Note: Descriptions are shown in the official language in which they were submitted.
_ CA 02366193 2001-10-10
Method and Device for Tapping Molten Metal from
Metallurgical Vessels
The invention relates to a method and a device for tapping molten
metal, preferably molten steel, from metallurgical melting vessels,
such as, for example, electric arc furnaces, via a tapping hole
arranged in the melting vessel bottom wherein the molten metal is
covered by molten slag at the time of tapping.
When carrying out thermal metallurgical processes on metals or
their alloys in a metallurgical melting vessel, upon completion of
these processes the metals are present in molten liquid form,
covered by molten slag. In order to separate the molten metal from
the molten slag, in known metallurgical melting vessels a tapping
hole is arranged, if possible, at a corner part of the melting
vessel bottom via which the molten metal is removed downwardly into
a smelting crucible.
When the molten bath level drops, a swirl (vortex) is created at
the tapping hole which extends at a slant to the melting vessel
wall. When the molten bath level drops farther, a hollow vortex
results in the end which entrains and swirls also parts of the
molten slag floating on the molten metal so that the originally
present separation between molten metal and molten slag is no
longer present and the molten slag will be removed together with
the molten metal through the tapping hole in the downward
direction.
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The oxidic slag transported in this way together with the molten
metal into the smelting crucible entrains oxygen and leads, for
example, to excess consumption of aluminum for the required
deoxidation, of synthetic slag for the uptake of the oxides and
calcium for the modification of the oxidic inclusions. The
oxidation product alumina (A1203) worsens the casting properties,
and the oxygen from Fe0 in the slag makes desulfurization and
degassing more complicated.
On the other hand, for example, the clean steel treatment of a
molten steel in the secondary metallurgy is considerably favored
for a reduced slag contents in the molten metal, which plays an
important role, in particular, for the production of "ultra-low
carbon" steels for flat products.
In order to reduce the described entrainment of the slag during
tapping of the molten metal, different methods and devices have
become known.
It is known from DE 33 27 671 C2 to lower a cone-shaped flow body
(with the cone tip facing downwardly) from above in the downward
direction by means of a lifting device to a location closely above
the tapping hole . With this measure the vortex now flows about the
shaped body and is thus bound in such a way that swirling of the
slag no longer takes place. This known process represents also a
relatively expensive and complex method because the shaped body is _
subject to wear in the molten bath and, therefore, must be
exchanged frequently.
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In DE 298 08 318 U1 it is finally suggested to arrange around the
tapping hole in the melting vessel bottom gas-permeable, truncated
cone-shaped porous plugs via which, from below, a gas - counter to
the flow direction of the molten metal - is blown into the molten
metal. With this measure the formation of a vortex above the
tapping hole is to be prevented.
In U.S. 5,203,909 it is suggested to blow a gas jet, by means of a
blowing lance, in the area of the tapping hole from above onto the
molten slag by which the molten slag is pushed away from the metal
surface. A disadvantage of this known method is the forcing of the
molten slag into the molten metal, resulting, in particular, when
a vortex is formed in the molten metal, in the risk of entrainment
of slag.
Finally, EP-A-0 321 861 describes a tiltable metallurgical vessel
in which above the tapping hole at the inner side of the sidewall
at least one blowing device, preferably a burner, is provided by
which the molten slag can be blown away from the surface of the
molten metal. In addition it is provided to supply gas bubbles
below the bath surface into the molten metal by means of a known
porous plug device, arranged laterally in the melting vessel wall
directly above the tapping hole, in order to also displace, inter
alia, the molten slag from the surface of the molten metal. A
disadvantage of this known method is that the rising gas bubbles
cause undesirable turbulences in the molten metal which favor slag
entrainment.
In addition to the described entrainment of the molten slag as a
result of its swirling with the molten metal there is also the
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possibility of a direct contact of the molten slag with the tapping
hole. This trailing slag flow will result when during tilting
back, in the case of a tiltable metallurgical melting vessel, for
example, in the case of the electric arc furnace, despite a high
tilting speed, the molten slag flows back quickly across and past
the molten metal.
It is therefore an object of the invention to provide a tapping
system which reduces with operational safety the slag entrainment
and also the trailing slag flow with simple means without greater
apparatus expenditure and without great operating costs.
This object is salved for metallurgical melting vessels with a
tapping hole arranged in the melting vessel bottom with the
characterizing features of claim 1.
With the feature according to the invention of arranging in
immediate vicinity of the tapping hole a nozzle through which a
neutral gas or a corresponding gas mixture can be blown into the
metallurgical melting vessel in the form of a jet of high impulse
energy, the molten slag is removed (blown away) from the molten
metal surface in the area of the tapping hole. This ensures that,
despite the formation of a vortex in the molten metal, slag
entrainment can no longer take place and, moreover, a trailing slag
flow is successfully prevented by this method with a
correspondingly energy-rich gas jet.
