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DAMAGE OF REFRACTORY BRICKS LINED IN CEMENT ROTARY KILN

Makoto Ono and Hisao Kozuka

Technical Research Laboratory, Mino Yogyo Co. Ltd., Japan

1. INTRODUCTION

We have been producing all kinds of refractories raging from fireclay, high alumina, basic refractories to monolithic refractories since the day of the establishment in 1918. We supply high quality products to industries such as cement, lime, steel, nonferrous metal, ceramics, chemical paper & pulp and environmental sanitation. In the cement industry, we pioneered the development and commercial production of basic bricks. Our developed magnesia-spinel bricks called MIC have been installed mostly in the transition zone and cooling zone of cement rotary kilns in Japan and have shown excellent durability due to their superior resistance to deterioration. Furthermore, MgO-CaZrO3 bricks called ECOS-C have been developed in consideration of the environmental risks caused by used magnesia-chrome bricks. They have been installed in the burning zone and have shown excellent corrosion resistance to cement clinker melt.

In this paper, we summarize the damage of refractory bricks lined in cement rotary kilns and how to

improve the performance from these damages. Additionally, influence of waste used in cement

production on refractory linings is presented.

2. WEARING MECHANISM OF REFRACTORIES IN A CEMENT ROTARY KILN

Fig. 1. Typical cement kiln with precalciner

Fig.1 shows a typical precalciner kiln and its zoning in a rotary kiln. Pulverized cement raw materials

are charged from the top of preheater and charged into a rotary kiln after heat-exchanged with

combustion gas. In the rotary kiln, raw materials react with each other under high temperature

conditions and forms cement clinker that is discharged to the cooler.

Cement containing around 65 mass% CaO is a basic material. Basic bricks that show superior

chemical stability to cement raw material are used for the high temperature area in the rotary kiln, such

as burning zone where the material reaches around 1450oC, transition zone and cooling zone, which

are located before and after the burning zone respectively. Temperatures in transition zone and cooling

zone are lower than that in burning zone and the amount of formed liquid phase is a little in cement

raw material. Accordingly, cement coating frequently repeats sticking or dropping off on the basic

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Thermal shock during heating-up and coating off

Unstable coating caused by fluctuation of operation and mechanical

stress from shell

Expose to the flame without coating

Infiltration and reaction with foreign elements like SOx, cement materials

Observed phenomenaCracking caused by thermal shock

• Erosion of bricks

• Cracking caused by structural change

Kiln Operation

Slow heating-up schedule Stable operation

Brick properties

Increase flexibility to deter crack propagation

•Increase resistance to thermal load

•Strengthen the bonding

Countermeasures

Thermal shock during heating-up and coating off

Unstable coating caused by fluctuation of operation and mechanical

stress from shell

Expose to the flame without coating

Infiltration and reaction with foreign elements like SOx, cement materials

Observed phenomenaCracking caused by thermal shock

• Erosion of bricks

• Cracking caused by structural change

Kiln Operation

Slow heating-up schedule Stable operation

Brick properties

Increase flexibility to deter crack propagation

•Increase resistance to thermal load

•Strengthen the bonding

Countermeasures

bricks in transition zone and cooling zone. On the other hand, cement coating is stably adhered on the

basic bricks in burning zone, where the amount of formed liquid phase is sufficient. Thus, atmosphere

and thermal conditions vary with the position in a rotary kiln and wearing mechanism of refractory

bricks also varies according to the lined position.

Refractory bricks lined in a rotary kiln are worn by the synergistic effect of thermal, chemical and

mechanical actions. These stresses are complicated and it is difficult to distinguish the wearing causes

in actual conditions. Hereafter, we simplify the wearing mechanism as much as possible for

understanding easily.

Fig. 2. Wearing mechanism in burning zone

2.1. Wearing Mechanism in Burning Zone

In burning zone, half melted cement raw material firmly sticks on basic bricks is called coating.

