CA1085168A - Apparatus for the manufacture of glass - Google Patents

Abstract

METHOD AND APPARATUS FOR THE MANUFACTURE OF GLASS

ABSTRACT
This invention relates to a process and an apparatus for the manufacture of fused glass, suitable for molding, wherein the total time of manufacture is reduced to about one hour. The process accelerates the homogenization and refining of glass by eliminating unfused particles and gas bubbles which are the main factors limiting the production rate of industrial glass. This is accomplished by increasing the temperature of the molten glass to produce foaming throughout its mass while maintaining its viscosity below 1,000 poises. This divisional application is specifically directed to an improvement in all apparatus for rapidly refining glass of the type having an elongated continuous channel in which a molten vitreous mass is introduced at one end thereof, flowed horizontally therethrough and subjected to an expansion of at least 50% of its initial molten volume by foaming the molten mass throughout its entire volume as the mass moves along the channel from its one end to the other end, said improvement comprising heating means disposed longitudinally along said channel and in said foaming zone below the surface of said vitreous mass for rapidly heating the flowing vitreous mass to cause its entire volume to pass totally to the foam state and expand by at least 50% of its initial volume.

Description

10~

SU~RY Ol~ T~IE: INV~;,MTION
-The vitreous material undergoing trea-tment is first melted ~o form a molten mass having a viscosity below about 1000 poises. The molten material is then foamed throughout its mass. This foaming results in an expansion of the molten vitreous material, by volume, of at least about 1.5 and pre-ferably between about 2 and 3. The foamed material is permit-ted to subside while maintaining its viscosity below 1000 poises.
BRIEF DESCRIPTION OF TEIE DRAWINGS
Figure 1 is a schematic view, partially in longi-tudinal section, of the entire installation;
Figure 2 is a cross section along line II-II of Figure 1;
Figure 3 is a top view oE a variation oE the reEining channel;
Figure 4 is a longitudinal section along line IV-IV
of Figure 3.
DETAILED DESCRIPTION
The vitrifiable mixtures of raw materials which can be employed in the process of the present invention are of the type commonly used in the manufacture of glass. Examples of a numb~r oE these mix-tures appear in Table II.
The present invention requires that the molten material be foamed throughout its mass. To initiate the in-tense and complete foaming required a number of steps may be ; taken. For example, foaming agents can be incorporated in-to the raw materials. The foaming agents give rise, in the temp-erature range, corresponding to the desired viscosities, to the formation of gas bubbles inside the glass.
It is also recommended that a refining agent be present, at least in the final phase, for the gases produced wg/J, :
.. . .

~L0~35~8 by these refining agen~s are soluble in glass, and their sol-ubility in the molten glass increases as its temperature decreases. Thus, af~er the elimination of most of the gases, the refining agents aid in the readsorption of the bubbles which remain on cooling.
The foaming agents are selected such that they do not induce foaming of the vitreous material until that material has reached a desired temperature, which temperature is main-tained in the refining channel. The following foaming agents are useful in the process according to the present invention :
arsenic compounds, such as arsenic trioxide ; antimony compounds such as antimony trioxide ; sulfur compounds, such as sodium sulfate ; and halogen salts such as potassium chloride. Other agents use~ul in the process will be apparen~ to those skilled in the art.
Another method of ensuring the thorough foaming of the molten mass is to subject the batch to rapid uniform heating during the foaming operation of about 20C per minute or more.
Such heating can be obtained in various ways, possibly combined, capable of acting within the batch, for example, submerged burners, submerged resistors, direct Joule effect or high-frequency induction. If desired, this foaming can be initiated or reinforced by mechanical actionusing an ultrasonic generator.
In a discontinous melting installation, these heating means are employed at a time when the vitreous batch contains a large number of solid or gaseous nuclei and a sufficient amount of foaming agents to ensure an expansion of at least 1.5, and preferably above 2 times the normal volume of the mass in the unfoamed molten state.
In a continuous melting installation similax heating means can be employed. The predefined time sequence corresponds to the rate of treatment of the vitroous massO

