TWI843517B - Direct-reduction iron melting method, solid iron and method for producing solid iron, civil engineering and construction material and method for producing civil engineering and construction material, and direct-reduction iron melting system - Google Patents

Abstract

The present invention provides a direct-reduction iron melting technology that can efficiently remove the ore component from the direct-reduction iron. The technology comprises: a direct reduction step of heating an iron ore or a mixture of an iron ore and a component adjusting material and contacting it with a reducing material to obtain the direct-reduction iron; a melting step of melting the direct-reduction iron in an induction heating melting furnace to obtain molten iron; a slag discharge step of discharging the slag generated in the melting step to the outside of the melting furnace; and a refining step that can be implemented as required; wherein the loading temperature of the direct-reduction iron melted in the melting step into the induction heating melting furnace is a temperature that falls after the end of the direct reduction step. to atmospheric temperature, and the melting step comprises: a first step of blowing gas into the molten iron during part of or the entire period of the melting step, and may further comprise one or more steps selected from the following steps (1) to (3) as required, (1) a second step of adding a slag composition regulator, (2) a third step of supplying heat to the slag from a heat source installed on the induction heating melting furnace, and (3) a fourth step of supplying one or more reducing solids or gases.

Description

Direct-reduction iron melting method, solid iron and method for producing solid iron, civil engineering and construction material and method for producing civil engineering and construction material, and direct-reduction iron melting system

The present invention relates to a method for melting direct-reduced iron for removing a ore component contained in the direct-reduced iron, solid iron and a method for producing solid iron using the method, civil engineering and construction materials and a method for producing civil engineering and construction materials, and a system for melting direct-reduced iron.

In recent years, the demand for expanding the use of cold iron sources (recycled waste) in the steelmaking industry has been increasing. In order to build a circular society, the recycling and reuse of iron resources is indispensable. In addition, based on the consideration of preventing global warming, the need to reduce _CO2 emissions is also indispensable to increase the use of recycled waste. Recycled waste is different from iron _oxide ( _Fe2O3 ), which is iron ore, and does not require a reduction process during the smelting process, so _CO2 emissions can be reduced. Therefore, it is a way to increase the use of cold iron sources.

The blast furnace-converter process is a steelmaking process in which the raw material, namely iron ore (Fe ₂ O ₃ ), is loaded into a blast furnace together with the reducing material, namely coke (carbon source), to melt into molten iron with a carbon concentration of about 4.5 to 5 mass%, and then the molten iron is loaded into a converter to oxidize and remove impurities, namely C, Si, and P. When using a blast furnace to produce molten iron, in order to reduce the iron ore, etc., about 500 kg of carbon source is required for each ton of molten iron. In addition, about two tons of CO ₂ gas is generated. In contrast, if iron recovery waste is used as a raw material to produce molten steel, it is not necessary to use the carbon source required for reducing the iron ore. Therefore, even if we only consider the heat energy required to melt recycled iron waste, by replacing each ton of molten iron with each ton of recycled iron waste, we can reduce _CO2 emissions by about 1.5 tons. From the above situation, it can be seen that in order to take into account the dual considerations of reducing greenhouse gas emissions and maintaining production activities, the use of recycled waste must be increased.

However, the supply and demand of recycled iron waste, especially high-grade recycled iron waste that is indispensable for making high-grade steel, is insufficient, so the demand for reducing iron to replace recycled waste is increasing. Reduced iron is produced by reducing iron ore. However, reduced iron does not need to have a high C concentration in iron like molten iron produced by the blast furnace-converter process, so only a small amount of carbon source is needed, and _CO2 emissions can be reduced by about 0.2 tons per ton of iron. Furthermore, the reducing material does not use a carbon source, but a carbonized hydrogen-based gas such as hydrogen or natural gas, so _CO2 emissions can be further reduced.

However, the composition of iron ore, the raw material for reducing iron, varies depending on where it is mined. The composition of iron ore is mainly evaluated based on the Fe content and the vein content. Table 1 shows an example of the composition of iron ore.

The Fe content is expressed as the total iron content T.Fe (mass %) in the iron ore. The larger this value is, the more Fe content there is, and therefore the higher the value as a raw material. The ore content is expressed as the total content of oxides other than Fe in the iron ore, most of which are SiO ₂ and Al ₂ O ₃ , and the rest are CaO and MgO, etc., which contain about 0.1 mass %. In the process of refining iron ore into iron, the ore components are considered impurities and are removed. Therefore, the more ore there is, the lower the Fe content is, and the transportation cost and refining cost per unit of Fe will increase.

Furthermore, the properties of reduced iron produced from iron ore, such as metallization rate and composition, vary depending on the brand of iron ore used, the type and unit of raw material modifiers mixed in, the type and unit of reducing materials, the reduction temperature, and the type of reduced iron production equipment. Table 2 shows an example of the composition of reduced iron.

Generally speaking, in the process of manufacturing reduced iron, some CaO is added as a raw material component adjuster to form slag together with the vein component contained in the iron ore, thereby ensuring the strength of the reduced iron. If the amount of slag is suppressed to a minimum and the metallization rate of the reduced iron is higher, the load on the subsequent transportation, melting, and refining processes will be smaller. On the other hand, this kind of reduced iron must use high-grade iron ore, so not only the raw material cost will increase, but also the cost of the reduction treatment used to increase the metallization rate will increase. These are technical issues that need to be solved. Therefore, the industry is still looking forward to the early development of a process that can efficiently separate the slag from the reduced iron produced from cheap low-grade iron ore to increase the metallic iron content.

In the past, some people have proposed a technical solution for a method of producing a reduced metal by removing the vein contained in such an ore as slag. For example, the technical solution of patent documents 1 and 2 is a method of producing a reduced metal by loading a raw material containing metal onto a solid reducing material layer already accumulated on the furnace bed of a mobile furnace, performing a heat reduction treatment, and forming a molten state at least once to separate the metal from the slag. [Prior technical document] [Patent document]

[Patent Document 1] Japanese Patent Publication No. 2000-292069 [Patent Document 2] Japanese Patent Publication No. 2004-204293 [Non-patent Document]

[Non-patent document 1] Slag Atlas, 2nd ed., Verlag Stahleisen GmbH, Duesseldorf, (1995), 105, 126.

[The problem the invention is trying to solve]

However, the following technical issues still need to be solved in the known technology. The technology disclosed in Patent Documents 1 and 2 is based on the premise that a carbon-based solid raw material is used as a reducing material when separating metal from slag. Therefore, if a process using a hydrogen-based reducing material for reduction treatment is adopted, which is considered to be the mainstream in the future from the perspective of reducing _CO2 emissions, it is considered that the separation efficiency of metal and slag is very low. Specifically, the temperature of the atmosphere gas in a movable furnace such as a rotary furnace is generally about 1300°C. At this temperature, the reason why the reduced metal is in a molten state is believed to be that the carbon-based solid raw material carburizes into the metal, causing the metal's melting point to drop. Therefore, the process using hydrogen-based reducing materials for reduction treatment will not cause the metal's melting point to drop due to carburization. Therefore, it is predicted that the liquid phase ratio of the load is low and the metal and slag are not easily separated.

In other words, it is sometimes difficult to remove the vein stone by only using the reduction process, and the vein stone must be separated from the produced reduced iron. Specifically, when using an induction heating melting furnace to melt the reduced iron, it is necessary to prevent the slag generated from the vein stone from hardening.

