Title of Invention

FLAT LEAD GLASS PRODUCED BY FLOATING ON A METAL BATH

Abstract The invention relates to a process for manufacturing flat glass rich in lead oxide, comprising the continuous floating, in a float plant with a neutral gaseous atmosphere, of a glass comprising at least 30% lead oxide by weight on a bath of molten metal having a higher density than that of the glass. The invention allows flat glass rich in lead, useful for protection against X-rays, to be produced.
Full Text FLAT LEAD GLASS PRODUCED BY FLOATING ON A METAL BATH
The invention relates to the production of a flat glass
rich in lead oxide by floating on a bath of molten
metal.
It is known to produce flat glass of the soda-lime-
silica type by floating on a bath of molten metal,
generally based on tin. The soda-lime-silica glass has
a density much less than that of tin (the density of
this glass is about 2.5, whereas the density of molten
tin is about 7) and the floating of the glass takes
place correctly without the top rolls pressing in too
pronounced a manner into the ribbon of glass floating
on the liquid metal. To avoid oxidation of the tin, the
atmosphere of the float plant is usually maintained in
a reducing medium, generally thanks to the presence of
hydrogen in the gaseous atmosphere above the glass and
metal. This gaseous atmosphere is generally composed of
nitrogen containing 7 to 15% hydrogen by volume.
Glass rich in lead oxide, to which the present
application applies, contains at least 30% lead oxide
PbO by weight. It is used more particularly when
protection against X-rays has to be provided,
especially in the medical or nuclear field. Such glass
is intended to be incorporated into partitions, windows
and handling boxes, and is reputed not to be produced
by the float method because of its high density,
approaching that of tin. This is because there is a
fear that the ribbon will be pressed into the molten
float metal, in particular owing to the effect of the
top rolls, and there is also the fear of exchange
between the lead of the glass and the environment of
the float plant, the tin, the refractory or the
atmosphere. Such exchange could result in the lead
oxide being reduced, to produce metallic lead, which
could evaporate and then condense in droplets, falling
on the glass and marking it (the formation of "top

lead" on the surface of the glass).
US 4 015 966 clearly teaches the difficulty of devising
ways of floating compositions comprising lead, antimony
or arsenic oxides to such a point that it recommends,
for the float process, for X-ray-absorbent compositions
to contain in particular none of these oxides. That
document also recommends carrying out the float process
in a reducing atmosphere. For all the reasons mentioned
above, lead-rich glass is usually manufactured in a
batch process by "in-pot" melting, followed by casting
on a table, followed by polishing or smoothing in order
to achieve a flat glass. However, this process is
particularly tedious and expensive. Flat glass 6, 8 and
12 mm in thickness can thus be produced. However, it is
not known how to produce thinner glass, such as that
with a thickness of 3.5 mm, as the glass with such a
small thickness is too fragile for the process
described above.
EP 525 555 teaches that lead oxide in a glass is to be
prescribed when the glass is to be produced by the
float process. EP 592 237 teaches that lead glass
cannot be produced by the float process.
US 5 073 524 and US 2 223 118 teach compositions rich
in lead.
As other documents of the prior art, mention may be
made of US 5 221 646 and US 4 876 480.
In addition to the abovementioned problems associated
with the presence of lead in such glass, the Applicant
has observed, from float tests under standard float
conditions for soda-lime-silica glass, that a grayish
film inevitably forms on the surface of the glass that
is in contact with the atmosphere of the float plant.
The Applicant has discovered that this film is a thin
layer of metallic lead and has also discovered that

this film could be avoided by removing the reducing
nature of the gaseous atmosphere of the plant (absence
of hydrogen). Thus, in the process according to the
invention, it is preferred to use a neutral atmosphere,
that is to say one that is neither oxidizing nor
reducing, such as a nitrogen atmosphere. In practice,
it is difficult to achieve an absolute zero oxygen
content, but it is recommended to reduce the oxygen
content of the atmosphere as low as possible in order
to limit oxidation of the tin of the float bath. The
gaseous atmosphere in the float plant preferably
contains less than 5 ppmv (parts by million by volume)
of oxygen. It may essentially be composed of nitrogen.
Apart from the particular composition of the glass and
of the neutral atmosphere in the float plant, the
latter is itself identical to the float plants usually
employed for more conventional glass, such as that of
the soda-lime-silica type.
The invention therefore relates to a process for the
manufacture of a ribbon of glass rich in lead oxide
produced by the float process, in which the ribbon,
formed on a bath of molten metal, even one based on
tin, progresses by floating on this bath, said ribbon
being removed from the bath when it has solidified
sufficiently.
Glass rich in lead oxide to which the present
application relates contains at least 30% and even at
least 45%, or indeed at least 60% by weight of lead
oxide PbO. Such glass generally contains up to 75% and
more particularly up to 70% by weight of lead oxide
PbO. Such glass may also include barium oxide BaO,
barium being a heavy element that can also stop X-rays.
The glass may for example comprise 2 to 20% BaO by
weight. In general, the glass comprises silica in an
amount from 25 to 35% by weight. The glass also
generally comprises K2O and/or Na2O, the sum of the mass

