GB2253844A

GB2253844A - Throat

Info

Publication number: GB2253844A
Application number: GB9106175A
Authority: GB
Inventors: Geoffrey Evans; Robert Alan Williams
Original assignee: Pilkington Glass Ltd
Current assignee: Pilkington United Kingdom Ltd
Priority date: 1991-03-22
Filing date: 1991-03-22
Publication date: 1992-09-23
Also published as: GB9106175D0

Abstract

A throat (4) located between a glass melting zone (2) and a refining zone (3) of a glass melting tank (1) for producing high quality clear float glass on a float bath has a refractory roof (8), two refractory side walls (9) (Figure 2) and a refractory bottom (6) and is designed to resist corrosion with very little contamination of the glass by having at least the roof comprise a fusion cast glass contact refractory which contains between 10 and 90% by weight Cr2O3 and preferably between 25 and 30% by weight Cr2O3. The sidewalls also preferably comprise such refractory. <IMAGE>

Description

THROAT This invention relates to a throat located between a glass melting zone of a glass melting tank and a float bath for the production of high quality clear float glass, the throat comprising a refractory roof, two refractory side walls and a refractory bottom. The invention further relates to a method of producing high quality clear float glass by use of a glass melting tank including such a submerged throat.

Throats provide a convenient way to draw glass from a melting zone whilst retaining unmelted batch floating on the surface of the glass in the melting zone. Throats have for many years been used for melting container glass and fibrous or wool glasses. The flow of glass through the throat induces a high degree of corrosion of the refractory blocks making up the roof and sidewalls of the throat, which then limits the life of the glass melting tank. Various fusion cast refractory blocks containing Cur203 have been used for the roof and sidewalls of throats, but it has always been recognised that although such refractories have a high resistance to corrosion, they introduce an unacceptable risk of contamination of clear glass due to the strong propensity of Cur203 dissolved from the refractory to give 23 green coloured or streaky glass.Such refractories have therefore been confined to applications in which the green colouration imparted by Cur203 was not objectionable e.g.

23 some fibrous or wool glasses and some container glasses as stated in GB 2 024 799A. In particular they have not been used for throats in glass melting tanks feeding glass to float baths.

US Patent 4 336 339 attempts to provide a refractory with improved corrosion resistance by using > 90% ZrO2 without any Cr203. This improved corrosion resistance is only significant for fibrous or wool glasses and the refractories are very expensive. They do not provide an economically viable solution to the problems of extending throat lifetimes in a way that does not give rise to high Cur203 levels or colour contamination of high quality 23 clear float glass.

According to the present invention a throat located between a glass melting zone of a glass melting tank and a float bath for the production of high quality clear float glass, comprises a refractory roof, two refractory side walls and a refractory bottom, in which at least the roof of the throat comprises a fusion cast glass contact refractory which contains between 10 and 90% by weight Cr203. Desirably the sidewalls also comprise such a fusion cast refractory.

For the production of the highest quality and least coloured glass we prefer to use refractories between 20 and 35% by weight Cr203. We most prefer to use refractories containing between 25 and 30% by weight Cr203 as these provide the twin benefits of low colouration and improved throat life even at low loads where the ratio of the tonnage of glass melted to the surface area of the Cur 203 containing refractory is low.

Advantageously the refractory also contains Al203, Zr02 and Si02 for the best balance of properties, such as thermal shock resistance with the desired properties of corrosion resistance and low Cr203 release.

Also according to the invention is a method of making high quality clear float glass including passing molten glass from a melting zone of a glass melting tank to a refining zone of the glass melting tank by means of a throat, wherein the throat includes a fusion cast refractory containing more than 10% by weight of Cur 0 and the Cur203 level in 23 23 the final glass product is less than 12 parts per million.

Advantageously the Cur 0 level in the product due to pick 23 up from the refractories is less than 6 parts per million.

Preferably the refractory contains between 20 and 35% by weight Cr203 and most preferably between 25 and 30% by weight Cur 0 . Advantageously the refractory further 23 comprises Al203, Zr02 and Si02.