The present invention was developed in view of the above circumstances, and its purpose is to provide: a direct-reduction iron melting method and a direct-reduction iron melting system that can efficiently remove the ore component from the direct-reduction iron. Another purpose is to provide: high-purity solid iron and a method for producing solid iron using the melting method, and civil engineering and construction materials and a method for producing civil engineering and construction materials that utilize its by-products. [Technical means for solving the problem]

The inventors of the present invention have discovered a creative idea: when using an induction heating melting furnace to melt direct reduced iron, by promoting heat conduction between molten iron and slag and/or by controlling the composition of the slag to inhibit the hardening of the slag, the slag can be more easily discharged out of the furnace, thereby completing the present invention.

The present invention can effectively solve the above-mentioned problems. The method for melting direct-reduced iron provided by the present invention is characterized in that it comprises: a direct reduction step of making iron ore or a mixture of iron ore and component adjusting material contact with reducing material under heating to obtain direct-reduced iron; a melting step of melting the direct-reduced iron using an induction heating melting furnace to obtain molten iron; a slag discharge step of discharging the slag generated in the melting step to the outside of the melting furnace; and a refining step of refining the molten iron obtained in the melting step according to the needs; wherein the direct-reduced iron melted in the melting step is loaded into the induction heating furnace. The loading temperature in the melting furnace is within the range from the temperature after the completion of the direct reduction step to the atmospheric temperature. The melting step comprises a first step of blowing gas into the molten iron during part or all of the melting step, and may further comprise one or more steps selected from the following steps (1) to (3) as required, (1) a second step of adding a slag composition regulator, (2) a third step of supplying heat to the slag from a heat source installed on the induction heating melting furnace, and (3) a fourth step of supplying one or more reducing solids or gases.

In addition, a better solution of the direct reduced iron melting method provided by the present invention is as shown in the following (a), (b) and (c): (a) In the aforementioned first step, the height H (m) from the position of the gas supply nozzle used to blow the aforementioned gas into the aforementioned molten iron to the aforementioned molten iron bath surface is expressed by the following formula (1), and the aforementioned gas is blown into the aforementioned molten iron under the condition of satisfying the following formula (2), H=1.27×W _DRI /(ρ ₁ D ² )×(%T.Fe) _DRI /100-h...Formula (1) H＞0.18×(ρ _g Q ² /ρ ₁ N ² d ² ) ^1/3 ...Formula (2) Wherein, ρ _g : density of supply gas (kg/m ³ ), ρ ₁ : density of molten iron (kg/m ³ ), Q: gas supply rate (Nm ³ /min), N: number of gas supply nozzles (-), d: diameter of gas supply nozzle (m), D: inner diameter of induction heating furnace (m), W _DRI : weight of reduced iron supplied to induction heating furnace (kg), (%T.Fe) _DRI : total iron content contained in reduced iron (mass %), h: height from the bottom of induction heating furnace to the position of gas supply nozzle (m). (b) In the second step, the type and amount of the slag composition modifier are adjusted so that the composition of the slag generated in the melting step has a ratio of CaO concentration (%CaO) to _SiO2 concentration (% _SiO2 ), that is, alkalinity, 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 amount of the reducing solid or gas are adjusted so that the total iron content (% _T.Fe ) of the composition of the slag generated in the melting step is less than 20 mass%.

The present invention provides a method for producing solid iron that can effectively solve the above-mentioned problems, characterized in that molten iron produced by any of the above-mentioned direct reduction iron melting methods is solidified to form solid iron. In addition, the solid iron provided by the present invention is solid iron produced by the production method, characterized in that the total iron content T.Fe is 93 mass % or more, and the total oxide content is 3 mass % or less.

The present invention can effectively solve the above-mentioned problems. The manufacturing method of civil engineering and construction materials provided by the present invention is characterized by comprising: a melting step of melting direct reduced iron in an induction heating melting furnace to obtain molten iron; a slag discharge step of discharging the slag generated in the above-mentioned melting step to the outside of the above-mentioned melting furnace; and a cooling and hardening step of cooling and hardening the slag discharged in the above-mentioned slag discharge step to make a raw material for civil engineering and construction materials; wherein, The loading temperature of the direct-reduction iron melted in the above-mentioned melting step into the above-mentioned induction heating melting furnace is within the range from the temperature after the above-mentioned direct-reduction step is completed to the atmospheric temperature. The above-mentioned melting step includes: a first step of blowing gas into the above-mentioned molten iron during part of or the entire period of the melting step, and may further include one or more steps selected from the following steps (1) to (2) as required, (1) is a second step of adding a slag composition regulator, and (2) is a third step of supplying heat to the slag from a heat source installed on the furnace of the above-mentioned induction heating melting furnace. Furthermore, the civil engineering and construction material provided by the present invention is a civil engineering and construction material manufactured by the manufacturing method, and is characterized in that, in terms of mass %, the ratio of CaO concentration (%CaO) to SiO ₂ concentration (%SiO ₂ ), that is, alkalinity, falls within the range of 0.5 to 2.0, and the Al ₂ O ₃ concentration (%Al ₂ O ₃ ) falls within the range of 10 to 25 mass %.

The present invention can effectively solve the above-mentioned problems. The invention provides a direct reduction iron melting system, which is characterized by comprising: a direct reduction furnace for producing direct reduction iron by heating an iron ore or a mixture of an iron ore and a component adjusting material in contact with a reducing material; an induction heating melting furnace for melting the direct reduction iron to produce molten iron; a slag discharge mechanism for discharging slag generated in the induction heating melting furnace to the outside of the melting furnace; and a refining device for refining the molten iron melted in the induction heating melting furnace according to needs; wherein the direct reduction iron melted in the induction heating melting furnace is directly The molten iron is loaded into the induction heating melting furnace at a temperature ranging from the temperature after the reduction treatment to the atmospheric temperature. The induction heating melting furnace has the function of blowing gas into the molten iron during a part of the melting of the direct reduction iron or the whole period, and can have one or more functions selected from the following (1) to (3) as required, (1) the function of adding a slag component regulator, (2) the function of supplying heat to the slag from a heat source installed on the induction heating melting furnace, and (3) the function of supplying one or more reducing solids or gases. In addition, a better solution for the direct reduction iron melting system provided by the present invention is to have two or more of the above-mentioned induction heating melting furnaces for one direct reduction furnace. [Effect of invention]

According to the direct-reduction iron melting method and/or direct-reduction iron melting system provided by the present invention, when the direct-reduction iron is melted by using an induction heating melting furnace, gas is blown into the molten iron, and at least one of the following steps can be implemented as required: supplying gas into the molten iron at an appropriate flow rate, controlling the composition of the generated slag, and supplying heat to the slag from a heat source installed on the furnace. In this way, the slag can be kept in a flowing state, so that the metallic iron in the direct-reduction iron can be melted while the slag can be separated. In particular, the produced direct-reduction iron is kept in a high temperature state and loaded into the induction heating melting furnace, so that the time and heat energy required for melting can be reduced.

The solid iron manufacturing method and solid iron provided by the present invention are very desirable because the molten iron is first separated from the slag and then solidified, so that high-purity solid iron can be manufactured. In addition, the civil engineering and construction material manufacturing method and civil engineering and construction material provided by the present invention recycles by-products, especially after adjusting the composition, so that the by-products can be effectively utilized.

The following will specifically describe the implementation of the present invention. The following implementation is only an example of a system and/or method that embodies the technical concept of the present invention, and is not intended to limit the constituent elements of the present invention to the following constituent elements. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the scope of the patent application.

