AU601109B2 - Ceramic foam - Google Patents

Description

AUSTRALIA4 Patents Act 66 0 f

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority 09 006-I 9 6 o 0 006 00 0 0 0 0 00 0000 0 C 6 o 00 0 00 0 00 00 0 0 0 0 00 0 0 0 0 C 0'! Related Art: I Th Curnct cont.il,~;'t~ amcndine-~ Sricv sc2:0on '9 anld is cornctitu APPLICANT'S RE'. Div. of 41243/85 Name(s) of Applicant(s): Address(es) of Applicant(s): SWISS ALUMINIUM LTD.

Ch-3965 Chippis, Switzerland.

Actual Inventor(s): Jerry W. BROCKMEYER Address for Service is: PHILLIPS, ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia, 3000 Complete Specification for the invention entitled: CERAMIC FOAM The following statement is a full description of this invention, including the best method of performing it known to applicant(s): P 19/3/84 CERAMIC FOAM The present invention is drawn to an improved ceramic foam material and process for making same and, more particularly, a ceramic foam material having superior mechanical, thermal and chemical properties than ceramic foam materials heretofore known.

It is known in the art to employ porous ceramic foam materials to filter molten metal, especially aluminium, as described for example in U.S. Patent Numbers 3,893,917, 3,947,363, 3,962,081, 4,024,056, 4,024,212, 4,075,303, 4,265,659, 4,342,644 and 4,343,704. The production material for these filters comprises primarily a phosphate bonded refractory material, having certain other additions, which has been fired to a temperature of about 2000 F. in order to mature the bond. See the process described in U.S. Patent Number 3,962,081. While this type of refractory is suitable for use in the aluminium industry and easily withstands most aluminium alloys which are typically cast at about 1300 0 it is unsuitable for other potential applications due to its low strength and poor chemical durability. Naturally, it would be highly desirable to develop a material which maintains the favourable properties of the ceramic foam materials heretofore known, namely, high porosity, low pressure drop, high geometric surface area and tortuous flow path, but which overcomes the above-noted difficulties of strength and chemical durability. In addition, it would be desirable to develop a single material which could be relatively simply produced and used in a number of applications.

Accordingly, it is a principal object of the present invention to provide an improved ceramic foam material and process for making same.

According to the present invention there is provided a process for preparing a ceramic foam filter for filtering molten metal which comprises: providing a reticulated, organic polymer foam; impregnating said foam with an aqueous slurry of a thixotropic ceramic RA _9-2-

C,

composition including a phosphate binder in an amount of at least drying and heating said impregnated polymer foam to remove the organic component therefrom; and firing at an elevated firing temperature of at least 3020 F. to volatilize the phosphate producing a refractory material having less than 2% of phosphate as P 2 0 5 and sintering the refractory material to produce a fired material having individual grains of ceramic tightly coalesced to minimize porosity between the grains such that the microporosity is less than The improved ceramic foam filter of the present invention is prepared from a ceramic slurry containing at C 9-a S 39 -2aleast 8% phosphate binder and is for use in filtering molten metal. The filter is characterized by having an open cell structure with a plurality of interconnected voids surrounded by a web of said ceramic. The filter is further characterized by being substantially phosphate free, having less than 2% of phosphate as P 2 0 5 and having individual grains of ceramic tightly coalesced to minimize porosity between the grains, i.e., microporosity. The said microporosity is less than about In accordance with the process of the present invention a hydrophobic, reticulated organic polymer foam, preferably polyurethane foam, is impregnated with an aqueous slurry of a thixotropic ceramic composition including a phosphate binder in an amount of at least 8% and preferably greater than 10%, dried and heated to remove the organic component therefrom, and fired at an elevated temperature to volatilize the phosphate producing o e o o a substantially phosphate-free material and sintering the refractory to produce a material having individual grains 0 of ceramic tightly coalesced to minimize porosity between the grains. In the preferred embodiment the firing is at a temperature of about greater than or equal to 3020°F., and preferably about greater than or equal to 3050°F., so as to volatilize the phosphate and then sinter the refractory material. The resultant ceramic foam material is as indicated above substantially free of phosphate which is susceptible to chemical attack. In addition, the resultant material is characterized by an increase in strength properties and chemical. durability. The preferred refractory material is4a lu ia; however, other refractory material such as zirconia, silicon carbide or other materials suitable for use with a phosphate binder can be used. Naturally, specific temperatures will vary from system to system.

