Process for the operation of electric furnaces by means of continuous electrodes

Abstract

Claims

Oct. 8, 1963 F lawn-: 1; 3,106,595 PROCESS FOR THE OPEI RATION CTRIC FURNACES BY MEANS OF CONTINUOUS ECTRODES Filed Nov. 5, 1959 INVEN 1 OR United States Patent 3,106,595 PRQCESS FOR THE QPERATIUN OF ELECTRIC FURNACES BY MEANS OF CONTINUOUS ELECTRODES Fricdhert Hitter, Euschhoi ulcer Konigswinter, Germany, assignor to Knapsack-Griesheim Aktiengesellschaft, Knapsaclr, near Cologne, Germany, a corporation of Germany Filed Nov. 5, 1959, Ser. No. 351,013 Claims priority, application Germany Nov. 5, 1958 ll Claim. (ill. 13-9) The present invention relates to a process for the operation of electric furnaces by means of continuous electrodes provided with a metal shell and a metal reinforcement, said electrodes being baked in the furnace in which they are utilized. Said process consists in fitting the bottom edges of the current supplies in the unbaked part of the electrodes about 1 to 5, possibly also 2, or 3, or 4 meters above the uppe end of the baked part of the electrodes, in conducting the current only through the metal shell and the metal reinforcement to the baked part of the electrodes, and in maintaining in these metal parts a current density not exceeding 2.5 amperes/rnm. of the cross-section of the metal, for example the crosssection of iron. The process furthermore consists in that an electrode is used of which the metal shell and the metal reinforcement have a cross-section which, depending on the current density, sufiices to prevent these metal parts from becoming hotter than +500 C. in the course of the operation when the current passes through them. In addition, the specific current load exerted on the total electrode does not exceed 4 amperes/cm. of the crosssection of the electrode. The general make-up of a furnace and electrode assembly which illustrates the state of the prior art but not the improvement as found in the present invention is disclosed in US. Patent 1,440,724 issued to Soderberg. It has already been proposed to fit the current supplies in the unbaked part of the continuous electrodes, and it has, on the other hand, also been endeavored to utilize the metal shell and the metal reinforcement of continuous electrodes for the current supply from the unbaked part to the baked part of the electrode; however, up to now the current supply has been positioned as close as possible to the baked part of the electrode or the current supply has been mounted on that part of the electrode that had already been baked. It was thus attempted to keep the passage from the electrode supports to the lower end of the electrode as short as possible and, in order to attain this object, electrode supports projecting into the furnace were used in which the current supply was titted in the closed furnace beneath the cover of the furnace. Now, I have found that it is possible to dispense with such electrode supports projecting into the furnace and to install the current supplies far above the upper end of the baked part of continuous electrodes by maintaining in the metal shell and the metal reinforcement a current density not exceeding 2.5 amperes/rnm? of the cross-section of the metal, for example the cross-section of iron. By limiting the current load, it is possible to conduct the total current supplied through the metal shell and the metal reinforcement to the baked part of the electrode 7 3,106,595 Patented Oct. 8, 1963 ice when observing, moreover, the measure of maintaining the cross-section of the metal shell and the metal reinforcement such that these metal parts do not become hotter than +500 C. when the current passes through them in the course of the operation. When the current density is higher than about 2.5 ampe-res/mrn. of the cross-section of the metal, for example 4 to 5 amperes/mm. of the cross-section of the metal, the metal shell and the metal reinforcement become so hot due to the current passing through them and due to the heat passing up from the furnace that their breaking strength is impaired so that it occurs that the electrode breaks in the vicinity of the baking Zone due to its own weight. When maintaining the cross-section of the metal shell and the metal reinforcement great enough that both metal parts do not become unduly hot even in the case of a current density of, for example, 4 to 5 amperes, the cross-section of the shell and the reinforcement would, in the case of using iron, become so large that too much iron would get into the melt of the electric furnace which would render it impossible to work in an economic manner. Thus, according to the invention, the cross-section of the metal shell and the metal reinforcement must be kept small. It need only be such that a current density not exceeding 2.5 amperes/mm. of the cross-section of the metal, for example the cross-section of iron, does not cause an undue heating. The specific current load for the total electrode must not exceed 3 to 4 a-mperes/cm. of the cross-section of the electrode in order that the baking zone is as close as possible to