Patent Application
20120240725
The present invention relates to a carbon composite agglomerate used as a material for a moving hearth furnace for producing reduced iron, and a method for producing reduced iron using such a carbon composite agglomerate.
Various techniques for producing reduced iron by reducing carbon composite agglomerates through heating with a rotary hearth furnace have been proposed (for example, refer to Patent Literatures 1 to 3).
Such reduced iron produced is used as an iron material for a blast furnace, a converter, an electric furnace, or the like. The reduced iron is demanded to have a C content as high as possible for improving the energy efficiency of such a furnace and a strength as high as possible for not producing powder. As an indicator for evaluating the strength of reduced iron usable for such furnaces, crushing strength is generally used. In the case of a material for a blast furnace, as a desirable crushing strength for bearing handling from discharging from a rotary hearth furnace to charging into a blast furnace and the load pressure of charged materials in the blast furnace, 180 kgf/briquette (about 1760 N/briquette) or more is demanded. In the case of a material for a converter or an electric furnace, since it is not necessary to consider load pressure in such a furnace, a crushing strength smaller than that in the case of a raw material for a blast furnace will suffice. However, in view of using reduced iron in general-purpose applications, the establishment of a technique that achieves a C content as high as possible and a crushing strength of 180 kgf/briquette (about 1760 N/briquette) or more has been demanded.
Accordingly, an object of the present invention is to provide a carbon composite agglomerate used as a material for a moving hearth furnace for producing reduced iron, the carbon composite agglomerate providing reduced iron having a sufficiently high carbon content and an increased crushing strength; and a method for producing reduced iron using such a carbon composite agglomerate.
An invention according to Claim 1 is a carbon composite agglomerate for producing reduced iron, the carbon composite agglomerate being used as a material for a moving hearth furnace for producing reduced iron, wherein, in the carbon composite agglomerate, a total content of SiO2, Al2O3, CaO, and MgO is 7 to 15 mass %, a MgO content is 0.1 to 6 mass %, a mass ratio of Al2O3/SiO2 is 0.34 to 0.52, a mass ratio of CaO/SiO2 is 0.25 to 2.0, and a C content is such that 1 to 9 mass % of C remains in reduced iron produced from the carbon composite agglomerate.
An invention according to Claim 2 is the carbon composite agglomerate for producing reduced iron according to Claim 1, having a porosity of 37.5% or less.
An invention according to Claim 3 is the carbon composite agglomerate for producing reduced iron according to Claim 1 or 2, wherein an average grain size d50 of a carbonaceous material in the carbon composite agglomerate measured by a laser diffraction scattering grain size distribution measurement method is 30 μm or less.
An invention according to Claim 4 is the carbon composite agglomerate for producing reduced iron according to any one of Claims 1 to 3, at least containing ironmaking dust.
An invention according to Claim 5 is a method for producing reduced iron by reducing the carbon composite agglomerate for producing reduced iron according to any one of Claims 1 to 4, through heating with a moving hearth furnace, wherein the moving hearth furnace is divided into a plurality of zones in a moving direction of a hearth and a final zone of the plurality of zones has an oxidizing atmosphere.
An invention according to Claim 6 is the method for producing reduced iron according to Claim 5, wherein the oxidizing atmosphere of the final zone has a gas oxidation degree OD of 1.0 or more, and
OD=(CO2+H2O+2O2)/(CO2+H2O+O2+CO+H2)
[where a unit of CO2, H2O, O2, CO, and H2 is vol %].
According to the present invention, by making the slag component composition of a carbon composite agglomerate and the amount of C remaining in a reduced iron product obtained by reducing the carbon composite agglomerate be in specific ranges, reduced iron having a sufficiently high carbon content and an increased crushing strength can be produced.
a) is a sectional view illustrating the internal structure of a carbon composite briquette reduced in a reducing atmosphere and
Hereinafter, the present invention will be described further in detail.
A feature of the present invention is to make the slag component composition and carbon content of carbon composite briquettes be in specific ranges. As a result, a reduced iron product that is more suitable as an iron material for a blast furnace, an electric furnace, a converter, or the like, has a sufficiently high carbon content, and has an increased crushing strength can be obtained.
Specifically, the following carbon composite briquettes are preferably used. In the carbon composite briquettes, the total content of SiO2, Al2O3, CaO, and MgO is 7 to 15 mass %; the MgO content is 0.1 to 6 mass %; the mass ratio of Al2O3/SiO2 is 0.34 to 0.52; and the mass ratio of CaO/SiO2 is 0.25 to 2.0 (more preferably 0.25 to 1.5, particularly preferably 0.25 to 1.0). In addition, the C content of the carbon composite briquettes is adjusted such that 1 to 9 mass % of C remains in a reduced iron product obtained by reducing the carbon composite briquettes.
