Untitled record

FRENCH REPUBLIC

5^e ADDITION

MINISTRY OF INDUSTRY

IN PATENT OF INVENTION

INDUSTRIAL PROPERTY SERVICE

2V ° 1.156.530 <

> PV n ° 870.884 International classification ^8θ·⁴⁶³07 <11:05 p.m. b

Method and apparatus for working with an arc.

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Said company: UNION CARBIDE CORPORATION residing in the United States of America.

(Main patent taken on July 24, 1956, in the name of a company known as: Union Carbide &amp; Carbon Corporation.)

Requested August 16, 1961, at 4 p.m.¹¹ 45^m, in Paris.

Issued by order of March 25, 1963.

(Official Bulletin of Industrial Property, No. 18 of 1963.) (Patent application filed in the United States of America on August 17, 1960, under No. 50.194, in the name of Mr. Robert M. Gage.)^re addition n ° 71.172. 3^e addition n ° 72.427.

The present invention relates to an arc melting furnace improved according to the main patent and in particular an arc melting furnace according to the second addition No. 72.076 of June 26, 1957 to said main patent, comprising an improved electrode system .

In the aforementioned second addition, an electric arc furnace has been described in which the electrode system comprises one or more arc torches intended to generate a stable beam of a substantially ionized gas directed towards the material to be melted. According to this previous addition, an arc torch suitable for use in such an oven comprises a rod-shaped electrode and an element containing a passage intended to conduct an arc formed between the rod-shaped electrode and a second electrode, in same time as a gas stream to the material to be melted. The second electrode can be constituted by the material being melted or, according to a variant, by the rod-shaped electrode of one or more additional arc torches connected to the supply circuit. Thus, for example, when a three-phase current is used, each of the separate phase wires can be connected to a respective arc torch, or two of the phase wires can be connected to torches and the third to the crucible containing the material to be melt. In general, the same connections can be used which are conventionally used with graphite furnace electrodes · to connect the rod-shaped electrodes to arc torches.

2^e addition n ° 72.076.

4^e addition n ° 76.013.

The present invention proposes to provide an electric arc furnace comprising one or more arc torches, in which the torches have an improved construction, so as to reduce erosion to a great extent, even under the most severe conditions of temperature. of metal working and splashing.

In particular, the present invention constitutes an improvement or a variant of the invention according to the second addition of the main patent and comprises an arc furnace in which the arc torch or torches comprise an electrode body cooled by a fluid which supports at its anterior end a refractory electrode, the nozzle having a relatively large intake portion in the vicinity of the anterior end of the electrode body and a constricted discharge portion, and the refractory electrode extending into the constricted discharge portion, but not beyond.

In the accompanying drawings:

Figure 1 is a cross section of an embodiment of the arc torch for use in the improved arc furnace according to the present invention;

x Figure 2 is a cross section of another embodiment of the arc torch; and

Figure 3 is a partial cross section of yet another embodiment of the arc torch.

The improved apparatus of the present invention allows the use of refractory metal electrodes in melting and refining furnaces of a

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Booklet price: 2 francs

2191 073 290 1 [80.463 / 1.156.530] - metal, substantially without consumption of electrodes (less than 1 to 2 g / hour for a tungsten electrode with a diameter of 1.9 cm) even at current intensities greater than About 4,000 amps, while achieving arc operation stability that has not been achieved with any existing arc furnace. This device generally consists of a refractory metal electrode having a diameter between 3.2 mm and 5 cm, usually the cathode for operation with direct current, supported by an electrode holder cooled by water and disposed in the central passage of a water-cooled nozzle having a diameter between 4.8 mm and 7.6 cm used to protect the cathode from contamination by the melt and by the atmosphere of the furnace. The tip of the cathode comes either at the outlet of the nozzle or is slightly indented. Means are also provided for passing a stream of shielding gas at a flow rate of between 0.14 and 2.8 m.³/ hour or more if desired, around the cathode to exit through the nozzle passage and to form an effluent arc jet and provide additional protection for the cathode.

