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453
Portland cement manufacturing is an energy intensive operation that involves pyroprocessing of
raw materials, referred to as the kiln feed, at extremely high temperatures in rotary kilns. The kiln
feed primarily consists of limestone with some additions of clay, sand, and iron oxide that chemi-
cally interact to form cement clinker. The kiln feed is alkaline in nature; however, the raw materials
often contain species that can generate corrosive reactants in the form of solids as well as gases. In
a dynamic rotary kiln where these reactions occur at temperatures between 1250°C to 1450°C, a
refractory lining that can withstand high temperatures, alkalinity, and corrosive conditions is
absolutely essential. Refractory requirements for wet and dry kiln processes, and for kilns with
cyclone preheaters and precalciners, differ significantly. In all situations, the refractories must have
good hot strength, resistance to abrasion, compatible chemical composition, and sound thermal
characteristics. This chapter discusses the importance of refractories, their types, and applications
in cement manufacturing. A cement rotary kiln with refractory lining is shown in Figure 3.7.1.
*President, RefrAmerica, Inc., 205, Sunset Drive, Suite 2, Butler, Pennsylvania 16001, E-mail: [email protected]
Figure 3.7.1. Rotary kiln containing refractory lining.
Chapter 3.7
by Ricardo Araujo Mosci*
Refractories in CementManufacturing
Page 2
ROLE OF REFRACTORIES
Refractories are ceramic materials capable of withstanding elevated temperatures without signifi-
cant deterioration. The role of refractories in cement kilns is multiple:
1. To protect the steel shell against heat –
Material and gas temperature inside the
rotary kiln surpass the maximum work-
ing temperature recommended for
carbon steel. Without refractories the
kiln shell would be destroyed by heat. As
a result, as soon as the refractory lining
fails, the kiln must be shut down for
lining repair. The overheated areas on
the kiln shell are commonly known as
“hot spots” or “red spots.” Figure 3.7.2
illustrates a refractory failure that
created a red spot on the shell.
2. To protect the kiln shell against abrasion – Cement clinker is very abrasive and without refrac-
tories the steel shell would be damaged by abrasion.
3. To minimize heat loss through the kiln shell – Part of the heat supplied to the kiln system is
lost as radiation through the steel shell. Refractories reduce heat loss because of their relatively
low thermal conductivity.
4. To control the flow of material through the kiln – The kiln load travels under the combined
action of kiln rotation and slope. Cam linings, dams, tumblers, and trefoils in the kiln oppose
material flow allowing some control of material residence time.
5. To promote heat transfer to the kiln load – Tumblers, trefoils, and profiled linings induce
material tumbling and mixing, which in turn promote heat transfer from gas to solids, from
refractory to load, and within the load itself through agitation and surface renewal.
TYPES OF REFRACTORIES FOR CEMENT KILNS
Refractories used in the kiln, cooler, and preheater are supplied either as pressed and fired brick,
unshaped as monolithic products, or in pre-cast, pre-fired shapes. The rotary kiln is almost
entirely lined with bricks, while the preheater, cooler, and gas ducts are usually lined with castables,
plastics, or pre-cast shapes held in place by metal or ceramic anchors attached to the shell.
Bricks are classified in four major groups according to their composition: basic, high alumina, fire-
clay, and special materials.
Innovations in Portland Cement Manufacturing454
Figure 3.7.2. Red spots on the kiln shell.
Page 3
Basic Bricks
Basic bricks have magnesia or dolomite as their major component, and a secondary mineral such
as alumina, zircon, or spinel as a minor component. In most products the major component
concentration varies between 60% and 95% by mass.
Natural sources of magnesia or dolomite of refractory quality are found in very few countries
around the world. Most of the magnesia used in cement kiln brick comes from seawater or brine
deposits. These are called synthetic magnesia or periclase sinters.
Magnesia alone is not used in kiln brick manufacture because of its poor thermal cycling proper-
ties. For this reason, magnesia is blended with a secondary mineral before it is pressed and fired
into bricks. The secondary mineral confers thermal shock properties or modified chemical proper-
ties to the brick.
Chromium ores were widely used as secondary minerals in magnesia brick, but disposal problems
caused by hexavalent chromium rendered these products an unattractive option in most countries.
As a result, magnesia-chromium products were replaced with magnesia-alumina spinel products.
Although magnesia-alumina spinel products do not present the excellent coatability of their pre-
decessors, their thermal spalling resistance and alkali resistance far exceed those of chromium-
containing products. Magnesia-spinel products also resist reducing conditions better than
magnesia-chromium products.
The magnesia-alumina spinel content in
commercial bricks varies from 3% to 18% by
mass. Higher magnesia products are usually
more refractory but have higher thermal
conductivity than lower magnesia products of
similar porosity. Higher spinel bricks are more
susceptible to chemical attack and fluxing than
lower spinel bricks, but they exhibit better
coatability and higher resistance to thermal
spalling. Figure 3.7.3 depicts a fluxed magnesia-
spinel brick. The spinel itself can be sintered or
fused, causing major differences in brick price and performance. Fused oxides are less reactive and
less expensive than sintered oxides. It becomes clear from the previous facts that refractories
cannot be compared only on the basis of their chemical or physical composition.
In order to improve the coatability of magnesia-spinel products, some manufacturers in Japan and
Europe replace the magnesia-alumina spinel with iron-alumina spinel in different forms and
455Refractories in Cement Manufacturing
Figure 3.7.3. Overheated magnesia-spinelbrick.
Page 4
concentrations. Iron oxides and spinels have been extensively used for this purpose. Trials were
also run with oxides of manganese.
