Note: Descriptions are shown in the official language in which they were submitted.
--\
1082515
The invention reLates to the melting, refining and casting of
molten metals and alloys and, more particuLarLy, to an improved method
and apparatus for the continuous withdrawal and discharge of molten metal
from a rotary furnace through its axiaL end opening.
Rotary furnaces used for meLting metals on a batch basis are
normaLly tapped through a tap hoLe Located in the waLI of the furnace. The
furnace rotation is stopped, the tap hole opened by a ram or Lance, and
the metaL tapped out quickLy into a ladle. For continuous operation,
discharge can be accompLished by overfLowing over the Lip of an axiaL
discharge opening, but the generaL discharge rate is subject to changes of
the charge repLenishment rate and is aLso subject to surges from other
sources. An eLaborate furnace tiLt-incLine mechanism can be empLoyed
to secure a degree of controL, but is cumbersome in use. Further, slag
and metaL normaLLy discharge simuLtaneousLy during Lip discharging and
special means are necessary for slag-metal separation, either at the
discharge Lip or foLLowing discharge.
: .1
Siphoning of metaLs into an externaL withdrawaL chamber by
, appLication of controLLed suction is known practice, for exampLe, the weLL
; known cLosed LadLe siphon for withdrawaL of moLten aLuminum from
20 aLuminum reduction pots. These techniques provide for filling up
individuaL LadLes, moLds, or other batch vesseLs, and are not weLL suited
to continuous operation.
. -- .
It is therefore a principaL object of the present invention to
,~ accomplish the continuous withdrawal of molten metals at a controLLed
,,J
,; 25 rate from within rotary furnaces through axiaL discharge openings
essentially independentLy of the repLenishment rate from charging or other
causes of smaLL metaL level changes within the furnace.
-2-
1082~1S
It is another object of the present invention to withdraw molten
metal from a furnace separately from the slag, thus providing the
opportunity for convenient discharge of sLag by separate means.
It is another object of the present invention to limit reoxidation
5 of molten metal by exposure to the externaL atmosphere during withdrawal,
transfer and discharge from a process vessel.
StiLL another object of this invention is to provide a method of
metaL withdrawal from a furnace on a continuous basis particularly
suitable for a continuous casting directly folLowing the withdrawal operation.
; 10 A further object of the present invention is to provide a method
which removes dissolved gases from the molten metal.
r~ It is a still further object of the invention tomaintain a constant
1 and controlled flow rate of discharged metal irrespective of surges or
;` periodic interruptions in the furnace charging rate to enable close control
15 of final metal composition by continuous aLLoy addition at a controLLed rate
varied according to flow rate of withdrawaL of the moLten metal.
The invention comprises, in a method for continuous melting
~1 and refining in a rotary furnace in which a charge of liquid metal is
,~ confined within the furnace by retaining it behind an annular dam restriction
. .
20 of the discharge opening, the combination of the foLLowing steps:
, (a) maintaining an externaL metaL withdrawal vesseL adjacent to the discharge
' end of the rotary furnace incorporating an encLosed chamber as part of the
vesseL;
` (b) maintaining a negative suction pressure within the enclosed chamber;
-3--
'" 1082915
(c) maintaining a discharge siphon tube inserted down into the metaL
within the rotary furnace and connecting into the enclosed chamber;
(d) alLowing the molten metal to fLow from the metaL bath within the
furnace through the siphon tube into the encLosed chamber at a rate
5 controlled by the magnitude of the negative suction pressure; and
(e) allowing the metal to flow out and discharge concurrently from a
submerged opening in the enclosed chamber under the influence of gravity,
and at an average rate corresponding to the rate of withdrawal from the
furnace through the siphon tube.
The invention provides for continual withdrawal at approximately
the overaLL average charge rate by maintaining a controlled and substantially
constant negative suction pressure within the encLosed chamber, enabling
the continuous subsequent addition of alloys and deoxidizers at a similarly
constant and controlled rate to maintain a corresponding close control
of the final composition of the cast product. The invention thus overcomes
; control problems stemming from intermittent and/or periodic furnace
charging, as from a charging box inserted into the furnace and dumped at
spaced time intervaLs, by eliminating the resuLting surges and providing
a measured fLow for control of the quantity of aLloy additions.
