Size Reduction – Pharmaceutical Engineering B. Pharm Third Semester PDF Notes

Size Reduction

Size Reduction - Pharmaceutical Engineering B. Pharm Third Semester PDF Notes

Learning objectives  

At the end of this lecture student will be able to: 

  • Explain various applications of size reduction  
  • Discuss the factors affecting size reduction  
  • Discuss various mechanisms of size reduction  
  • Explain the laws governing size reduction  
  • Discuss on the construction and working of a ball mill  
  • Discuss the process of Fluid energy mill and Hammer mill operation  
  • Explain the construction and working of Edge runner and End runner mill  

Size reduction  

  • Size refers to physical dimension of an object  
  • Reduction refers to decrement or the process of decreasing the size  

size reduction

Introduction  

In the materials processing industry, size reduction or comminution is usually carried out in order to:  

Increase the specific surface area because, in most reactions involving solid particles, the rate of reactions is directly  proportional to the area of contact with a second phase  

Break a material into very small particles in order to separate the valuable amongst the two constituents  Achieve intimate mixing  

Definition and Objectives  

Definition  

Size reduction (comminution and grinding)  

It is the process of reducing the particle size of a substance to a finer state  

Milling  

When the particle size of solids is reduced by mechanical means  

Objectives  

  • To produce smaller particles (in the preparation of suspensions or to facilitate the mixing of powders) 
  • To increase surface area (to increase adsorptive properties)  
  • In drugs that are crushed to expose cells prior to extraction  
  • Reduce the bulk of a material, since shipping charges  
  • may be based on volume  

Advantages  

Content uniformity  

Uniform flow  

Effective drying  

Increases specific surface area or viscosity  Uniform mixing and drying  

Improve rate of absorption  

Smaller the particles greater is absorption  Improve dissolution rate.  

Disadvantages  

Drug degradation  

Contamination  

Factors Affecting Size Reduction  

The pharmaceutical industry uses a great variety of materials, including chemical substances, animal tissues, and vegetable drugs which may be hard (seeds), fibrous (barks or roots) or spongy (peels).  

Thus, the methods of size reduction are numerous, and selection of the suitable method involves the material properties that may influence the process.  

The properties that affect size reduction include:  

Hardness  

Physiological effect  

Stickiness  

Purity required  

Softening temperature  

Toughness  

Material Structure  

Bulk density  

Abrasiveness  

Ratio of feed size to product size  

Moisture content  

Hardness  

It is a surface property of the material  

– It is frequently confused with a property named strength  

– Thus, it is possible for a material to be very hard, but if it is brittle also, then size reduction may present no special  problems  

– An arbitrary scale of hardness has been devised known as Moh’s Scale;  

 Moh’s Scale = 1 is for graphite  

 Moh’s Scale < 3 is for soft material  

 Moh’s Scale > 7 is for hard material  

 Moh’s Scale = 10 is for diamond  

The harder the material the more difficult it is to reduce in size  

Toughness  

Toughness of a material is sometimes more important than hardness  

A soft but tough material may present more problems in size reduction than a hard but brittle substance  For example it is difficult to break rubber than a stick of blackboard chalk  

Toughness is encountered in many pharmaceutical materials, particularly in fibrous drugs, and is often related to  moisture content ((the toughness of a `green’ twig and the brittleness of a dry one)  

Abrasiveness  

Abrasiveness is a property of hard materials (particularly those of mineral origin)  

It may limit the type of machinery that can be used  

During the grinding of some very abrasive substances the final powder may be contaminated with more than 0.1 % of  metal worn from the grinding mill  

Stickiness  

It is a property that cause difficulty in size reduction, because material may adhere to the grinding surfaces, or the  meshes of screens and become choked.  

Pharmaceutical substances that are gummy or resinous may be troublesome, particularly if the methods used for size  reduction generate heat.  

Solve this problem by complete dryness and the addition of inert substances (as addition of kaolin to sulphur) 

Softening temperature  

During size reduction process sometimes heat is generated which may cause some substances to soften, and the  temperature at which this occurs can be important  

Waxy substances, such as stearic acid, or drugs containing oils or fats are examples that may be affected  

Some methods can be used to overcome this like cooling the mill, either by a water jacket or by passing a stream of air  through the equipment  

Material structure  

Some substances are homogeneous in character  

Mineral substances may have lines of weakness along which the materials splits to form flake-like particles  Vegetable drugs have a cellular structure often leading to long fibrous particles  

Moisture Content  

Moisture content influences a number of properties that can affect size reduction, for example, hardness, toughness or  stickiness  

In general materials should be dry or wet and not merely damp  

Usually, less than 5 percent of moisture is suitable if the substance is to be ground dry or more than 50 if it is being  subjected to wet grinding  

Physiological effect  

Some substances are very potent (as hormone drugs) and small amounts of dust may have an effect on the operators  To solve this problem:  

 Use enclosed mills to avoid dust  

 Special air extraction systems are desirable  

 Wet grinding can eliminates the problem entirely.  

Purity required  

Certain types of size reduction apparatus cause the grinding surfaces to wear, thus such methods must be avoided if a  high degree of purity of product is needed  

Density  

When all other factors being equal, the output of the machine is related to the bulk density of the substance.

Ratio of feed size to product size  

Machines that produce a fine product require a small feed size. Thus, it may be necessary to carry out the size reduction  process in several stages with different equipment; for example, preliminary crushing, followed by coarse grinding and  then fine grinding  

Mechanism of size reduction  

Cutting  

The material is cut by means of a sharp blade or blades  

Compression  

The material is crushed by application of pressure  

Impact  

Occurs when the material is stationary and is hit by an object moving at high speed or when the moving particle strikes a  stationary surface. In either case, the material shatters to smaller pieces  

Attrition  

The material is subjected to pressure as in compression, but the surfaces are moving relative to each other, resulting in  shear forces which break the particles  

Mechanism of Size Reduction  

Stage 1  

It vary with the nature of the material and each drug may require separate treatment.  

Stage 2  

Fracture occurs preferentially along lines of weakness.  

Stage 3  

Fresh surfaces may be created or existing cracks and fissures may be opened up, the former requiring much more  energy  

Stage 4  

Size reduction is a very energy-inefficient process as only a small percentage of the energy supplied is utilized in subdividing the particles.  

Stage 5  

Much of the energy is spent in overcoming friction and inertia of machine parts, friction between particles and deforming the particles without breaking them. This energy is released as heat.  

Laws Governing Size Reduction 

Assumption:  

Energy required to produce a change dL in a particle of a typical size dimension L is a simple power function of L:    

 dE/dL = KLn (1)  

where  

dE – the differential energy required  

dL – the change in a typical dimension  

L – the magnitude of a typical length dimension  

K and n are constants  

Kick’s Law  

“This theory states that the energy used in deforming a set of particles of equivalent shape is proportional to ratio of  change in size”.  

Putting n = -1 and integration gives,  

dE/dL = K / L  

On rearrangement gives

dE = K dL / L  

Energy required to reduce a material in size is directly proportional to the size reduction ratio dL/L.  Upon integration, we get  

E = K ln (L initial / L final)  

Kick also assumed that  

K = Kkf

where  

Kk is Kick’s constant  

fc is the crushing strength of the material  

Therefore,  

E = Kkfc ln (Linitial / Lfinal)  

Kick’s law implies that the specific energy required to crush a material, for example from 10 cm down to 5 cm, is the same as the energy required to crush the same material from 5mm to 2.5 mm.  

For compression of large particles kick’s theory is useful.  

Rittinger’s assumption  

“According to this theory energy E required for size reduction of unit mass is directly proportional to the new surface area produced”.  

