JP7636730B2 - Method for melting direct reduced iron, method for producing solid iron, and method for producing civil engineering and construction materials - Google Patents

Description

The present invention relates to a method for melting direct reduced iron to remove gangue contained in the direct reduced iron, solid iron and a method for producing solid iron using this method, as well as civil engineering and construction materials and a method for producing civil engineering and construction materials.

In recent years, there has been an increasing demand for the expanded use of cold iron sources (scrap) in the steel industry. In order to build a recycling-oriented society, recycling of iron sources is necessary and unavoidable. Moreover, from _the perspective of preventing global warming, the increase in scrap usage is essential in order to meet the demand for reducing _CO2 emissions. Unlike iron ore, which is iron oxide ( _Fe2O3 ), scrap does not require a reduction step in the smelting process, making it possible to reduce _CO2 emissions. Therefore, the usage of cold iron sources is steadily increasing.

The blast furnace-converter process is a steelmaking process in which iron ore (Fe ₂ O ₃ ), a raw material, 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%, and the molten pig iron is then 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 CO ₂ gas is generated. On the other hand, when molten steel is produced using iron scrap as a raw material, the carbon source required for the reduction of iron ore is not required. Therefore, even when the energy required to melt iron scrap is taken into consideration, replacing 1 ton of molten pig iron with 1 ton of iron scrap leads to a reduction in CO ₂ 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.

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, but there is no need to make the C concentration in the produced iron high as in the blast furnace-converter process, and since no excessive carbon source is used, this leads to a reduction in _CO2 emissions of about 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.

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

The Fe content is expressed as the total amount of iron in the iron ore, T.Fe, and the larger this value, the more Fe is contained, so the higher _the value 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 , and also contains about 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, and the transportation cost and smelting cost per unit Fe amount will increase.

In addition, 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 component adjuster mixed in, the type and unit consumption of the reducing material, the reduction temperature, and the type of reduced iron manufacturing equipment. Table 2 shows an example of the component composition of reduced iron.

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 of a burden the subsequent transportation, melting, and refining processes will be. However, such reduced iron requires the use of high-grade iron ore, which poses issues such as increased raw material costs and increased reduction processing costs to increase the metallization rate. For this reason, it is desirable to develop a process that efficiently separates slag from reduced iron with a high slag content produced using cheap, low-grade iron ore as a raw material, thereby increasing the metallic iron content.

A method for producing reduced metals has been proposed that removes the gangue contained in such ores 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 material, and melting the material at least once to separate the metal and slag.

JP 2000-292069 A JP 2004-204293 A

Slag Atlas, 2nd ed., Verlag Stahleisen GmbH, Duesseldorf, (1995), 105, 126.

However, the conventional technology has the following problems.
The techniques disclosed in Patent Documents 1 and 2 are premised on the use of a carbonaceous solid raw material as a reducing agent when separating 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 material carburizes 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 that it is expected that the liquid phase rate of the charge will not increase and separation of metal and slag will not proceed easily.

In other words, in the reduction process, it can be difficult to remove 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.

The present invention has been made in consideration of the above circumstances, and aims to propose a method for melting direct reduced iron that efficiently removes gangue from direct reduced iron. In addition, it aims to provide high-purity solid iron and a method for producing solid iron using the melting method, as well as civil engineering and construction materials that utilize by-products and a method for producing civil engineering and construction materials.

The inventors discovered that when melting directly reduced iron in an induction melting furnace, it is possible to promote heat transfer between the molten iron and slag and to suppress solidification of the slag by controlling the composition of the slag, making it easier to discharge the slag from the furnace, and thus completed the present invention.

The method for melting direct reduced iron according to the present invention, which advantageously solves the above problems, comprises a melting step of melting direct reduced iron in an induction melting furnace to obtain molten iron, a slag discharge step of discharging slag generated in the melting step out of the induction melting furnace, and, optionally, a refining step of refining the molten iron obtained in the melting step, wherein 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.

The method for melting direct reduced iron according to the present invention comprises 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), wherein, ρ _g : density of supplied 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 : diameter of gas supply nozzle (m), D : inner diameter of induction furnace (m), W _DRI : weight of reduced iron supplied into induction furnace (kg), (% T.Fe) _DRI : total iron concentration (mass %) contained in reduced iron, and h : height (m) from the bottom of the induction furnace to the position of the gas supply nozzle,
(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 generated 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)

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 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.

