US3321300A

US3321300A - Degassing of metals or alloys

Info

Publication number: US3321300A
Application number: US388203A
Authority: US
Inventors: Worner Howard Knox
Original assignee: Conzinc Riotinto Ltd
Current assignee: Conzinc Riotinto of Australia Ltd; Conzinc Riotinto Ltd
Priority date: 1963-08-13
Filing date: 1964-08-07
Publication date: 1967-05-23
Anticipated expiration: 1984-05-23
Also published as: GB1023413A

Description

United States Patent 3,321,306 DEGASSING F METALS OR ALLOYS Howard Knox Werner, New Lambton, New South Wales, Australia, assigner, by mesne assignments, to Conzinc Riotinto of Australia Limited, Melbourne, Victoria, Australia, a corporation of Victoria Filed Aug. 7, 1964, Ser. No. 388,203 Claims priority, application Australia, Aug. 13, 1963, 34,188/ 63 Claims. (Cl. 75-49) This invention relates to improvements in the degassing of molten metals and alloys, and is concerned particularly with a method whereby the degassing is effected continuously.

Several methods are known for removing occluded or otherwise incorporated gases in molten metals or alloys. Practically all of these methods, even those designated continuous, are designed for degassing steel within or as delivered from individual ladles. They are therefore, in reality, semi-continuous or intermittent in Operation.

This present invention has been developed primarily for the continuous degassing of metals or alloys which are either produced continuously -or are delivered continuously from a holding furnace or from a continuous melting furnace. The method and apparatus of the invention can, however, be used in a semi-continuous manner for degassing a metal or alloy which is being transferred continuously from a ladle or other holding vessel to another holding vessel or furnace or to the tundish of a continuous casting machine. rIhe invention is particularly well suited to the degassifcation of molten steel being delivered continuously from a holding furnace to the tundish of a continuous casting unit.

The principles utilised in this invention are developed from those involved in a syphon comprising an inverted U-tube or the like having `an inlet leg connected to a first vessel containing molten metal or alloy to be degassed, an outlet leg delivering degassed metal or alloy to a second vessel, and a source of vacnum or reduced pressure connected to the U-tube.

The molten metal or alloy to be degassed is caused to fiow continously under the action of a vacuum or partial vacuum, from the first vessel, which may -be part of a holding furnace or a launder from a continuous production furnace, up over the hump of the inverted U syphon, to the second vessel, which may be part of the tundish of a continuous casting machine or a launder or a holding furnace.

As suction is applied, atmospheric pressure on the liquid in the first vessel forces it up into the inlet leg until the metal height therein plus the pressure in the unit balances the atmospheric pressure. The height of the hump is appropriately arranged to be slightly lower than the height to which the metal is lifted, so that the metal ows -across the hump and down the outlet leg of the inverted U-tube. With the vacuum Vheld more or less constant and the level of metal in the inlet leg maintained steady, the inverted U-tube then functions like a syphon, and the flow of metal in the syphon continues so long as and at the same rate as the metal is withdrawn from the second vessel into the casting machine or other device.

We have discovered that the rate of evolution of dissolved and ocoluded gases from the molten metal or alloy in the system above described bears a fairly direct relationship to the amount of surface area of liquid metal exposed to the vacuum or reduced pressure, yand the present invention is concerned, inter alia, with methods of and means for substantially increasing both the 'icc amount of liquid surface exposed to the vacuum and the time of exposure.

In one general form the invention is a method of degassing metals or alloys which comprises elevating the molten metal or alloy under reduced pressure, up a barometric inlet leg into a degassing chamber, subjecting the molten metal or alloy to turbulence and cascading :action to the degassing chamber by flowing it under gravity down an extended generally sloping surface having one or more recesses in which a pool or pools of molten metal or alloy are contained, the molten metal or alloy being caused to fiow through the said pool or pools, and causing the degassed molten metal or alloy to flow out of the degassing chamber through the Ibarometric outlet leg.

