GB2097777A

GB2097777A - Ceramic foam

Info

Publication number: GB2097777A
Application number: GB8113867A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1981-05-06
Filing date: 1981-05-06
Publication date: 1982-11-10

Abstract

A method of improving the strength and refractiveness of a ceramic foam made by impregnating a flexible organic reticulated foam with a ceramic slurry consists of immersing the fired foam in an aqueous sol of Aluminium Hydrate or Zirconium Hydroxide and subsequently drying and refiring the foam.

Description

SPECIFICATION Improvements in or relating to ceramic foam This invention relates to the manufacture of ceramic foam and in particular to ceramic foam material useful for the filtration of liquid metals.

The benefits of liquid metal filtration are well recognised by those skilled in the art.

Presence of inclusions in castings is known to reduce their performance by impairing their fabricability and machinibility and by reducing the mechanical properties of the metal.

It has been well known in the past to filter liquid metal through a variety of types of filter all of which have suffered disadvantages.

Most such filters consist of either screens or perforate plates, a loosely packed bed of solids, a bonded porous medium or particular types of ceramic foam structure.

Screen filters have been used extensively, however they suffer the disadvantage that the size of the particles which may be filtered from the melt are limited to the relatively large apertures formed within the screen or plate.

Loosely packed bodies of solids are also well known and usually consist of loosely packed beds of refractory balls, however such beds suffer from a major disadvantage known as channeling. Since the bed is a loosely packed aggregate of granules there is a strong tendency for the liquid metal to push the granules aside and form low resistance paths for the metal to flow through. Once a channel is formed it usually continues to become larger and large amounts of inclusions then pass through the channel with the liquid metal.

Bonded porous filters have usually consisted of refractory particles sintered together by a suitable bonding agent for example clay or enamel. The main disadvantage of this type of filter is the difficulty in obtaining consistency of porosity in production. Furthermore it is almost impossible to manufacture such filters having a porosity of more than 30% by volume. It is therefore necessary to have a relatively large head of liquid metal to ensure passage of the metal through the filter at a reasonable rate.

Porous ceramic foam filters are also well known from the following U.S. Patents Nos.

3,090,094, 3,097,930, 3,893,917, 4,024,212, 3,962,081 and British Patent Nos. 916,784, 1,537,549 and 1,388,912.

Porous ceramic foam filters are usually manufactured using a flexible organic reticulated foam as a precursor material which is coated with a ceramic slurry. The foam is subsequently dried and then fired to burn out the organic foam and sinter the ceramic thereby producing a porous reticulated ceramic foam.

Filters of this type have been used for the filtration of both low and high temperature melting alloys however they have all suffered a disadvantage in that they are relatively weak and are liable to shed loose ceramic particles into the liquid metal.

An object of the present invention is to provide a ceramic foam filter in which the aforementioned problems are substantially eliminated.

According to the present invention a method of making a ceramic foam filter consists of impregnating a flexible organic reticulated foam with a ceramic slurry, drying and firing the impregnated foam and subsequently immersing the foam in an aqueous sol of Aluminium Hydrate or Zirconium Hydroxide and thereafter further firing the foam.

The ceramic slurry may comprise 40 to 60% Tabular Alumina, 15 to 20% Fused Alumina, 5 to 20% Dorset Ball Clat, 5 to 15% Nepheline Syenite, 2.5 to 7.5% Dextrin and water.

Preferably the ceramic slurry contains substantially 52% Tabular Alumina, 17% Fused Alumina, 14% Dorset Ball Clat, 10.5% Nepheline Syenite and 5-2% Dextrin.

Furthermore the slurry may contain 0 to 1 % Chromium Oxide together with a bacterial inhibitor.

Preferably the ceramic is fired up to a temperature of 1460 to 1470 C and after immersion in the aqueous solution of Zirconium Hydroxide or Aluminium Hydrate is further fired up to a temperature of 1560 to 1570 C.

Preferably the aqueous sol of Aluminium Hydrate or Zirconium Hydroxide contains up to 10% solids.

Furthermore the Aluminium Hydrate or Zirconium Hydroxide contains up to 20% colloidal particles.

