Method for melting metal raw material

Abstract

Provided is a method for melting a metal raw material, wherein a condition for melting a metal raw material partially or entirely composed of reduced iron by using a submerged arc furnace (SAF) is set and stable energization is performed, thereby making it possible to melt a large amount of the metal raw material. This method for melting a metal raw material by using an electric furnace includes stacking and accommodating molten iron, molten slag, a metal raw material, and an auxiliary raw material in the electric furnace, and discharging the molten iron and the molten slag, wherein when an electric current is supplied to heat the molten metal by an arc or through the molten slag and the metal raw material, the in-furnace resistivity of the electric furnace is in the range of 0.01-0.02 (Ωm).

Description

Method for melting metal raw materials

[0001] The present invention relates to a method for melting metal raw materials, and more particularly to a method for melting metal raw materials that can produce molten metal by melting metal raw materials containing reduced iron in an electric furnace or a submerged arc furnace.

[0002] In recent years, CO ２ In the steel industry, CO emissions are also being reduced. ２ As an alternative to the blast furnace process, which involves a large amount of iron emissions, the direct reduction (DR) process for producing iron sources has been attracting attention. In the direct reduction (DR) process, for example, iron-containing agglomerates are reduced in a shaft furnace to produce direct reduced iron (DRI). This reduced iron or iron scrap or other iron source raw materials are charged into an electric arc furnace (EAF), submerged arc furnace (SAF), or the like, where they are heated and melted, and the slag is separated to produce molten iron.

[0003] In Japan, studies are underway to apply the direct reduction (DR) method to low-grade iron ore produced in Australia, India, etc. However, it is difficult to melt and process reduced iron produced from low-grade iron ore using current electric arc furnaces (EAFs). Therefore, the use of submerged arc furnaces (SAFs) is being considered.

[0004] In light of this technical background, various molten metal production processes using submerged arc furnaces (SAFs) have been proposed. These molten metal production processes include a method for producing molten iron using the so-called flat bath operation (FBO). For example, the production of molten iron using a submerged arc furnace (SAF) is carried out using the flat bath operation (FBO). The flat bath operation (FBO) employs a melting process in which raw materials and auxiliary materials, including metallic materials such as scrap, pig iron, direct reduced iron (DRI), or hot briquetted iron (HBI), are continuously fed into the submerged arc furnace (SAF) without opening the furnace lid, or the raw materials are charged using small buckets. During the charging of the raw materials, electrodes are inserted into the molten slag in the SAF, and an arc is generated between multiple electrodes or between the electrodes and the molten iron. An electric current is passed through the raw materials and slag to heat and melt the raw materials by resistance heating or heat transfer.

[0005] One problem with submerged arc furnaces (SAFs) is the inefficient heat transfer to materials located away from the electrodes. Incomplete metal melting, resulting from delayed melting of the metal material, can lead to problems such as concentration gradients, unreliable temperature measurement, unreliable process control, and excessively hot molten metal tapping. To address this temperature non-uniformity, electromagnetic stirring of the molten metal (disclosed in U.S. Pat. No. 5,623,499) and gas stirring of the molten slag (disclosed in U.S. Pat. No. 5,623,499) have been developed.

[0006] Furthermore, Patent Document 3 proposes a technique for obtaining molten metal by arc-heating pre-reduced metal. The method for producing molten metal described in Patent Document 3 uses a stationary, non-diameter, uniform melting furnace that melts pre-reduced metal by arc heating mainly using radiant heat, and this melting furnace has an arc heating electrode and a pre-reduced metal supply mechanism. The method for producing molten metal described in Patent Document 3 aims to obtain a refractory dissolution index RF, defined by a predetermined relational expression, of 400 MWV / m ２ The molten iron is melted at a temperature below this.

[0007] JP-T-2020-505579 A JP-A-2002-317918 JP-A-2003-105415

[0008] However, the prior art has the following problems. That is, the technology described in Patent Document 1 is merely a method for producing molten metal applied to a so-called electric arc furnace. Also, the technology described in Patent Document 2 proposes an electric furnace for a waste melting furnace, and does not intend to mass-produce molten metal. From such a technical point of view, a problem has been to apply a submerged arc furnace (SAF) to the production of molten metal and to develop an electric furnace operation method that takes into account the productivity of molten metal. Also, the technology described in Patent Document 3 proposes a method for producing molten metal that reduces the refractory dissolution index RF to 400 MWV / m ２In order to melt molten iron while maintaining the resistance value inside the electric furnace at or below 100 kJ / cm 2 , the resistance value inside the electric furnace is set to an extremely low value. Therefore, even if the molten metal manufacturing method described in Patent Document 3 is applied to melt reduced iron, a short circuit occurs between the electrodes PCD. As a result, the technology described in Patent Document 3 has the problem that it is not possible to stably energize the electric furnace, and therefore it is not possible to sufficiently melt reduced iron.

[0009] Furthermore, melting reduced iron made from low-grade iron ore using a submerged arc furnace (SAF) is difficult for the following reasons. When melting reduced iron using a submerged arc furnace (SAF), the conditions inside the SAF vary significantly depending on the voltage and current settings for generating the arc used for the melting process. For example, if the voltage value for generating the arc used to melt reduced iron using a submerged arc furnace (SAF) is too high, the arc discharge becomes large. As a result, the radiant heat from the arc discharge increases, significantly reducing the thermal efficiency of the molten iron.

