Haith supply new effluent treatment system at Whatley Haith supply new effluent treatment system at Whatley Haith supply new effluent treatment system at Whatley Haith supply new effluent treatment system at Whatley

Haith supply new effluent treatment system at Whatley

20th March 2009

After one year of continuous use a scalpings washing plant installed at Hanson UK's Whatley Quarry continues to successfully extract clean aggregate from scalpings for onward sale.

Nestled amidst the Somerset countryside on the eastern Mendip Hills, just outside Frome, Hanson's Whatley Quarry - one of the largest quarries in the UK - produces on average 70,000 tonnes of Carboniferous Limestone a week. The rail-linked quarry, which supplies markets across southern England, operates on a five day production basis starting 6am and normally running until midnight.

Around 16% of face material coming through the quarry's main crushing plant ends up as scalpings and the perpetually increasing scalpings pile was causing a problem at the quarry because production would be halted while this material was removed, which of course led to double handling.

Hanson wanted a system that could wash the scalpings to extract clean aggregate so it would not merely keep the scalpings pile under control but also provide product for onward sale.

In the autumn, new plant was installed at the quarry for this purpose and today the system continues to provided a solution that has proved itself to be successful not only in achieving Whatley Quarry's requirements but also in working reliably on an arduous, double shift basis.

The system processes up to 220 tph of scalpings (up to 1500 tonnes a day) to produce a 4 mm washed sand; the popular 30-5 mm aggregate that goes straight to concrete plants almost as soon as it's produced; 20-5 mm, which has found a big market in the Stratford City and Olympics developments in London and also 40-20 mm oversized material that can either be sold for sub-base material but is more often put back through the plant to screen it down to the smaller sizes.

Hanson tasked Centristic to design and build the scalpings washing plant at Whatley and the company acted as main contractor on the project, which saw the system being integrated to the site's existing plant. SP Services carried out the erection while the electrics were designed and installed by Tucker EMS, with the control system produced by Batching & Blending.

Installation of the plant was completed at the end of September 2007 and it has operated for much of the time since then on double shift - operating on almost a 24/7 basis - successfully recovering clean stone and sand from the quarry's scalpings. 

Effluent Treatment

Wash water and silt emanating from the aggregate washing and sand system is treated by the Effluent Treatment System, supplied to Whatley Quarry by Haith Industrial, which provides high quality treated water that can be used by the system and produces de-hydrated silt that can be transported and handled more easily than wet silt.

Designed to meet Whatley Quarry's specific requirements the plant can tolerate variances in the influent yet remain easy to operate. A silt or slurry buffer tank receives and stores the silt from the thickener at a rate dictated by the washing process. Standing between the 13 m diameter Thickener and Filter Presses it buffers the silt volumes generated during the latter part of the pressing cycle when the feed to the press is dramatically reduced. While these small variances can be accommodated in the Thickener, larger silt volumes cannot without the likelihood of a blockage. The inclusion of a buffer tank increases the system's flexibility and robustness by allowing the clarification process to proceed even when the press is on a routine or enforced maintenance outage.

The Thickener rakes are driven by a hydraulic system that suspends the rakes. Such a set-up means submerged bearings and bushes are not needed.

The fully automatic Overhead Beam Filter Presses incorporates an automatic cloth washer system, which negates periodical manual washing.

Effluent emanating from the washing plant is pumped to the Sieved Bend mounted above the feed launder to the Thickener. The Sieve Bend is fitted with a 3mm aperture wedge wire deck, which removes particles in excess 1.5 mm. Any grits removed are captured and held in a hopper at the base of the screen deck.

An effluent delivery pump feeding the Thickener delivers the pre-prepared flocculent solution stored in a multi-compartment powder system equipped with a 75 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 shortcutting and so ensures all product is fully matures before use improving.

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.  It also ensures that the compacted solids are directed towards the cone outlet and underflow pump suction.

Clarified water flows by gravity into a new clean water storage tank fitted with two pumps - one to provide treated water for the automatic cloth washing system on the Filter Press and the other a process pump to return treated water back to the aggregate washing plant (Supplied by others).

The thickened underflow is extracted from the Clarifier via two centrifugal pumps, each one sized for the operating silt yield. Operating in duty/assist the second pump is called to run if yields greater than the design 30 tph are being delivered to the thickener. Each pump delivers the silt via dedicated pipes into the buffer tank. The control system links these pumps via a pressure transducer within the thickener drive to continually monitoring the torque / load on the rake system, which is an indication of the silt density.  Pre-set limits on this control allow silt of a predictable, consistent specific gravity to be discharged into the balance tank automatically.

Level probes on the buffer tank prevent overfilling and also provide dry run protection for the press feed pumps.

From the buffer tank two Filter Press feed pumps operating on a command from the press PLC deliver to the feed ports of two Filter Presses and be constantly monitored by inline pressure switches. On reaching a pre-set line pressure the pumps stop and the presses enter its automatic sequence for opening and discharging the cakes using an overhead beam plate shifting device.

Once the cake has been completely discharged, the presses close and another pressing cycle automatically restarts.

All filtrate from the press is discharged by gravity into the treated water tank. Dirty water generated when the cloth washing system is in operation flows to a wash water tank fitted with a transfer pump, which will pump it to the Thickener feed launder.

The silt cake discharged from the press falls by gravity into a three-sided concrete bund below. The cake changes all the time, depending on how the material comes out of the quarry though more often than not it's emitted as nice solid cake. However the quarry has yet to find a use for this material.

From the control room the system is monitored on the computer graphical interface and can adjust throughput rates at key parts of the system as required.