Note: Descriptions are shown in the official language in which they were submitted.
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KOFF 0109 PCA
IIVVIPROVED YIELD METAL POURING SYSTEM
Technical Field
The present invention pertains to a process
for pouring metal. More particularly, the present
invention pertains to a process for pouring metal having
a layer of slag disposed thereon, whereupon a high yield
of slag-free metal may be obtained.
Background Art
In the smelting and refining of metals, the
occurrence of a lower density layer of slag atop the
metal surface is common. In many cases, this floating
slag layer is purposely provided as a sink for impuri-
ties which might otherwise remain in the refined metal,
and to prevent oxidation of molten metal in the presence
of atmospheric gases. However, as important as the
layer of slag may be for achieving its intended purpos-
es, entrainment of slag in metal poured from the furnace
or ladle results in a product which must be downgraded,
reworked or scrapped.
In the basic oxygen process, for example, the
furnace charge consists of scrap steel of varying
amount, generally from 15-30% by weight, but up to 450
by weight with preheating, onto which a layer of molten
pig iron is poured. Ferrosilicon and other ingredients
are added and oxygen injected through a lance. The
combination of oxygen with iron, silicon, and other
ingredients forms a slag which rises to and covers the
surface. The slag comprises nominally about 13 weight
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percent of the furnace contents, or about 28% by volume.
The slag not only serves to retain impurities and
further prevent unwanted oxidation, but also serves to
keep oxygen and other gases in solution, to avoid
effervescence from the melt.
The geometry of basic oxygen and other furnac-
es varies somewhat, but generally consist of a cylinder
with a concave or flat bottom, together termed the
"barrel", surmounted by a "cone" whose diameter tapers
toward the upper end of the furnace mouth.
In pouring steel from the furnace, past
attempts to avoid slag entrainment have included tilting
the furnace on its pivot or trunion and decanting the
lighter slag from the steel. This method has not proven
successful, however, as the hot slag and molten metal
were found to adversely affect the refractory lining
along the mouth of the furnace. Moreover, it is diffi-
cult to remove the slag completely without some molten
steel pouring over and coating the rim. Thus, steel is
now almost universally withdrawn by gravity flow through
a taphole, located generally at the intersection of the
cone and barrel of the furnace.
Pouring the steel through the taphole thus
described has the advantages of avoiding damaae,ro the
rim of the furnace mouth and the risk of forming~a"layer
of steel. thereupon. It has the further advantage that
the protective slag layer remains floating on the
surface of the liquid steel, shielding it from the
atmosphere as well as avoiding effervescence. However,
as the level of steel in the furnace diminishes, a
vortex is created which draws slag into the metal being
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KOFF 0109 PCA -3-
poured. To avoid this, numerous preventative methods
have been devised.
In U.S. Patent No. 4,431,169, an elongated
stopper mounted on a boom is inserted proximate the
taphole, and lowered to block the pour when the amount
of metal remaining is low. The boom is then raised a
small amount, allowing a slow pour of metal from the
furnace without creating a vortex. This method utilizes
relatively expensive control apparatus and is subject to
a great deal of error, because the mouth of the taphole
cannot be seen. The error is compounded when the mouth
of the taphole has been eroded from use. Moreover, a
slight error in the timing of the retraction of the plug
from the taphole can allow slag to be entrained in the
steel being poured, or worse, could cause blockage of
the taphole by steel which has cooled too much due to
the slower pour speed with the taphole mouth partially
blocked.
In U. S. Patent No. 4, 799, 650, a "dart" closure
having a higher specific gravity than slag but lower
than steel has an elongated hexahedral extension which
acts as a vortex inhibitor. When the level of steel
decreases to an amount determined by the geometry and
density of the device, the elongated extension of the
device enters and obstructs the taphole, preventing
further pour of steel and slag. Such devices are of
lesser usefulness in conventional side-tapped furnaces
where the depth of metal above the taphole is limited,
thus permitting the device to descend sideways such that
the extension passes by the taphole and thus cannot
obstruct the taphole at the appropriate time. Moreover,
not only is a substantial amount of steel retained in
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KOFF 0109 PCA -4-
the furnace when the dart enters the taphole, but also
the dart is difficult to remove from the taphole. A
device having a tetrahedral shape but without the
elongated extension is a distinct improvement, as taught
by U.S. Patent No. 5,044,610. This device restricts
vortex formation, and allows an increased pour of metal
before the device obstructs the taphole. However, even
with the '610 device, some slag may yet be entrained in
the steel, especially if the operator-controlled furnace
tilt angle is far from optimum.
