GB1585195A - Electric arc furnace and process for melting particulate charge therein - Google Patents

Description

(54) AN ELECTRIC ARC FURNACE AND PROCESS FOR MELTING PARTICULATE CHARGE THEREIN (71) We, TIBUR METALS LIMITED, a corporation organised and existing under the laws of the Dominion of Canada, of Toronto, Canada, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be.

performed to be particularly described in and by the following statement: This invention relates to electric arc furnaces. More specifically it relates to electric arc furnaces in which at least one of the electrodes has an axial passage of appropriate size so that a gas, such as argon, may be introduced into the interior of the arc at an appropriate rate to produce a stabilized, "extended arc." The term "extended arc" refers to the linear distance over which an arc may travel because of the electrical characteristics of the advised gas.

In an article on "Arc-furnace Melting" by J. A. Charles and A. G. Cowen which was published on February 12, 1960, in Iron and Coal Trades Review (pp.353-358) the authors describe the feeding of argon through hollow electrodes in an arc furnace used for melting.

The dperation is compared with the use of solid electrodes and identical conditions apparently including electrode spacing is used as with the solid electrodes. While certain improvements were noted with argon, no attempt was made to produce an extended arc.

A later article (J. Ravencroft "Electrical Review" pp. 413-418, September 14, 1962) describes the testing of argon introduction through electrode openings in a 5 ton arc furnace. The authors conclude "As shown from the results, furnace performance, as assessed by melting rate, electricity consumption to clearmelt, and electrode consumption, was not improved by using argon during melting on the 5 ton furnace"...."There appears to be no practical application for argon in the production furnace, because the increase in arc power which results from an increase of electrical conductivity in the arcs can easily be attained in practice (if desired) by increasing the current settings of the electrode regulator".

United States Patent No. 3,783,167 discloses a mechanical device designed to produce an extended arc zone by moving an electrode or a plasma gun in a closed path so that the resultant arc with a second annular electrode would be moved into a greater area.

United States Patent No. 3,834,895 shows a process using a plasma arc furnace. A gas such as argon is fed into the furnace through an annular passageway surrounding either the electrode or a hopper feeding particulate material into the plasma. In this case, particulate iron-bearing material is dropped into the plasma from a hopper situated immediately above the furnace. The patentee stresses as novel the feature of collecting the melted iron at the bottom of the furnace and removing it in a molten form.

United States Patent No. 2,909,422 uses a hollow electrode in an arc furnace to introduce inert gases, such as argon, to reduce hydrogen pickup, to minimize updraft in the hollow electrode and to reduce the heating period of the furnace. Patentee states "The stream of gas material leaving the bore of the furnace electrode impinges directly upon the surface of the melt, and its velocity produces a stirring or agitating effect in the melt region under the electrode, thereby shortening the reaction time between the melt ingredients and the slag, such as in the case of steel melting, and considerably shortening the refinement period" "By supplying argon through the electrode bore, the moisture in the furnace atmosphere is displaced in the arcing region, which being the hottest part of the furnace is the most likely place for the hydrogen transfer mechanism to occur.An increased rate of argon flow will naturally displace more moisture laden air from the arcing region and consequently will be more likely to prevent the incorporation of hydrogen in the melt".

United States Patent No. 3,105,864 discloses an electric arc furnace having an electrode with an axial opening and an outer coating on the electrode of higher melting point than the remainder of electrode so that when gas is passed through the axial opening the tip of the opening will wear into an enlarged tip or cone at the exit end of the electrode. Patentee is obviously unaware of the possibility of stabilizing and extending the arc by controlling the rate of gas flow and the linear velocity of gas passing through the opening.

According to the present invention there is provided a process for the melting of a charge of metallic or other material in a refractory lined vessel comprising the steps of; positioning the charge below and in close proximity to a first electrode having an axial passage therein extending through at least a major portion of the length of the electrode and communicating with the interior of the vessel; applying electrical power to the first electrode of sufficient intensity to produce an arc between the first electrode and a second electrode extending into the interior of the vessel or between the first electrode and the charge; flowing a selected gas through a predetermined size passage of the first electrode and into the interior of the arc and at a flow rate and linear velocity to create a stabilized and extended arc; increasing the distance between the first electrode and the second electrode or between the first electrode and the charge so as to extend the length of the arc, and controlling the power of the arc by controlling the rate of gas flow by increasing or decreasing the gas flow and/or the use of a particular gas composition.

