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US20130047787A1 - Carbon-material-containing iron oxide briquette composition, method for producing the same, and method for producing direct reduced iron using the same - Google Patents

Carbon-material-containing iron oxide briquette composition, method for producing the same, and method for producing direct reduced iron using the same Download PDF

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Publication number
US20130047787A1
US20130047787A1 US13/583,979 US201113583979A US2013047787A1 US 20130047787 A1 US20130047787 A1 US 20130047787A1 US 201113583979 A US201113583979 A US 201113583979A US 2013047787 A1 US2013047787 A1 US 2013047787A1
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Prior art keywords
slag
carbon
cao
direct reduced
nco
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US13/583,979
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English (en)
Inventor
Motohiro Horiguchi
Kazutaka Kunii
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority claimed from JP2010070175A external-priority patent/JP5503364B2/ja
Priority claimed from JP2010236743A external-priority patent/JP5503495B2/ja
Application filed by Kobe Steel Ltd filed Critical Kobe Steel Ltd
Assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) reassignment KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HORIGUCHI, MOTOHIRO, KUNII, KAZUTAKA
Publication of US20130047787A1 publication Critical patent/US20130047787A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B1/00—Preliminary treatment of ores or scrap
    • C22B1/14—Agglomerating; Briquetting; Binding; Granulating
    • C22B1/24—Binding; Briquetting ; Granulating
    • C22B1/242—Binding; Briquetting ; Granulating with binders
    • C22B1/244—Binding; Briquetting ; Granulating with binders organic
    • C22B1/245—Binding; Briquetting ; Granulating with binders organic with carbonaceous material for the production of coked agglomerates
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B13/00—Making spongy iron or liquid steel, by direct processes
    • C21B13/0046—Making spongy iron or liquid steel, by direct processes making metallised agglomerates or iron oxide
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B13/00—Making spongy iron or liquid steel, by direct processes
    • C21B13/0066—Preliminary conditioning of the solid carbonaceous reductant
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B13/00—Making spongy iron or liquid steel, by direct processes
    • C21B13/008—Use of special additives or fluxing agents
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B13/00—Making spongy iron or liquid steel, by direct processes
    • C21B13/10—Making spongy iron or liquid steel, by direct processes in hearth-type furnaces
    • C21B13/105—Rotary hearth-type furnaces
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B3/00—General features in the manufacture of pig-iron
    • C21B3/04—Recovery of by-products, e.g. slag
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
    • Y02W30/00—Technologies for solid waste management
    • Y02W30/50—Reuse, recycling or recovery technologies

Definitions

  • the present invention relates to a carbon-material-containing iron oxide briquette composition used as a feedstock for a moving hearth reduction furnace for producing direct reduced iron, a method for producing the same, and a method for producing direct reduced iron using the same.
  • the following technique is known: a technique for producing direct reduced iron in such a way that a carbon material which serves as a reduction agent and which contains coal and/or a carbon powder such as powder generated in the production of coke, water, and a binder are added to and mixed with iron ore and a powdery iron oxide-containing material such as dust containing a large amount of iron oxide generated in an ironmaking/steelmaking process; the mixture is formed into pellets or briquettes; and a compacted body (carbon-material-containing iron oxide briquette composition) thereof is dried and is then heated in a rotary hearth furnace such that a reduction reaction occurs. Since the compacted body is made of fine powder, the fine powder is generated from the unreduced compacted body.
  • the compacted body contains carbonaceous matter unused for reduction. Therefore, carbon, particles of direct reduced iron, and oxide components contained in the iron oxide-containing material remain in the furnace in the form of dust in some cases because of breakage due to impact when direct reduced iron is discharged from the furnace. Furthermore, while the discharged direct reduced iron is being transported with a transfer system such as a belt conveyor, fine powder is caused by the friction between the direct reduced iron particles or by drop impact due to connections of conveyor belts.
  • Portions of the direct reduced iron pulverized or crushed during discharge are suspended in a gas stream in the furnace and may possibly adhere to internal walls of the rotary hearth furnace and the wall of a waste gas duct. If the discharged direct reduced iron is pulverized or crushed into dust during transportation, then the dust needs to be used as a raw material again and a reduction in yield is caused in a step of melting the direct reduced iron in an electric furnace or a blast furnace into pig iron.
  • the following technique is desired: a technique for suppressing or preventing the occurrence of pulverization in the course of reduction in a furnace or during discharge out of the furnace and transportation.
  • direct reduced iron is used as an iron source for blast furnaces, steel converters, electric furnaces, and the like and therefore is required to have high carbon content for the purpose of increasing the energy efficiency of a furnace.
  • the following methods have been proposed: a method in which pulverization is suppressed in such a manner that the basicity (CaO/SiO 2 ) ratio is adjusted to 0.3 to 0.6 using a Ca-containing compound such as CaCO 3 as a raw material for adjusting the basicity of slag for the purpose of compacting direct reduced iron particles by the fusion and fixation of a slag component and thereby the crushing strength of direct reduced iron is increased and a method in which the basicity is adjusted to 1.4 to 1.6 and a briquette composition containing 10% to 20% carbon is heated at 1,250° C. to 1,350° C. (see Patent Literatures 1 and 2).
