METHOD FOR PRODUCING REDUCED IRON

TECHNICAL FIELD

This disclosure relates to a method for reducing an iron containing material using a movable hearth furnace, and particularly to a method which produces reduced iron from iron ore with a high content of zinc.

BACKGROUND

Crude steel producing methods are roughly classified into a blast-furnace-converter method which produces pig iron from iron ore and refines pig iron into steel, and an electric furnace method which melts scrap and refines the molten scrap into steel. With the advent of newly industrialized countries including China, the crude steel production quantity has been rapidly increasing on a worldwide basis. Particularly, the supply and demand of iron ore used in the blast-furnace-converter method is tight. Hence, the cost of iron ore is increasing and, at the same time, it is becoming more and more difficult to obtain high-quality iron ore.

Further, besides the above-mentioned methods, there has been also known a method for producing reduced iron using a movable hearth furnace. A movable hearth furnace method is one of methods for producing reduced metal represented by reduced iron. In the movable hearth furnace method, iron ore, a solid reducing material and the like are charged on a hearth which moves in the horizontal direction, and the iron ore is reduced by a solid reducing material with radiation heat from above, a reduced material is melted and separated to slag and metal as reduced iron product. (see JP 11-335712 A, JP 11-172312 A, for example).

On the other hand, demand for zinc is rapidly increasing on a worldwide basis in the same manner as iron ore thus giving rise to a drawback that the cost of zinc is increasing. Although various methods have been used to refine zinc, in general, zinc oxide is formed by calcination of sulfide ore, the formed zinc oxide is refined by a wet or a dry refining method thus producing zinc metal. Zinc also has a drawback that a zinc raw material such as sulfide ore and zinc oxide is in short supply.

Under such circumstances where there exists the drawback that resources such as an iron raw material and a zinc raw material are in short supply, we focused on iron ore which contains a larger quantity of zinc compared to ordinary iron ore. It is desirable that such iron ore with a high content of zinc is also used as a raw material in a blast-furnace-converter method. However, a raw material with a high content of zinc is scarcely used. The main reason is that zinc vaporized from the ore remains in the inside of a blast furnace as a material stuck to a furnace wall. A zinc component contained in sintered ore as zinc oxide or zinc sulfide is charged into the blast furnace. Although zinc compounds charged into the blast furnace are reduced to zinc and evaporated in the furnace, zinc is oxidized again and deposited in a part of the furnace where the temperature is low and the oxygen potential is high. Zinc oxide is deposited easily on an inner wall of a shaft of the blast furnace so that coke or ore fine coagulated with zinc oxide adhere to the inner wall thus immobilizing a charged material. Such an immobilized portion makes the unstable descending of the burden in blast furnace and inducing troubles such as “bridging” or “slip”.

Although zinc is a component which becomes a factor in causing trouble in a blast furnace operation in this manner, zinc is also a valuable metal. Zinc is an indispensable metal as a battery raw material, or as a plating material which enhances corrosion resistance of a surface of a steel sheet or the like, for example. As described previously, zinc metal is generally produced such that zinc oxide is prepared by calcination of sulfide ore, and zinc oxide is refined by a wet or a dry refining method. Recently, there has been also proposed a method in which crude zinc oxide is prepared by refining iron-making process dust, and crude zinc oxide is used as a zinc refining raw material.

For example, in the case of crude zinc oxide with a zinc concentration exceeding 10 mass %, crude zinc oxide of high concentration can be prepared by applying a treatment such as a Waelz method or the like, and such crude zinc oxide can be used as a zinc refining raw material. Further, in the case of crude zinc oxide with a zinc concentration exceeding 50 mass %, such crude zinc oxide can be directly used in zinc refining such as an ISP method or the like.

The application field where crude zinc oxide is used differs largely depending on zinc concentration in crude zinc oxide recovered in this manner, wherein, as a matter of course, the larger the zinc concentration in crude zinc oxide, the higher an economical value of the crude zinc oxide becomes. However, there has been no proposal with respect to a method for producing reduced iron which is compatible with both the production of reduced iron and the production of crude zinc oxide at a high concentration.

It could therefore be helpful to provide a method for producing reduced iron which can effectively make use of iron ore with a high content of zinc.

SUMMARY

We thus provide a method for producing reduced iron having the following aspects.

[1]. A method for producing reduced iron comprising:

preparing a mixed raw material which includes iron ore (X) containing high zinc iron ore (A) which contains 0.01 mass % or more of zinc and 50 mass % or more of iron, and a carbonaceous solid reducing material;

a mixed raw material charging step of charging the mixed raw material on a hearth of a movable hearth furnace;

and

a reducing step of reducing the mixed raw material charged on the movable hearth by supplying heat to the mixed raw material from above the hearth to obtain a reduced product.

[2]. The method according to [1], wherein the high zinc iron ore (A) contains 0.01 to 0.5 mass % of zinc and 50 to 70 mass % of iron.

[3]. The method according to [1], wherein the high zinc iron ore (A) has a mixing ratio of 10 to 100 mass % with respect to the iron ore (X).

[4]. The method according to [1], wherein the mixed raw material charging step comprises charging an agglomerated mixed raw material on the movable hearth.

[5]. The method according to [1], wherein the reducing step comprises reducing the mixed raw material at a heating temperature of 1200° C. or more.

[6]. The method according to [5], wherein the heating temperature is 1250° C. or more and less than 1400° C.

[7]. The method according to [1], wherein the reducing step comprises reducing the mixed raw material charged on the movable hearth by supplying heat to the mixed raw material from above the hearth, to produce the reduced iron in a state where the mixed raw material is not melted or the mixed raw material is melted only partially.

[8]. The method according to [1],

further comprising a recovering step of recovering crude zinc oxide from dust generated in the movable hearth furnace,

and

wherein the step of preparing the raw material comprises preparing a mixed raw material which contains iron ore (X) containing high zinc iron ore (A) which contains 0.01 mass % or more of zinc and 50 mass % or more of iron, zinc-containing dust, and a carbonaceous solid reducing material.

[9]. The method according to [8], wherein the mixed raw material has an average zinc concentration of 0.45 mass % or more.

[10]. The method according to [9], wherein the average zinc concentration is 0.45 to 0.60 mass %.

[11]. The method according to [8], wherein the zinc-containing dust is at least one dust selected from a group consisting of dust generated in a blast furnace, dust generated in a converter and dust generated in an electric furnace.

[12]. The method according to [1],

further comprising a recovering step of obtaining the recovered dust by recovering dust generated in the movable hearth furnace, and

wherein the step of preparing the raw material comprises preparing a mixed raw material which contains iron ore (X) containing high zinc iron ore (A) which contains 0.01 mass % or more of zinc and 50 mass % or more of iron, the recovered dust, and a carbonaceous solid reducing material.

[13]. The method according to [1], further comprising:

recovering dust generated in the movable hearth furnace;

charging the recovered dust on the movable hearth; and

supplying heat from above the hearth to obtain crude zinc oxide generated in the movable hearth furnace.

[14]. The method according to [1], further comprising a melting step of melting the reduced product.

[15]. The method according to [1],

further comprising a melting step of melting the reduced product, and

the mixed raw material contains iron ore (X) containing high zinc iron ore (A) which contains 0.01 mass % or more of zinc and 50 mass % or more of iron, a carbonaceous solid reducing material, and a slag-making material.

[16]. The method according to [14], wherein the melting step comprises melting the reduced product at a heating temperature of 1400° C. or more.

[17]. The method according to [16], wherein the heating temperature is 1450° C. or more and 1500° C. or less.

