# Patent

## Title
Sintered product produced from a zircon-based charge

## Abstract

The present invention relates to a sintered product elaborated from a starting charge containing 75-99% of zircon, in mass percentage based on the oxides and having the following average weight chemical composition, in mass percentages based on the oxides

60%≦ZrO2≦72.8%,

27%≦SiO2≦36%,

0.1%≦B2O3+GeO2+P2O5+Sb2O3+Nb2O5+Ta2O5+V2O5,

0.1%≦ZnO+PbO+CdO,

B2O3+GeO2+P2O5+Sb2O3+Nb2O5+Ta2O5+V2O5+ZnO+PbO+CdO≦5%,

0%≦Al2O3+TiO2+MgO+Fe2O3+NiO+MnO2+CoO+CuO≦5%,

other oxides: ≦1.5%,

    
    
        for a total of 100%.
    
    


Notably used in a glass furnace.

## Background
TECHNICAL FIELD
The invention relates to novel sintered materials produced from zircon, a method for their making and their use in a glass furnace.
TECHNOLOGICAL BACKGROUND
Among refractory products, a distinction is made between molten and cast products and sintered products.
Unlike sintered products, fused and cast products most often include a very abundant intergranular glassy phase which will fill the lattice of the crystallized grains. The problems encountered by sintered products and by fused and cast products in their respective applications, and the technical solutions adopted for solving them, are therefore generally different. Moreover, because of the significant differences between the making processes, a composition developed for making a fused and cast product cannot a priori be used as such for making a sintered product and vice versa.
Sintered products are obtained by mixing suitable raw materials and then shaping this mixture in the green state and baking the resulting green form at a sufficient temperature and during sufficient time in order to obtain sintering of this green form.
Sintered products depending on their chemical composition and their preparation method are intended for very diverse industries.
A sintered product adapted to a particular application will therefore not have a priori the properties required for it being used in another application where the temperature, corrosion or abrasion conditions are different.
For instance, U.S. Pat. No. 3,899,341 describes sintered products elaborated from zircon (50-90%) and from zirconia. The zirconia is partly stabilized in order to limit elastic deformations of the products, which lead to cracks. The products of U.S. Pat. No. 3,899,341 are however designed to be used in contact with molten steel. They are therefore not a priori suitable for being used in contact with molten glass.
Among the sintered products, the dense products elaborated from zircon (zirconium silicate: ZrO2.SiO2, or ZrSiO4) and optionally zirconia (free zirconium oxide ZrO2) may be used in applications where they are directly in contact with molten glass, in particular in the case of non-alkaline glasses.
EP 952 125 thus describes sintered products intended for glass furnaces and elaborated from zircon (5-40%) and from zirconia. These products further contain titanium, aluminium and yttrium oxides with which large blocks may be elaborated without any cracks. The SiO2 content of these products is less than 14%. Their ZrO2 content is larger than 82%.
WO 02/44102 describes isopipes used for making glass sheets. Isopipes include more than 95% by mass of zircon and exhibit insufficient creeping behavior, unless they also include between 0.2 and 0.4% of titanium oxide. In order to illustrate the state of the art, WO 02/44102 cites U.S. Pat. No. 5,124,287.
U.S. Pat. No. 5,124,287 describes compositions containing from 75 to 95% of zircon and of titanium oxide, intended to be in contact with molten glass. It indicates that the presence of titanium oxide is favorable for densifying the obtained products after sintering. In the final product, the zirconia should be non-stabilized and it is therefore preferable to use non-stabilized zirconia in the starting mixture. The use of stabilized zirconia, for example with stabilizers of zirconia such as yttrium or calcium oxides, is however not redhibitory, the heating of the mixture leading to destabilization of zirconia.
WO 2006/073841 describes refractory materials intended for the glass industry. These materials based on zircon, may include Y2O3. They always include at least 1% P2O5 or V2O5.
SU 1 020 404 describes a mixture intended for coating refractory products based on zircon.
The appearance of new glasses, such as alkali-free glasses, entails an increase in the melting and/or shaping temperatures for glass. Consequently, traditional materials based on zircon see their performances decrease. Indeed, traditional materials based on zircon tend to exudate when they are subject to temperatures above 1,550° C. Exudation is all the more significant since the temperature and the exposure time to these temperatures increase. This phenomenon is particularly damageable because it may then be accompanied by formation of defects in the glass, increased corrosion by the glass and by bubbling phenomena.
Therefore, there is a need for a product having very good resistance to exudation and which may be used in glass furnaces. The present invention is aimed at meeting this need.
SUMMARY OF THE INVENTION
For this purpose, the invention proposes a sintered product elaborated from a starting charge containing 75-99% zircon, in mass percentages on the basis of the starting charge, and having the following average weight chemical composition, in mass percentages based on the oxides:

60%≦ZrO2≦72.8%,

27%≦SiO2≦36%,

0.1%≦B2O3+GeO2+P2O5+Sb2O3+Nb2O5+Ta2O5+V2O5,

0.1%≦ZnO+PbO+CdO,

B2O3+GeO2+P2O5+Sb2O3+Nb2O5+Ta2O5+V2O5+ZnO+PbO+CdO≦5%,

0%≦Al2O3+TiO2+MgO+Fe2O3+NiO+MnO2+CoO+CuO≦5%,

other oxides: ≦1.5%,

for a total of 100%.

This sintered refractory product has excellent resistance to exudation, in particular relatively to the products described to this day. Advantageously, it also has a high density, either equivalent or higher than that of tested known products.
Preferably, the product according to the invention further has one or more of the following optional characteristics:

    
    
        The Ta2O5 content is larger than 0.3%, preferably larger than 0.5%, more preferably larger than 0.8%, in mass percentages based on the oxides.
        The Ta2O5 content is less than 3%, preferably less than 2.5%, more preferably less than 2%, in mass percentages based on the oxides.

0.1%≦GeO2+P2O5+Nb2O5+Ta2O5, and/or

0.1%≦ZnO, and/or

GeO2+P2O5+Nb2O5+Ta2O5+ZnO≦5%, and/or

0%≦Al2O3+TiO2+MgO+Fe2O3+NiO+MnO2+CoO+CuO≦5%,

        in mass percentages based on the oxides.
        The ZnO content is larger than 0.3%, preferably larger than 0.5%, more preferably larger than 0.8%, in mass percentages based on the oxides.
        The ZnO content is less than 3%, preferably less than 2.5%, more preferably less than 2%, in mass percentages based on the oxides.
        The product according to the invention includes at least one oxide, and preferably at least two oxides, from Al2O3, TiO2, MgO, Fe2O3, NiO, MnO2, CoO and CuO. Preferably the content of at least one of these oxides, or even each of at least two of these oxides, is larger than 0.05%, preferably larger than 0.10%, more preferably larger than 0.15%, still preferably larger than 0.3%, in mass percentages based on the oxides. Preferably, 0.05%<Al2O3+TiO2, preferably 0.9%<Al2O3+TiO2, more preferably 1%<Al2O3+TiO2. Also preferably, Al2O3+TiO2<3%, preferably Al2O3+TiO2<2%, in mass percentages based on the oxides.
        The product has the following average weight chemical composition, in mass percentages based on the oxides:

60%≦ZrO2≦72.4%,

27%≦SiO2≦36%,

0.2%≦B2O3+Nb2O5+Ta2O5≦2.5%,

0.2%≦ZnO≦2.5%,

0.2%≦Al2O3+TiO2+MgO+NiO+MnO2+CoO+CuO≦3%,

other oxides: ≦1.5%,

        for a total of 100%.
        The product has the following average weight chemical composition, in mass percentages based on the oxides:

