CIGAR’S HIGH-GRADE HURDLE

* to sink a vertical shaft to 510 m;

* to excavate two horizontal drifts from the shaft to the orebody; and

* to test the three mining methods from the drifts, generating 5,000 tonnes of ore.

Since Cigar Lake ore is so high grade (averaging up to 12% uranium), a daily production of just 100 tonnes from underground would maintain an annual output of 4,600 tonnes uranium, which is the current annual uranium production at the Key Lake mine (average grade 2% uranium). Key Lake, southwest of Cigar Lake, is the world’s largest single uranium producer. (For comparison, a typical underground operation with an annual output of 2,000 tonnes uranium and an ore grade of 0.1% uranium requires the daily production of roughly 8,000 tonnes of ore.) The amount of underground development needed to gain access to and mine the Cigar Lake deposit would also be relatively limited.

Test mining will be conducted from the two drifts, which will be 50 vertical metres apart, one above the ore zone and one below it. Preparations for shaft-sinking began in late 1987, soon after Cigar Lake received regulatory approvals (ministerial approval; construction and operating permits from Saskatchewan Environment and Public Safety; and an Underground Exploration Permit from the Atomic Energy Control Board). The shaft is now more than 300 m deep.

Situated 150 m south of the orebody, the shaft is circular, with a finished inside diameter of 4.9 m. The first underground station was cut at the 210-m level and will house underground pumps and serve as a drill platform. Two more stations will be cut later this year to provide access to the orebody at the 420-m and 470-m levels.

The shaft, fully lined with concrete, is designed to be water-tight and to withstand the expected hydrostatic head. The lining thickness will range from 0.3 m near the top to one metre at the bottom. Discharge of mine water, estimated to be as high as 800 cu m per hour, will be handled by co-cm-diameter pipelines in the shaft.

Drill-testing ahead of the advancing face is serving to evaluate the ground and water conditions to be encountered as shaft-sinking progresses, and this procedure will be continued during drifting. Grout curtains will be used when needed to minimize the inflow of groundwater to the mine. Ground Stabilization and Radiation Control

To be acceptable, the mining method for Cigar Lake must deal satisfactorily with the problems of ground instability, high radiation levels, hauling and storage of broken ore, and water control. Each of these concerns could be resolved individually with standard mining technology, but together they will require an unconventional or semi-conventional mining method.

It is expected that mine openings in massive clay and in the ore zone will require pre-excavation support. All permanent mine openings, such as the shaft , ventilation raise and main haulageways, are situated in relatively competent rock outside the alteration zone. All stope access drifts and the stoped areas will be backfilled after mining.

Mining access to the alteration zone will likely require preconditioning by grouting or freezing before further excavation, as well as the installation of permanent ground support or backfill.

Prior to any extensive underground development, operators plan to control the flow of water into the test mine workings by sealing off the surrounding rock fractures with grout and reducing the inflow of water to a manageable level.

Cigar Lake will employ a standard uranium mine water treatment system using barium chloride to form a solid precipitant of radium and a clarifier to settle out the solids, which will be confined in a separate area. The treatment plant and mine pumping systems will be designed to handle any unexpected increases in mine water levels.

Ground instability due to mining excavation may be governed by rock mass structure and stress and the size of the opening, together with the anticipated volume of groundwater and the impact of dewatering. This instability must be evaluated by appropriate in situ testing and monitoring systems. The design of mine ventilation and the mine water treatment system must take into account the potential of groundwater to carry radon gas.

Weak ground conditions and the high clay content of the ore and surrounding rock exclude mining methods like blasthole or shrinkage stoping, which require broken ore storage and recovery from chutes and drawpoints. Any method using conventional drilling and blasting techniques would be unacceptable because of ground instability, and the need for workers to remain in contact with the ore for lengthy periods of time. Three Methods

“Among mining experts, there is a range of opinion on the technical feasibility of mining the Cigar Lake deposit,” says Dick Williams, uranium adviser at Energy, Mines and Resources Canada. “Although high- grade uranium deposits have been mined from the surface at Cluff Lake in northern Saskatchewan and at Nabarlek in Australia’s Northern Territory, no company has attempted to exploit a high-grade deposit like Cigar Lake by underground methods. In addition to the radiation hazards, the incompetence of the host rock at Cigar Lake causes major engineering problems. The combination of high grade and depth, together with significant ground control problems poses a unique challenge to the project developers.”

The three potential mining methods were chosen to meet this challenge. The raise boring and cut-and- fill methods are based on proven mining technology, modified to suit the unique conditions at Cigar Lake. The blind boring method adapts techniques and equipment used for drilling large-diameter vertical shafts and ventilation raises. Raise Boring

Raise boring involves conventional equipment and proven underground mining technology. The drill is situated in the drift above the ore zone. A vertical pilot hole is drilled from the upper drift into the bottom drift below the ore zone. A large-diameter reaming head is attached to the drill string and the pilot hole is reamed upwards to full size. The cuttings fall to the bottom drift, where they are loaded into transport containers. These covered containers will be hauled by truck to the shaft, loaded into the cage, and hoisted to surface. After completion of reaming, the hole is plugged and backfilled with cement.

The equipment for these operations will be designed and operated to provide maximum shielding for the miners. The cutterheads used on the raise boring machines will be designed to accommodate rock types at Cigar Lake and, in particular, must be able to efficiently cut the Cigar Lake clay. The mean, unconfined, compressive strength of Cigar Lake rocks, varies from 4.3 MPa in the clay zone to 22.5 MPa in the ore zone and 67.9 MPa in the fresh sandstone.

Ore-handling is PMa difficult aspect of mining with a raise borer. The collection of the raise bore cuttings, control of spillage, contaminated water- handling, clean-up and transport will result workers being exposed to some degree of radiation. However, it is believed the radiation can be safely controlled by the design of the collection area, the use of remote-controlled load-haul-dump machines and other equipment, as well as adequate ventilation. Undercut-and-fill

This method is widely used in underground mines, particularly in applications where ground is very unstable. The basic concepts of undercut-and-fill can be adapted to mining at Cigar Lake. The method involves the use of remote-controlled equipment beneath a concrete cap. Ore is mined from top to bottom. The equipment must be capable of both mining and lo
ading the ore.

After every two metres of advance, the mining equipment is removed and replaced by remote-controlled shotcreting equipment which gives a sufficient thickness of reinforced shotcrete to provide the ground support for the walls. When each drift in ore is completed, the equipment is removed and the excavation is filled with cemented backfill.

Blind boring involves setting up the drilling equipment on a level above the ore zone and drilling a circular opening down through the ore zone. The walls of the opening will be maintained by the slurry, or by preconditioning methods such as freezing and grouting. After completion of drilling, the hole is filled with cemented backfill. Drilling mud is used to help support the walls and to remove the drill cuttings for transport to surface by either hauling or pumping. The drill mud is treated to remove the cuttings and is then recycled.

A variation of the blind boring method calls for removing the cuttings pneumatically. The walls of the mining excavation must be strengthened by preconditioning and the opening must be kept dry to lift the cuttings left behind by the boring machine.

The blind drilling method has been used on surface for excavation of shafts in hard and soft rock operations in the U.S., and in civil applications for trenching in unconsolidated materials (including canals and building foundations). The use of such equipment underground would represent a new departure. However, the Midwest Joint Venture, operated by Denison Mines, is planning to test the blind boring method at its underground uranium property, northeast of Cigar Lake (see accompanying story). Blind boring equipment would have to be redesigned for these applications. Stephen Salaff, PhD, is a Toronto-based freelance writer of energy-related issues.

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