Quarrying
feeders being utilised simultaneously, allowing the optimum blend of raw feed stock to be achieved. In order to efficiently handle the variable feed stock, (generally 100 mm down), each feeder is driven by twin unbalanced vibrator motors, with variable feed rate control allowing infinite regulation between approximately 30 -100% of the pre-set maximum, achieved via electrical frequency inverters.
“The Skako feeders can be run either manually or automatically,” explains Mike. “They can be run as a pair or individually. At the computer screen we can vary the percentage each feeder is working at the touch of the button on the computer control screen. This is how we blend the material.” The main feed is fed to a large scalpings screen where +100mm is rejected from the top deck to an external storage bay, whilst the lower deck screens +40 to 100mm clean stone for crushing and -32mm feed material to the wet screen house. Oversize 40 -100mm goes back through a crusher house where there are two Sandvik CH430 crushers: one for coarse material and the second for fine. There is a splitter screen and two bins, one on top of each crusher, the 40 mm in one and the oversize 40-100 mm goes into another bin which is crushed separately. That is re-fed onto the main feed conveyor, which goes up to the wet screen house comprising two double deck screens where washing takes place on both decks of each screen to ensure clean single size 10 and 20mm gravel, there is also a + 20mm re-feed conveyor return to the crusher house. The Sandvik crushers replace the old plant’s mobile, diesel- powered crusher. They automatically power down when not needed, enabling a power saving of 132,000 kW a year. Sandvik’s ASRI control system looks after the wear rate, which can be displayed on a daily basis allowing planned changes of wear parts. Two metal detectors on the conveyors ensure anything metal is kept out of the crushers.
The washing process
In the wash house the plant separates the single size material and flumes down the water/sand product into the Linatex sand processing and screen dewatering system, which washes and dewaters the sand fraction to reduce the silt content in the final product. It extracts fines using cyclone circuits and dewaters the product on a screen, achieving a 10.5-11% moisture content. It includes a large effluent tank and a pump to handle the dirty water. Wash water and fines emanating from the aggregate washing and sand system are treated by the effluent treatment
system, supplied by Haith Industrial, which provides high quality treated water that can be used by the system. The feed of material and water is flumed down to the centre of a feed regulating sump. From the base of the sump a pump draws the slurry and feeds two sets of extended sand separators to maximise sand/water separation, each set feeding onto two de-watering vibration screens. Fines water from the separator is returned to the effluent tank. The underflow from the dewatering screen sumps is then re-circulated back to the regulating sump. Dirty water collected in the effluent tank is pumped into the Haith thickener for treatment.
Effluent treatment
Wash water and silt emanating from the aggregate washing and sand system is treated by an effluent treatment system, supplied by Haith, which provides high quality treated water that can be used by the system.
Interlocked with the effluent delivery pump feeding the thickener delivers the pre-prepared flocculent solution stored in a multi-compartment powder system equipped with a 25 kg storage hopper. Once charged with flocculent powder the system makes up flocculent to pre-set solution strength on demand. The multi-compartment system prevents short cutting and so ensures all product is fully matured before use. The addition of flocculent is required to achieve solids liquid separation within the thickener giving rise to a thickened slurry underflow being discharged at the base of the cone shaped vessel and clear supernatant being discharged over the weir plates fixed to the inner face of the peripheral gallery on top of the thickener. The thickener internal rake system runs continually and maximises solids compaction by releasing the entrained layers of water that may be trapped in the sludge blanket. They also ensure that the compacted solids are directed towards the cone outlet and underflow pump suction.
The clarified water flows by gravity into a new clean water storage tank fitted with two pumps, one being main feed water to the screen house and the other required for feeding high pressure water to the bottom of the thickener cone to act as a stirring agent if required.
The thickened underflow is extracted from the clarifier via a centrifugal pump, sized to manage the thickened fines. The control system links this pump via a pressure transducer within the thickener drive to continually monitoring the torque / load on the rake system, which is an indication of the fines density. Using pre-set limits on this control enables fines of a predictable,
34 Solids & Bulk Handling • March 2010
www.solidsandbulk.co.uk
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