November 2010

Engineering Exchange

Dredging up business: Innovative equipment takes on challenging environments

By Heather Ednie

Holding pond

Re-established holding pond in foreground with terraced dredged tailings storage cells on the slope of thickened tailings deposit  | Photo taken by Guy Talbot of Talbot Surveys

Over the course of 44 years of operation, tailings solids had collected in the holding ponds at Xstrata Copper’s Kidd Operations in Timmins, Ontario. By 2008, upwards of 10 feet of material had accumulated and was threatening the operation’s capacity to manage and treat water flows from storms and spring melt. Fortunately for Kidd, Consolidated Giroux Environment, a dredge contractor with the ability to modify its machinery to handle just such a challenge, was able to provide a quick solution.

The Kidd Operations, including both the underground mine and large base metals processing facilities, began in 1966. “The tailings have evolved over the years,” says David Yaschyshyn, superintendent, environment and hygiene, Xstrata Copper Kidd Operations. “We started with conventional slurry discharge and then moved to thickened tailings in 1973. In fact, we pioneered along with E.I. Robinsky Associates the first thickened tailings site in North America.”

Today, the tailings management area spans 1,250 hectares, half of which is used for active tailings deposition. The operation utilizes a low-density sludge treatment system, whereby lime is added to treat the acidic tailings water. As the elevated tailings deposit has an average slope of two per cent, water run-off can carry some tailings with it. “With all the weather extremes in this part of the world — freeze, thaw, rain storms, drought, spring snowmelt — it leads to erosion of the tailings,” Yaschyshyn adds. “This will be ongoing until the tailings deposit is closed. Erosion carries tailings solids into our two water treatment plant holding ponds, which, over time, will fill in if not managed.”

Remediating the holding ponds

Aiming to improve the pond capacity at Kidd, Golder Associates determined that dredging the ponds was the most reliable and cost-efficient option, based on previous dredging experience on site. The firm then prepared the design reports and construction drawings to be used, and provided periodic monitoring services to review the project’s progress. Consolidated Giroux was contracted to dredge the ponds, which, complicating the situation even further, also contained old trees, stumps and sunken timber that needed to be removed along with the tailings.

To deal with such a situation, the company designed and built the AquaMaster, a self-propelled barge that operates as support equipment for the larger dredges. It possesses a number of practical features, including spuds, stabilizers and a hydraulic boom that can be retrofitted with a sludge press, debris bucket, excavator bucket, cutting talon, tree shearer or jackhammer. “The most important feature, for Kidd Operations, was the guillotine shear,” explains Clarence Giroux, president, Consolidated Giroux. “With it, we can cut a 14-inch diameter tree 20 feet underwater.”

In addition to the innovative modifications it made to machinery, Giroux retrofitted all of its dredges with double-walled fuel and hydraulic tanks to prevent spills. It also equipped all of its dredges, barges and the AquaMaster with biodegradable hydraulic oil.

As Giroux dredged the holding ponds, they removed the trees and other large debris with the AquaMaster, and hauled the material to shore using a transport barge equipped with a hydraulic moving floor. A modified 915 Mud Cat was then used to remove the tailings from the pond and return them to the tailings cone. This machine, modified by Giroux, is made of stainless steel to prevent corrosion in such an acidic environment. It also features a 350-hp engine (the standard is 238 hp) that drives a screw pump instead of the typical closed-face impeller pump, which enables the machine to handle tree branches and heavy debris lying within the settled tailings.

In the future, work in the holding ponds will be done on a more regular basis. “Ongoing maintenance will prevent the ponds from filling in and needing such a large cleaning,” Yaschyshyn adds.

Routine dredging of settling ponds

Due to the cold weather in the winter, dredging is limited to the ice-free period from May to November. Part of the water treatment process requires the addition of lime to the acidic water (to create a hydroxide) in the on-site settling ponds, leaving a hydroxide sludge and some unspent lime behind. Giroux dredges these settling ponds to remove the hydroxide and unspent lime, and returns it to the tailings area for storage. Dredging is routinely carried out once a year, unless a situation calls for a higher frequency. “For example, Giroux may dredge the entire pond one year, then just do half of it the following,” says Yaschyshyn. “The frequency is based on our needs.”

The water in the settling ponds tends to have a high pH level, due to the addition of lime. This basic environment is also corrosive, attacking materials such as aluminum and brass. As such, Giroux ensures that the equipment it uses in those ponds is not made of, or equipped with, any of those materials. As well, the basic environment can leave a brownish coating of scale — up to one-eighth inch — on all the equipment, which must be water or sandblasted off.

Another challenge for Giroux is having to move materials over a long distance. “For a normal dredging job, we typically move materials about 4,000 feet,” Giroux explains. “At Kidd, it’s in the vicinity of 7,000 to 8,000 feet, requiring a lot of extra pipeline and booster stations to push the materials through. We have about 30,000 feet of high-density polyethylene (HDPE) pipeline on site — it offers a good abrasion resistance.”

A perfect fit

Giroux operates 14 dredges for their various contracts and, over the years, they have been proactive in ensuring the equipment best suits the job and meets stringent environmental standards. “Every dredge that we have bought has been modified in our shop to fit the specific job we are working on, or to meet our safety standards,” Giroux explains.

Consolidated Giroux has turned out to be the right size fit for Kidd Operations’ requirements. “They are not too big, not too small,” Yaschyshyn adds. “There are some very large dredging outfits in the Alberta oil sands and elsewhere. In our experience, Giroux is dependable and obviously environmentally conscious. Finally, here at Xstrata Copper, we’re very safety conscious and Giroux has been maturing with our safety requirements. They’ve adopted our standards and procedures.”

Consolidated Giroux is happy to follow the safety guidelines set forth by Xstrata Copper. “We’ve trained our guys accordingly,” says Giroux. “And we are proud of our good safety record there.”

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