CA1142984A

CA1142984A - Arc furnace wall of graphite blocks surrounded by oxide bricks and composite refractory element for constructing said wall

Info

Publication number: CA1142984A
Application number: CA000341758A
Authority: CA
Inventors: Michel Ayme-Jouve; Marc Esnoult
Original assignee: Societe Europeenne des Produits Refractaires SAS
Current assignee: Societe Europeenne des Produits Refractaires SAS
Priority date: 1978-12-15
Filing date: 1979-12-12
Publication date: 1983-03-15
Also published as: DE2963194D1; ES486940A1; MX152576A; FR2444244A1; EP0012681A1; ES247509Y; JPS5582287A; JPS5919272B2; ES247509U; FR2444244B1; EP0012681B1

Abstract

D.9117 IMPROVED METHOD FOR CONSTRUCTING METALLURGICAL
ELECTRIC FURNACES AND A COMPOSITE REFRACTORY
ELEMENT FOR CARRYING OUT THE METHOD.

Michel AYME-JOUVE and Marc ESNOULT
SOCIETE EUROPEENNE DES PRODUITS REFRACTAIRES

ABSTRACT OF THE DISCLOSURE

A method for constructing at least certain areas of the upper part of the refractory wall of an electric arc metallurgical furnace which is not in prolonged contact with the liquid metal bath, according to which blocks of an oxide-based refractory material and graphite blocks are juxtaposed alternately in such a way that each graphite block is surrounded by blocks of the oxide-based refractory material. The invention allows to increase the service life of the furnace wall, the graphite blocks ensuring a cooling effect.

Description

This invention relates to an improved method for constructing metallurgical arc furnaces, particularly -the parts of such furnaces which are not in prolonged con-tact with the liquid metal bath, and also to a composite refractory element used in the method.
For several years, the power of arc furnaces in relation to their capacity, or specific power expressed in nominal KVA power of their transformers per ton of li~uid steel, has been increasing and has often exceeded 500 KVA
per ton (UHP furnace~, the power used sometimes even reaching 750 KW per ton during the melting period. This development has brought about the necessity for new methods of lining the furnace walls.
The mos-t used areas of the linings of arc furnaces (the slag line, the runner and above all, those parts of the wall which are opposite the three electrodes) are of-ten com-posed of ~used cast magnesia-chromium oxide refractory blocks~
Blocks of this type are sold under -the trademark CORHART C~104 by the applicants in Europe and by the company Corhart Refractories in the United States of America.
These elec-tro-melted refractory blocks are made in the form of blocks cut in ingots, the shrinkage of which, upon solidification, is dispersed in the state of a macroporosity having pores with a diameter of 1 to 10 mm, their total porosity being from 16 to 20% with a very low microporosity which, combined with their very high crystalline cohesion, makes them superior to other refractory materials, such as fired basic refractory materials, even based on resintered fused grains.

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The use of these blocks to form the most used areas of the linings of arc furnaces, combined with less superior refractory materials to form the areas which are less used, generally makes it possible to equalize the wear of the linings for a wide range of furnace power and capacity, thus optimizing the productivity and cost of the refractory materials.
When working conditions become too severe and in particular when the specific power used during the melting period exceeds 500 KW per ton, the use of such refractory blocks no longer makes it possible to obtain satisfactory service lives.
In fact, particularly at the "hot points" of the lining, the des-truction o~ fused cast refractory blocks occurs too rapidly.
This destruction results especially from chemical attack at high temperatures by fumes and splashes of slag and from scaling of the working faces under the effect of rapid temperature variations, these two phenomena occurring at varying rates and in varying proportions depending on the method of use of the furnace.
The aim of this invention is to provide an improved method of constructing metallurgical arc furnaces, in particular the upper part o the wall of these furnaces which is not in prolonyed contact with the liquid metal bath~ which provides a significantly extended service life for the noted part by decreasing the working temperature of the refractory material forming this part.
Another aim of this invention is to provide a composite refractory element to be used in carrying out -the method according to the invention.
The invention is based on the discovery that, in constructing at least certain areas of the upper part of the ....

