Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B11/00—Making pig-iron other than in blast furnaces
  • C21B11/08—Making pig-iron other than in blast furnaces in 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 one or more solid carbon reducing agents and a rotary hearth furnace to achieve continuous direct iron reduction.
• DRI direct reduced iron
• the third process termed the "MAUMEE” process herein, is similar to the INMETCO process, but utilizes wet or dry briquettes formed under high pressure with or without binder material.
• the MAUMEE process uses very expensive briquetting machines to form dry or nearly dry briquettes that, depending upon the starting material, may or may not require one or more binders to achieve briquettes strong enough to withstand handling steps onto the hearth of a rotary furnace.
• the binders are required in such large proportions in order to make a wet or dry agglomerate of adequate strength and durability to avoid breakage during handling ahead of the drier or rotary hearth furnace, and to avoid exfoliation or explosion in the case of the INMETCO process where wet balls are introduced directly onto the hot hearth of the rotary hearth furnace.
• agglomerates typically green balls but in some processes briquettes, are charged into a rotary hearth furnace, where the iron ore in the agglomerate is reduced to yield "sponge iron.”
• the term “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, normally is then densified by briquetting, shipped and melted to extract the reduced elemental iron from contaminants such as silica and sulfur, which are tightly bound to the elemental iron in the sponge iron product.
• All three processes, the FASTMET process, the INMETCO process and the MAUMEE process are accompanied by very large capital and operating costs, which are associated in part with the necessity of binder materials and/or expensive equipment, and processing steps to form the iron ore and other starting materials into agglomerates.
• the iron ore typically must be ground to a fairly stringent size specification before agglomeration.
• the balling or briquetting step requires large capital costs due to the need for proper machinery for forming and sizing the agglomerates and adding the large amounts of binders needed to form balls or briquettes.
• the agglomeration step typically requires the addition of moisture to the starting materials.
• One manner of alleviating this problem is to grind the ore and the reductant to a much finer particle size distribution.
• an agglomerate made for this process using course starting materials i.e., where about 80% of the particles are less than about lOO ⁇ m in diameter
• the mixture to be formed into the agglomerate must typically include from about 0.5 to about 0.6% binder by weight (typically about half organic binders and about half bentonite clay).
• binder typically about half organic binders and about half bentonite clay.
• relatively fine starting materials are to be used (i.e., where about 80% of the particles are less than about 44 ⁇ m in diameter)
• the process requires a somewhat smaller proportion of binder.
• grinding itself is associated with added costs, and only reduces the problem, but does not eliminate it.
• the MAUMEE process for certain starting materials, may be able to function without binders; however, the resulting briquettes are more fragile than even dry green balls. Therefore, this process requires special and more costly material handling equipment and practices. Furthermore, the MAUMEE process requires high capital costs due to the need for expensive briquetting equipment, and high operating costs due to wear of the molds used in the briquetting machines. All of these considerations, along with the need for intermediate bins and material handling equipment, are associated with large capital and operating costs in reduction processes using green balls, dry pellets, briquettes or other agglomerates as the feedstock. Therefore, there is a great need in the art for a direct reduction process which avoids the need for agglomeration of starting materials.
• surge bin refers to a container which holds material to be introduced onto the hearth prior to the introduction thereof.
• the surge bin may, for example, have coupled thereto a simple table feeder and a conveyor for delivering the feed material directly to the hearth feed system; and provides a buffering function, enabling the reduction process to continue even if one or more process steps for preparing feed material is interrupted due to, for example, equipment failure or planned maintenance.
• direct iron reduction processes known in the prior art, it is not feasible to employ such a surge bin because the wet green balls on the bottom of the bin will squash, deform and break due to the load of balls above them. Dry balls, although stronger than wet balls, will suffer a similar consequence, but to a lesser extent than wet balls.
• Halting of the reduction process has extremely disadvantageous economic implications.
• these disadvantages include a decrease of annual output; an increased energy cost associated with thermal efficiency (it is costly to maintain the temperature of a rotary hearth furnace which is not in use, yet it is also very expensive to reheat the furnace prior to resuming the reduction process); and interruptions in the process are known to decrease the quality of the output material. Therefore, the inability to utilize surge bins is a substantial disadvantage of prior art processes with respect to annual output, process efficiency and product quality.