The manufacturing method for civil engineering and construction materials according to the present invention, which advantageously solves the above-mentioned problems, is a manufacturing method for civil engineering and construction materials, comprising: a melting step of melting direct reduced iron in an induction melting furnace to obtain molten iron; a slag discharge step of discharging slag generated in the melting step to the outside of the melting furnace; and a cooling and solidification step of cooling and solidifying the slag discharged in the slag discharge step to obtain a civil engineering and construction material raw material, wherein the melting step includes a first step of blowing gas into the molten iron for a part or the entire period 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. In addition, 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 _CaO concentration (%CaO) to 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%.

According to the method for melting direct reduced iron of the present invention, when the direct reduced iron is melted in an induction melting furnace, gas is blown into the molten iron, and optionally at least one of the following is performed: 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 above the furnace. This makes it possible to maintain the slag in a fluidized state and separate the slag while melting the metallic iron in the direct reduced iron.

According to the manufacturing method for solid iron and the solid iron of the present invention, the molten iron and slag are separated and the molten iron is solidified, which is preferable because it is possible to manufacture solid iron with a high purity. In addition, according to the manufacturing method for 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, making it possible to effectively utilize the by-products.

1 is a graph in which the compositions of slag obtained from the reduced iron compositions shown in Table 2 are plotted on an Al ₂ O ₃ —CaO—SiO ₂ ternary phase diagram. 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—SiO ₂ —FeO ternary phase diagram.

The following is a detailed description of the embodiments of the present invention. The following embodiments are intended to exemplify compositions and methods for embodying the technical ideas of the present invention, and are not intended to specify the configuration as 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.

The inventors conducted their 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 the induced current. However, because the slag is not directly heated, there was an issue in that the slag that floats to the top of the molten iron solidifies due to differences in specific gravity, making it difficult to charge additional reduced iron.

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 when blowing gas into the bath during melting of reduced iron in an induction melting furnace, 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 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, thereby making it possible to more suitably 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.

By keeping the slag fluid in this way, the additional reduced iron is not obstructed by the solidified slag when it is charged, and the volume of the induction melting furnace can be used more effectively. Furthermore, because the slag is fluid, it is easy to separate the molten iron and slag, for example by allowing it to overflow 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 from which the slag is discharged by tilting the furnace body, from the standpoint of handling the high-temperature slag and repairing damaged areas.

The first embodiment of the present invention was obtained from the above-mentioned studies and comprises a melting step of melting directly reduced iron in an induction melting furnace to obtain molten iron, and a slag discharge step of discharging the slag produced in the melting step outside the induction melting furnace, the melting step including a first step of blowing gas into the molten iron for a part or the entire duration of the melting step, and optionally further including 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.

Second Embodiment
Next, the optimization of the gas injection conditions in the first step was examined. 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 generated and floated slag. 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 a pipe. For example, since oxidizing gases such as oxygen and carbon dioxide oxidize the molten iron and reduce the iron yield, inert gases such as Ar and _N2 are preferable.

However, if the amount of gas supplied is too large, the gas passes through as a continuous phase up to the bath surface of the molten iron, which is called 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 (mass%) contained in reduced iron,
h: Height from the bottom of the induction furnace to the gas supply nozzle position (m)
Represents.

On the other hand, if the amount of gas injected is too small, the stirring effect of the molten iron due to the floating of the gas will be small, and the heat transfer from the molten iron to the slag will be small, so it is preferable to supply gas at a total gas supply amount per unit of molten iron of at least 0.01 ^Nm3 /(min·t-molten iron).

The second embodiment of the present invention was obtained from the above-mentioned considerations, 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 surface of the molten iron bath is expressed by the above formula (1), and the gas is injected into the molten iron so as to satisfy the above formula (2).

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 Al ₂ O ₃ -CaO-SiO ₂ or CaO-SiO ₂ -FeO is 100%, and the compositions are plotted on the respective ternary phase diagrams shown 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 (atomic weight of Fe).

