EP1032547A1

EP1032547A1 - Molten metal filtration

Info

Publication number: EP1032547A1
Application number: EP98954598A
Authority: EP
Inventors: Steven Ray; Edwin Paul Stankiewicz
Original assignee: Foseco International Ltd
Current assignee: Foseco International Ltd
Priority date: 1997-11-28
Filing date: 1998-11-13
Publication date: 2000-09-06
Also published as: CA2311810A1; AU1165399A; WO1999028273A1

Abstract

Molten metal is filtered using a filter (10, 28, 44) comprising a porous carbon foam substrate coated substantially throughout with a refractory metal or refractory compound formed by chemical vapour deposition, and having a filtration porosity of less than 90 %, and preferably at least 75 %, and a ligament solid fraction at least 0.95, using apparatus comprising a holding vessel (1, 21, 40) having an inlet (2, 41) and an outlet (3, 22, 42) for the molten metal, at least one filter housing, the or each housing containing a filter (10, 28, 44), means for holding and sealing the filter or filters (10, 28, 44) in place, and means (5, 38) for preheating the holding vessel (1, 21, 40) and the filter or filters (10, 28, 44) prior to using the apparatus for filtering molten metal.

Description

MOLTEN METAL FILTRATION

This invention relates to the filtration of molten metals, and especially to the filtration of molten aluminium and molten aluminium alloys, and the invention will be particularly described with reference to those metals.

Molten metals, and in particular aluminium have been filtered for many years by means of a variety of filtration devices, and in the aluminium industry it is now common practice to filter molten aluminium and molten aluminium alloys prior to casting. Although various types of filter have been used in the past the filters which are now most commonly used are ceramic foams, which are produced by impregnating polyurethane foam with a slurry containing refractory material and a binder, and then drying and firing the impregnated foam, and filters of bonded particulate material made by mixing together refractory material and a binder, forming the mixture to a desired shape by pressing, and heating the formed shape.

Ceramic foam filters and their use in the filtration of molten metal are described in a number of patents, for example US 3090094, US 3893917, US 3947363, US 4024056, US 5190897 and US 5520823. US 4024056 describes the application of a bevelled, removable ceramic foam filter plate having pore sizes of approximately 2 - 18 pores per linear centimetre or 5 - 45 pores per linear inch (ppi), and this type of filter is used extensively in the filtration of molten aluminium.

US Patent No. 3524548 describes a rigid, porous filter medium for aluminium made by bonding together particles of refractory material with a vitreous material which is resistant to molten aluminium. The filter is used in the form of a plate or a tube. US Patent No. 3747765 describes rigid filter tubes which are used for filtering molten metal, and which are made from a similar admixture of refractory particles bonded together with a prefused vitreous binder.

US Patent No. 4964993 describes molten metal filters which consist of at least one cylindrical, close-ended porous ceramic filter element in a vertical orientation connected to an essentially horizontal porous ceramic sealing plate filter element. The vertical filter elements may be made from bonded particulate refractory material or they may be in the form of a ceramic foam.

Ceramic foam filters have been commercially exploited successfully for many years because of their ease of use, low cost and small unit footprint relative to bonded particulate filters. However, their pore size is larger than the pore size of bonded particulate filters, and consequently filtration efficiencies are lower. Ceramic foam filters are extensively used as fine as approximately 24 pores per centimetre or 60 ppi. However as the pore size decreases they become more difficult to make because it becomes more difficult to impregnate the finer pore size polyurethane foam with the slurry of refractory material. An increased number of blocked pores are formed and consequently the porosity available for filtration is reduced. This results in a reduction of flow rate per unit surface area, and can also cause application problems during priming or a reduction in filtration capacity.

Bonded particulate filters offer a number of advantages over ceramic foams when used as filters for molten aluminium. They are generally of higher strength and higher density, and since they have lower porosity, have a more tortuous flowpath and do not contain blocked pores, they give a greater filtration efficiency in terms of removal of oxide and other inclusions from the aluminium.

However, the high density and low porosity (typically 30 - 45 %) of bonded particulate filters can be disadvantageous, and have limited their commercial exploitation. As bonded particulate filters are more dense than ceramic foams they require significantly more preheating in order to achieve complete priming of the filters at the start of filtration and optimum filtration performance, and it is very difficult to achieve the required preheat quickly and uniformly. Some bonded particulate filters also have other disadvantages which result from their preheating requirements. For example, when used in the form of plates in aluminium cast-house filtration equipment, it is necessary to preheat the plates to permit molten aluminium to flow through the plates without solidifying. This preheating of the plates is usually done by direct flame impingement on the bottom face of the plates and because some bonded particle filter plates have poor thermal shock resistance, spalling of the filter plates can result.

The high density of bonded particulate filters also makes changing bonded particulate filter elements a very labour intensive operation. The low porosity not only limits flow rates per unit of surface area, but it also increases interstitial metal velocity within the pores of the bonded particulate filter. In order to overcome the problem of the limited flow rate it is necessary to utilise bonded particulate filter systems having a very high surface area. This is normally accomplished using bundles of long tubes which require the use of a box or container with a prohibitively large footprint.

In Japanese Patent Application Laid-Open No. 1-141884 it has been proposed to use as a filter for molten metal a carbon foam substrate which has been coated with silicon carbide by chemical vapour deposition. The coated material has a porosity of 90 to 99 %, and it is stated that when filtering molten aluminium blockage of the pores and loss of aluminium are excessive if the porosity is less than 90 %, and that if the porosity exceeds 99 % efficiency of impurity removal is poor, and the strength of the foam is insufficient.

It has now been found that, in contrast to what is taught in Japanese Application Laid-Open No. 1-141884, superior filters for filtering molten metal, and in particular molten aluminium, can be produced by depositing a refractory metal or refractory compound on a carbon foam substrate by chemical vapour deposition, if the filters have a filtration porosity of less than 90 % and a ligament solid fraction of at least 0.95.

According to the present invention there is provided a method of filtering molten metal comprising providing a filter comprising a porous carbon foam substrate coated substantially throughout with a refractory metal or refractory compound formed by chemical vapour deposition, the filter having a filtration porosity as hereinafter defined of less than 90% and a ligament solid fraction as hereinafter defined of at least 0.95, and causing molten metal to flow through the filter so as to remove inclusions contained in the molten metal.

In the context of this invention a ligament is defined as one of the substantially solid interconnected struts which form the structure of a foam.

Filtration porosity is defined as the percentage of the total volume of a foam filter body which is open, interconnected space exterior to the ligaments, and available for molten metal filtration.

The void fraction is the total volume of void space within a foam filter body divided by the volume outlined by the foam filter body, and void space in a foam filter body is any volume which is not solid. For example for a foam filter body having dimensions of 100 mm x 100 mm x 100 mm, and containing 850,000 mm³ of void space, the void fraction is 850,000/(100 x 100 x 100) or 0.85.

Void fraction can be expressed in the following manner :

Void fraction = 1 - density of foam filter body theoretical density of foam ligament

For example, a 100 mm x 100 mm x 100 mm foam filter body having a mass of 420 g has a density of 0.42 g/cm³. The theoretical density of the ligament if the foam is an alumina foam is 2.8 g/cm³. Therefore the void fraction is 1 - (0.42/2.8) or 0.85.

The ligament solid fraction is defined as the volume of solid material forming the ligament divided by the volume outlined by the ligament. For example, a filter with dimensions 100 mm x 100 mm x 100 mm, having a ligament volume of 150,000 mm³ and a ligament solid volume of 75,000 mm³ has a ligament solid fraction of 0.50.

Preferably the filters used in the method of the invention have a filtration porosity of at least 75%, more preferably at least 80%.

The carbon foam is preferably a reticulated carbon foam, and the porous carbon substrate may be formed by pyrolysis of a porous organic substrate. For example, a polyurethane foam may be pyrolised, and if desired the polyurethane foam may first be impregnated with a resin or a similar organic material. European Patent Application Publication No. 747124A describes a suitable method for the production of a carbon foam in which polyurethane foam material is impregnated with a resin, and the resin impregnated foam is then pyrolised at a temperature of 600 to 1200 °C to convert the polyurethane and resin to carbon.

