Since the advent and rapid diffusion of fused cast
refractories in the first half of the 20th century, there have
not been any equally significant revolutions in the development
of new refractories for glass furnaces.
The "fused cast era" laid the foundation for the evolution
of glassmaking technologies and the development of new classes
of advanced glasses that would not have been achievable without
the availability of glass-contact refractories capable of
extremely low glass-defect cession.
In almost 100 years of using fused cast glass refractories,
there have been significant improvements in furnace design and
glass chemistry. Better furnace handling practice and repair
techniques have considerably increased furnace life. In
addition to improving glass quality, the combined effect of
these advances has been to increase the productivity of a
typical furnace campaign by around two orders of magnitude.
Considering fused cast as a general class of refractories,
characterised by a common manufacturing practice (ceramic
foundry technology), its development trajectory has been one of
evolving chemistries from the initial aluminas and
alumina-zirconia-silica (AZS) families, to different
chrome-bearing compositions, high-zirconia and alumina-magnesia
spinels.
Different fused cast chemistries have filled application
niches for various glass compositions and for different
sections of furnaces, both in glass-contact and superstructure
products, bringing the advantage of extremely dense (low
porosity) refractories which exhibit high refractoriness,
chemical stability and resistance to abrasion.
With the exception of the development of the last generation
of large sintered pre-cast shapes, which are gaining a
significant success in superstructure applications of soda-lime
container furnaces, there has not been any significant newcomer
in the non-fused cast glass furnaces lining or glass-contact
arenas.
Until a radically new glass-smelting practice is developed,
fused cast refractories will remain the leading refractory
products in glassmaking.
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Schematic of submerged combustion melting
process
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Source: US Dept. of Energy
2006
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SCM
One of the most promising innovations with the potential to
shake up the glass refractories segment is not a new class of
refractory, but rather a new fusion technology known as
submerged combustion melting (SCM). This involves a new kind of
smelter that works through combustion inside the melted glass
batch.
Tests of this process have shown that SCM smelters can
significantly improve thermal efficiency and also display a
number of other advantages, including a very minor dimensional
impact, rapid start-up and shut-down and the simplicity of the
required change in chemistry.
Due to their design, SCM furnaces use a very minor quantity
of refractories per unit of product. SCM technology was first
theorised in the 1960s, but serious work on this process by
research institutions and private industry did not begin until
around 15 years ago. Today, only a handful of factories in
Belarus and Ukraine currently employ this innovative fusion
technique to produce glassrock.
The reason for such slow implementation of SCM technology is
down to one large drawback intrinsic to this method of fusion.
The smelted glass extracted from the furnace’s
fusion tank has a high occurrence of gas blisters, making it
unsuitable for use in many large end use segments such as
containers, tableware and flat glass, not to mention in
quality-sensitive applications like flat panel display and
optical glass.
This explains why SCM furnaces are currently only being used
to produce glassrock, but with some refining, the technology
could be used to make higher quality types of glass. However,
in order to make it suitable for use in the major glass
segments, the technique requires several stages of advanced
refining, which could ultimately cancel out many of its cost
and simplicity advantages.
It is therefore unlikely that SCM technology will become
established in container, tableware, flat and speciality
glassmaking in the medium term.
Crown refractories
Crown refractories for glassmaking are the main application
for silica wedges, which form a "crown" in the glass furnace.
One of the main problems associated with this silica bricks
application is "silica rat holing", a phenomenon whereby
alkali-rich vapour condenses in the cool areas of the silica
crown, causing corrosion. A great deal of effort has been
devoted to increasing the physical and chemical quality of
silica refractories to withstand this corrosion. Research has
also been conducted on developing adequate insulation for the
silica refractories, so as to move the sulphate condensation
zone outside the furnace’s dense silica layer.
These measures have reduced, but not eliminated, the need of
hot repairs to the silica crown during a furnace’s
campaign life.
The advent of oxyfuel fusion technology, the take-up of
which has been boosted by manufacturers’ responses
to anti-pollution legislation, has created an opportunity for
AZS and alumina-based fused-cast refractories to be installed
in silica furnace crowns, although the popularity of this
technique has been curbed by the higher energy cost associated
with oxygen generation and a much higher crown weight.
Nevertheless, a significant number of oxyfuel furnaces use high
quality silica wedges instead of fused cast blocks in crowns,
in spite of the rat holing problem.
The utilisation of other refractories such as zircon and
spinel-based materials and other refractories in crowns is
limited to furnaces making borosilicate, frits and some other
specialty glasses. It is not anticipated that there will be any
major shifts in these usage patterns in the medium term.
