Raw materials preblending


Good day everyone
today we will discuss the topic of pre blending or pre
homogenizing. From what I could observe pre blending is something many of our
technicians and engineers don’t pay much attention to but pre blending is
actually the first key process to ensure the uniformity of our raw materials this
will give us stability in kiln operation and proper quality of our clinker and
cement. What can be done in the plant to improve pre blending effectiveness is
very simple; no need for investments or difficult processes just paying attention
to the way we work we’ll get you through a great deal of improvements. Pre
blending is somehow like making a cake and eating it afterwards. We prepare layers
of chocolate marmalade pastry and then we cut slices for everyone to enjoy a
similar taste of the cake. We do the same with pre blending different qualities of
raw materials from the quarry are layered and then thinly cut to get a
mixture of these layers to feed the raw mill. The purpose is to premix the good
and the not-so-good materials we have in the quarry this will also help us
increase our quarry life. We will focus on the longitudinal pre blending system
and check what factors influence the system performance and what you can do
to improve efficiency in your system. As a rule of thumb raw material components
with standard deviation fluctuations of calcium oxides higher than one percent
should be pre blended. In addition to pre homogenizing there are other reasons for
installing and using pre blending systems. For instance it is a fully
automatic raw material storage of large capacity, it decouples the quarry
operation from the plant avoiding multiple shifts operation in the quarry,
in particular working at night. We can do selective quarying and pre mixing of
different raw material components using the good and the not so good components
as I mentioned earlier. The longitudinal pre blending system is known as batch
blending since one pile has to be formed while the other is reclaimed and
then we change. The raw material from the quarry is stacked in layers to a pile
the one on the left here and reclaimed in right angles to the layers in thin
slices on the other pile. There are various combinations of pile structure and
extraction processes let’s have a closer look to some of these processes. This
Chevron system is the most common and simplest pre blending type. Longitudinal
layers are placed by a to-and-fro stacker movement over the pile until the
full height is reached then this material will be afterwards reclaimed in
slices transversal to the pile by means of a front acting reclaimer. Another
stacking method is so called Windrow stacker several longitudinal layers are
stacked parallel over the whole width of the pile in such a way that they form a
triangle after completing the full height in
other words the stacker not only makes longitudinal but also transversal
movements forming a pattern of different rows. Reclaiming is always done with a
front reclaimer system. You may start asking yourself in this case the investment
costs are surely higher compared to the simpler Chevron system so where and why
will the windrow system be applied? Think about it … and this is yet another stacking system the stacker moves stepwise in longitudinal
direction the next step is only executed after completing the cone up to the full
pile height. Reclaiming is done laterally with a side reclaimer front reclaimer
should not be used with a conical shell stacking method … and this is another
question for you think about it. The conical shell method
is normally used in case of sticky materials and when the homogenizing
efficiency is not that important in other words when we already have very
homogeneous qualities of raw materials in the quarry. The stacking in
the homogenizing pit could be both either windrow stacking in longitudinal
or transversal on layers for best homogenizing effect the transversal stacking is preferred. The homogenizing pit is
applied for small pre blending capacities or sticky materials requiring
a good homogenizing efficiency. Due the high investment cost the
homogenizing pit is not normally used where longitudinal Chevron system can be
used. Reclaim is performed by a bucket chained excavator lifting and
discharging the material onto a bridge and a belt as seen here. So far we talked
about the overall systems let’s go into the details of stacking machines and
arrangements. For stockpiling the material there are two types of machines:
first the so called throw-of carriages or “tripper”, fed by a belt conveyor installed under the roof of the storage building this is one of
them the reversing or reversible belt and this is the second arrangement the
tripper belt the other type is the side stacker because of the counterweight
needed to balance the stacker this machine is wider and required more space
within the storage building. What would you think would be the biggest
advantage of this type of stacker compared to the tripper type? That is
another question for you… We talked so far about stackers let’s move to reclaimers. This bridge type front acting reclaimer equipped with an extracting
scraper chain is the most applied reclaimer machine. The homogenizing
effect is good because it slices off the whole cross section of the pile the
output rate is constant and easy to control only a small amount of clearance
is needed for the machine inside the storage hall. The working direction of
the machine can be changed quite simply. For feeding the exit belt
conveyor the machine requires on the discharge side a feeding table or
delivery chute. The machine moves continuously into the pile and the
scraper chain is filled progressively towards the discharge and because of
this it has to tendency to move slower at the end where it is fully loaded and
faster on the other side because there’s no material there. Provision must be
taken to keep the bridge Square to the belt system. There are different
types of chain arrangements in this case the horizontal chain with its discharge
chute here the inclined scraper chain notice a more complex discharge chute over a wall and a combination of the previous two, a
horizontal chain with an inclined discharge end. There are other types of
Reclaimers such as this one the side reclaimer it is equipped with a boom
scraper chain that can be lowered to the size of the pile it is typically used
for reclaiming additives stored in the same pre blending hall. It has low
homogenizing efficiency and therefore should be used with conical shell
stacking for materials with quality standard deviations lower than 1% CaO
