Abstract:
A method for making surface-coated reduced iron includes the step of coating the surface of reduced iron with a tar emulsion. Preferably, the tar emulsion includes at least one hydrocarbon-based material selected from the group consisting of natural petroleum tar, coal tar, pitch, asphalt, liquefied coal, and residual oil from petroleum refining; a surfactant; and water. More preferably, the tar emulsion contains 60 to 80 percent by mass of the hydrocarbon-based material, 0.1 to 1 percent by mass of the surfactant, and the balance being substantially water.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a technique for preventing reoxidation of reduced iron, for example, produced by a direct reduction process, during storage and transportation, and a technique for melting reduced iron efficiently.  
           [0003]    2. Description of the Related Art  
           [0004]    Reduced iron (sponge iron) produced by a direct reduction process has numerous open pores formed in the direct reducing process, and when the reduced iron is exposed in the air for a long period of time, the inner surfaces of the open pores are easily oxidized (reoxidized). If such reoxidized reduced iron is melted as feed material to melting process, a slag with a high iron oxide content is formed on the surface of the molten iron, thus decreasing the yield of iron. Additionally, when the sponge iron having numerous open pores is used, boiling may occur during melting, or the carbon content in the molten iron may become unstable because a portion of carbon is consumed by the reduction of iron oxides in the reduced iron and in the slag. Moreover, if the iron oxide content in the slag is increased, the viscosity of the slag is decreased, and the loss of the refractory lining of the melting furnace is increased.  
           [0005]    Therefore, in order to prevent the reoxidation of the reduced iron, various methods have been proposed. For example, Japanese Unexamined Patent Application Publication No. 50-116343 discloses a method in which the surface of reduced iron is treated with a medium containing an appropriate amount of oxygen to form a passive film composed of dense oxides on the surface of the reduced iron; Japanese Unexamined Patent Application Publication No. 52-107248 discloses a method for preventing oxidation by applying a dispersion liquid, in which graphite is diluted with an alcohol or water, to the surface of reduced iron; Japanese Examined Patent Application Publication Nos. 60-17803, 58-3003, 60-15681, etc. disclose methods in which CaO and iron oxides are applied to the surface of reduced iron so that the surface is coated with hydrates of CaO; Japanese Unexamined Patent Application Publication No. 47-5806 discloses a method in which air or nitrogen to which ammonia is added is brought into contact with reduced iron; Japanese Examined Patent Application Publication No. 59-17164 discloses a method in which reduced iron is immersed in a solution of an alkali metal silicate and is then dried by heating; Japanese Examined Patent Application Publication No. 60-12404 discloses a method for coating reduced iron with a hydraulic cement; Japanese Unexamined Patent Application Publication No. 55-128515 discloses a method for coating reduced iron with a chromate; Japanese Unexamined Patent Application Publication No. 52-123312 discloses a method for cooling reduced iron in a molten salt; Japanese Unexamined Patent Application Publication No. 58-71315 discloses a method for preventing ignition or heating up by controlling the discharging temperature after reduction; Japanese Unexamined Patent Publication Nos. 59-170213, 52-78610, etc. disclose methods in which reduced iron is processed by hot press molding (hot briquetting) using dies to decrease its specific surface areas, and thereby oxidation is prevented; and Japanese Unexamined Patent Publication No. 55-119116 discloses a method for preventing oxidation by continuous rolling.  
           [0006]    However, except for the hot pressing (hot briquetting) method, the effects of the conventional methods described above are not stable, and the resultant products do not sufficiently last during vessel transportation and long storage. With respect to the hot pressing (hot briquetting) method, dedicated facilities are required to perform the method, and moreover, since heat treatment must be performed at a high temperature of 800 to 900° C., the facilities are subjected to significant wear and tear, resulting in an increase in the maintenance costs and other operational problems.  
