FI73373C - FOERFARANDE OCH ANORDNING FOER MINIMERING AV SKUMBILDNING VID SMAELT METALLS FRIA FALL IN I FORMAR, TRAOG OCH MOTSVARANDE KAERL. - Google Patents

Description

1 73373

A method and apparatus for minimizing the formation of foam when molten metal falls freely into molds, troughs and similar containers. -Forfarande och anordning för minimering av skumbildning vid smält metals from fria fall in i formar, träg och motsvarande Kari.

The present invention relates to a device for minimizing the formation of foam on the upper surface of molten metal when metal is poured into molds or similar containers or when molten metal falls freely from an oven to a trough or when metal falls from one trough to another.

When molten metal, such as zinc, is poured or dropped freely, a considerable amount of metal foam is formed on the top surface of the molten metal. This foam is normally peeled from the surface of the molten metal. This work is laborious, requires workers close to molten metal and produces large amounts of metal foam that needs to be recovered.

Experiments performed by the applicant when pouring molten zinc into molds have shown that the formation of foam was due to a falling stream of molten metal trapping air with it. In this case, air moves below the surface of the molten metal, forming air bubbles in the molten metal. Due to oxidation, a thin but tough zinc oxide Ivo is formed on the inner surface of the bubbles. These bubbles rise to the surface and when they protrude on the surface it can be seen that the outer surface of the bubbles also oxidizes and in the case of large bubbles (diameter about 12.7 mm) it can be said that part of the bubble top can solidify immediately despite the metal material remaining below it. liquid for several minutes longer.

Realizing that the foam formed on molten metal is due to the formation of a zinc oxide film on the surface of the bubbles, the Applicant investigated several methods by which bubbles could either be released or prevented from forming. As a result of this study, the Applicant has realized that foam formation can be minimized by keeping molten metal when filling a mold or molten 2 73373 metal falling freely into troughs or other vessels in a substantially non-oxidizing atmosphere, preventing the falling stream from capturing enough oxygen to molten metal to form too much oxygen. bubbles with an oxidized film which do not disintegrate as they rise to the surface of the molten metal but instead form an undesired foam on that surface.

The non-oxidizing atmosphere is preferably formed with an inert gas such as nitrogen and may contain a small amount of oxygen, e.g. up to 2% without forming excessive amounts of bubbles.

The above method can be performed by placing a lid on a container of molten metal and making an opening therein for the passage of molten metal and means for supplying non-oxidized gas under the lid.

The above method could also be performed in a continuous slab casting machine comprising a series of adjacent and top-open ingot molds mounted on an endless chain conveyor by arranging a canopy over the casting machine at a filling station and supplying non-oxidized gas under the canopy. Seals would be required at the inlet and outlet ends of the hood and on its sides to prevent excessive loss of non-oxidized gas. The hood should, of course, have a sealed opening to deliver molten metal through one of its walls.

The above method could further be carried out in a continuous slab casting machine by means of the device disclosed in EP application 88 071, in which the device comprises a canopy or housing placed at short distances above the moving molds and arranged to cover two adjacent molds. At the top of the housing there is an opening for conducting the melt into the mold as well as an opening on the side for conducting non-oxidized gas under the housing. However, this device does not allow adequate distribution of the non-oxidized gas in the molds.

The device according to the invention comprises a cover plate which is

Il: 3 73373 placed at a predetermined distance above a plurality of vessels or ingots and having an opening for pouring molten metal into each vessel as they pass under the opening. The lid plate extends a predetermined distance to the front and rear of said opening and has a predetermined number of longitudinally spaced holes for supplying non-oxidized gas through the lid plate to progressively form an oxidized atmosphere in the containers as they approach the lid opening and maintaining this atmosphere in the vessels.