As used herein the term chemical vapour deposition includes both chemical vapour deposition and chemical vapour infiltration. In chemical vapour deposition solid phase refractory materials are nucleated and grown from gas phase precursors, for example a coating of silicon carbide may be deposited on a porous carbon substrate by the decomposition of methyltrichlorosilane. The chemical vapour deposition may be carried out by known techniques, and suitable methods of coating carbon substrates by chemical vapour deposition are described in US Patents Nos. 5154970, 5283109 and 5372380 and in European Patent Application Publication No. 747124A.

The refractory metal or refractory compound which is coated on to the porous carbon substrate by chemical vapour deposition to produce the filter must be compatible with the molten metal to be filtered, i.e. it must be resistant to the molten metal, and it must not contaminate the molten metal. Refractory metals which can be applied to porous carbon substrates by chemical vapour deposition to produce filters for use in the method of the invention include tungsten and molybdenum. Suitable refractory compounds include silicon carbide, silicon nitride, zirconium carbide, zirconium nitride, zirconium boride, niobium carbide, niobium nitride, niobium boride, titanium carbide, titanium nitride, titanium boride, hafnium carbide, hafnium nitride and hafnium boride. The preferred coating material is silicon carbide as it has suitable physical properties, such as relatively low density and a high thermal conductivity, and enables filters for use in the method of the invention to be made economically.

Particularly when the method of the invention is practised using a reticulated carbon foam, which has been coated with a refractory metal or metal compound by chemical vapour deposition, the filter has a number of advantages over conventional ceramic foam filters made by impregnating an organic foam with a slurry of particulate refractory material.

The deposition of coated refractory metal or metal compound is preferably a minimum of 0.25 g/cm³. The strength of the individual ligaments and of the filter body itself are dependent on the amount of solid matter coated on to the ligaments. The nature of the chemical vapour deposition process is such that small variations in coating density throughout the carbon foam may result, and if the intended coating density is less than 0.25 g/cm³ these variations may cause the filter to have insufficient strength of corrosion resistance to withstand the industrial environment.

Individual ligaments can be broken in handling if they are not coated with a sufficient amount of refractory metal or refractory compound, and a filter body can be broken in handling if it does not have sufficient strength. It has been found in practice that filters which are to be used to filter molten aluminium for greater than one week need to have a minimum crush strength of 35 kg/cm² (500 lb/in²), and the filters used in the method of the invention meet this requirement. An important feature of the filter used in the method of the invention is that, in the as manufactured state, essentially all the void fraction of the filter body is available for molten metal flow, and the void fraction is the same as the above defined filtration porosity. Higher accessible porosity results in higher molten metal flow rate or lower interstitial velocity at a constant pore size and surface area. A lower interstitial velocity is associated with a higher capture efficiency of inclusions in molten metal and therefore greater overall filtration efficiency. The higher porosity of the filter also results in a greater storage volume for captured inclusions, and this means that a single filter is capable of filtering a greater throughput of metal before clogging of the filter, and the need to replace the filter, occurs.

This is in contrast to traditional ceramic foam filters which have a number of voids contained in the hollow centre of the ligament, and as microporosity with the ligament itself. These portions of the void fraction are inaccessible to molten metal flow because of the high pressure which would be required to force molten metal into such fine capillaries, and so are not included in the filtration porosity. Additionally, many voids can become inaccessible to molten metal flow due to blocked areas being formed in the ceramic foam. This further decreases the filtration porosity compared with the void fraction.

Figures 1 and 2 of the accompanying drawings show a comparison of a traditional ceramic foam filter and a chemical vapour deposited refractory metal or refractory compound coated carbon foam filter as used in the method of the invention.

