CA1160017A - Process for minimizing foam formation during free falling of molten metal into moulds, launders or other containers - Google Patents
Process for minimizing foam formation during free falling of molten metal into moulds, launders or other containersInfo
- Publication number
- CA1160017A CA1160017A CA000358379A CA358379A CA1160017A CA 1160017 A CA1160017 A CA 1160017A CA 000358379 A CA000358379 A CA 000358379A CA 358379 A CA358379 A CA 358379A CA 1160017 A CA1160017 A CA 1160017A
- Authority
- CA
- Canada
- Prior art keywords
- molten metal
- zinc
- foam
- moulds
- oxygen
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
- B22D21/02—Casting exceedingly oxidisable non-ferrous metals, e.g. in inert atmosphere
- B22D21/027—Casting heavy metals with low melting point, i.e. less than 1000 degrees C, e.g. Zn 419 degrees C, Pb 327 degrees C, Sn 232 degrees C
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
- B22D27/003—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting by using inert gases
Landscapes
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Manufacture And Refinement Of Metals (AREA)
Abstract
ABSTRACT
A process for minimizing foam formation on the top surface of molten metal during free falling of molten metal, such as molten zinc, into moulds, launders or other containers is disclosed. The process comprises the step of maintaining the molten metal during the free falling thereof under an essentially non-oxidizing atmosphere, so as to prevent entrainment of sufficient oxygen into the molten metal by the falling stream to form excessive quantities of bubbles having a tenacious oxidized film and which do not collapse when they float to the surface of the molten metal but rather produce undesirable foam on the surface of molten metal.
A process for minimizing foam formation on the top surface of molten metal during free falling of molten metal, such as molten zinc, into moulds, launders or other containers is disclosed. The process comprises the step of maintaining the molten metal during the free falling thereof under an essentially non-oxidizing atmosphere, so as to prevent entrainment of sufficient oxygen into the molten metal by the falling stream to form excessive quantities of bubbles having a tenacious oxidized film and which do not collapse when they float to the surface of the molten metal but rather produce undesirable foam on the surface of molten metal.
Description
PROCESS FOR MINIMIZING FOAM FO~TION
.
DUR,ING FREE~FALLING OF MOLTEN METAL
INTO MOULDS, L~UNDER';: OR OTHE~ CONTAINERS
This invention relates to a process for mini-mizing foam formation on the top sur~ace of molten metal during pouring of the metal :in~o moulds or similar con-tainers, or during free falling of molten metal from a furnace to a launder, or cascading of the metal from launder to launder.
During pouring or free falling of molten metal, such as zinc, a considerable amount of meta.llis,foam is formed on the top surface of the molten metal. This foam is normally skimmed off the surface of the molten metal manually. The task is arduous, requires workers to be in close proximity to molten metal and produces significant amounts of skimmings from which the metal must be reclaimed.
Experiments carried out by the applicant during pouring of molten zinc into moulds have revealed that the formation of foam was due to the entrainment of air by the falling stream of molten metal~ Hence air is trans-ported below the molten metal surfaoe to form air bubbles within the molten metal~ Due to oxidation, a thin but tenacious zinc oxide film is formed on the inside surface of the bubbles'O These bubbles rise to the surface and, as they emerge at the surfaoe, it is observed that the outer surface of the bubbles is also oxidized and, in the case of larye bubbles (.approximately ~: inch diameter), the emerging upper skin of the bubble may freeze immediately although the metal pool beneath it remains li~uid for several minutes longer.
Having realizad that the foam ~ormed on top of the molten metal is due to the formation of bubbles consisting of a tenacious ZillC oxide film on the surface of the bu~bles, applicant investlgated several methods by which the bubbles could be either released or prevented from forming. As a result of such in~estigation, applicank has discovered, in accordance with the in~ent~on, that foam formation may be minimized by maintaining the molten metal, during filling of the mould, or frea falling of the molten metal into launders or other containers, under an essentially non-oxidizing atmosphere with respect to the molten metal so as to prevent entrainment of sufficient oxygen into the molten metal by the falling stream to form an excessive amount of bubbles having a tenacious oxidized film and which do not collapse when they float to the surface of the molten metal but rather produce undesirable foam on such surfaceO
The non-oxi izing atmosphere is preferably provided by an inert gas, such as nitrogen, and it can contain a small quantity of oxygen~ e.g. up to 2%, without producing ex~essive quantities of bubbIesO
The a~ove disclosed method may be carried out by placing a cover over the top of the molten metal container, such cover having an aperture therein for passage of the molten metal and means for introducing a non-oxidizing gas under the cover. The invention could . .
