Tailings ponds should never be tragedies

Three years ago, Brazil suffered the worst environmental disaster in its history. In the southeastern state of Minas Gerais, the Fundão tailings dam that belonged to the Samarco Mineração S/A mining company collapsed, releasing a massive tailings pond full of mine waste into the surroundings.

More than 33 million cubic metres of iron-ore tailings slurry were let go, taking the lives of 19 people and transforming more than 650 kilometres of rivers (the Gualaxo do Norte, do Carmo, and Doce) with the red-colored tailings that reached the Atlantic Ocean at the river´s delta. The incident decimated what had been the primary source of water and food for several communities and untold numbers of wildlife.

The Samarco incident was epic in its proportions and consequences. But sadly, tailings failures are not uncommon. Just a year before Samarco, a dam failure at the Mount Polley mine in British Columbia, Canada released tailings into a nearby lake. A 2017 United Nations report says there have been 40 incidents like these in the past decade alone.

“Tailings” are one of the main uneconomic by-products of mineral processing. Stored in ponds, some of them stretching for hundreds of hectares and visible from space, they’re called "tailings" because they represent the final, “tail-end” of the process. Extracting many of our most sought-after metals and minerals via the flotation method uses tremendous amounts of water. After processing, the used water and other wastes are stored in these large ponds. But the massive amounts of water these processes need is not the only environmental consideration. Processing metals and minerals introduces an element of chemistry that fundamentally changes the nature of the source material. Some of the reagents and hazardous chemicals used in the process can be released within the tailings ponds or drained from them as they settle and break down.

The nature of these tailings designs is a high risk from a physical-stability perspective. “Upstream” locations are chosen whenever possible because they’re less expensive to build and manage. With today’s technology, we might think that newer, state-of-the-art tailings ponds would curtail much of the risk, but no. The older a tailings pond, the less likely it is to fail. As time passes and the tailings sit in these ponds, the particulate matter settles to the bottom and solidifies. Water rises to the top where it can evaporate or be reused. Time also allows the pond's water pressure to equalize and stablilize, as the entire structure can be threatened if the settling process is rushed.

Fortunately, we’re now able to reduce the risk of these dewatering tailings and produce materials that have a higher solids content. Consistencies range from ultra-thickened to filtered cake, coming together in a stack landform that the industry refers to as a “dry stack.” Filtered tailings require dewatering equipment to remove the moisture, so they are rarely the low-cost option. But only from a CAPEX perspective. If closure costs are included, dewatering can become very cost-competitive. Also, substantially reducing the amount of liquid that needs to recirculate dramatically cuts the threat of spills because solids are much easier to control and deal with than liquids. It should be noted that the effectiveness of the dry stack depends on the size of the ore pieces being processed. The finer the particles, the more time needed to dewater them.

There are always complex loading and stress issues. So first and foremost, a good tailings program needs checks and balances to avoid failures and mitigate issues. It’s critical that the construction phase follows all the necessary recommendations to avoid stretching the design capability unnecessarily. Sometimes, that tiny bit of incremental stress on the system is all that’s needed to invite a catastrophic failure. Mines´ tailings are at their greatest risk of failure exactly when the operation is at its peak of productivity. During close-out and at end-of-life, the location usually has had ample time to become stable and threats of tailings failures are substantially reduced.

Tailings can have the best plans and the best construction. But those may not be enough if Mother Nature has other ideas. Consider, for example, the effects that earthquakes, hurricanes, floods, and freshet can have. Today, climate change issues are growing stronger and more numerous. They’re making conditions more extreme and edging these ponds ever closer to the brink of failure, especially those designed decades ago when environmental changes were not as abrupt.

Experts are still trying to repair the damage caused by the Samarco and Mount Polley failures. But we’ve made progress in prevention. The industry has begun to establish best practice guidelines for tailings and a determination to making sure tragedies like these don’t continue happening.

We can no longer allow cost to be the main deciding factor in how we manage tailings waste. The risks are too dire and the consequences too disastrous, first for the local communities and physical environment, but also for the responsible parties who could face financial ruin and even criminal prosecution. Dewatered tailings technologies, amongst which is dry stacking, are some of the best available solutions the industry has today. They can go a long way to ensure that catastrophic tailings failures start to become things of the past.