High Alumina Refractories:
The term high-alumina brick refers to refractory brick having an alumina
(AI2O3) content of 47.5% or higher. This descriptive title distinguishes them
from brick made predominantly of clay or other aluminosilicates which have
an alumina content below 47.5%. High-alumina brick are classified by their
alumina content according to the following ASTM convention. The 50%,
60%, 70% and 80% alumina classes contain their respective alumina
contents with an allowable range of plus or minus 2.5% from the respective
nominal values. The 85% and 90% alumina classes differ in that their
allowable range is plus or minus 2% from nominal. The final class, 99%
alumina, has a minimum alumina content rather than a range, and this value
is 97%.
Chemistry and phase mineralogy:
For alumina-silica brick, refractoriness is generally a function of alumina
content. The refractoriness of 50% alumina brick is greater than fireclay
brick and progressively improves as alumina content increases up to 99+%.
This relationship is best described by the Al 2O3 -SiO2 phase diagram. The
primary mineral phases present in fired high-alumina brick are mullite and
corundum which have melting points of 3362°F (1850°C) and 3722°F
(2050°), respectively. However, since phase equilibrium is seldom reached,
particularly in the fired refractory, the Al2O3-SiO2 diagram cannot be strictly
applied. For example, a 70% alumina product might contain a combination
of a bauxite aggregate of about 90% alumina, with various clay minerals
containing less than 45% Al2O3. When fired, the brick could contain a range
of phases which includes corundum (alumina), mullite, free-silica and glass.
In addition to Al2O3 -SiO2 content, the presence of certain impurities is
critical in determining refractoriness. Most naturally occurring minerals
contain amounts of alkalies (Na2O, K2O and Li2O), iron oxide (Fe2O3) and
titania (TiO2). Alkalies can be particularly harmful since they ultimately
react with silica to form a low melting glass when the brick are fired or reach
high temperature in service. Both Fe2O3 and TiO2 will react with Al2O3 and
SiO2, to form lower melting phases. Therefore, within any class of highalumina brick, the raw materials and their associated impurities impact on
the quality of the product and performance in service. In addition to the
melting behavior of brick, several other properties are affected by
composition.
Slag Resistance:
High-alumina brick are resistant to acid slags, that is, those high in silica.
Basic components in slag, such as MgO, CaO, FeO, Fe2O3 and MnO2 react
with high-alumina brick, particularly brick high in silica. As Al2O3 content
increases, slag resistance generally improves.
Creep or Load Resistance:
This property is most affected by melting point and, therefore, is likely to be
directly related to Al2O3 content. Impurities, such as alkalies, lime, etc., have
a significant effect on creep resistance. Mullite crystal development is also
particularly effective in providing load resistance.
Density:
Alumina has a specific gravity of 3.96 and silica, in its various forms, ranges
in specific gravity from 2.26 to 2.65. In refractories formulated from both
alumina and silica, bulk density increases with alumina content. Other
physical, chemical and thermal properties will be discussed within the
following sections concerning high-alumina brick.
Types of high-alumina brick:
50% Alumina Class:
As previously mentioned, a brick classified as a 50% alumina product has an
alumina content of 47.5% to 52.5%. Chemically, such brick are not greatly
different from superduty fireclay brick which can contain up to 44%
alumina. Brick within the 50% alumina class are often upgraded versions of
fireclay brick with the addition of a high-alumina aggregate. Compositions
of this class are designed primarily for ladles. These 50% alumina class
brick have low porosity and expand upon reheating to 2910°F (1600°C) desirable features for ladle applications since they minimize joints between
brick, giving a near monolithic lining at service temperature. These brick are
also characterized by low thermal expansion and good resistance to spalling.
Many high-temperature industries use them as backup brick. 50% alumina
products based on high purity bauxitic kaolin, and other ingredients in the
matrix, provide exceptional load-bearing ability, alkali resistance and low
porosity. These qualities make such brick an excellent choice for preheater
towers and calcining zones of rotary kilns.
