At such a rich grade (each tonne of ore yields roughly 53.5 lb of U3O8 while the mines in Ontario yield maybe 1.8 lb), the mine is profitable even at a strip ratio that hovers in the 40:1 range. When The Northern Miner Magazine visited the property in late April, the top of the Deilmann orebody had been exposed. The blackish basement rock, the top of which delineates the unconformity between the Aphebian basement and overlying Athabasca Sandstone, was visible at the pit bottom. The orebody itself strikes roughly east-west with a length of 900 m, a width of 30 to 50 m and a thickness of 40 m at the unconformity. Average thickness of the Athabasca Group sediments and glacial overburden is nearly 90 m and ore shoots extend to a depth of 200 m below surface. Most of the ore is at the unconformity.
Site Operations Manager Josef Spross said a feasibility study had revealed that, though probably economic, an underground mine would have had to contend with a fracture system running through the centre of the Deilmann causing serious ground instability. Radioactivity in a confined underground space also contributed to the decision to mine via open pit. The Key Lake Fault zone, as this friable ground is called, is clearly visible in the pit wall. Where there have been water problems, the operators have draped fabric, looking for all the world like a grey curtain, across the crumbling wall to shore it up. Initially, the sandstone walls are hard and competent, Spross said. But locally, as water and oxygen work at it, deterioration can be heavy. The over- all wall angle in sandstone is 40:45 degrees . In the unconsolidated glacial till overburden, the angle is a shallower 32 degrees .
Mining, which began in January, 1989, is done in 8-m benches. The ore, however, is excavated in two 4-m lifts for the sake of selectivity, Spross said. Radiometric down-the-hole probes are used for grade evaluation for blastholes. A map, developed from probing resutls, helps grade control direct the shovels to separate ore and waste. An experienced shovel operator can also “eyeball” the ore contacts. The mine has two primary drill rigs — a Schramm Rotadrill C450H (sold by Cubex Ltd. of Winnipeg) and an Atlas Copco Rotamec 2202. The machines drill up to 8 3/4-inch-diameter holes. A secondary drilling unit, a traxxon Model T750, is used for drilling boulders and ramp preparation. In ore, the drill pattern is 5×5 m; in waste, 7×5 m.
Two O & K shovels (an RH40 5-cu-m and an RH75 seven-cu-m machine) excavate ore and waste, which is hauled away in six 50-tonne 773B Caterpillar diesel trucks. Rounding out the equipment parade are two Caterpillar 988B loaders, two D8L Caterpillar dozers, a Liebherr shovel, and eight DJB D-44 4-wheel drive, 40-tonne trucks. The DJBs are used for sand stripping. C-I-L supplies blasting agents. The new shovels and haulage trucks are furnished with on-board computers to monitor all critical conditions, such as engine pressure and temperature, speeds, the number of trips per shift, downtime and so on.
Said Edward Warren, maintenance superintendent: “Our people can take a lap-top computer into the trucks and assess the previous 12 hours of running time right down to the gears that the truck has run in.” The on-board computers will also alert operators (and in extreme cases shut down the truck) when undesirable pressures and/or temperatures threaten the machine.
Before the trucks haul the ore to the crusher, they slip under radioactive sensors that evaluate ore grade in each load. Sub-economic ore is stockpiled separately from the wast dump and ore stockpile. Unlined waste piles yet too low grade for the mill is identified and dumped on a special pile. This material, grading 0.05% to 0.19%, may be milled when uranium prices reb ound, Spross said. (Obviously, in the world of uranium mining the terms “high grade” and “low grade” are relative. At U.S. and Ontario operations, 0.1% would be classed as uranium-rich rock.)
Pit equipment cabs are lined with lead and air filtered to fend off unsafe radioactivity from the pit. Other precautions are taken in the mine shop, where equipment is hosed down in a 2-bay decontamination unit before they enter the mechanical shop for repairs, overhaul or scheduled maintenance. Equipment availability in the mine is better than 80%.
At the time of our visit, mining had moved ahead of milling to the point that additional mine ore wouldn’t be required until later in the year. The mill processes approximately 700 tonnes per day. The haul distance from the pit to the ore stockpile is roughly 1.5 km. At the stockpile, the varying grades of ore (2.7% to 0.7%) are blended by a Caterpillar loader so that a consistent millfeed that ranges between 2.2% to 2.4% is produced. A grizzly screens out oversize (larger than 40 cm) and about 85% of the ore reports to a 4.2×2.0-m semi-autogenous grinding (sag) mill manufactured by Kennedy Van Saun Corp. The remaining 15% of the ore is crushed in a gyratory crusher. The sag mill is driven by a 670-kw Toshiba engine. A Skega-manufactured rubber liner in the sag was found to be more economic and lighter in handling than conventional liners.
When the plant was commissioned in 1983, run-of-mine ore reported to a gyratory crusher. High clay content, however, soon gummed up the original grinding circuit and severely pinched daily production. The SAG mill solved that problem. Post-SAG ore is fed to the mill in a 50% slurry form. (In the july issue, there will be a feature on Key Lake’s mill.)
In developing the Gaertner and the Deilmann orebodies, Key Lake Mining Corp. (a joint venture between Cameco, A Canadian Mining and Energy Corp, 662/3%, and Uranerz Exploration and Mining, 331/3%) dealt with heavy inflows of water. Twenty- nine wells were drilled into and around the Gaertner pit and another 87 in and around the Deilmann pit in order to lower groundwater levels. As mining progresses in Deilmann, the Pleuger submersible pumps with up to 10-inch diameters are continuously pumping water out of the pit. A dewatering crew maintains and repositions the pumps as required to handle water inflows.
Electrical requirements for the pumps, the mill and all other surface facilities are met by five 2,150-kw- per-hr diesel generators manufactured by Hawker-Siddley. A 900-kw-per-hr Caterpillar diesel serves as backup in case of black starts. The fuel to run the gensets and mine machinery is trucked in from Saskatoon, a 640-km trip north, and costs about $3.9 million a year. At an annual cost of $1.3 million, propane for steam generation and building heat is another costly item. Lime used in the milling process costs an annual $3.5 million.
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