Towards Greener Pyrotechnics
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Towards greener pyrotechnics Prof Walter W Focke, Ollie del Fabbro, Cheryl Kelly, Kolela Illunga, Thilo van der Merwe, Elmar Muller and Maria Atanasova Green chemistry is an initiative AEL Mining Services, a leading temperatures that exceed 2 000 °C developer, producer and supplier as the aluminium is oxidised to adopted by chemists and of commercial explosives, initiating alumina and pure iron metal is systems and blasting services for formed. Pyrotechnics fi nd widespread chemical engineers striving mining, quarrying and construction use in civilian, military and mining markets in Modderfontein, is also applications, generating sound, light, towards the sustainable committed to this ideal. It has made smoke and high heat. The commercial manufacture of chemical signifi cant progress in developing detonators used in mining and green chemistry pyrotechnic systems quarrying depend on pyrotechnic products. It is a philosophy and manufacturing methods. As part compositions for the exact timing of of its efforts in this regard, it has sequential blasting events. This is that encourages the partnered with the Institute of Applied achieved by the highly reproducible Materials (IAM) at the University of burn rates of the pyrotechnic design, development and Pretoria. composition, which is fi lled into the delay element assemblies. implementation of chemical The main focus of this research collaboration over the last 10 years Ultimately, this detonator products and processes that was to fi nd greener alternatives to subassembly comprises an reduce or eliminate the use lead- and other heavy metal-based ignition source, a small-diameter compounds currently in use in shock tube containing the compacted and generation of substances tube detonators. This includes lead- pyrotechnic composition and an based primary explosives, as well as ignition transfer system. Following that are hazardous to the lead-based delay compositions. ignition, a combustion wave travels down along the tube at a environment and to human The National Research Foundation constant velocity. This ensures the (NRF) has also supported this transmission of the initiation impulse health. Green chemistry for research through its Technology to the detonator in a precisely for Human Resources in Industry adjustable time interval. The delay pyrotechnics is currently an Programme (THRIP). While the elements are currently manufactured intensive global research research was primarily of an by pressing the pyrotechnic academic nature, it has led to several composition into aluminum tubes. effort. patents in addition to numerous The automated fi lling and pressing scientifi c publications, conference process requires powders with good contributions, and master’s and free-fl ow behaviour. At present, bachelor’s research dissertations. pyrotechnic time delay formulations Many students’ studies received are based on silicon as fuel, in generous fi nancial support from AEL combination with red lead oxide Mining Services. Selected students (fast-burning for short time delays) have also excelled by winning or barium sulphate (slow-burning for innovation awards for their research. long time delays). Pyrotechnics Ultimately, lead and its compounds in detonator time delays must be A conventional pyrotechnic phased out owing to environmental composition is a mixture of a metal and health concerns. The AEL team fuel and an oxidiser that is capable has found that bismuth oxide (Bi2O3), of a self-sustained exothermic redox prepared by thermal decomposition reaction. The reaction results in the of bismuth subcarbonate, may oxidation of the metal to form a more provide suitable fast-burning stable oxide and the reduction of the compositions with silicon as fuel metallic oxide to the free metal. The (155 mm s-1 with 20% Si). thermite reaction between aluminium Alternatively, the Si-Sb6O13 system and iron oxide powders is a popular yielded slow-burning compositions example. This reaction generates that could be slowed down even ESSAYS 119 INNOVATE 8 2013 Thermite reactions are diffi cult to start, as they require very high temperatures for ignition. For example, the ignition temperature of the Al-CuO thermite, when comprising micron-sized particles, is ca. 940 °C. The IAM team has discovered that the ignition temperature can be signifi cantly reduced when the binary Si–Bi2O3 system is added as sensitiser. Although even lower ignition temperatures are possible when the reagents are nano-sized powders, the AEL team has found that this system is capable of initiating high explosives even when the particles are only micron-sized. Dry mixing of pyrotechnic powders Figure 1: Scanning electron micrograph of copper antimonite crystal is a hazardous operation. AEL