or the equivalent as we have seen the purpose of heavier blending system is to
reduce quality variations of our raw materials the graph shows an example of
what comes into the pre blending hall and what we expect to have after going
through the pre blending process notice that in the longitudinal system because
we’re working in batches there will be notches changes of qualities whenever we
start a new pile. To calculate the efficiency of preblending we would have
to compare what comes in to what goes out of the system this would require
taking numerous representative samples of material and run them all through
chemical analysis to determine the pre blending efficiencies. There are simpler
methods using mathematical models to determine the homogenizing efficiencies
for this we use the primary chemical material fluctuations the number of
layers and the geometry of the pre blending system. The model says that the
efficiency can be defined as the ratio of input fluctuations over output
fluctuations and in turn this can be assumed to be
almost equal to the square root of the number of layers and how do we calculate
the number of layers well it is the cross section of the pile times the
stacker speed divided by the stacking rate. Notice that this formula applies
only when we have normally distributed layers remember normal distribution, the
symmetric bell curves. From this formula you can already start thinking what must
be done to keep your efficiency as high as possible in the longitudinal preblending system. There are additional issues which will influence the efficiency such as the so called end cone you see here the situation when we
changed position of the stacker reclaimer we start consuming a new pile
we face the issue of the end cone the reclaimer will at first consume a
limited number of layers there will be no blending effect at this
point one of the remedies is to stack with your so-called variable reversing
points. When forming a new pile the stacker will slowly be moving towards
the outer pile end, we call these reversing points because of this on the
inner side all layers will be open to the reclaimer while on the outer side
the layers thickness will change according to the movement of the stacker
again improving the blending effectiveness theoretically the number of these
stacker reversing points should be equal to the number of layers practically this
is not feasible a good number of reversing points will give better
blending results of course an appropriate number of stacker reversing
points will be in the range of 12 to 15 for a pile height of approximately 10 meters Another issue which will deteriorate the blending efficiency is seen on this picture what is the problem according to you?? segregation of material of course segregation can also be seen on this picture where the
coarse fraction of the material rolls down the pile while the fines remain on top we
can notice a second issue on this picture what is it? it is the fact that
the pile is not uniform with different heights on top indicating an improper
movement of the stacker and/or an inconsistent throughput of material
coming from the quarry or from the secondary crusher as coarse particles
tend to roll down the slope of a pile mainly course metal is reclaimed at the
beginning. This coarse material will have different chemical characteristics
than the average of the pile on this graph you can see the percentage of end
cones versus the pile and I also provide the formula so you can calculate your
situation there are few things you can do to reduce these adverse conditions
for instance focus to produce a high length width ratio for the piles this
will reduce the volume of end cones A minimum value of length width of three
should be aimed at which correspond to a volumetric proportional of about 15% of
end cones. For length width ratios of two the proportion of end cones will go to 20%. What to do about end cones? Well end cones at the far end of the pile can be left
standing or be only partially reclaimed of course this does not alter the situation at the near-end cone besides we could not use the complete
effectiveness of the stockpile volume another solution would be to recirculate the encode material to the second stockpile
being formed this procedure is time and energy consuming and can be
used only in extreme cases you can also use the reversal point as previously
mentioned in this way the reclaimer will not consume only segregated material
when starting the new pile. Finally make sure that the stacker boom starts very
low and is automatically lifted when the pile grows this way, segregation will
be reduced to a minimum. I would like to say something about this integrated pre
blending system. With the technology available today we can and should make
the best possible usage of our longitudinal pre blending system. You see
on this graph an example of such an integrated system where two or more
components are dumped on the same pile in a controlled manner, to attain a predefined chemical value this is done with a PGNAA, an
online analyzer which continuously monitors the material led to the pre
blending pile Corrections will have to be done to the feeding system of the raw
mill. of course. Before concluding this session I would like to remind you that
I have asked a few questions and here they are again. The first one is why and
where would a windrow pre blending system be used instead of the simpler Chevron system? Second question… Why should front reclaiming not be used in
the conical shell method? Third question what is the biggest advantage of the
side stacker compared to the reversing belt method and finally last question
what can be done in your plant in terms of production planning and execution
to improve the blending effectiveness and obtain sustainable
results? I will not give the answers now so you can think about it I will publish
a new video later on giving the answers I would like to conclude reminding you
what we’ve done so far we focused on the longitudinal pre blending system seeing
the different types of stackers and reclaimers and seeing how to maintain a
proper blending efficiency in our systems in particular dealing with the
famous or “infamous” en cones I hope that you could benefit from
this short information session I suggest you discuss with your
colleagues and supervisors and find out what can be done in your plan to improve
your blending effectiveness Thank you for your attention
and write me some comments and suggestions in case you have some bye
bye for now and till next time

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