US3395996A

US3395996A - Method and apparatus for inhibiting the flow of impurities of a molten bath to the glass sheet thereon

Info

Publication number: US3395996A
Application number: US438878A
Authority: US
Inventors: Loukes David Gordon
Original assignee: Pilkington Brothers Ltd
Current assignee: Pilkington Group Ltd
Priority date: 1964-03-13
Filing date: 1965-03-11
Publication date: 1968-08-06
Anticipated expiration: 1985-08-06
Also published as: DE1471995A1; LU48178A1; NL6503210A; DK108404C; BE661141A; GB1103276A; FR1426668A; ES310435A1; ES315907A1; DE1471995B2

Description

Aug. 6, 1968 D. cs. LOUKES 3,395,995

METHOD AND APPARATUS FOR INHIBITING THE FLOW OF IMPURITIES OF A MOLTEN BATH TO THE GLASS SHEET THEREON Filed March 11, 1965 3 Sheets-Sheet 1 amjfifijpgwenior B WWW 440M; #W

1968 D. 5. LOUKES 3,395,996 FOR INHIBIT METHOD AND APPARATUS ING THE FLOW OF IMPURITIES LAS OF A MOLTEN BATH TO THE G S SHEET THEREON I Filed March 11, 1965 5 Sheets-Sheet 2 F/GZQ. I: q /2\ wt dq nvenlor Home" United States Patent 3,395,996 METHOD AND APPARATUS FOR INHIBITING THE FLOW 0F IMPURITIES OF A MOLTEN BATH TO THE GLASS SHEET THEREON David Gordon Loukes, Prescot, England, assignor to Pilkington Brothers Limited, Liverpool, England, a corporation of Great Britain Filed Mar. 11, 1965, Ser. No. 438,878 Claims priority, application Great Britain, Mar. 13, 1964, 10,801/64 11 Claims. (Cl. 65-27) ABSTRACT OF THE DISCLOSURE In the float process for the manufacture of flat glass on a molten metal bath physical barriers are located in the bath to inhibit transfer of impurities from the exposed molten metal surface to the glass/metal interface.

This invention relates to the manufacture of fiat glass.

In one method of manufacture of flat glass in ribbon form, the glass is supported on a bath of molten metal, for example molten tin or a tin alloy having a specific gravity greater than the glass. The bath is so constituted as to have all the characteristics as fully described in US. patent specification No. 2,911,759.

In a method of manufacturing flat glass in ribbon form on a bath of molten metal, the surface of the bath which is exposed at each edge of the glass ribbon is liable to take up impurities, for example oxygen and/or sulphur or compounds of these elements, which may react with the molten metal of the bath to produce contaminants for the glass.

It is for this reason that it has been proposed to maintain in the headspace over the bath a plenum of protective atmosphere constituted by a non-oxidising gas or mixture of gases which will not chemically react to any substantial extent with the molten metal of the bath to produce contaminants for the glass.

However, even when a plenum of protective atmosphere is maintained over the molten metal, it is found that very small quantities of impurities, for example oxygen, do come into contact with the surface of the molten metal. The presence of small quantities of oxygen in the atmosphere over the molten metal may, for example, be due to an infiltration of the gaseous impurity against the plenum of protective atmosphere occasioned by a failure of the seal between the headspace and the normal atmosphere.

The presence of, for example, traces of oxygen and sulphur as impurities in the molten metal is liable to result in the uptake of particulate or dissolved contaminants by the bottom surface of the glass ribbon on the bath when these impurities migrate to the interface between the glass ribbon and the molten metal, either by direct transfer of the impurities due to circulation of the molten metal of the bath or even by diffusion of the impurities through the molten metal of the bath.

It is a main object of the present invention to improve the qualities of the lower surface of the glass ribbon which is in contact with the molten metal of the bath during the manufacture of the glass ribbon on the bath, for example by the process described and claimed in US. patent specification No. 3,083,551.

Accordingly the present invention provides in the manufacture of flat glass during which the glass is advanced in ribbon form on a bath of molten metal, the step of impeding the transfer of impurities in the molten metal to the interface between the glass ribbon and the molten metal from parts of the bath surface which are exposed to the atmosphere over the bath, and thereby reducing ice the uptake of contaminants by the bottom surface of the glass ribbon.

Conveniently the transfer of impurities in the molten metal to the interface between the glass ribbon and the molten metal from exposed parts of the bath surface is achieved by providing in the molten metal of the bath one or more physical barriers which prevent a direct flow of molten metal from the exposed parts of the bath surface to the interface between the molten metal and the glass ribbon.