The inventors of the present invention have examined how to remove vein ore from reduced iron, based on the premise that the reduced iron is first melted in an induction heating melting furnace. The characteristic of heating and melting reduced iron in an induction heating melting furnace is that the metallic iron component in the reduced iron can be directly heated by the induction current. However, on the other hand, since the slag component cannot be directly heated, there is a problem that "the slag floating on the molten iron hardens due to the difference in specific gravity, making it difficult to load additional reduced iron."

<First embodiment> Therefore, the inventors of the present invention have explored what conditions are suitable for separating the ore component after reducing iron is melted in an induction heating melting furnace as slag. As a result, it was found that when reducing iron is melted in an induction heating melting furnace, if gas is blown into the molten iron, the solidification of the slag can be suppressed, and the melting of the reducing iron and the separation of the slag can be carried out efficiently. In addition, a new idea was discovered, that is, by performing at least one of the following three methods: supplying an appropriate flow 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, the slag can be kept in a flowing state, and the metallic iron in the reduced iron can be melted more appropriately, and the slag can be separated. The so-called "keeping the slag in a flowing state" here means that the slag is in a state of circulation all the time, showing red heat and high temperature.

By keeping the slag in a flowing state in this way, when additional reduced iron is loaded, it will not be blocked by the solidified slag, and the volume of the induction heating melting furnace can be effectively utilized. Furthermore, because the slag is in a flowing state, the slag can be discharged by using a slag scraper mechanism that allows the slag to overflow from the upper part of the furnace, making it easier to separate the molten iron and the slag. From the perspective of subsequent treatment of high-temperature slag and/or repair of damaged areas, it is better to limit the place (location) where the slag is discharged by tilting the furnace body when separating the slag.

Reduced iron can be obtained by heating the raw material, i.e., iron ore, or a mixture of iron ore and a composition adjusting material in a furnace such as a rotary furnace or a direct reduction furnace, while contacting the reducing material. The composition adjusting material may be quicklime containing CaO, and the reducing material may be carbon powder and/or reducing gas, i.e., _H2 , CO, and/or _CH4 , etc., as a solid carbon source. The temperature at which the reduced iron is loaded into the induction heating melting furnace is within the range from the temperature after the reduction treatment in the direct reduction furnace to the atmospheric temperature. Since the high-temperature reduced iron is loaded into the induction heating melting furnace, the time and heat energy required for melting can be reduced, so it is more suitable. Therefore, the temperature of the reduced iron when loaded into the induction heating melting furnace is preferably higher than the atmospheric temperature, more preferably above 300°C, and it is best to control the temperature drop caused by the process of transferring the reduced iron from the direct reduction furnace to the induction heating melting furnace to the minimum required.

The first embodiment of the present invention is obtained based on the above-mentioned review, and comprises: a direct reduction step of producing direct-reduced iron by bringing iron ore or a mixture of iron ore and a composition adjusting material into contact with a reducing material under heating; a melting step of melting the direct-reduced iron by using an induction heating melting furnace to produce molten iron; a slag discharge step of discharging the slag generated in the melting step to the outside of the melting furnace; wherein the direct-reduced iron melted in the melting step is loaded into the induction heating melting furnace at a loading temperature of , which falls within the range from the temperature after the end of the direct reduction step to the atmospheric temperature, and the melting step includes: a first step of blowing gas into the molten iron during a part of or the whole of the melting step, and may further include one or more steps selected from the following steps (1) to (2) as required, (1) is a second step of adding a slag composition regulator, and (2) is a third step of supplying heat to the slag from a heat source installed on the furnace of the induction heating melting furnace.

<Second embodiment> Next, the optimization of the gas injection conditions was examined. If the gas is supplied to the molten iron, the molten iron will be stirred due to the floating of the gas, and the generated and floating slag will be subjected to heat conduction from the molten iron. As a result, the temperature of the slag will rise, thereby improving the fluidity of the slag. The gas supplied can be of any type as long as it does not liquefy when supplied using a pipe. For example, oxidizing gases such as oxygen and/or carbon dioxide will oxidize the molten iron and reduce the yield of iron. Therefore, it is appropriate to use an inert gas such as Ar or _N2 .

However, if the amount of gas supplied is too much, the gas will continue to rise through the bath surface of the molten iron (the upper surface of the molten iron) and the so-called "empty gallery phenomenon" will occur. If the "empty gallery phenomenon" occurs, the spitting of the molten iron will increase significantly, and the stirring efficiency and reaction efficiency of the supplied gas will be reduced, so the heat transfer effect of the molten iron on the slag will also be reduced. The inventors of the present invention conducted various experiments with different conditions. As a result, an original idea was discovered: when blowing gas into molten iron, if the height H (m) from the position of the gas supply nozzle to the molten iron bath surface is set to satisfy the relationship of the following formula (1), and if the gas is blown into the molten iron under the conditions of the following formula (2), the "empty gallery phenomenon" can be avoided. H=1.27×W _DRI /(ρ ₁ D ² )×(%T.Fe) _DRI /100-h…Formula (1) H＞0.18×(ρ _g Q ² /ρ ₁ N ² d ² ) ^1/3 …Formula (2) Wherein, ρ _g : Density of supply gas (kg/m ³ ), ρ ₁ : Density of molten iron (kg/m ³ ), Q: Gas supply rate (Nm ³ /min), N: Number of gas supply nozzles (-), d: Diameter of gas supply nozzle (m), D: Inner diameter of induction heating furnace (m), W _DRI : Weight of reduced iron supplied to the induction heating furnace (kg), (%T.Fe) _DRI : Total iron concentration contained in the reduced iron (mass %), h: The height from the bottom of the induction heating furnace to the gas supply nozzle position (m).

On the other hand, if the gas injection amount is too small, the stirring effect of the gas floating on the molten iron will be reduced, and the heat transfer of the molten iron to the slag will also be reduced. Therefore, it is advisable to supply gas at a rate of at least ^0.01Nm3 /(per minute and per ton of molten iron).

The second embodiment of the present invention is obtained based on the above-mentioned review. With respect to the first embodiment, in the first step, the height H (m) from the position of the gas supply nozzle used to blow the gas into the aforementioned molten iron to the molten iron bath surface is set to a relationship that satisfies the above-mentioned formula (1), and the gas is blown into the molten iron under the conditions that satisfy the above-mentioned formula (2).

<Third to Fifth Embodiments> Next, the optimization of the composition of slag was examined. The fluidity of slag varies greatly depending on the composition of slag. For the example of the reduced iron composition shown in Table 2, the slag components contained therein are converted to the composition after the total of the three components of _Al2O3 - _CaO - _SiO2 or CaO- _SiO2 -FeO is converted to 100%, and the components are respectively marked on the ternary phase diagram, as shown in Figures 1 and 2. Each ternary phase diagram is referenced to Non-Patent Document 1. The FeO concentration in the reduced iron here is 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 (atomic weight of Fe).