60% Alumina Class:
The 60% alumina class is a large, popular class of products. These brick are
used in the steel industry, as well as rotary kilns. Brick in this class are made
from a variety of raw materials. Some are produced from calcined bauxitic
kaolin and high purity clay to provide low levels of impurities. As a result of
firing to high temperature, these brick have low porosity, excellent hot
strength and creep resistance, and good volume stability at high
temperatures.
70% Alumina Class:
This is the most frequently used high- alumina product class because of its
excellent and cost-effective performance in multiple environments.
Applications include the steel-industry and cement and lime rotary kilns.
Most brick in this class are based on calcined bauxite and fireclay. Brick are
usually fired to fairly low temperatures to prevent excessive expansion in
burning which causes problems in final brick sizing. Expansion is caused by
reaction of the siliceous ingredients with bauxite to form mullite. The brick
typically undergo large amounts of secondary expansion when heated. This
is advantageous in reducing the size of joints between brick and providing a
tight vessel structure, e.g., a rotary kiln. A higher cost and higher quality
alternative to producing a 70% alumina brick is represented by brands based
on high purity calcined bauxitic kaolin. These brick have superior high
temperature strength and refractoriness and significantly lower porosity than
typical products based on calcined bauxite. Due to their more homogeneous
structure, they show somewhat less expansion on reheating than bauxitebased products.
80% Alumina Class:
These products are based primarily on calcined bauxite with additions of
various amounts of other fine aluminas and clay materials. They are usually
fired at relatively low temperatures to maintain consistent brick sizing. Most
brick in this class have about 20% porosity, good strength and thermal shock
resistance. Because they are relatively inexpensive, perform well and are
resistant to most slag conditions present in steel ladles, they are used
extensively in steel ladle applications.
90% and 99% Alumina Classes:
These brick contain tabular alumina as the base grain and may include
various fine materials such as calcined alumina, clay and fine silica. As these
brick generally have low impurity levels, alumina and silica typically make
up 99% of the chemical composition. Usually, the only mineral phases
present are corundum and mullite. Properties such as high hot strength,
creep and slag resistance benefit from this purity level.
Alumina-Chrome Brick:
Alumina-chrome brick consist of combinations of the two oxides fired to
develop a solid-solution bond. A wide range of products are available
depending upon Cr2O3 content. These include a 90% Al2O3-10% Cr2O3
product based on high purity sintered alumina and pure chromic oxide. The
solid- solution developed in firing results in brick with exceptional cold
strength, hot strength and load-bearing ability. In addition, the solid-solution
bond between alumina and chromic oxide is inert to a wide variety of slags.
Brick with higher Cr2O3 content are also available. Based on a special fused
grain high in chromic oxide, these products are selected for the most extreme
cases of high temperature and corrosiveness.
Mullite Brick:
In brick of this special category, the mineral phase mullite predominates.
The alumina content varies from about 70% to 78% and the brick can
contain a major portion of either sintered grain or fused mullite grain. These
brick are typically fired to high temperature to maximize mullite crystal
development.
Phosphate-Bonded Brick:
Phosphate-bonded brick can be produced from a variety of high-alumina
calcines, but typically they are made from bauxite. A P2O5 addition, such as
phosphoric acid or various forms of soluble phosphates, reacts with available
alumina in the mix. After the pressing operation, brick are cured at
temperatures between 600°F and 1000°F (320°C and 540°C) which sets a
chemical bond of aluminum phosphate. They may even be fired at higher
temperatures to develop a combination chemical and ceramic bond.
Phosphate-bonded brick are characterized by low porosity and permeability
and very high strength at intermediate temperatures between 1500°F (815°C)
and 2000°F (1090°C). Phosphate-bonded brick are widely used in the
mineral processing industries, particularly in applications such as nose rings
and discharge ends of rotary kilns where excellent abrasion resistance is
required.
Alumina-Carbon Brick:
In this class, brick are bonded by special thermosetting resins that yield a
carbonaceous bond upon pyrolysis. A wide variety of compositions are
possible based on the various high-alumina aggregates now available.
Graphite is the most common carbonaceous material, although silicon
carbide is used, as well.