GB2282764A

GB2282764A - Filter

Info

Publication number: GB2282764A
Application number: GB9420766A
Authority: GB
Inventors: Michael White
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-10-14
Filing date: 1994-10-14
Publication date: 1995-04-19
Anticipated expiration: 2014-10-14
Also published as: GB9420766D0; GB2282764B; GB9321179D0

Abstract

A filter for molten metal comprises a stack of ceramic gratings 21 - 23 whose slots are partly obstructed by ribs 62 spaced from the upper and lower faces of the gratings to define tortuous passages though the stack. Adjacent gratings may have their slots aligned as shown or staggered or oriented at right angles (Fig 4) and may be welded together. Gratings may be flat or frusto-conical. <IMAGE>

Description

FILTER The present invention relates to a filter and has particular reference to a refractory filter for filtering molten metal.

Refractory filters are employed in casting molten metals; typically a refractory filter is incorporated in the runner system of casting apparatus to filter molten metal as it is poured into a mould for casting.

The purpose of the refractory filter is to separate slag and other impurities from the metal itself. It will be appreciated that such a refractory filter should desirably satisfy three principal criteria: first of all the filter should, of course, provide satisfactory filtration; secondly, the filter should provide as little resistance to.the flow of molten metal through the filter as possible so that molten metal can be poured reasonably quickly into the mould and thirdly, the total void volume in the filter should be as little as possible as the metal contained in the filter on cooling and setting the molten metal is wastage.

EP-A-0165025 discloses a refractory filter for molten metal comprising a stack of filter elements; each element has a plurality of apertures formed therein which extend between upper and lower surfaces of the element. The elements are arranged in the stack such that the apertures in one element are staggered with respect to the apertures in a neighbouring element to provide a tortuous fluid path throught the filter.

We91/12062 also discloses a ceramic filter for molten metal which comprises a stack of filter elements; each element consists of a rectangular frame and a plurality of spaced parallel bars which extend from one side of the frame to an opposite side of the frame, the bars thus define a plurality of spaced elongate apertures. The elements in the stack are arranged such that the elongate apertures in one element are staggered with respect to the elongate apertures in a different element to provide a tortuous path for molten metal through the filter.

It will be appreciated by a person skilled in the art that in order to maximise the tortuosity (and thus filtration potential) of a filter of the type disclosed in EP-A-0165025 or W091/12062 it is desirable that the elements are stacked such that each aperture in one element is overlapped completely by a solid part intermediate two neighbouring apertures of a different element in the stack. In such an arrangement, however, the apertures in the one element and corresponding solid parts in the other element must be spaced one from the other in order to provide an open fluid path through the filter (it will be understood that if the solid part of the second element butts directly on the first mentioned element, the apertures in the first element will be blocked by the solid parts of the second element).

In EP-A-0165025, this spacing is obtained by providing a recess in one surface of each element so that, in the stack, the solid inter-aperture parts of one element are spaced from the solid parts of its neighbouring elements; in service therefore molten metal flows through the apertures in one element into a recess intermediate the next successive element and then into the apertures in said next successive element and so on through the stack. In accordance with WO91/12062, the spacing between an aperture in one element and the solid part of another element is provided by the interposition of spacing elements wherein the spaced bars extending across the frame are oriented at 900 with respect to the bars of its neighbouring elements (see Figures 2 and 4 of WO91/12062). In an alternative embodiment of Who91/12062 (see Figure 3) the elements are arranged in the stack such that the bars of one element are disposed parallel to the bars of its neighbouring elements and are staggered such that the bars of the one element only partially occlude the inter-bar apertures of its neighbouring elements.

The requirement for spacing the filter elements of EP-A-0165025 and We91/12062 in which the solid inter-aperture parts of one element overlap completely with the apertures of another element represents a considerable disadvantage, as the spacing increases the overall "dead space" of the filter and the concommitant wastage of metal during the casting process. Furthermore, in We91/12062 the spaced parallel bars of the spacing elements oriented at 900 with respect to their respective neighbouring elements reduces the cross-sectional open area of the filter transverse the direction of fluid flow and thus, in service, has the effect of choking the flow of molten metal through the filter.

There is therefore a general requirement for a refractory filter for filtering molten metal which encloses less "dead space" and allows a greater rate of fluid flow through the filter in service as compared with the prior art filters exemplified by EP-A-0165025 and WO91/12062.