Another important member of the basic brick group is dolomite brick. Due to their compatibility
with clinker minerals, dolomite bricks have good affinity for coating, making them an excellent
choice for burning zone applications. Most dolomite bricks receive additions of zirconia or other
secondary minerals to improve their thermal shock properties and also to delay brick infiltration
with clinker melt and alkali salts. Some modern dolomite bricks include additions of magnesia,
while others include additions of pitch or tar to decrease brick permeability and reduce its suscep-
tibility to chemical attack. Dolomite products offer the lowest direct cost among all basic brick, but
their application has been confined mostly to the burning zone where the clinker coating is more
stable. The most adverse property of dolomite products is their risk of hydration, requiring special
care in packaging, handling, and storage. For this reason dolomite products are usually confined to
areas not too far from the manufacturing plant. Thanks to special vacuum packaging and brick
treatment, the shelf life of dolomite products has increased considerably in recent years.
High-Alumina Bricks
High-alumina products used in cement kilns vary in alumina content from 50% to 85% by mass.
Depending on the type of raw material used in their manufacture, high-alumina products of the
same class present wide differences in properties and cost. To assume that all 70% alumina bricks
are just commodities is a risky generalization. For instance, a mullite-based product presents
greater thermal shock resistance than its bauxite counterpart. Similarly, a 60% andalusite brick
resists alkali attack much better than its 60% bauxite equivalent.
Higher alumina products, such as 80% or 85% are sometimes used in the discharge zone of the
kiln because of their superior mechanical strength and abrasion resistance. These bricks sometimes
contain 1% to 3% phosphorus pentoxide to improve their hot strength and abrasion resistance.
Phosphate additions have a tremendous impact on the stress-strain behavior of the product. Some
phosphate-bonded bricks, as they are called, are oven-cured rather than fired at high temperatures.
These products present better dimensional tolerances than their fired equivalents and are called
chemically-bonded brick, as opposed to clay-bonded or ceramic-bonded.
In the calcining zone of the kiln, 70% alumina is the preferred choice unless alkali attack is so
severe that a lower alumina product is required. As a general rule, the resistance to alkali attack
increases as the alumina content decreases.
Fireclay Bricks
According to their alumina content, refractoriness, and porosity, fireclay products are classified as
high duty, super duty and semi-silica. Although widely used in the upper part of the calcining zone
in the past, high-duty and super-duty bricks are gradually being replaced by high-alumina and
Innovations in Portland Cement Manufacturing456
Page 5
semi-silica products. One of the reasons is the difficulty of keeping fireclay brick tolerances within
acceptable limits. Another reason is loss of strength at higher operating temperatures. During kiln
upsets, fireclay products tend to react with the kiln load, leading to abrupt lining failure.
Semi-insulating products, although very low in mechanical strength, have the unique ability to
react with alkali vapors in the kiln to form a thin glaze that protects it from abrasion and chemical
attack. Their low thermal conductivity significantly reduces kiln shell temperature, a major advan-
tage over tires. For best performance, these lightweight products must be installed with mortar.
When selecting a semi-insulating product, attention must be paid to its thermal expansion proper-
ties. Some products shrink at temperatures above 1000°C, a great risk in precalcining kilns, for
instance.
Insulating bricks form a unique class of products. They are used only as backup linings for denser
products in the preheater, cooler, and tertiary air duct. They lack the mechanical strength and
refractoriness necessary to be used directly as the work lining.
Carbide and Zircon Bricks
In very special situations, zircon and silicon carbide bricks are used in the kiln with the purpose of
minimizing ring formation in the calcining zone. Zircon bricks react with the clinker liquid phase
when installed too close to the burning zone. Similarly, silicon carbide bricks promptly react with
oxygen at high temperatures, especially in the presence of alkali or steam. Moreover, silicon carbide
products have high thermal conductivity and low thermal expansion, making it difficult to get a
tight lining in service. These disadvantages restrict the use of carbide and zircon brick to the
calcining zone of the kiln.
REFRACTORY CLASSES
According to their shape, refractories are classified as:
Pressed and fired products: This class includes bricks, tiles, and blocks. Bricks are preferably
used in uniform sections such as the kiln, cylindrical vessels in the preheater, and straight walls in
the kiln hood and cooler. Special pressed shapes such as tiles and large blocks have been replaced
with more cost-effective castable alternatives.
Monolithic products: This class includes castables, plastic, gunning mixes, shotcrete, and
mortars. Monolithic products are used in complex areas such as ducts, ceilings, and curved
surfaces. These products require efficient anchoring systems for good performance. Although
faster to install than brick, monolithic products require careful curing and dryout before being put
into service.
457Refractories in Cement Manufacturing
Page 6
Specialties: This class includes calcium silicate boards, ceramic fibers, and mineral wool. These
insulating materials are used as backup linings, usually not thicker than 125 mm. In temperatures
above 1100°C, fiber insulators do not have sufficient stability to be used. In these applications they
are replaced with insulating firebrick or castables.
BRICK SHAPES
In order to fit the kiln radius, bricks must be tapered.
Figure 3.7.4 shows a typical wedge shape. When only one
brick shape is used to line the kiln, it is called a one-shape
system. When two shapes of different tapers are used, it is
called a two-shape system. Shapes for different bricks and
brick systems are given in Tables 3.7.1 to 3.7.4.
Innovations in Portland Cement Manufacturing458
Figure 3.7.4. A typical magnesia-spinel wedge.
