BALL MILL | FINE CRUSHING | COARSE GRINDING


Grinding 

The fine crushing or grinding means size less than 6mm and up to 200 mesh. The usual meaning of grinding is comminution of an ore particles that has already been reduced in size by crushing to less then 6mm size, which is know as grinding.

BALL MILL

Ball mill can be classified according to the
1. Shape of the mill
2. Methods if discharge of the ground ore.
3. Weather the grinding is conducted dry or wet.

According to the shape of the mill is two types
1. Cylindrical conical mills - Harding mill

2. Cylindrical mill - This mills are being 3 types by the mode of discharge of the ground product.
* Through a screen along the cylindrical shell
* By free overflow from the axis of the mill
* Through a great extending as a diaphragm across the full section of the mill.

Ball load :

The ball load is defined as the volume of the ball mill that is occupied by the grinding media without ore or water in it.

● The ball load should be more then 30% of the total volume. In general ball are between 30 - 50% of the volume of the mill.

● When mill operate for the first time ball of various sizes are charge in to the shell. During grinding the balls themselves wear and are constantly replaced by new balls of largest size.

Ball mill Construction :

It has three important section,
Ball milk | fine crushing | grinding
Ball mill

1. Cylindrical shell :

It is the rotating hollow cylindrical shell, partially filled with the balls. The ore to be crushed is fed through the trunnion at one end and the product is discharge through a similar trunnion at the other end. It shell is made up of high strength steel which axis is either horizontal or small angle to the base. It between 4 - 4.25 m and dia 3mtrs.

2. Inner surface or liners

Due to grinding impact and attrition the interior of the balls mill is lined with replaceable wear resisting liners. The liners are usually manganese alloy steels, stone or bubbles. Least wear takes place on the rubber lined interior.

3. Balls or grinding media

The ball are usually cast steel unless otherwise stated in some cases Flint ball may be used. The diameter of the grinding media varies from 1 - 5 inch's. The optimum size of ball is proportional to the square root of the feed size. The ball are in range of 450gms to 1250gms per tons of ore crushed.

Theory of Ball Mill Operation :

It depends on the batch type or continuous

In this mill the grinding media and the ore to be ground are rotated around the axis of the mill. Due to the friction between lining balls, lining ore lumps both are carried up along the inner wall of the shell nearly to the top by which grinding media falls on the particles with heavy impact, which is utilized in reducing the size of the lumps as the rotation of the mill continued. This process attribute the following forces during working.
1. Cataracting (impact of the ball on the
Ball mill grinding operation
Ball load 
particles)
2. Cascading (attrition between the ball and particles)
3. Inter particle collision and rubbing
4. Wear forces at the lining of the mill

It depends on the rotation of speed of the mill

If the mill operates at a low speed balls will a carried up along the inner wall to a certain height, but not large enough to give an impact force. So they roll over each other or slip over. This type of operational condition is known as cascading of the mill.
● If the speed is raised, the balls start moving up further along the inner wall and suddenly fall form a greater height imparting an impact force at the bottom of the mill, which is known as cataracting.
● If the speed of rotation becomes too high the balls are carried out over and again all along the inner lining as if they are sticking to the inner wall and hardly any grinding is called centrifuging of the mill.
● If the speed is too low cataracting not occurs, ball or particle roll down slowly and leading to cascading of the mill and only limited grinding is possible.
● At other in extreme at very high speed the mill centrifuge where littel or no grinding is done.
Theory of ball mill operation
Theory of ball mill

Characteristics of Ball Mill :

It depends on the
1. Speed of the mill
2. Ball load
3. Reduction ratio
4. Capacity
5. Energy consumption

Advantage of Ball Mill :

1. The cost of installation is low.
2. The ball mill can use inert atmosphere and therefore can be used for grinding explosive materials.
3. Grinding medium is cheap.
4. The mill is suitable for grinding materials with any degree of hardness.
5. It can be operated in batch or continuously.
6. It can be used for open or close circuit.

Critical Speed of the Mill :

The minimum speed at which centrifuging occurs in ball mill known as the critical speed of the ball mill. So the operating speed must be less then critical speed or the minimum speed at which the ball are carried along the inner wall of the shell is also known as critical speed.
Calculation of the Critical Speed or Load :
1. Let the radius of the cylindrical ball mill 'R'
2. Grinding media radius be 'r'
When cylinder is rotating the ball is acted upon by centrifugal force and gravitational force simultaneously. The speed at which the outer most ball loses contact with the inner wall of the mill depend upon the balance between gravitational and centrifugal forces.

Fc = Centrifugal force
Fg = Gravitational force (mg)
'g' is the acceleration due to gravity
Fc = MV×MV / (R - r )
V2 = us the linear speed of the media.
Expressing linear speed interms of rotation speed.
V = 2π(R - r)N, where N is the rotational speed.
      And (R - r) is the radius of rotation.

V×V = 4π×π . N×N(R - r)
Fc = m.4π.π . N×N(R -r)

The centrifugal force components of gravity force.
Mg = MgCos¢ = M[4π.π.N.N(R-r)]
Or Cos¢ = 4π×π . N×N(R - r)(R - r) / g
Applying the condition of critical speed ¢ = 0, Cos ¢ = 1 and N become critical speed = Nc

[ at X position gravitational force = Centrifugal force]
I.e.      Mg = m.4π.π. Nc.Nc(R - r)
     Or Nc.Nc = g / 4π.π(R - r)           R >> r

● Ball mill is rotated at 65 - 80% of the theoretically using value of 2π, g = (980cm/sec2) and R and r in cms.

Approximately the value Nc = 423/√D
D = dia of mill
(D = 2R)      R>>r or Nc = 54.2 / √R - r if R and r are expressed in feet.

Open Circuit and Closed Circuit Grinding

● Grinding is only comminution and has nothing to do the product practice size in many mills the feed is broken into particles of the satisfactory size by passing it once through the mill.

● When no attempt is made to return the over sized particular to the machine for further size reduction the mill is said to be operating in open circuit.

● On the other hand if the partially crushed material is screened and the oversized is returned back to be recrushed and the underside product is given as the feed to the next machine for further size reduction search this continued operation to achieved the desire product particle size is termed as closed circuit grinding.

● This method of grinding operation is generally obtained as it has been found to be
1. economical
2. utilise the capacity of each equipment efficiency
3. avoid unnecessary regrinding
Flow diagram of open circuit and close circuit grinding
Flow diagram


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