WHEN Mats Eklund goes metal prospecting he takes his life in his hands. But it’s not raging torrents or grizzly bears he has to worry about. His main concerns are crumbling warehouses and frenzied rush-hour traffic.
An environmental engineer at Linköping University in Sweden, Eklund is one of a new breed of prospectors who prefer to head downtown rather than out into the wild. Their target is “urban ore” – forgotten supplies of metals that lie in and under the city streets.
Eklund has spent several years tracing abandoned cabling and long-lost pipework beneath roads and pavements. His fieldwork suggests that many cities contain precious seams of valuable metals locked up in discarded subterranean infrastructure. Meanwhile, others are finding richer sources that lie on or flow beneath our city streets and which could turn out to be a prospector’s dream come true. Are city dwellers really sitting on a bonanza? If so, where are the most valuable veins, and how might we tap them?
Advertisement
Of course, reusing old metals is just a form of recycling, and there’s nothing new in that. According to , metal smiths were turning scrap copper and bronze into statues more than 2000 years ago.
But our reliance on recycled metals is different now. In the last decade or so, expanding economies in Asia and South America have pushed demand for raw materials to new heights, while the quality of many metal ores, including copper, is falling, as the richest sources are mined out. With extraction and refining costs rising, metal recycling has become an economic, and environmental, imperative.
Despite these pressures, many forms of metal scrap remain untapped, simply because there is no easy way to harvest them. Globally about 30 per cent of aluminium is not reused, while for copper the figure is more than 50 per cent. These metals are used by high-tech industries, particularly in electronics, in which materials are often finely dispersed. This makes recovering used metals technically challenging. More than 300 tonnes of gold and 7000 tonnes of silver are used globally in electronic goods each year, yet just 15 per cent of each is recovered. As a result, a bin full of waste electronics can be 50 times richer in metals than many mined ores.
With much of this electronic scrap going back underground as landfill, is it time to start mining these sites? Certainly no one is yet mining landfill on an industrial scale. Christer Forsgren, technical director at international recycling company Stena Metall, says the problem is the mix of materials these dumps contain. In one experiment, the firm drilled test pits and measured metal content across a Swedish dump site. There was plenty of copper. The problem, says Forsgren, is that extractors must pay to rebury all the unwanted waste. “Otherwise it would be economic for us to extract the metal.”
For now, there are other, more lucrative seams to be raided. Tom Graedel, an industrial ecologist at Yale University, has studied what he describes as “comatose” stocks – metals that are out of sight, locked up in obsolete infrastructure or dispersed in the environment. Like the stuff in landfill, this metal can be difficult to recover, and so far there has been little attempt to map precisely where the richest sources can be found. But one thing is certain, says Graedel: given that urban areas tend to be more prosperous and have more infrastructure than rural ones, most of this is likely to be found in cities. So that is where prospectors are starting to look (see diagram).
They are already hitting pay dirt, so to speak, in the gutter. Get rid of the leaves and old wrappers, and those mucky brown street sweepings hold value, if you know how to extract it. At the University of Birmingham, UK, Angela Murray and her colleagues have set up a company called which aims to recover the platinum group metals (PGMs) platinum, palladium and rhodium, from street sweepings. These metals come from the catalytic converters that help road vehicles reduce exhaust pollution. While the engine is running, minuscule fragments flake off and spray out of the exhaust pipe. As much as 70 per cent of the precious metals in these converters can end up as road dust
“Those mucky brown street sweepings hold value, if you know how to extract it”
The richest platinum deposits on Earth, mined in South Africa, contain between 2 and 10 parts per million (ppm) of platinum in their ore, says Murray. “Street sweepings contain around 1 part per million and they are much easier to mine.” She calculates that PGMs worth more than are swept from UK roads each year and dumped.
To get at this stuff, Murray and her colleagues dry and sieve the road dust, and then apply a recovery process that they have patented – a combination of magnetic, electrostatic and gravity-based separation. This increases the concentration of precious metals in the dust to more than 10 ppm, says Murray, and after developing the gravity technique the team hopes to achieve levels of about 60 ppm, which is suitable for smelting.
