US2680144A - Electric furnace - Google Patents

Description

J n 1. 4 R. A. WILKINS ET AL 2,680,144

ELECTRIC FURNACE Filed April 14 1951 3 Sheets-Sheet l w Jo4 9W W v MALI/aw w June 1. 1954 w l s ETAL 2,680,144

ELECTRIC FURNACE Filed April 14, 1951 3 Sheets-Sheet 2 mZon-fi m? a E arzfi E1 0 land v M'LALM ,5-

June 1. 1954 R. A. WILKlNS ET AL 2,680,144

ELECTRIC FURNACE Filed April 14, 1951 s Sheets-Sheet s Q mm 17 w jiawvamhr Patented June 1, 1954 UNITED STATES PATENT OFFICE ELECTRIC FURNACE Maryland Application April 14, 1951, Serial No. 221,028

Claims.

Our invention relates to electric furnaces which, amon other uses, may be used for practising the methods Of smelting zinc described in co-pending application of Richard A. Wilkins and Kenneth A. Phillips Serial Number 220,5'7 i, filed April 12, 1951. The present application from the aspect of a zinc smelting electric resistor furnace having a graphite partition for protecting graphite heating resistors of the furnace is a continuation-in-part of applicants co-pending application Serial Number 93.954, filed May 18, 1949.

In the method described by the Wilkins and Phillips application Serial Number 220,574 above referred to roasted zinc ore concentrate containing zinc oxide and impurities is mixed in granular form with reaction carbon in granular form and the mixture charged to a hearth in an operatively closed chamber to form on the hearth a mass of the charge having an upper downwardly sloping free surface which at its lowermost portion is substantially in the plane of the hearth. This reaction carbon preferably is in the form of coke or anthracite coal rendered granular by a crushing or grinding operation, or coke breeze, or bituminous coal fines. The mixture is fed to the chamber from below the sloping surface of the mass of charge and adjacent its higher portion so that the charge being fed forces the material of the mass to this sloping surface substantially throughout its entire extent and thus also acts to move away from the body of said mass the material of the mass adjacent the lower edge of said sloping surface. On the mass of charge on the hearth heat is radiated downward on said sloping surface to heat the surface portion of the mass to a temperature which will react the zinc oxide at such portion with the carbon. The zinc produced is in the form of a vapor which, together with the carbon monoxide and other gases formed by the reaction and heat, escapes upward from the free surface portion of the mixture. These escaping vapors and gases agitate the upper surface portion of the mixture and cause the excess coke, coal and other solid particles, with molten slag and iron droplets adhering to them, to gravitate down the sloping surface to its lower edge portion for removal from the chamber. The zinc vapors and gases which escape upward from the mass may be discharged from the chamber into a condenser where they are cooled for condensing the vaporous zinc to liquid zinc.

The furnace according to the present invention comprises electric heating resistors, which are 2 preferably of graphite, for radiating heat downward on the charge. These resistors are heated to incandescence, and, when of graphite or the like, are susceptible to chemical attack by dust and gases emanating from the charge if such dust and gases are of such composition, as, for example, Zinc oxide dust and carbon dioxide, that they will react with carbon. The present invention has among its objects protecting the resistors from such attack.

In the drawings:

Fig. 1 is a central longitudinal vertical section, on the line I-I of Fig. 2, with parts in elevation, of a furnace according to the invention;

Fig. 2 is a section on the line 22 of Fig. 1; and

Fig. 3 is a section on the line 3-3 of Fig. 2, with parts omitted.

Referring to the drawings, the furnace comprises an open top casing l of sheet metal, preferably steel, having side walls 3, 5, I and 9 and a bottom wall ll all welded together to form a fluid tight construction. As shown, this casing is provided with a cover 53 comprising a reinforced metal plate [5 the bottom surface of which rests on the upper edges of the side walls of the casing. The cover is removably secured to the casing by the bolts ll welded at their lower end portions to brackets [9 (Fig. 2) secured to the stiffening bars 2| for the side walls 7 and 9 of the casing, to which latter the inner edges of these bars are welded.

