Untitled record

SWEDEN

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PATENTS AND REGISTRATION OFFICE

PUBLISHING WRITING No. 358 802 m «ci H 05 b 7/00

Patent Application. No 8316/61 Received 17 VIII 1961 Validity Day 17 VIII 1961

Ans. generally available on 1 VII 1968

Ans. published and the pamphlet published on 6 VIII 1973

Priority requested, from 17 VIII 1960 (USA, 50,194)

UNION CARBIDE CORPORATION, NE »YORK, NY USA

Inventor: RM Gage

Agent: PU Brain

Electric arc furnace

Supplement to 342 122

The invention relates to an improved electric arc melting furnace and in particular to an arc melting furnace according to patent 342122, which comprises an improved electrode device.

According to patent 342122, an electric arc furnace is disclosed in which the electrode device comprises one or more arc burners arranged to provide a stable jet of substantially ionized gas directed at the melting material. According to this patent, an arc burner which is suitable for use in such an oven, a rod-shaped electrode and a member containing passages arranged to conduct an arc, formed between the rod electrode and a second electrode, together with a gas flow to the melting material. The second electrode may consist of the material melted or alternatively the rod electrode of one or more arc burners connected to the power circuit. Thus, for example, when using three-phase power, each of the separate phase lines can be connected to, respectively. eh arc burners or two of the phase wires can be connected to arc burners and the third to the crucible containing the melting material. In general, the same compounds commonly used with graphite furnace electrodes can be used in conjunction with the rod electrodes of the arc burners.

The present invention relates to an electric arc furnace, which. includes one or more arc burners, in which the burners are designed as an improved construction, whereby erosion is significantly reduced even under the most difficult operating conditions of temperature and metal splashes.

The invention thus relates to an electric arc furnace according to one or more of the claims according to patent 342122, characterized in that the arc burner or arc burners of the furnace comprise a liquid-cooled nozzle and a liquid-cooled central electrode (82) having a top (80) of refractory metal thereon. extends into the narrowest portion (104) of the nozzle outlet opening and terminates slightly behind the front end thereof.

In the accompanying drawing, Figure 1 shows a cross section of one embodiment of the arc burner used in the improved arc furnace according to the invention, Figure 2 a cross section of another embodiment of the arc burner and Figure 3 partly a cross section of another embodiment of the arc burner.

With the improved device according to the invention, it is possible to use refractory metal electrodes in metal melting and crushing furnaces with substantially no electrode loss (less is 1-2 g / hour for a tungsten electrode with a diameter of 1.9 cm) even at current levels above about 4000 amperes. and with a stable arc, which has not yet been achieved with existing arc furnaces. This device generally consists of a refractory metal electrode with a diameter of 3.2 mm to 1 cm, usually the cathode for direct current operation, carried by a water-cooled electrode holder and located within a central passage of diameter 4.8 mm to 7.6 cm of a water-cooled nozzle. , which was used to protect the cathode from pollution from the melt and furnace atmosphere. The tip of the cathode is either flattened with the nozzle outlet or slightly cut out or chamfered. Means are also provided for conducting a protective gas flow between about 0.14 to 2.8 m 2 / hour. or possibly more around the cathode and out through the nozzle passage to form an outgoing arc and for further shielding of the cathode.

The invention is described in detail below with reference to the drawing. Pig. 1 shows an embodiment of an arc burner for use in the improved furnace of the invention and suitable for power levels of about 500-2,000 kW in furnaces for 1-10 tonnes. The cathode 80, suitably constructed of refractory metals, such as tungsten, tantalum, niobium, molybdenum, containing minor quantities of emitting materials, such as thorium oxide and yttrium oxide, is supported by electrode holders 82 through the press fit, solder or thread connection 84. In order to remove excess heat from the cathode 80 and prevent it from melting, the electrode holder 82, which is made of high thermal conductivity material, such as copper, is cooled by high speed fluid coolant. Conveniently, the electrode 80 is not in direct contact with the cooling fluid but is separated by a wall portion of the sleeve 86 of the electrode holder 82. In this way, no cooling fluid can leak into the arc region if the connection 84 is incorrect. Cooling fluid enters through inlet 88 and through central passage 90 into cooling tube 92 and exits through annular passage 94 between cooling tube 92 and bore 96 of electrode holder 82. and then through outlet 98. For successful work with currents from about 3,000 to 10,000 amps, cathode 80 has a diameter of about 1.3 to 2.5 cm. According to an example of cooling the cathode, about 38 liters of water per minute (10 gpm) at a pressure of 2.8 kg / cm 2 is passed to the electrode holder 82 through the central passage passage 90 with a diameter of about 1.3 um and an annular outlet. 94 with a width of 1.6 mm.

