EP4656742A1

EP4656742A1 - Method for producing molten iron

Info

Publication number: EP4656742A1
Application number: EP23926416.1A
Authority: EP
Inventors: Futoshi Ogasawara; Nobuhiko Oda; Koichi Tsutsumi; Yoshie Nakai
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-03-07
Filing date: 2023-11-10
Publication date: 2025-12-03
Also published as: KR20250126114A; JPWO2024185210A1; CN120813708A; JP7772242B2; WO2024185210A1; TW202436631A; TWI881534B

Abstract

Provided is a method for melting a cold iron source with high productivity and with a reduced electric power unit consumption. Specifically, provided is a method for producing molten iron by melting a cold iron source with electric energy, using an electric furnace including a melting chamber and a preheating chamber, the method including arranging a burner in the melting chamber, the burner including an injection hole that injects fuel, and an injection hole that injects a combustion-supporting gas, the burner being adapted to inject a flame from the injection hole toward molten iron in the melting chamber; blowing in a powdery or powdered auxiliary raw material such that the auxiliary raw material passes through the flame formed by the burner; and when introducing an exhaust gas generated through combustion of the burner into the preheating chamber to preheat the cold iron source in the preheating chamber, adjusting a supply rate of the fuel used for the burner or the auxiliary raw material, thereby preheating the cold iron source in the preheating chamber to a predetermined temperature.

Description

Technical Field

The present invention relates to a method for melting a cold iron source with high productivity and with reduced electric power unit consumption.

Background Art

In recent years, from the perspective of preventing global warming, the iron and steel industry has also developed methods to reduce CO₂ gas emissions by reducing fossil fuel consumption. In conventional integrated steel plants, molten pig iron is produced by reducing iron ore with carbon. To produce such molten pig iron, about 500 kg of a carbon source is required per ton of molten pig iron for processes including iron ore reduction. Meanwhile, producing molten steel using a cold iron source such as iron scrap or solid reduced iron as the main raw material does not require a carbon source used for iron ore reduction, but only a sufficient amount of heat energy to melt the cold iron source. This can significantly reduce CO2 emissions.
An operation using a high content of a cold iron source is often carried out using an electric furnace, such as an arc furnace or an induction melting furnace. In such cases, most of the heat for melting the cold iron source is provided by electric power. For example, in a typical operation of an arc furnace, the following methods are adopted to increase productivity and reduce the electric power unit consumption. 1) An auxiliary burner is arranged on a furnace wall or in a slag removal port to promote the melting of a cold iron source at a cold spot, for example. 2) A so-called oxygen-enriched operation for providing oxidization heat for iron is performed by supplying oxygen through an oxygen-gas supply lance.
However, the oxygen-enriched operation has a problem of causing reduced yield due to the oxidation loss of iron. Moreover, when an auxiliary burner is used, a burner flame is formed in the upper portion of the furnace body above the surface of molten iron. This results in low heat imparting efficiency to the molten iron in the furnace, with most of the heat supplied being discharged as sensible heat in an exhaust gas. Therefore, even if the electric power unit consumption can be reduced, the effect of reducing the total amount of energy input, including fuel, is small. Against this background, there has been a demand for a heat supply means that can impart heat to molten iron and a cold iron source in a furnace with high efficiency.
As a high-efficiency heat supply means, for example, Patent Literatures 1 and 2 respectively disclose a method of installing a lance for supplying powdery particulate ore, other than a top-blowing lance used for supplying an oxidizing gas, in an iron-bath-type smelting reduction furnace. This method involves equipping the distal end of the lance with a circulation hole for ore and also providing a burner that features an injection hole for introducing fuel and oxygen, allowing ore to be supplied in such a way that it can pass through the flame produced by the burner. The patent literatures indicates that as ore heated in the flame transfers heat to the molten iron within the furnace, the efficiency of utilizing the combustion heat of the burner improves significantly; and that heat transfer from the gas produced by the burner combustion in the burner flame to the powdery particulate material results in a reduction in the temperature of the gas generated through burner combustion, leading to lowering the exhaust gas temperature.