Table 3.7.1. VDZ Shape System, B Series, Without Cardboard Spacers
Shape LC SC H Shape LC SC HB-216 78.0 65.0 160 B-222 78.0 65.0 220
B-316 76.5 66.5 160 B-322 76.5 66.5 220
B-416 75.0 68.0 160 B-422 75.0 68.0 220
B-616 74.0 69.0 160 B-622 74.0 69.0 220
P-160 95.0 85.0 160 P-220 95.0 87.0 220
P-161 71.0 63.0 160 P-221 71.0 65.0 220
B-218 78.0 65.0 180 B-325 78.0 65.0 250
B-318 76.5 66.5 180 B-425 76.5 66.5 250
B-418 75.0 68.0 180 B-625 74.5 68.5 250
B-618 74.0 69.0 180 B-725 74.0 69.0 250
P-180 95.0 87.0 180 P-225A 95.0 87.0 250
P-181 71.0 65.0 180 P-225B 71.0 65.0 250
B-220 78.0 65.0 200
B-320 76.5 66.5 200
B-420 75.0 68.0 200
B-620 74.0 69.0 200
P-200 95.0 87.0 200
P-201 71.0 65.0 200
RING CALCULATIONS
Number of Shape 2:
Number of Shape 1:
N = *LC * (D - 2H) - ( *D*SC )
LC *SC - LC *SC2
1 1
1 2 2 1
π π
N = *D - (LC *N )LC
12 2
1
π
198SC
LC
H
Page 7
459Refractories in Cement Manufacturing
Table 3.7.2. ISO Shape System, Without Cardboard Spacers
Shape LC SC H Shape LC SC H216 103 86.0 160 222 103 80.0 220
316 103 92.0 160 322 103 88.0 220
416 103 94.5 160 422 103 91.5 220
516 103 96.5 160 522 103 94.0 220
616 103 97.5 160 622 103 95.5 220
716 103 98.3 160 722 103 96.5 220
816 103 98.5 160 822 103 97.3 220
16 A 83 77.5 160 22 A 83 75.5 220
16 B 93 87.5 160 22 B 93 85.5 220
218 103 84.0 180 225 103 77.0 250
318 103 90.5 180 325 103 85.5 250
418 103 93.5 180 425 103 90.0 250
518 103 95.5 180 525 103 92.7 250
618 103 97.0 180 625 103 94.5 250
718 103 97.7 180 725 103 95.5 250
18 A 83 77.0 180 825 103 96.5 250
18 B 93 87.0 180 25 A 83 74.5 250
25 B 93 84.5 250
220 103 82.0 200
320 103 89.0 200
420 103 92.5 200
520 103 94.7 200
620 103 96.2 200
720 103 97.0 200
820 103 97.8 200
20 A 83 76.2 200
20 B 93 86.2 200
RING CALCULATIONS
Number of Shape 2:
Number of Shape 1:
N = *103*(D - 2H) - ( *D*SC )
103*SC -103*SC2
1
2 1
π π
N = *D - (103*N )
1031
2π
198
H
SC
103
Page 8
Innovations in Portland Cement Manufacturing460
Table 3.7.3. Rotary Kiln Wedges and Arches
Shape LC SC L HRKW 1 X 4.0 3.687 6.0 9.0RKW 1 4.0 3.531 6.0 9.0RKW 2 4.0 3.25 6.0 9.0
2/3 split 2.656 2.187 6.0 9.03/4 split 3.0 2.437 6.0 9.0
W 1 3.5 3.25 6.0 9.0W 2 3.5 3.062 6.0 9.0W 3 3.5 2.625 6.0 9.0
2/3 split 2.343 2.062 6.0 9.03/4 split 3.0 2.625 6.0 9.0
7 A 1 3.5 3.25 9.0 7.57 A 2 3.5 3.062 9.0 7.5
2/3 split 2.343 2.093 9.0 7.53/4 split 2.625 2.343 9.0 7.5
RKA 1 4.0 3.687 9.0 6.0RKA 2 4.0 3.5 9.0 6.0
2/3 split 2.656 2.343 9.0 6.03/4 split 3.0 2.625 9.0 6.0
6 A 1 3.5 3.25 9.0 6.06 A 2 3.5 3.062 9.0 6.0
2/3 split 2.343 2.156 9.0 6.03/4 split 2.625 2.388 9.0 6.0
RING CALCULATIONS
Number of Shape 2:
Number of Shape 1:
N = *LC * (D - 2H) - ( *D*SC )
LC *SC - LC *SC2
1 1
1 2 2 1
π π
N = *D*(LC *N )
LC1
2 2
1
π
L
H
SC
LC
Table 3.7.4. Rotary Kiln Blocks
Shape H SC
9 x 9 x 4 9 Var.
6 x 9 x 4 6 Var.
Keys: Cut to size 9 4
H
SC
Page 9
The one-shape family of bricks includes: 1) RKA, rotary kiln arches with 152-mm and 190-mm
(6-in. and 71⁄2-in.) lining thickness, 2) RKW, rotary kiln wedges with 229-mm (9-in.) lining thick-
ness, 3) RKB, rotary kiln blocks with 152-mm and 229-mm (6-in. and 9-in.) lining thickness.
The two-shape family of bricks includes: 1) VDZ shapes with160-mm, 180-mm, 200-mm, 220-mm,
and 250-mm lining thickness, 2) ISO shapes (160-mm, 180-mm, 200-mm, 220-mm, 250-mm
lining thickness, 3) Arch combinations with 152-mm and 190-mm (6-in. and 71⁄2-in.) lining thick-
ness, and 4) Wedge combinations with 229-mm (9-in.) lining thickness.
This multiplicity of shapes is unnecessary and increases manufacturing and inventory costs
considerably. The reason is the high cost of dies and tools, coupled with press set-up time, which
delays production at the brick plant.
