Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D99/00—Subject matter not provided for in other groups of this subclass
  • F27D99/0001—Heating elements or systems
  • F27D99/0006—Electric heating elements or system
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27B—FURNACES, KILNS, OVENS OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
  • F27B3/00—Hearth-type furnaces, e.g. of reverberatory type; Electric arc furnaces ; Tank furnaces
  • F27B3/10—Details, accessories or equipment, e.g. dust-collectors, specially adapted for hearth-type furnaces
  • F27B3/24—Cooling arrangements
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/52—Manufacture of steel in electric furnaces
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D11/00—Arrangement of elements for electric heating in or on furnaces
  • F27D11/02—Ohmic resistance heating
  • F27D11/04—Ohmic resistance heating with direct passage of current through the material being heated
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D99/00—Subject matter not provided for in other groups of this subclass
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B7/00—Heating by electric discharge
  • H05B7/02—Details
  • H05B7/12—Arrangements for cooling, sealing or protecting electrodes
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/52—Manufacture of steel in electric furnaces
  • C21C5/5229—Manufacture of steel in electric furnaces in a direct current [DC] electric arc furnace
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the present invention refers to an electrode for direct current continuous arc furnaces, used in particular as anode on the bottom of said furnaces.
• DC EAFs Direct current continuous arc furnaces
• the electric arc is discharged between at least a graphite electrode arranged at the top (cathode) and at least a bottom electrode (anode) arranged at the bottom in the hearth of the furnace.
• the passing of the current allows the formation of the electric arc whose effects of radiation and convection make the iron scrap to melt.
• the direct current arc furnace advantageously permits to have a lower consumption of electric energy, a lower consumption of electrodes and of refractories, uniform and fast melting of the scrap iron (because of the great length of the arc obtainable), a decrease in the noise and in the mechanical stresses and good stewing of the liquid metal bath. Furthermore, the variations of reactive power and the "flicker" effect are significantly less.
• continuous direct current arc furnaces have a top electrode or cathode, associated with the crown of the furnace and which extends inside the furnace itself, and a plurality of lower electrodes or bottom electrodes or anodes incorporated in the refractory hearth of the furnace for closing the electric circuit.
• the anodes are one of the most delicate components as they are crossed by currents having very high intensity and are subject to significant thermal stresses and magnetic forces.
• Such bottom electrodes are, for example, made in the shape of metal bars, incorporated in the refractory hearth of the furnace, which partially extend at the bottom end thereof outside of the furnace itself.
• the number of said bars and the arrangement thereof, which is symmetrical with respect to the middle of the furnace, depends on the power of the furnace and on the conformation of the hearth thereof.
• said metal bars may be divided into a plurality of billets having very small diameter, which are bottomly fixed on a common plate, generally air-cooled and connected to the power supply by means of water-cooled pipes.
• each electrode unit in place of the billets, may consist of a plurality of metal tabs welded on a common metal support and arranged in cooperation with other electrode units so as to form a ring, which is concentric to the furnace.
• the bar type electrodes may be made entirely in steel or in steel and copper.
• the top steel part of said bars being in contact with the melted metal bath, it melts up to a certain height. According to the efficiency of the cooling, the bar has a top liquid part and a lower solid part, divided by a separation zone.
• the main problem is the one of developing a cooling system capable of ensuring a solid lower part, along the height of the bar, which is as extended as possible also under conditions of high electric and thermal load conducted by said bottom electrodes.
• a first solution provides the use of a bimetallic steel-copper anode in the shape of billet, equipped with channels for the passage of a fluid in order to cool down the copper part, similarly to a continuous casting crystallizer.
• the heat exchange mechanism is the one of forced convection with a single-phase fluid (water in the liquid state). The movement of the cooling fluid substantially occurs parallel to the surface to be cooled down, which requires a certain speed and a certain dimension of the channels to ensure an adequate heat exchange.
• a second solution provides the use of a bimetallic steel-copper anode equipped with a cooling system, within the copper part of the anode, which uses a two-phase cooling fluid (liquid-gaseous) by means of drop atomization and the successive boiling thereof when they come in contact with the surface to be cooled down.
• the phase transition (the so-called "boiling") permits to efficiently remove the heat but, disadvantageously, only up to a critical temperature. There is a rapid decay of the heat exchange properties beyond this critical temperature which therefore leads to poor system reliability. If the complete perforation should occur of the base of the anode, a series of barriers is provided for blocking the leakage of the melted fluid into the housing of the cooling system, thus however involving increased construction complexity.
• Another object of the invention is to obtain efficiency of the cooling action of the bottom electrode, made in the shape of bimetallic billet, highly superior to that possible to date, by optimizing the heat exchange by means of a particular geometry of the cooling channels.
