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How are aggregates produced?

Introduction
     
Crushed rock is usually produced from quarries that are much larger and deeper than sand and gravel pits. Crushed rock quarries typically have outputs in the range 250 000 tonnes per annum up to about 5 million tonnes per annum. Sand and gravel operations have outputs in the range 100-300 000 tonnes per annum.
 
In the UK the term mine is defined by law (Mines and Quarries Act 1954) as any mineral extraction activity occurring below ground. Surface extraction is known as quarrying. At present, due to the lower costs involved, almost all aggregates are produced by quarrying. Methods used to extract rock, excavate unconsolidated sands and gravels, and process the material to the required particle sizes are relatively simple involving various stages of blasting, digging, crushing, sieving and washing.

Methods and equipment employed in the extraction of aggregates depend primarily on the type of deposit and source rock being worked, put simply, the degree of consolidation or induration of the deposit. Other controlling factors include the physical properties of the rock: density, impact strength and abrasivity, and also the rate and scale of production required.

Crushed rock aggregate is produced from 'hard rocks' which range from fairly soft sandstone and limestones to highly indurated, crystalline rocks which requires fragmentation by drilling and blasting before it can be dug from a loose pile. Crushed rock is usually produced from quarries that are much larger and deeper than sand and gravel pits. Crushed rock quarries typically have outputs in the range 250 000 tonnes per annum up to about 5 million tonnes per annum. Investment in plant and equipment can, therefore, be very large.

Typically sand and gravel operations have outputs in the range 100–300 000 tonnes per annum. Sands, gravels and some bedrock deposits may be excavated directly from the ground by suitably powered machines without blasting.
 

Subsequent secondary fragmentation, washing, and grading will depend on the quality or grade of aggregate required for the end use.

From whatever source aggregates are produced - hard rock quarries, land based sand and gravel quarries or marine sources there are four basic operations in quarrying prior to secondary crushing, grading, etc.
  • Overburden removal and face preparation
  • Extraction
  • Digging, loading and hauling
  • Primary fragmentation
For information on hard rock click here

For information on soft rock click here


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