wg/ ! ~

10~
To aid the foaming process, it is also recommended that the vitreous mass con~ains a large number of nuclei, such as unmelted particles or small gas bubbles, capable of inducing the foaming. When obtaining it through direct melting of raw materials, the nuclei should be distributed throughout the mol-ten mass at a concentration of at least 10 visible nuclei per cc. Furthermore, it is desirable that the raw materials be agglomerated or sintered. The sintering makes it possible to preheat the materials before actual meltin~. This melting is accomplished by a brief and intense heat transfer (less than 10 minutes) while simultaneously keeping the temperature of the materials below the foaming temperature. This permits the maintenance of a high number of nuclei consisting of unrnelted particles and gas bubbles in the vitreous mass introduced into th¢ ~otal foaming stage. The rapid melting of the sintered raw mat~rials can be accomplished in various ways, for example, by subjecting these materials to hok gases at a controlled temperature, which gases are driven at high speed and have a large exchange capacity. The granules can be introduced dir-ectly into the stream of the gas. The raw materials can takeany number of forms, for examp]e, granules, balls, pellets or strips. The thickness of the layer of raw materials can also vary and can be the size of smallest oE the sintered materials undergoing melting.
To assure the presence oE suEficient nuclei, outside nuclei, for example, cullet or colored cullet can be added to the raw materials. In relation to the usual glass refining processes, it is important to note that the present invention, requiring the presence of gas producing agents and foamlng nuclei, can employ unrefined vitreous materials. It has been discovered that 1 to 2 mm grains originating from the limestone and dolomite in the material introduced in the refining tank, wg/~ t, ~LO~5~
are totall~ dkJcstecl at the end of the total ~oaming phase.
The process according to the invention is thereforo not depend-ent on the u5e of a vitreous hatch of hiyh quality.
In continous manufacturinq installations it is important to avoid upstream currents or currents which exist downstream of the direction of flow of the glass throuyh the refining vesselO For example, currents of thermal origin often exist or are even deliberately created in the usual melting furnace. The currents tend, in the process according to pre- -~
sent invention, to mix glasses in different stages of produc-tion~ These undesirable currents may be eliminated by usiny baffles, dams, bottlenecks or cascades stationed alony the course followed by the vitreous mass undergoing treatment.
It is advantagcous for the wid-th of the channel in which the molten stream Elows to be n~rrow in relation to its lenyth, the ratio between the two being about 1:5 or less.
Another parameter that also affects the product is the thick-ness of the stream of flowing glass. In the example given below the height of the glass in the channel varies from ~ to 7 cm. In larger installations a height of 10 to 20`cm or more can be used provided the height of -the channel walls is suf-ficient to ensure total expansion ancl damaging currents are avoided.
In order to increase the maximum velocity of the gases in relation to the materials being heated, the materials should be maintained in a slow moving thin layer. In practice, this is obtained by directing the flow of the hot gases in a direction approximately perpendicular to the inclined surface on which the granules fall. A layer of granules is easily fixed on that surface and within a few minutes becomes a vit~
reous batch ready to undergo total foaming. The sur~ace on which the thin-layer melting is accomplished can be the inner w~/~ O

1(~8S~
wall of a cyclone furnace, a rotary drum combined with a scra-per to remove the vitreous batch or the inclined surace on which the vitreous batch flows whilc being formed. The rate of flow can be regulated by the surface's slope, by the temp-erature which affects the viscosity of the batch.and, con-sequently, the adhesion of the granules to that surface, or by the direction and/or concentration of the gas jets. The exam-ple below describes both the process and the device of the present invention.
The installation represented in Figure 1 comprises a channel 1 in which the molten vitreous material circulates from right to left while undergoing foaming. The refi.ning channel is also shown in Figure 2. Channel 1 is formed from a .7 mm thick sheet of 10% rhodium-alloyed platinum. Its leng~h is 1.5 m. Both the width and the dep~h are 15 cm. ~t both ends, the channel con~ains connections 2 supplying it with electric current delivered by alternating current generator 3, the voltage of which is adjustable from 0 to 10 V for a power of up to 25 kVA (2500 A maximum). Connections 2 are rhodium-alloyed platinum plates 10 mm thick, 20 cm long and 10 cm high. They are held between two copper jaws 4, cooled by water circulation (not shown) and to which are attached current lead-ins 5. At its lower end, the channel contains a draw pipe 6.
The draw pipe is welded to the bottom of the channel and heated by a rhodium-alloyed platinum resistor 7 wound on an insulating tube surrounding pipe 6. A coc~ 8 containing a rhodium-alloyed ~.
platinum needle valve allows for the gradual closing of pipe 6.
Above the drawing hole, the channel is provided with a rhodium-plated platinum dam 9 wllicll is welded to the walls of the chan- . .
nel and leaves a free passage 9a only 20 mm high at the ~ottom of the channel. The molten material flows under dam 9 before exiting through draw pipe 6. At the opposite end of the channel W9~. n .

