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The study
of the granulometry along the ball mill





1

Introduction





In order to
have a good idea of the ball mill's efficiency (linings, ball charge,
diaphragms, ventilation), we have to do a


granulometric
analysis inside the mill.


This
analysis includes three steps:




The sampling campaign inside the mill




The sieving of the samples in the laboratory




The results analysis with the curve and its interpretation





For this
presentation, we consider a classical 2 chambers mill as example.





2

The Sampling Campaign






First Chamber



As described in the Mill Inspection Procedure, we need the
following:






Procedure:




Take 4 samples (inlet, 2 intermediate samples, at the
intermediate diaphragm) of ± 3kg each




Ideally for each sample, take materiel in three points as
shown on the drawing here below







Dig a hole 30 cm deep removing the balls at the sampling point
and collect the sample with the collector.






Don't reject the unground particles from the sample!






Second Chamber



We need the following tools:









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Procedure:




Take 7 samples (1st at inlet, 5 intermediate samples at equal
distance and the last at the outlet diaphragm) of 0,5



1kg each




As the 1st chamber, take materiel in three points as shown on
the drawing here below







Dig a hole 30 cm deep removing the balls at the sampling point
and collect the sample with the collector near the



intermediate diaphragm (where there are the biggest balls in
case of classifying lining). In the area of the smallest



balls, use the shovel with the sieve and a bag below the sieve
to retain the material






Don't reject the unground particles from the sample!







Send all the samples to the laboratory





3

The Sieving in the laboratory







We recommand the following sieves: 50mm, 25mm, 10mm, 5mm,
2,5mm, 1mm, 0,5mm, 212µ, 90µ.




Of course, similar sieves can be used.




Some plants adopt smaller sieve like 63µ,
45µ or/and 32µ because cement quality reason




As an example, we will take the first sample of the 1st
chamber:



Weight of the sample: 3,105 kg (3105gr)



We will sieve manually for the biggest sieves (50mm, 25mm,
10mm and 5mm) and with the whole sample.



Why the whole sample? Because the sample is generally a
mixture of some big particles with fine particles. Then,



it is difficult to homogenize the sample and to reduce it
representatively.






Results:



1st sieving (with 50mm sieve): 326gr of material are retained
 It means that we have 326gr higher than 50mm



2nd sieving (with 25mm sieve): 236gr of material are retained
 It means that we have 236gr between 25mm and 50mm



3rd sieving (with 10mm sieve): 226,7gr of material are
retained  It means that we have 226,7gr between 10mm and 25mm



4th sieving (with 5mm sieve): 124,2gr of material are retained
 It means that we have 124,2gr between 5mm and 10mm



Summary of this first screening phase:




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2192,1gr of the sample are lower than 5mm



This part of the sample will be homogenized manually or with a
homogenizer and we only will take a small part for the



second step of the sieving. It depends of the laboratory and
the sieving device, but it is often between 100 and 500gr.



The sieving device will be a sieve shaker (vibrating) or
better, an air jet sieving for the possible agglomerated particles.






For our example, let's take 100gr of material.



Summary of the second step:






Now, we have to link both steps in order to have the complete
result of the sample 1 of chamber 1.



For that, we use the follwing equation:










With:



A = total residue cumulated in gr on a sieve



B = passing of the 1st step in gr



C = initial weight of the second step sample in gr



D = residue cumulated in gr on a sieve for the 2nd step in gr



E = residue cumulated of the last sieve of the 1st step in gr



Results on the table below:






We do the same procedure for all the samples taken inside the
mill.






The interpretation of the results is following on the next
page




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