CA1046279A - Apparatus for circulating molten metal - Google Patents

Abstract

ABSTRACT

Apparatus for circulating molten metal in a furnace comprises an electromagnet the coil of which is adapted for energisation from an alternating current source and the core of which is elongate and projects from both ends of the coil.
The core is disposed so that 25% to 50%, preferably 37%, of its length is at one axial side of the mid-point of the coil, and the part of the core projecting at the said one axial side of the coil has a coating or casing of a refractory material. The apparatus may be mounted with its core horizontal in a side wall of the furnace but it is preferred to dispose the core vertically with the heat shield close to the surface of the molten metal. A duct member may surround said part of the core and may be shaped to direct the molten metal laterally. A partition or other form of baffle may be disposed externally of the duct between the inlet and outlet of the duct to prevent the molten metal leaving the outlet of the duct from passing straight back to the inlet of the duct.

Description

10~279 The present invention relates to the circulation of molten metal in a melting or holding furnace.
In furnaces for melting light and 'neavy metal scrap it i~ de~irable to employ a ~eans for rapidly circulatin~ the metal in the furnace. For ex~mple, metal circulation can be effectively used for drawing solid aluminium swarf into the body of molten metal at the charging point without causing excessive di~turbance of the metal surface.
Submer~al of solid swarf by mechanical ~tirrers or the like can lead to very high melting los~es through oxidation of metal at the disturbed ~urface.
It has already been proposed to circulate molten metal in a furnace by means of an electromagnetic circulator.
The principal ob~ect of the present invention is to provide an improved electromagnetic device for thiR purpose.
According to this invention there i8 provided means for circulating molten metal in a furnace, comprising an induction coil adapted for energisation from an alternating current source, an elongate core having the coil disposed about it and pro~ecting from both end~ of the coil, said core being so di~posed in relation to the coil that 25-50% of the length of that core is at one axial side of the mid-point of the coil, a refractory casing or coating protecting the part of the core projecting at said one axial ~ide of the coil, and `~

104~Z79 a heat resistant shield protecting the coil at least at its side nearer said part of the core.
Preferably 37% of the length of the core is at said one axial side of the mid-point of the coil.
Some embodiments of the invention will now be described with reference to the accompanying drawings in which:
Figure 1 shows one form of circulator according to the invention, which is to circulate metal into and out of an open side well of a furnace, Figure 2 shows an alternative arrangement of circulator, Figure 3 shows a portable form of circulator, which may be conveniently raised and lowered in accordance with change of metal level, Figure 4 ~hows a further circulator arranged in a side ~cket of a furnace, and Figure 5 shows a circulator arranged horizontally.
The apparatus in each of Figures 1 to 3, includes an elongate core 2 arranged vertically and ~urrounded by a coil 5 which is above the liquid metal level. 25Z-50%, preferably 37Z, of the length of the core is below the midpoint of the coil, and most of thls part of the core i8 below the li~id;metal level in u~e of the apparatus.
The unit shown in Figure 1 i~ intended to operate at a generally fixed bath depth, as shQwn. A cylindrical nose piece 1 ~ ~ ~

