FD Birds

THE SOLUTION

The dumptruck feed hopper (40-tonne capacity) is fed via an access ramp. Its heavy-duty design and 6in tipping grid accepts ‘as-dug’ material while removing excess oversize. Feed is then conveyed to a twin-deck R2M108 (20ft x 6ft) rinsing screen. This separates oversize, intermediate aggregate and sand. 

The oversize is stockpiled while the –5mm slurry passes to a CDE MultiSands plant. The Multi-Sands plant comprises two CDE 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 two integrated C-Series quick-dry, high-frequency dewatering screens. 

The complete classification plant is controlled by a PLC program that 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 bends to remove larger lignite particles; the underflow from these is then passed back to the Multi-Sands plant while the dewatered lignite is stockpiled in a bay.

Wash water from the sand-classification process passes to an adjacent T-08 high-rate thickener. This efficient unit recycles as much of the wash water as possible, with minimal use of polyelectrolyte, and reduces waste volume from 400m3/h of dirty water to approximately 25m3/h of sludge at around 600g/litre solids concentration. The need for large settling lagoons is negated and the problems associated with such ponds — water loss through evaporation and run-off, risk of contaminating local waterways, health and safety, back excavation etc — are significantly reduced.

As noted, the primary rinsing screen carries forward the +5mm material. This carries a large amount of lignite and clays which need to be removed. CDE installed a Rotomax log-washer at this point to offer the highest level of attrition at feed rates of up to 80 tonnes/h. 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 groundwater and wash-down water, facilitating site housekeeping and maintenance. This slurry is recycled to the primary rinsing screen, improving fluidization and reducing fresh-water requirements.

Cleaned aggregates from the Rotomax are conveyed to an air-pulsed ‘allmineral’ jig. 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 fluidize 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 by a separate PLC and can be easily adjusted and optimized according to the feed characteristics. Lignite removed by the jig is dewatered on a static screen.