Citation List

Patent Literature

Patent Literature 1: Japanese Patent Laid-Open No. 2007-138207
Patent Literature 2: Japanese Patent Laid-Open No. 2008-179876

Summary of Invention

Technical Problem

However, the conventional method has the following problems.
When heating with the burner alone without supplying powdery material into the burner flame, the heat imparting efficiency of the burner combustion heat to the molten iron within the furnace is low, as described above. Then, the amount of sensible heat in the exhaust gas increases to discharge heat outside the furnace. As described in Patent Literatures 1 and 2, adding the powdery particulate material allows the burner combustion heat to be transferred to the powdery material, lowering the temperature of the gas produced by the burner combustion. However, a low supply rate of the powdery particulate material corresponds to a small heat transfer amount, resulting in low heat imparting efficiency to the molten iron in the furnace and a small extent of temperature drop of the combustion gas. As a condition for high heat imparting efficiency to molten iron in a furnace and a reduced temperature of an exhaust gas, Patent Literature 2 specifies that the powder-fuel ratio S/Q should be 0.3 or more during a smelting reduction process, provided that S (kg/min) represents a supply rate of a powdery material, and Q (MJ/min) represents a heat quantity from a burner fuel per unit time. That is, Patent Literature 2 specifies that it is necessary to supply powdery particulate material in a sufficient amount relative to burner combustion heat.
This means that the amounts of heat generated by the burner and heat that can be imparted to the molten iron in the furnace are limited by the amount of the powdery particulate material that can be supplied during the refining process. When an excess of powdery particulate material is added in the refining process compared to the amount of powdery auxiliary raw material originally required, additional sensible heat is required to heat the excess powdery particulate material to the temperature of molten iron. This results in a heat loss that is greater than the amount of heat supplied by the burner.
The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an electric furnace with a high-efficiency heat supply means, and a method for melting a cold iron source with high productivity and reduced electric power unit consumption.

Solution to Problem

A method for producing molten iron according to the present invention that advantageously solves the above problems involves melting a cold iron source with electric energy using an electric furnace provided with a melting chamber and a preheating chamber, the method being characterized by

disposing a burner in the melting chamber, the burner including an injection hole that jets fuel and an injection hole that blows a combustion-supporting gas, to emit a flame from the injection hole toward a furnace content in the melting chamber;
blowing in a powdery or powdered auxiliary raw material such that the auxiliary raw material passes through the flame formed by the burner; and
when introducing an exhaust gas generated by combustion by the burner into the preheating chamber to preheat the cold iron source in the preheating chamber, adjusting a supply rate of the fuel used for the burner or the auxiliary raw material, thereby preheating the cold iron source in the preheating chamber at a predetermined temperature.

Note that the method for producing molten iron according to the present invention may include the following features, for example, which are considered to be more preferable solution means.

(a) An upper limit of a preheated temperature of the cold iron source is 1200°C.
(b) Provided that a heat quantity from the fuel used for the burner per unit time is Q (MJ/min) and a supply rate of the auxiliary raw material is S (kg/min), a powder-fuel ratio S/Q (kg/MJ) is set to be 0.10 or more but 0.50 or less.
(c) The electric furnace includes a vertical preheating chamber.

Advantageous Effects of Invention

According to the present invention, a powdery particulate material is supplied via a burner flame, and the powdery particulate material is heated in the flame and converted into a heat transfer medium. This process enables the efficient utilization of burner combustion heat to heat a cold iron source and molten iron within a melting chamber of an electric furnace, thereby reducing the amount of electric power used. Further, a high-temperature exhaust gas generated by burner combustion is introduced into a preheating chamber located at a different position from the melting chamber where the burner is arranged, and is used to heat a cold iron source filling the preheating chamber. Accordingly, the sensible heat of the exhaust gas can also be effectively used. By charging the cold iron source preheated with the sensible heat of the exhaust gas into the melting chamber, it is possible to melt a predetermined amount of a cold iron source with a lower unit consumption of electric power compared to when a non-preheated cold iron source is added. This can also reduce the tapping interval of the electric furnace, thus increasing productivity. As the sensible heat of the exhaust gas generated from the burner combustion heat can be utilized to preheat the cold iron source, the amount of heat released to the outside of the system can be reduced, which can increase the thermal efficiency.
Moreover, the supply rate of the fuel used for the burner or the auxiliary raw material is adjusted to preheat the cold iron source in the preheating chamber at a predetermined temperature. This prevents excessive use of the auxiliary raw material and can ensure proper allocation of the burner combustion heat for both imparting heat to the molten metal in the melting chamber and preheating the cold iron source in the preheating chamber. As a result, the burner combustion heat can be efficiently utilized to melt the cold iron source.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a schematic longitudinal sectional view illustrating an AC arc furnace, which includes a melting chamber and a preheating chamber, as an electric furnace according to an embodiment of the present invention.
[Fig. 2] Fig. 2 is a schematic longitudinal sectional view of the distal end portion of a burner lance used in the foregoing embodiment.