           [0007]    With respect to the methods for forming protective films at ambient temperatures to prevent reoxidation, inorganic oxides, such as CaO, calcined lime, cement, water glass, and graphite, are used. In such methods, except for the methods using water glass and graphite, rigid oxidation-preventing films are formed using the hydration reactions and also iron is prevented from being oxidized by maintaining the alkaline environment. However, since water glass is highly hygroscopic and is neutralized by reaction with carbon dioxide in the air, the effect thereof does not substantially last long.  
           [0008]    In the method using graphite, by covering the surface of reduced iron with a dense graphite film, oxygen is prevented from penetrating into the reduced iron. However, in practice, it is very difficult to form a dense film using graphite, and the resultant film has many interstices. Therefore, it is not possible to obtain a satisfactory oxidation-preventing effect.  
           [0009]    Additionally, since the graphite film is electrically conductive, even when reduced iron is melted in an electric furnace, melting operation can be performed without any problems. However, in the methods in which the inorganic films other than the graphite film are formed, since the films are nonconductive, the melting reaction of the reduced iron is significantly hampered, resulting in a decrease in the melting efficiency.  
           [0010]    In order to overcome the problems described above, the applicant of the present invention has disclosed a method for coating reduced iron with an organic film-forming material, such as pitch, asphalt, coal tar, liquefied coal, or coal residual oil (Japanese Unexamined Patent Publication No. 8-260172). However, since most of these materials have high viscosities at ambient temperatures, it is difficult to coat the surface of the reduced iron thinly and homogeneously with the materials, even if appropriately heated. Therefore, in order to form the oxidation-preventing film, the organic film-forming material usually must be dissolved or dispersed in an appropriate solvent to decrease the viscosity, and then the surface of the reduced iron is coated by an immersion method, a spray method, or the like, followed by drying under reduced pressure or drying by heating to remove the solvent. Since these materials still have high viscosities even if dissolved in solvents, it is not possible to form a completely homogeneous film on the surface of the reduced iron, and thus it is not possible to prevent reoxidation sufficiently. Moreover, the solvents to be used are expensive, and for safety purpose, recovery facilities to recover the volatized solvents must be installed, resulting in an increase in the cost of equipment.  
         SUMMARY OF THE INVENTION  
         [0011]    It is an objective of the present invention to provide surface-coated reduced iron having a reliable and long-term oxidation-preventing effect, in which the increased cost of equipment and the operational problems associated with the hot pressing (hot briquetting) method are solved and in which the insufficient oxidation-preventing effect and the insufficient durability associated with the conventional surface-coating methods are improved. It is another objective of the present invention to provide a method for making surface-coated reduced iron, in which the reduced iron can be reliably prevented from being reoxidized for a long period of time by simple surface treatment. The further objective of the present invention is to provide a method for melting such surface-coated reduced iron efficiently.  
           [0012]    In one aspect of the present invention, in surface-treated reduced iron, the surface of the reduced iron is coated with a tar emulsion.  
           [0013]    Preferably, the aforementioned tar emulsion contains at least one hydrocarbon-based material selected from the group consisting of natural petroleum tar, coal tar, pitch, asphalt, liquefied coal, and residual oil from petroleum refining; a surfactant; and water.  
           [0014]    Preferably, the aforementioned tar emulsion contains 60 to 80 percent by mass of the hydrocarbon-based material, 0.1 to 1 percent by mass of the surfactant, and the balance being substantially water.  
           [0015]    In another aspect of the present invention, a method for making surface-coated reduced iron includes the step of coating the surface of the reduced iron with a tar emulsion.  
           [0016]    In the method for making surface-coated reduced iron, preferably, the aforementioned tar emulsion contains at least one hydrocarbon-based material selected from the group consisting of natural petroleum tar, coal tar, pitch, asphalt, liquefied coal, and residual oil from petroleum refining; a surfactant; and water.  
           [0017]    In the method for making surface-coated reduced iron, preferably, the aforementioned tar emulsion contains 60 to 80 percent by mass of the hydrocarbon-based material, 0.1 to 1 percent by mass of the surfactant, and the balance being substantially water.  
