Raw Materials of Refractories:
Refractories are a category of technical ceramics. Industrial refractories are
almost all complex combinations of high-melting crystalline oxides, plus a
few carbides, carbon and graphite. Polycrystalline ceramics are
characteristically brittle, and far less strong in tension than in compression.
They are subject to considerable variability in strength, resulting from local
variations in their microstructure and their lack of ductility. They exhibit
high- temperature creep or plastic deformation, but almost always by a
mechanism different from that operating in metals. Their mechanisms of
conduction of heat and electricity are also different from those of metals.
Their elastic moduli are generally quite high: this and their brittleness make
them somewhat prone to failure under thermal stress and shock. Compositions
can be chosen having selectively high chemical resistance (but rarely
immunity) to oxidizing or reducing agents, acids or bases, metals or salts,
whether liquid or gaseous. Shaped refractory objects are typically made by
particulate forming processes. Their thermal maturing or sintering produces a
wide variety of phase compositions and microstructural features. Most fired
industrial refractories have experienced “liquid-phase” sintering; and for this
and other reasons they contain porosity. Both the lower-melting
intercrystalline phases and the porosity influence mechanical and thermal
properties, and both have much to do with penetration by corrosive media.
Only a few refractories are made essentially pore-free and without selective
phase melting. The variety of phase compositions and structures sought in
refractory manufacture is thus greater than in metals, and in some ways
different in kind. The manufacturing methods themselves are more varied,
and mostly different in kind. The fundamental connections made between
properties and structure are in some ways unique to nonmetallic materials.
And finally, there is an intense concern with the preparation and
characterization of particulate starting materials, of pivotal importance in
ceramic synthesis but infrequently of any interest at all in metal manufacture.
These starting materials are often minerals, as opposed to high-purity
synthetic chemicals. Thus the technology of refractory materials and their
properties alone is itself a multidisciplinary matter. Refractory materials are
best comprehended on a foundation of ceramic science and engineering or of
materials science and engineering. Specifically, we shall be dealing mostly in
inorganic chemistry, kinetics and thermodynamics; solutions and phase
equilibria; the rudiments of nonmetallic crystalline and vitreous structure; the
thermomechanical properties of solids including fundamentals of elasticity,
plastic flow, and fracture; and concepts of heat, heat transport and electrical
transport not to mention also, ceramic synthesis.
BASIC REFRACTORIES:
The principal raw materials used in the production of basic refractories are
dead-burned and fused magnesites, dead-burned dolomite, chrome ore, spinel
and carbon. In recent years, the trend has shifted to developing highly
engineered basic refractories. This has resulted from attempts to address the
rapidly evolving needs of the metallurgical and mineral processing industries
that use basic refractories. One result of this effort has been the development
of technology to address specific wear mechanisms by employing special
additives in the refractory composition. These additives generally constitute
less than 6% of the total mix, although levels at 3% and below are probably
the most common. Examples of these special additives include zirconia,
which is sometimes used to improve the spalling resistance of burned basic
refractories. As carbon has become an important constituent in the
formulation of composite magnesite- carbon refractories, metallic additives,
such as powdered aluminum, magnesium or silicon have been used to
improve hot strength and oxidation resistance. Small boron carbide (B4C)
additions also can improve the oxidation resistance of certain magnesitecarbon compositions. These compositions are used in special applications
such as bottom blowing elements of basic oxygen furnaces.
FIRECLAY MATERIALS:
Refractory fire clays consist essentially of hydrated aluminum silicates with
minor proportions of other minerals. The general formula for these aluminum
silicates is Al2O3.2SiO2.2H2O, corresponding to 39.5% alumina (AI2O3),
46.5% silica (SiO2) and 14% water (H2O). Kaolinite is the most common
member of this group. At high temperatures, the combined water is driven
off, and the residue theoretically consists of 45.9% alumina and 54.1% silica.
However, even the purest clays contain small amounts of other constituents,
such as compounds of iron, calcium, magnesium, titanium, sodium,
potassium, lithium and usually some free silica. Of greatest importance as
refractories are flint and semi-flint clays, plastic and semi-plastic clays, and
kaolins. Flint clay, also known as “hard clay”, derives its name from its
extreme hardness. It is the principal component of most super duty and highduty fireclay brick made in the United States. Most flint clays break with a
conchoidal, or shell-like, fracture. Their plasticities and drying shrinkages,
after they have been ground and mixed with water, are very low; their firing
shrinkages are moderate. The best clays of this type are low in impurities and
have a Pyrometric Cone Equivalent (PCE) of Cone 33 to 34-35. Deposits of
flint and semi-flint clays occur in rather limited areas of Pennsylvania,
Maryland, Kentucky, Ohio, Missouri, Colorado and several other states.
Plastic and semi-plastic refractory clays, often called “soft clays” or “bond
clays”, vary considerably in refractoriness, plasticity and bonding strength.
Drying and firing shrinkages are usually fairly high. The PCE of clays of this
type ranges from Cone 29 to Cone 33, for the most refractory varieties, and
from Cone 26 to Cone 29 for many clays of high plasticity and excellent
bonding power. Kaolins consist essentially of the mineral kaolinite. They
usually are moderately plastic and have extremely high drying and firing
shrinkages. Siliceous kaolins shrink less and bauxitic kaolins shrink more
than kaolins which consist almost wholly of kaolinite. Refractory kaolins
generally have a PCE of Cone 33 to 35; less pure varieties with a PCE of
Cone 29 to 32 are common. Among the largest deposits of refractory kaolin
are those which occur in Georgia and Alabama. Most commercial deposits of
flint and plastic refractory clay occur in sedimentary strata in association with
coal beds. Usually, individual occurrences are relatively small and of
irregular form.
HIGH ALUMINA:
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%. There are several other special classes of highalumina products worth noting:
• Mullite brick - predominantly contains the mineral phase mullite (3Al2O3
.2SiO2) which, on a weight basis, is 71.8% Al2O3 and 28.2% SiO2.
• Chemically-bonded brick - usually phosphate-bonded brick in the 75% to
85% Al2O3 range. An aluminum orthophosphate (AlPO4) bond can be formed
at relatively low temperatures.
• Alumina-chrome brick - typically formed from very high purity, highalumina materials and chromic oxide (Cr2O3). At high temperatures, alumina
and chromia form a solid solution which is highly refractory.
• Alumina-carbon brick - high-alumina brick (usually bonded by a resin)
containing a carbonaceous ingredient such as graphite.
SILICA REFRACTORIES:
The raw material used in the manufacture of silica refractories consists
essentially of quartz in finely crystalline form having the proper
characteristics for conversion to the high temperature crystal modifications of
silica. To assure the highest commercial quality in the refractory product, the
mineral must be washed to remove natural impurities.