• the present invention overcomes the aforementioned problems by teaching a process for achieving direct reduction of iron which avoids agglomeration steps and instead involves charging particulate starting materials onto a rotary hearth furnace. By utilizing inventive methods, the above-described problems associated with the formation of and reduction of agglomerates are avoided, along with the large capital and operating costs associated therewith .
• the present invention provides methods for achieving direct reduction of iron by introducing a mixture of a particulate iron oxide composition and a particulate carbonaceous reductant onto the hearth of a rotary hearth furnace.
• the resulting sponge iron may be discharged from the hearth using, for example, one or more water-cooled discharge screws or plows, and passed through sealed refractory-lined chutes into, for example, insulated bottles, a hot briquetting machine to agglomerate and densify the product, a nitrogen-purged hot conveyor for transport to further processing steps, a hot pneumatic conveyance system or a cooler.
• a method for producing direct reduced iron which includes providing a mixture of a particulate iron oxide composition and a particulate carbonaceous reductant; the mixture being substantially free from agglomeration; positioning the mixture onto a rotary hearth furnace; subjecting the mixture to heat and reducing conditions to reduce a substantial portion of the iron oxide, thereby producing sponge iron; and discharging the sponge iron from the rotary hearth furnace.
• the mixture When positioning the mixture onto a rotary hearth furnace, it is preferred that the mixture be positioned in a substantially uniform layer.
• the mixture additionally includes water and/or an additive selected in accordance with the invention.
• the layer is compacted, preferably using one or more rollers, thereby bringing the particles of the various starting materials into closer contact with one another.
• the roller or other compacting device includes texturing features to impart a textured surface to the layer, thereby increasing the surface area of the layer.
• Inventive processes thereby eliminate the need to grind the iron oxide composition to pelletizing size, the need to use expensive binders and equipment to pelletize or briquette the starting materials, the need to dry green pellets, and the need to carefully handle fragile wet green balls, wet briquettes, dry briquettes or dried pellets prior to introducing them onto the hearth.
• FIG. 1 provides a top plan view of a portion of a rotary hearth furnace according to a preferred aspect of the invention, the portion including the feed zone, and having depicted therein a feed drum for introducing feed material onto the hearth, a leveling plow and a single compacting drum.
• FIG. 2 provides a side elevational view of a portion of a rotary hearth furnace according to a preferred aspect of the invention, the portion including the feed zone, and having depicted therein a feed drum for introducing feed material onto the hearth, a leveling plow and a single compacting drum.
• FIG. 3 provides a top plan view of a portion of a rotary hearth furnace according to a preferred aspect of the invention, the portion including the feed zone, and having depicted therein an oscillating conveyor for introducing feed material onto the hearth, a leveling screw and a single compacting drum.
• the present invention provides improved methods for the direct reduction of iron oxides. These methods eliminate the need for many processing steps and make possible the use of advantageous equipment and materials, thereby greatly reducing capital and operating costs associated with conventional methods of direct reduction of iron.
• the present invention involves iron reduction methods wherein the starting materials charged to a rotary hearth furnace are in particulate form, and are not bound together or "agglomerated," as is required in prior art carbonaceous direct reduction methods which utilize a rotary hearth furnace. This feature of the present invention eliminates the need for the cost-intensive steps associated with agglomerating the starting materials and thereafter reducing the agglomerates.
• One step that is minimized or in some cases eliminated is the step of grinding iron ore to obtain particles having useful sizes for pelletization. Additionally, since no balling, briquetting or agglomeration is required prior to the hearth, no binding agents, balling disks, balling drums, roll screens or briquetting machines are required in inventive processes and capital and operating costs are therefore reduced. Furthermore, since there is no wet agglomeration prior to the hearth, the need for a dryer in the process is eliminated, thereby decreasing capital and operating costs, and the risk of green ball explosion is eliminated, which can occur in methods which involve placing wet green balls onto a hot hearth. Because the moisture content of the mixture placed onto the hearth according to the invention is less than one third that of green balls taught in the prior art, the unit productivity of the hearth furnace is maximized.
