Greco lecture 2: Mechanical Methods Flashcards
What is milling?
Mechanical method used to give you reduction in particle size
What is the principle method that’s followed for size reduction?
- Cut
- Compression
- Impact
- Attrition
Describe the cutting process?
- Material is entered through a funnel which is cut up by many knives
- The particle goes through a screen which allows it to go through if it’s small enough size
Describe the compression method?
- A pressure is applied which breaks up the material
- Roller mill: cross section of cylinders attached to an engine that ensure it roll
- Good for breaking up crystalline material
- Can be traditionally done via pestle and mortar
Describe the impact method?
- Material is hit by rotating hammers
2. Particles are hit by a moving surface or the particles hit a moving surface
Describe the attrition method?
- Pressure and friction
- Roller mills that are attached to a engine which rotate at different speeds
- Ointment with a powder dispersed would use this milling method
Describe how impact and attrition methods can be combined together?
- Main chamber filled with balls that reduces the size of the particles- ball mill
- Factors which affect efficiency of it is the amount of material and the speed at which the mill rotates
- Subject to gravity as they roll upwards they are pushed down again
What is the problem that can occur if too much material is added to the impact and attrition ball mill method?
Decreases chance of the material heating the balls is held to a minimum this is called the cushioning effect
What is the problem that can occur if too little material is added to the impact and attrition ball mill method?
Balls will start heating among themselves which leads to a wearing of equipment
How do you decide to choose which appropriate mill to use?
- Particle size you want
- Characteristics:
- Cutter mills: elastic, fibrous material like roots/wood
- Attrition: ointments, solids in suspensions and paste
- Impact: brittle materials - Other factors:
- Cost
- time
- stability of the ingredients
- Specific milling conditions
What is particle size analysis?
Approximating the size of the particles by comparing it to a spheres diameter
What is the projected area diameter?
The diameter of the sphere who’s area is the same as the particle
What is the projected perimeter diameter?
The diameter of the sphere who’s perimeter is the same as the particle
What are the two direct methods for working out particle size?
- Sieving
2. Microscopy
How are sieve sizes expressed?
Based on the sieve aperture diameter measured in micro meters that are the size of squares
Explain how a 100g powder is sieved?
A large column contains many sieves that each individually filter the powder
Explain how light microscopy works?
- A drop of diluted suspension is placed on a microscope slide
- Particles are measured in relation to the equivalent diameter chosen
- Images are 2D and range between 1 to 1000 micrometers
Explain what electron microscopy is and the advantages/disadvantages?
- Scanning electron microscopy
- Provides a 3D image and gives information on shape
- Advantage:
3D images gives information on shape - Disadvantages:
Expensive, high level of operator expertise
Explain what a coulter counter is and does?
- Measures the volume of particles from 0.1 to 1000 micrometers
- Emerged in solution filled with electrolytes
What are the indirect methods to measure particle size?
- Sedimentation rate
2. Permeability
How does the laser light scattering method work?
- Laser light interacts with particles
- Light is diffracted by particles at an angle that is inversely proportional to the volume of particles
- Detector analyses the radiation diffracted by the particles
How does the sedimentation method work?
Diameter of particles estimated based on sedimentation rate
