FR2684667A1 - Process for the manufacture of a rigid insulating refractory material and material thus obtained - Google Patents

Abstract

Process for the manufacture of a rigid insulating refractory material capable of working in contact with a molten metal, in which: - first of all, a base refractory ceramic material is impregnated with a solution of a precursor of a refractory oxide; - then, the impregnated ceramic material is dried; - and finally this dried material is pyrolysed; characterised: - in that the precursor is a rare-earth salt or sol; - in that the base refractory ceramic material is a highly porous material which has a porosity of between 75 and 95%; - and in that pyrolysis is carried out at a temperature of between 800 and 1000@C for two to six hours, so that the walls of the pores of the base ceramic material are covered with a fine protective layer of rare-earth oxides. Applications: moulds for casting iron- or copper-based alloys.

Description

PROCESS FOR THE MANUFACTURE OF A REERACTIVE MATERIAL
RIGID INSULATION AND MATERIAL THUS OBTAINED.

 The invention relates to a process for the manufacture of a porous and rigid insulating refractory material, capable of working in contact with a molten metal, in particular molten alloys of iron or copper; these refractory materials are likely to be used during the casting of these alloys. It also relates to the rigid insulating porous refractory material thus obtained.

 It is well known to use refractory materials in contact with a molten metal, in particular during the casting of steel or copper alloys. It is important that this material can resist without damage, without alteration, or corrosion at temperatures of more than 1000 "C, in particular from 1400 to 16000C (in the case of steel).

 In practice, brick or concrete are mainly used, that is to say dense and massive materials. However, as we know, these materials, because of their low thermal insulating nature, are not very suitable for use in continuous casting processes. This results in great difficulty in controlling the heat losses and therefore the temperature of the metal, which is however essential in the continuous casting processes in order to obtain a regular metallurgical quality.

 There are also known ceramic insulating materials, used essentially to minimize the heat losses of an enclosure, such as an oven, due to their very low thermal conductivity. Unfortunately, these products, if they are well refractory and insulating, are easily wetted and penetrated by molten steel, and especially by copper alloys. The metal not only ruins the insulating character by penetrating into the pores of the material, but moreover, it tends to corrode the ceramic by oxidizing on contact with it.

 In the document FR-A-I 438 091, it has been proposed to improve the whiteness of the porcelain by impregnating it, in particular under vacuum, with zirconia acetate, then by drying and baking it. During firing, the zirconia is fixed by silicate bonds, which gives the ceramic an improved whiteness.

 In document US-A4 568 652, it has been proposed to reduce the harmful effects of the impurities contained in refractory bauxites with low alumina content (42 to 70%) and of low porosity (porosity less than 20%), by impregnating them of a solution containing at most ten percent (10%) of a metal capable of forming a refractory oxide, such as chromium, iron, calcium, molybdenum, zirconium, for example under vacuum, then, after drying at 110 C, heating the impregnated material at least to 1450 C. The oxides formed then react with the impurities to increase the refractoriness of the base material and reduce the external wettability by glass or slag.

 The invention overcomes these drawbacks. It relates to a process for preparing an insulating refractory material, porous and rigid, neither wet nor penetrated, nor corroded by a molten molten metal, in particular molten steel or a molten copper alloy, easy to machine and to use, and suitable in particular for use in the various processes for continuous or pressure casting of the molten metal, in particular by allowing the cooling rates of the metal to be controlled.

This process for the manufacture of an insulating, rigid refractory material capable of working in contact with a molten metal based on molten iron or copper in which:
first of all, a refractory ceramic material is impregnated with
base, with a solution of a refractory oxide precursor;
then, this ceramic material thus impregnated is dried;
and finally, this dried material is pyrolysed, is characterized:
- in that the precursor is a rare earth salt or soil;
- in that the basic ceramic material is a highly porous material having a porosity of between 75 and 95%;
- And in that one pyrolyses at a temperature between 800 and 1000 C for two to six hours, so that the walls of the pores of the basic ceramic material are covered with a thin protective layer of rare earth oxides.

