|The iconic C shaft of Giant mine.|
Giant mine is an underground mine a few kilometres outside Yellowknife in the Northwest Territories. It started operations in 1949 and ended major operations in 1999. During the 50 years of operations, it produced over 7 million ounces of gold, making it one of the most productive gold mines in Canadian history. However, the large majority of the gold mineralization was refractory (i.e. hard to recover by traditional methods) as the gold was encapsulated in arsenopyrite minerals. Thence, in order to recover gold, the additional step of roasting was necessary, a process that oxidizes arsenopyrite to transform it into iron oxides, arsenic trioxide and sulphur dioxide:
2FeAsS + 5O2 = Fe2O3 + As2O3 + 2SO2
The iron oxides (maghemite and hematite) would retain the gold and be processed through cyanide leaching, but the arsenic trioxide and sulphur dioxide would escape straight out the roaster stack.
While sulphur dioxide emissions is the main industrial contributor to acid rain, the largest concern with this process would have to be the production of arsenic trioxide. It is one of the most toxic forms of arsenic, a mere few grams of which can kill a human adult at once!
Surprisingly, the creation and release of arsenic trioxide in the environment was not considered in the mine development plans. So during the first two years of operations, arsenic trioxide was let to vent out of the roaster with no recovery mechanisms, resulting in the emissions of over 7 tonnes per day of arsenic trioxide directly into the air.
|Arsenic trioxide emissions from the Giant mine roaster (Wrye, 2008). tpd stands for tonnes per day.|
|The smoke coming out of the stack is mainly arsenic trioxide dust.|
These 20,000 tonnes, however, represent only about 8% of all the arsenic trioxide ever produced at Giant mine, the other 92%, or 237,000 tonnes, were captured by precipitators and were stored in select underground mine chambers.
|A schematic diagram of arsenic trioxide dust management from 1951-1999.|
These chambers were originally selected because they were within permafrost areas. However, five decades of mining altered the geothermal gradient and none of these chambers are now frozen, which is a real problem since arsenic trioxide is soluble in water. Therefore, without prompt action, we could see 237,000 tonnes of arsenic trioxide dissolving and dispersing in the surrounding groundwaters.
|Biomats found on bulkheads of paste backfills in Giant mine. These biomats are made of arsenite oxidizing bacteria, feeding on the high concentrations of arsenic in the leaching waters.|
|The first storage chamber successfully frozen.|
|The roaster complex was bagged to avoid spreading contaminated dust during demolition .|
|Demolishing the roaster|
|Contaminated building materials are stored in ship containers.|
|Ship container are stored next to tailings ponds, waiting permanent storage in underground chambers.|
The remediation plan is estimated to cost $900 Million, paid for by the Canadian tax payer. It is a complex and comprehensive plan that manages the most pressing issues, avoiding the tragic release of hundreds of thousands of tonnes of arsenic in the environment, and cleaning up a very large portion of the mine property. However, the freezing system and the planned water treatment plant will require maintenance in perpetuity, a cost and burden bequeathed to generations yet unborn.
Furthermore, no one knows how the frozen chambers will fare with an increasingly retreating permafrost and the effects of climate change being felt very seriously in these latitudes.
The remediation plan also does not include cleaning up contaminated areas beyond the mine property. The 20,000 tonnes of arsenic trioxide dust that were emitted from the roaster stack spread in the region, contaminating local soils and lakes. The extent of this regional surface contamination is still being studied.
Here is a video that further explores the topic:
Bissen, M., Frimmel, F. H. (2003). Arsenic—a Review. Part I: Occurenc, Toxicity, Speciation, Mobility. Acta Hydrochim. Hydrobiol., 37(1). 9-18.
Indian and Northern Affairs Canada (INAC) (2007). Giant mine remediation plan. Report of the Giant mine remediation team-Department of Indian Affairs and Northern Development as submitted to the Mackenzie Valley Land and Water Board (MVLWB).
Wrye, L. (2008) Distinguishing between natural and anthropogenic sources of arsenic in soils from the Giant Mine, Northwest Territories, and the North Brookfield Mine, Nova Scotia. M.Sc.Thesis, Queen’s University, Kingston, ON.