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Mobile units inertise underground mines

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Air Liquide Australia says its new Floxal inert gas generating system is an effective mobile inertising solution that prevents gas explosions in underground mines.

Its success has led to recent goaf inerting applications at Dartbrook, German Creek, Oaky North and Oaky No.1 underground mines.

It was also used earlier this year at Rio Tinto’s Kestrel Mine near Emerald to inertise the roadway when gas readings approached explosive range.

After its initial successful application at Oaky Creek, the Floxal AMSA system was used with increasing efficacy in central Queensland’s Bowen Basin and in NSW where Floxal mobile units used surface boreholes to pump nitrogen gas underground into each mine.

The first two mobile units can inertise underground goaf atmosphere at 500m3/hr with 97% inert gas purity.

Until the advent of the Floxal, nitrogen system mine inertising in Australia relied on combustion technology that produces boiler exhaust as an inert gas.

Air Liquide national product manager Sajimon Joseph explained to Australian Mining that unlike other gas inerting systems, Floxal units filter compressed air across hollow polymer membrane fibres causing the nitrogen to separate from oxygen and other components of the atmospheric air.

The Air Liquide hollow membrane fibre allows some gases to pass through the wall faster than others. Nitrogen is concentrated along the length of the fibre. Different parameters, including pressure and temperature, influence the concentration of the inert gas. Thousands of fibres are grouped into a single bundle and the compressed air is filtered and compressed while heated to a constant temperature of 45ºC.

Software tools are used to model the optimum supply.

“While Floxal units can produce nitrogen at a purity of up to 99.9% in coal mine applications, for optimum results nitrogen purity is set at 97%,” Joseph said.

“Results, encouragement from the coal industry and working with leading scientists from research faculties in Austalia, have led Air Liquide to progressive stages of development of the Floxal system to suit underground coal mine applications.”

Joseph told Australian Mining that the company has now increased the capacity of the Floxal system substantially to 1934m3/hr specifically for longwall applications, and that a number of these units will soon be available to contracting coal mines for goaf inertising.

“There are substantial differences between Floxal, which uses electricity, and other boiler systems that use water and up to 6000L of expensive diesel fuel daily. Air Liquide’s new 1934m3/hr Floxal system requires no more than 805kW of electricity.

“The Floxal inert gas generating unit does not run on diesel or need cooling water. The environmental advantages are evident considering that its waste product is oxygen and the system does not emit carbon monoxide,” Joseph said.

In a further comparison, Joseph pointed to the dangers to underground workers from the possible presence of carbon monoxide generated by flue-gas generators, and previous incidents of carbon monoxide “masking” the heating profile of the goaf.

By contrast with the boiler system, which produces nitrogen at approximately 1 bar, the Floxal unit produces nitrogen at a pressure of about 9 barg. This enables nitrogen gas to be reticulated through the mine for large distances while the unit can be located close to the power source.

“Nitrogen at ambient temperature and at pressure with no CO and acids with a dew point of -60ºC fits very well with the long cherished dream of ventilation officers and coal mine managers. Furthermore, while using a 6" pipe the nitrogen produced by the Floxal unit can cover up to 92km. The gas reticulated in a 4" polypropylene conduit can travel up to12.5km.”

According to Joseph, the Floxal system would enable the mines to proactively inert potentially explosive areas. He said this offered mine managements the option of semi-permanent Floxal installations capable of generating nitrogen gas as required to maintain the goafs in inert condition, and even to add Floxal facilities as the longwall progressed.

Joseph said operating remotely and without an operator in permanent attendance, the Floxal inert gas generating system enabled preventive maintenance by using a Tele-monitoring system to monitor the nitrogen flow and performance.

More info: (07) 3246 6363

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