Reactions of silicon with sulfate-based oxidizers used in pyrotechnic time delay compositions

Abstract

Chemical time delay detonators are used to control blasting operations in mines and quarries. Slow-burning Si-BaSO4 pyrotechnic delay compositions are employed for long time delays. However, soluble barium compounds may pose environmental and health risks. Hence it is necessary to consider replacing the barium sulfate with an alternative green oxidant that has similar burn properties. Anhydrous calcium sulfate was identified as a suitable, inexpensive alternative green oxidant. The initial part of the investigation focused on characterising the burn properties of the Si-CaSO4, as well as proposing a viable reaction mechanism for this composition. Thermochemical calculations indicated that stoichiometry corresponds to a composition that contains ca. 30 wt.% silicon (Si). Combustion was only supported in the range 30 70 wt.% Si. In this range the bomb calorimeter data and burn tests indicate that the reaction rate and energy output decrease with increasing silicon content. The compositions were filled into rigid aluminium elements and assembled into full detonators. Burn rates ranged from 6.9 to 12.5 mm s?1. The reaction product was a complex mixture that contained crystalline phases in addition to an amorphous calcium silicate phase. A reaction mechanism consistent with these observations is proposed. Slow-burning Si-BaSO4 pyrotechnic delay compositions are employed commercially for intermediate to long time delays. However, there is very little information on this composition available in the open literature. The reactivity of this composition was therefore characterised and compared with that of Si-CaSO4. The Si-BaSO4 composition supported combustion in the range of 20 to 60 wt.% Si in the bomb calorimeter. However, burning was only sustained between 20 and 40 wt.% Si in rigid aluminium tubes. The burn rates varied between 8.4 and 16 mm s?1. These values are comparable to those for the Si-CaSO4 system (6.9 12.5 mm s?1). However, the CaSO4-based formulations tended to have a higher energy output and produced a more pronounced transient pressure response than the barium sulfate compositions. Both the calcium sulfate- and barium sulfate-based formulations were insensitive to impact, friction and electrostatic discharge stimuli. The reaction products were a complex mixture that contained crystalline phases in addition to an amorphous phase. Although barium sulfate is insoluble in water and decidedly non-toxic, the reaction products produced by the Si-BaSO4 compositions were found to contain water-soluble barium compounds. This ranged from 50 to 140 mg Ba per gram of barium sulfate reacted. The burn rates of delay compositions used in detonators can be modified by varying a range of parameters in addition to the stoichiometry. With this in mind, the effect of additives and fuel particle size distribution on the burn rate of the silicon-calcium sulfate pyrotechnic delay compositions was investigated. The burn rate decreased with increase in fuel particle size, while the enthalpy remained constant. The addition of fuels to a base composition of 30 wt.% Si-CaSO4 increased the burn rate, with an increase from 12.5 mm s-1 to 43 mm s-1 being recorded on the addition of 10 wt.% Al. Ternary mixtures of silicon, calcium sulfate and an additional oxidiser generally decreased the burn rate. The exception was bismuth trioxide which increased it. The Si-CaSO4 formulation was found to be sensitive to the presence of inert material as the addition of as little as 1 wt.% of fumed silica stifled combustion in the aluminium tubes.