As is clear from FIG. 1, there are compositions with a lower melting point than the slag components contained in reduced iron. However, when the slag composition has a high concentration of SiO ₂ , the melting point is low but the viscosity is high and the fluidity is reduced. Therefore, it is preferable to set the basicity (hereinafter referred to as slag basicity), which is the ratio of the CaO concentration (%CaO) to the SiO ₂ concentration (%SiO ₂ ) on a mass basis, 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 Al ₂ O ₃ 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, SiO ₂ , and Al ₂ O ₃ 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%, so there is a problem that the amount of slag generated during the melting of reduced iron increases as the amount added increases, so quicklime is preferable. Examples of substances containing _SiO2 include silica stone, coal ash, and steelmaking slag. Alternatively, metal Si or silicon sludge may be added to utilize _SiO2 generated by reaction with _the iron oxide remaining in the reduced iron. Examples of substances containing _Al2O3 include natural stones such as corundum and bauxite, as well as metal Al and aluminum dross, _{which may be added to utilize Al2O3 generated} by reaction with the iron oxide remaining in the reduced iron.

The third embodiment of the present invention was obtained from the above-mentioned studies, and in addition to the first or second embodiment, 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 Al ₂ O ₃ concentration is in the range of 10 to 25 mass% for the composition of the slag generated in the melting step.

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 methods: 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.

The fourth embodiment of the present invention was obtained from the above-mentioned considerations, 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 reducing solids or gases so that the composition of the slag generated in the melting step has a total iron concentration (% T.Fe) of 20 mass% or less.

<Third step>
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 installing a heating source on the furnace to supply heat to the slag. The heating 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. The burner heating may be any liquid fuel such as heavy oil, or a gaseous fuel such as CO, _H2 , or hydrocarbons, or a combination 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 therein, 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.

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.

The dephosphorization reaction requires an oxygen source and a CaO source, as represented by the following formula (A).
2 [P] + 5/2・O ₂ (g) + 3CaO (s) = 3CaO・P ₂ O ₅ (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.

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 carry out the dephosphorization process at a low temperature. Considering this, it was concluded that it would be advantageous to lower the molten iron temperature within a range that does not affect the process.

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. In addition, when an iron oxide source is used, a cooling effect on the pure oxygen gas is obtained by the heat absorption when the iron oxide source is reduced to metallic Fe or forms molten slag in the form of iron oxide.

Next, since calcium carbonate contained in limestone absorbs heat when it decomposes into CaO and _CO2 , it is possible to cool the molten iron by using limestone as a CaO source. 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 problems 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 in consideration of the desired cooling effect and stable operation.

The occurrence of spitting varies depending on the freeboard (height from the top surface of the molten iron to the top end of the vessel) of the vessel where the dephosphorization treatment is performed 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 lance height according to the operating conditions of the dephosphorization treatment. In addition, in order to provide agitation to the molten iron, it is preferable 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 SiO ₂ carried over in the slag removal step and the type and amount of CaO source added. If necessary, a SiO ₂ source such as silica stone or ferrosilicon, or a CaO source such as quicklime may be added.

If the slag basicity is low, the amount of phosphorus removed during the dephosphorization process will be small. If the slag basicity is high, some of the slag will solidify and adhere to the refractory material when the molten iron temperature drops, making it difficult to remove the slag after dephosphorization. This can lead to problems such as abnormal reactions when molten iron is charged in the next process, or residual slag being mixed into the generated slag and causing components to become mismatched. In addition, since such dephosphorization processes using air generate large amounts of high-temperature exhaust gas, the exhaust heat can be recovered using a boiler or the like.

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 produce molten steel. It may also be solidified once in a mold to produce solid iron, which is then transported to the demand area and remelted and refined to produce 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 the equipment is newly installed, it will require huge costs and will not be possible to utilize existing equipment. In the latter case, it is 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 production and solidification of reduced iron in the country where the iron ore is produced, transport the solidified iron to the demand area, and remelt and refine it. In this case, in addition to being able to utilize 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.

The sixth embodiment of the present invention was obtained from the above-mentioned considerations, 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 removal step.