refractory wall of a metallurgical arc furnace which are not in prolonged contact with the li~uid metal bath but which can be in contact for short periods, for example when tiltiny the furnace for pouring, by means o~ alternately arranged blocks of an oxide-based refractory material, which is a relatively bad conductor ofheat, ana blocks of graphite, which is a good conductor of heat, it was possible to considerably reduce the wearing of -the noted areas and thus to extend the life of the refractory wall.
This invention provides a method for constructing at least certain areas of the upper part of the refractory wall of an electric arc ~urnace which is no-t in prolonged contact with the liquid metal bath, involving the use of cooling elements of a material which is a good conductor of electricity, charac-terized in that blocks of an oxide-based refractory material and graphite blocks are juxtaposed alternately in rows, these blocks extending from the internal face to the external face of the refractory wall and the blocks forming each row being offset with respect to the blocks forming -the adjacent rows so that each graphite block is surrounded by blocks of oxide-based refractory material.
According to a preferred method o~ use, lt is ~urthermore provided that at least one sheet of steel is placed between the blocks in a given row and the blocks in the adjacent rows and between at least certain of the blocks in the same row, for example between each pair of adjoining blocks in one row and the pairs of adjacent blocks in the row.
This invention also relates to a composite refractory element suitable for carrying out the preferred method of the invention, which comprises at least one pair of adjoining blocks made up of different refractory materials extending from the inner face to the outer face of the element and which is lined with a sheet of steel on at leas-t two of its adjacent faces extending from the inner face to the outer face, characterized in that one of the pair of blocks is composed of an oxide-based refractory material and the other block is made o~ graphite.
Preferably, the steel sheet covers the four faces of the com-posite element extending from the inner face to the outer face.
The expression "at least one pair of blocks" means that the composite element can comprise more than one pair o~
blocks, for example 2, 3 or 4 pairs.
However, most frequently composite elements comprising one single pair of blocks will be used and, consequently, when composite elements are mentioned in the following description, this means elements of one pair of blocks. The use of composite elements with several pairs of blocks can, however, be advantageous in certain particular cases where the user wishes to put in position panels of relatively large dimensions in one action.
The terms "inner face" and "outer face" re~er, respectively, to the surface of the element facing the interior of the furnace and that which is facing the ex-terior of the furnace.
Composlte refractory elements have been known for a long time for the construction of metallurgical furnaces. ~or example, British Patent Specification No. 925,646 describes composite elemen-ts covered with a metallic sheet, one portion of which is formed by a fired refractory material, for example fired chromium oxide/magnesia, and the other portion of which is ,3~L

formed by a raw refractory material, for example raw magnesia/chromium oxide. Composite elements of a similar type are also described in British Patent Specification No. 901,285.
However, to the knowledge of the applicants, it has never been proposed to use a combination of blocks made of an oxide-based refractory material and graphite blocks.
The invention can be used with all oxide-based refractory materials suitable for the construction of metallur-gical furnaces, in particular those whose wear is caused mainly by cracking and scaling due to temperature variations in the furnace or by corrosion by splashes from the slag and fumes at high temperatures which are present in the atmosphere of the furnaceO The invention is particularly effective with fused cast refractory materials of magnesia/chromium oxide, such as those described in U.S. Patent Specifications Nos. 2,599,566 and 2,690,974. An example of a fused cast refractory material of this type, commercially available in the form of blocks and widely used in the construction of metallurgical furnaces, is CORHART C.l0~, manufactured in the United States of America by CORHART ~EFRACTORIES CO. and, in France, by the SOCIETE
EUROPEENNE DES PRODUITS REFRACTAIRES. However, this invention can also be used with oxide-base~ agglomerated refractory materials, that is, refractory materials obtained by firing at high temperature oxide-based particles moulded in the form of blocks, or sintered oxide-based refractory materials. An example of such a ma-terial is that produced from fused cast refractory grains of the magnesia/chromium oxide type mentioned above. These materials are well known to a specialist and it is not necessary to go into further detail.

