Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2

• Description: Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2
• State: CO
• Date: 3/1/2005
• Title: Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2
• Water Supply Pro - Doc Type: Publication

ON THE GROUND c(onenued)
New Approach to Address
Acid Rock Drainage
Gregory J. Olson and Thomas R. Clark
— Little Bear Laboratories Inc., and Terry L
Mudder — TIMES Ltd.
Mine remediation costs are dramatically
increased when acid rock drainage (ARD)
is present. Consequently, mining engineers
have learned it is best to inhibit ARD
production before it begins through proper
waste management and source control.
ARD results from uncontrolled oxidation
of metal sulfides, primarily pyrite (FeS2)
in waste rock, tailings, and spent ore. Both
oxygen and microorganisms are important
in this process. Pyrite exposed to oxygen
and water slowly oxidizes, producing
ferrous sulfate and sulfuric acid. As the
pH becomes more acidic, microorganisms
greatly accelerate the oxidation of ferrous
sulfate to ferric sulfate, a strong oxidant
that readily oxidizes more pyrite and
other metal sulfides. The end result is
production of drainage
waters that have low
pH, high concentrations
of dissolved solids, and
toxic metals.
With funding from the
U.S. Environmental
Protection Agency's
Small Business Innovative
Research Program and
a consortium of eight
international mining
companies, Little Bear
Laboratories Inc. and Placement of waste rock in test pits at Red Dog Mine, Alaska. A
TIMES Ltd. are evaluating loaded and treated pit in the foreground shows wire leads from a
thermistor placed in a vertical pipe for temperature measurements.
a method for ARD source
control that focuses on stopping the
growth of iron - oxidizing microorganisms
and minimizing the chemical oxidation of
pyrite and other sulfide minerals.
Controlling Biooxidation of Iron
Iron - oxidizing bacteria at low pH are
extremely sensitive to thiocyanate
10 • March /April 2005 • Southwest Hydrology
(SCN-), with only a few parts per million
in solution being lethal. Thiocyanate is a
selective biocide, and is much less toxic
to "normal" microflora and higher forms
of life. It is relatively stable at low pH
and under anaerobic conditions, and at
neutral pH under aerobic conditions it is
biodegraded to carbon dioxide, sulfate,
and nitrate. Thiocyanate is a component
of wastewater solutions at many precious -
metal mines, therefore these solutions
could potentially be used for ARD control
following selective treatment to remove
constituents other than thiocyanate, such
as cyanide and metals.
Controlling Chemical Oxidation
Even if all biooxidation of iron is
inhibited, pyrite and other metal sulfides
may still be oxidized by oxygen and ferric
iron. However, this chemical oxidation can
be greatly reduced by adding phosphate
to sulfidic mine waste. Phosphates
neutralize acidity and precipitate ferric
iron and aluminum ions as phosphates.
The removal of iron and aluminum from
solution as phosphates precludes acid
production by precipitation of these metals
as hydroxides.
Furthermore, iron phosphate precipitation
may also coat unreacted pyrite, further
preventing its oxidation. Phosphate is a
relatively benign and inexpensive chemical
available worldwide.
In laboratory tests, combined thiocyanate