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rajuram sharma 16/07/2013 12:20:06

Query regarding calculation of C3S in clinker when using pet Coke as a fuel

Dear Experts,

We are using 100% Pet Coke as fuel in our one plant and around 40% in our second plant.At present SO3 % in first plant clinker is 1.80% and in case of second it is 1.00%. We have also checked water soluble Sulfates in both the clinker, which are as under:- First plant clinker water soluble Sulfates = 0.90 % and second plant clinker water soluble Sulfates = 0.50%. At present we are using this formulae for calculation of C3S

=4.071*(Cao-FCao)-7.60*SiO2+1.43*Fe2O3+6.718*Al2O3) as per bogues formulae which we use when we are using 100% Coal as fuel.

Now my query regarding calculation of C3S is:-(a) Should we use the same formulae as we are using at present or we should use the under mentioned formulae for the calculation of C3S in clinker:- C3S = 4.071*(Cao-FCao)-(7.60*SiO2+1.43*Fe2O3+6.718*Al2O3+2.852*SO3).(b) If we use C3S = 4.071*(Cao-FCao)-(7.60*SiO2+1.43*Fe2O3+6.718*Al2O3+2.852*SO3) in clinker should we take total SO3 or only water soluble sulfates.Please let me know that whether correction of SO3 should be taken for calculation of C3S when pet Coke is used as fuel and reference literature for modified bogues formulae:-

C3S = 4.071*(Cao-FCao)-(7.60*SiO2+1.43*Fe2O3+6.718*Al2O3+2.852*SO3)

Kindly do the needful and revert back as soon as possible.
With Best Regards,

Ted Krapkat 17/07/2013 03:44:23

Hello Rajuram,

This question is a difficult one to answer because of the complex ways in which SO3 may be incorporated into clinker. Basically it will depend on the SO3/Alkali molar balance of the clinker.

The equation;- C3S = 4.071*(CaO-FCaO) - 7.60*SiO2 - 6.718*Al2O3 - 1.43*Fe2O3 - 2.852*SO3 is not really meant to be applied to clinker. It was originally designed to be used to calculate the C3S content in early OPC which contained only clinker and gypsum. In that case, the term 2.852*SO3 was used to correct for the CaO present in the form of gypsum (CaSO4 ) ie.;-

The CaO due to CaSO4 is calculated as 56.1/80.1 * SO3 = 0.7 * SO3. If this value is subtracted from the total CaO in the clinker C3S equation we get;-

C3S = 4.071 * (C – 0.7 * SO3) - 7.6 * S – 6.72* A – 1.43 * F

C3S = 4.071 * C – (4.071 * 0.7 * SO3) - 7.6 * S – 6.72* A – 1.43 * F

C3S = 4.071 * C – 2.85 * SO3 - 7.6 * S – 6.72* A – 1.43 * F

or C3S = 4.071 * C - 7.6 * S – 6.72* A – 1.43 * F – 2.85 * SO3

But there is no guarantee that all, or even any, of the total SO3 analysed in clinker is present as CaSO4. In fact, CaSO4 is rarely found in industrial OPC clinkers... even those with quite high sulphur contents. This is related to the way SO3 reacts within the kiln system;-

If there are alkalis present, sulphur preferentially forms alkali sulphates. As the SO3/Alkali ratio increases above 1.0 ,the excess SO3 starts to react with K2SO4 and CaO to form calcium langbeinite (2CaSO4.K2SO4);-

2CaO + 2SO3 + K2SO4 ----> 2CaSO4.K2SO4

If the SO3/Alkali ratio increases further, SO3 increasingly becomes incorporated into the silicate minerals, particularly C2S. C2S can contain up to 2% SO3 in solid solution, whereas C3S can only contain about 0.5% SO3. But since the C3S content of most clinkers is ~4 times higher than the C2S content, the amount of SO3 tied up in each mineral is about the same, ~0.3%, which means a total of ~0.6% SO3 (maximum) in solid solution.

It is only after all of the alkalis have been converted to Na2SO4, K2SO4 & 2CaSO4.K2SO4, and the silicate minerals are saturated in SO3 that any excess SO3 can form CaSO4.

In summary;-

Only if significant amounts of CaSO4 and 2CaSO4.K2SO4 are present in your clinker would you need to use a modified Bogue equation such as ;-

C3S = 4.071*(CaO-FCaO) - 7.60*SiO2 - 6.718*Al2O3 - 1.43*Fe2O3 - 2.852*SO3

And this situation would most likely only occur if your clinker SO3/Alkali molar ratio was significantly greater than 2.

So, unless you have an exceptionally high SO3/alkali ratio, you should probably keep using the normal Bogue equations.

Regards,
Ted.

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