60%≦ZrO2≦72.4%,

27%≦SiO2≦36%,

0.2%≦Nb2O5+Ta2O5≦2.5%,

0.2%≦ZnO≦2.5%,

0.2%≦Al2O3+TiO2≦2%,

other oxides: ≦1.5%,

        for a total of 100%.
        The <<other oxides>> are impurities. Preferably, the content of <<other oxides>> is less than 1.2%, preferably less than 1%, more preferably less than 0.7%, preferably less than 0.5%, and still preferably less than 0.2%, in mass percentages based on the oxides.
        The P2O5 content is less than 1%, preferably less than 0.5%.
        The yttrium oxide Y2O3 content is less than 0.5%, preferably less than 0.3%, in mass percentages based on the oxides.
        The zircon content in the starting charge is larger than or equal to 80%, in mass percentage based on the starting charge.
        The zirconia content in the product is larger than 5%, preferably larger than 10%, and/or less than 15%, in mass percentages based on the oxides.
        More than 90% by number, or even substantially 100% by number of the grains have a size less than 50 microns, preferably less than 30 microns, preferably less than 20 microns, even preferably less than 10 microns.
        The apparent porosity of the product is less than 5%, preferably less than 2%, preferably less than 1%, preferably less than 0.5%.
        The product has an apparent density larger than 4.0 g/cm3, preferably larger than 4.1 g/cm3, preferably larger than 4.2 g/cm3.
        The product has the shape of a block, preferably having a mass larger than 5 kg, preferably larger than 10 kg.
    
    


The invention also relates to a method for making a sintered product including the following steps:

    
    
        a) mixing raw materials in order to form a starting charge,
        b) shaping a green part from said starting charge,
        c) sintering said green part so as to obtain said sintered product,

remarkable in that the starting charge is determined so that said product is according to the invention.

    
    


Preferably, a method according to the invention may further have one or more of the following optional characteristics:

    
    
        In step a), zircon is added and optionally zirconia in amounts such that the total of the zircon and zirconia contents account for at least 95% of the starting charge, in mass percentage based on the starting charge.
        One or more of the oxides Al2O3, TiO2, MgO, Fe2O3, NiO, MnO2, CoO, CuO, B2O3, GeO2, P2O5, Sb2O3, Nb2O5, Ta2O5, V2O5, PbO, ZnO and CdO are voluntarily (i.e. systematically and methodically) added, in amounts guaranteeing that the sintered product obtained in step c) is according to the invention.
        At least 50%, preferably at least 80%, preferably at least 90%, more preferably substantially 100%, by mass of the zinc is added as oxide ZnO.
        The phosphoric acid content is less than 1%, preferably less than 0.8%, even preferably less than 0.5%.
        The raw materials used in step a) have a median size (D50) less than 50 μm, or even less than 40 μm, or even less than 20 μm, or even less than 5 μm, or even less than 4 μm.
    
    


The invention also relates to the use of a refractory product according to the invention, made or capable of having been made in accordance with a method according to the invention, in a glass furnace, in particular in areas of the furnace in contact with molten glass. The invention also relates to such a glass furnace.
DEFINITIONS
In the present description and according to custom, in the starting charge or in the product, the molecules of ZrO2 which are not associated with SiO2 molecules for forming zircon are called <<zirconia>>. In the same way, the molecules of SiO2 which are not associated with ZrO2 molecules for forming zircon are called <<silica>>. The ZrO2 content of the product according to the invention corresponds to the ZrO2 content of zircon and to zirconia. The SiO2 content of the product according to the invention corresponds to the SiO2 content of zircon and to silica.
In a chemical composition, the oxide contents relate to the overall contents for each of the corresponding chemical elements, expressed as the most stable oxide, according to the customary convention of the industry; sub-oxides and optionally nitrides, oxynitrides, carbides, oxicarbides, carbonitrides, or even metal species of the aforementioned elements are therefore included.
In the sums of mass contents of the oxides Al2O3, TiO2, MgO, Fe2O3, NiO, MnO2, CoO, CuO, B2O3, GeO2, P2O5, Sb2O3, Nb2O5, Ta2O5, V2O5, PbO, ZnO and CdO, used for defining the chemical composition of the product, one or more of the mentioned oxides may be absent.
The raw materials containing zirconium oxide ZrO2 also contain small amounts of HfO2 (1.5%-2%). <<ZrO2>> conventionally designates zirconium oxide ZrO2 and these traces of HfO2 introduced with ZrO2.
Unless indicated otherwise, all the percentages are mass percentages based on the oxides. In particular, in the starting charge, the zircon percentage is expressed on the basis of the total mass of the starting charge.
By <<impurities>>, are meant unavoidable constituents, necessarily introduced into the raw materials or resulting from reactions with these constituents. In a particular embodiment, P2O5 is an impurity.
The <<median size>> D50 of a set of particles is the size dividing the particles of this set into first and second populations equal in mass, these first and second populations only including particles having a size which is larger or smaller than the median size, respectively.
The size of a grain is its larger dimension when observed in a polished section.


