WO2007065874A1

WO2007065874A1 - Method for producing flat glass by the float method and spout lip for the float method

Info

Publication number: WO2007065874A1
Application number: PCT/EP2006/069258
Authority: WO
Inventors: Jörg WITTE; Janusz Zborowski; Joachim Disam; Katharina LÜBBERS; Andreas Roters; Günter Fiederling
Original assignee: Schott Ag
Priority date: 2005-12-05
Filing date: 2006-12-04
Publication date: 2007-06-14
Also published as: DE112006003321A5; DE112006003321B4

Abstract

A description is given of a float method for producing special glass, for example borosilicate glass, alkali-free glass, aluminosilicate glass or precursor glass for glass ceramic, in which method the glass is poured onto the float bath over a spout lip of pressed, in particular isostatically pressed, and sintered refractory material. Suitable as materials are ZrO2, Al2O3 or ZrSiO4, wherein the content of Fe2O3 in the ZrO2 and ZrSiO4 refractory material is less than 0.08% by weight and in the Al2O3 refractory material is at most 0.2% by weight. The pressed and sintered refractory material is distinguished in particular by a uniform grain size distribution and a homogeneous microstructure.

Description

Process for the production of flat glass using the float process and pouring lip (spout lip) for the float process

The invention relates to a method for producing

Flat glass using the float process and a spout lip for the float process.

The manufacture of flat glass using the float process has been well known since the last century. Liquid glass, which is brought out of the work tub by means of a gutter, is allowed to flow onto a bath of molten metal, generally tin. The flow rate of the glass is regulated by a movable slide (tweel), with which, among other things, the glass thickness is also adjusted. Seen behind the glass in the direction of flow

The spout lip is located on the slide, from which the glass melt flows continuously onto the metal melt, where it is formed into a dimensionally stable glass band and solidifies, the solidified glass band then being removed from the metal bath.

The wear of the pouring lip and associated flat

Washings in their surface lead to defects in the stripped glass ribbon. This can include fixed glass defects (crystalline inclusions), different thicknesses or waviness in the glass ribbon.

Blistering can also occur at the FF material / glass melt interface. This is not permissible, particularly in the case of very high-quality thin glasses, and greatly reduces the yield. If too many glass defects are produced by a worn lip stone, which have a very negative impact on economic production, the lip stone must be replaced, which is usually associated with considerable effort. Quartz glass or melt-cast Al ₂ O _{3 is described} as suitable as lip material for soda-lime glass (DE 27 47 549 C3). Melt-cast lips with a high proportion of Al ₂ O ₃ and ZrO ₂ (trade name Zac) are also known from DE 21 22 702 A1. Although these pouring lips are very useful for soda-lime glass and have a service life of up to 3 years, they are

Requirements made by aggressive and higher melting glasses have grown inadequately and are subject to high wear. The consequences of this wear are ripples in the glass and an increased number of holes and strips on the underside of the glass.

JP 33 85 539 describes a spout lip which consists of HZFC material (HZFC: High Zirconia Fused Cast) with a ZrO ₂ content of 85-97% and the rest of a glassy material. These materials reduce the number of stripes and holes on the underside of the glass ribbon and the service life of the Spout Lip in the manufacture of high-melting glasses can be increased to around a year. A disadvantage of the HZFC material is its low resistance to temperature changes, so that when the temperature changes, flaking in the surface can easily occur, in extreme cases even cracks, which lead to quality defects in the glass. Another disadvantage is the high price for HZFC.

A ZrSiO ₄ refractory material for the overflow fusion process is known from US 2005/0130830 A1, to which Fe ₂ O ₃ (in addition to the impurities already present) has been added in order to improve the deflection behavior. However, this material shows a significantly increased interaction with the glass melt at high glass temperatures from approximately 1250 ^° C. DE 42 43 538 describes a zircon material which has good corrosion resistance to boric acid-containing glass melts at high temperatures. This material is made by adding phosphorus compounds and TiÜ ₂ as sintering aids.

The disadvantage of this material is its low level

Resistance to temperature changes.

