Wear Mechanisms in Cement Rotary Kilns
Cement
ICTM • R. Krischanitz • March 2013
Wear Mechanisms Chemical wear Alkaline salt infiltration Clinker melt infiltration (due to improper raw meal composition) REDOX Reactions Hydration
Thermal wear “Overheating” (mostly of kiln feed – clinker melt infiltration, rarely of brick) Thermal shock
Mechanical wear
2
Kiln shell deformation Excessive ovality Lining thrust Abrasion by clinker 48 Wear Mechanisms Improper Installation
Mechanical conditions
Factors Influencing the Refractory Performance
- Ovality - Deformed kiln shell Refractory
Mechanical condition of kiln
Process
Process
Refractories
- Selected material - Quality of product - Bricks vs castables - Installation 3 48
Wear Mechanisms
Refractory Lifetime
- Burnability of kiln feed - Kiln system - Fuel(s), burner - Production programme - Process Instabilities - etc.
Predominant Wear Mechanisms in Rotary Kilns CBZ
thrust
abrasion thermal load / overheating
thermal load (no coating)
UTZ
most critical areas
most critical areas
Outlet/LTZ
SZ
CZ
mechanical load
thermal shocks (unstable coating) chemical load (alk. salt infiltration) chemical load (alkali bursting)
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IZ
Chemical attack
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Wear Mechanisms
Wear Relevant Elements Periodic Table of the Elements
alkalise Na2O, K2O
SO3 Cl
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Wear Mechanisms
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 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 7 48
Wear Mechanisms
Wear Relevant Elements of Alternative Fuels cal. value [MJ/kg] 42 40 37 36 34 30 - 38 33 30 25 - 32 16 - 22 16 - 21 16 - 20 19 13 - 18 16 16 16 15 15 10 7 - 20 4-8 2 - 16
Fuel
Light oil Heavy oil Natural gas Rubber waste Anthracite Waste oil Petcoke Hard coal Waste tires Petrochemical residue Lignite Landfill gas PVC Fuller's earth Asphalt sludge Scrap wood, sawdust Rice husks Domestic refuse Cardboard, paper waste Dried sewage sludge Waste wood (contaminated) Hazardous waste Oil shale Animal meal (++) high input of wear-relevant elements (+) considerable input of wear-relevant elements (0) minor input of wear-relevant elements 8 48
Wear Mechanisms
Sulfur + ++
wear-relevant elements Chlorine Alkalis Phosphorous
++ + ++ ++ ++ +
0
++
0
+ +
+
+
+
++
0 +
+
+ + + + 0
+ 0 + +
+
++ + + ++ ++
++
+
++
+
++
++
Alternative fuels tend to increase the input of wear relevant elements into the system!
Kiln Cycles
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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 ASR >1 dens. + loss of flexibility
ASR ~1
Corrosion
ASR <1
dens. + loss of flexibility
Corrosion
dens. + loss of flexibility
Corrosion
X
x
X
X X
Magnesia Spinel
X
Magnesia Chromite
X
X
1)
X
x
X
Alumina / Fireclay
X
X 2)
X
x
X
1)
corrosion of the chromite 2) alkali bursting 10 48
Wear Mechanisms
Na 2O K 2O Cl + − 94 71 Balanced alkali/sulphur ratio ASR = 62 SO 3 ASR ~0,8 to 1,2 80
Wear Process: Alkaline Salt Infiltration
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Wear Mechanisms
Alkaline Salt Infiltration Chemical analysis: MgO Al2O3 SiO2
81,90% 9,41% 1,55%
CaO
3,22%
MgO Al2O3 SiO2 CaO
77,90% 7,46% 0,32% 0,62% MgO Al2O3 SiO2 CaO
88,90% 8,72% 0,42% 0,78%
K2O Na2O SO3
2,01% 0,26% 2,15%
Cl
0,05%
K2O Na2O SO3 Cl
7,04% 0,45% 7,79% 0,05% K2O Na2O SO3 Cl
0,26% 0,05% 0,52% 0,05%
densification of the microstructure and loss of thermo-mechanical brick properties (flexibility) 12 48
Wear Mechanisms
crack formation at the interface between infiltrated and not infiltrated brick area
Corrosion of Brick Bonding
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Wear Mechanisms
Corrosion of Calcium-Silicatic Brick 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.
