EP0088701A1 - Process and device for casting a molten non ferrous metal - Google Patents

Abstract

The invention relates to the casting of a non-ferrous metal from a metal distribution tank (1) to receiving molds L in the form of a free jet. According to the method, using a cover (12) covering the receiving molds (7B, 7C), an isolation zone is created with respect to the atmosphere and, using a ramp (23 ) supply and distribution of a liquid inert gas, this zone is inert before / and during casting. The process applies in particular to the continuous casting of zinc in an ingot mold.

Description

The subject of the invention is a method and an installation for casting in the form of a free jet of a non-ferrous metal capable of presenting the problem of the formation of a solid metal foam at the foot of the casting jet. It applies more particularly to the casting of zinc in an ingot mold.

As is known, during the casting of metals in the form of a free jet, an attempt is made to protect the casting jet against oxidation by ambient air. Among the many methods of protecting casting jets used to date, mention may be made of the process of casting metal into molds, described in French patent No. 2,165,769, according to which, on the one hand, the inerting of the surface of the molten metal during its heating by injection of an inert liquefied gas by means of a lance passing through the cover of the crucible for heating said metal, and, on the other hand, the prior inerting of the mold in which the casting will take place. This process has the disadvantage that there is a risk of oxidation during the transfer to the open air of the mold from its inerting station to its casting station, and also during the actual casting.

This problem of the protection of the casting jet arises in a particular way during the casting of non-ferrous metals capable of causing the formation of a solid metallic foam at the foot of the casting jet, and more especially during the casting of zinc in an ingot mold.

In general, it is well known that at any place where there is a fall of liquid zinc in free jet, in particular during the casting of zinc in an ingot mold, a solid metallic foam is formed at the foot of the casting jet which then spreads over the surface of the liquid bath contained in the mold.

As is known, the formation of this foam comes from the oxidation of zinc. Indeed, at the point of impact of the casting jet, bubbles are created due to the high surface tension of the zinc (≃780 dynes / cm). However, the air entrained by the jet causes rapid surface oxidation of these bubbles. As zinc oxide (Zn0 melting temperature ≃ 1950 ° C) is solid at casting temperature (470 ° C to 520 ° C) and, moreover, its calorific capacity is significantly higher than that of zinc (ZnO : Cp = 12 cal.mole-1 T ―1 Zn: Cp = 7,5 cal.mole ^-1 T ^-1 ), there is stabilization and solidification bubbles immediately. The solid metal foam thus formed is in the form of a very thin laminate which comprises approximately 98% of metal and represents 1 to 1.5% of the cast metal.

To avoid the defects formed by the oxides coming from this foam in the zinc ingots subsequently obtained, it is necessary to remove the surface layer of foam from the surface of the bath by manual skimming, generally with a squeegee, before complete solidification. metal in the mold. Thus, in addition to the disadvantage of a non-negligible loss of starting metal, the formation of zinc foam represents investments in manpower and time which harm the profitability of the whole process.

It is known, in order to avoid the formation of zinc foam, to carry out the casting with installations of different design, for example installations in which the casting takes place at the source. But, as we know, source casting is more expensive and difficult to carry out than falling casting.

The subject of the present invention is a method of casting a non-ferrous metal in the liquid state, which makes it possible to avoid the formation of solid metallic foam, in falling casting installations of the usual type.

The method of casting in the form of a free jet of a non-ferrous metal, in particular zinc, from a reserve of said liquid metal to a receptacle for receiving said metal in accordance with the invention, is characterized in that at least one covers a receptacle by an isolation wall so as to form an isolation zone with respect to the atmosphere, said isolation zone is inerted beforehand by introduction into said zone of an inert gas liquefied at a rate such as l atmosphere in the internal cavity of said receiving container has an oxygen content of less than 1%, and preferably less than 0.1%, said receiving container provided with the insulating wall is placed just below the liquid metal reserve , a direct passage is established between said reserve and the receiving container so that the liquid metal pouring jet crosses its entire height over the isolation zone, while maintaining the injection of liquefied inert gas in the upper part of said isolation zone ment at a flow rate such that the atmosphere in the vicinity of the jet of liquid metal has an oxygen content lower than the above values.