As can be seen from Figure 1, there are components with lower melting points than the slag components contained in the reducing iron. However, if the slag component is a high concentration of _SiO2 , although it has a low melting point, it has a high viscosity and reduces fluidity. Therefore, in terms of mass%, it is advisable to set the ratio of CaO concentration (%CaO) to _SiO2 concentration (% _SiO2 ), that is, the alkalinity (hereinafter referred to as slag alkalinity) to 0.5 or more. In addition, from the perspective of lowering the melting point of the slag, the slag alkalinity is set to less than 2.0, and _{the Al2O3} concentration is set to a range of 10-25 mass%. As a means for adjusting the composition of this slag group, it is preferable to add one or more substances containing CaO, _{SiO2 and Al2O3} as a raw material component adjuster when manufacturing reduced iron or a slag component adjuster when melting reduced iron. The substances containing CaO can be: limestone, slaked lime, quicklime, steelmaking slag, etc. In addition, if limestone and/or slaked lime are used, the temperature will decrease due to the endothermic reaction during decomposition. The CaO concentration of steelmaking slag is only 40-50% by mass. The amount of addition must be increased, which will lead to the problem of increasing the amount of slag generated when reducing iron is melted. Therefore, it is better to use quicklime. The substances containing _SiO2 can be: silica, coal ash, steelmaking slag, etc. In addition, _SiO2 generated by reacting the iron oxide components remaining in the reduced iron with the addition of metal Si and/or silica mud can also be used. Materials containing _Al2O3 include natural stones such as corundum and/or alumina, and _Al2O3 generated by reacting the iron oxide components remaining in the reduced iron with the addition of metal Al _and / _or aluminum slag.

The third embodiment of the present invention is obtained based on the above-mentioned review. With respect to the first or second embodiment, in the second step, the type and amount of the slag component adjuster are adjusted with respect to the composition of the slag generated in the melting step so that the alkalinity of the slag falls within the range of 0.5 to 2.0 and _{the Al2O3} concentration falls within the range of 10 to 25 mass %.

As can be seen from FIG. 2, the slag component contained in the reduced iron contains a high concentration of FeO, which is effective from the viewpoint of lowering the melting point of the slag to ensure fluidity. However, in this state, the yield of Fe is low, and the slag is further reduced so that the total iron content (%T.Fe) in the slag after separation by melting is less than 20 mass%, more preferably less than 10 mass%, and more preferably less than 5 mass%. The reduction means may be one of the following (a), (b), or (c), or a combination thereof: (a) supplying at least one solid of reducing materials such as C, Al, Si, etc. to the generated and floating slag; (b) using at least one reducing gas such as CO, _H2 , hydrogen carbide, etc. as the gas supplied to the molten iron; (c) increasing the raw unit of reducing materials at the time of producing reduced iron.

The fourth embodiment of the present invention is obtained based on the above-mentioned examination, and is a fourth step of supplying one or more reducing solids or gases in the melting step, in any one of the first to third embodiments. In addition, the fifth embodiment of the present invention is a fourth embodiment in which the type and supply amount of the reducing solid or gas are adjusted so that the total iron content (%T.Fe) of the components of the slag generated in the melting step falls below 20 mass %.

<Third step> In order to suppress the temperature drop of the slag generated by the melting of the reduced iron in the induction heating melting furnace, it is preferable to further include a third step of providing a heat source on the furnace to supply heat to the slag. The heat source may be, for example, a means of directly heating the slag such as jet heating, electric heating using an electrode, and induction heating by immersing a conductor in the slag, but the means are not limited thereto and a plurality of means may be used in combination. The jet heating may be liquid fuel such as fuel oil, gas fuel such as CO, _H2 , hydrogen carbonate, or a combination of these fuels. The conductive body immersed in the slag can be any object that can generate heat by induced current. However, for cost considerations, an iron rod, a carbon rod, etc. can be kept immersed in the slag, or reduced iron particles having an apparent density equivalent to that of the generated slag can be added from above and retained in the slag.

<Sixth embodiment> If the raw material, i.e., the impurities of the direct-reduced iron, contains phosphorus, it is better to remove the phosphorus from the molten iron. In addition, there is also a case where a desired component is added to the molten iron. The sixth embodiment of the present invention is developed and completed in response to this expectation.

As shown in the following formula (A), the dephosphorization reaction requires an oxygen source and a CaO source. 2[P]+5/2・O ₂ (g)+3CaO(s)=3CaO・P ₂ O ₅ (s)・・・Formula (A) Wherein, [P] represents phosphorus in the molten iron. For example, as a method of removing phosphorus from the molten iron as impurities, an oxygen source and a CaO source can be supplied to the molten iron produced in the melting step or the molten iron after the slag is discharged in the slag discharge step.

The oxygen source for dephosphorization treatment is generally pure oxygen gas. However, considering that dephosphorization reaction is an exothermic reaction, it is more advantageous to perform dephosphorization treatment at a low temperature. Therefore, it is concluded that it is more advantageous to lower the temperature of the molten iron as long as it does not cause problems in the treatment.

As a result of the review, we found a new idea: by supplying air or iron oxide sources such as iron ore and/or iron rust film as an oxygen source, it is possible to cool the molten iron and dephosphorize it sufficiently. If air is used, the sensible heat of nitrogen contained in the air is dissipated, and a cooling effect on pure oxygen gas can be obtained. In addition, if an iron oxide source is used, when the iron oxide source is reduced to metallic Fe or forms molten slag in the form of iron oxide, it absorbs heat, thereby obtaining a cooling effect on pure oxygen gas.

Secondly, since calcium carbonate contained in limestone absorbs heat when it decomposes into CaO and _CO2 , if limestone is used as a CaO source, molten iron can also be cooled. The same cooling effect can be obtained by supplying carbonates such as raw dolomite, but if the ratio of CaO in the auxiliary raw materials is reduced, the amount of auxiliary raw materials required to be added will increase, the amount of slag generated will increase, and the time required for addition will increase. In actual operation, it is advisable to first consider the cooling effect and stable workability you want to seek, and then adjust the type and amount of auxiliary raw materials you want to add.

Depending on the free space of the container used for dephosphorization (the height from the upper surface of the molten iron to the upper end of the container) and/or the shape of the nozzle of the upper gas lance, the splashing behavior will be different. Therefore, it is appropriate to adjust the supply rate of pure oxygen or air and/or the height of the lance in accordance with the actual working conditions of the dephosphorization. In addition, in order to stir the molten iron, inert gas may be blown in. A multi-hole plug and/or a lance may be provided to blow inert gas. The alkalinity of the slag is preferably in the range of 1.5 to 4.0, which can be adjusted by adjusting: the amount of slag containing more _SiO2 left after the deslagging step, the type and amount of the added CaO source. _SiO2 sources such as silica and/or ferrosilicon and/or CaO sources such as quicklime can also be added as needed.

In addition, if the alkalinity of the slag is too low, the amount of phosphorus removal achieved by the dephosphorization treatment will also be reduced. If the alkalinity of the slag is too high, when the temperature of the molten iron decreases, part of the slag will solidify and adhere to the refractory, making it difficult to remove the slag after the dephosphorization treatment. As a result, problems such as abnormal reactions when loading molten iron during the next dephosphorization treatment or residual slag mixing with newly generated slag and causing the range of dephosphorization slag components may occur. In addition, this dephosphorization treatment using air will produce a large amount of high-temperature exhaust gas, so boilers can also be used to recover the exhaust heat.

