Melted product with a high zirconium content

Abstract

A fused-cast refractory product including, as mass percentages on the basis of the oxides and for a total of 100% of the oxides: ZrO.sub.2+HfO.sub.2: remainder to 100%, with HfO.sub.25%; SiO.sub.2: 1.5% to 7.5%; Al.sub.2O.sub.3: 1.0% to 3.0%; CaO+SrO: 1.2% to 3.0%; Y.sub.2O.sub.3: 1.5% to 3.0%; Na.sub.2O+K.sub.2O: <0.15%; B.sub.2O.sub.3: <1.0%; P.sub.2O.sub.5: <0.15%; Fe.sub.2O.sub.3+TiO.sub.2: <0.55%; oxide species other than ZrO.sub.2, HfO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, Na.sub.2O, K.sub.2O, B.sub.2O.sub.3, CaO, SrO, Y.sub.2O.sub.3, P.sub.2O.sub.5, Fe.sub.2O.sub.3 and TiO.sub.2: <1.5%.

Claims

1. Fused-cast refractory product comprising, as mass percentages on the basis of the oxides and for a total of 100% of the oxides: ZrO.sub.2+HfO.sub.2: remainder to 100%, with HfO.sub.25%; SiO.sub.2: 1.5% to 7.5%; Al.sub.2O.sub.3: 1.0% to 3.0%; CaO +SrO: 1.2% to 3.0%; Y.sub.2O.sub.3: 1.5% to 3.0%; Na.sub.2O [+] and K.sub.2O: <0.15%; B.sub.2O.sub.3: <1.0%; P.sub.2O.sub.5: <0.15%; Fe.sub.2O.sub.3+TiO.sub.2: <0.55%; oxide species other than ZrO.sub.2, HfO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, Na.sub.2O, K.sub.2O, B.sub.2O.sub.3, CaO, SrO, Y.sub.2O.sub.3, P.sub.2O.sub.5, Fe.sub.2O.sub.3 and TiO.sub.2: <1.5%.

2. Product according to claim 1, in which the mass content of Y.sub.2O.sub.3 is less than or equal to 2.5%.

3. Product according to claim 1, in which the mass content of Y.sub.2O.sub.3 is greater than or equal to 1.7%.

4. Product according to claim 1, in which the mass content of CaO+SrO is less than or equal to 2.5%.

5. Product according to claim 1, in which the mass content of CaO+SrO is greater than or equal to 1.3%.

6. Product according to claim 1, in which the mass content of CaO+SrO is greater than or equal to 1.8%.

7. Product according to claim 1, in which the mass content of silica SiO.sub.2 is greater than or equal to 2.5%.

8. Product according to claim 1, in which the mass content of silica SiO.sub.2 is less than or equal to 7.0%.

9. Product according to claim 1, in which the mass content of silica SiO.sub.2 is less than or equal to 6.0%.

10. Product according to claim 1, in which the mass content of Al.sub.2O.sub.3 is less than or equal to 2.5%.

11. Product according claim in which the mass content of Al.sub.2O.sub.3 is less than or equal to 2.3%.

12. Product according to claim 1, in which the mass content of Al.sub.2O.sub.3 is greater than or equal to 1.5%.

13. Product according to claim 1, in which the mass content of B.sub.2O.sub.3 is less than or equal to 0.5%.

14. Product according to claim 1, in which: the mass content of Na.sub.2O +K.sub.2O is less than or equal to 0.10%, and the mass content of iron oxide and/or titanium oxide, Fe.sub.2O.sub.3+TiO.sub.2, is less than 0.4%, and the mass content of P.sub.2O.sub.5 is less than or equal to 0.05%, and the total mass content of the oxide species other than ZrO.sub.2, HfO.sub.2, SiO.sub.2, Al.sub.2O.sub.3 , Na.sub.2O, K.sub.2O, B.sub.2O.sub.3, CaO, SrO, Y.sub.2O.sub.3, P.sub.2O.sub.5, Fe.sub.2O.sub.3 and TiO.sub.2 is less than 1.0%.

15. Product according to claim 1, in which the ratio of the mass content of silica to the content of boron oxide SiO.sub.2 /B.sub.2O.sub.3 is greater than or equal to 8.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) In a fused-cast product according to the invention, the high content of zirconia ZrO.sub.2 makes it possible to meet the requirements in terms of high corrosion resistance without coloration of the glass produced, and without generation of defects that are harmful to the quality of this glass.

