A ball mill is a type of grinder or mill used to mill the materials into fine powder. This ball mills are especially useful for grinding hard materials.
Stainless steel balls are mostly used as balls or grinding material in the ball mills. Steel has the ability to pulverize almost any material. This ball mills are generally used within a laboratory. 1,2
Devices for shaking materials along with hard balls might be old, but it was not until the industrial revolution and the invention of steam power that a machine could be built. It is reported to have been used for grinding flint for pottery in 1870.
This process was developed by Benjamin and his coworkers at the International Nickel Company in the late of 1960. It was found that this method termed mechanical alloying could successfully produce fine, uniform dispersions of oxide particles in nickel-base super alloys that could not be made by more conventional powder metallurgy methods. 4,5
Ball mill is one type of milling machine which is used to mill or grind the materials such as large sized particles either active drug ingredients or excipients and agglomerates or clump materials.
Balls used in this milling machine are made of stainless steel, rubber, porcelain metal etc.
Mostly stainless steel balls are widely used. 1,2
BASIC PARTS, CONSTRUCTION, PRINCIPLE AND WORKING OF BALL MILL
The basic parts involved in the ball mill are as follows:
The metallic cylinder which is coated with different materials is helpful in the mechanism of attrition.
The balls which are made of stainless steel, rubber etc which are used in the reduction of the materials by the mechanism of both impact and attrition.
A ball mill consists of a horizontally rotating hollow cylindrical vessel or shell with the length slightly greater than its diameter which is mounted on a metallic frame in such a way, that it can be rotated on its longitudinal axis. The axis of the shell or vessel may be either horizontal or at a small angle to the horizontal. The cylindrical vessel contains balls that occupy 30-50% of the mill volume. The grinding media is the balls, which may be made of steel (chrome steel), stainless steel or rubber which the weight of the balls is kept constant. The ball size depends on the feed and the diameter of the mill.
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 rubber lined mills. 1-4
It works on the principle of impact and attrition. Size reduction is done by impact and attrition as the balls drop from near the top of the shell.
Working principle of ball mill
The steps involved in the working process are as follows
- Initial stage
- Intermediate stage
- Final stage
- Completion stage.
The powder particles are get flattened by the collision of the balls. It leads it changes in the shapes of individual particles or cluster of particles being impacted repeatedly by the milling balls with high kinetic energy.
Significant changes occur in comparison with those in the initial stage.
Reduction in particle size takes place. The microstructure of the particle also appears to be more homogenous in microscopic scale than those at the initial and intermediate stages.
The powder particles possess an extremely deformed metastable structure.
The speed of the rotation is more important. At a low speed the mass of the ball will slide or roll over each other and only negligible amount of size reduction takes place. At a high speed the balls will be thrown out to the walls by centrifugal force and no grinding will occur. But at 2/3rd of the speed the centrifugal speed force just occurs with the result that the balls are carried almost to the top of the mill and then fall in.
By this way the maximum size reduction is effected by the impact of particles between the balls and by attrition between the balls. After the suitable time the material is taken out and passed through a sieve to get powder of the required size. 1-8
Low speed High speed Correct speed
Working of ball mill
There are four types of ball mills as follows
- Pebble ball mill
- Vibrating ball mill
- Drum ball mills
- Horizontal rotary ball mills
Pebble ball mill
Pebble mills are sometimes called as jar mill or pot mill which works on the principle of attrition and impact.
The grinding is effected by placing the substance in the cylindrical vessel or jar vessels that are lined by the procelainor other hard substance containing pebbles ir balls. The cylindrical vessel revolves horizontally on their long axis and the tumbling of the pebbles over one another and agnaist the sides of the cylinder produce pulverization with a minimum loss of material.
Vibrating ball mill
Vibrating ball mill also works on the principle of attrition and impact. It consist of mill shell containing a charge of balls similar to that of ball mills. In this case the shell vibrates due to some frequency rather than rotated. 1,2
- Ball mill is an efficient tool for grinding many brittle materials into fine powder.
- It is also used in milling of the sticky materials.
- Different materials are used as media, including ceramic balls, flint pebbles and stainless steel balls. An internal coating effect and balls used reduces the material to a fine powder. It grinds the particles size to 100 to 5 mm or less can be obtained.
- Grinding can be carried for wetting and drying powder at low speed in ball mills.1-4
- The hard and abrasive materials can also be grinded in the ball mills for pharmaceutical purpose.
- stainless steel balls produces the ophthalamic and parenteral products as there have less contamination due to wear.
- ball mill is used for the milling of pigments , insecticides in industrial purpose.
- Ball mills are also used in manufacture of black powder.
- Ball mills are used extensively in the mechanical alloying process.
- The ball mill is a key equipment to grind the crushed materials and cement, silicate, new type building material, refractory material, fertilizer, ore dressing of ferrous metal and non-ferrous metal, glass ceramics, etc.
- In mineral, cement, refractory, chemical industry the ball mill is mainly used to grind materials etc. When the wet ball mill works water and other liquid will be added into materials to increase flowability of materials to increase the capacity. When dry ball Mill works capacity is affected because material flowability is reduced.
- The ball mill is used for grinding materials such as coal, pigments, and felspar for pottery.
- Ball Mill is widely used in metal and nonmetal mines, building materials and other industrial sector.
- Also used in milling of dyes.
- It can produce very fine particles.
- It can be used for continuous operation, if sieve or classifier is attached to the mill.
- It is capable of grinding a large variety of materials of different characters and different degree of hardness.
- It is suitable for wet or dry grinding processes.
- It can be used to grind the toxic materials as it can be used in a completely enclosed form.
- The cost of installation, power and grinding medium is low.
- It is suitable for both batch and continuous operation.
- The ball mill is a very noisy machine.
- Wear occurs from the balls as well as from the casing which may result in contamination of the product.
- Low working efficiency.
- Large total weight is of hundreds of tons, so one must be very great investment.
- Too much large sized particles requires greater investment in the reduction process.
- High production cost.
- Some raw materials become damaged by steel balls.
- Difficult in time creating powder from sensitive and flammable substances.
- Lieberman, Herbert A, Lachman, Leon (2009), Industrial pharmacy: Pharmaceutical dosage form-milling. Banglore, Pune. ISBN: 978-81-239-1679-8.
- Remington, Alfonso and Gennoro, Science And Practise Of Pharmacy- powders. 20th edidtion.
- Mehta RM, Pharmaceutics-I: Size reduction (1996), Page no: 93-94.