Size Reduction

Introduction to Size Reduction

  • Particle Size Reduction:
    • Also known as comminution, essential in industries like mining, pharmaceuticals, and materials processing.
    • Objectives include:
    • Achieving desired particle sizes.
    • Improving chemical reactions.
    • Enhancing material recovery.
    • Facilitating better handling.
  • Energy Consumption:
    • Size reduction is energy-intensive, utilizing 5% of all electricity generated.
    • Current practices achieve less than 1% efficiency.
  • Key concepts include understanding fracture mechanisms, energy requirements, and comminution equipment selection.

What is Size Reduction?

  • Definition: The process of breaking down solid materials into smaller particles.
  • Importance:
    • Create specific particle sizes and shapes.
    • Increase surface area for reactions.
    • Liberate valuable minerals.

Energy Inefficiency in Size Reduction

  • Methods: Size reduction can be achieved through:
    • Mechanical grinding, crushing, cutting, or milling.
    • Significant energy consumption involved.

Particle Fracture Mechanisms

  • Understanding Lattices:
    • Sodium chloride lattice consists of sodium ions (Na⁺) and chloride ions (Cl⁻).
    • The forces at play:
    • Attractive Forces: Pull ions towards each other.
    • Repulsive Forces: Prevent ions from coming too close, maintaining crystal stability.
  • Interatomic Distances:
    • Interatomic distance plays a critical role in compression and tension within atomic structures.

Applying Hooke's Law

  • Hooke's Law: States strain is directly proportional to applied stress.
  • Young's Modulus: Defines the relationship between stress and strain.
  • Elastic vs. Plastic Deformation:
    • Elastic: Deformation is temporary, material returns to original shape.
    • Plastic: Permanent deformation once threshold exceeds yield stress.

Theoretical Approaches to Fracture

  • Material Strength/Yield Stress: Strength dictated by attractive and repulsive forces between ions.
  • Fracture Mechanisms:
    • Overestimated Strength: Assumes all bonds break simultaneously.
    • Underestimated Strength: Considers only bonds about to break.

Strain Energy

  • Definition: Energy stored due to deformation under tension.
  • Guided by the area under the stress-strain graph.

Stress Concentration Factors

  • The concentration factor compares maximum stress in a structure to nominal stress.
  • Importance of Irregularities: Microscopic cracks and dislocations play a role in particle breakage.

Crack Propagation Criteria

  • Factors influencing crack spread:
    1. Strain energy released must surpass surface energy created.
    2. Availability of crack propagation mechanisms.

Energy & Power Requirements in Size Reduction

  • Postulates of Energy Requirements:
    • Rittinger’s Law: Energy proportional to area of new surface created.
    • Kick’s Law: Energy proportional to volume ratio of feed to product particle.
    • Bond’s Law: Energy proportional to the inverse square root of particle size.

Operations in Size Reduction

  1. Crushing:
    • E.g., jaw crushers, gyratory crushers, cone crushers.
  2. Grinding:
    • E.g., ball mills, rod mills, hammer mills.
  3. Cutting:
    • Sharp knife cuts large materials into smaller pieces.
  4. Attrition:
  5. Impact:
  6. Compression: Uses force to crush materials.

Factors Affecting Size Reduction Method Selection

  • Size of Feed & Product: Critical to choosing the appropriate mill.
  • Material Properties: Hardness, abrasiveness, toughness, and cohesivity affect processing.

Carrier Medium

  • The medium can be a gas or liquid influencing particle behavior in mills.
  • Common Fluids: Air for dry grinding, water or oil for wet grinding.

Modes of Operation

  • Batch vs. Continuous:
    • Efficiency, throughput, and economic considerations guide the choice between modes.

Milling Types

  1. Open Circuit: Single pass through the mill.
  2. Closed Circuit: Product subjected to classification with oversize returning to the mill.