Method for melting direct reduced iron, method for manufacturing solid iron, method for manufacturing civil engineering and construction materials, and system for melting direct reduced iron

Abstract

The present invention provides a feature for melting direct reduction iron in which gangue is efficiently removed from the direct reduction iron. This invention comprises: a direct reduction step for bringing iron ore or a mixture of iron ore and a component adjustment material into contact with a reducing agent under heating and obtaining direct reduction iron; a melting step for melting the direct reduction iron in an induction melting furnace and obtaining molten iron; a slag discharging step for discharging slag produced in the melting step out of the melting furnace; and optionally, a refining step. The charging temperature at which the direct reduction iron melted in the melting step is charged into the induction melting furnace is in the range between the temperature after the end of the direct reduction step and the atmospheric temperature. The melting step includes a first step for blowing a gas into the molten iron for a part or all of the period of the melting step, and optionally includes one or more steps selected from 1) a second step for adding a slag component adjustment agent, 2) a third step for supplying heat from a heat source installed above the induction melting furnace to the slag, and 3) a fourth step for supplying one or more types of reducing solid or gas.

Description

[Technical field]

[0001] The present invention relates to a method for melting direct reduced iron for removing gangue contained in direct reduced iron, solid iron and a method for producing solid iron using this method, civil engineering and construction materials and a method for producing civil engineering and construction materials, and a system for melting direct reduced iron. [Background technology]

[0002] In recent years, there has been an increasing demand for the expanded use of cold iron ore (scrap) in the steelmaking industry. Recycling of iron ore is essential to building a recycling-oriented society. Furthermore, from the perspective of preventing global warming, increasing scrap usage is essential to meet the demand for reducing CO2 emissions. Unlike iron ore, which is iron oxide (Fe2O3), scrap does not require a reduction process in the smelting process, making it possible to reduce CO2 emissions. As a result, the usage of cold iron ore is steadily increasing.

[0003] The blast furnace-converter process is a steelmaking process in which iron ore (Fe2O3) is charged into a blast furnace together with coke (carbon source), a reducing agent, to produce molten pig iron with a C concentration of about 4.5-5% by mass, and then the molten pig iron is charged into a converter to oxidize and remove impurities such as C, Si, and P. When producing molten pig iron in a blast furnace, about 500 kg of carbon source is required per ton of molten pig iron for the reduction of iron ore. About 2 tons of CO2 gas is generated. On the other hand, when producing molten steel using scrap iron as a raw material, the carbon source required for the reduction of iron ore is not required. Therefore, even when considering the energy required to melt scrap iron, replacing 1 ton of molten pig iron with 1 ton of scrap iron leads to a reduction in CO2 emissions of about 1.5 tons. From the above, it is necessary to increase the amount of scrap used in order to reduce greenhouse gas emissions and maintain production activities at the same time.

[0004] However, due to the tight supply and demand of iron scrap, especially high-quality iron scrap that is essential for the production of high-grade steel, there is a growing need for reduced iron instead of scrap. Reduced iron is produced by reducing iron ore. However, reduced iron does not require a high carbon concentration in the iron produced, as is the case with the blast furnace-converter process, and does not use an excessive carbon source, which leads to a reduction in CO2 emissions of approximately 0.2 t per ton of iron. In addition, by using a hydrocarbon gas such as hydrogen or natural gas as the reducing agent instead of a carbon source, it is possible to further reduce CO2 emissions.

[0005] The composition of iron ore, the raw material for reduced iron, varies depending on where it is mined. The composition of iron ore is mainly evaluated based on the Fe content and gangue content. Table 1 shows an example of the composition of iron ore.

[0006] [Table 1]

[0007] The Fe content is expressed as the total amount of iron in the iron ore, T.Fe (mass%); the higher this value, the more Fe is contained, and therefore the more valuable it is as a raw material. The gangue content is expressed as the sum of oxides other than Fe in the iron ore, most of which are SiO2 and Al2O3, with the remainder containing around 0.1 mass% CaO and MgO. In the process of smelting iron ore to obtain iron, gangue components are removed as impurities, so the more gangue there is, the lower the Fe content becomes, and the transportation costs and smelting costs per unit of Fe increase.

[0008] Furthermore, the properties of reduced iron made from iron ore as a raw material, such as the metallization rate and composition, vary depending on the brand of iron ore used, the type and unit consumption of the raw material composition adjuster mixed in, the type and unit consumption of the reducing material, the reduction temperature, and the type of reduced iron production facility. Table 2 shows an example of the chemical composition of reduced iron.

[0009] [Table 2]

[0010] In the production of reduced iron, it is common to add CaO as a raw material component adjuster and form slag with the gangue contained in the iron ore to ensure the strength of the reduced iron. The less slag there is and the higher the metallization rate of reduced iron, the less burden there is on the subsequent transportation, melting, and refining processes. On the other hand, such reduced iron requires the use of high-grade iron ore, which increases the cost of raw materials and the cost of reduction treatment to increase the metallization rate. Therefore, it is desirable to develop a process that can efficiently separate slag from reduced iron with a high slag content produced using cheap, low-grade iron ore and increase the metallic iron content.

[0011] A method for producing reduced metals has been proposed in which gangue contained in such ores is removed as slag. For example, Patent Documents 1 and 2 propose a method for producing reduced metals by charging raw materials containing metal-containing substances onto a solid reducing material layer deposited on the hearth of a moving hearth furnace, heating and reducing the raw materials, and melting the raw materials at least once to separate the metal and slag. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] JP 2000-292069 A [Patent Document 2] JP 2004-204293 A [Non-patent literature]

[0013] [Non-Patent Document 1] Slag Atlas, 2nd ed., Verlag Stahleisen GmbH, Duesseldorf, (1995), 105, 126. Summary of the Invention [Problem to be solved by the invention]

[0014] However, the conventional technology has the following problems. The techniques disclosed in Patent Documents 1 and 2 are premised on the use of carbonaceous solid raw materials as reducing agents to separate metal and slag. Therefore, in a process in which reduction is performed using a hydrogen-based reducing agent, which is expected to become mainstream in the future from the viewpoint of reducing CO2 emissions, it is expected that the efficiency of separation of metal and slag will decrease. Specifically, the furnace atmosphere temperature of a moving hearth furnace such as a rotary hearth furnace is usually about 1300°C. It is believed that the reason why the reduced metal melts at this temperature is because the carbonaceous solid raw materials carburize the metal, lowering the melting point of the metal. Therefore, in a process in which reduction is performed using a hydrogen-based reducing agent, the melting point of the metal does not drop due to carburization, so the liquid phase rate of the charge does not increase, and it is expected that separation of metal and slag will not proceed easily.

[0015] That is, in the reduction process, it may be difficult to remove the gangue, and it is necessary to separate the gangue from the produced reduced iron. Specifically, when the reduced iron is melted in an induction melting furnace, it is necessary to prevent the slag generated from the gangue from solidifying.

[0016] The present invention has been made in view of the above circumstances, and has an object to provide a method and system for melting direct reduced iron that efficiently remove gangue from direct reduced iron, as well as a method for producing high-purity solid iron and a method for producing civil engineering and construction materials that utilize by-products, using the melting method. [Means for solving the problem]

[0017] The inventors discovered that, when melting direct reduced iron in an induction melting furnace, promoting heat transfer between the molten iron and slag and controlling the composition of the slag can suppress solidification of the slag, making it easier to discharge the slag from the furnace, and thus completed the present invention.