Iron in the hole
Others are eyeing up street waste too. In January, UK waste management company Veolia Environment started up a street sweepings recycling plant in Warwickshire, where its engineers are developing techniques for concentrating the metal riches.
Chasing specks of rare and precious metals may be worthwhile, but for base metals like iron, copper and aluminium you need larger, more accessible deposits to make extraction profitable. Eklund thinks he has found such comatose deposits.
He and his colleagues have spent several years totting up the value of disused copper, iron, steel and aluminium hibernating under the tarmac in Swedish cities. Years of development since the industrial revolution, along with frequent technological upgrades, mean that webs of disused metal lie beneath these urban centres. “We’ve seen some streets that are so full of old pipes and cables that it hampers development,” Eklund says.
“Frequent technological upgrades mean that ourcity centres lie over a web of disused metal”
His team has gathered data from utility companies about the distribution of active pipes and cables in three cities: Gothenburg, Linköping and Norrköping. They also trawled through old maps, dating from 1850, to identify inactive pipes and cables. In Linköping they found that was inactive, while in Gothenburg and Norrköping, . The amount depends on the city’s history, says Eklund. Cities that went through a long period of industrialisation and with layouts that have changed in the last century are likely to have a greater proportion of obsolete metalwork, he says.
The researchers estimate that there is about 90,000 tonnes of copper, worth a cool $630 million, in inactive power cabling alone beneath Swedish cities. Using this scrap metal instead of ore would save the equivalent of 360,000 tonnes of CO2 since extracting the stuff takes less energy than mining and refining ore. Yet Sweden’s total comatose copper stocks are likely to be far higher – Eklund’s colleague Joakim Krook suggests there could be as much as 400,000 tonnes underground and says this stock will rocket in the next few years as old networks are retired.
In the case of Norrköping – Sweden’s first major industrial city – Eklund and his colleagues have turned their data into a treasure map. Hotspots are old industrial areas with abandoned cotton factories and the DC power lines that once served them, together with networks of disused tramlines, gas pipes and drains. “In one redevelopment site there was nearly 28 times as much aluminium and three times as much iron in hibernation, compared to the city average,” says Eklund. In places with a rich industrial heritage – like many UK cities – Eklund reckons that more than a quarter of all pipes and cables may be inactive.
Moving deeper beneath the streets, some prospectors are getting their hands really dirty. Bernd Lottermoser, a modern day metal-hunter from the University of Exeter, UK, foresees a golden future… for sewage.
In 1995 an Australian gold exploration company called Echidna Mining acquired the exploration rights for the Werribee sludges – a stockpile of sewage waste from Melbourne that has been accumulating since 1898. Test samples revealed reasonable concentrations of gold – at about half the level of the average gold ore deposit. Much of it originates from industrial effluents from dental practices, the electronics industry and jewellery manufacturers. Some also comes from medicines used to treat arthritis and cancer. “Small amounts come from the abrasion of gold jewellery worn while you wash dishes or take a shower,” says Lottermoser. His tests show that gold, silver and zinc can be extracted from the sludge, using acid leaching with sodium cyanide. However, environmental concerns coupled with the cost makes such extraction uneconomic for all but the most concentrated deposits.
But with the right waste, the riches spill out. A waste sludge treatment facility in Nagano prefecture, Japan, has been extracting gold since 2009. Situated in an industrial district containing many precision-equipment manufacturers, the sludge flowing through the drains has gold at concentrations of up to 2.9 kilograms per tonne, nearly 50 times greater than conventional gold ores. “In this case the cost of the cyanide treatment is outweighed by the gold they recover,” says Lottermoser. “Eventually sewage will become a viable resource in many places, as gold ores continue to decline in concentration.”
With this in mind, Lottermoser and his colleagues are looking for cheaper and more benign extraction methods. One of the reagents they are investigating is thiourea, which degrades more rapidly and is less toxic than sodium cyanide.
