Patent Publication Number: US-2016237514-A1

Title: Method for manufacturing iron nuggets

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing iron nuggets by heating agglomerates including iron oxide, for example, iron ore, and a carbon-containing reducing agent (also referred to hereinbelow as “carbonaceous reducing agent”) and reducing and melting the iron oxide contained in the agglomerates. 
     BACKGROUND ART 
     For example, the technique disclosed in Patent Literature 1 is known as a method for manufacturing iron nuggets by heating agglomerates including iron oxide and a carbonaceous reducing agent. This literature indicates that in a method for manufacturing a solid metal product from carbon including a metal-bearing compound, the surface of a molded product including carbon and the metal-bearing compound is coated with a treatment substance and the covered molded product is supplied on a furnace hearth and heated. It is also indicated that the coating layer includes a carbonaceous compound. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: U.S. Pat. No. 6,214,087 
     SUMMARY OF INVENTION 
     The agglomerates charged on the hearth of a movable-bed heating furnace are heated by radiation heat or gas heat transfer created by a heating burner provided in the furnace, and iron oxide contained in the agglomerates is reduced by the carbonaceous reducing agent, thereby generating iron nuggets. However, where a heating burner is used as a heating means, a flow of atmosphere gas is created inside the furnace. Since the atmosphere gas includes oxidizing gases such as carbon dioxide and water vapors, the reduced iron obtained by heating and reducing the agglomerates and the iron nuggets obtained by melting and aggregating the reduced iron can be re-oxidized by the oxidizing gas. Where the reduced iron or iron nuggets is re-oxidized, the amount of FeO in the slag, which is formed as a byproduct during the generation of the reduced iron, increases and the ratio of sulfur amount (S) in the slag to the sulfur amount [S] in the iron nuggets (hereinafter, referred to as sulfur distribution ratio and represented by (S)/[S]). Where the amount of sulfur in the iron nuggets increases, the quality of the iron nuggets is degraded. Further, FeO in the slag reacts with carbon [C] contained in the generated semi-molten iron and molten iron, thereby causing decarburization and reducing the amount of carbon in the iron nuggets. Following the decarburization reaction, a large number of fine CO gas bubbles are present in the slag, thereby causing significant expansion. As a result, intense slag foaming occurs and the foam covers the iron nuggets in a semi-molten state and molten state which is being aggregated. The resultant problem is that heat supplied from the upper section of the heating furnace is shielded, the reaction time is greatly increased, and productivity is decreased. Yet another problem is that where slag foaming is initiated, the iron nuggets assume an irregular shape, the iron nuggets are insufficiently separated from part of the slag, and iron nuggets quality is degraded. 
     The present invention has been created with the foregoing in view and it is an objective thereof to provide a method for manufacturing iron nuggets by which reduced iron obtained by heating and reducing agglomerates, or iron nuggets obtained by melting and aggregating the reduced iron can be prevented from reoxidation inside a movable hearth heating furnace and quality of the reduced iron can be improved. 
     In order to resolve the above-described problems, the present invention provides a method for manufacturing iron nuggets by charging and heating agglomerates including iron oxide and a carbonaceous reducing agent on a hearth of a movable hearth heating furnace, reducing and melting the iron oxide in the agglomerates, and then discharging obtained iron nuggets to the outside of the furnace and recovering the iron nuggets, wherein the agglomerates have a coating layer, including a fluid carbonaceous material, on the surface. 
     At least one selected from the group consisting of bituminous coal, subbituminous coal, and lignite can be used as the coal material. The average thickness of the coating layer is preferably greater than 0.30 mm. 
     The agglomerates are obtained by agglomerating a mixture including iron oxide and the carbonaceous reducing agent in a first pelletizer to form core portions, and then forming the coating layer including a fluid carbonaceous material in a second pelletizer on the surface of the obtained core portions. 
     It is preferred that a top portion of the coating layer be not lower than a top portion of the iron nuggets while the agglomerates are heated. 
     It is preferred that the coating layer become a shell-shaped coke while the agglomerates are heated. The agglomerates are preferably charged to form a single layer on the furnace hearth. It is preferred that the carbonaceous reducing agent be placed on the furnace hearth before the agglomerates are charged on the furnace hearth. 
     It is preferred that a C amount in the iron nuggets be 2.5 mass % or more. It is preferred that an S amount in the iron nuggets be 0.120 mass % or less. 
     According to the present invention, when iron nuggets are manufactured by heating, reducing, and melting the iron oxide contained in agglomerates, the agglomerates are used which have a coating layer including a fluid carbonaceous material on the surface of core portions including iron oxide and a carbonaceous reducing agent. Therefore, while the agglomerates are heated, the coating layer is swelled and modified, the so-called coking proceeds, and a petal-like shell-shaped coke is formed. The shell-shaped coke acts as a windbreak wall that prevents the atmosphere gas from oxidizing the core portions and protects the core portions. As a result, the reoxidation of the reduced iron obtained by heating and reducing the agglomerates, or the iron nuggets obtained as a result of melting and aggregating the reduced iron is suppressed and the increase in the FeO amount in a slag which is a byproduct generated in the production of the iron nuggets is suppressed. Therefore, the amount of sulfur contained in the iron nuggets can be reduced and the quality of the reduced iron can be increased. Furthermore, since the amount of FeO in the slag does not increase, the decarburization of carbon [C] contained in the produced semi-molten iron and molten iron can be suppressed and the amount of carbon in the iron nuggets can be increased. In addition intense slag foaming can be also prevented. Therefore, the generation of iron nuggets of irregular shape can be prevented, the separation of the iron nuggets and slag is improved, and the iron nuggets quality can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the process of heating the agglomerates charged on the hearth of the heating furnace. 
         FIG. 2  is a schematic diagram illustrating the stage ( 4 ) in  FIG. 1  in greater detail. 
         FIGS. 3 ( 1 ) to  3 ( 3 ) are photographs of agglomerates taken at the time of actual heating of the agglomerates in the heating furnace. 
         FIG. 4 ( 1 ) is a photograph, taken under an optical microscope, of the cross section of the reduced iron recovered in the later phase of solid reduction.  FIG. 4 ( 2 ) is a photograph obtained by image processing of the photograph in  FIG. 4 ( 1 ). 
         FIG. 5 ( 1 ) is a photograph, taken under an optical microscope, of the cross section of the reduced iron recovered immediately prior to melting and aggregation.  FIG. 5 ( 2 ) is a photograph obtained by image processing of the photograph in  FIG. 5 ( 1 ). 
         FIG. 6 ( 1 ) is a photograph illustrating a state in which the iron nuggets after the completion of melting and aggregation are covered by intensely foamed slag when the agglomerates of the related art which have no coating layer are heated.  FIG. 6 ( 2 ) is a photograph of the recovered iron nuggets.  FIG. 6 ( 3 ) is a photograph of the recovered slag. 
         FIG. 7 ( 1 ) is a photograph illustrating the state, after the completion of melting and aggregation, of agglomerates obtained by forming a coating layer including a fluid carbonaceous material on the surface of the core portion.  FIG. 7 ( 2 ) is a photograph of the recovered iron nuggets.  FIG. 7 ( 3 ) is a photograph of the recovered slag. 
         FIGS. 8 ( 1 ) to  8 ( 4 ) are schematic diagrams illustrating the cross section of the petal-like shell-shaped coke formed while heating the agglomerates when the thickness of the coating layer was changed. 
