Refractory Selection for Fore Hearths and Feeders

$Page 1: Refractory Selection for Fore Hearths and Feeders$
RREEFFRRAACCTTOORRYY SSEELLEECCTTIIOONN FFOORR

FFOORREEHHEEAARRTTHHSS AANNDD FFEEEEDDEERRSS

22070 Locate Varesino (COMO) ITALY - Via Delle Grigne 12/A Tel. +39 0331 823195 - Fax +39 0331 823198

e-mail: [email protected] - web site: www.sigmaref.it

2

INDEX OF SUBJECTS

1. REFRACTORY SELECTION FOR FOREHEARTHS AND FEEDERS..................... 3

2. MANUFACTURING PROCESSES ........................................................................... 4

2.1. Slip-casting ..................................................................................................... 4 2.2. Vibro-casting ................................................................................................... 5

3. ADVANTAGES AND DISADVANTAGES OF THE TWO PROCESSES .................. 6

3.1. Slip-casting ..................................................................................................... 6 3.2. Vibro-casting ................................................................................................... 7

4. THERMAL SHOCK RESISTANCE ......................................................................... 14

5. CHOOSING THE PROCESS ................................................................................. 14

6. CONCLUSIONS ..................................................................................................... 22

SLIP-CASTING AND VIBRO-CASTING COMPARISON............................................... 23

3

1. REFRACTORY SELECTION FOR FOREHEARTHS AND FEEDERS

Refractory selection will recognize the operating conditions within the

forehearth and feeder and this will determine the material properties necessary

for optimum performance.

The refractory manufacturing process will also be considered as this will have a

significant influence on product stability throughout the forehearth campaign.

The operating conditions, including thermal and physical stress and chemical

attack, for the various components within the forehearth and feeder will be

analysed separately in order to identify the appropriate refractory composition

and manufacturing process.

4

2. MANUFACTURING PROCESSES

Slip-casting and vibro-casting are the main manufacturing processes to

produce forehearth and feeder sintered refractories.

2.1. Slip-casting

Raw materials are mixed together with water and then poured into a porous

plaster mould.

Water is then absorbed by the plaster and, after opening the mould, the piece

can be dried and fired.

2.2.

5

2.2 Vibro-casting

Raw materials are mixed together with an alcohol binder and then transferred

to a wooden mould which is placed on a vibrating table.

After a prescribed period of vibration, the item is dried and fired.

VIBRATING TABLE MIXER WOODEN MOULD

6

3. ADVANTAGES AND DISADVANTAGES OF THE TWO PROCESSES

3.1. Slip-casting

Clay, which is used in the slip casting process as a binder, brings alkalies and

impurities to the mix, thus increasing the volume of the glassy phase.

The glassy phase is the “weak link” within the refractory structure resulting in

enhanced glass corrosion and attack by volatiles leading to reduced

“refractoriness under load”.

From an aesthetic point of view, the slip-casting process results in a smooth

and blemish-free surface.

7

3.2. Vibro-casting

Chemical bonding is achieved with alcohol binders as opposed to the use of

clay and this results in a significant improvement in chemical and physical

characteristics in the environment of the forehearth and feeder.

With a much reduced glassy phase, the refractory has a higher “refractoriness

under load” and an enhanced resistance to volatiles and molten glass.

Here below is a typical “refractoriness under load” curve of a chemically

(alcohol?) bonded zircon-mullite refractory.

-0,250,000,250,500,751,001,251,501,752,002,252,502,753,003,253,503,754,004,254,504,755,005,255,505,756,006,256,506,757,00

2510

020

030

040

050

060

070

080

090

0

1000

1100

1200

1300

1400

1500

1600

Temperature °C

DL/

L0*E

-3

8

SAMPLES BEFORE TESTING

9

CRUCIBLE FOR STATIC TEST

10

SAMPLES AFTER TESTING

11

The glassy phase is more susceptible to attack by glass or volatiles than a

tightly interlocked lattice structure.

The above photos illustrate clearly the difference in glass corrosion between the

slip-cast and vibro-cast zircon-mullite material in a static test at 1300°C

SLIP-CAST ZIRCON-MULLITE

VIBRO-CAST ZIRCON-MULLITE



12

Here below is the technical data of this test.

