Principle, Construction, Working, Uses, Merits and Demerits of Hammer Mill, Ball Mill and Fluid Energy Mill

Hammer Mill

Principles

Hammer mills operate on a straightforward principle. To crush a material, one needs a suitable engine, crushing hammers, and knives. A fast-moving rotor's hammer impacts a stationary powder bed, which is created by impact. Hammer mills crush materials between hammers and a casing, then sieve them until they are fine enough to go through a sieve at the bottom. The hammer mill can handle both fibrous and brittle materials, but fibrous materials may require projecting parts on the casing to allow for cutting motion.

Construction

In Figure, hammer mill construction is shown. There are two types of hammer mill: horizontal and vertical shaft. Abrasion-resistant materials are used for impact surfaces, such as haustellate or carboy, which are then coated with steel or stainless steel to make them abrasion-resistant. Pharmaceuticals can be handled with stainless steel hammers.

The shape of a hammer can vary. The stirrup and the bar are the two most basic shapes. Tablet granulation involves hammers with bar shapes. It is possible to have flat or sharp edges on each side of the hammer blades. Swing or rigid hammers are available. Having a free-swinging screen means the clearance between hammers and screens will increase if excessive buildup takes place in the mill. An enclosed chamber surrounds a mesh screen or grid that allows the material to pass through. It does not have woven screens. A metal sheet with holes or slots varying in thickness is used as a screen.

Working

Particles enter the crushing chamber through the feeding mechanism. Hammer mills may use gravity or a metering system for feeding, depending on the hammer design. In applications where product uniformity is important, metering systems are recommended since they eliminate all variables that may affect the quality of the output product. Here, a pneumatic rotary valve found between the crushing chamber and the feeding hopper serves as a good example. The feeding of particles into crushing chambers occurs entirely because of gravity.



Turning on and off the machine is controlled by the control box. The motor and feeding system can also be controlled. Several pharmaceutical milling machines are equipped with a display panel so users can monitor all processes. The mill operates at high speeds, ranging between 2,500 and 60,000 revolutions per minute. Hammers typically rotate anticlockwise or clockwise on horizontal shafts. The rotation of the rotor could affect this.

Uses

• Processes wet or dry granules and disperses powders in pharmaceutical industries.
• Herbal medicines, sugar, and pharmaceutical raw materials can be ground with the machine.
• Medicinal plants' barks, leaves, and roots can be powdered using this chemical.
• Active Pharmaceutical Ingredients (API), excipients, etc., are milled using this process.

Merits

• This method can produce specified top sizes without the use of a closed-circuit crushing system.
• The self-classification of the algorithm causes a reasonable number of size distributions with the least amount of fines.
• There is little space taken up by it
• Impact from blunt hammers is the best way to fracture hard materials.
• Grinding a variety of materials is possible with this machine
• Installation and operation of the machine are straightforward, and its operation is continuous.
• Maintaining and cleaning it is easy.
• The price is reasonable
• Constructing it locally is easier due to its ease of manufacture.

Demerits

• Abrasive and very hard materials should not be ground finely due to excessive wear.
• Due to excessive heat generated by mill fouling, sticky or plastic-like low-melting materials cannot be processed in the mill.
• It is possible to damage the mill by not controlling the feed rate.
• Foreign materials such as stones and metals are allowed to enter the material due to inadequate garbling processes
• It is possible for the screen to become clogged.

Ball Mill

Principles

Ball mills, sometimes referred to as pebble mills or tumbling mills, are milling machines that take the form of cylindrical vessels containing balls mounted on a metallic frame that can revolve around its longitudinal axis. Depending on the feed and mill size, balls with varying diameters occupy 30 - 50 % of the mill volume. By breaking down coarse feed materials, smaller balls reduce the void spaces between the balls, resulting in finer products. The material is ground by both impact and attrition in ball mills.