According to a variant of the invention, a set of ingot molds is made to pass, arranged transversely side by side, under said reserve of liquid metal, and each of the ingot molds is inerted beforehand before passing under the jet of liquid metal. According to one embodiment, at least two adjacent molds of said set are covered by the isolation wall.

• - un bac répartiteur à orifice de coulée,
• - des moyens d'isolement par rapport à l'ambiance solidarisés audit bac répartiteur autour dudit orifice de coulée et s'étendant vers le bas de façon à recouvrir au moins un récipient récepteur, comportant des moyens d'alimentation et de distribution d'un gaz inerte liquéfié raccordés à un réservoir de gaz inerte liquéfié.

The invention also relates to an installation for casting a non-ferrous metal implementing the process considered. This installation is characterized in that it comprises:

• - a distribution tank with pouring orifice,
• - means of isolation from the atmosphere secured to said distributor tank around said pouring orifice and extending downwards so as to cover at least one receiving container, comprising means for supplying and distributing a liquefied inert gas connected to a liquefied inert gas tank.

As will be understood, the creation of the isolation zone with respect to the atmosphere and the introduction into this zone of a liquefied inert gas prevents any oxidation of the liquid metal, and therefore the formation of solid metallic foam. Indeed, there is no stabilization or solidification of the bubbles formed which therefore remain liquid and are eliminated in the metal bath. Thus, thanks to the process of the invention, any loss of metal is avoided during casting and ingots free of foam are obtained at the end of the process.

• - la figure 1 est une vue en perspective d'une installation de coulée d'un métal non ferreux selon l'invention ;
• - la figure 2 est une vue en coupe transversale partielle et agrandie selon II/II de la figure 1.

The characteristics and advantages of the invention will appear in the following description, given without limitation, with reference to the accompanying drawings in which:

• - Figure 1 is a perspective view of an installation for casting a non-ferrous metal according to the invention;
• - Figure 2 is a partial and enlarged cross-sectional view along II / II of Figure 1.

Referring to Figures 1 and 2, a zinc casting installation comprises a metal distribution tank (1) in the shape of a rectangular parallelepiped, comprising a bottom (2) and four side walls (3a, 3b) and (4a, 4b) . The bottom (2) of the tank (1) has a slot (5). A chute (6) for supplying liquid zinc, connected to a holding furnace (not shown in the figure) and slightly inclined, opens out at the upper part of the wall (3a) of the distribution tank (1).

Metal ingot molds L, placed transversely side by side and integral with two endless transmission chains (8) by means of pivot axes (16), travel in the direction of the arrow F under the distribution bin (1) and successively take the positions (... 7A, 7B, 7C, 7D, ...). These ingot molds L are trough-shaped with a rectangular bottom (9) and four side walls (10a, 10b) and (11a, llb) slightly inclined.

A metal cover (12), in the shape of a rectangular parallelepiped has a horizontal upper plate (13) and four vertical side walls (14a, 14b) and (15a, 15b). The upper plate (13) is pierced with a rectangular opening having the dimensions just necessary to fit therein the lower part of the distribution tank (1). The cover plate (13) (12) is fixed by means of four metal connection plates (18) using bolts (19) to the walls (3a, 3b) and (4a, 4b) of the distribution tank (1).

The cover (12) completely overhangs the two ingot molds (7B) and (7C), and its side walls (14a, 14b) and (15a, 15b) arrive just flush with the upper edges of these ingot molds. More precisely, the lower edge (20) of the walls (14a, 14b) stops just above the upper edges (21) of the walls (10a, 10b) of the molds (7B) and (7C) and the edge lower (22) of the walls (15a, 15b) stops just above the upper edges (28) of the walls (11a, 11b) of the molds (7B) and (7C). Thus, the cover (12) forms a quasi-hermetic protective cover for the internal cavity of the molds (7B) and (7C) while not preventing them from scrolling.

