Patent Grant
7641712
The present invention relates to the reduction of metal bearing material (e.g., the reduction of iron bearing material such as iron ore).
Many different iron ore reduction processes have been described and/or used in the past. The processes may be traditionally classified into direct reduction processes and smelting reduction processes. Generally, direct reduction processes convert iron ores into a solid state metallic form with, for example, use of shaft furnaces (e.g., natural gas-based shaft furnaces), whereas smelting reduction converts iron ores into molten hot metal without the use of blast furnaces.
Many of the conventional reduction processes for production of direct reduced iron (DRI) are either gas-based processes or coal-based processes. For example, in the gas-based process, direct reduction of iron oxide (e.g., iron ores or iron oxide pellets) employs the use of a reducing gas (e.g., reformed natural gas) to reduce the iron oxide and obtain DRI. Methods of making DRI have employed the use of materials that include carbon (e.g., coal, charcoal, etc.) as a reducing agent. For example, coal-based methods include the SL-RN method described in, for example, the reference entitled “Direct reduction down under: the New Zealand story”, D. A. Bold, et al., Iron Steel International, Vol. 50, 3, pp. 145 and 147-52 (1977), or the FASTMET® method described in, for example, the reference entitled “Development of FASTMET® as a New Direct Reduction Process,” by Miyagawa et al., 1998 ICSTI/IRONMAKING Conference Proceedings, pp. 877-881.
Another reduction process in between gas-based or coal-based direct reduction processing and smelting reduction processing may be referred to as fusion reduction. Fusion reduction processes have been described in, for example, the reference entitled “A new process to produce iron directly from fine ore and coal,” by Kobayashi et al., I&SM, pp. 19-22 (September 2001), and, for example, in the reference entitled “New coal-based process, Hi-QIP, to produce high quality DRI for the EAF,” by Sawa et al., ISIJ International, Vol. 41 (2001), Supplement, pp. S17-S21. Such fusion reduction processes, generally, for example, involve the following generalized processing steps: feed preparation, drying, furnace loading, preheating, reduction, fusion/melting, cooling, product discharge, and product separation.
Various types of hearth furnaces have been described and/or used for direct reduction processing. One type of hearth furnace, referred to as a rotary hearth furnace (RHF), has been used as a furnace for coal-based production. For example, in one embodiment, the rotary hearth furnace has an annular hearth partitioned into a preheating zone, a reduction zone, a fusion zone, and a cooling zone, located along the supply side and the discharge side of the furnace. The annular hearth is supported in the furnace so as to move rotationally. In operation, for example, raw material comprising a mixture, for example, of iron ore and reduction material is charged onto the annular hearth and provided to the preheat zone.
After preheating, through rotation, the iron ore mixture on the hearth is moved to the reduction zone where the iron ore is reduced in the presence of reduction material into reduced and fused iron (e.g., metallic iron nuggets) with use of one or more heat sources (e.g., gas burners). The reduced and fused product, after completion of the reduction process, is cooled in the cooling zone on the rotating hearth for preventing oxidation and facilitating discharge from the furnace.
Various rotary hearth furnaces for use in direct reduction processes have been described. For example, one or more embodiments of such furnaces are described in U.S. Pat. No. 6,126,718 to Sawa et al., issued 3 Oct. 2000 and entitled “Method of Producing a Reduced Metal, and Traveling Hearth Furnace for Producing Same.” Further, for example, other types of hearth furnaces have also been described. For example, a paired straight hearth (PSH) furnace is described in U.S. Pat. No. 6,257,879B1 to Lu et al., issued 10 Jul. 2001, entitled “Paired straight hearth (PSH) furnaces for metal oxide reduction,” as well as a linear hearth furnace (LHF) described in U.S. Provisional Patent Application No. 60/558,197, filed 31 Mar. 2004, published as US 2005-0229748A1, and entitled, “Linear hearth furnace system and methods regarding same.”
Natural gas-based direct reduced iron accounts for over 90% of the world's DRI production. Coal-based processes are generally used to produce the remaining amount of direct reduced iron. However, in many geographical regions, the use of coal may be more desirable because coal prices may be more stable than natural gas prices. Further, many geographical regions are far away from steel mills that use the processed product. Therefore, shipment of iron units in the form of metallized iron nuggets produced by a coal-based fusion reduction process may be more desirable than use of a smelting reduction process.
Generally, metallic iron nuggets are characterized by high grade, essentially 100% metal (e.g., about 96% to about 97% metallic Fe). Such metallic iron nuggets are desirable in many circumstances, for example, at least relative to taconite pellets, which may contain 30% oxygen and 5% gangue. Metallic iron nuggets are low in gangue because silicon dioxide has been removed as slag. As such, with metallic iron nuggets, there is less weight to transport. Further, unlike conventional direct reduced iron, metallic iron nuggets have low oxidation rates because they are solid metal and have little or no porosity. In addition, generally, such metallic iron nuggets are just as easy to handle as iron ore pellets.
One exemplary metallic iron nugget fusion process for producing metallic iron nuggets is referred to as ITmk3. For example, in such a process, dried balls formed using iron ore, coal, and a binder, are fed to furnace (e.g., a rotary hearth furnace). As the temperature increases in the furnace, the iron ore concentrate is reduced and fuses when the temperature reaches between 1450° C. to 1500° C. The resulting products are cooled and then discharged. The cooled products generally include pellet-sized metallic iron nuggets and slag which are broken apart and separated. For example, such metallic iron nuggets produced in such a process are typically about one-quarter to three-eighths inch in size and are reportedly analyzed to include about 96 percent to about 97 percent metallic Fe and about 2.5 percent to about 3.5 percent carbon. For example, one or more embodiments of such a method are described in U.S. Pat. No. 6,036,744 to Negami et al., entitled “Method and apparatus for making metallic iron,” issued 14 Mar. 2000 and U.S. Pat. No. 6,506,231 to Negami et al., entitled “Method and apparatus for making metallic iron,” issued 14 Jan. 2003.
Further, another metallic iron nugget process has also been reportedly used for producing metallic iron. For example, in this process, a pulverized anthracite layer is spread over a hearth and a regular pattern of dimples is made therein. Then, a layer of iron ore and coal mixture is placed and heated to 1500° C. The iron ore is reduced to metallic iron, fused, and collected in the dimples as iron pebbles and slag. Then, the iron pebbles and slag are broken apart and separated. One or more embodiments of such a process are described in U.S. Pat. No. 6,270,552 to Takeda et al., entitled “Rotary hearth furnace for reducing oxides, and method of operating the furnace,” issued 7 Aug. 2001. Further, for example, various embodiments of this process (referred to as the Hi-QIP process) that utilize the formation of cup-like depressions in a solid reducing material to obtain a reduced metal are described in U.S. Pat. No. 6,126,718 to Sawa et al.
Such metallic iron nugget formation processes, therefore, involve mixing of iron-bearing materials and pulverized coal (e.g., a carbonaceous reductant). For example, either with or without forming balls, iron ore/coal mixture is fed to a hearth furnace (e.g., a rotary hearth furnace) and heated to a temperature reportedly 1450° C. to approximately 1500° C. to form fused direct reduced iron (i.e., metallic iron nuggets) and slag. Metallic iron and slag can then be separated, for example, with use of mild mechanical action and magnetic separation techniques.
Other reduction processes for producing reduced iron are described in, for example, U.S. Pat. No. 6,210,462 to Kikuchi et al., entitled “Method and apparatus for making metallic iron,” issued 3 Apr. 2001 and U.S. patent application Ser. No. US2001/0037703 A1 to Fuji et al., entitled “Method for producing reduced iron,” published 8 Nov. 2001. For example, U.S. Pat. No. 6,210,462 to Kikuchi et al. describes a method where preliminary molding of balls is not required to form metallic iron.
However, there are various concerns regarding such iron nugget processes. For example, one major concern of one or more of such processes involves the prevention of slag from reacting with the hearth refractory during such processing. Such a concern may be resolved by placing a layer of pulverized coke or other carbonaceous material on the hearth refractory to prevent the penetration of slag from reacting with the hearth refractory.
Another concern with regard to such metallic iron nugget production processes is that very high temperatures are necessary to complete the process. For example, as reported, such temperatures are in the range of 1450° C. to about 1500° C. This is generally considered fairly high when compared to taconite pelletization carried out at temperatures in the range of about 1288° C. to about 1316° C. Such high temperatures adversely affect furnace refractories, maintenance costs, and energy requirements.
Yet another problem is that sulfur is a major undesirable impurity in steel. However, carbonaceous reductants utilized in metallic iron nugget formation processes generally include sulfur resulting in such an impurity in the nuggets formed.
Further, at least in ITmk3 processes, a prior ball formation process utilizing a binder is employed. For example, iron ore is mixed with pulverized coal and a binder, balled, and then heated. Such a preprocessing (e.g., ball forming) step which utilizes binders adds undesirable cost to a metallic iron nugget production process.
Still further, various steel production processes prefer certain size nuggets. For example, furnace operations that employ conventional scrap charging practices appear to be better fed with large-sized iron nuggets. Other operations that employ direct injection systems for iron materials indicate that a combination of sizes may be important for their operations.
