Note: Descriptions are shown in the official language in which they were submitted.
10~;0658
The present invention relates to glass melting and more par-
ticularly to a method of refining mol-ten glass. The inven-tion
also relates to improved apparatus for use in refining molten
glass.
In a known method of manufacturing glass in a continuous pro-
cess, raw materials are fed in at one end of a glass melting tank
to form a blanket floating on an existing bath of molten glass.
The rate of feeding is sufficient to maintain a constant glass
depth in the tank whilst molten glass progressively flows towards
10 the opposite end of the tank known as the working end, from which -
molten glass is taken away for use in a forming process. The blan~
ket of raw materials is converted to molten glass as it passes
through a melting zone at one end of the tank by heat which may
come for example from burning fuel supplied from burners situated
at spaced intervals in the side walls above the glass level or
from electrical heating devices. The molten glass passes from
the melting zone into a refining zone where heat is also applied
above the molten glass. In the refining zone bubbles of gas still
remaining in the glass are encouraged to escape or go into solution
in the glass. The glass passes from the refining zone into a con-
ditioning zone adjacent the working end of the tank. In the con-
ditioning zone the glass is homogenised and brought to a suitable
thermal condition for use in the forming process. Normally a canal
leads from the working end of the tank to a forming process.
To obtain glass free from gas bubbles and refractory corrosion
.
products and to extend the life of the glass melting tank it is
desirable to ensure that the glass in contact with the refractory
. .
is sufficiently cold. This is in part achieved by the convection
flows set up in the molten glass within the tank, the flows causing
glass to return from the colder to hotter areas in the lower sec-
tions of the tank. This can involve four or more times as much
glass returning in the return flow within the tank as is being
supplied to the forming process in the upper forward moving layers
~ .
-- 1 -- 'F
~ 0~6)658
of molten glass. In this way additional heat is required to bring
the return flowing glass to the temperature required in the forward
flow.
It is an object of the present inven-tion to provide an impro-
ved method of refining glass in which the ratio of forward flow to
return flow in the refining zone is much larger than normal.
The present invention provides a method of refining molten
glass in a refining zone of a tank containing molten glass, comp-
rising supporting molten glass on a heat conducting support mate-
rial forming the base of the refining zone, heating the upperregions of the molten glass so that the temperature of the upper
region is sufficient to effect refining, removing heat from the
lower region of the molten glass through said suppor-t material
forming the base of the refining zone to minimise undesirable in-
teraction with the support material, causing the upper regions of
the glass to flow forwardly through the refining zone from an inlet
to the zone towards an outlet from the zone with a small return
flow in the lower regions of the glass, controlling the depth of
glass in the zone, the length of the zone and the temperature dif-
~20 ference between the glass at the inlet and outlet of the zone sothat the ratio of return flow to forward flow is between 1/6th and
1/2.
In the case of normal flat glass compositions the temperature
of the upper region of the glass may be at at least 1460C and by
,
use of the present invention the quantity of glass melt refined
per unit area at that tempèrature is increased owing to the large
.
proportion of forward flow through the refining zone. Furthermore,
the glass leaves the refining zone having spent a controlled time
being refined and as the quantity of glass circulating is less, the
quality of the glass melt is improved as the quantity of glass com-
ing into contact with refractory material in the refining zone is
considerably reduced. This also means that a reduction in the re-
fining area may be made e.g. by a reduction in the width of the
- - 2 -
.... ~ . ~ ~ . ... . . ..
~ 1060658
-
known refiner, or an increase in output could be accommodated using
the same tank.
It is preferable to maintain the return flow in the refining
zone out of contact with refractory material at the base of the
refining æone and preferably the molten glass is supported on a
layer of molten metal at the base of the refining zone. Preferably
the molten metal comprises tin. Other metals and alloys including
alloys of tin can be used.
Although the ratio of return flow to forward flow may be
varied, the preferred ratio is 1/4.