A ramp (23) for supplying and distributing a liquefied inert gas is fixed under the upper plate (13) in parallel. to the wall (14a), in the upstream part of the cover (12) relative to the direction of movement F of the molds L. This ramp (23) is of the conventional phase separator type, comprising a degassing slot (29) at its upper part and provided, at its lower part and at regular intervals, with injection nozzles (24) of liquid inert gas oriented downwards. The ramp (23) is connected to a tank of liquefied inert gas (25) via a conduit (26) and a dip tube (27). The supply and distribution ramp (23) is preferably located in the upstream part of the cover (12) relative to the direction of movement F, as shown in the figures, but it can also be placed in the downstream part of the cover (12); there may also be two supply and distribution ramps for liquefied inert gas, one in the upstream part and the other in the downstream part of the cover.

A vertical duct (30) crosses the downstream part of the upper plate (13) relative to the direction of movement F and opens slightly below said plate (13). This duct (30), fitted with a pump (32), is connected to an oxygen analyzer (31).

A preheating ramp (33), disposed parallel to the wall (14a) of the cover (12) and above the ingot mold (7A), is fixed to the wall (14a) by means of metal rods (34) fitted with metal rings (35). This preheating ramp (33) consists of a pipe (36) connected to sources of fuel gas and oxidant gas (not shown in the figures) and provided, at regular intervals, with burners (37) oriented towards the internal cavity of the mold (7A).

The distribution tank (1) contains a bath of liquid zinc (40) which flows through the slot (5) in the form of a jet J to form a bath of liquid zinc (41) in the ingot mold (7C).

The operation of the installation according to the invention is as follows. The distributor tank (1) is continuously supplied with liquid zinc, coming from a holding furnace, using the chute (6); impurities, in particular the oxides formed during transport to air in the chute (6), remain on the surface of the liquid metal bath (40) thus formed in the distributor (1). and the pure liquid zinc settles at the bottom of the distributor (1) and flows through the slot (5) in the mold which is in position (7C).

Using the chains (8), the ingot molds L are scrolled under the cover (12) in the direction of the arrow F. Before moving under the cover (12), each ingot mold L, in position (7A) , is preheated to a temperature above 100 ° C by means of the preheating ramp (33). When this ingot mold L then arrives in position (7B), it receives the jets of a liquefied inert gas, such as argon or nitrogen, injected by the nozzles (24) of the supply ramp (23 ). Part of this liquefied inert gas vaporizes and spreads in the space defined by the cover (12) and the molds in position (7B) and (7C); the other part remains liquid and forms a liquid layer (42) on the bottom of the mold (7B) which will be found on the surface of the metal bath (41) when the mold is in position (7C). Thus, an inert gas atmosphere is created in the internal cavity of the mold L in position (7B) as well as above the surface of the bath (41) and around the jet J in the following mold (position (7C)) . Then, the ingot mold L arrives in position (7C) where it receives the jet J of liquid zinc and gradually fills while being maintained in an inert gas atmosphere as just explained above. When the ingot mold L arrives in position (7D), it contains a bath of liquid zinc, the surface of which is free of solid metal foam.

Throughout the process, the oxygen content of the atmosphere in the vicinity of the pouring jet J and of the surface of the metal bath (41) is continuously monitored using the oxygen analyzer (31), and the injection rate of the liquefied inert gas in the supply rail (23) is adjusted so that this oxygen content is less than 1% and preferably 0.1%.

The possible preheating of each ingot mold in position (7A) is intended, in certain cases, to avoid excessive cooling of the latter when it is in position (7B) as well as of the bath of liquid metal which it will contain when it is in position (7C), cooling which comes from the frigories provided by the liquefied inert gas.

There is shown in Figures 3 and 4 attached, two cross-sectional views of another embodiment of an installation for casting a non-ferrous metal implementing the method according to the invention.

