GRAPHITIZED CARBON BONDED FILTER WITH OXIDATION RESISTANT

COATING FILTER

The present invention claims priority to US provisional application number

60/788,918, which is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to ceramic filters for use in the processing of molten

metal, more specifically, to an oxidation-resistant coating for carbon bonded filters.

BACKGROUND OF THE INVENTION

The processing of molten metals preferably removes exogenous intermetallic

inclusions such as impurities from the raw materials, slag, dross and oxides which

form on the surface of the melt, and small fragments of refractory materials that are

used to form the chamber or vessel in which the molten metal melt is formed.

Removal of these inclusions forms a homogenous melt that insures high

quality of products especially in the casting of steel, iron and aluminum metals.

Currently, ceramic filters are widely used due to their high ability to withstand

extreme thermal shock, their resistance to chemical corrosion, and their ability to

withstand mechanical stresses.

One method of producing these ceramic filters includes preparing a slurry or

paste by mixing ceramic powder with suitable organic binders and water. The slurry is

used to impregnate polyurethane foam which subsequently is dried and then fired at a

temperature in the range of from 1000 to 1700 ⁰ C. Firing burns off the combustible

material to produce a porous body. U.S. Pat. No. 2,360,929 and U.S. Pat. No.

2,752,258 may serve as examples for the common procedure. The resultant filter

typically comprises a random distribution of irregular interconnecting passages

An alternative method produces an open pore filter comprising a series of

parallel ducts passing through the material. The method includes pressing a damp

ceramic powder and organic binder into a mold containing perpendicular pins. A

perforated structure is thus obtained which can be in the form of a disk or block. The

perforated article is then fired at a temperature in the range of from 1000 to 1700° C.

depending on the final application to produce a perforated disc. During firing a ceramic

and/or glassy bond is developed. A honeycomb filter may also be produced via

extrusion.

WO 01/40414 A describes the use of a pressurized mold. This patent depends

on regulating the pressure inside the mold to obtain porous structure that is not a fully

open cell structure. The claim of filtration usage is one of many usages and there is no

proof that the filter was ever actually used to metal filtration. Also only aluminum was

mentioned for filtration since such filter is too weak for steel filtration. The patent

describes only a carbon filter without any ceramic. The process of making the filter is

based on regulating the pressure inside the mold. This process is difficult to control.

U.S. Pat. No. 4,514,346 uses phenolic resin to react with silicon at high

temperature to form silicon carbide. There is no carbon bonding involved. This patent

is for making porous silicon carbide only. Temperature in excess of 1600° C. is used to

obtain silicon carbide. The process is non-aqueous and produces closed cell pores,

which has little or no utility in filtration.

GB-A 970 591 concerns a process for producing high density, low permeability

graphite articles. The process uses an organic solvent, namely furfuryl alcohol as

solvent and not water. Binder in the form of pitch is used at 25%. Ceramic compounds

are not described as part of the process or resultant article. Final heating is in excess of

2700° C. The porosity is closed cell rather than open porosity.

U.S. Pat. No. 3,309,433 describes a method for manufacturing high density

Graphite using hot pressing as a means to obtain high density graphite articles for nuclear applications. It used special material called dibenzanthrone to bind the graphite. It has no useful application in metal filtration field and does not include any ceramic in the process. Process temperatures can reach 2700° C. EP 0 251 634 Bl describes an appropriate process for making defined porous ceramic bodies having smooth walled cells formed by the pore formers, and pores with round edges, which interconnect the cells.

U.S. Pat. No. 5,520,823 relates to filters for aluminum only. The bonding is obtained using borosilicate glass. Firing is carried out in air and a considerable amount of graphite is lost because of oxidation. Filters used for aluminum filtration are usually fired at around 1200° C. while those intended for the use of iron are fired at temperatures of 1450° C. and for steel at above 1600° C.