DETAILED DESCRIPTION OF AN EMBODIMENT
The zircon may be provided by zircon sand or by a chamotte of dense products with strong zircon content.
According to the invention, it is absolutely necessary that the starting charge includes at least 75%, preferably at least 80% of zircon. The advantageous properties of the product according to the invention would indeed not be obtained if the amounts of SiO2 and ZrO2 brought by zircon according to the invention were provided as silica and zirconia.
Preferably, the total of the zircon and zirconia contents accounts for at least 95% of the starting charge.
Zirconia has significant dilatometric variations due to its crystallographic state transition at high temperature. In order to limit these dilatometric variations, in particular on large blocks, it is necessary to limit the zirconia content. The starting charge should therefore contain less than 25% of zirconia, which is ensured with a zircon content of at least 75%.
According to the invention, the presence of at least one oxide selected from B2O3, GeO2, P2O5, Sb2O3, Nb2O5, Ta2O5, or V2O5, in particular Ta2O5 and Nb2O5, and still particularly Ta2O5, and simultaneously at least one oxide selected from ZnO, PbO, and CdO, in particular ZnO, improves the resistance to exudation of the refractory product based on zircon.
With the additions of at least one oxide selected from Al2O3, TiO2, MgO, Fe2O3, NiO, MnO2, CoO, CuO, it is possible, if necessary, to improve the density of the products according to the invention. These additions are therefore particularly advantageous in applications where the product is placed in a corrosive environment, and notably in contact with molten glass. The inventors have discovered that, in the products of the invention, and in particular in the products including Ta2O5, high density may however be obtained when these additions, and in particular of Al2O3 and of TiO2, are limited to low contents, in particular less than 3%, less than 2%, or even less than 1%, or even less than 0.5%, in mass percentages based on the oxides.
Preferably, at least 1% silica is added to the starting charge so as to promote starting of the densification at lower temperatures than those required for densifying pure zircon.
The <<other oxides>> are oxides such as Na2O or Y2O3. The Na2O content (which promotes dissociation of zircon) should be minimized. Preferably, these oxides are impurities brought by the raw materials, and they are not required constituents but are only tolerated. At contents less than 1.5%, it is considered that the effect of these <<other oxides>> does not substantially change the obtained result.
Preferably, the content of each of the oxides Na2O and Y2O3 in the sintered product according to the invention is less than 0.5%, preferably less than 0.3%, more preferably less 0.15% in mass percentages based on the oxides.
Sintered refractory blocks may be made according to a method conventionally including steps a)-c). Optionally, this method includes prior to step a), a step for milling zircon sources and other raw materials. With this step, it is possible to attain conventional particle size characteristics required for good subsequent densification of the material. In particular, the thereby prepared powders may have a median size (D50) less than 50 μm, or even less than 40 μm, or even less than 20 μm, or even less than 5 μm, or even less than 4 μm.
In step a), all the raw materials are dosed so that the mixture has the desired average weight chemical composition, and then mixed in the presence of deflocculant(s) and/or binder(s) conventionally used in the zircon sintering methods, for example phosphoric acid.
The mixture of raw materials may optionally be atomized before passing to step b).
In step b), the mixture is then shaped, for example by isostatic pressing, in order to form blocks of a desired size.
Other techniques such as slip-casting, uniaxial pressing, gel casting, vibrocasting or a combination of these techniques may be used.
In step c), the green part is sintered in air at atmospheric pressure and at a temperature comprised between 1,400° C. and 1,700° C., so as to form a dense refractory block.
Of course, the present invention is not limited to the embodiments described provided as illustrative and non-limiting examples.
EXAMPLES
The following non-limiting examples are given with the purpose of illustrating the invention.
In these examples, the following raw materials used have been selected, the given percentages being mass percentages:

    
    
        Micronized zircon having the following average chemical analysis (by weight): ZrO2+HfO2: 66%, SiO2: 33%, Al2O3: 0.3%, P2O5: 0.3%, Fe2O3: 0.07%, TiO2: 0.08%, and less than 0.2% of other compounds such as Y2O3. The particles have a median size (D50) of 2.5 μm.
        A solution of phosphoric acid H3PO4 at 80%.
        Zinc oxide containing more than 99% of ZnO, of which the median size (D50) of the particles is about 3 μm.
        Titanium oxide containing about 95% of TiO2 and whereof the median size of particles is 2.3 μm.
        Alumina, of which the median size of the particles is about 3 μm.
        Germanium oxide containing about 99% of GeO2 and of which the median size of the particles is about 3 μm.
        Tantalum oxide containing about 99.85% of Ta2O5 and of which the size of the particles is less than 44 μm.
    
    


Sintered refractory blocks were made according to a method conventionally including the following steps:

    
    
        a) mixing the raw materials in order to form the starting charge,
        b) shaping a green part from said mixture,
        c) sintering said green part.
    
    


In step a), all the raw materials were dosed so that the mixture has the desired average weight chemical composition, and then mixed in the presence of sintering agents, deflocculants, and/or binders, conventionally used, for example phosphoric acid.
The mixture of the raw materials may optionally be atomized before passing to step a).
In step b), the mixture was then shaped by isostatic pressing in order to form cylindrical green parts with a diameter of 200 mm and a height about 200 mm.
In step c), the green parts were then sintered in air, at atmospheric pressure and at a sintering temperature of 1,600° C. with the plateau being maintained for 20 hrs.
In order to measure the exudation level, samples as bars of product with a square section with a side length of 25 mm and a height of 75 mm were taken and subjected to a heating at 1,700° C. for 24 hours. At the end of this period, the volume of the sample was measured. The volume of the sample after heating is called “Vm”, the volume of the sample before heating “Vi” and the difference “Va” between both volumes (volume increase) Va=Vm−Vi. The ratio Va/Vi is called the <<exudation index>> noted as <<EI>> in Table 1.
The portions of the raw materials used (mass percentages) as well as the apparent porosity (in %), the density and the “EI” index of the obtained sintered products are given in Table 1.














TABLE 1











6
7



1
2
3
4
5
(comparative)
(comparative)






















Zircon
98.75
98.5
97
97.3
97.33
98.5
98.6


ZnO
0.25
0.5
2.5
2.5
2.5




H3PO4




0.17




Al2O3





0.8
0.3


Ta2O5
1
1
0.5






GeO2



0.2





TiO2





0.7
1.1


Apparent
4.14
4.71
0.12
0.79
1.23
0.29
0.25


porosity









Apparent
4.11
4.10
4.31
4.28
4.28
4.32
4.39


density









(g/cm3)









EI
3%
6%
3%
5%
3%
12%
9%









Table 1 clearly shows the very good resistance to exudation of the products according to the invention. The density of these products remains equivalent to that of the tested known products.
These products are therefore well adapted for being used in glass furnaces.