It is necessary for the cover plate to extend a certain distance in front of the mold filling opening to progressively form the required non-oxidizing atmosphere, and likewise the cover plate must have a back to maintain the necessary non-oxidizing atmosphere. The length of the front end is determined by the speed of the conveyor belt, the volume of the vessel, the vessel to cover the gap, and the effect of these factors on the volume of purge gas required to provide the desired atmosphere. The length of the outlet head is determined by the pneumatic resistance required to prevent the return of air to the container to be filled.

With a conveyor belt speed of 5 cm per second and filling of about 25 kg ingot molds, it has been found that oxygen concentrations of less than 0.5% can be achieved with cells up to 7.6 mm and gas flow rates below 57 Nm / h using a plate equal to five molds. width (three before and two after the mold opening).

The invention will now be explained by way of example with reference to the accompanying drawings, in which:

Figure 1 shows an apparatus for performing laboratory tests using a solid slab mold.

Figure 2 is an example of an apparatus used to cast molten metal into individual molds in a continuous ingot casting machine.

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Figure 3 is a section along the line 3-3 in Figure 2.

Figure 4 is a section along line 4-4 of Figure 2.

Figure 5 is a graphical representation of the effect of shoulder sizing change on oxygen concentration in the mold atmosphere with fixed nitrogen consumption.

Figure 6 is a graphical representation of the effect of the change in nitrogen consumption on the oxygen content in the mold atmosphere with two different solder dimensions.

Figures 7 and 8 show embodiments of a mold to reduce the gas flow requirement.

Figure 9 shows another possible embodiment which reduces the gas flow but comprises a non-abrasive seal.

The laboratory apparatus shown in Figure 1 includes a bottom casting vessel 10 used to feed molten zinc into the slab mold 12 through a lid 14 that closes the top of the mold. The bottom of the container is provided with an opening which is aligned with the corresponding opening in the lid 14 and said base is fixed to the lid in some suitable way, such as by welding. The opening in the bottom of the vessel is closed by a plug valve 16 which can be opened if desired to pour molten metal into a mold. The lid is sealed to the mold by an O-ring 18. A dimensioned nitrogen inlet duct 20 and an exhaust duct 22 are provided through the lid to maintain a suitable non-oxidizing atmosphere on the upper surface of the mold.

The preparatory test was performed by filling the vessel with molten zinc and then opening the valve to fill the mold. Quite a large amount of foam formed on the surface of the molten metal. The same procedure was repeated without locking the fact that the covered mold was purged with nitrogen before opening the valve to fill the mold. The mold was opened shortly after filling before solidification to cause the metal to solidify in air. There was no foam on the surface of the molten metal.

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A series of casting experiments were performed while varying the casting temperature of the metal and the oxygen content of the nitrogen. The effects of the change in the casting temperature of the metal were observed in the normal zinc casting temperature range of 460-530 ° C. Purification or purging was performed at gas flow rates of 20 1 / min. For 1 minute with nitrogen atmospheres ranging from commercial purity to 2% oxygen concentrations. Foamless surfaces were obtained for the slabs with nitrogen oxygen concentrations ranging from 0 to about 2? Ό: ϋη. It was also found that the effects of temperature and oxygen interact with respect to the fabrication of acceptable tile surfaces. The conditions for obtaining acceptable surfaces are summarized as follows: (a) A commercially pure nitrogen atmosphere at temperatures below 450 ° C. These conditions provide a shiny, crystalline surface that is visible through a completely transparent oxide film. At temperatures above 45 ° C, a phenomenon commonly referred to as "discoloration" was observed, with colors ranging from o to dark purple.

(b) Oxidation at about 2% oxygen in the temperature range of 450 to 475 ° C. These conditions produce a uniform silver-gray oxide film on the ingot surface.

