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Canada's “Rock to Road”
Magazine
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Unique portable screening / washing combination performs for Ontario producer The Novar pit, located 15 km north of Huntsville, is the main source of aggregates for Ministry of Transportation of Ontario (MTO) contract 99-221. This $36.5 million road building job is being completed by the Bot Construction Ltd.-E. & E. Seegmiller Ltd. joint venture and involves the construction of two new lanes as well as the rebuilding of existing lanes on Highway 11 (See Aggregates & Roadbuilding July/August 2000). Asphalt paving has begun on the job and the pit is producing fine and coarse asphalt aggregates for 40 000 tonnes of Heavy Duty Binder Course (HDBC) and 7000 tonnes of HL4 hot mix.
CFS/Sizetec portable cyclone and dewatering tower was designed and built in-house by E&E Seegmiller subsidiary, Automatic Welding Ltd. Ron Allan, manager of Northern Operations for Kitchener, Ont.-headquartered E. & E. Seegmiller, reports that the Novar deposit is variable and contains a high ratio of abrasive granite minerals. "Our goal here is to maximize the yield of both fine and coarse asphalt aggregate sizes while meeting the specified crushed material content. As the HDBC aggregates require 100 per cent crushed particle content for both fine and coarse aggregates, we decided to produce both HDBC and HL4 aggregates to this standard." The resulting process utilises a full-size portable spread to produce crushed feedstock with a controlled top size of either 19 mm for the HDBC or 16 mm for the HL4 mix. This feedstock is then hauled to a separate set up where a dual vibration screen operates dry and separates the material into three fractions; coarse (19 mm x 4.75 mm), chip (4.75 mm x 3 mm) and sand (3 mm minus). The coarse and chip fractions are stockpiled, while the sand fraction is conveyed to a nearby portable wash tower. Here, the sand is first slurried and then pumped through a cyclone. The product stream from the cyclone passes over a dewatering screen and the resulting semi-dry manufactured sand is conveyed to stockpiles.
Double-deck bivi-TEC screen enables very fine material to be screened dry without blinding. Two Superior 914 mm x 18.3 m conveyors stockpile coarse and chip-size materials. The dual vibration screen is a double deck bivi-TEC unit, and at 1.9 m wide x 6.33 m, slightly longer than a standard unit to ensure that feed material is distributed evenly across its top screen deck. More commonly seen in slag and mining applications, the screen is designed with the decks inclined at different angles to provide optimum screening efficiency for the respective fractions. For this application, it has slotted polyurethane screen panels with 4.75 mm wide x 8 mm openings on the top deck and 3 mm x 8 mm openings on the bottom deck. Rubber blocks connect the floating frame to the screen body, while the body itself is mounted on rubber springs. According to the manufacturer, the unique feature of this unit is the dual vibration induced by its single drive. During operation, rotating unbalanced weights induce circular motion in the screen body as well as horizontal motion in the floating screen frame. As a result, the polyurethane screen panels are alternately tensioned and relaxed, thereby flipping the material as it passes down the screen. This action transfers acceleration forces of up to 50Gs to the material, enabling very fine material to be screened dry without blinding. Two Superior 914 mm x 18.3 m conveyors stockpile product near the screen, while a similar sized conveyor feeds crushed granite sand to the custom built portable wash unit. The sand is first mixed with water in a slurry box and then pumped up to the inlet of a Clarification and Flotation Systems (CFS) 660 mm diameter cyclone. The cyclone’s job is to reduce the unwanted 200 mesh minus fines content in the feed material from about 15 per cent to 4 per cent or less. To do this, Seegmiller’s unit is designed to cut the feed material stream between the 150 and 200 mesh sizes. Its design capacity is 73 tonnes/h at a 25 per cent feed solids content, or expressed in terms of inflow rate, 3674 l/ min of water and 4106 l/min of slurry.
Overview of the screening and washing operations at the Novar pit. The 3 mm minus from the biviTEC screen is conveyed to the CFS/Sizetec portable wash tower by a Superior 914 mm x 18.3 m belt. Most of the 200 minus mesh fraction is carried in suspension through the cyclone’s overflow, while a mixture of the heavier sand product, water, and residual fines discharges through the cyclone’s underflow onto the dewatering screen below. There, the solids content of the product is increased from 70 per cent to about 90 per cent by the 1.83 m x 3.66 m CFS/Sizetec intensive dewatering screen, fitted with 0.3 mm x 12 mm slotted opening polyurethane screens. Meanwhile, water and any product passing through the dewatering screen is recovered in the slurry box below, where it is mixed with the incoming dry feed and recycled through the cyclone. A Georgia Iron Works (GIW) 10 x 8 pump provides pumping capacity for the wash system. Company president Tim Seegmiller ex-plains how each piece of equipment in the set up optimizes the performance of the others. "Firstly, the performance of the high frequency screen is improved by feeding it with a pre-screened and top size controlled material. With one belly dump truck on turnaround, feedstock is supplied to the screen at an average rate of 175 tonnes/h. We estimate that the screen separates 19 mm minus feedstock into 55 per cent coarse aggregate, 10 per cent chip and 35 per cent sand. The fine sizes can be efficiently dry screened because the bottom deck is only seeing crushed stone fines and not being asked to remove dirt fines at the same time. "Secondly, the removal of the chip fraction at the high frequency screen facilitates asphalt production, as the chip can now be metered back into the asphalt in accordance with the mix design requirement of 7.9 per cent. It also means that the cyclone can be tailored to optimize sand production using a close graded feed size that does not contain the coarser material.
Left: The dewatering screen is fitted with 0.3 mm x 12 mm slotted opening polyurethane media. Right: The cyclone and dewatering screen combination produces sand with solids content of 90% and 200 mesh minus content of 4% or less. "As an added bonus, the wear life of the cyclone and associated pipe work is improved, because the abrasive chip is not passing through the wet system. To further reduce wear, the cyclone is fitted with a 22 mm-thick wear resistant liner of poured urethane. "We have also found that the closed circuit of the cyclone and dewatering screen combination gives effective product recovery, as any product that passes through the dewatering screen is mixed with the incoming dry sand feed, recycled back to the cyclone and rescreened in a closed circuit. "Finally, the dewatering screen produces a semi-dry material that can be conveyed to stockpile. The result is minimal wastewater run off from the stockpile, allowing us to use small freshwater and slurry ponds, unlike other systems where product with a high moisture content is stockpiled." The wash unit was designed and built in-house by Automatic Welding Ltd., a wholly-owned subsidiary of E& E Seegmiller Ltd. By Andy Bateman, Engineering Editor
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