Size Reduction

 Chapter -2 

SIZE REDUCTION

 INTRODUCTION 

• Pharmaceutical powders are classified as monodispersed (particles of same size) as well as polydisperse (particles of different sizes). Particles of monodispersed type are ideal for pharmaceutical purposes whereas polydisperse powders create considerable difficulties in their processing for production of dosage forms. In order to obtain uniform size particles powders are to be reduced in their size by a process called size reduction. Size reduction is defined as a process of reducing large solid unit masses (vegetable or chemical substances) into small unit masses, coarse particles or fine particles. Comminution is the generic term used for size reduction and includes different operations such as crushing, grinding, milling, mincing, and dicing. Size reduction is not enough to obtain monodispersed particles, but they are to be further processed by size separation. There is numerous equipment available for size reduction of solids and semisolids to improve performance or to meet specifications. The size-reduction equipment is often developed empirically to handle specific materials. For size reduction knowledge of properties of the material to be processed is essential. The most critical characteristic that enables size reduction is hardness. 

• Size reduction involves techniques to create new surfaces and increase surface area by adding energy proportional to the bonds holding the feed particles together. Other important feature is flow properties because many size-reduction equipment works on continuous mode and often have choke points at which bridging occurs leading to flow interruption. Some of the important applications of size reduction include grinding polymers for recycling, facilitating separation of grain components, boosting the biological availability of medications and producing particles of an appropriate size for a given use. Size reduction may aid other processes such as expression and extraction.

 Some of the major applications of size reduction in pharmaceutical field are:

• In size reduction of dosage forms such as capsules, insufflations, suppositories and ointments require particle size to be below 60 µm size. 

• The therapeutic effectiveness of certain drugs can be increased by reducing the particle size. 

• The mixing of solid ingredients is easier if they are reduced to same particle size. 

• In case of suspensions particles being finer reduces rate of sedimentation. 

• The stability of emulsions is increased by decreasing the size of the oil globules.

• Particle size reduction in formulations such as ophthalmias and those meant for external application to the skin could help to reduce irritation of the skin area to which they are applied. 

• The rate of drug absorption depends on particle size. The smaller the particle size, quicker and greater will be rate of absorption. 

• The physical appearance of semisolids can be improved by reducing its particle size. 

Advanced applications of size reduction are:

• Properties of agglomerates: The breakage behaviour of agglomerates after milling help to investigate effects of the formulation and the mill settings. Both the size of the particles before granulation and the amount of binder used determine the breakage behaviour. These parameters have an influence on the strength of the granule to be milled.

• Bioavailability enhancement: The bioavailability of poorly soluble drugs is often intrinsically related to drug particle size. Particle size of pharmaceutical powders for use in inhalation is critical. It dictates areas of the respiratory tract that are to be targeted in order for the product to have the greatest efficacy. Micritization of solids causes size reduction of a drug substance suitable for an inhalation formulation; it can provide consistent results and is a very cost-effective way of achieving small particle sizes.

• Handling the powder: Powder handling and processing tends to be problematic because powders exhibit properties similar to both solids and liquids. Normally, they are surrounded by air and the degree of aeration can affect the way the powder behaves. Many common manufacturing problems are attributed to powder flow, including non-uniformity (segregation) in blending, under- or over-dosage, inaccurate filling, obstructions and stoppages. Size reduction help to minimize all those problems.

• Supercritical fluid technology: Supercritical fluid technology offers the possibility to produce dry powder formulations suitable for inhalation or needle-free injection. It facilitates controlled particle formation in fine form at near-ambient temperatures and integrates particle formation and solvent removal into a single step.      

• Precipitation: Precipitation with compressed antisolvents is used for generation of monodispersed ultra-fine particles. The drug is first dissolved in a solvent, and this solution is mixed with a miscible antisolvent. Mixing processes vary considerably. Precipitation of amorphous material may be favoured at high super saturation when the solubility of the amorphous state exceeded. It utilizes supercritical carbon dioxide as the antisolvent, but the solution jet is deflected by a surface vibrating at an ultrasonic frequency atomizing the jet into much smaller droplets. The advantage is that it can be used for production of organic-solvent free particles, has mild operating temperatures for processing biological materials and is easier for microencapsulation of drugs for controlled release of the therapeutic agents.

•  Nanotechnology: Wet milling of active drug in the presence of surfactant causes defragmentation. The obtained nanosuspension has increased dissolution rate due to larger surface area exposed, while absence of Ostwald ripening is due to the uniform and narrow particle size range obtained, which eliminates the concentration gradient effect.

OBJECTIVES 

•  The main objective of size reduction is to produce smaller particles from larger ones. Smaller particles are the desired product either because of their large surface area or because of their shape, size, and number. The energy efficiency of the operation can be related to the new surface formed by the reduction in size. The shape features of particles, both alone and in mixtures, are important for product evaluation after size reduction. In actual processing, using particular equipment does not produce a uniform product, whether the feed is uniformly sized or not. The product normally consists of a mixture of particles, which may contain a wide variety of sizes and even shapes. Some types of equipment are designed to control the magnitude of the largest particles in their products, but the fine sizes are not under such control. In some equipment, fines are minimized, but they cannot be totally eliminated.

