Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method for f~eding casting
additives such as casting powdcr to the surface of or into a bath of
molten metal such as it exists in molds.
It is common practice in casting engineering to provide
additives to the surface of the molten metal in the mold. Conventionally,
operating personnel manually performed the feeding in one form or another.
Alternatively, pneumatic devices are used, being possibly of a variety
which is combined with other types of mechanical and conveying devices.
Also, a high degree of automation has been introduced in this field. Manual
feeding is rather primitive and not very accurate. Moreover, participating
personnel are bothered to a considerable extent by the dust as it develops
during the application of the casting powder. Another drawback is the
lack of uniformity in the application which results in rather poor surface
texture of the casting, particularly in the case of continuous casting.
The German printed patent applications Nos. 2,653,306 and
2,651,266 describe pneumatic and pneumatic-mechanical feeders for casting
powder. These devices are of a rather complicated construction and produce
a considerable amount of flying dust. The dust development can be reducedp
for example, in a device as shown in the German application No. 2,653,306 by
placing a container or the like as a shield between the pneumatic and the
mechanical sections. Such a shield, however, does not completely eliminate
the development of flying dust. It should be noted further that pneumatic
transport of casting powder inherently involves the production of flying
dust and, as an unfortunate side effect, the components and constituents of
the powder begin to physically separate so that the powder is in a rather
non-homogeneous state. Another factor to be considered is the required
separation of the conveyor gas from the casting powder. The separation
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equipment is quite extensive, and the entire application and feeding
equipment experiences a considerable thermal load and temperature dif-
ferential which enhances wear. Still another drawback of the known auto-
mated methods and devices is that ~he casting powder is applied in a point-
like manner via jets emerging from nozzles. This, again, is a factor
which contributes to the lack of uniformity in the application resulting
in non-uniform slag layers on top of the molten metal bath.
It is an object of the present invention to pro~ide a method
which permits the controlled application of casting additi~es onto the
surface of or into a bath of molten me~al and in a manner which ensures
a substantially uniform distribution across the surface.
According to the present invention, there is provided
method of feeding casting powder to molten metal in a mold, comprising
the steps of providing a supply of powder; causing a mass of the powder
to be disposed in a fluidized state by means of a pressurized gas; and
causing and permitting the fluidized powder mass to flow into the mold,
the gas pressure being insufficient to form a powder dust cloud.
Thus, in accordance with the present invention, the casting
powder is fluidized, i.e., a fluidized bed of casting powder is established
under conditions which permit the fluidized powder to flow out of the bed
and ontoJ or into, the bath of molten metal to which such powder is to be
applied. The fluidized state is established by providing a bed of cas~ing
powder which is then subjected to a flow of gas in such a manner that the
powder becomes fluidized and flows out of the bed. Application of the
fluidizing agent is limited to such an extent that, with certainty, a
powder cloud is not produced. It is found to be of particular advantage
to meter the application of fluidized casting powder to the bath of molten
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metal by causing fluidization of a bed of powder in an intermittent fashion
preferably on Q periodic basis.
The primary function of the application of a gas is to convert a
heaped quantity of po~der into a fluid, of course, without melting it. An
inert gas may be used in order to avoid any unwanted side reactions because
the casting powder is usually a blend of different components. In such
cases, however, the casting powder may be subjected to a specific chemical
reac*ion just prior to its application and; it is therefore, of advantage
to employ a fluidization gas which will, in fact, produce and/or enhance
such a chemical reaction.
The method in accordance with the present invention avoids the
disadvantages and the drawbacks of known casting powder applications outlined
above. It is emphasized that the state in which the powder is applied differs
on one hand from a mere powder heap, i.e.S a firm bed of powder, and on the
other hand from a flying dust cloud uhich is the result of pneumatic con-
veyance. The decisive aspect is that gas is applied a~ such a rate that the
powder behaves like a liquid. Hence, the application of fluidizing gas must
be limited to such an extent that a cloud of dust is not produced.
While the specification concludes with claims particularly point-
ing out and distinctly claiming the subject matter uhich is regarded as theinvention, it is believed that the invention, and its objects and features
will be better understood from the follouing description of at present pre-
ferred embodiments and described in connection with the accompanying drawings
in which:
Figure 1 shows several gTaphical representations for explaining
the method and function of the present invention;
Figure 2 is a perspective vieu of a casting powder applicator by
means of which the inventive method can be practiced
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Figure 3 is a top view of a portion of a machine for continuous
casting in which two applicators of the type shown in Figure 2 are attached
to a tundish; and
Figure 4 is a side view of the device shown in Pigure 3.