The increased service life of the refractory walls of arc furnaces constructed according to the invention and/or with the composite elements of the invention results from two distinct effects:
a) A mutual protection seems to be established between the oxide-based refractory blocks and the graphite blocks, the graphite blocks holdiny back the flakes which form in the oxide-based blocks and in this way delaying their fall into the molten bath. Moreover, owing to the fact that the oxide-based refractory blocks surround each graphite block, the slag splashes form a coating of slag on the surface of the graphite blocks exposed to the atmosphere of the furnace which protects the graphite from the effects of oxidation. It is, in fact, known that furnace walls made solely of graphite offer poor resistance to oxidizing atmospheres at high temperatures. The maintenance of a coating oE slag on the exposed surface of the graphi-te blocks is made possible, since the slag does not usually adhere to the graphite, by the presence of oxide-based refractory blocks around each graphite block. In fact, the slay splashes aclhere to the oxlde-based blocks and end up hy forming bridges covering the surface of the graphite.
b) Owing to the good thermal conductivity of graphite, heat can be rapidly removed from the furnace, particularly when temperature peaks occur during the operation of the furnace.
Also the graphite blocks remove heat from the adjacent oxide-based blocks. All -this makes it possible -to reduce the extent of the effect of the thermal fluxes to which the oxide-based refractory hlocks are subjected and therefore slows down the ~2".~

scaling process of the blocks. This cooling effeck can be controlled by adjusting the ratio cross-section of each graphite block cross-section of each oxide-based refractory block, as this is obvious. In fact, the larger the surface occupied by the graphite blocks, the more pronounced the cooling effect is.
In practice, it has been determined that the width of each graphite block should be approximately between 10 and 50 mm, preferably between 20 and 40 mm. In fac-t, it is difficult to obtain graphite blocks or plates of sufficient mechanical strength with a width (or thickness) of less than 10 mm.
Moreover, a width greater than 50 mm is generally unnecessary and makes the graphi-te blocks more vulnerable to oxidation attack. With regard to the oxide-based refractory blocks, it has been found that their width should be approxima-tely between 80 and 150 mm. Beyond 150 mm, the cooling effect exerted by the graphite blocks on the oxide-based refractory blocks becomes insufficient, especially at the centre of the oxide-based blocks.
Below a width of 80 mm the oxide-based blocks become too expensive or inconvenient to use.
The height oE the oxide-based refractory blocks and the graphite blocks can be, for example, from approximately 75 to 250 mm, while their length (from the inner face to the outer face) can be, for example, from approximately 300 to 600 mm.
To obtain the best results, care should be taken that a good thermal contact exists between each graphite block and the oxide-based block or blocks with which it is in direct contact. For this purpose, these blocks must be in close contact and there should be no particles or other scrap between these blocks.

As has been men-tioned above, an intermediary steel sheet is preferably used. Although the tempera-tures present in the arc furnace are sufficient to melt the steel, it has been observed that only a limited attack is produced on the intermediary steel sheet at a depth of a few centimetres (2 to 4 cm for example), due to the fact that this sheet is to some extent protected by the blocks between which it is sandwiched. The purpose of this sheet is to connect the blocks together~ In fact, in use, the steel of the sheet melts in the area of the inner face of the blocks, thus forming a very compact brickwork. A cer-tain reaction can also take place between the oxide-based blocks and the steel sheet.
In the case where composite elements according to the invention (with one pair of blocks) are used to construct at least certain areas of the upper part of the refractory wall, it is sufficient, in order to place a steel sheet between adjacent elements, having juxtaposed and stacked up a plurality of these elements, to line each element with a steel sheet on two adjacent surfaces extending from the inner face to the outer face. These sheets can be fixed to the element by ~n appropriate me-thod, for example by sticking or by mechanical bonding.
However, the lining can also be placed on three surfaces or four surfaces of the element extending from the inner face to the outer face. In this case, it will be appreciated thatr after construction of the refractory wall, there will be at least certain areas where there exists a double thickness of sheets between the elements.
Moreover, in the case of the preferred composite element according to the invention lined with a continuous steel shee-t on its four surfaces extending from the inner face to the outer face, the steel sheet plays a protective role in relation -to the blocks making up the said element during its handling and transport, as well as serving the purpose of holding the blocks positioned in close contact with each other~ which facilitates the positioning of the element during construction of the refractory wall of the furnace and ensures good thermal contact between the blocks. In this case also it will not be necessary to provide means for fixing the sheet on to the element as it can be folded around the element so that it firmly encompasses the two blocks making up the latter.
The thickness of the sheet is not very critical. By way of example it can be from 0.5 to 2 mm~