Opinions on which shape is best for a given kiln differ considerably from plant to plant and from
person to person within the same plant in a quite subjective way.
Two-shape brick systems or brick combinations fit the kiln shell better than single shape systems be-
cause kiln shells are not perfectly circular. By changing the ratio of the brick combination, the mason
can line over imperfections without any major problem. However, when using one-shape brick over
distorted areas, the mason has to shim the brick with steel plates, thus adding unnecessary stress to
the lining. The claim that one-shape brick lines faster than a two-shape combination is questionable
and lining performance should never be sacrificed at the expense of speed of installation.
Whenever choosing a brick shape system, some practical rules apply:
1. Basic brick usually expands more than high-alumina brick, thus requiring a larger number of
joints per ring. As a consequence, small shapes such as VDZ, RKA, and RKW are more suitable
for basic brick than large shapes.
2. Larger kilns require bricks with more taper such as ISO and RKB.
3. At kiln tires, where shell ovality is high, smaller shapes should be used because they generate
more flexible linings, with a larger number of radial joints.
4. Against brick retainers, larger shapes should be used because they offer less chance for lining
movement against the steel bar.
5. Smaller bricks are less stressful on the masons than the heavier ISO or RKB bricks. This factor
is particularly important for crews working on 12-hour shifts.
461Refractories in Cement Manufacturing
Page 10
One last point to consider when choosing a
brick shape is the lining thickness. There is
no proven correlation between lining thick-
ness and performance. Sometimes a 160-
mm lining lasts twice as long as a 220-mm
lining in a given kiln for the simple reason
that the radial stress that crushes the brick
increases with the lining thickness. The ideal
initial lining thickness for any kiln is the
minimum thickness necessary to reach
acceptable shell temperatures. Figure 3.7.5
shows a special shape designed to reduce
kiln shell temperature.
REFRACTORY PROPERTIES OF PRACTICAL IMPORTANCE
Refractory properties are controlled through standard tests performed by specialized laboratories.
Some of the simpler tests can be performed at the consumer plant, but high temperature and some
physical properties require high cost specialized equipment and trained people for tests to be prop-
erly performed. Manufacturer data sheets include some of these properties but not necessarily the
most important ones for cement kiln application.
Coatability
In order to perform well in the burning
and transition zones, bricks must develop
and keep a stable clinker coating. Figure
3.7.6 is a micrograph of an unusual type
of coating. Without coating, even the
most refractory products would not resist
temperatures above 1500°C in the pres-
ence of fluxes. Although several labora-
tory coatability tests have been
developed, their correlation with actual
kiln conditions is usually weak.
Other properties being constant, brick coatability decreases in the following order:
dolomite, magnesia-chrome, magnesia-alumina spinel, alumina, zirconia, silicon carbide.
Brick coatability is also affected by the degree of mineral impurities contained in the brick. For
instance, a magnesia-spinel brick containing larger amounts of iron, alumina, silica, and lime coats
Innovations in Portland Cement Manufacturing462
Figure 3.7.5. Special brick design to reduce shelltemperature.
Figure 3.7.6. Micrograph of coating showing differ-ent crystal species, sizes, and orientations.
Page 11
better than a similar brick made with high purity raw materials. Some brick manufacturers add
iron to their brick in order to promote coating formation.
Excessive coating formation is as harmful to the lining as no coating. High coatability materials are
sometimes deeply infiltrated with clinker minerals. During kiln upsets when the heavy coating
falls, it carries with it considerable amounts of brick. Under heavy coating, less permeable products
with lower chemical affinity for coating should be employed.
Permeability
Permeability is a measure of the brick resistance to infiltration with gases and liquids. For the same
service conditions, high-permeability products become more deeply infiltrated with liquids than
low-permeability products. The infiltrates can react with the brick components or just condense
inside the brick. During kiln shutdown or upon coating loss, the infiltrated brick spalls off.
Permeability becomes critically important when the concentration of alkali and sulfur in the raw
materials and fuels is high, particularly when burning waste-derived fuels in the burning zone.
Thermal Conductivity
Thermal conductivity is a function of material composition and manufacturing. For a given
composition, a better pressed, less permeable product has higher thermal conductivity than a less
pressed, more porous product. In the absence of coating, magnesia-spinel products have higher
thermal conductivity than their magnesia-chrome and alumina counterparts. Shell temperatures
of 400°C and higher are not uncommon in the upper transition zone of precalcining kilns, even
with a new lining. Artificial ways to reduce the thermal conductivity of a basic brick, such as air
gaps or pockets filled with ceramic fiber, present two adverse consequences: they increase the depth
of brick infiltration with volatiles, and they promote kiln shell corrosion behind the lining.
The use of a two-component lining such as a dense brick installed over an insulating brick is not
used in the kiln for the previous reasons and also because of the mechanical instability of the
lining.
Abrasion Resistance
Refractory abrasion occurs mostly past the burning zone toward the cooler, where clinker is
already formed and the lining has no coating. The burner pipe lining, the kiln discharge zone, the
nosering, the cooler bull nose, and curbs are the most affected areas.
Abrasion resistance is measured in the laboratory, and test results correlate well with kiln applica-
tions. The test consists of blasting a pre-measured piece of refractory with a granular abrasive
under controlled conditions and calculating the volume loss afterwards.
463Refractories in Cement Manufacturing
Page 12
Reversible Thermal Expansion
Brick thermal expansion is important because it governs the mechanical stability of the lining in
service. Basic brick usually expands more than alumina brick and fireclay in the entire temperature
range. That is why in the absence of coating, basic linings have less tendency to move against the
kiln shell than alumina or fireclay products. Brick spiraling and shifting usually occurs in the
alumina and fireclay sections. Since at their respective temperatures basic brick expands more than
the kiln shell, cardboard spacers pre-glued to the brick are used to compensate for the expansion
difference. Too many spacers create gaps in the lining and should be avoided. Too few spacers cause
the lining to spall off in a dish-like pattern. Thermal expansion and lining shifting are better
controlled with the use of mortar.