         FIG. 9 ( 1 ) is a photograph taken immediately after the heating and reduction treatment of No. 4 in Table 5.  FIG. 9 ( 2 ) is a photograph taken immediately after the heating and reduction treatment of No. 5 in Table 5.  FIG. 9 ( 3 ) is a photograph taken immediately after the heating and reduction treatment of No. 6 in Table 5. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The inventors have conducted a comprehensive research aimed at the prevention of the reoxidation of iron nuggets obtained by heating, reducing, and melting iron oxide contained in agglomerates, and at the quality improvement of iron nuggets. In particular, the research was focused on reducing the amount of sulfur contained in the iron nuggets, increasing the amount of carbon contained in the iron nuggets, preventing the occurrence of iron nuggets of irregular shape, and improving the separation of the iron nuggets and slag. The results obtained have demonstrated that the abovementioned problems can be resolved by using agglomerates having a coating layer including a fluid carbonaceous material on the surface of core portions including iron oxide and a carbonaceous reducing agent. This finding led to the creation of the present invention. 
     Initially, the mechanism preventing the reoxidation of iron nuggets and improving the iron nuggets quality in the method for manufacturing iron nuggets in accordance with the present invention will be explained with reference to the drawings.  FIG. 1  is a schematic diagram illustrating the process of heating the agglomerates charged on the hearth of the heating furnace.  FIG. 2  is a schematic diagram illustrating the stage ( 4 ) in  FIG. 1  in greater detail.  FIG. 3 ( 1 ) is a photograph illustrating the swelling and coking state of the coating layer immediately after the agglomerates have been charged into the heating furnace.  FIGS. 3 ( 2 ) and  3 ( 3 ) are photographs of the petal-like shell-shaped coke, iron nuggets inside thereof, and slag grains, the photographs being taken after the agglomerates having a coating layer on the surface of core portions have been heated, melted, and aggregated. 
     In the manufacturing method of the present invention, the agglomerates having a coating layer including a fluid carbonaceous material on the surface of core portions including iron oxide and a carbonaceous reducing agent are charged onto the hearth of a movable hearth heating furnace and heated. The schematic diagram of the agglomerates charged into the heating furnace is depicted in  FIG. 1 ( 1 ). In  FIG. 1 ( 1 ), the reference numeral  1  stands for a core portion,  2 —a coating layer, and  3 —an agglomerate. 
     The core portion  1  includes iron oxide and a carbonaceous reducing agent and may optionally include a flux or a binder. The composition of the core portion  1  is the same as in the related art and will be described hereinbelow in detail. 
     The coating layer  2  includes a fluid carbonaceous material and may optionally include a binder. The composition and thickness of the coating layer  2  will be described hereinbelow in detail. 
     Inside the movable hearth heating furnace, the temperature is usually raised and held at about 1350° C. to 1550° C. with a heating burner. Where the agglomerate  3  is charged on the hearth of the heating furnace, the agglomerate  3  is heated by gas heat transfer and radiation heat created by the heating burner. At this time, the coating layer  2  is fluidized, swells as a whole, as depicted in  FIG. 1 ( 2 ), rapidly solidifies, and forms a shell-shaped coke. Cracks are initiated at the top portion of the shell-shaped coke, but the coke is continuous as a whole and forms a shell-shaped spherical body. 
     The state in which the coating layer  2  is swelled by heating and cracks are initiated in the coating layer  2  in the top portion of the agglomerate is depicted in  FIG. 3 ( 1 ). As indicted in  FIG. 3 ( 1 ), a large number of cracks have appeared in the coating layer  2 , but it is continuous as a whole and forms a shell-shaped spherical body. Since the shell-shaped spherical body is constituted by solid coke, it excels in thermal conductivity. Therefore, where the shell-shaped spherical body is heated by radiation heat inside the heating furnace, the core portion  1  is also heated by heat transfer. 
     Where the heating is continued, as depicted in  FIG. 1 ( 3 ), the reduction of iron oxide under the effect of the carbonaceous reducing agent proceeds in the core portion  1  and solid reduced iron is formed. At this time, the reduction of iron oxide constituting the core portion  1  proceeds from the top side of the core portion  1  and reduced iron  4  is generated. 
     Where the heating is further continued, as depicted in  FIG. 1 ( 4 ), iron oxide constituting the core portion  1  is sufficiently reduced and separated into iron nuggets  6  constituted by reduced iron and slag  7  produced as a byproduct when the iron nuggets  6  are generated. The state assumed at this time is depicted in  FIG. 3 ( 3 ). 
     Meanwhile, in the coating layer  2  that covers the core portion  1 , as depicted in  FIG. 1 ( 3 ) and  FIG. 1 ( 4 ), the shell-shaped spherical body is formed around the core portion  1 , and this coating layer  2  is gradually oxidized, consumed, and thinned by the oxidizing gas included in the atmosphere gas. At this time, the top portion of the coating layer  2  is oxidized and consumed faster than the bottom portion and is gradually lost. Therefore, when the thickness of the coating layer decreases, an opening is formed in the top portion, as depicted in  FIG. 1 ( 3 ) and  FIG. 1 ( 4 ). The photograph taken at this time is presented in  FIG. 3 ( 2 ). As depicted in  FIG. 3 ( 2 ), the shell-shaped coke formed by the coating layer  2  in which the opening has been formed in the top portion has assumed a petal-like shape. Where the coating layer becomes thicker, the formed shell-shaped coke also becomes thicker, the upper portion of the shell-shaped coke is not opened, and the reaction is completed in a state in which the core portion is enclosed in the coke. Therefore, it is clear that a more effective action is produced to prevent the reoxidation induced by the atmosphere gas. Both the case in which the upper portion of the shell-shaped coke is open and the case in which it is not open are included in the scope of the present invention. 
     As depicted in  FIG. 1 ( 4 ), the shell-shaped coke which is derived from the coating layer  2  and in which an opening is formed in the top portion is formed around the iron nuggets  6  such as to enclose the iron nuggets  6 . Therefore, the shell-shaped coke acts to prevent the reduced iron, which has been obtained by heating and reducing the core portion, and the iron nuggets, which has been produced by melting and aggregating the reduced iron, from reoxidation by the atmosphere gas inside the heating furnace. This action will be explained hereinbelow in detail with reference to  FIG. 2 . 
     The flow of the atmosphere gas is shown by arrows in  FIG. 2 . With the manufacturing method in accordance with the present invention, while the solid reduction of iron oxide contained in the core portion  1  is completed and the melting and aggregation advance, as depicted in  FIG. 2 , the shell-shaped coke derived from the coating layer  2  is formed such as to enclose the reduced iron, which has been obtained by heating and reducing the core portion, or the iron nuggets, which has been produced by melting and aggregating the reduced iron  6 , and the slag  7 . Therefore, the atmosphere gas in the heating furnace is unlikely to come into direct contact with the reduced iron, which has been obtained by heating and reducing the agglomerates in which the coating layer is present on the surface of the core portions, or the iron nuggets  6  produced by melting and aggregating the reduced iron. Further, the atmosphere gas includes carbon dioxide gas (CO 2  gas) and moisture (H 2 O), and where the carbon dioxide gas comes into contact with the shell-shaped coke derived from the coating layer  2 , the carbon dioxide gas is reduced by the shell-shaped coke and, as represented by Equation (1) below, carbon monoxide gas (CO gas) is generated. Further, where the moisture contained in the atmosphere gas comes into contact with the shell-shaped coke derived from the coating layer  2 , the moisture is reduced by the shell-shaped coke and, as represented by Equation (2) below, hydrogen gas (H 2 ) and carbon monoxide gas (CO gas) are generated. As a result, the reduction degree RD of the atmosphere gas on the periphery of the shell-shaped coke derived from the coating layer  2  increases, thereby preventing the reoxidation of the reduced iron, which has been obtained by heating and reducing the agglomerates in which the coating layer is present on the surface of the core portions, or the iron nuggets  6  produced by melting and aggregating the reduced iron. The reduction degree RD of the atmosphere gas is determined by Equation (3) below. 
       CO 2 +C=2CO  (1)
 
