US3840688A

US3840688A - Electric furnace having specifically located tap hole

Info

Publication number: US3840688A
Application number: US00454196A
Authority: US
Inventors: S Tomioka
Original assignee: Japan Metals and Chemical Co Ltd
Current assignee: Japan Metals and Chemical Co Ltd
Priority date: 1973-02-28
Filing date: 1974-03-25
Publication date: 1974-10-08
Anticipated expiration: 1991-10-08

Abstract

A method and an electric furnace for making ferro alloy, in which molten product metal and molten slag are simultaneously removed through a common tap hole bored on a wall of the furnace at a position above the level of a molten product layer in the furnace. The spacing between the tap hole and the top level of the molten product metal layer is related to the weight of the starting materials in the furnace. The molten product metal layer protects the bottom wall of the furnace against corrosion.

Description

United States Patent [191 [11] 3,840,688 Tomioka [45] 74 {541 ELECTRIC FURNACE HAVING 3,588,309 6/1971 Yasukawa..: 13/9 x SPECIFICALLY LOCATED TAP HOLE [75] Inventor: Shigenori Tomioka, Tokyo, Japan [73] Assignee: Japan Metals & Chemicals Co.,

. v Ltd., Tokyo, Japan [22] Filed: Mar. 25, 1974 [21] Appl. No.: 454,196

Related US. Application Data [62] Division of Ser. No. 336,855, Feb. 28, 1973.

[52] US. Cl. 13/9 [51] Int. Cl...... F27b 14/00 [58] Field of Search 13/9, 35, 33 [56] References Cited UNITED STATES PATENTS 3,465,085 9/1969 Yonemochi 13/9 X Primary Examiner-R. N. Envall, Attorney, Agent, or Firm-Ladas, Parry, Von Gehr, Goldsmith & Deschamps [57] ABSTRACT A method and an electric furnace for making ferro alloy, in which molten product metal and molten slag are simultaneously removed through a common tap hole bored on a wall of the furnace at a position above the level of a molten product layer in the furnace. The spacing between the tap hole and the top level of the molten product metal layer is related to the weight of the starting materials in the furnace. The molten product metal layer protects the bottom wall of the furnace against corrosion.

6 Claims, -2 Drawing F igure's PATEN] EU OCT 81374 ELECTRIC FURNACE HAVING SPECIFICALLY LOCATED TAP HOLE This is a division of application Ser. No. 336,855, filed Feb. 28, 1973.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of making ferro alloy by maximizing slag extraction at the time of tapping the ferro alloy from a furnace while leaving such an amount of metallic ferro alloy in the furnace which is suitable for protection of the furnace bottom wall. This invention also relates to an electric furnace which is particularly suitable for carrying out the aforesaid method of making ferro alloy.

2. Description of the Prior Art Conventionally, ferro alloy is made by using an electric furnace, e;g., an Heroult arc furnace, in which ores and carboneous reducing agents are placed. One or more slagmaking agents are sometimes added in the furnace. The product metal is removed from the furnace either together with slag through a common tap hole, or the product metal and slag are separately removed from the furnace through different holes.

For instance, US. Pat. No. 3,167,420, that was issued to Alfred Gordon on Jan. 26, 1965, teaches a process in which product metal and slag are removed through a common tap hole. Such a process, however, has a shortcoming in that, if entire molten metal is removed from the furnace together with the slag without 2 allowing operators at both the tap hole and the slagremoving hole; and that the operators must be trained for operations at the opposite sides of the furnace. The process of the Canadian Patent No. 865,816 also has shortcoming in that the tip of electrodes are located at comparatively shallow positions and cause large heat dissipation, that fresh slags which are left after removal of molten slag through the slag-removing hole tends to retard the reaction of forming the ferro alloy, and that the stability of the operation is poor.

