WO2021045173A1

WO2021045173A1 - Self-baking electrode and electrode paste for self-baking electrode

Info

Publication number: WO2021045173A1
Application number: PCT/JP2020/033511
Authority: WO
Inventors: 正浩森; 博一杉森
Original assignee: Ｊｆｅマテリアル株式会社
Priority date: 2019-09-06
Filing date: 2020-09-04
Publication date: 2021-03-11
Also published as: JP7024144B2; JPWO2021045173A1; JP2022059594A; BR112022003085A2

Abstract

Provided is a self-baking electrode which can be used as an electrode in an electric furnace used to produce low-carbon ferro-chromium. The self-baking electrode (10) is formed by filling a cylindrical case (11) with an electrode paste (12), and sintering/solidifying the electrode paste (12) by Joule heating by conducting electricity and by the conductive heat from the electric furnace. The current density of the case is set to 100A/cm² to 400A/cm², inclusive, and the self-baking electrode (10) is used as an electrode in an electric furnace used to produce low-carbon ferro-chromium.

Description

Self-firing electrode and electrode paste for self-firing electrode

The present invention relates to a self-firing electrode of an electric furnace for producing low-carbon ferrochrome and an electrode paste for the self-firing electrode.

Low carbon ferrochrome, which is an Fe—Cr alloy having a Cr of 60% by mass or more and a C of 0.1% by mass or less, is generally produced by a method of reducing chromium ore with silicon. As a specific manufacturing method, the first step of melting chrome ore and fresh lime in an electric furnace and the melting raw material (hereinafter referred to as primary slag) melted in the first step are poured into a ladle and this ladle is used. It is provided with a second step of charging low carbon ferrochrome as a reducing agent, stirring the mixture, and causing a reduction reaction to produce low carbon ferrochrome and secondary slag (see Patent Document 1).

By the way, a self-firing electrode is used as an electrode of an electric furnace for producing alloy iron such as high carbon ferromanganese (see Patent Document 2). The self-firing electrode is formed by filling a tubular case with the electrode paste and firing and solidifying the electrode paste by Joule heat generated by energization and conduction heat from an electric furnace. The self-firing electrode is characterized in that it is cheaper than a firing electrode (an electrode pressed and fired outside an electric furnace).

Japanese Unexamined Patent Publication No. 1-225743 Japanese Unexamined Patent Publication No. 2000-1505017

It is desirable to use self-firing electrodes in electric furnaces for producing low-carbon ferrochrome. However, in the low carbon ferrochrome production method, high voltage and low current operation in which the tip position of the electrode is raised is performed. This is to prevent the electrode from being immersed in the melting raw material in the electric furnace. This is because chromium has a very high affinity for carbon, and once the carbon chromium of the electrode and carbon are bonded, carbon cannot be removed later. However, when high-voltage and low-current operation is performed, Joule heat generated by energization is insufficient, firing of the electrode paste is insufficient, and hollowing out (a phenomenon in which the solidified electrode falls from the case) occurs in the self-firing electrode. For this reason, firing electrodes have been used in electric furnaces for producing low-carbon ferrochrome.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a self-firing electrode that can be used as an electrode of an electric furnace for producing low-carbon ferrochrome and an electrode paste for the self-firing electrode. To do.

In order to solve the above problems, one aspect of the present invention is formed by filling a tubular case with an electrode paste and firing and solidifying the electrode paste with Joule heat generated by energization and conduction heat from an electric furnace. a self baking electrode which is in, and set the case current density 100A / cm ² or more 400A / cm ² or less, which is a self baking electrode using said self firing electrode as an electrode of a low-carbon ferrochrome production for electric furnaces.

Another aspect of the present invention is an electrode paste for a self-firing electrode of an electric furnace for producing low carbon ferrochrome, wherein the electric specific resistance of the solidified electrode obtained by firing the electrode paste is 30 μΩm or more and 80 μΩm or less, and the heat of the solidified electrode. This is an electrode paste in which the content of artificial graphite in the electrode paste is set to 30% by mass or more and 70% by mass or less so that the conductivity is 8.0 W / m · K or more.

According to one aspect of the present invention, since the case current density is ^{lowered to 400 A / cm 2} or less, it is possible to suppress the fusing (in other words, wear) of the case below the electrode holder and hold the solidified electrode by the case. it can. Therefore, it is possible to prevent the solidified electrode from coming off. If the case current density is ^{less than 100 A / cm 2} , the electrode paste will be insufficiently fired, so that even if the case below the electrode holder is suppressed from fusing, the solidified electrode may be hollowed out.

According to another aspect of the present invention, the thermal conductivity can be made higher than that of the conventional self-firing electrode without making the electrical resistivity of the solidified electrode so low as that of the conventional self-firing electrode. Although the Joule heat generated by energization is slightly reduced, the heat of the electric furnace is efficiently transferred to the electrode paste through the solidified electrode, so that the electrode paste can be fired. Further, since the thermal conductivity of the solidified electrode is high, the temperature gradient inside and outside the solidified electrode becomes gentle, and the thermal shock resistance is improved.

It is a process drawing of the manufacturing method of low carbon ferrochrome. It is a figure which shows the equipment used in the manufacturing method of low carbon ferrochrome. It is a vertical cross-sectional view of the self-firing electrode of one Embodiment of this invention. It is a horizontal sectional view of the case of the self-firing electrode of this embodiment. It is a graph which shows the relationship between the case thickness of the self-firing electrode and the case current density.

Hereinafter, the self-firing electrode and the electrode paste for the self-firing electrode of the electric furnace 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 present invention can be embodied in various forms and is not limited to the embodiments described herein. The present embodiment is provided with the intention of allowing those skilled in the art to fully understand the scope of the invention by making sufficient disclosure of the specification.
(Manufacturing method of low carbon ferrochrome)

First, a method for producing low-carbon ferrochrome will be explained. FIG. 1 is a process chart of a method for producing low carbon ferrochrome. FIG. 2 is a diagram showing equipment used in a method for producing low carbon ferrochrome.

As shown in FIG. 1, the method for producing low-carbon ferrochrome is a first step (hereinafter referred to as primary slag) in which a mixture of chromium ore and quicklime as a medium solvent is dissolved in an electric furnace to produce a dissolved raw material (hereinafter referred to as primary slag). S1) is provided.

As shown in FIG. 2, chrome ore and quicklime are stored in the hopper 4. Chromium ore and quicklime are charged into the electric furnace 1 and melted in the electric furnace 1. The primary slag is discharged from the hot water outlet 1a of the electric furnace 1 to the reaction vessel 2.

Next, as shown in FIG. 1, the reaction vessel 2 from which the primary slag has been discharged is charged with the auxiliary raw materials silicochrome, chasing chromium ore, and recovered silicochrome as reducing agents. Then, the reaction vessel 2 is stirred by bottom-blowing an inert gas to reduce the oxide in the chromium ore to generate low-carbon ferrochrome and secondary slag (S2). This reduction step is the second step. The recovered silicochrome is silicochrome recovered in the third step described later. It should be noted that two pans may be used instead of the reaction vessel 2 for which the inert gas is bottom-blown, and stirring may be performed by relaying between the two pans.

The molten low-carbon ferrochrome produced by the reduction reaction is cast into a mold to become a product. On the other hand, the secondary slag produced by the reduction reaction is separated from the molten metal of low carbon ferrochrome and then discharged to the electric furnace 3 (see FIG. 2). Instead of the electric furnace 3, a reaction vessel having a gas bottom blowing device for blowing an inert gas may be used.

Next, ferrosilicon as a reducing agent is charged into the electric furnace 3 in which the secondary slag is charged, and the ferrosilicon as a reducing agent is reacted with the chromium oxide remaining in the secondary slag to recover the silicochrome and the tertiary slag (rejected). Slag) is generated (S3). The recovered siliconochrome is charged into the reaction vessel 2 together with the auxiliary material siliconochrome. Since a part of the auxiliary raw material silicochrome is replaced by the recovered silicochrome, the auxiliary raw material silicochrome is reduced.

It should be noted that the low carbon ferrochrome may be produced only in the first step and the second step without providing the third step.
(Self-firing electrode)

The self-firing electrode according to the embodiment of the present invention will be described. FIG. 3 is a vertical cross-sectional view of the self-firing electrode 10 of the present embodiment. The self-firing electrode 10 of the present embodiment is used as the electrode 6 of the electric furnace 1 or the electrode 5 of the electric furnace 3.

The self-firing electrode 10 includes a tubular case 11 and an electrode paste 12 charged into the case 11. The self-firing electrode 10 is formed by charging the electrode paste 12 into the case 11 and firing the electrode paste 12 by the Joule heat of the current flowing through the electrode holder 13 and the conduction heat from the electric furnace 1. The electrode paste 12 changes in this order from the top, in the order of the electrode paste 12, the molten paste 14 in which the electrode paste 12 is melted, and the solidified electrode 15 in which the molten paste 14 is solidified. Reference numeral 16 denotes a current-supplying conductive copper pipe connected to the

electrode holder

13, 17 is a water-cooled jacket for protecting the

case

11, and 18 is a cylinder of a pushing device.

During the operation of the electric furnace 1, the solidifying electrode 15 and the case 11 are consumed from the tip thereof. The case 11 is pushed down by the pushing device according to the wear of the solidifying electrode 15 and the case 11. At this time, the electrode holder 13 is kept in a fixed position by the cylinder 18 of the pushing device, and the case 11 slides downward in the electrode holder 13. A new case 11 is added to the upper part of the case 11 in response to the pushing down of the case 11. Since the pushing down of the case 11 and the addition of the case 11 by the pushing down device are known techniques, further description thereof will be omitted.

FIG. 4 shows a horizontal sectional view of the case 11. The case 11 includes a tubular main body 11a. Case current density, 100A / cm ² or more 400A / cm ² or less, desirably set to 150A / cm ² or more 400A / cm ² or less. Here, the case current density is represented by (current supplied to the self-firing electrode 10) / (cross-sectional area of the case 11). Sectional area of the case 11 is represented by ^{π × D 2/4-π} × (D-2t) 2/4. D is the outer diameter of the case 11, and t is the thickness of the case 11. The cross-sectional area of the case 11 is the cross-sectional area of only the main body 11a, and does not include the cross-sectional area of the rib 11b.

According to the self-firing electrode 10 of the present embodiment, the case current density is ^{lowered to 400 A / cm 2} or less, so that the case 11 under the electrode holder 13 is suppressed from being blown (in other words, consumed), and the solidified electrode is formed by the case 11. 15 can be held. Therefore, it is possible to prevent the solidified electrode 15 from being hollowed out. If the case current density is ^{less than 100 A / cm 2} , the electrode paste 12 will be insufficiently fired. Therefore, even if the case 11 under the electrode holder 13 is suppressed from being blown, the solidified electrode 15 may be hollowed out.

Electrode current density of the self-baking electrode 10 is set to 4.0A / cm ² or more 8.0A / cm ² or less. The electrode current density is represented by (current supplied to the self-firing electrode 10) / (area of the self-firing electrode 10). The area of self-baking electrode 10 is expressed by π × ^D 2/4. D is as described above.

If the electrode current density is ^{less than 4.0 A / cm 2} , the firing may be insufficient and the solidified electrode 15 may be hollowed out. If the electrode current density ^{exceeds 8.0 A / cm 2} , over-baking may occur and center breakage (a phenomenon in which the solidified electrode 15 breaks) may occur.

The thickness t of the case 11 of the self-firing electrode 10 of the present embodiment is set to be larger than 2.0 mm and 5 mm or less, preferably 2.5 mm or more and 4.5 mm or less, and more preferably 3.0 mm or more and 4.5 mm or less. .. Here, the thickness t of the case 11 is the thickness t of the main body 11a.

The thickness of the case of the self-firing electrode of the conventional electric furnace for ferroalloy production is set to 0.8 mm or more and 2.0 mm or less. According to the self-firing electrode 10 of the present embodiment, since the thickness of the case 11 is made thicker than 2.0 mm, the case 11 under the electrode holder 13 is more suppressed from being blown (in other words, consumed) and solidified by the case 11. The electrode 15 can be held. Therefore, it is possible to prevent the solidified electrode 15 from being hollowed out. If the thickness of the case 11 is too thick, the current distribution of the case 11 (the current distributed to the case 11 among the currents supplied from the electrode holder 13) becomes large, and the current distribution of the electrode paste 12 (supplied from the electrode holder 13). Of the current generated, the current distributed to the electrode paste 12) becomes smaller, making it difficult for the electrode paste 12 to be fired. Therefore, the thickness of the case 11 is set to 4.5 mm or less.

On the inner surface of the

case

11, 4 or more and 8 or less ribs 11b are provided at equal intervals in the circumferential direction. The ribs 11b have a role of helping the electrode paste 12 to be fired and a role of preventing the electrode paste 12 and the solidified electrode 15 from being hollowed out. The width B / electrode radius (D / 2) of the rib 11b is set to 0.1 or more and 0.8 or less, preferably 0.3 or more and 0.5 or less. The thickness of the rib 11b is set to 3.0 mm or more and 4.5 mm or less.

If the width B of the rib 11b is wide, cracks are likely to occur from the portion of the solidifying electrode 15 where the rib 11b has disappeared, which causes troubles such as chipping of the solidifying electrode 15. On the other hand, if the width B of the rib 11b is narrow, the current flowing into the electrode holder 13 may be concentrated in the case 11 and the case 11 may be melted due to an overcurrent. Therefore, the width B / electrode radius (D / 2) of the rib 11b is set to 0.1 or more and 0.8 or less.

When the number of ribs 11b is large, the current flowing through the ribs 11b increases due to the decrease in the electrical resistance of the ribs 11b, which has the effect of lowering the firing zone. On the other hand, the reduction of the total contact resistance between the rib 11b and the carbon makes it easier for the current to flow into the carbon, which also has the effect of raising the firing zone. When both of these are balanced, the firing zone is stable without change. The optimum number of ribs 11b is 4 or more and 8 or less.
(Electrode paste for self-firing electrode)

The electrode paste 12 is produced by mixing a carbonaceous binder such as artificial graphite, calcined charcoal, pitch coke and a binder. The electrode paste 12 is fired while descending inside the case 11. The electrode paste 12 is melted by Joule heat of an electric current and conduction heat from the electric furnace 1, and volatile components are removed to become a solidified electrode 15.

Calcined charcoal is smokeless charcoal heat-treated at high temperature by electric roasting. Anthracite is a natural, highly carbonized coal. By heat-treating anthracite, calcined charcoal with good conductivity and excellent corrosion resistance can be obtained. The calcined charcoal becomes the base of the aggregate raw material for electrodes.

Artificial graphite is artificially produced graphite compared to natural coal such as anthracite. Artificial graphite is obtained by adding pitch and tar to coke and graphitizing them at a high temperature of, for example, about 3000 ° C. Artificial graphite has a high thermal conductivity. By blending a large amount of artificial graphite with the electrode paste 12, the thermal conductivity of the solidified electrode 15 is increased. In the present embodiment, the content of artificial graphite in the electrode paste 12 is 30 so that the electrical resistivity of the solidified electrode 15 is 30 μΩm or more and 80 μΩm or less and the thermal conductivity of the solidified electrode 15 is 8.0 W / m · K or more. Set to mass% or more and 70 mass% or less.

Artificial graphite is generally produced by adding a binder to pitch coke and / or oil coke and performing a heat treatment (for example, a heat treatment at about 3000 ° C.). As described above, the heat source for firing the electrode paste 12 is Joule heat of the current flowing through the electrode paste 12 and conduction heat from the electric furnace 1. When the content of artificial graphite is increased, the resistivity heat generation becomes small because the resistivity of the artificial graphite itself is small, and it becomes difficult to generate heat due to Joule heat. However, since the thermal conductivity is high, the conduction heat from the electric furnace 1 is easily transferred to the electrode paste 12, the difference in thermal expansion of each part in the self-firing electrode 10 is small, and the solidified electrode 15 is less likely to be damaged. Become. In addition, the thermal shock resistance is improved when the operation has a large load fluctuation. Considering the balance between electrical resistivity, thermal conductivity, and thermal shock resistance, the content of artificial graphite is set to 30% by mass or more and 70% by mass or less, preferably 40% by mass or more and 60% by mass or less.

When the content of artificial graphite is reduced to less than 30% by mass, the thermal shock resistance is lowered, and the solidified electrode 15 is broken and cracked due to the heat shock in the batch operation in the low carbon ferrochrome production method. When the content of artificial graphite is more than 70% by mass, the electrical resistivity of the solidified electrode 15 becomes small, and the low current operation in the low carbon ferrochrome production method is combined, so that insufficient firing occurs.

Pitch coke has a good affinity with the binder and enhances the mechanical strength of the solidified electrode 15 after firing. Pitch coke is a raw material having good conductive properties because it is easily graphitized.

The amount of binder to be blended is determined by the type of raw material used and the particle size of the aggregate. The blending amount of the binder is determined in consideration of the fluidity of the electrode paste 12 at the time of melting and the supply of the binder to the solidified electrode 15.

During the operation of the electric furnace 1, a force acts on the solidifying electrode 15 from the charged raw material. Therefore, the solidified electrode 15 also needs to have a high Young's modulus. Table 1 shows the characteristic value of the solidified electrode 15 and a compounding example of the electrode paste 12 for obtaining this characteristic value.

