwise required if high sulfur, high gangue DRI is used in iron-making or steel-making operations.
  • Specular hematite concentrate containing less than about 5% silica and more than about 64% elemental iron by weight is ground by either a ball mill or roll press to a nominal particle size distribution of at least about 50% passing 200 mesh.
  • the ground iron ore concentrate is admixed with low volatile subbituminous coal containing about 77% fixed carbon by weight on a dry basis, which has been dried and ground to a size of about 80% passing 200 mesh.
  • High calcium limestone in powder form is also admixed at a dosage of 1% of the dried mixture.
  • Binders including .15% Peridur, .025% Perispray and .3% Bentonite are also admixed to the mixture.
  • the entire mixture is wetted to 10% water content in an intensive mixer and fed to one or more balling disks or drums in closed circuit with roll screens.
  • the mixture is agglomerated into green balls with a size distribution of about 90% between three (3) mm and six (6) mm in diameter.
  • the wet green balls are conveyed to a rotary grate drier where they are dried to less than about 1% water using hot gases from the rotary hearth off gas system.
  • Petroleum coke with a sulfur content of 4% by weight is dry ground to about 90% minus ten mesh and admixed with high calcium limestone which has been dry ground to about 80%> minus 200 mesh.
  • the two ingredients are admixed in an intensive mixer at a weight blend of about 70% pet coke, about 25% limestone and about 5% water.
  • the mixture constitutes the external reductant matrix material.
  • the matrix material is belt conveyed to a feed bin located near the pellet drier.
  • a feeder admixes the matrix material to the dry green balls by depositing the matrix material onto the belt conveyor that subsequently receives the dry balls from the drier discharge chute.
  • the dry green balls fall onto the moist matrix material that softens the impact of the fragile dry balls.
  • the composite of matrix and dry balls is conveyed and transferred onto an oscillating conveyor that distributes the furnace charge evenly across the width of a rotary hearth furnace.
  • the furnace charge is spread into a fairly uniform layer across the width of the hearth by one or more leveling screws to a thickness of about 15mm.
  • the charge material then enters the reaction zones of the rotary hearth.
  • the mixture is rapidly heated by natural gas and/or coal fired burners to approximately 1300°C. Residence time in the furnace is approximately 20 minutes. Metallization in excess of 90% is achieved with a unit productivity of the hearth of more than 70kg of DRI per useful square meter of hearth area per hour.
  • the mixture is removed from the hearth using a water cooled screw, discharged to a refractory lined buffer bin and then to insulated bottles for transfer to a water cooled rotary cooler with trommel discharge screens where the mixture is cooled in an inert atmosphere and to separate the ash, unspent coal fines calcium sulfate and unspent lime and limestone from the sponge iron pellets.
  • the clean sponge pellets are then either further cooled for storage, transported to the EAF for melting, or pre-cooled for hot briquetting.
  • EXAMPLE TWO Magnetite concentrate from Minnesota or Michigan with a silica content between about 1.5% and about 6.0% is ground to at least about 80% passing 325 mesh, and filtered to a moisture content of about 10%. It is then admixed with an internal reductant comprising metallurgical coal from British Columbia that has been dry ground to about 80% passing 200 mesh, together with a dosage of about 2% dry ground limestone, about .3% bentonite, about .1% Peridur and about .025% Peridur.
  • the balling mix recipe by weight is about 77% iron ore concentrate, about 10% reductant, about 10.575% water, about 2% limestone, and about .425% binders.
  • the thoroughly blended mixture is conventionally conveyed and discharged to a balling drum closed with a roll screen.
  • the balling circuit produces wet green balls with a size distribution about 90% between 8mm and 10mm in diameter.