Quality Original

sample width at glass level

Sample width at glass level after the test

Percentage of corrosion at glass level

Original sample width at 3 cm from

glass level

Sample width at 3 cm from

glass level after the test

Percentage of corrosion at 3 cm from glass level


12,4 mm 10,3 mm 16,9% 12,4 mm 11,3 mm 8,8%


13,0 mm 6,7 mm 48,4% 13,0 mm 10,5 mm 19,2%

13

The disadvantage of the vibro-casting process is the aesthetic appearance. The

surface is not smooth, since it is very difficult to eliminate all the air trapped

between the outside surface and the wooden mould.

AESTHETIC DEFECTS

However, the presence of surface “imperfections” does not detract from the

performance of the product and the absence of the “contaminating” clay

bonding material makes for improved performance.

14

4. THERMAL SHOCK RESISTANCE

This is an important aspect to be considered when selecting the manufacturing

method.

The main factors which influence the thermal shock resistance are thermal

expansion, thermal conductivity and glassy phase.

The desired thermal expansion and thermal conductivity characteristics can

be achieved by the appropriate choice of refractories, however, the use of clay

bonding material will reduce thermal shock resistance at low temperatures.

5. CHOOSING THE APPROPRIATE PROCESS

Whether to slip-cast or vibro-cast not only depends on the operating

environment but also on the geometry of the article.

A complicated shape may dictate the use of the slip-casting process as the mix

is more mobile than that of the vibro-cast process and therefore able to

completely fill the mould.

15

On the other hand, there will be a limit for slip-casting articles above a certain

size due to the inability to absorb sufficient water into the plaster.

However, the overriding factor to consider when choosing either slip or vibro-

casting processes is the relative performance of the product in the environment

of the forehearth and feeder.

Let us consider the different refractory items for feeders and forehearths,

assuming the glass is soda-lime.

2,5 m

16

Items Operating factors Refractory properties Quality options Main Manufacturing

processes Plungers �� Thermo-technical stress

(rapid temperature change)

�� Type of glass

�� Temperature

�� Chemical attack (glass flow)

�� Thermal shock resistance

�� Glass corrosion resistance

�� Chemical analysis

�� Thermal conductivity

�� Thermal expansion

�� Glassy phase

�� Apparent porosity

�� Sintered zircon-mullite

�� Sintered zircon

�� Sintered fused silica

�� Slip-casting and vibro-casting

Tubes �� Thermo-technical stress (fast temperature change)


�� Temperature







�� Glassy phase






Stirrers �� Thermo-technical stress (fast temperature change)


�� Temperature

�� Chemical attack (fast glass flow)






�� Glassy phase






Orifice rings �� Thermo-technical stress (fast temperature change)


�� Temperature







�� Glassy phase


�� Sintered high alumina


�� Slip-casting

17

Items Operating factors Refractory properties Quality options Main Manufacturing

processes Channels �� Type of glass

�� Temperature





�� Volume stability


�� Sintered high alumina (99% Al2O3)

�� AZS fused cast

�� High alumina fused cast

�� Vibro-casting

�� Electromelting

Forehearth

superstructures


�� Temperature

�� Chemical attack (gas and vapour)

�� Compression in hot conditions

�� Gas and vapour resistance

�� Gas permeability

�� Creep in compression



�� Volume stability

�� Sintered sillimanite


�� Vibro-casting

18

ORIFICE RINGS

Slip-casting manufacturing process is better than vibro-casting as it fills up

completely the moulds of these complicated shapes.

In this case slip casting is an unavoidable choice.

As far as the quality is concerned, we must take into consideration the

frequency of job changes: if there are many, the most used quality is high

alumina: on the other hand, in the case of few job changes, zircon-mullite

quality is suggested.

In addition it is possible to place inserts in the orifice ring holes.

Quality Al2O3 SiO2 Fe2O3 ZrO2 Bulk

Density Apparent Porosity

Reversible Thermal

Expansion % % % % Kg/dm3 % % (°C)

SLIP-CAST

HIGH ALUMINA 76 ÷ 78 19 ÷ 21 0,2 ÷ 0,4 2,50 ÷ 2,60 20 ÷ 22 0,55 (1000)

SLIP-CAST

ZIRCON MULLITE 64 ÷ 66 14 ÷ 16 0,3 ÷ 0,5 17 ÷ 19 2,95 ÷ 3,05 20 ÷ 22 0,61 (1000)

19

PLUNGERS, TUBES AND STIRRERS

As these feeder expendables will be subject to both thermal shock and glass

corrosion, the vibro-casting manufacturing method is suggested.