with a cloQed lower end ~urrounds and protects the core 2, and extend~ into an elbow-shaped duct 3. Both the nose piece 1 and the duct 3 are made from a suitable material which can either be a non-magnetic metallic or ceramic refrac~ory. The duct is keyed into position in a side well of the bath, ~nd i8 open at its top below the liquid ~urface and has a ~ide outlet 10. The circulator unit comprising the core, coil ~nd no~e piece 1 is shown in an operating position to which it has been lowered from the top of the duct. The nose piece lies concentrlc with the upwardly extending part of the duct and form~ with the duct an annular channel 4 for the passage of molten metal.
The coil 5 is protected by a heat barrier or shield 6 from possible damage due to heat and splashing of ~olten metal. The--circulator unit rest~ on brick piers built into the furnace but not shown. The coil 5 and the heat barrier 6 are ~o positioned that they stand clear from the top of the duct 3, ~o tha~ there i~ a 360 annular inlet 7 around the core for the ingre~s of molten metal into the annular channel 4. A partltion wall 9 arranged at the entry to a side well to which outlet 10 lead~
ensure~ that the circulating metal forced to leave through the outlet 10 cannot take the direct route back to the inlet 7.
The protrusion of the core 2 above the surface of the molten metal and above the coil serve~ the dual purpo~e~ of improving the magnetic efficiency of the device and of providing a heat 104~279 exchange ~urface for cooling by forced air or water for example thus leading heat away from the coil 5. Coolant can also be supplied through a channel 11 in the centre of the core 2. Cooling of the core is designed to keep its temperature below its Curie point.
In cert~in cases the nose piece 1 can be dispensed with, and the core can be protected either by a refractory wash or by a rammable refractory material. In such a case, the core can be regarded as semi-expendable and this ha~ two advantages over a core protected by a separate nose piece:
(a) the magnetic circuit i~ brought closer to the molten metal and hence results in an increased efficiency; and (b) it eliminates the need for using a ceramic no~e piece, which is advantageous bec~u~e tha core is much cheaper than the nose piece.
When the coil i8 energised by connection with an A.C.
source a secondary current is induced in the molten metal ~urrounding the casing and the interaction between the primary field and the field of the induced current leads to a force on the molten metal in a direction substantially lengthwise of the casing ~nd away from the coil.
Since the force on a notional ring of molten metal Jurrounding the core is an inverse function of its di-~tance lO'~Z ~'~
from the corc and the coil it is neces3ary for the purpose of getting good circulation in relation to the electric power input to maximise the performance of the device by careful design of the induction coil and core. We have ound experimentally that the force on a metal ring surrounding the core varies considerably according to the length of the core which pro~ects from the oppo~ite side of the coil. This i5 due to the fact that altering the location of the coll along the core alters the pattern of the flux emanating from the core and changes the area through which it ~preads. Further, since the reluctance of the magnetic flux is inversely proportional to the area through which it is made to spread, the reluctance can be decreased by encouraging the flux to ~pread over a ~ide area. The maximum area of the spread of flux i8 found to occur when the amount of core on the ring side from the mid-point of the coil i8 about 37Z of its total length. Thi~ i~
therefore the position where the force on the notional metal ring is a maximum; in other words, the pro~ecting tail end of the core should be longer than the length of core within the refractory ca~ing for maximum force. In general it may be said that the length of the core on the ring or metal side of the mid-point of the coil should be 25-50% of the overall length of the core.

lO~tjZ79 In order to ensure that the maximum force is applied to the notional ring of metal ln directions parallel to the longitudinal axis of the core, the line~ of magnetic force emanating at right angles to the longitudinal surface~ ~f the core should also inter~ect the ring of metal without having changed direction, i~e. the direction of the lines of magnetic flux through the notional ring of molten metal should be at right angles to the longitudinal axi~ of the core. This can be aohie~ed by arranging the geometry of the coil in relation tD the radial thicknes~ of the notional ring of molten metal, for example by making the outside diameter of the coil greater than that of the notional ring of molten metal.
The flux i~ thu~ con~trained to travel along a longer path round the coil than when the diameter of the coil is smaller.
In an example in which the outer diameter of the ring of metal wa~ found to be 40 cm., the outside diameter of the coil was made 56 cm.
A portable-type circulator which may be operated at various bath depths may be constructed as a modification of the built-in- type circulator ~ust de~cribed. In this modification the duct 3 is connected by means of connecting bars to either the nose piece 1 or the heat barrier 6, thus enabling the entire unit to be lifted into and out of the metal. The annular channel 4 and the 360 annular inlet 7 remain as 104~275 described. Similarly, the protrusion of the core 2 above the metal level and coil performs the ~ame functions a~ described previously.
As an example of the invention, a device constructed as shown in thi~ Figure 1 embodiment W~8 located in the side-well of a 23,000-kg (50,000-lb.) furnace. The prim~ry winding was a 230-mm diameter, 90 turn coil. The 920-mm long, 50 x 50-mm laminated steel core extended a third of it~ length into the molten metal. Both the coil and the core were air cooled. A
number of tri~l~ were conducted, and the re~ults of one of these trial~ i~ summari3ed below:-Molten aluminium capacity of the furnace 23,000 kg.
(including side well)........................ .(50,000 lb.) Quantity of metal in the furnace at the 18.000 kg.
time of trials .............................. (40,000 lb.) Depth of metal in the side well ............. .600 mm.

Average metal temperature (mea~ured in the O
~i~e well) .................................. .725 C.