[18]. The method according to [1],

further comprising a melting step of melting the reduced product; and a recovering step of recovering crude zinc oxide from dust generated in the movable hearth furnace; and

[19]. The method according to [18], wherein the mixed raw material has an average zinc concentration of 0.45 mass % or more.

[20]. The method according to [19], wherein the average zinc concentration is 0.45 to 0.60 mass %.

[21]. The method according to [18], wherein the zinc-containing dust is at least one dust selected from a group consisting of dust generated in a blast furnace, dust generated in a converter and dust generated in an electric furnace.

[22]. The method according to [1],

further comprising a melting step of melting the reduced product; and a recovering step of obtaining the recovered dust by recovering dust generated in the movable hearth furnace, and

wherein the step of preparing the raw material comprises preparing a mixed raw material which contains iron ore (X) containing high zinc iron ore (A) which contains 0.01 mass % or more of zinc and 50 mass % or more of iron, the recovered dust, a carbonaceous solid reducing material, and a slag-making material.

[23]. The method according to [1], further comprising:

melting the reduced product;

recovering dust generated in the movable hearth furnace;

charging the recovered dust on the movable hearth; and

obtaining crude zinc oxide from dust generated in the movable hearth furnace by supplying heat from above the hearth.

[24]. The method according to [1], further comprising, prior to the mixed raw material charging step, a carbon material charging step of charging a carbon material on the movable hearth for stacking the mixed raw material on the carbon material charged on the movable hearth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a rotary hearth furnace used in example 1.

FIG. 2 is a schematic view showing a facility flow used in example 1.

FIG. 3 is a schematic view showing the facility flow used in example 1 (using recovered dust).

FIG. 4 is a schematic view showing a facility flow used in example 1 (using recovered dust).

FIG. 5 is a graph showing a change of zinc concentration with respect to a mixing ratio of iron ore with a high content of zinc in a mixed raw material in example 1.

FIG. 6 is a schematic view showing a rotary hearth furnace used in example 2.

FIG. 7 is a schematic view showing a facility flow used in example 2.

FIG. 8 is a schematic view showing the facility flow used in example 2 (using recovered dust).

FIG. 9 is a schematic view showing the facility flow used in example 2 (using recovered dust).

FIG. 10 is a graph showing a change of zinc concentration with respect to a mixing ratio of iron ore with a high content of zinc in a mixed raw material in example 2.

EXPLANATION OF SYMBOLS

1: rotary hearth furnace, 2: furnace body, 2a: preheating zone, 2b: reducing zone, 2c: melting zone, 2d: cooling zone, 3: rotary hearth, 4: mixed raw material, 5: burner, 6: charging device, 7: discharge device, 8: cooling device, 11: ore hopper, 12: coal hopper, 13: slag-making material hopper, 14: mixer, 15: rotary hearth furnace, 16: reduced iron discharge opening, 17: bag filter for collecting discharge duct, 18: powder transport lorry, 19: suction fan, 20: stack, 21: recovered dust transport conveyor, 21a: first recovered dust transport conveyer, 21b: second recovered dust transport conveyer, 22: first recovered dust storage hopper, 23: dust yard

DETAILED DESCRIPTION
Example 1

A method for producing reduced iron includes a step of preparing a mixture of raw material, a mixed raw material charging step, a reducing step, and a melting step.

In the step of preparing the mixed raw material, the mixed raw material is prepared by mixing iron ore which contains iron ore with a high content of zinc which contains 0.01 mass % or more of zinc and 50 mass % or more of iron, a carbonaceous solid reducing material, and a slag-making material. In the mixed raw material charging step, the mixed raw material is charged on a movable hearth. In the reducing step, the mixed raw material charged on the movable hearth is reduced by supplying heat from above the hearth. In the melting step, a reduced product is melted.

We discovered the idea of using iron ore with a high content of zinc and using a movable hearth furnace to effectively make use of an iron component and a zinc component contained in iron ore with a high content of zinc. A method for producing small masses of molten iron using the movable hearth furnace is one of methods for producing reduced iron. Such a small mass of molten iron is referred to as “iron pebble”. In this method, iron ore, a solid reducing material and the like are charged on a hearth which moves in the horizontal direction, iron ore is reduced by heating iron ore with radiation heat transfer from above, a reduced product is melted on the hearth, and iron pebble which is reduced iron is produced by separating slag and metal from each other.

The movable hearth furnace is a furnace which applies heat to the hearth moving horizontally. The horizontal movement of hearth typically has the form of the rotational transfer as shown in FIG. 1. The movable hearth furnace having the form of the rotational transfer is particularly referred to as a rotary hearth furnace. In example 1, iron pebble as reduced iron is produced by applying reduction and melting treatment to iron ore with a high content of zinc using such a movable hearth furnace, particularly using the rotary hearth furnace. Hereinafter, example 1 is explained in conjunction with a case where the rotary hearth furnace is used as the movable hearth furnace.

Iron ore with a high content of zinc used in example 1 is iron ore which exhibits high zinc content compared to iron ore used as a normal blast furnace raw material, and contains 0.01 mass % or more of zinc and 50 mass % or more of iron in general. Although there is no upper limit with respect to zinc content and iron content in iron ore with a high content of zinc used in example 1, the upper limit of zinc content and iron content is naturally determined because iron ore with a high content of zinc is an iron ore. The upper limit of zinc content is approximately 0.5 mass % or less, for example, while the upper limit of iron content is approximately 70 mass % or less, for example. Further, content of alkali component in iron ore with a high content of zinc is usually 0.08 mass % or more in terms of oxide such as Na₂O or K₂O. It is preferable to be the content of alkali component in iron ore to 1 mass % or less. With such content of alkali component, clogging of gas discharge system can be effectively prevented.

Although example 1 is directed to a technique which produces iron pebble using such iron ore with a high content of zinc, iron ore with a high content of zinc may be also used in mixture with usual iron ore at the time of reducing iron ore with a high content of zinc by the rotary hearth furnace. Even when iron ore with a high content of zinc is used in mixture with usual iron ore, it is possible to preferably acquire the advantageous effects of example 1 by mixing approximately 10 mass % or more of iron ore with a high content of zinc with respect to the whole ore.

One example of a rotary hearth furnace used in example 1 is explained in conjunction with FIG. 1. The rotary hearth furnace 1 is, as shown in FIG. 1, constituted such that a rotatably movable hearth 3 is covered with a furnace body 2 which is partitioned into a preheating zone 2a, a reducing zone 2b, a melting zone 2c and a cooling zone 2d. A raw material 4 which is made of iron ore with a high content of zinc and a solid reducing material, for example, is charged on the rotary hearth 3. As the raw material 4, a mixed raw material produced by mixing iron ore with a high content of zinc, a carbonaceous solid reducing material and a slag-making material is used. The mixed raw material may be agglomerated as explained hereinafter. The furnace body 2 which covers the rotary hearth 3 is composed with refractories. Further, to protect hearth refractories, there may be a case where a carbon material is charged on the hearth 3, and the raw material 4 is stacked on the carbon material. Further, burners 5 are mounted on an upper portion of the furnace body 2, and iron ore in the mixed raw material 4 on the rotary hearth 3 is reduced using fuel combustion heat generated by the burners 5 as a heat source. In FIG. 1, a charging device 6 charges the raw material onto the rotary hearth 3, a discharge device 7 discharges a reduced product, and numeral 8 indicates a cooling device. Further, although an ambient temperature in the furnace body 2 is set to a temperature around 1300° C., the temperature in the melting zone is usually controlled to a high temperature around 1450° C.

