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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
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
VIBRO-CAST ZIRCON-MULLITE
12,4 mm 10,3 mm 16,9% 12,4 mm 11,3 mm 8,8%
SLIP-CAST ZIRCON-MULLITE
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)
���� 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
Stirrers ���� Thermo-technical stress (fast temperature change)
���� Type of glass
���� Temperature
���� Chemical attack (fast 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
Orifice rings ���� Thermo-technical stress (fast 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 high alumina
���� Sintered zircon-mullite
���� Slip-casting
17
Items Operating factors Refractory properties Quality options Main Manufacturing
processes Channels ���� Type of glass
���� Temperature
���� Chemical attack (glass flow)
���� Glass corrosion resistance
���� Chemical analysis
���� Apparent porosity
���� Volume stability
���� Sintered zircon-mullite
���� Sintered high alumina (99% Al2O3)
���� AZS fused cast
���� High alumina fused cast
���� Vibro-casting
���� Electromelting
Forehearth
superstructures
���� Type of glass
���� Temperature
���� Chemical attack (gas and vapour)
���� Compression in hot conditions
���� Gas and vapour resistance
���� Gas permeability
���� Creep in compression
���� Chemical analysis
���� Apparent porosity
���� Volume stability
���� Sintered sillimanite
���� Sintered zircon-mullite
���� 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.
Quality Al2O3 SiO2 Fe2O3 ZrO2 Bulk
Density Apparent Porosity
Reversible Thermal
Expansion % % % % Kg/dm3 % % (°C)
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.
Quality Al2O3 SiO2 Fe2O3 ZrO2 Bulk
Density Apparent Porosity
Reversible Thermal
Expansion % % % % Kg/dm3 % % (°C)
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.
Quality Al2O3 SiO2 Fe2O3 ZrO2 Bulk
Density Apparent Porosity
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