| AMD occurs when sulphide-bearing minerals in rock are exposed
to air and water, changing the sulphide sulphur to sulphuric acid. This
acid can dissolve heavy metals found in waste rock and tailings such as
lead, zinc, copper, arsenic, selenium, mercury, and cadmium, into ground
and surface water. Certain bacteria, naturally present, can significantly
increase the rate of this reaction. AMD and heavy metals pollution can poison
ground and drinking water. AMD can destroy aquatic life and habitat. Silver
is one ore body commonly mined that poses an AMD risk. Acid generation results
from exposure to air and water. This means that the more surface area of
rock exposed, the greater the amount of acid. During the mining process,
hundreds, sometimes thousands of tons of rock are dug up and crushed each
day. The acid then leaches through the ground and releases heavy metals
such as lead, zinc, copper, arsenic, selenium, mercury and cadmium. Acid
mine drainage can develop at several points throughout the mining process:
in underground workings, open pit mine faces, waste rock dumps, tailings
deposits, and ore stockpiles. Acid generation can last for decades, centuries,
or longer, and its impacts can travel many miles downstream. Roman mine
sites in Great Britain continue to generate acid drainage 2,000 years after
mining ceased. |
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New technology which uses microorganisms for the recovery of silver from
metal-containing streams is currently being implemented. This technology
is targeted at using naturally-occurring, sulfate-reducing bacteria for
the recovery of silver from ores, instead of the more traditional -- and
more toxic -- cyanidation process. Aerobic bacteria catalyze bio-oxidation
of low-grade, hard to treat (refractory), sulfidic silver ore. This step
is currently being adopted by silver producers for freeing silver from base-metal
sulfides (e.g., pyrite). A waste stream from the bio-oxidation step is used
in a second step. Next, naturally-occurring, sulfate-reducing bacteria are
used to convert the dissolved sulfate in acid mine drainage into a dissolved
bisulfide leaching agent and to neutralize the acid mine drainage. These
bacteria can use wood alcohol, grain alcohol or vinegar as food, and they
are also capable of consuming hydrogen produced by the gold dissolution
process. Silver dissolves in the bisulfide solution and is recovered with
activated carbon or zinc dust. If needed, excess sulfur can be recovered
as a byproduct. By using the natural sulfur cycle, the process provides
a complete solution to the silver recovery problem. This process has several
benefits. It is more environmentally friendly--the bisulfide leaching agent
used is about 200 times less toxic than cyanide. Since its invention in
1899, cyanidation and its variants have been the processes of choice for
extraction of silver from oxidized ores. Cyanide, however, is legendary
in its toxic potency. Furthermore, with increasing awareness for the environment
worldwide, containment, treatment costs and time spent on environment impact
studies associated with cyanidation plants have skyrocketed. These factors
have raised the economic hurdle necessary to justify a working mine. Environmentally
acceptable alternatives could broaden the definition of an attractive mine,
through reduction of the economic and environmental risks. In addition,
preliminary results indicate chemical reagent costs could be 80 percent
lower than cyanide.
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