Wear Phenomena of Refractories

$: Wear Phenomena of Refractories$
Dr. Johannes Södje

Wear of refractories, particularly when using waste fuels and their influence on the brick life

Lifetimeof

refractories

Refractoryinstallation

mechanical

Production-quality

Installationdraw

Raw materialquality

chemicalthermal

storagekiln

burningconditions

Refractoryselection

Influences on the part of the cement producer

Influences on the part of the producer

Stresses on the Refractory Lining in Rotary Cement Kilns

loads on refractory linings

Thermal influences

Typical burning conditions

Clinker melt infiltrations

Concave/sloping erosion

Thermal shocks

Typical burning conditions

Clinker melt infiltration

Densification of the brick‘s hot face caused by clinker melt infiltration

- increased thermal influences

- variations of the kiln feed composition

Concave/sloping erosion (brickwork erosion)

constant thermal overloading of the brickwork

direct flame impactcoating free operating conditions with standard bricks with low refractoriness

Thermal shock influences

Too fast heating up or cooling down

Loss of coating

Optimizing:

Taking heating procedure into consideration/cooling down slowly

Chemical influences

Salt infiltration

Corrosion of chrome spinel

Silicate corrosion

Redox burning conditions

“Alkali Spalling“

Corrosion of the kiln shell

Hydration of basic bricks

Salt infiltration:Volatile elements and their principal compounds(without heavy metals)

Sulphur compoundSO2, SO3, S2-

Potassium oxideK2O

Sodium oxideNa2O

ChlorineCl2, Cl-

Salt infiltration: alkali sources

Kiln feed raw material

Clay and mica minerals

Feldspar (Na,K)AlSi3O8

Plagioclase (Na,Ca) (Si,Al) Al2Si2O8

Additives (ashes, bentonite, etc.)

Alternative combustibles

Salt infiltration: chlorine sources

Coal 0.01 - 0.3 % Cl

Lignite 0.1 - 0.13 % Cl

Animal meal 0.6 - 1.6 % Cl

Plastics »PVC«

20 % of the worldwide production of»plastics«

CH2=CHCl 30 % Cl

Salt infiltration: sulphur sources

Kiln feed raw material: FeS2, PbS, ZnS, CaSO4, CaSO4·H2O

Oil 0.2 - 2.5 %

Pitch/Tar 1 - 6 %

Coal 0.1 - 12 % SO3 in the ash

Petrol coke 5 - 8 %

Lignite 0.2 - 15 % SO3 in the ash

Concentrations of salts in the cement kiln system Circulation of volatile compounds

alkalis + Cl- alkalis + SO2/SO3

80

7162943

22

SO

ClONaOK

ASM−+

=

1

KCl + K2SO4

<1

KCl + K2SO4 + SO3 free

>1

KCl + K2SO4 + K2O free

Calculation of the alkali-sulphate modulus (ASM) in the cement clinker

Salt infiltration, balanced alkali-sulphate modulus (ASM ~1)

Densification of different brick horizons

Infiltration of gaseous alkali chloride and/or sulfate components, condensation and densification of the brick texture

Sulfate salts mostly in the lower transition zone and burning zone togetherwith alkali chlorides in the upper transition zone

Salt infiltration, balanced alkali-sulphate modulus (ASM ~1)

Chrome spinel corrosion, surplus of alkalis in kiln atmosphere (ASM >1)

Corrosion of chrome spinel in the presence of free alkalis at higher temperature Formation of toxic, hexavalent alkali chromate sulfate(yellow efflorescences = water soluble)

Contamination of ground water, mason eczema

Silicate corrosion, surplus of sulphur in kiln atmosphere (ASM <1)

Silicate corrosion reduces the refractorinessand the structural flexibility

2 C2S + MgO + SO3 CaSO4 + C3MS2

C3MS2 + MgO + SO3 CaSO4 + 2 CMS

CMS + MgO + SO3 CaSO4 + M2S

Possibly operational difficulties in the kiln system and to the clinker composition caused by the high sulphur content in the kiln gas atmosphere and/or surplus of sulphur in kiln atmosphere (ASM <1)

Formation of coating rings

and/or build-ups in the calcining

zone, inlet section, cyclones etc.

caused by the formation of

anhydrite (CaSO4), double sulfate salts

(calcium langbeinite, syngenite),

and spurrite phases (sulfate spurrite).

Possibly operational difficulties in the kiln system and to the clinker composition caused by the high sulphur content in the kiln gas atmosphere and/or surplus of sulphur in kiln atmosphere (ASM <1)

Sulphur will be retained as sulfates in the clinker (approach or exceed 2 %),

reducing the add of gypsum at the clinker grinding stage. The retarding of

hydration and setting of cement will be changed.

