The coal industry has observed with interest the first two years of longwall production at the Moranbah North mine in Queensland’s Bowen Basin. While its production during this period has been impressive at 4.1 million ROM tonnes in calendar 1999 and 4.6Mt last year, the mine has yet to realise its enormous potential. Lessons learnt over the past two years have it well positioned as it continues to lift its production targets.
Moranbah North was part of Anglo American’s $1.5 billion purchase of Shell Coal’s mining interests last year. It was described by its previous owners as the crown jewel among the Australian coal assets.
According to Steve Rowland, general manager of Moranbah North Coal, the original desktop work-up of the mine set an annual production target of 3.5Mt. “By the time Moranbah North was officially opened a little over two years ago, it was stated that the mine would produce and reliably deliver over 4Mt per annum to international customers. That goal has been achieved.”
Behind the scenes, some observers had begun predicting possible production of 7-8Mtpa. Last year’s target of 6.9Mt looked ambitious, however, had the mine not encountered a range of geological and equipment challenges, this figure may well have been exceeded.
The end result was below plan, but still well above the previous year’s achievement.
“In the two-plus years to date there have been at least seven occasions when geological issues and related equipment constraints have seriously impacted on production,” Rowland said. “In addition, the longwall was brought to a halt by an industrial dispute for two days in December 2000 and on another occasion (for 30 hours) by a frog which fatally tangled with the surface power supply.”
Given the range of problems the mine encountered during its first two years, the fact that it is already one of the highest tonnage longwall producers in Australia is a remarkable achievement.
“The techniques devised to meet the geological and equipment challenges already have had a beneficial impact on the future operations of the mine. I have no doubt that our longwall production teams are among the most skilled underground miners in Australia and, in my opinion, among the best in the world. I have the background and experience here and overseas to unequivocally make that claim,” Rowland said.
“As an example of their high skill levels in operating equipment and routinely dealing with our ground conditions, when the longwall undermined a zone of sandstone in the latter part of 2000, our operators still averaged 96,000t a week production.”
As the first longwall block mined beneath a sandstone channel in 1999, cyclical weighting and associated convergence was encountered. Some 800m of the remainder of the block was successfully extracted under difficult mining conditions. Studies have indicated that as mining proceeds down-dip the influence of the sandstone channel will diminish.
“Numerical modelling indicates that stress conditions associated with increasing depth will assist in breaking up the sandstone,” said Bob Gallagher, Moranbah North technical manager. “Moranbah North believes that the conditions experienced in undermining the first longwall block are the worst that will be encountered. This conclusion is supported by experience in the second block where the channel is of greatest thickness and closest proximity to the seam. Our face management plan and operational controls have proven successful.”
Moranbah North’s production target for 2001 is about 7Mtpa and early indications suggest the mine is well on track to achieve that. In the first two weeks of 2001, the mine produced 240,000t of ROM coal. This is in stark contrast to early 2000, when weak ground conditions encountered at the outset of the second longwall block were responsible for sustained production losses resulting from roof falls, loading out of stone and associated AFC chain damage.
Extraction of the current 5Mt panel will be completed by mid-March 2001. Prior to bolt-up, the new face will be fully commissioned and ready to receive the chocks and an overhauled shearer. The third longwall panel, where cutting is due to commence cutting in April, contains about 5.7Mt over a 3.4km retreat distance. The planned base production rate is 155,000t per week. Block length increases to 4.2km, or just under 7Mt, for successive panels.
The most important operational change during 2000 has been the introduction of non-stop, 24-hour operation through 12-hour shifts, seven days a week. “If we stand still for any substantial period of time we experience a general degradation in the integrity of the immediate roof,” Gallagher said.
“The decision (to change to continuous operation) derives benefits of increased productivity through improved mining conditions — and also from a yield perspective, as the amount of stone from minor guttering and localised instability is minimised.”
The high productivity longwall operation puts pressure on mine development. Both Moranbah North and contracted Allied Mining crews develop gateroads. “Three-heading development has been introduced at the mine, necessitated by ventilation and operational requirements,” Gallagher said. “We anticipate making some significant productivity gains based on cycle analysis. Results this year have improved already.”