The above method can also be carried out in a continuous slab casting machine, such as a Sheppard casting machine, if several molds are mounted on an endless chain conveyor. In such machines, a non-oxidizing atmosphere can be formed by a hood surrounding the casting machine at the filling station. The molten metal would be fed from the furnace to the ladle located inside the dome and from the ladle to the molds as they passed the filling station. A dimensioned nitrogen inlet duct as well as an exhaust duct would be provided through the dome to form an unoxidized atmosphere in the dome. The nitrogen-i1 ma circumference must be stored in the dome under a slight overpressure so that the surrounding oxidizing atmosphere outside the dome cannot penetrate the dome through the inlet and outlet cells of the mold. However, seals would be required at the inlet and outlet ends of the hood to prevent excessive 73373 6 loss of nitrogen gas.

2 shows a series of closely spaced open top to the ingot mold 30, which is mounted to the endless conveyor ketjukul-32, the flow rate is about 5 cm per second in the arrow A direction. All ingot molds have flat tops. The fixed cover plate 34 is located close to but a predetermined distance D from the top surface of the molds and covers a predetermined number of molds before and after a metal pouring or casting station mounted on the cover plate. The metal casting station is conventional in construction, comprising a trough 36 terminating in a downcomer 38 for feeding molten metal to the ladle 40. The ladle 40 is tilted at regular intervals to pour the metal sequentially into each mold through a ladle opening 42 in the cover plate. A separator 44 is located at the end of the trough to trap any slag on the surface of the molten metal.

According to the quart 3, the cover plate 34 is provided with a predetermined number of gas inlet ducts 46 and inert gas is supplied to these ducts or cavities through the front piping 48 and the main piping 30. Inert gas is fed to the first mold under the plate through three gas inlet ducts for quick cleaning of the molds and to the remaining molds under the plate through one row of channels to progressively reduce the oxygen content and keep it in the casting position below a predetermined value. Also included is an oxygen piping 32 for supplying inert gas to the ladle housing 54 and the downcomer housing 56. Lid strips 58 are located in the gaps between the molds to prevent excessive gas leakage through these gaps.

According to Figure 4, the width of the plate 34 is equal to that of the molds 30. The inert gas protrudes into the molds from the gas passages or cells 46 and flows out through the slots in the sides and ends of the cover plate. Lid width and container width

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7 73373 compared to the interior of the vessel depend on the pneumatic resistance needed to ‘prevent the return of air to the molds.

It has been found that by adjusting the length of the cover plate 34, the number and location of gas discharge openings, the gas flow rate and the gap between the molds and the cover plate, an inert atmosphere can be maintained below the pouring opening 42 without the need for contact seals. In order to facilitate the study of the distance between the gas inlets, the dimensioning of the gap and the flow rate of the inert gas (nitrogen), a device for simulating or simulating a conventional casting machine was designed in the laboratory. The number of gas inlet ducts or openings in the cover plate was adjusted so that at any time there were at least three and at most four gas openings above the moving mold. The length of the cover plate was such that every moment under the cover plate were located five molds (three before and two after the drain hole 22).

The experiments were performed by generating a predetermined nitrogen flow rate through the cover plate and then passing the molds past the cover plate at the same speed as a conventional casting machine conveyor (3 cm per second). Each mold was progressively cleaned as it came under the cover plate. A sample of the mold atmosphere was taken from the center of the mold by pumping the sample to an oxygen analyzer as the mold approached the pouring orifice. The experiments were performed to determine the oxygen content in the mold atmosphere with (a) changes in mold and lid spacing and constant nitrogen flow rates, and (b) changes in nitrogen flow rates and constant spacing. The results of these experiments are shown in Figures 5 and 6. The flow rate of the front piping in the experiment shown in Figure 5 was about 5.7 Nm / h and the flow rate of the main piping was about 42 Nm / h. In the experiment shown in Figure 6, the flow rate of the front piping was set to about 5.7 3

Nm / h and the flow rate in the main pipeline varied between 14.2-85 Nm3 / h.

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The results of the experiments shown in Figure 5 showed that the oxygen content increased very ‘rapidly with intermediate dimensions greater than 6.35 mm. More importantly, this curve shows that less than 0.5% oxygen can be achieved with gaps up to 7.6 mm with a gas consumption 3 that is economically acceptable (about 57 Nm / h). Figure 6 shows the effect of variable nitrogen consumption on the oxygen content in the mold atmosphere with two different intermediate dimensions of 2.5 4 inm and 3.17 m m.