The most difficult area in which to control
thermal expansion is around the burner pipe
where the steel shell, metal anchors, and
refractory lining expand and shrink at differ-
ent rates. Figure 3.7.7 illustrates a heavy duty
anchor used in nose rings, tumblers and
chains. Pre-engineered expansion joints do
not work well because no prediction can be
made as to where the lining is going to crack.
Elastic Modulus
The elastic modulus is one of the most
important properties for kiln brick. Being a ratio between stress and strain, it determines how elas-
tic or inelastic a brick is under mechanical stress. Figure 3.7.8 exemplifies the consequences of
severe mechanical stress on the lining. Since
the kiln shell is not rigid, the lining must be
able to absorb the ovality stress generated
around tire areas. Another property that
depends on the elastic modulus is the thermal
spalling resistance of the product. Products
with low elastic modulus resist thermal
spalling and mechanical stress better than
products with high elastic modulus. This
important property is seldom displayed in
commercial product data sheets.
Innovations in Portland Cement Manufacturing464
Figure 3.7.7. Heavy duty metal anchor.
Figure 3.7.8. Brick damaged by mechanicalstress.
Page 13
Chemical Composition
Refractory chemistry is important because it helps determine if a given product is compatible with
certain applications. Although the chemical composition alone is not a suitable criterion to define
product usage, it is an important tool for predicting material compatibility, coatability, elastic
behavior, and resistance to thermal shock.
Dimensional and CosmeticProperties
Brick dimensions are critical to product
installation and performance (see Figure
3.7.9). Chord tolerances should be kept within
0.6% from the nominal value, and brick taper,
defined by the chord difference, should be
kept within 1 mm from the nominal value.
Taper deviations larger than 1 mm require the
use of turning shims during installation.
Depending on how large the deviation is,
sometimes the brick cannot be installed even
with the help of correction shims or mortar.
The reason for such narrow tolerances is the
cumulative effect of the deviation over the
large number of bricks required per ring.
Brick measurement is a simple task; it should always be performed by the end user or its desig-
nated inspector. All it requires is a tape measure for general dimensions and a caliper for chord and
warpage measurement.
Warped brick is a serious manufacturing
defect (see Figure 3.7.10). If the brick
surfaces are concave or convex, the bricks
only touch each other in a few spots, thus
generating high mechanical stress in serv-
ice. Sometimes the warpage is so severe
that the bricks crack during the keying of
the rings. The use of mortar during instal-
lation can minimize the risks.
Brick asymmetry is a manufacturing defect caused during pressing. Asymmetric bricks should
never be installed in the kiln, even with the help of shims or mortar.
465Refractories in Cement Manufacturing
Warpage Assymetry
SC
LC
H
L
Turning Diameter = 2 x H x LC/(LC - SC)
Figure 3.7.9. Important brick dimensions.
Figure 3.7.10. Brick defects.
Page 14
All brick surfaces but the hot face must be free of imperfections, sintering, and sand grains. Any
material attached to the brick surface creates tiny gaps in the lining. As the kiln rotates, these gaps
accumulate and may lead to lining loosening and brick loss.
There are many more specific and generic properties of refractories such as density, porosity,
modulus of rupture, cold crushing strength, thermal expansion, and refractoriness. However, from
a strictly functional point of view, the ones previously described are the most important.
REFRACTORY WEAR
No matter how good or how suitable a refractory is, sooner or later it deteriorates and fails in serv-
ice, forcing a kiln shutdown. Refractory wear increases with temperature and time. Some failures
are progressive and the kiln can be shut down following a standard procedure before any damage is
done to the shell. However, some failures are unpredictable, giving the operator almost no chance
to protect the kiln shell. Most sudden brick losses bring permanent damage to the kiln shell,
particularly if they coincide with power failures and heavy rain.
Modern kilns are equipped with shell temperature scanners that enable the kiln operator to see
where the coating or the refractory lining is thin, before any damage is done to the shell. Scanners
detect the so-called hot spots before they turn into damaging red spots. It is very important to
instruct the kiln operators, through written procedures, on what to do during such emergencies.
Refractories fail at different times, in different
kiln zones, and the failure mechanisms usually
fall into one of three categories: 1) thermal
stress including overheating and thermal shock,
2) mechanical stress including compression,
shearing, and pinch spalling, and 3) chemical
attack including alkali bursting, redox, hydra-
tion, and fluxing, as shown in Figure 3.7.11.
Most refractory failures are caused by a combi-
nation of two or more stress factors, such as a
chemical reaction followed by brick melting, or
lining densification followed by structural spalling.
Experience has demonstrated that the great majority of refractory failures are caused by poor kiln
maintenance and unstable kiln operation. It is a well-recognized fact that stable kilns have refrac-
tory performance superior to unstable kilns of the same type and size.
Innovations in Portland Cement Manufacturing466
Figure 3.7.11. High-alumina brickdestroyed by fluxing.
Page 15
In the thermal stress category, lining overheating
and sudden coating loss are the most common
causes of brick failure. Overheating can be caused
by many different factors such as feed starvation,
excess fuel, kiln stoppages with the burner on, slow-
ing the kiln down for long periods of time, defective
burner pipe, and massive ash ring formation in the
upper transition zone. Figures 3.7.12 and 3.7.13
show massive ring formation in the kiln. Lining
overheating can be restricted to a given kiln zone or
even to a few rows of brick within the zone. Sudden
coating loss, for instance, submits the lining to
damaging thermal shock.