The molten metal or alloy is subjected to turbulence or cascading action during its passage through the degassing zone. The molten metal or alloy in flowing over the extended surface in the degassing zone is subjected to reduced pressure over an extended period, and maximum removal of dissolved or occluded gases from the metal or alloy is thereby effected. After its passage through the degassing zone the lmolten metal or alloy is preferably caused to cascade in the form of a thin film or in droplets within the outlet leg of a syphon.

In a further form the invention is apparatus for degassing metals or alloys which comprises a first vessel containing molten metal or alloy to be degassed, an inlet leg of ya syphon unit dipping into the said molten metal or alloy, a second vessel containing degassed molten metal or alloy, an outlet leg of the lsyphon unit dipping into the degassed molten metal or alloy, an intermediate member disposed between and communicating with the inlet leg and outlet leg, a degassing zone in the intermediate member, means for connecting the degassing zone to a source of reduced pressure, an extended generally sloping surface in the intermediate member over which the molten metal or alloy is fiowed under gravity, and one or more recesses in the extended generally sloping surface in which a pool or pools of molten metal or alloys are contained.

The syphon unit preferably comprises an inverted U-tube with the open lower ends of its inlet leg -and outlet leg adapted to dip respectively into the untreated molten metal or alloy in the first vessel (which may be a ladle, feed launder, holding furnace, or other vessel) and into the degassed molten metal or alloy in the second vessel (which may -be a'casting machine tundish, a launder, a holding vessel or the like). The outlet leg is preferably longer than the inlet leg, for reasons which will be apparent from the ensuing description. The space inside the U-tube is connected to :a source of vacuum or reduced pressure. The flow of molten metal or alloy through the syphon unit may -be continuous or intermittent.

The degassing zone of the unit may comprise an extended surface-which slopes generally from the inlet leg and which is provided with one or more recesses containing pools of molten metal or alloythrough which the molten metal or alloy iiows or cascades, the said metal or alloy being thereby subjected to turbulence so that a large surface area of said metal or alloy is exposed to reduced pressure yduring the passage of the metal or alloy over said extended surface, and the residence time of the metal or alloy in the degassing zone is thus relatively long. The lower internal surface of the hump portion of the U-tube comprising the degassing zone may be formed with transverse ridges, which both slow down the rate of flow of molten metal or alloy from the degassing zone or cascading action in the flowing liquid.

Means are also preferably provided to effect an overflow of molten metal or alloy from the degassing zone into the outow leg, and this means may comprise an overflow lip or Weir at the exit end of the said extended surface over which the molten metal or alloy is caused to cascade in the form of a thin film or in droplets as it falls to the level of the degassed molten metal or alloy in the outlet leg.

In another form the invention consists of apparatus as above described together with means for admitting into one or both of the inlet and outlet legs of the inverted U-tube a gas which has negligible solubility in and no deleterious effect on the molten metal or alloy.

The term tube as used in this specification is intended to include not only tubes hav-ing circular or oval cross sections, but also those having other cross sections, for example square or rectangular. Furthermore the tube may have `appropriate vacuum-tight joints at or near the junction ofthe inlet and outlet legs, respectively with the hump section of the inverted U, or at the other desired positions.

Different forms of the invention are shown in the accompanying drawings, wherein:

FIGURE 1 is a view in sectional elevation of one form of apparatus for the continuous degassing of molten metals or alloys,

FIGURE 2. is a view in sectional elevation of another form of such apparatus, and

FIGURE 3 is a view in sectional elevation of a further form of such apparatus.

In all of the illustrations the same reference numerals are used to indicate like or corresponding parts.

Referring to FIGURE 1, the reference numeral 5 indicates a vessel, such as a launder or holding furnace, containing molten metal or alloy 6 which is to be degassed. The vessel 7, which may be the tundish of a continuous casting machine, contains degassed metal or alloy 8.

The degassing unit comprises an inverted tubular syphon structure 9 consisting of an inlet leg 10 the lower end of which dips into the molten metal 6 in the vessel 5', a degassing zone 11 for-med within an intermediate member 12 which is connected to the upper end of the inlet leg 10, and an outlet leg 13, of greater length than the inlet leg 10, which is connected at its upper end to the lower or outlet end of the degassing zone 11, the lower end of the outlet leg 13 being arranged to dip into the degassed molten metal 8 in the vessel 7.