DE 102 09738 A1 describes a spout lip which consists of a SiO ₂ ceramic which is provided on its side which comes into contact with the glass flow with a metal layer made of molybdenum or tungsten. A ceramic layer is also provided between the SiO ₂ ceramic and the metal layer, which is intended to prevent the back attack by diffusing oxygen on the metal. The

Handling this spout lip with its metal layer sensitive to air oxidation is difficult, especially when the spout lip is hot, e.g. in the event of a repair, must be replaced.

DE 103 08 031 A1 describes a pouring lip

Magnesium aluminate spinel described, which is coated on its top with a noble refractory metal, generally platinum or a platinum alloy. A gas-tight diffusion barrier is also arranged between the metal and the spinel. The

Metal coating must be made very precisely so that when

Heating the pouring lip no folds or distortions occur in the precious metal, which lead to stripes on the underside of the glass ribbon and thus to the loss of production. Below the lip stone there is a nozzle for purging this area with nitrogen in order to avoid that the forming gas from the float bath reduces components or impurities in the refractory material which react with the precious metal to form low-melting alloys or lead to embrittlement of the precious metal. In both cases, this means damage to the spout lip, which leads to glass defects. If the If nitrogen purging fails, protection against the reducing float bath atmosphere is no longer provided.

Because of the problems mentioned and because of the high costs, lipstones without a metal coating are still preferred.

However, it has been shown that melt-cast lipstones often cause problems when they come from one

melt cast block are worked out or if they are to be ground to exact dimensions for high precision of the glass ribbon to be produced.

It is known per se that melt-cast ceramic materials due to the zonal solidification during the manufacturing process

Show inhomogeneities in the structure. E.g. point

melt-cast AZS materials (Al ₂ O ₃ -ZrO ₂ -SiO ₂ ) with a proportion of 41% by weight ZrO ₂ in the area of the mold bottom have a significantly higher proportion of zirconium oxide than in the head of the stone. The

Difference can be more than 10% by weight. To the

As a result of the rapid cooling, mold walls usually form a very fine-grained structure. The solidification is delayed in the interior of the solidifying block, so that a coarse structure is usually formed there, which is partially interspersed with pores and in which the glass phase enriched with the impurities originating from raw materials is present in larger quantities, see Fig. 1 and Fig. 2

(light microscopic images of grinding). Figure 1 shows the fine structure of an HZFC material that forms near the mold wall in a melt-cast block, while Figure 2 shows the much coarser structure inside the same block. One can clearly see the inhomogeneities in the structure and the enrichment of the (black appearing) glass phase ("worm tracing"). Polyvalent impurities such as TiO ₂ and Fe ₂ O ₃ in particular can lead to blistering in glass contact. If the lip stone is carved out of a melt-cast block, it is

unavoidable that areas are exposed that differ in the structure and composition of the glass phase. In places with a locally increased glass phase content, there is increased corrosive removal. Areas in the refractory material that have pores or pore accumulations are places of increased corrosion, which can also contribute to the formation of glass defects. These disadvantages are more common with higher melting glasses.

A reduction in the proportion of the glassy phase is only possible up to a certain limit, since the glassy phase is necessary for the casting and a refinement of the coarse structure through faster cooling new problems, e.g. Tension in the casting, entails. It is therefore necessary to pour the desired shape of the spout lip as precisely as possible in order to achieve a

To keep reworking properties as low as possible.

The object of the invention is to find a process for the production of flat glass by the float process, with which very high quality qualities can be achieved even when using higher melting glasses, and a spout lip

to provide, which can be produced with a highly precise surface shape.

This object is achieved by the method according to claim 1 and the pouring lip according to claim 8.

It was found that a casting lip (lip stone, spout lip) made of pressed and sintered refractory material from the group ZrO ₂ , AI ₂ O _3, ZrSiO ₄ , with an Fe ₂ O ₃ content of less than

0.08% by weight if the refractory material is made of ZrO ₂ or ZrSiO ₄ and less than 0.2% by weight if the refractory material is made of Al ₂ O ₃ , because these materials have a very uniform structure with regard to grain size, glass phase and pore distribution. In the case of ZrO ₂ and ZrSiO ₄ materials, an Fe ₂ O ₃ content of at most 0.05% by weight is preferred, and in the case of Al ₂ O ₃ refractory materials of at most 0.1, in particular at most 0.08% by weight. %.

