Size Reduction: Objectives, Mechanisms & Laws governing size reduction, factors affecting size reduction, principles, construction, working, uses, merits, and demerits of Hammer mill, ball mill, fluid energy mill, Edge runner mill & End runner mill.

Size Reduction (Pulverization, Comminution, Diminution, Milling, Grinding)

• Definition: Size reduction is a mechanical unit operation to change large solid mass into smaller unit mass, coarse and fine particle size.

• The example size reduction process is divided into two categories. These are the size reduction of solid masses and the size reduction of liquids. If the drug is in solid form then the process is called Pulverization, Comminution, Diminution, Milling, or Grinding. If the drug is in liquid form then the process is called emulsification or atomization.

• Objectives:

• The main aim of size reduction is to increase the surface area of drug particles or liquid globules.

  • 1. To increase surface area

  • 2. Extraction: To increase the rate of extraction of active constituents from crude drugs,

  • 3. Drying: To increase the rate of drying

  • 4. Mixing: To form a uniform mixture

  • 5. To decrease segregation of particles after uniform mixing

  • 6. Dissolution: To increase the rate of dissolution, except for hydrophobic drugs

  • 7. Drug Absorption: To increase the rate of drug absorption

  • 8. To increase the therapeutic effectiveness of the drug.

  • 9. Physical Appearance: To improve the physical appearance of the dosage form.

  • 10. Powder: To make mixing of powder easy and homogeneous.

  • 11. Suspension: To reduce the rate of sedimentation in suspension.

  • 12. Emulsion: To improve the stability of emulsion.

  • 13. Semisolid Dosage form: To make semisolid preparations for external use free from grittiness.

• Disadvantages of size reduction

  • 1. Drug degradation: An increase in surface area promotes chemical reactions such as hydroxylation and oxidation. The heat generated during mechanical size reduction further promotes the decomposition of drugs. Thermo-labeled drugs are more sensitive to degradation during milling.

  • 2. Poor mixing: Decreases in size reduction increases the kinetic energy of the molecules. This increases the cohesive property of the fine particles. Fine articles tend to form aggregate. This hinders the uniform mixing during pharmaceutical processes.

  • 3. Poor wettability: An increase in surface area promotes the adsorption of air. This inhibits the wettability of drug molecules during pharmaceutical processes.

  • 4. Contamination: Particles from the grinding surface may enter into powder during grinding/milling.

• Mechanism (Principles) of Size Reduction:

• A. Mechanical methods.

• There are four mechanisms of size reduction for the mechanical method

  • a. Cutting: Material size is changed into smaller pieces by using instruments with sharp edges such as sharp blades, scissors, sharp Knife, and root cutters. On a large scale, a cutter mill is used. etc. It is used for the fibrous and waxy solids.

  • b. Compression: Two opposing rigid forces apply pressure to crush the solid particles into smaller sizes, such as jaw crusher, roller, etc. It is mainly used for brittle materials such as rock, coal, coarse particles larger than 3cm, etc. Example roller mill.

  • c. Impact: One moving object at high speed hits another stationary object producing impact. This impact breaks either a moving object or a stationary object. On a small scale: Hammer and large-scale hammer mill, disintegrator, and fluid energy mill.

  • d. Attrition: It utilizes compression and friction to break particles into smaller sizes. Application of pressure by two surfaces on material causes compression and the generation of shear force by moving these surfaces produces shear force to generate friction. On a small scale: mortar and pestle. On a large scale: roller mill, ball mill, fluid energy mill.

• Amorphous or crystalline solid particles have flaws lines. Flaw lines are weak areas of the solid particles. Application of force converts flaws lines into cracks. This leads to breaking solid particles into smaller particles.

• B. Precipitation method: The substance is dissolved in a solvent. Dissolved solute is precipitated by the addition of another solvent into the solution. In this case, both solvents are miscible but solute is soluble in one solvent and insoluble in another solvent. Inorganic chemicals such as calcium carbonate and magnesium carbonate particle size can be reduced by precipitation.

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• Factors affecting size reduction

• 1. Hardness: The hardness of the substances affects the size reduction process. Harder materials require more force to break. It is more difficult to break hard materials than soft materials.

• 2. Toughness: Toughness is the ability of materials to absorb energy before their breaking. Fibrous materials have moisture and are tougher in nature. It is more difficult to reduce the size of soft but tough materials than hard but brittle materials. For example rubber is soft but tough. Glass is hard but brittle.

• 3. Stickiness: Sticky materials adhere to each other; adhere with the grinding surface and sieves. This inhibits the grinding process and clogs the mills. It is difficult to grind resinous and gummy materials. The heat generated during milling creates more problems in grinding sticky materials. Complete dryness of materials helps to grind materials.

• 4. Slipperiness: Slippery materials slide through the mill during size reduction without their proper grinding.

• 5. Moisture content: Too much moisture makes the materials clumpy. Too little moisture affects the flow property of materials during grinding. Moisture contents affect several properties of materials such as hardness, toughness, and stickiness. Ideal moisture content depends upon the nature of the materials and the product desired. 5% moisture in dry grinding and 50% moisture in wet grinding are suitable for size reduction.

• 6. Temperature: The size reduction process generates heat. Waxy materials become soft on temperature rise.

• 7. Abrasiveness: Hard and gritty materials damage the mill during size reduction. This affects the efficiency of the mill and the quality of the product.

• 8. Physiological effect: Mill generates dust during size reduction. Dust may affect the health of the operator especially if it is a potent drug. The enclosed mill is used to avoid this problem

• 9. Feed size/Feed rate: Feed size affects the size of product particles. The feed size depends upon the capacity of the mill. Large feed size decreases the efficiency of mill