Rotary hearth furnace, its operating method, and reduced iron manufacturing method

Abstract

To provide a rotary hearth furnace that reduces facility cost, maintenance effort and fuel consumption while suppressing rotational disturbance.SOLUTION: A rotary hearth furnace 100 includes a rotary hearth 12, an annular furnace body 11 having outer and inner sidewalls 11a and 11b, a plurality of main burners 10 provided side by side along the circumferential direction at the outer and inner sidewalls 11a and 11b, and a plurality of hearth burners 13a and 13b for heating the gaps 32a and 32b. The main burners 10 have a first main burner 10A that burns at an air ratio less than 1.0 and a second main burner 10B that burns at an air ratio of 1.0 or more. The number of hearth burners 13a and 13b provided closer to the first main burner 10A is larger than the number of hearth burners 13a and 13b provided closer to the second main burner 10B.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a rotary hearth furnace, a method for operating the same, and a method for producing reduced iron. [Background technology]

[0002] In order to reuse iron oxide contained in steelmaking waste, a process is known in which the steelmaking waste is mixed with a reducing agent and a binder, granulated, and then charged into a rotary hearth furnace, where it is heated and reduced to produce reduced iron (see, for example, Patent Document 1). The steelmaking waste used as the raw material in this process may contain zinc oxide. Zinc fumes generated by heating and reduction volatilize in a high-temperature atmosphere, but when they enter the gap between the rotary hearth and the side wall of the annular furnace body, they are cooled by cold radiation from the water seal and precipitate as a solid. As these precipitates grow, they clog the gap and cause rotation problems.

[0003] Therefore, Patent Documents 2 and 3 propose a technique of providing burners around the entire circumference to heat the gap between the outer peripheral surface of the rotary hearth and the outer peripheral fixed wall, and the gap between the inner peripheral surface of the rotary hearth and the inner peripheral fixed wall. This prevents zinc vaporized in the furnace from being cooled and precipitated in each gap, and from adhering to the inner and outer peripheral surfaces of the rotary hearth and the inner and outer peripheral fixed walls. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-281617 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-223556 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-253765 Summary of the Invention [Problem to be solved by the invention]

[0005] Although it is effective to install hearth burners to heat the gaps in order to prevent deposits of zinc and other materials from adhering to the gaps, the use of a large number of hearth burners increases the cost of equipment, the amount of maintenance work, and the amount of fuel consumed. Also, it has been found that some of the deposits that form in the gaps are brittle and easily fall off.

[0006] Therefore, the present disclosure provides a rotary hearth furnace and an operating method thereof that can reduce equipment costs, maintenance work, and fuel consumption while suppressing rotational obstructions of the rotary hearth. The present disclosure also provides a method for producing reduced iron that can stably produce reduced iron at low production costs by using such a rotary hearth furnace. [Means for solving the problem]

[0007] The present disclosure provides a rotary hearth furnace comprising: an annular rotary hearth; an annular furnace body having an outer peripheral side wall arranged along the outer peripheral surface of the rotary hearth; and an inner peripheral side wall arranged along the inner peripheral surface of the rotary hearth; a plurality of main burners arranged in a line circumferentially on the outer peripheral side wall and the inner peripheral side wall; and a plurality of hearth burners for heating the gap between the outer peripheral surface and the outer peripheral side wall and the gap between the inner peripheral surface and the inner peripheral side wall, wherein the main burners include a first main burner that burns at an air ratio of less than 1.0 and a second main burner that burns at an air ratio of 1.0 or more, and the number of hearth burners arranged closer to the first main burner than the second main burner is greater than the number of hearth burners arranged closer to the second main burner than the first main burner.

[0008] The rotary hearth furnace has two main burners: a first main burner that burns at an air ratio of less than 1.0 and a second main burner that burns at an air ratio of 1.0 or greater. The number of hearth burners located closer to the first main burner, which has an air ratio of less than 1.0, is greater than the number of hearth burners located closer to the second main burner, which has an air ratio of 1.0 or greater. This reduces the number of hearth burners located near the second main burner, thereby reducing equipment costs, maintenance efforts, and fuel consumption. Furthermore, by providing hearth burners that heat the gaps near the first main burner, the formation of deposits containing metallic zinc as a primary component in the gaps can be suppressed. Meanwhile, deposits containing zinc oxide as a primary component may form in the gaps near the second main burner. Since deposits containing zinc oxide as a primary component are brittle and easily detach from the adhering surface, they do not cause rotational problems. Therefore, according to the rotary hearth furnace of the present disclosure, it is possible to reduce the number of hearth burners, thereby reducing equipment costs, maintenance work, and fuel consumption, while suppressing rotational interference of the rotary hearth.

