AU450421B2 - Method for granulating a glass batch - Google Patents

Description

l,4 0 It is -known to replace the alkali cont ent1, which usually

consists of sodium carbonate, of a conventional glass batch, wholly

or partially by caustic soda in the form of soda lye, in order to

prevent the segregation of the batch and the formation of dust

in the interior of the furnace, and also in order to melt the

.glass batch ingredients more rapidly whilst reducing the fuel

consu.mption. Since sodium carbonate is only soluble to a slight extent in 501 liquid soda lye, this soda lye was used for moisten-

ing the batch and at the same time as a partial replacement for

sodium carbonate..

However, the comparatively high proportion of water

contained in a 501%caustic soda solution requires a very careful determining of the mixtures, in order on the one hand to achieve

a satisfactory wetting for purposes of dust prevention, and on

the other hand in order to avoid an undesirable viscosity. In- dustrially, it is important for it to be possible for the raw

materials which are necessary for the preparation of the batch,

to be supplied in an exact manner by automatic measuring-out

of the quantities, without it being necessary, when the quantities

are being determined, to have to take into account simultaneously

the trickling properties of the batch ingredients which are to be

granulated. The invention therefore arose from the problem of avoid-

ing the disadvantages which are involved by the use of caustic

soda solution, and of achieving a granulate of the glass batch

which is capable of trickling in a satisfactory manner, even when

different recipes for the raw material are used, so that no undes-

i.rablre viscosity occurs, and using sodium hydroxide which is avail-

able in large quantities as a by-product, particularly from

sodium chloride electrolysis.

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17. 240/70

It is known that the conventional glass batches cannot be granulated because of the high proportion of sand or' can only be granulated with the addition of calcium hydroxide (hydrate of lime). An undesirable reaction takes place between soda ash and calcium hydroxide, resulting in the formation of sodium hydroxide and calcium carbonate.* The necessary.addition of water causes the batch to become strongly hygroscopic, and the aqueous sodium hydroxide which is formd has a corrosive effect. The invention provides a method for granulating a glass batch, which uses a cauastic soda melt with only a very small proportion of water, as an at least partial replacement of the usual sodium oxide producing constituents. The caustic soda melt may be added as a melt or formed in situ from solid caustic soda and water vapour. Preferably the water content of the batch is fixed at less than suitably, at approximately 3% or less. This can be achieved by using, for example, a caustic soda melt, of, at most, 15% water content, e.g. with an added quantity of approx- imately 18% caustic soda to the entire glass batch. The caustic soda melt is suitably introduced into the glass batch at a temperature of over 1800, and is mixed therewith. The heat contained in the caustic soda melt is sufficient to pre- vent an immediate setting of the melt during the mixing and granu- lation. The solidification and hardening of the grains or pellets of the granulate takes place simultaneously with the cooling of the batch ingredients, without the need arising for

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17, 240no an expensive drying process. A particular advantage of the use of liquid molten caustic soda is that such a strong binding with the other constituents of the batch takes place that even com

paratively coarse glass sands can be safely processed to form soli

pellets. This is of particular importance, because coarse-

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.17. 240 no grained sands are usually rejected in the glass industry on

account of the long time which it requires to melt them down or fuse them.

However, the use of pure ca ustic soda, without the

addition of water, is not recommended, because in the case

of a low moisture content in the entire batch, such as approx-

imately 2% at a temperature below 300*Ct the caustic soda would

immediately harden on contact with the cold sand, and it would

not be possible to make a mixture. However, maintaining temp-

eratures above 300' is hardly worth attempting from the economic

point of view.