In addition, the molten iron produced by the above-mentioned implementation method can also be directly refined into molten steel with the necessary components by entering the next process. It can also be poured into a mold to solidify to produce solid iron, and then transported to the required location, and then melted and refined to become molten steel. In the former case, there is no need to implement the solidification and remelting process. Therefore, although it has an advantage in energy saving, it must be continuously configured: the reduction iron manufacturing plant, the induction heating melting furnace, and the refining equipment. If you want to set it up in an existing steelmaking plant, there will be site restrictions. Or, if everything is newly set up, not only will it cost a lot, but it will also be impossible to use existing equipment. In the latter case, the reduced iron manufacturing plant, induction heating melting furnace and solidification equipment, remelting equipment and refining equipment can be separated. For example, the process from the manufacture of reduced iron to solidification can be implemented in the country where the iron ore is produced, and the solidified iron can be transported to the demand location for remelting and refining. In this case, not only can the existing refining equipment be used, but the solidified iron can also be transported in a state where the weight of the vein component contained in the iron ore is reduced, which is also helpful in reducing transportation costs. As for which process to choose, it is only necessary to consider the location of the business unit and/or its equipment and other factors to make an appropriate choice.

The sixth embodiment of the present invention is obtained based on the above-mentioned review, and is any one of the first to fifth embodiments, and further comprises a refining step of refining the molten iron obtained in the melting step. The refining step is preferably carried out after the slag removal step.

<Seventh embodiment> The seventh embodiment of the present invention is to solidify the molten iron obtained by the melting method of direct reduction iron provided by any one of the first to sixth embodiments to make solid iron. The total iron concentration T.Fe of the solid iron is preferably above 93 mass %, and the total oxide component is preferably below 3 mass %. In addition, although the shape and/or size of the casting mold used to solidify the solid iron is not limited, it is preferred to solidify the solid iron into granular solid iron with a particle size range of 10 to 100 mm in consideration of various factors such as the convenience of subsequent cargo loading and unloading, packaging, transportation, and supply to the equipment used.

<Eighth Embodiment> The eighth embodiment of the present invention is to utilize the slag produced as a by-product as a civil engineering material. That is, it further comprises a cooling and hardening step of cooling and hardening the slag discharged in the slag discharge step of the first embodiment to make it a raw material for civil engineering materials. The slag after cooling and hardening falls within the above alkalinity range and has various particle size distributions depending on the cooling method. It can be subjected to additional particle size adjustment treatment such as crushing and classification treatment according to needs, and can be used as a material utilizing its characteristics. For example, if the discharged slag is subjected to water crushing treatment, it becomes fine glass, and the specific surface area falls between ^0.35m2 /g and less than ^0.50m2 /g, so it can be used as a cement raw material (binder). In addition, if it is slowly cooled in the atmosphere and its particle size is adjusted according to its intended use, it can be used as a road base material and/or concrete aggregate. Therefore, the cooling and hardening method is to be selected by the business unit in accordance with the purpose of use of the discharged slag.

<Ninth embodiment> The ninth embodiment of the present invention is a direct reduction iron melting system applicable to the first to eighth embodiments. This embodiment comprises: a direct reduction furnace for producing direct reduction iron by bringing iron ore or a mixture of iron ore and a component adjusting material into contact with a reducing material under heating; an induction heating melting furnace for melting the direct reduction iron to produce molten iron; and a slag discharge mechanism for discharging the slag generated in the induction heating melting furnace to the outside of the melting furnace. As for the direct reduction furnace, a vertical furnace type, a fluidized bed type, a rotary furnace bed type, etc. can be used. Most of these furnaces use a method of continuously supplying raw materials and continuously producing reduced iron. On the other hand, because the induction heating melting furnace uses a batch method of processing, by setting up a facility that combines multiple induction heating melting furnaces for a reduced iron manufacturing plant, it is possible to supply reduced iron to other induction heating melting furnaces while one induction heating melting furnace is melting reduced iron, thereby reducing the waiting time for processing. Therefore, it is advisable to set up two or more induction heating melting furnaces for one direct reduction furnace. As for the slag discharge mechanism, a slag discharge port can be provided to discharge the slag from the upper part of the induction heating melting furnace; or a furnace body tilting mechanism can be provided; or a slag scraping mechanism can be provided to scrape out the slag.

It is preferred that the direct reduction furnace and the induction heating melting furnace are arranged close to each other so that the direct reduction iron to be melted by the induction heating melting furnace is loaded into the induction heating melting furnace at a temperature ranging from the temperature after the direct reduction step to the atmospheric temperature. It is preferred that the direct reduction iron is transported to the induction heating melting furnace in a manner that suppresses the temperature drop.

As a mechanism for blowing gas into the molten iron in the induction heating melting furnace, a gas supply nozzle may be provided at the furnace bottom and/or the furnace wall. The gas may also be blown into the molten iron from an immersion type nozzle. In order to add slag composition adjusting materials, a hopper may be provided at the top of the induction heating melting furnace. In addition, in order to heat the slag, the heat source described in the third step above may be provided. [Example]

(Example 1) A mixture of iron ore of the composition shown in Table 3, quicklime as a composition adjuster, and carbon powder as a reducing material was loaded into a rotary furnace with a production capacity of 5 tons/hour. The amount and ratio of fuel gas and oxygen supplied to the heating burner were controlled under the conditions of a treatment temperature of 1000°C ± 20°C and an oxygen partial pressure P _O2 in the range of logP _O2 of -14 to -15 to carry out reduction treatment to generate direct reduced iron. The amount of quicklime added was calculated so that the alkalinity (in mass %, the ratio of (%CaO)/(%SiO ₂ )) was controlled to 1. The operating conditions of this equipment (rotary furnace bed) are set to 90 minutes from loading to discharge, and 45 minutes after the loaded sample has passed, the temperature of the sample is measured and the gas composition is analyzed at the location where the sample exists.

The concentrations of carbon monoxide (CO) and carbon dioxide (CO ₂ ) in the sampled gas are measured using an infrared gas analyzer, and the oxygen partial pressure P _O2 is calculated using the following formula from the ratio of each concentration (partial pressure), i.e. the measured value of the CO/CO ₂ ratio. 2CO ₂ (g)=2CO(g)+O ₂ (g) ・・・Formula (B) ΔG ^○ =134300-40.74×T(cal/mol) ・・・Formula (3) K=exp(-ΔG ^○ /RT)=(P _CO /P _CO2 ) ²・P _O2・・・Formula (4) Wherein, T is the reaction temperature (K), K is the equilibrium constant of the chemical reaction in formula (B); R is the gas constant (cal/(K・mol)); P _CO and P _CO2 are the partial pressures of CO and CO ₂ when performing gas composition analysis.

The direct reduced iron obtained by this treatment was cooled to about 25°C and analyzed. The results showed that it had a composition equivalent to that of the reduced iron C shown in Table 2.

An induction heating melting furnace with an inner diameter of 0.9m and a height of 1.8m from the furnace bottom to the lower end of the soup discharge chute was used. 0.5 tons of seed soup (pre-melted molten iron) was melted in advance, and reduced iron prepared in the above manner and cooled to room temperature was added to the furnace. The molten iron was not allowed to overflow from the furnace body. After confirming that the melting was in progress and the raw material pile height in the furnace had dropped, reduced iron was repeatedly added from the hopper located at the top until the total amount of reduced iron added reached 5.0 tons. The induction heating melting furnace is constructed as follows: six bottom blowing nozzles are set at equal intervals at positions with a pitch circular diameter (PCD) of 0.3m and 0.6m at the bottom of the furnace, and the same flow of gas can be supplied to the nozzles combined according to demand through the gas supply head. A hopper for supplying auxiliary raw materials is set above the furnace, and auxiliary raw materials can be supplied in units of 10kg at the required time point in the processing process. The temperature of the molten iron in the furnace is measured at an appropriate number of times, and the input power of the induction heating melting furnace is controlled and the supply speed of the reduced iron and auxiliary raw materials is adjusted in a way that the temperature is maintained at 1600±20℃.

