Fused cast refractories are well
known in all glass market segments; these refractories
represent the major structural component of each glass smelting
furnace in all glass contact and most superstructure areas.
In spite of being among the most
recently developed type of refractories (invented and
introduced in USA and Europe in the first half of the
20th century), fused cast refractories have rapidly
grown into a forced choice, owing to indisputable
technical advantages relevant to campaign length, furnace
productivity and glass quality.
After less than a century since
their initial development, fused cast refractories are still an
unquestioned choice when designing a glass furnace and -
until the next generational change - no alternatives
exist. These refractory products have been introduced in
applications other than glass but, so far, well over 90%
continue to be used within the various segments of glass
applications.
After more than 50 years of this
markets expansion, there has been a progressive growth
slowdown owing to market saturation, reduced downstream
applications and, as a consequence of improved furnace design
and fusion technologies, a continuous reduction of the specific
refractory usage (kg/tonne of glass produced).
For example, in 1950, 15kg of total
refractories were used per tonne of glass produced, and now
this has declined to around 5kg/tonne (steel uses around
16kg/tonne - see IM October 2009: Into the fire).
The most recent decade saw a
stagnation (in some areas a recession) in the refractory
markets of the western world, and at present only Far East
markets still manifest clear signs of growth: in general terms,
there is a remarkable overcapacity at a global level and the
profitability of this business has moved from ancient highs to
the present lows, typical of commodities under pressure of
low-cost manufacturers (yes, China!).
Nowadays, the fused cast sold
market is estimated to total around 100,000 tpa, comprising:
80% alumina-zirconia-silica (AZS) type; 15% alumina products;
and 5% of various specialties (chrome-alumina, zirconia, MA
spinels).

Western AZS furnace set-up.
RHI AG
AZS refractories
AZS refractories are produced under
different chemistries, mostly ranging 32-41% ZrO2,
11-15% SiO2 content and 1-1.7% Na2O.
About 0.5% includes various minor components and the complement
is Al2O3.
About 80,000 tonnes of AZS
refractories are sold every year with an estimated average
composition of 49% Al2O3, 36%
ZrO2, 13% SiO2, and 1.5% Na2O.
We can easily infer the amount of mineral utilisation, based on
the following considerations:
-
Zirconia content comes almost exclusively from zircon
sand (ZrSiO4) directly as a batch component
and indirectly through the utilisation of synthetic
baddeleyite (monoclinic zirconia or FMZ). This is also a
batch component, mostly produced via thermal dissociation
of zircon sand in the presence of carbon in an electric
arc furnace (EAF).
-
The necessity of utilising a de-silicon form of zircon in
AZS formulations comes from the fact that almost all AZS
chemistries must have a ratio
ZrO2/SiO2 well above 2.03, which
represents the zircon stoichiometry. Natural baddeleyite
was once utilised as a batch component, but the
commercial availability of this mineral is now less
practical and therefore almost all of the zirconia
utilised in AZS fused cast refractories comes from zircon
sands, for a global quantity a bit above 43,000 tpa.
-
For alumina content the main sources are various types of
Bayer alumina. Minor amounts come with zircon sand
impurities and some is utilised as a fluxing agent in the
preparation of de-silicon zircon (a kind of FMZ with
variable amounts of residual silica) and comes into the
batch with this component. In total, the amount of Bayer
alumina utilised - directly and indirectly - is
around 39,000 tpa.
-
Na2O is a minor component of fused cast
refractories. It is added to the AZS composition as a
minor component of alumina, but mostly as
Na2CO3, with global tonnages
reaching around 2,000 tpa.
AZS phase diagram Budnikov-Litvakovskii

