MXPA97007406A - Cooling system for electrodes in hornosde electric arc electric current - Google Patents

Abstract

A cooling system for electrodes (11) in direct current electric arc furnaces, the electrodes (11) comprising a part (11a) subjected to the hot environment of the furnace, this part being made of graphite in the case of a cathode, and copper in the case of anode, the part (11a) being associated with a metal part (11b) by means of a joint (13) that includes inside it a gap (14), which comprises at least in correspondence with the metal part (11b), a cooling circuit (20) with at least one delivery channel (15), a return channel (16), and a heat exchanger (18), using the liquid metal circuit (20) as a cooling fluid, also including an element for the forced circulation of the cooling fluid, which consists of a hydromagnetic pump (1

Description

COOLING SYSTEM FOR ELECTRODES IN DIRECT CURRENT ELECTRIC ARC FURNACES This invention relates to a cooling system for electrodes in direct current electric arc furnaces, as stipulated in the main claim. The invention is applied to cool electrodes in direct current electric arc furnaces used to melt metal alloys. Although the cooling system can be applied both to the electrodes associated with the crown of the furnace (cathodes), and to the electrodes of the bottom of the furnace (anodes), in the following description, for reasons of practicality, we will refer to the application of the system to an electrode that works like a cathode. At present, the electrodes of electric arc furnaces, when functioning as a cathode, are generally composed of two main parts: a lower part made of graphite, and an upper part made of a metallic material, which also has a Support function, associated with the oven electrode support arm. When the electrode is functioning as an anode, the part of graphite is replaced by a part of copper, but the following description can be applied in the same way, with appropriate transpositions and adaptations. The two parts of the cathode are limited to each other by an intermediate joint, usually threaded, made of an electrically conductive material to allow the passage of electric current. During the casting cycle, the graphite part reaches very high temperatures, and is consumed progressively, so that from time to time new segments of graphite are added. These temperatures cause a flow of heat from the graphite part to the metal part of the electrode, which can cause damage to its structure, apart from causing heat dispersion that is useful in the casting process that is being carried out. Moreover, excessive overheating of the intermediate joint can compromise the mechanical stability of the connection between the two electrode parts. For this reason, the electrode needs a cooling system that acts in correspondence with the metal part, and that can remove a large part of the heat that migrates from the graphite part to the metal part. However, this cooling system must be achieved in such a way that the following three results are obtained concurrently: uniformity and temperature control of the metallic connection in order not to subject it to mechanical stresses, which make it unstable; a good electrical contact that reduces the Joule effect to a minimum; an increase in heat resistance in order to reduce energy losses, and to reduce the temperature of the mechanical connection. These results are all obtained together if the cooling system in its entirety, either due to the material used, or due to the structure, or due to the dynamics of its operation, makes it possible to obtain a low heat conductivity, and at the same time , a high electrical conductivity. However, it is well known that this is difficult to achieve, since materials that have a high heat conductivity are also good electrical conductors, and vice versa. Some known solutions in the state of the art include the use of traditional cooling systems that use water, which, however, have not been considered satisfactory by those operating in the field. European Patent Number EP-A-0, 682, 463, for example, teaches the cooling of the anode by means of forced recirculating water, which is not electrically conductive. This solution has given excellent results, but it is not considered to be definitely satisfactory.