produced by the “green” synthesis process invented by the AEL team has recently commercialised the much safer slurry spray-drying technology. This innovative method modelling and Raman yields free-fl owing granules, as it spectroscopy. It is creates almost perfect spherical evident that this particle agglomerates. In addition material has a highly to the acceptable fl ow properties, unusual crystal this process also yields well-mixed structure, as seen in compositions from dispersions Figure 2. Only half of containing different powders, and the crystal positions provides control over the particle available for the size distribution of the near-spherical copper atoms are agglomerates. Unfortunately, the essentially occupied. silicon powder fuel used in these This explains the compositions reacts with water, insensitivity of liberating hydrogen gas that poses a compositions based Figure 2: The CuSb O structure represented risk for explosion. 2 4 on this oxidant to as a ball-and-sticks model accidental ignition by The AEL research team developed friction and impact. further by adding fumed silica: a different ways to suppress this composition obtained by adding 10% Thermites are pyrotechnic hydrogen generation. These include fumed silica (add-on basis) to a 10% compositions that undergo reactions the addition of a noble ion salt, for example, copper (II) ions, to Si–90% Sb6O13 composition that still that are extremely exothermic and burned reliably at a burn rate of run with self-sustaining oxygen introduce a competing cathodic 2.3 mm s-1. content. The energy density of reaction, which is controlled air some stoichiometrically balanced oxidation of the silicon powder Early on, a novel oxidant, copper thermites is comparable to that before slurrying and adding organic corrosion inhibitors. It was found that antimonite (CuSb2O4), was identifi ed of conventional high explosives. as a candidate oxidant for use in time Recently, it was discovered that silane surface modifi cation of silicon delays for mining detonators with thermites comprised of nano- is the most effective method for the desirable properties. The AEL team sized powders can generate suppression of hydrogen evolution recently developed a “green” effl uent- temperature shocks capable of while maintaining or even improving free process for the synthesis of this initiating high explosives. Unlike silicon reactivity in a typical compound. The process yields small the lead-based primary explosives, pyrotechnic composition. well-formed crystals, with a cuboid they are relatively insensitive to habit as shown in Figure 1. The friction, shock and electric spark. Contrary to popular belief, a form crystal structure of this material was Consequently, much effort is of metal rust can react with the recently determined via modelling expended worldwide to explore the neat metal to generate very high of the powder X-ray diffraction use of thermites as replacements for temperatures. Manganese dioxide data and confi rmed via molecular primary explosives. powders, mixed with manganese ESSAYS 120 INNOVATE 8 2013 metal powder, showed reliable burning over a wide stoichiometric range. The unusual aspect of this system is that the fuel and the oxidant share a common metal. They combine to form the more stable intermediate oxide (MnO), releasing considerable quantities of heat in the process. Investigations showed that this system is suitable for time delay applications. A major advantage is the fact that hydrogen evolution from the manganese metal in aqueous slurries can be suppressed completely. The methods currently used for measuring the burn rates of pyrotechnic time delay compositions are destructive in nature and pose Figure 3: The progression of the pyrotechnic reaction in a metal tube as visual- some safety risks. The AEL team ised by the infrared camera at various times (t1–t6) after ignition of the sample recently implemented a new method. A high-speed infrared camera is used and allows the real-time recording of surface temperature Ilunga, K, Del Fabbro, O, Yapi, L and Focke, WW. Yen, NH and Wang, LY. 2012. Reactive metals in 2011. The effect of Si-Bi2O3 on the ignition explosives: Review. Propellants, Explosives and profiles. In combination with a of the Al-CuO thermite. Powder Technology, Pyrotechnics, 37:143–155. mathematical model, these can 205:97–102. be used to infer the temperature Ilunga, K, Muller, E, Del Fabbro, O, Yapi, L and Focke, WW. 2012. The effect of Si-Bi2O3 on profiles inside the element and to the ignition of an Al-CuO thermite. The 38th estimate average and instantaneous International Pyrotechnics Seminar, Westin burn speeds. Figure 3 shows typical Denver Downtown Hotel, Denver, Colorado, USA, 10–15 June 2012. images recorded during a burn test Kalombo, L, Del Fabbro, O, Conradie, C