In one way of carrying out the invention, the caustic

soda melt is mixed int o the other batch constituents at a temp-

erature of over 180 0C, and the mixture is thereby granulated. The mixing of the batch constituents with the caustic soda

melt is suitably carried out in a mixer with an eccentrically

arranged tool system, at elevated temperatures, and the mixture

is granulated in the same machine, under cooling. A known mixing

machine of this type consists e.g. of a mixing disc rotating

about a central axis, and therein a system of tools rotating

in the opposite direction about an eccentric axis, e.g. mixing

blades, but preferably rake-liLke or huok-like tools. If the

heat introduced by the caustic soda melt is not sufficient to

prevent setting of the mass before the mixed product has been

transformed into granulate form, heating may also take place

during the homogenising process. During the subsequent cooling,

pellet formation then follows in the same machine. In another way of carrying out the invention, the batch

constituents are mixed with the hot caustic soda melt in a mixing

machine, and the mixture, while still hot, is granulated after 45 04 2

17,240 no

removal from the mixing machine. A counterflow mixer (ribbon

blender) is preferably used for mixing of the type just described

but the tool system is equipped with blades. The crumbly,

still warm mixed product may then be conveyed into a granu-

lating drum which is adapted to be driven at such a rotational

speed that the centrifugal force acting on the material in the

apex of the granulating drum is greater than its own actu-

ual weight. A tool arranged in drum draws the material away from

the centrifugal force and guides it in thedirection of the inter-

ior of thegranulating drum. Located in the direction or rota- tion of the drum, if desired, before the deflecting device, there may be a plulverising tool which acts upon the granulate. In another way of carrying out the invention, the g-ass batch, while still warm and after reaction with the NaOH melt, is taken from the mixer and is conveyed to a screen, from which the particles of the batch, which pass through, fall onto a cool surface, e.g. a cooling bath, where the particles of thepatch harden into irregularly shaped granules. -The mat- erial which has passed through the screen can be handled as a granulate without a fine portion. The screen used in this process is preferably a wide meshed oscillating screen of for example, 10 mm mesh width. The use of other forms of granulating apparatus also lies within the scope of the invention, such as for example an oblique-disc granulator, preferably with a high speed rotor. If the water content of the batch is adjusted to a comparatively low value of, e.g. 2 to with the use of a highly concentrated caustic soda melt, the mixture in the mixer a 450.421

17. 2 40 no

is precipitated exclusively in the form of fine grains whose grain size is below 5 mm, up to about 80% having a grain size of about 1.5 to 1 mm. Such a fine grains can be introduced into the melting furnace, however coarser grains in the form of granules or pellets are preferable. Experience has shown, however, that if a fine grained batch with a very low moisture content is rolled e.g. in a granulator, granules or pellets are not formed. However, it is possible togranulate such a fine grained batch allowing it, without rolling, to harden into flat structures such as e.g. sheets or flat cakes which are then broken. In this case,

the batch, while still hot, is compressed in a state of rest before the hardening. In the specification and claims "in a state of rest" means the avoidance of any kind of mutual motion or rol- ling of the particles. It is possible to carry out this compress- ion of a layer of the still hot mixture which has been emptied from the mixer during translational onwards movement on a conveyor belt. A pressure roller may be arranged above the conveyor belt, for compressing the still warm mixture. After cooling and setting, the layer then may be broken up into scales oqgrains, in accordance with whether it has the thickness of a sheet or of a flat cake. A compression of long duration or a high compression pressure leads to a product which has less tendency to crumble. The compression may also be carried out by other means such as vibrating, stamping or another pressing apparatus. In all cases, contact with the air should be avoided as much as possible during the cooling, setting, and granulating of the

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17,240/no

product because the product easily absorbs water, due to the caustic soda content. If desired, suitable air cooling may be achieved be leading atmospheric air over or through the caustic soda which is to be added to the glass batch. This tre- atment may be so arranged that the caustic soda absorbs the quantity of water which is desired for the addition, and the air is dried, so that it may be used as cooling air. The method according to the invention leads to a very stable easily trickling granulate which may be conveyed to the melt without the formation of dust. Despite thecaustic soda content, the granulate is, surprisingly, so slightly hygroscopic that, during the time which is necessary in industry, it can be stored in a silo or any other intermediate container, before being introduced into the melting furnace for the production of glass. Examples.

1. A batch comprising sand and powdered limestone was used for thetests. 60% 12 parts by weight of sand, and 20% 4 parts by weight of powdered limestone, including small constit- uents such as colouring agents and the like, were weighed, and the usually customary 20% 4 parts by weight of soda were re- placed by caustic soda. In the firsest, a 66% caustic soda solution was used, at a temperature of about 700 C. In place of the 20% soda proportion, 22% of a 66% caustic soda solution was introduced. After the addition of this solution at a temperature of about 0 C, the mixture had a temperature of about 30 350 C and showed no inclination towards granulate formation in a mixing granulator.