Putting n = -2, and integration gives  

dE/dL = K / L2 

which on rearrangement  

dE = K dL / L2 

Energy required to reduce a material in size is directly proportional to the surface area change.  Upon integration, we get

E = K ln (1 / Lfinal – 1 / Linitial)  

Using K = KRf

where  

KR is Rittinger’s constant  

fc is the crushing strength of the material  

Therefore,

E = KRfc ln(1 /Lfinal – 1 / Linitial

E=KR(Sn– Si) – simple equation  

Rittinger’s Law implies that the energy required to reduce L for a mass of particles from 10 cm to 5 cm would be the  same as that required to reduce, for example, the same mass of 5 mm particles down to 4.7 mm.  

Applications  

Applicable to brittle materials undergoing fine mlling. This theory ignore deformation before fracture

Bond’s law  

bonds law

Energy Utilization  

One of the first important investigations into the distribution of the energy fed into a crusher was carried out by OWENS  who concluded that energy was utilized as follows:  

  • In producing elastic deformation of the particles before fracture occurs  
  • In producing inelastic deformation which results in size reduction  
  • In causing elastic distortion of the equipment  
  • In friction between particles, and between particles and the machine  
  • In noise, heat and vibration in the plant  
  • In friction losses in the plant itself  

Owens estimated that only about 10 per cent of the total power is usefully employed.  

Classification of Size Reduction Equipment’s or Mills  

1. CRUSHERS  

Edge runner mill  

End runner mill 

2. GRINDERS  

Impact Mill – Hammer mill  

Rolling compression – Roller mill  

Attrition mills – Attrition mill  

Tumbling Mills – Ball mill 

3. ULTRAFINE GRINDER  

Fluid energy mill 

4. CUTTING MACHINES  

Cutter mill  

Ball Mill / Pebble Mill/ Tumbling Mill  

The mechanisms of impact and attrition can be combined in two forms of mill.  

 In the ball mill the particles receive impacts from balls or stones and are subjected to attrition as the balls slide over  each other.  

Principle of operation  

The ball mill consists of a hollow cylinder mounted in such a way that it can be rotated on its horizontal axis with a  rotational frequency depend upon the diameter of the mill ≈ 0.5 r.p.s  

ball mill

  • The cylinder may be of metal, porcelain or of rubber, to reduce abrasion.  
  • The balls may be of metal, porcelain or stones (pebble mill).  
  • The cylinder contains balls that occupy 30 to 50 per cent of the mill volume.  

The cylinder may be of metal, porcelain or of rubber, to reduce abrasion. The balls may be of metal, porcelain or stones  (pebble mill).  

The ball size being dependent on the size of the feed and the diameter of the mill. Usually a mill 1 m in diameter will use  balls of 75 mm, in practice, the balls are damaged, so that a range of sizes from 20 mm upwards are used. This gives a  better product, since the larger balls crush the feed and the smaller ones form the fine product.  

Importance factors in the operation of the ball mill  

The amount of the material in the mill is of importance: too much exerting a reduced effect too little leading to loss of  efficiency and to abrasion.  

The speed of rotation  

At low speeds, the mass of balls will slide or roll over each other and negligible size reduction will occur 

The speed of rotation  

At high speeds, the balls will be thrown out to the wall by centrifugal force and no grinding will occur 

At about two-thirds of the speed at which centrifuging just occurs, movement takes place as shown in Fig. (c), that is, the  balls are carried almost to the top of the mill and then fall in a cascade (tumble) across the diameter of the mill.  

By this means, the maximum size reduction is effected by impact of the particles between the balls and by attrition  between the balls.  

Advantages  

It is capable of grinding a wide variety of materials of differing character and of different degrees of hardness  It can be used in a completely enclosed form; which makes it especially suitable for use with toxic materials  

It can be used for both batch and continuous operation, and a classifier can be used in conjunction with the mill, so that  particles of suitable size are removed while oversize particles are returned..  

