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Gallagher's Invest Nearly £2 Million in New Powerscreen Plant.

Hermitage Quarry near Maidstone, Kent has recently been the scene of further major development work; owned and operated by the Gallagher Group, a near £2 million investment in a new 350tph Powerscreen Washing Plant and Silt Management System is the most recent addition.

Gallagher now operates the largest Kent Ragstone quarry and the deposit comprises an inter-bedded 50 – 50 mix of Ragstone and Hassock. The Hassock is a relatively soft sandy material with a high, but variable, silt content that is separated from the ragstone by a trommel screen before crushing. Prior to the introduction of the Aggregates Levy, Hassock was sold in large volume as a low priced constructional fill material but the addition of the Levy resulted in this no longer being a viable option. Consequently, Hassock became a waste product to be disposed. Obviously, this is in direct conflict with the principles of the Aggregates Levy.

From the environmental view-point it seemed to Gallagher to be a nonsense that the introduction of the Levy should result in an increase in the extraction rate at the quarry in order to maintain a similar level of sales “out of the gate.”

Following extensive trials, Gallagher found that by washing the silt content from the waste Hassock, the resultant materials might prove suitable in other applications. At the same time, trials were undertaken on washing the fines residue from the on-site recycling operation and that too proved encouraging. Ever mindful of customer comments, Gallagher were aware that whilst Kentish Ragstone can produce a compliant Type 1 sub-base material in periods of extended heavy rain, the level of silt in the product can make it weather susceptible and difficult for it’s customers to lay. Again, removing silt by washing will eradicate the problem. With all this in mind, Gallagher decided that a washing plant would resolve some of the problems and result in the improvement in quality of its materials.

The decision was made and work on the civils undertaken by Gallagher Contractors commenced in December 2003 and involved the placement of some 5000 cubic metres of ready mixed concrete supplied by Gallagher Concrete.

Gallagher then passed the baton to Powerscreen Washing Systems of Coventry, a company with considerable experience and the leading exponent in aggregate washing within the Industry.

Aggregate Washing at Hermitage Quarry.

Powerscreen Washing Systems designed and installed a washing plant that interfaced with the existing crushing plant. The -100mm material from the existing trommel is conveyed via a 26m radial conveyor to a 30 ton hopper. This conveyor can be raised hydraulically and radially so that the Hassock can be stockpiled separately if necessary. The hopper has a remote controlled 4.5m long tipping grid and can also be fed directly by a mobile loading shovel.

From the hopper the material feeds onto an 80 ft long M100 conveyor. All material is then delivered into a double deck static 20 x 6 rinsing screen which separates the oversize, with the underflow passing to a Fines Master 160 which produces a combined total of 100 tonnes/h of soft and sharp sand. This efficient machine retains all the sand and separates the -75 micron silt and water. It comprises a twin bucket wheel, a centrifugal slurry pump, twin hydro cyclones, a collection tank and a high frequency 12 x 5 ft dewatering screen mounted on a single chassis to produce the grades of sand. This product with low moisture content is then delivered via two powered radial conveyors onto their respective stockpiles.

The remaining midsize material passing through the rinser is fed into a Powerscreen Powerscrub 180R logwasher, capable of producing up to 200 tonnes/h of contaminant-free sand and gravel. The Powerscrub unit comprises an inclined trough fitted with two counter-rotating axles with Hardox 500 exchangeable blades and an elevated spray-bar system. The unit mixes incoming feed while scrubbing and washing the material at the same time. Any roots, wood or plastic is discharged with the water over two sieve bends and removed.

Mid-size material passing directly through the Powerscrub is then fed onto an 80 ft M100 conveyor into 20 x 6 triple deck part rinser, fitted with spray bars the material is given a final wash before it is screened to four separate powered radial conveyors providing stockpiles of 6/10/14/20mm graded product.

To minimise sand losses from the plant all the -5mm water/sand/silt underflow from both screens and the logwasher is fed via Warman sump and pumped to the Powerscreen Fines Master 160.

The production process at Hermitage quarry has benefited from a consistent sand product grading even though the feed material varies. Plant Safety and operation is very important to Gallagher’s, hence all conveyors are powered radials, fitted with walkway along one side and round the head drum.

Silt Management.

The Silt Management system combines technology and experience from Tecnoidea, Parnaby, CIBA and Powerscreen Washing Systems.

The system is designed to clarify and reuse all the waste water and produce a manageable silt cake. All water and silt from the Powerscreen Fines Master is pumped across the site to a sieve bend with 3mm aperture sieve bends. This removes any oversize and protects the cloths on the belt presses.

The water and silt is collected in the primary pit and pumped via two ABRA pumps to two 7m diameter deep cone thickeners. Flocculent is automatically mixed in two Tecnoidea mixing stations and automatically dosed to the thickeners. The clarified water weirs over the top of both thickeners and is piped to a collection tank to be pumped back up to the washing plant. Top -up water for the wash plant comes from two bore hole pumps adjacent to the clarified water tank.

The underflow from both thickeners is collected in a 150m3 sludge tank fitted with three stirrers to maintain agitation. From here it is pumped using two Warman horizontal pumps, inverter controlled to the two 3m wide Parnaby belt presses. The silt management system was specifically designed and sized to minimise operator input and maintain low running costs. The system includes automatic flocculent mixing and dosing plant with a control system that minimises use of flocculent; automatic anti-clog system for the thickeners discharge valves, support framework, structure, walkways and access platforms.

The complete system is fully automated and only requires an operator to add flocculent powder occasionally and remove the pressed silt cake using a loading shovel. As the sludge is fed into the thickening tanks from the Powerscreen plant the feed is monitored every 50 seconds. This allows the system to automatically adjust the feed rate of the flocculent solution thereby guaranteeing clean water for re-washing and minimising the amount of flocculent required. Once in the thickener the silt settles to the bottom of the cone and is automatically discharged into the sludge tank when the density is 35% solids. This is constantly monitored so that a consistent feed is continuously pumped to the belt presses.

PSWS in sizing the water treatment plant for this application had to consider such factors as the silt content of the raw material, settlement rate, throughput and site drainage.

The flocculent mixing and dosing system has a continuous supply of filtered clean water from another two bore hole pumps and automatically mixes a batch of flocculent solution when the level drops. An operator only has to add the powder to a storage bin.