Iron ore with a high content of zinc contains a gangue component although an amount of gangue component differs depending on the source. Further, coal, coal char or coke which is a typical example of a carbonaceous solid reducing material contains an ash component. Accordingly, in a movable hearth furnace method which performs only reducing operation, the inclusion of gangue and ash component into reduced iron as a product is unavoidable with different from a blast-furnace-converter method. By reducing and melting the raw material on the hearth of the rotary hearth furnace, it is possible to readily separate metallic product as high density iron pebble and slag which is a residue from each other.

In the iron pebble obtained by example 1, slag component is separated due to reduction and melting as described above. Hence, in a state after iron pebble is discharged from the rotary hearth furnace, it is possible to achieve an apparent density of iron pebble to 5000 kg/m³or more without compression or the like. Usually, a particle size of iron pebble as a product is set to 3 mm or more and 100 mm or less after sieving step.

In applying reduction treatment to iron ore with a high content of zinc using the rotary hearth furnace, it is assumed that iron ore with a high content of zinc is mixed with a carbonaceous solid reducing material and a slag-making material, and the mixture is charged on the rotatably movable hearth. Coal, coke, graphite or the like can be used as the carbonaceous solid reducing material, and lime powder, dolomite, serpentine or the like can be used as the slag-making material.

When iron ore with a high content of zinc is lumpy ore, iron ore with a high content of zinc is crushed into fine ore having a particle size of 10 mm or less, and thereafter, the fine ore is mixed with the carbonaceous solid reducing material, and the mixture is charged on the rotary hearth and is reduced.

When iron ore with a high content of zinc is ore powder (particle size: 3 mm or less), iron ore with a high content of zinc is agglomerated together with a carbonaceous solid reducing material and a slag-making material thus allowing the use of iron ore with a high content of zinc as pellets incorporating a carbon material therein. Dust scattering from the agglomerated raw material is little at the time of heating and hence, zinc concentration in dust can be enhanced. In the same manner, iron ore with a high content of zinc may be molded by compression thus enabling the use of iron ore with a high content of zinc as briquettes. Further, at the time of granulating iron ore with a high content of zinc, an inorganic binder such as bentonite and an organic binder such as molasses or cornstarch are mixed into iron ore with a high content of zinc so as to increase strength of the iron ore agglomerate. These pellets or briquettes can be also used after vaporizing moisture. On the other hand, it is also effective to use iron ore with a high content of zinc in an original fine form. By using iron ore with a high content of zinc as an original fine raw material, costs for facilities, electric power, binders and the like necessary for producing agglomerates become unnecessary thus contributing to the economical improvements.

A heating temperature at the time of reducing and melting iron ore with a high content of zinc using the rotary hearth furnace is preferably set to 1400° C. or more. It is more preferable to set the heating temperature to 1450° C. or more. By setting a maximum temperature in the rotary hearth furnace to 1450° C. or more, the inside of the furnace and the raw material which is reduced and melted in the furnace arrive at a high temperature. Particularly, by setting the temperature of the raw material in a molten state to 1450° C. or more, the melts can ensure sufficient fluidity so that a gangue component in metal iron can be easily removed thus realizing the production of iron pebble having favorable properties.

By charging a carbon material on the hearth and by stacking the mixed raw material containing iron ore with a high content of zinc on the carbon material, it is possible to prevent the metal or slag in a molten state from eroding the refractories of the hearth. Since the iron component is taken in the refractories when the refractories are eroded, it is possible to reduce loss of the iron component by preventing the erosion of the refractories of the hearth thus contributing to the enhancement of productivity of iron pebble.

Dust contained in an exhaust gas generated in the rotary hearth furnace is recovered. Zinc in the dust is concentrated compared to iron ore with a high content of zinc and hence, the dust can be used as a raw material of crude zinc oxide. FIG. 2 is a schematic view of a general facility flow of a rotary hearth furnace which performs such dust recovery.

In FIG. 2, iron ore, coal and a slag-making material which are discharged from an ore hopper 11, a coal hopper 12, and a slag-making material hopper 13 are mixed together by a mixer 14 (also using a pelletizer or the like when necessary) thus preparing a mixed raw material. The mixed raw material is reduced and melted by heating in the rotary hearth furnace 15 thus producing reduced iron, and reduced iron is discharged from a reduced iron discharge opening 16. An exhaust gas generated in the rotary hearth furnace 15 is sucked by a suction fan 19 and is discharged from a stack 20. During such discharging of the exhaust gas, dust is recovered by an bag filter 17. The recovered dust is transported to the outside using a powder transport lorry 18 or the like. In the mixed raw material, by setting a mixing amount of iron ore with a high content of zinc to approximately 10 mass % or more of the whole ore, it is possible to set zinc concentration in the recovered dust to 1 mass % or more.

The dust which is recovered from the exhaust gas generated in the rotary hearth furnace as described above (hereinafter referred to as “first recovered dust”) is treated again with heat supplied from above the hearth in the rotary hearth furnace, and the dust generated in the rotary hearth furnace is recovered thus obtaining crude zinc oxide. Dust which is produced by recovering the dust generated at the time of treating the first recovered dust again in the rotary hearth furnace is referred to as “second recovered dust” hereinafter. Although it is sufficient to treat only the first recovered dust at the time of treating the first recovered dust in the rotary hearth furnace, from a view point of accelerating a reduction reaction, a small quantity of carbonaceous solid reducing material or a small amount of a slag-making material may be mixed into the first recovered dust (mixing ratio of the carbonaceous solid reducing material or the slag-making material being 2 mass % or less with respect to the first recovered dust). By refining the first recovered dust again in the rotary hearth furnace in such a manner, it is possible to concentrate zinc in the first recovered dust as described hereinafter. When the zinc concentration in the first recovered dust is a predetermined quantity or more, it is possible to increase a production quantity of iron pebble by mixing a carbonaceous solid reducing material, a slag-making material and iron ore into the first recovered dust. In treating the first recovered dust by mixing iron ore into the first recovered dust, an iron ore mixing quantity can be increased by using iron ore with a high content of zinc in keeping the constant zinc concentration of second dust and hence, a larger quantity of iron pebble can be produced while concentrating zinc in the dust. Accordingly, the mixing of iron ore with a high content of zinc is preferable.

The zinc concentration in the dust can be carried out in such a manner, for example, as shown in FIG. 3, that the first recovered dust in a dust yard 23 is transported using the powder transport lorry 18 or the like, the first recovered dust is heated in the rotary hearth furnace 15, an exhaust gas generated by heating is sucked, and dust is recovered by the bag filter 17. Alternatively, the zinc concentration in the dust can be performed by providing a first recovered dust storage hopper 22 parallel to the hoppers 11 to 13 for storing the mixed raw material as shown in FIG. 4. This facility is, compared to the facility shown in FIG. 2, additionally provided with a recovered dust transport conveyer 21 and the first recovered dust storage hopper 22. The recovered dust transport conveyer 21 is bifurcated into a first recovered dust transport conveyer 21a and a second recovered dust transport conveyer 21b, wherein the first recovered dust is transported to the first recovered dust storage hopper 22 using the first recovered dust transport conveyer 21a, the first recovered dust is heated by the rotary hearth furnace 15 and is reused, and the second recovered dust is discharged using the second recovered dust transport conveyer 21b as a product. Since the second recovered dust discharged by the second recovered dust transport conveyer 21b is fine powder, the second recovered dust is transported using the powder transport lorry 18 or the like, for example.

In mixing iron ore in the dust, the first recovered dust is stored in the first recovered dust storage hopper 22, a small amount of carbonaceous solid reducing material, a small amount of slag-making material and a small amount of iron ore are mixed into the dust, a mixed raw material is used as a raw material fed to a rotary hearth furnace, and is recovered as the second recovered dust at the time of reducing and melting the raw material by heating in the rotary hearth furnace 15.