Extending continuously around the upper edge portions of the side walls of the casing are angle-irons 23 the edges of the horizontal webs of which are welded to the casing side walls in a fluid tight manner to form a trough 25 which is filled with sand, oil or the like and into which projects a flange 2'! extending continuously around the edge of the cover plate 55 at its under side in fluid tight welded relation thereto. In this way the cover is secured to the casing in a fluid tight manner.

Internally the casing 4 contains a structure which forms a furnace chamber having, an upper portion 29 and a lower portion 3|. The bottom of the lower portion 3| is built up of hard carbon blocks 33 to form a hearth 35, which hearth, as shown by Fig. 2, is of arcuate cross-section. The blocks 33, as shown by Fig. 1, extend from one end of the furnace chamber to a point spaced from its opposite end to form a well or pit 3? having a bottom wall 39 formed by the hard carbon blocks 4|.

At opposite longitudinal sides of the hearth the blocks 33, as shown by Fig. 2, abut the wedge- 3 shaped blocks a3 a row of which extends for the entire length of the chamber. On the upper fiat surfaces of these wedge-shaped blocks are supported at opposite sides of the chamber the elongated horizontal graphite slabs ,55, similar slabs l! of graphite being positioned at opposite ends of the chamber. These slabs 35 and il are grooved at their upper sides as shown at 49, these grooves communicating with each other at the four corners of the chamber. The grooves in the slabs 45 receive the lower edge portions of vertically positioned horizontally extending graphite slabs El, and the grooves in the slabs ll receive the lower edge portions of vertically positioned horizontally extending graphite slabs 53, the slabs 5i and 53 abutting at the four corners of the chamber. Supported by each of the slabs 5! and the end portions of the slabs 3 at their upper edges are graphite bars 55 which,

as shown by Fig. 3, extend for the length of the chamber, while supported by each of the slabs 53 at their upper edges are graphite bars 5! which extend transversely of the chamber from one bar 55 to the other in abutting relation therewith, the bars 55 and 5'! being grooved at their lower sides, as shown at 59, for receiving the upper edges of the slabs 5i and 53;. Resting on the upper sides of the bars 55 are vertically positioned horizontally extending graphite slabs 6|, While resting on the bars 5'! and the end portions of the bars 55 are vertically positioned horizontally extending graphite slabs 653, these slabs 5i and 53 abutting each other at the four corners of the chamber, and the upper sides of the bars being grooved, as shown at 55, for receiving the lower edges of the slabs. The roof of the chamber is formed by the laterally abutting elongated graphite slabs l5? which rest at their opposite end portions on the top edges of the slabs M, the edge portions of the slabs El adjacent the slabs 63 also resting on the top edges of the latter.

The carbon blocks 33 and ll forming the hearth 35 and the bottom wall 39 of the well or pit 31, respectively, are supported on a layer formed by courses of firebricks 69, which layer at the sides of the wedge-shaped blocks 3 and adjacent the longitudinal ends of the casing l is built upward to support and back the slabs 35 and il and to form firebrick layers H adjacent the side walls of the casing. 011 the tops of the firebrick layers l'l rest the elongated graphite slabs 73 which close the space surrounded by said layers. The space between these slabs and the furnace chamber roof slabs ill, and the space between the firebrick layers H and the slabs and bars forming the interior side walls of said chamber, are filled with a mass of heat insulating material 75 as, for example, carbon beads or a mixture of the same and lampblack or broken up charcoal.

Between the slabs 73 and the cover 53 of the casing l, and between that casing and the fire-- brick layers adjacent its sides and bottom, are inserted heat insulating packing layers 16 of material capable of resiliently yielding under high pressure to permit the furnace casing and the interior walls of the furnace to expand and contract independently of each other so as to prevent rupture of the casing.