1085~
plungin~ resistor 10 is providccl. The resistor consists of a U-shapcd rhodium-alloyed platinum pl~te 0.7 mm thic~ and 20 cm lon~. Resistor 10 corresponds to the shape of the interior section of channel 1. The lower part of plunging resistor 10 is drillcd with evenly distributed holes, the dimensions of which are designed to reduce by approximately 25% the area available for passage o~ electric current. The purpose of this is to localize the dissipation of electric power and to improve the stirring of the vitreous mass in the course of foaming. Plunging resistor 10 is supplied with electric current by alternating current generator 11 (Figure 2) wi~h adjust-able voltage from 2 to 3 V and a power of 5 kVA. Refining channel 1 is completely surrounded by heat insu]ation cover 12-12a consisting of alumina bric~s lines with unsealed insul-ating bricks. By the con-trolled rcmoval of insulation, one is able to determine th~ temperature curve of th~ material along the channel.
The refining channel is fed at its upper end with a vitreous batch formed in melting furnace 13 by means of junc-tion 14 containing inclined hearth 15. Hearth 16 of melting furnace 13 is also inclined. Steel pipes 17 cross hearths 15 and 16 perpendicular to the plane of symmetry of the system.
In order to regulate the temperature of the hearths cooling fluids are passed through these pipes. ~rches 18 and 19 of junction 14 and furnace 13 respectively are also covered with insulating bric~s. Furnace 13 and junction 14 are heated, on one side, by burners ~0 which cross the arçh and are dir-ected perpendicular to the hearths to which they correspond.
On the other side, they are heated by burners 21 crossing the base of stack 22 of the furnace and stationed so that their flames converge in the area of hearth 16 where the granular material is introduced. These burners are of the type commonly w9/~ ' ~08$~
called "intcnsive," i.e., the rate of ejection of the gases is greater than the rate of fuel combustion. The fl~me is caught in the combustion chamber created in the arch. These burners can be fed with a mixture of propane, air and/or oxygen from a mixer ~not shown) with a capacity of 600,000 calories per hour. The flames escape through stack 22 crossing heat ex-changer 23 in which gravity causes the pre-sintered vitrifiable mixture to flow backward. The gases exhausted in heat exchang-er 23 as well as those coming directly from stack 22 (through bypass 2~) enter dust-separating cyclone 25. The circulation and discharge of the gases are assured by fan 26. Heat exchanger 23 is made of refractory steel and contains a double wall in which is placed a powdery heat-insu]ating material such as kieselguhr. The introduction into the furnace of vitrifiable raw materials, sintered and preheated in exellanger 23, is assured by clistributing drum 27. The rate oE rotation of drum 27 regulates the feed to the furnace.
; In the melting operation the vitrifiable raw material used is a material sintered in an extrusion press which sup-plies compacted bars 7 mm in diameter. ~ suitable composition of the vitrifiable materials for producing 90 kg of glass is:
Sand (250 ~m) 60 kg Limestone (100 ~m) 8.5 kg Dolomite ~1 mm 1~.5 kg Feldspar (500 ~m) 5.5 kg Dense sodium carbonate 6.8 kg Caustic soda with 50~ NaOH20.2 kg Fine sodium sulfate 0.9 kg The granules can-be dried in a ventilated electric oven at 250C, and stored away from moisture without other precautions.

ws/J ~

:lL085~
Exchanger 23 is fed at the top with cold granules which are progressively heated to a temperature r~nging between 500 and 600C at distributing drum 27. Simultaneously, the gases ~ntering the exchanger at 750C are mixed with cold air admitted through hole 28 and are sucked toward cyclone 25 at a temperature of about 200C. The granules deli~ered by distributor 27 fall directly on hearth 16 in the zone of cov-ergence of burners 21. They are rapidly converted into a vit-reous mass which flows over hearth 16 at an average rate of 10 cm per minute. Upon arrival at llearth 15, the temperature of the batch is 1300C. Hearth 15 -transfers the ma-terial very rapidly, due to its steeper slope and without notable heating, to the inlet of refining channel 1. Corrosion oE hearths 15 and 16 is rendered negligible by limiting the temperature of their surEace to approximately 800C. This is accomplished by the cooling fluid in pipcs 17. The temperature in the arches of these regions, however, is about 1450C.
On falling into refining channel 1, the material is subjected to rapid heating by contract with the bottom and side walls of the channel and with submerged resistor 10, the temperature of which is maintained at about 1530C. For a flow of 52 kg of glass per hour, the electric power dissipated is 28 k~A in the channel proper and ~ kVA in the submeryed resistor. Due to the intense heating of the glass, upon cros-sing the submerged resistor 10, a swelling of the mass occurs so that the thic~ness of the batch about 4 cm abo~e the sub-merged resistor is 13 to 14 cm.
A probe inserted at the bottom of the channel, immediately below submerged resistor 10, shows that the vitreous mass has passed totally to the foam stateO At a temperature o about 1520C downstream of resistor 10, a constant rate of swelling by foaming is obtained over appro~imately a 1 m W9~J ~. !