 In other words, the invention consists in immersing a highly porous refractory ceramic material in a soil or in a solution of rare earth salts, then, after drying, in pyrolyzing this solution or this soil, so as to transform this precusor made of refractory oxide, and thus to form a thin layer of oxide which covers the walls of the pores (or fibers) of the basic ceramic material, which nevertheless remains highly porous therefore insulating, which is rigid and refractory.

 One could not think that the deposit of a thin layer of refractory oxide by the pyrolysis of a salt or a suitable soil on the walls of the pores of a ceramic material prevents the wettability of this material by a molten metal , ferrous or copper-based, whereas until then this material was easily wetted by these metals. In other words, this selection makes it possible to effectively solve a problem which has been posed for a long time, in particular for molten alloys based on copper.

 By "rare earths" is meant the elements of the periodic table also known by the name "lanthanides" or "lanthanides", such as yttrium, cerium, praseodymium, etc. It is important that these precursors are in a liquid form capable of impregnating the base material. It may be a sol, that is to say a pseudo-solution of a chemical precursor of a refractory oxide based on rare earths, or a solution, preferably aqueous, of this precursor.

 Advantageously, the solution or the soil can contain a mixture of several precursors of oxides of different rare earths, which has the advantage of considerably lowering the cost of this raw material, since such mixtures are more readily available.

 In practice, the most suitable rare earths are chosen from the group comprising yttrium and cerium.

 According to a first fundamental characteristic of the invention, the basic ceramic material must have a porosity of between 75 and 95%. This porosity is preferably close to 85%.

 In the prior art, only dense refractory materials were used, materials which do not have the desired insulating character, since their porosity is less than 15%. This basic ceramic material must also have sufficient refractoriness to withstand the temperature of the molten metal, in particular steel. In practice, ceramics with a fibrous structure or ceramic foams with at least partially open cells are used.

 It has been observed that if the porosity of the basic ceramic material is less than 75%, the insulating character of the final product is too greatly reduced for the intended use, and moreover the machinability decreases rapidly. Similarly, if the initial porosity exceeds 90%, the mechanical strength of the material becomes too weak to contain the pressure of the molten metal, machining also becomes difficult due to lack of rigidity, and finally the insulating character decreases again quickly below an optimum density threshold.

Advantageously, in practice:
the soluble rare earth salt is an organic salt such as in particular an acetate, or even a formate, a propionate or a nitrate, or even an aqueous sol of rare earth hydroxide; it is important that the salt used is easily soluble in water, in particular at room temperature, to penetrate well into the very heart of the porous material and thus fill all the pores and also to better respect the environment; it is important that the oxide formed is the only solid residue from pyrolysis in order to preserve the refractarity of the basic product as much as possible;
- The concentration of the aqueous impregnation solution is between 5 and 23% by weight of residual oxide after pyrolysis, and preferably between 15 and 20%, to obtain a thin continuous layer of oxides lining the walls of the pores; if the concentration is less than 5%, no significant result is obtained; if this concentration is less than 15%, the thin layer of oxides will not always be continuous, which will subsequently alter the non-wettability by the molten metal; if this concentration exceeds 25%, the impregnation solution not only is not commercially available, but would be too viscous to easily penetrate and impregnate the pores; it has been observed that good results are obtained with a concentration of between 18 and 20% by weight of solution or soil;
- the impregnation is carried out by simple soaking or full bath immersion at room temperature; due to the high porosity of the base material, it is not useful, as in the prior art, to penetrate the solution by suction or by the action of vacuum;
- drying takes place in an oven for several hours at 110-130 C, this time being a function of the volume of the part to be dried;
- The impregnation solution may contain known additions, in order to accelerate, avoid or minimize the migration of the precursors to the surface during the drying phase;
- the solution or the impregnation soil is formed of a mixture of precursors, for example of yttrium and other rare earths in the report
Y203 / REO = 99%;
- Pyrolysis is carried out between 800 and 1000 C, for two to six hours, in particular four hours, in the presence of air to facilitate the formation of oxides; it is completely unnecessary to heat to temperatures as high as 1450 "C, as in the prior art mentioned in the preamble, since one does not seek to chemically combine impurities with the oxides formed, but simply to form a very thin homogeneous and protective layer (of the order of a pressure gauge) of oxides on the walls of the pores of the basic ceramic material.