A previously described metallic iron nugget production method that starts with balled feed uses balled iron ore with a maximum size of approximately three-quarter inch diameter dried balls. These balls shrink to iron nuggets of about three-eighths inch in size through losses of oxygen from iron during the reduction process, by the loss of coal by gasification, with loss of weight due to slagging of gangue and ash, and with loss of porosity. Nuggets of such size, in many circumstances, may not provide the advantages associated with larger nuggets that are desirable in certain furnace operations.
The methods and systems according to the present invention provide for one more various advantages in the reduction processes, e.g., production of metallic iron nuggets. For example, such methods and systems may provide for controlling iron nugget size (e.g., using mounds of feed mixture with channels filled at least partially with carbonaceous material), may provide for control of micro-nugget formation (e.g., with the treatment of hearth material layers), may provide for control of sulfur in the iron nuggets (e.g., with the addition of a fluxing agent to the feed mixture), etc.
One embodiment of a method for use in production of metallic iron nuggets according to the present invention includes providing a hearth including refractory material and providing a hearth material layer on the refractory material (e.g., the hearth material layer includes at least carbonaceous material or carbonaceous material coated with Al(OH)3, CaF2 or the combination of Ca(OH)2 and CaF2). A layer of a reducible mixture is provided on at least a portion of the hearth material layer (e.g., the reducible mixture includes at least reducing material and reducible iron bearing material). A plurality of channel openings extend at least partially into the layer of the reducible mixture to define a plurality of nugget forming reducible material regions (e.g., one or more of the plurality of nugget forming reducible material regions may include a mound of the reducible mixture that includes at least one curved or sloped portion, such as a dome-shaped mound or a pyramid-shaped mound of the reducible mixture). The plurality of channel openings are at least partially filled with nugget separation fill material (e.g., the nugget separation fill material includes at least carbonaceous material). The layer of reducible mixture is thermally treated to form one or more metallic iron nuggets (e.g., metallic iron nuggets that include a maximum length across the maximum cross-section that is greater than about 0.25 inches and less than about 4.0 inches) in one or more of the plurality of the nugget forming reducible material regions (e.g., forming a single metallic iron nugget in each of one or more of the plurality of the nugget forming reducible material regions).
In various embodiments, the layer of a reducible mixture may be a layer of reducible micro-agglomerates (e.g., where at least 50 percent of the layer of reducible mixture comprises micro-agglomerates having a average size of about 2 millimeters or less), or may be a layer of compacts (e.g., briquettes, partial-briquettes, compacted mounds, compaction profiles formed in layer of reducible material, etc.).
Yet further, the layer of a reducible mixture on the hearth material layer may include multiple layers where the average size of the reducible micro-agglomerates of at least one provided layer is different relative to the average size of micro-agglomerates previously provided (e.g., the average size of the reducible micro-agglomerates of at least one of the provided layers is less than the average size of micro-agglomerates of a first layer provided on the hearth material layer).
In addition, a stoichiometric amount of reducing material is the amount necessary for complete metallization and formation of metallic iron nuggets from a predetermined quantity of reducible iron bearing material. In one or more embodiments of the method, providing the layer of a reducible mixture on the hearth material layer may include providing a first layer of reducible mixture on the hearth material layer that includes a predetermined quantity of reducible iron bearing material and between about 70 percent and about 90 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof, and providing one or more additional layers of reducible mixture that includes a predetermined quantity of reducible iron bearing material and between about 105 percent and about 140 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof.
In yet another embodiment of the method, thermally treating the layer of reducible mixture includes thermally treating the layer of reducible mixture at a temperature less than 1450 degrees centigrade such that the reducible mixture in the nugget forming reducible material regions is caused to shrink and separate from other adjacent nugget forming reducible material regions. More preferably, the temperature is less than 1400° C.; even more preferably, the temperature is below 1390° C.; even more preferably, the temperature is below 1375° C.; and most preferably, the temperature is below 1350° C.
Yet further, in one or more embodiments of the method, the reducible mixture may further include at least one additive selected from the group consisting of calcium oxide, one or more compounds capable of producing calcium oxide upon thermal decomposition thereof (e.g., limestone), sodium oxide, and one or more compounds capable of producing sodium oxide upon thermal decomposition thereof. In addition, in one or more embodiments, the reducible mixture may include soda ash, Na2CO3, NaHCO3, NaOH, borax, NaF, and/or aluminum smelting industry slag. Still further, one or more embodiments of the reducible mixture may include at least one fluxing agent selected from the group consisting of fluorspar, CaF2, borax, NaF, and aluminum smelting industry slag.
Another method for use in production of metallic iron nuggets according to the present invention includes providing a hearth that includes refractory material and providing a hearth material layer on the refractory material (e.g., the hearth material layer may include at least carbonaceous material). A layer of reducible micro-agglomerates is provided on at least a portion of the hearth material layer, where at least 50 percent of the layer of reducible micro-agglomerates comprise micro-agglomerates having a average size of about 2 millimeters or less. The reducible micro-agglomerates are formed from at least reducing material and reducible iron bearing material. The layer of reducible micro-agglomerates is thermally treated to form one or more metallic iron nuggets.
In one or more embodiments of the method, the layer of reducible micro-agglomerates is provided by a first layer of reducible micro-agglomerates on the hearth material layer and by providing one or more additional layers of reducible micro-agglomerates on the first layer. The average size of the reducible micro-agglomerates of at least one of the provided additional layers is different relative to the average size of micro-agglomerates previously provided (e.g., the average size of the reducible micro-agglomerates of at least one of the provided additional layers is less than the average size of micro-agglomerates of the first layer).
Further, in one or more embodiments of the method, the first layer of reducible micro-agglomerates on the hearth material layer includes a predetermined quantity of reducible iron bearing material and between about 70 percent and about 90 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof, and the provided additional layers of reducible micro-agglomerates include a predetermined quantity of reducible iron bearing material and between about 105 percent and about 140 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof.
Yet further, in one or more embodiments of the method, providing the layer of reducible micro-agglomerates includes forming the reducible micro-agglomerates using at least water, reducing material, reducible iron bearing material, and one or more additives selected from the group consisting of calcium oxide, one or more compounds capable of producing calcium oxide upon thermal decomposition thereof, sodium oxide, and one or more compounds capable of producing sodium oxide upon thermal decomposition thereof. Further, the reducible micro-agglomerates may include at least one additive selected from the group consisting of soda ash, Na2CO3, NaHCO3, NaOH, borax, NaF, and aluminum smelting industry slag or at least one fluxing agent selected from the group consisting of fluorspar, CaF2, borax, NaF, and aluminum smelting industry slag.
In one preferred embodiment, a method for use in production of metallic iron nuggets comprising the steps of: providing a hearth comprising refractory material; providing a hearth material layer on the refractory material, the hearth material layer comprising at least carbonaceous material coated with one of Al(OH)3, CaF2 or the combination of Ca(OH)2 and CaF2; providing a layer of a reducible mixture on at least a portion of the hearth material layer, at least a portion of the reducible mixture comprising at least reducing material and reducible iron bearing material; the reducible mixture comprising at least one additive selected from the group consisting of calcium oxide, one or more compounds capable of producing calcium oxide upon thermal decomposition thereof, sodium oxide, and one or more compounds capable of producing sodium oxide upon thermal decomposition thereof; forming a plurality of channel openings extending at least partially into the layer of the reducible mixture to define a plurality of nugget forming reducible material regions having a density less than about 2.4; at least partially filling the plurality of channel openings with nugget separation fill material comprising at least carbonaceous material; and thermally treating the layer of reducible mixture at a temperature of less than 1450° C. to form one or more metallic iron nuggets in one or more of the plurality of the nugget forming reducible material regions is provided.
Yet another method for use in production of metallic iron nuggets according to the present invention includes providing a hearth that includes refractory material and providing a hearth material layer on at least a portion of the refractory material (e.g., the hearth material layer may include at least carbonaceous material). A reducible mixture is provided on at least a portion of the hearth material layer (e.g., the reducible mixture includes at least reducing material and reducible iron bearing material). A stoichiometric amount of reducing material is the amount necessary for complete metallization and formation of metallic iron nuggets from a predetermined quantity of reducible iron bearing material. In one embodiment, providing the reducible mixture on the hearth material layer includes providing a first portion of reducible mixture on the hearth material layer that includes a predetermined quantity of reducible iron bearing material and between about 70 percent and about 90 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof, and providing one or more additional portions of reducible mixture that comprise a predetermined quantity of reducible iron bearing material and between about 105 percent and about 140 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof. The reducible mixture is then thermally treated to form one or more metallic iron nuggets. For certain applications, the hearth layer might not be used, or the hearth layer might not contain any carbonaceous material.
In one embodiment of the method, a plurality of channel openings extend at least partially into the reducible mixture and define a plurality of nugget forming reducible material regions, and further where the channel openings are at least partially filled with nugget separation fill material.
In yet another embodiment of the method, providing the first portion of a reducible mixture on the hearth material layer includes providing a first layer of reducible micro-agglomerates on the hearth material layer and where providing one or more additional portions includes providing one or more additional layers of reducible micro-agglomerates on the first layer, where the average size of the reducible micro-agglomerates of at least one of the provided additional layers is different relative to the average size of micro-agglomerates previously provided.
In another embodiment, providing reducible mixture on the hearth material layer includes providing compacts of the reducible mixture. For example, a first portion of each of one or more compacts includes a predetermined quantity of reducible iron bearing material and between about 70 percent and about 90 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof, and one or more additional portions of each of one or more of compacts includes a predetermined quantity of reducible iron bearing material and between about 105 percent and about 140 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof.
