Darrington Quarries Ltd is part of the international infrastructure, environmental services and energy group, FCC and is a long established quarrying company operating in Yorkshire with six limestone quarries situated close to the A1, from Doncaster in the south to Tadcaster in the north. With an additional sand quarry at Hensall the company offer a full range of limestone aggregates for road construction, house building, concrete production and civil engineering. Additionally, Darrington Quarries also offer recycled aggregates and certificated green tree topsoil, with inert materials being accepted at their Barnsdale Bar Quarry for recycling and restoration applications.

Last year the company instigated a procurement process for a water and mud treatment plant facility to be built at their Darrington Quarry site. After very competitive bidding Finedoor Ltd were accepted as the most comprehensive and competitively priced tender to supply a SOTRES Washing and Water Treatment Plant.

Awarded the turnkey project Finedoor were then tasked to supply a plant within 6 months which would process a maximum of 450 m³/h (99,000 gallons/h - 1650 gallons/min) rate of waste water from the washing plant, treated with dry solids concentration in a sample taken at 72 gr/liter. With a designated working week of an 8 hour day, 5 day week and 200 days/pa; the production facility is required to produce dry mud (easy to load), and clarified water to re-use on the washing plant.

Designed for low maintenance and running costs Finedoor were responsible for the construction of the plant, including all steel erection, installation of machinery, electrical works and commissioning. The electrical works being sub contracted out to Specialist Electrical Systems Ltd. Additionally, as the plant was constructed to current CDM regulations, Finedoor took the role of CDM co-ordinator for the design and construction of the plant and the civil works.

Plant Flow:

Waste water is supplied into a feed box which links the washing plant and the 13 metre diameter thickener. The waste water is charged with suspended matter (of a particle size less than 80 microns) and is introduced into the feed mixing box which is subjected to high agitation with the flocculent injected at the same time. This very efficient mixing of the waste water with a highly diluted flocculent solution provides a significant reduction in flocculent usage. The flocculated waste water is then poured into the centre of the thickener via a tripod pipe. The flocculants formed settling to the bottom of the tank forming a sludge bed.

This sludge is then brought into the concentration cone by the peripheral scraper arm and blades. A central scraper concentrates the sludge into the cone, maintaining movement, thus eliminating any water suction by the pump. When the sludge achieves the right concentration the pump automatically commences extraction. The overflowing clarified water is then collected into the peripheral trough and stored in a buffer clarified water tank and subsequently pumped back to the washing plant for reuse. A tunnel below the thickener allows the sludge pump to be situated directly underneath the central cone allowing a greater extraction of extracted sludge. This pump maintains a homogeneous mixing of the slurry and avoids any blocking concentration in the cone. At the end of the production day the silo can be fully emptied.

A fresh water tank which is fed from an outside source for the requisite top-up (50m3/h), feeds the flocculent apparatus. The subsequent overflowing is then compensated by the fresh water injection consequently replacing the loss of water in the sludge and the aggregates.

The sludge extracted from the thickener then feeds the 200m³ buffer silo which in turn allows the storage of sludge due to variation in the quantity, or characteristic of the material. A pump installed at the bottom of the silo cone keeps the sludge mixed, avoiding excessive concentration in the cone. An ultrasonic probe then indicates the quantity of sludge in the silo and will only allow the commencement of the pressing cycle if there is sufficient sludge to feed the press.

The Filmac filter press is a 1500 mm square plates machine with a cloth washing system; complete with plate shifting facility (shaking system) the plates are pulled on four points, two high and two low allowing the adjustable system to be efficiently shaken.

The water treatment plant is driven from a dedicated control room with a finger touch PLC. Situated alongside is the Controlfloc Flocculent Dosing Apparatus. Equipped with two tanks this enables an alternated flocculent preparation with a low concentration and ideal maturation.

This apparatus allows the waste water sampling in supply phase already mixed with the flocculent and measures the sedimentation time. This enables the adjustment of the flocculent dosing following the product variation to treat. An inverter allows the PLC to make this automatically varying the flocculent pump velocity with the PLC.

The installation has been very successful and following one of the worst winters in the UK in living memory is now in full production. John Dickinson - Engineering Manager, commented, "Initial fire up was good just about every section of the plant worked first time. This last winter highlighted some problems with frost protection, but Finedoor worked to repair the damage as quick as the parts could arrive. Frost protection has been extended and since this was completed the plant as performed as expected.''