▲603The Journal of The South African Institute of Mining and Metallurgy NOVEMBER 2004

Introduction

Corrosion as far as cement is concerned isusually associated with its application inconcrete and specifically the degradation ofrebar in the concrete structure. The subject ofreinforcement corrosion is well and widelydescribed1-8, but descriptions of corrosion ofthe structures producing cement seldom appearin literature. Just as there are numerousvariables influencing the corrosion of rebar inthe cement composite called concrete, thecorrosion occurring in rotary cement kilns is acomplex phenomenon that depends on manydifferent processes and environmentalconditions.

A rotary cement kiln does not present ahomogeneous environment on the inside;rather, it can be seen as a complex chemicalreactor with various temperature zones anddiffering container wall constituents in thedifferent temperature zones9. To complicatematters, the equipment design (the number ofpre-heaters, for example), raw materialscomposition and operating conditions can allinfluence the corrosion occurring in a rotarycement kiln.

Of all the variables of interest, the chemicalcomposition of the raw meal being used andspecifically its chloride, sulphate and alkalicontents, and the type of refractory used in thekiln are probably the two most important onesin terms of their contribution to corrosion

occurring inside the kiln. Alkalis are normallyintroduced into the kiln through raw materials,while sulphur and chlorine originate mainlyfrom the fuel10. Chlorides and sulphates ofalkalis form low melting eutectics, withmelting points between 770ºC and 1080ºC11.Alkali vapours can be deposited on the rawmeal being pre-heated and are then recycledback into the furnace, thereby increasing inconcentration under circulation. Chlorinecombines preferentially with the alkalis,whereas sulphate often combines with CaO inthe bricks and infiltrates through the refractorylining; sulphates can even combine with theMgO in the refractory bricks during theinfiltration process12. In both cases theincrease in molar volume of the products thatare formed will lead to a decrease in brickstrength and subsequent increased infiltrationof the lining. Magnesia spinel bricks areknown to be sensitive to alkali attack in thetransition zone or further away from theburning zone13. Kiln stoppages and unstableoperating conditions, in combination with theoxidizing atmosphere that exists in the kiln,are aggravating factors that can further saltinfiltration into the refractory and ultimatelyhasten corrosion that occurs on the kilnshell11,14.

This paper describes a case study ofcorrosion of the kiln shell of two differentrotary cement kilns and illustrates the differentnatures of the observed corrosion. One of thekilns was a long dry-process furnace with onlyone pre-heater at the feed-end; the other was ashort kiln fitted with five pre-heater stages.

A case study of high-temperaturecorrosion in rotary cement kilnsby J.H. Potgieter*, R.H.M. Godoi†, and R. van Grieken†

Synopsis

This paper describes two cases of corrosion that occurred in dry-process, rotary, cement-clinker kilns. In the case of the kiln withfive pre-heater stages, the attack occurred primarily from sulphatesthat probably originated in the raw feed and fuel being used in theplant and that penetrated the refractory lining. The corrosion wassimilar to ash-deposit corrosion often observed in steam boilers. Inthe second case the plant consisted of a long dry kiln with a singlepre-heater fitted to the feed-end. In this case the corrosion could beascribed to alkali chloride penetration through the refractory-bricklining and subsequent hot corrosion of the kiln shell.

* School of Process and Materials Engineering,University of the Witwatersrand.

† Micro and Trace Analysis Centre, Department ofChemistry, University of Antwerp, Belgium.

© The South African Institute of Mining andMetallurgy, 2004. SA ISSN 0038–223X/3.00 +0.00. Paper received May 2004; revised paperreceived Oct. 2004.

A case study of high-temperature corrosion in rotary cement kilns

Examination

Sites and materials

The corrosion scales examined in this investigation wereremoved from the shells of two kilns during maintenancewhen the refractory bricks in the transition zones werereplaced. In both cases dolomite bricks were used. Thecorrosion scales are brittle and porous, and their coloursvaried from brown to black; in some areas salt crystals wereobserved. The scales have a layered appearance. In both kilnscorrosion occurred in the transition zone about 30 m fromthe coated clinkering zone. The length of the first kiln (kiln1) is 85 m in total. To preheat the raw meal entering the kiln,five pre-heaters are fixed at the feed-end of the kiln. Thesecond furnace (kiln 2) is a rotary kiln about 140 m in lengthand it has a single pre-heater stage at the feed-end.

Analysis

The chemical composition of the corrosion products wasanalysed by means of X-ray fluorescence (XRF) and theirphase compositions by means of X-ray diffractometry (XRD).Microanalysis and mappings were undertaken by means ofenergy dispersive X-ray spectrometry (EDX); images wererecorded by scanning electron microscopy (SEM).

