Published in March 2005 Australian Longwall Magazine
Located 10km east of Lithgow in New South Wales, thermal coal is mined from the 2.5-4.5m Katoomba seam. Overlaying the Katoomba seam is the Caley formation, made up of sandstone, mudstone, shale and then sandstone again, making for strong roof conditions – excellent for development, but not ideal for longwalling.
Clarence began its life as a bord and pillar operation in 1980 before turning to longwall mining under Oakbridge ownership in 1990. While longwalling, the mine experienced numerous problems - the longwall could mine out 100-150m at a time before a fall, causing loading and windblast.
Oakbridge decided to close the mine in 1998 and 350 people lost their jobs. The mine was bought by Centennial for $18.8 million in August 1998 and 82 people were re-employed. It reopened as a bord and pillar operation less than a year later.
What developed was one of the most economical mines in Australia. Clarence has consistently increased production from a level of 1.01 million tonnes in 1999-2000 to 1.54Mt in 2003-04. Budgeted production for 2004-05 has been set at 2.25Mt.
During its Oakbridge period, Clarence employed full pillar extraction. However, like longwalling, the roof would bridge and hold up for expanses up to 250m wide. Centennial recognised these problems when it took control and opted for a partial extraction system.
Crews develop a seven heading layout with 33m by 33m pillars. Once a boundary is reached, the crew backs out of the panel by removing half of the outside three pillars and leaving the two centre pillars intact. An 11m cut out is then taken from the pre-driven roadway. Approximately 11,000t is extracted per pillar row during development and about 5500t is extracted from each pillar row coming back.
Partial extraction means about 50% of the coal can be taken. When Clarence used full pillar extraction it could take about 75% of the coal, yet partial extraction has proven itself to be more economically viable, according to Clarence mine manager Richard Tacon.
“There are a lot of restrictions where we can mine with full extraction, with about a third of our lease off-limits. In the remaining lease area there is an additional 15Mt recoverable by partial extraction at a lower percentage than mining with full extraction,” he said.
Restrictions applying to full extraction relate to subsidence damage. Clarence undermines state forest in one of Australia’s natural wonders, the Blue Mountains, famous for its pagoda sandstone formations and cliff faces. Through mining with partial extraction, Clarence has minimised subsidence to 20mm average over the lease.
Besides partial extraction being cheaper, the system also helped solve Clarence’s water problems. Full extraction causes greater subsidence, breaking into aquifers higher up in the sequence, allowing that water to infiltrate the workings. Because partial extraction minimises subsidence, cracking and movement in the upper strata does not occur. Tacon said since partial extraction began, water-make in the mine had not increased.
“We pump 18 megalitres a day now, but if we had continued with the longwall or full extraction it could be 25 megalitres plus by now,” Tacon said. The water is pumped to the mine treatment plant and a third of the water is then pumped into the Lithgow drinking supply, with the remainder discharged into a local creek.
Part of Clarence’s success can also be attributed to its adoption of the super place change. In late-2003, following the successful introduction and trial of a new super place change mining unit, production rose by 20% above budget. In doing so, the mine twice broke unit and shift records and set new daily, weekly and monthly records.
Super place change employs two Joy 12CM12 continuous miners and a Joy multi-bolter. The miners enter and cut out 15m each with the bolter oscillating between the two working areas. The bolting crew uses a minimum of four DSI 1.8m spot bolts every two metres. Tacon said Clarence was able to employ this method because of the excellent roof conditions and low bolting density.
“The system used to be one continuous miner and one bolter, and the bolter just followed the miner around – we were cutting-constrained. By adding another shuttle car, another miner and two more people, we have increased productivity by 25%,” he said.
Tacon said lessons learnt at Clarence could be adopted by longwall development crews, on the proviso consistent lengthy cuts could be taken without any roof falls.
“I have worked two-heading place change at Newstan in another life and it worked there when you could do consistent 15m cuts, but unless you go to a three or four heading maingate development then you are not going to see the full effect of a super place change,” Tacon said.
Clarence’s equipment fleet consists of six Joy 12CM12 continuous miners, two Joy multibolters and 10 Joy 15SE shuttle cars. The mine recently took delivery of a Joy 15SE32 shuttle car and will have three more delivered over the next few months.
Clarence currently has 10 years of mine life left, but have a further 150Mt of reserves under application, ensuring Clarence can look forward to a long future of high productivity.
