CA1073188A - Method for recovering argon as contained in the blowing gases coming from an a.o.d.-converter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE.
In order to recover the costly gas argon from the ex-hausts of an AOD converter as used in refining stainless steels, a method is disclosed which provides at least one gas blanket around the exhaust gas stream. The blanketing gas is preferably carbon dioxide, but also argon itself and steam, or their mixtures, can be used, it being re-quired that the blanketing gas be easily separable from argon in order to recover the latter and to recycle it.
Scrubbing and purification stages can also be additionally provided.

Description

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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 ~73~

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 ~731~

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 ~L~'73~

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 ~731~

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 '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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Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method for recovering the argon as contained in exhaust gases from an A.O.D. 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 jacket of a blanketing gas 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 argon is separated from the exhaust gases.

2. A method according to claim 1, which includes the steps of:
i) suctioning from the converter mouth, exhaust gases comprising argon and carbon monoxide, with simultaneous protection of the forced suction by means of at least one jacket of said blanketing gas, carbon dioxide, argon, steam and their admixtures;
ii) cooling the gas and stripping the gas-borne dusts, iii) separating CO2, and iv) separating the components lighter than argon.

3. A method according to claim 2, wherein said exhaust gases further comprise carbon dioxide.

4. A method according to claim 2 or 3, wherein said blanketing gas is fed to a zone surrounding the forced suction under a slight overpressure relative to the pressure of the surrounding ambient air.

5. A method according to claim 2 or 3, wherein said separating of CO2 comprises a scrubbing with a solvent means for absorbing CO2.

6. A method according to claim 2 or 3, wherein said separating of the components lighter than argon consists of a scrubbing with cuprammonium, effective to separate CO from the gases emerging from step iii), the gas being sub-sequently subjected to washing for removal of NH3 and to a hydrogen-removing treatment.

7. A method according to claim 2 or 3, wherein said separating in step iv) comprises a low temperature fractionation of the previously dried gas coming from the separation step iii) under such working conditions that all the components lighter than argon are collected in a head fraction.

8. A method according to claim 1, wherein the suctioning of the exhaust gases coming from the converter is protected by a double jacket of first and second blanketing gases, the inner jacket being formed by a first blanketing gas inert relative to the converter exhaust gases, selected from said group, and an outer jacket of nitrogen gas.

9. A method according to claim 8, wherein said first blanketing gas is carbon dioxide.

10. A method for recovering argon from exhaust gases of an A.O.D. converter characterized by the steps of: forcing suction of the exhaust gases under the protection of a blanketing gas which can easily be separated from argon and selected from the group consisting of carbon dioxide, argon, steam and their admixtures; scrubbing the resulting gases to cool the gases and stripping the dust therefrom, scrubbing the thus obtained gases with a CO2-absorbing liquor, washing the thereby obtained gases with a cuprammonium solution effective to strip CO from the gases, scrubbing out ammonia from the resultant gases, treatment of the gases for removing hydrogen by combustion, drying the gases and recycling recovered argon to the A.O.D. converter.

11. A method according to claim 10, wherein said blanket-ing gas is CO2 and the CO2 stripped from the gases is recycled to said protection of the suction of the converter exhaust gases

12. A method for recovering argon from the exhaust gases of an A.O.D. converter, characterized by the steps of forcing suction of the exhaust gases under the protection of a gas, which can easily be separated from argon, selected from CO2, argon, steam and their admixtures, scrubbing the gases for cooling same and stripping the dusts, scrubbing the gases with a CO2-absorbing means, drying the gases, fractioning the gases at a low temperature with separation from argon of all the components lighter than argon, and recycling recovered argon to the A.O.D. converter.