[0010] On the other hand, if the voltage value for generating such an arc is too low, the distance between the molten iron and the electrodes used to generate the arc becomes too close in the submerged arc furnace (SAF). As a result, the surface of the molten iron in the submerged arc furnace (SAF) changes, causing unstable heating of the molten iron. Furthermore, there is a problem in that raw materials enter the electric circuit formed by the electrodes and the molten iron in the submerged arc furnace (SAF), causing unstable heating of the molten iron. Furthermore, when using a submerged arc furnace (SAF) to melt reduced iron, there has been no report on the appropriate voltage range to be applied when melting reduced iron.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for melting metal raw materials, which is capable of melting a large amount of metal raw materials by setting conditions for melting metal raw materials containing reduced iron as a part or the whole thereof using a submerged arc furnace (SAF) and stably passing current through the furnace.

[0012] Therefore, the present inventors conducted various experiments to solve the above-mentioned problems and found that, by setting the resistivity inside a submerged arc furnace (SAF) when melting a metal raw material that is entirely or partially composed of reduced iron and stably supplying current to the electric furnace, it is possible to melt a large amount of metal raw material that is entirely or partially composed of reduced iron. The present invention was made based on the above-mentioned findings and is summarized as follows.

[0013] That is, the method of the present invention for melting metal raw materials, which advantageously solves the above-mentioned problems, is a method of melting metal raw materials using an electric furnace, in which molten iron, molten slag, metal raw materials, and auxiliary materials are stored in a stacked manner inside the electric furnace, and the molten iron and molten slag are discharged separately and continuously, characterized in that when an electric current is supplied to the molten metal by an arc or via the molten slag and metal raw materials to heat it, the resistivity inside the electric furnace is set to a range of 0.01 to 0.02 (Ω m).

[0014] The method for melting metal raw materials according to the present invention comprises the steps of: (a) melting the metal raw materials by flat bath operation; (b) the metallization rate of the metal raw materials is 60% or more; and (c) the current density flowing through the electrodes of the electric furnace is 50 to 70 (A / m ２ (d) the electrodes are of a self-baking type; (e) the electric furnace is either a three-phase AC type having three electrodes or a single-phase type having multiple pairs of electrodes; (f) the metal raw materials and auxiliary raw materials are supplied to between the electrodes in a mixed state or individually; (g) some or all of the metal raw materials are reduced iron; (h) the basicity of the slag is in the range of 1.0 to 1.5, where the basicity of the slag is the ratio of SiO in the slag on a mass basis. ２ (i) the electric furnace is an arc furnace, and at least one selected from the distance from the interface of the molten iron to the arc electrode, the arc voltage, and the arc current is adjusted so that an arc voltage E (V) that is a voltage to ground and an arc current I (A) per arc electrode provided in the arc furnace satisfy the following relational expression (i): (j) A preferred solution is that the electric furnace is a submerged arc furnace.

[0015] According to the present invention, a submerged arc furnace (SAF) is used as the electric furnace, and conditions for melting a metal raw material that is entirely or partially composed of reduced iron are set, and a stable current is supplied to the electric furnace, thereby making it possible to melt a large amount of the metal raw material.

[0016] 1 is a schematic diagram showing an outline of an electric furnace used to implement the method for melting metal raw materials according to the present embodiment. 2 is a schematic diagram showing the state of electrodes and the periphery of the electrodes when current is applied to an electric furnace used to implement the method for melting metal raw materials according to the present embodiment. 3 is a schematic diagram showing an outline of a single-phase AC submerged arc furnace used to implement the method for melting metal raw materials according to the present embodiment.

[0017] [First Embodiment] A method for melting metal raw materials according to the first embodiment will be described. The method for melting metal raw materials according to this embodiment is a method for melting metal raw materials using an electric furnace, in which molten iron, molten slag, metal raw materials, and auxiliary materials are stacked and accommodated inside the electric furnace, and the molten iron and molten slag are discharged. The method is characterized in that when an electric current is supplied to the molten metal by an arc or via the molten slag and metal raw materials to heat it, the resistivity inside the electric furnace is set to a range of 0.01 to 0.02 (Ω·m). Each step included in the method for melting metal raw materials according to this embodiment will be described below.

[0018] 1 is a schematic vertical cross-sectional view illustrating the configuration of a submerged arc furnace used in the method for melting metal raw materials according to this embodiment. As shown in Fig. 1, the submerged arc furnace 1 includes a furnace wall 2, a plurality of electrodes 3, and a transformer (not shown).

[0019] The submerged arc furnace 1 may be equipped with an electromagnetic stirrer (not shown) made of a linear motor. The submerged arc furnace 1 may also be configured to inject gas bubbles by blowing gas through a bottom-blowing tuyeres (not shown). Thus, the submerged arc furnace 1, which is an electric furnace suitable for the method for melting metal raw materials according to this embodiment, has stirring means such as the electric stirrer 6 and the bottom-blowing tuyeres 7, thereby stirring the molten iron (hot metal) P and conducting heat by arc discharge to the interface with the molten slag (molten slag) S, thereby melting the metal raw materials containing reduced iron in part or entirely. The submerged arc furnace 1 may be an electric furnace that is approximately circular or rectangular in shape when viewed from above.