The syphon structure 9 may be constructed of metal and lined with refractory material.

The upper wall of the intermediate member 12 is provided with an aperture to which is connected a pipe 14 which is connected to a source of vacuum or reduced pressure.

The lower sloping surface 22' of the intermedi-ate member 12 is formed with a series of small transverse ridges 21, and a lip or overflow 16 is provided at the lower outlet end of the said intermediate member 12.

Gas inlet tubes

22 and 23 are provided by which an inert gas may be admitted into the inlet leg and outflow leg 13 respectively.

A baflie or deflecting plate 24 is provided in the degassing zone 11 which serves to prevent the in-rushing metal from splashing or spraying too far into the evacuated degassing zone 11 and possibly being sucked into the pipe 14 leading to the vacuum pumping system. If necessary, a further bafe plate 25 can be arranged adjacent to the entry to pipe 14.

A sight tube 26 is tted in the wall of the syphon unit 9, by means of which the splashing and cascading action of the molten metal can be observed.

In operation, a vacuum is applied through the pipe 14 to the interior of the syphon structure 9 and molten metal or alloy 17 to be degassed is elevated in the inlet leg 10 to a height h. The molten metal ows over from the upper end of the inlet leg into the degassing zone 11 and the metal 18 is then caused to cascade in succession over the ridges 21 under gravity until it reaches the lip or overflow 16, and the said lmetal then flows over said lip 16 and falls in the form of a lm or droplets 19 or both until it enters the body 20` of molten metal in the outlet leg 13.

During the ow of the molten metal over the ridges 21 `and through the pools of metal formed in the recesses between the ridges 21 and during its free fall in the outlet leg 13, and said metal is subjected to turbulence and/or cascading action which ensures that a large surface area of the molten metal is exposed to the reduced pressure, and that the molten metal is exposed to such reduced pressure for a substantial period, that is, there is a relatively long residence time of the metal in the degassing zone 11. By this means it has been found possible to substantially improve the efiiciency of the degassing operation.

The amount of free fall of the molten metal in the outlet -leg 13 depends on the length or height of the said outlet leg relative to that of the inlet leg 10. In the degassing of Imolten steel the inow leg 10 will usually be between 5 and 5.5 feet long, thus allowing a barometric leg height h of 56 to 57 inches when the vacuum in the system is equivalent to between 0.9. and 0.5 torr. The outflow leg 13 can conveniently be between 7 and l0 feet long. Allowing for the outlet leg 13 being immersed 0.5 foot to .1 foot in the molten steel in vessel 7 and a barometric height h of 4.75 feet, the fall of films or droplets of molten metal in the outlet leg 13 is approximately between 1.5 and 4 feet.

In this specification the term inert gas shall be understod to include not only gases like argon and helium, which are classied as inert in classical chemistry, but any other gas which has negligible solubility in the metal or alloy to be degassed and which, furthermore, will not react detrimentally with it.

When inert lgas is admitted into the inlet leg 10, it has the effect of reducing the apparent density of the metal 17 in this leg and so causes it to rise to a higher level and at greater velocity than would otherwise be the case. Under such circumstances, the metal may spray out of the opening from the inlet leg 10 into the zone 11 in the form of tine droplets. This also assists the degassing, which is also assisted by the washing action that a second gas may exert in relation to the gas or gases which it is de-v sired to remove from the liquid metal or alloy.

An inert gas admitted to the outlet leg 13 as for example at 23, acts as a type of scavenger, washing out and carrying upwards into the zone 11 any small amounts of residual occluded gas or gases which may have resisted removal by the action of cascading under vacuum in zone 11.

The inert gas may be admitted through the walls of the

refractory legs

10 and 13 by means of fluid cooled apertures (not shown). Alternatively, the inert gas may be admitted through a separate refractory pipe as shown at 23 in FIGURE l.

