Mining has always played an important role in Alaska's past and will almost
certainly impact its future.
Our state is rich in mineral resources that may be located near communities or
other valued areas. Mining developments can impact lands that have traditionally
been used for other purposes.
Mines can also have economic and social impacts on people and communities.
Educating yourself and your community about the full cycle of a mining development.
and the opportunities for individual and community participation
can help to assure that your needs and concerns are properly addressed.
Understanding potential benefits or impacts is important to the decision
making.
Mining operations may be conduced at the surface or underground.
Common surface mining methods such as strip mining and open pit mines,
both used in Alaska, are appropriate where economically marketable deposits
of minerals are found near the surface.
There are many different ways to extract minerals from the ore that
contains them.
Mines in Alaska employ several common processes for surface mining
operations.
However, all lines are unique and it is important to know the particulars of any
proposed project.
The Fort Knox Goldmine, located about twenty five miles northeast of Fairbanks,
Alaska
is an active surface or open pit goldmine.
The deposit is very low grade. A small amount of actual gold in a large amount
of rock,
only approximately 0.024 ounces of gold for every ton of
material
This low grade is accompanied by very low sulfur content, meaning less potential
for chemical reactions that produce sulfuric acid and potential acid rock
drainage, ARD.
ARD is an environmental concern at mines where the ore contains sufur bearing minerals.
minerals. For more information on ARD
visit the American Geological Institute website at http://www.agiweb.org/environment/publications/metals.html
The surface mining process at Fort Knox begins with the removal of the soil and
unmineralized rock overlaying the ore deposit.
Topsoil and other usable materials are stored and will be used in stabilizing
slopes and revegetating the area,
reclamation
when the minus closed.
Waste rock and materials from the overburden and other digging
activities are placed in dumps located away from the pit
out of the way of future activities.
These dumps will be contoured, seated, and allowed to restore themselves.
Open pits are dug downward in benches that are connected by gently sloping ramps.
Bench size is determined by the size of the equipment used
and the larger and deeper the pit,
the larger the equipment that is generally used.
At Fort Knox, working benches at the pit bottom are several hundred feet wide and
thirty three feet high
accommodating up to 240 ton haulage trucks and
large excavating equipment.
Each day explosives are detonated in blast holes, fracturing the gold bearing rock.
The broken ore with the highest gold content
is hauled to a crusher. This goes around. This sets the size of the rock.
This open beam is six inches so the rock comes down,crushes each rock. and comes down through the bottom.
Here the ore is crushed into pieces less than six to eight inches across.
The lower-grade ore is hauled directly to a large pile known as the heap leach pad.
Material on the heap is irrigated with a very dilute cyanide solution
that seeps through the pile and dissolves the gold.
The goal bearing solution from the heap is pumped back to the plant where the
goal is recovered.
The higher grade ore that has been crushed is ground up further in a mill.
Milling pulverizes into sand-sized smaller pieces, then is mixed with water to
make a slurry.
This slurry is about consistency of beach sand.
Some of the gold can be separated directly from this slurry,
but some needs to be removed by leaching.
The slurry is thickened and passed through tanks where a cyanide solution
dissolves the remaining gold.
Eventually the gold bearing solution from the leach tanks is combined with that
from the heap leach pad.
The gold bearing solution is then separated from the solids.
The gold precipitated out of solution by an electrical process onto plates.
The gold is collected from the plates, melted, and poured into molds to form gold
bars.
The gold bars produced at Fort Knox average 90 percent gold,
six to eight percent silver and two to four percent other impurities.
These bars are the main product of gold mines.
The non gold bearing material leftover from processing the slurry
is a waste called tailings.
To ensure that this waste is safely disposed and meets federal and state
standards, Fort Knox removes the residual cyanide.
Cyanide easily breaks down into harmless compounds in the presence of oxygen
and sunlight.
After the tailing are detoxified, they are disposed of in a man-made lake.
Water from the tailings slurry is then collected and circulated back to the mill for
reuse.
Water used in ore processing at Fort Knox never leaves the mine site.
Fort Knox is one example of a modern mining operation.
All mines have unique characteristics depending upon their environmental
setting
and the nature of the ore they produced,
however, all operate in a cycle from exploration, to production, and eventually
closure when the mining stops.
Understanding the processes and procedures used in a mines life-cycle
prepares you for the participation in the permitting and decision making
processes.
These processes are there to guide exploration,
development, production,
and close,
and are in place to ensure that the things you and your community value
are protected.
UAF mining extension provides research based information and resources,
balancing mining impacts
and opportunities for the environment and neighboring communities.
The purpose of this project is to inform stakeholders and the interested public about
the processes and procedures involved in mineral development in Alaska.
With an increased understanding, you and other concerned parties are better
prepared to effectively participate in the permitting
and evaluation process
designed to protect the environment and identified community values.