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

RHI Bulletin >2>2007, pp. 3538

Josef Nievoll, Susanne Jrg, Klaus Dsinger, and Juan Corpus

Sulphur, Spurrite, and RingsAlways a Headache for the Cement Kiln Operator?

IntroductionMany 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 seri-ous 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. Some-times 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 SulphurIt should be remembered that spurrite is used widely without distinguishing between true spurrite (Ca5(SiO4)2CO3) and the more ubiquitous, but structurally unrelated calcium silico sulphate (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 C5S2

S.

The relationship between ring formation, sulphur, and sul-phospurrite 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 atmos-phere, therefore favouring ring formation. Other factors influ-encing the sulphur cycle are the flame shape and the burner position.

Composition of RingsThe chemical analyses were carried out by X-ray fluores-cence 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 investi-gated by light microscope and scanning electron micro-scope, combined with energy-dispersive X-ray analysis. Additionally, X-ray diffraction was carried out. The spectra evaluation was done according to the international data-base.

Nine samples from three different suspension preheater kilns (Kilns AC) were studied chemically and microscopi-cally (Table I). From Kiln A five samples from two kiln cam-paigns were analysed. From Kiln B the ring which formed between running metre (rm) 4751 had a thickness of

Sulphospurrite (2C2S.CaSO4) is the mineral phase responsible for ring formation in the pre-heating 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 antistick-ing refractories.

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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 2530%.

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 tempera-ture scanning of the kiln shell) with the recordings of tem-perature and gas composition at the kiln inlet.

The presence of liquid Ca-langbeinite (K2SO4.2CaSO4) would also explain the good adherence of the sulphospurrite crys-tals on the refractory substrate and its subsequent rapid growth. Without molten Ca-K-sulphate, transport of nonvol-atile 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 system-atic 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 mechani-cally soon after the installation. This product line was, therefore, abandoned about 1015 years ago. Zircon con-taining 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 23 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 alu-mina 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 29.536.5). 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 doc-uments one headache less in clinker production. Meanwhile RESISTAL B50ZIS bricks have also been installed in the pre-heating zone of the second precalciner kiln at this plant.

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. Residu-al thickness after 18 months operation was 160190 mm.

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

1

2

Refractory Materials Against Ring FormationThe 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 con-tain 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 fol-lowing reaction:

4 Ca2SiO4 + K2SO4.2CaSO4 2 (2Ca2SiO4.CaSO4) + K2SO4

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