Slurry of activity1 October 2013
After almost a decade of construction, and in spite of increasing project costs, jet-boring operations are expected to begin at Canada’s Cigar Lake in early 2014. By Caroline Peachey
At the site of the Cigar Lake uranium mine in Canada's uranium-rich Athabasca Basin, around 660 km north of Saskatoon, work is continuing slightly behind a revised schedule set in 2009, after flooding of the underground workings in 2006. Following the receipt of an eight-year operating licence from the Canadian Nuclear Safety Commission (CNSC) in June, operator Cameco expects mining to begin in the first quarter of 2014.
The second of four jet boring systems, which will use high-pressure water jets to carve out cavities in the orebody, throughout the life of the mine, has been delivered to the site and assembled underground. Work is also progressing well at surface facilities (which include the ore slurry load-out building, maintenance facility and freeze plant), as well as on the underground infrastructure (see figure 2) needed for mining to begin.
"People at the site are getting excited about the prospect of starting the mine. At the same time, we are very determined to do this right - safely, cleanly and to the quality standards we set for ourselves," said Steve Lowen, general manager, Cigar Lake project.
When mining does start, ore slurry collected through the jet boring systems will be treated in underground grinding and thickening circuits before being pumped to the surface as slurry. There, it will be loaded into Type IP-2 transport containers and trucked to AREVA's McClean Lake mill, 70km northeast of the mine. First product from the mill is expected before the end of the second quarter of 2014, according to Cameco.
The 1042-hectare Cigar Lake uranium mine is one of the world's highest grade uranium deposits with proven and probable reserves of 216.7 million pounds U3O8 at an average grade of 18.3%. Production is expected to span 15 years and will be shared proportionally between 50%-owner and operator Cameco, and co-investors AREVA (37%), Idemitsu Canada Resources (8%) and TEPCO Resources (5%). The Cigar Lake deposit is located at depths of between 410 and 450 metres in geologically-challenging soft and water-saturated Athabasca sandstone.
"Athabasca brings all the challenges that a typical underground operation would have... On top of that we have a fairly complex geotechnical situation underground, the presence of water at pressure at depth, as well as high-grade uranium with the radiation management challenges that brings," said Grant Goddard, vice-president, Saskatchewan mining north, Cameco.
The Cigar Lake mine experienced three groundwater inflow events during construction, the most notable of which occurred in October 2006, and prompted a re-evaluation of the project (see also below).
In order to enhance ground stability and mitigate potential water inflows, Cameco will freeze the ore and rock around the mine by circulating brine at -30°C, in a low-pressure closed loop (see also Figure 1) from a depth of around 480m.
The orebody is frozen, and production tunnels are mechanically excavated about 40 metres below the orebody, then reinforced using yielding ground supports, a system adapted from the tunnel support methods used in the Alps. The jet boring system (JBS) will then drill a pilot hole into the ore from below. After removing the drill head, the jet-boring nozzle will be inserted into the hole, and will bore through the rock using high-pressure rotating jets of water, creating a cylindrical cavity. Resulting ore will be washed down the pilot hole into a slurry car where it will be combined with additional water.
In a process unique to the Cigar Lake project, slurry will be pumped from the car into two large storage tanks, where the ore will settle and run-off water will be recycled back into the process (see Figure 2).
The subsequent underground processing operations, which include the ball mill, where the ore is broken down to the consistency of fine sand, the clarifier where the slurry is thickened, storage pachucas and ore slurry hoist pumps, are similar in function to those used at Cameco's other project McArthur River.
The underground operation includes the use of appropriate thickness concrete/steel enclosures to provide radiation shielding from the high-grade slurry, as well as ventilation systems to capture radon, and the direct routing of air flowing through the process areas to an exhaust system.
Cigar Lake was first discovered in 1981, and the construction of a test mine followed around a decade later, leading to the subsequent decision to employ the innovative jet boring system to mine the deposit. A prototype of the current JBS unit has already been trialled at Cigar Lake in two separate test mining periods, one in 1992 and a larger scale test in 2000, where four cavities were bored in waste rock and four cavities in ore.