           [0018]    In the method for making surface-coated reduced iron, preferably, the reduced iron is produced by reduction with a gaseous reductant containing hydrogen and/or carbon monoxide or a solid reductant containing a carbonaceous material.  
           [0019]    In the method for making surface-coated reduced iron, preferably, the coating step is performed by controlling the temperature of the reduced iron at 250° C. or less.  
           [0020]    In another aspect of the present invention, in a method for melting the surface-coated reduced iron including surface-coated reduced iron in the form of powder and/or chips in a smelting furnace to produce molten iron, the surface of the reduced iron being coated with a tar emulsion, the method includes the step of injecting the surface-coated reduced iron in the form of powder and/or chips into a molten iron bath formed in the smelting furnace and/or into a slag formed on the molten iron bath.  
           [0021]    In the method for melting the surface-coated reduced iron, preferably, the injecting step is performed using a gaseous medium.  
           [0022]    As described above, the present invention is characterized by that in order to prevent the oxidation of reduced iron, the surface thereof is coated with a tar emulsion (emulsified tar).  
           [0023]    The tar emulsion is produced by adding a small amount of the surfactant and an appropriate amount of water to the hydrocarbon-based material, such as natural petroleum tar, coal tar, pitch, asphalt, liquefied coal, and residual oil from petroleum refining, followed by suspending (emulsifying) the hydrocarbon-based material. The viscosity of the tar emulsion is significantly lower than the viscosity of the hydrocarbon-based material itself. Therefore, the tar emulsion forms homogeneous and flawless protective films even if the film thickness is smaller compared to the graphite and other inorganic films described above. Moreover, the tar emulsion enters the numerous open pores of spongy reduced iron or the like and forms films on the inner surfaces of the open pores, thereby effectively preventing the surface oxidation of the reduced iron. When the organic film-forming material described above is used, the material must be dissolved or dispersed in a solvent to decrease the viscosity. However, since the tar emulsion contains the surfactant and water, handling is significantly facilitated, and the costs are also greatly reduced. As the tar emulsion, any one of the hydrocarbon-based materials described above may be used, and as necessary, two or more of these materials may be used. In order to decrease the viscosity of the tar emulsion and in order to perform the emulsifying step while the content of the hydrocarbon-based material is kept high to a certain extent, preferably, the tar emulsion contains 60 to 80 percent by mass of the hydrocarbon-based material, 0.1 to 1 percent by mass of the surfactant, and the balance being substantially water.  
           [0024]    The reduced iron to be coated with the tar emulsion is not particularly limited to its production process. For example, the reduced iron may be produced by being reduced by a gaseous reductant containing hydrogen and/or carbon monoxide in a shaft reduction furnace, a fixed bed reduction furnace, a fluidized bed reduction furnace, or the like. Alternatively, the reduced iron may be produced by being reduced by a solid reductant containing a carbonaceous material in a rotary hearth reduction furnace, a rotary kiln reduction furnace, or the like. The form and/or shape of the reduced iron are not particularly limited. For example, the reduced iron may be in the form of pellets, lumps, sheets, and the like, as it is reduced, or may be in the form of briquettes formed by hot pressing (hot briquetting) after reduction, or may be in the form of powder and chips generated by handling during or after reduction. A mixture of two or more of these forms may be acceptable.  
           [0025]    Since the tar emulsion has a sufficiently low viscosity at ambient temperatures and can be thinly and homogeneously applied to the surface of the reduced iron as it is, any conventional method may be applied to coat the surface of the reduced iron with the tar emulsion. For example, the reduced iron may be immersed in the tar emulsion, or the reduced iron may be sprayed with the tar emulsion. After the application of the tar emulsion, the water in the tar emulsion may be removed by drying by heating (with heat), solar drying, or the like, as desired.  
