This invention, relates to a method of filtering liquid metal and to a liquid metal filter.

Ceramic foam filter cartridges are customarily sold in rectangular shapes with tapered edges. They are, conventionally, wrapped with a flexible gasket and forced into a tapered rectangular hole in a filter box of well known type. Their life is usually limited to a single filtration operation. At the end of the filtration the cartridge is removed from the filter box and replaced with a new one. The cartridge usually has very little inherent strength while still hot at the end of its life. Attempts to remove it while still hot frequently result m it breaking, which causes the task to become more laborious. A common way of avoiding this problem is to make a hole through the cartridge then wait perhaps 15-30 minutes while the filter cools. A residue of the filtered liquid alloy remains in the filter cartridge and, once this has solidified, it strengthens the cartridge. Then the cartridge can be lifted out cleanly by means of a tool inserted through the hole. While this method enables the cartridge to be removed conveniently, a delay may be caused in the start of the subsequent filtration operation.

In spite of their inherent operational disadvantages such filter cartridges have been in use for some years and are currently widely employed as described.

Our granted US patent 4872908 discloses the provision of a porous container the base of which either contains or consists of a suitable substrate. Particles that are stable with respect to the liquid metal are disposed

within the container so that they are upstream of the substrate when the container is disposed in a liquid metal fluid path. The particles are then carried by the metal flow into the substrate to create a filter. When the liquid metal is an aluminium alloy the particles may be of alumina in powder form or incorporated in a metal matrix composite body either in the form of a semi-solid slurry or as a cast product thereof in the form of granules or briquettes.

It is an object of .the present invention to provide ah improved filter assembly and an improved method of filtering liquid metal. The invention may also incorporate some of' the more sophisticated techniques disclosed in US 4872908.

According to one aspect of the present invention there is provided a ceramic foam filter cartridge having upper and lower surfaces and edges tapering inwardly from the upper to the -lower surface characterised in that a structure including a reinforcing member is cast into the cartridge with part of the structure extending externally of the upper surface thereof. ^'

Another aspect of the present invention provides a method of filtering liquid metal in a filter box having a tapered aperture to receive a ceramic foam filter cartridge comprising the steps of: -

1) providing an annular frame having an upper end and lower end, downwardly tapered walls located between the ends, the frame being of a material capable of -withstanding the temperature of the liquid metal to be filtered, the tapered walls having an internal shape and size approximating to the external shape

and size of a ceramic foam filter cartridge and the frame having at least one part extending outwardly of the upper end thereof.

2) wrapping a flexible gasket around the outer surface of the tapered walls,

3) locating a ceramic foam filter cartridge in the frame as a force fit with its outer edges in intimate contact with the inner surface of the tapered walls so as to create a filter assembly comprising the cartridge, the frame and the gasket,

4) locating the filter assembly m the aperture of the filter box,

5) passing liquid metal through the filter assembly and thereafter

6) utilizing said part to remove the filter assembly bodily from the filter box.

Preferably the filter assembly is removed from the filter box while the cartridge still contains some liquid metal.

The invention also provides a liquid metal filter assembly for use in carrying out the method of the preceding paragraph. Such liquid metal filter assembly may comprise an annular frame having an upper end and a lower end downwardly tapered walls located between the ends, the frame being of a material capable of withstanding the temperature of the liquid metal to be filtered and the frame having at least one part extending outwardly of the upper end thereof, a ceramic foam filter cartridge having tapered edges located within the frame with its edges in

intimate contact with the tapered walls, means to retain the cartridge within the frame and a flexible gasket wrapped externally around the tapered walls.

Several aspects of the present invention will now be described by way of example with reference to the accompanying drawing in which:-

Figure 1 is. a vertical section through a filter cartridge incorporating cast-in handles,

Figure 2 is a similar view of a cartridge incorporating reticular reinforcement

Figure 3 shows another ^' arrangement showing cast-in walls for the cartridge and

Figure 4 shows diagrammatically and partly in section a filter assembly located in a filter box.

Referring to Figure 1 a ceramic foam filter cartridge 1 is of rectangular shape in plan view having upper and lower surfaces 2 and 3 and four edges such as 4 and 5 which taper inwardly from the upper surface to the lower surface. A plurality of inverted U-shaped handles 6 with limbs 7 carrying feet 8 are cast into the cartridge so that the feet and the lower part of the limbs are embedded in the cartridge while the remainder of the handles extend outwardly of the upper surface 2 to be readily accessible for installing the cartridge in a filter box (not shown) and removing it therefrom.

We have found that by careful location of the handles the problem set out in paragraph .2 of page 1 of this specification may be overcome without hindering the

desired liquid metal flow through the filter cartridge. When the cartridge is to be used to filter a liquid aluminium alloy the handles may be of steel covered with a suitable refractory composition or may be cast from a refractory incorporating metallic reinforcement.

Figure 2 shows another arrangement in which a reticulated reinforcement 9 extends across substantially the whole of the cartridge. This reinforcement may be used in conjunction with one or more handles 6 or part of the reinforcement may itself extend outwardly of the upper face 2 to constitute a handle.

The reinforcement 9 may be a coarse wire grid covered with a refractory material or it may be cast in a refractory material that may itself incorporate metallic reinforcement. The reinforcement 9 may alternatively be of inherently flexible material such as a coarsely woven glass or ceramic fibre mesh.