In accordance with this invention it has been found that improvements in operation and in ecomony for the melting or reaction of a charge, such as iron, steel, gas, etc. can be effected by the arc furnace design and process for operation which are described herein.

The improved design is based primarily on the stabilized extended arc made possible by the incorporation of at least one electrode having an axial opening through which an appropriate gas, such as argon, is fed into the arc. The size of the opening in the electrode or electrodes is selected to give an appropriate rate of gas flow there through by which the extended arc is stabilized. The furnace power may be controlled by modulating the rate of gas flow or by modifying the composition of the gas supplied.

It has been found that this stabilized extended arc operation results in (1) reduced electrode consumption, (2) improved power factor, (34 improved heat transfer, (4) improved power control, (5) reduced refractory wear, and (6) reduced acoustical and electrical noise.

The presence of the gas in the interior of the arc allows the arc to be extended by increase of the distance between arcing electrodes or between an electrode and the melt in the hearth, and imparts to the extended arc a stability not attainable by other means. Prior art arcs are erratic in performance. must be maintained at relatively short lengths, and are not capable of being extended to the degree and with the stability attained in the furnace of this invention.

In establishing the stabilized extended arc, the electrode is spaced from a second electrode or from material in the hearth the short distance normally used in initiating an arc with solid electrodes. Then the electrical power is applied to initiate the arc, following which the gas flow through the electrode is started. The resulting ionization of the gas in the arc lowers the electrical resistance and the electrode is then withdrawn at least double and generally many times more the original length of the arc. In a typical case the normal arc with solid electrodes could be about 1/2" to 1" whereas the extended stabilized arc of this invention may go up to 6 inches in length.

If the gas is introduced in any manner other than through the axial opening in the electrode the resultant arc does not have the stabilized extending effect accomplished by feeding in through the axial opening. When the arc is between two or more electrodes. it is only necessary to have an opening in one electrode although it may be preferred to have passages in more than one or even in all the electrodes. When the arc is between a series of two or more electrodes and conductive material in the hearth. it is preferable to have axial passages in each of the electrodes.

The number of electrodes and the arrangements thereof will vary according to the size and capacity of the furnace. For example it is possible to have a considerable number of electrodes set in a horizontal plane with half the electrodes extending inwardly from one side of the furnace and the other half extending inwardly from the opposite side. With the arcs extending between each opposing pair of electrodes, a considerable stabilized.

extended arc may be formed of a large horizontal pattern. Where it may be desirable to extend the arc area vertically it is possible to arrange pairs of electrodes horizontally opposed from each other but at different heights so that one or more pairs may be vertically positioned over one or more other pairs and so that the resultant stabilized extended arc may have a greater vertical dimension.

Among the advantages of this furnace are: 1. Reduced electrode consumption - Once the arc has become stabilized and the extended spacing effected, the electrode consumption is reduced drastically.

2. Improved power factor - As soon as stable operation is achieved, it is no longer necessary to maintain a large series reactance in the power circuit and therefore the power factor can be raised from the usual 70-75% to substantially unity.

3. Improved heat transfer - With the wide electrode tip spacing made possible by this technique the arc tends to become soft and diffused over a much greater volume, as compared to the very intense and relatively small volume of the conventional arc.

4. Power control - In addition to the methods available in conventional arc furnace operation for the modulation of power by changing the electrode tip spacing and by changing the voltage applied to the electrodes, power - may also be modulated, without moving the electrode or changing the voltage, merely by changing the rate of gas flow to the arc, or by changing the composition of the gas fed to the arc.

5. Lower refractory wear - With the softer nature of the arc, there is much less attack on the refractory as well as less "burning" or volatilization of the charge.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevational view of an essentially spherical furnace, Figure 2 is a vertical sectional view taken along the line 2-2 of Figure 1; Figure 3 is a vertical sectional view of another modification of the furnace according to the present invention; Figure 4 shows oscillographic recordings for current, power and arc vqltage for conventional arc mode of operation in a furnace as shown in Figure 1; Figure 5 shows corresponding oscillographic recordings for the stabilized extended type of operation of the present invention.

