Underground » Detection and Prevention of Fires and Explosions
Stone dust has long been used as an incombustible material spread over the walls and floor of coal mines to act as a heat shield and to arrest a propagating coal dust explosion. The stone dust (limestone) is raised into suspension along with the coal dust in an explosion and blocks the transfer of heat from one particle to another. The practice of applying stone dust is known as stone dusting.
Recent research has shown that legislative requirements for incombustible material in the practice of stone dusting to suppress potential coal dust explosions have been too low. Recent changes to the Queensland Coal Mining Safety and Health Regulations have increased these levels to reflect results of this research.
The use of a more efficient inertant could allow a reduction in these recently increased requirements.
The use of rock phosphate as an alternative to stone dust is investigated here, as it is readily available by virtue of the fact that the fertilizer industry currently imports it from worldwide sources. The recent development of the Phosphate Hill deposit near Mt Isa by QMC also makes the use of this material attractive.
Rock phosphate was chosen for investigation as a result of explosibility testing of superphosphates for the fertilizer industry. The addition of 50% rock phosphate to sulphur substantially reduced the explosion pressures when ignited.
Various forms of rock phosphate were tested against stone dust with regard to their ability to inert coal dust explosions. In all cases the rock phosphate showed no appreciable advantage in inerting ability.
The cost of using such an alternative would appear to be 50% greater.
Ammonium phosphate, which is a processed form of rock phosphate, did show a significant advantage in inerting level, when compared to stone dust, with only one third of the amount of inert material required for full inerting of a coal dust explosion. This material however is not suitable for general stone dusting due to its increased solubility in water. It could be used in bag barriers which are protected from moisture.