As described previously, although the second recovered dust which contains crude zinc oxide differs in application field depending on zinc concentration, the zinc concentration in the second recovered dust which is produced by the above-mentioned method exceeds 10 mass % and hence, it is possible to form the second recovered dust into crude zinc oxide of high concentration by an intermediate treatment such as a Waelz method, and crude zinc oxide can be used as a zinc refining raw material.

With respect to the mixed raw material at the time of recovering the first recovered dust, even when the whole iron ore is not iron ore with a high content of zinc, provided that the average zinc concentration in the iron ore is 0.005 mass % or more, it is possible to set the zinc concentration in the second recovered dust obtained by treating the mixed raw material in the rotary hearth furnace to 50 mass % or more. When the zinc concentration in the obtained recovered dust is 50 mass % or more, the intermediate treatment becomes unnecessary so that the second recovered dust can be directly used as crude zinc oxide which is used in zinc refining. Accordingly, it is preferable to set the zinc concentration in the obtained recovered dust to 50 mass % or more.

As described above, by treating the first recovered dust=) again in the rotary hearth furnace, the zinc concentration in the second recovered dust is enhanced thus also bringing about the economical improvements. In addition to such advantageous effects, this method has an advantageous effect that a cost for constructing an additional facility (intermediate treatment facility) for dust treatment becomes unnecessary, and an advantageous effect that a cost for transporting the generated dust to an intermediate treatment facility becomes unnecessary.

In the above-mentioned method, the dust contained in the exhaust gas generated in the rotary hearth furnace is recovered and is used. However, it is also possible to use zinc-containing dust other than the first recovered dust by mixing the zinc-containing dust into iron ore with a high content of zinc at the time of reducing iron ore with a high content of zinc by the rotary hearth furnace. By mixing dust which exhibits the higher zinc concentration than iron ore with a high content of zinc, with respect to dust which is recovered from the exhaust gas generated in the rotary hearth furnace as described above, it is possible to obtain dust which contains crude zinc oxide at high concentration.

As described previously, although the recovered dust which contains crude zinc oxide differs in the application field depending on the zinc concentration, the zinc concentration in the recovered dust which is produced using the zinc-containing dust exceeds 10 mass % irrespective of whether the recovered dust is produced inside the movable hearth furnace process or the outside the process. Accordingly, it is possible to form the recovered dust into crude zinc oxide of high zinc concentration by an intermediate treatment such as a Waelz method, and crude zinc oxide can be used as a zinc refining raw material.

Although the zinc-containing dust which is used in a mixed form with iron ore with a high content of zinc is not particularly limited, it is possible to use dust generated in steel making industry such as dust generated from a blast furnace, dust generated from a converter, or dust generated from an electric furnace, for example.

Provided that the average zinc concentration in the mixed raw material is 0.45 mass % or more, the zinc concentration in the recovered dust obtained by the treatment using the rotary hearth furnace can be increased to 50 mass % or more. When the zinc concentration in the obtained recovered dust is 50 mass % or more, the intermediate treatment becomes unnecessary so that the recovered dust can be directly used as crude zinc oxide which is used in zinc refining. Accordingly, it is preferable to set the zinc concentration in the obtained recovered dust to 50 mass % or more.

As described above, by using the iron ore with a high content of zinc in a state where the zinc-containing dust is mixed into the iron ore with a high content of zinc at the time of reducing the iron ore with a high content of zinc in the rotary hearth furnace, it is possible to enhance the zinc concentration in the recovered dust, and economy is also enhanced.

Another example is explained in detail hereinafter.

A mixed raw material containing iron ore with a high content of zinc, a carbonaceous solid reducing material and a slag-making material is charged on a hearth of a rotary hearth furnace, a temperature of the mixed raw material is elevated by heating while moving the mixed raw material in the furnace by rotating the hearth, air or air to which oxygen is added is blown into the furnace, and CO or H₂generated by a reduction reaction is subject to secondary combustion.

An exhaust gas generated by the secondary combustion is cooled and, thereafter, dust contained in the exhaust gas is recovered. On the other hand, the mixed raw material which remains on the hearth is completely melted into a liquid and, thereafter, the melts are cooled and solidified, and iron pebble is obtained by separating slag from the iron. By heating the mixed raw material on the movable hearth, it is possible to acquire following advantageous effects.

a) Metal iron is formed due to a reaction between iron oxide in ore and carbon in the carbonaceous solid reducing material.

b) An iron component is subject to a carburization reaction and a gangue component (SiO₂, Al₂O₃, MgO or the like) is mixed with a basic component such as CaO, Na₂O as exemplified by lime powder, dolomite, serpentine or the like so that a melting point of the mixed raw material is lowered and is melted.

c) By keeping the mixed raw material in a molten state, it is possible to separate the mixed raw material into a molten metal iron component (metal) and a molten gangue component (slag).

Due to such advantageous effects, it is possible to produce iron pebble which is reduced iron and can be used in the same manner as pig iron.

On the other hand, a zinc component in ore is present as zinc oxide. The zinc component is subject to reduction and evaporation due to the carbonaceous solid reducing material, is transported to an exhaust gas, is subject to oxidation and cohesion with cooling performed simultaneously, is separated from the exhaust gas, and is recovered as dust. Zinc is concentrated in the dust so that the dust directly becomes a raw material for zinc refining or becomes a raw material for zinc refining after performing a re-refining step.

Since the rotary hearth furnace is a furnace which is not provided with a filling layer, there is no possibility of the occurrence of a phenomenon observed with respect to a blast furnace, that is, a phenomenon where a zinc component contained in the raw material is adhered to a furnace wall thus giving rise to the adhesion of coke or ore, the immobilization of a filling material or the like. Accordingly, the operation of the furnace is not obstructed.

When the rotary hearth furnace is heated, a zinc component is evaporated and transported to the exhaust gas. Simultaneously, a part of the mixed raw material charged on the hearth is scattered and is mixed into the recovered dust. Accordingly, the zinc concentration in the recovered dust is determined based on an amount of zinc component which is evaporated and an amount of scattered mixed raw material. The higher the zinc concentration in the mixed raw material, the higher the zinc concentration in the recovered dust becomes. According to our studies, a scattered amount of mixed raw material is substantially constant in a normal operation, and it is confirmed that the scattered amount of mixed raw material is around 0.5 mass % of the mixed raw material charging amount. Further, the higher the zinc concentration in the dust, the higher a value of the dust as the zinc raw material becomes. Accordingly, the dust with high zinc concentration can be recovered thus realizing the more effective utilization of iron ore with a high content of zinc.

Further, by using the recovered dust as the whole or a part of the mixed raw material to be charged on the rotary hearth furnace, it is possible to recover the dust with higher zinc concentration by further concentrating zinc in the dust with high zinc concentration.

Experiment 1

To confirm the validity of our methods, in a rotary hearth furnace similar to the rotary hearth furnace shown in FIG. 1, an experiment of producing iron pebble is carried out using iron ore with a high content of zinc and general iron ore with low content of zinc. Further, dust generated in the rotary hearth furnace is recovered and the measurement of zinc concentration in the recovered dust is also performed. The specification of the rotary hearth furnace is shown in Table 1. In Table 2, T-Fe implies total Fe.

TABLE 1

furnace center diameter
7 m

furnace width
1 m

heating temperature
1300 to 1500° C.

The compositions of the iron ores used in the experiment are shown in Table 2.

TABLE 2

(mass %)

T-Fe
FeO
SiO₂
Al₂O₃
Zn

ore A
63.0
23.2
4.28
1.13
0.050

ore B
64.0
22.3
4.1
1.00
0.001

The ore A is iron ore with a high content of zinc, and the ore B is general ore with low content of zinc. Although an amount of gangue component and an amount of iron component are substantially equal between the ore A and the ore B, the zinc concentration in the ore A is approximately 50 times as high as the zinc concentration in the ore B.

