FIELD OF INVENTION

The invention relates to a device for holding/transferring molten metal. The invention also relates to a method of fabricating a device for holding/transferring molten metal.

BACKGROUND OF THE INVENTION

A variety of vessels are used in the handling of molten metals for transferring the molten metal from one location to another. Examples of such vessels include ladles, chutes, pipes, tundishes and channels for molten metal transfer, such as, furnace spouts and launders. Hand ladles are commonly used for transporting the molten metal from the melting pot to the mold or the die casting machine. Several design factors are taken into consideration while designing a molten metal handling and/or transferring vessel. The vessel should be able to withstand thermal shock of introduction of molten metal. More importantly, the vessel should have a good non-wettability property, in that, the non-ferrous molten metal such as aluminium, magnesium, tin, zinc, lead, brass, bronze, silver etc., does not easily stick to the inner wall of the vessel.

Such metal handling and processing devices have been conventionally made using stainless steel, cast iron. Such devices have low corrosion resistance to the molten non-ferrous metal. The molten non-ferrous metal wets and reacts with stainless steel, cast iron etc. and the purity of the non-ferrous metal to be conveyed/held gets deteriorated. As the non-ferrous metal wets/reacts with the surface metal of the device, the freeze-off builds up and volume flow rate of molten metal reduced. Therefore, frequent cleaning of the pipe reduces the operation efficiency of the process.

Also, conventionally, ceramic castable have been used as inner liners in such molten metal transferring/handling devices. Ceramic castables do not have a good thermal shock resistance as well as does not have a good wear resistance to the molten metal. Therefore, it does not have a very good working life wherein the temperature gradient is few hundred degrees due to the intermittent flow of non- ferrous molten metal such as aluminium, zinc, lead, magnesium during the operation process. Moreover, due to the poor abrasion resistance of conventional ceramic castable the quality of molten metal deteriorates.

Graphite-lined pipes are disadvantageous as it gets oxidized with increase in temperature and has high thermal conductivity and poor abrasion properties. Therefore, the life cycle and energy efficiency of the graphite pipe used for molten metal transfer is less.

It is also known to use aluminium titanate lined ceramic ladles as a measuring device to move a predetermined amount of molten aluminium alloy from a molten metal holding furnace to a molding machine. The ladle is formed from an aluminium titanate ceramic material that has superior low thermal expansion and thermal shock resistant properties. It is well known that aluminium titanate ceramic has low thermal expansion and superior thermal shock resistant properties.

However, aluminium titanate cannot be directly lined onto the metal surfaces of the ladles or chutes since there would be a differential thermal expansion between the metal and the aluminium titanate lining and is very likely to break out of the metal surface of the ladles or chutes during use at very high operating temperatures.

Therefore, there remains a challenge to retain the non-wettability property and the mechanical strength of the molten metal handling vessel and to overcome the problems associated with the lining of the vessels as mentioned above.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the invention to provide a molten metal holding/transferring device having little or no wettability, good corrosion resistance and wear resistance to the non-ferrous molten metal such as aluminium, magnesium, tin, zinc, lead, brass, bronze, silver or the like and thermal shock resistance to the high temperature gradient because of intermittent flow of the molten metal. The device should also have low thermal conductivity so that molten metal temperature is maintained within the device and delays the freeze-off and improves the energy efficiency of the process.

It is also an object of the invention to provide a molten metal holding/transferring device with increased mechanical strength and durability.

It is a further object of the invention to provide a method of fabricating a molten metal holding/transferring device.

SUMMARY OF THE INVENTION

The present invention provides a molten metal holding/transferring device comprising an inner wall lining of thermal shock resistant, low thermal conductive ceramic material to form the working area of the device in contact with the molten metal. This ceramic material of the inner wall is non-wettable to the molten metal and is a good thermal insulator. The ceramic material is lined on an intermediate wall comprising a monolithic ceramic castable with the help of a bonding agent. The ceramic castable has similar thermal coefficient of expansion as that of the ceramic material of the inner wall. The bonding agent also has similar thermal coefficient of expansion as that of the ceramic inner wall and the monolithic ceramic castable over the period of application temperature in view of strong bonding compatibility. The ceramic castable is lined on a metal outer wall, preferably by means of mechanical anchoring.

In a preferred embodiment the ceramic material of the inner wall comprises aluminium titanate since aluminium titanate has near-zero coefficient of expansion that makes it dimensionally very stable with change in temperature.

In a preferred embodiment, the ceramic material of the inner wall comprises silicon nitride, fused-silica shapes, sodium zirconium phosphate (NZP) or borate- bonded/glass bonded/self-bonded hexagonal boro-nitride.

