Patent Application
20050193862
The present invention relates to a method for manufacturing sponge iron used as a material in manufacturing iron powder and a method for manufacturing reduced iron powder with the sponge iron manufactured by the method.
The reduced iron powder is used in the form of powder as-is, and also used as a material for a sintered product such as a mechanical component and a magnetic material.
The present invention also relates to an apparatus for charging a material of sponge iron manufactured by the method for manufacturing a sponge iron, and relates to high-purity sponge iron that can be manufactured by the method.
Sponge iron is manufactured by the following procedure: Iron oxide powder 2 and reducing-agent powder 3 are alternately charged in the form of coaxial cylinders into a cylindrical heat-resistant reaction container 1 (sagger) that can be equipped with a lid at the bottom. The charged iron oxide powder 2 and reducing-agent powder 3 are heated (indirectly heated) at 1050° C. to 1200° C. in the reaction container 1 with a tunnel furnace or the like. The iron oxide powder 2 in the reaction container 1 is reduced (roughly reduced) and is sintered by the heat treatment, thus resulting in metallic iron that is in the form of a sponge, i.e., sponge iron.
The iron oxide powder 2 includes iron ore powder and powder produced by crushing mill scale. The reducing-agent powder 3 includes coke powder and coal powder. Lime powder or the like may be added to the reducing-agent powder 3, if necessary.
The above-described techniques are disclosed in “Tekkou binran”, third edition, vol. 5, pp. 457-459 (in particular, page 457, right column, line 10-13) and Japanese Unexamined Patent Application Publication No. 2002-241822.
In a known technique for manufacturing sponge iron as shown in
When the reaction container 1 is heated after the materials are charged, in an early stage, a carbon dioxide (CO2) gas formed by allowing oxygen that is present in the voids of the charged layer of the reducing agent to react with carbon in the reducing agent and formed by the decomposition of limestone added to the reducing agent reacts with carbon in the reducing agent according to chemical equation (1) to generate carbon monoxide (CO), which is a reducing gas, in the charged layer of the reducing-agent powder 3 (reducing-agent layer).
C+CO2→2CO equation (1)
The CO gas thus generated reaches from the reducing-agent layer to a charged layer of the iron oxide powder 2 (iron-oxide layer). Then, iron oxide is reduced as the generation of a CO2 gas according to the following chemical equation (2):
FeOn+nCO→Fe+nCO2 equation (2)
The generated CO2 gas diffuses into the iron-oxide layer including partially-reduced iron oxide and reaches the reducing-agent layer again. Then the CO2 gas reacts with carbon in the reducing-agent layer to generate a CO gas according to equation (1). This resulting CO gas diffuses into the iron-oxide layer again and reacts with unreduced iron oxide according to equation (2) to produce iron as the generation of a CO2 gas.
As a result, all iron oxide powder 2 charged in the reaction container 1 is reduced to iron powder by repeating the reactions according to equations (1) and (2) at certain intervals. At the same time of this reduction reaction, reduced iron particles are sintered to form cylindrical sponge iron (sintered body).
An amount of CO gas required for reducing all iron oxide is theoretically 1 in molar ratio according to equation (2) ((the number of moles of carbon atoms in the CO gas)/(the number of moles of oxygen atoms in the iron oxide)). Hence, an amount of reducing agent required for reducing all iron oxide is 1.0 in molar ratio ((the number of moles of carbon atoms in the reducing agent)/(the number of moles of oxygen atoms in the iron oxide)). Hereinafter, (the number of moles of carbon atoms in the reducing agent)/(the number of moles of oxygen atoms in the iron oxide) is referred to as “(the carbon content)/(the oxygen content) (molar ratio)”.
In the above-described process for reducing, diffusion of the CO and CO2 gases which are generated in the reaction container 1 into the iron oxide powder 2 and reducing-agent powder 3 is a main rate-determining factor in the reduction reaction. However, in a process having the structure charged as shown in
For example, in a manufacturing step for an industrial-scale production with a tunnel furnace for heating, a long time required for the reduction decreases reaction efficiency (gas use efficiency); hence, it takes several days from charging materials to drawing a product, thus leading to low productivity. Furthermore, heating energy consumption required for the reduction is significantly large.
In a process having a charged structure as shown in
Therefore, a combination of the thickness of the layer of the iron oxide powder 2 and the reduction time is uniquely determined so that the largest yield can be achieved. There are problems with a low degree of flexibility in adjusting the yield as well as the limitation of the yield.
In addition, in a process for charging as shown in
Furthermore, to hold shape of the layer of the iron oxidepowder 2 in a firing stage, it is necessary to excessively charge the reducing-agent powder 3 into a portion between the reaction container 1 and the iron oxide powder 2 and into inside of the cylindrical iron-oxide layer.
In the above-described circumstances in a known process, there is a problem in that a large amount of reducing-agent powder 3 is required, i.e., at least 2.0 in molar ratio, thus resulting in poor unit requirement of a reducing agent.
In addition, the lower portion of a cylindrical iron-oxide layer can swell under its own weight. Hence, there is a problem in that iron oxide in the swelling portion is insufficiently reduced within a predetermined reduction time, thus remaining an unreduced portion.
It is an object of the present invention to advantageously solve various problems described above of the known art. That is, it is an object of the present invention to provide a method for manufacturing sponge iron, wherein the method has high productivity and can easily adjust the yield.
It is another object of the present invention to provide an apparatus for charging materials into a reaction container, wherein the apparatus is advantageously used when the above-described method for manufacturing is performed.
The inventors have conducted intensive research, and found that the above-described problems can be advantageously solved by devising a charged form of iron oxide powder and reducing-agent powder in a reaction container. Consequently, the present invention has been completed.
That is, a first aspect of the present invention, a method for manufacturing sponge iron includes a charging step of charging iron oxide powder and reducing-agent powder into a reaction container; and a reducing step of reducing the iron oxide powder in the reaction container to produce a mass of sponge iron by heating from the outside of the reaction container, wherein, in the charging step, the iron oxide powder and the reducing-agent powder are charged such that alternating layers of the iron oxide powder and the reducing-agent powder are formed and such that each of the layers is in the form of a helix.
In the above-described first aspect of the present invention, suitable conditions described below are preferably applied alone or in any combination.
(1) In the charging step, the iron oxide powder and the reducing-agent powder are charged such that layers composed of the reducing-agent powder are disposed on an inner side-surface of the reaction container (referred to as “peripheral portion”) and disposed at a central portion along the vertical central axis and such that the alternating layers that are in the form of helices are disposed at a portion (referred to as “intermediate portion”) other than the portion of the layers disposed on the inner side-surface and at the central portion. The peripheral portion and the central portion along the vertical central axis correspond to a circumferential portion and a central portion, respectively, in horizontal sectional view of the container. The intermediate portion is preferably in the form of a cylinder or a column. When the reaction container is in the form of cylinder, the vertical central axis corresponds to the center of the cylinder.
(2) The iron oxide powder is composed of at least one selected from the group consisting of an iron ore, mill scale, and iron oxide powder recovered from waste pickle liquor.
(3) The reducing-agent powder is composed of at least one selected from the group consisting of coke, char, and coal.
(4) A source of a carbon dioxide gas is added to the reducing-agent powder. The source of a carbon dioxide gas preferably includes limestone (including calcined limestone). In this case, the reducing-agent powder to which the powder of the source of a carbon dioxide gas is added is charged.
(5) The heating temperature is 1000° C. to 1300° C. in the reducing step.
(6) In the charging step, the thicknesses of the layers of the iron oxide powder and the reducing-agent powder are variable when forming the layers that are in the form of helices. Variably controlling includes the following meanings: A different thickness of at least any one of the layers can be set in each reaction container. A thickness of at least any one of the layers can be varied with position of the reaction container 1.
(7) In the charging step, the amounts of iron oxide powder and reducing-agent powder in the reaction container are controlled such that the molar ratio of the carbon content in the reducing-agent powder to the oxygen content in the iron oxide powder is at least 1.1. The molar ratio is preferably 1.15 or more and more preferably 1.2 or more.
(8) In the charging step according to suitable conditions (1) and (7), the amounts of iron oxide powder and reducing-agent powder in the charged portion having layered structure are controlled such that the molar ratio of the carbon content in the reducing-agent powder to the oxygen content in the iron oxide powder is at least 0.5. The term “charged portion having layered structure” represents a cylindrical region formed of helically deposited layers of iron oxide powder and reducing-agent powder. The region usually corresponds to a portion other than “layers composed of the reducing-agent powder” described in (1).
A second aspect of the present invention is a method for manufacturing reduced iron powder, the method including the steps of pulverizing sponge iron manufactured by the method according to the first aspect; reducing the resulting pulverized iron; and repulverizing the resulting reduced iron.
Suitable conditions (1) to (8) in the first aspect of the present invention can be applied in any combination.
A third aspect of the present invention is sintered sponge iron having a helical form. The sponge iron preferably has high purity, i.e., has a metallic iron content of at least 97 percent by mass. In the first aspect of the present invention, even a mass of the high-purity sponge iron having a weight of 100 kg or more can be manufactured by, for example, particularly applying suitable condition (7) and subjecting to reduction treatment for sufficient time.
A fourth aspect of the present invention is an apparatus for charging materials used to manufacture sponge iron into a container, the materials being iron oxide powder and reducing-agent powder, the apparatus including a charger capable of rotating and vertically moving in the container when the charger is disposed in the container; an outlet for the iron oxide powder and an outlet for the reducing-agent powder, these outlets being provided at the bottom of the charger and capable of rotating together with the charger.
For the method for manufacturing sponge iron according to the first aspect of the present invention, the fourth aspect of the present invention is preferably employed to charge the iron oxide powder and the reducing-agent powder such that alternating layers of the iron oxide powder and the reducing-agent powder are disposed and such that each of the layers is in the form of a helix.
In the fourth aspect of the present invention, the opening areas of the outlet for the iron oxide powder and the outlet for the reducing-agent powder is preferably variable. Such a structure can be preferably used to particularly satisfy suitable condition (6).
In the fourth aspect of the present invention, the charger preferably includes a cylindrical main body having a diameter of up to 85% of the inside diameter of the container; and a lower end composed of part of a cylinder, the horizontal section of the cylinder being a circle having a diameter of 90% to 95% of the inside diameter of the container, wherein the horizontal section of the lower end has the shape of a sector including the center of the circle and part of the circumference of the circle, or has a shape including the sector. Such a structure can be preferably used to reduce the thickness of the layer composed of the reducing-agent powder disposed at the peripheral portion described in suitable condition (1). Furthermore, even when a projection composed of an adherent is produced in the reaction container, the above-described charger can be disposed without interference.
1, 11 reaction container (sagger)
2, 12 iron oxide powder
3, 13 reducing-agent powder
14 apparatus for charging materials
14
a, 14d partition wall
14
b rotatable charging cylinder
14
c cut-out section
15 outlet for iron oxide powder
16 outlet for reducing-agent powder (used for alternating charging)
16
a outlet for delivering reducing-agent powder to the peripheral portion
16
b outlet for delivering reducing-agent powder to the central axial portion
17 iron-oxide-powder holding section
18 reducing-agent-powder holding section
19
a, 19b presser plate
a opening height
[Method and Apparatus for Charging Materials]
The present invention is characterized by a method for charging materials. The materials are iron oxide powder and reducing-agent powder. Limestone and the like may be added to the reducing agent, if necessary.
As shown in
By employing a method for interwound helical charging, iron oxide powder and reducing-agent powder can be charged simultaneously and continuously. Therefore, a constant thickness of each layer (the amount of charged powder) can be obtained. Consequently, the thickness ratio of a reducing agent layer to an iron oxide layer can also be maintained constant. This thickness ratio can be set to a desired ratio in each reaction container depending on a purpose and circumstances.
In addition, the thickness ratio can also be changed to a desired value at any time.
Consequently, the method for interwound helical charging is useful as a method that is conducive to the improvement of productivity and yield.
The apparatus for charging materials 14 preferably has a structure described below.
The apparatus for charging materials 14 mainly consists of a rotatable charging cylinder 14b (charger) that is inserted into the reaction container 11. The cylindrical main body of the rotatable charging cylinder 14b is separated by a partition wall 14a into two compartments. The iron oxide powder 12 and the reducing-agent powder 13 are charged into the two compartments, i.e., an iron-oxide-powder holding section 17 and a reducing-agent-powder holding section 18, respectively (each of the material powder is not shown in the corresponding holding section). Furthermore, an outlet for iron oxide powder 15 and an outlet for reducing-agent powder 16 are provided as openings of the holding sections 17 and 18, respectively, at the lower end (bottom or the neighborhood of the bottom) of the rotatable charging cylinder 14b. The degree of opening of each outlet (for example, opening height a), that is, the opening area can be preferably adjusted by a gate such as a sliding gate (not shown). The position and the direction of each outlet may be determined according to need. The openings can be provided at any face selected from among the undersurface, the side face, and on a cut-out section provided at the undersurface of the rotatable charging cylinder 14b. Each of the material powder charged into the corresponding holding section is preferably delivered by its own weight in principle.
Modification of this structure includes a structure in which each of the cut-out sections for iron oxide powder and reducing-agent powder has the shape of a sector that is about one quarter of a circle in horizontal section and that is diagonally opposite each other. In this case, at least part of the outlet 15 and at least part of the outlet 16 are preferably provided at side faces, which are corresponding to a straight line of the sector, in the same plane through the axis of the rotatable charging cylinder 14b (a state illustrated in a cross-sectional view of
To surely charge material powder up to the circumferential portion of the reaction container 11 under control, the rotatable charging cylinder 14b preferably has a diameter close to the inner diameter of the reaction container 11. However, the reaction container is repeatedly used, and a plurality of cylinders may be stacked to form a reaction container. Hence, for example, reduced iron and ash in a reducing agent can adhere to inside of the reaction container to form a projection. In addition, the container can slightly incline by strain caused by repeated use. Therefore, the lower end of the rotatable charging cylinder 14b having a diameter very close to the inner diameter of the reaction container 11 can come in contact with the reaction container 11, thus causing damage.
The purpose of bringing the lower end of the rotatable charging cylinder 14b closer to the inner diameter of the reaction container 11 is that openings extending from near the center to near the circumference of the reaction container are used as the outlets. Hence, if the positions of the outlets are modified, the lower end of the rotatable charging cylinder 14b need not have the shape of the perfect circle in horizontal section. A sector that is part of this circle (virtual circle) or a shape including at least the sector is adequate for the lower end.
A desired central angle of a sector may be used. The central angle is preferably about 180° (i.e., semicircle) or less in achieving a satisfactorily compact lower end. More preferably, the maximal diameter of a horizontal section of a cut-out section is smaller than the diameter of the virtual circle.
The virtual circle of the lower end desirably has a diameter closer to the inner diameter of the reaction container in view of productivity, and preferably has a diameter of about 90% or more of the inner diameter of the reaction container. On the other hand, the virtual circle of the lower end desirably has an adequately small diameter in view of operation, and preferably has a diameter of about 95% or less of the inner diameter of the reaction container.
The rotatable charging cylinder 14b preferably has a diameter of about 85% or less of the inner diameter of the reaction container. Leaving a clearance for horizontal displacement in the container is preferable in order to avoid contact. From the viewpoint of ensuring the pathway of material powder charged, the main body of the rotatable charging cylinder has a diameter of about 30% or more of the inner diameter of the reaction container.
Interwound helical charging with such an apparatus for charging materials 14 is performed as follows: Opening areas (the degrees of openings) of the outlets 15 and 16 are adjusted. The rotatable charging cylinder 14b is then inserted into the reaction container 11 from above. By moving upward the rotatable charging cylinder 14b at a constant speed while rotating the rotatable charging cylinder 14b (that is, rotating the outlets 15 and 16), the materials are charged (alternating charging) via the outlets such that the ratio of the thickness of the layer of iron oxide powder and the thickness of the layer of reducing-agent powder is a constant and such that the layers are wound with each other. In this way, alternating layers of the iron oxide powder 12 and the reducing-agent powder 13, which are in the form of helices, is provided in the reaction container 11.
Materials are fed into the holding sections 17 and 18 before charging or during charging into a reaction container, according to need.
As shown in
The reducing-agent layer at the peripheral portion can be necessarily provided from the viewpoint of preventing the interference between the rotatable charging cylinder 14b of the apparatus for charging materials 14 and the reaction container 11 and preventing the seizure at the contact regions between the reaction container and the iron oxide powder. The reducing-agent layer at the central axial portion can be provided for handling reasons when removing sponge iron from the container. In such a case, since a layer composed of a reducing agent alone is provided at the peripheral portion or the central axial portion, paths of the reaction gases are formed; hence, the gases diffuse in the container readily and uniformly. As a result, the effect of improving the reaction rate can be expected. In addition, the reducing-agent layer provided at the peripheral portion can also prevent a product from adhering to the wall of the container. Therefore, these reducing-agent layers are preferably provided while optimizing the radial thickness of the layer in view of the yield of a reducing agent and the molar ratio of (the carbon content)/(the oxygen content) and the like, if necessary.
In a cylindrical container, a layer provided at the peripheral portion preferably has a radial thickness of about 2.5% to about 5% of the inner diameter of the container. The layer provided at the central axial portion preferably has a diameter of about 250 mm or less.
For example, to provide a reducing-agent layer at the peripheral portion, an opening is provided at the side of the rotatable charging cylinder 14a, and then reducing-agent powder may be delivered to form a layer at the peripheral portion. Furthermore, to provide a reducing-agent layer at the central axial portion, a central tube having an opening at its bottom is further provided at a position where the partition wall 14c is provided, and then reducing-agent powder may be delivered from the opening to form a layer at the central axial portion.
These openings may be connected to the outlet 16 for providing a helical layer or may be isolated.
As shown in
In the present invention, when interwound helical charging is performed with the above-described apparatus, it is preferable to variably control the thicknesses of the iron oxide layer and reducing-agent layer. That is, the thickness of each layer is preferably maintained constant in each reaction container. However, it is preferable to be able to adjust the thickness, for example, to optimize the thickness depending on a material.
