11 Wear Mechanism March 2013 Print Version

Wear Mechanisms in Cement Rotary Kilns

Cement

ICTM • R. Krischanitz • March 2013

Wear MechanismsChemical wear

� Alkaline salt infiltration

� Clinker melt infiltration(due to improper raw meal composition)

� REDOX Reactions

� Hydration

482 Wear Mechanisms

Thermal wear

� “Overheating”

(mostly of kiln feed – clinker melt infiltration, rarely of brick)

� Thermal shock

Mechanical wear

� Kiln shell deformation

� Excessive ovality

� Lining thrust

� Abrasion by clinker

� Improper Installation

Factors Influencing the Refractory Performance

Refractory

Mechanical condition

of kiln

- Ovality

- Deformed kiln shell

Mechanical

conditions

483 Wear Mechanisms

Refractory Lifetime

Process

- Burnability of kiln feed

- Kiln system

- Fuel(s), burner

- Production programme

- Process Instabilities

- etc.

- Selected material

- Quality of product

- Bricks vs castables

- Installation

Refractories

Process

Predominant Wear Mechanismsin Rotary Kilns

Outlet/LTZ CBZ UTZ SZ CZ IZ

thrust

most critical areas

most critical areas

48

mechanical loadthrust

thermal load / overheating

thermal load

(no coating)

thermal shocks (unstable coating)

chemical load (alkali bursting)

abrasion

chemical load (alk. salt infiltration)

most critical areas

most critical areas

Chemical attack

485 Wear Mechanisms

Wear Relevant Elements

� alkalise Na2O, K2O

Periodic Table of the Elements

486 Wear Mechanisms

� alkalise Na2O, K2O

� SO3

� Cl

Enrichment of Volatile Elements

by evaporation / condensation between kiln and preheater originating from:

� Raw meal

SO3:

as sulphate: gypsum CaSO4 x 2H2O and anhydrite CaSO4

as sulphide: pyrite FeS2, organic compounds

487 Wear Mechanisms

2

Cl-:

introduced by alkaline salts as halite NaCl or sylvine KCl

Alkalis (Na2O, K2O):

as interlayer cations in clay minerals and in feldspars

endmembers orthoklas KAlSi3O8, albit, NaAlSi3O8, anorthit CaAl2Si2O8

plagioclase solid solution Ab-An

alkalifeldspars solid solution Or-Ab

� Or fuel �

Wear Relevant Elements ofAlternative Fuels

Sulfur Chlorine Alkalis Phosphorous

Light oil 42 +

Heavy oil 40 ++

Natural gas 37

Rubber waste 36 ++ 0

Anthracite 34 +

Waste oil 30 - 38 ++ + +

Petcoke 33 ++ 0

Hard coal 30 ++ +

Waste tires 25 - 32 + +

Fuelcal. value

[MJ/kg]

wear-relevant elements

488 Wear Mechanisms

Waste tires 25 - 32 + +

Petrochemical residue 16 - 22 +

Lignite 16 - 21 ++ 0 +

Landfill gas 16 - 20

PVC 19 + + +

Fuller's earth 13 - 18

Asphalt sludge 16 ++ 0

Scrap wood, sawdust 16 + +

Rice husks 16

Domestic refuse 15 + ++ ++

Cardboard, paper waste 15 +

Dried sewage sludge 10 + + + ++

Waste wood (contaminated) 7 - 20 ++

Hazardous waste 4 - 8 + ++

Oil shale 2 - 16 +

Animal meal 0 + ++ ++

(++) high input of wear-relevant elements

(+) considerable input of wear-relevant elements

(0) minor input of wear-relevant elements

Alternative fuels tend to

increase the input of wear

relevant elements into the

system!

Kiln Cycles

489 Wear Mechanisms

Consequences of Alkali Salt Infiltration

There are two effects in case of alkaline salt infiltration

1. Densification of the microstructure � Reduction of structural flexibility

2. Depending on alkali sulphur ratio (ASR) corrosion of brick bonding – loss of

bonding strength

4810 Wear Mechanisms

bonding strength

dens. + loss

of flexibilityCorrosion

dens. + loss


dens. + loss


Magnesia Spinel X X x X X

Magnesia Chromite X X 1) X x X X

Alumina / Fireclay X X 2) X x X

1) corrosion of the chromite

2) alkali bursting

ASR >1 ASR ~1 ASR <1

Balanced alkali/sulphur ratio ASR

ASR ~0,8 to 1,2

80

719462

3

22

SO

ClOKONa−+

=

Wear Process:Alkaline Salt Infiltration


Alkaline Salt InfiltrationChemical analysis:

MgO 81,90% K2O 2,01%

Al2O3 9,41% Na2O 0,26%

SiO2 1,55% SO3 2,15%

CaO 3,22% Cl 0,05%


� densification of the microstructure and loss

of thermo-mechanical brick properties (flexibility)