In the upper portion 29 of the furnace chamber is positioned a graphite resistor grid comprising a row of parallel elongated heating resistor bars '11 of graphite. The opposite end bars of the row have extensions 79 projecting through the furnace walls in insulated relation thereto, and at the exterior of the furnace these two extensions are provided with terminals BI for the cables for energizing the resistors, the resistors being connected at opposite ends for series flow of current through them. The resistors are suspended from the roof slabs tl in electrically insulated relation thereto and the rest of the furnace by the elongated graphite bars 83. The construction of the resistors and their insulated suspension is more fully described in Poland United States Patent 2,472,613, issued June '7, 1949, and in pending United States application of Richard A. Wilkins Serial Number 162,220, filed May 16, 1950, and need not be further described here. The resistor when heated become incandescent and radiate heat downward toward the hearth. They also act to heat the graphite roof or" the chamber to incandescence to cause it also to radiate heat downward toward the hearth.

As shown, interposed between the resistor grid and the hearth, in out-of-contacting relation with said grid and its suspension, is a row of laterally abutting elongated bars 85, of circular crosssection, formed of graphite which is extremely heat refractory and an excellent conductor of heat. These bars form a partition dividing the furnace chamber into its upper and lower portions 29 and 3!. They serve to protect the resistors from attack by zinc oxide dust, vapors and gases emanating from the heated charge. For electrical and economic reasons it is highly desirabl to protect the graphite resistors and graphite resistor suspensions from deterioration, and the relatively inexpensive bars not only act mechanically to protect them from deterioration but do so chemically for the reason that the bars, like the resistors and resistor suspensions, are heated to incandescence and any carbon dioxide emanating from the charge and tending to leak by the partition formed by the bars must contact with the latter and by reaction with them be converted to carbon monoxide, which is inert with respect to graphite, before it can reach the resistors and resistor suspensions. The circular crosssection of the bars gives the upper and lower sides of the partition a, corrugated shape which increases the area of those sides and therefore increases the area presented by the lower side of the partition for contact with carbon dioxide and increases the facility with which the partition absorbs the heat from above and radiates such heat downward toward the hearth.

As shown, the bars 55 are formed to provide shelves ill on which the ends of the bars 85 rest, these shelves being inclined downward from one bar 52 to the other so that when the bars 85 are placed on the higher ends of the shelves they will roll downward to the lower ends of the shelves and there rest against the abutments 89 formed by the bars 5'! at said lower ends. The bars 85 may be inserted and removed from the furnace without cooling it. For this purpose the walls of the furnace are provided with openings in one of which adjacent the higher end of the inclined shelves 8? is positioned a graphite sleeve Ell (Fig. 4) and in another of which adjacent the lower end of those shelves is positioned a graphite sleeve $3. These sleeves are received at their inner ends in the enlarged diameter portions of openings ill in the adjacent bar 55. The sleeves are normally plugged with removable stoppers in the form of bars 99 preferably of hard carbon which is a relatively poor heat conductor, these bars being preferably of such length as to extend for the full length of the sleeves so as to minimize heat losses from the chamber. As shown, those ends of the sleeves 9I and 93 which project from the outer side of the furnace are surrounded by the water jacketed metal sleeves I60 welded at one end to the casing I, which sleeves I keep the outer ends of the carbon bars 99 cool enough to be conveniently handled. Conveniently the plugs are provided at their outer ends with tapped holes I02 for detachably securing to them a rod having a threaded end portion adapted to be screwed into said holes so that the plugs by use of such rod may be pulled from the sleeves 9i and 93 when desired. Upon removal of the plug from the sleeve 93 the bar 85 at the lowermost ends of the inclined shelves 81 may be pushed out of the furnace through such sleeves by means of a metal rod inserted and pushed through the graphite sleeve IIlI of small diameter bore positioned in an opening in the furnace wall opposite the sleeve 93 in alignment therewith, this sleeve IBl communicating with an opening I93 formed in the adjacent bar 55. The bore of the sleeve iIlI may be normally plugged with a stopper in the form of a carbon bar similar to one of the bars 99. Ordinarily however, due to the small diameter of the bore of the sleeve IBI, it will suflice if such bore is closed by a metal cap I05 screwthreaded on or otherwise removably carried by a metal sleeve I0! which surrounds the outwardly projecting end of the sleeve IBI and is welded to the adjacent wall of the casing I. For supporting the bars 85, when they are being inserted and removed from the furnace chamber, the bars 5? are each shown as formed with a horizontal shelf M9 on which the bars slide for preventing them from tilting and falling downward into the lower portion of the furnace chamber.

