Wet Shredding Dry Downstream System
The Wet Shredding Dry Downstream System has been developed to meet the stringent environmental pressures now placed on scrap processors, whilst continuing to maintain clean, good commercial quality, frag and non-ferrous products.
The overriding principal of the total system is a combination of controlled water injection into the shredder with either an air cleaning or magnet cleaning separation system within the downstream.
The mill water injection system is designed to add controlled amounts of water into the shredder housing in proportion to the material loading at any one time.
This is achieved by first sensing the mill motor current and feeding this signal into a Programmable Logic Controller (PLC).
The PLC then controls a pump and linear valve so that the water entering the mill body is proportional to the mill current and hence the material load being fragmentised.
The water is introduced into the body of the mill in a specific position, chosen to achieve maximum atomisation and effect of the water, whilst minimising the potential damage to the injector pipes and avoiding any difficulties during maintenance duties.
The aim of the system is to use the minimum amount of water necessary, hence steam is immediately produced within the mill from the heat of the shredding operation. The shredding process could therefore also be referred to as “damp shredding”.
No Free Water is created, with all its associated problems in terms of product quality, material handling and disposal costs of the ASR fraction.
Further controls are added into the PLC program and thus the quantity of water injected into the shredder is in proportion to:-
- The volume of material being shredded.
- The climatic/weather conditions at the time of shredding.
- The existing moisture content of the material being processed.
Within the downstream system, a combination of air extraction, magnetics and further screening separate the fragmentised material exiting the shredder into three main product fractions:-
- Ferrous steel (frag)
- Non-ferrous metals / non ferrous residue
- Residual ‘dirt’ (ASR)
The specific combination of air and magnetic separation is dependant on the chosen overall method of downstream processing.
The two main methods offered by R.E.D. Ltd incorporate either air cleaning systems or magnetic cleaning systems. The two approaches are described in greater detail below.
a) Air Cleaning Systems
Within an air cleaning downstream system air extraction is initially used to remove the lighter dirt fraction (ASR), from the ferrous and non ferrous residue products, at the head of the shrinkage conveyor from the shredder.
The extracted light fraction is then passed through the air cleaning module to remove the dirt fraction from the airstream.
The air cleaning module consists of one plenum (an expansion chamber) and either one, two or four cyclones (dependant on the plant production rate) to process and separate the dirt fraction. The complete module is powered by a high efficiency fan.
The air cleaned ferrous and non ferrous residue products fall through the air extraction cleaning chute onto the magnet feed shaker conveyor for separation over a rotary drum magnet.
The ferrous steel is lifted over the magnet and passed forward onto the picking conveyor for the removal of any contaminants within the picking station.
The non ferrous residue product falls from the end of the magnet feed shaker onto a collection conveyor. Further upgrading of this product can be achieved using eddy current separators to recover the non ferrous metals.
Eddy current separators can also be incorporated onto the dirt (ASR) fraction for recovery of non ferrous metals within this stream.
b) Magnetic Cleaning Systems
Within a magnetic cleaning downstream system the total material exiting the shredder is initially passed directly to the drum magnet feed shaker from the shrinkage conveyor.
The ferrous steel product is again lifted over the rotary drum magnet, however at least two, in-line, magnets are normally utilised to provide effective separation of the ferrous product from the residual materials.
It is also recommended that a smaller air cleaning module is incorporated after the magnet assembly to provide residual cleaning of the ferrous product.
The frag is normally also passed through a picking station in line with an air cleaning downstream system.
The complete non ferrous and dirt (ASR) mix falls from the magnet feed shakers onto a collection conveyor.
Standard practice is to then recover the non ferrous metals from this stream over eddy current separators.
A network of conveyors connects the complete downstream system, with the layout and specification individually designed to suit the client’s operating requirements.
Further processing and separation equipment can also be incorporated into the downstream, including trommels, overband magnets, picking stations and metal separators.
The Wet Shredding Dry Downstream System can be installed as either a new plant or as part of an existing installation and is compatible with any model of shredder from 1,000 to 10,000 HP drive capacity.
Benefits
The incorporation of the Wet Shredding Dry Downstream System provides many commercial and environmental benefits to the operator.
With particular reference to an air cleaning downstream system, dust emissions from the system to atmosphere are significantly reduced.
R.E.D. Ltd can guarantee dust levels in the air from the fan chimney to be less than 10mg/m3, a figure that is well below any current environmental limit on emissions set throughout Europe.
Other benefits, of either downstream type, include:-
- Reduced fire risk within the shredder
- Inhibited effects of explosions within the shredder, in terms of both force and noise
- Reduced shredder dust emissions
- Reduced conveyor transfer point dust creation
- Reduced capital and operating costs when items of plant such as mill extraction systems, wet arresters, venturi scrubbers and other filters are removed from a downstream system
- Clean, commercial quality, products