US Pat. Appl. Pub. No. US 2005/0229746, the entire contents of which are hereby incorporated by reference, offers a solution to the problems of the above- referenced filters. The invention disclosed relates to a ceramic filter suitable for molten metal filtration comprising a ceramic powder and fibers bonded by a network of graphitized carbon. The term "graphitizable" means that the carbon bonding obtained by pyrolysis of the carbon precursor can be converted into a graphite-like bond on heating to an elevated temperature in a reducing atmosphere. Graphitizable carbon is distinguished from that of a glassy carbon by the fact that it is impossible to convert glassy carbon to a graphite like bond no matter how high temperature it was heated to. It was also disclosed that adding up to 20%, preferably up to 10% by weight, of fibers to the filter recipes contribute to a significant improvement in the performance of the filters. The improvement is mainly due to increase mechanical strength, improve stiffness, higher impact resistance and better thermal shock. The improvement results

in increased filtration capacity, better mechanical integrity, and less contamination to the steel casting. Due to the outstanding mechanical strength of the carbon bonding in combination with fibers at high temperature, no softening or bending takes place during the process of metal casting. As discussed in the reference, the carbon-bonded filters, unlike oxide-bonded filters, cannot withstand pre-heating in an oxidizing atmosphere because the carbon can oxidize and subsequently volatize, resulting in excessive loss of hot strength and impact resistance. Pre-heat must occur at less than 1000 ⁰ C.

Unfortunately, certain process require a pre-heat of over 1200 ⁰ C and sometimes up to 1800 ⁰ C. Graphite filters must withstand the pre-heat process without loss of mechanical properties and filtering capacity.

SUMMARY OF THE INVENTION

The present invention relates to carbon-bonded molten metal filters. The filters are glazed with an oxidation-resistant glaze. BREIF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a graph of weight loss versus time for carbon-bonded filters with and without glazing.

DETAILED DESCRIPTION OF THE INVENTION

In a first embodiment, the invention relates to a ceramic filter suitable for molten metal filtration comprising a ceramic powder and fibers bonded by a network of graphitized carbon. US application number 10/5169,443 is hereby incorporated by reference.

The term "graphitizable" means a carbon precursor that can be converted to graphite- or graphite-like bonding. Conversion can occur by pyrolizing the carbon precursor in a non-oxidizing environment at elevated temperature. Graphitizable

carbon is distinguished from glassy carbon by the fact that a glassy carbon cannot be

converted to a graphite-like bond regardless of the temperature to which it is heated.

During the continuous work to improve the quality and the performance of

carbon bonded filters, the inventor now discovered that adding up to 20%, in

particular up to 10%, by weight of fibers to the filter recipes contribute to a significant

improvement in the performance of the filters. The entire The improvement is mainly

due to increased mechanical strength, improve stiffness, higher impact resistance and

better thermal shock. The improvement manifests by increase filtration capacity, better

mechanical integrity and less contamination to the steel casting. Due to the

outstanding mechanical strength of the carbon bonding in combination with fibers at

high temperature, no softening or bending can take place during the process of metal

casting. This contributes to an even cleaner metal cast.

The filter comprises graphitized carbon and a refractory ceramic. The

graphitized carbon should be present as a network of up to 15% by weight of the filter,

preferably up to 10% by weight, even more preferred in an amount of at least 2% by

weight up to 5% by weight.

Traditionally, fibers are added to ceramic and composite materials in order to

improve mechanical strength and gives stiffness to the articles. These fibers could be

either metal fibers, organic fibers such as polyester fibers, viscose fibers, polyethylene

fibers, polyacrylonitrile (PAN) fibers, aramid fibers, polyamide fibers, etc., or

ceramic fibers such as aluminosilicate fibers, alumina fibers or glass fibers, or carbon

fibers which consist of 100% carbon. All these types of fibers are used to a different

degrees in ceramic to give added advantaged to the properties of ceramic such as high

mechanical strength, high impact resistance and better thermal shock.

Addition of any of the types of fibers to the carbon bonded filters of the prior

art causes a significant improvement in the mechanical strength of the filters as well as improvement in the impact resistance and thermal shock. The improvement in strength could be as much as three time (i.e. from 0.5 MPa to 1.5 MPa). Impact resistance and thermal shock resistance also increase accordingly. As a result of this improvement, the carbon filters can now at least double their filtration capacity. For example a carbon filter having dimensions 100 mm x 100 mm x 20 mm can normally filter 100 kg of steel. The same filter but with 5% added ceramic filters has a capacity to filter 200 kg of steel. In particular ceramic fibers and carbon fibers are thermally stabile and do not change their physical properties when they are incorporated in the filter. Organic fibers, on the other hand are converted during firing of the filters to carbon fibers, that is, they pyrolize. This is considered to be beneficial with respect to ceramic or metal fibers.