[0018] The method for melting direct reduced iron according to the present invention, which advantageously solves the above problems, comprises the steps of: Raw materials containing CaO Ingredient adjustment Agent the direct reduced iron is charged into the induction melting furnace at a temperature ranging from the temperature after the direct reduction step to atmospheric temperature, and the melting step includes a first step of blowing gas into the molten iron for a part or the entire duration of the melting step, and optionally includes one or more steps selected from 1) a second step of adding a slag component adjuster, 2) a third step of supplying heat to the slag from a heat source installed above the induction melting furnace, and 3) a fourth step of supplying one or more types of reducing solids or gases.

[0019] The method for melting direct reduced iron according to the present invention includes the steps of: (a) In the first step, a height H (m) from a position of a gas supply nozzle for injecting the gas into the molten iron to the surface of the molten iron bath is expressed by the following formula (1), and the gas is injected into the molten iron so as to satisfy the following formula (2), where ρ g : Density of the supply gas (kg / m 3 ), ρ l : Density of molten iron (kg / m 3 ), Q: Gas supply rate (Nm 3 / min), N: number of gas supply nozzles (-), d: gas supply nozzle diameter (m), D: inner diameter of induction furnace (m), W DRI : Weight of reduced iron supplied to the induction furnace (kg), (%T.Fe) DRI : total iron concentration in reduced iron (mass%), h: height from the bottom of the induction furnace to the gas supply nozzle position (m), (b) In the second step, the type and amount of the slag component adjuster are adjusted so that the composition of the slag generated in the dissolution step has a basicity, which is the ratio of the CaO concentration (%CaO) to the SiO2 concentration (%SiO2) on a mass basis, in the range of 0.5 to 2.0 and an Al2O3 concentration (%Al2O3) in the range of 10 to 25 mass%; (c) In the fourth step, the type and supply amount of the reducing solid or gas are adjusted so that the total iron concentration (%T.Fe) of the slag produced in the melting step is 20 mass% or less; This is thought to be a more preferable solution. H=1.27×W DRI / (ρ l D 2 )×(%T.Fe) DRI / 100-h···(1) H>0.18×(ρ g Q 2 / ρ l N 2 d 2 ) 1 / 3 (2)

[0020] The method for producing solid iron according to the present invention, which advantageously solves the above problems, is characterized in that molten iron obtained by any of the above methods for melting direct reduced iron is solidified to produce solid iron. The solid iron according to the present invention is produced by the method and is characterized in that the total iron concentration T.Fe is 93 mass% or more and the total of oxide components is 3 mass% or less.

[0021] The present invention provides a method for producing civil engineering and construction materials that advantageously solves the above-mentioned problems, comprising: a melting step of melting direct reduced iron in an induction melting furnace to obtain molten iron; a slag removal step of discharging slag produced in the melting step outside the melting furnace; and a cooling and solidification step of cooling and solidifying the slag removed in the slag removal step to obtain a raw material for civil engineering and construction materials, wherein the temperature at which the direct reduced iron melted in the melting step is charged into the induction melting furnace is in a range from the temperature after the completion of the direct reduction step to atmospheric temperature, and the melting step includes a first step of blowing gas into the molten iron for a part or the entirety of the melting step, and optionally further includes one or more steps selected from 1) a second step of adding a slag component adjuster, and 2) a third step of supplying heat to the slag from a heat source installed above the induction melting furnace. Furthermore, the civil engineering and construction material according to the present invention is a civil engineering and construction material manufactured by the manufacturing method, and is characterized in that the basicity, which is the ratio of the CaO concentration (%CaO) to the SiO2 concentration (%SiO2) by mass, is in the range of 0.5 to 2.0, and the Al2O3 concentration (%Al2O3) is in the range of 10 to 25 mass%.

[0022] The present invention provides a direct reduced iron smelting system that advantageously solves the above problems, comprising: Raw materials containing CaO Ingredient adjustment Agentthe direct reduced iron is charged into the induction melting furnace at a temperature ranging from the temperature after completion of the direct reduction process to atmospheric temperature, and the induction melting furnace has a function of blowing gas into the molten iron for a part or the entire period during which the direct reduced iron is melted, and optionally has one or more functions selected from the group consisting of 1) a function of adding a slag composition adjuster, 2) a function of supplying heat to the slag from a heat source installed above the induction melting furnace, and 3) a function of supplying one or more types of reducing solids or gases. Incidentally, it is considered that a more preferable solution for the direct reduced iron melting system according to the present invention is to provide two or more induction melting furnaces for one direct reduction furnace. Effect of the Invention

[0023] According to the method and system for melting direct reduced iron of the present invention, when melting direct reduced iron in an induction melting furnace, gas is blown into the molten iron, and optionally at least one of supplying an appropriate flow rate of gas into the molten iron, controlling the composition of the generated slag, and supplying heat to the slag from a heat source installed on the furnace is performed, thereby making it possible to maintain the slag in a fluidized state and to separate the slag while melting the metallic iron in the direct reduced iron. In particular, by charging the produced direct reduced iron at a high temperature into the induction melting furnace, it is possible to reduce the time and energy required for melting.

[0024] According to the method for producing solid iron and the solid iron of the present invention, the molten iron and the slag are separated and the molten iron is solidified, so that it is possible to produce solid iron with a high purity, which is preferable. Also, according to the method for producing civil engineering and construction materials and the civil engineering and construction materials of the present invention, by-products are recovered, and in particular, the component composition is adjusted before recovery, so that the by-products can be effectively utilized. [Brief description of the drawings]

[0025] [Figure 1] 1 is a graph in which the compositions of slag obtained from the reduced iron compositions shown in Table 2 are plotted on an Al2O3-CaO-SiO2 ternary phase diagram. [Diagram 2] FIG. 1 is a graph in which the compositions of slag obtained from the reduced iron compositions shown in Table 2 are plotted on a CaO-SiO2-FeO ternary phase diagram. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The following is a detailed description of the embodiments of the present invention. The following embodiments are merely examples of systems and methods for embodying the technical ideas of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical ideas of the present invention can be modified in various ways within the technical scope described in the claims.

[0027] The inventors conducted research on the premise that gangue would be removed from reduced iron by first melting the reduced iron in an induction melting furnace. The heating and melting of reduced iron in an induction melting furnace is characterized by the fact that the metallic iron in the reduced iron is directly and efficiently heated by induced current. However, since the slag is not directly heated, there is an issue that the slag that floats on the molten iron due to the difference in specific gravity solidifies, making it difficult to charge additional reduced iron.

[0028] First Embodiment Therefore, the inventors have conducted a search for suitable conditions for melting reduced iron in an induction melting furnace and separating the gangue as slag. As a result, they have found that if gas is blown into the bath when melting reduced iron in an induction melting furnace, the solidification of the slag is suppressed, and the reduced iron can be efficiently melted and the slag separated. Furthermore, they have found that the slag can be kept in a fluidized state by performing at least one of the following: supplying an appropriate flow rate of gas into the molten iron, controlling the composition of the generated slag, and supplying heat to the slag from a heat source installed on the furnace, and thus it is possible to more effectively melt the metallic iron in the reduced iron and separate the slag. Here, the state in which the slag is fluidized refers to a state in which the slag is entirely red-hot and high-temperature slag is constantly circulating.

