Particle-Size-Reduction PDF

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Particle-Size-Reduction summary...

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Particle Size Reduction

Inhaled particles can need to be micronized (0.1-20microns). Particle size affects flow of material.        

Nanoparticles, molecularised, fine, intermediates. Comminution – particle size reduction Milling, we don’t get one size we get a particle size distribution. Ph. Eur and BP describe particle size based on sieve analysis. Cutting, chopping, crushing, grinding, milling, micronizing and trituration. Only 1-2% of energy causes particles to reduce in size. Mechanical properties relevant ; elastic, plastic deformation Deformation of initial cracks, deformation of machinery parts (if powder is very hard for example. Knives can become blunt), interparticle friction. Static energy. Heat, sound, vibration. Particle-machine wall friction.

Mills generally have an inlet and an outlet. Cutter mill. Screen is like a sieve/mesh which keeps particles of a certain size from passing through. The material is discharged through the screen when particle size is sufficiently small. Cutter mill, stationary knives & rotating knives. Clearance so that particles can get reduced in size. Can also mill by roller mill. Hammer mill. Material fed in through a screw (or by gravity). Hammers attached to central shaft, material is impacted by hammers. Most particles undergo brittle fracture & get fractured less with each hit. Stress is high due to the angular velocity of the hammers rotating. Cone mill eg Quadro Co-mill Mesh is around sides as a cone. Feed material in. Impeller rotates and when particles are small enough they pass through the mesh. The screen can be smooth or ‘cheese grate’. The thicker the screen, the smaller the particle. Changing between thickness of screens is important. Angle that particle travels. Fluid energy mill – uses friction between particles & energy from air to cause particles to break. Solids in airstream, introduce turbulence by fluid inlet jets. Particles moving in random manner, particle-particle interactions at high speed. Particles travel around and the centrifugal forces act on them causing the heavier particles to move to the edge and the smaller particles to stay on the inside, and they get removed through a classifier which removes fine particles & fluid. Pin mill – impact & attrition method. Type of mill you pick depends on material. Material with low melting point – Milling can increase heat. We will need a cooling element in mill – dry ice, or cool outside of mill. Hardness to mill – bond strength -elastic, ductile, plastic? Sticky? Agglomerate? Is it flammable? We have a lot of energy, fine powder, sparks in milling. Explosion risk! Remove oxygen (fuel) and use an inert gas such as nitrogen.

The shape, roughness, particle size & particle size distribution of the milled material also affects the equipment chosen....