• In comminuted products, the term “diameter” is generally used to describe the characteristic dimension related to particle size. The shape of an individual particle is conveniently expressed in terms of the sphericity (s), which is independent of particle size. For spherical particles ‘s’ equals unity, while for many crushed materials its value lies between 0.6 and 0.7. There are different types of particle size distributions, and no single distribution applies equally well to all comminuted products, particularly in the range of coarser particle sizes. For finer particles, however, the most commonly found distribution follows a log-normal function, which is the most useful among the different types of functions. Thus, in pharmaceutical practice the objective of this operation is to: 

1. Increase the surface area to enhance the rate of a physical or chemical process. 
2. Perform separation of two constituents in cases where one is dispersed in small isolated pockets.
3. Meet stringent specifications regarding the sizes of commercial products. 
4. Accomplish intimate mixing of solids in a solid-solid operation since the mixing is more complete if the particle size is small.
5. Improve dissolution rate, solubility, binding strength and dispersion properties. 
6. Increase the therapeutic effectiveness of certain drugs by reducing the particle size for example, the dose of griseofulvin is reduced to half than that of originally required.
7. Improve mixing of several solid ingredients. 
8. Improve physical appearance of products.  
9. Enhance flowability, improve compression and dose uniformity. 
10. Enhance stability of dispersed system for example, stability of emulsions is increased by decreasing the size of the oil globules

MECHANISMS AND LAWS GOVERNING SIZE REDUCTION 

• The mechanism of size reduction depends upon the nature of the material and each material requires separate treatment. Generally, fracture occurs along the lines of weakness. During size reduction fresh surfaces are created or existing cracks and fissures are opened up, wherein the former requires more energy. There may be a tendency that after processing agglomerates of particles are formed. Size reduction is an energy inefficient process because small amount of the energy required in subdividing the particles. In fact, lot of the energy is spent in overcoming friction and inertia of machine parts and the friction between particles and deforming the particles without breaking them. This energy is released as heat

Laws Governing Size Reduction Process

• One of the mechanisms of size reduction called grinding is very inefficient and thus it is important to use energy as efficiently as possible. It is not easy to calculate the minimum energy required for a given size reduction process. Fortunately, there are certain theories which are useful in approximately calculating energy requirement. Although number of theories have been put forth to predict the energy requirements, but none give accurate results.

• Equation is a statement of Kick's Law. It states that the specific energy required to crush a material, for example, from 10 cm to 5 cm. The same energy is required to crush the same material from 5 mm to 2.5 mm. Thus, in simple terms Kicks law can be stated as energy required to reduce the size of a given quantity of material is constant for the same reduction ratio regardless of the original size.     

• Griffith theory : The Griffith theory states that the amount of force to be applied depends on the crack length and focus of stress at the atomic bond of the crack apex.

• Bond’s law : Bond's law states that energy used to reduce particle size is proportional to the square root of the diameter of the particle produced.

• For the grinding of coarse particles wherein the increase in surface area per unit mass is relatively small, Kick's Law is a reasonable approximation. For size reduction of fine powders where large areas of new surfaces are being created better fits the Rittinger's Law. 

• Size reduction of pharmaceutical products involves reduction mechanism consisting of deforming the material pieces until it breaks or tears. This deformation may be achieved by applying diverse forces. The types of forces commonly used in size reduction process are compression, impact, attrition or shear and cutting. In this operation more than one type of force is usually acts. Summarizes these types of forces and examples of some of the mills commonly used in the pharmaceutical industry.

Compression 

• In this mechanism, the material is crushed by application of pressure. Compressive forces are used for coarse crushing of hard materials. Coarse crushing implies reduction to a size of about 3 mm.

Impact 

• Impact occurs when the material is more or less stationary and is hit by an object moving at high speed or when the moving particle strikes a stationary surface. In both the cases the material is crushed into smaller pieces. Usually both will take place, since the substance is hit by a moving hammer and the particles formed are then thrown against the casing of the machine. Impact forces can be regarded as general-purpose forces and may be associated with coarse, medium and fine grinding of a variety of materials.

Attrition

•  In 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. Shear or attrition forces are applied in fine pulverization, when the size of products can reach the micrometer range. Sometimes a term referred to as ultra-fine grinding is associated with processes in which the sub-micron range of particles is attained.

 Cutting

• Cutting reduces the size of solid materials by mechanical action (sharp blade/s) by dividing them into smaller particles. Cutting is used to break down large pieces of material into smaller pieces and definite shape suitable for further processing, such as in the preparation of powders and granules.  

