JP7655908B2 - Fixed electric furnace operation method - Google Patents

Description

The present invention relates to a method for operating a fixed electric furnace used in the production method of low-carbon ferrochromium, etc.

Low-carbon ferrochrome is an Fe-Cr alloy with 60% or more by mass of Cr and 0.1% or less by mass 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 are melted in an electric furnace as raw materials, the molten metal is poured out of the furnace, and a reducing agent is added to the molten metal to produce low-carbon ferrochrome.

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 large amount of 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 and stop the current flow, which causes problems such as a decrease in the current flow rate and large heat loss when the furnace is shut down.

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 pours molten metal from a spout 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.

Japanese Unexamined Patent Publication No. 64-52013

However, in conventional methods of operating fixed electric furnaces, the surface level of the molten metal after tapping drops to the height of the gate. This means that a sufficient amount of remaining molten metal cannot be secured, and there are problems with 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 position of the tip of the electrode also drops to the height of the gate, causing the gate to become too hot and eroding the refractory at the gate.

The present invention has been made in consideration of the above problems, and aims to provide a method for operating a fixed electric furnace that enables stable operation.

In order to solve the above problems, the present invention provides a method for operating a fixed electric furnace in which molten raw materials are poured from a spout, the method comprising: grasping the surface level of the molten metal in the fixed electric furnace when pouring; and controlling the surface level of the molten metal after pouring to a predetermined height or higher than the spout.

According to the present invention, the amount of remaining molten metal in a fixed electric furnace can be secured, so fluctuations in the solubility of the raw materials charged after tapping and fluctuations 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 sprue can be secured, so erosion of the refractory material at the sprue can be suppressed.

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. FIG. 2 is a vertical cross-sectional view of a fixed electric furnace. FIG. 1 is a circuit diagram of a fixed type electric furnace. 4A and 4B are vertical cross-sectional views of a fixed electric furnace (FIG. 4A shows the surface level of the molten metal after it has been poured out, and FIG. 4B shows the surface level of the molten metal before it has been poured out). 1 is a graph showing the basis for determining a predetermined height of the molten metal after tapping. 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). 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.

Below, a method of operating a fixed electric furnace according to an embodiment of the present invention will be described in detail with reference to the attached drawings. However, the method of operating a fixed electric furnace according to the present invention can be embodied in various forms and is not limited to the embodiment described in this specification. This embodiment is provided with the intention of enabling those skilled in the art to fully understand the scope of the invention by providing sufficient disclosure in the specification.

1 is a process diagram of a method for producing low-carbon ferrochrome to which the fixed electric furnace operating method of this embodiment is applied. As shown in FIG. 1, in the method for producing low-carbon ferrochrome, first, chromium ore as 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. The molten metal is then tapped into a reaction vessel from the sprue of the fixed electric furnace (S1).

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

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 in the following (2) and (3) to produce slag.
CaO+SiO ₂ →CaO・SiO ₂ …(2)
2CaO+SiO ₂ →2CaO・SiO ₂ …(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.

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.

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

Figure 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, 4c are inserted into the furnace body 3. The electrodes 4a, 4b, 4c are arranged on a circumference in a plan view with 120 degree intervals in the circumferential direction. The tips of the electrodes 4a, 4b, 4c are buried in the raw materials 11. 10a, 10b, 10c are holders that hold the electrodes 4a, 4b, 4c and pass electricity through the electrodes 4a, 4b, 4c. It is also possible to provide the gate 2 at the bottom 7 of the furnace.

By passing electricity 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 electricity passing through the electrodes 4a, 4b, and 4c.

In Figure 2, 8 is the steel shell, 9 is the refractory material, and 14 is the self-lining layer (ore dissolution layer). Refractory material 9 is provided on the inside of steel shell 8. 9a and 9b are refractory material 9 bricks, and 9c is a stamp of refractory material 9. A self-lining layer 14 is formed on the inside of refractory material 9. 13a and 13b are thermocouples that measure the temperature of refractory material 9 at the sprue.

Figure 3 is a circuit diagram of a 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 raises 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, voltage detector 24, motor 25, electrode lifting control device 26, and 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 raised and lowered.

The electrode lifting control device 26 uses the real current and real voltage of the electrodes 4a, 4b, and 4c as input signals, and controls the lifting and lowering of the electrodes 4a, 4b, and 4c so that the ratio of the real current to the real voltage (real current/real voltage) becomes the set value (set current/set voltage). When the ratio of the real current to the real 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 raise the electrodes 4a, 4b, and 4c, and conversely, when the ratio of the real current to the real 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 a constant value, the tip positions of the electrodes 4a, 4b, and 4c rise and fall in accordance with the molten metal surface level h.

