Learn about the mechanism of size separation, including sieving, air classification, centrifugal separation, and gravity separation with examples and applications in pharmaceuticals.

Size separation, also known as classification or screening, is a crucial process in pharmaceutical manufacturing and industrial applications. It helps in separating particles based on their size using different techniques like sieving, air classification, centrifugal separation, and gravity separation. This article explains the mechanisms of size separation, their working principles, and applications.

Sieving Mechanism

Sieving is the most common size separation method using a sieve or screen with uniform openings.

Steps in Sieving

1. Feeding – The mixture of particles is placed onto the sieve.
2. Vibration – The sieve is shaken to allow smaller particles to pass through.
3. Separation – Large particles remain on the sieve, while smaller ones pass through.
4. Collection – The separated fractions are collected accordingly.

Factors Affecting Sieving

Particle Size – Smaller particles pass easily.

Sieve Opening Size – Controls the separation efficiency.

Vibration Intensity – Improves efficiency.

Moisture Content – Wet particles may clog the sieve. Air Classification Mechanism

This method separates particles based on size and density using air flow. It is useful for fine particles that cannot be sieved efficiently.

Steps in Air Classification

1. Feeding – Particles are introduced into the air stream.
2. Acceleration – Air velocity pushes particles forward.
3. Separation – Lighter particles are carried away, while heavier ones settle.
4. Collection – Separated fractions are collected in different chambers.

Factors Affecting Air Classification

Air Velocity – Determines particle movement.

Particle Density – Heavier particles settle faster.

Particle Shape – Spherical particles move easily in the air stream.

Centrifugal Separation Mechanism

This method uses centrifugal force to separate particles by size and density. It is commonly used in cyclone separators and centrifugal sifters.

Steps in Centrifugal Separation

1. Feeding – Particles enter a rotating chamber.
2. Rotation – Centrifugal force acts on particles.
3. Separation – Larger particles move outward, smaller ones stay in the center.
4. Collection – Different fractions are collected separately.

Factors Affecting Centrifugal Separation

Rotational Speed – Higher speed improves separation.

Particle Size & Density – Larger particles experience more force.

Chamber Design – Affects separation efficiency.

Gravity Separation Mechanism

This method relies on natural settling of particles due to gravity. It is mainly used for coarse particles.

Steps in Gravity Separation

1. Feeding – Particles enter a settling chamber.
2. Settling – Heavier particles settle faster, while lighter ones remain suspended.
3. Collection – Heavy particles collect at the bottom, while lighter ones are removed separately.

Factors Affecting Gravity Separation

Particle Size & Density – Larger, denser particles settle first.

Settling Time – More time leads to better separation.

Fluid Viscosity – High viscosity slows the process.

Applications of Size Separation

● Pharmaceutical Industry – Tablet manufacturing, powder processing.
● Food Industry – Flour milling, spice processing.
● Mining & Metallurgy – Ore classification, sand separation.
● Chemical Industry – Polymer processing, pigment classification.

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