Liquid purification or separation – Processes – Separating
Patent
1994-11-14
1996-05-28
Spear, Frank
Liquid purification or separation
Processes
Separating
210496, 2105101, B01D 3700
Patent
active
055208231
DESCRIPTION:
BRIEF SUMMARY
This invention relates to filters for filtering light metals, by which term is to be understood aluminium, magnesium, alloys of these two metals, and alloys of either or both of these metals with other metals in which alloys, aluminium and/or magnesium predominate. In particular the invention relates to filters of relatively small size for use in dies for the gravity diecasting of aluminium or aluminium alloys.
It is well known to use open-cell ceramic foams to filter molten light metals. Such ceramic foam filters are generally made by impregnating an organic foam, usually polyurethane foam, with an aqueous slurry containing the material which is to form the filter and a binder, removing excess slurry, drying to remove water, and firing the dried impregnated foam to burn off the organic foam to form a ceramic foam.
Using the above method of manufacture individual filters are produced of the required shape and size, or alternatively a large piece of ceramic foam is produced and cut or machined to form individual filters. Both methods of manufacture suffer from disadvantages.
When individual filters of a given nominal size are produced directly from organic foam of a similar size to the filters their dimensional variation is such that problems arise in use, particularly in the gravity die casting of aluminium and its alloys, in which the filter is inserted in a recess in the die. As oversize filters cannot be used because they cannot be inserted in the recess it is common practice to compensate for the dimensional variations by deliberately making the filter slightly too small for the size of the recess. As a result small gaps remain between the edge of the filter and the wall of the recess, and because metal can penetrate through the gaps and bypass the filter, filtration efficiency is reduced. If the filter is loose, crushing of the filter can occur when the die is closed and this can lead to particles of ceramic material from the filter passing through the filter, and into the casting cavity of the die, thus resulting in inclusions in the casting.
When individual filters are produced from large pieces of ceramic foam by cutting or machining, manufacturing costs are high and productivity rates low, due to the nature of the materials from which the filters have hitherto been made. As the materials are brittle fracture of the foam tends to occur when the foam is cut or machined.
U.S. Pat. No. 3,524,548 describes a rigid, porous, filter medium for molten aluminium comprising particles of refractory material and a vitreous bonding agent of the type described in U.S. Pat. No. 2,997,402. The vitreous or glassy bonding agent consists essentially of boron oxide, calcium oxide and aluminium oxide and contains not more than 10% by weight of silica.
British Patent 1428437 describes a filter medium for molten aluminium made by shaping, drying and calcining at a temperature not lower than 1000.degree. C. a mixture comprising aggregate particles of fused alumina, sintered alumina, silicon carbide and/or silicon nitride, a combustible material, a caking material, water and an inorganic bonding material having a chemical composition of 10 to 50% by weight of silica and 5 to 20% by weight of boric oxide, at least 70% by weight of the inorganic bonding material being a non-vitreous substance. The inorganic bonding agent is prepared by mixing and pulverising at least 70% by weight of a natural or synthetic non-vitreous substance and not more than 30% by weight of a vitreous substance such as a commercially available glaze for porcelain.
EP 0507463A describes reticular foam filters which can be machined or cut to size from large pieces of filter material and which have a composition comprising graphite, wollastonite, silica, an alkali phosphate glass and monoaluminium orthophosphate.
It has now been found that the filters described in EP 0507463A can be improved if a borosilicate glass is used instead of the alkali phosphate glass and the monoaluminium orthophosphate.
According to the invention there is provided a filter for
REFERENCES:
patent: 2997402 (1961-08-01), McDonald et al.
patent: 3524548 (1970-08-01), McDonald et al.
patent: 5114472 (1992-05-01), Eckert et al.
patent: 5169542 (1992-12-01), Kaettlitz et al.
Jones David L.
Stoetzel Reinhard
Swift Martin S.
Foseco International Limited
Spear Frank
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