Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method ~or recovering
argon as contained in the gases coming from the re~ining
converter of stainless steels which works after the ar-
gon-oxygen decarburizing method (A O.D. method), which,
as is well known, is one of the most ad~anced methods
for refining stainless steels.
More particularly, this method permits to decarburize
down to extremely low levels, such as those provided for
stainless steels, the molten steel coming from an electric
furnace, by blowing into the liquid bath a mixture of oxy-
gen and argon.
The major fraction of oxygen reacts with carbon and l~
forms C0 whereas argon is an inert and, by lowering the
C0 partial pressure, permits to obtain extremely high de-
carburisation levels (residual carbon down to 0.02 - 0.05%
by weight) without having to increase the bath ~emperature
to prohibitive levels, the temperature being conversely
maintained at 1600C-1750C. Due to these comparatively
moderate temperatures, the percentage of oxidized chromium
which goes with the slags is maintained in the vicinity of
10%. With no argon being present, in order that the same
degree of decarburization may be obtained, it would be re-
.
quired that the temperature be increased to at least 1850C
1900C, and, under these conditions, a considerable chromium
loss is experienced, which amounts to about 40~ of the charge.
Usually~ the charge coming from an electric furnace
has a carbon content comprised between 0.8% and 1.2% by
weight; it is thus apparent that the diluting effect of
argon is especially important in the final refining stage.
Just for this reason, the refining operation is carried
out through four or five sequential stages in which the
molar ratios 02/Ar are decreased, from the first stage to
the last, from a value of 4 to about 0.5.
In the following Table there are reported, by way
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of example, the operative conditions provided for reducing
the carbon contents from 1 2% by weight to 0~05% by weight
according to the A O.D. method
Blowing 0 /Ar Blown-in Steel composition Blowing Temp.
stages ratio gases ~Percentage) t~me
Nor.cu. C Si Mn Cr mins.
meters
per ton
of steel ;
2 Ar
0 - - 1.2 0.5 ~.5 19 - 1450 ;
I 4/1 ~ 2.75 0.54 0.1 0.4 18.7 18 80 1602
II 3/1 4.1 1.37 0.28 0.05 0.3 18.40 7.4S 1650
III 2/1 1.92 o.96 0.20 0 05 0.2 18.10 3.90 1690
IV 1/1 2.2 2.2 0 10 0 05 0.2 17.75 6.04 1724
V 1/2 1.42 2.89 0.05 .S 0.2 17.50 5.78 1745
The complete duration of the cycle, including the
dead times, is about 100 to 110 minutes.
The argon contents in the blowing gases varies from
a minimum o~ about 18% by volume in the first stage, to a
maximum of 64~ by volume in the last stage
Usually, the gases emerging from a converter, which
are a mixture of C0 and argon, are drawn-in into~lspecial-
ly provided duct, diluted with a large amount of air to ~`
ensure the burning of carbon monoxide by the agency of
atmospherical oxygen, stripped of the dust and then dis-
charged in the atmosphere.
This fact involves problems connected with the high
temperatures as generated by the combustion of C0.
The results which can be achieved with the A.O~D.
method are, as outlined above, considerable from a metal-
lurgical standpoint, while their drawback is the use o~
large amounts of argon which, due to the high cost of the
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.
latter; have a not negligible bearing on the economical balance
of the oparationO
Another defect of the conventional method is that the
gases emerging from a converter are drawn with an open hood, that
which involves, of necessity, the simultaneous drawing-in of
external air. As a result, the gas admixture as drawn in by
the converter is supplemented with air and, above all, with the
nitrogen contained therein. ~ow, the gaseous mixture intended
for being used in the A.O.D. method cannot contain nitrogen and
hydrogen but in trace quantities in order to prevent the possible
formation of nitrides and hydrides with the molten metals~
The principal object of the present invention is to
provide a nove} method fox treating the gases emerging from a ;~
converter, said method allowing to recover a predominant fraction
of argon in a state of high purity, while permitting the possible
and prefarable recycling thereof towards the A O,D, converter,
the waste of argon being thus drasticall~ reduced. It is aLso
possible to recover a high percentage of the C0 contained in
the gases, the economical advantages being not negligible.