Description of Embodiments

Hereinafter, an embodiment of the present invention will be specifically described. Note that the drawings are only schematic and may differ from the actual ones. In addition, the following embodiment only illustrates examples of an apparatus and a method for embodying the technical idea of the present invention. Thus, the configuration of the present invention is not limited thereto. That is, the technical idea of the present invention may be changed in various ways within the technical scope recited in the claims.
Fig. 1 is a schematic longitudinal sectional view illustrating an overview of an AC arc furnace 101 as an electric furnace according to an embodiment of the present invention, and illustrates a configuration pattern of the operation of the AC electric arc furnace. In the present embodiment, the electric furnace 101 includes a melting chamber 1 for melting iron scrap, which is a cold iron source x, through arc heating, and a preheating chamber 2 for preheating the iron scrap x to be supplied to the melting chamber 1.
The upper portion of the melting chamber 1 is covered with an openable/closable furnace lid 4 with a water-cooled structure. A plurality of electrodes 5 are inserted into substantially the central portion of the melting chamber 1 from above through the furnace lid 4. Iron scrap is melted by striking arcs between the electrodes 5 in an arc heating portion A. The electrodes 5 are typically made of graphite, for example, and are configured to be movable up and down. Agitation may be performed by blowing gas from the bottom of the furnace.
The shaft-type (vertical) preheating chamber 2 is provided on and connected to the melting chamber 1 at a position distant from the arc heating portion A. The preheating chamber 2 is in vertical communication with the melting chamber 1. The upper portion of the preheating chamber 2 is provided with an openable/closable scrap charging port 20. The upper side portion of the preheating chamber 2 is provided with an exhaust port 21, and an exhaust duct 6 is connected to the exhaust port 2. The exhaust duct 6 is connected to a suction blower (not shown). Through suction by the suction blower, a high-temperature exhaust gas generated in the melting chamber 1 flows into the preheating chamber 2, passes through the preheating chamber 2, and is then discharged through the exhaust duct 6. A dust collector (not shown) is installed at the position of the exhaust duct 6.
A bottom-opening type supply bucket 13 suspended from a traveling carriage 16 is provided to be movable above the preheating chamber 2, and the iron scrap x is charged into the preheating chamber 2 from the supply bucket 13 via the scrap charging port 20.
A gate 22 is installed at the lower portion of the preheating chamber and separates the melting chamber 1 and the preheating chamber 2. The gate 22 is provided with a through-hole to allow a high-temperature exhaust gas in the melting chamber 1 to be introduced into the preheating chamber. The gate is opened as necessary to charge the iron scrap x in the preheating chamber 2 into the melting chamber. The iron scrap x within a space portion 1a is spontaneously pushed toward the arc heating portion A under the weight of the iron scrap x filling the preheating chamber 2 and the space portion 1a.
An extruder (pusher) that pushes the iron scrap x filling the space portion 1a toward the arc heating portion A of the electrodes 5 may be installed in the melting chamber 1 so as to face the space portion 1a below the preheating chamber 2. Preferably, the extruder 3 is provided so as to be movable back and forth in the direction of the arc heating portion A (the direction of the furnace center in the present embodiment) through a sidewall of the melting chamber 1, and is driven by a driving device (not shown) to push, with its a distal end, the iron scrap x in the space portion 1a toward the arc heating portion A.
In the present embodiment, a burner lance 9 is ascendably and descendably inserted into the melting chamber 1 through a burner lance insertion hole provided in the furnace lid 4. Although Fig. 1 illustrates an example in which the burner lance 9 is ascendably and descendably inserted perpendicularly through the furnace lid, the present invention is not limited thereto. The burner lance 9 may be inserted obliquely into the furnace from above a furnace wall. In addition, the form of the burner is not limited to the ascendable and descendable lance, and the burner may be configured such that its nozzle portion is fixed to the furnace lid or the furnace wall. In addition, the burner may be provided with an oxygen blowing function for blowing oxygen from the burner. The burner lance 9 emits a burner flame 9a toward the surface of the furnace content, such as the cold iron source x and molten iron m, contained in the melting chamber 1.
Note that an oxygen blowing lance and a carbonaceous material blowing lance may be inserted into the melting chamber 1 from above through the furnace lid 4.
A carbonaceous material containing, for example, one or more of coke, char, coal, charcoal, and graphite may be blown into molten slag s through the carbonaceous material blowing lance, using air, nitrogen, or the like as a carrier gas. In addition, oxygen may be supplied (injected) through the oxygen blowing lance so that the oxygen can be blown into the molten iron m by pushing aside the molten slag.
Instead of pure oxygen, an oxygen-containing gas (e.g., a mixed gas of pure oxygen and air) may be blown in through the oxygen blowing lance.
The melting chamber 1 has a taphole 11 at the furnace bottom on the side opposite to the side where the preheating chamber 2 is provided. In addition, a slag outlet 12 is provided in a sidewall above the taphole 11. The taphole 11 and the slag outlet 12 are closed by a taphole door 14 and a slag outlet door 15, respectively, to prevent the plugging sand or a mud agent filled therein from leaking out.
