By Andy Bateman, Engineering Editor

The Edgar sand and gravel operation is located about 20 km northeast of Barrie and produces a full range of construction aggregates.  Pit superintendent Roger McGillvary explains that the pit is projected to produce about 700 000 tonnes this year and, like other operations in the group, is experiencing strong demand for aggregates from ready mix and precast concrete producers. As a result, recent plant improvements have focused on optimizing its output of quality concrete aggregates.

The two new portable wash units are currently part of a high capacity spread that is producing washed 22 mm x 4.75 mm coarse concrete aggregate and 4.75 mm minus classified fine concrete aggregate. Additional equipment includes three crushers and two dry screens, together with associated surge bins, process conveyors and stockpile conveyors. Both of the new portable wash units incorporate a twin screw fine material washer. On the first unit, a wet triple deck screen is mounted over the washer, while the second unit has a classifying tank mounted over a similar washer. The screen-over-screw plant is producing coarse concrete aggregate and sand feed, while the tank-over-screw plant is producing classified concrete sand.

The deposit here contains 250 mm minus material, of which between 30 per cent and 50 per cent is coarse material (6.35 mm and larger), depending on location within the pit.   At the face, a Caterpillar 988F wheel loader loads a 32.5-tonne capacity Volvo A35D 6x6 articulated dump truck – a combination that allows raw feed to be blended from several locations and the deposit’s yield of finished product to be optimized. After a haul of between 100 m and 300 m, the A35D discharges into a 50-tonne capacity receiving hopper where a vibrating grizzly scalps off 102 mm minus feed material. A Hewitt-Robins feeder then regulates the flow of larger material to a second grizzly, where any remaining carry over fines are removed.  The feed is then reduced to 152 mm minus by a 30x42 Pioneer jaw crusher and conveyed to the primary screen, an 8x20 Simplicity triple deck inclined unit fitted with 38 mm, 22 mm and 4.75 mm screen cloths.  Material small enough to pass through the screen’s bottom deck bypasses the rest of the crushing and screening circuits and is conveyed directly to the second wash unit.

The fine aggregate fractions are discharged periodically into the twin Eagle 1.1 m x 9.75 m unit below the tank, where the material is washed again. In the final step, dewatered, classified sand is discharged onto the first of a series of field conveyors leading to the product stockpile.

Power for the portable spread is provided by two Caterpillar generator sets – a 725 kW generator is driven by a 3412 diesel engine while a 350 kW generator is teamed with a 3406 diesel engine. For aggregate washing, the system currently utilises a common pond for fresh and grey water, although a separate fresh water pond may be constructed for next year’s operating season. A Berkley pump with a capacity of up to 3000 gpm pumps fresh water to the wash units, while a Linatex 10 x 8 unit handles grey water.

Following the successful addition of the wash plants, plans are in hand to remove excess crushing capacity from the spread, thereby reducing its overall size and delivering cost savings. The initial feeder and the ElJay 54-in Rollercone crusher will be removed, while the primary jaw crusher’s setting will be adjusted to compensate for the cone’s removal.

In operation, McGillvary reports that the wash units are meeting the design requirement of consistent product grading for both coarse and fine aggregates. On the first unit, flop gates on the wet screen discharge chutes allow any excess of a particular size fraction to be diverted from the product stream if required, thereby providing close control of the coarse aggregate gradation. Over on the second unit, the double washing provided by the classifier and screw washer, together with the classifier’s fraction blending system are delivering consistent sand grading and cleanliness. In this application, the tank/screw combination is removing 200 mesh fines that comprises between 2 per cent and 10 per cent of the 4.75 mm minus feed.  According to the manufacturer, the classifying tank has three functions – it removes excess water from the screw washer feed, classifies the material by removing excess material in certain mesh sizes, and retains unwanted fines. The screw washer, rated at up to 318 tonnes of product per hour, requires a specific flow rate of water so that there is sufficient time for the coarser particles in the feed to settle out while the undesirable fines are carried out in suspension over its weir.

The classifier’s Eagle Autospec Mk V control system monitors the gradation of the feed entering the classifying tank, sensing variations in the build up rate of material over each of the tank’s settling stations. In response, the quantity of material discharged from each station is adjusted to match the desired product grading, a system that is said to maximize product yield and produce the consistent fineness modulus sought by concrete producers.

For transport, the wash units have been engineered to minimise overall height and are the result of discussions between Ed Lamb, Walker Industries operations manager, and engineers at Strongco Equipment’s Mississauga facility. The spray bars above the wash screen fold down for transportation, while the fines collecting pan under the screen has nearly flat side slopes. Water jets have been fitted inside the pan to flush any fine material from its corners and prevent any material build up. As a further height saving measure, both wash units are mounted on a chassis with deep drop sections on the side frames, enabling the wash tank to be mounted as low as possible while still main-taining adequate ground clearance for road travel.

The wash units are not the only equipment at the Edgar Pit that have been designed and built with portability in mind. Extensive use has been made of the company’s new JG stackable field conveyors.  Built by Baker Fabrication, about 40 of these conveyors are already in service in the group and their box frame design allows up to six conveyors to be stacked on a flat bed at once, for a total of length of just under 110 m of conveyor per load. These units carry 905 mm wide belts and have been manufactured in 18.3 m and 35.1 m lengths, with the longer units folding for transport.

Baker Fabrication is the Markdale-based division of Georgian Aggregates & Construction Inc., itself a wholly owned subsidiary of Walker Industries Holdings Ltd.

The wash units were supplied by the Aggregate & Paving Division of Strongco Equipment, custom fabricator and the local distributor for Eagle Iron Works and Cedarapids products.

Riverwood Supply & Manufacturing Inc. of Orillia supplied the fresh water and slurry pumps for the washing system.