Preferably heat is extracted from the bottom of the refining
; zone by circulating cooling air below the refining zone so as to
maintain the molten metal where necessary at a temperature where
it will not react with the glass. -
The invention also provides a glass melting tank having a
melting zone at one end of the tank into which glass forming mate-
rials are fed, a conditioning zone in which molten glass is brought
to a desired thermal condition prior to discharge from the tank, -
.,
a refining zone between the melting and conditioning zones, and
~ 20 means for applying heat to the upper surface of the glass in the
7 melting and refining zones, the refining zone having a base and
bein~ pro~ided with a layer of molten tin at its base on which
the molten. ~lass is supported, and cooling means located below the
, b~se of the tank adjacent the refining zone for removing heat from
the lower region of molten glass in the refining zone.
Preferably refractory barriers are located at opposite ends
of the refining zone, the barriers upstanding from the base of the
refining zone so as to contain the molten tin solely within the
refining zone.
Conveniently the cooling means comprises means for circulating
cooling air below the refining zone.
The time necessary for satisfactory refining is to some extent
dependent on the temperature at which refining is carried out. In
.
B - 3 ~
.. ~ . . .
~L060~;58
the case of soda-lime-silica glass it is desirable to use a refin-
ing temperature of at least ~460C. ~hen using a refining tempera-
ture of 1460C and above for the flow of glass leaving the refining
zone, i.e. the load flow for the conditioning zone, one may need
to maintain a minimum temperature differential between the surface
of the glass melt and the interface between the glass melt and the
molten metal of substantially 200C. It is only the load flow of
glass which will eventually enter the forming process which needs
to reach the refining temperature, in this case 1460C. This
offers the facility to control the molten glass/molten tin inter-
face temperature at a value, dependent upon the chemical properties
of the glass, at which the risk of significant interaction bet-
ween the glass and metal is minimised. This temperature differen-
tial is achieved by providing cooling means below the refining zone
to remove heat at the rates of the order of ~0 Kws/sq.m.
The glass return flow within the refining zone recirculates
at a relatively high temperature compared with previous proposals
, ~ ,.
and this reduces the amount of reheating required when the glass
again moves forward with the forward flow.
The amount of heat required in the refining operation in
accordance with the present invention is reduced due to the reduc-
tion of quantlty of glass in the return flow. The overall thermal
efficiency is improved when the temperature differential between
the surface and the support material is minimised. The heat which
is extracted from the base sf the refining zone can be reused, for
example by use in the regenerator chambers or for preheating batch
material fed to the tank.
..
Preferably the melting zone of the tank is arranged to hold
molten glass at a greater depth than the refining zone. In this
way greater recirculation of molten glass may occur within the
melting zone. It is also preferred that the melting end is pro-
vided with a filling pocket into which batch material is fed.
The raw materials may be fed in pellet form and may be pre-
:
.... . . . . . ~ . - .
1060658
heated by hot gas. The pellets may be heated to about 700C and
fed onto the surface of the existing batch of molten glass by con-
ventional means.
Preferably the conditioning zone is shallower than the refin-
ing zone and is preferably arranged so that the molten glass flow
through the conditioning zone is entirely in the forward direction.
The present i~vention is also applicable to refining molten
glass in an independent refining unit separate from the glass
melting tank. In this case molten glass should be fed to the
refiner in a substantially stone-free condition. The use of a
refining unit independent of the melting unit means that it is
possible to arrange the supply of molten glass from more than one
source, thus enabling problems of operation and life arising in
the melting unit to be solved without necessarily affecting the , ,
entire production. In the case of a conventional tank where melt-
ing zone, refining zone and conditioning zone are inter-related, a
problem in one zone cannot be separated from the overall operation
of the tank.
One embodiment of the invention will now be described by way
of example and with reference to the accompanying drawing which
shows a longitudinal section through a glass melting tank in accor-
dance with the present invention. In this example a tank 11 has a
melting zone 12, a refining zone 13 and a conditioning zone 14.
The melting zone 12 has a filling pocket 15 at the melting end of
the tank. ~he conditioning zone 14 leads to the working end of the ~ '
tank from which conditioned glass is discharged to a canal 9 lead-
ing to a forming process. The tank is formed of refractory mate- ,, '-
rial and as is shown in the drawing, the base of the tank is step- ~ ' '
ped so that the melting zone provides the greatest depth of glass. ~ '
The base of the tank has an upward step 16 at the downstream end
of the meltlng zone so that the refining zone 13 is shallower than ''
the melting æone. Similarly the downstream end of the re~ining
zone has,an upward step 17 so that the conditioning zone 14 is
- 5 -
'' .