Therefore, an object of the present invention is to mitigate the aforesaid difficulties of the conventional process and furnaces of making ferro alloy, by providleaving anymelt in the furnace, the variation of thermal energy in the furnace will become excessively large and there will be caused considerable corrosion of the furnace bottom wall. As a result, the electricpower consumptionper unit amount of the metal product increases, and the refining efficiency becomes low. The yield of the process also becomes low. To mitigate such shortcoming, it has'been proposed to dispose the tap hole at a position which is located above the level of the molten metal product, but such modification is not satisfactory, too. More particularly, even when the tap hole is located at a comparatively high position, the level of the molten metal product varies during the refining process and uniform tapping rate of the molten product metal cannot be achieved. Thus, the operation of the furnace may become instable. Furthermore, when the surface level of the molten product metal varies rapidly, the furnace bottom wall may be exposed to materials other than the molten product metal which tend to corrode the. furnace bottom wall.

It has been proposed to provide a slag-removing hole .in a refining furnace which is separated from a tap hole for removing molten product metal, as shown in Canadian Pat. No. 865,816 that was issued to Jutaro Yonemochi on Mar. 9, 1971. In this case, a special techniquefor sealing the metal tap hole'is required, because the metal tap hole is required to bear load due to the weight of slag in the furnace. The special sealing technique may include the use of a special sealing paste and complicated operation of the furnace requiring extra training of operators. The application of the paste to the metal tap hole needs special care, in order to ensure the safety of the operators. The slagremoving hole is usually located at a diametrically opposite position to the metal tap position,so that various precautions must be taken: namely, that extra floor space is necessary for ing an improved method and a furnace therefor.

SUMMARY OF THE INVENTOIN bored through the furnace side wall at a position which is above the level of the top surface of the residual molten metal pool. The vertical distan between the top sur face level of the residual molten metal pool and the tap hole is such that a metal extruding pressure corresponding to the weight of the starting materials at the core portion of the furnace is applied to the tap hole.

According to the present invention, there is provided a method of making ferro alloy by using a refining electric furnace, comprising steps of loading the furnace -with a selected amount of solid mixture consisting of starting material ore and solid reducing agents; forming a molten metal layer at the bottom of the furnace while forming different layers on the molten metal layer, which different layers include a molten slag layer, a slag-soaked coke layer, a half-molten starting material layer, and a layer of mixture of solid starting materials; and removing the molten metal product together with as much slag as possible through a tap hole bored through the side wall of the furnace, said tap hole being located at a position-above a lowest allowable top surface level H, of the molten metal layer, asmeasured from the inner bottom surface of the furnace, bya distance H which corresponds to an extruding pressure applied to the molten metal layer by the weight of the starting materials for removal from the furnace, whereby the ferro alloy production is continuously carried out while protecting the bottom surface of the furnace with the molten metal layer.

The present invention also relates to a refining electric furnace comprising a vessel having side and bottom walls and an inlet opening through which starting materials and carboneous reducing agents'are placed in the furnace, electrodes movably carried by a support so as to be selectively dipped in the starting materials placed in the vessel, and a tap hole bored through the side wall of the housing at a height H (cm) from inner surface of the bottom wall of the vessel, the electrodes being adapted to provide electric currents through the starting material in the furnace so as to form a mixed solid starting material layer of mean thickness H (cm), a half-molten starting material layer of mean thickness H (cm), a molten starting material layer, a carboneous of the furnace, said height H of the tap hole satisfying the following relations,

H, P,/s X 1000 here,

P an extruding pressure being applied to the molten metal,

H maximum head (cm) applicable to the molten metal layer for removal thereof from the vessel through the tap hole,

H minimum height (cm) of top level of molten product metal layer to be formed on the vessel bottom wall, as measured from the inner surface of the bottom wall,

Sg,: mean apparent density (g/cm of the mixed solid starting material layer,

Sg mean apparent density (g/cm of the halfmolten starting material layer,

Sg apparent density (g/cm) of the molten product metal, and

K a constant.