This invention does away with the defects enumerated
above and is mainly characterized in that the exhaust gases
emerging from the converter are protected against the external
air by at least one jacket of a blanketing gas which, when ad-
mixed with argon, may be easily and quickly separated from the
argon.
According to the invention there is provided a method
for recovering the argon a~ contained in exhaust gases fro~ an
A~O~Do converter for refining stainless steels in which said
gases are drawn in from the converter mouth, characterized in
that a forced exhaustion of the gases is protected with respect
to the external air by at least one ja¢ket of a blanketing gas
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selected from the group consisting of carbon dioxide, argon,
steam and admixtures thereof, which, when admixed with argon,
can easily and quickly be separated therefrom, whereafter the -Af
argon is separated from the exhaust gases.
More particularly, the present invention provides a
method for recovering argon from the exhaust gases of an
A.O~D. converter, said method comprising the steps of:
-drawing from the converter mouth gases composed by
argon, carbon monoxide and possibly carbon dioxide, while
protecting the suction with a blanketing gas which can
easily be separated from argon and selected from the group
consisting of carbon dioxide, argon, steam and their
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.,, ~..
. . ., , , ~, .
:
1~3~88
mixtures;
- cooling the gases and stripping the dusts entrained
thereby,
- stripping the carbon dioxide;
- stripping the components lighter t;han àrgon.
Basically, thus, the present invention provides for
the use of at least one blanketing gas which prevents the
entrance of air in correspondence with the movable duct,
kept under a negative pressure, through which the gases
emerging from the converter mouth are conveyed.
More particularly, according to a first embodiment,
the method according to the present invention provides for
surrounding the suction of the exhaust gases of the AOV
con~erter by a jacket of a blanketing gas such as defined
hereinabove, more particularl~ CO2 and carrying out the strip-
ping of CO from the gases by scrubbing, subsequent to the
absorption of CO2, with a cuprammonium solution, that which
permits to obtain, on completion of the purification stage, ;;
a-~ee~e~in the order of 90% of the argon as initially con-
tained in the exhaust gases, and the reuse of said argon
with a content of impurities which is b~low the range es-
tablished for the use in feeding the converter, for the re-
cycle to the same converter or to another converter instal-
led in parallel with respect to the former. ;
Stated another way, this first embodiment rec~vers ~;
argon having a content of nitrogen and hydrogen which ful-
fils the requirements imposed for feeding the converter, in
that the shield afforded by the gas blanket prevents the
inlet of air and thus of nitrogen and oxygen
Of course, by repeatedly recycling the recovered argon,
an increase may be experienced due to the accumulation of im-
purities~ especially nitrogen.
This fact, however, detracts nothing from the es-
sential advantage of the present invention, that is, that of
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permitting at least one recycling of the argon a~ contained
in the exhaust gases of the AOD converter
More particularly, this embodiment of the method ac- ;
cording to the present invention can find a profitable ap-
plication in tho~e installations which comprise more than
one converter, and in such a case the argon as recovered
from the exhaust gases of the first con~erter, can be ~ant
to feed the next converter for a stage which provides for a
higher 02/Ar ratio
According to another embodiment of the method of this
invention, the stripping of CO from the exhaust gases of a
converter~ after a scrubblng to remove CO2 and drying of the
gases, takes place by a low-temperature fractionation, from
which a lighter fraction is obtained, which contains CO and
other noxious impuritie3, especially nitrogen and hydrogen,
whereas the other fractlon~ the tailings, is substantially
formed by argon having the expected degree of purity.
This embodiment, especially suited to the closed-loop ro-
cycling of argon to the same converter, is, moreover, ex-
empt rom the above indioated difficulty of the possible
acoumula~ion of undesirabl~ or noxious components
An additional advantage of this embodiment lies in
that the blanketing gas can also contain nitrogen and hy-
drogen, which are stripped before the recycling of argon.
