Date: Thursday, 3 December 2015

Time: 16:00–17:00

Venue: Room 2-46, Engineering 2 Building 

Presenter: Shepherd Masimba Tichapondwa, Department of Chemical Engineering

Abstract

Chemical time delay detonators are used to control blasting operations in mines and quarries. Slow-burning Si-BaSO₄pyrotechnic delay compositions are commercially employed for intermediate to long time delays. However, soluble barium compounds may pose environmental and health risks. Hence it is necessary to consider replacing the barium sulphate with an alternative ‘green’ oxidant that has similar burn properties. Anhydrous calcium sulphate was identified as a suitable, inexpensive alternative ‘green’ oxidant. The initial part of the investigation focused on characterising the burn properties of the Si-CaSO_4, as well as proposing a viable reaction mechanism for this composition. Although the Si-BaSO₄ delay compositions is used commercially very little information about its reaction mechanism and potential reaction products is available in the open literature. The reactivity of this composition was therefore characterised and compared with that of Si-CaSO₄.

Bomb calorimeter data and burn tests indicated that the reaction rate and energy output for both compositions decreased with increasing silicon content. The formulations had comparable burn rates, with ranges of 6,9–12,5 mm s-1 and 8,4–16 mm s-1 recorded for the CaSO₄ and BaSO₄ based compositions, respectively.  Both 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 sulphate 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 reacted barium sulphate. 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 sulphate pyrotechnic delay compositions was also investigated. The burn rate decreased with increase in fuel particle size, while the enthalpy remained constant. Addition of fuel additives to a base composition of 30 wt.% Si-CaSO₄increased the burn rate, whilst oxidizer additives decreased it. The formulation was found to be sensitive to the presence of inert material since the addition of as little as 1 wt.% of fumed silica stifled combustion in the aluminium tubes.

For more information please contact Prof Walter Focke at 083 326 6549.