Demolition used to be waste material destined for landfill sites, but now contractors must pay money to dump this material in landfill sites. Government incentives to reuse this material are now prevalent and so it has become cost-effective recycle demolition waste to make new construction products. Demolition material is made up of concrete normally containing reinforcing steel bars (rebar). The rebar adds strength when used in the construction industry, but must be removed from within the concrete. Other material can include kerb stones, flag stones, floor beams and lintels, concrete blocks, bricks and decorative stonework. The collective term for this construction material is ?rubble?.
IMPACTOR OR JAW?
An impactor is very useful for separating different types of material bonded together either naturally (eg conglomerate rock such as Breccia) or artificially (eg concrete). If we take the example of concrete, the force of the impactor rotor can be used to shatter the bond of the cement filler material, leaving some of the base aggregate intact.
The second benefit of an impactor is the high reduction possible in one pass. Many larger pieces of concrete can be fed to the crusher and reduction ratios of 10:1 are not uncommon. The impact process creates a lot of fines material which can be incorporated with other clean aggregates into useful products.
Impactors are not always used for demolition. In many demolition sites, the rebar that may be present in the demolition material is too large in diameter or too long for an impactor to process. Also, for concrete produced with a basalt or granite base aggregate, the wear seen on the blow bars upon crushing may be too high to be cost-effective. This is because the concrete was produced using aggregates with a high percentage silica content. In these applications a jaw crusher may be more suitable.
SCREENING CRUSHED DEMOLITION PRODUCT
The high fines content produced by an impactor normally needs to be screened off. Small mesh sizes require large screening areas. For fine screening, the ideal conditions are small stroke movements at higher frequencies. A horizontal screen with an elliptical screening action can be suited to this type of screening because material is less likely to blind over.
The second issue with fine screening is the moisture content of the material. Stockpiles of crushed demolition material act like sponges when left for long periods in wet weather. The smaller material has a larger surface area to hold water and sticks together in lumps. When the material is screened, it then blinds over the woven wire mesh, effectively reducing the aperture size. This reduces the size of product that can pass through the mesh, reduces the throughput capacity of the screen plant and also propagates the effect of blinding further.
CASE STUDY: DEMOLITION OF BRICKS AND CONCRETE
A Finlay 984 mobile horizontal screen was used to scr een 0-7mm, 7-20mm, 20-40mm, and 40-120mm products. The feed material was mainly bricks and some concrete crushed to a 0-120mm size with an impactor. This machine worked with a capacity of up to 250 tph, depending on the amount of fines and the humidity of the material.
The concrete feed material originally used cement to bind the concrete material together. When this is crushed the fines produced are as good as sand. So the screened 0-7mm material is used to mix with cement.
The 7-20mm and 20-40mm is split so that air blowers can remove any small pieces of wood or plastic. This material is then mixed again and recycled with other aggregates and formed into new concrete products. The plastic and wood collected is then used in a biofuel processing plant.
The 40-120mm product is sold to a brick manufacturing company where they process the material further into a very fine dust and then create bricks again.
Finlay 984 set-up
Mesh sizes used were 45mm, 22mm and 8mm on the top, middle and bottom decks respectively. One advantage of using the horizontal screen is that the mesh size required is normally that of the product required. There is no adjustment necessary for the effective open area for incline screens. However, to ensure the products met the required specification for this material, small adjustments were necessary.
The bottom deck mesh is normally the first to blind over because of the small mesh size. The horizontal screen combines the elliptical screening motion with the adjustable stroke and speed to minimise this.
The Finlay 984 was delivered with two-plug weights fitted and a 45? stroke angle as standard. Screen speed can be adjusted between 730 and 875 rpm.
For screening large material or scalping, three-plug weights can be added to increase the stroke length, and the stroke angle can be decreased to 30? to throw the material across the screen further. Normally the screen speed is reduced in this set-up.
For fine screening, all the plug weights can be removed and the stroke angle increased to 60? to keep the material on the screen for longer. If the screen speed is increased it should give more efficient screening of a smaller product.
Source: Finlay Screening & Crushing