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September/October 2005 Issue

• Inland Aggregates boosts Edmonton wash plant output
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Inland Aggregates boosts Edmonton wash plant output

By Andy Bateman, Engineering Editor

Inland Aggregates’ Villeneuve pit, 25 km north west of Edmonton, is recognised as one of the region’s principal aggregate sources. The new 700 tonnes/h capacity plant, believed to be Canada’s biggest aggregate wash plant, is now in its first full operational season at Villeneuve following a May 2004 completion date.
   Regional aggregate demand has reportedly doubled over the last ten years and the predecessor plant, built in 1967, was unable to meet this rising demand despite numerous upgrades and supplementary production by a portable wash plant. Brian Puchala, Inland’s operations superintendent for Northern Alberta, also cites limited local availability of aggregates as a factor in the demand equation, estimating that the Villeneuve pit alone supplies some 80 per cent of the quality concrete aggregates going into the Edmonton market. With the new plant in full production, Inland now looks well positioned to benefit from continued regional growth driven by multiple infrastructure, housing and commercial construction projects already under construction or at the planning stage.
   The scope of the project included the replacement of the existing wash plant and the installation of a new automated load out system. Project planning began in 2002, with construction beginning in November 2003 and mostly completed over winter 2003-2004, despite some challenging Edmonton winter weather. Reported benefits of the new system include doubling of plant capacity and improved removal of any deleterious materials, while the integrated load out system provides improved truck loading efficiency and reduced loading costs.
   Major components of the new wash plant include a double deck primary screen, triple deck secondary screen, ten stage sand classifying tank, single deck dewatering screen and single deck fine sand screen, as well as two log washers and their associated dewatering screens. In typical operating mode, 25.4 and 23.8 mm minus feed material is conveyed to the wash plant from two reclaim tunnels under the wash plant surge pile. Material arriving at the plant is premixed with water in a slurry box before passing over a Metso 8x24 double deck primary screen. The screen is fitted with 5 mm urethane screens on the top deck and 0.8 mm to 2 mm urethane screens on the bottom deck. Sand passing the top deck and retained on the bottom deck is directed to the plant’s sand classifying tank, while fine material and water passing the bottom deck first travels down a 30.5 m gold recovery chute before rejoining the sand going to the classifying tank. Masonry sand is captured by four shorter outer chutes and dewatered on a single dewatering screw before being stockpiled by a separate stacking conveyor. Meanwhile, all coarse (5 mm or larger) feed material retained on the primary screen’s top deck travels down a chute to a Cedarapids 7x20 triple deck wet secondary screen where it is separated into three finished coarse products plus a fines fraction. Aggregate retained and rinsed on the secondary screen’s 23.8 mm top deck is stockpiled as 25.4 mm product, while aggregate retained and rinsed on the 15.9 + 17.5 mm urethane middle deck (20 mm coarse concrete aggregate) is discharged into a washed rock bin ahead of a log washer and a Tyler Tycan 5x10 screen. At the same time, aggregate retained and rinsed on the 6.3 mm urethane bottom deck (14 mm coarse concrete aggregate) is discharged into a second washed rock bin, also ahead of a log washer and its associated Tyler Tycan 5x10 screen. (If required, the two coarse sizes can be stockpiled together as a blended 5-20 mm product). Meanwhile, any fine material rinsed off the larger particles and passing the secondary screen’s 6.3 mm bottom deck joins the rest of the sand going to the classifying tank.
   All sand fractions entering the Eagle Iron Works (EIW) 10-station classifying tank are sorted into three sizes — coarse sand, fine sand and waste sand. The sand fractions typically reporting to the first five stations of the classifier are the coarse fractions of concrete sand and these are directed to a Tyler Tycan 8x24 inclined single deck dewatering screen. Meanwhile finer sand fractions reporting to the classifier’s six through ten stations are directed to a Tyler Tycan 8x24 single deck fine sand screen. Here, product passing the screen’s 0.8 mm opening is pumped initially to a holding tank before being pumped to a Krebs cyclone mounted over the 8x24 dewatering screen. The underflow of the cyclone joins the coarse sand fraction from the classifier and the resulting specification concrete sand is dewatered before being stockpiled.
   Automated controls on the new plant utilise programmable logic control (PLC) technology and include belt motion sensors, water measuring sensors, variable frequency drives, production scaling systems, plant interlocks and pump protection.
