JP7013111B2 - Method for manufacturing low carbon ferrochrome - Google Patents

Description

The present invention relates to a method for producing low carbon ferrochrome.

Low carbon ferrochrome is an Fe—Cr alloy having a Cr of 60% by mass or more and a C of 0.1% by mass or less, and is used as a Cr additive for special steels, particularly stainless steel. As a method for producing low carbon ferrochrome, the Peran method has been used for a long time. In the Peran method, chromium ore and quicklime are used as raw materials, and a melting raw material obtained by dissolving them in an electric furnace is discharged, and a reducing agent is added to the melting raw material to produce low-carbon ferrochrome.

In the Peran method, a tilting electric furnace is used as the electric furnace, in which the molten raw material is discharged while tilting. However, when the primary slag is discharged using a tilting electric furnace, the surface of the molten material is exposed to the atmosphere. Therefore, there is a problem that heat is largely dissipated from the surface of the hot water to the atmosphere and the thermal efficiency is poor. Further, since it is necessary to pull out the electrode that hinders the tilting to stop the energization, there is a problem that the energization rate decreases and the heat loss due to the shutdown increases.

In order to solve this problem, the applicant proposes to use a fixed electric furnace instead of the tilting electric furnace (see Patent Document 1). In a fixed electric furnace, the molten material is discharged from the hot water outlet without tilting the furnace. The whole amount of the dissolved raw material is not discharged, but remains as a pool at the bottom of the furnace after the hot water is discharged. If a fixed electric furnace is used, the solubility (thermal efficiency) of the raw material is improved by the effect of the hot water pool. Further, when the dissolved raw material is discharged, the surface of the molten metal can be covered with the undissolved raw material, so that heat dissipation from the surface of the molten metal to the atmosphere can be prevented.

Japanese Unexamined Patent Publication No. 64-52013

However, the dissolved raw material in which chromium ore and quicklime are dissolved has a high viscosity and a high melting point, and has a problem that it is easy to blow up. When the dissolved raw material is blown up, the blown-up dissolved raw material is solidified and must be redissolved, resulting in heat loss.

Therefore, an object of the present invention is to provide a method for producing low carbon ferrochrome that can suppress the blow-up of the dissolution raw material, increase the solubility, and reduce the electric power intensity.

In order to solve the above problems , one aspect of the present invention is to use chrome ore and fresh lime as raw materials, dissolve them in an electric furnace to generate hot water, and add a reducing agent to the dissolved raw materials to reduce carbon. In the method of producing ferrochrome, a fixed electric furnace that leaves a pool of hot water in the electric furnace is used, and the ratio of the set current of the electrode to the set voltage (set current / set voltage) is 30 A / V or more and 150 A / V or less. This is a method for producing low carbon ferrochrome, which is set to 5 and the average power density on the current flow surface of the fixed electric furnace is set to 500 kW / m ² or more and 3000 kW / m ² or less .

The phenomenon of blowing up the dissolved raw material is caused by the electrode being immersed in the dissolved raw material. When the electrode is immersed in the dissolved raw material, the carbon of the electrode reacts with the dissolved raw material to generate CO gas, and the CO gas blows up the dissolved raw material. According to one aspect of the present invention, the solubility of the raw material is improved by the effect of the hot water pool in the fixed electric furnace, so that the ratio between the set current and the set voltage of the electrode when controlling the elevation of the electrode is made small. That is, it can be set to a high voltage and a low current. Therefore, the distance between the lower end of the electrode and the surface of the molten material can be increased, and the electrode can be prevented from being immersed in the dissolved material.
Further, since the solubility of the raw material is improved by the effect of the hot water pool in the fixed electric furnace, the average power density (power density for melting the raw material) on the current transmission surface can be set small. Therefore, heat concentration and super heat in the core portion can be alleviated, and it is possible to prevent a bumping-up phenomenon from occurring in the molten raw material.

It is a process drawing of the manufacturing method of the low carbon ferrochrome of one Embodiment of this invention. It is a vertical sectional view of the fixed electric furnace used in the manufacturing method of low carbon ferrochrome of this embodiment. It is a circuit diagram of the said fixed type electric furnace. It is a cross-sectional view of the said fixed type electric furnace. It is a phase diagram of Cr ₂ O ₃ -Mg O-Al ₂ O ₃ . It is a graph which shows the transition of the furnace body temperature.

