Method for operating an electric arc furnace

The present invention relates to a method for operating an electric arc furnace as well as to an electric arc furnace and to an electrode for an electric arc furnace.

Background of the invention

Electric arc furnaces are used in metallurgy for processing, particularly melting of metallic material like metal scrap or direct reduced iron (DRI, also referred to as sponge iron). An electrode of the electric arc furnace is usually made of graphite and is energised in order to produce an electric arc between the electrode and the metallic material. The electric arc furnace can comprise e.g. one electrode of that kind if direct current is used or three electrodes if alternating current is used.

In order to make electrically non-conducting air inside the furnace conducting, the molecular nitrogen N ₂ of the air is usually dissociated into atomic nitrogen N, which is then ionised. By means of this conducting plasma the electric arc can be generated and can reach temperatures up to several thousand Kelvin such that the metallic material can be melted.

In a melting phase most of the metallic material inside the furnace is still in a solid state. In the course of this melting phase the electric arc drills holes into the solid metallic material. The electric arc can thus also melt material at the bottom of these holes and the radiation of the arc can also melt material at the sides of these holes. Thus, maximum energy yield and little losses of energy can be achieved. Once the metallic material is completely melted a flat bath is reached. In the course of a flat bath phase the electric arc is surrounded by a foaming slag in order to minimise energy losses. In the case of stainless steel, a foam slag is not used due to the required chemical analysis.

Operation of an electric arc furnace of that kind can involve high costs. With rising and varying energy prices, the costs for energising the electrodes can be a significant factor of the economic efficiency of the furnace. Further, wear of the graphite electrodes can also yield high costs and can make it necessary that the electrodes have to be replaced in certain intervals. Wear of the graphite electrodes usually comprises sublimation at the tip of the electrode and burn-off at its sides due to oxidation.

It is desirable to improve operation of an electric furnace, particularly to reduce the costs of the operation.

Disclosure of the invention

According to the invention, a method for operating an electric arc furnace as well as an electric arc furnace and an electrode for an electric arc furnace with the features of the independent claims are proposed. Advantageous further developments form the subject matter of the dependent claims and of the subsequent description.

The electric arc furnace can particularly be used to process a metallic material, especially to melt a metallic material, e.g. metal scrap or direct reduced iron or sponge iron. Metallic material to be processed is particularly loaded into the furnace.

In the course of the present method, an argon rich gas is injected into the furnace, particularly a specific amount of the argon comprising gas. The term "argon rich gas" is particularly to be understood as a gas or a mixture of gases, wherein an amount of argon is larger than the amount of any other gas. In particular, the amount of argon in the argon rich gas is at least 50%, particularly at least 75%, more particularly at least 90%, more particularly at least 95%.

An electrode unit comprising at least one electrode of the furnace is energised such that at least one electric arc is generated, particularly between the at least one electrode and the metal material to be processed. For this purpose, the at least one electrode is expediently connected with a corresponding energy supply, e.g. by means of a transformer, a high current cable, etc. The at least one electrode is particularly made of graphite of a graphite material.

In correspondence with the present method, an electric arc furnace according to the present invention comprises an injection unit configured to inject an argon rich gas into the furnace and an electrode unit with at least one electrode configured to be energised such that at least one electric arc is generated. Advantageous and preferred embodiments of the method and the electric arc furnace according to the present invention shall arise from the present description in analogous manner.

By injecting the argon rich gas into the furnace according to the present invention operation of the electric arc furnace can significantly be improved and costs can particularly be reduced, as will be explained hereafter.

In common electric arc furnaces, first of all, the electrically non-conducting air inside the furnace has to be made conducting in order to generate an electric arc. For this purpose, molecular nitrogen N ₂ is dissociated into atomic nitrogen N, a reaction for which 9.8eV of energy is needed per nitrogen molecule. The atomic nitrogen N is then ionised, for which 14.5eV of energy per nitrogen atom is necessary. Thus, per nitrogen molecule at least 24.3eV are necessary to create an electrically conducting plasma.

In contrast to nitrogen, argon is a one atomic gas, i.e. argon does not have to be dissociated. Thus, atomic argon only has to be ionised in order to create a conducting plasma. 15.8eV of energy are necessary to ionise one argon atom, i.e. only 65% of the energy in case of using nitrogen (N ₂ ).

According to the invention an argon rich gas is injected into the furnace through a bore inside the at least one electrode. It has been found that the amount of argon rich gas to be injected into the furnace depends on the size of the electrode. Therefore, the diameter of the bore has to be less than 10% of the diameter of the electrode. Such dimensioning helps to achieve the required gas amounts and gas velocities.

For a non-circular cross section of the electrode or of the bore, the term "diameter" shall mean the diameter of a circle which has the same size as the non-circular cross- sectional area.

According to preferred embodiments the diameter of the bore is less than 8%, less than 6%, less than 4% or less than 2% of the diameter of the electrode.

It has been found that the diameter of the bore through the electrode should be less than 50 mm, less than 40 mm, less than 30 mm or less than 20 mm. The invention proposes to use an electrode with a borehole and the argon rich gas is supplied via the borehole. The electrode preferably has a cylindrical shape with a longitudinal bore in the direction of its cylinder axis.

Hollow electrodes have been used in the past for supplying particulate material into an electric arc furnace. The holes in these hollow electrodes have diameters of 100 mm or more to make sure that the particulate material does not block the hole through the electrode. However, such prior art electrodes are not suitable for injecting argon rich gas according to the invention since the argon demand takes on dimensions that do not allow the economic operation of the system.

During the development of the present invention the inventors carried out tests with prior art hollow electrodes in combination with argon and lime. The results showed that, due to the size of the hole and due to argon losses, the required argon quantities were so high that the costs exceeded the benefits. Due to this fact an electrode with a small bore was proposed, which is different from the classical hollow electrode. This borehole cannot be used for the application of solids since they would block the bore. The inventive system has two major advantages: On the one hand, the argon demand is reduced due to the small borehole and the associated low argon losses and on the other hand, higher pressures can be used. At the low pressures which can be achieved with the prior art hollow electrodes the argon rich gas is diffusely distributed at the electrode tip. With the inventive system described here, the higher pressure leads to an exact supply of argon rich gas in the area where it is needed.

By injecting the argon rich gas into the furnace according to the present invention, argon expediently substitutes the air inside the furnace, especially in an area between the electrode unit and the metallic material. This area in which the electric arc is to be created is therefore enriched and particularly filled with argon. Thus, a smaller amount of energy has to be provided in order to create a plasma and thus to ignite the electric arc compared with common electric furnaces without the injection of the argon rich gas. Therefore, the electric energy necessary in order to generate and operate the electric arc and thus the costs for energising the electrode unit and for operating the furnace can be reduced. The electrodes can therefore be energised with lower currents in order to create and operate the electric arc. Thus, wear of the electrodes, particularly sublimation at their tips can expediently be reduced. Further, by injecting the argon rich gas into the furnace, the atmosphere inside the furnace in an area surrounding the at least one electrode is expediently enriched with argon and oxygen is suppressed in this area. Therefore, wear of the electrodes particularly in the form of burn-off at their sides due to oxidation can expediently be reduced. The intervals in which the electrodes need to be replaced can therefore be increased and costs can thus be reduced.

Furthermore, electric resistance of the electric arc is reduced such that the electric arc is created more efficient and more steady, which improves the operation of the furnace and increases its productivity.

Advantageously, the argon rich gas is injected into the furnace at a location at a tip of the at least one electrode of the electrode unit. Thus, this location at which the electric arc shall be generated can be enriched or filled with argon in order to improve and simplify ignition and operation of the electric arc.

Preferably, the argon rich gas is injected into the furnace by means of the electrode unit. The injection unit is preferably at least partially integrated into the electrode unit. The electrode unit is therefore enhanced to perform a further function additionally to creating the electric arc. Further, the argon rich gas can thus be injected at the location at the tip of the at least one electrode in order to enrich or fill the area where the eclectic arc is to be generated with argon.

The argon rich gas is injected into the furnace through a bore inside the at least one electrode of the electrode unit. The bore inside the at least one electrode of the electrode unit is configured to be connected with a gas supply for supplying the argon rich gas. Therefore, no additional elements have to be arranged inside in order to inject the argon rich gas. Further, by injecting the argon rich gas by means of the electrodes itself the argon rich gas can easily be injected at the location at the tip of the electrodes.

Each electrode particularly has a cylindrical shape with the bore extending along the entire length of the electrode in axial direction. Expediently, each electrode of the electrode unit comprises a bore of that kind. The corresponding gas supply especially comprises one ore several tanks or gas cylinders in which the argon rich gas is stored, usually under pressure (typically between 15 and 300 bar), as well as pipes from the tank(s) to the bores. These pipes can e.g. be attached to an end of the electrodes by means of covers which can expediently be connected to the electrodes. For example, the pipes can be arranged at a lift column and/or a support arm which are provided for fastening the electrodes.

The invention further relates to an electrode for an electric arc furnace of that kind, wherein the electrode is configured to be energised such that an electric arc is generated and wherein the electrode comprises a bore configured to be connected with a gas supply for supplying an argon rich gas. Advantageous and preferred

embodiments of this electrode according to the present invention shall arise from the present description of the method and the electric arc furnace according to the present invention in analogous manner.

Advantageously, an amount of the argon rich gas injected into the furnace is controlled. This control can expediently be integrated into a control mechanism of the furnace and can for example be performed by a control unit for controlling operation of the furnace. Particularly, the amount of argon rich gas injected into the furnace can be controlled in order to improve the operation of the furnace and to increase its productivity.

Preferably the amount of the argon rich gas injected into the furnace is controlled in dependence of a state of the furnace and/or in dependence of a state of the at least one electric arc. The amount of argon rich gas is thus particularly controlled such that the electric arc is created and operated efficiently and steadily, thus improving the operation of the furnace and increasing its productivity. For example, if the electric arc becomes unstable, the amount of injected argon rich gas will expediently be increased to support and stabilise the electric arc, that is the electric arc is dynamically controlled.

Advantageously, the electrode unit comprises one electrode, especially when direct current is used to energise the electrode. Alternatively, the electrode unit preferably comprises three electrodes, particularly when alternating current is used to energise the electrodes, especially by means of a three-phase electric power source. Preferably, argon is injected as the argon rich gas. In particular, argon with a purity of at least 90% is injected as the argon rich gas, particularly with a purity of at least 95%, more particularly with a purity of at least 99%. For example, argon 5.0 can be injected, i.e. argon with purity of 99.999%. Particularly, argon 6.0 can be injected with a purity of 99.9999%. Further, argon 7.0 can be injected, i.e. argon with a purity of 99.99999%.

It is however also possible to inject a mixture of gases as the argon rich gas, wherein an amount of argon in the gas mixture is at least 50 %, particularly at least 75%, more particularly at least 90%.

The argon is introduced into the furnace at a pressure of more than 8 bar and preferably at a pressure between 8 and 15 bar. Thereby, the argon is injected into the furnace with a velocity which is high enough so that the argon comes exactly to the point where it is needed to support the electric arc. The positive effect of the argon on the electric arc and on the plasma is achieved very fast and the argon losses are considerably reduced.

Further advantages and embodiments of the invention will become apparent from the description and the appended figures.

It should be noted that the previously mentioned features and the features to be further described in the following are usable not only in the respectively indicated combination, but also in further combinations or taken alone, without departing from the scope of the present invention.

In the drawings

Fig. 1 schematically shows a preferred embodiment of an electric arc furnace

according to the present invention in a sectional side view and

Fig. 2 schematically shows a preferred embodiment of a method according to the present invention as a block diagram.

Detailed description of the figures Fig. 1 schematically shows a preferred embodiment of an electric arc furnace 100 according to the present invention in a sectional side view.

The furnace 100 comprises a main body 101 in which a metallic material 102 to be processed can be loaded, e.g. metal scrap. Further, the furnace 100 comprises an electrode unit 1 10 with at least one electrode 1 1 1. In the present example, the electrode unit 1 10 comprises three electrodes 1 1 1 , particularly of identical construction. A mechanical holding element is provided for holding the electrodes 1 1 1 in position comprising a lift column 104 and a support arm 105. The support arm 105 can further be used to energise the electrodes 1 1 1. For this purpose the support arm 105 can be connected with a high current cable 106. This cable 106 can be connected with further electric elements e.g. with a transformer and a power source, particularly a three-phase electric power source.

When the electrodes 1 1 1 are energised, electric arcs 103 can be created between the electrodes 1 1 1 and the metallic material 102. These electric arcs 103 can reach temperatures of several thousand Kelvin such that the metallic material 102 can be melted. Fig. 1 exemplarily shows a melting phase, during which most of the metallic material 102 is still in a solid state and in the course of which the electric arcs 103 drill holes into the solid metallic material 102.

Further, an injection unit is provided in order to inject an argon rich gas into the furnace 100. This injection unit is partially integrated into the electrode unit 1 10 in that each electrode 1 1 1 comprises a bore 1 12. Each electrode 1 1 1 has a cylindrical shape with the bore 1 12 extending along the entire length of the electrode 1 1 1 in axial direction. The outer diameter of each electrode is, for example, 500 mm and the diameter of each bore is 48 mm.

