At the 530 metre level: Operator Al Turtle adds a rod to a Robbins 73RM-H raiseborer at the Cameco mine site.

The riches at
McArthur river

McArthur River is by far the world's richest uranium deposit, with current proven and probable reserves of 255 million pounds (98,084 tU) at an average grade of 22% uranium and indicated resources of 228 million pounds (87,699 tU) at an average grade of 12%. It is expected that McArthur River will produce for at least 20 years.

Located in the vast and remote wilderness of northern Saskatchewan, the mine started operations last December as the new flagship of Cameco Corporation and is expected to reach full production of 18 million pounds of uranium annually by the end of 2002.

The uranium grades at McArthur River are up to 100 times higher than average grades elsewhere in the world and raiseboring is the primary production method at the mine.

The discovery of the deposit in 1988 was followed by further investigation which culminated in a detailed environmental impact statement in late 1995. The statement concluded that the McArthur River project would bring to the region the benefits of training and jobs, with negligible adverse environmental and health impact.

Non-entry mining, with employees located away from the orebody. >> click to enlarge

The venture, which is 70% owned by Cameco and 30% owned by COGEMA Resources Inc., received the go-ahead to develop the deposit in 1997, following a series of public hearings.

Three shafts

There are three concrete-lined shafts at the site. The first, the Pollock shaft, with a 5.5 m inside diameter, was sunk to a depth of 684 m to allow for production of the orebody between the 530 and 640 m levels. The second shaft, with a 6 m inside diameter, is used for mine exhaust ventilation and as an emergency exit. The third shaft has been sunk to 542 m with a 6 m inside diameter. Construction of a ventilation drift from the 530 m level is in progress. This shaft will be the main ventilation intake for the underground mine and will also be a secondary emergency exit when completed.

The ore is currently being extracted using four Atlas Copco raise borers - one Robbins 73RM-H machine and three 53RH-EX units using reaming heads and cutters supplied by Secoroc. This fleet is the backbone of a revolutionary, non-entry mining technique developed by McArthur River engineers.

The rigs will be used for raiseboring (the main mining method), as well as boxhole boring, allowing the ore to be mined while employees are safely located away from the orebody and from possible exposure to radiation.

Brian Jamieson, formerly General Manager at McArthur River and now Vice President of Mining for Cameco, says: "This is non-entry mining in the truest sense."

Raiseboring is the mining method chosen in areas where it is possible to set up the machine in waste rock above the ore on the 530 m level. A 311 mm diameter pilot hole is drilled through waste rock and about 60 metres of ore, then more waste rock, to a lower chamber on the 640 m level. The pilot bit is removed from the drill string on the 640 m level, and the reaming head is attached.

Remote control

Next, the 2.4 m (sometimes 3.05 m) diameter reamer, is pulled up through the pilot hole to the top of the orebody by the raiseboring machine in the upper chamber. Rock chips drop into the ore containment chute (OCC) on the 640 m level. This OCC fits tight up against the back roof and funnels the cuttings into the bucket of a tele-remote controlled LHD unit parked underneath. Waste rock is taken to a waste pass where it is hauled conventionally up the Pollock shaft. Ore is scanned for grade and then taken to the underground crusher, the SAG (semi-autogenous grinding) mill.

Mine technician Tobi Ann Long checks the temperature of a freeze hole with a thermometer. It can be - 40°C, with frost build-up creating "snowmen" on the pipes.

Once a raise is complete, the OCC is removed and the open hole is sealed off at the bottom by a 5 m thick plug of concrete. The plug sets within a few days and lower-strength concrete is used to backfill the cavity. The next two raises are skipped over while a raiseboring machine mines the fourth panel. The backfill cavities are dealt with later. In this way, 95% of the ore is extracted.

Boxhole boring can be carried out in a similar way to the raiseboring operation, except that the pilot hole is drilled up through the ore from the 640 m level. A 2.4 m diameter reaming head will ream up into the ore, which will drop into the chute and will then be handled in the same way.

Water problem

The presence of groundwater at the McArthur River site has posed a problem for the project team. The sandstone in the footwall of the ore zones contains groundwater, whereas the basement rock is dry. Consequently, every drill hole that passes through the ore from the basement rock to the sandstone has to be grouted off so that the water does not flow into the underground chambers.

Frozen solution

Although this conventional method of water control worked well for shaft sinking and mine development, additional precautions were required for production mining. A system was installed which removes heat from the ground by circulating cold liquid through a network of pipes installed in the rock, thereby freezing the area around the ore and stopping water inflows.

Comments Brian Jamieson, "Control of the water is the key to successful mining at the site, as the water is under hydrostatic pressure and can also contain radon gas."

In both mining methods, the ore goes into the ore chute after it is mined and is then processed underground. It is crushed in the SAG mill, mixed with water, thickened and pumped to the surface as a slurry. This is another unique and very significant aspect of the McArthur River operation. It is one of the first instances in the world of successful hydraulic transportation of a hard rock ore to the surface.

Once on the surface, this slurry is loaded into radiation-shielded storage tanks. It is then transported by lorry over an 80 km, all-weather road to the Key Lake site for milling.

The ore is reduced to the consistency of fine sand in the grinding mill at the 640 metre level.

The tanks, pipes and shipping containers used for transporting the material during the various stages of the operation were specially constructed to shield against radiation.

With an annual capacity of 18 million pounds, Key Lake is currently the world's largest uranium milling operation. Although its two open pits have been mined out, stockpiled ore is used to blend down the high-grade McArthur River ore for processing.