       H 2 O+C=H 2 +CO  (2)
 
       RD=[(CO+H 2 )/(CO+H 2 +CO 2 +H 2 O)]×100  (3)
 
     According to the manufacturing method of the present invention, while the agglomerates  2  in which the coating layer is present on the surface of the core portions are heated, the reduced iron, which has been obtained by heating and reducing the agglomerates, or the iron nuggets, which have been produced by melting and aggregating the reduced iron, is sufficiently protected from an oxidizing gas by the shell-shaped coke derived from the coating layer  2 , and the reoxidation of the reduced iron or iron nuggets can be prevented. While the agglomerates are heated, the shell-shaped coke derived from the coating layer  2  assumes a petal-like shape, and although the height of the shell-shaped coke derived from the coating layer  2  is not constant and part of the coke is lost, the effect of the present invention still can be obtained. 
     By contrast, where only the core portions  1  which do not have the coating layer including a fluid carbonaceous material is heated in a heating furnace, as in the conventional process, although the reaction of the core portions  1  as a whole advances in the solid reduction phase, since the core portions  1  themselves are directly exposed to the atmosphere gas, the reduced iron, which has been obtained by heating and reducing the core portions  1 , or the iron nuggets, which has been produced melting and aggregating the reduced iron, is partially re-oxidized by the oxidizing gas contained in the atmosphere gas. 
     The photos of the reduced iron obtained by heating only the core portion  1  which does not have the coating layer including a fluid carbonaceous material are shown in  FIGS. 4 and 5 .  FIG. 4  is a photograph, taken under an optical microscope, of the cross section of the reduced iron recovered in the later phase of solid reduction.  FIG. 5  is a photograph, taken under an optical microscope, of the cross section of the reduced iron recovered immediately prior to melting and aggregation. Further, in  FIGS. 4 and 5 , ( 1 ) are microscopic photographs of the cross sections, and ( 2 ) are schematic diagrams which show, by shading, the reduced portions and re-oxidized portions in the cross section depicted in ( 1 ). 
     As depicted in  FIGS. 4 ( 2 ) and  5 ( 2 ), it is clear that in the upper portion of the reduced iron, part of the generated metallic iron is re-oxidized into FeO. 
     The FeO generated by the reoxidation is rapidly melted in the slag which is separated and generated in the melting and aggregation phase, thereby increasing the concentration of FeO in the slag. Further, when the FeO is melted in the slag, it reacts with carbon [C]contained in the generated semi-molten iron and molten iron, thereby causing decarburization. Therefore, a large number of fine CO gas bubbles are present in the slag and the slag is greatly expanded. As a result, intense slag foaming is induced and the foam covers the semi-molten and molten iron nuggets which is in the process of aggregation. The resultant problem is that heat supplied from the upper section of the heating furnace is blocked, the reaction time increases, and the productivity decreases. Yet another problem arising when slag foaming occurs is that the iron nuggets assume an irregular shape, the iron nuggets and part of the slag cannot be sufficiently separated, and quality of the iron nuggets is degraded. The oxidizing gas is generated by combustion with the combustion burner which is used for heating in the heating furnace, combustion of the combustible gas generated by the reduction reaction, and leakage of the air from the outside into the heating furnace. 
     Further, where agglomerates are used which have the coating layer including a fluid carbonaceous material, as in the manufacturing method of the present invention, since the reduced iron, which has been obtained by heating and reducing the agglomerates, or the iron nuggets, which has been produced by melting and aggregating the reduced iron, is prevented from reoxidation, and the iron nuggets and slag are individually melted and fused and separated from each other in the melting and aggregation phase. As a result, slag foaming is not induced. 
     As explained hereinabove, the most important feature of the manufacturing method of the invention is that agglomerates are used which have a coating layer including a fluid carbonaceous material on the surface of core portions including iron oxide and a carbonaceous reducing agent. 
     The fluid carbonaceous material, as referred to herein, means a carbonaceous material demonstrating thermal softening ability at 350° C. to 400° C. The “carbonaceous material demonstrating thermal softening ability”, as referred to herein, means a carbonaceous material with a softening and melting point of 350° C. to 400° C., when the softening and melting point of carbonaceous material is measured by the method specified by ISO 10329 (2009). 
     It is preferred that at least one coal selected from the group consisting of fluid bituminous coal, fluid sub-bituminous coal, and fluid brown coal be used as the fluid carbonaceous material, and two or more of the same may be used. Among those carbonaceous materials, bituminous coal is more preferred. Carbonaceous materials also include anthracite, but anthracite is not fluid. Therefore, even when anthracite is included in the coating layer  2 , a shell-shaped spherical body is not formed around the iron nuggets. Therefore, the core portion is exposed to the atmosphere gas in the heating furnace, and the reduced iron, which has been obtained by heating and reducing the agglomerates, or the iron nuggets, which has been produced by melting and aggregating the reduced iron, is re-oxidized. 
     The average thickness of the coating layer  2  is not particularly limited, but the thickness, for example, more than 0.30 mm is preferred. Where the average thickness of the coating layer  2  is more than 0.30 mm, the effect of suppressing the reoxidation of the iron nuggets is further enhanced and a petal-like outer shell can be formed. Such a thickness also effectively increases the strength of the coating layer  2  and also increases the strength of the entire agglomerate. Where the average thickness of the coating layer  2  is equal to or less than 0.30 mm, the strength of the coating layer  2  decreases and the thickness of the shell-shaped spherical body (i.e., petal-like coke) formed by heating of the coating layer  2  decreases. As a result, the heating process is accompanied by oxidation and consumption, and the shape of the iron nuggets is difficult to maintain till the iron nuggets are melted and aggregated. Therefore, the average thickness of the coating layer  2  is more preferably 0.50 mm or more, even more preferably 0.70 mm or more, and still more preferably 1.00 mm or more. The upper limit of the average thickness of the coating layer  2  is not particularly limited, but where the thickness is too large, the amount of carbonaceous material used increases, the amount of iron contained in all of the agglomerates decreases, and productivity decreases. Such a thickness is also cost inefficient. Therefore, the average thickness of the coating layer  2  is preferably 2.00 mm or less, more preferably 1.80 mm or less, and still more preferably 1.50 mm or less. 
