Sulphospurrite (Ca5Si2(SO4)O8, 2C2S.CaSO4), also known as calcium silico-sulphate, is the primary mineral phase responsible for ring formation in the preheating zone of cement rotary kilns. Samples from rings in three different kilns were analyzed chemically and microscopically to understand sulphospurrite crystallization and growth. SiO2 impregnated alumina bricks were found to effectively stabilize dicalcium silicate (C2S) and impede sulphospurrite crystallization in a more economic way than existing refractories containing zircon or silicon carbide.

1. > 35
Figure 1. Hot meal flowing out through the kiln inlet seal.
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?
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 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 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 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 Rings
The 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 A–C) 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) 47–51 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.

2. RHI Bulletin >2>2007
36 <
30 inch (762 mm), divided into three layers (Figure 2). Each
layer showed internally a more or less pronounced fine lay-
ering.
Conventional light microscopy revealed the highly porous
nature of the ring material and the internal layering, result-
ing 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-stoichio-
metric 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 ske-
let-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.
Kiln A B C
Kiln dimensions Ø 4.20 x 72 m Ø 5.18 x 82 m Ø 4.45 x 70 m
Ring position (rm) 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
Chemical analysis (wt.%)
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
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.
Figure 2. Ring in Kiln B, rm 47, with location of samples from
outer layer (1), middle layer (2), and inner layer (3).
Figure 3. Plate-like crystals of sulphospurrite (Kiln A, rm 42,
outer layer).
Figure 4. Polished section from Kiln B, middle layer, with needle
shaped sulphospurrite crystals (1). For further explanation see
text.
1
1
2
3

3. RHI Bulletin >2>2007
> 37
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 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 10–15 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 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 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 29.5–36.5). 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 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 160–190 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 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 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

4. RHI Bulletin >2>2007
38 <
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-stoichiomet-
ric 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 lin-
ing and subsequent rapid growth of the ring in a SO3 rich
kiln atmosphere is probably enhanced by liquid Ca-K-sul-
phate.
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 inhib-
iting sulphospurrite formation. Contrary to zircon and SiC
containing refractory materials, which also show antistick-
ing 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, josef.nievoll@rhi-ag.com