For better understanding thereof an embodiment of the invention will now be more particularly described by way of example only.

The ceramic material consists of a slurry of the following composition: 52.5% Tabular Alumina 300 mesh, 17% Fused Alumina 5y micron powder, 14% Dorset Ball Clay, 10-15% Nepheline Syenite, 5-25% Dextrin, 0.5% Chromium Oxide, 750 mis of distilled or demineralised water together with a suitable mould inhibitor such as for example Panacide.

To manufacture a ceramic filter in accordance with the present invention, pieces of polyurethane foam of the appropriate pore size are cut to a size approximately 3+% larger than that of the finished filter, this is done to allow for shrinkage during the manufacturing process. Cutting of the polyurethane foam can be accurately accomplished by using a hot wire type cutter.

The slurry is mixed to the consistency of a smooth cream and applied to the polyurethane foam. The surplus is squeezed out onto a glass plate using a hand roller. The foam is then allowed to dry on a perforate tray. Because the burning of polyurethane produces noxious and toxic fumes the coated filters are carbonised at a low temperature of approximately 450 C under an exhaust hood until all smoke is expelled.

The filter is then fired in an electrically heated furnace from cold up to a temperature of 1460 to 1470 C for approximately 5 hours and then allowed to cool and is then ready for use.

Reticulated type ceramic foam has a skeletal structure with a web thickness of approximately 10% of the pore size and may be considered as a series of fine rods; joined at their ends to form a three dimensional polygonal matrix. It may be considered as a fibrous filter having its layers fixed apart by the diameter of the pore. Depending upon the type of foam used as a precursor the structure may have a void volume as high as 95%. The high void volume thus causes a minimum restriction to flow whilst retaining a uniform pore structure to give good condiions for specific particle removal from molten metal.

Filtration of molten metal may now be accomplished using the filter in its present state however it will be relatively weak and is liable to shed loose particles which would result in undesirable inclusions within the filtered metal.

To eliminate the aforementioned problem the ceramic foam filter is therefore subjected to further treatment consisting of immersing the filter in an aqueous sol of fibrous or nonfibrous Aluminium Hydrate or Zirconium Hydroxide containing approximately 20% coloidal particles. The filter is thereafter drained and dried in air and subsequently fired at a temperature of between 1560 and 15704C and thereafter allowed to cool.

It will be appreciated that whilst the described embodiment of the present invention relates to the manufacture of a ceramic foam filter particularly suitable for filtering molten metal. However such a material could be used for a variety of purposes such as for example the manufacture of refractory bricks or tiles or catalyst supports for use in exhaust systems etc.

Claims

1. A method of making a ceramic foam filter consists of impregnating a flexible organic reticulated foam with a ceramic slurry, drying and firing the impregnated foam and subsequently immersing the foam in an aqueous sol of Aluminium Hydrate, or Zirconium Hydroxide and thereafter further firing the foam.

2. A method as claimed in claim 1 in which the ceramic slurry comprises 40 to 60% Tabular Alumina, 15 to 20% Fused Alumina, 5 to 20% Dorset Ball Clay, 5 to 15% Nepheline Syenite, 2.5 to 7.5% Dextrin and water.

3. A method as claimed in claim 1 in which the slurry comprises substantially 52% Tabular Alumina, 17% Fused Alumina, 14% Dorset Ball Clay, 10.5% Nepheline Syenite and 5 to 25% Dextrin.

4. A method as claimed in claim 1 in which the slurry contains 0 to 1.0% Chromium Oxide together with a bacterial inhibitor.

5. A method as claimed in claim 1 inwhich the ceramic foam is first fired up to a temperature of 1460 to 1470'C and after immersion in the aqueous sol of Zirconium Hydroxide or Aluminium Hydrate is further fired up to a temperature of 1560 to 1 570'C.

6. A method as claimed in claim 1 inwhich the aqueous sol of Aluminium Hydrate or Zirconium Hydroxide contains up to 10% solids, the solid portion of which contains up to 20% colloidal particles.

7. A ceramic foam made by the method as claimed in any preceding claim suitable for use as a liquid metal filter.

8. A ceramc foam substantially as hereinbefore described by way of example only and with reference to the accompanying drawings.