[0009] The annular furnace space formed by the furnace body and the rotary hearth has a first region in which a plurality of first main burners that burn at an air ratio of less than 1.0 are arranged along the circumferential direction, and a second region in which a plurality of second main burners that burn at an air ratio of 1.0 or more are arranged along the circumferential direction, and the number of hearth burners provided in the first region may be greater than the number of hearth burners provided in the second region. This makes it possible to sufficiently reduce the number of hearth burners provided in the second region while sufficiently suppressing the formation of deposits containing metallic zinc as a main component in the first region. There may be no hearth burners in the second region.

[0010] All of the hearth burners may be located closer to the first main burner than the second main burner. Alternatively, hearth burners may be located only in the first region. This allows for a further reduction in the number of hearth burners. This further reduces equipment costs, maintenance work, and fuel consumption.

[0011] In a plan view of the rotary hearth furnace, the distance between a hearth burner that is provided closer to the first main burner than the second main burner and the first main burner that is closest to the hearth burner may be 1.5 m or less. By providing the hearth burners at such a distance, it is possible to sufficiently prevent deposits containing metallic zinc as a main component from being formed in each gap.

[0012] The height of the main burners relative to the top surface of the rotary hearth may be 1.5 m or less. This makes it easier for zinc contained in deposits that form in the gaps near the second main burner to oxidize. This makes the deposits more brittle and more likely to fall off. Furthermore, since the gaps near the first main burner are heated not only by the hearth burners but also by the first main burner, the temperature rises and deposits are less likely to form. This allows for more stable operation.

[0013] The rotary hearth furnace may produce reduced iron by heating briquettes containing electric furnace dust and carbonaceous material. Electric furnace dust tends to have a higher metallic zinc content than other steelmaking dusts. However, even when briquettes containing electric furnace dust with a high zinc content are used, rotational problems in the rotary hearth can be suppressed, and stable operation can be continued at low cost.

[0014] The present disclosure provides a method for operating a rotary hearth furnace comprising: an annular rotary hearth; an annular furnace body having an outer peripheral sidewall arranged along the outer peripheral surface of the rotary hearth; and an inner peripheral sidewall arranged along the inner peripheral surface of the rotary hearth; a plurality of main burners arranged in a line circumferentially on the outer peripheral sidewall and the inner peripheral sidewall; and a plurality of hearth burners for heating the gap between the outer peripheral surface and the outer peripheral sidewall and the gap between the inner peripheral surface and the inner peripheral sidewall, the method comprising a step of burning a first main burner that is a part of the main burners at an air ratio of less than 1.0 and burning a second main burner that is also a part of the main burners at an air ratio of 1.0 or more, wherein the number of hearth burners that are arranged and burning closer to the first main burner than the second main burners is greater than the number of hearth burners that are arranged and burning closer to the second main burner than the first main burners.

[0015] The method for operating the rotary hearth furnace includes a step of firing the first main burner among the main burners at an air ratio of less than 1.0 and firing the second main burner at an air ratio of 1.0 or more. The number of hearth burners located near the first main burner and firing with an air ratio of less than 1.0 is greater than the number of hearth burners located near the second main burner and firing with an air ratio of 1.0 or more. This reduces the number of hearth burners firing near the second main burner, thereby reducing equipment costs, maintenance work, and fuel consumption. Furthermore, by firing and heating the hearth burners, the formation of deposits containing metallic zinc as a primary component in each gap near the first main burner can be suppressed. Meanwhile, deposits containing zinc oxide as a primary component may form in each gap near the second main burner. However, deposits containing zinc oxide as a primary component are brittle and easily detach from the attachment surface, and therefore do not cause rotational problems. Therefore, according to the method of operating a rotary hearth furnace disclosed herein, it is possible to reduce the number of hearth burners while suppressing rotational obstructions of the rotary hearth, thereby reducing equipment costs, maintenance work, and fuel consumption.

[0016] The above-described operating method may include a step of introducing briquettes containing electric furnace dust and carbonaceous material onto the rotary hearth. Electric furnace dust tends to have a higher zinc content than other steelmaking dusts. However, the above-described operating method can suppress rotational problems in the rotary hearth and continue stable, low-cost operation even when the briquettes contain electric furnace dust with a high zinc content.

[0017] The present disclosure provides a method for producing reduced iron, the method comprising the steps of: heating briquettes containing iron oxide and carbonaceous material using any one of the rotary hearth furnaces described above; and reducing the iron oxide to obtain reduced iron. Because the rotary hearth furnace is used in this production method, reduced iron can be produced stably and at low production cost.

