Fixed electric furnace operation method

Abstract

Provided is an operation method of a stationary electric furnace capable of stable operation. In an operation method of a stationary electric furnace (1) in which a molten raw material (11) is tapped from a sprue (2), a melt surface level (h) of a molten metal (12) in the stationary electric furnace (1) is ascertained, and the melt surface level (h) of the molten metal (12) after tapping is managed to be equal to or higher than a predetermined height (h1) that is higher than the sprue (2).

Description

[Technical field]

[0001] The present invention relates to a method for operating a stationary electric furnace used in a method for producing low-carbon ferrochromium and the like. [Background technology]

[0002] Low carbon ferrochrome is an Fe-Cr alloy containing 60% by mass or more of Cr and 0.1% by mass or less of C, and is used as a Cr additive for special steels, especially stainless steels. The Perrin process has long been used as a method for producing low carbon ferrochrome. In the Perrin process, chromium ore and quicklime as raw materials are melted in an electric furnace, the molten metal is tapped from the electric furnace, and a reducing agent is added to the tapped molten metal to produce low carbon ferrochrome.

[0003] In the Perrin process, a tilting electric furnace is used, which pours molten metal while tilting. However, when pouring molten metal using a tilting electric furnace, the surface of the molten metal is exposed to the atmosphere. This causes a problem of high heat dissipation from the surface to the atmosphere, resulting in poor thermal efficiency. In addition, it is necessary to pull out the electrodes that interfere with the tilting to stop the current flow, which causes problems such as a decrease in the current flow rate and a large heat loss when the furnace is stopped.

[0004] To solve this problem, the applicant has proposed using a fixed-type electric furnace instead of a tilting-type electric furnace (see Patent Document 1). A fixed-type electric furnace taps molten metal from a sprue located higher than the bottom of the furnace without tilting the furnace. The use of a fixed-type electric furnace can solve the above problem. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 64-52013 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the conventional method of operating a fixed electric furnace, the surface level of the molten metal after tapping drops to the height of the gate. This makes it impossible to secure a sufficient amount of remaining molten metal, which causes problems such as fluctuations in the solubility of the raw materials charged after tapping and fluctuations in the tapping temperature of the molten metal. In addition, as the surface level of the molten metal after tapping drops to the height of the gate, the tip position of the electrode also drops to the height of the gate, which causes the gate to become hot and erodes the refractory of the gate.

[0007] The present invention has been made in consideration of the above problems, and has an object to provide a method for operating a fixed type electric furnace that enables stable operation. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention provides a method for operating a fixed type electric furnace in which molten raw materials are poured from a gate, the method comprising: grasping the surface level of the molten metal in the fixed type electric furnace at the time of pouring; and controlling the surface level of the molten metal after pouring to be at least a predetermined height that is higher than the gate. Effect of the Invention

[0009] According to the present invention, the amount of remaining molten metal in the fixed electric furnace can be secured, so that the fluctuation in the solubility of the raw materials charged after tapping and the fluctuation in the tapping temperature of the molten metal can be suppressed. In addition, the distance from the tip of the electrode, which is at a high temperature, to the gate can be secured, so that the erosion of the refractory material at the gate can be suppressed. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a process diagram of a method for producing low-carbon ferrochromium to which a method for operating a fixed electric furnace according to one embodiment of the present invention is applied. [Diagram 2] FIG. 2 is a vertical cross-sectional view of a fixed electric furnace. [Diagram 3] FIG. 1 is a circuit diagram of a fixed type electric furnace. [Figure 4] FIG. 4(a) is a vertical cross-sectional view of a fixed electric furnace (FIG. 4(a) shows the surface level of the molten metal after pouring, and FIG. 4(b) shows the surface level of the molten metal before pouring). [Diagram 5] 1 is a graph showing the basis for determining a predetermined height of the molten metal after pouring. [Figure 6] 6 is a graph showing a transition of the electrode tip position (FIG. 6(a) shows a conventional example, and FIG. 6(b) shows an example of the present invention). [Figure 7] 1 is a graph comparing the solubility of raw materials, the tapping temperature of molten metal, and the temperature of the refractory material at the gate between a conventional example and an example of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, a method for operating a fixed electric furnace according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, the method for operating a fixed electric furnace according to the present invention can be embodied in various forms and is not limited to the embodiments described in this specification. This embodiment is provided with the intention of enabling a person skilled in the art to fully understand the scope of the invention by fully disclosing the specification.