Hereinafter, reasons for the numerical limitations on such parameters will be described.
The total content of SiO2, Al2O3, CaO, and MgO in carbon composite briquettes substantially equals to the slag component content of the carbon composite briquettes. When the slag component content of carbon composite briquettes is excessively low, the effect of promoting sintering of metallic iron by melting of the slag component is not provided. When the slag component content of carbon composite briquettes is excessively high, reduced iron obtained by reducing the carbon composite briquettes has an excessively high slag content, which inhibits the sintering reaction of metallic iron to decrease the strength of the reduced iron. In addition, the reduced iron has a low iron grade. The total content of SiO2, Al2O3, CaO, and MgO in carbon composite briquettes is preferably in the range of 7 to 15 mass %.
Note that all the carbon composite briquettes used in heating reduction tests described below had a total content of SiO2, Al2O3, CaO, and MgO in the range of 7 to 15 mass %.
When the MgO content increases, the melting point of slag increases and the amount of slag melted decreases and hence the strength-development action for reduced iron described below is not sufficiently exhibited. Accordingly, the upper limit of the MgO content is defined as 6 mass %. On the other hand, since ironmaking dust unavoidably contains the MgO component, the lower limit of the MgO content is defined as 0.1 mass %.
Note that all the carbon composite briquettes used in the heating reduction tests described below had a MgO content in the range of 0.1 to 6 mass %.
The inventors of the present invention first investigated the influence of the slag component composition on the crushing strength of a reduced iron product by performing the following heating reduction tests.
Blend materials having various slag component compositions were prepared by adjusting blending proportions of iron ore and an ironmaking dust mixture obtained by mixing ironmaking dusts including blast-furnace dust. The blend materials were formed into carbon composite briquettes having the shape of a pillow and a volume of 6 to 7 cm3 with a twin-roll briquetting machine. The briquettes were dried with a dryer so as to have a water content of 1 mass % or less. Examples of the chemical composition of the dried briquettes (hereafter, referred to as “dry briquettes”) are shown in Table 1. In the table, “T. C” represents the total carbon content; “T. Fe” represents the total iron content; “M. Fe” represents a metallic-iron content; “T. Fe” includes Fe2O3, FeO, and “M. Fe”; Na, K, and Pb are not present in the form of atoms, but are present in the form of oxides and the like.
About 44 g of the dry briquettes were placed on an alumina tray, inserted into a horizontal heating furnace heated at 1300° C. and having an atmosphere in which one of the two gases of N2 (100% by volume) and CO2/N2 (30%/70% by volume) was passed at a flow rate of 3 NL/min, held until the dry briquettes were reduced, subsequently cooled to room temperature in N2 gas atmosphere, and taken out from the furnace. The reduced briquettes were subjected to the measurement of crushing strength. Since a predetermined amount of CO-containing gas is generated from briquettes during the reduction, the heating reduction test in which N2 (100%) gas was passed to provide a reducing gas atmosphere around the briquettes simulates reduction conditions in a reducing atmosphere in an actual rotary hearth furnace; and the reduction test in which the CO2-containing gas was passed to provide an oxidizing gas atmosphere around the briquettes simulates reduction conditions in the presence of combustion exhaust gas in an actual rotary hearth furnace.
The measurement results are illustrated in
The reason for which the crushing strength of reduced iron is thus increased by making the mass ratios of the slag component of carbon composite briquettes, that is, CaO/SiO2 and Al2O3/SiO2 be in the specific ranges is probably as follows.
As illustrated in
As is clear from
As illustrated in
From the test results, CaO/SiO2 of carbon composite briquettes is particularly preferably in the range of 0.25 to 1.0. However, even when an excessive amount of CaO is present in the carbon composite briquettes, a portion of CaO melts and CaO/SiO2 of molten slag can satisfy the range of 0.25 to 1.0. Thus, sintering of metallic iron is promoted by the same action as that described above and the strength of reduced iron is developed. Accordingly, the preferred range of CaO/SiO2 is defined as the range of 0.25 to 2.0 (more preferably 0.25 to 1.5).
The composition of the slag component of carbon composite briquettes can be adjusted by, for example, adjusting blending proportions of a plurality of ironmaking dusts having different slag component compositions and iron ore, or adjusting the amount of CaO source added such as limestone or burnt lime.
