WO1997018339A2

WO1997018339A2 - Filtration device for filtering molten metal

Info

Publication number: WO1997018339A2
Application number: PCT/GB1996/002779
Authority: WO
Inventors: Philip Graham Enright
Original assignee: N-TEC Ltd
Current assignee: N-TEC Ltd
Priority date: 1995-11-11
Filing date: 1996-11-11
Publication date: 1997-05-22
Anticipated expiration: 1998-05-11
Also published as: GB9523124D0; GB2306896A; AU7579396A; GB9623382D0; WO1997018339A3; GB2306896B

Abstract

A filtration device for filtering molten metal comprising a body defining a cavity for receiving molten metal, an outlet for the molten metal from the body, and means for holding a filter between the cavity and outlet for concentrating inclusions in the metal in the cavity, characterised in that the body comprises refractory fibrous material. Use of such a filtration device enables the rapid melting of a solid metal sample in the device to form a representative liquid metal sample which can then be filtered.

Description

FILTRATION DEVICE FOR FILTERING MOLTEN METAL

The invention relates to a filtration device comprising a sample holder and filter for

example for separating inclusions such as oxides and borides from fluids such as

molten metal. In particular the invention relates to filtration devices to enable

characterisation of molten aluminium.

It is known to provide devices for filtering a sample of molten metal on-line in a

foundry. The filtration process allows the molten metal to pass through the device

whilst, at any same time, concentrating any inclusions above a filter in the device. By

filtering a known quantity of molten metal, it is possible to quantify the concentration

of the inclusions present in the foundry process. This quantification can be achieved

after filtration for example by optical analysis and quantitative metallography of the

inclusions which have built up on the filter surface during filtration.

In such systems, it is known to provide a crucible made of refractory material such as

plumbago having a filter in its lower end made of porous graphite, alumina or silicon

carbide for example. It is also known to provide an over-pressure above the filter

using a pressure chamber or an under-pressure below the filter using a suction tube and

reservoir, in order to draw the molten metal through the filter into a catchment

container.

Several problems exist in the known systems such as that crucibles made of plumbago ,-„„„ O 97/18339

2 require preheating to a temperature in the order of or above the molten metal

temperature pπor to filtration and therefore do not lend themselves to easy handling

in operation. Preheating is generally earned out m a separate furnace which results

in a cumbersome and not fully portable operation. Additionally, it is generally the case

that only molten metal samples are processed because of difficulties and

inconsistencies associated with remelting previously solidified metal samples since

using the procedures of the pπor art, it is not possible to guarantee the results of the

test. That is, additional impunties are added in remelting, for example if earned out

in a first crucible in a furnace before transferπng the sample to a heated plumbago

crucible.

An object of the invention is therefore to avoid or at least mitigate these problems ol

the pπor an. Accordingly, one aspect of the invention provides a filtration device, or

crucible, for filteπng molten metal compnsmg a body defining a cavity for receiving

molten metal, an outlet from the body for the molten metal, and means for holding a

filter between the cavity and outlet for concentrating inclusions in the metal in the

cavity, characteπsed in that the body compnses refractory fibrous matenal.

Beneficially, the fibrous matenal can be a ceramic such as alummo-sihcate fibres

which do not absorb very much energy from the molten metal and therefore do not

need to be preheated pnor to filtration. Furthermore, because of the degree of thermal

insulation provided by these crucibles, there is no radiated heat from their outside

surfaces and therefore the filtration unit can be more compact and handled more easily and conveniently.

To ensure that the pressurising gas does not permeate through the body and base of the

fibrous crucible and thereby adversely affect the pressurizing of the molten metal

through the filter, it is preferable that the body comprises an air impermeable layer

such as a masked or coated region. This mask or coating should be thick enough to

prevent gas leakage and preferably extend at least across the base of the crucible such

as to form a complete fillet at the junction between the filter and the base of the body

of the crucible and possibly for some distance up the outside edges, or at least to a

point beyond any gasket adjacent the base of the crucible. The coating is preferably

thin enough and of a material such that it does not crack during drying and generally

provides an impermeable and continuous surface layer. A layer in the order of or less

than 1mm thick of diluted Fraxbond is prefened.