  • the following method has also been proposed for the purpose of preventing pulverization due to bursting in a furnace: a method in which the (CaO/SiO 2 ) ratio is adjusted to 0.5 to 1.5, the sum of the amounts of water of crystallization and volatile matter is adjusted to 10.5% by mass or less, and the content of adsorbed water is adjusted to 1.0% or less (see Patent Literature 7).
  • this method is not applicable to a raw material containing an oxide containing more than 10.5% by mass of water of crystallization. It takes a long time to reduce the content of water to 1% by mass or less.
  • the following method has also been proposed: a method in which (CaO ⁇ MgO)/T.Fe is adjusted to 0.1 or less, (CaO ⁇ MgO)/SiO 2 is adjusted to 2 or less, the size of an iron oxide material is adjusted to 50 ⁇ m or less, the ratio of carbon dioxide to carbon monoxide in a reduction zone is adjusted to 0.3 to 1, the degree of metallization of direct reduced iron is adjusted to 50% to 85%, and residual carbon in the direct reduced iron is adjusted to 2% by mass or less (see Patent Literature 8).
  • an iron oxide material with a size of more than 50 microns needs a pulverization step, those having a larger particle size take a longer crushing time, and direct reduced iron containing a large amount of carbon cannot be obtained.
  • the present invention has an object to provide a carbon-material-containing iron oxide briquette composition which is not pulverized in a moving hearth reduction furnace or does not cause the accumulation of powder when direct reduced iron is obtained by heat-treating the carbon-material-containing iron oxide briquette composition in the moving hearth reduction furnace and which can reliably prevent a reduction in yield due to the pulverization of the obtained direct reduced iron during transportation, an object to provide a method for producing the same, and an object to provide a method for producing direct reduced iron using the same.
  • the present invention includes embodiments below.
  • a carbon-material-containing iron oxide briquette composition is used as a feedstock for moving hearth reduction furnaces for producing direct reduced iron.
  • the amount of a blended carbon material is such that the solidus temperature of an Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the briquette composition is 1,300° C. or lower and residual carbon in direct reduced iron produced by heat-treating the briquette composition in the moving hearth reduction furnace at a temperature which is higher than the solidus temperature thereof and which is lower than the liquidus temperature of the ternary slag accounts for 6% by mass or less.
  • a method for producing a carbon-material-containing iron oxide briquette composition used as a feedstock for a moving hearth reduction furnace for producing direct reduced iron includes a step of adjusting the blending ratio of an iron oxide-containing material, a carbon material, and an auxiliary material such that the solidus temperature of an Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the briquette composition is 1,300° C. or lower and residual carbon in direct reduced iron by produced by heat-treating the briquette composition in the moving hearth reduction furnace at a temperature which is higher than the solidus temperature thereof and which is lower than the liquidus temperature of the ternary slag accounts for 6% or less.
  • the auxiliary material used is at least one of a calcium oxide-containing substance and a silicon dioxide-containing substance.
  • a method for producing direct reduced iron by heat-treating the carbon-material-containing iron oxide briquette composition specified in Item (1) or a carbon-material-containing iron oxide briquette composition produced by the method specified in Item (2) or (3) in the moving hearth reduction furnace includes a step of adjusting the heat treatment temperature within a temperature range which is higher than the solidus temperature and which is lower than the liquidus temperature such that the carbon use efficiency ⁇ c defined by the following equation ranges from 0.08 to 0.12:
  • NCO and NCO 2 are the total molar quantity of CO and the total molar quantity of CO 2 , respectively, generated from the carbon-material-containing iron oxide briquette composition during the heat treatment.
  • a carbon-material-containing iron oxide briquette composition used as a feedstock for moving hearth reduction furnaces for producing direct reduced iron contains a pre-melt slag in which the pre-melt slag solidus temperature T S ⁇ P of an Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 is 1,300° C. or lower.
  • the pre-melt slag solidus temperature T S ⁇ P is 1,200° C. or lower.
  • the pre-melt slag is at least one of a blast furnace slag and a steelmaking slag.
  • the pre-melt slag is at least one of a blast furnace slag and a steelmaking slag.
  • a method for producing the carbon-material-containing iron oxide briquette composition specified in Item (5) or (7) includes a step of adjusting the blending ratio of the pre-melt slag such that the total-slag solidus temperature T S ⁇ S of the Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the briquette composition is 1,300° C.
  • the melt ratio of the Al 2 O 3 —CaO—SiO 2 ternary slag in the direct reduced iron is defined as the mass ratio of a portion of the Al 2 O 3 —CaO—SiO 2 ternary slag in the direct reduced iron to the direct reduced iron, the portion being converted into a liquid phase at the heat treatment temperature.
  • a method for producing the carbon-material-containing iron oxide briquette composition specified in Item (6) or (8) includes a step of adjusting the blending ratio of the pre-melt slag such that the total-slag solidus temperature T S ⁇ S of the Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the briquette composition is 1,200° C.
  • the melt ratio of the Al 2 O 3 —CaO—SiO 2 ternary slag in the direct reduced iron is defined as the mass ratio of a portion of the Al 2 O 3 —CaO—SiO 2 ternary slag in the direct reduced iron to the direct reduced iron, the portion being converted into a liquid phase at the heat treatment temperature.