  • the wet balls are conveyed to an furnace charge conveyor upon which a matrix mixture has previously been deposited, the furnace charge material then depositing the charge onto an oscillating conveyor which deposits the entire furnace charge onto the hearth.
  • the matrix material comprises Powder River basin coal dry ground to about 80% minus 50 mesh and dry ground limestone at a size of about 80% minus 100 mesh, these ingredients being intensively mixed in a mixer at a blend ratio of about 80% coal, about 12% limestone and about 8% water all by weight.
  • the matrix material is weigh fed to the furnace charge conveyor such that the ratio of carbon to iron in the combined furnace charge is about 3.2 to about 10 on a dry weight basis.
  • the matrix material is placed onto the belt conveyor that subsequently receives the green balls from the roll screen so that the balls fall onto the matrix
  • the entire furnace charge consisting of wet green balls imbedded in the external reductant matrix material is belt conveyed to an oscillating conveyor that spreads the charge across the full hearth width of the rotary hearth reduction furnace. Using one or more leveling screws the mixture is spread into an even layer approximately 20mm thick.
  • the charge layer enters the reaction zones of the rotary hearth furnace where it is heated slowly at first to drive off moisture, then rapidly to about 1200 ° C.
  • the residence time on the hearth is about 35 minutes.
  • An oscillating, water-cooled plow and screw remove the sponge iron balls and ash byproduct off the hearth in the discharge zone.
  • the plow oscillates with an amplitude of about six inches with an out stroke velocity one half that of the return stroke so that the plow gets back to the start position before the inner 6 inch material travels behind the plow on the out stroke.
  • the hot sponge iron and ash byproduct are processed through an inert gas cyclone which separates the byproduct ash from the DRI and cools it to briquetting temperature.
  • the purified DRI is then transported to a hot briquetting machine where the sponge iron is densified into briquettes for merchant sales.
  • Hematite fines or magnetite concentrate at about 4% moisture from South America or Australia are grounded using a high pressure roll press to about 50% minus 200 mesh and then admixed with a coal blend of Indonesian and British
  • the moist ground ore and dry coal are mixed with dolomitic hydrated lime at a ratio of about 89% ore, about 10% coal and about 1% dolomitic hydrated lime, by dry weight.
  • the blend is intensively mixed with water and binders in a pug mill before belt conveyance to a balling disk and roll screen circuit.
  • the green balls with an average diameter of about 7mm are dried in a traveling grate-style drier using air heated by a heat exchanger that transfers heat from the rotary hearth off gases.
  • Pet coke and low- cost coal are ground to about 80% minus 10 mesh and admixed with about 5% hydrated lime and water to a moisture of about 5%.
  • the matrix material is deposited on a conveyor that subsequently receives the dried balls from the traveling grate drier. The dried balls fall from the drier discharge onto the moist matrix material.
  • the entire charge is conveyed to an oscillating conveyer that spreads the mix across a 7.0 meter wide hearth. High temperature alloy plows level the bed to about 15mm.
  • the entire hearth loading zone is shrouded, sealed and charged with nitrogen gas to eliminate flashing as the carbonaceous mixture cascades down onto a hearth with a surface temperature of about 1000°C.
  • Reaction time is from about 20 minutes to about 30 minutes.
  • the DRI bed is removed in the discharge zone by a water-cooled discharge screw with a diameter of about 36 inches.
  • DRI with average metallization of about 93%> together with ash by-product is passed into nitrogen filled bottles lined with refractory.
  • the bottles then dicharge to a pneumatic transport system feeding one or more cyclones where the pellets are separated from the ash and calcium sulfate.
  • the pellets are subsequently charged hot into an EAF furnace for conversion to molten steel, or hot briquetted for sales, or storage for later uses.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Manufacture Of Iron (AREA)
  • Manufacture And Refinement Of Metals (AREA)

