CN1446764A - Appts. for conveying glass melt via overflow brick (outlet end) when producing float glass - Google Patents

Abstract

The invention relates to a device for conveying molten glass through an overflow brick (liquid outlet) in the production of float glass, the device conveys the melted glass from a glass melt tank to the overflow brick through an adjustable feeding channel, and It is guided via its surface covered with a metal layer onto the tin surface of the float bath. According to the invention, at least the surface of the overflow brick in contact with the glass melt is covered with a refractory metal layer of precious metal, not only the service life of the overflow brick is greatly improved, but also the surface quality of the float glass is improved due to the improved surface properties. optimization. Also, the materials of the metal layer and the overflow brick have well-matched coefficients of thermal expansion, so as to avoid formation of wrinkles and cracks in the metal layer.

Description

Device for conveying molten glass through overflow bricks (discharge end) in the production of float glass

The invention relates to a device for conveying glass solution through an overflow brick (liquid outlet) in the production of float glass. It is guided onto the tin surface of the float bath using a surface covered with a metal layer.

In the production of float glass according to the float process, the glass is generally melted in a glass melting tank, "refined" without bubbles in a refining unit, homogenized and pretreated in the feed channel between the glass melting tank and the float casting tank (Adjust to the viscosity required by molding). The pretreated glass melt floats on the liquid tin in the floating tank through the overflow brick (liquid outlet) at the end of the feeding channel. The glass melt forms glass ribbons in the float bath.

In this respect, the distance from the bottom edge of the overflow brick to the tin surface, as well as the surface properties of the overflow brick, are crucial for the forming process in the float tank and thus for the quality of the float glass.

If the glass melt comes into contact with overflow brick surfaces with cracks, holes or corroded sections, surface defects (crossovers) in the float glass can result.

If the distance from the bottom edge of the overflow brick to the tin surface in the float tank is too large due to the increase of corrosion of the overflow brick, it will change the delivery conditions of the glass melt to the float tank, such as the ratio of backflow: positive flow, Then wet back bubbles appear.

In known arrangements, the overflow bricks are made of refractory ceramic material. Slurry-cast and sintered silica glass ceramics (fused quartz) or melt-cast Al ₂ O ₃ are generally used as overflow brick material. This material is fully sufficient in the production of calcium-sodium oxide glass, and the service life of the overflow brick is 2 to 3 years.

In the production of corrosive glasses with high melting points, like borosilicate glass (less alkali and no alkali) and alumino-silicate glass, the service life of overflow bricks made of this material is only 6 months at most. Therefore, experiments have been made on overflow bricks to increase their service life by using other materials. It has therefore been proposed to use molten casting materials containing 85% to 97% by weight of ZrO ₂ and to use such overflow bricks in the production of borosilicate glass, in particular low-alkali borosilicate glass. The service life of the overflow brick can thus be increased to about 1 year (JP 06-345467 A).

A disadvantage of the previously known overflow brick materials is the known consequences (deformation, etc.) on the quality of the glass due to the limited service life due to the corrosion of the glass melt. Furthermore, any corrosion of ceramic materials is associated with bubble formation and dissolution of corrosion products, which likewise deteriorate glass quality.

The materials listed in JP 06-345467 A for overflow bricks have relatively good corrosion resistance, but are sensitive to temperature changes. When the temperature changes, the surface is prone to peeling off, and in extreme cases even cracks with the well-known quality defects of float glass. Furthermore, this material is very expensive.

When forming overflow bricks with a metal layer, particular consideration must be given to protecting the metal layer from attack by forming gases from the reduction of the float bath. In addition, when selecting the material for the overflow brick, it must be ensured that the thermal expansion coefficients of the overflow brick and the metal layer material are similar.

If common materials are used for the overflow brick, such as fused quartz, Al ₂ O ₂ or zirconium, then the overflow brick has a significantly lower thermal expansion coefficient than the same metal layer. When used at a temperature of 1200°C to 1400°C, it causes wrinkles and cracks to form in the metal layer, and in extreme cases causes overflow bricks to fall off.