Formation of dusty clinker

Excess of sulphur in the clinker reduces the viscosity of clinker melt and the surface

tension of the liquid phases, which results in clinker structure loosening, and a

greater proportion of clinker dust is formed.

Redox burning conditions

Local reduction and/or redox burning conditions caused byincomplete combustion of fuels (coarse coal, ashes on the lining’s surface)

Basic brick grades with alpine magnesia (crystalline magnesia) are sensitiveto the high Fe2O3 content

“Alkali Spalling“

Thin hot face spallings due to brittleness of the brick texture

Alkalis react with the brick components from fireclay and high alumina bricksforming alkali alumina silicates (feldspar, feldspathoids). This formation isaccompanied by volume increase.

alkali spalling

alumina content, refractoriness, mechanical resistance

alkali spalling due to the formationof new minerals

wea

r by

alka

lis

Kiln shell corrosion

Migration and efflorescences of salts between brickwork and kiln shell

Chemical attack of salts under kiln operating conditions (high thermal corrosion)

Depending on the alkali-SO3-ratio and oxygen partial pressure bi- and trivalent iron oxides and/or iron sulphides are formed.

Hydration

Cracks from the brick surface into the brick‘s internal texture

Basic bricks are sensitive to humidity and must be stored and protected against humidity, rain and sea water.

MgO reacts with water to brucite (Mg(OH)2) which is accompanied by volume increase (~ 53%).

Tropical/sub-tropical climate conditions accelerate this reaction.

Hydration

Mechanical influences

Thermal expansion

Loosenings of the lining

Kiln shell deformation (ovality)

Grooves in the lining

Pressure loads on the kiln retaining ring

Thermal expansion

Convex spallings on the longitudinal joints

Too little allowance for expansion leadsto higher pressure within the brickwork.Convex spallings finally occur.

Insufficient expansion space

Frequent kiln stops after the burnout of the cardboards

Loosenings of the lining

Loosenings of the brickwork due to brickwork movements

Spiral twisting, tilting of bricks, shearing cracks, abrasion markson the brick‘s cold face

Wrong installation, frequent kiln stoppages, kiln shell deformations and ovalities

Kiln shell deformation (ovality)

Local strong spallings in the tire section, the surrounding brickwork is intact.

Increased ovality in the tire section causes tensions and loosenings of the lining.

The brick‘s hot face is more loaded (spallings).

Groove formation

Increased wear parallel to the kiln axis caused by groove formation, the otherbrickwork is intact (spallings of 2 - 3 brickwork width)

Brickwork rings are closed too tightly, damage of the key bricks using a wronghammer

More than one iron plate within the brickwork rings

High pressure on the retaining ring section

Increased pressure of the brickwork onto the retaining ring. Shearing tensionsoccur within the brickwork leading to cracks and spallings of brick parts.

Instable, deformed kiln shell or ovality increase this wear.

mechanicalinfluences

chemicalinfluences

thermalinfluences

vapour explosion

monolithicwear

Wear influences onto the monolitic lining

Vapour explosion (100 °C to ~600 °C)

Too fast heating up during setting/hardening and after heating up of the monolithic lining

Thermal influences

Thermal shocks, thermal overloading, thermal spots

Chemical influences

Reaction with the kiln feed (clinker dust), thermochemical attack by harmful components (alkali and sulfur compounds)

Mechanical influences

Abrasion, dust erosion (clinker dust), influences due to anchor, kiln shell and metallic construction

Wear influences onto the monolitic lining

Vapour explosion (100 °C to ~600 °C)

Riser shaft to calcinator/inlet chamber Cooler banks

Thermal influences

Cooler banks (tension cracks caused by thermal shock)

Chemical influence

Kiln hood back wall (alkali attack -> alkali spalling)

Chemical influence

Corrosion of metallic anchors

Mechanical influence

Damper in the tertiary air duct (high abrasion caused by strong air stream)

Thermal, chemical and mechanical influence

Burner lance

A change of refractory wear, particularly when using alternative or waste fuels in the cement rotary kiln


Fuel oil and gas as primary energy since approx. 1960

Changeover to coal after the petroleum crisis in 1973/74

Use of alternative fuels since mid eighties

Coating in the burning zone

The magnesia-chromite bricks used in the hot zones and the high alumina and fireclay bricks in the other sections perform good regarding lifetime and wear resistance

outlet zone burning zone safety zone

Lining and coating zone of kilns using fuel oil and gas

Coating in the burning zone and particularly in the transition zones

Magnesia-chromite bricks in the burning zone showing a usual wear

Reduced service life of the magnesia-chromite lining in the transition zones (salt infiltration, formation of alkali chromates, redox burning conditions)

uppertransition zone

burning zonelower transition zone

outlet zone

safety zone

Lining and coating zone of kilns using coal and fuel oil

Salt infiltrations, alkali chromate formation and reducing burning conditions in case of magnesia-chromite bricks