[0020] 1, a submerged arc furnace 1 includes a furnace wall 2, a plurality of electrodes 3, and a transformer (not shown). The refractory material constituting the furnace wall 2 is not particularly limited as long as it has excellent fire resistance, heat insulation, and heat retention properties, and may be primarily made of one or more materials selected from carbon, magnesia, alumina, etc.

[0021] A plurality of electrodes 3 are installed inside the submerged arc furnace 1. The electrodes 3 may be configured as an electrode group consisting of three electrodes 3, or as an electrode group consisting of multiple pairs of electrodes 3. When the submerged arc furnace 1 has three electrodes 3, the submerged arc furnace 1 is a three-phase AC submerged arc furnace. On the other hand, when the submerged arc furnace 1 has multiple pairs of electrodes 3, the submerged arc furnace 1 is a single-phase AC submerged arc furnace.

[0022] The electrode 3 may be a sintered electrode or a self-sintered electrode. The type of electrode 3 is not particularly limited, and a graphite electrode used as an electrode in an electric steelmaking furnace capable of melting iron scrap to produce steel can be used. Graphite electrodes are preferred because they can be used even when the temperature inside the submerged arc furnace 1 is about 1600°C. Furthermore, graphite electrodes are preferred because they can be used even when the temperature at the tip of the graphite electrode is about 3000°C. The use of graphite electrodes as the electrode 3 is preferred because a large amount of current can be passed through the submerged arc furnace 1.

[0023] The electrode diameter D and length of the electrode 3 vary depending on the raw metal melting capacity of the submerged arc furnace 1, the power supply, and the like. Here, the electrode diameter D of the electrode 3 is defined as follows depending on the type of electrode. When the electrode 3 is a sintered graphite electrode, the diameter of the sintered electrode is taken as the electrode diameter D. When the electrode 3 is a self-sintering electrode, the electrode diameter D is taken as the inner diameter of the casing into which the electrode paste, which is the raw material for the self-sintering electrode, is charged. For example, when the raw metal melting capacity of the submerged arc furnace 1 is 20 to 60 t / h, the electrode diameter D of the electrode 3 is 1000 to 1800 mm. The length of the electrode 3 may be any length that ensures the length necessary for the up and down movement of the electrode 3 depending on the height of the submerged arc furnace 1, the molten metal holding capacity, and the like.

[0024] The submerged arc furnace 1 is equipped with a transformer. The transformer converts the voltage applied between the electrodes formed by the multiple electrodes 3 into an appropriate voltage. The transformer has a tap changer terminal on the high-voltage side of the transformer. The output voltage of the transformer can be switched and adjusted using the tap changer terminal. In addition, the current flowing between the electrodes formed by the multiple electrodes 3 can be adjusted by changing the height of the electrodes 3 inserted into the molten slag S stacked inside the submerged arc furnace 1.

[0025] The method for melting metallic raw materials according to this embodiment is carried out by inserting multiple electrodes 3 into molten slag S stacked in a submerged arc furnace 1, generating an arc 5 between the multiple electrodes 3 or between the electrodes 3 and molten iron (molten pig iron) P, and using radiant heating from the arc 5 to heat and melt raw materials M consisting of raw metal raw materials 4 and auxiliary raw materials that have been introduced into the submerged arc furnace 1. The method for melting metallic raw materials according to this embodiment is also carried out by inserting multiple electrodes 3 into the molten slag S stacked in the submerged arc furnace 1, and using resistance heating from a current flowing between the electrodes 3, raw materials M, slag S, and molten iron P to heat and melt raw materials M consisting of raw metal raw materials 4 and auxiliary raw materials.

[0026] In the method for melting metal raw materials according to this embodiment, it is preferable to continuously produce molten metal by introducing the metal raw material 4 from the top of the submerged arc furnace 1 and melting the raw material M containing the metal raw material 4. That is, the method for melting metal raw materials according to this embodiment can continuously discharge molten iron P and slag (molten slag) S obtained by dissolving the raw material M containing the metal raw material 4 separately. For example, the method for melting metal raw materials according to this embodiment can melt the raw material M containing the metal raw material 4 by flat bath operation. Furthermore, in the method for melting metal raw materials according to this embodiment, the molten iron P obtained by dissolving the raw material M containing the metal raw material 4, the slag (molten slag) S, and the auxiliary raw materials may be charged into the electric furnace in this order, or the charging order of the metal raw material 4 and the auxiliary raw materials may be changed as appropriate. This allows the raw material M containing the metal raw material 4 to be melted without employing flat bath operation.

[0027] In this case, the submerged arc furnace 1 preferably has a discharge port for the molten metal and a discharge port for the slag (molten slag) S. The discharge ports for the molten metal and the slag (molten slag) S are preferably arranged in different directions relative to the center of the hearth. Each discharge port is preferably filled with plugging material, and when a predetermined amount of molten metal has been retained, the plugging material is pierced to open the outlet, thereby discharging the molten metal.