   Automation aside, the judgement of operators Graham Evans and Dannis Sadiq remains critical to good productivity, as variations in feed material from 70 per cent to 30 per cent coarse material require frequent adjustments to input feed rate and choice of surge pile draw down side. Still on productivity, Puchala points out that the premixing of dry feed with water in the initial slurry box has a significant impact on the screening efficiency of the primary screen, estimating that the screening rate of the same screen fed with dry material would only be about 50 per cent of that achieved with slurry feed.
  All finished products from the new plant are subject to rigorous testing, with belt sampling of every finished product conducted every two hours. In addition, one combined sample from each shift for each product is sent to the company’s laboratory for testing of gradation, presence of deleterious materials, fineness modulus and particle shape. Test results are relayed back to the plant operators for any necessary adjustments.
   The separate screening of the coarse and fine sand fractions downstream of the classifying tank provides control over two important aspects of concrete sand quality. Firstly, it allows excess 315 and 630 micron material in the feed, known locally the “Edmonton Hump “, to be removed by the fine sand screen. In addition, the fine sand screen provides an opportunity for any residual coal fragments to float off with the screen’s wash water. Product quality measures in the coarse product process flow include two Metso log washers, installed as a precautionary measure to break up any ironstone fragments that may be present. When the log washers are not required, as is typically the case, the material flow bypasses the log washers and their respective dewatering screens. Coal and ironstone are both considered deleterious materials in concrete because of their high water absorption. If present in sufficient quantities, either can cause popouts in finished concrete as they expand in wet or freezing conditions.
   Like the process flow, the plant’s water management system shows close attention to detail. In many operations, all the grey water from a wash plant is pumped directly to a nearby settling pond. Here, a two-stage approach is used, with all grey water first discharged into a sump alongside the plant. The bottom of the sump is ramped, allowing a loader to remove excess fines when necessary, thereby reducing the fines load on both the pumping system and the operation’s main settling pond. After initial settlement in the sump, grey water is pumped some 1600 m from the sump to the settling pond by twin Metso 10x12 pumps. Water in the settling pond is subsequently recovered and returned to the fresh water pond for reuse via a series of weirs and ponds.
  In terms of overall wash plant performance so far, Puchala reports an average plant throughput rate of about 700 tonnes/h, yielding approximately 580-620 tonnes/h of finished product. Based on an operating season of about six months from mid-April to mid-October, the plant has a target output of 1.6 million tonnes of finished product in this first full season. This aggressive target includes around-the-clock running during peak periods, with back-to-back 12-hour shifts and two operators per shift.
  Integral to the wash plant is its huge stockpiling capacity and automated load out for 20 mm coarse aggregate, 14 mm coarse aggregate and concrete sand as the plant’s three main finished products. Three conveyors, each 760 mm wide and 106.7 m long and collectively known as the triple transfer, convey these products from the plant to a transfer point. From there, they are conveyed on high level structure to their respective stockpiles. The coarse product stockpiles have a capacity of 125 000 tonnes each, while the sand stockpile can stockpile 250 000 tonnes of product. Product stockpiling is completed by reversible shuttle conveyors above the stockpiles which in turn discharge onto reversible flinger conveyors.
  For truck loading, finished product is transferred by conveyor to 100 tonnes capacity surge bins over three separate 30.5m long Accurate scales. The system is fully automated and, by using a programmed swipe card, loads a truck within two minutes. Within each surge bin, level sensors indicate when to recharge the bin automatically from each product stockpile’s reclaim belt.
  Puchala, who has been involved with the project from the initial concept design stage through construction, commissioning and full production is well qualified to comment on the plant’s performance and is pleased with what he sees so far. “This project ensures that our production system will carry us well into the 21st century as a leader in our industry.”
   Inland Aggregates Limited is part of the North American Lehigh Cement Company. Lehigh is fully owned by HeidelbergCement, one of the world’s leading producers of cement, concrete and building materials.

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September/October 2005 issue

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