Resources

Sizing
Sizing screens use vibrating meshes with specific size apertures to separate oversize material from undersize material. Water is often added to fine material to allow different grain sized materials to be separated down to
0.5 mm. Up to three decks are used in each screen to produce several size fractions in a single operation. A cascade of screens may be used in series so that material that is too coarse for one set of decks is passed onto the next screen allowing a great variety of different sized materials to be sorted. Materials that are too big to fit through any of these screens are passed back into the tertiary crusher and may be cycled between the sizing screens and the crusher a number of times.

Classification
For sizing material less that 0.5 mm in diameter, the process of classification is used. Classification uses centrifugal force to separate materials of different mass. A mixture of material and water is rotated in a cone; while larger particles are gathered in the centre of the cone, small or low-density particles are thrown out to the sides and removed from the top. This allows marketable sand to be separated from silt and clay waste.

Storage bays for different sized grades of aggregate.

Silt handling
Pumped slurries contain a maximum of 40% silt by volume. Once the water has been removed from the aggregate products it is important to clean the silt and clay from suspension so that the water can be recycled or released. This separation is most simply achieved by holding the dirty water in silt lagoons allowing silt to settle and clean water to be recovered. In the UK, limits on water abstraction and effluent discharge, combined with space restrictions and concerns about lagoon stability have increased pressure for maximum recirculation of water without the use of lagoons.

Thickeners, shallow, large diameter circular tanks with conical bases, and filter-presses are commonly used. Thickeners have slowly rotating rakes which aid sedimentation and transport solid material down toward a central discharge point. Clarified water overflows at the top of the tank and is recycled. The process is assisted by the use of flocculants, which cause particles to clump together and settle faster. The sludge from the bottom of the tank is either sent to a silt lagoon or directed to a filter press. A filter press will squeeze the sludge to remove the remaining water and produce a waste material called filter cake, which may be used in site restoration.

A sizing screen.

Storage
Finished aggregate products are stored in ground level stockpiles and bays or in elevated silos and bins. Stockpiles may require a large area of land depending on the size of the operation. Some storage bays have feeders installed below ground allowing material to be rapidly reclaimed by conveyor.

Fine aggregates will have been processed through the classifier as a suspension in water and will therefore have to be dewatered before sale. Free drainage of water from heaps of aggregate is a common feature of quarries, especially those handling sand and gravel. The cost is low if sufficient space is available, but disadvantages include the visual impact and potential for contamination from windborne distribution of dust.

Value-added products
Aggregate is a high volume, low value material. It is therefore common in larger quarries for the operator to add value to a portion of his production by installing an asphalt plant or ready mix concrete plant. Quarries may also be suitable locations for cement works, block making plants, lime kilns or for the production of industrial powders such as fillers in paint, paper, plastics or animal feed. Over time the quarry may develop several industries that are visually contained within the quarry walls, with a positive impact on local employment and economy.

Readymix concrete, produced on site, awaiting delivery.

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