ES2260496T3 - ELECTRODE, IN PARTICLES FOR ELECTRIC AND SIMILAR ARC OVEN, AND CORRESPONDING METHOD OF OPERATION. - Google Patents

Abstract

An electrode (10), in particular for electric arc steel furnaces and the like, which comprises: a cylinder-shaped columnar body (16) of conductive material, partially coated with a protective layer (17) of insulating material, and a axial cavity (19) formed in the bottom part (26) of said columnar body (16), means (20, 23) for projecting the ejector fluid (22) towards the metal bath of the furnace, said fluid comprising a combustible component rich in heat decomposable carbon, characterized in that said means for the projected fluid comprise: a sliding lance (20) housed within said axial cavity (19) that ends with a nozzle head for the projection of said fluid which produces a cloud that protects the tip (14) of the electrode (10) and covers the surface of the electrode with a solid layer (27); and nozzles placed on the cylinder-shaped surface of the electrode (10) for the projection of another fluid (24) as a bottom of the columnar body (16) to interact with the surface, to perform a protective action.

Description

Electrode, in particular for ovens electric arc steelmakers and the like, and operation method correspondent.

The present invention relates to an electrode and to the corresponding method of operation, directed in particular to steel arc furnaces and similar.

For steel plants manufacturing steel electric furnaces are used that melt the raw material as p. eg, metal scrap and the like, by heat from (directly or indirectly) of one or more AC electric arcs or DC

These arcs are generated by the effect of voltage applied between the graphite electrodes and the charge of the metal.

These electric ovens employ electrodes whose bottom end, defined as electrode tip, is located at a distance from the metal (scrap or bath) under fusion which increases proportionally to the value of the voltage applied between the electrodes and charge.

Conventional electrodes are subjected during operation to wear phenomena of the tip (wear of the tip) and on the side wall of the electrode (wear by oxidation).

As can be inferred from different technical literature, among which p. eg, "The Electric Arc Furnace" - 1997, published by the International Iron and Steel Institute , oxidation wear depends on the temperature and the atmosphere surrounding the same electrodes, while the wear of the tip increases proportionally with the value of the current density that passes through the electrodes.

In general, the graphite wear of a electrode of a steelmaking electric furnace can valued as follows:

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oxidizing wear: at the rate of approximately 50 - 70% of total wear for blast furnaces productivity; Y

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tip wear: the remaining 30% - 50%.

These wear, when they are too high, they also imply frequent losses of time necessary to Restore electrode functionality.

Wear and waste of time, together with electricity consumption they represent the most cost important of steel manufacturing.

In order to reduce the problems stated above, and in particular oxidation wear, the electrodes were modified by cooling their outer surface by water sprayers. However, the latter did not give result when the part of the internal electrode cannot be reached at oven.

For metallurgical and process purposes, electrodes were also developed that had a coaxial conduit to them for the introduction of solid materials into the bath and directly into the electric furnace.
co.

These solid materials, such as p. eg the coal, have been used to generate foamy slags and to effect the metallurgical reduction of the oxides present in the chemical composition of slag produced during manufacturing of steel.

However, the introduction of these materials solid causes other problems, tending in fact to make the electric arc operation is unstable, as well as causing often the obstruction of the optional axial cavity of the electrode to from which these materials are introduced.

The patent US-A-3,730,961 (Bryce) describes a method for ignition in an oven where a fuel oil or a gas it is introduced through the electrode body in the space between the electrode and the oven charge.

The patent US-A-4,827,487 (Brotzmann et al.) describes an electric arc furnace where a fuel carbonaceous is introduced through one or more hollow electrodes.

The technical problem underlying this invention is to provide new electrodes that allow solve the aforementioned problems, in particular allowing to reduce the wear of the same and therefore the maintenance interventions

This problem is solved by an electrode. as defined in claim 1 of the appendix.

The main advantage of the electrode and the corresponding operating method in accordance with this invention is that it allows a significant reduction of wear of both the tip and the oxidizing zone of the conductive component of the bottom (graphite) of the electrode, increasing with this the useful life and reducing the number of changes of the same.

The present invention will be described hereinafter. according to a preferred version of it, provided to non-limiting example title and with reference to the drawings annexes, in which:

* Figure 1 is a schematic view of a DC electric oven, which incorporates a monoelectrode according to the present invention;

* Figure 2 is a schematic view of the electrode of figure 1 in a longitudinal section thereof;

* Figure 3 shows an enlarged detail of the electrode shown in figure 2.

* Figure 4 shows an enlarged detail of the electrode tip from which two fluids are injected simultaneously.

* Figure 5 shows a change diagram of temperature of a refrigerated electrode according to the present invention.

With reference to figure 1, an oven Electric arc steel is indicated by 1.

In particular, oven 1 is a DC oven monoelectrode consisting of a so-called operational bath section flat, and continuously fed, p. eg, with metal from scrap 2, by means of a conduit 3.

The oven 1 comprises the bottom part of the receptacle 4 or box, which consists of belly 5, which houses the metal bath 12 in which the scrap metal 2 is discharged, a conductive hearth 6 for the passage of the current, whereby the resulting molten steel is grouped, and the upper part 7 of the receptacle or box. The latter forms the vault of the chamber of fusion 8 and has an opening 9 suitable for insertion of a electrode 10. In the present version, electrode 10 acts as cathode (negative pole) and the conductor members 11 of the hearth conductive act of anode (positive pole).

Electrode 10 and conductive members 11 are electrically connected to generator 13.

Due to the voltage effect, an arc is produced electrical 15 between the tip 14 of the bottom of the electrode 10 and the bath surface 12. The electrode 10 comprises first, means for expelling at least one fluid into the bath of metal 12 contained in the electric oven 1, which will be detailed more ahead.

With reference to Figure 2, electrode 10 in its upper part comprises a cylinder-shaped columnar body 16 constructed of graphite or a metallic conductive material. In this last case, columnar body 16 is cooled with water and partially coated with a protective layer of material insulating ceramic 17.

In any of the different versions, the bottom portion 26 of electrode 10, including tip 14, is built of graphite.

In addition, in the bottom part 26, the electrode 10 is provided with an axial cavity 19, the first means for ejection of a fluid comprising a nozzle 20 housed in its inside. The nozzle 20, cooled with water by means of a cooling duct 18 running through it, is sliding inside the cavity 19.

Between the nozzle 20 which has a minimum diameter of 40 mm and the axial cavity 19 of the electrode 10 there is a vacuum 40 with a clearance that ranges from 0.5 to 2.0 mm. With respect to the fund, the lance 20 ends in a nozzle head 21 with a shape suitable to give the desired characteristics to the dynamics of the fluid, and preferably those of a compact jet to a fluid 22 downloaded from it. The distance between the head of the nozzle 21 and the part of electrode 10 consisting of the end of the tip 14 closer to the metal bath, ranges from 100 to 1000 mm and is such that fluid 22 does not undergo any chemical or physical change important within electrode 10.

The fluid 22 comprises at least one heat-decomposable carbon-rich fuel component, in particular a gaseous hydrocarbon selected from the group comprising methane, ethane, propane, butane and mixtures thereof, and is referred to as "reactive fluid "for being able to carry out a reduction / fuel action, with metallurgical functions analogous to those of coal usually found inside an oven
electric.

The fluid 22 flows out of the ejector nozzle, piercing the end section of the electrode axial cavity  at an adequate speed, heating up, but not enough slowly to undergo substantial chemical transformations, until reaching tip 14 of electrode 10, whereby it takes place its molecular breakage by the heat provided by the arc electric.

