The overriding criterion for the selection of McArthur River's mining method is the high grade of the uranium orebody. Reserves total an estimated 416 Mlb (188,660 0 averaging 15% U308, and 15% U308 requires remote mining! Furthermore, as general manager Brian Jamieson points out', "the ore will be diluted to 4% U308 using the very low grade special waste stored on surface at Cameco's Key Lake. This is one of the few cases where a mining operation purposely dilutes the ore. By doing so the existing milling facility at Key Lake can be simply modified, and the future liability of the special waste stored on surface is eliminated."
It is an extraodinarily rich deposit located in the uranium-rich Athabasca Basin of northern Saskatchewan, Canada. Using gold equivalents, Brian Jamieson compares it to a gold deposit containing a total of 23 Moz, at a grade of 18 oz/t
Using data from the underground drilling programme, feasibility studies determined that an annual production rate of 18 Mlb U308 was achievable, from a mining rate of just 125 t/d. Brian Jamieson continues: "From a practical point of view, this small tonnage requirement allows for an extremely high degree of engineering control of the extraction progress." Aside from the necessity of remote mining, the other two key criteria in choosing the mining method are ground conditions in and around the ore zone and the presence or lack of groundwater near it. The openings in the ore zone must be stable to minimise operating difficulties.
Brian Jamieson notes: "It typically takes ten to 15 years to bring a uranium project into production, making it a very expensive proposition." It has been estimated that some C$450 million will have been spent at McArthur River before any uranium is produced. Construction began this summer and the production startup will be in 1999. At prices prevailing at the beginning of the year, the contained value of the McArthur River ore is about US$4,600/t.
The orebody was discovered in 1988 by surface drilling, and shaft sinking began in 1993. Underground development at a depth of 530 m began in August 1994. The preliminary feasibility study was prepared after the underground drilling programme was completed in June 1995. In December that year Cameco filed an Environmental Impact Statement (EIS) which it believes is one of the most comprehensive ever undertaken for a uranium project (17 volumes with almost 13,000 pages). At the end of February this year, the panel examining uranium mining in northern Saskatchewan recommended that the project be approved by the federal and provincial governments. That approval was given in May and final regulatory approval for construction was given on August 22.
The orebody lies between 500 and 600 m below surface and is adjacent to a major fault (P2) identified during regional exploration. The P2 Fault extends northeast for more than 10 km, but uranium ore has only been defined for more than 1 km. Ore is found near a geological unconformity where younger sandstone overlies older Pelite basement rock.
The ore defined by underground drilling has been subdivided into four distinct zones based upon the relative location to P2. The main Pelite ore zone or Pod #2, about 100 m long, 20 m thick and averaging 60 m high, was one of the biggest finds in Canada in 1995. More than 300,000 t of ore containing more than 150 Mlb Of U308 was defined during the underground programme. This discovery contains a similar amount of uranium to Cameco's Key Lake mine that has been in operation for 15 years. The Pelite ore zone is the first mining target and the average mined ore grade is expected to be 15-20% U308. Also in 1995, Cameco discovered another good ore zone called Pod#1, approximately 80,000 t of ore grading 20% for some 35 Mlb U308.
Besides these high grade ore zones, there is the Quartzite ore zone, adjacent to the Pelite ore. Lower grade, at 1-2% U308 it has, as yet, not been well defined by drilling. Ore is also found along P2 Fault and has been subdivided as P2 Nose ore and PS Fault ore. The surface drilling consisted of very few holes so Cameco considers the potential to discover additional uranium to be excellent. All this adds up to the estimated reserve of 416 Mlb at 15% U308, 85% south of the shaft and only 15% to the north.
Mine design focussed on the need to control radiation exposure. Radon emission is controlled by a dual ventilation system. Fresh primary air flow is always present in active work areas, with a secondary exhaust system to remove contaminated air from particular sources. During all mining and processing stages the ore is fully contained. Gamma radiation is controlled using the principles of shielding, distance and time. Heavy wall steel pipes, thick vessel walls, concrete and sometimes lead sheeting are used as standard practice. Also, because of the relatively low tonnages, equipment operating time is short and maintenance life is long.
Monitoring of both the workplace and employees will measure the performance of the features designed to protect workers from potential radiation sources. This monitoring will help ensure that radiation exposure levels continue to be safe and kept to a minimum. A number of radiation monitoring methods will be used.
Regular air samples will be taken to assess all major work locations. Continuous monitors with signals like a red-amber-green traffic light will advise workers of the radon levels within an area. Workers will wear thermoluminescent dosimeters to measure individual radiation doses over time. Those working in areas of increased radiation risk will wear direct-reading dosimeters that provide daily radiation readings.
Cameco has an excellent record in radiation protection, achieved as operator and majority owner of the world's two largest, high-grade uranium mines currently operating, at Key Lake and Rabbit Lake.