The ceramic foam is prepared from an open cell, preferably hydrophobic flexible foam material having a plurality of interconnected voids surrounded by a web of 39 said flexible foam material. Typical material which may A 135L -3- T C be used include the polymeric foams such as polyurethane foams, and the cellulosic foams. Generally, any combustible organic plastic foam may be used which has resilience and ability to recover its original shape. The foam must burn out or volatilize at below the firing temperature of the ceramic material which is employed.

The aqueous ceramic slurry which is employed should be thixotropic, have a relatively high degree of fluidity and be comprised of an aqueous suspension of the ceramic intended for use in the material. Typical ceramic materials which may be employed include a~lf FrF zirconia and silicon carbide or mixtures of ceramics.

While 4 -l-uminium is preferred as the refractory material, any refractory material suitable for use with a phosphate binder and having a sintering temperature greater than the .o phosphorous volatilization temperature can be used. While o. aluminium orthophosphate is the preferred binder, naturally others can be used if desired, such as for S° example, phosphoric acid, aluminium phosphate, alkali metal phosphates as sodium hexametaphosphate, etc. The slurry should contain at least 8% and preferably greater than 10% phosphate binder by weight of solids.

Detailed procedures for preparing ceramic foams for molten metal filters are described in U.S. Patent Numbers 3,962,081, 4,075,303 and 4,024,212, the disclosures of which are hereby incorporated by reference.

The flexible foam material is impregnated with the aqueous ceramic slurry so that the fiber-like webs are coated therewith and the voids are filled therewith.

Normally, it is preferred to simply immerse the foam in the slurry for a short period of time sufficient to ensure complete impregnation of the foam.

The impregnated foam is then compressed to expel a portion of the slurry while leaving the fiber-like web portion coated therewith and with a plurality of blocked pores throughout the body to increase flow path tortuosity, homogeneously distributed throughout the ceramic body rather than grouped together. In a 3 9 continuous operation one may pass the impregnated foam through a preset roller to effect the desired expulsion of slurry from the foam and leave the desired amount impregnated therein. Naturally, this may be done manually by simply squeezing the flexible foam material to the desired extent. At this stage the foam is still flexible and may if desired be formed into configurations suitable for specific filtration tasks, into curved plates, hollow cylinders, etc. It is necessary to hold the formed foam in position by conventional means until the organic substrate is decomposed, or preferably until the ceramic is sintered. The impregnated foam is then dried by any suitable means, such as air drying, accelerated drying at a temperature of from 1000 to 700 C. for from minutes to 6 hours, or by microwave drying. Air drying may be achieved in from 8 to 24 hours. After drying, the material is heated at an elevated temperature to sinter the ceramic coating on the fiber-like webs leaving the plurality of blocked pores as described above.

In accordance with the present invention, the dried impregnated material is heated to an elevated temperature to sinter the refractory and volatilize the phosphate to produce a substantially phosphate-free material having individual grains tightly coalesced to minimize porosity between the grains, with a microporosity less than oo about Preferably the firing is at a temperature of at least 3020 0 preferably at least 3050 0 F. and held at oo that temperature for from 15 minutes to 10 hours in order to volatilize the web of flexible foam, volatilize the phosphate and sinter the ceramic.

oo, o30 The resultant product is a porous, fused ceramic 00 0 o, foam material which is substantially free of phosphate, has less than 2% of phosphate as P 2 0 5 and is characterized by superior mechanical, thermal and chemical properties than ceramic foam materials heretofore known.

The ceramic foam is characterized by having an open cell structure with a plurality of interconnected voids surrounded by a web of ceramic. The foam has individual grains of ceramic tightly coalesced to minimize porosity 39 between the grains, microporosity. The said 5135L c' 0 0 0 o a 0 0 o0 0 0 o S 0 0 0 00 G t 0 microporosity is less than about The aforesaid structure results in increasing desirable physical properties, such as increased mechanical, thermal and chemical properties. In addition, the aforesaid processing results in the entire structure shrinking somewhat and thereby resulting in a reduction of the macroporosity of gross porosity of the entire filter, for example, a filter processed in accordance with the present invention will have its macroporosity reduced from about 90% without the present processing to 85% with the present processing.

In accordance with the present invention, the specific features thereof will be more readily understandable from a consideration of the following comparative data.

Samples were prepared by impregnating a hydrophobic, reticulated polyurethane foam material with a ceramic slurry containing as solids 98% 4 an--i-u- am and 2% montmorillonite as a rheological aid. About 30% aluminium 20 orthophosphate binder in a 50% aqueous solution based on total weight was added. The samples were dried and fired at temperatures of 2000 0 2870 0 29800F., 3020 0 3050 0 and 3090°F. for 5 hours. The samples were then subject to a chemical analysis, compressive strength tests and chemical resistance tests.