[0018] The briquettes used in the above process may contain electric furnace dust containing iron oxide. Electric furnace dust tends to have a higher zinc content than other steelmaking dusts. However, even if the zinc content is high, it is possible to suppress rotational interference in the rotary hearth and produce reduced iron stably at low cost. [Effects of the Invention]

[0019] It is possible to provide a rotary hearth furnace and an operating method thereof that can reduce equipment costs, maintenance work, and fuel consumption while suppressing rotational obstructions of the rotary hearth. By using such a rotary hearth furnace, it is possible to provide a method for producing reduced iron that can stably produce reduced iron at low production costs. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a diagram schematically illustrating an embodiment of a rotary hearth furnace and a reduced iron manufacturing apparatus. FIG. [Figure 2] FIG. 2 is a horizontal cross-sectional view of the rotary hearth furnace of FIG. 1. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. 2. [Figure 5] FIG. 2 is a horizontal cross-sectional view of the rotary hearth furnace of FIG. 1. [Figure 6] 1 is a graph showing the relationship between the air ratio and the thickness of deposits. [Figure 7] FIG. 2 is a horizontal cross-sectional view of a rotary hearth furnace used in the experimental examples. [Figure 8] FIG. 2 is a horizontal cross-sectional view of a rotary hearth furnace used in a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings where appropriate. However, the following embodiments are merely examples for explaining the present disclosure and are not intended to limit the present disclosure to the following content. In each drawing, the same elements or elements having the same functions are designated by the same reference numerals, and redundant explanations will be omitted where appropriate.

[0022] Fig. 1 is a diagram schematically illustrating one embodiment of a rotary hearth furnace and a reduced iron manufacturing apparatus including the rotary hearth furnace. The reduced iron manufacturing apparatus 200 in Fig. 1 includes a rotary hearth furnace 100 that heats briquettes 22 containing electric furnace dust and carbonaceous material to produce reduced iron. In Fig. 1, a part of the furnace body 11 of the rotary hearth furnace 100 is cut away to show its internal structure.

[0023] The rotary hearth furnace 100 includes an annular rotary hearth 12, an outer peripheral sidewall 11a arranged along the outer peripheral surface 12a of the rotary hearth 12, an inner peripheral sidewall 11b arranged along the inner peripheral surface 12b of the rotary hearth 12, and an annular furnace body 11 having a ceiling wall 11c. A plurality of main burners 10 are arranged in a circumferential direction on the outer peripheral sidewall 11a of the rotary hearth furnace 100. A plurality of main burners 10 are also arranged in a circumferential direction on the inner peripheral sidewall 11b of the rotary hearth furnace 100. Supply pipes (not shown) for supplying fuel gas and combustion air may be connected to the main burners 10. The ratio of fuel gas to combustion air may be configured to be independently adjustable for each main burner 10.

[0024] FIG. 2 is a horizontal cross-sectional view of the rotary hearth furnace 100 of FIG. 1. The horizontal cross-sectional view of FIG. 2 shows a cross-section taken below the main burner 10, passing through the outer peripheral side wall 11a and the inner peripheral side wall 11b. However, in FIG. 2, the main burner 10 located above the cross-section is also shown to show the positional relationship between the hearth burners 13a, 13b and the main burner 10. Seven main burners 10 are arranged in a circumferential direction on the outer peripheral side wall 11a. Four main burners 10 are arranged in a circumferential direction on the inner peripheral side wall 11b. The number of main burners 10 arranged on the outer peripheral side wall 11a and the inner peripheral side wall 11b is not particularly limited. The intervals between adjacent main burners 10 may be the same or different.

[0025] Some of the multiple main burners 10 are first main burners 10A that burn at an air ratio of less than 1.0, and the other multiple main burners 10 are second main burners 10B that burn at an air ratio of 1.0 or more. The air ratio is the ratio of the amount of air actually supplied to the amount of air theoretically required to completely burn the fuel (theoretical air amount). When the air ratio is 1.0, the fuel is burned with the amount of air required for complete combustion. Since the first main burner 10A burns at an air ratio of less than 1.0, there is a shortage of oxygen, allowing the reduction reaction to proceed sufficiently. From the viewpoint of sufficiently promoting the reduction reaction of iron oxide, the air ratio of the first main burner 10A may be 0.95 or less, or may be 0.9 or less. On the other hand, from the viewpoint of suppressing incomplete combustion (the amount of carbon monoxide generated), the air ratio of the first main burner 10A may be 0.7 or more, or may be 0.8 or more.

