Heap leaching: What is it and why does the Eagle mine in the Yukon use it?

Following a landslide that spilled four million tonnes of ore at the Eagle mine in central Yukon last month — half of which escaped containment — several questions have been raised about the heap leach extraction method used by the mining company. 

So what exactly is heap leaching?

"The most common way to separate gold is to leach it: that is to say, you can filter it, keep the liquid rich in gold and dispose of the solid," said Jean-François Boulanger, a mineral engineering professor at Université du Québec in Abitibi-Témiscamingue.

"Almost all of the gold mines on the planet operate this way."

There are two main ways to extract ore through leaching: the heap method, which is used by the Eagle mine; and tank leaching, which is the most commonly used method in Canada, Boulanger said. 

Like a chocolate chip cookie

To explain heap leaching, Claude Bazin, an engineering professor at Laval University, said to imagine it like a chocolate chip cookie, with the ore as the cookie dough and the gold as the chocolate chips. 

"If you have an ore that has this behaviour, you can do what's called heap leaching — that is to say, you take your entire pile of rock and you send a solution which will dissolve the chocolate chips selectively and which will leave the rest of the cookie be," he said. 

This method, although used very little in Canada, is used in other countries like the U.S. and in Chile. 

"It's often in the middle of a very flat place, in the desert. We put a membrane beneath a pile 10 or 15 metres high, and if the pile ever seems to collapse like an under-baked cake, it won't go far," Boulanger said. 

The tank leaching method, on the other hand, extracts gold in an enclosed space, but it also requires more work, particularly to pulverize the ore. 

"In Quebec, we grind gold very, very finely and mix it with water before sending it into large vats. We add the cyanide and stir, then the reaction takes place calmly," Bazin said. 

In both methods, a cyanide-sodium solution is used to extract the ore and make it turn from solid to liquid. 

"Cyanide is really a fairly magical product, which is extremely selective for gold, which will not affect — or almost not affect — other metals," Boulanger said, adding there aren't many other products that are as selective. 

In its technical report, Victoria Gold explains that once a solution filters through the ore pile to recover the gold, it then passes into an absorption-desorption-recovery plant and then through a carbon absorption circuit. 

"The barren solution discharged from the final carbon column is pumped to the barren solution tank," the report said. 

Then by adding sodium cynanide, the cycle can continue. 

Since the landslide, mining activities have been suspended at Eagle. The containment basins for contaminated water are full and water is being recirculated onto the heap leach while storage capacity is increased. 

"So, in simple terms: the water is being recirculated between the ponds and the heap leach facility. Nothing is being circulated for the purposes of recovering gold," a spokesperson for the department of Energy and Mines said in an email. 

Neutralizing cyanide and containing contaminated water 

In his most recent report, the inspector named by the government of the Yukon to be in charge of the Victoria Gold file, Sevn Bohnet, said there are "reasonable grounds" to believe that water management at the Eagle mine violates the Yukon Water Act. 

According to the document, the mine must be able to safely contain 50,000 cubic metres of contaminated water, the equivalent of 20 Olympic pools. To do this, a pond was dug, but it does not meet government standards. 

"The size of the excavated area appears to be significantly less than 50,000 [cubic metres] and in the absence of a liner, any contaminated water deposited in the excavation can potentially infiltrate the granular materials and result in the deposit of waste to groundwater and the downstream environment," Bohnet wrote. 

The government demanded the mining company have a plan to treat the contaminated water by July 24, one month after the landslide, and that it provide the government with a list of reactive chemicals it would to use to do this. 

"We can't remove cyanide. We destroy it," Bazin said.

"We use oxygen to do this or even oxidants like hydrogen peroxide. Peroxide is H202, so it breaks down into H2O (water) and oxygen. The peroxide produces oxygen which helps break down the cyanide. It's a very aggressive oxygen that quickly destroys cyanide," Bazin added. 

Victoria Gold still has not indicated how it will treat the contaminated water. In order to protect groundwater, however, the department of Energy and Mines requires that a series of vertical wells be installed.