Show it:

Fig. 1: Microstructure / structure of HZFC material, fine crystalline

Area;

Fig. 2: Microstructure / structure of HZFC material in the area of so-called "worm tracing": inhomogeneities in the structure enrichment of the glass phase;

Image 3: Microstructure / structure of pressed and sintered

Aluminum oxide, very fine-grained and homogeneous structure;

Image 4: Microstructure / structure of pressed and sintered

Zirconium oxide, very homogeneous structure;

Figure 5: Microstructure / structure of melt-cast aluminum alumina, relatively coarse crystalline structure;

Figure 6: Bubble formation behavior of pressed and sintered

Material compared to melt-cast aluminum alumina (T = 1380 ° C / alkali-free glass); Figure 7: Bubble formation rate on zirconium silicate material with different Fβ ₂ θ ₃ content in contact with a glass melt at 1400 ⁰ C.

Figure 3 shows a polished section of a pressed and sintered α-aluminum oxide and Figure 4 shows a polished section of a pressed and sintered zirconium oxide. One can clearly see that the structure is much finer and more homogeneous than the melt-cast bodies.

The production of refractory ceramic products by pressing and sintering masses of appropriate composition is known per se and is also carried out in part on an industrial scale. The casting lip according to the invention can also be produced by these well-known processes.

The pressing can be uniaxial, but isostatic pressing is preferred, with which better results are achieved with complicated shapes. The hot isostatic pressing (HIP process) is also possible with good success, but is generally not popular due to the size of the casting lips to be pressed for economic reasons.

The material to be pressed should have such a grain size distribution that a very even structure is created during pressing. The smaller the grain size of the material to be pressed, the more homogeneous the structure of the body made from it and the better its mechanical properties. In practice, however, larger grains are often used, especially in the case of very large bodies, in a mixture with finer material in order to achieve the

Reduce sintering shrinkage. The material to be pressed can, if necessary. Known sintering aids such as SiO ₂ , TiO ₂ and stabilizers such as oxides from the group of lanthanides, in particular Y ₂ O ₃ or also CaO or MgO, can be added.

Stabilization is customary in particular in the case of a ZrO ₂ ceramic and up to 10% by weight of Y ₂ O _{3 are} added for full stabilization. However, partially stabilized compositions with a correspondingly lower proportion of stabilizing oxides are also common.

After sintering, the material has an open porosity of up to 12%. An open porosity of up to 7%, in particular up to 2%, is preferred. Open porosities of around 0.5% can be achieved.

Materials with higher porosity have a better stability against temperature changes, materials with a lower porosity have a lower blistering potential and a higher corrosion resistance. Blistering can e.g. caused by air released from the pores. Since at the

chemical corrosion (dissolution in the glass mass) of the spout lip always cut new pores, a spout lip with small pores / lower pore content has a clear advantage. The person skilled in the art must therefore, depending on the operating conditions of his float glass process or his spout lip, choose the most favorable compromise for his process between the properties such as pore fraction / pore size and the resistance to temperature changes. In general, it is preferred that zirconium silicate materials for use as spout lip have a maximum density of 4.15 g ^· cm ^"3 with the corresponding porosity in order to ensure good resistance to temperature changes

guarantee. To avoid possible repercussions on the glass, the materials from which the spout lip is made should be as pure as possible. Impurities cannot always be completely excluded. TiO ₂ or P ₂ O ₅ can generally be tolerated in each case up to about 1% by weight. The P ₂ O ₅ content should preferably not exceed 0.4% by weight and the TiO ₂ content should not exceed 0.8% by weight, in particular 0.5% by weight. However, the total amount of the above-mentioned impurities should not exceed 1% by weight. Also prove to be isostatic

Pressed, sintered materials containing zirconium oxide or zirconium silicate with a proportion of ZrO ₂ of more than 60% and aluminum oxide with a proportion of Al ₂ O ₃ of at least 90% in glass contact are suitable.