The strength and the blow rate of the gas jet are controlled by a
valve station which is connected with a compressed gas container or
a compressed gas generating plant.
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In order to prevent that undesirable components such as, for
example, oxygen, can reach the molten metal together with the gas
during the step of blowing gas in, a gas or gas mixture is used
that behaves in a neutral way with respect to the further use and
processing of the molten metal, such as, for example, a noble gas.
According to an advantageous configuration of the invention, the
valve station is connected with a measuring and control system by
which the starting point, the duration, and the intensity of the
step of blowing gas in is automatically monitored and controlled.
As parameters for this automatic control
- the height of the molten bath level in the metallurgical
melting vessel,
- the tilting angle and the tilting speed of the metallurgical
melting vessel,
- the tapping weight of the molten metal in the smelting
crucible
can be used by means of corresponding measuring devices. In this
way, an optimal blowing-in of gas can be adjusted with high
operational safety to the respective process situation during the
tapping process.
Depending on the location of the tapping hole one or more nozzles
are arranged in the area of the tapping hole such that reliably a
complete blowing away of the slag layer from the metal surface is
achieved.
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The size, shape, number, and arrangement (in the lateral melting
vessel wall and/or in the melting vessel cover, angle to the
melting vessel bottom) of the nozzles are matched to the size and
configuration of the metallurgical melting vessel as well as to the
size of the bath surface, wherein the outlet openings of the
nozzles can be arranged above and/or below the bath level.
Further advantages, details, and features of the invention will be
explained in more detail in the following by means of embodiments
shown in the schematic drawings.
It is shown in:
Fig. 1 a vertical section of the metallurgical melting vessel,
Fig. 2 a detail of the metallurgical melting vessel in vertical
section,
Fig. 3 a block diagram for the device according to the
invention.
In Fig. 1 a metallurgical melting vessel 5 at the point in time of
tapping is illustrated. This embodiment shows a conventional
electric arc furnace wherein the electrodes are not illustrated.
In the left corner a tapping hole 10 is arranged in the melting
vessel bottom 13 through which the molten metal 2 flows with a
pouring flow 6 into a Smelting crucible 11 arranged underneath the
metallurgical melting vessel 5. The smelting crucible 11 is
arranged on a crucible carriage 8 and the weighing cells 7 arranged
thereon by which the amount of metal flowing into the smelting
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crucible 11 can be measured continuously. Above the molten metal
2 molten slag 1 floats into which the vortex 4 generated by the
tapping process extends almost down to the bath level 15.
In immediate vicinity of the tapping hole 10 in the lateral melting
vessel wall 12 two nozzles 3 are arranged whose nozzle outlet
openings 14 are oriented from above and from below against the slag
layer 1. The gas supply lines 25 (Fig. 3) through which the
nozzles 3 are loaded with the gas to be blown in and also the valve
station 16 (Fig. 3) as well as the measuring and control system 20
(Fig. 3) are not illustrated in Fig. 1.
Fig. 2 shows in an enlarged detail the tapping portion of the
melting vessel 5 which is in a tilted position. In this embodiment
a nozzle 3 is arranged almost parallel to the melting vessel bottom
13 and slightly upwardly slanted in the lateral melting vessel wall
12. As illustrated schematically in Fig. 2, the molten slag 1
above the molten metal 2 is forced back and away from the tapping
hole 10 by the gas jet 9 to such an extent that the molten slag
cannot come into contact with the vortex 4 and slag entrainment or
trailing slag flow through the tapping hole 10 cannot take place.
In Fig. 3 it is illustrated in a block diagram in which way the
melting vessel 5 is functionally connected with the valve station
16 and the measuring and control system 20. The measuring pulses
which are generated by the weighing cells 7 as a result of the
pouring flow 6 flowing into the smelting crucible 11 and the
measuring pulses (tilting position, molten bath level) measured at
the melting vessel 5 are supplied via the measuring lines 19, 22
into the measuring and control system 20. From this measuring and
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control system 20, control pulses required for controlling the
compressed gas are then sent via the control line 21 to the valve
station 16. The compressed gas, available at the valve station 16
by means of a compressed gas container 18 and/or a compressed gas
generating plant 17 via the supply lines 23, 24, is then - via the
valve station 16 controlled by means of the measuring and control
system 20 - blown into the metallurgical melting vessel 5 through
the gas supply line 25.
The invention is not limited to the metallurgical melting vessels
(electric arc furnace/EAF) illustrated in the Figures but also
useable for other metallurgical melting vessels in which the
tapping hole is arranged at the bottom of the melting vessel and
where there is a risk of slag entrainment or a trailing slag flow
through the tapping hole during tapping.
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