Download Fireclay Refractories: Refractory fireclay consists essentially of

Fireclay Refractories:
Refractory fireclay consists essentially of hydrated aluminum silicates with
minor proportions of other minerals. As defined by the American Society for
Testing Materials (ASTM), there are five standard classes of fireclay brick:
superduty, high-duty, medium-duty, low-duty and semi-silica. These classes
cover the range from approximately 18% to 44% alumina, and from about
50% to 80% silica. A blend of clays is commonly used in the manufacture of
high-duty and superduty fireclay brick. Flint clays and high-grade kaolin
impart high refractoriness; calcined clays control the drying and firing
shrinkages; plastic clays facilitate forming and impart bonding strength. The
character and quality of the brick to be made determines the relative
proportions of clays used in a blend. Superduty fireclay brick have good
strength and volume stability at high temperatures and an alumina content of
40% to 44%. Some superduty brick have superior resistance to cracking or
spalling when subjected to rapid changes of temperature. High-duty fireclay
brick are used in large quantities and for a wide range of applications.
Because of their greater resistance to thermal shock, high- duty fireclay brick
can often be used with better economy than medium-duty brick for the linings
of furnaces operated at moderate temperatures over long periods of time but
subject to frequent shutdowns. Medium-duty brick are appropriate in
applications where they are exposed to conditions of moderate severity.
Medium-duty brick, within their serviceable temperature ranges, can
withstand abrasion better than many brick of the high-duty class. Low-duty
fireclay brick find application as backing for brick with higher refractoriness,
and for other service where relatively moderate temperatures prevail. Semisilica fireclay brick contain 18% to 25% alumina and 72% to 80% silica, with
a low content of alkalies and other impurities. With notable resistance to
shrinkage, they also have excellent load-bearing strength and volume stability
at relatively high temperatures.
Brick Classifications:
1. Superduty:
The outstanding properties of superduty fireclay brick are refractoriness,
alumina content of 40% to 44%, strength, and volume stability at high
temperatures. Many superduty brick have good resistance to cracking or
spalling when subjected to rapid changes of temperature. Their refractoriness,
in terms of their PCE values, may not be less than 33. In the class of
superduty fireclay refractories are several modifications, including brick
which are fired at temperatures several hundred degrees higher than the usual
product. The high firing temperature enhances the high-temperature strength
of the brick, stabilizes their volume and mineral composition, increases their
resistance to fluxing and renders them practically inert to disintegration by
carbon deposition in atmospheres containing carbon monoxide gas.
2. High-Duty:
The PCE value of high-duty fireclay brick may not be less than 31 1/2, and
ordinarily varies from 31 1/2 to 32 1/2 -33.
3. Medium-Duty and Low-Duty:
Fireclay brick of the medium-duty class have PCE values of 29 to 31. The
PCE values of low-duty fireclay brick cover the range from 15 to 27-29.
Fireclay Brick Manufacture:
Most fireclay brick are made from blends of two or more clays. Some brick,
especially those of the low-duty class, are made of a single clay. The mixes
for superduty and high-duty brick commonly contain raw flint and bond clays,
with or without calcined clay. In making brick of kaolin and various other
clays, a large proportion of the mix is precalcined to control firing shrinkage
and stabilize the volume and mineral composition of the product. In making
fireclay brick, the particles of ground clay must include a range of graded
sizes, each in proper proportion. The clays are typically ground in a “dry
pan,” which is a rotating, pan-shaped grinding mill having slotted openings in
the bottom. The batches are screened to the desired sizes and thoroughly
mixed with a small but closely controlled amount of water. The moistened
batch is then fed to a mechanically or hydraulically operated press in which
the brick are formed under pressure. In a modification of the power-press
process, certain physical properties are enhanced by the application of a high
vacuum during the forming of the brick. Brick made in this way typically
have a more homogeneous texture and are harder, stronger, less porous and
more dense than those made without vacuum. As a consequence, they are
more resistant to impregnation and corrosion by slags and to penetration by
gases. The extrusion process is sometimes used for making special shapes. In
making extruded brick, clays are ground in a dry pan, mixed wet or dry in a
mixer, brought to the proper consistency in a pug mill, and extruded through
the die of an auger machine in the form of a stiff column. The air is removed
from the clay before extrusion by a deairing system within the auger machine
chamber. The column is cut into brick by means of wires. The brick are then
typically repressed to give them sharp corners and edges and smooth surfaces.
Many intricate special shapes are formed in vertical piercing-and-forming
presses, in which blanks from the extrusion machine are completely reshaped.
Brick formed by any of the processes described above are dried in tunnel or
humidity driers. The temperature of firing depends upon the maturing
temperatures of the clays, and often upon the service for which the brick are
intended. In firing the brick, several necessary ends are accomplished: free
and combined water are driven off; iron and sulfur compounds and organic
matter are oxidized, and the gases formed are eliminated; mineral
transformations and changes in volume are affected; and finally, the particles
of clay are ceramically-bonded together into mechanically strong brick.
