Looking ahead to the 1990s, some Canadian mines may very well be driving drifts, crosscuts and raises with a technology completely different from anything in use today. While drilling and blasting have become more or less the conventional way to make openings in rock, and even though full-face tunnel-boring machines and water-assisted soft-rock roadheaders have made their debuts in hard-rock Canadian mines in this decade, these methods of breaking rock are not the most efficient. For the time being, however, they are the most practical because engineers know they work. One of the unconventional approaches which may eventually compete with these methods, though, is the use of ultrasonic energy, or high- power (100 kW), high-frequency (10 to 100 kHz) mechanical impulses to break rock. The technology for using this form of energy is being investigated at the Centre in Mining and Mineral Exploration Research (cimmer) in Sudbury, Ont., by physicists Dr G. A. Rubin and Dr M. F. Leach. Financial assistance is coming from the Natural Sciences and Engineering Research Council of Canada (nserc), Inco Ltd. and the Ministry of Energy, Mines and Resources.
With less than $200,000, the researchers have set up a small but sophisticated research lab in the basement of the Fraser Science Building of Laurentian University. Their equipment includes a power generator with a rated output of up to 10 kw (a one-of- a-kind machine manufactured by a U.S. company, eni Power Systems) and six transducers (ranging from 10 kHz to 100 kHz) built at the Sonar PowerLabs, Ohio State University. Much of the circuitry for the data-acquisition equipment was done by Research Assistant Douglas Maenpaa.
The research team’s objective is to test the theory of using high-frequency energy to fracture and weaken small samples of hardrock (with compressive strengths of 400 MPa or more). In two years of work they have shown that electrical energy, converted into high-frequency mechanical pulses, when transmitted to a small rock core sample, one inch in diameter and about 10 inches long, will break the rock. But the important finding is that the amount of energy required (10,000 Joules per sq m) is a fraction (one thousandth) of that which miners use today by drilling and blasting (10 billion j per sq m). More than 100 samples of greenstone, granite and norite have been subjected to a few hundred watts of 10-kHz ultrasound in the cimmer lab. A total of 29 of the first 48 samples fractured under load at input powers of only 200 watts or less. The researchers have not been able to apply higher input powers because of inadequate insulation in the rudimentary lab.
If this ultrasonic technology can be successfully scaled up to break rock over a surface area the size of a drift, for example, the cost of making underground openings in rock would be significantly reduced. Furthermore, the technology could lend itself well to automation and continuous mining systems.
However, Dr Rubin stresses that ultrasonics are being investigated only with regard to rock breakage; the investigations began at a U.S. Bureau of Mines lab in Minneapolis, Minn., but were discontinued in 1975 because of a shortage of funds. Development of an actual mining machine that uses ultrasonics is a long way off. But it could conceivably consist of a transducer head which would be pressed against a rock surface and a conventional rock-gathering and conveying system to remove broken rock.
“I am not qualified enough technically to say how a mining machine will look if we prove ultrasonic energy can be used economically on a large scale to make excavations in rock,” Rubin says. “But we have established a timetable whereby we hope to be able to break large samples of rock in the laboratory and then move the equipment out into the field for large- scale testing. Only then would we be ready to take it to an equipment manufacturer to design a machine that would do the job.”
The cimmer team has received $10,000 from nserc (the first of three instalments), $20,000 from the Canadian Centre for Mineral and Energy Technology and $10,000 from Inco for 1988 to complete the testing of large samples of rock in the laboratory. The money will be used to insulate transducers so that a higher power input can be applied and to pay graduate students to do the test work. Without insulation, a high-pitched sound, which can be very painful to the ear, can be heard throughout the science building when tests are run. Ideally, the whole lab would be insulated and a viewing window installed so that tests could be run remotely.
Barring any failures of major pieces of equipment, testing of larger samples in the lab should be completed by early fall, 1988. Also, preliminary site investigations in one of Inco’s open pits near Sudbury could be completed by the time the snow flies again in 1988. That means the researchers will not be able to do field trials until, at least, the summer of 1989.
Dr Rubin would like to see the Canadian Centre for Mining Automation and Robotics at Ecole Polytechnique and McGill University in Montreal become involved in the project. The reason, he says, is that the technology lends itself well to automation.
Once the research project reaches the field-testing stage, there should be enough unanswered questions about the technology to keep at least six graduate students busy for about two years, Rubin says. That work will require major financing — more than is available through nserc. The figure needed to develop the design criteria for a mining machine would be in the order of $200,000 to $300,000, Rubin estimates. That money would probably have to come from either mining companies or a major mining equipment manufacturer. Company representatives from both Boart Canada and Longyear Canada, two major international drill manufacturers, have visited the cimmer research lab.
Some of the things being investigated by the cimmer team are the mechanism of energy absorption, the lateral effects of ultrasonics, the acoustic impedance of the testing machine and energy reflectance.
Even if ultrasonic technology can not be advanced to the point of developing machines to drive openings in rock, using it as a replacement for mechanical rock-crushers would be a major benefit to the mining industry.
“There are many possibilities,” Rubin says, “but first we have to prove it works.”
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