Hereinafter, the method for producing low carbon ferrochrome according to the embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, the method for producing low carbon ferrochrome of the present invention can be embodied in various forms, and is not limited to the embodiments described in the present specification. The present embodiment is provided with the intention of allowing those skilled in the art to fully understand the scope of the invention by adequately disclosing the specification.

FIG. 1 is a process diagram of a method for producing a low carbon ferrochrome according to an embodiment of the present invention. As shown in FIG. 1, in the method for producing low-carbon ferrochrome of the present embodiment, first, chromium ore and fresh lime as a medium solvent are used as raw materials, and these are melted in a fixed electric furnace to produce a melting raw material. Then, the molten material is discharged into the reaction vessel from the outlet of the fixed electric furnace (S1).

Next, silicochrome as a reducing agent and chasing chromium ore are added to the reaction vessel from which the dissolved raw material is discharged, and the reaction vessel is stirred by bottom-blowing an inert gas (S2). It should be noted that two pans may be used as reaction vessels, and stirring may be performed by relaying between the two pans.

The reduction reaction between chromium oxide and silicon in the chromium ore proceeds as follows.
Cr ₂ O ₃ + 3/2Si → 2Cr + 3/2SiO ₂ ... (1)
Here, the liberated SiO ₂ reacts with quicklime as described in (2) and (3) below to form slag.
CaO + SiO ₂ → CaO ・ SiO ₂ … (2)
2CaO + SiO ₂ → 2CaO ・ SiO ₂ … (3)
(2) When slag is generated as in (3), the amount of free SiO ₂ in (1) decreases, and the reduction reaction in (1) proceeds from left to right.

The molten metal of low carbon ferrochrome produced by the reduction reaction is cast into a mold to form a product. The low carbon ferrochrome of the 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. On the other hand, the slag produced by the reduction reaction is separated from the molten metal of low carbon ferrochrome.

In the above embodiment, silicon is used as the reducing agent, but a silicon-based reducing agent such as metallic silicon may be used in addition to silicon. Further, 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 may be used. Further, a mixture of these reducing agents may be used.

FIG. 2 is a vertical sectional view of a fixed electric furnace used in the method of the present embodiment. A fixed electric furnace 1 is used as the electric furnace. Three electrodes (two electrodes 4a and 4b are omitted in FIG. 2 are shown) are inserted into the furnace body. The tips of the electrodes 4a and 4b are embedded in the raw material 11 of chromium ore and quicklime charged from the raw material chute 5. By energizing the electrodes 4a and 4b, the raw material 11 is melted to form the dissolved raw material 12. After forming the melting raw material 12, the mud material 6 filled in the hot water outlet 2 is removed, and the hot water of the melting raw material 12 is discharged from the hot water outlet 2. When the dissolved raw material 12 is discharged, the surface of the dissolved raw material 12 is covered with the undissolved raw material 11. After the hot water is discharged, the level of the dissolution raw material 12 drops to the level of the hot water outlet 2, but the dissolution raw material 12 remains as a hot water pool below the level of the hot water outlet 2. The charging, melting, and hot water discharge of the raw material 11 are repeatedly performed with the electrodes 4a and 4b energized. 7 is the bottom of the furnace and 8 is the iron skin. A refractory material 9 is provided inside the iron skin 8. 9a and 9b are bricks of refractory 9, and 9c is a stamp of refractory 9. Reference numerals 10a and 10b are electrode holders. In the above embodiment, the hot water level of the dissolving raw material 12 after hot water is adjusted to the level of the hot water outlet 2, but the hot water level of the melting raw material 12 after hot water is set to a predetermined level higher than that of the hot water outlet 2. You may match.

h1 is the furnace depth, that is, the distance from the furnace bottom 7 to the lower surface of the upper ring 14. h2 is the depth of the hot water pool, that is, the distance from the bottom 7 to the level of the molten metal 12 after hot water is discharged (in the present embodiment, the distance from the bottom 7 to the hot water outlet 2). The ratio (h2 / h1) of the pool depth h2 and the furnace depth h1 is set to 0.2 or more and 0.6 or less. This is because if it exceeds 0.6, the amount of hot water discharged becomes too small. On the other hand, if it is less than 0.2, the amount of remaining hot water is too small, and the effect of collecting hot water (the effect of improving the solubility of the raw material 11) is small.

