Uranium mining in Saskatchewan
McClean Lake: Jet boring reaches small pods
The McClean Lake mining operation, 700 km north of Saskatoon on the eastern rim of the Athabasca Basin, is a joint venture of Cogema (70% and operator), Denison Mines (22.5%), and OURD Canada (7.5%). Production began in 1999 after construction of the JEB mill and the JEB and Sue C open pits at a cost of $426 million. Reserves at both those pits are now exhausted, and the JEB pit has been turned into a tailings impoundment area. Mining resumed in mid-2005 at the Sue A pit.
A $60-million project is underway to expand the JEB mill capacity to 12 million lb/year of uranium oxide. The additional capacity will be used to process Cigar Lake ore as well as ore from new sources at McClean Lake.
The next mining target is the McClean North Orebody. It is one of several small “pods” of ore lying too deep or too far from existing pits to be recovered by conventional open pit methods. The McClean North deposit, approximately 160 m below surface, contains only 5,077 tonnes of ore grading 1.79% U3O8. If it were to be mined from underground, the very poor ground conditions would require freezing the area first. The expense of freezing would make recovery of such a small deposit uneconomic.
Instead, the Cogema engineers and a consulting team from the drilling and jet-grouting industry have come up with a method of “jet boring” to mine the deposit. They built a facility to conduct a test program in 2005 and are now ready to pursue the test, which will hopefully answer a number of questions.
Sylvain Eckert, Cogema’s manager of mine projects, explained how jet boring works.
An access hole is drilled though the overburden and overlying barren sandstone to the top of the ore zone. A reverse circulation drill was used during tests, but Eckert said they hope to use a hammer drill in the future because it is faster and probably more cost-effective. After the hole is drilled, a casing is installed. Five such holes will be drilled in the first half of 2006, and COGEMA has applied for regulatory approval to drill 15 more in the last half of the year.
A mining tool with a drill bit on its nose is lowered to the orebody. The tool has a side-mounted nozzle though which water is forced horizontally at 69 MPa (10,000 psi). As the tool bores through the ore, the water widens the opening and breaks up the rock. The slurry is forced to the surface through an air lift system.
Eckert said the size of the rock particles will depend on the strength of the rock. He anticipates that the slurry will contain pieces nominally 25-mm in diameter.
On the first pass, the mining tool drills through the ore zone, while the nozzle fires water on the side of the tool, thus expanding the diameter of the borehole. After this first pass, the arms of the mining tool are opened, thus bringing the jet nozzle closer to the face of the rock, hence expanding the reach of the jet nozzle. The arms of the mining tool can then be further opened to bring the cavity to its final diameter, which is expected to be 3.5 m.
When the ore slurry reaches the surface, it is sent to a separation plant. It is pumped over shaker tables to remove the coarse portion and through cyclones. Coarse ore is trucked to the JEB mill in conventional off-road 100-t haul trucks (which may or may not need to be covered), and stockpiled on the ore pad.
Fines from the shakers and cyclones report to a mud tank and are then pumped to a sedimentation pond to which fresh water is added. The overflow goes to the clarification pond. Reclaimed water from the two ponds is pressurized with a triplex pump and the addition of compressed air before it is pumped down the annulus of the well.
Cigar Lake: Exceptional grade requires an exceptional method
Cameco‘s Cigar Lake Development, about 660 km north of Saskatoon on the eastern side of the Athabasca Basin, was discovered in 1981, and construction of a test mine was approved in 1990. The developers are nothing if not patient: construction of a commercial mine only began in January 2005 and production is targeted for 2007. With grades of over 19% U3O8 and containing 231.5 million lb of yellowcake, it is a prize — the world’s largest undeveloped high-grade uranium deposit — worth the waiting. The mine will ramp up to full production of 18 million lb of U3O8 annually over three years, with the first phase of mining to last 15 years.