In a preferred embodiment, the monolithic ceramic material of the intermediate wall comprises fused silica.

In a preferred embodiment, the bonding agent comprises organic/inorganic silicate, inorganic phosphate or combinations thereof. More preferably, the bonding agent may be a heat resistant mortar.

Optionally, the bonding agent includes alumina and/or its fibres. In addition to the mechanical anchoring, ceramic fibre paper and/or wool may be provided as insulation between the intermediate wall and the metal outer wall to minimize the stress that may be caused due to thermal expansion mismatch and to improve the thermal efficiency.

In a preferred embodiment, the inner wall lining of thermal shock resistant, low thermal conductive ceramic material to form the working area of the device in contact with the molten metal is constructed in the form of tiles/sleeves having a self-locking arch profile.

In a further preferred embodiment, the inner wall of the device may comprise two layers of thermal shock resistant, low thermal conductive ceramic structures, such as aluminium titanate with a bonding agent interface between the two layers. The second layer of aluminium titanate is provided with staggered joint to prevent the possible percolation of molten metal through the joint to reach the metal outer wall. The bonding agent material forming the interface between the first layer and the second layer of the inner wall has a similar co-efficient of expansion as that of the aluminium titanate forming the first and second layers of the inner wall

The device for holding/transferring molten metal fabricated according to this invention improves the non-wettability property of the device due to the inner wall lining made of thermal shock resistant, low thermal conductive ceramic material, for example, aluminium titanate. This increases the volume accuracy of the molten metal that is taken from the furnace to the die casting mold or similar operations Further due to the intermediate layer formed by a ceramic castable, preferably mechanically anchored to the metal outer wall, the device has increased strength and durability to withstand numerous cycles of holding and transferring of molten metal at a very high temperature of more than 1300°C. The bonding agent having similar co-efficient of expansion as that of the inner ceramic wall and the intermediate ceramic castable wall ensures that the bonding is held strong enough over the period of application temperature. Further, it overcomes the problems associated with the lining of the vessels and increases the durability of the lining during use at very high operating temperatures.

According to a method of fabricating a molten metal holding/transferring device according to this invention, the outer metal wall is prepared and cleaned using a wire brush or by sand blasting or other similar processes. Mechanical anchor elements, which may be V-joints or Y-joints, are anchored at different portions of the metal surface by welding or similar techniques. A monolithic ceramic castable is cast on the metal outer wall using the anchor elements to form an intermediate wall. A layer of bonding agent is then applied on the ceramic castable and tiles/sleeves of thermal shock resistant, low thermal conductive ceramics structures are affixed on the bonding agent to form an inner wall and working area of the device, which would be in contact with the molten metal. The monolithic ceramic castable, the bonding agent and the thermal shock resistant, low thermal conductive ceramic structures have similar coefficient of thermal expansion.

In a preferred embodiment, the ceramic tile/sleeve structures of the inner wall are formed from aluminium titanate. Optionally, the tile/sleeve structures may comprise silicon nitride, fused-silica shapes, sodium zirconium phosphate (NZP) and borate-bonded/glass bonded/self-bonded hexagonal boro-nitride.

Preferably, the monolithic ceramic castable comprises fused silica.

In a preferred embodiment, the bonding agent comprises organic/inorganic silicate, inorganic phosphate or a combination thereof More preferably, the bonding agent may be heat resistant mortar.

In a preferred embodiment, the bonding agent includes alumina and/or its fibres.

The method comprises an optional step of providing a ceramic fibre paper and/or wool layer as insulation between the intermediate wall and the metal outer wall to minimize the stress that may be caused due to thermal expansion mismatch and to improve the thermal efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to the embodiments shown diagrammatically in the drawings wherein:

Figure 1 shows cross-section of a device for holding/transferring molten metal according to a preferred embodiment of this invention.

Figure 2 shows the mechanical anchors or joints used according to an embodiment of the present invention.

Figure 3 shows the device according to this invention with the mechanical anchoring,

Figure 4 shows a single tile structure of the inner wall of the device according to this invention.

Figure 5 shows the arrangement of the tiles/sleeves forming the inner wall of the device according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary embodiment of the present invention is illustrated in figure 1 wherein the device for holding/transferring molten metal is shown to be a chute (100). A chute is a device that is adapted to discharge molten metal from a furnace into a container or ladle under the effect of gravity. Chutes are also used to discharge molten metal from the ladle into a charge furnace under the effect of gravity. It is to be understood that any device that holds and/or transfers molten metal is within the ambit of this invention.

