WO1993014045A1

WO1993014045A1 - Method for the fabrication of a refractory insulating rigid material with high open porosity, and material thus obtained

Info

Publication number: WO1993014045A1
Application number: PCT/FR1992/001198
Authority: WO
Inventors: Alain Grandin De L'eprevier
Original assignee: Produits Cellulosiques Isolants - Procelis
Priority date: 1992-01-14
Filing date: 1992-12-17
Publication date: 1993-07-22
Also published as: FR2686081B1; JPH06506437A; EP0580832A1; CA2104379A1; AU3357193A; FR2686081A1

Abstract

Method for the fabrication of a refractory insulating rigid material with high open porosity which comprises mixing ceramic powders with a ceramisable binding composition, shaping said mixture by casting and dripping and finally, after drying, pyrolysing and ceramising the resulting product. The method is characterized in that the ceramic powders have sizes lower than one hundred and fifteen micrometers and, once packed, have an expanded density lower than 1.5; in that the binding composition contains from 20 to 150 grams per liter of dry ceramisable extracts; and in that, in the mixture, the weight ratio between the ceramisable binding material and the ceramic powder is comprised between 0.7 et 10. Application: casting molds for molten metals.

Description

PROCESS FOR THE MANUFACTURE OF AN INSULATING REFRACTORY MATERIAL. RIGID AND WITH A HIGH OPEN POROSITY. AND MATERIAL THUS OBTAINED.

The invention relates to a new process for the manufacture of an insulating, rigid refractory material with high open porosity; it also relates to isotropic rigid insulating refractory materials obtained in this way.

Insulating refractory materials based on brick or concrete are known, the densities of which are between 0.5 and 1.5, but the purity or nature of which often limits the refractant at temperatures of 1200 ° C. and above. In addition, in these products, the porosity is closed, therefore not accessible, which greatly limits their applications.

In recent years, fibrous insulating materials have been proposed, in particular in the form of sheets, which have the drawback of not being rigid. In addition, although these products are generally quite chemically pure, their temperature stability is often very insufficient and these products are not isotropic.

In the document EP-A-0 145 621 of the Applicant, a method has been described for manufacturing a rigid and stable insulating fibrous part. However, the play is not entirely isotropic and its density ^is limited to at most 0.4, which gives it a strength deemed sufficient for some applications, such as in particular for casting molten metals, such as steel or copper. The invention overcomes these drawbacks. It relates to a process for the manufacture of a rigid insulating refractory material, the density of which is greater than that of the insulating ceramic materials known to date, which has good mechanical resistance comparable to that of insulating brick, but whose porosity is completely open as in fibrous insulators whose chemical purity they share.

The invention also relates to a method for manufacturing such materials which are dimensionally stable and isotropic.

This process for the manufacture of a rigid insulating refractory material with high open porosity, which consists of:

. mixing ceramic powders with a ceramizable binder composition,. then, to form this mixture by pouring and draining,

. and finally, after drying, pyrolyzing and ceramising the product obtained, is characterized:

- in that the ceramic powders have dimensions of less than one hundred and fifty (150) micrometers and, once stacked, have an expanded density of less than 1.5;

- In that the binding composition contains from 20 to 150 grams per liter of ceramizable dry extracts;

- And in that, in the mixture, the weight ratio between the binding material and the ceramic powder is between zero five (0.5) and one and a half (1.5). Advantageously, in practice:

- The ceramic powders have dimensions close to one hundred (100) micrometers and have an alumina content of at least 40%, preferably greater than 90% and even 95%; - The ceramic powders have a form factor or an internal porosity such that their expanded density is between 0.5 and 1.2; in practice, the powders are, for example, prepared by grinding ceramic fibers carefully selected for their chemical composition; - The binding composition is a liquid or a pseudo-liquid and comprises from 50 to 80 grams per liter of ceramizable dry extracts;

- The dry ceramizable extract is of the silica-alumina type and contains less than 0.5% of impurities;

- The mixture is stirred and then poured into shape and simultaneously is drained through a filter screen;

- The product obtained is dried at 110-130 ° C for at least twenty-four hours, then pyrolysis at a temperature of at least 980 ° C, preferably 1200 ° C, for example for six hours and more.

