The present invention relates generally to fume control in steel
making operations and more particularly to fume control in the strand casting of
steel to which fume-emitting ingredients are added.
Examples of fume-emitting alloying ingredients are lead and bismuth
which are added to molten steel to improve the machinability properties of the
solidified steel product.
In the strand casting of steel, molten steel is introduced from a
ladle into a tundish from where the molten steel is directed into a casting mold
where at least an outer shell of solidified steel is formed.
The fume-emitting ingredients may be added to the molten steel in
the ladle, or they may be added to the stream of molten steel flowing from the
ladle to the tundish. Aside from the ladle, fumes may be emitted from the molten
stream between the ladle and the tundish and from the molten steel in the tundish.
In the strand casting process, the partially solidified steel moves
downstream from the casting mold into a spray chamber in which the steel is sprayed
with water to cool the steel and further solidify it. The solidified steel then
moves into a run-out chamber located at the downstream end of the spray chamber.
Relatively clean gases, devoid of fumes from the fume-emitting gases, are generated
in the spray chamber and in the run-out chamber.
After the run-out chamber, the solidified steel strand moves to a
torch-cutting station located immediately downstream of the run-out chamber where
the strand is cut into pieces. Torch-cutting of the strand generates fumes from
the fume-emitting ingredients in the solidified steel strand. These fumes must
be prevented from escaping into the work place environment surrounding the strand
casting equipment because the fumes can pose a health hazard. In the case of lead,
the law restricts the quantity of lead bearing material which may be present in
the work place environment as dust or fumes to no more than 50 micrograms per cubic
The fumes emitted from the molten steel, or from the strand during
the torch-cutting step, are at least initially in the form of lead or bismuth vapors
which may then react with the atmosphere to form oxides of lead or bismuth. In
accordance with the present invention, it matters not whether the fumes from the
fume-emitting ingredients are in the form of metallic vapors or the oxides thereof.
Both forms are equally undesirable.
Gases carrying fumes collected from steel making operations are normally
passed through a bag house which removes the fumes from the carrying gases which
are then exhausted to the atmosphere minus the fumes.
At the torch-cutting station, water sprays are used to wash scale
and dross resulting from the torch-cutting step into a flume located beneath the
steel strands at the torch-cutting station. Fumes generated during the torch-cutting
step are removed from the torch-cutting locale by exhaust ducts. Because of the
water sprays employed at the torch-cutting station, the gases exhausted from this
location are wet and cool. It is undesirable to process wet, cool gases through
a bag house because the moisture in such gases can precipitate in the bag house
and interfere with the ability of the bag house to perform its fume-removing function.
The invention as claimed provides a method and apparatus for severely
restricting the amount of toxic fume which can escape from the strand casting operation
into the surrounding work place environment.
The stream of molten steel is enclosed in a shroud as it passes between
the ladle and the tundish. The tundish is covered and has an opening through which
the molten steel may enter the tundish. A movable exhaust hood is positioned between
the ladle and the tundish with an exhaust inlet located immediately adjacent the
opening in the tundish. Baffles are provided to confine any fumes emitted through
the opening in the tundish to the vicinity of the exhaust inlet.
After the tundish has been emptied of essentially all the steel that
can be drained therefrom, it continues to emit some toxic fumes as it cools because
of a residue of molten steel remaining in the tundish or sticking to the walls
thereof. In accordance with the present invention, the tundish is moved from a
casting to a non-casting position, together with its associated exhaust hood,
and the fumes which continue to be emitted from the tundish while the latter is
in its non-casting position, are collected through its associated exhaust hood.
The exhaust gases collected from the tundish while it is in its casting
position, during the casting operation, before the tundish is emptied, are relatively
hot and dry compared to the gases collected at the torch-cutting station. In accordance
with the present invention, the hot, dry gases from the tundish are mixed with
the cool, wet gases from the torch-cutting station, at a location upstream of the
bag house, to raise the temperature of the gases collected at the torch-cutting
location to a temperature above the dew point thereof to prevent precipitation
within the bag house of moisture from the gases.
