Terex Washing Systems’ washing plant is helping a third-generation company feed sand and gravel products to concrete sites in Cork, Ireland.
Keohane Readymix’s plant in the Irish county of Cork processes natural sand and gravel deposits. The material, while fairly free-flowing, can get sticky and contain clay or organics like roots.
The feed material is 60:40 sand to gravel, predominantly minus 150mm but can be up to 300-400mm.
Keohane’s old wash plant was worn out, and they needed a new installation that could handle their unique requirements.
Terex Washing Systems (TWS) was selected to design the plant and began to analyse a sample of Keohane’s as-dug material and used this to propose the plant’s design.
Terex Washing Systems designs and manufactures a range of feeders, from H9 to H30, including static grids available in multiple spacings. The hopper volume of the H30 is 30m3 with an internal breaker bar to relieve the load on the belt. The feeder’s heavy-duty 1050mm wide belt is supported by impact rollers and is driven by a direct drive gearbox with variable speed control.
The Aggresand 206 is an integrated sand washing plant with a main conveyor powered by dual 11W electric motor and gearbox combinations, with alignment rollers to keep the belt centred and prevent belt damage. The main conveyor transfers material from the feeder to the main rinsing screen, where it is rinsed and screened on all three decks with polyurethane media, X spraybars, and ball valves. The rolling chute, a popular design feature on the Aggresand 206, splits the aggregates into three products, and a simple flip of the blending plate can blend mid-overs and mid-fines into a single product.
The ring-main controls the water delivery, and the washbox, which briefly contains the feed material while introducing a high volume of water to fluidise the sand and gravel, is key to maximising the rinsing screen’s performance. The high-capacity sealed washbox is supplied with water independently controlled for volume. The Aggresand has been designed with multiple valves that allow the drainage of the complete water system at the end of each day, preventing frost pockets that can cause blockages and damage.
20 x 6 Screen
This rinsing screen is a 20×6 three-deck box. Powered by a single 22kW electric motor, it operates at a fixed angle of 18°, ensuring optimal speed and throw for effective screening across all three decks. The screen is equipped with spraybars on each deck, which have staggered nozzle patterns to ensure efficient rinsing of all material. The spraybars feature individual ball valves that can be shut off as needed, with each spraybar being statically mounted to increase its lifespan and prevent additional weight and stresses on the screenbox.
Polyurethane media is used on all three decks, providing maximum wear life in washing applications. The screen is designed to wash sand through the bottom deck, with three different aggregate sizes being collected at the end of the screen and directed to a rolling chute. The rolling chute can be used to split the aggregate into three products, with oversize material going over the top, mid-overs going over the middle section, and midfines dropping into the closest chute to the screen. Alternatively, the mid-overs and mid-fines can be blended into a single product by flipping a blending plate. The oversize product can also be blended by repositioning the plates at the end of the chute.
The screen’s roll-away chute provides access to the discharge end of the screenbox, with a drop-in, lightweight flooring providing a safe platform for inspection and maintenance. Keohane is currently running the screen with 55mm apertures on the top deck, 24mm apertures on the middle deck, and a split of 2mm and 5mm on the first and second sections of the bottom deck to produce two grades of sand. The oversize material is directed to a stockpile at the back, while the mid-overs are discharged to the right. The 5mm-24mm aggregate is transferred via an extended conveyor to the AggreScrub 150.
Main collection sump tank
The main collection sump tank is one of the critical components of the plant. Its primary function is to collect the water used in the washing process and provide a central point for the collection of fluidised fine material.
Water from the wash plant flows into the sump tank.
Underground water storage
Unique for Keohane, the reservoir for recycled water used in the wash plant is located beneath the plinth to safe footprint on the overall layout. The water recycling system employed reduces the amount of freshwater required for the operation. This subterranean tank helps to maintain the proper balance of water in the system and prevents overflow during heavy rain or excessive water use.
Pumps and maintenance
Two types of pumps are used for handling sand and water – coarse sand pumps and fine sand pumps.
Typically, the pumps used in a wash plant are natural red rubber-lined to provide resistance against abrasive wear and tear caused by the sand particles. Terex has lined all the pipework to protect against wear and tear from the abrasive nature of the sand particles. The lining is a wear resistant rubber that can withstand the harsh conditions of the wash plant environment.
Regular maintenance of pumps is crucial to ensure their smooth operation and prevent breakdowns that can cause production downtime. During maintenance, the pumps are rolled out of their positions, dismantled, and inspected for any signs of wear and tear. Worn parts are replaced, and the pumps are reassembled, aligned, and reinstalled back into position.
Cyclones and underflow blending
The sand and water slurry is pumped to two rubber lined cyclones. The larger 660 Cyclone receives the higher volume coarse fraction, while the 500 Cyclone receives the lower volume 0-2mm fraction. Inside the cyclones, silts and clays are separated from the sand and discharged out the top of the cyclone, while the in-spec sand is discharged via the spigot into the underflow collection box.