• particulate starting materials including one or more particulate iron oxides, one or more particulate carbonaceous reducing agents, and, optionally, one or more additives selected in accordance with the present invention and water, are mixed, and placed in particulate form on the hearth of a rotary hearth furnace. Thereon, the particulate starting materials are exposed to appropriate reaction conditions to achieve direct reduction of the iron oxide into sponge iron.
• 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 sponge iron is then discharged from the rotary hearth furnace for subsequent processing or merchant sale.
• 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 a 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, ground lump ores, iron oxide pellet fines, hematite iron ore, specular hematite concentrate, earthy hematite, magnetite iron ore, magnetite concentrate, limonite, limonite concentrate, ilmenite, ilmenite 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 it is readily understood by a skilled artisan that additional compositions or combinations thereof which have iron oxide therein may find advantageous use according to the present invention.
• the particulate iron oxide composition used in accordance with the invention need not be ground to pelletizing particle sizes (for example, typically 65% smaller than 44 microns); however, iron ore concentrates or fines that are already ground to pelletizing size, for example due to benef iciation requirements, may also be advantageously used in the present invention.
• the iron oxide composition may be ground to enhance reducibility if the size distribution is too large to achieve good reduction efficiency. If grinding is desired to improve reduction or for other reasons, it may preferably be achieved using a roll press or a ball mill, or other grinding devices known in the art.
• the iron oxide composition is beneficiated to remove therefrom undesirable contaminants such as, for example, silica, alumina and sulfur.
• the particulate iron oxide composition has a size distribution whereby substantially all of the particles are less than about 5mm in diameter and at least about 90% of the particles have a particle size of less than about 2mm. More preferably, substantially all of the iron oxide particles are less than about 2mm in diameter and at least about 90% of the particles have a particle size of less than about lmm.
• the average particle size of the particulate iron oxide composition is less than about lmm in diameter, more preferably less than about 500 ⁇ m, still more preferably less than about 250 ⁇ m, and most preferably less than about 100 microns.
• Suitable iron oxide compositions for use according to the present invention include magnetite concentrates from Minnesota and Michigan, specular hematite concentrates from Eastern Canada, hematite fines from Brazil, hematite fines from Australia, hematites from India, iron ores from Sweden and iron ores from South 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, Hamersley Iron, BHP or MBR.
• a starting material needed to practice the present invention is a particulate carbonaceous reductant.
• a particulate carbonaceous reductant suitable for use in the present invention is one having a sufficient amount of reactivity, fixed carbon and volatile matter therein to advantageously react with the iron oxide composition under suitable reaction conditions to produce sponge iron.
• Examples of particulate carbonaceous reductants which are suitable for use in accordance with the invention include coal, coke, coke braize, pet coke, graphite and char. Again, it is not intended that this list be limiting, but only that it provide examples of useful carbonaceous reductants. It is well within the purview of a skilled artisan to select additional materials having sufficient reactivities, volatile matters, and fixed carbon therein to be advantageously used in accordance with this invention.
• the carbonaceous reductant has a particle size distribution whereby substantially all of the particles are less than about 5mm in diameter with at least about 90% of the particles having a diameter of at most about 2mm. More preferably, substantially all of the reductant particles are less than about 2mm with at least about 90% of the particles having a diameter of at most about lmm.
• the average particle size of a carbonaceous reductant is less than about lmm in diameter, more preferably less than about 500 microns, still more preferably less than about 200 microns, and most preferably less than about 74 microns. It is preferred that the carbonaceous reductant be finely ground, and it is well known in the art that these materials are relatively easy to grind. A suitable carbonaceous reductant may be readily obtained from sources well known to those skilled in the art .
• the carbonaceous reductant is beneficiated prior to being mixed together with the other starting materials.
• a preferred carbonaceous reductant, coal commonly includes about 0.1-4% sulfur by weight and about 1-12% ash by weight as contaminants.
• the sulfur must be removed from the metallic iron in very costly processing steps.