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Wear Mechanisms
ASR > 1: Alkali Attack on Alumina Bricks Physical 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 (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 15 48
Wear Mechanisms
ASR > 1: Alkali Attack on Alumina Bricks
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Wear Mechanisms
Examples of Alkali Spalling ⇐ Alkaline spalling of andalusite bricks in the cooler front wall after 1 month.
Alkaline spalling of castables ⇒ 17 48
Wear Mechanisms
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 18 48
Wear Mechanisms
Thermal load
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Wear Mechanisms
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 thermomechanical properties leads to crack formation and finally spalling. 20 48
Wear Mechanisms
Wear Process: Clinker Melt Infiltration
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Wear Mechanisms
Overheating of High Alumina Bricks in 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.
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Wear Mechanisms
Overheating of SiC Mullite Bricks in the Safety Zone
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Wear Mechanisms
Wear Process: Effect of Frequent Thermal Shocks
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Wear Mechanisms
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 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.
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Wear Mechanisms
Too Fast Heating Up
Overstress at hot face!
Open gap at cold face
Spalling of brick heads of magnesia-chromite bricks due to too fast heating up. 26 48
Wear Mechanisms
Combination of Wear Mechanisms CBZ after 5 months In practice there is often a combination of several wear mechanisms as this example demonstrates
Overheating at the hot face: Chemical analysis: 0.09% Cl, 0.67% SO3, 1.44% K2O, 2.08% Na2O, 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
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Wear Mechanisms
Mechanical load
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Wear Mechanisms
Reasons for Mechanical Load • Kiln shell torsions or deformations ⇒
Scratch marks on kiln shell
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Wear Mechanisms
• and instable lining ⇐
• Excessive lining thrust ⇒
Kiln shell Deformations Permanent Due to Hot Spot
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Wear Mechanisms
Kiln Shell Deformations: Reversible Deformation Due to too High Clearance
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Wear Mechanisms
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. 32 48
Wear Mechanisms
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. 33 48
Wear Mechanisms
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. 34 48
Wear Mechanisms
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). 35 48
Wear Mechanisms
Influence of Tyre Ovality
Higher mechanical stresses within the tyre section lead to significantly lower residual thicknesses especially in case of simultaneous present chemothermal load, as often present ion the UTZ. 36 48
Wear Mechanisms
Reasons for Mechanical Load Wrong Installation
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Wear Mechanisms
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. 38 48
Wear Mechanisms
Hydration
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Wear Mechanisms
Maximum Shelf Life 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. 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. 40 48
Hydration
Hydration of Magnesia Bricks
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. 41 48
Hydration
Hydration of Magnesia Bricks Bricks with radial cracks have lost their mechanical strength and must be discarded
When knocked with a steel hammer, hydrated bricks sound dull and break easily 42 48
Hydration
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)2 under increase in volume of 115%
MgO + H2O ↔ Mg(OH)2
Brucite crystals on top of periclase (SEM) 43 48
Installation of Rotary Kiln Bricks
How to Check for Hydration Typical indications: network like cracks (radial) bulged surface (ruler test) dull sound (sound test with hammer) loose or crumbly structure
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Hydration
Lab Test: Differential Thermogravimetry (DTG) Gewicht, Abdampfrate vs. Temperatur 0
10
-0,05 0
File: Datum: Nummer: Probe:
2562.TG 01.16.2003 4154-6 ANKROM-B65-R1
Einwaage (mg): Meßbereich (g): Bemerkung:
8233 0,2 1K/min 10l Luft/h
-0,1 -10
-20
-0,2
-0,25
-30
Abdampfrate in ppm/min
Gewichts%
-0,15
Lossofofwater water 100°C Loss at 100° C Loss at about Lossofofcristallwater crystal water at about 350° C, due to to degeneration of 350°C, due degeneration of brucit Mg(OH)2.
of brucit Mg(OH)2
-0,3 -40 -0,35 -50 -0,4
-0,45 0
100
200
300
400
500
600
700
800
900
-60 1000
Grad Celsius
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Hydration
difficult to detect because already low amounts of brucite, which is analytically difficult to identify, can lead to formation of cracks
Wet Bricks 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 radiated from the kiln shell. After drying, check bricks carefully for crack formation.
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Hydration
Wet Lining Sections
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New lining sections which have become wet have to be removed and replaced by dry bricks. Hydration
Measures to Avoid Hydration Stick to the RHI storage recommendations (storage under roof in well ventilated areas). 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.
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Hydration
Thank you for your attention!
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Wear Mechanisms