MAGPURE®93/95 ALMAG®A1 ALMAG SLC®

ALMAG®85

REFRAMAG®85

FERROMAG®90

Development of magnesia-spinel and magnesia-zirconia bricks

Main properties:

high resistance to alkali attack (no corrosion of the MA spinel or the zirconia)

insensitive to reducing or redox conditions

MAGNUM®SMAGNUM®95

Complex and instable coating situation and various flames

Lining lifetime clearly reduced, particularly in the transition zones(massive salt infiltrations, local overheating, local redox burning conditions)

outlet zone

lower transitionzone

burningzone I

burningzone II

transitionzone

transitionzone

safetyzone

Lining and coating zone using fuels with 50 % usual fuels and 50 % alternative fuels

Influence of secondary fuels on the refractory lining and kiln system

Stronger chemical and physical attack against the basic bricks

Risk of irregular temperature profile in the kiln (local thermal overload)

Risk of formation of local reducing atmosphere

Stronger chemical attack against the aluminous and fireclay bricks

Stronger attack against the kiln/cyclone shell and the anchor system

Percentage of the different wear types

Up-to-date studies show that more than 60 % of the wear cases are caused bysalt infiltrations (alone or in combination with other attacks)

23 % mechanical/thermomechanical influences and salts

23 % thermochemical influences and salts

23 % overheating

17 % salts

8 % mechanical/thermomechanical influences1 % redox conditions5 % other

> 60 %

Increase of salt infiltrations and accumulation of other harmful substances in the structure, balanced alkali-sulphate modulus (ASM ~1)

Increase of salt infiltrations and accumulation of other harmful substances in the structure, balanced alkali-sulphate modulus (ASM ~1)

Use of alternative additives with the kiln feed(e. g.: condensation of lead sulfide (PbS))

2 C2S + MgO + SO3 CaSO4 + C3MS2

C3MS2 + MgO + SO3 CaSO4 + 2 CMS

CMS + MgO + SO3 CaSO4 + M2S

K2O + 3 SO3 + 2 MgO K2Mg2[SO4]3

Silicate corrosion, surplus of sulphur in kiln atmosphere (ASM <1)

refractoriness and the structural flexibilityare reduced

Local overheating and redox burning conditions

Redox burning conditions and salt infiltrations

Formation of oldhamite (CaS), K2S, K2S3, KFeS2 under reducing atmosphere

Sulfate salts reform in oxidizing atmosphere, and lead to the brick‘s destruction.

Typical smell of H2S during kiln stop (smell of foul eggs)

Strong reducing and redox burning conditionscarbon disintegration, Boudouard reaction (CO2 + C <-> 2CO)

16

carbon horizon

“Alkali Spalling“ in the aluminous bricks

Structural embrittlement and spalling of thin layers.

The alkalis react with the components of fireclay and aluminous bricks underformation of alkali containing alumina silicates (feldspars, feldspathoids).This formation is accompanied by a volume increase.






Stronger attack against the kiln/cylcone shell and the anchor system

Effect of kiln shell corrosion and cyclone steel shell

Corrosion of metallic anchors (refractory areas installed with castables), formation of alkali chromate (K2CrO4), surplus of alkalis in kiln atmosphere (ASM >1)

Alkali chromate condensation in magnesia spinel lining

arcanite (blue efflorescences)

arcanite (green efflorescences)

alkali chromate

Requirements to refractory material and its installationin Cement Rotary Kilns fired with secondary fuels

Minimization of infiltration and corrosion of refractory components

Optimization of the structural texture by higher flexibility and high structural strength

Renewable sealing of the refractory’s hot face

Optimization of installation technology, especially in the static area of cementkiln system including a flexible and fast installation of high grade refractory concretes and bricks

Innovative insulating and protection options for the anchoring system and steelshell to reduce or even prevent different corrosion mechanism of shell and anchor

19

Refratechnik Cement’s refractory solution

Basic refractory brick grades established concept AF-series: ALMAG AF, REFRAMAG AF, TOPMAG AF new developments TOPMAG A1, FERROMAG F1, FORMAG 88

Non-basic materialbrick gradesestablished concept AR-series: KRONAL 50 AR, KRONAL 63 ARnew development KRONAL 60 AR

refractory concreteshigh grades LCC, and LCC-AR product rangehigh grade and fast application JC- and MCG-technology

Installation technology AR-lining concept with integrated refractory design including wear lining, insulating concept, and anchor system

20

Wear Phenomena of Refractories

Documents

Transcript of Wear Phenomena of Refractories