Construction

Ball mills consist of hollow cylinders suspended on shafts and rotating about horizontal axes. Metal, porcelain, or rubber can be used to make the cylinder. A ball or pebble is placed within the cylinder. The cylinder's volume is occupied by balls to the extent of 30 to 50%. Depending on the feed size and the cylinder diameter, the diameter of the balls will differ. These balls are between 2cm and 15cm in diameter. Stainless steel, porcelain, and metal can be used for the balls. A ball is used to grind food.

Working

Materials for grinding are kept in hollow cylinders. The material fills a volume of 60%. The cylinder is filled with balls and then closed with a lid. Rotation occurs in the mill. Rotation speed must be considered. Low speed will only cause a very small reduction in size because the ball mass will slide or roll over one another. Because of centrifugal force, the balls will be thrown against the cylinder wall at high speeds and will not grind against it. A ball mill's critical speed is 2/3rds speed, which is where centrifugation occurs. A ball mill's critical speed is 2/3rds speed, which is where centrifugation occurs. As a result, the particles between the balls impact each other and attrition occurs between them, causing the size reduction to be maximum. In most cases, the attrition between the balls occurs at 0.5 cycles per second (cps).

Uses

It ranges from 5 to 100 mm.
Parenteral and ophthalmic products can be manufactured.

Merits

• It is possible to obtain a very fine powder.
• Dry and wet grinding methods are equally suitable.
• As closed cylinders are used for grinding, it is possible to grind toxic substances.
• Sterile conditions can be maintained with the use of closed cylinders.
• Compared to other mills, ball mills are inexpensive to install, operate, and maintain.
• Batch and continuous operation are both possible.
• Grinder media are inexpensive.
• It is inexpensive to install and produce.

Demerits

• Ball mills are extremely noisy machines, especially when metal cylinders are used.
• Ball milling takes a long time.
• Ball mills are unable to grind the tacky, fibrous material. These materials might deposit on the sides of the mill, causing problems.
• A possibility of contamination of the product exists due to wear on the balls and the inside surface of the cylinder.

Fluid energy mill

Principle

A floating energy mill is also known as a micronizer or a jet mill, because it has a hollow toroid. It has a diameter range of 20 mm to 200 mm, depending upon the height of the loop. Attrition and impaction are used to grind particles together. High-pressure nozzles at the bottom of the loop inject fluids or milling gases, usually air or inert gas, as a high-pressure jet. As the powder particles are being accelerated, high-velocity jets are released. The kinetic energy of the air and turbulence create particles between two and ten micrometers in size resulting from particle-to-particle collisions and particle-to-wall contact. When the fluidized effect takes place, the particles are transported to the size classifier, where they are retained until sufficiently small to be removed.

Construction

There are loops of pipe whose diameter ranges from 20 to 200 mm. This loop can reach a height of up to 2 meters. The bottom of the pipe is fitted with several nozzles. There are generally between two and six grinding nozzles. At the collection point, there is a classifier. This is usually accomplished using compressed air.

Working

A nozzle at the bottom of the loop injects air at very high pressure. Compressed air with a pressure between 600 kilopascals and 1.0 megapascals is commonly used. Due to the high velocity of the air, turbulence is produced. By feeding the solids into the stream, turbulence is created. Turbulence causes particles to collide and attrit between each other. By incorporating a classifier into the process, the system collects only fine particles for processing as products, while larger particles are sent back into the air stream for further reduction in size. To feed the mill, raw materials have been reduced in size and screened to 100 meshes. A 5mm or smaller product is produced.

Uses

Griofulvin (a drug used to fight fungal infections), antibiotics, etc., can be ground in this manner when fine powder is required.

Merits

• Due to the lack of heat generated in the mill, thermolabile substances can be processed.
• This produces smaller particles than conventional grinding methods.
• It does not contain any contamination.
• Nitrogen can be used in place of normal air for oxygen-sensitive materials or for materials sensitive to moisture.

Demerits

• Milling soft, fibrous materials is not recommended.
• Highly expensive.

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