Figure 3 shows an arrangement in which the cartridge 1 is provided with rigid walls 10 of ceramic material cast around the edges of the cartridge 1. This may be done either in a two stage casting operation for the cartridge or a frame constituted by the walls could be pre-cast and the cartridge subsequently cast within it.

Referring to Figure 4 a conventional filter box 11 is disposed intermediate the length of a launder, one part of which is shown at 12. The box 11 has an upper chamber 13 having a base 14 formed with a downwardly tapered rectangular hole 15 into which a standard filter cartridge (not shown) is usually disposed. The hole 15 communicates with a lower chamber 16 that extends upwardly at 17 and thence to a launder part (not shown)

which is aligned with the launder part 12. It will be understood that the box 11 is rectangular in plan having a greater lateral dimension than the launder part 12 and it will be assumed that liquid metal flows in the direction of the arrows -A- and -B- into and out of the box 11, passing through any filter device located in the hole 15.

In its simplest form the present invention provides a filter assembly to be located in the hole 15. This assembly comprises an annular metal frame 18 of rectangular shape having four downwardly tapered walls

19, 20, 21 and 22 extending between upper and lower ends. The upper ends extend outwardly to form lips indicated by broken lines at 23 and 24. The frame is of a size and shape to fit loosely into the hole 15 so that it may be firmly located in the hole with the intermediary of a flexible gasket 25. The thickness of the gasket 25 has been exaggerated in the drawing for clarity.

Prior to disposition of the frame 18 in the hole 15 a rectangular ceramic foam filter 26 is disposed as a force fit in the frame 18 and is held in place by suitable retainers. These retainers can be tabs such as 27 bent out from the walls of the frame. The filter 26 is forced through the bottom of the frame to ensure that no gas bubble build up can occur during the filtering operation.

It will be understood that the filter 26 is of a shape and size that would conventionally fit in the hole 15 with the intermediary of a gasket 25.

The lips such as 23 and 24 are provided with handles which are not shown in the drawing but may be the same as

the handles 29 to be referred to subsequently.

The filter assembly thus far described may be manufactured and sold as a unit for incorporation in a standard filter box 11. After a filtering operation and before significant cooling of the filter 26 the assembly may readily be bodily removed from the box 11 using the handles referred to above. This removal may take place while at least some of the metal lodged in the filter is still liquid. The assembly is strong enough to maintain the integrity of the filter during its removal so that the time delay between filtering operations can be significantly reduced.

If desired the basic structure of filter assembly described above may be elaborated to take advantage of the disclosure of our US Patent 4872908. Thus the axial length of the frame 18 is increased significantly above the location of the lips 23 and 24 (which are omitted). The walls 8, 9, 10 and 11 have upper parts 18a, 19a, 20a and 21a and formed with two rows of apertures 28 and their upper ends are formed with outwardly turned lips such as 23a and 24a. Handles 29 are disposed around the lips 23a and 24a to permit the assembly to be located in and removed from the box 11.

A wire mesh cage 30 has an upper surface 31, a shoulder 32 along one edge, a downwardly turned wall 33 and a corrugated lower wall 34. The wall 34 comprises a series of inclined platform 35 and at its end opposite the shoulder 32 the adjacent platform 35 is extended to meet the upper surface 31 via a rounded part 36. The ends of the cage parallel with the plane of the accompanying drawing are open to permit slabs 37 to be located on the platforms 35 in overlapping lattice-like relationship.

The case containing the slabs is disposed in the top of the filter assembly with the shoulder 32 resting on the lip 24a and the rounded part 36 resting against the upper wall part 20a. The lower part of the cage may be spaced 30-40 mm from the filter 26.

The slabs 37 may be of- a metal matrix composition of an alloy compatible with the liquid metal to be filtered and particulate - alumina as disclosed in our above numbered US Patent 4872908 and their lattice-like disposition ensures that the slabs are ashed over by liquid metal flow, quickly melted and their pre-wetted particulate content deposited evenly over the filter 26 and extending partially therein to constitute an additional filter cake for the liquid metal flowing along the launder part 12.

When a particular filter operation is completed the holes 28. enable the launder part 12 to be substantially emptied of liquid metal and the filter assembly including the cage 30 emoved from the box 11 using the handles 29 while any debris from the filter cake is trapped within the walls 18a, 19a, 20a and 21a and not deposited in the flow path of liquid metal .

It is appreciated that the lowermost row of holes 28 would permit some liquid metal to by-pass the slabs 37 at the start of a filtering operation. However the additional filter cake would be formed quickly so a small amount of liquid metal not passing therethrough can be accepted. If, for example the launder leads to a casting machine it is expected that the beginning of each cast is cut off and remelted.

In a modification (not shown) the walls 18a and 20a could be reduced in height and the walls 19a and 21a carried

further upwardly, if desired to a position outside the box 11. In such arrangement the upper row of holes 28 would not be needed and the handles 29 would be more readily accessible.

In all the above arrangements of the basic structure of Figure 4 the frame 18, handles 29 and cage 30 may be of a suitable steel capable of withstanding the temperature of the liquid metal being filtered and these parts may conveniently be coated with a layer of refractory material .

It will also be understood that the arrangement of Figure 3 could be used as described m connection with Figure 4.