Lecture 11 - Mechanical processing (Ch. 7.1-7.2 & 9.7) Flashcards
What is the concept of mechanical treatment of waste and which kinds of processes are include in this treatment?
Mechanical treatment of waste –> unit operations that alter the physical but not the chemical characteristics of the waste
Processes:
- Size reduction
- Conditioning and concentration
- Sorting processes
- Compaction
What are the objectives of size reduction?
- homogenization of particle size distribution
- material liberation (separates some materials from each other)
- increases surface area
- mixes the waste
- refuse bag opening
Which types of equipment are used for size reduction?
- Hammer mills
- Cutters / shredders
- Impact crushers
- Cascade mills
- Jaw crushers
Explain the concept of a hammer mill and what it is used for.
Hammer mills are the most commonly used units for crushing end-of-life vehicles, construction and demolition waste,
commercial waste, and paper waste. Hammer mills have either horizontal or vertical shafts with flexible blades. The
blades may rotate as fast as 1200 rotations per minute (rpm) and at speeds up to 70 m/s.
What are the objectives of Conditioning, concentration and sorting?
Separation of a waste stream in 2 or more according to characteristics like weight, density, shape, magnetism, conductivity, colour, chemical composition or a combination of these
What are the occupational health risks of manual sorting?
o Physical injuries by contact with the waste
o Exposure to toxic substances
o Ergonomic damage due to repetitive work
o Infections, allergies
o Noise exposure
What are the limitations/disadvantages of manual sorting?
o Limitation on particle size (around 50 mm)
o Impossibility or limitations in identifying some materials
o High cost due to wage levels in developed countries
What is screening used for? Mention types of screens.
Screening separates material of various sizes into specific particle size ranges. If a specific particle size is characteristic for a specific material fraction, screening may also be used for separation of materials.
Types: Trommel, disc, bucket, oscillating
The screen recovery is influenced by various factors characterizing the process. Mention these factors.
- Machine dependent factors: screen width and length, screen angle and form, vibration frequency and amplitude, and revolutions.
- Screen surface dependent factors: surface type, opening size, and surface material.
- Waste dependent factors: feed input, surface dampness, particle size distribution, and fiber content.
Explain the concept of an air classifier and mention different types.
o Separation according to a particle’s falling velocity in an air stream (shape and specific gravity)
o Light and less dense materials (paper, plastic etc) are carried by the air stream
o Heavy materials (metals, stones etc) fall to the bottom or are left on the conveyor
Types: Zigzag air classifier, suction hood, cross-flow air classifier.
Explain the concept of ballistic separators
o criteria for separation is the size, density, 2D vs. 3D and rigidness of the waste particles
o three output fractions
o An example could be the separation of beverage cartons (3D) from plastics (with 2D shape)
- The broken-up material, depending on specific gravity and form, is transported up or down the deck. The heavy particles move to the lowest level because of the downward movement of the deck and thereby become the heavy fraction. Because of the movement of the vibrating elements and the intense rearranging that occurs, the lighter particles, such as paper, cardboard and plastic foil, move in the opposite direction toward the highest level of the deck and form the light fraction. Falling through the deck’s perforated bottom, the third, fine fraction is produced.
Explain concept of magnetic separators and mention different types.
o ferrous metal particles are attracted by strong permanent or electro-magnets
o types: head pulley magnet, drum magnet, over-band self cleaning magnets.
Explain the concept of an eddy current separator
o Eddy currents are a manifestation of electromagnetic induction occurring when a magnetic field is applied to a conductor.
o The separation occurs by the induction of eddy currents in non-magnetic metal particles by the use of a fast-alternating magnetic field.
o Eddy currents are created when conductive objects are located in or exposed to a spatially or temporally alternating magnetic field. Eddy current flow in closed loops within the conductor. According to Lenz’s law, the induced electric current produces a magnetic field opposite of the field to which it is exposed. A force is produced against the conductive object, which hurls the object out of the magnetic field. Less conductive objects require less force. With increasing density, greater hurling force is necessary due to the mass inertia of individual objects.
Explain the concept of Electrostatic separators
o the particles are first charged, depending on material they will acquire either a positive, negative or no charge
o the charged particles are fed into a separating zone of high intensity electric field and will migrate toward the attracting electrode
Explain the concept of density separation.
o The waste particles are fed into a fluid, the density of which can be adjusted usually, heavier particles will sink while lighter particles will float (e.g. used for separating plastic types).
