We present an apparatus based on hot-wire ignition in a pressurized enclosure and an associated methodology to measure the minimum pressure required to induce sustained combustion in water-based emulsion explosives. This method improves product characterization to allow one to use them more safely during pumping and mixing operations.
This manuscript describes a protocol to measure the minimum pressure required for sustained burning of water-based emulsion explosives. Pumping water-based emulsion explosives for blasting applications can be very hazardous, as demonstrated by a number of pump accidents around the globe in the last decades, including some that resulted in fatalities. In Canada, the recognition of this hazard has led to the development of pumping guidelines that were endorsed by both the explosives industry and the Explosive Regulatory Division of the Canadian government. In these guidelines, it was noted that the minimum burning pressures (MBP) measured in a laboratory would provide a good guide to characterize the behaviour of these products in pumping systems. The same guidelines also call for the design of pump systems that prevent, whenever possible, pressures from exceeding the MBP of the product being pumped. At the time of publication of these guidelines, a methodology existed for measuring such MBP values but it had never been validated to measure the MBP of ammonium nitrate water-based emulsions (AWEs). AWEs are now used much more widely than any other water-based explosives and precursors in on-site bulk loading operations.
The Canadian Explosives Research Laboratory (CanmetCERL) has been conducting research over the last ten years to develop a validated testing protocol to measure and interpret representative MBP values for AWEs. The test, as it is performed today, will be described and the critical components will be justified by reference to recent published data. Results of MBP measurements, for a range of AWE products, will be presented. Inclusion of the MBP test in the test standards for the authorization of high explosives in Canada will also be discussed.
The ammonium nitrate water-based emulsion (AWE) explosive was invented in 1961. It consists of microscopic droplets of a liquid oxidizer solution surrounded by a continuous oil phase. The first stable and practically useful emulsion blasting explosive was developed by Harold F. Bluhm in the USA (1969) 1,2. However, the successful commercialization of this type of explosive did not really happen before the beginning of the 1980s.
With the large scale of modern mining operations and the advent of fast bulk explosive loading methodology, very large volumes of AWE explosives have to be manufactured and transported. One tanker load typically transports 20 tons of AWE and many such truck loads are usually necessary to load only one blast. Accidental initiation of such large quantities of explosives would be particularly disastrous and, therefore, a good knowledge of their hazardous properties is required to design corresponding safe handling systems. While it is well known that emulsions are relatively insensitive to mechanical events (i.e. impact and friction events), accidental explosions have still been reported 3 while handling this type of explosive, particularly in pumping applications.
It has been known since the 1970's 4 that a minimum ambient pressure is required for self-sustained combustion to take place into water-based explosives. This latter value has usually been termed the "Minimum Burning Pressure" (MBP). From a safety point of view, knowledge of this threshold could allow manufacturers to better estimate safe operating pressures for various handling equipment.
The Department of Natural Resources of the Government of Canada has published "Guidelines for the Pumping of Water-Based explosives" 5, which state that using pumping pressures well below the MBP of the emulsions or watergels is a good safety practice. It should be noticed that these guidelines were designed with the collaboration of most commercial manufacturers and that, in the USA, the Institute of Makers of Explosives (IME) has also published very similar guidelines 6. However, in these documents, there was no description or prescription on how the MBP should be measured.
In the last decades, only a few studies related to MBP measurements have been reported. Chan et al. 4 reported the results of MBP measurements for watergel explosives, which are also ammonium nitrate and water-based. They have concluded that the MBP can have a strong dependency on several formulation factors such as water content, presence of chemical sensitizers or metallic powders. In another study, Wang 7 described a 2.5 L pressure vessel pressurized with N2 and used a Bruceton up-and-down method to determine the MBP for basic AWEs. With this system, MBP values of the order of 15 MPa were measured for a basic emulsion having a water content of 16 mass %.