In the mechanical stress category, brick crush-
ing is the most common problem (Figure
3.7.8). The main reasons for brick crushing
are: 1) installation problems such as too many
shims, gaps, and misalignment, 2) excessive
shell ovality, 3) kiln misalignment (doglegs) as
shown in Figure 3.7.14, and 4) improperly
designed brick retainers.
In the chemical attack category, brick reaction
with clinker melt and alkali salts is the most
common problem. The spinel phase in
magnesia-spinel products reacts with clinker
minerals to form low-melting compounds.
Dolomite reacts with sulphur and chlorine in a destructive way. Alumina bricks react with silica
and potassium, forming compounds that burst the brick out. The intensity of the chemical attack
increases with temperature, time, and
proximity to the burning zone. The
disposal of alternative fuels in cement
kilns has intensified chemical attack to
the lining in all kiln zones, including the
preheater and cooler.
467Refractories in Cement Manufacturing
Figure 3.7.12. View inside the burningzone of a kiln. A ring can be seen in thebackground.
Figure 3.7.13. Spurrite ring formation in thecalcining zone.
Figure 3.7.14 – Lining destroyed by kiln misalignment.
Page 16
REFRACTORY APPLICATIONS
Choosing which product to use in each kiln area is perhaps one of the most difficult tasks refrac-
tory suppliers and users face. The number of refractory products available to consumers today
surpasses the hundreds. To add to the complexity of the problem, the type of fuels disposed of in
cement kilns has also increased considerably, from agricultural wastes to plastic residues. The
major problem in refractory selection is the lack of information about the kiln conditions.
Modern kiln systems are lined with refractories in four different areas: 1) preheater including inlet,
feed shelf, precalciner, and cyclones, 2) rotary kiln, 3) hood including burner pipe, kiln door, and
tertiary air intake, and 4) cooler including walls, bull nose, curbs, and tertiary air intake.
Preheaters
Preheaters are lined with brick, castables, and combinations of brick and castables. The best
performances are obtained from brick linings because bricks have more uniform properties, are
fired at high temperatures, and do not require the use of anchors. Bricks also yield more flexible
linings than castables because of the larger number of joints in the brickwork. Perhaps the major
negative in brick linings is that they require more time and skill to be properly installed.
Preheater linings consist of two layers of materials: a dense layer, also called the work layer, over a
layer of insulating material such as insulating castable, firebrick, or fiberboard. The insulation
must be efficient to minimize heat loss through radiation because the surface area of the preheater
vessels and ducts is large. The combined lining thickness rarely exceeds 250 mm, with the dense
layer usually taking from 50% to 75% of the total lining thickness.
The higher vessels and ducts in the preheater only require fireclay, low-alumina brick, or castable.
Toward the kiln, where temperatures are higher, the lower stages 3, 4, and 5 work under high
concentrations of chlorine, potassium, and sulfur. In these areas the refractory must be less perme-
able and more resistant to alkali infiltration and attack. Care must be taken not to sacrifice chemi-
cal resistance for refractoriness because the maximum temperature in this part of the kiln rarely
exceeds 900°C.
Another important requirement for refractories in this area is their ability to repel buildups. As
alkali sulfate and alkali chloride vapors progressively condense as salts on the lining surface, they
reduce gas and solids flow, thus reducing preheater efficiency. If these buildups are not removed
periodically through air or water blasting, they can completely block gas and material passage, thus
forcing a kiln shutdown. Buildup removal with water requires high resistance to thermal shock
from the lining. The material of choice for these areas is 60% or 70% alumina, low-cement
castable, held in place by a combination of metal and ceramic anchors. If buildups are severe,
zirconia or silicon carbide containing castables are much better alternatives since they do not hold
Innovations in Portland Cement Manufacturing468
Page 17
buildups strongly. The use of silicon carbide in this application is not recommended if buildup
removal is done with a water blast. Water promotes carbide oxidation at high temperatures.
Precalciners
For the precalciners, the riser duct, and the feed shelf, the same recommendations apply. In
buildup areas the monolithic lining should not be gunned, shotcreted, or rammed, for maximum
coating repellency.
The lining inside cyclones and the flash calciner is usually a combination of brick on the cylin-
drical surfaces and castables on the conical sections, roof, vortex finders, and inlet chamber. The
calciner lining must also resist reducing conditions created by incomplete fuel combustion.
Rotary Kilns
Modern rotary kilns can be safely lined with just two types of bricks: 1) high alumina in the calcin-
ing and discharge zones, and 2) magnesia-spinel in the lower transition, upper transition, and
burning zones. Many new kilns have been successfully commissioned with this simplified lining
configuration, with good results. Some other kilns are lined with dolomite brick in the burning
zone, magnesia-spinel brick in both transition zones, and high alumina in the calcining and
discharge zones. The choice between dolomite and magnesia spinel in the burning zone depends
upon a series of factors such as coating stability, type of fuels injected in the burning zone, insuf-
flation of sodium carbonate or calcium chloride in the burning zone, and also the kiln run factor.
The advantage of magnesia-spinel products over dolomite in the burning zone is their better
resistance to structural spalling during coating loss or removal. The disadvantages are higher direct
cost, lower coatability, and spinel reactivity with clinker melt.
The length and relative position of each kiln zone is a function of several factors such as kiln type,
kiln dimensions, cooler type, fuel properties, burner design and position in the kiln, raw mix burn-
ability, coating stability, and amount of liquid phase at different temperatures. Empirical rules such
as defining zone length as a multiple or fraction of the kiln diameter should be avoided because
they do not correlate with actual kiln conditions.