1(~85~
length. This corresponds to a sojourn of about 15 minutes.
Over~the ncxt 10 to 15 cm, the foam subsides very rapidly and the vitreous mass becomes per~ectly refined glass at dam 9, where the temperature is no more than about 1~50C. l`he refined glass which has passed under dam 9 i5 drawn off through pipe 6. The level of the material in the channel is kept constant by regulating its delivery through pipe 6 using cock 8.
In the example just ~escribed, from the time a pre-heated granule falls on hearth 16 of the melting furnace and the time when the refined glass corresponding to that granule is drawn off through pipe 6`only 30 minutes el~pses. The device is capable, without changing its dimensions, of sup-plying yreater flows of refined glass, for exampl~, 100 kg per hour, provided the rate o~ foamirlcJ is reduced. For an iden-t~ical vitrifiable composition, the quantity oE fine sodium sulfate introduced in the vitrifiable mixture is reduced to 0.7 kg per 100 kg of glass produced. Under those conditions, the initial height of the batch above resistor 10 is 7 cm, and expands to about 1~ cm for an expansion of 2. Regardless of the method employed (discontinuous or continuous), for a given increase in temperature and a given vltrifi~ble mixture, having an identical sodium sulfate content, the refining time remains constant.
Foaming of the vitreous batch throughout its mass, which constitutes the essential characteristic of this inven-tion, has never been heretofore proposed as making it possible to accelerate the process of melting, refining and homogeniza-tion of fused glass.
The following tables give examples of the manufacture of five glasses of common type by the process according to the invention. Parts are by weight unless otherwise indic:ated.

W~

il Ob~S16~
Table I furnishes an analysis of those glasses expressed in percentages by weight of oxides. The fusion described in the foregoing example was glass No. 1.
Table II furnislles the composition by weight of five vitrifiable mixtures suitable for manufacture of the glass in question.
Table III indicates the characteristics of the pro-cess as applied to the five glasses.

ws~J~?

~!L0~35~
T~BLE I
COMPOSITION OF T~IE GL~SSES

_ No of Glasses Oxides 1 2 3 4 5 SiO270.7 73.7 29.5 56.0 63.0 A123 1.3 1.2 2.4 0.05 2.95 Fe23 0.25 CaQ 10.3 0.5 0.17 0.05 7.35 MgO 3.3 0.25 3.1 BaO 0.15 2.5 Na2014.0 4.8 ~.45 4.2 14.1 K20 0.3 2.55 11.0 0.8 PbO 48.9 27.4 B203 17.3 16.S5 5.9 Sb23 Q-7 A52~3 0.7 TABLE II
VITRIFIABLE MIXTURES

No. of Glasses com~onents 1 2 3 4 5 Sand 67.0 72.2 26.65 56.3 56.1 Limestone9.47 Dolomite16.2 1.45 13.6 Feldspar6.13 Phonolite 12.4-Kaolin 3.2 6.35 Sodium carbonate 7.58 1.5 6.65 19.65 Potassium carbonate 2.35 16.15 Barium carbonate 0.2 3.25 .),, , ;

~)851~

Lead oxide (PbO) 49.0 28.0 Boric aci.d 12.7 30.0 Borax 15.65 ' Rasorite 5.6 Calcined colemanite 8.55 50% Caustic soda22.5 Sodium sulfate 1.0 1.3 Sodium ni~rate 0.5 1.. 5 1.0 Potassium chloride 1.5 Antimony trioxide . 1.0 Arsenic trioxide 2.0 -TABLE III
CHAR~CTE~IST:tCS OF TR~AI'MEN'l' No. of Glasses 1 2 3 4 5 Preliminary melting temperature (C)1350 1400 1050 1250 1300 Rate of expansion heating (C/min) 25 25 30 35 ~ 25 Expansion starting temperature (C)1400 1450 1100 1300 1430 Expansion 3 2-3 2-3 2-3 2-3 Time of expansion until clarification (in minutes) 10 15 8 5 4 Clarification temp-erature (C) 1520 1550 1260 1480 1480 Figures 3 and 4 describe an alternate device,having a refining crucible in which the glass is heatecl by direct Joule effect. This device is not useful in the manufacture of the lead glasses of Examples 3 and 4, but is more economical than the previous method, because it uses molybdenum electrodes.