 It has been observed that if the pyrolysis is carried out at a temperature below 800 ° C., it is incomplete and a greater or lesser proportion of carbon or the like remains in the pores (depending on the nature of the precursor). if the temperature exceeds 1000 "C, the incipient oxides tend to combine with the support rather than crystallize in a thin and continuous layer.

 The finished refractory product obtained is highly porous (porosity of 75% and more), and therefore highly insulating. Despite this high porosity, this material remains perfectly rigid. As it has good mechanical strength, it is therefore easy to machine. According to an original characteristic, the walls of its pores or of the fibers constituting this ceramic material, are covered with a thin regular and homogeneous layer of rare earth oxides d thickness of the order of a nanometer. Unexpectedly, despite the high proportion of pores, and therefore of openings, the molten metal does not penetrate the material and does not pass through it, whereas it is known that the molten metal wets and easily penetrates porous ceramic materials. In this way, this material can be used for the continuous casting or the casting under pressure of ferrous alloys, of steel in particular or especially of copper alloys.

 It is believed that the thin layer of oxides formed on the walls of the pores or on the fibers creates surface tension forces which repel the molten metal and thus prevent it from penetrating the pores.

Compared to the known state of the art and mentioned in the preamble, the invention differs
on the one hand, by the choice of precise operating conditions, namely
highly porous ceramic refractory base material, choice
a soluble salt or rare earth, and pyrolysis to
temperatures between 800 and 10000C;
on the other hand, by the results obtained, namely high porosity, therefore
insulating character; rigidity therefore machinability, and finally not
wettability by molten metals, especially steel or
copper-based alloys.

The manner in which the invention can be carried out and the advantages which ensue therefrom will emerge more clearly from the exemplary embodiments which follow.
Example 1:
As a basic porous refractory ceramic material, a plate sold by the Applicant under the registered trademark "PROCELIT 160", having dimensions of 650 x 320 x 30 mm, resistant to 16000C, is used. This plate of porous refractory ceramic material has a composition having 85% of alumina and 15% of silica, and a density close to 0.3 (that is to say a porosity of 92%).

 At room temperature, this plate is immersed for approximately ten minutes (until the last bubbles have disappeared) in an aqueous solution containing 20% by weight of yttrium acetate. The porous plate absorbs about 2.5 times its dry weight of solution.

 To completely remove the water, it is then dried in a ventilated oven at 1300C for twenty-four hours.

 Pyrolysis is then carried out in an electric oven at 800 ° C. for four hours. During this pyrolysis, yttrium acetate is transformed into yttrium oxide, and simultaneously water and residual carbon dioxide are eliminated in the atmosphere.

 A plate is obtained having the same dimensions as the starting plate, consisting of a composite ceramic material with high porosity (80% porosity), the walls of the pores of which are covered with a thin layer of yttrium oxide (thickness of the order of a nanometer, in particular between 0.2 and 0.4 nanometers), which also favorably increases the rigidity of the material.

This plate, for a density close to 0.45 (against 0.3), and a compressive strength of 1.5N / mm2, has an average composition by weight, of:
- 68% alumina (against 85%);
-12% silica (against 15%);
- 20% yttrium.

 Despite its very high porosity, this plate to which the molten steel does not adhere, does not show any infiltration when it is subjected to a pressure resulting from a layer of 300 mm in height of molten steel for one hundred and twenty minutes and more.

 These rigid plates are easy to machine and are perfectly suited for the production of weights, nozzles or other parts for the continuous or pressure casting of steel and for the casting of copper alloys.

 The yttrium oxide deposited on the wall of the pores slightly affects the refractoriness of the plate, but above all prevents molten molten steel from entering the pores of the material and / or corroding it, despite the high porosity of the finished plate.