• a further object of the present invention is simultaneously ensuring that the conditions are kept of optimal thermal and electrical conduction at the junction zone between the cooled part and the uncooled part of the billet, thus obtaining improved furnace operation in terms of production efficiency, increased duration of the electrode, increased reliability and safety.
• an electrode for continuous direct current arc furnace for melting metals adapted to be accommodated in a bottom of said furnace, which, according to claim 1 , comprises
• bimetallic bar defining a longitudinal axis X, comprising along the longitudinal extension thereof a first steel portion, a first end of which is adapted to come in contact with a metal bath within the furnace, and a second copper portion welded to a second end of said first steel portion;
• cooling means comprise
• a collector placed at a first end of the cooling means, having an end wall provided with a plurality of channels which put said collector into communication with a first portion of said gap which is proximal to said first steel portion;
• the collector has a cross section area of at least 1 .5 times the cross section area of the first pipe with respect to said longitudinal axis
• a second aspect of the present invention provides a cooling process of the aforesaid electrode which, according to claim 13, comprises the following steps:
• the solution of the invention uses the heat exchange by convection mechanism by using a single-phase fluid, preferably water in the liquid state.
• the cooling action is advantageously double.
• Primary cooling is generated by means of moving the cooling liquid substantially in perpendicular direction with respect to the wall to be cooled down, thus taking advantage of the strike ("impinging") of the jets to remove heat.
• the characteristic of the perpendicularity of the confined jets permits to free the distance of the cooling system from the primary surface of the anode to be cooled down, thermal exchange being equal. Therefore a clearance or greater distance may be provided between said primary surface and the outlet section of the injection holes of the cooling liquid so that the mechanical deformations of the anode, under the effect of the high current, do not influence on the efficiency of the cooling, as instead occurred in the first solution of the state of the art.
• secondary cooling is generated by moving the cooling liquid substantially in direction parallel to the surface to be cooled down.
• Such a secondary cooling already manifests itself in the zone of the plate or holed cap, since after having struck the curved surface of the electrode jacket, the liquid of the jets tends to lap it until it reaches the vertical walls of the meatus or gap between electrode jacket and the cooling lance, in which the liquid then descends vertically in direction parallel to the corresponding surface of the anode, thus going towards the discharge section.
• the cooling system of the electrode of the invention operates as a single-phase
• Said system may operate indifferently with high or low outlet pressure.
• the number of holes of the plate or holed cap depends on the surface to be cooled down. Injection nozzles of the cooling liquid may also be provided.
• bimetallic steel-copper billet permits to ensure an excellent electrical conductivity and to extend the cooling effects in direction of the liquid steel bath. This permits to keep the solid-liquid interface of the steel as far as possible from the cooled zone.
• An efficient cooling system permits to further improve this aspect by significantly increasing the safety factors.
• Fig. 1 shows a first side view of the electrode according to the invention
• Fig. 2 shows a sectional view of the electrode in Fig. 1 , along plane A-A;
• Fig. 2a shows a sectional view of a first part of the electrode in Fig. 1 , along plane A-A;
• Fig. 2b shows a sectional view of a second part of the electrode in Fig. 1 , along plane ⁇ - ⁇ ;
• Fig. 3 shows a bottom view of the electrode in Fig. 1 ;
• Fig. 4 shows a sectional view of the electrode in Fig. 3, along plane B-B;
• Fig. 5 shows a view of the electrode of the invention incorporated within the hearth of the arc furnace
• Fig. 6a shows a sectional side view of a component of the cooling system of the electrode of the invention
• Fig. 6b shows a top view of the component in Fig. 6a.
• a first embodiment of an electrode for continuous direct current arc furnaces is represented, globally indicated with reference numeral 1 , in particular a bottom electrode or anode to be incorporated within the refractory hearth of said furnaces.
• the electrode 1 object of the present invention, comprises:
• the third copper portion 6 has a variation of section, in particular an enlargement from the top towards the bottom of the cross section thereof with reference to Fig. 2.
• Figures 1 and 2 show a steel-steel welding cordon 8 between first portion 4 and second portion 5; a steel-copper cordon or welding line 9 between second portion 5 and third portion 6; a copper-copper welding cordon 10 between third portion 6 and fourth portion 7.
• the fourth copper portion 7 substantially consists of a cylindrical tube.
• the third copper portion 6 at its bottom includes a recess 13, made in the wider portion of the third portion 6.
• Second steel portion 5 and third copper portion 6 define the so- called “spacer” to conveniently space the cooling zone of the electrode from the steel part of billet 2 which will become liquid during the operation of the arc furnace.