Referring to Figures 3 and 4, a zinc casting installation comprises a distribution tank (50) in the shape of a rectangular parallelepiped having a bottom (51) and four side walls (52). The bottom (51) of the tank (50) has a slot (53). A chute (54) for supplying liquid zinc opens into the upper part of the distribution tank (50).

Metal ingot molds (55), placed transversely side by side, pass in the direction of arrow G, following a slightly ascending slope thanks to a transport ramp (not shown in the figures), and successively take the positions (... 55A, 55B, 55C, 55D, ...). These molds (55) are trough-shaped with a rectangular bottom (56) and four side walls (57) slightly inclined.

A metal cover (59) bearing on protection plates of the transport ramp (not shown in the figures) and overhanging at least one ingot mold (55) is equipped with a ramp (60) for supplying inert gas. This ramp (60) is, on the other hand, connected to a wall (52) of a distribution tank (50) by a skirt (61), made of refractory fabric for example. An injection pipe for a liquefied inert gas (62) opens out above the internal cavity of one of the molds (55). A skirt (63), fixed to the bottom (51) of the distributor tank (50), descends vertically into the internal cavity of the next mold.

A metal cover (64) bearing on three of the side walls (52) of the tank (1) is equipped with a ramp (65) for supplying inert gas. A skirt (67) connects the ramp (65) to a metal cover (68) which overhangs the chute (54). A vertical partition (58), which stops at a distance from the bottom of the chute (54), stops the zinc foam formed on the surface of the liquid zinc during its transport in the open air to the chute and forces the zinc pure to pass from the bottom. A skirt (69) connects a side wall (52) of the tank (50) and the bottom of the chute (54).

The operation of the installation shown in Figures 3 and 4 is as follows. FIG. 3 represents the respective position of the elements of the installation when filling an ingot mold (55C) with liquid zinc; FIG. 4 represents the respective position of the elements of the installation during the passage from an ingot mold filled with liquid zinc (55C) to the next empty ingot mold (55B).

When filling the ingot mold (55C) (fig. 3), the distribution tank (50) is in a horizontal position. It is supplied with liquid zinc by the chute (54); thanks to the protection zone constituted by the cover (68), the skirt (67) and the cover (64), and the inerting of said protection zone by an inert gas (ramp (65), the surface of the bath liquid zinc (70) contained in the chute (54), the jet of liquid zinc (71) flowing from the chute (54) into the tank (50) and the surface of the bath of liquid zinc (72) contained in the tank (50), are perfectly clean, that is to say ie free of dross and mosses due to possible oxidation of the zinc. On the other hand, the jet of liquid zinc (73) and the surface of the bath of liquid zinc (74) contained in the ingot mold (55C), are protected against any oxidation thanks to the confinement zone constituted by the cover (59) , the bottom (51) and the inerting of said confinement zone with an inert gas (ramp 60) and an inert liquefied gas (cane 62).

Once the ingot mold (55C) is filled, the set of ingot molds is scrolled so as to bring the following ingot mold 55B) under the distribution tray (50). When passing from the mold (55C) to the mold (55B) (see FIG. 4), the tank (50) is put in an inclined position by tilting about an axis (66) so that the level of the bath liquid zinc (72) is such that the zinc can not flow through the slot (53). During the entire operation of moving the ingot molds until the ingot mold (55B) has taken the position occupied previously by the ingot mold (55C), the inerting of the protective zone is maintained (cover (68), skirt ( 67) and cover (64)) and of the confinement zone (cover (59), bottom (51) and skirt (63)), as described above. Thanks to the flexibility given to the assembly by the presence of skirts (61), (67), (69), (63), the isolation of the protection zone and of the confinement zone is maintained without problem during the passage from one mold to another. Thus, thanks to the method of the invention, clean liquid zinc is obtained at all points between the chute (54), the distribution tank (50) and the jet (73); and, when filling an ingot mold with this clean liquid zinc, it is protected against any oxidation by inerting of the confinement zone. It is thus certain to completely eliminate the formation of zinc foam on the surface of the bath contained in the molds once filled.