~61~ 17 also be carried out on a continuous slab casting machine by providing a hood over th.e casting machine at th.e filling station and would have an opening for introducing the molten metal through. one of the walls of the hood.
The invention will now be disclosed with reer-ence to an apparatus used to carry out laboratory ex-periments which is illustrated in the accompanying drawing.
The apparatus comprises a bottom-pouring tun-dish 10 which is used to feed molten zinc into a slab mould 12 through a cover 14 closing the top of the mould.
The ~ottom of the tundish is provided with an opening registering with. a corresponding opening in the cover 14 and is sealed to the cover by any suita~le means such as by welding. The openiny in the bottom of the tundish is closed ~y a plug valve 16 wh.ich may be opened when it is desired to pour molten metal into the mould. The cover is sealed to the mould by 'O' ring 18. A m~tered nitrogen inlet 20 and exhaust vent 22 are provided through the cover to maintain a suitable non-oxidizing atmosphere - 20 within and a~ove the mould.
A simple experimental procedure was carried out involving filling the tundis~ with molten zinc, purging the covered mould and the space above it with nitrogen and then opening the ~alve to fill the mould. The mould was uncovered soon after filling, ~efore solidification to allow freezing of metal in air. A series of casting trials were carried out while varying the metal casting temperature and the concentration o~ oxygen in t~e nitrogen atmosphere~ .
.
6~17 The effects of varying metal casting tempera-ture were obser~ed in the normal zinc casting temperature range o 440-530Co Purging was carried out at gas flow rates of 20 l/min. for one minute with nitrogen atmospheres ranging from commercial purity to concentrations o 2%
oxygen. Foam-free sur~aces on slab ingots were obtained with concentrations of oxygen in nitrogen varying from 0 to about 2%. It was also obsèrved that the effects of temperature and oxygen are interactive as far as the production of acceptable slab surface is concerned. The conditions which produce acceptable surfaces on slab ingots are summarized below:
(a2 Commercially~pure nitrogen atmosphere at temperatures less than 450C. These conditions produce a brilliant, crystalline surface which is visible through a fully transparent oxide film. At temperatures higher than 450C the phenomenon commonly referred to as "colouration" in which colours ranging from straw to dark purple was observed.
0 Cbl Oxygen concentrations of approximately 2~ at tempera-tures in the range of 450-475C. These conditions result in a smooth sil~ery oxide ~ilm at the ingot surfaceO
The invention may also be carried out on a continuous slab casting machine, such as the Sheppard casting machine which has a number of moulds mounted on an endless conveyor chain. On such machines, the non-oxidizing atmosphere may be provided within a hood surrounding the cas~ing machine at the filling statlon. The hood would normally extend over a num~er of moulcls so as to permit the molten m~tal in the moulds to cool below a predetermined tem-perature before emerging from the hood in order to control the formation of zinc oxide on the surface of the metal.
Molten metal would be fed from a furnace to a pouring ladle located inside the hood and from the pouring ladle into the moulds as they move past the filling station under the non-oxidizing atmosphere formed inside the hood and within the moulds. A metered nitrogen inlet and exhaust vent would be provided through the hood to ~orm the non-oxidizing atmosphere in the hood. The nitrogen atmosphere within the hood must be maintained at a slight positive pressure such that the ambient oxidizing atmos-phere outside the hood cannot enter the hood through themould entrance and exit ports~ Curtains are preferably pro~ided where the moulds enter and exit the hood to prevent excessive loss of nitrogen gas.
Although the method in accordance with the present inventi~on has been disclosed in association with a speciic apparatus, it is to be understood that it could be carried out by other apparatus including various types of continuous casting machines and that the invention is not limited to carrying out the novel method with the apparatus disclosed. Also, in some casting operations, the molten metal contains on its surface a su~stantial amount of floating i:mpurities called "dross". Such dross is usually not removed separately from the foam but rather forms a part of the total skimmings which are manuall~ xemoved from the surface of the molten metal after pouring into moulds, launders or similar containers. In the present casting operation, such dross would have to be removed prior to pouring into moulds, launders or other containers.
.
DUR,ING FREE~FALLING OF MOLTEN METAL
INTO MOULDS, L~UNDER';: OR OTHE~ CONTAINERS
This invention relates to a process for mini-mizing foam formation on the top sur~ace of molten metal during pouring of the metal :in~o moulds or similar con-tainers, or during free falling of molten metal from a furnace to a launder, or cascading of the metal from launder to launder.