Each bore 1 12 is connected with a gas supply 120 for supplying the argon rich gas.

This gas supply 120 can comprise a corresponding gas tank 121 and a common pipe 122, which can e.g. be arranged at the lift column 104 and the support arm 105. From the common pipe 122 three individual pipes 123 branch off to the three electrodes 1 1 1 . Although only one tank is exemplarily shown in Fig. 1 it is to be understood that an expedient number of tanks can be provided. For example, also a bundle of gas cylinders can be provided as the gas tank 121. Particularly, each individual pipe 123 is connected with a cover 124, which is connected to the corresponding electrode 1 1 1 such that the argon rich gas can be conducted into the bore 1 12 of the corresponding electrode 1 1 1 , as indicated by reference sign 1 13.

Thus, the argon rich gas 1 13 is injected into the furnace 100 through the electrodes 1 1 1 itself. When ejected out of the bores 1 12, argon rich gas 1 13 is therefore injected into the furnace 100 at location at the tip of the electrodes 1 1 1 .

Preferably, argon is injected as the argon rich gas 1 13, preferably argon with a purity of at least 99.9%, e.g. argon 7.0 with a purity of 99.99999%. Argon is therefore injected into the furnace 100 in the area between the electrodes 1 1 1 and the metallic material

102, in which the electric arcs 103 shall be created. This area is thus enriched and particularly filled with argon. When the electrodes 1 1 1 are energised the argon in this area is thus ionised and an electrically conducting plasma is created such that the electric arcs 103 can be ignited.

Since argon is a one atomic gas it does not have to be dissociated. Argon thus only has to be ionised in order to create a conducting plasma. For the ionisation of argon 15.8eV of energy are necessary. In contrast to that, in a common electric arc furnace, into which no argon is injected, molecular nitrogen N ₂ of air is usually ionised in order create plasma. However, first of all the molecular nitrogen N ₂ has to be dissociated into atomic nitrogen N, which is afterwards ionised. For this dissociation 9.8eV of energy is needed and for the ionisation 14.5eV of energy is needed. Thus, in a common electric arc furnace, at least 24.3eV are necessary to create an electrically conducting plasma.

Therefore, in the present electric arc furnace 100 a smaller amount of energy has to be provided in order to create plasma compared to common electric arc furnaces into which no argon is injected. Electric energy to generate and operate the electric arcs

103, costs for energising the electrodes 1 1 1 and for operating the furnace 100 are reduced compared to common electric arc furnaces.

Further, since the electrodes 1 1 1 can therefore be energised with lower currents in order to create and operate the electric arcs 103, wear of the electrodes 1 1 1 , particularly sublimation at their tips can expediently be reduced. Since the atmosphere inside the furnace 100 in an area surrounding the electrodes 1 1 1 is enriched with argon and oxygen is suppressed in this area, wear of the electrodes 1 1 1 in the form of burn- off at their sides due to oxidation can be reduced. Furthermore, since electric resistance of the electric arcs 103 is reduced the arcs 103 can be created and operated more efficient and steadier.

By means of the present invention, operation of the furnace 100 can therefore be improved, its productivity can be increased and costs can be reduced.

In order to further improve operation and productivity, the amount of the argon rich gas injected into the furnace is preferably controlled. For this purpose, a control unit 130 of the furnace is, in particular in terms of a computer program, configured to perform a preferred embodiment of a method according to the invention, which is shown in Fig. 2 as a block diagram.

In step 201 the metallic scrap is loaded into the furnace 100. In step 202, injection unit is activated and argon 1 13 is injected into the furnace. In step 203 the electrodes 1 1 1 are energised such that the plasma is created and that the electric arcs 103 can be ignited.

After the electric arcs 103 are generated, the amount of argon 1 13 injected into the furnace 100 is controlled in step 204, in particularly in dependence of a state of the furnace 100 and in dependence of a state of electric arcs 103.

For example, if the electric arcs 103 become unstable, the amount of injected argon 1 13 will be increased to support and stabilise the electric arcs 103. The electric arcs 103 can thus be created and operated efficiently and steadily, thus further improving the operation of the furnace 100 and increasing its productivity. Reference list

100 electric arc furnace

101 main body of the electric arc furnace

102 metallic material, metal scrap

103 electric arc

104 lift column

105 support arm

106 high current cable

1 10 electrode unit

1 1 1 electrode

1 12 bore

1 13 argon rich gas

120 gas supply

121 gas tank

122 common pipe

123 individual pipe

124 cover

130 control unit

201 method step

202 method step

203 method step

204 method step