A mixed raw material is prepared by mixing ore, coal which constitutes a carbonaceous solid reducing material and lime which constitutes a slag-making material. Table 3 shows the composition of coal used in the experiment, and Table 4 shows blending of mixed raw materials used in the experiment. In Table 3, FC implies fixed carbon, VM indicates a volatile component, and Ash implies an ash component.

TABLE 3

(mass %)

FC
VM
Ash
SiO₂
Al₂O₃

coal
85
7
8
4.5
2.5

TABLE 4

(kg/t-iron)

ore A
ore B
coal
lime

blending 1
—
1562
344
171

blending 2
1587
—
343
179

blending 3
145
1420
345
171

The rotary hearth furnace is operated using the mixed raw material having blending 1 to 3 shown in Table 4 under conditions shown in Table 5. A raw material state is shown in columns, wherein a case where a layer having a thickness of 50 mm is formed on the hearth using coal as a carbon material and, then, a mixed raw material is charged on the coal layer is referred to as “carbon material present at lower layer”, a case where a mixed raw material having a layer thickness of approximately 10 mm is charged on the hearth without agglomerating the mixed raw material is referred to as “powder”, and a case where a mixed raw material is formed into pellets having a particle size of 10 to 15 mm by agglomerating the mixed raw material is referred to as “agglomerate”.

TABLE 5

oper-
temper-

carbon
raw

ation
ature
blend-
material at
material

No.
(° C.)
ing
lower layer
state
Remarks

1
1400
1
none
powder
comparison

example

2
1460
1
none
powder
comparison

example

3
1400
2
none
powder
example

4
1400
3
none
powder
example

5
1460
2
none
powder
example

6
1460
2
present
powder
example

7
1460
2
none
agglomerate
example

Table 6 indicates a result of zinc concentration in dust and an iron component recovery ratio when iron pebble is produced under the conditions shown in Table 5.

TABLE 6

zinc
iron compo-

operation
concentration
nent recov-
treatment

No.
in dust
ery ratio
time
remarks

1
0.20%
97.0%
14 minutes
comparison

example

2
0.19%
97.0%
13 minutes
comparison

example

3
7.8%
97.0%
13.5 minutes
example

4
1.0%
97.0%
13.5 minutes
example

5
8.1%
98.0%
13 minutes
example

6
7.9%
99.0%
12 minutes
example

7
9.6%
99.5%
12 minutes
example

In Table 6, the operation No. 3 is one of our examples where iron ore with a high content of zinc is used. The zinc concentration in the dust is elevated to 7.8 mass %.

The operation No. 4 is the example where approximately 10 mass % of iron ore with a high content of zinc is mixed into the general ore. Also in this case, the zinc concentration in the dust is elevated to 1.0 mass % or more.

The operation No. 5 is the example where the heat treatment is applied to the mixed raw material at a high temperature of 1450° C. or more, and it is found that the treatment time is shortened so that the productivity is enhanced.

The operation No. 6 is the example where, in addition to the operation No. 5, a carbon material is laid on the hearth, and the mixed raw material is stacked on the carbon material. The recovery ratio of iron component is elevated.

The operation No. 7 is the example where, in addition to the operation No. 5, an agglomerated raw material is used. The zinc concentration in the dust is elevated.

Next, the recovered dust is recycled.

In carrying out an experiment for producing iron pebble using iron ore with a high content of zinc and general iron ore with low content of zinc by a facility substantially equal to the facilities explained in conjunction with FIG. 1 and FIG. 4, the relationship between the zinc concentration in iron ore and the zinc concentration in recovered dust is investigated. In the investigation, the ore A which is iron ore with a high content of zinc, and the ore B which is general iron ore are used in a mixed form, and the zinc concentration is continuously changed thus providing operations No. 11 to No. 19. Dust generated by the first-time treatment in the rotary hearth furnace (first recovered dust) is recovered, and the heat treatment is applied to a total amount of the first recovered dust in the rotary hearth furnace at a temperature of 1460° C. for 13 minutes, and the generated dust (second recovered dust) is recovered.

Table 7 and FIG. 5 show the zinc concentration in iron ore in the mixed raw material, the measurement result of zinc concentration in the first recovered dust which is dust generated by the first-time treatment in the rotary hearth furnace and becomes a raw material of the second-time treatment in the rotary hearth furnace, and the measurement result of zinc concentration in the second recovered dust which is the final product dust.

As can be understood from Table 7 and FIG. 5, it is understood that when the zinc concentration in the ore in the mixed raw material becomes 0.005 mass % or more, the zinc concentration in the second recovered dust which constitutes product dust exceeds 50 mass % and it is possible to obtain a raw material which can be directly used in the zinc refining such as an ISP method.

TABLE 7

zinc concentration
second

ore A
mixed raw
first
second
recovered

mixing
material
recovered
recovered
dust basic

operation
ratio
ore
dust
dust
unit

No.
mass %
mass %
mass %
mass %
kg/t-Fe
remarks

11
0.0
0.001
0.16
23
0.067
comparison

example

12
3.0
0.002
0.41
41
0.096
example

13
5.0
0.003
0.57
47
0.116
example

14
6.0
0.004
0.65
50
0.125
example

15
7.5
0.005
0.77
53
0.140
example

16
8.0
0.005
0.81
54
0.145
example

17
10
0.006
0.97
57
0.164
example

18
15
0.008
1.36
62
0.213
example

19
20
0.011
1.76
66
0.261
example

Next, a raw material which is prepared by mixing iron ore with a high content of zinc and zinc-containing dust is used.

The composition of the zinc-containing dust used in the experiment is shown in FIG. 8. Dust generated in a converter is used as the zinc-containing dust.

TABLE 8

T•Fe
FeO
SiO₂
Al₂O₃
Zn

zinc-containing dust
48
16.29
1.49
1.17
1.4

In carrying out an experiment for producing iron pebble using iron ore with a high content of zinc and the zinc-containing dust by a facility substantially equal to the facility explained in conjunction with FIG. 1, the relationship between the zinc concentration in the mixed raw material and the zinc concentration in recovered dust is investigated. In the investigation, the ore A which is iron ore with a high content of zinc and the zinc-containing dust are used in a mixed form, and the zinc concentration is continuously changed thus providing operations No. 21 to No. 25. The heat treatment is applied to mixed raw material in a rotary hearth furnace at a temperature of 1460° C. for 13 minutes, and the generated dust is recovered.

Table 9 shows a measurement result of a mixing ratio and the zinc concentration in the zinc-containing dust in the mixed raw material and the zinc concentration in the recovered dust.

TABLE 9

zinc-containing
zinc concentration

dust mixing
mixed raw
recovered

operation
ratio
material
dust

No.
mass %
mass %
mass %
Remarks

21
0
0.04
8
example

22
10
0.17
28
example

23
20
0.31
41
example

24
30
0.45
50
example

25
35
0.51
53
example

From Table 9, it is understood that the zinc concentration in the recovered dust is elevated along with the elevation of the mixing ratio of the zinc-containing dust in the mixed raw material, and when the zinc concentration in the mixed raw material becomes 0.45 mass % or more, the zinc concentration in the recovered dust which constitutes product dust exceeds 50 mass % and it is possible to obtain a raw material which can be directly used in the zinc refining such as an ISP method.

Example 2

A method for producing reduced iron according to example 2 includes a step of preparing a mixed raw material, a mixed raw material charging step, and a reducing step.