 

FACTORS AFFECTING SIZE REDUCTION 

 Hardness

• Hardness is a surface property of the material and is frequently confused with strength. It is possible that material is very hard posing a size reduction difficult. If material is brittle then size reduction may present no special problems. An arbitrary scale of hardness has been devised known as Moh’s Scale. A series of mineral substances has been given hardness numbers between 1 and 10, ranging from graphite to diamond. Up to 3 are known as soft and can be marked with the fingernail. Hardness above 7 are designated as hard and cannot be marked with a good pen knife blade, while those between 3 and 7 are described as intermediate. In general, the harder the material the more difficult it is to reduce in size.

Toughness 

• Toughness of a material is generally much more of importance than the hardness. A soft but tough material may present more problems in size reduction than a hard material. For example, tough material like rubber is difficult to break than brittle substance, for example, stick of blackboard chalk. Toughness is encountered in fibrous drugs and is often related to moisture content. Sometimes material toughness can be reduced by treating them with a liquefied nitrogen at a temperature lower than −100 to −150 °C. The method has additional advantages that there is a reduction in the decomposition of thermolabile materials, in the loss of volatile materials, in the oxidation of constituents, and in the risk of explosion.

Abrasiveness    

• Abrasiveness is a property of hard materials (minerals) that limits the mill to be used for size reduction. 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

• Stickiness of material causes considerable difficulty in size reduction. This type of materials may adhere to the grinding surfaces, or choke the meshes of the sieve. Usually the size reduction equipments produce heat. Gummy or resinous substances may be troublesome to reduce in size as their hardness changes with generation of heat and becomes sticky. Sometimes the addition of inert substances such as kaolin to sulphur may reduce stickiness

Slipperiness

• Slipperiness is the reverse of stickiness. This property also gives rise to size reduction difficulties, since the material acts as a lubricant and lowers the efficiency of the grinding surfaces. While size reduction material slips creating problem in milling. 

Softening Temperature

• During size reduction process sometimes, heat is generated which may cause some substances to soften, and thus the temperature at which this occurs is important. For example, waxy substances such as stearic acid or drugs containing oils or fats may find difficulties in size reduction with reduction in their functionalities. This can be overcome by cooling the mill, either by a water jacket or by passing a stream of cold air through the equipment. Another alternative is to use liquid nitrogen.

Material Structure 

• Materials used in pharmaceuticals are of wide variety with some are homogeneous but the majority show some special structures.

• For example, mineral substances have lines of weakness. Along the lines of weakness these materials splits in to forms like flakes, while vegetable drugs have a cellular structure that often leads to long fibrous particles. Thus, the resulting product at particular operating conditions may vary in their size. The energy required to perform this operation may vary.

Moisture Content 

• Moisture content of substances influences a number of properties that can affect size reduction. These properties include hardness, toughness or stickiness etc. In general, for size reduction materials should be dry or wet and not entirely damp. Usually, less than 5 % 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 and small amounts of fines generated have an effect on the operator’s health. To avoid these fines, mills must be enclosed; in addition, exhaust systems should be provided. If possible wet grinding is performed to entirely eliminate the problem. 

Purity Required    

• Some of the size reduction equipments cause wear and tear of the grinding surfaces. Use of these equipments must be avoided whenever high degree of purity of product is needed. Similarly, some of those equipments are so complex that they are unsuitable for cleaning between batches of different materials.

Ratio of Feed Size to Product Ratio

• Machines that produce a fine may need to carry out the size reduction in several stages with different equipments. For example, preliminary crushing followed by coarse grinding and then fine grinding. In such cases feed size is needed to be controlled in order to perform reduction efficiently

Bulk Density 

• The capacities of most batch mills depend on volume. These mills usually demand solid materials by weight rather than volume. The output of the mill is related to the bulk density of the substance. Higher the bulk density more is the product

PRINCIPLES, CONSTRUCTION, WORKING, USES, MERITS AND DEMERITS OF SIZE REDUCTION EQUIPMENTS 

• From the beginning of time, humans have found it necessary to make little pieces out of big ones. It was a slow, laborious process for many centuries. The first breakthrough was a hammer which worked better than ever, in fact, it's still one of the most widely used tools in size reduction. As we know size reduction requires adding energy to a material to make large pieces smaller, the output depends on energy utilized. Different types of size reduction equipment are available and each has its own mechanism of reduction. The right equipment for the task is the one that can add energy most efficiently for the application. There are many different size reduction equipments available to make little pieces out of big ones. Particle size-reduction equipment includes impact crushers and milling machines such as ball mills, hammer mills, pulverizes and grinders. Materials processed fall into broad categories including abrasive, non-abrasive, wet or dry, sticky and friable. Several factors mentioned earlier helps to select the correct equipment for each unique application. These equipments are classified into three classes according to the nature of the forces applied.