The invention will now be described, by way of example only, and with reference to the accompanying drawings, of which: Figure 1 is a longitudinal section through a submerged throat; Figure 2 is a cross section taken at Il-Il in figure 1; and Figure 3 is a plot of the measured level of Cur203 23 in the glass vs time in weeks.

Figure 1 shows a glass melting tank 1 having a melting zone connected to a refining zone 3 by means of a submerged throat 4. The throat 4 is shown sunken below the level of the bottom of the melting zone 5, but it could be level with it or above it. The bottom of the melting zone 5, the bottom of the throat 6 and the bottom of the refining zone 7 are made from a conventional fusion cast AZS refractory such as ER1711 or ER1681 obtained from SEPR of Le Pontet. The roof of the throat and the side walls of the throat 9, shown in figure 2 are made from a chrome containing fusion cast AZS refractory sold by SEPR under the designation ER2161.An analysed composition of ER2161 is: Cr 0 28 Al203 31.4 Zru 3 26.4 sio2 11.9 Fe 0.15 Mgu In this example the glass can be melted by means of electrodes 10 or by fuel firing through ports 11. The invention is equally applicable to glass melted by means of fuel fired radiant heating or combinations of electric and fuel fired heating.

A float tank was fed with glass melted in a glass melting tank containing a throat with chrome containing refractories. The flow of glass through the throat was 2 equivalent to 5.7 tonnes/day/dm of cross sectional area, which is a typical loading for such a construction. The chrome level in the finished glass product over a period of time is shown in figure 3.

Initially the Cr203 level in the final glass product was unacceptably high at about 25 ppm, compared with the normal level of 6 ppm when Cr203 free refractories are used. However, in line with our experimental work the level eventually dropped to about 10 ppm which is acceptable for high quality clear float glass. By high quality clear float glass we mean glass with a white light transmission (at 4 mm) of at least 88%, preferably at least 89%, without discernable discrete faults and colour bands.

Although chromium containing AZS refractories are preferred the invention is not limited to them; the analysis of another suitable, but less preferred, refractory is: Monofrax E (Carborundum) Cr 77 Al203 5-6 Zrl 2.0 SiO2 2.0 Fe 6.4 Mg 3 8.0 Likewise, the invention is not limited to any particular design of throat; it could be built in line with the wall of the melting zone or behind it. It could be submerged, sunken or level with the bottom of the melting zone. The throat roof can be horizontal or inclined and the side walls parallel or tapered. The throat bottom can also be made of chrome containing refractory.

Claims

1. A throat located between a glass melting zone of a glass melting tank and a float bath for the production of high quality clear float glass, comprising a refractory roof, two refractory sidewalls and a refractory bottom, in which at least the roof of the throat comprises a fusion cast glass contact refractory which contains between 10 and 90% by weight Cr203.

2. A throat according to claim 1 in which the roof of the throat comprises glass contact refractory which contains between 20 and 35% by weight Cur203

3. A throat according to claim 2 in which the roof of the throat comprises glass contact refractory which contains between 25 and 30% by weight Cr203.

4. A throat according to any preceding claim in which the glass contact refractory further comprises Al203, Zr02 and Six2.

5. A method of making high quality clear float glass including passing molten glass from a melting zone of a glass melting tank to a refining zone of the glass melting tank by means of a throat, wherein the throat includes refractories containing more than 10% by weight of Cur 0 and the Cur 0 level in the 23 23 final product is less than 12 ppm.

6. A method according to claim 5 wherein the throat includes refractories containing between 20 - 35% by weight Cr203.

7. A method according to claim 6 wherein the throat includes refractories containing between 25 - 30% by weight Cr203.

8. A method according to any one of claims 5 to 7 wherein the refractory further comprises Al203, Zr02, Six2.

9. Method according to any one of claims 5 to 8 wherein the level of Cr203 in the final product due to pick up from the refractories is less than 6 ppm.

10. Glass formed on a float bath, the glass containing between 1 and 6 ppm of Cr203 due to pick up from refractories in a throat.