According to a first alternative embodiment, the confinement zone overhanging at least one ingot mold is inert, either in the filling position of an ingot mold or in the position of passage from one ingot mold to another, both by injection of an inert gas liquefied upstream, relative to the direction of movement G, of the casting slot (53) and injection of an inert gas downstream of said slot (53); in this case, the quantity of liquefied inert gas injected is from 75% to 100% of the total quantity of gas used. According to a second alternative embodiment, the confinement zone is inerted by injection of an inert liquefied gas only upstream of the casting slot (53). According to a third alternative embodiment, the confinement zone is inerted by injection of an inert liquefied gas both upstream and downstream of the casting slot (53).

The inert gas used can be, for example, nitrogen or argon.

The invention applies more particularly to the continuous casting of zinc in an ingot mold, but it could also be applied to the casting of any non-ferrous metal in free fall liable to present the problem of the formation of solid metallic foam at the foot of the casting jet, such as lead or lead and calcium alloys, or zinc alloys.

Claims (11)

1. - Method of casting in the form of a free jet of a non-ferrous metal, in particular of zinc, capable of presenting the problem of the formation of a solid metallic foam at the foot of the casting jet, of a reserve of said liquid metal to a receptacle for said metal, characterized in that at least one receptacle is covered by an isolation wall so as to form an isolation zone with respect to the atmosphere, said zone is previously inerted isolation by introduction into said zone of an inert gas liquefied at a rate such that the atmosphere in the internal cavity of said receiving container has an oxygen content of less than 1 X, and preferably less than 0.1 X, said receiving container provided with the isolation wall just below the reserve of liquid metal, a direct passage is established between said reserve and the receiving container so that the pouring jet of the liquid metal crosses over its entire height the isolation area, all t by maintaining the injection of liquefied inert gas into the upper part of said isolation zone at a rate such that the atmosphere in the vicinity of the jet of liquid metal has an oxygen content lower than the aforementioned values. 2. - Method according to claim 1, according to which a set of molds is scrolled, arranged transversely side by side, under said reserve of liquid metal, characterized in that each of the molds is inerted beforehand before passing under the jet of liquid metal. 3. - Method according to one of claims 1 or 2, characterized in that there is covered by the insulating wall at least two adjacent molds of said set. 4. - Method according to one of claims 1 to 3, characterized in that each of the molds is preheated before it passes through the isolation zone. 5. - Method according to one of claims 1 to 4, characterized in that the inert gas is nitrogen or argon. 6. - Installation for casting a metal implementing the method according to one of claims 1 to 5, characterized in that it comprises: - a distribution tank (1) with a pouring hole in its bottom, - means of isolation from the atmosphere (12) secured to said distributor tank (1) around said pouring orifice and extending downward so as to cover at least one receiving container, comprising supply means and distribution (23) of a liquefied inert gas, connected to a tank (25) of liquefied inert gas. 7. - Installation according to claim 6, characterized in that the means for supplying and distributing a liquefied inert gas are constituted by a supply and distribution ramp (23) of the type forming phase separator with orifice liquid discharge (24) in the lower zone and gas discharge orifices (29) in the upper zone, placed inside said isolation means (12). 8. - Installation according to one of claims 6 or 7, characterized in that it comprises drive and guide means (8) for a set of receptacles (L) arranged transversely side by side in a train susceptible to scroll under said isolation means (12). 9. - Installation according to one of claims 6 to 8, characterized in that it comprises preheating means (33) upstream of the liquid metal reserve (1) relative to the direction of movement of said train. 10. - Installation according to one of the claims 6 to 9, characterized in that it comprises means for measuring the oxygen content (31) inside the isolation means (12), in the vicinity of the pouring hole of the distribution tank (1). 11. - Installation characterized in that the isolation means (12) consist of a metal cover in the shape of a rectangular parallelepiped, the horizontal upper plate (13) of which has a rectangular opening having the dimensions just necessary to fit the lower part therein. of the distribution tank (1) and the four vertical plates (14a, 14b, 15a, 15b) of which arrive just flush with the upper edges of at least one receiving container (L).

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