Another aspect of the invention is to use the thermally insulating and non-electrically

conducting nature of the crucibles to enable the rapid induction melting of a solid

metal sample (charge) in the crucible to form a representative liquid metal sample

which can then be pressure filtered. It is preferable to provide a lid to the system in

order to physically confine the charge during melting and to control the degree of

turbulence in the system caused by the electro magnetic forces generated in the melting

cycle. The advantage of such a confined rapid melting procedure is that the quality of the

sample with respect to oxide films and other solid inclusions in the metal is not

changed significantly during the remelting process and therefore the inclusion

measurements after remelting a solid charge are essentially the same as measurements

taken from an initially liquid charge taken from the same starting metal. Also, the

temperature control in the system is very good because of the low heat capacity of the

crucible.

A further aspect of the invention provides a filter comprising, for example, alumina

granules, preferably bonded with silica, silica carbide or sintered metal wherein

preferably the surfaces of the filter which contact the filtration material in use are

unfinished and not ground. Such a filter is preferably included in a container

according to the earlier aspect of the invention. An aspect of the invention therefore

provides a filtration device comprising a crucible and a filter according to the other

aspects of the invention.

A yet further aspect of the invention provides a method of making a filtration device

comprising the steps of any of the individual processes identified in the following

description.

Embodiments of the invention will now be described, by way of example only, with

reference to the accompanying drawings in which; Figure 1 is a schematic plan view from above of a filtration device according

to the invention;

Figure 2 is a schematic, cross-sectional, side elevation view of the device

shown in figure 1 fitted in part of a filtration system;

Figure 3 is a plan view from below of the device shown in figures 1 and 2; and

Figure 4 is a cross-sectional view of the lower end of the device shown in

figures 1 , 2 and 3.

A filtration device 10 according to the invention is shown in drawings 1 to 4

comprising a crucible 12 having an open upper end and a lower end 14 containing a

relatively naπower outlet 19. The inner surface of lower region 14 comprises a

relatively large aperture 16 whilst the outer surface comprises a naπower aperture 18

which define the outlet 19 from the crucible 12. Additionally, the apertures provide

a recess region comprising an annular ledge 20 on which a filter 22 can be placed

thereby presenting a first surface 24 to the inside of crucible 12 and a second surface

26 to the outlet-side. Filter 22 can be held in position in aperture 16 using a refractory

cement 28, preferably Fraxbond, which is available from Carborundum Coφoration,

is used. Additionally, the inner edges of the filter are sealed to prevent flow of molten metal

around the filter. A film 30 is applied to the inner surface of crucible 12. The film

should be a refractory material impermeable to the molten metal such as aluminium

under pressure and preferably contains flake or platelets together with refractory

cement. Prefened materials contain mica or kaolin and one such material is micawash

available from Microfine Minerals Limited.

Beneficially, as well as masking the edge regions of filter 22 to prevent flow of molten

metal around the filter, the film 30 enables the active area of filtration of surface 24

to be masked off to a desired shape, such as circular, and size.

Furthermore, a lowermost film 32 is used to mask the lower region 14 of crucible 12

thereby restricting movement of fluid such as air through the bottom end of the

crucible. This film should be continuous and extend at least across the base and

possibly part way up the side walls. It should form a complete fillet at the junction

with the filter 22. The film 32 can extend over the lower radius of the crucible to a

point 32o shown in figure 4, thereby restricting any downward flow of gas through the

base 14 of crucible 12. Preferably film 32 is made of diluted Fraxbond.

Preferably, the crucible is made of a fibroOus refractory material such as alumino¬

silicate. A prefened material is procal available from Foseco since it is inert to molten

aluminium and iron and has a very low thermal conductivity and low heat capacity. Such material provides a crucible which is readily manually handleable (albeit

preferably with gloves) even when full of molten metal at temperatures up to 1000°C.