In U.S. Patent No. 5,203,909, as the amount of
steel diminishes, a lance providing a pressurized jet of
air or inert gas is positioned above the surface of the
metal/slag interface, thereby literally blowing the slag
away from the taphole. Correct positioning of the lance
is necessary, however, and the use of large quantities
of inert gas such as argon increases cost.
In U.S. Patent No. 4,718,644, a slag sensor is
disclosed for mounting on a non-ferromagnetic taphole
nozzle. The sensor comprises electromagnetic coils
located on opposing sides of the nozzle, and detect the
presence of slag by measuring eddy currents and magnetic
fields in the material flowing through the nozzle.
Unfortunately, such devices do not alert the operator at
the time when slag first is entrained in the molten
metal, as during this transitional period when both slag
and metal exit the nozzle, the relatively large amount
of metal is enough to support large eddy currents and
magnetic fields. By the time the proportion of slag
increases to such an extent that slag is detected, a
significant amount of slag has already passed through
the taphole and into the ladle. Moreover, the electric
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KOFF 0109 PCA -5-
motors and reduction gearing which provide the driving
force for tilting the large and heavy furnace is only
responsive on the order of several degrees of tilt per
second. Even if the slag sensor could alert the opera-
s for to the onset of slag entrainment, the inertia of the
furnace would yet allow for slag entrainment before the
furnace is tilted back to a position where the taphole
is above the slag.
Despite the many attempts to maximize metal
yield whsle minimizing slag entrainment, the predominant
technology in use today is a combination of vortex-
reducing floats having a specific gravity between that -
of slag and that of steel, and operator control of the
tilt of the furnace to regulate flow of steel through
the side-mounted tapho,le . As both the slag and steel
are intensely hot, emitting enormous amounts of both
visible and infrared radiation, the operator cannot
easily determine visually the level of steel hidden
below the slag layer.
Furnace linings, in general, are quite thick,
for example in excess of two feet thick with an addi-
tional "safety" lining of from 6-9 inches . Such linings
are replaced after from 5000 to 6000 heats. During the
initial campaigns, the thickness of such linings and
attendant volume of the furnace prevent the furnace from
being tilted past 97-98 ° during the last portion of a
pour. As the furnace matures, the lining is eroded and
the pour angle increases until it reaches a value of
from 110-111°.
Determination of the slag content of the
furnace is thus not only difficult, but moreover, this
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KOFF 0109 PCA -6-
determination is tendered more difficult by the natural
wear and erosion of the furnace refractory interior. In
addition, the error is compounded by the normal variance
in observation and reaction, particularly with respect
to differences in operator experience and skill level,
attentiveness, and the like. Thus, the industry still
awaits a satisfactory solution to slag entrainment and
yield maximization.
Summary Of The Invention
The present invention provides for maximum
metal yield and minimum slag pour and/or entrainment
through sensing the amount of metal poured from the
furnace, sensing the amount of slag remaining in the
furnace, sensing the tilt angle of the furnace and
determining from these objective measurements, the
correct tilt angle of a side-tapped furnace.
The present invention improves the yield of
metal from a tilting, side-tapped furnace while minimiz-
ing slag entrainment. The present invention also
improves metal yield and minimizes slag entrainment by
providing objective indicia of pour parameters, thus
minimizing operator error, and in one embodiment of the
present invention, automated control of metal pour is
provided.
The above objects and other objects, features,
and advantages of the present invention are readily
apparent from the following detailed description of the
invention when taken in connection with the accompanying
drawings.
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KOFF 0109 PCA -7-
Brief Description Of The Drawings
FIGURE 1 illustrates a side-tapped furnace
containing slag and molten metal discharging metal into
a metal-receiving ladle.