However, once the coating drops off due to alternation of operation and/or stresses from the kiln shell,

the bricks are exposed to thermal shock and molten cement raw material. Thermal spalling occurs in

the lined bricks. When the cement melts infiltrated into the bricks, the bonding structure is destroyed

by the reaction of bricks with the cement melts and reacted area is washed out by rotating materials.

Thus, the bricks are worn by erosion. In the meantime, infiltrations of the cement melts and alkali salts

fill the pores at the hot face and subsequently densified the brick near the hot face. When the bricks are

exposed to temperature fluctuation, structural spalling takes place easily. These are summarized in

Fig. 2.

2.2. Wearing Mechanism in Transition Zone

In transition zone, surface temperature of bricks fluctuates because the coating repeatedly sticks to and

drops off from the bricks. The main cause of the wear of basic bricks in transition zone is deterioration

of bonding due to frequent temperature changes. The deterioration takes place easily in magnesia-

chrome bricks, because the contained iron oxide is transferred from ferric to ferrous, and vice versa

depending on the temperature change caused. It is said that magnesia-spinel bricks demonstrate less

deterioration and longer durability in the zone, because they do not contain iron oxides. However,

magnesia-spinel bricks consist of multiple components with different thermal expansion rate. The

temperature fluctuation also destroys the bonding between different components and consequently

magnesia-spinel bricks are deteriorated gradually around the hot face.

When solid fuel containing high sulfur is used, the atmosphere in transition zone could become

reduced locally and rich in SOx. MgO and CaO contained in basic bricks may react with infiltrated

SOx, causing MgSO4 and CaSO4 to be formed. Then, the sulfates react with K2SO4 or KCl, forming

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Temperature deviation and mechanical action caused by on and ffcoating

Deposit of alkali salts such as KCl, K2SO4, etc. change the structure and weaken bonding

t thMechanical stress caused by fatigued shell (l

ovality, shell runout, etc.)

Utilization of solid fuel

Reducingt h

Severe SOxinfiltration

(locally) Joint steel plate loss due

High SOxatmosphere to reaction with SOx

Observed phenomena

•Deterioration of bonding t thSevere spallingcaused by structural change, mechanical stress

and/or tensile stress caused by joint plate l

Kiln OperationExchange kiln shell Lower shell ovality

Brick properties•Strengthen the

b di•Maintain the flexibility to prevent the crack ti•Lower the porosity to minimize the

i filt tiIntroduce 0.2 mm thick joint steel l t

Countermeasures

Temperature deviation and mechanical action caused by on and ffcoating

Deposit of alkali salts such as KCl, K2SO4, etc. change the structure and weaken bonding

t thMechanical stress caused by fatigued shell (l

ovality, shell runout, etc.)

Utilization of solid fuel

Reducingt h

Severe SOxinfiltration

(locally) Joint steel plate loss due

High SOxatmosphere to reaction with SOx

Observed phenomena

•Deterioration of bondingt thSevere spallingcaused by structural change, mechanical stress

and/or tensile stress caused by joint plate l

Kiln OperationExchange kiln shell Lower shell ovality

Brick properties•Strengthen the

b di•Maintain the flexibility to prevent the crack ti•Lower the porosity to minimize the

i filt tiIntroduce 0.2 mm thick joint steel l t

Countermeasures

K2SO4·2CaSO4 and low melting components. These moved to the low temperature side from the hot

face. The movement of K2SO4·2CaSO4

Fig. 3. Wearing mechanism in transition zone

could destroy the bonding and causes texture deterioration around the hot face. Furthermore, when

steel plates are used as a joint material for basic bricks, the plates are not oxidized sufficiently and

form FeO-FeS eutectic under these conditions. The eutectic is liquefied near the hot face and infiltrates

the bricks. This leads to open spaces being generated in the infiltrated areas of steel plates. Cracks

begin to form at the edge of the remaining steel plate and spread to the bricks. This phenomenon also

wears the bricks. These are summarized in Fig. 3.

