Method for melting metal raw materials

Abstract

To provide a method for melting metal raw materials, which can melt a large amount of metal raw materials by setting conditions for melting metal raw materials that partially or entirely contain reduced iron using a submerged arc furnace (SAF) and conducting stable current flow. A method for melting metal raw materials using an electric furnace, which contains molten iron, molten slag, metal raw materials, and auxiliary materials in a stacked manner inside the electric furnace and discharges the molten iron and molten slag, and when an electric current is supplied to the molten metal by an arc or via the molten slag and metal raw materials to heat them, the resistivity inside the electric furnace is set to a range of 0.01 to 0.02 (Ω·m).

Description

[Technical field]

[0001] The present invention relates to a method for melting a metal raw material, and more particularly to a method for melting a metal raw material capable of producing molten metal by melting a metal raw material containing reduced iron in an electric furnace or a submerged arc furnace. [Background technology]

[0002] In recent years, CO 2 The steel industry is also required to reduce CO emissions. 2 As an alternative to the blast furnace process, which produces a large amount of iron, the direct reduction (DR) process is 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) or submerged arc furnace (SAF) and heated to melt, 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. It is difficult to melt and process the 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 this technical background, various molten metal production processes using a submerged arc furnace (SAF) have been proposed. Such molten metal production processes include a method for producing molten iron by the so-called flat bath operation (FBO). For example, the production of molten iron by a submerged arc furnace (SAF) is carried out in a flat bath operation (FBO). The flat bath operation (FBO) employs a melting process in which raw materials and auxiliary materials including metal 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, in the submerged arc furnace (SAF), electrodes are inserted into the molten slag, an arc is generated between multiple electrodes or between the electrode and the molten iron, and 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 (SAF) is the inefficiency of heat transfer to the material located away from the electrode, where incomplete metal melting due to delayed melting of the metal material leads to problems such as concentration gradients, unreliable temperature measurements, unreliable process control, and excessively hot molten metal tapping. To solve this uneven temperature problem, electromagnetic stirring of the molten metal as disclosed in US Pat. No. 5,399,433 and gas stirring of the molten slag as disclosed in US Pat. No. 5,499,433 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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2020-505579 [Patent Document 2] JP 2002-317918 A [Patent Document 3] JP 2003-105415 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, the conventional techniques have the following problems. That is, the technique described in Patent Document 1 is merely a method for producing molten metal applied to a so-called electric arc furnace. Also, the technique described in Patent Document 2 proposes an electric furnace for a waste melting furnace, and does not intend mass production of molten metal. From such a technical point of view, the problem was to apply a submerged arc furnace (SAF) to the production of molten metal and to develop an operation method of the electric furnace taking into account the productivity of molten metal. In addition, the technology described in Patent Document 3 achieves a refractory dissolution index RF of 400 MWV / m 2 In order to melt molten iron while maintaining the resistance value inside the electric furnace at or below this value, the resistance value inside the electric furnace is set to an extremely low value. Therefore, even if the method for producing molten metal 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 supply electricity to the electric furnace, and thus it is not possible to sufficiently melt reduced iron.

[0009] Furthermore, it is difficult to use a submerged arc furnace (SAF) to melt reduced iron made from low-grade iron ore for the following reasons. When using a submerged arc furnace (SAF) to melt reduced iron, the conditions inside the SAF vary greatly 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, and the thermal efficiency of the molten iron decreases significantly.

[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 for generating 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 the heating of the molten iron to become unstable. Furthermore, there is a problem in that the heating of the molten iron becomes unstable if raw materials enter the electric circuit formed by the electrodes and the molten iron in the submerged arc furnace (SAF). Furthermore, there have been no reports on the appropriate voltage range to be applied when melting reduced iron when using a submerged arc furnace (SAF) to perform melting treatment of reduced iron.

[0011] The present invention has been made in consideration of the above circumstances, and has an object 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 in whole or in part using a submerged arc furnace (SAF) and stably passing current through the melting process. [Means for solving the problem]

[0012] Therefore, the present inventors conducted various experiments to solve the above problems, and discovered that by setting the internal resistivity of a submerged arc furnace (SAF) when melting metal raw materials containing all or part of reduced iron and stably passing current through the electric furnace, it is possible to melt a large amount of metal raw materials containing all or part of reduced iron. The present invention was made based on the above findings, and the gist of the present invention is as follows.