  • a method for producing direct reduced iron by heat-treating the carbon-material-containing iron oxide briquette composition specified in any one of Items (5) to (8) or a carbon-material-containing iron oxide briquette composition produced by the method specified in Item (9) or (10) in the moving hearth reduction furnace, the amount of a carbon material blended in the carbon-material-containing iron oxide briquette composition being adjusted, includes a step of adjusting the heat treatment temperature within a temperature range which is higher than the total-slag solidus temperature T S ⁇ S and which is lower than the total-slag liquidus temperature T L ⁇ S such that the carbon use efficiency ⁇ c defined by the following equation ranges from 0.08 to 0.12 to obtain direct reduced iron with a carbon content of 6% by mass or less:
  • NCO and NCO 2 are the total molar quantity of CO and the total molar quantity of CO 2 , respectively, generated from the carbon-material-containing iron oxide briquette composition during the heat treatment.
  • direct reduced iron which contains a sufficient amount of carbon and which has increased crushing strength can be produced using a carbon-material-containing iron oxide briquette composition in which the solidus temperature of an Al 2 O 3 —CaO—SiO 2 ternary slag in the carbon-material-containing iron oxide briquette composition is adjusted to 1,300° C. or lower and in which raw materials are blended such that residual carbon in direct reduced iron that is produced by heating the carbon-material-containing iron oxide briquette composition at a temperature which is higher than the solidus temperature thereof and which is lower than the liquidus temperature thereof accounts for 6% by mass or less.
  • a pre-melt slag in which the solidus temperature of an Al 2 O 3 —CaO—SiO 2 ternary slag is 1,300° C. or lower, preferably 1,200° C. or lower, is blended in a carbon-material-containing iron oxide briquette composition. Therefore, when the carbon-material-containing iron oxide briquette composition is heat-treated in a moving hearth reduction furnace, a portion of the pre-melt slag is readily melted and the sintering reaction of metallic iron is promoted, whereby direct reduced iron with increased crushing strength can be produced.
  • FIG. 1 is a phase diagram of an Al 2 O 3 —CaO—SiO 2 ternary slag for illustrating the relationship between the slag composition and solidus temperature of a carbon-material-containing iron oxide briquette composition according to a first embodiment.
  • FIG. 2 is a phase diagram of an Al 2 O 3 —CaO—SiO 2 ternary slag for illustrating the relationship between the slag composition and solidus temperature of a carbon-material-containing iron oxide briquette composition according to a second embodiment.
  • FIG. 3 is a vertical sectional view outlining a compact high-frequency rapid heating furnace used in a reduction test.
  • FIG. 4 includes illustrations each schematically showing a heating pattern used in a reduction test, FIG. 4( a ) shows a heating pattern used in Experiment 1, and FIG. 4( a ) shows a heating pattern used in Experiment 2.
  • FIG. 5 is a phase diagram of an Al 2 O 3 —CaO—SiO 2 ternary slag for illustrating the relationship between the slag composition and solidus temperature of carbon-material-containing iron oxide pellets used in a reduction test of Experiment 1.
  • FIG. 6 is a phase diagram of an Al 2 O 3 —CaO—SiO 2 ternary slag for illustrating the relationship between the slag composition and solidus temperature of carbon-material-containing iron oxide pellets used in a reduction test of Experiment 2.
  • a first embodiment of the present invention is characterized in that the component composition of slag in a carbon-material-containing iron oxide briquette composition (hereinafter also simply referred to as “briquette composition”) and the amount of a carbon material blended therein are within a specific range.
  • briquette composition carbon-material-containing iron oxide briquette composition
  • This allows product direct reduced iron which is an iron source suitable for blast furnaces, electric furnaces, steel converters, and the like; which contains a sufficient amount of carbon; and which has increased crushing strength to be obtained.
  • the amount of the blended carbon material is such that the solidus temperature of an Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the briquette composition is 1,300° C. or lower and residual carbon in direct reduced iron produced by heat-treating the briquette composition in a moving hearth reduction furnace (for example, a rotary hearth reduction furnace) at a temperature which is higher than the solidus temperature thereof and which is lower than the liquidus temperature thereof accounts for 6% or less.
  • a moving hearth reduction furnace for example, a rotary hearth reduction furnace
  • Al 2 O 3 —CaO—SiO 2 ternary slag is designated is that components of slag present in the briquette composition are principally Al 2 O 3 , CaO, and SiO 2 .
  • the solidus temperature is the temperature at which slag begins to melt partially. If the liquidus temperature of slag is specified and the briquette composition is heated at a temperature higher than the liquidus temperature, then the slag components are completely melted at once and therefore a large number of voids are formed in the briquette composition to prevent the sintering of metallic iron; hence, high-strength direct reduced iron cannot be obtained.
  • the solidus temperature of slag is specified and the briquette composition is heated at a temperature higher than the solidus temperature, then a solid-liquid coexistence state in which the slag components are not completely melted but are partially melted is obtained, resulting in that the formation of voids due to the melting of slag can be suppressed and the sintering of metallic iron can be promoted.
  • the strength development of direct reduced iron is not due to the formation of a slag phase but is due to the sinter structure of metallic iron.
  • the reason why the solidus temperature is set to 1,300° C. or lower is that in the case of producing direct reduced iron in a rotary hearth reduction furnace, the fact that the heating temperature is often 1,300° C. or lower is taken into account. Furthermore, the reason why the solidus temperature is more preferably 1,200° C. or lower is that the sintering of metallic iron can be promoted in such a manner that a solid-liquid coexistence state is created at the initial stage of heat treatment.
  • the solidus temperature of the Al 2 O 3 —CaO—SiO 2 ternary slag can be determined, for example, as described below.
  • FIG. 1 is a phase equilibrium diagram (SLAG ATLAS 2nd Edition (1995), Verlag Stahleisen Gmbh, p. 105) of a CaO—SiO 2 —Al 2 O 3 ternary composite oxide commonly used.
  • T L liquidus temperature
  • T S solidus temperature
  • T S solidus temperature
  • T S solidus temperature of the ternary composite oxide
  • the amounts of an iron oxide-containing material, the carbon material, and auxiliary materials blended in the briquette composition may be adjusted such that the composition of the ternary slag in the briquette composition is within the above region. Since the amount of the carbon material blended in the briquette composition is finally determined depending on the amount of residual carbon in direct reduced iron as described below, the composition of the ternary slag is adjusted by principally adjusting the amounts of the iron oxide-containing material and auxiliary materials blended.
  • the composition of the ternary slag can be adjusted by adjusting the blending ratio of steelmaking dust and iron ore which are iron oxide materials and which have different slag component compositions and the amounts of an added calcium oxide-containing substance such as limestone or caustic lime and/or a silicon dioxide-containing substance such as silica stone, the calcium oxide-containing substance and/or the silicon dioxide-containing substance being an auxiliary material.
  • the heat treatment temperature means the maximum atmospheric temperature in the moving hearth reduction furnace.
  • residual carbon in direct reduced iron is limited to 2% by mass or less.
  • the strength of direct reduced iron with higher carbon content can be ensured by properly controlling the melting of the slag components as described above. Where the carbon content exceeds 6% by weight, the coalescence of metallic iron is inhibited and therefore the crushing strength of direct reduced iron is reduced (see Table 5 in an example below).
  • residual carbon in direct reduced iron is preferably low.
  • higher C content is preferred; hence, residual carbon in direct reduced iron is preferably more than 2% by mass and more preferably 3% by mass or more.
  • the amount of residual carbon in direct reduced iron can be adjusted by adjusting the amount (carbon content) of the carbon material blended in the carbon-material-containing iron oxide briquette composition and can also be adjusted by adjusting, for example, the blending ratio of blast furnace dust with high carbon content or the amount of the carbon material, such as coal or coke breeze, in the course of producing the carbon-material-containing iron oxide briquette composition.
  • the amount of the carbon material blended in the carbon-material-containing iron oxide briquette composition can be set in accordance with a thought below.
  • the volatile matter is removed at 500° C. to 600° C. in the course of heating the briquette composition in the rotary hearth reduction furnace.
  • the volatile matter hardly contributes to the reduction of iron oxide.
  • the temperature of the briquette composition increases to reach about 700° C., the reduction reaction of iron oxide is substantially initiated by fixed carbon as well known.
  • the mass Xc of fixed carbon in the briquette composition can be regarded as the sum of the mass Xc T of carbon needed to completely reduce iron oxide and zinc oxide into metals and the mass Xc R of residual carbon in direct reduced iron after reduction as shown in Equation (2) below.
  • Equation (3) the mass Xc T of carbon needed to completely reduce iron oxide and zinc oxide into metals can be estimated from Equation (3) below.
  • X 0 is the sum of the mass of oxygen in iron oxide in the carbon-material-containing iron oxide briquette composition and the mass of oxygen in zinc oxide therein and ⁇ c is carbon use efficiency (described below in detail).
  • Equation (3) The reason why the reduction of zinc oxide is taken into account in addition to iron oxide in Equation (3) is that in the case of using steelmaking dust as a raw material, a sizable amount of zinc oxide is contained therein and a sizable mass of carbon is needed to reduce the zinc oxide contained therein.
  • the content of an oxide of a non-ferrous metal such as lead or an alkali metal is less than that of iron oxide or zinc oxide and therefore is negligible.
  • Equation (3) means that as the proportion of a CO 2 gas component in a CO—CO 2 gas mixture generated from the reduction reaction increases, the mass of carbon needed to completely reduce iron oxide and zinc oxide into metals decreases.
  • the carbon use efficiency ⁇ c in heat-treating the carbon-material-containing iron oxide briquette composition in the rotary hearth reduction furnace can be determined in such a manner that a test in which direct reduced iron is prepared by heat-treating the carbon-material-containing iron oxide briquette composition in an inert gas atmosphere in a compact high-frequency rapid heating furnace used in an example below is performed and the composition of gas, containing CO and CO 2 , generated from the briquette composition in the test is analyzed.
  • the carbon use efficiency ⁇ c varies depending on the heat treatment temperature and ranges from 0.08 to 0.12.
  • the carbon use efficiency ⁇ c is set between 0.08 and 0.12 depending on the heat treatment temperature of a furnace actually used, the mass Xc T of carbon needed to reduce iron oxide is calculated using Equation (3), and the mass Xc of fixed carbon in the briquette composition is then calculated using Equation (2).
  • the amount of the carbon material blended in the briquette composition can be set on the basis of this calculation result.
  • the heat treatment temperature may be adjusted within a temperature range which is higher than the solidus temperature and which is lower than the liquidus temperature such that the carbon use efficiency ⁇ c defined by reprinted Equation (1) below ranges from 0.08 to 0.12.
  • NCO and NCO 2 are the total molar quantity of CO and the total molar quantity of CO 2 , respectively, generated from the carbon-material-containing iron oxide briquette composition during the heat treatment.