of K2O and Na2O generally ranging from 0.2 to 1% by
weight.
Such glass generally has a density ranging from 4 to 6
and more particularly a density ranging from 4.3 to
5.5.
In the manufacture of a ribbon of float glass for the
purpose of producing sheets of flat glass, molten glass
is poured onto a bath of molten metal, generally tin or
a predominantly tin-based alloy, where it forms said
continuous ribbon, which is progressively cooled and
extracted by means of extractor rolls that convey it
into an annealing furnace called a lehr. The region
covering the glass ribbon while it is running along the
bath of molten metal comprises heating systems and
cooling systems that are provided for conditioning the
temperature and, more precisely, the viscosity of the
glass in order to allow it to be drawn to the desired
thickness and then solidified.
The thickness of the glass ribbon is determined by the
tensile force exerted on the glass ribbon, on the one
hand, by the extractor rolls and possibly, on the other
hand, by the action of knurled top rolls that act on
the upper edges of the glass ribbon. This is because,
for a given output, that is to say for a certain
quantity of glass leaving the furnace per given unit of
time, the thickness of the ribbon of float glass
depends on the speed of said glass ribbon in the layer.
While the glass ribbon is running along the bath of
molten metal, said ribbon will therefore undergo a
drawing in order to reduce its thickness. This drawing
is obtained, for outputs of between 2 and 200 tonnes
per day and more particularly 5 to 100 tonnes per day,
with ribbon speeds of between 0.1 and 20 meters per
minute and more particularly 0.2 and 10 meters per
minute. It is also known that such speeds of the glass

ribbon running along the bath of molten metal cause,
beneath said ribbon, a tin current directed toward the
colder exit end of the bath, which may be termed the
downstream current. The metal entrained by the ribbon
following this downstream current butts against the
exit wall of the bath and then, by reflection
therefrom, it forms a return current directed toward
the upstream end of the bath, which may be termed the
upstream current. This upstream current is particularly
substantial between the edges of the glass ribbon and
the side walls of the bath. To limit the effect of this
upstream current, the bath may be equipped with tweels
as proposed by US 4 217 125. The bath may also be
equipped with recirculation circuits for recirculating
the tin from the downstream end toward the upstream end
of the bath via pipes external to the bath (see
GB 1 166 648 and US 3 658 504) or even inside the bath
(see abstract JP 59121125 and US 3 790 361) .
Thus, the invention relates to a process for
manufacturing flat glass rich in lead oxide, comprising
the continuous floating, in a float plant with a
neutral gaseous atmosphere, of a glass comprising at
least 30% lead oxide by weight on a bath of molten
metal having a higher density than that of the glass.
For the case in which tin oxide would nevertheless
slowly form because of the very slight contamination of
the atmosphere with oxygen, an elimination means for
eliminating this oxide which floats on the surface of
the metal bath may be provided. This elimination means
may be a device for eliminating the solid floating
matter, such as skim pockets downstream of the bath or
the dedrossing means described in US 4 046 549. This
means may also involve recirculation of the tin via an
external pipe passing through a purification unit
intended to remove or reduce the tin oxide formed, for
example by hydrogen sparging, as described in
FR 1 323 711, followed by elimination of the hydrogen