As shown in FIG. 2, the surface of the molten metal 12 is covered by the unmelted raw materials 11 that have 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 to match 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. The method of grasping the surface level h will be described later. It is also possible to perform constant impedance control 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 a constant distance, and to take this constant distance into account when grasping the surface level h.

An example of a method for determining the surface level h of the molten metal 12 will be described. The height x of the bottom end position of the monitorable holders 10a, 10b, 10c 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 bottom 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 = p - x - l. This allows the molten metal surface level h of the molten metal 12 to be determined. The electrode length 1 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 1 may take into account the amount of depression of the self-sintering electrodes.

Figure 4 (a) shows the surface level h of the molten metal 12 after tapping. In the operating method of 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 tapping is stopped. This ensures the remaining amount of molten metal. The method for determining the predetermined height h1 will be described later.

After the molten metal is tapped, the raw materials 11 are recharged into the fixed electric furnace 1 as shown in Figure 2. The raw materials 11 may be charged all at once or in portions. The amount of raw materials 11 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.

Figure 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. When pouring, the surface s of the molten metal 12 may be covered with unmelted raw material 11 as shown in Figure 2, or it may not be covered as shown in Figure 4(b).

An example of a method for determining the predetermined height h1 shown in Figure 4(a) is explained. Figure 5(b) shows a graph of the change in 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 dispensed. This results in the formation of a triangular waveform.

As shown in the area surrounded by the dashed ellipse in Figure 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 Figure 4 (a)), the refractory temperature of the gate rises, as shown in Figure 5 (a). For this reason, the predetermined height h1 of the molten metal 12 after pouring is set to be at least 100 mm, preferably at least 200 mm, 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 Figure 5 (b). Of course, h1 is not limited to 200 mm, and may be determined appropriately depending on the furnace shape and operating conditions.

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 equivalent to the target tapping rate (t/tap). In this example, the stroke amount of the electrodes equivalent 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 also not limited to 800 mm and may be determined appropriately depending on the furnace shape and operating conditions.

The method of operating a fixed electric furnace according to this embodiment has been described above. However, the present invention is not limited to being embodied in the above embodiment, and may be embodied in other embodiments without changing the gist of the present invention.

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

In addition, in the above embodiment, an example was described in which the fixed electric furnace 1 is used in a method for producing low-carbon ferrochrome, but the fixed electric furnace 1 can also be used in a method for producing high-carbon ferrochrome, metallic chromium, silicochrome, etc.

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 predetermined height h1 of the molten metal 12 after tapping was set to 200 mm from the center C of the gate 2, and the predetermined height h2 of the molten metal 12 before tapping was set to 800 mm from the center C of the gate 2.

Figure 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 managed. Figure 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 12 is managed. The horizontal axis is Tap, and the vertical axis is the electrode tip position (mm). Since there are three electrodes, Figures 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 (height of 200 mm from the center C of the gate 2), and h2 is the specified height before the molten metal is tapped (height of 800 mm from the center C of the gate 2).

As shown in Figure 6 (a), in the conventional example, the surface level h of the molten metal after tapping was below the management standard h1. 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 caused heat loss.

On the other hand, as shown in Figure 6 (b), in 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.

Figure 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 sprue between a conventional example and an example of the present invention.

The top row shows a graph of 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 example of the present invention, the variance in the power consumption rate was suppressed. As shown in the right column, compared to the conventional example, in the example of the present invention, 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.

The middle section shows a graph of 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 example, the variation in the tapping temperature of the molten metal was suppressed. As shown in the right column, compared to the conventional example, in the present invention example, the average tapping temperature of the molten metal was increased from 1931°C to 1954°C, and the standard deviation was 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 in the product can be stabilized.

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.

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

1... fixed type electric furnace 2... sprue 7... furnace bottom 11... raw material 12... molten metal h... molten metal surface level h1... predetermined height of molten metal after pouring h2... predetermined height of molten metal before pouring C... center of sprue

Claims (5)

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. A method for operating a fixed electric furnace as described in claim 1, characterized in that the surface level of the molten metal is grasped based on the tip position of the electrode of the fixed electric furnace. A method for operating a fixed electric furnace as described in claim 1 or 2, characterized in that the specified height is constant. 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. A method for operating a fixed electric furnace as described in any one of claims 1 to 4, characterized in that the surface level of the molten metal before pouring is controlled to a predetermined height.

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