During pouring or free falling of molten metal, such as zinc, a considerable amount of meta.llis,foam is formed on the top surface of the molten metal. This foam is normally skimmed off the surface of the molten metal manually. The task is arduous, requires workers to be in close proximity to molten metal and produces significant amounts of skimmings from which the metal must be reclaimed.
Experiments carried out by the applicant during pouring of molten zinc into moulds have revealed that the formation of foam was due to the entrainment of air by the falling stream of molten metal~ Hence air is trans-ported below the molten metal surfaoe to form air bubbles within the molten metal~ Due to oxidation, a thin but tenacious zinc oxide film is formed on the inside surface of the bubbles'O These bubbles rise to the surface and, as they emerge at the surfaoe, it is observed that the outer surface of the bubbles is also oxidized and, in the case of larye bubbles (.approximately ~: inch diameter), the emerging upper skin of the bubble may freeze immediately although the metal pool beneath it remains li~uid for several minutes longer.
Having realizad that the foam ~ormed on top of the molten metal is due to the formation of bubbles consisting of a tenacious ZillC oxide film on the surface of the bu~bles, applicant investlgated several methods by which the bubbles could be either released or prevented from forming. As a result of such in~estigation, applicank has discovered, in accordance with the in~ent~on, that foam formation may be minimized by maintaining the molten metal, during filling of the mould, or frea falling of the molten metal into launders or other containers, under an essentially non-oxidizing atmosphere with respect to the molten metal so as to prevent entrainment of sufficient oxygen into the molten metal by the falling stream to form an excessive amount of bubbles having a tenacious oxidized film and which do not collapse when they float to the surface of the molten metal but rather produce undesirable foam on such surfaceO
The non-oxi izing atmosphere is preferably provided by an inert gas, such as nitrogen, and it can contain a small quantity of oxygen~ e.g. up to 2%, without producing ex~essive quantities of bubbIesO
The a~ove disclosed method may be carried out by placing a cover over the top of the molten metal container, such cover having an aperture therein for passage of the molten metal and means for introducing a non-oxidizing gas under the cover. The invention could . .
~61~ 17 also be carried out on a continuous slab casting machine by providing a hood over th.e casting machine at th.e filling station and would have an opening for introducing the molten metal through. one of the walls of the hood.
The invention will now be disclosed with reer-ence to an apparatus used to carry out laboratory ex-periments which is illustrated in the accompanying drawing.
The apparatus comprises a bottom-pouring tun-dish 10 which is used to feed molten zinc into a slab mould 12 through a cover 14 closing the top of the mould.
The ~ottom of the tundish is provided with an opening registering with. a corresponding opening in the cover 14 and is sealed to the cover by any suita~le means such as by welding. The openiny in the bottom of the tundish is closed ~y a plug valve 16 wh.ich may be opened when it is desired to pour molten metal into the mould. The cover is sealed to the mould by 'O' ring 18. A m~tered nitrogen inlet 20 and exhaust vent 22 are provided through the cover to maintain a suitable non-oxidizing atmosphere - 20 within and a~ove the mould.
A simple experimental procedure was carried out involving filling the tundis~ with molten zinc, purging the covered mould and the space above it with nitrogen and then opening the ~alve to fill the mould. The mould was uncovered soon after filling, ~efore solidification to allow freezing of metal in air. A series of casting trials were carried out while varying the metal casting temperature and the concentration o~ oxygen in t~e nitrogen atmosphere~ .
.
6~17 The effects of varying metal casting tempera-ture were obser~ed in the normal zinc casting temperature range o 440-530Co Purging was carried out at gas flow rates of 20 l/min. for one minute with nitrogen atmospheres ranging from commercial purity to concentrations o 2%
oxygen. Foam-free sur~aces on slab ingots were obtained with concentrations of oxygen in nitrogen varying from 0 to about 2%. It was also obsèrved that the effects of temperature and oxygen are interactive as far as the production of acceptable slab surface is concerned. The conditions which produce acceptable surfaces on slab ingots are summarized below:
(a2 Commercially~pure nitrogen atmosphere at temperatures less than 450C. These conditions produce a brilliant, crystalline surface which is visible through a fully transparent oxide film. At temperatures higher than 450C the phenomenon commonly referred to as "colouration" in which colours ranging from straw to dark purple was observed.