It is equally suitable for wet or dry grinding processes. Grinding medium is cheap  

It can produce very fine powders  

The cost of installation and production is low  

Disadvantages  

Wear occurs, principally from the balls, but partially from the shell and this may result in the contamination of the  product; with abrasive materials this may exceed 0.1%  

Soft or sticky materials may cause problems by caking on the sides of the mill or by holding the balls in aggregates  The ball mill is a very noisy machine, particularly if the casing is of metal, but much less if rubber is used 

Applications  

Ball mills are applicable to a wide variety of materials  

Large ones being used for grinding ores prior to manufacture of pharmaceutical chemicals  

Small versions for the final grinding of drugs or for grinding suspensions.  

Fluid Energy Mill /Jet Mill/ Ultrafine Grinder 

Principle 

Material reduced in the size by attrition & impact. The feed stock is suspended within a high velocity air stream. Milling  takes place because of high velocity collision b/w the suspended particles  

Fluid Energy Mill /Jet Mill/ Ultrafine Grinder 

Construction  

It consists of a loop of pipe which has a diameter of 20 to 200 mm, depending on the overall height of the loop, may be  up to about 2 m  

There is a feed inlet and a series of nozzles for the inlet of air or an inert gas  

It also has an outlet with a classifier which allows the air to escape but prevents to pass until they become sufficiently  fine  

Working  

Solids are introduced into the stream and, as a result of the high degree of turbulence, impacts and attritional forces  occur between the particles  

A classifier is incorporated in the system, so that particles are retained until sufficiently fine  

The feed to the mill needs to be pre-treated to reduce the particle size to 100 mesh, enabling the process to yield a  product as small as 5 μm or less  

Advantages  

The particle size of the product is smaller than that produced by any other method of size reduction  

Expansion of gases at the nozzles leads to cooling, counteracting the usual frictional heat which can affect heat-sensitive  materials  

Since the size reduction is by inter-particulate attrition there is little or no abrasion of the mill and so virtually no  contamination of the product  

For special cases with very sensitive materials it is possible to use inert gases  

Having a classifier as an integral part of the system permits close control of particle size and of particle size distribution  The method is used where especially fine powders are required, as antibiotics, sulphonamides and vitamins 

Disadvantage  

Energy consuming  

High head space  

Avoid coarse materials into the chamber.  

The fed device may be clogged with the clump materials Special feeding devices should be provided for the feeding of  the materials  

Use of compressed air results in generation of static electricity  

Material recovered in the collection bags is difficult or impossible to remove by the normal blow back procedures 

Hammer Mill  

On the small scale, size reduction by Impact can be carried out by the shattering of brittle substances with a hammer or  with a pestle and mortar.  

Principle  

A hammer mill is essentially a steel drum containing a vertical or horizontal rotating shaft or drum on which hammers  are mounted  

The hammers are free to swing on the ends of the cross or fixed to the central rotor  

The rotor is spun at a high speed inside the drum while material is fed into a feed hopper  

The material is impacted by the hammer bars and is thereby Shredded and expelled through screens in the drum of a  selected size  

Hammer Mill  

Construction  

The hammer mill consists of a central shaft to which four or more hammers are attached  

These are mounted with swivel joints, so that the hammers swing out to a radial position when the shaft is rotated  The lower part of the casing consists of a screen through which material can escape  

The screen can be changed according to the particle size required  

Working  

Material is fed into the mill grinding chamber through the feed chute  

It repeatedly is struck by ganged hammers which are attached to a shaft rotators at high speed inside the mill chamber  

The material is crushed or shattered by a combination of repeated hammered impacts, collisions with the walls of the  grinding chamber and particle on particle impacts  

Perforated metal screens or bar grates covering the discharge opening of the mill retain coarse material for further  grinding while allowing properly sized materials to pass as finished product  

Advantages  

Easy to install, dismantle and clean  

Scale up problems are minimum  

Various types of feed stock can be handled using screen of different sized  

It occupies less space  

It is versatile  

Operated in a closed environment to avoid dust and explosion hazard  

The product can be controlled by variation of rotor speed, hammer type, and size and shape of mesh  

Operation is continuous. No surfaces move against each other, so that there is little contamination of the product with  metal abraded from the mill  

Disadvantages  

The screens may get clogged  

Product degradation due to heat building  

Wearing of mill with abrasive materials  

Unsuitable for sticky, fibrous and hard materials  

Applications  

Fine to moderate grinding of powders  

Particle size may vary from 10 to 400 mm  

Nonabrasive, brittle materials can be used as feed stock  

Milling dry materials, wet slurries, ointments etc. 

