Hello Raju,
To answer your questions;-
1. Yes, it's true that the first oxidised form of sulphur in the kiln and preheater is SO2. However, as long as there is sufficient excess O2 in the kiln the following reactions can take place;-
K2O + SO2 + 1/2 O2 ---> K2SO4
Na2O + SO2 + 1/2 O2 ---> Na2SO4
CaO + SO2 + 1/2 O2 ---> CaSO4
Since SO2 +1/2 O2 = SO3, and the fact that in the cement industry we usually express compounds in terms of oxides, it has become commonplace to refer to SO3 rather than SO2 + 1/2 O2. Some actual SO3 may exist in the kiln, by reaction between SO2 and O2,
but only in trace amounts, since the reaction needs a higher concentration of oxygen than is normally found in kiln gases, as well as needing to be catalysed by vanadium pentoxide or platinum, both of which are very rare in cement raw mixes.
2. It is recommended to run the kiln with an excess of O2 in the kiln inlet so that there is always some oxygen present to convert the alkalis into alkali sulphates (as per the equations above). Alkali Sulphates are far less likely to decompose back into
oxides and SO2 than are the sulphites (eg K2SO3), which would be formed if there was a lack of O2 in the kiln inlet. This allows the alkalis and sulphur to escape the kiln in the form of sulphates, thus reducing the recirculating load at the kiln inlet and
reducing the likelihood of rings and buildups.
3. The incorporation of SO3 into the silicate minerals is not strictly a chemical reaction. It is more correctly a substitution of the SiO2 molecule by the SO3 molecule in the crystal lattice. This is called a solid solution. A solid solution can have different
amounts of lattice substitution depending on the equilibrium conditions surrounding the crystal, whereas a chemical reaction has fixed molar amounts of reactants and products.
SO3 incorporated into C2S stabilises the C2S crystal lattice which tends to inhibit further reaction of C2S with CaO to form C3S, thus reducing the amount of C3S in the clinker. Stabilised C2S is also less reactive towards hydration. Both of these effects reduce
the clinker's strength potential. The degree of strength reduction is dependent on the amount of SO3 present in the C2S lattice, up to its saturation point, which is ~2% SO3 for C2S.
Hope this helps you.
Regards,
Ted.