In the embodiment shown in Figures 1 and 2, an outer spherical shell 1 and 1' is in two parts held together by flanges 2 and 2' and fastened by bolts 3 and 3'. This shell has a refractory inner lining 4. An electrode 5 has a passage 6 therein through which a gas such as argon is fed into the interior of the furnace as fed to the electrode from a supply 7 through connector 8. The gas (argon) stream is directed to an opposing electrode 9 which may or may not have a passage therein. In either case the gas from the electrode passage or passages is fed into the interior of an arc 10. The positioning of the electrodes so as to increase or decrease the distance between them is effected by rotation of either or both of two sprocket wheels 11 whose teeth fit into notches on the bottom of insulating supports 12.

The furnace is attached to and supported by flanges 13 and a base 14. The furnace may be tilted on the base 14 by pressure applied upward on arms 15 which are fastened to the furnace shell by flanges 16. In a tilted position molten material may be poured from the furnace through opening 17. Power is supplied to the electrodes by a power source 18.

In the modified furnace illustrated in Figure 3, three electrodes 5 as shown extend vertically downward toward a hearth 19. Shell 1 has an inner refractory lining 4 and is supported by a flange 13 on a base 14. The electrodes are held by insulating supports 12 which are in turn supported by bolts 20. The positioning of the electrodes to shorter or greater distances from the hearth 19 and the contents thereof is effected by the turning of bolts 20. Each of the electrodes as shown has a passage 6 therethrough to which a gas supply (not shown) may be connected. Power is supplied through power source 18.

Figure 4 shows oscillographic recordings for current, power and arc voltage for conventional arc mode of operation in a furance similar to that shown in Figure 1, while Figure 5 shows corresponding oscillographic recordings for the stabilized extended arc operation of this invention with an argon flow rate of 3 cu. ft. per hour and power of 15 kw.

The electrodes are preferably made of carbon or graphite, although other suitable materials may be used such as tungsten and the size may be whatever is appropriate to accommodate the size and design of furnace being used. The electrode passage must be present in one and may be in more or all of the electrodes. The electrodes may be positioned horizontally, vertically or inclined and they may be arranged so that the arcing is effected between two or more electrodes or between the electrode or electrodes and the metal in the collecting hearth. The size of the electrode passage or openings is determined in such a manner as to give the desired gas flow rate and linear velocity. The desired overall gas flow rate and linear velocity will vary according to the size of the furnace, the production capacity of the furnace, the nature of the particulate feed material and the nature of the gas.The electrodes are fastened in such a manner that the spacing for the arc may be adjusted for initiating arcing and maintaining the extending arc as well as to adjust the arc gap to compensate for consumption of the electrodes.

In addition to argon, various other gases may be used such as helium, nitrogen, carbon monoxide, methane, chlorine, etc., as well as mixtures thereof. However each gas differs somewhat in the effect produced and the appropriate rate of feed should be adjusted accordingly. The specific gas may be tested very easily to determine the appropriate rate of flow to produce the desired effect in stabilizing the extended arc.

The construction of the furnace proper may be similar to that of conventional arc furnaces using a refractory material where there will be exposure to extremely high temperatures- or molten metal and where desired for extra strength an outer shell of steel or other appropriate metal may be used. The thickness of the refractory and the size of the furnace hearth will vary according to the design capacity of the furnace. Furnaces having capacities. up to 400 tons of metal and even higher may be used. The power sources are similar to those used in other electric arc furnaces.

The invention is illustrated by the following examples which are intended merely for purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it may be practiced. Unless specifically indicated otherwise, parts and percentages are given by weight.

Example I A series of heats are performed in a small Detroit Rocking furnace having a design similar to that shown in Figures 1 and 2. The furnace is spherical in shape and has an inside diameter of 6-1/4 inches. The electrodes are 3/4 inch diameter graphite electrodes. One of the electrodes has an opening bored along its longitudinal axis. and these tests are for determining the effect of variations in the size of this opening. An input is used of approximately 15 KW maximum from a 575 volt, single phase, 60. Hertz supply through a step down water-cooled transformer. The electrode adjustments are made manually and argon is the gas fed into the electrode opening. Since arc resistance is one of the most important criteria in furnace performance this is measured for different rates of gas flow and for variations in the electrode spacing. The results of three sets of tests with three different sizes or openings are reported below in Tables A, B and C. It is also noted that rate of electrode consumption is drastically reduced. For these particular flow rates the electrode opening of 0.0625!' gives the best performance.

TABLE A Diameter of electrode opening = 0.0320" The gas used is argon.

Gas Arc Arc Arc Arc Resist.