The cross sectional view of the chute is illustrated in figure 1. The inner wall (2) of the chute (100) comprises a thermal shock resistant, very low thermally conductive ceramic. A suitable ceramic material like aluminium titanate maybe used for the inner wall.
Aluminium titanate (Al2TiO5 or Al2O3.TiO2) is known to have near-zero thermal expansion coefficient that makes it dimensionally very stable with change in temperature. It has also got high thermal shock resistance. Further, aluminium titanate has excellent non-wetting properties with respect to most of the non-ferrous molten metals because of it chemical composition and unique micro structure. Therefore, it prevents metal adhesion thereby increasing the accuracy of quantity of molten metal supplied, simplifies cleaning process of the device thereby extending the machine availability in between cleaning cycles. It also helps in avoiding the costly non-adhesive coating that need to be applied on the components to reduce metal adhesion. Aluminium titanate is known to have one of the lowest thermal conductivity that will help in reducing temperature I gradient and delays molten metal solidification and maintains quality of the products resulting in higher yields with better thermal management in the process. Therefore, aluminium titanate lined devices helps in improving the process reliability, yield level and energy efficiency. Table I below provides the typical property comparison between aluminium titanate and conventionally used graphite.

TABLE 1

As is evident ceramics manufactured from aluminium titanate exhibit extremely good resistance to thermal shock and good resistance from adhesion to molten metals. This is due to largely to the very low thermal expansion coefficient, which arises from significant anisotropy in the material's properties. In addition to generating almost zero thermal expansion, this has the effect of causing microcracks to form in the sintered material resulting in relatively low strength of the material. One way of usually minimizing the strength degradation is to add appropriate quantities of oxides such as SiO2, ZrO2 and MgO etc. and combinations thereof Ceramics made from aluminium titanate are shown to exhibit significantly longer operating life than competing materials such as calcium silicate and fused silica.

The inner wall (2), according to a preferred variation of the present invention, may also comprise silicon nitride, fused silica, sodium zirconium phosphate (NZP) or borate-bonded/glass-bonded/self-bonded hexagonal-boro-nitride. The irmer wall (2) of the chute (100) forms the working area that comes into contact with the molten metal.

The chute as illustrated in figure 1 has an intermediate wall (4) that is anchored onto the outer wall (6). The outer wall (6) can be made from metal such as steel, similar to the known molten metal holding and/or transfer devices, or from other rigid durable materials. The intermediate wall (4) comprises a monolithic ceramic castable having similar thermal coefficient of expansion as that of the ceramic material of the irmer wall. The monolithic ceramic castable for the intermediate wall (4) is preferably, but not limited to, fused silica-based castable. Fused silica based castable exhibits good strength, volume stability and good thermal shock resistance. Furthermore, fused silica has almost similar thermal coefficient of expansion as that of the aluminium titanate of the inner wall (2), The typical properties of a castable such as fused silica castable are given below in Table 2.

TABLE 2

The intermediate wall (4) comprising fused silica is preferably mechanically anchored onto the outer metal wall (6), which will be explained in detail in the succeeding paragraphs.

Mechanical anchors (8a, 8b, 8c) of the types shown in figure 2 may be fixed by means of welding or other techniques onto the metal surface. The mechanical anchors (8a, 8b, 8c) may be a V-joint or a Y-joint made of suitable durable and corrosion resistant material. The bases (10) of these mechanical anchors (8a, 8b, 8c) are welded onto the inner surface (6a) of the outer metal wall (6) at suitable positions.

Figure 3 shows an exemplary anchoring of the mechanical anchors (8a, 8b, 8c) on the inner surface (6a) of the outer metal wall (6) of the chute. The mechanical anchors (8a, 8b, 8c) provide the necessary rigidity and strength that is required to hold the ceramic castable that would subsequently be cast on the metal outer wall (6). It is possible to use the same type of anchor (either 8a or 8b or 8c) on the entire inner surface (6a) of the outer metal wall (6). Alternatively, it is also possible to weld combinations of different types of anchors illustrated as 8a, 8b and 8c on the different sides on the irmer surface (6a) of the outer metal wall (6), which is illustrated in figure 3.

The monolithic ceramic castable material is cast on the metal outer wall (6) having the mechanical anchors (8a, 8b, 8c) to form the intermediate wall (4). Upon casting, the castable is dried for 12 to 24 hours under normal air circulation and is cured at room temperature or by a heat treatment process. The typical heat treatment schedule can be as follows. The cast-component is heat-treated at about 25˚ C per hour uptol50°C. It is then soaked at 150˚ C for around 6 hours. Subsequently, it is soaked at a temperature from 25°C to 350˚ C for around 2 to 4 hours. The cast-component is then further heat treated to a temperature of 600°C to 1000°C. This process would form the ceramic castable intermediate wall (4) firmly fixed with the metal outer wall (6) using the mechanical anchors (8a, 8b, 8c).