In accordance with one aspect of the present invention therefore there is provided a refractory filter comprising a stack of filter elements, each element having a plurality of openings formed therein which extend between upper and lower surfaces of the element; characterised in that an opening in one of the elements has barrier means disposed therein spaced from at least one of the element surfaces, partially to occlude the opening and to define a juxtaposed straight-through fluid path; the arrangement being such that the barrier means of the one element at least partially overlays the straight-through path of an opening of at least another of the elements so as to define a tortuous fluid path through the filter.

The barrier means constitutes a barrier to the flow of fluid (e.g. molten metal) through the filter; the position of the barrier means to overlay the straight-through path of another element in the stack forces fluid flowing through the filter in service to follow a tortuous path around the barrier means. The spacing of the barrier means from at least one of the element surfaces allows the barrier means of one element to overlap completely the straight-through path of a corresponding aperture in an abutting element such that the barrier means in the one element is spaced from the solid part of the other element to provide an open fluid path between the opening of the other filter element and the straight-through path of the one element.

The provision of barrier means in accordance with the present invention spaced from at least one surface of the filter element therefore allows the solid part of one element to overlap completely with the straight-through path of an abutting element without the requirement for spacing means between the two elements. In accordance with the present invention therefore a more compact filter may be obtained having optimum tortuosity and rate of fluid flow through the filter in service as compared with the prior art filters described above.

In some embodiments, said barrier means may be spaced from said upper and lower surfaces of the filter.

Typically, the depth of said barrier means may be between a quarter to three-quarters the overall depth of the opening. In some embodiments, the depth of the barrier means may be about half the depth of the opening.

Typically, the barrier means may occlude between about a quarter and three-quarters of the cross-sectional area of the opening transverse the direction of fluid flow through the filter; in some embodiments, the barrier means may occlude about half the cross-sectional area of the opening.

It will be appreciated that the configuration of the barrier means is immaterial save in so far as the barrier means has to provide an effective barrier to the flow of fluid through the opening. In some embodiments, however, said barrier means may be rectilinear in cross-section, typically rectangular.

Typically, said barrier means may be formed integrally with the element.

In some embodiments, each opening may be elongate in a direction generally transverse the direction of fluid flow through the filter.

Typically, each element is substantially planar although it is envisaged that other filter configurations such, for example, as pyramidal, conical and frustoconical may be used. In some embodiments, each element may be generally rectilinear in configuration.

In one embodiment, each element comprises a rectangular frame formed integrally with a plurality of spaced parallel bars extending from one side of the frame to an opposite side of the frame thereby to define a plurality of elongate spaced openings.

Typically, each element has between five and ten elongate openings formed therein. Said barrier means may, in some embodiments, extend continuously from one end of the elongate opening to the other end of the opening. Alternatively, said barrier means may be discontinuous in the opening.

In one embodiment, said upper and lower surfaces of each element may be continuous and constitute stacking surfaces such that the upper and lower surfaces of each element in the stack are substantially flush with the surfaces of the neighbouring elements, with substantially no gap therebetween. It will be appreciated by a person skilled in the art that in such an arrangement the tensile strength of the filter may be significantly greater as compared with a filter having spaces or recesses intermediate neighbouring elements.

Said filter may comprise between two and ten stacked elements, typically about two to five elements.

The elements may be stacked such that the bars of one element are disposed in line with the bars of a neighbouring element such that the tortuosity in the filter is provided only by the barrier means.

Typically, the bars of one element are disposed in line with the bars on all the other elements.

Alternatively, however, the bars of one element may be staggered with respect to the bars of another element in the stack such that, in addition to the barrier means, the bars of the one element may partially overlap the openings in the other element; in this way, the bars may contribute to the tortuosity of the filter.

Typically, the elements in the stack are identical one to the others.

In some embodiments, the bars of one element may be oriented at 900 with respect to the bars of its neighbouring elements; the bars of the one element may thus partially occlude the cross-sectional open area of the filter transverse the direction of fluid flow.

The inclusion of an element having bars oriented in this manner may therefore act as a choke on the rate of fluid flow through the filter in service.

Said filter elements may be manufactured from any suitable refractory material known to persons skilled in the art such, for example, as a silicate or aluminosilicate. In one embodiment the filter elements may be manufactured by pressing in a mould and then firing an aluminosilicate composition having the following analysis: Silica SiO2 56% Alumina A1203 40% Titanium dioxide Ti02 1% Ferrous oxide Fe203 1% Quicklime CaO 0.15% Magnesia MgO 0.2% Sodium Oxide Na20 0.25% Potassium Oxide K 0 1.4% 2 Said filter may be contained in a holder with the filter elements loosely assembled therein; alternatively, said elements may be joined one to the others using a suitable adhesive or by silica welding the elements of the stack.