To form a crucible according to the invention, the alumino-silicate fibres are preferably

bonded with an organic binder, moulded into shape and fired to fix or remove the

organic binder. The crucible is then dipped to impregnate it with a rigidizer such as

sodium silicate and fired in order to give it sufficient mechanical strength in use such

as to support the molten metal and facilitate manual handling before and after the

filtration process.

In a prefened form the filter comprises alumina granules, or girconia granules, bonded

with silica, or silicon carbide, or sintered metal such as stainless steel. Preferably the

filter surfaces 24 and 26 are unfinished and not ground since this is found to enhance

consistency of flow characteristics. The use of the above materials for the filter and

crucible is beneficial not least since one or more of the materials is inert with respect

to aluminium, magnesium, iron and nickel based materials. Accordingly, molten

metals consisting mainly of these elements can be filtered using the present devices.

Preferably the average pore size in the filter is between 50 and 200 micrometres (μm)

and more preferably in the order of 90 micrometres (μm).

Referring to figure 2, it can be seen that in use the device can be inserted into a

filtration system FS comprising upper lid L, base B and gaskets G positioned against the upper and lower ends of crucible 12. The gaskets can be attached to the crucible

for example by using an adhesive paste. Accordingly, lowermost gasket G can be

added at the time of forming film 32. Molten metal may then be added to crucible 12

or alternatively a solidified sample within a crucible 12 can be heated to a desired

temperature using an induction coil heater, before allowing the metal M to pass

through filter 22 through outlet O of system FS. In the event that an overpressure is

used to effect filtration, fluid commumcation to a compressor can be provided via an

inlet I. In the event that molten metal is drawn through filter 22 by an under pressure,

mlet I acts as a vent to equalise pressure above the molten metal M within the crucible

12.

Additionally, especially if for example the upper end of crucible 12 is sealed against

a gasket duπng filtration, it is possible to provide an aperture or apertures in the upper

wall of the crucible to allow fluid flow into the crucible.

In order to form a filtration device according to the invention from basic matenals, it

is possible to manufacture a crucible 12 as descπbed above or purchase a procal 6

crucible from Foseco, i.e. a crucible consisting pnncipally of alumino silicate fibre and

silica, and to manufacture a filter as described above. A clean crucible without any

internal or external cracks should be used wherein ledge 20 in the base area should be

formed and preferably made flat. Cement such as Fraxbond 715 should then be added

to apermre 16 for fixing the filter in place. For uniform results the filter and the oπentation thereof should be made consistent. Excess cement should be removed from

around the filter such as in aperture 18 and no bubbles should be present since this can

lead to failure after drying. Region 28 of cement should fill the gap between the filter

22 and crucible 12 without unduly coveπng too much of the filter. The crucible should

then be left to dry, for example for 2 hours at ambient temperature. Lower film 32 can

then be added using an air impermeable paste such as Fraxbond 715 with 10 per cent

water which is painted on the base of the crucible and allowed to dry. The masking

surface 30 can then be painted on the inside of crucible 12 using for example one part

Mica mixed with three parts water. The inner surface of crucible 12 should be painted

together with the adhesive region 28. Preferably a stencil is placed over the centre of

filter 22 to prevent painting the wash over the central region of the filter. The crucible

should then be left to dry at ambient conditions pnor to inspection followed by an

oven-dry mg process at 150°C for half and hour. The method according to the

invention includes a method of forming a filtration device compnsing any one of the

above steps or any combination of the individual steps.

Claims

1. A filtration device for filtering molten metal comprising a body defining a cavity for receiving molten metal, an outlet for the molten metal from the body, and means for holding a filter between the cavity and outlet for concentrating inclusions in the metal in the cavity, characterised in that the body comprises refractory fibrous material.

2. A filtration device according to claim 1 wherein, the fibrous material comprises a ceramic such as alumino-silicate fibres.

3. A filtration device according to claim 1 or 2 wherein the body comprises a fluid impermeable Iayer such as a masked or coated region, adjacent the outlet

4 A filtration device according to claim 3 wherein the fluid impermeable Iayer is thick enough to prevent gas leakage therethrough.