�crack formation at the interface between

infiltrated and not infiltrated brick area

MgO 77,90% K2O 7,04%

Al2O3 7,46% Na2O 0,45%

SiO2 0,32% SO3 7,79%

CaO 0,62% Cl 0,05%

MgO 88,90% K2O 0,26%

Al2O3 8,72% Na2O 0,05%

SiO2 0,42% SO3 0,52%

CaO 0,78% Cl 0,05%

Corrosion of Brick Bonding


Corrosion of Calcium-SilicaticBrick Bonding

2Ca2SiO4 + SO3 + MgO � Ca3Mg(SiO4)2 + CaSO4

Ca3Mg(SiO4)2 + SO3 + MgO � 2CaMgSiO4 + CaSO4

CaMgSiO4 + SO3 + MgO � Mg2SiO4 + CaSO4


The corrosion of the calcium-silicatic brick bonding leads to a severe

loss of the bricks bonding strength. The new formed phases are

present as isolated particles within the pores and do not contribute to

the brick bonding.

The consequences are crack formation and finally spalling of hot

face brick parts.

ASR > 1: Alkali Attack on Alumina BricksPhysical attack:

- Deposition of alkali compounds in the open pores

(densification of microstructure)

Chemical attack: ∆V up to + 36%

- Incorporation of alkali oxides into glassy phase up to saturation


- Incorporation of alkali oxides into glassy phase up to saturation

(fireclay bricks)

- Reaction with cristobalite, quartz and mullite at T > 600°C, formation

of orthoklase (KAS6), albite (NAS6), leucite (KAS4) and nepheline

(NAS2) at T > 930°C: Volume increase up to 36%

- Formation of β-alumina (KA11) and K2O.Al2O3 at T 1000-1050°C:

Volume increase up to 20%

- Spalling of shells even at small temperature changes due to the

increased thermal expansion of the reaction layers in

comparison to mullite.

α nepheline ~ 3 α mullite

ASR > 1: Alkali Attack on Alumina Bricks


Examples of Alkali Spalling

⇐ Alkaline spalling of

andalusite bricks in

the cooler front wall

after 1 month.


Alkaline spalling of

castables ⇒

Alkali Attack: Failure of Steel Shell due to Expansion of Alumina Refractory

The strong volume

increase related with

alkali bursting can even

lead to damages of the

steel shell.


Calciner lifted by 15cm

steel shell.

Thermal load


Clinker Melt Infiltration


Increased clinker melt due to unfavourable clinker composition or

overheating of the kiln feed. Clinker melt infiltration is observed

only at the hot face, mostly adjacent to a thick clinker coating. The affected brick

microstructure is severely densified and the matrix heavily corroded. Often also a coagulation

of the matrix and the formation of coarse pores can be observed. The loss of thermo-

mechanical properties leads to crack formation and finally spalling.

Wear Process:Clinker Melt Infiltration


Overheating of High Alumina Bricksin the Outlet Zone

High alumina bricks after 7 months in

operation. Formation of gehlenite C2AS,

anorthite CAS2, nepheline NaAlSiO4

and other low melting Ca-alumosilicatic

phases at the hot face in reaction with

the kiln feed.


the kiln feed.

Overheating of SiC Mullite Bricksin the Safety Zone


Wear Process:Effect of Frequent Thermal Shocks


Thermal Shocks

An increased load by thermal shocks occurs mostly in the initial phase of kiln

operation, when the operation condition are not stabile yet.

Thermal shocks can effect the lining only in case of missing coating,

particularly in case of loss of a thick coating area. The fall off of clinker


particularly in case of loss of a thick coating area. The fall off of clinker

coating always implies also a certain mechanical load, which is

superimposed by the thermal-shock stress.

Spalling of hot face brick parts are the consequence.

Thermal shocks are especially severe in case that the microstructure has

been pre-damaged or degenerated by thermo-chemical influences, as

infiltration of clinker melt or alkaline salts.

Too Fast Heating UpOverstress at

hot face!


Spalling of brick heads of magnesia-chromite

bricks due to too fast heating up.

Open gap

at cold face

Combination of Wear MechanismsCBZ after 5 months

Overheating at the

hot face:

Chemical analysis:

0.09% Cl,

0.67% SO3,

1.44% K2O,

2.08% Na2O,

2.08% CaO,

In practice there is often a combination

of several wear mechanisms as this

example demonstrates


2.08% CaO,

0.74% SiO2,

5.09% Al2O3

Alkaline salt attack

behind the hot

face (black, etched

by water).

Chemical analysis:

0.77% Cl,

2.47% SO3,

3.00% K2O,

1.28% CaO

Mechanical load


Reasons for Mechanical Load

• Kiln shell torsions or

deformations ⇒⇒⇒⇒

4829 Wear Mechanisms• Excessive lining thrust ⇒⇒⇒⇒

• and instable

lining ⇐⇐⇐⇐Scratch marks on kiln shell

Kiln shell DeformationsPermanent Due to Hot Spot


Kiln Shell Deformations: Reversible Deformation Due to too High Clearance


Kiln Shell Deformations: Kiln Shell Constriction Due to too Low Clearance


Too low gap can lead to strangulation of the kiln shell within the tire during the

heating up procedure. Therefore it is important to monitor the tyre creep during

the heating up procedure. To avoid any risk of kiln shell constriction and lining

damage, keep tyre creep above 8 mm/rev during heating up and the

temperature difference between shell and tyre above 150°C.