Figure 1 is a diagrammatic representation of a section of a ligament of a traditional ceramic foam filter produced by impregnating a polyurethane foam with a ceramic slurry and firing to burn out the foam. The filter has a hollow ligament core(A) and a ceramic ligament (B) containing voids and having a solid fraction of 0.50. Figure 2 is a diagrammatic representation of a section of a ligament of a filter for use in the method of the invention produced by coating a carbon foam substrate with a refractory metal or refractory compound by chemical vapour deposition. The filter has a solid carbon ligament core (C) and an essentially void free ligament coating of refractory metal or refractory compound (D) having a minimum ligament solid fraction of 0.95.

In addition to reducing the filtration porosity, voids in the ligament also reduce the strength of the ceramic foam filter body.

In order to be useful in an industrial filtration process, a filter body must be strong and durable in order to withstand handling, installation and use. On the other hand to operate efficiently as a filter, the filter body must have a sufficiently high filtration porosity to operate at industrial flow rates with low pressure drops and a minimum filtration system footprint. All other factors being the same, the greater the void fraction of the filter body the lower its strength.

In the filters used in the method of the invention the strength of the filter body is maximised at a given void fraction as 100% of the void fraction is incorporated in the filtration porosity.

For example, in producing a silicon carbide filter for use in the method of the invention, a carbon foam substrate with nominally 3% volume solid or 97% filtration porosity and essentially void free carbon ligaments is coated with a uniformly distributed, essentially void free silicon carbide coating to a minimum amount of 0.32 g/cm³. A void free solid volume of chemical vapour deposited silicon carbide has a density of 3.2 g/cm³, and a silicon carbide coating of 0.32 g/cm³ is a 10% solid by volume of silicon carbide. The total volume percent of solids is the 3% carbon and the 10% silicon carbide. The filter body has a void fraction of 0.87 all of which is accessible to molten metal so the filter has a filtration porosity of 87%. The filter contains no binders, no sintering aids (because no firing or sintering of the filter is necessary after its production) and no impurities. The filter therefore has improved corrosion resistance in contact with molten metal and a higher strength to weight ratio. Compared with a conventional ceramic foam filter the filter is both stronger and has a lower density. As the density of the filter is lower the filter is more readily preheated.

As a result of these advantages, when using the method of the invention, it is possible to utilise a filtration system having higher filtration efficiency and longer life, when compared to a system which contains ceramic foam filters or bonded particulate filters. It may also be possible to design the filtration system such that it is smaller and more compact.

The filters used in the method of the invention may be in the form of plates, but they are preferably in the form of tubes. In the aluminium industry a plurality of tubes is commonly referred to as a cartridge. Although plates are commonly used to filter molten aluminium they are disadvantageous because, in order to provide sufficient filter surface area to enable large quantities of molten aluminium at an acceptable rate of throughput, it is necessary to make the filter plates very large, and the furnace or vessel in which the filtration is carried out, also has to be large in order to accommodate the filter plates. For this reason, there is a desire to use filters whose shape provides a greater surface area, and which require relatively less space to accommodate them, and tube or cartridge filters are advantageous in these respects.

Tube or cartridge filters made from conventional ceramic foam have achieved little commercial use because such filters are difficult to make, they have relatively low strength, they have blocked pores, they are difficult to preheat and to wet with the molten metal, and they are only suitable for applications in which they are in contact with molten metal for a relatively short time. Therefore, tube or cartridge filters, which are used commercially in the aluminium industry, are usually bonded particulate refractory filters. Carbon foams, coated with a refractory metal or refractory compound by chemical vapour deposition, and having a filtration porosity of less than 90 % and a ligament solid fraction of at least 0.95, suffer from none of these disadvantages, and that is why they are particularly suitable for use in the method of the invention as tubes or cartridges.

The method of filtering molten metal may be practised, for example, in filtration apparatus comprising a holding vessel having an inlet and an outlet for the molten metal, at least one filter housing, the or each housing containing a filter comprising a porous carbon foam substrate coated substantially throughout with a refractory metal or refractory compound formed by chemical vapour deposition and having a filtration porosity of less than 90% and a ligament solid fraction of at least 0.95, means for holding and sealing the filter or filters in place, and means for preheating the holding vessel and the filter or filters prior to using the apparatus for filtering molten metal.