3. APPEARANCES OF USED BRICKS CAUSED BY DIFFERENT WEARING CAUSES AND

THEIR COUNTERMEASURES

3.1. Wearing Caused by Thermal Shock

Fig.4 shows the relationship between the service period and the wear amount of magnesia-chrome

bricks in burning zone. The wear progresses with the service period, of which wear rate is about

50mm/1000h in Fig.4, but the graph presents the linear line does not start from the zero point, that is,

approximately 70mm of wearing takes place initially without correlation with service period. The

initial wearing is usually around 30mm, but 70mm in Fig.4 is quite big. This type of wearing could

occur during heating-up after shutdown. Basic bricks in the burning zone are exposed to the direct

flame during initial heating-up and cracks may generate by spalling in the brick (Fig. 5). Coating is

hard to stick on the smooth surface of new bricks and cracks easily generate in new bricks as Fig.5 in

rapid heating-up schedule. Thus, heating-up plan and starting-up procedure is important for the basic

bricks in burning zone to form coating promptly for decelerating wearing.rate of magnesia-chrome

brick used in burning zone

3.1.1. Countermeasures

In order to prevent the cracking and spalling at the beginning stage of operation, we think that bricks

need to have sufficient flexibility. We introduce the idea of “damage resistance parameter” to

assessquantitatively the flexibility. Our developed procedure is measure damage resistance parameter

simply so as to compare the resistance to crack propagation, namely, how the sample brick resist the

damage by cracking. The concept of damage resistance stands on the idea that elastic distortion

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energy, stored in the sample until fracture caused by stresses take place, is shifted to fracture energy

consumed by crack propagation.

Fig. 4. Relation between service period and wear rate of magnesia-chrome brick used in burning zone

Fig. 5. Cracked magnesia-chrome bricks by rapid heating-up

3.1.1.1. Method of Measuring Damage Resistant Parameter[1]

Damage resistance parameter is calculated from the load strain curve which is obtained from three

points bending strength measurement. The parameter is affected by the sample shape, bending strength

measurement condition, etc. Our measuring condition is as follows:

• Sample size: 120(l)×40(w)×25(t) (mm)

• Span distance: 100mm

• Loading condition: Cross head speed: 0.5mm/min

• Load: Measured by load cell

• Load-strain curve: Obtained with X-Y (time) plotter

• Chart speed: 120mm/min

0 1000 2000 30000

50

100

150

Wea

r am

ou

nt

(mm

)

Servis period (h)

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3.1.1.2. Calculation of Damage Resistance Parameter

Fig. 6 shows the load-strain curve obtained with X-Y plotter in three points bending test. In this

figure, point (a) is the border of elasticity strained part (load increases in a straight line) and plastic

deformed (non-linear) part. Surface area ‘A’ which goes from the beginning to point (a) is regarded as

elasticity strained energy. Point (b) is a point of 0.3MPa load. 0.3MPa was decided by judging that a

load less than 0.3MPa has no significance. Surface area ’B’ is regarded as fracture energy from the

point (a) to the point (b). We calculate damage resistance parameter using the following formula.

Damage resistance parameter = (A+B)/A

Damage resistance parameter for basic bricks using rotary kilns should be more than 2 so as to prevent

cracking caused by thermal and mechanical stresses. Then, heating-up speed should be as slow as

possible.

3.2. Wearing Caused by Erosion

The wear of basic bricks in burning zone occurs while the coating drops off, because the coating stuck

on basic brick protects the brick from thermal shock, chemical reaction with the melt of cement raw

material, etc. Erosion by molten cement raw material will occur on the basic bricks sometimes under

high thermal load conditions. Appearance of eroded basic bricks is shown in Fig. 7. Appearance of

the eroded brick is shown in Fig. 8.

Fig. 6. Load-strain curve to measure damage resistance parameter

Fig. 7. Appearance of eroded basic bricks

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Fig. 8 Appearance of eroded basic brick (Magnesia-chrome)

3.2.1. Countermeasures

In order to prevent the erosion of basic bricks, it is very important for the bricks to form coating

promptly. We studied the mechanism of erosion and developed MgO-CaZrO3 bricks ECOS-C having

good corrosion resistance.

