In the 12 months to June 2001, Kestrel produced 4.35 million tonnes of raw coal, placing it among the top five Australian producers for the year.
What is noteworthy about this achievement is that its mining equipment is rated at roughly half of the power of the other top performers, such as Moranbah North (5.15Mtpa) and South Bulga (4.21Mtpa). For calendar 2001, Kestrel is on schedule to produce 4.5Mt ROM and is budgeted to produce 5.5Mt ROM in calendar 2002.
The strategy is all about getting the most out of the equipment, Kestrel general manager Dan Teal said. Beginning around 18 months ago the speed at which the equipment was running was gradually increased. Shearer speed was running at 40% of total power from the tailgate to the maingate, cutting uni-di, and at 80% of total power on the way back.
Power output was gradually increased by 2% increments. At 55% and 95% of total power, equipment reliability was found to drop off significantly, and the longwall has been optimised at 50% and 90%. To further increase speed the mine is trialing a half-web cutting cycle, according to longwall superintendent Paul Bettinzoli.
The approach of running the equipment at peak load helped deliver a record month in August this year of 619,040t of raw coal. (Annualised, this is over 7Mt). In addition, longwall availability is running at about 72%.
Mining in the current eastern area is progressively exploiting a thinner seam. Kestrel's two Long-Airdox Electra 1000 shearers are capable of a cutting height of 2.7m-2.9m, but as the seam has thinned in the current mining area to 2.3m-2.4m more stone is being cut.
Efforts are underway to lower the profile of the equipment to reduce stone in product. (Dilution is currently around 35%). Kestrel’s products are 64% premium grade coking coal at 6.5% ash and 36% steaming coal.
The mine has two full sets of longwall gear, including chocks, AFC and shearer, which enables the mine to produce coal continuously without stoppages during longwall changeouts. Typically, once the current longwall face has begun bolt-up the second face has already begun mining. Managing coal flow during this transition period is always something of a challenge.
Main challenges to mining are the weak floor (5MPa) and roof (10-25MPa) conditions, similar to those experienced by the nearby BHP Billiton Crinum longwall operation. Where weak floor is a problem around 300mm of coal is left in the floor. Roof support typically ranges between 4-8x2.1 AX bolts per metre with full steel mesh based on the strata conditions. Some hard coal does occur making it hard to cut. Other mining issues include high rib emissions requiring gas capture drilling.
Kestrel's workforce totals 193 employees, including 22 coal preparation plant workers. Productivity per man year measures around 26,300t. Six crews work on development using ABM20 bolter/miners. Average development rates are 20m per 10-hour shift.
Teal said the main challenge at the mine at present is to get the balance right between technical and cultural issues. (Teal plans to present a paper on this specific issue at the 5th Annual Australian Longwall Mining Summit in Yeppoon next year.)
Mining in the current part of the lease is due to be completed at the end of 2003, after which mining will start in the new Ti Tree area in early 2004. In this area the seam is 3m-3.24m high and dips from 220m to around 400m in depth.
The mine has over 30 years of reserves, with 80% recovery. Measured coking reserves total 352Mt and measured thermal reserves amount to 35Mt.
Technical services manager Greg Niewenhuis said infill drilling has confirmed the existence of three major structures in the Ti Tree area, all less than seam thickness. In recent years the mine plan has undergone a radical rethink. Recent modelling undertaken by geomechanics consulting company Strata Control Technology (SCT) modelled horizontal stresses in order to optimise the mine plan.
"Ideally horizontal stress should be parallel to the layout of a longwall block," Niewenhuis said. "The orientation of the blocks has been changed by 75 degrees from the original ARCO layout."
Yet to be determined is whether the mine will continue to operate a single, though wider, longwall face or go to a two-longwall operation. At this stage, the first two blocks in the new area are planned to be 250m wide, which is the current face width, but thereafter will be extended to 310m and possibly 330m. At their longest, panels will be 4.5km long. Work by SCT has also modelled pillar size, which will be designed 5m-10m wider than in the current area.
Niewenhuis said Ti Tree would deliver a step-change to Kestrel in terms of production. A gradual equipment upgrade is being implemented to increase capacity. The mine recently placed an order to upgrade the control system of the second set of roof supports to the DBT PM4 system, after the other set was fitted earlier with the PM4.
A new 3000tph Joy AFC (1100mm wide) was delivered to the mine in August/September this year. The new AFC includes a live automatic chain tensioner which the manufacturer says increases sprocket and chain life. In future, the AFC will be upgraded to 4000-5000tph capacity to accommodate increased output from Ti Tree.
After the completion of the first two blocks in Ti Tree the mine plans to purchase a new AFC, new conveyor and new shearer. Roadway development of the new area has begun.
