Size Reduction: Objectives, Mechanisms & Laws Governing Size Reduction and Factors Affecting Size Reduction

Objectives of Size Reduction

• A particle's size can be decreased by reducing its surface area, resulting in smaller particles and increasing solution rates.
• Increasing the rate of extraction: Size reduction directly affects the rate of extraction. Particles with smaller sizes penetrate menstruum more readily, speeding up the extraction process.
• For many pharmaceutical dosage forms, effective mixing requires smaller particle sizes.
• Increasing rate of absorption is associated with decreasing particle size. Therefore, reducing the size of a medication helps to increase bioavailability, such as in the case of Griseofulvin.
• By reducing particle size, you can increase drying speed and increase effective surface area.
• Filtration is possible when particles are separated according to their sizes.
• Dispersions and emulsions that contain smaller particles have greater stability.

Mechanisms of Size Reduction

Impact

When hit by moving platforms or materials at high speeds, the material is reduced in size.

Cutting

As the material comes into contact with the sharp edges of the blades, the material shrinks.

Compression

When a stationary platform is in contact with a moving heavy platform, the material is crushed to reduce its size.

Attrition

Pressure and shear force come together to reduce the size of the material when it comes between two moving platforms.

Laws Governing Size Reduction

Rittinger

It was Rittinger's belief that the amounts of energy required for size reduction were directly correlated with surface area rather than length dimensions.

K=KR fc, n= -2


where KR is known as Rittinger's constant, and integration yields:


The specific surface (surface area per unit mass) of particles is proportional to 1/L, so equation (2) says that lowering L for a mass of particles from 10 cm to 5 cm requires the same amount of energy as lowering that mass to 4.7 mm for a mass of 5 mm particles.

Kick

Kick: dL/L is supposed to represent the size reduction ratio, which determines the energy required to reduce a material's size.

K=KkfC, n= -1

KK stands for Kick's constant, while fc stands for the material's crushing strength.

On integration it gives,

Bond

Bond has proposed an intermediate path, in which he hypothesizes that n = -3/2, which leads to:



Microns are used to measure L.

A Work Index (Ei) represents the amount of energy needed to reduce the mass of a material to a particle size of 100 micrometers from an arbitrarily large particle size.

The reduction ratio is denoted by q, where q = L1/L2.

Factors Affecting Size Reduction

Toughness

It is sometimes more important to have tough materials than hard ones. A soft but robust material may provide greater issues in terms of size reduction than a hard but brittle one. For example, breaking a rubber is harder than breaking a blackboard chalk stick.

Stickiness

When a substance is sticky, it is difficult to reduce its size because it adheres to the surfaces of the grinder or it may clog the screen meshes.

Hardness

It is a material surface attribute. The hardness of a substance is assessed using a contrivance known as the Moh's Scale, which ranges from 1 to 10. Materials with harnesses ranging from 1 to 3 are considered soft (for example, talc and waxes), materials with harnesses ranging from 4 to 7 are considered intermediate (for example, limestone and bauxite), and materials with harnesses ranging from 8 to 10 are considered hard (Quartz and diamond). The more difficult it is to reduce the size of a substance, the harder it is to shrink its size.

Abrasiveness

Abrasiveness is a hard material quality. During the grinding of abrasive substances, the resultant powder may contain more than 0.1 percent of the worn metal from the grinding mill.

Slipperiness

It is the inverse of stickiness. It can also reduce the efficiency of grinding surfaces by acting as a lubricant and reducing grindability.

Moisture content

Size reduction will depend on a number of qualities, such as hardness, toughness, and viscosity, depending on the moisture content. In general, the materials must be dry or moist, not just damp. If the product is to be ground dry, less than 5% moisture is usually sufficient, but more than 50% moisture is required for wet milling.

Softening temperature

Occasionally, heat is created during the size reduction process, which can cause some material to soften, and the temperature at which this occurs may be significant. Waxy compounds such as stearic acid, as well as medications containing oils or fats, are examples of chemicals that may be impacted. A water jacket or air stream blowing through machinery may be able to mitigate this problem.

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