Quarrying

Efficiency boost

Hanson’s replacement marine sand and gravel processing plant at Frindsbury Wharf improves safety and boosts efficiency

Based on the River Medway in Kent, Hanson’s replacement marine sand and gravel processing plant at Frindsbury Wharf is providing increased productivity with significantly reduced energy use. The operation supplies materials for concrete production such as 10, 20, and 40 mm aggregates including sharp washed sand to predominately ready mixed concrete outlets within a 20 mile radius, and also the Aggregate Industries bagging plant situated on an adjacent site.

The site was conceived in 1989 out of marshland and features a 200m cofferdam with a jetty that receives cargoes of sea- dredged aggregate from Hanson’s own fleet; predominately from the Norfolk coast and the English Channel. Hanson had been operating the old plant since March 1990.

“It was basically mobile kit stuck up on stilts which corroded quite quickly,” says Mike Smith, unit manager at Frindsbury. The old plant was fed by a loading shovel, which carried materials to be processed 120 metres to the plant feed hopper with crushing of larger sizes involving mobile equipment. This meant that materials were first processed then stored, crushed and then processed again.

So five years ago, Hanson submitted applications for the current fully automated plant that eliminates the double handling. Planning was achieved a year later and the installation was completed in March 2009.

With the historic dockyards as well as residents just across the river, Hanson held an open day to explain the benefits that the new plant would deliver, such as less dust and noise and the fact that operation would no longer have to operate 24 hours a day due to its increased productivity. “Aesthetically, the site has improved tremendously,” says Mike. “Furthermore, it’s state-of- the-art in terms of conservation of electricity and conservation of water. From that we achieved planning quite quickly and we still kept the 24 hour operation ability in the unlikely event that this is required however it’s unlikely because the capacity of the new plant is substantially higher than the old plant.” The old plant operated at approximately 150 tph; at its maximum the new plant will produce 550 - 600 tph, depending on operation mode and the material going through it. “Different dredging areas give a different make-up of the product we are trying to screen – from very fine to very course,” says Mike. The principal contractor for the project was Centristic, who designed, manufactured, erected and commissioned the structures, chutework, hoppers, bins and conveyors, supplied the three Skako Comessa feeders and installed the free issue Sandvik crushers and screens. A Linatex sand plant and Haith thickener system were separately contracted to Hanson. The civils were designed by the Millward Partnership and constructed by Firgrove, while the electrics installed by IES.

The process operation

Various grades of raw marine aggregates are transported to Frindsbury by ship - 4500-5000 tonnes of product at a time - and ground stockpiled for plant processing. It takes about 4.5 hours to discharge around high water.

The three Skako feeders carry out the principle discharge functions. In addition to the feeder trough the design includes the integral hopper bottom section, incorporating skirt plates, bed depth regulation gate, and pivoting helical coil spring suspension system. All the feeders are galvanised to suit the ‘salty' corrosive environment and the abrasive feed material. Both the hopper bottom sections and feeder trays are lined with Hardox 400 replaceable liner plates. To achieve the optimum blend of the variable raw material feed stock, the primary feed (through a ground hopper) is partitioned into two, with a Skako feeder located under each side, each arranged to centrally discharge onto the main plant feed conveyor.

Each of the two primary dump hopper discharge feeders is 1000 mm wide and individually capable of discharging the required feed rate. However, normal operation involves both

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