     The thickness of the coating layer  2  may be measured by observing the cross section of the agglomerates under an optical microscope. 
     Described hereinabove are the agglomerates which characterize the manufacturing method of the present invention. 
     A method for manufacturing iron nuggets in accordance with the present invention will be explained hereinbelow. 
     A method for manufacturing iron nuggets in accordance with the present invention includes, in the order of description: 
     a step for agglomerating a mixture including iron oxide and a carbonaceous reducing agent to form core portions (hereinafter also referred to as a “core portion forming step”); 
     a step for forming a coating layer including a fluid carbonaceous material on the surface of the obtained core portions (hereinafter also referred to as a “surface coating step”); 
     a step for charging the obtained agglomerates on the hearth of a movable hearth heating furnace, heating, and reducing and melting the iron oxide in the agglomerates (hereinafter also referred to as a “reducing and melting step”); and 
     a step for discharging the obtained iron nuggets to the outside of the furnace and recovering the iron nuggets (hereinafter also referred to as a “recovering step”). 
     [Core Portion Forming Step] 
     In the core portion forming step, core portions of the agglomerates are manufactured by agglomerating a mixture including iron oxide and a carbonaceous reducing agent. 
     Specific examples of iron oxide sources that can be used as the iron oxide include iron ore, iron sand, steelmaking dust, non-ferrous smelting residue, and steelmaking wastes. 
     A carbon-containing reducing agent, for example, coal or coke, can be used as the carbonaceous reducing agent. When coal is used, fluid coal may be used or non-fluid coal may be used. 
     A flux may be additionally compounded with the abovementioned mixture. The flux, as referred to herein, fuses together with the gangue in the iron oxide source or with the ash component in the carbonaceous reducing agent and adjusts the melting point or fluidity of the final slag. 
     For example, a CaO-supplying substance, a MgO-supplying substance, an Al 2 O 3 -supplying substance, a SiO 2 -supplying substance, and fluorite (CaF 2 ) can be used as the flux. For example, at least one selected from the group consisting of CaO (quicklime), Ca(OH) 2  (hydrated lime), CaCO 3  (limestone), and CaMg(CO 3 ) 2  (dolomite) can be used as the CaO-supplying substance. At least one selected from the group consisting of CaMg(CO 3 ) 2  (dolomite), a MgO powder, Mg-containing substances extracted from natural ores or seawater, and MgCO 3  may be compounded as the MgO-supplying substance. For example, an Al 2 O 3  powder, bauxite, boehmite, gibbsite, and diaspore can be compounded as the Al 2 O 3 -supplying substance. For example, a SiO 2  powder or quartz sand can be used as the SiO 2 -supplying substance. 
     A binder may be further compounded as a component other than the iron oxide, carbonaceous reducing agent, and flux to the abovementioned mixture. 
     For example, a polysaccharide such as starch, e.g., corn starch and wheat flour, can be used as the binder. 
     The flux is sometimes referred to hereinbelow as an additive. 
     The iron oxide, carbonaceous reducing agent, and optionally compounded additive and binder may be mixed using a mixer of a rotary container type or a stationary container type. 
     The mixture obtained in the mixer is agglomerated to manufacture core portions of agglomerates. The average diameter of the core portions is not particularly limited, but recommended to be, for example, 18 mm to 22 mm. 
     A first pelletizer which is used when agglomerating the mixture can be, for example, a pan pelletizer, a cylindrical pelletizer, a twin-roll briquette molding machine, and an extruder. 
     The shape of the core portions is not particularly limited and may be, for example, a pellet-like or briquette-like shape. 
     [Surface Coating Step] 
     In the surface coating step, a coating layer including a fluid carbonaceous material is formed on the surface of the core portions obtained in the core portion forming step. 
     When the coating layer is formed, a binder may be included in addition to the fluid carbonaceous material. The above-described binders can be used as the binder. 
     The binder included in the coating layer and the binder included in the core portions may be of the same or different types. 
     For example, a pan pelletizer or cylindrical pelletizer can be used as a second pelletizer to be used when forming the coating layer including a fluid carbonaceous material. 
     The first pelletizer and the second pelletizer may be of the same or different types. 
     The size of the agglomerates in which the coating layer including a fluid carbonaceous material is formed on the surface of the core portions is not particularly limited, but it is preferred that the maximum particle size be 50 mm or less. Where the particle size of the agglomerates is too large, the granulation efficiency is degraded. Further, where the agglomerates become too large, heat transfer to the lower portions of the agglomerates is degraded and productivity decreases. The lower limit value of the particle size of the agglomerates is about 5 mm. 
     The agglomerates may be also dried by heating in the heating furnace in the below-described reducing and melting step, but it is recommended that the drying be performed before the reduction and melting step. Further, the coating layer may be formed once the core portions have been dried after the granulation, but it is preferred that the drying be performed after the coating layer has been formed on the surface of the core portions. 
     [Reducing and Melting Step] 
     In the reducing and melting step, the agglomerates obtained in the surface coating step are charged onto the hearth of a movable hearth heating furnace and heated, thereby reducing and melting the iron oxide in the agglomerates and forming iron nuggets constituted by reduced iron. 
     The movable hearth heating furnace, as referred to herein, is a heating furnace in which the furnace hearth moves as a belt conveyor inside the furnace. Examples of such furnaces include a rotary hearth furnace and a tunnel furnace. In the rotary hearth furnace, the external shape of the furnace hearth is designed in a circular or donut shape such that the start point and end point of the furnace hearth are at the same position, the iron oxide contained in the agglomerates charged on the furnace hearth is heated and reduced in one rotation cycle inside the furnace to generate reduced iron which is then melted and aggregated to generate iron nuggets and slag. Therefore, in the rotary hearth furnace, a charging means is provided for charging the agglomerates into the furnace on the upstreammost side in the rotation direction and a discharging means is provided on the downstreammost side in the rotation direction. Because of a rotating structure, the upstreammost side is actually the directly upstream side of the charging means. The tunnel furnace is a heating furnace in which the furnace hearth moves in the linear direction inside the furnace. 