Of course, the ~ract~onation at low temperature can be ap-
plied in the present ca~e with advantage in view of the
comparatively high content~ of argon in the exhau~t gases
in question~ 90 that the operation is largely more advan-
tageou~ over the usual production of a similar amoùnt of
argon by fractionation of air,
According to a further alternative modification of
the seoond embodiment aforesaid, the protection of the suc-
tion of the exhaust gases from the AOD converter is carried
out by means of two gases, that i~, by providing a ~econd
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jacket of blanketing gas surrounding the one mentioned above, :
so as to ensure that also the blanketing gas which forms the
first jacket, is almost entirely recovered together with the
exhaust gases and is also preferablvv reused in a closed loop.
This fact finds ~k~ particular ~application in the
case in which the gas forming the first jacket, that is the
gas which is drawn in together with the exhaust gases, is ~ ;~
carbon dioxide; additionally, it is prevented in this case .
that about the converter an unbreathable atmosphere is formed,
or, at any rate, a noxious environmentO Simultaneously, this fact .
makes possible the use of nitrogen as a second blanketing ::.
..~ :
jacket, by thus utilizing a by-product of the production of
argon and oxygen whlch is readil~v and economically available
without disturbing the recover of argon from the converter
exhaust gases :~.
Lastly, a still further advantage which is common to
all the embodiments of the present invention is that the
blanketing gas forms a protective blanket for the internal :~
, . .
walls of the suction hood, said walls being otherwise sub-
jected to a considerable heating both due to the high temper- ; :
atur~ of the exhaust gases and, the irradiation due to the .
molten charge lying in the AOD converter. ~
As outlined above, the blanketing gas should be com- ~ :
paratively cheap and easily separable from argon so as not
to influence the decarburization reaction in the converter.
It has been found that carbon dioxide is responsive to both ~;
these requirements and can even:be obtained from the blow~
ing gases which contain a fair amount of it.
It should be noted that carbon dioxide can also serve
for the duct scavenging stages, before and after the blow-
ing stages, in order to prevent the ormation of explosive
mixtures. . ;.
In addition to carbon dioxide, it is possible to use
as the blanketing gas a fraction of the recovered argon, at
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the expen~es o~ a o~rtain lo~ o~ same, or ~team whioh oan
be utill2ad in oonnection with the s~cond e~mbodiment a~ de-
fined herein~bove, wher~in the presence o~ hydrogcn value~
in the argon can be acceptable ~ince hydro~ren a~ ~uch i9
removed by ~ractlonation ~as a matter of fact, ~team reactA
with CO at high temperatures ~uch as tho~e o~ the g~ses
emerging ~rom the converter 3 thu~ pro~ucing hydrogen:
H20 ~ CO;~ C02~ H2) .
Ctherwise~ in the ca~e of the first embodiment of the
method o~ this invention, small amounts o~ hydrogen can be
remo~edbya catalytlc combu~tion treatment (the so called
DEO~O prooe~), carried out after the separation of CO.
ln the following, an exemplary disclo~ure o~ the rnethod
ls given with reference to tha accompanying drawing, which
shows a diagram oE an installation Eor carrying the method
lnto con~truotive praotice in connection with the ~irst em-
bodiment as outlined above.
The oxygen and the argon which are required ~or the
decarburi~ing step are blown in the e~tablished amount~ and
during the pre~cribed times, at the bottom of the AOD con~
verter 1, through duot~ such a~ 2 and 3 The gases emerg-
ing rom the converter are collected by a movable duct 4
whioh i~ kept under a negative pressure and is equipped
with a slidable masklng sleeve 5, who~e task is to shield
the opening existing between the converter mouth and the
duct.
In the interior of the sleeve -there i~ introduced,
as the blanketing gas and through radial nozzles 6, CO2 in
excess over that which oan be drawn in through the above
indicated opening By so doing, air will be prevented
rom entering the duct 4 and becoming admixed with the gase~
A~ ~rawn in ~ro~ the ccnverter. A certain excess of CO2 as
b~own in the sleg~e will become dii~ipat~d.
The gases drawn in from the converter, upon cooling
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and scrubbing with water in the Venturi s~rubber 7, which
permits to strip the mekal dusts entrained by the gases,
are conveyed by an exhauster 8 to a gas-holder 9 From the
latter the gases are drawn by a compressor lO,compressed to
a pressure of 20~30 kgs/sq.cm. and sent tc> the purification
station.