Following is a description by way of example only and with reference to the accompanying informal drawings of methods of carrying the present invention into effect.

In the drawings: Figure 1 is a perspective view or a tilter in accordance with the present invention.

Figure 2 is a sectional view on the line II - II of Figure 1.

Figure 3 is a plan view of a filter element included in the filter of Figure 1.

Figure 4 is a schematic plan view of a different filter in accordance with the present invention.

A refractory filter (10) in accordance with the present invention is constituted by a stack of substantially identical filter elements (21, 22, 23).

In the embodiments shown in Figures 1, 2, 3 and 4 three filter elements are used; it will be understood however that between about 2 - 10 elements may be used. Each element (21, 22, 23) is formed from a refractory material. It will be appreciated that any suitable refractory material known to persons skilled in the art may be used; in one embodiment, however, the refractory material has the following composition: Silica Si02 56% Alumina A1203 40% Titanium dioxide Ti02 1% Ferrous oxide Fe203 1% Quicklime CaO 0.15% Magnesia Mg0 0.2% Sodium Oxide Na2O 0.25% Potassium Oxide K 0 1.4% 2 Each element (21, 22, 23) is moulded as a unitary construction from the above aluminosilicate composition by first of all forming a "green" pressing in a suitable mould and then firing the "green" pressing at a temperature of about 1120by. After firing, the elements (21, 22, 23) are stacked as hereinafter described and connected together by a ceramic welding technique; in an alternative embodiment, the elements may be bonded together using a suitable high temperature adhesive.

Each element (21, 22, 23) comprises a planar, rectangular frame (40) and a plurality of spaced parallel bars (42) which extend from one side (44) of the frame (40) to an opposite side (46). In the embodiment shown in Figures 1, 2 and 3 of the accompanying drawings, each filter element (21, 22, 23) is formed with six bars (42) which define seven spaced apertures (60). It will be appreciated, however, that the number of bars can be varied to provide a desired degree of filtration capacity and flow rate through the filter.

The upper and lower surfaces (48, 50) respectively of each bar (42) are flush respectively with the upper and lower surfaces (52, 54) of the frame (40). The upper surface of the frame and the upper surfaces of the bars constitute a substantially planar, continuous upper stacking surface; the lower surface of the frame and lower surfaces of the bars are substantially planar and constitute a continuous lower stacking surface. In the assembled filter, the upper stacking surface of each element (22) butts flush on the lower stacking surface of its upper neighbouring element (21); the lower stacking surface of the element (22) butts flush on the upper stacking surface of its lower neighbouring element (23).

The spaced parallel bars (42) define a plurality of spaced elongate apertures (60); each aperture (60) extends between the upper and lower stacking surfaces of the element.

Each bar (42) is formed integrally with an elongate rib (62) which extends continuously along the length of the bar (42) from the one side (44) of the frame to the other side (46) of the frame and is disposed in the opening (60) spaced from the upper and lower surfaces of the element. The rib (62) is generally rectangular in cross-section and extends outwardly from one side of the bar (42) to a point about half way across the opening (60) to define a juxtaposed straight-through path (64) between the extremity of the rib (62) and the adjacent bar (42). It will be appreciated that the present invention is not limited to the use of rectangular section ribs; in some embodiments, the ribs may be triangular or semi-circular in cross-section.

The elements (21, 22, 23) are arranged in the stack such that each element is oriented at an angle of 1800 with respect to its neighbouring elements.

The ribs (62) of each element therefore extend in the opposite direction from the ribs (62) on each neighbouring element (see Figure 2) such that each rib (62) on one element substantially overlays the straight-through path (64) in a corresponding opening (60) in a juxtaposed element; in service therefore molten metal is forced to follow a tortuous path through the filter. It will be understood that the tortuosity of the filter can be increased by providing ribs (62) which extend more than half way across their respective openings; the tortuosity can be decreased by reducing the extent to which each rib (62) occludes its respective opening (60).

It will be noted from Figure 2 that in the embodiment shown the elements (21, 22, 23) of the filter are stacked such that the bars on one element (22) are disposed in line with the bars on its neighbouring elements (21, 23); that is to say that the side walls of each bar (62) are disposed flush with the side walls of a corresponding bar in a neighbouring element. In an alternative embodiment (not shown), however, the elements can be arranged such that the bars (42) of one element are staggered with respect to the bars of a neighbouring element whereby to introduce a further degree of tortuosity in the filter.

