Frozen Block Method

There are 14 underground chambers and stopes (mined out cavities) that contain the arsenic trioxide dust. 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 freezing will occur in stages over a number of years to ensure that the chambers and surrounding rock are completely frozen. 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.

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 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 cold air during the winter. The system uses pressurized carbon dioxide (CO₂). The CO₂ is in a gas state underground, but changes into a liquid when it reaches the colder surface air. Because the liquid is heavier than the gas, it drops back down underground, where it is warmed up and becomes a gas again. Because of this continuous cycle, thermosyphons do not require an external source of power.

Thermosyphons are commonly used in the North to keep ground frozen. For example, thermosyphons are used in the parking lot of the NWT Legislative Assembly to prevent thawing of the natural permafrost. They are also used to maintain frozen core dams at the BHP Ekati Diamond Mine in the NWT.

The Freeze Optimization Study

While the remediation plan proceeds through the regulatory process, the Giant Mine Remediation Project Team is conducting a Freeze Optimization Study at Giant Mine on one of the chambers. This study is a small-scale version of the ground freezing process which is proposed in the remediation plan. The Project Team will use the results to inform future design decisions for the frozen block method. The optimization study has also been able to provide information about the frozen block method such as power requirements, rate of freezing, as well as more accurate cost estimates.

Construction began in June 2009 and the study became operational in early 2011. Since then, the rock surrounding the chamber under study has frozen faster than predicted. The study provides real time information on the speed of freezing and what types of technologies and methods work best in the northern environment. By varying the freezing process, the project team can monitor what happens when changes occur with the system; for instance, to study how fast the rock freezes under various combinations of passive and active cooling processes.

Stay tuned for updates on the Freeze Optimization Study from the Project Team at community meetings, in the media and on this website.

Frozen Block Method