6. Unit Operations in Pharmacy: Types, Principles, and Applications

UNIT OPERATIONS

Unit operations are basic physical steps used in pharmaceutical and chemical processes. They involve physical changes rather than chemical reactions and are widely applied in manufacturing, processing, and formulation. Common examples include evaporation, filtration, crystallisation, drying, mixing, size reduction, and separation. In pharmacy, unit operations are important because they help convert raw materials into safe, effective, stable, and usable dosage forms. A strong understanding of these operations is essential for students of pharmacy, especially in industrial pharmacy and pharmaceutical engineering.

Unlike unit processes, which involve chemical reactions such as bromination, sulphonation, or halogenation, unit operations focus on physical treatment of materials. This distinction is important because many pharmaceutical steps depend on mechanical or thermal processing rather than chemical transformation. For example, reducing the particle size of a drug, drying a wet mass, or filtering a solution are all unit operations. These steps affect product quality, stability, appearance, dissolution rate, and manufacturing efficiency.

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CLASSIFICATION OF UNIT OPERATIONS

1. Material Handling and Transportation / Fluid Flow Process

This group includes operations in which fluids or materials are transferred from one place to another. It is an essential part of plant operation because raw materials, semi-finished products, and finished products must move efficiently through different stages of production. Proper fluid flow also helps maintain process control and reduces waste.

• Pumping.
• Compression.
• Fluidisation.

2. Mechanical Unit Operations

Mechanical unit operations mainly deal with the physical transformation of solids. These operations are widely used in pharmacy for preparing powders, granules, and uniform mixtures. They influence particle size, surface area, flow properties, and compressibility, all of which are important in dosage form development.

• Size reduction.
• Size enlargement.
• Mixing.
• Agitation.
• Blending.

3. Mass Transfer Operations

Mass transfer operations involve movement of materials from one phase to another. They are used when a component needs to be separated, concentrated, absorbed, extracted, or transferred between liquid, gas, or solid phases. These operations are common in the purification and processing of pharmaceutical substances.

• Evaporation.
• Distillation.
• Absorption.
• Extraction.
• Leaching.

4. Heat Transfer Operations

Heat transfer operations are used whenever heat must be added, removed, or distributed through a material. These operations are essential in drying, evaporation, sterilisation, and many other pharmaceutical processes. The rate of heat transfer strongly affects energy use and product quality.

• Conduction.
• Convection.
• Radiation.

SIZE REDUCTION

Size reduction is the process of reducing the particle size of a substance to a finer state of subdivision. It may also be called comminution, grinding, or milling. In pharmaceutical practice, size reduction is one of the most useful mechanical operations because it improves surface area, mixing, extraction, dissolution, and product uniformity. It is commonly applied to crude drugs, powders, granules, and many bulk solids before they are used in formulations.

The extent of size reduction depends on the nature of the material and the desired final product. Some substances are easy to crush, while others resist breakage because of toughness, elasticity, moisture, or stickiness. The choice of equipment therefore depends on particle size, hardness, and product requirements. In pharmaceutical manufacturing, improper size reduction can lead to poor flow, non-uniform blending, and reduced therapeutic performance.

Objectives of Size Reduction

• To increase the surface area of the drug.
• To enhance solvent penetration capacity.
• To increase therapeutic efficacy.
• To facilitate the mixing process and yield a uniform product.
• To increase the stability of emulsions.
• To improve the physical appearance of ointments, pastes, and creams.
• To ensure stability of suspensions.
• To minimise irritation caused by coarse particles in ophthalmic preparations.
• To increase the rate of drying of wet masses during milling.
• To reduce the bulkiness of drugs.

Factors Affecting Size Reduction

• Toughness: Crude drugs with high water content or fibrous nature are more difficult to reduce than hard and brittle substances.
• Hardness: Soft materials are easier to reduce than hard materials.
• Stickiness: Resinous or gummy materials stick to grinding and sieve surfaces, making the process difficult.
• Moisture content: Moisture changes the physical properties of the material and may increase stickiness or toughness.
• Softening temperature: Wax-like materials may soften due to heat generated during milling.
• Purity required: Wear and tear of grinding surfaces may contaminate the powder if very high purity is needed.
• Material structure: Vegetable drugs with cellular structure often produce fibrous particles.
• Physiological effect: Potent drugs should be processed in closed mills to avoid dust exposure.
• Ratio of feed size to product size: Smaller feed size is needed for fine output.
• Bulk density: Influences the amount of material handled during the process.

