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Canada’s “Rock
to Road” Magazine
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Flexible screen cures blinding headaches at Dundas Quarry Faced with clogged screen cloths while producing fine sized aggregates, an Ontario aggregate producer has turned to flexible screens with impressive results. Dolomitic limestone screenings are in demand by several customers of Lafarge Canada Inc.’s Dundas quarry, including a precast paving stone manufacturer who utilises its 4.75 mm minus screenings as well as HL3 (13.2 mm x 6.75 mm) and 6.75 mm clear sizes. Many aggregate producers have found that effective dry screening at the fine end of the size range can be a challenge, particularly for sizes 4.75 mm (# 4 mesh, 3/ 16-in) and smaller. The production team at Dundas encountered this problem after two new screens went into production in August 2000. These 1.83x6.1 m (6x20) Tyler triple-deck units are manufacturer’s stock items, operating in an open air (ambient) set up. Screen feed material, sized at 22 mm minus and 100 per cent crushed, is conveyed to each screen at an average rate of 350 tonnes/h, of which 70 tonnes (20 per cent) is sized at 4.75 mm or smaller. The screens were originally fitted with conventional oil tempered woven wire screen cloths having 14 mm, 8 mm and 4.75 mm openings on the top, middle, and bottom decks, respectively. The bottom deck cloths had slotted openings that were 52 mm long (in the same direction as the material flow) and 4.75 mm wide.
One of two Tyler 6x20 triple-deck product screens at the Dundas Quarry fitted with Flex-Mat screen panels. It soon became apparent that changes were required to prevent blinding of the bottom deck with fine material. On some days, output of 4.75 mm minus product fell off rapidly as the bottom deck screen became clogged and the material passed over, instead of through, the screen. In addition to the lost production, the carry over resulted in remixing of the 4.75 mm minus fraction with 8 mm minus material being screened by the middle deck above. When this occurred, the resulting mixture had to be diverted to Dundas’ wash plant for further processing to avoid contaminating existing product stock-piles with off grade material. Production superintendent Ward Woolsey reports that the weather was a factor in screen efficiency. On hot dry days, the screens would occasionally run a full shift without excessive blinding. The toughest conditions were cool wet weather, when the moisture content of the feed material might rise as high as 8 per cent, causing the screens to blind over soon after start up. Either way, screen cleaning became a regular event. A few weeks later the Dundas production team agreed to install test sections of Major Wire’s Flex-Mat screen as a possible cure for the blinding problem. Quarry manager Gary Proctor admits to being sceptical at the time that the Flex-Mat screens would deliver on the manufacturers’ claims for performance and cost effectiveness, particularly given a price tag of between two and a half and three times that of a conventional screen. Two Flex–Mat screen test panels were placed in the upper positions (at the feed end) of each screen, while the woven screens were left in place in the lower positions for comparison purposes. The test went well and Flex-Mat screen panels were subsequently installed throughout the bottom deck of each screen. Each side-tensioned panel is 1.83 m wide and 1.22 m long, for a total of five panels in a 6.1 m long deck. Flex–Mat’s "D" (for diamond) wire pattern was chosen for this application. Here, each wire is crimped in a zig–zag pattern across the full width of the screen, so that adjacent touching wires present a diamond shaped opening to the material flow. Longitudinal support for the individual vertical wires is provided by bonded polyurethane strips. The spacing of the strips is tailored to suit each application, so that the strips bear directly on the rubber cap of each deck frame’s crown (support) bars. In this case, the Flex–Mat screens were supplied with strips at 305 mm centres to match the crown bar spacing of the Tyler screens. As crown bar spacing varies with screen size and manufacturer, each application is measured and the screen panels are supplied with the appropriate strip spacing.
The Flex-Mat screen panels fitted to the bottom deck of these Tyler 6x20 triple-deck screens are delivering 70 tonnes/h of 4.75 mm minus screenings per screen. Flex-Mat screens are sometimes incorrectly described as high frequency screens. This is not the case, as the wires vibrate at a similar frequency to the screen frame itself. However, it is true to say that the wires in the Flex–Mat panels have increased wire movement compared to a conventional woven screen, where the whole screen cloth moves in unison with the screen frame. Woolsey reports that the screening system is working well with 4.75 mm minus product being produced to specification at a consistent rate of 70 tonnes/h per screen, noting that these improvements have been achieved without any changes to the original set up apart from the change in screen media. The Flex-Mat panels have performed well so far, with no unscheduled downtime or replacements in nearly 18 months of continuous operation. There are also no reported instances of broken wires, or any measurable increase in produce size that would indicate excessive wire wear. The Dundas team did find that there is a learning curve with the new screens. Although durable and wear resistant in operation, they require more careful treatment when it comes to installation and maintenance. When first installed, they must be set square on the screen deck and evenly tensioned at the sides to ensure that the polyurethane strips align exactly with the deck crown bars for maximum support. In terms of maintenance, crews should avoid placing heavy objects on the screens when working on the upper decks or screen frame, as this can damage the relatively small diameter cross wires. Plant operators have also found that the appearance of the screens at the end of a production run can be deceptive. While the screens may appear to be blinded, they are actually clear and any residual material will be thrown off the screen on next start up. Building on successful experience with the bottom decks, "D" type screens have also been fitted to the middle decks on both screens. In addition, work is in progress on a Flex-Mat design suitable for Dundas’ PEP screen. By Andy Bateman Aggregates and Roadbuilding Contractor |
