A DENSE FILL PROJECT

Backfilling methods are certainly garnering plenty of attention these days. At Kidd Creek, the use of fly ash as a partial substitute for mor e expensive cement in backfill has been used successfully. At Inco’s Sudbury operations, engineers are experimenting with paste backfill. As well, Falconbridge is in the midst of a backfill project at a Sudbury operation. But perhaps one of the more advanced projects is at Placer Dome’s South Porcupine mine, where Special Projects Engineer Dale Churcher is refining a technique incorporating high-density or paste backfill. (He calls it, simply, dense fill.) Up until the 1950s, stopes at the Dome were left open or filled with sand. Since then, mined-out areas have been filled with conventional hydraulic backfill using the coarse fraction from classified tailings or unconsolidated waste rock that was quarried on surface and dumped into the mine through access raises. In 1984, Dome first tested dense fill, putting 160,000 tons of the material into a test stope. A second stope was filled later with 100,000 tons of dense fill; and now, because of the success of the first two stopes, the company is preparing a third. At the 1,100 level, this mined- out stope will accommodate 250,000 to 400,000 tons of dense fill. After the material has settled and consolidated, miners will be able to mine out a nearby stope.

With dense fill, costly bulkheads are not required to dam up the larger stope openings. And this is its chief advantage over hydraulic fill. The dense fill, with an angle of repose of 68 degrees , forms a plug at the drift opening. It also has the advantage of a higher compressive strength and deformational modulus because of its lower moisture content and superior grading. The key piece of equipment is called a Tailspinner, manufactured by Joy Manufacturing. The Tailspinner is a proprietary centrifuge, 61 cm in diameter and 81 cm high. It consists of a hollow centre shaft, an upper bowl and lower bowl and a peripheral bowl seal. The centrifuge takes the full-stream mill tailings as 50% solids in slurry form, which has been pumped underground through boreholes and pipes (laterally) to the work station. It centrifugally removes the water, bringing the slurry to about 78% solids, or about the consistency of brick mortar, Churcher says. The Tailspinner is a continuous feed process, as opposed to a batch process. Cement and fly ash are added at the mixing screw stage and this mixture is pumped into the stopes.

In the Dome project, three centrifuges, cement feeders and hopper, feed tanks and pumps, and electrical and motor control panels have been set up in a large excavation underground on the 1,100 level. (A Zimpro hydraulic displacement pump will send the ultra-fines, a byproduct of the process, to surface.)

“We see this as being an important step in mining. You can cut costs and pro duce a better (backfill) product,” Churcher says. This particular project will allow Dome to mine an ore zone in the hangingwall near the stope that will be backfilled. The federal government, through the Canada Centre for Mineral and Energy Technology (CANMET), is providing financial help for the applied research associated with the project. — 30 —