537 posts
Re: Raw Meal fineness x kiln output
faslan:
First question can you describe the quality effect of fineness deeply?
Raw meal fineness directly effects the clinker burnability (and hence mineralogy) in the following way;-
Because of the physical limitations of burning temperature and residence time, and the fact that the reactions occurring in the kiln are either solid/solid (<~1250oC) or solid/liquid (>~1250oC), raw meal fineness directly influences the relative quantities of clinker minerals produced. ie. finer particles will react faster and therefore more completely in the given time.
Conversion of C2S into C3S by reaction with CaO occurs by diffusion of CaO through the liquid phase. Therefore the rate of reaction is dependent on the diffusion rate of CaO through the liquid. This diffusion rate and the burning zone residence time together determine the critical distance that dissolved CaO has to travel through the liquid for complete reaction. Therefore particles have to be smaller than this critical distance to allow CaO diffusion and complete reaction in the time available in the kiln.
So, if the quartz particles are too coarse (>~40um) , frequent large C2S clusters will be produced at the expense of C3S... leaving a porportionate amount of unreacted free lime.
And, if the calcite particles are too coarse(>~100um), localized CaO over-saturation produces a "wall" of C3S crystals around the particle and the remaining CaO cannot readily diffuse out through this barrier and react with nearby silica... again resulting in unreacted free lime.
So, raw mix which contains coarse quartz and/or calcite particles will result in high free lime clinker which is higher in C2S and lower in C3S compared to the predicted values calculated by the standard Bogue equations.
Second what can you tell about the allowable values of coarse particles (calcite quartz)
My posts in similar threads located at the following links might have the information you need;-
Basically, you should keep the calcite and quartz upper particle sizes as low as practically and economically feasible.
I'm trying to persuade the plant staff to increase the fineness from 7-8 to 11-12 and even more according to kiln. I'm relating the upper-transition zone unstable coating to somehow the very fine raw with the raw mix composition. Can you help me? or am i wrong?
Sorry, Firat. I don't quite understand your question.
What are the values 7-8 and 11-12? Are they raw meal +90um residue measurements? If so, then going from 7-8% >90um to 11-12% would be decreasing the fineness (ie.gringing the meal coarser) Is that what you are proposing?
Also, coating stability is related more to the chemistry of the raw mix, which largely determines the quantity & properties of the liquid phase, as well as the temperature in the burning zone and the stability and shape of the flame, rather than raw meal fineness.
Large extremes of raw meal fineness may have an influence on clinker nodulisation, but I doubt that there would be any great effect on coating stability in the range of particle finenesses you are suggesting.
Regards,
Ted.
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38 posts
Re: Raw Meal fineness x kiln output
Dear Ted,
Sorry for the unidentified question. The 7-8% and 11-12% are 90 um residue measurements.
What I've read from the Holderbank's notes unstable coating, ring & ball formations are affected by the very fine particles below 20um.
Also another problem that could be result of fine raw meal is segregation. I've checked the ratio of LSFofparticles<32um/LSFkilnfeed to be 1,3. (132,7/101,8).
I just want to ask what could be possible problems with very fine (if 7-8% residue on 90um can be called as 'very fine') raw meal.
I've checked the burning pipe and there was a problem with the burner. We've fixed it. The combustion gets better less CO formation and higher O2 and NOx, the build-up at riser duct has decreased etc.
However still unstable coating is problem (especially at the upper-transition zone). The chemistry of raw mix is stable and the composition doesn't look like to be problematic. The temperature of the burning zone is being read by a pyrometer and during normal operating conditions it is 1380-1430.
100 % petcoke so there is no ash. ASR is around 1,1.
If you want I can send the pictures of scanner also. Thank you inadvance.
Best Regards,
Firat
537 posts
Re: Raw Meal fineness x kiln output
What I've read from the Holderbank's notes unstable coating, ring & ball formations are affected by the very fine particles below 20um.
Hello Firat,
Yes, I have seen such reports from Holcim. I have one by Dr. Jim Imlach on 'Coating, Ring and Build-up Formation in Cement Kilns', where he says that "in many cases the absence of particles under a critical size (e.g. 5um) ensures freedom from deposition." However, he is talking about fine particle agglomeration as a mechanism for the formation of buildups.