Reasons for mechanical load III


Not only the tyre clearance can influence the ovality

values also other factors such as the alignment of the kiln

axis, permanent kiln shell deformations or misalignment of

the support rollers can lead to increased ovality values.

Recommended Tyre Creep and Ovality

The ovality of the kiln shell depends on the tyre clearance, the

distance between kiln shell and tyre. The higher the clearance the

higher also the ovality. The acceptable clearance depends on the

diameter of the kiln.


Ideal situation under hot conditions (on the example of a 4,8m Ø kiln):

max. clearance = kiln Ø [mm] /1000

(4800mm Ø � 4,8mm clearance)

rec. creep = tyre clearance x π (4,8 x PI = 15,1)

The ideal creep value for a 4,8m diameter kiln should be around

15mm/rev.

Possible Consequences


Increased ovality values and the thereby caused excessive mechanical load

can lead to severe damages of the refractory lining (crack formation, spalling

and spiralling).

Influence of Tyre Ovality


Higher mechanical stresses within the tyre section lead to significantly lower

residual thicknesses especially in case of simultaneous present chemo-

thermal load, as often present ion the UTZ.

Reasons for Mechanical Load Wrong Installation


Signs of Mechanical Overload


Formation of vertical cracks

(white and red arrows) and a

crumbly microstructure

(circles) at the cold face as

well as scratches (yellow

lines) at the cold face are

clear signs of increased

mechanical load.

Hydration


Fireclay and alumina bricks are not susceptible to hydration and

can be stored indefinitely.

Mortar should not be stored at customer´s warehouse for more

than 12 months.

Maximum Shelf Life

4840 Hydration

than 12 months.

Magnesia bricks are susceptible to hydration and should

therefore not be stored for more than 12 months. Risk of

hydration is higher tropical conditions and for bricks made from

high purity, synthetic sintered magnesia. Under such conditions

a further reduction of storage time can be necessary.

Basic bricks should be installed shortly before kiln heat up,

earliest 4 weeks before heat up.

Hydration of Magnesia Bricks

4841 Hydration

The damage by hydration of unused magnesia bricks is characterized by one or

several cracks in the brick and may lead to its partial sandlike decomposition.

Bricks with radial cracks have

lost their mechanical strength

and must be discarded

Hydration of Magnesia Bricks

4842 Hydration

When knocked with a steel

hammer, hydrated bricks sound

dull and break easily

Hydration

� Hydration of periclase (MgO), key factors:

� High humidity

� Temperature range of 40°C to 120°C

� Time

� Transformation of periclase to brucite Mg(OH)2under increase in volume of 115%

4843

MgO + H2O ↔ Mg(OH)2

Brucite crystals on top of periclase (SEM)

Installation of Rotary Kiln Bricks

How to Check for Hydration

Typical indications:

� network like cracks (radial)

� bulged surface (ruler test)

4844 Hydration

� bulged surface (ruler test)

� dull sound (sound test with hammer)

� loose or crumbly structure

Lab Test: Differential Thermogravimetry (DTG)

File: 2562.TG

Datum: 01.16.2003

Nummer: 4154-6

Probe:

Einwaage (mg): 8233

Meßbereich (g): 0,2

Bemerkung: 1K/min 10l Luft/h

ANKROM-B65-R1

-0,15

-0,1

-0,05

0

Gewicht, Abdampfrate vs. Temperatur

-10

0

10

4845 Hydration

Loss of water at 100° C

Loss of cristallwater at about

350° C, due to degeneration of

of brucit Mg(OH)2.

-0,45

-0,4

-0,35

-0,3

-0,25

-0,2

Gew

ichts

%

-60

-50

-40

-30

-20

0 100 200 300 400 500 600 700 800 900 1000

Grad Celsius

Abdam

pfrate

in p

pm

/min

difficult to detect because already low amounts of brucite,

which is analytically difficult to identify, can lead to formation of cracks

Loss of water 100°C

Loss of crystal water at about

350°C, due to degeneration

of brucit Mg(OH)2

� Magnesia bricks which have become wet, must be stacked

openly and ventilated at ambient air temperatures until dried

completely.

� Do not use hot air, do not expose wet bricks to the heat

Wet Bricks

4846 Hydration

� Do not use hot air, do not expose wet bricks to the heat

radiated from the kiln shell.

� After drying, check bricks carefully for crack formation.

Wet Lining Sections

4847 Hydration

New lining sections which have become wet have

to be removed and replaced by dry bricks.

� Stick to the RHI storage recommendations (storage under roof in

well ventilated areas).

� Avoid long storing in countries with critical climate, supply of basic

Measures to Avoid Hydration

4848 Hydration

� Avoid long storing in countries with critical climate, supply of basic

lining material if possible just in time shortly before lining.

� Avoid shipments during rainy season.

� Special brick packing with use of desiccants.

Thank you for your attention!


www.rhi-ag.com

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11 Wear Mechanism March 2013 Print Version