Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 270

TABLE 6.3
(Continued)

Technology (and Type*) Short Description Basic Reagents Basic Products

Pregnant Pulp Air Stripping (P) Air stripping from pregnant pulps Air HCN

Reverse Osmosis (P) Physical removal of cyanide and its complexes by a

semipermeable membrane process under pressure

H 2 SO 4

CN

Flotation (P) Adsorption of precipitated CN particles onto fine air bubbles FeSO 4 , Surfactant Fe 4 [Fe(CN 6 )] 3

High Rate Thickeners (P) Fast thickening and recycling of CIP tailings none

CN

Source:
Environment Australia 1998

There are several reasons why natural degradation may not be appropriate as the sole
means of cyanide control. These include:
●

An excess of water, requiring discharge without sufficient residence time to achieve
the required degradation;

●

The risk that the tailings storage could be overtopped, discharging cyanide-bearing
solution, before sufficient degradation is achieved; and

●

The risk that animals, including livestock or wildlife including water birds are
attracted to the tailings ponds and are poisoned as a result. This risk is particularly
high in arid areas where surface water occurrences are usually rare.

Cyanide, being relatively unstable when exposed to sunlight, is generally not persistent
in the environment; however, there are some situations in which cyanide can persist. One
example is in groundwater; away from sunlight, cyanide may persist for many years if the
solution remains alkaline and there are no other reactions to form complex metal precipi-
tates. A case of persistent cyanide is described in Case 6.3.
Dilution is not commonly used on its own but may be used in combination with other
processes, or to provide an additional factor of safety. In the event of an accidental spill of

Away from sunlight, cyanide may
persist for many years if the
solution remains alkaline and
there are no other reactions to
form complex metal precipitates.

CASE 6.3
Cyanide and Groundwater

Montana is the only state in the USA to have implemented
a ban on cyanide leaching of gold ores. Three primary
reasons have been given for the decision to phase-out
existing projects and prohibit new developments involving
cyanide leach mining in Montana:
1. Open pit, cyanide leach mines threaten the pri-
vate property rights of neighbouring landowners (e.g.
Landowners downstream of the Golden Sunlight mine were
forced to sell their properties to Placer Dome Corp. after
their drinking water well was contaminated with cyanide)
2. Open pit, cyanide leach mines expose Montana tax-
payers to the costs of reclamation and leave liabilities for

future generations (e.g. Pegasus Gold Corp. declared
bankruptcy in 1997 leaving the State with insuffi cient
funds to reclaim the Zortman/Landusky mine, Montana's
largest gold mine. The State has estimated that water
treatment will have to occur at the mine site in
perpetuity).
3. Open pit, cyanide-leach mines consistently contami-
nate Montana's water resources with cyanide and other
pollutants placing human and environmental health at
risk. Since 1982, there have been 50 cyanide releases at
Montana mines, releasing millions of gallons of cyanide
solution into Montana's soil, surface and groundwater

resources. Cyanide can persist for very long periods of
time in groundwater because the sunlight and oxygen
needed to break it down to less harmful substances are
largely absent. Groundwater contamination is the most
prevalent form of cyanide contamination at Montana's
open pit cyanide leach mines because the liner systems
designed to prevent this type of occurrence are not imper-
meable and are prone to structural damage (punctures
or tears).

Source: Montana Environmental Information
Center (MEIC), [e-mail]