Mag-chro ECOS-C

Fig. 9. Appearance of used mag-chro brick and ECOS-C bricks

Fig. 9 shows the appearance of magnesia-chrome bricks and developed ECOS-C bricks lined in

adjacent rings in burning zone. Hot face of magnesia-chrome brick was eroded by the cement liquid

phase and the remaining thickness was 120mm. ECOS-C bricks, however, showed smooth surface and

the remaining thickness was 170mm. According to postmortem analysis, hot face of magnesia-chrome

brick became very dense, the cement minerals and reacted minerals were identified in the dense layer.

These results indicate the bricks were worn by corrosion. In the ECOS-C bricks, the cement minerals

and reacted minerals were not identified over the whole region. The bricks could be worn by peeling

phenomenon. Thus, developed ECOS-C bricks showed no erosion and were able to achieve the lining

life improvement.

3.3. Wearing Caused by Spalling

In the transition zone, basic bricks are worn by spalling as Fig. 10. The causes of spalling are various,

such as mechanical stress caused by shell ovality, joint plate loss, and deterioration of the bonding by

the reaction between bricks and sulfur oxides.

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3.3.1. Spalling Caused by Mechanical Stress

When the bricks are spalled near the tire area or a retainer ring, the spalling would be caused by the

mechanical stress. Some kilns show the cross-sectional deformation of the kiln shell known as ovality

near the tire area. As shown in Fig.11, the larger ovality, the shorter service life of the brick lining.2)

The bricks are severely spalled as Fig.10.

Fig. 10. Appearance of transition zone bricks

Countermeasures

The maintenance of tire ovality within the acceptable range by replacing filler bars under tire has the

positive effect on brick wearing.

3.3.2. Spalling Caused by Joint Steel Plates Loss

1.0-1.6mm thick steel plates are inserted as joint steel plates between basic bricks lined in cement

rotary kilns in Japan. Joint steel plates are oxidized and bonded to the bricks by the reaction with the

brick materials. Then, the individual brick ring becomes one-piece structure and the lateral movement

of the bricks as lining displacements will be minimized. Recently, due to the increased use of high

sulfur content solid fuels, the atmosphere in the kilns has become locally reducing. Under these

conditions and especially when in the presence of sulfur, steel plates are not oxidized sufficiently and

form Fe-FeS eutectic. The lowest melting point of Fe-FeS eutectic is 940oC. The eutectic is liquefied

and infiltrates into the bricks and leads to open spaces being generated in the filtrated area of steel

plates. A cross-section of used bricks taken from transition zone is shown in Fig. 12 [6]. Cracks begin

to form at the edge of remaining steel plate by the shear stress and spread to the bricks. Then, the

brick life is shortened by spalling.

Countermeasures

We could prevent cracking caused by this phenomenon by introducing 0.2mm joint steel plates. The

use of 0.2mm plates does not cause cracking when the plate is lost, because of the less opening

between the bricks.

3.3.3. Spalling Caused by Deterioration of the Bonding

Brick texture deteriorated near the hot face and cracks were generated below the deteriorated layer.

Densification was observed from the crack to the shell side. PbS that had to be exposed under the

reducing atmosphere was identified in the dense layer. K2SO42CaSO4 and KCl were identified in the

dense layer. Hot modulus of rupture at 1200oC of the specimen obtained from the different position

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Fig. 11. Relation between kiln jovialities and brick wearing speeds

Fig. 12. a. Sections of magnesia-spinel bricks infiltrating joint steel plate

Fig. 12. b. Section of deteriorated magnesia- spinel brick

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So as to prevent the deterioration, minimize the infiltration of foreign materials by lower the porosity.

Then, bonding strength is strengthened to resist the deterioration. However, flexibility of the brick is

from the hot face was measured. All specimens showed lower HMOR value than the original. HMOR

value at 0-120mm from the hot face were noticeably lowered.