## Claims

1. A sintered product elaborated from a starting charge containing 75-99% of zircon, in mass percentage based on the oxides and in which raw materials are powders having a median size of less than 5 μm, said sintered product having the following average weight chemical composition in mass percentages based on the oxides:


60%≦ZrO2≦72.8%,



27%≦SiO2≦36%,



0.1%≦B2O3+GeO2+P2O5+Sb2O3+Nb2O5+Ta2O5+V2O5,



0.1%≦ZnO+PbO+CdO,



B2O3+GeO2+P2O5+Sb2O3+Nb2O5+Ta2O5+V2O5+ZnO+PbO+CdO≦5%



0%≦Al2O3+TiO2+MgO+Fe2O3+NiO+MnO2+CoO+CuO≦5%,



other oxides: ≦1.5%,

for a total of 100%.

2. The sintered product according to claim 1, wherein


0.1%≦GeO2+P2O5+Nb2O5+Ta2O5, and/or



0.1%≦ZnO, and/or



GeO2+P2O5+Nb2O5+Ta2O5+ZnO≦5%.

3. The sintered product according to claim 1, wherein the Ta2O5 content is larger than 0.3% and less than 3%, in mass percentages based on the oxides.

4. The sintered product according claim 1, wherein the Ta2O5 content is larger than 0.8%, in mass percentage based on the oxides.

5. The sintered product according to claim 1, wherein the ZnO content is larger than 0.3% and less than 3%, in mass percentages based on the oxides.

6. The sintered product according to claim 1, wherein the ZnO content is larger than 0.8%, in mass percentage based on the oxides.

7. The sintered product according to claim 1, including more than 0.05% of at least one oxide from Al2O3, TiO2, MgO, Fe2O3, NiO, MnO2, CoO and CuO, in mass percentage based on the oxides.

8. The sintered product according to claim 1, including more than 0.3% of at least two oxides from Al2O3, TiO2, MgO, Fe2O3, NiO, MnO2, CoO and CuO, in mass percentage based on the oxides.

9. The sintered product according to claim 1, having a chemical composition such that 0.05%<Al2O3+TiO2<3%, in mass percentage based on the oxides.

10. The sintered product according to claim 1, having a chemical composition such that 0.9%<Al2O3+TiO2, in mass percentage based on the oxides.

11. The sintered product according to claim 1, having an average weight chemical composition, in mass percentages based on the oxides, such that:


60%≦ZrO2≦72.4%,



27%≦SiO2≦36%,



0.2%≦B2O3+Nb2O5+Ta2O5≦2.5%,



0.2%≦ZnO≦2.5%,



0.2%≦Al2O3+TiO2+MgO+NiO+MnO2+CoO+CuO≦3%,



other oxides: ≦1.5%,

for a total of 100%.

12. The sintered product according to claim 1, having an average weight chemical composition in mass percentages based on the oxides, such that:


60%≦ZrO2≦72.4%,



27%≦SiO2≦36%,



0.2%≦Nb2O5+Ta2O5≦2.5%,



0.2%≦ZnO≦2.5%,



0.2%≦Al2O3+TiO2≦2%,



other oxides: ≦1.5%,

for a total of 100%.

13. The sintered product according to claim 1, wherein the content of <<other oxides>> is less than 0.5%, in mass percentage based on the oxides.

14. The sintered product according to claim 1, wherein the yttrium oxide Y2O3 content is less than 0.5%, in mass percentage based on the oxides.

15. The sintered product according to claim 1, wherein the zircon content is larger than 5%, in mass percent based on the oxides.

16. A method of working with molten glass, comprising contacting the molten glass with the sintered product of claim 1 within a furnace configured for working with molten glass.

17. The method of claim 16, wherein the sintered product has a high resistance to exudation, which reduces the formation of defects in a glass product made from the molten glass.