Yet further, in another embodiment of the method, the compacts may include at least one of briquettes (e.g., three layer briquettes), partial-briquettes (e.g., two layers of compacted reducible mixture), balls, compacted mounds of the reducible mixture comprising at least one curved or sloped portion, compacted dome-shaped mounds of the reducible mixture, and compacted pyramid-shaped mounds of the reducible mixture. In one preferred embodiment, the partial-briquettes comprise full briquettes cut in half. The reducible mixture may even be multilayered balls of reducible mixture. In one embodiment, the mounds have a density of about 1.9-2, the balls have a density of about 2.1 and briquettes have a density of about 2.1. In one embodiment, the reducible material has a density less than about 2.4. In a preferred embodiment, the reducible material has a density between about 1.4 and 2.2.
Still further, yet another method for use in production of metallic iron nuggets is described herein. The method includes providing a hearth that includes refractory material and providing a hearth material layer on at least a portion of the refractory material. The hearth material layer includes at least carbonaceous material. Reducible mixture is provided on at least a portion of the hearth material layer. The reducible mixture includes: reducing material; reducible iron bearing material; one or more additives selected from the group consisting of calcium oxide, one or more compounds capable of producing calcium oxide upon thermal decomposition thereof, sodium oxide, and one or more compounds capable of producing sodium oxide upon thermal decomposition thereof; and at least one fluxing agent selected from the group consisting of fluorspar, CaF2, borax, NaF, and aluminum smelting industry slag. The reducible mixture is thermally treated (e.g., at a temperature less than about 1450 degrees centigrade) to form one or more metallic iron nuggets.
In one or more embodiments of the method, the reducible mixture may include at least one additive selected from the group consisting of calcium oxide and limestone. In other embodiments of the method, the reducible mixture may include at least one additive selected from the group consisting of soda ash, Na2CO3, NaHCO3, NaOH, borax, NaF, and aluminum smelting industry slag. Yet further, the hearth material layer may include carbonaceous material coated with Al(OH)3, CaF2 or the combination of Ca(OH)2 and CaF2.
Yet further, in one or more embodiments of the method, the reducible mixture may include one or more mounds of reducible mixture including at least one curved or sloped portion; may include reducible micro-agglomerates or multiple layers thereof having different composition; may include compacts such as one of briquettes, partial-briquettes, balls, compacted mounds of the reducible mixture comprising at least one curved or sloped portion, compacted dome-shaped mounds of the reducible mixture, and compacted pyramid-shaped mounds of the reducible mixture; or may include balls (e.g., dried balls) or multiple layered balls.
A system for use in production of metallic iron nuggets is also described herein. For example, one embodiment of a system according to the present invention may include a hearth comprising refractory material for receiving a hearth material layer thereon (e.g., the hearth material layer may include at least carbonaceous material) and a charging apparatus operable to provide a layer of a reducible mixture on at least a portion of the hearth material layer. The reducible mixture may include at least reducing material and reducible iron bearing material. The system further includes a channel definition device operable to create a plurality of channel openings that extend at least partially into the layer of the reducible mixture to define a plurality of nugget forming reducible material regions and a channel fill apparatus operable to at least partially fill the plurality of channel openings with nugget separation fill material (e.g., the nugget separation fill material may include at least carbonaceous material). A furnace is also provided that is operable to thermally treat the layer of reducible mixture to form one or more metallic iron nuggets in one or more of the plurality of the nugget forming reducible material regions.
In one or more embodiments of the system, the channel definition device may be operable to create mounds of the reducible mixture that include at least one curved or sloped portion (e.g., create dome-shaped mounds or pyramid-shaped mounds of the reducible mixture).
In still yet another method for use in production of metallic iron nuggets, the method includes providing a hearth including refractory material and providing a hearth material layer (e.g., at least carbonaceous material) on at least a portion of the refractory material. Reducible mixture is provided on at least a portion of the hearth material layer. The reducible mixture includes at least reducing material and reducible iron bearing material. A stoichiometric amount of reducing material is the amount necessary for complete metallization and formation of metallic iron nuggets from a predetermined quantity of reducible iron bearing material. At least a portion of the reducible mixture includes the predetermined quantity of reducible iron bearing material and between about 70 percent and about 90 percent of said stoichiometric amount of reducing material necessary for complete metallization thereof. The method further includes thermally treating the reducible mixture to form one or more metallic iron nuggets.
In one embodiment of the method, providing reducible mixture on at least a portion of the hearth material layer includes providing one or more layers of reducible mixture on the hearth material layer. A plurality of channel openings are defined that extend at least partially into the layer of the reducible mixture and define a plurality of nugget forming reducible material regions. Further, the channel openings are at least partially filled with nugget separation fill material (e.g., carbonaceous material).
Yet further, in one or more embodiments of the method, the reducible mixture may include one or more mounds of reducible mixture including at least one curved or sloped portion; may include reducible micro-agglomerates or multiple layers thereof having different composition; may include compacts such as one of briquettes (e.g., single or multiple layer briquettes), partial-briquettes, balls, compacted mounds of the reducible mixture comprising at least one curved or sloped portion, compacted dome-shaped mounds of the reducible mixture, and compacted pyramid-shaped mounds of the reducible mixture; or may include balls (e.g., dried balls) or multiple layered balls.
Yet further, in one or more embodiments of the method, the reducible mixture may include one or more additives selected from the group consisting of calcium oxide, one or more compounds capable of producing calcium oxide upon thermal decomposition thereof, sodium oxide, and one or more compounds capable of producing sodium oxide upon thermal decomposition thereof. Further, the reducible mixture may include at least one additive selected from the group consisting of soda ash, Na2CO3, NaHCO3, NaOH, borax, NaF, and aluminum smelting industry slag or at least one fluxing agent selected from the group consisting of fluorspar, CaF2, borax, NaF, and aluminum smelting industry slag.
Yet further, one embodiment of the method may include providing compacts, and yet further providing additional reducing material adjacent at least a portion of the compacts.
In a further embodiment of the invention, a reducible mixture comprising: reducing material; reducible iron bearing material; one or more additives selected from the group consisting of calcium oxide, one or more compounds capable of producing calcium oxide upon thermal decomposition thereof, sodium oxide, and one or more compounds capable of producing sodium oxide upon thermal decomposition thereof; and at least one fluxing agent selected from the group consisting of fluorspar, CaF2, borax, NaF, and aluminum smelting industry slag is provided.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.
One or more embodiments of the present invention shall generally be described with reference to
It will be apparent to one skilled in the art that elements or process steps from one or more embodiments described herein may be used in combination with elements or process steps from one or more other embodiments described herein, and that the present invention is not limited to the specific embodiments provided herein but only as set forth in the accompanying claims. For example, and not to be considered as limiting to the present invention, the addition of one or more additives (e.g., fluorspar) to the reducible mixture may be used in combination with the provision of the reducible mixture as micro-agglomerates, the nugget separation fill material in the channels may be used in combination with provision of the reducible mixture as micro-agglomerates, the molding process for forming the channels and mounds of reducible mixture may be used in combination with nugget separation fill material in the channels and/or with provision of the reducible mixture as micro-agglomerates, etc.
Further, various metallic iron nugget processes are known and/or have been described in one or more references. For example, such processes include the ITmk3 process as presented in, for example, U.S. Pat. No. 6,036,744 to Negami et al. and/or U.S. Pat. No. 6,506,231 to Negami et al.; the Hi-QIP process as presented in, for example, U.S. Pat. No. 6,270,552 to Takeda et al. and/or U.S. Pat. No. 6,126,718 to Sawa et al.; or other metallic nugget processes as described in, for example, U.S. Pat. No. 6,210,462 to Kikuchi et al., U.S. Patent Application No. US2001/0037703 A1 to Fuji et al., and U.S. Pat. No. 6,210,462 to Kikuchi et al. One or more embodiments described herein may be used in combination with elements and/or process steps from one or more embodiments of such metallic nugget processes. For example, and not to be considered as limiting to the present invention, the addition of one or more additives (e.g., fluorspar) to the reducible mixture and/or any reducible mixture described herein may be used in combination with the provision of the reducible mixture as a preformed ball, as the reducible mixture used to fill dimples in a pulverized carbonaceous layer, as part of one or more compacts (e.g., briquettes), or may be used in one or more other various molding techniques as part of such metallic iron nugget formation processes. As such, the concepts and techniques described in one or more embodiments herein are not limited to use with only the metallic iron nugget process described generally herein with reference to
The present invention as will be described in further detail herein may be used, for example, to provide one or more of the following benefits or features. For example, the present invention may be used to control the metallic iron nugget size as described herein. Conventional dried balls as feed mixtures lead to iron nuggets of small sizes in the order of ⅜ inches. Use of the mounds of reducible mixture (e.g., trapezoidal and dome-shaped mounds with channels filled partially with carbonaceous material) can increase the iron nugget size to as large as 4 inches across. Various shapes of mounds (e.g., trapezoidal mounds) may require a longer time to form fully fused iron nuggets than dome-shaped mounds of equal size.