[0028] In the method for melting metal raw materials according to this embodiment, a raw material M containing a metal raw material 4 is charged into a submerged arc furnace 1. The reduced iron contained in the metal raw material 4 may be produced in advance by a direct reduction method, or commercially available reduced iron may be purchased and used. The metal raw material 4 contains reduced iron in part or entirely, and may also contain small amounts of iron scrap and scale in addition to the reduced iron. The mass ratio of reduced iron to the metal raw material 4 is preferably 5 to 100%.

[0029] The method for melting a metal raw material according to this embodiment has a technical feature in that the metallization rate of the metal raw material contained in the electric furnace is set to 60% or more. Here, the metallization rate (DM) of the metal raw material is defined by the following relational expression (1) using the mass of total iron (T.Fe) and the mass of metallic iron (M.Fe) contained in the metal raw material.

[0030]

[0031] In the method for melting a metal raw material according to this embodiment, if the metallization rate of the metal raw material 4 is 60% or higher, the unit power consumption of the electric power required to melt the metal raw material 4 is low, resulting in excellent energy efficiency. The upper limit of the metallization rate of the metal raw material 4 is not particularly limited. If the metallization rate of the metal raw material 4 is too high, the metal raw material 4 introduced from above the slag (molten slag) S in the submerged arc furnace 1 may be electrified in the slag (molten slag) S, which may reduce the resistance heat, which is undesirable.

[0032] Furthermore, in the method for melting metal raw materials according to this embodiment, if the metallization rate of the metal raw material 4 is 60% or more, the amount of molten FeO contained in the by-produced slag (molten slag) S is reduced, thereby improving the iron yield. Also, if the metallization rate of the metal raw material 4 is 60% or more, it is possible to prevent the refractory material constituting the furnace wall 2 of the submerged arc furnace 1 from melting.

[0033] From this perspective, in the method for melting a metal raw material according to this embodiment, the metallization rate of the metal raw material 4 is preferably 80% or more, and even more preferably 95%. In the method for melting a metal raw material according to this embodiment, if the metallization rate of the metal raw material 4 exceeds 95%, the metal raw material 4 contained in the raw material M introduced above the slag (molten slag) S in the submerged arc furnace 1 may be electrified in the slag (molten slag) S, which may reduce the resistance heat, which is undesirable. Therefore, the upper limit of the metallization rate of the metal raw material 4 is preferably set to about 90%.

[0034] If the metal raw material 4 charged into the submerged arc furnace 1 contains carbon, the unreduced FeO contained in the metal raw material 4 can be efficiently reduced within the submerged arc furnace 1. In order to achieve such an efficient reduction effect, the carbon content of the metal raw material 4 is preferably 50% or more of the theoretical carbon amount required to reduce the unreduced FeO.

[0035] Furthermore, the specific gravity of the metal raw material 4 is 1.7 g / cm ３ 7.0g / cm or more ３A carbon concentration of 1.5 to 4.5 mass % (concentration in the molten metal) is not particularly limited. For example, when the carbon concentration is determined according to the molten FeO concentration, a carbon concentration of 1.5 to 4.5 mass % (concentration in the molten metal) is preferred in order to effectively reduce the molten FeO content in the molten metal.

[0036] In this way, the method for melting metal raw materials according to this embodiment can reduce the thermal energy required to melt the metal raw materials 4 by setting the metallization rate of the metal raw materials 4 charged into the submerged arc furnace 1 to 60% or more, and can melt large amounts of metal raw materials 4 by stably supplying electricity to the submerged arc furnace 1.

[0037] The method for melting metal raw materials according to this embodiment uses a metal raw material 4 containing reduced iron as a part or the whole thereof, and a molten slag S formed on molten iron P as an auxiliary raw material, the basicity of which is (CaO / SiO ２ At least one or both of a slag former for adjusting the basicity (CaO / SiO ２ ) is the SiO in the slag on a mass basis ２ is the ratio of CaO to

[0038] In the method for melting a metal raw material according to this embodiment, the metal raw material 4 containing, in whole or in part, reduced iron and the auxiliary raw material may be mixed and supplied to the electrode 3. By mixing the metal raw material 4 containing, in whole or in part, reduced iron and the auxiliary raw material and supplying the mixture to the electrode 3, the metal raw material 4 can be melted efficiently, which is preferable.

[0039] Furthermore, in the method for melting metal raw materials according to this embodiment, the metal raw material 4 containing reduced iron in part or in whole and the auxiliary raw materials may be separately supplied to the electrode 3. By separately supplying the metal raw material 4 containing reduced iron in part or in whole and the auxiliary raw materials mixed together and supplied to the electrode 3, it is possible to charge into the submerged arc furnace 1 a predetermined amount of the metal raw material 4 containing reduced iron in part or in whole, or the auxiliary raw materials required according to the melting state of the metal raw material 4, thereby facilitating the melting of the metal raw material 4. When the metal raw material 4 and the auxiliary raw materials are separately mixed and supplied to the electrode 3, the order in which they are supplied to the electrode 3 is not particularly limited and can be appropriately determined taking into account the melting state of the metal raw material 4. The metal raw material 4 may be supplied before or after the auxiliary raw materials.