[0012] Fig. 1 is a process diagram of a method for producing low-carbon ferrochrome to which the method for operating a fixed electric furnace according to the present embodiment is applied. As shown in Fig. 1, in the method for producing low-carbon ferrochrome, first, chromium ore as a raw material and quicklime as a flux are charged into a fixed electric furnace, and the raw materials are melted in the fixed electric furnace to produce molten metal. Then, the molten metal is tapped into a reaction vessel from a sprue of the fixed electric furnace (S1).

[0013] Next, silicochrome as a reducing agent and additional chromium ore as necessary are added to the reaction vessel from which the molten metal was poured, and inert gas is blown into the reaction vessel to stir (S2). Note that stirring may be performed by a relay ring that transfers the molten metal between two ladles.

[0014] The reduction reaction between chromium oxide and silicon in chromium ore proceeds as follows: Cr2O3+3 / 2Si→2Cr+3 / 2SiO2…(1) Here, the liberated SiO2 reacts with quicklime as shown below in (2) and (3), producing slag. CaO+SiO2→CaO·SiO2…(2) 2CaO+SiO2→2CaO·SiO2…(3) When slag is formed as in (2) and (3), the amount of free SiO2 in (1) decreases, and the reduction reaction of (1) proceeds from left to right.

[0015] The molten low-carbon ferrochrome produced by the reduction reaction is poured into a mold to become the product. The low-carbon ferrochrome product contains 60% by mass or more of Cr, 1.0% by mass or less of Si, and 0.1% by mass or less of C. Meanwhile, the slag produced by the reduction reaction is separated from the molten low-carbon ferrochrome.

[0016] The silicochrome of the reducing agent may be silicochrome recovered from the slag generated by the reduction reaction, or silicochrome from outside the system. In addition to silicochrome, a silicon-based reducing agent such as metal silicon may be used. Furthermore, in addition to the silicon-based reducing agent, an aluminum-based reducing agent such as aluminum or an aluminum alloy, a magnesium-based reducing agent such as magnesium or a magnesium alloy, or a calcium-based reducing agent such as calcium or a calcium alloy, or a mixture of these reducing agents may be used.

[0017] FIG. 2 is a vertical cross-sectional view of a fixed electric furnace 1. A gate 2 is provided at a position higher than the bottom 7 of the furnace body 3. Filling material 6 such as mud material is filled into the gate 2. Chromium ore and quicklime are charged as raw materials 11 into the furnace body 3 through a raw material chute 5. Three electrodes 4a, 4b, and 4c are inserted into the furnace body 3. The electrodes 4a, 4b, and 4c are arranged on a circumference in a plan view at intervals of 120 degrees in the circumferential direction. The tips of the electrodes 4a, 4b, and 4c are buried in the raw materials 11. 10a, 10b, and 10c are holders that hold the electrodes 4a, 4b, and 4c and pass electricity through the electrodes 4a, 4b, and 4c. The gate 2 can also be provided at the bottom 7 of the furnace.

[0018] By passing a current through the electrodes 4a, 4b, and 4c, the raw materials 11 melt, producing the molten metal 12. After the molten metal 12 is produced, the filler 6 filled in the gate 2 is removed with a drill or the like, and the molten metal 12 is poured out from the gate 2. The charging, melting, and pouring of the raw materials 11 are performed with current passing through the electrodes 4a, 4b, and 4c.

[0019] In Fig. 2, 8 is a steel shell, 9 is a refractory, and 14 is a self-lining layer (ore dissolving layer). The refractory 9 is provided inside the steel shell 8. 9a and 9b are bricks of the refractory 9, and 9c is a stamp of the refractory 9. A self-lining layer 14 is formed inside the refractory 9. 13a and 13b are thermocouples that measure the temperature of the refractory 9 at the sprue.