The invention will now be described in detail with reference to the drawings. Figure 1 shows an embodiment of an arc torch for use in the improved oven of the invention suitable for power levels between 500 and 2000 kW for ovens with a capacity of 1 to 10 tonnes . The cathode 80, preferably constructed of refractory metals such as tungsten, tantalum, columbium and molybdenum containing small amounts of emissive material, such as thorium oxide and yttrium oxide, is supported by a door -electrode 82 by means of a press wedge, welded or screwed 84. In order to remove excess heat from the cathode 80 and prevent it from melting, the electrode holder 82, made of a highly heat conductive material such as copper, is cooled by a high speed fluid coolant. It is preferable that the electrode 80 does not come into direct contact with the cooling fluid, but that they are separated by a wall of the cap 86 of the electrode holder 82. In this way, the coolant cannot escape into the arc area in the event of a defect in the seal 84. The coolant enters through the inlet 88 and passes through the central passage 90 in the tube. 92 and escapes through an annular passage 94 between the cooling tube 92 and the bore 96 of the electrode holder 82, then through the outlet 98. For successful operation at intensities between 3,000 and 10,000 amps approximately, the cathode 80 has a diameter between 1.3 and 2.5 cm. As an example of the cathode cooling used, water is admitted under a gauge pressure of

2.8 kg / cm³ into the electrode holder 82 through the central inlet passage 90 having a diameter of 1.3 cm and an annular discharge passage 94 having a diameter of 1.6 mm.

It should be noted that even if very efficient fluid cooling is used for the metal electrode, this electrode can be strongly damaged by resistance heating at high intensities, if the length of the electrode is excessive. For operation at high currents greater than about 3000 amperes, it is preferable that the length of the electrode is between 1/4 to twice the diameter of the electrode. This corresponds to the total length of the electrode which carries the arc current and includes both the exposed length and the conductive part supported by the electrode holder.

The cathode 80 is protected from contamination by splashing of the metal and by atmospheric gas from the furnace, mainly by the nozzle 100 which is arranged around and in the vicinity of the arcing tip 102 of the cathode 80. When cathodes having a diameter between 1.3 and 2.5 cm are used, the central passage 104 of the nozzle 100 must have a diameter between 14 and 38 mm. As the size of the cathode increases to a diameter of 5 cm, the passage 104 should increase to a diameter of 7.6 cm.

The nozzle 100 is cooled by water flowing at high speed, that is to say greater than

30.5 meters / second from the inlet 106 in the annular passage 108 to exit through the annular passage 110 and the outlet 112. As an example of successful operation between 3,000 and 10,000 amps approximately, water is admitted under a pressure of 14 kg / cm³ in the cooling passages of the annular nozzle which have a width of approximately 0.4 mm. This particular cooling arrangement, which is used in conjunction with the concentric tubes 114, 116 and 118 of the torch body 120, provides suitable cooling of the nozzle and the torch body to prevent melting or other heat damage from the torch. arc or metal contamination from the oven. In large metal melting furnaces in which high currents and voltages are used, there is a greater risk of forming a double arc by means of the nozzle towards the metal bath, especially when the arc length is less than about 10 cm, for example when operating at 200 volts. Therefore, it is preferable that the nozzle has an insulating jacket of refractory material 122 along the lower part to protect it against this double arc formation and the damage resulting from the nozzle. It is desirable that the arcing end 102 of the cathode 80 is placed at a distance of between 3.2 and 6.3 mm inside the passage 104 to increase the stability of the arc and decrease erosion of the cathode.

If the recoil is 9.5 mm or more, the arc tends to jump towards the nozzle, then towards the metal bath. This double arc formation destroys the nozzle. If the cathode extends an appreciable distance beyond the nozzle, an excessive and uneconomical amount of the shielding gas is necessary to protect the cathode.

To further protect the cathode 80 from contamination, a gas stream flows from the inlet 124 via the annular passage 126, from top to bottom along the cathode 80 and exits through the passage 104 of the nozzle. This gaseous current, which has a flow rate of between 2.83 and 4.25 rir per hour when arc intensities between 3,000 and 5,000 amperes are used, provides plasma arc stability of l arc which passes concurrently through passage 104. As the current increases to 10,000 amps, it may be desirable to increase the flow rate of the shielding gas. During certain high-intensity metal smelting operations, it may be advantageous to use a flow rate of up to 28.3 m³ per hour of shielding gas to properly protect the cathode. Inert gases are preferred, such as argon and helium, but other gases such as hydrogen and nitrogen can also be used to provide additional heat, particularly during melting. The gas flow must be capable of having a minimum speed of between 1.5 and 3 meters / second along the cathode, in order to compensate for the circulation of gases from the oven in the area of ​​the electrode caused by the pumping of the arc.

When using arc intensities between about 500 and 4,000 amperes, a massive cathode 80 can be used. However, when the current increases substantially above 4,000 amps, erosion of the cathode begins to become important. In this range of intensities, it is necessary to use a hollow cathode of the type shown in FIG. 2. An inert gas, such as argon, then flows through the tube 140 at the center of the cathode 80, as well as around her.

