Led by Dr. Vassilios Kazakidis, an associate professor of mining engineering at Laurentian University, the research team was asked by Xstrata to investigate the use of frozen backfill as an alternative to using cement, which would have to be transported to the remote mine site at great expense.
Raglan, 1,540 kilometres north of Rouyn-Noranda, is a fly-in operation that relies on port facilities at Deception Bay to transport supplies and concentrate shipments to and from Quebec City during the ice-free shipping season.
Its location in permafrost with underground mine temperatures below freezing makes frozen backfill possible. Kazakidis, who did similar work earlier in his career in Alaska, was tasked with determining the best mixture of materials for optimum strength and the logistics for producing it. A mixture of waste rock and tailings from Raglan’s concentrator worked best.
The NORCAT experimental mine was a perfect location to carry out the research, said Kazakidis. With mine access via an adit, researchers were able to produce columns of frozen backfill in moulds using snow-making machines just outside the mine. The samples were then transported inside by forklift and placed in a large walk-in freezer to raise the temperature to minus six or seven degrees Celsius, the temperature underground at Raglan. Strength testing was done in the mine using a press and instrumentation to measure the material’s load bearing capacity.
The research, funded by Xstrata and the Natural Sciences Engineering and Research Council of Canada, will continue at Raglan this summer to test the logistics of producing the frozen backfill.
“Mixtures of tailings and snow have been used before in mines located in permafrost, but there have been very few applications of rockfill and snow, so we needed to improve our technical knowledge in this area,” said Kazakidis. “The challenge was to make the snow in a way that could be easily replicated underground at Raglan.”
Because heat is dissipated when water is chilled and sprayed onto a mixture of rock and tailings, the frozen backfill will have to be produced away from stopes before being introduced into a raise and directed to the intended destination.
Fortunately, Xstrata no longer uses brine for drilling and wetting muck piles. Though useful in preventing water from freezing, its presence in the mine and in the tailings from the concentrator would have jeopardized the use of frozen backfill. Instead, the company is warming the air in areas when and where water is required.
In August 2006, an investment of $250 million was announced to increase nickel ore production at Raglan from 1 million tonnes per year to 1.3 million tonnes. The company is also spending $50 million to upgrade its port facilities at Deception Bay, 100 km north of the mine site. Concentrate from Raglan is shipped from Deception Bay to Quebec City, and from Quebec City to Xstrata’s smelter in Sudbury.