As mentioned earlier, the absence of clay bonding material and the tightly

interlocking lattice structure optimises thermal shock resistance and chemical

stability.

The suggested quality is zircon-mullite fired at high temperature and with a

chemical analysis similar to that of AZS fused cast refractories.



Reversible Thermal


VIBRO-CAST

ZIRCON-MULLITE 53 ÷ 55 13 ÷ 15 0,1 ÷ 0,2 29 ÷ 31 2,95 ÷ 3,10 19 ÷ 21 0,57 (1000)

20

CHANNEL BLOCKS

Channel blocks operate at effectively a constant temperature in contact with

the glass, and therefore the most important property they must have is a high

glass corrosion resistance.

Vibro-cast blocks, with minimal glassy phase, no added clay and high firing

temperatures, offer the best solution in regard to corrosion resistance.

The recommended qualities are zircon-mullite or high alumina, both fired at

high temperatures.



Reversible Thermal


VIBRO-CAST

ZIRCON-MULLITE 49 ÷ 51 23 ÷ 25 0,2 ÷ 0,3 21 ÷ 23 2,70 ÷ 2,80 20 ÷ 22 0,55 (1000)

VIBRO-CAST

HIGH ALUMINA 97 ÷ 99 1 ÷ 2 0,05 ÷ 0,1 2,95 ÷ 3,05 17 ÷ 19 0,80 (1000)

21

FOREHEARTH SUPERSTRUCTURES

The superstructure blocks are not subjected to thermal shock or glass contact,

but to a strong compression in hot conditions and to gas and vapour chemical

attack.

With this in mind, it is essential to minimise the glassy phase in order to

achieve the desired level of “refractoriness under load” and to minimise

degradation of the refractory structure.

This is particularly relevant to structural stability for large and heavy

superstructure blocks.

When considering soda-lime glass, the recommended quality is sillimanite but,

in colouring forehearths, particularly in the area of the stirrers, it is necessary

to install a special zircon-mullite quality, fired at high temperature.

22

In the area of the frit feeding, AZS fused cast refractories are suggested.



Refr.ness under load

% % % % Kg/dm3 % (°C)

VIBRO-CAST

SILLIMANITE 61 ÷ 63 35 ÷ 37 0,8 ÷ 1,0 2,35 ÷ 2,45 21 ÷ 23 ≥ 1600

VIBRO-CAST

ZIRCON-MULLITE 39 ÷ 41 26 ÷ 28 0,2 ÷ 0,4 31 ÷ 33 2,90 ÷ 3,0 20 ÷ 22 ≥ 1630

VIBRO-CAST

ZIRCON-MULLITE 57 ÷ 59 13 ÷ 15 0,2 ÷ 0,4 26 ÷ 28 3,0 ÷ 3,1 16 ÷ 18 ≥ 1630

6. CONCLUSIONS

Two manufacturing processes are normally used to produce refractories for

feeders and forehearths: slip-casting and vibro-casting.

Slip-cast products are predominantly used for feeder expendables whilst vibro-

cast items are mainly used in the forehearth sub and superstructure.

However, vibro-cast tubes, stirrers and plungers are increasingly being used as

a direct result of proven superior production performance. That is, increased

life and/or improved ware pack percentage.

As far as the sintered refractories for forehearths and feeders are concerned,

the recommended qualities are high alumina (up to 99% Al2O3), sillimanite and

zircon-mullite, all without clay as a binder and fired at high temperatures.

Advantages and disadvantages of the two processes are shown in the

comparison table.

23

SLIP-CASTING AND VIBRO-CASTING COMPARISON SLIP CASTING

Items Advantages (compared to vibro-casting)

Disadvantages (compared to vibro-casting)

Orifice rings

Plungers

Tubes

Stirrers

Spouts

�� Better aesthetic appearance

�� Lower thermal shock resistance

�� Lower glass corrosion resistance

VIBRO-CASTING (USING A NO CLAY BINDER)

Items Advantages (compared to slip casting)

Disadvantages (compared to slip casting)

Plungers

Tubes

Stirrers

Spouts

Spout covers

Channels

Superstructure blocks

�� Better thermal shock resistance

�� Better glass corrosion resistance

�� Better gas and vapour resistance

�� Worse aesthetic aspect

Documents

Refractory Selection for Fore Hearths and Feeders