Just one of these developments might be recorded[49][50] because it is typical of the interaction between the mining and chemical industries and scientists. At Mt Isa Mines massive ore pillars -each some 1800 million cubic metres -had to be removed. To achieve this in a single operation, explosive had to be pumped from a central tunnel into hundreds of 30 m long boreholes arranged in sequences of vertical planes and converging in each place to the central tunnel (Fig. 11). The boreholes thus radiated from the tunnel, downwardly, horizontally and upwardly. Watergels were indicated because of residual water in some holes; the many holes running upwardly had to retain the explosive for the prolonged period of preparation of the blast. This could only be achieved by pumping the watergel upwardly and causing it to gel instantaneously. Mt Isa invited ICI Australia Research Lab to collaborate on the project. New chemistry was needed; new engineering to pump viscous explosives hundreds of metres underground; and new electronic detonators were invented by the ICI team to detonate the charges in quick-fire succession, at intervals of milliseconds, since a single blast would have damaged buildings (a chimney stack!) above ground. Preparation of the blast by Mt Isa engineers and of the explosives system by ICI took several years, cost about seven million dollars and 350 tonnes of explosives brought down a pillar 225 m high, 100 m wide and 80 m long; 1.3 million tonnes of rock in a single blast! It was just one event; it symbolised the third explosives revolution, the science industry interaction and the maturity to which the technology had grown in Australia.

11 Charging 'up-holes' with ammonium nitrate slurry which gels instantaneously in-situ at Mount Isa mines. Insert shows drill hole pattern in pillar 225×100×80 metres.

ICI's watergel chemistry ('Molanal'), including the material used in this gigantic blast, had really reverted, at least semi-empirically, to the teaching of Bowden, Mulcahy, Vines and Yoffe (see pp 645-6) that air bubbles were a critical factor in sensitivity; aluminium particles enhanced their presence; crosslinking agents trapped them and attempts were even made to introduce them with plastic foams.

The last advances in the evolution, still progressing in the eighties, were emulsion explosives, aqueous solutions of oxidizing agents intimately dispersed in an oil matrix. DuPont introduced such bulk emulsions, (Tovex E') based, however, on a classified explosive, methylamine nitrate, and ICI developed a series of AN based emulsions, 'Powergels'. Evolution turned full cycle in the late seventies and eighties when AN watergels were packaged into tubes similar in appearance to traditional gelignite but based on paint fine aluminium, gas-bubble-sensitised blasting agents (ICI, Ireco) or methylamine nitrate (DuPont). Intimate contact, high density, improved waterproofing and suitability for both bulk and packaged explosives produced outstanding performance in the field. Emulsions also provided the ICI Australia team with the opportunity to make better and controlled use of the Bowden-Mulcahy (adiabatic compression) gas bubble effect discussed earlier. Using special surfactants they were able to incorporate gas and control and retain the gas bubble size. This led to the world's first gassed emulsion explosive 'Powergel 2510'.

Australian Academy of Technological Sciences and Engineering