Terrapure promotes centrifuges for slime pit cleanup
Alternative to using a loader for slime removal
Terrapure Environmental has developed an innovative way of cleaning slimes from sump pits to make the disposal of wastewater and solids more economical and environmentally friendly. It’s a service Terrapure is offering its mining clients as a way to prevent overflowing – and in some cases, hardening of – slime pits.
The idea came about as a result of one of those hardened pits.
“The sumps got so hard, they had to bring in labour to jackhammer and pickaxe the material so they could get it out,” said Todd Smith, Terrapure vice-president of environmental solutions. That’s when he came up with the idea of using a centrifuge to separate solids from the water at the pit before it hardens. “We do it with a centrifuge above ground with tailings ponds, so why wouldn’t we do it underground?” he asked.
Normally, when solids fill the sump pits, a scoop tram removes the slimes. This wet, heavy mess is then transported to an underground reservoir to dry out (through seepage) over six to eight months. The process is costly, inefficient, time consuming and takes up a lot of room in the mine.
Terrapure’s approach is to use a centrifuge to separate the water and solids on the spot. Terrapure – then the industrial division of Newalta – offered a proof of concept in Vale’s Coleman Mine in Sudbury in 2013. A three-person crew took six days to process slimes into stackable cakes with an average of 60 per cent solids. The water removed was so pure (99 per cent, according to a Terrapure case study) that it could be reused in any of the mine’s processes.
The solids can also be tested for mineral content. Depending on the metal and the concentrations, these could be recovered, boosting the cost efficiency of the process even further.
“This is the first application of a centrifuge underground that I’m aware of in the world,” Smith said. “Certainly, all the mines I’ve talked to have never seen it.”
Smith said that there are two approaches for using a centrifuge. One is to simply replace the scoop tram with a centrifuge team to periodically clean out the slime pits. Another is to have a dedicated centrifuge at the sump pit.
“It’s an ongoing process, so those sumps never fill up. As material goes in, we pump it through the centrifuge and remove it immediately,” Smith said.
A third possible approach is to separate the solids and liquids at the source, which would reduce wear and tear on pumps and contribute to the mine’s overall efficiency.
The technology is nothing new, just its application. Centrifuges are freely available on the market. The only thing different with the Alfa Laval Lynx 418 that Terrapure uses is the stand, which is modified to take into account uneven ground. So why can’t mines simply install their own centrifuges? The process takes some expertise, and what Terrapure brings to the table is experience, Smith said.
“Operating a centrifuge is really an art and a science,” Smith said. “Not anybody can walk up to the machine, turn it on, and start producing. Our experienced guys are worth their weight in gold.”
Smith said that some customers have tried to do it themselves, but have then asked Terrapure to take over the operation because they can’t get it to work properly.
Although the process is proven, Smith said that it has been difficult to sell customers on the service because it is another external cost – a cost that companies in a down market don’t see as essential. However, there is some real money to be saved in terms of recovered wastewater, reclaimed minerals and equipment wear-and-tear, Smith said.
A similar approach could be used for other areas of mining operations. For example, separating solids and liquids from tailing ponds would be more environmentally friendly and reduce the risk of a breach like the one at the Mount Polley mine in 2014, Smith said. Solids from tailing ponds could be used as material for the paste plant and then mine backfill.
“Rather than the tailings sitting in a pond for the hundreds of years, now it’s being used as backfill in the mining operation. You get that double benefit of environmental footprint reduction and reuse of the material,” Smith said.
“Instead of paying for material to backfill, they get it for free.”
Terrapure Environmental launched in 2013 when Newalta sold off its industrial division to Revolution Acquisition LP (Revolution), a Birch Hill Equity Partners company.