(2) In a product obtained by melting, HfO.sub.2 is not chemically dissociable from ZrO.sub.2. In the chemical composition of such a product, ZrO.sub.2+HfO.sub.2 denotes the total content of these two oxides. However, according to the present invention, HfO.sub.2 is not deliberately added to the starting feedstock. HfO.sub.2 therefore only denotes traces of hafnium oxide, this oxide always being naturally present in zirconia sources in contents generally less than 2%.

(3) For the sake of clarity, the content of zirconia and of trace of hafnium oxide may be denoted without preference by ZrO.sub.2+HfO.sub.2 or by ZrO.sub.2, or alternatively by zirconia content.

(4) The presence of silica SiO.sub.2 is necessary and especially allows the formation of an intergranular vitreous phase that is capable of efficiently accommodating the variations in volume of zirconia during its reversible allotropic transformation, i.e. during its passage from the monoclinic phase to the tetragonal phase. The content of silica SiO.sub.2 should, however, be limited to maintain very good corrosion resistance by limiting the content of vitreous phase to the benefit of the zirconia.

(5) The presence of alumina Al.sub.2O.sub.3 promotes the formation of a stable vitreous phase and improves the flowability of the products in the mold. An excessive content leads to instability of the vitreous phase (formation of crystals), which has a negative impact on the feasibility, in particular in the presence of boron oxide.

(6) The presence of boron oxide is optional. This element makes it possible to improve the feasibility of the products. In one embodiment, the mass content of B.sub.2O.sub.3 is thus greater than or equal to 0.3%, or even greater than or equal to 0.4%, or even greater than or equal to 0.5%.

(7) The presence of boron oxide does, however, have an effect an adverse effect on the formation of zircon in the product. In another embodiment, a mass content of boron oxide B.sub.2O.sub.3 that is as low as possible is preferred.

(8) The simultaneous presence of yttrium oxide Y.sub.2O.sub.3 and of the oxides CaO and/or SrO, in contents according to the invention, makes it possible, in combination with the other constituents, to obtain a refractory product that is particularly suitable for use in contact with molten glass. This refractory product in particular has good corrosion resistance and a resistance to thermal cycling that is high and substantially constant over time. These contents also make it possible to preserve good feasibility.

(9) The alkaline oxides Na.sub.2O and K.sub.2O may migrate into the molten glass and do not make it possible to ensure a lasting effect. The mass content of Na.sub.2O+K.sub.2O must therefore be limited.

(10) According to the invention, Fe.sub.2O.sub.3, TiO.sub.2 and P.sub.2O.sub.5 are known to be harmful and their content must preferably be limited to traces introduced as impurities with the starting materials.

(11) The other oxide species are the species that are not listed above, namely species other than ZrO.sub.2, HfO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, Na.sub.2O, K.sub.2O, B.sub.2O.sub.3, CaO, SrO, Y.sub.2O.sub.3, P.sub.2O.sub.5, Fe.sub.2O.sub.3 and TiO.sub.2. In one embodiment, the other oxide species are limited to species whose presence is not particularly desired and which are generally present as impurities in the starting materials.

(12) The presence of the other oxide species does not substantially modify the results obtained, provided that their content remains less than 1.5%.

(13) Conventionally, in a fused-cast product, the oxides represent more than 98.5%, more than 99%, or even substantially 100% of the mass of the product. This is likewise the case in a product according to the invention.

(14) Any conventional process for manufacturing molten products based on zirconia that are intended for applications in glass melting furnaces may be used, provided that the composition of the starting feedstock makes it possible to obtain products that have a composition in accordance with that of a product according to the invention.

(15) In particular, a product according to the invention may be conventionally manufactured according to steps a) to c) described above.

(16) In step a), the starting materials are chosen so as to ensure the contents of oxides in the finished product. A person skilled in the art knows perfectly how to determine the composition of a starting feedstock as a function of the composition of the desired product.

(17) In step b), the melting is preferably performed by means of the combined action of a fairly long electric arc, which produces no reduction, and of blending promoting the reoxidation of the products.

(18) To minimize the formation of nodules of metallic appearance and to avoid the formation of slits or cracks in the final product, it is preferable to perform the melting under oxidative conditions.