[0013] That is, the method for melting metal raw materials according to the present invention, which advantageously solves the above-mentioned problems, is a method for melting metal raw materials in an electric furnace, in which molten iron, molten slag, metal raw materials, and auxiliary raw 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 the metal raw materials to heat them, the internal resistivity of 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 includes the steps of: (a) melting said metal raw material by flat bath operation; (b) the metallization rate of the metal raw material is 60% or more; (c) The current density flowing through the electrodes of the electric furnace is 50 to 70 (A / m 2 ) range, (d) the electrode is of the self-baking type; (e) The electric furnace is either a three-phase type having three electrodes, or a single-phase type having multiple pairs of electrodes; (f) supplying the metal raw material and auxiliary raw material, either mixed or individually, between the electrodes; (g) a part or all of the metal raw material is 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 2 The ratio of CaO to (i) 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);

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

[0016] [Figure 1] 1 is a schematic diagram showing an outline of an electric furnace used to carry out a method for melting metal raw materials according to an embodiment of the present invention. [Diagram 2] 1 is a schematic diagram showing the state of the electrodes and the periphery of the electrodes when electricity is applied to an electric furnace used for carrying out a method for melting metal raw material according to an embodiment of the present invention. FIG. [Diagram 3] 1 is a schematic diagram showing an overview of a single-phase AC submerged arc furnace used to carry out a method for melting metal raw materials according to an embodiment of the present invention. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] [First embodiment] A method for melting metal raw materials according to a 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 raw materials are stored in a stacked manner inside the electric furnace, and the molten iron and molten slag are discharged, 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] Fig. 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 the present 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 blow gas bubbles from a bottom-blowing tuyeres (not shown). In this way, 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, and is therefore capable of stirring the molten iron (molten pig iron) 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 in whole. The submerged arc furnace 1 may be an electric furnace having a substantially circular shape when viewed from above, or an electric furnace having a rectangular parallelepiped shape.

[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, and may be mainly composed of one or more types selected from carbon, magnesia, alumina, etc.

[0021] A plurality of electrodes 3 are installed in the submerged arc furnace 1. The electrodes 3 may be configured as an electrode group consisting of three electrodes 3, or may be configured 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 becomes 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 becomes a single-phase AC submerged arc furnace.

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

[0023] The electrode diameter D and length of the electrode 3 vary depending on the metal raw material melting processing capacity of the submerged arc furnace 1, the power supply, etc. Here, the electrode diameter D of the electrode 3 is defined as follows depending on the type of the 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 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 metal raw material melting processing 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 can ensure the length required 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, etc.

[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 changing terminal on the high voltage side of the transformer. The output voltage of the transformer can be adjusted by switching using the tap changing 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 metal raw materials according to this embodiment is carried out by inserting a plurality of electrodes 3 into the slag (molten slag) S stacked in the submerged arc furnace 1, generating an arc 5 between the plurality of electrodes 3 or between the electrodes 3 and molten iron (molten pig iron) P, and heating and melting raw materials M consisting of raw metal raw materials 4 and auxiliary raw materials that have been put into the submerged arc furnace 1 by radiant heating from the arc 5. The method for melting metal raw materials according to this embodiment is carried out by inserting a plurality of electrodes 3 into the slag (molten slag) S stacked in the submerged arc furnace 1, and heating and melting raw materials M consisting of raw metal raw materials 4 and auxiliary raw materials by resistance heating of a current flowing between the electrodes 3, raw materials M, slag (molten slag) S, and molten iron P.

[0026] In the method for melting metal raw materials according to the present embodiment, it is preferable to continuously produce molten metal by feeding 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 the present embodiment can continuously discharge the molten iron P and slag (molten slag) S obtained by dissolving the raw material M containing the metal raw material 4 separately, and can melt the raw material M containing the metal raw material 4 by, for example, a flat bath operation. Furthermore, in the method for melting metal raw materials according to the present 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, and the raw material M containing the metal raw material 4 can be melted without adopting a 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 with respect to the center of the hearth. It is preferable to fill each discharge port with plugging material, and when a predetermined amount of molten metal can be held, the plugging material is penetrated and opened to discharge the molten metal.

[0028] In the method for melting metal raw materials according to this embodiment, raw material M containing 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 in whole, and may contain small amounts of iron scrap and scale in addition to the reduced iron. The mass ratio of reduced iron in the metal raw material 4 is preferably 5 to 100%.

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

[0030]

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[0031] In the method for melting a metal raw material according to the present embodiment, when the metallization rate of the metal raw material 4 is 60% or more, the unit power consumption of the power required to melt the metal raw material 4 is low, and the energy efficiency is excellent. 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 charged 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 not preferable.

