Aggregate Production Overview
Extraction
Extraction is the removal of
mineral from the ground and its
transfer to a processing plant (which may be at a fixed location or present as a
mobile plant in the
quarry workings). The four basic operations of extraction include
overburden removal,
drilling and
blasting,
secondary fragmentation, and
digging,
loading and
hauling.
The methods and equipment used depend primarily on the type of deposit and source rock being worked. The key factor is the degree of consolidation of the deposit, but other considerations include the physical properties of the rock (density,
impact strength and abrasiveness) and the rate and scale of production required.
Production plant processing
Production of hard rock
aggregate involves screening (scalping) to remove
fines and waste material followed by
crushing and screening to produce material with specified size grades (Animation 1 and Fig. 3). Crushing is carried out to reduce the size of the as quarried mineral from large blocks (up to a metre across) to a size finer than 20 to 50 mm. This size reduction is carried out in stages, typically with a low size
reduction ratio (<6:1) and is characterised by the use of certain types of crushing equipment, as shown in Table 1:
| Production Technology |
Table 1 |
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| Quarry process plant crushing stages: typical equipment and products |
Crushing stage
|
Crushing equipment |
Maximum feed size
(mm) |
Maximum crushed
product size
(mm) |
| Primary |
Jaw crusher
Gyratory crusher |
700-1000 |
100-300 |
| Secondary |
Cone crusher
HSI crusher
Jaw crusher (rarely) |
100-250 |
20-100 |
| Tertiary |
Cone crusher
VSI crusher |
14-100 |
10-50 |
Quaternary
(& subsequent stages)
|
VSI crusher
Cone crusher |
10-40 |
10-20 |
The particle size of the crushing product will determine the yield of the saleable
product. The particle shape requirements of the products and the rock type will affect the type and range of plant used. For further details
click here.
Animation 1. Quarry process line
Figure 3. Hard rock processing
Production of sand and
gravel involves washing and
scrubbing to remove clay, separation of the sand fraction by screening, grading of the gravel, sand classification and
dewatering, and crushing of any oversize gravel to produce a saleable product (Photo 4). Washing removes
silt and
clay (material finer than 0.063mm), which is present either as surface coatings or as clay-bound agglomerates that need to be broken down. The silt and clay content of a sand and gravel deposit is an important parameter in determining its economic viability; the silt content should be lower than 25% and the clay should be much lower as it is harder to remove. Some gravels containing clay (hoggin) may be sold "as dug" for constructional fill. The silt and clay is removed using cyclones and settled out in lagoons, from which process water is recovered.
Dry screening is also used to produce building sand at some locations. Blending of sands from the same or different sites can be carried out to meet consumer particle-size requirements; this also enables maximum use of the mineral resource. Washing plants are being built at some hard rock quarries to upgrade the quality of the
scalpings and fines. For further details click here.
Figure 4. Sand and gravel processing
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Products
Products from quarries include
aggregate,
asphalt,
industrial minerals,
lime (agricultural and industrial),
mortar and
ready mixed concrete. Aggregates are the primary output from quarries in the UK; they are an end product in themselves and are used as the raw material in the manufacture of other construction products such as ready-mixed concrete, asphalt, lime and mortar. Crushed rock sands may also be produced; often referred to as
manufactured sand. Aggregates are used in road construction, as railway ballast, in private housing, public infrastructure and industrial construction. A factsheet on aggregate supply in the UK is available to download from:
www.mineralsuk.com/britmin/mpfaggregates.pdf.
The quality of quarry products used in the UK is controlled by the
European Standards for Aggregates for concrete, mortar, asphalt and road construction. Information on these standards is available from the QPA website:
www.qpa.org/prod_agg01.htm. The introduction of these European standards in 2004 raised the possibility that production practice may have to be modified. However, it is apparent from discussions with the UK quarrying industry that few alterations to their existing production practices have been required. The main changes were to product designations and the range of test sieves used in the quality control laboratories. Largely, existing products were accommodated by the new standards with minor process changes such as the size of screen apertures used; to some extent the new standards allow the industry greater flexibility and options in production of saleable aggregate.
Quarry products are defined by their particle-size grading; the product designations are based on the respective lower (d) and upper (D) sieve sizes, which are expressed as d/D. All-in products are essentially ungraded, crushed material. The exception is light gradings of armourstone which are specified in weight (kg). The following are the current product size designations (BS EN 12620 for concrete aggregate, BS EN 13043 for asphalt aggregate, BS EN 13242 for road construction, BS EN 13383 for armourstone and BS EN 13450 for railway ballast), with their equivalent designations in the older
British Standards (BS 882 for concrete aggregate and BS 63 for asphalt aggregate) and Network Rail's specification for railway ballast in brackets:
- Armourstone
Coarse gradings: 45/125, 63/180, 90/250, 45/180 and 90/180
Light gradings: 5 to 40 kg, 40 to 200 kg, 10 to 60 kg, 60 to 300 kg and 15 to 300 kg
- Graded products
4/40 (40 to 5 mm graded), 4/20 (20 to 5 mm graded) and 2/14 (14 to 5 mm graded)
- Railway ballast
31.5/50 (28 to 50 mm railway ballast) and 31.5/63 (not used in UK)
- Single sized aggregate
20/40 (40 mm single sized), 20/31.5 (28 mm single sized), 14/20 or 10/20 (20 mm single sized depending on end-use), 8/14 or 6.3/14 (14 mm single sized depending on end-use), 6.3/10 & 4/10 (10 mm single sized depending on end-use), 2.8/6.3 or 2/6.3 (6 mm single sized depending on end-use) and 2/4, 1/4 or 1/3 (3 mm single sized depending on end-use)
- All-in aggregate
0/40 (40mm all-in), 0/20 (20mm all-in), 0/10 (10 mm all-in) and 0/6.3 (5 mm all-in)
- Sand products / Fine aggregate
0/4 (coarse sand), 0/4 or 0/2 (medium sand depending on end-use) and 0/2 or 0/1 (fine sand depending on end use)
Products are typically stored in stockpiles (open air or covered), product bays, bins or silos (Photos 5, 6 & 7).
 Photo 5. Limestone aggregate stockpile
 Photo 6. Washed gravel
 Photo 7. Washed concrete gravel
Quarry fines and waste
The production of
crushed rock aggregate will produce a certain proportion of quarry waste, including
quarry fines (<4 mm). The amount of waste is governed by the geology, nature of the rock, product specifications,
extraction and production processes, and to some extent its location with respect to potential markets and market economics.
Quarry wastes are produced from
overburden/ interburden materials, from washing of
sand and
gravel to remove fines, and from scalping, crushing and dry screening. Quarry wastes are largely localised at the sites where the mineral is worked. It can comprise up to 15 to 20% of the
excavated rock; at some sites it may exceed 50% of the excavated rock after
processing.
Quarry fines and scalpings can be difficult to sell and may become a waste product. They may be considered a resource (some scalpings are clean enough to meet
Type 1 sub-base specifications and all waste has the potential to be used as low grade, low value constructional fill), although some can only be sold after additional processing.
For further details click here.
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