     The agglomerates are preferably heated at a temperature of 1350° C. or higher. Where the heating temperature is below 1350° C., the reduced iron and slag are unlikely to melt and a high productivity sometimes cannot be obtained. Therefore, it is preferred that the heating temperature be 1350° C. or higher, more preferably 1400° C. or higher. However, where the heating temperature exceeds 15500° C., the exhaust gas temperature rises, large-scale exhaust gas treatment equipment needs to be used and the equipment cost rises. Therefore, it is preferred that the heating temperature be 1550° C. or less, more preferably 1500° C. or less. 
     It is preferred that the agglomerates be charged in a single layer on the furnace hearth. Where the agglomerates are charged on the furnace hearth in two or more layers, the agglomerates in the lower layer are not sufficiently heated, reducing and melting thereof are insufficient, and iron nuggets are difficult to produce. The one layer, as referred to herein, means that the agglomerates are not stacked in the vertical direction with respect to the furnace hearth, and gaps may be present between the agglomerates in the transverse direction. Thus, the agglomerates may be charged sparsely. Further, the agglomerates may partially overlap each other, but such partial overlapping will not cancel the effect of the present invention. 
     It is preferred that the carbonaceous reducing agent be laid as a bedding material on the furnace hearth prior to charging the agglomerates onto the furnace hearth. By laying the bedding material, it is possible to protect the furnace hearth. 
     It is preferred that the particle size of the bedding material be 3 mm or less so as to prevent the agglomerates or the melt thereof from submerging. The lower limit of the particle size of the bedding material is preferably 0.5 mm or more so as to prevent the bedding material from being scattered by the combustion gas of the burner. 
     [Recovering Step] 
     In the recovering step, the iron nuggets obtained in the reducing and melting step is discharged to the outside of the furnace, and the iron nuggets are recovered. 
     When the iron nuggets are discharged to the outside of the furnace, since the slag generated as a byproduct and the bedding material are included in addition to the iron nuggets, the iron nuggets may be recovered outside the furnace by using, for example, a sieve or a magnetic separator. 
     With the manufacturing method in accordance with the present invention, it is possible to manufacture iron nuggets with a C content of 2.5 mass % or more. Further, with the manufacturing method in accordance with the present invention it is possible to manufacture iron nuggets with an S content of 0.120 mass % or less. 
     The present application claims priority to Japanese Patent Application No. 2013-198980, filed on Sep. 25, 2013. The entire contents of the description of Japanese Patent Application No. 2013-198980 are incorporated by reference in the present application. 
     The present invention is explained in greater detail hereinbelow on the basis of examples thereof, but the present invention is not intended to be limited to those example, and it goes without saying that the present invention can be implemented by applying modifications within a range compatible with the aforementioned and below-described spirit, and those modifications are all included in the technical scope of the present invention. 
     Examples 
     Experimental Example 1 
     In the present experimental example, agglomerates having on the surface a coating layer including a fluid carbonaceous material and agglomerates having no coating layer were prepared, the agglomerates were heated in a heating furnace, and it was investigated whether or not the reoxidation of the obtained iron nuggets was suppressed. 
     Initially the agglomerates including iron oxide and a carbonaceous reducing agent were manufactured. 
     An iron ore of the composition presented in Table 1 below was used as the iron oxide. In Table 1, T. Fe means “total iron”. The iron ore to be used was ground such that the ore with a particle size of 44 μm or less constituted 67 mass %. 
     A carbonaceous material with the composition presented in Table 2 was used as the carbonaceous reducing material. In Table 2, T. C means “total carbon” and F. C means “fixed carbon”. The carbonaceous material to be used was ground such that the coal with a particle size of 75 μm or less constituted about 55 mass %. 
     Then, a binder, an additive, and an appropriate amount of water were compounded with the mixture including the iron ore and carbonaceous material, followed by agglomeration in a first pelletizer, and granulation into green pellets serving as core portions. Wheat flour was used as the binder. Limestone, dolomite, and fluorite were used as the additives. A pan pelletizer was used as the first pelletizer. The average diameter of the green pellets was 21 mm. The compounding ratio of the iron ore, carbonaceous material, binder, and additives is shown in Table 3. 
     Some of the obtained green pellets were charged into a drier and heated for about 1.0 h at 160° C. to 180° C. to remove the adhered water, thereby producing spherical dry pellets. 
     Other obtained green pellets were used, without drying, to form a coating layer including a fluid carbonaceous material on the surface thereof. Fluid bituminous coal was prepared as the fluid carbonaceous material, the green pellets were charged into a second pelletizer, a mixture prepared by mixing bituminous coal and a small amount of binder (wheat flour) was then supplied into the pelletizer, and a coating layer was formed on the surface of the core portions. A pan pelletizer was used as the second pelletizer. The green pellets obtained by forming the coating layer on the surface of the core portions were cut, the cross section was observed under an optical microscope, and it was confirmed that the average thickness of the coating layer was 1.0 mm. The green pellets in which the coating layer was formed on the surface were then charged into a drier and heated for about 1.0 h at 160° C. to 180° C. to remove the adhered water, thereby producing spherical dry pellets (i.e., agglomerates). 
     Then, the spherical dry pellets in which the coating layer was not formed and the spherical dry pellets in which the coating layer was formed were charged into a heating furnace (test furnace) maintained at about 1450° C. and heated, and the iron oxide in the dry pellets was reduced and melted. 