Therefore, according to a further feature of the invention there is provided apparatus for filtering molten metal as hereabove described.

The filters used in the above apparatus may be filter plates but in a preferred embodiment filter tubes are used, and the filters are preferably substantially horizontally disposed tubular shaped filter elements.

The sealing means should be capable of sealing the filters in place so as to ensure that all the molten metal passes through the filters without exerting excessive force on the filters, and it is desirable that the configuration of the housing and the sealing means used are such that the filters may be readily installed and removed. The heating means may be for example one or more gas fired burners or electrical heaters, which may be located in a lid which is fitted to the top of the holding vessel. It is important that the filters are located in the holding vessel in relation to the inlet such that when the holding vessel is filled with molten metal via the inlet the height of the surface of the molten metal in the holding vessel above the filters is sufficient to prime the filters, i.e. to initiate flow of molten metal through the filters.

Since the inner surface of the vessel must withstand molten metal such as aluminium and should not contaminate the metal, the vessel should either be made from or be lined with a refractory material which meets those requirements. In order to reduce heat loss through the walls of the vessel a lining of refractory heat-insulating material may be installed between the shell of the vessel and the refractory lining. Similarly, to reduce heat losses the lid may also be lined with a thermally insulating material.

Examples of suitable refractory materials for forming the inner, metal contacting surface of the vessel are fused silica based castable materials having a density of approximately 1.8 g/cm³. Examples of thermally insulating materials for the walls of the vessel and for the lid are refractory castable materials having a density of below 1.4 g/cm³ and also ceramic fibre containing materials in block, blanket or module form.

The invention is illustrated with reference to Figures 3 to 6 of the accompanying drawings in which :-

Figure 3 is a horizontal section through molten metal filtration apparatus according to the invention containing a plurality of filter tubes

Figure 4 is a vertical section through the apparatus of Figure 3

Figure 5 is a detail of the molten metal filtration apparatus of Figure 3 showing how the filter tubes are located in the apparatus and held in place and

Figure 6 is a vertical section through another embodiment of molten metal filtration apparatus according to the invention. Referring to Figures 3 to 5, an apparatus for filtering molten metal comprises a refractory lined vessel 1 , an inlet 2 for the molten metal, and outlet 3 for the molten metal, and a refractory lined lid 4 having heating means 5 located therein. The interior of the vessel 1 is divided into an inlet chamber 6 and an outlet chamber 7 by a refractory baffle plate 8 extending across and near to the top of the vessel 1 and having an aperture 9 connecting the inlet chamber 6 and the outlet chamber 7. An array of eleven filter tubes 10 in the form of pyrolised reticulated carbon foam which has been coated with silicon carbide by chemical vapour deposition, and having a filtration porosity of 87% and a ligament solid fraction of 0.995, is located near the base of the vessel 1 so that the filter tubes 10 are in three rows, one above the other. One end of each of the filter tubes 10 is inserted in a recess in a refractory plate 11 , and the opposite end of each of the filter tubes is located in an apertured recess in a refractory plate 12 so that the apertures in the recesses are aligned with the aperture 9 in the baffle plate 8. The filter tubes 10 are held and sealed in position without exerting excessive force on the filter tubes 10 by holding means 13 which fits over the filter tubes 10 near their end which is adjacent the aperture 9 in the baffle plate 8. The location of the filter tubes 10 in the vessel 1 is such that the height of the top of the vessel 1 above top row of the filter tubes 10 is sufficient for the filters to be primed when the inlet chamber 6 is filled with molten metal.

Before the apparatus is used to filter molten metal the interior of the vessel 1 and the filter tubes 10 are preheated by the heating means 5, which may be for example a gas fired burner or an electrical heater. In the case of molten aluminium filtration it is necessary for the preheating to be sufficient for the inner surface of the filter tubes 10 to be heated to at least 600 °C. The heating means 5 also maintains the temperature of the molten metal on the vessel 1.