Another form of the invention is shown in FIGURE 2. In this form, one or more pools 27 of molten metal is -or are formed in the degassing zone 11 by appropriate depress-ions or recesses 28 in the sloping internal lower surface of the member 12. If these pools 27 have an appreciable total volume, they effectively increase the residence time of the liquid metal 18 in the degassing zone 11 and so permit greater time for gas bubble nucleation and gas bubble removal. The surfaces of the refractory `depressions 28 are preferably either porous or roughened, :since it is found that micro bubbles are more read-ily nucleated at micropores in the refractory surface than they are within the liquid metal itself or at a smooth solid surface.

As an aid to degassing of low carbon steel, finely particulate carbon 29 may be fed through an appropriately sealed tubular -powder charging device 30 into the molten stream of metal as it leaves the inlet leg 10 and flows into the zone 11. The carbon particles 29 become stirred into the bath of molten metal and in addition to providing micro-porous solid surfaces on which gas bubbles can nucleate, the carbon reacts with any oxygen ions in the Ibath forming carbon monoxide. The evolution of this -g-as helps to sweep hydrogen and nitrogen out of the molten steel.

In the form of the invention shown in FIGURE 3 a series of pools or Arecesses 27 are formed in stepewise formation by means of refractory weirs or walls 31. Apertures 32 are formed in the walls 31 through which the molten metal is caused to cascade as shown at 33 from each pool or recess 27 into the next succeeding lower pool or recess 27. The molten metal then cascades or overflows over the lip v16 into the outlet leg 13 as described above.

The dimensions of the inverted U-shaped degassing structure are determined by the quantity lof liquid metal to be degassed in any unit of time and also by the lift produced by the vacuum applied. Naturally, with any given degree of vacuum, the lift will be greater with a lower specific gravity metal. Thus, in the case of aluminium, and Without inert gas admission, the lift would need to be over 21/2 times that required for molten steel. For this reason, the application of this invention to degassing of aluminium requires ample insulation around the legs and 13 and the intermediate member 12, otherwise lheat losses may be too great. The use of chlorine gas injection into the inlet leg 10, and possibly also into the outlet leg 13, makes possible a considerable reduction -in the height necessary for adequate degassing of aluminium.

'I'Io counteract heat losses it is possi-ble, and may in some cases be desirable, to apply heat electrically to the molten metal as it flows up the inlet leg 10 and/or as it passes through the zone 11. This can conveniently be done by means of induction coils or by resistance elements surrounding or appropriately embedded in the refractory material of the degassing unit.

The refractory material used should he a dense variety and appropriate to the metal or alloy being degassed. To assist in forming an adequate seal, the refractory is conveniently encased in steel or other appropriate metal, except at the bottom ends of the

legs

10 and 13 where they dip into the molten untreated metal 6 and the degassed metal 8, respectively. It is appropriate to iirst dip these lower ends of the inlet and outlet =

legs

10 and 13 into a ladle or vessel (not shown) containing an appropriate slag or glass. This coating helps both to protect and to seal the refractory at these positions.

The inverted U-shaped degassing unit may be supported along the length of the intermediate member 12 by a steel cage (not shown). Alternatively, rings (not shown) may be welded to the steel casing at appropriate positions so that hooks and chains (not shown) can be attached for the purpose of lowering the degassing equipment into position and adjusting its height and the angle of inclination of the intermediate member 12 during operation.

When degassing -is complete or some repair operation necessitates terminating the flow, a valve (not shown) in the vacuum line 14 can be slowly opened, and air gradually admitted to the degassing unit, allowing the metal in the inlet and

outlet legs

10 and 13 to iiow back into the

vessels

5 and 7 respectively. In the form of the invention incorporating pools 27 in the degassing zone 11, (see FIGURE 2) it may be necessary to tilt the degassing unit to discharge the metal remaining in the pools 27 before the apparatus is lifted out of position.

In starting up the ydegassing unit it is of course necessary to have a quantity of the liquid metal or alloy 8 in the launder, tundish or other vessel 7 .so that an effective seal is achieved in the outow leg 13.