Such a change in thickness of each layer can be achieved by adjusting at least any two selected from, for example, the rotation speed and the rising speed of the rotatable charging cylinder 14b and the degrees of openings of the outlets 15 and 16. In particular, the adjustment of the degrees of openings of the outlets 15 and 16 by, for example, opening and closing gates is preferable because a stable operation can be achieved without the reductions of diffusibility and the yield and the extension of reduction time.
The thickness of each layer can be varied continuously or discontinuously in theory with the height of the upright reaction container 11, for example, can be varied at the bottom, the middle, and the upper portion of the reaction container 11. The present invention does not exclude such an application. An example of an application includes that the thickness of the iron oxide layer is increased at the upper portion where the reduction tends to readily proceed.
An iron oxide layer and a reducing-agent layer, which are provided in the form of helices, preferably have a thickness of at least about 5 mm. The sum of the thicknesses of the iron oxide layer and the reducing-agent layer is preferably at least about 10 mm and more preferably at least 40 mm. Excessively small thickness readily results in an abnormal layer structure because of the fluctuation of the thickness of each layer. The lower limit of the thickness of each layer is more preferably at least about 10 mm. The lower limit of the sum of the thicknesses of the layers is more preferably at least about 30 mm.
On the other hand, excessively large thickness increases a time required for the reduction treatment and reduces the material-efficiency. Hence, each of the layers preferably has a thickness of about 100 mm or less. The sum of the thicknesses of the layers (one iron oxide layer and one reducing-agent layer) preferably is about 200 mm or less. The upper limit of the thickness of each layer is more preferably about 80 mm. The upper limit of the sum of the thicknesses of the layers is more preferably about 150 mm.
The ratio between an iron oxide layer and a reducing-agent layer is usually expressed not by the thickness but by (the carbon content)/(the oxygen content) (molar ratio). A preferable ratio will be described below.
The above-described apparatus for charging materials is an example. That is to say, in an apparatus for charging iron oxide powder and reducing-agent powder into a reaction container, the apparatus preferably includes a charger capable of rotating and vertically moving; and an outlet for the iron oxide powder and an outlet for the reducing-agent powder, these outlets being provided at the charger and capable of rotating together with the charger. The apparatus can charge the iron oxide powder and the reducing-agent powder from the outlets in the form of a double helix by putting the charger into the reaction container and then moving the charger upward while rotating the charger.
The charger advantageously has, for example, a cylindrical shape, but is not limited to this. The charger may have a tubular shape whose cross-section is in the form of, for example, a sector, a star, or a multilobal according to the shape of a reaction container. The holding sections need not be provided by separating the inside of the charger with a partition wall. Any shape and position of each holding section may be used. The iron-oxide-powder holding section and the reducing-agent-powder holding section need not have the same capacities.
A fixed or movable guide plate and/or a presser plate are preferably provided around the outlets 15 and 16 in order to guide material powder to the direction desired.
[Material Powder]
In a method for manufacturing sponge iron according to the present invention, materials charged into a reaction container include at least iron oxide powder and reducing-agent powder. The iron oxide powder preferably includes a powdered iron ore or powdered mill scale generated in a hot-rolling step of steel. A pickling step of removing, for example, oxides formed on the steel products with an acid such as hydrochloric acid results in a waste acid (pickle liquor). An iron oxide powder obtained by roasting this pickle liquor is also preferable as the material. Such an iron oxide powder preferably has an average particle size of about 0.05 to about 10 mm.
Furthermore, finer iron oxide powder having a particle size smaller than that of the above-described iron oxide powder, for example, hematite powder that is industrially controlled so as to have a specific surface area of at least 2 m2/g and a particle size of at least 0.01 μm is added to the mill scale and/or the iron ore to produce a mixture. The resulting mixture is preferably used for the material because the mixture improves the quality of sponge iron.
Reducing-agent powder includes so-called carbonaceous powder containing carbon. The carbonaceous powder preferably includes, for example, coke powder, char (a kind of high-volatile charcoal), coal powder (noncaking coal is preferable), anthracite powder, and charcoal. From the viewpoint of the efficient reduction, the carbonaceous powder preferably has a carbon content of 60% or more. Reducing-agent powder preferably has an average particle size of about 0.05 to about 10 mm.
There is no problem in that reducing-agent powder containing powder that is a source of a carbon dioxide gas is used as a material for reducing-agent layers, according to need. The source of a carbon dioxide gas preferably includes limestone (including hydrated lime).
[Reducing Step]
The iron oxide powder 12 and the reducing-agent powder 13 (including a source of a carbon dioxide gas added and mixed) are charged into the reaction container 11 with an apparatus for charging materials 14 shown in, for example,
The reaction container 11 into which the iron oxide powder 12, the reducing-agent powder 13, and, if necessary, limestone and the like is charged is placed on, for example, a truck and is disposed at a furnace such as a tunnel furnace. Then, the reduction is performed by heating the materials charged into the container for a predetermined time with the container. This reduction is called a “rough reduction”. The purity target (metallic iron content in sponge iron after the reduction) is determined depending on an application of the reduced iron powder and is at least about 90 percent by mass, and in an application that requires high purity, at least about 97 percent by mass. The purity target has no upper limit. However, the purity achieved within the allowable costs is about 99.5 percent by mass at the maximum under the present conditions.
Unsatisfactory heating temperature for the reduction leads to the insufficient reduction of iron oxide, thus decreasing the purity of the resulting sponge iron. The lower limit of the heating temperature is preferably about 1000° C. On the other hand, excessively high heating temperature excessively sinters sponge iron simultaneously with the reduction to harden. As a result, electric power consumption can be increased when roughly pulverizing or manufacturing costs can be increased due to wear and tear on a pulverizing tool. The upper limit of the heating temperature is preferably 1300° C. Consequently, the heating temperature is preferably in the range of 1000° C. to 1300° C.