           [0026]    When the surface of the reduced iron is coated with the tar emulsion, the temperature of the reduced iron is controlled, preferably, at 250° C. or less, and more preferably, between 100 and 250° C. By controlling the temperature of the reduced iron at 250° C. or less, the water in the coated tar emulsion is not immediately removed by vaporization due to the sensible heat of the reduced iron, and the viscosity of the tar emulsion is kept low for a certain period of time, and thereby the surface of the reduced iron is easily coated with the tar emulsion thinly and homogeneously during that period. By controlling the temperature of the reduced iron at 100° C. or more, after the reduced iron is coated with the tar emulsion, the water in the tar emulsion is entirely or partially removed by vaporization, and thereby drying by heating (with heat), solar drying, or the like is not necessary after coating, or the time of drying is shortened. As a result, the cost of drying facilities can be minimized and the area of the yard for solar drying can be decreased.  
           [0027]    With respect to the reduced iron of which surface is coated with the hydrocarbon-based material after the water is removed as described above, homogeneous and flawless protective films are formed even if the thickness is small, the inner surfaces of many open pores are also coated, and thereby the surface oxidation of the reduced iron is significantly and effectively prevented.  
           [0028]    The amount of hydrocarbon-based material in the tar emulsion to be deposited may be appropriately adjusted depending on the open porosity of the reduced iron, the temperature conditions during transportation and storage, etc. The amount of hydrocarbon-based material to be deposited is generally in the range of 0.01 to 10 percent by mass of the reduced iron, and more generally, in the range of 0.1 to 5 percent by mass. That is, if the amount of coating is insufficient, the function as the oxidation-preventing film is not exhibited sufficiently, and when the amount of coating is excessive, the oxidation-preventing effect is not increased, resulting in heat loss in the step of melting the reduced iron or the large amount of pyrolytic gas generation.  
           [0029]    When the surface-coated reduced iron is melted in a smelting furnace, such as a converter or an electric furnace, to produce molten iron (e.g., hot pig iron, and molten steel), the film of the hydrocarbon-based materials is easily pyrolyzed and the carbon residue contributes to the reduction of the residual iron oxides in the reduced iron and to the carburization of the molten iron. On the other hand, the pyrolytic gas generated is removed to exhaust gas, thus not adversely affecting the composition of the molten iron. Since the film is electrically conductive, melting with an electric furnace or the like is not adversely affected.  
           [0030]    When the surface-coated reduced iron contains the iron in the form of powder and chips, the portion of powder and chips is melted preferably by being injected into the molten iron bath or into the slag formed on the molten iron bath. Thereby, it is possible to prevent the fly loss of dust into the exhaust gas, which occurs when the entire feed material is fed by gravity from the top of the smelting furnace as is commonly performed, thus improving the yield of iron. In addition, CO gas, which is generated by the reduction of the iron oxides by the carbon residue, stirs the iron bath and creates foamy slag so as to accelerate the melting greatly, resulting in a significant reduction in the melting time. The portion of powder and chips is preferably injected by a gaseous medium via lances and tuyeres provided on the smelting furnace. Thereby, since the gaseous medium is added to the CO gas generated by the reduction, stirring of the molten iron and foaming of the slag are further activated. As the gaseous medium, an inert gas, such as N 2  or Ar, or a reducing gas, such as CO, H 2 , CH 4 , or a mixture of these gases, is preferably used. Thereby, the molten iron is prevented from being decarburized and the melting is accelerated. In addition, since heat (electric power) required for melting is greatly reduced, the production cost for molten iron (e.g., hot pig iron, and molten steel) can be reduced.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 is a process chart showing the steps of producing surface-coated reduced iron in an embodiment of the present invention; and  
         [0032]    [0032]FIG. 2 is a graph showing a change in the metallization over time with respect to the individual examples of reduced iron in a weather resistance test. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    [0033]FIG. 1 is a process chart showing the steps of producing surface-coated reduced iron in an embodiment of the present invention. As shown in FIG. 1, in step  1 , reduced iron A in the form of pellets or the like is produced by reduction in a shaft reduction furnace  1   a  or a rotary hearth controlling furnace  1   b . The reduced iron A may be subjected to hot pressing (hot briquetting) to form briquettes in step  2 . In step  3 , the reduced iron A, in the form of pellets or the like as produced or in the form of briquettes, is cooled to 250° C. or less by injection of inert gas, indirect water cooling, direct water spraying, or the like. In step  1 , reduced iron in the form of powder may be produced using a fluidized bed controlling furnace (not shown in the drawing) instead of the shaft controlling furnace  1   a  or the rotary hearth controlling furnace  1   b , and the reduced iron in the form of powder may be formed into briquettes by hot pressing (hot briquetting) in step  2 .  
