Beginning in the late 1880s, many of the larger Nova Scotia gold mines shifted from using the mercury amalgamation process to using a cyanidization process for extracting gold from their crushed gold ore. The main reason for this was that cyanidization was more efficient and allowed for greater gold recovery. Typically, the mercury amalgamation process extracted between 70-80% of the gold from the ore while the cyanide method recoveries were generally in the low 90% range. Cyanidization was also cheaper and was able to handle a greater volume of ore.

As with the mercury amalgamation process, with cyanidization the gold ore was crushed to sand-size in the mill. Since gold is very inert chemically, only a few chemical reagents will dissolve it. One of the most efficient is cyanide mixed with an oxidant, a mixture that readily dissolves gold. Following crushing, the gold ore was passed through the cyanide reagent which dissolved the gold. This gold-rich cyanide mix was then treated chemically to precipitate the gold and the cyanide was returned to the mill for reuse. Historically, once the gold was removed from the ore, the remaining material – called tailings – was transported to the nearby tailings pond. Unfortunately, these tailings contained a residue of the cyanide which exacted a toll on the surrounding environment, especially any aquatic wildlife. However, although cyanide is highly toxic, it readily decomposes to a non-toxic state under oxidizing conditions. Exposure to air and sunlight tended to render the cyanide residue in the tailings non-toxic as time passed.

Globally, cyanide is used in many industrial processes and most gold mines use the cyanidization extraction process. Modern gold mining operations, by law, employ methods to oxidize and decompose any cyanide residue that exists in their mill tailings prior to their discharge into the engineered tailings disposal ponds. Also by law in Canada, mining operations must reduce cyanide levels in their tailings pond water to below the allowable Canadian drinking water quality guideline level before any of the water is allowed to be discharged into the environment.
  
Art Gallery of Nova Scotia, Dian Day, Susan Sellers, Rita Wilson
1880 - 2013
Nova Scotia, CANADA
© 2013, Art Gallery of Nova Scotia. All Rights Reserved.

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