     The heating furnace had a highly oxidizing atmosphere to simulate the actual furnace. More specifically, the oxidizing gas was carbon dioxide, and the gas atmosphere in the furnace was a mixed atmosphere including 40 vol % of carbon dioxide and 60 vol % of nitrogen. As a result, when the dry pellets were charged into the heating furnace, the coating layer was swelled, the carbonaceous material contained in the coating layer was coked around the core portions, and petal-like outer shells were formed. The petal-like outer shells acted as windbreak walls preventing the atmosphere gas from coming into contact with the core portions. 
     The iron nuggets obtained after the iron oxide was reduced and melted in the heating furnace was discharged to the outside of the furnace and the iron nuggets were recovered. At this time, the slag which was generated as a byproduct when the iron nuggets were produced was also recovered. The compositions of the obtained iron nuggets and slag are presented in Table 4 below. 
     The ratio (sulfur distribution ratio) of the S amount (S) contained in the slag to the S amount [S] contained in the iron nuggets is also presented in Table 4 below. 
     The following conclusions can be made based on the results shown in Table 4. 
     (No Coating Layer) 
     When the coating layer was not formed, the amount of FeO in the slag was as large as 6.53 mass %, as shown in Table 4. As a result, the sulfur distribution ratio was 1.56, the S amount contained in the iron nuggets was 0.171 mass %, and the quality of the iron nuggets could not be improved. 
     The reason why the amount of FeO in the slag has increased is considered as follows. In the solid reduction phase, in the dry pellet in which the coating layer has not been formed, although the reduced iron is generated from the top of the pellet, part thereof is re-oxidized by the oxidizing gas in the atmosphere (Fe+CO 2 =FeO+CO), the generated FeO is melted in the molten slag, and high-FeO molten slag is produced. As a result, in the subsequent melting and reduction reaction within the melting and aggregation phase, a reaction (decarburization reaction) is induced between the FeO in the slag and [C] in the molten iron nuggets, and an intense slag foaming effect is demonstrated. The resultant drawback is that since the decarburization reaction is an endothermic reaction, the transfer of heat to iron is greatly delayed and the reaction time is significantly extended. Since the foamed slag covers the semimolten iron in the course of aggregation and blocks heat radiation from above, this is one more reason why the heat transfer to iron is significantly delayed and the reaction time is greatly extended. Although molten iron nuggets and molten slag are eventually formed, since the slag is greatly foamed and a high content of FeO in the slag is still maintained, [S] in the produced iron nuggets becomes 0.171 mass %. Further, [C] in the produced iron nuggets becomes 2.49 mass % which is less than the target value 2.5 mass %. The resultant effect is that the quality of iron nuggets serving as a product is greatly degraded. Another adverse result is that the intense slag foaming prevents the temperature of the iron nuggets from rising, the complete aggregation does not take place within a predetermined reaction time, iron nuggets of irregular shape which has taken in part of the slag at a high ratio is formed, and from the standpoint of shape, the value of the iron nuggets as a product is greatly reduced. 
       FIG. 6 ( 1 ) is a photograph of iron nuggets after completion of melting and aggregation.  FIG. 6 ( 2 ) is a photograph of the recovered iron nuggets.  FIG. 6 ( 3 ) is a photograph of the recovered slag. 
     In the present experimental example, a sufficient reaction time (i.e., in-furnace residence time) is ensured. As a result, it is clear that, as depicted in  FIG. 6 ( 2 ), although the recovered iron nuggets has irregular shape, the iron nuggets and slag are separated. However, from the standpoint of productivity of the actual equipment, it is difficult to ensure a sufficient in-furnace residence time and the product actually needs to be discharged to the outside of the furnace before the aggregation is entirely completed. The unavoidable consequences include further deterioration of iron nuggets shape and decrease in iron nuggets product quality and yield which occurs because the product is discharged to the outside of the furnace in a state in which the separation of part of the slag and metal is insufficient. 
     (With Coating Layer) 
     Where the dry pellets on which the coating layer has been formed are charged into the heating reaction field, the carbonaceous material contained in the coating layer is rapidly coked. A very characteristic effect confirmed at this time is that large cracks appear in the coating layer, but the coating layer does not peel off or fall down, and as depicted in  FIG. 2  a coke wall is formed and the core portion is enclosed therein. It has been found that this coke wall advances the reduction reaction of the iron oxide contained in the core portion, and while the upper part thereof is gradually oxidized and consumed by the atmosphere gas (C+CO 2 =2CO, C+H 2 O=CO+H 2 ), the coke wall can assume a petal shape with the removed upper portion. Furthermore, where the solid reduction is completed, the iron nuggets, which have been generated inside the core portion, and other oxides are aggregated while melting in the bottom portion inside the petal-like coke wall, the molten iron nuggets and molten slag are separated, and the reaction is completed. 
     Thus, the petal-like coke wall plays a very significant role of protecting the core portion from the oxidizing atmosphere gas, and a significant difference is confirmed with the reaction behavior of the conventional dry pellets, in which the coating layer is not formed, in that the reaction of the core portion is completed while the reoxidation thereof by the atmosphere gas is significantly suppressed over substantially the entire phase from the solid reaction phase to the melting and aggregation phase. 
       FIG. 7 ( 1 ) is a photograph illustrating the state after the completion of melting and aggregation.  FIG. 7 ( 2 ) is a photograph of the recovered iron nuggets.  FIG. 7 ( 3 ) is a photograph of the recovered slag. 
     As a result, as shown in  FIGS. 7 ( 2 ) and  7 ( 3 ), the method of the present invention makes it possible to obtain iron nuggets of substantially the same shape and also ensures good separation from the slag which is also recovered. Further, as indicted in Table 4, the amount of FeO contained in the slag is 0.29 mass %, and it is clear that the reoxidation of the iron nuggets is suppressed. In addition, the sulfur distribution ratio is 14.64, and the amount of S contained in the iron nuggets has been reduced to 0.059 mass %. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Composition of iron ore (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 T. Fe 
                 FeO 
                 SiO 2   
                 CaO 
                 Al 2 O 3   
                 MgO 
                 MnO 
                 TiO 2   
               