Flow Length Current Voltage Power of Arc CFH INCH AMP VOLTS KW OHMS 1 240 58 14 .242 3 244 62 15 .254 5 280 57 15 .22 7.5 1" 280 60 15 .214 10 280 60 15 .214 15 280 57 15 .22 3 220 69 14 .314 5 196 72 14 .368 7.5 204 72 14 .353 10 2" 220 69. 14.75 .314 15 235 68 15 .288 3 140 82 10.5 .585 5 3" 144 80 12.0 .555 TABLE B Diameter of electrode opening is 0.0625" The gas used is argon.

Gas Arc Arc Arc Arc Resist.

Flow Length Current Voltage Power of Arc CHF INCHES AMPS VOLTS KW OHMS 3 312 47 15.0 .15 5 360 36 14.0 .1 7.5 1" 358 36 13.5 .1 10 350 43 15.0 .123 15 350 42 14.5 .117 3 268 61 15 .228 5 300 55 15 .183 7.5 288 57 15 .198 10 2" 300 54 15 .180 20 296 50 15 .17 3 192 72 15.5 .375 5 220 73 15 .332 7.5 3" 208 66 15 .318 10 248 62 15.5 .25 15 260 66 15.5 .23 20 276 57 15.2 .205 TABLE C Diameter of electrode opening is 0.125" The gas used is argon.

Gas Arc Arc Arc Arc Resist.

Flow Length Current Voltage Power of Arc CFH INCHES AMP VOLTS KW OHMS 3 208 68 13 .327 5 288 62 15 .216 7.5 1" 350 56 15 .187 10 296 52 15 .176 15 296 50 15 .169 20 320 46 15 .144 5 160 80 15.5 .5 10 2" 260 60 15 .231 15 284 59 15 .206 20 292 56 15 .194 3 160 80 13 .5 5 192 72 14 .375 7.5 3" 200 72 15 .36 10 200 72 14.5 .36 15 188 74 14 .394 20 188 74 14 .294 Example II The furnace and conditions of Example I are used in a series of tests using an electrode having an opening of 0.0625 inch and variations in gas, gas flow rate and arc length. The results are tabulated below in Tables D-F.

TABLE D Nitrogen Arc Gas Arc Arc Arc Arc Length Flow Current Voltage Power Resist.

INCH CFH AMPS VOLTS KW OHMS 3 288 57 14.5 .2 5 260 58 14.5 .22 1" 7.5 272 54 14.5 .2 10 280 55 15 .2 15 292 53 15 .18 20 304 52 15 .17 3 240 58 15 .242 5 284 55 15 194 2" 7.5 284 55 15 .194 10 238 54 15 .187 15 264 59 15 .224 20 280 56 15 .2 3 256 64 15 .25 5 256 62 15 .242 3" 7.5 232 68 15 .293 10 252 64 15 .254 15 256 63 15 .246 20 260 62 15 .238 TABLE E Helium Arc Gas- Arc Arc Arc Arc Length Flow Current Voltage Power Resist INCH CFH AMPS VOLTS KW OHMS 3 264 59 15 .244 5 300 54 15 .18 1" 7.5 312 52 15 .165 15 312 49 15 .156 3 296 56 15 .189 5 304 54 15 .178 7.5 280 59 15 .21 2" 10 284 56 15 .198 15 272 59 15 .217 20 264 60 15 .225 3 264 63 15 .239 5 272 60 15 .22 3" 7.5 244 65 15 .255 10 256 63 15 .246 15 248 63 15 .254 20 200 60 15 .30 TABLE F Carbon Monoxide Arc Gas Arc Arc Arc Arc Length Flow Current Voltage Power Resist.

INCH CFH AMPS VOLTS KW OHMS 3 248 65 15 .262 5 248 64 15 .258 7.5 260 61 15 .235 3" 10 252 64 15 .254 15 220 70 15 .318 20 208 71 14.5 .342 3 140 81 12.5 .58 5 186 76 14.5 .41 3-1/2" 7.5 190 75 14 .395 10 196 71 14 .368 15 156 79 12 .506 3 100 87 9.5 .87 5 148 80 13 .54 7.5 152 77 12.5 .506 3-3/4" 10 156 76 12 .497 15 156 77 13 .495 20 156 78 11.5 .5 25 156 78 11.5 .5 30 148 79 12 .534 40 128 84 9.5 .656 Example III A commercial electric arc furnace of 3 tons capacity and a power supply of about 2,000 kw at 25 hertz is modified in accordance with this invention and operated to melt charges of iron metal. A 1/4 inch diameter hole is drilled along the longitudinal axis of a number of 7-inch diameter graphite electrodes of 60-inch length.Six of these are coupled in pairs to assemble into three electrodes of 10 feet each and installed in the furnace with the outer ends connected to an argon supply with appropriate fittings. Five batches of approximately 2.96 tons each are made with the argon flow adjusted at 2 to 3-1/2 cubic feet per minute to give the most effective results. Flow at a rate of 1 cu. ft. per minute has little effect on the arc, and increasing the flow in excess of 4 cu. ft. per minute does not show any appreciable additional change. The results are compared with furnace operation using solid electrodes.

