Economic necessity was a catalyst for some lateral thinking at the Hunter Valley mine, which is set to become the first Australian mine to adopt the Advancing Retreat Method of longwall mining.
A “rather challenging situation” is how Harry Adams, managing director of Gympie Gold, remembers the first few months after the company took over ownership of the old Ellalong mine in the New South Wales Hunter Valley in mid-1998.
“We had closed the Ellalong pit in the midst of a contract year with no established panel to continue production,” he recalled. “We had a vacant site with no workforce. We had no contracts to supply coal and we had a site that needed to be rehabilitated around the wash-plant.”
Gympie appointed contractor Colrok to operate the mine and by August 1998 longwall extraction of the mostly completed panel 13A was underway.
“Then we switched the workforce to quick development of the panel west of the lease (SL1), which was very quickly proved up. That panel is now half extracted,” Adams said.
Mining of SL1 has occurred at a rate of about 20,000 run-of-mine (ROM) tonnes per week, with production for the year to December 31 1999 having reached 1.1Mt of ROM coal. Once the mine’s start-up was successfully fast-tracked, attention shifted to the longer term future. A strategic decision was taken to develop the next panel, Southland 2, in the adjacent Bellbird South lease, some 3.5km away. The best part of $10 million was spent driving two roadways through the southern part of Bellbird and up into the northern part of the lease, in the process bypassing some panels that could have been extracted, Adams said. “We did that to get to a zone that could be laid out for a 10-year mine plan. It contained larger reserves where you could sequence out 10 years of production.”
The Bellbird resource is also low in sulphur, which will improve coal quality, and the seam is 5-8m thick.
“Cutting in a thicker seam means initially we can pick our horizon within the seam to select which coal to cut,” Adams said. “In the Ellalong pit the seam was usually 3-3.5m and extracting the full seam section meant there was no flexibility as to ply selection. When extracting 6m-plus you can play around with it and you leave a fair bit of coal in the roof.”
With the strategic decision in place to begin mining Bellbird, the race was on to develop SL2 ahead of the completion of SL1. The ultimate length of SL2 would be heavily dependent on development performance. However, while Southland has more than doubled its single heading development rates, drivage to Bellbird last year proved more difficult than first envisaged. Several faults were encountered which slowed progress and caused a rethink of the design of block SL2. There was simply not enough working capital available to fully develop SL2 to 2.2km if production continuity was to be maintained.
Management had to decide whether to shorten the panel length or maximise the recovery of reserves, but face discontinuity in production. Buying in other coal was not a feasible solution because of the specifications of Greta Seam coal.
The Southland management team, headed by chief executive John Leach, began examining alternative options.
“The Advancing Retreat Method (ARM) arose out of a comment made by a non-mining person from Southland Coal saying, ‘why can’t the longwall go forward?’” said Southland manager Tim Jackson. “Then we all got our heads together and the rest is history. Some of us were saying if it’s so simple why didn’t we try it years ago?”
The idea is deceptively simple and is basically a combination of conventional retreating and advancing methods. The ARM premise is to compartmentalise a single block into three sub-panels which are each developed and then extracted in sequence. While development of the second sub-panel continues, mining of the already established sub-panel can begin. Longwall extraction continues on retreat with the continuous miner completing development in another section of the same panel.
Southland turned to specialist consulting group AMC Resource Consultants which provides the mine with various technical and mine planning services. AMC consultant Dave Meldrum undertook a financial and operational evaluation of the ARM method, which had not been used before in Australia.
Describing ARM, Meldrum said: “The longwall extracts in a direction away from the supporting main headings which is typical of the advancing method. The longwall, however, operates in an environment typical of the retreating method. In order to provide this retreating environment a three heading maingate is driven. Longwall production travels inbye on the zone maingate conveyor to the inbye extent of the zone and then across to the system maingate conveyor which is installed in the heading adjacent to the next longwall block.”
Meldrum said the main advantages of ARM were a reduction in working capital, improved cashflow and production continuity.
In order to support longwall ventilation and coal clearance from the tailgate development, slant drives are driven across the longwall block. The longwall must mine through these roadways, probably the riskiest part of the exercise.
Jackson said both passive and active roof support would be used in the slant roads, including cuttable cans. While geotechnical modelling suggested such secondary support of the slant roads was adequate, Jackson said if conditions warranted, backfill could be introduced.
He said the mine also had initial success with coal augering trials, though some geotechnical issues and equipment problems had yet to be resolved. A section of the mine had been set aside for auger mining which would commence as soon as Mines Department approvals were received.
Mining of SL1 is due to be completed by the end of May. Management then faces the logistical nightmare of moving the longwall 3.5km underground to Bellbird. The old 1.5km rail system is being converted to a rubber-tyred-vehicle road. Mining of SL2 using ARM is scheduled to start in July.