Page 10 - HUB-4 Magazine Issue 85

Washing &
The Water Treatment Plant:
The fully integrated water treatment plant at Kirks Quarry, consists of a 10mØ thickener which has a 230m3 volume capacity with a low flocculent consumption. The plant has the facility to dose, flocculants, coagulants, and anti-foaming polymers, to facilitate the processing of more difficult applications.
An 8mØ clarified water tank provides a 125m3 capacity with a ground-level technical room housing the flocculant dosing unit.
95% of the water used in the washing process can be re-used. The dirty water is collected in a sump before being treated and allowed to settle in the thickener. The thickened sludge is then sent to a buffer tank to control the density of the sludge prior to being compressed within the 1500mm x 1500mm x 180 plate filter press enabling the last remaining water to be sent back into the system. DCA are then able to use the 30mm filter cakes that are expelled from the filter press.
The filter press is housed in a bespoke building with walkways around all sides. It operates up to 21 bar feed pressure and features a rapid plate opening and closing system with an automatic shaker system and electric control panel complete with PLC.
The water treatment plant at Kirks Quarry receives the “dirty water” from the washing process with a double target: Clarifying the water to be able to recycle it and concentrate the sludge.
Flocculation principle:
Flocculation consists in mixing the “dirty water” (i.e. water + dry solids in suspension) with a synthetic polymer having a high molecular weight forming a long macromolecular chain (flocculent). This polymer, characterized by its superficial electric tension, acts on the surface tension of the fines to gather them and form “flocs” which could then have an acceptable settling speed.
Preparation and dosing of the flocculant: DOSAFLOC:
The flocculant is delivered as a white powder soluble in water and producing a highly viscous solution. Its preparation at a concentration rate between 2 and 4 g/l is made in 2 stages:
• Afirstdilutionoftheflocculantdeliveredfromapowder hopper by means of an Archimedes screw distributor and homogenization in a primary tank fitted with a low-speed agitator.
• Transferintoasecondtank(formaturation)before injection and dosing of the flocculent solution is made through a pump fitted with a frequency variator.
Conditioning of the dirty water:
The “dirty water” coming from the cyclone overflow is mixed with the flocculant solution prepared by the Dosafloc. An
additional and final dilution of the flocculant solution through cyclonic spreaders enables to optimize this mixing in the flocculation box fitted with baffle plates. Once prepared the water is directed by gravity into the central feed shaft.
Flocculation Controller:
Sequentially, a sample of flocculated water is taken from the central shaft and the settling speed is measured by the Controlfloc, a glass tube fitted with optical cells. This information is then transferred to the PLC which automatically adjusts the flocculant dosing pump’s flow (according to the settings entered during the commissioning stage) in order to optimize the flocculation efficiency and consumption.
After each measure, the glass tube is automatically rinsed. Samples are taken with a vacuum system to avoid using a pump which would distort the measure by breaking the flocs. The optic cells also enable the ability to determine the turbidity of the recycled water and therefore to adjust the coagulant dosing.
Sludge concentration and evacuation:
The sludge settles quickly at the bottom of the 10m diameter thickener where it is concentrated and gathered into the pumping cone thanks to the slow movement of the scraper.
The sludge is then drawn by a pump located alongside the thickener tank (thus offering an easy access for maintenance operation). The choice of the most suitable sludge pump is depending on the requirements in terms of flows and final consistency, as well as the site constraints (pumping distance, geometric head...)
The sludge pumping cycles are controlled according to the measure of the resisting torque applied on the scraper interpreted by the PLC. This gives a reliable information about the quantity and consistency of the sludge at the bottom of the thickener and also offers a safety factor in case of “building up” inside the thickener. If an overload is detected by the invertor, the scraper is automatically and fully lifted to its upper position and then progressively lowered down to dilute the sludge.
Transfer and pumping of recycled waters:
The recycled overflowing waters are discharged in the recycled water storage tank, where the main water pump installed alongside the tank sends it back the washing plant.
An addition of the fresh water is necessary to compensate the losses due to the residual humidity of the dewatered sand and aggregate.
Electrics:
The whole plant is controlled by PLC with touch screen to display the synoptic and to give access to the setting. The main switch and control panel is located inside the technical room.
Screening cover story
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www.hub-4.com March/April - Issue 85