The danger of tailings dams

By P.M.

MORE THAN 300 people are missing, with many already confirmed dead, after millions of tons of muddy sludge engulfed homes and roads when a tailings dam at a mine in Brazil collapsed suddenly on January 25th. The dam near Brumadinho is owned by Vale, Brazil’s largest mining company. The cause of its bursting is being investigated, though it was inspected only four months ago and nothing appears to have indicated that it was about to fail. Over the years similar dams have given way at other mines around the world. One, jointly owned by Vale and BHP (an Anglo-Australian giant) near the town of Mariana in Brazil, collapsed in 2015, and potential penalties being discussed could exceed $40bn.

There are about 3,500 tailings dams around the world. Unlike the dams used to build reservoirs or hydroelectric projects, tailings dams are not usually made from reinforced concrete or stone. They are mostly constructed from tailings, the waste material left over from mining operations. As lots of water and sometimes chemicals are used to extract the minerals and metals, the tailings largely consist of wet sludge. Depending on the type of mine, these tailings can be toxic. Miners used to dump them into rivers, but in recent decades, with environmental awareness growing, the sludge has come to be stored behind dams, where the solid material settles, allowing the water in the sludge to be recovered and reused.