There is a substantial delay between the time the molten steel from
the ladle first enters the tundish and the time the strand is first subjected to
the torch-cutting operation. This delay period can be one hour, for example. The
hot, dry gases generated at the tundish during this delay period are circulated
through the bag house to preheat the bag house prior to the introduction therein
of exhaust gases collected at the torch-cutting station. Preheating the bag house
assists in preventing the precipitation therein of moisture in the gases collected
at the torch-cutting station.
After the tundish has been essentially emptied, the temperature of
the exhaust gases collected therefrom is substantially lower than the temperature
of the exhaust gases collected from the tundish while it contained substantial
amounts of molten steel. As a result, the gases collected from the tundish at this
stage may not be hot enough to prevent precipitation in the bag house of moisture
from gases collected at the torch-cutting station, when the latter are mixed with
the gases from the tundish.
The present invention compensates for this heat deficiency by utilizing
the clean gases generated at the run-out chamber located immediately upstream of
the torch-cutting station. These gases, consisting essentially of hot air, are
relatively hot and dry compared to the gases generated at the torch-cutting station.
By mixing the hot, dry gases from the run-out chamber with the cool, wet gases
from the torch-cutting station, precipitation of moisture in the bag house is
prevented. The location of the run-out chamber, where the relatively hot, dry gases
are generated, is sufficiently close to the torch-cutting station so that the
hot, dry gases retain sufficient heat at the time they are mixed with the gases
from the torch-cutting station to maintain the temperature of the mixed gases
above the due point thereof when the mixed gases enter the bag house. Moreover,
because the gases from the run-out chamber are relatively dry, the percentage of
water in the mixed gases is substantially less than the percentage of water in
the gases from the torch-cutting station.
Large droplets of moisture, initially carried by the cool, wet gases
collected from the torch-cutting station, are removed by passing these gases through
a cyclone separator located upstream of the location where the gases from the
torch-cutting station are mixed with gases from other locations in the strand casting
operation. The fumes which are controlled in accordance with the present invention
may be either metallic vapors or oxides of the fume-emitting ingredients, or both.
Other features and advantages are inherent in the method and apparatus
claimed and disclosed or will become apparent to those skilled in the art from
the following detailed description in conjunction with the accompanying diagrammatic
- Fig. 1 is a schematic flow diagram of a strand casting operation;
- Fig. 2 is a perspective of an embodiment of apparatus in accordance with the
- Fig. 3 is a fragmentary, vertical sectional view illustrating a portion of
the strand casting equipment illustrated schematically in Fig. 1;
- Fig. 4 is a fragmentary perspective of a portion of one embodiment of apparatus
in accordance with the present invention; and
- Fig. 5 is a fragmentary, vertical sectional view of a bag house bag in accordance
with an embodiment of the present invention.
Fig. 1 illustrates a strand casting operation wherein molten steel
from a ladle 10 is introduced through a shroud 11 into a tundish 12 from which
the molten steel passes through tundish nozzles 13 into a casting mold 14 wherein
the steel is at least partially solidified. The steel then moves along an arcuate
path through a spray chamber 15 of conventional construction employing conventional
water spray nozzles to cool the steel as it moves along the arcuate path. Located
at the downstream end of spray chamber 15, and separate and discrete therefrom,
is a run-out chamber 16 from which emerges a solid steel strand 17 which passes
over rollers 18 to a torch-cutting station comprising a cutting table 19 having
an open top and associated with a torch-cutting device 20 of conventional construction
which moves back and forth along a path at 21 to cut strand 17 into a multiplicity
of pieces, e.g. steel billets. Conventional water sprays (not shown), normally
associated with such a torch-cutting device, are employed at the torch-cutting
Fig. 3 shows tundish 12 located in a casting position directly below
ladle 10. Tundish 12 comprises a top cover 24 having an opening 25. Extending from
the bottom of ladle 10 toward tundish opening 25 is a conduit 26 for directing
molten steel from ladle 10 through tundish opening 25. Enclosing conduit 26 is
a tubular, outer shroud 27 extending from the bottom of ladle 10 through opening
25 in the top 24 of tundish 12. Shroud 27 encloses both conduit 26 and the stream
of molten steel directed by the latter into tundish 12 and helps protect the stream
of molten steel from the atmosphere outside the stream of molten steel.