[0040] Furthermore, when the metal raw material 4 and the auxiliary material are supplied to the electrode 3, it is possible to appropriately select whether to supply the metal raw material 4 and the auxiliary material to the electrode 3 as a mixture, or to supply the metal raw material 4 and the auxiliary material separately to the electrode 3, taking into consideration the molten state of the metal raw material 4, the time for which the submerged arc furnace 1 is energized, etc.

[0041] The slag-forming material contained in the auxiliary raw materials is limestone (CaCO ３ ), quicklime (CaO), and SiO ２ The source is silica (SiO ２ The slag former may be in the form of a lump, powder, or granules, and is preferably used by mixing with the metal raw material 4 containing reduced iron in part or in whole. By using the slag former, the basicity (CaO / SiO ２ ) is preferably in the range of 1.0 to 1.5. When the basicity of the slag (molten slag) S is in this range, the composition of the slag (molten slag) S is similar to that of blast furnace slag, making it suitable for reuse as roadbed materials such as cement. In addition, the total iron content in the slag (molten slag) S is preferably 5.0% by mass or less. If the total iron content in the slag (molten slag) S is 0.1 to 5.0% by mass, this is preferred because it improves the iron yield.

[0042] Coke or coal can be used as the carbonaceous material. Biomass charcoal may also be used as the carbonaceous material. The carbonaceous material may be in a lump or powder form, and is preferably used together with or mixed with the raw material M containing the metal raw material 4, for example, by granulation. The C content in the molten iron P is preferably adjusted to a range of 2.0 to 5.0 mass%. If the C content in the molten iron P is within this range, the molten metal obtained by melting the metal raw material 4 can be used as pig iron directly or as a steelmaking raw material in the next process, which is preferable.

[0043] The method for melting metal raw materials according to this embodiment uses electric energy to melt the metal raw materials 4. Therefore, the method for melting metal raw materials according to this embodiment uses less CO 2 than the method using the heat of combustion of carbon, such as the blast furnace method. ２ The effect of reducing the amount of discharged metal can be obtained. Note that the electric energy used to melt the metal raw material 4 may be electric energy generated by renewable energy.

[0044] The method for melting metal raw materials according to this embodiment is characterized in that the submerged arc furnace 1 used as the electric furnace has an in-furnace resistivity ρ in the range of 0.01 to 0.02 (Ω·m). A submerged arc furnace 1 having an in-furnace resistivity ρ of 0.01 (Ω·m) or more is preferable because stable current flow can be ensured without causing a short circuit between the electrodes formed by the multiple electrodes 3. A submerged arc furnace 1 having an in-furnace resistivity ρ of 0.02 (Ω·m) or less is preferable because it can reduce the power consumption required to melt the metal raw material 4 that contains reduced iron in part or entirely.

[0045] Figure 2 is a schematic diagram showing the state of the electrodes and their surroundings when current is applied to an electric furnace used in carrying out the method for melting metal raw materials according to this embodiment. As shown in Figure 2, the arc current generated from the electrode 3 flows in a hemispherical shape with a current application radius r from the center point of the cross section. Here, when the current application radius r is significantly larger than the electrode diameter D of the electrode 3, the furnace resistivity ρ (Ω m) of the submerged arc furnace 1 used as the electric furnace can be calculated using the following relational expression (2):

[0046] In the above relational expression (2), ρ is the resistivity (Ω·m) inside the electric furnace, and U ρ is the electrode-to-ground voltage (V), D is the electrode diameter (m), and I is the effective value of the current per electrode (A / electrode).

[0047] Furthermore, in relational expression (2), the effective value I (A / electrode) of the current per electrode can be directly measured. Then, using the following relational expression (3), the current density Je (A / m ２ ) can be calculated using the measured effective value I (A / electrode) of the current per electrode.

[0048] In the above relational expression (3), J ｅ is the current density (A / m ２ ) and D is the electrode diameter (m).

[0049] When the submerged arc furnace 1 has three electrodes 3, the submerged arc furnace 1 is a three-phase AC submerged arc furnace. ρ is the electrode-to-ground voltage (V), and when the submerged arc furnace 1 is a three-phase AC submerged arc furnace, the effective value of the line voltage between the electrodes formed by the three electrodes 3 is V L Then, the voltage between the electrode and ground U ρ (V), and the effective value V of the line voltage between the electrodes formed by the electrode 3 L (V) can be expressed by the following relational expression (4).

[0050]

[0051] Furthermore, when the submerged arc furnace 1 has a plurality of pairs of electrodes 3, the submerged arc furnace 1 becomes a single-phase AC submerged arc furnace. ρ is the electrode-to-ground voltage (V), and when the submerged arc furnace 1 is a single-phase AC submerged arc furnace, the effective value of the line voltage between the electrodes formed by multiple pairs of electrodes 3 is V L Then, the voltage between the electrode and ground U ρ (V), and the effective value V of the line voltage between the electrodes formed by the electrode 3 L(V) can be expressed by the following relational expression (5).

[0052]

[0053] In the relational expressions (4) and (5), the effective value of the line voltage between the electrodes is V L is smaller than the line voltage on the secondary side of the transformer by the amount of voltage reduction due to the voltage drop that occurs in the electrical circuit formed in the submerged arc furnace 1. However, the amount of voltage reduction due to the voltage drop is not relatively large in the electrical circuit formed in the submerged arc furnace 1 and can therefore be ignored.