1. Class I: The size reduction is accomplished by application of continuous pressure and this class includes equipment for coarse crushing. 
2. Class II: The reduction is affected by blow or impact. An example of impact is breaking of a brittle lumpy material by throwing against a wall when the material breaks into pieces due to sudden release of force.
3. Class III: In the third type shearing forces are applied by grinding or abrasion and this class gives fine grinding. There is no sharp demarcation line between first two classes since some mills use a combination of these forces for size reduction. Thus, a broad classification could be a crushing, impact and grinding mills

•  The materials to be reduced in their size must be thoroughly dried to avoid accumulation in the mill. The necessary care must be employed to avoid possible jamming with wet material and to prevent agglomeration of the particles after size reduction. Crystalline inorganic and organic medicinal compounds which are isolated by normal precipitation and crystallization methods must be thoroughly dried. Vegetable drugs, due to their wide variation in physical state, may require an initial reduction to small pieces. Materials like camphor and spermaceti, the particles of which tend to cohere as quickly as they are produced, need wetting with alcohol before size reduction to avoid this difficulty. On the small-scale initial size reduction of vegetable drugs may be done by slicing, rasping or contusion. Slicing or cutting may be done both transversely and longitudinally so that the tissues may be laid open as completely as possible for quicker drying of the material. Rasping or grating can be done with a nutmeg grater and is mainly used for soaps and waxes that are normally required in coarse state. Bruising is accomplished by beating the drug in heavy motor using pestles whose shape and material of construction vary. They are made of iron, marble, porcelain, glass, steel etc. The bottom surfaces of the mortar and pestle may be shallow or round. Shallow mortars give more grinding effect and are more efficient for size reduction of dry materials and for preparation of fine emulsions.

Hammer Mill

• A hammer mill is an essential machine in the pharmaceutical and food processing industries. It can be used to crush, pulverize, shred, grind and reduce material to suitable sizes. In a hammer mill, swinging hammer heads are attached to a rotor that rotates at high speed inside a hard casing. 

Principle: 

• The working principle of hammer mill is simple to understand. The principle is illustrated in Fig. 2.2(a). It only requires choosing an appropriate motor, crushing hammers/knives and material to be crushed. It operates on the principle of impact between rapidly moving hammers mounted on rotor and the stationary powder bed. The material is crushed and pulverized between the hammers and the casing and remains in the mill until it is fine enough to pass through a sieve which forms the bottom of the casing. Both brittle and fibrous materials can be handled in hammer mills, though with fibrous material, projecting sections on the casing may be used to give a cutting action.

Construction:  

• Hammer mill has five main parts. Fully assembled pharmaceutical hammer mill is showed in Normally, the number of parts may vary depending on the complexity of the machine design. Every part in the hammer mill plays an integral role in the overall working of hammer mills. However, the milling process mainly takes place in the crushing chamber. It consists of a stout steel casing in which a central shaft is enclosed to which four or more swinging hammers are attached. When the shaft is rotated by motor the hammers swing out to a radial position. On the lower part of the casing a sieve of desired size is fitted which can be easily replaced according to the particle size required. The material is crushed and pulverized between the hammers and the casing and remains in the mill until it is fine enough to pass through a sieve. Some mills consist of projecting sections on the casing used to give a cutting action if fibrous materials are to be processed. The hammer mills are available in various size, designs and shapes. In pharmaceutical industry they are used for grinding dry materials, wet filter cakes, ointments and slurries etc.

Working:

• Feeding mechanism refers to the process by which particles enter the crushing chamber. Depending on the design of the hammer in mill machine, it may use either gravity or a metered feeding system. Metered feeding systems are used when product uniformity is a major concern as they eliminate all possible variables that may cause output product inconsistencies. A good example in this case is the pneumatic rotary valve found between the feeding hopper and the crushing chamber. In the gravity feeding system, the milling machines solely depend on the gravitational force that helps to feed particles into the crushing chamber.

• Users can switch ON/OFF the machine from the control box. Operator may control the feeding system or motor speed. Some pharmaceutical milling machines come with a display panel where users can monitor all processes. This mill operates at a high speed that may vary from 2,500 to 60,000 r.p.m. In most cases, hammers are mounted on horizontal shafts where they may rotate either clockwise or anti-clockwise. This may depend on the direction of the rotor rotation. A rotor is the rotating shaft coupled to an electric motor. The hammers come in different styles and shapes. Hammer mill’s crushing tools may be coupled directly to a motor or driven by a belt. As opposed to direct connection, the belts can cushion the motor from shock and allows for accurate speed adjustment. The output of a pharmaceutical hammer mill varies broadly. Normally, the size of the particles depends on the sieve variation. These hammer mills may have over 12 different types of sieves meshes. Pharmaceutical materials that enter the systems are reduced to very small particle due to the rotating hammers.

The basic working steps are as follows:

• Introducing material through the feed hopper: Materials with suitable physical properties that have been cut to the right size are selected. Depending on the design of the hammer mill it will move into the crushing chamber either by gravity or controlled/metered process.

• In the crushing chamber: The ganged hammers or chopping knives hit the material severally. These components rotate at high-speed reducing materials to a desired size. Only particles whose diameter conforms to that of the sieve size passes through sieves. Otherwise, the hammers continue to hit these materials until they are reduced to the required size. Basically, within this chamber, the material is hit by a repeated combination of knives/hammer impact and collision with the wall of the milling chamber. Moreover, collision between particles plays an instrumental role in this size reduction process. It is adviced for not to open the crushing chamber when the machine is operating.