FIG. 3 is a circuit diagram of a fixed electric furnace. 21 is a breaker, 22 is a transformer, 4a, 4b, 4c are electrodes, 3 is a furnace body, 23 is a current detector that detects the current (actual current) flowing through the electrode 4c, and 24 is a furnace grounded to the electrode 4c. A voltage detector that detects the voltage (actual voltage) between the body 3 and 25 is a motor of an electrode elevating device that raises and lowers the electrode 4c, 26 is an electrode elevating control device that controls the elevating and lowering of the electrode 4c, and 27 is a motor 25. It is an inverter that supplies power. Although simplified in FIG. 3, the current detector 23, the voltage detector 24, the motor 25, the electrode elevating control device 26, and the inverter 27 are actually provided for each of the three phases. The three electrodes 4a, 4b, and 4c are controlled to move up and down one by one.

The electrode elevating control device 26 uses the actual current and the actual voltage of the electrodes 4a, 4b, and 4c as input signals, and the ratio of the actual current to the actual voltage (actual current / actual voltage) becomes a set value (set current / set voltage). The electrodes 4a, 4b, and 4c are controlled to move up and down as described above. When the ratio of the actual current to the actual voltage becomes larger than the set value, that is, when the impedance drops, the electrode elevating control device 26 outputs a speed signal for raising the electrodes 4a, 4b, 4c to the inverter 27. On the contrary, when the ratio of the actual current to the actual voltage becomes smaller than the set value, that is, when the impedance becomes large, the speed signal for lowering the electrodes 4a, 4b, 4c is output to the inverter 27. Controlling the ratio of the actual current to the actual voltage, that is, controlling the impedance to be constant in this way is generally called constant impedance control.

If the set value for controlling the elevation of the electrodes 4a, 4b, 4c, that is, the ratio of the set current to the set voltage (set current / set voltage) is increased, the distance between the electrodes 4a, 4b, 4c and the molten metal surface can be reduced. In some cases, it becomes a heat-concentrated type and the solubility is improved, but the dissolution efficiency is conversely lowered and the dissolution zone becomes smaller due to the super heat more than necessary. The ratio of the set current to the set voltage (set current / set voltage) is small, that is, high, to 30 A / V or more and 150 A / V or less because the solubility of the raw material is improved by the effect of the hot water pool in the fixed electric furnace. Set to voltage and low current. As a result, the distance between the lower ends of the electrodes 4a, 4b, 4c and the molten metal surface of the dissolving raw material 12 is increased, and the electrodes 4a, 4b, 4c are prevented from being immersed in the dissolving raw material 12. If it is less than 30 A / V, the solubility of the raw material 11 decreases. If it exceeds 150 A / V, the ratio of the set current becomes higher than the set voltage, and the electrodes 4a, 4b, 4c may be immersed in the dissolution raw material 12.

Table 1 shows the results of examining the effects of the ratio of the set current to the set voltage (A / V) on the immersiveness of the electrodes and the blow-up phenomenon. As shown in Table 1, when the A / V exceeds 150 A / V, the degree of immersion of the electrode in the dissolved raw material becomes large and the blow-up phenomenon occurs frequently, whereas when the A / V is 150 A / V or less, the immersion of the electrode is conspicuous. And the blow-up phenomenon does not occur and it becomes good. Further, if it is less than 30 A / V, the immersion of the electrode and the blow-up phenomenon do not occur, but the dissolution becomes insufficient. The optimum ratio of the set current to the set voltage is 30 A / V or more and 150 A / V or less.

FIG. 4 is a cross-sectional view of the fixed electric furnace 1. 3 is the furnace body, 31 is the furnace wall, 4a, 4b, 4c are the electrodes, 32a, 32b, 32c are the current flow planes (also called reaction zones), R is the diameter of the inner diameter of the furnace wall, and d is the diameter of the arrangement circle. , D is the diameter of the electrode, and r is the radius of the current flow plane. The diameter d of the arrangement circle is set to 1.0 m or more and 2.2 m or less. Three electrodes 4a, 4b, 4c are arranged at equal intervals on the arrangement circle. The radii r of the three current transmission surfaces 32a, 32b, and 32c are the same. The radius r of the current flow-through surfaces 32a, 32b, 32c is set so that the adjacent current flow-through surfaces 32a, 32b, 32c are in contact with each other.

The average power density (kW / m ² ) on the current flow plane is expressed by power / 3πr ² . Electric power (kW) = √3 ・ IV ・ cos ψ. Here, I is the actual current, V is the actual voltage, and ψ is the power factor. 3πr ² is the area of the current flow plane. If the average power density on the current transmission surface is large, the solubility of the raw material 11 is improved. However, since the solubility of the raw material 11 is improved by the effect of the hot water pool in the fixed electric furnace 1, the average power density on the current transmission surface is set as small as 500 kW / m ² or more and 3000 kW / m ² or less. As a result, the heat concentration and super heat in the core portion can be alleviated, and the bumping-up phenomenon can be prevented from occurring in the melting raw material 12. If it is less than 500 kW / m ² , the core portion will be insufficient in heat and the hot water will be adversely affected. When it exceeds 3000 kW / m ² , heat concentration and super heat are generated in the core portion, and a bumping-up phenomenon of the melting raw material 12 occurs.

Table 2 shows the results of examining the effect of the average power density on the current transmission surface on the blow-up phenomenon in the A and B furnaces having different diameters. As shown in Table 2, when the average power density exceeds 3000 kW / m ² , the blow-up phenomenon occurs frequently, whereas when the average power density is less than 3000 kW / m ² , the blow-up phenomenon does not occur and becomes good. Further, if it is less than 500 kW / m ² , the blowing-up phenomenon does not occur, but the dissolution becomes insufficient. The optimum average power density is 500 kW / m ² or more and 3000 kW / m ² or less.

As shown in FIG. 2 again, when the raw material 11 is melted in the fixed electric furnace 1, the mass ratio (MgO / Al ₂ O ₃ ) of MgO and Al ₂ O ₃ of the melting raw material 12 is 0.5 or more. Adjusted to 5 or less. Specifically, when the melting raw material 12 is discharged from the fixed electric furnace 1, the composition of the melting raw material 12 is analyzed, and the mass ratio of MgO and Al ₂ O ₃ of the melting raw material 12 (MgO / Al _{2O 3} ) is determined. The raw material 11 is adjusted so as to be 0.5 or more and 1.5 or less. The chromium ore of the raw material 11 includes a chromium ore having a high MgO content and a chromium ore having a _high content of _Al2O3 . When the mass ratio of MgO and Al ₂ O ₃ is low, the amount of chromium ore having a high Mg O content is increased, and when the mass ratio of Mg O and Al ₂ O ₃ is high, the content of Al ₂ O ₃ is high. Increase the amount of ore compounded. If the chrome ore alone cannot be adjusted, _add an MgO source and / or an _Al2O3 source. Then, a chromium spinel-type self-flying layer 51 shown by diagonal lines in the figure is formed inside the refractory material 9. The self-flying layer 51 prevents the refractory material 9 from being melted. If it is less than 0.5, the self-flying layer 51 is severely melted and a sufficient self-flying layer 51 is not formed. If it exceeds 1.5, the raw material is insufficiently dissolved and the dissolution zone becomes small. In addition, the viscosity of the dissolving raw material 12 becomes high, which causes an adverse effect on hot water discharge.

When a partially melted portion 52 is generated in the refractory due to partial overpower or the like, a chromium ore having a high MgO content and / or an MgO source is concentrated in the vicinity of the partially melted portion 52. Enter. Then _, the mass ratio of MgO and _Al2O3 of the melting raw material 12 is adjusted to the above range, and the chromium spinel type self-flying layer 51 is intensively formed in the vicinity of the partially melted portion 52. As a result, the partially melted portion 52 is repaired.

FIG. 5 is a phase diagram of Cr _{2O 3-MgO-Al 2 O 3 .} As shown in FIG. 5, if Cr ₂ O ₃ : MgO: Al ₂ O ₃ of the dissolution raw material 12 is adjusted to a molar ratio of 38.5%: 44.7%: 16.8%, a chromium spinel 53 is formed. can do.

Table 3 shows the composition of the dissolution raw material 12. If MgO: Al _{2 O 3 of the dissolution raw material 12 is adjusted to 1: 1 by mass ratio, MgO: Al 2 O 3 of the} dissolution raw material 12 can be adjusted to 44.7%: 16.8% by molar ratio. can. The mass of Cr ₂ O ₃ of the dissolution raw material 12 is sufficiently larger than that of Mg O and Al ₂ O ₃ , and it is easy to adjust the molar ratio as described above.