Computer modelling of radiation exposures in McArthur River's proposed facilities indicates that during operations, workers should have combined doses which are less than one quarter of the proposed dose limits.
Raiseborers are the primary production machines. The first step in the mining process will be to establish drifts above and below the orebody. These will be mined totally in waste rock to protect workers from the high grade ore. The raiseborer then drills a 300 mm diameter vertical pilot hole from the upper to the lower drift. One key to achieving good ore recovery will be accurate drilling of the pilot hole. Cameco is confident this can be achieved. Brian Jamieson points out that: "Raiseboring machines, with good initial set up practices readily achieve 0.3 to 0.5% deviation." The pilot hole length will vary from 35 to 120 m.
Once the pilot hole is completed, the raise borer then pulls the reaming head up through the barren waste into the ore zone. Raise diameters will be 2.4 to 3 m and their heights will vary from 20 to 110 m. Each raise will yield between 150 and 1,100 t of ore, producing 30,000 to 500,000 lb of U308. The chips excavated by the reaming head fall to the bottom of the raise.
These high grade chips will enter an ore chute similar in design to that developed at Cameco's Cigar Lake project during boxhole boring trials (Cigar Lake is expected to be in production a year after McArthur River). Any by-product dust will be eliminated by using water sprays and dust scrubbers. The chute is directly connected to the secondary exhaust system by rigid ducting, and provides gamma radiation shielding.
Once each raise stope is completed, the hole will be backfilled with low strength concrete. This is accomplished from the upper drift, via the pilot hole. A breather hose introduced into the pilot hole will remove air displaced during the placement of the backfill and deliver it to the mine's exhaust ventilation system.
Depending on height and diameter, it will take from seven to 21 days to bore and fill each raise. However, the boring time in ore will only be one to five days. The other elements of the production cycle (pilot hole drilling, reaming in waste and backfilling) will take up the rest of the production cycle.
By overlapping the raises up to 95% of the ore will be extracted. Backfill will be allowed 21 days to set before an overlapping raise is mined. This will mean mining a considerable amount of backfill and along with the waste rock, is expected to constitute up to 25% of the material extracted.
It is expected that the daily production requirement can be provided by one raiseborer in ore for one shift per day. This will be accomplished by raiseboring as little as 8 m/d to extract 125 t of ore at a rate of up to 50 t/h. Approximately 6 Mlb U308/y can be extracted by one raiseborer mining an estimated 15,000 t of ore. As many as four stopes will be available at any time, allowing selectivity of the daily production source. About 80 raises will be bored at McArthur River each year, by four raisebore machines.
There are two alternatives to conventional raiseboring that may be used where necessary. In boxhole boring, the reaming head mines up into the ore, with the operator controlling it remotely from the bottom drift. In remote boxhole stoping blastholes would be drilled from the upper level to hole through into the boxhole. These holes would than be charged and the ore blasted into the boxhole. Boxhole boring and remote boxhole boring will be used in areas where access is available only from below the ore zone. Raiseboring is the preferred method, provided drifts can be developed above and below the ore zone.
Ore collected in the ore chute below each raise will then pass through a roller crusher immediately below the chute. It will then fall through another borehole, down into a cone crusher. From there, passing through a surge bin, it will report to the grinding circuit. Following grinding, the ore slurry will be thickened before being pumped to surface. Throughout the underground comminution process, full containment of the ore is achieved within a casing, a chute, a pipe, a bin or a process machine.
Control rooms will be established to monitor a wide range of processes and conditions from remote locations. Radon gas detectors will continuously feed data back to the control rooms so that changes in operating conditions can be immediately identified. Additonally, control rooms will have the capability to start, stop or modify certain process equipment without the need for direct operator intervention.
Numerous video cameras at strategic process locations will provide direct visual feedback to control room operators. A minewide radio communication system will also be established.
On a surface, the slurry will be further thickened and then transferred to shipping containers. Once filled, the exteriors of these containers will be washed before loading onto trucks for the 80-km haul to the Key Lake mill. Cameco is building an all-weather haul road following the existing powerline corridor serving Key Lake.
An average of eight truck round trips between McArthur River and Key Lake will be required each day. Each truck will carry four special steel containers, each filled with 5.5 t of ore slurry. The containers are constructed from 30-mm thick steel surrounded by a steel
frame, making them exceptionally strong and break resistant, and providing shielding from exposure to radiation. The containers meet several design standards, including those of Transport Canada and the Atomic Energy Control Board. Containers will be handled remotely unless they are empty and clean. Water used to clean the trucks and the containers will either be recycled or treated before discharge to the environment.
The Key Lake mill currently produces 14 Mlb U308 annually. This capacity is to be expanded to 18 Mlb/y. McArthur River ore received at Key Lake will be diluted with crushed and ground special waste (Key Lake's low-grade ore) to obtain an average blended grade of about 4% U308. This will minimise radiation exposure in the mill and eliminate the future liability of the special waste which is currently stored on surface. Testwork at Key Lake has shown this blended millfeed can be accommodated with only minor modifications to the milling circuits. The ore receiving plant at Key Lake provides remotely controlled handling of the ore containers, vehicle and container washing, storage for empty containers, ore slurry storage and a pump to move the ore slurry to the blending areas.
