Aggregate extraction and processing
Sand and gravel deposits commonly are excavated from pits utilizing conventional earth-moving equipment. Mining crushed stone generally requires drilling and blasting of solid bedrock (also referred to as ledge or ledge-rock), which breaks the rock into rubble of a size suitable for crushing. Crushed stone and sand and gravel commonly are obtained from dry pits or quarries, but in some settings may be mined from water-filled excavations using dredges mounted on barges, or with draglines.
Sand and gravel or rock rubble at the mine face are transported by truck or conveyor to a processing plant. The material is crushed, passed over a screening device, and sorted according to size. The crushing, screening, and sorting process is repeated until the proper mix of particle sizes is reached. Sand and gravel may or may not be crushed, depending on the size of the largest gravel particles and the desired product. Depending on the specifications of the final product, the processed material may be washed to remove dust. Sand may be screened from the mixture and processed separately. After screening, sorting, and washing (if necessary), the sand and different size gravel/rock particles are moved by conveyors to separate stockpiles where they are stored until sold and shipped.
Most aggregate is sold in bulk. Upon sale, aggregate is loaded on trucks, railcars, barges, or freighters for transport to a destination. Aggregate is a high-bulk, low-value commodity, and transportation can add substantially to the cost at the point of use. For example, the cost of transportation of aggregates in the European Union is about 13% of the total cost of the aggregate.
The method of transport depends on a number of factors including volumes of material, distance to the point of use, delivery schedules, and access to rail or water transport systems. Trucks are by far the most flexible and most common means of transporting aggregate. They can be loaded and unloaded at many locations using a variety of techniques and can accommodate most delivery schedules. Rail and barge are much less flexible because they utilize fixed route systems following strict schedules and require considerable investment capital in terms of loading facilities, off-loading facilities, and distribution yards. Trains and barges achieve economy by moving large volumes of aggregate long distances on regular schedules.