Low carbon ferrochrome was produced according to the production process diagram of FIG. Table 4 shows (1) the amount of raw materials used, (2) the amount of electricity used in the fixed electric furnace, (3) the amount of hot water discharged from the dissolved raw materials, (4) the amount of auxiliary raw materials used, and (5) the amount of low-carbon ferrochrome. The production amount of slag and (6) dissolution power intensity are shown.

Table 5 shows the operating conditions of the fixed electric furnaces (A furnace and B furnace having different diameters).

As shown in Example 1, the ratio of the set current of the electrode to the set voltage (set current / set voltage) was set to 85 A / V, and the average power density on the current transmission surface was set to 1200 kW / m ² . No noticeable blow-up of the dissolved raw material occurred. As shown in Example 2, even when the set current / set voltage was set to 145 A / V and the average power density on the current transmission surface was set to 1150 kW / m ² , no conspicuous blow-up of the dissolution raw material occurred. .. As shown in Example 3, when the set current / set voltage was set to 60 A / V and the average power density on the current transmission surface was set to 2350 kW / m ² , no conspicuous blow-up of the dissolved raw material occurred.

On the other hand, as shown in Comparative Example 1, when the ratio of the set current and the set voltage of the electrode (set current / set voltage) was set to 160 A / V, the electrode was immersed in the dissolved raw material and CO gas was generated. The blow-up of the dissolved raw material occurred frequently.

As shown in Comparative Example 2, when the average power density on the current transmission surface was set to 3100 kW / m ² , heat concentration occurred in the core portion, and the molten raw material was frequently blown up.

As shown in Comparative Example 3, when a tilting electric furnace is used, heat is largely dissipated from the molten metal surface to the atmosphere, and the energization rate is lowered. there were.

Adjust the raw material so that the mass ratio (MgO / Al ₂ O ₃ ) of the molten raw material MgO and Al _{2O 3 is} 0.5 or more and 1.5 or less, and chrome spinel type self-flying layer inside the refractory. Formed. As shown in FIG. 2, temperature sensors 17a, 17b, 17c were attached to the side wall of the furnace body 3 and the furnace bottom 7, and these temperatures were measured. A plurality of temperature sensors 17a are provided on the upper side of the side wall of the furnace body 3 at equal intervals in the circumferential direction. A plurality of temperature sensors 17b are provided at the lower part of the side wall of the furnace body 3 at equal intervals in the circumferential direction.

FIG. 6A shows the transition of the furnace body temperature. As shown in FIG. 6A, the furnace body temperature was stable on both the side wall and the bottom of the furnace body and maintained a substantially constant value.

FIG. 6B shows the transition of the furnace body temperature when a partially melted portion is generated in the refractory. The temperature of the furnace body (particularly the bottom of the furnace) rose due to the occurrence of a partially melted portion in the refractory. Therefore, as shown by A and B in FIG. 6B, chromium ore having a high MgO content was intensively charged in the vicinity of the partially melted portion for 2 days, and MgO and Al ₂ were charged. The mass ratio of _O3 was adjusted to the above range. The partially melted part was repaired, and the temperature of the furnace body dropped and became stable.

This specification is based on Japanese Patent Application No. 2019-162550 filed on September 6, 2019. All this content is included here.

1 ... Fixed electric furnace 2 ... Hot water outlets 4a, 4b, 4c ... Electrodes 7 ... Furnace bottom 8 ... Iron skin 9 ... Refractory 11 ... Raw material 12 ... Melting raw material 51 ... Self-flying layer h1 ... Furnace depth h2 ... Hot water pool depth

Claims (2)

In a method of producing low-carbon ferrochrome by using chromium ore and fresh lime as raw materials, hot water is discharged from the dissolved raw material obtained by dissolving them in an electric furnace, and a reducing agent is added to the dissolved raw material.
Using a fixed electric furnace that leaves a pool of hot water in the electric furnace after hot water is discharged,
Set the ratio of the set current of the electrode to the set voltage (set current / set voltage) to 30 A / V or more and 150 A / V or less .
A method for producing low carbon ferrochrome in which the average power density on the current flow surface of the fixed electric furnace is set to 500 kW / m ² or more and 3000 kW / m ² or less . The low carbon according to claim 1, wherein the ratio (hot water pool depth / furnace depth) between the hot water pool depth and the furnace depth of the fixed electric furnace is set to 0.2 or more and 0.6 or less. Ferrochrome manufacturing method.

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