Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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APPARATUS AND METHOD FOR THE SECONDARY REFINEMENT OF
METALS
TECHNICAL FIELD
This invention relates to an apparatus and a method for the secondary
refinement of metals, particularly steel.
BACKGROUND ART
In the case of the steel industry, ladle furnaces are used extensively for
controlling the temperature of the steel coming from a steel furnace and for
refining it prior to casting through continuous casting machines. In such a
refining process, a steel casting ladle is positioned under a hood, and
electric
arc heating applied to the upper surface of the steel in the ladle while argon
gas
is bubbled through the steel to distribute the heat throughout the contents of
the
ladle.
By the introduction of fluxing agents such as calcined lime, silica, alumina
etc to
the steel, a layer of slag is formed which floats on top of the steel.
The aforesaid bubbling of argon gas is usually effected by introducing such
gas through one or more porous elements located on the bottom of the ladle.
Such argon gas bubbling is indispensable for ensuring good mass and heat
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transfer conditions between the steel and the slag. In~ some cases
electromagnetic stirring may be used for obtaining additional benefits.
Steel ladles are vertically disposed cylindrical vessels, of which the depth
to
diameter ratio is normally larger than 1, and smaller than 2. It will be
appreciated that one of the main reasons for having such vessels so vertically
disposed, is to maintain the surface area of the bath as small as possible and
hence retain the energy of the metal bath inside it.
The positioning of the aforesaid porous elements in such a vessel is also of
importance in order to obtain the best stirring effects. It is also known that
the
employment of vessels with smaller depth to diameter ratios, gives rise to
poorer mixing conditions, and that in shallower vessels, such as basic oxygen
vessels, several of such porous elements are required to obtain effective
mixing.
A further disadvantage found with such a vertically disposed vessel is that
the
surface to volume ratio of the metal bath in the vessel is relatively small.
Furthermore, since sufficient freeboard must be maintained in the ladle in
order
to minimise spillage during stirring, it is difficult to remove from such
ladle slag
that has been saturated by impurities. If this is for example done by tilting
the
ladle, the metal-slag surface area is increased, thereby reducing the
thickness
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of the slag Payer. Furthermore, where paddles are used for scraping the slag
towards the lip of the ladle, the substantially circular configuration of the
surface
of the slag layer puts a restriction on the width of such a paddle in the
sense
that a paddle which is wide enough to extend over the whole width of the slag
layer near such lip, is too narrow in a position further away from such Pip,
thus
allowing slag to escape around its ends.
OBJECT OF THE INVENTION
It is an object of this invention to provide an apparatus and method for the
secondary refinement of a metal, particularly steel, with which the aforesaid
problems may be overcome or at least minimised.
DISCLOSURE OF THE INVENTION
According to the invention apparatus for use in the secondary refinement of a
metal, particularly steel, includes an elongated vessel of which the length to
width ratio is greater than two, the vessel being provided with at least one
channel type inductor adapted for heating the vessel, and the vessel being
adapted in operation to be located with its long side substantially
horizontally
disposed.
It will be appreciated that while so horizontally disposed, the aspect ratio
of a
metal/slag bath inside the vessel, i.e. the ratio of the length of its surtace
to the
width of its surtace, will be larger than two. It will further be appreciated
that the
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surface area of such a metallslag bath will be much larger than what the case
is
with the conventional vertically disposed vessel so that a much larger contact
area is available for reaction with chemicals added to the 'bath for the
secondary refining of the metal and/or for the removal of sulphur from such
bath andlor for the interaction between the metal bath and its overlying slag
layer.
Further according to the invention the at least one inductor is disposed
substan-
tially along the centre of the bottom side of the vessel when it is so
horizontally
disposed.
Applicant has found that the fact that the bath in such a vessel is heated
from
below, coupled to the fact that a relatively low metallo static head is
present,
together with the resulting convection streams, ensure that a sufficiently
strong
stirring action is created in the vicinity of the inductors) to cause
effective
slaglmetal interaction and homogeneous distribution of the refined metal and
impurity saturated slag.
Furthermore, the fluxes and alloys which are required in the refining process
can be added in an at feast semi-continuous stream to the surface of the metal
bath directly above the inductor(s), i.e. where the most active stirring
motion is
achieved.
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It will be appreciated that any such fluxes etc which are required in the
metal
bath can also in conventional manner be introduced to the bath through one or
more lances passing from above into the bath.
Furthermore, if required, additional stirring of the metal bath may be
effected by
introducing argon andlor nitrogen gas into the bath by means of one or more
lances or porous element stirring devices located in the vessel.
Also, any additional heating which may be required, may be effected in
conventional manner by means of one or more plasma torches andlor three
phase electric arc electrodes.
Further according to the invention the vessel is of round cylindrical
configuration in cross section.
