Nitrogen plant started up at Pike River Mine 

13 December 2018

The nitrogen-producing plant crucial to the safe re-entry and recovery of the Pike River Mine was yesterday started up for the first time.

“We started pumping nitrogen through pipes at the mine portal for a brief time before the weather closed in and we had to take our monitoring guys off the hill above the portal and shut the plant down. What we’re looking at with the start-up is how long it takes to push nitrogen through the mine,” Chief Operating Officer Dinghy Pattinson says.

“We’ll be running the plant every week day through to our Christmas closedown.  All our operations are based on the success of this very important piece of hired equipment, so today was pretty important to the project.”

Mechanical Superintendent Matt Coll pushed the button to start the plant.

“Matt’s the one who will be managing the nitrogen plant throughout the re-entry and recovery operation, it’s his baby from today,” Agency Chief Executive Dave Gawn says.

There was no fanfare, not even a hiss and a roar when the plant was switched on.

“Considering how important this is to our efforts, it’s surprising how quiet it is. But it’s definitely good to take this next step before Christmas,” Mr Pattinson says.

The Pike River Mine workings and drift tunnel are currently full of methane, a gas that is dangerous when mixed with fresh air.  The Pike River Recovery Agency’s re-entry plan to conduct a safe manned re-entry is based on filling the tunnel with fresh air, so people re-entering the drift can breathe normally without breathing apparatus.  To do that, the methane will first be purged from the mine workings and tunnel.  

Two 4.5km polythene pipelines are being laid up the hill above the portal, so nitrogen will be able to be pumped into boreholes at the top end of the mine.  Two new boreholes will be drilled to assist that process in the new year.  A further borehole will also be drilled to monitor gases in the mine.

Mining regulations stipulate that there must be emergency portal doors that are able to be sealed remotely, and the installation of these will begin next week. 

The technical explanation of ventilating the mine:

The purging process is as follows:

• Flush methane from the mine using nitrogen injection from the portal seal.  Methane will be discharged from the mine through boreholes. The purpose of doing this is to establish and maintain a non-flammable inert atmosphere over the roof fall at the end of the drift access tunnel, which has the potential to re-ignite if exposed to fresh air. (While the current atmosphere of 96% methane is both non-flammable and inert, it can form a potentially explosive atmosphere when mixed with fresh air.) 

Throughout the drift re-entry phase the roof fall area near the end of the 2.3km drift access tunnel will be maintained as a nitrogen-rich atmosphere through the injection of nitrogen down the same boreholes used to flush the methane.  This will prevent fresh air being present over the roof fall through the provision of a higher static pressure in the mine workings than that in the drift.  The nitrogen will leave the drift through a borehole on the portal / drift tunnel entrance side of the roof fall.