Description of the Preferred Embodiments
The process and apparatus of the subject
invention are for use in side-tapped furnaces of conven-
tional design. Use in bottom-tapped furnaces is not
contemplated. by the term "side-tapped furnace" is
meant a furnace having a tap-hole located on the side of
the furnace through which molten metal exits as the
furnace is tilted. Such furnaces are generally con-
structed of steel and refractory lined, although for
some metals other than steel, a non-lined furnace may be
satisfactory. Such side-tapped furnaces generally
contain approximately 250 tons of steel, and are pivoted
on trunions or other means. Tilting of the furnace is
controlled by electric motors or hydraulic means.
By the term "slag" is meant "slag or dross",
i.e. a mixture of molten metal oxides and other materi-
als, often containing significant amounts of alkali and
alkaline earth metals and silicon, which occurs of
necessity or design above a layer of molten met,a''.iri a
furnace. The term "slag" used herein is the normal
commercial meaning, and is not intended to have a
different meaning other than that naturally ascribed to
it by those in the metallurgical arts.
In order to perform the process of the subject
invention, it is necessary to ascertain the values of
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KOFF 0109 PCA -8-
certain parameters associated with the metal smelting or
refining process.
The period of time which is critical during
the end of the furnace pour is the period wherein about
15% or less of metal residuum is present. At least
during this time or a portion thereof, the amount of
steel poured from the furnace is used to back-calculate
the metal residuum. This value, together with the
optimal tilt angle, is used to control the pour during
this stage of the process. In conventional methods of
pouring steel, the period toward the end of the pour is
very problematic, the operator frequently allowing the
pour to continue, believing only steel to be entering
the taphole, while in reality, a mixture of slag and
steel is being poured. The subject invention substan-
tially avoids this situation.
The critical tilt range is the range on either
side of the angle for any given furnace design wherein
the taphole is lowest with respect to gravity, the
"verticulum angle", and which must be subject to fine
adjustment as the metal residuum, i.e. the amount of
metal remaining in the furnace toward the end of the
pour, reaches a low value and the danger of vortex
formation and slag entrainment increases. The critical
tilt range is approximately 5 degrees on either side of
the verticulum angle, but may vary beyond this point in
a newly lined furnace. The optimal tilt angle is the
angle within the critical tilt range which is capable of
supplying liquid metal through the taphole with minimal
vortex formation and minimal slag entrainment for any
given amount of metal residuum. The optimal tilt angle
is best ascertained historically.
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KOFF 0109 PCA -9-
Each furnace is generally campaigned for
approximately fifty heats before the taphole refractory
is replaced, and many thousands of heats before relining
the entire furnace. A given furnace design will liter-
s ally produce tens or hundreds of thousands of heats
prior to replacement. Recommended tilt angles are in
general known even prior to the first campaign, as the
furnace geometry, as described below, may be used to
estimate the correct tilt angle. Refinement of the
recommended tilt angle to the optimal tilt angle may be
accomplished by monitoring the vortex and slag entrain-
ment during the early runs of the first campaign.
The interior geometry is fixed during the
design of the furnace and is therefore known with
accuracy. Moreover, simple measurements made following
the lining or relining of the furnace with refractory
material may be used to fine tune the designed internal
geometry. From the internal geometry and tap location,
the volume of the metal pool above the taphole may be
calculated for varying degrees of tilt within the
critical tilt range, i . a . , a range of tilt of plus or
minus about 2-5° from the position determined to be the
optimal angle of pour as the pool of metal in the
furnace diminishes.
The internal geometry may also be used to
calculate the height of the slag above the taphole at
various weight percent slag concentrations and at
varying metal content. In normal operation, the slag
does not decrease in volume as the metal flows out
beneath it, but the height of the slag layer above the
taphole will change in a predictable but irregular
fashion which can be calculated from the slag volume
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KOFF 0109 PCA -10-
(determined by the slag weight percent and total charge)
together with the internal geometry presented to the
slag at each degree of tilt and with a given metal
residuum below the slag.
The calculations of slag height and metal
height can be accomplished by one skilled in geometry,
and do not present any particular problem. Preferably,
using modern CAD/CAM computer programs, the various
volumes and heights are calculated automatically.