[0054] As described above, the method for melting metal raw materials according to this embodiment is based on the power transmission method adopted by the submerged arc furnace 1 and the effective value V of the line voltage between the electrodes formed by the plurality of electrodes 3 provided in the submerged arc furnace 1. L Considering the relationship with (V), the electrode-to-ground voltage U ρ (V) is calculated. Then, the calculated electrode-to-ground voltage U ρ Based on the value of (V), the furnace resistivity ρ (Ω·m) of the submerged arc furnace 1 can be set.

[0055] On the other hand, the electric furnace used in the method for melting metal raw materials according to this embodiment may be an arc furnace. When an arc furnace is used to carry out the method for melting metal raw materials according to this embodiment, it is preferable to adjust at least one selected from the distance from the interface of molten iron present inside the arc furnace to the arc electrodes provided in the arc furnace, the arc voltage E (V) which is the voltage to ground, and the arc current I (A) per arc electrode. Specifically, these values ​​can be set so that the arc voltage E (V) which is the voltage to ground and the arc current I (A) per arc electrode provided in the arc furnace satisfy the following relational expression (i):

[0056] In the method for melting metal raw materials according to this embodiment, arc stability can be maintained and the metal raw materials can be melted satisfactorily by ensuring that the arc voltage E (V), which is the voltage to ground, and the arc current I (A) per arc electrode of the arc furnace satisfy the above-mentioned relational expression (i). In relational expression (i), if the value obtained by dividing the arc voltage E (V), which is the voltage to ground, by the square root of the arc current I (A) per arc electrode exceeds 2.0, arc stability cannot be maintained when melting the metal raw materials, which is undesirable. Note that, in relational expression (i), the lower limit of the value obtained by dividing the arc voltage E (V), which is the voltage to ground, by the square root of the arc current I (A) per arc electrode can be appropriately set depending on the output of the arc furnace and the stirring conditions of the molten iron. The lower limit is preferably 0.5 or greater.

[0057] As described above, according to the invention of the first embodiment, conditions for melting a metal raw material containing reduced iron as a part or the whole thereof using a submerged arc furnace (SAF) or an electric arc furnace are set, and a stable current is applied to the electric furnace, thereby making it possible to melt a large amount of the metal raw material.

[0058] Second Embodiment A method for melting metal raw materials according to a second embodiment will be described. The method for melting metal raw materials according to this embodiment is the same as the method for melting metal raw materials according to the above embodiment except that the current density flowing through the electrodes of the electric furnace is 50 to 70 (A / m ２ The characteristic features of the method for melting a metal raw material according to this embodiment will be described below.

[0059] In the method for melting metal raw materials according to this embodiment, the current density of the current flowing through the electrodes of the electric furnace is 50 to 70 (A / m ２ The current density of the electric current flowing through the electrodes of the electric furnace is in the range of 50 (A / m ２ ) or more, it is possible to set the in-furnace resistivity of the electric furnace to ensure current flow stability, and to produce a self-baking electrode by baking the carbon paste. ２) or less, it is possible to produce a self-sintering electrode by sintering the carbon paste while maintaining a high value of the resistivity inside the electric furnace, which is preferable.

[0060] The electrodes provided in the electric furnace used in the method for melting metal raw materials according to this embodiment may be either sintered electrodes or self-sintered electrodes. That is, the electrodes provided in the electric furnace may be either sintered or self-sintered. When the electrodes are self-sintered, they may also be called Zetaberg electrodes.

[0061] For example, a self-baking electrode may consist of a vertically disposed electrode casing that extends through an opening in the roof or canopy of an electric furnace. The upper end of the electrode casing is open to allow the addition of unbaked carbonaceous electrode paste (carbon paste) used as the electrode material. When heated, the unbaked carbonaceous electrode paste softens and melts, and is then sintered into a solid carbon electrode by the heat released into the carbonaceous electrode paste in the region where an electrical operating current is supplied to the electrode. The self-baking electrode (Zetaberg electrode) manufactured in this manner can melt the metal feedstock containing reduced iron by generating the arc discharge necessary to melt the metal feedstock containing reduced iron in part or in whole. The tip of the electrode casing may be configured so that the self-baking electrode formed by sintering is exposed.

[0062] In the method for melting metal raw materials according to this embodiment, the current density of the current flowing through the electrodes of the electric furnace is set to 50 to 70 (A / m ２ ) in the above range, when the electrodes provided in the electric furnace are self-sintering type electrodes, the unsintered carbonaceous electrode paste (carbon paste) is likely to be sintered, and a self-sintering type electrode (Zetaberg electrode) having excellent performance can be produced.

[0063] In this way, the method for melting metal raw materials according to this embodiment can reduce the thermal energy required to melt the metal raw materials and smoothly manufacture self-baking electrodes by setting the current density of the current flowing through the electrodes of the electric furnace, thereby enabling stable operation of the electric furnace equipped with self-baking electrodes.