With the method and the furnace, according to the present invention, operation of making ferro alloy can be carried out continuously for an extended period of time, while protecting bottom wall of the furnace against corrosion. It has been found that the operation of the furnace of the present invention is very stable. At the end of the molten metal removal, a certain amount of molten metal is left at the bottom of the vessel for providing a molten metal pool, so that the temperature variation between before and after the molten metal removal is minimized, as compared with the complete removal of the entire molten metal. The comparatively stable temperature and the removal of the slag together with the molten metal result in a high yield of the ferro alloy per unit refining time. With the tap hole disposed at the aforesaid position relative to the bottom of the vessel, the head of the molten metal at the tap hole becomes negligible at the end of the tapping operation so that the tape holecan easily be sealed, without causing any danger to the operator. The distribution and balance of the different layers in the furnace are improved with the tap hole thus disposed, whereby the molten metal can be tapped substantially at a uniform rate.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the invention, reference is made to the accompanying drawing, in which:

FIG. 1 is a schematic vertical sectional view of a conventional refining furnace, illustrating the manner in which starting materials are refined therein; and

FIG. 2 is a schematic vertical sectional view of a refining electric furnace, according to the present invention, shown in operating conditions.

Like parts are designated by like numerals and symbols throughout the different figures of the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT When ferro alloy is made by using an electric refining furnace, it is believed that different starting materials, such as semi-refined layers, molten metal layers, and molten slag layers, are distributed as shown in FIG. 1. The distribution of FIG. 1 is assumed, based on operating experiences and inspection of the furnace at the time of maintenance work.

More particularly, a molten metal layer 1 and a molten slag layer 2 are so formed in an electric furnace 20 that the two layers lie horizontally at the bottom portion of the furnace, with the metal layer placed at a lower position due to the difference of the density. Three further different layers float on the molten slag layer 2; namely, a coke bed 3 which is soaked with molten slag, a half-molten starting material layer 4, and a solid layer of starting material mixture. It has been believed that the solid layer 5 of the starting material mixture is gradually heated, dehydrated, calcined, and preliminarily reduced. The lower portion of the solid layer 5 is gradually melted to form the half-molten layer 4, which is then reduced when coming in contact with the coke bed 3, so that it is divided into a molten slag layer 2 and a molten metal layer 1. As a tap hole 6 is opened, both the molten metal and the molten slag are discharged through the tap hole 6, until the top surface level of the molten metal layer 1 coincides with the level of the tap hole 6 at the end of the discharge. If a large extruding or discharging pressure should be applied to the molten metal and slag layers 1 and 2, those layers would be completely discharged, so that the inner surface of the bottom wall would be exposed to corrosive substances and the risk of corrosion of the bottom wall is increased.

On the other hand, the inventor has carried out a series of studies on the dynamics of materials in the furnace during the refining operation, and he has confirmed the result of such studies by inspection of a refining furnace which had been used continuously for more than one year. Based on such result, the inventor has worked out a new method of making ferro alloy and new dimensions for an electric refining furnace. Conventional practice of selecting different dimensions of a refining electric furnace is based on a theory which is developed from planar studies of the furnace, such as studies by Morcammer and Kelley. The conventional planar studies do not pay due attentions to more dynamic factors of the electric furnace: for instance, the relation between furnace depth and tap hole location, and the effect of molten metal layer. The inventor has carried out studies on the dynamic operation of a refining electric furnace, and designed and prepared a new refining electric furnace based on the findings of such studies. The new refining electric furnace thus prepared proved to be able to provide molten ferro alloy at a substantially uniform rate with a high yield. The yield of a process using the furnace of the present invention is improved, because the amount of nonreduced components in the slag is reduced. The uniformity of the delivery of the molten ferro alloy is particularly suitable for continuous operation of such furnace.