Glass-contact refractories
The biggest application area for fused cast refractories,
mostly AZS and aluminas, is in smelters used to produce glass
for large volume markets, like containers and flat glass. Some
speciality glass production also uses high-zirconia fused cast
(HZFC) refractories, typically in the making of FPD glass
(particularly with Corning’s fusion technology,
but also in the float process), due to the requirement for the
glass to be of very high quality.
A few attempts have been made to use unshaped or pre-cast
materials in these smelters, but the application of such
materials is only an option for minor hot repairs, since at
present there are no corrosion-resistant sinter materials
comparable to fused cast. Exceptions to this are high chrome
vibro-cast or iso-pressed shapes, which are suitably
corrosion-resistant but have the drawback of causing glass
discoloration proportional to the glass’ contact
with the refractory surface. These high chrome materials are
however a viable option for fibreglass and glass wool, where
the colour of the glass is not a concern.
The bottom of glass fusion and refining tanks are typically
lined with AZS or alpha-beta alumina fused cast tiles. In a few
cases, tiles made of sintered AZS or zircon iso-pressed
refractories can be used as an alternative to the fused cast
material.
Glass contact reinforcements (cross-walls, DH corners,
throat inlets/covers) used to be a typical application for
high-zirconia AZS (41% ZrO2). In recent decades, as
a consequence of moves to extend furnace campaign life by means
of reinforcing weak points, it has become more common to
install special refractories with greater wear resistance than
high zirconia AZS in the most stressed spots of furnaces.
Among the beneficiaries of this trend were chrome-bearing
refractories. Several years ago, attention centred on using
chrome-bearing fused cast materials such as AZS-Cr (AZS doped
with chrome) and ACr (chrome-corundum).
More recently, with the development of the iso-pressing
shaping technique for large blocks, high chrome (escolaite),
iso-pressed and sintered blocks have successfully replaced
fused cast refractories in extremely stressed applications,
such as cross walls. These have the advantage of a very
homogeneous structure compared with the intrinsically
inhomogeneous texture and composition of a fused cast body,
prolonging the refractory’s life within the
furnace.
Chrome-based materials still have the drawback of causing
glass discolouration. The presence of even a few parts per
million of chrome in a refractory strongly colours glass it
comes into contact with, meaning these materials are of limited
use in making white, extra white and some special glasses as
well as tableware, illumination, float and some borosilicate
tubing glass.
As an alternative to chrome-based refractories,
AZS-molybdenum (Mo) composite fused cast blocks have emerged
from the R&D departments of some major western refractory
producers. In these materials, a molybdenum sheet is embedded
into the fused cast block, a few millimetres beneath the
surface. After the wearing down of the AZS surface layer, the
molybdenum sheet is directly exposed to the glass contact,
providing long term protection to the rest of AZS block.
Molybdenum is used because it is a refractory metal with
extremely high resistance to glass corrosion.
However, these advanced composite blocks are difficult and
expensive to produce and can contain hidden intrinsic defects,
such as poor positioning of the molybdenum sheet. Inappropriate
heating of the blocks can cause them to crack, allowing oxygen
to get to the metal. Any of these defects can lead the blocks
to fail, with problematic consequences for the
furnace’s operation.
As a result of these risks, AZS-Mo refractories are not
widely used. Protecting the refractory blocks via various
means, including platinum external cladding of AZS fused cast
blocks, is an option to mitigate the chances of refractory
failure, but generally only for borosilicate glass production,
where the value of the final product makes the additional cost
worthwhile.
Superstructures
Shortly after the introduction of fused cast bricks in the
first half of the twentieth century, aluminas (both alpha-beta
and beta forms) and AZS fused cast (generally low-zirconia with
32% ZrO2) replaced the silica, silico-alumina and
zircon sinter bricks and shapes previously used in glass
furnaces.
The fused cast blocks succeeded in prolonging the campaign
life of furnaces, although AZS refractories tended to have the
unwanted side effect of causing exudate to drip into the glass,
potentially creating alumina/zirconia-enriched defects. This
persistent problem motivated glassmakers and refractories
manufacturers to develop improved AZS fused cast with reduced
tendency to exudate. This was achieved increasing the oxidation
level of the smelted ceramic components of the blocks via
oxygen injection, reducing impurities in the raw materials and
contamination from electrode graphite, ultimately developing
special low-exudation chemistries.
In the last decade, a new generation of sinter materials
produced with a pre-cast and firing technology have been
installed in a number of furnaces in place of fused cast
blocks. The main drawback of these sinter materials in this
application is their susceptibility to erosion via nephelitic
conversion (the formation of nephelite from the
brick’s components under thermal and chemical
aggression) and subsequent spalling of the converted layer.
Since they were first introduced, advanced formulations and the
use of special raw materials have significantly reduced
corrosion rates so that, in several cases these materials can
now withstand the full extent of furnaces’
life.