The object of the present invention is to provide a device of the above-mentioned type, wherein the overflow brick has a high temperature resistance up to 1400 ℃, has a high corrosion resistance to different glass melts, and does not produce undesirable contact with the glass melt. The resulting interaction is insensitive to temperature changes and does not release corrosion products into the glass melt.

This object is achieved according to the invention in that at least the surface of the overflow brick which comes into contact with the glass melt is covered with a special non-noble metal refractory metal layer.

The surface of the overflow brick in contact with the glass melt with a metal layer made of non-noble refractory metal has excellent corrosion resistance, thereby increasing the service life of the overflow brick. In addition, corrosion products are prevented from entering the glass melt, and the coefficient of thermal expansion is matched more favorably to the material chosen for the overflow brick. In this way, platinum or a platinum alloy can be chosen for the metal layer and magnesium aluminate-spinel for the overflow brick, and a similar thermal expansion state can be achieved. Therefore, formation of wrinkles and cracks in the metal layer can be easily avoided. The coefficient of thermal expansion is in the range of 10 to 14×10 ⁻⁶ K ⁻¹ .

As metal layers, alloys containing platinum and 10% by weight rhodium and alloys containing platinum and 5% by weight gold have proven to be very suitable.

To protect the platinum from the reducing atmosphere from the float bath, gaseous nitrogen at a temperature of 800 to 1200° C. is supplied through nozzles under the overflow brick.

The device is particularly suitable for use in float plants for the production of substrate glass for flat panel displays according to the float process.

It is precisely platinum and its alloys that have outstanding corrosion resistance to borosilicate and aluminosilicate glasses.

In the case of borosilicate glass, the composition contains

55%-65% _SiO2 by weight

12%-20% by _{weight Al2O3}

≥5% by weight _{of B2O3}

0-5% by weight of BaO

3-9 wt% CaO

1-5% by weight of MgO

1-5% by weight of SrO is particularly suitable, and the overflow brick formed according to the present invention has a high service life without wrinkles and cracks on the surface, which is of great significance for improving the surface quality of the float glass ribbon.

The invention is explained in more detail below with the aid of an exemplary embodiment shown in the drawing. in:

Figure 1 shows a float plant for the production of float glass in the transition zone from a glass melt tank to a float tank with overflow bricks, and

Figure 2 shows a detail of the transition zone with overflow bricks in a float plant with only one barrier plate.

As shown in the schematic diagram of FIG. 1 , the glass is melted in the glass melt tank 10 and delivered to the feed channel 11 , wherein the amount of refined glass melt 12 can pass through the overflow brick 20 at the end of the feed channel 11 The configured glass volume adjustment device 14 (Front Tweel) is adjusted.

The feed channel 11 is equipped with a cut-off device 13 (Back Tweel), by means of which the delivery of the glass melt 12 to the overflow brick 20 can be completely interrupted.

As shown more clearly in FIG. 2, the amount of glass melt 12.1 conveyed above the overflow brick 20 is metered by the glass quantity regulating device 14 (Front Tweel). The top of the overflow brick 20 is covered with a metal layer 22 . This layer is made of platinum or a platinum alloy. It is fastened by means of platinum or platinum alloy pins to the side bricks that limit the overflow brick 20 laterally. Here, the fastening point is arranged such that it does not come into contact with the glass melt 12 . 1 flowing on the covered overflow brick 20 . At this point it should be emphasized that, within the scope of the invention, the metal layer 22 can either be formed as a separate component, as is known, or it can be formed integrally with the overflow brick 20 . It is believed that the metal layer 22 is evaporated or otherwise coated on the overflow brick 20 .