The addition of a second binder composition of refractory oxide precursor chosen from the chemical groups containing zirconium and yttrium can be carried out either in the mixture before stirring, or before pyrolysis, or even after ceramization, which requires a new cycle of drying, pyrolysis and ceramization. The purpose of adding this second composition is essentially to modify certain properties such as, for example, the surface tension forces which induce non-wettability or the catalytic effects by formation of discrete deposits. The invention also relates to the rigid insulating refractory material obtained. This material based on bonded ceramic powders, is characterized in that it is isotropic and contains powders or fibers whose dimensions are less than one hundred and fifty (150) micrometers, preferably close to one hundred (100) micrometers, and contains at least 80% alumina; this material has an open porosity of at least 70% and a mechanical strength allowing it to withstand a pressure of at least thirty centimeters of molten metal at 1550 ° C, and finally, a flexural modulus between 2 and 10 MPa and a thermal conductivity at 1000 ° C of the order of 0.3 W / mK.

It is important that the powders or fibers have a dimension of less than one hundred and fifty (150) micrometers and preferably between ten (10) and one hundred and fifty (150) micrometers. It was observed that if this dimension was less than ten (10) micrometers, products were obtained which were much too dense and not very insulating. On the other hand, if the dimensions are greater than one hundred and fifty (150) micrometers, the material becomes too porous and consequently has insufficient mechanical strength. In addition, it becomes anisotropic. Good results are obtained with fibers the dimensions of which are close to one hundred (100) micrometers, or with porous powders so that, when expanded, their density is less than 1.5, and preferably between 0.5 and 1,2, failing which the final product obtained will no longer be sufficiently insulating.

It is also important that these ceramic powders contain at least 40% of alumina, the percentage being able to be much higher (95% and more) depending on the envisaged applications. According to a characteristic of the invention, the ceramic powders are ground fibers with lengths of less than 150 micrometers. This provides the finished product with regular open porosity, excellent dimensional stability at high temperatures and an isotropic structure significantly superior to that of conventional isometric crystalline powders.

Binders in liquid form must be pure and compatible with ceramic powders. As binding compositions, materials of common use are used for this type of application. Mention may be made of mineral binders such as for example those based on aluminum silicate, magnesium, calcium or zirconium, aluminum phosphate. It is also possible to use gelling solutions (sol / gel) comprising an appreciable proportion of silica and alumina. One can also use organic binders which have the advantage of power. destroy itself completely during the pyrolysis treatment.

In practice, use is essentially made of silica-aluminous sol-gel binders, or even of zirconia and / or yttrium oxide.

The liquid binder composition must have sufficient rheological properties to properly coat the ceramic powders. Alternatively, the binder composition may also contain plasticizers.

As already said, the binding composition contains from 20 to 150 grams per liter of dry extracts, preferably between 50 and 80 grams per liter. According to another characteristic of the invention, the ratio between the binder composition and the powder must be between 0.7 and 10 to be pourable. It has been observed that if this ratio is less than 0.7, the mixture obtained is not pourable because it is too pasty. On the other hand, if this ratio exceeds 10, the mixture becomes too liquid to be homogeneously pourable.

The binder and the powder are mixed by any known method. The mixture is then poured in a known manner either in the form of a blank which will subsequently be machined, or in the form of a block. This mixture is then drained by gravity, by overflow after sedimentation, or by the process which simultaneously consists of vibrating, pressing and draining as described in document EP-A-0 145 621 of the Applicant.

Once the material has been poured, it is dried in particular in a dry oven at 110-130 ° C for twenty-four to thirty-six hours until a constant weight is obtained, then in known manner, it is pyrolysed at more than 950 ° C, preferably at 1200 ° C.

In a preferred variant, the ceramic powders are obtained by grinding ceramic fibers by any known means, until the desired particle size is obtained so that when these powders are put in heaps, their bulk density is less than 1.5.

In another variant, the powders are agglomerated into porous granules, so that the bulk density is less than 1.5.

In an advantageous variant, the dried and cooked material is then impregnated with a precursor solution of an oxide, such as yttrium acetate, a rare earth acetate, zirconium acetate, or a precursor solution of a meta-catalyst such as platinum or nickel chloride, etc. After pyrolysis, this precursor lines the pores of the material without plugging them and thus gives the material either properties of non-wettability by molten metals, such as steel or copper, or gives the material the desired catalytic properties.

The manner in which the invention can be implemented and the advantages which ensue therefrom will emerge more clearly from the embodiments which follow.

Example 1:

Ceramic fibers containing 97% of alumina are ground, so as to obtain powders of dimensions close to 100 microns and of bulk density when put in a heap of 0.7.

Five kilos of this powder are mixed at room temperature with five kilos of a liquid silico-aluminous binder comprising 60 grams per liter of dry extracts, the latter containing by weight 80% alumina and 20% silica. We stir slowly.