In addition, the melting process was implemented by changing the gas flow rate supplied from the nozzle and the type and amount of the auxiliary materials added. The quicklime used as the auxiliary material is the quicklime after the limestone is baked at high temperature to remove _CO2 , and the CaO concentration is nearly 100% by mass. The silica is the fine silica obtained by crushing the silica collected from the crushing site, and the _SiO2 concentration is about 98% by mass, containing only a small amount of _Al2O3 and MgO. The alumina is the alumina obtained by crushing the ore imported as the raw material for refining Al, _{and the Al2O3} concentration is about 50% by mass, and the rest contains crystal water, _SiO2 , _TiO2 , etc., which are considered impurities. After the melting process, the furnace body was tilted to completely discharge the molten iron in the furnace, and the weight of the discharged molten iron was weighed. In addition, the slag was taken and crushed to less than 53μm for chemical analysis. The obtained values are shown in Table 4-1 and Table 4-2 together with the treatment conditions. In addition, for comparison, the melting process was also carried out under the condition that the gas was not supplied from the bottom of the furnace.

The criteria for judging the fluidity of slag indicated in Table 4-2 are as follows: Observe the surface of the slag in the furnace through the window on the furnace. If the slag is red-hot as a whole and the high-temperature slag is always in a circulating state, it is marked as ○; if a part of the slag is in a charred solid state, but the solid slag is always in a moving state on the surface of the molten iron, it is marked as △; if the slag is charred as a whole and only the cracked part is red-hot and is in a stagnant state, it is marked as ×. Here, under the condition of slag fluidity marked as ○, the time from the start of adding 5.0 tons of reduced iron to complete melting and the start of tipping the furnace body to pour out the molten iron is within 90 minutes. In contrast, under the condition of slag fluidity marked as △, the time consumed is longer than 90 minutes. This difference is because part of the slag has become solid, and it takes a longer time for the slag to move to the soup outlet in the slag discharge step. When the slag fluidity is marked as ×, even if additional reduced iron and auxiliary materials are added, they are only accumulated on the surface of the solidified slag and do not enter the interior of the molten iron. Only the reduced iron initially loaded into the furnace is melted.

For treatments No. 1 to 19, the evaluation of slag fluidity is marked as ○ or △, while the evaluation of slag fluidity for treatment No. 20 is marked as ×. Treatment No. 20 is considered to be due to insufficient heat supply from molten iron to slag, as gas was not supplied from the bottom of the furnace, resulting in solidification of the slag. Treatments No. 1 to 19 all supplied gas, so that regardless of the composition of the slag, the slag fluidity was maintained, and slag discharge was possible. In addition, it can be seen from Table 4-1 and Table 4-2 that when the conditions of the above formulas (1) and (2) are met, the alkalinity C/S of the slag falls within the range of 0.5 or more and _2.0 or less, and _{the Al2O3} concentration in the slag converted into the ternary system of CaO- _SiO2 - _Al2O3 falls within the range of 10 mass% or more and 25 mass% or less, the slag has particularly good fluidity.

In treatments No. 13 to 15, the amount of gas supplied from the bottom of the furnace was too much, so the aforementioned "empty corridor phenomenon" occurred, and the splashing of molten iron was very significant. As a result, the amount of metal in the poured molten iron was lower than that of other invention examples. In treatment No. 16, too much quicklime was added, and as a result, the alkalinity C/S of the slag exceeded 2.0, the melting point of the slag increased, resulting in poor fluidity and a longer slag discharge time. In treatment No. 17, too much silica was added, and as a result, the alkalinity C/S of the slag was less than 0.5, the viscosity of the slag increased, resulting in poor fluidity and a longer slag discharge time. Treatment No. 18 is the result of adding too much alumina, so the concentration of Al ₂ O ₃ in the ternary system of CaO-SiO ₂ -Al ₂ O ₃ exceeds 25% by mass, the melting point of the slag rises, resulting in poor fluidity and a longer slag discharge time. Treatment No. 19 is the result of adding too much quicklime and silica, the concentration of Al ₂ O ₃ in the ternary system of CaO-SiO ₂ -Al ₂ O ₃ is diluted to less than 5% by mass, the melting point of the slag rises, resulting in poor fluidity and a longer slag discharge time.

In addition, it was confirmed that the auxiliary raw materials are not limited to quicklime, silica, and alumina, and there is no problem in controlling the slag composition using other materials other than the above. It was also found that the auxiliary raw materials are better added in stages from the beginning of melting to the end of melting. The reason is believed to be that if a large amount of auxiliary raw materials is added immediately after the beginning of melting, it will hinder the contact between reduced iron and reduced iron, or between reduced iron and seed soup (pre-melted iron), and the efficiency of induction heating will be poor, resulting in an increase in the required melting time. In addition, another problem is that at this time, slag has already been generated after the melting is completed. Depending on the composition, some of the slag has already solidified. Even if auxiliary raw materials are added, they are just placed on top of the already solidified slag, which does not help to lower the melting point of the slag.

Even without seeding soup (pre-melted iron), it is possible to melt reduced iron. However, if there is seeding soup (pre-melted iron), heat can be supplied from the seeding soup (pre-melted iron) that has been induction heated to the solid reduced iron. As a result, the time required to melt the reduced iron can be shortened. Therefore, it is better to have seeding soup (pre-melted iron). It is particularly effective to have a seed soup (pre-melted iron) of about 5 mass % or more relative to the reduced iron put in. However, if the seed soup (pre-melted iron) is too much, the proportion of the volume occupied by the induction heating melting furnace increases, resulting in a decrease in the amount of reduced iron that can be melted. Therefore, it is advisable to set the proportion of the seed soup (pre-melted iron) to less than 70 mass % of the reduced iron put in. In addition, the seed soup (pre-melted iron) can be made by remelting recycled waste and/or large iron blocks with a large apparent density. In addition, a part of the molten iron that has been melted in the previous melting process can also be left in the furnace in advance.

Furthermore, a process of changing the position and/or combination of the nozzles is implemented. Regardless of the position and/or combination, as long as the amount of gas supplied to each nozzle meets the conditions of the above formula (2), the fluidity of the slag can be ensured and the splashing of the molten iron is very small. In contrast, if the conditions of formula (2) are not met, the splashing of the molten iron increases and the metal yield deteriorates.

In addition, it was confirmed that there is no problem if the nozzle is not located at the bottom of the furnace but at the side of the furnace. However, the distance h from the nozzle located at the side of the furnace to the bottom of the furnace is large. That is, if the height value H is too small, the gas supply rate that meets the formula (2) will be reduced, and it will be difficult to effectively transfer the heat of the molten iron to the slag. Therefore, it is better to select the position of the nozzle at the bottom of the furnace or near the bottom of the furnace as much as possible.

(Example 2) The reduction treatment of the FeO component contained in the generated slag is carried out using direct reduced iron produced by the same method as in Example 1 and the same induction heating furnace. CO, _H2 and _CH4 gases are supplied from the bottom blowing nozzle as reducing materials, and the supply time of the gas reducing materials is fixed at 90 minutes. In addition, the reduction treatment of the generated slag from the top is carried out by adding solid C, metal Al and metal Si as solid reducing materials. Here, if the reduction treatment is carried out with metal Al and metal Si, the alkalinity of the generated slag and the concentration of _Al2O3 will change. Therefore, quicklime or _silica is added as a by-raw material to adjust the alkalinity of the _slag and the concentration of _Al2O3 . After the reduction treatment, the furnace body was tilted to completely discharge the molten iron in the furnace, and the weight of the discharged molten iron was weighed. In addition, the slag was taken and crushed to less than 53μm for chemical analysis. The obtained values are shown in Table 5-1 and Table 5-2 together with the treatment conditions.