It should be noted that even if a high degree of fluid cooling is used for the metal electrodes, this electrode can be seriously damaged by resistance heating at high current levels, if the electrode length is large. When operating at a high power over about 3,000 amps, it is advisable that the electrode length be about 1/4 to 2 times the diameter of the electrode. This is the total electrode length supporting the arc current piece and includes both the exposed length and the conductive portion supported by the electrode holder.

The cathode 80 is protected from contamination by metal splashes and furnace atmospheric gas by the nozzle 100 located around and in the vicinity of the arc tip 102 of the cathode 80.

2.5 cm, the central passage 104 of the nozzle 100 should have a diameter of about 14 to 38 mm. As the cathode size increases up to about 5 cm in diameter, passage 104 will increase to about

7.6 cm in diameter.

The nozzle 100 is cooled by high speed flowing water, ie. greater than 30.5 m / sec, from the inlet 106 into the annular passage 108 and out through the annular passage 110 and the outlet 112. As an example of a successful work at about 3,000 to

000 amps are supplied with water at a rate of 150 l / min and an o pressure of 14'kg / cm to the annular nozzle cloud Inings passages,

358802 ⁴ which is about 0.4 mm wide. This particular cooling device, used in conjunction with the concentric tubes 114, 116 and 118 of the burner body, provides proper cooling of the nozzle and burner body to prevent melting or other damage caused by arc heat or furnace metal contamination. In large metal furnaces, where high currents and voltages prevail, there is an increased danger of doubling the arc between the nozzle and the metal bath, especially if the arc length is less than about 10 cm, e.g. when operating at 200 volts. Therefore, it is preferred that the nozzle has a refractory insulating liner 122 along the lower portion for protection against such dual arc arcing and consequent nozzle damage. Conveniently, the arc tip 102 of the cathode 80 is located about 3.2 to 6.3 mm inside the passage 104 to increase arc stability and decrease cathode erosion. If the reset is about 9.5 mm or more, there is a tendency for the arc to jump to the nozzle and then to the metal bath. Such double arc will destroy the nozzle. If the cathode extends a distance outside the nozzle, an uneconomically large quantity of protective gas is required to protect the cathode.

To further protect the cathode 80 from contamination, a gas flow from the inlet 124 flows through the annular passage 126 down the cathode 80 and out through the nozzle passage 104. Such a gas flow of the order of 2.83 to 4.25 m 2 / hour, if the arc currents are about 3,000 to 5,000 amps, providing arc stability of the arc plasma that is simultaneously passed through the passage 104. As the current increases toward 10,000 amperes, it may be appropriate to increase the shielding gas flow. In some metal smelting operations using high current, it may be desirable to use as much as 28.3 m5 / hour of shielding gas to properly protect the cathode. Inert gases, such as argon and helium, are preferred, but other gases such as two-atomic hydrogen and nitrogen can also be used to provide extra heat, especially during the melting. The gas flow must be such that a minimum velocity of 1.5 to 3 m / sec is provided along the cathode with the intention of counteracting the circulation of furnace gases into the electrode region caused by arc pumping.

Using arc currents of about 500 to 4,000 amps, a solid (homogeneous) cathode 80 is used. In this current range, it is necessary to use a hollow cathode of the kind shown in Fig. 2. Inert gas, such as argon, then flows through the tube 140 at the center of the cathode 80 as well.

as around it.

In this embodiment, the central water conduit 90 is modified with a further tube 14-0 at the center thereof with suitable connections at each end to provide protective gas and conduit through the cathode 80. This combination reduces the electrode loss to less than 1-2 g / h for an electrode with a diameter of 19 mm at high current levels.

