1. MILLING

2. Milling
Raw materials often occur in sizes that are too large to be used and, therefore, they must be reduced in size. This size-reduction operation can be divided into two major categories depending on whether the material is a solid or a liquid. If it is solid, the operations are called grinding and cutting, if it is liquid, emulsification or atomization.
MILLING,GRINDING AND CUTTING
Milling, Grinding and cutting reduce the size of solid materials by mechanical action, dividing them into smaller particles.
In the milling process, materials are reduced in size by fracturing them. The material is stressed by the action of mechanical moving parts in the milling machine and initially the stress is absorbed internally by the material as strain energy. When the local strain energy exceeds a critical level, which is a function of the material, fracture occurs along lines of weakness and the stored energy is released.

3. Some of the energy is taken up in the creation of new surface, but the greater part of it is dissipated as heat.
Time also plays a part in the fracturing process and it appears that material will fracture at lower stress concentrations if these can be maintained for longer periods. Milling is, therefore, achieved by mechanical stress followed by rupture.
The force applied may be compression, impact, or shear, and both the magnitude of the force and the time of application affect the extent of grinding achieved. For efficient grinding, the energy applied to the material should exceed, by as small a margin as possible, the minimum energy needed to rupture the material . Excess energy is lost as heat and this loss should be kept as low as practicable.

4. Impact milling.
Impact milling occurs when a hard object that applies a blunt force across a wide area hits a particle to fracture it. This milling action is produced by a rotating assembly that uses blunt or hammer-type blades. Another type of impact mill is a jet mill. A jet mill uses compressed gas to accelerate the particles, causing them to impact against each other in the process chamber. Impact mills can reduce both fine powders and large chunks of friable material, such as pharmaceutical powders, sugar, salt, spices, plastics, coal, limestone, chemical powders, and
fertilizer down to 50 microns with mechanical impact mills and less than 10 microns with jet mills.
Attrition milling.
In attrition milling, non-degradable grinding media continuously contact the material. This milling action is typically produced by a horizontal rotating vessel filled with grinding media and tends to create free-flowing, spherical particles. Attrition mills can reduce 1,000-micron (20-mesh) particles of friable materials, such as chemicals and minerals down to less than 1 micron. One type is the media mill (also called a ball mill).

5. Knife (Shear Milling) milling.
In knife milling, a sharp blade applies high, head-on shear force to a large particle, cutting it to a predetermined size to create smaller particles and minimize fines. This milling action is produced by a rotating assembly that uses sharp knives or blades to cut the particles. Knife mills can reduce 2-inch or larger chunks or slabs of material, including elastic or heat-sensitive materials such as various foods, rubber, and wax down to 250 to 1,200 microns.
Direct-pressure (compression) milling.
Direct-pressure milling occurs when a particle is crushed or pinched between two hardened surfaces. Two rotating bars or one rotating bar and a stationary plate generally produce this milling action. Direct pressure mills typically reduce 1-inch or larger chunks of friable materials such as minerals, chemicals, and some food products down to 800 to 1,000 microns.
Most mills use a combination of these principles to apply more than one type of force to a particle. This allows mill manufacturers to custom-design their equipment based on a material’s characteristics and enables mills to reduce various materials.

6. Hammer Mill
It is type of impact mill. Hammer mills work on the principle that most materials will crush or pulverize upon impact using a simple three step operation:
Material is fed into the mill's chamber typically by gravity.
2. The material is struck by ganged hammers (rectangular pieces of hardened steel) which are attached to a shaft which rotates at high speed inside the chamber. The material is crushed or shattered by the repeated hammer impacts, collisions with the walls of the grinding chamber, as well as particle on particle impacts.
3. Perforated metal screens, covering the discharge opening of the mill retains coarse materials for further grinding while allowing the properly sized materials to pass as finished product. Varying the screen size, shaft speed, or hammer configuration can dramatically alter the finished size of the product being ground. Faster speed, a smaller screen, and more hammers result in a finer end product. Each component can be changed individually or in any combination to produce the precise grind required.