[0020] FIG. 3 is a circuit diagram of the fixed electric furnace 1. 21 is a circuit breaker, 22 is a transformer, 4a, 4b, and 4c are electrodes, 3 is a furnace body, 23 is a current detector that detects the current (actual current) flowing through the electrode 4c, 24 is a voltage detector that detects the voltage (actual voltage) between the electrode 4c and the grounded furnace body 3, 25 is a motor for an electrode lifting device that lifts and lowers the electrode, 26 is an electrode lifting control device that controls the lifting and lowering of the electrode 4c, and 27 is an inverter that supplies power to the motor 25. Note that FIG. 3 is simplified, and in reality, the current detector 23, the voltage detector 24, the motor 25, the electrode lifting control device 26, and the inverter 27 are provided for each of the three (three-phase) electrodes 4a, 4b, and 4c. The three electrodes 4a, 4b, and 4c are individually controlled to be lifted and lowered.

[0021] The electrode lifting control device 26 receives the actual current and actual voltage of the electrodes 4a, 4b, and 4c as input signals, and controls the elevation of the electrodes 4a, 4b, and 4c so that the ratio of the actual current to the actual voltage (actual current / actual voltage) becomes a set value (set current / set voltage). When the ratio of the actual current to the actual voltage becomes larger than the set value, i.e., when the impedance decreases, the electrode lifting control device 26 outputs a speed signal to the inverter 27 to lift the electrodes 4a, 4b, and 4c, and conversely, when the ratio of the actual current to the actual voltage becomes smaller than the set value, i.e., when the impedance increases, the electrode lifting control device 26 outputs a speed signal to the inverter 27 to lower the electrodes 4a, 4b, and 4c. In this way, by controlling the impedance to be constant, the tip positions of the electrodes 4a, 4b, and 4c rise and fall in accordance with the molten metal surface level h.

[0022] As shown in FIG. 2, the surface of the molten metal 12 is covered by the unmelted raw material 11 that has been charged. For this reason, the surface level h of the molten metal 12 cannot be visually confirmed. However, by performing constant impedance control, the tip positions of the electrodes 4a, 4b, and 4c can be made substantially the same as the surface s of the molten metal 12, and the tip positions of the electrodes 4a, 4b, and 4c can be raised and lowered according to the surface s. Therefore, the surface level h of the molten metal 12 can be grasped based on the tip positions of the electrodes 4a, 4b, and 4c. A method for grasping the surface level h will be described later. Note that constant impedance control may be performed so that the distance from the tip positions of the electrodes 4a, 4b, and 4c to the surface s of the molten metal 12 is constant, and this constant distance may be taken into consideration when grasping the surface level h.

[0023] An example of a method for grasping the surface level h of the molten metal 12 will be described. The height x of the lower end position of the holders 10a, 10b, 10c that can be monitored from the reference height N is detected. The electrode length l from the holders 10a, 10b, 10c to the tip positions of the electrodes 4a, 4b, 4c is measured in advance, and the electrode length l is added to the height x of the lower end position of the monitored holders 10a, 10b, 10c. The height of the tip positions of the electrodes 4a, 4b, 4c and the height of the molten metal surface s are approximately the same, and the distance p from the reference height N to the center C of the gate 2 is known, so the molten metal surface level h from the center C of the gate 2 can be expressed as h=pxl. In this way, the molten metal surface level h of the molten metal 12 is grasped. The electrode length l may take into account the consumption of the electrodes 4a, 4b, and 4c, or, if self-sintering electrodes are used for the electrodes 4a, 4b, and 4c, the electrode length l may take into account the amount of depression of the self-sintering electrodes.

[0024] Fig. 4(a) shows the surface level h of the molten metal 12 after tapping. In the method of operating the fixed electric furnace 1 of this embodiment, the surface level h of the molten metal 12 is grasped when tapping, and the surface level h of the molten metal 12 after tapping is controlled to a predetermined height h1 that is higher than the gate 2 (more precisely, higher than the upper end of the gate 2). In other words, when the surface level h of the molten metal 12 drops to the predetermined height h1, the gate 2 is closed and the tapping is stopped. This ensures the remaining amount of molten metal. The method of determining the predetermined height h1 will be described later.

