re Raw mix adjustment for courser quartz and calcite

Dear Mr. Ted & RDS

If we do not have microscopy provision to find out quartz & calcite than how can we find out the same in XRF or gravimetric or seiving analysis.

Actually we are also having free lime 1.8 to 3% in clinker.

Kiln feed +90 mic is 14-15% & +212 mic is 1.5 to 1.8%

We do not have margin in VRM to grind finer due to drop in output.

What is the limit of quartz & calcite in kiln feed which will give high free lime or lower limit where not much impact on free lime?

We are using 80% Petcoke + 20% South African fuel mix in both firings with ILC system .

SO3 in hot meal is also not much 2-3% , SO3 in clinker is around 0.8%.

LSF in KF is 93.5-94%

SM in KF 2.25-2.26

Fuel Mix fineness is 3.5-4% on 90 mic.

Well I feel that coarse Quartz & Calcite could be one reason for high free lime in clinker .But how to check them in lab.

Daram

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re Raw mix adjustment for courser quartz and calcite

Hello COE,

Thanks for the clarification of your C3S calculation. Due to your high free lime content, your actual C3S is some 10% lower than the potential C3S. This would probably have a significant effect on cement strengths, and to compensate you are probably having to grind your cement a lot finer than is normally necessary.

It is obvious that your high free lime is related to coarse quartz (and possibly even coarse calcite). If it is not possible to reduce the coarse quartz content of your raw materials, then the only real alternative is to grind the slurry finer.

There is really no way to improve this situation significantly by adjustments to the raw mix chemistry.  Increasing the quantity of the liquid phase or changing its viscosity will be of no use since the rate limiting step in your situation is not the rate of diffusion of CaO within the liquid phase, it is lack of sufficient surface area of quartz particles for solid/liquid reaction.

If filtration becomes a problem due to finer particles you can always try a flocculating agent such as lime (Ca(OH)2),  which is very cheap and effective.  The optimum lime dosage rate will depend on the rheology of the clays in your slurry, but should only be around 2-5kg of Ca(OH)2 per tonne of dry slurry solids 

I don't believe that the changes to the raw mix chemistry that you indicated will result in a significant improvement in free lime.

 

Regards,

Ted.

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re Raw mix adjustment for courser quartz and calcite

Hello Daram,

You can estimate the coarse silica (quartz) in any size fraction gravimetrically by determination of insoluble residue in that fraction. (ie treatment with hot, dilute hydrochloric acid, followed by filtration, drying and weighing.)

You can also use XRF analysis of the size fraction to give you the SiO2 content of that fraction. Wet sieving of the raw meal is important in this instance since dry sieving leaves a significant amount of ultra-fine clay particles on the surface of the larger silica particles.

Your kiln feed 90 micron sizing is not too bad, although it would be better if you could achieve <1% on 212 micron.   With LSF = ~94 and SM = ~2.3, your kiln feed has good chemical burnability, and your fuel mix residue is also very good.

For more information on this subject, please see the following thread;-

http://www.cemnet.com/Forum/thread/108999/effect-of-free-silica-quartz-silica-in-kilnfeed.html

 

Another source of free lime is from buildups falling regularly into the kiln inlet and passing down through the kiln with the centres of the lumps relatively unburnt. Have you noticed anything lime this?

 

Regards,

Ted.

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