Published in September 2008 Australian Longwall Magazine

Yancoal Australia’s Austar mine, located near Cessnock in New South Wales’ Hunter Valley, has successfully increased coal recovery from a historical 47% using normal longwall methods to more than 85% with the introduction of LTCC.

In the first panel, the mine recovered 1.5 million tonnes with an 88% recovery rate of the seam. The dilution of the caved material was 9.8%.

The second panel to date is showing a recovery rate of greater than 88%.

In a presentation at the Longwall Summit in Mackay, Austar mining engineer Glenn Sobey told delegates that while the actual width of panel one was 150m and panel two was 220m, with the caving this was equivalent to a 275m and 400m wide traditional longwall face respectively.

Sobey said the first panel was a steep learning curve for the crews where they had to perfect the cutting sequence to successfully balance the cutting and caving with the capacity of the belt system. They also had to learn how to advance the complicated gate ends (there are seven gate end shields with six legs).

Sobey said the ground conditions on panel one were “generally well managed” with minimal impact on production.

One of the biggest challenges facing the mine was the longwall move.

The rear armoured face conveyor was recovered off the first face at three cut-through in the tailgate. To allow access to the AFC the tailgate end was supported and the goaf was consolidated with Wilson Mining Services’ Fenoflex resin injection.

The rear AFC drive was removed with a Myne Dozer while the AFC chain was removed into a bin on a chock hauler.

The pans were recovered with a Beltor Mine Puller by disconnecting them at the maingate, pulling the entire length in one go, and breaking the pans at the tailgate into sets for transport.

In the rear AFC recovery an adaptor pan was fabricated to connect the rack bars to the pan line. The adaptor pan uses dog bones to connect to the rear pan line and has a clevis to attach the rack bars from the puller.

Sobey said it was the first time a rear AFC pan line had been recovered in Australia, and to his knowledge the first time an AFC pan line had been recovered using a Beltor Puller.

To recover the longwall, Huesker mesh and Techflex were used to stabilise the roof and face prior to removing the longwall equipment.

The crew heavily cable-bolted the gate ends and an E frame was used during shield recover.

Sobey said the roadway and gate end shields recovery was a world first.

He said the transport of the gate end shields was particularly challenging due to the size of the equipment, with the shields weighing in at 35 tonnes. Fully collapsed for transport the shields are 7.2m by 2.2m by 1.8m and when fully extended are 10m in length.

Spontaneous combustion was also a major consideration in the longwall recovery. Given the mine’s history, where a spon com event resulted in a fire in 2003 under the previous owners, Austar has had to be ever vigilant in monitoring and preventing spon com.

There was high potential for spon com during recovery of the longwall due to the time taken to remove the complicated and large equipment; the high-sulphur coal left in the goaf; and the ventilation pressure across the goaf during recovery.

Of these factors, the Austar team determined ventilation pressure as the factor they could best control. A plan was put in place to target a ventilation pressure of less than 200Pa across the recovery face.

To maintain this, the mine used positive pressure chambers at the maingate seals to prevent leaking into the goaf and oxygen flow across the face. Oxygen ingress was also further diluted by the injection of nitrogen into the goaf at five cut-through, 250m inbye the recovery face.

An open airway was maintained across the face which reduced the ventilation pressure across the goaf.

Taking some lessons from the first panel’s installation road and roof monitoring results, Austar redesigned its support for the second panel. The crew installed 10m, 60-tonne cables on the first pass and updated the trigger action response plan to reflect actual results.

With the new pattern, Sobey said the mine has seen a 40% reduction in support density while maintaining roof integrity.

The second panel was started in November 2007, but has not been without its hiccups. The maingate roadway has negotiated cavities and the tailgate roadway conditions have proven difficult.

During the development of the A2 maingate roof conditions were very poor with guttering and cavities at the face. As the longwall retreated, steel beams packed with timber were used to form false roofs in the cavities. These roofs allowed the roadway shield to set to the roof and walk under the cavities.

The tailgate conditions have also been a tribulation. Double abutment loading on the tailgate roadway has seen conditions deteriorate.

“While Austar has used PUR as a proactive measure, the recent conditions of the tailgate corner have required the use of Rocsil and Fenoflex to consolidate the face and fill roof cavities ahead of the tailgate shields,” Sobey said in his presentation.

The mine has also increased standing support to 100m-plus outbye the face. Additional roof support has also been installed.

A review of the second panel since the longwall ramped up in November has indicated a weighting cycle of about 80m, with a more pronounced cycle every 160m with marked deterioration of the tailgate roadway.

Sobey said prevention of the conditions was extremely difficult due to the nature of the strata and the capacity constraints of the outbye belt system.

To tackle the conditions Austar is reinforcing with passive and active support those areas identified by its modelling as at risk of weighting. It is also proactively using PUR in problem areas before they begin to deteriorate.

Looking ahead, Austar is working on a number of long-term projects to further the success of the mine. The team is looking at redesigning the maingate and tailgate ends to simplify the operation of the equipment. While the redesign is still in the conceptual phase, the aim will be to reduce the number of hydraulic legs on the gate end shields and simplify the roadway support.

The mine also plans to re-hand the longwall between panels three and four. Currently the maingate is on the updip side of the block as the crew works its way uphill but in areas two and three the crew will work downhill so will need to re-hand the wall to deliver coal to the downdip-side maingate. Austar E Frame during the A1 recovery.