Still further according to the invention the vessel is provided towards at
least
one of its shorter ends with an opening through which a paddle or the like can
be inserted for scraping the spent slag layer from the metal bath in the
vessel.
It will be appreciated that because the slag surface is of substantially
rectangular configuration in plan, a paddle can be used which substantially
spans the whole width of the slag layer over substantially the whole length of
the slag layer.
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By pushing or pulling such a paddle along the slag layer, the spent slag can
be
removed from the top of the metal bath in a very effective manner.
To facilitate such removal of the slag, at least one of the said shorter ends
of
the vessel may be provided with a recessed outlet.
Preferably the width of the paddle is slightly less than the width of the
vessel,
the narrow gaps so left between the ends of the paddle and the walls of the
vessel ensuring free movement of the paddle in the vessel.
Preferably, air or nitrogen may be blown into such gaps to prevent the slag
from
escaping through the gaps when the paddle is moved along the slag layer.
The invention also includes within its scope a method for the secondary
refining
of a metal using the apparatus disclosed above.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of apparatus according to the invention will now be described
by way of example with reference to the enclosed drawings in which:
Figure 1 is a diagrammatic side view of apparatus according to the
invention;
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Figure 2 is a diagrammatic cross sectional view of the apparatus of
figure 2, as seen on line I : I in figure 1;
Figure 3 is a diagrammatic plan view of the apparatus of figure 1 as
seen in the direction an-ow A in figure 1.
BEST MODE OF CARRYING OUT THE INVENTION
In a preferred embodiment of the invention apparatus for carrying out the
secondary refinement of a metal, such as steel for example, comprises a
horizontally disposed refractory lined cylindrical vessel 10 of which the
length to
diameter ratio is greater than 2. In one embodiment of the invention the
length
of vessel 10 may, for example, be in the order of 8m and its diameter in the
order of 2m, the intention being that it will be filled with liquid metal 11
to a
depth in the order of 1 m.
It will be appreciated that although vessel 10 is shown in the drawings as
being
of circular configuration in cross section, it can be of any other suitable
configuration in cross section such as, for example, substantially square,
triangular, etc.
Vessel 10 is provided along its upper wall with an inlet 12 through which
fluxes
or the like may be charged to it. Vessel 10 is also provided along the centre
of
its bottom wall with at least one channel type inductor 13 of 3MW capacity.
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The upper wall of vessel 10 is also provided along each of its opposite ends
with an opening 14 through which an elongated slag paddle 15 may be inserted
which is operated by means of a handle 16, the width of paddle 15 being
slightly less than the diameter of vessel 10.
Openings 14 are provided with lids (not shown).
Vessel 10 is also provided with a lance 17 which passes from above into metal
bath 11 and through which argon andlor nitrogen may be passed into bath 11
for stirring purposes.
If required, vessel 10 may also be provided with one or more porous stirring
elements which can be fed with argon andlor nitrogen in order to assist in the
stirring of bath 11. Oniy one of these is shown in dotted lines at 18 in
t:lgure 1
of the drawings.
In operation the molten metal to be refined is charged to vessel 10 through
one
of the openings 14 to form a bath with a depth in the order of 1m, and the
required fluxes introduced through inlet 12.
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Inductors) 13, when switched on, not only serves) to heat up bath 11, but
through convection and electromagnetic forces also causes) the stirring of
bath
11.
If required, the aforesaid stirring may be supplemented by passing argon
andlor nitrogen through lance 17 (or elements) 18) into bath 11.
As a result of the fluxes melting and reacting with each other, a slag layer
19 is
formed on top of bath 11 which then reacts with the molten metal in bath 11.
It will be appreciated that because of the horizontal disposition of vessel
10, the
surface of the slaglmetal bath has the aforesaid advantageous aspect ratio
and, because bath 11 is mainly heated from below, all the advantages referred
to earlier relating to the efficiency of the chemical refining of the metal
and
stirring of the bath can be achieved with apparatus according to the
invention.
When the spent slag layer 19 has to be removed from vessel 10, paddle 15 is
inserted through one of the openings 14, and the slag in layer 19 raked or
pushed out through recessed outlets (not shown) provided in the said outer
ends of vessel 10.
The width of paddle 15 is slightly less than the cross sectional diameter of
vessel 10 so that when paddle 15 engages slag layer 19, narrow gaps 20 are
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left between the outer ends of paddle 15 and the inside wall of vessel 10 to
allow paddle 15 relatively free to-and-fro movement in vessel 10.
By blowing air or nitrogen into gaps 20, slag from layer 19 is prevented from
escaping through such gaps 20 during such movement of paddle 15.
It will be appreciated that with such an arrangement the disadvantages
referred
to earlier when such a slag layer has to be removed from the vessel, are
overcome or at least minimised.
It will be appreciated further that there are no doubt many variations in
detail
possible with an apparatus and method for the secondary refinement of metals
according to the invention without departing from the spirit andlor scope of
the
claims.