Edge Runner Mill / Roller Stone Mill 

Principle

The size reduction is done by crushing due to heavy weight of the stones and the shearing force which is involved during  the movement of these stones  

Edge Runner Mill / Roller Stone Mill 

Construction  

It consist of two heavy rollers and a bed made of stones or granite. The roller have a central shaft and they revolve on its  axis. The roller are mounted on the horizontal shaft and move around the bed  

Working  

The material to be grounded is put on the bed and with the help of the scrapper it is kept in the path of the stone wheels  

The material is ground for a definite period and then it is passed through the sieves to get the powder of the required  size.  

Applications  

Very fine particles sized materials can be obtained by this edge runner mill  

Edge runner mill is used for grinding most of the drugs to fine powder, but it requires more floor space than the other  commonly used mills  

It is used to crush or grind all types of the drugs 

Advantages  

It is mostly used for all types of the drugs  

Very fine particle size is produced  

The major advantage of this mill is that it requires less attention during operation  

The various groups of elements and combinations of such elements produce a machine which operates with greater  efficiency  

Utilizes less power, does not require any frequent clean up, it does not require any particular adjustment, it is simple in  structure  

Disadvantages  

It is not use for sticky materials  

Noise pollution  

High energy consumption and time consuming  

End Runner Mill / Mechanical Mortar and Pestle

Principle  

Size reduction is done by crushing or compression due to heavy weight of steel pestle. Shearing stress is also involved  during the movement of mortar and pestle  

End Runner Mill / Mechanical Mortar and Pestle  

Construction  

  • A steel mortar is fixed to a flanged plate  
  • Underneath the flanged plate, a beveled cog fitting is attached to a horizontal shaft with a pulley 
  • Dumb bell shaped pestle is flat rather than round  
  • The pestle carries a hinged arm for emptying and cleaning  

Working  

The material to be ground is placed in the mortar  

The scraper puts the material in the path of the pestle  

The mortar revolves at high speed  

The pestle is placed in the mortar  

The rotating mortar causes the pestle to revolve  

Thus size reduction is achieved by shearing as well as crushing  

The material is collected and passed through a sieve to get the powder of desired size  

Uses  

Suitable for fine grinding  

Disadvantage  

Not suitable for unbroken drugs  

Summary  

  • Size reduction is the process of reducing the particle size of a substance to a finer state  
  • The factors that affect size reduction process are hardness, toughness, stickiness, moisture content, ratio of feed to  product ratio etc.  
  • Moh’s scale is used to quantify the hardness of a substance  
  • The applications of size reduction include efficient mixing of different ingredients, increased drying and improved  absorption  
  • Different mechanisms of size reduction includes cutting, compression, impact and attrition 
  • Laws governing size reduction include Kick’s Law, Rittinger’s Law, and Bond’s Law  
  • Ball mill mechanism of operation include both impact and attrition  
  • Optimum speed of ball mill operation at which maximum efficiency of size reduction obtained is termed as Critical  speed  
  • Fluid energy mill utilizes both impact and attrition mechanism of size reduction  
  • The feed to the Fluid energy mill should be pre size reduced  
  • It is used for size reduction of thermolabile substances like antibiotics, vitamins etc.. 
  • Hammer mill utilizes impact as the mechanism of size reduction process  
  • Optimum speed of ball mill operation at which maximum efficiency of size reduction obtained is termed as Critical  speed 
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