When the amount of C remaining in a reduced iron product obtained by reducing carbon composite briquettes is excessively small, in the case of using the reduced iron product as an iron material for a blast furnace, a converter, an electric furnace, or the like, the action of remaining carbon serving as a reductant for reducing unreduced iron oxide (FeO and the like) remaining in the reduced iron product is insufficient. In addition, the effect of increasing the melting speed of metallic iron due to carburization of metallic iron is not provided. On the other hand, when the amount of C remaining in a reduced iron product is excessively large, a large amount of carbon grains remaining in the reduced iron inhibit bonding between metallic iron grains and hence the strength of the reduced iron becomes insufficient. The amount of C remaining in a reduced iron product obtained by reducing carbon composite briquettes is preferably in the range of 1 to 9 mass %.
Note that the C contents of all the reduced iron products obtained by reducing the carbon composite briquettes used in the heating reduction tests described above were in the range of 1 to 9 mass %.
The amount of C remaining in a reduced iron product can be adjusted by adjusting the amount of a carbonaceous material (carbon content) of carbon composite briquettes: for example, in the production of carbon composite briquettes, by adjusting the blending proportion of blast-furnace dust having a high carbon content or adjusting the amount of a carbonaceous material added such as coal or coke powder.
The carbon content Xc of carbon composite briquettes should be specifically set with the following formula (1).
Xc=XcT+XcR (1)
where XcT=(12/16)·Xo; XcT represents a theoretical C amount necessary for completely reducing iron oxide and zinc oxide in the carbon composite briquettes to the metals; XcR represents the amount of C remaining in reduced iron when the iron oxide and zinc oxide have been completely reduced to the metals with the theoretical C amount XcT; and Xo represents the total amount of oxygen of iron oxide and oxygen of zinc oxide in the carbon composite briquettes.
In the formula (1), in addition to reduction of iron oxide, reduction of zinc oxide is considered. This is because, when ironmaking dust is used as a material, the ironmaking dust contains a relatively large amount of zinc oxide and reduction of the zinc oxide requires a relatively large amount of C. Compared with iron oxide and zinc oxide, the contents of oxides of other nonferrous metals such as lead and alkali metals are low and hence these oxides are not considered.
The theoretical C amount is defined on the premise that reduction of 1 mole of oxygen of iron oxide or zinc oxide requires 1 mole of carbon. However, in actual reduction of carbon composite briquettes with a moving hearth furnace, CO gas is generated by reduction (direct reduction) of iron oxide or zinc oxide with carbon and the CO gas causes reduction (gas reduction) of iron oxide or zinc oxide to proceed; accordingly, the amount of carbon required for reduction of 1 mole of oxygen of iron oxide or zinc oxide is less than 1 mole. On the other hand, since carbon composite briquettes are heated by combustion with burners in a moving hearth furnace, the combustion gas consumes a portion of a carbonaceous material (carbon) in the carbon composite briquettes and the portion is not used for reduction of iron oxide or zinc oxide. As a result, the decrease in the C consumption due to the gas reduction substantially cancels out the increase in the C consumption due to burner combustion gas. Accordingly, the theoretical C amount can be regarded as a C amount actually required for reduction.
As described above, the presence of wustite (FeO) in carbon composite briquettes is desirable for facilitating melting of the slag component. Accordingly, in the production of reduced iron by reducing the carbon composite briquettes through heating with a moving hearth furnace such as a rotary hearth furnace, the rotary hearth furnace is preferably divided into a plurality of zones in the moving direction of the hearth and the final zone of the plurality of zones preferably has an oxidizing atmosphere. By making the final zone have an oxidizing atmosphere, as illustrated in
Specifically, the final zone is preferably made to have an oxidizing atmosphere having a gas oxidation degree OD of 1.0 or more.
OD=(CO2+H2O+2O2)/(CO2+H2O+O2+CO+H2)
[where the unit of CO2, H2O, O2, CO, and H2 is vol %].
Herein, the gas oxidation degree OD is defined as follows: OD=(CO2+H2O+2O2)/(CO2+H2O+O2+CO H2) [where the unit of CO2, H2O, O2, CO, and H2 is vol %] because, when the atmosphere contains the O2 component, the O2 component has an oxidizing capability for metal elements that is twice that of the CO2 component and the H2O component (for example, reaction formulae Fe+CO2═FeO+CO, Fe+H2O═FeO+H2, and 2Fe+O2═2FeO show that 1 mole of CO2 or H2O can oxidize 1 mole of Fe, whereas 1 mole of O2 can oxidize 2 moles of Fe).
The gas oxidation degree of the atmosphere of the final zone can be adjusted by changing, for example, the air-fuel ratio of a burner.