Stahl and Eisen, volume 110, number 8, August 14, 1990, "Kuehlung von lichtbogenofenelelektroden durch waermerohre" provide cooling of the electrodes by removing the heat produced by evaporation, and the subsequent condensation of the refrigerant, and then put back into circulation by means of closed tubes. In other words, it uses systems to transport heat through the vapors of the refrigerant, which can be water or monatomic liquid metals (Hg, K, Na). When vapors are used, it uses the latent heat of the cooling fluid that evaporates and condenses. European Patent Number EP-A-0, 223, 991 provides the cooling of oxygen injection nozzles in a converter, by means of a molten liquid metal, which is circulated by means of a remote circulation pump, co-operating the metal liquid with a remote heat exchanger. However, a circuit like this can not be adopted in association with electrodes for electric ovens, given the problems of installation and maintenance that would cause, and also the problems related to the carrier currents and parasitic currents that would come into play in this circuit. Accordingly, the present applicants have designed, tested and incorporated this invention to overcome the drawbacks of the state of the art, and to achieve other advantages. This invention is stipulated and characterized in the main claim, while the dependent claims describe variants of the idea of the main mode. The purpose of the invention is to provide a cooling system for electrodes in direct current electric arc furnaces, which ensures an efficient cooling action on the metallic part of the electrode, and which can also control and limit the flow of the desired values to the desired values. heat that, from the part subjected to high temperatures, either the graphite part of the cathode or the copper part of the anode, is transmitted to this metal part, maintaining substantially the electroconductivity characteristics of the electrode itself. A further purpose of the invention is to ensure a low temperature of the connection between the graphite or copper part and the metal part, in order to obtain a high mechanical stability of the connection joint. Another purpose of the invention is to obtain this cooling without compromising the electrical conductivity of the electrode. An additional purpose is to achieve a lower energy consumption in the oven feed.

The cooling system according to the invention can be used, with the appropriate adaptations, both with electrodes functioning as a cathode, and with electrodes that function as an anode. The invention includes a closed circuit cooling system that uses molten metal, not its vaporous state, to transport heat, and therefore uses the perceptible heat transported by the cooling fluid. The metal used as a cooling fluid, in accordance with the preferred embodiment, is a lead and bismuth eutectic (eg, 55 percent lead and 45 percent bismuth). According to a variant, tin, sodium, potassium or lithium can be used as a cooling fluid; in this case also, the metal is kept in its molten state, and not as a vaporized metal. The closed circuit comprises channels up and down for the cooling fluid, and at least one element that functions as a heat exchanger. In accordance with the invention, at least some of these channels cooperate with a hydro agnatic pump activated by the passage of a current. According to a further embodiment, inside the connection joint between the graphite part and the metal part of the electrode, there is a hole of the appropriate shape, where the cooling fluid circulates. The metal that functions as the cooling fluid, in one embodiment of the invention, at room temperature, is in its solid state, and is melted by the effect of the heat generated by the electric arc, the passage of the electric current (effect de Joule), and the heat exchange with the internal part of the furnace in the different steps of the casting process. According to a variant, the metal is already in its liquid state at room temperature. According to a further variant, the metal at room temperature is in its solid state, and consists of small burrs or other granular bodies of small dimensions. According to a variant, a first channel, or delivery channel, cooperates with the hydromagnetic pump, which delivers the liquid metal to the channel, which develops in correspondence with the side walls of the metal part of the electrode. According to another embodiment, the outer side walls of the delivery channel are made of two metal sleeves closely associated with each other, one of which (internal or external) is made of copper or its alloys, and the other of which (the external or internal) is made of metal or its alloys. In accordance with another variant, a second channel, or return channel, is developed from where the hydromagnetic pump sucks in the liquid metal in an area near the axis of the electrode. According to one embodiment of the invention, the delivery and return channels for the circulation of the liquid metal communicate directly with the internal gap of the joint. According to a variant of this mode, between the two environments, that is, between the delivery and return channels and the internal gap, there is a grid with suitable conveyor / diverter elements for directing the liquid metal from the delivery channel towards the hollow, and from the hollow towards the return channel. In correspondence with the heat exchanger, the liquid metal releases heat to the external environment, cools, and returns to the desired temperature. According to this method, the liquid metal flow coming from the delivery channel, apart from cooling the walls of the metallic part of the electrode and the walls of the joint, also removes the heat coming from the graphite part thereof. electrode. According to a further embodiment of the invention, between the circulation channels of the liquid metal and the hollow inside the joint, there is a separating element suitable for creating two different cooling circuits. In a possible solution, the metal of the two circuits is the same. According to a variant, in the two cooling circuits there are two different cooling fluids. In this case, in accordance with the preferred embodiment of the invention, a lead and bismuth eutectic is circulated in the main circuit, while lead or sodium circulates in the hole. According to a variant, laminar diaphragms made of a highly electroconductive material (for example, copper) configured substantially parallel to the axis of the electrode are mounted on the separating element. According to a further variant, the gap of the joint has, in cooperation with the graphite part and substantially in correspondence with the axis of the electrode, a conveyor insert of an elongated shape that develops upwards. This conveyor insert, made of a metal with greater properties of electroconductivity than those of the joint, is the main passage for the electric current from the graphite part to the metal part of the electrode. The passage of electrical current through the conveyor, and from the conveyor to the metal part of the electrode, in accordance with the variation in current density, causes the formation of vortices inside the hole, which cause the liquid metal to elevate in correspondence with the walls of the conductor insert, and then descend along the internal walls of the gap. This circulation inside the gap is in a direction opposite to that of the metal in the main circuit, which makes it possible to obtain a considerable level of uniformity of the temperatures in the mechanical connection. The side walls of the conveyor insert are electrically protected, in order to concentrate the flow of current at both ends. In the mode that includes the separator element, the liquid metal inside the gap is used to make the temperature of the joint uniform, while the liquid metal that circulates in the channels has a cooling function, and also serves to remove the liquid. flow of heat that comes from the graphite part of the electrode. According to the invention, in the area where the graphite part and the metal part are joined, the joint includes an air ring suitable for transporting the flow of electric current, and consequently, the heat, to a corresponding position with the central area of the electrode.

