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TABLE OF CONTENTS Aug 5 - 11, 1991 Volume 77 Number 22 - 0 comments

`Plasma blasting' could revolutionize mining

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By: by David Scott
In the earliest days of mining, hard rock was broken by "firesetting," the technique of lighting a wood fire at the tunnel face and then dousing the heated rock with cold water. The rock cracked and it was then a simple matter to pry the fractured rock away from the face. The net result was five centimetres of advance.

A new technique, called plasma blasting, is analagous to that ancient method but could revolutionize blasting technology in the mining industry. This time the fire is an electric spark carrying a jolt in excess of 3.5 billion watts (3.5 gigawatts) and the dousing cold water is replaced by a weak solution of copper sulphate.

The result is a cleaner, safer, lower-cost method of blasting than conventional methods in use today.

This scenario with its revolutionary implications for the mining industry is far removed from the fantasizing of Jules Verne. In fact, Noranda (TSE) expects to have a unit working underground within two years using the system. This unit will be breaking boulders too big to be handled otherwise. In a further two years, Noranda plans to have another series of machines that will tunnel through solid rock.

The technology has been explored by the military with particular interest in its possibilities as an artillery explosive. The supergun, given considerable publicity prior to the Iraq-Kuwait crisis and designed by Canadian scientist Gerald Bull, is believed to utilize the same plasma forces which are 3-5 times greater than those that can be safely produced by conventional explosives.

Noranda has been researching plasma blasting at its Technology Centre at Pointe-Claire, Que., near Montreal, for the past five years. A team of six professionals headed by J.H. Nantel has developed the system to the point that operational machines can now be planned.

In preliminary work, the team carried out testing on some of the hardest and toughest rocks to be found in Canada. The rocks ranged in strength from 175 to 415 MPa (25,000-60,000 psi).

In its essentials, the method consists of drilling holes 2-3 ft. apart and 1-3 ft. deep. A small quantity of electrolyte is placed in the holes (for example, 5% copper sulphate solution with bentonite added to make it more viscous), followed by the blasting electrode. The blasting electrode is a one of a kind spark plug: an outer electrical conductor and an internal conductor separated by insulating material.

When the 3.5 gigawatts of electrical energy are delivered to the electrode, the copper sulphate solution instantly vaporizes, forming a high-temperature, high-pressure plasma which shatters the surrounding rock.

While the peak electrical power is enormous, its duration is measured in microseconds. The dollar cost for the energy is consequently a fraction of a cent. Two or three blasts per minute should be possible in practice, according to Nantel, and factoring in moves, delays and normal day-to-day contingencies, 400 blasts per 8-hour shift will be a possibility. If each blast should break only one ton of rock at a time, the implications for the mining industry will be revolutionary. Also, there will be no need to withdraw from the work place before blasting and no delay for ventilation to disperse blasting fumes.

Normal underground earmuffs mute the sound of plasma blasting at a distance of 10 metres. There are no noxious gases and fly rock is not a problem. The power of the blast can be adjusted to suit the rock being broken and in like manner, the production of fly rock can be minimized by reducing the power input.

The main problem for Nantel's team has been perfecting the blasting electrode, finding metals that will not crack and insulation that will withstand the gigapascals of energy released by each detonation. Electrodes have now been developed that will last dozens of blasts and the alternative of one-use throwaway electrodes is under investigation.

The capacitors are 2x1x1 metres in size and good for about half a million shots. While the capacitors are readily available, Noranda designed its own switches to handle the half million amps that are routinely used. The company also designed the specialized power cable and undertook its manufacture and has applied for a number of other patents to protect the developing technology.

The implications of plasma blasting are without bounds: it means the end of cyclical, stop-go mining procedures. In essence, it ushers in the era of continuous mining.

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