450.421

17. 240 no

Obviously this is due to the fact that the moisture content of 6% is too high for practical used. In a second test, a soda lye prepared with about

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17. 2 4 0 no caustic soda was used at a temperature of approximately 200 0C.

Again, 12 parts by weight sand and 4 parts by weight powdered

limestone (including small constituents) were pre-mixed and

subsequently the mixture was reacted with 3.6 parts by weight of-

the 8% caustic soda melt. The temperature of the entire nixture amounted to about .700 C and themPoisture content to less than

Whereas the mixture in the first test, which was made with a 66% caustic soda solution, was a moist viscous mass, the

mixture obtained in the second test hardened, on cooling, to

form. a hard crust. In order to granulate this mixture, it'was placed at a temperature of about ?00C on a shaking screen with

m mesh width, under which was arranged a sheet metal plate, spaced away at a distance of about 1.5 m. After a short time,

the particles of the mass which fell through the shaking screen

hardened on the metal plate to form solid, irregularly shaped

grains, with dimensions of about 8 10 mm. This granular material could be further processed, in similar manner to a

granulate without a fine portion.

Comparison showed that with a caustic soda solution even of a concentration of 66%, a satisfactory granulation cannot

be achieved;-on the other hand such a granulation is successfulIly

achieved if a caustic soda melt is introduced with a water content

below It is also significant that if the concentrated soda lye is used, granulation is not possible despite the use of a

fine sand, whereas further tests have shown that if one uses a

caustic soda melt with a low water content, even coarser sand may

be introduced to achieve a granulate-like product.

2. 5 parts by weight of sodium hydroxide are melted with parts by weight of water, and are added, at a temperature of

about 24~0 0 to a previously prepared mixture consisting of 6.7 to 450.4 21

17. 240 no of parts by weight of powdered limestone and 20 parts by weight one another sand; and the constituents are intimately mixed with 0 at a temperature of about 70 C. There is thus obtained an easily

trickling fine-grain mixed product which is brought onto a con- 10 mm. veyor belt and formed into a layer at a thickness of about

The layer is rendered uniform by means of a stationary roller

arranged above the layer, and under the blight pressure it

hardens to form a flat cake. The conveyor belt is suitably

cooled with air or liquid. A breaking-apparatus e.g. a pressure

roller with strong bearing pressure, or a spiked roller, may

be arranged at the end of the conveyor belt for pulverising the

flat cake to a grain size of 0.5 to 5 mm, or also to obtain

larger grains.

0.421

Claims (4)

17 2400no THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of preparing a granulated glass batch having

a water content of less than 5% characterised in that a caustic soda melt having a water content of up to 15% at a temperature of at least 180C is added to the other batch constituents with mixing, the resulting batch being gran7laditdduring cooling.

2. A method according to claim 1, characterised in that the mixing of the batch constituents with the caustic soda melt is carried out in a mixer with eccentrically arranged tool system at elevated temperatures, and the mixture is granulated in the same machine, with cooling.

3. A method according to claim 1, characterised in that the batch constituents are combined with the caustic soda melt in a mixing machine, and the still hot mixture iskranulated with cool- ing, after having been removed from the mixing machine.

4. A method according to claim 3, characterised in that the granulating is effected by means of a screening apparatus. A method according to claim 4, characterised by the use of a coarse-mesh shaking screen which is arranged above a cooling surface. 6. A method according to claim 1, characterised in that thcaustic soda melt is formed in situ from solid caustic soda and water vapour. 7. A method according to claim 1, characterised in that the caustic soda is brought to melting by mixing it with heated sand, and this pre-mixture is mixed with the other batch constit- uents, if necessary with the addition of water.

12 450.421

172 40no

DATED this 19th day of mAY, 1974. WILHELM EIRICH GUSTAV EIRICH

Patent Attorneys for the Applicant

SPRUSON FERGUSON.

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