Freeze walls for uranium

Brian Mattie from Cameco outlines successful methods for extracting uranium at McArthur River Mine in Sasketchwan, Canada.

Having pioneered a unique raise bore mining method for uranium production, Cameco uses Maptek Vulcan™ for planning and production as well as critical geotechnical modelling.

Saskatchewan Province, Canada hosts some of the world’s most significant and unique uranium deposits. The McArthur River deposit was discovered in 1988, and with ore grades 100 times the world average, is now the world’s largest high-grade uranium mine. The operation mines 150 to 200 tonnes of ore per day, producing more than 18 million pounds of uranium each year.

The orebody is formed at a plunging fault between basement rock and the Athabasca sandstone, which is saturated with water. The contact between the basement and the sandstone marks the unconformity.

Stringent mine design is the cornerstone of Cameco’s development and production philosophy at McArthur River.

Freeze wall methods are used to control the groundwater and minimise the risk of shaft flooding. An underground milling facility turns the ore into a thick slurry which is pumped to the surface and transferred remotely to trucks.

Ore zones are planned several years in advance to ensure a seamless transition between zones, with no dip in production. This accommodates the necessarily slow process of installing the pipes that distribute the freezing brine solution to isolate ore from the water bearing sandstone.

Development of freeze drifts at the 520 and 590 levels enables ore from Zone 4 North to be extracted. This involves developing a raise bore and extraction chambers as well as access ramps under freeze protection.

Designing the freeze wall at the 520 level incorporates a 10m offset to the unconformity contact. Before development can begin, a diamond drill is used to drill a very tight pattern into the face where pipes will be installed. Any signs of water are plugged with high pressure grouting to avoid surprises as development progresses.

Drilling also confirms the location of the unconformity. All drill cuttings are captured and pumped to the milling circuit to avoid loss of any of the high grade ore. Negative ventilation is used extensively at McArthur River as part of the radiation controls. Rigid ducts suck the air into the heading so the workers always have fresh air at their backs.

Heavy steel bars are installed to avoid rock failures during excavation. Mechanical excavation minimises ground disturbances and proceeds in short rounds, 2.5m at a time, before shotcreting, bolting and screening to increase stability. Adhering to strict design criteria and development methods provides confidence that the ore can be extracted safely.

Careful planning and accurate mine design is required to ensure freeze pipes are not damaged when developing close to the freeze walls. Special measures are also required at the 590 level close to the unconformity. Instead of drill and blast methods, a row header and excavator cuts through rock. A 15m offset is created to account for back of drift development.

Holes around 120m long are drilled upwards from the 590 level, with horizontal holes drilled along the 520 level. Holes are spaced about 2m apart to carry the 18-inch pipes, through which brine at -35° C will be pumped from the freeze plant on the surface. Freezing can take up to 6 months.

The as-built holes are surveyed to correct the orientation of planned adjacent holes. Once the ore zone is isolated, the raise hole is developed. A 12-inch hole is followed by the 3m diameter reamer which is pulled up through the pilot hole.

Loose ore is collected from the bottom of the hole with a remote-controlled scoop tram that delivers it to the underground processing facility. Each raise removes about 200 tonnes of ore. The raises are developed next to each other in a honeycomb pattern.

Maptek had developed a specific technique in Vulcan for calculating the recoverable ore tonnage and grade from the honeycomb arrangement of raise bore stopes.

Sometimes the raise bore is required to mine through insitu ore and at the same time, intersect regions of cement fill present in the previously mined primary stopes. Calculating the diluted ore tonnages and grades, as well as sequencing the mining and backfilling of these stope arrangements, is performed by Vulcan’s Cubic Reserving option.

Careful design and development practice, aided by the best technology, overcomes the challenges of mining in this high risk environment. McArthur River has been able to confidently control high-pressure groundwater, stabilise weak rock and eliminate the risk of radiation exposure.

Thanks to Brian Mattie, Cameco
Presented at 2012 North America Users Conference