Miner keeps close watch on cesium underground
Cesium is another one of those elements that has found a multitude of uses since it was discovered in mineral water more than a 150 years ago in Durkheim, Germany. Among them, it increases the electrical conductivity of vacuum tubes, but it also helps us keep time because it’s used to help ensure the accuracy of the world’s atomic clocks. Plus, it’s used to regulate cell phone transmissions and the information flow on the Internet.
But in a somewhat more unusual application, cesium formate brine is also used to lubricate drilling equipment on North Sea drilling rigs where prior to its use, an average of one accident occurred on almost every gas well built in the North Sea.
Will Brits, Facility Manager for TANCO Mine’s cesium operation at Bernic Lake in Manitoba, said the problem was that conventional, solids-based drilling fluid affects well stability and control but: “Cesium formate brines are naturally very heavy and with no solids in the fluids so the risk of losing well stability and control is much reduced. As a result, there have been no well control safety incidents in over 150 deep gas well operations using cesium formate brines.”
And Brits should know. His parent company Cabot Corporation owns and operates TANCO mine. According to TANCO’s mill metallurgist Claude Deveau, the mine shifted its attention to cesium when a “boom and bust market” reduced the demand for tantalum used in high quality electronic capacitors. Meantime, demand for lithium used to cast automobile engines also tanked along with the auto industry in 2008/09. “That,” says Deveau, “really had an impact on the operations here.”
Making the most of what you’ve got…
Today, cesium ores are routinely shipped to Russia for the generation of electricity employing electrically conducting fluids (magnetohydrodynamics). Getting to the ore required sinking a single shaft and two ventilation raises into the pegmatite 60m below Bernic Lake. Actual mining is carried out using the room and pillar method, and interestingly enough, says Deveau, actually benefits from the mine’s hard rock foundation. “This technique is best utilized in flat lying ore bodies in an environment where you don’t have a lot of stresses from the surrounding rock,” he explains.
Deveau adds it’s because the ground is so competent that the rooms at TANCO are so large. Originally the rooms at TANCO were 16m square. Eventually rock mechanics studies indicated the room size could be increased to more than 22m square by shaving the pillars. Ore extraction itself is done in three phases: first the top level, followed by extraction at the lower level. Currently TANCO is mining the sill in between the two levels.
In typical room-and-pillar mining, material is extracted across a flat plane, creating horizontal arrays of rooms and pillars. How much material can be mined depends on the nature of the material, height of the pillars and roof conditions. But the biggest factors, says Deveau, is safety.
“You have to consider the width to height ratio of the pillars, back conditions and geological structures, stress and anticipated movement after mining. All this is critical to the stability of the mine.”
The roof at TANCO averages 20m above the current working levels and in places reach 30m. Due to the nature of the ore and the R & P mining method, rock bolting is rarely required, but when it is, says Deveau, TANCO is careful to get it right.
“Rock types like spodumene are very hard and very difficult to get supported as it has a very dominant cleavage zone. After blasting the bolters are sent to support the back when blasting haulages and drifts. It is critical to ensure that the person doing the bolting has scaled the area properly.”
The back of the mine is also carefully monitored using custom designed Giraffe aerial lifting devices; ventilation is downcast through the original Jack Nutt shaft and then upcast through the decline to produce a fresh air volumes exceeding 3,400 m3 per minute.
In any R and P mining project, pillars of material are left to support the roof overburden while the open areas are extracted. The big question at TANCO: Could crews extract the pillars themselves? The answer depends upon the original selection of optimum pillar size. Generally, the size of the room and pillars should be about equal. Make them too small and the mine will collapse; make them too large and you end up leaving large quantities of valuable material behind.
Over the years, “rock docs” have analyzed the stresses the pillars at TANCO would need to withstand in order to support the roof and allow partial pillar extraction. Their conclusion: pillar extraction could occur, but within limits. “This is not a caving method,” says Deveau. “You have the lake on top of the mine so essentially you have to ensure the integrity of those pillars and the back of the mine. Everything’s got to support itself.”
Meeting the human challenge…
TANCO crews rely on a single MacLean bolter to secure the integrity of the mine’s roof, says Deveau. Mining is carried out using two machines: an Atlas Copco hydraulic jumbo for drifting, and a single boom Atlas Copco Simba long-hole rock drill for benching. Standard features on the Atlas Copco include an articulated carrier with four-wheel drive powered by a Deutz diesel engine, front and rear mounted stingers and mechanized rod handling system.
“We also use 30-tonne ore trucks to transport ore to the transfer raise. From the loading pocket it’s hoisted to surface with the use of a double skip system. Those two ore bins on surface have a storage capacity of 450 tonnes.”
Where crushers have changed over the mine’s 60-year history, Deveau says the processing side of the operations remained pretty much the same: tantalum was processed through gravity concentration and spodumene primarily through flotation. By contrast, pollucite is ground down to 75 microns and then subjected to acid leaching and other chemical processing to produce cesium chemicals.
Deveau says 80 people oversee the entire mining operation, from engineers to equipment operators to support staff. Fully equipped machine and diesel repair shops, and equipment repair bays, maintain the mining and processing equipment at peak efficiency, and a large inventory of spare parts is held in the company warehouse.
Still, things haven’t always run smoothly for staff. The end of tantalum production at the mine last year forced TANCO to cut its $1.5 million payroll by laying off 28 unionized workers, including miners, hoistmen and mechanics. Meantime, environmental issues were also raised. A local wilderness committee complained to the Manitoba government that TANCO’s decision to reduce the pillar width at the mine from 15 m down to 7.5m significantly lowered their strength. This, they argued, risked collapse of the entire mine and significant environmental damage to the lake.
So far the government’s Conservation and Water Stewardship branch has resisted calls to mitigate potential environmental problems by taking over the operations. For its part, TANCO applied for environmental permits to dam and drain the lake above the mine, in order to relieve pressure on the pillars so that mining could continue.
More immediately, stress meters installed at strategic pillars within the mine are updated hourly. “In the event of a catastrophic failure, changes in stress within the pillars will be noticed and the situation can be dealt with appropriately by evacuating the mine,” says Deveau. Extensometers installed at strategic places within the back of the mine measure microscopic movement within the bedrock.
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