FIG. 2 is a conceptual view of inside conditions of an electric refining furnace according to the present invention during a process of making ferro alloy. A furnace 20 comprises side walls 9 and a bottom wall 8. A mixed solid starting material layer 5 is formed in the furnace 20 by charging it through an inlet opening 13. Solidified layers 7 of starting materials may be deposmetal layer 1 which are located vertically below elec-.

trodes 10. The electrodes are movably carried by a support 12. Slag-soaked coke layers 3 are formed on the molten slag layers 2, so as to cover the latter. In the embodiment of FIG. 2, the central portion of the molten metal layer 1 carries neither one of the molten slag layer 2 and the coke layer 3. Thus, the mixed layer 11 at the central portion of the molten metal layer 1 is covered with a half-molten starting material layer 4, which layer 4 also covers the molten slag layers 3. A comparatively thick solid starting material layer 5 spreads over the entire span of the halfmolten starting material layer 4. In FIG. 2, the effective thickness of the solid starting material layer 5 at the furnace core and the effective thickness of the half-molten starting material layer 4 at the furnace core are designated by H, (cm) and H (cm), respectively. Thus, the surface of the molten metal layer 1 is loaded with the pressure caused by the solid starting material layer 5 of the thickness H, and the half-molten starting material layer 4 of the. thickness H irrespectively of whether it is directly beneath the furnace core or beneath the electrodes 10. As a result, the top surface of the molten metal layer 1 becomes flat, and the flatness of the molten metal layer 1 is confirmed by calculation based on measured loads and inspection of the inner surface of the furnace at the time of its maintenance.

The inventor has conducted analytical studies on the dynamics of long-term operations of different kinds of electric furnaces for making ferro alloy, which furnaces were rated from 2,500 KW to 22,000 KW. As a result, the following conclusion was derived.

I The pressure P (Kg/c'rn which is applied to thetop surface of the molten metal layer 1 by the weight of the starting materials, is given by the following equation here,

1 Sg,: mean apparent density of the mixed solid starting material layer 5 (g/cm),

SG mean apparent density of the half-molten starting material layer 4 (g/cm),

I-I,: mean thickness of the mixed solid starting material layer 5 at the furnace core (cm), and

H mean thickness of the half-molten starting material layer 4 at the furnace core (cm).

If the height of the top level of the molten product metal layer 1 as measured from the inner surface of the bottom wall 8 at the end of tapping, which height may also be referred to as a minimum height of the top level of the molten product metal layer 1, is represented by H, (cm), a maximum head H (cm) applicable to the molten product metal layer 1 for removal thereof through the tap hole 6 can be given by the following equation (2). r

here, Sg represents apparent density of the molten product.

The maximum head H means that, in dynamic operation of the furnace 20, a molten product metal layer of thickness of up to (H H.,) as measured from the inner surface of the bottom wall 8 can be removed through the tap hole 8.

Referring to FIG. 2, H, represents the height of the top surface of the molten product layer l from the inner surface of the bottom wall 8 at the beginning of a tapping operation, and H represents distance between the tap hole 6 and the top surface of the layer 1 at the beginning of the tapping operation.

The thicknesses H and H' 'of the starting

material layers

5 and 4 may somewhat vary depending on the operating conditions of the furnace and the melting points and sizes of the starting materials, but the inspection of the furnace inside at the maintenance work provded that such thicknesses are within the following ranges.

Furnace capacity Hz/(Hf z) Less than 5.000 Kw 30 35% 5,000 m 10,000 KW 27 32% Above 10.000 KW 25 30% here, K is a proportionality constant.

Based on operating experiences and the inspection of the inside of the furnace, the inventor has found out that the value of the constant K of the equation (3) should be 30 to 120, preferably 35 to '90. When the constant K of 30 to is used, the amount of the prod uct metal in the layer 1 corresponds to a product of refining operation for 6 to 24 hours. When the constant K is less than 30, the bottom wall 8 of the furnace 20 may be corroded, and the starting material layers may be lowered too far and the tapping through the hole 6 may be hampered thereby. On the other hand, if the constant K is greater than 120, solidified phases of the starting material and product metal may be formed at the central portion of the bottomwall 8 of the furnace 20. Such solidified phases of the starting materials and product metal also tend to hamper the tapping of the molten product metal through the tap hole 6. Accordingly, the constant K should be in the range of 30 to 1 20. I

In selecting the value of K, due care should be taken so as to provide relatively constant values of the minimore such tap holes may be bored through the side wall 9 of the furnace, so as to stabilize the refining operation in the furnace.

The invention will now be described in further detail by referring to examples.