[0064] As described above, according to the method for melting metal raw materials according to the second embodiment, the current density of the current flowing through the electrodes of the electric furnace is set to 50 to 70 (A / m ２ ) range, the unsintered carbonaceous electrode paste, which is the raw material for the electrodes used in the electric furnace, can be favorably sintered, and self-sintering electrodes can be easily manufactured. The method for melting metal raw materials according to the second embodiment can reduce the thermal energy required for melting reduced iron, and by stably energizing the electric furnace, a large amount of metal raw materials can be melted and processed.

[0065] [Third embodiment] A method for melting metal raw materials according to a third embodiment will be described. The method for melting metal raw materials according to this embodiment is characterized in that, in the method for melting metal raw materials according to the above embodiments, the electric furnace is either a three-phase AC type having three electrodes, or a single-phase type having multiple pairs of electrodes. Below, the characteristic features included in the method for melting metal raw materials according to this embodiment will be described.

[0066] 3 is a schematic diagram showing an outline of a single-phase AC submerged arc furnace used to carry out the method for melting metal raw materials according to this embodiment. As shown in FIG. 3, the single-phase AC submerged arc furnace is composed of three pairs of electrodes 3, and the total number of electrodes 3 is six. Here, in the method for melting metal raw materials according to this embodiment, the electrode-to-ground voltage U required to set the in-furnace resistivity ρ (Ω·m) is ρ (V) is the effective value V of the line voltage between the electrodes formed by the three pairs of electrodes 3. L That is, the electrode-to-ground voltage U required for a single-phase AC submerged arc furnace to melt the metal raw materials containing reduced iron can be expressed by the above-mentioned relational expression (5). ρ (V) is the effective value V of the line voltage between the electrodes formed by the three pairs of electrodes 3. L It becomes 1 / 2 of.

[0067] That is, in the method for melting metal raw materials according to this embodiment, the effective value V of the line voltage between the electrodes is set according to the configuration and output power of the submerged arc furnace used in the method for melting metal raw materials. L Based on the electrode-to-ground voltage U ρ By calculating (V), the resistivity inside the submerged arc furnace can be set, and stable current flow by arc discharge can be realized.

[0068] Here, the method for melting metal raw materials according to this embodiment uses less CO compared to the blast furnace method, which uses the heat of combustion of carbon. ２ Therefore, the method for melting metal raw materials according to this embodiment is expected to be a means for melting and treating large amounts of metal raw materials. Therefore, the submerged arc furnace used in the method for melting metal raw materials according to this embodiment is preferably a large-sized electric furnace equipped with a large number of electrodes so that a large amount of metal raw materials can be melted and treated.

[0069] From this technical viewpoint, the electric furnace used in the method for melting metal raw materials according to this embodiment may be a single-phase AC submerged arc furnace, and the single-phase AC submerged arc furnace may have a rectangular parallelepiped shape. By using a single-phase AC submerged arc furnace with a rectangular parallelepiped shape, it is possible to install a large number of pairs of electrodes inside the arc furnace. Furthermore, a rectangular parallelepiped single-phase AC submerged arc furnace with multiple pairs of electrodes may be prepared as a single single-phase AC submerged arc furnace unit, and a large-scale metal raw material melting system may be constructed by combining multiple single-phase AC submerged arc furnace units. A large-scale metal raw material melting system may enable the melting and processing of extremely large amounts of metal raw materials.

[0070] On the other hand, when the submerged arc furnace is a three-phase AC submerged arc furnace, the electrode-to-ground voltage U required for the submerged arc furnace to melt the metal raw material is ρ (V) is the effective value V of the line voltage between the electrodes formed by the three electrodes 3 L This becomes 1 / √3 of the above.

[0071] As described above, according to the invention of the third embodiment, it is possible to construct a rectangular parallelepiped electric furnace equipped with a large number of electrodes in response to the future increase in the size of electric furnaces, and by stably supplying electricity to the electric furnace, it is possible to melt and process large amounts of metal raw materials.

[0072] [Other Embodiments] The present invention has been described above with reference to the embodiments, but the present invention is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the technical scope of the present invention. Furthermore, systems or devices that combine separate features included in each embodiment in any way are also included in the technical scope of the present invention.

[0073] The effects of the present invention will be specifically explained below based on examples, but the present invention is not limited to these examples.

[0074] (Invention Example 1) Using the method for melting metal raw materials according to the present invention, a submerged arc electric furnace was used to melt a metal raw material containing, in whole or in part, reduced iron. In this example, either a three-phase AC submerged arc electric furnace A or a single-phase AC submerged arc electric furnace B shown in Table 1 was used. In Invention Example 1, a single-phase AC submerged arc electric furnace B was used. Specifically, a 4- to 6-ton scale submerged arc electric furnace A or submerged arc electric furnace B, as shown in FIG. 1, was used to melt a metal raw material containing, in whole or in part, reduced iron. Table 1 summarizes the specifications of the three-phase AC or single-phase AC electric furnace used as the arc furnace. In this example, the metal raw material containing, in whole or in part, reduced iron was a 4- to 6-ton scale submerged arc electric furnace A or submerged arc electric furnace B. Fe (metallic iron) 83-88% by mass, metallization ratio (M.Fe / T.Fe) 94%, C (carbon) concentration 1.0-1.5% by mass, apparent bulk density 2.5-3.5t / m ３ The thickness was 5 to 20 mm.