After a bar 85 is inserted through the sleeve 9i at the higher ends of the inclined shelves 8? it may be readily rolled off the adjacent shelf I09 onto such inclined shelves by use of a suitable metal bar (not shown) inserted through the sleeve 9|, such sleeve having a pointed end which may be caused to enter the space between the bar 85 and adjacent vertical wall of the adjacent bar 5'! for prying the bar off said shelf I99. It will be understood that upon removal of a bar 95 at the lower ends of the inclined shelves 8? the remaining bars will roll down said shelves to place the next consecutive bar opposite the sleeve 93 and thus leave a space at the higher ends of such shelves into which another bar 85 may be inserted through the sleeve 9! when the stopper bar 99 in that sleeve is removed. In this way the bars may be removed for inspection and a damaged bar replaced by a new bar.

As further shown, the furnace is formed with a tap hole II I communicating with the well or pit 31, this tap hole being normally closed by a removable plug II3 of fireclay or the like, upon removal of which the material which enters the well from the hearth may be tapped therefrom as a liquid mass or be raked therefrom depending upon the temperature of such mass and the amount of solid particles contained therein.

Also, preferably, the exterior of the casing I at its lower portion is surrounded by a water jacket I I5 which has the effect of cooling the exposed surface of the hearth and well 31 so that there will be formed thereon a rather thin layer of solid iron or slag which protects the carbon blocks forming those surfaces from destruction. The cooling effect of the water jacket is so coordinated with the thickness and heat conductivity of the materials forming the adjacent portion of the furnace walls that when molten slag or iron contacts the surfaces of the hearth and well it will solidify and its thickness increase until its surface temperature reaches the melting point of the slag or iron, whereupon the protective layer thus formed will not build up farther, or, if the slag or iron when it first contacts these surfaces of the hearth and well eats them away, the consequent slight thinning of the blocks presenting those surfaces will cause such surfaces to reach a temperature which will solidify the slag or iron contacting them, which solidified slag or iron will build up to form a protective layer until the thickness of such layer is such that its exposed surfaces are at their melting points.

As shown, the vertical wall of the furnace adjacent the end of the hearth opposite the well 31' comprises a sleeve II! of hard carbon having a tapered bore II9. At its outer end this bore is provided with a metal sleeve extension I2I communicating with the horizontal conduit I23 at the exterior of the furnace. This conduit preferably communicates at one end with the bottom discharge opening of a hopper (not shown). Extending lengthwise of the interior of the conduit is an elongated rotary screw conveyor I31 driven by an electric motor (not shown). The mixture of oxidized concentrate and coke or coal may be entered into the hopper so as to discharge into the conduit I23 through which the screw conveyor forces the material and causes it to pass through the bore I I 9 to feed such material to the furnace and form on the hearth a pile or mass I3! (Fig. 1) having a downwardly sloping free upper surface I39 the slope of which is determined by the angle of repose of the material. This material builds up at its higher end to above the bore H9, and the incoming material from said bore acts to force the material toward the sloping surface of the pile substantially throughout the entire extent of said surface and to move the residue at the lower end of the pile along the hearth in a direction which is away from said bore.