Using a similar pressurized vessel test, Hirosaki et al. 8 have reported the results of some MBP measurements for AWE explosives. They have noted that the nature (i.e. glass or resin) of the micro-spheres being used to sensitize the explosives also has a strong influence on the results. More recently, Turcotte et al. 9 have developed a system similar to that of Wang and Hirosaki et al. and have attempted to use it to measure the MBP of some AWEs. However, they have found many possible problems that may lead to erroneous MBP determinations. In particular, it was noted that the ignition source geometry (nichrome wire coil) had never been properly validated for AWEs. In 2008, Turcotte et al. 10 and Chan et al. 11, have developed both an apparatus based on a calibrated ignition wire system and an associated methodology to measure the MBP of AWEs. They have also used the facility to study the ignition characteristics of typical AWEs, measured the energy requirements to obtain reliable ignitions 12 and studied the influence of physical characteristics and ingredients on the MBP of a wide variety of AWE explosives 13,14. This MBP measurement technique is presently being proposed as a standard test within the United Nation Transport of Dangerous Goods (UN TDG) Tests and Criteria for the classification for transport of AWEs 15.
NOTE: The materials and equipment used here are listed in the Table of Materials.
1. Preparation of Ignition Wire Assemblies
NOTE: Wearing nitrile gloves is recommended for this operation.
2. Sample and Test Cell Preparation
NOTE: Wearing nitrile gloves is recommended for this operation.
3. Loading Sample in Pressure Vessel
4. Performing a Test
5. Data Analysis
NOTE: See Figure 6 for an example of a graph showing an analyzed MBP experiment.
6. Cleaning up
Typical raw signals from a test resulting in a fully propagated event (i.e. "go") are shown in Figure 6. The ignition current (blue curve) is seen to come on at t0 = 0 and to stay on until the NiCr wire burns at tb = 19.1 s. The computed average ignition current (i.e. average of all data points between t0 and tb is Ihw = 10.59 A. On the pressure record (red curve), the first sign of clear departure from the initial baseline is observed to occur at tp0 = 17.3 s. The computed average initial pressure (i.e. average of all data points between t0 and tp0) is Pi = 4.924 MPa (700 psig). From tp0, the pressure is seen to rapidly increase to a maximum of Pmax = 6.095 MPa (870 psig) at ts = 33.7s. At this point the burning front has reached the internal wall of the cell and the pressure quickly decreases as combustion ceases.
The MBP measurement protocol presented here has been developed through a careful study of the many physical effects that can influence the outcome of the measurements. Through the publication of several documents, MBP data on a very wide variety of AWE formulations have been presented, thus establishing the usefulness and reproducibility 16 of the proposed measurement protocol.
In particular, the preponderant effect of the water content on the MBP of AWE formulations has been clearly demonstrated. This can be seen in Figure 5 showing the MBP data for five AWE formulations with water content varying between 11.7 and 24.8 mass percent (%). For these five emulsions, the oxidizer solution consisted of only ammonium nitrate and water while the oil phase (oil + surfactant) amount and composition was kept fixed. It can be observed that, for each measurement, a series of 12 to 16 tests were performed. For each measurement, the two short horizontal bars indicate the pressure interval between of the highest "no-go (or partial)" event and the lowest "go" event, as specified in the above protocol. This illustrates well the strong dependence of the MBP of these particular formulae on the water content. From Figure 5, it can also be observed that the scatter in the MBP data is much higher for the two formulae with lowest water content (EM4 and EM5). Since these formulae contained only ammonium nitrate in their oxidizer solution (no other salts), they have relatively high crystallization temperatures and, as such, may be more prone to crystallization upon manipulation. This could induce a certain degree of non-uniformity in the samples and, therefore, a more important scatter in the data.
Figure 1: Complete ignition wire assembly. Please click here to view a larger version of this figure.
Figure 2: Typical MBP test cell with installed ignition assembly and emulsion sample. Please click here to view a larger version of this figure.
Figure 3: Test cell. (A) Assembled test cell just before introduction of emulsion sample through the slit. (B) View of test cell from one open end with neoprene stopper removed, showing the details of the NiCr wire running along the axis of the stainless-steel cell. Please click here to view a larger version of this figure.
Figure 4: Visual observations and pressure recordings. (A) Typical visual observations for a "Go" (left) and "No-Go" (right) events. (B) Typical pressure records for "Go" & "No-Go" events. Please click here to view a larger version of this figure.
Figure 5: Summary of results for the MBP measurements of ammonium nitrate/water AWE formulae. Please click here to view a larger version of this figure.
Figure 6: Example of an analyzed MBP experiment. Please click here to view a larger version of this figure.
Our work demonstrated that the linear hot-wire geometry with 0.5 mm diameter NiCr straight wire and 10 to 16 A ignition current was adequate to ignite AWEs with water contents up to 25 mass %. For high viscosity formulations (such as packaged emulsion products), horizontal and vertical configurations provide almost identical results 17. However, for low viscosity formulae (such as bulk emulsion products) gravity effects in vertical configuration induce emulsion flow which disturbs the ignition process. In these cases, the horizontal configuration was found to provide valid and reproducible results 17. It should be noticed that the MBP values obtained in the present work for high water content emulsions are much lower than those reported by Wang 7 for similar products. This difference is probably due to the fact that, in his case, the ignition source had a coil geometry, which is less efficient to transfer energy to the emulsion, compared with the straight cylindrical geometry used in the present work. Also, if the coil is made out of too small diameter wire or if the loops are too close to each other, the ignition coil may burn prematurely, before the emulsion can be ignited. In such a case, it is very likely that failure to ignite may have been confused with failure to propagate.
As an example, for a typical surface bulk AWE such as EM6 (17.4 % water, Figure 5), the MBP measured with the present cylindrical geometry is 8.2 MPa. The MBP quoted by Wang for a similar product with less water (16.0 % water), using the coil geometry, was 15.2 MPa 7, which is almost twice as high. Moreover, using the coil geometry with the same nichrome wire used in the present work, it has been found that a similar emulsion with 16.8 % water could not be ignited to sustained combustion even at initial pressure up to 15.8 MPa 9. In comparison, the emulsions investigated in the present work, which had water contents as high as 24.8 %, could all be ignited to sustained combustion at pressures below 15 MPa.
As expected, the data obtained in the present work clearly demonstrates that water content is the major ingredient controlling the MBP of AWEs. The role of several other ingredients has also been investigated in some detail. However, many unexpected effects of some ingredients (sodium nitrate and glass microspheres, for examples) have been evidenced 14 and more research would be required to fully understand how their presence affects the ignition and propagation of combustion in these AWE systems.
The test, as described in the above protocol, has been added to the requirements for the authorisation of high explosives in Canada by the Explosives Regulatory Division of Natural Resources Canada 18. It became an authorisation test for the acceptance of explosives handled using pumps or augers. This test has also been proposed as an alternative to the UN TDG Series 8c Test (Koenen test) 19 for AWEs. The acceptance of the test is currently pending further discussion within the informal correspondence group being led by Canada 20. This group consists of seven international Competent Authorities and four non-Government Organizations. More detailed information on the above protocol can be obtained by contacting the authors.
The authors have nothing to disclose.
The development of the testing protocol reported in this publication results from a joint research project between Natural Resources Canada (CanmetCERL, Explosives R&D Section) and Orica Mining Services. Permission of Orica Mining Services to publish non-proprietary information on this subject is fully acknowledged. The participation of CanmetCERL's Analytical Section to the physical characterization of the various AWEs prepared throughout the present work is also gratefully acknowledged.
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oscilloscope | Any storage oscilloscope with 2 input channels (0-10 V), 12k samples per channel and acquisition frequency of 10 ms/sample. | ||
Precision Shunt Resister | Canadian Shunt Industries | LA-20-100 | (20 A, 100 mV) Enclosed in custom box http://www.cshunt.com/pdf/la.pdf |
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