The natural occurrence of zirconium as zirconium oxide or as zirconium silicate always contains a small proportion (usually 1.5-2.5% by weight) of hafnium oxide or silicate. When specifying the zirconium oxide or zirconium silicate content, the zirconium oxide content including the hafnium oxide content is therefore always understood.

A pressed and sintered aluminum oxide with a proportion of up to 20% by weight, in particular up to 10% by weight of zirconium oxide, can also be used.

If, due to local conditions due to stray electrical currents, increased electrochemical bubble formation on the spout lip can be expected, materials with low electrical conductivity, e.g. Zirconium silicate or σ-alumina preferred as material for the spout lip.

With regard to the bubble formation behavior, pressed and sintered ceramics, for example made of zirconium oxide or of aluminum oxide, can be used have lower blistering potential compared to melt-cast α / β-alumina, which is very often used as a spout lip material. Figure 5 shows a section through a fused-cast σ // - alumina is seen clearly the comparatively relatively coarse crystalline structure in which θ next aR-Al ₂ ₃ crystals also / -.? Al ₂ O ₃ crystals and small areas of Glass phase are recognizable. This material has a higher compared to pressed and sintered material

Bubble formation potential. The difference in terms of

Blistering behavior between melt-cast alumina material and pressed and sintered σ alumina material in contact with alkali-free glass at a temperature of 1380 ⁰ C is shown in Figure 6. Blister formation was determined by the method of Dunkl et al .: A Novel Method for the Determination of the Blistering Rate at the Refractory / Glass Interface; UNITECR '89: Proc. Unified Int Techn. Conf. on Refractories, Annaheim CA, 1-4 Nov. 1989, Westerville, 1989, 795-806.

Figure 7 shows the positive influence of a low Fβ ₂ θ ₃ content on the blistering rate of zirconium silicate materials in contact with a glass melt at a temperature of 1400 ⁰ C.

A particular advantage of the spout lip found is that it can be processed by grinding and / or polishing without impairing its good properties. It is therefore possible to make a spout lip from the full, i.e. work out a pressed and sintered block of material and it is also possible to bring a pressed and sintered spout lip to the desired size by milling, grinding and / or polishing or to produce a particularly smooth and flat surface. A spout lip with a cut to size (milled) and / or polished

Surface is created because of the particularly good quality of the Glass ribbon particularly preferred. Treatment of the surface by grinding and / or polishing is generally only required for the surface that comes into contact with the liquid glass.

The spout lip found is particularly suitable for the production of glass, which has particularly high requirements with regard to

Surface quality are set and / or the higher

Melting point has as soda-lime glass.

Such glasses are in particular borosilicate glasses, alkali-free glasses, aluminosilicate glasses, aluminolithium silicate glasses and precursor glasses for glass ceramics.

The Spout Lip is particularly suitable for the production of

Borosilicate glass with a composition of (in% by weight on an oxide basis): 55-65 SiO ₂ , 12-20 Al ₂ O ₃ , maximum 5 B ₂ O ₃ , 0-5 BaO, 3-9 CaO, 1-5 MgO and 1-5 SrO; for the production of display glass with a composition of 55-70 SiO ₂ , 12-20 Al ₂ O ₃ , 5-15 B ₂ O ₃ , 0-6 BaO, 0-12 CaO, 0-7 MgO, 0-10 SrO. It is also particularly suitable for producing various green glasses for glass ceramics, for example with 55-69 SiO ₂ , 19-25 Al ₂ O ₃ , 3-5 Li ₂ O, 0-1, 5 Na ₂ O, 0-1, 5 K ₂ O, ∑ Na ₂ O + K ₂ O, 0.2-2, MgO 0.1-2.2, CaO 0-15, SrO 0-1, 5, BaO 0-2.5, ∑ MgO + CaO + SrO + BaO below 6, ZnO 0-1, 5, TiO ₂ 1-5, ZrO ₂ 1-2.5, SnO ₂ O- below 1, ∑ TiO ₂ + SrO ₂ + SnO ₂ 2.5- 5, P ₂ O ₅ 0-3 or one