7. Advantages
It is rapid in action, and is capable of grinding many different types of materials.
The product can be controlled by variation of rotor speed, hammer type, and shape and size of mesh.
Operation is continuous.
No surface moves against each other, so that there is little contamination of the product with metal abraded from the mill.
Disadvantages
The high speed of operation causes generation of heat that may affect thermo labile materials or drugs containing gum, fats or resins. The mill may be water cooled, if necessary, to reduce this heat damage.
The rate of feed must be controlled.
Because of high speed the mill is susceptible to damage by foreign matters.
Application
Milling of barks, leaves, roots, crystals, filter cakes, granules (hammer with cutting edge).

11. Cutter mill
The mechanism of grinding is cutting. In such mills rotating knives are provided which rotates against the stationary knives in the casing, the lower portion is provided with screen, which can be adjusted..
Such mills are used to reduced soft materials, the particle size obtain is coarser. Commonly roots, peels or woods are crushed prior to extraction.

12. Ball Mill
A ball mill, a type of grinder, is a cylindrical device used to grind (or mix) materials like ores chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used for media, including ceramic balls, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously, fed at one end and discharged at the other. Large to medium ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the making of black powder but can't be used in the making of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. High quality ball mills are potentially expensive and can grind mixture particles ball mills are effective grinding systems in the general range of 500 microns down to 5 microns.
Grinding media
There are many types of grinding media suitable for use in a ball mill, each material having its own specific properties and advantages. Common in some applications are stainless steel balls. While usually very effective due to their high density and low contamination of the material being processed, stainless steel balls are unsuitable for some applications, including:
Black powder and other flammable materials require non-sparking lead antimony, brass, or bronze grinding media

14. Design:
Diameter = 1m
Balls = % of mill volume
Balls Diameter = 75 mm, 150 mm, 20 mm
Material of construction = porcelain, rubber, metal
Advantages:
Capable of grinding wide variety of material.
It can be used in a completely enclosed form, which makes it suitable for toxic materials.
It produces very fine powders
Suitable for wet and dry milling.
Disadvantages:
Wear and tear may occur which may results in product contamination.
Soft or sticky material may cause problem by caking on the side walls or holding the balls.
It makes a lot of noise specially in case of metallic material of construction.

15. Fluid Energy Mill
The mill consist of loop of pipe with a diameter of mm and loop height normally 2 m. a fluid is injected at high pressure through nozzles at the bottom, giving rise to high velocity with turbulent circulation of the fluid. solids are introduced into the stream as a result of high degree of turbulence, impact and attrition forces occur between the particles.
Advantages:
The particle size of the product is smaller than that produced by any other method.
Expansion of gases at the nozzles leads to cooling , thus counteracting the usual frictional heat.
As there is no moving part, no chance of material abrasion so product contamination is avoided.
Inert gases can be used in special cases.
Having a classifier as an integral part, it permits close control of particle size.
Disadvantages:
1. The particles develop electrostatic charges which are difficult to remove from mill.

18. Colloid Mill
A colloid mill is a machine that is used to reduce the particle size of a solid in suspension in a liquid, or to reduce the droplet size of a liquid suspended in another liquid. This is done by applying high levels of hydraulic shear to the process liquid. It is frequently used to increase the stability of suspensions and emulsions.

19. The rotor and stator are cone shaped, and have three stages of increasingly fine serrations, or grooves. The stator can be infinitely adjusted to obtain the desired gap setting between the rotor and stator. The grooves change directions in each stage for increased turbulence
Colloid mills are specialized machines used for milling, homogenizing, dispersing and breaking down of agglomerates into small size particles. The primary function of this machine is to breakdown larger particles into uniform sized small particles. The raw material is fed into a hopper and made to pass between the rotor and stator elements . It usually operates at 3000 to rpm, and the clearance can be adjusted between 50 and 500 µm.

21. Triple Roller Mill
Triple roller Mill is used to grind the pharmaceutical and other ingredients for the preparation of smooth and free from grain ointments, pastes, paints, printer's ink etc.
A Triple Roll Mill is a machine that uses the shear force created by three horizontally positioned rolls rotating at opposite directions and different speeds relative to each other to mix, refine, disperse, or homogenize viscous materials fed into it.

22. The pasty substance is fed into the hoper, where it is drawn between the feed and center rollers. When pre-dispersed, the substance sticks to the bottom of the center roller, which transports it into the second gap. In this gap the paste is dispersed to the desired degree of fineness. The scraper system removes the finished product.
The roller mills have an output of 2 to 20 kilogram per hour depending on the consistency of the product to be produced and the roller size.