Silica Refractories:
Silica refractories are well adapted to high temperature service because of
their high refractoriness, high mechanical strength and rigidity at temperatures
almost up to their melting points, as well as their ability to resist the action of
dusts, fumes and acid slags. The American Society for Testing Materials
(ASTM) divides silica brick into Type A and Type B based on the brick’s flux
factor. Flux factor is determined by adding the alumina content and twice the
total alkali content. The Type A class includes silica brick with a flux factor
of .50 or below; Type B includes all silica brick with a flux factor above .50.
Both classes require that brick meet the following criteria: Al 2 O 3 less than
1.5%; TiO 2 less than 0.2%; Fe 2 O 3 , less than 2.5%; CaO less than 4%;
and average modulus of rupture strengths not less than 500 psi. This system
for classifying silica brick was preceded by a less exact system which still is
referenced today. Under the earlier system, non- insulating silica brick were
either of conventional or superduty quality. Insulating silica brick were
classified only as superduty. Brick classified as superduty silica brick could
not contain more than a total of 0.5% alumina, titania and alkalies.
Manufacture of 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. After being formed, the
brick must be fired at a temperature high enough to convert the quartz into
forms of silica that are stable at high temperatures. In the firing and cooling
process, refractories must pass through several critical temperature ranges;
consequently, it is necessary to maintain a carefully planned time-temperature
schedule during the firing process. A proper schedule assures the production
of strong, well-bonded brick which attain their normal permanent expansion
of 12% to 15% by volume.
Effect of Aluminas and Alkalies:
After firing, silica brick contain a small proportion of silicates in the body that
is otherwise crystalline silica. Upon being reheated to high temperatures,
these silicates melt and form a small amount of liquid. As the temperature
rises, the liquid increases because the silica also melts, at first slowly and then
more rapidly, especially above 2900°F (1600°C). When relatively small
amounts of silicate liquid are present, the solid crystalline portion of the brick
forms a rigid skeleton, with liquid merely present between the solid particles,
and the brick as a whole retains its rigidity even under load. When larger
amounts of liquid develop at higher temperatures, the bond weakens and the
brick may lose its rigidity.
When silica brick contain the usual 2% to 3.5% of lime, the percentage of
liquid formed at high temperatures increases almost in direct proportion to the
total amount of alumina, titania and alkalies present. The temperature of
failure under load decreases correspondingly. Individually, these oxides and
alkalies vary appreciably in their effects on temperature of failure, but their
total concentration is the significant factor. When the sum of alumina, titania
and alkalies is less than 0.5%, the temperature of failure under a load of 25
pounds per square inch is 50°F (28°C) to 90°F (50°C) higher, than for brick
containing a total of 1% of these oxides. For this reason, brick classified as
superduty must contain no more than a total of 0.5% alumina, titania and
alkalies.
Characteristic Properties:
Among the important properties of silica brick are their relatively high
melting temperatures, i.e., approximately 3080°F (1695°C) to 3110°F
(1710°C); their ability to withstand pressure of 25 to 50 pounds per square
inch at temperatures within 50°F (28°C) to 100°F (56°C) of their ultimate
melting points; high resistance to acid slags; constancy of volume at
temperatures above 1200°F (650°C); and virtual freedom from thermal
spalling above 1200°F (650°C). At high temperatures, the thermal
conductivity of most silica brick is somewhat higher than that of fireclay
brick. At temperatures below 1200°F (650°C), silica brick have less resistance
to thermal shock. They are readily attacked by basic slags and iron oxide at
high temperatures in a reducing atmosphere.
Silica Brick Products:
Certain superduty silica brick have been developed to meet the demand for a
silica refractory that would permit higher furnace temperatures, give longer
life and reduce maintenance costs. These brick contain no more than 35%
alumina plus alkalies and titania. Superduty silica brick are used with
excellent results in the superstructures of glass-tank furnaces. For many years,
conventional quality silica brick have been regarded as the standard. The
properties responsible for the excellent service record of this brick are rigidity
under load at high temperatures, high resistance to spalling above 1200°F
(650°C), high mechanical strength, resistance to abrasion, resistance to
corrosion by acid slags and uniformity of size. Improved versions of
conventional quality silica brick are available having better resistance to high
temperature thermal shock. A lightweight silica brick with a bulk density of
65 to 70 pounds per cubic foot (1,041 to 1,121 kg/m 3 ) is suitable for use up
to 3000°F (1650°C). At a mean temperature of 1200°F (650°C), its insulating
value is excellent. Lightweight silica brick are used largely for the insulation
of silica brick constructions, especially the crowns of glass-tanks. They are
also ideal for the construction of tunnel kiln crowns, and their properties are
conducive to arches having a wide span.