The beneficial effect of the addition of fibers depends on the amount of fibers added, length of the fibers, nature and type of fibers added. The higher the level of fibers added the stronger the filter become. However very high level of fibers is not desirable because it has a negative effect on the rheology of the slurry. Best results are obtained from incorporating carbon fiber followed by ceramic fibers. On the other hand, carbon fibers are the most expensive while organic fibers are the cheapest. Organic fibers are the most economic to use since they are added at much lower level than either carbon or ceramic fibers (less than 2%). However, organic fibers interfere with the rheology of the slurry more than the ceramic or the carbon fibers. The form of fibers is either chopped or bulk fibers to be added during mixing of the filter ingredients. No extra mixing technique is required.

The filters according to the present invention preferably contain 0.1 to 20% by weight, in particular 1 to 10% by weight of said fibers, more particularly 5%.

The fibers used according to the present invention preferably have a length

from 0.1 mm to 5 mm.

In one embodiment of the present invention the carbon bonded ceramic filters

are produced in a first process comprising the steps:

a) impregnating a foam made of thermoplastic material with a slurry

containing fibers, a graphitizable carbon bonding precursor, ceramic

powder, and optionally other additives,

b) drying, optionally followed by one or two coatings of the same slurry in

order to increase the mass, followed by final drying,

c) firing the impregnated foam in non-oxidizing and/or reducing atmosphere

at a temperature in the range of from 500 to 1000° C, in particular from

600° C. to 700° C, whereby the carbon bonding precursor is converted at least partially or

fully to a bonded network of graphitized carbon.

In this process the thermoplastic material used for the foam to be impregnated

with the slurry preferably contains or consists of polyurethane.

It is advantageous to mix fibers, carbon bonding precursor prior to

impregnating the foam with ceramic powder, water, organic binder, and additives to

control the rheology, which in one embodiment of the invention may be present in an

amount of up to 2 parts by weight, preferably in a range of from 0.1 to 2 parts by

weight.

In another embodiment of the present invention a second type of carbon

bonded ceramic filter is produced by a process comprising the steps

a) pressing a semi-damp mixture comprising fibers, ceramic powder and a

graphitizable bonding precursor, and optionally other additives in a

hydraulic press,

b) pressing to obtain a perforated article in the shape of a disk or a block,

c) firing the perforated article in non-oxidizing and/or reducing atmosphere

at a temperature in the range of from 500° C. to 1000° C, in particular

from 600° C. to 700 ⁰ C, whereby the carbon bonding precursor is converted partially or fully to a

bonded network of graphitized carbon.

The source of the carbon bond, that is, the carbon bond precursor is preferably

high melting pitch (HMP) because it offers optimal properties with respect to

workability, cost and product quality. However, it must be noted that other carbon

bond precursors can also be used to produce carbon bonded materials, such as

synthetic or natural resins and sinterable carbon as long as it is graphitizable and

converted to a bonded network of graphitized carbon upon firing according to the

present invention. Thus, synthetic resin binders that form a glassy carbon which

cannot be converted to graphite may not be considered as carbon bond precursors as

the product suffers from low oxidation resistance, low mechanical strength, high

brittleness and lower heat resistance.

Also, for economical as well as ecological reasons the carbon bond precursor

should be compatible with water. However, organic- solvent based carbon bonding

precursors may be used as well.