[0029] By keeping the slag in a fluid state in this way, the additional reduced iron is not hindered by the solidified slag when it is charged, and the volume of the induction melting furnace can be effectively utilized. Furthermore, since the slag is fluid, it is easy to separate the molten iron and the slag by overflowing it from the top of the furnace or by using a slag dragger to remove the slag. When separating the slag, it is preferable to limit the area where the slag is discharged by tilting the furnace body from the viewpoint of handling the high-temperature slag and repairing damaged areas.

[0030] Reduced iron is produced by mixing iron ore, which is the raw material, or iron ore and raw material Ingredient adjustment Agent The direct reduction can be obtained by contacting a mixture of the above with a reducing agent while heating it in, for example, a rotary hearth furnace as a direct reduction furnace. raw material Ingredient adjustment AgentAs the reducing agent, quicklime containing CaO can be used, and as the reducing agent, carbonaceous powder and reducing gases such as H2, CO and CH4 can be used as a solid carbon source. The temperature of the reduced iron charged into the induction melting furnace is in the range from the temperature after the reduction treatment in the direct reduction furnace to the atmospheric temperature. It is preferable to charge high-temperature reduced iron into the induction melting furnace because it can reduce the time and energy required for melting. Therefore, the temperature of the reduced iron charged into the induction melting furnace is preferably higher than the atmospheric temperature, more preferably 300°C or higher, and even more preferably the temperature drop is kept to the minimum required for transportation from the direct reduction furnace to the induction melting furnace.

[0031] The first embodiment of the present invention was obtained from the above-mentioned investigations, and is a method for producing iron ore or iron ore and raw material Ingredient adjustment Agent the direct reduced iron is brought into contact with a reducing material under heating to obtain directly reduced iron; the direct reduced iron is melted in an induction melting furnace to obtain molten iron; and the slag generated in the melting step is discharged outside the melting furnace, wherein the temperature at which the direct reduced iron melted in the melting step is charged into the induction melting furnace is in the range from the temperature after the end of the direct reduction step to atmospheric temperature, and the melting step includes a first step of blowing gas into the molten iron for a part or the entirety of the melting step, and optionally further includes one or more steps selected from 1) a second step of adding a slag component adjuster, and 2) a third step of supplying heat to the slag from a heat source installed above the induction melting furnace.

[0032] Second Embodiment Next, we investigated the optimization of the gas injection conditions. When gas is supplied into the molten iron, the molten iron is stirred as the gas floats up, and heat is transferred from the molten iron to the slag that is generated and floats up. As a result, the temperature of the slag rises, improving the fluidity of the slag. Any type of gas can be supplied as long as it does not liquefy when supplied through piping. For example, oxidizing gases such as oxygen and carbon dioxide oxidize the molten iron and reduce the iron yield, so inert gases such as Ar and N2 are preferable.

[0033] However, if the amount of gas supplied is too large, the gas passes through as a continuous phase all the way to the bath surface of the molten iron, which is known as blow-by. When blow-by occurs, the scattering (spitting) of the molten iron increases significantly, and the stirring and reaction efficiency of the supplied gas decreases, so the effect of heat transfer to the slag decreases. The inventors conducted various experiments under different conditions. As a result, they found that the height H (m) from the position of the gas supply nozzle for injecting the gas into the molten iron to the molten iron bath surface can be expressed by the following formula (1), and that blow-by can be avoided by injecting the gas into the molten iron so as to satisfy the following formula (2). H=1.27×W DRI / (ρ l D 2 )×(%T.Fe) DRI / 100-h···(1) H>0.18×(ρ g Q 2 / ρ l N 2 d 2 ) 1 / 3 (2) Here, ρ g : Density of the supply gas (kg / m 3 ), ρ l : Density of molten iron (kg / m 3 ), Q: Gas supply rate (Nm 3 / min), N: Number of gas supply nozzles (-), d: gas supply nozzle diameter (m), D: Inner diameter of induction furnace (m), W DRI : Weight of reduced iron supplied to the induction furnace (kg), (%T.Fe) DRI : total iron concentration in reduced iron (mass%), h: Height from the bottom of the induction furnace to the gas supply nozzle position (m) Represents.

[0034] On the other hand, if the amount of gas injected is too small, the stirring effect of the molten iron due to the gas floating will be small, and the heat transfer from the molten iron to the slag will be small. Therefore, the amount of gas injected should be at least 0.01 Nm 3It is preferable to supply gas at 10000 / min·t of molten iron.

[0035] The second embodiment of the present invention was obtained from the above-mentioned investigations, and in addition to the first embodiment, in the first step, the height H (m) from the position of the gas supply nozzle for injecting gas into the molten iron to the molten iron bath surface is expressed by the above formula (1), and the gas is injected into the molten iron so as to satisfy the above formula (2).

[0036] Third to Fifth Embodiments Next, optimization of the slag composition was investigated. The fluidity of slag depends greatly on the slag composition. For the reduced iron composition examples shown in Table 2, the slag components contained in the slag are converted so that the sum of the three components Al2O3-CaO-SiO2 or CaO-SiO2-FeO is 100%, and the compositions are plotted on the respective ternary phase diagrams in Figures 1 and 2. Non-Patent Document 1 was used as a reference for the respective ternary phase diagrams. Here, the FeO concentration in the reduced iron was calculated by multiplying the difference between the total iron concentration T.Fe and the metallic iron concentration M.Fe by 71.85 (molecular weight of FeO) / 55.85 (Fe atomic weight).

[0037] As is clear from FIG. 1, there exists a composition with a lower melting point than the slag components contained in reduced iron. However, when the slag composition has a high concentration of SiO2, the melting point is low but the viscosity is high and the fluidity is reduced. Therefore, it is preferable to set the basicity, which is the ratio of the CaO concentration (%CaO) to the SiO2 concentration (%SiO2) on a mass basis (hereinafter referred to as slag basicity), to 0.5 or more. In addition, from the viewpoint of lowering the melting point of the slag, it is preferable to set the slag basicity to 2.0 or less and the Al2O3 concentration to a range of 10 to 25 mass%. As a means for adjusting the slag composition, it is preferable to add a substance containing one or more of CaO, SiO2, and Al2O3 as a raw material component adjuster when producing reduced iron or as a slag component adjuster when melting reduced iron. The substance containing CaO may be any of limestone, slaked lime, quicklime, steelmaking slag, etc. In addition, limestone and slaked lime cause a temperature drop due to an endothermic reaction during decomposition, and steelmaking slag has a CaO concentration of about 40 to 50 mass%, so there is a problem that the amount of slag generated during the melting of reduced iron increases as the amount added increases. As a substance containing SiO2, silica stone, coal ash, steelmaking slag, etc. are considered. In addition, by adding metal Si or silicon sludge, SiO2 generated by reaction with the iron oxide remaining in the reduced iron may be utilized. As a substance containing Al2O3, in addition to natural stones such as corundum and bauxite, metal Al or aluminum dross may be added to utilize Al2O3 generated by reaction with the iron oxide remaining in the reduced iron.

