REFRACTORY PRODUCT HAVING A HIGH CONTENT OF ZIRCONIA

Abstract

Fused cast refractory product including, as weight percentages on the basis of the oxides and for a total of 100%: ZrO.sub.2: balance to 100%, Hf.sub.2O: <5%, SiO.sub.2: 8.1% to 12.0%, B.sub.2O.sub.3: 0.20% to 0.90%, Na.sub.2O+K.sub.2O: 0.40% to 0.80%, Al.sub.2O.sub.3: 0.3% to 2.0%, Y.sub.2O.sub.3: <2.0%, Fe.sub.2O.sub.3+TiO.sub.2: <0.6%, and other species: <1.5%.

Claims

1. A fused cast refractory product comprising, as weight percentages on the basis of the oxides and for a total of 100%: ZrO.sub.2: balance to 100% Hf.sub.2O: <5% SiO.sub.2: 8.1% to 12.0% B.sub.2O.sub.3: 0.20% to 0.90% Na.sub.2O+K.sub.2O: 0.40% to 0.80% Al.sub.2O.sub.3: 0.3% to 2.0% Y.sub.2O.sub.3: <2.0% Fe.sub.2O.sub.3+TiO.sub.2: <0.6% other species: <1.5%.

2. The refractory product as claimed in claim 1, wherein: 83.0%<ZrO.sub.2+HfO.sub.2<92.0%; and/or 8.4%<SiO.sub.2<11.5%; and/or 0.25%<B.sub.2O.sub.3<0.75%; and/or 0.45%<Na.sub.2O+K.sub.2O<0.75%; and/or 0.6%<Al.sub.2O.sub.3<1.9%; and/or 0.5%<Y.sub.2O.sub.3<1.9%; and/or Fe.sub.2O.sub.3+TiO.sub.2<0.4%; and/or other species <1.2%.

3. The refractory product as claimed in claim 2, wherein: 84.0%<ZrO.sub.2+HfO.sub.2<90.0%; and/or 8.8%<SiO.sub.2<11.0%; and/or 0.40%<B.sub.2O.sub.3<0.70%; and/or Na.sub.2O+K.sub.2O<0.65%; and/or 0.8%<Al.sub.2O.sub.3<1.7%; and/or 0.7%<Y.sub.2O.sub.3<1.7%; and/or Fe.sub.2O.sub.3+TiO.sub.2<0.3%; and/or other species <1.0%.

4. The refractory product as claimed in claim 3, wherein: 85.0%<ZrO.sub.2+HfO.sub.2<90.0%; and/or 9.1%<SiO.sub.2<10.8%; and/or B.sub.2O.sub.3<0.60%; and/or 0.9%<Al.sub.2O.sub.3; and/or 1.0%<Y.sub.2O.sub.3<1.6%; and/or Fe.sub.2O.sub.3+TiO.sub.2<0.2%; and/or species other than ZrO.sub.2, Hf.sub.2O, SiO.sub.2, Y.sub.2O.sub.3, B.sub.2O.sub.3, Al.sub.2O.sub.3, Na.sub.2O, K.sub.2O, TiO.sub.2 and Fe.sub.2O.sub.3: <0.5%.

5. The refractory product as claimed in claim 1, comprising, as weight percentages on the basis of the oxides: SiO.sub.2: 8.5% to 11.0% B.sub.2O.sub.3: 0.30% to 0.80% Na.sub.2O+K.sub.2O: 0.40% to 0.70% Al.sub.2O.sub.3: 0.6% to 2.0%.

6. The refractory product as claimed in claim 1, comprising, as weight percentages on the basis of the oxides: SiO.sub.2: 8.5% to 10.8% B.sub.2O.sub.3: 0.30% to 0.70% Na.sub.2O+K.sub.2O: 0.40% to 0.70% Al.sub.2O.sub.3: 1.0% to 1.8%.