Proceeding now to the detailed description of the drawings,
we turn first to the diagrams of Figure 1. The diagram of Figure lA shows
particularly the pressure loss ~P of a gas over the height H of a bed of
fluidizable powder, plotted against the velocity Vg of such a gas. For rea-
sons of scaling, the diagram is drawn on a log-log scale. The diagram
reveals three ranges denoted respectively F, FL, and FLW.
The range F represents a range of low gas speeds in which the
powder bed remains unfluidized. In other words, the height of the bed is
so large or, for reasons of the powder consistency, the pressure drop is so
low that for a given velocity, the flowing gas cannot lift the powder against
gravity. The log-log relationship is a proportional one, the direct function
of the relation being that of a parabola accordingly.
Upon reaching a critical velocity VgU, fluldization begins (range
FL). This phenomenon is represented by the fact that the ratio ~P over H
remains almost constant with increasing gas speed. Specificially, as th0
speed of flow of the gas through the powder increases, and the bed becomes
fluidized, the height of the bed increases as the particles ar0 being lifted
by the upflow of gas; but that increase is proportional to the increase in
pressure drop so that the ratio P/H remains, ind~ed, constant.
This state o fluidization i5 maintained until a second critical
velocity Vg bas been reached. Figure 1 reveals that in the range FLW,
established by gas velocities higher than Vg , the characteristic merges into
a parabola being representative of the fact that the relationship between
gas velocity and pressure drop now follows the Bernoulli equation, Moreover,
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the gas flow at these high speeds converts the powder into a flying dust cloud.
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-g~~ Figure lB shows a curve-P~ illustrating the corresponding distribu-
tion of density in relation to gas speed. The two diagrams of Figures lA and
lB are drawn in ali~nmentJ with corresponding velocity scale values, to permit
the same identification of the speed ranges ~, FL, and PLW. A bed of un~
fluidized powder being flown through by gas at a velocity below vgU changes
the overall apparent density very little. ~owever, once that lower and criti-
cal velocity has been exceeded, the density drops with increasing speed. This
decrease in density isJ of course, the equivalent to an increase in the height
of the bed.
The figure lB shows also a curve dJ representing the density of
some dust that develops during the 1uidization, above the fluid bed. The
density of the powder drops with speed and the density of the dust increases.
When the two curves P and d merge, the fluid bed is converted into a flying
dust cloud, where density decreases with speed.
The curve in Figure lC is drawn solely for the purpose of illus-
trating an in~eresting thermodynamic analogy. The ordinate shows the density
of a material which remains solid at temperatures below melting point Ts.
The density M of the material declines at higher temperature, whileJ in a
20 closed system, vapor or steam develops uhose density (s) increases with tem-
perature. At the critical temperature Tg J those twv curves merge and only
steam remains whose density continues to drop with temperature above Tg.
In accordance with the method of the present invention, a quan-
tity of powder is to be con~erted into a fluidized bedO Suitable equipment
is used ~or permitting powder to be fluidized and to be applied in the
fluidized state, i.e., to flow towards the top or surface of a bath of molten
metal. Figure 2 illustrates such a device. The device includes a bin 1,
containing a supply o casting pouder and being open at the top to permit the
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contents to be replenished. A duct 3 with a front opening extends laterally
from bin 1, there being a transition zone 13 established between the generally
vertically oriented main portion of the bin, and the duc~ 3 proper. This
assembly is as far as a powder is concerned closed by a gas permeable parti~ion
4, underneath of which is provided a plenum chamber 6 which is closed except
for the permeability of the partition 4J constituting the top of that plenum
chamber, further excepting an inlet 5 for pressurized gas. This particular
applicator is disposed so that the front opening of the duct 3 is positioned
above the open top of a mold cavity 7, being for example the cavity of a mold
for continuous casting.
The bin 1 is filled with powder which will pour into the transi-
tion zone 13 as well as into the duct 3. As pressurized gas is applied to
plenum 6, one establishes basically three beds. The first bed encompasses that
portion of the powder in the bin 1 directly underneath the top opening thereof.
Next to that portion is the transition zone, followed by a zone for a bed of
low height in duct 3.
As pressurized gas is applied to the plenum chamber 6, that gas
penetrates little into the rather high bed in bin 1 and its state as an un-
fluidized powder bed is not altered. Looking at Figure 1, it can be seen that
the gas will have a low speed into the bed underneath the bin opening. The
height is large and the flow resistance substantial, resulting in a rapid pres-
sure drop. In fact, little gas flows into that portion of the powder and
fluidization will not occur. On the other hand, the gas pressure as applied
17~m b er~
~_~ to the plenum 4~xuy~ is chosen so that the height of duct 3 is sufficiently
low for establishing a fluidized bed in duct 3. The transition zone 13, there-
fore, is that portion in which powder is moved from the solid type bed into
the fluidization zone. The parameters, moreover, are chosen so that fluidized
powder rather than a fLying dust cloud emerges from the froDt opening of duct 3.