[0038] The third embodiment of the present invention was obtained from the above-mentioned investigations, and in addition to the first and second embodiments, in the second step, the type and amount of the slag component adjuster are adjusted so that the slag basicity is in the range of 0.5 to 2.0 and the Al2O3 concentration is in the range of 10 to 25 mass% for the composition of the slag generated in the melting step.

[0039] As is clear from Fig. 2, the slag contained in the reduced iron contains a high concentration of FeO, which is effective in lowering the melting point of the slag and ensuring its fluidity. However, if left as is, the Fe yield will be low, so it is preferable to reduce the slag so that the total iron concentration (%T.Fe) in the slag separated by melting is 20 mass% or less, more preferably 10 mass% or less, and even more preferably 5 mass% or less. The reduction method may be any of the following: a) supplying a solid containing at least one reducing agent such as C, Al, or Si to the generated and floated slag; b) using a gas containing at least one reducing gas such as CO, H2, or hydrocarbon as a gas to be supplied into the molten iron; or c) increasing the reducing agent consumption rate at the time of producing the reduced iron, or a combination of these methods.

[0040] The fourth embodiment of the present invention was obtained from the above-mentioned investigation, and in addition to any one of the first to third embodiments, the melting step includes a fourth step of supplying one or more types of reducing solids or gases. Furthermore, the fifth embodiment of the present invention, in addition to the fourth embodiment, adjusts the type and supply amount of the reducing solids or gases so that the total iron concentration (%T.Fe) of the slag generated in the melting step is 20 mass% or less.

[0041] <3rd process> In order to suppress a drop in the temperature of the slag produced by melting the reduced iron in the induction melting furnace, it is preferable to have a third step of supplying heat to the slag by installing a heat source on the furnace. The heat source is not limited to burner heating, current heating using electrodes, induction heating by immersing a conductor in the slag, or other means capable of directly heating the slag, and a combination of multiple means may be used. Burner heating may be performed using any liquid fuel such as heavy oil, or a gaseous fuel such as CO, H2, or hydrocarbons, or a combination of these may be used. The conductor immersed in the slag may be any material that generates heat by induced current, but from the perspective of cost, an iron rod, a carbon rod, or the like may be immersed in the slag and held there, or reduced iron particles prepared to have a bulk density equivalent to that of the produced slag may be added on top and allowed to remain in the slag.

[0042] Sixth Embodiment Direct reduced iron, which is a raw material, may contain phosphorus as an impurity, and it is preferable to remove phosphorus from the molten iron. Also, it may be preferable to add desired components to the molten iron. The sixth embodiment of the present invention has been developed in response to such demands.

[0043] The dephosphorization reaction requires an oxygen source and a CaO source, as shown in formula (A) below. 2[P]+5 / 2·O2(g)+3CaO(s)=3CaO·P2O5(s) ···(A) Here, [P] represents phosphorus in the molten iron. For example, as a method for removing phosphorus as an impurity from the molten iron, an oxygen source and a CaO source can be supplied to the molten iron obtained in the melting step or to the molten iron after the slag is discharged in the slag removal step.

[0044] Pure oxygen gas is generally used as the oxygen source for dephosphorization. However, since the dephosphorization reaction is an exothermic reaction, it is advantageous to perform the dephosphorization treatment at a low temperature. Considering this, we have concluded that it is advantageous to lower the molten iron temperature within the range that does not affect the treatment.

[0045] As a result of the investigation, it was found that sufficient dephosphorization is possible while cooling the molten iron by supplying air or an iron oxide source such as iron ore or mill scale as the oxygen source. When air is used, a cooling effect on the pure oxygen gas is obtained by the heat removal proceeding as the sensible heat of the nitrogen gas contained in the air. When an iron oxide source is used, a cooling effect on the pure oxygen gas is obtained by absorbing heat when the iron oxide source is reduced to metallic Fe or forms molten slag in the form of iron oxide.

[0046] Secondly, calcium carbonate contained in limestone absorbs heat when it decomposes into CaO and CO2, so by using limestone as a CaO source, it becomes possible to cool the molten iron. A similar cooling effect can be obtained by supplying carbonates such as raw dolomite, but if the CaO ratio in the auxiliary raw materials is low, the amount of auxiliary raw materials to be added increases, which leads to operational issues such as an increase in the amount of slag generated and a longer time required for addition, so it is preferable to adjust the type and amount of auxiliary raw materials to be added taking into consideration the desired cooling effect and stable operation.

[0047] The occurrence of spitting depends on the freeboard (height from the top of the molten iron to the top of the vessel) of the dephosphorization vessel and the shape of the nozzle of the top blowing lance, so it is preferable to adjust the supply rate of pure oxygen or air and the height of the lance according to the operational status of the dephosphorization treatment. In addition, in order to provide agitation to the molten iron, it is recommended to blow in an inert gas. The inert gas is preferably blown by installing a porous plug or an injection lance. The slag basicity is preferably in the range of 1.5 to 4.0, and is adjusted by the amount of slag containing a large amount of SiO2 carried over from the slag removal step and the type and amount of CaO source added. If necessary, a SiO2 source such as silica stone or ferrosilicon, or a CaO source such as quicklime may be added.

[0048] If the slag basicity is low, the amount of phosphorus removed during the dephosphorization process is small. If the slag basicity is high, when the temperature of the molten iron drops, part of the slag solidifies and adheres to the refractory material, making it difficult to remove the slag after the dephosphorization process. This can lead to problems such as abnormal reactions when the molten iron is charged in the next process, or residual slag being mixed into the generated slag and causing component deviation. In addition, since a large amount of high-temperature exhaust gas is generated in such a dephosphorization process using air, it is acceptable to recover the exhaust heat using a boiler or the like.

[0049] Furthermore, the molten iron obtained in the above embodiment may be refined as it is to obtain the necessary components in the next process to obtain molten steel. The molten iron may be solidified once in a mold to produce solid iron, and then transported to the demand area, remelted, and refined to obtain molten steel. The former is advantageous in terms of energy because it does not require the solidification and remelting processes, but it is necessary to continuously install the reduced iron production plant, induction melting furnace, and refining equipment, and there are space restrictions when installing it at an existing steelworks. Alternatively, if all of them are newly installed, in addition to requiring huge costs, it becomes impossible to utilize the existing equipment. In the latter case, it becomes possible to separate the reduced iron production plant, induction melting furnace, and solidification equipment from the remelting equipment and refining equipment, and for example, it is possible to perform the reduced iron production to solidification in the country where the iron ore is produced, transport the solidified iron to the demand area, and perform remelting and refining. In this case, in addition to being able to utilize the existing refining equipment, the solidified iron is transported in a state in which the weight of the gangue contained in the iron ore is reduced, which also leads to a reduction in transportation costs. The process configuration to be selected can be appropriately selected taking into consideration the location of the business establishment, the facilities owned, etc.

[0050] The sixth embodiment of the present invention was obtained from the above-mentioned investigations, and in addition to any one of the first to fifth embodiments, further includes a refining step of refining the molten iron obtained in the melting step. The refining step is preferably performed after the slag removing step.