Full-scale construction began at Cigar Lake in 2005, with production originally planned for 2007. However, a catastrophic water inflow in October 2006 flooded the mine and prompted a step back and reanalysis of the project. A second inflow was also experienced in August 2008, during the remediation operations.
The October 2006 inflow, caused by a rockfall in the development area on the 465 m level of the main workings in shaft 1, created a connection between the mine workings and the saturated sandstone. This resulted in an inflow of some 800-900 cubic metres per hour, which exceeded the mine's water pumping and treatment capacity at the time.
"After the flooding in Cigar Lake in October 2006, we had to take a step back and look at how we were running our operations," said Tim Gitzel, Cameco president and CEO.
Since then, Cameco has employed a motto 'assurance of success,' and a deliberate process of 'checking and rechecking' before proceeding with any work during mine recovery, remediation and construction operations. The mine was also subjected to numerous geotechnical and engineering studies to inform underground development work. This led the development of a new strategy to handle potential water inflows and a new mine plan, which will now see all JBS production undertaken from the 480-metre level. Prior to 2006, the plan was to freeze the orebody from the 480-metre level, with production work occurring at the 465-metre level.
Cameco has also increased the installed pumping capacity at Cigar Lake to 2500 cubic metres per hour, which is sufficient to pump at least one and a half times the maximum estimated inflow from the underground workings. New infrastructure includes a revised water collection system that reroutes potential inflow water from the 480-metre level through boreholes to the main water collection sumps at the 500-metre level; underground pumps (700 m3/hr) on the lowest level of the mine; a contingency dewatering pump (800m3/hr), as well as borehole pumps (1000 m3/hr). Additional water treatment and storage capacity has also been installed on the surface.
Capital costs for the Cigar Lake project are expected to rise by between 15% and 25% relative to the previous estimate, Cameco said in August. This would put Cameco's share at a possible $1.38 billion (compared with the previous figure of $1.1 billion) by the end of the construction phase in 2015.
Spending this year is expected to increase by an even bigger percentage -- around 43% -- with Cameco's share of 2013 capital costs now expected to reach $260 million.
"One of the biggest cost drivers is escalation," Bob Steane, Cameco senior vice-president and chief operating officer told investors in August, referring to price pressures felt by the mining industry in general. Other factors include scope changes at the mine and mill and the inclusion of some capital costs that will be incurred subsequent to the mining of the first ore that were not previously included.
Scope changes at Cigar Lake have included the installation of additional piping and freeze capacity. At the JEB, McClean Lake mill changes relate to capacity expansion from 3640 to 5900 tU/year, which was authorized by regulator CNSC late last year (2012). Steane said that the facility is still being designed, but that there have been changes in some of the equipment sizes, building geometry, and site infrastructure.
Despite the additional investment required in the JEB mill, Cameco's 2011 agreement with AREVA to mill all Cigar Lake ore at McClean Lake (rather than McClean Lake and Rabbit Lake) has reduced the Cigar Lake operating costs estimates from around $23.1 per pound in 2010 to $18.6 per pound, where they remain today.
For Cameco, Cigar Lake is a major part of a strategy to increase production from around 22 million pounds in 2012 to 36 million pounds per year by 2018, when it is expected to provide around a quarter of the company's production.
Speaking in an early August results call Cameco president and CEO Tim Gitzel described Cigar Lake as "a great replacement" for some of the 24 million pounds of low-enriched uranium (LEU) down-blended from Russian HEU that is ending in 2013, and that Cameco plans to ramp up as planned.
The ramp-up schedule that would have seen Cameco's share of output from Cigar Lake rise from 0.3 million pounds in 2013 to 1.8 million in 2014, 5.5 million in 2015 and 8.2 million pounds by 2017. However, in September Cameco said that during commissioning of the underground ore handling facilities in the mine, it identified 'additional work' that will delay jet boring in ore until the first quarter of 2014. Cameco said that it will revise its five-year production forecast in its 2013 annual report, when "further progress" on commissioning of the mine and mill is made.
Looking further ahead, exploration drilling and feasibility work to move identified mineral resources at Cigar Lake into the category of reserves will also be carried out through June 2021, which could extend the current projected life of mine beyond 2027.