Kiln feed
We have been told ‘Chewing of your food can change your life’ as digestion begins with it. And truly applicable to kiln also.
Smaller particles reacts faster in the kiln but it is not the same as cement grinding, where a narrow range of P.S.D. is normally maintained to control distribution range between 5 to 50 µm. But the size distribution for raw meal is quite wide as upper limit of size fraction can be as high as 125 µm.
Very fine grinding of raw materials feed with increased surface area may help in early volatizetion of minor components causing unwanted build-up at bottom stage cyclones.
In addition, very fine particles in kiln feed will bring down P.H. Cyclones efficiency (precipitation time of the raw mix in a cyclone accelerates with the square of the particle diameter) and too much dust in gas flow will have an adverse effect on heat exchange rate, resulting in higher exhaust gas temp and heat loss.
Particles fineness plays a major role at the last eight meters (may vary a little with kiln length) at discharge end of the kiln where conversion of C2S into C3S takes place. Particles travel time within this critical distance favors only the smaller particles that can complete the reaction to produce clinker between the desired f-CaO.
If the raw meal feed to the kiln is too coarse, it will increase the heat load of the kiln, decrease the decomposition rate of raw meal during the sintering process.
Reactivity and burnability are two fundamental characteristics that affects the through put of the plant and ultimately fuel consumption. Other than chemical compositions, the factors that affecting the burnability of cement raw mix is - mix granulometric composition. Mix B.I. (burnability index) is related to mix chemical as well as granulometric compositions as -
CaO1400˚C =[0.343*(LSF-93) + 2.74*(SR-2.3)] + [0.83*Q>45µ + 0.10*C>125µ]
The second part of the burnability equation (second bracket) represents the contribution to the burnability from the mineralogy and fineness of the raw mix. This is to be noted here that the quartz fineness is more significant than fineness of calcite, as indicated by its larger coefficient in the equation.
The influence of bigger quartz particle is approximately five times greater than that of calcite on clinkerization.
There is no such fixed optimum range of raw meal (kiln feed) fineness and its changeable with the type of raw material use and its individual reactivity and hardness (minerology).
Homogeneity of the raw mix plays an important role in determining the optimal feed meal fineness to achieve product quality and kiln output.
Here homogeneity is not intended for kiln feed sample to sample variation but it is the variation in the chemical composition in different size fraction samples by monitoring two major minerals (calcite and quartz) in raw mix.
This is observed, there was a reasonable chemical differences in first sample (RM-1) with the increase in sieve opening. Where as in second sample (RM-2) high increase in SiO2 % is detected with descending sieve size. Similarly, an opposite effect is reflected with CaO %.
A simple explanation of the above finding could be of hard siliceous component such as quartz presence in sample RM-2 is the reason for accelerated siliceous grains in lower number sieve residue.
Interestingly, P.S.D. of sample RM-2 is within a narrow range compare to sample RM-1 (exhibited at the bottom part of the chart) but major share of required silica particles are accumulated at the upper side of the permissible size fractions. This may lead to hard burning of sample RM-2 and presence of % f-CaO in clinker as made is expected to be more.
So the high mineralogical differences in raw material ground requires higher product fineness (as it is the safe requirement for mix type RM-2) and the mill feed materials with like gridibility index may have the liberty to target a little higher residue.
This would be beneficial for periodically re-confirmation of product fineness target (generally this is checked on B.S. 170 mesh as % residue obtained) to check chem composition/L.S.F. of sieve residue and comparing with designed mix value(s) — if overall analysis of collected sample remains close to designed value.
This is convenient to adjust target residue to avoid hard burning of mix where as changing mineralogical compositions of raw material use is near impossible.
Optimal fineness range may vary from 10 to 25% on +90 µm after minerological analysis for hard to soft burning mix.
This may not be the out of place to mention- few manufacturing units are running with as high as +40% residue on 90 microns and rock used might be resembling the required cement composition needed. But this is very unusual today as availability of cement grade limestone is now very limited and mostly existing stone for manufacturing, requires suitable processing before feeding to the kiln.
The scope for minerological analysis by optical microscopic to know calcite and quartz grain size and free silica present in raw materials is not always available in all process lab but XRF analysis of sieve residue can provide a fair idea about meal fineness requirement.
The materials to grind if not minerologically favorable, it may be again manageable by right adjustments of the classifier included in mill system.
The sample as represented above, the particles cut-size is found 61 µm or the size is having equal probability to be in reject or in product material. And when lower sieves (between 38 to 45 µm opening) residue represents high in siliceous particles, one may think of further reducing the separator cut-size — preferably adjusting drag air or cage speed in case separate fan is not in circuit. And the same sample analysis also may allow bigger cut-size if calcite grain is found sufficiently present.
Selecting lower cut size for a mill feed where mineralogical difference is more, will allow hard siliceous components to re-entering the mill for further reduction.
Even though, increase in feed material hardness will inevitably reflect in mill circulating load - still controlled operation always ensure product quality. Further more, mill runs to fulfill kiln requirement and its not just the opposite.
An ideal meal fineness is found necessary for better clinkerization and at same time avoiding over fine material grind from raw mill as meal fineness and mill output both are interdependent.
Generally, it is a regular process to monitor raw mill product fineness sometimes hourly or once in eight hours. A composite sample as day’s average could be analyzed in given format -
It may provide us a valued information regarding meal fineness and its probable effect on burning, also a cumulated study of everyday’s report can highlight of any change in raw material hardness, particularly in limestone.