  • the carbon use efficiency ⁇ c can be varied in such a manner that the heat treatment temperature is adjusted between the solidus temperature and the liquidus temperature. As, for example, the heat treatment temperature is increased, a carbon solution reaction (C+CO 2 ⁇ 2CO) is promoted in the briquette composition; hence, the carbon use efficiency ⁇ c tends to decrease.
  • the rotary hearth furnace is exemplified as a kind of moving hearth reduction furnace. Instead, a linear furnace may be used.
  • a second embodiment of the present invention is characterized in that a pre-melt slag in which the solidus temperature (pre-melt slag solidus temperature) T S ⁇ P of an Al 2 O 3 —CaO—SiO 2 ternary slag is preferably 1,300° C. or lower and more preferably 1,200° C. or lower is blended in a carbon-material-containing iron oxide briquette composition (hereinafter also simply referred to as “briquette composition”).
  • briquette composition carbon-material-containing iron oxide briquette composition
  • the pre-melt slag is used is that whereas conventional techniques take a long time to form slag in the case of using, for example, auxiliary materials such as limestone (a CaO source) and silica stone (an SiO 2 source) to adjusted slag components in the carbon-material-containing iron oxide briquette composition because CaO and SiO 2 themselves have a high melting temperature (melting point) and therefore a liquid phase is unlikely to be generated from these auxiliary materials, the pre-melt slag, which itself is already slag, has a melting temperature and therefore a liquid phase is generated in a short time to promote the sintering of metallic iron.
  • auxiliary materials such as limestone (a CaO source) and silica stone (an SiO 2 source)
  • Al 2 O 3 —CaO—SiO 2 ternary slag is designated is that components of the pre-melt slag are principally Al 2 O 3 , CaO, and SiO 2 .
  • the solidus temperature is the temperature at which slag begins to melt partially. If the liquidus temperature of the pre-melt slag is specified and the briquette composition is heated at a temperature higher than the liquidus temperature, then the pre-melt slag is completely melted at once and therefore a large number of voids are formed in the briquette composition to prevent the sintering of metallic iron; hence, high-strength direct reduced iron cannot be obtained.
  • the solidus temperature of the pre-melt slag is specified and the briquette composition is heated at a temperature higher than the solidus temperature, then a solid-liquid coexistence state in which the pre-melt slag is not completely melted but is partially melted is obtained, resulting in that the formation of voids due to the melting of the pre-melt slag can be suppressed and the sintering of metallic iron can be promoted.
  • the strength development of direct reduced iron is not due to the formation of a slag phase but is due to the sinter structure of metallic iron.
  • the reason why the solidus temperature is set to 1,300° C. or lower is that in the case of producing direct reduced iron in a rotary hearth reduction furnace, the fact that the heating temperature is often 1,300° C. or lower is taken into account. Furthermore, the reason why the solidus temperature is more preferably 1,200° C. or lower is that the sintering of metallic iron can be promoted in such a manner that a solid-liquid coexistence state is created at the initial stage of heat treatment.
  • the solidus temperature (pre-melt slag solidus temperature) T S ⁇ P of the Al 2 O 3 —CaO—SiO 2 ternary slag can be determined, for example, as described below.
  • FIG. 2 is a phase equilibrium diagram (SLAG ATLAS 2nd Edition (1995), Verlag Stahleisen Gmbh, p. 105) of a CaO—SiO 2 —Al 2 O 3 ternary composite oxide commonly used.
  • T L liquidus temperature
  • T S solidus temperature
  • T S solidus temperature
  • T S solidus temperature
  • the pre-melt slag may be selected and blended such that the composition of the ternary slag in the pre-melt slag blended in the briquette composition is within the above region.
  • a plurality of pre-melt slags which preferably have a pre-melt slag solidus temperature T S ⁇ P of 1,300° C. or lower and more preferably 1,200° C. or lower and which have different compositions may be blended in combination.
  • a blast furnace slag and a steelmaking slag can be used as the pre-melt slag.
  • the steelmaking slag include steel converter slags, hot-metal pretreatment slags, and electric furnace slags.
  • the blending ratio of the pre-melt slag is preferably adjusted such that the solidus temperature (total-slag solidus temperature) T S ⁇ S of the Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the briquette composition in which the pre-melt slag is blended is 1,300° C. or lower, preferably 1,200° C.
  • the total-slag solidus temperature T S ⁇ S is limited to 1,300° C. or lower, preferably 1,200° C. or lower, as described above; hence, a liquid state can be maintained even if a liquid phase (melted slag) generated from the pre-melt slag reacts with a gangue component in an iron oxide-containing material and ash contained in a carbon material to cause a change in slag composition when the briquette composition is heated in the furnace.
  • the total-slag solidus temperature T S ⁇ S can be determined by substantially the same method as that used to determine the pre-melt slag solidus temperature T S ⁇ P .
  • the melt ratio of the Al 2 O 3 —CaO—SiO 2 ternary slag in direct reduced iron is controlled within the range of 1% to 20%; hence, an appropriate amount of slag is melted, whereby the sintering reaction of metallic iron can be reliably promoted. That is, when the melt ratio thereof is less than 1%, the amount of the melt of slag is too small and therefore the sintering of metallic iron does not proceed sufficiently. In contrast, when the melt ratio thereof is more than 20%, the amount of the slag melt is excessive and therefore the strength development mechanism of cooled direct reduced iron changes from the sintering control of metallic iron to slag bonding control.
