PICTURE yourself at Cadia Hill gold mine. A horn sounds, warning you that the brown-grey rock face in front of you is about to explode. When the dust settles, monster trucks move in to scoop up hundreds of tonnes of boulders and transport their cargo towards a giant conveyor belt rising high into the air. You watch as boulders a metre across reach the end of the line and fall through a hollow metal cylinder. Hazard signs offer the only clues to what is happening inside: microwaves are zapping the boulders, leaving them criss-crossed with microscopic cracks. It then takes only a few sharp taps for the rock to shatter into a mix of gold dust and powder.
At least, that鈥檚 the scene that Sam Kingman hopes to witness in a few years鈥� time at mines like Cadia Hill, in New South Wales, Australia. Kingman, based at the University of Nottingham, UK, heads an international team of engineers investigating how microwaves can help to pulverise rocks. It is a process that could improve the yield of precious metals and minerals. And by slashing the mining industry鈥檚 electricity use it could cut energy bills by hundreds of millions of dollars per year and help curb global warming.
Today鈥檚 gold miners no longer hunt for nuggets. They are looking for microscopic grains that amount no more than 4 to 6 grams of each tonne of rock. Diamonds are a thousand times rarer still. To find the valuable grains, mines use giant grinding mills to chew their way through rock blasted from the ground. Standing like giant tumble-dryers up to 12 metres across, they process up to 250,000 tonnes a day.
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But the machines are not only hungry for ore. Smashing up rocks consumes immense amounts of energy 鈥� up to 5 per cent of all the electricity generated worldwide, according to some estimates. With global electricity consumption reaching over 14,300 billion kilowatt-hours in 2002, grinding rocks guzzles as much energy as 820 million 100-watt light bulbs a year.
鈥淪mashing up rocks consumes up to 5 per cent of all the electricity generated worldwide鈥�
There is plenty of room for improvement, however, because the mills are incredibly inefficient. Most of the energy is wasted as noise and heat as steel rods or balls inside them smash their way through the ore. Only a weedy 1 per cent actually goes into fracturing the rock to expose the desired metals and minerals. And it is a problem that is only getting worse as mining companies exhaust richer sources and turn to poorer deposits. In 1800, for example, a typical copper ore contained more than 10 per cent metal. By 2000 that had dropped to less than 1 per cent.
Researchers have been searching for decades for ways to extract the valuable minerals more readily. To make rocks easier for the grinding mills to digest, they have tried weakening them first by heating them, or bombarding them with sound waves. But every attempt has failed to take off commercially because it uses up more energy than it saves in the subsequent grinding process.
In 1984 a team led by Tzong Chen at the Canadian government鈥檚 CANMET Mining and Mineral Sciences Laboratories in Ottawa suggested that microwaves might provide a more promising approach. While simple heating raises the temperature of an entire rock, Chen and his team showed that microwaves can target the mineral of interest. The grains of mineral heat up and expand, fracturing the rock along the boundaries between the different materials in an ore, and so making it easier to free the precious mineral inside. Just how much energy is needed depends on the mineral鈥檚 dielectric properties 鈥� a measure of its ability to convert microwave energy into heat.
Chen and his colleagues noted that many of the commercially important minerals such as metal oxides and sulphides heat up strongly, while the unwanted material surrounding them is relatively transparent to microwaves.
At least, that鈥檚 the theory. 鈥淧eople started messing around putting minerals in microwaves in the 1980s,鈥� says Kingman, 鈥渂ut there wasn鈥檛 a lot of understanding behind it.鈥� He got involved in the mid-1990s as a PhD student at the University of Birmingham, UK, where he started zapping rocks in domestic microwave ovens. His work confirmed that microwaves did indeed weaken the structure of the ores, but it also showed that it would take more than 20 kilowatt-hours of energy to treat a tonne of rock. Since it does not take any more than that to grind a tonne of untreated ore, the process looked like an uneconomic dead end.
鈥淎 short blast of microwaves makes rocks easier for the grinding mills to digest鈥�
Yet Kingman was reluctant to let the idea go. After completing his PhD, he moved to the University of Nottingham where he joined forces with microwave modelling specialists at Stellenbosch University in South Africa and engineers at the British company E2V Technologies, based in Chelmsford, Essex, which makes magnetrons that generate powerful microwaves for radar and medical applications.
Kingman and his colleagues soon worked out why domestic microwave ovens are so inefficient when it comes to weakening rocks. In an oven, microwaves bounce around at random and this leads to an unpredictable pattern of hot and cold spots 鈥� not too much of a problem for heating food, but useless for homing in on minerals in rocks. They realised that they needed to redesign the microwave cavity so that microwaves entering it would reflect backwards and forwards to reinforce each other and set up a standing wave. This would cause the microwave intensity to peak at a known point in the chamber, so that rocks passing through this super-hotspot would experience intense heating.
Kingman鈥檚 colleagues calculated that they didn鈥檛 need a massive temperature rise to fracture a rock along its grain boundaries. A temperature difference across the boundary of just a few degrees would do, and they reckoned they would be able to achieve this by firing microwaves at a rock for as little as a microsecond.
The microwave cavity that Kingman鈥檚 team now has running in his lab exposes the rock to short, powerful bursts of microwaves, and this helps to improve the efficiency of the process. 鈥淲e鈥檝e cut the amount of energy we use from more than 20 kilowatt-hours per tonne of ore to less than 0.4 kilowatt-hours per tonne,鈥� Kingman says.
Finally, the figures are starting to make commercial sense. Kingman鈥檚 team has found that their microwave process weakens the rocks enough to halve the amount of energy needed to grind the toughest untreated ores. And that saving has caught the eye of some of the world鈥檚 biggest mining companies, which are funding Kingman鈥檚 group to develop the idea further. 鈥淭his project is extremely important,鈥� says Lucy Esdaile, manager of technology transfer at Anglo-Australian mining company Rio Tinto. 鈥淭he technology is very significant in terms of our costs and, of course, carbon dioxide emissions.鈥�
The biggest of the two microwave cavities in Kingman鈥檚 lab is 40 centimetres across and runs at 60 kilowatts. It is capable of treating 3 to 4 tonnes of ore an hour, though Kingman only runs it for a few seconds at a time. 鈥淚t costs a lot to ship rock over from Australia, so you don鈥檛 want tonnes of the stuff lying around,鈥� he says. In three years鈥� time, the team hopes to have an industrial-scale plant that can process around 100 tonnes of rock an hour at a working mine.
But a microwave unit big enough to process the hundreds of thousands of tonnes a day produced from a mine like Cadia Hill is still many years away. Mining is a conservative industry with low profit margins, and it takes time to phase in new technologies. 鈥淚t is a huge financial commitment to change technology,鈥� Esdaile warns. 鈥淭he average time for a new process to be developed is 15 to 20 years, but the work has come a long way already and I estimate that we are 10 years away from a commercial system.鈥�
As our appetite for metals and other valuable minerals continues to grow, a short blast of microwaves may be just what is needed to show that mining need not cost the Earth.