Further, for example, micro-agglomeration may be used to minimize dust losses in feeding furnaces (e.g., rotary or linear hearth furnaces); micro-agglomerates may be placed in layers over a hearth layer with respect to size, feed composition (e.g., stoichiometric percentage of coal may vary), etc.; and compaction of feed mixtures after placing them on a hearth layer (or, in one or more embodiments, compaction before placement on the hearth, such as, to form briquettes including one or more layers) may be desirable in view of the high CO2 and highly turbulent furnace gas atmospheres, particularly in a linear hearth furnace as described herein.
Yet further, for example, the present invention may be used to control micro-nugget formation. As described herein, use of excess coal beyond the stoichiometric requirement for metallization of a reducible feed mixture, and use of excess lime beyond a predetermined slag composition (e.g., a Slag Composition (L)) for the feed mixture, has led to an increased amount of micro-nuggets.
As described further herein, for example, Slag Composition (L), as shown on the CaO—SiO2—Al2O3 phase diagram of
The slag compositions are abbreviated by indicating the amounts of additional lime used in percent as a suffix, for example, (L1) and (L2) which represents that 1% and 2% by weight of lime was added to the feed mixture, respectively, over that of Slag Composition (L). In other words, the feed mixture includes an additional 1% and 2% by weight of lime, respectively, than the feed mixture at Slag Composition (L). Further, for example, the slag compositions are further abbreviated herein to indicate the existence of other elements or compounds in the feed mixture. For example, the amount of chemical CaF2 (abbreviated to CF) added in percent is indicated as a suffix, for example, (L0.5CF0.25) represents that the feed mixture includes 0.25% by weight of CaF2 with Slag Composition of (L0.5).
The use of hearth layers, including coke-alumina mixtures as well as Al(OH)3-coated coke, may be used to reduce such micro-nugget formation as described herein. Further, for example, addition of certain additives, such as fluorspar to the feed mixture may reduce the amount of micro-nuggets produced during processing of the feed mixture.
Still further, for example, as described herein, the present invention may be used to control the amount of sulfur in iron nuggets produced according to the present invention. It is common practice in the steel industry to increase the basicity of slag by adding lime to slag under reducing atmosphere for removing sulfur from metallic iron, for example, in blast furnaces. Increasing lime from Slag Composition (L) to (L1.5) and (L2) may lower sulfur (e.g., from 0.084% to only 0.058% and 0.050%, respectively, as described herein) but increases the fusion temperature as well as the amount of micro-nuggets generated, as described herein. The use of fluxing additives that lower the slag fusion temperature, such as fluorspar, was found to lower not only the temperature of iron nugget formation, but also to decrease sulfur in the iron nuggets, and, in particular, to be effective in decreasing the amount of micro-nuggets.
With increasing fluorspar (FS) addition, for example, sulfur in iron nuggets at Slag Compositions (L1.5FS0.5˜4) and (L2FS0.5˜4) was lowered steadily to as low as 0.013% and 0.009%, respectively, at fluorspar addition of 4%, as described further herein. The use of soda ash, particularly in combination with fluorspar, was effective in lowering sulfur in iron nuggets, but the use of soda ash tended to increase the amount of micro-nuggets also as described further herein.
As shown in block 12 of
Generally, hearth 42 includes a refractory material upon which material to be processed (e.g., feed material) is received. For example, in one or more embodiments, the refractory material may be used to form the hearth (e.g., the hearth may be a container formed of a refractory material) and/or the hearth may include, for example, a supporting substructure that carries a refractory material (e.g., a refractory lined hearth).
In one embodiment, for example, the supporting substructure may be formed from one or more different materials, such as, for example, stainless steel, carbon steel, or other metals, alloys, or combinations thereof that have the required high temperature characteristics for furnace processing. Further, the refractory material may be, for example, refractory board, refractory brick, ceramic brick, or a castable refractory. Yet further, for example, a combination of refractory board and refractory brick may be selected to provide maximum thermal protection for an underlying substructure.
In one embodiment of the present invention, for example, a linear hearth furnace system is used for furnace processing such as described in U.S. Provisional Patent Application No. 60/558,197 filed 31 Mar. 2004, published as US 20050229748A1, and the hearth 42 is a container such as a tray (e.g., such as shown in
With further reference to block 14 of
As used herein, carbonaceous material refers to any carbon-containing material suitable for use as a carbonaceous reductant. For example, carbonaceous material may include coal, char, or coke. Further, for example, such carbonaceous reductants may include those listed and analyzed in the tables (in terms of % by weight) shown in
For example, as shown in
The hearth material layer 44 includes a thickness necessary to prevent slag from penetrating the hearth material layer 44 and contacting refractory material of hearth 42. For example, the carbonaceous material may be pulverized to an extent such that it is fine enough to prevent the slag from such penetration. As recognized by one skilled in the art, contact of slag during the metallic iron nugget process 10 produces undesirable damage to the refractory material of hearth 42 if contact is not prevented.
As shown by block 16 of
In one embodiment, the hearth material layer 44 has a thickness of more than 0.25 inches and less than 1.0 inch. Further, in yet another embodiment, the hearth material layer 44 has a thickness of less than 0.75 inches and more than 0.375 inches.
Further, with reference to block 18 of
As used herein, iron-bearing material includes any material capable of being formed into metallic iron nuggets via a metallic iron nugget process, such as process 10 described with reference to
At least in one embodiment, such iron-bearing material is ground to −100 mesh or less in size for processing according to the present invention. The various examples presented herein use iron-bearing material ground to −100 mesh unless otherwise specified. However, larger size iron-bearing material may also be used. For example, pellet screened fines and pellet plant wastes are generally about 0.25 inches in nominal size. Such material may be used directly, or may be ground to −100 mesh for better contact with carbonaceous reductants during processing.
In a preferred embodiment, for compacts containing coal at 80% of the stoichiometric amount, mounds of reducible material have a density of about 1.9-2.0, balls have a density of about 2.1 and briquettes have a density of about 2.1. Further, the reducible mixture has a density of less than about 2.4. In one preferred embodiment, the density is between about 1.4 and about 2.2.
One or more of the chemical compositions of iron ore shown in the table of
As used herein, the reducing material used in the layer of reducible mixture 46 includes at least one carbonaceous material. For example, the reducing material may include at least one of coal, char, or coke. The amount of reducing material in the mixture of reducing material and reducible iron bearing material will depend on the stoichiometric quantity necessary for completing the reducing reaction in the furnace process being employed. As described further below, such a quantity may vary depending on the furnace used (e.g., the atmosphere in which the reducing reaction takes place). In one or more embodiments, for example, the quantity of reducing material necessary to carry out the reduction of the iron-bearing material is between about 70 percent and 90 percent of the stoichiometric quantity of reducing material necessary for carrying out the reduction. In other embodiments, the quantity of reducing material necessary to carry out the reduction of the iron-bearing material is between about 70 percent and 140 percent of the stoichiometric quantity of reducing material necessary for carrying out the reduction.
At least in one embodiment, such carbonaceous material is ground to −100 mesh or less in size for processing according to the present invention. In another embodiment, such carbonaceous material is provided in the range of −65 mesh to −100 mesh. For example, such carbonaceous material may be used at different stoichiometric levels (e.g., 80 percent, 90 percent, and 100 percent of the stoichiometric amount necessary for reduction of the iron-bearing material). However, carbonaceous material in the range of −200 mesh to −8 mesh may also be used. The use of coarser carbonaceous material (e.g., coal) may require increased amounts of coal for carrying out the reduction process. Finer ground carbonaceous material may be as effective in the reduction process, but the amount of micro-nuggets may increase, and thus be less desirable. The various examples presented herein use carbonaceous material ground to −100 mesh unless otherwise specified. However, larger size carbonaceous material may also be used. For example, carbonaceous material of about ⅛ inch (3 mm) in nominal size may be used. Such larger size material may be used directly, or may be ground to −100 mesh or less for better contact with the iron-bearing reducible material during processing. When other additives are also added to the reducible mixture, such additives if necessary may also ground to −100 mesh or less in size.
Various carbonaceous materials may be used according to the present invention in providing the reducible mixture of reducing material and reducible iron-bearing material. For example, eastern anthracite and bituminous coals may be used as the carbonaceous reductant in at least one embodiment according to the present invention. However, in some geographical regions, such as on the Iron Range in Northern Minnesota, the use of western sub-bituminous coal offers an economically attractive alternative, as such coals are more readily accessible with the transportation systems already in place, plus they are low in cost and low on sulfur. As such, western sub-bituminous coals may be used in one or more processes as described herein. Further, an alternative to the direct use of sub-bituminous coals may be to carbonize, for example, at 900° C., the sub-bituminous coal prior to its use.
In one embodiment, the reducible mixture 46 has a thickness of more than 0.25 inches and less than 2.0 inches. Further, in yet another embodiment, the reducible mixture 46 has a thickness of less than 1 inch and more than 0.5 inches. The thickness of the reducible mixture is generally limited and/or dependent upon the effective heat penetration thereof and increased surface area of the reducible mixture that allows for improved heat transfer (e.g., dome-shaped reducible mixture as described herein).