Moreover, once the calculated values are known, the
variables of furnace tilt, metal residuum volume, slag
volume, etc. may be input to standard computer programs
to derive an equation of closest fit. Alternatively,
the various parameters may be plotted versus tilt angle,
one set of plots for each",furnace charge.
In order to determine the optimal tilt angle,
a tilt indicating means must be installed on the fur-
nace. Means to indicate tilt are, in general, well
known, and include rotary variable capacitance sensors,
inductive sensors, DC servo motor sensors, and the like.
Preferably, the sensor should be capable of measuring
the degree of tilt of the furnace within ~ 1°, more
preferably ~ 0.5°, and most preferably within 15 minutes
over the critical tilt range. During the first series
of furnace runs, the vortex formation and slag entrain-
ment is noted along with the furnace tilt angle as
provided by the tilt indicating means. The tilt angle
may be presented in an analog manner, but preferably is
presented in the form of a digital output or readout.
The furnace metal output is measured. As the
charge of the furnace is generally known within ~ 2%,
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KOFF 0109 PCA -11-
the metal output is a useful measure which may be used
to calculate the metal residuum. The metal output may
be measured in a variety of ways. For example, the
ladle into which the metal pours may be provided with a
scale or load cell and tared prior to the pour. As the
metal pour continues, the weight of the ladle less its
tare weight provides the amount of steel poured.
Alternatively, the steel in the ladle may be viewed with
a video camera and the image compared to stored images
using procedures similar to those used in conventional
image recognition robotic techniques. By subtracting
this amount from the charged amount, the metal residuum
weight and volume may be calculated.
Alternatively, the height of steel in the
ladle, in conjunction with the ladle geometry, may be
used to provide the amount of steel poured. Although
the height of liquid steel may be gauged by the opera-
tor, it is desirable that the height be measured by
means which avoid operator judgment and participation,
such as the video means described above. Other such
means are available, and include microwave sensors where
the reflection of microwaves from the hot steel surface
is monitored and converted to a distance measurement;
conductive probes which may be lowered onto the liquid
steel surface, the onset of conductivity used to trigger
an electrical circuit which calculates height based on
the position of the sensor from a reference point.
Similar calculations based on height of a sensor from a
reference point may be used for a variety of non-con-
tatting sensors, for example hall-effect sensors,
capacitance sensors, and inductive sensors. A conduc-
tive electrode sensor is disclosed in U.S. Patent No.
4,413,810. Although designed for use in measuring the
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KOFF 0109 PCA -12-
position of the slag/steel interface in the furnace,
this type of device is even more suitable for measuring
steel height in a ladle. A further device is disclosed
in U.S. Patent No. 4,544,140. A suitable microwave
sensor which may be used to monitor steel height in a
ladle or slag height in the furnace is disclosed by
Tezuka et al., "M-Sequence Modulated Microwave Level
Meter and Its Application", 1994 Steelmaking Conference
Proceedings, pp. 181-185.
In the preferred control process according to
the subject invention, the amounts of the various
charges of slag forming ingredients, scrap steel, and
pig iron are inputted to a computer and used to either
calculate or determine, from a look-up table, the volume
of steel and slag to be produced. Alternatively, these
amounts may be provided to the computer by an operator
from independent calculation or table. However, it is
appropriate to link the computer program to the charging
process by monitoring the weights of the various charges
and applying the digitized output of the weight sensors
directly to the computer input, thus again minimizing
operator involvement.
The beginning of each pour may be operator
controlled or may be controlled by computer. Since the
amount of steel is large at this point, deviati~r~ from
the recommended tilt angle is not particularly critical,
as long as the tilt is enough to position metal over the
tap and not so much as to spill slag over the rim. As
the pour proceeds, the amount of steel in the ladle is
determined and the amount input to the computer. When
the amount of steel poured is within 80-90 percent of
the charge, the computer takes over control of the
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KOFF 0109 PCA -13-
electrically or electrohydraulically driven tilt means.
For each incremental percent of total charge, for
example each additional 0.2 to 1.0 weight percent, the
computer determines either by calculation from a suit-
s able equation or from a look-up table, the optimal tilt
angle for the metal residuum remaining, and signals the
electric motor or electrohydraulic system to cause the
furnace to attain the determined tilt.