[0051] Seventh Embodiment The seventh embodiment of the present invention is to solidify the molten iron obtained by the method for melting direct reduced iron according to any one of the first to sixth embodiments to obtain solid iron. It is preferable that the total iron concentration T.Fe of the solid iron is 93 mass% or more and the total of oxide components is 3 mass% or less. There is no limitation on the shape or size of the mold in which the solid iron is solidified, but it is preferable to solidify the solid iron into granules with a particle size in the range of 10 to 100 mm in consideration of the subsequent loading, packaging, transportation, and supply to the facility for use.

[0052] Eighth Embodiment The eighth embodiment of the present invention utilizes the slag as a by-product as a civil engineering and construction material. That is, the eighth embodiment further includes a cooling and solidification step in which the slag discharged in the slag discharge step of the first embodiment is cooled and solidified to be used as a civil engineering and construction material raw material. The cooled and solidified slag has a basicity within the above range, and can have various particle size distributions depending on the cooling method. By carrying out additional particle size adjustment processing such as crushing and classification as necessary, it can be used as a material that utilizes its characteristics. For example, when the discharged slag is granulated with water, it becomes fine glass-like and has a specific surface area of ​​0.35 m. 2 / g or more 0.50m 2 / g, it can be used as a cement raw material (binder). It can also be used as a roadbed material or concrete aggregate by slowly cooling it in the air and adjusting the particle size according to the intended use. In this way, the business operator can select the appropriate cooling and solidification method depending on the intended use of the discharged slag.

[0053] <Ninth embodiment> The ninth embodiment of the present invention is configured as a direct reduced iron melting system suitable for application to the first to eighth embodiments. raw material Ingredient adjustment AgentThe direct reduction furnace includes a direct reduction furnace that contacts the mixture with a reducing material under heating to obtain directly reduced iron, an induction melting furnace that melts the directly reduced iron to obtain molten iron, and a slag discharge mechanism that discharges the slag generated in the induction melting furnace to the outside of the melting furnace. The direct reduction furnace may be a shaft furnace, a fluidized bed furnace, a rotary hearth furnace, or the like. Most of them are systems in which raw materials are continuously supplied and reduced iron is continuously produced. On the other hand, since the induction melting furnace is a batch type process, by combining multiple induction melting furnaces with one reduced iron production plant, it is possible to supply reduced iron to other induction melting furnaces while melting reduced iron in one induction melting furnace, thereby reducing the waiting time for processing. Therefore, it is preferable to provide two or more induction melting furnaces for one direct reduction furnace. As the slag discharge mechanism, a slag discharge port that allows slag to overflow from the top of the induction melting furnace and discharge it, a mechanism for tilting the furnace body, or a slag drucker that scrapes out slag can be installed.

[0054] It is preferable that the direct reduction furnace and the induction melting furnace are arranged close to each other so that the direct reduced iron melted in the induction melting furnace is charged into the induction melting furnace at a temperature ranging from the temperature after the completion of the direct reduction step to the atmospheric temperature. It is preferable that the direct reduced iron is transported to the induction melting furnace while suppressing a temperature drop.

[0055] Gas can be blown into the molten iron in the induction melting furnace by installing a gas supply nozzle at the bottom or wall of the furnace. Gas can also be blown in through a submerged lance. Slag composition adjustment Agent A hopper can be provided at the top of the induction melting furnace for the addition of slag. Also, the heat source described in the third step above can be installed to heat the slag. EXAMPLES

[0056] Example 1 A mixture of iron ore with the composition shown in Table 3, quicklime as a component adjuster, and carbonaceous powder as a reducing agent was charged into a rotary hearth furnace with a production capacity of 5 tons / hr. The treatment temperature was 1000°C ± 20°C, and the oxygen partial pressure P O2 is logPO2 Direct reduced iron was produced by controlling the amounts and ratios of fuel gas and oxygen supplied to the heating burner so that the temperature was in the range of -14 to -15. The amount of quicklime added was determined so that the basicity (ratio of (%CaO) / (%SiO2) by mass) was about 1. In this facility (rotary hearth furnace), the operating conditions were set so that the time from charging to discharging was 90 minutes, and the temperature was measured and the gas composition was analyzed at the location where the charged sample was located 45 minutes after the sample was charged.

[0057] [Table 3]

[0058] The concentrations of carbon monoxide (CO) and carbon dioxide (CO2) in the collected gas are measured using an infrared gas analyzer, and the oxygen partial pressure P is calculated from the measured CO / CO2 ratio, which is the ratio of the respective concentrations (partial pressures). O2 was calculated from the following formula: 2CO2(g)=2CO(g)+O2(g) ···(B) ΔG 〇 =134300-40.74×T(cal / mol) ···(3) K = exp(-ΔG 〇 / RT)=(P CO / P CO2 ) 2 P O2 (4) Here, T is the reaction temperature (K), K is the equilibrium constant of the chemical reaction in Eq. (B), R is the gas constant (cal / (K mol)), and P CO , P CO2 are the partial pressures of CO and CO2 in gas composition analysis.

[0059] The direct reduced iron obtained by this treatment was cooled to about 25° C. and then analyzed. The composition was found to be equivalent to that of reduced iron C shown in Table 2.

[0060] In an induction melting furnace with an inner diameter of 0.9 m and a height from the bottom of the furnace to the bottom of the tapping trough of 1.8 m, 0.5 t of seed molten iron was melted in the furnace, and the reduced iron prepared as described above and cooled to room temperature was added so as not to overflow from the furnace body. After it was confirmed that the melting progressed and the stack height in the furnace had decreased, the reduced iron was added from the top from a hopper installed at the top, and this was repeated until the total amount of reduced iron added reached 5.0 t. At the bottom of the furnace, six bottom blowing nozzles were installed at equal intervals at positions with PCD (Pitch Circle Diameter) of 0.3 m and 0.6 m, and the same flow rate of gas could be supplied to each of the nozzles in any combination via a gas header. A hopper capable of supplying auxiliary materials was installed on the furnace, and it was possible to cut out 10 kg at any time during processing. The temperature of the molten iron in the furnace was measured appropriately, and the output of the induction melting furnace or the supply speed of the reduced iron and auxiliary materials was adjusted so that it was 1600 ± 20 °C.

[0061] The melting process was carried out by changing the gas flow rate from the nozzle or the type and amount of the auxiliary materials added. The quicklime used as the auxiliary material was made by roasting limestone at high temperature to remove CO2, and the CaO concentration was almost 100 mass%. Silica was collected from a quarry and crushed, and the SiO2 concentration was about 98 mass% and it contained small amounts of Al2O3 and MgO. Bauxite was imported ore as a raw material for aluminum smelting and crushed, and the Al2O3 concentration was about 50 mass% and the remaining impurities included water of crystallization, SiO2, TiO2, etc. After the process, the furnace was tilted, the molten iron in the furnace was completely discharged, and the weight of the discharged molten iron was weighed. In addition, the slag was collected and crushed to 53 μm or less for chemical analysis. The obtained values ​​are shown in Tables 4-1 and 4-2 together with the process conditions. For comparison, the process was also carried out under conditions where no gas was supplied from the bottom of the furnace.