Countermeasures

necessary for using rotary kilns. Thus, dense magnesia-spinel brick MIC-FD, which was able to

achieve the lining life improvement, was developed.

Typical properties of the basic bricks are shown in Table 1.

Table 1. Typical properties of basic bricks

Brand ECOS-C MIC-FD TTX-P

Material base MgO-CaZrO3 Magnesia-spinel Magnesia-chrome

Apparent porosity [%] 15.4 13.8 16.9

Bulk density [g/cm3] 3.11 3.04 3.04

Cold crushing strength

[MPa]46 47 52

HMOR [MPa]

at 1000oC 5 8.3 5.4

at 1400 oC 3.6 4.7 4

Damage resistance parameter 3.2 2.6 3.3

Chemical composition [mass%]

SiO2 0.3 0.2 1.8

Al2O3 17.5 7.2

MgO 82.3 81.8 74.2

CaO 5.8

ZrO2 11.3 Cr2O3 11.3

4. INFLUENCE OF WASTE USE ON REFRACTORY LININGS

In Japanese cement industry, the use of waste has increased recently. At present, approximately 27

million tpa is employed in kilns as raw material or fuel. The greater utilization of waste, the more

adverse effects are on lined refractories. These effects may be attributed to the change of heat balance

and an increase in chloride and/or sulfate. The advantages of using waste in the cement industry

include:

• High temperature kiln burning rapidly composing waste, while the remaining compounds are

absorbed into the cement. Therefore, cement production has a large capability to render even

hazardous waste harmless.

• Large amounts of waste can be treated.

• Use of waste can result in cost savings.

• As seen in Table 2, more than 361kg of waste and byproducts are disposed of safety in 1 ton

of cement production. The use of waste and byproducts has increased rapidly (272kg in 1996

to 361kg in 2002) in an ongoing effort to achieve the target amount of 400kg by 2010.

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Table 2. Annual usages of waste and byproducts in the cement industry

Unit: ×103 ton

Kinds of wastes 1996 1997 1998 1999 2000 2001 2002 Purpose of usage

Waste and reclaimed oil 263 276 318 338 359 353 352 Fuel, Raw meal

Waste tires 259 258 282 286 323 284 253 Fuel

Waste plastic - 21 29 58 102 171 211 Fuel

Blast furnace slag 13,892 12,684 11,353 11,449 12,162 11,915 10,474 Raw meal, Additive

Steel making slag 1,246 1,207 1,061 882 795 935 803 Raw meal

Nonferrous metal slag 1,430 1,671 1,161 1,256 1,500 1,236 1,039 Raw meal

Shale clay derived from coal 1,772 1,772 1,104 902 675 574 522 Raw meal, Fuel

Ash and dust 441 543 531 625 734 943 874 Raw meal

Fly ash (coal) 3,402 3,517 3,779 4,551 5,145 5,822 6,320 Raw meal

Sludge 930 1,189 1,394 1,744 1,906 2,235 2,286 Raw meal, Fuel

Others 3,351 3,462 3,359 3,493 3,658 3,593 4,104

Total 26,986 26,600 24,371 25,584 27,359 28,061 27,238 Fuel, Raw meal

Cement Production 99,267 92,558 82,569 82,181 82,373 79,119 75,479

Unit amount of waste 272 287 295 311 332 355 361 (Kg/ton)

5. CONCLUSION

The damages of refractory bricks lined in cement rotary kilns and the countermeasures of each damage

are discussed. Further investigations on the brick wearing and its countermeasures in a rotary cement

kiln are still underway to continually improve the performance.

REFERENCES

1. Y. Kajita, S. Kariya, H. Kozuka, T. Honda and S. Ota Development a method for quantitative

assessment of flexibility and its application for evaluating Mg-CaZrO3 bricks, Proceedings of

UNITECR'97 [1] 337-345, 1997.

2. Y. Tsuchiya, E. Nakajima, H. Takenouchi, and T. Honda, Lining service life and kiln ovality,

WORLD CEMENT 27[4] 28-31, 1996.