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~2~3~4 thick. A sh~et of or~inary quality ~teel oan be used.
~ he composite elemsnt ~ccording to the inventio~
can be in paralleiepiped form or in the form of an arch-stone~ especially in the case where it is to be used in the construction of a curved wall.
~ he description which follows, referring to the accompanying drawing,illustrates further the in~en-tio~
Figure 1 is an elevation showing a portion of a refractory wall constructed according to the method o~ the inventio~.
~igure 2 is a view of the inner ~ace of a composite element according to the invention comprising one pair of blocks.
Figure 3 is a horizontal section through the element shown in Figure 2.
~igure 4 is an elevation showing a portion of a refractor~ wall constructed by means o~ elements ~imilar to those in ~igures 2 and 3.
In ~igure 1 there can be seen a portio~, designated by the general reference numeral 1, of a refractory wall9 for example a portion o~ the wall op-posite an arc furnace electrode. This portion 1 is ob-tai~ed b~ the simple juxtaposition of oxide-based blocks 2 of electro-melted re~ractory material and graphite blocks ~ arranged alter~ately9 the blocks in a given row being offset with respect to the blooks in tha adjacent rows so that any graphite block 3 is surro~nded by blocks 2 o~ fu~ed cast material.
~igures 2 and 3 show a composite elemen-t aocording to the invention~ give~ the general ~e~erence numeral 11. ~his element i~ made up of an oxide-based block 12 of electro-melted refraotory material, for example of "CO~AR~ CO 104"j the composition by weight of which is a~q follows : MgO : 55~ Cr203 : 20~ 5~o~
~eO : 12~5~ A123 7~ ~i2 2 j5~9 CaO : 1~5%, ~i2 : 0-5%~ other~ : 0~5~o7 and a graphite block 13~
the boay ~ormed by the blocks 12 and 13 being surrou~ded, apart from on its inne~ and outer faces, by a sheet of ordinar~ s~eel 14 with a thickness o~ 1~5mm. In other words, the sheet 14 covers the surfaces of the whole body ex-tending ~rom the inner face to the outer ~aceO
.
The block 12, which is obtained by sawing an ingot o~
greater size, is in the ~orm of a wedge or arch-stone, while the block 13 is of substantiall~ constant thick-ness, bei~g obtained by cut-ting a plate. ~herefore, in total, the element is in the form of an arch-stone and is ~uitable for the construction o~ a cylindrical wall, ~y way o~ ex~mple, such a composite element can ha~e the following dimensions : length ~ (from the inner faoe to the outer ~ace) : 450mm, height h : 152mm, a~erage width 1 : 130mm (average width la 0~ block 12 : 100mm; width lb o~ block 13 : 30mm, with la + lb = l)o ~he widths li and le of the ele~ent at the inner face and outer fa¢e 9 25 respectively, will obviously be selected according to the radiu~ of the cylindrical wall to be co~structed~
Alternati~ely, o~ course~ parallelepiped elementc can be made (li - le~ i~ the wall to be co~s~
tructed i~ fla~.
~0 ~he wa-l~ obtained according to ~he inventlon have an e~ceptionally hi~h and appreciably u~iform - 12 ~

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resista~ce to corrosion during uoeO
Comparative te~ts for oorro~ion i~ ~ metallur-gical aro ~HP furnace with a capacity of 100 - 1~0 ton~
and a power o~ 80 MVA have given the following results :
5 ~ype of bloc~s forming~orrosion (in mm) per the wall cas~i~g ~10G~S of normal.CORH~RT
C. 104 (density 3015), without steel sheet 6 10 ~locks of normal COREAR~
C~ 104, with steel sheet 5 ~locks o~ dense COR~AR~
C0 104 ~density 3.45), without steel sheet - 15 The composite elements shown in ~igures 2 and ~ 1 to 2 These results s'now ~ery clearly the adva~tages obtai~ed from the inYention, that is the achieving of refractory walls, or at least certain desired areas o~
these walls, which present an inoreased re~istance to corro3io~3 and thereby the possibility of extended operati~g periods for the furnaoes and9 there~ore~
increased productivity from themO