[0062] [Table 4-1]

[0063] [Table 4-2]

[0064] In Table 4-2, the slag fluidity was judged as follows: ◯: the surface of the slag in the furnace was red hot overall, and high-temperature slag was constantly circulating, △: at least a part of the slag was black and solid, and the solid slag was constantly moving on the surface, and ×: the slag was black overall, and only cracked parts were red hot, and the slag was stagnant. Here, under the condition of ◯, it took 90 minutes from the start of adding 5.0 t of reduced iron until it completely melted and tilting for pouring began, whereas under the condition of △, it took longer than 90 minutes. This difference is because part of the slag was solid, and it took time for the slag to move to the tapping hole in the slag removal step. Under the condition of ×, even if reduced iron and auxiliary materials were added, they accumulated on the solidified slag surface and did not penetrate into the inside of the molten iron, and only the reduced iron that had been charged from the beginning could be melted.

[0065] The slag fluidity of the treatments No. 1 to 19 was evaluated as ○ or △, whereas the slag fluidity of the treatment No. 20 was evaluated as ×. In treatment No. 20, gas was not supplied from the bottom of the hearth, so it is considered that the heat supply from the molten iron to the slag was insufficient and the slag solidified. In treatments No. 1 to 19, gas was supplied, so the slag fluidity was maintained regardless of the slag composition, and it was possible to remove the slag. However, as is clear from Tables 4-1 and 4-2, the slag fluidity was particularly good when the above formulas (1) and (2) were satisfied, the slag basicity C / S was in the range of 0.5 to 2.0, and the Al2O3 concentration in the slag converted to a ternary system of CaO-SiO2-Al2O3 was in the range of 10% by mass to 25% by mass.

[0066] In the cases of No. 13 to 15, the amount of gas supplied from the hearth was too large, which caused the above-mentioned blow-by phenomenon and significant scattering of molten iron. As a result, the amount of metal tapped was also lower than in the other examples of the invention. In the case of No. 16, an excess of quicklime was added, which resulted in the slag basicity C / S exceeding 2.0, the melting point of the slag increasing, the fluidity decreasing, and the slag draining time increasing. In the case of No. 17, an excess of silica was added, which resulted in the slag basicity C / S being less than 0.5, the viscosity of the slag increasing, the fluidity decreasing, and the slag draining time increasing. In the case of No. 18, an excess of bauxite was added, which resulted in the Al2O3 concentration calculated as the CaO-SiO2-Al2O3 ternary system exceeding 25 mass%, the melting point of the slag increasing, the fluidity decreasing, and the slag draining time increasing. In process No. 19, excess quicklime and silica were added, resulting in dilution of the Al2O3 concentration in the CaO-SiO2-Al2O3 ternary system to less than 5 mass%, which increased the melting point of the slag, reduced its fluidity, and lengthened the draining time.

[0067] It was confirmed that auxiliary materials are not limited to quicklime, silica, and bauxite, and that there is no problem in controlling the slag composition with other substances mentioned above. It was found that it is better to add auxiliary materials step by step from the start of melting to the completion of melting. This is because adding a large amount of auxiliary materials immediately after the start of melting inhibits contact between the reduced iron particles or between the reduced iron and the seed molten metal, which leads to an increase in melting time due to a decrease in the efficiency of induction heating. In addition, slag has already been generated after the completion of melting, and depending on the composition, the slag may solidify, and even if auxiliary materials are added, they will sit on top of the solidified slag, which does not contribute to lowering the melting point of the slag.

[0068] Although the melting of reduced iron will proceed without the use of seed water, the use of seed water is preferable because the time required to melt the reduced iron is shortened as a result of heat being supplied from the induction-heated seed water to the solid reduced iron. It is particularly effective to have seed water of about 5% by mass or more relative to the reduced iron to be charged, but as the amount of seed water increases, the proportion of the seed water in the induction melting furnace increases and the amount of meltable reduced iron decreases, so the proportion of seed water is preferably 70% by mass or less relative to the reduced iron to be charged. In addition, the seed water may be used to newly melt scrap or large lump ingots with high bulk density, or it may be a part of the molten iron melted in the previous melting process that is left in the furnace.

[0069] Note that the process was performed with different nozzle positions and combinations. Regardless of the position or combination, when the amount of gas supplied per nozzle satisfied the above formula (2), the fluidity of the slag was ensured and the scattering of molten iron was small. In contrast, when formula (2) was not satisfied, the scattering of molten iron increased and the metal yield decreased.

[0070] It was also confirmed that the nozzle position can be on the side of the furnace instead of the bottom of the furnace. However, if the distance h from the nozzle on the side of the furnace to the bottom of the furnace is large, i.e., if H is small, the gas supply rate that satisfies formula (2) decreases, making it difficult to effectively transfer the heat of the molten iron to the slag, so it is preferable to position the nozzle at or near the bottom of the furnace as much as possible.

[0071] Example 2 The same induction furnace as that for the direct reduced iron produced in the same manner as in Example 1 was used to carry out a process for reducing the FeO content contained in the produced slag. CO, H2 and CH4 were supplied from the bottom nozzle as gas reducing agents, and the supply time of the gas reducing agents was constant at 90 minutes. In addition, solid C, metal Al and metal Si were added from above as solid reducing agents to the produced slag. Here, when reduction was performed with metal Al and metal Si, the basicity and Al2O3 concentration of the produced slag changed, so quicklime or silica stone was added as an auxiliary material to adjust the slag basicity and Al2O3 concentration. After the process, the furnace body was tilted, the molten iron in the furnace was completely discharged, and the weight of the discharged slag was weighed. In addition, the slag was sampled and crushed to 53 μm or less for chemical analysis. The obtained values ​​are shown in Tables 5-1 and 5-2 together with the process conditions.

[0072] [Table 5-1]

[0073] [Table 5-2]

[0074] It can be seen that under the conditions of supplying reducing gas in Process Nos. 21 to 27, the amount of metal tapped increased and the T.Fe concentration in the slag decreased compared to Process No. 4 in Example 1. In other words, it was revealed that the gas reducing agent effectively contributed to the reduction of FeO in the generated slag. In addition, when comparing Process Nos. 23 to 27, it can be seen that by increasing the amount of reducing gas (CH4) supplied, the T.Fe in the slag decreased and the amount of metal tapped increased.

[0075] Next, even under the conditions of adding a solid reducing agent in Processes No. 28 to 30, the amount of metal tapped was increased and the T.Fe concentration in the slag was decreased compared to Process No. 4 in Example 1. It was revealed that the solid reducing agent, like the reducing gas, effectively contributed to the reduction of FeO in the generated slag.

[0076] Various evaluations were carried out by changing the type and combination of reducing materials, the amount added, and the timing of addition, but in all cases, the amount of metal discharged increased and the T.Fe concentration in the slag decreased compared to conditions where no reducing material was added. For the purpose of this technology to produce molten iron, it is preferable to reduce the slag so that the T.Fe concentration in the slag is 20 mass% or less, more preferably 10 mass% or less, and even more preferably 5 mass% or less by supplying reducing materials to improve the iron yield. However, the utilization efficiency of the supplied reducing materials decreases as the reduction proceeds, so it is important to decide the processing method taking into consideration the composition and price of the reduced iron, auxiliary materials, and reducing materials.