It can be seen that in the conditions of supplying reducing gas in treatments No. 21 to 27, the amount of metal soup (molten iron) increased and the T.Fe concentration in the slag decreased compared with treatment No. 4 of Example 1. In other words, it can be proved that the gas reducing material is very helpful for the reduction of FeO in the generated slag. In addition, when comparing treatments No. 23 to 27, it can be seen that the amount of reducing gas (CH ₄ ) supplied increased, so the T.Fe in the slag decreased and the amount of metal soup (molten iron) increased.

In addition, it is also known that even in the conditions of adding solid reducing materials in treatments No. 28 to 30, the amount of metal soup (molten iron amount) increases and the T.Fe concentration in the slag decreases compared with treatment No. 4 of Example 1. It can be proved that, like reducing gas, solid reducing materials are very helpful for the reduction of FeO in the generated slag.

In addition, various evaluations were conducted for conditions such as the type and combination of reducing materials, the amount of addition, and the timing of addition. Compared with the conditions without adding reducing materials, the amount of metal soup (molten iron) increased and the T.Fe concentration in the slag decreased. With respect to the purpose of the present invention to produce molten iron, the yield of iron is improved by supplying reducing materials, and it is best to reduce the slag in a manner that can reduce the T.Fe concentration in the slag to less than 20 mass%, more preferably to less than 10 mass%, and more preferably to less than 5 mass%. However, as the reduction progresses, the utilization efficiency of the supplied reduced materials will decrease. Therefore, it is important to consider the composition and price of reduced iron, auxiliary raw materials and reduced materials before deciding which treatment method to choose.

(Example 3) The high-temperature direct-reduced iron produced by the rotary furnace bed shown in Example 1 is directly loaded into the induction heating melting furnace for melting. That is, the direct-reduced iron with a temperature of about 1000°C when it comes out of the rotary furnace bed is directly loaded into the induction heating melting furnace similar to Examples 1 and 2 for melting. At this time, the temperature of the reduced iron decreases during the transfer process from the rotary furnace bed to the induction heating melting furnace, and the temperature of the reduced iron just loaded into the induction heating melting furnace is about 900°C. Even reduced iron in a high temperature state can be inductively heated without any problem, and the time required for temperature rise can be shortened by more than 30 minutes compared with reduced iron of the same composition and the same treatment conditions. Here, in the melting process of reduced iron directly loaded in a heated state, according to the aforementioned gas injection conditions and the composition of the slag, the fluidity of the slag can be ensured and stable treatment can be performed. In addition, at the time of manufacturing reduced iron, if the slag alkalinity and _Al2O3 concentration are previously prepared, at the time of melting in the induction heating melting furnace, it is sufficient _to supply gas in accordance with the conditions of formula (2), and there is no need to add auxiliary raw materials.

(Example 4) The molten iron obtained in Examples 1 and 2 is first adjusted in temperature and then transferred to a pot-shaped container. At this time, of the slag generated when the vein contained in the reduced iron is melted in the induction heating melting furnace, the slag at a ratio of about 10 kg/ton of molten iron is transferred to the pot-shaped container together with the molten iron, and the remaining slag is transferred to other slag containers. Then, the pot-shaped container is moved to the dephosphorization treatment equipment, and the type and amount of the oxygen source and lime source supplied are changed to perform dephosphorization treatment. The dephosphorization treatment equipment has: an upper gas spray gun, a hopper for separate supply of auxiliary raw materials, and a bottom-blowing multi-pore plug. From the upper gas jet system, a gas containing pure oxygen or air can be supplied at a rate of about 1Nm ³ /(per minute and per ton of molten iron). There are three hoppers for the separate supply of auxiliary materials, which are filled with iron ore, quicklime (CaO), and calcium carbonate (CaCO ₃ ), respectively, and can be supplied at a rate of about 10kg/minute. Gas can be supplied from the bottom-blowing multi-pore plug system. In this embodiment, pure Ar gas is supplied at a rate of about 0.1Nm ³ /(per minute and per ton of molten iron).

The melting temperature in the induction heating melting furnace was adjusted so that the temperature of the molten iron before dephosphorization was maintained at about 1590℃. The temperature was measured and sampled using a melting type thermocouple temperature gun at the time point just before the upper gas jet was lowered and the time point after the upper gas jet was raised after the treatment was completed, which was regarded as before and after the dephosphorization treatment. The sampled samples were cut and ground, and then the carbon concentration [C] and phosphorus concentration [P] in the molten iron were evaluated using the emission spectrophotometry method based on the pre-made reference measurement line. In addition, the solidification temperature of molten iron at the time of temperature measurement and sampling with a melting type thermocouple temperature gun can be measured, and the solidification temperature _Tm of molten iron after dephosphorization treatment can also be actually measured.

The dephosphorization treatment starts when the upper gas gun starts to descend. After the upper gas gun reaches a predetermined height, the supply of oxygen gas source and the addition of auxiliary materials begin. After the supply of a predetermined amount of oxygen gas source and auxiliary materials is completed, the dephosphorization treatment is completed when the upper gas gun rises to the standby position. This period is considered the dephosphorization treatment time _tf (minutes).

After dephosphorization, the pot-shaped container is tipped over and the slag on the molten iron is removed by a slag scraper. A portion of the removed slag is collected for chemical analysis. Then, the pot-shaped container is lifted up by a crane and tipped over to transfer the molten iron to the casting feed bucket. The molten iron flows down from the casting feed bucket and collides with the platform to form droplets of molten iron. The molten iron falls into a cooling water tank and solidifies to produce iron pellets. The particle size of the produced iron pellets is 0.1 to 30 mm. The particle size distribution is: +0.1mm-1mm accounts for 17.2% by mass, +1mm-10mm accounts for 31.3% by mass, +10mm-20mm accounts for 38.8% by mass, and +20mm-30mm accounts for 12.7% by mass. Here, "+N-M" means: above the screen of the N-size screen and below the screen of the M-size screen.

The temperature _Tf of the molten iron after dephosphorization is adjusted to be lower than the temperature _Ti of the molten iron before dephosphorization, and the slag alkalinity C/S is adjusted to be in the range of 1.5 to 4.0, and the temperature _Tf of the molten iron after dephosphorization is adjusted to be 20°C or higher than the solidification temperature _Tm of the molten iron. As a result, the phosphorus concentration [P] _i of the molten iron before dephosphorization is about 0.12 mass%, and the phosphorus concentration [P] _f of the molten iron after dephosphorization is reduced to about 0.02 to 0.04 mass%. In addition, solid iron can be produced without any hindrance to the productivity of iron particles.

Furthermore, it was confirmed that, as long as the molten iron produced by the invention examples of Examples 1 and 2, including the granular iron produced in the above manner, was solidified, solid iron having a total iron content T.Fe of 93 mass % or more and a total oxide content of 3 mass % or less could be produced regardless of the size and shape of the casting mold. The size and shape of the casting mold can be changed in accordance with the future use of the solid iron, but in consideration of various factors such as the convenience of subsequent cargo loading and unloading, packaging, transportation, and supply to the use equipment, it is preferred to solidify the solid iron into granular solid iron within the range of 10 to 100 mm.