. .
; 20 FoLlowing from this, the invention is also particularly suited to
the situation where charge material is continually introduced into the charge
end of the rotary furnace and the negative suction pressure is regulated
to effect metal withdrawal at an average rate corresponding to the rate of
suppLying charge material, thus maintaining a substantially constant
average level of metal within the furnace. The method provides the
convenient facility for periodically varying the suction pressure and hence
the average metal level above and below this average level, thus effecting
--4--
108291S
the periodic discharge of slag over the annuLar dam restriction of the
discharge opening. This practice can greatly facilitate sLag removal,
particularLy if the slag is viscous or of inconsistent consistency.
The invention may employ either of two different methods of
discharging from the enclosed withdrawal chamber. In the first method,
the withdrawal vessel incorporates an open launder communicating with
the enclosed chamber via the submerged discharge opening; and the metal
flows into the launder and is aLlowed to discharge by overflowing a weir-type
side discharge lip at a rate corresponding to the flow rate into the Launder
through the submerged opening. In the second method, the enclosed chamber
contains a bottom nozzle and the metal is discharged by gravity directly
out through this nozzle opening. This nozzle is beneficial for emptying the
vessel when pouring i5 completed and is also normally equipped with a slide
, .
~, -gate device for opening and closing, which facilitates the evacuation of the
15 vessel to initiate the flow at start-up.
v -
. Various other objects, features and adva~ltages of the method
. and apparatus of this invention will become apparent from the foLlowing
., detaiLed description and claims, and by referring to the accompanying
drawings. Figure 1 is an elevation view, diagramatic partLy in section, of
t
20 the process and apparatus in operation, and Figure 2 is another embodiment
of the withdrawal vessel illustrating an alternative method if discharge.
;,
,, .
;.,
, Referring to the drawings, the rotary furnace comprises a body
consisting of a cylindrical steel shelL 1, lined with refractory material 2,
and is rotated on trunnions mounted on an appropriate foundation (not shown).
25 The furnace is fired with gas or oil by a charge end burner 3 supplying heat
for melting, and a discharge end burner 4 reguLating the temperature prior
--5--
1082915
to discharge. The furnace charge 5 is fed into the annular charge opening
via charge chute 6, entering directLy ~a partially molten metal bath in
the embodiment iLLustrated. Waste gases are colLected within charge hood
7 and passed through appropriate ducting, with heat exchangers and dust
colLectors as required. The metal bath 8 extends aLong the length of the
furnace and is normaLLy covered by a Layer of sLag 9 which can be aLLowed
to overfLow through the annuLar discharge opening. The annuLar dam 11
restricts the transfer of metaL between the meLting zone near the charge
end and the refining zone near the discharge end and therefore assists in
stabilizing the metal composition within the refining zone in preparation
for discharge.
The metal withdrawal vesseL 12 consists of an outer steel
sheLL Lined with refractory materiaL and incLuding an encLosed chamber 13.
A siphon tube 14 is arranged to extend LateralLy into the furnace and is bent
downward at the inlet end to penetrate through the slag with the inlet end
; kept immersed in the metaL during operation. The siphon tube extends
LateralLy from the furnace at a sLight incLine to faciLitate drainage on
compLetion of an operating campaign and is connected at its discharge end
into the upper part of the encLosed chamber. It is supported by an
- 20 appropriate flange support 16 aLlowing convenient connection and removal,
the joint effecting a sealed connection. The siphon tube is generally made
of high quality refractory-ceramic materiaL such as fused alumina silicate,
and the molten metaL fLows by suction through a lc,ngitudinaL centraL opening
to discharge into the encLosed chamber 13. The sLag is then discharged from
J n ~ ~
~25 the furnace separateLy, normalLy by overflowing over the Lip of ~dam
10.
.