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The pressure drop responsihle for fluidization is, of course,
measured in the vertical. ~owever, a lateral pressure drop is suparimposed
because the gas will follow the path of least resistance and will flow
upward to a small extent only; the dominant gas flow is into and through
duct 3, out of the opening thereof. As a consequence/a lateral pressure
drop develops, causing the fluidized powder to flow into and through duct
3. Powder will pour out of the bin and into the transition zone to become
fluidized therein.
The height of the bed in duct 3 is given by the height of the
duct and the pressure drop is basically given by the di~ferential between
ambient and plenum pressure. The latter is adjusted so that the resulting
gas speed remains below V .
The gas pressure in plsnum 6 may be regulated in the feeder
path for the gas to, thereby vary the amount of poNder flowing out. The
front opening of duct 3 may be a variable one for the same reason. The
rather wide ranges for speeds causing and maintaining ~luidization permit
the rate of powder application to be controlled through speed and gas pres-
sure control. In some cases, it may be desired to introduce intermittent
control for the gas flow for metering the powder application in an ON OFF
fashion. The temperature of the slag layer on top o the bath in the mold
may be a control parameter. Decisive in all these instances is that the
powder, when flowing out of duct 3, will flow in the fluidized state, well
below the critical speed of dust cloud development.
If the gas used to fluidize the powder is an inert gas, for ex-
ample argon, no reaction will occur between any reactive component o~ the
casting powder and the gas. Due to the reduced density of the fluidized
bed in duct 3 as compared with the density of the powder in bin 1, one has,
in fact, established a condition in which the casting powder is being
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applied under conditions o$ a reduced thermRl conduc~ivity. Frequently,
carbon has been added to a casting powder primarily for purposes of loosen-
ing up the powder as it i5 applied in the conyentional manner and for pur-
poses of enhancing thermal isolation. The fluidization, as envisioned
here, permits a reduction or even an elimination of the carbon con~ent in the
casting powder because the fluidized powder has already a significantly reduc-
ed conductivity.
For similar reasons, one may now use a casting powder which is
either free from gas-releasing constituents such as borax, or has only a
reduced content thereof. The fluidization by means o a gas and particular-
ly by means of argon makes it unnecessary to provide the casting powder
with such an inherent thermal insulation.
Applying the casting powder in the fluidized state ensures a very
uniform coverage of the bath of molten metal with the powder. This, in turn,
improves the surface texture and its overall uniformity and quality as far
as the casting emerging through the bottom of cavity 7 is concerned. The
inventive method, however, permits in addition that a supplemental use can
be made of the fluidizing gas. This is particularly so when a reaction is
needed between a casting add7tive in general and another medium just prior to
application, or if the molten metal is to be protected from the uncontrolled
entrance of oxygen.
For example, the gas to be used for fluidization may be an
oxidizing gas, such as air enriched with oxygen. Oxygen will react with
the casting powder particles, or with particular constituents thereof, such
as carbon. The quantity of gas so applied can be controlled because a
rather wide range of speeds and velocities is available for the fluidization
so that sufficient quantities of the fluidization agent can be provided to
react in a particular manner with the casting po~der, or a portion thereof.
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For e~ample, an organic gas may be used to react with a por~ion of the
casting powder for the purpose of releasing carbon to the casting powder
if such a supplemental carbon is needed for any reason.
Figures 3 and 4 illustrate how two such fluidization devices
and casting powder applicators can be used in conjunction with a mold for
continuous casting; the two applicators are attached to a tundish 8 by
means of holders 2 and to both sides of an outlet pipe 9. Molten metal
such as molten steel is fed through pipe 9 into the interior of the cavity
of mold 7 for the discharge of the molten steel underneath the surface of
the bath. A slag layer on top of the molten bath is replenished by the two
powder applicators, causing powder to flow in liquid-like fashion onto slag
10. The liquid slag, in turn, runs along the mold wall cavity during con-
tinuous casting.
The applicators 1 may be adjustable in their position in that
they can be shifted along the three transverse a~es of a coordinate system,
and they may well be mounted for pivoting in the hozizontal as well as above
a vertical axis.
If the method is practiced in regular mold casting, the desir-
able quantity of casting powder may be provided in a container in which a
fluidized state of the powder is maintained through the application of gas
2~ under pressure. This container is placed into the mold and will be destroy-
ed by the hot metal upon filling so that the fluidized casting powder is
directly applied into the interiOr of such a mold and the additives will be
flushed, or otherwise move up, to the surface of the bath upon filling of the
mold. The period of time between charging such a container and introducing
it into the molten metal should not exceed three minutes.
The invention is not limited to the embodiments described above,
but all changes and modifîcations thereof not constituting depar~ures from
the spirit and scope of the invention are intended to be included.
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