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Extracting the Gold

Detail of a black and white photograph showing a man cleaning the plate amalgamator of a stamp mill.

In the hard rock gold mines of Nova Scotia, the gold is extracted from the ore first by pulverizing the rock and then using chemicals to remove the gold from the other minerals.

Mercury

Once the ore is crushed into fine sand by the stamp mills it is flushed down a copper plate that is coated with a thin film of mercury on the surface.

Flow diagram showing the mercury-gold extraction process

Diagram showing mercury-gold extraction process.

A blanket can be used to capture gold missed by the mercury-coated copper plate. The ripples of the blanket catch the heavier gold as it is washed over the surface, while the lighter sand is washed away. The washings from these blankets are added to an amalgam barrel, where the gold forms an amalgam with the mercury. The resulting slurries from both the amalgam plate and amalgam barrel are then boiled, evaporating the mercury, leaving behind the gold. Some mercury boils off into the air and some is recaptured and used again.

The impure gold that remains is heated in a furnace to melt the gold into bars or ingots. The slag that remains in the furnace may be sent through the extraction process again to ensure no gold is lost. The sand that is left behind following the blanket and amalgam barrel steps are called tailings and dumped.

The resulting slurry is then boiled, evaporating the mercury, leaving behind the gold. Some mercury boils off into the air and some is recaptured and used again.

The mercury sticks to the gold to form an amalgam – an alloy of mercury and gold in this case - allowing very small quantities of gold to be captured. The resulting slurry is then boiled, evaporating the mercury, leaving behind the gold. Some mercury boils off into the air and some is recaptured and used again.

Diagram showing how the mercury amalgam was boiled to separate the gold from the mercury, which was collected for re-use.

Diagram showing how the mercury amalgam was boiled to separate the gold from the mercury. The mercury was collected for re-use.

A mercury amalgam retort (a sturdy iron pot in which mercury-gold amalgam is boiled to separate the gold from the mercury) is sealed except for a pipe attached to the top of the retort. This pipe allows the mercury vapour to escape. A water jacket around the pipe allows the mercury to cool and condense and the resulting liquid mercury can be collected for re-use. Cold water continually circulates: cold water enters the water jacket through the inlet while warm water leaves by the outlet, allowing the pipe to maintain a consistent temperature. The gold remains behind in the retort.

Photograph of a rusted cast iron container called a 'mercury separator'.

Mercury Separator. Cast iron container called a 'separator', used to separate gold from mercury.

MOI, I90.15.17 a-c

About 10-25% of all mercury used was lost in tailings. Tailings are the fine rock particles and water left over after the gold has been removed.

Photograph of two men collecting mercury from a retort.

Decanting (recollecting) mercury from a retort (at right) in a mill at the Oldham Gold District.

N.S. Department of Natural Resources Historical Mine Photo Collection:

Cyanide

In the late 1800s, cyanide started to be used to concentrate and capture gold from ore in a similar process to the mercury extraction, but is more efficient and loses less gold to the tailings. Once the rock is pulverized, it is mixed with water to create a slurry and a cyanide solution is added. This aqueous cyanide solution dissolves the gold and can then be extracted using a number of different methods.

Black and white photograph of the Seal Harbour Gold Mines Ltd. cyanide mill circa 1930s.

Cyanide mill at the Seal Harbour Gold Mines Ltd. Mine, Lower Seal Harbour Gold District in the 1930's.

N.S. Department of Natural Resources Historical Mine Photo Collection.

The first application in a Canadian gold mine of the cyanide-leach process of gold extraction was at Brookfield Mine in 1893 and it is a process still used today.