Australian coal mines have been faced with serious problems concerning the use of explosives, which has stemmed from the limited use of explosives for specific tasks within a mine.

Because explosives are used on an “as needed” basis and permitted explosives have a limited shelf life, there is a dire supply situation. Often when mines do wish to use explosives, they are not immediately available. Whilst most jobs can be scheduled with sufficient lead time, some situations such as outburst mitigation can require an immediate response.

The need for research into explosives was recognized this year in an ACARP grant, with the University of New South Wales being awarded funding for Permitted explosives for the 21st century. The project will analyse and determine a set of selection criteria based on current best practice to ensure that the risks posed by explosives use in underground mines can be better managed. The criteria will reduce the reliance on research completed about 50 years ago with a new and more valid set of tests.

Guideline will be developed to allow mining operations to select appropriate explosives for use in underground coalmines. Such a guideline would include the criteria for the selection and use of an appropriate explosive as well as the reasons why a particular methodology is safe. The eventual outcome of project will allow for greater advance rates when trying to de-gas in outburst prone zones allowing for fewer delays in longwall development.

Situations that often require blasting in an underground coal mine include excavating the roof for overcasts, conveyor overpasses, conveyor weight towers, underground crusher stations and interseam connections. It can be used to excavate the floor for sumps, underpasses, coal storage bins and interseam connections.

Blasting can also be used to excavate the coal seam when machinery is unavailable, a location is inaccessible to machinery, to remove an igneous intrusion that is too hard to be cut or if the gas content is too great for mining by machinery to proceed.

In these locations the presence of strata that can contain methane gas is quite likely and the presence on coal dust is also a possibility. The need for an explosive that can be detonated safely in these environments is of paramount importance.

University of New South Wales senior lecturer Duncan Chalmers has looked into the Australian state of affairs of explosives use, supply and future in his paper Reducing and controlling blasting risks and presented the question “where to from here?”.

This is certainly a valid question given that the only accepted research into explosives in underground coal mines was completed about 50 years ago. Dubbed the “Buxton tests” after the laboratory location in Buxton England, the tests determined the suitability of explosives that could be used.

The tests defined five categories of explosives that could be used for blasting in coal mines, ranging from P1 to P5 (P = permitted). P1 used for evacuation in stone do not present the underground coal industry with too many concerns.

The area Chalmers believes holds the greatest concerns is in delay firing of coal. This is due to restrictions in the use of a P5 explosive – which are all nitroglycerine based, such as Dynagex and Wincoal.

“Unfortunately both of these products have a limited shelf life and have been manufactured in Europe. The importation of explosives is via a special dangerous goods ship. This leads to long delays between shipments and may lead to the product being at sea for extended periods. This coupled with the limited usage and the shelf life results in quantities of this explosive deteriorating,” Chalmers said.

“With the drying up of supply, those underground mines that needed to use explosives as part of their Outburst Management Plan were faced with a dilemma. With no suitable permitted explosive available would it be possible to utilize a P1 emulsion explosive as a suitable substitute?” he asked.

It is a generally accepted view that there are threshold gas contents in coal, below which an outburst will not occur. These thresholds are dependent upon the constituent seam gases. Where a coal contains gas quantities above these thresholds, mining cannot continue, and the gas is drained. When the gas content will not lower below the threshold, despite extensive drilling and gas drainage efforts, these mines have resorted to drilling and firing to develop their roadways (grunching).

Once the supply of P5 explosives was exhausted mines with seam gas consisting predominantly of carbon dioxide and incapable of forming an explosive mixture in air, have been able to utilize the permitted P1 emulsion. According to Chalmers, this was seen as a stopgap measure at best.

“Where the seam gas is predominantly methane they have not been able to use emulsion explosive and have adopted other strategies. One mine had to mine into the roof and tunnel over the high gas content to be able to continue working. Others are looking at driving large diameter drill holes through these areas to facilitate gas drainage,” he said.

One option that has been explored but deemed too expensive was inertising the atmosphere using nitrogen generators.

With these options out of the picture, the opportunity to use P1 explosive was brought into question in a situation where carbon dioxide fell with a proportionate rise in methane. This spurred another set of questions - What is the limiting ratio of methane/ carbon dioxide? Can this limit be safely exceeded? What effect can the blasting fumes have on the explosibility of the gas mixtures?

Some short term solutions have been offered. Work done by Hughes and Raybould, Coward, US Bureau of Mines, and Mackenzie-Wood indicate a ratio of 77/23 carbon dioxide/methane ratio is the limiting factor when mixed with air. Seam gas with methane content below this level would not be capable of passing through the explosive triangle. A slight safety margin of 80% CO2 was suggested and P1 emulsion could be used in place of the P5 explosive.

Chalmers said work done into the use of explosives to combat oil well fires by inertising the atmosphere through the violence of the blast could be applied to lower the CO2/methane ratio. If sufficient fumes were liberated into the atmosphere and mixed with the emitting seam gas then it might lower the ratio. If it is released into the atmosphere as blasting continues then it will also dilute any methane that is present.

“By varying the quantity of blast fumes present in the atmosphere the relationship between the amount of CO2 and blast fumes is determined and conditions for blasting in coal where seam gases rich in carbon dioxide can be developed.” Continues, click here.