Fume-emitting ingredients, such as lead or bismuth, are introduced
into the stream of molten steel through a tube 28 extending at a downward angle
through the wall of tubular shroud 27. Another tube 29 communicates with the interior
of shroud 27 for introducing a pressure-regulating gas into the interior of shroud
27. The apparatus illustrated in Fig. 3 is described in greater detail in U.S.
application Serial No. 731,077 filed May 6, 1985, and the disclosure thereof is
incorporated herein by reference.
The introduction of fume-emitting ingredients into the molten steel
entering tundish 24 generates fumes at tundish 24 and in shroud 27. These fumes
can escape through that part of tundish opening 25 not occupied by the cross section
of shroud 27. These fumes are prevented from polluting the work place environment
by apparatus illustrated in Figs. 1, 2 and 4. To collect the fumes generated in
the tundish and the shroud, an exhaust hood 32 is located between ladle 10 and
tundish 12 (Fig. 1). Exhaust hood 32 has an inlet 33 which is located adjacent
top opening 25 of tundish cover 24 (Fig. 4). Exhaust inlet 33 has an arcuate shape
conforming to the shape of that part of tundish top opening 25 where exhaust inlet
33 is located. As shown in Fig. 4, tundish top opening 25 has an irregular shape
to accommodate tilting of shroud 11 to facilitate the positioning of the shroud
in opening 25.
Extending from exhaust conduit 32, on opposite sides of inlet 33,
are a pair of baffles 34, 35 which are normally located adjacent tundish opening
25 when exhaust inlet 33 is similarly located. Baffles 34, 35 extend between the
bottom of ladle 10 and tundish top cover 24. Baffles 34, 35 perform the function
of substantially confining toxic fumes from tundish 12 and shroud 11 to the vicinity
of exhaust inlet 33.
Baffles 34, 35 are mounted on hood 32, at 36 and 37 respectively
(Fig. 4), for pivotal movement of the baffles, relative to hood 32, toward and
away from each other. This facilitates positioning of the baffles to perform their
intended function. As shown in Fig. 4, baffle 34 comprises a bottom portion 38
for covering at least part of top opening 25 on tundish 12. Extending upwardly
from bottom portion 38 is a wall portion 39.
Referring to Fig. 2, exhaust conduit 32 is connected to one end of
a piston rod 42 reciprocable within an air actuated cylinder 43 for moving exhaust
conduit 32 relative to tundish opening 25, back and forth along a horizontal path,
between an extended, operative position adjacent opening 25 and a retracted, displaced
position relatively remote from opening 25.
Exhaust hood 32 has an outlet end 44 communicating with the inlet
end 45 of a coupling 46 when exhaust hood 32 is in its operative position. Coupling
46 has an outlet end 47 for communicating with another coupling 48 in turn communicating
with a conduit 49.
Tundish 12 is part of an assembly also comprising exhaust hood 32,
piston rod 42 and cylinder 43, and coupling 45, as well as supporting framework
(not shown). This assembly is mounted on a car having wheels 52, 52 for moving
the assembly from a casting position (solid lines in Fig. 2) to a non-casting position
(dash-dot lines in Fig. 2).
For a time after it has been drained of all the molten steel which
can be withdrawn therefrom, the tundish continues to emit toxic fumes. At this
stage, the tundish must be moved from the casting position, where fume collection
is available, to the non-casting position so that other parts of the strand casting
equipment can be readied for the next cast.
The tundish is often preheated at the non-casting position, before
the start of the strand casting operation. When a tundish has been previously used
for the strand casting of molten steel containing fume-emitting ingredients, there
is a residue in the refractory lining of the tundish which, during preheating,
will vaporize and emit fumes.
