UNIT OPERATIONS _ UNIT PROCESSES2 PDF

Hammer Mill [Also called comminuting mill or impact mill] Disc rotating at high speeds (up to 8000rpm) - fitted with a balanced number of hammers. direct impact Sieve mesh Approx. 40% of energy input Cooling jacket...

Hammer Mill [Also called comminuting mill or impact mill] Disc rotating at high speeds (up to 8000rpm) - fitted with a balanced number of hammers. direct impact Sieve mesh Approx. 40% of energy input Cooling jacket will produce new surface area – majority manifests as heat Cyclone separator 8 Cyclone separator – Reverse-flow cyclone – Particles in air or liquid suspension are introduced tangentially into the cylindrical upper section where high fluid velocity creates a vortex which throws solid particles out onto the walls – Particles are forced down the conical section under the influence of fluid flow – At the tip of the conical section the vortex of fluid is above the critical velocity at which it can escape through a narrow outlet and so it forms an inner vortex which travels back up the cyclone and out through a central outlet, “vortex finder” – Coarser particles separate out from the fluid stream and fall out of the cyclone (at the bottom) through the product outlet (“underflow outlet”) – Smaller particles (fines) remain entrained in the fluid stream and leave through the vortex finder (“overflow outlet”) – Could use series of cyclones with different flow rates or dimensions 9 1  Hammer Mill Hammer Mill, opened to front Screen inserts 10 Hammer heads - flat, knife or file-edged face Can produce v. fine powder (approx. 100 micron) and good control over particle size produced Very rapid process Continuous process Mixing, wet-grinding and ointment milling may also be performed Cutting Mill Swing Hammer Mill 11 2  “Comil” The feed material is introduced into the conical screen chamber and the rotating impeller imparts a vortex flow pattern to it. The material is then forced to the screen surface by centrifugal acceleration. The material is size reduced and instantaneously discharged through the screen openings. Impact and shear stress The finished product is discharged at the bottom of the milling chamber. Particle size can be optimised by screen, impeller and/or speed selection 12 Rotor Mill Rotor with a series of hardened steel blades Rotates at high speeds (>15,000 rpm) within a circumferential sieve ring Size reduction to < 100 micron 13 3  Pin Mill 2 horizontal steel plates fitted with concentric rings of pins on opposing faces. Stress: impact and attrition Fineness of product may be altered by using different pin arrangements Milling temperatures are low : good for thermolabile materials Soft, non-abrasive powders 14 Ball Mill Rotating hollow cylinder half-filled with balls of stainless steel, porcelain, flint Cylinder : stainless steel, chrome or porcelain Stress: impact and attrition 15 4  Low speeds : "cascading" - attrition predominates Higher speeds : "cataracting" - impact more important ✓ ✓ x 16 Alter size of product by changing ball size, mill speed and diameter Wet grinding Can totally enclose mill for sterile/aseptic grinding Fill with inert gas - oxidisable material Milling times can be long 17 5  Modifications on the simple ball mill, e.g. : 1. Conical ball mill 2. Planetary ball mill 18 Fluid Energy Mill/Microniser/Jet Mill Fluid Energy Mill: Type 1 19 6  Venturi feeder 20 Fluid Energy Mill : Type 2 21 7  TCD Microniser hopper filter Fluid pressure controls Venturi feed Milling chamber cyclone separator 22 Milling chamber 23 8  Grind material to < 5mm diameter Attrition between particles Advantages: – Very small particle (low-micron) size with a fairly narrow size distribution – Speed of operation fast – Flexibility - suitable for most materials. – Possible to vary particle size Particle size depends on : » size of feed material to be ground » rate at which the feed material enters the chamber » pressure at which fluid is introduced – Very little heat produced - expanding gas (i.e. the fluid) cools the system : good for heat sensitive materials – Inert gases can be used for oxidizable materials – Machine is easy to clean or sterilise – Dustless operation Disadvantages: – Large capital cost to install – Running costs high 24 Factors Influencing Selection of Grinding Machinery Must consider physical properties of feed and product and relevant operating parameters 1. Purpose for which powder is required 2. Physical form of material to be ground 3. Crystal or polymorphic form of material 4. Moisture content of material 5. Material stability : oxygen, heat, metals 6. May require aseptic conditions 7. Health hazards - dust 25 9