In the step of preparing the mixed raw material, the mixed raw material is prepared by mixing iron ore which contains iron ore with a high content of zinc which contains 0.01 mass % or more of zinc and 50 mass % or more of iron and a carbonaceous solid reducing material. In the mixed raw material charging step, the mixed raw material is charged on a movable hearth. In the reducing step, the mixed raw material charged on the movable hearth is reduced by supplying heat from above the hearth, and the reduced iron is produced in a state where the mixed raw material is not melted or the mixed raw material is melted only partially.

We discovered the idea of using a movable hearth furnace for effectively making use of an iron component and, further, a zinc component contained in iron ore with a high content of zinc. A method for producing reduced iron using the movable hearth furnace is one of methods for producing reduced iron. In this method, iron ore, a solid reducing material and the like are charged on a hearth which moves in the horizontal direction, and iron ore is reduced by heating iron ore with radiation heat transfer from above thus producing reduced iron.

The movable hearth furnace is a furnace which applies heat to the hearth moving horizontally. The horizontal movement of hearth typically has the form of the rotational transfer as shown in FIG. 6. The movable hearth furnace having the form of the rotational transfer is particularly referred to as a rotary hearth furnace. In example 2, reduced iron is produced by applying reduction treatment to iron ore with a high content of zinc using such a movable hearth furnace, particularly using the rotary hearth furnace. Hereinafter, example 2 is explained in conjunction with a case where the rotary hearth furnace is used as the movable hearth furnace.

Iron ore with a high content of zinc used in example 2 is iron ore which exhibits high zinc content compared to iron ore used as a normal blast furnace raw material, and contains 0.01 mass % or more of zinc and 50 mass % or more of iron in general. Although there is no upper limit with respect to zinc content and iron content in iron ore with a high content of zinc used in example 2, the upper limit of zinc content and iron content is naturally determined because iron ore with a high content of zinc is an iron core. The upper limit of zinc content is approximately 0.5 mass % or less, for example, while the upper limit of iron content is approximately 70 mass % or less, for example Further, content of alkali component in iron ore with a high content of zinc is usually 0.08 mass % or more in terms of oxide such as Na₂O or K₂O. It is preferable to be the content of alkali component in iron ore to 1 mass % or less. With such content of alkali component, clogging of gas discharge system can be effectively prevented.

Although example 2 is directed to a technique which produces reduced iron using such iron ore with a high content of zinc, iron ore with a high content of zinc may be also used in mixture with usual iron ore at the time of reducing iron ore with a high content of zinc by the rotary hearth furnace. Even when iron ore with a high content of zinc is used in mixture with usual iron ore, it is possible to preferably obtain the advantageous effects of example 2 by mixing approximately 10 mass % or more of iron ore with a high content of zinc with respect to the whole ore.

One rotary hearth furnace used in example 2 is explained in conjunction with FIG. 6. The rotary hearth furnace 1 is, as shown in FIG. 6, constituted such that a rotatably movable hearth 3 is covered with a furnace body 2 which is partitioned into a preheating zone 2a, a reducing zone 2b and a cooling zone 2d. A mixed raw material 4 which is formed by mixing iron ore with a high content of zinc and a carbonaceous solid reducing material is charged on the rotary hearth 3. The mixed raw material 4 may be also granulated as explained hereinafter. The furnace body 2 which covers the rotary hearth 3 is composed with refractories. Further, to protect the hearth refractories, there may be a case where a carbon material is charged on the hearth 3, and the mixed raw material 4 is stacked on the carbon material. Further, burners 5 are mounted on the furnace body 2, and iron ore in the mixed raw material 4 on the rotary hearth 3 is reduced using fuel combustion heat generated by the burners 5 as a heat source. In FIG. 6 a charging device 6 charges the raw material onto the rotary hearth 3, a discharge device 7 discharges a reduced product, and numeral 8 indicates a cooling device. In general, the furnace temperature is suppressed to approximately 1300° C. This is because the suppression of the furnace temperature is effective for prolonging the lifetime of furnace body refractories. Although our method does not positively melt the mixed raw material, the partial melting of the mixed raw material in the reducing step also falls within the scope of example 2.

Iron ore with a high content of zinc contains a gangue component although an amount of gangue component differs depending on the source. Further, coal, coal char or coke which is a typical example of a carbonaceous solid reducing material contains an ash component. Accordingly, in a movable hearth furnace method which performs only reducing operation, the inclusion of gangue into reduced iron which is a product is unavoidable thus giving rise to a possibility that an ash component attributed to a reducing material is also adhered to the product or is mixed into the product. Accordingly, the gangue component and the ash component are not sufficiently separated from the reduced iron obtained by example 2 and hence, the reduced iron obtained by example 2 is in a state where apparent density (however, in a state immediately after the reduced iron is discharged from the rotary hearth furnace and before the compression or the like is performed) is less than 5000 kg/m³.

In applying reduction treatment to iron ore with a high content of zinc using the rotary hearth furnace, it is assumed that iron ore with a high content of zinc is mixed with a carbonaceous solid reducing material, and is charged on the rotatably movable hearth. Coal, coke, graphite or the like can be used as the carbonaceous solid reducing material.

When iron ore with a high content of zinc is lumpy ore, iron ore with a high content of zinc is crushed into fine ore having a particle size of 10 mm or less and thereafter, the fine ore is mixed with the carbonaceous solid reducing material, and the mixture is charged on the rotary hearth and is reduced.

When iron ore with a high content of zinc is ore powder (particle size: 3 mm or less), iron ore with a high content of zinc is agglomerated together with a carbonaceous solid reducing material thus allowing the use of iron ore with a high content of zinc as pellets incorporating a carbon material therein. Dust Scattering from the agglomerated raw material is little at the time of heating and hence, zinc concentration in dust can be enhanced. In the same manner, iron ore with a high content of zinc may be molded by compression thus enabling the use of iron ore with a high content of zinc as briquettes. Further, at the time of agglomerating iron ore with a high content of zinc, an inorganic binder such as bentonite and an organic binder such as molasses or corn starch are mixed into iron ore with a high content of zinc so as to increase strength of the iron ore agglomerate. These pellets or briquettes can be also used after vaporizing moisture.

On the other hand, it is also effective to use iron ore with a high content of zinc in an original fine form. By using iron ore with a high content of zinc as an original fine raw material, costs for facilities, electric power, binders and the like necessary for producing agglomerates become unnecessary thus contributing to the economical improvements.

A heating temperature at the time of reducing iron ore with a high content of zinc using the rotary hearth furnace is preferably set to 1250° C. or more. By setting a maximum temperature in the rotary hearth furnace to 1250° C. or more, the inside of the furnace and the raw material which is reduced in the furnace arrive at a high temperature. By setting the maximum temperature in the rotary hearth furnace to 1250° C. or more, a reduction action is accelerated so that reduced iron can be manufactured at a high speed. In example 2, although an upper limit of the heating temperature is a temperature (less than 1450° C.) at which the mixed raw material is not completely melted, the upper limit is controlled to a temperature less than 1400° C. in a usual operation.

By charging a carbon material on the hearth and by stacking the mixed raw material containing iron ore with a high content of zinc on the carbon material, it is possible to prevent the mixed raw material in a partially molten state from eroding the refractories of the hearth. Since the iron component is taken in the refractories when the refractories are eroded, it is possible to reduce loss of the iron component by preventing the erosion of the refractories of the hearth thus contributing to the enhancement of productivity of reduced iron.

Dust contained in an exhaust gas generated in the rotary hearth furnace is recovered. Zinc in the dust is concentrated compared to iron ore with a high content of zinc and hence, the dust can be used as a raw material of crude zinc oxide. FIG. 7 is a schematic view of a general facility flow of a rotary hearth furnace which performs such dust recovery.