Following the molecular demolition of fluid 22 by the heat of the electric arc, the arc 15 becomes in an AP plasma arc (at very high temperature, varying from 15,000 at 20,000 ° C) due to the presence of the gas in the ionic state, which when it exits the tip 14 of electrode 10 it oxidizes in the atmosphere from the oven, contributing thermally to the latter and allowing with it a saving of energy in the manufacture of the steel.

In order to allow all the fluid 22 reach the tip of the electrode without backing into space, there is a suitable annular system, which provides a seal airtight at the top of the electrode itself by which Insert the spear.

With reference to figure 2, electrode 10 (versions of graphite and metallic material) also includes some second means 23 for the expulsion of a second fluid 24. Said means 23 are placed on the cylinder-shaped surface of electrode 10, at the bottom of the body 16.

The second fluid 24 stands out for its properties antioxidants, and can understand or be replaced by carbon-based combustible compositions, such as p. eg oil fuel, coal dust and other carbides such as carbides from calcium, silicon, aluminum.

Combustible oils may comprise the gas oil, diesel oil, oils, light oils from the oil refining or even drainage oils derived from the lubrication of mechanical components, sludge, blade oil, emulsions containing carbon and hydrogen.

Finally, the ejected fluid 24 can fully or partially understand water.

Said fluid interacts with the surface of graphite in the form of a cylinder, so that it performs an action protective that strongly reduces the oxidation of graphite on the body surface 26.

In addition, together with said reducing agents (produced from carbon-based materials, containing hydrogen), fluid 24 may also comprise oxides optionally suspended, highly stable, such as p. e.g., CaO, MgO, Al 2 O 3 or carbonates thereof, whose function is the of coating the graphite surface of the electrode with a layer of protective material Due to the high oven temperature, this fluid thus formed, sliding along the surface of the electrode allows its fluid phase to evaporate by depositing the solid fraction on the electrode, thereby forming a oxygen impermeable coating, called a barrier antioxidant "27.

This prevents contact with the oxidizing atmosphere optionally present in the oven.

The second means 23 for expulsion, consists in a ring of periferal nozzles that are fed separately with respect to the nozzle 20, by means of a loop of adequate feeding indicated by 25 in figure 2.

With reference to figures 2, 3 and 4, appreciate how through the nozzle head 21 the first one is ejected  fluid 22, which reacts with the metal bath and with the atmosphere that is located above it in an environment with a very high temperature

The nozzle head 21 may have a single inlet, from which a fluid is injected, optionally a mixture formed with at least one reactive fluid component, or may comprise a plurality of outputs, each corresponding to a reactive fluid expelled and fed separately from the other fluids.

The nozzle head 21 can vary its position inside the axial hole 19 in connection with the wear of the tip 14. Therefore, the nozzle 20 must be slidable at length of the axial cavity 19 although stationary with respect to the electrode.

Purely by way of example, Figure 4 represents a section of the tip of electrode 10 and the head of nozzle 20 when it is desired to simultaneously inject a second fluid 24 (e.g., oil), and a first fluid 22 (gas) within the electrode cavity, keeping the two still separate fluids at the tip of the spear.

Injection of the first and second fluid produces a region 28 that houses chemical transformations, in the projection of the nozzle head 21 between the tip 14 of the electrode 10 and the surface of the bath 12, and a suction volume 29, generated by the reaction gas of the two fluids it carries solid / liquid particles captured by the free surface of the 12 molten bath and solid particles produced by demolition Molecular of the fluids. Receding along the electrode, said suction volume 29 deposits said particles on the surface thereof, forming a layer of solid material that has  a function analogous to that of said antioxidant barrier 27 covering the entire surface of the bottom part of electrode 10 which comprises tip 14. With reference to figures 2, 3 and 4, of hereinafter the operation method of the new one will be described objective electrode of the present invention, clarifying the main technical characteristics thereof subdivided of according to the aspects concerning electrode wear, electric arc length, metallurgy and ecology.

Electrode wear

In order to reduce oxidation wear of the electrode, due to the presence of air or oxygen, it They performed the following operation steps.