(19) Preferentially, use is made of the long-arc melting process described in French patent 1 208 577 and its additions 75893 and 82310.

(20) This process consists in using an electric arc furnace whose arc spurts out between the feedstock and at least one electrode distanced from this feedstock and in regulating the length of the arc so that its reductive action is minimized, while at the same time maintaining an oxidative atmosphere above the molten bath and blending said bath, either by the action of the arc itself, or by sparging an oxidizing gas (for example air or oxygen) into the bath, or alternatively by adding to the bath substances that release oxygen such as peroxides or nitrates.

(21) The melting may in particular take place at a temperature above 2300 C., preferably between 2400 C. and 2500 C.

(22) A product of the invention thus manufactured is constituted of zirconia grains surrounded by a vitreous phase.

(23) The zirconia may be monoclinic to more than 80%, more than 85%, more than 90%, more than 99% or substantially 100%, as a mass percentage.

(24) The vitreous phase may comprise more than 50%, or even more than 70%, of silica, between 5% and 20% of B.sub.2O.sub.3 and between 1% and 20% of alumina, as mass percentages on the basis of the vitreous phase. The silica, B.sub.2O.sub.3 and alumina may represent more than 95%, more than 97%, or even substantially 100% of the mass of the vitreous phase.

EXAMPLES

(25) The nonlimiting examples that follow are given for the purpose of illustrating the invention.

(26) In these examples, the following starting materials were used: zirconia mainly containing, on average by mass, 98.5% of ZrO.sub.2+HfO.sub.2, 0.2% of SiO.sub.2 and 0.02% of Na.sub.2O, zircon sand containing 33% silica SiO.sub.2, alumina of AC44 type sold by the company Pechiney and containing on average 99.4% of alumina Al.sub.2O.sub.3, boron oxide with a purity of greater than 99%, yttrium oxide with a purity of greater than 99%, calcium carbonate containing about 56% CaO, strontium carbonate containing about 70% SrO

(27) The starting materials were melted at a temperature of between 2000 and 2500 C., via the standard process of melting in a Hroult arc furnace, under oxidative conditions, and the molten material was then cast in molds to obtain blocks in 220 mm450 mm150 mm format.

(28) The reference examples are the products ER-1195 and SCIMOS CZ, mentioned above.

(29) For all the products obtained, crystallographic analysis reveals zirconia crystals surrounded by a vitreous phase typically comprising more than 70% silica.

(30) Chemical analysis of the products obtained is given in Table 1. It is a mean overall chemical analysis, given as mass percentages on the basis of the oxides. The impurity content is about 0.3% as a mass percentage on the basis of the oxides; the remainder to 100% corresponds to the zirconia content.

(31) The resistance to thermal cycling is determined by means of a test that consists in subjecting a sample (of dimensions 30 mm30 mm30 mm) to twenty cycles between 800 C. and 1250 C. (Test A) and between 500 C. and 1250 C. (Test B). At each cycle, the sample is maintained for 1 hour at 800 C. or at 500 C., respectively, and the temperature is then brought to 1250 C. over 1 hour and is maintained at this value for 1 hour.

(32) The value Vv indicated in Table 1 corresponds to the increase in volume, given as a percentage, between the start and the end of the test. In this table, ND means not determined.

(33) Zirconia and the impurities constitute the remainder to 100%, the content of impurities being about 0.3%.