  • the melt ratio preferably ranges from 5% to 18%.
  • the upper limit and lower limit of the melt ratio may be arbitrarily combined into the range of the melt ratio.
  • the melt ratio of the Al 2 O 3 —CaO—SiO 2 ternary slag in direct reduced iron is defined as the mass ratio of a portion of the Al 2 O 3 —CaO—SiO 2 ternary slag in the direct reduced iron to the direct reduced iron, the portion being converted into a liquid (that is, a liquid phase) at the heat treatment temperature.
  • the carbon material in the briquette composition contains ash, of which the composition affects the determination of the composition of a total briquette composition ternary slag below. Since the amount of the carbon material blended in the briquette composition is finally determined depending on the amount of residual carbon in direct reduced iron as described below, the composition of the Al 2 O 3 —CaO—SiO 2 ternary slag (hereinafter simply referred to as “total briquette composition ternary slag”) that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the total briquette composition is adjusted by principally adjusting the amounts of the iron oxide-containing material, the pre-melt slag, and auxiliary materials blended.
  • the composition of the total briquette composition ternary slag can be adjusted by adjusting the amount of the added pre-melt slag and further adjusting the amounts of an added calcium oxide-containing substance such as limestone or caustic lime and a silicon dioxide-containing substance such as silica stone as required, the calcium oxide-containing substance and/or the silicon dioxide-containing substance being an auxiliary material, depending on the blending ratio of steelmaking dust and iron ore which are iron oxide materials and which have different slag component compositions.
  • the amount of the carbon material blended in the briquette composition is preferably adjusted such that the carbon use efficiency ⁇ c defined by an equation below ranges from 0.08 to 0.12 and such that residual carbon in direct reduced iron obtained by heat-treating the briquette composition at the heat treatment temperature, which is higher than the total-slag solidus temperature T S ⁇ S and is lower than the total-slag liquidus temperature T L ⁇ S , accounts for 6% by mass or less.
  • the heat treatment temperature means the maximum atmospheric temperature in the moving hearth reduction furnace.
  • the reason why the heat treatment temperature is adjusted to a temperature which is higher than the total-slag solidus temperature T S ⁇ S and which is lower than the total-slag liquidus temperature T L ⁇ S is that the sintering of metallic iron is promoted by partially liquefying slag.
  • the composition of slag may be selected such that the total-slag liquidus temperature T L ⁇ S is higher than 1,300° C.
  • residual carbon in direct reduced iron is limited to 2% by mass or less.
  • the strength of direct reduced iron with higher carbon content can be ensured by properly controlling the melting of the slag components as described above. Where the carbon content exceeds 6% by weight, the coalescence of metallic iron is inhibited and therefore the crushing strength of direct reduced iron is reduced. From the viewpoint of ensuring the strength of direct reduced iron, residual carbon in direct reduced iron is preferably low. However, from the viewpoint of improving the energy efficiency of blast furnaces, steel converters, electric furnaces, and the like, higher C content is preferred; hence, residual carbon in direct reduced iron is preferably more than 2% by mass and more preferably 3% by mass or more.
  • the amount of residual carbon in direct reduced iron can be adjusted by adjusting the amount (carbon content) of the carbon material blended in the carbon-material-containing iron oxide briquette composition and can also be adjusted by adjusting, for example, the blending ratio of blast furnace dust with high carbon content or the amount of the carbon material, such as coal or coke breeze, in the course of producing the carbon-material-containing iron oxide briquette composition.
  • the amount of the carbon material blended in the carbon-material-containing iron oxide briquette composition can be set in accordance with a thought below.
  • the volatile matter is removed at 500° C. to 600° C. in the course of heating the briquette composition in the rotary hearth reduction furnace.
  • the volatile matter hardly contributes to the reduction of iron oxide.
  • the temperature of the briquette composition increases to reach about 700° C., the reduction reaction of iron oxide is substantially initiated by fixed carbon as well known.
  • the mass Xc of fixed carbon in the briquette composition can be regarded as the sum of the mass Xc T of carbon needed to completely reduce iron oxide and zinc oxide into metals and the mass Xc R of residual carbon in direct reduced iron after reduction as shown in Equation (2) below.
  • Equation (3) the mass Xc T of carbon needed to completely reduce iron oxide and zinc oxide into metals can be estimated from Equation (3) below.
  • X 0 is the sum of the mass of oxygen in iron oxide in the carbon-material-containing iron oxide briquette composition and the mass of oxygen in zinc oxide therein and ⁇ c is carbon use efficiency (described below in detail).
  • Equation (3) The reason why the reduction of zinc oxide is taken into account in addition to iron oxide in Equation (3) is that in the case of using steelmaking dust as a raw material, a sizable amount of zinc oxide is contained therein and a sizable mass of carbon is needed to reduce the zinc oxide contained therein.
  • the content of an oxide of a non-ferrous metal such as lead or an alkali metal is less than that of iron oxide or zinc oxide and therefore is negligible.
  • Equation (3) means that as the proportion of a CO 2 gas component in a CO—CO 2 gas mixture generated from the reduction reaction increases, the mass of carbon needed to completely reduce iron oxide and zinc oxide into metals decreases.
