Sulphur, Spurrite, and Rings in cement kilns.pdf

RHI Bulletin >2 > 2007, pp. 35–38

Josef Nievoll, Susanne Jörg, Klaus Dösinger, and Juan Corpus

Sulphur, Spurrite, and Rings—Always a Headache for the Cement Kiln Operator? Sulphospurrite (2C2S.CaSO4) is the mineral phase responsible for ring formation in the preheating zone of cement rotary kilns. Samples from rings of three different kilns have been analysed chemically and studied microscopically to explain sulphospurrite crystallization and growth. SiO2 impregnated alumina bricks impede sulphospurrite crystallization by stabilizing dicalcium silicate (C2S) very efficiently and are much more economic than existing antisticking refractories. Introduction Many cement rotary kilns are plagued by rings in the inlet or preheating section. The effects of rings are well known [1]: >> The flow of the kiln feed is restricted; with sufficient height, hot meal is retained until the kiln inlet and flows out through the kiln inlet seal (Figure 1), posing a serious safety risk and damaging the kiln inlet seal. >> Increase of the pressure drop, augmenting thus the energy consumption of the induced draft (ID) fan. >> Increase of gas velocities in the ring area, entraining thus more dust into the kiln gas. These effects destabilize the clinker burning process. The ultimate consequence may be an unplanned kiln stop and the subsequent cleaning of the kiln. Despite its operational impact, the ring material is rarely examined in detail. Sometimes it is analysed chemically or by X-ray diffraction to confirm its “spurrite” nature; publications of microscopic analyses are very limited [2,3]. The present paper summarizes the results of several studies on rings from different kilns, which were carried out at the RHI Technology Center as a customer service in order to improve the kiln operation. It deals only with “spurrite” rings from the inlet and preheating zone of rotary kilns and,

given the complex compositions found, should serve as a starting point for further investigations.

Spurrite and Sulphur It should be remembered that “spurrite” is used widely ­without distinguishing between true spurrite (Ca5(SiO4)2CO3) and the more ubiquitous, but structurally unrelated calcium silico­sulphate (Ca5(SiO4)2SO4), which is sometimes called ­sulphospurrite [4,5]. In the German literature the latter is ­frequently referred to as Sulfatspurrit [2,6], therefore sulpho­ spurrite will be used further on in this paper. Besides the structural formula (see above) sulphospurrite is also written – as Ca5Si2(SO4)O8, 2C2S.CaSO4, and C5S2S. The relationship between ring formation, sulphur, and sulphospurrite in modern precalciner and suspension preheater kilns is quite obvious: Rings form easily when a pronounced excess of sulphur over alkalis in the kiln atmosphere exists. In most kilns, the excess sulphur is introduced by the fuel (e.g., when firing sulphur-rich petcoke), but rings are also observed in kilns fired with sulphur-free fuels (e.g., natural gas). In this case, the sulphur excess in the kiln atmosphere results from the lack of alkalis in the raw meal. Reducing conditions and raw meals of difficult burnability are also known to increase the amount of sulphur in the kiln atmosphere, therefore favouring ring formation. Other factors influencing the sulphur cycle are the flame shape and the burner position.

Composition of Rings The chemical analyses were carried out by X-ray fluorescence after dissolution of the sample in Li2B4O7 (according to DIN 51001); sodium and potassium were analysed by inductive coupled plasma-optical emission spectrometry (ICP-OES), sulphur and carbon using a LECO analyser, and chlorine by titration with silver nitrate. For the mineralogic investigation polished sections were prepared and investigated by light microscope and scanning electron microscope, combined with energy-dispersive X-ray analysis. Additionally, X-ray diffraction was carried out. The spectra evaluation was done according to the international database.

Figure 1. Hot meal flowing out through the kiln inlet seal.