The present invention provides for the capture of toxic fumes emitted
from the tundish when the latter is in the non-casting position. Referring to Fig.
2, exhaust hood 32 is normally retracted to its displaced position (dash-dot lines
above cylinder 43) when the tundish and associated equipment are moved from the
casting to the non-casting position. Hood 32 is moved back to its operative position,
wherein inlet 33 is adjacent opening 25 in tundish 12, when the assembly is at
the non-casting position so as to capture fumes escaping through opening 25. Located
in tundish top 24, on opposite sides of opening 25, and spaced from opening 25,
are a pair of exhaust vents 53, 54 each covered by a respective plate 55, 56 when
the tundish is in its casting position (solid lines in Fig. 2). However, when
the tundish is in its non-casting position (dash-dot lines in Fig. 2), the cover
plates, 55, 56 are removed from over exhaust vents 53, 54, and fumes escaping
through these vent openings are exhausted through additional hoods 57, 58 located
at the non-casting position.
Exhaust hoods 32, 57 and 58 are all employed to exhaust fumes from
tundish 12 when the latter is in its non-casting position, either during a preheating
operation or after a casting operation while the tundish continues to emit toxic
Exhaust hoods 57, 58 each communicate with a respective branch conduit
60, 61 each communicating with a main conduit 62 communicating with a coupling
63 in turn communicating with a connecting conduit 64 which communicates with
conduit 49. Conduit 49 is employed to remove fumes generated at the tundish when
the latter is in its casting position (solid lines in Fig. 2).
Exhaust hood 32 typically has a cross sectional area sufficient to
provide a 7,000 ft./min. (2134 m/min.) capture velocity in the vicinity of tundish
opening 25 when the tundish is in the casting position. This will maintain the
toxic fumes in the work place environment surrounding tundish opening 25 below
the required maximum of 50 micrograms per cubic meter. The rest of the exhaust
system downstream of hood 32 also has a capacity sufficient to maintain these
Referring to Figs. 1 and 2, located below the torch-cutting station
at table 19 is a flume 66 for collecting the dross and scale which falls from slab
17 through the open top of table 19 during the torch-cutting step. Flume 66 also
collects water which falls from above as a result of the water sprays (not shown)
which accompany the torch-cutting step. Flume 66 has a pair of opposite sides
99,100 on each of which is located a plurality of exhaust outlets 67, 67 communicating
with an exhaust manifold 68 communicating with a conduit 69. Fumes generated by
the torch-cutting step are drawn into flume 66 and exhausted therefrom through
exhaust outlets 67, 67.
Flume 66 has a bottom 70 which slopes downwardly in a downstream
direction. This causes the water which drops into flume 66 to flow in the downstream
direction, creating a downstream current, to wash downstream the scale and dross
which falls into flume 66. The current in flume 66 also causes some of the fumes
drawn into flume 66 to be carried towards the downstream end 74 of flume 66, and
at least part of these fumes avoid removal through exhaust outlets 67, 67. To prevent
these fumes from escaping into the work place environment surrounding downstream
flume end 74, an exhaust hood 72 is provided immediately downstream of, and above,
the downstream end 71 of table 19 (Fig. 2). Exhaust hood 72 communicates with
a conduit 73 in turn communicating with conduit 69 which, as noted above, also
connects to exhaust manifolds 68, 68. Exhaust hood 72 will also collect any fumes
generated by a sample cut-off device (not shown) normally located adjacent downstream
end 71 of table 19.
In summary, exhaust outlets 67, 67 collect gases at a location directly
below the torch-cutting station, and exhaust hood 72 collects gases at the downstream
end of the torch-cutting station, a location immediately downstream of the furthest
downstream position to which torch-cutting device 20 moves as it performs the torch-cutting
step. As shown in Fig. 2, exhaust hood 72 is located above exhaust outlets 67,
Gases collected at exhaust hood 72 and exhaust outlets 67, 67 are
conducted by conduit 69 to a cyclone separator 75 wherein large droplets of moisture
are separated from the gases which then exit through the top of separator 75 into
a conduit 76.
