Classifiers Flashcards
What is classification?
A method of separating a mixture particles into two or more products according to their properties.
What is wet classification?
- Separation of coarse particles from fine particles by liquid fluidization.
- Coarse particles move faster than fine particles at equal density. High density particles move faster than low density particles at equal size.
- Particles sizes between 10 and 1000 micrometers.
Name 6 types of wet classification processes, please?
- Gravity Settling Tank
- Cone classifier
- Double cone classifier
- Hydro cyclone classifier
- Spiral classifier
- Rake classifier
What is dry classification?
- Separation by air fluidization.
- Size, geometry and density of the particles are the deciding factors.
How does a gravity settling tank work? Pros/Cons?
A slow moving suspension enters the classifier. Particles settle into different sections on the bottom of the tank. Coarse particles settle first and fine particles are carried away in the overflow.
- Low instillation cost/ No complex mechanism needed/ simple instillation.
- No sharp separation/ Separation efficiency is low
How does a Spitzkasten work?
Basically the same as a gravity settling tank, but the size of the material collected in each unit is also based on the rate of the suspension being fed, the upward velocity of the fluid and the diameter of the vessel.
How does a cone classifier work? Pros/Cons?
Feed is fed from the top and centre of a point down cone. Coarse particles collect at the tip of the cone and fine particles wash away with the outlet.
- Low instillation cost
- Approximate separation/Large feed difficult
How does a double cone classifier work? Pros/Cons?
A cone, with the bottom cut off is is placed point down, on top of a cone tip with a smaller diameter than that which has just been cut off. This creates a ring/annular shaped gap in the cone. The feed is deposited into the cones and fluid is pumped up from the bottom. As the feed flows into the annular space, fine particles will flow with the fluid into the overflow and coarser particles will settle against the fluid flow and will be removed periodically.
- Separation efficiency is comparatively high.
- Complex construction so expensive
-
How does a Hydrocyclone classifier work? Applications?
Uses centrifugal force to classify solids by mass. High mass particles flow to the outer wall, then out of the underflow. Low mass particles move closer to the centre and out of the overflow.
- Classification in grinding circuits
- Dewatering/Dethickening
- Desliming/Washing
- Enrichment of heavy minerals
How does a magnetic/electrical property based classifier work?
A large magnetic drum rotates under the feed. Magnetic material is attracted to the drum, then knocked into a container by a scraper. Non magnetic material falls off the drum and into a separate container.
How does a spiral classifier work?
Consists of a semi cylindrical trough at an incline with a spiral conveyer in the middle. A slurry is fed in near the middle of the trough. Fine particles have no time to settle and so overflow. Coarse particles settle at the bottom, and are dragged up and out of the trough by the conveyer.
How does a sieve classifier work?
A sieve allows small particles through. It is vibrated either by a mechanical or electromagnetic device.
How does froth flotation work?
Separates particles on if they are hydrophilic or hydrophobic. Fine air bubbles are blown through a suspension and hydrophobic particles attach to these, then float to the surface. The froth can then be removed. A froth promoter is often required such as ethyl xanthate.
Particles fall into which 5 categories?
- Naturally occurring (e.g. wheat)
- Size reduced naturally occurring (e.g. coal)
- Manufactured and used in processes
- Product
- By product waste
What 3 ways can you form a product?
- Crystallisation
- Precipitation
- Co-precipitation