Nine samples from three different suspension preheater kilns (Kilns A–C) were studied chemically and microscopically (Table I). From Kiln A five samples from two kiln campaigns were analysed. From Kiln B the ring which formed between running metre (rm) 47–51 had a thickness of > 35

RHI Bulletin >2 > 2007

Kiln

A

Kiln dimensions

B

Ø 4.20 x 72 m

C

Ø 5.18 x 82 m

Ø 4.45 x 70 m

40

42 outer layer (hot face)

42 middle layer

42 inner layer (cold face)

52

47 outer layer (hot face)

47 middle layer

47 inner layer (cold face)

45

MgO

1.78

1.18

1.30

1.18

0.94

1.02

0.88

0.76

1.13

Al2O3

4.84

4.17

4.75

4.12

3.45

4.91

4.91

4.91

4.82

SiO2

18.20

19.60

20.90

19.70

20.90

19.10

18.70

18.60

19.90

CaO

67.30

63.20

63.10

62.70

60.70

64.70

64.30

63.60

57.80

Fe2O3

3.12

3.01

3.32

3.13

5.41

3.49

3.56

4.12

2.93

Loss on ignition

4.11

3.22

7.57

3.89

9.52

4.88

1.42

9.84

4.35

K2O

0.36

0.94

0.74

0.89

1.43

1.31

1.78

2.27

1.31

Na2O

0.08

0.16

0.17

0.16

0.19

0.41

0.53

0.61

0.65

Ring position (rm)

Chemical analysis (wt.%)

Cl

0.05

0.09

< 0.05

0.11

0.58

0.06

0.15

0.12

0.39

SO3

4.34

5.41

1.77

5.29

4.89

4.99

4.89

8.03

10.10

Table I. Chemical composition of ring materials from three different suspension preheater kilns.

30 inch (762 mm), divided into three layers (Figure 2). Each layer showed internally a more or less pronounced fine layering. Conventional light microscopy revealed the highly porous nature of the ring material and the internal layering, resulting from varying densities. The mineral phases were best studied using scanning electron microscopy because of the small grain size (< 10 μm) and the frequently off-stoichiometric composition. A fresh surface from within the outer layer of Kiln A showed plate-like crystals of sulphospurrite (Figure 3). In polished section, sulphospurrite appeared as thin needles (Figure 4). C4AF (brownmillerite (Ca4Al2Fe2O10) formed skelet-like, relative coarse crystals. Free lime (CaO) was easily recognizable by its relief, caused by the incipient hydration. C2S formed round to elongate grains, Ca-langbeinite (K2SO4.2CaSO4), as a low melting phase, filled pores and interstices.

Figure 3. Plate-like crystals of sulphospurrite (Kiln A, rm 42, outer layer).

3 2 1

Figure 2. Ring in Kiln B, rm 47, with location of samples from outer layer (1), middle layer (2), and inner layer (3).

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1

Figure 4. Polished section from Kiln B, middle layer, with needle shaped sulphospurrite crystals (1). For further explanation see text.

RHI Bulletin >2 > 2007 In the polished section from the inner layer from Kiln A ­(Figure 5) sulphospurrite was partly decomposed into C2S, probably because of cooling down beneath the temperature of the lower stability of sulphospurrite. Yeelimite [Ca4(Al6O12) (SO4)] forms the lowest melting sulphate phase. While sulphospurrite was found in all samples, spurrite (2C2S.CaCO3) could only be identified in four samples; its contribution in forming rings seemed to be much less than sulphospurrite whose calculated content was between 10 and 60%; typical values are around 25–30%. As mentioned earlier, the analysed samples showed an internal stratification that probably reflects the variations in composition and temperature of the kiln gases. Thus, it would be interesting to compare ring growth (via temperature scanning of the kiln shell) with the recordings of temperature and gas composition at the kiln inlet.

1

2

Figure 5. Relicts of sulphospurrite surrounded by C2S (1) and yeelimite (2). Sample from Kiln A, inner layer.

The presence of liquid Ca-langbeinite (K2SO4.2CaSO4) would also explain the good adherence of the sulphospurrite crystals on the refractory substrate and its subsequent rapid growth. Without molten Ca-K-sulphate, transport of nonvolatile CaO and SiO2 to the growing sulphospurrite crystals would be too slow. The role of Ca-langbeinite in sulpho­ spurrite formation is also supported by small, but systematic amounts of K2O in the sulphospurrite composition, probably substituting SiO2. Zircon bricks were the first refractories installed specifically to combat ring formation. The drawback of these bricks was their brittleness, so that the lining was crushed mechanically soon after the installation. This product line was, therefore, abandoned about 10–15 years ago. Zircon containing castables are, however, still part of the product range, but are for obvious reasons not appropriate for the rotary kiln. SiC containing high alumina bricks have for 2–3 years gained some reputation as a ring-inhibiting material, but customers are complaining about the high price. The latest material to provide ring-inhibiting properties are SiO2 impregnated alumina bricks [7], which are significantly cheaper than SiC containing bricks. SiO2 impregnated alumina bricks also have a technical advantage: While in zircon and in silicon carbide bricks the required SiO2 is bound in the silicate and carbide structure, respectively, it is not fixed in a crystalline structure in the SiO2 impregnated alumina bricks and is therefore more readily available for impeding sulphospurrite formation. A photograph of a precalciner kiln (4.0 m diameter x 65 m long, 2300 tonnes per day) which is fired with petcoke and liquid waste fuel and where with conventional alumina bricks always rings formed, is shown in Figure 6. On SiO2 impregnated alumina bricks (i.e., RESISTAL B50ZIS) no rings formed in the preheating zone (rm 29.5–36.5). For people not familiar with cement rotary kilns the picture showing a clean, smooth lining surface may seem unspectacular; but for the plant engineers it documents one headache less in clinker production. Meanwhile RESISTAL B50ZIS bricks have also been installed in the preheating zone of the second precalciner kiln at this plant.