The vaporous zinc admixed with the gaseous products of the reaction escape from the furnace chamber through an opening I49 in the furnace wall, from which opening they are led through a conduit I5I to a condenser (not shown) where the vaporous zinc is condensed to liquid zinc.

For insuring that no reaction of the zinc oxide and carbon will occur in the bore I I9 the portion of the water jacket I I5 at the end of the furnace adjacent that bore is extended upward to form a portion I58 that surrounds the sleeve extension IZI of the bore, and by absorbing heat from the adjacent portion of the furnace walls acts to keep the material in said bore and extension at a temperature below that at which any substantial re action between the zinc oxide and carbon occurs. For preventing zinc vapors from being forced from the furnace chamber through the charge into the bore H9 and charging conduit I23, and into said bore and conduit when the charge is being initially entered into the preheated furnace or when the mass of charge on the hearth is otherwise at a low level, in which bore and conduit such vapors would condense to troublesome solid metallic zinc and interfere with the charging operation, the block or sleeve II? is shown as formed with a passage I59 (Fig. 1) which opens into the bore II9 of said block. Through this passage an inert gas, such as nitrogen, may be fed to the bore II9 from a supply pipe ISI communicating with said passage, the flow of nitrogen being controlled by a valve itS in saidpipe. The amount of nitrogen so admitted to the bore He may be regulated by the valve to cause the pressure in that bore to be slightly higher than that in the furnace chamber so as to prevent backflow of the zinc vapors into said bore and the conduit i215. Nitrogen from this passage may also be admitted to the furnace chamber and hence to the condenser for displacing the air contained in them prior to heating the resistors and furnace chamber preliminary to initiating the smelting operation.

It will be understood that within the scope of the appended claims wide deviations may be made from the forms of furnaces described, without departing from the spirit of the invention.

We claim:

1. Apparatus for recovering zinc from zinciferone material comprising walls forming a chamber, a partition extending across said chamber for dividing it into upper and lower parts, which lower part is adapted to contain a mixture of zinc vapors and gases containing substance reactive with graphite, the partition being formed of elongated laterally contacting round bars and of such extent as to prevent communication between said parts of said chamber except such as may unavoidably occur through such interstices as may exist between said bars, a generally horizontal heating resistor grid of graphite in said upper part of said chamber for radiating heat on said partition for heating it to incandescence, said grid comprising a row of interconnected elongated resistors, the end resistors of said row having extensions projecting through said walls in electrically insulated relation thereto, means suspending said grid in said upper part of said chamber in spaced relation to said partition and in electrically insulated relation to said walls, the bars forming said partition being of graphite for rendering the partition highly heat conductive and for reacting with said substance of said mixture for rendering such of said mixture as may unavoidably pass through the interstices between said bars chemically inert with respect to attack on said grid.

2. Apparatus according to claim 1 in which parallel shelves are formed on a pair of opposite side walls, respectively, of the chamber, on which shelves the opposite end portions of the round bars forming the partition rollingly rest, the walls of the chamber having provision for successively entering said bars into the chamber and placing them on said shelves at one portion of the latter and for removing them from said shelves to the exterior of the chamber at another portion of said shelves.

3. Apparatus according to claim 1 in which parallel shelves are formed on a pair of opposite side walls, respectively, of the chamber, on which shelves the opposite end portions of the round bars forming the partition rollingly rest, the walls of the chamber having provision for successively entering said bars into the chamber and placing them on said shelves at one portion of the latter for removing them from said shelves to the exterior of the chamber at another portion of said shelves, the shelves being downwardly inclined from the first mentioned portion thereof to the last mentioned portion thereof for causing said bars when placed on said shelves at said first mentioned portion to roll down said shelves to said last mentioned portion.