It is possible, and in some applications advantageous, to maintain a protective layer of slag on the molten metal 6 or alloy 6 in the vessel 5. If the level ofthe entry tothe inlet leg 10 is Well down in the molten metal 6, no slag will be sucked in with the metal and thus slag-free and degassed metal 8 will be discharged into the vessel 7.

The advantages of the invention are:

(i) Its relative simplicity,

(ii) Its applicability vto the degassing of molten metal or alloy being produced continuously,

(iii) Its suitability for interposition between a metalproducing furnace and a continuous casting machine,

(iv) Its ease of removal and replacement.

In connection with (iii), it may in some cases be advantageous to have two or more degassing units operating in parallel, these being fed from the one production furnace and discharging the degassed product into the one continuous casting machine. Should one degassing unit need to be replaced, there is a continuing delivery of degassed metal or alloy via the other unit(s).

The following example illustrates the invention:

Low carbon (0.12% C) semi-killed steel melted under Oxy-oil burners and deliverately contaminated with hydrogen and nitrogen was degassed in a degassing unit of the type shown in FIGURE 1 with the degassing zone 11 being 3 feet long and with a vacuum of 0.5 torr and a free fall in the outow leg of approximately 3 feet.

Before degassin-g, the steel contained 0.011% nitrogen and 5 parts per million of hydrogen. After degassing, the steel contained 0.003% nitrogen and 2.5 parts per million of hydrogen. With larger equipment, high vacuum and by use of inert gas injection, the nitrogen and hydrogen contents can be reduced to substantially lower values.

I claim:

1. A method for degassing molten metallic material which comprises elevating the molten metallic material under reduced pressure up a barometric inlet leg into a reduced pressure degassing chamber, subjecting the molten metallic material to turbulence and cascading action for exposing a large surface area to the reduced pressure in the degassing chamber by ilowing it under gravity in a relatively thin iilm over at least one ridge in the surface of the bottom of the degassing chamber, holding the thus exposed molten material for a relatively long time in said degassing chamber by flowing it into at least one recess therein which has an appreciable total volume to form a pool of molten metallic material for permitting greater time for gas bubble nucleation and gas bubble removal, the molten metallic material being caused to iow through the said pool, and causing the degassed molten metallic material to flow out of the recess over a further ridge and out of the degassing chamber through the barometric outlet leg.

2. A method according to claim 1 wherein the metallic material is caused to overow from the degassing zone into the barometric outlet leg and to fall freely in the form of droplets or film into the molten metallic material in said barometric outlet leg.

3. Apparatus for degassing molten metallic material which comprises a lirst vessel containing molten metallic material to be degassed, a syphon unit having an inlet leg dipping into the said molten metallic material, a second vessel containing degassed molten metallic material, the syphon unit having an outlet leg dipping into the degassed molten metallic material in the second vessel, said syphon unit having an intermediate member disposed between and communicating with the inlet leg and outlet leg, a degassing zone in the intermediate member, means on said intermediate member for connecting the degassing zone to a source of reduced pressure, an extended generally sloping surface in the intermediate member over which the molten metal or alloy is owed under gravity, said intermediate member having at least two ridges thereon and at least one recess of appreciable total volume in the extended generally sloping surface between said ridges in which a pool of molten metallic material is contained.

4. Apparatus according to claim 3 and having means in said intermediate member for causing the molten metallic material to cascade into and out of each pool.

5. Apparatus according to claim 3 and having a plurality of ridges on the extended surface between which the recess is formed.

6. Apparatus according to claim 3 wherein the outlet leg of the syphon unit is longer than the inlet leg.

7. Apparatus according to claim 3 and having means in said intermediate member for causing the molten metallic material to cascade in the form of droplets or ilm within the outlet leg onto the molten metal or alloy in said outlet leg.

8. Apparatus according to claim 3 and having an overflow weir at the lower end of the extended generally sloping surface over which the molten metallic material cascades into the outlet leg.

9. Apparatus according to claim 3 and having means connected to said inlet leg and said outlet leg for admitting inert gas to the metal or alloy in the inlet leg and outlet leg.

