|Asphalt sand with a solids content of over 90 per cent is produced by a Derrick W56 Hi-Capacity dewatering screen and two 457 mm maximum density separators.
The dolomitic limestone quarry at Port Colborne has operated since the 1950s and, under its last owners, virtually all of its output was shipped across Lake Erie to Cleveland, Ohio, with minimal supply to the local market. Moreover, its production process was not geared to producing specification aggregates and its limited product range consisted only of granular base materials and pipe bedding. That situation has rapidly changed since Rankin took ownership, with the wash plant just one of several new process developments.
Manager Tim Cassibo explains that the wash plant’s rinsing screens separate coarse aggregate sizes while the separators and dewatering screen produce asphalt sand. Two internal requirements played an important part in the design and selection of the sand plant. The first of these was a low fines content in the finished product, allowing only two per cent passing the 75-micron (200-mesh) sieve or well under the OPSS specified limit of five per cent. The aim here was to minimize dust generation at the Thorold asphalt plant during the aggregate drying and heating process, thereby minimizing dust collection and handling costs. Secondly, product coming off the sand plant itself must have a solids content of at least 88 per cent (i.e., 12 per cent or less moisture by weight) for relatively dry conveying and stockpiling.
|The wash plant’s twin Terex Simplicity 6 by 20 triple deck rinsing screens separate granular A feed into coarse sized products and sand slurry.
|A 135 m long overhead conveyor delivers granular A feed from the main processing plant to the new wash plant.
|A spray bar on a dewatering screen? The spray action helps meet a product fines content of just two per cent to reduce dust generation at the hot mix plant.
The fine material slurry is gravity fed to a custom two-chambered sump assembly and consists of some 8,330-9,500 l/m (2,200-2,500 usgpm) of water and 180-220 tonnes-h of minus 3.2 mm. From there, it is pumped up to two Derrick 457 mm MD (maximum density) separators where the delivered velocity provides a “cut” point at the 200-mesh (75-micron) level. The underflow from the separators falls onto a Derrick W56 series, Hi-Capacity dewatering screen fitted with urethane screen media in a 1.4 mm and 1.0 mm combination. By the time the sand product discharges off the screen, its solids content has increased from 64 per cent to between 90 and 92 per cent, allowing it to be conveyed to stockpile. At the same time, fine material and water passing through the dewatering screen falls into the primary side of the sump and cyclone overflow is returned to the sump’s secondary side. Pumping capacity for the sand plant includes a Georgia Iron Works 254 mm by 305 mm centrifugal slurry pump while a Berkley 152 mm pump maintains the fresh water supply to the Terex Simplicity screens.
Cassibo estimates that the wash plant’s overall product split from the Granular A feed is 30 per cent 19 mm clear, 40 per cent HL3, 12 per cent 3.2 mm chip and 18 per cent minus 3.2 mm sand. About 92 per cent of the sand feed is saleable product while some 8 per cent is lost to the settling ponds. Cassibo adds that an ultra fines recovery unit can be added at a later date if necessary and would recover as saleable product about 50 per cent of the fines currently going to the ponds.
Novel features on the wash plant include V troughs on both the Simplicity screens and the dewatering screen. On the coarse screens, the V trough imparts a tumbling action to the material to help the cleaning process. The trough also protects the screen steel against wear and premature failure from the constant action of the water jets. On the dewatering screen, an odd place for a spray bar, one would initially think, the V trough plays a similar role, except that here the combination of spray jet and tumbling action helps to drive out the extra few percentage points of unwanted fines from the finished product.
The Derrick HI-Capacity dewatering screen is described by its manufacturers as a true high G-force, linear-motion dewatering screen with a maximum production capacity of 275 tonnes/h, with the 5 x 14 unit yielding 5.8m2 of available screen surface. It is driven by two 5 hp Derrick electro-mechanical vibratory drives, with the 10 hp duty total generating 7.3 g. Screen specifications include 100 per cent urethane encapsulating the screen frame, urethane-coated draw bars and Kevlar-lined urethane screen media.
With offices in St. Catharines and Hamilton, Rankin Construction Inc. specializes in a wide range of heavy civil construction including road, sewer and water main, marine and bridge construction projects. Diverse company activities also include wind turbines, design-build and engineer projects, marine and environmental work and subdivisions. n