In this embodiment, the central passage 90 reserved for water is modified so as to present an additional tube 140 at its center with appropriate connections at each end to supply a shielding gas and. a passage through cathode 80. This combination reduces the consumption of the electrode to less than 1.2 g per hour for an electrode having a diameter of 19 mm at high current intensities.

Another embodiment of the present invention which is particularly useful in the power range between 20 and 100 kW for ovens with a capacity of 45 kg or less, is shown in Figure 3. In this embodiment, the cathode 80 has a diameter between 3.2 and 6.3 mm and the passage 104 of - [80.463 / 1.156.530] the nozzle has a diameter between 7.9 and 13 mm. Due to the reduction in the size of the equipment, the water cooling applied to the body 120 of the torch and to the nozzle 100 via the annular cooling passages 108 and 110 can also be used to cool the holder. electrode 82 and cathode 80. The electrode holder 82 is thus arranged in thermal and electrical contact with the sleeve 128 of the electrode holder, that is to say that it is directly cooled by the fluid contained in the passage 108 . The nozzle 100 and the body 120 of the torch are electrically isolated from the sleeve 128 of the electrode holder by an insulator 130. The protective gas of the torch flows via longitudinal passages 132 in the electrode holder 82 and exits through the passage 104 of nozzle 100. This gas stream has a flow rate of between 0.42 and 1.4 m³/ hour for operation in the range of 20 to 100 kW.

In the embodiment shown in Figure 1, electrical energy is supplied directly to the electrode holder via connection 134, while the torch body is isolated from the electrode holder 82 by an insulator 136 and a insulated spacer 138. The main arc between cathode 80 and the melt can be initiated in several ways. The cathode 80 could be advanced through the nozzle 100 until it comes into contact with the electrode constituted by the molten mass, then withdrawn to form the arc. A preferred method consists in maintaining an auxiliary arc of a relatively low intensity between the cathode 80 and the nozzle 100. This auxiliary arc thus supplies a sufficient quantity of ionized gas to strike the main arc.

In the embodiment of FIG. 3, the electrical energy is sent either directly to the electrode holder 82 or more conveniently via the sleeve 128 of the electrode holder, then to the electrode holder 82. The arc can be struck by methods analogous to those of FIGS. 1 and 2, the external part of the body 120 of the torch preferably being electrically insulated so as to minimize the problems of double arcing.

The following examples clearly illustrate the utility of the present invention.

Example- 1. - The cathode apparatus of the type represented in FIG. 1 is used. A tungsten electrode having a diameter of 1.9 cm, containing 1% of thorium oxide, is placed in a nozzle made of cooled copper. by water, having a diameter of 2.5 cm. The tip of the cathode is set back a distance of 3.6 mm from the outlet of the nozzle. Argon gas passes at a flow rate of 14 m³/ hour around the cathode and out through the nozzle. An arc of 76 to 123 volts and 3,100 to 6,000 amperes is maintained over an arc length of between 6.3 and 7.6 cm between the cathode and a rotary graphite anode cooled by water. The 19-minute test [80.463 / 1.156.530] - resulted in a weight loss of the cathode of only 0.4 g.

Example 2. The cathode of the type shown in FIG. 2 is used. A tungsten cathode having a diameter of 2.5 cm and containing thorium oxide and having a central passage reserved for gas with a diameter of 9 , 5 mm is placed in a water-cooled copper nozzle, having a diameter of 3.2 cm. The tip of the cathode is set back 6.3 mm from the outlet of the nozzle. Argon gas passes at a flow rate of 14 m³ per hour around the cathode and out through the nozzle, while 3.4 m³ per hour of argon pass through the central passage of the cathode. An auxiliary arc of 180 amps is struck by a high frequency ignition means between the cathode and the nozzle. This auxiliary arc provides the means for striking an arc at 60 volts, 2,000 amps, a length of 6.3 cm between the cathode and a cylindrical billet of cast iron having a diameter of 56 cm and a height of 46 cm, surrounded by water-cooled copper coils. The length of the arc is increased to 12.7 cm and the power supply increased to 4,000 amps and 115 volts. After 6 minutes, the arc melted a part having a diameter of 25 cm at the top of the metal anode. Although a considerable amount of molten metal was splashed onto the body of the torch, the cathode was not significantly damaged.