Fig. 1 illustrates a state in which iron scrap is charged as the cold iron source x, and the supply of electric current is started to melt the cold iron source x. During the process, a powdery auxiliary raw material 9b is blown from the burner lance 9 through the burner flame 9a to promote the melting of the cold iron source x. This operation is preferably performed with fuel composed mainly of a hydrogen gas produced with renewable energy, such as sunlight, wind power, or water power. The fuel mainly composed of a hydrogen gas refers to a hydrogen gas or a hydrogen-rich gaseous fuel. The hydrogen-rich gaseous fuel can be a mixed gas of a hydrogen gas and a methane gas, a natural gas, or a petroleum gas. From the perspective of reducing CO₂ emissions, the mixed gas preferably contains 50 vol% or more of a hydrogen gas.
Although the AC arc furnace 101 including three electrodes is used as the electric furnace in the above embodiment, the electric furnace may be a DC arc furnace including an upper electrode and a lower electrode. When the arc furnace 101 is used as the electric furnace, the electrodes 5 and arcs are present in the central portion of the furnace body, so that the arrangement position of the burner lance 9 is limited. With the burner of the present embodiment, it is possible to reduce the temperature of the burner flame 9a by appropriately blowing in the powdery auxiliary raw material 9b even when fuel mainly composed of a hydrogen gas is used as described below. Thus, the operation can be performed without causing wear of the water-cooled panel of the furnace wall, refractories of the hearth, or the like.
Fig. 2 illustrates a schematic view of a distal end portion 30 of the burner lance 9 as one configuration example of the burner lance 9 used in the above embodiment. A powder supply pipe 31 with an injection hole is arranged in the center, and a fuel supply pipe 32 and a combustion-supporting gas supply pipe 33 each having an injection hole are arranged in this order around the powder supply pipe 31. An outer shell 35 with a cooling water passage 34 is provided on the outer side thereof. A fuel gas 36 and a combustion-supporting gas 37 are supplied through the injection hole provided in the outer peripheral portion of the powder supply pipe 31, so that the burner flame 9a is formed. Then, the powdery auxiliary raw material 9b injected through the powder supply pipe 31 is heated in the burner flame 9a. Accordingly, the powdery auxiliary raw material 9b becomes a heat transfer medium, which can increase the heat imparting efficiency of the flame to the furnace content, such as the cold iron source x and the molten iron m. Consequently, the amount of electric power can be reduced. As the combustion-supporting gas 37, not only pure oxygen, but also a mixed gas of oxygen and CO₂ or an inert gas; air; or oxygen-rich air is applicable. Further, an inert gas or a combustion-supporting gas can be used as a gas for carrying the powdery auxiliary raw material 9b as a powdery material.
In the method for producing molten iron according to the present embodiment, for example, the cold iron source x, such as iron scrap, is first charged as a main raw material into the melting chamber 1 and the preheating chamber 2 in the AC arc furnace 101 illustrated in Fig. 1 from the supply bucket 13. After the cold iron source x is first charged into the melting chamber 1, the supply of an electric current is started. Then, the burner lance 9 installed in the upper portion within the furnace is inserted into the melting chamber 1 so that the cold iron source x is heated with electric power and with the combustion heat of the burner flame 9a. An exhaust gas flows into the preheating chamber 2 and is used to preheat the cold iron source x in the preheating chamber 2.
When the first-charged cold iron source x has melted further and reached a flat bath state (a state in which the cold iron source x is immersed in the molten iron m even if the cold iron source x has unmelted portions), slag is removed through the slag outlet 12 as appropriate. The gate 22 is then opened to charge the cold iron source x in the preheating chamber 2 into the melting chamber 1. After the charging of the cold iron source x in the preheating chamber 2, an additional cold iron source x is charged into the preheating chamber 2 from above the preheating chamber 2. From the perspective of securing productivity, it is preferable to continue the supply of an electric current as well as the burner heating while the cold iron source x is being additionally charged from the preheating chamber 2 and while the cold iron source x is being newly charged into the preheating chamber 2. The number of additional charges may be three or more.
The inventors examined the heat imparting efficiency to the furnace content using the AC arc furnace including the melting chamber or the preheating chamber illustrated in Fig. 1 and a typical AC arc furnace without a preheating chamber, by varying the flow rate of a fuel gas and the supply rate of a powdery material. When using the AC arc furnace with a preheating chamber, the preheated temperature of the cold iron source was also examined. Herein, the ratio of the supply rate S (kg/min) of the powdery auxiliary raw material 9b to the heat quantity Q (MJ/min) per unit time from the fuel 36 used for the burner lance 9 is represented by a powder-fuel ratio S/Q.
As a result, in the AC arc furnace with a preheating chamber, when a sufficient amount of powdery particulate material was supplied relative to the heat quantity from a fuel gas, specifically, when the powder-fuel ratio S/Q was in the range of 0.30 to 0.50 (kg/MJ), the temperature of a burner flame achieved 1500°C or below. Also, an effect of increasing the heat imparting efficiency to the furnace content, an effect of reducing the electric power unit consumption, and an effect of increasing productivity were confirmed. These effects were found to be greater than those when using the AC arc furnace without a preheating chamber, under the same powder-fuel ratio. This is attributed to the fact that the sensible heat of the burner combustion gas effectively contributed to preheating the cold iron source in the preheating chamber, thereby further reducing the sensible heat of the exhaust gas discharged to the outside of the furnace. If the powder-fuel ratio S/Q exceeds 0.50 (kg/MJ), that is, if the heat quantity from fuel is extremely small relative to the supply amount of the powdery particulate material, the effects of reducing the electric power unit consumption and increasing productivity may be diminished.