• the copper part and the steel part of the billet 2 which will remain solid are welded together to ensure the passage of current and the thermal continuity.
• the cooling means 3 are accommodated, at least partially, in the longitudinal cavity 50 defined by the inside of the cylindrical tube 7 and by said recess 13 of the third copper portion 6.
• Spacer 5, 6 and cylindrical tube 7 define the so-called electrode jacket.
• the cooling means 3 comprise a cooling lance substantially cylindrical 1 1 in preferably metal material, in turn comprising the following elements:
• cylindrical tube 12 having an outer diameter slightly smaller than the inner diameter of the cylindrical tube 7;
• a convex or substantially flat lid 14 provided for closing a first end of said tube 12, provided in the outer wall thereof with a plurality of channels 20, e.g. in shape of simple through holes;
• the tube 12 is preferably coaxial to said conveyer pipe 19.
• the lid 14 is associated with an annular element 16, in the central hole 18 of which the conveyer pipe 19 is inserted.
• the central hole 18 has a flare towards the collector 17.
• the collector 17 has a cross section area of at least 1 ,5 times the cross section area of the pipe 19 with respect to said longitudinal axis X.
• the cross section of the collector 17 is equal to at least double the cross section of the pipe 19.
• the diameter of the collector 17 is preferably equal to 1 ,5 times the diameter of the pipe 19. In case of sections other than circular, the same relationship is preferably between the respective equivalent diameters.
• the annular element 16 is integrally fixed, e.g. by means of welding, to the cylindrical tube 12.
• the same lid 14 may be integrally fixed, e.g. by means of welding, above said annular element 16, thus defining the collector 17 therewithin.
• the holes 20 are made in the thickness of the lid 14 by putting the inner collector 17 in communication with the exterior of the cooling means.
• the lid 14 may have the shape of a hemispheric cap or of a dome more or less squashed at the top or of a substantially flat plate, according to the shape of the inner central surface of the electrode to be cooled down.
• the profile of the recess 13 substantially corresponds to the outer profile of the lid 14.
• the conveyer pipe 19 protrudes from the cylindrical tube 12 at the second end of the latter, that is from the opposite side with respect to the lid 14, and is connected to an inlet flange 21 of the cooling water.
• the lance 1 1 is accommodated within the electrode jacket.
• the shape of the recess 13 is such as to receive the lid 14 of the lance 1 1 .
• a predetermined clearance or distance H is provided between the recess 13 and lid 14, at the inner central or primary surface 23 of the electrode to be cooled down, which, preferably, decreases from the inner side or secondary surface 24 of the electrode.
• the distance H between the primary surface 23, also called “wetted surface”, and the corresponding surface of the lid 14, that is between the primary surface 23 and the outlet section of the holes or nozzles 20, is in the range between 5 and 30 mm, preferably between 6 and 12 mm. In a preferred variant, the distance H is equal to 8 mm. This distance H corresponds to the width of a first portion of the gap between cavity 50 and cooling means 3 or lance 1 1 .
• the width of the meatus 25 between tube 7 and tube 12 is preferably between 2 and 12 mm, said width of the meatus 25 corresponding to the width of a second portion of the gap between cavity 50 and cooling means 3 or lance 1 1 .
• Such a meatus 25 is connected to a discharge pipe 26 of the cooling water provided with an outlet flange 27.
• the diameter "d,” of the holes or nozzles 20 is advantageously in the range between 1 and 10 mm, preferably between 1 and 5 mm. In a preferred variant, the diameter "d,” is equal to 3 mm.
• the spacing between the holes 20, indicated with L d is a function of the diameter of the holes, preferably but not necessarily equal to a multiple of said diameter.
• the distribution may be uniform or not uniform on the surface of the lid 14.
• the spacing L d is in the range between 3 and 15 times the diameter d, of the holes 20, preferably in the range between 6 and 1 1 times the diameter of the holes. In a preferred variant, L d is equal to 31 ,5 mm.
• the distribution criteria of the holes on the lid 14 is based on an optimal coverage of the primary surface 23 to be cooled down by part of the whole of high-efficiency cooling regions generated by the strike of the individual jets.
• the holes 20 are made in the lid 14 so as to have a longitudinal axis thereof substantially perpendicular to the plane tangent to the outlet section thereof, that is substantially perpendicular to the corresponding portion of primary surface 23.
• the primary surface 23 or "wetted area" may be planar or curved and the extension thereof in terms of equivalent diameter D eq depends on the diameter of the billet 2 and has a maximum value of about 700 mm, preferably in the range between 250 and 600 mm. In a preferred variant, D eq is equal to 550 mm.