         [0034]    The reduced iron A in the form of pellets or the like, or in the form of briquettes is transferred to step  4 , in which the surface of the reduced iron A is coated with a tar emulsion B. In step  4 , for example, as shown in FIG. 1, a narrow container  4   a  provided with a screw feeder  4   b  is filled with the tar emulsion B in an appropriate quantity, the reduced iron A is continuously fed from the inlet into the container  4   a  and is immersed in the tar emulsion B for a predetermined period by the screw feeder  4   b , and the reduced iron A is then discharged from the outlet of the container  4   a . As the tar emulsion B, for example, an emulsified natural petroleum tar, which is commercially available as fuel for thermal powder generation, containing approximately 70 percent by mass of a natural petroleum tar, 0.3 to 0.5 percent by mass of a surfactant, and the balance being substantially water, may be used. As necessary, an appropriate amount of water may be added thereto to adjust the concentration of the hydrocarbon-based material.  
         [0035]    By appropriately changing the concentration of the tar emulsion B, the temperature of the reduced iron A to be immersed into the tar emulsion B, the immersing time, etc., it is possible to adjust the amount of the tar emulsion B to be deposited on the reduced iron A.  
         [0036]    Additionally, if reduced iron A cooled to 250° C. or less is used, instead of the immersion method described above, a method may be used in which the tar emulsion A is sprayed on the reduced iron A while the reduced iron A is moved by a belt conveyor. In such a method, the amount of the tar emulsion B to be deposited on the reduced iron A can be adjusted by the concentration of the tar emulsion B, the temperature of the reduced iron, the spraying time, etc.  
         [0037]    Additionally, step  3  (reduced iron-cooling step) and step  4  (tar emulsion-applying step) are not necessarily performed continuously. The reduced iron A may be completely cooled to ambient temperature in step  3 , and after storing for a certain period or after transportation, coating treatment may be performed in step  4 .  
         [0038]    Next, reduced iron coated with the tar emulsion (surface-coated reduced iron) A 1  is transferred to step  5  to dry/remove the water in the tar emulsion. In step  5 , for example, the surface-coated reduced iron A 1  is placed in a layer on a moving, endless grate, and a drying gas of which temperature is set at 90 to 200° C. is passed through the layer of the surface-coated reduced iron A 1 . As the drying gas, for example, an exhaust gas produced by burning fuel, such as natural gas, heavy oil, or pulverized coal, may be used. Alternatively, exhaust gas from the reducing furnace in step  1  may be used by mixing with air and adjusting the temperature of the mixture. Additionally, by controlling its cooling temperature in step  3  at a highest possible temperature below 250° C., since the water in the tar emulsion B can be dried/removed by the sensible heat of the reduced iron A when the surface of the reduced iron A is coated with the tar emulsion B in step  4 , step  5  may be omitted or reduced.  
         [0039]    As described above, by removing the water by drying from the coated tar emulsion, the hydrocarbon-based materials, such as a natural petroleum tar, remain on the surface of the reduced iron to form a homogeneous protective film, and thereby even if the surface-coated reduced iron is stored and transported in the air for a long period of time, reoxidation can be prevented.  