These curves show that no increase is achieved by increasing the use of the atmosphere above 57 Nm / h and that acceptable oxygen concentrations are easily achieved with very low (28 Nm ^ / h) gas consumption. The intermediate heights of 2.54 mm and 3.17 mm used in these experiments are practical values that can be achieved and maintained in current casting machines.

Lower tolerances that should be sought when designing future casting machines can result in an oxygen content of less than 0.1% with economically acceptable consumption.

After the said test tests had been carried out, the equipment shown in Figure 2 was installed on a slab ingot in Canadian Eleet rolytic Zinc Limited,

Valleyfield, Quebec, Canada, in order to demonstrate under factory conditions that foam-free tiles can be made by casting liquid zinc in a nitrogen atmosphere.

Atmospheric experiments were initially performed using a machine but without casting liquid metal. These experiments showed that the oxygen concentrations can be kept at the casting station in the range of 0.3-0.5% and that no increase is achieved by increasing the flow rate of 3 oxygen above 57 Nm / h.

Zinc liquefaction was then started by directing preheating flames into the trough and ladle. Nitrogen was initially supplied at a rate of 7.1 Nm / h to the Kist o and at a rate of ¢ 2 3

Nm / h is used to cover the main pipes of the cover plate and then successively to the housings of 3 ladles and downcomers at a speed of 7.1 Nm / h.

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The oxygen content stored at the casting station ranged from 0.35 to 0.45 S.

A significant reduction in the foam in the casting tiles was already observed before the initial steps were completed. Complete disappearance of the foam was achieved after the closure and cleaning of the ladle and trough housings had been completed.

The surfaces of the slabs were found to be clear and slag-free. Transparent oxide membranes similar to those obtained in laboratory experiments using an oxygen concentration range of 0.2 to 0.5% were observed in the leaves.

Figures 7, 8 and 9 show embodiments for reducing gas loss between molds and reducing gas flow requirements as a result. In Fig. 7 the edges of the molds are thicker than the mold shown in Fig. 2 and this also increases the flow resistance in the gap 60 between the molds. In Fig. 8 the edges of the molds are shaped so that the gap 62 is horizontal to prevent direct gas flow from the lid openings. This structure is in a way similar to that achieved with the cover strip 11a 58 of Figure 2, but this structure is more resistant to abrasion and tearing. Figure 9 shows another method for reducing the gas flow, which method uses a seal 64 between the molds. This option is possible because this seal is not abrasive.

Although the method according to the invention has been presented in connection with a particular device, it is clear that it can be carried out with other devices, including various types of continuous casting machines, and that the method is by no means limited to the device presented for carrying out the invention.

Claims (3)

10 73373 A device for use in a continuous casting machine comprising a series of adjacent top open containers (30) mounted on an endless chain conveyor (32) to minimize foaming on the surface of molten metal as it falls freely into the containers, characterized in that the device comprises a cover plate (34) disposed at a predetermined distance above a plurality of such vessels (30) and having an opening (42) for casting molten metal into each vessel as they pass under the opening (42), the lid plate (34) extending a predetermined distance in front of said opening (42) and rearwardly and has a predetermined number of longitudinally spaced holes (46) and means for holding the molten metal during its free fall in a substantially non-oxidized atmosphere, including means for supplying non-oxidized gas through the lid plate (34) to form an non-oxidized atmosphere in the containers (30) as they approach the opening (42) of the cover plate and said atmosphere to be stored in containers as they pass through said opening. Device according to claim 1, characterized in that said atmosphere is an inert atmosphere. Device according to claim 1, characterized in that it comprises a sealing member (58, 64) in or on the gaps (60, 62) between the containers (30) to prevent excessive gas leakage through these gaps. II