EXAMPLE 1 Starting materials:

Manganese ore (containing 43.8% of 100 Kg manganese) Coke Kg Lime stone 6 Kg Qperating conditions:

Secondary voltage v 135 V Secondary current 60,000 to 65,000 A During the operation, it was found that H 340 cm, H 140 cm, Sg 1.4 (g/cm, dry), and Sg 2.1 (g/cm). The dimensions H H and the mean densities Sg Sg were as defined in FIG. 2 and the equations (1) and (2). Accordingly, the pressure P caused by the starting materials will be given by P (Sg, H, Sg z)/1,00O (1.4 X 340 2.1 X 140)/l,000 0.770 (Kg/cm The minimum thickness H of the molten product metal layer and the height H of the tap hole 6 were designed to be 35 cm and 160 cm, as measured from the inner surface of the bottom wall of the furnace, respectively. The mean density of the molten product metal was 6.0 (g/cm).

After one year of continuous operation with the electric furnace, no corrosion of the furnace bottom wall was found, and no solidifed metalswas found at the central portion of the inner surface of the furnace bottom wall. The molten product metal and the molten slag were obtained substantially at constant rates. The operation proved to be stable. The electric power consumption per unit weight of the product was 2,060 KWH/ton, and the yield of manganese was 80.5 percent, which were better than those with conventional furnaces.

The chemical composition of the product obtained was as follows.

Manganese Carbon Silicon Iron 74.2% 7.13% 0.35% Balance Power consumption ratYper tQFoFprEHuct Yield of Manganese Thus, the performance of the furnace with the design according to the presentinvention proved to be better than that of conventional design. Furthermore, it was found at the time of maintenance inspection that the furnace bottom wall was corroded by 40 cm after the latter reference operation. The reference operation was not stable.

EXAMPLE 2 A half-sealed three-phase electric furnace of 9,500 KW of Heroult type was prepared, in accordance with the principles of the present invention. Silicomanganese according to the stipulations of JIS (Japanese Industrial Standard Class 2 was prepared by using the electric furnace, with the following starting materials and operating conditions.

Starting materials:

Manganese ore (containing 30.3% of 100 Kg manganese) Manganese slag 150 Kg Silica 50 Kg Coke 43 Kg Operating conditions:

Secondary voltage 135 V Secondary current 50,000 to 52,000 A During the operation, it was found that H 175 cm, H cm, Sg 1.35 (g/cm, dry), Sg 2.0 g/cm and P, 0.386 (Kglcm The dimensions H,, I-1 and the mean densities Sg,, Sg were as defined in FIG. 2 and the equations (1) and (2).

The minimum thickness H, of the molten product metal layer and the height H of the tap hole 6 were designed to be 20 cm and cm, as measured from the inner surface of the bottom wall of the furnace, respectively. The mean density of the molten product metal was 5.9 (g/cm").

After two years of continuous operation with the electric furnace of Example 2, the following performance characteristics were obtained.

Electric power consumption rate Yield of manganese Composition of the product 3,510 KWH/ton 86.3%

The operation of the furnace of Example 2 proved to be stable, and it was generally better than that of conventional furnaces.

For comparison, the furnace of Example 2 was operated with the tap hole height H cm, for half a year prior to the operation of the design of the present invention. The performance during this half a year was as follows: namely, the electric power consumption rate per unit weight of the product was 3,740 KWH/ton; the manganese yield was 80.4 percent; the furnace bottom was raised and the excessive and insufficient outputs of the molten product metal were experienced in the first 2 to 3 months after the start of the operation and it was stabilized only after modifying the at a height H (gi r minnensur al g th b ttgm wall of the vessel, the electrodes being adapted to provide electric currents through the starting materials in the furnace so as to form a mixed solid starting material layer of mean thickness H, (cm), a half-molten starting material layer of mean thickness H (cm), a molten starting material layer, a carboneous material layer, and a molten product metal layer in the furnace in said order from the upper portion thereof, said mean thicknesses being taken at a central portion of the furnace,

said height H of the tap hole satisfying the following conditions,

H l-l H P (Sgyl-l Sg 'H2)/1,00O H (P /Sg X 1000 H K-P here,

P extruding pressure being applied to the molten metal,

H maximum head (cm) applicable to the molten product metal layer for removal thereof from the vessel through the tap hole,

l-l minimum height (cm) of top level of molten product metal layer to be formed on the vessel bottom wall, as measured from the inner surface of the bottom wall,

