Ball Mill Working Principle, Construction, Applications, Advantages and Disadvantage

A ball mill is an engineering device used to grind metal, rock, and other materials into fine powder. It consists of a horizontal axle, a rotating shaft, and a vertical sifter screen. The horizontal axle is connected to a power source and holds the body of the mill. 

A ball mill uses balls to crush rocks into dust. The body of the ball mill is made of steel or cast iron and lined with cloth or paper for absorption. Holes are drilled in the center of each end of the ball and secured with screws. 

Principle

It works on the principle of impact, i.e. size reduction is done by impact as the balls drop from near the top of the shell.

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Construction

A ball mill consists of a hollow cylindrical shell rotating about its axis. The axis of the shell may be either horizontal or at a small angle to the horizontal. It is partially filled with balls. The grinding media is the balls which may be made of steel, stainless steel, or rubber. The inner surface of the cylindrical shell is usually lined with an abrasion-resistant material such as manganese steel or rubber. Less wear takes place in the rubber-lined mills. The length of the ball is approximately equal to its diameter.

The balls occupy about 30 - 50% of the volume of the mill. The diameter of the ball used is/lies between 12 mm and 125 mm. The optimum diameter is approximately proportional to the square root of the size of the feed. The shell is rotated at low speed through a drive gear (60 to 100 rpm) and in a large ball mill, the shell might be 3 meters in diameter and 4.25 meters in length.

The ball mill may be operated in a batch or continuous fashion, wet or dry. In a continuously operated mill, as shown in the diagram, the outlet is normally covered with a coarse screen to prevent the escape of the balls.

Ball Mill Working Principle

In the case of a continuously operated ball mill, the material to be ground is fed from the left through a 60° cone, and the 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 then they cascade down from near the top of the shell. In doing so, the solid particles in between the balls are ground and reduced in size by impact.

The mill contains balls of various sizes. As the shell rotates, the large balls segregate near the feed end and small balls segregate near the product end/discharge. The initial breaking of the feed particles, therefore, is done by the largest balls dropping the largest distance and small particles are ground by small balls dropping for a much smaller distance. If the rate of feed is increased, a coarser product will be obtained, and if the speed of rotation has increased the fineness for a given capacity increases.

During grinding, balls themselves wear and are constantly replaced by new ones so that the mill contains balls of various ages and thus of various sizes. Ball mills produce 1 to 50 ton/h power of which about 70 to 90 percent would pass a 200 mesh screen and the energy requirement of a ball mill is about 16 kWh/t.

In the case of the batch-operated mill, a measured quality of a solid to be ground is charged into the mill through an opening in the shell. The opening is then closed and the mill is rotated for several hours. It is then stopped and the product is discharged.

Ball Mill Applications 

The ball mill is used for grinding materials such as coal, pigments, and felspar for pottery.

Grinding can be carried out either wet or dry but the former is carried at low speeds.

The advantages of wet grinding include lower power consumption (20 to 30% less than it dry grinding) increased capacity, reduction in the formation of fines, facilitating the removal of the product, and no dust formation. 

The disadvantage of wet grinding includes the necessity to dry the product and high wear on the grinding medium (about 20 percent higher as compared to dry grinding). 

The factors influencing the size of the product in a ball mill 

(a) 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.

(b) Properties of the feed material: With a hard material, a smaller size reduction is achieved.

(c) Weight of balls: With a heavy charge 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 higher density. Optimum grinding conditions are obtained when the volume of the balls is equal to 50% of that of the mill. So the variation in the weight of balls is done by using materials of different densities.

(d) Speed of rotation of the mill: At low speeds, the balls simply roll over one another, and little grinding is obtained while at very high speeds, the balls are simply carried along the walls of the shell, and little or no grinding takes place. So for effective grinding, the ball mill should be operated at a speed equal to 50-70% of the critical speed.

(e) Level of the material in the mill: A low level of material in the mill results in a reduction in power consumption. If the level of material is raised, the crushing action increases, and power is wasted by the production of undersized material in an excessive quantity. 

Advantages of the Ball Mill 

(i) The cost of installation is low.

(ii) The cost of power required is low.

(iii) It is suitable for materials of all degrees of hardness.

(iv) It is suitable for batch as well as continuous operation.

(v) It can be used for grinding certain explosive materials since it can be used in an inert atmosphere.

(vii) The grinding medium is cheap.

Disadvantages of Ball Mill 

(i) A ball mill is a slow and costly way to produce certain materials.

(ii) Because of the high cost, a small amount of material may represent a large price per pound.

(iii) Ball mills are only useful for certain types of materials.

Critical and Optimum Speed of Ball Mill

The balls are projected across the mill depending upon the speed of rotation. At low speeds of operation, the balls simply roll over each other resulting in little crushing action. If the mill is operated at slightly higher speeds, the balls will be carried up further inside the mill, and greater will be the power consumption. But at the same time, as the balls fall down from higher distances, the greater will be the impact at the bottom, and the large will be the capacity of the mill.

If the mill is operated at very high speeds, the balls are carried right around in contact with the side of the mill, and the mill is said to be centrifugal.

The minimum speed at which centrifuging occurs is called the critical speed of the mill, and under these conditions, centrifugal force will be exactly balanced by the weight of the ball. Little or no grinding takes place when a mill is centrifuging. If the mill is to operate practically, the operating speed must be less than the critical speed.

The speed at which the outermost balls break contact with the wall depends on a balance between the centrifugal and gravitational force. This can be shown with the help of images. Consider the ball at point B on the periphery of the mill. Let R and r be the radii of the mill and ball respectively. R-r represents the distance between the center of the ball and the axis of the mill. Let α be the angle between OB and the vertical.

The forces acting on the ball are:

1. The force of gravity, mg where m is the mass of the ball and 

2. The centrifugal force, mv²/(R - r), where v is the peripheral speed.

The component of gravity opposing the centrifugal force (centripetal component) is (mg) cos α. As long as the centrifugal force exceeds the centripetal component of the force of gravity, the particle will not lose contact with the wall. As the angle α decreases, the centripetal force increases, and unless the speed crosses the critical value, a stage is reached where the opposing forces are equal and the ball is ready to fall away. The angle at which the said phenomenon occurs is found by equating the opposing forces:

mg cos α = mv/(R - r) -------01

cos α = v²/(R - r)g ---------02

A relation between the peripheral speed and the speed of rotation is given by the equation

v = 2π N (R - r) ---------03

Putting the value of v from equation (03) into equation (01), we get 

cos α = 4π² N² (R - r)/g -------04 

At the critical speed: α = 0, δ cos α = 1, and N becomes the critical speed Nc.

cos α = 1 = 4π² N²c (R - r)/g 

N²c = g/4π² (R - r) 

Critical speed formula of ball mill

Nc = 1/2π √g/R - r 

The operating speed/optimum speed of the ball mill is between 50 and 75% of the critical speed.

Also Read: Hammer Mill Construction and Working Principal

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane