August 2010

Engineering Exchange

Innovation at work: Novel tailings management allows Minto to proceed on schedule

By H. Ednie

Tailings 

Tailings discharge from the process plant ready for load-out to the disposal site


As permitting and regulations grow more complex, tailings management has become a major discipline unto itself. An effective tailings management plan can make, or break, a mine plan. With so many variables to consider, companies are turning to tailings experts to guide the development of such systems to suit their operations.

When Capstone Mining Corporation acquired Minto Explorations Ltd. in June 2005, the acquisition included the partially constructed, but inactive, Minto Mine operation. Within two years, the once-dormant site reached commercial production and since then, the mine has increased production by more than 100 per cent — all with the help of an effective tailings management plan.

A high-grade copper-gold mine located 240 kilometres north of Whitehorse, in the Yukon, Minto Mine had all permits in place before economic pressures forced its previous owners to cease construction. The original mine plan included partial dewatering of the tailings at the mill. The resulting paste tailings would flow down the valley where a dam would catch water run-off and recirculate some of it back to the mill as process water.

Capstone asked EBA Engineering Consultants to review the tailings management plan in 2006. “We told them the plan was fundamentally flawed,” recalls Don Hayley, director of Arctic resource projects for EBA’s Arctic Practice, who has over 40 years of experience on engineering projects in the Arctic. “You have to understand how the water content in the paste tailings affects the angle at which it will flow.”

Minto Mine is located in a valley with an average along-valley gradient of approximately five per cent. At the planned 67 per cent solids content, the slope on the paste tailings would be 5.5 per cent — very close to the valley slope. If the water content ran high, the slope angle would drop off rapidly, to four per cent or less. “In such a case, the tailings would continue to travel all the way down the valley to the dam, compromising the dam and water supply reservoir,” he explains.

Hayley led a team seeking an alternative tailings management plan. Two options were considered. The first consisted of building an interior rockfill dyke up slope where the tailings would come to rest, and the water would continue to flow down the valley. Although a sound option, such dyke construction was not included in the existing permit and would therefore have required an additional permitting process, resulting in a substantial loss of time for the development of the mine — upwards of two years.

The second option considered was dry stack tailings. This method, increasingly popular in the North, includes removing most of the excess water from the tailings, leaving a fine sandy soil to be trucked and placed. “In fact, dry stack tailings are not totally dry, but dry enough to behave like engineered embankment fill,” says Hayley. “They therefore have their own inherent strength, and can be trucked and placed, remaining stable on their own. Notably, this method easily met the existing permit requirements.”

The dry stack tailings method is advantageous as additional dams are not required. Also, at mine closure, it simply requires covering the stacked tailings with a layer of waste rock; there should be minimal requirement for further maintenance and monitoring.

The disadvantages of the method are with respect to operating costs. At Minto Mine, an additional building had to be constructed to accommodate massive filter presses for dewatering. This process consists of compressing  the tailings slurry, allowing water to run out on a series of filters. The dryer materials fall through the bottom onto a conveyor belt to be stacked for transportation to site.

“The cost associated with constructing the separate building and acquiring and maintaining the filter presses is substantial, and you need redundancy in the system so that it always meets output volume,” admits Hayley. “So, yes, the operating cost is higher than some traditional tailings management methods. However, as permits become more restrictive —especially those related to water management — such alternative methods are increasingly attractive by reducing the environmental footprint, and there is always an offsetting reduction in capital cost for dam construction.”

A dry stack tailings storage facility (DSTSF) site was located over a gently sloping area southeast of the tailings filter building, relatively close to the mill. Its location minimizes potential impacts on Minto Creek, and will contain approximately 3,800,000 m3 of tailings.

Minto Mine has come a long way since the first Arctic operation’s use of the dry stack tailings method. Some mines, like Xstrata Nickel’s Raglan Mine in northern Quebec, create permafrost within the stacked tailings, although that is not in the cards for Minto.

As plans to extend the mine life continue to evolve, the limited extra space for stacking the tailings had to be addressed. Luckily, a solution was readily available. “Minto’s plans include continuing to a new pit once the first is mined-out,” Hayley says. “So, the first pit will be a repository for tailings and some waste rock.” This was no stroke of luck, but the result of a well-laid-out tailings management plan.

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