Size Reduction
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
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 = Kkfc
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 = KRfc
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
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
- 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
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
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
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
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
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