[0026] The second main burner 10B burns at an air ratio of 1.0 or higher, which promotes the oxidation of zinc and reduces the metallic zinc content in deposits. It also suppresses the generation of carbon monoxide, reducing the cost of treating exhaust gas. From the viewpoint of further promoting the oxidation of zinc and further reducing the carbon monoxide content in exhaust gas, the air ratio of the second main burner 10B may be greater than 1.0, or even 1.1 or higher. On the other hand, from the viewpoint of improving thermal efficiency, the air ratio of the second main burner 10B may be 1.3 or lower, or even 1.2 or lower.

[0027] In this embodiment, six first main burners 10A and five second main burners 10B are provided. The number of first main burners 10A and second main burners 10B is not particularly limited, and neither is the ratio of their numbers. However, as shown in FIG. 2, the first main burners 10A are preferably provided adjacent to each other in the circumferential direction. The second main burners 10B are preferably provided adjacent to each other in the circumferential direction. By providing the first main burners 10A and the second main burners 10B adjacent to each other without alternating, the furnace space 70 of the rotary hearth furnace 100 can be divided into a first region that promotes a reduction reaction and a second region that promotes an oxidation reaction. Only the first main burners 10A may be provided in the first region, and only the second main burners 10B may be provided in the second region.

[0028] The rotary hearth 12 rotates in the circumferential direction P relative to the furnace body 11. The rotary hearth 12 is provided with running wheels (not shown) on its underside and is supported so as to be rotatable in the circumferential direction P of the furnace body 11. A gap 32a is provided between the outer peripheral surface 12a of the rotary hearth 12 and the outer peripheral side wall 11a of the furnace body 11. That is, the outer peripheral surface 12a of the rotary hearth 12 and the outer peripheral side wall 11a of the furnace body 11 are separated by the gap 32a. A gap 32b is provided between the inner peripheral surface 12b of the rotary hearth 12 and the inner peripheral side wall 11b of the furnace body 11. That is, the inner peripheral surface 12b of the rotary hearth 12 and the inner peripheral side wall 11b of the furnace body 11 are separated by the gap 32b.

[0029] A plurality of hearth burners 13a for heating the gap 32a are provided below the gap 32a. A plurality of hearth burners 13b for heating the gap 32b are also provided below the gap 32b. The hearth burners 13a, 13b may be connected to supply pipes (not shown) for supplying fuel gas and combustion air. The hearth burners 13a, 13b provided in the gaps 32a, 32b are arranged side by side in the circumferential direction. The intervals between the plurality of hearth burners 13a are not uniform, but are unevenly spaced. The intervals between the plurality of hearth burners 13b are also not uniform, but are unevenly spaced.

[0030] The number of hearth burners 13a, 13b provided closer to the first main burner 10A than the second main burner 10B is greater than the number of hearth burners 13a, 13b provided closer to the second main burner 10B than the first main burner 10A. There is no limit to the number of each. For example, the number of hearth burners 13a, 13b provided closer to the second main burner 10B than the first main burner 10A may be zero. That is, as shown in FIG. 2, all hearth burners 13a, 13b may be provided closer to the first main burner 10A than the second main burner 10B.

[0031] 3 is a cross-sectional view taken along line III-III in FIG. 2. That is, FIG. 3 shows a portion of a vertical cross section of the rotary hearth furnace 100 taken along the radial direction of the rotary hearth furnace 100, passing near the first main burner 10A. A gap 32a between the outer peripheral surface 12a of the rotary hearth 12 and the outer peripheral sidewall 11a is sealed by a water seal 17 between a seal plate 15 connected to the lower end of the outer peripheral sidewall 11a and a seal plate 16 connected to the lower end of the outer periphery of the rotary hearth 12. Although not shown in FIG. 3, a gap 32b between the inner peripheral surface 12b of the rotary hearth 12 and the inner peripheral sidewall 11b of the furnace body 11 is also sealed in the same manner as the gap 32a. In this way, an internal furnace space 70 isolated from the outside air is formed inside the rotary hearth furnace 100.

[0032] A hearth burner 13a is provided above the water seal section 17 (below the gap 32a). Therefore, a high temperature can be maintained even if the gas flowing through the furnace space 70 enters the vicinity of the gap 32a. This prevents deposits containing metallic zinc as a main component from forming on the outer peripheral surface 12a and the outer peripheral side wall 11a that form the gap 32a. Similarly, this prevents deposits containing metallic zinc as a main component from forming on the inner peripheral surface 12b and the inner peripheral side wall 11b that form the gap 32b. This effectively prevents the rotation of the rotary hearth 12 from being impeded by deposits containing metallic zinc.

[0033] The height H of the main burner 10 (first main burner 10A) based on the maximum height of the upper surface 12A of the rotary hearth 12 is preferably 1.5 m or less. This increases the temperature in the gap 32a (32b), making it difficult for deposits containing metallic zinc as a main component to form. This allows the rotary hearth furnace 100 to continue operating more stably. The height H is calculated as the difference between the center line of the main burner 10 and the maximum height of the upper surface 12A.