• Outlet of the milling chamber: The outlet of the milling chamber has perforated metal sieves (bar gates). Depending on the size and design of the metal sieve, it allows the required size of particles to pass through while retaining coarse material. The material that passes through is basically the finished product called output. 

Factors determining output and capacity of hammer mill: 

• Basically, there are three aspects that determine the particle size of a hammer mill. These include hammer or cutting knife configuration, shaft speed and sieve size. In most cases reducing large particles into small size may result in a fine or coarse finish. To obtain a fine particles key aspects such as size, a fast rotating rotor speed, small sieve size and large or/and more crushing knives/hammers are preferred. For coarse finished output key aspects are few or/and small crushing hammers/knives, slow rotor speed and large sieve. The overall capacity of hammer mills depends on many critical aspects that include characteristics of materials to be crushed, nature or type of the crushing hammers or knives, number of rows of crushing hammers or knives and feed size. 

Merits:  

• It is rapid in action, and is capable of grinding many different types of materials. 

• They are easy to install and operate, the operation is continuous. 

• There is little contamination of the product with metal abraded from the mill as no surface move against each other. 

• The particle size of the material can be easily controlled by changing the speed of the rotor, hammer type, shape and size of the sieve. 

Demerits: 

• The high speed of operation causes generation of heat that may affect thermolabile materials or drugs containing gum, fat or resin. The mill may be water-cooled to reduce this heat damage.

• The rate of feed must be controlled carefully as the mill may be choked, resulting in decreased efficiency or even damage. 

• Because of the high speed of operation, the hammer mill is susceptible to damage by foreign objects such as stones or metal in the feed. Magnets may be used to remove iron, but the feed must be checked visually for any other contamination.

Uses: 

• Fibrous materials can be handled in hammer mills by cutting edges. 

• Brittle material is best fractured by impact of blunt hammers. 

• It is capable of producing intermediate grades of powders of almost all substances.

• Powdering of barks, leaves, roots, crystals and filter cakes. 

• Useful for granulation where the damp mass is cut in to granules by the hammers.

Ball Mill 

• The general idea behind the ball mill is an ancient one that it was used for grinding flint for pottery. A pharmaceutical ball mill is a type of grinder used to grind and blend materials while manufacturing various dosage forms. The size reduction is done by impact as the balls drop from near the top of the shell. Ball mills are used primarily for single stage fine grinding, regrinding, and as the second stage in two stage grinding circuits. According to the need ball mill can be either for wet or dry designs. Ball mills have been designed in standard sizes of the final products between 0.074 mm and 0.4 mm in diameter.     

Principle:  

• The size reduction in ball mill is a result of fragmentation mechanisms (impact and attrition) as the balls drop from near the top of the shell. Mixing of feed is achieved by the high energy impact of balls. The energy levels of balls are as high as 12 times the gravitational acceleration. Rotation of base plate provides the centrifugal force to the grinding balls and independent rotation of shell to make the balls hit the inner wall of the shell. Since the shell is rotating in alternate (one forward cycle and one reverse cycle) directions a considerable part of grinding take place in addition to homogenous mixing. The operating principle of the ball mill consists of following steps. In a continuously operating ball mill, feed material is fed through the central hole into the drum (shell) and moves there along with grinding media (balls).

• The material to be ground is fed from hopper at a 60° angle and the product is discharged through a 30° angle. As the shell rotates the balls are lifted up on the rising side of the shell and cascade down (or drop down on to the feed) from near the top of the shell. The material grinding occurs during impact of falling grinding balls and abrasion of the particles between the balls. The discharge of ground material is performed through the central hole in the discharge cap (mills with center unloading the milled product) or through the grid (mills with unloading the milled product through the grid). In ball mill depending on the rotational speed following possible modes of the grinding media motion could be achieved.

1. Low speed: Speed mode with a rolling of grinding balls without flight. 
2.  Mixed mode (Cascade mode motion): Speed mode with a partial rolling and a partial flight of grinding balls.
3. High speed: Speed mode with circular motion of balls with no fall.  

• In ball milling the speed of the rotation is more important. At a low speed, the mass of the ball slides or rolls over each other with inefficient output. At a high speed, the balls are thrown out to the walls by centrifugal force. Since at this speed there is absence of any impact or attrition no grinding occurs. Compression by the ball against the shell wall is not enough for comminution. But at 2/3rd of the speed (50 to 80% of the critical speed), the centrifugal speed force just occurs with the result that the balls are carried almost to the top of the mill and then fall to the bottom. By this way the maximum size reduction is effected by the impact of particles between the balls and by attrition between the balls. After the suitable time the material is taken out and passed through a sieve to get powder of the required size. Ball mills are very effective for grinding smooth, aqueous or oily dispersions by wet grinding since it gives particles of 10 microns or less.