               
                   
               
               
                 63.28 
                 1.38 
                 5.39 
                 0.08 
                 0.70 
                 0.05 
                 0.67 
                 0.12 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Composition of carbonaceous material (mass %) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 T. C 
                 F. C 
                 Volatiles 
                 Ash 
               
               
                   
                   
               
               
                   
                 86.18 
                 77.96 
                 15.46 
                 6.58 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Compounding ratio (mass %) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Iron ore 
                 Carbonaceous material 
                 Binder 
                 Additive 
               
               
                   
                   
               
               
                   
                 70.23 
                 16.36 
                 1.10 
                 12.31 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                 Composition of 
                 Composition of slag 
                   
               
               
                   
                 iron nuggets (mass %) 
                 (mass %) 
                 (S)/ 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 C 
                 Si 
                 Mn 
                 S 
                 T. Fe 
                 FeO 
                 S 
                 [S] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 No Coating 
                 2.49 
                 0.03 
                 0.04 
                 0.171 
                 5.95 
                 6.53 
                 0.267 
                 1.56 
               
               
                 layer 
               
               
                 With Coating 
                 3.45 
                 0.07 
                 0.44 
                 0.059 
                 1.35 
                 0.29 
                 0.864 
                 14.64 
               
               
                 layer 
               
               
                   
               
            
           
         
       
     
     Experimental Example 2 
     In the present experimental example, the agglomerates were manufactured by changing the thickness of the coating layer formed on the surface of core portions, the produced agglomerates were heated in a heating furnace, and it was investigated whether or not the reoxidation of the obtained iron nuggets was suppressed. 
     Initially, green pellets in which the coating layer was formed on the surface of core portions were manufactured according to the procedure of Experimental Example 1 by changing the thickness of the coating portion. The core portions on which the coating layer was formed were cut and the thickness of the coating layer was checked by observing the cross section under an optical microscope. As a result, the average thickness of the coating layer was 0.30 mm to 2.00 mm. 
     The green pellets obtained by forming the coating layer on the surface of the core portions were charged into a drier and heated for about 1.0 h at 160° C. to 180° C. to remove the adhered water, thereby producing spherical dry pellets (i.e., agglomerates). 
     Then, the spherical dry pellets were charged into a heating furnace (test furnace) maintained at about 1450° C. and heated, and the iron oxide in the dry pellets was reduced and melted. The heating furnace had a highly oxidizing atmosphere to simulate the actual furnace. More specifically, it was a mixed atmosphere including 40 vol % of carbon dioxide and 60 vol % of nitrogen. As a result, when the dry pellets were charged into the heating furnace, the coating layer swelled, the carbonaceous material contained in the coating layer was coked around the core portions, and petal-like outer shells were formed. The height of the petal-like outer shell differed among the samples, but all of the outer shells still acted as windbreak walls preventing the atmosphere gas from coming into contact with the core portions. 
     The iron nuggets obtained after the iron oxide was reduced and melted in the heating furnace was discharged to the outside of the furnace and the iron nuggets was recovered. At this time, the slag which was generated as a byproduct when the iron nuggets were produced was also recovered. The compositions of the obtained iron nuggets and slag are presented in Table 5 below. 
     Meanwhile, as a comparison example, Table 5 presents the results obtained when spherical dry pellets were manufactured by directly charging green pellets, in which the coating layer was not formed on the surface, into the drier and drying under the conditions same as those when the coating layer was formed on the surface. 
     The obtained spherical dry pellets were heated under the conditions same as those when the coating layer was formed on the surface, and the iron oxide in the dry pellets was reduced and melted. The compositions of the obtained iron nuggets and slag are presented in Table 5 below. 
     The ratio (sulfur distribution ratio) of the S amount (S) contained in the slag to the S amount [S] contained in the iron nuggets is also presented in Table 5 below. 
     The following conclusions can be made based on the results as shown in Table 5. 
     In No. 8, the coating layer was not formed on the surface of the core portions. Therefore, the reoxidation of the iron nuggets obtained by reduction could not be prevented, the amount of FeO contained in the slag increased to 6.53 mass %, and the sulfur distribution ratio decreased to 1.56. As a result, the amount of S contained in the iron nuggets increased to 0.171 mass % and the quality of the iron nuggets could not be improved. 
     By contrast, in Nos. 1 to 7, the coating layer was formed on the surface of the core portions. Therefore, the reduced iron obtained by reduction of the iron oxide contained in the agglomerates, or the iron nuggets was prevented from the reoxidation in the heating furnace, the amount of FeO contained in the slag reduced to 0.18 mass % to 2.23 mass %, and the sulfur distribution ratio increased to 41.64 to 2.96. As a result, the amount of S contained in the iron nuggets decreased to 0.022 mass % to 0.139 mass %, and the quality of the iron nuggets could be improved. Further, as clearly indicated in Table 5, the amount of FeO contained in the slag tends to decrease and the sulfur distribution ratio tends to increase with the increase in the thickness of the coating layer. Therefore, it is clear that the amount of S contained in the iron nuggets can be decreased by increasing the thickness of the coating layer. In particular, in Nos. 1 to 6, the amount of S contained in the iron nuggets could be suppressed to 0.120 mass % or less. 
     Meanwhile, in No. 8 in which the coating layer was not formed on the surface of the core portions, the amount of carbon contained in the iron nuggets was as low as 2.49 mass %, whereas in No. 1 to 7 in which the coating layer was formed on the surface of the core portion, the amount of carbon contained in the iron nuggets increased to 2.65 mass % to 3.52 mass %, and it is clear that the quality of the iron nuggets can be improved by forming the coating layer on the surface of the core portions. 
     It was also understood that a large height of the petal-like outer shell formed after the heating and reduction treatment tends to be maintained as the average thickness of the coating layer increases. 
       FIGS. 8 ( 1 ) to  8 ( 4 ) are schematic diagrams illustrating the height of the petal-like wall surface which has been formed while heating the agglomerates and remained after the iron nuggets have been obtained when the thickness of the coating layer was changed.  FIG. 8 ( 1 ) illustrates the case in which the average thickness of the coating layer was, for example, 1.30 mm to 2.00 mm.  FIG. 8 ( 2 ) illustrates the case in which the average thickness of the coating layer was, for example, 0.80 mm to 1.20 mm.  FIG. 8 ( 3 ) illustrates the case in which the average thickness of the coating layer was, for example, 0.60 mm to 0.80 mm.  FIG. 8 ( 4 ) illustrates the case in which the average thickness of the coating layer was, for example, more than 0.30 mm and equal to or less than 0.50 mm. In  FIG. 8 , the reference numeral  2  stands for the coating layer,  6 —iron nuggets,  7 —slag. 
       FIG. 9 ( 1 ) is a photograph taken immediately after the heating and reduction treatment of No. 4 in Table 5.  FIG. 9 ( 2 ) is a photograph taken immediately after the heating and reduction treatment of No. 5 in Table 5.  FIG. 9 ( 3 ) is a photograph taken immediately after the heating and reduction treatment of No. 6 in Table 5. 
     In No. 7 in which the average thickness of the coating layer was 0.30 mm, small-scale slag foaming has occurred, but in No. 6 in which the average thickness of the coating layer was 0.50 mm, no slag foaming has occurred. Meanwhile, in No. 8 in which the coating layer was not formed on the surface of the core portions, very intense slag foaming has occurred. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
             