Methods / Mechanism of Size Reduction

• Cutting: Material is cut with sharp blades, as in a cutter mill.
• Compression: Pressure is applied to crush material into smaller particles, as in a roller mill.
• Impact: A fast-moving object strikes stationary material or moving particles strike a surface.
• Attrition: Relative movement between surfaces produces shear forces and gradual reduction, as in a fluid energy mill.

EQUIPMENT FOR SIZE REDUCTION

Several mills are used in the pharmaceutical industry for reducing particle size. The most common are hammer mill, ball mill, fluid energy mill, and disintegrator. Among these, hammer mill and ball mill are widely discussed because they are common, practical, and easy to understand. Each machine works on a different principle and is selected according to the nature of the material and the required fineness.

Hammer Mill

Principle

The hammer mill works on the principle of impact. In this method, a high-speed moving object strikes a stationary substance and breaks it into smaller particles. The repeated impact produces pulverisation or grinding. This makes the hammer mill suitable for brittle and dry materials that need rapid size reduction.

Construction

• It may have either a horizontal or vertical shaft.
• Hammers are made of hardened or stainless steel.
• The impact surface is made of wear-resistant materials such as haystellite and carboloy.
• Hammer shapes include stirrup and bar type; bar-shaped hammers are preferred for granulating tablets.
• Hammer edges may be flat or sharp, or both.
• Hammers may be rigid or swing type.
• The chamber contains a removable screen or grid made of perforated metal sheets.

Working

The material to be reduced is fed into the hopper connected to the drum. The rapidly rotating hammers strike the particles and break them into smaller pieces. The powder is collected below the screen, which allows only particles of suitable size to pass. Because the hammers are not fixed, the machine can operate continuously with a reduced chance of choking. The resulting product is usually coarse to moderately fine.

Applications

• Used in wet or dry granulation.
• Grinding pharmaceutical raw materials, herbal medicines, and sugar.
• Making powders of barks, leaves, and roots having medicinal properties.
• Milling active pharmaceutical ingredients and excipients.

Ball Mill

Principle

The ball mill works on the principle of impact and attrition. Size reduction takes place when the balls drop from the near top of a rotating hollow metal cylinder. The falling balls crush the material by impact, while the movement of balls against each other and against the cylinder produces attrition. This combination makes the ball mill suitable for fine grinding.

Construction

• Hollow cylinder: Made of metal with chromium lining and mounted on a frame for rotation around its longitudinal axis. About 30 to 50 percent of the volume is occupied by steel balls.
• Balls: Made of metal coated with chromium, or sometimes lined with rubber or porcelain. Their weight remains constant, and their size depends on the mill diameter and the feed material.

Working

• The cylinder is filled with the drug substance and rotated.
• The speed of rotation determines the action of the balls.
• At low speed: Balls roll and slide over each other, producing very little size reduction.
• At high speed: Centrifugal force pushes the balls toward the walls, so grinding does not occur effectively.
• At correct speed: Balls rise and then fall, producing cascading action and maximum size reduction by impact and attrition.
• After the desired time, the material is removed and passed through a suitable sieve to obtain the required particle size.

Applications

• Used for regrinding operations.
• Widely used in cement, silicate products, building materials, fire-proof materials, chemical fertilisers, black and non-ferrous metals, glass, and ceramics.
• Can grind ores and other materials by wet or dry process.

SUMMARY TABLE: SIZE REDUCTION METHODS

Method	Description	Example Equipment
Cutting	Material is cut with sharp blades	Cutter mill
Compression	Pressure is applied to crush material	Roller mill
Impact	High-speed object strikes stationary material	Hammer mill
Attrition	Shear forces from relative movement of surfaces	Fluid energy mill

COMPARISON: HAMMER MILL vs BALL MILL

Feature	Hammer Mill	Ball Mill
Principle	Impact	Impact and attrition
Speed effect	High-speed rotating hammers	Correct cascading speed required
Product particle size	Coarse to moderately fine	Fine to superfine
Operation	Continuous, less choking	Batch or continuous

Overall, unit operations form the backbone of pharmaceutical manufacturing. Among them, size reduction is one of the most important because it improves the physical characteristics of solid materials and helps in the successful preparation of many dosage forms. Both hammer mill and ball mill are standard examples that show how different mechanical principles are used to obtain the desired particle size. A clear understanding of these operations helps pharmacy students connect theory with industrial practice and understand how medicines are prepared on a large scale.