Construction:  

• The basic parts of ball mill are a shell, balls and motor ball mill is also known as pebble mill or tumbling mill. It consists of a hollow cylindrical shell (drum) containing balls mounted on a metallic frame such that it can be rotated along its longitudinal axis. The axis of the shell may be either horizontal or at a small angle to the horizontal. It is partially filled with balls. The grinding media is the balls, which may be made of chrome steel, stainless steel or ceramic. The balls which could be of different diameter occupy 30 - 50% of the mill volume and its size depends on the feed and mill size. The large balls tend to break down the coarse feed materials and the smaller balls help to form the fine product by reducing void spaces between the balls. Usually the grinding media balls weight is kept constant. The ball size depends on the feed and the diameter of the mill. The inner surface of the cylindrical shell is usually lined with an abrasion-resistant material such as manganese steel or rubber. Less wear takes place in rubber lined mills. The metallic cylinder which is coated with different materials is helpful in the mechanism of attrition. The length of the mill is approximately equal to or slightly greater than its diameter.

• An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously to 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. High quality ball mills are potentially expensive and can grind mixture particles to as small as 0.0001 mm, enormously increasing surface area and reaction rates.

Factors determining efficiency of ball mill:

The degree of milling in a ball mill is influenced by;

• Residence time of the material in the mill chamber. 
• The size, density and number of the balls. 
• The nature (hardness) of the balls and material to be grinded. 
• Feed rate and feed level in the vessel. 
• Rotation speed of the cylinder. 

Working: 

• Several types of ball mill exist. They differ to an extent in their operating principle. They also differ in their maximum capacity of the milling shell, ranging from 0.010 liters for planetary ball mill, mixer mill or vibration ball mill to several 100 liters for horizontal rolling ball mills. The steps involved in the working process of ball mill are as follows:

1. Initial stage: The powder particles are get flattened by the collision of the balls. It leads it changes in the shapes of individual particles or cluster of particles being impacted repeatedly by the milling balls with high kinetic energy. 
2. Intermediate stage: Significant changes occur in comparison with those in the initial stage. 
3. Final stage: Reduction in particle size takes place. The microstructure of the particle also appears to be more homogenous in microscopic scale than those at the initial and intermediate stages.
4. Completion stage: The powder particles possess an extremely deformed metastable structure.   

Types of ball mill: 

• There are various types of ball mills used for different applications amongst which first two are commonly used in pharmaceutical practice. These includes Pebble ball mill, Vibrating ball mill, Drum ball mills, Jet-mills, Bead-mills, Horizontal rotary ball mills,

1. Pebble ball mill: Pebble mills are sometimes called as jar mill or pot mill which works on the principle of attrition and impact. The grinding is affected by placing the substance in the cylindrical vessel or jar vessels that are lined by the porcelain or other hard substance containing pebbles or balls. The cylindrical vessel revolves horizontally on their long axis and the tumbling of the pebbles over one another and against the sides of the cylinder produce pulverization with a minimum loss of material. 
2. Vibrating ball mill: Vibrating ball mill also works on the principle of attrition and impact. It consists of mill shell containing a charge of balls similar to that of ball mills. In this case the shell vibrates due to some frequency rather than rotated. 

Uses:

• Small capacity ball mills are used for the final grinding of drugs or for grinding suspensions. 

• The high-capacity ball mills are used for milling ores prior to manufacture of pharmaceutical chemicals.

• Ball mills are an efficient tool for grinding many brittle and sticky materials into fine powder.

• The hard and abrasive as well as wet and dry materials can be grinded in the ball mills for pharmaceutical purpose. 

• Powders for ophthalmic and parenteral products can be reduced in size. 

• Ball mill is used for the milling of pigments and insecticides for industrial purpose.

• Ball mills are also used in manufacture of black powder.

• Blending of explosives is an example of an application for rubber balls. 

• For systems with multiple components, ball milling has been shown to be effective in increasing solid-state chemical reactivity. 

• Ball milling has been shown effective for production of amorphous materials

Merits:

• It produces very fine powder (particle size less than or equal to 10 microns).

• It is suitable for milling toxic materials because of its design as a completely enclosed form. 

• It is used in milling highly abrasive materials. 

• Strong adaptability to the fluctuation of the physical property of the materials such as granularity, water content and hardness. 

• Ball mill has a big crushing ratio and high production capacity.  

• It has simple design, ease of examination and change of abraded spare parts.

• Reliable operation, simple maintenance and management.

• It can be used for continuous operation, if sieve or classifier is attached to the mill.

• It is capable of grinding a large variety of materials of different characters and different degree of hardness. (x) It is suitable for wet as well as dry grinding processes.

• The cost of installation, power and grinding medium is low. 

• It is suitable for both batch and continuous operation. 

• Suitable for grinding material with high hardness. 

•  (xiv) The shape of the final products is circular. 

•  (xv) No contamination in the powder with ceramic ball.

•  (xvi) The capacity and fineness can be adjusted by adjusting the diameter of the ball. 

Demerits:

(i) Contamination of product may occur as a result of wear and tear of the balls and partially from the casing. 

