Underground » Detection and Prevention of Fires and Explosions
Current Australian stone dusting regulations have their research origins as far back as the 1920s. However, changing mining practices have progressively increased the generation of coal dust and reduced its particle size. As particle size affects the explosibility of coal dusts, current inerting levels have therefore been rendered obsolete and an increase to levels discussed in this report is recommended.
Testing a range of Australian coals and dust particle sizes shows the minimum incombustibles requirement is between 65% and 90% depending mainly on the initiation energy and stone dust particle size. Assuming current stone dust sizes, and without mine specific testing, the incombustible content of settled dust should be increased to a minimum of 90% to markedly change the effectiveness of this risk management strategy. Even at 90% there is little or no margin of safety to cover all credible circumstances, including the presence of flammable gas. This inerting level represents the total inerts concentration; that is, it includes the ash and moisture contents of the cost dust. This value could be difficult to consistently achieve in some high production mines.
The effectiveness of stone dust used in Australian mines can be improved by reducing its particle size and/or adding chemical suppressants. Further research is required to determine if these strategies could offset the need for increased dusting, and even reduce the level of inertisation, as suggested by this report.
The results from the limited local testing of the effect of stone dust particle size on inerting levels shows a broad improvement with decreasing particle size. This is probably related to the finding that of the order of only 2% of dust dispersed by the pressure front has particle sizes >250?m, irrespective of the particle size distribution of the settled dust from which it was raised. More extensive research undertaken in Canada and the United States confirms that the use of finer stone dust particle sizes increases effectiveness and is therefore recommended. This overseas research suggests the use of stone dust with an optimal particle size distribution, as detailed in the report, could reduce the total inerts requirement.
This study has been unable to generate a strong relationship between dust explosibility and volatile content as found in the UK and USA. The report discusses the likely conclusion that Australian coal dusts should be considered to behave differently in a flame zone compared to Northern Hemisphere dusts. This is because there is considerable variability in the macerals analysis and the grain size of vitrinite in Australian coals. The same conclusion has been drawn in relation to South African coals. Accordingly, in the absence of additional testing, the current regulatory based linkage should be abandoned in favour of a single value, or a value linked to the presence of flammable gas.
The concept of mine-wide explosion protection solely by general dusting appears to be seriously flawed. This study has shown explosion with Australian coal dusts can be propagated at 90% inerts, a level that will be difficult to consistently achieve. These results suggest a secondary explosion protection strategy is required to reduce the irks to as low as reasonably achievable. This could be in the form of more effective passive barriers, triggered barriers at the face or outbye, or more likely, some combination of all of these.