537 posts
re how to increase C3S & reduce freelime in clinker
Hello XXX,
At 0.8% SO3 in the clinker your sulphur input does not appear to be exceptionally high. (unless you have a kiln bypass system)
What is the molar SO3/Alkali ratio of your clinker?
At an SO3/Alkali ratio of 1, SO3 preferentially reacts with K2O and Na2O to form simple alkali sulphates. As the SO3/Alkali ratio increases, the excess SO3 forms calcium langbeinite (2CaSO4.K2SO4) which melts at 1011oC and is stable up to 1300oC.
At even higher SO3/Alkali ratios, SO3 next starts to become incorporated in the clinker minerals, particularly the silicates (up to about 2% with respect to C2S and ~0.5% with respect to C3S). It is only at very high SO3 levels and high SO3/Alkali ratios that any remaining SO3 is free to form anhydrite (CaSO4).
In my experience, anhydrite does not appear in industrial clinkers unless the total SO3 content of the clinker is greater than ~3% and the molar SO3/Alkali ratio is >3.6. Therefore, I doubt that decomposition of CaSO4 is the cause of your increased free lime. However, I agree with Arthur that you should have your clinker microscopically examined to determine why the free lime is high. Microscopy will tell you if the free lime is due to inhomogeneity of the kiln feed, for example, or oversize particles of quartz or calcite, or decomposition of C3S due to very slow cooling, or underburning, or a host of other causes.
Also, I don't think your idea of increasing the petcoke ratio in fuel from 80 to 90% is likely to reduce your free lime. This would increase the LSF of the clinker, since less coal ash would be incorporated into the clinker, potentially increasing the free lime further.
Is your free lime continuously high (ie in every sample), or does it 'spike' up suddenly at frequent intervals?
Do the brown centres of your large clinker nodules appear underburnt or over-burnt?
Regards,
Ted.
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1 posts
537 posts
re how to increase C3S & reduce freelime in clinker
Hello GSR,
To increase C3S you will need to increase the LSF of the clinker, either by increasing the LSF of the raw meal or using fuel with a lower ash content. If this causes an unwanted increase in free lime, you will have to grind the raw meal finer to counteract this.
Regards,
Ted.
79 posts
re how to increase C3S & reduce freelime in clinker
- C3S up to ±67% in grey cement clinker can be achieved by raising the lime saturation of the clinker towards 100 per cent. Beyond 100 per cent lime saturation the additional lime will be present as free lime and there will be no additional C3S
- With AM higher than 1.7 there is more C3S present than indicated by the Bogue calculation. The higher the alumina modulus above 1.7, the greater is the increase in the C3S content above the Bogue.
- High alkali content of clinker acts as an inhibitor to C3S formation in the kiln.
- High potassium oxide in excess of sulphate is particularly problematic as a modified C2S mineral is formed which cannot be converted to C3S. This can effectively block the formation of C3S in the kiln, resulting in high free lime.
- Phosphate (P2O5) above 0.5 per cent in clinker leads to progressive decomposition of C3S to form a solid solution of tri-calcium phosphate and C2S
- Fluoride(F-) causes a widening of the primary phase field of C3S and allows clinkers with lime saturation up to 104 per cent to be fully combined with significantly higher C3S content.
chari