The Key Lake mill is owned two-thirds by Cameco and one-third by Uranerz, while McArthur River is owned 56% by Cameco, 28% by Uranerz and 16% by Cogema.
Tailings are produced from three uranium process operations, CCD circuit, bulk neutralisation and radium removal. These are combined, the pH is adjusted with lime and the slurry is fed to a Supaflo high-compression thickener. This thickener, selected due to the viscous nature of the underflow, has a very high torque rated hydraulic drive head (DBS SH-60-20-4) with a peak torque of 1.6 million Nm. Feed dilution was effected with the 'autodilution' feature of the Flocmiser Feedwell.
After start-up of the unit it was quickly apparent that the flocculated combined tailings were very susceptible to 'doughnut' formation. This, combined with other control and mechanical factors, created some problems during the early commissioning stages of the thickener. During this period, over flocculation and control difficulties twice resulted in plugging of the thickener, requiring shut down and clean out. These events proved the strength of the drive and mechanism in their ability to completely rotate the sludge bed without damage.
Co-operative efforts by Cameco and Outokumpu Mintec personnel corrected initial set up and control problems, and established operational procedures for the thickener operators. Specific targets for these procedures were to avoid 'doughnut' formation, maintaining flocculant dosage within prescribed limits and keeping the underflow density within the designed operating range.
A steady bearing was added to protect against unbalanced loads resulting from 'doughnut' formation and the collapse of any 'doughnut'formed. Minor modifications were also made to the flocculant spargers and the size of the feed pipe inlet to the feedwell was reduced. Also, frequent raise/lower cycles (once an hour) were scheduled for the rake mechanism to avoid solids build-up on the rakes.
Since March 1996, this Supaflo high compression unit has operated without problems, readily achieving and exceeding the underflow density target of 35-40% at very low 10 g/ t flocculant dosage.
The mill tailings will be deposited in the mined-out Deilmann open pit at Key Lake. While the panel that reviewed the project supports this disposal method, it is of the opinion that monitoring of this facility will be required for a "much longer time" than suggested in the EIS.
Bernard Michel, Cameco's chair, president and chief executive officer commented: "We are pleased with the general recommendations of the panel and acknowledge their view that monitoring requirements, particularly after decommissioning, should not be underestimated. We have stated that decommissioning plans should not be finalised too early as our experience shows that these plans often need to be revised as the decommissioning date approaches. This ensures that the final plans will conform with the environmental guidelines of the day; accommodate changes at the site and incorporate the best known practices."
The tailings will be piped 4 km to Deilmann. The already existing pipeline is located within a prefabricated concrete pipe trench and is equipped with regularly spaced collection sumps. A system monitors the flow rates and the sumps and sets off an alarm in the case of a problem. The current tailings handling system uses a design similar to the pervious surround method which has been used successfully at Rabbit Lake since 1985 (MM April 1994, pp.203-208).
As long as ore from the Key Lake orebodies is being milled, the resulting tailings will be placed in a highly permeable envelope of crushed rock and sand in the mined-out pit. Water contained in the tailings will seep to the bottom of the pit, where it will be collected, pumped and returned to the mill for recycling.
Water removal and settling allows the tailings to compact. Like the Rabbit Lake system, groundwater flowing towards the tailings in the Deilmann tailings management facility will take the path of least resistance, travelling in the envelope and around, not through, the compacted tailings.
The envelope of rock and sand will be constructed at the bottom of the pit and up the lower portions of the sides of the pit to an elevation sufficient to allow the placement of all the future tailings from the Key Lake orebodies. Above this level, natural and different geological conditions eliminate the need for the pervious envelope and the tailings system will be modified for the McArthur River tailings. The water level in the pit will be allowed to rise above the level of the consolidated Key Lake tailings. The tailings from the milling of the combined McArthur River ore and Key Lake special waste will be deposited, under the water and on top of the Key Lake tailings, from a floating barge. By adjusting the rate of pumping from the wells surrounding Deilmann, the water would continue to flow at a controlled rate toward the pit. Contaminated water below the tailings would continue to be pumped out for treatment.
Computer modelling and testing have confirmed that the density of submerged tailings is comparable with the density of tailings deposited on surface. The method proposed also offers the advantage of eliminating the risk of airborne radioactive dust and reducing radon emissions. It also prevents the build-up of permafrost layers, thereby promoting more even year-round settling and consolidation of the tailings and reducing the time needed for decommissioning.
1. Jamieson, Brian, McArthur River, CIM Meeting, Vancouver, January 1997.
2. The McArthur River project Environmental Impact Statement, Executive Summary.