Furthermore, even when the heat quantity from fuel was excessively large relative to the supply amount of the powdery particulate material, specifically, when the powder-fuel ratio S/Q was below 0.30 (kg/MJ), the effects of reducing electric power and increasing productivity were observed. Typically, in an electric furnace without a preheating chamber, operating at a powder-fuel ratio S/Q in such a range results in an excessively high exhaust gas temperature, which in turn reduces the heat imparting efficiency to the furnace content and diminishes the effects of reducing electric power unit consumption and improving productivity. Meanwhile, in an electric furnace with a preheating chamber, while the burner combustion heat has a lower efficiency of contributing to the heating of the powdery particulate material, resulting in high temperatures of the burner flame and the exhaust gas as 1500°C or higher, the cold iron source in the preheating chamber can be preheated. This can reduce the sensible heat of the exhaust gas discharged to the outside of the furnace. That is, it was confirmed that the burner combustion heat can be utilized with high efficiency without restricting the supply of the powdery particulate material required for a refining process. When operating at an even lower powder-fuel ratio S/Q, the preheated temperature of the cold iron source in the preheating chamber increased. When the preheated temperature exceeded 1200°C, cold iron source materials in the preheating chamber began to weld together, potentially making it difficult to discharge them from the preheating chamber. Therefore, the upper limit of the preheated temperature of the cold iron source in the preheating chamber is preferably set to 1200°C. In addition, a study revealed that the powder-fuel ratio S/Q of 0.10 (kg/MJ) or more can achieve the preheated temperature of the cold iron source in the preheating chamber of 1200°C or lower. Although the lower limit of the preheated temperature of the cold iron source in the preheating chamber is not limited, from the perspective of increasing thermal efficiency, the preheated temperature should exceed the temperature of the cold iron source at the time when it is charged into the preheating chamber. It is preferably 300°C or higher and further preferably over 500°C. To regulate the preheated temperature of the cold iron source in the preheating chamber to an appropriate temperature, it is preferable to adjust the flow rate of air entrained from the surroundings. This can be achieved, for example, by controlling the flow rate of an air-exhaust ventilator or by adjusting the size of the opening of the electric furnace on the side of the furnace body.
A slag forming material, which is the powdery or powdered auxiliary raw material 9b, dust, etc., may be used as the powdery material. To efficiently heat the powdery material in a burner flame, it is necessary to increase the specific surface area of the powdery material, and the particle size thereof is preferably approximately 100 µm or less. When the particle size of the auxiliary raw material is larger, it is preferable to reduce the particle size to approximately 100 µm or less, for example, by grinding. Herein, the particle size is expressed by the 50% passing rate in terms of volume.
As the cold iron source x, it is preferable to use iron scrap or solid reduced iron. The solid reduced iron is produced through a reduction process that uses a reducing agent with reduced CO₂ emissions and contains approximately 10 to 20 mass% of gangue derived from iron ore, such as SiO₂ and Al₂O₃, depending on the grade. When the solid reduced iron is melted, these components are found on the surface of the molten iron m as slag s. Slag s has a composition with a high melting temperature as it is and is likely to solidify and adhere to the furnace wall, which may cause operational problems.
In response, it is preferable to use lime as the powdery auxiliary raw material 9b to be supplied after burner heating, as it can control the basicity, i.e., the CaO/SiO₂ ratio in mass of the slag s, to approximately 1.0. This can achieve a lower melting temperature of the slag s and thus suppress the solidification of the slag s. Furthermore, since heat is provided to the slag s from the heated powdery material, an effect of promoting slag formation can be obtained. After the slag s is formed in such a manner, the slag removal port may be opened to remove the slag during melting or before tapping.
Any electric furnace is applicable as long as it can melt the cold iron source with electric energy to obtain molten iron. For example, in addition to the AC or DC arc furnace described above, an arc furnace such as a submerged arc furnace may be used, in which heating is performed by submerging a Söderberg self-baking electrode or the like in the slag. In addition, an indirect resistance furnace may be used, in which an object to be heated is heated with radiation from a heating element provided in the furnace, convection in the furnace, and conductive heat transfer. A plasma arc melting furnace may be also used.
The molten iron m melted in the present embodiment has a composition corresponding to the metal composition of the iron scrap or solid reduced iron used as the main raw material, and is typically molten steel with a relatively low C content. To adjust the components, additional processes may be performed in the same electric furnace where the melting was performed, including the addition of alloy, and finishing decarburization and dephosphorization by oxygen refining. Further, secondary refining, such as molten steel desulfurization and vacuum degassing, may be performed after tapping. Subsequently, a semi-finished product, such as a cast slab, is produced through a casting step such as continuous casting.