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of constructing at least a portion of the refractory wall of a metallurgical electric arc furnace, said method comprising the steps of:
(a) providing a multiplicity of refractory blocks, some of which are made of an oxide-based material and some of which are made of substantially pure graphite, each of the refractory blocks having a rectangular cross section, the refractory blocks made of an oxide-based material each having a width of between 80 and 150 mm and the refractory blocks made of substantially pure graphite each having a width of between 10 and 50 mm;
(b) positioning some of said refractory blocks of step (a) in a particular row along a part of the refractory wall of the metallurgical electric arc furnace which is not intended to be in prolonged contact with the liquid metal bath in the furnace so as to alternate in side-by-side relationship refractory blocks made of oxide-based material with refractory blocks made of substantially pure graphite, each of the refractory blocks extending from the inner face to the outer face of the refractory wall; and (c) positioning other of said refractory blocks of step (a) in another row adjacent said particular row so as to alternate in side-by-side relationship refractory blocks made of an oxide-based material with refractory blocks made of substantially pure graphite, each of the refractory blocks extending from the inner face to the outer face of the refractory wall, and the refractory blocks in said another row being offset with respect to the refractory blocks in said particular row such that each refractory block made of substantially pure graphite is surrounded by a refractory block made of an oxide-based material.

2. The method as defined in claim 1 including positioning at least one sheet of steel between adjacent rows of refractory blocks.

3. The method as defined in claim 2 including positioning at least one sheet of steel between at least some of the adjacent blocks of each row.

4. The method as defined in claim 1 wherein the refractory blocks made of oxide-based material are fused cast refractory blocks of the magnesia/chromium oxide type.

5. The method as defined in claim 4 wherein the refractory blocks made of oxide-based material have the following composition, by weight: MgO: 55%, Cr2O3: 20.5%, FeO: 12-5%, Al2O3: 7%, SiO2: 2.5%, CaO: 1.5%, TiO2: 0.5%, others: 0.5%.

6. A method of constructing at least a portion of the refractory wall of a metallurgical electric arc furnace, said method comprising the steps of:

(a) providing a multiplicity of composite refractory elements which comprise adjacent refractory blocks alternately made of an oxide-based material and of substantially pure graphite, each of the refractory blocks having a rectangular cross section, the refractory blocks made of an oxide-based material each having a width of between 80 and 150 mm and the refractory blocks made of substantially pure graphite each having a width of between 10 and 50 mm;
(b) positioning some of said composite refractory elements of step (a) in a particular row along a part of the refractory wall of the metallurgical electric arc furnace which is not intended to be in prolonged contact with the liquid metal bath in the furnace so as to be in side-by-side relationship, each of the composite refractory elements extending from the inner face to the outer face of the refractory wall; and (c) positioning other of said composite refractory elements of step (a) in another row adjacent said particular row, each of the composite refractory elements extending from the inner face to the outer face of the refractory wall, and the composite refractory elements in said another row being offset with respect to the composite refractory elements in said particular row such that each refractory block made of substantially pure graphite is surrounded by a refractory block made of an oxide-based material.

7. The method as defined in claim 6 including positioning at least one sheet of steel between adjacent rows of refractory composite elements.

8. The method as defined in claim 6 wherein each refractory composite element includes a sheet of steel between each refractory block made of an oxide-based material and each refractory block made of substantially pure graphite.

9. The method as defined in claim 7 wherein each refractory composite element includes a sheet of steel on at least one of its sides which face other refractory composite elements.

10. A composite refractory element which has an inner face and an outer face and four side surfaces which extend from its inner face to its outer face, said element including at least one pair of adjoining refractory blocks and at least two steel sheets respectively forming two adjacent side sur-faces of the element, each pair of adjoining refractory blocks consisting of a refractory block made of an oxide-based material and a refractory block made of substantially pure graphite, each of said refractory blocks having a rectangular cross section with a width and height dimension, the width of each refractory block made of an oxide-based material being between 80 and 150 mm and the width of each refractory block made of substantially pure graphite being between 10 and 50 mm.

11. The composite refractory element as defined in claim 10 including four steel sheets which form the four side surfaces of the element.

12. The composite refractory element as defined in claim 10. wherein the width of the refractory block made of substantially pure graphite of each pair of adjoining refractory blocks is between 20 and 40 mm.

13. The composite refractory element as defined in claim 10 wherein each refractory block made of an oxide-based material is a block of fused cast refractory material of the magnesia/chromium oxide type.

14. The composite refractory element as defined in claim 13 wherein each refractory block made of an oxide-based material has the following composition, by weight: MgO: 55%, Cr2O3: 20.5%, FeO: 12.5%, Al2O3: 7%, SiO2: 2.5%, CaO: 1.5%, TiO2: 0.5%, others: 0.5%.