In further embodiments these processes use a slurry (for the production of a

carbon bonded ceramic filter of the first type) or a semi-damp mixture (for the

production of the carbon bonded ceramic filter of the second type) that comprises:

a) fibers in the range of 0.1 to 20% by weight,

b) a graphitizable carbon bonding precursor in the range of from 2 (5) to 15 (25)

parts by weight, c) ceramic powder in the range of from 0 (20) to 95 (80) parts by weight, anti- oxidation material d) in the range of from 0 to 80 parts by weight, graphite in the range of from 0 to 90 parts by weight, e) organic binder in the range of from 0 to 10, in particular 0.2 to 2 parts by weight and, f) dispersion agent in the range of from 0 to 4, in particular 0.1 to 2 parts by weight. Water is added in a quantity as required. For the purpose of slurry-preparation,

20 to 70 parts by weight are necessary depending on the nature of the ceramic filler materials. For the semi-damp mixture used for pressing, water is necessary in an amount of from 2 to 10 parts by weight, depending of the nature of the ceramic filler materials. The ceramic powder may comprise zirconia, silica, alumina, brown fused alumina, magnesia, any type of clay, talcum, mica, silicon carbide, silicon nitride and the like or any mixture thereof. Graphite may also be used as a substitute for ceramic powder.

Preferred anti-oxidation materials according to the present invention are metallic powder such as steel, iron, bronze, silicon, magnesium, aluminum, boron, zirconium boride, calcium boride, titanium boride and the like, and/or glass fits containing 20 to 30% by weight of boric oxide.

Organic binders that are preferred according to the present invention are green binders such as polyvinyl alcohol (PVA), starch, gum arabic, sugar or the like or any combination thereof. These binders may be added to improve the mechanical

properties of the fillers during handling prior to firing. Starch and gum arabic may

also be used as thickening agent.

Preferred dispersion agents according to the present invention are

Despex.RTM., ligninsulphonate or the like, or any combination thereof which help to

reduce the water level in the slurry and improve the rheology.

In a further embodiment of the present invention the slurry or semi-damp

mixture may comprise a plasticizer such as polyethylene glycol (preferred molecular

weight: 500 to 10000) in the range of from 0 to 2 parts by weight, preferably 0.5 to 1

part by weight and/or an anti-foam agent such as silicon anti-foam in the range of

from 0 to 1 part by weight, preferably 0.1 to 0.5 parts by weight.

In accordance with the present invention, the carbon-bonded filters further

comprise an external glaze. External means at least some portion of the exposed

surface area of the filter. The exposed surface area would include the faces of the

filter and the interstices of the filter through which the molten metal is expected to

flow. The faces include the inlet and outlet faces of the filter.

Glaze means any material that substantially reduces oxidation of the filter

during preheating in an oxidizing atmosphere. The glaze may include a barrier that

resists diffusion of oxygen to the graphitized portion of the filter. Alternatively or in

combination, the glaze may include a barrier that scavenges oxygen before the oxygen

can reach the graphitized portion of the filter. Oxygen scavengers include, for

example, aluminum, magnesium and silicon. The glaze is preferably applied to the

filter after graphitization of the carbon precursor. The glaze may be applied to the

filter by immersion, spraying, brushing, and electrostatic methods. The applied glaze

may be dried to remove excess water. Optionally, the glaze may be fired.

In one embodiment, after the graphitized filter is produced in accordance with

one of the above embodiments, the filter is dipped in a glaze material. The filter is left in the glaze long enough for the glaze to substantially cover the external surface of the filter. This may take, for example, about thirty (30) seconds. The filter is removed from the glaze material and centrifuged to ensure that the pores of the filter are not clogged by the glaze material and to remove any excess glaze. The filter is dried in accordance with ASTM 028.

In an alternative embodiment, the glaze material is sprayed on the filter instead of dipping the filter in the glaze material. In a further alternative embodiment, the glaze material is applied by electrostatic application. Any suitable glaze material known to those skilled in the art may be used in accordance with the present invention. A glaze of the following composition may be used:

Material wt. %

Tarmac 87.2.126 69.95% Dextrine 1.61%

Darvan 7 1.34%

Primal E- 1801 Acrylic 2.40%

Deionized water 25.00%

Obviously, numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described. While this invention has been described with respect to certain preferred embodiments, different variations, modifications, and additions to the invention will become evident to persons of ordinary skill in the art. All such modifications, variations, and additions

are intended to be encompassed within the scope of this patent, which is limited only by the claims appended hereto.