Features Aggregates Profiles
New life for an old quarry

John MacLellan is operations superintendent for Rankin Construction
Inc. and explains that the acquisition of a quarry in Port Colborne,
Ont., in early 2007 marks the company’s first entry into the aggregate
production business. Significant investment has been made to expand the
quarry’s capacity and product range, including aggregates for the
company’s hot mix asphalt plant in nearby Thorold.

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.

John MacLellan is operations superintendent for Rankin Construction Inc. and explains that the acquisition of a quarry in Port Colborne, Ont., in early 2007 marks the company’s first entry into the aggregate production business. Significant investment has been made to expand the quarry’s capacity and product range, including aggregates for the company’s hot mix asphalt plant in nearby Thorold. A new wash plant is producing coarse and fine asphalt aggregates to tight specifications and consists of twin rinsing screens, twin high-capacity separators and a dewatering screen.

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 wash plant was commissioned in 2008 and has been set up some
distance from the main stationary plant to provide ample real estate
for inventory storage and water management. Wash plant products include
19 mm clear, HL3 (minus 16 mm plus 4.75 mm), 3.2 mm chip and asphalt
sand. In operation, Granular A crushed stone feed is conveyed to the
plant on a 135 m long overhead conveyor and distributed by a splitter
box onto a pair of Terex Simplicity 6 x 20 triple deck rinsing screens.
Screen media here includes 16 mm stainless steel top decks, 6.3 mm
urethane on the middle deck and 3.2 mm urethane on the bottom decks.
Coarse products off these screens are stockpiled and the mixture of
fine (minus 3.2 mm) material and water passing the screens’ bottom
decks goes forward for further processing.

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