Examples

Cold iron source melting tests were performed using an AC arc furnace (A) without a preheating chamber, and an AC arc furnace (B) including a melting chamber and a preheating chamber with a configuration similar to that illustrated in Fig. 1. Scrap was used as the cold iron source, with a total charge amount set at 100 tons.
Each electric furnace had a furnace lid provided with a burner lance including a fuel supply line and an oxygen supply line, and the distal end portion of the burner lance was formed to have a multiple-pipe structure similar to that illustrated in Fig. 2. A propane gas was used as a burner fuel. Comparisons were made between the following three cases: a case where no burner was used, a case where the furnace content was heated by a burner flame alone with a burner fuel supplied but without a powdery material supplied, and a case where powdery lime was blown into a burner flame. The tapping temperature was set at 1650°C.
After the supply of an electric current began, the melting of the first-charged cold iron source progressed, reducing the height of the charged materials in the furnace until a space was formed in the upper portion of the furnace. At this point, the burner lance was lowered to apply heat from the burner flame as well. An argon gas was used as a carrier gas to supply a powdery material. A total of 50 kg/t of powdery lime was supplied into the electric furnace at a supply rate of S=100 kg/min. A propane gas as a fuel gas was supplied at a flow rate varying in the range of 2.2 to 14 Nm³/min for each heating process of the electric furnace. The powder-fuel ratio S/Q was found to be 0.08 to 0.51 kg/MJ. In addition, an oxygen gas was supplied as a combustion-supporting gas to burn propane as the fuel gas in each heating process. When the first-charged cold iron source melted further and reached a flat bath state (a state in which the cold iron source is immersed in the molten iron even if the cold iron source has unmelted portions), slag was removed through the slag removal port. The electric current supply and burner operation were then stopped, and the furnace lid was opened to charge the cold iron source for the second time and beyond. After the second charge of the cold iron source, the electric current supply was resumed to perform a similar operation as after the first charge. Thus, molten steel at 1650°C was obtained and tapped into a ladle.