7. The refractory product as claimed in claim 1, comprising, as weight percentages on the basis of the oxides: SiO.sub.2: 9.1% to 11.0% B.sub.2O.sub.3: 0.30% to 0.70% Na.sub.2O+K.sub.2O: 0.40% to 0.70% Al.sub.2O.sub.3: 1.1% to 1.8%.

8. The refractory product as claimed in claim 1, wherein CaO+MgO+BaO+SrO<0.60%.

9. The refractory product as claimed in claim 1, having the shape of a block, all the dimensions of which are greater than 10 mm.

10. A glass melting furnace comprising a block made of a refractory product as claimed in claim 1.

11. The glass melting furnace as claimed in claim 10, wherein the block is positioned in the superstructure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Other features and advantages of the invention will become more apparent on reading the detailed description which follows and on examining the appended drawing in which FIG. 1 [FIG. 1], described in the preamble, schematically represents a half cross section of a glass melting furnace.

DETAILED DESCRIPTION

[0082] In the fused cast products according to the invention, the high content of ZrO.sub.2 makes it possible to meet the requirements of high corrosion resistance without generating defects that are detrimental to the quality of the glass.

[0083] The hafnium oxide, HfO.sub.2, present in the product according to the invention is the hafnium oxide naturally present in the sources of ZrO.sub.2. Its content in a product according to the invention is therefore less than 5%, generally less than 2%.

[0084] The presence of SiO.sub.2 enables in particular the formation of an intergranular glassy phase capable of effectively accommodating the deformations of the zirconia backbone. On the other hand, the addition of SiO.sub.2 should not exceed 12% since this addition is carried out at the expense of the zirconia content and may therefore be detrimental to the corrosion resistance.

[0085] The presence of Al.sub.2O.sub.3 is in particular useful for the formation of a stable glassy phase and for the good castability of the molten material in the mold. However, the addition of Al.sub.2O.sub.3 should be limited since too high a weight content may lead to an instability of the glassy phase (formation of mullite crystals), in particular in the presence of boron oxide.

[0086] The simultaneous presence of B.sub.2O.sub.3 and of Na.sub.2O+K.sub.2O makes it possible to improve the feasibility of the products. B.sub.2O.sub.3 has an effect an unfavorable effect on the formation of zircon in the product, which may result in a detrimental effect on the resistance to thermal cycling. The weight content of boron oxide B.sub.2O.sub.3 should therefore remain limited.

[0087] The weight content of Na.sub.2O+K.sub.2O is preferably limited in order to limit the fly-off of the raw materials, in particular of the boron oxide. In a product according to the invention, it is considered that the oxides Na.sub.2O and K.sub.2O have similar effects.

[0088] In one embodiment, at least one of the contents of Na.sub.2O and of K.sub.2O is greater than 0.30%, preferably greater than 0.35%, preferably greater than 0.40%.

[0089] According to one particular embodiment: [0090] SiO.sub.2: 8.5% to 10.8% [0091] B.sub.2O.sub.3: 0.30% to 0.70% [0092] Al.sub.2O.sub.3: 1.0% to 1.8%
at least one of the contents of Na.sub.2O and of K.sub.2O being greater than 0.30%, preferably greater than 0.35%, preferably greater than 0.40%.

[0093] The weight content of yttrium oxide Y.sub.2O.sub.3 should be limited to preserve a good feasibility.

[0094] According to the invention, the weight content of Fe.sub.2O.sub.3+TiO.sub.2 is less than 0.50%, preferably less than 0.30%. Preferably, the weight content of P.sub.2O.sub.5 is less than 0.05%. Specifically, these oxides are harmful and their content should be limited to traces introduced as impurities with the raw materials.

[0095] The “other species” are the oxide species which are not listed above, namely the species other than ZrO.sub.2, Hf.sub.2O, SiO.sub.2, Y.sub.2O.sub.3, B.sub.2O.sub.3, Al.sub.2O.sub.3, Na.sub.2O, K.sub.2O, TiO.sub.2 and Fe.sub.2O.sub.3. In one embodiment, the “other species” are limited to species whose presence is not particularly desired and which are generally present as impurities in the raw materials.