In FIG. 7, iron ore and coal which are discharged from an ore hopper 11 and a coal hopper 12 are mixed together by a mixer 14 (also using a pelletizer or the like when necessary) thus preparing a mixed raw material. The mixed raw material is reduced by heating in the rotary hearth furnace 15 thus producing reduced iron, and the reduced iron is discharged from a reduced iron discharge opening 16. An exhaust gas generated in the rotary hearth furnace 15 is sucked by a suction fan 19 and is discharged from a stack 20. During such discharging of the exhaust gas, dust is recovered by an bag filter 17. The recovered dust is transported to the outside using a powder transport lorry 18 or the like. In the mixed raw material, by setting an amount of iron ore with a high content of zinc to approximately 10 mass % or more of the whole ore, it is possible to set zinc concentration in the recovered dust to 1 mass % or more.

The dust which is recovered from the exhaust gas generated in the rotary hearth furnace as described above (hereinafter referred to as “first recovered dust”) is treated again with heat supplied from above the hearth in the rotary hearth furnace, and the dust generated in the rotary hearth furnace is recovered thus obtaining crude zinc oxide. Dust which is produced by recovering the dust generated at the time of treating the first recovered dust again in the rotary hearth furnace is referred to as “second recovered dust” hereinafter. Although it is sufficient to treat only the first recovered dust at the time of treating the first recovered dust in the rotary hearth furnace, from a view point of accelerating a reduction reaction, a small amount of carbonaceous solid reducing material may be mixed into the first recovered dust (mixing ratio of the carbonaceous solid reducing material being 2 mass % or less with respect to the first recovered dust). By refining the first recovered dust again in the rotary hearth furnace in such a manner, it is possible to concentrate zinc in the first recovered dust as described hereinafter. When the zinc concentration in the first recovered dust is a predetermined quantity or more, it is possible to increase a production quantity of reduced iron by mixing a carbonaceous solid reducing material and iron ore into the first recovered dust. In treating the first recovered dust by mixing iron ore into the first recovered dust, an iron ore mixing quantity can be increased by using iron ore with a high content of zinc in keeping the constant zinc concentration of second dust and hence, a larger quantity of reduced iron can be produced while concentrating zinc in the dust. Accordingly, the mixing of iron ore with a high content of zinc is preferable.

The concentration of zinc in the dust can be carried out in such a manner, for example, as shown in FIG. 8, that the first recovered dust in a dust yard 23 is transported using the powder transport lorry 18 or the like, and the first recovered dust is heated in the rotary hearth furnace 15, an exhaust gas generated by heating is sucked, and dust is recovered by the bag filter 17. Further, the concentration of zinc in the dust can be performed by providing a first recovered dust storage hopper 22 parallel to the hoppers 11, 12 for storing the mixed raw material as shown in FIG. 9. This facility is, compared to the facility shown in FIG. 7, additionally provided with a recovered dust transport conveyer 21 and the first recovered dust storage hopper 22. The recovered dust transport conveyer 21 is bifurcated into a first recovered dust transport conveyer 21a and a second recovered dust transport conveyer 21b, wherein the first recovered dust is transported to the first recovered dust storage hopper 22 using the first recovered dust transport conveyer 21 a, the first recovered dust is heated in the rotary hearth furnace 15 and is reused, and the second recovered dust is discharged using the second recovered dust transport conveyer 21b as a product. Since the second recovered dust discharged by the second recovered dust transport conveyer 21b is fine powder, the second recovered dust is transported using the powder transport lorry 18 or the like, for example.

In mixing iron ore in the dust, the first recovered dust is stored in the first recovered dust storage hopper 22, a small amount of carbonaceous solid reducing material and a small amount of iron ore are mixed into the dust, a mixed raw material is used as a raw material fed to a rotary hearth furnace, and is recovered as the second recovered dust at the time of reducing the raw material by heating in the rotary hearth furnace 15.

As described previously, although the second recovered dust which contains crude zinc oxide differs in application field depending on zinc concentration, the zinc concentration in the second recovered dust which is produced by the above-mentioned method exceeds 10 mass % and hence, it is possible to form the second recovered dust into crude zinc oxide of high concentration by an intermediate treatment such as a Waelz method further comprises, and crude zinc oxide can be used as a zinc refining raw material.

With respect to the mixed raw material at the time of recovering the first recovered dust, even when the whole ore is not ore with a high content of zinc, provided that the average zinc concentration in the ore is 0.005 mass % or more, it is possible to set the zinc concentration in the second recovered dust obtained by treating the mixed raw material in the rotary hearth furnace to 50 mass % or more. When the zinc concentration in the obtained recovered dust is 50 mass % or more, the intermediate treatment becomes unnecessary so that the second recovered dust can be directly used as crude zinc oxide which is used in zinc refining. Accordingly, it is preferable to set the zinc concentration in the obtained second recovered dust to 50 mass % or more.

As described above, by treating the first recovered dust again in the rotary hearth furnace, the zinc concentration in the second recovered dust is enhanced thus also bringing about the economical improvements. In addition to such advantageous effects, this method has an advantageous effect that a cost for constructing an additional facility (intermediate treatment facility) for dust treatment becomes unnecessary, and an advantageous effect that a cost for transporting the generated dust to an intermediate treatment facility becomes unnecessary.

In the above-mentioned method, the dust contained in the exhaust gas generated in the rotary hearth furnace is recovered and is used. However, it is also possible to use zinc-containing dust other than the first recovered dust by mixing the zinc-containing dust into iron ore with a high content of zinc at the time of reducing iron ore with a high content of zinc in the rotary hearth furnace. By mixing dust which exhibits the higher zinc concentration than iron ore with a high content of zinc, with respect to dust which is recovered from the exhaust gas generated in the rotary hearth furnace as described above, it is possible to obtain dust which contains crude zinc oxide at high concentration.

One mode of example 2 is explained in detail hereinafter.

A mixed raw material containing iron ore with a high content of zinc and a carbonaceous solid reducing material is charged on a hearth of a rotary hearth furnace, a temperature of the mixed raw material is elevated by heating while moving the mixed raw material by rotating the hearth in the furnace, air or air to which oxygen is added is blown into the furnace, and CO or H₂generated by a reduction reaction is subject to secondary combustion.

An exhaust gas generated by the secondary combustion is cooled and, thereafter, dust contained in the exhaust gas is recovered. The mixed raw material which remains on the hearth is sufficiently reduced thus obtaining the reduced iron.

When the rotary hearth furnace is heated, a zinc component is evaporated, and is transported to the exhaust gas. Simultaneously, a part of the mixed raw material charged on the hearth is scattered and is mixed into the recovered dust. Accordingly, the zinc concentration in the recovered dust is determined based on an amount of evaporated zinc component and an amount of scattered mixed raw material. The higher the zinc concentration in the mixed raw material, the higher the zinc concentration in the recovered dust becomes. According to our studies, a scattered amount of mixed raw material is substantially constant in a normal operation, and it is confirmed that the scattered amount of mixed raw material is around 0.5 mass % of the mixed raw material charging amount. Further, the higher the zinc concentration in the dust, the higher a value of the dust as the zinc raw material becomes. Accordingly, by carrying out example 2, the dust with high zinc concentration can be recovered thus realizing the more effective utilization of iron ore with a high content of zinc.

Experiment 2

To confirm the validity of example 2, in a rotary hearth furnace similar to the rotary hearth furnace shown in FIG. 6, an experiment of producing reduced iron is carried out using iron ore with a high content of zinc and general ore with low content of zinc. Further, dust generated in the rotary hearth furnace is recovered and the measurement of zinc concentration in the recovered dust is also performed. The specification of the rotary hearth furnace is shown in Table 10.