[0034] FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. 2. That is, FIG. 4 shows a portion of a vertical cross section of the rotary hearth furnace 100 taken along the radial direction of the rotary hearth furnace 100, passing near the second main burner 10B. No hearth burner 13a is provided below the gap 32a below the second main burner 10B. Therefore, the gas that enters the vicinity of the gap 32a is cooled by cold radiation from the water seal 17. As a result, zinc oxide contained in the gas precipitates, and a deposit 75 containing zinc oxide as a main component is formed on the outer circumferential surface 12a and the outer circumferential sidewall 11a that form the gap 32a. Although not shown in FIG. 4, the gas that enters the vicinity of the gap 32b is also cooled by cold radiation from the water seal. As a result, a deposit containing zinc oxide as a main component is formed on the inner circumferential surface 12b and the inner circumferential sidewall 11b that form the gap 32b.

[0035] The height H of the main burner 10 (second main burner 10B) based on the maximum height of the upper surface 12A of the rotary hearth 12 is preferably 1.5 m or less. This makes it easier for the metallic zinc contained in the deposits that form in the gap 32a (32b) near the second main burner 10B to oxidize. This makes the deposits more brittle and more likely to fall off. This allows the rotary hearth furnace 100 to continue operating more stably.

[0036] The deposits 75 containing zinc oxide as the primary component are more brittle than deposits containing metallic zinc as the primary component. Therefore, they easily fall off the outer peripheral surface 12a, the outer peripheral sidewall 11a, the inner peripheral surface 12b, and the inner peripheral sidewall 11b. Therefore, even if the deposits 75 form, they do not interfere with the rotation of the rotary hearth 12, allowing for stable operation. The rotary hearth furnace 100 does not have hearth burners 13a and 13b that heat the gap 32a (32b) below the second main burner 10B, so the number of hearth burners can be reduced. This reduces equipment costs, maintenance costs, and fuel consumption.

[0037] FIG. 5 is a horizontal cross-sectional view of the rotary hearth furnace 100, similar to FIG. 1. In FIG. 5, the main burner 10 is shown above the cross-section to illustrate the positional relationship between the hearth burners 13a and 13b and the main burner 10. For ease of explanation, the introduction section 21 and the discharge section 60 are omitted from FIG. 5. As shown in FIG. 5, the annular furnace space 70 in the rotary hearth furnace 100 is divided into two sections: a first section 71 in which the first main burners 10A are arranged circumferentially, and a second section 72 in which multiple second main burners are arranged circumferentially. The first section 71 and the second section 72 have a substantially C-shaped cross-sectional shape. Imaginary boundary lines B1 and B2, which are the boundaries between the first region 71 and the second region 72, are drawn as line segments connecting the midpoints of the first and second main burners 10A, 10B that are adjacent to each other and provided on the outer peripheral sidewall 11a, and the midpoints of the first and second main burners 10A, 10B that are adjacent to each other and provided on the inner peripheral sidewall 11b, in a horizontal cross section such as that shown in Fig. 5. In this case, the line segments are drawn so as to connect midpoints that face each other across the furnace space 70.

[0038] In this embodiment, no hearth burners 13a, 13b are provided in the second region 72. This allows for sufficient reductions in equipment costs, maintenance costs, and fuel consumption. However, this is not limiting, and in a modified example, hearth burners 13a, 13b may also be provided in the second region 72. It is sufficient that the number of hearth burners 13a, 13b provided closer to the first main burner 10A than the second main burner 10B is greater than the number of hearth burners 13a, 13b provided closer to the second main burner 10B than the first main burner 10A. It is preferable that the number of hearth burners 13a, 13b provided in the first region 71 is greater than the number of hearth burners 13a, 13b provided in the second region 72. This makes it possible to sufficiently suppress the generation of deposits containing metallic zinc as a main component in the first region 71, while also making it possible to sufficiently reduce the number of hearth burners 13a, 13b provided in the second region 72.

[0039] The hearth burners 13a, 13b are provided in the first region 71 closer to the first main burner 10A than the second main burner 10B. The distance D between each hearth burner 13a (13b) and the nearest first main burner 10A may be 1.5 m or less, or may be 1.0 m or less. By providing the hearth burners 13a, 13b with such a distance D, it is possible to sufficiently prevent deposits containing metallic zinc as a main component from forming in the gaps 32a, 32b. The distance D can be measured when the rotary hearth furnace 100 is viewed from above.