According to a variant, on the underside of the joint in contact with the graphite part, there is a metallic element that melts at a low temperature (for example, lead). This element, which is passed through by electric current, melts and therefore increases its volume; It raises the joint and places it between the two parts of the joint itself, thus improving the mechanical connection and the passage of the current. The system according to the invention, therefore, makes it possible to cool the walls of the metal part of the electrode, and contrasts the rise of the heat flow coming from the graphite part, maintaining the electroconductive characteristics of the electrode unchanged, and consequently, without causing functional imbalances of the furnace. The system according to the invention also makes it possible to make the temperature of the joint uniform, and keep it within the appropriate values, this in order to guarantee the mechanical stability of the connection between the graphite part and the metal part; moreover, this stability increases as a result of the interstices between the walls of the joint that are filling. According to the invention, the intensity of the cooling of the side walls of the metal part of the electrode, and the level of the temperature of the joint, can be varied by intervening in the operating cycle of the hydromagnetic pump and / or in the position and / or on the characteristics of the heat exchanger. In one embodiment of the invention, the entire cooling system can be replaced to review operations to restore, maintain or replace the metal used. The attached figures are given as a non-restrictive example, and show some preferred embodiments of the invention as follows. Figure 1 shows the longitudinal section of an electrode in an electric arc furnace, which uses the cooling system in a first embodiment of the invention. Figure 2 shows detail "A" of Figure 1. Figure 3 shows the longitudinal section of an electrode in an electric arc furnace, which uses the cooling system in a second embodiment of the invention. Figure 4 shows the detail "B" of Figure 3. In the attached figures, the number 10 indicates the cooling system for the electrodes 11 in direct current electric arc furnaces. In the case shown, the electrode 11 functions as a cathode, has a graphite part at the top, and a metal part 11b below, which also functions as a support member associated with its own element 12 for attaching to the electrode support arm . In this case, the structure of the metal part 11b is composed of two metal sleeves closely associated with each other, one of which, either 111b or 211b, is made of copper or its alloys, and the other of which, 211b or 111b is made of iron or its alloys. This configuration of the metal part 11b is convenient, because it gives high characteristics both of mechanical strength and also of electrical conductivity; the metallic part 11b, moreover, is covered, on its lower part, on the outside, by a refractory layer 11c. According to the invention, the graphite part Ia and the metal part 11b are associated with each other by means of a threaded joint 13, which has inside a hollow 14 filled with metal, with a desired casting temperature. This recess 14 cooperates with a circuit 20 containing the same metal, and comprising a delivery channel 15 that develops along the inner side wall 111b of the metal part 11b, a return channel 16 that develops in a central position with respect to the metal part llb, a hydromagnetic pump 17 communicating with the two channels 15, 16, and a heat exchanger 18 mounted outside on the metal part llb, and associated with the delivery channel 15. In this case, the delivery channel 15 develops in an annular manner, while the return channel 16 extends coaxially with respect to the metal part llb of the electrode. According to the invention, the heat developed inside the furnace causes the metal contained inside the gap 14 and the circuit 20 to melt, in the event that this metal is in its solid state at room temperature. Once the metal is completely melted, it enters circulation under the effect of the hydromagnetic pump 17. According to another embodiment, the metal is already in its liquid state at room temperature. According to another embodiment, the metal is in its solid state, and consists of small burrs or other granular bodies with a small dimension, which facilitates and accelerates the first step of the casting. The hydromagnetic pump 17 sucks in the molten metal from the return channel 16, and sends it, through the delivery