[0032] Furthermore, in the method for melting metal raw materials according to the present 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 decreases, improving the iron yield. Also, if the metallization rate of the metal raw material 4 is 60% or more, the melting damage of the refractory constituting the furnace wall 2 of the submerged arc furnace 1 can be suppressed.

[0033] From this viewpoint, in the method for melting a metal raw material according to this embodiment, the metallization rate of the metal raw material 4 is more 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%, it is not preferable because the metal raw material 4 contained in the raw material M charged from above the slag (molten slag) S in the submerged arc furnace 1 may be electrified in the slag (molten slag) S, causing a decrease in resistance heat. Therefore, it is preferable that the upper limit of the metallization rate of the metal raw material 4 is about 90%.

[0034] When 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 in the submerged arc furnace 1. In order to obtain 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 metal raw material 4 is 1.7 g / cm 3 More than 7.0g / cm 3 If the carbon concentration in the molten metal is less than 1.5%, the metal raw material 4 charged into the submerged arc furnace 1 is not captured on the slag (molten slag) S, and is efficiently melted in the slag (molten slag) S. A molten metal recarburizer such as a carbon material may be charged together with the metal raw material 4. This is preferable because the molten FeO concentration in the slag (molten slag) S can be reduced by increasing the carbon concentration in the molten metal. The carbon concentration in the molten metal is not particularly limited, and for example, when the carbon concentration is determined according to the molten FeO concentration, the carbon concentration is preferably 1.5 to 4.5 mass% (concentration in the molten metal) in order to exert the effect of reducing molten FeO.

[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 material 4 by setting the metallization rate of the metal raw material 4 charged into the submerged arc furnace 1 to 60% or more, and can melt and process a large amount of metal raw material 4 by stably passing electricity through the submerged arc furnace 1.

[0037] The method for melting a metal raw material according to the present embodiment uses a metal raw material 4 that contains reduced iron in part or in whole, and a molten slag S formed on the molten iron P as an auxiliary raw material having a basicity (CaO / SiO 2 In order to adjust the basicity (CaO / SiO 2 ) is the SiO in the slag by mass 2 is the ratio of CaO to

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

[0039] In the method for melting a metal raw material according to the present embodiment, the metal raw material 4 containing partially or entirely reduced iron and the auxiliary raw materials may be separately supplied to the electrode 3. By separately mixing and supplying the metal raw material 4 containing partially or entirely reduced iron and the auxiliary raw materials to the electrode 3, it is possible to charge into the submerged arc furnace 1 only a predetermined amount of the metal raw material 4 containing partially or entirely reduced iron required in accordance with the melting state of the metal raw material 4, or the auxiliary raw materials, and thus promote the melting of the metal raw material 4, which is preferable. In addition, when the metal raw material 4 and the auxiliary raw material are mixed and supplied to the electrode 3 separately, the order of supplying them to the electrode 3 is not particularly limited and can be appropriately set taking into consideration the molten state of the metal raw material 4. The metal raw material 4 may be supplied before the auxiliary raw material, or the metal raw material 4 may be supplied after the auxiliary raw material.

[0040] Furthermore, when the metal raw material 4 and the auxiliary raw material are supplied to the electrode 3, whether to supply the metal raw material 4 and the auxiliary raw material to the electrode 3 in a mixed state or to supply the metal raw material 4 and the auxiliary raw material to the electrode 3 separately can be appropriately selected, taking into consideration the molten state of the metal raw material 4, the time for which current is passed through the submerged arc furnace 1, and the like.

[0041] The slag-forming material contained in the auxiliary raw materials is limestone (CaCO 3 ) and quicklime (CaO), and SiO 2 The source is silica (SiO 2 The slag former may be in the form of a lump, powder, or granule, 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 approximately the same as that of blast furnace slag, which is 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 mass% or less. If the total iron content in the slag (molten slag) S is 0.1 to 5.0 mass%, this is preferable because it improves the iron yield.

[0042] The carbonaceous material may be coke or coal. Biomass charcoal may be used as the carbonaceous material. The carbonaceous material may be in a lump or powder form, and is preferably used, for example, after granulation, together with or mixed with the raw material M containing the metal raw material 4. 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 it is as pig iron, or as a steelmaking raw material in the next process, which is preferable.

[0043] The method for melting metal raw materials according to the present embodiment uses electric energy to melt the metal raw materials 4. Therefore, the method for melting metal raw materials according to the present embodiment uses less CO 2 The effect of reducing emissions 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 in-furnace resistivity ρ of the submerged arc furnace 1 used as the electric furnace is set to a range of 0.01 to 0.02 (Ω·m). If the in-furnace resistivity ρ of the submerged arc furnace 1 is 0.01 (Ω·m) or more, stable current flow can be ensured without causing a problem of short circuit between the electrodes formed by the multiple electrodes 3, which is preferable. If the in-furnace resistivity ρ of the submerged arc furnace 1 is 0.02 (Ω·m) or less, power consumption required for melting the metal raw material 4 containing reduced iron in part or in whole can be saved, which is preferable.