Abstract

Cette invention a trait à des techniques permettant de produire du fer à faible teneur en soufre résultant d'une réduction directe, en continu et de façon productive. L'une des réalisations préférées porte sur la production d'agglomérats constitués d'une composition d'oxyde de fer et d'une composition de réducteur carboné interne à teneur en soufre relativement faible. Cette technique consiste à mettre les agglomérats en contact étroit, sur la sole d'un four rotatif (1), avec une matrice particulaire comportant un réducteur carboné externe, à soumettre le mélange à des conditions réductrices afin de réduire une partie importante de l'oxyde de fer et, partant, de produire un préréduit et un sous-produit sous forme de cendres renfermant du soufre, à retirer du four à sole rotatif ces deux substances et à enrichir le préréduit pour le séparer des cendres à haute teneur en soufre, ce qui permet d'obtenir un préréduit purifié.
PCT/US1998/012347 1997-06-20 1998-06-17 Technique de reduction de fer a faible teneur en soufre a l'aide d'un four a sole rotatif Ceased WO1998059079A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU81433/98A AU8143398A (en) 1997-06-20 1998-06-17 Low sulfur iron reduction process using a rotary hearth furnace

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US88002797A 1997-06-20 1997-06-20
US08/880,027 1997-06-20

Publications (1)

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WO1998059079A1 true WO1998059079A1 (fr) 1998-12-30

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6630010B2 (en) * 2000-03-30 2003-10-07 Midrex International B.V. Zurich Branch Method of producing metallic iron
WO2009052066A1 (fr) * 2007-10-15 2009-04-23 E. I. Du Pont De Nemours And Company Procédé de réduction de minerais utilisant des matériaux à base de carbone ayant une faible teneur en soufre et produit de métallisation issu de celui-ci contenant de l'oxyde de titane et du fer
US7628839B2 (en) 2004-12-07 2009-12-08 Iwao Iwasaki Method and system for producing metallic iron nuggets
CN105969927A (zh) * 2016-06-15 2016-09-28 江苏省冶金设计院有限公司 一种转底炉直接还原铁水淬装置及方法
CN116282119A (zh) * 2023-03-29 2023-06-23 华新水泥股份有限公司 一种制球实现磷石膏高脱硫率的方法

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3443931A (en) * 1965-09-10 1969-05-13 Midland Ross Corp Process for making metallized pellets from iron oxide containing material
US3770417A (en) * 1971-02-23 1973-11-06 Salem Corp Simultaneous production of metallized ores and coke

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3443931A (en) * 1965-09-10 1969-05-13 Midland Ross Corp Process for making metallized pellets from iron oxide containing material
US3770417A (en) * 1971-02-23 1973-11-06 Salem Corp Simultaneous production of metallized ores and coke

Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6630010B2 (en) * 2000-03-30 2003-10-07 Midrex International B.V. Zurich Branch Method of producing metallic iron
US7628839B2 (en) 2004-12-07 2009-12-08 Iwao Iwasaki Method and system for producing metallic iron nuggets
US7632335B2 (en) 2004-12-07 2009-12-15 Nu-Iron Technology, Llc Method and system for producing metallic iron nuggets
US7641712B2 (en) 2004-12-07 2010-01-05 Nu-Iron Technology, Llc Method and system for producing metallic iron nuggets
US7695544B2 (en) 2004-12-07 2010-04-13 Nu-Iron Technology, Llc Method and system for producing metallic iron nuggets
US8158054B2 (en) 2004-12-07 2012-04-17 Nu-Iron Technology, Llc Method and system for producing metallic iron nuggets
WO2009052066A1 (fr) * 2007-10-15 2009-04-23 E. I. Du Pont De Nemours And Company Procédé de réduction de minerais utilisant des matériaux à base de carbone ayant une faible teneur en soufre et produit de métallisation issu de celui-ci contenant de l'oxyde de titane et du fer
AU2008312639B2 (en) * 2007-10-15 2012-11-08 The Chemours Company Fc,Llc Ore reduction process using carbon based materials having a low sulfur content and titanium oxide and iron metallization product therefrom
US8372179B2 (en) 2007-10-15 2013-02-12 E I Du Pont De Nemours And Company Ore reduction process using carbon based materials having a low sulfur content and titanium oxide and iron metallization product therefrom
CN105969927A (zh) * 2016-06-15 2016-09-28 江苏省冶金设计院有限公司 一种转底炉直接还原铁水淬装置及方法
CN116282119A (zh) * 2023-03-29 2023-06-23 华新水泥股份有限公司 一种制球实现磷石膏高脱硫率的方法
CN116282119B (zh) * 2023-03-29 2024-10-29 华新水泥股份有限公司 一种制球实现磷石膏高脱硫率的方法

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