Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS FOR MAINTAINING AN ~TMOSPHERE AROUND
A PREDETERMINED PORTION OF AN ENDLESS DISC~ETE
O~ECT CONVEY~R
This invention relates to an apparatus for maintaining
a particular atmosphere around a predetermined portion of
a train of closely spaced open top containers mounted on
an endless conveyor chain.
In many continuous processes involving discrete
ohjects, it is desirable to maintain a particular
atmosphere around the process coveyance for some portion
of the process. An example is the casting of molten
metal into discrete ingot moulds on a continuous ingot
moulding machine where it is desirable to pour the metal
under an inert atmosphere to avoid oxidation of the
metal as disclosed in Canadian Patent Application no.
358,379 ~iled August 15, 1980. As it is disclosed in the
above patent application, this may be done by placing a
hood surrounding the casting machine at the metal pouring
station and maintaining an inert atmosphere inside the
hood. However, seals must be provided at the hood entrance
and exit as well as on the sides of the haod to prevent
excessive loss of inert gas~
It is difficult to maintain an atmosphere on endless
conveyors since the methods of sealing must be capable
of coping with the continuous wear caused by movement
of the conveyor. This probl~m is particularly severe
in molten metal operations where the high temperature
inv~lved detrimentally affect the life of contact sealing
materials.
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It is therefore the object of the present invention
to provide a reliable, maintenance free apparatus for
maintaining an atmosphere around a predetermined portion
of an endless conveyor, especially with minimum sealing
temperature restrictions.
The apparatus, in accordance with the present
invention, for maintaining a particular atmosphere
above a predetermined portion of a train of closely
spaced open top containers mounted on an endless
conveyor chain, comprises a cover plate located at a
predetermined distance above a numher of such containers
and extending before and after such predetermined portion.
The cover plate has a predetermined number of ports
therein for feeding a gas through the cover plate to
progressively develop a particular atmosphere in the
containers as they approach the predetermined portion
; of the endless conveyor and to maintain such atmosphere
in the containers as they pass such predetermined portion.
The entrance Iength of the cover plate before the
predetermined portion develops the required atmosphere
in the predetermined portion while the exit length of
the cover plate is necessary to maintain the required
atmosphere. The entrance length is determined by the
conveyor line speed, the container volume, the container
to cover gap and the influence of these factors on th~
volume of purging gas required to obtain the desired
atmosph~re. The exit length is det~rmined by the
pneumatic resistance requlred to prevent back flow of
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air into the predetermined porti.on. The width of the
cover plate as we].l as the width of the container
with res.pect to the container cavity are also dependent
on the pneumatic resistance required to prevent back flow
of air into the predetermined portion.
In an embodiment of the invention the containers
are ingot moulds and means are provided for casting
- molten metal in each mould through an opening in the
cover plate at such a predetermined portion of the train
of ingot moulds. The gas atmosphere is non oxidizing and
preferably provided by a nitrogen gas.
At a conveyor line speed of 2 in/sec., and usin~ 56
: pounds ingot moulds~ it has been found that the length
of the plate is preferably equal to that required to
cover fi~e moulds (three be~ore and two after the mould
filling point). rrhe distance between the plate and the
top of the moulds was less than 0.3 inch and the gas
flow rate less than 2~000 SCFH.
The invention will now be disclosed, ~y way of
example, with reference to the accompanying drawings
in which:
Figure 1 is an example of an apparatus in accordance
with the present invention used in the casting of molten
metal into discrete ~oulds on a continuous ingot
moulding machine;
Figure 2 is a view taken along line 2-2 of
Figure l;
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Figure 3 is a view taken along line 3-3 of
Figure. 1:
Figure 4 i5 a graph illustrating the effect of
varying gap dimension on oxygen level in mould
atmosphere for a fixed nitrogen consumption;
Figure 5 is a graph illustrating the effect of
~arying nitrogen consumption on oxygen level in mould
atmosphere for two diffexent gap dimensions;
Figures 6 and 7 show mould refinements to reduce
the gas flow requirement;
Figure 8 shows another possible refin~Pnt which
reduces gas flow but involves a non rubbing. seal;
Figure 9 illustrates how the ~asic principle of
the invention can be used to maintain an atmosphere
for handling discrete not necessarily metallic objects
of varying sixes.
Referring to Figure 1 of the drawings, there is
shown a train of closely spaced open top ingot moulds
30 mounted on an endless conveyor chain 32 moving at a
line speed of about 2 in/sec. in the direction of
arrow A. The ingot moulds all have flat top surfaces.
A stationary cover plate 34 is mounted adjacent to
but spaced ~y a predetermined distance D from the top
surface of the moulds and covers a predetermined number
of moulds before and after a metal pouring station
which is mounted on the top of the cover plate. The metal
pouring statlon is a conventional design comprising
a launder 36 which ends with a downspout 38 used to
feed molten metal into a ladle 40. The ladle 40 is
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intermittently pivoted to successi~ely pour metal into
each mould through a pouring slot 42 in t~e cover plate.
A trap 44 is positioned at the end of the launder to
capture d~oss which may be floating on the surface
of the molten metal.
As shown in Figure 2, the cover plate 34 is provided
with a predetermined number of gas inlet ports 46 and an
inert gas is fed into such ports through a front manirold
48 and a main manifold 50. Inert gas is fed to the front
mould entering under the plate through three gas inlet
ports to rapidly purge the moulds, and to the rem~in;ng
moulds u~der the plate through a single row of ports to
progressively lower and maintain the oxygen level at the
pouring station below a predetermined value. An auxiliary
manifold 52 is also-provided for feeding inert gas to the
ladle enclosure 54 and the downspout enclosure 56. Cover
strips 58 are placed on the gaps between the moulds so as to
prevent excessive leakage of gas through such gaps.