In use molten metal enters the inlet chamber 6 via inlet 2, is filtered through filter tubes 10, passes into the outlet chamber 7 through the aperture 9 in the baffle plate 8, and exits the vessel via outlet 3. Referring to Figure 6, an apparatus for filtering molten metal comprises a refractory lined vessel 21 having an inlet for the molten metal (not shown), and an outlet 22 for the molten metal, and a refractory lined lid 23. The interior of the vessel 21 is divided into an outlet chamber 24 and an inlet chamber 25 by a refractory baffle plate 26 extending across the vessel 21 and having an aperture 27 connecting the outlet chamber 24 and the inlet chamber 25. Four filter tubes 28 in the form of pyrolised reticulated carbon foam which has been coated with silicon carbide by chemical vapour deposition, and having a filtration porosity of 87% and a ligament solid fraction of 0.995, are located in line near the base of the vessel 21 in inlet chamber 25. One end of each of the filter tubes 28 is inserted in a recess in a refractory plate 29, and the opposite end of each of the filter tubes is located in an apertured recess in a refractory plate 30 so that the apertures in the recesses are aligned with the aperture 27 in the baffle plate 26. The filter tubes 28 are held and sealed in position without exerting excessive force on the filter tubes 28 by holding means 31 which fits over the filter tubes 28 near their end which is adjacent the aperture 27 in the baffle plate 26. The location of the filter tubes 28 in the vessel 21 is such that the height of the top of the vessel 21 above the filter tubes 28 is sufficient for the filters to be primed when the inlet chamber 25 is filled with molten metal.

The refractory lined lid 23 has two apertures 32, 33 which are in line with apertures 34, 35 in the top of the vessel located above the chambers 24, 25 respectively, and two plugs 36, 37. Before the apparatus is used the plugs 36, 37 are raised so that they close the apertures 32, 33 in the lid 23, and the interior of the lid 23 and of the vessel 21 are preheated by means of gas burner 38. After the preheating operation the plugs 36, 37 are lowered so as to close the apertures 34, 35 in the top of the vessel.

In use molten metal enters the inlet chamber 25, is filtered through the filter tubes 28, passes through the aperture 27 in the baffle plate 26 into the outlet chamber 24, and exits the vessel 21 via the outlet 22. During filtration molten metal in the vessel 21 is maintained at the desired temperature by means of immersion heaters 39 located in the inlet chamber 25 and outlet chamber 24.

The following Examples will also serve to illustrate the invention :-

EXAMPLE 1

In practice it has been found that filtration tubes which are to be used for the filtration of molten aluminium on a commercial basis must have a minimum crush strength of greater than 35 kg/cm² (500 lb/in²). A series of experiments was carried out to establish the relationship between crush strength and filtration porosity.

Carbon foam substrates of nominally 97% filtration porosity and having void free carbon ligaments were coated with various levels of silicon carbide by chemical vapour deposition, to produce plate filters having a ligament solid fraction of 0.995 and a range of filtration porosities. A number of coated plates was produced for each level of silicon carbide, and hence each porosity. A section was cut from each plate and its crush strength was determined. The ligament solid fraction was measured by mounting and polishing a section of the plates, and inspecting the ligaments visually.

The silicon carbide solid by volume (%), the filtration porosity (%) and the minimum crush strength for each level of silicon carbide coating are shown in the table below.