The preferred computer controlled tilting
furnace is advantageously provided with manual override
capability to allow operator control should an unusual
event, for example computer malfunction, sensor failure, -
or the like occur. The advantage of the most preferred
embodiment is that operator control and judgment are
both substantially eliminated, thus producing consis-
tent, repeatable pours which maximize clean metal yield.
Preferably, the process further includes use
of the vortex inhibitors as disclosed in U. S. 5, 044, 610 .
As the metal residuum in the furnace decreases, the
vortex inhibitor self-aligns with the taphole, decreas-
ing vortex formation while also partially throttling the
flow of steel during the last seconds of pour, allowing
time for the furnace tilt to be adjusted to cut off the
flow at the proper point. As the flow diminishes in
response to the change in tilt angle, nozzle located
slag sensors may now become useful, especially in
conjunction with a nozzle gate or valve.
In a further preferred process, operator
judgment is substantially eliminated but operator
control maintained. In one embodiment of this further
preferred process, the amount of metal poured, and thus
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KOFF 0109 PCA -14-
the metal residuum remaining is determined. This
determination may be relayed to the operator in the form
of an alphanumeric display device, a computer screen, a
labeled LED device, analog gauge, or other means. In a
preferred embodiment of this further process, the metal
residuum is translated to an optimal tilt angle either
by the operator personally, i.e. by a look-up table or
tilt angle versus residuum graph, or by inputting the
poured metal parameter to a computer and calculating the
tilt angle. The operator then adjusts the tilt of the
furnace by manual control until the furnace tilt angle
sensor indicates that the correct tilt angle has been
achieved. Although the operator must control the
process, the judgment of the operator is not involved.
The refractory furnace lining will be gradual-
ly eroded over time, thus changing the interior volume
of the furnace. In the most preferred process according
to the subject invention, the increase in internal
volume and any change in interior geometry will be
factored into the computer program used to calculate the
tilt angle. For example, historical knowledge of the
changes to be expected may be factored into the program.
Moreover, during the furnace down time while the refrac-
tory in the taphole is replaced, inspection and/or
measurement of the erosion in the refractory lining may
be made.
The height of slag for a given furnace charge
is also reflective of the change in furnace internal
volume, as for a given charge, the height of the slag
layer will decrease as the volume of the furnace in-
creases. The height of the slag layer will vary more as
the metal residuum decreases, as the change in,volume at
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KOFF 0109 PCA -15-
this point of the process is greater relative to the
initial volume of a newly lined furnace.
If a computer is not to be used to calculate
the optimal tilt angle, but operator look-up tables or
graphs are to be used instead, a separate set of look-up
tables or graphs may be provided for successive incre-
ments of heats, for example a separate set of tables
and/or graphs for each fiftieth heat or some other
appropriate incremental value.
Measurement of slag height may be accomplished
by known means, for example, by microwave techniques or
the capacitively coupled antenna of U.S. Patent No.
4,880,212. Incorporation of the change in interior
volume to the means of determining tilt angle, particu-
larly by actual measurement of slag height, permits an
unprecedented level of control of clean metal output.
In Figure 1, a side-tapped furnace 101 has a
furnace lining 103 and contains molten metal 105 and
slag 107. Shown at 109 is a vortex inhibitor as dis-
closed in U.S. Patent No. 5,044,610. The axis of the
supporting trunnions is shown at 111. Associated with
this axis is tilt angle sensing means 113 whose output
is inputted into computer control device 115. Input
also to computer control device 115 are the output of
slag height sensor 117 which detects the height 118 of
slag 107, and metal output sensor 119 which measures the
amount of metal delivered into ladle 121. A further
input shown at 123 can be used to supply charge informa-
tion or other data to the computer. At 125 is a tilt
driving means responsive to a tilt angle adjusting
output flowing through output line 127. The furnace
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KOFF 0109 PCA -16-
spout is shown at 129 ~ delivering metal stream 131 to
ladle 121. The dashed lines in the Figures indicate
interrelationships between the various sensors, tilt
adjusting means, etc.
