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A step toward a permanent solution to the arsenic remediation puzzle
UPDATE: Watch the recording of the August 9 mine waste management live stream here.
UPDATE: The Bachelor of Mining Engineering Technology program is hosting a free webinar on mine waste management with Professor Ghahreman, August 9, 2017 at 11:30am EDT. Register here.
Giant Mine beat at the economic heart of the Northwest Territories for more than half a century. Some seven million troy ounces of gold were scratched from the earth there, just a few kilometers north of Yellowknife, before the mine closed for good in 2004.
Among the many hazards left behind are 15 subterranean chambers containing more than 213,000 tons of potentially lethal arsenic trioxide dust. That’s technically enough poison to kill everyone, everywhere, globally. It’s reasonably safe and contained where it is under dry conditions but it’s highly soluble in water, so any moisture leaking or condensing into those chambers means potentially toxic and sustained arsenic contamination of the water table. Aside from the immediate mortal danger of arsenic poisoning, long exposure to elevated levels can have horrible and life-shortening effects, including cancer.
Clean-up of the Giant Mine site is expected to take decades and cost Canadian taxpayers more than $1 billion. For now, at least, the plan is to use refrigeration equipment to cool the rock surrounding the chambers to ensure any water freezes in place. That’s not a forever solution.
Mine operators have wrestled with arsenic toxicity in mining waste for decades. It’s not new. The modern approach for mitigating arsenic leeching is to deal with toxic waste products as they’re produced. For arsenic, that means mixing waste rich in arsenic trioxide with hydrogen peroxide. Hydrogen peroxide is consumed as arsenic trioxide is oxidized into arsenic pentoxide. Arsenic pentoxide is still pretty nasty stuff on its own but when chemically bonded to certain other molecules, the arsenic ions in it – the part that causes cell death in living organisms – become chemically unavailable. The arsenic is effectively immobilized in molecules that can’t really hurt us.
It’s a complex and expensive process for mine operators but, as the Giant Mine example shows, it’s an investment in a sustainable future for safe mining and the natural environment. Still, the cost to immobilize all the arsenic stored at Giant Mine could run into the billions of dollars.
One of the largest expenses associated with contemporary methods of arsenic immobilization is the hydrogen peroxide typically consumed as an oxidant. Though readily available, at about $2,000 per ton, the cost adds up quickly. Some mining operations in far-flung places aren’t close to a ready supply, adding even greater transportation costs. There’s also a significant amount of waste in the process, with typically only about 70 percent efficiency of the oxidation reaction.
But one Queen’s research team is perfecting a process that eliminates the need for mine operators to buy, ship and store hydrogen peroxide for arsenic immobilization.
“Instead of hydrogen peroxide, we just use air and that does the job for us,” says Queen’s mining engineering professor Ahmad Ghahreman. “You put activated carbon into a column and then pump in the mine-waste solution and air. Your only reagent from this process is oxygen from the air. The activated carbon is only a catalyst, so you don’t have to add to or change it, ever. We have been running a column in the lab for about six months and our oxidation ratio has been always above 99 percent.”
The activated carbon can be made from coconut husks and Ghahreman says the preparation is a one-time cost of about $2,500 per ton. There are additional one-time costs for retooling existing operations: reconfiguring or adding the column reactors. These costs are paltry compared with the relentless need for hydrogen peroxide consumed in the contemporary processes.
Not only is it a great life-long cost-saving option for existing mining operations but if Ghahreman’s method were applied to the mountain of arsenic at Giant Mine, the cost for each ton of arsenic oxidized would be less than half than under the current process. Cleaning up Giant Mine would still be like eating an elephant with a pie fork, but the final costs for a permanent solution would likely be a small fraction of those projected now. It seems almost too simple: Substitute consumable hydrogen peroxide with non-consumable inputs and save thousands of dollars per ton.
How did Ghahreman and his team come up with such a seemingly simple and elegant solution?
“The idea for this process came from a paper published in 1936,” says Ghahreman. “I always tell my students that good chemistry is in older papers. When we started looking at this we had no idea what the chemistry is behind this process. Later on, one of my students did a very good job of understanding how it works. It turns out that activated carbon and oxygen from the air combine to produce in situ a hydrogen peroxide species in water, in solution. Because we produce that hydrogen peroxide in situ, we don’t have to pay for it. We just pay for the air. That’s it.”
Ghahreman and his research team has a patent for the process in place and are looking for industry partners to install the reactors in their existing operations.
“There are a number of companies interested in process but getting someone to go first is the key,” he says. “Very likely a Canadian company will be soon to be the first to use it. My hope is someday that I could apply this process for the arsenic problem we have in the NWT for the sake of Canada’s environment. Hopefully that happens someday.”
Learn more about the Robert M. Buchan Department of Mining.