• the DRI product of conventional direct reduction processes commonly includes therein about 100-500 times more sulfur than that which may be present in iron used for making steel. This presence of sulfur in the DRI product is a major shortcoming of solids-based direct reduction processes in the prior art.
• preferred embodiments of the present invention involve grinding of the carbonaceous reductant before it is mixed with other starting materials, and then purifying the reductant with respect to fixed carbon content by removing ash and sulfur using beneficiation processes known in the art.
• suitable benef iciation techniques include flotation separation, heavy media separation, magnetic separation, rare-earth magnetic separation, chemical leaching, bio-chemical treatment and screening separation. This "front-end" beneficiation of the reductant allows the production of cleaner metallic iron and reduces the need for many costly purifying steps after the iron is metallized according to the invention.
• additive which catalyzes the direct reduction reaction and/or desulfurizes the iron in the DRI product.
• additive is intended to refer to a composition that, when present in an inventive feed material mixture according to the invention, enhances the rate of the reduction reaction, or, in other words, "catalyzes” the reduction of the iron oxide, and/or desulfurizes the iron in the DRI, therefore facilitating purification of the iron in downstream refining processes. It has been discovered by the present inventor that many additives advantageously perform both of these functions.
• Suitable additives for use in the advantageous 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. According to particular aspects of the invention, multiple additives may be blended prior to mixing with the other starting materials.
• additives may be obtained in the preferred powdered form from a wide variety of commercial outlets well known to a person skilled in the art
• Many additives, such as those described above, are detrimental to carbonaceous direct iron reduction processes taught in the prior art which utilize binders to form agglomerates or briquettes, because the additives interfere with the binders' ability to give the agglomerate adequate dry strength and durability. Therefore, if an additive 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 present inventor has discovered that hydrated or slaked lime is used in accordance with the invention to 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 hydrated lime is especially advantageous in certain embodiments of the invention, i.e. those wherein the resulting metallic iron is to be used for steelmaking, because the use of this additive reduces the amount of lime which must be added to the metallized iron in downstream steel-making operations.
• the moisture content of the mixture advantageously imparts the following characteristics to inventive mixtures: (1) elimination of dust problems, (2) improved compaction characteristics of the mixture on the bed, (3) prevention of the ignition of the mixture upon contact with a hot hearth, (4) prevention of segregation of mixture components which occurs, for example, due to the large differences in density between the oxide, carbon and additive components, and (5) delay of coal volatization so that, if the rotary hearth furnace flue is located at the beginning of the feed 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.
• the moisture content of a given mixture may be optimized according to the invention to minimize the drying load born by the hearth.
• the water content of the mixture is less than about 14% by weight, more preferably less than about 6% by weight, and most preferably less than about 4% by weight.
• Starting materials are preferably mixed intensely according to the invention in appropriate proportions to maximize the efficiency of the overall direct reduction process.
• iron oxide compositions such as iron ores
• carbonaceous reductants may have widely varying amounts of fixed carbon present therein
• the proportions of iron oxide composition to carbonaceous reductant are selected according to the invention based upon the amount of iron in the iron oxide composition and the amount of fixed carbon in the carbonaceous reductant.
• the ratio of carbon to iron in the mixture is selected to optimize reduction of the iron oxide without wasting reductant. It is within the purview of a skilled artisan to determine the amount of fixed carbon in the reductant and the amount of iron in the iron oxide composition, and to stoichiometrically determine the weight proportions of these two components needed to achieve optimal reduction. In this regard, it is expected that about 50% of the fixed carbon in a reductant according to the invention will effectively perform a reducing function on the hearth.
• the ratio of fixed carbon in the reductant 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.
• an inventive mixture comprises less than about 0.1% additive and from about 0.5% to about 14% water, both by weight.
• a mixture according the invention comprises from about 0.1% to about 10% additive by weight; and from about 0.5% to about 14% water by weight.
• the particulate iron oxide composition is hematitic or magnetitic virgin iron ore or concentrates thereof;
• the carbonaceous reductant is one or a blend of more than one low or medium volatile bituminous or subbituminous coals;
• the additive is one or a blend of more than one of the following compositions: 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 mixture comprises from about 65% to about 85% virgin ore; from about 10% to about 30% low to medium volatile bituminous or subbituminous coal; from about 0.5% to about 8% additive; and from about 1% to about 10% water, 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 carbonaceous reductant is from about 60% to about 80%.