5. A filtration device according to claim 3 or 4 wherein the fluid impermeable Iayer extends at least across the base of the device to form a complete fillet at the junction at the outlet between the filter and the base of the body

6. A filtration device according to claim 5 wherein the fluid impermeable Iayer extends some distance up the outside edges of the body.

7. A filtration device according to claim 6 wherein the device compnses a gasket on the outside of the body proximal the outlet, and the fluid impermeable Iayer extends some distance beyond the gasket up the outside edges of the body

8. A filtration device according to any of claims 3 to 7 wherein the fluid impermeable Iayer is in the order of or less than 1 mm thick.

9. A filtration device according to any of claims 3 to 8 wherein the fluid impermeable Iayer comprises diluted Fraxbond.

10. A filtration device according to any preceding claim wherein the filter holding means compnses a ledge in the body, which ledge is preferably annular and preferably substantially smooth.

11. A filtration device according to claim 10 wherein the filter is fixed to the holding means by a cement such as Fraxbond 715.

12. A filtration device according to any preceding claim wherein the filter has a first surface operably presented to the cavity, which surface is partially masked to provide an active filtration area in use.

13. A filtration device according to claim 12 wherein the mask comprises refractory material impermeable to molten metal.

14 A filtration device according to claim 13 wherein the film comprises flake or platelets and a refractory cement.

15. A filtration device according to any of claims 12 to 14 wherein the film comprises mica or kaolin.

16 A filtration device according to any of claims 13 to 15 wherein the mask extends beyond the filter onto the body surface adjacent thereto

17. A filter for inhibiting the flow of inclusions in molten metal, comprising sintered granules, such as alumina or zirconia.

18. A filter according to claim 17 compnsing a granule bonding agent such as silica, silica carbide or sintered metal.

19. A filter according to claim 17 or 18 compnsing a first surface which operably contacts the molten metal, the surface comprising an active region which operably allows flow of molten metal thorugh the filter and which region is unfinished.

20. A filter according to claim 19 wherein the active region is unground after sintering.

21. A filter according to claim 20 or 21 wherein the active region is defined by a film on the first surface, which film is impermeable to molten metal.

22. A filter according to claim 21 wherein the film comprises platelet mateπal such as mica or kaolin

23. A filter according to any of claims 17 to 22 comprising a second surface through which molten metal leaves the filter, the second surface compnsing a region which is unfinished and preferably unground.

24. A filter according to claim 23 wherein the active region of the second surface is defined by a film of platelet material such as mica or kaolin.

25. A filter according to any of claims 17 to 24 or independently thereof, wherein the average pore size is in the region of 50 to 200 (μm) and preferably about 90 (μm).

26. A method of making a filtration device for separating inclusions from molten metals, comprising the step of fabricating a body, defining a cavity for the molten metal, of refractory fibrous matenal.

27. A method according to claim 26 wherein the body is formed by bonding the fibrous material together using a binder which is fixed for example during firing.

28. A method according to claim 25 or 26 wherein the fibrous material is alumino silicate and the binder is an organic material.

29. A method according to claim 26, 27 or 28 further compnsing the step of rigidisiπg the body by impregnation of a ngidizer.

30. A method according to any of claims 26 to 29 comprising the step of forming means within the body for holding a filter.

31. A method according to the previous claim comprising the step of fixing a filter in the holding means using an adhesive.

32. A method according to any preceding claim comprising the step of sealing the body adjacent an outlet for the molten metal thereby to inhibit flow of fluids, such as air, through the body. 33 A method according to any of claims 26 to 32 comprising the step of temporaπly masking an active region of the filter while applying a film to the filter, thereby operably to allow molten metal to flow through the active area but not the film

34 A method according to the preceding claim compnsing the step of extending the film across the body adjacent to the filter

35 Use of a filtration device according to any of claims 1 to 16 to enable the rapid melting of a solid metal sample in the device to form a representative liquid metal sample which can then be filtered.

36 A filtration device for filtering molten metal comprising a body defining a cavity for receiving molten metal, an outlet from the body for the molten metal, and a filter positionable adjacent the outlet to concentrate inclusions in the molten metal above the filter which filter is in accordance with any of claims 17 to 25