Refractory Materials Against Ring Formation The following refractory materials are reported to inhibit ring formation or at least to reduce ring stability: >> Zircon bricks >> Andalusite-SiC bricks >> Mullite-SiC bricks >> SiO2 impregnated alumina bricks >> Zircon containing castables >> SiC containing castables The common feature of all these materials is that they contain a component that at operating conditions makes SiO2 available for the following chemical reaction: 2 (2Ca2SiO4. CaSO4) + SiO2 → 5 Ca2SiO4 + 2 SO3 With SiO2, the thermodynamically more stable C2S is formed instead of sulphospurrite. In absence of SiO2, the formation of sulphospurrite may occur according to the following reaction: 4 Ca2SiO4 + K2SO4.2CaSO4 → 2 (2Ca2SiO4. CaSO4) + K2SO4

Figure 6. Surface of preheating zone in a precalciner kiln after 18 months operation. Sulphospurrite ring formation was impeded by RESISTAL B50ZIS, a SiO2 impregnated alumina brick. Residual thickness after 18 months operation was 160–190 mm.

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RHI Bulletin >2 > 2007

Conclusion Sulphospurrite (2C2S.CaSO4) was confirmed to be the main ring-building mineral phase in the samples investigated from three different suspension preheater kilns. Due to the minute size of the mineral phases and their off-stoichiometric composition, scanning electron microscopy combined with an energy-dispersive analysis system is the most appropriate way to study ring material. Crystallization of sulphospurrite crystals on the surface of the refractory lining and subsequent rapid growth of the ring in a SO3 rich kiln atmosphere is probably enhanced by liquid Ca-K-sulphate. SiO2 impregnated alumina bricks are an economic solution to eliminate sulphospurrite rings. The SiO2 residue from the impregnation plays the key role in stabilizing C2S and inhibiting sulphospurrite formation. Contrary to zircon and SiC containing refractory materials, which also show antisticking properties, in SiO2 impregnated alumina bricks the silica is not bound crystallographically and is thus more readily available for impeding sulphospurrite formation.

References [1] Dover, P. Practical Solutions to Kiln and Preheater Build-Up Problems. Proceedings Cemtech, Lisbon, Portugal, 2003; pp. 121–131. [2] Bonn, W. and Lang, T. Brennverfahren. ZKG International. 1986, 39, 105–114. [3] Palmer, G. Ring formations in cement kilns. World Cement. 1990, December, 538–543. [4] Taylor, H.F.W. Cement Chemistry; Academic Press: London, 1990; p 475. [5] Choi, G. and Glasser, F.P. The Sulphur Cycle in Cement Kilns: Vapour Pressures and Solid-Phase Stability of the Sulphate Phases. Cement and ­Concrete Research. 1988, 18, 367–374.

[6] Weisweiler, W. and Dallibor, W. Bildung von Sulfatspurrit 2C2S.CaSO4 aus Rohmehlkomponenten und Klinkerbestandteilen. ZKG International. 1987, 40, 430–433. [7] Nievoll, J. and Monsberger, G. The Performance of Specially Impregnated Alumina Bricks. RHI Bulletin. 2004, 2, 11–14.

Authors Josef Nievoll, RHI AG, Industrial Division, Vienna, Austria. Susanne Jörg, RHI AG, Technology Center, Leoben, Austria. Klaus Dösinger, RHI AG, Industrial Division, Vienna, Austria. Juan Corpus, RHI REFMEX, Ramos Arizpe, Mexico. Corresponding author: Josef Nievoll, [email protected]

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