MELTED MAGNESIUM ALUMINATE GRAIN RICH IN MAGNESIUM

Abstract

A fused grain is essentially composed of a matrix of a magnesium aluminum oxide of MgAl.sub.2O.sub.4 spinel structure and/or of the MgOMgAl.sub.2O.sub.4 eutectic structure, and of inclusions essentially composed of magnesium oxide. The grain has the following overall chemical composition, as percentages by weight, expressed in the form of oxides: more than 5.0% and less than 19.9% of Al.sub.2O.sub.3, Al.sub.2O.sub.3 and MgO together represent more than 95.0% of the weight of the grain. The cumulative content of CaO and of ZrO.sub.2 is less than 4000 ppm, by weight.

Claims

1. A fused grain essentially composed of a matrix of a magnesium aluminum oxide of MgAl.sub.2O.sub.4 spinel structure and/or of the MgOMgAl.sub.2O.sub.4 eutectic structure, and of inclusions essentially composed of magnesium oxide, said grain having the following overall chemical composition, as percentages by weight, expressed in the form of oxides: more than 5.0% and less than 19.9% of Al.sub.2O.sub.3, Al.sub.2O.sub.3 and MgO together represent more than 95.0% of the weight of said grain, wherein a cumulative content of CaO and of ZrO.sub.2 is less than 4000 ppm, by weight.

2. The fused grain as claimed in claim 1, wherein Al.sub.2O.sub.3 represents more than 8.0% by weight.

3. The fused grain as claimed in claim 1, wherein the cumulative content of CaO and ZrO.sub.2 is less than 3000 ppm.

4. The fused grain as claimed in claim 1, wherein the cumulative content of CaO and ZrO.sub.2 is less than 2500 ppm.

5. The fused grain as claimed in claim 1, wherein the grain does not comprise an alumina Al.sub.2O.sub.3 phase.

6. The fused grain as claimed in claim 1, wherein impurities in the grain are essentially CaO, ZrO.sub.2, Fe.sub.2O.sub.3, SiO.sub.2, Na.sub.2O, MnO.sub.2.

7. The fused grain as claimed in claim 1, comprising less than 2000 ppm of CaO.

8. The fused grain as claimed in claim 1, comprising less than 200 ppm of ZrO.sub.2.

9. The fused grain as claimed in claim 1, wherein Al.sub.2O.sub.3 and MgO together represent more than 99.0% of the weight of said grain.

10. The fused grain as claimed in claim 1, wherein the matrix is composed of separate regions of spinel structure and of the MgOMgAl.sub.2O.sub.4 eutectic structure.

11. The fused grain as claimed in claim 1, comprising fine inclusions essentially composed of calcium and zirconium oxides, a largest dimension of which is less than 2 micrometers.

12. A mixture of fused grains as claimed in claim 1.

13. A ceramic material obtained by sintering fused grains as claimed in claim 1.

14. The ceramic material as claimed claim 13, comprising inclusions essentially composed of calcium and zirconium oxides, the number of which is less than 100 per 10 000 square micrometers, on an electron microscopy photograph.

15. A process for manufacturing grains as claimed in claim 1, comprising: a) mixing the starting materials in order to form a starting feedstock, b) melting the starting feedstock until a molten liquid is obtained, c) cooling said molten liquid so that the molten liquid is completely solidified to form a solid mass, d) grinding said solid mass so as to obtain a mixture of said fused grains.

16. The fused grain as claimed in claim 11, wherein the largest dimension of the fused grain is less than 1 micrometer.

Description

EXAMPLE 1

[0057] In this comparative example, a mixture is prepared from the following commercial starting materials: [0058] a magnesium oxide powder having the following chemical analysis (in percentages by weight): MgO99%; CaO0.1%; Na.sub.2O0.1%; SiO.sub.2<0.05%; ZrO.sub.20.02%; Fe.sub.2O.sub.30.02%; [0059] an aluminum oxide powder with Al.sub.2O.sub.399% (by weight) having the following impurities (in percentages by weight): Na.sub.2O: 0.23%, CaO<0.02%, MgO<0.05%, Fe.sub.2O.sub.3<0.02%, SiO.sub.2<0.05%, ZrO.sub.2<0.02%, TiO.sub.2<0.02%.

[0060] The mixture is formed solely from these two commercial powders mixed in an MgO/Al.sub.2O.sub.3 weight ratio of 84.3/15.7; then co-ground until a median particle size dso, as measured by laser particle size analysis, of the order of 3.4 micrometers is obtained.

[0061] The elemental analysis by X-ray fluorescence of the mixture thus obtained makes it possible to determine, with a relative uncertainty of the order of 1%, the concentrations of oxides. The mixture has a content of aluminum oxide of 15.7 weight percent and a content of magnesium oxide of 84.2 weight percent. The impurities detected are calcium (700 ppm of CaO equivalent) and sodium (400 ppm of Na.sub.2O equivalent); the other species being below the detection thresholds of the measurement device, including iron (<200 ppm of Fe.sub.2O.sub.3 equivalent), silicon (<500 ppm of SiO.sub.2 equivalent), zirconium (<500 ppm of ZrO.sub.2 equivalent) and titanium (<120 ppm of TiO.sub.2 equivalent).