A layer of bonding agent (3) is applied on the ceramic castable upon cooling of the castable in case of curing by heat-treatment process. The material for bonding agent is chosen to have a similar co-efficient of expansion as that of the monolithic ceramic castable forming the intermediate wall (4) and the thermal shock resistant, low thermal conductive ceramic structures forming the inner wall (2). The bonding material should have the property to retain its strength over the working temperature and to set well at room temperature.

Accordingly, in a preferred embodiment, the bonding material may be an organic/inorganic silicate, inorganic phosphate or a combination thereof. In a more preferred embodiment the bonding material may be a heat resistant mortar.

In a further preferred embodiment, ceramic fibres, such as alumina, but not limited to the same, is incorporated in the bonding material to maintain the thermal expansion similar to that of the inner wall (3) made of thermal shock resistant, low thermal conductive ceramic structures, such as aluminium titanate, as well as to minimize the heat transfer, thereby providing good thermal insulation to the device.

Due to the low thermal conductivity of the material of the inner wall (2) the differentiation in thermal expansion at the interface between the monolithic ceramic castable forming the intermediate wall (4) and the metal outer wall (6) is minimal. Insulation wool may also be placed between the monolithic ceramic castable intermediate wall (4) and the metal outer wall (6) to minimize the stress because of thermal expansion mismatch, if any. This construction ensures excellent heat management and efficiency of the device

The inner wall (2) comprising the thermal shock resistant, low thermal conductive ceramic structures is then formed on the bonding agent. In a preferred embodiment as shown in figures 4 and 5, the inner wall (2) is designed to be in the form of tiles or sleeves. Other structures and designs of the inner wall can be envisaged within the scope of this disclosure. Figure 4 shows a single tile-like structure (12) made of aluminium titanate. Each tile has a preferred length (L), thickness (THK), top width (TW), bottom width (BW), side angle (SA), taper angle (TA) and thickness angle (Q°). The aluminium titanate tiles (12) are arranged such that they form an arch shaped profile as shown in figure 5. These tiles are arranged with each other in such a way so as to form a self-locking arrangement. A partial view of the irmer wall (2) self-locking tile arrangement in a chute is shown in figure 5. The tile arrangement comprises a plurality of sections marked by numerals 14a, 14b, 14c, 14d and 14e. Each section comprises a plurality of tiles (12) having substantially same thickness but different widths and heights to form a conical arrangement in the inner wall (2). Only as an example of the structure, the tiles of the sections I4a-14e have the preferred dimensions given in table 3 below to form the arrangement in the inner wall (2) of the chute.

TABLE 3

As can be observed, the bottom width of the tiles in section 14a is the top width of the tiles in section 14b. Similarly the bottom width of the tiles in section 14b is the top width of the tiles in section 14c. Likewise, the bottom width of the tiles in section 14c is the top width of the tiles in section 14d and the bottom width of the tiles in section 14d is the top width of the tiles in section 14e. This ensures the accuracy of the arch-type self-locking profile of the tile arrangement in the inner wall (2).

In a preferred embodiment, the inner wall (2) of the chute as illustrated in figure 1 may comprise two layers (2a, 2b) of thermal shock resistant, low thermal conductive ceramic structures, such as aluminium titanate with a bonding agent interface (5) between the two layers (2a, 2b). The second layer (2b) of aluminium titanate is provided with staggered joint to preyent the possible percolation of molten metal through the joint to reach the metal outer wall (6). The bonding agent material forming the interface (5) between the first layer (2a) and the second layer (2b) of the inner wall has a similar co-efficient of expansion as that of the aluminium titanate forming the first and second layers (2a, 2b) of the inner wall (2). Accordingly, in a preferred embodiment, the bonding material may be an organic/inorganic silicate, inorganic phosphate or a combination thereof In a more preferred embodiment the bonding material may be a heat resistant mortar

The second layer (2b) of the inner wall is very similar in structure and arrangement as that of the first layer (2a) explained in relation to figures 4 and 5, in that they form a self-locking arch-type profile and for sake of brevity will not be explained once again. However, the dimensions of the tiles of the second layer I (2b) would be slightly different from that of the tiles of the first layer (2a). In particular, the top width and the bottom width of the tiles of the second layer (2b) would be slightly larger than those of the first layer (2a). Only as an example, the tiles of the second layer (2b) have the preferred dimensions given in table 4 below.