(Example 1 of self-firing electrode)
Low carbon ferrochrome was produced according to the production process diagram of FIG. A self-firing electrode was used as the electrode of the electric furnace. Table 2 shows the specifications of the self-firing electrode.

In Examples 1 and 2 of the present invention in which both the case current density and the electrode current density satisfy the scope of the present invention, troubles such as hollowing out and bending of the self-firing electrode do not occur, and stable operation of the electric furnace can be performed. I was able to do it. In the comparative example in which both the case current density and the electrode current density are out of the range of the present invention, since the case current density is large, the case is blown and the self-firing electrode is hollowed out. Moreover, since the electrode current density is small, the firing of the electrode paste is also insufficient.

(Example 2 of self-firing electrode)
The effect of the case current density of the self-firing electrode on the fusing of the case and the firing of the electrode paste was tested. ^{As shown in Table 3, when the case current density exceeds 400 A / cm 2} regardless of the electrode diameters of 60 cm, 70 cm, and 80 cm, the case is blown. On the other hand, when the case current density was 400 A / cm ² or less, the case did not melt. However, when the case current density ^{approached 100 A / cm 2} , the firing of the electrode paste was slightly insufficient.

As shown in Table 3, when the case thickness is 2.0 mm, the case current density tends to exceed ^{400 A / cm 2.} When the case thickness is 2.5 mm or more, the case current density ^{tends to be 400 A / cm 2} or less. When the case thickness is 5 mm or more, the case current density approaches ^{100 A / cm 2, especially when the electrode diameter is 80 cm.}

FIG. 5 is a graph showing the relationship between the case thickness and the case current density in Table 3. As shown in FIG. 5, there is a negative correlation between the case thickness and the case current density.

(Example 3 of self-firing electrode)
The effect of the electrode current density of the self-firing electrode on the firing of the electrode paste was tested. As shown in Table 4, when the electrode current density was ^{less than 4.0 A / cm 2} , the firing of the electrode paste was insufficient, and hollowing out occurred in the self-burning electrode. When the electrode current density was 4.0 A / cm ² or more and 8.0 A / cm ² or less, the electrode paste was fired well, and no hollowing out occurred in the self-fired electrode. When the electrode current density ^{exceeded 8.0 A / cm 2} , the electrode paste was over-fired, and the self-fired electrode was broken.

(Example of electrode paste for self-firing electrode)
The electrode paste was placed in the case of the self-firing electrode and fired. Table 5 shows the specifications of the electrode paste.

In the example of the present invention in which the content of artificial graphite, the resistivity, and the thermal conductivity satisfy the range of the present invention, the electrode paste could be sufficiently fired. The self-firing electrode did not have any troubles such as hollowing out and breaking, and the electric furnace could be operated stably. In the comparative example in which the content of artificial graphite and the thermal conductivity are out of the range of the present invention, the content of artificial graphite in the electrode paste is low and the thermal conductivity is also low, so that the electrode paste is insufficiently fired. In addition, hollowing out occurred in the self-firing electrode.

This specification is based on Japanese Patent Application No. 2019-162551 filed on September 6, 2019. All this content is included here.

10 ... Self-firing electrode 11 ... Case 11b ... Rib 12 ... Electrode paste

Claims

A self-firing electrode formed by filling a tubular case with an electrode paste and firing and solidifying the electrode paste by Joule heat generated by energization and conduction heat from an electric furnace.
Set the case current density 100A / cm 2 or more 400A / cm 2 or less,
A self-firing electrode using the self-firing electrode as an electrode of an electric furnace for producing low-carbon ferrochrome.
Self baking electrode according to claim 1, characterized in that setting the electrode current density of the self-baking electrode to 4.0A / cm 2 or more 8.0A / cm 2 or less.
The self-firing electrode according to claim 1 or 2, wherein the thickness of the case is larger than 2.0 mm and 5.0 mm or less.
The self-firing electrode according to any one of claims 1 to 3, wherein the rib width / electrode radius of the self-firing electrode is 0.1 or more and 0.8 or less.
An electrode paste for self-firing electrodes of electric furnaces for producing low-carbon ferrochrome.
The content of artificial graphite in the electrode paste is 30 so that the electrical specific resistance of the solidified electrode obtained by firing the electrode paste is 30 μΩm or more and 80 μΩm or less, and the thermal conductivity of the solidified electrode is 8.0 W / m · K or more. An electrode paste set to mass% or more and 70 mass% or less.