Apparatus required: - Laboratory model Ball Mill.
Objective: To carry out the fine grinding of ore by using ball mill, that means the product size less than 6mm and up to 200 mesh (74µm).
Theory: Ball mills may be continuous or batch type. 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 angle, the inner wall of the shell nearly to the top, from where the grinding media is thus utilized in reducing the size of the particles as the reduction of the mill is continued. In fact the grinding process in the ball mill is attributed to following force working simultaneously on the particles.
The working forces are:
a. Cascading (attraction between the particles)
b. Cataracting (Impact of the ball on the particle)
c. Inter particle collision and rubbing.
d. Wear forces at the lining of the mill
Effective grinding depends on the rotational speed of the mill. If the mill operate at a low speed ball will be carried up along the inner wall to certain height, but not large enough to give an impact force. Rather, they roll over each other or slip over. This type of operational condition is known as “cascading”.
If the speed is raised, the balls start moving up further along the inner wall and suddenly fall from a greater height imparting an impact force at the bottom of the mill. The impact is largely responsible for most of the grinding. This condition is known as “Cataracting”.
If the speed of the rotation becomes too high, the balls are carried over and over again all along the inner lining as if they are sticking to the inner wall and there is hardly any grinding. This condition is known as centrifuging of the mil
Constructional Features of Ball Mill:
Ball mill can be classified according to the
1. Shape of the mill
2. Method of discharge of the ground ore
3. Weather the grinding is conducted dry or wet.
1. Shape of the mills:
a. Cylinder- conical mills- Harding mill (Where feed & discharge ends is fixed.)
b. Cylindrical mill – The usual ball mills.
Mechanical Construction: A ball mill has three important sections as
a. Cylindrical Shell
b. Inner surface or liners
c. Balls or grinding media.
It is the rotating hollow cylinder partially filled with the balls. The ore to be crushed is feed through the turnnion at one end and the product is discharge through a similar turnnion at the other end. The material for the hollow shell is usually high strength steel. The steel axis is either horizontal or at smaller angle to the base. Large ball mills have a length of 4-4.25m, diameter of 3mts using balls of 25-125mm size.
Inner surface or liners: As the grinding process involve impact and attraction the interior of the ball mails is lined with replaceable wear resisting liners. The liners are usually manganese alloy steels, stones or rubber. Least wear takes place on rubber lined interior. As the coefficient of friction between balls and steel liner is large, the balls are taken to a higher height along the inner wall of the steel and dropped down on the ore to be crushed with a larger impact force resulting in better grinding.
Balls or grinding media: The balls are usually cast steel unless otherwise stated in some cases flint balls may be used. The diameter of the grinding media varies from 1”-5”. The optimum size of the ball is proportional to the square root of the feed size. The ball wear is usually in the range of 450gms to 1250gms per tons of ore crushed and the liner wear range form 0.50-205 gms per ton of ground ore.
Working Principle Ball Mill:
i. Ball mill is a horizontal cylinder, in and out of the hollow shaft and grind the first parts. According to the particle size of grinding material selection, material by hollow shaft ball mill feed end into the cylinder body. When the ball mill cylinder rotation, grinding medium because of inertia and centrifugal force effect, the effect of friction, making it nearby cylinder on the cylinderliner.When was brought to a certain height, thrown by its own gravity, the whereabouts of the grinding medium like projectile of cylinder body of the material to break.
ii. Material composed of feeding device through the hollow shaft spiral evenly into the mill first. The warehouse with ladder plate or corrugated plate, built in all kinds of steel ball, cylinder rotate falling after the ball to a certain height, the centrifugal force is generated on the material and grinding effect. Material after the first storehouse to coarse grinding, the single plate into the second warehouse every storehouse. Within the warehouse with a flat plate with steel ball, will further grinding material. Powder through the unloading Bi board, finish grinding homework.
iii. Cylinder in the process of turning, grinding medium has slipped phenomenon. In the process of slide to the material to grinding effect, in order to effectively use the grinding effect, the teaching material particle size and a large, generally 20 grinding fine time, the mill cylinder body with every storehouse board separated into two parts, namely become two bin. The material is steel ball break into the first storehouse. Material into the second warehouse, steel section for material grinding, fine grinding qualified material from the discharge end of the hollow shaft discharge, material of feed particles small mill, such as sand ii slag, coarse fly ash. Mill barrel cannot set partition, into a single warehouse tube mill. Grinding medium can also use the steel section.
Particle size before crushing
weight after 5min
weight after 10 min
Weight after 15min
Reduction ratio after 15 min
Reduction Ratio: The reduction ratio that can be obtained by use of ball mill is large compared to reduction ratios obtainable with primary or secondary crusher. Instead of 5-8 it may range from 50-100 for a ball mill classifier circuit. If reduction ratio is high along with large capacity it is more economical to arrange ball mills in series. The first is a coarse grinder having reduction ratio of 20 and the last one is fine grinder having reduction ratio of 5.
Capacity: The capacity of the ball mill depends upon its size, hardness of the ore, reduction ratio attempted. Ball mills yield 1-50 ton/hr of powder with 90% passing through 200# (mesh) screen.