EXAMPLE 2

Two plate filters 30.5 cm x 30.5 cm x 2.5 cm thick were prepared, one having a pore count of approximately 26 pores per centimetre (ppc) or 65 ppi and the other having a pore count of approximately 32 ppc or 80 ppi, both having filtration porosity of 87% and a ligament solid fraction of 0.995. Reticulated polyurethane foam was pyrolised to produce carbon foam and the carbon foam was machined to the required size. The carbon foam was then coated with silicon carbide by chemical vapour deposition to produce the filter. A filter was inserted in a 50 cm deep filter bowl, and the filter and bowl were preheated using a gas torch. It was found that due to their high thermal conductivity and low thermal mass the filters heated up almost instantly. Approximately 3175 kg of molten aluminium alloy 6061 were then poured into the filter bowl and passed through the filter. In the test using the 32 ppc or 80 ppi filter the molten aluminium alloy was deliberately contaminated by the addition of 5 to 50 micron size alumina. In the case of the 26 ppc or 65 ppi filter it took about 1 hour for all the molten aluminium alloy to be filtered, corresponding to a flow rate of approximately 0.05 kg/min/cm². In the case of the 32 ppc or 80 ppi filter a flow rate of approximately 0.03 kg/min/cm² was used. In both tests samples of the alloy were taken after filtration, and tested for the presence of inclusions using an inclusion concentration technique. Both samples were completely free of inclusions.

EXAMPLE 3 A tube filter having an external diameter of 4 inches and an internal diameter of 2 inches was produced by pyrolising reticulated polyurethane foam having a pore count of 26 ppc or 65 ppi, and coating the resulting carbon foam with silicon carbide produced by chemical vapour deposition so as to produce a filter having a filtration porosity of 87% and a ligament solid fraction of 0.995. The tube filter was cut to a length of 205 mm and tested as a filter for molten aluminium in a filter box as shown in vertical section in Figure 7 of the accompanying drawings.

The filter box 40 which was lined with refractory material had an inlet chamber 41 and an outlet chamber 42 separated by a refractory baffle plate 43. The filter tube 44 was positioned horizontally near the bottom 45 of the filter box 40 with one end inserted in refractory plate 46 and the other end inserted in a recess 47 in the baffle plate 43. A fibre gasket (not shown) was placed around the ends of the filter tube 44 to seal the filter tube 44 in place, and prevent bypass of molten aluminium. The baffle plate 43 had an aperture 48 with which the end of the filter tube 44 was aligned. The height of the inlet chamber 41 above the filter tube 44 was 240 mm. The inside of the filter box 40 and the filter tube 44 were preheated by means of two gas burners inserted in the inlet chamber 41 and the outlet chamber 42 until the filter tube glowed red. Care was taken to avoid direct flame impingement from the burner in the inlet chamber 41 on to the filter tube 44. After preheating the surface of the filter tube 44 showed no evidence of cracking.

Molten aluminium alloy of purity 99.5 % was poured into the filter box 40 from a tilting furnace of 800 kg capacity at a temperature of 746 °C. The molten alloy started to prime the filter tube 44 when the height of the molten alloy above the filter tube 44 reached approximately 200 mm, as indicated by air trapped in the filter tube 44 escaping to the surface, and the appearance of molten alloy in outlet chamber 42. Once a steady state was reached the difference in head height of the molten alloy in the two chambers was approximately 25 mm. Three billets were cast in sequence. Each billet was approximately 3.5 m long and 190 mm in diameter. The fill speed was 95 mm/minute which equates to a flow rate of about 8 kg/minute for the total of 800 kg of molten alloy. There was about 15 minutes between casts so the filter tube 24 was immersed in the molten alloy for a total of 2.5 hours.

After the third billet had been cast the filter tube 24 was removed from the filter box 20 and inspected. There was no evidence of any form of degradation in the filter material. The effectiveness of the filter material in removing inclusions from the molten aluminium alloy was confirmed by sectioning the filter tube, and examining the section microscopically.

Claims

1. A method of filtering molten metal comprising providing a filter comprising a porous carbon foam substrate coated substantially throughout with a refractory metal or refractory compound formed by chemical vapour deposition, and causing molten metal to flow through the filter so as to remove inclusions contained in the molten metal, characterised in that the filter has a filtration porosity as hereinbefore defined of less than 90% and a ligament solid fraction as hereinbefore defined of at least 0.95.

2. A method according to Claim 1 characterised in that the filter has a filtration porosity of at least 75%.

3. A method according to Claim 2 characterised in that the filter has a filtration porosity of at least 80%.

4. A method according to any one of Claims 1 to 3 characterised in that the carbon foam substrate is a reticulated carbon foam.