Comparisons were made between process conditions in terms of the electric power unit consumption, the processing time of the electric furnace, and the efficiency of imparting burner combustion heat. The electric power unit consumption is an index value obtained by dividing the amount of electric power used for each process condition by the amount of electric power used for process No. 1. The processing time of the electric furnace corresponds to the time (minutes) from the start of the electric current supply to the start of tapping. The efficiency of imparting burner combustion heat is the ratio of the amount of heat imparted to the furnace content to the total heat quantity from burner fuel. For the AC arc furnace (B), the temperature of the scrap in the lower portion of the preheating chamber immediately before the charging of the scrap into the furnace was measured with a radiation thermometer, and the measured temperature was determined as the preheated temperature of the cold iron source. Table 1 shows the results. Table 1 also shows the types of electric furnaces and the features of the burners.

[Table 1]

No.	Electric furnace type	Burner				Electric power unit consumption index	Processing time of electric furnace	Efficiency of imparting burner combustion heat	Preheated temperature of cold iron source	Remarks
		Use	Supply rate S of powdery material	Supply rate of propane	Powder-fuel ratio S/Q	Electric power unit consumption index	Processing time of electric furnace	Efficiency of imparting burner combustion heat	Preheated temperature of cold iron source
		Used/Not used	kg/min	Nm³/min	kg/MJ	-	min	%	°C
1	A	Not used	-	-	-	1.00	80	-	-	Conventional Example
2	A	Used	-	2.2	-	1.00	80	15	-	Comparative Example
3	A	Used	100	2.2	0.51	0.95	76	70	-	Comparative Example
4	A	Used	100	2.8	0.40	0.90	72	70	-	Comparative Example
5	A	Used	100	3.2	0.35	0.85	68	70	-	Comparative Example
6	A	Used	100	3.7	0.30	0.80	64	70	-	Comparative Example
7	A	Used	100	4.0	0.28	0.79	63	55	-	Comparative Example
8	A	Used	100	4.4	0.25	0.79	63	40	-	Comparative Example
9	A	Used	100	5.6	0.20	0.78	62	30	-	Comparative Example
10	A	Used	100	11.1	0.10	0.77	62	20	-	Comparative Example
11	A	Used	100	12.3	0.09	0.77	62	18	-	Comparative Example
12	A	Used	100	14.0	0.08	0.77	62	16	-	Comparative Example
13	B	Used	100	2.2	0.51	0.90	72	80	500	Invention Example
14	B	Used	100	2.8	0.40	0.85	68	80	550	Invention Example
15	B	Used	100	3.2	0.35	0.80	34	80	580	Invention Example
16	B	Used	100	3.7	0.30	0.75	60	80	620	Invention Example
17	B	Used	100	4.0	0.28	0.71	57	80	650	Invention Example
18	B	Used	100	4.4	0.25	0.67	54	79	740	Invention Example
19	B	Used	100	5.6	0.20	0.64	51	79	950	Invention Example
20	B	Used	100	11.1	0.10	0.60	48	78	1180	Invention Example
21	B	Used	100	12.3	0.09	0.57	46	77	1210	Invention Example
22	B	Used	100	14.0	0.08	0.55	44	77	1250	Invention Example

