Open Cut » Drilling & Blasting
The aim of this project was to develop new bulk explosive products that can completely eliminate NOx fume emissions. One of the key outcomes has been the development of explosives that use Hydrogen Peroxide (HP) as the main oxidiser together with a fuel phase that incorporates a sustainable and renewable fuel source. As part of the product development process, Hybrids of HP with other nitrates such as Ammonium Nitrate (AN) Calcium Nitrate (CN) and Sodium Nitrate (SN) have also been investigated and shown to perform as conventional non-ideal AN-based explosives.
HP/CN and HP/SN mixtures for example have shown advantages associated with more flexibility in the density range; and robustness leading to improved stability. Research with these new products opened specific questions about the potential formation of NOx given by the presence of nitrogen in the formulation, this is particularly the case for products such as HP/CN and HP/SN hybrids. These mixtures do not have a second source of nitrogen such as the NH4+ found in AN-based products.
The objective of this project was to quantify the NOx (or NO2) fumes produced by the detonation of HP-based hybrids products, where a portion of the HP is replaced by nitrates such as ammonium nitrate, calcium nitrate and sodium nitrate.
A blasting chamber was designed and commissioned to measure the production of NOx and other gases from a selected range of HP hybrid formulations. The commissioning of the blast chamber was completed in two stages by initially detonating small explosive charges and incrementally increasing to larger charge weights. Using small charges ensured the risk of damaging the chamber and any potentially unforeseen hazards were minimised. The primary objective was to monitor the post-blast gases and this required the chamber to be a closed system that could contain the overpressure generated from each blast.
A comprehensive experimental program was designed and implemented within a limited timeframe. HP-based hybrid gel formulations were prepared and tested at five different densities. In hybrid products part of the HP was replaced with AN, CN or SN. The HP-based product is a gel, with an emulsion explosive-like consistency. Two phases, oxidiser and fuel phases, are used to produce the product. These phases were mixed and then chemically gassed to achieve the target density. Samples were sensitised (gassed) and the gassing solution used was either KMnO4 1.0, 0.5% or 0.25% w/w. Control samples of ANFO and emulsion (single salt) were also tested for comparison purposes. Samples and tests consisted of 100 grams of product in glass beakers with a 51 mm internal diameter. This charge size allowed a short ventilation time from the blast chamber and produced gas concentrations within the ranges of the gas monitor.
The HP-based gels were manufactured with an oxygen balance (OB) of 0.0±0.5. It was expected that CO presence would be minimal but testing showed otherwise. The CO presence was comparable to the AN-based products tested. Analysis showed that the presence of the plastic from the booster would have contributed to the CO measurements. Although only 12 grams of plastic is used to form the booster casing, the violent reaction caused during a detonation would result in the decomposition of the plastic material and will contribute to the gaseous products. Analysis also indicated that NH3 measurements may have come from the production of N2 which continues to react with excess hydrogen produced by the booster's plastic casing, forming NH3.
The formation of NOx was detected in HP/AN and HP/CN based hybrids explosives; and can be explained through basic analysis of the chemistry. HP-based (gel only) explosives, as expected, did not produce NOx. HP/SN hybrids did not produce NOx and although encouraging, it was difficult to explain. Analysis of the temperatures during the reaction taking place with HP/SN hybrids it was hypothesised that a lower temperature is achieved and thus prevented the formation of NOx. As part of future work it is important to extend testing in this area to fully understand the absence of NOx from these specific hybrid mixtures. It is also important to highlight the general trends observed which indicated a lower relative concentration of NOx in HP hybrid products when compared to the control AN based products (ANFO, emulsion and emulsion blend).
A key recommendation from this work is to transition to in situ trials as quickly as possible to further evaluate the performance of HP hybrids and any potential explosive rock interaction that could influence both detonation and the formation of expected gas by-products such as CO; and to a lesser extent NOx. Based on current results, the HP/SN-based hybrid explosive seems to be an ideal candidate for field applications to reduce NOx fumes and due to the high energy but lower VOD, could be well suited to softer more friable ground conditions such as those found in pre-stripping and inter burden shots.