TABLE 4

The process of manufacturing the pre-engineered shapes of aluminium titanate lining involves mixing and milling together the inorganic and organic raw materials required to form aluminium titanate. Slurry is prepared from the resulting mix and the same is spray-dried. Upon spray drying, the shapes of the tiles are formed by green machining process of cutting and grinding to form the self-locked shapes with respective side angles. The tiles thus formed are sintered and then applied as a liner on the bonding agent applied on the pre-cast/pre-cast pre-heat-treated monolithic castables to develop the arch-type self-locking profile of the working area of the molten metal holding/transferring device.

The lining thickness for composite lining to maintain the desired cold-face temperature for a given hot-face temperature is appropriately determined. It is known that the basic relation of heat flow by conduction is the proportionality between the rate of heat flow across an isothermal surface and the temperature gradient at the surface, represented by the equation:

Where,

A = area of isothermal surface

B = distance measures normally to surface

q = rate of heat flow

TH = hot face temperature

Tc = cold face temperature

K = co-efficient of thermal conductivity.

For a multilayer solid composite under steady state of heat flow the equation can be rewritten as:

Where,

BA, BB and BC are distances measures normally to each of the multiple surfaces.

Based on the above equations, the lining thickness for the aluminium titanate lining is appropriately determined for the device for holding/transferring molten metal according to this invention.

While the above paragraphs explain the various embodiments of the invention, it is apparent that numerous modifications and variations can be made without departing from the scope and spirit of this invention.

WE CLAIM

1. A molten metal holding/transferring device (100) comprising a metal outer wall, an inner wall (2) comprising thermal shock resistant, low thermal conductive ceramic material; an intermediate wall (4) anchored to the metal outer wall, said intermediate wall (4) comprising monolithic ceramic castable material having similar thermal coefficient of expansion as that of the ceramic material of the inner wall (2); and a bonding agent (3) between the inner wall (2) and the intermediate wall (4) said bonding agent (3) having similar thermal coefficient of expansion as that of the ceramic material of the inner wall (2) and the ceramic castable of the intermediate wall (4).

2. The device as claimed in claim 1, wherein the inner wall (2) comprises two layers (2a, 2b) of thermal shock resistant, low thermal conductive ceramic material with a bonding agent as an interface (5) between the two layers (2a, 2b).

3. The device as claimed in claim 1, wherein the intermediate wall is mechanically anchored to the metal outer wall.

4. The device as claimed in claim 1 or 2, wherein the ceramic material of the inner wall is chosen from a group consisting aluminium titanate, silicon nitride, fused-silica shapes, sodium zirconium phosphate and borate-bonded/glass bonded/self-bonded hexagonal boro-nitride.

5. The device as claimed in claim 1, wherein the monolithic ceramic material of the intermediate wall comprises fused silica.

6. The device as claimed in claim 1 or 2, wherein the inner wall is in the form of tiles/sleeves having a self-locking arch profile.

7. The device as claimed in claim 1 or 2, wherein the bonding agent is chosen from a group consisting of organic/inorganic silicate, inorganic phosphate or a combination thereof and heat resistant mortar.

8. The device as claimed in claim 7, wherein the bonding agent includes alumina and/or its fibres.

9. The device as claimed in claim 1, wherein ceramic fibre paper and/or wool is provided between the intermediate wall and the metal outer wall.

10. A method of fabricating a molten metal holding/transferring device comprising the steps of:

preparing and cleaning an outer metal wall of the device;

connecting anchor elements to the metal surface;

casting a monolithic ceramic castable to the metal outer surface using the anchor elements to form an intermediate wall;

applying a layer of bonding agent on the intermediate wall; and

affixing tiles/sleeves of thermal shock resistant, low thermal conductive ceramics structures on the bonding agent to form an inner wall of the device, wherein the monolithic ceramic castable, the bonding agent and the thermal shock resistant, low thermal conductive ceramic structures have similar coefficient of expansion.

11. The method as claimed in claim 10, wherein the ceramic structures of the inner wall is chosen from a group consisting aluminium titanate, silicon nitride, fused-silica shapes, sodium zirconium phosphate and borate-bonded/glass bonded/self-bonded hexagonal boro-nitride.

12. The method as claimed in claim 10, wherein the monolithic ceramic castable comprises fused silica.

13. The method as claimed in claim 10, wherein the bonding agent is chosen from a group consisting of organic/inorganic silicate, inorganic phosphate or a combination thereof and heat resistant mortar.

14. The method as claimed in claim 13, wherein the bonding agent includes alumina and/or its fibres.

15. The method as claimed in claim 10, comprising the step of providing a ceramic fibre paper and/or wool between the monolithic ceramic castable and the outer metal wall.