Maximum velocity of metal (measured ~n the side well) .................................. .29 cm/~ec.
N~ximum flow rate ........................... .26,000 lb./min.
Power input to the coil ..................... .5kW
Power factor ................................ .0~22 Show~ in Figure 2 is a circulator unit of the portabletype, having a core 2, coil 5, shield 6 and nose piece 1 as in the Figure 1 con~ruction, and having a straight duct 23 104bi279 connected to the nose piece 1 by means of cross rods (not shown).
The whole circulator unit, including duct 23, is mounted for both vertical ~nd horizontal movement ad~acent a trough 24 inclined a~ 20 to the horizontal in a ramp unit 25, located in the side well of the furnace. It will be seen that metal leaving the bottom end of the duct 23 in a vertically downward dlrection will be given a horizontal component of motion to produce metal circulation as a result of striking the bottom of the trough.
If a change of metal level occurs the unit may be ad~u~ted both vertically and horizontally so as to ensure that the lower end of the duct 23 i8 in close proximity with the inclined trough 24, whilst the heat shield 6 remains in close proximity wlth the surface of the body of molten metal to achieve maximum efficiency.
With this portable arrangement it is found in some circumstances possible to omit the surrounding duct 25, relying on the impact of metal on the floor of the trough to produce circulstion. Figure 3 shows a further modified form of apparatus according to the invention. As in Figure~ 1 and 2, the circulator includes a core 2 surrounded by a coil 5 at a position ~uch that abo~t a third of the core i~ below the mid-point of the coil, and the part of the core below the coil is 104~279 surrounded by a protective nose piece 1. The nose piece may have a length of 38 cm. and the length of the as~ociated core may be 105-140 cm. An angled duct 3 having a horizontal outlet 10 surround~ the lower part of the nose piece 1 and defines therewith an annular channel 4 with a 360 inlet 7 at its upper end. A heat shield 6 surrounds the coil and extends beneath it to protect it againæt possible damage due to heat and splashing of molten metal. In this embodiment, there are bolted on each side of the heat shield 6 aprons 8 and 9 which also ~erve to support the duct 3 and hold it clear below the heat æhield 6. The apron 8 shown on the right haæ a width in exces~ of the diameter of the outlet 10 and provides a bsffle reducing the chance~ of metal pasging from the outlet 10 ~traight back to the inlet 7 of the duct 3.
Stainle~s teel str~p~ 11 are bolted to the shield and to a cruciform unit 12 also of ~tainle~s steel and extending across the top of the core 2. An eyebolt 14 is fixed to the unit 12.
The straps ll æupport the full weight of all the other componentæ shown and are the means by which the unit can be liftet in and out of the metal or ad~usted as to height according to the depth of metal in the furnace.
Preferably the nose piece 1, the duct 3, the heat shield 6 and the aprons 8 and 9 are made from fused silica while those areas of these partæ which may come into contact with molten _ g _ 104~Z'79 metsl are preferably coated with a heat-re~istant wash.
Thermal insulation such as is shown at 13 on the outside of the lower part of the core, at the bottom of the noqe piece 1, and beneath the straps 11 ~here they turn under the core, may be provided for protection purposes. The coil 5 is suitably water cooled and a~ in previou~ embodiments the core can have a central channel through which coolant air may be passed.
In the embodiment of Figure 4, a circulator according to the invention i8 shown in a side pocket 40 of a furnace 41.
A cylindrical ch~mber 42 18 provided in the pocket, and a core 45 with no~e piece 43 extends downwardly into thi~
chamber, to define an annular vertical pa~sage therewith. As in the previou~ embodiment~, the core i8 surrounded by a coil 51 above the chamber, and thus above metal level, the coil being protected by a shield 44. Half of the core i~ below the mid-point of the coil. A lifting arrangement including ~traps 46 end a ring 47 is attached to the core and shield 44.
The chamber 42 in the pocket communicates with the main part of the furnace 41 via inclined ports 48 and 49, opening respectively ad~acent the top and bottom of the chamber 42. Theæe ports are offset in that they open into the furnace in different directions in order to provide appropriate thorough circulation of molten metal in the furnace, and to reduce recirculation from port 49 to port 48.