Zircon characteristics
Zircon sand is essentially the only
natural mineral, utilised in form of sand with relatively
strict technical specifications. Years ago, when the AZS
fused-cast sector was a very profitable speciality refractory
market, the only quality of zircon sand utilised was
premium grade A, with a very low content of iron
and titanium dioxide, low alumina content, de-dusted and
without sulphur contamination from fossil fuels.
This zircon is a speciality sand
not normally utilised for foundry and most sinter refractories,
representing a fraction of the overall zircon sand market and
in the topmost price range. These sands were once mostly coming
from the west coast of Australia and the USA (only for domestic
AZS fused cast manufacturers).
Pricing cyclicality followed the
major sands market relevant to other end user segments (such as
ceramics), always floating on top of the standard grade
price.
In the last couple of decades, with
the progressive draining of the AZS fused-cast profitability
margin, most major historical manufacturers have initiated a
critical review of the real technical need of utilising
premium A grade versus intermediate or
standard grade.
The result is that, today, most
western players are using a blend of premium and standard
grade, where the alumina content is not a binding factor
(alumina is indeed a major component that can be balanced),
while the main limit in impurities is the sum
Fe2O3 + TiO2.
This sum should stay under 0.30% to
prevent deterioration of some technological properties like the
tendency of releasing defects (blistering) to the glass in
contact and the discharge of a glassy exudate from the hot face
at temperatures above 1,200¡C, which is also cause of
glass contamination.
The utilisation of these lower
quality sands, obviously financially motivated, is possible
when the process control is very effective and other sources of
contamination (eg. reprocessed scrap crushing) are well
controlled; this also means that security margins
in the manufacturing are reduced and accidental out-of-spec
episodes more possible.
Additional sources of zircon sands
have become available in Australia (west and east coasts),
South Africa, South America and, more recently, a number of
different exotic sources have come online, while
the suppliers market has undergone progressive
consolidation.
Some historical mines, like in the
USA, have been depleted, while others have been replaced by
different sites and characteristics. This overall change in the
sources configuration has increased the necessity to
control other parameters like the content of radioactive
materials, a peculiar characteristic of heavy sands, to be
considered against different countries environmental and
health regulations.
Overall, the content of uranium and
thorium has been capped at a maximum of 500ppm. As a matter of
fact, several zircon sand sources fail against this
criterion.
Another point to check carefully,
when considering a new source of zircon sand for this specific
application, is the presence of very minor amounts of some
contaminants (eg. copper, chrome), normally not taken into
consideration but possible, in principle, of imparting
discolouration to the refractory and to the glass in contact
(which is the worst outcome) even when present at levels of a
few ppm.
Though this problem must be a very
rare occurrence for zircon sands it was, for example, a serious
issue with natural baddeleyite from South Africa, which
contained traces of copper (and phosphates) at levels
sufficient to impart a light blue colour to the white glass in
contact with refractories produced utilising that
mineral.
Alumina properties
Bayer alumina, or alumina
trihydrate, is a processed material and therefore, in general
terms, more controllable than a natural mineral.
Acting on numerous variables in the
manufacturing process, alumina is commercially available in a
wide range of grades including low-calcined, high-calcined,
gamma or alpha, medium or low soda content, with variable
reactivity, specific surface, ultimate crystal size and so
forth.
The largest quantities are supplied
to the metallurgical market for aluminium production, but a
large range of speciality grades are dedicated to catalysts,
fillers, abrasives, ceramics, refractories and fused cast
refractories (see IM March 2011: ATH - a finely divided
market).
Similarly to zircon sands, for the
first half a century of fused-cast manufacturing the only
alumina considered usable (particularly in Europe) was a kind
of speciality grade, often produced in dedicated batches, under
strict quality parameters and, obviously, offered by a few
selected suppliers at a considerable price premium.
The commoditisation of AZS
fused-cast manufacturers has pushed hard towards revaluating
the possibility to utilise more standard alumina (up to the
metallurgical type) or a blend of qualities, with the aim to
reduce the batching cost and to secure multiple and reliable
vendor configurations. The consolidation of the
non-metallurgical alumina market, and the rationalisation of
its manufacturing structure, has also led to the closure of
minor speciality production capacity, forcing the R&D
structure of users toward alternative sourcing.
Today, most fused-cast
manufacturers utilise various types of Bayer alumina, though in
the west the major source is still in the range of
high-calcined alpha alumina, and some amount of reactive
qualities.
Manufacturers have learned how to
utilise cheaper materials, but this is only possible by
rearranging some of the process parameters. An easy and
uncontrolled replacement of one quality with another can still
be the cause of expensive deteriorations in process yield; the
effect of aluminas physical properties on the process
viability (and particularly on the development of cracking in
the finished blocks) is still under investigation to understand
more clearly the cause and effect relationship, but the
evidence of such correlation is statistically proven.

De-silicon zircon bubbles, a processed raw
material for AZS fused cast
The China factor
One major driver of the
commoditisation of fused-cast raw materials has been the
progressive introduction of low cost producers, mostly
originating in China, which have developed their own technology
and a basic know-how.
Low cost products penetrate the
western market through hard price leveraging and an aggressive
commercial approach, destroying the profitability of the
business on a global basis and largely contributing to the
aforementioned changes.
Since the major competitiveness
factor for low cost material manufacturers is labour, the
relative impact of energy and raw materials on the costing is
enhanced. These manufacturers (which do not enjoy any advantage
on raw materials and energy costs), therefore, are forced to
buy zircon sand and alumina mostly based on price.
While major western manufacturers
belong to large industrial groups and enjoy enough volume
leverage to negotiate procurement of the main raw materials
directly from primary producers, the low cost producers are
extremely fragmented and usually buy zircon sand from traders
and stock holders which (in addition to marking-up prices), can
have problems guaranteeing a stable primary source and,
therefore, quality level.
It is worth mentioning that these
low cost fused-cast materials, under technically led
procurement, could have marginal chance of penetration in the
west, but nowadays most glassmakers (and particularly the large
corporations) have definitely switched from technical to
financially-driven procurement.
This is forcing organisations to
buy refractories under a strong price drive, in spite of
assuming some unknown level of risk; this is also part of the
commoditisation process.
In conclusion, the transformation
of AZS fused cast refractories from speciality to commodity
products, and the emergence of low cost manufacturers, has
brought along an evolution in the quality of industrial
minerals utilised.
For traditional western
manufacturers, as well for low-cost producers, the average
quality of products has not improved. Most of the R&D
activity, instead of being orientated towards newer and better
products and services, has targeted the possibility of
utilising lower quality raw materials; in some cases reducing
the safety margin between the requested and offered technical
performance of the refractories.
The more glassmakers enjoy getting
short-term financial advantages, the less they will permit
further development of products and services in the area of
refractories; this will push towards the utilisation of cheaper
commodities in the area of raw materials, in an overall
downward spiral.
To see again development of better
and different materials, on both ends of the supply chain
(industrial minerals to refractories to glassmakers), we may
have to wait for a new global renaissance, fuelled by an
improved economy, a better balanced approach to business and a
longer-term investment policy. It is hard to say when (and if)
this will happen!
Contributor: Dr P. Carlo Ratto, independent
consultant at Fused Cast Technologist. Dr Ratto has over 35
years of experience in the technology, R&D, manufacturing,
marketing and customer support areas for fused cast
refractories, working with companies such as RHI and
Vesuvius.