before reintroduction.
Moreover, so as to remove other metals, such as iron,
sparging with steam may also be effected.
If the tin is made to circulate outside the plant in
order to treat it, it may also be treated in order to
remove its iron. This iron elimination is desirable as
too high an iron concentration in the tin results in
the formation of solid particles of an iron/tin alloy
that will become encrusted in that face of the glass
which is in contact with the bath, producing defects.
This iron elimination process is carried out by
precipitation, by cooling. By cooling between 250 and
450°C, especially between 270 and 350°C, FeSn2 is
precipitated. The iron may also be chemically separated
from the tin in a suitable reactor.
Moreover, since the tin of the bath of molten metal
risks becoming laden with lead, provision may be made
for draining off the tin via a pipe in order to bring
it to a treatment station for removing the lead. This
elimination may be carried out by chemical
reprocessing.
Thus, the invention also relates to a float process
comprising a treatment station (or unit) especially for
purifying the tin of the float bath (the bath of molten
metal) associated with said bath.
The tin, optionally stripped of the lead and/or the
iron, is then reinjected into the bath. If necessary,
the treated tin is reheated before it is reinjected if
its temperature has dropped during the treatment. The
reheating brings the temperature of the tin to be
reinjected close to that of the bath at the reinjection
point.
The float plant is supplied at the upstream end with

molten glass. This molten glass is prepared by melting
batch materials in at least one melting furnace located
upstream of the float plant. These batch materials may
comprise: cullet, Pb2O3, silica, sand, Zr-rich sand (the
precursor for ZrO2) , barium carbonate (the precursor
for BaO), KOH (the precursor for K2O), etc.
By way of example of a melt furnace, it is possible to
use two tanks in series, the second tank being at a
lower temperature than the first and being supplied
with lead oxide. In particular, the device may be used
with the two following tanks in series:
the first being equipped with submerged burners
heating the furnace to between 1100 and 1300°C, which
is supplied for example in particular with cullet, Ba
carbonate and, optionally, other batch materials, all
these representing, for example, between 30 and 70% of
the total output, a flue allowing the gases formed to
be discharged;
the second being equipped with SnO2 electrodes
keeping the furnace between 800°C and 1200°C, and not
including fume extraction, the tank being supplied with
the composition coming from the first tank and with
lead oxide, and optionally with other batch materials,
the composition thus obtained feeding the float plant.
Introducing lead oxide into the second, cooler tank
with no flue prevents the lead oxide from flying
around. This is because lead oxide is noxious and it is
necessary to prevent it from evaporating and
penetrating into the external environment. The second
tank may be equipped with a stirrer for thermal and
compositional homogenization. It is also possible for
it not to be equipped with a stirrer, but in this case
provision is nevertheless made for homogenizing (in
terms of temperature and composition) the glass, in the
flow channel that takes the glass to the float plant,
said channel then being provided with at least one
stirrer, or in a mixing cell provided with at least one

stirrer placed between the second tank and the float
plant.
Thus, according to a variant, before the float plant,
the glass may be melted in a furnace that includes at
least one submerged burner. In particular, the furnace
may comprise at least two tanks in series, the second
tank being fed with lead oxide. The first tank may
especially be equipped with at least one submerged
burner and may be fed with the batch materials other
than lead oxide. In particular, the second tank is
advantageously at a lower temperature than the first
tank.
The glass is generally at a temperature of between 700
and 900°C upstream of the float plant and between 400
and 600°C downstream of the float plant. Overall, it
may be stated that the float glass is between 400 and
900°C and more particularly between 500 and 800°C. The
temperature of the bath of molten metal is lower than
the temperature of a bath of molten metal of a float
plant for a soda-lime-silica glass containing no lead.
It may be considered that the temperature of the glass
is substantially the same as that of the bath at the
same point.
The glass manufactured by the process according to the
invention is in general enriched with tin on that side
that has been in contact with the bath of molten metal,
for example with a concentration possibly up to 2% by
weight, for example 0.01 to 2% by weight, at least over
the first micron of the skin, and even over the first
few microns of the skin. The tin content forms a
concentration gradient that decreases from the surface
of the glazing toward the core.
The process according to the invention allows the
production of flat glass with a thickness ranging from
1 to 25 mm in thickness, especially between 3 and 14 mm