104~Z 79 Access port~ 52 and 53 are provided in the outer wall of the pocket for cleaning and maintenance purpose~. The lower of these access port~, 53, i~ shown with a bung 54.
In use molten metal flow~ in through the upper port 48, so a~ to surround the core, and i~ forced downward~ electro-megnetically ~o a~ to flow out of port 49 into the furnace.
A plurelity of such pockets and circulators can be provided if necess~ry, and ~rranged ~o as to en~ure circulation of molten me~al throughout the furnace. The circulator~ can be lifted out of the pockets during cleaning.
Referring now to Figure 5 of the drawing~, an arrangement is shown in which the core is mounted horizontally in a side well of the furnace. The casing 61, formed of refractory material, ha8 a tubular nose piece 62, which is of generally square ~ection. The wall thickness of the nose piece 62 is about 6.3 cm. and the internal wldth of the 8pace within the nose piece 62 i~ about 15 cm., 80 th~t a simple laminated core 63~ 10 cm x 10 cm can be fitted into it without contact with the wall~. The coil 64 encircling the core is disposed in a rece~s in the wall of the furnace. The core is kept a~ cool as possible without administering any thermal shock to th~! refractory no~e piece 62. The cooling of the core $~ i5 for the purpose of keeping it below its curie point.
The interior of the ~pace 65 in the no~e piece 62 i8 lined 104~279 with a blan~et of heat insulating material, guch as the very efficient material sold under the name "Fiberfrax". The tail end 66 of core 63 and the external surface of coil 64 are cooled by air circulation, so that the extended tail 66 serves a dual purpose of improving the magnetic efficiency of the device end of providing a heat exchange surface.
The coil 64 may itself be water-cooled.
The base of ca~ing 61 is kept thin 80 that the distance between the coil 4 and the molten metal in the furnace 18 a minimum and hence the force on the metal i8 increased. Further, the casing 61 is designed 80 that the length of its flange in contact with the surrounding refractory wall of the furnace is as large as possible.
The nose piece 62 i8 preferably, but not essentially, surrounded by a tunnel member 68, which acts as a mean~ of directing the metal flowing aw~y from the end of the nose piece 62 and as a means for protecting the nose piece from accidental damage. The passage through the tunnel member 8 i~ preferably of square section and is arranged to provide a passage about 5 cm. between itself and the nose piece.
The utility of the apparatus of Figure 5 in a melting furnace is illustrated by the following ex~mple.
An electromagnetic circulator con~tructed as d-scribed above was installed in a melting furnace of the type having 104~;Z79 a side well to receive metal charged into the furnace.
The total capacity of the furnace was 50,000 lb~. and an aluminium charge of 40,000 lbs. was held at 715C. With this charge a flow rste of about lO,OOO lbs/min. at a velocity of about 7 cm~/~ec. was achieved in the side well at a power input of about 8 kw to the coil.
With this circulation rate it was found possible ~o melt aluminium scrap at a rate of about 10,000 lb~/hour, whereas without circulation the melting rate was about 5,000 lb~/hour.
In general, however, the arrangements of Figures 1 to 4 are preferred because i) maintenance and repair are easier, ii~ the nose piece can be made much thinner with resulting saving in cost and increase in efficiency and iii) the coil can be dispo~ed nearer the surface of the metal.

Claims (11)

1. Apparatus for circulating molten metal in a furnace, comprising an induction coil adapted for energisation from an alternating current source, an elongate core having the coil disposed about it and projecting from both ends of the coil said core being so disposed in relation to the coil that 25-50% of the length of the core is at one axial site of the mid-point of the coil, a refractory casing or coating protecting the part of the core projecting at said one axial side of the coil, and a heat-resistant shield protecting the coil at least at its side nearer said part of the core.

2. Apparatus for circulating molten metal according to claim 1 in which approximately 37% of the length of the core is at said one axial side of the mid-point of the coil.

3. Apparatus according to claim 1, wherein the said coating on the core comprises a refractory wash.

4. Apparatus according to claim 1, wherein the said coating on the core comprises a rammable refractory material.

5. Apparatus according to claim 1, further comprising an open ended duct having at least a part surrounding the core at said one side of the coil and defining a passge of annular cross-section with the encased or coated part of the core, the end of the duct nearer the coil being open for the inflow of molten metal over its full circumferential extent.

6. Apparatus according to claim 5, wherein the core is disposed vertically and wherein there is provided a ramp locatable in the furnace below the core and having therein an inclined trough aligned beneath the core.

7. Apparatus as claimed in claim 6, wherein the core, coil and shield are movable for adjustment horizontally and vertically with respect to the ramp unit.

8. Apparatus according to claim 5, wherein the core is disposed vertically and wherein the duct has upper and lower parts whereof the lower part extends at an angle to the upper part.

9. Apparatus according to claim 8 further comprising a partition extending between the lower part of the duct and the shield and serving as a baffle to prevent immediate recirculation of molten metal from the outlet back to the inlet of the duct.

10. Apparatus according to claim 1 wherein the refractory casing protecting said part of the core is about 38 cm. in length, the core length being 105-140 cm.

11. Apparatus according to claim 10, wherein the refractory casing comprises a central nose piece about the projecting part of the core and a base to be received in a side wall of the furnace.