w9/!(~

The crucible consist~ of a channel of refractory material 30, the interior rectangular cross section of which is about 25 s~uare centimeters. Its length is about 2 meters. The lower part contains a narrow funnel type portion 31 about 5 cen timeters above hearth 32 and reducing the width to a few centi-meters in order to conduct the glass to outlet 33 while avoiding any blind angles likely to create stagnation.
The hearth and wall of channel 31 as well as its arch (not shown) are of a material commonly employed in conventional glass melting furnaces, an alumina and zircon-base electrofused material. Cover 3~ consisting of briclcs of a light refractory material provides heat insulation. The heating of the glass pas-sing through the channel and the regulation of its temperature are assured by six pairs oE electrocles El to r!6 . 'l'hese elec-t~odes, distributed along the edcJas o the channcl, are mad~ of 3-eentime-ter plates and are arranged symmetrically in relation to the axis of the channel. They are distributed along the edges of this channel. Each pair of electrodes is connected to an independent adjustable electric power source. The current lead-ins of the electrodes horizontally cross the walls of the ehanneland make possible erosswise placement of the eleetrodes. The lead-ins are made of molybdenum.
The glass thiekness above outlet 33 is sufEieien-t to entirely submerge the electrodes and protect them from oxidation.
The eurrent lead-ins are protected by bathing their hot parts in a reducing atmosphere consisting, for example, town gas.
The glass has free passage around the electrodes along the hearth and side wallsO Passage of the eurrent from one electrode to the other produces active thermal conveetion whieh favors the erosswise homogeni~ation of the molten mass and eli-minates parasitic longitudinal eurrentsO The result approaches a uniform flow of glass called "piston" flow. ~ifferent w~ /~c, ~o~s~

temperaturc readings are tal:en at the points Tl to T7.
Table IV shows the characteristics of the electric power supply used in a refining operation simi]ar to that of the foreyoing example, i.e., in which the batch of glass, result-ing from preliminary melting of composition No. 1, is introducecl into the tank at point 1'1 at a temperature of about 1250 to 1300C and at a flow of approximately 50 kg/h.
TABL~ IV

Supply devices El E2 E3 E4 E5 E6 Rated characteristics Power (kVA) 20 20 6 6 6 6 Voltage (V) 80 80 60 60 60 60 Intensit~ (~) 250 250 100 100 100 100 Conclitions for a cl~livery of 50 kg/h (glass No. 1)~

Power supplied (kVA) 10 10 3 3 1 0 Temperature :
Measuring points T 11' 2 T 3 T 4 T 5 T 6 T 7 Values (C) 1300 20This application is a division of copending Canadian application Serial No. 233,203, filed August 14, 1975.

ws~

Claims (5)

THE EMBODIMENTS O THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The improvement in an apparatus for refining glass comprising:
(a) an elongated continuous flow refining channel in which a molten vitreous mass is introduced at one end thereof and flowed horizontally therethrough to an exit at its other end;
(b) a localized foaming zone extending along a predetermined length of said channel adjacent said one end thereof;
(e) first heating means disposed longitudinally along said channel in said foaming zone at a location spaced from the sides and bottom of said channel and below the surface of said vitreous mass for rapidly heating the flowing vitreous mass to cause its entire volume to pass totally to the foam state while in said foaming zone;
(d) second heating means disposed longitudinally along the remainder of the channel longitudinally of said foaming zone for heating the flowing vitreous mass; and (e) said first and second heating means being positioned relative to the sides and bottom of said channel and of a sufficient power to generate sufficient heat and produce active transverse thermal convection in said flowing vitreous mass along said channel preventing longitudinal currents tending to mix the vitreous mass of one area of said channel with that of another.

2. The improvement in the apparatus of claim 1 wherein:
(a) said first and second heating means comprise electrodes spaced from the sides and bottom of said channel and positioned below the surface of said vitreous material.

3. The improvement in the appratus of claim 2 wherein:
(a) the electrodes have a heating capacity sufficient to increase the temperature of the molten mass by at least 20°C per minute.

4. The improvement in an apparatus for refining glass comprising:
(a) an elongated continuous flow refining channel in which a molten vitreous mass is introduced at one end thereof and flowed horizontally therethrough to an exit at its other end;
(b) a localized foaming zone extending along a predetermined length of said channel adjacent said one end thereof;
(c) heating means disposed longitudinally along said channel and in said foaming zone below the surface of said vitreous mass for rapidly heating the flowing vitreous mass to cause its entire volume to pass totally to the foam state and expand by at least 50% of its initial volume.

5. The improvement in the apparatus of claim 4 wherein:
(a) the heating means includes a plurality of pairs of submerged electrodes horizontally positioned throughout the length of the channel in opposed relation along opposite sides of the channel and adjacent the bottom thereof.