The opportunity presented by these materials to provide
other advantages, like the possibility of producing extra-large
shapes such as single piece arches, reducing thermal
conductivity and preventing exudation, has increased
glassmakers’ interest in using and developing
these new pre-cast sinter refractories, although there is as
yet no industry-wide consensus on their effectiveness.
Regenerator packages
Regenerative furnaces continue to represent a popular design
for combustion energised furnaces which run on gas or oil.
These types of furnace include smelters for producing
container, float and tableware glass. For regenerative
furnaces, regenerator packages require large volumes (tonnes)
of refractories, comparable to those of glass-contact,
superstructure and crown.
The package’s main task is to recover or
exchange heat from combustion fumes in a chemically and
mechanically aggressive environment with a wide range thermal
cycling. Early refractories used to perform this role were
special silico-aluminas, before being replaced with high
temperature-fired periclase (mag-chrome and periclase-zircon).
Initially installed as bricks, these were eventually replaced
with tiles that greatly increased the surface area per volume
of refractory and with this the thermal exchange efficiency of
the package as a whole.
Some innovative producers of basic refractories have
developed products with specific chemistries to be installed in
different zones of the regenerator package according to the
type of furnace, based on an advanced understanding of their
corrosion mechanisms and thermal behaviour.
In the last few decades, one leading global manufacturer of
fused cast has attempted to shake up the regenerative package
segment by introducing a new family of fused cast shapes,
referred to as cruciforms, with chemistries ranging from AZS to
spinel.
A solid body of experimental evidence has demonstrated that
these products are competitive in performance terms with
traditional sinter checkers, so fused cast cruciforms have
successfully gained a significant share of the regenerative
package market. The proliferation of these products has however
been limited by their debatable overall financial benefits and
occasional instances of premature failure against their life
span expectations of two furnace campaigns.
Future for fused cast
Although challenger products have demonstrated considerable
improvements over incumbent fused cast refractory technology,
the various drawbacks of these materials outlined above when it
comes to their performance in glass furnaces, particularly with
regard to the large volume glass commodities, suggest that
fused cast materials will remain the dominant glass refractory
for some time.
This forecast is supported by the fact that the supply of
glass refractories, which was previously dominated by two major
Western manufacturers, has in the last decade diversified to
offer a range of products and services together with more
competitive prices.
Broadly speaking, there are now three main tiers of glass
refractory manufacturers.
Tier 1
The first tier of glass refractory manufacturers is
dominated at the top end by Western companies which have
developed market-leading technologies and services and whose
main manufacturing bases continue to be based in Western
countries.
These tend to be the most expensive suppliers, justifying
the prices they charge for their products and services by
ensuring a low level of technical risk associated with their
technology. However, in an increasingly competitive industry,
it is becoming more and more difficult for glassmakers to
justify procuring refractories from these top level
suppliers.
The lower half of this tier is characterised by Western
companies with premium technologies which have relocated their
main manufacturing operations to low-cost centres such as Asia.
Typically, the products manufactured in these cheaper locations
are not made to exactly the same standards as they are in
Western factories, particularly for AZS materials, but the
quality of these products is solid and still among the best in
the industry and their level of technical risk is among the
lowest.
As a result, products and services offered by these lower
first tier suppliers are generally more affordable than those
at the top end of this bracket.
Tier 2
The second tier of glass refractory makers comprises mainly
low-cost independent manufacturers, generally based in Asia and
producing less sophisticated products than first tier
suppliers. The extent of the differences between the technology
offered by this group compared to Western companies varies, but
can be widely divergent in the case of alumina fused cast
materials. The level of service offered by second tier
companies to glassmakers is regarded as average to poor and the
level of technical risk is higher, however the price of both
products and services offered by this layer of supplier is
significantly lower.
Accordingly, even some major Western glass producers will
occasionally buy materials from this tier, usually with the
safety net of having first tier level third party service
providers to hand.
Tier 3
These are low-cost independent manufacturers invariably
based in Asia and offering the most basic level of refractory
technology. Their low-cost manufacturing operations are
generally set up to serve domestic markets rather than foreign
customers.
Conclusion
The fused cast refractories sector is becoming increasingly
competitive and is ripe for a leap forward in technology.
Industry observation suggests that service provision is
developing more rapidly than product sophistication and a new
class of third party service providers is emerging to satisfy
the needs of glassmakers who are demanding more from the
performance of the refractories they use.
As glass producers are increasingly tempted by lower cost
products with higher levels of technical risk, better service
provision could exacerbate the shrinking market share of first
tier refractory suppliers.
*Dr P. Carlo Ratto has over 35 years of experience in
the fused cast refractory industry.
www.fusedcast.com