I can see how fine meal particles might be associated with rings and buildups above the upper transition zone, but once a liquid phase is formed, such fine particles would be the first to react and be taken up in the nodulisation/mineral reaction process.
So, as long as sufficient liquid phase was present, I really can't imagine how fine raw meal particles could have any significant effect on coating stability in the upper transition zone. Besides it is impossible to produce a kiln feed that has an absence of particles under 5um, or even 20um for that matter.
When talking about the causes of rings and buildups, there is often confusion between raw meal dust and clinker dust. Clinker dust can definitely influence the formation of rings, buildups and unstable coating. However clinker dust has nothing to do with the fineness of the raw meal, it comes from the cooler and, upon passing through the flame, it partially melts and/or picks up more molten material (coal ash) and is then transported by the kiln gases and deposits further up the kiln.
Clinker dust is associated with an excessive sulphur cycle or having too little liquid phase or an unstable kiln operation.
Also another problem that could be result of fine raw meal is segregation. I've checked the ratio of LSFofparticles<32um/LSFkilnfeed to be 1,3. (132,7/101,8).
This is quite normal. The finer fractions (<32um) are rich in the softer components such as limestone and argillaceous material. You will probably also find that this fraction has a slightly higher alumina ratio and lower silica ratio than kiln feed. As long as you minimise the entrainment of kiln feed in the gas flow at the kiln inlet (which is normal practice) this should not cause a problem.
I just want to ask what could be possible problems with very fine (if 7-8% residue on 90um can be called as 'very fine') raw meal.
I wouldn't consider 7-8% residue on 90um to be very fine. It is a bit on the fine side, but I have seen plants that grind finer without any diffuculties. In fact, 11-12% residue is fairly average for raw meal. So, it wouldn't hurt to increase your target to this level anyway. However, I doubt that an increase of 3 or 4 % on 90um residue alone will have any effect on kiln coating or buildups in the transition zone..
However still unstable coating is problem (especially at the upper-transition zone).
Firat, what exactly are the symptoms of this unstable coating problem and what sort of duration/frequency are we talking about?
100 % petcoke so there is no ash. ASR is around 1,1.
How do you calculate your ASR? And what are the levels of K2O, Na2O and SO3 in your raw meal and petcoke?
Even if your alkali and sulphur are in balance, high alkali suplhur levels can effect the surface tension of the liquid phase causing poor nodulisation and dusty clinker.
Regards,
Ted.
38 posts
Re: Raw Meal fineness x kiln output
Dear Ted,
Thanks inadvance for your detailed useful information. Now it is more clear to understand the clinker dust effect.
"This is quite normal. The finer fractions (<32um) are rich in the softer components such as limestone and argillaceous material. You will probably also find that this fraction has a slightly higher alumina ratio and lower silica ratio than kiln feed. As long as you minimise the entrainment of kiln feed in the gas flow at the kiln inlet (which is normal practice) this should not cause a problem."
this was again limited in a range of 0,8<Ratio<1,2 at Holcim notes. Anyway..
Anyway, the frequency of coating changes and sometimes increases to day by day. Let me briefly explain coating change. The kiln is 64*4,6 mt. Between 25-37 mt, there, we use a refractory with 85% MgO, 10-15% Al2O3 0,8%Fe2O3. 37-64mt we use Aluminate refractory. Between 25-37 mt we can see coating (sometimes ring formation) daily. In one day coating forms and the other days it spills. As a result the kiln is stable for one -or two days- then it works with low product rate in other day because of the coatings spillage.
The other problem is kiln inlet ring formation with a frequency of 2 weeks. It is because of the sulphur cycle. Because I had checked its chemical analyses and the K2O content was 3,5% and SO3 content 8%.
In sinter zone, we sometimes have ring formation with a frequency of 2-4 weeks. If you want I can send you the scanner's views in ppt format.
ASR=(Na2O/62+K2O/94)/(SO3/80)
In clinker (according to many measurments);
SO3:1,15 Na2O:0,5 K2O:0,73
Sulphur content in Petcoke is 4,5%.
Best wishes,
Firat