Rock to Road

Features Aggregates Technology
Multiple winning lines for new sand plant

September 10, 2008  By Administrator

For aggregate users, low moisture
content can be an important property of an aggregate operation’s sand
and other fine sized products

For aggregate users, low moisture content can be an important property of an aggregate operation’s sand and other fine sized products. Whether the sand is destined for concrete, asphalt or other applications, customers, perhaps not surprisingly, want to pay only for the sand and not for excess water. At the CBM McNally pit near Aberfoyle, Ont., a new sand production plant is producing drier concrete sand while simultaneously delivering cost savings to the producer.

John Pennings is the plant manager at the McNally pit and explains that the new sand system is part of an integrated crushing spread producing a full range of aggregates. Products from the pit are in strong demand, particularly fine and coarse concrete aggregates for the company’s ready mix operations. On the day of Aggregates & Roadbuilding’s visit, there was certainly little sign of any slowdown in the Ontario economy with a steady stream of highway trucks hauling product across a new scale.

The new sand production leg itself consists of a surge bin, twin rinsing screens, and twin dewatering screens with an integral separator. In operation, minus 24 mm feed material from the main spread first enters the ElRus surge bin which controls the flow of feed to a matched pair of Terex Simplicity 6×20 triple deck wet screens. The load spreading top decks of these screens are fitted with 16mm wire mesh while the middle decks are fitted with urethane screens having both 9.5 mm and 7.9 mm openings. Product from the top and middle decks are combined and conveyed to the crushing side of the plant for the production of coarse concrete aggregate. The screens’ bottom decks are fitted with urethane media having 3.2 mm openings. Chip (minus 7.9 mm plus 3.2 mm) passing the middle deck and retained on the bottom deck is stockpiled separately while a mixture of rinse water and minus 3.2 mm material passes through the bottom deck and into a slurry tank under the screens. Sand slurry from this tank is pumped by an LPT 10×8 pump to a McLanahan VD18 dewatering screen designated DS-1 coarse. The coarser sand fraction in the slurry discharges off this screen while a mixture of water and finer material falls through into a slurry tank below. From there, fine sand slurry is pumped by a second LPT 10×8 pump up to a McLanahan 920 mm diameter separator. Separator overflow containing mostly unwanted minus 200 mesh fines exits from the top of the cyclone while partially dewatered underflow discharges onto a second dewatering screen designated DS-2 fine. Coarse and fine sand from both dewatering screens is combined on a common product conveyor belt and stockpiled as a drip free cake. Although it has not been used so far, a gate located in the coarse dewatering screen’s discharge chute allows some of the coarse fraction to be diverted to a separate stockpile, thereby increasing the ratio of fine fractions in the main sand product.


Overall, Pennings is pleased with the new system on a number of counts: “The concrete sand produced by the previous system tended to be on the coarse side, making it necessary to adjust the gradation by the addition of fine sand. That step is no longer necessary as the new plant has improved fine sand fraction recovery to the point where the product stream meets specification without further adjustment. In addition to eliminating the costs of fine sand addition, improved fine sand recovery also means an increased ratio of saleable product and a similar reduction in the loss of saleable fractions going to waste.” Pennings adds that the reduced moisture content of sand from the new plant, typically about 7.5 per cent off the dewatering screens, benefits both customer and producer. For customers, drier sand product means that sand can be shipped soon after production at times of peak demand, thanks to reduced stockpile draindown time. For the producer, drier product translates into higher stockpiles which consume less real estate than the slumped stockpiles of wet product. Drier product also improves haulage efficiency with a higher sand to water ratio in each truckload. As Pennings points out, every 1 per cent reduction in moisture content means 10 tonnes more sand and 10 tonnes less water in every 1000 tonnes of product delivered. Finally, the plant is delivering expected productivity by averaging 200 tonnes/h of finished product. Even at this output rate, the plant is operating well within its maximum rated capacity of 300 tonnes/h.
As an interesting aside, Pennings points out that the surge bin installed at the head of the sand system serves multiple purposes. In normal operation, the surge bin helps to maintain a steady flow of material to the sand screens by absorbing any pulses or interruptions in the flow of feed material. That steady flow helps to improve product consistency as the rinsing and dewatering screens see a consistent material load. At the same time, the surge bin minimises the opportunity for screen or slurry tank overloading by preventing large surges of feed material reaching the screens. The design of the surge bin also allows the sand leg to be isolated from the rest of the production spread when required. In this operating mode, the bin’s feeder is turned off and feed material accumulates in the bin. Excess feed discharges down a chute on the back of the bin and from there conveyed back to a feed stockpile. Later on, the feed can be recovered and baled directly back into the surge bin by a wheel loader.

Technical details on the dewatering screens indicate that screen DS1 is a 6×12 VD 18 unit equipped with two 6-hp motors, while screen DS2 is a slightly smaller 5×12 VD 15 unit, also equipped with two 6 hp motors. Each screen is equipped with a feed box, discharge chute, underpan and TEMA Isenmann 305 mm x 305 mm urethane panels, while both are equipped with two sets of dams to allow for maximum water drainage and dewatering. On the coarse screen (DS1), the screen panels in first eight rows from the feed end have 2 mm apertures while the remaining four rows have 1 mm apertures. With these screen openings, the typical product gradation off DS1, measured as per cent passing each respective sieve size opening, is 100 per cent 6 mesh, 83 per cent 8 mesh, 50 per cent 16 mesh, 6 per cent 50 mesh, 2 per cent 100 mesh and 1 per cent 200 mesh.

On the fine screen (DS2), the whole deck is fitted with 0.3 mm aperture panels, resulting in a product gradation of 100 per cent 16 mesh, 42 per cent 50 mesh, 11 per cent 100 mesh, and 1 per cent 200 mesh. By comparison, the typical gradation of waste from the separator overflow (again, measured as per cent passing each sieve opening) demonstrates how unwanted fines are being removed from the finished product: 100 per cent 50 mesh, 96 per cent 100 mesh and 76 per cent 200 mesh. The typical water flow rate through the system is 8328 litres per minute at a solids feed rate to DS1 of 270 tonnes/h. If necessary, the water system has the capacity to provide an additional 1136 litres per minute. The finished sand product has a design moisture content of 15 per cent (85 per cent solids).

According to the LPT Group, the Velco Dewatering (VD) Screen was introduced 25 years ago and has stood up to the most arduous mining and aggregate duties during that time. Designed primarily to remove excess water from sand fractions, the dewatering screens provide drip-free discharge to improve housekeeping and inventory management. If required, a divided screen can accommodate two to three products at one time, allowing for separate products or desired blending. The screens are typically positioned after a screw washer, cyclone, or classifying tank, with their relatively dry product available for immediate load out.

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