GB2047106A - Removal of particulate matter from liquid metals - Google Patents

Abstract

A method and apparatus for removing non-metallic inclusions in molten metal comprises a combination of a bed of refractory granules 5 and one or more screens or pieces of screen made of woven refractory material 6 located in the flow path of the metal. <IMAGE>

Description

SPECIFICATION Improvements relating to the removal of particulate matter from liquid metals This invention relates to the removal of particulate matter from liquid metals by filtration and particularly although not exclusively to the filtration of light metals such as aluminium.

For production of both shaped castings and cast ingot for subsequent plastic working, it is generally desirable and often essential to filter the liquid metal used in order to control the content of particulate matter in the cast metal. Thus melts of light metals may contain non-metallic inclusions such as oxides and nitrides of aluminium and magnesium, together with intermetallic particles rich in titanium, manganese or zirconium. Such inclusions can give rise to various known defects in the final product.

To control inclusion content it has been proposed to pass the metal through filter materials of three kinds, namely: (a) porous ceramic plates or tubes, necessitating application of pressure to effect filtration, (b) beds of granules, (c) screens of perforated metal or woven fabric.

Filters of type (a) are highly effective but relatively expensive to use and require a considerable metallostatic head, eg 400-500mm, for satisfactory operation.

Granule beds of type (b) with particle size between 3 and 14 ASTM sieve size - ie between 0.055 and 0.25 in (1.41 - 6.35 mm) were recommended in British Patent No. 831 637. Such filter beds have proved very successful in removing many kinds of particulate material but they undergo blockage after a time.

Attempts have been made to reduce this problem by using a graded bed of granules in which the metal passes first through relatively coarse granules and then through progressively finer ones, whereby blockage of the fine granule is delayed.

With regard to screens (i.e. type (c)), it has been known for many years as a result of published work carried out at Massachusetts Institute of Technology that for best results (i) the screen must be immersed in the metal stream and (ii) the metal should pass upwards through the screen, and these features are incorporated in US Pat. No. 3 654 150 which envisages a screen comprising at least two contiguous layers of glass-cloth. The glass-cloth filter is supported by a heavy rigid grid on the down stream side of the filter.However, such large multilayer glass-cloth screens with rigid backing grids are not very convenient to use in practice and in general have given less satisfactory results than granule beds, in consequence of which they have not achieved widespread adoption within the industry, the use of glass-cloth being however retained for straining out oxide scum in launders and moulds. In the former case the glass-cloth is disposed substantially vertically and in the latter substantially horizontally with the liquid metal passing through the screen in a downward direction.

In British Patent No. 1 266 500 there is described a process of the continuous cleansing and degassing of liquid aluminium in which the metal is treated with nitrogen in the presence of a liquid salt flux and thereafter flows through a bed of coarse refractory granules, preferably balls of tabular alumina, at least some of which are coated with liquid flux.

According to one aspect of the present invention thre is provided a process for removing non-metallic inclusions in molten metal comprising flowing the metal through a bed of refractory granules, the bed incorporating at least one screen of woven refractory material located in and disposed transversely to the flow path of the metal. Preferably the granules are at least 3/1 in diameter and are of alumina.

Advantageously the metal flows through at least some part of the bed of granules in an upward direction and one or more screens are disposed in a substantially horizontal manner within this part of the bed.

The invention also provides a molten metal filter comprising a container having at least one flow path between an inlet and an outlet, a bed of refractory granules disposed within the flow path to subdivide the latter into a multiplicity of flow channels and at least one screen of woven refractory material located in and disposed transversely of the flow path.

Preferably the granules are at least 3/1 in diameter and are of alumina. If desired at least some of the granules are flux coated.

The woven refractory material may be of glasscloth or of cloth made of fused silica, aluminium silicate, carbon or boron nitride.

The screens may be incorporated in pieces of much smaller area than the cross sectional area of the flow path in a direction at right angles thereto and they may be disposed throughout the bed, but it is generally preferred to use pieces of somewhat greater area than the cross sectional area of the ball bed, so that the whole of the latter is covered with several inches of overlap all round. When the ball bed is being assembled, the granules may be fed into the container until a total depth of granules a few inches less than that finally desired has been obtained. The filter cloth may then be laid over the surface of the granules and additional granules added on top of the filter cloth to achieve the desired total depth of bed. In this way the edges of the cloth will be pressed snugly against the walls of the container and the cloth can be easily removed and replaced.

To facilitate further the initial passage of liquid metal through screens of fine weave, they can be pre-dipped in liquid salt fluxes, e.g. of the types disclosed in British Patent No. 1 266 500.

The above and other aspects of the present invention will now be described by way of example with reference to the accompanying drawings in which : Figures land 2 are partial sectional views of two chamber units of the type often used for carrying out metal treatment processes, and Figure 3 is a section of a launder.

In Figures 1 and 2 a container 1 is divided by a baffle wall 2 into two chambers 3 and 4, the exit chamber 4 being provided with a bed of refractory granules 5. In Figure 1 a screen of woven refractory material 6 is diposed in a single piece of larger area than the cross-section of the chamber 4, whilst in Figure 2 the woven refractory material 6 is provided as discreet pieces disposed at different levels.

In Figure 3 the launder 10 is conveniently provided with a baffle 11 to retain the balls 12 and a skimming baffle 13 may also be provided to retain surface oxide scum. Two screens 14 are shown embedded in the balls 12. In a launder, the process is of particular value in the treatment of metal which has been subjected to a degassing operation at a point upstream of that at which the present invention is applied.

The screens may be pre-dipped in liquid salt flux as disclosed in our British Patent No. 1 266 500.

Thus in carrying out the process of British Patent No. 1 266 500 glass-cloth screens may be incorporated within the ball-column in the outgoing chamber of the 2-chamber vessel, the screens being disposed approximately horizontally across the column at a distance of several inches below the top of the column.Several advantages result from such an arrangement: since the metal flows upward through the screen and the screen remains fully immersed in use, the screen is used to best advantage, no rigid grid is required to support the screen because it is much more effectively supported by the granules on each side of it; initial passage of the metal through the screen is facilitated by its being embedded in hot granules; local blockage of parts of the screen and channelling effects are eliminated or greatly reduced because of the many separate channels through the granule bed by which liquid metal can reach the screen; the life of the screen is increased because the incoming metal is effectively cleaned of most of its load of residual particulate matter by passage through that part of the granule bed lying upstream of the screen; moreover, for this latter reason and because of the greater ease of getting metal to pass through the filter, finer screens can be used.

Claims (9)

1. A process for removing non-metallic inclusions in molten metal comprising flowing the metal through a bed of refractory granules, the bed incorporating at least one screen of woven refractory material located in and disposed transversely to the flow path of the metal.

2. A process according to claim 1 in which the granules are at least #' in diameter and are of alumina.

3. A process according to claim 1 or claim 2 in which the metal flows through at least some part of the bed of granules in an upward direction and one or more screens are disposed in a substantially horizontal manner within this part of the bed.

4. A molten metal filter comprising a container having at least one flow path between an inlet and an outlet, a bed of refractory granules disposed within the flow path to subdivide the latter into a multiplicity of flow channels and at least one screen of woven refractory material located in and disposed transversely of the flow path.

5. A filter according to claim 4 in which the granules are at least 81' in diameter and are of alumina.

6. A filter according to claim 4 or claim 5 in which at least some of the granules are flux coated.

7. A filter according to any one of claims 4 to 6 in which the woven refractory material is of glass-cloth or of cloth made of fused silica, aluminium silicate, carbon or boron nitride.

8. A process substantially as herein described.

9. A molten metal filter substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.