When a tunnel furnace is used, the reaction container 11 (and iron oxide in the container) that is placed on a truck and moved in the furnace passes through a preheating zone, where the temperature is gradually increased, over a period of about 24 hours (preferably between 20 and 28 hours) and is retained in a firing zone at about 1000° C. to about 1300° C. for about 60 hours (preferably at least 36 hours and more preferably at least 56 hours; and preferably up to 72 hours and more preferably up to 64 hours). After passing through a cooling zone where the temperature is gradually reduced (preferably over a period of 20 to 28 hours), the reduction treatment is completed. The inlet temperature of the preheating zone and the outlet temperature of the cooling zone are preferably about 200° C. (about 20° C. to about 400° C.), while the outlet temperature of the preheating zone and the inlet temperature of the cooling zone are preferably about 900° C. (about between (the temperature of the firing zone)−450° C. and (the temperature of the firing zone)−50° C.), from the viewpoint of, for example, the protection of the reaction container (refractory).
Iron oxide is reduced with a reducing agent to produce a mass of sponge iron by such a thermal reduction reaction. The resulting sponge iron is necessarily a mass that is in the form of helix.
A larger height (the axial direction) of the resulting mass of sponge iron is preferable. However, in view of the limitation of the size of a reaction container and the reduction of thermal efficiency resulting from the large size of a reaction container when heightening a reaction container, a mass of sponge iron preferably has a height of about 2000 mm or less.
A method of the present invention can provide high-purity sponge iron having a purity of 97 percent by mass or more. When the purity is at least 97 percent by mass, the product characteristics of sintered components such as mechanical components and magnetic materials or of reduction iron powder that is used in the form of powder as-is are advantageously guaranteed. However, a method of the present invention has the advantage other than purity and thus is not limited to a method for manufacturing sponge iron having a purity of at least 97 percent by mass or having high purity. That is, a method of the present invention can be generally applied to a usually rough reduction providing sponge iron having a purity of at least 90 percent by mass. Components other than produced metallic iron generally contains iron oxide and impurities such as silicon (Si), manganese (Mn), phosphorous (P), and sulfur (S), the impurities being in an amount of up to one percent by mass in total.
After heating for the rough reduction, produced sponge iron is separated from a reducing agent and is removed from the reaction container 11. The resulting sponge iron removed from the reaction container 11 is roughly pulverized for a finishing reduction into powder generally having a particle size of about 150 μm or less, thus resulting in roughly reduced particles. Next, the roughly reduced particles are disposed in a finish-reducing furnace with a reducing atmosphere and are subjected to finishing reduction, and are then further pulverized, thus resulting in reduced iron powder.
[Ratio of Iron Oxide to Reducing Agent]
In charging materials into a reaction container, the ratio of the amount of iron oxide to the amount of a reducing agent (solid reducing agent) when the above-described interwound helical charging is performed, in particular, the ratio of carbon content in a reducing agent required for oxygen content in iron oxide has already been described above according to equation (2). That is, the ratio is determined based on the reduction reaction in which one carbon atom in a reducing agent reacts with one oxygen atom in iron oxide ((the carbon content)/(the oxygen content)=1.0 (molar ratio)). However, a reducing agent needs to generally have a carbon content larger than the oxygen content in iron oxide. In a known method, the carbon content in a reducing agent is excessively charged, that is, is 2.0 to 2.5 times the oxygen content in iron oxide ((carbon content)/(oxygen content)=2.0 to 2.5 (molar ratio)) because of the above-described reasons. In this case, a reduction ratio (the purity target of sponge iron) is at least 90 percent by mass and preferably at least 97 percent by mass in metallic iron.
The inventors have investigated the relationship between (the carbon content)/(the oxygen content) (molar ratio) and the time required for the reduction in a method for interwound helical charging by the following experiments.
As shown in
A reaction container used for the experiments has an inner diameter of 370 mm, and materials are charged such that the charged materials have a height of 1400 mm. Iron oxide powder and reducing-agent powder used were the same materials used in Example 1 described below. Reduction treatment is performed at a maximum temperature of 1150 ° C. A reduction time represents a retention time at this maximum temperature.
As shown in
Iron oxide layers having thicknesses of 15 mm (Experimental Example 4: cross (x)), 20 mm (Experimental Example 3: triangle), 30 mm (Experimental Example 2: square (▪)), and 50 mm (Experimental Example 1: rhombus (♦)) provided by horizontally alternating charging (as shown in
When the molar ratio is less than 1.2, it tends to prolong the reduction time. However, alternating from a process for charging in a cylindrical form to a method of alternating charging and the effect resulting from the reduction of the thickness of layers predominantly counteract the tendency of the prolongation of the reduction time. That is, more iron oxide can be charged by a method for helical charging. For example, in this example, a method for charging iron oxide having a thickness of 30 mm in an interwound helical form can charge substantially the same amount of iron oxide charged by a general process for charging in a cylindrical form. Therefore, in the experimental range where the molar ratio is 1.1 or more, the effect of the present invention is sufficiently obtained. In addition, when the molar ratio is 1.15 or more, the effect of the present invention is more sufficiently obtained because of a small degree of prolongation of the reduction time. As a matter of course, when the molar ratio is 1.2 or more, the reduction time is further shortened.
When the thickness of each iron oxide layer was 15 mm, the reduction time was substantially constant at a molar ratio of 1.6 or more. Resulting from repeated experiments under the different conditions, it was also found that, in an oxygen iron layer having a thickness of less than 20 mm, the following relationship holds:
(molar ratio)×(thickness of iron oxide layer (mm))=2.3 to 2.5 equation (3)
When the thickness of each iron oxide layer is less than 20 mm, by charging so as to satisfy equation (3), the determination of the thickness of each iron oxide layer necessarily leads to the reduction time, thus resulting in a stable operation and a stable quality of sponge iron produced. However, this relationship can be due to the difficulty in stably controlling thinner thickness of each reducing-agent layer rather than an essential relationship based on the rate of reaction. Hence, it is expected that the above-described limitation is relaxed as an improvement of a technique in controlling the thickness of layers.
From the viewpoint of the yield of a reducing agent, (the carbon content)/(the oxygen content) (molar ratio) preferably is not increased. When the molar ratio is less than 2.0, a method of the present invention has an advantage compared with a general process for charging in a cylindrical form. The molar ratio is preferably 1.8 or less.