Another embodiment of the invention which is particularly valuable for the power range of 20-100 kW for furnaces with a capacity of 4-5 'kg or less is shown in Fig. J. In this embodiment, the cathode 80 is about 5.2 to 6 , 5 mm in diameter and the nozzle opening 104 is about 7.9 to 15 mm in diameter. Due to the reduced size of the equipment, the water cooling provided for the arc burner body 120 and the nozzle 100 through cooling lines 108 and 110 can also be used to cool the electrode holder 82 and. the cathode 80. The electrode holder 82 is thus located in thermal and electrical contact with the electrode holder sleeve 128, which is directly cooled by fluid in the conduit 108. Nozzle 100 and arc burner body 120 are electrically insulated from electrode holder sleeve 128 by insulation 130. Protective burner gas flows through the longitudinal passages 132 of electrode holder 82 and out through passage 104 of nozzle 100. This gas flow is of the order of 0.42 to 1.4 m5 / hour when working in the range 20-100 kW.

In the embodiment shown in Fig. 1, the electrical power is applied directly to the electrode holder through connection 134 while the arc burner body is insulated from the holder 82 by the insulation 136 and the insulated spacer 138. The main arc from the cathode 80 to the melt can be initiated in several ways. The cathode 80 could be extended through the nozzle 100 until it contacts the melt electrode and is then retracted to form the arc. A suitable way is to maintain an auxiliary arc of relatively low power between the cathode 80 and the nozzle 100. Thus, such auxiliary arc provides sufficient ionized gas to initiate the main arc.

In that embodiment shown in Fig. 3, the electrical force is either directed directly to the electrode holder 82 or more conveniently through the electrode holder sleeve 128 and then to the electrode holder 82. The arc can be started in a manner similar to that of Figures 1 and 2. , and an outer portion of the arc burner body 120 is preferably electrically insulated so as to reduce the problems of double arcs.

The following examples illustrate the invention.

Example 1

The cathode device of Fig. 1 is used. A tungsten electrode, 'containing' 1% thorium oxide and with a diameter of 1.9 cm, was located completely inside a water-cooled copper nozzle with a diameter of 2.5 cm. The cathode tip was chamfered (cut out) 3.6 mm from the nozzle outlet. Argon gas at the rate of 14 m3 / hour was passed around the cathode and out through the nozzle. An arc of 7θ to 123 volts and 3,100 to 6,000 amperes was maintained over an arc length of 6.3 to 7.0 c® between the cathode and a rotating water-cooled graphite anode. The 19-minute test showed that the cathode had only lost 0.4 g in weight.

Example 2

The cathode device of Figure 2 was used. A thorium-containing tungsten cathode with a diameter of 2.5 cm and a central gas line with a diameter of 9.5 mm were located inside a water-cooled copper nozzle with a diameter of 3.2 cm. The nozzle tip was beveled (cut out) 6.3 mm from the nozzle outlet. Argon gas at the rate of 14 m3 / hour was passed around the cathode and out through the nozzle, while 3.4 m3 / hour of argon was conducted around the central passage of the cathode. An auxiliary arc of 180 amps was initiated by high frequency starter devices between the cathode and the nozzle. This auxiliary arc constituted the starting means for a 60-volt 2,000-ampere arc of length 6.3 cm between the cathode and a cylindrical cast iron blank with a diameter of 56 cm and a height of 46 cm, which was surrounded by water-cooled copper loops. The arc length was increased to 12.7 cm and the electrical power increased to 4000 amps and 115 volts. By the end of 6 minutes of work, the light bulb had melted a part with a diameter of 25 cm from the top of the metal anode. Although considerable quantities of molten metal had splashed onto the burner body, no significant damage was detected on the cathode. Example 3

