Arsenic Trioxide and the Frozen Block Method
The arsenic trioxide waste was created during the production of more than seven million ounces of gold between 1948 and 1999. The rock mined at Giant is rich in gold and arsenopyrite, a mineral that has a high arsenic content. When the ore was roasted to release the gold, arsenic was also released as a gas. As the gas cooled, it became arsenic trioxide waste. During the life of the mine, this process created 237,000 tonnes of arsenic trioxide waste, which was stored underground.
When the mine was still in operation, the original underground storage method relied on the area's natural permafrost, which worked as a frozen barrier. It was believed that when the time came to close Giant Mine, permafrost would reform around the storage chambers and stopes, and seal in the arsenic trioxide.
However, ongoing mining activity in the area caused the permafrost to thaw. This has resulted in water seeping in and out of some of the underground storage areas. All contaminated water from the mine is currently being collected and pumped to a water treatment facility above ground on site. The contaminants in the water are removed through a treatment process before the water is released into the environment.
There are 15 underground chambers and stopes (mined out cavities), 14 of which contain the arsenic trioxide waste. These will be carefully frozen to create an impenetrable barrier that will prevent water from entering the chambers and arsenic from leaving the chambers. The process used is referred to as the Frozen Block Method (below).
The department will ensure that the site is safely managed throughout the entire process, and long-term, regular monitoring of the chambers and stopes will continue after the freezing is complete.
How the frozen block will be created
Freezing the arsenic trioxide will occur in stages over a number of years to ensure that the chambers and surrounding rock are completely frozen.
The freezing will be achieved using a combination of active and passive freezing systems. The active freeze systems circulate cooled liquid through a series of underground pipes to freeze the designated areas around and within each of the chambers and stopes. This system is very similar to what is used to freeze ice in indoor rinks.
The passive systems will then keep the ground frozen. This will be done by using thermosyphons, which are tall, metal tubular devices that take the heat out of the ground and releases it into the air. The system uses pressurized carbon dioxide. The carbon dioxide is in a gas state underground, but changes into a liquid when it reaches the surface air. The liquid, which is heavier than the gas, drops back underground where it is warmed up and becomes a gas again and rises. Because of this continuous cycle, thermosyphons do not require an external source of power.
Thermosyphons are commonly used in the Northwest Territories to keep ground frozen. For example, thermosyphons are used in the parking lot of the Legislative Assembly to prevent thawing of the natural permafrost. They are also used to maintain frozen core dams at the BHP Ekati Diamond Mine.
Find out how the frozen block method will be implemented.
Why was the Frozen Block Method chosen? Was it the cheapest option?
The least expensive way to manage the arsenic trioxide waste would be to continually pump out the contaminated ground water and treat it through a water treatment plant. The safest way to manage the arsenic trioxide waste is to freeze it where it is, contain it and keep it where it cannot contaminate the underground water because the ground will be frozen solid. Taking the waste out and bringing it to surface is too risky - for the workers who would do it; for the nearby communities; and because it would have to be stored on surface creating another area of contamination.
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