According to this aspect, therefore, the present invention provides in the manufacture of flat glass during which the glass is advanced in ribbon form on a bath of molten metal, the step of maintaining in the molten metal of the bath one or more physical barriers to inhibit the transfer of impurities in the molten metal to the interface between the glass ribbon and the molten metal from parts of the bath surface which are exposed to the atmosphere over the bath, thereby reducing the uptake of contaminants by the bottom surface of the glass ribbon.

It will be understood that the method of the present invention does not set out to prevent entirely the presence of contaminants (for example the presence of compounds of the metal of the bat-h) in the bottom surface of the glass ribbon, but the present invention is concerned with keeping the quantities of such contaminants down below a level at which their presence does not manifest itself in such a way as to detract from. the flatness and surface brightness obtained by the manufacture of a flat glass ribbon on a bath of molten metal, either directly or when the glass is subjected to subsequent treatment.

Accordingly, during the manufacture of flat glass in ribbon form by a method according to the present invention, any contaminants for the glass initially present near the surface of the bath at the interface with the glass ribbon will enter the surface of the glass ribbon, but, owing to the physical separation provided between the parts of the bath near the surfaces of the molten metal which are exposed to the atmosphere over the bath and the part of the molten metal which supports the glass ribbon, there will be a tendency during the manufacture of the glass ribbon on the bath of molten metal for the concentration of impurities in the parts of the bat-h of molten metal near the interface bet-ween the glass and the molten metal to decrease.

Advantageously the method of the present invention may be used in conjunction with the maintenance, in the headspace over the bath, of a plenum of an atmosphere intended to be protective to the molten metal of the bath, that is to say an atmosphere comprising essentially a gas, which is inert to the molten metal, with a proportion of a reducing gas.

The present invention also comprehends apparatus for use in the manufacture of flat glass in ribbon form, comprising, in combination, an elongated tank structure including side and end walls, said tank structure containing a bath of molten metal, means for advancing glass in ribbon form along the bath towards an outlet from the bath, and, in the bath of molten metal, one or more physical barriers extending longitudinally of the tank structure and substantially preventing a direct circulation or diffusion of molten metal from those parts of the bath exposed at the edges of the glass ribbon to the interface between the glass ribbon and the molten metal over a substantial length of the elongated tank structure containing the molten metal.

The physical barriers should be present in the tank structure over a considerable length thereof, for example at least 40%50% of the length of the elongated tank structure.

Preferably screens arranged vertically in the bath on either side of the glass ribbon and extending from the floor of the bath to the surface of the molten metal may comprise the physical barriers in the bath of molten metal. The screens may be parallel to the ribbon of glass after the glass has stabilised at a final width and thickness, and particularly at the outlet end of the tank structure. At the input or hot end of the tank structure, the screens may be arranged to conform to the shape adopted by the hot glass on the bath, but the screens will be positioned in space relation from the edges of the glass body on the bath of molten metal.

The screens may be arranged in any other desired position in relation to the surface of the bath and to the floor of the tank structure.

As one example, the screens may extend from the floor of the tank structure up to a position near to, but below, the surface of the molten metal; in such a case, the barriers may be integral with the tank structure, but preferably they are arranged so as to be movable in relation to the longitudinal walls of the tank. Such movable screens or barriers may be formed from vertical screens which are denser than the molten metal of the bath, or they may be of a density equal to or lighter than the density of the molten metal, provided that the barriers are located and held in position, for example in the end wall of the tank structure.

The advantage of screens or barriers which do not break the surface of the molten metal is that the glass on the bast may meander from its intended position without coming into contact with the solid of the screen or barrier.

On the other hand, mobile screens or barriers can conveniently be employed by using a weight of material in the screen or barrier which is such that the barrier will float in a vertical position but breaking the surface of the molten metal. Advantageously the barrier is arranged to float vertically with its lower edge near to but not in contact with the floor of the tank structure. In the event of the glass wandering from its intended course and coming into contact with such a barrier which breaks the surface of the molten metal, the contact of the glass with the barrier will result in the barrier moving from the position in which it was placed, rather than the body of the glass being deformed by contact with the barrier.

Alternatively the vertical screens or barriers can be located in position by being held on arms located in the side walls of the tank structure above the level of the surface of the molten metal, in which case the density of the material of the screens or barriers is immaterial, and such an arrangement of the barriers is preferred at positions in the tank structure where the glass has adopted a stable ribbon form so that the chance of deviation of the glass from its intended path is substantially reduced, if not eliminated.