Results and discussion

The chemical compositions of the two corrosion scales asdetermined by XRF analysis are given in Table I. Thecorrosion products in the two samples differ from each other.In the sample from the short kiln with 5 pre-heaters (sample1) there is a high sulphate concentration, which seems toindicate a sulphur-dominant type of corrosion attack. In thesample from the long dry kiln with a single pre-heater(sample 2) the concentrations of both chloride and potassiumare high, which points to a chloride type of attack of the kilnshell.

Table II lists the phases identified in the two samples byXRD and one notes a difference in composition between thetwo scales. This analysis also points to two different types of

corrosion attack in the two kilns. The EDX spectra of eachcorrosion scale show that sample 1 has high sulphur andcalcium contents, while sample 2 has substantial amounts ofpotassium and chloride present in it. Figure 1 is a mapping ofthe surface of the corrosion scale of sample 1 as shown in thephotomicrograph of Figure 2. The backscattered image ofeach individual element mapped separately shows high levelsof Ca and S (probably SO42-) species.

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604 NOVEMBER 2004 The Journal of The South African Institute of Mining and Metallurgy

Table I

Chemical composition (%m/m) of investigatedcorrosion scales

Compound Sample 1 (%m/m) Sample 2 (%m/m)

SiO2 0.6 0.1

A12O3 0.4 0.1

Fe2O3 67.5 78.4

Mn2O3 0.6 1.3

TiO2 <0.1 <0.1

CaO 1.8 0.6MgO 2.2 <0.1P2O5 <0.1 <0.1

SO3 16.9 0.1

C6 2.3 7.5K2O 0.9 3.2

Na2O <1.0 1.4

LOI 7.0 6.7

Total 100.2 99.4

Table II

Qualitative phase composition of corrosion scales

Phase Sample 1 Sample 2

KCI (sylvite) — XFe2O3 (hematite) X X

Fe3O4 (magnetite) X X

FeS (troilite) X —FeS2 (pyrite) X —

CaSO4 (anhydite) X —

Figure 1—X-ray maps of corrosion scale from case 1 kiln

Figure 2—Photomicrograph (backscattered-electron image) ofcorrosion scale from the kiln in case 1

Figure 3 is a mapping of the corrosion scale of sample 2as shown in the photomicrograph of Figure 4. The X-ray mapimages of the individual elements plotted separately indicatehigh levels of K and Cl with respect to the other elements.This, together with the difference in corrosion-scalemorphologies shown in Figures 2 and 4, is an indication thatthe cause of corrosion in the two cases under investigation isdifferent.

Corrosion processes in cement kilns are often similar tothose occurring in steam boiler walls that are covered withdeposits15. In this respect, the kiln acts as a ‘wall deposit’. Inan oxidizing atmosphere, the iron from the steel shell willreact with oxygen to form an oxide scale. Generally this scalecontains different iron oxides with Fe2O3 (hematite) usuallybeing found at the scale-brick interface15. This is inagreement with results of this current investigation, whichobserved the presence of different types of iron oxides in thesamples of corrosion scale.

The X-ray diffractogram of sample 1 detected thepresence of various sulphur compounds. A microscopic

examination of the piece showed tiny salt crystals along thefissure boundaries. Although the composition of the fuel andthe raw mix used are not given in this investigation, it is wellknown that they are the most common possible sources ofsulphur10. Sulphur from the fuel can react in the oxidizingatmosphere of the kiln to produce acid gases according to thefollowing well-known reactions:

S(g) + O2(g)→SO2(g) [1]

2SO2(g) + O2(g)→2SO3(g) [2]

In the oxygen-restricted environment between therefractory lining and the kiln shell, it is conceivable that SO2or SO3 can act as the oxygen donors and react with ironaccording to the following reaction15:

5Fe(s) + 2SO2(g)→Fe3O4(s) + 2FeS(s) [3]

If one considers the possibility of an interaction betweenSO2 or SO3 and the dolomite component of the refractorybricks in the lining and the positive identification of CaSO4 byXRD, the observed corrosion products could be explained byan excess of sulphur oxides present in the kiln atmosphereand their penetration through the refractory lining.

In sample 2 the concentration of KCl in the corrosionscale is high. Whereas chlorine gas can react with the kilnshell, the alkali metal oxides K2O and Na2O cannot. Alkaliscan only penetrate the refractory brick lining as part of apotassium- or sodium-salt melt. The corrosion productscontain substantial KCl and in this particular instancetherefore the damage was caused by ‘hot corrosion’. In hotcorrosion a liquid phase normally takes part in the corrosionreaction. In boilers hot corrosion usually occurs between 800and 950ºC15, but in rotary cement kilns it is conceivable thatit can occur above a critical temperature of between 300 and340ºC11. Kiln shell temperatures in this plant, as measured byan infrared scanning device, usually fluctuate between 350and 380ºC. Chlorine build-up in the kiln seems to be the mainculprit, which causes the observed kiln corrosion. This build-up is made worse by the prevailing practice at this plant ofrecirculating kiln dust from the electrostatic precipitators, adust that is rich in alkalis originating from raw materials ofhigh alkali content.