As shown in
To determine the amount required of a reducing agent at the portion of the deposited layers of materials (an intermediated portion in the form of a cylinder) in a reaction container, the inventors conducted experiments whether any tendency was observed in reduction behavior with the ratio of the thicknesses of a reducing-agent layer and an iron oxide layer.
The experiment and the result will be described below.
That is, the molar ratio of the carbon content in a reducing agent to the oxygen content in iron oxide charged in a reaction container was fixed at 1.2. An experiment for changing the carbon content in a reducing agent to the oxygen content in iron oxide in a portion where the iron oxide and the reducing agent were disposed in the form of alternating layers excluding the reducing agent provided at a portion near the wall (peripheral portion) of the reaction container and at a central portion along the axial direction was performed.
This experiment was performed with a method for horizontal charging as in the above-described experiment.
As shown in
Consequently, to maximally take advantage of the effect obtained when (the carbon content)/(the oxygen content) (molar ratio) is 1.2 or more in the entire container, it was found that (the carbon content)/(the oxygen content) (molar ratio) was preferably at least 0.5 at the cylindrical intermediate portion, and the cylindrical intermediate portion being the charged portion being in the form of helices (interwound helices).
To verify these results, another experiment was performed as follows: The molar ratio of the carbon content in a reducing agent to the oxygen content in iron oxide at the cylindrical intermediate portion was fixed at 0.8. The amounts of a reducing agent charged into the peripheral portion and the central axial portion of the reaction container were varied.
As shown in
However, as described above, even if the molar ratio is less than 1.2, the effect of the present invention can be obtained if the molar ratio is 1.1 or more and preferably 1.15 or more.
In summary, in charging iron oxide and a reducing agent into the reaction container 11 in the form of alternating layers (such as interwound helical charging) according to the present invention, the ratio of the reducing agent to the iron oxide charged in the entire reaction container 11 that includes the peripheral portion, the cylindrical intermediate portion, and the central axial portion of the reaction container 11 is determined such that the molar ratio of the carbon content in the reducing agent to the oxygen content in the iron oxide is preferably at least 1.1, more preferably at least 1.15, and most preferably at least 1.2.
The thickness ratio of a reducing agent layer to an iron oxide layer at the cylindrical intermediate portion that is charged in the form of (interwound) helices is preferably determined such that the molar ratio of the carbon content in the reducing agent to the oxygen content in the iron oxide is at least 0.5.
In this example, experimental levels as shown in Table 1 were defined. Iron oxide and a reducing agent were charged into the reaction container 11 composed of silicon carbide (SiC) according to the experimental levels and then roughly reducing treatment was performed to produce sponge iron. Each of levels A to C and H is an example of a process for charging in a cylindrical form as shown in
In Table 1, 20% of the increment of the charge of Levels A and D represents that the sum of the thicknesses of layers composed of mill scale in the reaction container 11 was increased by 20%; 40% of the increment of the charge of Levels B and E represents that the sum of the thicknesses of layers composed of mill scale in the reaction container 11 was increased by 40%; and 60% of the increment of the charge of Levels C and F represents that the sum of the thicknesses of layers composed of mill scale in the reaction container 11 was increased by 60%. The conditions are described in detail in Table 2. Under these conditions, each Level was studied to determine a method for charging, a suitable thickness of a layer, and purity.
In this experiment, mill scale generated in a hot rolling step was dried, pulverized, and screened. The mill scale powder used included 40 percent by mass of particles that can pass through 60 μm mesh (it was analyzed that the mill scale powder had an average particle size within a range of 0.05 to 10 mm). A mixture of limestone powder and carbonaceous powder was used as a reducing agent that was an auxiliary material. The carbonaceous powder was produced by mixing coke and anthracite at the coke to the anthracite ratio of about 7:3. The coke used had an average particle size of 85 μm and the anthracite used had an average size of 2.4 mm. The content of the limestone powder having an average particle size of 80 μm in the entire reducing agent powder was about 14 percent by mass.
A reaction container was a cylindrical container having an inner diameter of 400 mm. For charging in a cylindrical form, iron oxide was charged so as to form a cylindrical shape having an outer diameter of 320 mm, having a thickness of each value represented in Table 2, and having a height of about 1500 mm (the axial direction). For helical charging, a reducing-agent layer was provided with a diameter of about 80 mm at the central axial portion and with a thickness of about 15 mm at the peripheral portion. Interwound charging was performed at the remaining cylindrical intermediate portion according to Table 2. The resulting charged cylindrical intermediate portion had a height of about 1500 mm. The molar ratio of the carbon content to the oxygen content in the entire container and at the cylindrical intermediate portion that is in the form of a cylinder were at least 1.2 and at least 0.5, respectively.
Horizontally alternating charging was performed in order to verify the efficiency of charging. That is, the charging was performed by the following procedure: An apparatus for charging materials used was the same as for interwound helical charging. The rotatable charging cylinder was rotated while charging any one of iron oxide powder or reducing-agent powder and was moved upward. Next, another powder was charged by the same way. This procedure was repeated. As shown in Table 1, the horizontally alternating charging cannot continuously charge and required a longer charging time than those of the charging in a cylindrical form and the interwound helical charging. The interwound helical charging had the shortest charging time.
Reaction containers 11 each being charged with materials according to the corresponding Level were placed on one truck and disposed in a tunnel furnace. The truck passed through a preheating zone over a period of about one day 200° C. to 900° C.) and a firing zone 1150° C.) over a period of about three days and then a cooling zone over 200° C. to 900° C.) a period of about one day. The truck was removed from the tunnel furnace, and sponge iron was removed from the container. The purity of the resulting sponge iron was measured. All resulting sponge iron weighed 200 kg or more.
The purity of sponge iron was given by converting the metallic iron content in a chemical composition determined by a method for analyzing oxygen.