A cathode device according to Fig. 1, consisting of 1.3 cm diameter thorium-containing tungsten electrode and located in a 1.9 cm diameter water-cooled copper nozzle, was used. The cathode tip was bevelled (recessed) 3.2 mm from the nozzle outlet. Argon gas at the rate of 5.1 to 5.4 m3 / hour was passed around the cathode and out through the nozzle. An auxiliary arc of 175 amps and 18 volts was held continuously between the cathode and the nozzle. This cathode structure was mounted in a casing for a metal melting crucible and extended into the crucible, the contents of which constituted the anode. The crucible had a diameter of 1.5 m and a depth of 1.5 m with a 30 cm thick refractory cladding. It contained 453 kg of steel scrap, 31.8 kg of pig iron and 32.7 kg of martin with a total load of 507.5 kg. Electrical connection to the crucible is made over 5 cm in diameter of large iron bars, j

which formed the bottom electrodes. One. An arc of 990 amps and 460 volts was initiated between the cathode and the metal bar. About 45 minutes later, the arc power was increased to 2 000 amps and 477 volts. Ten minutes later, the power was increased to 2,460-2,500 amps and 465 volts. After about 2 hours, the metal barge had completely melted. The experiment continued for about 4 hours, during which time the arc was periodically extinguished for temperature measurements and then returned. At the end of this time, the arc was extinguished and the molten charge was cast from the oven. Examination of the cathode and nozzle showed negligible damage or erosion.

Example 4

A cathode device according to Fig. 3, consisting of a 6.3 mm diameter tungsten electrode containing 2% thorium oxide, was mounted adjacent to the outlet of a water-cooled copper nozzle with the inner diameter of 7.9 mm. This device was mounted inside a furnace with an inner diameter of 30 cm and a depth of 20 cm, which had a 6.4 cm thick refractory cladding on the walls and a 40 cm thick cladding on the bottom. The oven contained 22.7 kg of punched soft iron. Argon gas at a rate of 0.96 m 2 / h was passed around the cathode and through the nozzle. An arc of about 25 to 50 volts and 500 amps was initiated by letting the cathode contact the charge and then withdrawn to a distance of about 43 mm. As the metal melted, the arc length gradually increased to about 6.4 cm. After 8 minutes, 0.88 no⁵/ hour hydrogen to the argon flow, resulting in a voltage of 80 to 440 volts. The temperature measurements recorded with an immersion thermocouple indicated that the charge had completely melted after about 30 minutes. The arc was then held for about 45 minutes, at which time the melt was poured. The arc was deliberately extinguished just before casting. Len's total required power for melting was similar to that used for similar processes using ovens, holding 22.7 kg.

Example 5

The etode of Fig. 2 was used. A thorium-containing tungsten cathode with a diameter of 2.5 cm and a central gas line with a diameter of 9.5 mm were located in a water-cooled copper nozzle with a diameter of 3.8 cm. The eatod tip was beveled or recessed 6.5 m from the nozzle outlet.

An arc of 9,000 amperes and 445 volts was held between the hollow thorium-containing tungsten cathode and a rotating water-cooled graphite anode, while 2.83 m 2 / hour of argon flowed through the hollow cathode and 8.49 m 5 / hour of argon were passed around the cathode. The trial continued during

358802 ⁸ minutes with a tungsten loss of only 0.079 grams (0.24 g / h). A very small hole at the periphery of the cathode passage was the only visible damage to the cathode surface. Operation with similar current conditions could not be maintained with previously known solid tungsten electrodes without complete electrode destruction.

In the above examples, the cathode has in all cases been used mainly in vertical position with respect to the anode. In many cases, it may be desirable to provide the cathode at an angle to the metal bath. This has the advantage that metal splashes are reduced against the burner and also that some are achieved. stirring effect on the bath. Metal splashes on the burner can also be reduced by moving the burner over the bath surface.

The above examples illustrate the ability of the device according to the invention to melt a metal batch in an oven. In all of these samples, no electrode loss and no pollution of the melt could be detected. Similarly, no significant damage could be detected on the burner body as such, due to metal splashes and the like. It is thus clear that the present device works satisfactorily for very long periods of time in a metal smelting furnace and thus is far superior to ordinary ball arc and hitherto known arc burners.

While certain preferred embodiments have been described above, certain modifications and modifications may also be made without departing from the scope of the invention. For example, the above description has been directed mainly at a high-power burner (500 - 2,000 kW) in 1-10 tons ovens and a low power burner (20 - 100 kW) is useful in furnaces holding 45.3 kg or less. Such burners according to the invention can likewise be used in connection with medium power ranges and furnace sizes. In addition, the device has been described in connection with a directional polarity for DC power, but this device can also be used for DC polarity with reverse polarity and AC power.

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