Sg mean apparent density (g/cm) of the mixed solid starting material layer,

Sg mean apparent density (g/cm) of the halfmolten starting material layer,

Sg apparent density (g/cm of the molten product metal, and

K a constant.

2. A refining electric furnace according to claim 1,

wherein said constant K is 30 to 120.

3. A refining electric furnace according to claim 1, wherein said constant K is 35 to 90.

4. A refining electric furnace according to claim 1,

wherein H /(H H is 0.30 to 0.35.

5. A refining electric furnace according to claim 1,

wherein H,j(H H is 0.27 to 0.32.

6. A refining electric furnace according to claim 1, wherein H /(H,+ H is 0.25 to 0.30.

lJNITED STATESBATENT OFFICE C ERTIFICATEVOF CORRECTION Pate $840588 Dated ctober-8, 1974- Invent It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the cover sheet, left column, below 211 A p- .;Nod 454,196"

should be added-*- -[3 0] Foreign Application Priority Data Maroh 6-, 1972 Japan. ..1.... n;..'...22213 72--*.

Signed and sealed this 21st day of January 1975.

ISEAL) Attest:

MCCOY M. GIBSON; JR. Attesting Officer C. MARSHALL DANN Commissioner of Patents STATES PATENT OFFICE. (LERTIFICATE' OF CORRECTION I D g October -8', 1974' Patent No. ,3

Inventor) Shigenori Tomioka 1' appears in the above-itlehtified patent It is certified that erro cted as shown below:

and that said Letters Patent are hereby corre On the cover sheet, left column, below [21] App'lyNo. 454,196"

should be added-5'- --[30] Foreign Application Priority Data' Match 6, 1972 Japan' ..;.".-..'...22213/72--'4 Signed and sealed this 21st day of January 1975.

ISEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents

Claims

1. A refining electric furnace comprising a vessel having side and bottom walls and an inlet opening through which starting materials and carboneous reducing agents are placed in the furnace, electrodes movably carried by a support so as to be selectively dipped in the starting materials placed in the vessel, and a tap hole bored through the side wall of the housing at a height H (cm) from inner surface of the bottom wall of the vessel the electrodes being adapted to provide electric currents through the starting materials in the furnace so as to form a mixed solid starting material layer of mean thickness H1 (cm), a half-molten starting material layer of mean thickness H2 (cm), a molten starting material layer, a carboneous material layer, and a molten product metal layer in the furnace in said order from the upper portion thereof, said mean thicknesses being taken at a central portion of the furnace, said height H of the tap hole satisfying the following conditions, H H3 + H4 P1 (Sg1.H1 + Sg2.H2)/1,000 H3 (P1/Sg3) X 1000 H4 K.P1 here, P1 : extruding pressure being applied to the molten metal, H3 : maximum head (cm) applicable to the molten product metal layer for removal thereof from the vessel through the tap hole, H4 : minimum height (cm) of top level of molten product metal layer to be formed on the vessel bottom wall, as measured from the inner surface of the bottom wall, Sg1: mean apparent density (g/cm3) of the mixed solid starting material layer, Sg2: mean apparent density (g/cm3) of the half-molten starting material layer, Sg3: apparent density (g/cm3) of the molten product metal, and K : a constant.

2. A refining electric furnace according to claim 1, wherein said constant K is 30 to 120.

4. A refining electric furnace according to claim 1, wherein H2/(H1 + H2) is 0.30 to 0.35.

5. A refining electric furnace according to claim 1, wherein H2/(H1 + H2) is 0.27 to 0.32.

6. A refining electric furnace according to claim 1, wherein H2/(H1+ H2) is 0.25 to 0.30.