The majority of size reduction equipment may be classified according to the way in which forces are applied namely; impact, shear, attrition, and shear-compression. A given mill may operate successfully in more than one class: a hammer mill may be used to prepare a 16-mesh granulation and to mill a crystalline material to a 120-mesh powder.
The mills used for size reduction of the granules can be divided into two primary categories based on the energy input into the process. Even though there are several high-energy mills available for size reduction, only a few are used in the pharmaceutical industry for the wet or dry sizing process. Milling is an extremely inefficient unit operation with only 1–2% of the applied energy being utilized in the actual size reduction. Milling efficiency is dependent on the characteristics of the material used and the type of mill employed.
|EXAMPLE||PRODUCT SIZE||TYPE OF MATERIAL||NOT USED FOR|
|Impact||Hammer mill||Moderate to fine||Brittle and dry material||Fibrous, sticky, low-melting
|Shear||Extruder and hand
|Dry material, hard, abrasive
|attrition||Oscillating granulator||Coarse to moderate||Dried granulation||Wet granulation, abrasive
|Shear compression||Conical screening mill||Moderate to coarse||Wet, dry granulation||Abrasive materials|
- Size reduction occurs primarily by shear.
- They are made of brass or stainless steel and consist of a woven wire cloth stretched in a circular or rectangular frame.
- They are available in sizes ranging from 4 to 325 mesh; however, for granulation, primarily mesh sizes from 4 to 20 are used.
- They are most widely used for sieve analysis or for size reduction of wet and dry granules in the early stages of formulation development.
- They consist of an oscillating bar contacting a woven wire screen, and the material is forced through the screen by the oscillating-rotary motion of the bar.
- Size reduction is primarily by shear with some attrition.
- Speed, rotary or oscillatory motion, and screen size are important variables to be considered during the sizing process.
- They are used primarily for size reduction of wet and dry granulations and, to some extent, for milling tablets and compacts that must be reprocessed.
- The narrow size distribution and minimum amount of fines are advantages during the size reduction of dry granulation.
- Heat-sensitive and waxy materials can be milled owing to the low heat generated during the sizing process.
- Low throughput rates and possible metal contamination from wearing down or broken screen are some of its limitations.
- It is primarily used for continuous wet granulation.
- Wet material is forced through a screen and the extruded material is dried in a tray or fluid bed dryer or can be spheronized to produce granules with a high degree of sphericity and then dried for controlled-release applications.
- Less dust generation and more uniformgranules are some of the advantages.
Hammer Mill:- The hammer mill is one of the most versatile and widely used mills in the pharmaceutical industry. The principle of size reduction in the hammer mill is one of high-velocity impact between the rapidly moving hammers mounted on a rotor and the powder particles. These mills can produce a wide range of particle sizes, even down to micrometer size. The particle shape, however, is generally sharper and more irregular than that produced by compression methods. The force imparted by the hammers and the screen opening size and shape control the degree of particle size reduction.
- They can be used for size reduction of wet or dry granulations and milling of raw materials.
- There is a wide range of interchangeable feed throats and variable feed screw systems available to optimize the feed rate.
- Hammers can rotate horizontally or vertically, based on the rotor configuration, and at variable speeds.
- Hammers can be fixed or free-swinging.
- Hammers with blunt or impact edges are preferred for pulverizing and knife or sharp edges are preferred for chopping or sizing of granules.
- Screen openings generally vary from 0.3 to 38 mm, with round or square perforations, diagonal or straight slots, or with a rasping surface.
- Feed rate and dryness of the granules are important variables relative to the material.
- Type of hammers, rotor speed, screen type, thickness, and opening size are important variables relative to the machine.
- Ease of setup, clean-up, minimum scale-up problems, and ability to handle a wide variety of size and type of feedstock are some advantages.
- Heat buildup, screen wear, and potential clogging of screens are some of the limitations.
- Integrated designs are available for dust containment.
Conical Screening Mill
- It is effective for dry (deagglomeration–delumping) and wet milling of soft to medium hard materials.
- The comminution chamber consists of an impeller rotating at variable speed, imparting a compression or shear force inside a conical screen.
- The impeller imparts a vortex flow pattern to the feed material, and the centrifugal acceleration forces the particles to the screen surface and up thecone (360 ) in a spiraling path
- The space between the impeller and the screen can be adjusted.
- The size and shape of the screen holes, screen thickness, impeller configuration, and mill speed are important variables.
- It is used for difficult-to-mill, heat-sensitive material and hard granules.
- Low heat and lower amounts of fines are produced compared with the hammer mill; hence, it produces a narrower particle size distribution.
- The impeller does not touch the screen; hence, chances of screen breakage and metal contamination are greatly reduced compared with an oscillating granulator.
- The dual action of conical screening mills (size reduction and mixing) makes this equipment more desirable than the use of traditional oscillators.
- Integrated designs are available that are attached to a high-shear granulator discharge, which provides a deagglomerated, lump-free product for the dryer.
- Centrifugal-impact mills and sieves are useful to minimize the production of fine particles, because their design combines sieving and milling into a single operation.
- Unlike the conical screening mills, these consist of a nonrotating bar or stator which is fixed within a rotating sieve basket. This action produces a very low product agitation and impact; hence, no heat is generated.
- The particles that are smaller than the holes of the sieve can pass through the mill without comminution; however, the larger particles are directed by centrifugal force to impact the stator.
- Older designs are not preferred because the likelihood of sieve-to-stator contact can result in metal particulates in the product. Newer designs eliminate metal-to-metal contact.
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