Example 3. A cathode analogous to that shown in FIG. 1, constituted by a tungsten electrode containing thorium oxide, having a diameter of 1.3 cm, is placed in a water-cooled copper nozzle having a diameter of 1.9 cm. The tip of the cathode is set back 3.2 mm from the outlet of the nozzle. Argon gas passes at a flow rate between 5.1 and 5.4 m³ per hour around the cathode and out through the nozzle. An auxiliary arc of 175 amps and 18 volts is maintained constantly between the cathode and the nozzle. This cathode structure is mounted in a cover intended for a metal melting crucible and extends into the crucible, the content of which constitutes the anode. The crucible has a diameter of 1.5 meters and a depth of 1.5 meters and includes a jacket of refractory material having a thickness of 30 cm. It contains 453 kg of steel scrap, 31.8 kg of pig iron and 22.7 kg of ingot iron for a total load of 507.5 kg. An electrical connection with the crucible is maintained by means of ingot iron rods with a diameter of 5 cm which form the lower electrodes. An arc of 990 amps and 160 volts is struck between the cathode and the metallic charge. Approximately 45 minutes later, the intensity of the arc is increased to 2,000 amperes and the voltage to 177 volts. Ten minutes later, the intensity is increased between 2,460 and 2,500 amps and the voltage at 165 volts. After about 2 hours, the metal charge is completely melted. The test was continued for about one hour, during which the arc was periodically extinguished to take the temperature of the bath, then re-lit. At the end of this time, the arc is extinguished and the molten charge is poured from the oven. Examination of the cathode and the nozzle reveals negligible damage or erosion.

Example 4. The cathode, analogous to that of FIG. 3, constituted by a tungsten electrode having a diameter of 6.3 mm and containing 2% of thorium oxide is mounted at the outlet of a nozzle in water-cooled copper with an internal diameter of 7.9 mm. This device is mounted in an oven having an internal diameter of 30 cm, a depth of 20 cm and comprising a jacket of refractory material with a thickness of 6.4 cm on the walls and a jacket with a thickness of 10 cm on the bottom. The oven contains 22.7 kg of stamped pieces of soft iron. Argon gas passes at a flow rate of 0.96 m³ per hour around the cathode and through the nozzle. An arc of approximately 25 to 50 volts and 500 amperes is struck by bringing the cathode into contact with the stamped parts, then by withdrawing it at a distance of approximately 13 mm. As the metal melts, the length of the arc is gradually increased to 6.4 cm. After 8 minutes, 0.88 m is added³/ hour of hydrogen to the current of argon, which gives a voltage between 80 and 110 volts. The temperature is measured by immersion of a thermocouple which indicates that the charge is completely melted after approximately 30 minutes. The arc is then maintained for approximately 15 minutes, at the end of which the molten mass is poured. The arc is intentionally extinguished just before payment. The total energy required for melting can compete with that of existing processes used with ovens with a capacity of 11.7 kg.

Example 5. - A cathode of the type represented in FIG. 2 is used. A tungsten cathode containing thorium oxide having a diameter of 2.5 cm having a central passage reserved for gases with a diameter of 9.5 mm is arranged in a water-cooled copper nozzle having a diameter of 3.8 cm. The tip of the cathode is set back 6.3 mm from the outlet of the nozzle. A 9,000 amp, 115 volt arc is maintained between the tungsten cathode containing hollow thorium oxide and a rotary water-cooled graphite anode, while a flow rate of 2.83 m³ per hour of argon flows through the hollow cathode and a flow of 8.49 m³per hour of argon passes around the cathode. The test is continued for 20 minutes to obtain a tungsten loss of only about 0.24 g per hour. A small puncture at the periphery of the cathode passage constitutes the only visible damage to the surface of the cathode. Operation under similar intensity conditions could not be maintained with previous solid tungsten electrodes, without complete destruction of the electrode.

The above examples have all used the cathode in a substantially vertical position relative to the anode. In many cases it may be desirable to arrange the cathode at an angle to the metal bath. This has the advantage of reducing metal splashing on the torch and also providing some agitation of the bath. Metal spatter on the torch could also be minimized by moving the torch over the surface of the bath.

The above examples show the ability of the device to melt a metal charge in an oven. In all these tests, there is practically no evidence of consumption of the electrodes, nor of contamination of the melt. There was also no significant damage to the body of the torch itself by splashing of the metal, etc. Thus, it can be seen that the present device operates satisfactorily for indefinite periods in a metal melting furnace and is therefore much superior to conventional charcoal arc and to the arc devices currently available.

Although certain preferred embodiments have been described and shown, it is obvious that certain modifications and substitutions could be made by a specialist without departing from the scope and spirit of the invention. For example, the above description mainly concerns a high-power torch (500 to 2,000 kW) for ovens with a capacity between 1 and 10 tonnes and a low-power torch (20 to 100 kW) for ovens having a capacity of 45.3 kg or less. It is obvious that the torches of the present invention can be used in conjunction with an intermediate power range and a corresponding oven size. In addition to the direct current of normal polarity described, this device is also useful with a direct current of reverse polarity and an alt ernating current.