 As the immersion treatment is carried out from a solution, good penetration to the core is obtained. In this way, this treatment can be carried out on blanks, which limits the loss of oxides during subsequent machining in finishing when very precise dimensions must be obtained.

Example 2:
The same PROCELIT 160 plate used as it is in contact with molten steel, has its porosity penetrated by steel over a depth of fifteen millimeters after three minutes of contact with the molten metal. This infiltration completely alters the insulating nature of the material and makes it unsuitable for this application.

Example 3:
The same PROCELIT 160 plate, this time impregnated with a salt of another metal, such as chromium, magnesium and / or any other metal whose refractory oxides are commonly used in the steel industry, is also penetrated by steel. melted and quickly put out of use. With the exception of aluminum, the oxide of which has no effect on the refractoriness and wettability, all have been found to be harmful.

Example 4:
The same plate prepared according to the method of Example 1, but pyrolyzed at 1450 "C, has a slightly reduced wettability by molten steel compared to Example 2. The nascent oxide layer reacted with the silica and the alumina of the substrate to form new chemical combinations and the surface tension forces which prevented the penetration of the metal into the pores are lost.

Example 5:
The same PROCELIT 160 plate is soaked according to the method of Example 1, but in a solution containing a mixture of precursors of oxides of all the different rare earths in a proportion Y203 / Oxides of Rare Earths total of 80% by weight measured on the dry pyrolized extract. Such a mixture allows a significant reduction in the cost of the raw material, since it is directly available in 50 liter drums ready for use, with a guaranteed concentration between 15 and 18% by weight of oxides per liter of mixture.

 After drying and pyrolizing at 9000C, the material has a density close to 0.40 and is completely impermeable to molten copper alloys. This plate can be used without damage to the channel or mold in contact with the molten metal.

 Under the same conditions, an untreated plate would be penetrated and destroyed by the molten metal in a few minutes.

 The mixture of oxides introduced does not appreciably affect the refractoriness of the plate, nor its insulating character, as in Example I.

The method according to the invention has many advantages, in particular compared to the solutions known to date. We can cite:
- simplicity of manufacture;
- the ease of finishing the machined parts, even with usual steel or carbide tools;
- the non-wettability of the material by steel or molten copper alloys, despite its high porosity, when it was known that these molten metals easily wetted porous ceramics
- the possibility of controlling with good precision the temperature of casting of the molten metal by considerably reducing the heat losses through the refractories;
- good resistance to thermal shock, which avoids preheating these parts before casting.

 In this way, these refractory materials are perfectly suitable for the continuous or pressure casting of steel or the casting of copper alloys.

Claims (5)

 1 / Process for the manufacture of an insulating, rigid refractory material capable of working in contact with a molten metal based on iron or copper, in which:  first of all, a refractory ceramic material is impregnated with  base, with a solution of a refractory oxide precursor;  . then, the impregnated ceramic material is dried;  and finally, this dried material is pyrolyzed; characterized:  - in that the precursor is a rare earth salt or soil;  - in that the basic refractory ceramic material is a highly porous material having a porosity of between 75 and 95%;  - And in that it is pyrolyzed at a temperature between 800 and 1000 "C for two to six hours, so that the walls of the pores of the basic ceramic material are covered with a thin protective layer of rare earth oxides.  2 / A method according to claim 1, characterized in that the impregnation solution is a solution of rare earth acetate at a concentration between 15 and 20% by weight.  3 / A method according to claim 1, characterized in that the impregnation is carried out by simple full bath immersion at room temperature, then in that the drying is carried out in an oven at 110 1300C.  4 / A method according to claim 1, characterized in that the solution or the impregnating sol is formed of a mixture of precursors.  5 / Porous refractory ceramic material, with high porosity, obtained by implementing the method according to claim 1, characterized:  - in that the material is rigid and has a porosity of at least eighty percent (80%), and;  - in that the wall of the pores is covered with a thin layer of rare earth oxides with a thickness of the order of a nanometer.