As show~ in Figure 3, the width of the plate 34 is
equal to that of moulds 30. Inert gas enters the moulds
at gas ports 46 and flows out through the gaps at the
sides and the ends of the cover plate.
It has been found that by controlling the lenyth
of the cover plate 34, the number and location of gas
emission ports, the gas flow rate and the gap between
the moulds and the cover plate, a desired inert atmosphere
can be maintained below the pouring slot 42 without
having to use contacting seals. To facilitate investigation
of the gas ~nlet port spacing, gap dimension and inert
gas Cnitrogen) ~low rate, an apparatus was designed in
the laboratory to simulate a conventional casting
machine. The number of gas inlet ports in the cover
plate was set so that, at any time there was a minimum
of three and a maximum of faur gas ports above a
traversing mould. The length of the cover plate was such
that at any time five moulds (three before and two after
pouring slot 22) were located under the cover plate.
The tests were carried out by establishing a pre~
determined nitrogen flow rate through the cover plate and
then travexsing the moulds past the cover plate at the
same speed as a conventional casting machine conveyor
(2 in/sec.l. Each mould was progressively purged as
it entered under the co~er plate. The mould atmosphere
was sampled in the centre of the mould by pumping a
sample to an oxygen analyser as the mould approached
the pouring slot.
Experiments were carried out to determine the oxygen
concentration in the mould atmosphere (al at varying
mould to cover gap dimensions and constant nitrogen flow
rates and (b) at varying nitrogen flow rates and constant
gap dimension. The results of these trials are presented
in Figures 4 and 5~ The front manifold flowrate in the
test shown in Figure 4 was about 200 SCFH and the main
manifold flowrate was about 1500 SCFH. In the test shown
in ~igure 5, the front manifold flowrate was fixed at
about 200 SCFH and the main manifold flowrate was varied
from 500 to 3~00 SCFH~
The results of the tests presented in Figure 4,
revealed that the oxygen leveI increased very rapidly
for gap dimensions greater than 0.25 in. More importantly,
this curve illustrates that less than 0O5~ oxygen can
be achieved with gaps up to 0.3 in with economically
feasible gas consumption (~2000 SCFH). The effect of
varying nitrogen consumption on the oxygen level in
the mould atmosphere is illustrated in Figure 5 for
10 two different gap dimensions of 0.10 and 0.125 in. From
these curves, it is evident that no gain is achieved
by increasing a-tmosphere usage beyond 2000 SCFH and that
acceptable oxygen levels are easily obtained with very
low ( laO0 SCFH) gas consumption. The gap heights of
- 15 0.10 and 0~125 in. used in these tests are practical
values which can be achieved and maintained on present
casting machines. Closer tolerances, which should be
aimed for in the design of future casting machines can
result in less than 0.1~ oxygen with economical usage.
Following completion of the above pilot plant tests,
equipment such as shown in Figure 1 was installed on a
slab ingot casting machine at Canadian Electrolytic Zinc
Limited, Valleyfield, Quebec, Canada to demonstrate,
under plant production conditions, that skimming-free
sla~s can be produced by pouring liquid zinc in a
nitrogen atmosphere.
Atmosphere tests were initially carried out with
the machine in opPration but without pouring liquid
metal. These tests indicated that oxygen levels could
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be maln-tained at the pouring station in the range of
0.3-0.5% and that no gain could be achieved by increasing
the nitrogen flow rate a~ove 2000 SCFH.
Liquid zinc was then started up with preheating
flames on the laundex and ladle. Nitrogen was first
delivered at 250 SCFH to the front manifold and at
1500 SCFEI to the main manifold of the cover plate and
successively to the ladle and downspout enclosures at
250 SCFH. The oxygen level maintained at the pouring
station was in the range of 0.35-0.45~.
Major reduction in the foam floating on cast slabs
was observed even before start-up procedures were completed.
Total absence of foam was achieved when the closure and
purging of the ladle and launder enclosures were
completed.
The slab ingot surfaces were seen to be bright and
dross-free. Transparent oxide films identical to those
obtained in the laboratory tests using oxygen levels in
the range of 0.2-~.5% were observed on the slabs.
Figures 6, 7 and 8 show refinements to reduce loss
of gas in bekween the moulds and so reduce the gas flow
requirements~ In Eigure 6, the edges of the moulds
are thicker than that shown in Figure l and this increases
the resistance to gas flow in the gap 60 between the moulds.
In Figure 7, the edges Qf the moulds are designed so
that the gap 62 is horizontal in order to prevent direct
flow of the gas from the cover ports. This design is in
a way equivalent to the cover strips 58 of Figure 1 but
is much more resistant ~o wear and tear. Figure 8 shows
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~nother method of reducing gas flow which involves
the use of a seal 64 in between the moulds. This alternative
is possible since this seal is non rubbing.
Although the in~ention has been disclosed with
reference to the casting of molten metal and with the
use of an inert gas, such as nitrogen, is is to be
understood that this is a non limiting example. The
apparatus in accordance with the present invention
could be used to maintain any atmosphere (inert or not)
around any discrete objects which are not ne~essarily
metallic, contained in a train of open top containers
as shown by discrete objects 66 in Figure 9 of the drawings.