:'
1'~)82915
The negative suction pressure, as required for metal with-
drawal, i8 maintained by a vacuum pump and evacuation system. The
manifoLd 17is traced with a cooLing coil 18 and connects the top of the
enclosed chamber 13 to a receiver tank 19 (condenser~ which settles out
5 dust and dirt as welL as the greater part of the moisture in the gases
removed from the metal. This tank is evacuated by the vacuum pump
20 via a valved manifold. The pressure in the system is regulated at a
controlled setting by the vacuum controller 21 connected into the receiver
tank, which is also equipped with an air bleed valve 22.
In the metaL withdrawaL vessel 12, Figure 1, the enclosed
chamber 13 connects through its bottom outlet opening 23 with an open
launder 24 which permits discharging of the molten metaL by overflowing
the weir-type discharge Lip 25. In operation, the metaL pressure head
represented by the difference in metal level 26 within the chamber and
level 27 in the launder, corresponds to the controlled negative suction
pres~ure. A~ thi~ difference is increased, the rate of metal withdrawal
through the siphon tube 14 also increases, according to the principles of
', Bernoulli's theorem. By maintaining a constant suction pressure, a
continuous flow of metal is maintained through the siphon tube into the
enclosed chamber, then the launder, finally discharging over the lip.
, "
In initial vacuum is required to start the flow, and therefore the
opening2~must be temporarily sealed off with a refractory plug until a
" ,f!,, !.
pressure head of metal equal to the vacuum is secured within chamber 13,
at which time the opening is cleared and continuous flow begins. Another
Q~C
method ~Ss starting flow is to fill the vessel with starting metaL by pouring
in via the open launder top, prior to creating the withdrawal vacuum.
1082~5
Regarding emptying at the end of a campaign, the withdrawal
vessel wiLL remain full of metal when ceasing withdrawal, and must be
emptied by tilting. The vessel may aLso incorporate a bottom nozzle and
slide gate for emptying as with the apparatus of Figure 2, but normally
discharging via the launder, with the bottom nozzle being used onLy for
emptying the vessel.
There are various forms of equipment and techniques which
may be empLoyed for supporting the withdrawal vessel, and for inserting,
positioning and withdrawing the siphon tube, which are not illustrated in
detail. The unit shown is mounted on a track-mounted carriage 28 riding
on wheeLs 29. The withdrawaL vesseL itself rests on pivot trunnions on
either side of the carriage as part of a cradle, with the pivot points at the
approximate centre of gravity of the withdrawal vesseL. The cradle (not
shown) aLso includes a locking-support device to fasten the vessel in the
fixed operating position shown. This arrangement is simple and efficient
when used in conjunction with an overhead crane and multiple withdrawal
vessel units.
Figure 2 shows an alternative withdrawal vessel unitj and method
of discharge whereby the metal is discharged by way of a nozzle 30 in the
` 20 bottom of the enclosed chamber rather than into an overflow launder. By
employing a slide-gate apparatus 31 for opening and cLosing the nozzle, as
now cornmonly used in ladles and continuous casting tundishes, this arrange-
ment has the advantage of convenient closure and seaLing to initiate flow on
starting vacuum, and also provides for compLete emptying on shut-down while
the withdrawaL unit remains in the operating position. The reLationship
between flow rate, nozzle size, siphon tube size and vessel height is governed
according to the principles of Bernoulli's theorem. When initiating the pour,
--8--
'
^` 108Z915
the metal pressure head within the vessel must be allowed to increase to
correspond approximately to the vacuum applied before opening the nozzle,
after which continuous and balanced flow can be maintained. Selection of
the diameter of the siphon tube and nozzle openings are governed by the
` 5 desired flow rate and suction pressure employed.
Final alloying and deoxidation of metal may be accomplished in
the launder 24 by lance injection or continuous feeding of sized material, or
the metal may be discharged into a hoLding furnace or heated tundish 32
where this is accomplished by feeding directly into this vessel via an
10 appropriate chute 33. The rate of feeding alloys is governed by the rate of
metal withdrawal, in fact, the control may be interlocked to vary with the
amount of negative suction pressure within the enclosed chamber.
It will be appreciated that a preferred embodiment of the method
and apparatus for metal withdrawal and discharge of a rotary furnace has
15 been described and illustrated and that variations and modifications may be
made by persons skilled in the art, without departing from the spirit and
scope of the invention defined in the appended claims.
:'
.:
_9