TABLE 10

furnace center diameter
7 m

furnace width
1 m

heating temperature
1300 to 1500° C.

The composition of the ores used in the experiment are shown in Table 11. In Table 11, T-Fe implies total Fe.

TABLE 11

(mass %)

T-Fe
FeO
SiO₂
Al₂O₃
Zn

ore A
63.0
23.2
4.28
1.13
0.050

ore B
64.0
22.3
4.1
1.00
0.001

A mixed raw material is prepared by mixing ore and coal which constitutes a carbonaceous solid reducing material. Table 12 shows the composition of coal used in the experiment, and Table 13 shows blending of mixed raw materials used in the experiment. In FIG. 12, FC implies fixed carbon, VM indicates a volatile component, and Ash implies an ash component.

TABLE 12

(mass %)

FC
VM
Ash
SiO₂
Al₂O₃

coal
85
7
8
4.5
2.5

TABLE 13

(kg/t-iron)

ore A
ore B
coal

blending 1
—
1562
344

blending 2
1587
—
343

blending 3
145
1420
345

The rotary hearth furnace is operated using the mixed raw material having blending 1 to 3 shown in Table 13 under conditions shown in Table 14. A raw material state is shown in columns, wherein a case where a layer having a thickness of 50 mm is formed on the hearth using coal as a carbon material and, then, a mixed raw material is charged on the coal layer is referred to as “carbon material present at lower layer”, a case where a mixed raw material having a layer thickness of approximately 10 mm is charged on the hearth is referred to as “powder”, and a case where a mixed raw material is formed into pellets having a particle size of 10 to 15 mm by agglomerating the mixed raw material is referred to as “agglomerate”.

TABLE 14

oper-
temper-

carbon
raw

ation
ature
blend-
material at
material

No.
(° C.)
ing
lower layer
state
remarks

1
1200
1
none
powder
comparison

example

2
1260
1
none
powder
comparison

example

3
1200
2
none
powder
example

4
1200
3
none
powder
example

5
1260
2
none
powder
example

6
1260
2
present
powder
example

7
1260
2
none
agglomerate
example

Table 15 indicates a result of zinc concentration in dust and an iron component recovery ratio when reduced iron is produced under the conditions shown in Table 14.

TABLE 15

zinc
iron compo-

operation
concentration
nent recov-
treatment

No.
in dust
ery ratio
time
remarks

1
0.18%
97.0%
12 minutes
comparison

example

2
0.17%
97.0%
11 minutes
comparison

example

3
7.6%
97.0%
11.5 minutes
example

4
1.0%
97.0%
11.5 minutes
example

5
7.9%
98.0%
10.5 minutes
example

6
7.8%
99.0%
10.5 minutes
example

7
8.8%
99.5%
10.5 minutes
example

In Table 15, the operation No. 3 is one of our examples where iron ore with a high content of zinc is used. The zinc concentration in the dust is elevated to 7.6 mass %.

The operation No. 5 is the example where the heat treatment is applied to the mixed raw material at a high temperature of 1250° C. or more, and it is found that the treatment time is shortened so that the productivity is enhanced.

The operation No. 6 is an example where, in addition to the operation No. 5, a carbon material is laid on the hearth, and the mixed raw material is stacked on the carbon material. The recovery ratio of iron is elevated.

The operation No. 7 is an example where, in addition to the operation No. 5, an agglomerated raw material is used. The zinc concentration in the dust is elevated.

Next, the recovered dust is recycled.

In carrying out an experiment for producing reduced iron using iron ore with a high content of zinc and general ore with low content of zinc by a facility substantially equal to the facilities explained in conjunction with FIG. 6 and FIG. 9, the relationship between the zinc concentration in ore and the zinc concentration in recovered dust is investigated. In the investigation, the ore A which is iron ore with a high content of zinc and the ore B which is general ore are used in a mixed form, the zinc concentration is continuously changed thus providing operations No. 11 to No. 19. Dust generated by the first-time treatment in the rotary hearth furnace (first recovered dust) is recovered, the heat treatment is applied to a total amount of the first recovered dust in the rotary hearth furnace at a temperature of 1260° C. for 13 minutes, and the generated dust (second recovered dust) is recovered.

Table 16 and FIG. 10 show the zinc concentration in iron ore in the mixed raw material, the measurement result of zinc concentration in the first recovered dust which is dust generated by the first-time treatment in the rotary hearth furnace and becomes a raw material of the second-time treatment in the rotary hearth furnace, and the measurement result of zinc concentration in the second recovered dust which is the final product dust.

TABLE 16

zinc concentration
second

ore A
mixed raw
first
second
recovered

mixing
material
recovered
recovered
dust basic

operation
ratio
ore
dust
dust
unit

No.
mass %
mass %
mass %
mass %
Kg/t-Fe
remarks

11
0.0
0.001
0.16
22
0.071
comparison

example

12
3.0
0.002
0.38
38
0.102
example

13
5.0
0.003
0.55
46
0.118
example

14
6.0
0.004
0.64
49
0.127
example

15
7.5
0.005
0.78
53
0.141
example

16
8.0
0.005
0.79
53
0.147
example

17
10
0.006
0.99
58
0.162
example

18
15
0.008
1.42
63
0.209
example

19
20
0.011
1.83
67
0.257
example

From Table 16 and FIG. 10, it is understood that when the zinc concentration in the ore in the mixed raw material becomes 0.005 mass % or more, the zinc concentration in the second recovered dust which constitutes product dust exceeds 50 mass % and it is possible to obtain a raw material which can be directly used in the zinc refining such as an ISP method.

Next, a raw material which is prepared by mixing iron ore with a high content of zinc and zinc-containing dust is used.

The composition of the zinc-containing dust used in the experiment is shown in FIG. 17. Dust generated in a converter is used as the zinc-containing dust.

TABLE 17

T•Fe
FeO
SiO₂
Al₂O₃
Zn

Zinc-containing dust
48
16.29
1.49
1.17
1.4

In carrying out en experiment for producing reduced iron using iron ore with a high content of zinc and the zinc-containing dust by a facility substantially equal to the facility explained in conjunction with FIG. 6, the relationship between the zinc concentration in the mixed raw material and the zinc concentration in recovered dust is investigated. In the investigation, the ore A which is iron ore with a high content of zinc and the zinc-containing dust are used in a mixed form, and the zinc concentration is continuously changed thus providing operations No. 21 to No. 25. The heat treatment is applied to mixed raw material in a rotary hearth furnace at a temperature of 1260° C. for 13 minutes, and the generated dust is recovered.

Table 18 shows a measurement result of a mixing ratio and the zinc concentration in the zinc-containing dust in the mixed raw material and the zinc concentration in the recovered dust.

TABLE 18

zinc concentration

Zinc-containing
mixed raw
recovered

operation
dust mixing ratio
material
dust

No.
mass %
mass %
mass %
remarks

21
0
0.04
8
example

22
10
0.17
29
example

23
20
0.31
42
example

24
30
0.45
51
example

25
35
0.51
55
example

As can be understood from Table 18, it is understood that the zinc concentration in the recovered dust is elevated along with the elevation of the mixing ratio of the zinc-containing dust in the mixed raw material, and when the zinc concentration in the mixed raw material becomes 0.45 mass % or more, the zinc concentration in the recovered dust which constitutes product dust exceeds 50 mass % and it is possible to obtain a raw material which can be directly used in the zinc refining such as an ISP method.

Number	Date	Country	Kind
2008-115674	Apr 2008	JP	national
2008-115675	Apr 2008	JP	national
2008-169804	Jun 2008	JP	national
2008-169805	Jun 2008	JP	national

METHOD FOR PRODUCING REDUCED IRON

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