[0077] Example 3 The high-temperature direct reduced iron produced in the rotary hearth furnace shown in Example 1 was directly charged into an induction melting furnace and melted. The direct reduced iron, which was about 1000°C when it left the rotary hearth furnace, was directly charged into an induction melting furnace similar to those in Examples 1 and 2 and melted. Here, the temperature of the reduced iron dropped during transportation until it was charged, and the temperature of the reduced iron immediately after charging was about 900°C. It was possible to induce heating of the reduced iron in a high-temperature state without any problems, and the time required for heating could be shortened by more than 30 minutes compared to the reduced iron of the same composition and the same treatment conditions. Here, even in the melting of the reduced iron directly charged in a heated state, the fluidity of the slag was ensured and stable treatment was possible when the above-mentioned gas injection conditions and slag composition were satisfied. In addition, if the above-mentioned conditions of slag basicity and Al2O3 concentration were satisfied at the time of producing the reduced iron, it was only necessary to supply gas under the conditions satisfying formula (2) at the time of melting in the induction melting furnace, and there was no need to add auxiliary materials.

[0078] Example 4 The molten iron obtained in Examples 1 and 2 was transferred to a ladle-type container after adjusting the temperature. At this time, of the slag generated during melting in the induction furnace due to gangue contained in the reduced iron, about 10 kg / t of molten iron was transferred to the ladle-type container together with the slag, and the remainder was transferred to a separate slag container. The ladle-type container was moved to a dephosphorization treatment facility, where the types and amounts of the oxygen source and lime source supplied were changed to perform dephosphorization treatment. The dephosphorization treatment facility has a top gas blowing lance, an auxiliary raw material cutting hopper, and a bottom blowing porous plug. Approximately 1 Nm of molten iron was blown from the top gas blowing lance. 3 Gas containing pure oxygen or air can be supplied at a rate of 10 kg / min / (min·t-molten iron). There are three auxiliary raw material extraction hoppers, each filled with iron ore, quicklime (CaO), and calcium carbonate (CaCO3), and each can be supplied at a rate of approximately 10 kg / min. Gas can be supplied from the bottom-blowing porous plug, and in this embodiment, the gas can be supplied at a rate of approximately 0.1 Nm 3 Pure Ar gas was supplied at a rate of 1000 ppm / min·t of molten iron.

[0079] The melting temperature in the induction melting furnace was adjusted so that the temperature of the molten iron before the dephosphorization treatment was approximately 1590°C. The timing before and after the dephosphorization treatment were defined as before and after the dephosphorization treatment, respectively, and temperature measurement and sampling were carried out using a sublance. The sampled samples were cut and polished, and the carbon concentration [C] and phosphorus concentration [P] in the molten iron were evaluated using a calibration curve prepared in advance by optical emission spectroscopy. It was also possible to measure the solidification temperature of the molten metal at the timing of the sublance temperature measurement and sampling, and the solidification temperature T of the molten iron after the dephosphorization treatment was determined. m was actually measured.

[0080] The start of the dephosphorization process was determined as the start of the downward movement of the top gas blowing lance, and after the top gas blowing lance reached a predetermined height, the supply of the oxygen gas source and the addition of the auxiliary materials were started. The end of the dephosphorization process was determined as the time when the top gas blowing lance had risen to the standby position after the supply of the predetermined amount of the oxygen gas source and the auxiliary materials was completed. The period during which the dephosphorization process was completed was determined as the treatment time t f (minutes).

[0081] After the dephosphorization process, the ladle-shaped vessel was tilted, and the slag on the molten iron was removed by a slag dragger. A portion of the removed slag was collected and subjected to chemical analysis. The ladle was then lifted and tilted by a crane, and the molten iron was transferred to a tundish. The molten iron was allowed to flow down from the tundish and collide with a platen, turning into droplets of molten iron. The molten iron was then dropped into a cooling water tank and solidified to produce granular iron. The particle size of the resulting granular iron was 0.1 to 30 mm. The particle size distribution was +0.1 mm-1 mm: 17.2 mass%, +1 mm-10 mm: 31.3 mass%, +10 mm-20 mm: 38.8 mass%, and +20 mm-30 mm: 12.7 mass%. Here, +NM means that the sieve is above the mesh size N and below the mesh size M.

[0082] Molten iron temperature after dephosphorization T f is the molten iron temperature before dephosphorization T i The slag basicity C / S is adjusted to be in the range of 1.5 to 4.0, and the molten iron temperature T after dephosphorization is adjusted to be lower. f The solidification temperature T of the molten iron m The temperature was adjusted to be 20°C higher than the normal temperature. As a result, the P concentration of the molten iron before dephosphorization [P] i : The P concentration of molten iron after dephosphorization is about 0.12 mass% [P] f The content of Cr was reduced to 0.02 to 0.04 mass %. In addition, solid iron could be produced without impeding the productivity of nugget iron.

[0083] It was confirmed that when the molten iron produced in the invention examples of Example 1 and Example 2, including the nugget iron produced above, is solidified, a solid iron having a total iron amount T.Fe of 93 mass% or more and a total oxide content of 3 mass% or less is obtained, regardless of the size and shape of the mold. The size and shape of the mold can be changed depending on the subsequent use, but it is preferable to solidify it into granules of 10 mm to 100 mm in size, taking into consideration loading, packaging, transportation, and supply to the equipment to be used.

[0084] The slag produced in the invention examples of Example 1 and Example 2 has the fluidity required for slag removal. The C / S ratio, which is the same as the basicity of the slag, is in the range of 0.5 to 2.0, and the Al2O3 concentration calculated as the CaO-SiO2-Al2O3 ternary system is in the range of 10 to 25 mass%. The molten slag is then granulated into fine glass particles, with a size of 0.35 m. 2 / g or more 0.50m 2 It was also confirmed that the molten slag can be used as a raw material for cement, with its particle size being less than several hundred mm, by slowly cooling the molten slag in air. It was also confirmed that the slag can be used as a roadbed material or concrete aggregate by crushing and classifying it to adjust the particle size appropriately.

[0085] In this specification, the unit of mass "t" is 10 3 kg, and the calorie unit "cal" is converted to "4.184J". Also, the volume unit "Nm 3 " indicates the standard state of the gas, which is defined in this specification as 1 atm (=101325 Pa) and 0°C. In the chemical formula notation, [M] indicates that element M is dissolved in the molten iron or reduced iron. [Industrial Applicability]

[0086] According to the method for melting direct reduced iron of the present invention, when direct reduced iron is melted in an induction melting furnace, gas is blown into the molten iron to increase the fluidity of the slag and separate the slag from the molten iron, and by melting the reduced iron at a high temperature in particular, energy can be saved and productivity can be improved, which is industrially useful.

