Mercury Management in Modern Precious Metals Mines

1
Mercury Management in Modern Precious Metals Mines

• Glenn Miller and Greg Jones
• Department of Natural Resources and Environmental
  Science
• University of Nevada, Reno

2
Purpose of the Study

• Determine the total amount of byproduct mercury
  produced and how it is managed
• Determine and analyze the atmospheric emissions
  both from the Toxic Release Inventory and the
  Voluntary Mercury Reduction Program in Nevada

3
Historic Use of Mercury in Precious Metals Mines

• Gold will dissolve in liquid mercury and this
  process was extensively used for recovery of gold
  from ground ore
• Generally utilized plates covered with mercury to
  which an ore slurry was passed over
• Estimated 1 lb of mercury lost for each ton of
  ore processed (Comstock)
• Tailings often eroded into streams or were
  deposited directly into streams
• Metallic mercury converted to ionic mercury and
  then methylated to make the toxic methyl mercury

4
Mercury Legacy from Historic Mines

• Elevated concentrations of mercury in sediments
• Elevated concentrations of mercury in fish, birds
  and other wildlife (Lahontan, Carson sink, Walker
  Lake, Sierra Nevada streams and others)
• Restrictions on the amount of fish that should be
  consumed by humans

5
Toxic Issues with Mercury

• Neurotoxicity
• Birth defects
• Kidney function

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Current Gold Mines

• Mercury is not used to extract gold or silver in
  Nevada mines (although still in developing
  countries)
• Mercury release to the environment is related to
  the co-incidence of mercury in many gold ores in
  Nevada, and release during ore processing
• Mercury is produced as a byproduct from gold
  mines in Nevada, and is the largest source of new
  mercury in the U.S.

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Byproduct Mercury Production
The Chemistry
Au O2 CN- Au(CN)2- (water soluble)
Hg O2 CN- Hg(CN)2
Hg(CN)42- (water soluble)
Both the gold and mercury cyanide complexes are
trapped on carbon and recovered during
processing Mercury is distilled (retorted) from
the gold and collected as liquid mercury and sold
by the flask (76 lbs)
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Previous records for Hg Productionfrom the
Nevada Division of Minerals
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Total Mercury By-Product Recovered for Sale
(pounds and metric tons) (provided by each
company)
total Newmont byprodut (Nev. Div. Of Minerals)
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How is Byproduct Managed

• Sold or transferred to three primary mercury
  recyclers
• Bethlehem Apparatus Co. Inc., Hellertown, PA.
• D.F Goldsmith Chemical and Metal Corp, Evanston,
  IL
• Mercury Waste Solutions, Inc., Mankato, MN
• Each of these companies is regulated by the EPA
• Current U.S. demand for mercury is 150-200
  tons/year Hg produced in NV is 30-50 tons
• Price of Hg increased over the past three years
  from 150/flask to approx. 750/flask
• Management and transfer is conducted in a safe
  and reliable manner
• There is no requirement for supplying data to the
  public on mercury sales

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Atmospheric Emissions of Mercury from Precious
Metals Mines in the U.S.

• Generally not measured until 1998
• Toxics Release Inventory was applied to precious
  metals mines
• Revealed that precious metals mines were some of
  the single largest sources of atmospheric mercury
  in the U.S.
• Voluntary Mercury Reduction Program (VMRP)
  initiated in 2001

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Mercury Air Emissions (lbs) from Precious Metals
Mines, 1998-2003 (TRI data)
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Ranking of States for Atmospheric Mercury Release
NV Mines 2003 (est) 4500 lbs
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Atmospheric Mercury Releases

• Nevada was the 2 state nationally in mercury
  release in 1998 (after Texas) and dropped to 7
  or 8 in 2003 and reduced its atmospheric
  emissions by 2/3
• Nevada still is the second largest source of
  atmospheric mercury west of the Mississippi
  (again after Texas)

15
VMRP Data from Jerritt Canyon (2001-2003)
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VMRP Data from Barrick Goldstrike
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Value of production up by 11 from 2002 due to
higher price
Top 7 years of all time
7.3 M oz in 2003, 82 of U.S. and 9 of world
production
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Estimate of Total Amount of Mercury Emitted from
Nevada Mines, 1980-present

• Assume an average of 12,000 pounds per year from
  1988-2001 (inclusive), and 5000 pounds for 2002
  to 2004 (inclusive), and ignore the years
  1980-1988.
• 12000 x 14 168,000 lbs
• 5000 x 3 15,000 lbs
• Total is 183,000 lbs or 83 metric tons (70-100
  tons)
• Contributions of Hycroft, Paradise Peak and Hog
  ranch are unknown

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What is the Significance of this Mercury Release

• Largely unknown, although it is much less than
  the estimated 6000-7000 metric tons released into
  the Carson River drainage
• Transport of mercury is still not well
  understood, although the areas around the sources
  will receive the greatest amount of fallout
• It is unclear if the elevated mercury in Idaho
  fish is related to mercury releases from the
  mines

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23 major gold mines (8 not on the Carlin trend
with gt 100 K oz production)
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Conclusions

• Management of byproduct mercury
• Byproduct mercury appears to be managed well
• Based on discussions with the recyclers, they
  feel strongly that a demand for mercury in the
  U.S. exists and byproduct mercury can be used to
  satisfy this need, primarily in lights and
  electrical equipment
• There is currently no legal method for reducing
  liability to the mining industry for storing
  mercury, while selling it into commerce
  eliminates the liability
• No need for any changes

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Conclusions

• Atmospheric release of mercury
• The gold mining industry previously was a major
  source of mercury release to the atmosphere
• Through the VMRP and other efforts by the
  industry, a 2/3 reduction has been realized,
  although mining sites still are some of the
  highest point sources
• Further reductions can be realized with current
  technology

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Recommendations

• Atmospheric mercury emissions from mines should
  be regulated by either the State or EPA. While
  successful, the VMRP remains voluntary, and does
  not require new sources to participate.
• More consistent and more mercury measurements
  should be required. Because of the complexity of
  the mercury emission sources, a systematic
  evaluation of the methods used to determine
  mercury emissions rates and concentrations should
  be undertaken.

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Recommendations (2)

• New systems for better mass balance are
  recommended for accurate assessments of mercury
  release. This includes more precise measurements
  of mercury in the ore, mercury in the process
  fluids, and mercury sent out to the tailings
  facilities. The amount of mercury in the ore
  should be accounted for in a life-cycle
  assessment.
• Byproduct mercury production and sales should be
  reported
• Fate studies for the mercury deposition need to
  be expanded. Is the mercury deposition regional
  or continental, or global? Who has the burden of
  showing where the mercury goes?