As shown in
Sponge iron was manufactured according to Inventive Examples 1 to 5 and Conventional Example 1. A method for charging as shown in
In this Inventive Example, an iron oxide layer having a thickness of 50 mm and a reducing-agent layer having a thickness of 50 mm were charged in the form of interwound helices. A cylindrical reaction container was used with a height of 1.8 m and with an inner diameter of 40 cm. A mixture of coke powder having a particle size of up to 1 mm and 16 percent by mass of limestone having an average particle size of about 95 μm was used as the reducing-agent powder. Pulverized mill scale having a particle size of up to 0.1 mm (after pulverizing, the mill scale was screened. The resulting mill scale included 40 percent by mass of particles that can pass through 60 μm mesh) was used as the iron oxide powder. Both mill scale powder and coke powder had an average particle size within a range of 0.05 to 10 mm.
An apparatus for charging materials as shown in
As a result of the charging, charged interwound helices each having 17 turns were obtained, wherein the iron oxide layer had a thickness of 50 mm and the layer of solid (powder) reducing agent had a thickness of 50 mm. The charged iron oxide weighed 339 kg.
In this Inventive Example, an iron oxide layer having a thickness of 35 mm and a reducing-agent layer having a thickness of 65 mm were charged in the form of interwound helices. Iron oxide and a solid reducing agent were charged with the same reaction container, material powder, and apparatus for charging materials as Inventive Example 1. The charging was performed as follows: The height of the opening of the outlet for iron oxide powder 15 was adjusted to 35 mm. The height of the opening of the outlet for reducing-agent powder 16 was also adjusted to 65 mm. The rotatable charging cylinder 14b was operated at a rotating speed of 4 rpm and at a rising speed of 400 mm/min.
As a result of the charging, charged interwound helices each having 17 turns were obtained, wherein the iron oxide layer had a thickness of 35 mm and the layer of solid reducing agent had a thickness of 65 mm. The charged iron oxide weighed 237 kg.
In this Inventive Example, an iron oxide layer having a thickness of 60 mm and a reducing-agent layer having a thickness of 40 mm were charged in the form of interwound helices. Iron oxide and a reducing agent were charged with the same reaction container, material powder, and apparatus for charging materials as Inventive Example 1. The charging was performed as follows: The height of the opening of the outlet for iron oxide powder 15 was adjusted to 60 mm. The height of the opening of the outlet for reducing-agent powder 16 was also adjusted to 40 mm. The rotatable charging cylinder 14b was operated at a rotating speed of 4 rpm and at a rising speed of 400 mm/min.
As a result of the charging, charged interwound helices each having 17 turns were obtained, wherein the iron oxide layer had a thickness of 60 mm and the layer of solid reducing agent had a thickness of 50 mm. The charged iron oxide weighed 406 kg.
In this Inventive Example, an iron oxide layer having a thickness of 25 mm and a reducing-agent layer having a thickness of 25 mm were charged in the form of interwound helices. Iron oxide and a reducing agent were charged with the same reaction container, material powder, and apparatus for charging materials as Inventive Example 1. The charging was performed as follows: The height of the opening of the outlet for iron oxide powder 15 was adjusted to 25 mm. The height of the opening of the outlet for reducing-agent powder 16 was also adjusted to 25 mm. The rotatable charging cylinder 14b was operated at a rotating speed of 4 rpm and at a rising speed of 200 mm/min.
As a result of the charging, charged interwound helices each having 34 turns were obtained, wherein the iron oxide layer had a thickness of 25 mm and the layer of solid reducing agent had a thickness of 25 mm. The charged iron oxide weighed 339 kg.
In this Inventive Example, an iron oxide layer having a thickness of 57.5 mm and a reducing-agent layer having a thickness of 50 mm were charged. Iron oxide and a reducing agent were charged with the same reaction container, material powder, and apparatus for charging materials as Inventive Example 1. The charging was performed as follows: The height of the opening of the outlet for iron oxide powder 15 was adjusted to 57.5 mm. The height of the opening of the outlet for reducing-agent powder 16 was also adjusted to 50 mm. The rotatable charging cylinder 14b was operated at a rotating speed of 4 rpm and at a rising speed of 430 mm/min.
As a result of the charging, charged interwound helices each having 16 turns were obtained, wherein the iron oxide layer had a thickness of 57.5 mm and the layer of solid reducing agent had a thickness of 50 mm. The charged iron oxide weighed 366 kg.
In this example, charging in a cylindrical form was performed according to a known process as shown in
Reduction treatment was performed with a tunnel furnace. A time required for the reduction was investigated.
Table 3 summarizes the results.
The time required for the reduction represents a retention time at a firing zone (1150° C.) in order to produce sponge iron having a purity of 95% or more. Production per hour represents a value obtained by dividing the weight of charged iron oxide by the time required for the reduction.
As shown in Table 3, the method of the present invention significantly improves productivity compared with the conventional process.
A layer composed of the reducing-agent powder 13 (coke powder) was deposited with a thickness of 30 mm at the bottom of the reaction container 11 with the apparatus for charging materials as shown in
Charging in the form of horizontal layers as shown in
charging in a cylindrical form as shown in
Next, the heat-resistant reaction container 11 containing materials was placed on a truck and passed through a tunnel furnace to heat and reduce iron oxide. The tunnel furnace having an entire length of 100 m was used, and the atmospheric temperature was adjusted to 1150° C. at the center zone having a length of 40 m. Table 4 summarizes the results of the operations for manufacturing sponge iron having a purity of 97 percent by mass under those conditions.
As is clear from Table 4, in this example of the present invention, the truck speed was 1.3 m/h compared with 1.1 m/h of the Conventional Example and was thus 18% faster than the Conventional Example. The amount of mill scale charged was 256 kg per container compared with 220 kg per container of the Conventional Example and was thus 16% greater than the Conventional Example. As a result, productivity was improved by as much as 38%. A quantity of heat per unit mass of iron oxide required for heating can be reduced from 11,470 MJ/ton to 8,820 MJ/ton by as much as about 30%.
Sponge iron was manufactured with an apparatus for charging materials as shown in
The rotatable charging cylinder can move to an opposite side when the front end lightly came into contact with the projection or the reaction container; hence, the rotatable charging cylinder was able to be inserted to the bottom of the reaction container with no problems, and there is no problem when charging materials, that is, 260 kg of iron oxide powder was charged with no problems (a layer composed of iron oxide had a thickness of 50 mm, and a layer composed of a reducing agent had a thickness of 30 mm).