When using the AC arc furnace (A) without a preheating chamber, in process No. 2 where the furnace content was heated by a burner flame alone, burner combustion heat was not effectively imparted to the furnace content, and the electric power unit consumption and the processing time of the electric furnace were substantially the same as those of process No. 1 where no burner was used. In each of processes No. 3 to 12 where powdery lime was heated in the burner flame, the electric power unit consumption and processing time of the electric furnace were reduced. This is because the powdery lime was heated in the burner flame, with some of the burner combustion heat being transferred to the furnace content. However, in the conditions (processes No. 7 to 12) where the heat quantity from fuel was excessively large relative to the supply rate of powdery lime, the efficiency of imparting burner combustion heat decreased, reducing the effect of reducing the electric power unit consumption and the processing time of the electric furnace. This is because the amount of heat transferred to the powdery lime has peaked, resulting in an increased ratio of the burner combustion heat discharged as the sensible heat of an exhaust gas.
In each of processes No. 13 to 22, in which powdery lime was heated in a burner flame in the AC arc furnace (B) including a melting chamber and a preheating chamber, the electric power unit consumption and the processing time of the electric furnace were reduced, as in processes No. 3 to 12, in which powdery lime was heated in a burner flame in the AC arc furnace (A) without a preheating chamber. The results of processes No. 13 to 22 were superior to those of processes No. 3 to 12 when comparisons were made based on the same powder-fuel ratio S/Q. In the conditions (processes No. 7 to 12) where the AC arc furnace (A) without a preheating chamber was used and the heat quantity from fuel was excessively large relative to the supply rate of powdery lime, the heat imparting efficiency of the burner was reduced, and the effects of reducing the electric power unit consumption and the processing time of the electric furnace reached their peak. Meanwhile, in each of processes No. 17 to 22 performed in the AC arc furnace (B) with a preheating chamber, the heat imparting efficiency of the burner was not reduced even at the same powder-fuel ratio S/Q. In addition, the effects of reducing the electric power unit consumption and the processing time of the electric furnace were observed as the heat quantity from the fuel increased.
This is because, even when the amount of heat transferred to the powdery lime reached its peak and resulted in a higher temperature of the burner combustion gas, under the condition that the heat quantity from the fuel was excessively large relative to the supply rate of the powdery lime, the gas was used to preheat scrap in the preheating chamber by introducing the gas into the preheating chamber. This reduced sensible heat of an exhaust gas. In addition, with an increase in the heat quantity from the fuel relative to the supply rate of the powdery material, the preheated temperature of the cold iron source in the preheating chamber increased. In some of the heating processes performed under the condition that the preheated temperature of the cold iron source was above 1200°C, a malfunction occurred such that the scrap in the preheating chamber welded together and thus could not be charged into the furnace even if the gate in the lower portion of the preheating chamber was opened. The preheated temperature of the scrap in the preheating chamber is preferably set to 1200°C or lower.
The unit "t" of mass used in this specification represents 10³ kg. Symbol "N" added to the unit of the volume of a gas represents the volume in the standard state, that is, at a temperature of 0°C and a pressure of 101325 Pa.

Industrial Applicability

According to the method for producing molten iron of the present invention, it is possible to melt a cold iron source using a heat source with increased heat imparting efficiency and reduced CO₂ emissions, and thus reduce the electric power unit consumption as well as environmental burdens, which is industrially advantageous. This method can be suitably applied to a process of a refining furnace or the like that needs a heat source with reduced CO₂ emissions and also needs the addition of a powdery auxiliary raw material.

Reference Signs List

1: melting chamber
1a: space portion
2: preheating chamber
3: extruder
4: furnace lid
5: electrode
6: exhaust duct
7: oxygen blowing lance
8: carbonaceous material blowing lance
9: burner lance
9a: burner flame
9b: (powdery) auxiliary raw material
10: furnace wall
11: taphole
12: slag outlet
13: supply bucket
14: taphole door
15: slag outlet door
16: traveling carriage
20: scrap charging port
21: exhaust port
22: gate
30: distal end portion (nozzle) of burner lance
31: powder supply pipe
32: fuel supply pipe
33: combustion-supporting gas supply pipe
34: cooling water passage
35: outer shell
36: fuel gas
37: combustion-supporting gas
38: cooling water
101: (AC) arc furnace (electric furnace)
x: cold iron source (iron scrap)
m: molten iron
s: (molten) slag
A: arc heating portion

Claims

A method for producing molten iron comprising melting a cold iron source with electric energy using an electric furnace provided with a melting chamber and a preheating chamber, characterized by
disposing a burner in the melting chamber, the burner including an injection hole that jets fuel and an injection hole that blows a combustion-supporting gas, to emit a flame from the injection hole toward a furnace content in the melting chamber;

blowing in a powdery or powdered auxiliary raw material such that the auxiliary raw material passes through the flame formed by the burner; and

when introducing an exhaust gas generated by combustion by the burner into the preheating chamber to preheat the cold iron source in the preheating chamber, adjusting a supply rate of the fuel used for the burner or the auxiliary raw material, thereby preheating the cold iron source in the preheating chamber at a predetermined temperature.
The method for producing molten iron according to claim 1, wherein
an upper limit of a preheated temperature of the cold iron source is 1200°C.
The method for producing molten iron according to claim 1, wherein,
provided that a heat quantity from the fuel used for the burner per unit time is Q (MJ/min) and a supply rate of the auxiliary raw material is S (kg/min), a powder-fuel ratio S/Q (kg/MJ) is set to be 0.10 or more but 0.50 or less.
The method for producing molten iron according to claim 1, wherein
the electric furnace includes a vertical preheating chamber.