CONTAINER MADE OF STAINLESS STEEL FOR FORMING SELF-BAKING ELECTRODES FOR USE IN LOW ELECTRIC REDUCTION FURNACES

Abstract: The present invention relates to a self-baking electrode for use in low electric reduction furnaces, and refers particularly to a container (1) for the formation of self-baking electrodes to be used in low electric reduction furnaces, allowing the manufacture of silicon alloys with iron content as low as 0.35 %. The container comprising a cylindrical casing (11) split in two parts containing therein a plurality of ribs (12) uniformly attached perpendicularly along the inner surface of the casing (11) lengthwise along the cylindrical casing, characterized by the fact that the cylindrical casing (11) and ribs (12) are made of stainless steel plates. The invention also relates to a method of forming a self baking electrode using this container as well as the electrode formed thereby. Finally, the invention relates to the use of a self-baking electrode formed in this container for manufacturing silicon alloys.

Description: CONTAINER MADE OF STAINLESS STEEL FOR FORMING SELF-BAKING ELECTRODES FOR USE IN LOW ELECTRIC REDUCTION FURNACES Technical Field The present invention relates to a self-baking electrode for use in low electric reduction furnaces, and in particular it refers to a container for forming self-baking electrodes to be used in low electric reduction furnaces. The invention also relates to a method of forming a self baking electrode using this container as well as the electrode formed thereby. Finally, the invention relates to the use of a self baking electrode formed in this container for manufacturing silicon alloys. Background Art Conventional self-baking electrodes are formed in a segmented cylindrical container (sections of casing) arranged vertically extending from the inside of the furnace stack until the uppermost height of the building thereof. The upper end of the cylindrical container is open in order to allow the addition of unbaked electrode paste, which when submitted to heating, due to the heat added in the area of supply of electric operating current to the electrode, softens, melts, discharges volatile products, and is thereafter baked into a solid carbon electrode. As the electrode is consumed in the furnace, the electrode is lowered and new sections of casing are installed at the top of the column, where the unbaked electrode paste is then added. A conventional electrode of this type is equipped with metallic ribs attached to the inner surface of the vertical casing, the ribs extending radially relative to the axis of the electrode. When a section of casing is installed at the top of the electrode column, its casing and its ribs are welded to the casing and the ribs of the already installed segment in order to obtain continuity of the ribs in the vertical direction. The ribs serve to support, conduct electric current, and heat into the electrode during the baking process. To compensate for the consumption of the electrode, the same is lowered into the furnace by means of the sliding mechanism. When conventional electrodes of this type are used, the electrode container casing and the inner ribs melt when the electrode is being consumed in the furnace. The metal content of the casing and the ribs is transferred to the product in the furnace. Since the container casing and the inner ribs usually are made from carbon steel, such self-baking electrodes can not be used in electric reduction furnaces for the production of high-grade silicon alloys, as the iron content in the produced material will become unacceptable. Already in the 1920's it was proposed to conduct heat into the self-baking electrodes through inserts of pre-baked carbon bodies in the unbaked electrode paste. In Norwegian patent NO 45408 there is disclosed a method for the production of self-baking electrodes wherein pre-baked carbon bodies are placed in the periphery of the electrodes and are kept in place by the unbaked electrode paste. The carbon inserts are not attached to the casing, but are merely kept in place by the unbaked electrode paste, and when the electrode is baked, by the baked electrode paste. In order to keep the carbon inserts in place before, during, and after the baking of the electrode paste, it is necessary that each casing be fully filled with hot liquid electrode paste when a new length of casing is installed at the top of the electrode column, since it is only the electrode paste that keeps the carbon inserts in place against the inner wall of the casing, which may render difficult the calcination of the central part of the electrode. Those carbon inserts will not function in the same manner as the ribs used in the conventional self-baking electrodes. The method in accordance with Norwegian patent NO 45408 has for these reasons not found any practical use. There have been proposed over the years, however, a number of modifications of the conventional self-baking electrodes not having inner ribs made of steel in order to avoid contamination of the silicon produced in the furnace caused by the iron product of the casing and the ribs. Thus, in Norwegian patent NO 149451 there is disclosed a self-baking electrode wherein the electrode paste contained in a casing devoid of ribs, is being baked above the location where the electric operating current is supplied, and wherein the casing is removed after baking, but before having been lowered down to the place where the electric operating current is supplied. An electrode is produced in this manner which has neither casing nor ribs. That kind of electrode has been used in low furnaces for the production of silicon, but nevertheless having the disadvantage when compared with conventional pre-baked electrodes in that costly equipment must be installed in order to bake the electrode and to remove the casing from the electrode. In US Patent 4,692,929 there is described a self-baking electrode to be used with electric furnaces for the production of silicon. The electrode comprises a permanent metal casing without ribs and a support frame for the electrode comprising carbon fibers, wherein the electrode paste is baked upon the support frame and wherein the baked electrode is being held by the support frame. That electrode has the disadvantage that special fastening equipment must be arranged above the top of the electrode in order to hold the same using the support structure comprising carbon fibers. Furthermore, it may be difficult to have the electrode slide downwards