Specs BETTER BLASTING

Misfires are a common problem in any blasting operation. But engineers at a U.S. National Laboratory in Albuquerque, N.M., have developed and tested a digital device which promises to greatly improve the no-fire characteristics of most analog bridge-wire detonators of the type widely used in the mining industry. This development, which could significantly improve reliability and predictability in blasting, was described by project leader Robert Bickes of the Sandia National Lab at the 3rd Canadian Symposium on Mining Automation, in Montreal in September. Bridge-wire detonators set off explosive charges by passing a low electrical current through a wire, heating it and the powder pressed against it. This usually happens within a few milliseconds. The silicon semiconductor device developed at Sandia can be heated in only a few tenths of microseconds (three orders of magnitude faster than bridge wires) with a low- energy current pulse of less than 3 mJ, which is about one-tenth the input energy of a bridge-wire device. The device produces a plasma discharge which heats the surrounding explosive.

The temperature of the plasma has been estimated at 5,500 K. It transfers heat to the powder bed by penetrating the bed and condensing on to the solid particles, heating them to their ignition temperature, according to Bickes. If this microconvective mechanism is correct, low-density powder would permit greater permeation of the plasma into the powder. This, in turn, would lead to a lower energy disposition per unit volume and a higher plasma energy. This means the current levels at which there is a no-fire condition are higher than for bridge-wire detonators.

The lab has also designed a low- voltage capacitor discharge unit firing set, which forms the basis of a digitally- controlled firing set. This digital set is a smart firing set which requires two passwords: one to charge the capacitor and the second, if correct, to discharge the current. Another firing set uses a digital delay circuit to accurately time delays longer than 10 milliseconds.

The device, which has passed all electrostatic discharging tests, has not been manufactured on a commercial scale. But affordable, high throughput assembly techniques could be used to manufacture the devices, Bickes says. Continuous Drill and Blast Researchers at the Massachusetts Institute of Technology (MIT) and Foster-Miller have developed what they call a uniq ue non-legwire electric blasting cap. This cap is a modified high-strength electric blasting cap, originally developed by IRECO for military applications. Electrical connections are made through a shunted contact assembly built into the rear end of the cap. When a spring-loaded button at the end of the cap is depressed, the shunt is opened and the cap may be ignited by applying a voltage between the capshell and the depressed button.

The cap costs a few cents and was developed for use by a continuous drill-load-blast machine, being developed by Foster-Miller. The objective is to develop a machine that stays at the face continuously.

Allan Fisk, division manager at Foster-Miller in Waltham, Mont., and Prof Carl Peterson of MIT developed the machine, which drills and blasts individual holes, one at a time. The system loads the explosive and detonator directly through the drill steel as the drill is withdrawn from the hole once the hole is drilled. Inexpensive, pneumatically loaded ammonium nitrate and fuel oil (ANFO) is used as a bulk explosive.

The concept is under development at Foster-Miller with funding from the U.S. Air Force, which is interested in the machine for excavating underground defence sites. The first part of the research showed the feasibility of loading explosive and a detonator through the drilling components. The next step is to develop and field test a complete module, drilling, loading and automated initiation.

Fisk and Peterson presented their results at the 3rd Canadian Symposium on Mining Automation held in Montreal in September. Copies of symposium proceedings are available from Andre Piche, Canadian Centre for Automation and Robotics in Mining, Box 6079, Station `A’, Montreal, Que. H3C 3A7. Phone (514) 340-4049.