kiln shell corrosion

Kiln shell corrosion

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Causes of kiln shell corrosion

• Composition of the kiln atmosphere

• Content of volatile components

• Alkalis ,chloidic,sulfur and vanadic compounds, water vapouretc are the main volatile components predominating in the kiln gas atmosphere of cement kiln systems.

• Alkalis and chloridic compounds are mainly taken into the kilnby the kiln feed and the sulfur and vanadic compounds by thefuel

• The combustion of alternate fuels and especially waste fuelsIncreases the concentrations of volatile and further harmfulcompounds within the kiln system accelerating shell corrosion.

• Attack by the back fly Ferrum Pedis.



Schematic mechanism of shell corrosion



Different shell corrosions

Three different kinds of kiln shell corrosion are observed inCement kiln systems

• Corrosion during kiln operation resulting in scaling of thekiln shell by oxidation of the metallic iron at elevatedtemperature ( so-called high temperature corrosion

• Rusting during longer kiln shut downs caused by condensationor absorption of humid ( moist air , water)

• Wear of oxidised kiln shell during longer kiln shut downs causedby small blackflies eating iron compounds ( so-called Ferrum Pedis syndrome)



Corroded kiln shell samples ( high temperature corrosion)



Corroded kiln shell samples( rusting during longer kiln shut downs



Corrosion of corroded kiln shell by Ferrum Pedis



Corroded kiln shell



Kiln shell corrosion• migration and efforescences of salts between brick work and kiln shell• chemical attack of salts under kiln operating conditions (high thermal corrosion)• depending on the alkali-sulfur ratio and oxygen partial pressure bi- and trivalent

iron oxides and / or iron sulfides are formed.



Pictures of corroded shell byOptical microscope



Corrosion

1. Scaling : Oxidation by dry gases at elevated temperature ( S , Cl )

2. Rusting : Oxidation by moist air or water of condensation at low temperatures ( KCl )



Corrosion ( Study )

• 12 Kilns at four plants located relatively close to one another have been studied.• Clear indications were found of a relationship between shell corrosion and the cement-making process,• Corrosion was found only in the eight precalciner kilns studied.• Suspension preheater kilns , showed no signs of corrosion.



Macroscopic description of the corrosion

• The fastest corrosion rates are found from end of the coating over a distance of 5 * the kiln dia towards the kiln inlet.• The rate of corrosion then gradually decreases towards the kiln inlet.• There is hardly any corrosion on the burner side of the coating.• Fig. 1 illustrates the usual pattern of the coating.



Corrosion scale morphology

and composition

Brick side : Brownish

Kiln shell side : Shiny dark brown or shiny black



Corrosion scale morphology

and composition

Three types of corrosion scales wereobserved

• Chloride-free, alkali-free scales

• Scales containing chloride without alkalis

• Scales containing both chloride and alkalis



Corrosion scale morphology and composition

Chloride-free, alkali-free scales • Alternating layers of iron oxides and sulphides

• The minerals found by X-ray diffraction were

1. Fe2O3 ( haematite )

2. Fe3O4 ( magnetite )

3. FeS2 ( pyrite )

4. FeS ( pyrrhotite )PDF created with pdfFactory Pro trial version www.pdffactory.com


Corrosion scale morphology and

compositionScales containing chloride without alkalis • High sulphur layers alternate with high oxygen layers

• With presence of chloride

• Quantitative microanalysis of corrosion powder

( % w/w ) : Iron : 74 , Sulphur : 15 , Chloride : 2.5,

Potassium : 0.5 PDF created with pdfFactory Pro trial version www.pdffactory.com


Corrosion scale morphology and composition

Scales containing both chloride and alkalis

•Quantitative microanalysis of corrosion powder

( % w/w ) : Iron : 64 , Sulphur : 14 , Chloride : 12

Potassium : 6.5



Mechanisms important to kiln shell corrosion

1. Oxidation

2. Sulphidization

3. Hot corrosion




Oxidation

In an oxidising atmosphere the iron from the

steel shell will react with oxygen to form

an oxide scale. Compound with highest oxygen

content , Fe2O3 being found at the scale-brick

interface , and the compound with highest iron

content , FeO at the metal-scale interface .




SulphidizationWhere no oxygen is present , SO2 takes over as the oxygen donor and a different reactionoccurs as follows :4 Fe + 2 SO2 ( g ) = Fe3O4 + FeS2

Suphidization is enhanced by the presence of chlorides , mainly because they affect the morphology of corrosion scale ,hindering the formation of a strong , protective oxide layer.




Sulphidization

1. High temperature hydrolysis of the thermally unstable alkali chlorides to form the more stable sulphates.

2. Followed by re-oxidation of hydrogen chloride gas ( by oxygen or SO2 ) at lower temperatures to elemental chlorine which attacks the metal.

2 KCl ( g) + H2O ( g ) + SO2 ( g ) + ½ O2 (g ) =

K2SO4 + 2 HCl ( g ) ( T < 900 0C )

2 HCl ( g ) + O2 ( g ) = Cl2 ( g ) + H2O ( T< 400 0C )




Sulphidization

1. In preheater kilns , chlorides evaporates during or shortly after calcination without substantial formation of hydrogen chloride gas.

2. Kilns with tertiary air duct, show delayed alkali chloride evaporation and, consequently, evaporation will be followed by more extensive hydrolysis of chlorides.




Sulphidization• Once Cl2 ( g ) is formed, it can reach the kiln shell

through the protective refractory bricks or through cracks and fissures in the lining, and will react with either the oxide-sulphide layers or, most likely, directly with the kiln shell according to the following reactions :- reaction with the oxide-sulphide layers

FeS + Fe3O4 + 4 Cl2 = 4 FeCl2 + SO2 + O2- reaction with the kiln shell

Cl2 + Fe = FeCl2PDF created with pdfFactory Pro trial version www.pdffactory.com



Hot corrosion

• Alkalis can probably only penetrate the lining as part of a liquid potassium and/or sodium salt melt.

• If corrosion products therefore contain substantial quantities of potassium or sodium the form of corrosion is termed as Hot corrosion, indicating that a liquid phase takes part in the corrosion reactions.



Measures to prevent kiln shell corrosion

1. Apply gas-tight coatings on the kiln shell.

2. Maintain a full size dense lining.

3. Keep a high oxygen surplus in the kiln.

4. To cut down on chloride and sulphur input to the kiln.

5. To increase kiln chloride valve : to reduce burning zone temperature.



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