- Unwanted tailings go into water that can threaten environment
- With iron ore, biggest issue is the high-volume produced
The dams that ruptured in Brazil Thursday, sending a wall of mud along the countryside, were built to contain the leftover materials from mining known as tailings, and disposing of them safely has long been a challenge for miners.
Typically mixed with water and stored in a slurry form, these scraps can pose threats to wildlife and water supplies if not properly contained. With iron ore mining, the biggest issue is generally one of volume: there is simply too much produced to dump safely into local waterways. That’s forced miners to find other ways to contain the tailings, including in artificial ponds secured by dams.
“You can’t just dump it in rivers,” said David Chambers, president of The Center for Science in Public Participation, a non-profit group based in Bozeman, Montana. “Because most of what you mine becomes waste.” The cheapest way to deal with the waste materials is to build a dam, he said.
Brazilian authorities today resumed a search for victims of the five-mile long mudslide in Minas Gerais state at an iron-ore project jointly owned by two of the world’s biggest miners, BHP Billiton Ltd. and Vale SA. One person is confirmed dead, four are injured and 13 are missing, according to the state fire department.
Vale expressed solidarity with those affected and said it has offered support and assistance to local authorities. The joint venture, Samarco Mineracao SA, has set a news conference for later today.
“We cannot at the moment confirm the causes or the extent of the incident or the number of victims,” Samarco CEO Ricardo Vescovi said in a video posted on the company’s Facebook page, adding that dams called Fundao and Santarem had ruptured between the towns of Mariana and Ouro Preto. “Our focus is on assuring people’s safety and protecting the environment.”
While tailings from iron-ore operations like this one typically don’t contain hazardous materials, according to Chambers, other types of mining do produce toxic leftovers.
Minerals such as cadmium, arsenic and zinc are naturally dangerous if poorly handled. And other types of sulfidic minerals, such as copper and nickel, contain large amounts of pyrite, which can produce sulfuric acid once exposed to air and water. Additionally, chemicals used by miners to extract specific metals, including cyanide, can sometimes find their way into tailing ponds.
In the past, tailings were frequently dumped into rivers or wetlands in the past. Today, it’s common to use them as backfill in underground mines, after removing the water to create a dry material, or to pump them into ponds contained by dams.
Dam failures have been one of the biggest environmental risks posed by mining. Unusual events, such as earthquakes or flooding, can cause tailings dams to fail, as can design or construction flaws. When the ponds become full, they’re closed, and the slurry is allowed to dry out, leaving a more stable land form, Chambers said. The ponds are most dangerous when the tailings are wet and therefore mobile.