1060658
shallower than the refining zone. The base of the refining zone
13 is formed of a high heat conducting refractory 18 supporting a
layer of molten tin 19 between the step 17 and an upstanding bar-
rier 20 adjacent the inlet to the refining zone. Below the base
18 of the refining zone 13 is an air circulating system for cir-
culating cooling air. This comprises a series of elongated aper-
tures 21 between refractory supports 22. The supports 22 are
mounted on a further refractory base 23.
In operation, glass forming material is fed into the pocket
15 in a conventional manner to form a blanket 24 lying on top of
the molten glass 25. Heat is applied to the melting zone by gas
burners mounted above the surface of the molten glass within the
melting zone and operating through ports 26. The glass circulates,
as shown by the arrows, in the melting zone so that there is a
substantial return flow as well as forward flow. The molten glass
progressively moves along the tank to the refining zone 13 and the
conditions of operation in the refining zone are arranged such that
the forward ~low through the refining zone is substantially greater
than the return flow, as is indicated by the arrows in the refining
zone. The conditioning zone is shallow and is operated so that all
the glass in the conditioning zone 14 moves in the forward direc-
tion towards the working end of the tank, with no return flow.
The temperature of the upper region of the glass in the re- :
fining zone is at about 1460C in order to carry out satisfactory
refining. To prevent interaction between the molten glass and the
tin in the refining zone, heat is extracted from the bottom of the
refining zone in order to lower the temperature of the glass in
contact with the molten tin. The temperature of the glass adjacent -
the tin can be of the order of 200C below the temperature of the
upper surface of the glass in the refining zone.
The cooling air which is circulated through the passages 21
below the refining zone extracts heat through the high conductivity
refractory 18 and molten tin 19 and the extracted heat may be re-
- 6 -
B :
.. . . . . . . .
1060658
u~ 1 by passing the cooling air to heat regenerators (not shown).
For a particular load, the ratio of forward to return flowwithin the ref`ining zone 13 is dependent on the temperature differ-
ential between the glass entering the refining zone at its inlet
and leaving the refining zone at its junction with the conditioning
zone as well as the depth of glass within the refining zone and the
length of the refining zone (that is the distance between the barr-
ier 20 and the step 17). In this particular example the temperature
difference between the inlet and outlet of the refining zone, the
depth of the refining zone and the length of the refining zone is
adjusted such that the ratio of return flow to forward flow is be-
tween l/6th and 1/2. Preferably the ratio is 1/4 as this provides
maximum thermal efficiency. The choice of these conditions to
attain a particular return flow is dependent on the original design
of the tank. Such a design is best determined by a practical model
work or by use of a theoretical computer model of the variables in
the type of tank envisaged. In the practical operation of the tank,
the final adjustment to control the return flow can be achieved by
varying the temperature difference between the inlet and outlet
temperatures of the molten glass, and less desirably the glass depth
by varying the tin depth in the refining zone. All the other vari-
ables are fixed by the design of the tank. The glass depth is -
normally of the order of two thirds of a metre and the length is
as indicated above a function of the residence time needed for re- ~ -
fining with a particular glass and the rate of flow of glass through
the system. These factors are all determinable by the man practis-
ed in the art on the basis of his knowledge of the proposed operat-
:::
ing load and the operating conditions of the tank. ;~
Although in the above example the base of the refining zone
is provided by molten tin, it may be possible in some cases to use
other molten metals, including alloys, or even other refractory -
material provided it is capable of avoiding excessive interaction
with the molten glass at the return flow temperature used in the
.
, . . :. :. ` ' ' .
.
106()658
refining zone.
The invention is not limited -to the details of the foregoing
example. For instance, the refining zone could be provided in a
separate refining unit.
Although the example shown has one conditioning zone supplied
by the refining and melting regions 13 and 12, it may be desirable
in some cases to provide two or more conditioning canals which
operate in parallel and are ~ed from a single melting or refining
unit. It may in some cases be desirable when all three zones form
a common unit to provide between each zone in the space above the
glass a shadow wall or a barrier made of a platinum sheet to re-
duce or prevent free flow of the atmosphere from one space to the
next.
- 8 -