         [0040]    The reduced iron A is subjected to various types of mechanical handling as the reduced iron A is transported from step  1  to step  5  in that order, and therefore the parts of the reduced iron A are broken to form power and chips. In the conventional reduced iron without surface coating, the powder and chips are easily reoxidized due to the large specific surface areas. Consequently, the metallization is decreased, and the quality is deteriorated. Heat generation by the oxidation may also result in ignition and firing. Therefore, the powder and chips must be removed by sieving or the like before storage and transportation, resulting in a decrease in the yield of reduced iron in the conventional method. In addition, the unit and apparatus for reusing the removed powder and chips must also be prepared separately. In contrast, in the present invention, the entire amount of reduced iron can be transferred to the tar emulsion-coating step without sieving. That is, although the surface-coated reduced iron produced through the tar emulsion-coating step contains powder and chips in addition to lumpy shaped reduced iron, such as pellets and briquettes, since the surfaces of the powder and the chips are also densely coated, their reoxidation is prevented, and the problems associated with the conventional reduced iron are not arisen.  
         [0041]    When the surface-coated reduced iron is melted in a smelting furnace, such as a converter or an electric furnace, to produce hot pig iron and molten steel, although the entire coated reduced iron together with powder and chips may be charged or poured by gravity from the top of the smelting furnace, a large amount of fly dust loss may be generated, the same as the conventional method. Therefore, preferably, by sieving out the powder and chips, only lumpy reduced iron is fed by gravity from the top of the smelting furnace, and the powder and chips are separately injected into the iron bath and the slag layer retained in the smelting furnace. Specifically, for example, a predetermined amount of powder and chips is discharged from a storage bin for storing the powder and chips, and is transferred to an injection hopper. The powder and chips are then directly injected into the iron bath or the slag layer via injection lances immersed in the iron bath or the slag layer or through tuyeres provided on the side wall or bottom of the smelting furnace, using high-pressure N 2  or high-pressure reduction gas which is produced by cooling a part of reduction gas for the reducing furnace, followed by pressurization. Alternatively, the powder and chips may be injected into the slag layer via injection lances provided with injection ports above the slag layer. By the direct injection into the iron bath or the slag layer or by the high-speed injection, even the powder is effectively caught by the iron bath and the slag layer, and thereby the fly loss of dust is significantly decreased. As described above, the surface-coated reduced iron in the form of powder and chips thus injected into the iron bath or the slag layer stirs the iron bath and foams the slag, and the melting rate of the reduced iron is greatly accelerated, resulting in a significant reduction in the melting time, thus significantly increasing the productivity.  
         [0042]    The present invention will be described in more detail based on the examples.  
       EXAMPLE 1  
       [0043]    In order to confirm the reoxidation-preventing effect in accordance with the present invention, with respect to reduced iron of which surface was coated with an emulsified natural petroleum tar (Example 1 of the present invention), reduced iron of which surface was coated with an organic film-forming material (Comparative Example 1), and untreated reduced iron without surface coating (Comparative Example 2), a weather resistance test was conducted under the same conditions.  
         [0044]    As the emulsified natural petroleum tar, Orimulsion (registered trademark) manufactured by Bitumenes Orinoco, S.A. was used. The emulsified natural petroleum tar contained, in percent by mass, 71±1% of a natural petroleum tar, 0.3 to 0.5% of a surfactant, and the balance being water, and had the following composition determined by elementary analysis: 59.0 to 60.5% of C, 7.2 to 7.8% of H, 0.43 to 0.56% of N, and 2.1 to 2.9% of S. The viscosity of the emulsified natural petroleum tar was 0.9 Pa·s or less at 30° C. In a container filled with the emulsified natural petroleum tar, reduced iron pellets (with an average particle size of 12 mm; powder and chips with a particle size of 6 mm or less being removed beforehand) produced with a shaft reduction furnace and cooled to ambient temperature was immersed. The surface-coated reduced iron pellets were collected from the container and kept for 10 minutes in a drier of which temperature was set at 90° C. to dry and remove the water. The amount of the hydrocarbon-based materials deposited after drying was approximately 3 percent by mass of the reduced iron.  