In addition to the reducing material (e.g., coal or char) and reducible iron-bearing material (e.g., iron oxide material or iron ore), various other additives may optionally be provided to the reducible mixture for one or more purposes as shown by block 20 of
For example, the additives shown in the table of
As discussed herein with reference to metallic iron nugget processes that differ in one manner or another from that described with reference to
With further reference to
As shown in
In the embodiment shown in
Further, as would be apparent from the description herein, depending upon the shape of the formed portions, or mounds 52, channel openings 50 would have shapes or configurations associated therewith. For example, if mound 52 was a pyramid structure, a truncated pyramid structure, or a trapezoidal-shaped mound, openings 50 may be formed in a V-type configuration. One or more of such different types of channel openings are described further herein with reference to
The channel openings may be formed using any suitable channel definition device. For example, one or more various channel definition devices are described with reference to
Further with reference to
At least in one embodiment, such pulverized material used to fill the channel openings is ground to −6 mesh or less in size for processing according to the present invention. At least in one embodiment, such pulverized material used to fill the channel openings is −20 mesh or greater. Finer pulverized material more than −20 mesh (e.g., −100 mesh) may increase the amount of micro-nugget formation. However, larger size materials may also be used. For example, carbonaceous material of about ¼ inch (6 mm) in nominal size may be used.
As shown in
With the channel openings 50 at least partially filled with nugget separation fill material 58, a formed layer 48 of reducible mixture (e.g., mounds 52) may be thermally treated under appropriate conditions to reduce the reducible iron-bearing material and form one or more metallic iron nuggets in the one or more defined metallic iron nugget forming reducible material regions 59 as shown in block 24 of
As further shown in
The mechanism of iron nugget formation during the thermal treatment (block 24) of the formed reducible mixture layer 48 is described herein with reference to
Such a process is quite different from the mechanism proposed and described which uses dried iron ore/coal mixture balls such as described in the Background of the Invention section herein. The mechanism used with the balls is reported to involve formation of direct reduced iron by the reduction of carbon-containing balls, formation of a dense metallic iron shell on the surface of the original round shape with molten slag separated from metal, and a large void space inside, followed by melting of the iron phase and separation of slag from molten metal.
The metallic iron nugget process 10 may be carried out by a furnace system 30 as shown generally in
A channel definition device 35 is then operable (e.g., manual and/or automatic operation thereof, typically automatic in commercial units or systems) to create the plurality of channel openings 50 that extend at least partially through the layer of the reducible mixture 46 to define the plurality of nugget forming reducible material regions 59. The channel definition device 35 may be any suitable apparatus (e.g., channel cutting device, mound forming press, etc.) for creating the channel openings 50 in the layer of reducible mixture 46 (e.g., forming the mounds 52, pressing the reducible mixture 46, cutting the openings, etc.). For example, the channel definition device 35 may include one or more molds, cutting tools, shaping tools, drums, cylinders, bars, etc. One or more suitable channel definition devices shall be described with reference to
The furnace system 30 further includes a channel fill apparatus 37 operable to at least partially fill the plurality of channel openings 50 with nugget separation fill material 58. Any suitable channel fill apparatus 37 for providing such separation fill material 58 into the channels 50 may be used (e.g., manual and/or automatic operation thereof). For example, a feed apparatus that limits and positions material in one or more places may be used, material may be allowed to roll down dome-shaped mounds to at least partially fill the openings, a spray device may be used to provide material in the channels, or an apparatus synchronized with a channel definition device may be used (e.g., channels at least partially filled as the mounds are formed).
With the formed reducible material 48 provided on the hearth material layer 44 and with nugget separation fill material 58 provided to at least partially fill the plurality of channel openings 50, a reducing furnace 34 is provided to thermally treat the formed layer of reducible mixture 48 to produce one or more metallic iron nuggets 63 in one or more of the plurality of nugget forming reducible material regions 59. The reducing furnace 34 may include any suitable furnace regions or zones for providing the appropriate conditions (e.g., atmosphere and temperature) for processing the reducible mixture 46 such that one or more metallic iron nuggets 63 are formed. For example, a rotary hearth furnace, a linear hearth furnace, or any other furnace capable of performing the thermal treatment of the reducible mixture 46 may be used.
Further as shown in
One or more different reducing furnaces may be used according to the present invention depending on the application of the present invention. For example, in one or more embodiments herein, laboratory furnaces were used to perform the thermal treatment. One will recognize that from the laboratory furnaces, scaling to mass production level can be performed and the present invention contemplates such scaling. As such, one will recognize that various types of apparatus described herein may be used in larger scale processes, or production equipment necessary to perform such processes at a larger scale may be used.
In the absence of any other information of the furnace gas composition of iron nugget processes, most of the laboratory tests described herein were carried out in an atmosphere of 67.7% N2 and 33.3% CO, assuming that CO2 in a natural gas-fired burner gas would be converted rapidly to CO in the presence of carbonaceous reductants and hearth layer materials by the Boudouard (or carbon solution) reaction (CO2+C=2CO) at temperatures higher than 1000° C., and a CO-rich atmosphere would prevail at least in the vicinity of the reducible materials.
While the presence of CO in the furnace atmosphere accelerated the fusion process somewhat as compared to a N2 only atmosphere, the presence of CO2 in furnace atmospheres slowed the fusion behaviors of iron nuggets. There was a pronounced effect of CO2 in furnace atmospheres on iron nugget formation at 1325° C. (2417° F.), wherein temperature was on the verge of forming fused iron nuggets. The effect of CO2 became less pronounced at higher temperatures and, in fact, the effect became virtually absent over 1400° C. (2552° F.). In the examples given herein, unless otherwise indicated, salient features of findings are provided as observed mainly in the N2 and CO atmosphere.
Two reducing furnaces used to arrive at one or more of the techniques and/or concepts used herein include laboratory test furnaces including, for example, a laboratory tube furnace, as shown in
The laboratory tube furnace 500 (
A typical temperature profile of the tube furnace when the temperature was set at 1300° C. (2372° F.) is shown as follows.
The constant temperature zone of 1 inch upstream from the middle of the furnace was sufficient to extend over a 4 inch long graphite boat 511.
Reduction tests were conducted by heating to a temperature in the range of 1325° C. (2417° F.) to 1450° C. (2642° F.) and holding for different periods of time with a gas flow rate, in many of the tests, of 2 L/min N2 and 1 L/min CO for atmosphere control. In certain tests, the atmosphere was changed to contain different concentrations of CO2. The furnace temperature was checked with two different calibration thermocouples and the readings were found to agree within 5° C.
For reduction tests, a graphite boat 511 was introduced in the water-cooled chamber 507, the gas was switched to either a N2—CO or N2—CO—CO2 mixture and purged for 10 minutes. The boat 511 was moved into and removed from the constant temperature zone. Then, iron nuggets and slag were picked out and the remainder separated on a 20 mesh screen, and the oversize and the undersize were magnetically separated. The magnetic fraction of the oversize included mainly metallic iron micro-nuggets, while the magnetic fraction of the undersize in most cases were observed to include mainly of coke particles with some magnetic materials attached, whether from iron ores or from iron-bearing impurities of added coal.
Further, a laboratory electrically heated box furnace 600 (
The temperature variation over a 6 inch long tray 606 was within a few degrees. The furnace 600 was preceded by a cooling chamber 608, 16 inch high×13 inch wide×24 inch long, with a side door 620 through which a graphite tray 606, 5 inch wide×6 inch long×1½ inch high with a thickness of ⅛ inch, was introduced, and a view window 610 at the top. A gas inlet port 614, another small view window 612, and a port 616 for a push rod to move a sample tray 606 into the furnace 600 were located on the outside wall of the chamber. On the side attached to the furnace, a flip-up door 622 was installed to shield the radiant heat from coming through. A ½ inch hole in the flip-up door 622 allowed the gas to pass through, and the push rod to move the tray 606 inside the furnace 600. At the opposite end of the furnace, a furnace gas exhaust port 630, a gas sampling port 632, and a port for a push rod 634 to move a tray 606 out of the furnace 600, were located.
To control the furnace atmosphere, N2, CO, and CO2 were supplied to the furnace 600 in different combinations via respective rotameters. Total gas flow could be adjusted in the range of 10 to 50 L/min. In most tests, graphite trays 606 were used, but in some tests, trays made of high-temperature fiberboards with a thickness of ½ inch were used. After introducing a tray 606 into the cooling chamber 608, the furnace was purged with N2 for 30 minutes to replace the air, followed by another 30 minutes with a gas mixture used in a test of either a N2—CO or a N2—CO—CO2 mixture before the sample tray 606 was pushed into the furnace.
Initially, the tray was pushed just inside of the flip-up door 622, held there for 3 minutes, then into the first chamber 602 for preheating, typically at 1200° C., for 5 minutes, and into the second chamber for iron nugget formation, typically at 1400° C. to 1450° C. for 10 to 15 minutes. After the test, the gas was switched to N2 and the tray 606 was pushed to the back of the door 622 and held there for 3 minutes, and then into the cooling chamber 608. After cooling for 10 minutes, the tray 606 was removed from the cooling chamber 608 for observation.
Then, iron nuggets and slag were picked out and the remainder separated on a 20 mesh screen, and the oversize and the undersize were magnetically separated. The magnetic fraction of the oversize included mainly metallic iron micro-nuggets, while the magnetic fraction of the undersize in most cases included mainly coke particles with some magnetic materials, whether from iron ores or from iron-bearing impurities of added coal. The magnetic fraction of +20 mesh was labeled and is referred to herein as “micro-nuggets,” and the −20 mesh was labeled and is referred to herein as “−20 mesh mag.”. As such, as used herein, micro-nuggets refers to nuggets that are smaller than the parent nugget formed during the process but too large to pass through the 20 mesh screen, or in other words the +20 mesh material.
