How Modern Aggregate Processing Equipment Works

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Sand and crushed stone are used in almost every road, railway line and building in the UK. Before they can be used, the raw material from a quarry, gravel pit or mining site must be crushed, washed and classified according to a certain specification.

The equipment that does this has changed a lot in the last twenty years. The plants now use more automation, recycle most of their water and handle recycled material as well as stone.

Crushing material down to size

Processing usually begins with destruction. A jaw crusher takes the largest rock and crushes it between a fixed plate and a moving plate, producing a coarse product that can go on a conveyor belt.

Cone crushers then further reduce the material by compressing it in a rotating chamber that fits hard abrasive stone such as granite.

Where a well-shaped, cubic product is needed, operators often use it Impact crushers. These throw material against breaker plates at high speed so that it breaks with natural weaknesses. The shape affects performance, as flaky or elongated stone requires more cement paste in the concrete and packs less tightly in a road base.

Washes out clay, silt and contaminants

Crushed or excavated material usually carries clay, silt and organic matter. Left in, these weaken concrete and stain finished surfaces, so they need to be washed.

In the aggregate washing plant brings several stages together in one river. The feed is first moistened and scrubbed, sometimes in a log washer or exclusion cell, where paddles rub particles against each other to remove the clay.

Slurry pumps then move the mixture to hydrocyclones, which use centrifugal force to separate sand from the light silt. Cyclone liners are made of rubber or polyurethane to resist wear and tear, and replacing them is one of the main on-site costs.

Finally, dewatering screens vibrate the fine-mesh sand until it is dry enough to store and sell. Some older sites still use bucket wheel settings for this step.

Grading material by size

Screening sets the product range. Vibrating screens with two or three blankets split material into separate sizes, and spray bars rinse each size as it travels along the plate.

Each product must fall within a grading curve, which is the size distribution set out in the relevant standard. Many decks now use polyurethane sheets instead of woven steel mesh because they last longer, run more quietly and can be replaced quickly when worn.

Setting up the slabs correctly on an aggregate screen is often the difference between a stock that meets specification and one that needs to be reprocessed or sold as low-value fill.

Recycling of water

Washing aggregates use large volumes of water, so most modern plants recycle it. Dirty water from the plant flows into a thickener where a flocculant clumps fine particles together and settles.

Clear water overflows from the top and returns to the plant, while the thickened sludge is pumped away. Many sites now put this sludge through a filter press, which presses it into a solid cake that can be moved with a loading shovel. This reduces the need for large settling lagoons.

Automation and Remote Monitoring

Programmable logic controllers manage the startup and shutdown so that conveyors, pumps and screens come on and off in the right order.

Sensors track engine loads, bearing temperatures, water flow and tank levels. Engineers can view this data remotely, which helps them spot a blocked chute or a failed bearing before it causes a lengthy shutdown.

Modular plants in recycled material

More plants are now built from modular, skid-mounted sections that are tested before delivery and joined on site. This shortens the installation and makes it easier to move the equipment when a deposit is prepared.

The same approach has helped the growth of recycled aggregate. Construction and demolition waste can be crushed, screened and washed to remove plaster, wood and plastic, with some plants using high water currents to remove light contaminants.

Fines recovery systems then capture sand that would otherwise be lost to settlement ponds, so more of the material can go back into new construction.

Choose the right setup

No two sites use exactly the same layout. The right combination of crushers, screens and washing stages depends on the feed material, the products the site has to sell and the available space and water.

Most operators work with a supplier of Mineral processing equipment to test samples of their material first. The results show which stages are needed and how the plant should be adjusted before any equipment is ordered.

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