Contents :
- Principle of Ball Mill
- Construction of Ball Mill
- Ball Mill Diagram
- Working of Ball Mill
- Critical Speed of Ball Mill
- Derivation of Critical Speed of Ball Mill
- Applications of Ball Mill
- Ball Milling Method
- High energy ball milling
- Wet Grinding
- Advantages and Disadvantages of Wet Grinding
Principle of Ball Mill :
Construction of Ball Mill :
• A ball mill is a rotating hollow cylinder. It can turn horizontally or at a slight angle. Inside, there are balls, usually made of steel, stainless steel, or rubber, which do the grinding.
• The inner surface of the cylindrical shell is usually lined with an abrasion-resistant material such as manganese steel or rubber. The length of the mill is approximately equal to its diameter.
• The balls occupy about 30 to 50 percent of the volume of the mill. The diameter of ball used is/lies in 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 (60-100 rpm) and in a large ball mill, the shell might be 3 m in diameter and 4.25 m 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 Fig. the outlet is normally covered with a coarse screen to prevent the escape of the balls.
Ball Mill Working :
• In case of continuously operated ball mill, the material to be ground is fed from the left through a 60o cone and the product is discharged through a 30o 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 (or drop down on to the feed), 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 ages and sizes since the balls continually wear by attrition and are replaced by new ones. 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 is done by the largest balls dropping from the largest distance and small particles are ground by small balls dropping from a much smaller distance. If the rate of feed is increased, a coarser product will be obtained and if the speed of rotation is increased (less than critical speed), the fineness for a given capacity increases.
• During grinding, balls themselves wear and are constantly replaced by new ones so that mill contains balls of various ages and thus of various sizes.
• In case of batch operated mill, a known quantity of material to be ground is charged into the mill through the opening in the shell. The opening is then closed and the mill is rotated for a predecided time. It is then stopped and the product is discharged.
Optimal Grinding Conditions:
Attaining the ideal grinding conditions in a ball mill is crucial for maximizing its efficiency. One key factor is the composition of the grinding medium, which is often overlooked. Selecting the right mix of ball sizes and materials can significantly enhance the grinding process. By varying the combination, you can achieve tailored results for particle size distribution and overall product quality. Experimenting with different media configurations allows operators to optimize the mill's performance for specific materials and desired outcomes. This approach empowers users to fine-tune their processes, unlocking the full potential of the ball mill's capabilities
Applications of Ball Mill:
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 (20-30% less than it for dry grinding), increased capacity, reduction in the formation of fines/dust, facilitates the removal of the product and no dust formation.
The disadvantages of wet grinding include necessity to dry the product and high wear on the grinding medium (about 20% higher as compared to dry grinding).
Factors influencing the size of the product :
(a) Feed rate : With a high feed rate, less size reduction is resulted 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 aball material of higher density. Optimum grinding conditions are obtained when the volume of the balls is equal to 50% 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 an effective grinding, the ball mill should be operated at a speed (optimum speed) equal to 50 to 75 percent of the critical speed.
(e) 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 increased, the cushioning action increases and power is wasted by the production of undersize material in an excessive quantity.
Advantages of the Ball Mill :
- Cost-Effective Setup: Low installation costs make ball mills an economical choice.
- Efficient Power Usage: Ball mills consume minimal power during operation, reducing expenses.
- Versatile Hardness Handling: Suitable for a wide range of material hardness levels.
- Batch and Continuous: Operates efficiently in both batch and continuous modes.
- Safe for Explosive Materials: Can grind explosive materials safely in inert atmospheres.
- Open and Closed Circuits: Adaptable to open and closed circuit setups.
- Affordable Grinding: Utilizes cost-effective grinding mediums.
Derivation of the critical speed of a ball mill
• The speed at which the outermost balls break contact with the wall depends on the balance between centrifugal force and gravitational force. This can be shown with the help of Fig. Consider the ball at point B on the periphery of the ball mill. Let R be the radius of the mill and r be the radius of the ball. R–r represents the distance between the centre of the ball and the axis of the mill. Let 'α' be the angle between OB and vertical through the point O. 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. Unless the speed crosses the critical value, a stage is reached where the above opposing forces are equal and the ball is ready to fall away from the wall. The angle at which the said phenomenon occurs is found out by equating the two opposing forces. Thus,
mg cos α = mv²/ (R – r) … (1)
cos α = v²/ (R – r) g ... (2)
The relationship between the peripheral speed and the speed of rotation is given by
v = 2π N (R – r) … (3)
Substituting the value of v from equation (3) into equation (1), we get
cos α = 4π²N²(R – r)/g ... (4)
At the critical speed : α = 0, and thus cos α = 1 and N becomes the critical speed Nc.
∴ cos α = 1 =4π²Nc²/(R – r)g … (5)
Nc² = g4π²/(R – r) … (6)
The operating speed/optimum speed of the ball mill is between 50 and 75 percent of the critical speed.
Also Read : Hammer Mill
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MCQs on Ball Mill
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FAQs on Ball Mill :
What is centrifuging in Ball Mill ?
If the mill is operated at very high speeds, the balls are carried right round in contact with the sides of the mill and the mill is said to be centrifuging.
What is Critical Speed of Ball Mill ?
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 the mill is centrifuging.
How do you find the critical speed of a ball mill ?
By using following relation you can find out the critical speed of ball mill. Nc = 1/2π.√g/R-r The operating speed/optimum speed of the ball mill is between 50 and 75 percent of the critical speed.