[0040] Returning to FIG. 1 , a method for operating the rotary hearth furnace 100 and a method for producing reduced iron using the rotary hearth furnace 100 and a reduced iron production apparatus 200 equipped with the rotary hearth furnace will be described below. The reduced iron production apparatus 200 includes a forming unit that forms a mixture obtained by kneading dust, a carbonaceous material, and a binder into briquettes 22 (formed bodies). The forming unit may be, for example, a double-roll forming machine. The dust may contain iron oxide, zinc oxide, and other components. The total iron content (T.Fe) of the dust may be, for example, 10 to 60 mass %, and the ZnO content may be 10 to 40 mass %. The carbonaceous material may be, for example, pulverized coal. The dust may be steelmaking dust or may contain at least one of electric furnace dust, blast furnace dust, converter dust, and sintering dust. Table 1 shows examples of the components of electric furnace dust and blast furnace dust.

[0041] [Table 1]

[0042] As shown in Table 1, electric furnace dust tends to have a higher zinc content than other dusts. The briquettes 22 may contain electric furnace dust. The rotary hearth furnace 100 and the reduced iron production apparatus 200 including the rotary hearth furnace 100 can suppress rotation obstructions of the rotary hearth furnace 100 and produce reduced iron stably and at low cost, even when electric furnace dust with a high zinc content is used as a raw material.

[0043] The briquettes 22 are transported by the transport section 24 and introduced onto the rotary hearth 12 from the introduction section 21 of the rotary hearth furnace 100 (introduction process). The introduction section 21 is, for example, configured with a vibrating sieve having slits. The briquettes 22 pass through the slits of the vibrating sieve and are introduced onto the rotary hearth 12. The furnace space 70 is heated to, for example, 1000 to 1300°C. The briquettes 22 introduced onto the rotary hearth 12 from the introduction section 21 are heated while moving inside the furnace body 11 as the rotary hearth 12 rotates (heating process). When the briquettes 22 contain iron oxide and zinc oxide, an oxidation-reduction reaction represented by the following reaction formula progresses as they are heated. Note that n may be any number, such as 1, 2, or 3. m may be any number, such as 1, 3, or 4.

[0044] Fe n O m +mC → nFe+mCO (1) Fe n O m +mCO → nFe+mCO2(2) ZnO+C → Zn+CO (3) ZnO + CO → Zn + CO2(4) C + O2 → CO2(5) C + CO2 → 2CO (6)

[0045] In the heating process, the first main burner 10A burns at an air ratio of less than 1.0, while the second main burner 10B burns at an air ratio of 1.0 or more. The iron oxide contained in the briquettes is reduced by the carbonaceous material, for example, according to the above formulas (1) and (2), to form reduced iron. Reduced iron containing reduced iron as a main component is discharged from the discharge section 60. In this manner, the process of obtaining reduced iron by heating the briquettes containing iron oxide and the carbonaceous material and reducing the iron oxide can be performed. After being cooled in the cooling section 62, the reduced iron may be used as a raw material for, for example, an electric furnace. The zinc oxide contained in the briquettes and the zinc produced by the reduction reactions of formulas (3) and (4) become fumes and circulate through the furnace space 70 of the rotary hearth furnace 100 together with combustion gases such as carbon dioxide and carbon monoxide.

[0046] The first main burner 10A shown in FIG. 2 has an air ratio of less than 1.0, so the metallic zinc fumes are not oxidized and are directly incorporated into the combustion gas. A portion of the combustion gas enters the vicinity of the gaps 32a and 32b. Hearth burners 13a and 13b are provided in the gaps 32a and 32b below the first main burner 10A, respectively. Therefore, the exhaust gas temperature is maintained near the gaps 32a and 32b. This prevents deposits containing metallic zinc as a primary component from adhering to the vicinity of the gaps 32a and 32b. Therefore, rotational problems of the rotary hearth 12 caused by deposits can be sufficiently prevented.

[0047] The second main burner 10B shown in FIG. 2 has an air ratio of 1 or more, so the zinc fumes are oxidized to zinc oxide. A portion of the combustion gas containing zinc oxide enters the gaps 32a and 32b. The hearth burners 13a and 13b are not provided in the gaps 32a and 32b below the second main burner 10B. Therefore, the combustion gas entering the gaps 32a and 32b is cooled, and as shown in FIG. 4, a deposit 75 containing zinc oxide as a main component is formed. Because the deposit 75 is brittle, it easily falls off due to vibration or the like. Therefore, it does not interfere with the rotation of the rotary hearth 12, and stable operation of the rotary hearth furnace 100 can be maintained.

[0048] 1, the combustion gas is discharged from an exhaust gas pipe 30 connected to the ceiling wall 11c. The exhaust gas discharged from the exhaust gas pipe 30 is cooled in a gas cooler .

[0049] In the recovery section 40, solids contained in the exhaust gas are captured and recovered. The recovery section 40 may have, for example, a bag filter. The recovered solids may include zinc and iron oxide in addition to zinc oxide. The exhaust gas obtained by removing the solids in the recovery section 40 is sucked in by a blower 45 and released into the atmosphere through a chimney 50.

[0050] In this manner, reduced iron can be produced by operating the rotary hearth furnace 100. In this production method, the rotary hearth furnace 100 and the reduced iron production apparatus 200 including the rotary hearth furnace 100 are used, so that reduced iron can be produced stably at low production costs. The reduced iron may contain components other than iron.

[0051] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. [Example]

[0052] The present disclosure will be described in more detail with reference to the following examples and comparative examples, but the present disclosure is not limited to the following examples in any way.

[0053] (Experimental Example 1) A rotary hearth furnace 110 shown in FIG. 7 was prepared. This rotary hearth furnace 110 was equipped with seven main burners 10 on the outer side wall and four on the inner side wall. The height of each main burner 10 (H in FIGS. 3 and 4 ) was 1.5 m, based on the maximum height of the upper surface of the rotary hearth 12. On the other hand, the rotary hearth furnace 110 was not equipped with hearth burners. Briquettes were prepared using electric furnace dust and carbonaceous material (pulverized coal). These briquettes were introduced onto the rotary hearth 12 in the rotary hearth furnace 110 through the introduction section 21 and heated to produce reduced iron. During production, the air ratio of only one main burner installed on the outer side wall 11 a was set to 0.7. The air ratios of the other 10 main burners were all set to 1.0. After 120 days of continuous operation, the thickness of deposits adhering to the gap 32 a below the main burners 10 with an air ratio of 0.7 was measured.

[0054] The thickness was measured as the sum of the thickness of the deposits on the outer peripheral surface 12a of the rotary hearth 12, which forms the gap 32a, and the thickness of the deposits on the inner surface of the outer peripheral sidewall 11a facing it (hereinafter simply referred to as "thickness"). Each thickness was measured as a length along the direction in which the outer peripheral surface 12a of the rotary hearth 12 faces the inner surface of the outer peripheral sidewall 11a. Measurements were performed in an area 3 m on both sides (1.5 m on each side) centered on the main burner 10, which had an air ratio of 0.7, and the maximum thickness was determined. The results are shown in Table 2.

[0055] The strength of the adhered material was checked by poking it with the tip of a tool. If the adhered material did not crumble when poked with the tip of the tool, it was rated as "A," and if the adhered material crumbled when poked with the tip of the tool, it was rated as "B." The results are shown in Table 2.

[0056] The deposits were analyzed by ICP atomic emission spectrometry. Based on the analysis results, the main components (most abundant components) of the deposits were determined. As a result, the main components of the deposits were as shown in Table 2.

[0057] (Experimental Examples 2 to 6) Reduced iron was produced in the same manner as in Experimental Example 1, except that the air ratio of one main burner provided on the outer peripheral side wall was changed as shown in Table 2. Then, the maximum thickness of the deposit was measured in the same manner as in Experimental Example 1. The strength and main components of the deposit were evaluated in the same manner as in Experimental Example 1. The results are shown in Table 2.

[0058] [Table 2]

[0059] The relationship between the air ratio and the thickness (maximum value) of the deposits shown in Table 2 is plotted in Figure 6. As shown in Figure 6 and Table 2, it was confirmed that the thickness of the deposits increases significantly when the air ratio is less than 1.0, which is a reducing atmosphere. It was also confirmed that the deposits formed in reductive combustion with an air ratio of less than 1.0 are strong and contain metallic zinc as the main component. On the other hand, in oxidative combustion with an air ratio of 1.0 or more, the thickness of the deposits was small. It was confirmed that this deposit contained zinc oxide as the main component and was brittle, so it easily fell off from the surface it was attached to.

[0060] (Experimental Example 7) Briquettes made from electric furnace dust and carbonaceous material (pulverized coal) were supplied to the rotary hearth furnace 110 used in Experimental Examples 1 to 6 to produce reduced iron. The briquettes were introduced into the rotary hearth furnace 110 from the inlet 21 and heated to produce reduced iron. As shown in FIG. 8, six of the main burners 10 were designated as first main burners 10A and five were designated as second main burners 10B. The air ratio of each of the first main burners 10A was 0.7, and the air ratio of each of the second main burners 10B was 1.1.