Conclusions

This investigation describes two different processesresponsible for kiln shell corrosion in rotary cement kilns.Current plant practices, the composition of the raw materialsand the type of refractory brick could all potentially havecontributed to the corrosion that occurred.

Recommendations

A possible solution to the observed corrosion in both casesmight be to use a denser brick to resist infiltration byaggressive chemical species. It is known that magnesia-zirconia bricks have good resistance to corrosion by alkalisalts, whereas magnesia-enriched dolomite bricks alsoperform well under such conditions13. Another worthwhileapproach might be to abandon the current practice ofrecycling kiln dust to the raw meal. As a last resort, achloride or alkali by-pass might be considered.

A case study of high-temperature corrosion in rotary cement kilns

▲605The Journal of The South African Institute of Mining and Metallurgy NOVEMBER 2004

Figure 3—X-ray maps of corrosion scale from case 2 kiln

Figure 4—Photomicrograph (backscattered-electron image) ofcorrosion scale from the kiln in case 2

A case study of high-temperature corrosion in rotary cement kilns

Acknowledgements

The authors wish to thank staff at VDZ (Verein DeutscheZementwerke) in Düsseldorf, Germany and the librarian atthe Technical Information and University Library ofHannover (TIB/UB), Germany, for their assistance in thisinvestigation. JHP wishes to thank the FWO-Vlaanderen(Fonds voor Wetenschappelijk Onderzoek) for financialassistance.

References

1. DEWAH, H.A.F., MASLEHUDDIN, M. and AUSTIN S.A. Long term effect ofsulfate ions and associated cation type on chloride-induced reinforcementcorrosion in Portland cement concretes. Cem. Concr. Comp., vol. 24, 2002,pp. 17–25.

2. TRITTART, J. PETTERSSON, K. and SORENSON, B. Electrochemical removal ofchloride from hardened cement paste. Cem. Concr. Res., vol. 23, no. 5,1993, pp. 1095–1104.

3. MONTEMOR, M.F, SIMÕES, A.M.P. and FERREIRA, M.G.S. Chloride-inducedcorrosion on reinforcing steel: from the fundamentals to the monitoringtechniques. Cem. Concr. Comp., vol. 25, 2002, pp. 491–502.

4. EL-GELANY, M.A. Short term corrosion rate measurement of OPC and HPCreinforced concrete specimens by electrochemical techniques. Mater.Struct., vol. 34, 2001, pp. 426–432.

5. THANGAVEL, K. and RENGASWAMY, N.S. Relationship betweenchloride/hydroxide ratio and corrosion rate of steel in concrete. Cem.Concr. Comp., vol. 20, 1998, pp. 283–292.

6. SOYLEV, T.A. and FRANÇOIS, R. Quality of steel-concrete interface andcorrosion of reinforcing steel. Cem. Concr. Res., 2003, vol. 33, 1998, pp.1407–1415.

7. OH, B.H., JANG, Y.S., and SHIN, Y.S. Experimental investigation of thethreshold chloride concentration for corrosion initiation in reinforcedconcrete structures. Mag. Concr. Res., vol. 55, no. 2, 2003, pp. 117–124.

8. REDDY, B., GLASS, G.K., LIM, P.J., and BUENFELD, N.R. On the corrosion riskpresented by chloride bound in concrete. Cem. Concr. Comp., vol. 24,2002, pp. 1–5.

9. POTGIETER, J.H. High temperature chemistry in a cement clinker kiln. S. Afr.J. Chem., vol. 50, no. 3, 1997, pp. 111–114.

10. SAXENA, J.P. Refractory Engineering and Kiln Maintenance in CementPlants. Tech Books International, New Dehli, India, 2003.

11. GROSSE-DALDRUP, H. and SCHEUBEL, B. Alternative fuels and their impact onrefractory linings. World Cement, vol. 27, no. 3, 1996, pp. 94–98.

12. NIEVOLL, J. Magnesia-spinel bricks for the cement industry. ZKG Int., vol.48, no. 3, 1995, pp. 146–156.

13. BONGERS, U. and STRADTMANN, J. Basic refractories for the burning andtransition zones of cement kilns. ZKG Int., vol. 48, no. 4, 1995, pp. 231–240.

14. KUNEMANN, H., NAEFE, H. and NAZIRI, M. Modern refractory solutions forcement kilns. World Cement, vol. 27, no. 6, 1996, pp. 54–60.

15. JONS, E.S. and ØSTERGARD, M.J.L. Investigations into shell corrosion ofrotary cement kilns. ZKG Int., vol. 52, no. 2, 1999, pp. 68–79. ◆

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606 NOVEMBER 2004 The Journal of The South African Institute of Mining and Metallurgy

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