Solid Hollow Electrodes Electrodes with Argon Electrode Consumption (Ibs. per 11 6 ton) Power Consumption (kw.hrs.per ton)604 567 Arc Stability - Improved Average Argon Consumption - 40 (cu. ft. per ton) There was an apparent reduction in the time required for melt-down and no arc flare or refractory wear observed. The more stable arc produced with the argon gave a more even and regular undulating pattern of power demand.

Although the invention has been described with reference to certain embodiments and by the use of various examples it is to be understood that the furnace and process herein disclosed and described may be utilized for the melting of metal, glass, and other materials and for various heat treatment of molten material within the scope of the appended claims.

WHAT WE CLAIM IS: 1. A process for the melting of a charge of metallic or other material in a refractory lined vessel comprising the steps of; positioning the charge below and in close proximity to a first electrode having an axial passage therein extending through at least a major portion of the length of the electrode and communicating with the interior of the vessel; applying electrical power to the first electrode of sufficient intensity to produce an arc between the first electrode and a second electrode extending into the interior of the vessel or between the first electrode and the charge; flowing a selected gas through a predetermined size passage of the first electrode and into the interior of the arc and at a flow rate and linear velocity to create a stabilized and extended arc; increasing the distance between the first electrode and the second electrode or between the first electrode and the charge so as to extend the length of the arc, and controlling the power of the arc by controlling the rate of gas flow by increasing or decreasing the gas flow and/or the use of a particular gas composition.

2. A process according to Claim 1 in which the arc is extended to at least double the length of arc produced without the presence of the stabilizing gas flow.

3. A process according to Claim 1 or 2 in which the arc-stabilizing gas is argon.

4. A process according to Claim 1 or 2 in which the arc-stabilizing gas is nitrogen.

5. A process according to Claim 1 or 2 in which the arc-stabilizing gas is helium.

6. A process according to Claim 1 or 2 in which the arc-stabilizing gas is carbon monoxide.

7. A process according to Claim 1-or 2 in which the arc-stabilizing gas is methane.

8. A process for the melting of a metallic charge substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. disclosed and described may be utilized for the melting of metal, glass, and other materials and for various heat treatment of molten material within the scope of the appended claims. WHAT WE CLAIM IS:

1. A process for the melting of a charge of metallic or other material in a refractory lined vessel comprising the steps of; positioning the charge below and in close proximity to a first electrode having an axial passage therein extending through at least a major portion of the length of the electrode and communicating with the interior of the vessel; applying electrical power to the first electrode of sufficient intensity to produce an arc between the first electrode and a second electrode extending into the interior of the vessel or between the first electrode and the charge; flowing a selected gas through a predetermined size passage of the first electrode and into the interior of the arc and at a flow rate and linear velocity to create a stabilized and extended arc; increasing the distance between the first electrode and the second electrode or between the first electrode and the charge so as to extend the length of the arc, and controlling the power of the arc by controlling the rate of gas flow by increasing or decreasing the gas flow and/or the use of a particular gas composition.

2. A process according to Claim 1 in which the arc is extended to at least double the length of arc produced without the presence of the stabilizing gas flow.

3. A process according to Claim 1 or 2 in which the arc-stabilizing gas is argon.

4. A process according to Claim 1 or 2 in which the arc-stabilizing gas is nitrogen.

5. A process according to Claim 1 or 2 in which the arc-stabilizing gas is helium.

6. A process according to Claim 1 or 2 in which the arc-stabilizing gas is carbon monoxide.

7. A process according to Claim 1-or 2 in which the arc-stabilizing gas is methane.

8. A process for the melting of a metallic charge substantially as herein described with reference to the accompanying drawings.