Glass ceramic precursor glass with a composition of SiO ₂ 55-75, Al ₂ O ₃ 15-30, Li ₂ O 2.5-6, ∑ Na ₂ O + K ₂ O less than e, ∑ MgO + CaO + SrO + BaO less than 6, B ₂ O ₃ O to less than 4, ∑ TiO ₂ + ZrO ₂ less than 2 or a glass ceramic precursor glass with a composition of SiO ₂ 60-72, Al ₂ O ₃ 18-28, Li ₂ O 3-6, ∑ Na ₂ O + K ₂ O 0.2-2, ∑ MgO + CaO + SrO + BaO less than 6, ZnO 0-1, 5, B ₂ O ₃ O-less 4, SnO 0.1-1, 5, ∑ TiO ₂ + ZrO ₂ less than 2, P ₂ O ₅ 0-3, F 0-2. Through the use of materials with small pore sizes and open porosities of less than 1%, which can be realized with the invention, the material also asserts itself

Grinding / polishing or pores exposed by corrosion in the

Glass contact does not pose any risk of blistering in the glass. Despite their low porosity, the isostatically pressed high zirconium oxide-containing materials have better temperature change behavior in relation to the change in bulk density and volume than conventional melt-cast HZFC material. A particular advantage over the materials previously used for Spout Lips is that sintered material made from very fine-grained raw materials can be used, which can be ground and / or polished to size while maintaining the excellent performance properties of the Spout Lip thus produced with regard to blistering and waviness .

Claims

1. Process for the production of flat glass according to the

Float process, in which molten glass is poured onto a bath of molten metal via a spout lip, the glass on the bath is formed into a continuous dimensionally stable glass band and the solidified glass band is continuously removed from the metal bath, characterized in that the Glass flows over a pouring lip made of pressed and sintered refractory material from the group ZrO ₂ , AI2O ₃ , ZrSiO ₄ , the content of Fe ₂ θ ₃ in the ZrO ₂ and ZrSiO ₄ refractory material being less than 0.08% by weight % and in the Al ₂ θ ₃ refractory material is at most 0.2% by weight.

2. The method according to claim 1, characterized in that the glass is allowed to flow over a ZrO ₂ or ZrSiO ₄ refractory material which has a maximum Fe ₂ O ₃ content

0.05 wt .-% has.

3. Process according to claims 1, characterized in that the glass is allowed to flow over an Al ₂ O ₃ refractory material which has an Fe ₂ O ₃ content of at most 0.1% by weight, in particular at most 0.08% .-%.

4. The method according to any one of claims 1 to 3, characterized

characterized in that the glass is allowed to flow over an isostatically pressed and sintered pouring lip.

5. The method according to at least one of claims 1 to 4, characterized in that the glass over a Pour lip with an open porosity of less than 12%.

6. The method according to claim 5, characterized in that the glass is allowed to flow over a pouring lip with an open porosity of less than 7%, in particular less than 2%.

7. The method according to at least one of claims 1 to 7,

characterized in that the glass is allowed to flow over a ground and / or polished pouring lip.

8. Spout lip for the float glass process, thereby

characterized in that the pouring lip consists of pressed and sintered refractory material from the group ZrO ₂ , Al ₂ O ₃ , ZrSiO ₄ , the content of Fe ₂ O ₃ in the ZrO ₂ and ZrSiO ₄ refractory material being less than 0.08% by weight .-% and in the Al ₂ O ₃ refractory material is a maximum of 0.2 wt .-%.

9. pouring lip according to claim 8, characterized in that the Fe ₂ O ₃ content in the ZrO ₂ - and the ZrSiO4 refractory material is at most 0.05 wt .-%.

10. pouring lip according to claim 8, characterized in that the Fe ₂ O ₃ content in the AI ₂ O ₃ refractory material maximum

0.1% by weight, in particular a maximum of 0.08% by weight.

11. spout lip according to one of claims 8 to 10,

characterized in that the pouring lip is made of isostatically pressed and sintered refractory material.

12. spout lip according to at least one of claims 8 to 11, characterized in that it has an open porosity of less than 12%.

13. pouring lip (spout lip) according to claim 12, characterized

characterized in that it has an open porosity of less than 7%, in particular less than 2%.

14. pouring lip according to at least one of claims 8 to 13,

characterized in that its side coming into contact with the glass has a surface which has been ground and / or polished to size after sintering.