Seventh Embodiment
In a seventh embodiment of the present invention, molten iron obtained by the method for melting direct reduced iron according to any one of the first to sixth embodiments is solidified to obtain solid iron. The total iron concentration T.Fe of the solid iron is preferably 93 mass% or more, and the total of oxide components is preferably 3 mass% or less. Although there is no limitation on the shape or size of the mold in which the solid iron is solidified, it is preferable to solidify the solid iron into granules having a particle size in the range of 10 to 100 mm, taking into consideration the subsequent loading, packing, transportation, and supply to the facility where the solid iron is used.

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 method further includes a cooling and solidifying step in which the slag discharged by 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 the above-mentioned basicity range, and can have various particle size distributions depending on the cooling method. If necessary, the slag can be used as a material that utilizes its characteristics by performing additional particle size adjustment processing such as crushing and classification. For example, if the discharged slag is water-granulated, it becomes fine glass-like and has a specific surface area of 0.35 m ² /g or more and less than 0.50 m ² /g, so that it can be used as a cement raw material (binding material). In addition, it can be used as a roadbed material or concrete aggregate by slowly cooling in the air and adjusting the particle size according to the intended use. In this way, the operator can appropriately select the cooling and solidifying method according to the intended use of the discharged slag.

Example 1
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 reduced iron C shown in Table 2 was added so as not to overflow from the furnace body. After it was confirmed that the melting proceeded 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, respectively, and the configuration allows gas to be supplied at the same flow rate to any combination of nozzles via a gas header. A hopper capable of supplying auxiliary materials was provided on the furnace, and cutting out in 10 kg units was possible at any timing during processing. The temperature of the molten iron in the furnace was appropriately measured, 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.

The melting process was carried out by changing the gas flow rate from the nozzle or the type and amount of the auxiliary raw material added. The quicklime used as the auxiliary raw material was made by roasting limestone at high temperature to remove _CO2 , and the CaO concentration was almost 100 mass%. The silica stone was collected at a quarry and crushed, and the _SiO2 concentration was about 98 mass%, and it contains small amounts _{of Al2O3} and MgO. The 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 crystal water, _SiO2 , _TiO2 , etc. After the process, the furnace body 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, and subjected to chemical analysis. The obtained values are shown in Tables 3-1 and 3-2 together with the process conditions. Here, for comparison, the process was also carried out under conditions where no gas was supplied from the bottom of the furnace.

In Table 3-2, 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 portion of the slag was blackened and solid, but the solid slag was constantly moving across the surface, and ×: the slag was black overall, and only cracked areas were red hot, and the slag was stagnant. Here, under the condition of ◯ for slag fluidity, it took less than 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 △ for slag fluidity, it took longer than 90 minutes. This difference was due to the fact that part of the slag was solid, and it took time for the slag to move to the tapping port in the slag removal step. Under the condition of × for slag fluidity, 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.

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 the treatment No. 20, gas was not supplied from the bottom of the furnace, so it is considered that the heat supply from the molten iron to the slag was insufficient and the slag solidified. In the 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 3-1 and 3-2, 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 Al ₂ O ₃ concentration in the slag converted to a ternary system of CaO-SiO ₂ -Al ₂ O ₃ was in the range of 10 mass% to 25 mass%.

In the cases of Nos. 13 to 15, the amount of gas supplied from the bottom of the furnace was too large, which caused the above-mentioned blow-through phenomenon and significant scattering of molten iron. As a result, the amount of metal tapped was also lower than in the other invention examples. In the case of No. 16, an excess of quicklime was added, which resulted in a slag basicity C/S of over 2.0, an increase in the melting point of the slag, a decrease in fluidity, and a longer slag draining time. In the case of No. 17, an excess of silica was added, which resulted in a slag basicity C/S of less than 0.5, an increase in the viscosity of the slag, a decrease in fluidity, and a longer slag draining time. In the case of No. 18, an excess of bauxite was added, which resulted in an Al ₂ O ₃ concentration calculated as a CaO-SiO ₂ -Al ₂ O ₃ ternary system of over 25 mass%, an increase in the melting point of the slag, a decrease in fluidity, and a longer slag draining time. In No. 19, as a result of adding excessive quicklime and silica stone, the Al ₂ O ₃ concentration calculated as a CaO-SiO ₂ -Al ₂ O ₃ ternary system was diluted to less than 5 mass%, the melting point of the slag increased, the fluidity decreased, and the slag removal time became longer.