Yet further, as previously described herein, a linear hearth furnace such as that described in U.S. Provisional Patent Application No. 60/558,197, entitled “Linear hearth furnace system and methods,” filed 31 Mar. 2004, published as US 20050229748A1, may also be used. A summary of the linear hearth furnace described therein is as follows. One exemplary embodiment of such a linear hearth furnace is shown generally in
Zone 728 is described as an initial heating and reduction zone. This zone may operate on two natural gas-fired 450,000 BTU burners 738 capable of achieving temperatures of 1093° C. Its walls and roof are lined with six (6) inches of ceramic fiber refractory rated to 1316° C. Its purpose is to bring samples to sufficient temperature for drying, de-volatilizing hydrocarbons and initiating the reduction stages. The burners are operated sub-stoichiometrically to minimize oxygen levels.
Zone 730 is described as the reduction zone. This zone may operate on two natural gas-fired 450,000 BTU burners 738 capable to achieve 1316° C. Its walls and roof are lined with 12 inches of ceramic fiber refractory rated to sustain constant operating temperatures of 1316° C. The reduction of the feed mixture occurs in this zone 730.
Zone 731 is described as the melting or fusion zone. This zone may operate on two natural gas-fired 1,000,000 BTU burners 738 capable to sustain this zone at 1426° C. The walls and roof are lined with 12″ of ceramic fiber refractory rated to sustain constant operating temperatures of 1426° C. The function of this zone is to complete the reduction, fusing the iron into metallic iron nodules or “nuggets”. In the event that this furnace is being used to make direct reduced iron or sponge iron, the temperatures in this zone would be reduced where complete reduction would be promoted without melting or fusion.
The final zone 734, or cooling zone, is a water-jacketed section of the furnace approximately eleven (11) feet long. A series of ports have been installed between the third zone and the cooling section so that nitrogen can be used to create a blanket. The purpose of this zone is to cool the sample trays 715 so that they can be safely handled and solidify the metallic iron nuggets for removal from the furnace.
Zones 728, 730, and 731 are controlled individually according to temperature, pressure and feed rate, making this furnace 712 capable of simulating several iron reduction processes and operating conditions. An Allen Bradley PLC micro logic controller 718 coupled to an Automation-Direct PLC for a walking beam mechanism 724 controls the furnace through a user-friendly PC interface.
The operation of the furnace under positive pressure allows the control of atmosphere in each of the zones to reduced oxygen levels (e.g., to 0.0%). Sample trays 715 are also filled with coke breeze or other carbonaceous hearth material layers to further enhance the furnace atmosphere. High temperature caulking was used to seal seams on all exposed surfaces to minimize air infiltration.
Feed rate is controlled by an Automation-Direct PLC controlled hydraulic walking beam mechanism 724 that advances the trays 715 through the furnace 712. This device monitors time in each zone and advances trays 715 accordingly with the walking beam mechanism 724 while regulating feed rate. Furnace feed rate and position of the trays is displayed on an operating screen through communication with the PLC. A pair of side-by-side, castable refractory walking beams extends the length of the furnace 712. They are driven forward and back with a pair of hydraulic cylinders operated through the PLC. The beams are raised and lowered through a second pair of hydraulic cylinders that push the beam assemblies up and down a series of inclines (wedges) on rollers. Activation of the beam mechanism moves them through a total of 5 revolutions or 30 inches per cycle, the equivalent of one tray.
Sample trays 715 are manually prepared prior to starting the test. Additional trays may be also used, covered with coke or a carbonaceous reductant to regulate the furnace atmosphere. A roll plate platform elevator 752, raised and lowered with a pneumatic cylinder, is designed to align sample trays 715 at the feed 720 of the furnace for tray insertion. Raising the elevator 752 pushes open a spring-loaded feed door, exposing the feed section of the furnace to the atmosphere to insert trays. Trays are inserted into the furnace once the proper height and alignment is achieved. An automated tray feeding system is used to feed sample trays with a pneumatic cylinder.
The walking beam 724 transports trays 715 to the opposite end 722 of the furnace where they are discharged onto a similar platform (roller ball plate) elevator 754. A safety mechanism has been installed to monitor the position of the hot trays at the discharge of the furnace. Discharge rollers drive the trays onto the platform elevator where they can be removed or re-inserted back into the furnace. The discharge rollers will not function unless trays are in position for discharge, platform elevator is in the “up” position, and the walking beams have been lowered to prevent hot trays from accidental discharge. Tiered conveyor rollers are located at the discharge of the furnace to remove and store sample pallets until cool. To re-enter trays back into the furnace, a return cart has been designed that transports hot trays, underneath the furnace, back to the platform elevator at the feed end.
The exhaust gas system 747 is connected to an exhaust fan 753 with a VFD controlled by the furnace PLC. Because the exhaust fan 753 is oversized for this application, a manually controlled in-line damper or pressure control 755 is used to reduce the capacity of the exhaust fan 753 to improve zone pressure control. As a safety precaution, a barometric leg into a level controlled water tank is installed between the common header and exhaust fan to absorb any sudden pressure changes. Exhaust gases are discharged from the fan 753 to a forty-foot exhaust stack 757. The exhaust ducts are refractory lined to the exterior walls of the furnace where they transition to high temperature stainless steel (RA602CA), fitted with water spray nozzles 749, used to cool the waste gases. The temperature of the water gases from each zone is controlled with an in-line thermocouple and a manually controlled water flow meter attached to each set of water sprays. The stainless ducts are followed by standard carbon steel once the gases are sufficiently cooled. A thermocouple in the common header is used to monitor the temperature of the exhaust gas and minimize heat to the exhaust fan bearings.
The sample trays or pallets 715 (as shown in
The above described furnace systems are given for exemplary purposes only to further illustrate the nugget formation process 10 and provide certain details on testing and results reported herein. It will be recognized that any suitable furnace system capable of carrying out one or more embodiments of a metallic iron nugget formation process described herein may be used according to the present invention.
As generally described with reference to
As shown in the multiple embodiments, one will recognize that the channel openings may be formed to extend through the entire reducible mixture layer to the hearth material layer or only partially therethrough. Further, one will recognize that the nugget separation fill material may entirely fill each of the channel openings or may only partially fill such openings.
The channel definition device 106 includes a first elongated element 108 and one or more extension elements 110 extending orthogonally from the elongated element 108. As shown by direction arrows 107, 109, the channel definition device 106 and/or the reducible mixture 102 may be moved along both x and y axes to move sufficient material of the reducible mixture to create the channel openings 104. For example, when element 108 and/or the reducible mixture 102 is moved in the direction represented by arrow 107, channels are created which are orthogonal to those created when the device 106 is moved in the direction 109. In one embodiment, the elongated element 108 need not move in the direction represented by arrow 107, as the layer of reducible mixture 102 is moving, for example, to the right at a constant speed such as in a continuous forming process shown in
The channel definition device 126 includes a first elongated rotating shaft element 128 that includes a plurality of spaced-part disc elements 127 mounted orthogonally relative to the elongated shaft element 128. In one exemplary embodiment, the disc elements 127 rotate in place to create grooves when the reducible feed mixture 122 moves in direction 133. In other words, bidirectional arrow 132 indicates rotation of the shaft element 128 and, as such the one or more disc elements 127 such that rotation of disc elements 127 (when the layer of reducible mixture 122 is moved in the direction 133) produces groove-shaped channels 124 in a first direction (i.e., in the direction of arrow 133). In one embodiment, the channel definition device 126 further includes one or more flat blades 130 connected to the rotating shaft element 128 between the disc elements 127. The flat blades 130 (e.g., two blades mounted 180 degrees apart as shown in
One will recognize that channel openings 124 extending in direction 133 may be created by the same or a different channel definition device as those created orthogonal thereto. For example, channel definition device 126 may be used to create channels 124 along direction 133, whereas the channel device 106, as shown with reference to
In other words, the channel definition device 146 includes an elongated element 148 extending along an axis about which the device 146 rotates. One or more mold surfaces 150 are formed at a location radial from axis 148. As shown in
As described herein, various channel definition devices may be used to form the mounds and associated channel openings according to the present invention. However, in one embodiment, dome-shaped or substantially spherical mounds, such as those shown in
The use of pressure or compaction may be combined with any of the described embodiments herein or as an alternative thereto. For example, and as described herein, in the formation of the channels or formation of the reducible mixture on the hearth material, compaction or pressure (e.g., pressing using one or more of the channel definition devices) may be used to alter the nugget formation process. Such compacted reducible mixture may be used alone or in combination with nugget separation fill material being provided in openings formed by compaction or pressure.
Further, for example, a compaction apparatus (e.g., a briquetting cylinder or roll or a briquetting press) may be used to optimize the size and/or shape of the nuggets formed. The compaction apparatus may, for example, be configured to imprint a pattern into a layer of reducible mixture (e.g., iron-bearing fines and a reducing material). The deeper the imprint, the greater would be the compaction in a particular area. Such compaction may result in greater throughput for the nugget formation process. Further, it may be possible to increase the size of nuggets to a point where solidification rates and other physical parameters restrict formation of metallic nuggets and slag separation.