 (ii) High machine noise level especially if the hollow cylinder is mode of metal, but much less if rubber is used. 

 (iii) It has relatively long milling time due to low rotary speed and thus has low working efficiency.

 (iv) It is difficult to clean the machine after use. 

 (v) High production cost and high unit electricity consumption. 

 (vi) Heavy equipment so very high one time capital investment. 

 (vii) Some raw materials may become damaged by steel balls.

 (vii) Not suitable for sensitive and flammable substances.

Fluid Energy Mill  

• Fluid energy mill is also known as pulverizer, micronizer or jet mill. It is used for fine grinding and for close particle size control. The reduction of the particles takes place by the attrition and impact mechanism by the air or inert gas introduced through the nozzles presents in the chamber. This mill is mainly used to grind heat sensitive materials to the fine powder.

Principle: 

• It operates on the principle of impact and attrition. The inlet and outlets are attached with classifier which prevents the particles to pass until they become sufficiently fine, It helps in determination of particle size and shape. The speed of air/inert gas is directly related with efficiency. Solids introduced into the stream through inlet result in high degree of turbulence, impact and attritional forces to occur between the particles. This erratic motion between the feed and air result in breakdown of particles. This mill involves no heat generation and hence used to grind heat sensitive materials.

Construction:   

• The main basic parts present in the fluidized energy mill are inlet, nozzles, classifier and hollow toroid (Loop). Through inlet the solid material is introduced into the chamber made of stainless steel. The air or the inert gas is introduced through nozzles into the chamber at the bottom of the loop. The cyclone separator called classifier is attached at the top from which the fine particles are collected. The loop of a pipe has a diameter of 20 to 200 mm depending on the overall height of the loop which may be up to about 2 meters. The high pressure of fluid exerts a high velocity circulation in the loop in a very turbulent manner. A classifier is incorporated in the system, so that particles are retained in loop until sufficiently fine.

• Fluidized energy mills are available in other subclasses. They have no moving parts and are primarily distinguished from one another by the configuration and/or shape of their chambers, nozzles and classifiers. They include tangential jet, loop/oval, opposed jet, opposed jet with dynamic classifiers, fluidized bed, moving target, fixed target and high pressure homogenizers.

Working: 

• The feed introduced in to fluid energy mill is pre-treated to reduce the particles size to the order of 100 meshes. This enables the process to yield a product as small as 5 micrometers or less. Despite this, mills capable of output up to 40 kg/h are also available. Air or inert gas is injected as a high-pressure jet through nozzles at the bottom of the loop.

 

• particle collision and attrition due to particle-wall contact resulting in particle size reduction up to 5 µm. Size-reduction in this mill also depends on the energy supplied by a compressed air that enters the grinding chamber at high speed. The fluidized effect carries particles to a classifier zone where the larger particles are retained until they become sufficiently fine. Fine particles are collected through a classifier. 

Types of fluid energy mills: 

There are two main classes of pulverizers (fluid energy mills)

• (i) Air Swept Pulverizer: In this mill the particles along with air are fed into the mill inlet. Air swept pulverizers uses air to transport particles to the pulverizing section of the apparatus. The beater plates support the hammers and distribute the particles around the periphery of the grinding chamber. The hammers grind the solid against the liner of the grinding chamber. The beater plates rotate between 1600 and 7000 rpm to reduce the size of the incoming particles. The classifier plate separates the fine product and exit through the discharge outlet. The larger material is back feed to the mill inlet through the recycle housing

• (ii) Air Impact Pulverizer: In air impact pulverizers superheated steam or compressed air produces the force that reduces the size of large particles. It results in the smashing of the particles into smaller particles. This pulverizer uses high speed air to pulverize the particles.  

The products from both air swept and air impact pulverizers produces particles which do not require further sieving or classifying. 

Factors determining efficiency of fluid energy mills:

1. The speed of air/inert gas. 
2. Feed rate and size.
3. The configuration of the mill. 
4. Design of the classifier. 
5. The position of the nozzle. 
6. The impact between the feed and air.  

Uses:

• Fluid energy mill is used when fine powders are required, for example, antibiotics, sulphonamides and vitamins. 

• Suitable for laboratories where small samples are needed. 

• The mill is used to grind heat sensitive material to fine powder.

• The major advantage is fine grinding of pigments, kaolin, zircon, titanium and calcium, alumina, ceramic frit, powder insecticides such as DDT, diatomaceous earth, feldspar, fluorspar, graphite, gypsum, iron ore, iron oxide, iron powder, limestone, polymers, rare earth ores carbon, talc etc. 

• It is the choice of mill when higher degree of drug purity is required

Merits:

• The particle size of the product is smaller than produced by any other method. 

• Expansion of gases at the nozzles leads to cooling, counteracting the frictional heat thus protecting heat-sensitive materials. 

• There is little or no abrasion of the mill and so no contamination of the product.

• To protect sensitive drugs from oxidative degradation this mill has facility to use inert gases.

• Presence of classifier permits control of particle size and particle size distribution. 

• Suitable for size reduction of materials capable of generating a static charge. 

• The process is suitable for friable, abrasive or crystalline materials. 

• Air needed is freely available. (ix) Homogeneous blend of large range of sizes available. 

• The equipment is easily sterilized. 

• At the end of milling product particle size between 2 and 10 µm is obtained.

Demerits: 

• This mill is energy consuming and the energy consumed per ton of milled product is high.

• High head space is required.

• Coarse feed size is not suitable.

•  The fed device may be clogged with the clump materials.

• Special feeding devices should be provided for the feeding of the materials. 

• The use of compressed air leads to generation of static electricity. 

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

• Tendency of forming aggregates or agglomerates after milling. 

• Generation of amorphous content due to high energy impact.

• Formation of unwanted ultra-fine particles.

Edge Runner Mill  

• The edge runner mill is also called as roller stone mill and is mechanized form of mortar and pestle-type compression comminution. It crushes the materials into fine powders by the rotating stones. The edge runner mill consists of two large rotating grinding wheels or stones turning slowly in a large bowl. In this mill material can be crushed or ground in a continuous operation.

Principle: 

• The edge runner mill mainly works on the attrition and impaction by which the crushing or grinding of the powder material takes place. The principle of size reduction by this mill are crushing due to heavy weight of the stones or metal and shearing force. Movement of stones     or metal causes size reduction. The Edge-runner mill has the pastel equivalent mounted horizontally and rotating against a bed of powders.

Construction:

• The basic parts of this mill consist of two heavy rollers and a bed made of stones or granite which is used for the grinding of the materials. The rollers are mounted on the central horizontal shaft and move around the bed in a shallow circular pan. 

Working: 

• The material to be milled is fed into the centre of the pan and is worked outwards by the action of the wheels. Scrapers are employed in scraping the material constantly from the bottom of the wheel vessel after which it is fed to the wheel where it gets crushed to powders. The material is ground for a definite period and then it is passed through the sieves to get the powder of the required size. An electric motor in the basement provides power for the mill to start the device as used in flour mills. This mill is used for crushing or grinding various materials without the danger of clogging or jamming of the machine by accumulation of the ore with greater efficiency and in high comparative quantities. The crushed powder is discharged through outlet at the bottom.

 

Uses:

• This mill is used for grinding most of the drugs to very fine powder.

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

• It is most preferred for size reduction of non-sticky materials

Merits: 

• Very fine particle sized materials can be obtained by this edge runner mill. 

• It has simple design. 

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

• The combinations of various elements in this machine makes it to operate with greater efficiency.

• Utilizes less power, and does not require frequent clean-up and particular adjustment. 

Demerits:  

• It requires more floor space than the other commonly used mills. 

• It is not suitable for sticky materials. 

• This mill produce lot of noise pollution.

End Runner Mill 

• End-runner mill similar to edge runner mill is a mechanized form of mortar and pestle type size reduction equipment. A heavy weight pestle rotates due to the friction of material present between mortar and pestle upon rotation of mortar driven by motor at the base.

Principle:  

• The principle of size reduction applied in these mills are crushing due to heavy weight of the stones or metal pestle and shearing force as a result of movement of these stones or metal. In the end-runner mill, a weighted pestle is turned by the friction of material passing beneath it as the mortar rotates under powder to be processed. 

Construction: 

• End-runner mills are the mechanized forms of mortar and pestle-type compression comminution. This milling equipment consists of pestle made of either stone or metal, connected by a shaft, Fig. 2.7. The pastel rotates at its axis in a shallow steel or porcelain mortar. The pestle is mostly dumb-bell shaped. The mortar is fixed to a flanged plate at the bottom. It also consists of scrappers at the centre and alongside of the circular pan. The pestle is mounted horizontally and rotating against a bed of powders. 

Working:

• The material to be milled is fed into the centre of the circular mechanical mortar (pan) and is worked outwards by the action of the wheels and mill is operated. The pestle rotates against a bed of powders. Mortar revolves at high speed and causes the pestle to revolve. Scrapers are employed in scraping the material constantly from the bottom of the wheel and are feed back to the wheel were it gets crushed further. Finally, pestle is raised from the mortar manually or automatically to facilitate emptying and cleaning.

Uses:  

• It is used to reduce fibrous crude drugs to a fine size.

• It used for grinding semisolid preparations such as ointments and pastes to fine size.

• It is used for uniform distribution of the contents in viscous dispersion medium. 

• It can be used for both wet and dry grinding of crude drugs.

Merits:  

• It has simple design and thus cleaning and maintenance is easy.

• It utilizes less electrical power.

• It produces fine and sometimes very fine particles. 

• Requires less attention during the milling operation.

• It has no problem of chocking or clogging as it has no sieves for size separation.

Demerits: 

• It runs only on batch operation. 

• It is not suitable for milling sticky materials. 

• Unsuitable for drugs which are hard and unbroken or in slightly broken condition. 

• Machine noise causes lot of noise pollution. 

• It requires scrapper adjustment intermittently.