(34) TABLE-US-00001 TABLE 1 Test Test Na.sub.2O + CaO + A B Example SiO.sub.2 Al.sub.2O.sub.3 B.sub.2O.sub.3 K.sub.2O Y.sub.2O.sub.3 SrO (Vv) (Vv) Na.sub.2O K.sub.2O CaO SrO ER1195 4.0 1.2 0.0 0.30 0.2 0.0 6 20 0.30 0.00 0.0 0.0 CZ 4.5 0.7 0.4 0.02 0.2 0.0 29 32 0.02 0.00 0.0 0.0 1 5.3 2.1 0.5 0.00 0.1 0.4 17 ND 0.00 0.00 0.4 0.0 2 4.4 1.2 0.5 0.00 0.1 2.5 48 ND 0.00 0.00 0.0 2.5 3 4.9 2.1 0.3 0.05 0.2 2.5 46 ND 0.05 0.00 2.5 0.0 4 5.2 2.1 0.3 0.02 0.0 2.1 36 ND 0.00 0.02 0.0 2.1 5 4.9 1.8 0.6 0.05 1.3 2.3 100 ND 0.05 0.00 2.3 0.0 6 5.2 2.0 0.4 0.05 1.4 2.2 48 ND 0.05 0.00 2.2 0.0 7 5.1 2.4 0.3 0.00 1.5 2.1 6 ND 0.00 0.00 0.0 2.1 8 5.0 2.2 0.3 0.00 1.6 2.0 6 ND 0.00 0.00 0.0 2.0 9 5.1 2.0 0.5 0.00 2.1 2.4 6 ND 0.00 0.00 2.4 0.0 10 5.1 2.2 0.2 0.01 2.1 1.9 6 9 0.00 0.01 0.0 1.9 11 4.3 2.1 0.1 0.00 2.1 2.3 6 9 0.00 0.00 2.1 0.2 12 4.9 2.1 0.2 0.12 2.2 2.2 6 9 0.07 0.05 2.1 0.1 13 5.2 2.2 0.2 0.10 2.2 1.9 6 9 0.07 0.03 0.0 1.9 14 2.8 2.5 0.1 0.00 2.2 2.6 6 9 0.00 0.00 2.3 0.3 15 4.6 2.1 0.1 0.01 2.2 2.2 6 9 0.00 0.01 2.0 0.2 16 5.1 2.0 0.6 0.05 2.3 2.3 6 9 0.05 0.00 2.3 0.0 17 5.1 2.3 0.5 0.00 2.5 2.3 6 9 0.00 0.00 1.3 1.0 18 5.2 2.3 0.5 0.00 2.6 2.3 6 ND 0.00 0.00 1.6 0.8 19 5.1 2.2 0.1 0.06 2.6 2.3 6 9 0.05 0.01 1.7 0.6 20 5.3 2.2 0.1 0.13 2.6 2.4 6 9 0.09 0.04 1.9 0.5 21 2.8 1.4 0.1 0.06 2.2 1.4 ND 9 0.05 0.01 1.4 0.0 22 3.2 1.7 0.2 0.00 2.0 1.4 ND 9 0.00 0.00 0.7 0.7 23 3.6 1.6 0.1 0.07 2.4 1.2 ND 9 0.06 0.01 0.3 0.9 24 5.2 2.1 0.1 0.06 2.7 2.2 ND 9 0.06 0.00 1.8 0.4 25 5.5 2.6 0.1 0.11 2.2 1.6 ND 9 0.09 0.02 1.6 0.0 26 4.2 2.0 0.8 0.02 1.9 2.8 ND 11 0.00 0.02 2.5 0.3 27 3.6 1.1 0.7 0.00 2.1 1.7 ND 16 0.00 0.00 0.9 0.8 28 1.5 1.0 0.0 0.06 1.6 1.6 ND 9 0.05 0.01 1.5 0.1 29 2.6 1.3 0.1 0.06 2.0 1.5 ND 9 0.05 0.01 1.5 0.0

(35) Comparison of Examples 4 and 6 shows that, in the presence of a content of alkaline oxides of less than 0.15%, an increase in the content of Y.sub.2O.sub.3 may be detrimental, the result in test A going from 36 to 48 when the content of Y.sub.2O.sub.3 goes from 0 to 1.4%. The inventors have, to their merit, discovered that a positive effect was possible beyond a content of 1.5%, especially provided that the CaO+SrO content is between 1.2% and 3.0%, as illustrated in particular by comparison of Examples 6 and 8.

(36) The results show that the tested products of the invention have very good resistance to thermal cycling, despite the low content, or even the absence, of Na.sub.2O and K.sub.2O.

(37) Examples 26 and 27 show that the resistance to thermal cycling is particularly improved when the ratio of the mass content of silica to the content of boron oxide SiO.sub.2/B.sub.2O.sub.3 is greater than or equal to 8, in particular when the content of silica is less than 4.5%.

(38) Without wishing to tie the invention to any theory, the inventors think that the composition of the products of the invention makes it possible to limit the formation of zircon. Thus, no formation takes place, around the zirconia crystals, of a crystalline phase having dilatometric behavior different from that of zirconia, in particular at about 1000-1150 C. The vitreous phase can then accommodate the volume deformations associated with thermal cycling.

(39) Needless to say, the present invention is not limited to the embodiments described and represented, which are given as nonlimiting illustrative examples.