New wash plant for NRL

DUO Equipment install full washing and water-management plant for Nottinghamshire Recycling Ltd

DUO Equipment have recently supplied and installed a complete new washing and water-management plant for Worksop-based Nottinghamshire Recycling Ltd (NRL), a sustainable waste-solution provider that processes construction and demolition waste, recycled aggregates, wood and green waste.

The new plant, which includes a Gennaretti centrifuge system, will provide NRL with the ability to divert waste material from landfill and to supply high-quality clean washed products to the local construction materials market.

In deciding which processing plant to purchase, NRL focused on the quality of the equipment and the ability to provide bespoke plant design and specifications for their particular site, as well as back-up service and support and a proven ability to process difficult materials.

Production process

Currently producing recycled aggregates at a rate of 100 tonnes/h, material arriving on site is fed by excavator into a static Powerscreen Chieftain 1400 feed hopper featuring a single-deck, live-head grid and an electric variable-speed feeder. Material from the hopper is fed on to an inclined conveyor equipped with a magnet and delivered on to the unit’s 10ft x 5ft static rinser screen. This is fitted with polyurethane deck modules and incorporates six top-deck and six bottom-deck spray bars with plastic nozzles.

This machine features a patented four-bearing, ‘free-floating’ screen box, which eliminates the need for any fine-tuning. Acting as the primary screen, the rinser facilitates the removal of the sand and silt component, which is fed to a conical sump where any lightweight contaminants overflow into a self-cleaning filter. The fine sand is pumped to an adjacent Finesmaster 120 Compact twin-grade sand plant, with the coarse sand being fed to the opposite side of the machine.

Meanwhile, material that passes over the rinser’s integral screen decks is fed on to a T4026 conveyor to stockpile, with a further direct feed into the latest design of Powerscreen recycling log-washer.

Capable of producing up to 120 tonnes/h of contaminant-free sand and gravel, the log-washer comprises an inclined trough with two counter-rotating axles, both equipped with high-tensile exchangeable blades, and an elevated spray-bar system. The unit mixes the incoming feed while milling and washing the material at the same time. Any lumps of earth or clay are broken down during this process and carried away in the wash water, while any lighter particles are floated off through specially designed outflow exits on to a 6ft x 2ft trash screen.

The scrubbed gravel is discharged on to an 8ft x 4ft dewatering screen fitted with polyurethane modules and three spray bars to provide a final rinse. The water and sand flows under gravity into the adjacent Finesmaster machine, while the clean material is sent, via a transfer conveyor, to a 10ft x 5ft double-deck dry screen which grades the material into 10mm, 20mm and 40mm sizes. These are conveyed to stock by three further radial stockpile conveyors.

During the production process, material from the bottom of the recycling log-washer is fed into the Finesmaster sand-recovery unit where it is classified at approximately 75 microns and dewatered. Comprising a 15in fines cyclone and a 24in coarse cyclone, a centrifugal slurry pump, a collection tank and a high-frequency 12ft x 5ft dewatering screen fitted with polyurethane dewatering modules mounted on a single carrier chassis, the Finesmaster produces one grade of coarse sand and one grade of fine sand for delivery to stockpile. The unit has the ability to blend the fine and coarse grades for maximum flexibility and produces a combined total of around 120 tonnes/h. Any residual silt is sent to the thickener via a filter and sump arrangement.

Water management

As water management was an essential part of the project, DUO Equipment, in conjunction with their partners Gennaretti, supplied a GHT 503 variable-speed centrifuge system capable of processing silt at up to15 tonnes/h.

In addition to the centrifuge, the water-management system includes a foul-water pump, automatic flocculant mixing and dosing station for the thickener system, a 7m diameter deep-cone thickener tank with pneumatic discharge valve, sludge and fresh water tanks, and all the associated support framework, walkways and access platforms.

Operating principle

The waste water and silt from the cyclones on the Finesmaster gravitate into an adjacent steel sump tank and pass through an in-line screw filter, to remove any organic material and polystyrene, before being pumped to the top of the deep-cone thickener.

Simultaneously, batching pumps automatically inject a flocculant solution into the delivery pipes to mix with the material. Once inside the thickener tank, the sludge drops to the bottom of the cone while clean water weirs over the top of the thickener into the clarified water tank for subsequent reuse by the wash plant.

Meanwhile, the underflow from the thickener discharges into a sludge tank equipped with an internal rake, before being pumped directly to the Gennaretti centrifuge system for dewatering before being stockpiled in an allocated bay. The clarified water from the centrifuge is pumped back into the system via the deep-cone thickener.

The entire process operates as a fully closed-loop system that allows for the recycling of as much water as possible back into the washing plant.

According to DUO Equipment, when selecting the best method of dewatering sludge from the washing plant, the inclusion of a Gennaretti decanter centrifuge was an obvious choice. Gennaretti units are compact in design, relative to their output capabilities, and efficiently process sludge with variable solids content, such as that arising from washing recycled aggregates.

Fintan McKeever, director – DUO Equipment, commented: ‘This unit has proved very successful in processing silts from recycled aggregates, sand and gravel, and limestone, and offers a continuous process incorporating an automatic variable-speed control to handle large fluctuations in feed material.’

For the NRL project, DUO Equipment delivered a fully automatic silt-management system that was specifically designed and sized for the process, in order to minimize operator input and to maintain low running costs.  

The company also took into account such factors as the silt content, clay content and settlement rate of the raw material. The flocculant mixing and dosing system is designed to maintain a continuous supply of clean water and automatically mixes a batch of flocculant solution when the level drops below a predetermined point. All the operator has to do is periodically add the flocculant powder to a storage bin.

According to Kevin Burgess, managing director of NRL, installation of the new washing plant and water-management system has been very beneficial. ‘The choice of plant was based on the need to utilize a closed-loop water-treatment system and the ability to produce a consistent product to very demanding market specifications,’ he commented.

‘We have a significant number of Powerscreen machines across all our recycling sites and we expect, and receive, guaranteed service support from DUO and Powerscreen. The new aggregate washing plant will help to maintain NRL’s position as one of the major suppliers of quality recycled aggregates in the Midlands.’