  • the carbon use efficiency ⁇ c in heat-treating the carbon-material-containing iron oxide briquette composition in the rotary hearth reduction furnace can be determined in such a manner that a test in which direct reduced iron is prepared by heat-treating the carbon-material-containing iron oxide briquette composition in an inert gas atmosphere in a compact high-frequency rapid heating furnace used in an example below is performed and the composition of gas, containing CO and CO 2 , generated from the briquette composition in the test is analyzed.
  • the carbon use efficiency ⁇ c varies depending on the heat treatment temperature and ranges from 0.08 to 0.12.
  • the carbon use efficiency ⁇ c is set between 0.08 and 0.12 depending on the heat treatment temperature of a furnace actually used, the mass Xc T of carbon needed to reduce iron oxide is calculated using Equation (3), and the mass Xc of fixed carbon in the briquette composition is then calculated using Equation (2).
  • the amount of the carbon material blended in the briquette composition can be set on the basis of this calculation result.
  • the heat treatment temperature may be adjusted within a temperature range which is higher than the total-slag solidus temperature T S ⁇ S and which is lower than the total-slag liquidus temperature T L ⁇ S such that the carbon use efficiency ⁇ c defined by reprinted Equation (1) below ranges from 0.08 to 0.12.
  • NCO and NCO 2 are the total molar quantity of CO and the total molar quantity of CO 2 , respectively, generated from the carbon-material-containing iron oxide briquette composition during the heat treatment.
  • the carbon use efficiency ⁇ c can be varied in such a manner that the heat treatment temperature is adjusted between the solidus temperature and the liquidus temperature. As, for example, the heat treatment temperature is increased, a carbon solution reaction (C+CO 2 ⁇ 2CO) is promoted in the briquette composition; hence, the carbon use efficiency ⁇ c tends to decrease.
  • the rotary hearth furnace is exemplified as a kind of moving hearth reduction furnace. Instead, a linear furnace may be used.
  • carbon-material-containing iron oxide briquette compositions were used in the form of pellets.
  • the dried pellets (carbon-material-containing iron oxide pellets) were heat-treated in a compact high-frequency rapid heating furnace (manufactured by Sekisui Medical Electronics, the type of an oscillator being MU-1700, the type of a furnace being UD-250) outlined in FIG. 3 .
  • a graphite tube coated with alumina was used to prevent graphite from being depleted by a CO 2 -containing gas generated by heating the pellets.
  • a heating pattern was as follows: as shown in FIG. 4( a ), heating was performed at a rate of 150° C./min from room temperature to 1,250° C. and a rate of 15° C./min from 1,250° C. to 1,320° C. and was stopped immediately after reaching 1,320° C. and quenching was performed using a He gas.
  • a heating atmosphere was supplied with an N2 gas at a rate of 3 NL/min and a cooling atmosphere was supplied with a He gas at a rate of 3 NL/min.
  • the liquid phase ratio is located between a solidus temperature and liquidus temperature in an equilibrium diagram and is defined as the mass percentage of a liquid in a solid-liquid mixture (that is, a solid phase-liquid phase mixture) (see paragraph of Japanese Unexamined Patent Application Publication No. 2005-48197).
  • the mass percentage (%) of a liquid in a solid-liquid mixture at the heat treatment temperature (1,300° C.) was calculated using the thermodynamic equilibrium calculation software “FactSage”.
  • the unit kgf of the crushing strength of direct reduced iron in this table corresponds to 9.80665 N.
  • FIG. 5 shows a diagram obtained by plotting the slag composition of each of the carbon-material-containing iron oxide pellets of Inventive Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-5 on the ternary phase diagram of FIG. 1 .
  • the carbon use efficiency ranges from 0.08 to 0.12.
  • the carbon-material-containing iron oxide pellets of Inventive Examples 1-1 to 1-4 meet requirements for the first embodiment of the present invention because the solidus temperature T S of an Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the pellets is 1,300° C. or lower and the amount of residual carbon in direct reduced iron obtained by heat treatment at 1,320° C., which is higher than the solidus temperature T S of the ternary slag and lower than the liquidus temperature T L thereof, is 6% by mass or less (more than 2% by mass).
  • the crushing strength of the direct reduced iron is high, more than 19 kgf/piece.
  • the carbon-material-containing iron oxide pellets of Comparative Example 1-5 do not meet the requirements for the first embodiment of the present invention because although the solidus temperature T S of an Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the pellets is 1,300° C. or lower, the amount of residual carbon in direct reduced iron obtained by heat treatment at 1,320° C., which is higher than the solidus temperature T S of the ternary slag and lower than the liquidus temperature T L thereof, exceeds 6% by mass.
  • the crushing strength of the obtained direct reduced iron is low, less than 3 kgf/piece.
  • a carbon-material-containing iron oxide briquette composition was used in the form of pellets.
  • the dried pellets (carbon-material-containing iron oxide pellets) were heat-treated in a compact high-frequency rapid heating furnace (manufactured by Sekisui Medical Electronics, the type of an oscillator being MU-1700, the type of a furnace being UD-250) outlined in FIG. 3 .
  • a graphite tube coated with alumina was used to prevent graphite from being depleted by a CO 2 -containing gas generated by heating the pellets.
  • a heating pattern was as follows: as shown in FIG. 4( b ), heating was performed at a rate of 150° C./min from room temperature to 1,250° C. and a rate of 15° C./min from 1,250° C. to 1,320° C. and was stopped immediately after reaching 1,320° C. and quenching was performed.