Easy access lids allow for visual inspection of the umbrella flow out the bottom of the cyclone. A simple blending box within the main underflow box provides a second opportunity to blend fine sand into the coarse sand.
Sand dewatering screen
The sand is then discharged onto a 14×6 high frequency dewatering screen, which uses polyurethane media and dividers to keep the material apart the entire length of the screen. By the time the sand reaches the end, the moisture content has been reduced to below 14 per cent. The dewatering screen’s underflow is returned to the central collection sump for maximum recovery of fines.
TWS designed a custom lined chute for Keohane’s site to use one standard radial conveyor for the fine fraction, and a radial conveyor for the higher volume 0-5mm sand. This chute provides the third and final opportunity for the Keohane to blend some of the fine sand into the coarse sand.
All dirty water reporting out the top of the cyclones is collected and discharged into the ground collection sump.
When there is a high concentration of silts and clays in the feed material, the aggregates can bind together, making it difficult to remove the clay through washing alone. The material analysis completed during the initial proposal stage determined that scrubbing of the material was required.
Scrubbing involves a process of stone-on-stone attrition, which breaks down conglomerates and frees the aggregates. TWS washers are designed to handle a feed rate of 60 to 200 tonnes per hour and produce a clean and marketable aggregate. The AS150 version is modular and can be easily transported and assembled. It comes equipped with a part-rinsing sizing screen, a high-volume trash screen, and a central collection sump, and offers easy access to all serviceable areas.
In this specific application, most of the conglomerates are found in the mid fines portion. The stock is diverted to the AS150 via the Aggresand extended conveyor. The challenge is to then liberate valuable aggregates and light material, such as organics or trash, from difficult-to-break-down fines and insoluble clay.
The Aggrescrub range features twin rotating bladed shafts with two dedicated motor gearboxes that introduce energy to the feedstock, causing the particles to collide and scrape against each other. This stone-on-stone attrition liberates conglomerates, which are transported across the water and expelled out of the rear weirs, and onto the trash screen where they are dewatered. The water then travels to the integrated sump tank.
As the aggregate exits the hull, it receives a final rinse to remove any 0-5mm carry over, and this is sent to the integrated sump. All the water used in the scrubbing process is contained in a closed loop, and the dirty water is collected in the integral sump. From here, it is pumped back along the extended conveyor to the Aggresand sump, where it enters the sand washing process. What remains is a pile of clean, marketable aggregate in distinct-sized stockpiles and a neat pile of trash and organics.
Flocculant dosing unit
The plant uses an FDU3000 flocculent dosing unit. The 3000-litre capacity unit is a stainless-steel construction split into three separate tanks, a mixing tank, maturing tank and storage tank.
During flocculant preparation clean potable water is pumped from a water storage tank situated outside the control room. Simultaneously dry flocculant powder is fed into the pressurised water entering the mixing tank flow by an auger mounted in the flocculant hopper. The flocculant solution is then thoroughly mixed using a mixer in the mixing tank.
Premixed flocculant is displaced and overflows from the mixing tank into the maturing tank via a weir, where it is further mixed and allowed to mature for between 45 -60 minutes.
Flocculant in the storage tank is then pumped via a mono pump into the slurry feed pipeline just before the static mixer, where it thoroughly mixed with the slurry feed, before entering the thickener.
The settling rate of the solids within the slurry are continuously monitored automatically by the dosing control unit. This allows the control panel to monitor and adjust the amount of flocculant used and therefore optimise consumption.
Deep cone thickener
This is where the dirty water, now dosed with flocculant, enters into the centre feed well. Immediately the floc begins working, causing the particles of silt and clay to bind together and sink to the bottom of the DC220. Here the clarified water weirs over the troughs and is channelled into a below ground water storage tank. The thickened sludge is compressed in the deep cone and the volume of sludge in the thickener is controlled via this sensor. When the level reaches its set point the sensor sends a signal to the control system which automatically opens the discharge valve. The thickened sludge is then transferred to the pond under gravity. A key advantage of the deep cone thickener is not requiring a sludge pump. The sludge can also reach a higher solids ratio, on average 50 per cent solids, meaning that a greater level of water in recovered. TWS installed flush out points at the exit position of the deep cone to allow the operator to easily flush out any blockages safely and quickly.
The thickened sludge is piped to the settlement ponds where is contained and easier to handle.
Central control room
TWS also incorporated a central control room with insulated panels lighting and heating. This is where the main panels and switch gear systems are located. The control panels have a touch screen system that allows operators to drill down into the system and look at an individual machines performance to optimise plant efficiency. When faults arise, these are highlighted on the large display screen and can be identified and rectified quickly.
The system also allows for Terex engineers to remotely dial in for troubleshooting and fault diagnostics. They can help with plant performance and change settings if feed material change or support are needed.
End to end
The plant was designed with custom features to meet Keohane’s specific needs.
The company provided a full turnkey solution that leveraged its wide product portfolio. All of the equipment used in Keohane’s Brinny plant comes from TWS’s product line. TWS’s decades of in-house experience helped ensure its equipment is applied properly and efficiently. •
Visit terex.com/washing to learn more.