5. A method according to any one of Claims 1 to 4 characterised in that the refractory metal is tungsten or molybdenum.

6. A method according to any one of Claims 1 to 4 characterised in that the refractory compound is silicon carbide, silicon nitride, zirconium carbide, zirconium nitride, zirconium boride, niobium carbide, niobium nitride, niobium boride, titanium carbide, titanium nitride, titanium boride, hafnium carbide, hafnium nitride or hafnium boride.

7. A method according to any one of Claims 1 to 6 characterised in that the carbon foam substrate is coated to a density of at least 0.25 g/cm³.

8. A method according to any one of Claims 1 to 7 characterised in that the filter has a minimum crush strength of at least 35 kg/cm².

9. A method according to any one of Claims 1 to 8 characterised in that the filter is in the form of a plate or a tube.

10. Filtration apparatus comprising a holding vessel (1 , 21 , 40) having an inlet (2, 41) and an outlet (3, 22, 42) for the molten metal, at least one filter housing, the or each housing containing a filter (10, 28, 44) comprising a porous carbon foam substrate coated substantially throughout with a refractory metal or refractory compound formed by chemical vapour deposition, means for holding and sealing the filter or filters (10, 28, 44) in place, and means (5, 38) for preheating the holding vessel (1 , 21 , 40) and the filter or filters (10, 28, 44) prior to using the apparatus for filtering molten metal, characterised in that the filter or filters (10, 28, 44) have a filtration porosity of less than 90% as hereinbefore defined and a ligament solid fraction of at least 0.95 as hereinbefore defined.

11. Apparatus according to Claim 10 characterised in that the filter or filters (10, 28, 44) have a filtration porosity of at least 75%.

12. Apparatus according to Claim 11 characterised in that the filter or filters (10, 28, 44) have a filtration porosity of at least 80%.

13. Apparatus according to any one of Claims 10 to 12 characterised in that the carbon foam substrate is a reticulated carbon foam.

14. Apparatus according to any one of Claims 10 to 13 characterised in that the refractory metal is tungsten or molybdenum.

15. Apparatus according to any one of Claims 10 to 13 characterised in that the refractory compound is silicon carbide, silicon nitride, zirconium carbide, zirconium nitride, zirconium boride, niobium carbide, niobium nitride, niobium boride, titanium carbide, titanium nitride, titanium boride, hafnium carbide, hafnium nitride or hafnium boride.

16. Apparatus according to any one of Claims 10 to 15 characterised in that the carbon foam substrate is coated to a density of at least 0.25 g/cm³.

17. Apparatus according to any one of Claims 10 to 16 characterised in that the filter or filters (10, 28, 44) have a minimum crush strength of at least 35 kg/cm².

18. Apparatus according to any one of Claims 10 to 17 characterised in that the filter or filters (10, 28, 44) are in the form of plates or tubes.

19. Apparatus according to Claim 18 characterised in that the vessel (1 , 21) is divided into an inlet chamber (6, 25) and an outlet chamber (7, 24) by a refractory baffle plate (8, 26) extending across the vessel (1), the baffle plate (8) having an aperture (9, 27) connecting the inlet chamber (6, 25) and the outlet chamber (7, 24), filters (10, 28) in the form of tubes are located near the base of the vessel (1 , 21) with one end of each filter tube (10, 28) inserted in a recess in a refractory plate (11 , 29), and the opposite end located in an apertured recess in a refractory plate (1 , 30), so that the apertures in the recesses are aligned with the aperture (9, 27) in the baffle plate (8, 26), and the filter tubes (10, 28) are held and sealed in position by holding means (13, 31) which fits over the filter tubes (10, 28).

20. Apparatus according to Claim 19 characterised in that the vessel (21) has a refractory lined lid (23) having two apertures (32, 33) in line with apertures (34, 35) in the top of the vessel located above chambers (25, 24) respectively, and two plugs (36, 37) which when raised close the apertures (32, 33) in the lid (23), and when lowered close apertures (34, 35) in the top of the vessel.