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An Overview of
Refractory Raw
Materials – Part 1
Alumina
Clays and Chamottes
Abstract
Manufacturing Process
As a leading mineral processing organisation Minelco supply a variety of
raw materials to the refractory industry. The purpose of this two-part
technical paper is to review the sourcing, properties and applications of
the major refractory raw materials supplied by Minelco. In Part 1 the focus
will be on alumina based products and Part 2 will concentrate on basic
(magnesia) raw materials.
Melvyn Bradley, Technical Manager, Minelco Ltd
Dr Tony Hutton, Assistant Technical Manager, Minelco Ltd
Blended clays calcined at 1500 – 1600°C
Alumina Content
40% – 70%
Other Chemistry
Low Iron (Fe ≈1%)
Up to ≈ 1600°C
Service Limit
Thermal Behaviour
Typically expand on heating
FIGURE 2
Introduction
There are several parameters that may determine the
type of raw material to use in a refractory product.
Figure 1 lists a few of the possible service
requirements that a designer may need to consider.
No single refractory material has ideal properties e.g.
Sintered alumina may have excellent corrosion
resistance but poor thermal shock properties. This
means a wide range of alumina raw materials need to
be made available to refractory organisations.
In general alumina based materials are defined by
their Al2O3 content, other elements such as Fe2O3,
light insulating aggregates, which are not discussed
in this paper, alumina refractory materials can be
classed under three headings; refractory clays,
bauxite and sintered/fused alumina. For each
classification the chemical & physical properties will
be discussed and examples of typical applications
highlighted. In addition, a brief note on how the
new REACh regulations affect alumina materials will
be made at the end of the article.
Chamotte (Fireclay)/Mullite
Refractory Clays and Chamotte's with varying
alumina contents are produced by blending alumina
• Maximum Service Temperature
• Hot Load Strength
• Chemical Attack Resistance
• Thermal Conductivity
• Thermal Shock Resistance
• CO Resistance
• Slag Resistance
• Volume Stability
FIGURE 1
SiO2, CaO, K2O and Na2O are important factors
during the selection process. Also, bulk density,
porosity and high temperature volume stability of a
raw material have a significant effect on the
properties of the final refractory product. Except for
rich raw mineral clays (commonly Kaolinite
materials) and calcining these at 1500- 1600°C. At
this temperature the clays react together to produce
the refractory material which is typically a mixture of
mullite, quartz, cristobalite and a glassy phase
which contains the impurities from the raw
materials. By controlling the amount of each mineral
clay in the original blend a refractory product with
the desired alumina content can be produced.
However for the higher alumina refractory clays,
Bauxitic minerals can be blended with the Kaolinte
materials or a blend of Bauxitic mineral and silica
sand can be used to produce a refractory with an
alumina content of 60 -70%. A general description of
these refractory Clays and Chamottes is shown in
figure 2.
The main sources of high quality refractory clays are
the USA, China, France and South Africa. Typical
properties and applications are shown in Figure 3. A
common feature of all sources of refractory clays is
low thermal expansion, and good thermal shock
resistance. Choosing the correct grade of clay is
dependent on the application, for instance the
MULCOA branded products are favoured for CO
resistant applications.
Within the 40-70% alumina range there is also
Andalusite, sourced in South Africa & France.
Andalusite is unique in that it does not require any
calcination, mining & beneficiation produces a
material with exceptional thermal shock properties.
Properties:
Low thermal expansion coefficient, low
thermal conductivity, low specific gravity,
CO resistant
Typical Applications:
Ladle safety lining, tundish safety lining,
coke oven, annealing furnace, reheating
furnace, blast furnace, hot stove, soaking
pit, cement kilns, incinerators
itself is classified by the end use and the material
requirements that his implies, Figure 4. Bauxite is an
aluminium hydroxide material and the term bauxite
covers a range of raw mineral forms, Gibbsite the
trihydrate form, AlO(OH)3, and diasopore and
beohmite both monohydrate forms, AlO(OH).
Refractory grade raw material bauxite must contain
high Alumina and very low alkali content, Figure 5.
The levels of Iron Oxide and Titania in the ore are
also very important as these impact on the high
temperature strength properties of the refractory. To
Metallurgical
Refractory*
Abrasive*
Cement
Chemical
Al2O3
50 - 55%
>85*
80 -88%
45 - 55%
55% (Min)
SiO2
0-15%
7.5% (Max)
4 - 8%
6% (Max)
5 - 18%
TiO2
0 - 6%
4% (Max)
2 - 5%
3% (Max)
0 - 6%
Fe2O3
5 - 30%
2.5% (Max)
2 - 5%
20 -30%
2% (Max)
Other Chemical
Refractory Grade Bauxite
The main use of the bauxite mined globally is in the
production of aluminium metals, however, the ore
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There are very limited sources of refractory bauxite
the predominant two being China and Guyana.
Although chemically similar the physical properties
of gibbsitic & diasporic bauxites are quite different.
Gibbsitic bauxite tends to expand slightly above
1600°C whereas diasporic bauxite will shrink at this
temperature. The Guyana branded bauxite RASC has
always been the favoured bauxite amongst refractory
FIGURE 3
High alumina, low
iron and silica and
very low alkali content required
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Gibbsitic form of
bauxite with very
low iron required
Diaspore (preferred)
Iron added to control Fe2O3:SiO2 ratio
in ferrosilicon slag
* These figures are for calcined material
FIGURE 4
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produce the bauxite material used in refractories the
ores are calcined at 1600 – 1650°C to produce dense
and more volume stable aggregate.