• the mixture comprises from about 73% to about 79% low silica iron ore fines or concentrate by weight; from about 18% to about 22% low to medium volatile bituminous or subbituminous coal by weight; from about .5% to about 3% powdered additive by weight; and from about 2% to about 4% water by weight.
• the components are preferably mixed in the dry form, with the optional addition of water to achieve the features discussed above.
• a fine-grained magnetite concentrate in wet filter cake form is used as a starting material, then little or no water would be added such that the filter cake moisture (typically 9-11%) is reduced by the addition of dry constituents to approximately 7% by weight.
• each component of the mixture is carefully weigh-fed into an intensive mixer or pug mill to thoroughly blend the mixture.
• a mixture prepared or obtained as described above may then be introduced onto a rotary hearth furnace for reduction to sponge iron.
• the moist, blended mixture may be conveyed on, for example, a belt conveyor, to a feed bin (also capable of functioning as a surge bin 20, as discussed more fully below) spanning the width of the hearth.
• the feed bin is designed to handle the optimized moisture content of the mixture without bridging.
• Steep wall slopes, bin vibrators, and/or live bin bottoms may preferably be used to aid in the gravity flow of a moist mixture.
• the moist mixture may preferably be removed from the bin through, for example, a slide gate 16 by either a drum drag feeder 11 (optionally equipped with cleats) or a star feeder (not shown).
• the feeder in this embodiment preferably spans the full width of the bin and the hearth.
• the feed area is preferably shrouded, sealed, and fed with nitrogen gas to purge air and eliminate the hazard of combustion of the carbon in the mixture upon contact with the hot hearth.
• Examples of these include (1) vibrating feeder-conveyors with feed openings cut in a tapered fashion to proportion feed between inside and outside radii of the hearth, (2) drag feeders with slide chutes with vane deflectors to distribute more material to the outer portion of the hearth and less to the inner portion, (3) star feeders with tapered sectors, and (4) tapered drum drag feeders.
• the moist, blended mixture may be conveyed to an oscillating conveyor 21.
• the oscillating conveyor from which the feed mixture drops onto the hearth may advantageously be moved back and forth repeatedly across the width of the hearth, thus introducing feed material onto the hearth as the hearth steadily moves thereunder.
• the oscillating conveyor 21 in a preferred embodiment is oriented as shown in FIG. 3, and a relatively even layer of feed material on the hearth is achieved by a movement of the conveyor back and forth with greater speed at the "inside" end of the stroke.
• the eccentric oscillation allows the oscillating conveyor 21 to feed the mixture across the width of the hearth.
• a mixture layer on the hearth may be leveled using, for example, a set of stationary or oscillating plows 12 or a leveling screw 22 that sweep excess material to the inside or outside of the hearth. Excess material may preferably be swept off the hearth and recycled to the feed system. It is also preferred that the mixture layer which enters the reaction zone of the rotary hearth furnace has a loose-layer thickness of less than about 100mm.
• the thickness is less than about 30mm, yet more preferably less than about 20mm and 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.
• 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 mixture layer is "plowed,” “churned” or “shaved” at one or more points within the reaction zone of the rotary hearth furnace.
• a large layer for example, up to about 100mm thick, is reacted in the first zone of a multi-zone furnace, then the top 5 to 20mm is shaved off the hearth by a discharge screw or plow. Thereafter, the remaining "fresh” material continues reacting in the next zone. This sequence can be repeated with a number of zones and shaving steps in one rotation of the circular hearth.
• the layer of the mixture which is introduced onto the hearth may be compacted using a compacting device before entry into the reaction zone.
• the mixture may be compacted on the hearth using one or more tapered drums 13 rotating by friction with the bed or using, for example, one or more rollers, tires or cylinders.
• the preferred thickness of the post-compacted layer is preferably less than about 30mm, more preferably less than about 15mm.