            
               
                   
                   
               
               
                   
                 Thick- 
                 Composition of 
                 Composition of slag 
                   
               
               
                   
                 ness 
                 iron nuggets (mass %) 
                 (mass %) 
                 (S)/ 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 (mm) 
                 C 
                 Si 
                 Mn 
                 S 
                 T. Fe 
                 FeO 
                 S 
                 [S] 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 2.00 
                 3.49 
                 0.08 
                 0.48 
                 0.022 
                 1.38 
                 0.18 
                 0.916 
                 41.64 
               
               
                 2 
                 1.50 
                 3.51 
                 0.08 
                 0.48 
                 0.034 
                 1.36 
                 0.19 
                 0.905 
                 26.62 
               
               
                 3 
                 1.30 
                 3.52 
                 0.08 
                 0.47 
                 0.042 
                 1.32 
                 0.21 
                 0.890 
                 21.19 
               
               
                 4 
                 1.00 
                 3.45 
                 0.07 
                 0.44 
                 0.059 
                 1.35 
                 0.29 
                 0.864 
                 14.64 
               
               
                 5 
                 0.70 
                 3.10 
                 0.04 
                 0.33 
                 0.080 
                 1.33 
                 0.49 
                 0.634 
                 7.93 
               
               
                 6 
                 0.50 
                 3.01 
                 0.04 
                 0.21 
                 0.117 
                 1.78 
                 1.16 
                 0.513 
                 4.38 
               
               
                 7 
                 0.30 
                 2.65 
                 0.03 
                 0.12 
                 0.139 
                 2.50 
                 2.23 
                 0.411 
                 2.96 
               
               
                 8 
                 0 
                 2.49 
                 0.03 
                 0.04 
                 0.171 
                 5.95 
                 6.53 
                 0.267 
                 1.56 
               
               
                   
               
            
           
         
       
     
     Experimental Example 3 
     In the present experimental example, the agglomerates were manufactured by using a non-fluid carbonaceous material for including in the coating layer to be formed on the surface of the core portions, the agglomerates were heated in the heating furnace, and it was examined whether or not the reoxidation of the obtained iron nuggets was suppressed. 
     Initially, green pellets in which the coating layer with an average thickness of 0.50 mm was formed on the surface of the core portions were manufactured according to the procedure of Experimental Example 1. In this case, anthracite was used as a non-fluid carbonaceous material instead of the fluid bituminous coal. The composition of the anthracite is presented in Table 6 below. 
     The green pellets obtained by forming the coating layer on the surface were charged into a drier and heated for about 1.0 h at 160° C. to 180° C. to remove the adhered water, thereby producing spherical dry pellets (i.e., agglomerates). 
     Then, the spherical dry pellets on which the coating layer was not formed and the spherical dry pellets on which the coating layer was formed were charged into a heating furnace (test furnace) maintained at about 1450° C. and heated, and the iron oxide in the dry pellets was reduced and melted. 
     The heating furnace had a highly oxidizing atmosphere to simulate the actual furnace. More specifically, it was a mixed atmosphere including 40 vol % of carbon dioxide and 60 vol % of nitrogen. 
     As a result, when the dry pellets were charged into the heating furnace, the coating layer swelled, but cracked in a tortoise shell shape and deposited on the core portions as thin debris. As a result, the petal-like outer shell constituted by coke could not be formed. The debris deposited on the core portions was falling down on the periphery of the core portions with the passage of time, and the top of the core portions was exposed to the atmosphere gas. 
     The iron nuggets obtained after the iron oxide was reduced and melted in the heating furnace was discharged to the outside of the furnace and the iron nuggets were recovered. At this time, the slag which was generated as a byproduct when the iron nuggets were produced was also recovered. The compositions of the obtained iron nuggets and slag are presented in Table 7 below. 
     The ratio (sulfur distribution ratio) of the S amount (S) contained in the slag to the S amount [S] contained in the iron nuggets is also presented in Table 7 below. 
     The following conclusions can be made on the basis of Table 7. Even in the case in which the coating material is formed on the surface of the core portions, when the carbonaceous material contained in the coating layer is not fluid, the reoxidation of the reduced iron obtained by heating and reducing the agglomerates, or the iron nuggets produced by melting and agglomeration of the reduced iron cannot be prevented and the amount of FeO contained in the slag cannot be reduced. As a result, the sulfur distribution ratio decreases, the amount of sulfur contained in the iron nuggets increases, and quality cannot be improved. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Composition of anthracite (mass %) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 T. C 
                 F. C 
                 Volatiles 
                 Ash 
               
               
                   
                   
               
               
                   
                 79.59 
                 79.69 
                 4.35 
                 15.96 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                   
                 Composition of iron 
                 Composition of slag 
                   
               
               
                 Thickness 
                 nuggets (mass %) 
                 (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 (mm) 
                 C 
                 Si 
                 Mn 
                 S 
                 T. Fe 
                 FeO 
                 S 
                 (S)/[S] 
               
               
                   
               
               
                 0.50 
                 2.64 
                 0.03 
                 0.13 
                 0.147 
                 2.18 
                 2.34 
                 0.404 
                 2.75 
               
               
                   
               
            
           
         
       
     
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               1  core portion 
               2  coating layer 
               3  agglomerates 
               4  reduced iron 
               6  iron nuggets 
               7  slag