[0096] Preferably, the product according to the invention is in the form of a block.

[0097] The total privacy of the product according to the invention is less than 15%, or indeed less than 10%, or indeed less than 5%, or indeed less than 2%, or indeed less than 1%.

[0098] A product according to the invention may be conventionally manufactured according to the steps a. to c. described below: [0099] a. mixing raw materials so as to form a feedstock, [0100] b. melting said feedstock until a molten material is obtained, [0101] c. solidifying said molten material, by cooling, so as to obtain a refractory product according to the invention.

[0102] In step a., the raw materials are chosen so as to guarantee the contents of oxides in the finished product.

[0103] In step b., the melting is preferably carried out by means of the combined action of a relatively long electric arc, which does not produce reduction, and of mixing that promotes the reoxidation of the products.

[0104] To minimize the formation of nodules of metallic appearance and to prevent the formation of slits or cracks in the final product, it is preferable to perform the melting under oxidizing conditions.

[0105] Preferentially, the long arc melting process described in French patent no. 1 208 577 and its additions nos. 75893 and 82310 is used.

[0106] This process consists in using an electric arc furnace in which the arc surges from between the feedstock and at least one electrode separate from this feedstock and in adjusting the length of the arc so that its reducing action is minimized, while at the same time maintaining an oxidizing atmosphere above the molten bath and by mixing said bath, either by the action of the arc itself, or by sparging into the bath an oxidizing gas (for example air or oxygen) or alternatively by adding to the bath substances that give off oxygen such as peroxides or nitrates.

[0107] In step c., the cooling is preferably performed at a rate of less than 20° C. per hour, preferably at a rate of about 10° C. per hour.

[0108] Any conventional process for manufacturing zirconia-based molten products intended for applications in glass melting furnaces may be used, provided that the composition of the feedstock makes it possible to obtain products having a composition in accordance with that of a product according to the invention.

[0109] In a product according to the invention, ZrO.sub.2 is substantially entirely (typically for more than 95% of its weight) in the form of zirconia and SiO.sub.2 and Al.sub.2O.sub.3 are substantially entirely (typically for more than 95% of their weights) in the glassy phase.

Examples

[0110] The following nonlimiting examples are given for the purpose of illustrating the invention.

[0111] In these examples, the following raw materials were used: [0112] zirconia Q1 containing on average 99% of ZrO.sub.2+HfO.sub.2, [0113] zircon sand containing on average 33% of SiO.sub.2 and 66% of ZrO.sub.2+HfO.sub.2, [0114] “Sable BE01 Bedouin” (BE01 Bedouin sand) silica containing on average 99% of SiO.sub.2, [0115] boron oxide containing on average 98% of B.sub.2O.sub.3, [0116] sodium carbonate containing on average 99.5% of Na.sub.2CO.sub.3 as source of Na.sub.2O, [0117] alumina of AC34 type containing on average 99% of Al.sub.2O.sub.3, [0118] yttrium oxide containing on average 99% of Y.sub.2O.sub.3.

[0119] The products were prepared according to the conventional arc furnace melting process, then cast in order to obtain blocks measuring 996 mm×203 mm×800 mm.

[0120] The chemical analysis of the products obtained is given in table 1; it is an average chemical analysis, given in weight percentages.

Resistance to the Thermomechanical Stresses

[0121] In order to study the ability of the products to withstand the thermomechanical stresses undergone by the blocks of superstructures, the inventors used Kingery's theory that connects the MOR/MOE ratio to the thermal shock resistance and Hasselman's theory which connects the energy of rupture to the thermal shock resistance. Furthermore, in linear elastic mechanics, the severity of the thermomechanical stresses is linked to the ratio between the modulus of rupture (MOR) and the modulus of elasticity (MOE). This ratio should be maximized, just like the energy of rupture, so that the products are resistant to cracking of thermomechanical origin. They are placed at 1000° C., which corresponds substantially to the temperature in the core of the block.