Kiln Hood
The next area of concern is the kiln hood, a transition chamber between the kiln and the clinker
cooler. Temperatures in the hood are higher than those in the preheater, and potassium attack is a
factor in refractory selection. Another factor of concern is the high concentration of abrasive
clinker dust that could penetrate behind the refractory lining, pushing it in until it collapses. The
most suitable material for hood walls is fireclay or low-alumina brick, followed by pre-cast, pre-
fired shapes, and cast-in-place linings. On average, shotcrete or gunning mixes do not outlast the
previous alternatives because they lack uniformity of properties.
469Refractories in Cement Manufacturing
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Although bricks invariably outlast monolithic products in this application, brick installation is
labor intensive and most plants avoid it. From a purely cost/benefit standpoint, brick lining is the
best alternative. Pre-cast, pre-fired shapes made with low-cement castables usually last from 3 to 10
years without maintenance in cooler walls. However, when specifying shapes for this application,
attention must be paid to all details during the design, manufacture, and installation of the shapes.
The back wall and the hood ceiling can be rammed with refractory plastic, gunned, sprayed, or
formed, and cast. The two main wear factors in these areas are clinker dust, alkali attack, and
anchor failure. In some kilns the back wall is equipped with air blasters to eliminate clinker
buildup. The use of silicon carbide materials in this application could be advantageous.
Burner Pipe
The burner pipe is usually lined with 75
to 100 mm of plastic or castable, held in
place by metal anchors. Anchor failure
and differential expansion are the most
frequent reasons for burner pipe failure
as displayed in Figure 3.7.15.
Consequently, it is important that the
metal anchors are the floating type.
Usually only the first 500 mm from the
tip of the burner become damaged in
service. This is the area that requires the
most attention during material selection
and installation.
Clinker Cooler
With modern, high-efficiency coolers, the secondary air temperature surpasses 1000°C, thus requir-
ing more refractory products around the hood. Metal anchor failure under thermal stress became
common, requiring stainless steel of higher grade and caliber. Ceramic anchors are required in the
hottest areas. In some extreme cases, basic brick has been successfully used in the hood.
In the cooler, the three sidewalls before the bull nose can be lined in many different ways. A cost-
effective alternative for this application is pre-cast blocks individually anchored through the cooler
shell as shown in Figure 3.7.16. Anchoring the blocks inside the shell defeats the main advantage of
this system: quick lining repair. The material of choice for the blocks is 70% alumina, low-cement
castable. If cooler buildups (“snowmen”) are severe, then the blocks can be cast with silicon
carbide to take advantage of its non-sticking properties. The air blasters are still required between
the carbide blocks.
Innovations in Portland Cement Manufacturing470
Figure 3.7.15. Damaged burner pipe lining.
Page 19
The bull nose is one of the most difficult areas for lining stability. Usually the wear mechanism is
dust penetration behind the lining, followed by anchor overheating and shearing. The best lining
alternative is interlocking pre-cast shapes anchored to a hollow box beam. Cold air is blown into
the box to cool down the anchoring system. The refractory material must resist constant abrasion
from hot clinker dust and frequent temperature changes.
At the grate level, refractory curbs are used to keep the clinker from eroding the walls. Curbs are
usually formed and cast in place, requiring careful heat up because of their massive size. Here, too,
pre-cast, pre-fired, high-alumina curbs make the best lining alternative. The use of 2% by weight
steel fibers in this application is highly recommended. The fibers increase the tensile and flexural
strength of the lining.
Walls in the cooler can be advantageously lined with inexpensive fireclay brick. Brick linings in the
cooler, when properly anchored and mortared, should last no less than 10 years without repairs.
The cooler roof is best lined with plastic or a good quality shotcrete mix, anchored by a combina-
tion of metal and ceramic anchors. Another cost effective alternative is to use pressed and fired
shapes directly suspended from steel beams.
REFRACTORY MAINTENANCE
Refractory maintenance involves the demolition and removal of damaged linings, the installation
of new linings, and, most importantly, the inspection and repair of existing linings.
In some cement plants the kiln is stopped for refractory replacement only when the lining fails.
This corrective maintenance practice is dangerous to the equipment and involves high costs and
risks such as:
471Refractories in Cement Manufacturing
Figure 3.7.16. Clinker cooler walls lined with pre-cast shapes.
Page 20
• Permanent damage to the kiln shell – Distorted kiln shells are difficult to re-brick. Kiln
misalignment caused by such deformations may induce frequent lining failures.
• Refractories ordered in emergency situations may not be the most suitable for the application,
and their delivered price is usually higher than programmed orders.
• After a few shutdowns, the kiln will be lined with many different products that will in turn
wear at different rates, causing other unexpected shutdowns.
• Every time the kiln is stopped for emergency repairs, the rest of the lining is subjected to addi-
tional thermal and mechanical stresses.
• Emergency shutdowns do not allow maintenance crews to work in other critical parts of the
kiln system such as rollers, seals, coal mill, pumps, clinker cooler, etc.
The preferred maintenance schedule is a well-programmed annual shutdown of at least three
weeks, preferably during spring when the weather is warm enough for proper castable installation
and curing. Moreover, maintenance costs tend to be lower during hot months than during winter.
Refractory maintenance is sometimes performed by house crews, sometimes sourced outside.
Brick contractors are usually better trained in the job than home crews because they do it more
often. Depending on the extension and complexity of the repair, house crews should not be
involved in refractory maintenance. For instance, cement plants are usually not equipped to gun or
shotcrete large volumes of monolithic materials, nor do they invest in brick demolition machines
that stay idle most of the year.
