refractories in cement manufacturing

$: Refractories in Cement Manufacturing$
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

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.

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.

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


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.


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.


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


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


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

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.


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


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.

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.


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.


Figure 3.7.7. Heavy duty metal anchor.

Figure 3.7.8. Brick damaged by mechanicalstress.

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.


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.

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.


Figure 3.7.11. High-alumina brickdestroyed by fluxing.

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.


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.

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


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.


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.


Figure 3.7.15. Damaged burner pipe lining.

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:


Figure 3.7.16. Clinker cooler walls lined with pre-cast shapes.

• 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.


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


• 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


• 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.

REFERENCES

Aliprandi, G., Matériaux Réfractaires et Céramiques Techniques, Paris: Éditions Septima, 1989.

Alper, Allen M., High Temperature Oxides, Part I, Academic Press, New York, 1970.

Chapman, Robert P., Recommended Procedures For Mechanical Analysis of Rotary Kilns, Fuller Co.,Bethlehem, Pennsylvania, 1985.

Ciullo, Peter A., Industrial Minerals and Their Uses, Westwood, Noyes Publications, New Jersey,1996.

Lapoujade, P., and Le Mat, Y., Traité Pratique Sur L’Utilisation des Produits Réfractaires, Éditions H.Vial, Dourdan, France, 1986.

Mosci, R. A., Understanding Clinker Liquid Phase, RefrAmerica Technical Publication 01, Butler,Pennsylvania, April 1998.

Mosci R. A., Stop Kiln Shell Corrosion, RefrAmerica Technical Publication 02, Butler, Pennsylvania,November 2000.

Mosci, R. A., Eliminate Calcining Zone Rings, RefrAmerica Technical Publication 03, Butler,Pennsylvania, April 2001.

Mosci, R. A., Nose Ring Lining, RefrAmerica Technical Publication 04, Butler, Pennsylvania,November 2000.

Mosci, R. A., “Modular Lining for Clinker Coolers,” 2000 IEEE – IAS/PCA Technical Conference,Salt Lake City, Utah, May 7-12, 2000, pages 211-218.

Mosci, R. A., “The Power of Clinker Chemistry and Microscopy,” World Cement, Vol. 30, No. 8,Champlain, New York, August 1999, pages 55-61.

Mosci, R. A., “Ceramic Coatings Stop Kiln Shell Corrosion,” World Cement, Vol. 28, No. 12,Champlain, New York, December 1997, pages 28-30.

Mosci, R. A., Dolomite or Mag-Spinel in the Burning Zone, RefrAmerica Technical Publication 08,Butler, Pennsylvania, May 2001.

Mosci, R. A., What is the Optimum Brick Thickness for My Kiln?, RefrAmerica Technical Publication09, Butler, Pennsylvania, May 2001.

Mosci, R. A., Silicon Carbide Versus Zirconia Against Buildups, RefrAmerica Technical Publication10, Butler, Pennsylvania, May 2001.

Mosci, R. A., Brick Installation in Rotary Kilns, RefrAmerica, Butler, Pennsylvania, 1995.


Mosci, R. A., Maximum Kiln Shell Temperature, RefrAmerica Technical Publication 12, Butler,Pennsylvania, July 2002.

Seidel, G., Huckauf, H., Stark, J., Technologie des Ciments, Chaux, Plâtre, Éditions Septima, Paris,1980.

Refractories Manual, 2nd Ed., American Foundrymen’s Society, Des Plaines, Illinois, 1989.

Schacht, Charles A., Refractory Linings, Marcel Dekker, Inc., New York, 1995.


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