The results are set forth hereinbelow.

The results clearly showed that samples fired at around 2000 0 F. contained substantially in excess of 2% phosphate as P 2 0 5 but samples fired at 3020°F. and above were substantially phosphate-free, contained less than 2% phosphate as P 2 05. In addition, in all cases the samples fired at 3020 F. and above were characterized by having individual grains of ceramic tightly coalesced to minimize porosity between the grains wherein the said microporosity was less than Tables I and II below show various properties at the various firing temperatures.

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0 0 000 0 00 0 c 00 0 0 0 00; 0 0 0 Ci 0 4I 0 0 o i o 00 0O TABLE I COMPRESSIVE STRENGTH, BULK DENSITY AND THICKNESS OF CERAMIC FOAMS FIRED AT VARIOUS TEMPERATURES COMPRESSIVE APPARENT FIRING NO. SAMPLES STRENGTH BULK DENSITY THICKNESS OF. TESTED kg/cm 2 psi gm/cm 3 lb/ft 3 cm in.

2000 8 10.28 146 0.31 19.2 5.10 2.01 2870 10 11.01 157 0.36 22.3 4.87 1.92 2980 9 9.70 138 0.33 20.4 4.82 1.90 3020 11 10.12 144 0.37 22.9 4.64 1.83 3050 10 14.30 203 0.37 22.9 4.41 1.74 3090 10 18.52 263 0.39 24.1 4.48 1.77 5136L

I

l~ The strength data shown in Table I, above, show a strong increase at temperatures above 3050 F. while acceptable strength is obtained at 3020 F. The data also show that the apparent density increases and the thickness decreases indicating shrinkage which in turn is related to the loss of porosity in the walls, i.e.

microporosity.

o ou o a a 0 39 5135L -8- 000 0 0 0 0 a 0 OC oC f 00 Cc 0 0 TABLE II CHEMICAL RESISTANCE OF CERAMIC FOAMS FIRED AT VARIOUS TEMPERATURES FIRING TEMP.

OF. WEIGHT LOSS in: NaOH 10% HNO 3 10% H 2

SO

4 10% HC1 10% CH 3

COOH

2000 SAMPLES DISINTEGRATED IN THE SOLUTIONS 2870 13.4 12.5 12.6 10.5 2980 10.4 10.5 10.6 10.6 1.4 3020 8.8 10.1 8.1 9.2 1.6 3050 3.6 4.9 4.4 5.5 1.1 3090 2.7 2.0 2.2 1.4 0.9 5136L r Chemical resistance of the ceramic was determined by immersing the pre-weighed filter samples in the specified solutions for 5-1/2 days. The samples were then removed from the solutions, rinsed, dried and re-weighed. The 2000 0 F. samples disintegrated or softened to the point that they could not be removed from the solutions intact, so no weight loss data was recorded. Weight losses for the other samples are as shown. There appears to be a dramatic change at 3020 0 F. and even more so at 3050°F.

Thus, as can be seen from the foregoing, a ceramic foam material having superior strength properties and chemical durability is obtained.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and 2 all changes which come within the meaning and range of equivalency are intended to be embraced therein.

39 39 5135L -i0r

Claims (7)

1. A process for preparing a ceramic foam filter for filtering molten metal which comprises: providing a reticulated, organic polymer .oam; impregnating said foam with an aqueous slurry of a thixotropic ceramic composition including a phosphate binder in an amount of at least drying and heating said impregnated polymer foam to remove the organic component therefrom; and firing at an elevated firing temperature of at least 3020°F. to volatilize the phosphate producing a refractory material having less than 2% of phosphate as P 2 0 5 and sintering the refractory material to produce a fired material having individual grains of ceramic tightly coalesced to minimize porosity between the grains such that the microporosity is less than o°0

2. A process according to claim 1 wherein the polymer .0 0"0 foam is a polyurethane foam.

3. A process according to claim 1 or 2 wherein the firing temperature is greater than 3020°F. and firing is conducted for from 15 minutes to 10 hours. oO° o

4. A process according to claim 1 or 2 wherein the firing temperature is at least 3050 F. and firing is "conducted for from 15 minutes to 10 hours. o ao

5. A process according to any one of claims 1 to 4, wherein said ceramic is alumina.

6. A process according to any one of claims 1 to 0 «wherein said phosphate is aluminium orthophosphate.

7. A ceramic foam filter produced by the process of any one of claims 1 to 6. DATED: 25 May 1990 PHILLIPS ORMONDE FITZPATRICK Attorneys for: SWISS ALUMINIUM LTD. -11-