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Ball Charge Sampling

























1

Introduction:





























It is always interesting
to know the ball charge gradation inside a ball mill.










Reasons are:














* to see the difference
with the theorical ball charge given by the supplier or somebody else







* to adjust the future
makeup












* to get a good idea of
the wear of the balls











* to decide if it is time
to change completely the ball charge










* to check if the
classification is good in second chamber











Experienced
engineers have a good feeling only having a look during the mill's visit but
results of a complete sampling is better





in order to convince
other people.













The problem number one is
the difficulty of the job. In fact it is very hard and long work...




















2

Method used in the mining
industry:


























Let us recall the method
often used in wet process.























2.1

Tools needed:




































A
tube in PVC with a diameter of 300mm and a length of 1000mm (this tube must
be resistant, a thickness of 810mm is necessary)






A piece of wood is
necessary between the tube and the hammer due to the impact









A big hammer to knock the
tube into the ball charge












A magnet to remove the
balls from the ball charge












A cord for lifting the
magnet and the balls












A solid bucket to carry
the balls in the laboratory























2.2

Procedure:





























Take
three samples along the mill, the first 1 meter after the mill inlet, the
second at the middle of the chamber and the third





1 meter before the mill
outlet. The samples points will be on the millaxis. See figure below:




















As the sampling campaign
of 3 samples take a long time, the procedure can be reduced to a single
sample






to be taken in the middle
the mill













Knock the tube into the
ball charge with the help of a piece of wood and the hammer (±2030cm).







After that, remove the
balls inside the tube with the magnet linked to the cord.








Put the balls inside the
bucket.












Normally, this operation
is repeated 4 or 5 times depending on the facility to enter the tube into the
charge.






Watch the following
drawings:





















One sample should weigh
between 500 and 600 kg












All this material is sent
in a room to be analysed























2.3

Comments:





























This
method is impossible to implement for the first chamber because the ball
charge is too coarse (balls from ø90mm to 60mm).






This
method could be implemented in the second chamber, especially in the second
part where there are not balls of 60, 50mm.





In any case, it is
necessary to empty the mill before to carry out the sampling campaign.


















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3

Method to be used in the
cement mill:























3.1

Tools needed:








































A solid bucket to carry
the balls in the laboratory












A shovel to take the
small balls in chamber 2























3.2

First chamber:





























Take 2 samples in two
points as shown on the sketch below:























For each sample, about
ten buckets of 20 kg each are sufficient.











Balls in surface are
accepted due to the difficulty to go deeper.











Balls are taken by hand.














All this material is sent
in a room to be analysed























3.3

Second chamber:




























Take 4 samples in four
points as shown on the sketch below:





















For each sample, about
five buckets of 20 kg each are sufficient for samples 1 (1/5) and 2 (2/5).








For each sample, about
two buckets of 20 kg each are sufficient for samples 3 (3/5) and 4 (4/5).








For samples 1/5 and 2/5,
balls in surface are accepted due to the difficulty to go deeper.









For samples 3/5 and 4/5,
take the balls deeper if possible .











Balls are taken by hand
if ø60, 50 and 40mm balls.












Balls are taken with a
shovel if small balls.












All this material is sent
in a room to be analysed





















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4

Analysis of the samples

























4.1

All we need:









































A balance


























4.2

Procedure:





























The
procedure here below is the one of the website
"www.themininggrindingoffice.com" but can be applied here also.






The balls are assembled
visually by sizes.












The biggest balls (from
90mm to 50mm) are weighed one by one.










For the lower dimensions,
we will assemble the balls of the same size by lots in order to save time.








The
balls from 50mm to 30mm are assembled by lots of 5. After, each lot of five
balls is weighed. In the calculation list which will





follow, each ball of one
lot will have the average weight of the lot (total weight of the lot divided
by 5).








The
balls from 30mm to 15mm are assembled by lots of 10. After, each lot of ten
balls is weighed. In the calculation list which will





follow, each ball of one
lot will have the average weight of the lot (total weight of the lot divided
by 10).







The
media lower than 15mm are often scraps and will be weighed together. A good
approximation of the number of small pieces





is enough.















It
is also interesting to count the balls which have a defect. This can be
achieved with balls having a diameter greater than 40mm.






One can see some pictures
of the lots by size here below:




































4.3

Example:





























The example below is the
one of a monochamber ball mill in the mining industry.









In this example, only one
sample in the middle of the mill has been taken










Total weight of the
sample: 567,9 kg













Number of balls: 480














Here below, the raw list
of this sample with the weight of the balls and the balls with defect in red






















































































































































































































































































































































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After the weighing, we
must classify the balls by category.











For
this, we convert the weight in diameter and we have the following categories
(with specific gravity of 7,8kg/dm3):





categories (with specific
gravity of 7,8kg/dm3):
























And we have the following
results:
























Important notice: In this sample,
there were not balls under 40mm and no scraps. The reason was that this mill
was always





working full of material. The pulp was going out by
the door at mill outlet.









Normally, a charge at the
equilibrium must have a gradation from the topup size (here 80mm) up to ±
10mm






(when the ball becomes a
chip and leaves the mill).












This mill was grinding
copper ore in Chile.












Ball mills in the mining
industry don't have classifying linings.






















4.4

Analysis of the results
and interpretation:


























For the first chamber of
a cement mill, the most important is to know if:









* there is really the
right percentage of the biggest balls (often 90mm)









* there is a big amount
of balls lower than the smallest dimension (often 60mm)









For the second chamber,
the most important is to know if:










* there is a satisfying
classification (segregation) of the balls






















4.5

Analysis of the results
and interpretation of the example here above:
























the
analysis of a ball mill in the mining industry is realized in order to know
if the ball charge has reached its equilibrium.






For
those who are interested, the analysis of the results and an interpretation
of the example here above is the following:






In our example, the top
size is 80mm and the top up is 100% of 80mm diameter ball.









In order to interpret the
sampling results, we have to compare it with the theory.









We take into account
sizes with 5mm interval as 80,75,70,65...etc











A
theorical charge at the equilibrium is a charge where all the intervals have
the same number of balls, except the top size





which
has only one half of interval (8077,5mm) and the 10mm size (12,510mm)
because we consider that a 10mm ball will





leave the mill














Let's calculate the
theoretical equilibrium charge as follows:

















We calculate the number
of balls as follows:



















where N = number of balls
























All the sizes have 68
balls except 80 and 10mm diameters which have 34 balls










Summary sheet:






























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we plot 3 charts in order
to compare the reality and the theory and we have:


















Comments on the chart
here above (Number):











We can see than there is
a dramatic decrease from 50mm.










There are not balls
smaller than 40mm.











Normally we must find
balls up to 1015mm inside the ball mill.



















Comments on the chart
here above (Weight):











On this graph, we can see
the same phenomenon.











The surface 2 represents
all the smaller balls which don't exist.










The surface 1 compensates
the surface 2 (there must be equals).




























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The graph here above is equivalent to the graph
"Weight"











Last
remark on this sample:
9% of the balls between 80 and 40mm have a defect. This percentage is
relatively high and





must be investigated


























Other example:





























In the
example here above, one can observe that the reality is closer to the theory,
especially for the weight chart

















5


Conclusion





























The method
presented here is not the only existing.






The most
important is to collect the necessary information to improve the grinding
performance.















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All rights reserved © 20122016 The Cement Grinding Office