It was confirmed that auxiliary materials are not limited to quicklime, silica, and bauxite, and that there is no problem with controlling the slag composition with the other substances mentioned above. It was found that it is better to add auxiliary materials in stages, 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, and increases the melting time due to a decrease in the efficiency of induction heating. In addition, slag has already been generated after melting is complete, and depending on the composition, the slag may solidify, and even if auxiliary materials are added, they may end up sitting on top of the solidified slag, which does not contribute to lowering the melting point of the slag.

Although the melting of reduced iron will proceed even without seed water, the presence 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 input, but as the amount of seed water increases, its proportion relative to the volume of the induction melting furnace increases and the amount of meltable reduced iron decreases, so it is preferable to set the proportion of seed water to 70% by mass or less relative to the reduced iron input. In addition, the seed water can be used to newly melt scrap or large lump bullion with high bulk density, or it can be used to leave some of the molten iron melted in the previous melting process in the furnace.

In addition, the process was performed by changing the nozzle position and combination. 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. However, when formula (2) was not satisfied, the scattering of molten iron increased and the metal yield decreased.

It was also confirmed that there is no problem with positioning the nozzle on the side of the furnace instead of the bottom. However, if the distance h from the nozzle on the side of the furnace to the bottom is large, i.e., if H is small, the gas supply rate that satisfies equation (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.

Example 2
The same induction furnace as in Example 1 was used to carry out a process for reducing the FeO content contained in the produced slag. CO, H ₂ and CH ₄ 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 to the produced slag as solid reducing agents from above. Here, when reduction was performed with metal Al and metal Si, the basicity and Al ₂ O ₃ concentration of the produced slag changed, so quicklime or silica stone was added as an auxiliary material to adjust the slag basicity and Al ₂ O ₃ concentration. After the process, the furnace body was tilted, the molten iron in the furnace was completely discharged, and the weight of the molten iron was weighed. In addition, the slag was sampled and crushed to 53 μm or less, and subjected to chemical analysis. The obtained values are shown in Tables 4-1 and 4-2 together with the process conditions.

It can be seen that under the conditions of supplying reducing gas in Process Nos. 21 to 27, 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. 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 the reducing gas (CH ₄ ) supplied, the T.Fe in the slag was reduced and the amount of metal tapped was increased.

Next, even under the conditions where solid reducing agents were added in Process Nos. 28 to 30, the amount of metal tapped increased and the T.Fe concentration in the slag decreased compared to Process No. 4 in Example 1. It became clear that the solid reducing agent, like the reducing gas, effectively contributed to the reduction of FeO in the generated slag.

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% or less, more preferably 10% or less, and even more preferably 5% or less by supplying reducing materials to improve iron yield. However, the utilization efficiency of the supplied reducing materials decreases as reduction progresses, 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.

Example 3
The molten iron obtained in Examples 1 and 2 was transferred to a ladle-shaped container after temperature adjustment. At this time, about 10 kg/t-molten iron out of the slag generated during melting in the induction melting furnace due to gangue contained in the reduced iron was transferred to the ladle-shaped container together with the slag, and the remainder was transferred to another slag container. The ladle-shaped container was moved to a dephosphorization treatment facility, and the type and amount of the oxygen source and lime source to be supplied were changed to perform dephosphorization treatment. The dephosphorization treatment facility has a gas top-blowing lance, a secondary raw material cut-out hopper, and a bottom-blowing porous plug. The gas top-blowing lance can supply gas containing pure oxygen or air at a rate of about 1 Nm ³ /(min.t-molten iron). There are three secondary raw material cut-out hoppers, each filled with iron ore, quicklime (CaO), and calcium carbonate (CaCO ₃ ), and each can be supplied at a rate of about 10 kg/min. Gas can be supplied from the bottom-blowing porous plug, and in this embodiment, pure Ar gas was supplied at a rate of about 0.1 Nm ³ /(min·t-molten iron).