  • a heating atmosphere was supplied with an N2 gas at a rate of 3 NL/min and a cooling atmosphere was supplied with a He gas at a rate of 3 NL/min.
  • the liquid phase ratio is located between a solidus temperature and liquidus temperature in an equilibrium diagram and is defined as the mass percentage of a liquid in a solid-liquid mixture (that is, a solid phase-liquid phase mixture) (see paragraph of Japanese Unexamined Patent Application Publication No. 2005-48197).
  • the mass percentage (%) of a liquid in a solid-liquid mixture at the heat treatment temperature (1,300° C.) was calculated using the thermodynamic equilibrium calculation software “FactSage”.
  • the melt ratio (%) of the Al 2 O 3 —CaO—SiO 2 ternary slag in direct reduced iron was determined by multiplying the total mass % of the Al 2 O 3 —CaO—SiO 2 ternary slag in direct reduced iron by the liquid phase ratio (%) divided by 100.
  • the unit kgf of the crushing strength of direct reduced iron in this table corresponds to 9.80665 N.
  • FIG. 6 shows a diagram obtained by plotting the composition of the Al 2 O 3 —CaO—SiO 2 ternary slag in each of the carbon-material-containing iron oxide pellets of Inventive Examples 2-1 to 2-7 and Comparative Examples 2-1 to 2-5 on the ternary phase diagram of FIG. 2 .
  • the carbon use efficiency ranged from 0.08 to 0.12.
  • the carbon-material-containing iron oxide pellets of Inventive Examples 2-1 to 2-7 all contain the pre-melt slag, in which the solidus temperature (pre-melt slag solidus temperature) T S ⁇ P of the Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 is 1,300° C. or lower.
  • the solidus temperature (total-slag solidus temperature) T S ⁇ S of an Al 2 O 3 —CaO—SiO 2 ternary slag that is determined from the content of Al 2 O 3 , CaO, and SiO 2 in the pellets is 1,300° C.
  • the amount of residual carbon in direct reduced iron obtained by heat treatment at the same temperature is 6% by mass or less (3% by mass or more).
  • the crushing strength of the direct reduced iron is high, more than 25 kgf/piece.
  • a steelmaking slag which serves as a pre-melt slag and which a solidus temperature T S ⁇ P of 1,200° C. or lower, which is more preferred is blended; the content of residual carbon is 3% or more; and the total-slag solidus temperature T S ⁇ S is 1,200° C. or lower, which is more preferred. This results in that the crushing strength of direct reduced iron is highest, more than 40 kgf/piece.
  • the carbon-material-containing iron oxide pellets of Comparative Example 2-4 do not meet the requirements for the second embodiment of the present invention because although a pre-melt slag having a pre-melt slag solidus temperature T S ⁇ P of 1,300° C. or lower is blended and the total-slag solidus temperature T S ⁇ S is 1,300° C. or lower, the melt ratio exceeds 20%. Therefore, the crushing strength of obtained direct reduced iron is very low, less than 4 kgf/piece.
  • direct reduced iron which contains a sufficient amount of carbon and which has increased crushing strength can be produced using a carbon-material-containing iron oxide briquette composition in which the solidus temperature of an Al 2 O 3 —CaO—SiO 2 ternary slag in the carbon-material-containing iron oxide briquette composition is adjusted to 1,300° C. or lower and in which raw materials are blended such that residual carbon in direct reduced iron that is produced by heating the carbon-material-containing iron oxide briquette composition at a temperature which is higher than the solidus temperature thereof and which is lower than the liquidus temperature thereof accounts for 6% by mass or less.
  • a pre-melt slag in which the solidus temperature of an Al 2 O 3 —CaO—SiO 2 ternary slag is 1,300° C. or lower, preferably 1,200° C. or lower, is blended in a carbon-material-containing iron oxide briquette composition. Therefore, when the carbon-material-containing iron oxide briquette composition is heat-treated in a moving hearth reduction furnace, a portion of the pre-melt slag is readily melted and the sintering reaction of metallic iron is promoted, whereby direct reduced iron with increased crushing strength can be produced.

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US10185792B2 (en) * 2015-12-22 2019-01-22 DOOSAN Heavy Industries Construction Co., LTD Method for predicting slagging production position and slagging production possibility in furnace
CN110750911A (zh) * 2019-10-25 2020-02-04 中冶赛迪重庆信息技术有限公司 一种高炉渣物理性能分析方法及分析系统

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JP2013249496A (ja) * 2012-05-30 2013-12-12 Kobe Steel Ltd 還元鉄とスラグの混合物の製造方法
JP2014084526A (ja) * 2012-10-26 2014-05-12 Kobe Steel Ltd 還元鉄の製造方法
JP6235439B2 (ja) * 2014-09-10 2017-11-22 株式会社神戸製鋼所 粒状金属鉄の製造方法
CN105039613A (zh) * 2015-08-25 2015-11-11 东北大学 一种从炼钢粉尘回收金属颗粒的方法

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US10185792B2 (en) * 2015-12-22 2019-01-22 DOOSAN Heavy Industries Construction Co., LTD Method for predicting slagging production position and slagging production possibility in furnace
CN110750911A (zh) * 2019-10-25 2020-02-04 中冶赛迪重庆信息技术有限公司 一种高炉渣物理性能分析方法及分析系统

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