in thickness. The thickness may therefore be less than
5 mm and even less than 4 mm.
On the molten metal, the ribbon may have a width
ranging from 0.90 to 6 meters and more generally from 1
to 4 meters. On leaving the forming plant, the ribbon
passes into a lehr for progressively cooling it, after
which the ribbon is cut longitudinally (cutting of
strips from two edges) and transversely into panels
comprising two principal faces and an edge. Each
principal face of these panels may for example have an
area ranging from 0.15 m2 to 20 m2.
Figure 1 shows an elongate tank for the manufacture of
glass by the float process. The tank has side walls 1
and end walls 2 and 3, at the entry and exit of the
tank, respectively. The tank, containing a bath of
molten tin 4, has a downstream part 5 of smaller width.
The molten glass is poured onto the bath at its entry
end, from a delivery channel 6 placed above the entry
wall of the tank. Temperature regulators (such as SiC
resistance heating elements), which are not shown in
the figures, are incorporated into the roof that covers
the bath. These regulators thermally condition the
glass, keeping it in the deformable state until the end
of the drawing zone. The bath comprises, in the
manufacture of the glass, several zones represented in
figure 1, which may be distinguished in the following
manner:
- a zone I, in which the glass spreads out after
being poured, upstream, onto the molten metal;
- a zone II, in which the glass ribbon being
formed undergoes longitudinal forces that are directed
toward the outside, under the action of the extractor
rolls 8 and the top rolls 9. The drawing of the glass
starts in this zone, the glass becoming thinner;
- a zone III in which the glass ribbon assumes its
final shape under the action of the extractor rolls 8.
The zones II and III together form the drawing zone;

and
- a consolidation zone IV in which the solidified
glass ribbon is progressively cooled.
After having been poured onto the bath of molten metal,
the glass freely spreads out as far as possible in zone
I. A ribbon 7 thus forms which moves toward the
downstream end under the effect of the traction by the
extractor rolls 8 external to the tank. The desired
thickness is then obtained by the combined action of
the traction by the extractor rolls 8 and of the
knurled top rolls 9, which are generally made of steel
and slightly oblique relative to the perpendicular to
the direction of advance of the ribbon. These top rolls
are connected via a shaft 10 to motors 11, which
generally drive them at speeds which differ depending
on their position and increase on going toward the
downstream end. These rolls apply, to the edges of the
glass ribbon being formed, forces that prevent the
glass ribbon from necking. The glass ribbon then
undergoes a drawing operation in the zone with these
top rolls. The glass ribbon is then brought to the
desired thickness by drawing due to the extractor
rolls.
Next, the ribbon passes into the lehr, in order for the
glass to be cooled very steadily and uniformly. The
ribbon is then cut into panels, conventionally by
transverse and longitudinal cutting.
The invention also relates to the use of the flat glass
according to the invention for protection against
X-rays.

WE CLAIM ;
1. A process for manufacturing flat glass rich in lead oxide, comprising
the continuous floating, in a float plant with a neutral gaseous
atmosphere, of a glass comprising at least 30% lead oxide by weight on a
bath of molten metal having a higher density than that of the glass.
2. The process as claimed in the preceding claim, wherein the neutral
gaseous atmosphere contains less than 5 ppmv oxygen.
3. The process as claimed in one of the preceding claims, wherein the
neutral gaseous atmosphere contains essentially nitrogen.
4. The process as claimed in one of the preceding claims, wherein the
temperature of the bath of molten metal is lower than the temperature of
a bath of molten metal in a float plant for a soda-lime-silica glass
containing no lead.

5. The process as claimed in the preceding claim, wherein the
temperature of the float glass is between 500 and 800°C.
6. The process as claimed in one of the preceding claims, wherein a
molten metal treatment station is associated with said bath.
7. The process as claimed in one of the preceding claims, wherein the
glass comprises at least 45% lead oxide by weight.
8. The process as claimed in the preceding claim, wherein the glass
comprises at least 60% lead oxide by weight.
9. The process as claimed in one of the preceding claims, wherein the
glass has a density ranging from 4 to 6.
10.The process as claimed in the preceding claim, wherein the glass has
a density ranging from 4.3 to 5.5.

11. The process as claimed in one of the preceding claims, wherein before
the float plant, the glass is melted in a furnace that includes at least one
submerged burner.
12. The process as claimed in the preceding claim, wherein the furnace
comprises at least two tanks in series, the second tank being fed with
lead oxide.
13. The process as claimed in the preceding claim, wherein the first tank
is equipped with at least one submerged burner and is fed with the batch
materials other than lead oxide.
14. The process as claimed in either of the two preceding claims, wherein
the second tank is at a lower temperature than the first tank.
15. A flat glass comprising at least 30% lead oxide PbO by weight,
enriched on one face with tin.