[0045] Fig. 2 is a schematic diagram showing the state of the electrodes and the periphery of the electrodes when current is applied to an electric furnace used in carrying out the method for melting metal raw materials according to the present embodiment. As shown in Fig. 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 passing area diameter r is much larger than the electrode diameter D of the electrode 3, the internal resistivity ρ (Ω·m) of the submerged arc furnace 1 used as the electric furnace can be calculated by the following relational expression (2).

[0046]

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

[0048]

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

[0050]

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

[0052]

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[0053] In 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 based on the voltage drop, which occurs due to a voltage drop in the electric circuit formed in the submerged arc furnace 1. However, since the amount of voltage reduction based on the voltage drop is not relatively large in the electric circuit formed in the submerged arc furnace 1, it can be ignored.

[0054] As described above, the method for melting metal raw materials according to the present embodiment is based on the power transmission method adopted by the submerged arc furnace 1 and the effective value V L Considering the relationship with (V), the electrode-to-ground voltage U ρ (V) is calculated. The calculated electrode-to-ground voltage U ρ Based on the value of (V), the internal 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 the present embodiment may be an arc furnace. When an arc furnace is used in carrying out the method for melting metal raw materials according to the present embodiment, it is preferable to adjust at least one selected from the distance from the interface of the molten iron present inside the arc furnace to the arc electrodes installed 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, the arc voltage E (V), which is the voltage to ground, and the arc current I (A) per arc electrode of the arc furnace can be set so that they satisfy the following relational expression (i):

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[0056] In the method for melting metal raw materials according to the present embodiment, 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 formula (i), thereby maintaining the arc stability and melting the metal raw materials well. In the relational formula (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, it is not preferable because it is not possible to maintain the arc stability when melting the metal raw materials. In the 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 more.

[0057] As described above, according to the invention relating to the first embodiment, by setting conditions for melting metal raw materials containing all or part of reduced iron using a submerged arc furnace (SAF) or an electric arc furnace and stably passing electricity through the electric furnace, it is possible to melt a large amount of metal raw materials.

[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 the present 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 internal resistivity of the electric furnace to ensure current flow stability, and to manufacture a self-sintered electrode by sintering the carbon paste. 2 ) or less, it is possible to produce a self-sintered 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 type electrodes.

[0061] For example, a self-baking electrode consists 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 green carbonaceous electrode paste (carbon paste) that is used as the electrode material. When heated, the green carbonaceous electrode paste softens and melts, and is then baked into a solid carbon electrode by the heat released into the carbonaceous electrode paste in the area that provides the electrical operating current to the electrode. The self-baking electrode (Zetaberg electrode) manufactured in this manner generates the arc discharge required to melt the metal raw material containing reduced iron, thereby making it possible to melt the metal raw material containing partially or entirely reduced iron. The tip portion of the electrode casing may be configured so that the self-sintered electrode formed by sintering is exposed.

[0062] In the method for melting metal raw materials according to the present 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 equipped 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 type electrode) having excellent performance can be produced.

[0063] In this way, the method for melting metal raw materials according to the present embodiment can reduce the thermal energy required for melting 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 the 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 satisfactorily sintered, and a self-sintered electrode 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 a large amount of metal raw materials can be melted and processed by stably passing current through the electric furnace.

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

[0066] Fig. 3 is a schematic diagram showing an overview 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 material 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 the metal raw material according to the present 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 the metal raw material. L Based on the electrode-to-ground voltage U ρ By calculating (V), the internal resistivity of the submerged arc furnace can be set, and stable current flow through arc discharge can be achieved.

[0068] Here, the method for melting metal raw materials according to the present embodiment uses less CO compared to the blast furnace method, which uses heat from the combustion of carbon. 2 Therefore, the method for melting metal raw materials according to this embodiment is expected to be useful as a means for melting and treating a large amount 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 as to be capable of melting and processing a large amount of metal raw materials.

[0069] From such a technical viewpoint, the electric furnace used in the method for melting metal raw materials according to the present embodiment may be a single-phase AC submerged arc furnace, and the shape of the single-phase AC submerged arc furnace may be a rectangular parallelepiped. By making the shape of the single-phase AC submerged arc furnace a rectangular parallelepiped, a large number of pairs of electrodes can be installed inside the arc furnace. Furthermore, a rectangular parallelepiped single-phase AC submerged arc furnace equipped with multiple pairs of electrodes can be prepared as one single-phase AC submerged arc furnace unit, and a large-scale raw metal melting system can be constructed by combining multiple single-phase AC submerged arc furnace units. A large-scale raw metal melting system can melt and process a very large amount of raw metal.