A different filter in accordance with the invention is shown in Figure 4. The filter of Figure 4 comprises three filter elements (21, 22, 23) of the type described above. The elements are stacked, however, such that the middle element (22) is oriented at 900 with respect to each of its neighbouring elements (21, 23), and the top and bottom elements in the stack (21, 23) respectively are oriented at 1800 with respect to each other. The tortuosity in the filter is provided substantially by the arrangement of the top and bottom elements (21, 23); the bars (42) of the intermediate element (22) partly obturate the openings in the top and bottom elements.

The middle element (22) therefore in service acts as a choke on the rate of fluid flow through the filter.

Claims

1. A refractory filter comprising a stack of filter elements, each element having a plurality of openings formed therein which extend between upper and lower surfaces of the element; characterised in that an opening in one of the elements has barrier means disposed therein spaced from at least one of the element surfaces, thereby partially to occlude the opening and to define a juxtaposed straight-through fluid path; the arrangement being such that the barrier means of the one element at least partially overlays the straight-through path of an opening of at least another of the elements so as to define a tortuous fluid path through the filter.

2. A filter as claimed in claim 1 wherein said barrier means is spaced from said upper and lower surfaces of the filter.

3. A filter as claimed in claim 1 or claim 2 wherein the depth of the barrier means is between a quarter to three quarters the overall depth of the opening.

4. A filter as claimed in claims 1, 2 or 3 wherein the depth of the barrier means is about half the depth of the opening.

5. A filter as claimed in any preceding claim wherein the barrier means occludes between about a quarter and three quarters of the cross-sectional area of the opening ransverse the direction of fluid flow through the filter.

6. A filter as claimed in any preceding claim wherein the barrier means occludes about half of the crosssectional area of the opening.

7. A filter as claimed in any preceding claim wherein the barrier means is rectilinear in cross-section, typically rectangular.

S. A filter as claimed in any preceding claim wherein the barrier means is formed integrally with the element.

9 A filter as claimed in any preceding claim wherein each opening is elongate in a direction generally transverse the direction of fluid flow through the Fi.lter.

10. A filter as claimed in any preceding claim wherein cach element is substantially planar.

11. A filter as claimed in any preceding claim wherein each element is generally rectilinear in configuration.

12. A filter as claimed in any preceding claim wherein each element comprises a rectangular frame formed integrally with a plurality of spaced parallel bars extending from one side of the frame to an opposite side of the frame, thereby to define a plurality of elongate spaced openings.

13. A filter as claimed in any preceding claim wherein each element has between 5 and 10 elongate openings formed therein.

14. A filter as claimed in any preceding claim wherein said barrier means extends continuously from one end of the elongate opening to the other end of opening.

15. A filter as claimed in any of claims 1 to 13 wherein said barrier means is discontinuous in the opening.

16. A filter as claimed in any preceding claim wherein each of said upper and lower surfaces is continuous and constitutes a stacking surface, the arrangement being such that the upper and lower surfaces of each element in the stack are substantially flush with the surfaces of the neighbouring elements, with substantially no gap therebetween.

17. A filter as claimed in any preceding claim comprising between 2 and 10 stacked elements, typically about 2 to 5 elements.

18. A filter as claimed in any preceding claim wherein the elements are stacked such that the bars of one element are disposed in line with the bars of a neighbouring elements such that the tortuosity in the filter is provided only by the barrier means.

19. A filter as claimed in any claims 1 to 17 wherein the bars of one element are staggered with respect to the bars of another element in the stack such that, in addition to the barrier means, the bars of the one element partially overlap the openings in the other element, such that the bars contribute to the tortuosity of the filter.

20. A filter as claimed in any preceding claim wherein the elements in the stack are identical one to the others.

21. A filter as claimed in any preceding claim wherein the bars of one element are oriented at 900 with respect to the bars of its neighbouring elements.

22. A filter as claimed in any preceding claim which is manufactured by pressing in a mould and then firing an aluminosilicate composition having the following analysis: Silica SiO2 56% Alumina A1203 40% Titanium dixoide TiO2 1% Ferrous oxide Fe203 1% Quicklime CaO 0.15% Magnesia MgO 0.2% Sodium Oxide Na2O 0.25% Potassium Oxide K20 1.4%

23. A filter as claimed in any preceding claim characterised by a holder having the filter elements assembled loosely therein.

24. A filter substantially as hereinbefore described with reference to and as illustrated in Figures 1, 2 and 3 or Figure 4 of the accompanying drawings.