channel 15, to the heat exchanger 18, where this liquid metal gives off heat to the external environment before reach hole 14 (Figure 1). In this step, the liquid metal flow 21 along the delivery channel 15, causes the side walls 111b, 211b of the metal part 11b to cool. In an intermediate position between the hollow 14 and the two channels 15, 16, there is a laminar grid 19 having, on its periphery, deviators 19a placed in such a way as to direct, at least partially, the flow 21 of the liquid metal arriving from the delivery channel 15, towards the hollow 14 (Figure 2). The laminar grid 19 has, in its central position, deviators 19b positioned in such a way as to direct the liquid metal flow 21 from the hollow 14 to the return channel 16. This direction of flow of the liquid metal flow 21 is opposite to the direction of flow. direction taken by the electric current and the consequent flow of heat that, from the graphite part Ia, extends towards the metal part 11b mainly through the walls 13a of the joint 13. This difference in the direction between the flow 21 of Liquid metal and heat flux, causes a reduction in heat dispersion, which is useful for the melting process to take place; it is also useful to cool the gasket 13, which allows the temperature of the gasket to be kept within appropriate values, with the advantage of the mechanical stability of the connection between the two parts Ia, llb. This mechanical stability is further increased by the inclusion of a plate 22 made of metal, which melts at a low temperature, for example, lead, between the lower face 13b of the joint 13, and the graphite part Ilia. When the electric current passes through, this plate 22 melts, increases in volume, and expands into the interstices 13c between the threads of the gasket 13 and the graphite part Ilia. According to the invention, in order to limit the passage of electric current, and consequently, the flow of heat, in the peripheral areas of the electrode 11, the gasket 13 has, in its outermost part, an air ring 13d to separate it from the graphite part lia. Due to the presence of this air ring 13d, the electric current and the relative heat flow extend mainly through the side walls 13a of the gasket 13, and consequently, in the area of greatest efficiency of the cooling system 10. According to the variant shown in Figures 3 and 4, the recess 14 does not communicate with the delivery channel 15 and the return channel 16, and is separated from them by means of a partition wall 23, in this case solid with the gasket 13. On this partition wall 23 there is a plurality of laminar diaphragms 24, made of a highly electroconductive material (such as copper), configured substantially parallel to the longitudinal axis of the electrode 11 and, in this case, extending from the upper face 23a of the partition wall 23 upwards to the hollow 14. In a position coaxial with respect to the joint 13, a conveyor insert 25 made of a material is solidly associated. highly electroconductive (for example, copper). This conveyor insert 25 is of an elongated shape, and extends upwards into the recess 14; in correspondence with the external side walls, it is coated with a layer of electrically insulated material 27. This configuration of the conveyor insert 25, causes the current flow 26 to take a preferential path; from the graphite part, it extends mainly through the conveyor insert 25, from one end to the other, until, near the top 25a of the conveyor insert 25, it expands into the gap 14. This causes the formation of swirling flows 21a of the liquid metal inside the recess 14, which then move upwards in correspondence with the conveyor insert 25, and descend again near the side walls 13a of the joint 13. The swirling flow 21a of the liquid metal makes it possible to cool the gasket 13, and also because it flows in the direction opposite to the flow in the main circuit 20, it makes the temperature uniform, releasing its heat towards the flow of the liquid metal 21 circulating in the gas. main circuit 20 through the laminar diaphragms 24. By using liquid metal as a cooling fluid, the system according to the invention can maintain substance Without changing the electroconductivity characteristics of the electrode 11, without causing imbalances with respect to the functionality of the electric furnace. The invention makes it possible to intensively exploit the effect of electromagnetic stirring of the cooling fluid, as generated by the electric current lines, whose effect is exasperated by the geometric arrangement of the parts.