• the compacting device 13 comprises a plurality of texturing devices for imparting various surface features or textures to the compacted layer, such as for example, grooves, windrows, holes or slots.
• the texturing devices enhance reaction efficiency, degree of metallization, uniformity of reaction and ease of removal of the reduced bed or compact from the hearth in the discharge zone. While it is not intended that the present invention be limited by any mechanism whereby it achieves its advantageous result, it is believed that such surface features or textures improve the efficiency of the reduction process by increasing the surface area of the mixture which is exposed to radiant heat in the reaction zone .
• the mixture In the reaction zone, the mixture, either in compacted or uncompacted form, 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 mixture is heated to a temperature of from about 1000°C to about 1500°C and for a period of time of from about 3 minutes to about 30 minutes.
• Conventional rotary hearth furnaces having multiple reaction zones may be advantageously used in accordance with the present invention. Since there is no threat of green ball explosion or breakage in inventive processes, such as those which may occur in other direct iron reduction systems known in the art by rapid heating of agglomerates, the mixture can be heated more rapidly than prior art. This feature, coupled with the lower moisture content of the mixture (preferably about 3%) and the elimination of void space on the hearth, provides the ability, using inventive methods, to greatly increase output of DRI at higher and more uniform metallization as compared to prior art reduction techniques.
• the reduced material (sponge iron), either in a compacted or uncompacted form, is removed from the hearth.
• the sponge iron is preferably removed using one or more water-cooled discharge plows or screws 17, and may then be discharged through refractory-lined chutes into, for example, insulated and nitrogen purged bottles or a nitrogen-purged hot conveyor or pneumatic conveyance system for transport to further processing steps.
• the product may be hot briquetted or cooled and prepared for merchant sale.
• the starting materials are not agglomerated, as is required in conventional rotary hearth direct reduction processes. Since no balling, briquetting or other agglomeration is performed, no binding agents, balling disks, balling drums, roll screens or briquetting machines are required in the process and capital and operating costs of the process are therefore substantially reduced. Additionally, since there is no wet agglomeration step prior to introducing the mixture onto the hearth, there is no need to undergo capital-intensive and costly drying steps prior to introduction of the starting materials onto the hearth. Also, due to the lower moisture content of the furnace charge according to the present invention, the drying task on the hearth is reduced substantially, the result being substantially greater unit productivity of the hearth. Furthermore, the avoidance of agglomerated starting materials enables a greater efficiency of metallization because the burden on the hearth has greater density and less air space which results from use of the agglomerates laid in a bed.
• the elimination of agglomeration steps also overcomes the problem of explosion of wet green balls when they are placed onto a hot hearth.
• prior art rotary hearth furnace processes required, in addition to very high dosages of binders, that the hearth be cooled prior to introduction of the agglomerates therein to avoid thermal shock to the wet green balls, thereby preventing green ball explosion.
• the consequence of this cooling is a substantial loss in thermal efficiency and unit productivity of the rotary hearth reduction furnace.
• the present invention avoids this efficiency and productivity penalty.
• the present invention provides as an option the preheating of the hearth prior to placement of the mixture onto the hearth thereby gaining productivity and efficiency in the direct reduction process.
• the preheated hearth aids in the metallization of the bottom materials of the mixture thereon, and thereby overcomes a limiting condition in the prior art of rotary hearth reduction where the bottom layer of agglomerates dictate the degree of metallization and productivity. Since there is no agglomeration associated with the reduction method of the present invention, the risks of agglomerate thermal shock and green ball explosion are eliminated.
• Another advantage of avoiding the formation of agglomerates is the very high cost of using such high concentrations of synthetic and bentonite clay binders.
• these binders are often the fourth largest cost component of the reduction process, behind the ore, the solid reductant and combustion fuel (with the cost of binders in some locations being greater than that of combustion fuel). Therefore, the avoidance of this cost component is a very desirable advantage of the invention, and provides a cost advantage of up to 15% of total cash costs of DRI production.
• preferred inventive methods advantageously may include a wide variety of additives, as described above.
• a surge bin 20 may be used.
