A dump truck feed hopper (40t capacity) is fed via an access ramp. Its heavy duty design and 6" tipping grid accepts 'as-dug' material whilst removing excess oversize. Feed is then conveyed to a twin deck R2m108 (20x6') rinsing screen. This separates oversize, intermediate aggregate, and sand. The oversize is stockpiled while the minus 5mm slurry passes to the CDE Multi-Sands Plant.
The Multi-Sands Plant comprises two of CDE's Counter Flow Classification Units (CFCUs) fitted with sieve bends, and three hydrocyclones. The CFCUs separate sand at 250 microns, overflowing fine particles and lignite from the main sand fraction. PLC controlled pneumatic pinch valves control the flow of sand from these units to the two integrated C-Series Quick-Dry high frequency dewatering screens. Finer cut-points are achieved with the cyclones. These are specified according to the flow volumes at different stages in the system and have effective silt separation points from 75 to 40 microns.
The complete classification plant is controlled by a PLC programme which the operator can easily manipulate to achieve the precise grade required for the two sand outputs at any time, usually concrete and building sands. The lignite free sand is offered at a typical 12% moisture content. Slurries containing lignite and fine sand particles are passed through sieve bend to remove larger lignite particles; the underflow from these is then passed back to the Multi-Sands Plant whilst dewatered lignite is stockpiled in a bay.
Wash water from the sand classification process passes to the adjacent T-08 High-Rate Thickener. This efficient Thickener recycles as much of the wash water as possible with minimal use of polyelectrolyte and reduces waste volume from 400m³/hr dirty water to approx. 25m³/hr sludge at circa 600g/litre solids concentration. The need for large settling lagoons is negated and the problems associated with such ponds are significantly reduced; water loss through evaporation and run-off, risk of contaminating local waterways, health and safety, back excavation etc.
As noted the primary rinsing screen carries forward the plus 5mm material. This carries a large amount of lignite and clays which need to be removed. CDE installed their SF82-7 Rotomax Log Washer at this point to offer the highest level of attrition at up to 80tph feed. This breaks down any clay-bound material and scrubs the agglomerated gravel. Liberated fine material and water from the Rotomax is passed to a ground sump fitted with a vertical Warman pump. The sump also collects ground and wash down water, facilitating site housekeeping and maintenance. This slurry is recycled to the primary rinsing screen, improving fluidisation and reducing fresh water requirements.
Cleaned aggregates from the Rotomax are conveyed to an air pulsed 'allmineral' jig; the first of its kind in the UK. This jigging unit removes lignite and any other light contaminants, achieving 0.01% lignite by weight in the final product. The principle of the jig is that particles stratify in pulsating water; the upward and downward currents fluidise and compact the grains into homogenous layers. Low density particles rise to the surface while denser particles settle to a lower level. The jig is controlled a separate PLC and can be easily adjusted and optimised according to the feed characteristics. Removed lignite is dewatered on a static screen.
Clean and lignite free aggregates are conveyed to a CDE twin-deck D2M60 (13x5') screen for final classification to 6mm, 10mm, and 20mm. The 6mm is passed to a final C-Series Quick-Dry dewatering screen to keep as much water as possible within the system. Each of the aggregate products is then stockpiled by 15m mobile conveyors.
