Not until the appearance of a white knight in the form of Indian smelter operator Sterlite Industries in April 1999 did stability and security return to a mine that has been in continuous production for more than a century.
Through a process of continuous improvement combined with capital cost containment, Copper Mines of Tasmania (CMT) has led to a record year in mine production in 2003.
However, one of the problems the operations is still grappling with is keeping mine and mill in sync: it seems when one is going well the other experiences problems. So while the mine has been averaging 250,000tpm the mill has not been able to keep pace, whereas last year the mill was burning along.
A step change in winning more tonnage from the mine was achieved three years ago when Barminco took over mining under a schedule of rates contract. Prior to its involvement development had gone well but production was lagging. Things came to a head when a good part of one of the levels was lost because the former contractor did not move quickly enough to beat the ground conditions while they were mining.
Barminco came in with new mobile equipment, which now consists of five Caterpillar R2900 loaders, eight 43t-payload Sandvik Tamrock Toro 50D trucks, a Tamrock Datasolo 1520 production drill rig and a Tamrock Powerclass jumbo.
According to CMT general manager Neal Valk, Barminco has developed a good understanding of the mine over the past three years and the mine has settled down well. Contract discussions between CMT and Barminco are underway at the moment.
The mine, now down 850m below the old open pit, works on a three to five year plan, although diamond drilling has confirmed the Prince Lyell orebody remains open at depth. Current reserves measure 5.89Mt at 1.3% Cu and 0.35g/t Au with resources of 23.04Mt at 1.41% Cu and 0.37g/t Au.
The operation produces 2000t of 29.8% copper concentrate with a moisture content of 9% each week. This is trucked up the road a short way to Melba Flats by TasRail who then rail it to Burnie on the state’s north west coast and load it on to a ship bound for Sterlites’s smelter in India every four to five weeks.
CMT had pushed mine off three levels but have reverted to two because it meant splitting ventilation between three sets of loaders with three lots of secondary ventilation. In the end it got just too hard and too much time was wasted adjusting the ventilation to match the location of the engine power. Working off two levels has also improved truck and loader utilisation, with the help of the workforce and supervisors.
Transverse mining
The most recent boost to production has come from a change in direction of extraction.
For the last eight years mining has been by longitudinal sub-level caving but CMT has moved to transverse sub-level caving. The changeover will take another nine months to complete.
Valk told Australian Mining that this major change was driven by drill and blast problems firing along strike and because in the narrow parts of the orebody operations were restricted to between three to five draw points on each level, which made two level production very difficult. With the changeover seven working draw points are available on each level, which has led greater consistency in blasting and to greater confidence from having more draw point available.
Transverse mining at Mt Lyell is not new, being used until 1994 when a new operator switched direction. The next six levels were mined longitudinally. The operator also changed other parameters, like making the levels further apart, which made blasting more difficult.
The goal of CMT, however, was always to ramp production up from 200,000tpm to 250,000tpm. To achieve this demanded mining be more productive more of the time, so earlier this year it was decided to change mining direction. The decision has paid off and now the technique is being refined. For example, initially it was decided to mine wider to improve blasting, but this has presented some ground problems so mining will narrow in future.
In November, work began on the next loop in the 5m x 4.9m decline to take it down to 1490mRL. Meanwhile 1515mRL level is under development and stoping is scheduled for March. Ventilation will be in the hangingwall.
Other initiatives include looking at better bolting systems to improve efficiencies.
Because of age problem, level development will be left later so it is younger when stoped. Valk says if a development is left open for 12 to 18 months, ground water rusts out the ground support.
Moisture problems
Despite these improvements in mining, progress towards a consistent 250,000tpm has been, at least temporarily, stymied by trouble at the mill.
While last year the plant did 2.7Mt it started having moisture problems in about February, which has meant a lot of blocked chutes, and build-up overloading the screens and crushers. Valk says the sticky dirt has inevitably led to some loss of focus as everyone has been in fire fighting mode, shovelling tail drums out, etc. It has, he says, “been a pretty miserable winter”.
These problems have led to the amassing of a 330,000t stockpile – good news no doubt for the bean counters with the rise in price of copper, but double handling issues for operations.
Valk says the stockpile has been built because as the mine is working so well, management wanted to maintain the momentum. It had also hoped to overcome the plant problems earlier.
Now, since October instead of trying to produce 250,000tpm and 200m development, this has been rescheduled to 200,000tpm and 250m development in an effort to reclaim the stockpile. CMT estimates that changes under way at the plant will see a return full-scale mining in April.
The plant has been the subject of continuous improvement since the production rate increased with the change of contractor in October 2000.
To help keep up, a quaternary crushing circuit run by an external contractor was installed and run by on a per tonne basis. This improved feed size to the mill and recovery.
Next in late 2001, an Outokumpu OK 100 tank cell was installed along with a regrind mill to improve the concentrate grade.
These relatively low capital outlays led the plant to have a cracker of a year in 2002 and inspired CMT to buy the quaternary circuit from the contractor and to automate most of the crushing circuit.
But the onset of the moisture problems quickly undid the improvements leading to a throughput decline from 230-240,000tpm to 180-200,000tpm in the worst months.
What was initially thought to be a passing problem has extended over several months and has reached a critical stage. CMT is now searching for a long-term solution and has considered increasing quaternary crushing capacity, wet screening and SAG milling.
Solutions
At least as a partial solution, it has plumped to increase in quaternary circuit capacity. In July it changed a crusher in the quaternary circuit to give more capacity and is currently installing a duplicate circuit in the quaternary.
The move to double the quaternary circuit capacity with the addition of another crusher and screen in parallel, will also provide more flexibility in the tertiary crushing circuit. This work will be completed in mid-December.
On another level the drilling of drain holes in different areas of the mine has proved too hit or miss to be really effective.
The search for solutions has also seen CMT adjust the mining sequence of the footwall and hangingwall to obtain a better blend of moisture and material.
Maintenance has improved with the provision of standby pumps. Also the installation of a fifth tailings pump has taken the pressure off the other pumps, which means the plant now closes every 10 weeks instead of every six.
Next, CMT is looking to install some new equipment in the flotation plant.
Valk says they have progressively worked through the plant from skip loading through to the filter plant designing out those things that make particular installations critical. A few critical points remain, for instance, the surge bin still has a fair amount of downtime from wear.
Another major improvement this year has been running fibre optic fibre right around the sprawling 15km2 site, which is now hooked up from the bottom of the mine to the concentrates shed.
This has helped with materials handling because it allows operators to see what is happening all along the long the line on their PC screens, instead of relying on phones as before. They can now see, for example, if the secondary circuit is working so they know they to keep hoisting.
This is all connected to the operation’s Citect control system that in addition to providing trends also allows sections of the plant to be fully automated; although operators still run some areas as they can best make adjustments for roughness of rock and the inevitable moisture.
Valk says “ our problem now is where do we go next”. Having tacked equipment on to overcome bottlenecks they have reached a point where wholesale changes are needed.
Although the copper price is good and 100% of the concentrate from Mt Lyell goes to parent company Stirlite’s smelter, Valk says this does not mean that they can produce at any cost. At present, he says, the production cost of $24.50/t is reasonable, the main challenge remains to make production consistent.
Meanwhile, CMT is considering developing the Western Tharsis, a blind orebody that has never before mined, but which could produce up to 1Mtpa. Access would be gained from the Prince Lyell shaft.
FOOTNOTE: Sterlite Industries, and therefore CMT, will soon become part of Vendenta Resources, a new float on the London stock exchange.