• a surge bin 20 is a container for holding feed material prior to introducing the material onto the hearth, and provides a buffer in the event that there is an interruption of some kind in a pre-hearth step of the overall reduction process. While a surge bin is not feasible (in the case of green ball processing) or is only minimally useful (in the case of dried agglomerates) in prior art carbonaceous direct reduction processes, a surge bin 20 is a useful tool in accordance with inventive processes.
• the surge bin 20 may preferably be placed immediately adjacent or above the loading region of the hearth, and the feed material may be introduced directly from the surge bin 20 onto the hearth or indirectly via a feeder and conveyor arrangement.
• a surge bin 20 allows a process according to the invention to run more continuously, even where a prior processing step is interrupted. This prevents the need to restart and reheat the furnace, as would be required if the process were intermittantly stopped, this greatly improving overall thermal efficiency of the process. Additionally, minimizing interruptions and, thus reducing "down time,” helps to optimize output of the hearth and also yields a higher quality product. All of these factors result in substantial advantages over reduction processes known in the prior art.
• Specular hematite concentrate containing less than 5% silica and more than 64% elemental iron by weight is ground by either a ball mill or roll press to a nominal particle size distribution of at least 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.
• Hydrated lime in powder form is also admixed at a dosage of 1% of the dry mixture. The entire mixture is wetted to 3.0% water content in an intensive mixer.
• the moist mixture is conveyed to and placed in a uniform and compacted layer of 15mm thickness on a rotary hearth furnace in a shrouded, sealed and nitrogen-purged feed zone.
• the mixture is rapidly heated by natural gas and/or coal fired burners to approximately 1300°C. Residence time in the furnace is approximately 10 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 bottle for transfer to an electric arc furnace to be hot charged into the EAF melt.
• Magnetite concentrate from Minnesota or Michigan with a silica content between 1.5% and 6.0% is ground to at least 80% passing 325 mesh, and filtered to a moisture content of about 10%. It is then mixed with a reductant mix of a dry ground blend of 50% pet coke and 50% high fixed carbon, low volatile coal together with a 2% dosage of powdered hydrated lime.
• the mix recipe by weight is 72% iron ore concentrate, 19% reductant mix, 7% water and 2% hydrated lime.
• the thoroughly blended mixture is conventionally conveyed and, using an oscillating conveyer, spread 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 25 mm thick.
• Hematite fines at about 4% moisture from South America or Australia are ground using a high pressure roll press to 50% minus 200 mesh and then admixed with a coal blend containing 75% fixed carbon and 17% volatiles.
• the coal blend has been dry ground to about 80% smaller than 100 microns.
• the moist ground ore and dry coal are mixed with dolomitic hydrated lime at a ratio of 77% ore, 22% coal and 1% dolomitic hydrated lime, by weight.
• the blend is intensively mixed in a pug mill before belt conveyance to an oscillating conveyer that distributes the mix across a 7.0 meter wide rotary hearth.
• High temperature alloy plows level the bed to 15mm and a tapered drum with conical points compacts the bed to about 10mm thickness with conical depressions in a pattern with cones spaced every 3cm with a conical base diameter of 8mm and a length of 10mm.
• the entire feed 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 1000°C.
• Reaction time including drying time is approximately 10 minutes to 12 minutes.
• the DRI bed is removed in the discharge zone by a water cooled discharge screw with a diameter of 36 inches. DRI with average metallization of 95% is passed into nitrogen filled bottles lined with refractory.
• the hot DRI is transported to a submerged arc furnace for melting and further refining.

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• Engineering & Computer Science (AREA)
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• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacture Of Iron (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Manufacture Of Metal Powder And Suspensions Thereof (AREA)

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• 1998
  • 1998-04-22 JP JP54618898A patent/JP2001522405A/ja active Pending
  • 1998-04-22 BR BR9809287-1A patent/BR9809287A/pt not_active Application Discontinuation
  • 1998-04-22 CA CA002288095A patent/CA2288095A1/en not_active Abandoned
  • 1998-04-22 EP EP98918362A patent/EP1017858A4/de not_active Withdrawn
  • 1998-04-22 KR KR1019997009804A patent/KR20010020219A/ko active Search and Examination

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