As a further alternative the surfaces of the molten metal exposed on either side of the ribbon of glass may be segregated by employing two constructions of screens or barriers, which have closed ends and act as boats which are maintained partially submerged on either side of the ribbon or glass. In this case, the walls of the boat will preferably break the surface of the molten metal because the boat-shaped barrier is filled or partially filled with molten tin in order to sink it to the desired level in the bath. Desirably the boat-shaped barried will be sunk to a level which it nearly touches the floor of the bath. Such a boat-shaped barrier can naturally be made of a range of materials, and is conveniently constructed as a number of lengths of material linked together.

As a still further alternative, a U-shaped channel member may be employed instead of the boat, and the U-Shaped channel member may be made of a material denser than the molten metal of the bath, so that the member rests on the floor of the tank structure containing the bath of molten metal.

In the preferred embodiments of apparatus according to the invention, the bath is a bath of molten tin, while the physical barriers may be made of carbon and maintained in position by weighted metal blocks positioned on the upper surfaces of the carbon barriers above the level of the molten metal. Other materials from which the barriers may be made when the bath is a bath of molten tin are refractory materials or spheroidal graphite cast iron. These materials are quoted merely as preferred examples of suitable materials for the physical barriers.

The present invention also comprehends a sheet of glass cut from a ribbon of fiat glass manufactured by the method described or on a bath of molten metal in apparatus in accordance with the present invention.

In order that the present invention may be more clearly understood, some preferred embodiments thereof will be described purely by way of example, with reference to the accompanying diagrammatic drawings in which:

FIGURE 1 is a plan view of a tank structure containing a bath of molten metal and including physical barriers in accordance with the present invention,

FIGURES 2a, 2b and 2c show alternative forms of construction for the physical barriers of FIGURE 1, and

FIGURE 3 is a plan view of another form of elongated tank structure showing the position of the barriers in accordance with the present invention.

In the drawings the same or similar parts are designated by like reference numerals.

With reference to FIGURE 1 of the acompanying diagrammatic drawings, there are shown end Walls 1 and side walls 2 of an elongated tank structure. The tank structure includes a bath 3 of molten tin and over the elongated tank structure there is a roof structure 4 (shown in FIGURE 2c) including corresponding side walls 5 and end walls 6. The roof structure 4 defines a headspace over the bath 3 of molten metal within which there is maintained an atmosphere intended to be protective to the molten tin of the bath, for example an atmosphere including a majority of nitrogen and a small proportion of a reducing gas such as hydrogen.

At the inlet end of the tank structure there is provided a spout 7 whose width is defined by side jarnbs 8, and in operation of the apparatus by one of the methods described in US. patent specification No. 3,083,551, molten glass is delivered from a lip 9 of the spout 7 to the surface of the molten metal of the bath 3.

Positioned on the surface of the bath in spaced lateral relation from the position of the spout 7 are restrictor tiles 10 of refractory material. As the molten glass delivered to the bath is advanced beyond the restrictor tiles 10, the molten glass is permitted to flow laterally without physical constraint, and a ribbon of glass 11 of suitable width and thickness is formed as the glass is advanced along the bath.

In accordance with the invention, two series of barriers or screens 12 are positioned in the bath 3 of molten tin and arranged to be spaced a small distance from the edges of the glass on the bath as compared with the distance from the barriers 12 to the side walls 2 of the tank structure.

The barriers 12, which at the inlet end of the bath near the spout are curved to follow a contour approximately the same as the outline of the glass ribbon on the bath advantageously extend the full depth of the molten tin comprising the bath 3, but particular arrangements of the barriers 12 will be described with reference to FIGURES 2a, 2b, 2c and 2d. However, in all cases the barriers 12 are such as to impede transfer of im purities in the molten metal from parts of the bath surface such as 13, which are exposed to the atmosphere over the bath, to the parts of the molten tin near the interface between the glass and the molten tin.

Referring now to FIGURE 2a, there is shown a partsectional view of one embodiment in which the barriers 12 are walls or screens resting on the floor 14 of the tank structure.

In FIGURE 2b of the accompanying drawings, there is shown a barrier 12 which is floating in the molten tin of the bath 3 and which is maintained in position by a support 15 from outside the tank structure.

FIGURE shows another arrangement in which the barrier 12 is completely submerged, and this arrangement of the barrier 12 is particularly suitable for use at the hot end of the bath.

Turning to FIGURE 3 of the accompanying drawings, there is shown a tank structure including a pocket 16 in the side Walls 2. The pocket 16 may, for example, be for the purpose of introducing into the bath 3 an additive for reacting with impurities in the molten tin by the method described in co-pending application No. 390,015, now Patent No. 3,330,635.