The purification station comprises~
- A carbonate-absorbing plant 11 wherein the ~2 con-
tents of the gases is absorbed by solvents according to
conventional methods (such as alkali-containing solutions
based on hot potassium carbonates). The high-purity C02
will be used as a blanketing gas through 6.
- An installation 12 for ab6orbing the CO values by
means of cuprammonium solutions. It is known, in fact,
that CO can easily be absorbed by aqueous solutions which
contain the cuprous copper-ammonia complex:
Cu ~- (NH3)2+ NH3+ CO = Cu + (NH3)3CO and that the solution
is regenerated according to conventional methods and permits
to obtain high-purity CO to be used as a fuel.
- A section 13 for the final scrubbing of the gas with
water or by the agency of absorbers so as to remove any
traces of ammonia as contained therein while permitting to
obtain high-purity argon which, through the tank 14, will be
recycled towards the AOD converter.
In order to prevent the accumulation of inert gases
other than argon it could be fitting to treat a pcrtion
of the gases only~ by discharging through 15 -the sca~enging
gas and a portion of the gas relative to the first blowing
stage, wherein the concentration of argon is comparatively
low.
Obviously~ there is provided in the installation a
topping-up line for ef-fecting the topping-up of the argon
~alues which had been unavoidably lost during progress of
purification.
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In the case in which the blanketing CO2 is already
available ln the installation, or if it iS desired to use
a different blanketing gas as outlined above, the discrete
decarbonating s-tage can be dipensed with (installation 11)
and CO2 can be removed simultaneously with the CO withdrawal
in the cuprammonium scrubbing stage 12, which, as is known,
is also capable of absorbing CO2.
According to still another embodiment of the method :~
according to the present invention, the purification of the
exhaust gases from the converter 1 goes on similarly to the
ahove indicated outline, with the exception that the instal-
lation 12 is a low-temperature fractionation column, which
i~ fed by the processed gas emerging from scrubbing for the
separation o~ CO2 and properly dried The column operates
in a wa~ which is wholly similar to those as used for frac-
tionation o-f air but with the paramount advantage of involv-
ing working conditions which are less drastic than those as
compulsorily required in the fractionation of air to obtain
argon, since in khe head fraction there are obtained, in
addition to CO, all the disturbing impurities and argon is
recovered in the tailings at the desired degree of purity.
Obviously, in the case of fractionation, the gas
scrubbing stage for removing as~monia and stripping hydrogen
and other impurities (installation 13) is no longer required.
As regards the separation of CO2, in addition to a
scrubblng with an absorption agent, other systems can be adop-
ted, such as molecular sieves, and in such a case -the drying
of gases prior to fractionation is no more necessary
Lastly, in the modification indicated above of the
second embodimentg the blanketing jacke-t as -ormed by the
-first gas is, in its turn? surrounded by a second protection
jacket of a second gas, whi.ch is involved in the subsequent
purification to a very small extent only, so that it has sub-
stan~ally no bearing on the progress thereof) but si.multaneously
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'l~q3~38 `: ~
prevents useless and costl~v losses of the first blanketing
gas while concurrently permitting to use an easily and cheap-
ly available gas, such as nitrogen
For the protection of the gas forming the first jacket
for protecting the suction from the AOD converter there can
be adopted several systems which can easily be envisaged by
those skilled in the art: for example, the sleeve S can be ~ -
shaped with a H-like cross-section with a central feed (per-
pendicularly to the axis of the suction hood) of the gas Eorm- ~^
ing the first suction and introduction at the two ends of the ~;
gas forming the second protection.
From the foregoing considerations it will be clear ; ~-
that the present invention is based on the principle of a
adopting a protect.ion of the exhaust gas from the converter
whlch is preferably inert relative to the exhaust gas while
being easily separable therefrom, thus enabling a large frac-
tion of the argon as initially blown into the converter to be
recovered and reused.
It is also understood that the blanketing gases are
fed to the working area which surrounds the converter mouth
under a slight overpressure, in the first place to prevent
the entrance of air and also the accidental seeping of ex-
haust gases and thus of argon.
;, ! ;
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