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Technical Cement Forum / MgO in raw mix

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Stephenb 07/10/2009 11:52:48

MgO in raw mix

Good day experts.

Normally, our raw mix has an upper limit on MgO of 0.75%. If MgO exceeds this by about 0.1%, we see operation changes in the kiln and slabbing usually occurs(wet kiln). My 2 questions:

(1) What causes this unstability in the kiln when the MgO increases? Is it a higher LP?

(2) What corrective action can be taken to minimize the MgO effect?

My thinking is that MgO makes th eslurry easier burning, and hence changes the normal burnability of the slurry, so as a corrective action i would increase the CaO to try to nulify the MgO effect. Is my logic correct?

Thanks.

Ted Krapkat 08/10/2009 06:52:29

Hi Stephen,

I am surprised that such a small increase in MgO (0.75% to 0.85%) could have such a marked process effect. Are you sure it is MgO related?

I doubt it could be liquid phase related because although MgO reports to the liquid phase (below about 2%), an increase of 0.1% would only increase the liquid phase by the same small amount, which is really insignificant.

I suspect that the problem may be located elsewhere, perhaps indirectly linked to the MgO increase, although not caused by it.

What is the nature of these slabs and where do they form? Are you able to obtain a sample of them?

And, are the slabs associated with any other events such as subsequent sudden high free lime in the clinker?

Regards,

Ted.

Stephenb 08/10/2009 12:48:27

Hi Ted.

The slabs usually comes from behind and in front of the flame. Slabbing doen not occur everytime the MgO increases though, only some of the times.

I have never closely looked at the free lime when this happens though. What would you suggest could be happening when you asked if i the free lime increased?

Thanks.

Ted Krapkat 09/10/2009 03:58:33

Thanks Stephen,

The fact that the slabbing occurs only some of the time when MgO increases confirms my suspicion that MgO may not be the real culprit here.

There are many reasons why these buildups might be forming and one of the best ways to find out is to try and get a sample of the slabs or, failing that, a sample of any lumps which may come out with the clinker just after a slab falls. Chemical and physical examination of these samples may indicate possible causes.

The reason I asked about the clinker free lime is that one possibility may be that these slabs are of unburnt material, which has built up quickly at temperatures below the sintering temperature. When such slabs release, large pieces can quickly enter the cooler without having a chance to completely burn out the free lime they contain.

Therefore the clinker exiting the kiln will be temporarily contaminated with this unburnt slab material and a sudden, sharp, short-term (2 - 3hrs) increase in clinker free lime may be noticed in the hourly clinker samples. (You may also see a corresponding increase in K2O and SO3 in the clinker depending on the composition of the slabs.

If the slabs are from an ash ring the free lime will probably be low, however the MgO, (as well as K2O or SO3) may be high, depending on the composition of your coal ash.

If you inspect the clinker exiting the cooler after one of these slabs has fallen out you may be able to pick out some of the largest pieces and get them tested chemically.

You may also examine the pieces and be able to determine if the buildups are soft, hard, or are made up of layers etc. Layering suggests the buildup may be caused by alternating kiln conditions such as flame, coal ash absorption, gas flow, slurry chemistry, reducing atmosphere etc. High K2O buildups are often soft, while buildups due to SO3 are generally quite hard.

Once you gather some more information like this you can start to narrow down the actual cause.

Good luck,

Ted.

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