10. Apparatus according to claim 3 and having means in said intermediate member for admitting particulate material for the degassing zone.

References Cited by the Examiner UNITED STATES PATENTS 1,554,368 9/1925 Rackoi et al. 266-34 X 1,921,060 8/ 1933 Williams 75-49 2,054,922 9/ 1936 Betterton et al. 266-34 2,587,793 3/1952 Waldron 266-34 X 3,022,059 2/ 1962 Aarders 75-49 3,202,409 8/ 1965 Farrer 266-34 3,230,074 1/1966 Roy et al. 266-34 X HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

H. W. TARRING, Assistant Examiner.

Claims

1. A METHOD FOR DEGASSING MOLTEN METALLIC MATERIAL WHICH COMPRISES ELEVATING THE MOLTEN METALLIC MATERIAL UNDER REDUCED PRESSURE UP A BAROMETRIC INLET LEG INTO A REDUCED PRESSURE DEGASSING CHAMBER, SUBJECTING THE MOLTEN METALLIC MATERIAL TO TURBULENCE AND CASCADING ACTION FOR EXPOSING A LARGE SURFACE AREA TO THE REDUCED PRESSURE IN THE DEGASSING CHAMBER BY FLOWING IT UNDER GRAVITY IN A RELATIVELY THIN FILM OVER AT LEAST ONE RIDGE IN THE SURFACE OF THE BOTTOM OF THE DEGASSING CHAMBER, HOLDING THE THUS EXPOSED MOLTEN MATERIAL FOR A RELATIVELY LONG TIME IN SAID DEGASSING CHAMBER BY FLOWING IT INTO AT LEAST ONE RECESS THEREIN WHICH HAS AN APPRECIABLE TOTAL VOLUME TO FORM A POOL OF MOLTEN METALLIC MATERIAL FOR PERMITTING GREATER TIME FOR GAS BUBBLE NUCLEATION AND GAS BUBBLE REMOVAL, THE MOLTEN METALLIC MATERIAL BEING CAUSED TO FLOW THROUGH THE SAID POOL, AND CAUSING THE DEGASSED MOLTEN METALLIC MATERIAL TO FLOW OUT OF THE RECESS OVER A FURTHER RIDGE AND OUT OF THE DEGASSING CHAMBER THROUGH THE BAROMETRIC OUTLET LEG.

3. APPARATUS FOR DEGASSING MOLTEN METALLIC MATERIAL WHICH COMPRISES A FIRST VESSEL CONTAINING MOLTEN METALLIC MATERIAL TO BE DEGASSED, A SYPHON UNIT HAVING AN INLET LEG DIPPING INTO THE SAID MOLTEN METALLIC MATERIAL, A SECOND VESSEL CONTAINING DEGASSED MOLTEN METALLIC MATERIAL, THE SYPHON UNIT HAVING AN OUTLET LEG DIPPING INTO THE DEGASSED MOLTEN METALLIC MATERIAL IN THE SECOND VESSEL, SAID SYPHON UNIT HAVING AN INTERMEDIATE MEMBER DISPOSED BETWEEN AND COMMUNICATING WITH THE INLET LEG AND OUTLET LEG, A DEGASSING ZONE IN THE INTERMEDIATE MEMBER, MEANS ON SAID INTERMEDIATE MEMBER FOR CONNECTING THE DEGASSING ZONE TO A SOURCE OF REDUCED PRESSURE, AN EXTENDED GENERALLY SLOPING SURFACE IN THE INTERMEDIATE MEMBER OVER WHICH THE MOLTEN METAL OR ALLOY IS FLOWED UNDER GRAVITY, SAID INTERMEDIATE MEMBER HAVING AT LEAST TWO RIDGES THEREON AND AT LEASST ONE RECESS OF APPRECIABLE TOTAL VOLUME IN THE EXTENDED GENERALLY SLOPING SURFACE BETWEEN SAID RIDGES IN WHICH A POOL OF MOLTEN METALLIC MATERIAL IS CONTAINED.