-

generation around the electrode of graphite, of a layer of reducing gas (or at least non-oxidizing) extended over the entire length of the electrode,

-

generation of a thin barrier of solid material 27, such as the graphite surface of the electrode, physically and chemically separated from the surrounding atmosphere.

The antioxidant barrier 27 may be formed by the oxides present in the second ejection fluid 24, or by the gas resulting from the molecular demolition of the fluid formed by gases / liquids discharged from the nozzle 20 when said gases / liquids interact with the plasma arc 15 and with the free surface of the bathroom, where there is usually a human waste. In the latter case, in order to generate the barrier solid antioxidant 27, the entry of solid substances into the fluid 24 out of the nozzle 20 through the inlet 30 is unnecessary since said substances are already remarkably present in the human waste.

The reaction of the fuel (gas) 22 of the fluid (oil) 24 with the slag, and the carbon derivative of the demolition of fuel 22 and fluid 24 generates an opaque volume of suction 29, which also protects the tip 14 of the electrode 10 of the radiant energy of the electric arc 15.

The chemical reactions that take place in the region 25 below the nozzle 14 of the electrode 10, are valid for all hydrocarbons of suitable ratios and are represented as follows, referring to the injection of methane fuel

(one)

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CH_ {4} \ C → 2H_ {2}

reaction endothermic

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(2)

 \ hskip0.5cm 

C + 2H_ {2} + 3/2 \ O_ {2} \ rightarrow CO + 2H2 {O

reaction exothermic

The first reaction (1) that determines the breakage (crack) of the carbon-hydrogen bonds allows the cooling of tip 14, reducing wear.

The second reaction (2) that takes place when the products of the first reaction (1) meet the oxygen present in the oven atmosphere or the oxygen contained in The metal bath due to the reduction in progress, contributes to heat the metal bath, more conveniently when this metal is The junk feed.

As the electric arc temperatures are very high, the oxygen required to perform the second reaction can be partially supplied by the oxides (FeO, SiO2, MnO, etc.) present in the slag, whereby the latter are reduce.

This produces positive metallurgical effects, such as desulfurization of steel, recovery of manganese, chromium and silicon, by reducing the corresponding oxides of the human waste.

Reactions 1 and 2 generate the volume of aspiration 29, which is opaque due to the presence of particles solid, and also limits the radiation of the plasma arc 15 towards tip 14, contributing to a lower temperature of the latter and therefore reduce the wear of graphite.

The water-cooled nozzle 20 contributes to reduce oxidation wear since the average electrode temperature.

At this lower electrode temperature corresponds to a lower electrical resistivity of the graphite of electrode 10 and therefore a lower heating thereof due Joule effect.

Computing an example considering merely the cooling and the Joule effect, shows that the average temperature of electrode 10 is around 250 ° C lower with respect to that of the uncooled electrode (figure 5).

With this lower temperature value, the corresponding electrical resistance of graphite as well as the heat output is approximately 5% lower.

Other electrode cooling can be achieved. 10 taking out the electrode at the end of each wash before reach the state of thermal equilibrium, and putting it in a container in a non-oxidizing atmosphere in which it is allowed to cool before using it again. In the interim, it will be used for production of a second electrode previously cooled or new and placed in the working position by a second arm with electrode, independent of the first.

Thus, before the reinsertion of the oven electrode can lower the temperature of the oven to values close to 600-800ºC, values that correspond to a resistivity approximately 20% lower than the of the electrode not removed and kept in the oven.

Therefore, the electrode in question allows achieve an increase in kiln productivity by increasing of the thermal power transferred to the metal bath and due to the option to use, keeping the intensities of current, higher voltages of the electric arc, and therefore higher electrical powers.