[0070] On the other hand, if 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 is 1 / √3 of the original square root.

[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 future increases in the size of electric furnaces, and by stably passing electricity through the electric furnace, it is possible to melt and process large amounts of raw metal materials.

[0072] [Other embodiments] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above-mentioned 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. In addition, systems or devices that combine the separate features included in each embodiment in any way are also included in the technical scope of the present invention. EXAMPLES

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

[0074] (Example 1) The method for melting metal raw materials according to the present invention was adopted, and a submerged arc type electric furnace was used to melt metal raw materials that were entirely or partially composed of reduced iron. In this example, either a three-phase AC submerged arc type electric furnace A or a single-phase AC submerged arc type electric furnace B shown in Table 1 was used. In invention example 1, a single-phase AC submerged arc type electric furnace B was used. Specifically, a 4-6 ton scale submerged arc type electric furnace A or a submerged arc type electric furnace B as shown in FIG. 1 was used to melt metal raw materials that were entirely or partially composed of reduced iron. Table 1 shows the specifications of the three-phase AC or single-phase AC electric furnaces used as the arc furnaces in the investigation. In this embodiment, the metal raw material containing reduced iron in part or in whole is a raw material having a T.Fe (total iron content) of 88 to 94 mass%, an M.Fe (metallic iron content) of 83 to 88 mass%, a metallization rate (M.Fe / T.Fe) of 94%, a C (carbon) concentration of 1.0 to 1.5 mass%, and an apparent bulk density of 2.5 to 3.5 t / m 3 The thicknesses used were 5 to 20 mm.

[0075] [Table 1]

[0076] A single-phase submerged arc electric furnace B was prepared, and the flat bath operating conditions were set to melt the metal raw material containing reduced iron in part or in whole. 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 amount of pre-charged materials was adjusted to 750 kg of molten iron and 1500 kg of slag. After the pre-charged materials were completely melted, raw materials were continuously charged, which were a pre-mix of the metal raw material containing reduced iron in part or in whole 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 molten iron and the composition of the slag. The maximum amount of molten iron was targeted to be 4 t, and was adjusted by the amount of raw material charged. The thickness of the molten iron was finally 140 mm.

[0077] When melting the metal raw material containing reduced iron in part or in whole, the line voltage was adjusted by tapping to adjust the power to a predetermined condition. The electrode height was adjusted to adjust the current value. Furthermore, the current density and the voltage to ground were adjusted so that the furnace resistivity was within a predetermined range. In the invention example 1, the conditions were set so that the furnace resistivity was 0.0115 (Ω·m). The flat bath operating conditions and the evaluation of the current stability in the melting of molten iron carried out in the invention example 1 are shown in Table 2. The molten steel temperature was in the range of 1400 to 1700°C. In addition, E / √I, which is the value obtained by dividing the arc voltage E (V), which is the voltage to ground, by the square root of the current I (A) per arc electrode, was calculated. Then, at least one of the electrode height, the arc voltage, and the 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 bus operation was possible due to stable power supply. ×: Flat bus operation was not possible due to unstable current flow caused by arc interruption or other malfunction.

[0078] [Table 2]

[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 for melting 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 resistivity inside the furnace 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 in 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 capable of melting large amounts of metal raw materials by stably passing electricity through a submerged arc furnace (SAF). [Industrial Applicability]

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

[0082] 1 Electric furnace (Submerged Arc Furnace, SAF) 2 Furnace wall 3 electrodes 4. Input reduced iron (metal raw material) 5. Arc P Molten iron (molten pig iron) S Molten slag M Raw material

Claims

1. A method for melting metal raw materials in an electric furnace, comprising storing molten iron, molten slag, metal raw materials, and auxiliary raw materials in a layered manner in an electric furnace, and discharging the molten iron and molten slag, comprising the steps of: When electric current is delivered to the molten metal by an arc or through the slag and raw metal, it heats the molten metal. The method for melting metal raw materials, wherein the electric furnace has an internal resistivity in the range of 0.01 to 0.02 (Ω·m).

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

3. 2. 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. 5. The method for melting metal raw materials according to claim 4, wherein the electrodes are of a self-sintering type.

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

7. 2. 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 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. 2. The method for melting metal 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) that 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): [0010]

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

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