[0075]

[0076] A single-phase submerged arc electric furnace B was prepared, and flat-bath operating conditions were set to melt metallic raw materials containing reduced iron in whole or in part. Specifically, in Example 1, shredder waste, carbonaceous material, and blast furnace slag were charged into the single-phase submerged arc electric furnace B, and electricity was applied to perform initial melting. The pre-charged amounts were adjusted to 750 kg of hot metal and 1,500 kg of slag. After the pre-charged materials were fully melted, a pre-mixed raw material consisting of metallic raw materials containing reduced iron in whole or in part as the main raw material and auxiliary raw materials such as carbonaceous material, lime, alumina, and MgO source for adjusting the C concentration in the hot metal and the slag composition was continuously charged. The maximum amount of hot metal was targeted to be 4 t, and was adjusted by the amount of raw material charged. The final thickness of the hot metal was 140 mm.

[0077] When melting metal raw materials containing reduced iron in part or in whole, the line voltage was adjusted with a tap to adjust the power to meet the specified conditions. The electrode height was adjusted to adjust the current value. Furthermore, the current density and voltage to ground were adjusted so that the furnace resistivity was within a specified range. In Example 1, the conditions were set so that the furnace resistivity was 0.0115 (Ω·m). Table 2 shows the flat bath operation conditions and the evaluation of current stability during melting of molten iron performed in Example 1. The molten steel temperature was set in the range of 1400 to 1700°C. Furthermore, E / √I, which is the arc voltage E (V), or the voltage to ground, divided by the square root of the current I (A) per arc electrode, was calculated. At least one of the electrode height, arc voltage, and arc current was adjusted so that E / √I was 2.0 or less. In Table 2, the current stability was evaluated according to the following criteria: ∘: Flat bath operation was possible with stable current flow. ×: Flat bus operation was not possible due to unstable current flow caused by arc interruption or other problems.

[0078]

[0079] (Invention Examples 2 to 7, Comparative Examples 1 to 5) A single-phase AC submerged arc electric furnace B was used as the electric furnace used to melt molten iron, and metal raw materials containing all or part of reduced iron were melted in the same manner as in Invention Example 1, except that the flat bath operating conditions were changed. In Invention Examples 2 to 7, the furnace resistivity was set to 0.012 to 0.017 (Ω m). On the other hand, in Comparative Examples 1 to 5, it was set to less than 0.010 (Ω m). The flat bath operating conditions and evaluation of current stability during the melting of molten iron performed in Invention Examples 2 to 7 and Comparative Examples 1 to 5 are shown in Table 2.

[0080] From the above, it has been confirmed that the method for melting metal raw materials according to the present invention is a technique that can melt large amounts of metal raw materials by stably passing electricity through a submerged arc furnace (SAF).

[0081] According to the method for melting metal raw materials of the present invention, a submerged arc furnace (SAF) is used as the electric furnace, and by stably energizing the electric furnace, a large amount of metal raw materials can be melted and processed, which contributes to the development of related industries such as the steelmaking industry and is extremely useful industrially.

[0082] 1 Electric furnace (submerged arc furnace, SAF) 2 Furnace wall 3 Electrode 4 Charged reduced iron (metal raw material) 5 Arc P Molten iron (molten pig iron) S Molten slag (molten slag) M Raw material

Claims

1. A method for melting metal raw materials in an electric furnace, which contains molten iron, molten slag, metal raw materials, and auxiliary materials in a layered manner inside the electric furnace and discharges the molten iron and molten slag, wherein when an electric current is supplied to the molten metal by an arc or via the molten slag and metal raw materials to heat it, the resistivity inside the electric furnace is set to a range of 0.01 to 0.02 (Ω m).

2. The method for melting metal raw materials according to claim 1, wherein the metal raw materials are melted by flat bath operation.

3. The method for melting a metal raw material according to claim 1, wherein the metallization rate of the metal raw material is 60% or more.

4. The current density flowing through the electrodes of the electric furnace is 50 to 70 (A / m ２ 2. The method for melting a metal raw material according to claim 1, wherein the temperature is within the range of 100° C.

5. The method for melting metal raw materials according to claim 4, wherein said electrodes are of the self-baking type.

6. The method for melting metal raw materials according to claim 1, wherein the electric furnace is either a three-phase AC type having three electrodes, or a single-phase type having multiple pairs of electrodes.

7. The method for melting a metal raw material according to claim 1, wherein the metal raw material and the auxiliary raw material are supplied between the electrodes either in a mixed state or individually.

8. The method for melting metal raw materials according to claim 7, wherein a part or all of the metal raw materials is reduced iron.

9. The basicity of the slag is in the range of 1.0 to 1.5, where the basicity of the slag is the ratio of SiO ２ The method for melting metal raw materials according to claim 1, wherein the ratio of CaO to CaO is 0.01 to 0.

01.

10. The method for melting metallic raw materials according to claim 1, wherein the electric furnace is an arc furnace, and at least one selected from a distance from an interface of the molten iron to the arc electrode, the arc voltage, and the arc current is adjusted so that an arc voltage E (V) which is a voltage to ground and an arc current I (A) per arc electrode provided in the arc furnace satisfy the following relational formula (i):

11. The method for melting metal raw materials according to claim 1, wherein the electric furnace is a submerged arc furnace.

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