In short, electrode wear, which it depends on the temperature and is proportional to the density of current that hits the tip of it and causes its heating, is reduced by virtue of the following three effects combined:

1. Larger diameter of tip 14 due to lower oxidation wear over the entire electrode
10.

2. Deposition on the electrode surface 10 and above tip 14, of the carbon generated by the demolition molecular of the injected fluids by means of the nozzle of the spear, which oxidizes instead of the graphite of the latter, in case of a reaction with an oxidizing atmosphere.

3. Tip cooling effect induced by the dissociation reactions of the injected fluid. These dissociation reactions, which are endothermic, reduce the effect of sublimation of graphite due to the very high temperatures of Electric arc.

Electric arc length

Injection of fluids through the electrode allows to reach 20% -60% of electric arc shortening fifteen.

In this way the electric arc 15 radiates less towards the refractory walls of oven 1, reducing damage produced to them, and also increasing the transmission of heat to the bathroom 12.

The mentioned results have been constantly searched in electric steelmaking furnaces during the operation steps of flat bath, by generating foaming slags or reducing electrical power, causing bath oxidation and decreased productivity, respectively.

In the case of carbon steels the electric arc shortening achieved by this method decreases  the need to use materials (C, CaCO 3, carbides) to generate foamy slags. In the case of steelmaking stainless, the generation of foaming slag is extremely difficult and expensive in terms of Cr production, the Cr being oxidized by the oxygen required by the generation. Thus, through this new method the desired shortening of the electric arc 15, simultaneously increasing performance in Cr.

Metallurgical aspects

Using the electrode object of the present invention the reaction products of the injected fluids, which consist mainly of hydrocarbon cracking gases, they have the effect of limiting with respect to a normal atmosphere, the air content of the arc atmosphere in a furnace foundry.

In this new atmosphere, nitrogen ions generated by the very high arc temperatures electrical exert a lower pressure (concentration) than exerted With an arc generated outdoors. This also causes the steel manufactured is exposed to a lesser extent in the area of impact of the electric arc, at the entrance of nitrogen.

The advantage of this effect is that the content Nitrogen finish of the steel manufactured in the electric furnace of foundry can reach values comparable to those achieved in the manufacture of integrated cycle steel (20 - 50 ppm).

In this way, the need for the denitrification treatments required to achieve the analytical objectives of most steels is reduced. In fact, for the latter the nitrogen content must be the lowest
possible.

In addition, the products of the entry of these fluids, since they are essentially constituted by substances able to combine with the oxygen present, they have a reduced effect on its interaction with the slag present on the bath of foundry 12.

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This effect produces the following consequences metallurgical:

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steel quality increase, in particular, by reducing the nitrogen content and sulfur;

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metal recovery (Cr, Yes, Faith, Mn) of the slag.

Therefore, a general increase of The quality of steel.

Ecological aspects

The use of the target electrode of the present invention has generally allowed to reduce the environmental impact, due to the reduced:

?

NO_ {x} generation, due to the low presence of nitrogen ions in the arc of plasma;

?

CO2 generation given that electric arc shortening reduces the need to generate carbon oxides to form foamy slags. In addition, it has decreased the need to introduce coal in order to effect slag reduction.

Example

In order to check the effects of a electrode according to the object of the present invention, is carried out tests in a mono electric bath electrode oven DC plane, testing different electrodes object of the present invention as well as traditional electrodes.

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Parameter operational Traditional electrode Electrode invention Example Power electric 0.5-60 MW 0.5-60 MW 1.5 MW Graphite electrode diameter 650 mm 650 mm 250 mm Electrode column length 8000 mm 8000 mm 4100 mm Fuel flow rate oil - - - 20 and 200 kg / hour 27 kg / hour Flow rate gas - - - 3 and 300 Nm3 / hour 15 Nm3 / hour Gas outlet speed from from - - - 0.5 and 50 m / sec 2.5 m / s the cavity of electrode Nozzle diameter of gas - - - 1-5 mm 3 mm 60 MW oven Wear of graphite tip \ approx 80 kg / hour \ approx 30 kg / hour \ approx 7 kg / hour Wear of graphite by oxidation \ approx 120 kg / hour \ approx 50 kg / hour Productivity 60 t / hour 60 t / hour - - - Steel quality (Mn, Yes, Cr) < > (Mn, Yes, Cr)> (Mn, Yes, Cr)>  \ hskip1.3cm '' (S_ {N} {2}) < > (S_ {N} {2}) < (S_ {N} {2}) < Smoke from NO_ {x} Present Absent Absent