4. A zinc smelting furnace formed to provide a reaction chamber, a partition formed of laterally abutting round bars of graphite extending across said chamber acting to divide it into upper and lower parts, said partition being of such extent as to prevent communication between such parts except such as may unavoidably occur through such interstices as may exist between said bars, means for entering into said lower part of said chamber a mixture of oxidized zinciferous material and reaction carbon, a heating grid comprising graphite resistors in said upper part of said chamber, means suspending said grid in electrically insulated relation to the rest of the furnace and in spaced relation to said partition for heating said partition to a temperature at which the graphite of which said bars are formed will react with carbon dioxide to reduce it to carbon monoxide and for causing said partition to conduct and radiate heat downward on such mixture to produce zinc vapors and reaction gases including carbon dioxide, means for discharging such vapors and gases from said lower part of saic chamber, said abutting graphite bars acting to reduce to carbon monoxide the carbon dioxide of such of said gases as tend to pass from said lower to said upper part of said chamber through the interstices between said abutting bars.

5. A zinc smelting furnace according to claim 4 in which parallel shelves are formed on a pair of the opposite side walls, respectively, of the re action chamber, the graphite bars being cylindrical and rollingly resting at their opposite end portions on said shelves, the furnace having provision for successively entering said bars into said chamber and placing them on said shelves at one of the ends thereof and for removing them fron said shelves to the exterior of the furnace at the opposite ends of said shelves.

6. A zinc smelting furnace according to claim 4 in which parallel shelves are formed on a pair of the opposite side walls, respectively, of the action chamber, the graphite bars being cylindrical and rollingly resting at their opposite end portions on said shelves, said parallel shelves be ing inclined to the horizontal, the furnace hav-- ing provision for successively entering the bars endwise into the furnace from its exterior for placing them on the higher portions of said shelves whereby successive bars so placed will roll down said shelves to build up the partition, and also having provision for successively removing said bars endwise from the lower portions of said shelves to the exterior of the furnace.

'7. An electric furnace having walls forming a furnace chamber, electric heating resistors in the upper portion of said chamber in electrically insulated relation to said walls for heating material entered into its lower portion, laterally abutting elongated bars of heat conductive heat re fractory material below said resistors in adjacent spaced relation thereto so as to be electrically insulated therefrom forming a partition separating the space containing said resistors from the space below it, said partition being of such extent as to prevent communication between said spaces except such as may unavoidably occur through such interstices as may exist between said bars.

8. An electric furnace having walls forming a furnace chamber, electric heating resistors of graphite in the upper portion of said chamber in electrically insulated relation to said walls for heating material entered into its lower portion, laterally abutting elongated bars of graphite below said resistors in adjacent spaced relation thereto so as to be electrically insulated therefrom forming a partition separating the space contaming said resistors from the space below it, said partition being of such extent as to prevent communication between said spaces except such as may unavoidably occur through such interstices as may exist between said bars.

9. An electric furnace comprising walls forming a furnace chamber, electric heating resistors in the upper portion of said chamber for heating material charged to its lower portion, parallel shelves formed on a pair of opposite side walls, respectively, of said chamber, which shelves are inclined to the horizontal, laterally abutting elongated round bars of heat conductive heat refractory material forming a partition between the resistors and the lower portion of said chamber, which bars at their opposite ends have cylindrical portions that rollingly rest on said shelves, the furnace having provision for entering said bars into said furnace from its exterior for placing them on the higher portions of said shelves whereby successive bars so placed will roll down said shelves to build up said partition.

10. An electric furnace according to claim 9 in which a wall of the furnace has an opening through which the bars may be slid endwise from the exterior of the furnace to place them on the shelves the higher portions of the latter, opposite walls of the furnace having openings at the lower portions of said shelves for pushing and removing the bars endwise from said shelves to the exterior of the furnace.

Cited in the file of this patent UNITED STATES PATENTS Number Name Date 950,904 Fitzgerald Mar. 1, 1910 1,105,538 Schwann July 28, 1914 1,110,359 Thierry Sept. 15, 1914 1,749,762 itsgerald Mar. 11, 1930 2,007,605 Heyroth et al. July 9, 1935 23171613 Poland June '2, 1949

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