In a uniform temperature environment, the areas of greater compaction should enhance heating and diffusion, thereby acting as the nucleation and collection site for metallic nuggets, providing a manner to locate where a nugget will form on the hearth. Further, it may be possible to use the added degree of freedom brought about by the compaction or pressure as a control parameter to counteract the negative effects of a non-uniform temperature profile across the hearth that may result as a consequence of furnace geometry (e.g., edge effects) and heat source location in the furnace. Yet further, in addition to use of pressure to control reaction rates (i.e., in the formation of metallic nuggets), diffusion rates of reducing gases can be varied by using pressure in combination with particle size, to control the pathways for gases entering the formed material. Likewise, solid state reaction rates of particulates, as governed by heat transfer and metallurgical diffusion mechanisms, can also be varied.
Various compaction profiles are shown in
Further,
As shown in one example according to
As shown in the example of
As shown therein, and also in corresponding
The thermal processing to form the iron nuggets was performed in the electric box furnace at a temperature of 1450° C. for 6 minutes. At 5.5 minutes, an iron nugget at the center showed a sign of being on the verge of full fusion. Accordingly, it could be concluded that 5.5 minutes was the minimum time required for full fusion with the molded pattern.
The example shown in
As shown in
The grooves in the reducible mixture of
As shown in
The above exemplary illustrations provide support for the provision of channel openings in the layer of reducible mixture to define metallic iron nugget forming regions (block 22), as described with reference to
Further, at least in one or more embodiments according to the present invention, the channel openings are filled at least partially with nugget separation fill material (e.g., carbonaceous material) (block 26) as described in the examples herein. With use of such channel openings 50 and nugget separation fill material 58 therein, as shown, for example, in
In one embodiment, and as shown in
Further, as shown and described with reference to
The hearth material layer 44 (e.g., the size distribution thereof) may influence the amount of mini-nuggets and micro-nuggets generated during the reduction processing of the layer of reducible mixture 46. For example, at least in one embodiment, the hearth material layer 44 includes a pulverized coke layer having a size distribution of +65 mesh fraction of the “as ground” coke. In another embodiment, +28 mesh fraction of “as ground” coke is used as the hearth material layer. With the use of mounds 52, such as shown in
Due to the presence of excess carbon, the micro-nuggets do not coalesce with the parent nugget in the nugget forming reducible material region 59 or among themselves. Such formation of micro-nuggets is undesirable and ways of reducing micro-nugget formation in processes such as those described according to the present invention are desirable.
While the hearth material layer 44 which may include pulverized coke may generate a large quantity of micro-nuggets when dome-shaped mound patterns are used, a pulverized alumina layer has been found to minimize their amount. Although the use of alumina demonstrates the role played by a carbonaceous hearth material layer 44 in generating micro-nuggets, pulverized alumina cannot be used as a hearth material layer 44 because of its reactiveness with slag.
In order to minimize the generation of micro-nuggets when channel opening defined mounds are processed according to the present invention, the effect of different types of hearth material layers 44 have been compared indicating that the hearth material layer, or carbonaceous material thereof, may be optionally modified (block 16 of
% micro nuggets=Wtmicro nuggets/(Wtnuggets+Wtmicro nuggets)×100
The results of one or more exemplary illustrative test embodiments are shown in the table of
For the “12 elongated domes” data shown in
For the “12 and 16 balls” data of
Two extremes of the effect of hearth layer materials are contrasted in the table of
The results when only coke and an equal weight (50:50) mixture of coke and alumina were used as the hearth layer, are compared. The amount of micro-nuggets was reduced to less than a half by the presence of alumina in the hearth material layer.
Further, pulverized coke was coated with Al(OH)3 by mixing 40 g of coke in an aqueous slurry of Al(OH)3, dried and screened at 65 mesh to remove excess Al(OH)3. The coke acquired 6% by weight of Al(OH)3. The Al(OH)3-coated coke was used as the hearth material layer. The amount of micro-nuggets notably decreased (3.9%).
Yet further, pulverized coke was coated with Ca(OH)2 by mixing 40 g of coke in an aqueous slurry of Ca(OH)2, dried and screened at 65 mesh to remove excess Ca(OH)2. The coke acquired 12% by weight of Ca(OH)2. The Ca(OH)2-coated coke was used as the hearth material layer. Apparently, the coating of Ca(OH)2 had essentially no effect on the generation of micro-nuggets (14.2%). It may be speculated that an addition of CaF2 to Ca(OH)2 in the coating would minimize the amount of micro-nuggets by lowering the fusion of high lime slag as in the case of Slag Composition L1.5FS0.5˜2, see
As described previously with reference to
The one or more fluxing agents that may be provided for use with the reducible mixture (block 208) may include any suitable fluxing agent, for example, an agent that assists in the fusion process by lowering the fusion temperature of the reducible mixture or increases the fluidity of the reducible mixture. In one embodiment, calcium fluoride (CaF2) or fluorspar (e.g., a mineral form of CaF2) may be used as the fluxing agent. Further, for example, borax, NaF, or aluminum smelting industry slag, may be used as the fluxing agent. With respect to the use of fluorspar as the fluxing agent, an amount of about 0.5% to about 4% by weight of the reducible mixture may be used.
Use of fluorspar, for example, as well as one or more other fluxing agents, lowers the fusion temperature of the iron nuggets being formed and minimizes the generation of micro-nuggets. Fluorspar was found to lower not only the nugget formation temperature, but also to be uniquely effective in decreasing the amount of micro-nuggets generated.
In an attempt to improve sulfur removal capacity of slag, as shall be described further herein, the level of lime or one or more other compounds capable of producing calcium oxide is typically increased beyond a composition (L), as shown on the CaO—SiO2—Al2O3 phase diagram of
Generally,
Generally,
Generally,
Although fluorspar is reported to be not particularly an effective desulfurizer in steelmaking slag,
Further with reference to
As further shown in
Soda ash is used as a desulfurizer in the external desulfurization of hot metal. Sodium in blast furnace feed materials recirculates and accumulates within a blast furnace, leading to operational problems and attack on furnace and auxiliary equipment lining. In rotary hearth furnaces, recirculation and accumulation of sodium is less likely to occur, and, as such, larger amounts of sodium may be tolerated in feed materials than in blast furnaces.
The table of
An addition of Na2CO3 without CaF2 decreased sulfur in iron nuggets as effectively as, or even more effectively than the CaF2, but the amount of micro-nuggets generated increased, as shown in
The table of
In previous and various metallic iron reduction processes, such as those using formed and/or dried balls as presented in the Background of the Invention section herein, carbonaceous reductants are typically added in an amount greater than the theoretical amount required to reduce the iron oxides for promoting carburizing of metallic iron in order to lower the melting point. The amount of carbonaceous reductant in the balls is thus claimed to include an amount required for reducing iron oxide plus an amount required for carburizing metallic iron and an amount of loss associated with oxidation.
In many of the processes described herein, the stoichiometric amount of reducing material is also necessary for complete metallization and formation of metallic iron nuggets from a predetermined quantity of reducible iron bearing material. For example, in one or more embodiments, the reducible mixture may include the predetermined quantity of reducible iron bearing material and between about 70 percent and about 125 percent of the stoichiometric amount of reducing material (e.g., carbonaceous reductant) necessary for complete metallization thereof (e.g., where the reducible feed mixture has a uniform coal content throughout the reducible mixture, such as when formed in mounds).
However, in one or more embodiments according to the present invention, use of the amount of carbonaceous reductant in the amount of the stoichiometric amount needed for complete metallization may lead to the break-up of the reducible mixture into mini-nuggets and the generation of a large amount of micro-nuggets, as shown in
As seen in
The control of the amount of reducing material in the reducible mixture based on the stoichiometric amount necessary to complete the metallization process (as well as the use of various additives described herein), may be applied to other nugget formation processes as well as the methods described with reference to
For example, compacts that employ 70% to 90% of carbonaceous reductant needed for complete metallization in a suitable reducible mixture may be used. For example, such compacts may have the appropriate additions of flux and limestone, and/or may further include auxiliary reducing agent on the hearth or partially covering the compacts to effectively provide nugget metallization and size control. In other words, the stoichiometric control described herein along with the variation in compositions (e.g., additives, lime, etc.) provided herein may be used with compacts (e.g., briquettes, partial-briquettes, compacted mounds, etc.). Use of compacts may alleviate any need to use nugget separation material as described with reference to
However, as described above, such data shown in
Linear Hearth Furnace Tests
The tests were run using a 40-ft. long, natural gas-fired linear hearth furnace including three heating zones and a cooling section like that described generally with reference to
The reducible feed mixture 228 on the tray 223 was formed in the shape of 6-segment domes for the laboratory box furnace tests, placed on a −10 mesh coke layer in each of the four quadrants of the tray 223 labeled as (1) through (4). Each of the domes in the 6×6 segment quadrant had the dimensions of substantially 1¾ inches wide by 2 inches long and were 11/16 inches high, and contained medium-volatile bituminous coal in indicated percentages (see various test examples below) of the stoichiometric amount and at the indicated (see various test examples below) Slag Composition.