through the permanent casing when the electrode is being consumed. In US Patent 4,575,856 there is disclosed a self-baking electrode having a permanent casing without ribs, wherein the electrode paste is being baked over a central graphite core and wherein the electrode is being held by the graphite core. That electrode has the same disadvantages as the electrode according to US Patent 4,692,929 and in addition the graphite core is prone to breakage when the electrode is subjected to radial forces. The methods cited above for the production of a self-baking electrode without ribs suffer from the disadvantage that they can not be used for electrodes with a diameter above 1.2 m without substantially increasing the probability of breakage. However, conventional self-baking electrodes are used that have diameters of up to 2.0 m. Description of the Invention Although the methods and apparatuses mentioned above for the production of self-baking electrodes are intended to avoid iron contamination in the product produced in low furnaces, there is still a need for a simple and reliable self-baking carbon electrode, able to overcome the disadvantages of the known electrodes. It is therefore an object of the present invention to provide a container for forming a self-baking carbon electrode which, when in operation, may allow the production of high- grade silicon alloys. Accordingly, the present invention refers to a self-baking carbon electrode produced in direct connection with the furnace wherein the same is consumed, comprising an outer casing made of an electrically conductive material (stainless steel), with inner ribs radially and vertically attached. Electrode paste is initially added to the casing in raw unbaked form. With the passage of the electric current through the same, it is baked and forms the solid electrode. The ribs are made of stainless steel plates with low iron content and with dimensions sufficient to withstand the weight of the electrode column. The assembly of the casings follows the same principle adopted for the conventional carbon steel casings. The ribs generally extend beyond both ends of the casing in order to allow the welding thereof and to ensure their continuity. In a preferred embodiment of the invention, the ribs extend on the order of about 20mm beyond the ends of the casing The present invention allows for a decrease in the contribution of "Iron" to the product through the casings compared to the traditional model (manufactured from carbon steel). This decrease can be on the order of 70% allowing the production of silicon alloys with "Iron" content down to 0.35 wt. %. As used herein, the expression ""Iron" content down to 0.35 wt. %" means that a specification for this material would list 0.35 wt. % as the maximum "Iron" content for the material. It is therefore an object of the present invention to provide a container for the formation of self-baking electrodes to be used in low electric reduction furnaces, comprising a cylindrical casing containing in the inside thereof a plurality of ribs perpendicularly attached along the inner surface of the casing in the longitudinal direction of the cylindrical casing, characterized by the fact that the cylindrical casing is made of stainless steel plates and the ribs are made of stainless steel plates. If desired, the container can be split in 2 parts. In a preferred embodiment, the container comprises creases and external blasting of the stainless steel plates used for the casing. In another preferred embodiment, the container comprises aluminum reinforcement rings mounted at the inner part of the stainless steel casing. In another preferred embodiment, the ribs have two folds, one at each end of the rib. In another embodiment, the fold in the rib next to the casing has grooves in order to allow the assembly of rings. In a further embodiment, the ribs are attached to the inside of the stainless steel casing by means of welding. In yet another embodiment, the container may comprise ribs provided with alternating circular holes offset from the horizontal axis passing through the center of the same. In yet another embodiment, the holes provided in the ribs are drawn back for additional support. It is another object of the invention to provide a method of forming a self baking electrode comprising adding unbaked electrode paste to an electrode container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs perpendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing and heating the paste by a method selected from heat supplied by a heater, heat generated by the introduction of electric energy, and a combination thereof. It is another object of the invention to provide an electrode produced by the above process. It is yet another object of the invention to provide a method for manufacturing silicon alloys with low iron content using a self baking electrode, the improvement comprising forming the self baking electrode in an electrode container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs perpendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing. Description of the Drawings The following drawings and descriptions provide a representative embodiment of the invention, but the limitations included therein are not meant to limit the invention or narrow the scope of the claims. Figure 1 is a cross-sectional view through the container for the formation of self-baking electrodes to be used in low electric reduction furnaces in accordance with the present invention, with the electrode placed inside the same. Figure 2 is an horizontal view taken along plane I - I of the container depicted in Figure 1. Figure 3 is an enlarged view of area "A" marked in Figure 2 and showing the attachment of the ribs to the stainless steel casing by means of welding. Figure 4A shows a front view of the casing and blasting. Figure 4B shows in detail the creases, grooves in the rib and the assembly position of the ring. Figure 5 depicts the fold and drawn back portions of the holes provided in the rib. Figure 6 shows the alternating and offset holes provided in the rib. As may be seen in Figure 1 , the self-baking electrode is formed by a cylindrical container (1), which is segmented in casing sections (1 '). The container (1) can extend from the inside of the furnace stack until the uppermost height of the building housing the same. The upper end of the cylindrical container (1) is open to allow the addition of unbaked electrode paste (2). The formation of the electrode takes