[0025] After tapping, the raw materials 11 are recharged into the fixed electric furnace 1 as shown in Fig. 2. The raw materials 11 may be charged all at once or in portions. The amount of raw materials 11 to be charged is determined according to the target tapping rate (kg / tap). When the charged raw materials 11 are melted, the surface level h of the molten metal 12 rises again.

[0026] Fig. 4(b) shows the surface level h2 of the molten metal 12 before pouring. The surface level h of the molten metal 12 before pouring is controlled to a predetermined height h2. That is, when the surface level h of the molten metal 12 rises to the predetermined height h2, the sprue 2 is opened by a drill or the like, and the molten metal 12 is poured. Note that when pouring, the surface s of the molten metal 12 may be covered with unmelted raw material 11 as shown in Fig. 2, or may not be covered as shown in Fig. 4(b).

[0027] An example of a method for determining the predetermined height h1 shown in Fig. 4(a) will be described. Fig. 5(b) shows a graph of the transition of the electrode tip position. The horizontal axis is Tap (time) and the vertical axis is the electrode tip position. In one tap, the electrode tip position gradually rises and then falls as the molten metal is poured out. This results in the formation of a triangular waveform.

[0028] As shown in the region surrounded by the dashed ellipse in FIG. 5(b), when the electrode tip position after pouring (i.e., the surface level h of the molten metal 12 after pouring) becomes less than 100 mm from the center C of the gate 2 (see FIG. 4(a)), the refractory temperature of the gate rises as shown in FIG. 5(a). For this reason, the predetermined height h1 of the molten metal 12 after pouring is set to be 100 mm or more, preferably 200 mm or more higher than the center C of the gate 2. In this example, in order to ensure the amount of molten metal to be poured, the predetermined height h1 is set to 200 mm as shown in FIG. 5(b). Of course, h1 is not limited to 200 mm, and may be determined appropriately depending on the furnace shape and operating conditions.

[0029] The predetermined height h2 of the molten metal 12 before tapping is determined by the sum of the predetermined height h1 of the molten metal 12 after tapping and the stroke amount of the electrodes 4a, 4b, and 4c corresponding to the target tapping rate (t / tap). In this example, the stroke amount of the electrodes corresponding to the target tapping rate of 11.5 (t / tap) was 600 mm. For this reason, the predetermined height h2 of the molten metal 12 before tapping was set to 600 mm + 200 mm = 800 mm. h2 is not limited to 800 mm, and may be determined appropriately depending on the furnace shape and operating conditions.

[0030] The method for operating a fixed electric furnace according to the present embodiment has been described above. However, the present invention is not limited to the above embodiment, and may be embodied in other embodiments without departing from the spirit and scope of the present invention.

[0031] In the above embodiment, the predetermined height h1 of the molten metal 12 after tapping is constant, but the predetermined height h1 may be changed for each Tap.

[0032] In the above embodiment, the fixed electric furnace 1 is used in a method for producing low-carbon ferrochrome. However, the fixed electric furnace 1 can be used in a method for producing high-carbon ferrochrome, metallic chromium, silicochrome, etc. EXAMPLES

[0033] The raw materials, chromium ore and quicklime, were melted using a fixed electric furnace as shown in Figure 2. The target tapping rate was set to 11.5 (t / tap), the prescribed height h1 of the molten metal 12 after tapping was set to 200 mm from the center C of the gate 2, and the prescribed height h2 of the molten metal 12 before tapping was set to 800 mm from the center C of the gate 2.

[0034] FIG. 6(a) shows the change in electrode tip position (i.e., the molten metal surface level h) in a conventional example in which the molten metal surface level is not controlled. FIG. 6(b) shows the change in electrode tip position (i.e., the molten metal surface level h) in an example of the present invention in which the molten metal surface level of molten metal 12 is controlled. The horizontal axis is Tap, and the vertical axis is the electrode tip position (mm). Since there are three electrodes, FIGS. 6(a) and (b) show the change in the tip positions of the three electrodes. C is the height of the center of the gate 2, h1 is the specified height after the molten metal is tapped (a height of 200 mm from the center C of the gate 2), and h2 is the specified height before the molten metal is tapped (a height of 800 mm from the center C of the gate 2).