MOR Measurement

[0122] The modulus of rupture (MOR) is the maximum stress measured at 1000° C. in air for a sample with dimensions of 150×25×15 mm.sup.3 placed in a 3-point bending assembly set up with a distance of 120 mm between the two lower supports, the descent rate of the punch providing the upper support, halfway along the length of the sample, being equal to 0.5 mm/min. The value of the MOR is an average resulting from three successive measurements.

MOE Measurement

[0123] To measure the MOE, use is made of the same assembly as the one described for the MOR and of a displacement sensor to monitor the displacement of the deflection of the sample and to determine the MOE, that is to say the ratio between the stress and the elastic strain caused by this stress.

Energy of Rupture Measurement

[0124] The energy of rupture is measured at 1000° C., in air, on a sample with dimensions of 150×25×25 mm.sup.3, and having in the middle thereof a triangular notch having an angle of 60° and a base of 25 mm, placed in a 4-point bending assembly with a distance of 120 mm between two lower supports and a distance of 40 mm between the two upper supports. The descent rate of the upper supports is equal to 20 μm/min.

Corrosion Resistance Measurement

[0125] The corrosion resistance (CR) is measured by spraying a powder consisting of 50% of glass cullet, 15% of silica, 5% of dolomite and 30% of sodium carbonate, at a rate of 180 grams per hour for a total of 20 kilograms over 4 samples with dimensions of 110×100×30 mm.sup.3 rotated (6 rpm) in a furnace at 1450° C. The corroded volume is measured by a 3D scan and this volume is related to the initial volume.

Exudation Measurement

[0126] The resistance to exudation (REx) is measured, in air, on a sample with dimensions of 100×100×20 mm.sup.3. The sample undergoes cycles during which it is brought to 1550° C. at a rate of 100° C. per hour then held at 1550° C. for 6 hours. REx is the percentage of the volume of the silicate phase which has escaped from the sample after two cycles (which is found either on the sample (increase in the volume of the sample), or in the bottom of the crucible), relative to the initial volume of the sample. It is placed at 1550° C., which corresponds substantially to the temperature on the face of the block exposed to the interior of the tank.

[0127] Examples 1 and 2 correspond respectively to a conventional AZS product and to a conventional product having a high zirconia content.

[0128] The balance corresponds to the content of ZrO.sub.2+HfO.sub.2 and also to the impurities (the content of which is always less than 0.5% in these examples).

TABLE-US-00001 TABLE 1 1* 2* 3 4 SiO.sub.2 15.0 4.3 10.1 8.5 Al.sub.2O.sub.3 50.9 1.2 1.7 1.6 Na.sub.2O 1.30 0.22 0.43 0.43 B.sub.2O.sub.3 <0.2 <0.2 0.47 0.26 Y.sub.2O.sub.3 <0.2 <0.2 0.51 1.19 MOR/MOE 1.80 1.40 3.16 2.13 Energy of rupture 0.4 kJ/m.sup.2 0.5 kJ/m.sup.2 4.0 kJ/m.sup.2 1.2 kJ/m.sup.2 CR 4.30% ND <0.5% ND REx 4.78% 0.77% 0.71% 0.16% *outside the invention

[0129] The tests show that, relative to the comparative product 2, the products 3 and 4 according to the invention have an improved resistance to thermomechanical stresses and a higher energy of rupture.

[0130] The degree of exudation of the products according to the invention is excellent.

[0131] As is clearly apparent, the invention therefore provides a product which has a remarkable mechanical performance in the environment of a glass melting furnace superstructure, and also a low exudation in operation.

[0132] Of course, the invention is not limited to the embodiments described and represented, which are provided solely by way of illustration.