It should be understood that analogs can be achieved advantages with different electrode configurations and with the adoption of the corresponding operation method described more above. A person skilled in the art can perform modifications and variants on the electrode described above, with in order to meet more and unexpected needs, coinciding all of them however, within the protective scope of this invention, as defined in the appendix claims.

Claims (15)

1. An electrode (10), in particular for electric arc steel furnaces and the like, which comprises: a cylinder-shaped columnar body (16) of conductive material, partially coated with a protective layer (17) of insulating material, and an axial cavity (19) formed in the bottom part (26) of said columnar body (16), means (20, 23) for projecting the ejector fluid (22) towards the metal bath of the oven, said fluid comprising a Carbon-rich fuel component decomposable by heat, characterized in that said means for the projected fluid comprise: a sliding lance (20) housed within said axial cavity (19) that ends with a nozzle head for projecting said fluid which produces a cloud that protects the tip (14) of the electrode (10) and covers the surface of the electrode with a solid layer (27); and nozzles placed on the cylinder-shaped surface of the electrode (10) for the projection of another fluid (24) as a bottom of the columnar body (16) to interact with the surface, to effect a protective action. 2. The electrode (10) according to the claim 1, wherein said fuel component rich in carbon is a gaseous hydrocarbon selected from the group formed by methane, ethane, propane, butane and mixtures thereof. 3. The electrode (10) according to the claim 1, wherein said nozzle head (21) comprises a plurality of output sections, each corresponding by at least one component projected and fed separately from the others. 4. The electrode (10) according to the claim 2, wherein said gaseous hydrocarbon is capable of perform a reducing / fuel action, with functions metallurgical similar to those of coal that are usually Found inside an electric oven. 5. The electrode (10) according to the claim 1, wherein said fluid comprises fuels liquids and / or solids. 6. The electrode (10) according to the claim 1, wherein said other fluid comprises a proportion of flow that is water. 7. The electrode (10) according to the claim 2, wherein the velocity of the hydrocarbon flow gas ranges from 3 to 300 Nm3 / hour, with an output speed of the gas from the axial cavity of the electrode, which ranges from 0.5 to 50 m / sec 8. The electrode (10) according to the claim 2, wherein the hydrocarbon outlet section gas is ≥80 mm2. 9. The electrode (10) according to the claim 3, wherein the nozzle head (21) produces a straight jet. 10. The electrode (10) according to the claim 3, wherein the distance between the nozzle (21) and the tip (14) of the electrode (10) ranges from 100 to 1000 mm. 11. The electrode (10) according to the claim 1, wherein the free space between the lance (20) and The cavity (19) ranges from 0.5 mm to 2.0 mm. 12. The electrode (10) according to the claim 11, wherein the lance (20) has a minimum diameter 40 mm 13. The electrode (10) according to the claim 1, wherein said other fluid (24) comprises Compositions of fuel based on carbon and / or carbides. 14. The electrode (10) according to the claim 13, wherein said fuel compositions a carbon base, include coal dust, combustible oils, to know, gas oil, diesel oil, oils, light oils from the oil refining, drainage oils derived from the lubrication of mechanical components, sludge, oils for blades, emulsions containing carbon and hydrogen; and where said carbides comprise calcium carbides, silica, aluminum. 15. The electrode (10) according to the claim 1, wherein said fluid and / or said other fluid they comprise highly stable oxides (CaO, MgO, Al 2 O 3) or carbonates thereof.