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product is a higher purity than BFA with an Al2O3
content of ~ 99.5%. WFA is also a denser than BFA
and with its large crystal size means that it has
excellent corrosion resistance to molten metal slag.
Like WFA, Sintered Alumina is also produced from
calcined alumina, however, in this case the material
is pelletized and then passed slowly through a
vertical rotary furnace, discharging a dense high
purity aggregate.
Due to the high purity of sintered/fused alumina the
refractory properties are superior to most other raw
materials and as such they are used in highly critical
applications, Figure 8. Generally BFA is used in
applications that require a superior performance to
bauxite but offers a cost advantage against
WFA/sintered alumina. Since they are both
manufactured from the same feedstock i.e. calcined
alumina, there are a number of similarities between
WFA and sintered alumina. However, the type of
production process used gives raise to different
properties which ultimately determines the most
suitable material for a variety of applications.
Properties:
High refractoriness, good slag resistance,
high specific gravity, high thermal conductivity, creep resistance, abrasion resistance
Typical Applications:
Slide gate, flow control, ladle impact pads,
launders, glass tanks, high temperature
kilns
Bauxites
Manufacturing Process
Mined ores calcined at 1600 - 1650°C
Alumina Content
82% – 92%
Other Chemistry
Two forms: Gibbsitic (S. America) + Diasporic (China)
Up to ≈ 1750°C
Service Limit
Thermal Behaviour
FIGURE 8
Typically shrink on heating
Another class of materials within this group are
Alumina-rich spinels. These normally contain 10-22%
MgO which is added to the alumina during the
sintering/fusion process. Alumina-rich spinels are
often used in high alumina based refractory products
to improve hot strength, slag resistance and thermal
shock resistance.
REACh
Initially when the Registration, Evaluation,
Authorisation and Restriction of Chemicals
legislation (REACh) was being discussed one area of
concern to the refractory industry was the status of
the calcination process. Under the original
interpretation of the REACh legislation it appeared
that calcination could be considered a chemical
modification and as such most, if not all, alumina
based refractories would need to be registered at
potentially very high costs. Thankfully because of the
description of “not chemically modified” as defined
in Article 3 (40) of Regulation (EC) No 1907/2006
(REACh), Figure 9, it can be shown that calcined
kaolins and bauxites are exempted from REACh
because to produce these refractory materials only
naturally occurring kaolinitic materials or only
natural bauxitic materials are heat treated and
although some impurities are removed in the process
the alumina material itself is not modified. There
does, however, remain some discussion and
disagreement on the REACh status of the higher
alumina refractory clays due to the way they can be
manufactured using a combination of kaolinic and
bauxitic materials or bauxitic materials and silica
sands and as such could be classified as synthetic
materials by REACh. Therefore some of these products
would require registration before they can be
released on to the market in Europe. Sintered and
Fused Alumina, as synthetic manufactured materials,
FIGURE 5
Properties:
High refractoriness, good mechanical
strength, moderate slag resistance
Typical Applications:
Ladle working lining, tundish furniture, Delta
sections, general purpose repair, aluminium
melting furnace, mineral processing
FIGURE 6
technologists and in same applications gives superior
performance to Chinese bauxite. However, in many
applications Chinese and Guyana bauxite can be used
interchangeably without any significant effect on
performance, Figure 6.
Sintered/Fused Alumina
The highest alumina content Refractories are
synthetic materials formed at very high
temperatures. There are three general categories for
these Sintered Alumina, Brown Fused Alumina (BFA)
and White Fused Alumina (WFA), Figure 7. BFA with
the lower alumina content of ~95% Al2O3 is formed
by fusing fired bauxite in an electric arc furnace. This
allows the removal of impurities as a ferroalloy. The
fused alumina is then allowed to cool and solidify
before processing into the desired size fractions. WFA
alumina is formed in the same way as the BFA,
however, the alumina source in this case is calcined
alumina from the Bayer process. In the Bayer process
Bauxite is treated with caustic soda to precipitate
out alumina hydroxide which is then heated at 12001300°C to produce the calcined alumina product. Due
to higher purity of the starting material the WFA
Sintered + Fused Alumina
Manufacturing Process
Sintered: Aluminium Oxide is sintered at up to ~2000°C
Brown Fused: Bauxite is fused/melted
White Fused: Calcined Alumina is fused/melted
Alumina Content
95% – >99%
Other Chemistry
Low levels of impurities
Up to ≈ 1850°C
Service Limit
Thermal Behaviour
Volume stable on heating
FIGURE 7
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“Not chemicallymodified substance:means a
substance whose chemical structure
remains unchanged, even if it has
undergone a chemical process or treatment,
or a physical mineralogical transformation,
for instance to remove impurities;”
FIGURE 6
do fall under the scope of REACh and therefore can
only be sold in Europe if they have been registered
by a manufacturer or importer.
Conclusion
To meet the ever increasing demands on refractories
it is vital that a wide variety of raw materials are
made available to refractory companies. Even within
each classification described in this article there can
be significant differences in the properties of
materials that on first glance appear very similar. As
a major supplier of refractory minerals it is important
that Minelco supply materials that meet all the
technical and legal requirements.
Acknowledgements:
Refractories Handbook, The Technical Association of
Refractories, Japan, June 1998
Practical Refractories, Dr J D Hancock, 1988
Industrial Minerals A Global Geology, P.W. Harben &
M. Kužvart, 1996