Cameco holds 50% of the project and is the operator. The other owners are Cogema Resources Inc. (37%), Idemitsu Uranium Exploration Canada Ltd. (8%) and Tepco Resources Inc. (5%).
Cigar Lake is an unconformity-type deposit lying about 450 m below surface. It has a high-grade (20.7% U3O8) core containing about two-thirds of the reserves. A lower-grade (16.9%) portion of the deposit lies to the west. A 500-m shaft was sunk and initial development included levels above and below the ore zone. A second 500-m deep shaft is under construction.
Several challenges must be overcome to mine Cigar Lake: poor ground conditions, groundwater control, potential exposure of miners to radiation, and ore handling and storage. Similar challenges have been overcome at Cameco’s McArthur River Mine (CMJ, June 1999), another high-grade uranium underground mine, 50 km to the southwest. The main advance at Cigar Lake is the use of a non-entry, high-pressure water jet boring mining method, instead of the raise-bore machine drilling used at McArthur River.
To solve the twin problems of poor ground and groundwater inflow, the area to be mined will be frozen. A tunnel-boring machine will cut levels approximately 40 m below the orebody. Freeze holes will be drilled upwards through the orebody on 2m centres between holes and 12-m centres between rows of holes. Freeze pipes will be installed in these holes. Then the ground between freeze rows will be frozen by continuously circulating brine chilled to -40C in the freeze pipes. When a 12-m-wide panel of the deposit reaches -10C, the ground is stabilized, and potential water inflow is eliminated.
A 4.25-m production level between the freeze tunnels and ore zone will then be bored to house the jet boring systems (JBS). Plans call for operating two JBS units at a time, recovering 100 to 150 tonnes of ore daily. A pilot hole will be drilled from the production level upwards through the ore zone. A destructible fibreglass casing will be installed in the pilot hole in the orebody area and a steel casing below it. The jet boring drill casing is then inserted inside the fibreglass casing, and the ore is ready for jet boring.
Very high-pressure water pumped through the jet boring casing destroys the fibreglass casing and mining of the soft, frozen ore will begin. The rotating jet of high-pressure water does four things at once: it mines the ore, reduces the rock size, cleans the boring equipment screen openings, and flushes ore from the cavity. The coarse slurry will flow to the production level through the outer steel casing. All elements of the freezing and ore extraction systems have already been successfully tested in the high-grade ore zone.
Each cavity bored through the orebody will be roughly cylindrical with a diameter of 4.5 to 5.0 m. Empty spaces will be filled with special concrete that sets quickly in the frozen conditions. Jet boring has proven effective in removing ore sections adjacent to those that are backfilled without significant dilution.
The ore collected on the production level will be crushed and ground in an underground ore-handling system very similar to the one operating at McArthur River since 1999. Ore particle size will be reduced in a small SAG mill on the 500-m level. The resulting slurry will be thickened and then pumped to surface by a positive displacement pump. The ore slurry will then be stored in pachuca tanks on surface prior to being loaded onto specially designed
trucks for the haul to the McClean Lake mill. Each truck load will contain up to 15 tonnes of high-grade ore, meaning that approximately 10 truck loads will be shipped from mine to mill per day. Due to milling limitations at McClean Lake, approximately 50% of the uranium from Cigar Lake will be shipped to the Rabbit Lake mill as a uranium-rich solution. Rabbit Lake will then continue the milling process to produce the final yellowcake product.
Variations on a technique
To bore or not to bore? The answer is “yes” at both McClean Lake and Cigar Lake. Although engineers at both mines have chosen “jet boring”, in practice they have adapted the technique to suit the individual orebodies. Both variations promise to be very successful.
The mines of northern Saskatchewan make Canada the world’s leading uranium producer. Canada supplied 29% of global demand, or 11.60 million tonnes of the metal in 2004. Here are two bright ideas – how to mine an orebody by neither pit nor underground method, and how to mine high-grade ore without miners – that Cogema and Cameco are pursuing in the Athabasca Basin.