the lower end of the electrode, i.e. that the baked part of the electrode is short. When admitting a higher current density, for example 6 to 7 amperes/cm. of the cross-section of the electrode, the electrode would in any case be baked right into the zone of the current supplies. Experience has shown that continuous electrodes can only be operated in a satisfactory manner when the baked part of the electrode is short, i.e. when its length does not exceed double the diameter of the electrode. When a longer section of the electrode has been baked, this long, baked trunk is not only exposed to an increased extent to mechanical transverse forces but, due to its higher weight, is too much of a load for the electrode shell and its iron reinforcement from which the baked trunk is suspended wholly or to a considerable extent. In comparison with the electrode supports projecting into the furnace which have been used hitherto, the process according to the invention has the advantage that the complicated cooling device being installed in closed furnaces below the furnace cover can be dispensed with and that the support need no longer project as closely as possible into the furnace but, on the contrary, can be fitted as high up as possible at the electrode. Apart from the fact that the current supplies are thus exposed to a lesser strain by the furnace heat and owing to the less complicated construction, the invention offers the advantage that the current supplies need only be adjusted at long intervals because, the longer the distance from the cover of the furnace to the current supply, the longer the intervals at which the current supplies on the electrode have to be adjusted. The current supply on 3 the electrode can thus travel downward with the electrode in the same manner as before for a considerably longer distance before it becomes necessary to pull the current supply up again because it has arrived at the bottom of the furnace cover. A further modification of the invention consists in using stationary current supplies and supporting, lifting and lowering devices engaging at the upper ends of the electrodes. In this manner the advantage is gained that the current supplies need no longer be adjusted at all because they no longer travel downward with the electrodes and are then pulled up again but are, for example, fixed opposite the cover of the furnace. When working according to this modification, the current supplies are suitably only used in this capacity. The supporting, lifting and lowering of the electrode is effected by devices engaging at the upper end of the electrodes, for example at the top section of the electrode shell. For the operation of furnaces by means of continuous electrodes according to the invention it is, furthermore, of considerable advantage that attention need not 'be paid to the state of baking of the electrodes. The baking zone of the electrode must, of course, remain beneath the cover of the furnace. It was to be expected that a current condensation to the outside would occur in the electrodes so that the cross-section of the iron shell and the iron reinforcement would not sufiice to conduct the current from the current supply to the baked part of the electrode without an excessive heating of the iron parts occurring. It was all the more surprising that, in spite of the skin effect, such a passage of current is possible without increasing the plate thi kness of the electrode shell even when the cross-section of the metal shell and the metal reinforcement is only as large as permitting a load of at most 2.5 amperes/rnan. of the cross-section of the metal. The single FIGURE of the drawing is an elevation partly in section showing the apparatus of the invention. The following example serves to illustrate the invention, but it is not intended to limit it thereto: Example In a closed electric furnace for the preparation of phosphorus, three continuous electrodes ll (only one being shown) of each 1350 mm. cross-section are arranged at the corners of an equilateral triangle whereby the distance between the axis of the electrodes is about 3000 mm. The interior of the furnace is sealed against the outside atmosphere by a furnace lid 2 and by stuffing rings 3 arranged around the electrodes. The electrodes have an iron jacket 5 of 2 mm. thickness of the sheet and have a total iron cross-section togethe of about 220 cm. The electrodes extend downwardly from the furnace lid about 3500 mm. average into the furnace interior (a plus b). The combustion zone in which the electrode substance is baked in the interior of the electrodes has its upper end about 500 mm. below the surface of the furnace lid b. The electrode portion 6 below this combustion zone is solid, but portion 7 above this combustion zone is not baked and is somewhat soft. The socket of the electrodes carrying the current consisting of, tl.8. of a ring of ten contact plates 8 pressed by a thrust collar 9 each of a length of 120 cm. e is arranged so that it is above the furnace lid, namely with its bottom edge about 1 to 5 rn. above the upper end of the