The gases entering conduit 69 contain moisture as a result of the
water sprays employed at the torch-cutting station. Accordingly, the gases in conduit
69 are relatively cool and wet compared to the gases exhausted from the tundish
into conduit 49. The gases exiting from cyclone separator 75 through conduit 76,
although stripped of large droplets of water, are still relatively wet and cool.
These gases are conveyed through a conduit 77 to a bag house 78 for removing from
the gases the toxic ingredients therein, e.g. oxides of lead and bismuth.
It is undesirable that gases entering a bag house have a temperature
below the dew point of the gases because this causes moisture in the gases to precipitate
in the bag house thereby interfering with the ability of the bag house to perform
its intended function. More particularly, in a bag house, dirty gases are drawn
through the walls of vertically extending fabric bags, from the outside to the
inside of the bags. As the gases pass through the fabric walls of the bags, they
are cleaned of dust particles which accumulate on the outside of the bag walls.
The cleaned gases entering the inside of the bags are conducted further downstream
and eventually exhausted to the atmosphere. Periodically, when the dust accumulating
on the outside of the bag walls gets too thick, the bags are shaken to dislodge
the dust. This is necessary because an overly thick dust layer will impede the
passage of gas through the bag. If the gases have a temperature below the dew
point thereof, moisture in the gases will precipitate on the outside of the bag
walls, causing the dust particles which accumulate there to cake, and this interferes
with the dislodgement of the dust particles from the bag walls. On the other hand,
if the temperature of the gases are above the dew point thereof, the moisture is
in the form of a vapor and it will pass through the bag walls with the cleaned
Raising the temperature of the cool, wet gases from the torch-cutting
station to a temperature above the dew point thereof is accomplished by mixing
these gases with the relatively hot, dry gases exhausted from the tundish. Mixing
of the gases also produces an H&sub2;0 percentage therein substantially less than
the H&sub2;0 percentage in the gases from the torch-cutting station just before
mixing. The increase in gas temperature and the decrease in H&sub2;0 percentage,
compared to the corresponding conditions in the gases from the torch-cutting station,
both contribute to reducing the likelihood of H&sub2;0 precipitation in the bag
Mixing of the gases begins at a junction 80 where conduit 76, containing
the relatively wet, cool gases from the torch-cutting station, joins conduit 49
containing the relatively hot, dry gases from the tundish. Conduits 76 and 49
join at junction 80 to form conduit 77. Junction 80, is upstream of bag house 78.
There is a substantial delay period between the beginning of the
molten steel introducing step at tundish 12 and the beginning of the torch-cutting
step at table 19. During this delay period, the hot, dry gases generated at tundish
12 are directed through conduits 49 and 77 into bag house 78 to preheat the bag
house before any fumes from a torch-cutting step are directed into the bag house.
This reduces the precipitation of moisture in the bag house when the gases from
the torch-cutting station are eventually directed therethrough. More particularly,
the gases used to preheat bag house 78 during the delay period are hotter than
the mixed gases which will enter the bag house after the delay period. Accordingly,
at least at the beginning of the time when the mixed gases enter the bag house,
the bag house is at a substantially greater temperature than the entering gases.
Eventually, of course, the temperature of the bag house will drop and approach
that of the mixed gases entering the bag house, but the temperature of the bag
house will not drop below the dew point of the entering mixed gases, which are
maintained at a temperature above the dew point thereof.
During the delay period, a damper 81 in conduit 76 is closed to prevent
cool gases from being drawn into conduit 77 at junction 80. During this period,
no fumes are being generated at the torch-cutting station because that station
Just as the introduction of molten steel into the tundish proceeds
for a substantial period before the beginning of the torch-cutting step, so also
the torch-cutting step proceeds for a substantial period after the conclusion
of the introduction of molten steel into the tundish. Thus there will continue
to be a substantial generation of cold, wet fumes at the torch-cutting station
during a time when there is a substantial diminution, if not a total cessation,
in the generation at the tundish of hot, dry gases which can be mixed with the
cool, wet gases at junction 80. To prevent the gases entering bag house 78 from
dropping below the dew point thereof, the wet, cool gases entering conduit 77
from conduit 76 are mixed with hot, dry gases from another source.