         [0045]    On the other hand, with respect to surface coating using the organic film-forming material, reduced iron pellets prepared in the same manner as described above were immersed in a solution prepared by adding 50 parts by mass of pitch to 50 parts by mass of kerosene and dissolving the pitch homogeneously. The surface-coated reduced iron pellets were collected from the solution and drying was performed under reduced pressure to remove the kerosene by vaporization. The amount of the organic film-forming material deposited was approximately 3 percent by mass of the reduced iron.  
         [0046]    In order to perform the weather resistance test under the same conditions as the actual long-term storage conditions, the individual test materials (reduced iron pellets) were stored in outdoor and were exposed to sunshine and rain at a reduced iron manufacturing plant in Venezuela. The test materials were placed in one layer in a bat provided with drain holes on the bottom. As for the climatic condition, the average annual precipitation was 911 mm, the average humidity was 76%, and the temperature was 17 to 41° C. A sample in a small quantity was taken from each test material every 5 days, and the total iron (T. Fe) and the metallic iron (M. Fe) were determined by chemical analysis. The metallization (M. Fe/T. Fe×100%) was calculated to find a change in the metallization over time for each test material. The results thereof are shown in FIG. 2. As is obvious from FIG. 2, with respect to the reduced iron pellets of which surfaces were coated with the organic film-forming material in Comparative Example 1, the decrease in the metallization was smaller and an reoxidation-preventing effect was exhibited compared to the untreated reduced iron pellets in Comparative Example 2. However, with respect to the reduced iron of which surface was coated with the emulsified natural petroleum tar in Example 1 of the present invention, the decrease in the metallization was much smaller and a larger reoxidation-preventing effect was exhibited. As is also clear from FIG. 2, compared to the initial metallization of 92% in the individual test materials at the start of the weather resistance test, after 30 days, the metallization of Comparative Example 2 decreased to 62%, and the metallization of Comparative Example 1 decreased to 78%, while Example 1 of the present invention retained the metallization of 87% which was sufficient for use in a smelting furnace in the subsequent step.  
       EXAMPLE 2  
       [0047]    In order to confirm the effect of the method for melting the surface-coated reduced iron in accordance with the present invention, a melting test for surface-coated reduced iron was performed using an electric arc furnace as the smelting furnace.  
         [0048]    In a container filled with the same emulsified natural petroleum tar as that used in Example 1 of the present invention, reduced iron containing powder and chips, which was produced in a shaft reduction furnace and cooled to ambient temperature was immersed. The surface-coated reduced iron was collected from the container and kept for 10 minutes in a drier of which temperature was set at 90° C. to dry and remove the water, and thus Sample 1 was prepared. A half of Sample 1 was separated by sieving into reduced iron in the form of powder and chips having a particle size of less than 6 mm and reduced iron in the form of lumps (mainly pellets) having a particle size of 6 mm or more, and thus sample 2 was prepared.  
         [0049]    An iron bath and a slag layer were preliminarily formed in the electric arc furnace. With respect to the case in which the reduced iron of Sample 1 which was not sieved was continuously fed by gravity into the electric arc furnace from above the iron bath (Comparative Example 3) and the case in which reduced iron lumps having the particle size of 6 mm or more were continuously fed by gravity from above the iron bath and the powder and chips having the particle size of less than 6 mm were continuously injected by N 2  gas through injection lances immersed in the iron bath (Example 2 of the present invention), the melting time and the power consumption required for melting the entire amount of the samples were measured.  
         [0050]    As a result, it has been confirmed that the melting time and the power consumption for Example 2 of the present invention were significantly decreased compared to Comparative Example 3. Additionally, by visual observation, it has been confirmed that the fly loss of dust in the melting method in Example 2 of the present invention was significantly decreased compared to the melting method in Comparative Example 3.