Explain the working, advantages and disadvantages of any one mill based on the principle of combined impact and attrition?
2 Answers
Answer :
Fluid Energy mill OR Ball mill works on the principle of combined impact and attrition.
Explain any one
FLUID ENERGY MILL
Working
1. The material which is to be size reduced is fed in the grinding chamber from the bottom through the feed inlet.
2. The air or inert gas is introduced with a very high pressure through nozzles.
3 .Due to high degree of turbulence, impact and attritional forces between the particles there is size reduction.
4. The air moves at a very high speed in elliptical part carrying with it fine particles that pass through the outlet in a classifier and are collected.
5. The large particles are carried by centrifugal force to the end whereby they are further exposed to the moving air.
6. The design of the mill provides for the internal classification of the particles whereby lighter, finer particles are discharged and heavier particles are retained due to effect of centrifugal force to be reduced to smaller size.
7 .Feed should be of 20 to 200 # size &mill produces particles of 1 to30 micron range to get a very fine powder even upto 5µ,the material is pre-treated to reduce the particle size to the order of 100# and then passed through fluid energy mill.
Advantages
1) No contamination of the product.
2) An arrangement is made for classification in mill.
3) Suitable for heat sensitive material such as vitamins & antibiotics.
4) It is used to grind the material to fine powder.
5) Up to 6000 kg of feed can be milled per hour.
Disadvantages (0.5x2=1mark)
1) Not suitable for soft, tacky and fibrous materials.
2) The equipment is expensive, because it needs additional accessories.
3) Pre-milling of material is necessary
BALL MILL
Working:(1 marks)
1) Cascading at a low speed the balls tumble, roll and jump down on the material. Negligible amount of size reduction will occur in this case.
2) Cataracting at an increased speed,the ball reaches the top of the mill and falls on the material.No size reduction will occur in this case.
3) Centrifuging about 2/3rd of the speed, the centrifugal force occurs with the result that the balls are carried just to the top of the mill and then fall in, by this way size reduction occurs at maximum rate by impact of material between the balls and by attrition between the balls and the surface.
After the required time the material is taken out & is passed through the sieve to get powder of required size.
Ball mill works on the principle of impact and attrition. There are three types of patterns as shown in figure:-
Advantages (0.5x2=1mark)
• It can produce very fine particle.
• It can be used continuous operation, if sieve or classifier is attached.
• It is capable of grinding a large variety of material of different character and of different degree of hardness.
• Suitable for wet and dry grinding.
• Used to grind toxic material.
Disadvantages: (0.5x2=1mark)
1) It is a very noisy machine
2) Not suitable for soft and sticky materials.
3) Contamination of product may occur due to particles generated due to wear from balls as well as casing.