*Basic knowledge for miners*
• Some basic technical skills are required to successfully and safely operate a cyanide leach process.
o The process needs to be controlled and there are relatively simple tests to do this (e.g., pH paper, blackening of zinc strips in sock, etc.)
• Under the correct conditions, cyanide will dissolve the gold from the rock leaving it in the water.
o To leach gold, there are four components required:
• water, cyanide, air (oxygen) and alkalinity (high pH).
• If one of these components is missing, the process will not work.
• While there is usually enough oxygen in the water, an alkaline such as lime must be added along with the cyanide
• There are three main types of processes that use cyanide to recover gold
o *heap leaching, vat leaching and agitated tank leaching.*
• After the gold is leached, it is recovered from the water (called the pregnant solution) using one of two processes: **zinc precipitation or carbon
adsorption* .
• Only a relatively small amount of cyanide is needed to recover gold,
usually less one kg of cyanide per tonne of rock
• Cyanide leaches coarse and very fine gold as well as gold that is attached
to the rock
• The process is quick; tank leaching normally takes less than one day
• Adding excess cyanide does not increase the amount of gold recovered
• Although cyanide selectively leaches gold, it can also react with copper, iron, silver and mercury. When these metals react with cyanide, they
consume it, meaning there is less cyanide available for leaching the gold
o Some ores require more cyanide because the cyanide reacts with other metals in the ore
Ores with copper minerals such as malachite and azurite cause the largest problems because the copper in these minerals reacts quickly with cyanide
• Free cyanide is very toxic and is readily absorbed through inhalation, ingestion or skin contact and is distributed throughout the body via blood.
Cyanide stops cells from absorbing oxygen, so death results from the
depression of the central nervous system
• Cyanide is highly toxic, and at high concentrations it will kill fish, birds and mammals (including humans).
o If used responsibly, the risk of cyanide poisoning can be minimized
• Again, if the leach is discharged to a stream there is a risk that it will kill fish, contaminate wells, poison drinking water and possibly kill people.
• It is important to contain cyanide waste in a sealed pond—recycling the cyanide containing water is good for the environment and helps leach more gold