• the bimetallic billet 2 of the bottom electrode 1 is incorporated in the refractory hearth 30 of a continuous direct current arc furnace.
• the billet 2 is surrounded by at least an order of annular refractory ferrules 31 .
• the top end 32 of the billet 2 is in contact with the liquid metal bath (not shown) within the furnace. Said contact with the liquid metal, together with the effect of the passing of high currents along the billet itself, determines the formation along the billet 2 of a top liquid part 33 and of a bottom solid part 34, separated by an interface zone 35.
• the cooling liquid of the electrode preferably but not necessarily water, is continuously introduced with a predetermined flow in the conveyer pipe 19, it flows along it until arriving at the central hole 18 of the annular element 16 thus reaching the collector 17 inside the lid 14.
• the central hole 18 of the first annular element 16 has a flare towards the collector 17 for minimizing load losses and recovering the highest pressure within the collector 17.
• the pressure of the cooling liquid within the collector 17 is in the range between 1 and 15 barg, preferably equal to about 12 barg.
• the liquid is perpendicularly injected onto the primary copper surface 23 through the plurality of the holes 20.
• the speed v jet of the liquid jets exiting the holes 20, which affects the local transfer of heat, has a maximum value of 50 m/s, preferably in the range between 25 and 30 m/s. In a preferred process variant, such a speed is equal to about 27 m/s.
• the speed of the liquid jets, continuously exiting the holes 20, is such to prevent any possibility of evaporation of the liquid in contact with the inner surface of the copper part of the electrode.
• the distribution of the holes 20 and the process parameters are such to obtain a maximum thermal flow through the copper part of the electrode equal to about 20 MW/m 2 .
• thermocouples 40 is provided ( Figures 3 and 4) installed in the copper slug of the anode, in particular the third section 6, to map the thermal flow which develops through the copper part of the electrode.
• thermocouples 40 are accommodated inclined at about 60° with respect to the longitudinal axis of the electrode and the front ends thereof are arranged close to said axis.
• This configuration of the thermocouples 40 has the advantage of permitting the continuous monitoring of the status of one of the most thermo-mechanically stressed zones.
• thermoresistances 41 are provided within the tube 7 and the copper section 6 of the electrode, as further control instrument of temperature and diagnostics.
• cooling system of the electrode of the invention permits to generate and discharge cooling liquid jets in a zone, or "top collector”, delimited at the top by the primary surface 23 to cool down.
• the cooling system of the electrode of the invention operates as a single-phase (only water, no air within the system) and closed system. Said system may operate indifferently with high or low outlet pressure.
• the configuration of the lid 14, cap or holed plate shaped, and of the zone between primary surface 23 and the same cap or holed plate is such to promote the substantially perpendicular strike of the liquid jets on the primary surface 23 and the successive flow descending towards the discharge of the cooling system where the cooling effect of the jets was completely converted into turbulent convection.
• the liquid jets strike on the "wetted area" 23 which is the cooled top area of the anode; the rest of the inside of the copper jacket of the electrode, that is the meatus 25, being cooled by the descending flow of liquid coming from said cooled top area (secondary cooling).
• a turbulent jet which perpendicularly strikes on a flat surface has generated, in the region close to the stagnation point of the jet, among the highest heat exchange coefficient values encountered in single-phase convection (no air).

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Plasma & Fusion (AREA)
• Chemical & Material Sciences (AREA)
• Manufacturing & Machinery (AREA)
• Organic Chemistry (AREA)
• Metallurgy (AREA)
• Materials Engineering (AREA)
• Vertical, Hearth, Or Arc Furnaces (AREA)
• Discharge Heating (AREA)
• Furnace Details (AREA)
• Control Of Resistance Heating (AREA)
• Resistance Heating (AREA)

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• 2009
  • 2009-12-15 IT ITMI2009A002192A patent/IT1396945B1/it active
• 2010
  • 2010-12-15 RU RU2012129987/02A patent/RU2516116C2/ru not_active IP Right Cessation
  • 2010-12-15 WO PCT/EP2010/069730 patent/WO2011073244A1/en not_active Ceased
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  • 2010-12-15 JP JP2012543712A patent/JP5475143B2/ja active Active
  • 2010-12-15 US US13/261,323 patent/US9335097B2/en active Active
  • 2010-12-15 CN CN201080056170.1A patent/CN102656415B/zh active Active
  • 2010-12-15 ES ES10803070.1T patent/ES2444620T3/es active Active
  • 2010-12-15 KR KR1020127018453A patent/KR101398663B1/ko active Active
  • 2010-12-15 EP EP10803070.1A patent/EP2513581B1/en active Active
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