When refractory maintenance is outsourced, then the plant must clearly define the scope of serv-
ices and responsibilities, prior to opening the job for bids. By having a written understanding of
what is to be expected from contractors, the plant can save considerable amounts of money and
maintenance time, while obtaining the best lining quality. The list of requirements should cover, at
a minimum, details such as what type of alloy is acceptable for metal anchors, what the welding
procedures will be, who will collect testimonials, what the allowable number of bricking shims per
ring is, etc. Any item or procedure that affects cost or quality must be properly defined.
One aspect of refractory maintenance commonly overlooked is the inspection and repair of
preheater, hood, and cooler linings. Open joints and cracks must be sealed in order to prevent hot
dust and gas penetration and destruction of the metal anchors.
REFRACTORY PROCUREMENT
Until recently, refractories were specified and purchased exclusively at plant level. This responsibil-
ity was usually shared between the production manager and the plant manager.
Innovations in Portland Cement Manufacturing472
Page 21
With the consolidation of cement plants into fewer holding groups, the buying leverage of cement
companies has increased considerably. For this reason many groups are centralizing their world-
wide purchasing offices, and at the same time they are limiting the number of refractory suppliers
to one or two per product line. The supplier selection criteria vary from group to group, but basi-
cally it involves pricing, logistics, and payment terms. Only a minority of cement groups includes
product quality, dimensional tolerances, amount of technical support, and post-service investiga-
tion in their agreements.
Time has proven that there are benefits and risks associated with consolidated purchasing. The
biggest benefit is volume leverage while the biggest risk is alienating the plants from the decision-
making process.
The direct impact of refractories on cement manufacturing costs is less than 2% provided the prod-
uct performs well. If the refractory lining fails prematurely, then cement manufacturing costs go up
considerably. Unlike commodities such as grinding media, paper bags, lubricants, gypsum or coal,
refractory failures cause kiln shutdown and sometimes permanent damage to the equipment. For
these simple reasons refractories cannot be treated as commodities. Each refractory brand has its
own set of physical and chemical properties that could make the difference between success and
failure. These differences arise from raw materials and equipment used in brick manufacture.
One of the procedures in corporate purchasing is to have all plants in the group define which
refractory products work for them, add up product tonnages, and then request bids from different
suppliers. This approach does not necessarily reduce the number of suppliers, although it increases
buying leverage. Another procedure is to restrict the total number of suppliers and have the plants
work with a restricted line of products. The risks in this case far exceed the benefits: the limited
line of products may not be sufficient to address individual differences between kilns such as raw
mix, fuels, burner type, thermal loading, dust loading, and mechanical stress. A benefit common to
both approaches is that the responsibility of carrying inventories is transferred to suppliers.
As a tool to force refractory suppliers to reduce their prices and costs, global purchasing is quite
efficient. However, several refractory manufacturers closed their operations or discontinued entire
product lines because their profits disappeared in a trade off for larger volumes. As the source of
supply dwindles, market forces drive refractory prices up again, thus closing a cycle. Time and
again, a free market without supplier exclusion and multiple products seems to be the best way to
reduce refractory costs without halting investment in new product development.
INNOVATIONS AND FUTURE TRENDS
During the last decade cement kilns went through a technological revolution. Some of the major
changes that affect refractory performance are:
• Increase in cooler efficiency, with a corresponding increase in secondary air temperature
473Refractories in Cement Manufacturing
Page 22
• Considerable increase in kiln slope
• Large increase in kiln specific loading
• Progressive reduction in kiln length
• Large increase in preheater and precalciner size
• Disposal of complex fuels and agro-industrial wastes in the kiln
• Higher demand for hard-to-burn clinkers such as low alkali and oil well
• Environmental limitations on disposal of used refractory
In response to the new challenges, the refractory industry developed new bricks and castables more
resistant to chemical attack, thermal shock, and mechanical stress. Some of the major accomplish-
ments in this area are:
• Magnesia-spinel products replacing magnesia-chrome
• Bricks with superior structural flexibility to absorb mechanical stress
• Bricks with reduced permeability to liquid and gas
• Bricks with better coatability
• Bricks with improved dimensional tolerances
• Castables with high flow ability and increased mechanical strength
• Shotcrete and gunning materials with very low rebound
• More efficient machines to remove and install refractories
• Instruments to accurately measure residual lining thickness
• Software to manage all aspects of refractory maintenance, allowing plants to exchange infor-
mation on a worldwide basis
• Laser instruments to align the brickwork in the kiln
The present and future trends in refractories for cement kilns are toward monolithic products. As
kilns become smaller and preheaters become larger, the brick business tends to shrink. Today the
kiln represents only 25% of the total volume of refractories installed in a new cement plant. With
future improvements in monolithic products and anchoring systems, that proportion will be
further reduced. Bricks are still laid by hand, one by one, limiting the installation speed to no more
than one meter per hour. Certain monolithic products can be shot in place at much higher speeds,
using a reduced number of people.
Despite the progress made by the refractory industry so far, there are several areas in cement kilns
that require additional product research and development. These include:
• More efficient buildup-repelling bricks and castables for preheater application
• Monolithic products for burning and transition zone application
Innovations in Portland Cement Manufacturing474
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• High-magnesia products with low modulus of elasticity
• High-magnesia products with lower thermal conductivity
• Dolomite products with higher resistance to hydration and spalling
• Chemically bonded, unfired basic and high-alumina brick
• Better lining systems for burner lances
• Explosion-proof monolithic products for fast firing.
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Innovations in Portland Cement Manufacturing476