The glass melt 12.1 reaches the float bath, which has a front wall 15 and a bath bottom part 17 and holds liquid tin 18 . The flowing glass melt 12.1 forms a glass ribbon 12.1 on the liquid tin 18, which is pulled out of the float bath by lifting rollers.

The distance from the bottom edge of the overflow brick 20 to the surface of the liquid tin 18 in the floating casting tank must be reasonable. Here, the overflow brick 20 can be arranged on the front wall 15 of the floating casting tank. A nozzle 21 is arranged under the overflow brick 20, and the transition between the overflow brick 20 and the floating casting tank can be carried out by gaseous nitrogen at a temperature of 800 to 1200°C. Parts of the platinum coating that are not covered by the glass melt thus prevent a reducing bath atmosphere.

The transition zone between the overflow brick 20 and the float trough is covered by means of the front cover 19 towards the material channel 11 downwards. Liquid tin 18 is formed in the float tank with front wall 15 and tank bottom part 17 as a liquid carrier for the floating glass ribbon 12.2, which is separated from the front wall 15 of the float tank by tiles 16.

The overflow brick 20 is composed of magnesium aluminate spinel, which has a similar coefficient of thermal expansion (10-14) x ^10-6 to the metal layer composed of platinum and 10 wt. K ^-1 . This suitable coefficient of thermal expansion ensures that at temperatures of 1200° C. to 1400° C., no wrinkles or cracks form in the metal layer and produces an excellent surface with increased surface quality for the floating glass ribbon 12 . On the bottom surface of the overflow brick 20, gaseous nitrogen at a temperature of 800 to 1200° C. is supplied through the nozzle 21 in order to protect the platinum from the reducing atmosphere from the float bath.

The underside of the metal layer may also have a gas-tight diffusion barrier in order to protect the metal layer from tin vapor and reduced forming gases coming out of the float bath.

The device according to the invention is particularly suitable for the production of glass substrates for flat panel displays by the float process. The borosilicate glass used for this preferably has the following composition:

55-65 wt% _SiO2

12-20 wt% Al ₂ O ₃

≥5% _by weight _B2O3

0-5% by weight of BaO

3-9 wt% CaO

1-5% by weight of MgO

1-5% by weight of SrO

Claims (9)

1. A device for transporting molten glass through an overflow brick (liquid outlet) when producing float glass, characterized in that at least the surface of the overflow brick (20) that is in contact with the molten glass (12.1) Covered with a specific refractory metal layer (22). 2. The device as claimed in claim 1, characterized in that the underside of the metal layer (22) which is not in contact with the glass melt (12.1) has a gas-tight diffusion barrier. 3. The device as claimed in claim 1 or 2, characterized in that the metal layer (22) consists of platinum or a platinum alloy. 4. The device as claimed in one of claims 1 to 3, characterized in that the overflow brick (20) consists of a material with a coefficient of thermal expansion (WAK) of 10 to 14×10 ⁻⁶ K ⁻¹ . 5. The device according to claim 4, characterized in that the overflow brick (20) is made of magnesium aluminate spinel, which has excellent thermal expansion characteristics, and the metal layer (22) made of platinum and its alloys Compatible with thermal expansion characteristics. 6. The device as claimed in claim 3, characterized in that the metal layer (22) consists of an alloy containing platinum and 10% by weight rhodium or platinum and 5% by weight gold. 7. The device as claimed in one of claims 1 to 6, characterized in that nitrogen at a temperature of preferably 800 to 1200° C. can be introduced via nozzles (21) underneath the overflow brick (20). 8. The device as claimed in one of claims 1 to 7, characterized in that it is used in the production of substrate glass for flat panel displays by the float process. 9. The device according to claim 8, characterized in that it is a borosilicate glass containing the following composition: 55-65 wt% _SiO2 12-20 wt% Al ₂ O ₃ ≥5% by weight _{of B2O3} 0-5% by weight of BaO 3-9 wt% CaO 1-5% by weight of MgO 0-5% by weight of SrO

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