Claims (22)

REIVI DICACIONES

1. A cooling system for electrodes (11) in direct current electric arc furnaces, the electrodes (11) comprising a part (Ha) subjected to the hot environment of the furnace, this part being made of graphite in the case of a cathode, and copper in the case of an anode, the part (Ha) being associated with a metal part (llb) by means of a joint (13), which includes inside it a gap (14), the system being characterized because it comprises, at least in correspondence with the metallic part (llb), a cooling circuit (20) with at least one delivery channel (15), a return channel (16), and a heat exchanger (18), using the circuit (20) Liquid metal as a cooling fluid, also including an element for the forced circulation of the cooling fluid, consisting of a hydromagnetic pump (17).

2. A cooling system as in claim 1, wherein the heat exchanger (18) is placed outside the metal part (llb).

3. A cooling system as in any of the preceding claims, wherein the delivery channel (15) and the return channel (16) cooperate in the lower part with the recess (14) inside the joint (13).

4. A cooling system as in any of the preceding claims, wherein, at the bottom of the recess (14), there is at least one conveyor insert (25) extending along the top of the recess (14).

5. A cooling system as in claim 4, wherein the conveyor insert (25) has, on its lateral periphery, a layer of electrically insulating material (27).

6. A cooling system as in any of the preceding claims, wherein, in cooperation with the gasket (13), there is an element (22) made of a low melting metal, which serves to disperse heat and clog the interstices (13c).

7. A cooling system as in any of the preceding claims, wherein, in the area of separation between the part (lia) and the metal part (lb), there is an air ring (13d).

8. A cooling system as in any of the preceding claims, wherein, the recess (14) has a laminar grid (19) with annular deviators (19a, 19b).

9. A cooling system as in claim 8, wherein the diverters (19a, 19b) include elements for directing and transporting the fluid.

10. A cooling system as in any of claims 1 to 7, inclusive, wherein, between the gap (14) and the delivery channel (15) and return (16) there is a separation wall (23).

11. A cooling system as in claim 10, wherein the partition wall (23) includes circular laminar diaphragms (24), which extend into the recess (14).

A cooling system as in any of the preceding claims, wherein, the outer wall of the delivery channel (15) includes a layer (111b or 211b) made of copper or its alloys, and a layer (211b or 111b) made of iron or its alloys.

13. A cooling system as in any of claims 1 to 12, inclusive, wherein, the metal used is a eutectic of lead and bismuth.

14. A cooling system as in any of claims 1 to 12, inclusive, wherein, the metal used is tin.

15. A cooling system as in any of claims 1 to 12, inclusive, wherein, the metal used is sodium.

16. A cooling system as in any of claims 1 to 12, inclusive, wherein, the metal used is potassium.

17. A cooling system as in any of claims 1 to 12, inclusive, wherein, the metal used is lithium.

18. A cooling system as in any of claims 1 to 17, inclusive, wherein, the metal, at room temperature, it is in its solid state.

19. A cooling system as in any of claims 1 to 17, inclusive, wherein, the metal, at room temperature, is in its liquid state.

20. A cooling system as in any of claims 1 to 17, inclusive, wherein the metal, at room temperature, is in a solid granular state.

21. A cooling system as in any of the preceding claims, wherein the delivery channel (15) is of a toric shape, and extends around the circumference of the metal part (llb) of the electrode (11).

22. A cooling system as in any of the preceding claims, wherein the return channel (16) extends centrally and axially to the electrode (11).