Claims

1. a direct reduction step of contacting iron ore or a mixture of iron ore and a raw material component adjuster containing CaO with a reducing material under heating to obtain directly reduced iron; a melting step of melting the direct reduced iron in an induction melting furnace to obtain molten iron; A slag discharge step of discharging the slag generated in the melting step outside the melting furnace; Optionally, the method further comprises a refining step of refining the molten iron obtained in the melting step, a charging temperature of the direct reduced iron melted in the melting step into the induction melting furnace is in a range from a temperature at the end of the direct reduction step to atmospheric temperature; The melting step includes a first step of blowing an inert gas into the molten iron during a part or the entire duration of the melting step, In the first step, a height H (m) from a position of a gas supply nozzle for injecting the inert gas into the molten iron to a surface of the molten iron bath is expressed by the following formula (1), and the inert gas is injected into the molten iron so as to satisfy the following formula (2): The gas supply nozzle is a nozzle or a submerged lance provided at the bottom or wall of the furnace, Additionally, optionally, 1) a second step of adding a slag component adjuster; 2) a third step of supplying heat to the slag from a heat source installed on the induction melting furnace; and 3) A fourth step of supplying one or more reducing solids or gases A method for melting direct reduced iron, comprising one or more steps selected from the following: H=1.27×W DRI / (ρ l D 2 )×(%T.Fe) DRI / 100-h...(1) H>0.18×(ρ g Q 2 / ρ l N 2 d 2 ) 1 / 3...(2) Here, ρ g : density of the supply gas (kg / m 3 ), ρ l : density of molten iron (kg / m 3 ), Q: gas supply rate (Nm 3 / min); N: number of gas supply nozzles (-), d: gas supply nozzle diameter (m), D: inner diameter of induction furnace (m), W DRI : Weight of reduced iron supplied to the induction furnace (kg), (% T.Fe) DRI: Total iron concentration (mass%) contained in reduced iron, h: Height from the bottom of the induction furnace to the gas supply nozzle position (m) Represents.

2. In the second step, The composition of the slag produced in the melting step is, on a mass basis, SiO ２ Concentration (%SiO ２ The basicity, which is the ratio of the CaO concentration (% CaO) to the Al concentration (% CaO), is in the range of 0.5 to 2.

0. ２ O ３ Concentration (%Al ２ O ３ 2. The method for melting direct reduced iron according to claim 1, wherein the type and amount of the slag component adjuster are adjusted so that the content of the slag component adjuster is in the range of 10 to 25 mass %.

3. In the fourth step, 2. The method for melting direct reduced iron according to claim 1, wherein the type and supply amount of the reducing solid or gas are adjusted so that the composition of the slag generated in the melting step has a total iron concentration (% T.Fe) of 20 mass% or less.

4. A method for producing solid iron, comprising solidifying molten iron obtained by the method according to any one of claims 1 to 3 to produce solid iron.

5. The method for producing solid iron according to claim 4, wherein the solid iron has a total iron concentration T.Fe of 93 mass% or more and a total of oxide components of 3 mass% or less.

6. A method for manufacturing a civil engineering and construction material, comprising: a direct reduction step of contacting iron ore or a mixture of iron ore and a raw material component adjuster containing CaO with a reducing material under heating to obtain directly reduced iron; a melting step of melting the direct reduced iron in an induction melting furnace to obtain molten iron; A slag discharge step of discharging the slag generated in the melting step outside the melting furnace; A cooling and solidifying step of cooling and solidifying the slag discharged in the slag removal step to obtain a civil engineering and construction material raw material, a charging temperature of the direct reduced iron melted in the melting step into the induction melting furnace is in a range from a temperature at the end of the direct reduction step to atmospheric temperature; The melting step includes a first step of blowing an inert gas into the molten iron during a part or the entire duration of the melting step, In the first step, a height H (m) from a position of a gas supply nozzle for injecting the inert gas into the molten iron to a surface of the molten iron bath is expressed by the following formula (1), and the inert gas is injected into the molten iron so as to satisfy the following formula (2): The gas supply nozzle is a nozzle or a submerged lance provided at the bottom or wall of the furnace, Additionally, optionally, 1) a second step of adding a slag component adjuster; and 2) A third step of supplying heat to the slag from a heat source installed on the induction melting furnace. A method for producing a civil engineering and construction material, comprising one or more steps selected from the following: H=1.27×W DRI / (ρ l D 2 )×(%T.Fe) DRI / 100-h...(1) H>0.18×(ρ g Q 2 / ρ l N 2 d 2 ) 1 / 3...(2) Here, ρ g : density of the supply gas (kg / m 3 ), ρ l : density of molten iron (kg / m 3 ), Q: gas supply rate (Nm 3 / min); N: number of gas supply nozzles (-), d: gas supply nozzle diameter (m), D: inner diameter of induction furnace (m), W DRI : Weight of reduced iron supplied to the induction furnace (kg), (% T.Fe) DRI: Total iron concentration (mass%) contained in reduced iron, h: Height from the bottom of the induction furnace to the gas supply nozzle position (m) Represents.

7. By carrying out the second step, the civil engineering and construction material is SiO ２ Concentration (%SiO ２ The basicity, which is the ratio of the CaO concentration (% CaO) to the Al concentration (% CaO), is in the range of 0.5 to 2.

0. ２ O ３ Concentration (%Al ２ O ３ 7. The method for producing a civil engineering and construction material according to claim 6, wherein the content of the polyolefin copolymer in the polyolefin copolymer is in the range of 10 to 25 mass%.

8. a direct reduction furnace for contacting iron ore or a mixture of iron ore and a raw material component adjuster containing CaO with a reducing material under heating to obtain directly reduced iron; an induction melting furnace for melting the direct reduced iron to obtain molten iron; A slag discharge mechanism that discharges slag generated in the induction melting furnace to the outside of the melting furnace; Optionally, further comprising a refining facility for refining the molten iron melted in the induction melting furnace, The direct reduced iron melted in the induction melting furnace is charged into the induction melting furnace at a temperature in the range from the temperature at the end of the direct reduction process to atmospheric temperature, the induction melting furnace has a function of blowing an inert gas into the molten iron during a part or the entire period during which the direct reduced iron is melted, The induction melting furnace is configured such that a height H (m) from a position of a gas supply nozzle for injecting the inert gas into the molten iron to a surface of the molten iron bath is expressed by the following formula (1), and the inert gas is injected into the molten iron so as to satisfy the following formula (2): The gas supply nozzle is a nozzle or a submerged lance provided at the bottom or wall of the furnace, Additionally, optionally, 1) The function of adding slag component adjusters; 2) A function of supplying heat to the slag from a heat source installed on the induction melting furnace; and 3) The function of supplying one or more reducing solids or gases. A direct reduced iron melting system having one or more functions selected from the above. H=1.27×W DRI / (ρ l D 2 )×(%T.Fe) DRI / 100-h...(1) H>0.18×(ρ g Q 2 / ρ l N 2 d 2 ) 1 / 3...(2) Here, ρ g : density of the supply gas (kg / m 3 ), ρ l : density of molten iron (kg / m 3 ), Q: gas supply rate (Nm 3 / min); N: number of gas supply nozzles (-), d: gas supply nozzle diameter (m), D: inner diameter of induction furnace (m), W DRI : Weight of reduced iron supplied to the induction furnace (kg), (% T.Fe) DRI: Total iron concentration (mass%) contained in reduced iron, h: Height from the bottom of the induction furnace to the gas supply nozzle position (m) Represents.

9. The system for melting direct reduced iron according to claim 8 , further comprising two or more induction melting furnaces for one direct reduction furnace.

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