A typical run of computer-controlled metal
pour is as follows. The weight of scrap steel, pig
iron, ferrosilicon, alloying ingredients, and slag
forming ingredients are input into the computer, either
from weighing sensors or by hand. The computer calcu-
lates the expected metal yield, slag yield, and slag
height. The operator then initiates the pour by tilting
the furnace to a predetermined tilt angle, indicated on
a computer monitor. As the furnace tilt passes a preset
value associated with the onset of a pour, for example
78° from vertical, the computer clean-metal pour system
is activated. The computer continually monitors the
output of the sensors which measure the degree of tilt,
the amount of metal poured into the ladle, and optional-
ly, the slag height, and uses an algorithm tailored to
the historically determined optimal tilt angles or
calculated from the furnace geometry, to calculate the
amount of metal residuum and to determine the optimal
tilt angle as a function of the metal residuum. The
computer then compares the calculated tilt angle with
the actual tilt angle and signals the tilt drive mecha-
nism to correct the actual tilt to the calculated tilt
by activating a reversible gear driven motor, servomo-
tor, or the valves of a pneumatic or hydraulic system
which tilts the furnace in the necessary direction. As
the metal pour is completed, the algorithm signals the
tilt adjusting means to restore the furnace to vertical,
or to remove the metal pouring spout located beneath the
taphole and replace it with a spout to direct slag from
CA 02166027 2004-08-17
-17-
the furnace. Except for the operator initiating the pour,
no operator judgment or participation is involved.
In a typical non-computer controlled run, the
amount of metal yield and slag yield are calculated by an
operator or determined from a look-up table based on raw
ingredient charge. The operator then activates the furnace
tilting means to begin the pour. Digital or analog readouts
indicate the current furnace tilt angle, percent of metal
poured, and optionally the slag height. At each ten percent
of metal poured until a metal pour of approximately 80
percent is realized, the operator consults a simple table
supplying tilt angles and adjusts the tilt by manually
activating the tilting means until the tilt angle readout
is the same as that recommended for the particular range of
metal pour. After about 80% of the pour is complete, the
operator consults a table for each additional 20 of the
pour, and adjusts the furnace tilt manually to the optimal
angle provided by a table or graph, which may also factor
in slag height and/or the change in volume expected for the
number of runs the furnace has experienced since relining.
Although operator participation is required, operator
judgment is eliminated, as the correct tilt angle is
determined from the table or graph, and the tilt angle
readout indicates when the actual tilt angle equals the
tilt angle recommended.
The process of the subject invention is
particularly useful when used in conjunction with a vortex
inhibiting device such as those disclosed in U.S. Patents
4,601,415, 4,871,148, and 5,044,610. When such devices are
utilized, they are ordinarily introduced into the
' .~ ~1660~~
KOFF 0109 PCA -18-
furnace at the command of the operator, who visually
determines the amount of steel remaining in the furnace .
If the vortex inhibitor is introduced too early, the
flow may be slowed enough to result in over-cooling of
the molten steel, resulting in an off-spec product. If
introduced too late, vortexing may already have allowed
slag to be entrained in the poured steel.
As both steel and slag volume changes with
differing amounts of charge, and as furnace lining
erosion may considerably change the volume and geometry
of the furnace, significant operator skill and experi-
ence is required to achieve any degree of uniformity in
terms of the correct time to introduce a vortex inhibi-
tor. When coupled with the potential for inattentive-
ness, the net result is considerable variation in steel
output and quality.
In the present invention, the steel output,
from which is calculated the metal residuum, optionally
in conjunction with the measurement of slag height as
indicative of lining erosion, may be used to give an
aural or visual signal to the operator when time for
introduction of a vortex inhibitor has been reached.
This combination process, involves a particularly high
degree of synergism, since the throttling effect of the
vortex inhibitor, in addition to reducing vortex-initi-
ated slag entrainment, also serves to increase the pour
time during the last few minutes of the pour such that
fine control of furnace tilt may be achieved despite the
large furnace inertia.
Having now fully described the invention, it
will be apparent to one of ordinary skill in the art
216~02~
KOFF 0109 PCA -19-
that many changes and modifications can be made thereto
without departing from the spirit or scope of the
invention as set forth herein.