The melting temperature in the induction melting furnace was adjusted so that the temperature of the molten iron before the dephosphorization treatment was about 1590°C. The timings before and after the dephosphorization treatment were defined as the timing before the top gas blowing lance was lowered and when the top gas blowing lance was lifted after the treatment, respectively, and temperature measurement and sampling were carried out using a sublance. The sampled samples were cut and polished, and the C concentration [C] and P concentration [P] in the molten iron were evaluated by optical emission spectroscopy from a calibration curve prepared in advance. It was also possible to measure the solidification temperature of the molten metal at the timing when the sublance temperature measurement and sampling were carried out, and the solidification temperature _Tm of the molten iron after the dephosphorization treatment was actually measured.

The start of the dephosphorization process was determined as the start of the downward movement of the top gas lance, and the supply of the oxygen gas source and the addition of the auxiliary materials were started after the top gas lance reached a predetermined height. The end of the dephosphorization process was determined as the time when the top gas 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 time during which the dephosphorization process was performed was determined as the processing time _tf (minutes).

After the dephosphorization process, the ladle-shaped vessel was tilted, and the slag on the molten iron was removed using a slag dragger. A portion of the removed slag was sampled and subjected to chemical analysis. The ladle was then lifted and tilted using 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 surface plate, 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%, +20 mm-30 mm: 12.7 mass%. Here, +N-M means that the sieve is above the mesh size N and below the mesh size M.

The slag basicity C/S was adjusted to a range of 1.5 to 4.0 while adjusting the molten iron temperature after dephosphorization _Tf to be lower than the molten iron temperature before dephosphorization _Ti , and the molten iron temperature after dephosphorization _Tf was adjusted to be 20°C or more higher than the solidification temperature _Tm of the molten iron. As a result, the P concentration [P] _i of the molten iron before dephosphorization: about 0.12 mass% was reduced to the P concentration [P] _f of the molten iron after dephosphorization: 0.02 to 0.04 mass%. Furthermore, solid iron could be produced without impeding the productivity of granular iron.

It was confirmed that when the molten iron produced in the invention examples of Examples 1 and 2, including the nuggets produced above, is solidified, a solid iron having a total iron content (T.Fe) of 93% by mass or more and a total oxide content of 3% by 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 considering loading, packaging, transportation, and supply to the equipment to be used, it is preferable to solidify it into granules of 10 mm to 100 mm.

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 Al ₂ O ₃ concentration calculated as a CaO-SiO ₂ -Al ₂ O ₃ ternary system is in the range of 10 to 25 mass%. It was confirmed that the molten slag can be made into a fine glass-like material by subjecting it to water granulation, with an area of 0.35 m ² /g or more and less than 0.50 m ² /g, and can be used as a cement raw material. It was also confirmed that the molten slag can be cooled slowly in the air to obtain slag lumps of several hundred mm or less, and that the slag can be used as a roadbed material or concrete aggregate by crushing and classifying the slag to an appropriate particle size adjustment.

In this specification, the unit of mass "t" represents 10 ³ kg. Furthermore, the N attached to the unit of volume "Nm ³ " represents the standard state of gas, which in this specification is 1 atm (=101,325 Pa) and 0° C. In the notation of chemical formulas, [M] represents that element M is dissolved in molten iron or reduced iron.

According to the method for melting direct reduced iron of the present invention, when the 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, thereby producing high-quality solid iron and by-products, which is industrially useful.

Claims (7)

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 out of the induction melting furnace;
Optionally, the method further comprises a refining step of refining the molten iron obtained in the melting step,
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 the 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 provided at the bottom or side of the furnace,
Furthermore, 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 method for melting direct reduced iron, comprising one or more steps selected from a fourth step of supplying one or more types of reducing solids or gases.
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. In the second step,
The method for melting direct reduced iron as described in claim _{1 ,} wherein the type and amount of the slag component adjuster are adjusted so that the composition of the slag generated in the melting step has a basicity, which is the ratio of _CaO concentration (%CaO) to _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%. 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. A method for producing solid iron, comprising solidifying the molten iron obtained by the method according to any one of claims 1 to 3 to produce solid iron. 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. A method for manufacturing a civil engineering and construction material, comprising:
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,
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 the 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 provided at the bottom or side of the furnace,
Furthermore, 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 above the induction melting furnace.
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. The method for producing a civil engineering and construction material according to claim 6, wherein by carrying out the second step, the civil engineering and construction material 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% _.

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