16. The glass as claimed in the preceding claim, wherein it comprises at
least 60% lead oxide by weight.
17. Process for protecting against X-rays, wherein the glass of one of the
preceding glass claims is incorporated into partitions or windows or
handling boxes.


The invention relates to a process for manufacturing
flat glass rich in lead oxide, comprising the
continuous floating, in a float plant with a neutral
gaseous atmosphere, of a glass comprising at least 30%
lead oxide by weight on a bath of molten metal having a
higher density than that of the glass. The invention
allows flat glass rich in lead, useful for protection
against X-rays, to be produced.

Documents:

01976-kolnp-2006-abstract.pdf

01976-kolnp-2006-asignment.pdf

01976-kolnp-2006-claims.pdf

01976-kolnp-2006-correspondence other.pdf

01976-kolnp-2006-correspondence others-1.1.pdf

01976-kolnp-2006-description (complete).pdf

01976-kolnp-2006-drawings.pdf

01976-kolnp-2006-form-1.pdf

01976-kolnp-2006-form-2.pdf

01976-kolnp-2006-form-3.pdf

01976-kolnp-2006-form-5.pdf

01976-kolnp-2006-international publication.pdf

01976-kolnp-2006-international search report.pdf

01976-kolnp-2006-pct form.pdf

01976-kolnp-2006-priority document.pdf

1976-KOLNP-2006-ABSTRACT 1.1.pdf

1976-KOLNP-2006-CLAIMS 1.1.pdf

1976-KOLNP-2006-CORRESPONDENCE.pdf

1976-kolnp-2006-correspondence1.1.pdf

1976-KOLNP-2006-DESCRIPTION (COMPLETE) 1.1.pdf

1976-KOLNP-2006-DRAWINGS 1.1.pdf

1976-kolnp-2006-examination report.pdf

1976-KOLNP-2006-FORM 1.1.1.pdf

1976-kolnp-2006-form 18.pdf

1976-KOLNP-2006-FORM 2.1.1.pdf

1976-KOLNP-2006-FORM 3.1.1.pdf

1976-kolnp-2006-form 3.pdf

1976-KOLNP-2006-FORM 5.1.1.pdf

1976-kolnp-2006-form 5.pdf

1976-KOLNP-2006-FORM-27.pdf

1976-kolnp-2006-gpa.pdf

1976-kolnp-2006-granted-abstract.pdf

1976-kolnp-2006-granted-claims.pdf

1976-kolnp-2006-granted-description (complete).pdf

1976-kolnp-2006-granted-drawings.pdf

1976-kolnp-2006-granted-form 1.pdf

1976-kolnp-2006-granted-form 2.pdf

1976-kolnp-2006-granted-specification.pdf

1976-KOLNP-2006-OTHERS.pdf

1976-kolnp-2006-others1.1.pdf

1976-KOLNP-2006-PETITION UNDER RULE 137.pdf

1976-kolnp-2006-reply to examination report.pdf

1976-kolnp-2006-translated copy of priority document.pdf

abstract-01976-kolnp-2006.jpg


Patent Number 246768
Indian Patent Application Number 1976/KOLNP/2006
PG Journal Number 11/2011
Publication Date 18-Mar-2011
Grant Date 15-Mar-2011
Date of Filing 14-Jul-2006
Name of Patentee SAINT-GOBAIN GLASS FRANCE
Applicant Address MIROIRS, 18 AVENUE D'ALSACE, F-92400 COURBEVOIE
Inventors:
# Inventor's Name Inventor's Address
1 RODRIGUEZ CAURTAS, RAMON SAINT-GOBAIN GLASS ESPANA AVDA DE LUGO, 100-APARTADO 88, 33400 AVILES, (ASTURIAS)
2 FERNANDEZ SUAREZ, JUAN-LUIS SAINT-GOBAIN GLASS ESPANA AVDA DE LUGO, 100 APARTADO 88, 33400 AVILES (ASTURIAS)
3 BOUIGEON, CHRISTIAN SOVIS OPTIQUE LIEU-DIT "LE GOUFFRE" 77640 JOUARRE
PCT International Classification Number C03B 18/20
PCT International Application Number PCT/FR2005/050094
PCT International Filing date 2005-02-14
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 0401499 2004-02-16 France