Prof.Dr.Salah Elmofty

Ph.D.professorMaterial Technology

Department of Mining Petroleum &Metallurgical,Faculty Of Engineering

E-mail : s_elmofty@hotmail.com

The main purposes of fine grinding during dressing of minerals:

1-Liberation of valuable minerals from waste minerals of the ore.

2-Grinding to size meeting specific process requirements (for flotation).

3-Grinding to meet commercial product requirements.

Principal draw backs of the grinding mills:

1-Fine grinding is a relatively expensive operation.

2-Fine grinding machines are inherently inefficient.

3-The production of excessive quantity of material is a waste both of

energy &of ore values

Theory of mill operation

.

π

π 2

MvMv

r= Mg cos N

22

N

Mg

M= Mass of ball

V= linear velocity

r= radius of the mill

n= number of revolutions per min.

v= 2 π rn

COS N = 42

gn r2

mv

r

2

Grinding action

-The grinding in ball mill is therefore caused due to:

1-Cascading action of grinding media.

2-By impact due to fall of grinding media by gravity.

3-Crushing by heavier and harder lamp.

4-Kinetic energy of falling and rolling grinding media.

-The grinding in ball mill based on:

1-Nature of the ore. 2-Feed size.

3-Pulp density. 4-Balls size

5-Liners design. 6-State of dispersion.

Critical speed of the mill

-The faster the mill is rotated,the higher the balls will be lifted.

-The minimum speed at which a mill charge will centrifuge is known as the critical speed of the mill and is given by formula:

√N =

54.19

R

N = Critical speed.

R=Radius of the mill.

Power consumption

-The energy that mill consumes is a function notonly of the speed but also of the ball load,the specific gravity of the ore and the dilution of the pulp.

The advantages of ball mill

(i ) Low power consumption for a givenreduction.

(ii ) Simple operation .

(iii ) , If over loaded it can be brought backto normal in a very short time owing to its free discharge and rapid forward propelling action.

Liners

-Various materials such as cast-iron , carbon and alloy steels are used for liners , manganese steel liners are most common in ore milling plants .

-Recently soft rubber is gaining important in the

line

Mill shell liners

Tube mill Vs ball mill

1 -Tube mills are cylindrical mills of relatively great length .

2 -Ball mills in general include all mills in which steel balls are used as grinding media.

3 -Tube mill 5’ 6” in diameter and 22’ long are standard.

Rod mill

-Rod mill is similar to ball mill in appearance and general principals .

-Rod mill differ from ball mill in its grinding action , in that the rods are kept a part by the coarsest particles.

-Consumption of rod and liners of energy is almost the same as in ball mill employed for a similar duty using steel balls .

Autogenous mill

-Grinding takes place only between ore and ore.

-Autogenous mills are tube mills in which bigger ore lump ( 4” to 6” size ) are used instead of flint

pebbles ..

Closed circuit and open circuit (a ) In closed circuit operations there is closer

approach to a uniform ratio of ball size to average particle size throughout the

length .(b ) Over grinding can be avoided in closed

circuit as there is continuous reduction in proportional of finished size in the mill.

(c ) Power consumption is lower in open circuit than closed circuit .

(d ) Time of grinding is lower in open circuit than closed circuit .

(e ) There is higher tonnage in open circuit than closed circuit .

Moussa El Mewafy

Hany Salah Ahmed

Emad El Dein

Ahmed lotfy

Assem Abd El naby

Thank You