After charging, the reduction was performed with no problems using a tunnel furnace in the same way as EXAMPLE 2. As a result, a mass of sponge iron having a helical shape was produced with a purity of 95 percent by mass.
Sponge iron was manufactured according to Inventive Examples 7 to 11, Comparative Example 2, and Conventional Example 3. A method for charging as shown in
In this EXAMPLE, iron oxide powder composed of mill scale and/or iron ore was pulverized and screened in order to adjust the particle size, and was then used as the main material. Reducing-agent powder composed of at least any one of a simple substance or a mixture of coke powder, char, coal powder, charcoal powder, and the like was pulverized and screened in order to adjust the particle size and was then used as a material. All materials had an average particle size of about 70 to 90 μm.
An apparatus was used with a rotatable charging cylinder as shown in
In this example, a general process for charging was employed as shown in
In this example, interwound helical charging was performed. An iron oxide layer had an outer diameter of 390 mm, an inner diameter of 60 mm, a thickness of 60 mm, and a helical shape. A reducing-agent layer had a thickness of 45 mm and a helical shape. The outer diameter and the inner diameter of the reducing-agent layer was the same as the iron oxide layer. The iron oxide layer and the reducing-agent layer were simultaneously formed. The molar ratio of (the carbon content in the reducing agent)/(the oxygen content in the iron oxide) was 0.8 in the cylindrical intermediate portion. (The carbon content)/(the oxygen content) (molar ratio) was 1.2 in the all charged materials. As a result, the amount of materials charged was increased by 35% compared with Conventional Example 3. However, the reduction time was as short as 60 hours. The resulting sponge iron did not adhere to the inner face of the container and was readily removed from the container.
In this example, interwound helical charging was performed. An iron oxide layer had an outer diameter of 365 mm, an inner diameter of 100 mm, a thickness of 60 mm, and a helical shape. A reducing-agent layer had a thickness of 28 mm and a helical shape. The outer diameter and the inner diameter of the reducing-agent layer was the same as the iron oxide layer. The iron oxide layer and the reducing-agent layer were simultaneously formed. The molar ratio of (the carbon content in the reducing agent)/(the oxygen content in the iron oxide) was 0.5 in the cylindrical intermediate portion. The molar ratio of the carbon content to the oxygen content was 1.2 in the all charged materials. As a result, the amount of materials charged was increased by 35% compared with Conventional Example 3. However, the reduction time was 59 hours. The resulting sponge iron did not adhere to the inner face of the container and was readily removed from the container.
In this example, interwound helical charging was performed. An iron oxide layer had an outer diameter of 350 mm, an inner diameter of 100 mm, a thickness of 60 mm, and a helical shape. A reduced iron layer had a thickness of 17 mm and a helical shape. The outer diameter and the inner diameter of the reducing-agent layer was the same as the iron oxide layer. The iron oxide layer and the reducing-agent layer were simultaneously formed. The molar ratio of (the carbon content in the reducing agent)/(the oxygen content in the iron oxide) was 0.3 in the cylindrical intermediate portion. The molar ratio of the carbon content to the oxygen content was 1.2 in the all charged materials. As a result, the amount of materials charged was increased by 35% compared with Conventional Example 1. However, the reduction time was 70 hours. The resulting sponge iron did not adhere to the inner face of the container and was readily removed from the container. However, the reduction time was comparable with that in Conventional Example 3 even in view of the increment.
In this example, interwound helical charging was performed. An iron oxide layer had an outer diameter of 375 mm, an inner diameter of 100 mm, a thickness of 60 mm, and a helical shape. A reducing-agent layer had a thickness of 45 mm and a helical shape. The outer diameter and the inner diameter of the reducing-agent layer was the same as the iron oxide layer. The iron oxide layer and the reducing-agent layer were simultaneously formed. The molar ratio of (the carbon content in the reducing agent)/(the oxygen content in the iron oxide) was 0.8 in the cylindrical intermediate portion. The molar ratio of the carbon content to the oxygen content was 1.5 in the all charged materials. As a result, the amount of materials charged was increased by 20% compared with Conventional Example 3. However, the reduction time was 59 hours. The resulting sponge iron did not adhere to the inner face of the container and was readily removed from the container. Inventive Example 7 with a low molar ratio of (the carbon content)/(the oxygen content) in the container represented higher production efficiency per reduction time compared with this example. However, this example represented excellent results compared with the Conventional Example.
In this example, interwound helical charging was performed. An iron oxide layer had an outer diameter of 395 mm, an inner diameter of 40 mm, a thickness of 60 mm, and a helical shape. A reducing-agent layer had a thickness of 45 mm and a helical shape. The outer diameter and the inner diameter of the reducing-agent layer was the same as the iron oxide layer. The iron oxide layer and the reducing-agent layer were simultaneously formed. The molar ratio of (the carbon content in the reducing agent)/(the oxygen content in the iron oxide) was 0.8 in the cylindrical intermediate portion. The molar ratio of the carbon content to the oxygen content was 1.1 in the all charged materials. As a result, the amount of materials charged was increased by 40% compared with Conventional Example 3. However, the reduction time was 78 hours. The resulting sponge iron did not adhere to the inner face of the container and was readily removed from the container. In this example, the reduction time was prolonged. The reduction time was comparable with that in Conventional Example 3 even in view of the increment.
Table 5 summarizes the results.
*(Weight of iron oxide (relative ratio))/(Reduction time (h))
As described above, according to the present invention, sponge iron can be manufactured with high productivity and high quality (for example, at a purity of 97% or more) by employing the technique of interwound helical charging. Furthermore, since a structure formed by charging materials into a reaction container can be changed to a desired structure easily and readily, quality, quantity, and a reduction time can be easily adjusted; hence, production efficiency can be significantly improved. As a result, high-purity sponge iron can be manufactured at low cost.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-024638 | Jan 2003 | JP | national |
| 2003-182533 | Jun 2003 | JP | national |
| 2003-286047 | Aug 2003 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP04/00866 | 1/29/2004 | WO | 11/10/2004 |