Two areas of consideration with regard to the products resulting from the linear hearth furnace tests were the amount of sulfur in the metallic iron nuggets formed by the process and the amount of micro-nugget formation. The laboratory tube and box furnace tests described herein indicated that Slag Composition (L1.5FS1) and the use of medium-volatile bituminous coal at 80% of the stoichiometric amount minimized sulfur in iron nuggets and minimized micro-nugget formation. However, linear hearth furnace tests revealed that unexpectedly high CO2 levels and highly turbulent furnace gas next to the feed being processed consumed much of the added coal (e.g., added reducing material which was added to the reducible iron bearing material) in Zones 1 and 2, and not enough reductant (e.g., reducing material) was left for carburizing and melting the metallic iron in the high temperature zone (Zone 3). Use of coal in the amount of 105 to 125 percent of the stoichiometric amount was necessary for forming fully fused metallic iron nuggets as shown by the Tests 14 and 17 provided below.
In linear hearth furnace Test 14, a pallet having an arrangement of different feed mixtures in 6-segment domes was used, such as generally shown in
In linear hearth furnace Test 17, a pallet having an arrangement of different feed mixtures in 6-segment domes was used, such as generally shown in
Iron nuggets formed in Tests 14 and 17 using coal additions of 105% to 125% of the stoichiometric amount and Slag Compositions of (L1.5FS1˜3).
Concentrations of CO, expressed as percentages of CO+CO2, were plotted in the equilibrium concentration diagrams of iron oxide reduction and carbon solution (Boudouard) reactions as shown in
Analytical results of iron nuggets and slags of linear hearth furnace Tests. 14 and 17 are given in
As shown in
In
As shown in
As shown in
The amounts of micro nuggets in the linear hearth furnace tests were also large, e.g., in the range of 10 to 15%, as summarized in
In view of the above, in one embodiment of the present invention, use of a feed mixture with a sub-stoichiometric amount of coal next to the hearth layer to minimize micro-nugget formation, which is overlaid by a feed mixture containing coal in excess of the stoichiometric amount to allow for the loss by the carbon solution reaction, is used. In other words, a stoichiometric amount of reducing material (e.g., coal) is necessary for complete metallization and formation of metallic iron nuggets from a predetermined quantity of reducible iron bearing material, the reducing material (e.g., coal) and the iron bearing material providing a reducible feed mixture for processing according to one or more embodiments described herein. For certain applications of a feed mixture with a sub-stoichiometric amount of carbonaceous material, the hearth layer might not be used, or the hearth layer might not contain any carbonaceous material.
One embodiment according to the present invention may include using reducible feed mixture that includes a first layer of reducible mixture on the hearth material layer that has a predetermined quantity of reducible iron bearing material but only between about 70 percent and about 90 percent of the stoichiometric amount of reducing material necessary for complete metallization thereof so as to reduce the potential for formation of micro-nuggets (e.g., such as suggested when the processing was accomplished using the box and tube furnaces). The predetermined quantity of reducible iron bearing material may be determined and varied dynamically at the time the reducible iron bearing material is placed on the hearth layer. Subsequently, one or more additional layers of reducible mixture that include a predetermined quantity of reducible iron bearing material and between about 105 percent and about 140 percent of the stoichiometric amount of reducing material necessary for complete metallization thereof would be used. As such, the reducible feed mixture would include layers of mixture having different stoichiometric amounts of reducing material (e.g., the stoichiometric percentage increasing as one moves away from the hearth layer).
As discussed above, in certain furnaces (e.g., such as natural gas fired furnaces with high CO2 and highly turbulent gas atmospheres), added carbonaceous material (e.g., coal) in feed mixtures (e.g., such as those reducible mixtures described herein) is lost by the carbon solution (Boudouard) reaction in certain zones of the furnace (e.g., pre-heating and reduction zones). To compensate for the loss, it may be necessary to add reducing material (e.g., carbonaceous material) in excess of the stoichiometric amount necessary for complete metallization thereof. However, also as described herein, such an addition of reducing material (e.g., coal) in excess of the stoichiometric amount may lead to formation of large amounts of micro-nuggets. Such micro-nugget formation appears to be related to the amount of reducing material in an area near the hearth layer that remains high during processing.
As indicated herein, an addition of the reducing material somewhat below the stoichiometric amount minimizes the formation of such micro-nuggets. As such, a feed mixture (e.g., a reducible mixture) with a sub-stoichiometric amount of reducing material (e.g., coal) next to the hearth layer overlaid with reducible mixture containing reducing material in excess of the stoichiometric amount necessary for complete metallization to minimize micro-nugget formation is described herein. Further, the loss of added reducing material (e.g., coal) during processing by the carbon solution reaction may be minimized by compaction of the reducible mixture in various ways (e.g., formation of compacts or briquettes from the reducible mixture).
In one embodiment, two layer balls having a size that is ¾ inch or less in diameter are made. With respect to ¾ inch or less diameter balls, for example, an outer layer having a thickness of, for example, 1/16 inch amounts to about 40 percent or more of the total weight of the ball in the outer layer, while a thickness of ⅛ inch amounts to about 60 percent or more of the total weight. As such, with this amount of the outer layer having a sub-stoichiometric amount of reducing material (e.g., between 70% and 90% of the stoichiometric amount necessary for complete metallization), the central core (i.e., inner portion) would need to be appreciably higher in reducing material (e.g., coal) content than, for example, when mounds including multiple layers are used (e.g., the central core may need to be higher than 125 percent of the stoichiometric amount necessary for complete metallization). In one embodiment, the interior of the ball is formed of reducible mixture containing reducing material in excess of 105 percent of the stoichiometric amount necessary for complete metallization but less than about 140 percent).
As shown in
In one embodiment, the compacts 302 are formed using a press such as that shown in
One will recognize that various shapes of the compacts may be used while still maintaining the benefit of having feed mixture with a sub-stoichiometric amount of reducing material (e.g., coal) next to the hearth layer to minimize micro-nugget formation. The configurations described herein are given for illustration only.
With further reference to
The micro-agglomerates are formed (block 252) with provision of reducible iron-bearing material (e.g., iron oxide material, such as iron ores) (block 260) and with the use of reducing material (block 256). Optionally, one or more additives (block 250) may be additionally mixed with the reducible iron-bearing material and the reducing material as described herein with regard to other embodiments (e.g., lime, soda ash, fluorspar, etc.). Water is then added (block 254) in the formation of the micro-agglomerates. For example, in one embodiment, a mixer (e.g., like that of a commercial kitchen stand mixer) may be used to mix all the components until they are formed into small micro-agglomerate structures.
Direct feeding of fine dried particles, such as taconite concentrates and pulverized coal, in gas-fired furnaces would result in a large quantity of the particles being blown out as dust by the movement of furnace gases. Therefore, micro-agglomeration of the feed mixture is desirable. For example, direct mixing of wet filter cakes of taconite concentrates and dry ground coal with optimum addition of water can generate micro-agglomerates by a suitable mixing technique such as Pekay mixers, paddle mixers, or ribbon mixers. Typical size distributions of micro-agglomerates as a function of different levels of moisture are shown in
Feeding of micro-agglomerates to hearth surfaces has several advantages. Micro-agglomerates can be fed to hearth surfaces without breakage, with minimal dust losses, and with uniform spreading over hearth surfaces. Then, micro-agglomerates, once placed on the hearth, may be compacted into mound-shaped structures as described herein (e.g., pyramidal shapes, rounded mounds, dome shaped structures, etc.)
The table of
The moisture content to provide desired properties for the micro-agglomerates will depend on various factors. For example, the moisture content of the micro-agglomerates will depend at least on the fineness (or coarseness) and water absorption behavior of the feed mixture. Depending on such fineness of the feed mixture, the moisture content may be within a range of about 10 percent to about 20 percent.
Any type of layering of the micro-agglomerate may be used. For example, the reducible micro-agglomerates may be provided by providing a first layer of reducible micro-agglomerates on the hearth material layer. Subsequently, one or more additional layers of reducible micro-agglomerates may be provided on a first layer. The average size of the reducible micro-agglomerates of at least one of the provided additional layers could be different relative to the size of the micro-agglomerates previously provided. For example, the size may be larger or smaller than the previously-provided layers. In one embodiment, feeding of micro-agglomerates in layers with coarser agglomerates at the bottom and with decreasing size to the top may minimize the mixing of iron ore/coal mixtures with the underlying heath material layer (e.g., pulverized coke layer), thereby minimizing the generation of micro-nuggets.
The use of reducible feed mixture layers having different stoichiometric amounts of reducing material may be advantageously used in combination with the use of micro-agglomerates as described herein. (e.g., the stoichiometric percentage increasing as one moves away from the hearth layer). For example, larger size micro-agglomerates (e.g., coarser agglomerates) along with lower stoichiometric percentages of reducing material may be used for material adjacent the hearth layer. Additional layers having higher stoichiometric percentages and micro-agglomerates of decreasing size (e.g., finer agglomerates) may then be provided to the coarser and lower percentage micro-agglomerates provided on the hearth layer.
All patents, patent documents, and references cited herein are incorporated in their entirety as if each were incorporated separately. This invention has been described with reference to illustrative embodiments and is not meant to be construed in a limiting sense. As described previously, one skilled in the art will recognize that other various illustrative applications may use the techniques as described herein to take advantage of the beneficial characteristics of the particles generated hereby. Various modifications of the illustrative embodiments, as well as additional embodiments to the invention, will be apparent to persons skilled in the art upon reference to this description.
This application claims priority from provisional application Ser. No. 60/633,886, filed Dec. 7, 2004. Application Ser. No. 60/633,886 is hereby incorporated by reference.
The present invention was made with support by the Economic Development Administration, Grant No. 06-69-04501. The United States government may have certain rights in the invention.
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