Microscopic miners
It is not just gold that makes waste a sparkling prospect. In 2009 a team at Cardiff University in the UK measured concentrations of nearly 200 parts per billion of in sludge from the city of Birmingham. Hospital effluent is particularly rich in platinum, as it is widely used in cancer drugs. The challenge, however, is to concentrate it in an economic way. Living organisms might make that possible (See “Natural born miners“).
Perhaps more of a challenge is how to extract pipes and cables without a lot of trouble and disruption, says Eklund. One solution, from Austrian company Kabel-X, is a no-dig extraction technique for telecoms cables, in which oil is pumped in between the outer jacket and inner core. This lubricates the cable and enables up to 400 metres of copper core to be winched out in one go. Finding lost stocks is also set to get easier, thanks to technology being developed by teams from five British universities. In a project called , the collaboration is developing a multi-sensor system capable of locating pipes and cables without excavation. At one test site – a water treatment works near Bristol – they have already mapped long-forgotten 19th century pipework.
Such technology raises the prospect that urban mining will soon be viable. “For some metals, such as copper, where the price has risen fivefold in two years, we are reaching a tipping point,” says Eklund. “Only about half of the metals we have taken from the Earth’s crust are currently being used,” he says.
This wasted resource is coming under increasing scrutiny. The first was held last year and in Europe politicians are now to harvest inactive stocks. Not that being in use is a barrier for some people, mind you. In the UK, for instance, active railway signal cables and domestic gas pipes have already become magnets for thieves.
In the long term, even litter-strewn grass verges alongside busy roads could prove lucrative. Platinum, palladium and rhodium are found in roadside soils at concentrations between 100 and 1000 times greater than normal, says biogeochemist Walter Wenzel at the University of Natural Resources and Life Sciences in Vienna, Austria. “It may be possible to identify plants that can extract these platinum group elements,” he says. Helen Parker, a chemist at the University of York, agrees: “Studies of grass growing at the side of the road show that it is naturally taking up platinum and palladium.” If metal prices keep rising, companies may yet vie for the rights to “farm” valuable elements alongside highways.
And who can guess what else may eventually be worth exploiting. Some optical fibres contain aramid polymers like Kevlar which are in growing demand for recycling. A company called Teijin has been converting old Kevlar into new polymers at a plant in the Netherlands since 2008. Further in the future, optical fibres doped with rare earth elements may also become a sought-after commodity – another seemingly commonplace strand in our infrastructure which could turn out to be the must-have urban ore of tomorrow.
Natural born miners
Metals are usually separated from their ores with strong acids or by melting the metals and tapping them off. Both processes are effective but damaging for the environment.
Living organisms might solve that problem. Bioleaching, for example, makes use of metal-munching bacteria. They need certain elements for structural or catalytic functions, so choose the right species and it will gather metal compounds for you.
Anna Kaksonen, from the Commonwealth Scientific and Industrial Research Organisation in Australia, is looking into ways of encouraging these bacteria to chomp through piles of electrical waste, to recover metals such as zinc and nickel.
Angela Murray of the University of Birmingham, UK, is working on this too. She is testing bacteria that have an enzyme which fixes platinum ions, converting them to metal nanoparticles that become attached to the bacterium’s surface. The idea is that when fully loaded, the bacteria can be collected, dried and ground up to make a ready-to-use powdered catalyst.
Some plants are also known to accumulate metals. Helen Parker and her colleagues at the University of York, UK, are experimenting with “hyper-accumulating” plants grown in soil rich in street sweepings and which can be harvested for palladium. “You get a very unhappy plant at the end, but once powdered it can be used directly as a catalyst,” she says. Eventually genetically modified plants could suck metals out of industrial waste land, road sweepings and even liquid wastes such as landfill runoff and sewage sludge.
For metals in solution, seaweed is also showing great promise. It can successfully capture gold, copper, nickel, zinc and platinum group elements. “Potentially it could be used to extract precious metals from solvent waste streams produced by industrial and pharmaceutical plants,” says Parker.
This article appeared in print under the headline “Mid-town miners”