place through the transformation of the raw unbaked electrode paste (2) into fluid paste (3), paste being calcined (4) and calcined paste (5) due to the heat supplied by the hot air blown-in (originating from fan (8) and from heater (7)), as well as by the heat generated by the introduction of electric energy through the contact plates (6), which are pressed against the electrode by pressure ring (9). The casing segments above the contact plates are enclosed by the protective shield (10) for a sufficient distance starting at, for example 2.5 cm above the contact plates. In Figure 2 there is depicted the container (1), seen in cross section along the plane I - 1 of Figure 1. As will be noted, the container (1) is comprised of a cylindrical casing (1 1), made of stainless steel plates, and which includes in the inside thereof a plurality of ribs (12) attached perpendicularly to the inner wall of the casing (11). Preferably, the ribs (12) are attached uniformly on the inner wall of the casing (11). The ribs (12) are made of stainless steel. Figure 3 shows an enlarged view of area "A" marked in Figure 2, showing the attachment of stainless steel rib (12) to the casing (11), which is also made of stainless steel, by means of welding. The drawn back portions of the holes contained in the ribs (12') are on alternating sides of the rib (12). Figure 4A is a front view of the casing with a stainless steel casing shell, showing the blasting as surface treatment of the casing (18). Figure 4B shows a detailed view of the creases (19), grooves (17) in the rib (12) and the position of assembly of the aluminum reinforcement ring (16). Figure 5 depicts the construction of stainless steel rib (12), inside view, and showing the drawn back portions (12'), the folds (20) and the point of attachment (21 ) of the rib (12) to the casing (11). Figure 6 is a front view of the ribs (12) in the position of attachment to the casing, wherein the holes are shown to be offset and alternating.

Claims: CLAIMS 1. A container for the formation of self-baking electrodes for use in low electric reduction furnaces, characterized by comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs peφendicularly attached along inner surface of the casing lengthwise of the cylindrical casing, wherein the outer surface of the cylindrical casing has creases and external blasting. 2. A container for the formation of self-baking electrodes for use in low electric reduction furnaces, characterized by comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs perpendicularly attached along inner surface of the casing lengthwise of the cylindrical casing, wherein aluminum reinforcement rings are mounted on the inside of the cylindrical casing. 3. The container according to claim 1, characterized by the fact that the ribs have a folded portion at each of its ends. 4. The container according to claim 2, characterized by the fact that the ribs have a folded portion at each of its ends. 5. The container according to claim 4, characterized by the fact that the fold in the rib next to the casing has grooves in order to allow the assembly of rings. 6. The container according to claim 1, characterized by the fact that the ribs have circular holes arranged alternately and offset from the horizontal axis that passes through the center of the same. 7. The container according to claim 2, characterized by the fact that the ribs have circular holes arranged alternately and offset from the horizontal axis that passes through the center of the same. 8. The container according to claim 1, characterized by the fact that the ribs has holes which are drawn back. 9. The container according to claim 2, characterized by the fact that the ribs have holes which are drawn back. 10. The container according to claim 1, characterized by the fact that the stainless steel ribs are attached to the inside of the stainless steel casing by means of welding. 11. The container according to claim 2, characterized by the fact that the stainless steel ribs are attached to the inside of the stainless steel casing by means of welding. 12. The container according to claim 1, characterized by the fact that it contains electrode paste. 13. The container according to claim 2, characterized by the fact that it contains electrode paste. 14. A method of forming a self baking electrode comprising: adding unbaked electrode paste to an electrode container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs peφendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing, wherein the outer surface of the cylindrical casing has creases and external blasting; and heating the paste by a method selected from heat supplied by a heater, heat generated by the introduction of electric energy, and a combination thereof. 15. A method of forming a self baking electrode comprising: adding unbaked electrode paste to an electrode container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs peφendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing, wherein aluminum reinforcement rings are mounted on the inside of the cylindrical casing; and heating the paste by a method selected from heat supplied by a heater, heat generated by the introduction of electric energy, and a combination thereof. 16. The electrode made by the process of claim 14. 17. The electrode made by the process of claim 15. 18. In a method for manufacturing silicon alloys with low iron content using a self baking electrode, the improvement comprising forming the self baking electrode in an electrode container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs peφendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing, wherein the outer surface of the cylindrical casing has creases and external blasting. 19. In a method for manufacturing silicon alloys with low iron content using a self baking electrode, the improvement comprising forming the self baking electrode in an electrode container comprising a stainless steel cylindrical casing containing therein a plurality of stainless steel ribs peφendicularly attached along the inner surface of the casing lengthwise of the cylindrical casing, wherein aluminum reinforcement rings are mounted on the inside of the cylindrical casing.

Inventors: DE ALBUQUERQUE HELIO CAVALCANT, DE ALBUQUERQUE, HELIO CAVALCANTE LOPES
Applicants: ALBUQUERQUE HELIO CAVALCANTE L, COMPANHIA BRASILEIRA CARBURETO, COMPANHIA BRASILEIRA CARBURETO DE CALCIO, DE ALBUQUERQUE, HELIO CAVALCANTE LOPES
Application year: 2000
Main classification: H05B7
IPC codes: H05B7