[0062] The composition of the mixture thus obtained is determined according to the following protocol:

[0063] Phase analysis: Analysis of the sample by x-ray diffraction makes it possible to identify the various crystalline phases of the sample. It is carried out using the EVA software and the PDF-2 database (2005 version) of the ICDD. The proportions of the various phases are then determined by the Rietveld method using the HighScore Plus 4.0 software (PANalytical B.V.). The .cif files of the Inorganic Crystal Structure Database: ICSD #9863 (for MgO periclase), #203212 (for Mg(OH).sub.2 brucite), #51687 (for Al.sub.2O.sub.3 corundum), and #22354 (for MgAl.sub.2O.sub.4 spinel) are used as starting point for the refinement. The Bragg peaks are modelled with pseudo-Voigt functions.

[0064] The phase analysis carried out on the mixture obtained at the end of the co-grinding shows that this mixture is essentially composed of an MgO periclase phase and an Al.sub.2O.sub.3 corundum phase.

[0065] The resistance to hydration of the mixture prepared as described above is measured according to the following protocol:

[0066] A preliminary phase analysis is carried out on the sample. The resistance to hydration is measured by placing 5 grams of the mixture in 25 cm.sup.3 of distilled water with stirring and at ambient temperature for two hours. The sample is then dried for 24 hours at 110 C. before a new phase analysis and a weighing. The comparison of the two phase analyses according to the protocol described above and of the weighings before and after the hydration test make it possible to determine the sensitivity of the sample to hydration according to the following two criteria:

[0067] the more the brucite phase appears, the more sensitive the sample,

[0068] the higher the mass is above 5 grams, the more hydrated the sample and therefore the more sensitive the sample to hydration.

[0069] The results obtained for the comparative sample according to this example 1 are given in table 1 below.

EXAMPLE 2

According to the Invention

[0070] In this example according to the invention, fused grains are prepared by melting starting from the commercial starting materials used in example 1, mixed in an MgO/Al.sub.2O.sub.3 weight ratio of the order of 85/15.

[0071] The powder mixture is this time melted in an arc furnace at a temperature of the order of 2100 to 2300 C. The molten liquid is solidified and cooled. The fused product is then ground until a powder of fused grains is obtained having a median particle size d.sub.50, as measured by laser particle size analysis, of the order of 2.9 micrometers and having a particle size distribution similar to that of example 1.

[0072] An elemental analysis by X-ray fluorescence of the fused grains thus obtained makes it possible to determine, as in example 1, the concentrations of the elemental oxides.

[0073] The grains have a content of aluminum oxide of 15.7 weight percent and a content of magnesium oxide of 84.2 weight percent. The impurities detected are calcium (1200 ppm of CaO equivalent); the other species being below the detection thresholds of the measurement device: iron (<200 ppm of Fe.sub.2O.sub.3 equivalent), silicon (<500 ppm of SiO.sub.2 equivalent), zirconium (<500 ppm of ZrO.sub.2 equivalent) and titanium (<100 ppm of TiO.sub.2 equivalent).

[0074] The phase analysis, carried out on the mixture of grains according to the protocol described above, shows that these grains are essentially composed of an MgO periclase phase and an MgAl.sub.2O.sub.4 spinel phase.

[0075] Provided in the appended figure is an electron microscopy photograph taken on a fused grain obtained according to the invention: a very particular structure is observed, in which zones 1 composed of the MgAl.sub.2O.sub.4 spinel and/or MgOMgAl.sub.2O.sub.4 eutectic phase encompass, in a matrix structure, grains 2 composed essentially of the MgO periclase phase, as was able to be demonstrated directly by elemental analysis using the Castaing microprobe (also known as electron probe microanalyser, EPMA). The darkest zones 3 on the photograph correspond to the porous zones of the structure.

[0076] The resistance to hydration of the fused grains thus prepared is measured according to the same protocols as described above in connection with example 1. The results obtained are given in table 1 below.

TABLE-US-00001 TABLE 1 Example 1 Example 2 initial mass 5 grams 5 grams Mass after hydration test 5.9 grams (+18%) 5.7 grams (+14%) Phases MgO magnesium 36% 50% after oxide hydration MgAl.sub.2O.sub.4 spinel 15% test Al.sub.2O.sub.3 alumina 12% Mg(OH).sub.2 brucite 52% 35%

[0077] The results given in table 1 above show the advantages linked to the use of the fused grains according to the invention in an aqueous-phase forming process since only a limited amount of brucite is detected. Yet, the observations made by the applicant company have shown that the presence of too high a proportion of this phase makes forming in the aqueous phase very difficult, even impossible, as is described below:

[0078] The mixture of co-ground powders from example 1 and also the powder of fused grains from example 2 were suspended under conventional aqueous-phase ceramic forming conditions: 60% by weight of ceramic powder were mixed, under magnetic stirring, with 40% by weight of demineralized water. 1 gram of DOLAPIX dispersant is added per 100 grams of ceramic powder.

[0079] In the suspension using the mixture of co-ground powders from example 1 as ceramic powder, a gel then forms. No forming operation was possible using this suspension.

[0080] After 45 minutes, the suspension using the powder of fused grains according to example 2 on the contrary did not form a visible gel. A forming of this suspension was able to be carried out without any difficulties.