The slag produced in the invention examples of Examples 1 and 2 must have the required fluidity for slagging. The alkalinity C/S of the slag is in the range of 0.5 to 2.0, and _{the Al2O3 concentration converted into the ternary system of CaO-SiO2-Al2O3 is in the} range of 10 to 25 mass%. It was also confirmed that by water-crushing the molten slag to make it fine glass, the specific surface area is in the range of 0.35 ^m2 /g or more and less than 0.50 ^m2 /g, and it can be used as a cement raw material. Furthermore, it has been confirmed that when molten slag is slowly cooled in the atmosphere, slag blocks of about several hundred mm or less can be produced, and when these blocks are crushed and classified to an appropriate particle size, they can be used as road base materials and/or concrete aggregates.

In this manual, the unit of mass "t" is expressed in 10 ³ kg (tons), and the unit of heat "cal" has been converted to "4.184J". In addition, the N added to the unit of volume "Nm ³ " represents the standard state of the gas. In this manual, the standard state of the gas is expressed in 1atm (=101325Pa) and 0℃. [M] in the chemical formula means that the element M has been dissolved in molten iron and/or reduced iron. [Industrial Applicability]

According to the direct reduced iron melting method of the present invention, when the direct reduced iron is melted in an induction heating melting furnace, gas is blown into the molten iron to improve the fluidity of the slag, and the slag and the molten iron are separated. In particular, by melting the high-temperature reduced iron, heat energy can be saved and productivity can be improved. Therefore, it has practical value in the industry.

[Fig. 1] is a graph showing the composition of the slag obtained from the reduced iron composition disclosed in Table 2, mapped onto the _Al2O3 - _CaO - _SiO2 ternary phase diagram. [Fig. 2] is a graph showing the composition of the slag obtained from the reduced iron composition disclosed in Table 2, mapped onto the CaO- _SiO2 -FeO ternary phase diagram.

Claims (10)

A method for melting direct-reduced iron comprises: a direct reduction step of producing direct-reduced iron by bringing iron ore or a mixture of iron ore and a composition adjusting material into contact with a reducing material under heating; a melting step of melting the direct-reduced iron using an induction heating melting furnace to produce molten iron; a slag discharge step of discharging the slag generated in the melting step to the outside of the melting furnace; and a refining step of refining the molten iron obtained in the melting step according to demand; wherein, The loading temperature of the direct reduction iron melted in the above-mentioned melting step into the above-mentioned induction heating melting furnace is within the range from the temperature after the above-mentioned direct reduction step to the atmospheric temperature. The above-mentioned melting step includes: a first step of blowing gas into the above-mentioned molten iron during part of or the entire period of the melting step, and may further include one or more steps selected from the following steps (1) to (3) as required, (1) a second step of adding a slag component regulator, (2) a third step of supplying heat to the slag from a heat source installed on the furnace of the above-mentioned induction heating melting furnace, and (3) a fourth step of supplying one or more reducing solids or gases. A method for melting direct reduced iron as described in claim 1, wherein, in the first step, a height H (m) from a position of a gas supply nozzle for blowing the gas into the molten iron to a surface of the molten iron bath is expressed by the following formula (1), and the gas is blown into the molten iron under the condition satisfying the following formula (2): H = 1.27 × W _DRI / (ρ ₁ D ² ) × (%T.Fe) _DRI / 100-h ... Formula (1) H > 0.18 × (ρ _g Q ² /ρ ₁ N ² d ² ) ^1/3 ... Formula (2) wherein, ρ _g : density of supply gas (kg/m ³ ), ρ ₁ : density of molten iron (kg/m ³ ), Q : gas supply rate (Nm ³ /min), N: Number of gas supply nozzles (-), d: Diameter of gas supply nozzle (m), D: Inner diameter of induction heating furnace (m), W _DRI : Weight of reduced iron supplied to induction heating furnace (kg), (%T.Fe) _DRI : Total iron concentration contained in reduced iron (mass %), h: Height from the bottom of induction heating furnace to the position of gas supply nozzle (m). A method for melting direct reduced iron as described in claim 1, wherein the type and amount of the slag composition modifier are adjusted in the second step so that the composition of the slag generated in the melting step, in terms of mass %, the ratio of CaO concentration (%CaO) to _SiO2 concentration (% _SiO2 ), that is, the alkalinity, falls within the range of 0.5 _to 2.0, and _{the Al2O3} concentration (% _Al2O3 ) falls within the range of 10 to 25 mass %. A method for melting direct reduced iron as described in claim 1, wherein, in the fourth step, the type and supply amount of the reducing solid or gas are adjusted so that the total iron content (%T.Fe) of the components of the slag generated in the melting step is less than 20 mass %. A method for producing solid iron comprises solidifying molten iron produced by the method described in any one of claims 1 to 4 into solid iron. A solid iron produced by the method described in claim 5, wherein the total iron concentration T.Fe is 93 mass % or more and the total oxide content is 3 mass % or less. A method for manufacturing civil engineering and construction materials comprises: a melting step of melting direct reduction iron in an induction heating melting furnace to obtain molten iron; a slag discharge step of discharging the slag generated in the aforementioned melting step to the outside of the aforementioned melting furnace; and a cooling and hardening step of cooling and hardening the slag discharged in the aforementioned slag discharge step to make a raw material for civil engineering and construction materials; wherein, the loading temperature of the direct reduction iron melted in the aforementioned melting step into the aforementioned induction heating melting furnace is within the range from the temperature after the completion of the aforementioned direct reduction step to the atmospheric temperature, The aforementioned melting step comprises: a first step of blowing gas into the aforementioned molten iron during a part of or the entire period of the melting step, and may further comprise one or more steps selected from the following steps (1) to (2) as required, (1) a second step of adding a slag composition regulator, and (2) a third step of supplying heat to the slag from a heat source installed on the furnace of the aforementioned induction heating melting furnace. A civil engineering and construction material is a civil engineering and construction material produced by the method as described in claim 7, wherein the ratio of CaO concentration (%CaO) to _SiO2 concentration (% _SiO2 ), i.e., alkalinity, is in the range of 0.5 _to 2.0, and _{the Al2O3} concentration (% _Al2O3 ) is in the range of 10 to 25% by mass. A direct reduction iron melting system comprises: a direct reduction furnace for producing direct reduction iron by bringing iron ore or a mixture of iron ore and a composition adjusting material into contact with a reducing material under heating; an induction heating melting furnace for melting the direct reduction iron to produce molten iron; a slag discharge mechanism for discharging slag generated in the induction heating melting furnace to the outside of the melting furnace; and a refining device for refining the molten iron melted in the induction heating melting furnace according to demand; wherein, The direct-reduction iron melted by the induction heating melting furnace is loaded into the induction heating melting furnace at a temperature in the range of the temperature after the direct reduction treatment is completed to the atmospheric temperature. The induction heating melting furnace has the function of blowing gas into the molten iron during a part of or the entire period of the melting of the direct-reduction iron, and may have one or more functions selected from the following functions (1) to (3) as required, (1) the function of adding a slag component regulator, (2) the function of supplying heat to the slag from a heat source installed on the induction heating melting furnace, and (3) the function of supplying one or more reducing solids or gases. A direct reduction iron smelting system as described in claim 9, wherein for one direct reduction furnace, two or more induction heating smelting furnaces are provided.