[0061] After 120 days of continuous operation, the interior of the rotary hearth furnace 110 was visually inspected to confirm the presence or absence of deposits. As a result, as shown in FIG. 8, the gaps 32a and 32b below the first main burner 10A were clogged with deposits 76. The deposits 76 primarily contained metallic zinc. Meanwhile, a small amount of deposits formed in the gaps 32a and 32b below the second main burner 10B, but the gaps 32a and 32b were not clogged. This deposit primarily contained zinc oxide and was brittle. This confirmed that rotational problems could be suppressed and stable operation could be maintained by heating only the gaps 32a and 32b below the first main burner 10A with a hearth burner, where the air ratio is less than 1.0. [Explanation of symbols]

[0062] 10...main burner, 10A...first main burner, 10B...second main burner, 11...furnace body, 11a...outer peripheral side wall, 11b...inner peripheral side wall, 11c...ceiling wall, 12...rotary hearth, 12A...upper surface, 12a...outer peripheral surface, 12b...inner peripheral surface, 13a, 13b...hearth burner, 15, 16...seal plate, 17...water seal section, 21...inlet section, 22...briquette, 24...conveyor section, 30...exhaust gas pipe, 32a, 32b...gap, 35...gas cooling section, 40...recovery section, 45...blower, 50...chimney, 60...outlet section, 62...cooling section, 70...inner furnace space, 71...first region, 72...second region, 75, 76...deposits, 100, 110...rotary hearth furnace, 200...apparatus for producing reduced iron.

Claims

1. A rotary hearth furnace for producing reduced iron, comprising: a circular rotary hearth; an annular furnace body having an outer peripheral side wall disposed along the outer peripheral surface of the rotary hearth and an inner peripheral side wall disposed along the inner peripheral surface of the rotary hearth; a plurality of main burners arranged side by side along a circumferential direction on the outer peripheral side wall and the inner peripheral side wall; A rotary hearth furnace comprising: a plurality of hearth burners for heating a gap between the outer peripheral surface and the outer peripheral side wall, and a gap between the inner peripheral surface and the inner peripheral side wall, the main burners include a first main burner that burns at an air ratio of less than 1.0 and a second main burner that burns at an air ratio of 1.0 or more; A rotary hearth furnace, wherein the number of the hearth burners provided closer to the first main burner than the second main burner is greater than the number of the hearth burners provided closer to the second main burner than the first main burner.

2. The annular furnace space formed by the furnace body and the rotary hearth has a first region in which a plurality of first main burners that burn at an air ratio of less than 1.0 are arranged along the circumferential direction, and a second region in which a plurality of second main burners that burn at an air ratio of 1.0 or more are arranged along the circumferential direction, 2. The rotary hearth furnace according to claim 1, wherein the number of the hearth burners provided in the first region is greater than the number of the hearth burners provided in the second region.

3. 3. The rotary hearth furnace according to claim 1, wherein all of the hearth burners are provided closer to the first main burner than the second main burner.

4. 4. The rotary hearth furnace according to claim 1, wherein, in a plan view, the distance between the hearth burner that is provided closer to the first main burner than the second main burner and the first main burner that is closest to the hearth burner is 1.5 m or less.

5. 5. The rotary hearth furnace according to claim 1, wherein the height of the plurality of main burners relative to the upper surface of the rotary hearth is 1.5 m or less.

6. A rotary hearth furnace as described in any one of claims 1 to 5, in which the reduced iron is obtained by heating briquettes containing dust and carbonaceous material.

7. a circular rotary hearth; an annular furnace body having an outer peripheral side wall disposed along the outer peripheral surface of the rotary hearth and an inner peripheral side wall disposed along the inner peripheral surface of the rotary hearth; a plurality of main burners arranged side by side along a circumferential direction on the outer peripheral side wall and the inner peripheral side wall; a plurality of hearth burners that heat a gap between the outer peripheral surface and the outer peripheral side wall and a gap between the inner peripheral surface and the inner peripheral side wall, a step of firing a first main burner, which is a part of the main burners, at an air ratio of less than 1.0 and a second main burner, which is a part of the main burners, at an air ratio of 1.0 or more; A method for operating a rotary hearth furnace, wherein the number of hearth burners that are arranged and combustible nearer to the first main burner than to the second main burner is greater than the number of hearth burners that are arranged and combustible nearer to the second main burner than to the first main burner.

8. A method for operating a rotary hearth furnace as described in claim 7, comprising a step of introducing briquettes containing dust and carbon material onto the rotary hearth.

9. A method for producing reduced iron, comprising the steps of heating briquettes containing iron oxide and a carbonaceous material using the rotary hearth furnace according to any one of claims 1 to 6, and reducing the iron oxide to obtain reduced iron.

10. The method for producing reduced iron according to claim 9 , wherein the briquettes used in the step contain electric furnace dust containing the iron oxide.

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