[0035] As shown in Figure 6(a), in the conventional example, the surface level h of the molten metal after tapping was below the standard h1 that should be managed. This caused a decrease in the solubility of the raw material and an avalanche of unmelted raw material, which resulted in an unsmooth electrode stroke and electrode hunting in region B, which in turn caused heat loss.

[0036] On the other hand, as shown in Fig. 6(b), in the example of the present invention, the surface level h of the molten metal 12 after pouring is set to h1, ensuring a sufficient amount of remaining molten metal. This makes the electrode stroke smoother, prevents electrode hunting and heat loss, and ensures efficient heat conduction.

[0037] FIG. 7 is a graph comparing the solubility of raw materials, the tapping temperature of the molten metal, and the temperature of the refractory material at the gate between the conventional example and the example of the present invention.

[0038] The top graph shows the solubility of the raw materials. The horizontal axis is days, and the vertical axis is the power consumption rate (amount of electricity per ton of molten metal). In the conventional example, the power consumption rate fluctuated up and down, whereas in the present invention example, the variation in the power consumption rate was suppressed. As shown in the right column, compared to the conventional example, in the present invention example, the average value of the power consumption rate was reduced from 1073 (kWh / t-slag) to 1058 (kWh / t-slag), and the standard deviation was also reduced from 23 to 15.

[0039] The middle graph shows the tapping temperature of the molten metal. The horizontal axis is days, and the vertical axis is the tapping temperature of the molten metal. In the conventional example, the tapping temperature of the molten metal fluctuated up and down, whereas in the present invention, the variation in the tapping temperature of the molten metal could be suppressed. As shown in the right column, compared to the conventional example, in the present invention, the average tapping temperature of the molten metal could be increased from 1931°C to 1954°C, and the standard deviation could be reduced from 28 to 17. If the tapping temperature of the molten metal can be stabilized, the smelting temperature in the reaction vessel can be stabilized, and the composition of the low-carbon ferrochrome product can be stabilized.

[0040] The lower part shows a graph of the temperature of the refractory at the gate. The horizontal axis is days, and the vertical axis is the refractory temperature at the gate. In the conventional example, the refractory temperature at the gate fluctuated up and down, whereas in the example of the present invention, the variation in the refractory temperature at the gate was suppressed. As shown in the right column, compared to the conventional example, in the example of the present invention, the average refractory temperature at the gate was reduced from 367°C to 300°C, and the standard deviation was reduced from 41 to 13. It was found that erosion of the refractory at the gate can be prevented by suppressing the refractory temperature at the gate.

[0041] This specification is based on Japanese Patent Application No. 2020-094637, filed on May 29, 2020, the entire contents of which are incorporated herein by reference. [Explanation of symbols]

[0042] 1. Fixed type electric furnace 2…Spout 7...hearth bottom 11...Raw materials 12…Molten metal h…Water level h1: The specified height of the molten metal after pouring h2: Predetermined height of molten metal before pouring C: Center of spout

Claims

1. A method for operating a fixed type electric furnace in which molten raw materials are poured from a spout, comprising the steps of: When pouring, the level of the molten metal in the fixed electric furnace is grasped, A method for operating a fixed type electric furnace, which controls the surface level of the molten metal after tapping to a predetermined height or higher than the sprue.

2. 2. The method for operating a fixed electric furnace according to claim 1, further comprising the step of determining a surface level of the molten metal based on a tip position of an electrode of the fixed electric furnace.

3. 3. The method for operating a fixed type electric furnace according to claim 1, wherein the predetermined height is constant.

4. The gate is provided at a position higher than the bottom of the fixed electric furnace, 4. The method for operating a fixed type electric furnace according to claim 1, wherein the predetermined height is 100 mm or more from the center of the gate.

5. 5. A method for operating a fixed type electric furnace according to claim 1, further comprising the step of controlling the surface level of the molten metal before the metal is poured out to a predetermined height.

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