0 Cbl Oxygen concentrations of approximately 2~ at tempera-tures in the range of 450-475C. These conditions result in a smooth sil~ery oxide ~ilm at the ingot surfaceO
The invention may also be carried out on a continuous slab casting machine, such as the Sheppard casting machine which has a number of moulds mounted on an endless conveyor chain. On such machines, the non-oxidizing atmosphere may be provided within a hood surrounding the cas~ing machine at the filling statlon. The hood would normally extend over a num~er of moulcls so as to permit the molten m~tal in the moulds to cool below a predetermined tem-perature before emerging from the hood in order to control the formation of zinc oxide on the surface of the metal.
Molten metal would be fed from a furnace to a pouring ladle located inside the hood and from the pouring ladle into the moulds as they move past the filling station under the non-oxidizing atmosphere formed inside the hood and within the moulds. A metered nitrogen inlet and exhaust vent would be provided through the hood to ~orm the non-oxidizing atmosphere in the hood. The nitrogen atmosphere within the hood must be maintained at a slight positive pressure such that the ambient oxidizing atmos-phere outside the hood cannot enter the hood through themould entrance and exit ports~ Curtains are preferably pro~ided where the moulds enter and exit the hood to prevent excessive loss of nitrogen gas.
Although the method in accordance with the present inventi~on has been disclosed in association with a speciic apparatus, it is to be understood that it could be carried out by other apparatus including various types of continuous casting machines and that the invention is not limited to carrying out the novel method with the apparatus disclosed. Also, in some casting operations, the molten metal contains on its surface a su~stantial amount of floating i:mpurities called "dross". Such dross is usually not removed separately from the foam but rather forms a part of the total skimmings which are manuall~ xemoved from the surface of the molten metal after pouring into moulds, launders or similar containers. In the present casting operation, such dross would have to be removed prior to pouring into moulds, launders or other containers.
Claims (9)
1. A process for minimizing foam formation on the surface of molten metal which forms a foam during free falling of the molten metal into moulds, launders or other containers, so as to eliminate the need for skimming, which comprises maintaining said molten metal during free falling thereof under an essential non-oxidizing atmosphere so as to prevent entrainment of sufficient oxygen into the molten metal by the falling stream to form an excessive amount of bubbles having a tenacious oxidized film and which do not collapse when they float to the surface of the molten metal but rather produce undesirable foam on said surface.
2. A process as defined in claim 1 wherein said non oxidizing atmosphere is an inert gas atmosphere.
3. A process as defined in claim 2, wherein the inert gas is nitrogen.
4. A process as defined in claim 1, 2 or 3, wherein the non-oxidizing atmosphere contains a small quantity of oxygen.
5. A process as defined in claim 1, 2 or 3, wherein the molten metal is zinc.
6. A process as defined in claim 1, wherein a metal body substantially free of surface foam is produced and allowed to solidify in air to a solid shape.
7. A process as defined in claims 1 or 6, wherein the molten metal is zinc and the gas atmosphere contains approximately 2% oxygen and the temperature of the molten zinc is maintained between 450° to 475°C.
8. A process as defined in claims 1 or 6, wherein the molten metal is zinc maintained at a temperature less than 450°C and the non-oxidizing atmosphere is essentially free of oxygen.
9. A process as defined in claims 1 or 6, wherein, when the molten metal contains on its surface floating impurities, said impurities are removed prior to free falling the molten metal into said container.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000358379A CA1160017A (en) | 1980-08-15 | 1980-08-15 | Process for minimizing foam formation during free falling of molten metal into moulds, launders or other containers |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000358379A CA1160017A (en) | 1980-08-15 | 1980-08-15 | Process for minimizing foam formation during free falling of molten metal into moulds, launders or other containers |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1160017A true CA1160017A (en) | 1984-01-10 |
Family
ID=4117659
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000358379A Expired CA1160017A (en) | 1980-08-15 | 1980-08-15 | Process for minimizing foam formation during free falling of molten metal into moulds, launders or other containers |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1160017A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4576220A (en) * | 1983-12-23 | 1986-03-18 | Noranda Inc. | Method and apparatus for maintaining an atmosphere around a predetermined portion of an endless discrete object conveyor |
| WO1997021510A1 (en) * | 1995-12-14 | 1997-06-19 | Commonwealth Scientific And Industrial Research Organisation | Ingot mould system |
-
1980
- 1980-08-15 CA CA000358379A patent/CA1160017A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4576220A (en) * | 1983-12-23 | 1986-03-18 | Noranda Inc. | Method and apparatus for maintaining an atmosphere around a predetermined portion of an endless discrete object conveyor |
| WO1997021510A1 (en) * | 1995-12-14 | 1997-06-19 | Commonwealth Scientific And Industrial Research Organisation | Ingot mould system |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |