Production at Carborough Downs stopped in May after elevated levels of carbon monoxide were detected at the longwall operation.

A directive issued by an inspector of coal mines required the orderly withdrawal of workers and the suspension of all underground operations while mine management dealt with the situation.

It also required mine management to review and validate the effectiveness of “trigger” levels used to activate its action response plan to deal with the type of gas situation.

"With the assistance of the Queensland government Mines Inspectorate, Carborough Downs mine management has brought the underground heating and gas issues under control and the situation at the mine has returned to normal,” the Natural Resources and Mines Department spokesman told ILN.

“The coal mine has been re-entered and production has resumed in some development panels of the mine.

“Other workers are underground and in the process of getting the coalface in the affected longwall ready for production to resume.

“The directive issued by a mines inspector on 29 May 2012 suspending all operations at Carborough Downs was lifted on 4 July 2012."

The mine used several inert gas generators to direct inert gas into the longwall goaf via surface-to-seam boreholes.

Both Floxal and Tomlinson boiler units are in use.

The Queensland Mine Rescue Service GAG unit was available if it was required.

There was a risk of methane accumulation in the goaf, the spokesperson said.

The mine enacted strategies to lower the oxygen content in the goaf to minimise any potential further temperature rise and potential ignition of methane.

It included bulldozing cracks on the surface of the mine above the longwall panel.

Ongoing mine atmosphere testing will continue to monitor for methane and other gases.

Risk of methane accumulation in the goaf needs to be evaluated as part of risk assessment and management procedures associated with recovery operations.

Elevated levels of carbon monoxide are believed to be associated with accelerated oxidation or self-heating of the coal seam in the longwall goaf.

ILN did not get a response from Vale at the time of writing.