More particularly, as slab 17 passes through run-out chamber 16,
the slab is still relatively hot. The slab is not subjected to spray cooling in
run-out chamber 16, so that the air within run-out chamber 16 is heated by slab
17, and that air is neither cooled nor moistened by water sprays. Thus, the gases
within run-out chamber 16 are relatively hot and dry compared to the gases exhausted
from the torch-cutting station.
The hot, dry gases in run-out chamber 16 are withdrawn through an
exhaust outlet at 83 communicating with a conduit 84 in turn communicating with
a connecting conduit 85 communicating with another conduit 87 which joins conduit
49 at a junction 88.
As previously noted, run-out chamber 16 is located at the downstream
end of spray chamber 15 and is immediately upstream of the torch-cutting station.
Run-out chamber 16 is sufficiently close to the torch-cutting station so that
the hot, dry gases withdrawn from run-out chamber 16 retain sufficient heat at
the time they reach junction 80, where they are mixed with the cold, wet gases
from the torch-cutting station, to maintain the temperature of the mixed gases
above the dew point thereof when the mixed gases enter bag house 78.
Connecting conduit 85 contains a damper 89, and conduit 84 contains
a damper 90 located downstream of the junction 91 between conduit 84 and connecting
conduit 85. Damper 89 is opened and damper 90 is closed when the hot, dry gases
from run-out chamber 16 are to be mixed with the cool, wet gases from the torch-cutting
station. Damper 89 is closed and damper 90 is opened when the gases from run-off
chamber 16 are not to be mixed with the gases from the torch-cutting station. In
that instance, the gases flowing through conduit 84 bypass bag house 78.
Clean gases from bag house 78 flow into an exhaust conduit 93 which
communicates with a pair of inlet conduits 94, 94 each leading into a respective
blower 95, each having an outlet conduit 96, communicating with a conduit 97 in
turn communicating with a stack 98.
Referring now to Fig. 5, bag house 78 contains a plurality of bag-type
filters each comprising a fabric sock 101 having an open top 104 and a closed bottom
105, and into which the gas passes from the outside forming a film of dust on
the sock which acts as a filtering medium. Bag house exhaust outlet 93 (Fig. 2)
is in communication with the open top 104 on each sock. Each sock 101 is supported
at the top 104 in a conventional manner (not shown). When the film of dust becomes
too thick, the exit end of the sock may be closed at 104 thereby shutting off
the gas flow, and the sock may be shaken or vibrated to drop the excess dust into
a collecting hopper at the bottom of the socks. Alternatively, the socks may be
"pulsed" by directing an air blast down through the open top 104 of each sock,
e.g. by reversing blowers 95, 95.
As noted above, if the temperature of the gas entering the bag house
drops below its dew point, moisture will precipitate on the fabric wall of the
sock, thereby forming a caked deposit of dust on the sock which would be extremely
difficult if not impossible to dislodge. In a preferred embodiment of the present
invention, illustrated in Fig. 5, this problem is avoided by lining the outside
of each sock 101 with a layer or membrane 102 of polytetrafluoroethylene (e.g.
Teflon). In effect, the sock has an inner layer 103 of fabric, and an outer layer
or membrane 102. This has two advantages. The membrane has pores which are so small
that it does a much more efficient job than the fabric of excluding dust particles
from passing into the interior of the sock. In addition, membrane 102 is much smoother
than the fabric so that, even if moisture does precipitate and cause caking on
the membrane, the caked material will not stick thereto but will slide off the
membrane when the sock is pulsed.
The foregoing detailed description has been given for clearness of
understanding only, and no unnecessary limitations should be understood therefrom,
as modification will be obvious to those skilled in the art.