The arrangement of the barriers 12 in such a process is such as to permit a direct flow of molten tin from the pocket 16 into the region of the bath 3 supporting the glass, but barriers 12a are provided at right angles to the movement of the glass down the bath 3 so that the additive may be channeled directly into that part of the bath which supports the glass ribbon, and the additive is not dispersed throughout the parts 13 of the bath which are exposed to the atmosphere over the bath. Also the transfer of impurities from the parts 13 of the bath to the interface between the glass ribbon and the molten tin is substantially prevented.

Alternatively the additive may be introduced into the molten metal of the bath by the method described and claimed in co-pending application No. 388,810, now Patent No. 3,351,447. When this method is adopted, the barriers 12 are preferably arranged in the manner illustrated in FIGURE 1 of the drawings, and the additive is introduced between the barriers 12 and the glass 11.

By the use of the invention as described herein, it is found that fiat glass is produced in which the presence of contaminants showing a harmful effect in subsequent use or treatment of the glass is substantially eliminated.

I claim:

1. In the manufacture of fiat glass during which the glass is advanced in ribbon :form on a bath of molten metal and a unitary plenum of protective atmosphere is maintained over the advancing ribbon and the exposed bath surface alongside the advancing ribbon, the step of maintaining in the molten metal of the bath one or more physical barriers extending at most just through the surface of the bath to inhibit the transfer of impurities in the molten metal to the interface between the glass ribbon and the molten metal from parts of the bath surface Which are exposed to said unitary plenum of protective atmosphere over the bath, thereby reducing the uptake of contaminants by the bottom surface of the glass ribbon without materially interfering with the free flow of the protective atmosphere to and from the area above the glass and the area to one side of such a barrier.

2. Apparatus for use in the manufacture of fiat glass in ribbon form, comprising, in combination, an elongated tank structure including side and end walls, said tank structure containing a bath of molten metal, a roof structure over the tank structure defining a unitary tunnel-like headspace over the molten metal bath, means for advancing glass in ribbon form along the bath towards an outlet from the bath, and, in the bath of molten metal, one or more physical barriers extending longitudinally of the tank structure and at most just extending through the surface of the bath which barriers substantially prevent a direct circulation or diffusion of molten metal from those parts of the bath exposed alongside the edges of the glass ribbon to the interface between the glass ribbon and the molten metal over a substantial length of the elongated tank structure containing the molten metal without materially interfering with the free flow of the protective atmosphere to and from the area above the glass and the area to one side of such a barrier.

3. Apparatus according to claim 2, wherein screens arranged vertically in the bath on either side of the glass ribbon extend from the floor of the bath to the surface of the molten metal to comprise the physical barriers in the bath of molten metal.

4. Apparatus according to claim 2, wherein screens arranged vertically in the bath on either side of the glass ribbon extend from the floor of the tank structure to a position near to, but below, the surface of the molten metal and comprise the physical barriers in the bath of molten metal.

5. Apparatus according to claim 2, wherein the barriers are comprised by screens floating vertically in the molten metal of the bath with the upper edges of the screens breaking the surface of the molten metal and the lower edges near to but not in contact with the floor of the tank structure.

6. Apparatus according to claim 2, wherein the barriers are located in position by being held on arms located in the side walls of the tank structure above the level of the surface of the molten metal.

7. Apparatus according to claim 2, wherein the barriers are comprised by constructions of screens as boats which are maintained partially submerged in the molten metal on either side of the glass ribbon.

8. Apparatus according to claim 2, wherein the barriers are comprised by U-shaped channel members which are partially submerged in the molten metal, with one vertical limb of the U on either side of the glass ribbon.

9. Apparatus according to claim 2, wherein the bath is a bath of molten tin and the physical barriers are made of carbon.

10. Apparatus according to claim 2, wherein the bath is a bath of molten tin and the physical barriers are made of refractory material.

11. Apparatus for use in the manufacture of flat glass in ribbon form, comprising, in combination, an elongated tank structure, said tank structure containing a bath of molten tin, a roof structure over the tank structure defining a headspace over the bath, means for advancing glass in ribbon form along the bath towards an outlet from the bath and in the bath physical barriers to metal flow extending longitudinally of the tank structure over a substantial length of the tank structure and on either side of the path of travel of the ribbon of glass, which barriers are made of spheroidal graphite cast iron.

References Cited UNITED STATES PATENTS 11/1959 Pilkington et a1 6565 7/1966 Javaux 65l82 X