The mixture is then poured into a mold, the bottom of which is lined with a perforated fabric to allow the excess binder to be drained.

Simultaneously, the mixture is vibrated to remove air bubbles.

This mixture, cast in the form of a block, is dried in an oven for twenty-four hours at 130 ° C. and pyrolysis for six hours at 1200 ° C.

An insulating refractory material is obtained, the density of which is 0.8, which has an alumina content greater than 85%, a flexural modulus of 3 MPa, a thermal conductivity at 1000 ° C. of 0.34 W / mK and a fully open porosity close to 80%.

This material has sufficient mechanical strength to contain a height of 20 cm of stainless steel melted at 1550 ° C without penetration of the metal into the pores.

This isotropic material is perfectly suitable for the production of molten metal casting molds. Example 2:

Example 1 is repeated.

When the mixture has been dried, it is impregnated with a concentrated zirconium acetate composition and the whole is dried so as to obtain a continuous deposit of zirconium salt on all the walls of the pores.

After pyrolysis, an insulating material of density 0.9 is obtained having the same physical characteristics as the material of Example 1, and having no surface attack by molten steels, even slight after several casting passes.

Example 3: Example 1 is repeated, but using powders close to fifty (50) micrometers. We get an expanded density of 1.15. The material obtained has mechanical characteristics up to 10 MPa. On the other hand, it is slightly less porous (70%) and therefore less insulating. It lends itself without problem to the treatment of Example 2.

Example 4:

Five kilos of 97% Al2θ3 fibers ground to one hundred (100) micrometers are mixed directly with five kilos of a concentrated and liquid binder composition of Yttrium salt.

The mixture is poured, vibrated and dried as in Example 1 and pyrolysis at 1500 ° C. A rigid machinable material with a density close to 0.7 is obtained, very refractory and very stable in size usable for thermal insulation between 1700 and 2000 ° C., as well as for the casting of copper alloys.

The rigid insulating refractory materials obtained in this way, based on bonded ceramic powders, are isotropic, moderately dense, rigid and therefore machinable and stable without sintering up to temperatures up to 1550 ° C. and above. These materials also have a totally open porosity, that is to say the pores of which are interconnected with one another and open onto the faces of the material.

In this way, these materials can be used, advantageously for the manufacture of casting molds for molten metals.

Claims

1 / Process for the manufacture of an insulating, rigid refractory material with high open porosity, which consists of:. mixing ceramic powders with a ceramizable binder composition,. then, to form this mixture by pouring and draining,. and finally, after drying, pyrolyzing and ceramising the product obtained, characterized: - in that the ceramic powders have dimensions of less than one hundred and fifty (150) micrometers and, once put in a heap, have an expanded density less than 1.5;

- in that the binding composition contains from 20 to 150 grams per liter of ceramizable dry extracts; - And in that, in the mixture, the weight ratio between the ceramizable binder and the ceramic powder is between zero seven (0.7) and one and a half (1.5).

2 / A method according to claim 1, characterized in that the ceramic powders have dimensions close to one hundred (100) micrometers and have an alumina content of at least 40%, preferably greater than 90% and even 95% .

3 / Method according to one of claims 1 and 2, characterized in that the ceramic powders have an expanded density between 0.5 and 1.2.

4 / Method according to one of claims 1 and 3, characterized in that the ceramic powders are prepared by grinding of elementary ceramic fibers. 5 / Method according to one of claims 1 to 4, characterized in that the binder composition is a liquid or a pseudo-liquid comprising from 50 to 80 grams per liter of ceramizable dry extracts.

6 / A method according to one of claims 1 to 5, characterized in that the ceramizable binder is of the silica-alumina type and contains less than 0.5% of impurities.

7 / A method according to one of claims 1 to 6, characterized in that the mixture is stirred and then poured into shape and simultaneously is drained through a filter screen.

8 / A method according to one of claims 1 to 7, characterized in that the poured mixture is dried for at least twenty-four hours at 110-130 ° C, then is pyrolysis at a temperature of at least 1000 ° C.

9 / Rigid insulating refractory material, based on bonded ceramic powders, obtained by the implementation of the method according to one of claims 1 to 8, characterized in that this material is isotropic and contains powders or fibers whose dimensions are close to one hundred (100) micrometers, and contain at least 80% of alumina, said material having an open porosity of at least 70%, a mechanical resistance making it possible to withstand at 1550 ° C a pressure of at least thirty centimeters of molten metal, a flexural modulus between 2 and 10 MPa, and a thermal conductivity at 1000 ° C close to 0.3 W / mK