The Embodiment 1 above describes an example in which the physical internal structure of the carbon composite briquettes is not particularly limited. As for the physical internal structure of carbon composite briquettes, in particular, by making the porosity of carbon composite briquettes be in a specific range, even when the amount of carbon remaining in a reduced iron product obtained by reducing the carbon composite briquettes is large, a sufficiently high crushing strength can be achieved with certainty.
Specifically, carbon composite briquettes having a porosity of 37.5% or less are preferably used.
Hereinafter, the reason for which the porosity of carbon composite briquettes is limited to 37.5% or less will be described.
The inventors of the present invention investigated the influence of various parameters on the crushing strength of reduced iron obtained by preparing carbon composite briquettes from ironmaking dust and reducing the carbon composite briquettes under the same test conditions as in Embodiment 2.
The inventors studied the reason why a high crushing strength is achieved in spite of a high C content and, as a result, have found that the porosity of carbon composite briquettes to be reduced influences the crushing strength.
Accordingly, as illustrated in
By making the porosity of carbon composite briquettes be the predetermined value or less, the distance between iron oxide grains in the carbon composite briquettes becomes short and bonding of metallic iron grains (sintering of metallic iron) after reduction is promoted, which probably results in a further increase in the strength of reduced iron.
When the porosity of carbon composite briquettes is made excessively low, bursting tends to occur during reduction. Accordingly, the lower limit of the porosity is preferably 25%.
The porosity of carbon composite briquettes is calculated from the apparent density and true density of carbon composite briquettes:
Porosity(%)=(1−[apparent density]/[true density])×100
where the apparent density of carbon composite briquettes represents the measurement value of the apparent density of dry briquettes; and the true density of carbon composite briquettes represents a weighted average value of true densities of individual materials forming carbon composite briquettes in terms of blending proportions.
In the case of using ironmaking dust as a material, since ironmaking dust has a very small grain size, it may be difficult to compact ironmaking dust. Depending on the type or blending proportion of ironmaking dust used, there are cases where it is difficult to make the porosity of carbon composite briquettes be 37.5% or less by standard forming techniques. In such cases, for example, the following technique may be employed (refer to Japanese Unexamined Patent Application Publication No. 2009-7667): under size after compaction with a briquetting machine is mixed as a recycled material with a new material and returned to the briquetting machine to compact the material to thereby increase the apparent density (that is, decrease the porosity) of carbon composite briquettes.
The Embodiments 1 and 2 above describe examples in which the grain size of a carbonaceous material contained in the carbon composite briquettes is not particularly limited. By making the grain size of such a carbonaceous material be in a specific range, the crushing strength of a reduced iron product obtained by reducing the carbon composite briquettes is ensured and the amount of carbon remaining in the reduced iron can be further increased.
Specifically, the average grain size d50 of a carbonaceous material in carbon composite briquettes measured by a laser diffraction scattering grain size distribution measurement method is preferably made 30 μm or less (more preferably, 10 μm or less).
For example, blast-furnace wet dust containing a large amount of carbon grains derived from coke powder or pulverized coal is used as ironmaking dust and the carbon grains of the blast-furnace wet dust are used as a carbonaceous material to prepare carbon composite briquettes. As for reduced iron obtained by reducing such carbon composite briquettes, it has been found that the amount of carbon remaining in the reduced iron can be made high while the crushing strength is ensured. The grain size distribution of the blast-furnace wet dust was measured by a laser diffraction scattering grain size distribution measurement method and the grain size distribution illustrated in
Accordingly, the average grain size d50 of a carbonaceous material in carbon composite briquettes measured by a laser diffraction scattering grain size distribution measurement method is preferably 30 μm or less, more preferably 10 μm or less.
The average grain size d50 of a carbonaceous material in carbon composite briquettes may be adjusted, for example, in the following manner. When blast-furnace wet dust is used as a portion of materials, the blending proportion of the dust is adjusted. When coal powder or coke powder is added as a carbonaceous material, the pulverization grain size of such a powder is adjusted.
In the above-described embodiments, as an example of the agglomerate form of carbon composite agglomerates, briquettes are described. Alternatively, pellets may be employed.
In the above-described embodiments, as an example of the furnace type of a moving hearth furnace, a rotary hearth furnace is described. Alternatively, a straight hearth furnace may be employed.
While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
This application is based on Japanese Patent Application No. 2009-169719 filed on Jul. 21, 2009, the entire contents thereof being hereby incorporated by reference.
The present invention is advantageous as a technique of producing reduced iron from ironmaking dust in ironmaking equipment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-169719 | Jul 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/062254 | 7/21/2010 | WO | 00 | 1/20/2012 |
