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Grinding media


















1

Introduction:











The
grinding media is a very important factor of the mills internals, not only on
a mill's efficiency point of view but also on




a wear point of view.











Generally, the ball
charge consists of grinding balls of several sizes and of different material
qualities.







Generally, balls between 125 and 30 mm are used.











The composition of the
ball charge depends on various factors, such as:








* Mill diameter










* Mill length










* Ratio L/D







* Mill speed






* Types of linings






* Mill circuit






* Grindability of the fresh feed






* Granulometry of the fresh feed






* Hardness of the fresh feed






* Product fineness







Concerning the ball quality, it depends of:






* Impact forces






* Friction forces between the balls and the liners










* Abrasiviness of materials










* Corrosion especially in wet process








2

Ball charges quality:











Three types of balls exist on the market:










* Forged balls










* Cast Iron balls










* High Chromium Cast Iron balls











A small table with the
advantages and disadvantages of these balls:




















See suppliers websites for composition

















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3

Wear rate:











The
wear of grinding media in a mill is a contentious subject and many theories
exist regarding the laws governing this process.





The wear of the balls is due to 4 reasons:










* Impacts










* Friction forces










* Abrasivity of the feed components










* Corrosion











Depending of these
reasons and the grinding medis quality, wear rates can be between 100 and
2000 gr/t.




4

Wear rate calculation:











Several theories have
made an important contribution to an understanding of the wear process.







Among the models and theories put forward are:










* The Bond Wear Model










* The Volume theory










* The Linear Wear theory





















For Dry ball mills (with grate
discharge):






























For wet ball milling:




















Where Ai is the abrasion
index of material determined from abrasion test.








Some figures of abrasion index herebelow:





















The volume theory:










According to this theory,
the rate at which the weight of any ball decreases is proportional to the
mass (volume) of a ball.





This can be presented mathematically as:





















The Linear Wear Theory:










The
most widely accepted approach to characterize the wear of grinding media in
ball mills is known as the Linear Wear Theory.




Its rate of lost weight
is directly proportional to its surface area exposed to impacts, friction
forces, abrasion and





corrosion phenomenon.










According
to the surface theory, the size frequency distribution is flat: the same
number of balls is found in every interval




when the size range is divided into equal widths.










It is also called charge in equilibrium.










This can be expressed by the following formula:




















Where:










δ = ball
consumption in kg/hr










m = ball weight in kg after t hours in the mill










Cm = surface
area of the ball in m2










Sb = constant
in kg/hr/m2










The surface at the
equilibrium can be calculated with the following expression:


















Where:










S = surface area of the ball charge in m2










Wb = total
weight of ball charge in tons










Δb = specific gravity of the ball in kg/dm3










db = diameter
of the ball in mm










ds = diameter of the
minimum ball permitted in mm (or must be removed or scrap)








Measurement of the filling degree (volume load):










Let's take the example of
a new ball charge inside the mill.









After x hours, we want to
know the wear rate of the grinding media.









We will measure the filling degree inside the mill.










It is better to empty the
mill before to realize this operation.










Wear rate in gr/t calculation:










Knowing the original
filling degree and the one after x hours, we calculate the number of tons
consumed.







In
order to know the wear rate, we have to divide the number of tons consumed by
the production during the x hours period.





Example:



























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Other wear rate unities with an example:










* gr/kwh:




















* mm/100h:



















* mm/1000tons produced:


















Hours required for the
smallest ball to reach the minimum ball diameter permitted:















5

Cylpebs:











Cylpebs are normally
cylindrical grinding media with a ratio length to diameter of 1.








Cylpebs from 45x45 up to
24x24 are normally used in the mining grinding rotary mills.








A picture of cylpebs:





















Cylpebs have the
following advantages in comparison to balls:









* A higher bulk density:
4,9 kg/dm3 against 4,54,65 for the balls.









* A higher surface area for the same weight:










A ball of 14mm diameter
has a weight of 11gr and a surface area of 616mm2








A cylpebs of 12x12 has a
weight of 11gr and a surface area of 679mm2 (+10%)








* A contact with an other
cylpebs can be a line against only 1 point for the ball.









For these reasons,
cylpebs are theorically better than balls for grinding process.








A great number of
industrial trials or laboratory tests have been carried out in the past and
again today but it never been





proved that cylpebs are really better than balls.











Other cylpebs type: the boulpebs.










Boulpebs are like cylpebs
but wit rounded cylinder bases.


























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