burned part d, that is about 0.5 to 4.5 m. above the upper surface of the furnace lid 0. The furnace is now charged with the composition known for the manufacture of phosphorus and consisting of crude phosphate, carbon and silicic acid raw material and with an electric current of about 55,000 'amperes per electrode. The voltage changes with the furnace efficiency desired in each case and amounts to several 100 '4 volts. In the course of this operation, the electrodes do not become excessively overheated. The furnace can be operated in the usual manner. It should be noted in this connection that the passage of the current need by no means solely bring about the heating but that the highest temperature occurs in the vicinity of the baking zone, is. within a zone in which the furnace heat passing up is also effective. The novel process offers the advantage that the electrode supports projecting into the furnace and their complicated cooling devices can be dispensed with, that the electrode supports supplying the current need only be adjusted at longer intervals than usual hitherto or need not be adjusted at all at the electrodes, furthermore the construction of the device serving to displace the electrode can be more simple and the electrode operated solely according to operational requirements, without having regard to its state of baking. In the process of the invention, the baked trunk of the electrode shall not exceed a length of about double the diameter of the electrode and thus not exceed a weight of, for example, 6 tons. in a certain zone of the electrode, i.e. where the supporting function is transferred from the iron shell to the solidified electrode, the electrode, the iron shell and the iron reinforcement are-when maintaining the aforesaid weight-just about in a position to support the baked trunk of the electrode with safety. The current load of the cross-section of the metal shell rnust, therefore, be kept below the aforesaid limit in order that the iron of the shell, in the critical range just above the baking Zone, does not become hotter than is necessary to bring about the solidification of the electrode mass. Thus a normal electrode shell will not be heated to a substantial degree upon the passage of 2.5 amperes/rnnm Only in the vicinity of a temperature of +500 C. would a current density higher than 2.5 amperes/rnm. endanger the solidity of the iron. The total load of the electrode may not exceed 3 to 4 amperes/cm. since, otherwise, there would be baked too long and too heavy a part of the electrode so that, according to experience, the stability of the continuous electrode would be impaired. The process for the operation of electric furnaces by means of continuous electrodes equipped with a metal shell and a metal reinforcement, said electrodes being baked in the furnace in which they are utilized, is thus characterized according to the present invention by the following features: (a) The current supplies are fitted in the unbaked part of the electrodes in a manner such that their lower edge is about 1 to 5 meters above the upper end of the baked part of the electrodes; (b) The current is only conducted through the metal jacket and the metal reinforcement to the baked part of the electrodes; (0) The current density in the metal shell and the metal reinforcement is adjusted such that it does not exceed 2.5 amperes/mnfi; (d) The proportion of the cross-section of the metal shell and the metal reinforcement to the cross-section of the total electrode is adjusted such that the specific current load exerted on the total electrode does not exceed 4 amperes/orm Finally, there are used stationary current supplies as well as supporting, liftingand lowering devices which engage the electrodes at their upper ends. I claim: In a process for operating electric furnaces with permanent electrodes equipped with metallic reinforcements, said electrodes being baked in the furnace in which they are used, wherein current is supplied to the unbaked part of the electrodes at a spaced distance above the upper end of the baked part of the electrodes and wherein said current is conducted only through the metallic reinforcements to the baked part of the electrode while maintaining the current density in the metallic reinforcements up to 4 amperes per mm the improvement which comprises supplying the current to the unbaked part of the electrodes at a distance of about 1 to 5 meters above the upper end of the baked part of the electrodes, maintaining the current density in the metallic reinforcements up to at most 2.5 amperes per rnrn and propc-rtioning the cross-sections of the metallic reinforcements to the crossof the entire electrode does not exceed the value of 4 amperes per cnr References Cited in the file of this patent section of the general electrodes so that the specific load 10 2,338,936 UNITED STATES PATENTS Soderberg Jan. 2, Sodenberg Jan. 24, Wisdom Sept. 4, Sem Mar. 12, Hagerup-Larssen Jan. 11,

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Patent Citations (5)

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