This unusual mining operation begins at the tailings pond next to the old Lake Shore mine headframe, in downtown Kirkland Lake. For six hours a day, shoppers in the nearby mall can watch the dredge Lady LAC churning away in a small pond of water just off shore. The “Lady LAC” is an all-hydraulic suction cutter head dredge, specially designed for the job by Kenner Marine of Louisiana.
“Our biggest problem has been bending spuds,” Dredge Foreman Alfred Burns admitted. The Lady has two of these 44-ft long, 12-inch-diameter, schedule 40 steel poles at the stern end of the 90-ft hull. One, which is called the working spud, is driven like a pile down into the tailings. It provides a pivot point from which the dredge operates while taking a cut. The other spud is called the walking spud. It is used when the boat moves to the next cut.
Two steel cables are anchored off the bow of the dredge, one on the port and one on the starboard side. Attached to a winch, these cables are used to swing the dredge about the pivot point provided by the spud.
To break up the finely-ground tailings below, the dredge is equipped with a rotating cutter head on the end of a hydraulic boom. The boom can be raised or lowered along an axis parallel to the keel of the dredge. In the lowered position, it can exert from 1,500 to 2,000 lbs of pressure per square inch on the 22-ft (6.5-m) thick tailings beds below. That’s more than enough to break them up, and in some cases is enough to bend a spud.
Two 6×6-inch Pekor suction pumps (one acting as a booster) transport the broken-up tailings material (at a density of about 70% solids) to the screening house on shore via a 6-inch-diameter pipe. From the holding tank below the screenhouse, the slurry is pumped via a 12-inch-diameter, insulated pipeline to the Macassa mill, 7,000 ft (2,100 m) distant. The overflow from the thickener in the mill returns to the tailings pond via another, parallel 12-inch line.
This arrangement allows the dredge to operate for only five to six hours during the single 12-hr shift, and thereby meet the daily quota required by the mill. For the rest of the shift, the dredge operator, his helper and the screenhouse operator move the dredge, perform routine maintenance on th e dredge or operate a backhoe. During our visit, they were piling up tailings that had been fouled badly with organic matter. This material was then being washed with a high- pressure water-monitor borrowed from another, nearby tailings project, operated by Eastmaque Gold Mines. The organic material remains in the pile and the tailings material flows back into the dredging pond. For logistical and safety reasons, the dredge likely will operate close to shore during the winter months. “We don’t want to have a long power cable in the water during the winter when there is a danger of it being cut by the ice,” Burns said. That 5-kv line provides power to four electric motors on-board the dredge.
In preparation for the winter operating season, the crew will be working to replace the original cutter head and the two suction pumps. The dredge should be down for only three days.
Not only will the Lady LAC be supplying enough tailings to the mill for the entire operating year, but Mill Superintendent Len Robinson is confident the mill can maintain recoveries in the mid-70s range. That wasn’t being achieved last January when gold recoveries were down in the 35% to 40% range. “The big problem was getting the fine grind we needed,” metallurgist Symes explained. Wright Engineers designed the milling circuit based on lab-scale tests conducted by the Ontario Research Foundation (which has since been renamed Ortech International), in Mississauga. That design called for the tailings, which range from 60% passing 200- mesh to 90% passing 400-mesh, to be ground fine enough so that 97% would pass through a 400-mesh screen. To achieve this, Wright had included a tower mill in the mill design.
Commonly used in the U.S. coal industry, these mills consist of a vertical cylinder filled about one-third with steel balls. Along the axis of the cylinder is a screw-type lifter which serves to rotate the steel balls. Tailings are fed into the bottom of the unit and ground between the balls as it travels upwards through the column. With 18 tons (16 tonnes) of steel balls in the unit, wear on the wear shoes is high, Robinson said. Every three months, they have to be changed. The unit, manufactured by Mineral Processing Systems of York, Pa., costs about the same as a ball mill to operate. But it experiences more downtime because of the high wear. At Macassa, about one-third of the tailings are treated in the tower mill; the other two-thirds are ground in a 3.2×4.9-m, 1,250-hp ball mill. The average grind is now about 96% passing 325-mesh.
Last winter, a problem was posed by freezing in the outside thickener. The tailings were pumped directly to the thickener from the screenhouse, then into the grinding circuit. Freezing and sanding the thickener out proved problematic. As a result, the entire tailings circuit was shut down for about six weeks until February, during which time the tailings circuit was redesigned. Tailings now are fed into a surge tank instead of the thickener. The tailings then go through the grinding circuit, and subsequently the thickener. The energy added to the slurry during the grinding stage is sufficient to prevent the thickener from freezing. Larger-capacity cyclones also were installed.
The tailings are leached for 60 hours in tanks separate from the Macassa ore circuit, then the two streams are combined in the carbon-in-pulp circuit. About one to 1.5 lbs of calcium cyanide and about five tons of lime are used per ton of tailings treated. The mill pours about 2,000 oz (62,200 g) of gold per week. For production to increase to 1,000 tons (910 tonnes) per day, LAC would have to operate the dredge for two 8-hr shifts as well as increase the capacity of the ball mill discharge pump, which feeds the cyclones, Symes said. “Such a system would not be guaranteed to achieve the same recoveries. There may be a tradeoff.”
About 17 people work in the Macassa mill each shift.
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