The document discusses the principles and operation of a ball mill. It describes how a ball mill works by rotating a hollow cylindrical shell partially filled with balls to grind materials by impact and attrition. The grinding medium is the balls, which can be made of steel, stainless steel, or rubber. The document outlines factors that affect grinding efficiency such as feed rate, ball size and weight, and rotation speed relative to critical speed.

1. Contents
 Principle
 Construction
 Working
 Applications
 Advantages
 Equations governing the operating speed

2.  Ball mill consist of a hollow cylindrical shell rotating about its
axis.
 Axis of the shell horizontal or at small angle to the
horizontal
 It is partially filled with balls made up of Steel,Stainless steel
or rubber
 Inner surface of the shell is lined with abrasion resistant
materials such as Manganese,Steel or rubber
 Length of the mill is approximately equal to its diameter
 Balls occupy about 30-50% of the volume.dia of the ball 12
mm-125 mm
 Shell is rotated through a drive gear (60-100 rpm) and large
mills, shell might be in 3m in dia and 4.25 m in length.
 Operation may be batch or continuous,wet or dry…in a
continuosly operated ball mill outlet is normally covered with
coarse screen to prevent the escape of the balls..

5.  Material to be ground is fed from the left through a 60
cone and product is discharged through a 30 cone to
the right.
 As the shell rotates the balls are lifted up on the rising
side of the shell and they cascade down from near the
top of the shell
 The solid particles in between balls are ground and
reduced in size by impact.
 As the shell rotates the large balls segregate near the
feed end and small balls segregate near the product end
 If the rate of feed is increased, coarser product will be
obtained and if the speed of rotation is increased the
fineness for a given capacity increases.

6.  During grinding, balls themselves wear and are
continuosly replaced by new ones so that mill contain
balls of various ages and thus of various ages and
thus of various sizes
 Ball mill produces 1 to 50 tonn/hr of powder of which
70 to 90 % will pass through a 200 mesh screen and
energy requirment of the ball mill is about 16
kwh/tonn
 In case of a batch operated mill a measured quantity
of a solid to be ground is charged to the mill through
the opening in the shell. The opening is closed and
the mill is rotated for several hours. It is then stopped
and the product is discharged

7.  The ball mill is used for grinding materials
such as coal,pigments,and fedspar for
pottery.
Grinding can be carried out in either wet or dry
but the former is carried out at low speeds.
The advantages of wet grinding are less power
consumption,increased capacity,no dust
formation etc…
Dis advantages are high wear on the grinding
medium and neccesity to dry the product

8.  The cost of installation is low
 The cost of Production is low
 It is suitable for materials of all degree of
hardness
 The grinding Meduim is cheap
 Used for the grinding of explosive materials
 Suitable for both batch &continuous
operations

9.  Feed rate:With a high feed rate,less size reduction is
effected since in this case the material is in the mill for
a shorter time.
 Properties of the feed material:With a hard material,a
smaller size reduction is achieved
 Level of the material in the mill:A low level of material in
the mill results into a reduction in the power
consumption.if the level of material is raised power is
wasted by the production of under size material

10. • Weight of the balls: With a heavy discharge of
balls,we get a fine product.We can increase the
weight of the charge by increasing the number of
balls or by using a ball material of high density
• Speed & rotation of Ball mill: low
speeds,the balls simply roll over one
another and little grinding is obtained
while at very high speeds the balls are
carried along the walls of the shell and the
mill is said to be centrifuging.The
minimum speed at which centrifuging
occurs is called critical speed.In practise
operating speed must be less than the
critical speed.

11. -Critical speed
R –radii of the mill
r- radii of the ball
g –acceleration due to gravity

12.  Critical speed of ball mill(in rps)=
 R=.45/2=.225m
 r=25/2=12.5m
 g=9.81m/s2
 Nc=1.08rps=64.8 rpm

13.  r=60/2=30mm
 R=800/2=400mm
 Nc=.82 rpm
 Critical speed=1.4*operating speed
 Operating speed =.82/1.4=.586 rps=35 rpm